Calculation of a Methane C-H Oxidative Addition Trajectory: Comparison to Experiment and Methane Activation by High-Valent Complexes Page: 340
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J. Am. Chem. Soc. 1994, 116, 340-347
Calculation of a Methane C-H Oxidative Addition Trajectory:
Comparison to Experiment and Methane Activation by
Thomas R. Cundari'
Contribution from the Department of Chemistry, Memphis State University,
Memphis, Tennessee 38152
Received March 4, 19930
Abstract: An effective core potential (ECP), parallel supercomputing study of methane activation by 14-electron,
Ir(PH3)2(X) complexes (X = H, Cl) is presented. Considerable weakening of the coordinated methane C-H bond
occurs upon formation of an r2-CH coordinated (X)(PH3)2Ir...HCH3 adduct. A more strongly bound adduct (with
greater weakening of the coordinated C-H bond) occurs when X = Cl versus X = H. The calculated Ir(PH3)2(H)
+ CH4 - Ir(PH3)2(H)2(Me) reaction enthalpy is -12.8 kcal mol-', and -41.6 kcal mol-1 for the chloro analogue. The
intrinsic reaction coordinate is calculated and compared to an experimental trajectory. Analysis of the wave function
along the intrinsic reaction coordinate (IRC) suggests that although donation of electron density from methane to metal
is essential for adduct formation, it is not until backdonation to a*CH increases that the C-H bond is activated and
cleaved. The electronic and molecular structure of the reacting system along the IRC suggest a two-stage mechanism:
substrate to complex donation is important in the early part of the reaction (electrophilic stage) while complex to
substrate backdonation is necessary later on (nucleophilic stage) for C-H scission. Finally, comparison of IRCs for
low- and high-valent methane-activating complexes shows similar topology in the early portion of the activation event;
differentiation between oxidative addition and u-bond metathesis occurs at the point at which there is a shift from the
electrophilic to nucleophilic stage of the reaction.
Selective C-H activation of methane by transition metal and
lanthanide complexes has been the subject of considerable study
as a result of its importance in catalytic methane conversion.2
Two main strategies have been pursued to effect concerted (as
opposed to nonconcerted radical3 or electrophilic4 activation) C-H
activation in homogeneous systems: oxidative addition5 and
u-bond metathesis.6 u-Bond metathesis involves 1,2-addition of
C-H across an appropriate metal-ligand bond (single or multiple)
and is effected by high-valent (do and df") complexes.6 Rep-
resentative examples of high-valent, methane-activated complexes
are Cp*2LuCH3,7 (NHSi')2Zr=NSi' (Si' = Si(t-Bu)3),8 and
thoracyclobutanes.9 Oxidative addition entails 1,1-addition of
C-H to a low-valent transition metal and results in an increase
in the formal oxidation state and coordination number of the
metal by two units.5 The 16-electron Cp*M(L) (M = Rh, Ir; L
= PR3, CO) intermediates,10 are perhaps the best known examples
of complexes for which oxidative addition is the putative C-H
* Abstract published in Advance ACS Abstracts, September 15, 1993.
(1) e-mail: cundarit@memstvxl .memst.edu.
(2) (a) Activation and Functionalization of Alkanes; Hill, C. L., Ed.;
Wiley: New York, 1989. (b) Saillard, J. Y. In Selective Hydrocarbon
Activation; Davies, J. A., Ed.; VCH: New York, 1990; p 207. (c) A good
discussion of the requirements for a successful methane conversion catalyst
from an industrial point of view can be found in: Parkyns, N. D. Chem. Br.
1990, 9, 841.
(3) (a) Khenkin, A. M.; Shilov, A. E. New J. Chem. 1989, 13, 659. (b)
Liu, H. F.; Liu, K. Y.; Liew, R. E.; Lunsford, J. H. J. Am. Chem. Soc. 1984,
(4) Olah, G. A. Acc. Chem. Res. 1987, 20, 422.
(5) Jones, W. D. In ref 2a, p 111.
(6) Rothwell, I. P. In ref 2a, p 151.
(7) Watson, P. L. J. Am. Chem. Soc. 1983, 105, 6491.
(8) Cummins, C. C.; Baxter, S. M.; Wolczanski, P. T. J. Am. Chem. Soc.
1988, 110, 8731.
(9) Fendrick, C. M.; Marks, T. J. J. Am. Chem. Soc. 1984, 106, 2214.
(10) (a) Janowicz, A. H.; Bergman, R. G. J. Am. Chem. Soc. 1982, 104,
352. (b) Hoyano, J. K.; Graham, W. A. G. J. Am. Chem. Soc. 1982, 104,
3723. (c) Jones, W. D.; Feher, F. J. J. Am. Chem. Soc. 1982, 104, 4240. (d)
A review of the research on 16-electron CpML C-H activators can be found
in ref 5 and: Jones, W. D.; Feher, F. J. Acc. Chem. Res. 1989, 22, 91.
Another family of low-valent, C-H-activating complexes are
14-electron species of the general form M(L)n(PR3)m, where L
is a univalent ligand such as hydrido, chloro, or an nl-carboxylate
group.1,12 As Crabtree has pointed out,''" 14-electron complexes
have advantages over 16-electron species from the point of view
of building a catalyst system. The hydrido(alkyl) complex, eq
la, is still electron-deficient (i.e., 16-electron), typically with
LnM + R-CH2-CH3 - LnM(H)(CH2--CH--R) (la)
LnM(H)(CH2-CH2-R) --* L(H)2(H2C=C(H)R) (lb)
Ln(H)2(H2C=C(H)R) - LnM(H)2 + H2C=C(H)R (lc)
LM(H)2 -- LnM + H2
vacant coordination sites, allowing for -H transfer to form an
18-electron bis(hydrido)olefin, eq ib, which can dissociate olefin
to yield a bis(hydrido) complex, eq 1c. The bis(hydrido) complex
can regenerate the reactive intermediate either by photolytic
dissociation of H2 or by addition of an H2 scavenger (e.g., tert-
butylethylene), eq id." Crabtreel","2a has developed an alkane
dehydrogenation catalyst starting from Ir(H)2(v2-carboxy-
late)(PR3)2; the reactive intermediate is thought to be the 14-
electron Ir(PR3)2(n'-carboxylate). Sakakura and Tanaka'2c have
studied catalytic alkane carbonylation by a Rh catalyst; the C-H
activating species is thought to be Rh(Cl)(PR3)2. Recently,
Schulz and Wernerl2d have studied the rearrangement between
vinyl(hydride) complexes and olefin isomers of Ir(PR3)2-
(11) Crabtree, R. H. In ref 2a, p 69.
(12) (a) Burk, M. J.; Crabtree, R. H. J. Am. Chem. Soc. 1987,109, 8025.
(b) Cameron, C. J.; Felkin, H.; Fillebeen-Khan, T.; Forrow, N. J.; Guittet,
E. J. Chem. Soc., Chem. Commun. 1986, 801. (c) Sakakura, T.; Tanaka, M.
Chem. Lett. 1987, 249, 1113. Sakakura, T.; Tanaka, M.J. Chem.Soc., Chem.
Commun. 1987, 758. (d) Schulz, M.; Werner, H. Organometallics 1992, 11,
0002-7863/94/1516-0340$04.50/0 1994 American Chemical Society
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Cundari, Thomas R., 1964-. Calculation of a Methane C-H Oxidative Addition Trajectory: Comparison to Experiment and Methane Activation by High-Valent Complexes, article, January 1994; [Washington, DC]. (digital.library.unt.edu/ark:/67531/metadc107777/m1/1/: accessed May 25, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT College of Arts and Sciences.