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

The structure and dioxygen-reactivity of copper(I) complexes 2R 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 1Phe of N,N-bis[2-(pyridin-2-yl)ethyl]amine tridentate ligand L1Phe and copper(I) complex 3Phe of N,N-bis(pyridin-2-ylmethyl)amine tridentate ligand L3Phe. Copper(I) complexes 2Phe and 2PhePh exhibited a distorted trigonal pyramidal structure involving a d–π interaction with an η1-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–π interaction in 2Phe and 2PhePh was weaker than that of the d–π interaction with an η2-binding mode in 1Phe but stronger than that of the η1 d–π interaction in 3Phe. Existence of a weak d–π interaction in 2Bn 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 1Phe > 2R > 3Phe. 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 1R < 2R < 3R. The structure of the resulting Cu2/O2 intermediate was also altered by the supporting ligand. Namely, oxygenation of copper(I) complex 2R at a low temperature gave a (μ-η2:η2-peroxo)dicopper(II) complex as in the case of 1Phe, but its O–O bond was relatively weakened as compared to the peroxo complex derived from 1Phe, and a small amount of a bis(μ-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 3Phe mainly afforded a bis(μ-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(μ-oxo) complex.