Electronic structure, geometry, and stability of organic cations, dications, and donor-acceptor complexes

Polyhalomethanes Combined with Lewis Acids in Alkane Chemistry

“…Polyhalomethyl cations are likely generated from polyhalomethanes in the presence of strong Lewis acids, as is suggested by the spectroscopy data for the polyhalomethane−SbF 5 systems in solution 122 and in the solid state, as well as by the established fact of aluminum halide-induced polyhalomethane ionization. , Both semiempirical and nonempirical calculations of the CCl 4 • n AlCl 3 and CBr 4 • n AlBr 3 ( n = 1−3) complexes and related CX 3 + , CHX 2 + , CHX 2+ , CX 2 2+ ions have been carried out. These studies indicated that the polyhalomethyl cations (X = Cl, Br, I, but not F), both free and incorporated in cationic CX 3 + Y - and dicationic CX 2 2+ Y 2 - (Y = AlBr 4 or Al 2 Br 7 ) complexes, are not carbenium ions but rather are halenium cations or cationic complexes containing positively charged halogen atoms directly bonded to the carbon, that is, X 2 CX + or X + CX + . , In other words, electron density transfer from the lone electron pairs of the halogen to the carbocation center ( Scheme ) is significant to the extent that a large positive charge is localized on the halogen atoms, with the C-atom being electroneutral or negatively charged. Similar results have been obtained by DFT and ab initio calculations. 122b,− Selected data on the charge distribution and bond lengths in the polyhalomethyl cations are presented in Tables and .…”

“…These halenium cations differ fundamentally from the classical carbenium (e.g., CF 3 + , CHF 2 + , CF 2 2+ ) and halonium ions 4 Calculated Atomic Mulliken Charges of CX 3 + , a CX 2 2+ , and Related Ions b q, auq, auspeciesXCspeciesXCH CF 3 + −0.04 ÷ −0.18 1.12 ÷ 1.55 HCF 2 + 0.07 0.54 0.31 CCl 3 + 0.34 ÷ 0.39 −0.02 ÷ −0.17 CBr 3 + 0.43 ÷ 0.51 −0.29 ÷ −0.53 HCCl 2 + 0.35 0.04 0.25 CI 3 + 0.51 ÷ 0.63 −0.52 ÷ −0.90 CF 2 2+ 0.47 1.06 HCBr 2 + 0.46 −0.17 0.25 CCl 2 2+ 0.94 0.12 CBr 2 2+ 1.04 −0.07 HCI 2 + 0.53 −0.29 0.23 CI 2 2+ 1.04 −0.08 a MP2/6-31G*, MP2/LANL2DZ 122b , MP2/VDZ+P, DFT, , SCF-RHF/6-31G, AM1 AM1 …”

“…As mentioned above (section 4.2), the CBr 3 + cation and the CBr 3 + AlBr 4 - cationic complexes generated from the CBr 4 •AlBr 3 systems contain strongly positively charged bromine atoms. The calculated structures of the CBr 3 + cation ( 79 ) and one of the two rotamers of CBr 4 •AlBr 3 ( 80 ) with the bridging Br atoms carrying large positive charges , are shown in Scheme .

115

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“…In addition to the mechanism shown in Scheme , another pathway for the reaction of propane with the CBr 3 + cation (reductive alkylation; Scheme ) has been demonstrated . The reductive alkylation pathway results in complex 85 , in which the Br + is coordinated to two geminal Br atoms of dibromoisobutane in a bidentate fashion.…”

“…The formation of solvate 92 in Scheme is the first step of the reaction of methane with CBr 3 + AlBr 4 - . TS 93 for the conversion of 92 to the final complex CH 3 Br•AlBr 3 •CHBr 3 ( 94 ) exhibits a cyclic planar structure. ,, One might view 93 as a transition state that is electrostatically stabilized by charge transfer. Although well-known in organic and organometallic chemistry, , such transitions states had no precedent in early reports on electrophilic reactions of alkanes.…”