Mechanistic Studies on the Reversible Hydrogenation of Carbon Dioxide Catalyzed by an Ir-PNP Complex

Structural inhomogeneity as a factor promoting the homogenous catalysis of CO2 hydrogenation by (PMe3)4RuH2

During homogenous catalysis by organometallic complexes, the dissociation of a ligand to produce an unsaturated site on the metal center is often invoked as the first step of activation, especially when photo-excitation is involved. In this theoretical study, we demonstrated that under mild conditions, a thermodynamically unstable yet dynamically favorable active intermediate could be produced by the inhomogeneity of the solvent distribution around the catalyst rather than by ligand dissociation. This occurred at the end of the first catalytic cycle when the product was eliminated. The empty site was immediately filled by one of the additive molecules aggregated around the reaction center even when the intermediate complex was unstable, producing a transient and more active catalyst. This process accounted for the accelerated reaction rate observed in the landmark CO₂ hydrogenation catalyzed by (PMe₃)₄RuH₂ in supercritical CO₂ when H₂O, MeOH, or HNMe₂ was added. This also suggests a new way to exploit the structural inhomogeneity around an organometallic complex for the design of superior catalysts.

Efficient CO2 Hydrogenation to Formate with Immobilized Ir-Catalysts Based on Mesoporous Silica Beads

The Nozaki Ir-based CO₂ hydrogenation catalyst was successfully immobilized on post-functionalized silica beads (d=200 μm) through click chemistry. This material hydrogenates CO₂ into formic acid with turnover numbers reaching 2.8×10⁴ in a batch reactor within 24 hours, paving the way towards the design of efficient heterogeneous catalysts for this transformation.

Transient and intermediate carbocations in ruthenium tetroxide oxidation of saturated rings

The ruthenium tetroxide-mediated oxidation of cyclopentane, tetrahydrofuran, tetrahydrothiophene and N-substituted pyrrolidines has been studied computationally by DFT and topological (analysis of the electron localization function, ELF) methods. In agreement with experimental observations and previous DFT calculations, the rate-limiting step of the reaction takes place through a highly asynchronous (3 + 2) concerted cycloaddition through a single transition structure (one kinetic step). The ELF analysis identifies the reaction as a typical one-step-two-stages process and corroborates the existence of a transient carbocation. In the case of pyrrolidines, the carbocation is completely stabilized as an energy minimum in the form of an iminium ion and the reaction takes place in two steps.

Cobalt-Catalyzed Aqueous Dehydrogenation of Formic Acid

Among the known liquid organic hydrogen carriers, formic acid attracts increasing interest in the context of safe and reversible storage of hydrogen. Here, the first molecularly defined cobalt pincer complex is disclosed for the dehydrogenation of formic acid in aqueous medium under mild conditions. Crucial for catalytic activity is the use of the specific complex 3. Compared to related ruthenium and manganese complexes 7 and 8, this optimal cobalt complex showed improved performance. DFT computations support an innocent non-classical bifunctional outer-sphere mechanism on the triplet state potential energy surface.

Synthesis and reactivity at the Ir-MeTpm platform: from κ1-N coordination to κ3-N-based organometallic chemistry

Reaction of [Ir(μ-Cl)(COE)2]2 (COE = cis-cyclooctene) with tris(3,5-dimethylpyrazol-1-yl)methane (MeTpm) affords [IrCl(κ1-N-MeTpm)(COD)] (1) (COD = 1,5-cyclooctadiene). The formation of 1 implies the transfer dehydrogenation of a COE ligand to give COD and COA (cyclooctane). A mechanistic proposal based on DFT calculations that explains this iridium promoted process has been disclosed. Additionally, reactivity studies have allowed the preparation and characterization, including determination of the molecular structures of a number of iridium complexes with the MeTpm ligand in κ1, κ2 or κ3-N coordination modes. Moreover, the first example of an Ir-cyclooctyl complex featuring hydride and carbonyl ligands, whose solid state structure has been determined by X-ray diffraction methods, is reported.

Theoretical analysis of glyoxal condensation with ammonia in aqueous solution

The reactions of glyoxal with ammonia, ammonium salts, and amines cause the formation of the secondary organic aerosol (SOA) components (imidazole and its derivatives) in the atmosphere. The interaction of glyoxal and ammonia in aqueous solution is a primary reaction for these processes, and the explanation of its mechanism will allow developing the methods to control the formation of the SOA components. A detailed mechanism for the formation of key intermediates, namely, ethanediimine, diaminoethanediol, and aminoethanetriol, required for the imidazole ring cyclization, is proposed, and its potential energy surface (PES) has been constructed. This mechanism includes the experimentally identified intermediate compounds and takes into account the conformational and hydration equilibria of glyoxal. The schemes are proposed for further conversion of the key intermediates to the products of condensation between glyoxal and ammonia in the aqueous solution, C-N cyclic oligomers, that were identified. The products are shown to correspond to low positions on the PES in terms of Gibbs free energy, from -30.8 to -68.3 kcal mol-1, which confirms the high probability of their formation. The preferable thermodynamic pathway for formation of the imidazole products does not comprise the conversion of the diimine intermediate with the participation of the proton, but rather the interaction of either the diaminoalcohol with glyoxal monohydrate or two monoamine derivatives between themeselves (aminoethantriol and aminohydroxyacetaldehyde).

Reactivity patterns for the activation of CO2 and CS2 with alumoxane and aluminum hydrides

Carbon dioxide is readily fixed when reacting with either alumoxane dihydride [{MeLAl(H)}2(μ-O)] (1) or aluminum dihydride [MeLAlH2] (2) (MeL = HC[(CMe)N(2,4,6-Me3C6H2)]2-) to produce bimetallic aluminum formates [(MeLAl)2(μ-OCHO)2(μ-O)] (3) and [(MeLAl)2(μ-OCHO)2(μ-H)2] (5), respectively. Furthermore, [(MeLAl)2(μ-OCHO)2(μ-OH)2] (4) is easily obtained upon the reaction of 3 or 5 with H2O. The stability of the unusual dialuminum diformate dihydride core observed in 5 stems from the proximity of the Al centers allowing the formation of two Al-HAl bridges and precluding further hydride transfer to the HCO2 moieties. Contrary to this behavior, 1 and 2 react with CS2 giving cyclic alumoxane and aluminum sulfides [(MeLAl)2(μ-S)(μ-O)] (6) and [{MeLAl(μ-S)}2] (7), respectively. The molecular structures of 3-7 were characterized by IR, Raman, solution or solid-state (MAS) NMR spectroscopy and mass spectrometry and for 4-7 were characterized by X-ray diffraction studies. NMR kinetic studies and DFT calculations suggest that the mechanisms for the formation of 6 and 7 involve the transfer of a hydride group forming transient aluminum thioformate intermediates which proceed to form Al-S-Al moieties through the cleavage of C-S bonds and insertion of a sulfur atom, followed by the elimination of thioformaldehyde.

Rational Design of Efficient Amine Reductant Initiators for Amine-Peroxide Redox Polymerization

Amine-peroxide redox polymerization (APRP) has been highly prevalent in industrial and medical applications since the 1950s, yet the initiation mechanism of this radical polymerization process is poorly understood so that innovations in the field are largely empirically driven and incremental. Through a combination of computational prediction and experimental analysis, we elucidate the mechanism of this important redox reaction between amines and benzoyl peroxide for the ambient production of initiating radicals. Our calculations show that APRP proceeds through S_N2 attack by the amine on the peroxide but that homolysis of the resulting intermediate is the rate-determining step. We demonstrate a correlation between the computationally predicted initiating rate and the experimentally measured polymerization rate with an R² = 0.80. The new mechanistic understanding was then applied to computationally predict amine reductant initiators with faster initiating kinetics. This led to our discovery of N-(4-methoxyphenyl)pyrrolidine (MPP) as amine reductant, which we confirmed significantly outperforms current state-of-the-art tertiary aromatic amines by ∼20-fold, making it the most efficient amine-peroxide redox initiator to date. The application of amines with superior kinetics such as MPP in APRP could greatly accelerate existing industrial processes, facilitate new industrial manufacturing methods, and improve biocompatibility in biomedical applications conducted with reduced initiator concentrations yet higher overall efficiency.

Solvation Entropy Made Simple

The entropies of molecules in solution are often calculated using gas phase formulas. It is assumed that, because implicit solvation models are fitted to reproduce free energies, this is sufficient for modeling reactions in solution. However, this procedure exaggerates entropic effects in processes that change molecularity. Here, computationally efficient (i.e., having similar cost as gas phase entropy calculations) approximations for determining solvation entropy are proposed to address this issue. The S_ω, S_ϵ, and S _ϵα models are nonempirical and rely only on physical arguments and elementary properties of the medium (e.g., density and relative permittivity). For all three methods, average errors as compared to experiment are within chemical accuracy for 110 solvation entropies, 11 activation entropies in solution, and 32 vaporization enthalpies. The models also make predictions regarding microscopic and bulk properties of liquids which prove to be accurate. These results imply that Δ H_sol and Δ S_sol can be described separately and with less reliance on parametrization by a combination of the methods presented here with existing, reparametrized, implicit solvation models.

A computational study on ligand assisted vs. ligand participation mechanisms for CO2 hydrogenation: importance of bifunctional ligand based catalysts

Bifunctional aminomethyl based Mn(i) catalysts favour a revised Noyori type mechanism for the CO₂ hydrogenation reaction.

Entropic corrections for the evaluation of the catalytic activity in the Al(iii) catalysed formation of cyclic carbonates from CO2 and epoxides

Entropic corrections are found to be crucial for evaluating the catalytic performance in solution for the reaction of CO₂ with epoxides.

Light-driven catalytic hydrogenation of carbon dioxide at low-pressure by a trinuclear iridium polyhydride complex

Under irradiation conditions, low-pressure and room-temperature hydrogenation of carbon dioxide (CO₂) has been achieved using a trinuclear iridium hexahydride complex 1.

CO2 hydrogenation by phosphorus–nitrogen PN3P-pincer iridium hydride complexes: elucidation of the deactivation pathway

PN³P–Ir pincer hydride complexes were synthesized and characterized as catalysts and key intermediates in the direct hydrogenation of CO₂ to formate under mild conditions.

Ir-catalyzed selective reduction of CO2 to the methoxy or formate level with HSiMe(OSiMe3)2

Ir-NSi-based catalysts allow controlling the selective reduction of CO₂ with HSiMe(OSiMe₃)₂ to afford methoxysilane or silyl formate.

Structural analysis of transient reaction intermediate in formic acid dehydrogenation catalysis using two-dimensional IR spectroscopy

The molecular structure of a catalytically active key intermediate is determined in solution by employing 2D IR spectroscopy measuring vibrational cross-angles. The formate intermediate (2) in the formic acid dehydrogenation reaction catalyzed by a phosphorus-nitrogen PN3P-Ru catalyst is elucidated. Our spectroscopic studies show that the complex features a formate ion directly attached to the Ru center as a ligand, and a proton added to the imine arm of the dearomatized PN3P* ligand. During the catalytic process, the imine arms are not only reversibly protonated and deprotonated, but also interacting with the protic substrate molecules, effectively serving as the local proton buffer to offer remarkable stability with a turnover number (TON) over one million.

Amine Induced Retardation of the Radical-Mediated Thiol-Ene Reaction via the Formation of Metastable Disulfide Radical Anions

The effect of amines on the kinetics and efficacy of radical-mediated thiol-ene coupling (TEC) reactions was investigated. By varying the thiol reactant and amine additive, it was shown that amines retard thiyl radical-mediated reactions when the amine is adequately basic enough to deprotonate the thiol affording the thiolate anion, e.g., when the weakly basic amine tetramethylethylenediamine was incorporated in the TEC reaction between butyl 2-mercaptoacetate and an allyl ether at 5 mol %, the final conversion was reduced from quantitative to <40%. Alternatively, no effect is observed when the less acidic thiol butyl 3-mercaptopropionate is employed. The thiolate anion was established as the retarding species through the introduction of ammonium and thiolate salt additives into TEC formulations. The formation of a two-sulfur three-electron bonded disulfide radical anion (DRA) species by the reaction of a thiyl radical with a thiolate anion was determined as the cause for the reduction in catalytic radicals and the TEC rate. Thermodynamic and kinetic trends in DRA formations were computed using density functional theory and by modeling the reaction as an associative electron transfer process. These trends correlate well with the experimental retardation trends of various thiolate anions in TEC reactions.

Predicting Hydride Donor Strength via Quantum Chemical Calculations of Hydride Transfer Activation Free Energy

We propose a method to approximate the kinetic properties of hydride donor species by relating the nucleophilicity (N) of a hydride to the activation free energy ΔG^⧧ of its corresponding hydride transfer reaction. N is a kinetic parameter related to the hydride transfer rate constant that quantifies a nucleophilic hydridic species' tendency to donate. Our method estimates N using quantum chemical calculations to compute ΔG^⧧ for hydride transfers from hydride donors to CO₂ in solution. A linear correlation for each class of hydrides is then established between experimentally determined N values and the computationally predicted ΔG^⧧; this relationship can then be used to predict nucleophilicity for different hydride donors within each class. This approach is employed to determine N for four different classes of hydride donors: two organic (carbon-based and benzimidazole-based) and two inorganic (boron and silicon) hydride classes. We argue that silicon and boron hydrides are driven by the formation of the more stable Si-O or B-O bond. In contrast, the carbon-based hydrides considered herein are driven by the stability acquired upon rearomatization, a feature making these species of particular interest, because they both exhibit catalytic behavior and can be recycled.

Mechanistic insights into HCO2H dehydrogenation and CO2 hydrogenation catalyzed by Ir(Cp*) containing tetrahydroxy bipyrimidine ligand: the role of sodium and proton shuttle

Catalytic HCO₂H dehydrogenation by Ir(Cp*) tetrahydroxy bipyrimidine is influenced not only by the protonation states but also by the involvement of Na⁺ and the availability of HCO₂H as a proton shuttle.

Computational study of the mechanism of amide bond formation via CS2-releasing 1,3-acyl transfer

A systematic computational study on CS₂-releasing 1,3-acyl transfer was performed for the first time and provided deeper mechanistic insights.

Flexible proton-responsive ligand-based Mn(i) complexes for CO2 hydrogenation: a DFT study

Significance of a flexible proton responsive ligand to the dihydrogen (H⋯H) bond for CO₂ hydrogenation.

DFT study of CO2 hydrogenation catalyzed by a cobalt-based system: an unexpected formate anion-assisted deprotonation mechanism

An unexpected formate anion-assisted deprotonation mechanism is unfolded by a DFT study of CO₂ hydrogenation catalyzed by a cobalt-based system.