Schiff Bases from TRIS and ortho‐Hydroxyarenecarbaldehydes: Structures and Tautomeric Equilibria in the Solid State and in Solution

Abstract

Three Schiff bases generated by conventional condensation of a simple aminopolyol (TRIS) and 5‐bromo‐ and 5‐iodosalicylaldehyde, and 2‐hydroxy‐1‐naphthaldehyde, have been fully characterized, both in solution and in the solid state. This study provides a complete analysis of imine–enamine equilibria, and sheds light onto the mechanism of hydrogen transfer, which has been controversial in the literature. Low‐temperature X‐ray diffraction accompanied by electron‐density maps have further been complemented by theoretical studies at the B3LYP/6‐31G* and M06‐2X/6‐311++G** levels. To ascertain the influence of crystal packing on tautomeric stability, the lattice has also been simulated by computation. This protocol involves the assessment of a supramolecular cluster around a core tautomer possessing either imine or enamine structures. Such an analysis, in full agreement with solid‐state data, reveals the greater stability of zwitterionic structures for the salicyl derivatives. In contrast, these substances show preferential imine forms in solution, whereas the naphthyl‐based compound exhibits a prevalent keto‐enamine structure in all cases.

Multifunctional imine-POSS as uncommon 3D nanobuilding blocks for supramolecular hybrid materials: synthesis, structural characterization, and properties

“…36,37 There is usually a rapid equilibrium in solution between imine and enamine (or zwitterionic) tautomers. Understanding proton tautomerization is of fundamental importance, not only in evaluating the structural and/or electronic parameters involved in this behavior, but also for its potential application in the design of molecular switches in which Schiff bases can serve as valuable sensors.…”

Removal of copper(II) ions from Aqueous Media by Chemically Modified MCM‐41 with N‐(3‐(trimethoxysilyl)propyl)ethylenediamine and Its 4‐hydroxysalicylidene Schiff‐base

“…Thus, in the pH range of 5–7, the adsorption capacity of Cu(II) ions onto MCM‐41‐NH 2 and MCM‐41‐N‐Hdhba increased with increasing the pH . This feature can be discussed on the bases of tatomerization of phenol‐imine {enol (Scheme a)}, enamine {keto (Scheme b)}, and zwitterion (Scheme c) forms ; that is, the deprotonated O(2) – and lone pair of NH 2 groups become ready to bind Cu(II) ions. The maximum adsorption capacity of Cu(II) ions onto MCM‐41‐NH 2 and MCM‐41‐N‐Hdhba sorbents was achieved at pH 5–7 and pH 6 was applied in the subsequent experiments.…”

Resonance‐Assisted Hydrogen Bonding as a Driving Force in Synthesis and a Synthon in the Design of Materials

“…These compounds can display interesting properties, such as photochromism,t hermochromism, coordination ability,a nd molecular recognition, and continue to attract wide interest because of their applications. [193][194][195][196][197][198][199][200] The structures of keto and zwitterionic forms can be regarded as extreme canonical forms of ar eso-nance hybrid, although experimentally derived bond lengths reflect the predominance of one form. [189][190][191][192] The presence of an ortho-OH group is as tructuralr equirement for the photochromism of these compounds and the proposed H-transfer mechanism.Areversible tautomeric equilibrium, OÀH···NÐNÀH···O, assisted by electrostatic differences betweent he oxygen and nitrogen atoms of the salicylideneimine fragment, promotes the delocalization of the aromatic p-electron system leadingt oaq uinoid form, which is also related to its canonical zwitterionic form, supported by CAHB (Scheme 19).…”

“…[193][194][195][196][197][198][199][200] The structures of keto and zwitterionic forms can be regarded as extreme canonical forms of ar eso-nance hybrid, although experimentally derived bond lengths reflect the predominance of one form. [199][200][201][202] Electron delocalization in these systems, which are susceptible to proton transfer,r epresents au seful concept to rationalize the structure and reactivity of av ariety of Schiff bases. In fact, most Schiff bases derived from salicylaldehydes and alkyl/arylamines adopt imine structures (enol), [189-192, 199, 201] but some derivatives have been reported with enamine skeletons (keto and zwitterionic forms)i nt he solid state.…”