Reduction and alkylation of rhodium porphyrins in alcohol solutions. Radiation chemical and photochemical studies

Radiolytic reduction of chlororhodium(III) tetramesitylporphyrin (ClRh{sup III}P) in alcohol solutions forms a transient Rh{sup II}P, which reacts to yield different products under different conditions. In alkaline 2-propanol the product is Rh{sup I}P{sup -}, in weakly acidic conditions HRh{sup III}P is formed, and under strongly acidic conditions the main products of radiolysis are the alkylated rhodium complexes, R-Rh{sup III}P. The latter products are formed by reaction of Rh{sup II}P with alkyl radicals (R{sup .}) that are produced in the irradiated solvent (R{sup .} = {sup .}CH{sub 3} and (CH{sub 3}){sub 2}C{sup .}OH in 2-propanol). UV photolysis of ClRH{sup III}P in acetone/2-propanol solutions led to formation of HO(CH{sub 3}){sub 2}CRh{sup III}P. One-electron reduction of CH{sub 3}-Rh{sup III}P occurs at the porphyrin ligand to produce a transient {Pi}-radical anion, CH{sub 3}-Rh{sup III}P{sup .-}. In alkaline solution, this transient eliminates {sup .}CH{sub 3} to form the stable Rh{sup I}P{sup -}, but in neutral or acidic solutions, it undergoes disproportionation, promoted by protonation on the macrocycle, to form CH{sub 3}-Rh{sup III}-chlorin and then CH{sub 3}-Rh{sup III}-isobacteriochlorin upon further reduction. In the presence of CO{sub 2}, the initial radiolytic reduction yields are increased. After extensive irradiation, however, the yields of reduction are decreased and a verymore » low yield of CO was found. 29 refs., 2 figs.« less