Manganese Catalysts Research Articles

Inert C-H Bond Transformations Enabled by Organometallic Manganese Catalysis.

Traditional organic synthesis relies heavily on the transformations of various preinstalled functional groups, such as cross-coupling reactions using organohalides and organometallic reagents. The strategy of C-H activation enables the direct formation of C-C/C-X (X = heteroatom) bonds from inert C-H bonds, which can enhance the atom- and step-economy of organic synthesis. To date, precious metals have overwhelmingly dominated the C-H activation field; however, the rarity and high cost of these metals necessitate the development of more sustainable catalysts. In this regard, catalysts based on manganese are highly desirable owing to the abundant reserve of manganese in the earth's crust and its economic benefits, low toxicity, and potentially unique reactivity. Although the first stoichiometric manganese-mediated C-H activation reaction was reported as early as 1970, manganese-catalyzed C-H activation reactions are largely underdeveloped. How to construct an efficient catalytic cycle for manganese in C-H activation reactions remains as a key issue to be addressed. In this Account, we summarize our recent advances in the manganese-catalyzed transformations of inert C-H bonds. To overcome the challenges associated with building manganese-based catalytic cycles, we developed two novel strategies, namely, synergy between manganese catalysts and bases and between manganese catalysts (with or w/o bases) and acids. By implementing the former strategy, we developed cooperative manganese/base catalytic systems that facilitate a new mode of C-H bond activation by manganese via a redox-neutral base-assisted deprotonation mechanism. As such, the requirement for the tedious preparation of MnR(CO)5 complexes (R = Me, Bn, Ph) in stoichiometric reactions was eliminated, and a series of manganese-catalyzed C-H activation reactions of arenes with various reaction partners having C≡C and C═C bonds were achieved. Through the latter strategy of synergy between manganese catalysts (with or w/o bases) and acids, we disclosed a "dual activation" mode for performing manganese-catalyzed C-H bond transformations, that is, merging C-H activation by manganese catalysts and C-X multiple bond activation by Lewis acids. Consequently, the scope of C-H substrates could be expanded to include challenging ketones and olefinic C-H compounds. Additionally, the range of reaction partners could be significantly broadened to include those bearing more polarized C═O, C═N, and C≡N bonds such as aldehydes, imines, and nitriles. Remarkably, the innate reactivity of different C-H bonds in ketones could be reversed by manganese catalysis, and the reactions could even be carried out at room temperature. Our findings provide guiding information for the future development of manganese-catalyzed C-H activation reactions and beyond. Related important contributions from other groups are mentioned, and the remaining challenges and future perspective in this emerging area are also presented.

Structural and surface changes of cobalt modified manganese oxide during activation and ethanol steam reforming reaction

Surface and structural changes of unmodified manganese and cobalt-manganese oxide during activation and ethanol steam reforming reaction conditions (ESR) were studied by means of X-ray diffraction, X-ray photoelectron spectroscopy, temperature-programmed reduction/oxidation (TPR/TPO) and transmission electron microscopy. It was shown that synthesis of cobalt manganese oxide by the redox precipitation method led to the formation of strongly dispersed cobalt ionic species within cryptomelane-based manganese oxide structure. Development of large cube-like MnO nanoparticles with spherical cobalt metallic crystallites decorated by manganese oxide on the high oxidation state and potassium species was observed during reduction. Cobalt manganese catalyst showed high initial activity and selectivity to H2 and CO2 in ethanol stem reforming reaction in the range of 390–480 °C. The drop of ethanol conversion and changes of selectivity with the time-on-stream were observed. An increase of reaction temperature led to intensification of deactivation phenomena. TEM studies evidenced coexistence of Co and CoOx nanoparticles formed under ethanol steam reforming conditions, partially covered by filamentous and encapsulating carbonaceous deposits.

Catalysing water oxidation using nature’s metal

In nature, a manganese catalyst is used for photosynthetic water oxidation, but efforts to develop artificial manganese-based counterparts have been hampered by the lability of manganese complexes. By using a bulky and hydrophilic ligand, a water-soluble Mn12 complex is found to be a stable and efficient water oxidation electrocatalyst.

A β-diketiminate manganese catalyst for alkene hydrosilylation: substrate scope, silicone preparation, and mechanistic insight.

The dimeric β-diketiminate manganese hydride compound, [(2,6-iPr2PhBDI)Mn(μ-H)]2, was prepared by treating [(2,6-iPr2PhBDI)Mn(μ-Cl)]2 with NaEt3BH. This compound was characterized by single crystal X-ray diffraction and found to feature high-spin Mn centres that exhibit strong magnetic coupling by EPR spectroscopy. Once characterized, [(2,6-iPr2PhBDI)Mn(μ-H)]2 was found to mediate the hydrosilylation of a broad scope of alkenes at elevated temperature. Aliphatic alkenes were found to undergo anti-Markovnikov hydrosilylation, while the hydrosilylation of styrenes using [(2,6-iPr2PhBDI)Mn(μ-H)]2 afforded Markovnikov's product. Importantly, this catalyst has also been employed for the cross-linking of industrially-relevant silicones derived from vinyl-terminated poly(dimethylsiloxane) and 1,2,4-trivinylcyclohexane with catalyst loadings as low as 0.05 mol%. To gain a mechanistic understanding of [(2,6-iPr2PhBDI)Mn(μ-H)]2-catalyzed olefin hydrosilylation, 4-tert-butylstyrene was added to [(2,6-iPr2PhBDI)Mn(μ-H)]2 and conversion to the monomeric Mn alkyl complex, (2,6-iPr2PhBDI)Mn(CH(CH3)(4- t BuPh)), was observed. Isolation of this secondary alkyl intermediate confirms that olefin insertion into the Mn-H bond dictates the observed regioselectivities. The importance of our mechanistic findings as they relate to recent advances in Mn hydrosilylation catalysis is described herein.

Selective catalytic oxidation of alkenes employing homobinuclear manganese(II) catalysts with TBHP

The two novel homobinuclear compounds [Mn2(II,II) (µ1,1-4-CH3-C6H4COO)2(phen)4](ClO4)2 (1) and [Mn2(II,II) (µ1,3-4-CH3-C6H4COO)2(bipy)4](ClO4)2 (2), where bipy = 2,2-bipyridine and phen = 1,10-phenanthroline, have been synthesized and characterized by elemental analyses and spectral methods (UV–Vis, FTIR, and X-ray). A single-crystal X-ray diffraction structure analysis of the compounds revealed that the manganese atom is octahedrally coordinated. In compound 1, the binuclear(II) structure is monodentate, bridged with one oxygen atom of carboxylate ligand in μ1,1 mode, and each Mn(II) center is coordinated with two phen ligands. In compound 2, the binuclear(II) structure is syn–anti bidentate, bridged with two oxygen atoms of carboxylate ligand in μ1,3 mode, and each Mn(II) center is coordinated with two bipy ligands. The Mn–Mn separation is 3.441 (1) Å and 4.450 (1) Å for 1 and 2, respectively. The catalytic potentials of these compounds have been tested for the oxidation reaction of various olefins (i.e., styrene, cyclohexene, ethyl benzene, 1-hexene, 1-octene). The oxidation reactions were carried out in the presence of catalytic amounts of 1 and 2 with a peroxide oxygen donor (TBHP = tert-Butyl hydroperoxide) in acetonitrile at 70 °C. On comparing the catalytic activities of 1 and 2, both catalysts showed good activity (∼100% conv. in 24 h) in the oxidation of studied alkenes, and excellent conversion was obtained for cyclohexene (∼100% conv. in 3 h; TON = 265 and TON = 257, respectively, for 1 and 2).

Oxidation of Vicinal Diols to α-Hydroxy Ketones with H2O2 and a Simple Manganese Catalyst.

α‐Hydroxy ketones are valuable synthons in organic chemistry. Here we show that oxidation of vic‐diols to α‐hydroxy ketones with H2O2 can be achieved with an in situ prepared catalyst based on manganese salts and pyridine‐2‐carboxylic acid. Furthermore the same catalyst is effective in alkene epoxidation, and it is shown that alkene oxidation with the MnII catalyst and H2O2 followed by Lewis acid ring opening of the epoxide and subsequent oxidation of the alkene to α‐hydroxy ketones can be achieved under mild (ambient) conditions.

Manganese Complexes for (De)Hydrogenation Catalysis: A Comparison to Cobalt and Iron Catalysts.

The sustainable use of the resources on our planet is essential. Noble metals are very rare and are diversely used in key technologies, such as catalysis. Manganese is the third most abundant transition metal of the Earth's crust and based on the recently discovered impressive reactivity in hydrogenation and dehydrogenation reactions, is a potentially useful noble-metal "replacement". The hope of novel selectivity profiles, not possible with noble metals, is also an aim of such a "replacement". The reactivity of manganese complexes in (de)hydrogenation reactions was demonstrated for the first time in 2016. Herein, we summarize the work that has been published since then and especially discuss the importance of homogeneous manganese catalysts in comparison to cobalt and iron catalysts.

Highly Selective Manganese(I)/Lewis Acid Cocatalyzed Direct C-H Propargylation Using Bromoallenes.

A manganese(I)/Lewis acid cocatalyzed direct C-H propargylation with high selectivity has been developed. BPh3 was discovered to not only promote the reactivity, but also enhance the selectivity. Secondary, tertiary, and even quaternary carbon centers at the propargylic position could be directly constructed. Both internal and terminal alkynes are easily accessible. The chirality was successfully transferred from an axially chiral allene to central chirality. The reactivity of the manganese catalyst in this reaction was found to be unique among transition metal catalysts.

A bioinspired soluble manganese cluster as a water oxidation electrocatalyst with low overpotential

The electrocatalytic oxidation of water is a challenging step towards the production of hydrogen as an alternative fuel. In nature, water oxidation is catalysed by a high oxidation state Mn4CaO x cluster. The corresponding industrial development of manganese catalysts for water oxidation is very attractive due to the low cost of this metal. A few manganese complexes have been previously explored as water oxidation catalysts using various chemical oxidants in homogeneous and heterogeneous systems. Efficient electrochemical water oxidation catalysed by a soluble manganese-oxo cluster, however, has not been achieved. Here, we report the synthesis and characterization of [Mn12O12(O2CC6H3(OH)2)16(H2O)4] (Mn12DH), a unique example within this class of compounds in being both highly soluble and stable in water. We demonstrate that Mn12DH, which is readily prepared from cheap and environmentally benign starting materials, is a stable homogeneous water oxidation electrocatalyst operating at pH 6 with an exceptionally low overpotential of only 334 mV. During photosynthesis, nature uses an enzyme with a manganese–calcium core for water oxidation. Here, the authors report the synthesis of a stable, water-soluble manganese cluster that acts as a homogeneous water oxidation electrocatalyst, displaying low overpotential and high Faradaic efficiency.

The Emergence of Manganese-Based Carbonyl Hydrosilylation Catalysts.

In recent years, interest in homogeneous manganese catalyst development has intensified because of the earth-abundant and nontoxic nature of this metal. Although compounds of Mn have largely been utilized for epoxidation reactions, recent efforts have revealed that Mn catalysts can mediate a broad range of reductive transformations. Low-valent Mn compounds have proven to be particularly effective for the hydrosilylation of carbonyl- and carboxylate-containing substrates, and this Account aims to highlight my research group's contributions to this field. In our initial 2014 communication, we reported that the bis(imino)pyridine-supported compound (Ph2PPrPDI)Mn mediates ketone hydrosilylation with exceptional activity under solvent-free conditions. Silanes including Ph2SiH2, (EtO)3SiH, (EtO)2MeSiH, and (EtO)Me2SiH were found to partially reduce cyclohexanone in the presence of (Ph2PPrPDI)Mn, while turnover frequencies of up to 1280 min-1 were observed using PhSiH3. This led us to evaluate the hydrosilylation of 11 additional ketones and allowed for the atom-efficient preparation of tertiary and quaternary silanes. At that time, it was also discovered that (Ph2PPrPDI)Mn catalyzes the dihydrosilylation of esters (by way of acyl C-O bond hydrosilylation) to yield a mixture of silyl ethers with modest activity. Earlier this year, the scope of these transformations was extended to aldehydes and formates, and the observed hydrosilylation activities are among the highest obtained for any transition-metal catalyst. The effectiveness of three related catalysts has also been evaluated: (Ph2PPrPDI)MnH, (PyEtPDEA)Mn, and [(Ph2PEtPDI)Mn]2. To our surprise, (Ph2PPrPDI)MnH was found to exhibit higher carboxylate dihydrosilylation activity than (Ph2PPrPDI)Mn, while (PyEtPDEA)Mn demonstrated remarkable carbonyl hydrosilylation activity considering that it lacks a redox-active supporting ligand. The evaluation of [(Ph2PEtPDI)Mn]2 revealed competitive aldehyde hydrosilylation and formate dihydrosilylation turnover frequencies; however, this catalyst is significantly inhibited by pyridine and alkene donor groups. In our efforts to fully understand how (Ph2PPrPDI)Mn operates, a thorough electronic structure evaluation was conducted, and the ground-state doublet calculated for this compound was found to exhibit nonclassical features consistent with a low-spin Mn(II) center supported by a singlet PDI dianion and an intermediate-spin Mn(II) configuration featuring antiferromagnetic coupling to PDI diradical dianion. A comprehensive mechanistic investigation of (Ph2PPrPDI)Mn- and (Ph2PPrPDI)MnH-mediated hydrosilylation has revealed two operable pathways, a modified Ojima pathway that is more active for carbonyl hydrosilylation and an insertion pathway that is more effective for carboxylate reduction. Although these efforts represent a small fraction of the recent advances made in Mn catalysis, this work has proven to be influential for the development of Mn-based reduction catalysts and is likely to inform future efforts to develop Mn catalysts that can be used to prepare silicones.

Metal-based mesoporous materials and their application as catalysts for the degradation of methyl orange azo dye

Cobalt, manganese and copper heterogeneous catalysts were prepared using MCM-41 and SBA-16 mesoporous supports. The synthesized materials were characterized by X-ray diffraction, chemical analysis, N2 sorptometry, vibrational spectroscopy and transmission electron microscopy. Their catalytic activity was studied in the oxidative degradation of methyl orange (MO) in aqueous solution, at pH 6, using persulfate (PS) anion as oxidant at 30°C. The level of degradation of the dye was monitored by UV–vis spectroscopy and Total Organic Carbon (TOC) analysis. The synthesized materials showed high activity, being the most effective catalysts those based on Co and Cu, achieving an efficient dye decolorization of >95% until 2h of reaction. After 24h of reaction a high degree of mineralization was achieved (>70%). Radical quenching studies showed that sulfate radicals are the main species formed by the catalyst/PS interaction. The supported catalysts exhibited stable performance during the reusability test, reaching an activity greater than 90% up to the fifth evaluated cycle of reuse for copper and cobalt based catalysts.

Electric current jump-starts diamine synthesis

To make vicinal diamines, in which neighboring carbons each have an attached amine group, chemists generally expose alkenes to harsh chemical oxidants, which can produce environmentally toxic by-products. Now, researchers have replaced those harsh reactants with a metal catalyst and some electric current to create a safer and less wasteful synthetic route to 1,2-diamines, functional groups that crop up in many pharmaceuticals and stereoselective catalysts (Science 2017, DOI: 10.1126/science.aan6206). The reaction combines abundant starting materials—alkenes and sodium azide—with a manganese catalyst inside a simple electrochemical cell that has a carbon anode and platinum cathode. It adds two equivalents of azide to the alkene C=C bond, generating a 1,2-diazide that can then easily be reduced to a 1,2-diamine. Although electrochemical setups are not common in synthesis labs, they are starting to attract more attention from organic chemistry research groups. One advantage of electrosynthesis is that it giv...

Manganese-Catalyzed Asymmetric Hydrosilylation of Aryl Ketones.

We disclose the synthesis of a series of manganese complexes of chiral iminopyridine oxazoline ligands and their application in the first manganese-catalyzed asymmetric ketone hydrosilylations. The most sterically hindered manganese catalyst bearing two CH(Ph)2 groups at the 2,6-ortho positions of the imino aryl ring and a tBu group on the oxazoline ring furnishes the secondary alcohols in high enantioselectivities and yields.

Manganese-Catalyzed Upgrading of Ethanol into 1-Butanol.

Biomass-derived ethanol is an important renewable feedstock. Its conversion into high-quality biofuels is a promising route to replace fossil resources. Herein, an efficient manganese-catalyzed Guerbet-type condensation reaction of ethanol to form 1-butanol was explored. This is the first example of upgrading ethanol into higher alcohols using a homogeneous non-noble-metal catalyst. This process proceeded selectively in the presence of a well-defined manganese pincer complex at the parts per million (ppm) level. The developed reaction represents a sustainable synthesis of 1-butanol with excellent turnover number (>110 000) and turnover frequency (>3000 h-1). Moreover, mechanistic studies including control experiments, NMR spectroscopy, and X-ray crystallography identified the essential role of the "N-H moiety" of the manganese catalysts and the major reaction intermediates related to the catalytic cycle.

Plasma Catalytic Removal of p-Xylene from Air Stream Using γ-Al2O3 Supported Manganese Catalyst

The abatement of dilute p-xylene (500 ppm) from a contaminated air stream (2 L min−1) by the non-thermal plasma in combination with an ozone decomposing catalyst (pelleted Mn/γ-Al2O3) was investigated. The reactor performance was evaluated in terms of p-xylene conversion, CO x selectivity and ozone emission with and without external heating. Generally, the removal of p-xylene gradually increased with increasing the applied voltage from 16 to 22 kV (peak height). As the catalyst was used (i.e., packed inside the plasma active region), the reactor was able to remove ca. 82% of p-xylene at 22 kV, compared to 54% done by plasma alone. Under the same plasma-catalytic condition, the removal of p-xylene was successfully achieved (ca. 96%) by heating up the reactor to 150 °C. Not only was the conversion improved, but also the mineralization of p-xylene was also enhanced considerably by the mild thermal heating, increasing the CO2 selectivity from ca. 41 to 59%. The ozone emission measurement showed that ozone formed in plasma was readily catalytically decomposed on Mn/γ-Al2O3 at above 16 kV, even without external heating. As revealed by the reactor infrared thermographic images, the plasma-induced surface temperature of the catalyst was far higher than the external heating temperature (i.e., 150 °C), facilitating the catalytic degradation of ozone and reducing the formation of byproducts. Additionally, no organic compounds have been found on the spent catalyst, avoiding the possibility of catalyst deactivation during the course of experiment.

Aerobic oxidation of α-pinene catalyzed by homogeneous and MOF-based Mn catalysts

Manganese catalysts were investigated for the liquid-phase aerobic oxidation of α‐pinene. First, the influence of reaction parameters such as time, solvent, temperature, oxidant flow rate and catalyst concentration on conversion, yield and selectivity were evaluated using Mn(III) acetate as homogeneous catalyst. Mn(III) acetate afforded pinene oxide as the main product along with small amounts of verbenol and verbenone. The optimized reaction conditions were likewise applied to a novel mixed-linker metal-organic framework (MOF) based on MIL-53-NH2(Al), which was successfully synthesized and modified in a two-step post-synthetic reaction using maleic anhydride and Mn(III) acetate. The performance of the Mn-containing MOF catalyst was directly compared to its homogeneous counterpart Mn(III) acetate, showing very similar activity in a mixture of diethyl carbonate/dimethylformamide (DEC/DMF) as solvent. In both cases, a conversion of 31% and the formation of 17% pinene oxide (55% selectivity) were observed after 6 h. The heterogeneous MOF catalyst was easily removed from the reaction mixture by filtration and reused for at least five catalytic cycles without significant loss of activity. A hot filtration test showed that the catalysis mainly proceeded heterogeneously, although minor contributions of homogeneous species could not be completely excluded.

Study of different effects on magnetic properties of MgO-supported Fe–Co–Mn oxides

A new catalyst is produced using Fe(NO3)3·9H2O, Co(NO3)2·6H2O and Mn(NO3)2·4H2O. The magnetic properties of the nanoparticles of “iron–cobalt–manganese catalysts (ICMC)” have been studied, with magnesium oxide as a support employing co-precipitation method. The effects of calcination and drying conditions such as temperature, time, and different percentage of MgO-supported Fe–Co–Mn oxides on the magnetic properties were investigated using vibrating sample magnetometer (VSM). Our results indicated that calcination at 600 °C can change magnetic phase of the sample, and calcination at 700 °C changed the sample from ferromagnetic to superparamagnetic by adding 15% MgO into the ICMC. By increasing the calcination temperature, the values of remnant magnetization (M r) and saturation magnetization (M S) were increased. By increasing the calcination time duration, values of the M r and coercivity (H C) were increased. Furthermore, when the percentages of MgO increased, the values of M r, M S, and H C were decreased. It was observed that most variables in the experiments affected the magnetic properties of the new catalysts.

Corrigendum: A Highly Active Manganese Catalyst for Enantioselective Ketone and Ester Hydrogenation.

Corrigendum: A Highly Active Manganese Catalyst for Enantioselective Ketone and Ester Hydrogenation.

An Approach to 3-(Indol-2-yl)succinimide Derivatives by Manganese-Catalyzed C-H Activation.

The manganese-catalyzed addition of C-2 position of indoles to maleimides has been achieved under additive-free conditions. The manganese catalyst exhibits excellent chemo- and regioselectivity, good functional group compatibility, and high catalytic efficiency. The substrate scope can also be extended to maleates, ethyl acrylate, 1,4-dihydro-1,4-epoxynaphthalene, pyrroles, and 2-phenylpyridine, which further demonstrates the universality of this straightforward approach.

Promoting a non-heme manganese complex catalyzed oxygen transfer reaction by both lewis acid and Brønsted acid: Similarities and distinctions

This work demonstrates that certain Lewis and Brønsted acids can sharply improve the oxygen transfer efficiency of a manganese(II) catalyst bearing non-heme ligand. In the absence of Lewis and Brønsted acids, oxidation of manganese(II) complex will generate di-μ-oxo-bridged dinuclear Mn2(III,IV) core which is very sluggish for olefin epoxidation. Adding non-redox metal ions as Lewis acid or Brønsted acid will both improve the catalytic epoxidation of olefin, and this improvement is dependent on the pKa of Brønsted acid, or the net charge of non-redox metals of Lewis acid. Mechanism study revealed that similar promotional effect by either Lewis or Brønsted acids was originated from a similar reaction pathway by dissociating aforementioned sluggish di-μ-oxo core. However, distinctions of reactive intermediate were also demonstrated for Lewis or Brønsted acids.