Does hydrohalic acid HX (X = F, Cl) form true N-protonated twisted amide salts? Effects of anions on the ion-pair interactions and on the amide moiety in N-protonated tricyclic twisted amide salts

Abstract

The geometries of N-protonated tricyclic twisted amide salts (I–VI) and amide cations I⁺ have been optimized at the DFT-D3(BJ) level using density functionals PBE and PBE0. The optimized structure of I and II at the DFT/PBE-D3(BJ) level of theory is in excellent agreement with the experimental structure of Stoltz’s 2-quinuclidone tetrafluoroborate salt (Nature 2006, 441, 731) and Aubé‘s medium-bridged tricyclic N-protonated amide salt (J. Am. Chem. Soc., 2010, 132, 8836). In the studied N-protonated amide salts (I–IV), the [BF₄]⁻ and [RSO₃]⁻ anions interact with the cationic fragment [I]⁺ through N–H⋯F and N–H⋯O hydrogen bonds, respectively. The hydrohalic acids HX (X = F, Cl) interact with non-protonated amides with very weak N⋯H–X and X–H⋯O(CO) hydrogen bonds. The ion pair dissociation energies of amides [I⁺][X⁻] (I, X = BF₄⁻, II, X = OTs⁻, III, X = MeSO₃⁻, IV, X = HSO₃⁻) are in the range 8.2–12.7 kcal mol⁻¹ and in the range 6.2–10.6 kcal mol⁻¹ in water solvents. Charge analysis is consistent with the formulation of the salts of twisted amides (I–VI) as [I]^q+[X]^q− with q = 0.81 in I, 0.72 in II, 0.66 in III, 0.70 in IV, 0.39 in V, 0.45 in VI. Only 0.10e and 0.23e charge flows from the non-protonated tricyclic amide to HF and HCl, respectively. The nature of N–H⋯F, N–H⋯O, N⋯H–F and N⋯H–Cl bonds has also been investigated using spectral studies (infrared frequencies, ¹H NMR chemical shifts, ¹⁴N nuclear quadrupole coupling constants) to gain theoretical insights.