Explorations of Multi-Carboranyl Thiols and Phosphines

In order to ascribe the impact of substituent groups on the bis-ortho-carborane, closo-{C2B10}-closo-{C2B10} multi-cluster system, the first sulfur-containing biscarborane derivatives were synthesized. Biscarborane dithiol, disulfide, bis(methyl)thioether, as well as multiple dithiolate salts were structurally characterized. The exceedingly long C–C bonds and shortened C–S bonds in biscarborane dithiol and its doubly deprotonated dithiolate salts are indicative of significant delocalization of electron density from the sulfur atoms into the cluster. Additionally, a new concept of ligand ambiphilicity is described where, instead of the involvement of only a single center in a ligand, this amphiphilicity relies on the redox behavior of the indirectly coordinated boron cluster scaffold. A copper(I) complex of a carborane-based triphosphine [tBuP-closo-{C2B10}-PhP-closo-{C2B10}-tBuP]Cu(CH3CN)BF4 coordinates an exogenous X anion (X = Cl– or N3–) at the phosphorus center of the ligand and not at the cationic metal site forming [tBuP(X)-nido-{C2B10}-PhP-closo-{C2B10}-tBuP]Cu(CH3CN)2. This nontraditional ligation behavior 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 exhibits reversible solvent dependence, thus representing an unusual case of metal-ligand anion tautomerism promoted by redox-behavior of the carborane backbone.