Spectral and photochemical behaviour of newly synthesized 2,2′-dipyridylamine complexes of Pt(II) and Pd(II) with various dioxolenes

Abstract

Eight new complexes of the formula [M(X-X)(DPA)] (where M≡Pd(II) or Pt(II), DPA is 2,2′-dipyridylamine and X-X are various dianions of catechol (CAT) and its derivatives, such as 4-methylcatechol (MCAT), 4-tert-butylcatechol (TBCAT), 3,4-dihydroxybenzoic acid (DHBA) and 3,4-dihydroxybenzaldehyde (DHB) were synthesized and characterized. The absorption spectra and electrochemistry of these complexes are discussed. The complexes were found to sensitize the oxidation of 2,2,6,6-tetramethyl-4-piperidinol in N,N-dimethylformamide (DMF) to the nitroxide radical. This photo-oxidation reaction involves singlet molecular oxygen (¹O₂) as an intermediate and its presence was confirmed by quenching studies using bis (diethyldithiocarbamato) nickel(II), a well-known ¹O₂ quencher. The ability of the complexes to photosensitize the above photo-oxidation reaction follows the order: [Pt(DHBA)(DPA)]>[Pt(DHB)(DPA)]>[Pt(CAT)(DPA)]⪢[Pd(CAT)(DPA)]>[Pd(DHBA)-(DPA)]>[Pd(MCAT)(DPA)]>[Pd(TBCA)(DPA)]>[Pd(DHB)(DPA)]. This order depends on the nature of the metal ion, the nature of the mixed ligands in the complexes and the distorted square planar geometry of the complexes.