When five are six : the myth of five-coordinate copper(II) in supramolecular chemistry

We have made a systematic investigation of {Cu(tpy)(bpy)} and {Cu(tpy)(phen)} complexes by determining the single crystal structures of [Cu(bpy)(tpy)][PF6]2, 4{[Cu(6-Mebpy)(tpy)][PF6]2}·Et2O·3H2O, [Cu(6,6′-Me2bpy)(tpy)][PF6]2·MeCN, [Cu(2,9-Me2phen)(tpy)][PF6]2, 2{[Cu(6-Phbpy)(tpy)][PF6]2}·2MeNO2·H2O, [Cu(6,6′-Ph2bpy)(tpy)][PF6]2 and 2{[(6,6′-Me2bpy)Cu(µ-tpySStpy)Cu(6,6′-Me2bpy)][PF6]4}·1.5MeCN·3H2O (bpy = 2,2′-bipyridine, 6-Mebpy = 6-methyl-2,2′-bipyridine, 6,6′-Me2bpy = 6,6′-dimethyl-2,2′-bipyridine, 2,9-Me2phen = 2,9-dimethyl-1,10-phenanthroline, 6-Phbpy = 6-phenyl-2,2′-bipyridine, 6,6′-Ph2bpy = 6,6′-diphenyl-2,2′-bipyridine, tpy = 2,2′:6′,2″-terpyridine, tpySStpy = 1,2-bis(2,2′:6′,2″-terpyridin-4′-yl)disulfide). Among these copper(II) complexes, only one possesses a true trigonal bipyramidal coordination environment and this is crystallographically imposed. A comparison is made between these newly determined structures and the seven related {Cu(tpy)(bpy)} and {Cu(tpy)(phen)} structures available in the Cambridge Structural Database. The bulk of these “five-coordinate” complexes exhibiting ostensibly square-based pyramidal geometries also show an additional interaction in the remaining axial site leading to a better description as their being six-coordinate.