| Nitrogenase is a complex metalloenzyme that reduces atmospheric dinitrogen to bioavailable nitrogen sources under normal temperature and pressure.Cage-like MoFe7S9C(R-homocitrate)(N-his)clusters have been confirmed as the active sites of iron molybdenum cofactors(FeMo-co)in nitrogenases,in which the octahedral coordination spheres of molybdenum are occupied by three sulfides,one nitrogen atom from the imidazolyl group of a histidine residue,and two oxygen atoms from coordinating R-homocitrate.R-homocitrate chelates to molybdenum with bidentate a-alkoxy andα-carboxy groups respectively,the protonation of α-alkoxy groups to α-hydroxy groups has also been suggested.The interaction of imidazole in the histidine residue with molybdenum shows a monodentate coordination mode in FeMo-co.It is proposed that homocitrate and molybdenum are inserted into the precursor of FeMo-co in the final step.The R-homocitrate with the long and flexible CH2CO2 arm can form a hydrogen bond with the NH group of the imidazole ligand,which may play a very important role in the process of reducing nitrogen.Little research has concentrated on molybdenum complexes with both α-hydroxycarboxylate and imidazole ligands,especially regarding Mo(Ⅲ/Ⅳ)complexes related to the redox process of FeMo-co.Herein we have investigated the coordination chemistry of mixed-ligand molybdenum complexes with a-hydroxycarboxylate and imidazole ligands,especially the coordination modes of molybdenum(Ⅳ/Ⅴ)complexes in low oxidation states.We synthesized a series of mixed-ligand molybdenum complexes 1～21 by a-hydroxycarboxylate ligands such as homocitrate and its homologues and N-heterocyclic ligands such as imidazole:[(Mo3SO3)(glyc)2(im)5]·im·H2O(1),Na2[(Mo3SO3)(R,S-lact)3(im)3]·10H2O(2),[(Mo6O10)(R,S-lact)2(im)10]·16H2O(3),Na6[(Mo204)3(R,S-mal)4]·5H2O(4),K2{Mo3O4(im)3[MoO3(Hcit)]2}·3im·4H2O(5),(Him)2{Mo3SO3(im)3[MoO3(Hcit)]2}·im·6H20(6),(Et4N)[Mo02Cl(H2cit)]·H20(7),trans-[(MoO)20(H2homocit)2(bpy)2]·4H2O(8),[(MoO)2O(H2homocit)2(phen)2]·5H2O(9),trans-[(MoO)20(glyc)2(bpy)2]·3H2O(10),trans-[(MoO)2O(lact)2(bpy)2]·3H2O(11),trans-[(MoO)20(Hmal)2(bpy)2](12),cis[(MoO)20(glyc)2(phen)2]·5H2O(13),trans-[(MoO)2O(glyc)2(phen)2](14),trans-[(MoO)20(laet)2(phen)2]·4H2O(15),trans-[(MoO)20(Hmal)2(phen)2]·4H2O(16),cis-MoO2(glyc)(bpy)(17),Na6[(MoO2)2O2Fe2(nta)4]·16H2O(18),Na6[(MoO2)2O2Al2(nta)4]·16H2O(19),Na6[(MoO2)2O2Cr2(nta)4]·16H2O(20),[(MoO4)FeⅡFeⅢ4(ida)8]n(21).The results of the thesis are summarized as follows:1.We adopted a two-step synthesis strategy to synthesize α-hydroxycarboxylic acid imidazole mixed ligand molybdenum complexes 1～4 as a simulated compound for FeMoco of nitrogenase to study the coordination environment of molybdenum in nitrogenase.a-Hydroxycarboxylates in 1 and 2 acted as bidentate ligands of Mo(IV)atoms through α-alkoxy and α-carboxy groups,while the imidazoles coordinated monodentately with nitrogen atoms.α-Alkoxy coordination with molybdenum[Mo-Oα-alkoxy 1.993(7)avA]in 1 and 2 are obviously shorter than those of FeMo-cofactors(Mo-Oav 2.272 A),which can serve as indirect evidence for the protonation of homocitrate in FeMo-co.Solid-state and solution 13C NMR resolances of carbon atoms is α-alkoxy groups appeared in a high-field region(71.6,77.4 ppm),indicating that a-alkoxy groups were easy to protonate.The lactates in 3 coordinated with Mo(IV)atoms through two oxygen atoms of a-carboxy groups,leaving the α-hydroxy group free.Malic acid acts as a tridentate ligand to coordinate with MoV withα-alkoxy,α-carboxyl and β-carboxyl to form a novel hexanuclear malate complex 4.2.The pentanuclear imidazole hydrocitrato molybdenum(Ⅳ/Ⅵ)complexes 5 and 6 have been obtained by hydrothermal conditions.Here the molybdenum atoms in 5 and 6 are+4 or+6,which is different from the common mixed-valence complexes in the adjacent valence state.It is worth noting that the α-alkoxy groups of the coordinated citric acid in 5 and 6 are protonated to become a-hydroxy groups,indicating O(α-hydroxy)atoms in molybdenum(Ⅳ)complexes with higher charge densities are easier to be protonated.The[MoⅥo2Cl(H2cit)]-ion is further captured by large cation(Et4N)+ to obtain 7 without protonation in α-alkoxy group.One of terminal oxygen in[MoⅥO3(H2cit)]2-can be replaced by chloride under strong acidic condition.The solution behaviors of 5 and 6 are discussed based on 1H and 3C NMR spectroscopies,showing no decomposition of the species.Cyclic voltammograms of 5 and 6 indicate that substitution of central sulfur atom in 6 is easily reduced than that of central oxygen atom.3.Homocitrato oxomolybdenum(V)complexes 8 and 9 have been obtained,and a series of dimeric N－heterocycle a-hydroxycarboxylato oxomolybdenum(V)complexes[(MoO)2OL2(bpy)2]·xH2O and[(MoO)2OL2(phen)2]·xH2O have also been synthesized in acidic media for comparisons.The α-Hydroxycarboxylates in the neutral complexes 8 and 10～16 chelate to molybdenum(V)with bidentate α-alkoxy and α-carboxy groups respectively.No protonation was observed for α-alkoxy groups even under acidic condition,which indicates that Mo(V)may have a low charge density and cannot be protonated.In 10 and 13,there are characteristic six-membered ring or twelve-membered ring water clusters,which play a very important role in structural stability.The two molybdenum atoms in 10 and 12 are paramagnetic independently as examples by EPR spectroscopies.4.The tetranuclear rectangular heterometallic complexes 18～20 have been synthesized in modular manners.Solution NMR spectra of 19 show no decomposition in solution and is not easily decomposed.21 having a 3D structure is also synthesized by a similar method,which is stable up to 250℃ and has a pore diameter of 3.3 A,and can selectively adsorb small amounts of CO2.The catalytic performance of 18 and 21 was tested by the catalytic reaction of H2O2 to degrade methyl orange.18 was structurally stable and had no catalytic ability.21 released Fe with stirring,and the catalytic efficiency increased significantly with time.The degradation rate reached 72.7%after 4 hours. |
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