New 2,2′:4,4":4′,4′"-quaterpyridyl transition metal complexes

The reaction of the previously reported 2,2':4,4":4',4"'-quaterpyridyl ligand, with d(6) Re-1 and d(8) Pd-11 and Pt-11 metal centres has been investigated. This has lead to the synthesis, isolation and characterisation of seven new complexes. The UV-Vis absorption spectra of the new complexes are dominated by intraligand and charge transfer bands. While the d(8) metal complexes are not emissive, the Re-1 complexes display broad unstructured luminescence that is consistent with emission from a Re(d) --> diimine(pi*) manifold. Electrochemical studies reveal that the d(8) metal complexes and one of the Re-1 complexes only display irreversible ligand based reductions. However the remaining two other Re-1 complexes display chemically irreversible metal based oxidations. The photophysical and electrochemical properties of the complexes have been rationalised by a consideration of the sigma- and pi-donor/acceptor characteristics of the ligands. (C) 2003 Elsevier B.V. All rights reserved.