Electron exchange kinetics in a tetrahedrally coordinated copper(II)/(I) couple

Abstract

The four-coordinate anion CuI(dpym)2- (Hdpym = 3,3′,5,5′-tetramethyl-4,4′- dicarboethoxydipyrromethene) can be prepared in solution in acetone either by electrochemical reduction of the known tetrahedral complex Cu II(dpym)20 (E0 = -290 mV vs. SCE) or by the quantitative reaction of 2Hdpym with Cu(CH3CN) 4+ in the absence of O2. The latter reaction does not go to completion in solvents that bind relatively strongly to CU I or that are poor proton acceptors. Ligand exchange between CU I(dpym)2- and excess Hdpym in acetone is "fast" in the 1H NMR timeframe, with k1 = 1.4 × 107 L mol-1 s-1 at 298 K (first order in each reactant), ΔH1‡ = 3.4 ± 0.6 kJ mol-1, and ΔS1‡ = -97 ± 3 J K-1 mol-1. In the absence of excess Hdpym, dissociation of CuI(dpym)2- in acetone remains negligible. Homogeneous electron exchange between CuI(dpym)2- and CuII(dpym)20 in acetone falls in the "slow" 1H NMR timeframe, with kex = 5.9 × 103 L mol-1 s-1, ΔH ex‡ = 48.5 ± 3.0 kJ mol-1, and ΔSex‡ = -10 ± 10 J K-1 mol-1, at ionic strength I ≈ 0.007 mol L-1 and 298 K, while for the same self-exchange on a Pt electrode the heterogeneous rate constant kel = 0.16 ± 0.04 cm s-1 at I ≈ 0.1 mol L-1 and 298 K, according to AC voltammetry. These values of k ex and kel are of the order expected for CUII/I couples in which no significant change in coordination number or geometry accompanies electron transfer.