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.