A Computational Investigation of the Insertion of Carbon Dioxide into Four- and Five-Coordinate Iridium Hydrides

First-Principles Insights into the Mechanism of CO2 Hydrogenation Reactions by Fe-PNP Pincer Complex

Using the state of the art theoretical methods, we have provided a comprehensive mechanistic understanding of the CO2 hydrogenation into HCOOH, H2CO, and CH3OH by 2,6-bis(diisopropylphosphinomethyl)pyridine (PNP)-ligated Fe pincer complex, featuring one CO and two H as co-ligands. For the computational investigation, a verified structural model containing methyl groups in place of the experimental isopropyl groups was used. Three catalytic conversions involving hydrogenation of CO2 into formic acid (HCOOH), HCOOH into formaldehyde and methanol were studied in different solvent medium. Our modelled complex appears to be a viable base-free catalyst for the conversion of CO2 into HCOOH and HCOOH into H2CO, based on the free energy profiles, which show apparent activation energy barriers of 16.28 kcal/mol and 23.63 kcal/mol for the CO2 to HCOOH and HCOOH to H2CO conversion, respectively. However, the computed results show that, due to the huge energy span of H2CO to CH3OH conversion, complete hydrogenation of CO2 into methanol could not occur under moderate conditions. Morpholine co-catalyst, which can lower the hydrogenation barrier by taking part in a simultaneous H-atom donation-acceptance process, could have assisted in completing this step.

Monomeric gold hydrides for carbon dioxide reduction: ligand effect on the reactivity

We analyzed the ligand electronic effect in the reaction between a [LAu(I)H]0/- hydride species and CO2, leading to a coordinated formate [LAu(HCOO)]0/-. We explored 20 different ligands, such as carbenes, phosphines and others, carefully selected to cover a wide range of electron-donor and -acceptor properties. We included in the study the only ligand, an NHC-coordinated diphosphene, that, thus far, experimentally demonstrated facile and reversible reaction between the monomeric gold(I) hydride and carbon dioxide. We elucidated the previously unknown reaction mechanism, which resulted to be concerted and common to all the ligands: the gold-hydrogen bond attacks the carbon atom of CO2 with one oxygen atom coordinating to the gold center. A correlation between the ligand σ donor ability, which affects the electron density at the reactive site, and the kinetic activation barriers of the reaction has been found. This systematic study offers useful guidelines for the rational design of new ligands for this reaction, while suggesting a few promising and experimentally accessible potential candidates for the stoichiometric or catalytic CO2 activation.

Ruthenium Complexes of a Triphosphorus-Coordinating Pincer Ligand: Ru-P Ligand-Substituent Exchange Reactions Driven by Large Variations of Bond Energies

The reaction of [(p-cymene)RuCl₂]₂ with the triphosphine ligand bis(2-di-tert-butylphosphinophenyl)phosphine (^tBuP^HPP) results in an unusual exchange reaction in which a chloride ligand and a phosphorus-bound H atom are exchanged ("H-P/Ru-Cl exchange") to give the (chlorophosphine)ruthenium hydride complex (^tBuP^ClPP)RuHCl [1^Cl-HCl; ^tBuP^ClPP = bis(2-di-tert-butylphosphinophenyl)chlorophosphine]. Density functional theory calculations indicate that the presumed initial product of metalation, (^tBuP^HPP)RuCl₂ (1^H-Cl2), undergoes an H-P/Ru-Cl exchange via sequential P-to-Ru α-H migration to give the intermediate (^tBuPPP)RuHCl₂, followed by Ru-to-P α-Cl migration to give the observed product 1^Cl-HCl (crystallographically characterized). Dehydrochlorination of 1^Cl-HCl under a H₂ atmosphere gives (^tBuP^ClPP)RuH₄ (1^Cl-H4), which then can undergo a second dehydrochlorination and addition of H₂ to give (^tBuP^HPP)RuH₄ (1^H-H4). This reaction may proceed via the reverse of the intramolecular exchange by 1^H-Cl2, i.e., loss of H₂ from 1^Cl-H4 to give 1^Cl-H2, which could undergo Cl-P/Ru-H exchange to give (^tBuP^HPP)RuHCl (1^H-HCl). Accordingly, the thermodynamics of Cl-P/Ru-H exchange are found to be highly dependent on the nature of the ancillary anionic ligand (H or Cl), which is not directly involved in the exchange. The origin of this thermodynamic dependence can be explained in terms of the high stability of complexes (^RP^XPP)RuHCl (X = H, Cl; R = Me, ^tBu), in which the hydride is approximately trans to a vacant coordination site and the central phosphine group is approximately trans to the weak-trans-influence chloride ligand. This conclusion has general implications for five-coordinate d⁶ complexes, both pincer- and nonpincer-ligated.

Visible-Light Photocatalytic Conversion of Carbon Dioxide by Ni(II) Complexes with N4S2 Coordination: Highly Efficient and Selective Production of Formate

The efficient and selective light-driven conversion of carbon dioxide to formate is a scientific challenge for green chemistry and energy science, especially utilizing visible-light energy and earth-abundant catalytic materials. In this report, two mononuclear Ni(II) complexes of pyridylbenzimidazole (pbi) and pyridylbenzothiazole (pbt), such as Ni(pbt)(pyS)₂ (1) and Ni(pbi)(pyS)₂ (2) (pyS = pyridine-2-thiolate), were prepared and their reactivities studied. The two Ni complexes were examined for CO₂ conversion using eosin Y as a photosensitizer upon visible-light irradiation in a H₂O/ethanol solvent. The photoreaction of CO₂ catalyzed by complexes 1 and 2 selectively affords formate with a high efficiency (14 000 turnover number) and a high catalytic selectivity of ∼99%. Undesirable proton reduction pathways were completely suppressed in the photocatalytic reactions with these sulfur-rich Ni catalysts under CO₂. Hydrogen photoproduction was also studied under argon. Their kinetic isotope effects and influence of solution pH for formate and H₂ production in the photocatalytic reactions are described in relation to the reaction mechanisms. These bioinspired Ni(II) catalysts with N/S ligation in relation to [NiFe]-hydrogenases are the first examples of early transition metal complexes affording such high selectivity and efficiencies, providing a future path to design solar-to-fuel processes for artificial photosynthesis.

Kinetic and mechanistic analysis of a synthetic reversible CO2/HCO2- electrocatalyst

[Pt(depe)2](PF6)2 electrocatalyzes the reversible conversion between CO2 and HCO2- with high selectivity and low overpotential but low rates. A comprehensive kinetic analysis indicates the rate determining step for CO2 reduction is the reactivity of a Pt hydride intermediate to produce HCO2-. To accelerate catalysis, the use of cationic and hydrogen-bond donor additives are explored.

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.

Acceleration of CO2 insertion into metal hydrides: ligand, Lewis acid, and solvent effects on reaction kinetics

The insertion of CO2 into metal hydrides and the microscopic reverse decarboxylation of metal formates are important elementary steps in catalytic cycles for both CO2 hydrogenation to formic acid and methanol as well as formic acid and methanol dehydrogenation. Here, we use rapid mixing stopped-flow techniques to study the kinetics and mechanism of CO2 insertion into transition metal hydrides. The investigation finds that the most effective method to accelerate the rate of CO2 insertion into a metal hydride can be dependent on the nature of the rate-determining transition state (TS). We demonstrate that for an innersphere CO2 insertion reaction, which is proposed to have a direct interaction between CO2 and the metal in the rate-determining TS, the rate of insertion increases as the ancillary ligand becomes more electron rich or less sterically bulky. There is, however, no rate enhancement from Lewis acids (LA). In comparison, we establish that for an outersphere CO2 insertion, proposed to proceed with no interaction between CO2 and the metal in the rate-determining TS, there is a dramatic LA effect. Furthermore, for both inner- and outersphere reactions, we show that there is a small solvent effect on the rate of CO2 insertion. Solvents that have higher acceptor numbers generally lead to faster CO2 insertion. Our results provide an experimental method to determine the pathway for CO2 insertion and offer guidance for rate enhancement in CO2 reduction catalysis.

Homogeneously Catalyzed Electroreduction of Carbon Dioxide-Methods, Mechanisms, and Catalysts

The utilization of CO₂ via electrochemical reduction constitutes a promising approach toward production of value-added chemicals or fuels using intermittent renewable energy sources. For this purpose, molecular electrocatalysts are frequently studied and the recent progress both in tuning of the catalytic properties and in mechanistic understanding is truly remarkable. While in earlier years research efforts were focused on complexes with rare metal centers such as Re, Ru, and Pd, the focus has recently shifted toward earth-abundant transition metals such as Mn, Fe, Co, and Ni. By application of appropriate ligands, these metals have been rendered more than competitive for CO₂ reduction compared to the heavier homologues. In addition, the important roles of the second and outer coordination spheres in the catalytic processes have become apparent, and metal-ligand cooperativity has recently become a well-established tool for further tuning of the catalytic behavior. Surprising advances have also been made with very simple organocatalysts, although the mechanisms behind their reactivity are not yet entirely understood. Herein, the developments of the last three decades in electrocatalytic CO₂ reduction with homogeneous catalysts are reviewed. A discussion of the underlying mechanistic principles is included along with a treatment of the experimental and computational techniques for mechanistic studies and catalyst benchmarking. Important catalyst families are discussed in detail with regard to mechanistic aspects, and recent advances in the field are highlighted.

Origin of ligand effects on reactivities of pincer-Pd catalyzed hydrocarboxylation of allenes and alkenes with formate salts: a computational study

Distortion induced by R substituents on PGeP-pincer ligands is the key factor affecting the reactivity of CO₂ insertion into allylpalladium and benzylpalladium intermediates.

Formylation or methylation: what determines the chemoselectivity of the reaction of amine, CO2, and hydrosilane catalyzed by 1,3,2-diazaphospholene?

DFT computations have been performed to gain insight into the mechanisms of formylation/methylation of amines (e.g. methylaniline (1a)/2,2,4,4-tetramethylpiperidine (2a)) with CO2 and hydrosilane ([Si]H2, [Si] = Ph2Si), catalyzed by 1,3,2-diazaphospholene ([NHP]H). Different from the generally proposed sequential mechanism for the methylation of amine with CO2, i.e. methylation proceeds via formylation, followed by further reduction of formamide to give an N-methylated amine, the study characterized a competition mechanism between formylation and methylation. The chemoselectivity originates from the competition between the amine and [NHP]H hydride to attack the formyloxy carbon of [Si](OCHO)2 (the insertion product of CO2 into [Si]H2). When the attack of an amine (e.g.1a) wins, the transformation affords formamide (1b) but would otherwise (e.g.2a) result in an N-methylated amine (2c). The reduction of formamide by [Si]H2 or [NHP]H is highly unfavorable kinetically, thus we call attention to the sequential mechanism for understanding the methylation of amine with CO2. In addition, the study has the following key mechanistic findings. The activation of CO2 by [NHP]H establishes an equilibrium: [NHP]H + CO2 ⇄ [NHP]OCHO ⇄ [NHP]+ + HCO2-. The ions play catalytic roles to promote formylation via HCO2- or methylation via[NHP]+ . In 1a formylation, HCO2- initiates the reaction, giving 1b and silanol byproducts. However, after the initiation, the silanol byproducts acting as hydrogen transfer shuttles are more effective than HCO2- to promote formylation. In 2a methylation, [NHP]+ promotes the generation of the key species, formaldehyde and a carbocation species (IM17+ ). Our experimental study corroborates our computed mechanisms.

Aliphatic Mn–PNP complexes for the CO2 hydrogenation reaction: a base free mechanism

Aliphatic amido Mn–PNP-based complexes were found to be promising for CO₂ hydrogenation reaction.

Insight into the electronic effect of phosphine ligand on Rh catalyzed CO2 hydrogenation by investigating the reaction mechanism

The effect of the outer coordination sphere of the diphosphine ligand on the catalytic efficiency of [Rh(PCH₂X^RCH₂P)₂]⁺ (X^R = CH₂, N–CH₃, CF₂) catalyzed CO₂ hydrogenation was studied. It was found that the hydricity of the metal hydride bond determined the activation energy of the rate determining step of the reaction.

Enhanced CO2 electroreduction efficiency through secondary coordination effects on a pincer iridium catalyst

An iridium trihydride complex supported by a bifunctional pincer ligand promotes the electrocatalytic reduction of CO₂ to formate in with excellent Faradaic efficiency and low overpotential.

Mechanistic insights into hydride transfer for catalytic hydrogenation of CO2 with cobalt complexes

The catalytic hydrogenation of CO₂ to formate by Co(dmpe)₂H can proceed via direct hydride transfer or via CO₂ coordination to Co followed by reductive elimination of formate.