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Switchable Catalysis

This overall scheme for switchable catalysis demonstrates the many different paradigms possible depending on the identity of A, B, C, and D.

A switchable catalyst is one which can be switched between two different states with different reactivities by a simple external stimulus, such as the presence of light or addition of another reagent. In most cases, a catalyst is only useful for a specific reaction or class of reactions and accordingly many different catalysts are used both industrial and in research settings. As industrial processes seek lower cost alternatives and chemists strive towards green chemistry, research efforts are being directed towards the development of switchable catalysts. These catalysts are desired to decrease costs either by providing one catalyst that can be employed for a variety of reactions or to employ switching in situ to quickly produce molecular complexity via tandem catalysis. Switchable catalysis is also a promising area of investigation for polymer chemistry as they may be employed to generate polymers of specific molecular weight or sequence-controlled polymers.

Biological Inspiration

Switchability is common amongst biological catalysts known as enzymes. Most enzymes are capable of catalyzing at least both the forward and backwards pathways of one reaction, and many are capable of catalyzing a reaction of many substrates or many different reactions. Allosteric enzyme's reactivity is modulated by the binding of small molecules. The resulting conformational changes in the enzyme structure change reactivity of the enzyme such as reducing or increasing its activity. This ability to change enzymes' catalytic activity is essential for biological functions and maintaining the complex machinery of life. Chemists often take inspiration from biology to devise new catalytic systems. Most synthetic catalysts do not have the ability to be modulated in this way. Thus, there is a large effort to develop switchable catalysts to improve catalytic systems by mimicking biology.

Light Induced Switching

Light activated systems and photoredox catalysts are becoming increasingly common as light is an abundant resource that can be harvested for chemical reactions. Many systems absorb light to break bonds, induce conformational changes, or reach excited states capable of electron transfer. Many systems have thus been devised to modulate activity based on the absorbance of a specific wavelength of light. A variety of photo switchable organic catalysts have been devised to exploit the light induced E/Z conformation change of nitrogen-nitrogen double bonds. For example, a 3,3'-dicarboxylic acid was found to catalyze the hydrolysis of nitrophenyl β-D-glucose when irradiated adopt a Z configuration. The E configuration, produced by irradiation with a different frequency, was inactive as a catalyst.

However, most examples of photo induced switches merely change the rate of catalysis with one state reacting faster than the other. Some notable examples modulate chirality of the product depending on the presence of light. A copper(I) cyclopropanation catalyst has been developed that produces chiral products in modest enantioselectivities when irradiated with one wavelength, and a nearly racemic mixture of products when irradiated with a different wavelength. Again the role of light in this example is to induce a conformational change in the ligand structure.

*Ref 12 Figure*

No prominent examples exist of photo switching between two different reactions, an extent goal of all areas of switchable catalysis.

pH Induced Switching

Many compounds can be protonated or deprotonated depending on pH, such as amines. This basic chemical reaction can be easily used to reversibly modulate a compound between two different states. In many cases, the protonated and deprotonated forms have different structural or electronic properties which can be exploited to modulate activity. A few notable pH switched catalysts are able to effect different transformations depending on the pH of the system. However, the generation of two different products depending on the switch is a less desirable goal than tandem catalysis, as in situ switching would generated a mixture of products.

Coordination Induced Switching

Most catalysts rely on a metal center with one or more ligands coordinated to it. The catalysts specific reactivity and selectivity is a result of the particular ligands coordinated to the metal. Thus, it is possible to modulate reactivity of a catalyst by the addition and removal of small ligands, which in some cases can be performed in situ. A similar motif is the induction of conformational changes in the catalyst by the addition or removal of metal cations. This binding can effect the coordination of the catalytically active metal center to change activity.

Redox Induced Switching

Most transition metals can adopt a variety of oxidation states, which generally have different reactivity from each other. However, changing the oxidation state of the catalytically active metal center is generally not considered in switchable catalysis as these can be thought of as completely different catalysts. For example, oxidizing or reducing the metal center may change how many ligands are bound to it, and many compounds cannot accordingly be reversibly oxidized/reduced in situ. Instead, the use of redox active ligands allows for modulation of the complex's redox state and overall charge without disrupting the catalytically active metal.

Mechanically Induced Switching

Mechanical force in the form of sonication can sometimes be used to cleave weak bonds such as coordination interactions between some metals and ligands. Specifically this has been used to dissociate polymer supported ligands to generate a catalytically active complex.

Other Types of Switching

Several other types of switching are possible, such as a switch induced by a change in reaction conditions, i.e. temperature, solvent, atmosphere, etc. However, these methods generally are more comparable to traditional catalysis. For example, a reaction will not reach completion if not enough of a reagent is present or the reaction is performed in an incompatible solvent.

Switchable Catalysis for Polymerization

Most examples of switchable catalysis thus far have demonstrated relatively simple on/off strategy. The few examples that go beyond this only demonstrate reactivity of one substrate towards two or more different products. While worthwhile proofs of concept, these developments are not synthetically useful. However, one of the leading areas of switchable catalysis is its use for polymerization control. On/off selectivity can be used usefully in polymerization reactions to create polymers of precise molecular weights, and has been demonstrated with a variety of systems such as photo switching and redox control. Switchable catalysis has also been used to switch activity between two different monomers to create sequence controlled block copolymers.

See Also

• Non-innocent ligand
• Green chemistry

Additional Reading

• Blanco, Victor; Leigh, David A.; Marcos, Vanesa."Artificial switchable catalysts". Chem. Soc. Rev. 44 (15): 5341–5370. doi:10.1039/c5cs00096c.