One-electron reduction of chromium(III) porphyrins. Formation of chromium (II) porphyrins or chromium(III) porphyrin {pi}-radical anions
- NIST, Gaithersburg, MD (United States)
- Howard Univ., Washington, DC (United States); and others
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{sup III}TSPP(L{sub 1}L{sub 2}) [chromium(III) tetrakis(4-sulfonatophenyl)porphyrin with two axial ligands L{sub 1} and L{sub 2}] was reduced to the {pi}-radical anion in water, alcohols, and N,N-dimethylformamide (DMF), where L{sub 1} and L{sub 2} are H{sub 2}O, OH{sup {minus}}, ROH, RO{sup {minus}}, or DMF. Cr{sup III}TPP(Cl)L (tetraphenylporphyrin) and Cr{sup 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{sup {minus}} was exchanged with OH{sup {minus}} (in the presence of KOH) or L was replaced with pyridine, DMF, or dimethyl sulfoxide (DMSO). Cr{sup III}OEP(Cl)L (octaethyporphyrin) in alcohol and Cr{sup III}MSP(OH)L (mesoporphyrin-IX) in aqueous alcohol were reduced to Cr{sup II}P even in the presence of base. With added pyridine, however, reduction took place on the ligand to form the Cr{sup III}P{sup {center_dot}{minus}} 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. 28 refs., 6 figs., 3 tabs.
- Sponsoring Organization:
- USDOE
- OSTI ID:
- 555059
- Journal Information:
- Journal of Physical Chemistry, Journal Name: Journal of Physical Chemistry Journal Issue: 11 Vol. 96; ISSN JPCHAX; ISSN 0022-3654
- Country of Publication:
- United States
- Language:
- English
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