Structure and dynamic features of an intramolecular frustrated Lewis pair

Di(mesityl)cyclohexenylphosphine undergoes hydroboration with Piers' borane [HB(C₆F₅)₂] to yield the cyclohexylene‐anellated frustrated Lewis pair 5 . This P/B pair splits H₂ with the formation of the product 4 and adds to the C?O double bond of phenyl isocyanate to yield 6 . In the crystal, compound 5 features a puckered four‐membered heterocyclic core structure with a long P? B bond (av. 2.197(5) Å). The activation energy of the P? B cleavage of the frustrated Lewis pair 5 was determined by dynamic ¹⁹F NMR spectroscopy at ΔG^≠(298 K)=12.1±0.3 kcal mol^?1.     

CO2 and formate complexes of phosphine/borane frustrated Lewis pairs

The reaction of a solution of B(C6F4H)3 and either iPr3P or tBu3P with CO2 afforded the species R3P(CO2)B(C6F4H)3 (R=iPr (1), tBu (2)). In a similar fashion the boranes, RB(C6F5)2 (R=hexyl, cyclohexyl (Cy), norbornyl), ClB(C6F5)2, or PhB(C6F5)2 were combined with tBu3P and CO2 to give the species tBu3P(CO2)BR(C6F5)2 (R=hexyl (3), Cy (4), norbornyl (5), Cl (6), Ph (7)). Similarly, the compounds [tBu3PH][RBH(C6F5)2] (R= hexyl (8), Cy (9), norbornyl (10)) were prepared by reaction of the precursor frustrated Lewis pair (FLP) with H2. Subsequent reactions of 9 and 10 with CO2 afforded the species [((C6F5)2BR)2(μ-HCO2)][tBu3PH] (R= Cy (11), norbornyl (12)). In related chemistry, combinations of the boranes RBG(C6F5)2 (R=hexyl, Cy, norbornyl) with tBu3P treated with an equivalent of formic acid gave [(C6F5)2BR(HCO2)][tBu3PH] (R=hexyl (13), Cy (14), norbornyl (15)). Subsequent addition of an additional equivalent of borane provides a second synthetic route to 11 and 12. Crystallographic studies of compounds 2-6 and 8-14 are reported and discussed. Further understanding of the FLP complexation and activation of CO2 is provided by computational studies.     

The Taming of Redox-Labile Phosphidotitanocene Cations

Tame d⁰ phosphidotitanocene cations stabilized with a pendant tertiary phosphane arm are reported. These compounds were obtained by one-electron oxidation of d¹ precursors with [Cp₂Fe][BPh₄]. The electronic structure of these compounds was studied experimentally (EPR, UV/Vis, and NMR spectroscopy, X-ray diffraction analysis) and through DFT calculations. The theoretical analysis of the bonding situation by using the electron localization function (ELF) shows the presence of π-interactions between the phosphido ligand and Ti in the d⁰ complexes, whereas dπ–pπ repulsion prevents such interactions in the d¹ complexes. In addition, CH–π interactions were observed in several complexes, both in solution and in the solid state, between the phosphido ligand and the phosphane arm. The d⁰ complexes were found to be light sensitive, and decompose through Ti−P bond homolysis to give Ti^III species. A naked d⁰ phosphidotitanocene cation has been trapped by reaction with diphenylacetylene, yielding a Ti/P frustrated Lewis pair (FLP), which was found to be less reactive than a previously reported Zr analog.     

Cycling between Molybdenum-Dinitrogen and -Nitride Complexes to Support the Reaction Pathway for Catalytic Formation of Ammonia from Dinitrogen

Cycling between molybdenum(I)-dinitrogen and molybdenum(IV)-nitride complexes was investigated under ambient reaction conditions. A kinetic study of the second-order reaction rate for the conversion of the molybdenum-dinitrogen complex into the molybdenum-nitride complex indicates that the formation of the dinitrogen-bridged dimolybdenum complex is involved in the rate-determining step. DFT calculations indicate that the molybdenum-dinitrogen complex transforms into the molybdenum-nitride complex via direct cleavage of the nitrogen-nitrogen triple bond of the bridging dinitrogen ligand of the dinitrogen-bridged dimolybdenum complex. The corresponding reaction of the molybdenum-nitride complex transforming into the molybdenum-dinitrogen complex proceeds via the ligand exchange of ammonia for dinitrogen at the dinitrogen-bridged dimolybdenum complexes. A new modified reaction pathway has been proposed based on the findings of our experimental and theoretical results.     

Rational Design of a Carbon‐Boron Frustrated Lewis Pair for Metal‐free Dinitrogen Activation

Molecular nitrogen (N₂) is abundant in the atmosphere and, found in many biomolecules, an essential element of life. The Haber–Bosch process, developed over 100 years ago, requires relatively harsh conditions to activate N₂ on the iron surface and generate ammonia for use as fertilizer or to produce other chemicals, leading to consumption of more than 2 % of the world's annual energy supply. Thus, developing “green” approaches for N₂ activation under mild conditions is particularly important and urgent. Here we demonstrate that a metal‐free N₂ activation could be favorable both thermodynamically and kinetically (with an activation energy as low as 9.1 kcal mol^?1) by using a carbon‐boron formal frustrated Lewis pair, which is supported by high‐level coupled cluster calculations. Mechanistic studies reveal that aromaticity plays a crucial role in stabilizing both the transition state and the product. Our findings highlight the importance of a combination of an N‐heterocyclic carbene with a methyleneborane unit in metal‐free N₂ activation, providing conceptual guidance for experimental realization.     

Reaction of a 6-dimethylaminopentafulvene with the Mes2PCH2CH2B(C6F5)2 frustrated Lewis pair

The frustrated Lewis pair Mes(2)PCH(2)CH(2)B(C(6)F(5))(2) reacts readily with 6-dimethylamino-6-methylfulvene at room temperature to yield the trans-1-[B(C(6)F(5))(2)]-2-[CH(2)CH(2)PMes(2)] disubstituted fulvene derivative 9 that features an internal N-B contact. Thermolysis (80 °C in toluene) results in a complete isomerization to the respective 1-[B(C(6)F(5))(2)]-3-[CH(2)CH(2)PMes(2)] isomer 10. Both compounds were characterized by using X-ray diffraction. A reaction scheme is formulated to rationalize the specific formation of these compounds, involving a retro-hydroboration/hydroboration sequence. The reaction of the 6-dimethylaminofulvene with HB(C(6)F(5))(2) yielded the corresponding parent compound 13 that was also characterized by X-ray diffraction.     

Probing the Charge Transfer in a Frustrated Lewis Pair by Resonance Raman Spectroscopy and DFT Calculations

A classical Lewis adduct derives from a covalent bond between a Lewis acid and a base. When the adduct formation is precluded by means of steric hindrance the association of the respective acid-base molecular system is defined as a frustrated Lewis pair (FLP). In this work, the archetypal FLP Mes₃P/B(C₆F₅)₃ was characterized for the first time by resonance Raman spectroscopy, and the results were supported by density functional theory (DFT) calculations. The charge transfer nature of the lowest energy electronic transition, from phosphine to borane, was confirmed by the selective enhancement of the Raman bands associated to the FLP chromophore at resonance condition. Herein, we demonstrate the use of resonance Raman spectroscopy as a distinguished technique to probe the weak interaction involved in FLP chemistry.     

Development of [3]ferrocenophane-derived N/B frustrated Lewis pairs for the metal-free catalytic hydrogenation of imines

A series of novel [3]ferrocenophane-derived N/B frustrated Lewis pairs (FLPs) were synthesized and successfully applied to the catalytic hydrogenation of imines in 71–93% yields. This approach could be easily conducted on gram scale and provided versatile synthetic route for the key intermediate of sertraline hydrochloride without heavy metal residues.     

Heterolytic outer-sphere cleavage of h(2) for the reduction of n(2) in the coordination sphere of transition metals-a DFT study

Cleaved by frustration, an H(2) molecule inserts into the void between the components of the frustrated Lewis pair 1, which consists of a dicationic ruthenium pincer complex as the Lewis acid and (tBu)(3) P as the Lewis base. The end-on-coordinated N(2) in 1 accepts the hydride, and this is the initial step, which could, in principle, lead to the complete reduction of N(2) to NH(3) .     

Solid-State Nuclear Magnetic Resonance Techniques for the Structural Characterization of Geminal Alane-Phosphane Frustrated Lewis Pairs and Secondary Adducts

The first comprehensive solid-state nuclear magnetic resonance (NMR) characterization of geminal alane-phosphane frustrated Lewis pairs (Al/P FLPs) is reported. Their relevant NMR parameters (isotropic chemical shifts, direct and indirect ²⁷Al-³¹P spin-spin coupling constants, and ²⁷Al nuclear electric quadrupole coupling tensor components) have been determined by numerical analysis of the experimental NMR line shapes and compared with values computed from the known crystal structures by using density functional theory (DFT) methods. Our work demonstrates that the ³¹P NMR chemical shifts for the studied Al/P FLPs are very sensitive to slight structural inequivalences. The ²⁷Al NMR central transition signals are spread out over a broad frequency range (>200 kHz), owing to the presence of strong nuclear electric quadrupolar interactions that can be well-reproduced by the static ²⁷Al wideband uniform rate smooth truncation (WURST) Carr-Purcell-Meiboom-Gill (WCPMG) NMR experiment. ²⁷Al chemical shifts and quadrupole tensor components offer a facile and clear distinction between three- and four-coordinate aluminum environments. For measuring internuclear Al⋅⋅⋅P distances a new resonance-echo saturation-pulse double-resonance (RESPDOR) experiment was developed by using efficient saturation via frequency-swept WURST pulses. The successful implementation of this widely applicable technique indicates that internuclear Al⋅⋅⋅P distances in these compounds can be measured within a precision of ±0.1 Å.     

Using the Secondary PH/BH Functional Groups of an Active Geminal Frustrated Lewis Pair for Carbon Monoxide Reduction and Reactions with Nitriles and Isonitriles

Reaction of the secondary alkynyl(Mes*)PH phosphane 2 with (Fmes)BH₂?SMe₂ gives the geminal PH/BH frustrated Lewis pair (FLP) 3 . The PH and the BH functions are jointly used in the reduction of carbon monoxide under mild reaction conditions to give the [P]‐CH₂‐O‐[B] product. A subsequent cycloaddition sequence results in the liberation of formaldehyde. The FLP 3 reacts with benzonitrile to give a P‐benzamidine, and it couples two isonitriles at the FLP framework.     

Why the hydrothermal fluorinated method can improve photocatalytic activity of carbon nitride

Carbon nitride(CN) photocatalysts have attracted much attention due to their excellent photocatalytic properties.And hydrothermal fluorination is a common method to improve the photocatalytic effect of CN photocatalyst.Here,the influence of the band gap was first revealed of fluorination and hydroxylation of CN photocatalyst based on the first theoretical principle.Here,the effect of fluorination and hydroxylation on the CN band gap was discussed for the first time using the first theoretical principle.With F atoms and OH doping,the band gap of CN was significantly improved,conduction band and valence band moved up.Then,F-CN photocatalyst with F atoms and OH was successfully synthesized by a hydrothermal fluorinated method.Next,the reasons why F-CN photocatalyst was more effective than that of traditional CN photocatalyst were fully discussed.From the photocatalytic effect of photocatalyst(12,593.2 μmolg^-1 h^-1to the morphology(super-small nanosheets),structure(homojunctions),composition(metal-free),specific surface area(54.1 m²/g),visible light absorption response(AQE is10.9% at 420 nm) and photo-induced carrier life(14.13 ns).Therefore,this work has a great guiding effect on the development of CN photocatalyst.     

Synthesis of carbon nitride hollow microspheres with highly hierarchical porosity templated by poly (ionic liquid) for photocatalytic hydrogen evolution

Hydrogen, as a sustainable and clean energy, has been considered as a promising candidate to replace fossil fuels. And it is meaningful to fabricate the photocatalysts to drive photocatalytic water splitting leading to hydrogen production. Herein, a facile approach was developed by the means of the template effect of poly (ionic liquid) and self-assembly of cyanuric acid and melamine through hydrogen bonds, to obtain carbon nitride hollow microspheres with highly hierarchical porosity. The influence of poly (ionic liquid) concentration on the structure and photocatalytic activity of as-prepared carbon nitride was investigated. The optimized carbon nitride hollow microspheres possessed the multiple porous channels and improved surface area (71 m²/g) due to the decomposition of poly (ionic liquid) and cyanuric acid-melamine supramolecular aggregates. Moreover, the as-prepared carbon nitride hollow microspheres exhibited a remarkable catalytic activity in the photocatalytic hydrogen evolution reaction under visible light irradiation. Especially, the sample CN-0.02 exhibits the highest hydrogen evolution rate (90.1 μmol h⁻¹). The outstanding photocatalytic activity is attributed to the high specific surface area, broad light absorption range and fast separation rate of photogenerated electron–hole pairs. This novel method opens up a new way toward the development of highly-active photocatalysts for water splitting.     

Development of silicon nitride fiber from Si-containing polymer by radiation curing and its application

A silicon nitride fiber (Si₃N₄) was synthesized from polycarbosilane (PCS) fiber by radiation application. PCS fibers were cured by electron beam (EB) irradiation in a helium gas atmosphere prior to the pyrolysis. The cured PCS fiber was converted to Si₃N₄ ceramic fiber with flexibility by nitridation in ammonia gas at a high temperature of 500–1000°C. The obtained Si₃N₄ fibre showed a high heat resistance up to 1300°C, a high tensile strength of 2 GPa and excellent electrical resistivity of 10¹³ Ω cm. The ceramic fiber was fabricated to cloth and applied for electric wire insulator. The wire cable is flexible and can be applied at a high temperature atmosphere of around 1000°C.     

Ammonothermal Synthesis,Crystal Structure,and Vibrational Properties of the Doubly Deprotonated Calcium Guanidinate,CaC(NH)3

We report the synthesis of the doubly deprotonated calcium guanidinate, CaC(NH)₃, from liquid ammonia and its crystal structure as determined from powder X-ray and neutron diffraction, and confirmed using CNH elemental analysis and infrared (IR) spectroscopy data. CaC(NH)₃ crystallizes in the hexagonal system with space group P6₃/m (no. 176) with Z = 2 and a = 5.2666(13) Å, c = 6.5881(6) Å and V = 158.25(4) ų. We also compare the structural similarities and differences of this phase with the isotypical strontium and ytterbium compounds.