Combined experimental and theoretical approach to understand the reactivity of a mononuclear Cu(II)-hydroperoxo complex in oxygenation reactions.
A copper(II) complex bearing a pentadentate ligand, [Cu(II)(N4Py)(CF(3)SO(3))(2)] (1) (N4Py = N,N-bis(2-pyridylmethyl)bis(2-pyridyl)methylamine), was synthesized and characterized with various spectroscopic techniques and X-ray crystallography. A mononuclear Cu(II)-hydroperoxo complex, [Cu(II)(N4Py)(OOH)](+) (2), was then generated in the reaction of 1 and H(2)O(2) in the presence of base, and the reactivity of the intermediate was investigated in the oxidation of various substrates at -40 degrees C. In the reactivity studies, 2 showed a low oxidizing power such that 2 reacted only with triethylphosphine but not with other substrates such as thioanisole, benzyl alcohol, 1,4-cyclohexadiene, cyclohexene, and cyclohexane. In theoretical work, we have conducted density functional theory (DFT) calculations on the epoxidation of ethylene by 2 and a [Cu(III)(N4Py)(O)](+) intermediate (3) at the B3LYP level. The activation barrier is calculated to be 39.7 and 26.3 kcal/mol for distal and proximal oxygen attacks by 2, respectively. This result indicates that the direct ethylene epoxidation by 2 is not a plausible pathway, as we have observed in the experimental work. In contrast, the ethylene epoxidation by 3 is a downhill and low-barrier process. We also found that 2 cannot be a precursor to 3, since the homolytic cleavage of the O-O bond of 2 is very endothermic (i.e., 42 kcal/mol). On the basis of the experimental and theoretical results, we conclude that a mononuclear Cu(II)-hydroperoxo species bearing a pentadentate N5 ligand is a sluggish oxidant in oxygenation reactions.
Kamachi
Takashi
T
Institute for Materials Chemistry and Engineering, Kyushu University, Fukuoka 812-8581, Japan.
Lee
Yong-Min
YM
Nishimi
Tomonori
T
Cho
Jaeheung
J
Yoshizawa
Kazunari
K
Nam
Wonwoo
W
D003300:Copper / Q000737:chemistry*; D018360:Crystallography, X-Ray / Q000379:methods; D004578:Electron Spin Resonance Spectroscopy; D004852:Epoxy Compounds / Q000737:chemistry; D005290:Ferric Compounds / Q000737:chemistry; D006861:Hydrogen Peroxide / Q000737:chemistry*; D007700:Kinetics; D008024:Ligands; D008958:Models, Molecular; D008968:Molecular Conformation; D009584:Nitrogen / Q000737:chemistry; D010084:Oxidation-Reduction; D012996:Solutions; D021241:Spectrometry, Mass, Electrospray Ionization / Q000379:methods; D013053:Spectrophotometry / Q000379:methods
D004852:Epoxy Compounds; D005290:Ferric Compounds; D008024:Ligands; D012996:Solutions; D003300:Copper; D006861:Hydrogen Peroxide; D009584:Nitrogen