Catalyst control of selectivity in the C–O bond alumination of biomass derived furans

Non-catalysed and catalysed reactions of aluminium reagents with furans, dihydrofurans and dihydropyrans were investigated and lead to ring-expanded products due to the insertion of the aluminium reagent into a C–O bond of the heterocycle. Specifically, the reaction of [{(ArNCMe)₂CH}Al] (Ar = 2,6-di-iso-propylphenyl, 1) with furans proceeded between 25 and 80 °C leading to dearomatised products due to the net transformation of a sp² C–O bond into a sp² C–Al bond. The kinetics of the reaction of 1 with furan were found to be 1st order with respect to 1 with activation parameters ΔH^‡ = +19.7 (±2.7) kcal mol⁻¹, ΔS^‡ = −18.8 (±7.8) cal K⁻¹ mol⁻¹ and ΔG^‡_{298 K} = +25.3 (±0.5) kcal mol⁻¹ and a KIE of 1.0 ± 0.1. DFT calculations support a stepwise mechanism involving an initial (4 + 1) cycloaddition of 1 with furan to form a bicyclic intermediate that rearranges by an α-migration. The selectivity of ring-expansion is influenced by factors that weaken the sp² C–O bond through population of the σ*-orbital. Inclusion of [Pd(PCy₃)₂] as a catalyst in these reactions results in expansion of the substrate scope to include 2,3-dihydrofurans and 3,4-dihydropyrans and improves selectivity. Under catalysed conditions, the C–O bond that breaks is that adjacent to the sp²C–H bond. The aluminium(iii) dihydride reagent [{(MesNCMe)₂CH}AlH₂] (Mes = 2,4,6-trimethylphenyl, 2) can also be used under catalytic conditions to effect a dehydrogenative ring-expansion of furans. Further mechanistic analysis shows that C–O bond functionalisation occurs via an initial C–H bond alumination. Kinetic products can be isolated that are derived from installation of the aluminium reagent at the 2-position of the heterocycle. C–H alumination occurs with a KIE of 4.8 ± 0.3 consistent with a turnover limiting step involving oxidative addition of the C–H bond to the palladium catalyst. Isomerisation of the kinetic C–H aluminated product to the thermodynamic C–O ring expansion product is an intramolecular process that is again catalysed by [Pd(PCy₃)₂]. DFT calculations suggest that the key C–O bond breaking step involves attack of an aluminium based metalloligand on the 2-palladated heterocycle. The new methodology has been applied to important platform chemicals from biomass.

Non-catalysed and catalysed reactions of aluminium reagents with furans, dihydrofurans and dihydropyrans were investigated and lead to ring-expanded products due to the insertion of the aluminium reagent into a C–O bond of the heterocycle.     

A Promising Electrochemical Platform for Dopamine and Uric Acid Detection Based on a Polyaniline/Iron Oxide-Tin Oxide/Reduced Graphene Oxide Ternary Composite

A ternary polyaniline/Fe₂O₃-SnO₂/reduced graphene oxide (PFSG) nanocomposite was prepared using a simple two-step hydrothermal treatment. The composite was applied as a glassy carbon electrode modifier (GCE) to enhance dopamine (DA) and uric acid (UA) detection. The ternary PFSG composite was compared with its binary precursor Fe₂O₃-SnO₂/reduced graphene oxide (FSG). The influence of the modified GCE electrodes on their performance as a sensing platform was determined. GCE/PFSG showed better sensing parameters than GCE/FSG due to the introduction of polyaniline (PANI), increasing the electrocatalytic properties of the electrode towards the detected analytes. GCE/PFSG enabled the detection of low concentrations of DA (0.076 µM) and UA (1.6 µM). The peak potential separation between DA and UA was very good (180 mV). Moreover, the DA oxidation peak was unaffected even if the concentration of UA was ten times higher. The fabricated sensor showed excellent performance in the simultaneous detection with DA and UA limits of detection: LOD_DA = 0.15 µM and LOD_UA = 6.4 µM, and outstanding long-term stability towards DA and UA, holding 100% and 90% of their initial signals respectively, after one month of use.     

Exploring benzylic gem-C(sp3)–boron–silicon and boron–tin centers as a synthetic platform

A stepwise build-up of multi-substituted C_sp³ carbon centers is an attractive, conceptually simple, but often synthetically challenging type of disconnection. To this end, this report describes how gem-α,α-dimetalloid-substituted benzylic reagents bearing boron/silicon or boron/tin substituent sets are an excellent stepping stone towards diverse substitution patterns. These gem-dimetalloids were readily accessed, either by known carbenoid insertion into C–B bonds or by the newly developed scalable deprotonation/metallation approach. Highly chemoselective transformations of either the C–Si (or C–Sn) or the C–B bonds in the newly formed gem-C_sp³ centers have been achieved through a set of approaches, with a particular focus on exploiting the synthetically versatile polarity reversal in organometalloids by λ³-aryliodanes. Of particular note is the metal-free arylation of the C–Si (or C–Sn) bonds in such gem-dimetalloids via the iodane-guided C–H coupling approach. DFT calculations show that this transfer of the (α-Bpin)benzyl group proceeds via unusual [5,5]-sigmatropic rearrangement and is driven by the high-energy iodine(iii) center. As a complementary tool, the gem-dimetalloid C–B bond is shown to undergo a potent and chemoselective Suzuki–Miyaura arylation with diverse Ar–Cl, thanks to the development of the reactive gem-α,α-silyl/BF₃K building blocks.

This work explores divergent reactivity of the benzylic gem-boron–silicon and boron–tin double nucleophiles, including the arylation of the C–B bond with Ar–Cl, along with a complementary oxidative λ³-iodane-guided arylation of the C–Si/Sn moiety.     

Can acyclic conformational control be achieved via a sulfur–fluorine gauche effect?

The gauche conformation of the 1,2-difluoroethane motif is known to involve stabilising hyperconjugative interactions between donor (bonding, σ_C–H) and acceptor (antibonding, σ*C–F) orbitals. This model rationalises the generic conformational preference of F–C_β–C_α–X systems (φ_FCCX ≈ 60°), where X is an electron deficient substituent containing a Period 2 atom. Little is known about the corresponding Period 3 systems, such as sulfur and phosphorus, where multiple oxidation states are possible. Conformational analyses of β-fluorosulfides, -sulfoxides and -sulfones are disclosed here, thus extending the scope of the fluorine gauche effect to the 3rd Period (F–C–C–S(O)_n; φ_FCCS ≈ 60°). Synergy between experiment and computation has revealed that the gauche effect is only pronounced in structures bearing an electropositive vicinal sulfur atom (S⁺–O^–, SO₂).     

Molecular Characteristics and Antioxidant Activity of Spruce (Picea abies) Hemicelluloses Isolated by Catalytic Oxidative Delignification

Spruce (Picea abies) wood hemicelluloses have been obtained by the noncatalytic and catalytic oxidative delignification in the acetic acid-water-hydrogen peroxide medium in a processing time of 3–4 h and temperatures of 90–100 °C. In the catalytic process, the H₂SO₄, MnSO₄, TiO₂, and (NH₄)₆Mo₇O₂₄ catalysts have been used. A polysaccharide yield of up to 11.7 wt% has been found. The hemicellulose composition and structure have been studied by a complex of physicochemical methods, including gas and gel permeation chromatography, Fourier-transform infrared spectroscopy, and thermogravimetric analysis. The galactose:mannose:glucose:arabinose:xylose monomeric units in a ratio of 5:3:2:1:1 have been identified in the hemicelluloses by gas chromatography. Using gel permeation chromatography, the weight average molar mass M_w of hemicelluloses has been found to attain 47,654 g/mol in noncatalytic delignification and up to 42,793 g/mol in catalytic delignification. Based on the same technique, a method for determining the α and k parameters of the Mark–Kuhn–Houwink equation for hemicelluloses has been developed; it has been established that these parameters change between 0.33–1.01 and 1.57–472.17, respectively, depending on the catalyst concentration and process temperature and time. Moreover, the FTIR spectra of the hemicellulose samples contain all the bands characteristic of heteropolysaccharides, specifically, 1069 cm⁻¹ (C–O–C and C–O–H), 1738 cm⁻¹ (ester C=O), 1375 cm⁻¹ (–C–CH₃), 1243 cm⁻¹ (–C–O–), etc. It has been determined by the thermogravimetric analysis that the hemicelluloses isolated from spruce wood are resistant to heating to temperatures of up to ~100 °C and, upon further heating, start destructing at an increasing rate. The antioxidant activity of the hemicelluloses has been examined using the compounds simulating the 2,2-diphenyl-2-picrylhydrazyl free radicals.     

Cerium/Ascorbic Acid/Iodine Active Species for Redox Flow Energy Storage Battery

In this study, we developed a novel cerium/ascorbic acid/iodine active species to design a redox flow battery (RFB), in which the cerium nitrate hexahydrate [Ce(NO₃)₃·6H₂O] was used as a positive Ce³⁺/Ce⁴⁺ ion pair, and the potassium iodate (KIO₃) containing ascorbic acid was used as a negative I₂/I⁻ ion pair. In order to improve the electrochemical activity and to avoid cross-contamination of the redox pair ions, the electroless plating and sol–gel method were applied to modify the carbon paper electrode and the Nafion 117 membrane. The electrocatalytic and electrochemical properties of the composite electrode using methanesulfonic acid as a supporting electrolyte were assessed using the cyclic voltammetry (CV) test. The results showed that the Ce (III)/Ce (IV) active species presented a symmetric oxidation/reduction current ratio (1.09) on the C–TiO₂–PdO composite electrode. Adding a constant amount of ascorbic acid to the iodine solution led to a good reversible oxidation/reduction reaction. Therefore, a novel Ce/ascorbic acid/I RFB was developed with C–TiO₂–PdO composite electrodes and modified Nafion 117–SiO₂–SO₃H membrane using the staggered-type flow channel, of which the energy efficiency (EE%) can reach about 72%. The Ce/ascorbic acid/I active species can greatly reduce the electrolyte cost compared to the all-vanadium redox flow battery system, and it therefore has greater development potential.     

Superhard boron suboxide (B6O): Crystal structure,synthesis, properties,applications, and materials based thereon

A review of reported data on the crystal structure, synthesis, and properties of boron suboxide B₆O, which is a superhard material (hardness of up to 55 GPa, as measured in single crystals), and composites based on it is presented. Methods for the synthesis of B₆O with a icosahedral structure and for the growing of crystals at high temperature and high pressure are described. The mechanical and operational characteristics of composite materials based on B₆O are summarized; the resistance of B₆O to oxidation is analyzed, and the properties of B₆O–diamond and B₆O–B₄C composite materials are described.     

The stability of graphene and boron nitride for III-nitride epitaxy and post-growth exfoliation

A challenging approach, but one providing a key solution to material growth, remote epitaxy (RE)—a novel concept related to van der Waals epitaxy (vdWE)—requires the stability of a two-dimensional (2-D) material. However, when graphene, a representative 2-D material, is present on substrates that have a nitrogen atom, graphene loss occurs. Although this phenomenon has remained a hurdle for over a decade, restricting the advantages of applying graphene in the growth of III-nitride materials, few previous studies have been conducted. Here, we report the stability of graphene on substrates containing oxygen or nitrogen atoms. Graphene has been observed on highly decomposed Al₂O₃; however, graphene loss occurred on decomposed AlN at temperatures over 1300 °C. To overcome graphene loss, we investigated 2-D hexagonal boron nitride (h-BN) as an alternative. Unlike graphene on AlN, it was confirmed that h-BN on AlN was intact after the same high-temperature process. Moreover, the overgrown AlN layers on both h-BN/AlN and h-BN/Al₂O₃ could be successfully exfoliated, which indicates that 2-D h-BN survived after AlN growth and underlines its availability for the vdWE/RE of III-nitrides with further mechanical transfer. By enhancing the stability of the 2-D material on the substrate, our study provides insights into the realization of a novel epitaxy concept.

A challenging approach, but one providing a key solution to material growth, remote epitaxy (RE)—a novel concept related to van der Waals epitaxy (vdWE)—requires the stability of a two-dimensional (2-D) material.     

Bottom-up Design of Bimetallic Cobalt–Molybdenum Carbides/Oxides for Overall Water Splitting

Earth-abundant transition-metal-based catalysts for electrochemical water splitting are critical for sustainable energy schemes. In this work, we use a rational design method for the synthesis of ultrasmall and highly dispersed bimetallic CoMo carbide/oxide particles deposited on graphene oxide. Thermal conversion of the molecular precursors [H₃PMo₁₂O₄₀], Co(OAc)₂ ⋅ 4 H₂O and melamine in the presence of graphene oxide gives the mixed carbide/oxide (Co₆Mo₆C₂/Co₂Mo₃O₈) nanoparticle composite deposited on highly dispersed, N,P-doped carbon. The resulting composite shows outstanding electrocatalytic water-splitting activity for both the oxygen evolution and hydrogen evolution reaction, and superior performance to reference samples including commercial 20 % Pt/C & IrO₂. Electrochemical and other materials analyses indicate that Co₆Mo₆C₂ is the main active phase in the composite, and the N,P-doping of the carbon matrix increases the catalytic activity. The facile design could in principle be extended to multiple bimetallic catalyst classes by tuning of the molecular metal oxide precursor.     

A Study of the Effect of Sintering Conditions of Mg0.95Ni0.05Ti3 on Its Physical and Dielectric Properties

Mg_0.95Ni_0.05TiO₃ ceramics were prepared by traditional solid-state route using sintering temperatures between 1300 and 1425 °C and holding time of 2–8 h. The sintered samples were characterized for their phase composition, micro-crystalline structure, unit–cell constant, and dielectric properties. A two-phase combination region was identified over the entire compositional range. The effect of sintering conditions was analyzed for various properties. Both permittivity (ε_r) and Q factor (Q_f) were sensitive to sintering temperatures and holding times, and the optimum performance was found at 1350 °C withholding time of 4 h. The temperature coefficient of resonant frequency (τ_f) in a range from −45.2 to −52 (ppm/°C) and unit–cell constant were not sensitive to both the sintering temperature and holding time. An optimized Q factor of 192,000 (GHz) related with a permittivity (ε_r) of 17.35 and a temperature coefficient (τ_f) of −47 (ppm/°C) was realized for the specimen sintered at 1350 °C withholding time of 4 h. For applications of 5G communication device (filter, antennas, etc.), Mg_0.95Ni_0.05TiO₃ is considered to be a suitable candidate for substrate materials.     

Calcium Carbonate@silica Composite with Superhydrophobic Properties

In this paper, spherical calcium carbonate particles were prepared by using CaCl₂ aqueous solution + NH₃·H₂O + polyoxyethylene octyl phenol ether-10 (OP-10) + n-butyl alcohol + cyclohexane inverse micro emulsion system. Then, nanoscale spherical silica was deposited on the surface of micron calcium carbonate by Stöber method to form the composite material. Scanning electron microscope (SEM), X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Thermogravimetric analysis (TGA) and X-ray photoelectron spectroscopy (XPS) were used to characterize the morphology and structure of the composite material. It is found that the surface of the composite material has a micro-nano complex structure similar to the surface of a “lotus leaf”, making the composite material show hydrophobicity. The contact angle of the cubic calcium carbonate, spherical calcium carbonate and CaCO₃@SiO₂ composite material were measured. They were 51.6°, 73.5°, and 76.8°, respectively. After modification with stearic acid, the contact angle of cubic and spherical CaCO₃ were 127.1° and 136.1°, respectively, while the contact angle of CaCO₃@SiO₂ composite was 151.3°. These results showed that CaCO₃@SiO₂ composite had good superhydrophobicity, and the influence of material roughness on its hydrophobicity was investigated using the Cassie model theory.     

Carbon–carbon bond activation by B(OMe)3/B2pin2-mediated fragmentation borylation

Selective carbon–carbon bond activation is important in chemical industry and fundamental organic synthesis, but remains challenging. In this study, non-polar unstrained Csp²–Csp³ and Csp²–Csp² bond activation was achieved by B(OMe)₃/B₂pin₂-mediated fragmentation borylation. Various indole derivatives underwent C2-regioselective C–C bond activation to afford two C–B bonds under transition-metal-free conditions. Preliminary mechanistic investigations suggested that C–B bond formation and C–C bond cleavage probably occurred in a concerted process. This new reaction mode will stimulate the development of reactions based on inert C–C bond activation.

Non-polar unstrained Csp²–Csp³ and Csp²–Csp² bond activation was achieved via B(OMe)₃/B₂pin₂-mediated fragmentation borylation, in which C–C bond activation occurred regioselectively at the C2-position in various substituted indoles.     

Preparation of Curcumin-Eudragit® E PO Solid Dispersions with Gradient Temperature through Hot-Melt Extrusion

Hot-melt extrusion (HME) has great advantages for the preparation of solid dispersion (SD), for instance, it does not require any organic solvents. Nevertheless, its application to high-melting-point and thermosensitive drugs has been rarely reported. In this study, thermally unstable curcumin (Cur) was used as a drug model. The HME process was systematically studied by adjusting the gradient temperature mode and residence time, with the content, crystallinity and dissolution of Cur as the investigated factors. The effects of barrel temperature, screw speed and cooling rate on HME were also examined. Solubility parameters and the Flory–Huggins method were used to evaluate the miscibility between Cur and carriers. Differential scanning calorimetry, X-ray diffraction, Fourier transform infrared spectroscopy, equilibrium solubility and in vitro and in vivo experiments were used to characterize and evaluate the results. An amorphous Cur SD was successfully obtained, increasing the solubility and release of Cur. In the optimal process, the mass ratio of Cur to Eudragit^® E PO (EPO) was 1:4 and the barrel temperature was set at a gradient heating mode (130 °C–135 °C–140 °C–145 °C–150 °C–155 °C–160 °C) at 100 rpm. Related pharmacokinetic test results also showed the improved bioavailability of the drug in rats. In a pharmacodynamic analysis of Sprague–Dawley rats, the Cmax and the bioavailability of the Cur-EPO SD were 2.6 and 1.5 times higher than those of Cur, respectively. The preparation of the amorphous SD not only provided more solubility but also improved the bioavailability of Cur, which provides an effective way to improve the bioavailability of BCS II drugs.     

Estimation of the upper limit of carbon concentration in boron carbide crystals

The existence of a boron carbide phase with ∼25 at % carbon was proven experimentally. To evaluate the maximum possible concentration of C atoms in boron carbide (B_{12 − x} C _x )(BC₂) crystals, we performed quantum-chemical calculations of (B_{12 − x} C _x )(BH₂)₆(CH₃)₆ model compounds (x = 0–4; the goal of calculations was to determine the upper limiting number of C atoms in the B_{12 − x} C _x icosahedron) by the density functional theory method (B3LYP, 6-31G** basis set, full geometry optimization). A comparison of the experimental and calculated data showed that the calculations of the model compounds reproduced the experimental dependences of the structural parameters of the icosahedron (mean bond length and volume) on the number of C atoms in it. The icosahedra were found to be stable at x ≤ 3. According to the results of the quantum-chemical calculations, the maximum carbon concentration in boron carbide was 33 at %, which corresponded to the composition B₁₀C₅ and the structural formula (B₉C₃)(BC₂).     

A faux hawk fullerene with PCBM-like properties

Reaction of C₆₀, C₆F₅CF₂I, and SnH(n-Bu)₃ produced, among other unidentified fullerene derivatives, the two new compounds 1,9-C₆₀(CF₂C₆F₅)H (1) and 1,9-C₆₀(cyclo-CF₂(2-C₆F₄)) (2). The highest isolated yield of 1 was 35% based on C₆₀. Depending on the reaction conditions, the relative amounts of 1 and 2 generated in situ were as high as 85% and 71%, respectively, based on HPLC peak integration and summing over all fullerene species present other than unreacted C₆₀. Compound 1 is thermally stable in 1,2-dichlorobenzene (oDCB) at 160 °C but was rapidly converted to 2 upon addition of Sn₂(n-Bu)₆ at this temperature. In contrast, complete conversion of 1 to 2 occurred within minutes, or hours, at 25 °C in 90/10 (v/v) PhCN/C₆D₆ by addition of stoichiometric, or sub-stoichiometric, amounts of proton sponge (PS) or cobaltocene (CoCp₂). DFT calculations indicate that when 1 is deprotonated, the anion C₆₀(CF₂C₆F₅)^– can undergo facile intramolecular S_NAr annulation to form 2 with concomitant loss of F^–. To our knowledge this is the first observation of a fullerene-cage carbanion acting as an S_NAr nucleophile towards an aromatic C–F bond. The gas-phase electron affinity (EA) of 2 was determined to be 2.805(10) eV by low-temperature PES, higher by 0.12(1) eV than the EA of C₆₀ and higher by 0.18(1) eV than the EA of phenyl-C₆₁-butyric acid methyl ester (PCBM). In contrast, the relative E _1/2(0/–) values of 2 and C₆₀, –0.01(1) and 0.00(1) V, respectively, are virtually the same (on this scale, and under the same conditions, the E _1/2(0/–) of PCBM is –0.09 V). Time-resolved microwave conductivity charge-carrier yield × mobility values for organic photovoltaic active-layer-type blends of 2 and poly-3-hexylthiophene (P3HT) were comparable to those for equimolar blends of PCBM and P3HT. The structure of solvent-free crystals of 2 was determined by single-crystal X-ray diffraction. The number of nearest-neighbor fullerene–fullerene interactions with centroid···centroid (⊙···⊙) distances of ≤10.34 Å is significantly greater, and the average ⊙···⊙ distance is shorter, for 2 (10 nearest neighbors; ave. ⊙···⊙ distance = 10.09 Å) than for solvent-free crystals of PCBM (7 nearest neighbors; ave. ⊙···⊙ distance = 10.17 Å). Finally, the thermal stability of 2 was found to be far greater than that of PCBM.     

Application of dicesium metaborate ion for the determination of boron by thermal ionisation mass spectrometry

Application of dicesium metaborate ion /Cs₂BO ₂ ⁺ / in Thermal Ionisation Mass Spectrometry /TIMS/ for the determination of boron present at sub ppm level in heavy water moderator as well as for the isotopic composition of boron in boron carbide is reported. Contamination of samples with natural boron while determining trace levels of boron in heavy water was checked by analysing SRM-952 isotopic reference material. The atom ratios of boron in B₄C were determined by directly fusing the material on the tantalum filament with Cs₂CO₃ as well as with Na₂CO₃ and also by following the conventional fusion procedures and the results were compared.     

Fabrication of Large-Area Molybdenum Disulfide Device Arrays Using Graphene/Ti Contacts

Two-dimensional (2D) molybdenum disulfide (MoS₂) is the most mature material in 2D material fields owing to its relatively high mobility and scalability. Such noticeable properties enable it to realize practical electronic and optoelectronic applications. However, contact engineering for large-area MoS₂ films has not yet been established, although contact property is directly associated to the device performance. Herein, we introduce graphene-interlayered Ti contacts (graphene/Ti) into large-area MoS₂ device arrays using a wet-transfer method. We achieve MoS₂ devices with superior electrical and photoelectrical properties using graphene/Ti contacts, with a field-effect mobility of 18.3 cm²/V∙s, on/off current ratio of 3 × 10⁷, responsivity of 850 A/W, and detectivity of 2 × 10¹² Jones. This outstanding performance is attributable to a reduction in the Schottky barrier height of the resultant devices, which arises from the decreased work function of graphene induced by the charge transfer from Ti. Our research offers a direction toward large-scale electronic and optoelectronic applications based on 2D materials.     

Boron(iii) β-diketonate-based small molecules for functional non-fullerene polymer solar cells and organic resistive memory devices

A class of acceptor–donor–acceptor chromophoric small-molecule non-fullerene acceptors, 1–4, with difluoroboron(iii) β-diketonate (BF₂bdk) as the electron-accepting moiety has been developed. Through the variation of the central donor unit and the modification on the peripheral substituents of the terminal BF₂bdk acceptor unit, their photophysical and electrochemical properties have been systematically studied. Taking advantage of their low-lying lowest unoccupied molecular orbital energy levels (from −3.65 to −3.72 eV) and relatively high electron mobility (7.49 × 10⁻⁴ cm² V⁻¹ s⁻¹), these BF₂bdk-based compounds have been employed as non-fullerene acceptors in organic solar cells with maximum power conversion efficiencies of up to 4.31%. Moreover, bistable resistive memory characteristics with charge-trapping mechanisms have been demonstrated in these BF₂bdk-based compounds. This work not only demonstrates for the first time the use of a boron(iii) β-diketonate unit in constructing non-fullerene acceptors, but also provides more insights into designing organic materials with multi-functional properties.

Boron(iii) β-diketonates have been demonstrated to serve as multi-functional materials in NFA-based OPVs and organic resistive memories.     

A–X⋯σ Interactions—Halogen Bonds with σ-Electrons as the Lewis Base Centre

CCSD(T)/aug-cc-pVTZ//ωB97XD/aug-cc-pVTZ calculations were performed for halogen-bonded complexes. Here, the molecular hydrogen, cyclopropane, cyclobutane and cyclopentane act as Lewis base units that interact through the electrons of the H–H or C–C σ-bond. The FCCH, ClCCH, BrCCH and ICCH species, as well as the F₂, Cl₂, Br₂ and I₂ molecular halogens, act as Lewis acid units in these complexes, interacting through the σ-hole localised at the halogen centre. The Quantum Theory of Atoms in Molecules (QTAIM), the Natural Bond Orbital (NBO) and the Energy Decomposition Analysis (EDA) approaches were applied to analyse these aforementioned complexes. These complexes may be classified as linked by A–X···σ halogen bonds, where A = C, X (halogen). However, distinct properties of these halogen bonds are observed that depend partly on the kind of electron donor: dihydrogen, cyclopropane, or another cycloalkane. Examples of similar interactions that occur in crystals are presented; Cambridge Structural Database (CSD) searches were carried out to find species linked by the A–X···σ halogen bonds.     

Stimuli Responsive Materials Supported by Orthogonal Hydrogen and Halogen Bonding or I···Alkene Interaction

Smart materials represent an elegant class of (macro)-molecules endowed with the ability to react to chemical/physical changes in the environment. Herein, we prepared new photo responsive azobenzenes possessing halogen bond donor groups. The X-ray structures of two molecules highlight supramolecular organizations governed by unusual noncovalent bonds. In azo dye I-azo-NO₂, the nitro group is engaged in orthogonal H···O···I halogen and hydrogen bonding, linking the units in parallel undulating chains. As far as compound I–azo–NH–MMA is concerned, a non-centrosymmetric pattern is formed due to a very rare I···π interaction involving the alkene group supplemented by hydrogen bonds. The Cambridge Structural Database contains only four structures showing the same I···CH₂=C contact. For all compounds, an ¹⁹F-NMR spectroscopic analysis confirms the formation of halogen bonds in solution through a recognition process with chloride anion, and the reversible photo-responsiveness is demonstrated upon exposing a solution to UV light irradiation. Finally, the intermediate I–azo–NH₂ also shows a pronounced color change due to pH variation. These azobenzenes are thereby attractive building blocks to design future multi-stimuli responsive materials for highly functional devices.