Synthesis and Characterization of Hypervalent Pentacoordinate Carbon Compounds Bearing a 7-6-7-Ring Skeleton
Graphical Abstract
Electronic tuning of ground-state structures for tetra- and penta-coordinate carbon species has been achieved by changing electronic properties of the equatorial aryl groups on the central carbon, confirmed by a correlation between Hammett para-substituent constant and solid-state structures of a series of cationic species based on a 7-6-7 tricyclic framework.
Abstract
To stabilize SN2 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 (σp+) correlates well with the bonding in these compounds. The methylthio-derivative with intermediate Hammett substituent constants (p-MeS; σp+=−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.
Conflict of interest
The authors declare no conflict of interest.
Open Research
Data Availability Statement
The data that support the findings of this study are available in the supplementary material of this article.
