REDUCTION AND ALKYLATION OF RHODIUM PORPHYRINS IN ALCOHOL-SOLUTIONS -RADIATION-CHEMICAL AND PHOTOCHEMICAL STUDIES

Radiolytic reduction of chlororhodium(III) tetramesitylporphyrin ((ClR hP)-P-III) in alcohol solutions forms a transient (RhP)-P-II, which re acts to yield different products under different conditions. In alkali ne 2-propanol the product is (RhP)-P-I-, in weakly acidic conditions ( HRhP)-P-III is formed, and under strongly acidic conditions the main p roducts of radiolysis are the alkylated rhodium complexes, R-(RhP)-P-I II. The latter products are formed by reaction of (RhP)-P-II with alky l radicals (R(.)) that are produced in the irradiated solvent (R(.) = (CH3)-C-. and (CH3)2(C)(.)OH in 2-propanol). UV photolysis of (ClRhP)- P-III in acetone/2-propanol solutions led to formation of HO(CH3)(2)C- (RhP)-P-III. One-electron reduction of CH3-(RhP)-P-III occurs at the p orphyrin ligand to produce a transient pi-radical anion, CH3-(RhP.-)-P -III. In alkaline solution, this transient eliminates (.)CH3 to form t he stable (RhP-)-P-I, but in neutral or acidic solutions, it undergoes disproportionation, promoted by protonation on the macrocycle, to for m CH3-Rh-III-chlorin and then CH3-Rh-III-isobacteriochlorin upon furth er reduction. In the presence of CO2, the initial radiolytic reduction yields are increased. After extensive irradiation, however, the yield s of reduction are decreased and a very low yield of CO was found. No reaction was detected between (RhP-)-P-I or (HRhP)-P-III and CO2, even under visible light illumination. Although this system is found to ca talyze homogeneous photochemical formation of H-2, no catalytic activi ty for CO2 reduction was found under the current experimental conditio ns.