Ir-Pincer dihydrogen complex catalyzed dehydrogenation of ammonia-borane differs from ethane dehydrogenation

INOR 15

Ankan Paul1, Charles B. Musgrave, chasm@stanford.edu2, and Paul Zimmerman1. (1) Chemical Engineering, Stanford University, 380 Roth Way Department of Chemical Engineering, Department of Chemical Engineering, Stanford, CA 94305-5025, (2) Department of Chemical Engineering, Stanford University, 380 Roth Way Department of Chemical Engineering, Stanford, CA 94305-5025
We report the first mechanistic pathway for ammonia-borane, (NH3BH3) dehydrogenation by a pincer [ 3-1,3-(OPtBu2)2C6H3]Ir(H2) complex in THF employing hybrid density functional theory in conjunction with the conductor like polar continuum solvent model (CPCM). The dehydrogenation pathways for NH3BH3, which proceed through concerted removal of two hydrogen atoms via a six-membered and five-membered transition states from NH3BH3 is distinctively different from the traditional dehydrogenation of ethane, a structural analogue of NH3BH3 by Ir-pincer complexes. Both the pathways involve simultaneous transfer of hydride (from B end of NH3BH3) and proton (from N end NH3BH3) to the transition metal catalyst and the barriers are below >18 kcal/mol. Surprisingly, no low barrier pathways were found for NH3BH3 dehydrogenation, which operated through N-H or B-H activation analogous to C-H activation as in the case of ethane dehydrogenation. We attribute these differences to the charge distribution on hydrogen atoms in NH3BH3. Additionally, similar pathways for NH3BH3 dehydrogenation by Ni-N-heterocyclic carbene complexes have been identified.

 

Theoretical and Computational Chemistry
9:00 AM-11:20 AM, Sunday, August 19, 2007 BCEC -- 203, Oral

Division of Inorganic Chemistry

The 234th ACS National Meeting, Boston, MA, August 19-23, 2007