Chromophore-Inspired Design of Pyridinium-Based Metal-Organic Polymers for Dual Photoredox Catalysis.

2,4,6-Triphenylpyrylium (TPT+ ) functions as a classic organic photocatalyst and exhibits a noteworthy absorption in the visible range, strongly oxidizing excited states, and a somewhat unstable structure. Inspired by the nuclear chromophore and dual catalysis strategy, herein, we report a universal photoredox platform constructed by TPT+ -mimic bridging ligands and reductive metal ions on the basis of metal-organic supramolecular systems for various organic couplings and molecular oxygen activation under visible-light irradiation. Significant photoinduced electron transfer and ligand-to-metal charge-transfer events are both integrated and regulated by the spatial and kinetic confinement effects of the structurally confined microenvironments, effectively improving the efficiency of electron transfer and radical-radical coupling processes in photocatalysis. This package deal provides a promising way for the design of novel photocatalysts and the development of versatile and sustainable synthetic chemistry.

Electron transfer in the confined environments of metal-organic coordination supramolecular systems.

The incorporation of electron transfer pairs in both ground and excited states (including electron transfer pairs in excited states and hydrogen or oxygen transfer pairs in ground states) into redox-active hosts with electronic acceptor or donor guests has led to development of a novel method of mimicking the photophysical properties and redox reactions of naturally occurring enzymatic systems. This occurs within the confined microenvironments of metal-organic capsules and metal-organic frameworks. These two types of coordination supramolecular host-guest systems can dock and separate electronic donor-acceptor pairs via closed through-space separation. Electron transfer within confined cavities, which is mainly controlled by spatial and kinetic effects, does not utilize a through-bond electron transfer pathway. In this review, we provide an overview of significant progress in the photophysical and catalytic applications of supramolecular host-guest systems with electron-transfer processes in confined environments. Special emphasis is placed on the action modes and regulatory factors that affect electron transfer between different components to produce enhanced photophysical or redox catalytic performance. Finally, the prospects for confined-environment electron transfer, its application to photophysics and catalysis, and the remaining challenges in this field are highlighted.

Theoretical Study on Hydrolytic Stability of Borohydride-Rich Hypergolic Ionic Liquids.

Hypergolic ionic liquids (HILs) are a new kind of green rocket fuels, which are used as potential replacements for toxic hydrazine derivatives in liquid bipropellants. These functional HILs can react with oxidizers and release a large amount of heat in a very short time, finally leading to ignition of the propellant system. Among them, most borohydride-rich HILs were very sensitive to water, but a few special examples displayed good hydrophobicity and remained very stable in air even after a month or more. However, the reasons behind their hydrolytic stability are unclear. In this study, several calculation methods including electrostatic potentials (ESPs), molecular orbital energy gaps, and interaction energy were used to explore the water stability of eight typical borohydride-rich HILs. The obtained results demonstrated that negatively charged anions with high absolute ESP values usually reacted more easily with positively charged water. The large molecular orbital energy gap with BPB-, BCNBCN-, CTB-, and BTB- indicates the high degree of difficulty of interactions between anions and water, leading to a better hydrolytic stability of borohydride-rich anions. During the analyses of interaction energy, the relatively water-sensitive borohydride-rich anions (BH4-, BH3CN-, etc.) generally had lower interaction energy with water than stable anions such as BPB- and BCNBCN-. Studies on their stepwise hydrolysis mechanism demonstrate that, in the case of all the reactions, the first step is the rate-determining step and high energy barrier values of anions correspond to good hydrophobicity. This study will help us understand the hydrolysis of borohydride-rich HILs and provide a guide for the development of new HILs with promising properties.

Synthesis and Properties of Triaminocyclopropenium Cation Based Ionic Liquids as Hypergolic Fluids.

A novel family of hydrophobic triaminocyclopropenium cation based ionic liquids have been synthesized, and their structures and physicochemical properties characterized by NMR and IR spectroscopy, elemental analysis, differential scanning calorimetry, and hypergolic tests. The experimental results showed that all of these ionic liquids exhibited the expected hypergolic reactivity with the oxidizer white fuming nitric acid. Among them, the hypergolic ionic liquid based on the cyanoimidazolylborohydride anion showed excellent integrated properties, including high decomposition temperature (194 °C), high density (0.95 g cm-3 ), moderate viscosity (44 MPa s), ultrafast ignition delay time (6 ms), and high specific impulse (301.9 s); this demonstrates its potential as an environmentally friendly alternative to toxic hydrazine derivatives.

Rational Design and Facile Synthesis of Boranophosphate Ionic Liquids as Hypergolic Rocket Fuels.

The design and synthesis of new hypergolic ionic liquids (HILs) as replacements for toxic hydrazine derivatives have been the focus of current academic research in the field of liquid bipropellant fuels. In most cases, however, the requirements of excellent ignition performances, good hydrolytic stabilities, and low synthetic costs are often contradictory, which makes the development of high-performance HILs an enormous challenge. Here, we show how a fuel-rich boranophosphate ion was rationally designed and used to synthesize a series of high-performance HILs with excellent comprehensive properties. In the design strategy, we introduced the {BH3 } moiety into the boranophosphate ion for improving the self-ignition property, whereas the complexation of boron and phosphite was used to improve the hydrolytic activity of the borohydride species. As a result, these boranophosphate HILs exhibited wide liquid operating ranges (>220 °C), high densities (1.00-1.10 g cm-3 ), good hydrolytic stabilities, and short ignition delay times (2.3-9.7 milliseconds) with white fuming nitric acid (WFNA) as the oxidizer. More importantly, these boranophosphate HILs could be readily prepared in high yields from commercial phosphite esters, avoiding complex and time-consuming synthetic routes. This work offers an effective strategy of designing boranophosphate HILs towards safer and greener hypergolic fuels for liquid bipropellant applications.

Visible-Light-Mediated Aerobic Oxidation of N-Alkylpyridinium Salts under Organic Photocatalysis.

Quinolones and isoquinolones exhibit diverse biological and pharmaceutical activities, and their synthesis is highly desirable under mild conditions. Here, a highly efficient and environmentally friendly visible light-mediated aerobic oxidation of readily available N-alkylpyridinium salts has been developed with Eosin Y as the organic photocatalyst and air as the terminal oxidant, and the reaction provided quinolones and isoquinolones in good yields. The method shows numerous advantages including mild and environmentally friendly conditions, high efficiency, tolerance of wide functional groups and low cost. Furthermore, 4-desoxylonimide with important pharmaceutical activities was effectively prepared by using our method. Therefore, the present method should provide a novel and useful strategy for synthesis and modification of N-heterocycles.

Iron-Catalyzed Photoredox Alcohol α-C-H Alkylation and Tandem Intramolecular Cyclization: Facile Access to Multisubstituted 2,3-Dihydrofurans and γ-Butyrolactones.

Multisubstituted furans occupy a pivotal position within the realms of synthetic chemistry and pharmacological science due to their distinctive chemical configurations and inherent properties. We herein introduce a tandem difunctionalization protocol of alcohols for the efficient synthesis of multisubstituted 2,3-dihydrofurans and γ-butyrolactones through the combination of photocatalysis and iron catalysis under mild conditions. Photoredox alcohol α-C(sp3)-H activation and Pinner-type intramolecular cyclization are two key processes. This method features significant convenience, economic benefits, and environmental friendliness.

A Modular Three-Component Approach for Site-selective Tandem Arene Thiophosphorylation.

Thiophosphates serve as pivotal reagents within the realms of both organic and inorganic synthesis, with their most notable applications observed in agricultural chemistry. This manuscript delineates a modular three-component synthetic strategy for site-selective arene C-H thiophosphorylation with thianthrenium salt, 1,4-diazabicyclo[2.2.2]octane-sulfur dioxide (DABSO), and diarylphosphine oxides as substrates. This approach facilitates the metal-free and green synthesis of a diverse spectrum of S-aryl phosphorothioates through C-H functionalization and late-stage modification showcasing practicality and broad applicability.

Copper-catalyzed aerobic oxidative C-H functionalization of substituted pyridines: synthesis of imidazopyridine derivatives.

A novel, efficient, and practical method for the synthesis of imidazopyridine derivatives has been developed through the copper-catalyzed aerobic oxidative C-H functionalization of substituted pyridines with N-(alkylidene)-4H-1,2,4-triazol-4-amines. The procedure occurs by cleavage of the N-N bond in the N-(alkylidene)-4H-1,2,4-triazol-4-amines and activation of an aryl C-H bond in the substituted pyridines. This is the first example of the preparation of imidazopyridine derivatives by using pyridines as the substrates by transition-metal-catalyzed C-H functionalization. This method should provide a novel and efficient strategy for the synthesis of other nitrogen heterocycles.

Difunctionalization of 1,3-Butadiene via Sequential Radical Thiol-ene Reaction and Allylation by Dual Photoredox and Titanium Catalysis.

Recently, radical difunctionalization of the feedstock 1,3-butadiene has become an attractive strategy for increasing molecular complexity. Herein, we present a novel approach that effectively combines radical thiol-ene chemistry with TiIII catalysis to enable a three-component aldehyde allylation using 1,3-butadiene as an allyl group source under visible light conditions. This sustainable and straightforward method has facilitated the rapid production of diverse allylic 1,3-thioalcohols with exceptional regio- and diastereoselectivity.

Visible-Light-Induced Radical Cascade Cross-Coupling via C(sp3)-H Activation and C-N/N-O Cleavage: Feasible Access to Methylenebisamide Derivatives.

Here we report facile and manipulable access to methylenebisamide derivatives via visible-light-driven radical cascade processes incorporating C(sp3)-H activation and C-N/N-O cleavage. Mechanistic studies reveal that a traditional Ir-catalyzed photoredox pathway and a novel copper-induced complex-photolysis pathway are both involved, contributing to activating the inert N-methoxyamides and rendering the valuable bisamides. This approach exhibits many advantages, including mild reaction conditions, broad scope and functional group tolerance, and competitive step economy. Given the mechanistic plenitude and operational simplicity, we believe this package deal paves a promising way for the synthesis of valuable nitrogen-containing molecules.

Photochemical Nozaki-Hiyama-Kishi Coupling Enabled by Excited Hantzsch Ester.

This work reports the first photochemical Nozaki-Hiyama-Kishi coupling enabled by bioinspired Hantzsch ester. The salient feature of this process is that commercially available and low-cost organic photoactive Hantzsch ester can serve as both an electron and a proton donor to reduce Cr/Ni to low-valent species and hydrolyze the CrIII-alkoxy bond, thus bypassing the use of stoichiometric metallic reductants and additives such as TMSCl and Cp2ZrCl2. The mild conditions and operationally easy method showed broad compatibility with various alkenyl triflates and aldehydes, including electron-poor pentafluorobenzaldehyde which failed under previous conditions.

Iron-Catalyzed Photoredox Functionalization of Methane and Heavier Gaseous Alkanes: Scope, Kinetics, and Computational Studies.

Herein, we report the development of the photocatalytic C-H functionalization of methane, ethane, and heavier gaseous alkanes with good yields and selectivity, broad scope (57 examples), mild conditions, and low cost. Kinetics and density functional theory calculations were investigated for the key photoinduced ligand-to-metal charge transfer and hydrogen atom transfer processes to reveal the detailed mechanism of iron photocatalysis. This work may bring novel ideas for feedstock upgrading and catalyst design.

Photocatalyst-Free Visible-Light Photoredox Dearomatization of Phenol Derivatives Containing Ketoximes: An Easy Access to Spiropyrrolines.

A novel and simple visible-light photoredox intramolecular dearomatization of phenol derivatives containing ketoximes leading to spiropyrrolines has been developed. The protocol uses readily available 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as the base and electron-donor, visible light as the light source, and the reaction was performed well at room temperature without need of a photocatalyst. Therefore, the present method should provide a useful strategy for synthesis of spiropyrrolines.

Fabrication of protonated g-C3N4 nanosheets as promising proton conductive materials.

Protonated g-C3N4 nanosheets were prepared by refluxing a mixture containing g-C3N4 and 95% H2SO4 at 80 °C for 1 hour. The obtained g-C3N4 nanosheets showed a similar layer structure to that of g-C3N4 and high water adsorption capacities. They also depicted superior conductivities reaching up to 1.44 × 10-2 S cm-1 at 80 °C and 98% RH, with promising features for proton conductivity materials.

Synthesis and hypergolic properties of flammable ionic liquids based on the cyano (1H-1,2,3-triazole-1-yl) dihydroborate anion.

A series of hypergolic ionic liquids (HILs) based on the cyano (1H-1,2,3-triazole-1-yl) dihydroborate anion were synthesized by introducing the {BH2-CN} moiety into the 1,2,3-triazole anion. This introduction allowed us to improve the self-ignition property and decrease the viscosity of ionic fuels. The synthesized series of HILs exhibited wide liquid operating ranges (>220 °C), high densities (>1 g cm-3), low viscosities (as low as 22.48 cP), and ignition delay (ID) times as short as 5 ms by using white fuming nitric acid (WFNA) as the oxidizer. Furthermore, these HILs can be ignited upon contact with a 90 wt% H2O2 oxidizer in the presence of iodine.

Combination of gem-dinitromethyl functionality and a 5-amino-1,3,4-oxadiazole framework for zwitterionic energetic materials.

A new design strategy for zwitterionic energetic materials was developed, and two gem-dinitromethyl functionalized 5-amino-1,3,4-oxadiazolate inner salts were synthesized and characterized. Compared with common energetic salt analogues, both zwitterionic energetic compounds exhibited higher densities, better detonation performances, and comparable mechanical sensitivities. This design strategy may stimulate the development of additional zwitterionic energetic materials.

Synthesis of Thermally Stable and Insensitive Energetic Materials by Incorporating the Tetrazole Functionality into a Fused-Ring 3,6-Dinitropyrazolo-[4,3-c]Pyrazole Framework.

A series of fused-ring energetic materials, i.e., 3,6-dinitro-1,4-di(1H-tetrazol-5-yl)-pyrazolo[4,3-c]pyrazole (DNTPP, compound 2) and its ionic derivatives (compounds 3-8), were designed and synthesized in this study. The molecular structures of compounds 2, 3, 6, 7·2H2O, and 8 were confirmed using single-crystal X-ray diffraction. Their physicochemical and energetic properties, such as density, thermal stability, heat of formation, sensitivity, and detonation properties (e.g., detonation velocity and detonation pressure), were also evaluated. The results indicate that DNTPP and most of its ionic derivatives are extremely thermally stable and insensitive toward mechanical stimuli. In particular, the thermal decomposition temperature of compound 3 is up to 329 °C, while compounds 7 and 8 are very insensitive (impact sensitivity: >20 J; friction sensitivity: >360 N). Compounds 2, 3, and 6 possess good comprehensive properties, including excellent thermal stability, remarkable low sensitivities, and favorable detonation performance. These features show that DNTPP and its ionic derivatives have considerable promise as thermally stable and insensitive energetic materials.

Revisiting the reactive chemistry of FOX-7: cyclization of FOX-7 affords the fused-ring polynitro compounds.

A novel fused-ring polynitro compound (5) and a cyclic compound featuring gem-dinitromethylene and nitroso groups (7) were synthesized from 1,1-diamino-2,2-dinitroethylene (FOX-7). Both compounds exhibit high density, decent thermal stability, high detonation performances, and low mechanical sensitivities. This newly developed derivatization strategy enriches the ever-expanding reactive chemistry of FOX-7.

A simple and versatile strategy for taming FOX-7.

A novel derivatization strategy was developed for the exciting reactive chemistry of FOX-7. The as-synthesized FOX-7-derived polynitro compounds exhibited high crystal densities, excellent detonation performances, and good impact and friction sensitivities. This study opens a new path for the ever-expanding chemistry of FOX-7.