Lewis Acid-Promoted Oxidative Addition at a [Ni-0(diphosphine)(2)] Complex: The Critical Role of a Secondary Coordination Sphere


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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 [Ni-0(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 [Ni-0(P2B4Cy)(2)] (P2B4Cy=1,2-bis(di(3-dicyclohexylboraneyl)-propylphosphino)ethane) is competent for room-temperature PhI cleavage to give [Ni-II(P2B4Cy)(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 [Ni-0(P2B4Cy)(2)], a "boron-trapped" 16-electron kappa(1)-diphosphine Ni(0) complex. Moreover, formation of [Ni-II(P2B4Cy)(Ph)(I)] is inherent to the P2B4Cy secondary coordination sphere: treatment of the Lewis adduct, [Ni-0(P2B4Cy)(2)(DMAP)(8)] with PhI provides [Ni-II(P2B4Cy)(2)(DMAP)(8)(I)]I via iodine-atom abstraction and not a [Ni-II(Ph)(I)(diphosphine)] compound - an unusual secondary sphere effect. Finally, the reactivity of [Ni-0(P2B4Cy)(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.
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Key words
boron, cross-coupling, diphosphine ligands, metal-ligand cooperativity, nickel
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