Antimony ligand creates a diversion to unmask platinum catalyst’s reactivity

Homogeneous transition-metal complexes typically need a jump start to become catalytically active. One strategy chemists employ is to use a silver salt or a Lewis acid activator such as a boron compound to pull off and sequester an anionic ligand from the catalyst metal. Di You and François P. Gabbaï of Texas A&M University have designed a new strategy for ligand abstraction by building an antimony Lewis acidic site directly into a platinum catalyst’s architecture, forming a self-activating catalyst system (J. Am. Chem. Soc. 2017, DOI: 10.1021/jacs.7b03287). Gabbaï’s group is known for synthesizing electron-deficient main-group compounds and studying their anion-binding and redox properties. While investigating antimony-platinum complexes, the researchers found that antimony’s ability to adopt variable coordination numbers, a property the Gabbaï group calls coordination noninnocence, enables antimony to shed loosely bound anionic ligands such as triflate and add stronger coordinating anions such as chloride. In the new antimony-platinum complex, this ability is triggered when a nucleophilic substrate approaches the electrophilic platinum reactive site. Subsequent spontaneous chloride migration from platinum to antimony unmasks platinum, leaving it exposed and catalytically active. The researchers found that the antimony-platinum complex can mediate enyne cyclization and hydro­arylation of propargyl aryl ethers without the need for adding a chloride abstracting reagent.