A Structural Strategy for Generating Rapid Electron-Transfer Kinetics in Copper(II/I) Systems

Abstract

Electron-transfer in low molecular weight copper(II/I) systems is generally accompanied by a large reorganization of the inner-coordination sphere. On the basis of recent kinetic studies involving Cu(II/I)-macrocyclic polythiaether complexes, it was hypothesized that forcing Cu(II) out of the macrocyclic cavity (i) decreases the changes in bond angles upon reduction and (ii) obviates any need for donor atom inversion. This should diminish the reorganizational barrier and, thereby, increase the electron self-exchange rate. This hypothesis has now been tested utilizing a somewhat soluble 12-membered macrocyclic tetrathiaether, oxathiane[12]aneS4 (L). Crystal structures of the CuIIL and CuIL complexes confirm that, whereas one Cu−S bond dissociates upon reduction, the remaining bond lengths and angles change only minimally. The free ligand, oxathiane[12]aneS4, C10H18OS4, crystallizes in the orthorhombic space group Pbca with Z = 8, a = 15.211(2) Å, b = 8.5113(9) Å, c = 20.548(3) Å. The CuIIL complex crystallizes as a 5-coordinate monomer with water as the apical ligand: [CuL(OH2)](ClO4)2·H2O, C10H22O11S4Cl2Cu, monoclinic P2(1)/c, Z = 4, a = 15.774(2) Å, b = 8.485(5) Å, c = 16.508(9) Å, β = 112.11(6)°. The CuIL complex crystallizes as a binuclear species: [(CuL)2NCCH3](ClO4)2·NCCH3, C24H42N2O10S8Cl2Cu2, in the triclinic space group P1̄ with Z = 4, a = 12.5917(2) Å, b = 13.0020(3) Å, c = 14.9285(3) Å, α = 68.356(1)°, β = 84.298(1)°, γ = 61.129(1)°. The kinetics of CuII/I(oxathiane[12]aneS4) reacting with four selected counter reagentstwo oxidants and two reductantsyield exceptionally large cross-reaction rate constants. Application of the Marcus cross relation yields calculated self-exchange rate constants ranging from 4 × 105 to 8 × 105 M-1 s-1 (median: 6 × 105 M-1 s-1) for this CuII/IL redox system at 25 °C, μ = 0.10. A comparable result of k11 = (8.4 ± 0.8) × 105 M-1 s-1 has been obtained by NMR line-broadening measurements (at 25 °C, corrected to μ = 0.10). This is the largest self-exchange rate constant ever reported for a low molecular weight Cu(II/I) system. Thus, elimination of donor atom inversion coupled with a constrained inner sphere appears to represent a feasible approach for accelerating electron transfer in Cu(II/I) macrocyclic systems.