Gaseous [H3C−Cl−Cl]+ Ions from the Reaction of Methane with Cl3+, the First Example of a New Dihalogenation Process: Formation and Characterization of CH3Cl2+ Isomers by Experimental and Theoretical Methods

Abstract

The structure, stability, and reactivity of the [C,H 3 ,Cl 2 ] ions from the reaction of Cl 3 with CH 4 were studied by structurally diagnostic mass spectrometric techniques and by computational methods. The ionic products were characterized as protonated dichloromethane [ClH 2 CÀClH] (1), the more stable isomer, that behaves exclusively as a Brùnsted acid, and the hitherto unknown [H 3 CÀClÀCl] ion (2), a chlorinating and methylating cation. Formation of 2 is the first example of a new class of electrophilic substitution, in which a single H atom is replaced by the diatomic [ClÀCl] dichlorinium ion. In addition, the proton affinity of dichloromethane was determined by experimental and theoretical methods to be 150.2 AE 2 and 151 kcal mol À1 , respectively.

H-Coupled Electron Transfer in Alkane C−H Activations with Halogen Electrophiles

“…For another simple model electrophile Cl 3 + (Scheme ) which serves as a source of “Cl + ” in C−H functionalizations, pathway B via TS5 is marginally more favorable (Scheme ). Because of the system size, the frequencies could only be computed at DFT, but the MP2 energies show analogous trends.…”

“… We now address the relevance of the H-coupled ET activations for other alkanes. Experimental data are available for the reactions of HCl 2 + and Cl 3 + with methane; , these lead to mixtures of [H 3 C−Cl−H] + and [H 3 C−Cl−Cl] + . The kinetically controlled formation of [H 3 C−Cl−Cl] + cannot be explained in terms of an electrophilic C−H insertion of Cl 3 + .…”

“…Mechanistic computational modeling is equally difficult as only intermediates − rather than transition structures can generally be located for these often highly exothermic reactions. Only a limited number of transition structures for the hydrogen exchange reactions for surface-adsorbed alkanes were found to date. − Some computations are available for the reactions of alkanes with relatively stable electrophiles such as H 3 O 2 + , NO + , ,, carbocations, ,,, and positively charged halogens. − However, there is no decisive evidence for 22,23 or against 24 an electrophilic 3c-2e activation mechanism, involving the attack on the C−H bonds, that is, via transition structures with triangular C−H−E moieties. In addition, our ,, and other − studies on hydrocarbon activations with electrophile-oxidizers show that electron-transfer (ET) processes cannot be ruled out and may actually be dominating.…”

Selective Alkane Transformations via Radicals and Radical Cations: Insights into the Activation Step from Experiment and Theory

“…Methane activation with strong electrophiles is one of the major achievements of superacid chemistry. , The original mechanistic rationalization is based on the direct attack of the electrophile on the C−H bonds. ,,, Such a mechanistic scenario, however, is difficult to verify computationally or to support experimentally, because the highly exothermic protonation of alkanes occurs without an activation barrier; only product distributions rather than transition structures can be analyzed. ,− The mechanisms for the methane activation with electrophiles which are more stable than the proton, were studied over the past decade for a number of reagents, e.g., carbocations, NO + , , H 3 O 2 + , , SO •+ , “F + ”, B, B 2 , BH 2 , Cl 3 + , Cl 2 H + , electrophilic Pt complexes, − MO n + species, − and bare metal ions. ,− The computational results show that the electrophilic attack is directed toward the atoms (C or H) rather than the C−H or C−C bonds; this is exemplified by the TS for the CH 4 activation with NO + 834 and FeO 2+ (Figure ) ) and the hydrocarbon moieties of the transition structures for methane activation with NO + and FeO + should be emphasized: the TSs resemble inner-sphere electron-transfer structures.…”