Thermodynamic Analysis of Metal–Ligand Cooperativity of PNP Ru Complexes: Implications for CO2 Hydrogenation to Methanol and Catalyst Inhibition

Electrocatalytic formate and alcohol oxidation by hydride transfer at first-row transition metal complexes

The electrocatalytic oxidation of carbon-based liquid fuels, such as formic acid and alcohols, has important applications for our renewable energy transition. Molecular electrocatalysts based on transition metal complexes provide the opportunity to explore the interplay between precise catalyst design and electrocatalytic activity. Recent advances have seen the development of first-row transition metal electrocatalysts for these transformations that operate via hydride transfer between the substrate and catalyst. In this Frontier article, we present the key contributions to this field and discuss the proposed mechanisms for each case. These studies also reveal the remaining challenges for formate and alcohol oxidation with first-row transition metal systems, for which we provide perspectives on future directions for next-generation electrocatalyst design.

Impact of the Methylene Bridge Substitution in Chelating NHC-Phosphine Mn(I) Catalyst for Ketone Hydrogenation

Systematic modification of the chelating NHC-phosphine ligand (NHC = N-heterocyclic carbene) in highly efficient ketone hydrogenation Mn(I) catalyst fac-[(Ph2PCH2NHC)Mn(CO)3Br] has been performed and the catalytic activity of the resulting complexes was evaluated using acetophenone as a benchmark substrate. While the variation of phosphine and NHC moieties led to inferior results than for a parent system, the incorporation of a phenyl substituent into the ligand methylene bridge improved catalytic performance by ca. 3 times providing maximal TON values in the range of 15000-20000. Mechanistic investigation combining experimental and computational studies allowed to rationalize this beneficial effect as an enhanced stabilization of reaction intermediates including anionic hydride species fac-[(Ph2PC(Ph)NHC)Mn(CO)3H]- playing a crucial role in the hydrogenation process. These results highlight the interest of such carbon bridge substitution strategy being rarely employed in the design of chemically non-innocent ligands.

Thermodynamic and Kinetic Activity Descriptors for the Catalytic Hydrogenation of Ketones

Activity descriptors are a powerful tool for the design of catalysts that can efficiently utilize H2 with minimal energy losses. In this study, we develop the use of hydricity and H- self-exchange rates as thermodynamic and kinetic descriptors for the hydrogenation of ketones by molecular catalysts. Two complexes with known hydricity, HRh(dmpe)2 and HCo(dmpe)2, were investigated for the catalytic hydrogenation of ketones under mild conditions (1.5 atm and 25 °C). The rhodium catalyst proved to be an efficient catalyst for a wide range of ketones, whereas the cobalt catalyst could only hydrogenate electron-deficient ketones. Using a combination of experiment and electronic structure theory, thermodynamic hydricity values were established for 46 alkoxide/ketone pairs in both acetonitrile and tetrahydrofuran solvents. Through comparison of the hydricities of the catalysts and substrates, it was determined that catalysis was observed only for catalyst/ketone pairs with an exergonic H- transfer step. Mechanistic studies revealed that H- transfer was the rate-limiting step for catalysis, allowing for the experimental and computation construction of linear free-energy relationships (LFERs) for H- transfer. Further analysis revealed that the LFERs could be reproduced using Marcus theory, in which the H- self-exchange rates for the HRh/Rh+ and ketone/alkoxide pairs were used to predict the experimentally measured catalytic barriers within 2 kcal mol-1. These studies significantly expand the scope of catalytic reactions that can be analyzed with a thermodynamic hydricity descriptor and firmly establish Marcus theory as a valid approach to develop kinetic descriptors for designing catalysts for H- transfer reactions.

Hydrogen Activation with Ru-PN3P Pincer Complexes for the Conversion of C1 Feedstocks

The hydrogenation of C1 feedstocks (CO and CO2) has been investigated using ruthenium complexes [RuHCl(CO)(PN3P)] as the catalyst. PN3P pincer ligands containing amines in the linker between the central pyridine donor and the phosphorus donors with bulky substituents (tert-butyl (1) or TMPhos (2)) are required to obtain mononuclear single-site catalysts that can be activated by the addition of KOtBu to generate stable five-coordinate complexes [RuH(CO)(PN3P-H)], whereby the pincer ligand has been deprotonated. Activation of hydrogen takes place via heterolytic cleavage to generate [RuH2(CO)(PN3P)], but in the presence of CO, coordination of CO occurs preferentially to give [RuH(CO)2(PN3P-H)]. This complex can be protonated to give the cationic complex [RuH(CO)2(PN3P)]+, but it is unable to activate H2 heterolytically. In the case of the less coordinating CO2, both ruthenium complexes 1 and 2 are highly efficient as CO2 hydrogenation catalysts in the presence of a base (DBU), which in the case of the TMPhos ligand results in a TON of 30,000 for the formation of formate.

Merging electrocatalytic alcohol oxidation with C-N bond formation by electrifying metal-ligand cooperative catalysts

Electrification of thermal chemical processes could play an important role in creating a more energy efficient chemical sector. Here we demonstrate that a range of MLC catalysts can be successfully electrified and used for imine formation from alcohol precursors, thus demonstrating the first example of molecular electrocatalytic C-N bond formation.This novel concept allowed energy efficiency to be increased by an order of magnitude compared to thermal catalysis. Molecular EAO and the electrification of homogeneous catalysts can thus contribute to current efforts for the electrocatalytic generation of C-N bonds from simple building blocks.

Structure and Thermodynamic Hydricity in Cobalt(triphosphine)(monophosphine) Hydrides

The mononuclear cobalt hydride complex [HCo(triphos)(PMe3)], in which triphos = PhP(CH2CH2PPh2)2, was synthesized and characterized by X-ray crystallography and by 1H and 31P NMR spectroscopy. The geometry of the compound is a distorted trigonal bipyramid in which the axial positions are occupied by the hydride and the central phosphorus atom of the triphos ligand, while the PMe3 and terminal triphos donor atoms occupy the equatorial positions. Protonation of [HCo(triphos)(PMe3)] generates H2 and the Co(I) cation, [Co(triphos)(PMe3)]+, and this reaction is reversible under an atmosphere of H2 when the proton source is weakly acidic. The thermodynamic hydricity of HCo(triphos)(PMe3) was determined to be 40.3 kcal/mol in MeCN from measurements of these equilibria. The reactivity of the hydride is, therefore, well suited to CO2 hydrogenation catalysis. Density functional theory (DFT) calculations were performed to evaluate the structures and hydricities of a series of analogous cobalt(triphosphine)(monophosphine) hydrides where the phosphine substituents are systematically changed from Ph to Me. The calculated hydricities range from 38.5 to 47.7 kcal/mol. Surprisingly, the hydricities of the complexes are generally insensitive to substitution at the triphosphine ligand, as a result of competing structural and electronic trends. The DFT-calculated geometries of the [Co(triphos)(PMe3)]+ cations are more square planar when the triphosphine ligand possesses bulkier phenyl groups and more tetrahedrally distorted when the triphosphine ligand has smaller methyl substituents, reversing the trend observed for [M(diphosphine)2]+ cations. More distorted structures are associated with an increase in ΔGH-°, and this structural trend counteracts the electronic effect in which methyl substitution at the triphosphine is expected to yield smaller ΔGH-° values. However, the steric influence of the monophosphine follows the normal trend that phenyl substituents give more distorted structures and increased ΔGH-° values.

Metal-Ligand Cooperativity Promotes Reversible Capture of Dilute CO2 as a Zn(II)-Methylcarbonate

In this study, a series of thiosemicarbazonato-hydrazinatopyridine metal complexes were evaluated as CO₂ capture agents. The complexes incorporate a non-coordinating, basic hydrazinatopyridine nitrogen in close proximity to a Lewis acidic metal ion allowing for metal-ligand cooperativity. The coordination of various metal ions with (diacetyl-2-(4-methyl-thiosemicarbazone)-3-(2-hydrazinopyridine) (H₂L¹) yielded ML¹ (M = Ni(II), Pd(II)), ML¹(CH₃OH) (M = Cu(II), Zn(II)), and [ML¹(PPh₃)₂]BF₄ (M = Co(III)) complexes. The ML¹(CH₃OH) complexes reversibly capture CO₂ with equilibrium constants of 88 ± 9 and 6900 ± 180 for Cu(II) and Zn(II), respectively. Ligand effects were evaluated with Zn(II) through variation of the 4-methyl-thiosemicarbazone with 4-ethyl (H₂L²), 4-phenethyl (H₂L³), and 4-benzyl (H₂L⁴) derivatives. The equilibrium constant for CO₂ capture increased to 11,700 ± 300, 15,000 ± 400, and 35,000 ± 200 for ZnL²(MeOH), ZnL³(MeOH), and ZnL⁴(MeOH), respectively. Quantification of ligand basicity and metal ion Lewis acidity shows that changes in CO₂ capture affinity are largely associated with ligand basicity upon substitution of Cu(II) with Zn(II), while variation of the thiosemicarbazone ligand enhances CO₂ affinity by tuning the metal ion Lewis acidity. Overall, the Zn(II) complexes effectively capture CO₂ from dilute sources with up to 90%, 86%, and 65% CO₂ capture efficiency from 400, 1000, and 2500 ppm CO₂ streams.

Performance of homogeneous catalysts viewed in dynamics

Effective assessment of catalytic performance is the foundation for the rational design and development of new catalysts with superior performance. The ubiquitous screening/optimization studies use reaction yields as the sole performance metric in an approach that often neglects the complexity of the catalytic system and intrinsic reactivities of the catalysts. Using an example of hydrogenation catalysis, we examine the transient behavior of catalysts that are often encountered in activation, deactivation and catalytic turnover processes. Each of these processes and the reaction environment in which they take place are gradually shown to determine the real-time catalyst speciation and the resulting kinetics of the overall catalytic reaction. As a result, the catalyst performance becomes a complex and time-dependent metric defined by multiple descriptors apart from the reaction yield. This behaviour is not limited to hydrogenation catalysis and affects various catalytic transformations. In this feature article, we discuss these catalytically relevant descriptors in an attempt to arrive at a comprehensive depiction of catalytic performance.

Translating aqueous CO2 hydrogenation activity to electrocatalytic reduction with a homogeneous cobalt catalyst

A molecular cobalt CO₂ hydrogenation catalyst was explored for electrocatalytic CO₂ reduction under aqueous conditions. The resulting pH-dependent selectivity between H₂ and HCO₂^- is rationalized with thermodynamic analysis and stoichiometric experiments.

Large changes in hydricity as a function of charge and not metal in (PNP)M–H (de)hydrogenation catalysts that undergo metal–ligand cooperativity

Ligand pKa and metal hydricity scale with one another in (de)hydrogenation catalysts that undergo metal–ligand cooperativity, irrespective of metal or ligand identity. Anionic hydrides are significantly more hydridic than their neutral counterparts.

Electrification of a Milstein-type catalyst for alcohol reformation

Novel energy and atom efficiency processes will be keys to develop the sustainable chemical industry of the future. Electrification could play an important role, by allowing to fine-tune energy input and using the ideal redox agent: the electron. Here we demonstrate that a commercially available Milstein ruthenium catalyst (1) can be used to promote the electrochemical oxidation of ethanol to ethyl acetate and acetate, thus demonstrating the four electron oxidation under preparative conditions. Cyclic voltammetry and DFT-calculations are used to devise a possible catalytic cycle based on a thermal chemical step generating the key hydride intermediate. Successful electrification of Milstein-type catalysts opens a pathway to use alcohols as a renewable feedstock for the generation of esters and other key building blocks in organic chemistry, thus contributing to increase energy efficiency in organic redox chemistry.

Homogeneous Hydrogenation of CO2 and CO to Methanol: The Renaissance of Low-Temperature Catalysis in the Context of the Methanol Economy

The traditional economy based on carbon-intensive fuels and materials has led to an exponential rise in anthropogenic CO₂ emissions. Outpacing the natural carbon cycle, atmospheric CO₂ levels increased by 50 % since the pre-industrial age and can be directly linked to global warming. Being at the core of the proposed methanol economy pioneered by the late George A. Olah, the chemical recycling of CO₂ to produce methanol, a green fuel and feedstock, is a prime channel to achieve carbon neutrality. In this direction, homogeneous catalytic systems have lately been a major focus for methanol synthesis from CO₂ , CO and their derivatives as potential low-temperature alternatives to the commercial processes. This Review provides an account of this rapidly growing field over the past decade, since its resurgence in 2011. Based on the critical assessment of the progress thus far, the present key challenges in this field have been highlighted and potential directions have been suggested for practically viable applications.

Correlating Thermodynamic and Kinetic Hydricities of Rhenium Hydrides

The kinetics of hydride transfer from Re(^Rbpy)(CO)₃H (bpy = 4,4'-R-2,2'-bipyridine; R = OMe, ^tBu, Me, H, Br, COOMe, CF₃) to CO₂ and seven different cationic N-heterocycles were determined. Additionally, the thermodynamic hydricities of complexes of the type Re(^Rbpy)(CO)₃H were established primarily using computational methods. Linear free-energy relationships (LFERs) derived by correlating thermodynamic and kinetic hydricities indicate that, in general, the rate of hydride transfer increases as the thermodynamic driving force for the reaction increases. Kinetic isotope effects range from inverse for hydride transfer reactions with a small driving force to normal for reactions with a large driving force. Hammett analysis indicates that hydride transfer reactions with greater thermodynamic driving force are less sensitive to changes in the electronic properties of the metal hydride, presumably because there is less buildup of charge in the increasingly early transition state. Bronsted α values were obtained for a range of hydride transfer reactions and along with DFT calculations suggest the reactions are concerted, which enables the use of Marcus theory to analyze hydride transfer reactions involving transition metal hydrides. It is notable, however, that even slight perturbations in the steric properties of the Re hydride or the hydride acceptor result in large deviations in the predicted rate of hydride transfer based on thermodynamic driving forces. This indicates that thermodynamic considerations alone cannot be used to predict the rate of hydride transfer, which has implications for catalyst design.

Nucleophilic Reactivity at a ═CH Arm of a Lutidine-Based CNC/Rh System: Unusual Alkyne and CO2 Activation

Reaction of an amido pincer complex [(CNC)*Rh(CO)] (1) (CNC* is the deprotonated form of CNC) with carbon dioxide gave a neutral complex [(CNC-CO₂)^Mes*Rh(CO)] (2), which is the result of a C-C bond-forming reaction between the deprotonated arm of the CNC* ligand and CO₂. The molecular structure of 2 showed a zwitterionic complex, where the CO₂ moiety is covalently connected to the former ═CH arm of the CNC* pincer ligand. The unusual structure of 1 allowed us to explore the reactivity of the CO₂ moiety with selected primary amines RNH₂ (benzylamine and ammonia), which afforded cationic complexes [(CNC)^MesRh(CO)][HRNC(O)O] (R = Bz (3), H (4)). Compounds 3 and 4 are the result of a C-N coupling between the incoming amine and the CO₂ fragment covalently connected to the pincer ligand in 2, a process that involves protonation of the "CH-CO₂" fragment in 2 from the respective amines. Once revealed the nucleophilic character of the ═CH fragment in 1, we explored its reactivity with alkynes, a study that enlightened a novel reactivity trend in alkyne activation. Reaction of 1 with terminal alkynes RC≡CH (R = Ph, 2-py, 4-C₆H₄-CF₃) yielded neutral complexes [(CNC-CH═CHR)^Mes*Rh(CO)] (R = Ph (5), 2-py (6), 4-C₆H₄-CF₃ (7)) in good yields. Deuterium labeling experiments with PhC≡CD confirmed that complex 5 is the product of a formal insertion of the alkyne into the C(sp²)-H bond of the deprotonated arm in 1. This structural proposal was further confirmed by the X-ray molecular structure of phenyl complex 5, which showed the alkyne covalently linked to the pincer ligand. Besides, this novel transformation was analyzed by DFT methods and showed a metal-ligand cooperative mechanism, based on the initial electrophilic attack of the alkyne to the ═CH arm of the CNC^Mes* ligand (making a new C-C bond) followed by the action of a protic base (HN(SiMe₃)₂), which is able to perform a proton rearrangement that leads to the final product 5.

Basic Promotors Impact Thermodynamics and Catalyst Speciation in Homogeneous Carbonyl Hydrogenation

Homogeneously catalyzed reactions often make use of additives and promotors that affect reactivity patterns and improve catalytic performance. While the role of reaction promotors is often discussed in view of their chemical reactivity, we demonstrate that they can be involved in catalysis indirectly. In particular, we demonstrate that promotors can adjust the thermodynamics of key transformations in homogeneous hydrogenation catalysis and enable reactions that would be unfavorable otherwise. We identified this phenomenon in a set of well-established and new Mn pincer catalysts that suffer from persistent product inhibition in ester hydrogenation. Although alkoxide base additives do not directly participate in inhibitory transformations, they can affect the equilibrium constants of these processes. Experimentally, we confirm that by varying the base promotor concentration one can control catalyst speciation and inflict substantial changes to the standard free energies of the key steps in the catalytic cycle. Despite the fact that the latter are universally assumed to be constant, we demonstrate that reaction thermodynamics and catalyst state are subject to external control. These results suggest that reaction promotors can be viewed as an integral component of the reaction medium, on its own capable of improving the catalytic performance and reshaping the seemingly rigid thermodynamic landscape of the catalytic transformation.

Biomimetic Metal-Free Hydride Donor Catalysts for CO2 Reduction

The catalytic reduction of carbon dioxide to fuels and value-added chemicals is of significance for the development of carbon recycling technologies. One of the main challenges associated with catalytic CO₂ reduction is product selectivity: the formation of carbon monoxide, molecular hydrogen, formate, methanol, and other products occurs with similar thermodynamic driving forces, making it difficult to selectively reduce CO₂ to the target product. Significant scientific effort has been aimed at the development of catalysts that can suppress the undesired hydrogen evolution reaction and direct the reaction toward the selective formation of the desired products, which are easy to handle and store. Inspired by natural photosynthesis, where the CO₂ reduction is achieved using NADPH cofactors in the Calvin cycle, we explore biomimetic metal-free hydride donors as catalysts for the selective reduction of CO₂ to formate. Here, we outline our recent findings on the thermodynamic and kinetic parameters that control the hydride transfer from metal-free hydrides to CO₂. By experimentally measuring and theoretically calculating the thermodynamic hydricities of a range of metal-free hydride donors, we derive structural and electronic factors that affect their hydride-donating abilities. Two dominant factors that contribute to the stronger hydride donors are identified to be (i) the stabilization of the positive charge formed upon HT via aromatization or by the presence of electron-donating groups and (ii) the destabilization of hydride donors through the anomeric effect or in the presence of significant structural constrains in the hydride molecule. Hydride donors with appropriate thermodynamic hydricities were reacted with CO₂, and the formation of the formate ion (the first reduction step in CO₂ reduction to methanol) was confirmed experimentally, providing an important proof of principle that organocatalytic CO₂ reduction is feasible. The kinetics of hydride transfer to CO₂ were found to be slow, and the sluggish kinetics were assigned in part to the large self-exchange reorganization energy associated with the organic hydrides in the DMSO solvent. Finally, we outline our approaches to the closure of the catalytic cycle via the electrochemical and photochemical regeneration of the hydride (R-H) from the conjugate hydride acceptors (R⁺). We illustrate how proton-coupled electron transfer can be efficiently utilized not only to lower the electrochemical potential at which the hydride regeneration takes place but also to suppress the unwanted dimerization that neutral radical intermediates tend to undergo. Overall, this account provides a summary of important milestones achieved in organocatalytic CO₂ reduction and provides insights into the future research directions needed for the discovery of inexpensive catalysts for carbon recycling.

Rational Design of Main Group Metal-Embedded Nitrogen-Doped Carbon Materials as Frustrated Lewis Pair Catalysts for CO2 Hydrogenation to Formic Acid

Developing efficient and inexpensive main group catalysts for CO₂ conversion and utilization has attracted increasing attention, as the conversion process would be both economical and environmentally benign. Here, based on the main group element Al, we designed several heterogeneous frustrated Lewis acid/base pair (FLP) catalysts and performed extensive first-principles calculations for the hydrogenation of CO₂. These catalysts, including Al@N-Gr-1, Al@N-Gr-2, and Al@C₂N, are composed of a single Al atom and two-dimensional (2D) N-doped carbon-based materials to form frustrated Al/C or Al/N Lewis acid/base pairs, which are all predicted to have high reactivity to absorb and activate hydrogen (H₂). Compared with Al@N-Gr-1, both Al@N-Gr-2 and Al@C₂N, especially Al@N-Gr-2, containing Al/N Lewis pairs exhibit better catalytic activity for CO₂ hydrogenation with lower activation energies. CO₂ hydrogenation on the three catalysts prefers to go through a three-step mechanism, i.e., the heterolytic dissociation of H₂, followed by the transfer of the hydride near Al to CO₂, and finally the activation of a second H₂ molecule. Other IIIA group element (B and Ga)-embedded N-Gr-2 materials (B@N-Gr-2 and Ga@N-Gr-2) were also explored and compared. Both Al@N-Gr-2 and Ga@N-Gr-2 show higher catalytic activity for CO₂ hydrogenation to HCOOH than B@N-Gr-2. However, the CO₂ hydrogenation path on Ga@N-Gr-2 tends to follow a two-step mechanism, including H₂ dissociation and subsequent hydrogen transfer. The present study provides a potential solution for CO₂ hydrogenation by designing novel and effective FLP catalysts based on main group elements.

Computational Study of CO2 Reduction Catalyzed by Iron(I) Complex at Different Spin States: Cooperativity of Hydrogen Bonding and Auxiliary Group Effect

To explore alternative approaches to the CO₂ reduction to formate and provide an insight into the spin state effect on the CO₂ reduction, we theoretically designed a kind of low-valence iron(I) model complex, whose doublet, quartet, and sextet states are denoted as ² Fe(I), ⁴ Fe(I), and ⁶ Fe(I), respectively. This complex is featured with an iron(I) center, which bonds to a 1,2-ethanediamine (en) and a 2-hydroxy-biphenyl group. Reaction mechanisms for the CO₂ reduction to formate catalyzed by this iron(I) model complex were explored using density functional theory (DFT) computations. Studies showed that the univalent iron(I) compound can efficiently fix and activate a CO₂ molecule, whereas its oxidized forms with trivalent iron(III) or bivalent iron(II) cannot activate CO₂. For the iron(I) compound, it was found that the lowest spin state ² Fe(I) is the most favorable for the CO₂ reduction as the reactions barriers involving ² Fe(I), ⁴ Fe(I), and ⁶ Fe(I) are 25.6, 37.2, and 35.9 kcal/mol, respectively. Yet, a photosensitizer-free visible-light-mediated high-low spin shift from ⁴ Fe(I) and ⁶ Fe(I) to ² Fe(I) is likely through the reverse intersystem crossing (RIC) because the ⁴ Fe(I) and ⁶ Fe(I) compounds have strong absorption in the visible-light range. Notably, the synergistic interaction between the hydrogen bonding from the auxiliary hydroxyl group in the 2-hydroxy-biphenyl moiety to CO₂ and an intermediate five-membered ring promotes the proton transfer, leading to the formation of the -COOH moiety from CO₂ and the Fe-O bond. With the addition of H₂, one H₂ molecule is split by the Fe-O bond and thus serves as H atom sources for both the CO₂ reduction and the recovery of the auxiliary hydroxyl group. The present theoretical study provides a novel solution for the challenging CO₂ reduction, which calls for further experimental verifications.

Bulky PNP ligands blocking metal-ligand cooperation allow for isolation of Ru(0), and lead to catalytically active Ru complexes in acceptorless alcohol dehydrogenation

We synthesized two 4Me-PNP ligands which block metal-ligand cooperation (MLC) with the Ru center and compared their Ru complex chemistry to their two traditional analogues used in acceptorless alcohol dehydrogenation catalysis. The corresponding 4Me-PNP complexes, which do not undergo dearomatization upon addition of base, allowed us to obtain rare, albeit unstable, 16 electron mono CO Ru(0) complexes. Reactivity with CO and H 2 allows for stabilization and extensive characterization of bis CO Ru(0) 18 electron and Ru(II) cis and trans dihydride species that were also shown to be capable of C(sp2)-H activation. Reactivity and catalysis are contrasted to non-methylated Ru(II) species, showing that an MLC pathway is not necessary, with dramatic differences in outcomes during catalysis between i Pr and t Bu PNP complexes within each of the 4Me and non-methylated backbone PNP series being observed. Unusual intermediates are characterized in one of the new and one of the traditional complexes, and a common catalysis deactivation pathway was identified.

Quantum Dot-Sensitized Photoreduction of CO2 in Water with Turnover Number > 80,000

Climate change and global energy demands motivate the search for sustainable transformations of carbon dioxide (CO₂) to storable liquid fuels. Photocatalysis is a pathway for direct conversion of CO₂ to CO, one step within light-powered reaction networks that could, if efficient enough, transform the solar energy conversion landscape. To date, the best performing photocatalytic CO₂ reduction systems operate in nonaqueous solvents, but technologically viable solar fuels networks will likely operate in water. Here we demonstrate catalytic photoreduction of CO₂ to CO in pure water at pH 6-7 with an unprecedented combination of performance parameters: turnover number (TON(CO)) = 72,484-84,101, quantum yield (QY) = 0.96-3.39%, and selectivity (S_CO) > 99%, using CuInS₂ colloidal quantum dots (QDs) as photosensitizers and a Co-porphyrin catalyst. At higher catalyst concentration, the system reaches QY = 3.53-5.23%. The performance of the QD-driven system greatly exceeds that of the benchmark aqueous system (926 turnovers with a quantum yield of 0.81% and selectivity of 82%), due primarily to (i) electrostatic attraction of the QD to the catalyst, which promotes fast multielectron delivery and colocalization of protons, CO₂, and catalyst at the source of photoelectrons, and (ii) termination of the QD's ligand shell with free amines, which capture CO₂ as carbamic acid that serves as a reservoir for CO₂, effectively increasing its solubility in water, and lowers the onset potential for catalytic CO₂ reduction by the Co-porphyrin. The breakthrough efficiency achieved in this work represents a nonincremental step in the realization of reaction networks for direct solar-to-fuel conversion.

Ascendancy of Nitrogen Heterocycles in the Computationally Designed Mn(I)PNN Pincer Catalysts on the Hydrogenation of Carbon Dioxide to Methanol

The development of sustainable catalysts to get methanol from CO₂ under milder conditions and without any additives is still considered an arduous task. In many instances, transition-metal-catalyzed carbon dioxide to formic acid formation is more facile than methanol formation. This article provides comprehensive density functional theoretic investigations of six new Mn(I)PNN complexes, which are designed to perform CO₂ to methanol conversion under milder reaction conditions. All these six catalysts have similar structural features except at terminal nitrogen, -N (1), where adenine-inspired nitrogen heterocycles containing pyridine and pyrimidine moieties are attached to instill an electron withdrawing effect on the central metal and thus to facilitate dihydrogen polarization during the catalyst regeneration. All these computationally modeled Mn(I)PNN complexes demonstrate the promising catalytic activity to get methanol through cascade catalytic cycles at 298.15 K. The metal-ligand cooperative (MLC) as well as noncooperative (NC) pathways are investigated for each catalytic cycle. The NC pathway is the preferred pathway for formic acid and formaldehyde formation, whereas methanol formation proceeds through only the MLC pathway. Different nitrogen heterocycles attached to the -N (1) terminal manifested a considerable amount of impact on the Gibbs free energies, overall activation energies, and computed turnover frequencies (TOFs). Among all the catalysts, SPCAT02 provides excellent TOFs for HCO₂H (500 151 h^-1), HCHO (11 912 h^-1), and CH₃OH (2 372 400 h^-1) formation at 50 °C. SPCAT04 is found to be a better catalyst for the selective formation of formic acid formation at room temperature than the rest of the catalysts. The computed TOF results are found reliable upon comparison with experimentally established catalysts. To establish the structure-activity relationship, the activation strain model and Fukui function calculations are performed on all the catalysts. Both these studies provide complementary results. The present study revealed a very important finding that a more electrophilic metal center could facilitate the CO₂ hydrogenation reaction robustly. All computationally designed catalysts could be cheaper and better alternatives to convert CO₂ to methanol under mild reaction conditions in an aqueous medium.

Anti-Electrostatic Main Group Metal-Metal Bonds That Activate CO2

There has been growing interest in the CO₂ capture and reduction by transition-metal-free catalysts. Here we performed a proof-of-concept study using an ab initio valence bond method called the block-localized wave function (BLW) method. The integrated BLW and density function theory (DFT) computations demonstrated that heterobimetallic Ae⁺/Al(I) (Ae represents alkaline earth metals Mg and Ca) Lewis acid/base combinations without transition metals can facilely capture and activate CO₂. There are two remarkable findings in this study. The first concerns the ionic nature of the metal-metal bonds. The experimentally synthesized low valent aluminum compound with a bidentate β-diketiminate (BDI) ligand, or (BDI)Al(I) in brief, is a Lewis base due to the lone pair on the aluminum cation though overall Al(I) is positively charged. Al(I) can form ionic metal-metal bonds with the alkaline earth metals of the positively charged Lewis acids (BDI)Ae⁺. This type of ionic metal-metal bonds is counterintuitive and antielectrostatic as both metals carry positive charges. The second finding is the CO₂ activation mechanism. (BDI)Al(I) can effectively bind and activate CO₂ by transferring one electron to CO₂, and the resulting complex can be best expressed as [(BDI)Al(I)]⁺[CO₂]^-. The participation of (BDI)Ae⁺ further enhances the capture and activation of CO₂ by (BDI)Al(I).

Alcohol-Assisted Hydrogenation of Carbon Monoxide to Methanol Using Molecular Manganese Catalysts

Alcohol-assisted hydrogenation of carbon monoxide (CO) to methanol was achieved using homogeneous molecular complexes. The molecular manganese complex [Mn(CO)2Br[HN(C2H4P i Pr2)2]] ([HN(C2H4P i Pr2)2] = MACHO- i Pr) revealed the best performance, reaching up to turnover number = 4023 and turnover frequency 857 h-1 in EtOH/toluene as solvent under optimized conditions (T = 150 °C, p(CO/H2) = 5/50 bar, t = 8-12 h). Control experiments affirmed that the reaction proceeds via formate ester as the intermediate, whereby a catalytic amount of base was found to be sufficient to mediate its formation from CO and the alcohol in situ. Selectivity for methanol formation reached >99% with no accumulation of the formate ester. The reaction was demonstrated to work with methanol as the alcohol component, resulting in a reactive system that allows catalytic "breeding" of methanol without any coreagents.

Temperature and Solvent Effects on H2 Splitting and Hydricity: Ramifications on CO2 Hydrogenation by a Rhenium Pincer Catalyst

The catalytic hydrogenation of carbon dioxide holds immense promise for applications in sustainable fuel synthesis and hydrogen storage. Mechanistic studies that connect thermodynamic parameters with the kinetics of catalysis can provide new understanding and guide predictive design of improved catalysts. Reported here are thermochemical and kinetic analyses of a new pincer-ligated rhenium complex (^tBuPOCOP)Re(CO)₂ (^tBuPOCOP = 2,6-bis(di-tert-butylphosphinito)phenyl) that catalyzes CO₂ hydrogenation to formate with faster rates at lower temperatures. Because the catalyst follows the prototypical "outer sphere" hydrogenation mechanism, comprehensive studies of temperature and solvent effects on the H₂ splitting and hydride transfer steps are expected to be relevant to many other catalysts. Strikingly large entropy associated with cleavage of H₂ results in a strong temperature dependence on the concentration of [(^tBuPOCOP)Re(CO)₂H]^- present during catalysis, which is further impacted by changing the solvent from toluene to tetrahydrofuran to acetonitrile. New methods for determining the hydricity of metal hydrides and formate at temperatures other than 298 K are developed, providing insight into how temperature can influence the favorability of hydride transfer during catalysis. These thermochemical insights guided the selection of conditions for CO₂ hydrogenation to formate with high activity (up to 364 h^-1 at 1 atm or 3330 h^-1 at 20 atm of 1:1 H₂:CO₂). In cases where hydride transfer is the highest individual kinetic barrier, entropic contributions to outer sphere H₂ splitting lead to a unique temperature dependence: catalytic activity increases as temperature decreases in tetrahydrofuran (200-fold increase upon cooling from 50 to 0 °C) and toluene (4-fold increase upon cooling from 100 to 50 °C). Ramifications on catalyst structure-function relationships are discussed, including comparisons between "outer sphere" mechanisms and "metal-ligand cooperation" mechanisms.

Bonding Energetics of Palladium Amido/Aryloxide Complexes in DMSO: Implications for Palladium-Mediated Aniline Activation

Thermodynamic knowledge of the metal-ligand (M-L) σ-bond strength is crucial to understanding metal-mediated transformations. Here, we developed a method for determining the Pd-X (X=OR and NHAr) bond heterolysis energies (ΔG_het (Pd-X)) in DMSO taking [(tmeda)PdArX] (tmeda=N,N,N',N'-tetramethylethylenediamine) as the model complexes. The ΔG_het (Pd-X) scales span a range of 2.6-9.0 kcal mol^-1 for ΔG_het (Pd-O) values and of 14.5-19.5 kcal mol^-1 for ΔG_het (Pd-N) values, respectively, implying a facile heterolytic detachment of the Pd ligands. Structure-reactivity analyses of a modeling Pd-mediated X-H bond activation reveal that the M-X bond metathesis is dominated by differences of the X-H and Pd-X bond strengths, the former being more influential. The ΔG_het (Pd-X) and pK_a (X-H) parameters enable regulation of reaction thermodynamics and chemoselectivity and diagnosing the probability of aniline activation with Pd-X complexes.

Catalytic coproduction of methanol and glycol in one pot from epoxide, CO2, and H2

An atom (100%) and energy-efficient approach to coproduce two commodity chemicals, methanol and glycol, has been demonstrated for the first time using H₂, CO₂, and epoxide as feeds. A basic medium used for CO₂ capture, polyethyleneimine (PEI₆₀₀), is shown to facilitate the formation of a key reaction intermediate, cyclic carbonates. Upon hydrogenation of cyclic carbonates in the presence of a homogenous Ru-PNP catalyst, a 1 : 1 mixture of methanol and glycol is produced. This approach has been demonstrated in one pot by adding all the required reactants directly or stepwise. The stepwise addition of reactants resulted in good yields (>95% for PG and 84% for methanol) and selectivity of products.

An atom (100%) and energy-efficient approach to coproduce two commodity chemicals, methanol and glycol, has been demonstrated for the first time using H₂, CO₂, and epoxide as feeds.

Hydrogenation and electrocatalytic reduction of carbon dioxide to formate with a single Co catalyst

A cobalt(i) complex is shown to be capable of both electrocatalytic reduction and hydrogenation of CO2 to formate. Several proposed intermediates are characterized and thus form the basis for a proposed mechanism that allows for the dual reactivity: reduction of CO2via H2 addition, and H+/e- equivalents. The work makes use of a novel tris(phosphino) ligand. When a pendent amine is attached to the ligand, no change in catalytic reactivity is observed.

Toward Combined Carbon Capture and Recycling: Addition of an Amine Alters Product Selectivity from CO to Formic Acid in Manganese Catalyzed Reduction of CO2

Owing to the energetic cost associated with CO₂ release in carbon capture (CC), the combination of carbon capture and recycling (CCR) is an emerging area of research. In this approach, "captured CO₂," typically generated by addition of amines, serves as a substrate for subsequent reduction. Herein, we report that the reduction of CO₂ in the presence of morpholine (generating mixtures of the corresponding carbamate and carbamic acid) with a well-established Mn electrocatalyst changes the product selectivity from CO to H₂ and formate. The change in selectivity is attributed to in situ generation of the morpholinium carbamic acid, which is sufficiently acidic to protonate the reduced Mn species and generate an intermediate Mn hydride. Thermodynamic studies indicate that the hydride is not sufficiently hydritic to reduce CO₂ to formate, unless the apparent hydricity, which encompasses formate binding to the Mn, is considered. Increasing steric bulk around the Mn shuts down rapid homolytic H₂ evolution rendering the intermediate Mn hydride more stable; subsequent CO₂ insertion appears to be faster than heterolytic H₂ production. A comprehensive mechanistic scheme is proposed that illustrates how thermodynamic analysis can provide further insight. Relevant to a range of hydrogenations and reductions is the modulation of the hydricity with substrate binding that makes the reaction favorable. Significantly, this work illustrates a new role for amines in CO₂ reduction: changing the product selectivity; this is pertinent more broadly to advancing CCR.

Hydrogenative Depolymerization of Nylons

The widespread crisis of plastic pollution demands discovery of new and sustainable approaches to degrade robust plastics such as nylons. Using a green and sustainable approach based on hydrogenation, in the presence of a ruthenium pincer catalyst at 150 °C and 70 bar H₂, we report here the first example of hydrogenative depolymerization of conventional, widely used nylons and polyamides, in general. Under the same catalytic conditions, we also demonstrate the hydrogenation of a polyurethane to produce diol, diamine, and methanol. Additionally, we demonstrate an example where monomers (and oligomers) obtained from the hydrogenation process can be dehydrogenated back to a poly(oligo)amide of approximately similar molecular weight, thus completing a closed loop cycle for recycling of polyamides. Based on the experimental and density functional theory studies, we propose a catalytic cycle for the process that is facilitated by metal–ligand cooperativity. Overall, this unprecedented transformation, albeit at the proof of concept level, offers a new approach toward a cleaner route to recycling nylons.

Recent Advances in the Chemistry of Metal Carbamates

Following a related review dating back to 2003, the present review discusses in detail the various synthetic, structural and reactivity aspects of metal species containing one or more carbamato ligands, representing a large family of compounds across all the periodic table. A preliminary overview is provided on the reactivity of carbon dioxide with amines, and emphasis is given to recent findings concerning applications in various fields.

Hydration of nitriles using a metal-ligand cooperative ruthenium pincer catalyst

The hydration of a broad range of aliphatic and (hetero)aromatic nitriles is reported via catalysis by metal–ligand cooperative Ru pincer complexes under very mild conditions.

Nitrile hydration provides access to amides that are important structural elements in organic chemistry. Here we report catalytic nitrile hydration using ruthenium catalysts based on a pincer scaffold with a dearomatized pyridine backbone. These complexes catalyze the nucleophilic addition of H₂O to a wide variety of aliphatic and (hetero)aromatic nitriles in ^tBuOH as solvent. Reactions occur under mild conditions (room temperature) in the absence of additives. A mechanism for nitrile hydration is proposed that is initiated by metal–ligand cooperative binding of the nitrile.