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 2(R) supported by N,N-bis(6-methylpyridin-2-ylmethyl)amine tridentate ligands L2(R) [R (N-alkyl substituent) = - CH2Ph (Bn), - CH2CH2Ph (Phe) and - CH2CHPh2 (PhePh)] have been examined and compared with those of copper(I) complex 1(Phe) of N,N-bis[2-(pyridin-2-yl) ethyl] amine tridentate ligand L1(Phe) and copper( I) complex 3(Phe) of N,N-bis( pyridin-2-ylmethyl) amine tridentate ligand L3(Phe). Copper( I) complexes 2(Phe) and 2(PhePh) exhibited a distorted trigonal pyramidal structure involving a d-pi interaction with an eta(1)-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 2(Phe) and 2(PhePh) was weaker than that of the d-pi interaction with an eta(2)-binding mode in 1(Phe) but stronger than that of the eta(1) d - pi interaction in 3(Phe). Existence of a weak d - pi interaction in 2(Bn) in solution was also explored, but its binding mode was not clear. Redox potentials of the copper(I) complexes (E-1/2) were also affected by the supporting ligand; the order of E-1/2 was 1(Phe) > 2(R) > 3(Phe). Thus, the order of electron-donor ability of the ligand is L1(Phe) < L2(R) < L3(Phe). This was reflected in the copper( I) - dioxygen reactivity, where the reaction rate of copper(I) complex toward O-2 dramatically increased in the order of 1(R) < 2(R) < 3(R). The structure of the resulting Cu-2/O-2 intermediate was also altered by the supporting ligand. Namely, oxygenation of copper( I) complex 2(R) at a low temperature gave a (mu-eta(2):eta(2)-peroxo) dicopper(II) complex as in the case of 1(Phe), but its O - O bond was relatively weakened as compared to the peroxo complex derived from 1(Phe), and a small amount of a bis(mu-oxo) dicopper(III) complex co-existed. These results can be attributed to the higher electron-donor ability of L2(R) as compared to that of L1(Phe). On the other hand, the fact that 3(Phe) mainly afforded a bis(mu-oxo) dicopper(III) complex suggests that the electron-donor ability of L2(R) is not high enough to support the higher oxidation state of copper(III) of the bis(mu-oxo) complex.