Structural and dynamic properties of propane coordinated to TpRh(CNR) from a confrontation between theory and experiment
- *Institut Charles Gerhardt (Unité Mixte de Recherche 5253, Centre National de la Recherche Scientifique, Université Montpellier 2, École Nationale Supérieure de Chimie de Montpellier, Université Montpellier 1), Chimie Théorique Méthologies Modélisations, cc 14, Université Montpellier 2, 34095 Montpellier Cedex 5, France; and
- ‡Department of Chemistry, University of Rochester, Rochester, NY 14627
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Edited by Jay A. Labinger, California Institute of Technology, Pasadena, CA, and accepted by the Editorial Board February 6, 2007 (received for review October 25, 2006)
Abstract
Density functional calculations with the B3PW91 functional have been carried out on the TpRh(CNMe) species [Tp = HB(pyrazolyl)3] as a model for Tp′Rh(CNCH2CMe3) [Tp′ = HB(3,5-dimethylpyrazolyl)3] in interaction with propane. Two σ complexes have been found as minima coordinated through either a methyl or a methylene
C
H bond, the former being more stable. The approach of the alkane to TpRh(CNMe) has been studied. Although no transition state
could be located, study of this path reveals the key importance of the partial decoordination of one pyrazole ring. The full
coordination of the alkane can only be achieved when the metal is essentially in a square pyramid coordination with one of
the three pyrazole groups only weakly interacting with Rh. The main reaction of the methyl σ complex is oxidative addition,
leading to the n-propyl hydride complex. In contrast, two reactions are found for the methylene σ complex: (i) oxidative addition to give the isopropyl complex and (ii) exchange between the secondary and primary C
H bonds to convert the methylene complex of propane into a methyl complex of propane. This latter reaction has a much lower
barrier than the oxidative addition at the methylene C
H bond. The results account well for most of the experimental results obtained from kinetic studies. Steric factors are found
to control the energy barriers between these various processes, disfavoring any process that brings the central carbon into
close proximity to Rh.
Footnotes
- †To whom correspondence should be addressed. E-mail: odile.eisenstein{at}univ-montp2.fr
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Author contributions: W.D.J. designed research; E.C. performed research; E.C. and O.E. analyzed data; and O.E. and W.D.J. wrote the paper.
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The authors declare no conflict of interest.
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This article is a PNAS Direct Submission. J.A.L. is a guest editor invited by the Editorial Board.
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↵ § The statistical preference for a pentane methyl vs. a cyclohexane methylene C
H bond is taken as 1:3 because there are two methyl groups vs. six methylene groups. We do not believe that a “per-hydrogen”
basis is as useful, inasmuch as methyl groups will always have three hydrogens and methylenes will always have two hydrogens.
- Abbreviation:
- TS,
- transition state.
- © 2007 by The National Academy of Sciences of the USA
