True and quasi-isomorphism in tetrakis(acetonitrile)coinage metal(i) salts

Abstract

Salts of the family [M(NCMe)₄]⁺X⁻ (M = Cu, Ag; X⁻ = [BF₄]⁻, [ClO₄]⁻, [PF₆]⁻ crystallize in the space groups Pna2₁ or P2₁2₁2₁ with very similar cell dimensions. This gives rise to two sets of true isomorphs (four members in Pna2₁, two in P2₁2₁2₁), and a solvated species (Cu/PF₆/MeCN) with a distorted P2₁2₁2₁ lattice. Across the two space groups and regardless of the choice of metal atom or anion, the underlying basic structures are virtually identical in the six unsolvated cases, consisting of alternate sheets of cations and anions, mainly held together by C–H⋯C,N and C,N⋯C,N interactions, and a columnar cation–anion motif perpendicular to the sheets maintained by C–H⋯F,O interactions. The pair of concomitant polymorphs found with Cu/BF₄ in both Pna2₁ and P2₁2₁2₁ can readily be described as being quasi-isomorphous. Using a Hirshfeld surface approach and a tri-cationic structural synthon extracted from the asymmetric units, the common underlying pattern of close intercomponent approaches has been analysed and quantified. The structure of the Ag/PF₆ complex is new; low-temperature redeterminations are recorded for the Ag/ClO₄ and the Cu/PF₆/MeCN complexes.