Interaction between molybdate and histidine and its reduction to molybdenum(V) dimer

Temperature-dependent reactions of molybdates with histidine in weak acidic solution resulted in the isolations of coordinated and uncoordinated products Na[MoΛO3(S-His)]·H2O (1) (HHis = histidine, C6H9N3O2) and K4(S-H2His)2(Mo7O24)·16H2O (2) respectively. The former 1 can be reduced by hydrazine hydrochloride for the preparation of molybdenum(V) histidine [Mo2O4(S-His)2]·3H2O (3) with dinuclear [Mo2O4]2+ core. The complex 2 shows strong hydrogen bonds between molybdate and histidine. The solution behaviors of the complexes were probed and compared between solid and solution 1H and 13C NMR spectra. Obvious dissociation of histidine in 1 has been observed for 35%, showing the equilibrium between the coordinated and free histidine groups. This may support the liability of coordinated imidazole in histidine with iron-molybdenum cofactor. Furthermore, solid CD spectra show 1 and 3 with positive and negative Cotton effects respectively.

Mixed salt of molybdate and histidine K4(S-H2his)2(Mo7O24)·16H2O could be converted to coordinated molybdenum(VI) histidine Na[MoΛO3(S-his)]·H2O, the latter is reduced easily to dimeric product of [Mo2O4(S-his)2]·3H2O and shows obvious decomposition in solution.Figure optionsDownload as PowerPoint slide