H2 Elimination Research Articles

Theoretical investigation on propane dehydrogenation over extraframework Ga hydride species trapped at Al-pairs in MFI zeolite

Ga/ZSM-5 is widely used to catalyze propane dehydrogenation (PDH). For the harsh operation conditions, the detailed active species, and the plausible reaction pathways that are responsible for the observed superior PDH performance remain unresolved for Ga/ZSM-5. Reduced extraframework Ga hydride species trapped at zeolite framework Al-pairs, including b[GaH]2+, [Ga]+-BAS, [GaH2]+-BAS, etc. are an important kind of Ga species known as active species for PDH. In this work, the PDH pathways these over these Ga species were investigated theoretically at temperatures and pressures relevant to experiments. We showed that dynamically generated undercoordinated b[GaH]2+ would exhibit significantly higher catalytic activity at PDH conditions as compared with other Ga hydride species. The most plausible PDH pathway over b[GaH]2+ is the carbenium pathway. The Ga species and BAS act in concert in promoting the H2 elimination and β-H transfer, and in turning PDH efficient. Ga hydride species may also form as intermediates along the PDH pathways as active sites for subsequent activation and conversion of propane, suggesting the evolution of Ga hydride species trapped by Al-pairs in Ga/ZSM-5 at operation conditions. The findings are expected to pave the way for better understanding of the experimentally observed operation condition dependent PDH performance of Ga/ZSM-5.

Mechanism of Dinitrogen Photoactivation by P2PPhFe Complexes: Thermodynamic and Kinetic Computational Studies.

The P2PPhFe(N2)(H)2 catalyst showed a significant ammonia yield under light irradiation. However, under thermal conditions, the hydrogen evolution reaction (HER) is favored over the nitrogen reduction reaction (N2RR), making P2PPhFe(N2)(H)2 an ideal system for studying the competition between both reactions. In this study, we used a series of computational tools to elucidate the photochemical reaction mechanism for the N2RR and thermal pathways leading to the HER with this catalyst. We calculated the energy profile for each transformation and estimated the rate constants for each step. Our results, which are consistent with experimental observations, indicate that photoinduced H2 elimination from P2PPhFe(N2)(H)2 promotes the formation of P2PPhFe(N2)2, which is on-path for N2RR. However, this elimination process is kinetically hindered due to high-energy barriers. Furthermore, our calculations reveal enhanced dinitrogen activation upon the conversion of P2PPhFe(N2)(H)2 to P2PPhFe(N2)2.

A Dehydrocoupling Approach Towards Titanocene Iminophosphane Complexes

Dehydrocoupling reactions represent a powerful tool for element‐element bonds formation upon H2 elimination. It was shown that transition metals (in low oxidation states or in the form of hydrides) can be coupled with two equivalents of primary pnictanes (R–PnH2; Pn = P, Sb) providing direct access to otherwise elusive η2‐dipnictene (R–Pn=Pn–R) complexes. Herein, we report on the reactivity of the common group IV metallocene precursors Cp2Ti(btmsa) and Cp2Zr(btmsa)(pyridine) (btmsa = C2(SiMe3)2) towards primary pnictanes (R–PnH2 (Pn = P or N) and secondary aminophosphanes (R’P(H)–N(H)R). We found that Cp2Ti(btmsa) reacts with secondary aminophosphanes (R’P(H)–N(H)R) to give η2‐iminophosphane titanocene complexes. The molecular and electronic structure of the new compounds were thoroughly investigated spectroscopically, crystallographically and computationally.

The Effect of β-Hydrogens on the Tropospheric Photochemistry of Aldehydes: Norrish Type 1, Triple Fragmentation, and Methylketene Formation from Propanal.

Wavelength and pressure dependent quantum yields (ϕ, QYs) of propanal photolysis have been measured for photolysis wavelengths, λ = 300-330 nm, and buffer gases of 3-10 Torr propanal and 0-757 Torr N2. Following laser photolysis, three photochemical pathways were established, using Fourier transform infrared spectroscopy of the stable end-products. Photolysis is dominated by the Norrish Type 1 reaction, which has been reported previously, but with inconsistent quantum yields. The propanal α-hydrogen leads to a 4-center elimination of H2, as observed in CH3CHO, here leading to methylketene. The presence of hydrogen attached to the β-carbon allows a new photochemical pathway: concerted triple fragmentation into CO + H2 + C2H4 via a 5-center transition state. Neither of these channels has been reported previously. No evidence for the previously reported C2H6 + CO, C2H4 + H2CO or CH3 + CH2CHO channels, nor for phototautomerization to 1-propenol (CH3CH═CHOH) was found. Modeling of the wavelength, pressure and collision partner dependence of the QYs allows us to reconcile the previous NT1a results and make recommendations for the quantum yields of all three channels under tropospheric conditions. The general impact of β-hydrogen atoms in the photochemistry of aldehydes is to open up new pathways from cyclic transition states and to reduce the importance of other photolysis or isomerization channels.

Divergent Reactivity of a Cyclic Bis-Hydridostannylene: A Masked Sn(I) Diradicaloid.

Herein, reactivity studies of a cyclic bis-hydridostannylene [(ADC)SnH]2 (1-H2) (ADC=PhC{(NDipp)C}2; Dipp=2,6-iPr2C6H3) with various unsaturated organic substrates are reported. Reactions of terminal alkynes (RC≡CH) with 1-H2 afford mixed acetylide-vinyl-functionalized bis-stannylenes via dehydrogenation and hydrostannylation. Treatment of 1-H2 with PhC≡CCH3 gives a unique distannabarrelene via dehydrogenative C(sp3)-H stannylation and hydrostannylation of the C≡CCH3 moiety. 1-H2 undergoes dehydrogenative [2+2]-cycloaddition reactions with diphenylacetylene, azobenzene, acetone, benzophenone, and benzaldehyde to form the 1,4-distannabarrelene derivatives. The elimination of H2 in these reactions suggests the masked-diradical property of 1-H2. In fact, these [2+2]-cycloaddition products are also accessible on treatments of the Sn(I) diradicaloid [(ADC)Sn]2 (1) with appropriate reagents. All compounds have been characterized by multinuclear NMR spectroscopy and single crystal X-ray diffraction. Moreover, the catalytic activity of 1-H2 has been shown for the hydroboration of unsaturated substrates.

Mesoporous Ternary Nanozymes with Anti‐Senescence and Anti‐Inflammation Activities for Atherosclerosis Management

AbstractNumerous anti‐inflammatory agents and corresponding therapeutic strategies have been explored for treating inflammation‐related atherosclerosis, which has struggled with compromised antiatherosclerotic efficacy due to the complex pathogenesis. The senescent endothelial cells participate fundamentally at all stages in atherosclerosis, providing the basis for developing senotherapies to blunt the deleterious effects of senescent cells. In this work, a mesoporous palladium‐boron‐phosphorus ternary nanozyme with intrinsic superoxide dismutase/catalase‐mimicking properties is rationally engineered to co‐load H2 (an anti‐inflammatory agent) within nanozyme's crystal lattice and 4,4′‐dimethoxychalcone (DIM, an anti‐senescence agent) within mesopores, which elicits strong anti‐inflammatory effect in pro‐inflammatory macrophages by elimination of reactive oxygen species (ROS), H2 anti‐inflammation and promotion of cholesterol efflux, while simultaneously exerts anti‐senescence capacity in endothelial cells by protection of DNA from ROS damage and degradation of the cellular senescent components via autophagy activation. Both in vitro and in vivo experimental results verify the synergy between nanozyme‐based anti‐inflammatory therapy and autophagy activation, demonstrating that such a precise regulation of vascular senescence is applicable to augment antiatherosclerotic efficiency, which provides the nanomedicine‐enabled and senotherapy‐based paradigm for atherosclerosis management.

Understanding Decomposition Mechanism of Acetone under Electric Pulsed Discharge: Generation of C3O+ and C3H4O+ ions

AbstractGas‐phase investigations on acetone, under the influence of electric pulsed discharge has provided distinct insight related to its decomposition. Time‐of‐flight mass spectrometry has been utilized to identify different products generated upon dissociation/ionization of acetone, under the influence of electric discharge. In addition to molecular (C3H6O+) and protonated acetone (C3H7O+) ions, predominant ions signal corresponding to C3H4O+, C3O+ and CH3CO+ were observed in the mass spectrum. Under discharge condition, acetone undergoes soft dissociation/ionization producing C3H4O+ and C3O+ ions, which are distinctive from usual fragment ions reported upon decomposition of acetone, in the gas‐phase. Though metastable in nature, significant ion yield has been obtained for C3O+ ions. Computational studies have been carried out to understand the mechanism of acetone decomposition in gas‐phase. Based on experimental and computational findings, generation of these ions (C3O+ and C3H4O+) has been ascribed to step‐wise H2 elimination reaction originating from acetone molecule, followed by ionization of resultant fragments under the influence of electric pulsed discharge. Present studies are of relevance for plasma pyrolysis of volatile organic compounds and laboratory astrochemical investigations.

Influence of bromination arrangement and level on the formation of polybrominated dibenzo-p-dioxins and dibenzofurans from pyrolysis and combustion of polybrominated diphenyl ethers: Mechanisms and kinetics

Controlling the emission of polybrominated dibenzo-p-dioxins and dibenzofurans (PBDD/Fs) presents a huge challenge in the thermo-chemical conversion of halogenated waste. It is crucial to understand the formation of PBDD/Fs to effectively control their emissions. This paper utilizes density functional theory calculations to comprehensively explore the mechanisms and kinetics of PBDD/Fs formation using 65 polybrominated diphenyl ethers (PBDEs) with varying bromination arrangements and levels. The study uncovers two new pathways for the formation of PBDFs from PBDEs, i.e., two-step H2 elimination and ether-ketone isomerization at ortho-C(H) sites. These pathways are applicable to PBDEs with specific bromination arrangements. In the case of some PBDEs, their initial energy barriers are comparable to the generally accepted direct HBr elimination. It is found that low-brominated PBDEs have a higher risk of forming PBDD/Fs, while high-brominated PBDEs are more likely to break ether bonds and generate polybromobenzenes. The study also investigates the effects of polymer materials and metals on the formation of PBDD/Fs from PBDEs to explain the high yield of PBDD/Fs in the co-treatment of PBDEs with polymers and metals. The involvement of polymers and metals significantly reduces the energy barriers for the formation of PBDD/F precursors, namely ortho-phenyl-type radicals, accelerating the formation of PBDD/Fs. This study markedly advances the present understanding of the transformation of PBDEs into PBDD/Fs and provides theoretical guidance for reducing pollutant emissions in the thermal treatment and cleaner conversion of PBDE-containing waste.

Mechanistic insights into the rhodium catalysed dehydrogenative cycloaddition of cyano-yne-allene substrates

DFT calculations explored the Rh-catalysed dehydrogenative cycloaddition of cyano-yne-allene substrates. The reaction involves cycloaddition, hydrogen shift, 6π electrocyclization, and acceptorless H2 elimination.

Germanium Analogue of the Parent Phosphine-Borane FLP Compound.

Diimino-carbene-supported germylone dimNHCGe does not react with BPh3 and does not activate dihydrogen in the FLP mode in the combination with this borane. However, it reacts with B(C6 F5 )3 to give the zwitterionic borate dimNHCGe-(C6 F4 )BF(C6 F5 )2 . This compound can be converted into the hydroborate dimNHCGe-(C6 F4 )BH(C6 F5 )2 (8) and further into [dimNHCGe-(C6 F4 )B(C6 F5 )2 ]+ (4). Compound 4 is a Ge/B analogue of Stephan's FLP parent P/B compound (C6 H2 Me3 )2 P-C6 F4 -B(C6 F5 )2 but unlike the latter cannot split dihydrogen. Moreover, attempts to prepare a Ge/B analogue of the zwitterion (C6 H2 Me3 )2 HP-C6 F4 -BH(C6 F5 )2 by protonation of borate 8 resulted in immediate elimination of H2.

Cationic Cobalt(II) Bisphosphine Hydroformylation Catalysis: In Situ Spectroscopic and Reaction Studies.

[HCo(CO)x(bisphosphine)](BF4), x = 1-3, is a highly active hydroformylation catalyst system, especially for internal branched alkenes. In situ infrared spectroscopy (IR), electron paramagnetic resonance (EPR), and nuclear magnetic resonance studies support the proposed catalyst formulation. IR studies reveal the formation of a dicationic Co(I) paramagnetic CO-bridged dimer, [Co2(μ-CO)2(CO)(bisphosphine)2]2+, at lower temperatures formed from the reaction of two catalyst complexes via the elimination of H2. DFT studies indicate a dimer structure with square-pyramidal and tetrahedral cobalt centers. This monomer-dimer equilibrium is analogous to that seen for HCo(CO)4, reacting to eliminate H2 and form Co2(CO)8. EPR studies on the catalyst show a high-spin (S = 3/2) Co(II) complex. Reaction studies are presented that support the cationic Co(II) bisphosphine catalyst as the catalyst species present in this system and minimize the possible role of neutral Co(I) species, HCo(CO)4 or HCo(CO)3(phosphine), as catalysts.

Conversion of primary alcohol to ester in presence of Ruthenium(II)-PNP pincer complex and comparison with isoelectronic (PNP)Os and (PNP)Rh+ complexes: A computational study

Mechanistic pathway of (PNP)Ru {PNP = bis[(2-dimethylphosphino)-ethyl]amine} mediated synthesis of an ester (2-methoxyethyl-2-methoxyacetate) from two equivalent of a primary alcohol (3-oxabutanol) is investigated using density functional theory. A ten-stage catalytic cycle is proposed. The key steps of this process is broadly described as the oxidation of first equivalent of primary alcohol into aldehyde with the elimination of H2 molecule, coupling of the aldehyde with second equivalent of alcohol to form an ester followed by the regeneration of the pincer catalyst with the liberation of second H2 molecule. Two isoelectronic pincer complexes (PNP)Os and (PNP)Rh+ are also considered to shed light on the variation of the energetics. Prior to H2 elimination, a η2-bonded metal-dihydride complex is obtained for (PNP)Ru and (PNP)Rh+, whereas pure Os-H bonds are observed in case of (PNP)Os. Theoretical catalytic efficiency and TOF values infer that (PNP)Ru is the best amongst the three pincer complexes considered here.

How thermal fluctuations influence the function of the FeMo cofactor in nitrogenase enzymes

The catalytic mechanism of N2 fixation by nitrogenase remains unresolved in how the strong N≡N bond is activated and why the reductive elimination of H2 is required. Here, we use density functional theory and physiologically relevant thermal simulations to elucidate the mechanism of the complete nitrogenase catalytic cycle. Over the accumulation of four reducing equivalents, we find that protons and electrons transfer to the FeMo cofactor to weaken and break its bridge Fe–S bond, leading to temporary H2S formation that exposes the Fe sites to weakly bind N2. Remarkably, we find that subsequent H2 formation is responsible for chemical activation to an N=N double bond accompanied by a low barrier for H2 release. We emphasize that finite temperature effects smooth out mechanistic differences between DFT functionals observed at 0 K, thus leading to a consistent understanding as to why H formation is an obligatory step in N2 adsorption and activation.

Synthesis and Coordination of a Bisphosphine-[NHC-borane] Compound: A Ligand Framework for Bimetallic Structure Featuring a Boron-Bridging Moiety.

We report herein the synthesis of a bisphosphine-[NHC-BH3] compound and its coordination toward gold. The ligand is shown to support a bimetallic structure bisphosphine-[NHC-BH3](AuCl)2. The abstraction of one chloride from the gold metal center triggers the activation of a BH3 fragment, leading to the reductive elimination of H2 and the formation of a dicationic Au42+ complex featuring Au centers at the +0.5 oxidation state, via a (μ-H)Au2 intermediate, characterized in situ at 183 K. The reactivity of Au4 with thiophenol led to the reoxidation of the gold metal centers to a (μ-S(Ph))Au2 complex. In the different complexes, borane fragment was shown to bridge the Au2 core via weak interaction with [BH], [BCl], and [BH2] moieties.

Pitfalls for POCOP-Type Palladium Pincer Complexes in Catalytic Reduction of CO2 with Catecholborane

Reduction of CO2 with catecholborane (HBcat) is known to be catalyzed by nickel hydride complexes supported by a bis(phosphinite)-based or POCOP-type pincer ligand. The objective of this research is to examine how changing the metal center from nickel to palladium would impact the outcome of CO2 reduction. Stoichiometric studies show that the palladium hydride complexes react rapidly with CO2, although the resulting palladium formate complexes can lose CO2 under reduced pressure or in chloroform. In the presence of HBcat, the formate complexes are converted back to the palladium hydride species while the formate group is reduced to CH3OBcat. This process is also accompanied by the formation of palladium bis(catecholato)borate complexes, which diverts some of the HBcat to yield B2(cat)3 and B2H6. For palladium hydrides stabilized by a POCOP-pincer ligand with relatively small phosphorus substituents (e.g., isopropyl or cyclopentyl groups), HBcat cleaves the P–O bonds in the ligand backbone to degrade the catalysts to secondary phosphine complexes. Without CO2, HBcat also undergoes H2 elimination with the Pd–H moiety to form palladium boryl complexes. These side reactions with HBcat and the palladium pincer complexes play profound roles in the catalytic reduction of CO2 with the borane.

Cooperative Rare-Earth/Lithium-Mediated Conversion of White Phosphorus.

The rare-earth/lithium cooperative effect on functionalization of white phosphorus has been investigated. The reaction of diazabutadiene-supported yttrium hydride chelated a LiPPh2 molecule (LY ⋅ THF)2 (μ-H)2 [μ-PPh2 (Li)] (1, L=N,N'-di(2,6-diisopropylphenyl)-1,4-diazabutadiene) with P4 gave two novel mixed Y/Li multinuclear polyphosphorus complexes (LY ⋅ THF)2 [cyclo-P3 ]Li(THF)3 (2) and [Li(THF)4 ]+ [(LY ⋅ THF)3 (norborane-P7 )Li(THF)]- (3), accompanied with the elimination of diphosphorus compound Ph2 PPPh2 (4) and H2 . However, the comparative reaction of yttrium hydride (LY ⋅ THF)2 (μ-H)2 with P4 afforded a trinuclear yttrium pyramid-P4 complex (LY ⋅ THF)3 (μ3 -P(PH)3 ) (5). Further investigations show that 5 cannot continuously react with LiPPh2 to form 2 and 3, and LiPPh2 reacted with P4 to form a Zintl-P7 lithium complex (TMEDA⋅Li)3 (Zintl-P7 ) (6) and 4. These results indicated that the cooperation of Y/Li for activation of P4 is a key for the formation of 2 and 3. All new compounds have been characterized by NMR spectroscopy and single-crystal X-ray diffraction studies.

Synthesis and Reversible H2 Activation by Coordinatively Unsaturated Rhodium NHC Complexes

AbstractWe report the synthesis of coordinatively unsaturated cationic rhodium complexes bearing the sterically encumbered electron‐rich NHC ligand IPr*OMe. The COD (1,5‐cyclooctadiene) complex [Rh(IPr*OMe)(COD)]BF4 adopts a tilted, pseudo‐square planar coordination geometry, where bonding to the ipso‐carbon of the NHC aryl substituent was observed in the solid state. Hydrogenation of this complex afforded a metastable dihydride complex [Rh(IPr*OMe)(H)2]BF4 with an unusual internal coordination to an arene of the ligand. In the absence of a hydrogen atmosphere, spontaneous reductive elimination of H2 afforded a rhodium complex [Rh(IPr*OMe)]BF4 with a single chelating ligand that stabilizes the highly unsaturated metal by two‐fold π‐face donation as suggested by NMR spectroscopy and computational studies. This unusual complex might serve as a versatile precatalyst for a variety of transformations.

From a Fluorenyl Substituted Ylide-Functionalized Phosphine to a Neutral Phosphide via P-C Bond Cleavage.

Bulky ylide-substituted phosphines have recently found application as potent ligands in homogeneous catalysis. The attempted synthesis of the ylide-substituted fluorenylphosphine YPh P(Cy)Flu [YPh =Cy3 P(Ph)C; Flu=9-methylfluorenyl] now resulted in the unexpected elimination of 9-methylenefluorene during the deprotonation step of the intermediary α-phosphino phosphonium salt to yield the secondary ylide-substituted phosphine YPh P(Cy)H. This phosphine underwent formal H2 elimination under basic conditions to form a cyclic phosphonium ylide with a P-C-P-C four-membered ring via deprotonation of one cyclohexyl group of the PCy3 moiety. Upon coordination to transition metals the secondary ylidylphosphine forms a neutral phosphide ligand by shift of the proton into the ylide-backbone and formation of zwitterionic metal complexes.

N2 binding to the E0-E4 states of nitrogenase.

Nitrogenase is the only enzyme that can convert N2 into NH3. The reaction requires the addition of eight electrons and protons to the enzyme and the mechanism is normally described by nine states, E0-E8, differing in the number of added electrons. Experimentally, it is known that three or four electrons need to be added before the enzyme can bind N2. We have used combined quantum mechanical and molecular mechanics methods to study the binding of N2 to the E0-E4 states of nitrogenase, using four different density functional theory (DFT) methods. We test many different structures for the E2-E4 states and study binding both to the Fe2 and Fe6 ions of the active-site FeMo cluster. Unfortunately, the results depend quite strongly on the DFT methods. The TPSS method gives the strongest bonding and prefers N2 binding to Fe6. It is the only method that reproduces the experimental observation of unfavourable binding to the E0-E2 states and favourable binding to E3 and E4. The other three methods give weaker binding, preferably to Fe2. B3LYP strongly favours structures with the central carbide ion triply protonated. The other three methods suggest that states with the S2B ligand dissociated from either Fe2 or Fe6 are competitive for the E2-E4 states. Moreover, such structures with two hydride ions both bridging Fe2 and Fe6 are the best models for E4 and also for the N2-bound E3 and E4 states. However, for E4, other structures are often close in energy, e.g. structures with one of the hydride ions bridging instead Fe3 and Fe7. Finally, we find no support for the suggestion that reductive elimination of H2 from the two bridging hydride ions in the E4 state would enhance the binding of N2.

Synthesis, Structures, and Thermal Stability of Dialkyl and Bis(amido) Zirconium(IV) Acen Complexes

AbstractReaction of one equivalent of H2(acen), H2(cis‐Cyacen) or H2(trans‐Cyacen) with [Zr(CH2SiMe3)4] at room temperature afforded [Zr(acen)(CH2SiMe3)2] (1), [Zr(cis‐Cyacen)(CH2SiMe3)2] (2) or [Zr(trans‐Cyacen)(CH2SiMe3)2] (3), respectively (acen=C2H4(NCMeCHC(O)Me)2; Cyacen=1,2‐C6H10(NCMeCHC(O)Me)2). These alkyl compounds are trigonal prismatic in the solid state, and whereas 1 and 3 decomposed without sublimation above 120 °C (5–10 mTorr), 2 sublimed in >95 % yield at 85 °C (5–10 mTorr). However, heating solid 2 at 88 °C under static argon for 24 hours resulted in extensive decomposition to afford H2(cis‐Cyacen) and SiMe4 as the soluble products. Compound 2 reacted cleanly with two equivalents of tBuOH to afford [Zr(cis‐Cyacen)(OtBu)2] (4), but excess tBuOH caused both SiMe4 and H2(cis‐Cyacen) elimination. The 1 : 1 reaction of H2(acen) with [Zr(NMeEt)4] did not proceed cleanly, and 8‐coordinate [Zr(acen)2] (5) was identified as a by‐product; this complex was isolated from the 2 : 1 reaction. A zirconium amido complex, [Zr(acen)(NMeEt)2] (6) was accessed via the reaction of 1 with two equiv. or excess HNMeEt, but decomposed readily in solution at room temperature. More sterically hindered [Zr(acen){N(SiMe3)2}2] (7) was synthesized via the reaction of [Zr(acen)Cl2] with two equivalents of Li{N(SiMe3)2}, but was also thermally unstable as a solid and in solution at room temperature. Compounds 1–3, 5 and 7 were crystallographically characterized.