Breaking bonds and breaking rules: inert-bond activation by [(iPr3P)Ni]5H4 and catalytic stereospecific norbornene dimerization.

The facile carbon atom abstraction reaction by [(iPr3P)Ni]5H6 (1) with various terminal alkenes to give [(iPr3P)Ni]5H4(µ5-C) (2) occurs via a common highly reactive intermediate [(iPr3P)Ni]5H4 (3), which was isolated by the reaction of 1 with norbornene. Temperature dependent 1H and 31P{1H} NMR chemical shifts of 3 are consistent with a thermally populated triplet excited state only 2 kcal mol-1 higher energy than the diamagnetic ground state. Complex 3 catalyzes the dimerization of norbornene to stereoselectively provide exclusively (Z) anti-(bis-2,2'-norbornylidene).

Efficient Copolymerization of Acrylate and Ethylene with Neutral P, O-Chelated Nickel Catalysts: Mechanistic Investigations of Monomer Insertion and Chelate Formation.

The efficient copolymerization of acrylates with ethylene using Ni catalysts remains a challenge. Herein, we report two neutral Ni(II) catalysts (POP-Ni-py (1) and PONap-Ni-py (2)) that exhibit high thermal stability and significantly higher incorporation of polar monomer (for 1) or improved resistance to tert-butylacrylate (tBA)-induced chain transfer (for 2), in comparison to previously reported catalysts. Nickel alkyl complexes generated after tBA insertion, POP-Ni-CCO(py) (3) and PONap-Ni-CCO(py) (4), were isolated and, for the first time, characterized by crystallography. Weakened lutidine vs pyridine coordination in 2-lut facilitated the isolation of a N-donor-free adduct after acrylate insertion PONap-Ni-CCO (5) which represents a novel example of a four-membered chelate relevant to acrylate polymerization catalysis. Experimental kinetic studies of six cases of monomer insertion with aforementioned nickel complexes indicate that pyridine dissociation and monomer coordination are fast relative to monomer migratory insertion and that monomer enchainment after tBA insertion is the rate limiting step of copolymerization. Further evaluation of monomer insertion using density functional theory studies identified a cis-trans isomerization via Berry-pseudorotation involving one of the pendant ether groups as the rate-limiting step for propagation, in the absence of a polar group at the chain end. The energy profiles for ethylene and tBA enchainments are in qualitative agreement with experimental measurements.

Ligand architecture for triangular metal complexes: a high oxidation state Ni3 cluster with proximal metal arrangement.

A new multidentate tetraanionic ligand platform for supporting trinuclear transition metal clusters has been developed. Two trisphenoxide phosphinimide ligands bind three Ni centers in a triangular arrangement. The phosphinimide donors bridge in µ3 fashion and the phenoxides complete a pseudo-square planar coordination sphere around each metal center. Electrochemical studies reveal two pseudo-reversible oxidation events at notably low potentials (-0.80 V and +0.05 V). The one electron oxidized species was characterized structurally, and it is assigned as a NiIII-containing cluster.

Dismantling of Vinyl Ethers by Pentanuclear [(iPr3 P)Ni]5 H6 : Facile Cooperative C-O, C-C and C-H Activation Pathways.

The [(iPr3 P)Ni]5 H6 cluster (1) and H2 C=CHOtBu react at room temperature to form the new pentanuclear NiH carbide [(iPr3 P)Ni]5 H4 (C)(CO) (3), along with an equivalent of isobutylene. This transformation requires the activation of multiple unreactive bonds, including C-H, C-C, and C(sp3 )-O bond cleavage. Analysis of the reaction mixture by 1 H NMR revealed the production of two additional paramagnetic species, assigned as [(iPr3 P)Ni]4 H4 (C-CH3 )NiOtBu (4 a) and [(iPr3 P)Ni]4 H4 (C-CH2 OtBu)NiOtBu (5 a), which arise from C(sp2 )-O bond cleavage and CH bond rearrangements. The reaction of 1 with H2 C=CHOSiMe2 CH2 Ph produced the isolable 4 a analogue [(iPr3 P)4 Ni5 ]H4 (CCH3 )(OSiMe2 CH2 Ph) (4 c). An isolable analogue of 5 a was obtained from the reaction of 1 with H2 C=CHOAd (Ad=1-admantyl), which produced [(iPr3 P)4 Ni5 ]H4 (CCH2 OAd)(OAd) (5 d). The utilization of both cluster faces and vertices for bonding substrate fragments in these transformations demonstrates the remarkable flexibility of the robust Ni5 H4 core in the cooperative activation of multiple C-O, C-C and C-H bonds.

Synthesis of Surface-Analogue Square-Planar Tetranuclear Nickel Hydride Clusters and Bonding to µ4-NR, -O and -BH Ligands.

Tetranuclear Ni complexes were synthesized with bonding to BH, NR, and O in atypical surface-like geometries. The previously reported electron-deficient cluster [( iPr3P)Ni]5H6 (1) reacts with N-methylmorpholine oxide to give [( iPr3P)Ni]4H4(µ4-O) (2), which contains O coordinated in the center of a square-plane arrangement of Ni atoms. Reaction of 1 with benzonitrile gave the square-planar tetranuclear Ni cluster [( iPr3P)Ni]4H4(µ4-NCH2Ph) (3), which contains an imido donor in a square-based-pyramidal geometry. This reaction also gives [( iPr3P)Ni(N≡CPh)]3 (4), with bridging benzonitrile ligands. Trimer 4 was independently synthesized from the reaction of Ni(COD)2, iPr3P, and PhC≡N. The addition of dihydrogen to a 1:1 mixture of [( iPr3P)2Ni]2(N2) and ( iPr3P)2NiCl2 yielded [( iPr3P)Ni]4(µ3-H)4(µ2-Cl)2 (5), with a tetrahedral Ni core, in contrast to the square-planar geometries of 2 and 3. The solid-state structure of 5 was determined using both X-ray and neutron diffraction. Reaction of 5 with LiBH4 gave [( iPr3P)Ni]4H4(µ4-BH)2] (6) via loss of LiCl and H2.

Versatile (η6-arene)Ni(PCy3) nickel monophosphine precursors.

Nickel monophosphine arene adducts have been proposed as highly reactive intermediates capable of difficult C-O bond activation steps in Ni catalyzed cross-coupling reactions. The addition of N-methylmorpholine N-oxide to arene solutions of ([Cy3P)2Ni]2N2 allows for the synthesis of stable (η6-arene)Ni(PCy3) complexes. The isolation of these species demonstrates their viability as intermediates and provides an experimental means to test the hypothesized importance of the Ni(PCy3) moiety in bond activation and catalysis.

Facile Deep and Ultradeep Hydrodesulfurization by the [((i)Pr3P)Ni]5H6 Cluster Compared to Mononuclear Ni Sources.

The pentanuclear nickel cluster [((i)Pr3P)Ni]5H6 facilitates the room-temperature hydrodesulfurization of dibenzothiophene, 4-methyldibenzothiophene, and 4,6-dimethydibenzothiophene. These reactions provide the new tetranuclear nickel hydride sulfide [((i)Pr3P)Ni]4(µ-H)4(µ4-S) (1). In comparison, the dinuclear dinitrogen nickel complex [((i)Pr3P)2Ni]2(µ-N2) undergoes oxidative addition of the C-S bonds of dibenzothiophene and 4-methyl dibenzothiophene to provide the metallacycles Ni3(P(i)Pr3)3C12H8S (2) and Ni3(P(i)Pr3)3C13H10S (3), respectively, but 4,6-dimethydibenzothiophene is unreactive, even with heating to 70 °C for a week. The reaction of [((i)Pr3P)Ni]5H6 with SP(i)Pr3 in toluene provided [((i)Pr3P)Ni]5H6(S) (4), which was observed and characterized by NMR spectroscopy. The addition of vinyltrimethylsilane to 4 provided the best synthetic route to 1, with ((i)Pr3P)Ni(η(2)-CH2═CHSiMe3)2 (5) generated as a byproduct.