Viewpoint Cite This: Inorg. Chem. 2018, 57, 4205−4221
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Chemical Transformations in Confined Space of Coordination Architectures Indranil Sinha* and Partha Sarathi Mukherjee* Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India ABSTRACT: The scholastic significance of supramolecular chemistry continues to grow with the recent development of catalytic transformations in confined space of supramolecular architectures. It has come a long way from a natural cavity containing molecules to modern smart materials capable of manipulating reaction pathways. The rise of selfassembled coordination complexes provided a diverse array of host structures. Starting from purely organic compounds to metalloligand surrogates, supramolecular host cavities were tuned according to the requirement of the reactions. The understanding of their participation in a reaction led to better usage of those assemblies for specific reaction sequences. Commencing from cyclodextrin, a wide range of organic molecules was used for cage-catalyzed organic transformations. However, difficulties in synthesis and a tedious purification procedure led chemists to choose a different pathway of metal−ligand coordination-driven self-assembly. The latter stood out as a potential replacement of the organic cages, overcoming the previous drawbacks. In the glut of different transitionmetal assemblies used for catalytic transformations, many of them showed chemo- and stereoselective products. However, the small cavity size in some of them led to premature failure of the reaction. In that context, “molecular barrels” showed good efficacy for the catalytic reaction sequence. The large cavity size and bigger orifice for intake of the substrate and easy release of the product made them a better choice for catalysis. Additionally these are mostly used in aqueous media, which reinforces the idea of green and environmentally nonhazardous chemistry. In this Viewpoint, we discuss the use of metal−ligand coordinationdriven self-assembled molecular containers used for catalysis with special emphasis on molecular barrels. This paper built on existing literature provides a thorough development of the fertile ground of the coordination architecture for catalysis and its future direction of propagation.
1. INTRODUCTION Supramolecular self-assembly has been a promising approach to generating an extensive range of molecular architectures that are often carried out in a one-pot reaction.1 A good knowledge of the self-assembled structures plays a key role in understanding the topology of the supramolecules.2,3 Over the last few decades subcomponent self-assembly strategies have looked very promising, in which very complex motifs were prepared from simple building blocks through the formation of dynamic covalent and coordination linkages.4,5 Other strategies involve the formation of molecular assemblies with distinct physical properties.6,7 A distinct difference between a synthetic chemist and a supramolecular chemist lies in their way of constructing a molecule. Synthetic chemists rely on concocting small functional groups to make the large target molecule by different reactions including catalysis, whereas supramolecular chemists look toward exploiting the intermolecular interaction as a second alternative for control over the reactions.8 They tend to mimic the macromolecular catalysts that bring changes to the encapsulated substrates via the preprogrammed hydrophobic cavities. The encapsulated substrate can be activated by the influence of different weak interactions or external stimuli. It is worth mentioning that such interactions are individually fragile and their formation is reversible. However, in aggregates, they show specific catalysis with a rate enhancement of 1017-fold and higher.9,10 © 2018 American Chemical Society
Enough effort has been put forward to design and synthesize proper supramolecular catalysts that show these remarkable properties.11 Different examples exist where chemists have designed well-defined hydrophobic cavities to mimic the enzyme’s active site. Early examples include cyclic oligomers and other covalent assemblies. They were tuned according to their size and chemical environment of the inner cavity.12 Literature shows examples of cucurbiturils13 and cyclodextrins14 as pure organic hosts. However, the main drawback of these hosts is their difficult synthesis and purification. In addition, covalent artificial hosts, namely, crown ether, calixarenes, and carcerands, were difficult to synthesize. There has been tremendous research on inventing suitable host structures even decades after their first application. Unique assemblies that would provide efficient supramolecular catalysis have been a major driving force in the field of self-assembly chemistry. In the quest for suitable host structures, scientists have been driven toward metal−ligand coordination assemblies. There are quite a few logical reasons for this. The difficulty of obtaining a properly preprogrammed geometry associated with the covalent host structures can easily be dodged, resulting in a well-defined, stable, and rigid geometry for the host. Mostly, this kind of coordination-driven self-assembly is the aftermath Received: December 5, 2017 Published: March 26, 2018 4205
DOI: 10.1021/acs.inorgchem.7b03067 Inorg. Chem. 2018, 57, 4205−4221
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Inorganic Chemistry of Lewis-basic donor and Lewis-acidic acceptor molecules.15,16 This acid−base chemistry is of great use in the development of specific metalloarchitectures and continues to be the basis of the metal-directed supramolecular self-assembly process. Later on, among the different self-assembly procedures, self-sorting has also gained much attention.17−19 This provides typical geometrical complementarity of the size and shape to preprogram the outcome of the self-assembly process. In that context, chiral self-sorting processes are very challenging because of variation of spatial orientation. This self-sorting eventually leads to the formation of homochiral or heterochiral assemblies.20 In this Viewpoint, we choose to highlight the recent developments of coordination-driven self-assembling discrete architectures that are used for catalytic purposes with the main focus on our recent developments of chemical transformations in confined space of “molecular barrels”. These molecular architectures are specifically designed in such a way that they inherit large internal cavities to facilitate the reaction inside them. In addition, large apertures for uptake of the reactant and easy elimination of the product make them better candidates for selective catalysis. We will discuss the relevant historical inventions of this host−guest chemistry with a special emphasis on those systems that are used for catalytic purposes. It is tailored in a way that highlights the diversity and interdisciplinary nature of the field as well as studies that explore the complex functionality of the system. For other structural aspects, readers are directed to previous reviews.8,21−23 This Viewpoint is based on the application of discrete coordination architectures as reaction vessels for catalytic organic transformations. To provide an appropriate context for the topic, here we mostly discuss the discrete coordination architectures used for catalysis. Examples of metal-free organic host systems are discussed, excellently elsewhere24−26 and readers are directed to them for additional knowledge. In this Viewpoint, we aim to start with a few organic architectures used for catalysis, then we move toward specific examples of catalysis in coordination architectures, and finally we discuss the molecular barrels used for catalysis.
hemicryptophanes were also used diversely for catalysis.33 The unique structure of hemicryptophane is made up of two portions, a reactive center at the bottom and a large cavity to accommodate the guest just above it. Several examples are present that show the encapsulation of a variety of compounds (charged species and neutral) into the cavity. Thus, it is quite expected that the hemicryptophanes are highly reactive because of their artificially protected, specific, rigid size and shape. This rigidity enforces a specific orientation of the inbound guest molecule, leading to higher reactivity. It is also the circumscribed structure that shields the substrate from the outer environment during the reaction, leading to an improved catalytic efficiency. The examples of organic supramolecular hosts discussed here enlighten both successes and failures. In most of the cases, we saw that the construction of an efficient catalyst is difficult because of strong substrate binding inside the cavity and troublesome product elimination from it. The outlook can be written as the host must possess a distinct inner cavity to ensure secure entry of the substrate and uncomplicated removal of the final product. We will revisit these challenges in discussing catalysis by supramolecular coordination cages.
3. METALLOARCHITECTURES AS HOSTS 3.1. Design and Synthesis. In search of supramolecular assemblies prone to molecular catalysis, chemists were intrigued toward discovering larger and sophisticated reaction vessels.25 The synthesis of larger reaction vessels can best be achieved by self-assembly. Apart from the tedious organic synthetic procedures, a complementary approach is the metal−ligand coordination-driven self-assembly to develop minuscule host structures to serve as reaction vessels. These coordination bonds are labile and allow the formation of most stable thermodynamically controlled assemblies and deliberately exclude the less favorable geometry. Mostly, the construction is rapid and specific. Starting from its first serendipitous invention by Saalfrank et al.,34 it has gained much attention from chemists. Saalfrank and co-workers actually studied the doubly metalated organic species for synthons. Diethyl malonate and oxalyl chloride were condensed in the presence of methylmagnesium bromide to result in ditopic bis(magnesium)malonate (Scheme 1). The product was a magnesium salt of the condensed diethyl malonate. However,
2. METAL-FREE SUPRAMOLECULAR ASSEMBLIES Supramolecular chemistry focusing on host−guest interactions started as a distinct discipline with crown ethers and their interactions with alkali metals.27,28 However, the weak hydrophobicity of the crown ether cavity made it a poor choice for the encapsulation of nonpolar guests.29,30 Thus, without any speculation, it was necessary to make a viable and hydrophobic cavity that could accommodate organic substrates. In this aspect, cyclodextrin was a prime choice. This watersoluble cyclic oligomer of α-D-glucopyranoside has a specific and well-defined hydrophobic cavity. Most commonly α-, β-, and γ-cylodextrins (6, 7, or 8 glucose units, respectively) were used for this purpose. The inner cavities were found to include a number of organic moieties.31,32 Another example is cucurbituril. This is a barrel-shaped macrocyclic oligomeric host molecule that usually prefers cationic substrates. The O atoms in the outside ring prefer to associate with positive charges for stabilization. Its unique structural features allowed chemists to use it as an organic barrel for catalysis.13 In spite of cucurbituril’s intriguing catalytic chemistry, major drawbacks associated with it are strong product inhibition due to the high affinity of the product inside the cavity and low catalytic turnover. In addition to cyclodextrins and cucurbiturils,
Scheme 1. Synthesis of the Ditopic Malonate Ligand as Mg2+ and NH4+ Salts and Schematic Structure of the Magnesium Coordination Complex 1, Showing Only One of Six Ligands for Claritya
a
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Color codes: green, Mg2+; blue, malonate ligand. DOI: 10.1021/acs.inorgchem.7b03067 Inorg. Chem. 2018, 57, 4205−4221
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Inorganic Chemistry
Figure 1. Octahedral cage (2) and bowl-shaped molecule (3) reported by Fujita et al. Adapted with permission from ref 44. Copyright 2006 AAAS.
upon structural elucidation of the salt by single-crystal X-ray diffraction analysis, a tetrahedral geometry could be seen. This serendipitous result left an impression that metal− ligand coordination could be used for the highly symmetric coordination cages with a well-defined inner cavity. This architecture was too small to encapsulate other substrates inside, but other larger polyhedra could be prepared for encapsulation. This observation led to a careful selection of the components. They were chosen in such a manner that the components contained symmetry elements of the polyhedral compound and were rigid enough to discard unwanted or unfavorable geometries. The labile metal−ligand bond will dominate formation of the thermodynamically most stable species.35−37 Use of the supramolecular architecture as molecular containers for various chemical transformations requires the careful preparation of host molecules that can bind the guest (substrate) molecules. The fascinating adamantoid structure reported by Saalfrank et al. inspired Fujita et al. to prepare a two-dimensional cationic “molecular square” that could accommodate a single molecule of 1,3,5-trimethoxybenzene in water, forming a 1:1 inclusion complex.38 This was a watersoluble complex having cis-blocked Pd(en)2+ centers (en = ethylenediamine) and a linear bipyridyl bridging ligand. The combination of these two led to 4:4 stoichiometry, forming a square. The Lewis-acidic metal center showed a 90° bite angle, enforcing a square geometry having a large cavity. This work was later extended to a family of different three-dimensional (3D) structures with different metal−ligand connectivity and inner cavity sizes.39 Different guests like nanoparticles40 to macromolecular ubiquitin41 were also incorporated within a variety of structures. These guest encapsulated structures were excellently reviewed previously.3,42,43 Herein we will only focus on the host−guest assemblies that have successfully been applied in chemical transformations. Fujita et al. extensively studied octahedral (2) and squarepyramidal (3) cages.44 They were appropriate for small guest molecules (Figure 1). In addition to Fujita et al.’s work, Caulder and Raymond designed M4L6 tetrahedra for supramolecular catalysis. The structure somewhat resembled that of Saalfrank’s adamantoid cage. Caulder and Raymond reported a tetrahedral anionic cage, [Ga4L6]12− [11; L = 1,5-bis(2,3-dihydroxybenzoylamino)naphthalene; Figure 2]. The ditopic ligand included two catecholate moieties separated by a naphthalene spacer. The reported cage formed instantly upon combination of the metal
Figure 2. Molecular container 11, which catalyzes aza-Cope electrocyclization of the allylenammonium cations. (For the best rate 854, R1, R2, R3 = H, i-Pr, H).51
salt and the deprotonated ligand in a 4:6 ratio.45 Topologically, cage 11 was identical with the previously reported 1 (Scheme 1), although the interior was smaller for the previous one. The judicious design of the anionic cage allowed encapsulation of a large number of monocationic (NMe4+)46 and neutral organic and inorganic [Co(Cp*)2+] species. Cationic guests were strongly bound in a wide range of polar solvents. The Coulombic interactions between the host and guest turned out to be enthalpically favorable processes, which helped in encapsulation. While guest encapsulation was enthalpically favored, solvent expulsion from the host cavity was entropically favored, which helped the encapsulation process in an aqueous medium.11 3.2. Catalysis in Molecular “Containers”. The invention of desired supramolecular host systems triggered research on catalysis in their cavity. Among the several kinds of reactions that could be done in covalent and hydrogen-bonded 4207
DOI: 10.1021/acs.inorgchem.7b03067 Inorg. Chem. 2018, 57, 4205−4221
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Inorganic Chemistry
Scheme 2. (a) Supramolecular Assembly 2 Catalyzing the DA Addition of 4 and 5, Resulting in 7, and (b) DA Addition of 4 and 5 Catalyzed by 3, Resulting in Regioisomer 6
absence of catalyst 2 with a poor yield of
