Homogeneous conversion of methane to methanol. 2. Catalytic activation of methane by cis- and trans-platin: A density functional study of the Shilov type reaction
K. Mylvaganam et al., Homogeneous conversion of methane to methanol. 2. Catalytic activation of methane by cis- and trans-platin: A density functional study of the Shilov type reaction, J AM CHEM S, 122(9), 2000, pp. 2041-2052
The C-H activation of methane catalyzed by cis- and trans-platin in aqueous
solution has been studied by density functional based computational method
s. By analogy with the Shilov reaction, the initial step is the replacement
of an ammonia ligand by methane, followed by the formation of a methyl com
plex and the elimination of a proton. The computations utilize the B3LYP hy
brid functionals, effective core potentials, and double-zeta to polarized d
ouble-zeta basis sets and include solvation effects by a dielectric continu
um method. In contrast with the Shilov reaction studied by Siegbahn and Cra
btree (J. Am. Chem. Sec. 1996, 118, 4442), in the platins the replacement o
f an ammonia ligand by methane is found to be effectively rate determining,
in that the energy barriers to C-H activation are comparable with those of
the initial substitution reaction, viz. similar to 34 and 44 kcal/mol;for
cis- and trans-platin, respectively. Several reaction pathways for C-H acti
vation and subsequent proton elimination were identified. For cis-platin th
e energy barriers associated with the oxidative addition and sigma-bond met
athesis type mechanisms were found to be comparable, while for trans-platin
oxidative addition is predicted to be strongly preferred over sigma-bond m
etathesis,which, interestingly,; also proceeds through a Pt(IV) methyl hydr
ide complex as reaction intermediate. In line with accepted ideas on trans
influence, the methyl and hydride ligands in the Pt(IV) complexes that aris
e in the oxidative addition;reactions were always found to be cis to each o
ther. On the basis of the population analyses on the Pt(TV) complexes it is
suggested that the Pt-H and Pt-CH3 bonds are best described as covalent bo
nds and, further, that the preference of the hydride and methyl anions to b
e cis to each other is a consequence of such covalent bonding. In light of
these findings, the energies of several methyl Pt(IV) hydride bisulfate com
plexes were also recalculated, with CH3 and H placed cis to each other. The
revised results provide evidence for the thermodynamic feasibility of oxid
ative addition of methane to catalysts such as [Pt(NH3)(2)(OSO3H)(2)] Or [P
t(NH3)(2)(OSO3H) (H2SO4)](+).