Racemic Compound FormationversusConglomerate Formation with [M(bpy)3](PF6)2 (M = Ni, Zn, Ru); Molecular and Crystal Structures

Abstract

Minor modifications in molecular size and shape induced by variation of the central atom drastically alter the crystallization behaviour of the hexafluorophosphate salts of [M(bpy)3]2+. With M = Ru, racemic solutions crystallize as true racemates in the β-type [P3¯c1, a = 10.6453(5), c = 16.2987(9) A, Z = 2], while for M = Zn the racemic structure cannot be obtained, as spontaneous resolution {γ-type [P31, a = 10.3873(5), c = 26.1309(16) A, Z = 3]} is the only crystallization path observed. For [Ni(bpy)3](PF6)2, crystals of both structural types can be obtained concomitantly from the same crystallization batch. The γ-phase is systematically twinned by merohedry. The dramatic and unpredictable change in the packing pattern of the very similar building blocks within this series arises from a delicate balance between the intermolecular forces. Distinct intermolecular interactions with different directionalities can be identified in the two structure types. Through careful analysis of the crystal and molecular structures, a trend with respect to molecular size and shape on going from [Ru(bpy)3](PF6)2 to [Zn(bpy)3](PF6)2 can be established. Even though one might speculate that increasing the size of the molecular cation should destabilize the β-type structure relative to the γ-type, no conclusive rationalization based solely on structural parameters can be given as regards why the relative order of the two local minima on the energy hypersurface should become inverted.