Synthesis, spectral, and structural characterizations of imidazole oxalato molybdenum(iv/v/vi) complexes

Abstract

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 [Mo^IV₃O₄]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 Mo^VIO₄(Mo^V₂O₄) center, where the [Mo^V₂O₄]²⁺ unit is linked by [Mo^VIO₄]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.