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 Δ E p/p ∼ 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 (BC 6 H 5 ) 2 )] ⇌ [Cu I (bib) 2 ] + + [Co III ((nox) 3 (BC 6 H 5 ) 2 )] + ((nox) 3 (BC 6 H 5 ) 2 = a clathrochelate ligand) yield K = 2.6 in acetonitrile at 25 °C with μ = 0.1 M (Et 4 NBF 4 ). Using the known E ° for the Co III/II couple yields E ° = 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 (BC 4 H 9 ) 2 )]. The reaction of [Cu II (bib) 2 ] 2+ with [Co II ((nox) 3 (BC 4 H 9 ) 2 )] obeys simple bimolecular kinetics with k = 87 M - 1 s - 1 . Similar behavior occurs with [Co II ((nox) 3 (BC 6 H 5 ) 2 )] and [Co II ((dmg) 3 (BC 4 H 9 ) 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 (Δ(Cu−N) ave = 0.07 Å) during redox turnover. However, molecular mechanics calculations with newly developed copper force fields demonstrate Δ G ‡ in 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.