Quinoline acylhydrazone ligands and their fe(ii) spin crossover complexes and methods of synthesis

Abstract

This invention discloses a quinoline hydrazone ligand, its Fe(II) spin-crossover complex, and a method for synthesis. The quinoline hydrazone ligand is ( E )-4-bromo- N' -(quinoline-8-methylene)benzoylhydrazine, with the structural formula shown, and the complex structure shown. The ligands of this invention have sufficiently strong coordination ability with metals. The complex is not easily oxidized in air, does not easily dissociate in solution, and has good acid resistance. The complex is in a low-spin state and is suitable for photoacid switching in solution, making it applicable to fields such as spin-crossed molecular switches and acid-base stimulation sensors.

Description

Technical Field

[0001] This invention belongs to the field of inorganic coordination chemistry and relates to a quinoline hydrazone ligand and its Fe(II) spin-crossed complex and its synthesis method. Background Technology

[0002] Spin crossover (SCO) is a concept in coordination chemistry that refers to elements in the fourth period of the periodic table that have 3d... 4 -3d 7 A spin-crossover phenomenon in mononuclear MnO4 complexes, characterized by a spin state transition or spin jump. III complex tuned by metal dithiolene counteranions,2014,43(9):3783-3791].

[0003] Complexes are composed of inorganic or organic ligands with multiple coordinating atoms (usually nitrogen and oxygen atoms) and a central metal ion. During coordination, the coordinating atoms on the ligands donate lone pairs of electrons, while the metal ion provides empty d orbitals. The two are bonded together by Lewis addition to form a coordinate bond. When the field strength of the ligand is a non-spherical symmetric field, such as an octahedral field, taking Fe(II) as an example, its five originally identical and degenerate d orbitals will split, forming two new electron orbitals. g orbit and t 2g The energy difference between two orbitals is called the crystal field splitting energy Δ. o Its magnitude depends on the magnitude of the ligand field strength; the greater the field strength, the larger Δ. o The larger the value, the smaller the value; conversely, the smaller the value. At this point, the d electrons of the central metal ion will be redistributed to the e electrons. g orbit and t 2g In the orbit, there exists an electron pairing energy P between d electrons, when Δ o When <P, all electrons are arranged in pairs at t. 2g On the orbit, a low-spin state LS (Low Spin,t) is formed. 2g 6 e g 0 ); when Δ o When the electron configuration is greater than P, electrons begin to align with the e-column. g On the orbit, a high-spin state HS (High Spin,t) is formed. 2g 4 e g 2 ); when Δ oWhen the electron is in the P orbital, it can easily transition between the two orbitals, achieving a spin-state transition.

[0004] Acylhydrazones are essentially Schiff bases. Schiff bases are a class of organic compounds containing a carbon-nitrogen double bond, with a nitrogen atom and a carbon atom connected by a double bond. Their general structural formula is R′R″C=NR, where R′ and R″ are hydrogen atoms or organic groups, and R is an organic group. Schiff bases are typically formed by the dehydration condensation reaction of a primary amine (-NH₂) with an aldehyde (-CHO) or ketone. Acylhydrazones, however, are formed by the reaction of an acylhydrazine compound (R₁CONHNH₂) with an aldehyde; this reaction is also known as the Schiff base reaction.

[0005] However, reports on quinoline hydrazones, both domestically and internationally, are extremely scarce, with fewer than 15 reported cases. Most of these reports involve quinoline hydrazones coordinating with other transition metal ions such as Zn(II), U(VI), Al(III), Cd(II), Pd(II), Ag(I), and Mg(II), used as chemical sensors in fluorescence and colorimetric methods to detect some of these metal ions or to study the antibacterial properties of quinoline hydrazone complexes against *Escherichia coli* and *Staphylococcus aureus*, rather than for studying spin-crossing properties. The only reported case of quinoline hydrazone coordinating with Fe(II) still uses a colorimetric chemical sensor to detect Fe(II), as shown below:

[0006] (A novel quinoline derivative as dual chemosensor for selectivesensing of Al 3+ by fluorescent and Fe 2+ by colorimetric methods, 2021,1231:129965).

[0007] In addition, photoacids are a class of spiropyran-type organic compounds. When exposed to light of a specific wavelength in protic solvents (water, methanol), their structure undergoes isomerization from open-ring to closed-ring form, releasing protons and increasing acidity. However, when exposed to darkness, their structure reversibly reverts from closed-ring to open-ring, while reclaiming protons and decreasing acidity. Summary of the Invention

[0008] The purpose of this invention is to provide a quinoline hydrazone ligand, its Fe(II) spin-crossed complex, and a synthetic method. This invention synthesizes a novel quinoline hydrazone ligand using quinoline-8-carboxaldehyde and 4-bromobenzoylhydrazine, and then combines it with Fe... 2+Coordination forms complexes, and the resulting quinoline hydrazone Fe(II) complexes have potential applications in areas such as spin-crossed molecular switches in solution and acid-base response sensors.

[0009] The technical solution for achieving the objective of this invention is as follows:

[0010] The quinoline hydrazone ligand is (E)-4-bromo-N'-(quinoline-8-methylene)benzoylhydrazide, with the following structural formula:

[0011]

[0012] The above-mentioned method for synthesizing quinoline hydrazone ligands uses the following reaction formula:

[0013]

[0014] The specific steps are as follows:

[0015] Using quinoline-8-carboxaldehyde and 4-bromobenzoylhydrazine as raw materials, and ethanol as the reaction solvent, the reaction was carried out under stirring and reflux. After the reaction was completed, the solvent was removed by rotary evaporation to obtain the quinoline hydrazone ligand KL-Br.

[0016] Preferably, the molar ratio of quinoline-8-carboxaldehyde to 4-bromobenzoylhydrazine is 1:1.

[0017] Preferably, the reflux reaction temperature is 90±5℃ and the reaction time is 12~24h.

[0018] This invention provides a quinoline hydrazone Fe(II) complex, which is a KL-Br-Fe(II) complex, with the following structural formula:

[0019]

[0020] The specific steps for synthesizing the above-mentioned quinoline hydrazone Fe(II) complex are as follows:

[0021] (1) Triethylamine was added dropwise to a methanol solution of quinoline hydrazone ligand KL-Br to deprotonate it;

[0022] (2) Dissolve Fe(ClO4)2·6H2O in methanol, then add L-ascorbic acid to dissolve Fe. 3+ Reduced to Fe 2+ ;

[0023] (3) Stir and mix the solution from step (2) and the solution from step (1) until they are homogeneous. Let them stand at room temperature to allow the solvent to evaporate slowly and precipitate KL-Br-Fe(Ⅱ) complex crystals.

[0024] Preferably, in step (3), the molar ratio of quinoline hydrazone ligand to Fe(ClO4)2·6H2O is 2:1.

[0025] Furthermore, the present invention also provides the application of the above-mentioned quinoline hydrazone Fe(II) complex in the preparation of spin-crossed molecular switches or acid-base stimulation sensors.

[0026] Compared with the prior art, the present invention has the following advantages:

[0027] (1) Quinoline hydrazone ligands are extremely soluble in common organic solvents (such as methanol). They can be completely dissolved at room temperature by sonication or gentle stirring without heating. They have a high saturation concentration and are easy to synthesize with high yield and high purity. This greatly facilitates the preparation of complexes and is also suitable for rapid and large-scale industrial production, further improving the synthesis efficiency.

[0028] (2) Quinoline hydrazone Fe(II) complexes, whether in crystalline or solution form, are not easily oxidized when exposed to air. The solution remains green for a long time, and the crystals remain dark green under a microscope without yellow spots. Furthermore, the preparation of the complexes does not require an oxygen-free environment. In contrast, Fe(II) complexes of other types of ligands are more easily oxidized. Therefore, this characteristic of quinoline hydrazone Fe(II) complexes not only simplifies the preparation process but also makes them very easy to store for extended periods, reducing operating and storage costs.

[0029] (3) Quinoline hydrazone Fe(II) complexes have a relatively small tendency to spontaneously dissociate in solution and are more acid-resistant than other types of Fe(II) complexes, meaning that they dissociate less in acid, making them suitable for studying their spin-crossing properties in solution.

[0030] (4) Quinoline hydrazone ligands have extremely strong coordination ability and the crystal growth rate of complexes is extremely fast. This applies not only to Fe(II) but also to other metals such as Zn(II). This greatly reduces the time cost of complex preparation and brings convenience to subsequent testing and research. Attached Figure Description

[0031] Figure 1 This is a schematic diagram of an existing colorimetric chemical sensor that uses quinoline hydrazone to coordinate with Fe(II) to detect Fe(II).

[0032] Figure 2 The image shows the 1H NMR spectrum of the quinoline hydrazone ligand KL-Br.

[0033] Figure 3 The infrared spectrum of the quinoline hydrazone ligand KL-Br.

[0034] Figure 4 The image shows the crystal structure of the KL-Br-Fe complex, where Fe: green; N: blue; O: red; Br: orange; C: gray; H: white.

[0035] Figure 5 The temperature-dependent molar magnetic susceptibility diagram of the KL-Br complex is shown.

[0036] Figure 6 PXRD stacking diagrams of measured, simulated and heteromorphic Zn(II) complexes of KL-Br complex.

[0037] Figure 7 Infrared stacking diagram of KL-Br and KL-Br-Fe.

[0038] Figure 8 Thermogravimetric curves of the KL-Br complex.

[0039] Figure 9 The images show the UV-Vis spectrum and on / off plot of the Fe(II) complex in solution under photoacid-driven spin state switching. Detailed Implementation

[0040] The present invention will now be described in further detail with reference to specific embodiments and accompanying drawings.

[0041] Example 1

[0042] 1. Synthesis of quinoline hydrazone ligands and their Fe(II) spin-crossed complexes:

[0043] (1) Synthesis steps of ligand KL-Br: Quinoline-8-carboxaldehyde (1.572 g, 10 mmol) and 4-bromobenzoylhydrazine (2.15 g, 10 mmol) were mixed and added to a 250 mL flask, and ethanol (50 mL) was added as a solvent. The mixture was then heated and stirred under reflux at 90 °C for 24 h. The solvent was then removed by a rotary evaporator to obtain a yellow solid with a yield of 95%. Its 1H NMR spectrum is shown below. Figure 2 As shown.

[0044]

[0045] (2) Synthesis steps of complex KL-Br-Fe(II): Weigh 0.2 mmol (71 mg) of KL-Br ligand into a beaker, add 20 mL of methanol and stir to dissolve it completely, and add 2 drops of triethylamine and mix well; weigh 0.1 mmol (36 mg) of Fe(ClO4)2·6H2O into a centrifuge tube, add 3 mL of methanol and shake to dissolve. At this time, the solution is yellow. Add 1 mg of L-ascorbic acid and continue to shake until the solution becomes colorless; pour the metal solution in the centrifuge tube into the beaker, stir and mix it thoroughly with the ligand, and seal the mouth of the beaker with plastic wrap. Finally, poke several small holes in the plastic wrap with a needle and place it at room temperature for several hours. Dark green blocky crystals precipitate at the bottom of the beaker, which is the complex KL-Br-Fe(II).

[0046] 2. Infrared characterization

[0047] Infrared spectral data were obtained using a Vacuum FT-IR Spectrometer instrument, with a measurement range of 4000–400 cm⁻¹. -1 The infrared spectrum of the ligand is as follows: Figure 3 As shown, 1650cm -1 The peak at 1600 cm⁻¹ is a strong characteristic peak of the C=O stretching vibration of amides, belonging to the amide I band, while the peak at 1600 cm⁻¹ is... -1 Nearby are the in-plane bending vibrations of the NH group on the amide ring, belonging to the amide II band; the skeletal stretching vibrations of the aromatic ring and the C=C stretching vibrations of the quinoline ring both belong to the 1600 cm band. -1 The characteristic peak at 1060 cm⁻¹ is observed in the KL-Br structure because the Br atom is directly bonded to the benzene ring. Simultaneously, both CX stretching vibrations and aromatic ring skeletal stretching vibrations exist, influencing each other. Therefore, the spectrum does not show a pure CX stretching vibration peak, but rather a peak attached to the benzene ring, as shown in the yellow region (bromobenzene) in the figure. -1 The peak at this point is the characteristic absorption peak of the KL-Br halogen atom, and the stretching vibration of the NH atom on the amide is located in the range of 3300–3020 cm⁻¹. -1 At this point, the basic structure of KL-Br has been roughly characterized. Other characteristic absorption bands on the infrared spectrum are mostly CH vibrations on aromatic and quinoline rings, as well as C=N and NN vibrations.

[0048] 3. Variable-temperature molar magnetic susceptibility

[0049] The variable-temperature molar magnetic susceptibility data were obtained using Quantum Design MPMS-3SQUID-VSM at 2K·min. -1 The scanning rate was measured under a magnetic field strength of 1T. The temperature measurement range for the KL-Br complex was 100–400K, and the results are as follows: Figure 5 As shown in the figure. The magnetic test results of the complex show that the KL-Br complex has no spin crossover phenomenon at all within the test temperature range, and Fe(II) always maintains a low spin. This indicates that temperature is not a factor affecting the spin state of the KL-Br complex. The magnetic susceptibility value in this temperature range can be reproduced in the subsequent cooling-heating cycle, which shows that it is reversible.

[0050] 3. Powder diffraction test

[0051] Powder diffraction (PXRD) data were obtained using a Rigaku Ultimate IV powder diffractometer. Figure 6The PXRD spectra of the KL-Br complex showed that the powder diffraction results matched the simulated single-crystal X-ray diffraction results, with almost no impurity peaks. This indicates that the structure of the obtained quinoline hydrazone complex is consistent with the measured single-crystal structure, ensuring the accuracy of other tests. The powder diffraction results of the Zn and Fe(II) complexes of the quinoline hydrazone ligands were also basically consistent, indicating that the coordination characteristics of Zn and Fe(II) are largely similar to those of the quinoline hydrazone ligands, belonging to isomorphous heterostructures, and they have the same molecular structure and crystal packing characteristics.

[0052] 4. Infrared stacking diagram

[0053] Figure 7 The image shows the infrared stacking of the KL-Br ligand and its complex, revealing that the pre-coordination free KL-Br ligand exists in a keto form, with the characteristic absorption band located at 1650 cm⁻¹. -1 The stretching vibration peak near C=O, known as the amide I band, shifts to lower wavenumbers after coordination, and the peak intensity decreases (blue area in the figure). Since the ligand requires deprotonation to coordinate with the metal, it exists in an enol form after coordination, with a peak intensity at 1350 cm⁻¹. -1 New CO will appear in the spectral bands of nearby complexes. - The absorption peaks (yellow area in the figure) can be used to identify these two characteristic absorption peaks as markers for judging and distinguishing the pre- and post-dislocation phases.

[0054] 5. Thermogravimetric analysis

[0055] Thermogravimetric data were recorded using a Mettler-Toledo TGA / SDTA851e thermal analyzer. Figure 8 The thermogravimetric curves show that the complex is stable below 300℃, with only minor mass loss caused by trace impurities adhering to the sample surface. This indicates that the KL-Br complex contains almost no solvent molecules and the sample is very dry. The KL-Br complex decomposes by 70% at around 320℃, and continues to decompose slowly by the remaining 10% upon further heating.

[0056] 6. UV spectra and on / off diagrams of Fe(II) complexes undergoing photoacid-driven spin state switching in solution.

[0057] 10 prepared with methanol -4 Add 3 mL of each mol / L complex solution to a quartz cuvette, add a small amount of iron powder to the cuvette to prevent oxidation, and perform the first measurement; then add 300–600 μL (1–2 eq) of 10 mol / L complex solution to the cuvette. -3 The photosensitive methanol solution was irradiated under a xenon lamp at a wavelength of 435 nm until the solution turned colorless or pale yellow, and a second measurement was performed; then 60–105 μL (2–3.5 eq) of the solution was added to the cuvette. -2The ligand methanol solution was allowed to stand in the dark until it turned green, and then measured a third time. The solution was then exposed to light and allowed to stand again, and this cycle of testing was repeated.

[0058] Depend on Figure 9 It can be seen that the initial strong absorption peak of the complex near 672 nm originates from Fe. Ⅱ (dπ)→L - (pπ * The absorption peak, representing a charge transfer transition, is characteristic of Fe(II) in its low-spin state, corresponding to the spin state of the quinoline hydrazone complex in the solid phase. Furthermore, the spin-state switching of the quinoline hydrazone Fe(II) complex in solution, driven by photoacid, is highly successful. Upon addition of photoacid and subsequent illumination, the released protons cause almost complete dissociation of the complex, resulting in a decrease in the initial low-spin strong absorption peaks, transforming them into high-spin peaks. After adding ligands and restoring the solution in darkness, the photoacid recaptures the protons, and the free ligands and metal re-coordinate, causing the high-spin peaks to revert to low-spin peaks. Repeated cycles of illumination and darkness demonstrate the excellent spin-state switching effect of the KL-Br complex, exhibiting stable cycling with no decay within the red dashed line range. Therefore, the spin-state switching effect of the quinoline hydrazone Fe(II) complex in solution is ideal.

Claims

1. A quinoline hydrazone Fe(II) complex, characterized in that, It is a KL-Br-Fe(II) complex, and its structural formula is: 。 2. The method for synthesizing the quinoline hydrazone Fe(II) complex according to claim 1, characterized in that, The specific steps are as follows: (1) Triethylamine is added dropwise to a methanol solution of quinoline hydrazone ligand KL-Br to deprotonate it, wherein the quinoline hydrazone ligand KL-Br is ( E )-4-bromo- N' -(quinoline-8-methylene)benzoylhydrazide, with the following structural formula: ； (2) Dissolve Fe(ClO4)2·6H2O in methanol, then add L-ascorbic acid to dissolve Fe. 3+ Reduced to Fe 2+ ; (3) Stir and mix the solution from step (2) and the solution from step (1) until they are homogeneous. Let them stand at room temperature to allow the solvent to evaporate slowly and precipitate KL-Br-Fe(Ⅱ) complex crystals.

3. The synthesis method according to claim 2, characterized in that, In step (1), the quinoline hydrazone ligand KL-Br is synthesized via the following reaction, the reaction formula of which is: ， The specific steps are as follows: Using quinoline-8-carboxaldehyde and 4-bromobenzoylhydrazine as raw materials, and ethanol as the reaction solvent, the reaction was carried out under stirring and reflux. After the reaction was completed, the solvent was removed by rotary evaporation to obtain the quinoline hydrazone ligand KL-Br.

4. The synthesis method according to claim 3, characterized in that, The molar ratio of quinoline-8-carboxaldehyde to 4-bromobenzoylhydrazine is 1:

1.

5. The synthesis method according to claim 3, characterized in that, The reflux reaction temperature was 90±5 ℃, and the reaction time was 12~24 h.

6. The synthesis method according to claim 2, characterized in that, In step (3), the molar ratio of quinoline hydrazone ligand KL-Br to Fe(ClO4)2·6H2O is 2:

1.

7. The application of the quinoline hydrazone Fe(II) complex according to claim 1 in the preparation of spin-crossed molecular switches or acid-base stimulation sensors.

Images