Photochemical behavior of platinum(II) and palladium(II) complexes of 4,4′-dimethyl-2,2′-bipyridine

Several mixed-ligand complexes of formula [MX2(MBPY)] {where M is Pd(II) or Pt(II); X is Cl-, I-, N-3(-) or NO2- and MBPY is 4,4'-dimethyl-2,2'-bipyridine} have been prepared. The UV-Vis spectra of these complexes were found to show a low-lying MLCT band and on irradiation at the MLCT band these complexes sensitize the oxidation of 2,2,6,6-tetramethyl-4-piperidinol (XH) in N,N-dimethylformamide (DMF) to 4-hydroxy-2,2,6,6-tetramethyl-1-piperidinoloxy free radical (XO). This photo-oxidation reaction involves singlet molecular oxygen (O-1(2)) as an intermediate and its presence was confirmed by quenching studies using bis(diethyldithiocarbamato)nickel(II) [Ni(DDTC)(2)], a well-known O-1(2) quencher. The ability of the complexes to photosensitize the above photo-oxidation reaction follows the order: [Pt(N-3)(2)(MBPY)] (2)similar to[Pt(NO2)(2)(MBPY)] (3)>[PtCl2(MBPY)] (4) > [PtI2(MBPY)] (l) > [Pd(NO2)(2)(MBPY)] (7) similar to[Pd(N-3)(2)(MBPY)] (6) > [PdCl2(MBPY)] (8) > [PdI2(MBPY)] (5), which reflects the nature of the metal ion and the nature of the ligands present in the complexes.