Calculation of a Methane C-H Oxidative Addition Trajectory: Comparison to Experiment and Methane Activation by High-Valent Complexes Page: 344
The following text was automatically extracted from the image on this page using optical character recognition software:
344 J. Am. Chem. Soc., Vol. 116, No. 1, 1994
Memphis State University
Ir(PH3)2(H)2(Me): TS for OA
Figure 2. Imaginary frequency for TS of oxidative addition of methane
energy Hessian which shows only one imaginary vibrational
frequency: 1168i cm-1 (for 6a) and 1124i cm-' (for 6b),
respectively (see Figure 2 for the hydrido model; the chloro
analogue is identical). Decomposition of the imaginary mode
into internal coordinate displacements shows the major component
to be C-H bond breaking (formation for the reverse), as one
would expect for a true oxidative addition TS (Figure 2).
H H-Ir-H = 149" H CI-lr-H H 156'
H-r-C = 167 CI-r-C = 163
Ir-H-C = 90* H P Ir-H-C = 93'
H 2.23A H C 221A H
Ir-P = 231A H Ir-P= 236A H
P-Ir-P- 169' P-Ir-P= 172
Activation barriers change dramatically upon the inclusion of
electron correlation. Enthalpies of activation for 3a - 6a and
3b - 6h are 20 and 9 kcal mol-', respectively, at the RHF level.
Calculation of energies at the MP2 level (using RHF-optimized
geometries) lowers barriers by 19 kcal mol-' for both cases, yielding
a negative calculated barrier for X = Cl. For the Rh(Cl)(PH3)2
+ CH4-- Rh(Cl)(PH3)2(CH3)(H) reaction, methane activation
barriers range from -2.3 to 7.7 kcal mol-1, compared with barriers
of 15-24 kcal mol-1 (depending on basis set size) at the RHF
level.'5 These changes in activation barrier might suggest that
a single-determinant description of the TS is not sufficient.
However, a multiconfiguration self-consistent field (MCSCF)
calculation" at the RHF transition state shows a very large
contribution (>95%) from the Hartree-Fock configuration, as
found for do methane activators.sa-< Probing the complete
reaction coordinate for oxidative addition at correlated levels
would be of interest, although computationally expensive. Ex-
periments indicate that C-H oxidative addition to low-valent
metals proceeds with small activation barriers (Bergman et al.35b
estimate a barrier of only 5 kcal mol-'), as found in this and
previous5-17'44 theoretical studies.
(55) The two MOs located on the Ir..H..-C "active site" are included in
the MCSCF active space as are their antibonding counterparts. This yields
a four-orbital/four-electron active space. The FORS (full optimized reaction
space)-MCSCF wave function is generated by constructing all possible singlet
configurations (20 in total). Schmidt, M. W.; Ruedenberg, K.; Elbert, S. T.;
Gilbert, M. M. Chem. Phys. 1982, 92, 1476.
The most interesting point to emerge from calculation of TS
geometries is the extent to which Ir-H and Ir-C bond lengths are
formed in the transition state. Relative to their values in the
product, the Ir-C and Ir-H bond lengths in TS 6a (2.23 and 1.60
A, respectively) are only 2% and 3% longer than those in product
4a; for the chloro derivative, Ir-C and Ir-H bond lengths in TS
4b (2.21 and 1.60 A, respectively) are 6% and 3%, respectively,
longer than those in product 4b. The C-H bond being activated
is further removed from equilibrium values than Ir-H and Ir-C:
+55% (6a) and +31% (6b). Crabtree" has deduced a late TS
for oxidative addition in 14-electron complexes on the basis of
selectivity patterns; kinetic products arise from activation at the
least hindered C-H bonds in the substrate, suggesting a short
M-C distance in the TS and steric hindrance between substituents
on C and ancillary ligands. The calculations support a "late"
transition state for oxidative addition; not only is the Ir-C
interaction substantial but Ir-H is as well. The tightness of
transition states 6a and 6b should make it profitable to control
the selectivity of alkane functionalization catalysts built around
these 14-electron X(M)(PR3)2 species, particularly through choice
of R groups on the phosphines.56
5. Intrinsic Reaction Coordinate. The intrinsic reaction
coordinate (IRC)21 yields dynamic information about chemical
interactions which dictate the ease of passage of reactants through
the TS and on to products. Crabtree et al.21 have analyzed the
crystal structures of agostic complexes and thus constructed a
trajectory for C-H oxidative addition. This experimental
trajectory is then used to follow the evolution of the C-H distance
and M---H---C angle for the C-H being oxidatively added. Since
the IRC is mathematically defined as the steepest descent path
(in mass-weighted Cartesian coordinates) from the TS to reactants
and products, it can also be calculated from first-principles
quantum mechanics,2' making it of interest to compare the well-
known experimental trajectory27 with a computational trajectory.
Calculated changes in bond lengths and angles along the IRC
for (PH3)2(H)Ir...H2CH2 -- [*] -- Ir(PH3)2(H)2(CH3) (3a --
6a - 4a) are shown in Figure 3a using the reaction coordinate
(rbp, eqs 3 and 4) defined by Crabtree et al.27 (r = 0.28 A; 1.27
dbp = [dMH2 + 2dCH2 - r(dMH2 + dH2 - dMC2)]1/2 (3)
rbp = dbp- 1.27 A
A is the covalent radius of Ir, rbp "is effectively the covalent
radius of the C-H bonding electrons.").27 In Figure 3b the same
bond length and bond angles are plotted using Sttal (eq 5), the
Total = [(x,(0) - xi(s))2 + (y(0) -yi(s))2 +
(zt(0) - z,(s))2]'/2 (5)
mass distance along the reaction coordinate, on the abscissa. The
values xi(0), yi(0), and z,(0) in eq 5 are the mass-weighted
Cartesian coordinates of atom i at the TS, while x#(s), y,(s), and
z1(s) are the mass-weighted Cartesian coordinates of atom i at
a subsequent point along the IRC. Proceeding from positive to
negative values of Stoa (Figure 3b) describes the oxidative addition
process, the TS of which is at Stotal = 0 bohr amul/2. Inspection
of Figure 3 shows that using either rbp or Stota as the reaction
coordinate yields nearly identical results for describing the C-H
oxidative addition trajectory. Thus, Stotal will be used as the
reaction coordinate in subsequent discussion.
The experimental trajectory27 derived by Crabtree and co-
workers is reproduced in Figure 4. It seems reasonable to ascribe
(56) Product selectivity for C-H activation by 16-electron CpML inter-
mediates is kinetically controlled at low temperatures (where the reverse
reaction, reductive elimination of hydrocarbons, is minimal) and thermody-
namically controlled as one approaches room temperature. Jones, W. D.
(Chemistry, Rochester). Personal communication.
Here’s what’s next.
This article can be searched. Note: Results may vary based on the legibility of text within the document.
Tools / Downloads
Get a copy of this page or view the extracted text.
Citing and Sharing
Basic information for referencing this web page. We also provide extended guidance on usage rights, references, copying or embedding.
Reference the current page of this Article.
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/m1/5/: accessed October 22, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT College of Arts and Sciences.