A copper(II) complex bearing a pentadentate ligand, [CuII(N4Py) (CF3SO3)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 CuII-hydroperoxo complex, [CuII(N4Py)(OOH)]+ (2), was then generated in the reaction of 1 and H2O2 in the presence of base, and the reactivity of the intermediate was investigated in the oxidation of various substrates at -40 °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 [CuIII(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 CuII-hydroperoxo species bearing a pentadentate N5 ligand is a sluggish oxidant in oxygenation reactions.