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

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 CuII(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 CuI or that are poor proton acceptors. Ligand exchange between CuI(dpym)2− excess Hdpym in acetone is "fast" in the 1H NMR timeframe, with k1 = 1.4 × 107 L mol−1 s−1at 298 K (first order in each reactant), ΔH‡1 = 3.4 ± 0.6 kJ mol−1, and ΔS‡1 = −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 ΔS‡ex = −10 ± 10 J K−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−1L−1 and 298 K, according to AC voltammetry. These values of Kex and Kel are of the order expected for CuII/I couples in which no significant change in coordination number or geometry accompanies electron transfer. Keywords: Electron transfer, copper complexes, ligand substitution kinetics, dynamic NMR.