Coordination capacity of Keggin anions as polytopic ligands: case study of [VNb12O40]15−

Abstract

Reaction of Na₉H₄[VNb₁₂O₄₀{NbO(CO₃)}₂] with [(C₆H₆)RuCl₂]₂ (molar ratio {VNb₁₂} : {(C₆H₆)Ru} = 1 : 4) in aqueous solution gives a mixture of [α-{(C₆H₆)Ru}₄VNb₁₂O₄₀]⁷⁻ and [α-{(C₆H₆)Ru}₃VNb₁₂O₄₀]⁹⁻. Direct acetone diffusion into mother liquor leads to crystallization of Na₆H[α-{(C₆H₆)Ru}₄VNb₁₂O₄₀]·41.25H₂O (1), characterized by single crystal X-ray diffraction (SCXRD). This anion has four organometallic fragments coordinated to the α-Keggin type [VNb₁₂O₄₀]¹⁵⁻ backbone in different manner. Three {(C₆H₆)Ru}²⁺ groups cap triangular faces and one group a rectangular face of [VNb₁₂O₄₀]¹⁵⁻. Equilibrated mixture of [α-{(C₆H₆)Ru}₄VNb₁₂O₄₀]⁷⁻ and [α-{(C₆H₆)Ru}₃VNb₁₂O₄₀]⁹⁻ was studied by ¹H DOSY NMR, HPLC-ICP-AES and HPLC-ESI-MS combined techniques. Direct chromatographic separation of these complexes results in unexpected transformation of both species into [α-{(C₆H₆)Ru}₅VNb₁₂O₄₀]⁵⁻, isolated and characterized as Na₅[α-{(C₆H₆)Ru}₅VNb₁₂O₄₀]·16H₂O (2). This anion contains five coordinated organometallic groups occupying both triangular and rectangular faces.