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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There has been a growing interest in the molecular design of bulky phosphane ligands, [1] which can facilitate the formation of catalytically active low-coordinate species by their steric properties. The tertiary phosphane complexes of divalent palladium are well recognized as important species because of their effectiveness in organic synthesis and as homogeneous catalysts.[2] Generally, the reactions of tertiary phosphanes with palladium(ii) halides produce the 2:1 complexes [PdX 2 (PR 3 ) 2 ] (X = halide), which may or may not be converted to the 2:2 complexes [(PdX 2 ) 2 (PR 3 ) 2 ] upon heating. [3] Although bulky phosphane ligands tend to favor the direct formation of 2:2 complexes, [3] there has been no report of a phosphane ligand bulky enough to form the 2:3 trinuclear complex [(PdX 2 ) 3 (PR 3 ) 2 ]. Most of the bulky phosphane ligands thus far reported have the substituents in the vicinity of the phosphorus center, which often results in facile intramolecular cyclometallation. For example, trimesitylphosphane (P(Mes) 3 ) reacts with palladium(ii) chloride under a wide range of reaction conditions to produce only the cyclometallated dinuclear complex.[4] Although such cyclometallation reactions have attracted much attention in view of the catalytic utility of the resulting metallacycles, [5] they are undesirable when one intends to synthesize a metal complex with the phosphane ligands intact. It is usually difficult, however, to increase the steric bulk of the phosphane ligands without an increase in the steric congestion around the phosphorus center.In the course of our study on bowl-shaped molecules, [6] we previously designed the triarylmethane, [7a] triarylsilane, [7b] and triarylgermane [7c] derivatives 1-3, in which the radially extended m-terphenyl units form a large cavity. It is expected that replacement of the central atom of these molecules by a phosphorus atom will produce very bulky phosphane ligands without severe steric congestion in the vicinity of the phosphorus center. Recently, Tatsumi and co-workers [8] and our group [9] independently reported the synthesis of triarylphosphane TRMP.[10] TRMP was applied to the highly efficient rhodium-catalyzed hydrosilylation of ketones by Tsuji et al. [11] In this communication, we report the synthesis and structure of the first example of the [(PdX 2 ) 3 (PR 3 ) 2 ]-type trinuclear complex by taking advantage of a novel bowlshaped phosphane TRIP, [10] in which the methyl groups of TRMP are replaced by isopropyl groups (Figure 1).TRIP was prepared by lithiation of the corresponding mterphenyl bromide 4[6e] followed by treatment with PCl 3 (Scheme 1). The 31 P NMR spectra (CDCl 3 ) of TRIP showed a signal at d = À6.60 ppm, which is almost the same as the chemical shifts of TRMP (d = À7.16 ppm) and triphenylphosphane (d = À6.0 ppm).[12] X-ray crystallographic analysis established the structure of TRIP (Figure 2), [13] in which the central phosphorus atom is embedded in a bowl-shaped cavity. The space-filling models of the crystal structures of ...

The selective synthesis of half-sandwich type complexes of Ni(II) and Co(II) with the less-hindered hydrotris(3,5-dimethyl-4-X-1-pyrazolyl)borate [X = Me (TpMe3; a series), H (TpMe2; b series), and Br (TpMe2,Br; c series)] ligands, [TpMe2,XMII(κ2-O,O′-L)] [L = NO3 (2), OAc (3)], has been achieved by dropwise addition of a THF solution of NaTpMe2,X to an excess amount of ML2 dissolved in MeOH. The solid state structures of the nitrato complexes 2 have been characterized by X-ray crystallography as their solvated form, 2·solv. (solv. = MeCN, MeOH), in which the metal centers have the six-coordinated octahedral geometry. Solvates are readily dissociated from the metal center upon dissolution in non-coordinationg solvents, such as CH2Cl2 and toluene, as well as drying of a solid sample to give the non-solvated five-coordinated species 2. The crystal structure of the non-solvated TpMe2Ni derivative 3bNi consists of a highly distorted trigonal bipyramidal nickel center.

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.