Synthesis, spectral, and structural characterizations of imidazole oxalato molybdenum(IV/V/VI) complexes

Substitutions of trans-Na(Him)[Mo2O4(ox)(2)(H2O)(2)]center dot H2O (1) and trans-(Him)(2)[Mo2O4(ox)(2)(H2O)(2)] (2) with imidazole result in the formation of the mixed-ligand molybdenum complexes cis-Na-2[Mo2O4(ox)(2)(im)(2)]center dot 4.5H(2)O (3), cis-K-2[Mo2O4(ox)(2)(im)(2)]center dot 3H(2)O (4), respectively (H(2)ox = oxalic acid; im = imidazole). Further reduction of cis-K-2[Mo2O4(ox)(2)(im)(2)]center dot 3H(2)O (4) gives a trinuclear molybdenum(IV) complex K(Him)[Mo3O4(ox)(3)(im)(3)]center dot 3H(2)O (5), which contains an incomplete cubane cluster [(Mo3O4)-O-IV](4+). Two novel trinuclear mixed-valence imidazole compounds [Mo3O8(im)(4)](im)center dot H2O (6) and [Mo3O8(im)(4)]center dot H2O (7) were obtained by the reduction of (Him)(4)[Mo8O26(im)(2)] (8). Both 6 and 7 contain a novel (MoO4)-O-VI((Mo2O4)-O-V) center, where the [(Mo2O4)-O-V](2+) unit is linked by [(MoO4)-O-VI](2-) anion. The Mo-Mo bond distances in 1-7 decrease with the decrease of oxidation state of molybdenum. Solid and solution NMR spectra show that imidazole molybdenum compounds 6-8 fully dissociate in solution, where solvated imidazole and imidazolium groups in 6 and 8 could be served as internal references in their solid C-13 NMR spectra. Furthermore, mixed-ligand molybdenum species 3 and 4 are stable in water. Stabilities of 3 and 4 in solution may be attributed to the strong coordination of bidentate oxalate and the formation of hydrogen bond. Dimers 2 and 4 display quasi-reversible redox process, while trimer 6 is irreversible. Bond valence calculations for 1-8 are consistent with their oxidation states of molybdenum atoms. Calculation of the oxidation state in recent structure of iron molybdenum cofactor [MoFe7S9C(R-homocit)] (FeMo-co) is 3.318.