Formation Of Famous Tantalum Complex Is Nailed Down

Pentaneopentyltantalum, TaR₅, where R is the neopentyl group CH₂C(CH₃)₃, may be one of the most important chemicals you have never heard of. It is now the confirmed precursor to the tantalum alkylidene R₃Ta=CHC(CH₃)₃, the first isolated and characterized transition-metal alkylidene complex, which in 1974 launched MIT chemist Richard R. Schrock on the road to the 2005 Nobel Prize in Chemistry for developing his version of olefin metathesis catalysts. Although R₃Ta=CHC(CH₃)₃ has been vigorously studied and appears in textbooks, its exact mechanism of formation has been a mystery. Researchers including Schrock have guessed that TaR₅ forms as an intermediate between the starting material TaR₃Cl₂ and R₃Ta=CHC(CH₃)₃. Zi-Ling (Ben) Xue of the University of Tennessee, Knoxville, reported fleeting glimpses of TaR₅ in experiments 14 years ago, but there hasn't been unambiguous evidence to support the sitings. Xue, Julia K. C. Abbott, and Liting Li now provide definitive proof from their studies on deuterated versions of the compounds in the reaction sequence, which are more stable and easier to characterize (J. Am. Chem. Soc. 2009, 131, 8246). One of Xue and coworkers' key observations confirms that R₃Ta=CHC(CH₃)₃ indeed forms via abstraction of an α-hydrogen atom from one neopentyl group that is carried away by a second neopentyl group. "It is nice to see a problem that has been out there for 35 years finally solved," Schrock says.