Systematic study of the substitution effect on the tetrel bond between 1,4-diazabicyclo[2.2.2]octane and TH 3 X.

A tetrel bond was characterized in the complexes of 1,4-diazabicyclo[2.2.2]octane (DABCO) with TH 3 X (T = C, Si, Ge; X= -Me, -H, -OH, -NH 2 , -F, -Cl, -Br, -I, -CN, -NO 2 ). DABCO engages in a weak tetrel bond with CH 3 X but a stronger one with SiH 3 X and GeH 3 X. SiH 3 X is favorable to bind with DABCO relative to GeH 3 X, 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 2 . 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 CH 3 X complex but an enhancing effect in the SiH 3 X complex and a negligible effect in the GeH 3 X 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 TH 3 X. The stability of both SiH 3 X and GeH 3 X complexes is primarily controlled by electrostatic interactions and polarization.