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New Ligand Works In A Pinch

ACS Meeting News: Tridentate pincer ligand is the first to use three carbon atoms as donors to a transition-metal cation

by Stephen K. Ritter
April 2, 2012 | A version of this story appeared in Volume 90, Issue 14

Catalyst builders have a new metal-binding ligand in their toolbox. Pincer ligands featuring three-way metal-binding motifs, such as NCN and PCP, are often used to construct transition-metal catalysts. But absent from this collection until now were examples of pincer ligands that feature a trianionic CCC metal-binding array. These missing ligands “were undoubtedly a consequence of the synthetic difficulties in attaching three alkyl or aryl groups from a single ligand to a metal, rather than any lack of desire to investigate such complexes,” Columbia University’s Gerard Parkin told C&EN. Parkin and graduate student Aaron Sattler overcame those difficulties when preparing a tantalum terphenyl pincer complex (J. Am. Chem. Soc., DOI: 10.1021/ja210404x). The researchers added 2,6-di-p-tolylphenyl to Ta(CH3)3Cl2 via a lithiation reaction. The central phenyl ring binds to tantalum, but the phenyl’s tolyl side groups remain free. In a subsequent cyclometallation step they added trimethylphosphine to induce elimination of methane, which allows the tolyl groups to bind to tantalum to form the pincer complex. The team also discovered it can replace the methyl and chloride ligands on the tantalum pincer complex with a benzene ligand, which binds to the metal through two of its carbon atoms.

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