REDUCTION OF THE BIS(2,9-DIMETHYL-1,10-PHENANTHROLINE)COPPER(II) ION BY SUBSTITUTED HYDROQUINONES

Anaerobic stopped-flow kinetic studies of the reduction of the bis(2, 9-dimethyl-l, 10-phenanthroline)copper(II) ion (Cu(dmp)22+) by 16 monosubstituted hydroquinones (H2Q-X) have been performed (µ= 0, 2 M (acetate)). In each case, the rate law was found to be d[Cu(dmp)2+]/dt = (k1 + (k2/[H+]))[H2Q-X] [Cu(dmp)22+] within the range pH 4-6 at 25 °C. The. data are consistent with a mechanism involving reduction of Cu(dmp)22+ by both the un-ionized and monoionized forms of H2Q-X. For hydroquinone, rate constants (25 °C) and activation parameters associated with reduction of Cu(dmp)22+ by H2Q and HQ- are 1.2 x 102 M-1 s-1 (∆H‡ = 15.6 kcal/mol, ∆S‡ = +3 cal/(mol deg)) and 2.5 X 107 M-1 s-1 ∆H‡ = 12.5 kcal/mol, ∆S‡ = +18 cal/mol deg), respectively. It is shown that Marcus theory for outer-sphere electron transfer accounts reasonably well for the dependence of rate on thermodynamic driving force in the reactions of both H2Q-X and (HQ-X)- with Cu(dmp)22+. Marcus theory calculations further demonstrate that the 105-fold reactivity difference between H2Q-X and (HQ-X)- is entirely attributable to the more favorable standard free energy exchange for the latter substrate. Finally, comparisons are made between the electron-transfer reactivities of substituted hydroquinones with Cu(dmp)22+ and the type 1 copper site in the metallo enzyme laccase. © 1979, American Chemical Society. All rights reserved.