High-valent transition-metal terminal oxygen complexes have long attracted much attention as a key reactive intermediate in the biological and abiological oxidation reactions. Tri-valent transition metal complexes with a terminal oxide or hydroxide ligand such as FeIII–OMe, MnIII–OH, and RuIII–OH have also been investigated as a functional model of lipoxygenases. Tolman and coworkers have also developed the mononuclear high-valent copper(III) complexes containing hydroxide or alkoxide terminal ligand using dianionic ligands, and explored their hydrogen atom abstraction reactivity.1,2
Recently, we have developed a series of tetrahedral copper(II) complexes, [CuII(TMG3tach)(L)]+ (1L), where TMG3tach is an N3-tridentate ligand consisting of cis,cis-1,3,5-triamino-cyclohexane (tach) and N,N,N′,N′-tetramethylguanidino (TMG) substituents and L is an anionic axial co-ligand (F–, Cl–, Br–, I–, MeO–, C6F5O–, C6F5S–, and ROO–) (Fig. 1).3,4 The copper(II)-halide complexes 1X (X = F–, Cl–, Br–, and I–) showed C–H and O–H activation reactivity, even though the metal ion has a normal copper(II) oxidation state but not a high-valent oxidation state such as copper(III). We concluded that such an oxidation reactivity is attributed to the tetrahedral geometry of the copper(II)-halide complexes which basically stabilizes the low-valent copper(I) oxidation state. In this study, we have also found that the copper(II)-methoxide complex 1OMe exhibited much higher C–H and O–H activation reactivity compared to 1X. The results will provide important insights into the oxidation mechanism of the mononuclear copper monooxygenases such as peptidylglycine α-hydroxylating monooxygenase (PHM), dopamine β-monooxygenase (DβM), and lytic polysaccharide monooxygenase (LPMO).
(1) Tolman, W. B. et al. J. Am. Chem. Soc. 2011, 133 (44), 17602-17605.
(2) Tolman, W. B. et al. J. Am. Chem. Soc. 2021, 143 (9), 3295-3299.
(3) Itoh, S. et al. Inorg. Chem. 2023, 62 (27), 10539-10547.
(4) Itoh, S. et al. Chem. Commun. 2024,