Tunable and Switchable Catalysis Enabled by Cation-Controlled Gating with Crown Ether Ligands

Abstract

Conspectus Catalysis has become an essential tool in science and technology, impacting the discovery of pharmaceuticals, the manufacture of commodity chemicals and plastics, the production of fuels, and much more. In most cases, a particular catalyst is optimized to mediate a particular reaction, continually producing a desired product at a given rate. There is enormous opportunity in developing catalysts that are dynamic, capable of responding to a change in the environment to alter structure and function. Controlled catalysis, in which the activity or selectivity of a catalytic reaction can be adjusted through an external stimulus, offers opportunities for innovation in catalysis. Catalyst discovery could be simplified if a single thoughtfully designed complex could work synergistically with additives to optimize performance rather than trying a multitude of different metal/ligand combinations. Temporal control could be gained to facilitate the execution of multiple reactions in the same flask, for example, by activating one catalyst and deactivating another to avoid incompatibilities. Selectivity switching could enable copolymer synthesis with well-defined chemical and material properties. These applications might sound futuristic for synthetic catalysts, but in nature, such a degree of controlled catalysis is commonplace. For example, allosteric interactions and/or feedback loops modulate enzymatic activity to enable complex small-molecule synthesis and sequence-defined polymerization reactions in complex mixtures containing many catalytic sites. In many cases, regulation is achieved by “gating” substrate access to the active site. Fundamental advances in catalyst design are needed to better understand the factors that enable controlled catalysis in the arena of synthetic chemistry, particularly in achieving substrate gating outside of macromolecular environments. In this Account, the development of design principles for achieving cation-controlled catalysis is described. The guiding hypothesis was that gating substrate access to a catalyst site could be achieved by controlling the dynamics of a hemilabile ligand through secondary Lewis acid/base and/or cation–dipole interactions. To enforce such interactions, catalysts sitting at the interface of organometallic catalysis and supramolecular chemistry were designed. A macrocyclic crown ether was incorporated into a robust organometallic pincer ligand, and these “pincer-crown ether” ligands have been explored in catalysis. Complementary studies of controlled catalysis and detailed mechanistic analysis guided the development of iridium, nickel, and palladium pincer-crown ether catalysts capable of substrate gating. Toggling the gate between open and closed states leads to switchable catalysis, where cation addition/removal changes the turnover frequency or the product selectivity. Varying the degree of gating leads to tunable catalysis, where the activity can be tuned based on the identity and amount of salt added. Research has focused on reactions of alkenes, p...

Tuning the Catalytic Activity of a Pincer Complex of Rhodium(I) by Supramolecular and Redox Stimuli

“…The NDI-containing bisimidazolium salt [1](I) 2 was prepared by condensation of N-(1-ethylpropyl)naphthalene-1,8naphtalimide-4,5-dicarboxylic anhydride and 2,6bis(imidazolyl)-pyridine-4-amine, and further N-quaternization with n-butyl iodide, as depicted in Scheme 1. The bisimidazolium salt [1](I) 2 was characterized by means of NMR spectroscopy and electrospray mass spectrometry (ESI-MS), and its molecular structure was determined by single crystal X-ray diffraction means. [9] Then, the rhodium pincer complex 2 was prepared following the Ag 2 O-trasmetallation strategy, by combining [1](I) 2 with [RhCl(CO) 2 ] 2 and Ag 2 O in the presence of Na(BAr F 4 ).…”

“…The bisimidazolium salt [1](I) 2 was characterized by means of NMR spectroscopy and electrospray mass spectrometry (ESI-MS), and its molecular structure was determined by single crystal X-ray diffraction means. [9] Then, the rhodium pincer complex 2 was prepared following the Ag 2 O-trasmetallation strategy, by combining [1](I) 2 with [RhCl(CO) 2 ] 2 and Ag 2 O in the presence of Na(BAr F 4 ). Complex 2 was characterized by NMR spectroscopy and electrospray mass spectrometry.…”

“…One of the greatest challenges of homogeneous catalysis is to find catalysts that are dynamic and capable of changing their structure and function in response to an external stimulus. [1] Stimuli-responsive catalysts may simplify catalyst discovery, as a single complex could work synergistically with additives to adjust its performance, rather than trying the tedious combinations of ligands and metals.…”

Reversible Modulation of the Electronic and Spatial Environment around Ni(0) Centers Bearing Multifunctional Carbene Ligands with Triarylaluminum

“…A judicious combination of multifunctional ligands and LAs is thus essential for constructing an LA-mediated system that can reversibly regulate the electronic and spatial environments around the metal centers. It should be noted that Fan et al have reported an example that relies predominantly on an electrostatic interactions , involving the reversible complexation of Na + and a crown-ether moiety included in a Rh complex that bears Aza-CrownPhos; the reactivity of the Rh complex can be regulated through the Na + -mediated interconversion . The authors evaluated the change in the spatial environment around the Rh center using multinuclear NMR spectroscopy and ESI mass spectrometry, even though a quantitative evaluation of how much the field expanded and diminished via the reaction was not discussed.…”

Switchable Organocatalysis from N‐heterocyclic Carbene‐Carbodiimide Adducts with Tunable Release Temperature**

“…Regulating catalytic activity is a grand challenge in homogeneous catalysis as synthetic chemists attempt to mimic enzymatic control over chemical reactions. [1,2] This challenge has led to the field of switchable catalysis, [2][3][4][5][6] which promotes advanced (spatio)temporal control over a variety of organic [5,[7][8][9] and polymerization [10][11][12][13] reactions. Reversible coordination is one common strategy to protect (i.e., "mask" the reactivity of) a catalyst in a stable, inactive state before an external stimulus-such as heat, light, or the addition of a chemical compound-converts the catalyst into its active form (Figure 1A).…”