Electrochemical cascade migratory versus ortho-cyclization of 2-alkynylbenzenesulfonamides.

Efficient control over several possible reaction pathways of free radicals is the chemical basis of their highly selective transformations. Among various competing reaction pathways, sulfonimidyl radicals generated from the electrolysis of 2-alkynylbenzenesulfonamides undergo cascade migratory or ortho-cyclization cyclization selectively. It is found that the incorporation of an extra 2-methyl substituent biases the selective migration of the acyl- over vinyl-linker of the key spirocyclic cation intermediate and thus serves as an enabling handle to achieve the synthetically interesting yet under-investigated cascade migratory cyclization of spirocyclic cations.

A Triplet Iron Carbyne Complex.

Tuning the spin state of metal carbynes, which have broad applications in organic synthesis and material science, presents a formidable challenge for modern chemists as the strong field nature of carbyne ligands dictates low-spin ground spin states (S = 0 or 1/2) for known metal carbynes. Through the oxidative addition reaction of a low-coordinate iron(0) N-heterocyclic carbene complex with the C-S bond of a thioazole-2-ylidene, we synthesized the first triplet (S = 1) metal terminal carbyne, an iron cyclic carbyne complex. Different from the classical metal carbynes, the triplet complex features an LXZ-type carbyne ligand and a weak Fe≡C triple bond, which endow it with the unique reactivity pattern of facile carbyne coupling, weak affinity toward nucleophiles, and facial addition reactions with electrophiles.

π-Aromaticity Dominating in a Saturated Ring: Neutral Aromatic Silicon Analogues of Cyclobutane-1,3-diyls.

The synthesis, structures, and reactivity of the first neutral 2π-aromatic Si4 rings [LSiSiAr(X)]2 (3: X = Br; 4: X = Cl; L = PhC(NtBu)2, Ar = 2,4,6-Me3C6H2) were described. Compounds 3 and 4 were obtained by 1,3-halogenation of tetrasilacyclobutadiene (LSiSiAr)2 (2), which was prepared by the reductive cross-coupling of trisilane (ArSiCl2)2SiHAr with two equiv of chlorosilylene LSiCl. The reaction of 3 with two equiv of PhLi yielded the corresponding substitution Si4 ring [LSiSiAr(Ph)]2 (5). Single-crystal X-ray diffraction analysis of 3 disclosed that it adopts both puckered (3a) and planar (3b) structures in the solid state, whereas 4 and 5 exhibit only a puckered structure. DFT calculations suggested that the puckered 3a features almost the same electronic structure with fully delocalized 2π planar 3b. The dominant 2π-aromaticity of 3 in a σ-frame has been demonstrated by DFT calculations, providing the first example of aromatics featuring both planar and puckered structures.

Simultaneously realizing enhancement of sensitivity for freshness monitoring and multinomial properties of carrageenan/konjac glucomannan/blueberry anthocyanin-based intelligent film by diatomite.

Here, highly sensitive blueberry anthocyanin (BBA)-induced intelligent indicating films were fabricated by incorporating a novel composite ingredient, diatomite (DA), into a matrix of konjac glucomannan (KGM), carrageenan (CAR) and BBA. We systematically investigated the effects of introducing DA and BBA on the structure, physical properties, colorimetric response, and practical application of the KGM/CAR film. Our findings revealed that the DA particles and BBA were well-distributed in the KGM/CAR matrix through hydrogen bonding interactions. This distribution significantly improved tensile strength, surface hydrophobicity, thermal stability, and barrier properties of the KGM/CAR film. Notably, the KGM/CAR-based intelligent film loaded with 6 % DA exhibited the most optimal properties. Furthermore, DA exhibited a hierarchical porous structure, enabling the KGM/CAR film to detect volatile amines with heightened sensitivity. When applied to monitor shrimp spoilage in transparent plastic packaging, the color of the composite film underwent remarkable changes from bright pink to bluish violet. These color changes correlated well with the total volatile basic nitrogen (TVB-N) and pH changes in the shrimp, as determined by standard laboratory procedures. Our work presents a promising approach to the development of high-performance and intelligent food packaging materials. These materials hold great potential for practical applications in the field of food packaging.

Synthesis and Reactivity of an Anti-van't Hoff/Le Bel Compound with a Planar Tetracoordinate Silicon(II) Atom.

For a long time, planar tetracoordinate carbon (ptC) represented an exotic coordination mode in organic and organometallic chemistry, but it is now a useful synthetic building block. In contrast, realization of planar tetracoordinate silicon (ptSi), a heavier analogue of ptC, is still challenging. Herein we report the successful synthesis and unusual reactivity of the first ptSi species of divalent silicon present in 3, supported by the chelating bis(N-heterocyclic silylene)bipyridine ligand, 2,2'-{[(4-tBuPh)C(NtBu)]2SiNMe}2(C5N)2, 1]. The compound resulted from direct reaction of 1 with Idipp-SiI2 [Idipp = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene]. Alternatively, it can also be synthesized by a two-electron reduction of the corresponding Si(IV) precursor 2 with 2 molar equiv of KC10H8. Density functional theory calculations show that the lone pair at the ptSi(II) resides almost completely in its 3pz orbital, very different from known four-coordinate silylenes. Oxidative addition of MeI to the ptSi(II) atom affords the corresponding pentacoordinate Si(IV) compound 4, with the methyl group located in an apical position. Remarkably, the reaction of 2 with [CuOtBu] leads to the regeneration of the bis(silylene) arms via Si-Si bond scission and induces the Si(II) → Si(IV) oxidation of the central Si(II) atom and concomitant two-electron reduction of the bipyridine moiety to form the neutral bis(silylene)silyl Cu(I) complex 5.

Predicting Small Molecule Activation including Catalytic Hydrogenation of Dinitrogen Promoted by a Dual Lewis Acid.

For decades, N2 activation and functionalization have required the use of transition metal complexes. Thus, it is one of the most challenging projects to activate the abundant dinitrogen through metal-free systems under mild conditions. Here, we demonstrate a proof-of-concept study on the catalytic hydrogenation of dinitrogen (with activation energy as low as 15.3 kcal mol-1 ) initiated by a dual Lewis acid (DLA) via density functional theory (DFT) calculations. In addition, such a DLA could be also used to activate a series of small molecules including carbon dioxide, formaldehyde, N-ethylenemethylamine, and acetonitrile. It is found that aromaticity plays an important role in stabilizing intermediates and products. Our findings provide an alternative approach to N2 activation and functionalization, highlighting a great potential of DLA for small molecule activation.

A bis(silylene)pyridine pincer ligand can stabilize mononuclear manganese(0) complexes: facile access to isolable analogues of the elusive d7-Mn(CO)5 radical.

Using the potentially tridentate N,N'-bis(N-heterocyclic silylene)pyridine [SiNSi] pincer-type ligand, 2,6-N,N'-diethyl-bis[N,N'-di-tert-butyl(phenylamidinato)silylene] diaminopyridine, led to the first isolable bis(silylene)pyridine-stabilized manganese(0) complex, {κ3-[SiNSi]Mn(dmpe)} 4 (dmpe = (Me2P)2C2H4), which represents an isolobal 17 VE analogue of the elusive Mn(CO)5 radical. The compound is accessible through the reductive dehalogenation of the corresponding dihalido (SiNSi)Mn(ii) complexes 1 (Cl) and 2 (Br) with potassium graphite. Exposing 4 towards the stronger π-acceptor ligands CO and 2,6-dimethylphenyl isocyanide afforded the related Mn(0) complexes κ2-[SiNSi]Mn(CO)3 (5) and κ3-[SiNSi]Mn(CNXylyl)2(κ1-dmpe) (6), respectively. Remarkably, the stabilization of Mn(0) in the coordination sphere of the [SiNSi] ligand favors the d7 low-spin electronic configuration, as suggested by EPR spectroscopy, SQUID measurements and DFT calculations. The suitability of 4 acting as a superior pre-catalyst in regioselective hydroboration of quinolines has also been demonstrated.

An Isolable 2,5-Disila-3,4-Diphosphapyrrole and a Conjugated Si=P-Si=P-Si=N Chain Through Degradation of White Phosphorus with a N,N-Bis(Silylenyl)Aniline.

White phosphorus (P4 ) undergoes degradation to P2 moieties if exposed to the new N,N-bis(silylenyl)aniline PhNSi2 1 (Si=Si[N(tBu)]2 CPh), furnishing the first isolable 2,5-disila-3,4-diphosphapyrrole 2 and the two novel functionalized Si=P doubly bonded compounds 3 and 4. The pathways for the transformation of the non-aromatic 2,5-disila-3,4-diphosphapyrrole PhNSi2 P2 2 into 3 and 4 could be uncovered. It became evident that 2 reacts readily with both reactants P4 and 1 to afford either the polycyclic Si=P-containing product [PhNSi2 P2 ]2 P2 3 or the unprecedented conjugated Si=P-Si=P-Si=NPh chain-containing compound 4, depending on the employed molar ratio of 1 and P4 as well as the reaction conditions. Compounds 3 and 4 can be converted into each other by reactions with 1 and P4 , respectively. All new compounds 1-4 were unequivocally characterized including by single-crystal X-ray diffraction analysis. In addition, the electronic structures of 2-4 were established by Density Functional Theory (DFT) calculations.

Asymmetric Construction of α,γ-Disubstituted α,ß-Butenolides Directly from Allylic Ynoates Using a Chiral Bifunctional Phosphine Ligand Enables Cooperative Au Catalysis.

A highly efficient construction of chiral γ-substituted α-allyl-α,ß-butenolides with up to >99% enantiomeric excess from readily available allylic ynoates is realized. In this asymmetric gold catalysis, the cationic gold(I) catalyst featuring a bifunctional phosphine ligand enables a four-step cascade which permits the conversion of a diverse array of allylic ynoates into valuable chiral α,γ-disubstituted α,ß-butenolides.

Unexpected White Phosphorus (P4 ) Activation Modes with Silylene-Substituted o-Carboranes and Access to an Isolable 1,3-Diphospha-2,4-disilabutadiene.

New types of metal-free white phosphorus (P4 ) activation are reported. While the phosphine-silylene-substituted dicarborane 1, CB-SiP (CB=ortho-C,C'-C2 B10 H10 , Si=PhC(tBuN)2 Si, P=P[N(tBu)CH2 ]2 ), activates white phosphorus in a 2 : 1 molar ratio to yield the P5 -chain containing species 2, the analogous bis(silylene)-substituted compound 3, CB-Si2 , reacts with P4 in the molar ratio of 2 : 1 to furnish the first isolable 1,3-diphospha-2,4-disilabutadiene (Si=P-Si=P-containing) compound 4. For the latter reaction, two intermediates having a CB-Si2 P4 and CB-Si2 P2 core could be observed by multinuclear NMR spectroscopy. The compounds 2 and 4 were characterized including single-crystal X-ray diffraction analyses. Their electronic structures and mechanisms were investigated by density functional theory calculations.

Theoretical Study on Reaction Mechanisms of Dinitrogen Activation and Coupling by Carbene-Stabilized Borylenes in Comparison with Intramolecular C-H Bond Activation.

Dinitrogen (N2 ) activation is particularly challenging due to the significantly strong N≡N bond, let alone the catenation of two N2 molecules. Recent experimental study shows that cyclic (alkyl)(amino)carbene (CAAC)-stabilized borylenes are able to tackle N2 activation and coupling below room temperature. Here we carry out density functional theory calculations to explore the corresponding reaction mechanisms. The results indicate that the reaction barrier for the dinitrogen activation by the first borylene is slightly higher than that by the second borylene. In addition, replacing the CAAC moiety of the borylenes with cyclic diaminocarbenes (CDACs) could make such dinitrogen activation and coupling more favorable thermodynamically. The reaction mechanisms of the intramolecular C-H bond activation of borylene have also been discussed, which is found to be favorable both thermodynamically and kinetically in comparison with N2 activation. Thus, adequate attention should be paid to the design of borylenes aiming at N2 activation. In addition, our calculations suggest that the CDAC moiety of the borylene could lead to a different product in terms of intramolecular C-H bond activation. All these findings could be useful for the development of dinitrogen activation as well as C-H bond activation by main group species.

Predicting Dinitrogen Activation by Five-Electron Boron-Centered Radicals.

Due to the high bond dissociation energy (945 kJ mol-1) and the large highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gap (10.8 eV), dinitrogen activation under mild conditions is extremely challenging. On the other hand, the conventional Haber-Bosch ammonia synthesis under harsh conditions consumes more than 1% of the world's annual energy supply. Thus, it is important and urgent to develop an alternative approach for dinitrogen activation under mild conditions. In comparison with transition metals, main group compounds are less explored for nitrogen activation. Here, we carry out density functional theory calculation to screen boron radicals for dinitrogen activation. As a result, the experimentally available seven-electron boron-centered radicals are found to be inactive to N2 activation, whereas some five-electron boron-centered radicals become favorable for dinitrogen activation, inviting experimental chemists' examination. The principal interacting spin-orbital analyses suggest that a five-electron boron-centered radical can mimic a transition metal on a synergic interaction with dinitrogen in the transition states.

Predicting Dinitrogen Coupling with a Series of Small Molecules Catalyzed by a Pincer Complex.

Due to consumption of more than 2% of the world's annual energy supply by Haber-Bosch process and the strongest triple bond (N≡N) in nature, directly coupling N2 with small molecules is particularly important and challenging, let alone in a catalytic fashion. Here we first demonstrate that a NNN-type pincer phosphorus complex could act as a catalyst to couple dinitrogen with a series of small molecules including carbon dioxide, formaldehyde, N-ethylidenemethylamine, and acetonitrile in the presence of diborane(4) under a mild condition by theoretical calculations. N2 fixation proceeds via a stepwise mechanism involving initial N2 activation by diborane(4), followed by intramolecular isomerization to a key intermediate (zwitterion). Such a zwitterion can be used to couple a series of small molecules with activation barriers of 23.5-25.2 kcal mol-1 . All these findings could be particularly useful for main group chemistry aimed at N2 activation.

Predicting Dinitrogen Activation via Transition-Metal-Involved [4+2] Cycloaddition Reaction.

As the strongest triple bond in nature, the N≡N triple bond activation has always been a challenging project in chemistry. On the other hand, since the award of the Nobel Prize in Chemistry in 1950, the Diels-Alder reaction has served as a powerful and widely applied tool in the synthesis of natural products and new materials. However, the application of the Diels-Alder reaction to dinitrogen activation remains less developed. Here we first demonstrate that a transition-metal-involved [4+2] Diels-Alder cycloaddition reaction could be used to activate dinitrogen without an additional reductant by density functional theory calculations. Further study reveals that such a dinitrogen activation by 1-metalla-1,3-dienes screened out from a series of transition metal complexes (38 species) according to the effects of metal center, ligand, and substituents can become favorable both thermodynamically (with an exergonicity of 28.2 kcal mol-1 ) and kinetically (with an activation energy as low as 13.8 kcal mol-1 ). Our findings highlight an important application of the Diels-Alder reaction in dinitrogen activation, inviting experimental chemists' verification.

Systematic Design of a Frustrated Lewis Pair Containing Methyleneborane and Carbene for Dinitrogen Activation.

Activation of atmospherically abundant dinitrogen (N2) by metal-free species under mild reaction conditions has been one of the most challenging areas in chemistry for decades. Very recent but limited progress in N2 activation by boron species, including two-coordinated borylene and methyleneborane and three-coordinated borole and borane, has been made toward metal-free N2 activation. Here, we systematically probe an experimentally viable frustrated Lewis pair (FLP) containing two moieties (methyleneborane and carbene) for N2 activation via density functional theory (DFT) calculations, which has proven to be an efficient approach for N2 activation in a thermodynamically and kinetically favorable manner. Aromaticity is found to play a crucial role in stabilization of the product. This study could be a valuable alternative for the development of metal-free N2 activation chemistry, highlighting great potential of FLP for N2 activation and functionalization.

Promotion of NH3-SCR activity by sulfate-modification over mesoporous Fe doped CeO2 catalyst: Structure and mechanism.

The mesoporous Fe doped CeO2 catalyst after modifying organic sulfate functional groups show an excellent activity with above 80% NOx conversion in a temperature range of 250-450 °C. These organic-like sulfate groups bound to the Fe-O-Ce species leads to the strong electron interaction between Fe3+-O-Ce4+ species and sulfate groups, which modifies the acidity and redox properties of catalyst. The strong ability of SË­O/S-O in sulfate groups to accommodate electrons from a basic molecule is a driving force in the generation of acidic properties, and thus promotes to produce new Brønsted acid sites. The bondage of Fe-O-Ce species obviously inhibits the creation of thermostable bidentate NO3- species. Besides, the redox cycles between Fe3+ and Ce4+ are disrupted, thus inhibiting NH3 oxidation at medium-high temperatures and resulting in the increase of NOx conversion. Furthermore, the in situ DRIFTS results show that for the fresh samples, the coordinate NH3 reacts not only with NO3- through L-H mechanism, but also with oxygen species to form NOx. Differently for sulfated sample, the coordinate NH3 might react with achieved NO2 instead of the oxygen species through E-R mechanism, meanwhile the NH4+ could react with the NO3- species through L-H mechanism.

Screening Borane Species for Dinitrogen Activation.

Activation of the strongest triplet bond in molecular nitrogen (N2) under mild conditions is particularly challenging. Recently, its fixation and reduction were achieved by highly reactive dicoordinated borylene species at ambient conditions, ripping the limits of harsh reaction conditions by metallic species. Less reactive species with a facile preparation could be desirable for next-generation N2 activation. Now density functional theory calculations reveal that tricoordinated boranes could be a potential candidate of N2 activation/functionalization. As composites of an intramolecular frustrated Lewis pair (FLP), optimal and realistic boranes are screened out to activate N2 in a significantly favorable manner (both thermodynamically and kinetically). The significant thermodynamic stabilities of the FLP-N2 adducts as well as the low activation barriers could be particularly interesting for the development of borane-based FLP chemistry applied in N2 activation.

Mechanistic Investigation into the Effect of Sulfuration on the FeW Catalysts for the Selective Catalytic Reduction of NOx with NH3.

Iron tungsten (FeW) catalyst is a potential candidate for the selective catalytic reduction (SCR) of NOx with ammonia because of its excellent performance in a wide operating window. Sulfur poisoning effects in SCR catalysts have long been recognized as a challenge in development of efficient catalysts for applications. In this paper, the impact of sulfuration on catalyst structure, NH3-SCR reaction performance and mechanism was systematically investigated through spectroscopic and temperature-programmed approaches. The sulfuration inhibited the SCR activity at low temperatures (<300 °C), while no evident effect was observed at high temperatures (≥300 °C). After sulfuration for FeW oxides catalyst, the organic-like with covalent S═O bonds sulfate species were mainly formed over the FeW catalysts. Combining TPD with in situ DRIFTS results, it was found that the Lewis and the Brønsted acidity were enhanced by the interaction between metal species and sulfate species due to the strong electron withdrawing effect of the S═O double bonds. The in situ DRIFTS study showed that the formation of NO2 was hindered, leading to the "fast-SCR" pathway was partly cut off by the sulfuration process and thereby the loss of SCR activity at low temperatures. However, the Langmuir-Hinshelwood reaction pathway between adsorbed NH3/NH4+ species and nitrate species was facilitated and dominated at high temperatures, making the as-synthesized FeW catalysts resistant to SO2 poisoning.

Superior Performance of Fe1-xWxOδ for the Selective Catalytic Reduction of NOx with NH3: Interaction between Fe and W.

Novel iron-tungsten catalysts were first developed for the selective catalytic reduction of NOx by NH3 in diesel exhaust, achieving an excellent performance with a wide operating temperature window above 90% NOx conversion from 225 or 250 to 450 °C (GHSVs of 30 000 or 50 000 h-1). It also exhibited a pronounced stability and relatively high NOx conversion in the presence of H2O, SO2 and CO2. The introduction of W resulted in the formation of α-Fe2O3 and FeWO4 species obtained by HRTEM directly. The synergic effect of two species contributed to the high SCR activity, because of the increased surface acidity and electronic property. The FeWO4 with octahedral [FeO6]/[WO6] structure acted as the Brønsted acid sites to form highly active NH4+ species. Combining DFT calculations with XPS and UV-vis results, it was found that the fine electron interaction between α-Fe2O3 and FeWO4 made the electron more easily transfer from W6+ sites to Fe3+ sites, which promoted the formation of NO2. Judging by the kinetics and SCR activity studies, the Fe0.75W0.25Oδ with an appropriate W amount showed the strongest interaction, and thereby the lowest activation energy of 39 kJ•mol-1 and optimal catalytic activity. These findings would be conducive to the reasonable design of NH3-SCR catalysts by adjusting the fabrication.