Polycyano Derivatives of some Organic Tri- and Hexacyclic Molecules Are Powerful Super- and Hyperacids in the Gas Phase and DMSO: Computational Study by DFT Approach

Abstract

B3LYP/6-311+G(2df,p)//B3LYP/6-31+G(d) calculations convincingly show that tricyclic organic compounds 1 and 2 offer scaffolds suitable for tailoring powerful neutral organic acids. Their cyanation at all C(sp(2))-H positions provide superacids as evidenced by their enthalpies of deprotonation ΔHacid(1bCN) = 257.2 and ΔHacid(2bCN) = 250.9 kcal mol(-1), which are close to the threshold of hyperacidity of 245 kcal mol(-1). On the other hand hexacyclic system 3 cyanated at all possible substitution sites along the perimeter except the acidic C(sp(3))-H position provides a true hyperacid with ΔHacid(3aCN) = 242.8 kcal mol(-1). The reason behind the dramatic acidifying effect is a vigorous anionic resonance effect in the corresponding conjugate bases assisted by a cooperative substituent effect of the CN groups. It is convincingly shown that compounds 1 and 2 represent rigid antiaromatic quasi-[12]annulene and aromatic quasi-[14]annulene par excellence, respectively, conforming to Hückel's (4n + 2)π electron count rule in the bridged polycyclic systems. Calculations of the pKa values in DMSO show that the polycyano derivatives are powerful superacids in solutions too.

A density functional theory study on the superacidity of sulfuric, fluorosulfuric, and triflic acid derivatives with two cyclopentadiene rings: ion pairs formation in the gas phase

“…Analogously, we expected a higher acidity for their cyclopentadiene derivatives (systems 4 and 5 ) compared with 3 . The O─H acids 4a and 5a with ∆ H acid of 225.5 and 229.9 kcal mol –1 are among the strongest acids studied in this work and can be classified as hyperacids, since their deprotonation enthalpies fall below the proton affinity of the paradigmatic proton sponge 1,8‐bis(dimethylamino)naphthalene PA (DMAN) = 245 kcal mol –1 proposed as the hyperacidity threshold . However, these tautomers are not stable.…”

“…[42] 3 | RESULTS AND DISCUSSION Figure 1 shows organosulfuric acid derivatives with two cyclopentadiene rings and compares their relative stabilities with respect to a possible prototropic tautomerism in neutral systems that was demonstrated as significantly affecting the acidity of similar compounds. [21][22][23][24][25]31] In 2 and 3, hydrogen atoms of the five-membered rings have been replaced by the electron-withdrawing F and CN moieties to enhance the resonance affect and promote the acidity. While the mentioned fluorine substitution did not alter the number of possible tautomers relative to 1, the introduction of the cyano group led to a larger number of tautomers due to the formation of the keteneimine C═C═NH functionality, which is usually more stable than the HC (sp 3 )─CN structural alternative.…”

“…While the mentioned fluorine substitution did not alter the number of possible tautomers relative to 1, the introduction of the cyano group led to a larger number of tautomers due to the formation of the keteneimine C═C═NH functionality, which is usually more stable than the HC (sp 3 )─CN structural alternative. [21][22][23][24][25]31] Isomers 1a, 2a, and 3a have two ─OH groups are O─H acids, in which their doubly bonded oxygen atoms (S═O) are both substituted by two cyclopentadiene rings. A comparison of relative energies in Figure 1 reveals that tautomers 1a, 2a, and 3a with two ─OH groups are, as a rule, the least stable of all compounds.…”

Superacidity of P(OH)3 and SO(OH)2 derivatives of cyclopentadiene and vinylcyclopentadiene in the gas phase: A computational DFT analysis

“…Cyclopentadiene has been used in designing a wide range of neutral organic acids . After deprotonation, the excess negative charge is delocalized over all atoms in the five‐membered ring followed by the formation of a 6 π‐electrons aromatic cyclopentadienyl fragment, C 5 H 5 − .…”

Superhalogen and Superacid

“…The idea of symmetric cage surrounded with electronwithdrawing groups is barely new. [2,[6][7][8][9][10][11][12] For example, one of the strongest currently known Brønsted acids is fluorocarborane acid H[CHB 11 F 11 ]. [13] Its anion is formed by a carborane cage surrounded with fluorine atoms and shows superhalogenic behavior.…”