Earth-abundant Metal Catalysts Research Articles

Highly Enantioselective Polymerization of β-Butyrolactone by a Bimetallic Magnesium Catalyst: An Interdependent Relationship Between Favored and Unfavored Enantiomers.

Herein, we report that (S,S)-prophenolMg2(μ-OnBu)(THF)2 ((S,S)-1, prophenol = (S,S)-2,6-bis[2-(hydroxydiphenylmethyl)pyrrolidin-1-ylmethyl]-4-methylphenol) is a highly enantioselective (kR/kS = 140) precatalyst for ring-opening polymerization of rac-β-butyrolactone (β-BL) to isotactic poly(3-hydroxybutyrate) (i-PHB), a high performance, biodegradable polyester. Precatalyst (S,S)-1 polymerizes (R)-β-BL with an inversion of stereochemistry to (S)-PHB with a m% (percentage of adjacent linkages with a meso configuration) of 98% at 41% conversion and Tm of 165 °C under a variety of conditions. Complex (S,S)-1 demonstrates unique polymerization kinetics, as it does not polymerize the preferred enantiomer, (R)-β-BL, alone. Mechanistic studies revealed that (S)-β-BL is needed to convert (S,S)-1 into the active enantioselective polymerization catalyst. To the best of our knowledge, (S,S)-1 produces i-PHB with the highest degree of isotacticity observed from a polymerization of rac-β-BL. This study informs the design and understanding of future enantioselective and earth-abundant metal catalysts for ring-opening polymerization of β-lactones.

Earth Abundant Transition Metal Catalysts: New and Efficient Tools for Hydrophosphination and Oxyphosphination of Alkenes and Alkynes

Hydrophosphination and oxyphosphination are two important topical reactions in order to prepare organophosphorus derivatives from unsaturated derivatives such as alkenes and alkynes in a more sustainable fashion. Noticeably, metal catalysed versions have shown great interest and efficiency. By contrast, the use of earth abundant transition metal based catalysts for such transformations is less reported, even if there is a growing interest during the last decade. This review article reports and highlights recent developments using manganese, iron, cobalt, nickel and copper based catalysts for hydro‐ and oxyphosphination, notably exhibiting the selectivity, functional group tolerance, milder conditions and catalyst design. Even if significant progresses were made, the scopes are still rather limited (mainly focused on activated olefins such as styrenes) and chemo‐ and stereo‐selectivity issues still have to be solved, notably for asymmetric transformations. Of interest, the use of visible light including blue one as activator emerged giving promising and stimulating results at ambient conditions.

Highly efficient Cu-Fe containing prussian blue analogs (PBAs) for excellent electrocatalytic activity towards overall water splitting

Hydrogen generation by water electrolysis is considered a clean, green, and renewable approach for sustainable energy demands and environmental management. The efficient use of earth-abundant transition metal catalysts is cost-effective and limits the usage of noble metal catalysts like IrO2, RuO2, and Pt/C. The PBAs (Prussian Blue Analogs) are a class of transition metal hexacyanoferrates that can be tailored for specific electrochemical reactions. Both iron and copper are well-proven transition metals for catalytic activities owing to their aptitude to achieve much-pronounced reactant transformations under minor circumstances, as well as their environmental friendliness, cyclability, and long-term durability at 10 mA cm−2 in 1.0 M KOH, the greenly synthesized Fe8Cu2CN (PBAs) on NF (Nickel foam) displays a low overpotential of 150 mV (118 mV dec−1) for HER (Hydrogen Evolution Reaction) and 290 mV (112 mV dec−1) for OER (Oxygen Evolution Reaction). Fe8Cu2CN displays ultra-stable behavior (150 h) with a slight current loss of 4.2 % for OER and 3.4 % for HER, respectively. The interaction of Fe, Cu, and CN (cyanide) enhances the rate of reaction kinetics. The designing of a water electrolyzer that delivers 10 mA cm−2 at 1.60 V is made possible with the help of the improved bifunctional catalyst Fe8Cu2CN/NF. The Fe8Cu2CN/NF//Fe8Cu2CN/NF exhibits long-last durability (over 180 h) with a negligible current reduction of 5.4 %. The improved effectiveness of the synthesized catalyst is demonstrated by the solar water splitting at 1.66 V. These findings suggest that Fe8Cu2CN/NF//Fe8Cu2CN/NF can be used to produce large quantities of H2 at very affordable prices.

Aerobic Oxidative Carboxylation of Styrene Over Cobalt Catalysts: Integrated CO2 Capture and Conversion.

The direct synthesis of cyclic carbonates through oxidative carboxylation of alkenes using CO2 and O2 offers a sustainable and carbon-neutral method for CO2 utilization, which is, however, still a largely unexplored field. Here we develop a single-atom catalyst (SAC) Co-N/O-C as the earth-abundant metal catalyst for the oxidative carboxylation of styrene with CO2 and O2. Remarkably, even using the flue gas as an impure CO2 and O2 source, desired cyclic carbonate could be obtained with moderate productivity, which shows the potential for integrated CO2 capture and conversion, leveraging the high CO2 adsorption capacity of Co-N/O-C. In addition, the catalyst can be reused five times without an obvious decline in activity. Detailed characterizations and theoretical calculations elucidate the crucial role of single Co atoms in activating O2 and CO2, as well as controlling selectivity.

Three-Component Radical Cross-Coupling: Asymmetric Vicinal Sulfonyl-Esterification of Alkenes Involving Sulfur Dioxide.

A novel catalytic system for radical cross-coupling reactions based on copper and chiral Pyridyl-bis(imidazole) (PyBim) ligandsis described. It overcomes the challenges of chemoselectivity and enantioselectivity, achieving a highly enantioselective vicinal sulfonyl-esterification reaction of alkenes involving sulfur dioxide. This strategy involves the use of earth-abundant metal catalyst, mild reaction conditions, a broad range of substrates (84 examples), high yields (up to 97% yield), and exceptional control over enantioselectivity. The reaction system is compatible with different types of radical precursors, including O-acylhydroxylamines, cycloketone oxime esters, aryldiazonium salts, and drug molecules. Chiral ligand PyBim is identified as particularly effective in achieving the desired high enantioselectivity. Mechanistic studies reveal that copper/PyBim system plays a vital role in C─O coupling, employing an outer-sphere model. In addition, the side arm effect of ligand is observed.

Bifunctional metal–acid sites on nickel boride catalysts: Phenol hydrodeoxygenation and water-promoted C = C hydrogenation

Modification of earth-abundant metal catalysts with optimized catalytic performance can be potential alternatives to precious metal catalysts for more sustainable processes. Using a chemical reduction method, nickel boride (Ni3B) alloy catalysts were prepared with a Ni/B ratio of 3. Ex-situ and in-situ characterization indicates that the structure of Ni3B changes from amorphous to crystalline with increasing pretreatment temperatures. The presence of B does not seem to alter the electronic properties of Ni. However, after pretreatment, oxidized B species (i.e., BOx and H3BO3) ex-solve from the bulk, leading to the formation of a B-enriched surface. In-situ titration and dehydration reaction experiments demonstrate the presence of acid sites, which play an important promoting role in the hydrogenation of C = C bonds. Moreover, Ni3B catalysts show a remarkable activity towards phenolic ring hydrogenation with the added ability to deoxygenate the secondary products via dehydration on the acid sites. Under water-free reaction conditions, the Ni3B catalysts exhibit similar hydrogenation activity to that of a pure Ni catalyst. Remarkably, the addition of small amounts of water leads to a > 100-fold increase on Ni3B catalysts, while it inhibits the reaction on bare Ni catalyst by blocking active sites. These observations highlight the cooperative role of bifunctional sites in Ni3B catalysts and solvent composition for C = C hydrogenation reactions.

Aerobic Catalytic Cross-Dehydrogenative Coupling of Furans with Indoles Provides Access to Fluorophores with Large Stokes Shift.

Sustainability in chemical processes is a crucial aspect in contemporary chemistry with sustainable catalysis as a vital parameter of the same. There has been a renewed focus on utilizing earth-abundant metal catalysts to expand the repertoire of organic reactions. Furan is a versatile heterocycle of natural origin used for multiple applications. However, it has scarcely been used in cross-dehydrogenative coupling. In this work, we have explored the cross-dehydrogentive coupling of furans with indoles using commonly available, inexpensive FeCl3 ⋅ 6H2 O (<0.25 $/g) as catalyst in the presence of so called 'ultimate oxidant' - oxygen, without the need for any external ligand or additive. The reactions were found to be scalable and to work even under partially aqueous conditions. This makes the reaction highly economical, practical, operationally simple and sustainable. The methodology provides direct access to π-conjugated short oligomers consisting of furan, thiophene and indole. These compounds were found to show interesting fluorescence properties with remarkably large Stokes shift (up to 205 nm). Mechanistic investigations reveal that the reaction proceeds through chemoselective oxidation of indole by the metal catalyst followed by nucleophilic trapping by furan.

Cu(ii)-catalyzed aerobic oxidative coupling of furans with indoles enables expeditious synthesis of indolyl-furans with blue fluorescence.

With the purpose of incorporating sustainability in chemical processes, there has been a renewed focus on utilizing earth-abundant metal catalysts to expand the repertoire of organic reactions and processes. In this work, we have explored the atom-economic oxidative coupling between two important electron-rich heterocycles - indoles and furans - using commonly available, inexpensive metal catalyst CuCl2·2H2O (<0.25$ per g) to develop an expeditious synthesis of indolyl-furans. Moreover, the reaction proceeded well in the presence of the so-called 'ultimate oxidant' - air, without the need for any external ligand or additive. The reaction was found to be scalable and to work even under partially aqueous conditions. This makes the methodology highly economical, practical, operationally simple and sustainable. In addition, the methodology provides direct access to novel indole-furan-thiophene (IFT)-based electron-rich π-conjugated systems, which show green-yellow fluorescence with large Stokes shift and high quantum yields. Mechanistic investigations reveal that the reaction proceeds through chemoselective oxidation of indole by the metal catalyst followed by the nucleophilic attack by furan.

Spin effect on redox acceleration and regioselectivity in Fe-catalyzed alkyne hydrosilylation.

Iron catalysts are ideal transition metal catalysts because of the Earths abundant, cheap, biocompatible features of iron salts. Iron catalysts often have unique open-shell structures that easily undergo spin crossover in chemical transformations, a feature rarely found in noble metal catalysts. Unfortunately, little is known currently about how the open-shell structure and spin crossover affect the reactivity and selectivity of iron catalysts, which makes the development of iron catalysts a low efficient trial-and-error program. In this paper, a combination of experiments and theoretical calculations revealed that the iron-catalyzed hydrosilylation of alkynes is typical spin-crossover catalysis. Deep insight into the electronic structures of a set of well-defined open-shell active formal Fe(0) catalysts revealed that the spin-delocalization between the iron center and the 1,10-phenanthroline ligand effectively regulates the iron center's spin and oxidation state to meet the opposite electrostatic requirements of oxidative addition and reductive elimination, respectively, and the spin crossover is essential for this electron transfer process. The triplet transition state was essential for achieving high regioselectivity through tuning the nonbonding interactions. These findings provide an important reference for understanding the effect of catalyst spin state on reaction. It is inspiring for the development of iron catalysts and other Earth-abundant metal catalysts, especially from the point of view of ligand development.

Mononuclear Non-Heme Manganese-Catalyzed Enantioselective cis-Dihydroxylation of Alkenes Modeling Rieske Dioxygenases.

The practical catalytic enantioselective cis-dihydroxylation of olefins that utilize earth-abundant first-row transition metal catalysts under environmentally friendly conditions is an important yet challenging task. Inspired by the cis-dihydroxylation reactions catalyzed by Rieske dioxygenases and non-heme iron models, we report the biologically inspired cis-dihydroxylation catalysis that employs an inexpensive and readily available mononuclear non-heme manganese complex bearing a tetradentate nitrogen-donor ligand and aqueous hydrogen peroxide (H2O2) and potassium peroxymonosulfate (KHSO5) as terminal oxidants. A wide range of olefins are efficiently oxidized to enantioenriched cis-diols in practically useful yields with excellent cis-dihydroxylation selectivity and enantioselectivity (up to 99% ee). Mechanistic studies, such as isotopically 18O-labeled water experiments, and density functional theory (DFT) calculations support that a manganese(V)-oxo-hydroxo (HO-MnV═O) species, which is formed via the water-assisted heterolytic O-O bond cleavage of putative manganese(III)-hydroperoxide and manganese(III)-peroxysulfate precursors, is the active oxidant that effects the cis-dihydroxylation of olefins; this is reminiscent of the frequently postulated iron(V)-oxo-hydroxo (HO-FeV═O) species in the catalytic arene and alkene cis-dihydroxylation reactions by Rieske dioxygenases and synthetic non-heme iron models. Further, DFT calculations for the mechanism of the HO-MnV═O-mediated enantioselective cis-dihydroxylation of olefins reveal that the first oxo attack step controls the enantioselectivity, which exhibits a high preference for cis-dihydroxylation over epoxidation. In this study, we are able to replicate both the catalytic function and the key chemical principles of Rieske dioxygenases in mononuclear non-heme manganese-catalyzed enantioselective cis-dihydroxylation of olefins.

Titanium-Catalyzed Selective N-Alkylation of Amines with Alcohols via Borrowing Hydrogen Methodology.

The N-alkylation of amines with alcohols using earth-abundant and nonprecious metal catalysts has gained considerable attention in the pharmaceutical industry. We described titanium-catalyzed synthetic protocol for N-alkylation of amines with alcohols via borrowing hydrogen or hydrogen autotransfer reactions. The methodology enables the selective monoalkylation of various substituted (hetero)aromatic amines in good to excellent yields (up to 97% yield). The importance of the protocol was further demonstrated by the applicability of earth-abundant metal catalysis and the synthesis of 32 N-alkylated amines. The work allows the utilization of titanium-based catalysts for various reactions to expand the nature blueprint in catalysis.

Fe(CO)5-catalyzed oxidative photolytic cyclization of ferrocenated en-yne-aldehydes: Synthesis of ferrocene appended furans

En-yne-aldehydes are important organic motifs due to having a high synthetic potential of cyclization. Herein, the ferrocenated en-yne-aldehydes were successfully transformed into ferrocene appended furans by oxidative, photolytic exo-cyclic ring closure reaction in the presence of an earth-abundant metal catalyst, i.e., iron pentacarbonyl. Along with the formation of the furan ring, the carbon adjacent to furan gets oxidized into a carbonyl group. The transformation takes just 30 min to accomplish the desired product. The formation of these unusual ferrocene appended furan products were confirmed by single crystal X-Ray diffraction studies, FTIR, 1H & 13C NMR, and HRMS spectrometry. The general scope of the reaction was established and a library of 11 new ferrocene-appended furan products were developed. Here, the established reaction protocol was found to be active for aryl, alkyl, cycloalkyl, and ferrocenyl-substituted ene-yne-aldehydes and produced good to significant yield of the respective furan products. To the best of our minds, this report serves as the first example of iron-catalyzed photolytic oxidative cyclization of ferrocenated en-yne-aldehydes, and the products are first time reported in this report.

Probing the Origins of Puzzling Reactivity in Fe/Mn-Oxo/Hydroxo Species toward C-H Bonds: A DFT and Ab Initio Perspective.

Activation of C-H bonds using an earth-abundant metal catalyst is one of the top challenges of chemistry, where high-valent Mn/Fe-oxo(hydroxo) biomimic species play an important role. There are several open questions related to the comparative oxidative abilities of these species, and a unifying concept that could accommodate various factors influencing reactivity is lacking. To shed light on these open questions, here, we have used a combination of density functional theory (DFT) (B3LYP-D3/def2-TZVP) and ab initio (CASSCF/NEVPT2) calculations to study a series of high-valent metal-oxo species [Mn+H3buea(O/OH)] (M = Mn and Fe, n = II to V; H3buea = tris[(N'-tert-butylureaylato)-N-ethylene)]aminato towards the activation of dihydroanthracene (DHA). The H-bonding network in the ligand architecture influences the ground state-excited state gap and brings several excited states of the same spin multiplicity closer in energy, which triggers reactivity via one of those excited states, reducing the kinetic barriers for the C-H bond activation and rationalizing several puzzling reactivity trends observed in various high-valent Mn/Fe-oxo(hydroxo) species.

Silica Modulation of Raney Nickel Catalysts for Selective Hydrogenation

Selective hydrogenation over earth-abundant metal catalysts is challenging but particularly valuable for practical applications in heterogeneous catalysis. Herein, we demonstrate that the catalytic selectivity of the commercial Raney nickel catalyst can be greatly tuned by modulation of the nickel surface by silica. Using quinoline hydrogenation as a model, we show that the silica-modified Raney nickel catalysts exhibit good activity, excellent selectivity, and long stability, whereas the undesired overhydrogenation reactions are effectively hindered. In contrast, the pristine Raney nickel catalyst shows inferior selectivity for the targeted product. Mechanistic studies confirm a positive role of silica to facilitate the desorption of 1,2,3,4-tetrahydroquinoline from the catalyst surface, thus enhancing the product selectivity.

Size sensitivity of supported Ni nanoparticles on carbon for the synthesis of secondary amines from acceptorless dehydrogenation-hydrogenation of primary amines

Earth-abundant metal catalysts, especially Ni, are highly favourable in heterogeneous reactions compared with noble metal ones. Herein, Ni nanoparticle catalysts supported on Vulcan carbon (VCX) support were prepared through a simple impregnation-reduction method and utilized for the acceptorless dehydrogenation of primary amines to secondary amines. The best performance was achieved with Ni(A)/VXC with Ni particle size of 11.9 nm at 150 °C. Smaller or larger particle sizes resulted in a lower TOF, indicating a structure-sensitive activity. This work may serve as inspiration for further investigation into acceptorless dehydrogenation using non-noble metal catalysts.

Base- and Additive-Free Carbon Dioxide Hydroboration to Methoxyboranes Catalyzed by Non-Pincer-Type Mn(I) Complexes.

Well-defined, bench stable Mn(I) non-pincer-type complexes were tested as earth-abundant transition metal catalysts for the selective reduction of CO2 to boryl-protected MeOH in the presence of pinacolborane (HBpin). Essentially, quantitative yields were obtained under mild reaction conditions (1 bar CO2, 60 °C), without the need of any base or additives, in the presence of the alkylcarbonyl Mn(I) bis(phosphine) complexes fac-[Mn(CH2CH2CH3)(dippe)(CO)3] [Mn1, dippe = 1,2-bis(diisopropylphosphino)ethane] and [Mn(dippe)(CO)2{(μ-H)2(Bpin)}] (Mn4), that is obtained by reaction of the bench-stable precatalyst Mn1 with HBpin via elimination of butanal. Preliminary mechanistic details were obtained by a combination of NMR experiments and monitoring of the catalytic reactions.

A Highly Active Cobalt Catalyst for the General and Selective Hydrogenation of Aromatic Heterocycles.

Nanostructured earth abundant metal catalysts that mediate important chemical reactions with high efficiency and selectivity are of great interest. This study introduces a synthesis protocol for nanostructured earth abundant metal catalysts. Three components, an inexpensive metal precursor, an easy to synthesize N/C precursor, and a porous support material undergo pyrolysis to give the catalyst material in a simple, single synthesis step. By applying this catalyst synthesis, a highly active cobalt catalyst for the general and selective hydrogenation of aromatic heterocycles could be generated. The reaction is important with regard to organic synthesis and hydrogen storage. The mild reaction conditions observed for quinolines permit the selective hydrogenation of numerous classes of N-, O- and S-heterocyclic compounds such as: quinoxalines, pyridines, pyrroles, indoles, isoquinoline, aciridine amine, phenanthroline, benzofuranes, and benzothiophenes.

Hydrothermal catalytic conversion of oleic acid to heptadecane over Ni/ZrO2

There is growing interest in the substitution of expensive noble metal catalysts with low-cost earth-abundant metals in applications targeting biofuels production from waste organic feedstocks. Here, nickel (Ni) catalysts supported on zirconium dioxide (ZrO2), both with and without copper (Cu) as a co-metal, were tested in hydrothermal reactions of unsaturated and saturated C18 fatty acids as models for waste oil feedstocks. In contrast to recent reports, this study showed no enhancement of nickel's activity for fatty acid conversion to alkane products when Cu was present. Ni/ZrO2 prepared by metal coprecipitation methods converted 100% of oleic acid with 25% selectivity to heptadecane after 5 h of reaction at 350 °C using methanol as a hydrogen donor source, increasing to 41% heptadecane after 20 h. Lower yields were observed with catalysts prepared by wet impregnation and using alternative hydrogen donor sources (glycerol, formic acid). Greater selectivity to heptadecane was also observed at higher temperatures (370 °C) and when the initial fatty acid had greater saturation. Longer term exposure to hydrothermal media led to metal sintering and catalyst deactivation. Findings support a path forward to the development of earth-abundant metal catalysts for the upgrading of waste organic feedstocks.

Natural attapulgite supported nano-Ni catalysts for the efficient reductive amination of biomass-derived aldehydes and ketones

The efficient synthesis of useful primary amines via reductive amination of biomass-based aldehydes and ketones over earth-abundant base metal catalysts is an attractive biomass value-adding technology yet facing lots of challenges. Herein, natural attapulgite (ATP) was applied as support for the fabrication of active Ni catalysts with different Ni loadings (5–30 ​wt%) by the deposition-precipitation method. The Ni/ATP-550 catalyst with 10–15 ​wt% Ni loadings was found to present the highest catalytic performance for the synthesis of valuable 5-amino-1-pentanol (5-AP) via reductive amination of biofurfural-derived 2-hydroxytetrahydropyran among a variety of commonly used oxide supports loaded Ni catalysts, as well as ATP supported nickel catalysts with other loadings, achieving 5-AP yield up to 94%. The intrinsic activity of the Ni/ATP catalysts was found to depend strongly on the Ni0 crystallite size and Ni0 fraction of the catalysts, which generally increased with increasing Ni0 crystallite size and fraction, owing probably to the hydrogenation of imine intermediate is a structure-sensitive reaction. The efficient 10Ni/ATP-550 catalyst also exhibited good activity and stability in the reductive amination of several other biomass-derived aldehydes and ketones to their corresponding primary amines with good to excellent yields (81%–99%). This work provided a clean and efficient natural ATP-supported non-noble metal nickel-based catalytic system for the reductive amination of aldehydes and ketones to synthesize high-value-added primary amines, which could be a promising candidate for the industrial production of amines.

Morphology Modulation and Phase Transformation of Manganese-Cobalt Carbonate Hydroxide Caused by Fluoride Doping and Its Effect on Boosting the Overall Water Electrolysis.

Increasing demands for pollution-free energy resources have stimulated intense research on the design and fabrication of highly efficient, inexpensive, and stable non-noble earth-abundant metal catalysts with remarkable catalytic activity for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Morphology control of the catalysts is widely implemented as an effective strategy to change the surface atomic coordination and increase the catalytic behavior of the catalysts. In this study, we have designed a series of Mn-Co catalyts with different morphologies on the graphite paper substrate to enhance OER and HER activities in alkaline media. The prepared catalysts with different morphologies were successfully obtained by adjusting the amount of ammonium fluoride (NH4F) in the hydrothermal process. The electrochemical tests display that the cubic-like Mn-Co catalyst with pyramids on the faces at a concentration of 0.21 M NH4F exhibits the best activity toward both OER and HER. The cubic-like Mn-Co catalyst with pyramids on the faces showed overpotentials of 240 and 82 mV at a current density of 10 mA cm-2 for OER and HER, respectively. Also, the cubic-like Mn-Co catalyst with pyramids on the faces required overpotentials of 319 and 216 mV to reach the current density of 100 mA cm-2 for OER and HER, respectively. The current density of this catalyst at η = 0.32 V was 701.05 mA cm-2 for OER, and for HER, the current density of the catalyst was 422.89 mA cm-2 at η = 0.23 V. The Tafel slopes of the Mn-Co catalyst with cubic-like structures with pyramids on the faces were 78 and 121 mV dec-1 for OER and HER, respectively. A two-electrode overall water electrolysis system using this bifunctional Mn-Co catalyst exhibited low cell voltages of 1.60 in the alkaline electrolyte at the standard current density of 10 mA cm-2 with appropriate stability. These electrochemical merits exhibit the considerable potential of the cubic-like Mn-Co catalyst with pyramids on the faces for bifunctional OER and HER applications.