IR-Spectroscopic and X‑ray-Structural Study of Vinyl-Type Carbocations in Their Carborane Salts

The butylene carbocation in its salts with anions CHB11F11– and CHB11Cl11– forms isomers CH2C+–CH2–CH3 (I) and
CH3–C+CH–CH3 (II), which were characterized here by infrared (IR)
spectroscopy and X-ray diffraction analysis. The strongest influence
on the structure of the cations is exerted by geometric ordering of
their anionic environment. In the crystalline phase, the cations uniformly
interact with neighboring anions, and the CC bond is located
in the middle part of the cations forming a −CHC+– moiety with the highest positive charge on it and
the lowest νCC frequency, at 1490 cm–1. In the amorphous phase with a disordered anionic environment of
the cations, contact ion pairs Anion–···CH2C+–CH2–CH3 form predominantly, with terminal localization of the CC
bond through which the contact occurs. The positive charge is slightly
extinguished by the anion, and the CC stretch frequency is
higher by ∼100 cm–1. The replacement of the
hydrogen atom in cations I/II by a Cl atom giving rise
to cations CH2C+–CHCl–CH3 and CH3–C+CCl–CH3 means that the donation of electron density from the Cl atom
quenches the positive charge on the C+C bond more
strongly, and the CC stretch frequency increases so much that
it even exceeds that of neutral alkene analogues by 35–65 cm–1. An explanation is given for the finding that upon
stabilization of the vinyl cations by polyatomic substituents such
as silylium (SiMe3) and t-Bu groups, the stretching CC
frequency approaches the triple-bond frequency. Namely, the scattering
of a positive charge on these substituents enhances their donor properties
so much that the electron density on the CC bond with a weakened
charge becomes much higher than that of neutral alkenes.