Synthesis and Characterization of Hypervalent Pentacoordinate Carbon Compounds Bearing a 7-6-7-Ring Skeleton

To stabilize S(N)2 transition state-like penta-coordinate carbon species, triaryl-substituted cationic carbon compounds bearing a moderately flexible 7-6-7-ring skeleton with sulfur donors were synthesized and characterized. Electronic effects of para substituents (R=Cl, F, H, CH3, SMe, OMe) of the two equatorial aryl groups bound to the cationic central carbon were investigated systematically along with a planar bidentate thioxanthene derivative. X-ray analysis on their solid-state structures showed that the parent (R=H), chloro-, fluoro- and methyl-derivatives were tetracoordinate carbon (sulfonium) structures, while the p-MeO and thioxanthenyl system were pentacoordinate carbocation structures. The Hammett substituent constants for the para substituents (sigma(+)(p)) correlates well with the bonding in these compounds. The methylthio-derivative with intermediate Hammett substituent constants (p-MeS; sigma(+)(p)=-0.60) showed a tetracooridnate solid-state structure, though solution UV-Vis properties suggested the presence of a penta-coordinate structure. These findings amount to the first unambiguous solution evidence of the hypervalent apical 3c-4e interactions in pentacoordinate carbon compounds.

Automated summary

In this study, triaryl-substituted cationic carbon compounds with a moderately flexible 7-6-7-ring skeleton and sulfur donors were synthesized and characterized to stabilize S(N)2 transition state-like penta-coordinate carbon species. The electronic effects of para substituents (R = Cl, F, H, CH3, SMe, OMe) and a planar bidentate thioxanthene derivative were systematically investigated and shown to have significant impact on the bonding in these compounds.