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

Structures, physicochemicalproperties, and reactivity ofthe whole series of mononuclear copper(II)-halide complexessupported by a TMG(3)tach tridentate ligand consisting of cis,cis-1,3,5-triaminocyclohexane (tach) and bulky and stronglyelectron-donating substituent N,N,N & PRIME;,N & PRIME;-tetramethylguanidine(TMG) have been explored to evaluate the structural importance inthe spectroscopic and electronic features as well as the reactivitytoward C-H bond activation reaction. Structures, physicochemical properties, and reactivityof the wholeseries 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 & PRIME;,N & PRIME;-tetramethylguanidine(TMG). The tach ligand framework with the bulky and strongly electron-donatingTMG substituents enforces the copper(II) complexes to take a tetrahedralgeometry, as inferred from the electron paramagnetic resonance (EPR)spectra, exhibiting relatively large g(z) and small A(z) values. The electronicabsorption spectra of 1 ( X ) agreedwith the simulation spectra obtained by time-dependent density functionaltheory (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-& BULL;, and in the case of 1 ( Br ), intramolecular hydroxylationof a methyl group of the TMG substituent took place under aerobicconditions, which may be caused by the reaction of the generated copper(I)complex and dioxygen (O-2), generating a reactive oxygenspecies. 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-orderrate constant as 1.4 x 10(-3) M-1 s(-1) at 25 & DEG;C. Such an HAA reactivity canbe attributed to the higher basicity of F- and/orlarge bond dissociation free energy of conjugate acid H-F aswell as the unstable copper(II) electronic state in the tetrahedralgeometry.