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.