AEROBIC PHOTOOXIDATION AND C-C BOND-CLEAVAGE OF THE ACETYLACETONATE LIGAND IN (2-ARYLAZO)ARYLPALLADIUM(II) COMPLEXES INDUCED BY VISIBLE-LIGHT

(Arylazo)aryl(acetylacetonato)palladium(II) complexes, [Pd(4-RC6H3N=NC 6H4R-4')(O,O-acac)] (R = H 1a, Me 1b or OMe 1c; Hacac = acetylacetone) , upon irradiation of their acetone solutions at room temperature by U V, or even long-wavelength visible (lambda > 500 nm) light, were readi ly oxidized by atmospheric oxygen to give the corresponding acetato de rivatives [Pd(4-RC6H3N=NC6H4R-4')(mu-O2-CMe)]2 (R = H 2a, Me 2b or OMe 2c), providing a model for some chemical and biological processes. Th e oxidation reaction of complex 1b was first order in the complex when [1b]0 less-than-or-equal-to 2 x 10(-3) mol dm-3. Complexes 1b and 1c were photooxidized at approximately twice the rate of complex la. In t he presence of a scavenger of C-centred radicals (CCl4) the rate of co mplex 2b accumulation was reduced. When pyridine was added or co-ordin ating solvents (acetonitrile or nitromethane) were used no photooxidat ion 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 w as 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 (ant enna) and the acac ligand.