Internal reorganization energies for copper redox couples:: The slow electron-transfer reactions of the [CuII/I(bib)2]2+/+ couple

[Cu-II(bib)(2)](2+) and [Cu-I(bib)(2)](+) (bib = 2,2'-bis(2-imidazolyl)biphenyl) have similar geometries, with the Cu(II) complex best described as pseudo-square planar and the Cu(I) complex pseudo-tetrahedral. The cyclic voltammograms in acetonitrile are only quasi-reversible, with Delta E-p/p similar to 103 mV, which leads to uncertainty in E-1/2 UV-vis measurements of the reaction [Cu-II(bib)(2)](2+) + [Co-II((nox)(3)(BC6H5)(2))] reversible arrow [Cu-I(bib)(2)](+) + [Co-III((nox)(3)(BC6H5)(2))](+) ((nox)(3)(BC6H5)(2) = a clathrochelate ligand) yield K = 2.6 in acetonitrile at 25 degrees C with mu = 0.1 M (Et4NBF4). Using the known E degrees for the Co-III/II couple yields E degrees = 0.274 V vs Ag/AgCl for [Cu-II/I(bib)(2)](2+/+). This, in turn, leads to K = 72 for reaction with [Co-II((nox)(3)(BC4H9)(2))]. The reaction of [Cu-II(bib)(2)](2+) With [COII((nox)(3)(BC4H9)(2))] obeys simple bimolecular kinetics with k = 87 M-1 s(-1). Similar behavior occurs with [Co-II((nox)(3)(BC6H5)(2))] and [Co-II((dmg)(3)(BC4H9)(2))] as reductants. A value of k(11) = 0.16 M-1 s(-1) is calculated for the [Cu-II(bib)(2)](2+)/[Cu-I(bib)(2)](+) Self-exchange reaction based on the Marcus cross-exchange relationship and the known electron-self-exchange rate constants for the Co(III)/Co(II) couples. This value of k(11) is remarkably small for a Cu(II/I) system, especially in view of the small structural change at copper (Delta(Cu-N)(ave) = 0.07 Angstrom) during redox turnover. However, molecular mechanics calculations with newly developed copper force fields demonstrate Delta G(in)double dagger to be the major factor that accounts for the small value of k(11). Much of this large reorganization energy appears to arise from angular distortions around copper, Nevertheless, the structural changes for the [Cu-II/I(bib)(2)](2+/+) couple are small relative to those for systems that have much greater self-exchange rate constants. These considerations underscore the difficulties in applying the entatic hypothesis to cuproprotein active sites.