Non-spectator behavior of a neutral phosphine ligand driven by a redox-active boron cluster

A new concept of ligand ambiphilicity that relies on the redox behavior of the indirectly coordinated boron cluster scaffold instead of the direct involvement of a single center in a ligand is introduced. A copper(i) complex of a carboranyl-based triphosphine, [ ^{t Bu}P-closo-{C₂B₁₀}-^PhP-closo-{C₂B₁₀}- ^{t Bu}P]CuPF₆, in acetonitrile solution coordinates an exogenous X^- anion (X^- = Cl^- or N₃ ^-) at the phosphorus center of the ligand and not at the cationic metal site to form [ ^{t Bu}P(X)-nido-{C₂B₁₀}-^PhP-closo-{C₂B₁₀}- ^{t Bu}P]Cu(CH₃CN)₂. This first example of a nontraditional ligation behavior for a trigonal phosphine is imparted by an internal two-electron event at the boron cluster, which renders a non-constrained electron-donating phosphine group into an electrophilic coordination site. The observed ligand-centered behavior is reversible and can be switched by a change in solvent system, thus representing an unusual case of metal-ligand anion tautomerism enforced by the redox behavior of the boron cluster backbone. This dynamic system offers new possibilities for changing the coordination environment and reactivity of a metal center and opens new perspectives in molecular switching and catalysis.