Methyl groups as unconventional Lewis bases in chalcogen bonding†

It is well known that methyl groups can behave as Lewis acids in non-covalent interactions. However, when methyl carbon atoms bonded with less electronegative atoms, the electron accumulation on the methyl groups makes it possible for them to act as Lewis bases. In the present work, we have theoretically investigated the chalcogen bonding interactions between methyl groups and a series of chalcogenadiazoles. The results show that the nucleophilicity of methyl groups in BeMe₂ and MgMe₂ enables them to connect with the σ-hole on the chalcogen atoms and form stable chalcogen-bonded complexes. The interaction energies range from −1.5 to −6.6 kcal mol⁻¹ and are closely related to the nature of the chalcogen atom. It can be found that the orbital interactions, primarily involving electron transfer from σ_C–M (M = Be, Mg) bonding orbitals to empty (Ch = S, Se, Te, Po) antibonding orbitals, play a crucial role in the attractive interactions. Further analysis reveals that the chalcogen bonding can be characterized as weak, closed-shell interactions, with significant contributions from electrostatics, dispersion, and polarization.