Том 68, № 6 (2023)

Synthesis methods, properties, and application fields of various boron-containing coumarin derivatives are summarized. Particular attention is paid to derivatives with various polyhedral boron clusters as substituents, such as carboranes, metallacarboranes, and the closo-dodecaborate anion. Derivatives of coumarin with simple organoboron compounds, boronic acids and their esters, are also considered.

A new method for the synthesis of imidazolium salts of the perchlorinated closo-decaborate anion (RMIM)2[B10Cl10] (R = H, n-Et, n-Bu, n-C8H17) has been proposed. The synthesis of these compounds can be carried out by simple metathesis reactions between the potassium salt of the decachloro-closo-decaborate anion K2[B10Cl10] and chlorides of imidazolium derivatives. The crystal packings and intermolecular interactions in compounds (EMIM)2[B10Cl10] and (BMIM)2[B10Cl10] have been studied using X-ray diffraction and Hirshfeld surface analysis. The [B10Cl10]2– anions in the crystal lattice of these compounds are linked by a network of weak Cl···Cl interactions with a distance between chlorine atoms of 2.246–3.623 Å.

The reaction between the 1,4-dioxane derivative of the closo-decaborate anion [2-B10H9O(CH2)4O]– with cyanide, hydrophthalate, and hydroiminoacetate ions has been studied. Alkali metal salts (K, Na, and Cs) of the closo-decaborate anion derivatives with pendant groups –NHCH2CH2NH2-, –OOC(o-C6H4)COOH-, –OOCCH2NHCH2COOH-, –OCH2CH2OH-, –CN-, –SCN-, and –SH- have been isolated. All compounds have been shown to have extremely low cytotoxicity (CT50 ~ 1000 μg/mL). It has been found that compounds Na2[B10H9O(CH2)2O(CH2)2SCN] and Na2[B10H9O(CH2)2O(CH2)2CN] exhibit activity and selectivity in vitro against the modern strain of SARS-CoV-2 coronavirus (IC50 312 and 625 μg/mL, respectively). In relation to influenza A virus and rabies virus, the compounds show weak antiviral activity at high concentrations (1250 μg/mL), i.e. show no selective effect on the reproduction of these viruses.

The reaction of nitrile derivatives of closo-dodecaborate anion (Et4N)[B12H11N≡C–R] (R = Me, Et, nPr, iPr) with water has yielded a series of (Et4N)[B12H11NH=C(OH)–R] iminols. It has been demonstrated that the hydrolysis products are in acid–base iminol–amide equilibrium, which can be controlled by changing the pH of a medium. The reaction products have been identified by 11В, 1Н, 13С NMR, IR spectroscopy, and ESI mass spectrometry. The structures of the [B12H11(Z-NH=C(OH)nC3H7)]− and [B12H11(E-NH–C(O)nC3H7)]2− anions have been determined by X-ray single-crystal diffraction.

The results of synthesis and study of magnetically-active iron(II) complexes with polynitrogen heterocyclic ligands and outer-sphere boron cluster anions have been considered. Derivatives of 1,2,4-triazole, tris(pyrazol-1-yl)methane, and 2,6-bis(imidazol-2-yl)pyridine have been used as ligands, decahydro-closo-decaborate, dodecahydro-closo-dodecaborate, decachloro-closo-decaborate, and 1,5,6,10-tetra(R)-7,8-dicarba-nido-undecaborates (R = H, Cl, Br) have been studied as outer-sphere anions. A number of iron(II) complexes showing spin crossover accompanied by thermochromism in the majority of cases has been obtained. Effect of ligand nature and cluster anion on temperature (Tc) and spin transition character has been considered. In particular, it has been shown that introduction of substituent increasing electron density over the system of conjugated three-centered two-electron bonds in cluster anion leads to increase in the field strength of ligand bound to anion via network of hydrogen bonds.

Phosphorescence data on boron difluoride β-diketonates of various structure have been systematized. Nonplanar boron difluoride molecules are characterized by the inversion of the S1 and T2 levels, which promotes efficient population of triplet levels and intense phosphorescence or delayed fluorescence of crystals. Planar molecules are characterized by a classical sequence of singlet and triplet levels and a coplanar arrangement of antiparallel molecules, which contributes to excimer delayed fluorescence.

Reactions of (Et3NH)2[B10Cl10] and organic ligands 2,2'-bipyridyl (Bipy) and 1,10-phenanthroline (Phen) in the acetonitrile–trifluoroacetic acid system have yielded compounds of composition (HL)2[B10Cl10]·3CH3CN (L = Bipy, Phen). The compounds have been characterized by IR spectroscopy, elemental analysis, and X-ray diffraction (CCDC nos. 2224377 and 2224378). It has been shown that in the presence of trifluoroacetic acid, protonation of organic ligands occurs with the formation of bipyridylium and phenanthrolinium salts, which are stabilized by the decachloro-closo-decaborate anion. It has been found that the cations participate in the formation of hydrogen bonds with the solvate molecules of acetonitrile, while only weak C–H…Cl and B–Cl…π interactions are observed for boron cluster anions.

We studied how the ratio of precursor titanium and boron powders influenced the combustion temperature and combustion velocity in the high-temperature self-propagating synthesis (SHS) mode, and the microstructure, phase composition, and mechanophysical properties of rods fabricated by SHS extrusion. The subject matters of the study were materials for which the as-batch phase compositions of products were TiB–(20–40) wt % Ti. The formation of boron solid solution in titanium was considered. Scanning electron microscopy (SEM), X-ray powder diffraction (XRD), and mechanical measurements implied the texture of the prepared materials (TiB whiskers were aligned in the direction in which the external pressure was applied). In all of the composites studied, the conductivity was close to the conductivity of undoped titanium; the electrical resistance increased slightly in response to increasing boron weight fraction. The three-point bending strength of the material increased by a factor of 1.7 at most as the weight fraction of boron solid solution in titanium increased from 20 to 40 wt %.

The processes occurring in the Ca–Ir–B system were studied. The elemental and phase compositions and morphology of the products formed in the reaction of iridium metal with calcium hexaboride in the 700–1600°C temperature range were investigated. The signs of the onset of the reaction are already visible at 700°C. The first reaction products are iridium borides IrB0.9 and IrB1.1, where IrB0.9 is formed as a low-temperature and metastable high-temperature polymorphs. An increase in the treatment temperature in the 1000–1600°C range leads to the formation of three ternary compounds, presumably, Ca3Ir8B6, CaIr4B4, and CaIr2B2; the first two of these phases were previously unknown. The unit cell parameters of the new compounds, Ca3Ir8B6 (based on Sr3Rh8B6) and CaIr4B4 (based on LaIr4B4), were determined. The results attest to the complexity of the processes occurring in the Ca–Ir–B system and the diversity of ternary boride phases. The ternary boride phases can be of interest by themselves for high-temperature materials science.

A new method for manufacturing composites comprising graphene oxide (GO) and zinc borate nanopowders is described. The method comprises ultrasonic stirring of precursor slurries followed by removal of water. Exposure to supercritical isopropanol provides a composite comprising reduced graphene oxide (RGO) and zinc borate nanopowder due to removal of oxygen functions from the graphene oxide structure, thereby providing a uniform distribution of zinc borate particles over the surface of reduced graphene oxide.