Calculation of a Methane C-H Oxidative Addition Trajectory: Comparison to Experiment and Methane Activation by High-Valent Complexes Page: 345
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A Methane C-H Oxidative Addition Trajectory
Sx xx x xx x +
XX XX0( ,K(X~c4
, O x wo
030 X" 19
- . .. . . .. ....... . . . . .. .. . . . .
S xx i ! o o
1 r I
0 0.2 0.4 0.6
(H)(PH)2Ir'HCH, -.> (H),(PH)Ir(CH,)
product - TS - adduct
J. Am. Chem. Soc., Vol. 116, No. 1, 1994 345
-3 -2 -1 0 1 2 3
S, (bohr * amut2)
Figure 3. (a, top) Calculated bond length and bond angle changes along
the IRC for conversion of the Ir(PH3)2(H)(v2-HCH3) adduct into
Ir(PH3)2(H)2(CH3) using rbp as the reaction coordinate. See text for
definition of rbp. (b, bottom) Calculated bond length and bond angle
changes along the IRC for conversion of the Ir(PH3)2(H)(92-HCH3)
adduct into Ir(PH3)2(H)2(CH3) using Stota as the reaction coordinate.
See text for definition of Stalt.
differences with computed ones to the fact that the experimental
trajectory is derived from consideration of a variety of complexes
(with intramolecular M.*.H-C interaction) while the computa-
tional geometry arises from a single set of reactants (with
intermolecular M**H-C interaction). However, the fact remains
that similar behavior for the C-H distance and M-H-C angle
is exhibited in both experimental (Figure 4) and computational
trajectories (Figure 3), which is pleasing given the important
contribution the Crabtree trajectory has made to the thinking on
C-H activation. The computational trajectory does have an
advantage in that it allows one to see important parts of the
reaction coordinate not sampled by crystal structures, particularly
in the region of the transition state.
Analysis of the changes in bond lengths and bond angles along
the intrinsic reaction coordinate (Figure 3b) reveals three main
points of interest.
(1) The Ir-H-C angle is nearly constant at 1050 for Stotl
1 bohr amu1l/2. The value obtained from the experimental reaction
coordinate is - 130. The experimental trajectory27 was con-
structed from intramolecular, agostic complexes whose ability to
sample the entire M**H-C conformational space is more restricted
than in an intermolecular case. The near degeneracy of the edge-
coordinated (2) and n2-CH (3a) adducts for Ir(PH3)2(H)(CH4)
suggests that distortion along the Ir-H-C mode is very soft at
this stage in the reaction. Steric hindrance is expected to be less
in a linear adduct than for an q2-CH coordination geometry.
However, an off-axis approach in the experimental and compu-
tational trajectories is consistent with a favorable orientation for
donation from <CH to a vacant metal orbital concomitant with
03c a n a
Figure 4. Plot of both H (the angle C-H-M in deg) and dcH (A) against
rep (A) from Table IV. Squares and hexagons refer to neutron data,
triangles and circles to X-ray data. Hexagons and circles refer to a-CH
bridges that are more constrained geometrically than $ and higher types
(squares and triangles); more weight is attached to the latter, particularly
with regard to H. Only when ri, falls appreciably below 1.0 does H fall
below 1300 and dCH rise significantly above the value in the free C-H
bond (ca. 1.08 A). The calculated value of H for a cis alkyl hydride
complex (entry 21 of Table IV) is also included; the corresponding value
of dCH (2.68 A) is too large to include in this plot. Figure 4 was reproduced
with permission from the ACS and is taken from ref 27.
backdonation from a filled metal orbital into r*CH; a linear (Ir-
H-C = 1800) approach does not allow for any backdonation.
(2) The Ir-H and Ir-C distances are nearly at the values they
assume in the oxidative addition product even before the C-H
bond has begun to be activated (at Statl 1 bohr amu1/2, Figure
3b). Thus, the main motion, and energetic expenditure, in the
region of the transition state (Stal = -1 bohr amut/2 to Stotal =
+1 bohr amul/2, Figure 3b) corresponds to cleavage of the C-H
bond, as decomposition47a of the imaginary frequency (Figure 2)
indicates.57 As mentioned above, a TS for C-H activation with
substantial M.*C bond formation has been inferred from
selectivity patterns." Analysis of the IRC indicates that M*.C
(and M...H) bond formation is substantial even before the TS.
(3) The sharp increase in the C-H bond length is correlated
with a sharp decrease in the Ir-H-C angle at Sttl m 1 bohr
amu11/2 (Figure 3b). Calculation of Mulliken atomic charges
shows that the "break point" at Storal - 1 bohr amu'/2 is roughly
coincident with a decrease in the charge on the methane fragment.
The calculated charge on the methane fragment is +0.14 in the
adduct (3a) and increases to a value of +0.45 in the TS region,
after which it decreases toward the value of +0.22 that is obtained
in the product (4a). Gordon and Gano58 have studied the IRC
for a main-group analogue of this reaction (H2E + CH4 - H3E-
CH3) and interpreted it in terms of a two-stage mechanism: an
electrophilic, early stage (dominated by donation from aCH to a
vacant EH2-based MO) followed by a nucleophilic, later stage
(dominated by backdonation from an occupied EH2 orbital to
*CH). In light of the Saillard-Hoffmann model of M/C-H
bonding, the positive charge on the methane fragment at all stages
between methane adduct and methyl(hydride) product indicates
that forward donation from a C-H bonding MO to a vacant
metal-based MO is greater than backdonation from an occupied
(57) Kubas et al. have concluded from analysis of neutron diffraction
structures of q2-H2 complexes "that the reaction coordinate for H-H bond
breaking shows relatively little change in H-H distance until bond rupture
is quite imminent, presumably when increased backbonding can no longer be
tolerated." These deductions are consistent with this analysis of the IRC for
the related C-H oxidative addition reaction. Kubas, G. J.; Burns, C. J.; Eckert,
J.; Johnson, S. W.; Larson, A. C.; Vergamini, P. J.; Unkefer, C. J.; Khalsa,
G. R. K.; Jackson, S. A.; Eisenstein, O. J. Am. Chem. Soc. 1993, 115, 569.
(58) Gordon, M. S.; Gano, D. R. J. Am. Chem. Soc. 1984, 106, 5421.
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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/m1/6/: accessed August 21, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT College of Arts and Sciences.