Neutral and Ionic Hydrogen Bonding in Schiff Bases

Abstract

Low-temperature, high-resolution X-ray studies of charge distributions in the three Schiff bases, the dianil of 2-hydroxy-5-methylisophthaldehyde, 3,5-dinitro-N-salicylidenoethylamine and 3-nitro-N-salicylidenocyclohexylamine, have been carried out. These structures exhibit interesting weak interactions, including two extreme cases of intramolecular hydrogen bonds that are ionic N(+)-H...O- and neutral O-H...N in nature. These two types of hydrogen bond reflect differences in geometrical parameters and electron density distribution. At the level of geometry, the neutral O-H...N hydrogen bond is accompanied by an increase in the length of the C(1)-O(1) bond, opening of the ipso-C(1) angle, elongation of the aromatic C-C bonds, shortening of the C(7)-N(2) bond and increased length of the C(1)-C(7) bond, relative to the ionic hydrogen bond type. According to the geometrical and critical point parameters, the neutral O-H...N hydrogen bond seems to be stronger than the ionic ones. There are also differences between charge density parameters of the aromatic rings consistent with the neutral hydrogen bond being stronger than the ionic ones, with a concomitant reduction in the aromaticity of the ring. Compounds with the ionic hydrogen bonds show a larger double-bond character in the C-O bond than appears in the compound containing a neutral hydrogen bond; this suggests that the electronic structure of the former pair of compounds includes a contribution from a zwitterionic canonical form. Furthermore, in the case of ionic hydrogen bonds, the corresponding interaction lines appear to be curved in the vicinity of the hydrogen atoms. In the 3-nitro-N-salicylidenocyclohexylamine crystal there exists, in addition to the intramolecular hydrogen bond, a pair of intermolecular O...H interactions in a centrosymmetric dimer unit.

Continua of Interactions between Pairs of Atoms in Molecular Crystals

“…In the present work, we aim to analyse relationships between parameters of electron density at the bond critical points in the previously studied DMAN [15] and its complexes [1,16,17] and a series of already studied [18] and some new Schiff bases N,N'-bis(2,5-dihydroxybenzylidene)-1,2-diaminobenzene (S4) and tris(3-formyl-5-methyl-2-salicylidenaminoethyl)amine (S5, see Figure 1). S5 forms molecular crystals consisting of two tripodal moieties which form different types of intramolecular hydrogen bonding with O-H···N hydrogen bonds in all three wings of the first moiety and [O À ···H-N + ] hydrogen bonding in the other moiety.…”

A Salicylbisimine Cage Compound with High Surface Area and Selective CO2/CH4 Adsorption

“…In the crystalline state two different types of imine bonds can be distinguished: one forms a hydrogen-bond-stabilized sixmembered ring, whereas the other does not. [24] The hydroxy groups are directed into the cavity of the cage, forming a slightly distorted octahedron with an average edge length of d(O···O) = 10.53 and a volume of roughly 550 3 . The cages self-assemble through p-p interactions between the phenolic arene rings, which are orientated coplanar to each other at a distance of 3.65 , which is a typical distance for p-p interactions (Figure 2).…”

Eine Salicylbisimin‐Käfigverbindung mit großer spezifischer Oberfläche und selektiver CO2/CH4‐Adsorption

“…Im kristallinen Zustand können zwei Arten von Iminbindungen unterschieden werden; eine bildet einen sechsgliedrigen Ring, der durch eine Wasserstoffbrücke geschlossen wird, die andere nicht. [24] Die Hydroxygruppen sind ins Innere des Hohlraums gerichtet und bilden ein leicht verzerrtes Oktaeder mit einer mittleren Kantenlänge d(O···O) = 10.53 und einem Volumen von ca. 550 3 .…”