| Nitrogenase catalyzes the reduction of dinitrogen to ammonia in the process of biological nitrogen fixation. In the past few decades, its catalytic mechanism and chemical simulation have been widely studied. The high resolution (1.16 (?)) X-ray structural analysis of the MoFe protein of nitrogenase reveals the FeMo-co (iron molybdenum cofactor) as a cage structure, MoFe7S9X(S-cys)(N-his)(homocit). The molybdenum atom is coordinated by three sulfur atoms, a nitrogen atom from histidine and two oxygen atoms from homocitrate. The homocitrate entity employs itsα-alkoxy andα-carboxy oxygen atoms chelating to the molybdenum atom. In addition, some organisms have alternative nitrogenases containing metal atoms like Fe and V, or Fe only. These alternative nitrogenases are inferred to have much similarity with Mo-nitrogenase in structure, but appear to be less efficient than the latter and are generally only expressed when Mo-nitrogenase is unavailable in the organism.Recent reference shows that potentially molybdenum and homocitrate are transferred into the NifEN protein in the last step. How do the molybdenum and homocitrate add to the precursor cluster and what are the detail composition and structure of molybdenum-homocitrate system remain unclear, as well as the molybdenum-hydroxycarboxylate system and vanadium-hydroxycarboxylate system.In order to further mimic the coordinative environment of molybdenum in FeMo-co and study theα-hydroxycarboxylato molybdenum and vanadium species in solutions, we have studied vanadium and molybdenum complexes 1-19 with citric acid, malic acid, citric acid, citramalic acid, homocitric acid as ligands, as well as 1,10-phenanthroline and 2, 2'-bipydine: [Hneo]4[V2O4(R-Hcit)(OH)][V2O4(S-Hcit)(OH)]·4H2O (1), [Ni(phen)3]2[V2O4(R-Hcit)(OC2H5)] [V2O4(S-Hcit)(OC2H5)]·4H2O (2), [V2O3(phen)3(Hcit)]·5H2O (3),[V2O3(phen)3(Hcit)2(phen)3O3V2]·12H2O (4), Na3(Hhomocit)·H2O (5),[V2O3(phen)3(R,S-H2homocit)]Cl·6H2O (6),[VO2(phen)2]2[V2O4(R,S-H2homocit)2]·4H2O·2C2H5OH (7),[V2O3(phen)3(mal)H(mal)(phen)3O3V2]Cl·14H2O (8), [VO(bpy)(R,S-H2cit)]·2H2O (9), [VO(phen)(R,S-H2cit)]·1.5H2O (10),[VO(phen)(R,S-H2cit)]·6.5H2O (11), [VO(bpy)(R,S-Hmal)]·H2O (12), [VO(phen)(R,S-Hmal)]·H2O (13), [VO(bpy)(R-Hcitmal)]·2H2O (14), [Co(phen)(R,S-H2cit)(H2O)]·3H2O (15), [Ni(phen)(R,S-H2cit)(H2O)]·3H2O (16), [(MoO2)2O(phen)(H2cit)(H2O)2]H2O (17), [(MoO)2O(bpy)2(H2cit)2]·4H2O (18), [MoO2(bpy)(H2cit)]·H2O (19). The results are summarized as follows:1, The anions of complexes 1 and 2 have asymmetric V2O2 core containing hydroxy or ethoxy bridges, respectively. The molar ratio of vanadium and citrate is 2: 1, the citrate acts as tridentate ligand via itsα-alkoxy,α-carboxy andβ-carboxy groups. 5 is trisodium homocitrate. 3,4,6 and 8 have asymmetric V2O3 core containing oxygen atom bridges. The molar ratio of vanadium and citrate is also 2: 1, theα-hydroxycarboxylates act as bidentate ligands via theirα-alkoxy andα-carboxy groups. The complex 7 has symmetric V2O core, which is a precursor of complex 6. The a-hydroxycarboxylates act as tridentate ligand via theirα-hydroxy,α-carboxy andβ-carboxy groups in complexes 9-16, and each metal formed MN2O4 configuration (M=V,Co,Ni). Complex 17 is asymmetric dinuclear citrato molybdate, the citrate chelates bidentately to molybdenum via its a-alkoxy and a-carboxy groups. Complex 19 is mononuclear citrato molybdate, formed MoN2O4 configuration, the citrate also chelates bidentately to molybdenum via itsα-alkoxy andα-carboxy groups. Symmetric dinuclear structure was found in molybdenum (V) complex 18. Each molybdenum atom is in MoN2O4 configuration, and the citrate also acts as bidentate ligand via its a-alkoxy andα-carboxy groups. The bidentate coordination modes of molybdenum or vanadium in these hydrocarboxylato complexes are similar to that of homocitrato molybdate in FeMo-co. Itseems that the complexes could be served as model complexes for exploring the coordination environment of molybdenum in nitrogenase.2, By studying the different hydrogen bond models and their relationship in complex 3 and 4, it is found that theβ-carboxy groups of citrate not only hydrogen bond to lattice water, but also form intramolecular and intermolecular hydrogen bonds each other. The species with intramolecular hydrogen bond can transformed into the species with intermolecular hydrogen bond. Thus, it is proposed that the homocitrate may participate in proton's transformation byγ-carboxy groups in the nitrogen fixation, but also withβ-carboxy groups. 3, The synthesis and characterization of asymmetric dinuclear and mononuclear vanadium complexes show that the pH value is very important for the products' formation and isolation. In some pH range, theα-alkoxy bridge was easy to break and form asymmetric dinuclear and mononuclear vanadium structure. The pH value also affects the speed of reduction of vanadium. The asymmetric dinuclear and mononuclear vanadium complexes can be transformed each other, e.g.:4, Dinuclear citrato molybdenum (Ⅴ,Ⅵ) complexes 17 and 18 were successfully synthesized and isolated, show that theβ-carboxy group of citrate can be replaced in dinuclear citrato molybdenum complexes. Meanwhile, the first mononuclear citrato molybdenum (Ⅵ) complex 19 was isolated and the citrate acted as bidentate ligand. It is proposed that the homocitrate and molybdenum were transferred into precursor cluster by mononuclear mode,while the homocitrate acts as bidentate ligand.
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