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 Mo₃O_xSe_4–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⁺(E_f°= 190 mV). Similarly the 5+ ion can be oxidised to the 6+ state (E_f°= 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.