Crystal supramolecular motifs: two-dimensional grids of terpy embraces in [ML2](z) complexes (L = terpy or aromatic N-3-tridentate ligand)

By analysis of crystal packing we have identified a crystal supramolecular motif that is a two-dimensional net of terpy embraces formed by metal compl exes [M(terpy)(2)](2+) (terpy = 2,2': 6',2"-terpyridyl) and similar meridio nal [M(N-3-tridentate)(2)] complexes. The terpy embrace involves two comple xes attracted by one offset-face-to-face (off) and two edge-to-face (ef) in teractions by the outer pyridyl rings of the ligand. In many crystals conta ining small monoanions there is a two-dimensional net of these embraces, in which each complex forms eight ef and four off interactions with its neigh bours. The principal axes of the complexes are normal to the layer, which i s exactly or approximately planar, and can occur with high (tetragonal) or low crystal symmetry. Grooves that occur on the layer surfaces, formed betw een parallel central pyridyl rings of the ligands, run in orthogonal direct ions on the two surfaces of each layer. Anions and solvent molecules in the crystals are usually disordered, in or near the grooves. The net attractiv e energy of the terpy embrace for a pair of CM(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. Hydro gen bonding involving NH functions of these ligands, solvent, and/or anions , does not in general disrupt the motif. In one instance where the [M(N-3-t ridentate)(2)] complex is uncharged there is mutual interpenetration of con tiguous layers. These infinite two-dimensional nets of octahedral metal com plex sites formed as crystal supramolecules are analogous to the two-dimens ional gridlike supermolecules formed by extended oligo-chelating ligands. O pportunities for crystal engineering are discussed.