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–.