Substitutions of trans-Na(Him)[Mo2O4(ox)2(H2O)2]·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-Na2[Mo2O4(ox)2(im)2]·4.5H2O (3), cis-K2[Mo2O4(ox)2(im)2]·3H2O (4), respectively (H2ox = oxalic acid; im = imidazole). Further reduction of cis-K2[Mo2O4(ox)2(im)2]·3H2O (4) gives a trinuclear molybdenum(IV) complex K(Him)[Mo3O4(ox)3(im)3]·3H2O (5), which contains an incomplete cubane cluster [MoIV3O4]4+. Two novel trinuclear mixed-valence imidazole compounds [Mo3O8(im)4](im)·H2O (6) and [Mo3O8(im)4]·H2O (7) were obtained by the reduction of (Him)4[Mo8O26(im)2] (8). Both 6 and 7 contain a novel MoVIO4(MoV2O4) center, where the [MoV2O4]2+ unit is linked by [MoVIO4]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 13C 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.
Dalton Transactions
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