Crystal Packing Modulation of the Strength of Resonance-Assisted Hydrogen Bonds and the Role of Resonance-Assisted Pseudoring Stacking in Geminal Amido Esters: Study Based on Crystallography and Theoretical Calculations

Abstract

A detailed experimental and theoretical investigation of a series of substituted geminal amido esters (ethyl (2E)-3-(arylamino)-2-(arylcarbamoyl)­prop-2-enoate, AME-1–8) leading to the identification of a unique angularly fused pseudotricyclic (S(6),S(6),S(6)) ring system stabilized by an intramolecular resonance-assisted hydrogen bond (RAHB) and a non-RAHB are presented in addition to weak intermolecular interactions. An analysis of X-ray and theoretical models reveals that the strength of the intramolecular RAHB (N1–H1 N ···O1) varies in a wide range (6.9–11.4 kcal mol–1) due to crystal-packing constraints arising from different aromatic ring substitutions. However, the effect is less significant and the strength differs only in a narrow range (8.2–9.9 kcal mol–1) in the case of non-RAHB. The downfield shift (δ ∼12.3) observed for the N–Haniline signal in 1H NMR spectra of AME-1–8 is due to the presence of intramolecular RAHB. A PIXEL energy analysis suggests that the molecular dimer formed by stacking of RAHB pseudorings is found to be strong (E tot = −14.4 to −17.9 kcal mol–1), and this dimer forms the basic motif in most of the structures reported herein. A detailed analysis of the isostructurality suggests that the basic motif exists in most of the structural combinations. The weak intermolecular C–H···O, C–H···Cl, and C–H···π interactions play a vital role in the stabilization of these crystal structures, as evaluated by PIXEL and Bader’s quantum theory of atoms in molecules approach (QTAIM). A lattice energy analysis suggests that the Coulombic contribution and total lattice energies are higher in the para-substituted compounds (AME-2, AME-5, and AME-8) in comparison to the other isomeric compounds. Further, the crystal packing of these compounds is analyzed on the basis of the energy frameworks. It shows that most of the crystals show similar 3D topologies, suggesting that these compounds may have similar mechanical behavior.

Synthesis, crystal structure, Hirshfeld surface analysis, and DFT studies of 2-bromo-4,6-bis(dibromoacetyl)resorcinol

“…In this S(6) ring motif stacked with another S(6) ring of the neighbouring molecule. Similar kinds of stacking interactions were observed in geminal amidoesters(Venkatesan et al 2021b(Venkatesan et al , 2021a) and other compounds(Blagojević and Zarić 2015). Apart from these two stacking pairs, the molecular pairs M 3 (E tot : -6.6 kcal mol -1 ) and M 4 (E tot : -4.6 kcal mol -1 ) are stabilized by C10-H10•••Br2 and C8-H8•••O4 = C9 interaction respectively.…”

Significant Stacking Interactions of Resonance-Assisted Hydrogen-Bridged (RAHB) Rings at Large Horizontal Displacements

“…Resonance-assisted hydrogen-bridged (RAHB) rings are structures formed by intramolecular hydrogen bonds, where hydrogen donor and acceptor atoms are connected by a π-conjugated system (Figure a). They exhibit synergistic effects between hydrogen bonding and resonance, which makes them very interesting from theoretical and experimental points of view − and also because of the possible application in synthesis − and material design. − Stacking interactions of resonance-assisted hydrogen-bridged (RAHB) rings were observed , in crystal structures from the Cambridge Structural Database (CSD) . Namely, 91% of mutual contacts between RAHB rings and 59% of contacts between RAHB and C 6 -aromatic rings are stacking contacts, while significant interaction strength was confirmed by ab initio calculations. , The strength of interactions depends on the composition of the RAHB ring and can be up to −4.7 kcal/mol for RAHB/RAHB interactions (Figure b) and up to −3.7 kcal/mol for RAHB/C 6 -aromatic interactions (Figure c) …”