Structures of photochemically prepared mixed-valence polyoxovanadate clusters: oblong [V₁₈O₄₄(N₃)]^14- , superkeggin [V₁₈O₄₂(PO₄)]^11- and doughnut-shaped [V₁₂B₃₂O₈₄Na₄]^{15 -} anions

Abstract

Three mixed-valence polyoxovanadates, Na₁₂H₂[V₁₈O₄₄(N₃ )]·30H₂O 1, K_8.5H_2.5[V₁₈O₄₂(PO₄ )]·19H₂O 2 and H₁₅[V₁₂B₃₂O₈₄Na₄ ]·13H₂O 3 have been photochemically synthesized and characterized by X-ray single-crystal analysis and magnetic susceptibility measurements. The [V₁₈O₄₄(N₃)]^14- anion has an oblong structure encapsulating a linear N₃^- anion with approximate D_2h symmetry. The extent of reduction is different from that in [V₁₈O₄₄(N₃)]^7- prepared by thermal decomposition of [V₁₉O₅₀]^17- in the presence of N₃^- at 75 °C. The [V₁₈O₄₂(PO₄)]^11- anion has a superkeggin structure with a PV₁₂O₄₀ Keggin unit capped by six VO moieties which lie above six rectangle faces of the T_d-distorted cuboctahedron defined by the V atoms in the Keggin unit. The [V₁₂B₃₂O₈₄Na₄]^{15 -} anion contains an edge-sharing dodecagonal array of OVO₄ square pyramids, sandwiched by two hexadecaborates to produce a cyclic doughnut-shaped framework. The crystallographic analysis indicates that the cavity produced by the cyclic [V₁₂B₃₂O₈₄]^19- anion is occupied by a tetragonal Na₄⁴⁺ moiety. The paramagnetic complexes 1–3 show the antiferromagnetic exchange interaction which is discussed in terms of the nearest V · · ·   V distances in the anion.

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