Versatility of the Cyano Group in Intermolecular Interactions

Abstract

Several cyano groups are added to an alkane, alkene, and alkyne group so as to construct a Lewis acid molecule with a positive region of electrostatic potential in the area adjoining these substituents. Although each individual cyano group produces only a weak π-hole, when two or more such groups are properly situated, they can pool their π-holes into one much more intense positive region that is located midway between them. A NH3 base is attracted to this site, where it forms a strong noncovalent bond to the Lewis acid, amounting to as much as 13.6 kcal/mol. The precise nature of the bonding varies a bit from one complex to the next but typically contains a tetrel bond to the C atoms of the cyano groups or the C atoms of the linkage connecting the C≡N substituents. The placement of the cyano groups on a cyclic system like cyclopropane or cyclobutane has a mild weakening effect upon the binding. Although F is comparable to C≡N in terms of electron-withdrawing power, the replacement of cyano by F substituents substantially weakens the binding with NH3.

Computational Evaluation of Me2TCCP as Lewis Acid

“…This is in line with an earlier report on the detrimental effect of −C≡N to −F mutation, despite a similar electron withdrawing effect of both −C≡N and −F. [ 18 , 27 ] Most informative is the analysis of the C 2 v symmetric [Me 2 TCCP⋅⋅⋅Cl − ] adduct which allows to distinguish the orbital interaction within the irreducible representations. The main contribution of −7.4 kcal mol −1 originates from the sigma donation (0.08 e − ) of the Cl − sp‐hybrid orbital into the accepting orbital of Me 2 TCCP which is a hybrid of a cyclopropane σ* orbital and four π* orbitals of the cyano groups.…”

“…This is in line with a recent report about the synergistic effect that multiple cyano substituents on alkanes have on the Lewis acidity of the molecule, and that sp 3 and sp carbon centres can work in concert. [27] …”

“…This is in line with a recent report about the synergistic effect that multiple cyano substituents on alkanes have on the Lewis acidity of the molecule, and that sp 3 and sp carbon centres can work in concert. [27] While the interaction energies reported in Table 1 and Table 2 give a clear picture of the relative stabilities of the various adducts in the gas phase, such calculations ignore solvation and entropy considerations. As such, these computations are of limited value to predict actual binding in nongaseous states such as solutions, the interior of proteins, or within cell-membranes.…”

Intermolecular noncovalent interactions with carbon in solution

“…1c and d, shows the positive blue region at the face of the C 1 -C 3 bond and at the face of the methyl groups, which can give rise to the formation of carbon bonds. Similar tetrel-bonded molecular complexes have been reported by Scheiner et al 47 If the nucleophile approaches the nitrile carbon atoms through the Bürgi-Dunitz trajectory, it can give rise to n / p* interaction. We chose pyridine-N-oxide (PNO) as the nucleophile to meet the experimental criteria.…”

Organic binary charge-transfer compounds of 2,2′ : 6′,2′′ : 6′′,6-trioxotriphenylamine and a pyrene-annulated azaacene as donors

“…The donors constitute extended, planar p-systems and hence should be well-suited for pstacking, [23][24][25] but also allow for interdonor hydrogen bonding involving the O or N heteroatoms and for C^N or C-F p-hole tetrel interactions. [26][27][28][29][30] TOTA is a nonplanar, electron-rich molecule with a low half-wave potential E 1/2 0/+ of 110 mV (in CH 2 Cl 2 /NBu 4 PF 6 against the ferrocene/ferrocenium standard couple FcH/FcH + = 0 mV) for its rst one-electron oxidation. In contrast, PAA shows its rst oxidation potential at E ox 0/+ = 1205 mV under the same experimental conditions and is hence a much weaker donor (Table 1, vide infra).…”