Co-H Bond Research Articles

Theoretical Insight into the Special Synergy of Bimetallic Site in Co/MoC Catalyst to Promote N2 -to-NH3 Conversion.

The catalytic mechanisms of nitrogen reduction reaction (NRR) on the pristine and Co/α-MoC(001) surfaces were explored by density functional theory calculations. The results show that the preferred pathway is that a direct N≡N cleavage occurs first, followed by continuous hydrogenations. The production of second NH3 molecule is identified as the rate-limiting step on both systems with kinetic barriers of 1.5 and 2.0 eV, respectively, indicating that N2 -to-NH3 transformation on bimetallic surface is more likely to occur. The two components of the bimetallic center play different roles during NRR process, in which Co atom does not directly participate in the binding of intermediates, but primarily serves as a reservoir of H atoms. This special synergy makes Co/α-MoC(001) have superior activity for ammonia synthesis. The introduction of Co not only facilitates N2 dissociation, but also accelerates the migration of H atom due to the antibonding characteristic of Co-H bond. This study offers a facile strategy for the rational design and development of efficient catalysts for ammonia synthesis and other reactions involving the hydrogenation processes.

The incorporation of cocatalyst cobalt sulfide into graphitic carbon nitride: Boosted photocatalytic hydrogen evolution performance and mechanism exploration

2D-layered graphitic carbon nitride (g-C 3 N 4 ) is regarded as a great prospect as a photocatalyst for H 2 generation. However, g-C 3 N 4 's photocatalytic hydrogen evolution (HER) activity is significantly restricted by the recombination of photocarriers. We find that cobalt sulfide (CoS 2 ) as a cocatalyst can promote g-C 3 N 4 nanosheets (NSs) to realize very efficient photocatalytic H 2 generation. The prepared CoS 2 /g-C 3 N 4 hybrids display highly boosted photocatalytic H 2 generation performance and outstanding cycle stability. The optimized 7%-CoS 2 /g-C 3 N 4 hybrids show a much improved photocatalytic H 2 generation rate of 36.2 ​μmol −1 ​h −1 , which is about 180 times as much as bare g-C 3 N 4 (0.2 ​μmol −1 ​h −1 ). In addition, the apparent quantum efficiency (AQE) of all the samples was computed under light at λ=370 ​nm, in which the AQE of 7%-CoS 2 /g-C 3 N 4 hybrids is up to 5.72%. The experimental data and the DFT calculation suggest that the CoS 2 /g-C 3 N 4 hybrid's excellent HER activity is attributable to the lower overpotential and the smaller Co-H bond activation energy for HER. Accordingly, the CoS 2 cocatalyst loading effectively boosts the photocatalytic performance of g-C 3 N 4 for H 2 evolution. The project promotes fast development of high-efficiency photocatalysts and low-cost for photocatalytic H 2 generation.

Catalytic conversion of polyoxymethylene with bio-derived substrates: kinetic modeling on solvent enhancement effect and experimental studies on reaction mechanism

Catalytic conversion of waste polyoxymethylene plastics to renewable and value-added chemicals represents a promising area in green chemistry. Previous studies in both academia and industry have focused on thermal depolymerization under harsh operating temperature and pressure. In this work, we have investigated the detailed reaction mechanism for cascade depolymerization and etherification of polyoxymethylene with bio-derived polyols into valuable oxygenates. As one of the highlights in this work, we proposed and validated a two-step mechanism involving depolymerization and nucleophilic attack COC bond. Kinetic modeling, IR and NMR spectra revealed mechanistic information for solvent-enhancement: the synergism of cation (Bi3+), anion (OTf-) and 1,4-dioxane (1) induces formation of intermolecular H bonding thus activating COH bond in polyol molecules, and (2) facilitates nucleophilic attack on carbocation in polyoxymethylene. The outcome of this work will provide useful fundamental data for rational design of industrial processes involving atomic efficient conversion of various polymers into value-added products.

Photoproduct formation in coenzyme B12-dependent CarH via a singlet pathway

The CarH photoreceptor exploits of the light-sensing ability of coenzyme B12 ( adenosylcobalamin=AdoCbl) to perform its catalytic function, which includes large-scale structural changes to regulate transcription. In daylight, transcription is activated in CarH via the photo-cleavage of the Co-C5' bond of coenzyme B12. Subsequently, the photoproduct, 4',5'-anhydroadenosine (anhAdo) is formed inducing dissociation of the CarH tetramer from DNA. Several experimental studies have proposed that hydridocoblamin (HCbl) may be formed in process with anhAdo. The photolytic cleavage of the Co-C5' bond of AdoCbl was previously investigated using photochemical techniques and the involvement of both singlet and triplet excited states were explored. Herein, QM/MM calculations were employed to probe (1) the photolytic processes which may involve singlet excited states, (2) the mechanism of anhAdo formation, and (3) whether HCbl is a viable intermediate in CarH. Time-dependent density functional theory (TD-DFT) calculations indicate that the mechanism of photodissociation of the Ado ligand involves the ligand field (LF) portion of the lowest singlet excited state (S1) potential energy surface (PES). This is followed by deactivation to a point on the S0 PES where the Co-C5' bond remains broken. This species corresponds to a singlet diradical intermediate. From this point, the PES for anhAdo formation was explored, using the Co-C5' and Co-C4' bond distances as active coordinates, and a local minimum representing anhAdo and HCbl formation was found. The transition state (TS) for the formation of the Co-H bond of HCbl was located and its identity was confirmed by a single imaginary frequency of i1592 cm-1. Comparisons to experimental studies and the potential role of rotation around the N-glycosidic bond of the Ado ligand were discussed.

Characterization of Spent Coffee Grounds in the Community as Supporting Materials for Renewable Energy

Coffee grounds are a by-product of the coffee brewing process. Currently, coffee grounds in the community are still untapped waste. Whereas spent coffee grounds has the potential to be converted into various high value bio-products that are environmentally friendly. This study aims to characterize coffee grounds waste which is popular in the community as a supporting material for renewable energy. This study uses a comparative method of 3 samples of Arabica coffee grounds (SCG-A), Robusta (SCG-R), and the Arabica-Robusta blend (SCG-AR) from coffee brands that are popular in Indonesian. Quantitative analysis was carried out by comparing the percentage of residual yield of the three samples. Qualitative characterization of coffee grounds was carried out using the FTIR 8300/8700 Spectrophotometer. The results of the three samples showed different rendemen values, namely 70% SCG-A, 60% SCG-R, and 80% SCG-AR. The FTIR test results showed that the three spent coffee grounds had the same functional group characteristics in the frequency range of 650–3900 cm-1. The detection of the hydroxyl functional group (-OH), the asymmetric strain of the CH bond of the methyl group (-CH3), and the stretching vibration of CO in the COH bond found in coffee grounds waste shows its potential as a supporting material for renewable energy if a further process is carried out in the form of pyrolysis/calcination at room temperature. 700◦C. Utilization of spent coffee grounds in the community can be done by establishing a Spent Coffee Grounds Bank (SCG Bank), educating the public so that they are willing to donate spent coffee grounds, and managing SCG as a supporting material for renewable energy.

Chemoselective and Tandem Reduction of Arenes Using a Metal-Organic Framework-Supported Single-Site Cobalt Catalyst.

The development of heterogeneous, chemoselective, and tandem catalytic systems using abundant metals is vital for the sustainable synthesis of fine and commodity chemicals. We report a robust and recyclable single-site cobalt-hydride catalyst based on a porous aluminum metal-organic framework (DUT-5 MOF) for chemoselective hydrogenation of arenes. The DUT-5 node-supported cobalt(II) hydride (DUT-5-CoH) is a versatile solid catalyst for chemoselective hydrogenation of a range of nonpolar and polar arenes, including heteroarenes such as pyridines, quinolines, isoquinolines, indoles, and furans to afford cycloalkanes and saturated heterocycles in excellent yields. DUT-5-CoH exhibited excellent functional group tolerance and could be reusable at least five times without decreased activity. The same MOF-Co catalyst was also efficient for tandem hydrogenation-hydrodeoxygenation of aryl carbonyl compounds, including biomass-derived platform molecules such as furfural and hydroxymethylfurfural to cycloalkanes. In the case of hydrogenation of cumene, our spectroscopic, kinetic, and density functional theory (DFT) studies suggest the insertion of a trisubstituted alkene intermediate into the Co-H bond occurring in the turnover limiting step. Our work highlights the potential of MOF-supported single-site base-metal catalysts for sustainable and environment-friendly industrial production of chemicals and biofuels.

Electronic and photolytic properties of hydridocobalamin.

Hydridocobalamin (HCbl), is a known member of the B12 family of molecules (cobalamins, Cbls) yet unlike other well-studied Cbls, little is known of the electronic and photolytic properties of this species. Interest in HCbl has increased significantly in recent years when at least three experimentally proposed mechanisms implicate HCbl as an intermediary in the photoreaction of coenzyme B12-dependent photoreceptor CarH. Specifically, cleavage of the Co-C5' bond of coenzyme B12 could lead to a β-hydride or β‑hydrogen elimination reaction to form HCbl. HCbl is known to be a transient species where the oxidation state of the Co is variable; Co(I)-H+ ↔ Co(II)-H↔Co(III)-H-. Further, HCbl is a very unstable with a pKa of ~1. This complicates experimental studies and to the best of our knowledge there are no available crystal structures of HCbl - either for the isolated molecule or bound to an enzyme. In this study, the electronic structure, photolytic properties, and reactivity of HCbl were explored to determine the preferred oxidation state as well as its potential role in the formation of the photoproduct in CarH. Natural bond orbital (NBO) analysis was performed to determine the oxidation state of Co in isolated HCbl. Based on the NBO analysis of HCbl, Co clearly had excess negative charge, which is in stark contrast to other alkylCbls where the Co ion is marked by significant positive charge. In sum, NBO results indicate that the CoH bond is strongly polarized and almost ionic. It can be described as protonated Co(I). In addition, DFT was used to explore the bond dissociation energy of HCbl based on homolytic cleavage of the CoH bond. TD-DFT calculations were used to compare computed electronic transitions to the experimentally determined absorption spectrum. The photoreaction of CarH was explored using an isolated model system and a pathway for hydrogen transfer was found. Finally, quantum mechanics/molecular mechanics (QM/MM) calculations were employed to investigate the formation of HCbl in CarH.

Photo-Initiated Cobalt-Catalyzed Radical Olefin Hydrogenation.

Outer‐sphere radical hydrogenation of olefins proceeds via stepwise hydrogen atom transfer (HAT) from transition metal hydride species to the substrate. Typical catalysts exhibit M−H bonds that are either too weak to efficiently activate H2 or too strong to reduce unactivated olefins. This contribution evaluates an alternative approach, that starts from a square‐planar cobalt(II) hydride complex. Photoactivation results in Co−H bond homolysis. The three‐coordinate cobalt(I) photoproduct binds H2 to give a dihydrogen complex, which is a strong hydrogen atom donor, enabling the stepwise hydrogenation of both styrenes and unactivated aliphatic olefins with H2 via HAT.

Atomically-ordered active sites in NiMo intermetallic compound toward low-pressure hydrodeoxygenation of furfural

Activation of oxygen-containing functional groups plays a key role in sustainable biomass upgrading and conversion. In this work, a NiMo intermetallic compound (IMC) catalyst was prepared based on layered double hydroxides (LDHs) precursors, which displayed prominent catalytic performance for furfural hydrodeoxygenation (HDO) to 2-methylfuran (2-MF) (yield: 99 %) at a rather low hydrogen pressure (0.1 MPa), significantly superior to NiMo alloy, monometallic Ni and other Ni-based catalysts ever reported. CO-IR, STEM, EXAFS and XANES give direct evidences that the atomically-ordered Ni/Mo sites in NiMo IMC determine the uniform bridging-type adsorption mode of CO bond in furfural whilst adsorption of furan ring is extremely suppressed. In situ FT-IR and DFT calculation further substantiate that ordered Ni-Mo bimetallic sites of IMC, in contrast to the random atomic sequence in NiMo alloy, facilitate the activation and cleavage of COH bond in the intermediate (furfuryl alcohol, FOL), accounting for the production of 2-MF. This work demonstrates the decisive effect of atomically-ordered active sites in IMC catalyst on activation of oxygen-containing functional groups and product selectivity, which can be extended to catalytic upgrading of biomass-derived platform molecules.

Regioselective Hydrosilylation of Olefins Catalyzed by Co-Iminobipyridine Complexes: The Role of Cyclohexyl Substituent on the Imino Nitrogen

Abstract Regioselective hydrosilylation of aliphatic olefins catalyzed by Co-iminobipyridine complexes, Co〈R〉, were investigated, where R indicates a substituent on the imino nitrogen in an iminobipyridine ligand (iminobypyridine = [2,2′-bpy]-6-C(Me)=N-R). Syntheses of two complexes, Co〈Mes〉 (Mes = 2,4,6-trimethylphenyl) and Co〈Cy〉 (Cy = cyclohexyl), and comparison of their catalytic activity for hydrosilylation of 1-octene with diphenylsilane revealed that the reaction system using Co〈Mes〉 produced a mixture of anti-Markovnikov and Markovnikov hydrosilylation products, whereas that using Co〈Cy〉 produced the anti-Markovnikov product selectively. Crystal structures of these complexes implied that a catalytically active species derived from Co〈Cy〉 has narrower active space for hydrosilylation than that from Co〈Mes〉. According to the Chalk-Harrod mechanism, there are two types of hydrosilylated products (anti-Markovnikov and Markovnikov products), which stem from the way of terminal olefin insertion into an M-H bond (that is 1,2- or 2,1-insertion). One of the intermediates derived from Co〈Cy〉 in the Chalk-Harrod mechanism has hydride and silyl ligands. In the step of olefin insertion into the Co-H bond, 1,2-insertion is more likely to occur from the steric point of view, leading to selective formation of the anti-Markovnikov product.

Cobalt-Catalyzed Regio- and Stereoselective Hydroboration of Allenes.

An efficient pincer-ligand-based cobalt-complex-catalyzed allene hydroboration affording Z-allylic boronates is described. The reaction demonstrates an excellent regio- as well as Z-stereoselectivity and a wide substrate scope that tolerates many functional groups. Based on solvent-assisted electrospray ionization mass spectrometry (SAESI-MS) studies, a rationale for the cobalt-catalyzed hydroboration involving the highly selective insertion of an allene into the Co-H bond to form Z-allylic cobalt intermediates is proposed.

Photodissociation of trifluoroacetic acid at 193 nm: Mechanism for formation of OH radical and stable products

Trifluoroacetic acid (TFA) is released in the atmosphere through its use in the chemical industry and as degradation product of chlorofluorocarbon (CFC) alternatives like hydrofluorocarbons and hydrochlorofluorocarbons. In the present study, we have investigated the OH formation dynamics in the photodissociation of TFA at 193 nm by Laser Photolysis-Laser Induced Fluorescence (LP-LIF) method, as well as stable product formation by GCMS and FTIR. It was found that, ∼26% of the available energy is partitioned into the relative translation of the photoproducts (f(T) = 0.26), which could be explained by presence of an exit barrier of ∼13 kcal/mol in OH formation channel. This result is very similar to OH formation from acetic acid (AA) and difluoroacetic acid (DFA), indicating fluorination at the side chain of aliphatic carboxylic acids does not significantly change the mechanism of COH bond scission. Our experimental results tallied with the theoretical studies, which suggested that the major OH formation channel in acetic acid and fluoroacetic acid is direct dissociation from the optically excited S1 state through an exit barrier, with some competition from the T1 state. However, quantum yield of OH formation from TFA (0.4) was found to be much smaller than AA (0.8), which is probably caused by higher reaction barrier in T1 state of TFA, compared to AA. CHF3, C2F4, C2F6, CO2, CO, CF3CFO, CF2O and hexafluoropropylene oxide (HFPO) were detected as the stable products of the photolysis of TFA. The theoretically optimized ground state dissociation channels showed significant difference between TFA and AA.

Selective hydrodeoxygenation of lignin β-O-4 model compounds and aromatic ketones promoted by palladium chloride with acidic CO2/MeOH system

Selective hydrogenolysis is a crucial challenge for lignin valorization. A PdCl2-catalyzed hydrogenolysis of COH bond of various lignin β-O-4 alcohol model compounds was developed for preparation of corresponding phenethoxybenzene in 78–98% isolated yield. Notably, the introduction of low-pressure CO2 (1 MPa) remarkably improves the reaction efficiency and selectivity of phenethoxybenzene. Neither over-reduction of aromatic ring nor further debenzylation was detected. This is understandable that the in situ methylcarbonic acid generated from CO2 and methanol acts as an acidic catalyst and enhances the leaving ability of hydroxyl group of β-O-4 alcohols. Besides, the present PdCl2/CO2/MeOH catalyst system proved to be effective for reductive cleavage of CO bonds of β-O-4 ketones, producing ethylbenzenes accompanied with phenols in high yields. Furthermore, this protocol could also be extended to selective hydrodeoxygenation of the carbonyl group to methylene in aromatic ketones. As a sound reaction medium, such in situ CO2/MeOH acidic system could be inherently neutralized by depressurizing CO2, providing feature advantages for simple post-processing and none waste disposal.

Hydrodeoxygenation of m-cresol over bimetallic NiFe alloys: Kinetics and thermodynamics insight into reaction mechanism

Developing mechanistic insights into the reaction network of hydrodeoxygenation (HDO) of lignin-derived compounds is key to rational design of high-performance catalysts for bio-oils upgrading. Herein, we present a comprehensive theoretical study on HDO of m-cresol, a model compound of phenolics, on Ni(1 1 1) and NiFe(1 1 1) surfaces using periodic density functional theory calculations and microkinetic modeling techniques, with a focus on the several competing reaction pathways including enol-keto tautomerization, hydrogenation, and dehydroxylation. Our results show that the activation of the COH bond of m-cresol and phenolic intermediates can be greatly promoted on oxophilic NiFe(1 1 1), evidenced by the elongated COH bond length and the enhanced dehydroxylation activity with respect to Ni(1 1 1). It is found that m-cresol HDO on NiFe(1 1 1) shares certain common features with that on Ni(1 1 1), but exhibits important differences that result in dramatic changes in selectivity. We show that the COH bond length of adsorbed phenolic intermediates can be used as a good descriptor for prediction on the COH bond scission reaction in HDO. Finally, microkinetic modeling augmented by degree of rate control analysis is applied to rationalize the experimentally-observed differences in product distributions over Ni and NiFe catalysts when kinetic factors still dominate.

Insight on mechanism of Sn modification in alumina supported RhSn catalysts for acetic acid hydrogenation to fuel-grade ethanol

Acetic acid hydrogenation has been carried out over Rh/Al2O3 and RhSn/Al2O3 with various Sn/Rh ratios and different loadings. Metallic Rh, Rh oxides and Sn oxides were detected after reduction of RhSn/Al2O3 at 623K. Spillover of hydrogen was observed in H2 desorption process due to the coexistence of metallic Rh and Sn oxides. The addition of Sn improved dissociation of COH bond and stabilized top adsorption of acetyl species during the adsorption of acetic acid at 548K. With the introduction of hydrogen, high Sn/Rh ratio suppressed methane production and generated more ethoxy species. High loadings of Sn and Rh can also improve catalytic performance of acetic acid hydrogenation to ethanol. In our experiments, 3Rh5Sn/Al2O3 achieved the selectivity to ethanol of 74% with the corresponding conversion value of about 98%.

Phototransformations of quinaldic acid: Theoretical and experimental study

A combined theoretical and experimental study was performed in order to clarify the mechanisms of phototransformations of a quinaldic acid solution in acetonitrile when exposed to UV radiation. The theoretical calculations were performed at the BLYP/aug-cc-pVDZ level. It was established that the most stable isomer of the compound in the solution is the one with an intramolecular H-bond between the H atom from the carboxylic group and the N atom from the quinoline ring. The major photoprocess is the breaking of the intramolecular H-bond, i.e. the rotation around the COH bond of the carboxylic group through the specroscopically active 1ππ* excited state. Rietveld method was applied to refine the crystal structure of the compound. The refinement lead to the following unit cell parameters: a = 9.76754 Å, b = 6.02724 Å, c = 28.11714 Å, β = 90.495°; and a space Group of P 2/c (Z = 4). The cell volume is 1655.23 Å3.

Electrochemical Detection of Transient Cobalt Hydride Intermediates of Electrocatalytic Hydrogen Production.

A large variety of molecular cobalt complexes are used as electrocatalysts for H2 production, but the key cobalt hydride intermediates are frequently difficult to detect and characterize due to their high reactivity. We report that a combination of variable scan rate cyclic voltammetry and foot-of-the-wave analysis (FOWA) can be used to detect transient Co(III)H and Co(II)H intermediates of electrocatalytic H2 production by [Co(II)(P(tBu)2N(Ph)2)(CH3CN)3](2+) and Co(II)(dmgBF2)2(CH3CN)2. In both cases, reduction of a transient catalytic intermediate occurs at a potential that coincides with the Co(II/I) couple. Each reduction displays quasireversible electron-transfer kinetics, consistent with reduction of a Co(III)H intermediate to Co(II)H, which is then protonated by acid to generate H2. A bridge-protonated Co(I) species was ruled out as a catalytic intermediate for Co(II)(dmgBF2)2(CH3CN)2 from voltammograms recorded at 1000 psi of H2. Density functional theory was used to calculate Co(III)-H and Co(II)-H bond strengths for both catalysts. Despite having very different ligands, the cobalt hydrides of both catalysts possess nearly identical heterolytic and homolytic Co-H bond strengths for the Co(III)H and Co(II)H intermediates.

С-ОН bond cleavage initiated by electron transfer: electroreduction of 9-fluorenol

Сyclic voltammetry, chronoamperometry, coulometry, electrolysis, digital simulation, quantum chemical calculations of 9-fluorenol as an example, were used to show that the electroreduction of aryl derivatives of methanol in 0.1M Bu4NClO4/DMF proceeds via the ECE mechanism (including the stages of radical anion formation and the COH bond cleavage in the radical anion) complicated by the reactions of the depolarizer with the anionic products. Among these reactions are the deprotonation of 9-fluorenol and its monoanions by hydroxide anion and fluorenyl anion. The thermodynamic parameters of the reactions have been estimated both theoretically and experimentally. It was found that the equilibrium constants of the fluorenyl anions deprotonation are close (C-anion) or higher (O-anion) than that of fluorenol. As a result the total equilibrium is shifted towards the side of the dianion of 9-fluorenone. The unusual ratio of the equilibrium constants was explained by lower basicity of π*-dianion compare with other anions.

Computational Characterization of Redox Non-Innocence in Cobalt-Bis(Diaryldithiolene)-Catalyzed Proton Reduction.

Localized orbital bonding analysis (LOBA) was employed to probe the oxidation state in cobalt-bis(diaryldithiolene)-catalyzed proton reduction in nonaqueous media. LOBA calculations provide both the oxidation state and chemically intuitive views of bonding in cobalt-bis(diaryldithiolene) species and therefore allow characterization of the role of the redox non-innocent dithiolene ligand. LOBA results show that the reduction of the monoanion species [1Br](-) is metal-centered and gives a cobalt(II) ion species, [1Br](2-), coordinated to two dianionic ene-1,2-dithiolates. This electronic configuration is in agreement with the solution magnetic moment observed for the analogous salt [1F](2-) (μeff = 2.39 μB). Protonation of [1Br](2-) yields the cobalt(III)-hydride [1Br(CoH)](-) species in which the Co-H bond is computed to be highly covalent (Löwdin populations close to 0.50 on cobalt and hydrogen atoms). Further reduction of [1Br(CoH)](-) forms a more basic cobalt(II)-H intermediate [1Br(CoH)](2-) (S = 0) from which protonation at sulfur gives a S-H bond syn to the Co-H bond. Formation of a cobalt-dihydrogen [1Br(CoH2)](-) intermediate is calculated to occur via a homocoupling (H(•) + H(•) → H2) step with a free energy of activation of 5.9 kcal/mol in solution (via C-PCM approach).

Cobalt catalyzed z-selective hydroboration of terminal alkynes and elucidation of the origin of selectivity.

A bis(imino)pyridine cobalt-catalyzed hydroboration of terminal alkynes with HBPin (Pin = pinacolate) with high yield and (Z)-selectivity for synthetically valuable vinylboronate esters is described. Deuterium labeling studies, stoichiometric experiments, and isolation of catalytically relevant intermediates support a mechanism involving selective insertion of an alkynylboronate ester into a Co-H bond, a pathway distinct from known precious metal catalysts where metal vinylidene intermediates have been proposed to account for the observed (Z) selectivity. The identity of the imine substituents dictates the relative rates of activation of the cobalt precatalyst with HBPin or the terminal alkyne and, as a consequence, is responsible for the stereochemical outcome of the catalytic reaction.