Macrocyclic Catalysts Research Articles

A Minireview on Porphyrin and Phthalocyanine Based Catalysts for Electrocatalytic Synthesis of Ammonia

AbstractAmmonia (NH3) is an important raw material in the chemical industry, but the synthesis of NH3 by the traditional Haber‐Bosch process will increase the carbon footprint. Therefore, it is necessary to develop sustainable routes for NH3 production. New NH3 production schemes, including nitrogen reduction (N2RR), nitrite reduction (NO2RR), and nitrate reduction (NO3RR), have been proposed. Porphyrins and phthalocyanines are macrocyclic compounds with a central metal ion coordinated with nitrogen. The metal centers in these catalysts play a crucial role in binding and activating nitrogen, nitrite, and nitrate. Their unique structure allows for effective electron transfer and catalytic activation in NH3 synthesis. Recently, metal porphyrin and phthalocyanine based catalysts have been demonstrated to be efficient in catalyzing N2RR, NO2RR, and NO3RR. Unfortunately, there is no review focusing on such macrocyclic catalysts for the electrocatalytic synthesis of NH3. In this review, we discuss the electrocatalytic reduction performances and summarize the key factors and reaction mechanisms that affect the catalytic performance of metal porphyrin and phthalocyanine based catalyst systems. This review helps to design more effective new electrocatalysts for NH3 synthesis.

Ammonium-Binding Bifunctional Aza-Crown Ether Catalysts for Substrate-Selective Hydroxyl Functionalization.

Herein, we describe a new bifunctional macrocyclic catalyst that employs multiple weak noncovalent interactions to enable substrate-selective O-silylation of ammonium alcohols over more reactive aliphatic alcohols with up to >20:1 substrate selectivity. Our catalytic strategy merges (i) the use of crown ethers as ammonium-binding receptors and (ii) the exploitation of N-methyl imidazole as a catalytic motif. Our collective mechanistic studies reveal the importance of receptor size, conformational preorganization, and the number of hydrogen-bonding acceptor units needed to achieve high selectivity within the macrocyclic binding pocket.

Chiral Bis-phosphate Macrocycles for Catalytic, Efficient, and Enantioselective Electrophilic Fluorination.

Incorporation of privileged catalytic scaffolds into a macrocyclic skeleton represents an attractive strategy to furnish supramolecular catalysis systems with enzyme-mimetic cavity and multi-site cooperation. Herein we reported the synthesis, structure, binding properties and catalytic application of a series of chiral bis-phosphate macrocycles toward the challenging asymmetric electrophilic fluorination. With a large, integrated chiral cavity and two cooperative phosphate sites, these macrocycles exhibited good inclusion toward 1,4-diazabicyclo[2.2.2]octane (DABCO) dicationic ammoniums through complementary ion-pair and C-H⋅⋅⋅O interactions, as confirmed by crystallographic and solution binding studies. In fluorocyclization of tryptamines with Selectfluor reagent which has a similar DABCO-based dicationic structure, only 2 mol% macrocycle catalyst afforded the desired pyrroloindoline products in moderate yields and up to 91 % ee. For comparison, the acyclic mono-phosphate analogue gave obviously lower reactivity and enantioselectivity (<20 % ee), suggesting a remarkable macrocyclic effect. The high catalytic efficiency and superior stereocontrol were ascribed to the tight ion-pair binding and cavity-directed noncovalent interaction cooperation.

Unveiling the Activity and Mechanism Alterations by Pyrene Decoration on a Co(II) Macrocyclic Catalyst for CO2 Reduction.

Mechanistic studies involving characterization of crucial intermediates are desirable for rational optimization of molecular catalysts toward CO2 reduction, while fundamental challenges are associated with such studies. Herein we present the systematic mechanistic investigations on a pyrene-appended CoII macrocyclic catalyst in comparison with its pyrene-free prototype. The comparative results also verify the reasons of the higher catalytic activity of the pyrene-tethered catalyst in noble-metal-free CO2 photoreduction with various photosensitizers, where a remarkable apparent quantum yield of 36±3 % at 425 nm can be obtained for selective CO production. Electrochemical and spectroelectrochemical studies in conjunction with DFT calculations between the two catalysts have characterized the key CO-bound intermediates and revealed their different CO-binding behavior, demonstrating that the pyrene group endows the corresponding CoII catalyst a lower catalytic potential, a higher stability, and a greater ease in CO release, all of which contribute to its better performance.

Molecular engineering binuclear copper catalysts for selective CO2 reduction to C2 products

Molecular copper catalysts serve as exemplary models for correlating the structure-reaction-mechanism relationship in the electrochemical CO2 reduction (eCO2R), owing to their adaptable environments surrounding the copper metal centres. This investigation, employing density functional theory calculations, focuses on a novel family of binuclear Cu molecular catalysts. The modulation of their coordination configuration through the introduction of organic groups aims to assess their efficacy in converting CO2 to C2 products. Our findings highlight the crucial role of chemical valence state in shaping the characteristics of binuclear Cu catalysts, consequently influencing the eCO2R behaviour. Notably, the Cu(II)Cu(II) macrocycle catalyst exhibits enhanced suppression of the hydrogen evolution reaction (HER), facilitating proton transfer and the eCO2R process. Furthermore, we explore the impact of diverse electron-withdrawing and electron-donating groups coordinated to the macrocycle (R = –F, –H, and –OCH3) on the electron distribution in the molecular catalysts. Strategic placement of –OCH3 groups in the macrocycles leads to a favourable oxidation state of the Cu centres and subsequent C–C coupling to form C2 products. This research provides fundamental insights into the design and optimization of binuclear Cu molecular catalysts for the electrochemical conversion of CO2 to value-added C2 products.

Chiral Macrocyclic Catalysts for the Enantioselective Addition of Lithium Acetylides to Ketones.

Alkynyl addition to carbonyl compounds is a valuable synthetic method for the preparation of versatile chiral alcohols that are widely found in pharmaceuticals and natural products. Although a variety of enantioselective variations have been reported, alkynyl addition to simple ketones remains an unmet challenge due to their low reactivity and difficult enantiofacial discrimination. Here, we report a method for the catalytic enantioselective addition of lithium acetylide to a variety of ketones using macrocyclic lithium binaphtholates as catalysts. These reactions generally suffer from facile aggregation of lithium species, which leads to less active and selective catalysts. The macrocyclic structure designed in this study prevents such aggregation, affording a monomeric and highly active catalyst that can furnish enantioenriched tertiary alcohols from a variety of ketones within 5-30 min. Moreover, the confined cavity and lipophilicity of the macrocycle confer substrate specificity on the system, demonstrating a multiselectivity similar to that of enzymatic reactions. Thus, these findings offer new insights into the rational design of small-molecule artificial enzymes that exhibit high levels of reactivity and multiselectivity.

Precious-Metal-Free CO2 Photoreduction Boosted by Dynamic Coordinative Interaction between Pyridine-Tethered Cu(I) Sensitizers and a Co(II) Catalyst.

Improving the photocatalytic efficiency of a fully noble-metal-free system for CO2 reduction remains a fundamental challenge, which can be accomplished by facilitating electron delivery as a consequence of exploiting intermolecular interactions. Herein, we have designed two Cu(I) photosensitizers with different pyridyl pendants at the phenanthroline moiety to enable dynamic coordinative interactions between the sensitizers and a cobalt macrocyclic catalyst. Compared to the parent Cu(I) photosensitizer, one of the pyridine-tethered derivatives boosts the apparent quantum yield up to 76 ± 6% at 425 nm for selective (near 99%) CO2-to-CO conversion. This value is nearly twice that of the parent system with no pyridyl pendants (40 ± 5%) and substantially surpasses the record (57%) of the noble-metal-free systems reported so far. This system also realizes a maximum turnover number of 11 800 ± 1400. In contrast, another Cu(I) photosensitizer, in which the pyridine substituents are directly linked to the phenanthroline moiety, is inactive. The above behavior and photocatalytic mechanism are systematically elucidated by transient fluorescence, transient absorption, transient X-ray absorption spectroscopies, and quantum chemical calculations. This work highlights the advantage of constructing coordinative interactions to fine-tune the electron transfer processes within noble-metal-free systems for CO2 photoreduction.

Precisely manipulated π–π stacking of catalytic exfoliated iron polyphthalocyanine/reduced graphene oxide hybrid via pyrolysis-free path

Metal macrocycles with well-defined molecular structures are ideal platforms for the in-depth study of electrochemical oxygen reduction reaction (ORR). Structural integrity of metal macrocycles is vital but remain challenging since the commonly used high-temperature pyrolysis would cause severe structure damage and unidentifiable active sites. Herein, we propose a pyrolysis-free strategy to precisely manipulate the exfoliated 2D iron polyphthalocyanine (FePPc) anchored on reduced graphene oxide (rGO) via π–π stacking using facile high-energy ball milling. A delocalized electron shift caused by π–π interaction is firstly found to be the mechanism of facilitating the remarkable ORR activity of this hybrid catalyst. The optimal FePPc@rGO-HE achieves superior half-wave potential (0.90 V) than 20 % Pt/C. This study offers a new insight in designing stable and high-performance metal macrocycle catalysts with well-defined active sites.

Efficient Transformation of Polymer Main Chain Catalyzed by Macrocycle Metal Complexes via Pseudorotaxane Intermediate

AbstractPost‐synthesis modification of polymers streamlines the synthesis of functionalized polymers, but is often incomplete due to the negative polymer effects. Developing efficient polymer reactions in artificial systems thus represents a long‐standing objective in the fields of polymer and material science. Here, we show unprecedented macrocycle‐metal‐complex‐catalyzed systems for efficient polymer reaction that result in 100 % transformation of the main chain functional groups presumably via a processive mode reaction. The complete polymer reactions were confirmed in not only intramolecular reaction (hydroamination) but also intermolecular reaction (hydrosilylation) by using Pd‐ and Pt‐macrocycle‐catalyzed systems. The most fascinating feature of the both reactions is that higher‐molecular‐weight polymers reach completion faster. Various studies suggested that the reactions occur in the catalyst cavity via the formation of a supramolecular complex between the macrocycle catalyst and polymer substrate like pseudorotaxane, which should be of characteristic of the efficient polymer reactions progressing in a processive mode.

Efficient Transformation of Polymer Main Chain Catalyzed by Macrocycle Metal Complexes via Pseudorotaxane Intermediate.

Post-synthesis modification of polymers streamlines the synthesis of functionalized polymers, but is often incomplete due to the negative polymer effects. Developing efficient polymer reactions in artificial systems thus represents a long-standing objective in the fields of polymer and material science. Here, we show unprecedented macrocycle-metal-complex-catalyzed systems for efficient polymer reaction that result in 100 % transformation of the main chain functional groups presumably via a processive mode reaction. The complete polymer reactions were confirmed in not only intramolecular reaction (hydroamination) but also intermolecular reaction (hydrosilylation) by using Pd- and Pt-macrocycle-catalyzed systems. The most fascinating feature of the both reactions is that higher-molecular-weight polymers reach completion faster. Various studies suggested that the reactions occur in the catalyst cavity via the formation of a supramolecular complex between the macrocycle catalyst and polymer substrate like pseudorotaxane, which should be of characteristic of the efficient polymer reactions progressing in a processive mode.

Impaired conjugation boosts CO2 electroreduction by Ni(ii) macrocyclic catalysts immobilized on carbon nanotubes

Attenuated conjugation in a Ni(ii) catalyst simultaneously facilitates the molecule–support interactions and the intrinsic activity, unexpectedly displaying a superior performance in CO2 electroreduction.

Lewis acid-driven self-assembly of diiridium macrocyclic catalysts imparts substrate selectivity and glutathione tolerance.

Molecular inorganic catalysts (MICs) tend to have solvent-exposed metal centers that lack substrate specificity and are easily inhibited by biological nucleophiles. Unfortunately, these limitations exclude many MICs from being considered for in vivo applications. To overcome this challenge, a strategy to spatially confine MICs using Lewis acid-driven self-assembly is presented. It was shown that in the presence of external cations (e.g., Li+, Na+, K+, or Cs+) or phosphate buffered saline, diiridium macrocycles spontaneously formed supramolecular iridium-cation species, which were characterized by X-ray crystallography and dynamic light scattering. These nanoassemblies selectively reduced sterically unhindered C[double bond, length as m-dash]O groups via transfer hydrogenation and tolerated up to 1 mM of glutathione. In contrast, when non-coordinating tetraalkylammonium cations were used, the diiridium catalysts were unable to form higher-ordered structures and discriminate between different aldehyde substrates. This work suggests that in situ coordination self-assembly could be a versatile approach to enable or enhance the integration of MICs with biological hosts.

A tape-reading molecular ratchet.

Cells process information in a manner reminiscent of a Turing machine1, autonomously reading data from molecular tapes and translating it into outputs2,3. Randomly processive macrocyclic catalysts that can derivatise threaded polymers have been described4,5, as have rotaxanes that transfer building blocks in sequence from a molecular strand to a growing oligomer6-10. However, synthetic small-molecule machines that can read and/or write information stored on artificial molecular tapes remain elusive11-13. Here we report on a molecular ratchet in which a crown ether (the 'reading head') is pumped from solution onto an encoded molecular strand (the 'tape') by a pulse14,15 of chemical fuel16. Further fuel pulses transport the macrocycle through a series of compartments of the tape via an energy ratchet14,17-22 mechanism, before releasing it back to bulk off the other end of the strand. During its directional transport, the crown ether changes conformation according to the stereochemistry of binding sites along the way. This allows the sequence of stereochemical information programmed into the tape to be read out as a string of digits in a non-destructive manner through a changing circular dichroism response. The concept is exemplified by the reading of molecular tapes with strings of balanced ternary digits ('trits'23), -1,0,+1 and -1,0,-1. The small-molecule ratchet is a finite-state automaton: a special case24 of a Turing machine that moves in one direction through a string-encoded state sequence, giving outputs dependent on the occupied machine state25,26. It opens the way for the reading-and ultimately writing-of information using the powered directional movement of artificial nanomachines along molecular tapes.

Mixed Metal Phthalocyanine-Modified Carbon Nanotubes for Bifunctional Oxygen Reduction and Evolution Reaction

In this ever-expanding world, researchers are focused on changing how we consume energy. Progress has been made over time, but the devices such as low-temperature polymer electrolyte fuel cells and metal-air batteries that can replace the conventional fossil fuel engines still use noble metal catalysts,1 which hinders the mass production and long-term usage of the devices. Transition metal macrocyclic (MN4-type) catalysts are good alternatives for the noble metal catalysts due to their low-price and promising electrocatalytic activity.2 Thus, the aim of this study was to develop the bifunctional ORR/OER electrocatalysts using mixed transition metal phthalocyanine-modified multi-walled carbon nanotubes (MWCNT) by simple pyrolysis method.3 Among the prepared bifunctional catalysts, FeCoN-MWCNT showed the superior electrocatalytic ORR activity in alkaline media with half-wave potential of 0.86 V vs RHE and FeNiN-MWCNT catalyst exhibited the exceptional OER activity with E j=10 mA cm–2 of 1.58 V (see Fig. 1a, b). Physical characterization of the prepared catalysts was investigated with (scanning) electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and Raman spectroscopy. Further, catalysts were tested in anion-exchange membrane fuel cells (AEMFCs) as cathode material and very good peak power density of 692 mW cm–2 was obtained for FeCoN-MWCNT (see Fig. 1c). FeNiN-MWCNT was tested in anion-exchange membrane electrolyzer (AEMEL) and showed comparable results with state-of-the-art RuO2 catalysts (Fig. 1d). Altogether, this study unveils the insight into potential ways of preparing the bifunctional ORR/OER electrocatalysts with transition metal macrocyclic-based compounds. References S. Hussain, H. Erikson, N. Kongi, A. Sarapuu, J. Solla-Gullón, G. Maia, A. M. Kannan, N. Alonso-Vante, K. Tammeveski, Int. J. Hydrogen Energy, 45, 31775-31797 (2020).A. Sarapuu, E. Kibena-Põldsepp, M. Borghei, K. Tammeveski, J. Mater. Chem. A, 6, 776–804 (2018).Y. Kumar, E. Kibena-Põldsepp, J. Kozlova, M. Rähn, A. Treshchalov A. Kikas, V. Kisand, J. Aruväli, A. Tamm, J. C. Douglin, S. J. Folkman, I. Gelmetti, F. A. Garcés-Pineda, J. R. Galán-Mascarós, D. R. Dekel, K. Tammeveski, ACS Appl. Mater. Interfaces, 13, 41507–41516 (2021). Figure 1

Bioinspired tetraamino-bisthiourea chiral macrocycles in catalyzing decarboxylative Mannich reactions.

A series of tetraamino-bisthiourea chiral macrocycles containing two diarylthiourea and two chiral diamine units were synthesized by a fragment-coupling approach in high yields. Different chiral diamine units, including cyclohexanediamines and diphenylethanediamines were readily incorporated by both homo and hetero [1 + 1] macrocyclic condensation of bisamine and bisisothiocyanate fragments. With the easy synthesis, gram-scale of macrocycle products can be readily obtained. These chiral macrocycles were applied in catalyzing bioinspired decarboxylative Mannich reactions. Only 5 mol % of the optimal macrocycle catalyst efficiently catalyzed the decarboxylative addition of a broad scope of malonic acid half thioesters to isatin-derived ketimines with excellent yields and good enantioselectivity. The rigid macrocyclic framework and the cooperation between the thiourea and tertiary amine sites were found to be crucial for achieving efficient activation and stereocontrol. As shown in control experiments, catalysis with the acyclic analogues having the same structural motifs were non-selective.

Fluorinated cobalt phthalocyanine axially coordinated to oxo functionalities on different dimensional carbon (1D–3D) for durable oxygen reduction reaction

The development of low-cost, durable and easily available electrocatalysts as alternatives to Pt-based catalysts for the oxygen reduction reaction (ORR) will promote the large-scale commercial application of fuel cells and metal-air batteries. Among the transition metal based N4-metallomacrocyclic molecules, the Co macrocycle catalyst usually exhibits superior long-term durability and stability than the Fe family. Developing efficient Co-N4 ORR electrocatalyst is of great significance. In the work, fluorinated cobalt phthalocyanine (CoPcF16) were anchored on various carbon materials in different structural dimensions (1–3D) including 1D carboxyl carbon nanotube (CNT), 1D carbon nanofibers (CNF), 2D nitrogen doped graphene (N-G), and 3D ordered mesoporous carbon CMK-3, which were explored and investigated as electrocatalysts for ORR. The results show that the molecular CoPcF16 anchored on theses carbon materials exhibited different electrocatalytic activity with the increased half-wave potential (E1/2) order of CMK-3>CNT>N-G>CNF, which is related to oxo functionalities (axial coordination to Co), porous structures and defective sites. The CoPcF16-carbons hybrids exhibited much lower tafel slope and comparable E1/2 than commercial Pt/C catalysts for ORR. The prepared catalysts also demonstrated excellent stability even after 5000 cycling. CoPcF16-carbons hybrids will be potential candidates to replace Pt-based catalysts for PEMFCs applications in future.

Enhancing the electrocatalytic activity of Fe phthalocyanines for the oxygen reduction reaction by the presence of axial ligands: Pyridine-functionalized single-walled carbon nanotubes

We have examined the electrocatalytic activity of iron phthalocyanine (FePc) and perchlorinated iron phthalocyanine 16(Cl)FePc for the oxygen reduction reaction (ORR) in alkaline medium with the two molecules either adsorbed on the external surface of single-wall carbon nanotubes (SWCNT) or covalently anchored via an axial pyridine ligand on pyridine-functionalized single-wall carbon nanotubes (py-SWCNT). Regardless of the particular phthalocyanine type, the ORR activity is higher when the substrate is py-SWCNT rather than SWCNT. The Tafel slopes for ORR are very similar for the two Fe macrocyclic complexes attached to SWCNTs in the two different configurations, suggesting a common rate-determining step for the ORR for all four catalysts. It is also observed that, for both the SWCNT and py-SWCNT supports, the ORR activity is higher for 16(Cl)FePc than for FePc. This is attributed to the electron-withdrawing effect of the peripheral and non-peripheral chlorine atoms in the macrocyclic ligand. While FeN4 macrocycles are known to be located on the strong binding side of a volcano correlation including several MN4 species, the chlorine substituents decrease the binding energy of O2 on the central Fe cation, thereby moving up the macrocycle catalyst towards the apex of the volcano correlation. Both the carbon surface and macrocyclic ligand effects were optimized with 16(Cl)FePc attached to py-SWCNT, which is more active than both FePc on SWCNT and FePc on py-SWCNT. Here we show that both the axial ligand and the electron withdrawing groups (-Cl) have a combined collaborative effect in increasing the catalytic activity for ORR of Fe-phthalocyanines confined on the external walls of single-wall carbon nanotubes.

Low-Cost Single Chamber MFC Integrated With Novel Lignin-Based Carbon Fiber Felt Bioanode for Treatment of Recalcitrant Azo Dye

A flow through anaerobic microbial fuel cell (MFC) was designed and optimized for efficient treatment of recalcitrant textile wastewater. The membrane-less MFC was first time fabricated with a unique combination of electrodes, a novel bioanode of synthesized lignin-based electrospun carbon fiber supporting a biofilm ofGeobacter sulfurreducensfor acetate oxidation and an air-breathing cathode, consisting of a pyrolyzed macrocycle catalyst mixture on carbon bonded by polytetrafluoroethylene (PTFE). The effects of different organic loadings of acetate along with Acid Orange (AO5), operation time and ionic strength of auxiliary salts (conductivity enhancers) were investigated and responses in terms of polarization and degradation were studied. In addition, the decomposition of the organic species and the degradation of AO5 along with its metabolites and degraded products (2-aminobenzenesulfonic acid) were determined by chemical oxygen demand (COD) analysis, UV-Vis spectrophotometry and high-performance liquid chromatography (UV-HPLC) techniques. SEM and TEM images were also used to find out the biocompatibility of the microbes on lignin-based electrospun carbon felt anode and the morphology of the cathode. Reduction and breakage of the azo bond of AO5 occurs presumably as a side reaction, resulting in the formation of 2-aminobenzenesulfonic acid and unidentified aromatic amines. Maximum current density of anode 0.59 Am−2and power density of 0.12 Wm−2were obtained under optimized conditions. As a result, decolouration of AO5 and chemical oxygen demand (COD) removal efficiency was 81 and 58%, respectively. These results revealed that the low-cost MFC assembly can offer significant potential for anaerobic decolouration of recalcitrant textile wastewater.

Alkene Epoxidations Mediated by Mn-Salen Macrocyclic Catalysts

Three new chiral Mn macrocycle catalysts containing 20 or 40 atoms in the macrocycle were synthetized and tested in the enantioselective epoxidation of cis-β-ethyl-styrene and 1,2-dihydronathalene. The effect of the presence of a binaphtol (BINOL) compound in the catalyst backbone has been evaluated, including by Density Functional Theory (DFT) calculations.

Macrocycles in Bioinspired Catalysis: From Molecules to Materials.

Macrocyclic compounds have been studied extensively as the host molecules in supramolecular chemistry. Their structural characteristics make macrocycles desirable in the field of molecular recognition, which is the key to high catalytic efficiencies of natural enzymes. Therefore, macrocycles are ideal building blocks for the design of bioinspired catalysts. This mini review highlights recent advances ranging from single-molecule to metal-organic framework materials, exhibiting multilevel macrocycle catalysts with unique catalytic centers and substrate-binding affinities.