Cu(I) Complexes of 3,3-Polymethylene Bridged Derivatives of 2,2-Bi-1,10-Phenanthroline

A series of 3,3ʹ-polymethylene-2,2ʹ-bi-1,10-phenanthrolines coordinate with Cu(I) to form dinuclear complexes [(CuL)2]^2+. As the 3,3ʹ-bridge is lengthened from two to four carbons, the ligand becomes more twisted about the 2,2ʹ-bond, favoring dinuclear coordination. The distance between the two copper atoms varies from 2.92 (angstrom) for the dimethylene-bridged system to 3.59 (angstrom) for the tetramethylene bridge. Favorable (pi)-stacking interactions occur between opposing ligands and promote complex formation. Competition experiments indicate that self-recognition is important and only homoleptic complexes are observed. Under equilibrium conditions, formation of the tetramethylene-bridged complex appears to be the most favored while the dimethylene-bridged system is least favored. The intensity of the long wavelength metal-ligand charge-transfer absorption band decreases as the 3,3ʹ-bridge is shortened. Interaction between the two copper centers is evidenced by a splitting of the oxidation wave, and this splitting increases as the Cu-Cu distance is decreased.