A new type of C+⋯Hδ−(C=) bond in adducts of vinyl carbocations with alkenes

By X-ray diffraction analysis and IR spectroscopy, it was established here that vinyl carbocations C₃H₅⁺/C₄H₇⁺ with carborane counterion CHB₁₁Cl₁₁⁻ form stable monosolvates C₃H₅⁺⋅C₃H₆/C₄H₇⁺⋅C₄H₈ with molecules of alkenes C₃H₆/C₄H₈. They contain molecular group =C⁺⋯H^δ−–C^δ+= with a new type of bond formed by the H atom of the H–C= group of the alkene with the C atom of the C⁺=C group of the carbocation. The short C⁺----C^δ+ distance, equal to 2.44 Å, is typical of that of X----X in proton disolvates (L₂H⁺) with an quasi-symmetrical X–H⁺⋯X moiety (where X = O or N) of basic molecule L. The nature of the discovered bond differs from that of the classic H–bond by an distribution of electron density: the electron–excessive H^δ− atom from the (=)C–H group of the alkene is attached to the C⁺ atom of the carbocation, on which the positive charge is predominantly concentrated. Therefore, it can be called an inverse hydrogen bond.