SPECTRAL AND PHOTOCHEMICAL BEHAVIOR OF NEWLY SYNTHESIZED 2,2'-DIPYRIDYLAMINE COMPLEXES OF PT(II) AND PD(II) WITH VARIOUS DIOXOLENES
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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-tertbutylcatechol (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 (O-1(2)) as an intermediate and its presence was confirmed by quenching studies using bis(diethyldithiocarbamato)nickel(II), a well-known O-1(2) 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(TBCAT)(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.
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