Calculation of a Methane C-H Oxidative Addition Trajectory: Comparison to Experiment and Methane Activation by High-Valent Complexes Page: 343
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J. Am. Chem. Soc., Vol. 116, No. 1, 1994 343
H with Cl yields an r/2-CH minimum (C, symmetry, 3b) for which
Ir**C and Ir***H distances are much shorter. The differences in
calculated geometries are reflected in a greater AHadd for
Ir(PH3)2(C1) versus Ir(PH3)2(H). Calculated AHadd values for
3a and 3b are -6.8 and -15.6 kcal mol-', respectively.46 Calculated
values for AHadd are in line with the experimental and compu-
tational estimates given above; more quantitative assessments
will require higher levels of theory.
Calculated intrinsic stretching frequencies47 show coordinated
C-H bonds to be much weaker than spectator C-H bonds: 2873
cm-1 versus 3014 - 16 cm-1 (3a) and 2680 cm-1 versus 3032 S
17 cm-' (3b). Note that the coordinated C-H is much weaker
in 3b than in 3a, consistent with bond lengths of the coordinated
C-H: 1.10 A in 3a; 1.13 A in 3b. The charge on the methane
fragment (from Mulliken population analyses) in 3a is +0.14
versus +0.25 for 3b. Within the Saillard-Hoffmanni3 model
two explanations seem feasible for the difference in charges.
Replacement of X = H with X = Cl makes lb a better acceptor
for cCH than Ia; lb is less effective at ir-backbonding to t*CH
than la. The Mulliken charge on Ir is -1.10 in la and -0.69 for
lb. A less negative charge on lb should lower in energy metal-
based MOs (as the calculations indicate); however, 3r-donation
from the chloro destabilizes the dir orbital perpendicular to the
IrP2X plane (the one backbonding to a*CH), making it roughly
the same energy in la (-0.314 hartree) and lb (-0.324 hartree).
The calculations thus point to la and lb being equivalent ir-donors
with the latter a better a-acceptor, leading to a more exothermic
AHadd and increased C-H weakening upon adduct formation for
3b. The weakening of coordinated C-H which occurs upon adduct
formation may assist in C-H scission, as suggested for inter-
molecular41 and intramolecular39*48 M..-H-C complexes. The
calculations show the adduct to be sensitive to electronic effects
induced by ancillary ligands, in addition to steric factors.15,17b,36
Tunable properties are attractive from the standpoint of designing
a catalyst precursor in which C-H bond weakening is maximized
in an adduct, particularly if the interactions carry through to the
transition state and lower the activation barrier.
3. Products. Products are 16-electron, five-coordinate Ir!"'
complexes-Ir(X)(H)(CH3)(PH3)2 (4). The phosphines remain
trans to each other with angles close to 1800. The geometry of
the complexes is closer toa square pyramid (SQP5) than a trigonal
bipyramid (TBP5). Frontier orbital arguments suggest that a d6
TBP5 complex will be Jahn-Teller unstable and distort to an
SQP5 geometry;25 furthermore, a Walsh diagram shows the
preferred Lapical-M-Las, angle to be near 900.25 The optimized
geometries of the products (4) bear out these predictions in all
The Ir-X bonds X = H, Cl, P) are in good agreement with
experimental models described above. The most interesting
changes in bond lengths highlight the trans influence of hydrido
and methyl ligands. The Ir-C1 bond in 4b, trans to methyl,
lengthens by 0.12 A (versus lb); the Ir-H bond in 4a, trans to
a methyl, lengthens by 0.10 , still within reported limits.32 Rausch
et al.49 quote Ir-C distances ranging from 1.982(26) to 2.202(4)
A depending on metal coordination environment. The Ir-C
(46) Modification of ancillary ligands can substantially affect the acidity/
basicity of a metal center. Replacement of hydride with chloride makes the
Os center significantly more acidic/less basic in CpOs(PPh3)2X. The enthalpy
of protonation by CF3SO3H is 23.2 kcal mol-' less exothermic for
CpOs(PPh3)2CI than CpOs(PPh3)2H. Rottink, M. K.; Angelici, R. J. J. Am.
Chem. Soc. 1992, 114, 8296.
(47) (a) Boatz, J. A.; Gordon, M. S. J. Phys. Chem. 1989, 93, 1819. (b)
Calculated stretching frequencies are typically 10% too high and thus scaled
by 0.9 to account for correlation and anharmonicity. Pople, J. A.; Schlegel,
H. B.; Krishnan, R.; DeFrees, D. F.; Binkley, J. S.; Frisch, M. J.; Whiteside,
R.; Hout, R. F.; Hehre, W. J. Int. J. Quantum Chem., Proc. Sanibel Symp.
1981, 15, 269. We have used a scaling factor of 0.925, since this best reproduces
the experimental C-H stretches of methane.
(48) (a) Crabtree, R. H. Chem. Rev. 1985, 85, 245. (b) Schrock, R. R.
Acc. Chem. Res. 1979, 12, 98.
(49) Rausch, M. D.; Clearfield, A.; Gopal, R.; Bernal, I.; Moser, G. A.
Inorg. Chem. 1975, 11, 2727.
H H H H
Ir P = 233A H Ir-P 236A H H
P-]r-P = 172 H P-Ir-P = 174' (
1I8 Ir 100. H) 2 39 85 H)
C 85 4 H OC 11 nt
l55A t 55a
(H H) (H
distance in the complex with a trans hydrido (Ir-C = 2.18 A, 4a)
is much longer than that with a trans chloro (Ir-C = 2.10 A, 4b),
in line with the trans influence order H- > CH3- > Cl-.29 Apical
H atoms in 4a and 4b are trans to a vacant coordination site and
show Ir-H bond lengths (1.55 A) nearly identical to that of the
Ir(PH3)2(H) reactant (1.58 A).
The Ir(PH3)2(H) + CH4 - Ir(PH3)2(H)2(Me) reaction
enthalpy is -12.8 kcal mol-1 versus separated reactants, while
that for Ir(PH3)2(C1) is -41.6 kcal mol-1. Theoretical estimates
for methane C-H oxidative addition are -15, -17, -33, and -36
kcal mol-1 for CpRh(CO), CpRh(PH3), CpIr(CO), and CpIr-
(PH3), respectively.'6 Koga and Morokuma'5 calculate reaction
energies for (Cl)(PH3)2 + CH4 - Rh(C1)(H)(CH3)(PH3)2 from
-17.3 to -24.4 kcal mol-'. Experimental Ir-H and Ir-R bond
strengths for Cp*Ir(R)2 are 74 and 56 kcal mol-' (R = Me).50
Combining these values with a methane C-H bond strength of
105 kcal mol-' yields an estimate of -25 kcal mol-' for
Cp*Ir(PMe3) + H3C-H - Cp*Ir(PMe3)(H)(CH3).51 Ir-C bond
enthalpies in Ir(Cl)2(CO)(PR3)2(C(O)Me) span a range of 12
kcal mol-' for a variety of PR3 ligands;52 Ir-C bond energies in
Ir(C)2(CO)(PPh3)(R) are spread over a 17 kcal mol-1 range
depending on the nature of R.53 Bond strengths of Ir-C are thus
very sensitive to the chemical environment, a contention further
supported by the large range of Ir-C bond lengths quoted by
Rausch et al.49 The stronger trans influence of hydrido versus
chloro leads one to predict that the Ir-C bond will be weaker in
4a than in 4b; thus, reaction 2 will be more exothermic for X =
Cl than for X = H, as found. The calculations suggest that the
relative trans influence of hydride versus chloride is great enough
to lead to a large difference in reaction exothermicities. Detailed
studies of transition metal-carbon bond enthalpies as a function
of trans ligand would be of great value in this regard.
4. Transition State. It is generally agreed5,11,14,54 that low-
valent complexes activate C-H bonds through a triangular
M-H-C oxidative addition transition state (TS), 5a. The
oxidative addition TS is quite distinct from the four-centered,
a-bond metathesis TS, 5b.6 Stationary points (6) with the
geometry shown in 5a are found. That 6a and 6b are transition
states at this level of theory is confirmed by calculation of the
(50) Stoutland, P. O.; Bergman, R. G.; Nolan, S. P.; Hoff, C. D. Polyhedron
1988, 7, 1429.
(51) Crabtree'" has proposed that C-H activation by 16-electron
Cp*M(PR3) (M = Ir, Rh) species will have greater thermodynamic driving
force versus that by 14-electron (X)Ir(PR3)2, because the former d8 complex
is forced into a nonplanar arrangement by Cp*.
(52) Simoes, J. A. M.; Beauchamp, J. L. Chem. Rev. 1990, 90, 629.
(53) Blake, D. M.; Winkelman, A.; Chung, Y. L. Inorg. Chem. 1975, 14,
(54) Rest et al. have proposed that the 16-electron C-H-activating species
produced upon photolysis of CpIr(CO)2 is not CpIr(CO) but rather
"3-Cpir(CO)2. Oxidative addition is still the preferred pathway for C-H
activation. Rest, A. J.; Whitewell, I.; Graham, W. A. G.; Hopkins, J. K.;
McMaster, A. D. J. Chem. Soc., Chem. Commun. 1984, 824.
A Methane C-H Oxidative Addition Trajectory
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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/m1/4/: accessed October 23, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT College of Arts and Sciences.