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 [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(P ₂B ^Cy ₄) ₂] (P ₂B ^Cy ₄=1,2-bis(di(3-dicyclohexylboraneyl)-propylphosphino)ethane) is competent for room-temperature PhI cleavage to give [Ni ^II(P ₂B ^Cy 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 [Ni ⁰(P ₂B ^Cy ₄) ₂], a “boron-trapped” 16-electron κ ¹-diphosphine Ni(0) complex. Moreover, formation of [Ni ^II(P ₂B ^Cy ₄)(Ph)(I)] is inherent to the P ₂B ^Cy ₄ secondary coordination sphere: treatment of the Lewis adduct, [Ni ⁰(P ₂B ^Cy ₄) ₂(DMAP) ₈] with PhI provides [Ni ^II(P ₂B ^Cy ₄) 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(P ₂B ^Cy 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.