72 research outputs found

    Hydrogenation of Carbon Dioxide Catalyzed by PNP Pincer Iridium, Iron, and Cobalt Complexes: A Computational Design of Base Metal Catalysts

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    The reaction mechanisms for hydrogenation of carbon dioxide catalyzed by PNP-ligated (PNP = 2,6-bis(di-iso-propylphosphinomethyl)pyridine) metal pincer complexes, (PNP)IrH3 (1-Ir), trans-(PNP)Fe(H)2CO (1-Fe) and (PNP)CoH3 (1-Co), were studied computationally by using the density functional theory (DFT). 1-Ir is a recently reported high efficiency catalyst for the formation of formic acid from H2 and CO2. 1-Fe and 1-Co are computationally designed low-cost base metal complexes for catalytic CO2 reduction. For the formation of formic acid from H2 and CO2 catalyzed by 1-Ir, 1-Fe, and 1-Co, the reaction pathways with direct H2 cleavage by OH– without the participation of the PNP ligand are about 20 kcal mol–1 more favorable than a previously postulated H2 cleavage mechanism that involves the aromatization and dearomatization of the pyridine ring in the PNP ligand. This finding reveals the essential role of the base, OH–, in the catalytic CO2 reduction cycle and suggests that the incorporation of strong bases and unsaturated ligands may be critical for new catalyst design in the area of hydrogen activation and low energy proton transfers. The calculated overall enthalpy barriers for the formation of formic acid from H2 and CO2 catalyzed by 1-Ir, 1-Fe, and 1-Co are 18.6, 21.9, and 22.6 kcal mol–1, respectively. Such low barriers explain the observed unprecedented high catalytic acitivity of 1-Ir and indicate that 1-Fe and 1-Co can be considered as promising low-cost catalyst candidates for fast hydrogenation of CO2

    Unexpected Direct Reduction Mechanism for Hydrogenation of Ketones Catalyzed by Iron PNP Pincer Complexes

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    The hydrogenation of ketones catalyzed by 2,6-bis­(diisopropylphosphinomethyl)­pyridine (PNP)-ligated iron pincer complexes was studied using the range-separated and dispersion-corrected ωB97X-D functional in conjunction with the all-electron 6-31++G­(d,p) basis set. A validated structural model in which the experimental isopropyl groups were replaced with methyl groups was employed for the computational study. Using this simplified model, the calculated total free energy barrier of a previously postulated mechanism with the insertion of ketone into the Fe–H bond is far too high to account for the observed catalytic reaction. Calculation results reveal that the solvent alcohol is not only a stabilizer of the dearomatized intermediate but also more importantly an assistant catalyst for the formation of trans-(PNP)­Fe­(H)2(CO), the actual catalyst for hydrogenation of ketones. A direct reduction mechanism, which features the solvent-assisted formation of a trans dihydride complex trans-(PNP)­Fe­(H)2(CO), direct transfer of hydride to acetophenone from trans-(PNP)­Fe­(H)2(CO) for the formation of a hydrido alkoxo complex, and direct H2 cleavage by hydrido alkoxo without the participation of the pincer ligand for the regeneration of trans-(PNP)­Fe­(H)2(CO), was predicted

    Mechanistic Insights into Ruthenium-Catalyzed Production of H<sub>2</sub> and CO<sub>2</sub> from Methanol and Water: A DFT Study

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    A density functional theory study of the reaction mechanism of the production of H2 and CO2 from methanol and water catalyzed by an aliphatic PNP pincer ruthenium complex, (PNP)­Ru­(H)­CO, reveals three interrelated catalytic cycles for the release of three H2 molecules: the dehydrogenation of methanol to formaldehyde, the coupling of formaldehyde and hydroxide for the formation of formic acid, and the dehydrogenation of formic acid. The formation of all three H2 molecules undergoes the same self-promoted mechanism that features a methanol or a water molecule acting as a bridge for the transfer of a ligand proton to the metal hydride in a key intermediate, trans-(HPNP)­Ru­(H)2CO

    A Self-Promotion Mechanism for Efficient Dehydrogenation of Ethanol Catalyzed by Pincer Ruthenium and Iron Complexes: Aliphatic versus Aromatic Ligands

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    A density functional theory study reveals that the dehydrogenation of ethanol catalyzed by an aliphatic PNP pincer ruthenium complex, (PNP)­Ru­(H)­CO {<b>1</b><sub><b>Ru</b></sub>, PNP = bis­[2-(diisopropylphosphino)­ethyl]­amino}, proceeds via a self-promoted mechanism that features an ethanol molecule acting as a bridge to assist the transfer of a proton from ligand nitrogen to the metal center for the formation of H<sub>2</sub>. The very different catalytic properties between the aromatic and aliphatic pincer ligand in ruthenium complexes are analyzed. The potential of an iron analogue of <b>1</b><sub><b>Ru</b></sub>, (PNP)­Fe­(H)­CO (<b>1</b><sub><b>Fe</b></sub>), as a catalyst for the dehydrogenation of ethanol was evaluated computationally. The calculated total free energy barrier of ethanol dehydrogenation catalyzed by <b>1</b><sub><b>Fe</b></sub> is only 22.1 kcal/mol, which is even 0.7 kcal/mol lower than the calculated total free energy barrier of the reaction catalyzed by <b>1</b><sub><b>Ru</b></sub>. Therefore, the potential of <b>1</b><sub><b>Fe</b></sub> as a low-cost and high-efficiency catalyst for the production of hydrogen from ethanol is promising

    Metal Hydride and Ligand Proton Transfer Mechanism for the Hydrogenation of Dimethyl Carbonate to Methanol Catalyzed by a Pincer Ruthenium Complex

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    The hydrogenation of dimethyl carbonate to methanol catalyzed by a PNN-ligated ruthenium complex (PNN)­Ru­(CO)­(H) was studied computationally using the density functional theory at the range-separated and dispersion-corrected ωB97X-D functional level in conjunction with an all-electron 6-31++G­(d,p) basis set (Stuttgart ECP28MWB basis set for Ru). A direct metal hydride and ligand proton transfer mechanism with three cascade catalytic cycles for the hydrogenation of dimethyl carbonate, methyl formate, and formaldehyde to methanol is proposed. The resting state in the catalytic reaction is the trans dihydride complex <i>trans-</i>(PNN)­Ru­(H)<sub>2</sub>(CO). Calculation results indicate that the rate-determining step in the whole reaction is the formation of the second methanol molecule through simultaneous breaking of a C–OCH<sub>3</sub> bond and transferring a ligand methylene proton to the dissociated CH<sub>3</sub>O<sup>–</sup> in the catalytic cycle for hydrogenation of methyl formate. The essential role of the noninnocent PNN pincer ligand is to split H<sub>2</sub> and assist methanol formation through the aromatization and dearomatization of the pyridine ring in the ligand. A new iron pincer complex, <i>trans-</i>(PNN)­Fe­(H)<sub>2</sub>(CO), is proposed and evaluated as a promising low-cost and high efficiency catalyst for this reaction

    Bioinspired Design and Computational Prediction of Iron Complexes with Pendant Amines for the Production of Methanol from CO<sub>2</sub> and H<sub>2</sub>

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    Inspired by the active site structure of [FeFe]-hydrogenase, we built a series of iron dicarbonyl diphosphine complexes with pendant amines and predicted their potentials to catalyze the hydrogenation of CO2 to methanol using density functional theory. Among the proposed iron complexes, [(PtBu2NtBu2H)­FeH­(CO)2(COOH)]+ (5COOH) is the most active one with a total free energy barrier of 23.7 kcal/mol. Such a low barrier indicates that 5COOH is a very promising low-cost catalyst for high-efficiency conversion of CO2 and H2 to methanol under mild conditions. For comparison, we also examined Bullock’s Cp iron diphosphine complex with pendant amines, [(PtBu2NtBu2H)­FeHCpC5F4N]+ (5Cp‑C5F4N), as a catalyst for hydrogenation of CO2 to methanol and obtained a total free energy barrier of 27.6 kcal/mol, which indicates that 5Cp‑C5F4N could also catalyze the conversion of CO2 and H2 to methanol but has a much lower efficiency than our newly designed iron complexes

    Computational Study of the Substituent Effects for the Spectroscopic Properties of Thiazolo[5,4‑<i>d</i>]thiazole Derivatives

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    Inspired by the structure and optical properties of N,N′-dialkylated/dibenzylated 2,5-bis­(4-pyridinium)­thiazolo­[5,4-d]­thiazole, we proposed a series of disubstituted thiazolo­[5,4-d]­thiazole derivatives as promising materials for multifunctional optoelectronic, electron transfer sensing, and other photochemical applications. Density functional theory study of the electronic structures and transition properties of those newly proposed molecules indicates that the electron-donating and electron-withdrawing groups introduced to the peripheral pyridyl ligands extend the distributions of molecular frontier orbitals, increase the electron density in thiazolo­[5,4-d]­thiazolea, and therefore lead to remarkable red-shifts of their absorption and emission peaks

    Mechanistic Insights into Iridium Catalyzed Disproportionation of Formic Acid to CO<sub>2</sub> and Methanol: A DFT Study

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    The disproportionation of formic acid to methanol catalyzed by a half-sandwich iridium complex, [Cp*Ir­(bpy-Me)­OH2]2+, was computationally investigated by using density functional theory. A newly proposed mechanism features three interrelated catalytic cycles, the dehydrogenation of formic acid to CO2 and H2, the hydrogenation of formic acid to formaldehyde with the formation of water, and the hydrogenation of formaldehyde to methanol. Methanol assisted proton transfer and direct C–O bond cleavage after hydroxyl deprotonation in two competitive pathways for the formation of formaldehyde are the rate-determining steps in the whole catalytic reaction. Calculation results indicate that the formation of formaldehyde from methanediol through direct cleavage of a C–O bond after hydroxyl deprotonation has a free energy barrier of 25.9 kcal/mol, which is 1.9 kcal/mol more favorable than methanol assisted proton transfer

    Bioinspired Design and Computational Prediction of Iron Complexes with Pendant Amines for the Production of Methanol from CO<sub>2</sub> and H<sub>2</sub>

    No full text
    Inspired by the active site structure of [FeFe]-hydrogenase, we built a series of iron dicarbonyl diphosphine complexes with pendant amines and predicted their potentials to catalyze the hydrogenation of CO2 to methanol using density functional theory. Among the proposed iron complexes, [(PtBu2NtBu2H)­FeH­(CO)2(COOH)]+ (5COOH) is the most active one with a total free energy barrier of 23.7 kcal/mol. Such a low barrier indicates that 5COOH is a very promising low-cost catalyst for high-efficiency conversion of CO2 and H2 to methanol under mild conditions. For comparison, we also examined Bullock’s Cp iron diphosphine complex with pendant amines, [(PtBu2NtBu2H)­FeHCpC5F4N]+ (5Cp‑C5F4N), as a catalyst for hydrogenation of CO2 to methanol and obtained a total free energy barrier of 27.6 kcal/mol, which indicates that 5Cp‑C5F4N could also catalyze the conversion of CO2 and H2 to methanol but has a much lower efficiency than our newly designed iron complexes
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