Efficient Catalyst for Acceptorless Alcohol Dehydrogenation: Interplay of Theoretical and Experimental Studies

Development of ruthenium complexes with S-donor ligands for application in synthesis, catalytic acceptorless alcohol dehydrogenation and crossed-aldol condensation

The reaction of [Ru(dmso)4Cl2] with a potassium salt of four xanthate (RO-C(S)S-; R = Me, Et, iPr and tBu) ligands (depicted as Ln; n = 1-4) in hot methanol afforded a group of mixed-ligand complexes of type [Ru(Ln)2(dmso)2]. The crystal structures of all the four complexes have been determined, which show that the xanthate ligands are bound to the metal center forming four-membered chelates and dmso is coordinated through sulfur and they are mutually cis. The relative thermodynamic stability of this cis and the other possible trans-isomers of these complexes has been assessed with the help of DFT calculations, which have revealed that the cis-isomer is the more stable isomer. The coordinated dmso in the [Ru(Ln)2(dmso)2] complexes could be easily displaced by chelating bidentate ligands (depicted as L') to furnish complexes of type [Ru(Ln)2(L')], as demonstrated through isolation of two such complexes, viz. [Ru(L3)2(bpy)] and [Ru(L2)2(phen)] (bpy = 2,2'-bipyridine and phen = 1,10-phenanthroline). The crystal structure of [Ru(L3)2(bpy)] has been determined and the structure of [Ru(L2)2(phen)] has been optimized by the DFT method. The electronic spectra of the four [Ru(Ln)2(dmso)2] complexes and the two derivatives ([Ru(Ln)2(L')]; n = 3, L' = bpy; n = 2, L' = phen), recorded in dichloromethane solutions, show intense absorptions spanning the visible and ultraviolet regions, which have been analyzed by the TDDFT method. The [Ru(Ln)2(dmso)2] complexes are found to serve as efficient catalyst precursors for the acceptorless dehydrogenation of 2-propanol followed by crossed-aldol condensation with substituted benzaldehydes (and related aldehydes), using tert-butoxide as the co-catalyst, producing dibenzylideneacetone derivatives in good yields.

Mechanistic Study of B(C6F5)3-Catalyzed Transfer Hydrogenation of Aldehydes/Ketones with PhSiH3 and Stoichiometric Water

A DFT study was performed on the mechanisms of B(C6F5)3-catalyzed transfer hydrogenation of aldehydes/ketones, using PhSiH3 and stoichiometric water. Path B2 includes a stepwise Piers SN2-Si process, H- transfer, and hydrolysis desilylation of siloxane, in which the hydrolysis desilylation step is rate-determining. Path C1 is first determined, involving a B(C6F5)3-catalyzed concerted addition step of 2H2O to carbonyl generating R1R2C(OH)2, a subsequent SN2-Si dehydroxylation step of R1R2C(OH)2 giving R1R2C=OH+ and (C6F5)3B-H-, and final H- transfer producing the respective alcohol R1R2CHOH. A B(C6F5)3-catalyzed H2 generation process (Path H0) is determined. Path B2 is the only mechanism for the stepwise method. Using a one-time one-pot feeding method, alkyl/aryl aldehydes, dialkyl ketones, and alkyl aryl ketones (1a-g) can be reduced into alcohols chemoselectively and effectively at room temperature. More than 1 equiv of water over substrates is necessary. Herein, Path C1 is the dominant transfer hydrogenation pathway, and the H2 generation is efficiently inhibited, by the competitive advantage of Path C1 and initial dominant existence of the complexes IM0 and IM1-x. The diaryl ketones (1h,1i) cannot be efficiently reduced into the respective alcohols using the one-time feeding one-pot method. The barriers of C-TS1-h/i are obviously higher than those of C-TS1-a-g, attributed to the electron-donating and space effects of the two aryls on carbonyl C. The possible Paths B2 and C1 of transfer hydrogenation have no competitive advantage with Path H0. The DFT results are consistent with the experiments.

Exploring Hydrogen Sources in Catalytic Transfer Hydrogenation: A Review of Unsaturated Compound Reduction

Catalytic transfer hydrogenation has emerged as a pivotal chemical process with transformative potential in various industries. This review highlights the significance of catalytic transfer hydrogenation, a reaction that facilitates the transfer of hydrogen from one molecule to another, using a distinct molecule as the hydrogen source in the presence of a catalyst. Unlike conventional direct hydrogenation, catalytic transfer hydrogenation offers numerous advantages, such as enhanced safety, cost-effective hydrogen donors, byproduct recyclability, catalyst accessibility, and the potential for catalytic asymmetric transfer hydrogenation, particularly with chiral ligands. Moreover, the diverse range of hydrogen donor molecules utilized in this reaction have been explored, shedding light on their unique properties and their impact on catalytic systems and the mechanism elucidation of some reactions. Alcohols such as methanol and isopropanol are prominent hydrogen donors, demonstrating remarkable efficacy in various reductions. Formic acid offers irreversible hydrogenation, preventing the occurrence of reverse reactions, and is extensively utilized in chiral compound synthesis. Unconventional donors such as 1,4-cyclohexadiene and glycerol have shown a good efficiency in reducing unsaturated compounds, with glycerol additionally serving as a green solvent in some transformations. The compatibility of these donors with various catalysts, substrates, and reaction conditions were all discussed. Furthermore, this paper outlines future trends which include the utilization of biomass-derived hydrogen donors, the exploration of hydrogen storage materials such as metal-organic frameworks (MOFs), catalyst development for enhanced activity and recyclability, and the utilization of eco-friendly solvents such as glycerol and ionic liquids. Innovative heating methods, diverse base materials, and continued research into catalyst-hydrogen donor interactions are aimed to shape the future of catalytic transfer hydrogenation, enhancing its selectivity and efficiency across various industries and applications.

On the mechanism of acceptorless dehydrogenation of N-heterocycles catalyzed by tBuOK: a computational study

The catalytic acceptorless dehydrogenation (ADH) of saturated N-heterocycles has recently gained considerable attention as a promising strategy for hydrogen release from liquid organic hydrogen carriers (LOHCs). Recently, a simple ^tBuOK base-promoted ADH of N-heterocycles was developed by Yu et al. (Adv. Synth. Catal. 2019, 361, 3958). However, it is still open as to how the ^tBuOK plays a catalytic role in the ADH process. Herein, our density functional study reveals that the ^tBuOK catalyzes the ADH of 1,2,3,4-tetrahydroquinoline (THQ) through a quasi-metal-ligand bifunctional catalytic channel or a base-catalyzed pathway with close energy barriers. The hydride transfer in the first dehydrogenation process is determined to be the rate determining step, and the second dehydrogenation can proceed directly from 34DHQ regulated by the ^tBuOK. In addition, the computational results show that the cooperation of a suitable alkali metal ion with the ^tBuO^- group is so critical that the ^tBuOLi and the isolated ^tBuO^- are both inferior to ^tBuOK as a dehydrogenation catalyst.

Catalytic Acceptorless Dehydrogenation (CAD) of Secondary Benzylic Alcohols into Value-Added Ketones Using Pd(II)-NHC Complexes

For the creation of adaptable carbonyl compounds in organic synthesis, the oxidation of alcohols is a crucial step. As a sustainable alternative to the harmful traditional oxidation processes, transition-metal catalysts have recently attracted a lot of interest in acceptorless dehydrogenation reactions of alcohols. Here, using well-defined, air-stable palladium(II)-NHC catalysts (A-F), we demonstrate an effective method for the catalytic acceptorless dehydrogenation (CAD) reaction of secondary benzylic alcohols to produce the corresponding ketones and molecular hydrogen (H2). Catalytic acceptorless dehydrogenation (CAD) has been successfully used to convert a variety of alcohols, including electron-rich/electron-poor aromatic secondary alcohols, heteroaromatic secondary alcohols, and aliphatic cyclic alcohols, into their corresponding value-added ketones while only releasing molecular hydrogen as a byproduct.

Catalytic Mechanisms of Transfer Hydrogenation of Azobenzene with Ammonia Borane by Pincer Bismuth Complex: Crucial Role of C=N Functional Group on the Pincer Ligand

Transfer hydrogenation of azobenzene with ammonia borane mediated by pincer bismuth complex 1 was systematically investigated through density functional theory calculations. An unusual metal-ligand cooperation mechanism was disclosed, in which the saturation/regeneration of the C=N functional group on the pincer ligand plays an essential role. The reaction is initiated by the hydrogenation of the C=N bond (saturation) with ammonia borane to afford 3^CN , which is the rate-determining step with Gibbs energy barrier (ΔG^≠ ) and Gibbs reaction energy (ΔG) of 25.6 and -7.3 kcal/mol, respectively. 3^CN is then converted to a Bi-H intermediate through a water-bridged pathway, which is followed up with the transfer hydrogenation of azobenzene to produce the final product N,N'-diphenylhydrazine and regenerate the catalyst. Finally, the catalyst could be improved by substituting the phenyl group for the tert-butyl group on the pincer ligand, where the ΔG^≠ value (rate-determining step) decreases to 24.0 kcal/mol.

Ligand-Promoted [Pd]-Catalyzed α-Alkylation of Ketones through a Borrowing-Hydrogen Approach

A new class of palladium complexes bearing bidentate 2-hydroxypyridine based ligands have been prepared and fully characterized. The applications of these new complexes towards ketone alkylation reactions with alcohols through a metal-ligand cooperative borrowing-hydrogen (BH) process were demonstrated.

Iridium-catalyzed α-selective deuteration of alcohols

The development of chemoselective C(sp3)-H deuteration is of particular interest in synthetic chemistry. We herein report the α-selective, iridium(iii)-bipyridonate-catalyzed hydrogen(H)/deuterium(D) isotope exchange of alcohols using deuterium oxide (D2O) as the primary deuterium source. This method enables the direct, chemoselective deuteration of primary and secondary alcohols under basic or neutral conditions without being affected by coordinative functional groups such as imidazole and tetrazole. Successful substrates for deuterium labelling include the pharmaceuticals losartan potassium, rapidosept, guaifenesin, and diprophylline. The deuterated losartan potassium shows higher stability towards the metabolism by CYP2C9 than the protiated analogue. Kinetic and DFT studies indicate that the direct deuteration proceeds through dehydrogenation of alcohol to the carbonyl intermediate, conversion of [IrIII-H] to [IrIII-D] with D2O, and deuteration of the carbonyl intermediate to give the α-deuterated product.

Regio-Selective C3- and N-Alkylation of Indolines in Water under Air Using Alcohols

We disclosed a regio-selective C-H and N-H bond functionalization of indolines using alcohols in water via tandem dehydrogenation of N-heterocycles and alcohols. A diverse range of N- and C3-alkylated indolines/indoles were accessed utilizing a new cooperative iridium catalyst. The practical applicability of this methodology was demonstrated by the preparative-scale synthesis and synthesis of a psychoactive drug, N,N-dimethyltryptamine. A catalytic cycle is proposed based on several kinetic experiments, series of control experiments and density functional theory calculations.

Modulation of H+/H- exchange in iridium-hydride 2-hydroxypyridine complexes by remote Lewis acids

A series of iridium hydride complexes featuring dihydrogen bonding are presented and shown to undergo rapid H⁺/H^- exchange (1240 s^-1 at 25 °C). We demonstrate that the H⁺/H^- exchange rate can be modified by post-synthetic modification at a remote site using BH₃, Zn(C₆F₅)₂, and [Me₃O][BF₄]. This route provides a complementary strategy to traditional methods that rely on pre-metalation modifications to a metal's primary sphere.

Mechanistic Insight into the 1,3,2-Diazaphospholene-Catalyzed Reductant (HBpin/NH3BH3)-Controlled Reaction of Allyl 2-Phenylacrylate: Claisen Rearrangement or Hydrogenation?

Mechanistic study on the 1,3,2-diazaphospholene (1)-catalyzed reduction reaction of allyl 2-phenylacrylate 4 with HBpin or ammonia borane (AB) was systematically performed by the density functional theory (DFT) method. When HBpin is employed as the reductant, the reductive Ireland-Claisen (IC) rearrangement reaction occurs. First, the active species P-hydrido-1,3,2-diazaphospholene 3 is generated through the metathesis reaction of 1 with HBpin. Next, the terminal C═C double bond of 4 is inserted into the P-H bond of 3 to produce 6a through the 1,2-addition (Markovnikov) step, which is followed by the pinB-H bond activation to afford key boron enolate 8. Then, 8 undergoes the [3,3] rearrangement that is followed by the alcoholysis reaction with methanol leading to the final product γ,δ-unsaturated carboxylic acid. The [3,3] rearrangement step is the rate-determining step with the Gibbs energy barrier (ΔG^≠) and Gibbs reaction energy (ΔG) of 23.9 and -27.5 kcal/mol, respectively. When AB is employed as the reductant, the transfer hydrogenation reaction occurs through two comparable pathways, 1,2- and 1,4-transfer hydrogenation pathways. The former pathway directly leads to the hydrogenation product with the ΔG^≠ and ΔG values of 22.4 and -27.7 kcal/mol, respectively. The latter pathway produces an enolate intermediate (rate-determining step, ΔG^≠/ΔG = 24.1/-0.3 kcal/mol) first, which then prefers to undergo the enol-keto tautomerism instead of the [3,3] rearrangement to afford the hydrogenation product. Obviously, the generation of the boron enolate plays a crucial role in the reductive IC rearrangement reaction because it prevents the enol-keto tautomerism.

Identification of active catalysts for the acceptorless dehydrogenation of alcohols to carbonyls

Acceptorless dehydrogenation into carbonyls and molecular hydrogen is an attractive strategy to valorize (biobased) alcohols. Using 2-octanol dehydrogenation as benchmark reaction in a continuous reactor, a library of metal-supported catalysts is tested to validate the predictive level of catalytic activity for combined DFT and micro-kinetic modeling. Based on a series of transition metals, scaling relations are determined as a function of two descriptors, i.e. the surface binding energies of atomic carbon and oxygen. Then, a volcano-shape relation based on both descriptors is derived, paving the way to further optimization of active catalysts. Evaluation of 294 diluted alloys but also a series of carbides and nitrides with the volcano map identified 12 promising candidates with potentially improved activity for alcohol dehydrogenation, which provides useful guidance for experimental catalyst design. Further screening identifies β-Mo2N and γ-Mo2N exposing mostly (001) and (100) facets as potential candidates for alcohol dehydrogenation.

A Proton-Responsive Pyridyl(benzamide)-Functionalized NHC Ligand on Ir Complex for Alkylation of Ketones and Secondary Alcohols

A Cp*Ir(III) complex (1) of a newly designed ligand L¹ featuring a proton-responsive pyridyl(benzamide) appended on N-heterocyclic carbene (NHC) has been synthesized. The molecular structure of 1 reveals a dearomatized form of the ligand. The protonation of 1 with HBF₄ in tetrahydrofuran gives the corresponding aromatized complex [Cp*Ir(L¹ H)Cl]BF₄ (2). Both compounds are characterized spectroscopically and by X-ray crystallography. The protonation of 1 with acid is examined by ¹ H NMR and UV-vis spectra. The proton-responsive character of 1 is exploited for catalyzing α-alkylation of ketones and β-alkylation of secondary alcohols using primary alcohols as alkylating agents through hydrogen-borrowing methodology. Compound 1 is an effective catalyst for these reactions and exhibits a superior activity in comparison to a structurally similar iridium complex [Cp*Ir(L² )Cl]PF₆ (3) lacking a proton-responsive pendant amide moiety. The catalytic alkylation is characterized by a wide substrate scope, low catalyst and base loadings, and a short reaction time. The catalytic efficacy of 1 is also demonstrated for the syntheses of quinoline and lactone derivatives via acceptorless dehydrogenation, and selective alkylation of two steroids, pregnenolone and testosterone. Detailed mechanistic investigations and DFT calculations substantiate the role of the proton-responsive ligand in the hydrogen-borrowing process.

Linear-Organic-Polymer-Supported Iridium Complex as a Recyclable Auto-Tandem Catalyst for the Synthesis of Quinazolinones via Selective Hydration/Acceptorless Dehydrogenative Coupling from o-Aminobenzonitriles

A linear-organic-polymer-supported iridium complex Cp*Ir@P4VP, which is designed and synthesized by the coordinative immobilization of [Cp*IrCl₂]₂ on poly(4-vinylpyridine), was proven to be an efficient heterogeneous autotandem catalyst for synthesizing quinazolinones via selective hydration/acceptorless dehydrogenative coupling from o-aminobenzonitriles. Furthermore, the synthesized catalyst was recycled five times without an obvious decrease in the catalytic activity.

N-Methylation of Amines with Methanol in the Presence of Carbonate Salt Catalyzed by a Metal-Ligand Bifunctional Ruthenium Catalyst [(p-cymene)Ru(2,2'-bpyO)(H2O)]

A ruthenium complex [(p-cymene)Ru(2,2'-bpyO)(H₂O)] was found to be a general and efficient catalyst for the N-methylation of amines with methanol in the presence of carbonate salt. Moreover, a series of sensitive substituents, such as nitro, ester, cyano, and vinyl groups, were tolerated under present conditions. It was confirmed that OH units in the ligand are crucial for the catalytic activity. Notably, this research exhibited the potential of metal-ligand bifunctional ruthenium catalysts for the hydrogen autotransfer process.

The catalytic dehydrogenation of ethanol by heterogeneous catalysts

In this review, recent advances in the catalytic dehydrogenation of ethanol to acetaldehytde with the release of hydrogen catalyzed by a heterogeneous catalyst aresummerized and discussed.

Proton-responsive naphthyridinone-based RuII complexes and their reactivity with water and alcohols

We report the synthesis and reactivity of RuII complexes with a new naphthyridinone-substituted phosphine ligand, 7-(diisopropylphosphinomethyl)-1,8-naphthyridin-2(1H)-one (L-H), which contains two reactive sites that can potentially be deprotonated by a strong base: an NH proton of naphthyridinone and a methylene arm attached to the phosphine. In the absence of a base, the stable bis-ligated complex Ru(L-H)2Cl2 (1) containing two NH groups in the secondary coordination sphere is formed. Upon further reaction with a base, a doubly deprotonated, dimeric complex is obtained, [Ru2(L*-H)2(L)2] (2), in which two of the four ligands undergo deprotonation at the NH (L), while the other two ligands are deprotonated at the methylene groups (L*-H) as confirmed by an X-ray diffraction study; intramolecular hydrogen bonding is present between the NH group of one ligand and an O-atom of another ligand in the dimeric structure, which stabilizes the observed geometry of the complex. Complex 2 reacts with protic solvents such as water or methanol generating aqua Ru(L)2(OH2)2 (3) or methanol complexes Ru(L)2(MeOH)2 (4), respectively, both exhibiting intramolecular H-bonded patterns with surrounding ligands at least in the solid state. These complexes react with benzyl alcohols to give aldehydes via base-free acceptorless dehydrogenation.

Dioxygen Activation by Internally Aromatic Metallacycle: Crystallographic Structure and Mechanistic Investigations

Mononuclear metal-peroxo species are invoked as the key intermediates in metalloenzymatic or synthetic catalysis. However, either transience or sluggishness reactivity of synthetic analogs of metal-peroxo species impedes our understanding of oxygen activation mechanism. Herein, we designed and characterized a dioxygen-derived mononuclear osmium-peroxo complex, in which the peroxo ligand is stabilized by internally aromatic metallacycle. We demonstrate that the osmium-peroxo species shows catalytic activity toward promoterless alcohol dehydrogenations. Furthermore, computational studies provide a new mechanism for the osmium-peroxo-mediated alcohol oxidation, starting with the concerted double-hydrogen transfer and followed by the generation of osmium-oxo species. Interestingly, the internally aromatic metallacycle also plays a vital role in catalysis, which mediates the hydrogen transfer from osmium center to the distal oxygen atom of Os–OOH moiety, thus facilitating the Os–OOH→Os=O conversion. We expect that these insights will advance the development of aromatic metallacycle toward aerobic oxidation catalysis.

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A dioxygen-derived mononuclear osmium-peroxo complex was characterized

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The peroxo ligand is stabilized by internally aromatic metallacycle

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O₂ activation involves the reversible aromatization-dearomatization

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A concerted double-hydrogen transfer mechanism for alcohol dehydrogenation

Recent Advances in Homogeneous Catalysis via Metal–Ligand Cooperation Involving Aromatization and Dearomatization

Recently, an increasing number of metal complex catalysts have been developed to achieve the activation or transformation of substrates based on cooperation between the metal atom and its ligands. In such “cooperative catalysis,” the ligand not only is bound to the metal, where it exerts steric and electronic effects, but also functionally varies its structure during the elementary processes of the catalytic reaction. In this review article, we focus on metal–ligand cooperation involving aromatization and dearomatization of the ligand, thus introducing the newest developments and examples of homogeneous catalytic reactions.

A cyclometalated Ir(III)-NHC complex as a recyclable catalyst for acceptorless dehydrogenation of alcohols to carboxylic acids

In this work, we have synthesized two new [C, C] cyclometalated Ir(iii)-NHC complexes, [IrCp*(C∧C:NHC)Br](1a,b), [Cp* = pentamethylcyclopentadienyl; NHC = (2-flurobenzyl)-1-(4-methoxyphenyl)-1H-imidazoline-2-ylidene (a); (2-flurobenzyl)-1-(4-formylphenyl)-1H-imidazoline-2-ylidene (b)] via intramolecular C-H bond activation. The molecular structure of complex 1a was determined by X-ray single crystal analysis. The catalytic potentials of the complexes were explored for acceptorless dehydrogenation of alcohols to carboxylic acids with concomitant hydrogen gas evolution. Under similar experimental conditions, complex 1a was found to be slightly more efficient than complex 1b. Using 0.1 mol% of complex 1a, good-to-excellent yields of carboxylic acids/carboxylates have been obtained for a wide range of alcohols, both aliphatic and aromatic, including those involving heterocycles, in a short reaction time with a low loading of catalyst. Remarkably, our method can produce benzoic acid from benzyl alcohol on a gram scale with a catalyst-to-substrate ratio as low as 1 : 5000 and exhibit a TON of 4550. Furthermore, the catalyst could be recycled at least three times without losing its activity. A mechanism has been proposed based on controlled experiments and in situ NMR study.

Effect of a Substituent in Cyclopentadienyl Ligand on Iridium-Catalyzed Acceptorless Dehydrogenation of Alcohols and 2-Methyl-1,2,3,4-tetrahydroquinoline

New iridium(III)-bipyridonate complexes having cyclopentadienyl ligands with a series of alkyl substituents were synthesized for the purpose of tuning the catalytic activity for acceptorless dehydrogenation reactions. A comparison of the catalytic activity was performed for the reaction of alcoholic substrates such as 1-phenylethanol, 2-octanol, and benzyl alcohol. The 1-t-butyl-2,3,4,5-tetramethylcyclopentadienyl iridium complex exhibited the best performance, which surpassed that of the 1,2,3,4,5-pentamethylcyclopentadienyl (Cp*) iridium catalyst in the dehydrogenation reaction of alcohols. The catalytic activity in the dehydrogenation of 2-methyl-1,2,3,4-tetrahydroquinoline was also examined. The highest efficiency was obtained in the reaction catalyzed by the same t-butyl-substituted cyclopentadienyl iridium complex.

Jahn-Teller Distorted Effects To Promote Nitrogen Reduction over Keggin-Type Phosphotungstic Acid Catalysts: Insight from Density Functional Theory Calculations

Molecular geometry, electronic structure, and possible reaction mechanism of a series of mono-transition-metal-substituted Keggin-type polyoxometalate (POM)-dinitrogen complexes [PW₁₁O₃₉M(N₂)] ^n- (M = Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, Tc, Ru, Rh, Pd, Ag, Cd, W, Re, Os, Ir, Pt, Au, and Hg) have been investigated by using density functional theory (DFT) calculations with M06L functional. The calculated adsorption energy of N₂ molecule, N-N bond length, N-N stretching frequency, and the NBO charge on the coordinated N₂ moiety indicate that Mo^II-, Tc^II-, W^II-, Re^II-, and Os^II-POM complexes are significant for binding and activation of the inert N₂ molecule. The degree of the N₂ activation can be classified into the "moderately activated" category according to Tuczek's sense [ J. Comput. Chem. 2006 , 27 , 1278 ]. Electronic structure and NBO analysis indicate that the terminal N atom of the coordinated N₂ molecule in these POM-dinitrogen complexes possesses more negative charge relative to the bridge N atom because Jahn-Teller distorted effects lead to an effective orbital mixture between σ_2s* orbital of N₂ and d _z² orbital of transition metal center. And the mono-lacunary Keggin-type POM ligand with five oxygen donor atoms serves as a strong electron donor to the bivalent metal center. Meanwhile, a catalytic cycle for direct conversion of N₂ into NH₃ has been systematically investigated based on a Re-POM complex along distal, alternating, and enzymatic pathways. The calculated free energy profile of the three catalytic cycles indicates that the distal mechanism is the favorable pathway in the presence of proton and electron donors.

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.