No abstract

Vital elixir for life and technology f r o m green plants beneath the sun.Dioxygen species are the various oxidation states and forms of diatomic oxygen, both as independent entities and as complexes in association with transition metal oxygen carriers. Included in this classification are the species 02, HOz., 02-., H202, H02-, and OZ2-. Because dioxygen is vital to many oxidation reactions and biological respiration processes, its oxidation-reduction chemistry has been the subject of numerous investigations. However, the mechanisms and energetics for the reduction of O2 and for the interconversion of the various dioxygen species have become known only during the last decade.Although the highly exothermic free energy (316 kJ/mol a t pH 7) for the four-electron reduction of O2(eq 1, NHE = normal hydrogen electrode) makes the O2 + 4H+ + 4e-2H20(1)Eo' = +0.82 V vs. NHE process attractive for energy transfer and storage,l mechanistic limitations preclude realization of this potential and energy for most redox reactions. Recent studies2s3 indicate that the primary electron-transfer step for the reduction of O2 is a one-electron process which is followed by chemical disproportionation reactions to yield H202 under acidic conditions (an overall two-electron process) and -OH under basic conditions (an overall four-electron process). The effective potential for this reduction is -0.33 V vs. NHE and is independent of pH. Hence, mechanistic restrictions on electron transfer to O2 cause the effective oxidizing energy of this species to be reduced by as much as 444 kJ/mol (relative to the thermodynamic value for eq 1).The reverse reaction of eq 1 (generation of 02 from H 2 0 ) represents the net oxidation process for photosystem I1 in green plant p h o t o~y n t h e s i s .~~~ Again the overall thermodynamics for the process is equivalent to that for eq 1, but the rate of the oxidation of H 2 0 to O2 is dependent upon the mechanistic pathway for the interconversion of various membrane-bound dioxygen species as well as for the formation of a diatomic oxygen species from two water molecules.6 An understanding of the energetics and dynamics for the redox reactions of various dioxygen species is important Donald T. Sawyer is Professor of Chemistry at the University of California, Riverside. His research interests include bioinorganic chemistry, electroanalytical chemistry, and coordination chemistry. Of primary interest are model systems for metaiioenzymes that contain manganese, vanadium, and molybdenum redox centers.John P. Wilshire received his B.S. degree (1973) and his Ph.D. degree from York University, Toronto, Canada, in 1977. He is now a Postdoctoral Research Associate at the University of California, Riverside. Dr. Wiishire's research interests are in the study of redox properties of biological systems and in the synthesis and characterization of analogues for inorganic complexes involved in oxygen transport mechanisms.

A study of the redox potentials of a range of metal phthalocyanines in non-aqueous media compared with analogous data for some porphyrin complexes provides evidence for extensive back donation in metallophthalocyanines. The potentials for some couples are exceedingly sensitive to axial ligation. It is suggested that this sensitivity may be used to 'tune' a redox couple for a specific purpose such as designing a reversible oxygen breathing system.

The electrochemical behavior of manganese(II) phthalocyanine dissolved in pyridine, dimethyl sulfoxide, or dimethylacetamide is reported, in the presence of perchlorate, chloride, and bromide supporting electrolyte anions. Electron-transfer couples representing net oxidations of manganese, and of the phthalocyanine ring, and two net reductions of the phthalocyanine ring are characterized by a range of electrochemical techniques, with emphasis on cyclic voltammetry. Heterogeneous rate constants are reported for several of these couples in the presence of perchlorate ion. All the couples show close to ideal reversible behavior except at higher scan rates for chloride and bromide as supporting electrolyte anions, where some deviation is observed. This system does not exhibit such sensitivity to environment as was previously observed with iron phthalocyanine.

Manganese(II) phthalocyanine fails to react with oxygen when dissolved in rigorously purified, dry pyridine. Reaction does occur, however, in pure '', '-dimethylacetamide to yield a solution of an oxygen adduct. The reaction may be reversed slowly by degassing, more rapidly upon exposure to bright white light or upon addition of an electron donor in vacuo. Addition of certain electron donors, in oxygen, causes conversion to the known PcMnm-0-MnnlPc. This oxy-bridged species may be reconverted to the oxygen adduct by reaction with oxygen. The oxygen adduct may be isolated as a solid. Analysis, thermodynamic measurements, infrared (oxygen-18 isotopic substitution) and electronic spectra, magnetism (5 = 3/2), and ESR appear consistent with the formulation (02)MnmPc, a bound superoxide.