Internal Reorganization Energies for Copper Redox Couples:  The Slow Electron-Transfer Reactions of the [Cu^II/I(bib)₂]^2+/+Couple

Abstract

[CuII(bib)2]2+ and [CuI(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 ΔE p/p ∼ 103 mV, which leads to uncertainty in E 1/2. UV−vis measurements of the reaction [CuII(bib)2]2+ + [CoII((nox)3(BC6H5)2)] ⇌ [CuI(bib)2]+ + [CoIII((nox)3(BC6H5)2)]+ ((nox)3(BC6H5)2 = a clathrochelate ligand) yield K = 2.6 in acetonitrile at 25 °C with μ = 0.1 M (Et4NBF4). Using the known E° for the CoIII/II couple yields E° = 0.274 V vs Ag/AgCl for [CuII/I(bib)2]2+/+. This, in turn, leads to K = 72 for reaction with [CoII((nox)3(BC4H9)2)]. The reaction of [CuII(bib)2]2+ with [CoII((nox)3(BC4H9)2)] obeys simple bimolecular kinetics with k = 87 M-1 s-1. Similar behavior occurs with [CoII((nox)3(BC6H5)2)] and [CoII((dmg)3(BC4H9)2)] as reductants. A value of k 11 = 0.16 M-1 s-1 is calculated for the [CuII(bib)2]2+/[CuI(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 ΔG ‡ 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 [CuII/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.

Electrochemical and homogeneous electron transfers to the Alzheimer amyloid-β copper complex follow a preorganization mechanism

“…Electron transfer is accompanied by considerable reshuffling of the coordination sphere from a four-coordination distorted square planar Cu II arrangement to an unusual two-coordination linear Cu I configuration. Although such twocoordination Cu I have been previously described (21-24), they are far less common than four-or three-coordination structures (25,26). Internal reorganization upon electron transfer is thus expected to be even larger in the first case than in the second.…”

“…Several artificial systems possessing similar entatic characteristics have been proposed (25,28,(50)(51)(52)(53). These entatic structures are the result of strong constraints exerted on the coordination sphere.…”

Electron Transfer by Copper Centers

“…This suggests that the internal reorganizational energy barriers have generally been larger than anticipated as noted by Stanbury and coworkers in their study on the electron-transfer kinetics of Cu II/I (bib) 2 . 6 Nonetheless, five systems exhibiting values of k 11 equal to or greater than 10 5 M -1 s -1 have been reported (Table 12). …”

Copper Guanidinoquinoline Complexes as Entatic State Models of Electron‐Transfer Proteins

“…obtained a k 11 value of 15 s −1 m −1 for a complex with a bispidine ligand . The Cu(bib) 2 system with only N‐donors, developed by the Stanbury group, achieves slow electron‐transfer rates of 0.024–0.49 s −1 m −1 . However, there are also copper complexes with high electron self‐exchange rates: the Cu thiaether‐bipyridine system of Rorabacher et al.…”