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

Abstract

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 = ClO4 - or SbCl6 -), 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 ΔH and ΔS 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.

Tryptophan-mediated charge-resonance stabilization in the bis -Fe(IV) redox state of MauG

“…Similar phenomena were reported for the radical derivatives of some other metalloporphyrins (29,31). Subsequent crystallization studies demonstrated that the cation porphyrin radicals formed tight dimeric complexes in a face-to-face manner with an interplanar separation distance smaller than 3.5 Å (35).…”

“…4B) (4,(28)(29)(30)(31)(32)(33). We term the former case with an odd number of spin/charge as type I CR and the latter case with an even number as type II CR.…”

“…Notably, both types of CR-transition phenomena may be observed from the same system and further distinguished by the relative positions of their CR bands (4,(31)(32)(33). Despite the inherent energy penalty caused by electrostatic repulsion in the doubly charged complex, the ΔE CR value of type II CR usually is larger than that of type I CR from the same system, which causes a blue-shift of the CR band of the (Π +• ) 2 complex compared with the (Π) 2 +• complex (4,(31)(32)(33). In principle, both types of CR complexes bear a close relationship to the conventional charge-transfer complexes derived from closed-shell diamagnetic precursors consisting of electron-rich donors and electron-poor acceptors.…”

Mechanism of protein oxidative damage that is coupled to long-range electron transfer to high-valent haems

“…These designations would indicate that the two Π systems ( i,e ., the hemes) share two holes. In contrast, the Compound II-like state would be considered a type I CR, (Π) 2 +• complex where the Π systems have a single hole distributed over both sites [37]. …”

Greatly Enhanced Intermolecular π‐Dimer Formation of a Porphyrin Trimer Radical Trications through Multiple π Bonds

“…[9] Thus, much attention has been focused on the electronic interactions between two or more porphyrins in the oxidized state from the point of view of bio-inspired chemistry [6,[10][11][12][13][14][15] in addition to supramolecular chemistry in the design of highly organized assemblies that consist of porphyrin p systems.…”