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

Abstract

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 Å) in which the pyridyl lone pair interacts with the σ* orbital of the disulfide fragment. The complexes [Cu(L–L)][PF ₆ ] and [Ag(L–L)][PF ₆ ] 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 °C between them (i.e. nearly mutually perpendicular). The electronic spectrum of the Cu ^I complex [Cu(L–L)][PF ₆ ], 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.