Characterization and Reactivity Studies of Mononuclear Tetrahedral Copper(II)-Halide Complexes.

Structures, physicochemical properties, and reactivity of the whole series of copper(II)-halide complexes ( 1 X ; X = F, Cl, Br, and I) were examined using a TMG 3 tach tridentate supporting ligand consisting of cis,cis -1,3,5-triaminocyclohexane (tach) and N , N , N ', N '-tetramethylguanidine (TMG). The tach ligand framework with the bulky and strongly electron-donating TMG substituents enforces the copper(II) complexes to take a tetrahedral geometry, as inferred from the electron paramagnetic resonance (EPR) spectra, exhibiting relatively large g z and small A z values. The electronic absorption spectra of 1 X agreed with the simulation spectra obtained by time-dependent density functional theory (TD-DFT) calculations on a slightly distorted tetrahedral geometry. 1 I and 1 Br gradually decomposed to generate the corresponding copper(I) complex and halide radical X • , and in the case of 1 Br , intramolecular hydroxylation of a methyl group of the TMG substituent took place under aerobic conditions, which may be caused by the reaction of the generated copper(I) complex and dioxygen (O 2 ), generating a reactive oxygen species. 1 X except 1 I showed hydrogen atom abstraction (HAA) reactivity toward 1,4-cyclohexadiene (CHD), where 1 F exhibited the highest reactivity with a second-order rate constant as 1.4 × 10 -3 M -1 s -1 at 25 °C. Such an HAA reactivity can be attributed to the higher basicity of F - and/or large bond dissociation free energy of conjugate acid H-F as well as the unstable copper(II) electronic state in the tetrahedral geometry.