B. Biswas et al., C-H bond activation of benzene and methane by M(eta(2)-O2CH)(2) (M = Pd orPt). A theoretical study, ORGANOMETAL, 19(19), 2000, pp. 3895-3908
C-H bond activations of benzene and methane by M(eta(2)-O2CH)(2) (M = Pd or
Pt) are theoretically investigated with density functional theory (DFT), M
P2-MP4(SDQ), and CCSD(T) methods. The C-H bond activation of benzene takes
place with activation energies (E-a) of 16.1 and 21.2 kcal/mol and reaction
energies (Delta E) of -16.5 and -25.8 kcal/mol for M = Pd and Pt, respecti
vely, to afford M(eta(2)-O2CH)(CsH5)(eta(1)-HCOOH), where MP4(SDQ) values a
re given hereafter and a negative Delta E value represents that the reactio
n is exothermic. The C-H bond activation of methane proceeds with E-a value
s of 21.5 and 17.3 kcal/mol and Delta E values of -8.3 and -13.3 kcal/mol f
or M = Pd and Pt, respectively, to afford M(eta(2)-O2CH)(CH3)(eta(1)-HCOOH)
. However, C-H bond activations of benzene and methane by Pd(PH3)2 need a l
arge E-a value, and these reactions are significantly endothermic: E-a = 26
.5 kcal/mol and Delta E = 22.1 kcal/mol for benzene and E-a = 34.7 kcal/mol
and Delta E = 31.5 kcal/mol for methane. Also, the C-H bond activation of
methane by Pt(PH3)2 needs a large E-a value (28.1 kcal/mol) with moderate e
ndothermicity (Delta E = 7.0 kcal/mol), while the C-H bond activation of be
nzene by Pt(PH3)(2) occurs with a moderate E-a value (17.3 kcal/mol) and a
negative Delta E value(-3.9 kcal/mol). From these results, the following co
nclusions are presented: (1) Pd(eta(2)-O2CH)(2) can perform easily the C-H
bond activations of benzene and methane but Pd(PH3)(2) cannot. This is beca
use the formate ligand assists the C-H bond activation through formation of
a strong O-H bond. (2) Pt(eta(2)-O2CH)(2) more easily performs the C-H bon
d activation of methane but much less easily the C-H bond activation of ben
zene than Pd(eta(2)-O2CH)(2), because the intermediate, Pt(II)-benzene comp
lex, is too stable. (3) Benzene more easily undergoes C-H bond activation t
han does methane. The higher reactivity of benzene is interpreted in terms
of M-C6H5 and M-CH3 bond energies and the bonding interaction of benzene pi
and pi* orbitals with M d orbitals. Analysis Df electron distribution impl
icitly indicates that the C-H bond activation by M(eta(2)-O2CH)(2) is chara
cterized to be heterolytic C-H bond fission, while the C-H bond activation
by M(PH3)(2) is characterized to be homolytic C- H bond fission.