Tetrakis(dimethyltin)-Containing Tungstophosphate [{Sn(CH₃)₂}₄(H₂P₄W₂₄O₉₂)₂]^28-: First Evidence for a Lacunary Preyssler Ion

We demonstrate for the first time that the superlacunary polyanion [H₂P₄W₂₄O₉₄]^22- reacts with electrophiles. One-pot reaction of this precursor polyanion with dimethyltin dichloride in aqueous acidic medium results in the hybrid organic−inorganic [{Sn(CH₃)₂}₄(H₂P₄W₂₄O₉₂)₂]^28- (1). Single-crystal X-ray analysis was carried out on K₁₇Li₁₁[{Sn(CH₃)₂}₄(H₂P₄W₂₄O₉₂)₂]·51H₂O (1a), which crystallizes in the tetragonal system, space group P4₂/nmc, a = b = 21.5112(17) and c = 27.171(3) Å, and Z = 2. Polyanion 1 is composed of two (P₄W₂₄O₉₂) fragments that are linked by four equivalent diorganotin groups. The unprecedented assembly 1 has D_2d symmetry and contains a hydrophobic pocket in the center of the molecule. The cyclic voltammetry pattern of 1 is constituted by a first broad, 16-electron reduction wave followed by a second large current intensity wave. No splitting of the first reduction wave could be obtained at moderate scan rate values, even though two well-separated oxidation processes are associated with it. The characteristics of the first wave are clearly different from those obtained for the polyanion precursor [H₂P₄W₂₄O₉₄]^22- and the related, wheel-shaped [H₇P₈W₄₈O₁₈₄]^33-, which is due to the {Sn(CH₃)₂} fragments in 1. However, no feature was observed in the voltammogram which could be associated with reduction of the Sn centers.