Spectral and photochemical behaviour of mononuclear and dinuclear α-diimine complexes of Pt(II) and Pd(II) with catechol derivatives

Abstract

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 (DDP) 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-dimethyl-formamide (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 ( 1O 2) as an intermediate and its presence is confirmed by quenching studies using bis(diethyldithiocarbamato)nickel(II) (Ni(DDTC) 2), a physical quencher of 1O 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)] ⩾ [Pd(DHB)(PHEN)] > [Pd(DHB)(BIQ)] ⩾ [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)}(DHB)] > [Pd(DHB)(BPY)]. These orders depend on the metal ion, the number of metal ions in the complex, the nature of the α-diimine ligand and the distortion from planar geometry of the metal complex.