SPECTRAL AND PHOTOCHEMICAL BEHAVIOR OF MONONUCLEAR AND DINUCLEAR ALPHA-DIIMINE COMPLEXES OF PT(II) AND PD(II) WITH CATECHOL DERIVATIVES

Mononuclear and dinuclear complexes with the formulae [M(DHB)(N-N)] (where M = Pd(II) or Pt(II); N-N = 2,2'-bipyridine (BPY), 2,2'-biquinoline (BIQ), 4,7-diphenyl-1,10-phenanthroline (DPP) or 1,10-phenanthroline (PHEN); DHB is the dianion of 3,4-dihydroxybenzaldehyde) and [{M(BPY)}(2)(THB)] (where M = Pd(II) or Pt(II); BPY is 2,2'-bipyridine; THB is the tetraanion of 3,3,4,4-tetrahydroxybenzaldazine) photosensitize the oxidation of 2,2,6,6-tetramethyl-4-piperidinol (XH) in N,N-dimethylformamide (DMF) to give 4-hydroxy-2,2,6,6-tetramethyl-4-piperidinyloxy, a nitroxide free radical (XO). This photo-oxidation reaction involves singlet molecular oxygen (O-1(2)) as an intermediate and its presence is confirmed by quenching studies using bis(diethyldithiocarbamato)nickel(II) (Ni(DDTC)2), a physical quencher of O-1(2). The ability of the mononuclear complexes to photosensitize the above photo-oxidation reaction follows the order: [Pt(DHB)(PHEN)] > [Pt(DHB)(DPP)] > [Pt(DHB)(BIQ)] > [Pt(DHB)(BPY)] > [Pd(DHB)(DPP)] greater than or equal to [Pd(DHB)(PHEN)] > [Pd(DHB)(BIQ) ]greater than or equal to [Pd(DHB)(BPY)]. Moreover, the dinuclear complexes, in comparison with the analogous mononuclear complexes, follow the order: [{Pt(BPY)}(2)(THB)]> [Pt(DHB)(BPY)] > [{Pd(BPY)}(2)(DHB)] > [Pd(DHB)(BPY)]. These orders depend on the metal ion, the number of metal ions in the complex, the nature of the alpha-diimine ligand and the distortion from planar geometry of the metal complex