Co-ordination chemistry of lacunary Lindqvist-type polyoxometalates: cubic vs. square-antiprismatic co-ordination

Abstract

In methanol, the sodium complex of the monovacant Lindqvist-type nitrosyl polyoxomethoxomolybdate [Mo₅O₁₃(OMe)₄(NO)]^3– reacted with a number of divalent or trivalent cations, including Ca²⁺, Sr²⁺, Ba²⁺, Ce³⁺, Eu³⁺ and Bi³⁺, to yield 2∶1 complexes of the type [X{Mo₅O₁₃(OMe)₄(NO)}₂]^n– which have been isolated as tetrabutylammonium salts. The crystal and molecular structures of [NBu₄]₃[Bi{Mo₅O₁₃(OMe)₄(NO)}₂]·3MeOH, [NBu₄]₃[Ce{Mo₅O₁₃(OMe)₄(NO)}₂]·3MeOH and [NBu₄]₄[Ba{Mo₅O₁₃(OMe)₄(NO)}₂] have been determined by single-crystal X-ray diffraction. In these three compounds and most probably in all other members of the series, the tetradentate inorganic porphyrin-like [Mo₅O₁₃(OMe)₄(NO)]^3– anions provide an eight-co-ordination environment for the central cation. However, there are striking differences between the complexes. Indeed while the environment is square antiprismatic in the isomorphous compounds of Bi and Ce, it is square prismatic in that of Ba. Infrared spectroscopy also provides evidence for a square prismatic environment in the complexes of Ca²⁺ and Sr²⁺. The related [W₄O₁₃(OMe)₄{Mo(NO)}]^3– species has been obtained by controlled degradation of [W₅O₁₈{Mo(NO)}]^3– in MeOH and isolated as [NBu₄]₂[{Na(MeOH)}W₄O₁₃(OMe)₄{Mo(NO)}]. Its bismuth(III) complex has been prepared and characterized. The molecular structure of [Bi{W₄O₁₃(OMe)₄Mo(NO)}₂]^3– is quite similar to that of [Bi{Mo₅O₁₃(OMe)₄(NO)}₂]^3–.