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

Citation
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
Citations number
49
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Chemistry & Analysis",Chemistry
Journal title
JOURNAL OF THE AMERICAN CHEMICAL SOCIETY
ISSN journal
0002-7863 → ACNP
Volume
122
Issue
9
Year of publication
2000
Pages
2041 - 2052
Database
ISI
SICI code
0002-7863(20000308)122:9<2041:HCOMTM>2.0.ZU;2-8
Abstract
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)](+).