ONE-ELECTRON REDUCTION OF CHROMIUM(III) PORPHYRINS - FORMATION OF CHROMIUM(II) PORPHYRINS OR CHROMIUM(III) PORPHYRIN PI-RADICAL ANIONS

One-electron reduction of chromium(III) porphyrins has been studied by radiolytic and electrochemical methods with several porphyrin ligands in various solvents. The absorption spectra of the first reduction products were monitored by pulse radiolysis within microseconds after the pulse. Two types of differential spectra were observed; intense broad absorptions at 600-800 nm ascribed to the pi-radical anions or weaker sharp absorption at various wavelengths due to chromium(II) porphyrins. Cr(III)TSPP(L1L2) [chromium(III) tetrakis(4-sulfonatophenyl)porphyrin with two axial ligands L1 and L2] was reduced to the pi-radical anion in water, alcohols, and N,N-dimethylformamide (DMF), where L1 and L2 are H2O, OH-, ROH, RO-, or DMF. Cr(III)TPP(Cl)L (tetraphenylporphyrin) and Cr(III)TMP(Cl)L (tetramesitylporphyrin), on the other hand, were reduced on the metal in neutral alcohol (L = ROH) but gave the pi-radical anions when either the Cl- was exchanged with OH- (in the presence of KOH) or L was replaced with pyridine, DMF, or dimethyl sulfoxide (DMSO). Cr(III)OEP(Cl)L (octaethylporphyrin) in alcohol and Cr(III)MSP(OH)L (mesoporphyrin-IX) in aqueous alcohol were reduced to Cr(II)P even in the presence of base. With added pyridine, however, reduction took place on the ligand to form the Cr(III)P.- species. Cyclic voltammetry and thin layer spectroelectrochemistry experiments were carried out to determine the redox potential and to confirm the above assignments. In conclusion, all chromium porphyrins studied are reduced on the metal center when this bears axial ligands that are weak electron donors, but stronger axial ligands on the Cr center direct the reduction toward the porphyrin pi-system.