The influence of co-ordination number on copper(I)–(II) redox interconversions. Part 1. Reduction of five-co-ordinate (±)-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetra-azacyclotetradecanecopper(II) complexes, [CuII(tet-b)X]+(X–= Cl–, Br–, or thiourea), with Cr2+, V2+, and [Ru(NH3)6]2+

Abstract

The 1 : 1 Cr²⁺ reductions of the five-co-ordinate [Cu^II(tet-b)X]⁺ complexes (X^–= Cl^–, Br^–, and thiourea) have been studied. At 25 °C, I= 0.20M(lithium p-toluenesulphonate), reactions are independent of [H⁺](0.01–0.20M) and rate constants (l mol^–1 s^–1) are 17.9 (Cl^–), 21.4 (Br^–), and 81 (thiourea). The Cr^III products are [CrCl]²⁺, [CrBr]²⁺, and S-bonded [Cr(thiourea)]³⁺. With V²⁺ as reductant rate constants (l mol^–1 s^–1, 25 °C) and activation parameters ΔH^‡(kcal mol^–1) and ΔS^‡(cal K^–1 mol^–1) are 0.055, 9.9, and –31.0 (Cl^–) and 0.020, 12, and –27 (thiourea), respectively. Less extensive studies with X^–= Br^– give a rate constant at 50 °C of ca. 0.03 l mol^–1 s^–1. Slow [Ru(NH₃)₆]²⁺ reduction of [Cu^II(tet-b)Cl]⁺, K⩽ 10^–3 l mol^–1 s^–1 at 20 °C rules out an outer-sphere process for the corresponding V²⁺ reduction and, since, the parameters obtained are not in the substitution-controlled range, suggests an inner-sphere electron-transfer-controlled process. The extreme reluctance of [Cu^II(tet-b)X]⁺ complexes to react by an outer-sphere process is noted.