Stable CI3+ salts and attempts to prepare CHI2+ and CH2I+

Abstract

The room-temperature stable CI₃⁺ salts [CI₃]⁺[pftb]⁻1 and [CI₃]⁺[al-f-al]⁻2 ([pftb]⁻ = [Al(OC(CF₃)₃)₄]⁻; [al-f-al]⁻ = [((CF₃)₃CO)₃Al-F-Al(OC(CF₃)₃)₃]⁻) were prepared in quantitative yields from purified CI₄ and the corresponding silver aluminates with total exclusion of light (NMR, IR, UV-VIS, X-ray diffraction). The isolated CI₃⁺ cation is trigonal planar with a sum of ∠(I–C–I) = 360.0° (1) and 359.9° (2). Attempts to prepare CHI₂⁺ and CH₂I⁺ salts from CHI₃ or CH₂I₂/Ag[pftb] mixtures remained unsuccessful; the reaction with CH₂I₂ leads to the formation of the adduct [Ag(CH₂I₂)₃]⁺[pftb]⁻3, while for HCI₃, dismutation with formation of 1 as well as 3 was observed. All particles were also calculated at the MP2/TZVPP level to predict the vibrational and electronic spectra as well as to calculate the Gibbs free energies of all reactions (ΔG°, gas phase and CH₂Cl₂ solution). Quantum chemical calculations were also used to investigate the stability of the [pftb]⁻ anion against the electrophilic attack of the CX₃⁺ and CH_nX_3–n⁺ cations (X = F–I, n = 1–3). The strength of the Lewis acidity of these cations and of the isoelectronic boron halides BX₃ and BH_nX_3–n have been established on the basis of their fluoride ion affinities (FIAs). The FIAs of the carbon and the boron containing compounds show opposite trends, with fluorinated halomethyl cations being stronger acids than their heavier congeners but iodinated holoboranes being stronger acids than their lighter homologues.