The crystal and molecular structures of two new phosphorochromones determined by single-crystal X-ray diffraction are presented. The existence of two tautomers stabilized by intramolecular N−H···O and O−H···N hydrogen bonds and environmental effects in crystals is observed as evidence of the proton transfer reaction. The proton transfer process within simple enaminones being analogues of the compounds studied by X-ray diffraction methods is investigated usingMP2/6-311++G** and MP4/6-311++G** levels of theory. The Bader theory is also applied in the analysis of the hydrogen bonding within investigated systems. The results indicate that the systems with N−H···O intramolecular hydrogen bonds are more stable than those containing O−H···N bonds.

The nature of the NO-bond in the N-oxide group was investigated by means of combined theoretical calculations (including QTAIM and NBO approaches) and statistical analyses of the contents of crystal structure databases. The N-O bond in the N-oxide group should be classified as the NO donating bond with an important contribution of ON back-donation (of the π-electron type, when available). The visualization of the Laplacian of electron density in the region of an oxygen valence sphere suggests the presence of two lone pairs for the imine-N-oxide group (characterized by effective ON back-donation). A detailed bonding analysis performed by means of natural resonance theory indicates that the N→O bond is of an order of magnitude clearly greater than 1. In addition, the stability of the N→O bond in various N-oxides was estimated. The analyses of the hydrogen- and halogen-bonded complexes of the N-oxides reveal strong Lewis basicity of the N-oxide group. The formation of H- and X-bonding leads to N→O bond elongation due to its structural, topological and spectroscopic characteristics. Moreover, in pyridine-N-oxide, the electron-withdrawing -NO2 group additionally stabilizes the N→O bond, whereas the opposite effect can be observed for the electron-donating-NH2 substituent. This is due to a substituent effect on the π-type ON back-donation. As a result, the oxygen atom in pyridine-N-oxide may change its availability during intermolecular interaction formation, as revealed in the interaction energy, which changes by about half of the estimated total interaction energy.

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A series of eleven cymantrene-and cyrhetrene-nucleobase conjugates together with the hitherto unreported N7-isomer of the ferrocene-adenine conjugate have been synthesised and characterized. The synthetic approach involved a Michael addition reaction of in situ generated acryloylcymantrene, acryloylcyrhetrene and acryloylferrocene with canonical nucleobases, such as thymine, uracil and adenine, respectively. The mechanism of these reactions was investigated by means of density functional theory (DFT) calculations.The respective products were characterized by spectroscopic methods and by electrochemical measurements. The molecular structure of one cymantrene-adenine conjugate (5) in the solid state was determined by single-crystal X-ray structure analysis, confirming the N9-substitution mode of the adenine moiety. It was found that the molecule adopts a bent conformation with the adenine and cyclopentadienyl planes in almost perpendicular orientation. The cymantrenyl nucleobases showed an irreversible redox behaviour which is associated with ligand exchange reactions of the radical cationic species. The newly synthesised compounds were also tested for their activity against the protozoan parasite Trypanosoma brucei and human myeloid leukaemia HL-60 cells. Some compounds showed promising antitrypanosomal activity while most of them were non-toxic to HL-60 cells. It was additionally found that cymantrene and cyrhetrene ketone nucleobases were more active than their alcohol congeners. These findings indicate the potential of cymantrene and cyrhetrene nucleobase conjugates as possible lead compounds for future antitrypanosomal drug development.

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We have investigated 44 crystal structures, found in the Cambridge Structural Database, containing the X 3 synthon (where X = Cl, Br, I) in order to verify whether three type II halogen−halogen contacts forming the synthon exhibit cooperativity. A hypothesis that this triangular halogen−bonded motif is stabilized by cooperative effects is postulated on the basis of structural data. However, theoretical investigations of simplified model systems in which the X 3 motif is present demonstrate that weak synergy occurs only in the case of the I 3 motif. In the present paper we computationally investigate crystal structures in which the X 3 synthon is present, including halomesitylene structures, that are usually described as being additionally stabilized by a synergic interaction. Our computations find no cooperativity for halomesitylene trimers containing the X 3 motif. Only in the case of two other structures containing the I 3 synthon a very weak or weak synergy, i.e. the cooperative effect being stronger than −0.40 kcal mol −1 , is found. The crystal structure of iodoform has the most pronounced cooperativity of all investigated systems, amounting to about 10% of the total interaction energy.

PAPER

The crystal and molecular structures of three new phosphorochromones determined by the X-ray diffraction method are presented. For all crystal structures, a similar pattern of centrosymmetric dimer is formed for which bifurcated hydrogen bonds exist with bifurcated acceptor O‚‚‚(H-N, H-C). For one of the crystal structures, there is additionally the intramolecular resonance-assisted H bond. The analysis of those interactions is performed in terms of their geometries and strengths. Additional calculations on simple model systems are performed to study the nature of bifurcated H bonds. The wave functions are applied for further analysis based on the Bader theory.