Complex and process

Abstract

The invention relates to ruthenium complexes which may find use in the field of organic chemistry. Specifically, the present invention relates to ruthenium complexes which may be used in the reduction or hydrogenation of esters to alcohols. The invention further relates to a process for reducing or hydrogenating an ester to form an alcohol using the ruthenium complexes of the invention.

Description

[0001] Complex and Process

[0002] Field of the Invention

[0003] The present invention relates to transition metal complexes and their use. More specifically, the present invention relates to ruthenium complexes which find use in the reduction of esters to form alcohols. The invention further relates to a process for reducing esters to form alcohols using the ruthenium complexes.

[0004] Background of the Invention

[0005] The reduction of esters is an essential transformation in the chemical industry as a route to primary alcohols. The reduction of esters has conventionally been carried out using reagents (in stoichiometric or excess quantities) such as sodium metal in ethanol (the Bouveault-Blanc reduction) or more recently with a metal hydride reagent, such as UAIH4 or NaBH4. These reduction reactions are, however, difficult to carry out effectively on a large scale, not least due to safety concerns associated with an extremely exothermic quenching step. As such, large scale catalytic reduction reactions use hydrogen gas. Cu- or Zn-based heterogeneous catalysts are used for ester reduction, primarily in the Natural Detergent Alcohol (NDA) market on very large scale. However, these methods require very high pressures and / or temperatures, in large scale, dedicated production facilities. The chemoselectivity for ester reduction compared to other sensitive functional groups can also be problematic in some cases using these methods.

[0006] The use of homogeneous catalysts for the reduction of esters has presented itself as an attractive alternative to harsh heterogeneous conditions. In particular, homogeneous ruthenium complexes comprising certain tridentate ligands (such as those disclosed in W02013 / 023307A1) have been applied with great success.

[0007] Ruthenium complexes comprising tridentate ligands (L) may typically be prepared from the ruthenium starting material, [RuCl2(PR3)3]. This starting material is prepared by reacting RuC with an excess of PR3as shown in Formula 1 :

[0008] RuC (OH) + 6 PR3^ RUCI2(PR3)3 + 2 PR3+ P(O)R3+ HCI

[0009] Formula 1

[0010] [RuCl2(PR3)3] may be reacted with a tridentate ligand (L) to produce a complex of general formula [Ru(L)(PR3)Ch] with the release of two further equivalents of PR3, as shown in Formula 2: RuCI2(PR3)3+ L [RU(L)(PR3)CI2] + 2 PR3

[0011] Formula 2

[0012] A disadvantage of the above-described method is the consumption of trialkyl or triaryl phosphines (PR3) which must be used in excess and which are lost to waste following synthesis of the ruthenium complex. For instance, PR3is produced as a by-product in 4 molar equivalents per mole of ruthenium complex formed. Furthermore, oxidation products of PR3, such as P(O)R3, are also formed, which, in addition to any excess PR3, may need to be handled as chemical waste. Accordingly, the overall atom economy of known methods is poor and result in wastage of expensive reagents (e.g. PR3) which are not incorporated into the final product.

[0013] There is therefore a need for ester hydrogenation catalysts which can be obtained in an environmentally friendly way with good atom efficiency.

[0014] Summary of the Invention

[0015] The present invention provides ruthenium complexes which may be used to provide ester hydrogenation catalysts, which can be produced with high atom efficiency and with minimal by-product formation.

[0016] In a first aspect of the present invention there is provided a ruthenium complex of formula (I) or formula (II):

[0017] [Ru(L)AZ]W (I)

[0018] [Ru(L)AZW] (II) wherein L is a tridentate ligand having formula (III): wherein: X is a nitrogen atom and when taken together with R1it forms an optionally substituted heteroaryl group when Rxis absent, or

[0019] X is -SRa, and R1and Rxare each independently selected from hydrogen, substituted or unsubstituted Ci-10-alkyl, substituted or unsubstituted C2-io-alkenyl, substituted or unsubstituted C2- -alkynyl, substituted or unsubstituted Ci-io-heteroalkyl, substituted or unsubstituted Ci- -alkoxy, substituted or unsubstituted Cs-w-cycloalkyl, substituted or unsubstituted Cs-w-cycloalkenyl, substituted or unsubstituted C2-io-heterocycloalkyl, substituted or unsubstituted Ce-w-aryl, and substituted or unsubstituted C4-io-heteroaryl;

[0020] Y is selected from -SRb, -PRaRb, -OPRaRb, and -NHPRaRb;

[0021] R2and Ry, if present, are each independently selected from hydrogen, substituted or unsubstituted Ci-10-alkyl, substituted or unsubstituted C2-io-alkenyl, substituted or unsubstituted C2- -alkynyl, substituted or unsubstituted Ci-io-heteroalkyl, substituted or unsubstituted Ci-io-alkoxy, substituted or unsubstituted Cs-w-cycloalkyl, substituted or unsubstituted Cs-w-cycloalkenyl, substituted or unsubstituted C2-io-heterocycloalkyl, substituted or unsubstituted Ce-w-aryl, and substituted or unsubstituted C4-w-heteroaryl;

[0022] R3a, R3b, R4a, and R4bare each independently selected from hydrogen, substituted or unsubstituted Ci-io-alkyl, substituted or unsubstituted C2-w-alkenyl, substituted or unsubstituted C2-w-alkynyl, substituted or unsubstituted Ci-w-heteroalkyl, substituted or unsubstituted Ci-w-alkoxy, substituted or unsubstituted Cs-w-cycloalkyl, substituted or unsubstituted Cs-w-cycloalkenyl, substituted or unsubstituted C2-w-heterocycloalkyl, substituted or unsubstituted Ce- -aryl, and substituted or unsubstituted C4-w-heteroaryl;

[0023] R5is selected from hydrogen, or substituted or unsubstituted Ci-s-alkyl;

[0024] Raand Rbare each independently selected from substituted or unsubstituted Ci-w-alkyl, substituted or unsubstituted C2-w-alkenyl, substituted or unsubstituted C2-w-alkynyl, substituted or unsubstituted Ci-w-heteroalkyl, substituted or unsubstituted Ci-w-alkoxy, substituted or unsubstituted Cs-w-cycloalkyl, substituted or unsubstituted Cs-w-cycloalkenyl, substituted or unsubstituted C2-w-heterocycloalkyl, substituted or unsubstituted Ce-w-aryl, and substituted or unsubstituted C4-w-heteroaryl, or Raand Rbtogether with the phosphorus atom to which they are attached form a heterocycle;

[0025] Z and W are anionic ligands each independently selected from the group consisting of carboxylates, alkoxides, hydride, and halides; and

[0026] A is a sulfoxide ligand.

[0027] It has surprisingly been found that ruthenium complexes of the invention, comprising a sulfoxide ligand, can be used to provide competent ester hydrogenation catalysts. Moreover, the ruthenium complexes of the invention may be produced using a more atom efficient process compared to analogous complexes where A is a phosphine ligand or a carbonyl ligand.

[0028] In a second aspect of the present invention there is provided a process for reducing an ester containing substrate to an alcohol containing substrate in the presence of a hydrogen source and a base using the ruthenium complex of formula (I) or the ruthenium complex of formula (II) of the first aspect of the invention.

[0029] In a third aspect of the present invention there is provided a process for preparing the ruthenium complex of formula (I) or formula (II) of the first aspect of the invention, the process comprising the step of reacting a ruthenium precursor complex with a tridentate ligand, L, as defined in relation to the first aspect of the invention.

[0030] Brief Description of the Drawings

[0031] Figure 1 shows a tridentate ligand of formula (III).

[0032] Figure 2 shows an X-ray crystal structure of [Ru(bis(2-(ethylthio)ethyl)amine)(dimethyl sulfoxide)Cl2], a complex according to the invention.

[0033] Definitions

[0034] The point of attachment of a moiety or substituent is represented by For example, -OH is attached through the oxygen atom.

[0035] “Alkenyl” refers to a straight-chain or branched unsaturated hydrocarbon group comprising at least one carbon-carbon double bond.

[0036] “Alkoxy” refers to an optionally substituted group of the formula alkyl-O- or cycloalkyl-O-, wherein alkyl and cycloalkyl are as defined below.

[0037] “Alkyl” refers to a straight-chain or branched saturated hydrocarbon group. The alkyl group may be unsubstituted. Alternatively, the alkyl group may be substituted. Unless otherwise specified, the alkyl group may be attached at any suitable carbon atom and, if substituted, may be substituted at any suitable atom. Typical alkyl groups include but are not limited to methyl, ethyl, n-propyl, iso-propyl, n-butyl, iso-butyl, sec-butyl, tert-butyl, n-pentyl, n-hexyl and the like. “Alkynyl” refers to a straight-chain or branched unsaturated hydrocarbon group comprising at least one carbon-carbon triple bond.

[0038] “Aryl” refers to an aromatic carbocyclic group. The aryl group may have a single ring or multiple condensed rings. The aryl group may be unsubstituted. Alternatively, the aryl group may be substituted. Unless otherwise specified, the aryl group may be attached at any suitable carbon atom and, if substituted, may be substituted at any suitable atom. Examples of aryl groups include, but are not limited to, phenyl, naphthyl, anthracenyl and the like.

[0039] “Cycloalkenyl” refers to an unsaturated, non-aromatic carbocyclic ring. The cycloalkenyl group therefore has at least one carbon-carbon double bond, but may have more. In certain embodiments, the cycloalkenyl group may have from 3-15 carbon atoms, in certain embodiments, from 3-10 carbon atoms, in certain embodiments, from 3-8 carbon atoms. The cycloalkenyl group may be unsubstituted. Alternatively, the cycloalkenyl group may be substituted. Unless other specified, the cycloalkenyl group may be attached at any suitable carbon atom and, if substituted, may be substituted at any suitable atom. Typical cycloalkenyl groups include but are not limited to cyclopropenyl, cyclobutenyl, cyclopentenyl, cyclohexenyl, and the like.

[0040] “Cycloalkyl” refers to a saturated carbocyclic hydrocarbon radical. The cycloalkyl group may have a single ring or multiple condensed rings. The cycloalkyl group may be unsubstituted. Alternatively, the cycloalkyl group may be substituted. Unless other specified, the cycloalkyl group may be attached at any suitable carbon atom and, if substituted, may be substituted at any suitable atom. Typical cycloalkyl groups include but are not limited to cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, adamantyl and the like.

[0041] "DMSO” refers to dimethylsulfoxide.

[0042] “Fatty acid” refers to a carboxylic acid with a long aliphatic chain (e.g. >6 carbon atoms), which can be either saturated or unsaturated. The aliphatic chain of the fatty acid may be branched or unbranched. In certain embodiments, the aliphatic chain of the fatty acid comprises 12 to 24 carbon atoms. In certain embodiments, the aliphatic chain of the fatty acid comprises 0 to 5 carbon-carbon double bonds.

[0043] “Fatty alcohol” refers to an alcohol with a long aliphatic chain (e.g. >6 carbon atoms), which can be either saturated or unsaturated. The aliphatic chain of the fatty alcohol may be branched or unbranched. In certain embodiments, the aliphatic chain of the fatty alcohol comprises 12 to 24 carbon atoms. In certain embodiments, the aliphatic chain of the fatty alcohol comprises 0 to 5 carbon-carbon double bonds.

[0044] “Halo”, “halide”, or “hal” refers to F, Cl, Br and I atoms or ions.

[0045] “Heteroalkyl” refers to a straight-chain or branched saturated hydrocarbon group wherein one or more carbon atoms are independently replaced with one or more heteroatoms (e.g. nitrogen, oxygen, phosphorus and / or sulfur atoms). The heteroalkyl group may be unsubstituted. Alternatively, the heteroalkyl group may be substituted. Unless otherwise specified, the heteroalkyl group may be attached at any suitable atom and, if substituted, may be substituted at any suitable atom. Examples of heteroalkyl groups include but are not limited to ethers, thioethers, primary amines, secondary amines, tertiary amines and the like.

[0046] “Heteroaryl” refers to an aromatic carbocyclic group wherein one or more carbon atoms are independently replaced with one or more heteroatoms (e.g. nitrogen, oxygen, phosphorus and / or sulfur atoms). The heteroaryl group may be unsubstituted. Alternatively, the heteroaryl group may be substituted. Unless otherwise specified, the heteroaryl group may be attached at any suitable atom and, if substituted, may be substituted at any suitable atom. Examples of heteroaryl groups include but are not limited to thienyl, furanyl, pyrrolyl, imidazolyl, pyrazolyl, thiazolyl, isothiazolyl, oxazolyl, isoxazolyl, triazolyl, thiadiazolyl, thiophenyl, oxadiazolyl, pyridinyl, pyrimidyl, benzoxazolyl, benzthiazolyl, benzimidazolyl, indolyl, quinolinyl and the like.

[0047] “Heterocycle” encompasses both heterocycloalkyl groups and heteroaryl groups.

[0048] “Heterocycloalkyl” refers to a saturated cyclic hydrocarbon group wherein one or more carbon atoms are independently replaced with one or more heteroatoms (e.g. nitrogen, oxygen, phosphorus and / or sulfur atoms). The heterocycloalkyl group may be unsubstituted. Alternatively, the heterocycloalkyl group may be substituted. Unless otherwise specified, the heterocycloalkyl group may be attached at any suitable atom and, if substituted, may be substituted at any suitable atom. Examples of heterocycloalkyl groups include but are not limited to epoxide, morpholinyl, piperadinyl, piperazinyl, thirranyl, pyrrolidinyl, pyrazolidinyl, imidazolidinyl, thiazolidinyl, thiomorpholinyl and the like.

[0049] “Substituted” refers to a group in which one or more hydrogen atoms are each independently replaced with substituents (e.g. 1 , 2, 3, 4, 5 or more) which may be the same or different. Examples of substituents include but are not limited to -halo, -C(halo)3, -R9, =0, =S, -0-R9, - S-R9, -NR9Rh, -CN, -N02, -C(O)-R9, -COOR9, -C(S)-R9, -C(S)OR9, -S(O)2OH, -S(O)2-R9, - S(O)2NR9Rh, -O-S(O)-R9and -CONR9Rh, such as -halo, -C(halo)3(e.g. -CF3), -R9, -O-R9, - NR9Rh, -CN, or -NO2. R9and Rhare independently selected from the groups consisting of H, alkyl, aryl, arylalkyl, heteroalkyl, heteroaryl, or R9and Rhmay be linked to form a heterocycloalkyl group. R9and Rhmay be unsubstituted or further substituted as defined herein.

[0050] As used herein, the term “wax ester” refers to an ester of a fatty acid and a fatty alcohol, wherein fatty acid and fatty alcohol are as defined above.

[0051] As used herein, the term “S / C” is an abbreviation for “substrate / catalyst” and is used to describe the catalyst loading employed in a reaction, i.e. it describes the molar ratio of ester- containing substrate and catalyst present in the reaction mixture. In the instance the ester- containing substrate contains more than one ester moiety, the S / C value is adjusted accordingly.

[0052] As used herein, unless otherwise specified, “mol%” describes the amount of moles of the specified material (e.g. a base) relative to the amount of moles of the ester containing substrate, as a percentage.

[0053] As used herein, the term “hydrogenation” refers to hydrogenation or reduction of a substrate (e.g. of an ester containing substrate) using a hydrogen source (e.g. molecular hydrogen, H2).

[0054] Detailed Description

[0055] Preferred and / or optional features of the invention will now be set out. Any aspect of the invention may be combined with any other aspect of the invention unless the context demands otherwise. Any of the preferred and / or optional features of any aspect may be combined, either singly or in combination, with any aspect of the invention unless the context demands otherwise.

[0056] According to a first aspect the present invention provides a ruthenium complex of formula (I) or formula (II):

[0057] [Ru(L)AZ]W (I)

[0058] [Ru(L)AZW] (II) In the ruthenium complex of formula (I) or formula (II) the Ru atom may have oxidation state of +2.

[0059] The ruthenium complex of the invention may be a ruthenium complex of formula (I). The ruthenium complex of the invention may be a ruthenium complex of formula (II).

[0060] As will be readily understood the tridentate ligand, L, is bonded to the Ru atom through three bonds. The three bonds are formed between the Ru atom and three coordinating atoms of the tridentate ligand, L, within the coordination sphere of the Ru atom.

[0061] In the ruthenium complex of formula (I) or formula (II), L is a tridentate ligand having formula (HI) wherein:

[0062] X is a nitrogen atom and when taken together with R1it forms an optionally substituted heteroaryl group when Rxis absent, or

[0063] X is -SRa, and R1and Rxare each independently selected from hydrogen, substituted or unsubstituted Ci-10-alkyl, substituted or unsubstituted C2-io-alkenyl, substituted or unsubstituted C2- -alkynyl, substituted or unsubstituted Ci-io-heteroalkyl, substituted or unsubstituted Ci- -alkoxy, substituted or unsubstituted Cs-w-cycloalkyl, substituted or unsubstituted Cs-w-cycloalkenyl, substituted or unsubstituted C2-io-heterocycloalkyl, substituted or unsubstituted Ce-w-aryl, and substituted or unsubstituted C4-io-heteroaryl;

[0064] Y is selected from -SRb, -PRaRb, -OPRaRb, and -NHPRaRb;

[0065] R2and Ry, if present, are each independently selected from hydrogen, substituted or unsubstituted Ci-10-alkyl, substituted or unsubstituted C2-io-alkenyl, substituted or unsubstituted C2- -alkynyl, substituted or unsubstituted Ci-io-heteroalkyl, substituted or unsubstituted Ci-w-alkoxy, substituted or unsubstituted Cs-w-cycloalkyl, substituted or unsubstituted Cs-w-cycloalkenyl, substituted or unsubstituted C2-io-heterocycloalkyl, substituted or unsubstituted Ce-w-aryl, and substituted or unsubstituted C4-w-heteroaryl; R3a, R3b, R4a, and R4bare each independently selected from hydrogen, substituted or unsubstituted Ci-10-alkyl, substituted or unsubstituted C2-io-alkenyl, substituted or unsubstituted C2-io-alkynyl, substituted or unsubstituted Ci-io-heteroalkyl, substituted or unsubstituted Ci- -alkoxy, substituted or unsubstituted Cs-w-cycloalkyl, substituted or unsubstituted Cs-w-cycloalkenyl, substituted or unsubstituted C2-io-heterocycloalkyl, substituted or unsubstituted Ce-w-aryl, and substituted or unsubstituted C4-io-heteroaryl;

[0066] R5is selected from hydrogen, or substituted or unsubstituted Ci-s-alkyl;

[0067] Raand Rbare each independently selected from substituted or unsubstituted Ci-10-alkyl, substituted or unsubstituted C2-io-alkenyl, substituted or unsubstituted C2- -alkynyl, substituted or unsubstituted Ci-w-heteroalkyl, substituted or unsubstituted Ci-m-alkoxy, substituted or unsubstituted Cs-w-cycloalkyl, substituted or unsubstituted Cs-w-cycloalkenyl, substituted or unsubstituted C2-io-heterocycloalkyl, substituted or unsubstituted Ce-w-aryl, and substituted or unsubstituted C4-w-heteroaryl, or Raand Rbtogether with the phosphorus atom to which they are attached form a heterocycle.

[0068] In the tridentate ligand having formula (III), where X is a nitrogen atom, X and R1together with the atoms to which they attach form an optionally substituted heteroaryl group. Preferably, the heteroaryl group may be a substituted or unsubstituted 5-membered or a substituted or unsubstituted 6-membered heterocyclic ring. More preferably, the heteroaryl group is a substituted or unsubstituted 6-membered ring. Even more preferably, the heteroaryl group is selected from substituted or unsubstituted pyridinyl, pyrrolyl, imidazolyl, pyrazolyl, thiazolyl, isothiazolyl, oxazolyl, isoxazolyl, triazolyl, tetrazolyl, thiadiazolyl, oxadiazolyl, pyrimidyl, benzoxazolyl, benzthiazolyl, benzimidazolyl, indolyl, and quinolinyl. Even more preferably still, the heteroaryl group is a substituted or unsubstituted pyridinyl ring. Most preferably, the heteroaryl group is an unsubstituted pyridinyl ring.

[0069] Alternatively, X may be -SRa.

[0070] In the tridentate ligand having formula (III), where X is -SRa, R1and Rxare each independently preferably selected from hydrogen, substituted or unsubstituted Ci-io-alkyl, substituted or unsubstituted Ci-w-heteroalkyl, and substituted or unsubstituted Ci-w-alkoxy. More preferably, R1and Rxare each independently selected from hydrogen, substituted or unsubstituted C1.5- alkyl, substituted or unsubstituted Ci-5-heteroalkyl, and substituted or unsubstituted C1.5- alkoxy. More preferably still, R1and Rxare each independently selected from hydrogen, methyl, ethyl, / so-propyl, and terf-butyl. Most preferably, R1and Rxare each hydrogen.

[0071] In the tridentate ligand having formula (III), R1and Rxmay be the same or different. In the tridentate ligand having formula (III), Y is preferably selected from -SRb, -PRaRb, and - NHPRaRb. Most preferably, Y is selected from -SRband -PRaRb.

[0072] In the tridentate ligand having formula (III), where X is a nitrogen atom and when taken together with R1it forms an optionally substituted heterocycle when Rxis absent, Y is preferably -PRaRb, -OPRaRb, or -NHPRaRb, for example -PRaRb, or -NHPRaRb. In the tridentate ligand having formula (III), where X is -SRa, Y is preferably -SRb.

[0073] In the tridentate ligand having formula (III), where Y is -OPRaRbor -NHPRaRbit will be understood that the phosphorus atom of Y is bonded to the Ru atom in the ruthenium complex of formula (I) or ruthenium complex of formula (II).

[0074] In the tridentate ligand having formula (III), R2and Ryare each independently preferably selected from hydrogen, substituted or unsubstituted Ci-10-alkyl, substituted or unsubstituted Ci-io-heteroalkyl, and substituted or unsubstituted Ci- -alkoxy. More preferably, R2and Ryare each independently selected from hydrogen, substituted or unsubstituted Ci-s-alkyl, substituted or unsubstituted Ci-5-heteroalkyl, and substituted or unsubstituted Ci-5-alkoxy. More preferably still, R2and Ryare each independently selected from hydrogen, methyl, ethyl, / so-propyl, and terf-butyl. Most preferably, R2and Ryare each hydrogen.

[0075] In the tridentate ligand having formula (III), R3a, R3b, R4a, and R4bare each independently preferably selected from hydrogen, substituted or unsubstituted Ci-10-alkyl, substituted or unsubstituted Ci-io-heteroalkyl, and substituted or unsubstituted Ci-w-alkoxy. More preferably, R3a, R3b, R4a, and R4bare each independently selected from hydrogen, substituted or unsubstituted Ci-s-alkyl, substituted or unsubstituted Ci-5-heteroalkyl, and substituted or unsubstituted Ci-5-alkoxy. More preferably still, R3a, R3b, R4a, and R4bare each independently selected from hydrogen, methyl, ethyl, / so-propyl, and terf-butyl. Most preferably, R3a, R3b, R4a, and R4bare each hydrogen.

[0076] As will be understood, in the tridentate ligand, L, neither of R3anor R3bare linked with either of R4aor R4bto form a ring structure.

[0077] In the tridentate ligand having formula (III), R5is preferably selected from hydrogen and methyl. Most preferably, R5is hydrogen. In the tridentate ligand having formula (III), Raand Rbare each independently preferably selected from substituted or unsubstituted Ci-10-alkyl, substituted or unsubstituted C1.10- heteroalkyl, substituted or unsubstituted Ci- -alkoxy, substituted or unsubstituted C3-10- cycloalkyl, substituted or unsubstituted Cs- -cycloalkenyl, substituted or unsubstituted C2-10- heterocycloalkyl, substituted or unsubstituted Ce-io-aryl, and substituted or unsubstituted C4- 10-heteroaryl. More preferably, Raand Rbare each independently selected from substituted or unsubstituted Ci-s-alkyl, substituted or unsubstituted Ci-s-heteroalkyl, substituted or unsubstituted Ci-s-alkoxy, substituted or unsubstituted Cs- -cycloalkyl, substituted or unsubstituted Ce- -aryl, and substituted or unsubstituted C4-io-heteroaryl. Most preferably, Raand Rbare each independently selected from methyl, ethyl, / so-propyl, terf-butyl, cyclohexyl, adamantyl, and phenyl. For example, Raand Rbmay each independently be selected from ethyl, / so-propyl, terf-butyl, and phenyl.

[0078] Raand Rbmay be the same or different. In preferred tridentate ligands having formula (III), Raand Rbare the same. For example, Raand Rbmay be the same and be selected from / so- propyl, terf-butyl, and phenyl.

[0079] In the tridentate ligand having formula (III), where X is -SRaand Y is -SRbit may be preferred that Raand Rbare not both terf-butyl.

[0080] In the tridentate ligand having formula (III), where Y is -PRaRb, -OPRaRb, and / or -NHPRaRbit may be preferred that Raand Rbare each independently selected from / so-propyl, terf-butyl, cyclohexyl, adamantyl, and phenyl. For example, Raand Rbmay each independently be selected from the group consisting of / so-propyl, terf-butyl, and phenyl. In the tridentate ligand having formula (III), where X is -SRaand Y is -SRbit may be preferred that Raand Rbare each independently selected from the group consisting of methyl, ethyl, n-propyl, n-butyl, pentyl, / so-propyl, and terf-butyl, preferably ethyl.

[0081] In preferred ruthenium complexes of formula (I) or formula (II), L may be a tridentate ligand having formula (Illa)

[0082] wherein:

[0083] Y is selected from -PRaRb, -OPRaRb, and -NHPRaRb;

[0084] R2and Ryare each independently selected from hydrogen, substituted or unsubstituted Ci- - alkyl, substituted or unsubstituted C2-io-alkenyl, substituted or unsubstituted C2-io-alkynyl, substituted or unsubstituted Ci-io-heteroalkyl, substituted or unsubstituted Ci- -alkoxy, substituted or unsubstituted Cs-w-cycloalkyl, substituted or unsubstituted Cs-w-cycloalkenyl, substituted or unsubstituted C2-io-heterocycloalkyl, substituted or unsubstituted Ce-w-aryl, and substituted or unsubstituted C4-w-heteroaryl;

[0085] R3a, R3b, R4a, and R4bare each independently selected from hydrogen, substituted or unsubstituted Ci-10-alkyl, substituted or unsubstituted C2-io-alkenyl, substituted or unsubstituted C2- -alkynyl, substituted or unsubstituted Ci-w-heteroalkyl, substituted or unsubstituted Ci-m-alkoxy, substituted or unsubstituted Cs-w-cycloalkyl, substituted or unsubstituted Cs-w-cycloalkenyl, substituted or unsubstituted C2-io-heterocycloalkyl, substituted or unsubstituted Ce-w-aryl, and substituted or unsubstituted C4-w-heteroaryl;

[0086] R5is selected from hydrogen, or substituted or unsubstituted Ci-s-alkyl;

[0087] Raand Rbare each independently selected from substituted or unsubstituted Ci-10-alkyl, substituted or unsubstituted C2-w-alkenyl, substituted or unsubstituted C2-w-alkynyl, substituted or unsubstituted Ci-w-heteroalkyl, substituted or unsubstituted Ci-w-alkoxy, substituted or unsubstituted Cs-w-cycloalkyl, substituted or unsubstituted Cs-w-cycloalkenyl, substituted or unsubstituted C2-w-heterocycloalkyl, substituted or unsubstituted Ce- -aryl, and substituted or unsubstituted C4-w-heteroaryl, or Raand Rbtogether with the heteroatom to which they are attached form a heterocycle; and each Rwis independently selected from the group consisting of hydrogen, substituted or unsubstituted Ci-w-alkyl, substituted or unsubstituted Ci-w-heteroalkyl, and substituted or unsubstituted Ci-w-alkoxy.

[0088] In the tridentate ligand having formula (Illa), Y is preferably selected from -PRaRb, and - NHPRaRb. More preferably, Y is -PRaRb. In the tridentate ligand having formula (Illa), R2and Ryare each independently preferably selected from hydrogen, substituted or unsubstituted Ci-10-alkyl, substituted or unsubstituted Ci-io-heteroalkyl, and substituted or unsubstituted Ci- -alkoxy. More preferably, R2and Ryare each independently selected from hydrogen, substituted or unsubstituted Ci-s-alkyl, substituted or unsubstituted Ci-5-heteroalkyl, and substituted or unsubstituted Ci-5-alkoxy. Even more preferably, R2and Ryare each independently selected from hydrogen, methyl, ethyl, / so-propyl, and terf-butyl. Most preferably, R2and Ryare each hydrogen.

[0089] In the tridentate ligand having formula (Illa), R3a, R3b, R4a, and R4bare each independently preferably selected from hydrogen, substituted or unsubstituted Ci-10-alkyl, substituted or unsubstituted Ci-io-heteroalkyl, and substituted or unsubstituted Ci- -alkoxy. More preferably, R3a, R3b, R4a, and R4bare each independently selected from hydrogen, substituted or unsubstituted Ci-s-alkyl, substituted or unsubstituted Ci-5-heteroalkyl, and substituted or unsubstituted Ci-5-alkoxy. More preferably still, R3a, R3b, R4a, and R4bare each independently selected from hydrogen, methyl, ethyl, / so-propyl, and terf-butyl. Most preferably, R3a, R3b, R4a, and R4bare each hydrogen.

[0090] As will be understood, in the tridentate ligand having formula (Illa), neither of R3anor R3bare linked with either of R4aor R4bto form a ring structure.

[0091] In the tridentate ligand having formula (Illa), R5is selected from hydrogen, or substituted or unsubstituted Ci-s-alkyl. Preferably, R5is selected from hydrogen and methyl. Most preferably, R5is hydrogen.

[0092] In the tridentate ligand having formula (Illa), Raand Rbare each independently preferably selected from substituted or unsubstituted Ci-10-alkyl, substituted or unsubstituted C1.10- heteroalkyl, substituted or unsubstituted Ci-io-alkoxy, substituted or unsubstituted C3-10- cycloalkyl, substituted or unsubstituted Cs- -cycloalkenyl, substituted or unsubstituted C2-10- heterocycloalkyl, substituted or unsubstituted Ce-io-aryl, and substituted or unsubstituted C4- 10-heteroaryl. More preferably, Raand Rbare each independently selected from substituted or unsubstituted Ci-s-alkyl, substituted or unsubstituted Ci-s-heteroalkyl, substituted or unsubstituted Ci-s-alkoxy, substituted or unsubstituted Cs- -cycloalkyl, substituted or unsubstituted Ce- -aryl, and substituted or unsubstituted C4-io-heteroaryl. More preferably still, Raand Rbeach independently selected from / so-propyl, terf-butyl, cyclohexyl, adamantyl, and phenyl. For example, Raand Rbmay each independently be selected from / so-propyl, tertbutyl, and phenyl. In the tridentate ligand having formula (Illa), Raand Rbmay be the same or different. Preferably, Raand Rbare the same. For example, Raand Rbmay be the same and be selected from / so-propyl, terf-butyl, and phenyl.

[0093] In the tridentate ligand having formula (Illa), each Rwis independently preferably selected from hydrogen, and substituted or unsubstituted C1-10 alkyl. More preferably, each Rwis independently selected from hydrogen, and substituted or unsubstituted C1.5 alkyl. Most preferably, each Rwis hydrogen.

[0094] In alternative preferred ruthenium complexes of formula (I) or formula (II), L may be a tridentate ligand having formula (I I lb) wherein:

[0095] R1and Rxare each independently selected from hydrogen, substituted or unsubstituted CMO- alkyl, substituted or unsubstituted C2-io-alkenyl, substituted or unsubstituted C2- -alkynyl, substituted or unsubstituted Ci-io-heteroalkyl, substituted or unsubstituted Ci- -alkoxy, substituted or unsubstituted Cs-w-cycloalkyl, substituted or unsubstituted Cs-w-cycloalkenyl, substituted or unsubstituted C2-io-heterocycloalkyl, substituted or unsubstituted Ce-w-aryl, and substituted or unsubstituted C4-io-heteroaryl;

[0096] R2and Ryare each independently selected from hydrogen, substituted or unsubstituted CMO- alkyl, substituted or unsubstituted C2-io-alkenyl, substituted or unsubstituted C2- -alkynyl, substituted or unsubstituted Ci-io-heteroalkyl, substituted or unsubstituted Ci-io-alkoxy, substituted or unsubstituted Cs-w-cycloalkyl, substituted or unsubstituted Cs-w-cycloalkenyl, substituted or unsubstituted C2-io-heterocycloalkyl, substituted or unsubstituted Ce-w-aryl, and substituted or unsubstituted C4-w-heteroaryl;

[0097] R3a, R3b, R4a, and R4bare each independently selected from hydrogen, substituted or unsubstituted Ci-10-alkyl, substituted or unsubstituted C2-w-alkenyl, substituted or unsubstituted C2-w-alkynyl, substituted or unsubstituted Ci-w-heteroalkyl, substituted or unsubstituted Ci-w-alkoxy, substituted or unsubstituted Cs-w-cycloalkyl, substituted or unsubstituted Cs-w-cycloalkenyl, substituted or unsubstituted C2-w-heterocycloalkyl, substituted or unsubstituted Ce- -aryl, and substituted or unsubstituted C4-w-heteroaryl; R5is selected from hydrogen, or substituted or unsubstituted Ci-s-alkyl;

[0098] Raand Rbare each independently selected from substituted or unsubstituted Ci-10-alkyl, substituted or unsubstituted C2-io-alkenyl, substituted or unsubstituted C2-io-alkynyl, substituted or unsubstituted Ci-io-heteroalkyl, substituted or unsubstituted Ci- -alkoxy, substituted or unsubstituted Cs- -cycloalkyl, substituted or unsubstituted Cs- -cycloalkenyl, substituted or unsubstituted C2-io-heterocycloalkyl, substituted or unsubstituted Ce- -aryl, and substituted or unsubstituted C4-io-heteroaryl.

[0099] In the tridentate ligand having formula (lllb), R1and Rxare each independently preferably selected from hydrogen, substituted or unsubstituted Ci-10-alkyl, substituted or unsubstituted Ci-io-heteroalkyl, and substituted or unsubstituted Ci- -alkoxy. More preferably, R1and Rxare each independently selected from hydrogen, substituted or unsubstituted Ci-s-alkyl, substituted or unsubstituted Ci-5-heteroalkyl, and substituted or unsubstituted Ci-5-alkoxy. More preferably still, R1and Rxare each independently selected from hydrogen, methyl, ethyl, / so-propyl, and terf-butyl. Most preferably, R1and Rxare each hydrogen.

[0100] In the tridentate ligand having formula (lllb), R2and Ryare each independently preferably selected from hydrogen, substituted or unsubstituted Ci-10-alkyl, substituted or unsubstituted Ci. -heteroalkyl, and substituted or unsubstituted Ci- -alkoxy. More preferably, R2and Ryare each independently selected from hydrogen, substituted or unsubstituted Ci-s-alkyl, substituted or unsubstituted Ci-5-heteroalkyl, and substituted or unsubstituted Ci-5-alkoxy. Even more preferably, R2and Ryare each independently selected from hydrogen, methyl, ethyl, / so-propyl, and terf-butyl. Most preferably, R2and Ryare each hydrogen.

[0101] In the tridentate ligand having formula (lllb), R3a, R3b, R4a, and R4bare each independently preferably selected from hydrogen, substituted or unsubstituted Ci-10-alkyl, substituted or unsubstituted Ci- -heteroalkyl, and substituted or unsubstituted Ci- -alkoxy. More preferably, R3a, R3b, R4a, and R4bare each independently selected from hydrogen, substituted or unsubstituted Ci-s-alkyl, substituted or unsubstituted Ci-5-heteroalkyl, and substituted or unsubstituted Ci-5-alkoxy. More preferably still, R3a, R3b, R4a, and R4bare each independently selected from hydrogen, methyl, ethyl, / so-propyl, and terf-butyl. Most preferably, R3a, R3b, R4a, and R4bare each hydrogen.

[0102] As will be understood, in the tridentate ligand having formula (lllb), neither of R3anor R3bare linked with either of R4aor R4bto form a ring structure. In the tridentate ligand having formula (lllb), R5is selected from hydrogen, or substituted or unsubstituted Ci-s-alkyl. Preferably, R5is selected from hydrogen and methyl. Most preferably, R5is hydrogen.

[0103] In the tridentate ligand having formula (lllb), Raand Rbare each independently preferably selected from substituted or unsubstituted Ci-10-alkyl, substituted or unsubstituted C1.10- heteroalkyl, substituted or unsubstituted Ci- -alkoxy, substituted or unsubstituted C3-10- cycloalkyl, substituted or unsubstituted Cs- -cycloalkenyl, substituted or unsubstituted C2-10- heterocycloalkyl, substituted or unsubstituted Ce-io-aryl, and substituted or unsubstituted C4- 10-heteroaryl. More preferably, Raand Rbare each independently selected from substituted or unsubstituted Ci-s-alkyl, substituted or unsubstituted Ci-s-heteroalkyl, substituted or unsubstituted Ci-s-alkoxy, substituted or unsubstituted Cs- -cycloalkyl, substituted or unsubstituted Ce- -aryl, and substituted or unsubstituted C4-io-heteroaryl. More preferably still, Raand Rbeach independently substituted or unsubstituted C1.5 alkyl. For example, Raand Rbmay each independently be selected from methyl, ethyl, n-propyl, n-butyl, pentyl, / so-propyl, and terf-butyl.

[0104] In the tridentate ligand having formula (lllb), Raand Rbmay be the same or different. Preferably, Raand Rbare the same. For example, Raand Rbmay be the same and be selected from ethyl, / so-propyl, and terf-butyl.

[0105] In the tridentate ligand having formula (lllb), it may be preferred that Raand Rbare not both terf-butyl.

[0106] It may be preferred that in the ruthenium complex of formula (I) or formula (II), L is a tridentate ligand having formula (Illa) and / or formula (lllb).

[0107] It may be preferred that in the ruthenium complex of formula (I) or formula (II), L is one or more tridentate ligands of formulae (lllc), (Hid), and / or (Hie),

[0108] wherein Raand Rbare each independently selected from substituted or unsubstituted Ci- - alkyl, substituted or unsubstituted C2-io-alkenyl, substituted or unsubstituted C2-io-alkynyl, substituted or unsubstituted Ci-io-heteroalkyl, substituted or unsubstituted Ci- -alkoxy, substituted or unsubstituted Cs-w-cycloalkyl, substituted or unsubstituted Cs-w-cycloalkenyl, substituted or unsubstituted C2-io-heterocycloalkyl, substituted or unsubstituted Ce-w-aryl, and substituted or unsubstituted C4-io-heteroaryl, or where the tridentate ligand is a tridentate ligand having formula (I He) or (Hie) Raand Rbtogether with the heteroatom to which they are attached form a heterocycle.

[0109] In the tridentate ligand having formula (I I Ic) or formula (I I le), Raand Rbare each independently preferably selected from substituted or unsubstituted Ci-10-alkyl, substituted or unsubstituted Ci-io-heteroalkyl, substituted or unsubstituted Ci-w-alkoxy, substituted or unsubstituted C3-10- cycloalkyl, substituted or unsubstituted Cs-w-cycloalkenyl, substituted or unsubstituted C2-10- heterocycloalkyl, substituted or unsubstituted Ce-w-aryl, and substituted or unsubstituted C4- w-heteroaryl. More preferably, Raand Rbare each independently selected from substituted or unsubstituted Ci-5-alkyl, substituted or unsubstituted Ci-5-heteroalkyl, substituted or unsubstituted Ci-5-alkoxy, substituted or unsubstituted Cs-w-cycloalkyl, substituted or unsubstituted Ce-w-aryl, and substituted or unsubstituted C4-w-heteroaryl. More preferably still, Raand Rbeach independently selected from / so-propyl, te / Y-butyl, cyclohexyl, adamantyl, and phenyl. For example, Raand Rbmay each independently be selected from / so-propyl, tertbutyl, and phenyl. In the tridentate ligand having formula (I I Ic) or formula (Hie), Raand Rbmay be the same or different. Preferably, Raand Rbare the same. For example, Raand Rbmay be the same and be selected from / so-propyl, te / Y-butyl, and phenyl.

[0110] In the tridentate ligand having formula (Hid), Raand Rbare each independently preferably selected from substituted or unsubstituted Ci-10-alkyl, substituted or unsubstituted C1.10- heteroalkyl, substituted or unsubstituted Ci- -alkoxy, substituted or unsubstituted C3-10- cycloalkyl, substituted or unsubstituted Cs- -cycloalkenyl, substituted or unsubstituted C2-10- heterocycloalkyl, substituted or unsubstituted Ce-io-aryl, and substituted or unsubstituted C4- 10-heteroaryl. More preferably, Raand Rbare each independently selected from substituted or unsubstituted Ci-s-alkyl, substituted or unsubstituted Ci-s-heteroalkyl, substituted or unsubstituted Ci-s-alkoxy, substituted or unsubstituted Cs- -cycloalkyl, substituted or unsubstituted Ce- -aryl, and substituted or unsubstituted C4-io-heteroaryl. More preferably still, Raand Rbeach independently substituted or unsubstituted C1.5 alkyl. For example, Raand Rbmay each independently be selected from methyl, ethyl, n-propyl, n-butyl, pentyl, / so-propyl, and terf-butyl.

[0111] In the tridentate ligand of formula (Hid), it may be preferred that Raand Rbare not both tertbutyl.

[0112] In the tridentate ligand having formula (Hid), Raand Rbmay be the same or different. Preferably, Raand Rbare the same. For example, Raand Rbmay be the same and be selected from ethyl, and / so-propyl.

[0113] It may be preferred that in the ruthenium complex of formula (I) or formula (II), L is a tridentate ligand having formula (I lie). It may be preferred that in the ruthenium complex of formula (I) or formula (II), L is a tridentate ligand having formula (Hid). It may be preferred that in the ruthenium complex of formula (I) or formula (II), L is a tridentate ligand having formula (Hie). It may be preferred that in the ruthenium complex of formula (I) or formula (II), L is a tridentate ligand having formula (I I Ic) or (Hie), formula (I I Ic) or (Hid), or formula (Hid) or (Hie).

[0114] In the ruthenium complex of formula (I) or formula (II), it may be more preferred that L is one or more tridentate ligands selected from the group consisting of:

[0115] In the ruthenium complex of formula (I) or formula (II), it may be most preferred that L is one or more tridentate ligands selected from the group consisting of:

[0116] Alternatively, in the ruthenium complex of formula (I) or formula (II), it may be most preferred that L is one or more tridentate ligands selected from the group consisting of: For the avoidance of doubt the abbreviations Ph, 'Pr, ‘Bu, and Et as used herein will be understood to refer to phenyl, / so-propyl, terf-butyl, and ethyl, respectively.

[0117] In the ruthenium complex of formula (I) or formula (II), W and Z are anionic ligands each independently selected from the group consisting of carboxylates, alkoxides, hydride, and halides.

[0118] In the ruthenium complex of formula (I) or formula (II), W may be a coordinated anionic ligand, i.e. the anionic ligand is bonded to the Ru atom within the coordination sphere (e.g. where the ruthenium complex is a ruthenium complex of formula (II)). Alternatively, W may be a non-coordinated anionic. By “non-coordinated anionic ligand”, we mean the anionic ligand is forced to the outer sphere of the Ru atom (e.g. where the ruthenium complex is a ruthenium complex of formula (I)). A non-coordinated anionic ligand, therefore, is dissociated from the Ru atom. The anionic ligand can be generally identified as coordinating or noncoordinating by analysing the X-ray crystal structure of the complex. In the ruthenium complex of formula (I) or formula (II), Z is a coordinated anionic ligand.

[0119] Where either W and / or Z are carboxylate, W and / or Z may be a carboxylate ligand of formula (IV) where R6may be selected from substituted or unsubstituted Ci-10-alkyl, substituted or unsubstituted C2-io-alkenyl, substituted or unsubstituted C2- -alkynyl, substituted or unsubstituted Ci-io-heteroalkyl, substituted or unsubstituted Ci- -alkoxy, substituted or unsubstituted Cs-w-cycloalkyl, substituted or unsubstituted Cs-w-cycloalkenyl, substituted or unsubstituted C2-io-heterocycloalkyl, substituted or unsubstituted Ce-w-aryl, and substituted or unsubstituted C4-io-heteroary.

[0120] In the carboxylate ligand having formula (IV), R6may be preferably selected from substituted or unsubstituted Ci-10-alkyl, substituted or unsubstituted Ci- -heteroalkyl, substituted or unsubstituted Ci-w-alkoxy, substituted or unsubstituted Ce-w-aryl, and substituted or unsubstituted C4-w-heteroaryl. More preferably, R6may be selected from substituted or unsubstituted Ci-s-alkyl, substituted or unsubstituted Ci-5-heteroalkyl, substituted or unsubstituted Ci-5-alkoxy, substituted or unsubstituted Ce- -aryl, and substituted or unsubstituted C4-io-heteroaryl. More preferably still, R6may be selected from methyl, ethyl, propyl, butyl, pentyl, or phenyl. Most preferably, R6may be methyl.

[0121] Where R6is methyl it will be understood that the carboxylate ligand of formula (IV), or carboxylate ligand of formula (IVb) as described below, may be referred to as an acetate (OAc) ligand.

[0122] As will be understood by the skilled person, carboxylate ligands may be coordinated to the Ru atom either as a monodentate or a bidentate ligand. In other words, carboxylate ligands may be coordinated either in an r|1or an r|2coordination mode.

[0123] Where W and / or Z are carboxylate ligands and coordinated to the Ru atom in a monodentate fashion it will be understood that they each of W and / or Z occupy one coordination site on the Ru atom and may be regarded as each being bonded to the Ru atom each by a single oxygen atom.

[0124] Where Z is carboxylate and coordinated to the Ru atom as a bidentate ligand it will be understood that it occupies two coordination sites on the Ru atom and may be regarded as being bonded to the Ru atom by the two oxygen atoms of the carboxylate group or by a delocalised Tt-system.

[0125] Where Z is coordinated to the Ru atom as a bidentate ligand, in an r|2coordination mode, Z may be regarded as having formula (IVb) wherein R6is as hereinbefore defined.

[0126] As will be understood, where Z is carboxylate, the nature of the ruthenium complex and coordination mode of the ligand W, will depend on whether Z is coordinated to the Ru atom in a monodentate or bidentate fashion. Accordingly, where Z is carboxylate and coordinated to the Ru atom as a bidentate ligand, in an r|2coordination mode, the ruthenium complex of the invention is a ruthenium complex of formula (I) where W is a non-coordinated anionic ligand; and where Z is carboxylate and coordinated to the Ru atom as a monodentate ligand, in an r|1coordination mode, the ruthenium complex of the invention is a ruthenium complex of formula (II) where W is a coordinated anionic ligand and is coordinated to the Ru atom in a monodentate fashion.

[0127] W and / or Z may be a halide. For example, W and / or Z may be F; Cl; Br, or I’. Preferably, W and / or Z may be Cl; Br, or I’. More preferably, W and / or Z may be Cl'.

[0128] W and / or Z may be an alkoxide.

[0129] Where either W and / or Z are alkoxide, W and / or Z may be an alkoxide ligand of formula (V) 0 7

[0130] O - R7

[0131] (V) wherein R7is substituted or unsubstituted C1.10 alkyl.

[0132] In the alkoxide ligand of formula (V), R7may be preferably selected from the group consisting of methyl, ethyl, / so-propyl, and terf-butyl.

[0133] W and Z may be the same or different. In certain complexes of the invention, W and Z are the same. In certain complexes of the invention, W and Z are different.

[0134] It may be preferred that W and / or Z are selected from the group consisting of acetate, chloride, methoxide, ethoxide, and hydride. It may be more preferred that W and / or Z are selected from the group consisting of acetate, chloride, and hydride.

[0135] In certain complexes of the invention, it may be preferred that W is halide (e.g. chloride) and Z is hydride. In certain complexes of the invention, it may be preferred that W is halide (e.g. chloride) and Z is halide (e.g. chloride). In certain complexes of the invention, it may be preferred that W is carboxylate (e.g. acetate) and Z is carboxylate (e.g. acetate). In certain complexes of the invention, it may be preferred that W is hydride and Z is carboxylate (e.g. acetate).

[0136] In the ruthenium complex of formula (I) or formula (II), A is a sulfoxide ligand.

[0137] A may be a sulfoxide ligand of formula (VI), wherein R8and R9are each independently selected from substituted or unsubstituted Ci-w alkyl, and substituted or unsubstituted Ce- aryl, or R8and R9are linked to form a ring structure.

[0138] In the sulfoxide ligand of formula (VI), R8and R9may be the same or different. Preferably, R8and R9are the same.

[0139] In the sulfoxide ligand of formula (VI), R8and R9are each independently preferably selected from methyl, ethyl, iso-propyl, n-propyl, n-butyl, terf-butyl, pentyl, hexyl, octyl, and phenyl. More preferably, R8and R9are each independently selected from methyl, ethyl, n-butyl, and phenyl. Most preferably, R8and R9are the same and are methyl. Where R8and R9are the same and are methyl it will be understood that A is dimethylsulfoxide (DMSO).

[0140] Where R8and R9are linked to form a ring structure, the sulfoxide ligand of formula (VI) may preferably be tetrahydrothiophene 1 -oxide.

[0141] As will be understood by the skilled person the sulfoxide ligand, A, may coordinate to the Ru atom in the complex of formula (I) or formula (II) by a bond from either the oxygen atom or the sulfur atom of the sulfoxide ligand.

[0142] For the avoidance of doubt, any complex depicted in a diagram or figure herein is intended to include all confirmations and isomers thereof (e.g. fac- / mer- isomers and cis- / trans- isomers) unless explicitly stated otherwise.

[0143] In a preferred aspect of the present invention the ruthenium complex of formula (I) or formula

[0144] (II) may be one or more selected from:

[0145] In another preferred aspect of the present invention, the ruthenium complex of formula (I) or formula (II) may be one or more selected from:

[0146] It may be preferred that the ruthenium complex of formula (I) or the ruthenium complex of formula (II) is not: In a second aspect of the invention there is provided a process for reducing an ester containing substrate to an alcohol containing substrate in the presence of a hydrogen source and a base using the ruthenium complex of formula (I) or the ruthenium complex of formula (II) as defined in the first aspect of the invention. The process of the invention may comprise the steps of: contacting an ester containing substrate, a hydrogen source, a base, and the ruthenium complex of formula (I) or the ruthenium complex of formula (II) to form a reaction mixture; and reacting the ester containing substrate with the hydrogen source to produce an alcohol containing substrate.

[0147] The process of the invention may be referred to as a hydrogenation process.

[0148] The ruthenium complex of formula (I) and the ruthenium complex of formula (II) are as defined in the first aspect of the invention.

[0149] The ester containing substrate may be any organic molecule comprising one or more ester moieties. For example, the ester containing substrate may be a methyl ester (e.g. methyl acetate), an ethyl ester (e.g. ethyl acetate), or a wax ester.

[0150] When the ester containing substrate is a monoester, the products of the process are alcohols. When the ester containing substrate is a lactone, the product of the process is a diol. When the ester containing substrate contains multiple ester moieties, the products of the process are a polyol and alcohols. In preferred processes, the process does not produce a hemiacetal by-product.

[0151] Typically, the hydrogen source may be molecular hydrogen (H2 gas). Typically, the molecular hydrogen is provided to the reaction mixture at elevated pressure. For instance, molecular hydrogen may be provided at a pressure of from 5 barg to 50 barg, such as from 10 barg to 40 barg.

[0152] The ruthenium complex of formula (I) or ruthenium complex of formula (II) may be present in the reaction mixture in an amount of from 0.001 mol% or more, from 0.002 mol% or more, from 0.003 mol% or more, or from 0.005 mol% or more relative to the ester containing substrate. The ruthenium complex of formula (I) or ruthenium complex of formula (II) may be present in the reaction mixture in an amount of from 0.5 mol% or less, from 0.1 mol% or less, from 0.05 mol% or less, or from 0.02 mol% or less relative to the ester containing substrate. For example, the ruthenium complex of formula (I) or ruthenium complex of formula (II) may be present in the reaction mixture in an amount of from 0.001 to 0.5 mol%, from 0.002 to 0.1 mol%, from 0.003 to 0.05 mol%, or from 0.005 mol% to 0.02 mol% relative to the ester containing substrate. As will be understood by the person skilled in the art, the molar percentages expressed above may also be expressed as a ratio of organic substrate to catalyst (S / C) ratio. For instance, 0.001 mol% of the ruthenium complex of formula (I) or ruthenium complex of formula (II) relative to the ester containing substrate corresponds to an S / C ratio of 100,000:1.

[0153] It is a surprising advantage of the process of the present invention that low loadings of the ruthenium complex of formula (I) or the ruthenium complex of formula (II) can be used whilst maintaining high catalytic activity.

[0154] The reaction mixture may comprise a solvent or may be carried out under so called “solvent free” conditions. It will be understood that where solvent free conditions are used the ester containing substrate may act as a liquid medium for the reaction mixture. Where a solvent is used the solvent may suitably selected from one or more of an alcohol, toluene, THF and Me-THF.

[0155] The reaction mixture further comprises a base. Suitable bases include metal alkoxides such as sodium methoxide (NaOMe), sodium ethoxide (NaOEt) and potassium terf-butoxide (KO‘Bu), or bases such as potassium phosphate (K3PO4). The base may be present in an amount of from 20 to 200 mol% relative to the ester containing substrate. For example, the base may be present in an amount of about 40 mol% or more, 50 mol% or more, 80 mol% or more, or 100 mol% or more relative to the ester containing substrate.

[0156] The step of reacting the ester containing substate with the hydrogen source may be carried out at elevated temperature. Typically, a temperature of from 20 to 140 °C may be chosen, such as from 30 °C to 100 °C, or from 30 °C to 70 °C.

[0157] The step of reacting the ester containing substrate with the hydrogen source may be carried out over any suitable time period. Typically, the reaction takes place over a time period of from 1 to 16 hours, such as from 2 to 10 hours.

[0158] In a third aspect of the present invention there is provided a process for preparing the ruthenium complex of formula (I) or formula (II) of the first aspect of the invention, the process comprising the step of reacting a ruthenium precursor complex with a tridentate ligand, L, as defined in relation to the first aspect of the invention. Suitably, the ruthenium precursor complex may be a ruthenium precursor complex of formula [Ru(A)4X2] where A is a sulfoxide ligand as defined in relation to the first aspect of the invention and X is a halide (e.g. Cl').

[0159] Suitably, one or more chloride ligands of the complex of formula [Ru(L)(sulfoxide)Cl2] may be replaced using methods known in the art. For example, chloride ligands may be substituted by one or more carboxylate ligands by treating [Ru(L)(sulfoxide)Cl2] with a silver compound (e.g. silver carbonate or silver oxide) and a carboxylic acid (e.g. acetic acid).

[0160] Suitably, the reaction of the ruthenium precursor complex with the tridentate ligand, L, is carried out in a solvent. Suitable solvents include toluene and THF, and mixtures thereof.

[0161] Suitably, the step of reacting the ruthenium precursor complex with the tridentate ligand, L, may be carried out at elevated temperature, for example at reflux.

[0162] The tridentate ligands, L, of the ruthenium complexes of the invention may be prepared according to the methods disclosed in W02013023307A1, W02014036650A1 , and W02014139030A1.

[0163] [RU(A)4X2] may be prepared according to the method disclosed in James et. al., Inorg. Nucl. Chem. Lett., 1971, 7, p.781. James et. al. reacts RuCh.n W with a sulfoxide ligand, A, (i.e. dimethylsulfoxide, DMSO) at 80 °C in the presence of hydrogen to directly form the ruthenium precursor complex. [Ru(A)4X2] may also be prepared according to the method of Bratsos, I. and Alessio, A. in Inorganic Syntheses, Vol. 35, Ed: Rauchfuss, T. B. Wiley, 2010. As will be understood, the anionic ligand, X, of the ruthenium precursor complex [RU(A)4X2] may be exchanged by methods known in the art. For instance, [Ru(A)4Ch] may be treated with NaHCCh and acetic acid (AcOH) to produce [Ru(A)4(OAc)2]. For instance, [RU(A)4(OAC)2] may then be used as a suitable ruthenium precursor to produce ruthenium complexes of formula (I) or formula (II) which comprise acetate (OAc) ligands.

[0164] As will be understood, sulfoxides (e.g. dimethylsulfoxide) are cheap, low molecular weight (versus PR3), and readily available, and may further serve as a solvent in preparing the ruthenium precursor complex.

[0165] Unlike methods of the prior art as discussed hereinabove, it is a surprising advantage of the present invention that a competent ester hydrogenation catalyst may be provided without the use of toxic reagents, such as PR3, and with no loss of the same to waste. Accordingly, the ruthenium complexes of the invention may be produced using a more atom efficient process, as compared to analogous complexes where A is a phosphine ligand or a carbonyl ligand, and with minimised chemical waste formation.

[0166] Examples

[0167] Materials

[0168] NaOEt, NaOMe, and K3PO4, the ester-containing substrate methyl decanoate, ethyl decanoate, and ethyl benzoate, and solvents are all commercially available, e.g. from Sigma Aldrich, Fisher Scientific, Alfa Aesar, Acros Organics etc. RuCh.n W is available from Johnson Matthey.

[0169] [Ru(DMSO)4Ch] was prepared according to the method disclosed in James et. al., Inorg. Nucl. Chem. Lett., 1971 , 7, p.781.

[0170] Preparation of ruthenium complexes of the invention

[0171] Ru(SNS)(DMSO) refers to a complex having the following structure:

[0172] Ru(SNS)(DMSO) was prepared according to the following procedure. To a Schlenk flask under argon was added [Ru(DMSO)4Cl2] (0.970 g, 2.002 mmol) and bis(2- (ethylthio)ethyl)amine (0.41 g, 2.1 mmol, “SNS”). Mesitylene (30 mL) was added and the mixture heated at reflux for 5 hours. The resulting solid was filtered and washed with mistylene (15 mL) and cyclohexane (3 x 20 mL) and dried under air. The solid was recrystalised from hot ethanol, and washed with diethyl ether (20 mL) and pentane (20 mL) and dried under air. The title compound was obtained as an orange / yellow solid (0.441 g, 39 % yield). An X-ray crystal structure of this complex is shown in Figure 2.

[0173] Ru(PNN)(DMSO) refers to a complex having the following structure:

[0174] Ru(PNN)(DMSO) was prepared according to the following procedure. To a Schlenk flask under argon was added [Ru(DMSO)4Ch] (0.516 g, 1.065 mmol) (1-(Diphenylphosphino)-2- (2'-pyridylmethylamino)ethane (0.382 g, 1.19 mmol, “PNN”). The solids were suspended in THF (18.5 mL) and toluene (25 mL). The solution was heated at reflux for a period of 18 hours to yield an orange solution. The crude product was precipitated by addition of diethyl ether (34 mL), recovered by filtration, and the crude product washed with deionised water (2 x 30 mL) and diethyl ether (30 mL). The title compound was obtained as an orange solid (0.475 g, 83% yield).

[0175] Ru(PNP)(DMSO) refers to a comparative complex not according to the invention having the following structure:

[0176] Ru(PNP)(DMSO) was prepared according to the following procedure. To a Schlenk flask under argon was added [Ru(DMSO)4Cl2] (0.662 g, 1.376 mmol) and bis[(2-di-i- propylphosphino)ethyl]amine in THF (4.55g of a 10 wt% solution in THF, 1.49 mmol, “PNP”). To the flask was added toluene (11 mL) and THF (15 mL) and the resulting solution heated at reflux for 18 hours. The resulting solid was filtered and the title compound obtained as an orange / yellow solid (0.759 g, 67 % yield).

[0177] Measurement methods

[0178] Gas chromatography (GC) measurements were conducted using a Varian 3900 or 3800 gas chromatograph system. Unless otherwise indicated, reaction conversions were determined by GC analysis.

[0179] General Procedure for Ester Hydrogenation

[0180] To an 8 mL vial, a ruthenium complex of the invention - Ru(SNS)(DMSO) or Ru(PNN)(DMSO) - was added, followed by base and 2-20 mmol of ester-containing substrate, and optionally a solvent. The vial was added to a Biotage Endeavor screening system, the stirring head sealed, and the reaction mixture purged with nitrogen. A purge sequence involved pressurizing to approximately 45 psi nitrogen and releasing the pressure (repeated 5 times). The reactor was pressurised with 400 psi hydrogen and heated to the desired temperature. Once the reaction time was complete, typically 16 hrs, the reaction was allowed to cool to room temperature. Nitrogen purge cycles (5 repeats) were then carried out to remove hydrogen. The reaction mixture was analysed via GC. Example 1

[0181] The general procedure for ester hydrogenation was used to investigate the hydrogenation of methyl decanoate at an S / C ratio of 1,000 / 1 at different temperatures and using different solvents. Methyl decanoate (0.42 mL, 2 mmol) and base (1 mmol, 50 mol% versus methyl decanoate) were used in each experiment. Experiments 11 to 14 were carried out in the absence of a base, whilst experiments 12 and 14 probed whether the addition of trifluoroacetic acid impacted conversion. The results from Example 1 are shown in Table 1.

[0182] Table 1

[0183] Experiment 1 shows that complexes of the invention are competent ester hydrogenation catalysts. Hydrogenation of methyl decanoate was accomplished at temperatures as low as 40 °C. Experiment 1 shows that the presence of a base was required in order for hydrogenation of methyl decanoate to occur.

[0184] Example 2

[0185] The general procedure for ester hydrogenation was used to investigate the hydrogenation of methyl decanoate at an S / C ratio of 10,000 / 1 or 50,000 / 1 at different temperatures in the absence of solvent (“solvent free” conditions). Methyl decanoate (4.25 mL, 20 mmol) and sodium methoxide (10 mmol, 50 mol% versus methyl decanoate) were used in each experiment. The results from Example 2 are shown in Table 2.

[0186] Table 2

[0187] Example 2 shows that under “solvent free” conditions ruthenium complexes of the invention are competent catalysts for the hydrogenation of methyl decanoate at temperatures as low as 40 °C. Moreover, even at S / C ratios of 50,000 / 1 ester hydrogenation was found to occur.

[0188] Example 3

[0189] The general procedure for ester hydrogenation was used to investigate the hydrogenation of ethyl dodecanoate at an S / C ratio of 44,000 / 1 at 40 °C in the absence of solvent (“solvent free” conditions). Ethyl dodecanoate (4.65 mL, 20 mmol) and sodium ethoxide (10 mmol, 50 mol% versus ethyl decanoate) were used in each experiment. The results from Example 3 are shown in Table 2.

[0190] Table 3

[0191] Example 3 shows that ruthenium complexes of the invention are competent catalysts for the conversion of ethyl dodecanoate under S / C ratios of 44,000 / 1 and temperatures as low as 40 °C. Example 4

[0192] The general procedure for ester hydrogenation was used to investigate the hydrogenation of ethyl benzoate at an S / C ratio of 50,000 / 1 at 40 °C in the absence of solvent (“solvent free” conditions). Ethyl benzoate (4.65 mL, 20 mmol) and sodium ethoxide (10 mmol, 50 mol% versus ethyl benzoate) were used in each experiment. The results from Example 3 are shown in Table 4.

[0193] Table 4

[0194] Example 4 shows that ruthenium complexes of the invention are effective ester hydrogenation catalysts for ester substrates comprising aryl groups at S / C ratios of 50,000 / 1 at 40 °C and show some activity at S / C of 100,000 / 1.

[0195] Example 5

[0196] The general procedure for ester hydrogenation was used to investigate the hydrogenation of various ester substrates at an S / C ratio of 50,000 / 1 or 10,000 / 1 at 40 °C. Using toluene (2 mL, 50% of total reaction volume) as a co-solvent. Substrate (4.25 mL, 20 mmol) and base (10 mmol, 50 mol% versus ethyl benzoate) were used in each experiment. The results from Example 5 are shown in Table 5.

[0197] Table 5

[0198] Example 5 shows that the presence of a toluene co-solvent does not impact the conversion to any marked degree indicating that the ruthenium complexes of the invention are particularly effective under solvent free conditions.

[0199] Example 6 (Comparative) Example 6 is a comparative example which shows the performance of a comparative catalyst, Ru(PNP)(DMSO). The conditions in Example 6 are directly comparable to those of Example 1. The general procedure for ester hydrogenation was used to investigate the hydrogenation of methyl decanoate at an S / C ratio of 1,000 / 1 at different temperatures and using different solvents. Methyl decanoate (0.42 mL, 2 mmol) and base (1 mmol, 50 mol% versus methyl decanoate) were used in each experiment. The results from Comparative Example 6 are shown in Table 6.

[0200] Table 6

[0201] Example 6 shows that the comparative catalyst, Ru(PNP)(DMSO), shows inferior conversions of methyl decanoate compared to where ruthenium complexes of the invention are used. For example, experiment 32 achieves a conversion of methyl decanoate of 30.7 %, whilst Ru(SNS)(DMSO) and Ru(PNN)(DMSO) achieve conversions of 86.1 % and 94.1 %, respectively. This result shows that it is surprising that ruthenium complexes of the invention, which comprise the tridentate ligand, L, form competent ester hydrogenation catalysts.

[0202] Moreover, the comparative complex, Ru(PNP)(DMSO) did not affect catalysis at temperatures of 40 °C under any tested conditions. This is in contrast to ruthenium complexes of the invention, which show activity for ester hydrogenation at this temperature. The invention may further be defined in relation to the following numbered clauses.

[0203] 1. A ruthenium complex of formula (I) or formula (II): [Ru(L)AZ]W (I) [Ru(L)AZW] (II) wherein L is a tridentate ligand having formula (III): wherein:

[0204] X is a nitrogen atom and when taken together with R1it forms an optionally substituted heteroaryl group when Rxis absent, or

[0205] X is -SRa, and R1and Rxare each independently selected from hydrogen, substituted or unsubstituted Ci-10-alkyl, substituted or unsubstituted C2-io-alkenyl, substituted or unsubstituted C2- -alkynyl, substituted or unsubstituted Ci-io-heteroalkyl, substituted or unsubstituted Ci- -alkoxy, substituted or unsubstituted Cs-w-cycloalkyl, substituted or unsubstituted Cs-w-cycloalkenyl, substituted or unsubstituted C2-io-heterocycloalkyl, substituted or unsubstituted Ce-w-aryl, and substituted or unsubstituted C4-io-heteroaryl;

[0206] Y is selected from -SRb, -PRaRb, -OPRaRb, and -NHPRaRb;

[0207] R2and Ry, if present, are each independently selected from hydrogen, substituted or unsubstituted Ci-10-alkyl, substituted or unsubstituted C2-io-alkenyl, substituted or unsubstituted C2- -alkynyl, substituted or unsubstituted Ci-io-heteroalkyl, substituted or unsubstituted Ci-io-alkoxy, substituted or unsubstituted Cs-w-cycloalkyl, substituted or unsubstituted Cs-w-cycloalkenyl, substituted or unsubstituted C2-io-heterocycloalkyl, substituted or unsubstituted Ce-w-aryl, and substituted or unsubstituted C4-w-heteroaryl;

[0208] R3a, R3b, R4a, and R4bare each independently selected from hydrogen, substituted or unsubstituted Ci-io-alkyl, substituted or unsubstituted C2-w-alkenyl, substituted or unsubstituted C2-w-alkynyl, substituted or unsubstituted Ci-w-heteroalkyl, substituted or unsubstituted Ci-w-alkoxy, substituted or unsubstituted Cs-w-cycloalkyl, substituted or unsubstituted Cs-w-cycloalkenyl, substituted or unsubstituted C2-w-heterocycloalkyl, substituted or unsubstituted Ce- -aryl, and substituted or unsubstituted C4-w-heteroaryl; R5is selected from hydrogen, or substituted or unsubstituted Ci-s-alkyl;

[0209] Raand Rbare each independently selected from substituted or unsubstituted Ci-10-alkyl, substituted or unsubstituted C2-io-alkenyl, substituted or unsubstituted C2-io-alkynyl, substituted or unsubstituted Ci-io-heteroalkyl, substituted or unsubstituted Ci- -alkoxy, substituted or unsubstituted Cs- -cycloalkyl, substituted or unsubstituted Cs- -cycloalkenyl, substituted or unsubstituted C2-io-heterocycloalkyl, substituted or unsubstituted Ce- -aryl, and substituted or unsubstituted C4-io-heteroaryl, or Raand Rbtogether with the phosphorus atom to which they are attached form a heterocycle;

[0210] Z and W are anionic ligands each independently selected from the group consisting of carboxylates, alkoxides, hydride, and halides; and

[0211] A is a sulfoxide ligand.

[0212] 2. A ruthenium complex of formula (I) or formula (II) according to clause 1 , wherein R2and Ryare each independently selected from hydrogen, substituted or unsubstituted C1.10- alkyl, substituted or unsubstituted Ci-io-heteroalkyl, and substituted or unsubstituted Ci- 10- al koxy.

[0213] 3. A ruthenium complex of formula (I) or formula (II) according to clause 1 or clause 2, wherein R2and Ryare each independently selected from hydrogen, substituted or unsubstituted Ci-5-alkyl, substituted or unsubstituted Ci-5-heteroalkyl, and substituted or unsubstituted Ci-5-alkoxy.

[0214] 4. A ruthenium complex of formula (I) or formula (II) according to any one of the preceding clauses, wherein R2and Ryare each independently selected from hydrogen, methyl, ethyl, / so-propyl, and terf-butyl.

[0215] 5. A ruthenium complex of formula (I) or formula (II) according to any one of the preceding clauses, wherein R2and Ryare each hydrogen.

[0216] 6. A ruthenium complex of formula (I) or formula (II) according to any one of the preceding clauses, wherein R3a, R3b, R4a, and R4bare each independently preferably selected from hydrogen, substituted or unsubstituted Ci-10-alkyl, substituted or unsubstituted C1.10- heteroalkyl, and substituted or unsubstituted Ci-w-alkoxy.

[0217] 7. A ruthenium complex of formula (I) or formula (II) according to any one of the preceding clauses, wherein R3a, R3b, R4a, and R4bare each independently selected from hydrogen, substituted or unsubstituted Ci-s-alkyl, substituted or unsubstituted Ci-5-heteroalkyl, and substituted or unsubstituted Ci-5-alkoxy.

[0218] 8. A ruthenium complex of formula (I) or formula (II) according to any one of the preceding clauses, wherein R3a, R3b, R4a, and R4bare each independently selected from hydrogen, methyl, ethyl, / so-propyl, and terf-butyl.

[0219] 9. A ruthenium complex of formula (I) or formula (II) according to any one of the preceding clauses, wherein R3a, R3b, R4a, and R4bare each hydrogen.

[0220] 10. A ruthenium complex of formula (I) or formula (II) according to any one of the preceding clauses, wherein X is a nitrogen atom and when taken together with R1it forms an optionally substituted heterocycle when Rxis absent.

[0221] 11. A ruthenium complex of formula (I) or formula (II) according to clause 10, wherein the heterocycle is a substituted or unsubstituted 6-membered heterocyclic ring, for example substituted or unsubstituted pyridinyl, pyrrolyl, imidazolyl, pyrazolyl, thiazolyl, isothiazolyl, oxazolyl, isoxazolyl, triazolyl, tetrazolyl, thiadiazolyl, oxadiazolyl, pyrimidyl, benzoxazolyl, benzthiazolyl, benzimidazolyl, indolyl, or quinolinyl.

[0222] 12. A ruthenium complex of formula (I) or formula (II) according to any one of clauses 1 to 9, wherein X is -SRaand R1and Rxare each independently selected from hydrogen, substituted or unsubstituted Ci-10-alkyl, substituted or unsubstituted C2-io-alkenyl, substituted or unsubstituted C2- -alkynyl, substituted or unsubstituted Ci-io-heteroalkyl, substituted or unsubstituted Ci- -alkoxy, substituted or unsubstituted Cs- -cycloalkyl, substituted or unsubstituted Cs- -cycloalkenyl, substituted or unsubstituted C2-io-heterocycloalkyl, substituted or unsubstituted Ce- -aryl, and substituted or unsubstituted C4-io-heteroaryl.

[0223] 13. A ruthenium complex of formula (I) or formula (II) according to clause 12, wherein R1and Rxare each independently selected from hydrogen, substituted or unsubstituted C1.10- alkyl, substituted or unsubstituted Ci-io-heteroalkyl, and substituted or unsubstituted Ci- 10- al koxy.

[0224] 14. A ruthenium complex of formula (I) or formula (II) according to clause 12 or clause 13, wherein R1and Rxare each independently selected from hydrogen, substituted or unsubstituted Ci-5-alkyl, substituted or unsubstituted Ci-5-heteroalkyl, and substituted or unsubstituted Ci-5-alkoxy. 15. A ruthenium complex of formula (I) or formula (II) according to any one of clauses 12 to 14, wherein R1and Rxare each independently selected from hydrogen, methyl, ethyl, isopropyl, and terf-butyl, preferably hydrogen.

[0225] 16. A ruthenium complex of formula (I) or formula (II) according to any one of the preceding clauses, wherein Y is selected from -SRb, -PRaRb, and -NHPRaRb, preferably -SRband - PRaRb

[0226] 17. A ruthenium complex of formula (I) or formula (I I) according to clause 10 or 11 , wherein Y is -PRaRb, -OPRaRb, or -NHPRaRb

[0227] 18. A ruthenium complex of formula (I) or formula (II) according to any one of clauses 12 to 15, wherein Y is -SRb.

[0228] 19. A ruthenium complex of formula (I) or formula (II) according to any one of the preceding clauses, wherein R5is selected from hydrogen and methyl, preferably hydrogen.

[0229] 20. A ruthenium complex of formula (I) or formula (II) according to any one of the preceding clauses, wherein Raand Rbare each independently preferably selected from substituted or unsubstituted Ci-10-alkyl, substituted or unsubstituted Ci-io-heteroalkyl, substituted or unsubstituted Ci- -alkoxy, substituted or unsubstituted Cs-w-cycloalkyl, substituted or unsubstituted Cs-w-cycloalkenyl, substituted or unsubstituted C2-io-heterocycloalkyl, substituted or unsubstituted Ce-w-aryl, and substituted or unsubstituted C4-io-heteroaryl.

[0230] 21 . A ruthenium complex of formula (I) or formula (II) according to any one of the preceding clauses, wherein Raand Rbare each independently selected from substituted or unsubstituted Ci-s-alkyl, substituted or unsubstituted Ci-5-heteroalkyl, substituted or unsubstituted C1.5- alkoxy, substituted or unsubstituted Cs-w-cycloalkyl, substituted or unsubstituted Ce-w-aryl, and substituted or unsubstituted C4-w-heteroaryl, preferably, from methyl, ethyl, / so-propyl, te / Y-butyl, cyclohexyl, adamantyl, and phenyl.

[0231] 22. A ruthenium complex of formula (I) or formula (II) according to clause 10 or 11 , wherein Raand Rbare each independently selected from the group consisting of / so-propyl, te / Y-butyl, cyclohexyl, adamantyl, and phenyl. 23. A ruthenium complex of formula (I) or formula (II) according to any one of clauses 12 to 15, wherein Raand Rbare each independently selected from the group consisting of methyl, ethyl, n-propyl, n-butyl, pentyl, / so-propyl, and terf-butyl, preferably ethyl.

[0232] 24. A ruthenium complex of formula (I) or formula (II) according to any one of the preceding clauses, wherein where X is -SRaand Y is -SRbRaand Rbare not both terf-butyl.

[0233] 25. A ruthenium complex of formula (I) or formula (II) according to clause 1 , wherein L is a tridentate ligand having formula (Illa) wherein:

[0234] Y is selected from -PRaRb, -OPRaRb, and -NHPRaRb;

[0235] R2and Ryare each independently selected from hydrogen, substituted or unsubstituted Ci- - alkyl, substituted or unsubstituted C2-io-alkenyl, substituted or unsubstituted C2- -alkynyl, substituted or unsubstituted Ci-io-heteroalkyl, substituted or unsubstituted Ci- -alkoxy, substituted or unsubstituted Cs-w-cycloalkyl, substituted or unsubstituted Cs-w-cycloalkenyl, substituted or unsubstituted C2-io-heterocycloalkyl, substituted or unsubstituted Ce-w-aryl, and substituted or unsubstituted C4-w-heteroaryl;

[0236] R3a, R3b, R4a, and R4bare each independently selected from hydrogen, substituted or unsubstituted Ci-10-alkyl, substituted or unsubstituted C2-io-alkenyl, substituted or unsubstituted C2- -alkynyl, substituted or unsubstituted Ci-w-heteroalkyl, substituted or unsubstituted Ci-m-alkoxy, substituted or unsubstituted Cs-w-cycloalkyl, substituted or unsubstituted Cs-w-cycloalkenyl, substituted or unsubstituted C2-io-heterocycloalkyl, substituted or unsubstituted Ce-w-aryl, and substituted or unsubstituted C4-w-heteroaryl;

[0237] R5is selected from hydrogen, or substituted or unsubstituted Ci-s-alkyl;

[0238] Raand Rbare each independently selected from substituted or unsubstituted Ci-w-alkyl, substituted or unsubstituted C2-w-alkenyl, substituted or unsubstituted C2-w-alkynyl, substituted or unsubstituted Ci-w-heteroalkyl, substituted or unsubstituted Ci-w-alkoxy, substituted or unsubstituted Cs-w-cycloalkyl, substituted or unsubstituted Cs-w-cycloalkenyl, substituted or unsubstituted C2-io-heterocycloalkyl, substituted or unsubstituted Ce-w-aryl, and substituted or unsubstituted C4-io-heteroaryl, or Raand Rbtogether with the heteroatom to which they are attached form a heterocycle; and each Rwis independently selected from the group consisting of hydrogen, substituted or unsubstituted Ci-10-alkyl, substituted or unsubstituted Ci-io-heteroalkyl, and substituted or unsubstituted Ci- -alkoxy.

[0239] 26. A ruthenium complex of formula (I) or formula (II) according to clause 25, wherein Rwis independently selected from hydrogen, and substituted or unsubstituted C1-10 alkyl.

[0240] 27. A ruthenium complex of formula (I) or formula (II) according to clause 25 or clause 26, wherein each Rwis independently selected from hydrogen, and substituted or unsubstituted C1.5 alkyl.

[0241] 28. A ruthenium complex of formula (I) or formula (II) according to any one of clauses 25 to 27, wherein each Rwis hydrogen.

[0242] 29. A ruthenium complex of formula (I) or formula (II) according to clause 1 , wherein L is a tridentate ligand having formula (I I lb) wherein:

[0243] R1and Rxare each independently selected from hydrogen, substituted or unsubstituted C1.10- alkyl, substituted or unsubstituted C2-io-alkenyl, substituted or unsubstituted C2- -alkynyl, substituted or unsubstituted Ci-io-heteroalkyl, substituted or unsubstituted Ci-io-alkoxy, substituted or unsubstituted Cs-w-cycloalkyl, substituted or unsubstituted Cs-w-cycloalkenyl, substituted or unsubstituted C2-io-heterocycloalkyl, substituted or unsubstituted Ce-w-aryl, and substituted or unsubstituted C4-w-heteroaryl;

[0244] R2and Ryare each independently selected from hydrogen, substituted or unsubstituted C1.10- alkyl, substituted or unsubstituted C2-w-alkenyl, substituted or unsubstituted C2-w-alkynyl, substituted or unsubstituted Ci-w-heteroalkyl, substituted or unsubstituted Ci-w-alkoxy, substituted or unsubstituted Cs-w-cycloalkyl, substituted or unsubstituted Cs-w-cycloalkenyl, substituted or unsubstituted C2-io-heterocycloalkyl, substituted or unsubstituted Ce-w-aryl, and substituted or unsubstituted C4-w-heteroaryl;

[0245] R3a, R3b, R4a, and R4bare each independently selected from hydrogen, substituted or unsubstituted Ci-10-alkyl, substituted or unsubstituted C2-io-alkenyl, substituted or unsubstituted C2-io-alkynyl, substituted or unsubstituted Ci-io-heteroalkyl, substituted or unsubstituted Ci- -alkoxy, substituted or unsubstituted Cs-w-cycloalkyl, substituted or unsubstituted Cs-w-cycloalkenyl, substituted or unsubstituted C2-io-heterocycloalkyl, substituted or unsubstituted Ce-w-aryl, and substituted or unsubstituted C4-w-heteroaryl;

[0246] R5is selected from hydrogen, or substituted or unsubstituted Ci-s-alkyl;

[0247] Raand Rbare each independently selected from substituted or unsubstituted Ci-10-alkyl, substituted or unsubstituted C2-w-alkenyl, substituted or unsubstituted C2-w-alkynyl, substituted or unsubstituted Ci-w-heteroalkyl, substituted or unsubstituted Ci-w-alkoxy, substituted or unsubstituted Cs-w-cycloalkyl, substituted or unsubstituted Cs-w-cycloalkenyl, substituted or unsubstituted C2-w-heterocycloalkyl, substituted or unsubstituted Ce- -aryl, and substituted or unsubstituted C4-w-heteroaryl.

[0248] 30. A ruthenium complex of formula (I) or formula (II) according to claim 29, wherein Raand Rbare not both terf-butyl.

[0249] 31 . A ruthenium complex of formula (I) or formula (II) according to clause 1 , wherein L is one or more tridentate ligands of formulae (lllc), (Hid), and / or (Hie), wherein Raand Rbare each independently selected from substituted or unsubstituted C1.10- alkyl, substituted or unsubstituted C2-io-alkenyl, substituted or unsubstituted C2-io-alkynyl, substituted or unsubstituted Ci-io-heteroalkyl, substituted or unsubstituted Ci- -alkoxy, substituted or unsubstituted Cs- -cycloalkyl, substituted or unsubstituted Cs- -cycloalkenyl, substituted or unsubstituted C2-io-heterocycloalkyl, substituted or unsubstituted Ce- -aryl, and substituted or unsubstituted C4-io-heteroaryl, or where the tridentate ligand is a tridentate ligand having formula (I He) or (Hie) Raand Rbtogether with the heteroatom to which they are attached form a heterocycle.

[0250] 32. A ruthenium complex of formula (I) or formula (II) according to clause 31 , wherein where the tridentate ligand is a tridentate ligand having formula (Hid) Raand Rbare not both terf-butyl.

[0251] 33. A ruthenium complex of formula (I) or formula (II) according to any one of the preceding clauses, wherein Raand Rbare the same or different, preferably the same.

[0252] 34. A ruthenium complex of formula (I) or formula (II) according to clause 1 , wherein the tridentate ligand having formula (HI) is one or more tridentate ligands selected from the group consisting of:

[0253] 35. A ruthenium complex of formula (I) or formula (II) according to clause 1 , wherein the tridentate ligand having formula (III) is one or more tridentate ligands selected from the group consisting of:

[0254] 36. A ruthenium complex of formula (I) or formula (II) according to clause 1 , wherein the tridentate ligand having formula (III) is one or more tridentate ligands selected from the group consisting of:

[0255] 37. A ruthenium complex of formula (I) or formula (II) according to any one of the preceding clauses, wherein Wand / orZ is a carboxylate ligand having formula (IV) or a carboxylate ligand having formula (IVb) where R6is selected from substituted or unsubstituted Ci-10-alkyl, substituted or unsubstituted C2-io-alkenyl, substituted or unsubstituted C2-io-alkynyl, substituted or unsubstituted Ci-io-heteroalkyl, substituted or unsubstituted Ci- -alkoxy, substituted or unsubstituted Cs- -cycloalkyl, substituted or unsubstituted Cs- -cycloalkenyl, substituted or unsubstituted C2-io-heterocycloalkyl, substituted or unsubstituted Ce- -aryl, and substituted or unsubstituted C4-io-heteroary. 38. A ruthenium complex of formula (I) or formula (II) according to clause 34, wherein R6is selected from methyl, ethyl, propyl, butyl, pentyl, or phenyl, preferably methyl.

[0256] 39. A ruthenium complex of formula (I) or formula (II) according to any one of the preceding clauses, wherein W and / or Z are halide, preferably F; Cl; Br, or I’.

[0257] 40. A ruthenium complex of formula (I) or formula (II) according to any one of the preceding clauses, wherein W and / or Z are an alkoxide ligand of formula (V)

[0258] 07

[0259] O - R7

[0260] (V) wherein R7is substituted or unsubstituted Ci-w alkyl, preferably methyl, ethyl, / so-propyl, and terf-butyl.

[0261] 41 . A ruthenium complex of formula (I) or formula (II) according to any one of the preceding clauses, wherein A is a sulfoxide ligand of formula (VI),

[0262] O

[0263] .S I.

[0264] R8^R9

[0265] (VI) wherein R8and R9are each independently selected from substituted or unsubstituted Ci-w alkyl, and substituted or unsubstituted Ce- aryl, or R8and R9are linked to form a ring structure.

[0266] 42. A ruthenium complex of formula (I) or formula (II) according to clause 41 , wherein R8and R9are the same or different.

[0267] 43. A ruthenium complex of formula (I) or formula (II) according to clause 41 or clause 42, wherein R8and R9are each independently selected from methyl, ethyl, iso-propyl, n-propyl, n-butyl, terf-butyl, pentyl, hexyl, octyl, and phenyl.

[0268] 44. A ruthenium complex of formula (I) or formula (II) according to any one of clauses 41 to 43, wherein R8and R9are each independently selected from methyl, ethyl, n-butyl, and phenyl. 45. A ruthenium complex of formula (I) or formula (II) according to any one of clauses 41 to 44, wherein R8and R9are the same and are methyl.

[0269] 46. A ruthenium complex of formula (I) or formula (II) according to any one of clauses 41 to 45, wherein R8and R9are the same.

[0270] 47. A ruthenium complex of formula (I) or formula (II) according to clause 1 , wherein the ruthenium complex of formula (I) or formula (II) is one or more selected from: 48. A ruthenium complex of formula (I) or formula (II) according to clause 1 , wherein the ruthenium complex of formula (I) or formula (II) is one or more selected from:

[0271] 49. A ruthenium complex of formula (I) or formula (II) according to any one of clauses 1 to 46, wherein the ruthenium complex of formula (I) or the ruthenium complex of formula (II) is not:

[0272] 50. A process for reducing an ester containing substrate to an alcohol containing substrate in the presence of a hydrogen source and a base using the ruthenium complex of formula (I) or the ruthenium complex of formula (II) as defined in any one of the preceding clauses. 51. A process for preparing the ruthenium complex of formula (I) or formula (II) according to any one of clauses 1 to 49, the process comprising the step of reacting a ruthenium precursor complex with a tridentate ligand, L, as defined in any one of clauses 1 to 49.

Claims

Claims1. A ruthenium complex of formula (I) or formula (II): [Ru(L)AZ]W (I) [Ru(L)AZW] (II) wherein L is a tridentate ligand having formula (III):wherein:X is a nitrogen atom and when taken together with R1it forms an optionally substituted heteroaryl group when Rxis absent, orX is -SRa, and R1and Rxare each independently selected from hydrogen, substituted or unsubstituted Ci-10-alkyl, substituted or unsubstituted C2-io-alkenyl, substituted or unsubstituted C2- -alkynyl, substituted or unsubstituted Ci-io-heteroalkyl, substituted or unsubstituted Ci- -alkoxy, substituted or unsubstituted Cs-w-cycloalkyl, substituted or unsubstituted Cs-w-cycloalkenyl, substituted or unsubstituted C2-io-heterocycloalkyl, substituted or unsubstituted Ce-w-aryl, and substituted or unsubstituted C4-io-heteroaryl;Y is selected from -SRb, -PRaRb, -OPRaRb, and -NHPRaRb;R2and Ry, if present, are each independently selected from hydrogen, substituted or unsubstituted Ci-10-alkyl, substituted or unsubstituted C2-io-alkenyl, substituted or unsubstituted C2- -alkynyl, substituted or unsubstituted Ci-io-heteroalkyl, substituted or unsubstituted Ci-io-alkoxy, substituted or unsubstituted Cs-w-cycloalkyl, substituted or unsubstituted Cs-w-cycloalkenyl, substituted or unsubstituted C2-io-heterocycloalkyl, substituted or unsubstituted Ce-w-aryl, and substituted or unsubstituted C4-w-heteroaryl;R3a, R3b, R4a, and R4bare each independently selected from hydrogen, substituted or unsubstituted Ci-io-alkyl, substituted or unsubstituted C2-w-alkenyl, substituted or unsubstituted C2-w-alkynyl, substituted or unsubstituted Ci-w-heteroalkyl, substituted or unsubstituted Ci-w-alkoxy, substituted or unsubstituted Cs-w-cycloalkyl, substituted or unsubstituted Cs-w-cycloalkenyl, substituted or unsubstituted C2-w-heterocycloalkyl, substituted or unsubstituted Ce- -aryl, and substituted or unsubstituted C4-w-heteroaryl;R5is selected from hydrogen, or substituted or unsubstituted Ci-s-alkyl;Raand Rbare each independently selected from substituted or unsubstituted Ci-10-alkyl, substituted or unsubstituted C2-io-alkenyl, substituted or unsubstituted C2-io-alkynyl, substituted or unsubstituted Ci-io-heteroalkyl, substituted or unsubstituted Ci- -alkoxy, substituted or unsubstituted Cs-w-cycloalkyl, substituted or unsubstituted Cs-w-cycloalkenyl, substituted or unsubstituted C2-io-heterocycloalkyl, substituted or unsubstituted Ce-w-aryl, and substituted or unsubstituted C4-io-heteroaryl, or Raand Rbtogether with the phosphorus atom to which they are attached form a heterocycle;Z and W are anionic ligands each independently selected from the group consisting of carboxylates, alkoxides, hydride, and halides; andA is a sulfoxide ligand.

2. A ruthenium complex of formula (I) or formula (II) according to claim 1 , wherein R2and Ryare each hydrogen.

3. A ruthenium complex of formula (I) or formula (II) according to claim 1 or claim 2, wherein R3a, R3b, R4a, and R4bare each hydrogen.

4. A ruthenium complex of formula (I) or formula (II) according to any one of the preceding claims, wherein X is a nitrogen atom and when taken together with R1it forms an optionally substituted heterocycle when Rxis absent.

5. A ruthenium complex of formula (I) or formula (II) according to claim 4, wherein the heterocycle is a substituted or unsubstituted 6-membered heterocyclic ring, for example substituted or unsubstituted pyridinyl, pyrrolyl, imidazolyl, pyrazolyl, thiazolyl, isothiazolyl, oxazolyl, isoxazolyl, triazolyl, tetrazolyl, thiadiazolyl, oxadiazolyl, pyrimidyl, benzoxazolyl, benzthiazolyl, benzimidazolyl, indolyl, or quinolinyl.

6. A ruthenium complex of formula (I) or formula (II) according to any one of claims 1 to 3, wherein X is -SRaand R1and Rxare each independently selected from hydrogen, substituted or unsubstituted Ci-10-alkyl, substituted or unsubstituted C2-io-alkenyl, substituted or unsubstituted C2- -alkynyl, substituted or unsubstituted Ci-io-heteroalkyl, substituted or unsubstituted Ci-w-alkoxy, substituted or unsubstituted Cs-w-cycloalkyl, substituted or unsubstituted Cs-w-cycloalkenyl, substituted or unsubstituted C2-io-heterocycloalkyl, substituted or unsubstituted Ce-w-aryl, and substituted or unsubstituted C4-w-heteroaryl.

7. A ruthenium complex of formula (I) or formula (II) according to claim 6, wherein R1and Rxare each independently selected from hydrogen, methyl, ethyl, / so-propyl, and terf-butyl, preferably hydrogen.

8. A ruthenium complex of formula (I) or formula (II) according to any one of the preceding claims, wherein Y is selected from -SRb, -PRaRb, and -NHPRaRb, preferably -SRband - PRaRb9. A ruthenium complex of formula (I) or formula (II) according to any one of the preceding claims, wherein Raand Rbare each independently preferably selected from substituted or unsubstituted Ci-10-alkyl, substituted or unsubstituted Ci-io-heteroalkyl, substituted or unsubstituted Ci- -alkoxy, substituted or unsubstituted Cs- -cycloalkyl, substituted or unsubstituted Cs- -cycloalkenyl, substituted or unsubstituted C2-io-heterocycloalkyl, substituted or unsubstituted Ce- -aryl, and substituted or unsubstituted C4-io-heteroaryl.

10. A ruthenium complex of formula (I) or formula (I I) according to claim 4 or 5, wherein Raand Rbare each independently selected from the group consisting of / so-propyl, terf-butyl, cyclohexyl, adamantyl, and phenyl.

11. A ruthenium complex of formula (I) or formula (II) according to claim 6 or claim 7, wherein Raand Rbare each independently selected from the group consisting of methyl, ethyl, n-propyl, n-butyl, pentyl, / so-propyl, and te / Y-butyl, preferably ethyl.

12. A ruthenium complex of formula (I) or formula (II) according to claim 1 , wherein L is a tridentate ligand having formula (Illa)wherein:Y is selected from -PRaRb, -OPRaRb, and -NHPRaRb;R2and Ryare each independently selected from hydrogen, substituted or unsubstituted Ci- - alkyl, substituted or unsubstituted C2-io-alkenyl, substituted or unsubstituted C2-io-alkynyl, substituted or unsubstituted Ci-io-heteroalkyl, substituted or unsubstituted Ci- -alkoxy, substituted or unsubstituted Cs-w-cycloalkyl, substituted or unsubstituted Cs-w-cycloalkenyl, substituted or unsubstituted C2-io-heterocycloalkyl, substituted or unsubstituted Ce-w-aryl, and substituted or unsubstituted C4-w-heteroaryl;R3a, R3b, R4a, and R4bare each independently selected from hydrogen, substituted or unsubstituted Ci-10-alkyl, substituted or unsubstituted C2-io-alkenyl, substituted or unsubstituted C2- -alkynyl, substituted or unsubstituted Ci-io-heteroalkyl, substituted or unsubstituted Ci-io-alkoxy, substituted or unsubstituted Cs-w-cycloalkyl, substituted or unsubstituted Cs-w-cycloalkenyl, substituted or unsubstituted C2-io-heterocycloalkyl, substituted or unsubstituted Ce-w-aryl, and substituted or unsubstituted C4-w-heteroaryl;R5is selected from hydrogen, or substituted or unsubstituted Ci-s-alkyl;Raand Rbare each independently selected from substituted or unsubstituted Ci-10-alkyl, substituted or unsubstituted C2-w-alkenyl, substituted or unsubstituted C2-w-alkynyl, substituted or unsubstituted Ci-w-heteroalkyl, substituted or unsubstituted Ci-w-alkoxy, substituted or unsubstituted Cs-w-cycloalkyl, substituted or unsubstituted Cs-w-cycloalkenyl, substituted or unsubstituted C2-w-heterocycloalkyl, substituted or unsubstituted Ce- -aryl, and substituted or unsubstituted C4-w-heteroaryl, or Raand Rbtogether with the heteroatom to which they are attached form a heterocycle; and each Rwis independently selected from the group consisting of hydrogen, substituted or unsubstituted Ci-w-alkyl, substituted or unsubstituted Ci-w-heteroalkyl, and substituted or unsubstituted Ci-w-alkoxy.

13. A ruthenium complex of formula (I) or formula (II) according to claim 12, wherein each Rwis hydrogen.

14. A ruthenium complex of formula (I) or formula (II) according to claim 1 , wherein L is a tridentate ligand having formula (I I lb)wherein:R1and Rxare each independently selected from hydrogen, substituted or unsubstituted Ci- - alkyl, substituted or unsubstituted C2-io-alkenyl, substituted or unsubstituted C2-io-alkynyl, substituted or unsubstituted Ci-io-heteroalkyl, substituted or unsubstituted Ci- -alkoxy, substituted or unsubstituted Cs-w-cycloalkyl, substituted or unsubstituted Cs-w-cycloalkenyl, substituted or unsubstituted C2-io-heterocycloalkyl, substituted or unsubstituted Ce-w-aryl, and substituted or unsubstituted C4-io-heteroaryl;R2and Ryare each independently selected from hydrogen, substituted or unsubstituted CMO- alkyl, substituted or unsubstituted C2-io-alkenyl, substituted or unsubstituted C2- -alkynyl, substituted or unsubstituted Ci-io-heteroalkyl, substituted or unsubstituted Ci-io-alkoxy, substituted or unsubstituted Cs-w-cycloalkyl, substituted or unsubstituted Cs-w-cycloalkenyl, substituted or unsubstituted C2-io-heterocycloalkyl, substituted or unsubstituted Ce-w-aryl, and substituted or unsubstituted C4-w-heteroaryl;R3a, R3b, R4a, and R4bare each independently selected from hydrogen, substituted or unsubstituted Ci-io-alkyl, substituted or unsubstituted C2-w-alkenyl, substituted or unsubstituted C2-w-alkynyl, substituted or unsubstituted Ci-w-heteroalkyl, substituted or unsubstituted Ci-w-alkoxy, substituted or unsubstituted Cs-w-cycloalkyl, substituted or unsubstituted Cs-w-cycloalkenyl, substituted or unsubstituted C2-w-heterocycloalkyl, substituted or unsubstituted Ce- -aryl, and substituted or unsubstituted C4-w-heteroaryl;R5is selected from hydrogen, or substituted or unsubstituted Ci-s-alkyl;Raand Rbare each independently selected from substituted or unsubstituted Ci-w-alkyl, substituted or unsubstituted C2-w-alkenyl, substituted or unsubstituted C2-w-alkynyl, substituted or unsubstituted Ci-w-heteroalkyl, substituted or unsubstituted Ci-w-alkoxy, substituted or unsubstituted Cs-w-cycloalkyl, substituted or unsubstituted Cs-w-cycloalkenyl, substituted or unsubstituted C2-w-heterocycloalkyl, substituted or unsubstituted Ce-w-aryl, and substituted or unsubstituted C4-w-heteroaryl.

15. A ruthenium complex of formula (I) or formula (II) according to any one of the preceding claims, wherein R5is selected from hydrogen and methyl, preferably hydrogen.

16. A ruthenium complex of formula (I) or formula (II) according to claim 1 , wherein L is one or more tridentate ligands of formulae (lllc), (Hid), and / or (Hie),wherein Raand Rbare each independently selected from substituted or unsubstituted C1.10- alkyl, substituted or unsubstituted C2-io-alkenyl, substituted or unsubstituted C2-io-alkynyl, substituted or unsubstituted Ci-io-heteroalkyl, substituted or unsubstituted Ci- -alkoxy, substituted or unsubstituted Cs- -cycloalkyl, substituted or unsubstituted Cs- -cycloalkenyl, substituted or unsubstituted C2-io-heterocycloalkyl, substituted or unsubstituted Ce- -aryl, and substituted or unsubstituted C4-io-heteroaryl, or where the tridentate ligand is a tridentate ligand having formula (I He) or (Ille) Raand Rbtogether with the heteroatom to which they are attached form a heterocycle.

17. A ruthenium complex of formula (I) or formula (II) according to any one of the preceding claims, wherein Raand Rbare the same.

18. A ruthenium complex of formula (I) or formula (I I) according to claim 1 , wherein the tridentate ligand having formula (III) is one or more tridentate ligands selected from the group consisting of:

19. A ruthenium complex of formula (I) or formula (II) according to any one of the preceding claims, wherein W and / or Z is a carboxylate ligand having formula (IV) or a carboxylate ligand having formula (IVb)(IV) (IVb) where R6is selected from substituted or unsubstituted Ci-10-alkyl, substituted or unsubstituted C2-io-alkenyl, substituted or unsubstituted C2-io-alkynyl, substituted or unsubstituted Ci-io-heteroalkyl, substituted or unsubstituted Ci- -alkoxy, substituted or unsubstituted Cs- -cycloalkyl, substituted or unsubstituted Cs- -cycloalkenyl, substituted or unsubstituted C2-io-heterocycloalkyl, substituted or unsubstituted Ce- -aryl, and substituted or unsubstituted C4-io-heteroary.

20. A ruthenium complex of formula (I) or formula (II) according to any one of the preceding claims, wherein W and / or Z are halide, preferably F; Cl; Br, or I’.21 . A ruthenium complex of formula (I) or formula (II) according to any one of the preceding claims, wherein A is a sulfoxide ligand of formula (VI),wherein R8and R9are each independently selected from substituted or unsubstituted Ci-w alkyl, and substituted or unsubstituted Ce- aryl, preferably R8and R9are both methyl, or R8and R9are linked to form a ring structure.

22. A ruthenium complex of formula (I) or formula (II) according to any one of claims 1 to 21, wherein the ruthenium complex of formula (I) or the ruthenium complex of formula (II) is23. A process for reducing an ester containing substrate to an alcohol containing substrate in the presence of a hydrogen source and a base using the ruthenium complex of formula (I) or the ruthenium complex of formula (II) as defined in any one of the preceding claims.

24. A process for preparing the ruthenium complex of formula (I) or formula (II) according to any one of claims 1 to 22, the process comprising the step of reacting a ruthenium precursor complex with a tridentate ligand, L, as defined in any one of the claims 1 to 22.

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