Switchable Catalysts Research Articles

Regulating Access to Active Sites via Hydrogen Bonding and Cation-Dipole Interactions: A Dual Cofactor Approach to Switchable Catalysis.

Hydrogen bonding networks are ubiquitous in biological systems and play a key role in controlling the conformational dynamics and allosteric interactions of enzymes. Yet in small organometallic catalysts, hydrogen bonding rarely controls ligand binding to the metal center. In this work, a hydrogen bonding network within a well-defined organometallic catalyst works in concert with cation-dipole interactions to gate substrate access to the active site. An ammine ligand acts as one cofactor, templating a hydrogen bonding network within a pendent crown ether and preventing the binding of strong donor ligands, such as nitriles, to the nickel center. Sodium ions are the second cofactor, disrupting hydrogen bonding to enable switchable ligand substitution reactions. Thermodynamic analyses provide insight into the energetic requirements of the different supramolecular interactions that enable substrate gating. The dual cofactor approach enables switchable catalytic hydroamination of crotononitrile. Systematic comparisons of catalysts with varying structural features provide support for the critical role of the dual cofactors in achieving on/off catalysis with substrates containing strongly donating functional groups that might otherwise interfere with switchable catalysts.

Photoswitchable coordination cages.

Stimuli-responsive behaviour is key to the design of smart materials, surfaces, nano-systems and effector molecules, allowing their application as switchable catalysts, molecular transporters, bioimaging probes or caged drugs. Supramolecular chemistry has embraced the widespread integration of photoswitches because of their precise spatiotemporal addressability and waste-free nature. In the vibrant area of discrete metal-mediated self-assembly, however, photoswitches are still rarely employed. Only recently has it been shown that embedding photoswitches into the organic backbones of coordination cages enables control of their host and material properties and thus unlocks the hitherto unexploited dynamic adaptivity of such systems. Here we discuss four cases where triggering ligand-integrated photoswitches leads to (1) control over disassembly/reassembly, (2) bi-stable switching between defined states, (3) interplay with thermal processes in metastable systems and (4) light-fuelled dissipative self-assembly. We highlight first clues concerning the relationship between fundamental photophysics and dynamic assembly equilibria and propose directions for future development.

Thermo-responsive polymer catalysts for polyester recycling processes: switching from homogeneous catalysis to heterogeneous separations

The use of thermo-responsive polymers in switchable catalysts enables the efficient and homogeneous chemical recycling of post-consumer poly(ethylene terephthalate) and the heterogeneous separation of catalyst in a single system.

Frontispiece: Rational design of allosteric switchable catalysts (EXP2 2/2022)

Toolbox of allosteric catalysts Allosteric regulation that controls the activities of natural enzymes motivates the exploration of expanded toolbox of switchable catalysts for chemical biology research. This review presents prospects of how allosteric scaffolds, ranging from single molecules to self-assemblies, could be designed into artificial switchable catalysts. The strategy of conformation-catalysis synergistics provides great inspirations and opportunities for fundamental research and biomedical applications.

Rational design of allosteric switchable catalysts.

Allosteric regulation, in many cases, involves switching the activities of natural enzymes, which further affects the enzymatic network and cell signaling in the living systems. The research on the construction of allosteric switchable catalysts has attracted broad interests, aiming to control the progress and asymmetry of catalytic reactions, expand the chemical biology toolbox, substitute unstable natural enzymes in the biological detection and biosensors, and fabricate the biomimetic cascade reactions. Thus, in this review, we summarize the recent outstanding works in switchable catalysts based on the allosterism of single molecules, supramolecular complexes, and self-assemblies. The concept of allosterism was extended from natural proteins to polymers, organic molecules, and supramolecular systems. In terms of the difference between these building scaffolds, a variety of design methods that tailor biological and synthetic molecules into controllable catalysts were introduced with emphasis.

Recent advances utilized in artificial switchable catalysis.

Developing “green” catalytic systems with desirable performance such as solubility, recyclability, and switchability is a great challenge. However, inspired by nature, the studies on synthesis and activity of artificial switchable metal catalysts and organocatalysts have become an intense, fervid, and challenging field of research. The peculiarity of these catalysts is that they can be generally triggered in the “on” or “off” states by several external stimuli such as light, heat, solvents, pH change, coordination events or ion influxes, redox processes, mechanical forces, or other changes in reaction conditions. A large number of review articles are available in these areas. However, most efforts are currently focused on the invention of new types of switchable catalysts with different forms of stimuli–response units incorporated within their architectures in order to achieve control over the catalytic activity and regio-, chemo- and stereocontrol of various chemical reactions. Thus, in this review, we begin with a brief introduction to switchable catalysts, followed by discussion of types of stimuli and the influence factors on their activities in the field of biomedical engineering, and catalysis as well as related catalytic mechanisms summarized and discussed. The emphasis is on the recent advances utilized in artificial switchable catalysis.

Light‐gated binding in double‐motorized porphyrin cages

AbstractMolecular motors change conformation under the influence of light and when attached to host molecules, they may find applications as sensors and switchable catalysts. Here, we present a porphyrin macrocyclic host functionalized with two motor appendages for future catalytic applications. The compound is formed as a mixture of six stereoisomers (three sets of enantiomers), which have been separated by (chiral) chromatography. 1H NMR and chiral spectroscopy revealed that in one set of enantiomers, the two motors interact with the cavity of the host (bound‐bound), whereas in a second set, one interacts and the other one does not (bound‐loose). In the third set, both motors do not interact with the host compound (loose‐loose). The motorized hosts bind guest molecules in the order: (loose‐loose) > (bound‐loose) > (bound‐bound). They can be switched with light to pseudoidentical diastereomers, leading to orthogonal behavior in the light‐gated binding of guest molecules. Although the photoisomerization of the diastereomer set loose‐loose significantly lowers the binding affinity for viologen guests, the opposite is true for the diastereomer set bound‐bound, that is, the binding affinity increases. For the diastereomer set bound‐loose, no influence on guest binding is observed as the effect of photoisomerization on the motors is cancelled out.Key Points We present a porphyrin cage compound with two light‐switchable molecular motors for future catalytic applications The double‐motorized porphyrin cage compound is formed as a mixture of six stereoisomers (three sets of enantiomers), which could be separated, and by photoswitching converted to pseudoidentical stereoisomers The stereoisomers display different binding affinities for guest molecules, which reverse on photoswitching, leading to orthogonal guest binding behavior

Ionic organic cage-encapsulated metal clusters for switchable catalysis

Summary The capability to build up stable, switchable catalysts for complex artificial systems that can mimic the responsiveness of biological systems to multiple triggers is highly desirable and challenging. Herein, we engineer such a system by physically locking catalytically active metal clusters (MCs) inside individual cationic molecular organic cages (I-Cages), where the responsive and task-specific counteranions of I-Cages impart MCs with a programmable gating effect. This allows for precise spatial and temporal control over chemical reactions by regulating accessibility of reagents to the MC sites. Following this strategy, we have successfully tailored catalytic activity of a series of model catalysis (e.g., methanolysis of ammonia borane, and the reduction of 4-nitroaniline) by employing light and pH stimuli, as well as their combination, for programmable-activity control.

Heterotrimetallic Carbon Dioxide Copolymerization and Switchable Catalysts: Sodium is the Key to High Activity and Unusual Selectivity.

A challenge in polymer synthesis using CO2 is to precisely control CO2 placement in the backbone and chain end groups. Here, a new catalyst class delivers unusual selectivity and is self‐switched between different polymerization cycles to construct specific sequences and desirable chain‐end chemistries. The best catalyst is a trinuclear dizinc(II)sodium(I) complex and it functions without additives or co‐catalysts. It shows excellent rates across different ring‐opening (co)polymerization catalytic cycles and allows precise control of CO2 incorporation within polyesters and polyethers, thereby allowing access to new polymer chemistries without requiring esoteric monomers, multi‐reactor processes or complex post‐polymerization procedures. The structures, kinetics and mechanisms of the catalysts are investigated, providing evidence for intermediate speciation and uncovering the factors governing structure and composition and thereby guiding future catalyst design.

Heterotrimetallic Carbon Dioxide Copolymerization and Switchable Catalysts: Sodium is the Key to High Activity and Unusual Selectivity

AbstractA challenge in polymer synthesis using CO2 is to precisely control CO2 placement in the backbone and chain end groups. Here, a new catalyst class delivers unusual selectivity and is self‐switched between different polymerization cycles to construct specific sequences and desirable chain‐end chemistries. The best catalyst is a trinuclear dizinc(II)sodium(I) complex and it functions without additives or co‐catalysts. It shows excellent rates across different ring‐opening (co)polymerization catalytic cycles and allows precise control of CO2 incorporation within polyesters and polyethers, thereby allowing access to new polymer chemistries without requiring esoteric monomers, multi‐reactor processes or complex post‐polymerization procedures. The structures, kinetics and mechanisms of the catalysts are investigated, providing evidence for intermediate speciation and uncovering the factors governing structure and composition and thereby guiding future catalyst design.

A Switchable Catalyst Duo for Acyl Transfer Proximity Catalysis and Regulation of Substrate Selectivity.

Enzymes are encoded with a gamut of information to catalyze a highly selective transformation by selecting the proper reactants from an intricate mixture of constituents. Mimicking biological machinery, two switchable catalysts with differently sized cavities and allosteric control are conceived that allow complementary size‐selective acyl transfer in an on/off manner by modulating the effective local concentration of the substrates. Selective activation of one of two catalysts in a mixture of reactants of similar reactivity enabled upregulation of the desired product.

Molecular motor-functionalized porphyrin macrocycles

Molecular motors and switches change conformation under the influence of an external stimulus, e.g. light. They can be incorporated into functional systems, allowing the construction of adaptive materials and switchable catalysts. Here, we present two molecular motor-functionalized porphyrin macrocycles for future photo-switchable catalysis. They display helical, planar and point chirality, and are diastereomers, which differ in the relative orientation of the motor and macrocyclic components. Fluorescence, UV-vis, and 1H NMR experiments reveal that the motor-functionalized macrocycles can bind and thread different variants of viologen guests, including a one-side blocked polymeric one of 30 repeat units. The latter feature indicates that the motor systems can find the open end of a polymer chain, thread on it, and move along the chain to eventually bind at the viologen trap, opening possibilities for catalytic writing on single polymer chains via chemical routes.

Application of Hemilabile Ligands to “At-Metal Switching” Hydrogenation Catalysis

The paper describes the development of switchable catalysts: i.e., precatalysts that are activated by a reagent and the resulting active catalyst can be shut off with a second reagent. A concept is introduced involving oxidative addition of a rhodium(I) catalyst with trityl chloride and reductive activation of dichlororhodium(III) phosphines with cobaltocene. Part 1 of the paper describes the development of the catalytic platforms, which are 2-diphenylphosphinoanisole (PPh2An) complexes of Rh and Ir. Part 2 describes the proof of concept as applied to the hydrogenation of styrene, including mechanistic investigations. The rhodium catalysts were developed from Rh2Cl2(C2H4)4, which was converted to Rh2Cl2(C2H4)2(κ1-PPh2An)2 and RhCl(κ1-PPh2An)(κ2-PPh2An). This charge-neutral chloride is a precursor to [Rh(κ2-PPh2An)2]BArF4 and the precatalyst [RhCl2(κ2-PPh2An)2]BArF4. The iridium catalysts were developed from Ir2Cl2(coe)4, which reacts with PPh2An to give IrClH(κ2-PPh2C6H4OCH2)(κ2-PPh2An). This cyclometalated complex behaves equivalently to IrCl(PPh2An)2. IrClH(κ2-PPh2C6H4OCH2)(κ2-PPh2An) readily reacts with H2 to form IrClH2(κ1-PPh2An)(κ2-PPh2An), which is a viable precursor to the off-state catalyst [IrCl2(κ2-PPh2An)2]BArF4. In part 2, we demonstrate that the complexes [MCl2(κ2-PPh2An)2]BArF4 (M = Rh, Ir) are inactive for styrene hydrogenation, in contrast with the other M-PAn compounds. Especially in the case of Rh, the hydrogenation is well controlled by the addition of selected reagents. Details of oxidative addition/reductive activation (OA/RA) are elucidated using cyclic voltammetry and stoichiometric chemical redox experiments.

Switchable Catalytic Polyoxometalate-Based Systems for Biomass Conversion to Carboxylic Acids.

We present the Keggin-type polyoxometalate H6[PV3Mo9O40] as a switchable catalyst being able to catalyze the transformation of both glucose and glyceraldehyde to formic acid (42%) and lactic acid (40%), respectively, within 1 h reaction time by simply changing the reaction atmosphere at 160 °C from oxygen to nitrogen in one reactor setup. In detail, we report the influence of different gas atmospheres and reaction temperatures on various vanadium-containing catalysts in the selective transformation of several biogenic substrates to carboxylic acids with a special emphasis on reaction pathways and switchability of the catalyst systems. All investigations were carried out in parallel using either an oxygen or a nitrogen atmosphere of 20 bar performing time-resolved experiments between 0.25 and 5 h and a temperature variation from 160 to 200 °C. Furthermore, a catalyst and a substrate variation led to the reaction system consisting of glyceraldehyde and the Keggin-type polyoxometalates (POM) H6[PV3Mo9O40] as the best switchable reaction system under the applied conditions. This study shows interesting potential for using both Keggin-type and Lindqvist-type POMs as switchable catalysts for selective biomass conversion to platform chemicals.

Switchable iridium hydride catalysts for controlling selectivity of alcohol oxidation

Novel pyridyl-triazolylidene iridium(III) hydride complexes have been synthesized through modification of the analogous iridium chloride complexes. The dehydration of alcohols was used to probe the catalytic potential of the iridium chloride compounds and the influence of the electronic modification on the pyridyl-triazolylidene ligand scaffolds. The incorporation of electron donor substituents on the triazolylidene heterocycle considerably enhanced the catalytic activity of the coordinated iridium center towards the catalytic dehydration of alcohols. Moreover, the iridium hydride compounds are switchable catalysts that perform either alcohol dehydration or dehydrogenation. Their selectivity was predictably triggered by the presence or absence of HPF6 in the catalytic reaction.

Switchable Catalysts for Chemical CO2 Recycling: A Step Forward in the Methanation and Reverse Water–Gas Shift Reactions

Advanced catalytic technologies are crucial to enable the transition towards a low-carbon industry with minimised CO2 emissions. This paper showcases the application of highly effective Ru-promoted...

Photoactivated Artificial Molecular Machines that Can Perform Tasks.

Research on artificial photoactivated molecular machines has moved in recent years from a basic scientific endeavor toward a more applicative effort. Nowadays, the prospect of reproducing the operation of natural nanomachines with artificial counterparts is no longer a dream but a concrete possibility. The progress toward the construction of molecular-machine-based devices and materials in which light irradiation results in the execution of a task as a result of nanoscale movements is illustrated here. After a brief description of a few basic types of photoactivated molecular machines, significant examples of their exploitation to perform predetermined functions are presented. These include switchable catalysts, nanoactuators that interact with cellular membranes, transporters of small molecular cargos, and active joints capable of mechanically coupling molecular-scale movements. Investigations aimed at harnessing the collective operation of a multitude of molecular machines organized in arrays to perform tasks at the microscale and macroscale in hard and soft materials are also reviewed. Surfaces, gels, liquid crystals, polymers, and self-assembled nanostructures are described wherein the nanoscale movement of embedded molecular machines is amplified, allowing the realization of muscle-like actuators, microfluidic devices, and polymeric materials for light energy transduction and storage.

Switchable Catalysts Used To Control Suzuki Cross-Coupling and Aza–Michael Addition/Asymmetric Transfer Hydrogenation Cascade Reactions

The development of a switchable strategy to control the catalytic action of dual active species is of great significance toward the precise manipulation of a cascade reaction. Herein, by combining ...

Reversible Structural Transformations of Metal-Organic Frameworks as Artificial Switchable Catalysts for Dynamic Control of Selectively Cyanation Reaction.

The synthesis of molecular-level artificial switchable catalysts, of which activity in different chemical processes can be switched by controlling different stimuli, has provided a new paradigm to perform mechanical tasks and measurable work. In this work, to obtain highly effective and regioselective artificial switchable catalysts, a hierarchical anion-pillared framework {(H3 O)[Cu(CPCDC)(4,4'-bpy)]}n (1; H3 CPCDC=9-(4-carboxyphenyl)-9H-carbazole-3,6-dicarboxylic acid, 4,4'-bpy=4,4'-bipyridine), including free [H3 O]+ ions as guest molecules, was constructed. Upon dissolve-exchange-crystallization behavior, fascinating reversible structural transformations proceeded between anion framework 1 and neutral 2D stair-stepping framework {[Cu(CPCDC)(4,4'-bpe)]}n (2; 4,4'-bpe=4,4'-vinylenedipyridine). Moreover, frameworks 1 and 2 can act as heterogeneous artificial switchable catalysts to selectively promote the direct cyanation reaction of terminal alkynes and azobisisobutyronitrile. The results indicated that 1 and 2 exhibited excellent selectivity to generate vinyl isobutyronitrile skeletons or propiolonitrile frameworks, respectively, as unique products. Furthermore, indicating paper, GC-MS, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy analysis demonstrated that the reversible structural transformations endowed 1 and 2 with well-defined platforms to stabilize the isobutyronitrile and CN sources through the different catalytic pathways.

Redox-Switchable Ring-Opening Polymerization with Ferrocene Derivatives.

Switchable catalysts incorporate stimuli-responsive features and allow synthetic tasks that are difficult or impossible to accomplish in other ways. They mimic biological processes in that they can provide both spatial and temporal control, unlike most reactions promoted by human-made catalyststhat usually occur according to carefully optimized conditions. In the area of switchable catalysis, redox-switchable ring-opening polymerization (ROP) has attracted much attention, emerging as a powerful strategy for the development of environmentally friendly biodegradable copolymers, especially those containing blocks with complementary properties. Controlling the sequence and regularity of each copolymeric building block can affect the material properties significantly since they are directly related to the respective microstructures. Such control can be exerted with a well-designed redox-switchable catalyst by timing the oxidation and reduction events. In highly selective systems, one form of the catalyst reacts with a monomer until the redox state of the catalyst is altered, at which point the alteredstate of thecatalyst reacts with another monomer. The reaction time may be varied from one cycle to another to generate various designer multiblock copolymers. The first instance of redox-mediated ROP was described by N. Long and co-workers in 2006. This example, as well as many early reported redox-switchable catalysts, could only achieve an on/off switch of activity toward a single monomer or substrate. However, our efforts brought on a general strategy for designing redox-switchable metal complexes that can catalyze different reactions in two oxidation states. In recent years, our contributions to this research field led to the synthesis of several multiblock copolymers prepared from biorenewable resources. This Account provides an overview of reported redox-switchable polymerization catalysts that allow for complementary reactivity in different oxidation states and highlights the potential of this strategy in preparing biodegradable materials. First, we define the field of redox-switchable catalysis and illustrate the design and significance of our ferrocene-chelating ligands, in which the oxidation state of iron in ferrocene can control the reactivity of the resulting metal complexes remotely. Next, we illustrate recent advances in the synthesis of new biodegradable copolymers including (1) how to tune the activity of the ROP catalysts by exploring various metal centers and ferrocene-based ligand combinations; (2) how to synthesize new multiblock copolymers of cyclic esters, epoxides, and carbonates by redox-switchable ROP; and (3) how to understand the mechanism of these reactions by discussing both experimental and theoretical investigations. By the application and development of redox-switchable strategies, various novel materials and reactions can be expected in the future.