Structure and dioxygen-reactivity of copper(I) complexes supported by bis(6-methylpyridin-2-ylmethyl)amine tridentate ligands

Abstract

The structure and dioxygen-reactivity of copper(I) complexes R supported by N,N-bis(6-methylpyridin-2-ylmethyl)amine tridentate ligands L2R[R (N-alkyl substituent)=-CH2Ph (Bn), -CH2CH2Ph (Phe) and -CH2CHPh2(PhePh)] have been examined and compared with those of copper(I) complex (Phe) of N,N-bis[2-(pyridin-2-yl)ethyl]amine tridentate ligand L1(Phe) and copper(I) complex (Phe) of N,N-bis(pyridin-2-ylmethyl)amine tridentate ligand L3(Phe). Copper(I) complexes (Phe) and (PhePh) exhibited a distorted trigonal pyramidal structure involving a d-pi interaction with an eta1-binding mode between the metal ion and one of the ortho-carbon atoms of the phenyl group of the N-alkyl substituent [-CH2CH2Ph (Phe) and -CH2CHPh2(PhePh)]. The strength of the d-pi interaction in (Phe) and (PhePh) was weaker than that of the d-pi interaction with an eta2-binding mode in (Phe) but stronger than that of the eta1 d-pi interaction in (Phe). Existence of a weak d-pi interaction in (Bn) in solution was also explored, but its binding mode was not clear. Redox potentials of the copper(I) complexes (E1/2) were also affected by the supporting ligand; the order of E1/2 was Phe>R>Phe. Thus, the order of electron-donor ability of the ligand is L1Phe<L2R<L3Phe. This was reflected in the copper(I)-dioxygen reactivity, where the reaction rate of copper(I) complex toward O2 dramatically increased in the order of R<R<R. The structure of the resulting Cu2/O2 intermediate was also altered by the supporting ligand. Namely, oxygenation of copper(I) complex R at a low temperature gave a (micro-eta2:eta2-peroxo)dicopper(II) complex as in the case of Phe, but its O-O bond was relatively weakened as compared to the peroxo complex derived from Phe, and a small amount of a bis(micro-oxo)dicopper(III) complex co-existed. These results can be attributed to the higher electron-donor ability of L2R as compared to that of L1Phe. On the other hand, the fact that Phe mainly afforded a bis(micro-oxo)dicopper(III) complex suggests that the electron-donor ability of L2R is not high enough to support the higher oxidation state of copper(III) of the bis(micro-oxo) complex.