Interaction of formaldehyde with nucleic acids and their structural components in the presence of amines. II. Structure of adenine derivatives formed in the reaction with formaldehyde and primary amine

Two products of modified adenine have been isolated from reaction of adenine with a mixture of formaldehyde and methylamine; their structures have been demonstrated by UV, 1H NMR and mass spectra data. The formal scheme of reaction can be described as simultaneous or sequential cycloaddition of dimethylol derivatives of methylamine to two reaction's positions of the adenine residue. This results in the formation of a new partly or completely hydrogenated six-membered 1,3,5-triazine cycle which has the atoms of adenine nitrogen in its 1,3-positions and nitrogen of methylamine in 5-position. This is used as a model for discussing the peculiarities of DNA-formaldehyde interaction in the presence of amine.     

Electrochemical CO2 Reduction with Atomic Iron‐Dispersed on Nitrogen‐Doped Graphene

Electrochemical reduction of CO₂ provides an opportunity to reach a carbon‐neutral energy recycling regime, in which CO₂ emissions from fuel use are collected and converted back to fuels. The reduction of CO₂ to CO is the first step toward the synthesis of more complex carbon‐based fuels and chemicals. Therefore, understanding this step is crucial for the development of high‐performance electrocatalyst for CO₂ conversion to higher order products such as hydrocarbons. Here, atomic iron dispersed on nitrogen‐doped graphene (Fe/NG) is synthesized as an efficient electrocatalyst for CO₂ reduction to CO. Fe/NG has a low reduction overpotential with high Faradic efficiency up to 80%. The existence of nitrogen‐confined atomic Fe moieties on the nitrogen‐doped graphene layer is confirmed by aberration‐corrected high‐angle annular dark‐field scanning transmission electron microscopy and X‐ray absorption fine structure analysis. The Fe/NG catalysts provide an ideal platform for comparative studies of the effect of the catalytic center on the electrocatalytic performance. The CO₂ reduction reaction mechanism on atomic Fe surrounded by four N atoms (Fe–N₄) embedded in nitrogen‐doped graphene is further investigated through density functional theory calculations, revealing a possible promotional effect of nitrogen doping on graphene.     

Uniform Lithium Deposition Assisted by Single‐Atom Doping toward High‐Performance Lithium Metal Anodes

For a long time lithium (Li) metal has been considered one of the most promising anodes for next‐generation rechargeable batteries. Despite decades of concentrated research, its practical application is still hindered by dendritic Li deposition and infinite volume change of Li metal anodes. Here, atomically dispersed metals doped graphene is synthesized to regulate Li metal nucleation and guide Li metal deposition. The single‐atom (SA) metals, supported on the nitrogen‐doped graphene can not only increase the Li adsorption energy of the localized area around the metal atomic sites with a moderate adsorption energy gradient but also improve the atomic structural stability of the overall materials by constructing a coordination mode of M‐N_x‐C (M, N, and C denoted as metal, nitrogen, and carbon atoms, respectively). As a result, the as‐obtained electrode exhibits an ultralow voltage hysteresis of 19 mV, a high average Coulombic efficiency of 98.45% over 250 cycles, and a stable Li plating/stripping performance even at a high current density of 4.0 mA cm^?2. This work demonstrates the application of SA metal doping in the rational design of Li metal anodes and provides a new concept for further development of Li metal batteries.     

Cation-π interaction of alkali metal ions with C24 fullerene: a DFT study

Using first principle calculations, we investigated cation-π interactions between alkali cations (Li(+), Na(+), and K(+)) and pristine C(24) or doped fullerenes of BC(23), and NC(23). The most suitable adsorption site is found to be atop the center of a six-membered ring of the exterior surface of C(24) molecule. Interaction energies of these cations decreased in the order: Li(+)?>?Na(+)?>?K(+), with values of -31.82, -22.36, and -15.68 kcal mol(-1), respectively. It was shown that the interaction energies are increased and decreased by impurity doping of B and N atoms in adjacent wall of adsorption site, depending on electron donating or receptivity of the doping atoms.     

Reconnoitring the current characteristics of the double C<Subscript>20</Subscript> fullerene molecular device in two probe configuration

It is worth remarking that the C₂₀ cage like isomer has been the topic of concentrated theoretical research. C₂₀ single fullerene molecular devices gained a lot of popularity in the field of nano research due to their superlative doping dependent conductive properties. In this work, the double fullerene device has been considered. Here double fullerene molecular junction is created when two C₂₀ fullerene molecules, one in pristine form and other in doped form, are positioned between gold electrodes. Doping was done firstly by second period elements, boron, nitrogen, oxygen, and fluorine and then by group 14 tetragens, silicon, germanium, tin, and lead. For both the cases current characteristics were investigated. Superior conductivity was observed in the boron doped double C₂₀ molecular device while the fluorine doped device was the least conducting. Further for group 14 doping, the silicon doped double C₂₀ device showed maximum current carrying feature, whereas, least value of current was noted in tin doped C₂₀ device.     

Supramolecular Polymerization Promoted In Situ Fabrication of Nitrogen‐Doped Porous Graphene Sheets as Anode Materials for Li‐Ion Batteries

A novel strategy of utilizing supramolecular polymerization for fabricating nitrogen doped porous graphene (NPG) with high doping level of 12 atom% as the anode material for lithium ion batteries is reported for the first time. The introduction of supramolecular polymer (melamine cyanurate) functions not only as a spacer to prevent the restacking of graphene sheets but also as a sacrificial template to generate porous structures, as well as a nitrogen source to induce in situ N doping. Therefore, pores and loose‐packed graphene thin layers with high N doping level are very effectively formed in NPG after the annealing process. Such highly desired structures immediately offer remarkably improved Li storage performance including high reversible capacity (900 mAh g^?1 after 150 cycles) with good cycling and rate performances. The effects of annealing temperature and heating rates on the final electrochemical performance of NPG are also investigated. Furthermore, the low cost, facile, and scalable features of this novel strategy may be helpful for the rational design of functionalized graphene‐based materials for diverse applications.     

Boron- and nitrogen-doped carbon nanotubes and graphene

Multi-walled, single-walled and double-walled carbon nanotubes as well as graphene can be doped with boron and nitrogen. B₂H₆ has been generally used as the boron source while NH₃ or pyridine is employed as the nitrogen source. Doping carbon nanotubes and graphene with boron and nitrogen brings about significant changes in the electronic structure and properties. Such doping not only results in desirable properties but also allows manipulation of properties for specific purposes. Doping with boron- and nitrogen-causes marked changes in the Raman spectra of the carbon nanostructures. In this article, we present the synthesis, characterization and properties of boron- and nitrogen-doped carbon nanotubes and graphene.     

Reaction of iron(III)-polyaminocarboxylate complexes with hydrogen peroxide: Correlation between ligand structure and reactivity

Reactions of iron(III) complexes with five polyaminocarboxylates and hydrogen peroxide in an alkaline solution were investigated. Iron(III) complexes of which the ring including two nitrogen and iron atoms is five-membered formed a well-known stable side-on peroxo adduct. On the other hand, iron(III) complexes which have a six-membered ring formed a short-lived side-on peroxo adduct and then changed to iron(II) complex and superoxide. Electrochemical measurements showed that the redox potentials of the iron complexes having a six-membered ring are higher than those of the complexes having a five-membered ring. These results indicate that the chelate size is an important factor for tuning the redox potential of the iron center and for the reactivity toward hydrogen peroxide.     

Selective Etching of Nitrogen‐Doped Carbon by Steam for Enhanced Electrochemical CO2 Reduction

Nitrogen‐doped carbon structures have recently been demonstrated as a promising candidate for electrocatalytic CO₂ reduction, while in the meantime the pyridinic and graphitic nitrogen atoms also present high activities for electroreduction of water. Here, an etching strategy that uses hot water steam to preferentially bind to pyridinic and graphitic nitrogen atoms and subsequently etch them in carbon frameworks is reported. As a result, pyrrolic nitrogen atoms with low water affinity are retained after the steam etching, with a much increased level of among all nitrogen species from 22.1 to 55.9%. The steam‐etched nitrogen‐doped carbon catalyst enables excellent electrocatalytic CO₂ reduction performance but low hydrogen evolution reaction activity, suggesting a new approach for tuning electrocatalyst activity.     

Electronic structure of boron-doped finite graphene sheets: unrestricted DFT and complete active space calculations

B3LYP and complete active space methods were applied to study graphene nanoribbons (GNRs) doped with boron atoms. The restricted B3LYP solutions were found to be unstable in all but two cases, and the complete active space calculations prove the multiconfigurational character of the ground state contributing with two most important configurations. The exception is the structure c4 where the system has single reference ground state in spite of the instability of the restricted wavefunction.

The distance between dopant atoms, their mutual position and their location within the nanoribbon impact the relative stability of doped nanoribbons. B doping does not modify the ionisation potentials of doped GNRs. However, it notably increases the electron affinity of the core-doped nanoribbons. The doping also has a notable impact on the reorganisation energy of the nanoribbons. The reorganisation energy of B-doped GNRs is higher than the corresponding reorganisation energy of pristine and nitrogen-doped GNRs.     

Computational study of energetic nitrogen-rich derivatives of 1,4-bis(1-azo-2,4-dinitrobenzene)-iminotetrazole

The heats of formation (HOFs), electronic structure, energetic properties, and thermal stabilities for a series of 1,4-bis(1-azo-2,4-dinitrobenzene)-iminotetrazole derivatives with different substituents and substitution positions and numbers of nitrogen atoms in the nitrobenzene rings were studied using the DFT-B3LYP method. All the substituted compounds have higher HOFs than their parent compounds. As the number of nitrogen atoms in the nitrobenzene ring increases, the HOFs of the derivatives with the same substituent rise gradually. Replacing carbon atoms in the nitrobenzene with nitrogen atoms to form N–N bonds is very helpful in improving their HOFs. Most of the substituted compounds have higher HOMO–LUMO gaps than the corresponding unsubstituted compounds. Substitution of the –NO₂, –NF₂, or –ONO₂ group and an increase in the number of nitrogen atoms in the nitrobenzene rings are useful for enhancing their detonation performance. The substituents’ substitution is not favorable for improving thermal stability. Considering detonation performance and thermal stability, five compounds may be considered potential candidates for high energy density compounds (HEDCs).     

Engineering of a Low‐Cost,Highly Active,and Durable Tantalate–Graphene Hybrid Electrocatalyst for Oxygen Reduction

The oxygen reduction reaction (ORR) is one of the most important reactions in renewable energy conversion and storage devices. The full deployment of these devices depends on the development of highly active, stable, and low‐cost catalysts. Herein, a new hybrid material consisting of Na₂Ta₈O_21?x/Ta₂O₅/Ta₃N₅ nanocrystals grown on N‐doped reduced graphene oxide is reported. This catalyst shows a significantly enhanced ORR activity by ≈4 orders of magnitude in acidic media and by ≈2 orders of magnitude in alkaline media compared to individual Na₂Ta₈O_21?x on graphene. Moreover, it has excellent stability in both acid and alkaline media. It also has much better methanol tolerance than the commercial Pt/C, which is relevant to methanol fuel cells. The high ORR activity arises not only from the synergistic effect among the three Ta phases, but also from the concomitant nitrogen doping of the reduced graphene oxide nanosheets. A correlation between ORR activity and nitrogen content is demonstrated. Deep insights into the mechanism of the synergistic effect among these three Ta‐based phases, which boosts the ORR's kinetics, are acquired by combining specific experiments and density functional theory calculations.     

1,4-Dihydroindeno[1,2-c]pyrazoles as potent checkpoint kinase 1 inhibitors: extended exploration on phenyl ring substitutions and preliminary ADME/PK studies

A study on substitutions at the four open positions on the phenyl ring of the 1,4-dihydroindeno[1,2-c]pyrazoles as potent CHK-1 inhibitors is described. Bis-substitution at both the 6- and 7-positions led to inhibitors with IC(50) values below 0.3nM. The compound with the best overall activities (36) was able to potentiate the anti-proliferative effect of doxorubicin in HeLa cells by at least 47-fold. Physicochemical, metabolic, and pharmacokinetic properties of selected inhibitors are also disclosed.     

Mechanism of the Dimroth rearrangement in adenosine.

An unambiguous mechanism for the Dimroth rearrangement has been established for adenosine by observation of isotopic [¹⁵N] nitrogen exchange between the exocyclic (N⁶) and endocyclic (1-) positions of the adenine ring which occurs simultaneously with the well documented methyl migration (from positions 1- to N⁶). The elucidation of this rearrangement by NMR techniques indicates that ring cleavage between the N₁-nitrogen and C₂-carbon atoms followed by 180° internal rotation, is the correct pathway.     

Benzylidene acetal structural elucidation by N.M.R. Spectroscopy: Application of carbon-13. N.M.R.-Spectral parameters

The ₁₃C-n.m.r. spectra of 19 2-phenyl-1,3-dioxolane, -1,3-dioxane and -1,3-dioxopane derivatives were examined and it was found that both the ₁₃C-n.m.r. chemical shift for the acetal carbon atom and the one-bond coupling constant between the acetal carbon atom and the acetal proton had values that could be used to distinguish between acetals having different ring sizes. In addition, the presence of axial substituents at positions 4 or 6 in substituted 2-phenyl-1,3-dioxane rings and 4 or 7 in substituted 2-phenyl-1,3-dioxepane rings could be readily detected. The structures of a number of carbohydrate examples were determined by using these two parameters and also the chemical shift of the acetal proton from ₁H-n.m.r. spectra. The use of all three parameters made assignment of benzylidene acetal ring-size unambiguous.     

One- and two-electron reduction of hydroxy-1,4-naphthoquinones and hydroxy-9,10-anthraquinones: The role of internal hydrogen bonding and its bearing on the redox chemistry of the anthracycline antitumour quinones

First and second half-wave reduction potentials of 11 1,4-naphthoquinones and 42 9,10-anthraquinones have been measured for solutions in dimethylformamide. The presence of hydroxy groups at the 5- and 8-positions of the 1,4-naphthoquinone nucleus, and at the 1-, 4-, 5- and 8-positions of the 9,10-anthraquinone (the α-positions) markedley raises both reduction potentials. Measurements on the corresponding methoxy- and acetoxyquinones indicate that internal hydrogen bonding in the α-hydroxyquinones makes a major contribution to stabilisation of the semiquinone, probably as a result of increased delocalisation due to exchange of the hydroxy hydrogen between the two neighbouring oxygen atoms. The bearing of this phenomenon on the mechanism of action off anthracycline antitumour quinones is discussed, and the stabilising effect on the semiquinone of hydroxy groups at the 1- adn 5-positions of the 9,10-anthraquinone nucleus is highlighted.     

New coordination polymer based on salpn Schiff base and azide bridging ligands: Synthesis and structural characterization of {Na[Co(μ-salpn)(μ1,1-N3)2]}n (H2salpn = N,N′-bis(salicylidene)-1,3-diaminopropane)

One-pot reaction of cobalt(II) nitrate hexahydrate Co(NO₃)₂ · 6H₂O with H₂salpn (N,N′-bis(salicylidene)-1,3-diaminopropane) in presence of a large excess of sodium azide (NaN₃) gives the new Co(III) compound {Na[Co^III(μ-salpn)(μ_1,1-N₃)₂]}_n (1), which was characterized by single crystal X-ray diffraction analysis. The crystal structure shows polymeric 1D complex generated by the hexadentate Schiff base salpn²⁻ and two crystallographically different azide ligands. The two nitrogen atoms of the salpn ligand are bonded to the cobalt(III) ion while each phenoxo oxygen atom is bonded to the same Co(III) ion and to two equivalent sodium ions. Each azide ligand acts with the end-on bridging coordination mode between Co(III) and Na(I) ions. The Co(III) ion adopts a distorted octahedral geometry arising from two oxygen and two nitrogen atoms of the salpn ligand and from two nitrogen atoms of the two crystallographically different azide ligands in trans positions. Such [Co(salpn)(N₃)₂] entities are connected each other by sodium ions through four oxygen atoms of two equivalent Schiff base ligands and two nitrogen atom of the two different azide ligands to generate the 1D structure of 1.     

One- and two-electron reduction of hydroxy-1,4-naphthoquinones and hydroxy-9,10-anthraquinones. The role of internal hydrogen bonding and its bearing on the redox chemistry of the anthracycline antitumour quinones

First and second half-wave reduction potentials of 11 1,4-naphthoquinones and 42 9,10-anthraquinones have been measured for solutions in dimethylformamide. The presence of hydroxy groups at the 5- and 8-positions of the 1,4-naphthoquinone nucleus, and at the 1-, 4-, 5- and 8-positions of the 9,10-anthraquinone (the alpha-positions) markedly raises both reduction potentials. Measurements on the corresponding methoxy- and acetoxyquinones indicate that internal hydrogen bonding in the alpha-hydroxyquinones makes a major contribution to stabilisation of the semiquinone, probably as a result of increased delocalisation due to exchange of the hydroxy hydrogen between the two neighbouring oxygen atoms. The bearing of this phenomenon on the mechanism of action of anthracycline antitumour quinones is discussed, and the stabilising effect on the semiquinone of hydroxy groups at the 1- and 5-positions of the 9,10-anthraquinone nucleus is highlighted.     

Coordination compounds of Zn(II) with several bidentate-NN′ and tridentate-NN′N nitrogen donor ligands

Reaction of ZnCl₂ with N-alkylaminopyrazole ligands 1-[2-(ethylamino)ethyl]-3,5-dimethylpyrazole (deae), 1-[2-(tert-butylamino)ethyl]-3,5-dimethylpyrazole (deat), bis-[(3,5-dimethylpyrazolyl)methyl]ethylamine (bdmae), and bis-[(3,5-dimethylpyrazolyl)ethyl]ethylamine (ddae) in ethanol yields [ZnCl₂(NN^′)] (NN^′ = deae (1), deat (2)), [ZnCl₂(bdmae) (3), and [ZnCl(ddae)]₂[ZnCl₄] (4). These Zn(II) complexes have been characterised by elemental analyses, conductivity measurements and IR and ¹H and ¹³C{¹H} spectroscopy. The NMR studies proved the flipping of the six-membered ring. The solid of complexes 1, 2, and 4 were determined by X-ray diffraction studies. The Zn(II) is coordinated to the deae and deat ligands by one nitrogen atom of the pyrazolyl group and one nitrogen atom of the amine, and to bdmae ligand by two nitrogen atoms of the pyrazolyl groups, along with two chlorine atoms in a tetrahedral geometry. With ddae ligand, the Zn(II) is coordinated by two nitrogens atoms of the pyrazolyl groups and one nitrogen atom of the amine, along with one chlorine atom, in a tetrahedral geometry.     

Synthesis of 1-homoaustraline

The 1,3-dipolar cycloaddition of a five-membered cyclic nitrone derived from malic acid and unsaturated D-threo-hexonolactone leads to a single adduct, which was transformed into 1-homoaustraline via a reaction sequence involving rearrangement of the six-membered lactone ring into the five-membered one, removal of the terminal carbon atom from the sugar chain, reduction of the lactone fragment into triol, protection of primary hydroxy groups, mesylation of the secondary one, cleavage of the N-O bond, and the intramolecular alkylation of the nitrogen atom.