Electron-Rich Phenoxyl Mediators Improve Thermodynamic Performance of Electrocatalytic Alcohol Oxidation with an Iridium Pincer Complex

Electron-rich phenols, including alpha-rac-tocopherol (ArOH)-O-1, 2,4,6,-tri-tert-butylphenol (ArOH)-O-3, and butylated hydroxytoluene (ArOH)-O-4, are effective electrochemical mediators for the electrocatalytic oxidation of alcohols by an iridium amido dihyride complex (P N P) Ir (H)(2) ( I r N 1, PNP = bis [ 2 - diisopropylphosphino)ethynamide). Addition of phenol mediators leads to a decrease in the onset potential of catalysis from -0.65 V vs Fc(+/0 )under unmediated conditions to -1.07 V vs Fc(+/0) in the presence of phenols. Mechanistic analysis suggests that oxidative turnover of the iridium amino trihydride (PNHP)Ir(H)(3) (IrH 2, PNHP = bis[2-diisopropylphosphino)ethyl]amine) to IrN 1 proceeds through two successive hydrogen atom transfers (HAT) to 2 equiv of phenoxyl that are generated transiently at the anode. Isotope studies and comparison to known systems are consistent with initial homolysis of an Ir-H bond being rate-determining. Turnover frequencies up to 14.6 s(-1 )and an average Faradaic efficiency of 93% are observed. The mediated system shows excellent chemoselectivity in bulk oxidations of 2-propanol and 1,2- benzenedimethanol in THE and is also viable in neat 2-propanol.