Supramolecular motifs: sextuple aryl embraces in crystalline [M(2,2′-bipy)3] and related complexes†

Abstract

Tris(bidentate ligand)metal complexes such as [M(bipy)₃] commonly associate in crystals as columns of complexes with collinear three-fold axes, in order that three heterocyclic rings of one complex can engage in six vertex-to-face interactions with three heterocyclic rings from the neighbour. This concerted supramolecular motif is called the sextuple aryl embrace (SAE), and is analogous to the sextuple phenyl embrace formed by XPh₃ and XPh₄ molecules. The three oxidation states of [Ru(bipy)₃]^z, z = 0, 2+, 3+, were found to crystallise with SAEs, as do most other [M(bipy)₃]^z complexes with small symmetrical anions. Expansion of crystal lattices to accommodate anions was found not to disrupt the infinite columns of SAEs. A pair of tris(bidentate ligand)-metal complexes must have opposite chirality to participate in an SAE, and in this way the SAE differs from the otherwise similar homochiral helicate complexes. A small number of tris(tridentate ligand) complexes demonstrate a similar crystal supramolecular motif.