Substituent Group Electronic Effects on Inversion and Proton-Exchange Rates at Chiral Nitrogen Centers for a Series of Bis(β-diketonato)(N-methylethylenediamine)cobalt(III) Complexes

Abstract

Abstract The rates of the base-catalyzed proton exchange (deuteration, kD) and inversion (epimerization, kep) at chiral nitrogen centers of [Co(Xacac)2(Me-en)]+ (Xacac: NO2acac=3-nitro-2,4-pentanedionate, Clacac=3-chloro-2,4-pentanedionate, CH3acac=3-methyl-2,4-pentanedionate; Me-en=N-methylethylenediamine) in aqueous solutions have been measured at 34.0 °C, and the results were compared with one another and with those of the corresponding acac complex (acac=2,4-pentanedionate) in order to investigate the substituent influence on the rates. The kD and kep values of the NO2acac complex are ca. 104 and ca. 106 times, respectively, larger than those of the CH3acac complex. The effect of substituents is electronic and the Hammett relationship was found to hold for both deuteration and epimerization rates. The reduction (Co3+/Co2+) half-wave potential, E1⁄2 obtained by rotating glassy carbon electrode voltammetry increases from −0.59 V (vs. SCE) for the CH3acac complex to −0.08 V for the NO2acac complex. A linear dependence was observed between E1⁄2 and the Hammett parameter.