First-Row Transition Metal (De)Hydrogenation Catalysis Based On Functional Pincer Ligands

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.

Comparing the Electronic Structure of Iron, Cobalt, and Nickel Compounds That Feature a Phosphine-Substituted Bis(imino)pyridine Chelate

It was recently discovered that (^Ph2PPrPDI)Mn (PDI = pyridine diimine) exists as a superposition of low-spin Mn(II) that is supported by a PDI dianion and intermediate-spin Mn(II) that is antiferromagnetically coupled to a triplet PDI dianion, a finding that encouraged the synthesis and electronic structure evaluation of late first row metal variants that feature the same chelate. The addition of ^Ph2PPrPDI to FeBr₂ resulted in bromide dissociation and the formation of [(^Ph2PPrPDI)FeBr][Br]. Reduction of this precursor using excess sodium amalgam afforded (^Ph2PPrPDI)Fe, which possesses an Fe(II) center that is supported by a dianionic PDI ligand. Similarly, reduction of a premixed solution of ^Ph2PPrPDI and CoCl₂ yielded the cobalt analog, (^Ph2PPrPDI)Co. EPR spectroscopy and density functional theory calculations revealed that this compound features a high-spin Co(I) center that is antiferromagnetically coupled to a PDI radical anion. The addition of ^Ph2PPrPDI to Ni(COD)₂ resulted in ligand displacement and the formation of (^Ph2PPrPDI)Ni, which was found to possess a pendent phosphine group. Single-crystal X-ray diffraction, CASSCF calculations, and EPR spectroscopy indicate that (^Ph2PPrPDI)Ni is best described as having a Ni(II)-PDI^2- configuration. The electronic differences between these compounds are highlighted, and a computational analysis of ^Ph2PPrPDI denticity has revealed the thermodynamic penalties associated with phosphine dissociation from 5-coordinate (^Ph2PPrPDI)Mn, (^Ph2PPrPDI)Fe, and (^Ph2PPrPDI)Co.

Fac-to-mer isomerization triggers hydride transfer from Mn(I) complex fac-[(dppm)Mn(CO)3H]

Low-temperature IR and NMR studies combined with DFT calculations revealed the mechanistic complexity of apparently simple reactions between Mn(I) complex fac-[(dppm)Mn(CO)₃H] and Lewis acids (LA = Ph₃C⁺, B(C₆F₅)₃) involving the formation of so-far elusive meridional hydride species mer-[(dppm)Mn(CO)₃H⋯LA] and unusual dearomatization of the Ph₃C⁺ cation upon hydride transfer.

Light-driven reduction of aromatic olefins in aqueous media catalysed by aminopyridine cobalt complexes

A catalytic system based on earth-abundant elements that efficiently hydrogenates aryl olefins using visible light as the driving-force and H₂O as the sole hydrogen atom source is reported. The catalytic system involves a robust and well-defined aminopyridine cobalt complex and a heteroleptic Cu photoredox catalyst. The system shows the reduction of styrene in aqueous media with a remarkable selectivity (>20 000) versus water reduction (WR). Reactivity and mechanistic studies support the formation of a [Co–H] intermediate, which reacts with the olefin via a hydrogen atom transfer (HAT). Synthetically useful deuterium-labelled compounds can be straightforwardly obtained by replacing H₂O with D₂O. Moreover, the dual photocatalytic system and the photocatalytic conditions can be rationally designed to tune the selectivity for aryl olefin vs. aryl ketone reduction; not only by changing the structural and electronic properties of the cobalt catalysts, but also by modifying the reduction properties of the photoredox catalyst.

A dual catalytic system based on earth-abundant elements reduces aryl olefins to alkanes in aqueous media under visible light. Mechanistic studies allow for rational tunning of the system for the selective reduction of aryl olefins vs ketones and vice versa.

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.

Sustainable catalysis with fluxional acridine-based PNP pincer complexes

Because of the widespread use of fossil fuels and the resulting global warming, development of sustainable catalytic transformations is now more important than ever to obtain our desired fuels and building materials with the least carbon footprint and waste production. Many sustainable (de)hydrogenation reactions, including CO2 reduction, H2 carrier systems, and others, have been reported using molecular pincer complexes. A specific subset of pincer complexes containing a central acridine donor with flanking CH2PR2 ligands, known as acridine-based PNP pincer complexes, exhibit special reactivities that are not imitable by other PNP pincer complexes such as pyridine-based or (R2PCH2CH2)2NH type ligands. The goal of this article is to highlight the unique reactivities of acridine-based complexes and then investigate how these reactivities allow these complexes to catalyse many sustainable reactions that traditional pincer complexes cannot catalyse. To that end, we will initially go over the synthesis and structural features of acridine complexes, such as the labile coordination of the central N donor and the observed fac-mer fluxionality. Following that, distinct reactivity patterns of acridine-based complexes including their reactivity with acids and water will be discussed. Finally, we will discuss the reaction systems that have been developed with acridine complexes thus far, including the notable selective transformations of primary alcohols to primary amines using ammonia, N-heteroaromatic synthesis from alcohols and ammonia, oxidation reactions with water with H2 liberation, development of H2 carrier systems, and others, and conclude the article with future possible directions. We hope that the systemic study presented here will aid researchers in developing further sustainable reactions based on the unique acridine-based pincer complexes.

Low-Valent Molybdenum PNP Pincer Complexes as Catalysts for the Semihydrogenation of Alkynes

Low-valent molybdenum PNP pincer complexes were studied as catalysts for the semihydrogenation of alkynes. For that purpose, tBu-substituted PNP complexes PNP^tBuMo(CO)₂ (6a) and PNP^tBuMo(CO)₃ (6c) and the NNP complex NNP^iPrMo(CO)₂(PPh₃) ((rac)-7) were synthesized and characterized. By utilizing the cyclohexyl-substituted complex PNP^CyMo(CO)₂(CH₃CN) (5a), several diphenylacetylene derivatives are transformed to the corresponding (Z)-alkenes with good to very good diastereoselectivities (up to 91:9). Mechanistic experiments indicate an outer-sphere mechanism including metal–ligand cooperativity.

Cobalt-N-Heterocyclic Carbene Complexes in Catalysis

Cobalt-NHC complexes have emerged as an attractive class of 3d transition metal catalysts for a broad range of chemical processes, including cross-coupling, hydrogenation, hydrofunctionalization and cycloaddition reactions. Herein, we present a comprehensive review of catalytic methods utilizing cobalt-NHC complexes with a focus on catalyst structure, the role of the NHC ligand, properties of the catalytic system, mechanism and synthetic utility. The survey clearly suggests that the recent emergence of well-defined cobalt-NHC catalysts may have a tremendous utility in the design and application of catalytic reactions using more abundant 3d transition metals.

Cooperative approaches in catalytic hydrogenation and dehydrogenation

Metal-ligand cooperativity (MLC) is an established strategy for developing effective hydrogenation and dehydrogenation catalysts. Metal-metal cooperativity (MMC) in bimetallic complexes is not as well understood, and to date has had limited implementation in (de)hydrogenation. Herein we use (de)hydrogenation processes as a platform to examine modes of cooperativity, with a particular focus on catalytic mechanisms. We investigate how lessons learnt from the extensive development of metal-ligand cooperative catalysts can aid the ongoing development of metal-metal cooperative catalysts.

Acceleration Effect of Bases on Mn Pincer Complex-Catalyzed CO2 Hydroboration

Herein, we report a comprehensive study of CO₂ hydroboration catalyzed by Mn pincer complexes. The traditional metal-ligand cooperation (MLC) mechanism based on the H-Mn-N-Bpin pincer complex is not viable due to the competing abstraction of the Bpin group from the H-Mn-N-Bpin complex by NaO^tBu. Instead, we propose an ionic mechanism based on the H-Mn-N-Na species with a low energy span (22.5 kcal/mol) and unveil the acceleration effect of bases. The X groups in the H-Mn-N-X catalyst models are further modulated, and the steric hindrance and H→B donor-acceptor interactions of the X group increase the energy barrier of the hydride transfer. The hydrogen bond and electrostatic interactions of the X group can accelerate the hydride transfer to HCOOBpin and HCHO molecules except for the nonpolar CO₂ molecule. Based on these discoveries, we designed a pyridine-based Mn pincer catalyst system, which could achieve CO₂ hydroboration in low-temperature and base-free conditions through a metal-ligand cooperation mechanism.

E-Selective Manganese-Catalyzed Semihydrogenation of Alkynes with H2 Directly Employed or In Situ-Generated

Selective semihydrogenation of alkynes with the Mn(I) alkyl catalyst fac-[Mn(dippe)(CO)₃(CH₂CH₂CH₃)] (dippe = 1,2-bis(di-iso-propylphosphino)ethane) as a precatalyst is described. The required hydrogen gas is either directly employed or in situ-generated upon alcoholysis of KBH₄ with methanol. A series of aryl-aryl, aryl-alkyl, alkyl-alkyl, and terminal alkynes was readily hydrogenated to yield E-alkenes in good to excellent isolated yields. The reaction proceeds at 60 °C for directly employed hydrogen or at 60-90 °C with in situ-generated hydrogen and catalyst loadings of 0.5-2 mol %. The implemented protocol tolerates a variety of electron-donating and electron-withdrawing functional groups, including halides, phenols, nitriles, unprotected amines, and heterocycles. The reaction can be upscaled to the gram scale. Mechanistic investigations, including deuterium-labeling studies and density functional theory (DFT) calculations, were undertaken to provide a reasonable reaction mechanism, showing that initially formed Z-isomer undergoes fast isomerization to afford the thermodynamically more stable E-isomer.

Combining metal-metal cooperativity, metal-ligand cooperativity and chemical non-innocence in diiron carbonyl complexes

Several metalloenzymes, including [FeFe]-hydrogenase, employ cofactors wherein multiple metal atoms work together with surrounding ligands that mediate heterolytic and concerted proton-electron transfer (CPET) bond activation steps. Herein, we report a new dinucleating PNNP expanded pincer ligand, which can bind two low-valent iron atoms in close proximity to enable metal-metal cooperativity (MMC). In addition, reversible partial dearomatization of the ligand's naphthyridine core enables both heterolytic metal-ligand cooperativity (MLC) and chemical non-innocence through CPET steps. Thermochemical and computational studies show how a change in ligand binding mode can lower the bond dissociation free energy of ligand C(sp3)-H bonds by ∼25 kcal mol-1. H-atom abstraction enabled trapping of an unstable intermediate, which undergoes facile loss of two carbonyl ligands to form an unusual paramagnetic (S = ) complex containing a mixed-valent iron(0)-iron(i) core bound within a partially dearomatized PNNP ligand. Finally, cyclic voltammetry experiments showed that these diiron complexes show catalytic activity for the electrochemical hydrogen evolution reaction. This work presents the first example of a ligand system that enables MMC, heterolytic MLC and chemical non-innocence, thereby providing important insights and opportunities for the development of bimetallic systems that exploit these features to enable new (catalytic) reactivity.

ChemSpaX: exploration of chemical space by automated functionalization of molecular scaffold

Exploration of the local chemical space of molecular scaffolds by post-functionalization (PF) is a promising route to discover novel molecules with desired structure and function. PF with rationally chosen substituents based on known electronic and steric properties is a commonly used experimental and computational strategy in screening, design and optimization of catalytic scaffolds. Automated generation of reasonably accurate geometric representations of post-functionalized molecular scaffolds is highly desirable for data-driven applications. However, automated PF of transition metal (TM) complexes remains challenging. In this work a Python-based workflow, ChemSpaX, that is aimed at automating the PF of a given molecular scaffold with special emphasis on TM complexes, is introduced. In three representative applications of ChemSpaX by comparing with DFT and DFT-B calculations, we show that the generated structures have a reasonable quality for use in computational screening applications. Furthermore, we show that ChemSpaX generated geometries can be used in machine learning applications to accurately predict DFT computed HOMO–LUMO gaps for transition metal complexes. ChemSpaX is open-source and aims to bolster and democratize the efforts of the scientific community towards data-driven chemical discovery.

This work introduces ChemSpaX, an open-source Python-based tool for automated exploration of chemical space of molecular scaffolds with a special focus on transition-metal complexes.

Evaluation of Manganese Catalysts for the Hydrogenative Deconstruction of Commercial and End-of-Life Polyurethane Samples

Polyurethane (PU) is a thermoset plastic that is found in everyday objects, such as mattresses and shoes, but also in more sophisticated materials, including windmills and airplanes, and as insulation materials in refrigerators and buildings. Because of extensive inter-cross linkages in PU, current recycling methods are somewhat lacking. In this work, the effective catalytic hydrogenation of PU materials is carried out by applying a catalyst based on the earth-abundant metal manganese, to give amine and polyol fractions, which represent the original monomeric composition. In particular, Mn-^Ph MACHO is found to catalytically deconstruct flexible foam, molded foams, insulation, and end-of-life materials at 1 wt.% catalyst loading by applying a reaction temperature of 180 °C, 50 bar of H₂ , and 0.9 wt.% of KOH in isopropyl alcohol. The protocol is showcased in the catalytic deconstruction of 2 g of mattress foam using only 0.13 wt.% catalyst, resulting in 90 % weight recovery and a turnover number of 905.

Metal-ligand cooperative approaches in homogeneous catalysis using transition metal complex catalysts of redox noninnocent ligands

Catalysis offers a straightforward route to prepare various value-added molecules starting from readily available raw materials. The catalytic reactions mostly involve multi-electron transformations. Hence, compared to the inexpensive and readily available 3d-metals, the 4d and 5d-transition metals get an extra advantage for performing multi-electron catalytic reactions as the heavier transition metals prefer two-electron redox events. However, for sustainable development, these expensive and scarce heavy metal-based catalysts need to be replaced by inexpensive, environmentally benign, and economically affordable 3d-metal catalysts. In this regard, a metal-ligand cooperative approach involving transition metal complexes of redox noninnocent ligands offers an attractive alternative. The synergistic participation of redox-active ligands during electron transfer events allows multi-electron transformations using 3d-metal catalysts and allows interesting chemical transformations using 4d and 5d-metals as well. Herein we summarize an up-to-date literature report on the metal-ligand cooperative approaches using transition metal complexes of redox noninnocent ligands as catalysts for a few selected types of catalytic reactions.

Well-defined manganese complex catalyzed dehydrogenative synthesis of quinazolin-4(3H)-ones and 3,4-dihydro-2H-1,2,4-benzothiadiazine 1,1-dioxides

The synthesis of N-heterocycles has been considered an emerging topic of chemical research due to its widespread usage in medicinal chemistry, materials science, and natural product synthesis.

Iron-catalysed quinoline synthesis via acceptorless dehydrogenative coupling

An iron-catalysed atom-economical and straightforward methodology for the synthesis of quinolines from α-2-aminoaryl alcohols and secondary alcohols is presented.

Facile synthesis of amides via acceptorless dehydrogenative coupling of aryl epoxides and amines

The synthesis of amides is significant in a wide variety of academic and industrial fields. We report here a new reaction, namely acceptorless dehydrogenative coupling of epoxides and amines to form amides catalyzed by ruthenium pincer complexes. Various aryl epoxides and amines smoothly convert into the desired amides in high yields with the generation of H₂ gas as the only byproduct. Control experiments indicate that amides are generated kinetically faster than side products, possibly because of the facile activation of epoxides by metal–ligand cooperation, as supported by the observation of a ruthenium-enolate species. No alcohol or free aldehyde are involved. A mechanism is proposed involving a dual role of the catalyst, which is responsible for the high yield and selectivity of the new reaction.

We report the ruthenium pincer complex catalyzed acceptorless dehydrogenative coupling of epoxides and amines to form amides. The reaction offers a facile and atom economical two-step strategy for transforming alkenes into amides.

Copper-catalyzed transfer methylenation via C(sp3)–C(sp3) bond cleavage of alcohols

Transfer Methylenation: A copper-catalyzed transfer methylenation via the cleavage of unstrained C(sp3)-C(sp3) bonds is developted. This is a de novo report for transfer hydrocarbylation between alcohols and carbonyl compounds.

Ultrafine PdCo bimetallic nanoclusters confined in N-doped porous carbon for the efficient semi-hydrogenation of alkynes

Ultrafine PdCo bimetallic nanoclusters with Co atom-modified Pd active sites were highly dispersed and confined in an m-NC material for selective semi-hydrogenation of alkynes to alkenes.

Sustainable synthesis of pyrazoles using alcohols as the primary feedstock by an iron catalyzed tandem C–C and C–N coupling approach

We report two new efficient iron-catalyzed synthetic strategies for multicomponent synthesis of tri-substituted pyrazoles using biomass-derived alcohols as the primary feedstock.

Redox-Neutral Dehydrogenative Cross-Coupling of Alcohols and Amines Enabled by Nickel Catalysis

Presented herein is a facile and straightforward synthetic method for the construction of amides via Ni/NHC-catalyzed amidation of alcohols with amines. The strategy exhibits various advantages over existing methods, including...

A general and practical bifunctional cobalt catalytic system for N-heterocycle assembly via acceptorless dehydrogenation

A novel and highly-efficient N-heterocycle assembly methodology catalyzed by a cobalt-N,N-bidentate complex via acceptorless dehydrogenation coupling of alcohols and amines has been established.

Steric and electronic effects in latent S-chelated olefin metathesis catalysts

In this work, the structure, latency, and activity of twelve sulfur-chelated ruthenium precatalysts were studied by systematically varying their ligand shell.

Spin-state crossover in photo-catalyzed nitrile dihydroboration via Mn-thiolate cooperation

The role of a phosphine-free SNS-pincer ligand in metal–ligand cooperative hydroboration catalysis was investigated. The bifunctional thiolate donor and spin-state change to high-spin Mn are crucial to accessing low-energy activation barriers.

Synthesis and reactivity of titanium ‘POCOP’ pincer complexes

Titanium ‘POCOP’ complexes have been made, and their ability to support further reactivity investigated, giving a rare isolable titanium chlorohydride.

Ligand-assisted nickel catalysis enabling sp3 C–H alkylation of 9H-fluorene with alcohols

A nickel catalysed chemoselective sp3 C–H alkylation of 9H-fluorene with alcohols is reported which follows a radical pathway employing the borrowing hydrogen route.

Ruthenium pincer complex catalyzed efficient synthesis of quinoline, 2-styrylquinoline and quinazoline derivatives via acceptorless dehydrogenative coupling reactions

The synthesis of N-heterocycles has been considered an emerging area of chemical research due to their extensive utilization in pharmaceuticals, materials science, and natural product synthesis.

Room temperature Z-selective hydrogenation of alkynes by hemilabile and non-innocent (NNN)Co(ii) catalysts

Room temperature chemo- and stereoselective hydrogenation of alkynes is described using a well-defined and phosphine-free hemilabile cobalt catalyst.

Cooperative B-H bond activation: Dual sites borane activation by redox active κ2-N,S-chelated complexes

Cooperative dual site activation of boranes by redox-active 1,3-N,S-chelated ruthenium species, mer-[PR₃{κ²-N,S-(L)}₂Ru{κ¹-S-(L)}], (mer-2a: R = Cy, mer-2b: R = Ph; L = NC₇H₄S₂), generated from the aerial oxidation of borate complexes, [PR₃{κ²-N,S-(L)}Ru{κ³-H,S,S′-BH₂(L)₂}] (trans–mer-1a: R = Cy, trans–mer-1b: R = Ph; L = NC₇H₄S₂), has been investigated. Utilizing the rich electronic behaviour of these 1,3-N,S-chelated ruthenium species, we have established that a combination of redox-active ligands and metal–ligand cooperativity has a big influence on the multisite borane activation. For example, treatment of mer-2a–b with BH₃·THF led to the isolation of fac-[PR₃Ru{κ³-H,S,S′-(NH₂BSBH₂N)(S₂C₇H₄)₂}] (fac-3a: R = Cy and fac-3b: R = Ph) that captured boranes at both sites of the κ²-N,S-chelated ruthenacycles. The core structure of fac-3a and fac-3b consists of two five-membered ruthenacycles [RuBNCS] which are fused by one butterfly moiety [RuB₂S]. Analogous fac-3c, [PPh₃Ru{κ³-H,S,S′-(NH₂BSBH₂N)(SC₅H₄)₂}], can also be synthesized from the reaction of BH₃·THF with [PPh₃{κ²-N,S-(SNC₅H₄)}{κ³-H,S,S′-BH₂(SNH₄C₅)₂}Ru], cis–fac-1c. In stark contrast, when mer-2b was treated with BH₂Mes (Mes = 2,4,6-trimethyl phenyl) it led to the formation of trans- and cis-bis(dihydroborate) complexes [{κ³-S,H,H-(NH₂BMes)Ru(S₂C₇H₄)}₂], (trans-4 and cis-4). Both the complexes have two five-membered [Ru–(H)₂–B–NCS] ruthenacycles with κ²-H–H coordination modes. Density functional theory (DFT) calculations suggest that the activation of boranes across the dual Ru–N site is more facile than the Ru–S one.

Redox-active ruthenium complexes supported by hemilabile κ²-N,S-chelated ruthenacycles undergo unusual dual site B–H bond activation through metal–ligand cooperation with free and bulky boranes.

Control of Catalyst Isomers Using an N-Phenyl-Substituted RN(CH2CH2PiPr2)2 Pincer Ligand in CO2 Hydrogenation and Formic Acid Dehydrogenation

A novel pincer ligand, ^iPrPN^PhP [PhN(CH₂CH₂PⁱPr₂)₂], which is an analogue of the versatile MACHO ligand, ^iPrPN^HP [HN(CH₂CH₂PⁱPr₂)₂], was synthesized and characterized. The ligand was coordinated to ruthenium, and a series of hydride-containing complexes were isolated and characterized by NMR and IR spectroscopies, as well as X-ray diffraction. Comparisons to previously published analogues ligated by ^iPrPN^HP and ^iPrPN^MeP [CH₃N(CH₂CH₂PⁱPr₂)₂] illustrate that there are large changes in the coordination chemistry that occur when the nitrogen substituent of the pincer ligand is altered. For example, ruthenium hydrides supported by the ^iPrPN^PhP ligand always form the syn isomer (where syn/anti refer to the relative orientation of the group on nitrogen and the hydride ligand on ruthenium), whereas complexes supported by ^iPrPN^HP form the anti isomer and complexes supported by ^iPrPN^MeP form a mixture of syn and anti isomers. We evaluated the impact of the nitrogen substituent of the pincer ligand in catalysis by comparing a series of ^iPrPN^RP (R = H, Me, Ph)-ligated ruthenium hydride complexes as catalysts for formic acid dehydrogenation and carbon dioxide (CO₂) hydrogenation to formate. The ^iPrPN^PhP-ligated species is the most active for formic acid dehydrogenation, and mechanistic studies suggest that this is likely because there are kinetic advantages for catalysts that operate via the syn isomer. In CO₂ hydrogenation, the ^iPrPN^PhP-ligated species is again the most active under our optimal conditions, and we report some of the highest turnover frequencies for homogeneous catalysts. Experimental and theoretical insights into the turnover-limiting step of catalysis provide a basis for the observed trends in catalytic activity. Additionally, the stability of our complexes enabled us to detect a previously unobserved autocatalytic effect involving the base that is added to drive the reaction. Overall, by modifying the nitrogen substituent on the MACHO ligand, we have developed highly active catalysts for formic acid dehydrogenation and CO₂ hydrogenation and also provided a framework for future catalyst development.

Controlled Synthesis of Well-Defined Polyaminoboranes on Scale Using a Robust and Efficient Catalyst

The air tolerant precatalyst, [Rh(L)(NBD)]Cl ([1]Cl) [L = κ³-(ⁱPr₂PCH₂CH₂)₂NH, NBD = norbornadiene], mediates the selective synthesis of N-methylpolyaminoborane, (H₂BNMeH)_n, by dehydropolymerization of H₃B·NMeH₂. Kinetic, speciation, and DFT studies show an induction period in which the active catalyst, Rh(L)H₃ (3), forms, which sits as an outer-sphere adduct 3·H₃BNMeH₂ as the resting state. At the end of catalysis, dormant Rh(L)H₂Cl (2) is formed. Reaction of 2 with H₃B·NMeH₂ returns 3, alongside the proposed formation of boronium [H₂B(NMeH₂)₂]Cl. Aided by isotopic labeling, Eyring analysis, and DFT calculations, a mechanism is proposed in which the cooperative "PNHP" ligand templates dehydrogenation, releasing H₂B═NMeH (ΔG^‡_calc = 19.6 kcal mol^-1). H₂B═NMeH is proposed to undergo rapid, low barrier, head-to-tail chain propagation for which 3 is the catalyst/initiator. A high molecular weight polymer is formed that is relatively insensitive to catalyst loading (M_n ∼71 000 g mol^-1; Đ, of ∼ 1.6). The molecular weight can be controlled using [H₂B(NMe₂H)₂]Cl as a chain transfer agent, M_n = 37 900-78 100 g mol^-1. This polymerization is suggested to arise from an ensemble of processes (catalyst speciation, dehydrogenation, propagation, chain transfer) that are geared around the concentration of H₃B·NMeH₂. TGA and DSC thermal analysis of polymer produced on scale (10 g, 0.01 mol % [1]Cl) show a processing window that allows for melt extrusion of polyaminoborane strands, as well as hot pressing, drop casting, and electrospray deposition. By variation of conditions in the latter, smooth or porous microstructured films or spherical polyaminoboranes beads (∼100 nm) result.

Computational Studies on the Mechanisms for Deaminative Amide Hydrogenation by Homogeneous Bifunctional Catalysts

The deaminative hydrogenation of amides is one of the most convenient pathways for the synthesis of amines and alcohols. The ideal source of reducing equivalents for this reaction is molecular hydrogen, though, in practice, this approach requires high pressures and temperatures, with many catalysts achieving only small turnover numbers and frequencies. Nonetheless, during the last ten years, this field has made major advances towards larger turnovers under milder conditions thanks to the development of bifunctional catalysts. These systems promote the heterolytic cleavage of hydrogen into proton and hydride by combining a basic ligand with an acidic metal centre. The present review focuses on the computational study of the reaction mechanism underlying bifunctional catalysis. This review is structured around the fundamental steps of this mechanism, namely the C=O and C–N hydrogenation of the amide, the C–N protonolysis of the hemiaminal, the C=O hydrogenation of the aldehyde, and the competition between hydrogen activation and catalyst deactivation. In line with the complexity of the mechanism, we also provide a perspective on the use of microkinetic models. Both Noyori- and Milstein-type catalysts are discussed and compared.

Chemically robust and readily available quinoline-based PNN iron complexes: application in C-H borylation of arenes

Iron catalysts have been used for over a century to produce ammonia industrially. However, the use of iron catalysts generally remained quite limited until relatively recently, when the abundance and low toxicity of iron spurred the development of a variety of iron catalysts. Despite the fact that iron catalysts are being developed as alternatives to precious metal catalysts, their reactivities and stabilities are quite different because of their unique electronic structures. In this context, our group previously developed a new family of quinoline-based PNN pincer-type ligands for low- to mid-valent iron catalysts. These chemically robust PNN ligands provide air- and moisture-tolerant iron complexes, which exhibit excellent catalytic performances in the C-H borylation of arenes. This feature article summarises our recent work on PNN iron complexes, including their conception and design, as well as related reports on iron pincer complexes and iron-catalysed C-H borylation reactions.

Manganese(I)-Catalyzed H-P Bond Activation via Metal-Ligand Cooperation

Here we report that chiral Mn(I) complexes are capable of H–P bond activation. This activation mode enables a general method for the hydrophosphination of internal and terminal α,β-unsaturated nitriles. Metal−ligand cooperation, a strategy previously not considered for catalytic H–P bond activation, is at the base of the mechanistic action of the Mn(I)-based catalyst. Our computational studies support a stepwise mechanism for the hydrophosphination and provide insight into the origin of the enantioselectivity.