The synthesis and structural characterization of a series of supramolecular complexes of bicyclic cationic pyridine-fused 1,2,4-selenodiazoles with various anions is reported. The binding of trifluoroacetate, tetrachloroaurate, tetraphenylborate, perrhenate, and pertechnetate anions in the solid state is regarded. All the anions interact with selenodiazolium cations exclusively via a pair of “chelating” Se⋯O and H⋯O non-covalent interactions, which make them an attractive, novel, non-classical supramolecular recognition unit or a synthon. Trifluoroacetate salts were conveniently generated via novel oxidation reaction of 2,2′-dipyridyl diselenide with bis(trifluoroacetoxy)iodo)benzene in the presence of corresponding nitriles. Isolation and structural characterization of transient 2-pyridylselenyl trifluoroacetate was achieved. X-ray analysis has demonstrated that the latter forms dimers in the solid state featuring very short and strong Se⋯O and Se⋯N ChB contacts. 1,2,4-Selenodiazolium trifluoroacetates or halides show good solubility in water. In contrast, (AuCl4)−, (ReO4)−, or (TcO4)− derivatives immediately precipitate from aqueous solutions. Structural features of these supramolecular complexes in the solid state are discussed. The nature and energies of the non-covalent interactions in novel assembles were studied by the theoretical methods. To the best of our knowledge, this is the first study that regards perrhenate and pertechnetate as acceptors in ChB interactions. The results presented here will be useful for further developments in anion recognition and precipitation involving cationic 1,2,4-selenodiazoles.
Supramolecular chemistry of chalcogenadiazoles
is attracting an
increasing attention due to its applications in materials chemistry.
Chalcogen bonding allows a fine-tuning of the self-assembly and, therefore,
modulation of physical properties when these compounds are employed.
Here, we report a facile preparation of a broad scope of 1,2,4-selenadiazoles
via coupling of 2-pyridylselenenyl halides with unactivated nitriles,
that represent a novel type of supramolecular building blocks which
eagerly engage in a variety of chalcogen bonding interactions. The
substituent-dependent propensity of novel selenadiazoles for the formation
of four-center Se···N chalcogen bonding is analyzed.
Other weak interactions, which in some cases outcompete the formation
of 2Se–2N squares, are described. The discovery of the adducts
derived from α-halogenated nitriles, which form robust dimers
featuring a very specific combination of 2Se–2N square, two
Hal···Hal, and two Se···Hal noncovalent
interactions, is presented.
The series of benzylic-substituted 1,2,4-selenodiazolium salts were prepared via cyclization reaction between 2-pyridylselenyl chlorides and nitriles and fully characterized. Substitution of the Cl anion by weakly binding anions promoted the formation supramolecular dimers featuring four center Se2N2 chalcogen bonding and two antiparallel selenium···π interactions. Chalcogen bonding interactions were studied using density functional theory calculations, molecular electrostatic potential (MEP) surfaces, the quantum theory of atoms-in-molecules (QTAIM), and the noncovalent interaction (NCI) plot. The investigations revealed fundamental role of the selenium···π contacts that are stronger than the Se···N interactions in supramolecular dimers. Importantly, described herein, the benzylic substitution approach can be utilized for reliable supramolecular dimerization of selenodiazolium cations in the solid state, which can be employed in supramolecular engineering.
The series of substituted 1,2,4-selenodiazolium tetraphenylborate complexes were synthesized via cyclization between 2-pyridylselenylchloride, followed by the anion metathesis, and fully characterized. The utilization of tetraphenylborate anion, a strong π-electron donor via its phenyl rings, promoted the formation of assemblies exhibiting selenium–π interactions. The chalcogen bonding (ChB) interactions involving the π-systems of the tetraphenylborate anion were studied using density functional theory (DFT) calculations, where “mutated” anions were used to estimate the strength of the Se···π chalcogen bonds. Moreover, molecular electrostatic potential (MEP) surfaces were used to investigate the electron-rich and poor regions of the ion pairs. The quantum theory of atoms-in-molecules (QTAIM) and the noncovalent interaction (NCI) plot methods based on the topology of the electron density were used and combined to characterize the ChBs. The investigation reported herein disclosed that the formation of symmetrical dimers can be broken by the introduction of a stronger π-acceptor and, consequently, forming stronger Se···π contacts with selenodiazolium cations.
The interaction of chitosan with 3-(chloromethyl)-[1,2,4]selendiazole[4,5-a]pyridin-4 bromide results in water-soluble, selenium-containing, cationic chitosan derivatives. Derivatives of chitosan with degrees of substitution of 0.15, 0.45, and 0.65 were obtained. These derivatives are characterized by a pronounced in vitro antibacterial activity against Staphylococcus aureus and Escherichia coli, and the antibacterial activity of the derivatives increases with an increase in their degree of substitution. The antibacterial activity of the highly substituted derivative is comparable to that of the conventional antibiotics ampicillin and gentamicin.
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