Metalloporphyrin Mixed-Valence π-Cation Radicals:  Solution Stability and Properties

Solution equilibria and optical spectra of several metallooctaethylporphyrin π-cation radicals have been examined in methylene chloride solution. The several π-cation radical species, [M(OEP • )]Y (M = Cu, Ni, Zn, Pd, or VO; Y = ClO 4 - or SbCl 6 - ), are found to dimerize to form [M(OEP • )] 2 2+ . These dimeric species are characterized by the appearance of a new, strongly concentration-dependent near-infrared absorption band. This band is found in the region of 900−960 nm for all compounds except for the vanadyl complex which absorbs farther to the red at 1375 nm. The equilibrium constants for the dimerization reaction (2[M(OEP • )] + ( K D ) [M(OEP • )] 2 2+ ) have been evaluated from multiple-wavelength, concentration-dependent absorption data. Equimolar solutions of these π-cation radicals and their analogous neutral M(OEP) derivatives react to form new binuclear species, [M(OEP •/2 )] 2 +, in which the single radical electron is delocalized over both porphyrin rings. These new species, for which we suggest the term “mixed-valence” π-cation radical and which bear a formal relationship to the oxidized special pair of photosynthetic reaction centers, also display a near-infrared absorption band. The near-IR band is however distinctly different from that of the π-cation radical dimers. Equilibrium constants for the formation of these new species (M(OEP) + [M(OEP • )] + ( K MV ) [M(OEP •/2 )] 2 + ), have also been determined. The values of K MV are 10−100-fold larger than the analogous dimerization constant K D . Values of Δ H and Δ S have been obtained for both equilibrium processes. The variations in K MV and K D values are largely the consequence of entropy differences that probably result from a much stronger solvation of the π-cation radical compared to the neutral porphyrin.