Methane coordination cations

Varied Methane Cations The methane molecular ion (methane radical cation, CI b -), the parent ion in MS, and the methane dication (CH42 ) are of great significance and have been the subject of experimental and theoretical studies. Its five-coordinate planar structure was suggested in the early calculations by Olah and Klopman, and indeed shown to be correct in subsequent more advanced studies. 

Bis(A7-azaindolyl)methane-coordinated complexes have been prepared and fully characterized, and addition of [H(OEt2)2] to the dimethylplatinum(ii) complex 200 forms the cationic methylplatinum complex with the ligand, whereas the reaction in benzene forms the cationic phcnylplatinum complex 201 via C-H bond activation of the solvent by the cation (Equation (52)). 

In this system both the reaction rate and selectivity can be direcdy attributed to the relative rates of CH activation of the CH bonds present in the reaction system by the poorly solvated [XHg] catalyst. Calculations show that the poorly solvated species, [XHg], reacts with a -29 kcal/mol barrier via a transition state in which methane is coordinated to a two-coordinate cationic mercury species, [XHgCH4], that loses a proton to the solvent to generate CHsHgX. This correlates well with the experimental activation barrier of -28 kcal/mol and the direct observation of [HgCH3] as an intermediate in the catalytic cycle. Both calculations and experimental investigations show that the [HgCH3] species readily reacts to generate methanol and the reduced catalyst, Hg2(II), is rapidly reoxidized by hot sulfuric acid. 

The mechanism for the reaction catalyzed by cationic palladium complexes (Scheme 24) differs from that proposed for early transition metal complexes, as well as from that suggested for the reaction shown in Eq. 17. For this catalyst system, the alkene substrate inserts into a Pd - Si bond a rather than a Pd-H bond [63]. Hydrosilylation of methylpalladium complex 100 then provides methane and palladium silyl species 112 (Scheme 24). Complex 112 coordinates to and inserts into the least substituted olefin regioselectively and irreversibly to provide 113 after coordination of the second alkene. Insertion into the second alkene through a boat-like transition state leads to trans cyclopentane 114, and o-bond metathesis (or oxidative addition/reductive elimination) leads to the observed trans stereochemistry of product 101a with regeneration of 112 [69]. 

The CH cation 1, protonated methane, is the parent of hypercoordinated carbocations containing a five coordinated carbon atom. It is elusive in solution and has not been observed by NMR spectroscopy but gas-phase infrared investigations have shown its fluxional structure which has been proven by ab initio molecular dynamic simulation.18... 

Teppen et al. [89] have used a flexible model for clay minerals that allows full movement of the M-O-M bonds in the clay structure, where M represents Si, Al, or other cations in the octahedral sheet. This model was used in MD simulations of interactions of hydrated clay minerals with trichloroethene [90, 91]. The simulations suggest that at least three distinct mechanisms coexist for trichloroethene sorption on clay minerals [90], The most stable interactions of trichloroethene with clay surfaces are by full molecular contact, coplanar with the basal surface. The second type more reversible, less stable is adsorption through single-atom contact between one chlorine atom and the surface. In a third mechanism, trichloroethene interacts with the first water layer and does not interact with clay surface directly. Using MC and MD simulation the structure and dynamics of methane in hydrated Na-smectite were studied [92], Methane particles are solvated by approximately 12-13 water molecules, with six oxygen atoms from the clay surface completing the coordination shell. 

This observation was explained in terms of the unusual electronic properties of pentaco-ordinated cations (Kollmar and Smith, 1970). The CHs species is best described as a hydrogen molecule with an abnormally long bond, 0-94 A, to which a deformed trivalent carbonium (carbenium) ion is coordinated. The energy of CHs was calculated to be 47 kcal mol-1 lower than that of H2 + CH3 (gas phase value) and two of the C—H bonds only of half the strength of a C—H bond in methane. This electronic structure, perhaps best being described as closely related to protonated hydrogen, is probably the reason for the reverse order of oxidation potentials observed. 

---