Assessment of the Electronic Factors Determining the Thermodynamics of "Oxidative Addition" of C-H and N-H Bonds to Ir(I) Complexes

Electrocatalytic Transfer Hydrogenation of 1-Octene with [(tBuPCP)Ir(H)(Cl)] and Water

Electrocatalytic hydrogenation of 1-octene as non-activated model substrate with neutral water as H-donor is reported, using [(tBuPCP)Ir(H)(Cl)] (1) as the catalyst, to form octane with high faradaic efficiency (FE) of 96 % and a kobs of 87 s-1. Cyclic voltammetry with 1 revealed that two subsequent reductions trigger the elimination of Cl- and afford the highly reactive anionic Ir(I) hydride complex [(tBuPCP)Ir(H)]- (2), a previously merely proposed intermediate for which we now report first experimental data by mass spectrometry. In absence of alkene, the stoichiometric electrolysis of 1 in THF with water selectively affords the Ir(III) dihydride complex [(tBuPCP)Ir(H)2] (3) in 88 % FE from the reaction of 2 with H2O. Complex 3 then hydrogenates the alkene in classical fashion. The presented electro-hydrogenation works with extremely high FE, because the iridium hydrides are water stable, which prevents H2 formation. Even in strongly alkaline conditions (Bu4NOH added), the electro-hydrogenation of 1-octene with 1 also proceeds cleanly (89 % FE), suggesting a highly robust process that may rely on H2O activation, reminiscent to transfer hydrogenation pathways, instead of classical H+ reduction. DFT calculations confirmed oxidative addition of H2O as a key step in this context.

A Deeper Insight into the Supramolecular Activation of Oxidative Addition Reactions Involving Pincer-Rhodium(I) Complexes

The factors governing the acceleration of the oxidative addition of methyl iodide to pincer rhodium(I)-complexes induced by coronene have been computationally explored in detail using quantum chemical methods. Both the parent reaction and the coronene-mediated process proceed via a stepwise SN2-type mechanism. It is found that the acceleration of the process derives from the formation of an initial supramolecular complex, mainly stabilized by electrostatic and π-π interactions, which significantly increases the electron richness of the complex. The impact of this effect on the reaction barrier has been quantitatively analyzed by applying the activation strain model in combination with the energy decomposition analysis method. In addition, the influence of other polycyclic aromatic hydrocarbons on the oxidative reaction has been also considered.

Supramolecular Control of the Oxidative Addition as a Way To Improve the Catalytic Efficiency of Pincer-Rhodium (I) Complexes

1 H NMR studies using a cationic complex with a pyridine-di-imidazolylidene pincer ligand of formula [Rh(CNC)(CO)]+ revealed that this compound showed high binding affinity with coronene in CH2 Cl2 . The interaction between coronene and the planar RhI complex is established by means of π-stacking interactions. This interaction has a strong impact on the electron-donating strength of the pincer CNC ligand, which is increased significantly, as demonstrated by the shifting of the ν(CO) stretching bands to lower frequencies. The addition of coronene increases the reaction rate of the nucleophilic attack of methyl iodide on the rhodium (I) pincer complex, and also has a positive effect on the performance of the complex as a catalyst in the cycloisomerization of 4-pentynoic acid. These findings highlight the importance of supramolecular interactions for tuning the reactivity and catalytic activity of square-planar metal complexes.

Progression of Hydroamination Catalyzed by Late Transition-Metal Complexes from Activated to Unactivated Alkenes

ConspectusCatalytic intermolecular hydroamination of alkenes is an atom- and step-economical method for the synthesis of amines, which have important applications as pharmaceuticals, agrochemicals, catalysts, and materials. However, hydroaminations of alkenes in high yield with high selectivity are challenging to achieve because these reactions often lack a thermodynamic driving force and often are accompanied by side reactions, such as alkene isomerization, telomerization, and oxidative amination. Consequently, early examples of hydroamination were generally limited to the additions of N-H bonds to conjugated alkenes or strained alkenes, and the catalytic hydroamination of unactivated alkenes with late transition metals has only been disclosed recently. Many classes of catalysts, including early transition metals, late transition metals, rare-earth metals, acids, and photocatalysts, have been reported for catalytic hydroamination. Among them, late transition-metal complexes possess several advantages, including their relative ease of handling and their high compatibility of substrates containing polar or sensitive functional groups.This Account describes the progression in our laboratory of hydroaminations catalyzed by late transition-metal complexes from the initial additions of N-H bonds to activated alkenes to the more recent additions to unactivated alkenes. Our developments include the Markovnikov and anti-Markovnikov hydroamination of vinylarenes with palladium, rhodium, and ruthenium, the hydroamination of dienes and trienes with nickel and palladium, the hydroanimation of bicyclic strained alkenes with neutral iridium, and the hydroamination of unactivated terminal and internal alkenes with cationic iridium and ruthenium. Enantioselective hydroaminations of these classes of alkenes to form enantioenriched, chiral amines also have been developed.Mechanistic studies have elucidated the elementary steps and the turnover-limiting steps of these catalytic reactions. The hydroamination of conjugated alkenes catalyzed by palladium, rhodium, nickel, and ruthenium occurs by turnover-limiting nucleophilic attack of the amine on a coordinated benzyl, allyl, alkene, or arene ligand. On the other hand, the hydroamination of unconjugated alkenes catalyzed by ruthenium and iridium occurs by turnover-limiting migratory insertion of the alkene into a metal-nitrogen bond. In addition, pathways for the formation of side products, including isomeric alkenes and enamines, have been identified during our studies. During studies on enantioselective hydroamination, the reversibility of the hydroamination has been shown to erode the enantiopurity of the products. Based on our mechanistic understandings, new generations of catalysts that promote catalytic hydroaminations with higher rates, chemoselectivity, and enantioselectivity have been developed. We hope that our discoveries and mechanistic insights will facilitate the further development of catalysts that promote selective, practical, and efficient hydroamination of alkenes.

The Janus face of high trans-effect carbenes in olefin metathesis: gateway to both productivity and decomposition

Ruthenium–cyclic(alkyl)(amino)carbene (CAAC) catalysts, used at ppm levels, can enable dramatically higher productivities in olefin metathesis than their N-heterocyclic carbene (NHC) predecessors. A key reason is the reduced susceptibility of the metallacyclobutane (MCB) intermediate to decomposition via β-H elimination. The factors responsible for promoting or inhibiting β-H elimination are explored via density functional theory (DFT) calculations, in metathesis of ethylene or styrene (a representative 1-olefin) by Ru–CAAC and Ru–NHC catalysts. Natural bond orbital analysis of the frontier orbitals confirms the greater strength of the orbital interactions for the CAAC species, and the consequent increase in the carbene trans influence and trans effect. The higher trans effect of the CAAC ligands inhibits β-H elimination by destabilizing the transition state (TS) for decomposition, in which an agostic MCB C_β–H bond is positioned trans to the carbene. Unproductive cycling with ethylene is also curbed, because ethylene is trans to the carbene ligand in the square pyramidal TS for ethylene metathesis. In contrast, metathesis of styrene proceeds via a ‘late’ TS with approximately trigonal bipyramidal geometry, in which carbene trans effects are reduced. Importantly, however, the positive impact of a strong trans-effect ligand in limiting β-H elimination is offset by its potent accelerating effect on bimolecular coupling, a major competing means of catalyst decomposition. These two decomposition pathways, known for decades to limit productivity in olefin metathesis, are revealed as distinct, antinomic, responses to a single underlying phenomenon. Reconciling these opposing effects emerges as a clear priority for design of robust, high-performing catalysts.

In ruthenium catalysts for olefin metathesis, carbene ligands of high trans influence/effect suppress decomposition via β-H elimination, but increase susceptibility to bimolecular decomposition.

Synthesis and Reactivity of Cobalt-Dinitrogen Complexes Bearing Anionic PCP-Type Pincer Ligands toward Catalytic Silylamine Formation from Dinitrogen

A series of cobalt(I)-dinitrogen complexes bearing anionic 4-substituted benzene-based PCP-type pincer ligands are synthesized and characterized. These complexes work as highly efficient catalysts for the formation of silylamine from dinitrogen under ambient reaction conditions to produce up to 371 equiv of silylamine based on the cobalt atom of the catalyst.

The continuum of carbon-hydrogen (C-H) activation mechanisms and terminology

As a rapidly growing field across all areas of chemistry, C-H activation/functionalisation is being used to access a wide range of important molecular targets. Of particular interest is the development of a sustainable methodology for alkane functionalisation as a means for reducing hydrocarbon emissions. This Perspective aims to give an outline to the community with respect to commonly used terminology in C-H activation, as well as the mechanisms that are currently understood to operate for (cyclo)alkane activation/functionalisation.

Structure-Based Relative Energy Prediction Model: A Case Study of Pd(II)-Catalyzed Ethylene Polymerization and the Electronic Effect of Ancillary Ligands

Rapidly mapping a reaction energy profile to understand the reaction mechanism is of great importance and highly desired for the discovery of new chemical reactions. Herein, a combination of density functional theory (DFT) calculations and regression analysis has been applied to construct quantitative structures-based energy prediction models, considering Pd(II)-catalyzed ethylene polymerization as an example, for rapid construction of the reaction energy profile. It is inspiring that only geometrical parameters of the reaction center of one species are capable of predicting the whole energy profile with high accuracy. The reaction energies of ethylene insertion and β-H elimination, which directly correlate with polymerization activity and the possibility of branch formation, were studied to elucidate the electronic effects of ancillary ligands. Further analyses of these models from the statistical and chemical points of view afforded useful information on the design of the catalyst ligand. The current work is expected to methodologically shed new light on rapidly mapping the energy profile of chemical reactions and further provide useful information for the development of the reactions.

Oxidative Addition of Water, Alcohols, and Amines in Palladium Catalysis

The homolytic cleavage of O-H and N-H or weak C-H bonds is a key elementary step in redox catalysis, but is thought to be unfeasible for palladium. In stark contrast, reported here is the room temperature and reversible oxidative addition of water, isopropanol, hexafluoroisopropanol, phenol, and aniline to a palladium(0) complex with a cyclic (alkyl)(amino)carbene (CAAC) and a labile pyridino ligand, as is also the case in popular N-heterocyclic carbene (NHC) palladium(II) precatalysts. The oxidative addition of protic solvents or adventitious water switches the chemoselectivity in catalysis with alkynes through activation of the terminal C-H bond. Most salient, the homolytic activation of alcohols and amines allows atom-efficient, additive-free cross-coupling and transfer hydrogenation under mild reaction conditions with usually unreactive, yet desirable reagents, including esters and bis(pinacolato)diboron.

Mechanism of atom economical conversion of alcohols and amines to amides using Fe(ii) pincer catalyst. An outer-sphere metal-ligand pathway or an inner-sphere elimination pathway?

In this present theoretical study, we investigated the reaction mechanism of atom-economical amide formation from alcohols and amines mediated by iron(ii) hydride complex (^iPrPNP)Fe(H)(CO) (^iPrPNP = N[CH₂CH₂(PiPr₂)]₂) using state-of-the-art density functional theory. Two scenarios of mechanistic pathways were considered, the inner-sphere and the outer-sphere pathways. In former case, the reaction of encounter complex of formaldehyde with amine is the rate-determining step with ΔG_{298 K} = 33.75 kcal mol⁻¹ while as in latter case dehydrogenation from trans-hydride is the rate-determining step having ΔG_{298 K} = 21.34 kcal mol⁻¹. Both the mechanistic scenarios operate through stepwise ionic pathways. The assessment of computational results demonstrate that inner-sphere pathway is energetically demanding and thus rendering outer-sphere pathway to be the most plausible mechanism of amide formation. Ligand modifications reveal that electron-withdrawing groups like CF₃ near N of PNP ligand reduce the catalytic efficiency of the catalyst. Furthermore, changing the isopropyl moiety of phosphine scaffold with CH₃ has a minimal impact on catalytic activity of the catalyst. Overall, our computational results provide new insights for the design and development of new Fe(ii) based pincer catalysts for atom economical amide formation from alcohols and amines.

The schematic representation depicting the difference in inner and outer-sphere pathways for amide synthesis from alcohols and amines mediated by Fe(ii) hydride complex.

Triple hydrogen atom abstraction from Mn–NH3 complexes results in cyclophosphazenium cations

All three H atoms of the NH₃ ligand of [Mn(depe)₂(CO)(NH₃)]⁺ are abstracted by an organic radical, giving a rare cyclophosphazenium cation; computations suggest that insertion of NH_x into a Mn–P bond provides a strong thermodynamic driving force.

Iridium-Catalyzed Isomerization of N-Sulfonyl Aziridines to Allyl Amines

The Crabtree's reagent catalyzes the isomerization of N-sulfonyl 2,2-disubstituted aziridines to allyl amines. The selectivity of allyl amine vs imine is very high (up to 99/1). The unprecedented isomerization takes place in mild conditions without activation of the catalyst by hydrogen. The mechanism has been studied computationally by DFT calculations; instead of the usual hydrogenation of COD, the catalytic species is formed by a loss of the pyridine ligand. Approaching of aziridine to this unsaturated species leads to a carbocation intermediate through a low energy barrier. A metal-mediated tautomerization involving sequentially γ-H elimination and N-H reductive elimination affords selectively the allyl amine. The readiness of the CγH bond to participate in the H elimination step accounts for the selectivity toward the allyl amine product.

Understanding the reaction mechanism of the oxidative addition of ammonia by (PXP)Ir(i) complexes: the role of the X group

An analysis of the electronic rearrangements for the oxidative addition of ammonia to a set of five representative (PXP)Ir pincer complexes (X = B, CH, O, N, SiH) is performed. We aim to understand the factors controlling the activation and reaction energies of this process by combining different theoretical strategies based on DFT calculations. Interestingly, complexes featuring higher activation barriers yield more exothermic reactions. The analysis of the reaction path using the bonding evolution theory shows that the main chemical events, N-H bond cleavage and Ir-H bond formation, take place before the transition structure is reached. Metal oxidation implies an electron density transfer from non-shared Ir pairs to the Ir-N bond. This decrement in the atomic charge of the metal provokes different effects in the ionic contribution of the Ir-X bonding depending on the nature of the X atom as shown by the interacting quantum atoms methodology.

Methane Activation by Tantalum Carbide Cluster Anions Ta2C4. J Phys Chem Lett 2017;8:605-610. [PMID: 28088857 DOI: 10.1021/acs.jpclett.6b02568]

Methane activation by transition metals is of fundamental interest and practical importance, as this process is extensively involved in the natural gas conversion to fuels and value-added chemicals. While single-metal centers have been well recognized as active sites for methane activation, the active center composed of two or more metal atoms is rarely addressed and the detailed reaction mechanism remains unclear. Here, by using state-of-the-art time-of-flight mass spectrometry, cryogenic anion photoelectron imaging spectroscopy, and quantum-chemical calculations, the cooperation of the two Ta atoms in a dinuclear carbide cluster Ta₂C₄^- for methane activation has been identified. The C-H bond activation takes place predominantly around one Ta atom in the initial stage of the reaction and the second Ta atom accepts the delivered H atom from the C-H bond cleavage. The well-resolved vibrational spectra of the cryogenically cooled anions agree well with theoretical simulations, allowing the clear characterization of the structure of Ta₂C₄^- cluster. The reactivity comparison between Ta₂C₄^- cluster and the carbon-less analogues (Ta₂C₃^- and Ta₂C₂^-) demonstrated that the cooperative effect of the two metal atoms can be well tuned by the carbon ligands in terms of methane activation and transformation.

A family of rhodium and iridium complexes with semirigid benzylsilyl phosphines: from bidentate to tetradentate coordination modes

Rh and Ir coordination of phosphines with 1 to 3 Si–H groups.

Rational design of model Pd(ii)-catalysts for C–H activation involving ligands with charge-shift bonding characteristics

A palladium(ii) complex with a bis-2-borabicyclo[1.1.0]but-1(3)-ene ligand having charge-shift bonding characteristics contributes to better performance for C–H bond activation.

Theoretical prediction of the synthesis of 2,3-dihydropyridines through Ir(iii)-catalysed reaction of unsaturated oximes with alkenes

The Ir(iii)-catalysed reaction of unsaturated oximes with alkenes was predicted, and the results indicate a more efficient synthesis of 2,3-dihydropyridines.

UV-light promoted C–H bond activation of benzene and fluorobenzenes by an iridium(i) pincer complex

UV irradiation of [Ir(2,6-(P^tBu₂CH₂)₂C₆H₃)(CO)] generates the transient and reactive 14 VE Ir(i) fragment {Ir(2,6-(P^tBu₂CH₂)₂C₆H₃)}, which subsequently undergoes C–H bond activation of benzene and fluorobenzenes.

Mechanism of Rh-Catalyzed Oxidative Cyclizations: Closed versus Open Shell Pathways

A conceptual theory for analyzing and understanding oxidative addition reactions that form the cornerstone of many transition metal mediated catalytic cycles that activate C-C and C-H bonds, for example, was developed. The cleavage of the σ- or π-bond in the organic substrate can be envisioned to follow a closed or an open shell formalism, which is matched by a corresponding electronic structure at the metal center of the catalyst. Whereas the assignment of one or the other mechanistic scenario appears formal and equivalent at first sight, they should be recognized as different classes of reactions, because they lead to different reaction optimization and control strategies. The closed-shell mechanism involves heterolytic bond cleavages, which give rise to highly localized charges to form at the transition state. In the open-shell pathway, bonds are broken homolytically avoiding localized charges to accumulate on molecular fragments at the transition states. As a result, functional groups with inductive effects may exert a substantial influence on the energies of the intermediate and transition states, whereas no such effect is expected if the mechanism proceeds through the open-shell mechanism. If these functional groups are placed in a way that opens an electronic communication pathway to the molecular sites where charges accumulate, for example, using hyperconjugation, electron donating groups may stabilize a positive charge at that site. An instructive example is discussed, where this stereoelectronic effect allowed for rendering the oxidative addition diastereoselective. No such control is possible, however, when the open-shell reaction pathway is followed, because the inductive effects of functional groups have little to no effect on the stabilities of radical-like substrate states that are encountered when the bonds are broken in a homolytic fashion. Whether the closed-shell or open-shell mechanism for oxidative addition is followed is determined by the ordering of the d-orbital dominated frontier orbitals. If the highest occupied molecular orbital (HOMO) is oriented in space in such a way that will give the organic substrate easy access to the valence electron pair, the closed-shell mechanism can be followed. If the shape and orientation of the HOMO is not appropriate, however, an alternative pathway involving singlet excited states of the metal that will invoke the matching radicaloid cleavage of the organic substrate will dominate the oxidative addition. This novel paradigm for formally analyzing and understanding oxidative additions provides a new way of systematically understanding and planning catalytic reactions, as demonstrated by the in silico design of room-temperature Pauson-Khand reactions.

Experimental and computational study of alkane dehydrogenation catalyzed by a carbazolide-based rhodium PNP pincer complex

A rhodium complex based on the bis-phosphine carbazolide pincer ligand was investigated in the context of alkane dehydrogenation and in comparison with its iridium analogue. (carb-PNP)RhH2 was found to catalyze cyclooctane/t-butylethylene (COA/TBE) transfer dehydrogenation with a turnover frequency up to 10 min-1 and turnover numbers up to 340, in marked contrast with the inactive Ir analogue. TONs were limited by catalyst decomposition. Through a combination of mechanistic, experimental and computational (DFT) studies the difference between the Rh and Ir analogues was found to be attributable to the much greater accessibility of the 14-electron (carb-PNP)M(i) fragment in the case of Rh. In contrast, Ir is more strongly biased toward the M(iii) oxidation state. Thus (carb-PNP)RhH2 but not (carb-PNP)IrH2 can be dehydrogenated by sacrificial hydrogen acceptors, particularly TBE. The rate-limiting segment of the (carb-PNP)Rh-catalyzed COA/TBE transfer dehydrogenation cycle is found to be the dehydrogenation of COA. Within this segment, the rate-determining step is calculated to be (carb-PNP)Rh(cyclooctyl)(H) undergoing formation of a β-H agostic intermediate, while the reverse step (loss of a β-H agostic interaction) is rate-limiting for hydrogenation of the acceptors TBE and ethylene. Such a step has not previously been proposed as rate-limiting in the context of alkane dehydrogenation, nor, to our knowledge, has the reverse step been proposed as rate-limiting for olefin hydrogenation.

Cationic mono and dicarbonyl pincer complexes of rhodium and iridium to assess the donor properties of PCcarbeneP ligands

The donor properties of five different PC_carbeneP ligands are assessed by evaluation of the CO stretching frequencies in iridium(i) and rhodium(i) carbonyl cations.