Supramolecular Stabilization of Metastable Tautomers in Solution and the Solid State

Evaluation of Concomitant Halogen and Pnictogen Bonds in Cocrystals of Imines Derived from 2-Nitrobenzaldehyde and 4-Haloaniline

Three imines have been prepared by condensation of 2-nitrobenzaldehyde and 4-haloanilines (halo = Cl, Br, and I) with functionalities that enabled them to act as both halogen and pnictogen bond donors; however, both interactions were found to be absent in the solid state. The prepared imines were further cocrystallized with 1,3-diiodotetrafluorobenzene and 1,3,5-triiodotetrafluorobenzene as halogen bond donors. Six novel cocrystals were prepared by means of liquid-assisted mechanochemical synthesis and by crystallization from solution. All six cocrystals were of 1:1 stoichiometry and comprised a N···I halogen bond between an iodine atom of the perhalogenated halogen bond donor and the imino nitrogen atom of the imine acting as an acceptor. Additionally, in all six cocrystals, the imine molecules were interconnected by NO2···NO2 pnictogen bonding interactions. Computational analysis has shown that the NO2···NO2 exhibits bond critical point electron densities in the region (4.897-8.306) × 10-3 e Å-3 and interaction energies of 23.6-27.7 kJ mol-1, whereas the N···I halogen bonds generally have higher critical point electron densities ((1.795-1.937) × 10-2 e Å-3), but the corresponding total interaction energies are lower (19.4-20.4 kJ mol-1). Statistical analysis of the appearance of NO2···NO2 contacts concomitantly with halogen or hydrogen bonds seems to indicate that there is a positive correlation between the presence of NO2···NO2 pnictogen bonding interactions and other directional interactions in crystal structures.

Incorporation of Photo- and Thermoresponsive N-Salicylidene Aniline Derivatives into Cobalt and Zinc Layered Hydroxides

In search of new multifunctional hybrid materials and in order to investigate the influence of chemical modification on the possible synergy between properties, the carboxylate and sulfonate derivatives of photo- and thermochromic N-salicylidene aniline were successfully inserted into Co(II)- and Zn(II)-based layered simple hydroxides, resulting in four novel hybrids: Co-N-Sali-COO, Co-N-Sali-SO3, Zn-N-Sali-COO, and Zn-N-Sali-SO3. All synthesized hybrids adopt a double organic layered configuration, which prevents the cis-trans photoisomerization ability of N-Sali-R molecules in the hybrids. However, the Zn hybrids exhibit fluorescence upon exposure to UV light due to the excited-state intramolecular proton transfer (ESIPT) mechanism. The thermally stimulated keto-enol tautomerization of N-salicylidene aniline in the hybrids was related with the changes in interlamellar spacings observed by temperature-dependent PXRD. This tautomerization process was prominently evident in the Co-N-Sali-SO3 hybrid (about 11% increase in d-spacing upon decreasing the temperature to -180 °C). Finally, the Co-N-Sali-R hybrids exhibit the typical magnetic behavior associated with Co(II)-based LSHs (ferrimagnetic ordering at TN = 6.8 and 7.7 K for Co-N-Sali-COO and Co-N-Sali-SO3, respectively). This work offers insights into isomerization in LSHs and the ESIPT mechanism's potential in new luminescent materials and prospects for designing new multifunctional materials.

TG-DSC and TG-FTIR Studies of Annelated Triazinylacetic Acid Ethyl Esters-Potential Anticancer Agents

To avoid problems associated with the storage and processing of newly developed potential medicines, there is a need to carry out thermal studies in the preclinical phase of drug development. The thermal behaviour and decomposition pathway of a whole novel class of patented potential molecular pharmaceutics, i.e., ethyl 2-[4-oxo-8-(R-phenyl)-4,6,7,8-tetrahydroimidazo[2,1-c][1,2,4]triazin-3-yl]acetates (1-6) were reported for the first time in inert and oxidative atmospheres. The experiments were conducted with the use of simultaneous thermogravimetry/differential scanning calorimetry (TG-DSC) and simultaneous thermogravimetry coupled with Fourier transform infrared spectroscopy (TG-FTIR). The decomposition pathways of compounds 1-6 were found to be different under oxidative and inert conditions. It was proven that the investigated molecules reveal higher thermal stability under a synthetic air atmosphere than under a nitrogen atmosphere, and their decomposition is preceded by the melting process. Among all the investigated compounds, only the meta-chloro derivative (4) was found to exhibit interesting polymorphic behaviour at a low heating rate (10 °C min^-1). It was proven that the oxidative decomposition process of the studied molecules proceeds in three overlapping stages accompanied by strong exothermic effects. Additionally, it was concluded that the title compounds were stable up to a temperature of 195-216 °C in an atmosphere of synthetic air, and their thermal stability decreased in the order of R at the benzene ring: 4-CH₃ > 3,4-Cl₂ > 4-Cl > H > 2-OCH₃ > 3-Cl.

Photo-and thermoresponsive N-salicylideneaniline derivatives: solid-state studies and structural aspects

Eight photo-and/or thermoresponsive salicylideneanilines were obtained by mechanosynthesis under solvent-free conditions and stabilized by external stimuli for their isolations in solid-state. Switching properties of these materials were investigated by various analytical techniques and spectroscopic data.

The crystal structure and characterization of a co-product in the synthesis of a Schiff base give evidence of its zwitterionic nature in the solid state

The zwitterionic form of a Schiff base in the solid state was found by the structural characterization of a coproduct and a mechanistic proposal of its formation.

Solid-state separation of hypoxanthine tautomers through a doping strategy

The solid-state separation of hypoxanthine tautomers was realized by a doping strategy. The doping forms of hypoxanthine in HAmG, AG β and AG α are N7-hypoxanthine, and in GM and dehydrated-GM are N9-hypoxanthine.

Tautomeric Equilibrium of an Asymmetric β-Diketone in Halogen-Bonded Cocrystals with Perfluorinated Iodobenzenes

In order to study the effect of halogen bond on tautomerism in β-diketones in the solid-state, we have prepared a series of cocrystals derived from an asymmetric β-diketone, benzoyl-4-pyridoylmethane (b4pm), as halogen bond acceptor and perfluorinated iodobenzenes: iodopentaflourobenzene (ipfb), 1,2-, 1,3- and 1,4-diiodotetraflorobenzene (12tfib, 13tfib and 14tfib) and 1,3,5-triiodo-2,4,6-trifluorobenzene (135titfb). All five cocrystals are assembled by I···N halogen bonds involving pyridyl nitrogen and iodoperfluorobenzene iodine resulting in 1:1 (four compounds) or 1:2 (one compound) cocrystal stoichiometry. Tautomer of b4pm in which hydrogen atom is adjacent to the pyridyl fragment was found to be more stable in vacuo than tautomer with a benzoyl hydroxyl group. This tautomer is also found to be dominant in the majority of crystal structures, somewhat more abundantly in crystal structures of cocrystals in which additional I···O halogen bond with the benzoyl oxygen has been established. Attempts have also been made to prepare an equivalent series of cocrystals using a closely related asymmetric β-diketone, benzoyl-3-pyridoylmethane (b3pm); however, all attempts were unsuccessful, which is attributed to more effective crystal packing of b3pm isomer compared to b4pm, which reduced the probability of cocrystal formation.

Determination of tautomeric preference of fenobam in solution by high-resolution NMR spectroscopy

In this work, tautomeric preference of fenobam in solution was investigated by homonuclear and heteronuclear solution nuclear magnetic resonance (NMR) spectroscopy. ¹ H-¹ H nuclear Overhauser effect spectroscopy (NOESY) spectrum revealed that fenobam in liquid state exists exclusively in one of the two possible tautomeric structures, which was confirmed by ¹ H-¹³ C HSQC and heteronuclear multiple bond correlation (HMBC) spectra. Moreover, difference between the two tautomeric structures was studied by theoretical calculations, which further proved the result obtained by the NMR experiments.

Microwave assisted synthesis, experimental and theoretical characterization and antibacterial activity screening of novel azomethine compounds containing thiophene and aminophenol functionality

Eight new azomethine compounds (3a-3h) containing thiophene and aminophenol functionality were synthesized in excellent yields by using conventional heating and microwave assisted synthesis methods. The structures of newly synthesized compounds were characterized by spectroscopic techniques such as UV-Vis, FTIR, ¹H and ¹³C NMR and elemental analysis. UV-Vis and ¹H NMR results show that all compounds prefer the phenol-imine tautomer form in solvent media. The chemical structure of 3a, 3b and 3g was also confirmed by single crystal X-ray diffraction method. The molecular conformations of 3a, 3b and 3g are stabilized by an N⁺-H⋯O^- type intramolecular hydrogen bond in zwitterionic form in the crystalline solid state. The optimized molecular structures, ¹H and ¹³C NMR chemical shift values, UV-Vis spectroscopic parameters, HOMO-LUMO energies, Mulliken (MPA) and natural (NBO) atomic charges, Natural bond orbitals (NBO), molecular electrostatic potential (MEP) maps and solvent accessible surfaces (SASs) for 3a-3h were calculated by using DFT/B3LYP/6-311G(2d,p) approach. The theoretical spectroscopic features obtained by DFT calculations show a very good agreement with the experimental data. In addition, the synthesized compounds (3a-3h) were screened for their antibacterial activities against Bacillus cereus (NRRL-B3711), Bacillus subtilis (NRRL-B4378), Escherichia coli (NRRL B-3008), Staphylococcus aureus (ATCC-6538) and Salmonella typhimurium (ATCC-13311). The results show that investigated compounds have either moderately active, slightly active or inactive among the tested microorganisms. 3a exhibited the stronger antibacterial activity against all test bacteria than other compounds. It also has been observed that compounds with relatively low HOMO-LUMO energy gaps exhibit better antibacterial effects.

Stoichiometry of adamantylamine–trinitrophloroglucinol salts controlled by solvate formation

The stoichiometry of salts formed by adamantylamine and trinitrophloroglucinol depends on the incorporation of the crystallisation solvent in the crystal structure.

The effect of solution environment and the electrostatic factor on the crystallisation of desmotropes of irbesartan

The experimental conditions necessary for stabilising irbesartan (IBS) tautomers in solution and selectively obtaining the desmotropic crystal forms are presented herein. 1H and 2H tautomers were stabilized in specific solution conditions and the 2H-tetrazole⋯imidazole interaction was confirmed by solution-state NMR. The results showed that highly polar and polarisable solvents (higher values of the electrostatic factor (EF)) lead to the crystallisation of IBS form B. Furthermore, the variations of pH in methanol, in turn, determined the crystallisation of desmotropes A and/or B.

The experimental conditions necessary for stabilising irbesartan (IBS) tautomers in solution and selectively obtaining the desmotropic crystal forms are presented herein.

Enantiopure Schiff bases of amino acid phenylhydrazides: impact of the hydrazide function on their structures and properties

New chiral Schiff bases have been synthesized and characterized with an important contribution from quantum chemical calculations at the DFT level of theory. They have been evaluated for their physical and biological properties.

Halogen and Hydrogen Bonding between (N-Halogeno)-succinimides and Pyridine Derivatives in Solution, the Solid State and In Silico

A study of strong halogen bonding within three series of halogen-bonded complexes, derived from seven para-substituted pyridine derivatives and three N-halosuccinimides (iodo, bromo and chloro), has been undertaken with the aid of single-crystal diffraction, solution complexation and computational methods. The halogen bond was compared with the hydrogen bond in an equivalent series based on succinimide. The halogen-bond energies are in the range -60 to -20 kJ mol^-1 and change regularly with pyridine basicity and the Lewis acidity of the halogen. The halogen-bond energies correlate linearly with the product of charges on the contact atoms, which indicates a predominantly electrostatic interaction. The binding enthalpies in solution are around 19 kJ mol^-1 less negative due to solvation effects. The optimised geometries of the complexes in the gas phase are comparable to those of the solid-state structures, and the effects of the supramolecular surroundings in the latter are discussed. The bond energies for the hydrogen-bonded series are intermediate between the halogen-bond energies of iodine and bromine, although there are specific differences in the geometries of the halogen- and hydrogen-bonded complexes.

Thymine cocrystals based on DNA-inspired binding motifs

Cocrystal design through DNA-type base pairing between a nucleic acid and complementary heterocycles.

Reversible crystal-to-crystal chiral resolution: making/breaking non-bonding S⋯O interactions

Unconventional non-covalent interactions are the key to reversible chiral resolution in polycrystalline samples of a small, achiral molecule.

Two tautomeric forms of 2-amino-5,6-dimethylpyrimidin-4-one

Derivatives of 4-hydroxypyrimidine are an important class of biomolecules. These compounds can undergo keto-enol tautomerization in solution, though a search of the Cambridge Structural Database shows a strong bias toward the 3H-keto tautomer in the solid state. Recrystallization of 2-amino-5,6-dimethyl-4-hydroxypyrimidine, C6H9N3O, from aqueous solution yielded triclinic crystals of the 1H-keto tautomer, denoted form (I). Though not apparent in the X-ray data, the IR spectrum suggests that small amounts of the 4-hydroxy tautomer are also present in the crystal. Monoclinic crystals of form (II), comprised of a 1:1 ratio of both the 1H-keto and the 3H-keto tautomers, were obtained from aqueous solutions containing uric acid. Forms (I) and (II) exhibit one-dimensional and three-dimensional hydrogen-bonding motifs, respectively.

Hydrous salts of 1-aminoethylidenediphosphonic acid and piperazidine: temperature induced reversible structural transformation in a humid environment

A series of hydrous salts constructed using AEDPH₄ and piperazidine (pip) were synthesized and structurally characterized. A reversible structural transformation circle is observed in single crystals of compounds 1–3.

Pharmaceutical crystalline complexes of sulfamethazine with saccharin: same interaction site but different ionization states

Sulfamethazine (SMT) can form either 1 : 1 salt or 1 : 1 cocrystal with saccharin (SAC). The two crystalline complexes possess the same main intermolecular interaction site except the locations of the acidic proton are different.

A structural, spectroscopic and theoretical study of an o-vanillin Schiff base derivative involved in enol-imine and keto-amine tautomerism

The potassium salt of the Schiff base derived from o-vanillin and taurine is involved in enol-imine (I) and keto-amine (II) tautomerism. Both tautomeric forms coexist in the crystal and they are stabilized by strong O–H⋯N and O⋯H–N intramolecular hydrogen bonds.

Design of Lead(II) Metal-Organic Frameworks Based on Covalent and Tetrel Bonding

Three solid materials, [Pb(HL)(SCN)2 ]⋅CH3 OH (1), [Pb(HL)(SCN)2 ] (2), and [Pb(L)(SCN)]n (3), were obtained from Pb(SCN)2 and an unsymmetrical bis-pyridyl hydrazone ligand that can act both as a bridging and as a chelating ligand. In all three the lead center is hemidirectionally coordinated and is thus sterically optimal for participation in tetrel bonding. In the crystal structures of all three compounds, the lead atoms participate in short contacts with thiocyanate sulfur or nitrogen atoms. These contacts are shorter than the sums of the van der Waals radii (3.04-3.47 Å for Pb⋅⋅⋅S and 3.54 Å for Pb⋅⋅⋅N) and interconnect the covalently bonded units (monomers, dimers, and 2D polymers) into supramolecular assemblies (chains and 3D structures). DFT calculations showed these contacts to be tetrel bonds of considerable energy (6.5-10.5 kcal mol(-1) for Pb⋅⋅⋅S and 16.5 kcal mol(-1) for Pb⋅⋅⋅N). A survey of structures in the CSD showed that similar contacts often appear in crystals of Pb(II) complexes with regular geometries, which leads to the conclusion that tetrel bonding plays a significant role in the supramolecular chemistry of Pb(II) .

Photochromism emergence in N-salicylidene p-aminobenzenesulfonate diallylammonium salts

N-Salicylidene p-aminobenzenesulfonate salts were prepared by in situ condensation of p-aminobenzenesulfonate diallylammonium salt and salicylaldehyde. Modulation of thermo- and photochromism was achieved by varying the alkyl chain length of the diallylammonium counter-cation. A structural-optical properties investigation reveals that both crystal packing and dihedral angle between aromatic rings of the N-salicylidene aniline switch are not sufficient to predict the occurrence of photochromism in the solid state. The available free space around the N-salicylidene p-aminobenzenesulfonate, in addition to the flexibility of the nearby environment, is shown to be of major importance for the cis→trans isomerisation to occur as well as for the stabilisation of the trans-keto form. Emergence of photochromic properties was determined from the diallylhexylammonium cation within the series of investigated counter-cations. High stability is observed for the trans-keto form of one polymorph of N-salicylidene p-aminobenzenesulfonate diallylhexylammonium salt (k = 2.4×10(-7)  s(-1)).

Isomerization of a lanthanide complex using a humming top guest template: a solid-to-solid reaction

SbPh₃Cl₂is used to effect a dramatic rearrangement of the ligand sphere and “pancake bonding” in Ce(hfac)₃(5′-Br-pyDTDA)₂by forming a 2 : 1 host–guest complex.