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

Abstract

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.

Oxidation mechanism of phenols by copper(ii)–halide complexes

“…To explore the redox reactivity of such tetrahedral copper( ii ) complexes, we have developed a series of copper( ii ) complexes, [Cu II (TMG 3 tach)X] + ( 1 X ), where TMG 3 tach is an N 3 -tridentate ligand consisting of cis,cis -1,3,5-triaminocyclohexane (tach) and N , N , N ′, N ′-tetramethylguanidino (TMG) substituents, and X is an anionic co-ligand (F − , Cl − , Br − , I − , MeO − , C 6 F 5 O − , C 6 F 5 S − , or ROO − ). 1–3 Reactivity studies of the halide complexes demonstrated that they undergo Cu II –X bond cleavage, and in the case of X = F − , Cl − , and Br − , they induce C–H bond activation of an external substrate, such as 1,4-cyclohexadiene (CHD) with a weak C–H bond (76.0 ± 1.2 kcal mol −1 ), 4 to give the corresponding copper( i ) complex and benzene as the oxidation product. 3 Such C–H bond activation reactivity of transition-metal halide complexes has been reported by using high-valent transition-metal halide complexes of Ni III , Pd IV , Cu III , and Au III , where the higher oxidation state of the metal ions induces homolytic cleavage of the metal–halide bond.…”

“…1–3 Reactivity studies of the halide complexes demonstrated that they undergo Cu II –X bond cleavage, and in the case of X = F − , Cl − , and Br − , they induce C–H bond activation of an external substrate, such as 1,4-cyclohexadiene (CHD) with a weak C–H bond (76.0 ± 1.2 kcal mol −1 ), 4 to give the corresponding copper( i ) complex and benzene as the oxidation product. 3 Such C–H bond activation reactivity of transition-metal halide complexes has been reported by using high-valent transition-metal halide complexes of Ni III , Pd IV , Cu III , and Au III , where the higher oxidation state of the metal ions induces homolytic cleavage of the metal–halide bond. 5–14 In the case of 1 X , on the other hand, the metal centre has a normal Cu II oxidation state, but not a high-valent metal ion such as Cu III .…”

“…Thus, we suspected that such reactivity of halide complexes can be attributed to their tetrahedrally distorted geometry, which induces Cu II –X bond homolysis to give Cu I and X˙, the latter of which formally abstracts a hydrogen atom from the substrate. 3 Unfortunately, however, the oxidation reaction of CHD by 1 X was too slow to perform a detailed kinetic analysis.…”