Exploring the chemistry of free CI3+: reactions with the weak Lewis bases PX3 (X = Cl–I), AsI3 and Et2O

Abstract

What is the preferred coordination site of CI₃⁺? Recent computational work suggested the iodine atoms of the Lewis acid CI₃⁺ to be more electrophilic than the classically expected carbon atom, e.g. the complex with water is of type I₂C–I⋯OH₂⁺ and not the classically expected I₃C–OH₂⁺. If this structure is correct, one may also anticipate reactions of CI₃⁺ as an I⁺ donor. Thus, we were interested in investigating the chemistry of CI₃⁺ in the room-temperature stable salt [CI₃]⁺[pftb]⁻ ([pftb]⁻ = [Al(OC(CF₃)₃)₄]⁻) with weak nucleophiles that i) mimic water (OEt₂) or ii) are electronically deactivated weak nucleophiles (PX₃, X = Cl–I; AsI₃). One question was: is it possible to obtain iodine-coordinated Lewis acid–base adducts of the CI₃⁺ cation? With Et₂O as a base, the cation behaves as a strong Lewis acid and cleaves the ether to give I₃C–OEt, C₂H₄ and [H(Et₂O)₂]⁺. By contrast PX₃ and AsI₃ coordinate to the CI₃⁺ cations and the adducts have classical, carbon-bound ethane-like structures, as proven by X-ray single-crystal diffraction, IR, UV-Vis and NMR spectroscopy. From variable temperature ¹³C NMR studies, it followed for the I₃C–AsI₃⁺ salt that the equilibrium between CI₃⁺ and AsI₃ is reversible and temperature dependent in solution. The I₃C–PI₃⁺ salt decomposes at room temperature giving PI₄⁺ and C₂I₄, likely through an iodine coordinated I₂C–I⋯PI₃⁺ intermediate. Thus CI₃⁺ may also act as an I⁺ donor. All reactions are in agreement with ab initio quantum chemical calculations at the MP2/TZVPP level and assignments of experimental spectra were aided by quantum chemistry.