Calculation of a Methane C-H Oxidative Addition Trajectory: Comparison to Experiment and Methane Activation by High-Valent Complexes Page: 342
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342 J. Am. Chem. Soc., Vol. 116, No. 1, 1994
to a vinyl; hydrocarbyls are known to have large trans influences.29
Ir-Cl bond lengths in Ir(H)(C1)(PPh3)2(3-C3H4Ph),3 Ir(H)2-
(C)(Ph2P(CH2)2CH=CH(CH2)2PPh2),31 and Ir(Cl)(Ph2P-
(CH2)2CH--CH(CH2)2PPh2)31 are 2.549(2), 2.510(5), and
2.391(2) A, respectively. The Ir-Cl bond length in lb is somewhat
shorter than that in experimental models, as one might anticipate
given its low coordination number and lack of a trans ligand.
Teller and Bau32 quote an average Ir-H bond length of 1.67 t
0.07 A for Ir-H bonds with a range 1.57(7)-1.82(17) A. The
sum of the single-bond metallic radius of Ir (1.27 A) and the
covalent radius of H (0.37 A or half the equilibrium bond length
of H2) is 1.64 A.33 Three iridium hydride complexes have been
characterized by neutron diffraction: Ir-H = 1.603(9) A (average
Ir-H in Ir(H)s(P-i-Pr3)2), 1.59 A ([H4IrPt(PEt3)4]+), and
1.594(3) A (H2Ir(SiEt3)2Cp*),34 all in good agreement with the
calculated value of Ir-H in Ia. Thus, calculated Ir-X bonds in
model reactants (1) are in accord with experimental data. Good
agreement between theory and experiment has been found in
previous works8 using ECPs and valence basis sets; it is satisfying
to find this trend continued for a new family of low-valent TM
2. Initial Interaction of Substrate and Reactant Complex.
Substantial interest has focused on loosely bound, alkane adducts
of TM complexes and their relevance to alkane activation.3-43
For silanes, s2-SiH adducts are stable enough to permit structural
analysis.41 Parkin and Bercaw42 have measured inverse kinetic
isotope effects for methane elimination from Cp*2W(CH3)(H)
and deuterated isotopomers, as have Gould and Heinekey43 for
[Cp2Re(H)(CH3)]+. Both findings have been interpreted in terms
of adduct formation. Bergman et al.35 have used time-resolved
infrared spectroscopy (TRIS) to probe alkane adducts of
CpRh(CO). Rayner et al. have performed gas-phase TRIS studies
of alkane interaction with W(CO)5.36 Burkey37 has used
photoacoustic calorimetry to measure enthalpies of adduct
formation between hydrocarbons and coordinatively unsaturated
species such as M(CO)5 (M = Cr, Mo, W). Thus, there is a
growing body of kinetic, spectroscopic, and thermochemical data
supporting the presence of alkane adducts of TM complexes. The
structure of these transients and their function, if any, in the
crucial bond scission step are less clear.
On the basis of TRIS studies, an adduct-binding enthalpy
(AHadd) of 10 kcal mol-1 has been estimated for alkanes and
CpRh(CO).35 Burkey has measured AHadd values as high as 17
kcal mol-' (Mo(CO)s + heptane) in solution.37 Gas-phase AHadd
values for Cr(CO)5 with alkanes and fluorinated alkanes range
(29) Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry, 5th ed.;
Wiley: New York, 1991; p 1300.
(30) Tulip, T. H.; Ibers, J. A. J. Am. Chem. Soc. 1978, 100, 3252. Tulip,
T. H.; Ibers, J. A. J. Am. Chem. Soc. 1979, 101, 4201.
(31) Clark, G. R.; Mazid, M. A.; Russell, D. R.; Clark, D. R.; Jones, A.
J. J. Organomet. Chem. 1979, 166, 109.
(32) Bau, R.; Teller, R. G. Stuct. Bondong (Berlin) 1981, 44, 1.
(33) Pauling, L. The Nature of the Chemical Bond, 3rd ed.; Cornell Press:
Ithaca, NY, 1961.
(34) Garlaschelli, L.; Kahn, S. I.; Bau, R.; Longoni, G.; Koetzle, T. F. J.
Am. Chem. Soc. 1985, 107, 7212.
(35) (a) Weiller, B. H.; Wasserman, E. P.; Bergman, R. G.; Moore, C. B.;
Pimentel, G. C. J. Am. Chem. Soc. 1989, 111, 8288. (b) Wasserman, E. P.;
Moore, C. B.; Bergman, R. G. Science 1992, 255, 315.
(36) Brown, C. E.; Ishikawa, Y.; Hackett, P. A.; Rayner, D. M. J. Am.
Chem. Soc. 1990, 112, 2530.
(37) Burkey, T. J. In Energetics of Organometallic Species; Simoes, J. A.
M., Ed.; Kluwer: Dordrecht, The Netherlands, 1992.
(38) Schaller, C. P.; Bennett, J. L.; Wolczanski, P. T. Abstracts ofPapers,
of the American Chemical Society, Washington, DC, Fall 1992; American
Chemical Society: Washington, DC, 1992; INOR 453.
139) Green, M. L. H.; Brookhart, M.; Wong, L. K. Prog. Inorg. Chem.
1988, 36, i.
(40) A computational study of methane adducts of high-valent, C-H-
activating complexes has been conducted. Cundari, T. R. Organometallics
1993, 12, 1998.
(41) A review of a-bonded X-H complexes can be found in: Crabtree, R.
H.; Hamilton, D. G. Adv. Organomet. Chem. 1988, 28, 299.
(42) Parkin, G.; Bercaw, J. E. Organometallics 1989, 8, 1172.
(43) Gould, G. L.; Heinekey, D. M. J. Am. Chem. Soc. 1989, 111, 5502.
Memphis State University
Ir(PH3)2(H) + CH4
Figure 1. Imaginary frequency for bifurcated (H)(PH3)21r.--H2CH2
from <5 (methane) to 12.2 kcal mol- (EtF).36 Ziegler et al.
calculate a linear geometry for Cp(L)M*..HCH3 (M = Rh, Ir;
L = PH3, CO); binding energies are = 13 kcal mol-l for CpIr(L)
and 6-7 kcal mol-1 for CpRh(L).16 Blomberg et al.44 find a
bifuracted Rh...H2CH2 adduct (binding energy = 8.5 kcal mol-')
lower in energy than a linear adduct by 3.4 kcal mol-'. Koga and
Morokuma'5 calculate a bifurcated (PH3)2(C1)Rh...H2CH2 ad-
duct as lowest in energy with many conformers close in energy.
Calculated binding energies for [(Cl)(PH3)2Rh]...[CH4] range
from -8.3 to -17.7 kcal mol-1 depending on level of theory.'
Note that the calculated quantities are binding energies and not
binding enthalpies (i.e., no correction for zero-point energy and
a temperature above absolute zero).45 However, on the basis of
our experience, binding enthalpies are higher (less negative) by
only a few kcal mol-' than binding energies.
Initial optimization of bifurcated Ir(PH3)2(H)...H2CH2 (2)
was carried out in C2 symmetry. Calculation of the energy Hessian
reveals a single imaginary frequency (112i cm-') for 2, corre-
sponding to the motion in Figure 1. Distorting along the imaginary
mode, followed by geometry optimization (in C, symmetry), yields
a minimum (3a) lower than 2 by only 0.1 kcal mol-1 at the RHF
( Ir H-C = 116
Ir-P = 2.31 P-r-P= 17136
P-lr-P- 169" PIr-P= 171
level. Thus, as Koga and Morokuma have clearly shown,15
different adduct geometries can be very close in energy. Reducing
optimization convergence criteria by an order of magnitude yields
no significant change in geometry (<1% changes in bond lengths
and bond angles) or energy (<0.1 mhartree). It seems likely that
at ambient temperatures a methane adduct will be fluxional,
sampling different coordination modes. Simple replacement of
(44) Blomberg, M. R. A.; Siegbahn, P. E. M.; Svensson, M. J. Phys. Chem.
1991, 95, 4313.
(45) Calculation of zero-point energies and the correction from 0 K to
some finite temperature is done with the equations on p 259 of ref 24 using
RHF vibrational frequencies.
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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/m1/3/: accessed May 23, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT College of Arts and Sciences.