Isolation and Oxidation-Reduction of Methylviologen Cation Radicals. Novel Disproportionation in Charge-Transfer Salts by X-ray Crystallography

Abstract

The totally reversible reduction of the well-known organic electron mediator methylviologen dication MV2+ to its cation radical MV•+ and thence to the neutral MV° is established by cyclic voltammetric studies in nonaqueous solutions. The use of aprotic solvents thus allows crystalline forms of MV•+ and MV° to be isolated and structurally characterized for the first time by X-ray crystallography. The structural parameters of MV° show it to be a nonaromatic, polyene-like dihydrobipyridine (similar to dipyran and dithiopyran) with excellent electron-donor properties. The crystalline methylviologen cation radical (as the 1:1 salt MV•+PF6-) exists as an infinite stack of structurally distinct cationic moieties that are designated as MVI and MVII. Structural parameters show that the MVI component is related to the oxidized MV2+, and MVII is related to the reduced MV°. The alternate stacking of MVI and MVII in MV•+PF6- is ascribed to an unique intracrystalline disproportionation that derives from strong charge-transfer interactions inherent to the methylviologen cation radical. The phenomenological relationship of such charge-transfer forces to the dimeric (MV•+)2 extant in concentrated aqueous solutions and to properties relevant to organic metals is advanced. © 1990, American Chemical Society. All rights reserved.