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

TL;DR

This paper reports a new type of chemical bond formed between vinyl carbocations and alkenes, resembling an inverse hydrogen bond.

Contribution

The discovery of a novel inverse hydrogen bond in adducts of vinyl carbocations with alkenes.

Findings

A new bond type was identified with a C+----Cδ+ distance of 2.44 Å.

The bond involves an electron-rich Hδ− atom from an alkene attaching to a carbocation's C+ atom.

This bond differs from classic hydrogen bonds in electron density distribution.

Abstract

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+⋯Hδ−–Cδ+= 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+----Cδ+ 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 Hδ− 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.

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