Calculation of a Methane C-H Oxidative Addition Trajectory: Comparison to Experiment and Methane Activation by High-Valent Complexes Page: 341
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A Methane C-H Oxidative Addition Trajectory
Computational studies of oxidative addition (and the reverse,
reductive elimination) have shed considerable light on mechanistic
details.2b Seminal among the contributions are the studies of
Saillard and Hoffmann'3 as well as Low and Goddard.'4 Koga
and Morokuma'5 have studied methane activation by Rh(Cl)-
(PH3)2 to form Rh(Cl)(H)(Me)(PH3)2. Density functional
methods have been used by Ziegler et al.16 to look at oxidative
addition of methane to 16-electron CpM(L) (M = Ir, Rh; L =
CO, PH3) species. Extended Hiickel theory has been used to
probe activation of ethylene C-H bonds by CpIr(PH3) and
As part of a continuing focus on transition metal bonding and
reactivity,'8 a study of the important C-H activation reaction,
eq 2, was undertaken. For the present, the focus is on C-H
Ir(PH3)2(X) + CH3-H - Ir(PH3)2(X)(CH3)(H) (2)
activation by 14-electron complexes of the form Ir(PH3)2(X),
where X = H and C1.11,12 In previous workisa the concentration
has been on high-valent C-H-activating systems operating through
u-bond metathesis. In order to come to a more global under-
standing of the C-H activation problem, we instituted research
on oxidative addition by 14-electron systems. This work also
constitutes further testing of the coupling of promising new
technologies (like parallel computing'9) and methods (such as
effective core potentials20) as answers to the challenges of
computational transition metal (TM) and lanthanide (Ln)
Calculations employ the quantum chemistry program GAMESS.'9
One important difference between this and previous work'8 is that the
GAMESS code has now been made to run in parallel on a variety of
platforms.19 For this research the iPSC/860 supercomputer (128 nodes
with 8Mb of memory per node) at Oak Ridge National Laboratories is
utilized for parallel computations. Calculations with correlated wave
functions employ the vectorized version of GAMESS and were run on
the Cray Y-MP/464 at the National Center for Supercomputer
Applications. The effective core potentials (ECPs) and valence basis
sets of Stevens et al.a are used for all heavy atoms, and the -31 IG basis
set is used for H. ECPs replace the innermost core orbitals for the TMs
and all core orbitals for main-group (MG) elements. Thus, the ns, np,
nd, (n + l)s, and (n + 1)p are treated explicitly for the d-block; for the
main-group, ns and np are treated explicitly. In the standard imple-
mentation, TM valence basis sets are quadruple- and triple-c for the sp
and d shells, respectively, while main-group elements have a double-"
valence basis. Basis sets for heavy, main-group elements are augmented
with a d polarization function in our work. Transition metal ECPs are
generated from all-electron, Dirac-Fock calculations and thus include
Darwin and mass velocity effects, while spin-orbit coupling is averaged
out in potential generation.20 No degradation in accuracy has been
found upon descending a transition metal triad toward heavier members
(for which relativistic effects will be most important).
(13) Hoffmann, R.; Saillard, J. Y. J. Am. Chem. Soc. 1984, 106, 2006.
(14) Low, J. J.; Goddard, W. A. J. Am. Chem. Soc. 1986, 108, 6115 and
(15) Koga, N.; Morokuma, K. J. Chem. Phys. 1990, 94, 5454.
(16) Ziegler, T. L.; Tschinke, V.; Fan, L.; Becke, A. D. J. Am. Chem. Soc.
1989, 111, 9177.
(17) (a) Stoutland, P. O.; Bergman, R. G. J. Am. Chem. Soc. 1985, 107,
4581. (b) Silvestre, J.; Calhorda, M. J.; Hoffmann, R.; Stoutland, P. O.;
Bergman, R. G. Organometallics 1986, 5, 1841.
(18) (a) Cundari, T. R.J. Am. Chem.Soc. 1992,114,10557. (b) Cundari,
T. R.; Gordon, M. S. J. Am. Chem. Soc. 1993, 115, 4210. (c) Cundari, T.
R. J. Am. Chem. Soc. 1992, 114, 7879. (d) Cundari, T. R. Int. J. Quantum
Chem., Proc. 1992 Sanibel Symp. 1992, 26, 793.
(19) (a) Schmidt, M. W.; Baldridge, K. K.; Boatz, J. A.; Jensen, J. H.;
Koseki, S.; Gordon, M. S.; Nguyen, K. A.; Windus, T. L.; Elbert, S. T.
GAMESS. QCPE Bulletin, 1990, 10, 52. (b) Schmidt, M. W.; Baldridge,
K. K.; Boatz, J. A.; Jensen, J. H.; Koseki, S.; Matsunaga, N.; Gordon, M. S.;
Nguyen, K. A.; Su, S.; Windus, T. L.; Elbert, S. T.; Montgomery, J.; Dupuis,
M. J. Comput. Chem. 1993, 14, 1347.
(20) (a) Krauss, M.; Stevens, W. J.; Basch, H.; Jasien, P. G. Can. J. Chem.
1992, 70, 612. (b) Stevens, W. J.; Cundari, T. R. J. Chem. Phys. 1993, 98,
J. Am. Chem. Soc., Vol. 116, No. 1, 1994 341
Geometries are optimized at the restricted Hartree Fock (RHF) level
for closed-shell singlets. Previous work'8 has shown that bond lengths
and angles for TM complexes (involving complexes in a variety of
geometries and oxidation states and of metals from the entire transition
series) are predicted to within 1-3% of experiment using the present
Vibrational frequencies are calculated at stationary points to identify
them as minima (zero imaginary frequencies) or transition states (one
imaginary frequency). Plotting the imaginary frequencies is used to assess
which TS connects which reactants and products. In some cases the
intrinsic reaction coordinate (IRC) is followed using the steepest descent
algorithm of Ishida et al.,21a with the stabilization method described by
Schmidt et al.21b
Although geometries are accurately predicted at the RHF level,
energetics will typically be poor if correlation is ignored. For species
described well at the RHF level, the correlation contribution can be treated
as a perturbation of the RHF energy and calculated using Moller-Plesset
second-order perturbation theory (MP2).22 Koga and Morokuma'5 used
a similar scheme in their work, as have Krogh-Jespersen et al. in studies
of oxidative addition.23 A simple RHF geometry/MP2 energy scheme
yielded good agreement with experimental data for high-valent C-H-
activating systems, both in terms of absolute numbers and trends.1sa-c
Results and Discussion
1. Reactants. The geometry of methane is well-known.24
Reactant complexes are three-coordinate Ir(X)(PH3)2 complexes
(X = H, Cl), models of the putative 14-electron, C-H-activating
species in dehydrogenation catalysts.81,12a,b As in previous
computational work'13,5- 7 we have focused on the lowest energy,
singlet surface. Geometries for bis(phosphino)Ir' chloride and
bis(phosphino)IrI hydride minima (C2 symmetry) are shown in
1. As expected for a d8 ML3 complex, the geometry is T-shaped.25
P-Ir-P - 174
C1-Jr-P = ST
Ir-P = 236A
CI-Ir-P = 88'
The cationic, five-coordinate complex trans-[Ir(PPh3)2(CO)3]-
[HSO4] has Ir-P bond lengths of 2.364(3) and 2.36393) A.26 In
"five-coordinate" [Ir(H)2(8-methylquinoline)(PPh3)2] [BF4] the
trans phosphines have Ir-P bond lengths of 2.314(3) and 2.312(3)
A; there is evidence for an agostic interaction in this complex
between the metal and a C-H on the methyl group in 8-meth-
ylquinoline.27 For [Ir(PPh3)2(CO)(8-methylquinoline)] [PF6] (an
Ir' formal oxidation state and coordination number of four), Ir-P
= 2.323(2) and 2.335(2) A.28 For six-coordinate Ir(P-i-
Pr3)2(CO)(H)(vinyl)(Cl),12d Ir-Cl = 2.479(2) A and Ir-P =
2.362(1) and 2.371(1) A for the trans phosphines; the Cl is trans
(21) (a) Ishida, K.; Morokuma, K.; Komornicki, A. J. Chem. Phys. 1977,
66, 2153. (b) Schmidt, M. W.; Gordon, M. S.; Dupuis, M. J. Am. Chem. Soc.
1985, 107, 2585. (c) Garrett, B. C.; Redmon, M. J.; Steckler, R.; Truhlar,
D. G.; Baldridge, K. K.; Bartol, D.; Schmidt, M. W.; Gordon, M. S. J. Phys.
Chem. 1989, 93, 2888.
(22) Moller, C.; Plesset, M. S. Phys. Rev. 1934, 46, 618.
(23) Abu-Hasanayn, F.; Krogh-Jespersen, K.; Goldman, A. S. Inorg. Chem.
1992, 32, 495 and personal communication.
(24) Pople, J. A.; Hehre, W. J.; Radom, L.; Schleyer, P. v. R. Ab-Initio
Molecular Orbital Theory; Wiley: New York, 1986.
(25) Albright, T. A.; Burdett, J. K.; Whangbo, M. H. Orbital Interaction
in Chemistry; Wiley: New York, 1985.
(26) Randall, S. L.; Thompson, J. S.; Buttrey, L. A.; Ziller, J. W.; Churchill,
M. R.; Atwood, J. D. Organometallics 1991, 10, 683.
(27) Crabtree, R. H.; Holt, E. M.; Lavin, M.; Morehouse, S. M. Inorg.
Chem. 1985, 24, 1986.
(28) Neve, F.; Ghedini, M.; De Munno, G.; Crispini, A. Organometallics
1991, 10, 1143.
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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/m1/2/: accessed April 24, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT College of Arts and Sciences.