Nickel complexes with hydrotris(pyrazolyl)borate ( = Tp R ) ligands catalyze alkane oxidation with organic peroxide meta-Cl-C 6 H 4 C(vO)OOH (= mCPBA). The electronic and steric hindrance properties of Tp R affect the catalyses. The complex with an electron-withdrawing group containing a less-hindered ligand, that is, Tp Me2,Br , exhibits higher alcohol selectivity. Higher selectivity for secondary over tertiary alcohols upon oxidation of methylcyclohexane indicates that the oxygen atom transfer reaction proceeds within the coordination sphere of the nickel centers. A reaction of the catalyst precursor, dinuclear nickel(II)bis(μ-hydroxo) complexes, with mCPBA yields the corresponding nickel(II)-acylperoxo species, as have been characterized by spectroscopy. Thermal decomposition of the nickel(II)-acylperoxo species in CH 2 Cl 2 yields the corresponding nickel(II)-chlorido complexes through Cl atom abstraction. Employment of the brominated ligand increases the thermal stability of the acylperoxo species. Kinetic isotope effects observed on decay of the nickel(II)-acylperoxo species indicate concerted O-O breaking of the nickelbound acylperoxide and H-abstraction from the solvent molecule. † Electronic supplementary information (ESI) available. See

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Transition-metal alkylperoxo species (M-OOR'; R' denotes alkyl group) are postulated as reactive intermediates in various oxidation reactions ranging from industrial processes to biological systems. [1] The alkylperoxo complexes of d 0 early transition-metal ions (groups 4-6), in particular, have attracted much attention, owing to their potential as oxygenating reagents for organic compounds (for example, olefin epoxidation). [2][3][4][5] Interest in the alkylperoxo complexes of late transition metals is also growing because of their industrial significance and biological relevance. [6][7][8] Nickel is recognized as one of the most useful metals in promoting various organic transformation reactions, owing its intrinsic redox properties. Recent developments in the chemistry of nickel dioxygen complexes have shed light on the applicability of nickel species as oxidizing agents. For example, dinuclear nickel(III) bis(m-oxo) compounds exhibit H-abstraction of aliphatic hydrocarbyl species, in a similar fashion to the M 2 (m-O) 2 species of high-valent Fe and Cu. [9] Furthermore, several nickel dioxygen complexes [9b, 10] and peroxide-based catalytic oxidation reactions with nickel complexes [11] have been reported. An nickel(II) alkylperoxo complex, reported by Suzuki and coworkers, was formed by O 2 oxygenation of the alkyl radical center on the metalsupporting ligand, which was itself generated through Habstraction by the Ni 2 (m-O) 2 species. [9a, 12] However, the structure and reactivity of nickel alkylperoxo complexes remains to be studied, to afford an insight into the interaction between nickel compounds and peroxides, the intrinsic reactivity of the resulting nickel peroxo species, and the properties of catalytic intermediates. Herein we report the molecular structure and the oxidizing activity of a nickel(II) alkylperoxo complex, [Ni II (OOtBu)(Tp iPr )] (1; Tp iPr = hydrotris(3,5-di-2-propylpyrazolyl)borate).Compound 1 was synthesized by the dehydrative condensation of a nickel(II) hydroxo complex, [(Ni II Tp iPr ) 2 (m-OH) 2 ] (2, Scheme 1), [13] with a stoichiometric amount of tertbutylhydroperoxide (TBHP). Addition of two equivalents of TBHP at 0 8C to a pentane solution of 2 resulted in an immediate change in the color of the solution, from green to orange, and the appearance of a strong band in the absorption spectrum at around 370 nm (e = 1400 cm À1 m À1 ). The molecular structure of 1 was revealed by single-crystal X-ray diffraction ( Figure 1). The nickel center of 1 is coordinated by three donor nitrogen atoms from the Tp iPr ligand. The Scheme 1. Synthesis and reactions of 1. TBHP = tert-butylhydroperoxide; Tp iPr = hydrotris(3,5-di-2-propylpyrazolyl)borate. Figure 1. The molecular structure of 1, with thermal ellipsoids set at 50 % probability. All hydrogen atoms are omitted for clarity.[*] Prof.