The π-Tetrel Bond and its Influence on Hydrogen Bonding and Proton Transfer

The positive region that lies above the plane of F₂ TO (T=C and Si) interacts with malondialdehyde (MDA), which contains an intramolecular H-bond. The T atom of F₂ TO can lie either in the MDA molecular plane, forming a T⋅⋅⋅O tetrel bond, or F₂ TO can stack directly above MDA in a parallel arrangement. The former structure is more stable than the latter, and in either case, F₂ SiO engages in a much stronger interaction than does F₂ CO, reaching nearly 200 kJ mol^-1 . The π-tetrel bond strengthens/weakens the MDA H-bond when the bond is formed to the hydroxyl/carbonyl group of MDA, and causes an accompanying inhibition/promotion of proton transfer within this H-bond; this effect is stronger for F₂ SiO. These same aspects can be tuned by substituents placed on any of the C atoms of MDA, although their effects are not fully correlated with the electron-withdrawing or electron-releasing properties of the substituent. A new type of π-π tetrel bond occurs when the π-hole on the T atom of F₂ TO approaches the middle carbon atom of MDA from above, and a similar configuration is also found between F₂ TO and benzene. Evidence for extensive C⋅⋅⋅C π-π tetrel bonding in crystal materials is presented.