LEWIS-BASE-CATALYZED DIIMINE-LIGAND-SUBSTITUTION REACTIONS AT COPPER(I)

Substitutions of 2,2'-biquinoline (biq) in Cu(biq)2+ by 2,9-dimethylphenanthroline (dmp) and 1,10-phenanthroline (phen), in acetone, were found to be catalyzed by Lewis bases, L. The catalyzed pathway involves an intermediate mixed-ligand complex Cu(biq)L+, in which L is rapidly replaced by the incoming diimine ligand. The individual rate constants for L = iodide and acetonitrile were evaluated. Analysis of the respective rate constants, together with the equilibrium constants, clearly shows that the better nucleophile, iodide, is the more efficient catalyst. The ligand dependences of the rate constants indicate that the mechanism of ligand-substitution reactions at tetrahedral Cu(I) is associative (I(a)). On the basis of the effect of acetonitrile-assisted substitution reactions in acetone, an attempt was made to obtain an approximate value of the corresponding rate constants in the pure solvent by extrapolation from low acetonitrile concentrations. The result agrees with the observation that substitution reactions at Cu(I) in acetonitrile are generally too fast for the stopped-flow time scale. The solvent-assisted pathway represents an important contribution to diimine-substitution reactions in acetonitrile, whereas in acetone this pathway is of minor importance.