Aerobic photooxidation and C–C bond cleavage of the acetylacetonate ligand in (2-arylazo)arylpalladium(II) complexes induced by visible light

Abstract

(Arylazo)aryl(acetylacetonato)palladium(II) complexes [Pd(4-RC₆H₃NNC₆H₄R-4′)(O,O-acac)](R = H 1a, Me 1b or OMe 1c; Hacac = acetylacetone), upon irradiation of their acetone solutions at room temperature by UV, or even long-wavelength visible (λ > 500 nm) light, were readily oxidized by atmospheric oxygen to give the corresponding acetato derivatives [Pd(4-RC₆H₃NNC₆H₄R-4′)(µ-O₂CMe)]₂(R = H 2a, Me 2b or OMe 2c), providing a model for some chemical and biological processes. The oxidation reaction of complex 1b was first order in the complex when [1b]₀⩽ 2 × 10^–3 mol dm^–3. Complexes 1b and 1c were photooxidized at approximately twice the rate of complex 1a. In the presence of a scavenger of C-centred radicals (CCl₄) the rate of complex 2b accumulation was reduced. When pyridine was added or co-ordinating solvents (acetonitrile or nitromethane) were used no photooxidation of complex 1 occurred. In the presence of PPh₃ an induction period was observed and the oxidation of 1 started only after all the PPh₃ was oxidized under irradiation into OPPh₃. Spectroscopic data suggested that these ligands inhibit this photochemical oxidation because they change the type of co-ordination of the azobenzene chromophore and/or the acac ligands inducing a disconnection between the chromophore (antenna) and the acac ligand.