Digest paperRecent developments on artificial switchable catalysis
Graphical abstract
Introduction
Science is driven by imagination and knowledge. For any future scientific development, while knowledge provides the necessary input, an imagination reflects the potentiality of the output. The 2016 Nobel Prize in Chemistry truly portrays it. Artificial molecular machines, that garnered this recognition, are synthetic molecules which perform various stimuli-controlled ‘machine-like’ tasks.1 Nature uses a bunch of smart biological machines for carrying out uncountable parallel processes with an unimaginable level of proficiency. This fascinating truth has inspired catalysis researchers also to explore their knowledge and imagination in designing and constructing artificial stimuli-controlled catalysts. Such inspiration derived from Nature is, of course, inherently associated with an obvious demand in modern-day catalysis as it plays a pivotal role in the synthesis of valuable chemicals and materials. Eventually, the research field of ‘artificial switchable catalysis’ has emerged with a goal of offering advanced and controllable catalytic output in terms of reaction-rate, chemo-/regio-/stereo-selectivity, tandem and orthogonality, and even bi-/multi-functionality etc.2
Typically, the artificial switchable catalysts are equipped with smart structural features that respond reversibly to external stimuli such as light, pH, redox event, metal-ion coordination, or any other physicochemical trigger, to induce an on/off-type regulatory function related to one or more chemical events during catalysis. With the help of this sophisticated chemistry, therefore, one can achieve a good degree of control over the outcome of catalytic reactions. The most common type of ‘control’ achieved so far in majority of the reported switchable catalysis is that on the reaction rate, that is, switching the catalysis ‘on’ (upregulation) or ‘off’ (downregulation) in response to the applied stimulus. However, examples of more advanced control such as modulation of other stereochemical outcomes or orthogonal control of different catalysts/catalytic processes are slowly emerging.
Although the field of ‘artificial switchable catalysis’ is young but the advances in this budding research area have already started to emerge. Leigh et al. elucidated the state-of-the-art developments in this field and elegantly covered various types of stimuli-switchable catalysis in an overall comprehensive review in 2015.2a A perspective on exclusively photoswitchable catalysis was also published in 2013 by Neilson and Bielawski.2b Similarly, an extensive review by Bielawski et al. in 2016 focusing exclusively on switchable polymerization catalysis highlighted the significance of this rapidly-growing field.2c Very recently, in 2017, Luisi et al. emphasized the important developments in switchable chiral catalysis along with the advantages of stimuli-driven dynamic control of enantioselectivity.2d Therefore, this Digest focusses only on some of the latest and novel developments in this field to underscore the power of artificial switchable catalysts both on the conceptual as well as application facets. It is to be mentioned here that several important works have been reported in literature in which only enhancement of catalytic activity induced by stimuli (e.g., redox, light, cation, anion, or guest molecules) was shown without reversible switching of the activity between on and off or high and low. These examples have been excluded from the discussion here.
Section snippets
Light-stimulated systems
Light has been an attractive stimulus to switch the state of a catalyst by either incorporating a photoresponsive function within it via simple structural manipulation or using a separate photochemically active co-catalyst as an additive. The advantages of using light include its non-invasive nature, choice of desired wavelength depending on the chromophoric unit, excellent spatiotemporal control, and ease of operation. The primary concept of light-induced switching of activity/selectivity of a
Redox-stimulated systems
Redox-switchable catalysts represent another popular class of smart catalysts to tune the activity and/or selectivity associated with various transition metal-catalyzed reactions. In general, a redox-active metalloligand is chosen for constructing the catalytically active metal complex.2(a), 2(c) The most commonly used redox-active motif is ferrocene (sometimes cobaltocene and quinone-based motifs), so that reversible oxidation/reduction of this group can lead to electronically poor/rich
pH-stimulated systems
pH of a solution is an important parameter, as the changes in pH can dramatically alter the property of the medium and physic-chemical responses of the species present in the solution. In natural systems, there are numerous processes controlled very smartly by changes in pH. In the artificial systems, switchable control of catalytic events as a function of pH change has been found to be challenging so far. Very few examples are known. Some early examples include ruthenium(II)-based olefin
Metal-ion coordination-stimulated systems
Like oxidation/reduction or addition of acid/base, metal-ion coordination (with ligands) also exerts profound electronic and/or geometrical influence to a molecular system. This simple fact was exploited judiciously to develop switchable catalysts wherein a coordination/decoordination event at the catalytic site or at a site remote from the catalyst could trigger an on/off action in catalytic processes. Many interesting examples have been reported related to the above two approaches. For
Electrochemically-stimulated systems
Electrochemical trigger is also emerging as a tool to switch catalytic processes. Recently, Prins and co-workers showed that a monolayer-functionalized gold nanoparticle-based supramolecular nanocatalyst (Au NP) could be reversibly activated or deactivated via electrochemical input.19 The electrochemical input in the form of a constant oxidative potential or reductive potential was applied alternatively to generate copper(II)-coordinated Au NP (active catalyst) or copper-free Au NP (inactive
Multi-stimuli systems
Finally, the target to apply multiple stimuli in a controlled and programmed manner has also reportedly been within reach. A very recent work by Johnson and co-workers reflected a successful and promising strategy toward this challenging task.20 They showed that how two stimuli – heat and light, were used in tandem to demonstrate a Logic-gated radical polymerization (Logic-CRP) with an on/off switchability function operated via an ‘AND’ gate (Fig. 16). Thus a combination of light ‘on’ and ‘low’
Conclusion
This Digest showed that the recent progress in the nascent field of artificial switchable catalysis is remarkable through a concise discussion on only the works developed during last couple of years. The earlier works, as discussed in several review papers, set the foundation for inventing new generation highly sophisticated and efficient switchable catalysts for multiple and complex tasks. Switchable tandem catalysis and network-catalysis with pre-programmed stimuli responses are emerging to
Acknowledgments
This activity is partially supported by the generous financial assistance by IISER Bhopal. The author thanks Shrivats Semwal, Dr. Ranjeesh T. K., Dr. Moumita Mondal, and Dr. Suraj K. Gupta for contributing to this field from our group.
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