Preparation, structure and properties of dinuclear, trinuclear incomplete cuboidal and cuboidal molybdenum-selenium cluster complexes
Abstract
Preparation of the di-µ-selenido-dimolybdenum(V) complex [Mo2O2(µ-Se)2(cys)2]2–(cys =L-cysteinate dianion), and the related aqua ion [Mo2O2(µ-Se)2(H2O)6]2+, has enabled the trimolybdenum(IV) incomplete cuboidal Mo3OxSe4–x4+(x= 0–3) complexes [Mo3(µ3-Se)(µ-Se)3(H2O)9]4+[Mo3(µ3-Se)-(µ-O)(µ-Se)2(H2O)9]4+, [Mo3(µ3-Se)(µ-O)2(µ-Se)(H2O)9]4+, and the mixed-valence (average oxidation state 3.25) cuboidal [Mo4(µ3-Se)4(H2O)12]5+ complex to be obtained for the first time. The crystal structures of Na[NMe4][Mo2O2(µ-Se)2(cys)2]·7H2O, [NMe4]5-(pts)5·14H2O (pts–= toluene-p-sulfonate) have been determined. Electrolytic reduction of [Mo4(µ3-Se)4(H2O)12]5+ in 2 M Hpts gives quantitatively the air-sensitive tetramolybdenum(III) analogue [Mo4(µ3-Se)4(H2O)12]4+(Ef°= 190 mV). Similarly the 5+ ion can be oxidised to the 6+ state (Ef°= 790 mV). Compared to analogous Mo-S clusters, Mo-Se clusters exhibit identical structures but with longer bond distances, red-shifted absorption bands, and in the case of the cuboidal aqua complexes less-positive reduction potentials. By analogy with [Mo3S4(H2O)9]4+, [Mo3MSe4(H2O)10]4+ heterometal cuboidal and related complexes are obtained. The edta(=N,N,N′N′-ethylenediamine-tetraacetate) complex of the 5+ cube [Mo4Se4(edta)2]3– has been prepared, and reduction potentials for the corresponding core 5+/4+ and 6+/5+ couples determined as –40 and 650 mV respectively.