Substituent effects on the structure of hexacoordinate carbon bearing two thioxanthene ligands

In order to elucidate the electronic nature of our recently reported first hexacoordinated carbon (12-C-6), density functional theory (DFT) calculations of sulfide precursor, sulfone derivative, and S -F derivative were carried out and compared with those of the reported S -Me hexacoordinated carbon. Computations on the hexacoordinated carbon, indicating that four attractive C–O interactions with the central hexacoordinate carbon atom exist, also revealed that the interactions consist of two different types of three-center four-electron bonds, which can be regarded as electron donation by the lone pairs of the oxygen atoms to the empty low-lying π*-orbitals of the allene. The optimized structures of the sulfide, sulfone, S -F, and the original S -Me suggested that the introduction of electron-withdrawing groups at the sulfur atoms would make the C–O attractive interactions stronger by a larger contribution of the carbon dication resonance structure. Thus, allene compounds (sulfide, sulfone, sulfonium) with two different thioxanthene ligands (one with 1,8-dimethoxy groups as in the S -Me compound and the other with 1,8-diphenoxy groups) were synthesized to confirm the predicted substituent effects on the C(central)–O interactions. Electron-withdrawing substituents at the sulfur atoms were found to give rise to strong C(central)–O attractive interactions; the average values of the four C–O distances were smaller as the electron-withdrawing ability of the sulfur atoms rose. Additionally, C(central)···OMe distances were shorter than the corresponding C(central)···OPh distances, reflecting the higher electron-donating ability of the oxygen atoms at these 1,8-positions of the thioxanthene skeleton.