Selective Catalytic Hydrogenations of Nitriles, Ketones, and Aldehydes by Well-Defined Manganese Pincer Complexes

Palladium(II) and Platinum(II) Complexes Bearing ONS-Type Pincer Ligands: Synthesis, Characterization and Catalytic Investigations

This work describes the synthesis of eight new Pd(II) and Pt(II) complexes with the general formula [M(TSC)Cl], where TSC represents the 4N-monosubstituted thiosemicarbazone derived from 2-acetylpyridine N-oxide with the substituents CH3 (H4MLO), C2H5 (H4ELO), phenyl (H4PLO) and (CH3)2 (H4DMLO). These complexes have been characterized by elemental analysis, molar conductivity, IR spectroscopy, 1H, 13C, 195Pt and ESI-MS. The complexes exhibit a square planar geometry around the metallic center coordinated by a thiosemicarbazone molecule acting as a donor ONS-type pincer ligand and by a chloride, as confirmed by the molecular structures of the complexes, [Pd(4ELO)Cl] (3) and [Pd(4PLO)Cl] (5), determined by single-crystal X-ray diffraction. The 195Pt NMR spectra of the complexes of formulae [Pt(4PLO)Cl] (6) and [Pt(4DMLO)Cl] (8) in DMSO show a single signal at -2420.4 ppm, confirming the absence of solvolysis products. Complexes 3 and 5 have been tested as catalysts in the Suzuki-Miyaura cross-coupling reactions of aryl bromides with phenylboronic acid, with yields of between 50 and 90.

Air-Stable Arene Manganese Complexes as Catalysts for the Syngas-Assisted Direct Reductive Amination, Cyanation of Aldehyde, and CO2 Fixation by Epoxide with High Functional Groups Tolerance

Manganese complexes [(arene)Mn(CO)3]+ were prepared in one step from arenes and Mn(CO)5Br. They were found to be efficient catalysts in the carbonyl cyanation with TMSCN, CO2 fixation by epoxides, and direct reductive amination in the presence of syngas. The amination reaction tolerated various reducible functional groups. The synergy of carbon monoxide and hydrogen in syngas provides high efficiency of the catalytic system. The developed protocols do not require an inert atmosphere, and the catalysts can be handled in air.

Au nanoparticle-catalyzed double hydrosilylation of nitriles by diethylsilane

We present the first example of Au-catalyzed reduction of nitriles into primary amines. In contrast to monohydrosilanes which are completely unreactive, diethylsilane (a dihydrosilane) is capable of reducing aryl or alkyl nitriles into primary amines under catalysis by Au nanoparticles supported on TiO2, via a smooth double hydrosilylation pathway. The produced labile N-disilylamines are readily deprotected by HCl in Et2O to form the hydrochloric salts of the corresponding amines in very good to excellent yields. The catalyst is recyclable and reusable at least in 5 consecutive runs.

Divergent Synthesis of Pyrazoles via Manganese Pincer Complex Catalyzed Acceptorless Dehydrogenative Coupling Reactions

This report detailed the synthesis of multi-substituted pyrazoles through the acceptorless dehydrogenative coupling (ADC) reaction catalyzed by a well-defined manganese(I)-pincer complex. Symmetrically substituted pyrazoles were synthesized by reacting 1,3-diols with hydrazines. Unsymmetrically substituted pyrazoles were selectively made via the ADC of primary alcohols with methyl hydrazones. Water and hydrogen are liberated as the green byproducts. The endurance of these methodologies has been presented by producing 30 substrates with varied functionalities. Model reactions were scaled up to demonstrate practicability. The reaction rate and order were measured to transparent the involvement of the reagents during catalysis. Control experiments elucidated the plausible reaction mechanisms.

Homogeneous Catalyzed Direct Conversion of Furfural to Gamma-Valerolactone

Herein, we report the direct conversion of biomass-derived furfural to γ-valerolactone (GVL) in a one-pot system, using the combination of Ru-MACHO-BH and a Brønsted acid (H3PO4). A GVL yield of 84 % is achieved under mild reaction conditions using 1 mol% of Ru-MACHO-BH and 3.8 M H3PO4(aq) at 100 °C for 7 hours.

A Phosphine-Oxide Cobalt(II) Complex and Its Catalytic Activity Studies toward Alcohol Dehydrogenation Triggered Direct Synthesis of Imines and Quinolines

Herein, a new pincer-like amino phosphine donor ligand, H2L1, and its phosphine-oxide analog, H2L2, were synthesized. Subsequently, cobalt(II) complexes 1 and 2 were synthesized by the reaction of anhydrous Co(II)Cl2 with ligands H2L1 and H2L2, respectively. The ligands and complexes were fully characterized by various physicochemical and spectroscopic characterization techniques. Finally, the identity of the complexes 1 and 2 was confirmed by single crystal X-ray structure determination. The phosphine ligand containing complex 1 was converted to the phosphine oxide ligand containing complex 2 in air in acetonitrile solution. Both complexes 1 and 2 were investigated as precatalysts for alcohol dehydrogenation-triggered synthesis of imines in air. The phosphine-oxide complex 2 was more efficient than the phosphine complex 1. A wide array of alcohols and amines were successfully reacted in a mild condition to result in imines in good to excellent yields. Precatalyst 2 was also highly efficient for the synthesis of varieties of quinolines in air. As H2L2 in 2 has side arms that can be deprotonated, we investigated complex 2 for its base (KOtBu) promoted deprotonation events by various spectroscopic studies and DFT calculations. These studies have shown that mono deprotonation of the amine side arm attached to the pyridine is quite feasible, and deprotonation of complex 2 leads to a dearomatized pyridyl ring containing complex 2a. The mechanistic investigations of the catalytic reaction, by a combination of experimental and computational studies, have suggested that the dearomatized complex, 2a acted as an active catalyst. The reaction proceeded through the hydride transfer pathway. The activation barrier of this step was calculated to be 26.5 kcal/mol, which is quite consistent with the experimental reaction temperature under aerobic conditions. Although various pincer-like complexes are explored for such reactions, phosphine oxide ligand-containing complexes are still unexplored.

Chemoselective Hydrogenation of α,β-Unsaturated Ketones Catalyzed by a Manganese(I) Hydride Complex

Here, we report the chemoselective hydrogenation of α,β-unsaturated ketones catalyzed by a well-defined Mn(I) PCNHCP pincer complex [(PCNHCP)Mn(CO)2H] (1). The reaction is compatible with a wide variety of functional groups that include halides, esters, amides, nitriles, nitro, alkynes, and alkenes, and for most substrates occurs readily at ambient hydrogen pressure (1-2 bar). Mechanistic studies and deuterium labeling experiments reveal a non-cooperative mechanism, which is further discussed in this report.

Manganese-catalyzed C-C and C-N bond formation with alcohols via borrowing hydrogen or hydrogen auto-transfer

Transition-metal-mediated "borrowing hydrogen" also known as hydrogen auto-transfer reactions allow the sustainable construction of C-C and C-N bonds using alcohols as hydrogen donors. In recent years, manganese complexes have been explored as efficient catalysts in these reactions. This review highlights the significant progress made in manganese-catalyzed C-C and C-N bond-formation reactions via hydrogen auto-transfer, emphasizing the importance of this methodology and manganese catalysts in sustainable synthesis strategies.

Synthesis, Characterization, and Catalytic Application of Colloidal and Supported Manganese Nanoparticles

Colloidal and supported manganese nanoparticles were synthesized following an organometallic approach and applied in the catalytic transfer hydrogenation (CTH) of aldehydes and ketones. Reaction parameters for the preparation of colloidal nanoparticles (NPs) were optimized to yield small (2-2.5 nm) and well-dispersed NPs. Manganese NPs were further immobilized on an imidazolium-based supported ionic phase (SILP) and characterized to evaluate NP size, metal loading, and oxidation states. Oxidation of the Mn NPs by the support was observed resulting in an average formal oxidation state of +2.5. The MnOx@SILP material showed promising performance in the CTH of aldehydes and ketones using 2-propanol as a hydrogen donor, outperforming previously reported Mn NPs-based CTH catalysts in terms of metal loading-normalized turnover numbers. Interestingly, MnOx@SILP were found to lose activity upon air exposure, which correlates with an additional increase in the average oxidation state of Mn as revealed by X-ray absorption spectroscopic studies.

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

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

Manganese Complex Catalyzed Sequential Multi-component Reaction: Enroute to a Quinoline-Derived Azafluorenes

The development of innovative synthetic strategies for constructing complex molecular structures is the heart of organic chemistry. This significance of novel reactions or reaction sequences would further enhance if they permitted the synthesis of new classes of structural motifs, which have not been previously created. The research on the synthesis of heterocyclic compounds is one of the most active topics in organic chemistry due to the widespread application of N-heterocycles in life and material science. The development of a new catalytic process that employs first-row transition metals to produce a range of heterocycles from renewable raw materials is considered highly sustainable approach. This would be more advantageous if done in an eco-friendly and atom-efficient manner. Herein we introduce, the synthesis of various new quinoline based azafluorenes via sequential dehydrogenative multicomponent reaction (MCR) followed by C(sp3)-H hydroxylation and annulation. Our newly developed, Mn-complexes have the ability to direct the reaction in order to achieve a high amount of desired functionalized heterocycles while minimizing the possibility of multiple side reactions. We also performed a series of control experiments, hydride trapping experiments, reaction kinetics, catalytic intermediate and DFT studies to comprehend the detailed reaction route and the catalyst's function in the MCR sequence.

Unlocking the potential of metal ligand cooperation for enantioselective transformations

Metal-ligand cooperation, in which both the metal and the ligand of a transition metal complex actively participate in chemical transformations leading to enhanced reactivity or selectivity in chemical reactions, has emerged as a powerful and versatile concept in catalysis. This Viewpoint discusses the development trajectory of transition metal-based complexes as catalysts in (de)hydrogenative processes, in particular those cases where metal-ligand cooperation has been invoked to rationalise the observed high reactivities and excellent selectivities. The historical context, mechanistic aspects and current applications are discussed with the suggestion to explore the potential of the MLC mode of action of such catalysts in enantioselective transformations beyond (de)hydrogenative processes.

Catalytic utility of PNN-based MnI pincer complexes in the synthesis of quinolines and transfer hydrogenation of carbonyl derivatives

This manuscript describes the synthesis of a triazolyl-pyridine-based phosphine, N-((diphenylphosphaneyl)methyl)-N-methyl-6-(1-phenyl-1H-1,2,3-triazol-4-yl)pyridin-2-amine, [2,6-{(PPh2)CH2N(Me)(C5H3N)(C2HN3C6H5)}] (1) (here onwards referred to as PNN) and its cationic and neutral MnI complexes and catalytic applications. The reaction of 1 with Mn(CO)5Br afforded a cationic complex [Mn(CO)3(PNN)]Br (2), which is highly stable in solid state, but in solution it gradually loses one of the CO groups to form a neutral complex [Mn(CO)2(PNN)Br] (3). Complex 2 on treatment with AgBF4 also yielded a cationic complex [Mn(CO)3(PNN)]BF4 (4). These complexes efficiently promoted the synthesis of quinoline derivatives via acceptor-less dehydrogenative coupling of 2-aminobenzyl alcohol and ketones, with complex 3 showing the highest activity with a very low catalyst loading (0.03 mol%) at 110 °C. Complex 3 (0.5 mol%) also showed excellent catalytic activity in the transfer hydrogenation of ketones and aldehydes to form respective secondary and primary alcohols.

Enantioselective Hydrophosphination of Terminal Alkenyl Aza-Heteroarenes

This paper presents a Mn(I)-catalysed methodology for the enantioselective hydrophosphination of terminal alkenyl aza-heteroarenes. The catalyst operates through H-P bond activation, enabling successful hydrophosphination of a diverse range of alkenyl-heteroarenes with high enantioselectivity. The presented protocol addresses the inherently low reactivity and the commonly encountered suboptimal enantioselectivities of these challenging substrates. As an important application we show that this method facilitates the synthesis of a non-symmetric tridentate P,N,P-containing ligand like structure in just two synthetic steps using a single catalytic system.

Theoretical Study on the Mechanism of the Hydrogenation of Azo (N═N) Bonds to Amines Catalyzed by Manganese

The mechanism of the transition metal manganese complex Mn(PhPNN)(CO)2Br (CA-4) that catalyzed the hydrogenation of the azo (N═N) bond to amines has been investigated using the PBE0 function. The results show that the whole reaction involves three basic processes: (1) the addition of H2 to CA gives IN2, which can hydrogenate the azo (N═N) bond at the later stage; (2) hydrogenation of azobenzene by IN2, which gives 1,2-diphenylhydrazine (PhNHNHPh); and (3) hydrogenation of 1,2-diphenylhydrazine by IN2, which affords aniline (PhNH2). The results suggest that the hydrogenation of CA and hydrogenation of azobenzene by IN2 to afford PhNHNHPh are easy to occur due to the low barriers, and the overall rate-determining step is the formation of IN11 and PhNH2 by breaking the N-N bond in the stage of hydrogenation of 1,2-diphenylhydrazine by IN2, with an energy barrier of 39.1 kcal/mol. The computed results are in good agreement with the experimental results. The mechanism of the azobenzene reaction catalyzed by manganese was analyzed by charge and orbital analysis in detail. The theoretical results provide a deeper understanding of the mechanism and fully explain the experimental facts.

Advances in Group VI Metal-Catalyzed Homogeneous Hydrogenation and Dehydrogenation Reactions

Transition metal-catalyzed homogeneous hydrogenation and dehydrogenation reactions for attaining plethora of organic scaffolds have evolved as a key domain of research in academia and industry. These protocols are atom-economic, greener, in line with the goal of sustainability, eventually pave the way for numerous novel environmentally benign methodologies. Appealing progress has been achieved in the realm of homogeneous catalysis utilizing noble metals. Owing to their high cost, less abundance along with toxicity issues led the scientific community to search for sustainable alternatives. In this context, earth- abundant base metals have gained substantial attention culminating enormous progress in recent years, predominantly with pincer-type complexes of nickel, cobalt, iron, and manganese. In this regard, group VI chromium, molybdenum and tungsten complexes have been overlooked and remain underdeveloped despite their earth-abundance and bio-compatibility. This review delineates a comprehensive overview in the arena of homogeneously catalysed (de)hydrogenation reactions using group VI base metals chromium, molybdenum, and tungsten till date. Various reactions have been described; hydrogenation, transfer hydrogenation, dehydrogenation, acceptorless dehydrogenative coupling, hydrogen auto transfer, along with their scope and brief mechanistic insights.

Activated Mn-MACHO Complexes Form Stable CO2 Adducts

Manganese(I) carbonyl complexes bearing a MACHO-type ligand (HN(CH2 CH2 PR2 )2 ) readily react in their amido form with CO2 to generate 4-membered {Mn-N-C-O} metallacycles. The stability of the adducts decreases with the steric demand of the R groups at phosphorous (R=isopropyl>adamantyl). The CO2 -adducts display generally a lower reactivity as compared to the parent amido complexes. These adducts can thus be interpretated as masked forms of the active amido catalysts and potentially play important roles as off-loop species or branching points in catalytic transformations of carbon dioxide.

Borrowing Hydrogen β-Phosphinomethylation of Alcohols Using Methanol as C1 Source by Pincer Manganese Complex

Herein, we report a manganese-catalyzed three-component coupling of β-H containing alcohols, methanol, and phosphines for the synthesis of γ-hydroxy phosphines via a borrowing hydrogen strategy. In this development, methanol serves as a sustainable C1 source. A variety of aromatic and aliphatic substituted alcohols and phosphines could undergo the dehydrogenative cross-coupling process efficiently and deliver the corresponding β-phosphinomethylated alcohol products in moderate to good yields. Mechanistic studies suggest that this transformation proceeds in a sequential manner including catalytic dehydrogenation, aldol condensation, Michael addition, and catalytic hydrogenation.

Manganese Transfer Hydrogenases Based on the Biotin-Streptavidin Technology

Artificial (transfer) hydrogenases have been developed for organic synthesis, but they rely on precious metals. Native hydrogenases use Earth-abundant metals, but these cannot be applied for organic synthesis due, in part, to their substrate specificity. Herein, we report the design and development of manganese transfer hydrogenases based on the biotin-streptavidin technology. By incorporating bio-mimetic Mn(I) complexes into the binding cavity of streptavidin, and through chemo-genetic optimization, we have obtained artificial enzymes that hydrogenate ketones with nearly quantitative yield and up to 98 % enantiomeric excess (ee). These enzymes exhibit broad substrate scope and high functional-group tolerance. According to QM/MM calculations and X-ray crystallography, the S112Y mutation, combined with the appropriate chemical structure of the Mn cofactor plays a critical role in the reactivity and enantioselectivity of the artificial metalloenzyme (ArMs). Our work highlights the potential of ArMs incorporating base-meal cofactors for enantioselective organic synthesis.

Synthesis, Characterization, and Reactivity of a (PPP) Pincer-Ligated Manganese Carbonyl Complex: Polarity Reversal Imparted by the Electrophilic Nature of a Planar Mn-P(NR2)2 Fragment

The bonding interactions of a synthesized pincer-ligated manganese dicarbonyl complex featuring an N-heterocyclic phosphenium (NHP+) central moiety are explored. The pincer ligand [PPP]Cl was coordinated to a manganese center using Mn(CO)5Br and 254 nm light to afford the chlorophosphine complex (PPClP)Mn(CO)2Br (2) as a mixture of halide exchange products and stereoisomers. The target dicarbonyl species (PPP)Mn(CO)2 (3) was prepared by treatment of 2 with 2 equiv of the reductant KC8. Computational investigations and analysis of structural parameters were used to elucidate multiple bonding interactions between the Mn center and the PNHP atom in 3. The generation of a product of formal H2 addition, (PPHP)Mn(CO)2H (4), was achieved through the dehydrogenation of NH3BH3, affording a 2:1 mixture of 4syn:4anti stereoisomers. The nucleophilic nature of the Mn center and the electrophilic nature of the PNHP moiety were demonstrated through hydride addition and protonation of 3 to produce K(THF)2[(PPHP)Mn(CO)2] (6) and (PPClP)Mn(CO)2H (5), respectively. The observed reactivity suggests that 3 is best described as a Mn-I/NHP+ complex, in contrast to pincer-ligated dicarbonyl manganese analogues typically assigned as MnI species.

Reactivity of [(PNP)Mn(CO)2] with Organophosphates

Organophosphorus nerve agents (OPAs) are a toxic class of synthetic compounds that cause adverse effects with many biological systems. Development of methods for environmental remediation and passivation has been ongoing for years. However, little progress has been made in therapeutic development for exposure victims. Given the postexposure behavior of OPA materials in enzymes such as acetylcholinesterase (AChE), development of electrophilic compounds as therapeutics may be more beneficial than the currently employed nucleophilic countermeasures. In this report, we present our studies with an electrophilic, 16-electron manganese complex (iPrPNP)Mn(CO)2 (1) and the nucleophilic hydroxide derivative (iPrPNHP)Mn(CO)2(OH) (2). The reactivity of 1 with phosphorus acids and the reactivity of 2 with the P-F bond of diisopropylfluorophosphate (DIPF) were studied. The role of water in both nucleophilic and electrophilic reactivity was investigated with the use of 17O-labeled water. Promising results arising from reactions of both 1 and 2 with organophosphorus substrates are reported.

Click-derived multifunctional metal complexes for diverse applications

The Click reaction that involves Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC) serves as the most potent and highly dependable tool for the development of many complex architectures. It has paved the way for the synthesis of numerous drug molecules with enhanced synthetic flexibility, reliability, specificity and modularity. It is all about bringing two different molecular entities together to achieve the required molecular properties. The utilization of Click chemistry has been well demonstrated in organic synthesis, particularly in reactions that involve biocompatible precursors. In pharmaceutical research, Click chemistry is extensively utilized for drug delivery applications. The exhibited bio-compatibility and dormancy towards other biological components under cellular environments makes Click chemistry an identified boon in bio-medical research. In this review, various click-derived transition metal complexes are discussed in terms of their applications and uniqueness. The scope of this chemistry towards other streams of applied sciences is also discussed.

Preorganized Nitrogen Sites for Au11 Amidation: A Generalizable Strategy toward Precision Functionalization of Metal Nanoclusters

Atomically precise metal nanoclusters have received tremendous attention due to their unique structures and properties. Although synthetic approaches to this kind of nanomaterial have been well developed, methods toward precision functionalization of the as-synthesized metal nanoclusters are extremely limited, hindering their interfacial modification and related performance improvement. Herein, an amidation strategy has been developed for the precision functionalization of the Au₁₁ nanocluster based on preorganized nitrogen sites. The nanocluster amidation did not change the number of gold atoms in the Au₁₁ kernel and their bonding mode to the surface ligands but slightly modified the arrangement of gold atoms with the introduction of functionality and chirality, thus representing a relatively mild method for the modification of metal nanoclusters. The stability and oxidation barrier of the Au₁₁ nanocluster are also improved accordingly. The method developed here would be a generalizable strategy for the precision functionalization of metal nanoclusters.

Manganese Salan Complexes as Catalysts for Hydrosilylation of Aldehydes and Ketones

Manganese has attracted significant recent attention due to its abundance, low toxicity, and versatility in catalysis. In the present study, a series of manganese (III) complexes supported by salan ligands have been synthesized and characterized, and their activity as catalysts in the hydrosilylation of carbonyl compounds was examined. While manganese (III) chloride complexes exhibited minimal catalytic efficacy without activation of silver perchlorate, manganese (III) azide complexes showed good activity in the hydrosilylation of carbonyl compounds. Under optimized reaction conditions, several types of aldehydes and ketones could be reduced with good yields and tolerance to a variety of functional groups. The possible mechanisms of silane activation and hydrosilylation were discussed in light of relevant experimental observations.

Manganese-Catalyzed Hydrogenative Desulfurization of Thioamides

Earth-abundant transition metal catalysis has emerged as an important alternative to noble transition metal catalysis in hydrogenation reactions. However, there has been no Earth-abundant transition metal catalyzed hydrogenation of thioamides reported so far, presumably due to the poisoning of catalysts by sulfur-containing molecules. Herein, we described the first manganese-catalyzed hydrogenative desulfurization of thioamides to amines or imines. The key to success is the use of MnBr(CO)₅ instead of commonly-employed pincer-manganese catalysts, together with simple NEt₃ and CuBr. This protocol features excellent selectivity on sole cleavage of the C=S bond of thioamides, in contrast to the only known Ru-catalyzed hydrogenation of thioamides, and unprecedented chemo-selectivity tolerating vulnerable functional groups such as nitrile, ketone, aldehyde, ester, sulfone, nitro, olefin, alkyne and heterocycle, which are usually susceptible to common hydride-type reductive protocols.

Rational Design of a Facially Coordinating P,N,N Ligand for Manganese-Catalysed Enantioselective Hydrogenation of Cyclic Ketones

DFT calculations on the full catalytic cycle for manganese catalysed enantioselective hydrogenation of a selection of ketones have been carried out at the PBE0-D3PCM //RI-BP86PCM level. Mn complexes of an enantiomerically pure chiral P,N,N ligand have been found to be most reactive when adopting a facial coordination mode. The use of a new ligand with an ortho-substituted dimethylamino-pyridine motif has been calculated to completely transform the levels of enantioselectivity possible for the hydrogenation of cyclic ketones relative to the first-generation Mn catalysts. In silico evaluation of substrates has been used to identify those likely to be reduced with high enantiomer ratios (er), and others that would exhibit less selectivity; good agreements were then found in experiments. Various cyclic ketones and some acetophenone derivatives were hydrogenated with er's up to 99 : 1.

Kinetics and Pathway Analysis Reveals the Mechanism of a Homogeneous PNP-Iron-Catalyzed Nitrile Hydrogenation

Nitrile hydrogenation via the in situ-generated PNP-Fe^II(H)₂CO (1) catalyst leads to a previously inexplicable loss of mass balance. Reaction kinetics, reaction progress analysis, in situ pressure nuclear magnetic resonance, and X-ray diffraction analyses reveal a mechanism comprising reversible imine self-condensation and amine-imine condensation cascades that yield >95% primary amine. Imine self-condensation has never been reported in a nitrile hydrogenation mechanism. The reaction is first order in catalyst and hydrogen and zero order in benzonitrile when using 2-propanol as the solvent. Variable-temperature analysis revealed values for ΔG_298 K^⧧ (79.6 ± 26.8 kJ mol^-1), ΔH^⧧ (90.7 ± 9.7 kJ mol^-1), and ΔS^⧧ (37 ± 28 J mol^-1 K^-1), consistent with a solvent-mediated proton-shuttled dissociative transition state. This work provides a basis for future catalyst optimization and essential data for the design of continuous reactors with earth-abundant catalysts.

Efficient Hydrogenation of N-Heterocycles Catalyzed by NNP-Manganese(I) Pincer Complexes at Ambient Temperature

Manganese-catalyzed hydrogenation reactions have aroused widespread interest in recent years. Among the catalytic systems described, especially PNP- and NNP-Mn pincer catalysts have been reported for the hydrogenation of aldehydes, ketones, nitriles, aldimines and esters. Furthermore, NNP-Mn pincer compounds are efficient catalysts for the hydrogenolysis of less reactive amides, ureas, carbonates, and carbamates. Herein, the synthesis and application of specific imidazolylaminophosphine ligands and the corresponding Mn pincer complexes are described. These new catalysts have been characterized and studied by a combination of experimental and theoretical investigations, and their catalytic activities have been tested in several hydrogenation reactions with good to excellent performance. Especially, the reduction of N-heterocycles can be performed under very mild conditions.

Metal/Non-Metal Catalyzed Activation of Organic Nitriles

Nitrile activation is a prominent topic in recent developments in chemistry, especially in organic, inorganic, biological chemistry, as well as in the natural synthesis of products and in the pharmaceutical industry. The activation of nitriles using both metal and non-metal precursors has attracted several researchers, who are exploring newer ways to synthesize novel compounds. Nitrile activation can be achieved by combining various catalytic double hydroelementation reactions, such as hydrosilylation, hydroboration, and hydrogenation of organonitriles using silanes, pinacolborane, and other sources of hydrogen. These methodologies have garnered considerable attention since they are effective in the reduction of organonitriles, whose end products are extensively applied in synthetic organic chemistry. In this review, we summarize the development of selective hydroborylation, hydrosilylation, dihydroborysilylation, and hydrogenation of organonitriles, as well as their reaction mechanisms and the role of metal complexes in the catalytic cycles. This review article explains various synthetic methodologies applied toward the reduction of organonitriles into corresponding amines.

Room temperature chemoselective hydrogenation of C[double bond, length as m-dash]C, C[double bond, length as m-dash]O and C[double bond, length as m-dash]N bonds by using a well-defined mixed donor Mn(i) pincer catalyst

Chemoselective hydrogenation of C[double bond, length as m-dash]C, C[double bond, length as m-dash]O and C[double bond, length as m-dash]N bonds in α,β-unsaturated ketones, aldehydes and imines is accomplished at room temperature (27 °C) using a well-defined Mn(i) catalyst and 5.0 bar H₂. Amongst the three mixed-donor Mn(i) complexes developed, κ³-(^R2PN³N^Pyz)Mn(CO)₂Br (R = Ph, ⁱPr, ^t Bu); the ^t Bu-substituted complex ( ^tBu2PN³N^Pyz)Mn(CO)₂Br shows exceptional chemoselective catalytic reduction of unsaturated bonds. This hydrogenation protocol tolerates a range of highly susceptible functionalities, such as halides (-F, -Cl, -Br, and -I), alkoxy and hydroxy, including hydrogen-sensitive moieties like acetyl, nitrile, nitro, epoxide, and unconjugated alkenyl and alkynyl groups. Additionally, the disclosed method applies to indole, pyrrole, furan, thiophene, and pyridine-containing unsaturated ketones leading to the corresponding saturated ketones. The C[double bond, length as m-dash]C bond is chemoselectively hydrogenated in α,β-unsaturated ketones, while the aldehyde's C[double bond, length as m-dash]O bond and imine's C[double bond, length as m-dash]N bond are preferentially reduced over the C[double bond, length as m-dash]C bond. A detailed mechanistic study highlighted the non-innocent behavior of the ligand in the ( ^tBu2PN³N^Pyz)Mn(i) complex and indicated a metal-ligand cooperative catalytic pathway. The molecular hydrogen (H₂) acts as a hydride source, whereas MeOH provides a proton for hydrogenation. DFT energy calculations supported the facile progress of most catalytic steps, involving a crucial turnover-limiting H₂ activation.

Manganese Promoted (Bi)carbonate Hydrogenation and Formate Dehydrogenation: Toward a Circular Carbon and Hydrogen Economy

We report here a feasible hydrogen storage and release process by interconversion of readily available (bi)carbonate and formate salts in the presence of naturally occurring α-amino acids. These transformations are of interest for the concept of a circular carbon economy. The use of inorganic carbonate salts for hydrogen storage and release is also described for the first time. Hydrogenation of these substrates proceeds with high formate yields in the presence of specific manganese pincer catalysts and glutamic acid. Based on this, cyclic hydrogen storage and release processes with carbonate salts succeed with good H₂ yields.

Paired electrocatalysis in 5-hydroxymethylfurfural valorization

5-Hydroxymethylfurfural (HMF) has aroused considerable interest over the past years as an important biomass-derived platform molecule, yielding various value-added products. The conventional HMF conversion requires noble metal catalysts and harsh operating conditions. On the other hand, the electrocatalytic conversion of HMF has been considered as an environmentally benign alternative. However, its practical application is limited by low overall energy efficiency and incomplete conversion. Paired electrolysis and highly efficient electrocatalysts are two viable strategies to address these limitations. Herein, an overview of coupled electrocatalytic HMF hydrogenation or hydrogen evolution reaction (HER) with HMF oxidation as well as the associated electrocatalysts are reviewed and discussed. In this mini-review, a brief introduction of electrocatalytic HMF upgrading is given, followed by the recent advances and challenges of paired electrolysis with an emphasis on the integration HMF electrohydrogenation with HMF electrooxidation. Finally, a perspective for a future sustainable biomass upgrading community based on electrocatalysis is proposed.

General Construction of Amine via Reduction of N=X (X = C, O, H) Bonds Mediated by Supported Nickel Boride Nanoclusters

Amines play an important role in synthesizing drugs, pesticides, dyes, etc. Herein, we report on an efficient catalyst for the general construction of amine mediated by nickel boride nanoclusters supported by a TS-1 molecular sieve. Efficient production of amines was achieved via catalytic hydrogenation of N=X (X = C, O, H) bonds. In addition, the catalyst maintains excellent performance upon recycling. Compared with the previous reports, the high activity, simple preparation and reusability of the Ni-B catalyst in this work make it promising for industrial application in the production of amines.

Switching between Hydrogenation and Olefin Transposition Catalysis via Silencing NH Cooperativity in Mn(I) Pincer Complexes

While Mn-catalyzed (de)hydrogenation of carbonyl derivatives has been well established, the reactivity of Mn hydrides with olefins remains very rare. Herein, we report a Mn(I) pincer complex that effectively promotes site-controlled transposition of olefins. This reactivity is shown to emerge once the N–H functionality within the Mn/NH bifunctional complex is suppressed by alkylation. While detrimental for carbonyl (de)hydrogenation, such masking of the cooperative N–H functionality allows for the highly efficient conversion of a wide range of allylarenes to higher-value 1-propenybenzenes in near-quantitative yield with excellent stereoselectivities. The reactivity toward a single positional isomerization was also retained for long-chain alkenes, resulting in the highly regioselective formation of 2-alkenes, which are less thermodynamically stable compared to other possible isomerization products. The detailed mechanistic analysis of the reaction between the activated Mn catalyst and olefins points to catalysis operating via a metal–alkyl mechanism—one of the three conventional transposition mechanisms previously unknown in Mn complexes.

Advances in Catalytic Routes for the Homogeneous Green Conversion of the Bio-Based Platform 5-Hydroxymethylfurfural

5-Hydroxymethylfufural (HMF) is an intriguing platform molecule that can be obtained from biomasses and that can lead to the production of a wide range of products, intermediates, or monomers. The presence of different moieties in HMF (hydroxy, aldehyde, furan ring) allows to carry out different transformations such as selective oxidations and hydrogenations, reductive aminations, etherifications, decarbonylations, and acetalizations. This is a great chance in a biorefinery perspective but requires the development of active and highly selective catalysts. In this view, homogeneous catalysis can lead to efficient conversion of HMF at mild reaction conditions. This Review discussed the recent achievements in homogeneous catalysts development and application to HMF transformations. The effects of metal nature, ligands, solvents, and reaction conditions were reported and critically reviewed. Current issues and future chances have been presented to drive future studies toward more efficient and scalable processes.

Hydrogenation of nitriles to primary amines catalyzed by an unsupported nanoporous palladium catalyst: understanding the essential reason for the high activity and selectivity of the catalyst

An efficient and highly selective heterogeneous catalyst system for nitrile hydrogenation was developed using unsupported palladium nanopores (PdNPore). The PdNPore-catalyzed selective hydrogenation of nitriles proceeded smoothly, without any additives, under mild conditions (low H₂ pressure and low temperature) to yield primary amines with satisfactory to excellent yields. Systematic studies demonstrated that the high activity and excellent selectivity of the PdNPore originated from its good Lewis acidity and porous structure. No palladium leached from the PdNPore during the hydrogenation reaction. Moreover, the catalyst was easily recovered and reused without any loss of catalytic activity. A deuterium-hydrogen exchange reaction clearly indicated that the present hydrogenation involves heterolytic H₂ splitting on the surface of the PdNPore catalyst.