Crystalline and solution chemistry of tetrameric and dimeric molybdenum(VI) citrato complexes

Reaction of citrate with molybdenum(VI) salts under controlled conditions of reactant ratios and pH values in aqueous solutions affords a family of molybdenum citrates. It includes tetranuclear complexes K2(NH4)2[(MoO2)4O3(Hcit)2]·5H2O (1), (NH4)5[(MoO2)4O3(Hcit)(cit)]·3H2O (2) and K2(NH4)4[(MoO2)4O3(cit)2]·7H2O (3), dinuclear complexes K2[(MoO2)2O(H2cit)2]·4H2O (5), (NH4)3[(MoO2)2O(H2cit)(Hcit)] (6), (NH4)14{[(MoO2)2O(Hcit)2][(MoO2)2O(Hcit)(cit)]2}·14H2O (8) and (NH4)6[(MoO2)2O(cit)2]·3.5H2O (9), as well as a mononuclear complex (NH4)4[MoO3(cit)]·2H2O (10) (H4cit = citric acid), where 2, 5, 6, and 8 are new type of citrato complexes. X-ray single-crystal structural analyses reveal that the molybdenum atom in these complexes is hexa-coordinated in an approximately octahedral geometry. Each citrate ligand uses its α-alkoxy, α-carboxy and one β-carboxy groups to act as a tridentate ligand while the other β-carboxy group is left free. Also, structural analyses show that some of the complexes possess strong hydrogen bonds that lead to one and two dimensional polymeric associations. All of these complexes have been investigated by 13C and 1H NMR spectroscopy. The NMR spectra demonstrate that these complexes except 1 and 8 obviously undergo decomposition in solution. Furthermore, four series of solutions with the molar ratio Mo:cit = 2:1, 1:1, 1:2, and 1:4 in the pH range 0.1–9.0 were investigated by 13C NMR spectroscopy. The solution NMR studies reveal that the state of equilibriums among different species [4,2]n−, [2,2]n−, [2,1]n−, [1,2]n− and/or [1,1]n− being established under each set of specified conditions.