Crystal supramolecular motifs: two-dimensional grids of terpy embraces in [ML2]z complexes (L=terpy or aromatic N3-tridentate ligand)

Abstract

By analysis of crystal packing we have identified a crystal supramolecular motif that is a two-dimensional net of terpy embraces formed by metal complexes [M(terpy)2]2+ (terpy=2,2′:6′,2″-terpyridyl) and similar meridional [M(N3-tridentate)2] complexes. The terpy embrace involves two complexes attracted by one offset-face-to-face (off) and two edge-to-face (ef) interactions by the outer pyridyl rings of the ligand. In many crystals containing small monoanions there is a two-dimensional net of these embraces, in which each complex forms eight ef and four off interactions with its neighbours. The principal axes of the complexes are normal to the layer, which is exactly or approximately planar, and can occur with high (tetragonal) or low crystal symmetry. Grooves that occur on the layer surfaces, formed between parallel central pyridyl rings of the ligands, run in orthogonal directions on the two surfaces of each layer. Anions and solvent molecules in the crystals are usually disordered, in or near the grooves. The net attractive energy of the terpy embrace for a pair of [M(terpy)2]2+ is calculated to be ca. 15 kJ mol-1: in the two-dimensional net the attractive cation···cation energy per cation is ca. 29 kJ mol-1. Inclusion of the anions associated with one layer increases the attractive energy per [M(terpy)2]2+ to the order of 130 kJ mol-1. A variety of ligands, which are minor or major modifications of terpy, also form this supramolecular motif. Hydrogen bonding involving NH functions of these ligands, solvent, and/or anions, does not in general disrupt the motif. In one instance where the [M(N3-tridentate)2] complex is uncharged there is mutual interpenetration of contiguous layers. These infinite two-dimensional nets of octahedral metal complex sites formed as crystal supramolecules are analogous to the two-dimensional gridlike supermolecules formed by extended oligo-chelating ligands. Opportunities for crystal engineering are discussed.