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Cation-Controlled Catalysis with Crown Ether-Containing Transition Metal Complexes

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Cation-Controlled Catalysis with Crown Ether-Containing Transition Metal Complexes ChemComm Cation-Controlled Catalysis with Crown Ether-Containing Transition Metal Complexes Journal: ChemComm Manuscript ID CC-FEA-01-2019-000803.R1 Article Type: Feature Article Page 1 of 15 Please doChemComm not adjust margins Chemical Communications FEATURE ARTICLE Cation-Controlled Catalysis with Crown Ether-Containing Transition Metal Complexes Received 00th January 20xx, Changho Yoo, Henry M. Dodge, and Alexander J. M. Miller* Accepted 00th January 20xx Transition metal complexes that incorporate crown ethers into the supporting ligands have emerged as a powerful class of DOI: 10.1039/x0xx00000x catalysts capable of cation-tunable reactivity. Cations held in the secondary coordination sphere of a transition metal www.rsc.org/ catalyst can pre-organize or activate substrates, induce local electric fields, adjust structural conformations, or even modify bonding in the primary coordination sphere of the transition metal. This Feature Article begins with a non-comprehensive review of the structural motifs and catalytic applications of crown ether-containing transition metal catalysts, then proceeds to detail the development of catalysts based on “pincer-crown ether” ligands that bridge the primary and secondary coordination spheres. substrate activation, and ligand conformational gating. The Introduction uncommon ability to tune the primary coordination sphere through cation–crown interactions will be explored in detail The discovery of crown ethers in 1960 is often considered to using the “pincer-crown ether” ligand framework, which mark the birth of supramolecular chemistry.1–3 Thousands of incorporates an aza-crown ether moiety into a meridional crown ethers have now been synthesized,4–7 tailored to host an tridentate organometallic ligand. array of ionic and neutral guests. Fundamental studies of cation–crown interactions helped reveal the potential utility of crown ethers in applications ranging from phase transfer Classifying transition metal complexes that catalysis to separations science and ion sensing. Increased structural and dynamic complexity, in particular the ability to incorporate features of crown ethers reversibly engage and disengage cation–crown interactions, In this review, the term “metalla-crown ether” is used to underpinned the development of molecular machines.8–10 describe the broad array of transition metal complexes that By the 1980s, crown ethers were being incorporated into incorporate structural features of crown ether macrocycles.12 transition metal complexes. Most designs featured distinct The similarly named “metallacrowns” are a distinct class of binding pockets, one for a transition metal and one for another cyclic polymetallic structures.22 Another related class of metal cation, leading to applications including ion pair complexes are metallomacrocycles, which can be defined in a recognition, dual activation of small molecules, fluorescent way that would include metalla-crown ethers,11 or in a way that sensing, and redox tuning. The synthesis and applications of focuses on polymetallic clusters.23 Because the nomenclature these structures have been comprehensively reviewed.11,12 describing transition metal complexes with crown ether This Feature Article considers the roles that crown ethers features is not deeply codified, this Perspective strives to clearly can play in catalysis, where there has been less activity define terms and maintain consistency wherever possible. compared to the aforementioned applications. While Among the large array of metalla-crown ethers, some supramolecular systems beyond crown ethers are outside the structural trends emerge based on the position of the scope of this Feature Article, there are several excellent reviews macrocycle relative to the transition metal center. Three that highlight the full structural diversity of supramolecular categories of transition metal complexes bearing crown ethers catalysis.13–21 Here, a non-comprehensive review of the can be defined: enchained, pendent, and bridging. Figure 1 structural designs of crown ether-containing transition metal illustrates the three classes in conceptual form. This section complexes will be provided at the outset. The various roles that compares the salient features of these different classes of crown ethers can play in cation-controlled catalysis will then be metalla-crown ethers. discussed, highlighting examples in substrate preorganization, Department of Chemistry University of North Carolina at Chapel Hill Chapel Hill, NC 27516-3290, United States E-mail: [email protected] This journal is © The Royal Society of Chemistry 20xx Chem. Commun., 2019, 00, 1-3 | 1 Please do not adjust margins Please doChemComm not adjust margins Page 2 of 15 ARTICLE Journal Name complexes were the focus of initial studies,25,26 and were used Enchained Pendent Bridging L L L as a platform for the development of NMR titration methods for L 40 L metalla-crown ethers. Related amine-containing macrocycles L L L L L L 50,51 L have also been explored. L L L In pendent metalla-crown ethers, the transition metal L L L L L L center is tethered to a macrocycle as shown in Figure 3. These L L transition metal cation substrate/ligand systems provide good control over spatial separation and orientation, with different designs holding the guest either in Figure 1. Schematic representation of different metalla-crown ether designs, close proximity or far from the transition metal center. Starting where L denotes Lewis basic sites. with the pioneering monodentate ligand of McLain,52,53 Enchained metalla-crown ethers are comprised of a single phosphine-based systems have been widely explored. Another large macrocycle that contains distinct transition metal and early crown ether-containing phosphine ligand was reported as cation binding sites. Enchained structures can be produced part of a phase transfer catalysis study.54,55 Such systems remain from a single organic macrocycle (or macrobicycle, also called under active study,56 with examples of cation-promoted 24 “lateral macrobicyclic” molecules) or through chelation of a migratory insertion57 and the development of tridentate bidentate poly(ether) to produce a metal-containing phosphine-based metalla-crown ethers.58 macrocycle. Figure 2 collects representative examples of enchained metalla-crown ethers. Phosphines Many of the earliest examples of metalla-crown ethers can O R2P O Ph2P N O 24–27 N P N be classified as enchained. Several examples with pyridine- O O 24,28 27,29–33 O O n PPh2 containing macrocycles and Schiff-base-containing Fe O O R2P O O n macrocycles marked this early period of development. Other n = 1 or 2 noteworthy systems are based on tetra-aza-macrocycles that PPh2 bind the transition metal center.34,35 Many of these studies Nitreogen based ligands aimed to establish the fundamental host–guest properties of O metalla-crown ethers and explore the bonding motifs in N O O N NH O heterobimetallic systems, including metal–metal N O O n N NH O O O 36,37 interactions. O O n = 1, 2, 3 O O Salen Phosphine O n O O O n N N O O N N O O O O O O R O O R P P O O O O O O O O O O X R R O O n N O O N n = 3, 4, 5 X X Pincer R N N X N NH HN O O X S S O O N N O O NH HN O O O O O O O HN X = N, CH O N N N N n O O O O n = 0, 1, 2, 3 X = N, C-OH n n n O O R = H, CH3 O O n = 0, 1 n = 0, 1 m m = 0, 1 Tetra-aza iPr N O O O O N N O O O O O n O O O O iPr O O O O n N N N n = 1, 2, 3 N N N N O Figure 3. Representative pendent metalla-crown ethers, where the grey sphere N represents a transition metal center and any additional ancillary ligands. n = 1, 2 Figure 2. Representative enchained metalla-crown ethers, where the grey sphere represents a transition metal center and any additional ancillary ligands. Several nitrogen donor ligands have been fitted with pendent crown ethers (Figure 3). Porphyrin-based systems59 Another noteworthy class of enchained metalla-crown were developed early on.60–64 Porphyrazine-based systems65–67 ethers feature bidentate phosphine ligands (to bind transition and phthalocyanine-based systems68–77 have also been studied metals) linked by a poly(ether) chain (to bind cations, Figure extensively. These systems have shown promising behavior 2).12,38–49 In many of the phosphine-based systems the free related to sensing, functional materials, and catalysis.59 Other ligand is not macrocyclic. It is not until the two phosphine- chelates such as bipyridine78–84 ligands and Schiff-base85–88 decorated ends of the poly(ether) bind to a transition metal that (salen-type) ligands have received special attention in the a binding pocket is presented. Molybdenum carbonyl development of pendent crown ether systems.89 Gilbertson has 2 | Chem. Commun., 2019, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Page 3 of 15 Please doChemComm not adjust margins Journal Name ARTICLE led the development of crown ether-appended pyridinediimine relatively rigid region for binding a transition metal ion and a ligands for applications in redox tuning and small molecule more flexible poly(ether) region for binding alkali or alkaline activation.90–92 earth metal cations. The diphosphine-based enchained systems In bridging metalla-crown ethers, one of the cation receptor are typically quite flexible. Pendent metalla-crown ethers can site donors (permanently or temporarily) binds to the transition have very rigid structures (as in the porphyrin-based systems) metal center (Figure 4). Some bridging systems can also be or quite flexible structures (typified by the phosphine-based classified as enchained, wherein one or more ether donor binds systems). The choice of flexibility in the design is often related the transition metal as observed for a few heavy metal to the desired application, with more rigid systems better suited complexes.93,94 for redox tuning or sensing and more flexible systems better The Miller group recently introduced “pincer-crown ether” suited for transition-metal-bound substrate interactions.
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