The crystallisation of polyoxo-molybdate and -tungstate anions with phenylated phosphonium and arsonium cations, in relation to the crystal packing and species in solution

Abstract

Polyoxometallate anions often occur as mixtures of species in solution. We have investigated the possible selectivity of crystallisation of these anions using arylated cations that commonly adopt cation dominated crystal packing motifs known as multiple aryl embraces. The crystallisation and crystal structures of the compounds (Ph₄P)₂[Mo₂O₇], (Ph₄P)₂[Mo₆O₁₉](CH₃CN)₂, (Ph₄P)₂[W₆O₁₉](DMSO)₂, (MePh₃As)₂[Mo₆O₁₉], (Ph₄P)₂(Na)₂[Mo₈O₂₆](CH₃CN)₂(H₂O), (Bu₄P)₃(NH₄)[Mo₈O₂₆], (Ph₄P)₄[W₁₀O₃₂] and (MePh₃P)₄[W₁₀O₃₂] are described, and solution compositions measured by electrospray mass spectrometry. The crystal packing of these and some related literature compounds is analysed and interpreted. One- and two-dimensional nets of embracing Ph₄P⁺ cations are general, with segregated cations and segregated anions, and it appears that Ph₄P⁺ cation arrays control the separations of anions, with solvent occupying any gaps between anions. An embracing pair of MePh₃As⁺ is cube-shaped, as is [Mo₆O₁₉]²⁻, and together they form a pseudo face-centered cubic array. In (MePh₃P)₄[W₁₀O₃₂] this fcc array is expanded by additional stuffed cations. The electrostatic energies of the ion arrays are discussed, in the context of the segregation of homo-charged ions, and the commensurabilities of the ion arrays. Using electrospray mass spectrometry of the solutions from which the crystals formed, we show that (Ph₄P)₂[Mo₂O₇] crystallises from a solution containing [Mo₈O₂₆]⁴⁻.