Manganese Catalysts Research Articles

Direct Synthesis of Cycloalkanes from Diols and Secondary Alcohols or Ketones Using a Homogeneous Manganese Catalyst.

A method for the synthesis of substituted cycloalkanes was developed using diols and secondary alcohols or ketones via a cascade hydrogen borrowing sequence. A non-noble and air-stable manganese catalyst (2 mol %) was used to perform this transformation. Various substituted 1,5-pentanediols (3-4 equiv) and substituted secondary alcohols (1 equiv) were investigated to prepare a collection of substituted cyclohexanes in a diastereoselective fashion. Similarly, cyclopentane, cyclohexane, and cycloheptane rings were constructed from substituted 1,4-butanediol, 1,5-pentanediol, and 1,6-hexanediol, and sterically hindered ketones following a (4 + 1), (5 + 1), and (6 + 1) strategy, respectively. This reaction provides an atom economic methodology to construct two C-C bonds at a single carbon center generating high-value cycloalkanes from readily available alcohols as feedstock using an earth-abundant metal catalyst.

Improved Bimetallic Cobalt–Manganese Catalysts for Selective Oxidative Cleavage of Morpholine Derivatives

Catalytic methods for the site-selective scission of C(sp3)–C(sp3) bonds remain scarcely explored in contrast to the vast literature on C–C coupling. In view of this, we report a means of oxidative...

Oxidative degradation of cresol isomers using ozone in the presence of SiO2-supported nickel, iron, manganese and vanadium catalysts

We report the catalytic ozonation as a viable tool in the oxidative degradation of cresol isomers. SiO2-supported metal catalysts with 2.5 and 7.5 wt.% (Ni, Fe, Mn, V) were employed in this ozone initiated oxidation reactions. All catalysts were characterized using various analytical techniques such as XRD, FT-IR, SEM-EDX, TEM, ICP-OES, and BET-surface area analyses. XRD studies revealed the existence of Fe as Fe2O3 and Mn as Mn3O4 on the supports’ surface. EDX results displayed a dispersion homogeneity of the metals over the surface of SiO2 support, and the ICP-OES analysis confirm the concentration of a loaded metal. The degradation of cresols (m-, o-, p-cresol) was investigated by aerating each substrate with ozone (0.123 mg/L) in a glass reactor from 6 to 24 h of reaction time at ambient reaction conditions. The catalytic activities of various metal loaded SiO2 catalysts in terms of conversion and selectivity were studied. The reaction products of various cresol isomers were identified using GC–MS analysis. m-Tolyl acetate (m-TA), 2,3-dihydroxytoluene (2,3-DT), diethyl maleate (DM) and diethyl oxalate (DO) were the identified products in the oxidative degradation of m-Cresol. Whereas, o-cresol yielded o-tolyl acetate (o-TA), 2,5-dihydroxytoluene (2,5-DT), DM and DO as products. p-Cresol produced p-tolyl acetate (p-TA), 3,4-dihydroxytoluene (3,4-DT), DM and DO as reaction products. The Ni(2.5%)/SiO2 catalyst possess excellent activity towards the conversion of m-cresol (85.0%), whereas, Fe(2.5%)/SiO2 led to highest conversion for both o-cresol (76.3%) and p-cresol (87.0%).

Molecularly Defined Manganese Catalyst for Low-Temperature Hydrogenation of Carbon Monoxide to Methanol.

Methanol synthesis from syngas (CO/H2 mixtures) is one of the largest manmade chemical processes with annual production reaching 100 million tons. The current industrial method proceeds at high temperatures (200-300 °C) and pressures (50-100 atm) using a copper-zinc-based heterogeneous catalyst. In contrast, here, we report a molecularly defined manganese catalyst that allows for low-temperature/low-pressure (120-150 °C, 50 bar) carbon monoxide hydrogenation to methanol. This new approach was evaluated and optimized by quantum mechanical simulations virtual high-throughput screenings. Crucial for this achievement is the use of amine-based promoters, which capture carbon monoxide to give formamide intermediates, which then undergo manganese-catalyzed hydrogenolysis, regenerating the promoter. Following this conceptually new approach, high selectivity toward methanol and catalyst turnover numbers (up to 3170) was achieved. The proposed general catalytic cycle for methanol synthesis is supported by model studies and detailed spectroscopic investigations.

The Evolution of Catalysis for Alkyd Coatings: Responding to Impending Cobalt Reclassification with Very Active Iron and Manganese Catalysts, Using Polydentate Nitrogen Donor Ligands

Autoxidation processes to achieve curing of alkyd resins in paints, inks, and coatings are ubiquitous in many applications. Cobalt soaps have been employed for these applications for many decades and most of the paint and ink alkyd resin formulations have been optimized to achieve optimal benefits of the cobalt soaps. However, cobalt soaps are under increased scrutiny because of likely reclassification as carcinogenic under REACH (Registration, Evaluation, Authorisation, and Restrictions of Chemicals) legislation in Europe. This is critical, since such coatings are available for regular human contact. Alternative manganese- and iron-based siccatives have been developed to address this need for over a decade. They often show very high curing activity depending on the organic ligands bound to the metal centers. Recently, new classes of catalysts and modes of application have been published or patented to create safe paints, whilst delivering performance benefits via their unique reaction mechanisms. Besides the use of well-defined, preformed catalysts, paint formulations have also been developed with mixtures of metal soaps and ligands that form active species in-situ. The change from Co-soaps to Mn- and Fe-based siccatives meant that important coating issues related to radical-based curing, such as skinning, had to be rethought. In this paper we will review the new catalyst technologies and their performance and modes of action, as well as new compounds developed to provide anti-skinning benefits.

Catalytic Cleavage of Amide C-N Bond: Scandium, Manganese, and Zinc Catalysts for Esterification of Amides.

Amide C-N bonds are thermodynamically stable and their fission, such as by hydrolysis and alcoholysis, is considered a long-challenging organic reaction. In general, stoichiometric chemical transformations of amides into the corresponding esters and acids require harsh conditions, such as strong acids/bases at a high reaction temperature. Accordingly, the development of catalytic reactions that cleave not only primary and secondary amides, but also tertiary amides in mild conditions, is in high demand. Herein, we surveyed typical stoichiometric transformations of amides, and highlight our recent achievements in the catalytic esterification of amides using scandium, manganese, and zinc catalysts, together with some recent catalyst systems using late-transition metal reported by other groups.

One-Step Synthesis of Manganese Doped Single Atom Catalysts for Oxygen Reduction Reaction in Polymer Electrolyte Membrane Fuel Cells

Current commercial catalysts for polymer electrolyte membrane fuel cells (PEMFCs) require platinum group metal (PGM) catalysts that lead to high costs, thus preventing viable commercialization of PEM fuel cells. Many investigations into PGM-free catalysts have led to the identification of single metal atom sites coordinated with 4 nitrogens as being highly active. One of the best performing metal centers in the acidic environment of the PEMFC is Iron (Fe). Highly active iron-based catalysts have been synthesized easily via a one-step method of doping Iron into a ZIF-8 framework; however, there have been membrane degradation concerns due to the Fenton reaction products that are generated by Fe. Manganese (Mn) has recently been proposed as a potential metal center for the highly active metal-N4 catalyst site as theory predicts that it will have good activity and increased stability1 . The best performing Mn-based catalysts have been synthesized from a Mn doped ZIF-8 structure, but due to the high oxophilicity of Mn and difficulty in incorporating Mn into the ZIF structure, a laborious Mn catalyst synthesis route has been adopted, which requires acid leaching, secondary doping, and multiple pyrolysis steps2. This causes problems in scaling up the synthesis and limits the active site density when compared to the Fe catalyst. We aim to use highly reactive Mn metal precursors in an air-free environment to incorporate Mn into the metal sites of ZIF. High temperature pyrolysis (1100°C) leads to manganese catalyst synthesis in one step. We show that using a highly active manganese borohydride precursor allows for the formation of a metal organic framework (MOF) when combined with 2-methylimidazole, which can subsequently be pyrolyzed to form the target catalyst. Using zinc borohydride as a secondary metal source we can also form bimetallic MOF structures with increased thermal stability and varied Mn loading. With this strategy Mn can be incorporated directly into the metal MOF sites with a wide range of Mn loading. The utilization of an air-free environment prevents metal oxide formation thus giving a facile one step synthesis of a manganese-based catalyst. (1) Liu, K.; Qiao, Z.; Hwang, S.; Liu, Z.; Zhang, H.; Su, D.; Xu, H.; Wu, G.; Wang, G. Mn- and N- Doped Carbon as Promising Catalysts for Oxygen Reduction Reaction_ Theoretical Prediction and Experimental Validation. 2018. (2) Li, J.; Chen, M.; Cullen, D. A.; Hwang, S.; Wang, M.; Li, B.; Liu, K.; Karakalos, S.; Lucero, M.; Zhang, H.; et al. Atomically Dispersed Manganese Catalysts for Oxygen Reduction in Proton-Exchange Membrane Fuel Cells. Nat. Catal. 2018, 1 (12), 935–945.

Effectiveness of fresh and used supported noble and non-noble metal based catalysts for industrial organic raffinate oxidation by catalytic wet air oxidation at low operating parameters

In the present study, the efficacy of fresh and used noble and non-noble metal based catalysts for degradation of industrial organic raffinate has been studied. The industrial organic raffinate containing refractory and toxic organic compounds was characterized and oxidized by catalytic wet air oxidation (CWAO) at very mild operating conditions and the results were evaluated in terms of chemical oxygen demand (COD) removal (%). The efficacy of catalysts was monitored for determining their potential for long runs by repeating experiments 3 times with same catalyst. The catalysts stability have been checked in terms of their carbonaceous deposition and leaching of active metals in reaction solution. The fresh and used catalysts were characterized and compared by different techniques such as surface area, field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), transmission electron microscopy (TEM) and thermo-gravimetric analysis (TGA). The biodegradability in terms of BOD/COD ratio has been calculated for industrial organic raffinate and CWAO effluent obtained after treatment. The results obtained with all catalysts were compared and it was observed that at low operating conditions the non-noble metal ceria promoted manganese catalyst have potential to replace the costly noble metal such as platinum.

Experimental Study on Catalytic Oxidation of Toluene with Manganese Catalysts and Ozone at Low Temperature

The single metal oxide catalyst of transition metal element Mn and the composite metal oxide catalysts of Mn-Co and Mn-Fe are prepared by impregnation method using activated carbon (AC) as carrier. The catalysts are characterized by x-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS). The performance of the catalysts for catalytic ozonation of toluene is tested in a fixed bed tubular reactor. The results show that the activity of the single metal oxide catalyst of Mn is lower than that of the composite metal oxide catalysts of Mn-Co and Mn-Fe, and the activity of Mn-Co is higher than that of Mn-Fe. The catalytic efficiency of three catalysts is Mn/AC< Mn-Fe/AC> Mn-Co/AC. In this study, the factors influencing the degradation of toluene by catalytic ozonation are analyzed, and the mechanism is explored.

Bioinspired Manganese and Iron Complexes for Enantioselective Oxidation Reactions: Ligand Design, Catalytic Activity, and Beyond

The development of efficient methods for the enantioselective oxidation of organic molecules continues to be an important goal in organic synthesis; in particular, the use of earth-abundant metal catalysts and environmentally friendly oxidants in catalytic asymmetric oxidation reactions has attracted significant interest over the last several decades. In nature, metalloenzymes catalyze a wide range of oxidation reactions by activating dioxygen under mild conditions. Inspired by selective and efficient oxidation reactions catalyzed by metalloenzymes, researchers have developed a number of synthetic model compounds that mimic the functionality of metalloenzymes. Among the reported biomimetic model compounds, tetradentate aminopyridine (N4) ligands have emerged as appealing frameworks because of their easy synthesis and facile diversification, and their complexes with metals such as Fe and Mn have proven to be versatile and powerful catalysts for a variety of (enantioselective) oxidation reactions. In this Account, we describe our efforts on the design of chiral N4 ligands and the use of their manganese and iron complexes in asymmetric oxidation reactions with H2O2 as the terminal oxidant, aiming to show general strategies for asymmetric oxidation reactions that can guide the rational design of ligands and relevant metal catalysts. In studies of manganese catalysts, the aryl-substituted (R,R)-mcp [mcp = N,N'-dimethyl-N,N'-bis(pyridine-2-ylmethyl)cyclohexane-1,2-diamine] manganese complexes exhibited high enantioselectivity in the asymmetric epoxidation (AE) of various olefins with H2O2 while requiring stoichiometric acetic acid as an additive for the activation of H2O2. To address this issue, we established bulkier N4 ligands for this catalytic system in which a catalytic amount of sulfuric acid enables the manganese-complex-catalyzed AE with improved stereocontrol and efficiency. In addition, this system was found to be active for the oxidative kinetic resolution of secondary alcohols. Further exploration of the structure-reactivity relationships has shown that aminobenzimidazole N4 ligands derived from l-proline, in which the conventional pyridine donors are replaced by benzimidazoles, act as promising ligands. These novel C1-symmetric manganese catalysts showed dramatically improved activities with unprecedented turnover numbers in the AE reactions. Notably, this class of manganese complexes can catalyze the oxidation of the C-H bonds of spirocyclic hydrocarbons and spiroazacyclic compounds in a highly enantioselective manner, providing ready access to chiral spirocyclic β,β'-diketones and spirocyclic alcohols. Remarkably, iron catalysts with these chiral N4 ligands are effective for AE of olefins, enabling rare examples of highly enantioselective syntheses of epoxides by the iron catalysts. Finally, mechanistic studies provide valuable insights into the roles of the carboxylic acid and sulfuric acid in the catalytic oxidation reactions. Thus, the results described in this Account have demonstrated the importance of tunability and compatibility of the ligands for the development of efficient oxidation catalysts with earth-abundant transition metals and environmentally benign oxidants, and we hope that our study will pave the way for the discovery of efficient oxidation catalysis.

Theoretical and experimental investigation of the mechanism of the catalytic ozonation process by using a manganese-based catalyst

ABSTRACT The mechanisms of heterogeneous catalytic ozonation using a manganese catalyst have been studied in this work. The effects of catalyst, pH and the radical scavengers on the degradation of a model pollutant (i.e. phenol) were evaluated. The contribution of hydroxyl radicals and molecular ozone on the oxidation of phenol was studied for both homogeneous and heterogeneous systems. It was observed that the contribution of hydroxyl radial was 11–44% at pH 3–9. The heterogeneous catalyst may pose a rather complex mechanism as compared to homogenous catalyst. In presence of manganese catalyst, the decomposition ozone is very fast and generates more hydroxyl radicals. This helps the rapid oxidation of phenol at a wide range of pH 3–9. The surface of the catalyst undergoes multiple oxidation reaction and plays the major role in generation of the hydroxyl radicals. A considerable amount of phenol degradation was observed even at low catalyst dose (i.e. 0.5 g L−1). The effect of catalyst on the ozone mass transfer in water was also studied in this work. It was observed that, the overall performance of the phenol degradation is dependent on both chemical reaction due to catalytic action and gas–liquid mass transfer rate.

Cover Feature: Manganese‐Mediated Formic Acid Dehydrogenation (Chem. Eur. J. 45/2019)

A manganese gas generator, a hybrid PNNOP-supported manganese catalyst capable of performing formic acid dehydrogenation to form carbon dioxide and hydrogen, was developed. Similar to the depiction, we report a first-row transition metal catalyst akin to a fire hydrant; this is a system capable of generating extreme gas pressures in a very short time period. This new ligand–metal system is also reusable, capable of performing numerous gas generation cycles. More information can be found in the Communication by A. M. Tondreau et al. on page 10557.

Mechanistic Studies of Hydride Transfer to Imines from a Highly Active and Chemoselective Manganate Catalyst.

We introduce a highly active and chemoselective manganese catalyst for the hydrogenation of imines. The catalyst has a large scope, can reduce aldimines and ketimines, and tolerates a variety of functional groups, among them hydrogenation sensitive examples such as an olefin, a ketone, nitriles, nitro groups, and an aryl iodo substituent or a benzyl ether. We could investigate the transfer step between imines and the hydride complex in detail. We found that double deprotonation of the ligand is essential and excess base does not lead to a higher rate in the transfer step. We identified the actual hydrogenation catalyst as a K-Mn-bimetallic species and could obtain a structure of the K-Mn complex formed after hydride transfer by X-ray analysis. NMR experiments indicate that the hydride transfer is a well-defined reaction, which is first order in imine, first order in the bimetallic (K-Mn) hydride, and independent in rate from the concentration of the potassium base. We propose an outer-sphere mechanism in which protons do not seem to be involved in the rate-determining step, leading to a transiently negatively charged nitrogen atom in the substrate which reacts rapidly with HOtBu (2-methylpropan-2-ol) to produce the amine. This is based on several observations, such as no dependency of the reaction rate on the HOtBu concentration, no observable manganese amide complex, and a high reaction constant in a conducted Hammett study. Furthermore, hydrogen transfer of the catalytic cycle was experimentally probed and monitored by NMR with subsequent quantitative regeneration of the catalyst by H2.

Investigating the Structure and Dynamics of Apo‐Photosystem II

AbstractPhotosynthetic water oxidation is a model for future technologies employing solar energy to split water into hydrogen and oxygen. Natural water oxidation is carried out by a special manganese catalyst, the water‐oxidizing complex (WOC), located in photosystem II (PSII) of cyanobacteria, algae, and plants. Hence, there is great interest in the molecular structure as well as structural changes during catalytic activity and assembly/disassembly of the WOC. In particular, the light‐driven assembly during photosystem II repair under physiological conditions is poorly understood, and structural information about manganese depleted PSII (apo‐PSII) is required as a starting point for improving this understanding. Recently Zhang et al. (eLife 2017;6:e26933) showed that the cavity harboring the WOC in PSII remains largely intact upon manganese‐depletion and suggested that deprotonation of hydrogen‐bonding pairs enables the charge‐compensated insertion of the manganese cations without any major change of the cavity structure. By computational methods we have further investigated the structure of apo‐PSII and show how it can be stabilized by protons localized at the terminal carboxylate groups inside the remaining cavity. Ab‐initio molecular dynamics simulations suggest that not more than two water molecules fill the void left by manganese depletion.

Corrigendum: Manganese Catalysts with C1 -Symmetric N4 Ligand for Enantioselective Epoxidation of Olefins.

In this Communication, there is an error in the Table 2, entry 20 on page 6751. In the case of asymmetric epoxidation of α-methylstyrene (2t), the GC with a CP-Chirasil-Dex CB column did not separate both enantiomers of the epoxide (3t). The wrong ee value was provided previously. The experiment was repeated and the ee value was determined by HPLC with an IC column (HPLC-separation conditions: Chiralpack IC, 25 °C, 220 nm, 98/2 hexane/i-PrOH, 1.0 ml min−1; tR=5.32 min, tR [ent]=5.68 min.), 74 % ee should be changed to 11 % ee, and a correct version of the Supporting Information is published online along with this Corrigendum. The yield of epoxide can be reduplicated, and even better (82% isolated yield). The authors apologize for this oversight. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Sustainable Alkylation of Nitriles with Alcohols by Manganese Catalysis.

A general and chemoselective catalytic alkylation of nitriles using a homogeneous nonprecious manganese catalyst is presented. This alkylation reaction uses naturally abundant alcohols and readily available nitriles as coupling partners. The reaction tolerates a wide range of functional groups and heterocyclic moieties, efficiently providing useful cyanoalkylated products with water as the only side product. Importantly, methanol can be used as a C1 source and the chemoselective C-methylation of nitriles is achieved. The mechanistic investigations support the multiple role of the metal-ligand manganese catalyst, the dehydrogenative activation of the alcohol, α-C-H activation of the nitrile, and hydrogenation of the in-situ-formed unsaturated intermediate.

Manganese‐Catalyzed Transfer Hydrogenation of Aldimines

AbstractThe reduction of imines to amines via transfer hydrogenation was achieved promoted by phosphine‐free manganese(I) catalyst. Using isopropanol as reductant, in the presence of tBuOK (4 mol %) and manganese complex [Mn(CO)3Br(κ2N,N‐PyCH2NH2)] (2 mol %), a large variety of aldimines (30 examples) were typically reduced in 3 hours at 80 °C with good to excellent yield.

Estimation of the effect of temperature, the concentration of oxygen and catalysts on the oxidation of the thermoanthracite carbon material

The effect of temperature, oxygen concentration and gas-air mixture flow rate on the oxidation efficiency of thermoanthracite carbon granular material with the formation of carbon monoxide and dioxide is considered. The modeled installation implies heating of the carbon material sample in a heat chamber with constant passing of the gas-air mixture obtained by adjusting the supply of air and carbon dioxide through the heating zone. The temperature range was 20–850 °C.It is found that noticeable oxidation of carbon material in a heat chamber during purging of gas-air mixtures with an oxygen content of 8–21 % begins at temperatures above 500 °C, and significant concentrations of carbon monoxide are formed at temperatures of 600–800 °C. It is determined that, at an oxygen concentration of 14 %, the content of carbon monoxide in flue gases is minimal in the chosen range of oxygen concentrations of 8–21 %.It is shown that the use of the gas mixture saturated with water vapor increases the efficiency of carbon monoxide reoxidation at temperatures of 650–850 °C, but when using the manganese catalyst, the catalytic effect of water vapor is not observed.The use of the manganese catalyst applied as microcrystals on the carbon material surface is proposed, which provides a significant reduction of CO concentrations at temperatures of 500–850 °C. The disadvantage of the process is a 1.6–2.0 times increase in carbon material losses due to the acceleration of coal oxidation to CO and CO2.The possibility to create certain conditions for the technological process of oxidation of thermoanthracite carbon granular material at enterprises to provide a reduction of the carbon monoxide content in oxidation products is presented

Enantioselective Epoxidation of β,β-Disubstituted Enamides with a Manganese Catalyst and Aqueous Hydrogen Peroxide.

Enantioselective epoxidation of β,β-disubstituted enamides with aqueous hydrogen peroxide and a novel manganese catalyst is described. Epoxidation is stereospecific and proceeds fast under mild conditions. Amides are disclosed as key functional groups to enable high enantioselectivity.

Lutidine-Based Chiral Pincer Manganese Catalysts for Enantioselective Hydrogenation of Ketones.

A series of MnI complexes containing lutidine-based chiral pincer ligands with modular and tunable structures has been developed. The complex shows unprecedentedly high activities (up to 9800 TON; TON=turnover number), broad substrate scope (81 examples), good functional-group tolerance, and excellent enantioselectivities (85-98 % ee) in the hydrogenation of various ketones. These aspects are rare in earth-abundant metal catalyzed hydrogenations. The utility of the protocol have been demonstrated in the asymmetric synthesis of a variety of key intermediates for chiral drugs. Preliminary mechanistic investigations indicate that an outer-sphere mode of substrate-catalyst interactions probably dominates the catalysis.