Reactions of carboxyl radicals generated by the photocleavage of complexes of iron(III) tetra(2-N-methylpyridyl)porphyrin with unsaturated and aromatic carboxylic acids in aqueous solution

Abstract

Anaerobic aqueous solutions of the complexes of iron(III) tetra(2-N-methylpyridyl)porphyrin with carboxylate anions, benzoate, phthalate and 2- and 4-methylbenzoate, have been photolysed with visible light (λ > 390 nm) to give solvent-caged iron(II) porphyrin and aroyloxyl radicals. Evidence is presented that the rate of build-up of the iron(II) porphyrin in the reaction is dependent on the partitioning of the caged aroyloxyl radicals between product formation (decarboxylation and hydrogen abstraction from the methyl group) and regeneration of the starting complex. The fastest reaction occurs with 2-methylbenzoate for which intramolecular hydrogen abstraction converts the initially formed 2-methylbenzoyloxyl into 2-carboxybenzyl radicals. The photolysis of the iron(III) porphyrin complexes of acrylic, methacrylic and dimethylacrylic acids shows analogous behaviour, with the extra possibility that the carboxyl radicals may also add to the alkenic group of the substrate molecules in the solvent cage. The rapid decarboxylation of phenylacetoxyl radicals results in the fast photoreduction of the iron(III) porphyrin by phenylacetic acid to iron(II) porphyrin and to benzyl radicals. Under aerobic conditions the reoxidation of the iron(II) to iron(III) allows the system to become photocatalytic in iron porphyrin. The stability of the catalyst to oxidative destruction is moderate but can be dramatically improved by anchoring the charged iron porphyrin by electrostatic interaction to a silica support.