Ketone Asymmetric Hydrogenation Catalyzed by P-NH-P′ Pincer Iron Catalysts: An Experimental and Computational Study

Co-Catalyzed Asymmetric Hydrogenation. The Same Enantioselection Pattern for Different Mechanisms

The mechanism of the recently reported catalyzed asymmetric hydrogenation of enyne 1 catalyzed by the Co-(R,R)-QuinoxP* complex was studied by DFT. Conceivable pathways for the Co(I)-Co(III) mechanism were computed together with a Co(0)-Co(II) catalytic cycle. It is commonly assumed that the exact nature of the chemical transformations taking place along the actually operating catalytic pathway determine the sense and level of enantioselection of the catalytic reaction. In this work, two chemically different mechanisms reproduced the experimentally observed perfect stereoselection of the same handedness. Moreover, the relative stabilities of the transition states of the stereo induction stages were controlled via exactly the same weak disperse interactions between the catalyst and the substrate.

The Mechanism of Markovnikov-Selective Epoxide Hydrogenolysis Catalyzed by Ruthenium PNN and PNP Pincer Complexes

The homogeneous catalysis of epoxide hydrogenolysis to give alcohols has recently received significant attention. Catalyst systems have been developed for the selective formation of either the Markovnikov (branched) or anti-Markovnikov (linear) alcohol product. Thus far, the reported catalysts exhibiting Markovnikov selectivity all feature the potential for Noyori/Shvo-type bifunctional catalysis, with either a RuH/NH or FeH/OH core structure. The proposed mechanisms of epoxide ring-opening have involved cooperative C-O bond hydrogenolysis involving the metal hydride and the acidic pendant group on the ligand, in analogy to the well-documented mechanism of polar double-bond hydrogenation exhibited by catalysts of this type. In this work, we present a combined computational/experimental study of the mechanism of epoxide hydrogenolysis catalyzed by Noyori-type PNP and PNN complexes of ruthenium. We find that, at least for these ruthenium systems, the previously proposed bifunctional pathway for epoxide ring-opening is energetically inaccessible; instead, the ring-opening proceeds through opposite-side nucleophilic attack of the ruthenium hydride on the epoxide carbon, without the involvement of the ligand N-H group. For both catalyst systems, the rate law and overall barrier predicted by density functional theory (DFT) are consistent with the results from kinetic studies.

Methyl Effects on the Stereochemistry and Reactivity of PPP-Ligated Iron Hydride Complexes

Iron dihydride complexes are key intermediates in many iron-catalyzed reactions. Previous efforts to study molecules of this type have led to the discovery of a remarkably stable cis-FeH₂ complex, which is supported by bis[2-(diisopropylphosphino)phenyl]phosphine (^iPrPP^HP) along with CO. In this work, the hydrogen on the central phosphorus has been replaced with a methyl group, and the corresponding iron carbonyl dichloride, hydrido chloride, and dihydride complexes have been synthesized. The addition of the methyl group favors the anti configuration for the Me-P-Fe-H moiety and the trans geometry for the H-Fe-CO motif, which is distinctively different from the ^iPrPP^HP system. Furthermore, it increases the thermal stability of the dihydride complex, cis-(^iPrPP^MeP)Fe(CO)H₂ (^iPrPP^MeP = bis[2-(diisopropylphosphino)phenyl]methylphosphine). The variations in stereochemistry and compound stability contribute greatly to the differences between the two PPP systems in reactions with PhCHO, CS₂, and HCO₂H.

Surface and Interface Coordination Chemistry Learned from Model Heterogeneous Metal Nanocatalysts: From Atomically Dispersed Catalysts to Atomically Precise Clusters

The surface and interface coordination structures of heterogeneous metal catalysts are crucial to their catalytic performance. However, the complicated surface and interface structures of heterogeneous catalysts make it challenging to identify the molecular-level structure of their active sites and thus precisely control their performance. To address this challenge, atomically dispersed metal catalysts (ADMCs) and ligand-protected atomically precise metal clusters (APMCs) have been emerging as two important classes of model heterogeneous catalysts in recent years, helping to build bridge between homogeneous and heterogeneous catalysis. This review illustrates how the surface and interface coordination chemistry of these two types of model catalysts determines the catalytic performance from multiple dimensions. The section of ADMCs starts with the local coordination structure of metal sites at the metal-support interface, and then focuses on the effects of coordinating atoms, including their basicity and hardness/softness. Studies are also summarized to discuss the cooperativity achieved by dual metal sites and remote effects. In the section of APMCs, the roles of surface ligands and supports in determining the catalytic activity, selectivity, and stability of APMCs are illustrated. Finally, some personal perspectives on the further development of surface coordination and interface chemistry for model heterogeneous metal catalysts are presented.

Iron Dihydride Complex Stabilized by an All-Phosphorus-Based Pincer Ligand and Carbon Monoxide

PNP-pincer-stabilized iron carbonyl dihydride complexes are key intermediates in catalytic hydrogenation and dehydrogenation reactions; however, decomposition through these intermediates has been observed. This inspires the development of a PPP-pincer system that may show improved catalyst stability. In this work, bis[2-(diisopropylphosphino)phenyl]phosphine (or ^iPrPP^HP) is used to react with FeCl₂ under a carbon monoxide (CO) atmosphere to yield trans-(^iPrPP^HP)Fe(CO)Cl₂. A subsequent reaction with NaBH₄ produces syn/anti-(^iPrPP^HP)FeH(CO)Cl or cis,anti-(^iPrPP^HP)Fe(CO)H₂, depending on the amount of NaBH₄ employed. The cis-dihydride complex shows catalytic activity for the conversion of PhCHO to PhCH₂OH (under H₂) or PhCO₂CH₂Ph (under Ar). It also catalyzes the dehydrogenation of PhCH₂OH to PhCHO and PhCO₂CH₂Ph, albeit with limited turnover numbers. A more efficient catalytic process is the dehydrogenation of formic acid to carbon dioxide (CO₂), which can operate under additive-free conditions. Mechanistic investigation suggests that the cis-dihydride complex undergoes protonation with formic acid to release H₂ while forming anti-(^iPrPP^HP)FeH(CO)(OCHO)·HCO₂H, in which the CO ligand has shifted and the formate is hydrogen-bonded to formic acid. The hydrido formate complex loses CO₂ under ambient conditions, completing the catalytic cycle by reforming the cis-dihydride complex.

Electronic insights into aminoquinoline-based PNHN ligands: protonation state dictates geometry while coordination environment dictates N-H acidity and bond strength

A variety of transition metal complexes bearing aminoquinoline PN^HH'-R ligands R = Ph (L1H), Cy (L2H) and their amido analogues are reported for rhodium(I) ([Rh(L1H)(PPh₃)]⁺1 and Rh(L1)(PPh₃) 2), cobalt(II) (Co(L2)(Cl) 3), and iron(II) ([Fe(L1H)₂]²⁺5, Fe(L1)₂6, and [Fe(C₅Me₅)(L1H)]PF₆7). The acid-base and redox properties of the amido complexes 2, 6, and their protio parent complexes 1, and 5 permit the determination of the pK_a and bond dissociation free energy (BDFE) of their N-H bonds while the ligand scaffold is coordinated to metal centres of square planar and octahedral geometry, respectively. From relative concentrations obtained by the use of ³¹P{¹H} NMR spectroscopy, a pK_a^THF value of 14 is calculated for rhodium complex 1, 6.4 for iron complex 5, and 24 for iron complex 7. These data, when combined with elecrochemical potentials obtained via cyclic voltammetry, allow the calculations of BDFE values for the N-H bond of 69 kcal mol^-1 for 1, and of 55 kcal mol^-1 for 5.

Discovery of Electronic Structure and Interfacial Interaction Features in Catalytic Activity

The selective transformation of inert bonds (C-H, C-O, C-C, C-F, etc.) via various catalysts is one of the most challenging areas, with applications in organic synthesis, materials science, and biological and pharmaceutical chemistry. The catalytic performance of homogeneous and heterogeneous catalysts can be rationally controlled in two ways: (i) electronic structure modulation of the active site, such as the metal center, ligands, and coordination modes, to improve the catalytic activity and stability and (ii) tuning intermolecular or interfacial interactions to promoting the reaction kinetics by accelerating the transmission of electrons between the catalyst and solvents or support. The rational design of catalysts based on adjustable features, such as metal (monometallic or bimetallic) active sites, crystal phase, ligands, solvents, and supports for inert bond activation under mild conditions remains a challenge. This Perspective summarizes the features of electronic structures, interfacial interactions, and their effects on molecular catalysis, metal-organic frameworks (MOFs), and natural mineral catalysis. The discovery of efficient catalysts could be promoted using machine-learning methods with high-performance descriptors. More attention should be paid to high-throughput quantum-chemical computations and experiments, automatic searches of chemical reaction pathways, and efficient machine-learning or deep-learning methods to accelerate catalyst design and synthesis in the future.

Proton-Promoted Nickel-Catalyzed Asymmetric Hydrogenation of Aliphatic Ketoacids

A robust and highly active homogeneous chiral nickel-phosphine complex for the asymmetric hydrogenation of aliphatic γ- and δ-ketoacids has been discovered. The hydrogenation could proceed smoothly in the presence of 0.0133 mol% catalyst loading (S/C=7500). The coordination chemistry and catalytic behavior of Ni(OTf)₂ with (S,S)-Ph-BPE were explored by ¹ H NMR and HRMS. The mechanistic studies revealed that a proton promoted the activation of the substrate C=O bond and controlled the stereoselectivity through hydrogen bonds. A series of chiral γ- and δ-alkyl substituted lactones were obtained in high yields with excellent enantioselectivities (up to 98 % yield and 99 % ee). In addition, this catalytic system also demonstrated that levulinic acid produced from a biomass feedstock was converted into chiral γ-valerolactone without loss of ee value.

Unraveling the Effect of Aromatic Groups in Mn(I)NNN Pincer Complexes on Carbon Dioxide Activation Using Density Functional Study

Carbon dioxide utilization is necessary to reduce carbon footprint and also to synthesize value-added chemicals. The transition metal pincer complexes are attractive catalysts for the hydrogenation of carbon dioxide to formic acid. There is a need to understand the factors affecting the catalytic performance of these pincer complexes through a structure-activity relationship study using computational methods. It is a well-established fact that aromatic functionalities offer stability and selectivity to transition metal catalysts. However, their impact on the performance of the catalysts is lesser known in the case of metal pincer complexes. Hence, it is necessary to investigate the catalytic performance of Mn(I)NNN pincer complexes with variably activated aromatic functionalities. In this context, 15 catalysts are designed by placing different types of aromatic rings at the pincer carbons and two terminal nitrogen of Mn(I)NNN pincer complexes. A benzene moiety, placed at C2-C3 carbons of Mn(I)NNN pincer complex with identical aromatic groups at the terminal nitrogen, is found to be most efficient toward CO₂ hydrogenation than the rest of the catalysts. On the other hand, when N,N-dimethyl aniline is placed at C2-C3 carbons of Mn(I)NNN pincer complexes, then the catalytic performance is significantly decreased. Thus, the present study unravels the impact of aromatic groups in Mn(I)NNN pincer complexes toward the catalytic hydrogenation of carbon dioxide.

C 1-Symmetric diphosphorus ligands in metal-catalyzed asymmetric hydrogenation to prepare chiral compounds

Asymmetric hydrogenation has remained an important and challenging research area in industry as well as academia due to its high atom economy and ability to induce chirality. Among several types of ligands, chiral bidentate phosphine ligands have played a pivotal role in developing asymmetric hydrogenation. Although C₂-symmetric chiral bidentate phosphine ligands have dominated the field, it has been found that several C₁-symmetric ligands are equally effective and, in many cases, have outperformed their C₂-symmetric counterparts. This review evaluates the possibility of the use of C₁-symmetric diphosphorus ligands in asymmetric hydrogenation to produce chiral compounds. The recent strategies and advances in the application of C₁-symmetric diphosphorus ligands in the metal-catalyzed asymmetric hydrogenation of a variety of CC bonds have been summarized. The potential of diphosphorus ligands in asymmetric hydrogenation to produce pharmaceutical intermediates, bioactive molecules, drug molecules, agrochemicals, and fragrances is discussed. Although asymmetric hydrogenation appears to be a problem that has been resolved, a deep dive into the recent literature reveals that there are several challenges that are yet to be addressed. The current asymmetric hydrogenation methods mostly employ precious metals, which are depleting at a fast pace. Therefore, scientific interventions to perform asymmetric hydrogenation using base metals or earth-abundant metals that can compete with established precious metals hold significant potential.

A phosphine-free Mn(I)-NNS catalyst for asymmetric transfer hydrogenation of acetophenone: a theoretical prediction

The density functional theory (DFT) method was employed to investigate the reaction mechanism of the hydrogen activation and asymmetric transfer hydrogenation (ATH) of acetophenone catalyzed by a well-defined phosphine-free Mn(I)-NNS complex. The calculation results indicate that the Mn-NNS complex has potential high catalytic hydrogenation activity. Meanwhile, the hydrogen transfer step of this reaction is proposed to be a concerted but asynchronous process, and the hydride transfer precedes proton transfer. This work also pointed out that the stereoselectivity of ATH catalyzed by the Mn(I)-NNS complex mainly originates from the noncovalent interaction between the substrate and the catalyst. Additionally, the catalytic activities of Mn-NNS complexes with different X ligands (X = CO, Cl, H, OMe, NCMe, CCMe, and CHCHMe) were compared, and the calculated total reaction energy barriers were all viable, which indicates that these Mn-NNS complexes show higher CO bond hydrogenation activity under mild conditions. This theoretical study predicts that the reactions catalyzed by complexes with H and NCMe ligands exhibit high stereoselectivity with enantiomeric excess (ee) values of 97% and 93%, respectively.

Trans Element-Hydrogen Bonds: A Distinctive Difference Between Transition Metals and Main Group Elements

The change in sign of the interaction force constant between element-hydrogen stretching modes of trans-dihydrides of the d block and p block elements is analyzed for the first time. As the transition metal M approaches group 12, the higher energy symmetric trans-H-M-H vibration ν_sym approaches the energy of the antisymmetric vibration ν_asym. Crossing to group 13 elements E, the trans-H-E-H vibration ν_sym increasingly drops below ν_asym. This reversal is attributed to the d orbital that participates in the H-M-H bonding but is nonbonding in the H-E-H compounds. DFT calculations are used to probe the energetics of isoelectronic triatomic [H-M-H]ⁿ⁺ and [H-E-H]^n- to reveal this trend and also to demonstrate that the magnitude of these interactions (ν_gap) increases down groups 11, 12, and 14 but remains fairly constant for group 13. They are also used to show that this reversal is seen in the transition state for hydride transfer to CO₂ from the model compounds trans-NiH₂(porphyrin) and trans-EH₂(porphyrin), E = Si and Ge in their singlet states.

Enantio- and Regioselective CuH-Catalyzed Conjugate Reduction of Yne-Allenones

A new asymmetric catalytic conjugate reduction of yne-allenones to synthesize enantioenriched cyclobuta[a]naphthalen-4(2H)-ones has been established that uses copper-bisphosphine complexes as catalysts and gives excellent regio- and enantioselectivities (≥99% ee) in most cases. This protocol tolerates a broad scope of substrates, exhibits high compatibility with various substituents, and gives excellent stereoselectivity, providing a catalytic and efficient entry to fabrication of synthetically important chiral 6-6-4 tricarbocyclic scaffolds.

Enantioselective direct, base-free hydrogenation of ketones by a manganese amido complex of a homochiral, unsymmetrical P–N–P′ ligand

Base-free direct hydrogenation of ketones using a Mn(PNP′)(CO)₂ complex is more enantioselective than that of a related base-activated iron complex.

Asymmetric hydrogenation catalyzed by first-row transition metal complexes

This review focuses on asymmetric direct and transfer hydrogenation with first-row transition metal complexes. The reaction mechanisms and the models of enantiomeric induction were summarized and emphasized.

Unraveling the catalytically preferential pathway between the direct and indirect hydrogenation of CO2 to CH3OH using N-heterocyclic carbene-based Mn(i) catalysts: a theoretical approach

The mechanistic investigation of direct vs. indirect CO₂ hydrogenation to methanol using single molecular NHC-based Mn(i) complexes.

A combined experimental and computational study to decipher complexity in the asymmetric hydrogenation of imines with Ru catalysts bearing atropisomerizable ligands

RuCl₂(P–OP)(N–N) complexes containing an atropisomerizable phosphine–phosphite and a chiral diamine are effective catalyst precursors for the asymmetric hydrogenation of N-aryl imines following an outer-sphere mechanism.

Organic synthesis with the most abundant transition metal–iron: from rust to multitasking catalysts

The promising aspects of iron in synthetic chemistry are being explored for three-four decades as a green and eco-friendly alternative to late transition metals. This present review unveils these rich iron-chemistry towards different transformations.

Facile H/D Exchange at (Hetero)Aromatic Hydrocarbons Catalyzed by a Stable Trans-Dihydride N-Heterocyclic Carbene (NHC) Iron Complex

Earth-abundant metal pincer complexes have played an important role in homogeneous catalysis during the last ten years. Yet, despite intense research efforts, the synthesis of iron PC_carbeneP pincer complexes has so far remained elusive. Here we report the synthesis of the first PC_NHCP functionalized iron complex [(PC_NHCP)FeCl₂] (1) and the reactivity of the corresponding trans-dihydride iron(II) dinitrogen complex [(PC_NHCP)Fe(H)₂N₂)] (2). Complex 2 is stable under an atmosphere of N₂ and is highly active for hydrogen isotope exchange at (hetero)aromatic hydrocarbons under mild conditions (50 °C, N₂). With benzene-d₆ as the deuterium source, easily reducible functional groups such as esters and amides are well tolerated, contributing to the overall wide substrate scope (e.g., halides, ethers, and amines). DFT studies suggest a complex assisted σ-bond metathesis pathway for C(sp²)–H bond activation, which is further discussed in this study.

Manganese-Catalyzed anti-Selective Asymmetric Hydrogenation of α-Substituted β-Ketoamides

A Mn-catalyzed diastereo- and enantioselective hydrogenation of α-substituted β-ketoamides has been realized for the first time under dynamic kinetic resolution conditions. anti-α-Substituted β-hydroxy amides, which are useful building blocks for the synthesis of bioactive molecules and chiral drugs, were prepared in high yields with excellent selectivity (up to >99 % dr and >99 % ee) and unprecedentedly high activity (TON up to 10000). The origin of the excellent stereoselectivity was clarified by DFT calculations.

A One-Step Preparation of Tetradentate Ligands with Nitrogen and Phosphorus Donors by Reductive Amination and Representative Iron Complexes

The synthesis and use of the first examples of unsymmetrical, mixed phosphine donor tripodal NPP₂' ligands N(CH₂CH₂PR₂)₂(CH₂CH₂PPh₂) are presented. The ligands are synthesized via a convenient, one pot reductive amination using 2-(diphenylphosphino)ethylamine and various substituted phosphonium dimers in order to introduce mixed phosphine donors substituted with P/P', those being Ph/Cy (2), Ph/ⁱPr (3), Ph/ⁱBu (4), Ph/o-Tol (5), and Ph/p-Tol (6). Additionally, we have developed the first known synthesis of a symmetrical tripodal NP₃ ligand N(CH₂CH₂PiBu₂)₃ using bench safe ammonium acetate as the lone nitrogen source (7). This new protocol eliminates the use of extremely dangerous nitrogen mustard reagents typically required to synthesize NP₃ ligands. Some of these tetradentate ligands and also P₂NN' ligands N(CH₂-o-C₅H₄N)(CH₂CH₂PR₂)₂ (P₂NN'-Cy, R = Cy; P₂NN'-Ph, R = Ph) prepared by reductive amination using 2-picolylamine are used in the synthesis and reactions of iron complexes. FeCl₂(P₂NN'-Cy) (8) undergoes single halide abstraction with NaBPh₄ to give the trigonal bipyramidal complex [FeCl(P₂NN'-Cy)][BPh₄] (9). Upon exposure to CO(g), complex 9 readily coordinates CO giving [FeCl(P₂NN'-Cy)(CO)][BPh₄] (10), and further treatment with an excess of NaBH₄ results in formation of the hydride complex [Fe(H)(P₂NN'-Cy)(CO)][BPh₄] (11). Our previously reported complex FeCl₂(P₂NN'-Ph) undergoes double halide abstraction with NaBPh₄ in the presence of the coordinating solvent to give [Fe(NCMe)₂(P₂NN'-Ph)][BPh₄]₂ (12). Ligand 3 can be coordinated to FeCl₂, and upon sequential halide abstraction, treatment with NaBH₄, and exposure to an atmosphere of dinitrogen, the dinitrogen hydride complex [Fe(H)(NPP₂'-ⁱPr)(N₂)][BPh₄] (13) is isolated. Our symmetrical NP₃ ligand 7 can also be coordinated to FeCl₂ and, upon exposure to an atmosphere of CO(g), selectively forms [FeCl(NP₃)(CO)][BPh₄] (14) after salt metathesis with NaBPh₄. Complex 14 can be treated with an excess of NaBH₄ to give the hydride complex [Fe(H)(NP₃)(CO)][BPh₄] (15), which can further be deprotonated/reduced to the Fe(0) complex Fe(NP₃)(CO) (16) upon treatment with an excess of KH.

Strategies and mechanisms of metal–ligand cooperativity in first-row transition metal complex catalysts

The use of metal–ligand cooperation (MLC) by transition metal bifunctional catalysts has emerged at the forefront of homogeneous catalysis science.

Unmasking the Ligand Effect in Manganese-Catalyzed Hydrogenation: Mechanistic Insight and Catalytic Application

Manganese-catalyzed hydrogenation reactions have attracted broad interest since the first report in 2016. Among the reported catalytic systems, Mn catalysts supported by tridentate PNP- and NNP-pincer ligands have most commonly been used. For example, a number of PNP-Mn pincer catalysts have been reported for the hydrogenation of aldehydes, aldimines, ketones, nitriles, and esters. Furthermore, various NNP-Mn pincer catalysts have been shown to be active in the hydrogenation of less-reactive substrates such as amides, carbonates, carbamates, and urea derivations. These observations indicated that Mn catalysts supported by NNP-pincer ligands exhibit higher reactivity in hydrogenation reactions than their PNP counterparts. Such a ligand effect in Mn-catalyzed hydrogenation reactions has yet to be confirmed. Herein, we investigated the origin and applicability of this ligand effect. A combination of experimental and theoretical investigations showed that NNP-pincer ligands on the Mn complexes were more electron-rich and less sterically hindered than their PNP counterparts, leading to higher reactivity in a series of Mn-catalyzed hydrogenation reactions. Inspired by the ligand effect on Mn-catalyzed hydrogenations, we developed the first Mn-catalyzed hydrogenation of N-heterocycles. Specifically, NNP-Mn pincer catalysts hydrogenated a series of N-heterocycles (32 examples) with up to 99% yields, and the corresponding PNP-Mn pincer catalysts afforded low reactivity under the same conditions. This verified that such a ligand effect is generally applicable in hydrogenation reactions of both carbonyl and noncarbonyl substrates based on Mn catalysis.

Cobalt-Catalyzed Asymmetric Hydrogenation of C=N Bonds Enabled by Assisted Coordination and Nonbonding Interactions

An efficient cobalt-catalyzed asymmetric hydrogenation of C=N bonds has been realized. Chiral hydrazines were obtained in high yields and with excellent enantioselectivities (95-98 % ee). The hydrogenation went smoothly at up to 2000 substrate/catalyst and on a gram scale. The success of this reaction relies on the presence of an NHBz group in the substrates, with the reactivity and enantioselectivity improved by an assisted coordination to the cobalt atom and a nonbonding interaction with the ligand. Furthermore, this reaction has practical applications for the synthesis of several useful chiral nitrogen-containing compounds.

Theoretical insights into CO2 hydrogenation to methanol by a Mn–PNP complex

Unravelling the role of an amide intermediate in co-catalyst-based sequential CO₂ hydrogenation reaction to methanol.

Physical insights into mechanistic processes in organometallic chemistry: an introduction

Mechanistic studies of late transition metal hydride homogeneous catalysts – 3d versus 4d metals.

PNN′ & P2NN′ ligands via reductive amination with phosphine aldehydes: synthesis and base-metal coordination chemistry

Phosphorus-donor “arms” are readily added to amines in order to enable sturdy base metal coordination.