Variable Structure of the Cu2S2 Core in Doubly Thiolate-Bridged Dicopper(I) Complexes. An X-Ray Crystallographic, Electrochemical and EPR Study of (dmp)2(o-CH3C6H4S)2Cu2, dmp = 2,9-Dimethyl-1,10-phenanthroline, and a Comment on the Nature of Phenanthroline-Containing “Cu(0)” Complexes

Abstract

The molecular and crystal structure of the dicopper(I) complex (dmp)2(μ-o-CH3C6H4S)2Cu2 (1), dmp = 2,9-dimethyl-1,10-phenanthroline, has been determined. With Cu/S/Cu and S/Cu/S bond angles between 80 and 100°, the planar structure of the central CuSCuS four-membered ring is much more symmetric for 1 as compared to the analogous complex 2 of unsubstituted 1,10-phenanthroline which has a folded Cu2S2 core with bond angles between 67 and 108°. This result illustrates a considerable structural flexibility of the LCu(μ-SR)2CuL entity which is being discussed as a possible model for CuA centers of cytochrome c oxidase or nitrous oxide reductase. Oxidation of both complexes 1 and 2 remains irreversible even at cyclovoltammetric scan rates of 1 V/s. However, the methyl substitution in complex 1 causes increased reversibility of the electronically coupled dmpbased reduction processes. Accordingly, the EPR spectrum of 1- · is characterized by hyperfine splitting from dmp- · and a relatively small 63,65Cu coupling of 0.53 mT. The stabilization of 2,9-disubstituted 1,10-phenanthroline radical anions by coordination to Cu(I) is also demonstrated by detection of a single broad EPR line at g 2.0028 for the formal “Cu(0)” complex Cu(dpp)2 = CuI(dpp-·)(dpp), dpp = 2,9-diphenyl-1,10-phenanthroline. While the EPR signal of Cu(dpp)2 loses intensity on cooling, possibly due to dimerization and spin-pairing, the distorted tetrahedral configuration with essential orthogonal arrangement of dpp π systems seems to disfavour an intramolecular electron hopping.