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

Abstract

The four-coordinate anion Cu I (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) 2 0 (E 0 = −290 mV vs. SCE) or by the quantitative reaction of 2Hdpym with Cu(CH 3 CN) 4 + in the absence of O 2 . 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 − excess Hdpym in acetone is "fast" in the 1 H NMR timeframe, with k 1 = 1.4 × 10 7 L mol −1 s −1 at 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 Cu I (dpym) 2 − in acetone remains negligible. Homogeneous electron exchange between Cu I (dpym) 2 − and Cu II (dpym) 2 0 in acetone falls in the "slow" 1 H NMR timeframe, with k ex = 5.9 × 10 3 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 k el = 0.16±0.04 cm s −1 at I ≈ 0.1 mol −1 L −1 and 298 K, according to AC voltammetry. These values of K ex and K el are of the order expected for Cu II/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.