Copper(I) and silver(I) complexes of a new tetrahedrally-enforcing ligand containing two bipyridyl binding sites linked by a diphenyl disulfide bridge

Conversion of the amino group of 6-(2-aminophenyl)-2,2'-bipyridine to a thiol resulted not in the expected N,N,S-terdentate chelating ligand HL [6-(2-sulfhydryl)-2,2'-bipyridine] but in the oxidised disulfide 2,2'-bis[6-(2,2'-bipyridyl)]diphenyldisulfide, L-L. This ligand contains two bipyridyl arms linked by a diphenyl disulfide bridge in such a way that they cannot be coplanar, but tend to co-ordinate to a single metal ion with a substantial dihedral angle between the two ligand planes giving a pseudo-tetrahedral geometry. The crystal structure of L-L . 0.5C(6)H(5)CH(3) shows the presence of near-linear N ... S-S interactions (non-bonded N ... S; 2.75 Angstrom) in which the pyridyl lone pair interacts with the sigma* orbital of the disulfide fragment. The complexes [Cu(L-L)][PF6] and [Ag(L-L)][PF6] were prepared and are isostructural, both having a distorted four-co-ordinate metal co-ordination environment in which the two chelating bipyridyl fragments have dihedral angles of between 70 and 80 degrees C between them (ie. nearly mutually perpendicular). The electronic spectrum of the Cu-I complex [Cu(L-L)][PF6], and the EPR spectrum of its one-electron oxidised product [Cu(L-L)](2+), both confirm that the rigidity of the ligand donor set does not permit much variation in the co-ordination environment about the copper ion. The ligand therefore imposes a relatively rigid pseudo-tetrahedral geometry on the metal centres.