Gas-Phase Characterization of Hypervalent Carbon Compounds Bearing 7-6-7-Ring Skeleton: Penta- versus Tetra-Coordinate Isomers

In this study, we afford explicit characterizations of the electronic and geometrical structures of recently reported hypervalent penta-coordinate carbon compounds by using gas-phase characterization techniques: photodissociation spectroscopy (PDS) and ion mobility-mass spectrometry (IM-MS). In particular for a compound with moderately electron-donating ligands, bearing p-methylthiophenyl substituents, the coexistence of tetra- and penta-coordinate isomers is confirmed, consistent with solution characterizations. It is in sharp contrast to the exclusive tetra-coordinate form (with normal valence of the central carbon atom) in the single crystal. This suggests that a non-polar environment makes the penta-coordinate structure thermodynamically most stable. This delicate difference between the tetra- and penta-coordinate structures, which depends on the environment, is a close reflection of the lower activation barrier of the S(N)2 reaction found in neutral solvent or gas-phase reactions.