Ligand-Centered Borenium Reactivity in Triaminoborane-Bridged Diphosphine Complexes

Borenium ions (i.e., three-coordinate boron cations) are known to promote a wide variety of stoichiometric and catalytic reactions because of their high Lewis acidity. As demonstrated by the growing number of chemically reactive borane ligands, there is considerable interest in developing ligands with highly electrophilic boron sites that promote multisite reactivity in metal complexes. However, there are currently few examples of ligand-centered borenium ions, especially with ligands that form coordination complexes with a wide range of metals. Here we report borenium-like reactivity on a highly versatile diphosphine ligand. Treating (^PhTBDPhos)NiCl₂ (1) with strong Bronsted acids such as HBF₄·Et₂O, HOTf, or HNTf₂ resulted in fluoride or chloride abstraction from BF₄^- or NiCl₂, respectively, to form trans N-H and B-X bonds on the ligand backbone. HCl addition to the bridgehead N-B bond is reversible, and the reactivity depends on the identity of the supporting counteranions, as observed when treating [(^PhTBDPhos)NiCl]₂X₂, where X = NTf₂^- (3), OTf^- (4), or BAr^F₄^- (5), with HCl. The reaction of 4 with HNTf₂ instead of HCl yielded NMR evidence of the latent borenium cation in solution and showed how poor nucleophiles such as triflate bind to the borenium ion in the solid state. Remarkably, replacing the chloride ligands in 1 with chelating and less-labile thiolates or catecholates, or changing the phosphorus substituents (phenyl to isopropyl), attenuates the reactivity on the ligand backbone. Density functional theory was used to quantify the reaction free energies, and the theoretical results corroborate the experimental observations. Given the broad utility of diphosphines in homogeneous catalysis and the known benefits of strong Lewis acid promotors in many catalytic reactions, we anticipate that the results will provide new opportunities for dual-site reactivity involving boron ligands and metals.