Ynone Co-Coordination at a Nickel Borane Complex: An Assessment of Secondary Coordination Sphere Effects.

Secondary coordination sphere ligand effects can be used to direct or organize small molecule substrates at a metal center. Herein, we assess the bifunctional ambiphilic diphosphine, tri-tert-butylboranyldiphosphinoethane (ttbbpe) and its ability to influence stereoselective substrate coordination, while appended to nickel. This report takes a synthetic/computational approach to test the impacts and limitations associated with ligand-directed substrate coordination using [Ni(ttbbpe)(η2:η2-COD)] (COD = 1,5-cyclooctadiene) and ynones (alkynes having an α-carbonyl group at the propargylic position) as model substrates.

Reactions of nickel boranyl compounds with pnictogen-carbon triple bonds.

The catalytic conversion of unsaturated small molecules such as nitriles into reduced products is of interest for the production of fine chemicals. In this vein, metal-ligand cooperativity has been leveraged to promote such reactivity, often conferring stability to bound substrate - a balancing act that may offer activation at the cost of turnover efficiency. This report describes the reactivity of a [(diphosphine)Ni] compound with pnictogen carbon triple bonds (R-C[triple bond, length as m-dash]E; E = N, P), where the diphosphine contains two pendant borane groups. For E = N, cooperative nitrile coordination is observed to afford {Ni}2 complexes displaying B-N interactions, whereas for E = P, B-P interactions are absent. This work additionally outlines a structure-activity relationship that uses nitrile dihydroboration as a model reaction to unveil the effect of SCS stabilization, employing [(diphosphine)Ni] where the diphosphine contains 0, 1, or 2 pendant Lewis acid groups.

Bis(1-bora-4-phosphorinane) ring closure at Cp*M (M = Fe, Co) complexes.

Bis(1-bora-4-phosphorinane) metal complexes have been synthesized using a Cp*M-protecting (M = FeIICl, CoI, Cp* = C5Me5-) strategy and structurally authenticated by NMR spectroscopy and single crystal X-ray diffraction. Synthesis of these scaffolds is highly sensitive to the identity of vinylphosphine precursor. This approach provides a new method for the generation of saturated P,B-containing main-group ring systems.

Competitive gold/nickel transmetalation.

Transmetalation is a key method for the construction of element-element bonds. Here, we disclose the reactivity of [NiII(Ar)(I)(diphosphine)] compounds with arylgold(I) transmetalating agents, which is directly relevant to cross-coupling catalysis. Both aryl-for-iodide and unexpected aryl-for-aryl transmetalation are witnessed. Despite the strong driving force expected for Au-I bond formation, aryl scrambling can occur during transmetalation and may complicate the outcomes of attempted catalytic cross-coupling reactions.

Lewis Acid-Promoted Oxidative Addition at a [Ni0 (diphosphine)2 ] Complex: The Critical Role of a Secondary Coordination Sphere.

Oxidative addition represents a critical elementary step in myriad catalytic transformations. Here, the importance of thoughtful ligand design cannot be overstated. In this work, we report the intermolecular activation of iodobenzene (PhI) at a coordinatively saturated 18-electron [Ni0 (diphosphine)2 ] complex bearing a Lewis acidic secondary coordination sphere. Whereas alkyl-substituted diphosphine complexes of Group 10 are known to be unreactive in such reactions, we show that [Ni0 (P2 BCy 4 )2 ] (P2 BCy 4 =1,2-bis(di(3-dicyclohexylboraneyl)-propylphosphino)ethane) is competent for room-temperature PhI cleavage to give [NiII (P2 BCy 4 )(Ph)(I)]. This difference in oxidative addition reactivity has been scrutinized computationally - an outcome that is borne out in ring-opening to provide the reactive precursor - for [Ni0 (P2 BCy 4 )2 ], a "boron-trapped" 16-electron κ1 -diphosphine Ni(0) complex. Moreover, formation of [NiII (P2 BCy 4 )(Ph)(I)] is inherent to the P2 BCy 4 secondary coordination sphere: treatment of the Lewis adduct, [Ni0 (P2 BCy 4 )2 (DMAP)8 ] with PhI provides [NiII (P2 BCy 4 )2 (DMAP)8 (I)]I via iodine-atom abstraction and not a [NiII (Ph)(I)(diphosphine)] compound - an unusual secondary sphere effect. Finally, the reactivity of [Ni0 (P2 BCy 4 )2 ] with 4-iodopyridine was surveyed, which resulted in a pyridyl-borane linked oligomer. The implications of these outcomes are discussed in the context of designing strongly donating, and yet labile diphosphine ligands for use in a critical bond activation step relevant to catalysis.

Preparation of a borane-appended Co(III) hydride: evidence for metal-ligand cooperativity in O-H bond activation.

Cobalt hydrides are known to mediate a number of important chemical transformations including proton (H+), hydride (H-), and hydrogen-atom (H˙) transfer. Central to the tunability of such frameworks is judicious ligand design, which offers the flexibility to alter fundamental properties relevant to reactivity. Herein, we report the preparation of one such cobalt(III) hydride: [Cp*CoIII(P2BCy4)(H)]BPh4 (Cp* = C5Me5-, P2BCy4 = 1,2-bis(di(3-dicyclohexylborane)propylphosphino)ethane) that is encircled by a boron-based Lewis-acidic secondary coordination sphere. The structure of this species is supported by synchrotron-radiation crystallography, evidencing a terminal Co(III) hydride with four sp2-hybridized boranes that invite Lewis base coordination. To this end, electrochemical reactivity studies performed using [Cp*CoIII(P2BCy4)Cl]+ or an "all-akyl" model, [Cp*CoIII(dnppe)Cl]+ (dnppe = 1,2-bis(di-n-propylphosphino)ethane) with benzoic or 4-pyridylbenzoic acid show divergent responses for protonation of electrochemically-generated Co(I) to give a Co(III) hydride. For [Cp*CoIII(P2BCy4)Cl]+, this process is complex, not only involving protonation, but also engagement of the pendant borane moieties in Lewis acid/base interactions. For protonation by benzoic acid, for example, borane-benzoate contacts are substantiated by variable temperature NMR spectroscopic measurements and theoretical calculations, pointing to a cooperative Co-H/B-O bond forming process. These data are discussed in the context of designing new molecular catalysts for ligand-assisted hydrogen evolution reactivity.