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, [tBuP-closo-{C2B10}-PhP-closo-{C2B10}-tBuP]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 [tBuP(X)-nido-{C2B10}-PhP-closo-{C2B10}-tBuP]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.