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, NO ₂; L2: N,N-bis[2-(2-pyridyl)ethyl]-2-methyl-2-phenylethylamine; L3: N,N-bis[2-(2-pyridyl)ethyl]-2,2-diphenylethylamine) were solved to demonstrate that all the copper(I) complexes involve an η² copper-arene interaction with the phenyl ring of the ligand sidearm. The Cu¹ 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 C₁-C₂ 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 ¹H and ¹³C 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 ΔH° and ΔS° 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 π orbital of the phenyl ring. The copper(I) complexes 1^x reacted with O₂ at -80°C in CH₂Cl ₂ to give the corresponding (μ-η²:η ²-peroxo)dicopper(II) complexes 4, the formation rates k _obs of which were significantly retarded by stronger d-π interaction, while complexes 2 and 3, which exhibit the strongest d-π interaction showed significantly lower reactivity toward O₂ under the same experimental conditions. Thus, the d-π 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.