72 research outputs found
Hydrogenation of Carbon Dioxide Catalyzed by PNP Pincer Iridium, Iron, and Cobalt Complexes: A Computational Design of Base Metal Catalysts
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
The hydrogenation of ketones catalyzed by 2,6-bisÂ(diisopropylphosphinomethyl)Âpyridine
(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++GÂ(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)ÂFeÂ(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)ÂFeÂ(H)2(CO), direct transfer of hydride to acetophenone
from trans-(PNP)ÂFeÂ(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)ÂFeÂ(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
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)ÂRuÂ(H)ÂCO,
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)ÂRuÂ(H)2CO
A Self-Promotion Mechanism for Efficient Dehydrogenation of Ethanol Catalyzed by Pincer Ruthenium and Iron Complexes: Aliphatic versus Aromatic Ligands
A density functional theory study
reveals that the dehydrogenation
of ethanol catalyzed by an aliphatic PNP pincer ruthenium complex,
(PNP)ÂRuÂ(H)ÂCO {<b>1</b><sub><b>Ru</b></sub>, PNP = bisÂ[2-(diisopropylphosphino)Âethyl]Âamino},
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)ÂFeÂ(H)ÂCO
(<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
The hydrogenation of dimethyl carbonate to methanol catalyzed
by
a PNN-ligated ruthenium complex (PNN)ÂRuÂ(CO)Â(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++GÂ(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)ÂRuÂ(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)ÂFeÂ(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>
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)ÂFeHÂ(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)ÂFeHCpC5F4N]+ (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
Inspired
by the structure and optical properties of N,N′-dialkylated/dibenzylated 2,5-bisÂ(4-pyridinium)ÂthiazoloÂ[5,4-d]Âthiazole, we proposed a series of disubstituted thiazoloÂ[5,4-d]Âthiazole 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 thiazoloÂ[5,4-d]Âthiazolea, 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
The
disproportionation of formic acid to methanol catalyzed by
a half-sandwich iridium complex, [Cp*IrÂ(bpy-Me)ÂOH2]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>
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)ÂFeHÂ(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)ÂFeHCpC5F4N]+ (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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