Mononuclear nickel complex incorporating a disulphide-based scaffold and applications thereof

Abstract

Described herein is a nickel(II) complex [Ni(L1)2S2·DMF] of structure I as below: Structure I The mononuclear nickel complex of structure I shows significant antibacterial activity against gram positive and gram negative bacteria. The complex further exhibits a selective affinity for Fluoride ions. The invention further relates to a process for manufacturing said nickel(II) complex [Ni(L1)2S2·DMF].

Description

Field of Invention: The current invention is in field of mononuclear complexes. More particularly, the current invention relates to a mononuclear nickel(II) complex, [Ni(L1)2S2•DMF], synthesized through the coordination of an ONS-donor ligand with nickel(II) perchlorate hexahydrate.Background of invention:The scientific community is struggling to keep up with the pace at which bacterial in The scientific community is struggling to keep pace with the rate at which bacterial infections are developing multidrug resistance. As of July 2019, there were forty-two antibiotic drug candidates in clinical trials. However, only eleven of these represented entirely new chemical entities, and among them, just one compound showed effectiveness against the notoriously resilient Gram-negative strains. Commercial development of new antibiotics is unlikely to replenish the antibiotic pipeline in the near future. The number of pharmaceutical companies actively pursuing antibiotic research continues to decline every year, largely due to the unfavourable return on investment and short treatment durations that limit commercial viability.This growing gap between resistant pathogens and effective therapies has prompted researchers to explore new directions beyond traditional organic antibiotics. One such direction is bioinorganic chemistry, a rapidly evolving field that investigates the biological applications of inorganic and organometallic molecules. The rationale lies in the unique properties of metals - including variable oxidation states, coordination flexibility, and redox activity - which allow them to interact with biological targets in ways organic molecules cannot.The idea of using metal-based compounds in medicine is not new. Historically, some of the earliest successful therapeutic agents were metal or metalloid complexes. Ehrlich's Salvarsan, containing arsenic, was introduced in 1909 as the first effective treatment for syphilis. Several decades later, Rosenberg's discovery of the platinum-based coordination complex cisplatin, followed by its derivatives oxaliplatin and carboplatin, revolutionized cancer therapy. These discoveries demonstrated the immense therapeutic potential of metal complexes and established their importance in modern medicine.Earlier, the use of organo-transition metal chelates as antiviral agents was discussed by David D. Busath et al. in Patent Application WO 2015 / 192037. However, WO '037 specifically focuses on such chelates for the treatment of influenza virus infections, not for coronaviruses.More recently, J. Karges and S. M. Cohen (ChemBioChem, 2021, 22, 2600; DOI: 10.1002 / cbic.202100186) provided detailed insight into the potential of metal complexes as antiviral agents against SARS-CoV-2. Their study explored gold (Au)-, selenium (Se)-, rhenium (Re)-, and bismuth (Bi)-based complexes, which were found to inhibit the main protease (Mpro) and papain-like protease (PLpro) of the virus - both of which are key enzymes in the viral replication cycle.Building upon these early foundations, contemporary research has extended the use of metal complexes from antibacterial and anticancer fields into antiviral therapy. A significant contribution in this direction is described in Indian Patent Application IN553352 (IN '352), filed by the same applicant. IN '352 discloses phenoxy-bridged transition metal coordination complexes, particularly those of Ni(II), characterized as dinuclear complexes. The complexes as claimed in IN '352 demonstrate inhibitory activity against the SARS-CoV-2 virus, suggesting their potential as antiviral agents.Collectively, these developments trace a clear scientific trajectory - from the decline of traditional antibiotics, through the historic success of metal-based therapies, to the present revival of bioinorganic compounds as versatile antiviral and antimicrobial candidates. The unique coordination chemistry and redox behaviour of transition metal complexes offer mechanisms of action distinct from conventional drugs. This renewed focus on metal-based therapeutics represents a rational and promising strategy to address the global antimicrobial resistance crisis and to develop new treatments for viral infections such as SARS-CoV-2.In view of above, the current inventors propose novel set of bioinorganic compounds which are mononuclear nickel complexes which exhibit anti-bacterial activity. Objects of Invention: The primary object of the invention is to provide such bioinorganic compounds which are mononuclear complexes of Nickel(II).In another object the invention intends to provide such complexes which show multiple activities, including anti-bacterial activity and ion sensing activity. Summary of the invention:In primary aspect, the invention provides mononuclear complexes of nickel(II). More particularly, the present invention provides a mononuclear Nickel(II) complex incorporating a disulphide-based scaffold.The complexes of the invitation exhibit a good antibacterial activity.The complexes of the invitation, further exhibit a selective negative ion affinity, more particularly, fluoride ions.In yet another aspect of the invention, the compound of the invention enhancing early-stage drug discovery in computer-aided drug discovery. Conventional approaches typically rely on different compounds for each of these tasks, leading to increased time, cost, and complexity. This drawback of the prior art has been mitigated by the current invention. In yet another aspect, the invention provides a process for synthesis of mononuclear Nickel(II) complex incorporating a disulphide-based scaffold.Brief Description of Drawings: Fig. 1 illustrates an ORTEP view of the mononuclear nickel(II) complex [Ni(L1)2S2•DMF]. Ellipsoids are drawn to encompass 50% probability level Fig. 2 illustrates graphical presentation of in-vitro antibacterial activity for disulfide-based ligand (L2H2S2) against Gram-positive bacteria (Bacillus Subtilis) with different drug concentrations.Fig. 3 illustrates graphical presentation of in-vitro antibacterial activity for disulfide-based ligand (L2H2S2) against Gram-negative bacteria (E. Coli) with different drug concentrations.Fig. 4 illustrates Graphical presentation of in-vitro antibacterial activity for nickel(II) complex, [Ni(L1)2S2·DMF] against Gram-positive bacteria (Bacillus Subtilis) with different drug concentrations.Fig. 5 illustrates graphical presentation of in-vitro antibacterial activity for nickel(II) complex, [Ni(L1)2S2·DMF] against Gram-negative bacteria (E. Coli) with different drug concentrations.Fig. 6 illustrates UV-Vis spectra of disulfide ligand (L2H2S2) with increasing TBAF (0.1-1.2 eq), showing spectral changes due to interaction with fluoride ions.Fig. 7 illustrates molecular docking of the bounded protein 5Y9P with the nickel(II) complex, [Ni(L1)2S2·DMF].Fig. 8 illustrates molecular docking of the bounded protein 1AHP with the nickel(II) complex, [Ni(L1)2S2·DMF].Detailed Description of the Invention:The invention has other advantages and features which will be more readily apparent from the following detailed description of the invention and the appended claims, when taken in conjunction with the accompanying drawings.The invention is described herein in detail with the help of figures appended at the end of the specification. The figures illustrate the preferred embodiment as well as other embodiments that define the scope of the present invention. However, it may be understood that the figures presented herein are intended to exemplify the scope of the invention only. The person skilled in art may note that by no means the figures limit the scope of the invention. Any variation in the drawings by any other person will be falling in the scope of the present invention.Throughout the specification and claims, the following terms take the meanings explicitly associated herein unless the context clearly dictates otherwise. The meaning of "a", "an", and "the" include plural references. The meaning of "in" includes "in" and "on." Referring to the drawings, like numbers indicate like parts throughout the views. Additionally, a reference to the singular includes a reference to the plural unless otherwise stated or inconsistent with the disclosure herein.The current invention describes a mononuclear nickel(II) complex [Ni(L1)2S2•DMF] which exhibits a significant antibacterial activity against various gram positive and gram negative bacteria.Accordingly, The invention focuses on the development of a new mononuclear nickel(II) complex, [Ni(L1)2S2·DMF], formed using a custom-designed disulfide-based ligand (L2H2S2).In accordance with one of the embodiments, the disulfide-based ligand (L2H2S2) is synthesized through a multi-step reaction sequence. Initially, 2-amino-4-chlorobenzenethiol undergoes oxidation with iodine (I2) and hydrogen peroxide (H2O2) in a basic ethanolic medium, leading to the formation of 6,6'-disulphanediyl bis(3-chloroaniline). This intermediate is then condensed with 3,5-dichlorosalicylaldehyde in the presence of magnesium sulfate (MgSO4) in ethanol, resulting in the formation of the final disulfide ligand, L2H2S2. The ligand contains functional groups capable of coordinating with nickel(II) ions, forming a stable mononuclear complex with octahedral geometry, as illustrated in Scheme 1Scheme 1A dimethylformamide (DMF) molecule is also coordinated to the nickel(II) center, enhancing the structural integrity and stability of the resulting mononuclear complex. This complex is designed for targeted applications, including antimicrobial assessment against Staphylococcus aureus and Escherichia coli, selective fluoride ion detection, and antibacterial activity evaluation through computational molecular docking studies involving the same bacterial strains. The synthesized compounds was verified using SC-XRD, NMR and FT-IR for the ligand, HRMS for the nickel(II) complex, and UV-Vis spectroscopy to study the ligand and its sensing with fluoride ions in DMF solvent. The complex exhibited significant antimicrobial activity, supported by theoretical studies. Coordination with the metal centre enhances the compound's bioactivity by promoting better cellular uptake and interaction with biological targets. In accordance with one of the embodiments, the invention comprises two main components: a novel disulfide-based ligand (L2H2S2) and its corresponding mononuclear nickel(II) complex [Ni(L1)2S2·DMF]. Each component is described in detail below:(1) A solution of 2-amino-4-chlorobenzenethiol in ethanol is oxidized using iodine and hydrogen peroxide under basic conditions to form 6,6'-disulfanediyl bis(3-chloroaniline) via disulfide bond formation. This intermediate is then condensed with 3,5-dichlorosalicylaldehyde in ethanol in the presence of anhydrous MgSO4, yielding the disulfide-based ligand (L2H2S2), which is purified by filtration and washing with cold ethanol.(2) To a stirred solution of ligand L2H2S2 in dimethylformamide (DMF), a stoichiometric amount of nickel(II) perchlorate hexahydrate [Ni(ClO4)2·6H2O] is added, followed by triethylamine (NEt3) as a base. The reaction mixture is allowed to stir at room temperature, enabling the deprotonated ligand to coordinate with the nickel(II) ion. The resulting complex features two ligands coordinated through donor atoms (nitrogen, oxygen and sulphur) to the metal centre, with one DMF molecule occupying an additional coordination site. This forms a mononuclear nickel(II) complex with an octahedral geometry. The product is allowed to crystallize by slow evaporation, yielding single crystals suitable for structural analysis Crystallographic data and structure refinement details for the nickel(II) complex [Ni(L1)₂S₂·DMF] are presented in Table 1.In accordance with one more embodiment, The disulfide-based ligand (L2H2S2) and nickel(II) complex, [Ni(L1)2S2·DMF] were evaluated for in vitro antibacterial activity against four bacterial strains: two Gram-positive (Bacillus subtilis and Staphylococcus aureus) and two Gram-negative (Escherichia coli and Salmonella typhi). The assessment involved determining the minimum inhibitory concentration (MIC) and the zone of inhibition. MIC is defined as the lowest concentration at which visible bacterial growth is completely suppressed, while the non-inhibitory concentration (NIC) refers to the lowest concentration that slows but does not stop bacterial growth. To determine these values, serial dilutions of both (L2H2S2) and nickel(II) complex, [Ni(L1)2S2·DMF] were prepared in a liquid growth medium. Each dilution was inoculated with a specific bacterial strain, incubated under appropriate conditions, and observed for bacterial growth. The MIC was identified as the lowest concentration that prevented visible growth, offering a quantitative measure of antibacterial efficacy.The modified Gompertz model was applied to analyze antibacterial activity data by adjusting parameters such as the lower asymptote (A), curve range (C), logarithm of the inflection point (M), and slope factor (B). This model provides a more accurate determination of the minimum inhibitory concentration (MIC) by fitting the experimental data more precisely. Figures 2-5 show the in vitro antibacterial effects of the disulfide-based ligand (L2H2S2) and nickel(II) complex [Ni(L1)2S2·DMF] at various concentrations against four bacterial strains. Both compounds demonstrated significant activity, especially against Gram-positive and Gram-negative bacteria, evidenced by low MIC and NIC values and larger inhibition zones. The increased lipophilicity of the metal complex likely enhances membrane permeability and uptake, improving its antibacterial efficacy against Gram-positive strains.Furthermore, in silico molecular docking studies were conducted to evaluate the inhibitory properties (binding affinity and inhibition constant) of the nickel(II) complex against Staphylococcus aureus RNase HII and E. coli targets (PDB IDs: 5Y9P and 1AHP). Stronger binding between the nickel(II) complex [Ni(L1)2S2•DMF] and receptor proteins was indicated by negative binding affinity values. Molecular docking into the functional dimer of Staphylococcus aureus RNase HII and E. coli targets (PDB IDs: 5Y9P and 1AHP) revealed binding affinities of -8.1 and -7.8 kcal / mol, with corresponding inhibition constants of 1.365 and 1.258 mM, respectively showing better performance compared to standard antibacterial drugs.To assess the impact of mutations on protein-ligand / metal interactions in S. aureus and E. coli, control docking experiments were conducted using their unmutated / unbound protein structures (PDB IDs: 5Y9P and 1AHP). The results are shown in Figures 7 and 8. Control docking with the unbound protein provides a reference, helping identify structural changes in the mutated protein that affect metal binding. Various types' of interactions such as hydrogen bonding, electrostatic interaction, hydrophobic interactions, π-π stacking, and metal coordination were observed. These results suggest good antimicrobial potential of the complex and support its further evaluation as a metal-based therapeutic agent. These findings provide valuable insights into the antimicrobial potential of the nickel(II) complex, highlighting strong intermolecular interactions as a promising therapeutic approach. In accordance with one of the embodiments, the nickel(II) complex [Ni(L1)2S2·DMF] of the invention exhibited selective affinity towards fluoride ion. Accordingly, The UV-Vis absorption spectrum of disulfide-based ligand (L2H2S2) highlights the interaction between a 10 μM and increasing equivalents of tetrabutyl ammonium fluoride (TBAF) from 0 to 1.2. In its unbound state (0 eq TBAF), the Schiff base exhibits two major absorption peaks around 330 nm and 470 nm, corresponding to distinct electronic transitions. Upon gradual addition of TBAF, the absorbance at ~470 nm intensifies while the peak near ~330 nm diminishes, indicating a fluoride-induced shift in the electronic distribution, likely through hydrogen bonding or de-protonation at functional groups adjacent to the imine moiety. The emergence of a well-defined isosbestic point near 400 nm confirms a clean transformation between two species the unbound and fluoride-bound (or deprotonated) forms-supporting a specific 1:1 interaction. After the addition of approximately 1.2 equivalent of TBAF, do not lead to any significant spectral changes, indicating that all available interaction sites on the disulfide-based ligand have been saturated. At this stage, additional fluoride ions remain unbound in solution and do not further influence the electronic structure of the sensor molecule. Fig. 6 shows the UV-Vis spectra of disulfide ligand in DMF upon addition of TBAF (0.1-1.2 eq) for fluoride ion sensing.Advantages of the Invention: The current invention provides a multi-functional, dual-purpose complex that is both an effective antimicrobial and a selective fluoride ion sensor.The bioinorganic complex of the invention shows strong binding to S. aureus and E. coli proteins. The bioinorganic complex of the invention demonstrates lower inhibition constants than standard antibacterial drugs.The bioinorganic complex of the invention Combines experimental validation and computational docking, providing mechanistic insights.Existing sensors and antimicrobial agents lack selectivity, are expensive, or pose toxicity issues. Easy to synthesize and scale, making it suitable for broader research and commercial use.Compatible with current fluoride ion sensing and antimicrobial platforms.Useful in bioinorganic chemistry, therapeutic drug design, and chemical sensing for environmental application. In an advantageous embodiment, the bioinorganic complex of the invention is more cost-effective than silver or gold-based complexes.In yet another advantageous embodiment, the bioinorganic complex of the invention is less toxic, environmentally safer alternative to other transition metal complexes. In yet another advantageous embodiment, the bioinorganic complex of the invention is stable and reproducible complex structure with clear ligand design. EXAMPLES:Several examples are set forth below to further illustrate the nature of the invention and the manner of carrying it out. However, the invention should not be considered as being limited to the details thereof.Example 1:Process for synthesis of mononuclear nickel(II) complex [Ni(L1)2S2·DMF]:Oxidizing a solution of 2-amino-4-chlorobenzenethiol in ethanol using iodine and hydrogen peroxide under basic conditions to form 6,6'-disulfanediyl bis(3-chloroaniline) via disulfide bond formation;condensing the intermediate of step 1 with 3,5-dichlorosalicylaldehyde in ethanol in the presence of anhydrous MgSO4, yielding the disulfide-based ligand (L2H2S2), purifying the disulfide-based ligand of step 2 by filtration and washing with cold ethanol;adding stoichiometric amount of nickel(II) perchlorate hexahydrate [Ni(ClO4)2·6H2O] to the stirred solution of ligand L2H2S2 in dimethylformamide (DMF) of step 3; triethylamine (NEt3) is further added to the solution of Step 3 as a base;stirring the reaction mixture of Step 5 at room temperature, enabling the deprotonated ligand to coordinate with the nickel(II) ion. The resulting complex features two ligands coordinated through donor atoms (nitrogen, oxygen and sulphur) to the metal centre, with one DMF molecule occupying an additional coordination site. This forms a mononuclear nickel(II) complex with an octahedral geometry. The product is allowed to crystallize by slow evaporation, yielding single crystals suitable for structural analysis Crystallographic data and structure refinement details for the nickel(II) complex [Ni(L1)₂S₂·DMF] are presented in Table 1.Table 1. Crystallographic data and structure refinement for nickel(II) complex, [Ni(L1)2S2DMF].Example 2: Antibacterial ActivityTable 2 summarizes the antibacterial activity and MIC values for both compounds against the tested Gram-positive and Gram-negative strains. Results are expressed as mean ± SEM, based on three or more independent experiments (n ≥ 3). MIC values were derived from the inflection point of the dose-response curve and the plateau regions, providing a consistent and reproducible evaluation of antimicrobial potential.MIC = 10M + 1 / BTable 2: Antibacterial activity and minimum inhibition concentration values of the disulfide-based ligand (L2H2S2) and nickel(II) complex, [Ni(L1)2S2·DMF] against gram positive and gram-negative bacteria.Example 3: Molecular Docking StudyTable 3: The molecular docking results and types of interactions disulfide-based ligand (L2H2S2) and nickel(II) complex, [Ni(L1)2S2·DMF] with the structure of Staphylococcus aureus and Escherichia Coli (PDB ID: 5Y9P and 1AHP) including the binding affinity, inhibition constant and different amino acid residues that interact with complexes.

Claims

1. A mononuclear nickel(II) complex [Ni(L1)2S2·DMF] of Structure I: Structure I wherein, the mononuclear complex exhibits an antibacterial activity against gram positive and gram negative bacteria.

2. The mononuclear complex as claimed in Claim 1, wherein MIC for gram positive bacteria is in range of 2.8 - 3.0.

3. The mononuclear complex as claimed in Claim 1, wherein MIC for gram negative bacteria is in range of 2.7 - 3.0.

4. The mononuclear complex as claimed in Claim 1, wherein the mononuclear complex inhibits 5Y9P and 1AHP proteins.

5. The mononuclear complex as claimed in Claim 4, wherein the inhibition constant (Ki) for 5Y9P is 1.365 mM.

6. The mononuclear complex as claimed in Claim 4, wherein the inhibition constant (Ki) for 1AHP is 1.258 mM7. The mononuclear complex as claimed in Claim 1, wherein the molecule has a well-defined octahedral geometry.

8. The mononuclear complex as claimed in Claim 1, wherein the molecule exhibits selective affinity towards Fluoride ions.

9. A process for synthesis of mononuclear complex as claimed in Claim 1, wherein the process comprises of: a. Oxidizing a solution of 2-amino-4-chlorobenzenethiol in ethanol using iodine and hydrogen peroxide under basic conditions to form 6,6'-disulfanediyl bis(3-chloroaniline) via disulfide bond formation; b. condensing the intermediate of step a with 3,5-dichlorosalicylaldehyde in ethanol in the presence of anhydrous MgSO4, yielding the disulfide-based ligand (L2H2S2); c. purifying the disulfide-based ligand of step b by filtration and washing with cold ethanol; d. adding stoichiometric amount of nickel(II) perchlorate hexahydrate [Ni(ClO4)2·6H2O] to the stirred solution of ligand L2H2S2 in dimethylformamide (DMF) of step c; e. triethylamine (NEt3) is further added to the solution of Step c as a base; f. stirring the reaction mixture of Step e at room temperature, enabling the deprotonated ligand to coordinate with the nickel(II) ion, and g. crystallizing the mononuclear complex of step f by slow evaporation, for yielding single crystals of mononuclear nickel(II) complex.