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)(2)CU-2, DMP=2,9-DIMETHYL-1,1O-PHENANTHROLINE, AND A COMMENT ON THE NATURE OF PHENANTHROLINE-CONTAINING CU(O) COMPLEXES

The molecular and crystal structure of the dicopper(I) complex (dmp)(2 )(mu-o-CH3C6H4S)(2)Cu-2 (1), dmp = 2,9-dimethyl-1,10-phenanthroline, h as been determined. With Cu/S/Cu and S/Cu/S bond angles between 80 and 100 degrees, the planar structure of the central CuSCuS four-membered ring is much more symmetric for 1 as compared to the analogous comple x 2 of unsubstituted 1,10-phenanthroline which has a folded Cu2S2 core with bond angles between 67 and 108 degrees. This result illustrates a considerable structural flexibility of the LCu(mu-SR)(2)CuL entity w hich is being discussed as a possible model for Cu, centers of cytochr ome 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 r eversibility of the electronically coupled dmp-based reduction process es. Accordingly, the EPR spectrum of 1(-.) is characterized by hyperfi ne splitting from dmp(-.) and a relatively small Cu-63.65 coupling of 0.53 mT. The stabilization of 2,9-disubstituted 1,10-phenanthroline ra dical anions by coordination to Cu(I) is also demonstrated by detectio n of a single broad EPR line at g 2.0028 for the formal ''Cu(O)'' comp lex Cu(dpp)(2) = Cu-I(dpp(-.))(dpp), dpp = 2,9-diphenyl-1,10-phenanthr oline. While the EPR signal of Cu(dpp)(2) loses intensity on cooling, possibly due to dimerization and spin-pairing, the distorted tetrahedr al configuration with essential orthogonal arrangement of dpp pi syste ms seems to disfavour an intramolecular electron hopping.