Homocitrato Molybdates And Their Homologues

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 recent high resolution (1.16 A) X-ray structural analysis of the MoFe protein of nitrogenase reveals the FeMo-co (iron molybdenum cofactor) as a cage structure, MoFe7S9N(S-cys)(N-His)(homocit). The molybdenum atom is coordinated with three sulfur atoms, a nitrogen atom from histidine and two oxygen atoms from homocitrate. The homocitrate entity employs its a-alkoxyl and a-carboxyl oxygen atoms chelating to the molybdenum atom. Substitution of polycarboxylic acids for homocitrate resulted in lower N2 reduction activity. In spite of the large volume of information available, it is still unknown for the role of homocitrate in substrate reduction.In another aspect, the chemistry of peroxomolybdates and peroxotungstates has received special attention due to their importance in a variety of industrial, pharmaceutical and biological processes. It is expected that stabilities of peroxomolybdates and peroxotungstates may be improved by the coordination of carboxylate ligands.In order to further mimic the coordinative environment of molybdenum in FeMo-co and understand the peroxo species in solutions, we have studied complexes 1-22 with glycolic acid, lactic acid, malic acid, citric acid, nitrilotriacetic acid, homocitric acid as ligands: K2[MoO2(glyc)2]H2O (1),(9), K6[Mo8016(glyc)6(Hglyc)2]-10H20 (10), K2.5(NH4)o.5[Mo2O4H(nta)2]-3H2O (11), Na2(NH4)[Mo2O4H(nta)2]-7H2O (12), (NH4)6[Mo2O4(cit)2]-3H2O (13), K3[Mo203(02)4(Ac)] (14), K2n[MoO(02)2(S-Hmal)]n-nH20 (15),K4[MoO(O2)2(cit)]-4H2O (16), K5[MoO(O2)2(Hcit)H(Hcit)(O2)2OMo]-6H2O (17), K3[W2H04(02)4]-H20 (18), K2[W203(02)4(H20)2]-2H20 (19),K6[W408(02)6(C03)]-6H20 (20), K4[W202(O2)4(^-tart)]-3H2O (21), K5[WO(O2)2(Hcit)H(Hcit)(O2)2OW]-6H2O (22). The results are summarized as follows:1 Anions of complexes 1-5 can be formulated as MO2L2 (M=Mo, W), in which each molybdenum or tungsten is coordinated by two c/s-oxo groups, two bidentate hydrocarboxylate ligands via their oc-alkoxyl and a-carboxyl groups. The anion of complex 6 can be typed as M2OsL2 and nitrilotriacetate uses its nitrogen atom and two oxygen atoms from carboxyl groups to coordinate molybdenum. Homocitrate acts as tridentate ligand in complexes 7-9. Structural analyses show 7 and 8 are tetrameric homocitrate molybdates, which represent the first examples of the synthetic homocitrate molybdate. Strong metal-metal bonds have been found in reductive molybdenum(V) complexes 10-13. In peroxomolybdates and peroxotungstates 14-22, hydrocarboxylate ligands offer their a-alkoxyl and a-carboxyl groups to coordinate the central metal. The bidentate coordination modes of molybdenum or tungsten in these hydrocarboxylato complexes are similar to that of homocitrato molybdate in FeMo-co. It seems that the complexes could be served as model complexes for exploring the coordination environment of molybdenum in nitrogenase.2 The distance of Mo-Oa.aikoxyi in hydrocarboxylato molybdates is sensitive to oxidation state. The lower the oxidation state, the longer the distance. However, the distance of M-Oa.carboxyi is less susceptible to the oxidation state. Thus, it is proposedthe activation of protonation of a-alkoxyl other than a-carboxyl maybe one of the important steps in biological nitrogen fixation. This is quite different from the cleavage of a-carboxyl from molybdenum Durrani proposed.3 The conversions of citratomolybdate, citralomolybdale and peroxocilralomolybdale can be carried out. The tridentate citrate in citratomolybdate can be transformed to a more stable bidentale ligand in peroxocitralomolybdate(VI). This is a clear demonstration that the p-carboxylate is relatively weak, thus it is deduced the homocitrate use a-alkoxyl...