Voltammetric oxidation of arene, cycloheptatriene, and cycloheptatrienyl tricarbonyl complexes of chromium

Abstract

The series of π-arene chromium tricarbonyl complexes [(π-C₆H_nMe_6–n)Cr(CO)₃](n= 0–6), [(π-C₆H₅R)Cr(CO)₃](R = Prⁱ, NH₂, NHMe, Nme₂, OMe, CO₂Me, and Cl), and [(π-C₆H₄RR′)Cr(CO)₃](R = Me, R′= Prⁱ; R = Me, R′= NH₂) undergo, voltammetrically, two one-electron oxidation processes. The E_½ value for the oxidation waves are dependent on the nature of the arene ring substituents but are independent of ring substituent positional isomerism. There is a linear correlation between E_½(complex) and E_½(free arene), and between E_½(complex) and the ionisation potentials of the complexes [(π-C₆H_nMe_6–n)Cr(CO)₃] and of the free arenes. The cycloheptatriene species [(π-C₇H₆RR′)Cr(CO)₃](R = H, R′= H, exo- or endo-Ph, and exo-CN; R = R′= OMe) may also be oxidised to [(π-C₇H₆RR′)Cr(CO)₃]⁺ and the species where R = H, R′=exo-CN and R = R′= OMe may be reduced in a one-electron step. The cycloheptatrienyl complexes [(π-C₇H₇R)Cr(CO)₃]⁺(R = H or OMe), undergo an irreversible one-electron reduction process.