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

Lewis acidic site pulls a chloride ligand away from platinum, forming a self-activating electrophilic catalyst

by Stephen K. Ritter
May 29, 2017 | A version of this story appeared in Volume 95, Issue 22

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.

A reaction scheme shows how an antimony-platinum complex rearranges to become an active platinum catalyst.


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