Synthesis and characterization of a series of [M2(β-SiW8O31)2](n-) clusters and mechanistic insight into the reorganization of {β-SiW8O31} into {α-SiW9O34}.

Lacunary polyoxometalates of low nuclearity are difficult to synthesize in isolation. We report the facile synthesis of six {M2(B-β-SiW8O31)2} clusters (M = Co/Mn/Ni/Zn/Cu(2+), Fe(3+)) that can be employed as building blocks for the formation of larger architectures. We show for the first time that such {B-β-SiW8O31} lacunae are capable of reorganizing into larger Keggin lacunary species even in the absence of an external source of tungstate. We hypothesize, based on electrospray ionization mass spectrometry evidence obtained, not only that such a transformation is only possible via an initial decomposition of the {SiW8} precursor into a {SiW6}-based intermediate but also that it is this {SiW6} species that acts as the template for the growth of the larger fragments.