π-π stacking-induced cooperativity in copper(I) complexes with phenanthroline ligands

The formation constants of mono- and bischelate copper(I) complexes with four phenanthroline derivatives have been determined by absorption spectrophotometry in acetonitrile and in a ternary acetonitrile/dichloromethane/ water 80/15/5 (v/v) mixture. The influence of electron-donating methyl and anisyl substituents in positions 2 and 9 of the phenanthroline core has been investigated using both symmetrical (dmp = 2,9-dimethyl-1,10-phenanthroline; dap = 2,9-di(p-anisyl)-1,10-phenanthroline) and unsymmetrical (map = 2-(p-anisyl)-1,10-phenanthroline; Memap = 2-(p-anisyl)-9-methyl-1,10-phenanthroline) ligands. The equilibrium constants show no significant dependence upon the solvent composition. The binding affinity of alpha,alpha'-diimine ligands and their methyl-substituted derivatives is governed by the sigma-donation of the nitrogen atoms. In contrast, anisyl substituents exert a destabilizing effect on the monochelate complexes likely due to steric hindrance, but favor the formation of the bischelate species. The resulting positive cooperativity was rationalized in terms of intramolecular pi-pi stacking interactions between the electron rich anisyl groups and the electron accepting phenanthroline moieties. Cyanide-assisted demetalation kinetic studies were carried out in order to gain further insight into the structural properties of the four bischelate complexes examined. The rate constants, which reflect subtle geometrical variations, span over more than 5 orders of magnitude and reveal an unexpected high accessibility of the copper(I) center in the unsymmetrical complexes Cu(map)(2)(+) and Cu(Memap)(2)(+).