| Nitrogenase catalyzes the reduction of dinitrogen to ammonia in the process of biologicalnitrogen fixation. In the past few decades, its catalytic mechanism and chemical simulationhave been widely studied. The high resolution (1.16 (?)) X-ray structural analysis of the MoFeprotein of nitrogenase reveals the iron molybdenum cofactor (FeMo-co) as a cage structure,MoFe7S9X(homocit) (X=C, N or O). The molybdenum atom is coordinated with three sulfuratoms, a nitrogen atom from histidine and two oxygen atoms from homocitrate. Thehomocitrate entity employs itsα-alkoxy andα-carboxy oxygen atoms chelating to themolybdenum atom. The studies indicated that substitution of hydroxypolycarboxylic acids forhomocitrate resulted in lower N2 reduction activity. However, it is still unknown for the roleof homocitrate in substrate reduction in FeMo-co. Recent reference shows that potentiallymolybdenum and homocitrate are transferred into the NifEN protein in the last step. How dothe molybdenum and homocitrate insert to the precursor cluster and how about thecomposition and structure of molybdenum-homocitrate system remain unclear. Thus, it isvery important to investigate the chemistry of molybdenum and homocitrate. However,complicated synthesis and high price of homocitrate makes studies on molybdenum withhomocitrate very difficult. Usually, hydroxycarboxylic acids such as citric, malic, lactic andglycolic acids are used instead of homocitrate in studies.Further investigations ofα-hydroxycarboxylato molybdenum and tungsten species insolutions will be helpful to understand the coordinative environment of molybdenum inFeMo-co. In this dissertation, we have studied molybdenum and tungsten complexes 1-23with citric acid, malic acid and citramalic acid as ligands: K2(NH4)2[(MoO2)4O3(Hcit)2]·5H2O(1), (NH4)4[(MoO2)4O3(Hcit)2]·8.5H2O (2), (NH4)5[(MoO2)4O3(Hcit)(cit)]·3H2O (3),(Him)3(NH4)3[(MoO2)4O3(Hcit)(cit)]Cl·2H2O (4), K2(NH4)4[(MoO2)4O3(cit)2]·7H2O (5),(Him)4(NH4)2 [(MoO2)4O3(cit)2]·6H2O (6), K2 [(MoO2)2O(H2cit)2]·4H2O (7),(NH4)3 [(MoO2)2O(H2cit)(Hcit)] (8),(NH4)14{[(MoO2)2O(Hcit)2] [(MoO2)2O(Hcit)(cit)]2}·14H2O (9),(NH4)11{[(MoO2)2O(cit)2H(cit)2O(MoO2)2]·4H2O (10), (NH4)6[(MoO2)2O(cit)2]·3.5H2O (11), K4(NH4)2[(MoO2)2O(cit)2]·5H2O (12), (NH4)4[MoO3(cit)]·2H2O (13),K8[(MoO2)4O3 (R-mal)2] [(MoO2)4O3 (S-mal)2]·10H2O (14),(Him)2K6 [(MoO2)4O3(R-mal)2] [(MoO2)4O3(S-mal)2]·8H2O (15),K8[(MoO2)2O(R-mal)2][(MoO2)2O(S-mal)2]·4H2O (16),K4[MoO2(S-Hmal)2][MoO2(R-Hmal)2]·4H2O (17), K4[(MoO2)4O3(R-cmal)2]·6H2O (18),K4[(MoO2)4O3(S-cmal)2]·6H2O (19), (NH4)4[(MoO2)4O3(S-cmal)2]·6H2O (20),K2[WO2(H2cit)2]·3H2O (21), (Him)10(NH4)2[(WO2)2O(Hcit)2]3·10H2O (22),(Him)8(NH4)13{[(WO2)2O(Hcit)(cit)H(cit)(Hcit)O(WO2)2][(WO2)2O(cit)2]2·14H2O (23). Theresults are summarized as follows:1. Complexes 1-6, 14, 15, 18-20 are tetrameric molybdates, the central metal molybdenumwith hydroxycarboxylic acids including citric, malic and citramalic acids in a molar ratio of2:1. The anions of these complexes have four molybdenum atoms and two ligands. Eachligand acts as a tridentate fashion via theirα-alkoxy,α-carboxy andβ-carboxy groupscoordinated to molybdenum. This coordination mode is the same as that of tetramerichomocitrate molybdenum complexes. Complexes 7-12, 16, 22 and 23 are dimeric molybdatesor dimeric tungstates, the central metal with citrate or malate in a molar ratio of 1:1. Theanions of these complexes have two metal atoms and two ligands. Each ligand acts as atridentate fashion via theirα-alkoxy,α-carboxy andβ-carboxy groups coordinated to eachmetal atom. Complex 13 is a monomeric molybdate. The citrate ligand coordinates tomolybdenum with itsα-alkoxy,α-carboxy andβ-carboxy groups in a tridentate mode.Complexes 17 and 21 are monomeric malate molybdate and citrate tungstate, respectively.The ligand uses itsα-alkoxy andα-carboxy oxygen atoms chelating to the central metal atom.The bidentate coordination mode of ligands with metal atom in these hydroxycarboxylatocomplexes is similar to that of homocitrato molybdate in FeMo-co. It seems that thecomplexes could be served as model complexes for exploring the coordinative environmentof molybdenum in nitrogenase.2. The synthesis and characterization of citrate molybdenum complexes indicates that pHvalue acts as a key factor to separate products. In a certain molar ratio of reagents, the degree of deprotonation does not affect the coordination mode of citrate in certain range of pH value.In addition, it is found thatβ-carboxy groups of citrate form very strong hydrogen bonds insome complexes, leading to the formation of dimer (complexes 3, 7, 8, 10), trimer (complex22), and tetramer (complex 23). These properties of hydroxycarboxylic acids make them as apossible proton source for proton transfer in nitrogenase.3. NMR studies show that most complexes are not stable and dissociated in solution, whilea few complexes are still stable in solution. In addition, NMR spectra of molybdate andcitrate system in solution indicate that the species in solution are dependent on the molar ratioof molybdenum with citrate and pH values.
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