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Published April 24, 2007 | Published
Journal Article Open

The role of alkane coordination in C–H bond cleavage at a Pt(II) center


The rates of CFormula H bond activation for various alkanes by [(N–N)Pt(Me)(TFEd3)]+ (N Formula N = ArFormula NFormula C(Me)Formula C(Me)Formula NFormula Ar; Ar = 3,5-di-tert-butylphenyl; TFE-d3 = CF3CD2OD) were studied. Both linear and cyclic alkanes give the corresponding alkene-hydride cation [(N–N)Pt(H)(alkene)]+ via (i) rate determining alkane coordination to form a CFormula H {sigma} complex, (ii) oxidative cleavage of the coordinated CFormula H bond to give a platinum(IV) alkyl-methyl-hydride intermediate, (iii) reductive coupling to generate a methane {sigma} complex, (iv) dissociation of methane, and (v) beta-H elimination to form the observed product. Second-order rate constants for cycloalkane activation (CnH2n), are proportional to the size of the ring (k ~ n). For cyclohexane, the deuterium kinetic isotope effect (kH/kD) of 1.28 (5) is consistent with the proposed rate determining alkane coordination to form a CFormula H {sigma} complex. Statistical scrambling of the five hydrogens of the Pt-methyl and the coordinated methylene unit, via rapid, reversible steps ii and iii, and interchange of geminal CFormula H bonds of the methane and cyclohexane CFormula H {sigma} adducts, is observed before loss of methane.

Additional Information

© 2007 by the National Academy of Sciences. Edited by Jack Halpern, University of Chicago, Chicago, IL, and approved February 15, 2007 (received for review December 11, 2006). Published online before print April 6, 2007, 10.1073/pnas.0610981104 We thank Dr. Travis Williams and Bo-Lin Lin for useful discussions and assistance in preparing the manuscript. This work was supported by British Petroleum as part of the Methane Conversion Consortium and a National Science Foundation Graduate Research Fellowship (to G.S.C.). Coordination Chemistry of Saturated Molecules Special Feature. Author contributions: G.S.C., J.A.L., and J.E.B. designed research; G.S.C. performed research; G.S.C. contributed new reagents/analytic tools; G.S.C., J.A.L., and J.E.B. analyzed data; and G.S.C., J.A.L., and J.E.B. wrote the paper. The authors declare no conflict of interest. This article is a PNAS Direct Submission. This article contains supporting information online at www.pnas.org/cgi/content/full/0610981104/DC1.

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