Polymorphs and pseudo-polymorphs: nine crystals containing [Fe(phen)3]2+ associated with [HgI4]2−

Abstract

Nine different solvated crystals of [Fe(phen)₃]²⁺ [HgI₄]²⁻ are described. They occur in six different crystal lattices (denoted A–F) and have the compositions A1-(acetone)(H₂O), A2-(acetone)(H₂O), A-(dmso)(H₂O), B-(acetone)₂, B-(dmf)₂, C-acetone, D-(H₂O)_1.5, E-(dmf)₂ and F-(ethanol)(CH₃CN). No unsolvated crystals were found in many crystallisation experiments. This set of nine solvates includes two sets of isomorphous crystals (A ×3, B ×2), and two pairs of strict dimorphs (B-(dmf)₂ and E-(dmf)₂, and A1-(acetone)(H₂O) and A2-(acetone)(H₂O)): the difference between A1-(acetone)(H₂O) and A2-(acetone)(H₂O) is subtle. Analyses of the crystal packing, in relation to crystallisation conditions, lead to the general interpretation that electrostatic energies between the doubly charged ions are the dominant influence, causing the ion arrays to be fairly regular, but varied by the local motifs between [Fe(phen)₃]²⁺ and [HgI₄]²⁻, which are influenced by the invariant flanged shapes of [Fe(phen)₃]²⁺ and [HgI₄]²⁻ and use I⋯phen-face and I⋯H–C phen-edge interactions. The known concerted embrace motifs between [Fe(phen)₃]²⁺ complexes do not occur in these crystals. The geometrical requirements of the motifs between [Fe(phen)₃]²⁺ and [HgI₄]²⁻ generate space in the crystals which is occupied by the solvent molecules, and it appears that the solvents best able to occupy these spaces form the more stable crystals.