Structure-directing effect of aqueous alkali metal ion (Li+, Na+ and K+) clusters on the polyoxoanion configuration in vanadium–molybdenum polyoxometalate solid solutions

Abstract

In a structural chemistry context, the dense packing and the structure-directing agent (SDA) often play a crucial role in the formation of the desired crystal structure. Herein, we present two isomorphous decavanadate-type polyoxometalates, M₂(NH₄)_4−x[V_10−xMo_xO₂₈]·10H₂O (M = Li and x = 0.13 for 1; M = Na and x = 0.95 for 2), prepared by solution evaporation under ambient conditions and characterized by the techniques of EDS, ICP-MS, TG, XPS, X-band EPR spectroscopy, and X-ray single crystal and powder diffractions. Both 1 and 2 crystallized in the space group P with analogous cell parameters, containing thermodynamically stable [M₂(H₂O)₁₀]²⁺ (M = Li or Na) clusters, NH₄⁺ ions and decavanadate-type polyoxoanions with V⁵⁺ and Mo⁶⁺ ions. The polyoxoanions and {M₂(H₂O)₁₀}²⁺ cluster cations, owing to their comparable dimensions, arrange alternately to form dense packing along the a + c, c- and a-axis directions. As earlier reported, however, [(H₂O)_0.3@K₆(H₂O)₁₂]H_4.45[PV_7.45Mo_4.55O₄₀]·11H₂O (3) crystallizes in a CsCl-type dense packing structure under analogous crystal growth conditions. In 3, the thermodynamically-stable (H₂O)_0.3@K₆(H₂O)₁₂ cluster cation and Keggin-type {Mo_4.55V_7.45PO₄₀}^10.45− polyoxoanion have comparable dimensions. This study clearly reveals the structure-directing effect of thermodynamically-stable aqueous alkali metal ion clusters, initially formed in the reaction mixture, on a polyoxoanion configuration in vanadium–molybdenum polyoxometalate solid solutions.