Photoinduced Reduction of Nitroarenes and Tandem C-N Cross-Coupling with Haloarenes

An efficient photocatalytic C-N cross-coupling of nitroarenes with haloarenes has been developed using simple and cheap Ni(acac)2 as a cocatalyst. The reaction is confirmed as a stepwise process: (1) metal free photoinduced reduction of nitroarenes into aniline derivatives and (2) photo- and Ni-catalyzed C-N cross-coupling of anilines with haloarenes. The reaction conditions are simple and mild, giving high-value diarylamines with good to excellent yields and good functional group tolerance.

Competition of Hydrogen Bonds and Coordinate Bonds Induces a Reversible Crystal Transformation

Achieving reversible molecular crystal transformation between coordinate aggregates and hydrogen bonded assemblies has been a challenging task because coordinate bonds are generally much stronger than hydrogen bonds. Recently, we have reported the incorporation of silver ions into the cyanuric acid-melamine (CAM) network, resulting in the formation of a 1D coordination polymer (crystal 1) through forming the κ₁N-Ag-κ₂N coordination bonds. In this work, we find crystal 1 will undergo reversible transformation to hydrogen bonded coordinate units (crystal 2) through the breaking of coordinate chains and then the addition of CAM hydrogen bonding motifs into the framework. Crystal 2 presents a pseudohexagonal arrangement comprised of the κ₁N-Ag-κ₂N units connected by two sets of the triple hydrogen bonds, which extends two-dimensionally and stacks into a layer-structured crystal. Light was shed on the tautomerization of CA and M ligands associated with the crystal transformations using single crystal X-ray diffraction and infrared spectroscopy by analyzing the bond lengths and vibrations. We also highlight that photoluminescence can be a useful tool to probe the tautomer conversions of conjugated molecules. Furthermore, crystal 1 demonstrates high flexibility and can be bent over 180° and recover to its original shape after stress release. Crystal 2, on the contrary, is brittle and shows distinct mechanical anisotropy along different crystal orientations, as unveiled by nanoindentation measurements. The elastic modulus is well correlated with the chemical bonding strength along each orientation, and it is noteworthy that the contribution of the triple hydrogen bonds is comparable to that of the coordination bonds.

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.

Energetic Tricyclic Polynitropyrazole and Its Salts: Proton-Locking Effect of Guanidium Cations

An axisymmetric polynitro-pyrazole molecule, 3,5-di(3,5-dinitropyrazol-4-yl)]-4-nitro-1H-pyrazole (5), and its salts (6-12) were prepared and fully characterized. These compounds not only show promising energetic properties but also show a unique tautomeric switch via combining different cations with the axisymmetric compound (5). Its salts (6-9) remain axisymmetric when the cations are potassium, ammonium, or amino-1,2,4-triazolium. However, when the cations are guanidiums, the salts (10-12) dramatically become asymmetric owing to the fixed proton. The introduction of guanidium cations breaks the tautomeric equilibrium by blocking the prototropic transformations and results in the switch-off effect to tautomerism. The structural constraints of ¹H NMR and ¹³C NMR spectra provide strong evidence for the unusual structural constraint phenomenon. These stabilized asymmetric tautomers are very important from the point of molecular recognition, and this research may promote further developments in synthetic and isolation methodologies for novel bioactive pyrazole-based compounds.

Halogen bonding in the co-crystallization of potentially ditopic diiodotetrafluorobenzene: a powerful tool for constructing multicomponent supramolecular assemblies

Halogen bonding is emerging as a significant driving force for supramolecular self-assembly and has aroused great interest during the last two decades. Among the various halogen-bonding donors, we take notice of the ability of 1,4-diiodotetrafluorobenzene (1,4-DITFB) to co-crystallize with diverse halogen-bonding acceptors in the range from neutral Lewis bases (nitrogen-containing compounds, N-oxides, chalcogenides, aromatic hydrocarbons and organometallic complexes) to anions (halide ions, thio/selenocyanate ions and tetrahedral oxyanions), leading to a great variety of supramolecular architectures such as discrete assemblies, 1D infinite chains and 2D/3D networks. Some of them act as promising functional materials (e.g. fluorescence, phosphorescence, optical waveguide, laser, non-linear optics, dielectric and magnetism) and soft materials (e.g. liquid crystal and supramolecular gel). Here we focus on the supramolecular structures of multicomponent complexes and their related physicochemical properties, highlight representative examples and show clearly the main directions that remain to be developed and improved in this area. From the point of view of crystal engineering and supramolecular chemistry, the complexes summarized here should give helpful information for further design and investigation of the elusive category of halogen-bonding supramolecular functional materials.

Systematic comparison of racemic and enantiopure multicomponent crystals of phenylsuccinic acid—the role of chirality

Comparison of binary cocrystals of chiral and racemic carboxylic acids showed that the introduction of chiral building blocks may lead to the formation of subclasses of multicomponent crystals with unique Z′′/Z^r values combined with complex protonation stages of the molecules.

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.

Conformational and Tautomeric Control by Supramolecular Approach in Ureido-N-iso-propyl,N'-4-(3-pyridin-2-one)pyrimidine

Ureido-N-iso-propyl,N’-4-(3-pyridin-2-one)pyrimidine (1) and its 2-methoxy pyridine derivative (1Me) has been designed and prepared. The conformational equilibrium in urea moiety and tautomerism in the pyrimidine part have been investigated by variable temperature and ¹H NMR titrations as well as DFT quantum chemical calculations. The studied compounds readily associate by triple hydrogen bonding with 2-aminonaphthyridine (A) and/or 2,6-bis(acetylamino)pyridine (B). In 1, the proton is forced to 1,3-tautomeric shift upon stimuli and keeps it position, even when one of the partners in the complex was replaced by another molecule. The observed tautomerism controlled by conformational state (kinetic trapping effect) opens new possibilities in molecular sensing that are based on the fact that reverse reaction is not preferred.

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.

Crystal engineering of a zwitterionic drug to neutral cocrystals: a general solution for floxacins

The transformation of zwitterionic Sparfloxacin (SPX) to the neutral form is achieved by cocrystallization. Neutral forms of drugs are important for higher membrane permeability, while zwitterions are more soluble in water. The twin advantages of higher solubility/dissolution rate and good stability of neutral SPX are achieved in a molecular cocrystal compared to its zwitterionic SPX hydrate. The amine-phenol supramolecular synthon drives cocrystal formation, with the paraben ester acting as a "proton migrator" for the ionic to neutral transformation.

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.

Controlled keto–enol tautomerism of coumarin containing β-ketodithioester by its encapsulation in cucurbit[7]uril

CB7 shifts the tautomeric equilibrium of CAM2, from the enol- to keto-form, whereas β-CD maintains the enol form.

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.

Finding a single-molecule receptor for citramalic acid through supramolecular chelation

We have demonstrated that the tritopic hydrogen-bond acceptor 1,3,5-(5,6-dimethylbenzimidazol-1-yl)-2,4,6-trimethylbenzene can act as a perfectly complementary receptor for citramalic acid. The solid-state structure of the cocrystal of the two components show that they form 1:1 pairs where each pair is held together by three near-linear O–H···N hydrogen bonds in a converging manner. The conformational flexibility of both species is apparently no hindrance to the formation of discrete dimeric “cups” wherein each species presents three hydrogen-bond donors/acceptors in a face-to-face orientation.

One hydrogen bond does not a separation make, or does it? Resolution of amines by diacetoneketogulonic acid

Diacetoneketogulonic acid was used to separate primary amines from their racemic modifications and the selectivity of the acid was rationalized by lattice energy calculations and analyzing the weak interactions around the captured amines.