Syntheses,Spectra And Structures Of Imidazole Carboxylato Oxovanadium Complexes And Their Applications In Chemical Simulations Of Vanadium Coordination Microenvironment In Iron-vanadium Cofactor

Nitrogenase is a metal enzyme that can reduce nitrogen to biologically available nitrogen resources at normal temperature and pressure.The chemical simulation of nitrogenase is mainly focused on the simulation of iron-molybdenum(vanadium)cofactor in the catalytic activity center of nitrogenase,which is expected to obtain more accurate spectral and structural characterization through the comparison of small molecule mimetics and nitrogenase macromolecular structure.So the purpose of chemical simulation and exploration of nitrogen fixation can be achieved further.The structure of the iron-vanadium cofactor is VFe7S8C(XO3)(R-homocit)(X＝C or N)with bidentate chelate coordination of homocitric acid,which is similar to that of the iron-molybdenum cofactor MoFe7S9C(R-homocit).The octahedral coordination environment of vanadium is occupied by three sulfur atoms,one imidazole nitrogen atom of histidine residue and two oxygen atoms from homocitric acid.Homocitric acid coordinates with vanadium through a-alkoxy or a-hydroxy group and a-carboxy groups.The imidazole group of histidine residue coordinates with vanadium monodentately.Homocitric acid and imidazole may have great influence on the nitrogen fixation process,nevertheless there are few reports about the vanadium analogue complexes of mixed ligands of a-hydroxycarboxylic acid and imidazole.Therefore,we mainly focus on the coordination chemistry of vanadium complexes with mixed ligands of hydroxycarboxylic acid and imidazole,which is helpful to understand the coordination microenvironment of vanadium in iron-vanadium cofactor.In this thesis,we synthesized a series of mixed ligand complexes 1～16 by carboxylic ligands such as tartaric acid,oxalic acid,malonic acid and N-heterocyclic ligands such imidazole-like ligands(imidazole,2-methylimidazole,4-methylimidazole):(Him)2[V2O2(R,R-H2tart)(R,R-tart)(im)2]·im(1),[V202(R,R-tart)(im)6].4H2O(2),(Him)2[V202(S,S-H2tart)(S,S-tart)(im)2]-im(3),[V202(S,S-tart)(im)6]·4H2O(4),K4[V202(R,R-tart)2]·3H2O(5),[VO(ox)(im)3]·H20(6),VO(ox)(2-mim)3(7),VO(ox)(4-mim)3(8),K2[VO(ox)2(2-mim)]·2H2O(9),(2-Hmim)[VO2(ox)(2-mim)2]·2-mim(10),[VO(mal)(im)3]·H2O(11),(2-Hmim)2[VO(mal)2(H20)](12),[VO(mal)(4-mim)3]·2H20(13),[V202(1,3-pdta)(bpy).2]·9H2O(14),[V2O2(1,3-pdta)(phen)2]·6H2O(15),[V202(edta)(phen)2]·11H2O(16)。The main results of this thesis are summarized as follows:1.We synthesized vanadyl imidazole tartrates 1-4 as model compounds for the chelated local structure of FeV-cofactor by adjusting the pH step by step.Tartaric acid hemiploid coordinates with vanadium through a-alkoxy and α-carboxyl groups,and imidazole coordinates with vanadium through N atom as monodentate ligand.It was found that the protonated V-Oα-t,ydroxy bond distance[2.235(6)av A]was significantly longer than the V-Oα-alkoxy bond distance[1.942(8)av A],and similar to the average value of V-O distances in FeV-cofactors(2.165,v A)of FeV-proteins，suggesting that the homocitric acid in the iron-vanadium cofactor might be protonated.Compared with the corresponding groups in free tartaric acid and imidazole,the resonance peaks of carbon-containing groups in complexes 1 and 2 showed obvious downfield shifts,which revealed the effect of coordination acting on the chemical environment of ligands.The signal peaks of the coordinated a-hydroxy groups have not been captured separately,indicating there existing a rapid exchange equilibrium between the protonation and deprotonation in solution,which has been suggested there is a coexistence of homocitrates coordinated with a-alkoxy(deprotonated)and a-hydroxy(protonated)groups in the extracted FeMo-confactor of nitrogenase.2.We have obtained and characterized a series of mononuclear imidazole oxalato oxovanadium and imidazole malonato vanadyl complexes by changing the reaction conditions such as pH value,temperature and ratio of ligands.Since that dicarboxylic acids tend to form a bidentate coordination mode with vanadium,and the monodentate imidazole ligands can smoothly occupy remaining coordination sites of vanadium,it is easy to form[VO(L1)(L2)3]·xH2O coordination mode.In addition,the different coordination modes of vanadium with oxalic acid and 2-methylimidazole can be transformed by changing the ratio of the two ligands and adjusting the pH value.Due to the asymmetric structure and weaker coordination ability ofα-hydroxycarboxylic acid such as glycolic acid and lactic acid,it is difficult to synthesis and separate a-hydroxycarboxylic acid imidazole mixed ligand vanadium complex.However,this part of work also provides reference for further research on the synthesis and separation of simple a-hydroxycarboxylic acid,especially glycolic acid or lactic acid-imidazole mixed ligand vanadium complexes.3.N-heterocycle aminopolycarboxylato oxovanadium complexes 14-16 were synthesized under hydrothermal conditions.1,3-propanediamine tetraacetic acid of 14 and 15 is more flexible with one more methylene,resulting that two amino nitrogen atoms bridge the two vanadium in a chain-like way to form "shoulder pole" binuclear structure.While ethylenediamine tetraacetic acid of 16 firmly "fixes" one vanadium ion by five-tooth chelation,and the other vanadium ion is coordinated to edta via the left carboxyl oxygen to form the asymmetric binuclear structure.In addition,the catalytic performances of complexes 14-16 were tested by the catalytic degradations of methyl orange(MO)with hydrogen peroxide.The catalytic degradation performance of complexes 14-16 may be related to the exposure of metal centers and their activation by hydrogen peroxide.