Calculation of a Methane C-H Oxidative Addition Trajectory: Comparison to Experiment and Methane Activation by High-Valent Complexes Page: 346
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346 J. Am. Chem. Soc., Vol. 116, No. 1, 1994
* M- High- versus Low-Valent
* M-H Methane Activation
MH .. ,, o o o -o6 -
A CH 0o o o -
+ . o ,
o" . '.
S h a
-1 0o 1 2
S , (bohr * aiu112)
Figure S. Calculated bond length and bond angle changes along the IRC
for conversion of the W(=NH)3(942-HCH3) adduct into W(=NH)2-
(NH2)(CH3) (filled points) and Ir(PH3)2(H)(12-HCH3) into Ir(PH3)2-
(H)2(CH3) (open points).
metal-based MO to a C-H antibonding MO all along the oxidative
addition IRC. The increasing positive charge on the methane
fragment as the adduct moves toward the TS indicates that
donation is increasing relative to backdonation; the decreasing
charge after the "break point" at Stotl = 1 bohr amu1/2 suggests
that the reverse situation applies in the later stages of oxidative
addition. Combining these threads leads to the conclusion that
although donation of electron density from methane to metal is
essential for formation of an adduct, it is not until significant
population of o*CH occurs that C-H is cleaved.59
One intriguing result which emerged from the experimental
reaction coordinate27 is that both low- and high-valent, agostic
complexes fit on the same trajectory, although occupying different
positions along the reaction coordinate. To more closely probe
the similarities between a typical u-bond metathesis IRC and the
present oxidative addition one, we have plotted both on the same
reaction coordinate (Figure 5). The u-bond metathesis IRC is
that for the reaction W(==NH)3 + CH4 - W(=NH)2(NH2)-
(CH3). The tungsten IRC shows the same features discussed at
length in previous computational studies of methane activation
by imido complexes.lsa-so A W example was chosen, since the
metallic single-bond radii of W (1.30 A) and Ir (1.27 A) are
roughly the same.33
The resemblance of the oxidative addition IRC (Figure 3a) to
the u-bond metathesis IRC (Figure 5) is striking and suggests
a good degree of similarity between C-H activation mechanisms,
particularly early on in the reaction coordinate in the vicinity of
the methane adduct (Stotal > 1 bohr amul/2, Figure 5).61 In a
study of methane adducts of high-valent complexes,4 72-CH
coordination was also found to be preferred. For both u-bond
metathesis and oxidative addition, M-C bond lengths in the
transition state (Stl = 0 bohr amu1/2) are close to the values
they assume in the product (Figure 5). Significant M*.C bond
formation has been inferred in the TS for C-H oxidative addition
by 14-electron complexes" from selectivity patterns. Extensive
M***C interaction in the u-bond metathesis TS (Figure 3a) should
also allow one to engineer an alkane functionalization catalyst
(59) (a) It is worth noting that all partitioning of the total electron density
is arbitrary, although one expects trends in properties to be more reliable than
absolute numbers. (b) Stretching the C-H bond and closing the M-H-C
angle should greatly increase the effectiveness of metal to Q*cH backdonation.
See ref 13, in particular Table II.
(60) Full details of the reaction W(--NH)3 + CH4 -- W(=NH)2-
(NH2)(CH,) are to be published in a future contribution. C-H activation
by high-valent, group IVB imido complexes is discussed in ref 18a.
(61) Labinger et al. have made a similar proposal based on experimental
results for Pt"-mediated alkane oxidations. Labinger, J. A.; Herring, A. M.;
Lyon, D. K.; Luinstra, G. A.; Bercaw, J. E.; Horvath, I. T.; Eller, K.
Organometallics 1993, 12, 895.
abrupt increase in C-H bond distance and a decrease in M-H-C
angle. Analysis of the wave functions shows that the methane
fragment which had been growing more positively charged up to
this point starts to accept electron density and become less
positively charged. In view of the Saillard-Hoffmann'3 model
built on a high-valent imido active species which will permit control
of selectivity through judicious choice of ancillary ligands. The
M-H distances show distinctly different behavior along the IRC
for the two mechanisms; this is not surprising, since in oxidative
addition the H ends up coordinated to the metal and in u-bond
metathesis it is coordinated to a ligated atom. For u-bond
metathesis the importance of the M-H interaction in moderating
the energetics of the transition state has been discussed.8sa-
However, in both cases the M-H distance in the TS is only slightly
longer than a typical metal-terminal hydride bond length.
The most interesting of the similarities between oxidative
addition and u-bond metathesis IRCs (Figure 5) is the same
correlation between the abrupt increase in C-H distances and
the sharp decrease in M-H-C angle. The same pattern of
increasing charge on the methane fragment, followed by a decrease
in the methane charge near this "break point" (Sto, 1 bohr
amu'/2) is seen for the high-valent system; however, it is not as
dramatic as in oxidative addition, as expected since backdonation
from the metal will be minimal for an electrophilic, do complex.
The frontier orbitals of this imido (and others we have studied'8)
are metal-nitrogen ir bonds, making them a potential source for
electron density. Thus, while the metal acts as an electrophile
in u-bond metathesis and oxidative addition (which the calcu-
lations indicate is essential for adduct formation), the difference
lies in the moiety which acts as nucleophile (donating to u*cH
which is crucial to cleavage of the C-H bond). The nucleophiles
in oxidative addition and u-bond metathesis are, of course, the
metal and a polarized metal-ligand bond which directs the H in
the C-H bond being activated to its eventual position. A two-
stage C-H activation process provides an explanation for the
similarity seen in the early part of the activation mechanism,
since it is here that electron donation from substrate to metal is
crucial for adduct formation for both types of methane-activating
complexes. More work is needed to probe these conclusions for
a wider variety of C-H-activating complexes; such research is
currently underway in our lab.
An effective core potential, parallel supercomputing study of
methane activation by oxidative addition was carried out for the
14-electron complexes Ir(PH3)2(X), X = H, Cl. Several con-
clusions have been reached which are summarized below.
(1) Effective core potentials have again shown their ability to
describe the bonding and structure of a transition metal system.'
The coupling of this new methodology with emerging technologies
like parallel computing will continue to expand the ability to
probe interesting chemistry in all regions of the Periodic Table.
Given the importance of the transition metals (and lanthanides)
in catalysis and advanced materials, such efforts are clearly
(2) Initial interaction of methane with the activating complexes
is in line with previous computation and experiment. The chloro
adduct is found to be more strongly bound (by =9 kcal mol-')
due to electronegative Cl making the complex a better acceptor
for uacH density.
(3) Metal-carbon bond formation is substantial in the TS, as
suggested by experiment." Additionally, Ir-H bonding in the
transition state is also substantial. Thus, the main energetic
expenditure in the TS region comes from scission of the C-H
(4) Analysis of the intrinsic reaction coordinate shows good
agreement with the experimental trajectory of Crabtree et al.2'
The most interesting observation is the correlation between an
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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/m1/7/: accessed June 24, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT College of Arts and Sciences.