Carbon dioxide hydrogenation catalyzed by a ruthenium dihydride: a DFT and high-pressure spectroscopic investigation

Reaction pathways during CO(2) hydrogenation catalyzed by the Ru dihydride complex [Ru(dmpe)(2)H(2)] (dmpe=Me(2)PCH(2)CH(2)PMe(2)) have been studied by DFT calculations and by IR and NMR spectroscopy up to 120 bar in toluene at 300 K. CO(2) and formic acid readily inserted into or reacted with the complex to form formates. Two formate complexes, cis-[Ru(dmpe)(2)(OCHO)(2)] and trans-[Ru(dmpe)(2)H(OCHO)], were formed at low CO(2) pressure (<5 bar). The latter occurred exclusively when formic acid reacted with the complex. A RuHHOCHO dihydrogen-bonded complex of the trans form was identified at H(2) partial pressure higher than about 50 bar. The trans form of the complex is suggested to play a pivotal role in the reaction pathway. Potential-energy profiles along possible reaction paths have been investigated by static DFT calculations, and lower activation-energy profiles via the trans route were confirmed. The H(2) insertion has been identified as the rate-limiting step of the overall reaction. The high energy of the transition state for H(2) insertion is attributed to the elongated Ru--O bond. The H(2) insertion and the subsequent formation of formic acid proceed via Ru(eta(2)-H(2))-like complexes, in which apparently formate ion and Ru(+) or Ru(eta(2)-H(2))(+) interact. The bond properties of involved Ru complexes were characterized by natural bond orbital analysis, and the highly ionic characters of various complexes and transition states are shown. The stability of the formate ion near the Ru center likely plays a decisive role for catalytic activity. Removal of formic acid from the dihydrogen-bonded complex (RuHHOCHO) seems to be crucial for catalytic efficiency, since formic acid can easily react with the complex to regenerate the original formate complex. Important aspects for the design of highly active catalytic systems are discussed.     

Hydrogenation of CO2 to Formic Acid with a Highly Active Ruthenium Acriphos Complex in DMSO and DMSO/Water

The novel [Ru(Acriphos)(PPh₃)(Cl)(PhCO₂)] [ 1 ; Acriphos=4,5‐bis(diphenylphosphino)acridine] is an excellent precatalyst for the hydrogenation of CO₂ to give formic acid in dimethyl sulfoxide (DMSO) and DMSO/H₂O without the need for amine bases as co‐reagents. Turnover numbers (TONs) of up to 4200 and turnover frequencies (TOFs) of up to 260 h^?1 were achieved, thus rendering 1 one of the most active catalysts for CO₂ hydrogenations under additive‐free conditions reported to date. The thermodynamic stabilization of the reaction product by the reaction medium, through hydrogen bonds between formic acid and clusters of solvent or water, were rationalized by DFT calculations. The relatively low final concentration of formic acid obtained experimentally under catalytic conditions (0.33 mol L^?1) was shown to be limited by product‐dependent catalyst inhibition rather than thermodynamic limits, and could be overcome by addition of small amounts of acetate buffer, thus leading to a maximum concentration of free formic acid of 1.27 mol L^?1, which corresponds to optimized values of TON=16×10³ and TOF_avg≈10³ h^?1.     

CO2 Fixation and Transformation by a Dinuclear Copper Cryptate under Acidic Conditions

CO₂ fixation and transformation by metal complexes continuously receive attention from the viewpoint of carbon resources and environmental concerns. We found that the dinuclear copper(II) cryptate [Cu₂L¹](ClO₄)₄ ( 1 ; L¹=N[(CH₂)₂NHCH₂(m‐C₆H₄)CH₂NH‐(CH₂)₂]₃N) can easily take up atmospheric CO₂ even under weakly acidic conditions at room temperature and convert it from bicarbonate into carbonate monoesters in alcohol solution. The compounds [Cu₂L¹(μ‐O₂COH)](ClO₄)₃ ( 2 ), [Cu₂L¹(μ‐O₂COR)](ClO₄)₃ ( 3 : R=CH₃; 4 : R=C₂H₅; 5 : R=C₃H₇; 6 : R=C₄H₉; 7 : R=C₅H₁₁; 8 : R=CH₂CH₂OH), [Cu₂L¹(μ‐O₂CCH₃)](ClO₄)₃ ( 9 ), and [Cu₂L¹(OH₂)(NO₃)](NO₃)₃ ( 10 ) were characterized by IR spectroscopy and ESI‐MS. The crystal structures of 2 – 6 and 10 were studied by single‐crystal X‐ray diffraction analysis. On the basis of the crystal structures, solution studies, and DFT calculations, a possible mechanism for CO₂ fixation and transformation is given.     

Ab initio static and molecular dynamics study of 4-styrylpyridine.

We report an in‐depth theoretical study of 4‐styrylpyridine in its singlet S₀ ground state. The geometries and the relative stabilities of the trans and cis isomers were investigated within density functional theory (DFT) as well as within Hartree–Fock (HF), second‐order Møller–Plesset (MP2), and coupled cluster (CC) theories. The DFT calculations were performed using the B3LYP and PBE functionals, with basis sets of different qualities, and gave results that are very consistent with each other. The molecular structure is thus predicted to be planar at the energy minimum, which is associated with the trans conformation, and to become markedly twisted at the minimum of higher energy, which is associated with the cis conformation. The results of the calculations performed with the post‐HF methods approach those obtained with the DFT methods, provided that the level of treatment of the electronic correlation is high enough and that sufficiently flexible basis sets are used. Calculations carried out within DFT also allowed the determination of the geometry and the energy of the molecule at the biradicaloid transition state associated with the thermal cis?trans isomerization and at the transition states associated with the enantiomerization of the cis isomer and with the rotations of the pyridinyl and phenyl groups in the trans and cis isomers. Car–Parrinello molecular dynamics simulations were also performed at 50, 150, and 300 K using the PBE functional. The studies allowed us to evidence the highly flexible nature of the molecule in both conformations. In particular, the trans isomer was found to exist mainly in a nonplanar form at finite temperatures, while the rotation of the pyridinyl ring in the cis isomer was incidentally observed to take place within ≈1 ps during the simulation carried out at 150 K on this isomer.     

Aqueous phase carbon dioxide and bicarbonate hydrogenation catalyzed by cyclopentadienyl ruthenium complexes

The water‐soluble ruthenium(II) complexes [Cp′RuX(PTA)₂]Y and [CpRuCl(PPh₃)(mPTA)]OTf (Cp′ = Cp, Cp*, X = Cl and Y = nil; or X = MeCN and Y = PF₆; PTA = 1,3,5‐triaza‐7‐phosphaadamantane; mPTA = 1‐methyl‐1,3,5‐triaza‐7‐phosphaadamantane) were used as catalyst precursors for the hydrogenation of CO₂ and bicarbonate in aqueous solutions, in the absence of amines or other additives, under relatively mild conditions (100 bar H₂, 30–80 °C), with moderate activities. Kinetic studies showed that the hydrogenation of HCO₃^? proceeds without an induction period, and that the rate strongly depends on the pH of the reaction medium. High‐pressure multinuclear NMR spectroscopy revealed that the ruthenium(II) chloride precursors are quantitatively converted into the corresponding hydrides under H₂ pressure. Copyright © 2007 John Wiley & Sons, Ltd.     

Transformation of Atmospheric CO2 Catalyzed by Protic Ionic Liquids: Efficient Synthesis of 2‐Oxazolidinones

Protic ionic liquids (PILs), such as 1,8‐diazabicyclo[5.4.0]‐7‐undecenium 2‐methylimidazolide [DBUH][MIm], can catalyze the reaction of atmospheric CO₂ with a broad range of propargylic amines to form the corresponding 2‐oxazolidinones. The products are formed in high yields under mild, metal‐free conditions. The cheaper and greener PILs can be easily recycled and reused at least five times without a decrease in the catalytic activity and selectivity. A reaction mechanism was proposed on the basis of a detailed DFT study which indicates that both the cation and anion of the PIL play key synergistic roles in accelerating the reaction.     

N-Aryl and N-Alkyl Carbamates from 1 Atmosphere of CO2

We have successfully isolated and characterized the zinc carbamate complex (phen)Zn(OAc)(OC(=O)NHPh) ( 1 ; phen=1,10-phenanthroline), formed as an intermediate during the Zn(OAc)₂/phen-catalyzed synthesis of organic carbamates from CO₂, amines, and the reusable reactant Si(OMe)₄. Density functional theory calculations revealed that the direct reaction of 1 with Si(OMe)₄ proceeds via a five-coordinate silicon intermediate, forming organic carbamates. Based on these results, the catalytic system was improved by using Si(OMe)₄ as the reaction solvent and additives like KOMe and KF, which promote the formation of the five-coordinated silicon species. This sustainable and effective method can be used to synthesize various N-aryl and N-alkyl carbamates, including industrially important polyurethane raw materials, starting from CO₂ under atmospheric pressure.     

Computational Design of Iron Diphosphine Complexes with Pendant Amines for Hydrogenation of CO2 to Methanol: A Mimic of [NiFe] Hydrogenase

Inspired by the active‐site structure of the [NiFe] hydrogenase, we have computationally designed the iron complex [P^tBu₂N^tBu₂)Fe(CN)₂CO] by using an experimentally ready‐made diphosphine ligand with pendant amines for the hydrogenation of CO₂ to methanol. Density functional theory calculations indicate that the rate‐determining step in the whole catalytic reaction is the direct hydride transfer from the Fe center to the carbon atom in the formic acid with a total free energy barrier of 28.4 kcal mol^?1 in aqueous solution. Such a barrier indicates that the designed iron complex is a promising low‐cost catalyst for the formation of methanol from CO₂ and H₂ under mild conditions. The key role of the diphosphine ligand with pendent amine groups in the reaction is the assistance of the cleavage of H₂ by forming a Fe?H^δ????H^δ+?N dihydrogen bond in a fashion of frustrated Lewis pairs.     

Effect of Sodium Cation on Metallacycle β‐Hydride Elimination in CO2–Ethylene Coupling to Acrylates

The catalytic conversion of carbon dioxide and olefins into acrylates has been a long standing target, because society attempts to synthesize commodity chemicals in a more economical and sustainable fashion. Although nickel complexes have been known to successfully couple CO₂ and ethylene for decades, a key β‐hydride elimination step has proven a major obstacle to the development of a catalytic process. Recent studies have shown that Lewis acid additives can be used to create a lower‐energy pathway for β‐hydride elimination and facilitate a low number of catalytic turnovers. However, the exact manner, in which the Lewis acid promotes β‐hydride elimination remains to be elucidated. Herein, we describe the kinetic and thermodynamic role that commercially relevant and weakly Lewis acidic sodium salts play in promoting β‐hydride elimination from nickelalactones synthesized from CO₂ and ethylene. This process is compared to a non‐Lewis acid promoted pathway, and DFT calculations were used to identify differences between the two systems. The sodium‐free isomerization reaction gave a rare CO₂‐derived β‐nickelalactone complex, which was structurally characterized.     

Mechanism of BPh3-Catalyzed N-Methylation of Amines with CO2 and Phenylsilane: Cooperative Activation of Hydrosilane

BPh₃ catalyzes the N-methylation of secondary amines and the C-methylenation (methylene-bridge formation between aromatic rings) of N,N-dimethylanilines or 1-methylindoles in the presence of CO₂ and PhSiH₃; these reactions proceed at 30–40 °C under solvent-free conditions. In contrast, B(C₆F₅)₃ shows little or no activity. ¹¹B NMR spectra suggested the generation of [HBPh₃]⁻. The detailed mechanism of the BPh₃-catalyzed N-methylation of N-methylaniline ( 1 ) with CO₂ and PhSiH₃ was studied by using DFT calculations. BPh₃ promotes the conversion of two substrates (N-methylaniline and CO₂) into a zwitterionic carbamate to give three-component species [Ph(Me)(H)N⁺CO₂⁻⋅⋅⋅BPh₃]. The carbamate and BPh₃ act as the nucleophile and Lewis acid, respectively, for the activation of PhSiH₃ to generate [HBPh₃]⁻, which is used to produce key CO₂-derived species, such as silyl formate and bis(silyl)acetal, essential for the N-methylation of 1 . DFT calculations also suggested other mechanisms involving water for the generation of [HBPh₃]⁻ species.     

Ab initio molecular dynamics free-energy study of microhydration effects on the neutral-zwitterion equilibrium of phenylalanine.

The structures and relative energies of the most stable conformers of both naked and microsolvated phenylalanine, Phe.(H(2)O)(n)(n=0-3), are calculated by density functional theory. For selected structures, coordination-constrained ab initio molecular dynamics simulations determine the proton-transfer mechanism connecting neutral and zwitterionic forms of Phe. The associated free-energy profiles are calculated by thermodynamic integration. While no zwitterionic free-energy minimum is found for naked Phe, microsolvation is found to stabilize the zwitterionic form. For cluster sizes n > or = 3, the proton-transfer equilibrium shifts towards the zwitterionic structure for specific proton-transfer pathways. The energetically most favourable interconversion path between the neutral and zwitterionic forms is through a H(2)O bridge with free-energy barriers as low as 14.4 kJ mol(-1) for Phe.(H(2)O)(3). The free energy required for breaking a carboxylic OH bond involved in intramolecular hydrogen bonding is typically lower than in the water-assisted case. However, the resulting zwitterion turns out to be unstable with respect to the backward proton-transfer reaction.     

Direct Assembly of 2‐Oxazolidinones by Chemical Fixation of Carbon Dioxide

The reaction of β‐ and γ‐haloamines with carbon dioxide to give pharmaceutically relevant 2‐oxazolidinones and 1,3‐dioxazin‐2‐ones, was found to proceed efficiently in the presence of a base and in the absence of catalyst. After optimization of reaction conditions, the system was successfully expanded to a variety of haloamines, even at multigram scale. The reaction was further studied in silico by DFT calculations.     

Highly Efficient,Selective, and Stable CO2 Electroreduction on a Hexagonal Zn Catalyst

Electrocatalytic CO₂ conversion into fuel is a prospective strategy for the sustainable energy production. However, still many parts of the catalyst such as low catalytic activity, selectivity, and stability are challenging. Herein, a hierarchical hexagonal Zn catalyst showed highly efficient and, more importantly, stable performance as an electrocatalyst for selectively producing CO. Moreover, we found that its high selectivity for CO is attributed to morphology. In electrochemical analysis, Zn (101) facet is favorable to CO formation whereas Zn (002) facet favors the H₂ evolution during CO₂ electrolysis. Indeed, DFT calculations showed that (101) facet lowers a reduction potential for CO₂ to CO by more effectively stabilizing a ^.COOH intermediate than (002) facet. This further suggests that tuning the crystal structure to control (101)/(002) facet ratio of Zn can be considered as a key design principle to achieve a desirable product from Zn catalyst.     

Fabrication of Ruthenium Nanoparticles in Porous Organic Polymers: Towards Advanced Heterogeneous Catalytic Nanoreactors

A novel strategy has been adopted for the construction of a copolymer of benzene–benzylamine‐1 (BBA‐1), which is a porous organic polymer (POP) with a high BET surface area, through Friedel–Crafts alkylation of benzylamine and benzene by using formaldehyde dimethyl acetal as a cross‐linker and anhydrous FeCl₃ as a promoter. Ruthenium nanoparticles (Ru NPs) were successfully distributed in the interior cavities of polymers through NaBH₄, ethylene glycol, and hydrothermal reduction routes, which delivered Ru‐A, Ru‐B, and Ru‐C materials, respectively, and avoided aggregation of metal NPs. Homogeneous dispersion, the nanoconfinement effect of the polymer, and the oxidation state of Ru NPs were verified by employing TEM, energy‐dispersive X‐ray spectroscopy mapping, cross polarization magic‐angle spinning ¹³C NMR spectroscopy, and X‐ray photoelectron spectroscopy analytical tools. These three new Ru‐based POP materials exhibited excellent catalytic performance in the hydrogenation of nitroarenes at RT (with a reaction time of only ≈30 min), with high conversion, selectivity, stability, and recyclability for several catalytic cycles, compared with other traditional materials, such as Ru@C, Ru@SiO₂, and Ru@TiO₂, but no clear agglomeration or loss of catalytic activity was observed. The high catalytic performance of the ruthenium‐based POP materials is due to the synergetic effect of nanoconfinement and electron donation offered by the 3D POP network. DFT calculations showed that hydrogenation of nitrobenzene over the Ru (0001) catalyst surface through a direct reaction pathway is more favorable than that through an indirect reaction pathway.     

The partial hydrogenation of benzene to cyclohexene by nanoscale ruthenium catalysts in imidazolium ionic liquids

The controlled decomposition of an Ru(0) organometallic precursor dispersed in 1-n-butyl-3-methylimidazolium hexafluorophosphate (BMI.PF(6)), tetrafluoroborate (BMI.BF(4)) or trifluoromethane sulfonate (BMI.CF(3)SO(3)) ionic liquids with H(2) represents a simple and efficient method for the generation of Ru(0) nanoparticles. TEM analysis of these nanoparticles shows the formation of superstructures with diameters of approximately 57 nm that contain dispersed Ru(0) nanoparticles with diameters of 2.6+/-0.4 nm. These nanoparticles dispersed in the ionic liquids are efficient multiphase catalysts for the hydrogenation of alkenes and benzene under mild reaction conditions (4 atm, 75 degrees C). The ternary diagram (benzene/cyclohexene/BMI.PF(6)) indicated a maximum of 1 % cyclohexene concentration in BMI.PF(6), which is attained with 4 % benzene in the ionic phase. This solubility difference in the ionic liquid can be used for the extraction of cyclohexene during benzene hydrogenation by Ru catalysts suspended in BMI.PF(6). Selectivities of up to 39 % in cyclohexene can be attained at very low benzene conversion. Although the maximum yield of 2 % in cyclohexene is too low for technical applications, it represents a rare example of partial hydrogenation of benzene by soluble transition-metal nanoparticles.     

Activation of CS2 and CO2 by Silylium Cations

The hydride-bridged silylium cation [Et₃Si−H−SiEt₃]⁺, stabilized by the weakly coordinating [Me₃NB₁₂Cl₁₁]⁻ anion, undergoes, in the presence of excess silane, a series of unexpected consecutive reactions with the valence-isoelectronic molecules CS₂ and CO₂. The final products of the reaction with CS₂ are methane and the previously unknown [(Et₃Si)₃S]⁺ cation. To gain insight into the entire reaction cascade, numerous experiments with varying conditions were performed, intermediate products were intercepted, and their structures were determined by X-ray crystallography. Besides the [(Et₃Si)₃S]⁺ cation as the final product, crystal structures of [(Et₃Si)₂SMe]⁺, [Et₃SiS(H)Me]⁺, and [Et₃SiOC(H)OSiEt₃]⁺ were obtained. Experimental results combined with supporting quantum-chemical calculations in the gas phase and solution allow a detailed understanding of the reaction cascade.     

Transfer hydrogenation of aryl ketones with homogeneous ruthenium catalysts containing diazafluorene ligands

Novel cationic ruthenium(II) complexes bearing a 4,5‐diazafluorene unit and p‐cymene as ligands have been synthesised. The complexes were characterised based on elemental analysis and Fourier transform infrared and nuclear magnetic resonance spectroscopies. The synthesised Ru(II) complexes were employed as pre‐catalysts for the transfer hydrogenation of aromatic ketones using 2‐propanol as both hydrogen source and solvent in the presence of NaOH. All complexes showed high catalytic activity as catalysts in the reduction of substituted acetophenones to corresponding secondary alcohols. The products of catalysis were obtained with conversion rates of between 80 and 99%. Among the seven new complexes investigated, the most efficient catalyst showed turnover frequencies in the range 255–291 h^?1 corresponding to 85 to 97% conversion, respectively. Copyright © 2016 John Wiley & Sons, Ltd.