Quantitative evaluation of d-π interaction in copper(I) complexes and control of copper(I)-dioxygen reactivity

Crystal structures of the copper(I) complexes 1(x), 2, and 3 of a series of tridentate ligands L1(x), L2, and L3, respectively (L1(x): p-substituted derivatives of N,N-bis [2-(2-pyridyl)ethyl]-2-phenylethylamine; X=H, Me, OMe, Cl, NO2; L2: N,N-bis[2-(2-pyridyl)ethyl]-2-methyl-2-phenylethylamine; L3: N,N-bis[2-(2-pyridyl)ethyl]-2,2-diphenyl-ethylamine) were solved to demonstrate that all the copper(I) complexes involve an eta(2) copper-arene interaction with the phenyl ring of the ligand side-arm. The Cut ion in each complex has a distorted tetrahedral geometry consisting of the three nitrogen atoms (one tertiary amine nitrogen atom and two pyridine nitrogen atoms) and C-1-C-2 Of the phenyl ring of ligand sidearm, whereby the Cu-C distances of the copper-arene interaction significantly depend on the para substituents. The existence of the copper-arene interaction in a nonpolar organic solvent (CH2Cl2) was demonstrated by the observation of an intense MLCT band around 290 nm, and the magnitude of the interaction was evaluated by detailed analysis of the H-1 and C-13 NMR spectra and the redox potentials E-1/2 Of the copper ion, as well as by means of the ligand-exchange reaction between the phenyl ring and CH3CN as an external ligand. The thermodynamic parameters DeltaHdegrees and DeltaSdegrees for the ligand-exchange reaction with CH3CN afforded a quantitative measure for the energy difference of the copper-arene interaction in the series of copper(I) complexes. Density functional studies indicated that the copper(I)-arene interaction mainly consists of the interaction between the d(z2) orbital of Cu-1 and a pi orbital of the phenyl ring. The copper(I) complexes 1x reacted with 02 at -80degreesC in CH2Cl2 to give the corresponding (mu-eta(2):eta(2)-peroxo)dicopper(II) complexes 4, the formation rates k(obs) Of which were significantly retarded by stronger d-pi interaction, while complexes 2 and 3, which exhibit the strongest d-pi interaction showed significantly lower reactivity toward O-2 under the same experimental conditions. Thus, the d-pi interaction has been demonstrated for the first time to affect the copper (I)-dioxygen reactivity, and represents a new aspect of ligand effects in copper (I)-dioxygen chemistry.