| 1. |
- Hansen, Nicolai S.B., et al.
(författare)
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Development and mechanistic investigation of the dehydrogenation of alcohols with an iron(iii) salen catalyst
- 2023
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Ingår i: Organic and biomolecular chemistry. - : Royal Society of Chemistry (RSC). - 1477-0520 .- 1477-0539. ; 21:23, s. 4794-4800
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Tidskriftsartikel (refereegranskat)abstract
- The iron(iii) salen complex (R,R)-N,N′-bis(salicylidene)-1,2-cyclohexanediamineiron(iii) chloride has been developed as a catalyst for the acceptorless dehydrogenation of alcohols. The complex catalyzes the direct synthesis of imines in good yields from different primary alcohols and amines with the liberation of hydrogen gas. The mechanism has been investigated experimentally with labelled substrates and theoretically with density functional theory calculations. In contrast to the corresponding manganese(iii) salen-catalyzed dehydrogenation, it has not been possible to identify a homogeneous catalytic pathway with the iron complex. Instead, poisoning experiments with trimethylphosphine and mercury indicated that the catalytically active species are heterogeneous small iron particles.
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| 2. |
- Wu, Haibo, et al.
(författare)
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Site- and Enantioselective Iridium-Catalyzed Desymmetric Mono-Hydrogenation of 1,4-Dienes
- 2021
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Ingår i: Angewandte Chemie International Edition. - : Wiley. - 1433-7851 .- 1521-3773. ; 60:35, s. 19428-19434
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Tidskriftsartikel (refereegranskat)abstract
- The control of site selectivity in asymmetric mono-hydrogenation of dienes or polyenes remains largely underdeveloped. Herein, we present a highly efficient desymmetrization of 1,4-dienes via iridium-catalyzed site- and enantioselective hydrogenation. This methodology demonstrates the first iridium-catalyzed hydrogenative desymmetriation of meso dienes and provides a concise approach to the installation of two vicinal stereogenic centers adjacent to an alkene. High isolated yields (up to 96%) and excellent diastereo- and enantioselectivities (up to 99:1 d.r. and 99% ee) were obtained for a series of divinyl carbinol and divinyl carbinamide substrates. DFT calculations reveal that an interaction between the hydroxy oxygen and the reacting hydride is responsible for the stereoselectivity of the desymmetrization of the divinyl carbinol. Based on the calculated energy profiles, a model that simulates product distribution over time was applied to show an intuitive kinetics of this process. The usefulness of the methodology was demonstrated by the synthesis of the key intermediates of natural products zaragozic acid A and (+)-invictolide.
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| 3. |
- Yang, Jianping, et al.
(författare)
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Combined Theoretical and Experimental Studies Unravel Multiple Pathways to Convergent Asymmetric Hydrogenation of Enamides
- 2021
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 143:51, s. 21594-21603
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Tidskriftsartikel (refereegranskat)abstract
- We present a highly efficient convergent asymmetric hydrogenation of E/Z mixtures of enamides catalyzed by N,P–iridium complexes supported by mechanistic studies. It was found that reduction of the olefinic isomers (E and Z geometries) produces chiral amides with the same absolute configuration (enantioconvergent hydrogenation). This allowed the hydrogenation of a wide range of E/Z mixtures of trisubstituted enamides with excellent enantioselectivity (up to 99% ee). A detailed mechanistic study using deuterium labeling and kinetic experiments revealed two different pathways for the observed enantioconvergence. For α-aryl enamides, fast isomerization of the double bond takes place, and the overall process results in kinetic resolution of the two isomers. For α-alkyl enamides, no double bond isomerization is detected, and competition experiments suggested that substrate chelation is responsible for the enantioconvergent stereochemical outcome. DFT calculations were performed to predict the correct absolute configuration of the products and strengthen the proposed mechanism of the iridium-catalyzed isomerization pathway.
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| 4. |
- Zheng, Jia, et al.
(författare)
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Iridium-catalysed enantioselective formal deoxygenation of racemic alcohols via asymmetric hydrogenation
- 2019
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Ingår i: NATURE CATALYSIS. - : NATURE PUBLISHING GROUP. - 2520-1158. ; 2:12, s. 1093-1100
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Tidskriftsartikel (refereegranskat)abstract
- Asymmetric hydrogenation of alkenes is one of the most powerful tools for the preparation of optically active compounds. However, to achieve high enantioselectivity, the starting olefin in most cases needs to be isomerically pure in either the cis or the trans form. Generally, most olefination protocols provide olefins as isomeric mixtures that are difficult to separate, and in many cases also generate lots of waste. In contrast, the synthesis of racemic alcohols is straightforward and highly atom-efficient, with products that are easier to purify. Here, we describe a strategy that enables rapid access to chiral alkanes via enantioconvergent formal deoxygenation of racemic alcohols. Mechanistic studies indicate an Ir-mediated elimination of water and subsequent in situ hydrogenation. This approach allows rapid and efficient assembly of chiral intermediates and is exemplified in the total synthesis of antidepressant sertraline and sigma(2) receptor PB 28.
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| 5. |
- Agarwala, Hemlata, et al.
(författare)
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Alternating Metal-Ligand Coordination Improves Electrocatalytic CO2 Reduction by a Mononuclear Ru Catalyst**
- 2023
-
Ingår i: Angewandte Chemie International Edition. - : Wiley. - 1433-7851 .- 1521-3773. ; 62:17
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Tidskriftsartikel (refereegranskat)abstract
- Molecular electrocatalysts for CO2-to-CO conversion often operate at large overpotentials, due to the large barrier for C−O bond cleavage. Illustrated with ruthenium polypyridyl catalysts, we herein propose a mechanistic route that involves one metal center that acts as both Lewis base and Lewis acid at different stages of the catalytic cycle, by density functional theory in corroboration with experimental FTIR. The nucleophilic character of the Ru center manifests itself in the initial attack on CO2 to form [Ru-CO2]0, while its electrophilic character allows for the formation of a 5-membered metallacyclic intermediate, [Ru-CO2CO2]0,c, by addition of a second CO2 molecule and intramolecular cyclization. The calculated activation barrier for C−O bond cleavage via the metallacycle is decreased by 34.9 kcal mol−1 as compared to the non-cyclic adduct in the two electron reduced state of complex 1. Such metallacyclic intermediates in electrocatalytic CO2 reduction offer a new design feature that can be implemented consciously in future catalyst designs.
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| 6. |
- Agarwala, Hemlata, et al.
(författare)
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An Elusive Intermediate Uncovered in the Pathway for Electrochemical Carbon Dioxide Reduction by Ruthenium Polypyridyl Catalyst - Combined Spectroscopic and Computational Investigation
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- A scrutinous study of the catalytic cycle for electrochemical CO2 reduction by the ruthenium 2,2:6,2-terpyridine (tpy) 2,2-bipyridine (bpy) class of catalysts is presented. An unprecedented 2-(C,O)-carboxycarboxylatoruthenium(II) metalacyclic intermediate, critical for C-O bond dissociation at low overpotentials, so far precluded from mechanistic considerations of polypyridyl transition metal complex catalysts, is unearthed by infra-red spectroscopy coupled to controlled potential electrolysis in corroboration with density functional theory (DFT) investigations. Thermodynamic and kinetic analyses of the intermediate reveal the important role of the structural flexibility of polypyridyl ligands and fine electronic tunability of the metal center, along with kinetic trans effect, in propelling catalysis at lower overpotentials. The choice of metal center, Ru in the present case, points to the fact that the requirement of an additional Lewis acid to enhance C-O bond dissociation, hence increase the catalytic rate or turnover, can be circumvented.
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| 7. |
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| 8. |
- Ahlstrand, David A., et al.
(författare)
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Csp(3)-H Activation without Chelation Assistance in an Iridium Pincer Complex Forming Cyclometallated Products
- 2017
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Ingår i: Chemistry - A European Journal. - : WILEY-V C H VERLAG GMBH. - 0947-6539 .- 1521-3765. ; 23:8, s. 1748-1751
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Tidskriftsartikel (refereegranskat)abstract
- Cyclometallation of 8-methylquinoline and 2-(dimethylamino)-pyridine in an iridium-based pincer complex is described. The C-H activation of 2-(dimethylamino) pyridine is not chelation assisted, which has not been described before for Csp(3)-H bonds in cyclometallation reactions. The mechanism of the cyclometallation of 2-(dimethylamino) pyridine was studied by DFT calculations and kinetic measurements.
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| 9. |
- Castner, Ashleigh T., et al.
(författare)
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Microscopic Insights into Cation-Coupled Electron HoppingTransport in a Metal-Organic Framework br
- 2022
-
Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 144:13, s. 5910-5920
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Tidskriftsartikel (refereegranskat)abstract
- Electron transport through metal-organic frameworks by ahopping mechanism between discrete redox active sites is coupled to diffusion-migration of charge-balancing counter cations. Experimentally determinedapparent diffusion coefficients,Deapp, that characterize this form of chargetransport thus contain contributions from both processes. While this is wellestablished for MOFs, microscopic descriptions of this process are largelylacking. Herein, we systematically lay out different scenarios for cation-coupledelectron transfer processes that are at the heart of charge diffusion throughMOFs. Through systematic variations of solvents and electrolyte cations, it isshown that theDeappfor charge migration through a PIZOF-type MOF,Zr(dcphOH-NDI) that is composed of redox-active naphthalenediimide(NDI) linkers, spans over 2 orders of magnitude. More importantly, however,the microscopic mechanisms for cation-coupled electron propagation arecontingent on differing factors depending on the size of the cation and its propensity to engage in ion pairs with reduced linkers,either non-specifically or in defined structural arrangements. Based on computations and in agreement with experimental results, weshow that ion pairing generally has an adverse effect on cation transport, thereby slowing down charge transport. In Zr(dcphOH-NDI), however, specific cation-linker interactions can open pathways for concerted cation-coupled electron transfer processes thatcan outcompete limitations from reduced cationflux.
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| 10. |
- Chen, Xiaoyu, et al.
(författare)
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Aggregation and Significant Difference in Reactivity Therein : Blocking the CO2-to-CH3OH Reaction
- 2021
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Ingår i: Organometallics. - : American Chemical Society (ACS). - 0276-7333 .- 1520-6041. ; 40:17, s. 3087-3093
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Tidskriftsartikel (refereegranskat)abstract
- A CoPc/CNT system has been only recently reported to transform CO2 to methanol via electrochemical reductions, despite the fact that catalyst has been studied extensively since the 1980s. The explanation of high methanol selectivity lies behind the fact that in the new report CoPc exists mainly as a monomer, while in earlier works aggregates dominate. Here, we have studied the reactivity of monomeric and dimeric CoPc by DFT. The mechanism involves rate-limiting CO2 association, with the C-O cleavage step having very similar activation free energy. Once the Co-CO-intermediate is formed, the reaction bifurcates with two possible paths: (1) CO dissociation or (2) one additional reduction follows a protonation to give the Co-CHO-intermediate, which then leads to methanol by further reactions. For the monomeric species at low reduction potentials, CO dissociation is favored, but the formation of Co-CHO-becomes competitive at more negative applied potentials. For the dimer, the CO dissociation is always favored, and the reduction needed to form the C-H bond is negative enough for it not to be observed. The more difficult reduction stems from repulsive interactions between the CoPc units and lower solvent stabilization of the charge in the aggregate.
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| 11. |
- Chen, Xiaoyu, et al.
(författare)
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Aggregation and the Siginificant Difference in Reactivity therein: Blocking the CO2-to-CH3OH Reaction Pathway
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- A CoPc/CNT system was recently reported to transform CO2 to methanol via electrochemical reductions, despite the catalyst has been studied since the 1980s, such observations were not reported earlier. A clue to the high methanol selectivity is that CoPc exist as mainly as monomers in the new report while in earlier works CoPc aggregates dominate. Here we have studied the reactivity of monomeric and dimeric CoPc by DFT. The mechanism involves rate limiting CO2 association, with the C-O cleavage step being having very similar activation free energy. Once the Co-CO- intermediate is formed the reaction bifurcates with two possible paths, CO dissociation or further reduction and protonation to give the Co-CHO- intermediate, which then leads to methanol by further reactions. For the monomeric species at low reduction potentials CO dissociation is favored, but the formation of Co-CHO- becomes competitive at more negative applied potentials. For the dimer the CO dissociation is always favored, and the reduction needed to form the C-H bond is negative enough for it not to be observed. The more difficult reduction stems from repulsive interac- tions between the Co-Pc units and lower solvent stabilization of the charge in the aggregate.
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| 12. |
- Chen, Xiaoyu, et al.
(författare)
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Deconstructing the Enhancing Effect on CO2 Activation in the Electric Double Layer with EVB Dynamic Reaction Modeling
- 2020
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Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 124:41, s. 22479-22487
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Tidskriftsartikel (refereegranskat)abstract
- The reactivities of the same molecular electrocatalyst under homogeneous and heterogeneous conditions can be dramatically different, highlighting that the reaction environment plays an important role in catalysis. For catalysis on solid electrodes, reactions take place in the electric double layer (EDL), where a strong electric field is experienced. In this work, empirical valence bond molecular dynamics (EVB-MD) was used to explore CO2binding in the EDL. It allows explicit descriptions of the solvent, electrolyte, catalyst–reactant, and the electrode surface material, as well as an unbiased description of the applied electric field. The strong local electric field concentrates cations, which in turn stabilizes the bound CO2. Furthermore, controlled computational experiments suggest that neither the electric field nor the cations alone can produce significant stabilization, but that the combination leads to a dramatic stabilization of the CO2 bound state.
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| 13. |
- Chen, Xiaoyu, 1993-
(författare)
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Theoretical Studies on CO2 Reduction Electrocatalysts
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The atmospheric CO2 concentration has increased by more than 20% since 1980s and has now reached the highest level than at any point in the past 800 000 years. Electrochemical CO2 reductions are receiving particular in- terest as the apparatus are relatively easy to maintain and cheap to operate. However, the direct reduction of CO2 into CO2 radical requires a very high over-potential, meaning a substantial waste in energy. In order to lower the over-potential required, a large number of catalysts has been synthesised and studied. Among these catalysts, three are studied in this work due to their interesting reactivities. We believe the further understanding gained in our studies will benefit the development of new and better catalysts.Ru(6-Mebpy)(tBu3-tpy) reduces CO2 at its first reduction potential and can therefore lower the over-potential required significantly. This observation is unique for Ru(tpy)(bpy) type of catalysts. Density functional theory (DFT) cal- culations revealed that the steric hindrance provided by the 6-methyl group weakens Ru-solvent interactions and hence allows solvent detachment to take place after only one reduction, which is otherwise not possible. Furthermore, we proposed a new mechanism for CO2 to CO reduction at the first reduc- tion potential and identified a cyclic intermediate by Infra-red spectroscopy in collaboration with experimentalists. Such intermediate was not reported pre- viously for Ru-based electrocatalysts.Co(TPP)/CNTs as a heterogenous catalyst exhibits superior reactivity as compared to in solution. DFT calculations with implicit solvent model ac- counts its enhanced reactivity to the increased proton concentration in water. The inverse-loading effect was studied by potential mean force (PME) sam- pling. Our results suggest that aggregation is triggered by the strong inter- molecular p - p interactions among the catalysts. Flatter nanotubes have better contact with Co(TPP) and hence reduces aggregation tendency. The same cat- alyst was also used as an example to study catalysis at interfaces in an electric field. Our full-explicit EVB -MD (Empirical Valence Bond-Molecular Dynam- ics) model illustrates that the electric double layer concentrates cations, which significantly stabilises polarised CO2 at a higher concentration and hence eases CO2 binding. Furthermore, we have also shown that either the electric field or the cations along provides only a minor, almost negligible stabilisation.In 2019, CoPc/CNTs was reported to be the first early-period transition metal complex that can catalyse CO2-to-CH3OH conversion at a decent yield. Literature search on previous work suggests that the presence of well-dispersed, monomeric CoPc is crucial to further reduce CO into CH3OH. We calculated the reaction profiles for both monomeric CoPc and dimeric CoPc, which is the simplest form of aggregates. Our DFT results demonstrate that after the formation of catalyst-CO- complex, monomers tend to go though further reac- tions to afford CH3OH while dimers tend to dissociate CO as reductions are slightly harder, which in turn, is raised from a less degree of solvation stabili- sation upon reductions.
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| 14. |
- Chen, Xiaoyu, et al.
(författare)
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Understanding the Enhanced Catalytic CO2 Reduction upon Adhering Cobalt Porphyrin to Carbon Nanotubes and the Inverse Loading Effect
- 2020
-
Ingår i: Organometallics. - : American Chemical Society (ACS). - 0276-7333 .- 1520-6041. ; 39:9, s. 1634-1641
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Tidskriftsartikel (refereegranskat)abstract
- Adhering a cobalt porphyrin (Co(TPP)) catalyst on a carbon nanotube (CNT) supporting material greatly enhances its reactivity and enables catalysis in water, which is otherwise impossible. However, the effect of solvent as well as supporting materials on catalysis is still elusive. On the basis of computational results we found that water as a reaction medium lowers the reductive potential required due to the stabilization of intermediates and transition states, and provides higher availability of protons. To understand the effect of the support materials, we combine computations and experiments and illustrate that the curvature of the nanotubes plays an essential role in aggregation through the competition between the Ï-πinteractions between the porphyrin rings as well as between the Co(TPP) and the nanotube, providing an insight into lessening the degree of aggregation.
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| 15. |
- Chen, Xiaoyu, et al.
(författare)
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Understanding the Mechanism of CO2 to CO Conversion by Ruthenium Polypyridyl Catalysts
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- A detailed mechanistic study of ruthenium 2,2:6,2-terpyridine (tpy) 2,2-bipyridine (bpy) class of catalysts is presented, with all three key stages (i.e. solvent dissociation, C-O bond cleavage and CO dissociation) discussed. DFT calculations together with kinetic studies revealed that the introduction of a methyl substituent on the bipyridine ligand eases solvent dissociation and hence allow catalysis to take place at the first reduction potential as the five coordinated Ru complex is easier to reduce. This highlights the importance of steric effect in catalyst-design. For C-O bond cleavage, DFT calculations suggest that proton acts as a much better oxide accepter compared to CO2, explaining the improved activity when water is added to the system. To further understand how the electronic nature of the ring substitutes affects the reactivity, we designed a hypothetical catalyst with fluorine substitutes and found out electron withdrawing groups lower the reductive potentials at a cost of harder solvent dissociation. For the final CO dissociation, due to the special nature of carbonyl ligands, neither steric nor electronic alternations can ease the step and here is where kinetic trans effect comes into play. In line with a recent experimental work, our DFT calculations showed that when a carbene group is trans to CO, the dissociation rate is increased dramatically.
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| 16. |
- Colozza, Noemi, et al.
(författare)
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Insights into Tripodal Tris(pyrazolyl) Compounds as Ionophores for Potentiometric Ammonium Ion Sensing
- 2022
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Ingår i: ChemElectroChem. - : Wiley. - 2196-0216. ; 9:18
-
Tidskriftsartikel (refereegranskat)abstract
- The decentralisation of accurate determination of the ammonium ion (NH4+) is relevant for environmental monitoring (i. e., nitrogen cycle) and certain clinical applications (e. g., kidney and liver diseases). Potentiometric ionophore-based sensors are one alternative for these purposes in terms of versatile implementation, though the potassium ion (K+) is known to be a major source of interference. We herein investigate the use of three different tripodal tris(pyrazolyl) compounds derived from 1,3,5-triethylbenzene as NH4+ ionophores. A complete set of potentiometric experiments together with theoretical simulations reveals suitable analytical performance while demonstrating a suppression of the K+ interference given the formation of an adequate cavity in the ionophore to host NH4+ over K+ in the membrane environment. The results support the use of these electrodes in the analytical detection of NH4+ in a wide range of samples with variable contents.
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| 17. |
- Daniel, Quentin, et al.
(författare)
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Rearranging from 6-to 7-coordination initiates the catalytic activity : An EPR study on a Ru-bda water oxidation catalyst
- 2017
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Ingår i: Coordination chemistry reviews. - : Elsevier. - 0010-8545 .- 1873-3840. ; 346, s. 206-215
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Tidskriftsartikel (refereegranskat)abstract
- The coordination of a substrate water molecule on a metal centered catalyst for water oxidation is a crucial step involving the reorganization of the ligand sphere. This process can occur by substituting a coordinated ligand with a water molecule or via a direct coordination of water onto an open site. In 2009, we reported an efficient ruthenium-based molecular catalyst, Ru-bda, for water oxidation. Despite the impressive improvement in catalytic activity of this type of catalyst over the past years, a lack of understanding of the water coordination still remains. Herein, we report our EPR and DFT studies on Ru-bda (triethylammonium 3-pyridine sulfonate)(2) (1) at its Ru-III oxidation state, which is the initial state in the catalytic cycle for the O-O bond formation. Our investigation suggests that at this III-state, there is already a rearrangement in the ligand sphere where the coordination of a water molecule at the 7th position (open site) takes place under acidic conditions (pH = 1.0) to form a rare 7-coordinated Ru-III species.
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| 18. |
- Daniel, Quentin, et al.
(författare)
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Water Oxidation Initiated by In Situ Dimerization of the Molecular Ru(pdc) Catalyst
- 2018
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Ingår i: ACS Catalysis. - : AMER CHEMICAL SOC. - 2155-5435. ; 8:5, s. 4375-4382
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Tidskriftsartikel (refereegranskat)abstract
- The mononuclear ruthenium complex [Ru(pdc)L-3] (H(2)pdc = 2,6-pyridinedicarboxylic acid, L = N-heterocycles such as 4-picoline) has previously shown promising catalytic efficiency toward water oxidation, both in homogeneous solutions and anchored on electrode surfaces. However, the detailed water oxidation mechanism catalyzed by this type of complex has remained unclear. In order to deepen understanding of this type of catalyst, in the present study, [Ru(pdc)(py)(3)] (py = pyridine) has been synthesized, and the detailed catalytic mechanism has been studied by electrochemistry, UV-vis, NMR, MS, and X-ray crystallography. Interestingly, it was found that once having reached the Ru-IV state, this complex promptly formed a stable ruthenium dimer [Ru-III(pdc)(py)(2)-O-Ru-IV(pdc)(py)(2)](+). Further investigations suggested that the present dimer, after one pyridine ligand exchange with water to form [Ru-III(pdc)(py)(2)-O-Ru-IV(pdc)(py)(H2O)](+), was the true active species to catalyze water oxidation in homogeneous solutions.
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| 19. |
- de Gracia Triviño, Juan Angel, 1989-, et al.
(författare)
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Removing the Barrier in O-O Bond Formation Via the Combination of Intramolecular Radical Coupling and the Oxide Relay Mechanism
- 2024
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Ingår i: Journal of Physical Chemistry A. - : American Chemical Society (ACS). - 1089-5639 .- 1520-5215. ; 128:19, s. 3794-3800
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Tidskriftsartikel (refereegranskat)abstract
- The Ru(tda) catalyst has been a major milestone in the development of molecular water oxidation catalysts due to its outstanding performance at neutral pH. The role of the noncoordinating carboxylate group is to act as a nucleophile, donating an oxygen atom to the oxo group, thereby acting as an oxide relay (OR) mechanism for O-O bond formation. A substitution of the carboxylates for phosphonate groups has been proposed, resulting in the Ru(tPaO) catalyst, which has shown even more efficient performance in experimental characterization. In this study, we explore the feasibility of the OR mechanism in the newly reported Ru(tPaO) molecular catalyst. We investigated the catalytic cycle using density functional theory and identified a variation of the OR mechanism that involves radical oxygen atoms in O-O bond formation. We have also determined that the subsequent hydroxide nucleophilic attack is the sole rate-limiting step in the catalytic cycle. All activation free energies are very low, with a free-energy barrier of 2.1 kcal/mol for O-O bond formation and 4.2 kcal/mol for OH- nucleophilic attack.
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| 20. |
- De Gracia Triviño, Juan Angel, et al.
(författare)
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The Role of Counterions in Intermolecular Radical Coupling of Ru-bda Catalysts
- 2022
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Ingår i: Topics in catalysis. - : Springer Nature. - 1022-5528 .- 1572-9028. ; 65:1-4, s. 383-391
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Tidskriftsartikel (refereegranskat)abstract
- Intermolecular radical coupling (also interaction of two metal centers I2M) is one of the main mechanisms for O-O bond formation in water oxidation catalysts. For Ru(bda)L-2 (H(2)bda = 2,2'-bipyridine-6,6'-dicarboxylate, L = pyridine or similar nitrogen containing heterocyclic ligands) catalysts a significant driving force in water solution is the hydrophobic effects driven by the solvent. The same catalyst has been successfully employed to generate -N2 from ammonia, also via I2M, but here the solvent was acetonitrile where hydrophobic effects are absent. We used a classical force field for the key intermediate [(RuN)-N-VI(bda)(py)(2)](+) to simulate the dimerization free energy by calculation of the potential mean force, in both water and acetonitrile to understand the differences and similarities. In both solvents the complex dimerizes with similar free energy profiles. In water the complexes are essentially free cations with limited ion paring, while in acetonitrile the ion-pairing is much more significant. This ion-pairing leads to significant screening of the charges, making dimerization possible despite lower solvent polarity that could lead to repulsion between the charged complexes. In water the lower ion pairing is compensated by the hydrophobic effect leading to favorable dimerization despite repulsion of the charges. A hypothetical doubly charged [(RuIN)-I-VI(bda)py(2)](2+) was also studied for deeper understanding of the charge effect. Despite the double charge the complexes only dimerized favorably in the lower dielectric solvent acetonitrile, while in water the separated state is more stable. In the doubly charged catalyst the effect of ion-pairing is even more pronounced in acetonitrile where it is fully paired similar to the 1+ complex, while in water the separation of the ions leads to greater repulsion between the two catalysts, which prevents dimerization.
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| 21. |
- Fan, Ting, et al.
(författare)
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The Ru-tpc Water Oxidation Catalyst and Beyond : Water Nucleophilic Attack Pathway versus Radical Coupling Pathway.
- 2017
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Ingår i: ACS Catalysis. - : AMER CHEMICAL SOC. - 2155-5435. ; 7:4, s. 2956-2966
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Tidskriftsartikel (refereegranskat)abstract
- Many Ru water oxidation catalysts have been documented in the literature. However, only a few can catalyze the O-O bond formation via the radical coupling pathway, while most go through the water nucleophilic attack pathway. Understanding the electronic effect on the reaction pathway is of importance in design of active water oxidation catalysts. The Ru-bda (bda = 2,2'-bipyridine-6,6'-dicarboxylate) catalyst is one example that catalyzes the 0-0 bond formation via the radical coupling pathway. Herein, we manipulate the equatorial backbone ligand, change the doubly charged bda(2-) ligand to a singly charged tpc- (2,2':6',2 ''-terpyridine-6-carboxylate) ligand, and study the structure activity relationship. Surprisingly, kinetics measurements revealed that the resulting Ru-tpc catalyst catalyzes water oxidation via the water nucleophilic attack pathway, which is different from the Ru-bda catalyst. The O-O bond formation Gibbs free energy of activation (AGO) at T = 298.15 K was 20.2 +/- 1.7 kcal mol(-1). The electronic structures of a series of Ru-v=O species were studied by density function theory calculations, revealing that the spin density of O-Ru=O of Ru-v=O is largely dependent on the surrounding ligands. Seven coordination configuration significantly enhances the radical character of Ru-v=O.
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| 22. |
- Guo, Yaxiao, et al.
(författare)
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Molybdenum and boron synergistically boosting efficient electrochemical nitrogen fixation
- 2020
-
Ingår i: Nano Energy. - : Elsevier Ltd. - 2211-2855 .- 2211-3282. ; 78
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Tidskriftsartikel (refereegranskat)abstract
- Ammonia production consumes ~2% of the annual worldwide energy supply, therefore strategic alternatives for the energy-intensive ammonia synthesis through the Haber-Bosch process are of great importance to reduce our carbon footprint. Inspired by MoFe-nitrogenase and the energy-efficient and industrially feasible electrocatalytic synthesis of ammonia, we herein establish a catalytic electrode for artificial nitrogen fixation, featuring a carbon fiber cloth fully grafted by boron-doped molybdenum disulfide (B-MoS2/CFC) nanosheets. An excellent ammonia production rate of 44.09 μg h–1 cm–2 is obtained at −0.2 V versus the reversible hydrogen electrode (RHE), whilst maintaining one of the best reported Faradaic efficiency (FE) of 21.72% in acidic aqueous electrolyte (0.1 M HCl). Further applying a more negative potential of −0.25 V renders the best ammonia production rate of 50.51 μg h–1 cm–2. A strong-weak electron polarization (SWEP) pair from the different electron accepting and back-donating capacities of boron and molybdenum (2p shell for boron and 5d shell for molybdenum) is proposed to facilitate greatly the adsorption of non-polar dinitrogen gas via N≡N bond polarization and the first protonation with large driving force. In addition, for the first time a visible light driven photo-electrochemical (PEC) cell for overall production of ammonia, hydrogen and oxygen from water + nitrogen, is demonstrated by coupling a bismuth vanadate BiVO4 photo-anode with the B-MoS2/CFC catalytic cathode.
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| 23. |
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| 24. |
- Juan Angel, de Gracia Triviño, 1989-, et al.
(författare)
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Combining intramolecular radical coupling with the Oxide Relay mechanism: Radical Oxide Relay mechanism
- 2022
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Annan publikation (populärvet., debatt m.m.)abstract
- The Ru(tda) catalyst has been a major milestone in the development of molecular water oxidation catalysts due to its outstanding performance at neutral pH. The role of the non-coordinating carboxylate group is to act as a nucleophile, donating an oxygen atom to the oxo group, acting as an oxide relay mechanism the the O-O bond formation. A substitution of the carboxylates for phosphonate groups has been proposed, the Ru(tPaO), and its experimental characterization has shown an even more efficient performance. In this study, we explore the feasibility of the oxide relay mechanism in the newly reported Ru(tPaO) molecular catalyst. We have explored the catalytic cycle using density functional theory and we have identified a variation of the oxide relay mechanism that involves radical oxygen atoms in the O-O bond formation. We have explored the origin of the radical character in this complex and we have identified the hydroxyl nucleophilic attack as the sole rate limiting step in the catalytic cycle. The barriers are very low in all the steps, the O-O bond formation has a free energy barrier of 2.1 kcal/mol and the OH- nucleophilic attack 4.2 kcal/mol.
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- Juan Angel, de Gracia Triviño, 1989-
(författare)
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From Molecular Catalysts to Hybrid Electrodes: A Theoretical Guide
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The industrial revolution thrived our society to great technological advancement and a shift from an agrarian to an industrial society. Besides this fact, the side effect has been the development of a society highly dependent on energy, and the main sources of energy are based on non-renewable fossil fuels. This issue calls for the quest for new renewable energy sources that can address the energy dependency minimizing its side effects of it. In this quest, hydrogen is a promising source due to its high energy capacity and clean sub-products.The first chapter of this thesis will revise more in deep this environmental issues and what is needed to implement sustainable hydrogen production by water splitting. As well, as how the water source is extremely relevant, and solutions for using seawater are required to scale up hydrogen production. Also, an introduction to molecular catalysts for water oxidation based on Ru will be exposed, including a historical perspective and the state of the art at this day. The first chapter will finish with the strategies explored in this thesis to overcome the limitations of molecular catalysts in water splitting devices i.e, stability and current density.This work uses an ample set of computational tools to explore the reactivity and supramolecular properties of molecular catalysts. The second chapter will start with the treatment of molecules as electronic systems utilizing molecular quantum mechanics. Wave function formalism and density functional formalism of molecular quantum mechanics will be exposed and explained to the extent that is needed to ground the results of this thesis. The next section will introduce the treatment of molecules as atomic systems employing molecular mechanics and how we derive relevant supramolecular effects such as hydrophobicity, means of attachment to electrode surfaces, solvent, and electric field effects. Finally, this chapter will revise the Empirical Valence Bond approach to study the reactivity dependence on the molecular environment.The last chapter will go over the results of this thesis that correspond to the annexed papers at the end of this work. Starting from the characterization of the oxide relay mechanism in the highly efficient catalyst Ru(tda) where a novel function for the non-coordinating carboxylate ligand is proposed. To increase the stability of the Ru(tda) an attachment to carbon surfaces has been proposed and proved to increase significantly the stability. A study of the oxide relay mechanism at the surface revealed that the water-excluded environment of the active site in the reactive intermediate does not affect the key steps of this mechanism, in agreement with the experimental results reported. Next, the Ru(bda) has been shown to effectively catalyze the formation of molecular nitrogen from ammonia in an apolar solvent. The Ru(bda) has been well studied for water oxidation due to its high efficiency and the key step has been identified as the dimerization of two complexes driven by the aqueous solvent. The study of the dimerization process in acetonitrile has revealed the crucial role of solvent in supramolecular effects since acetonitrile promotes complex-counterion pairing aiding the dimerization of the Ru(bda) and. To increase the current density is needed a strategy to increase the catalyst density at the surface. Oligomerization of the Ru(tda) has shown to be an effective strategy to increase the current density of the hybrid electroanode to levels that are comparable to commercial electrolyzers. The exploration of the ways of attachment to the carbon surface revealed high dependency on the metal center oxidation state, the solvent, and the electric field. Also, the reactivity of the oligomer has been explored using the Empirical Valence Bond approach, revealing that the O-O bond formation remains unaltered in the oligomer and the reactivity remains unaltered in this complex environment, in agreement with experimental results. Finally, the substitution of the carboxylates in the Ru(tda) by phosphonates (Ru(tPaO)) has been proved to double the efficiency of the molecular catalyst at neutral pH. Due to the similarities between carboxylates and phosphonates the oxide relay mechanism has been tested in the Ru(tPaO), revealing that the origin of the extreme reactivity comes from low barriers in all the steps. The O-O bond formation involves an intramolecular radical coupling lowering the activation barrier to 2.1 kcal/mol. This radical coupling revealed a variation of the oxide relay mechanism called the radical oxide relay mechanism.
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