Systematic study of the substitution effect on the tetrel bond between 1,4-diazabicyclo[2.2.2]octane and THX.

A tetrel bond was characterized in the complexes of 1,4-diazabicyclo[2.2.2]octane (DABCO) with THX (T = C, Si, Ge; X= -Me, -H, -OH, -NH, -F, -Cl, -Br, -I, -CN, -NO). DABCO engages in a weak tetrel bond with CHX but a stronger one with SiHX and GeHX. SiHX is favorable to bind with DABCO relative to GeHX, inconsistent with the magnitude of the σ-hole on the tetrel atom. The methyl group in the tetrel donor weakens the tetrel bond but an enhancing effect is found for the other substituents, particularly -NO. The substitution effect is also related to the nature of the tetrel atom. The halogen substitution from F to I has a weakening effect in the CHX complex but an enhancing effect in the SiHX complex and a negligible effect in the GeHX complex. The above abnormal results found in these complexes can be partly attributed to the charge transfer from the lone pair on the nitrogen atom of DABCO into the anti-bonding orbital σ*(T-X) of THX. The stability of both SiHX and GeHX complexes is primarily controlled by electrostatic interactions and polarization.