Synergistic Noncovalent Catalysis Facilitates Base-Free Michael Addition

Photoswitchable Catalysis by a Self-Assembled Molecular Cage

A heteroleptic [Pd2L2L'2]4+ coordination cage containing a photoswitchable azobenzene-derived ligand catalyzes the Michael addition reaction between methyl vinyl ketone and benzoyl nitromethane within its cavity. The corresponding homoleptic cages are catalytically inactive. The heteroleptic cage can be reversibly disassembled and reassembled using 530 and 405 nm light, respectively, allowing catalysis within the cage to be switched OFF and ON at will.

Origins of High-Activity Cage-Catalyzed Michael Addition

Cage catalysis continues to create significant interest, yet catalyst function remains poorly understood. Herein, we report mechanistic insights into coordination-cage-catalyzed Michael addition using kinetic and computational methods. The study has been enabled by the detection of identifiable catalyst intermediates, which allow the evolution of different cage species to be monitored and modeled alongside reactants and products. The investigations show that the overall acceleration results from two distinct effects. First, the cage reaction shows a thousand-fold increase in the rate constant for the turnover-limiting C-C bond-forming step compared to a reference state. Computational modeling and experimental analysis of activation parameters indicate that this stems from a significant reduction in entropy, suggesting substrate coencapsulation. Second, the cage markedly acidifies the bound pronucleophile, shifting this equilibrium by up to 6 orders of magnitude. The combination of these two factors results in accelerations up to 109 relative to bulk-phase reference reactions. We also show that the catalyst can fundamentally alter the reaction mechanism, leading to intermediates and products that are not observable outside of the cage. Collectively, the results show that cage catalysis can proceed with very high activity and unique selectivity by harnessing a series of individually weak noncovalent interactions.

Enzyme-like Acyl Transfer Catalysis in a Bifunctional Organic Cage

Amide-based organic cage cavities are, in principle, ideal enzyme active site mimics. Yet, cage-promoted organocatalysis has remained elusive, in large part due to synthetic accessibility of robust and functional scaffolds. Herein, we report the acyl transfer catalysis properties of robust, hexaamide cages in organic solvent. Cage structural variation reveals that esterification catalysis with an acyl anhydride acyl carrier occurs only in bifunctional cages featuring internal pyridine motifs and two crucial antipodal carboxylic acid groups. 1H NMR data and X-ray crystallography show that the acyl carrier is rapidly activated inside the cavity as a covalent mixed-anhydride intermediate with an internal hydrogen bond. Michaelis-Menten (saturation) kinetics suggest weak binding (KM = 0.16 M) of the alcohol pronucleophile close to the internal anhydride. Finally, activation and delivery of the alcohol to the internal anhydride by the second carboxylic acid group forms ester product and releases the cage catalyst. Eyring analysis indicates a strong enthalpic stabilization of the transition state (5.5 kcal/mol) corresponding to a rate acceleration of 104 over background acylation, and an ordered, associative rate-determining attack by the alcohol, supported by DFT calculations. We conclude that internal bifunctional organocatalysis specific to the cage structural design is responsible for the enhancement over the background reaction. These results pave the way for organic-phase enzyme mimicry in self-assembled cavities with the potential for cavity elaboration to enact selective acylations.

Non-Templated Assembly of D5h-Symmetric Pd5L10 Rings by Precise Ligand Angle Adjustment

We report a series of Pd(II)nL2n coordination rings for which nuclearity is controlled by the binding angle of the corresponding bis-monodentate bridging ligands. Judicious choice of the angle within a family of rather rigid ligands allowed for the first-time to synthesize a homoleptic five-membered Pd5L10 ring that does not require any template to form. We demonstrate that control over the ring size is maintained both in the solid-, solution-, and gas-phase. Two X-ray structures of five-membered rings from ligands with ideal angles (yielding a perfect pentagonal ring) vs. suboptimal angles (resulting in a highly distorted structure) illustrate the importance of the correct ligand geometry. A mathematical model for estimating the expected ring size based on the ligand angle was derived and DFT computations show that ring-strain is the major factor determining the assembly outcome.

Triel Bonds between BH3/C5H4BX and M(MDA)2 (X = H, CN, F, CH3, NH2; M = Ni, Pd, Pt, MDA = Enolated Malondialdehyde) and Group 10 Transition Metal Electron Donors

A systematic theoretical study was conducted on the triel bonds (TrB) within the BH3∙∙∙M(MDA)2 and C5H4BX∙∙∙M(MDA)2 (M = Ni, Pd, Pt, X = H, CN, F, CH3, NH2, MDA = enolated malondialdehyde) complexes, with BH3 and C5H4BX acting as the electron acceptors and the square-coordinated M(MDA)2 acting as the electron donor. The interaction energies of these systems range between -4.71 and -33.18 kcal/mol. The larger the transition metal center M, the greater the enhancement of the TrB, with σ-hole TrBs found to be stronger than π-hole TrBs. In the σ-hole TrB complex, an electron-withdrawing substituent on the C opposite to the B atom enhances the TrB, while an electron-donating substituent has little effect on the strength of TrB in the Pd and Pt complexes but enhances the TrB in the Ni-containing complexes. The van der Waals interaction plays an important role in stabilizing these binary systems, and its contribution diminishes with increasing M size. The orbital effect within these systems is largely due to charge transfer from the dz2 orbital of M into the empty pz orbital of B.

Diastereoselective Self-Assembly of Low-Symmetry Pdn L2n Nanocages through Coordination-Sphere Engineering

Metal-organic cages (MOCs) are popular host architectures assembled from ligands and metal ions/nodes. Assembling structurally complex, low-symmetry MOCs with anisotropic cavities can be limited by the formation of statistical isomer libraries. We set out to investigate the use of primary coordination-sphere engineering (CSE) to bias isomer selectivity within homo- and heteroleptic Pdn L2n cages. Unexpected differences in selectivities between alternative donor groups led us to recognise the significant impact of the second coordination sphere on isomer stabilities. From this, molecular-level insight into the origins of selectivity between cis and trans diastereoisomers was gained, highlighting the importance of both host-guest and host-solvent interactions, in addition to ligand design. This detailed understanding allows precision engineering of low-symmetry MOC assemblies without wholesale redesign of the ligand framework, and fundamentally provides a theoretical scaffold for the development of stimuli-responsive, shape-shifting MOCs.

Exo-cage catalysis and initiation derived from photo-activating host-guest encapsulation

Coordination cage catalysis has commonly relied on the endogenous binding of substrates, exploiting the cavity microenvironment and spatial constraints to engender increased reactivity or interesting selectivity. Nonetheless, there are issues with this approach, such as the frequent occurrence of product inhibition or the limited applicability to a wide range of substrates and reactions. Here we describe a strategy in which the cage acts as an exogenous catalyst, wherein reactants, intermediates and products remain unbound throughout the course of the catalytic cycle. Instead, the cage is used to alter the properties of a cofactor guest, which then transfers reactivity to the bulk-phase. We have exemplified this approach using photocatalysis, showing that a photoactivated host-guest complex can mediate [4 + 2] cycloadditions and the aza-Henry reaction. Detailed in situ photolysis experiments show that the cage can both act as a photo-initiator and as an on-cycle catalyst where the quantum yield is less than unity.

A Lantern-Shaped Pd(II) Cage Constructed from Four Different Low-Symmetry Ligands with Positional and Orientational Control: An Ancillary Pairings Approach

One of the key challenges of metallo-supramolecular chemistry is to maintain the ease of self-assembly but, at the same time, create structures of increasingly high levels of complexity. In palladium(II) quadruply stranded lantern-shaped cages, this has been achieved through either 1) the formation of heteroleptic (multi-ligand) assemblies, or 2) homoleptic assemblies from low-symmetry ligands. Heteroleptic cages formed from low-symmetry ligands, a hybid of these two approaches, would add an additional rich level of complexity but no examples of these have been reported. Here we use a system of ancillary complementary ligand pairings at the termini of cage ligands to target heteroleptic assemblies: these complementary pairs can only interact (through coordination to a single Pd(II) metal ion) between ligands in a cis position on the cage. Complementarity between each pair (and orthogonality to other pairs) is controlled by denticity (tridentate to monodentate or bidentate to bidentate) and/or hydrogen-bonding capability (AA to DD or AD to DA). This allows positional and orientational control over ligands with different ancillary sites. By using this approach, we have successfully used low-symmetry ligands to synthesise complex heteroleptic cages, including an example with four different low-symmetry ligands.

Picking the lock of coordination cage catalysis

The design principles of metallo-organic assembly reactions have facilitated access to hundreds of coordination cages of varying size and shape. Many of these assemblies possess a well-defined cavity capable of hosting a guest, pictorially mimicking the action of a substrate binding to the active site of an enzyme. While there are now a growing collection of coordination cages that show highly proficient catalysis, exhibiting both excellent activity and efficient turnover, this number is still small compared to the vast library of metal-organic structures that are known. In this review, we will attempt to unpick and discuss the key features that make an effective coordination cage catalyst, linking structure to activity (and selectivity) using lessons learnt from both experimental and computational analysis of the most notable exemplars. We will also provide an outlook for this area, reasoning why coordination cages have the potential to become the gold-standard in (synthetic) non-covalent catalysis.

Exploiting reduced-symmetry ligands with pyridyl and imidazole donors to construct a second-generation stimuli-responsive heterobimetallic [PdPtL4]4+ cage

A new sequential metalation strategy that enables the assembly of a new more robust reduced symmetry heterobimetallic [PdPtL4]4+ cage C is reported. By exploiting a low-symmetry ditopic ligand (L) that features imidazole and pyridine donor units we were able to selectively form a [Pt(L)4]2+ "open-cage" complex. When this was treated with Pd(ii) ions the cage C assembled. 1H and DOSY nuclear magnetic resonance (NMR) spectroscopy and electrospray ionisation mass spectrometry (ESIMS) data were consistent with the quantitative formation of the cage and the heterobimetallic structure was confirmed by single crystal X-ray crystallography. The cage C was shown to bind anionic guest molecules. NMR studies suggested that these guests interacted with the cavity of the cage in a specific orientation and this was confirmed for the mesylate ion (MsO-) : C host-guest adduct using X-ray crystallography. In addition, the system was shown to be stimulus-responsive and could be opened and closed on demand when treated with appropriate stimuli. If a guest molecule was bound within the cage, the opening and closing was accompanied by the release and re-uptake of the guest molecule.

Programmed guest confinement via hierarchical cage to cage transformations

Taking inspiration from Nature, where (bio)molecular geometry variations are exploited to tune a large variety of functions, supramolecular chemistry has continuously developed novel systems in which, as a consequence of a specific stimulus, structural changes occur. Among the different architectures, supramolecular cages have been continuously investigated for their capability to act as functional hosts where guests can be released in a controlled fashion. In this paper, a novel methodology based on the use of phenanthrenequinone is applied to selectively change the binding properties of a tris(2-pyridylmethyl)amine TPMA-based cage. In particular, subcomponent substitution has been used to change structural cage features thus controlling the inclusion ratio of competing guests differing in size or chirality.

Anionic Templates Drive Conversion between a ZnII9L6 Tricapped Trigonal Prism and ZnII6L4 Pseudo-Octahedra

This work introduces the use of 8-aminoquinoline subcomponents to generate complex three-dimensional structures. Together with a tris(formylpyridine), 8-aminoquinoline condensed around Zn^II templates to produce a tris(tridentate) ligand. This ligand is incorporated into either a tricapped trigonal prismatic Zn^II₉L₆ structure or a pair of pseudo-octahedral Zn^II₆L₄ diastereomers, with S₄ and D₂ symmetries. Introduction of a methyl group onto the aminoquinoline modulated the coordination sphere of Zn^II, which favored the Zn^II₉L₆ structure and disfavored the Zn^II₆L₄ assembly. The tricapped trigonal prismatic Zn^II₉L₆ architecture converted into a single Zn^II₆L₄ cage diastereomer following the addition of a dianionic 4,4'-dinitrostilbene-2,2'-disulfonate guest. Four of these guests clustered tightly at the four windows of the Zn^II₆L₄ cage, held in place through electrostatic interactions and hydrogen bonding, stabilize a single diastereomeric configuration with S₄ symmetry.

Tetramine Aspect Ratio and Flexibility Determine Framework Symmetry for Zn8 L6 Self-Assembled Structures

We derive design principles for the assembly of rectangular tetramines into Zn8 L6 pseudo-cubic coordination cages. Because of the rectangular, as opposed to square, geometry of the ligand panels, and the possibility of either Δ or Λ handedness of each metal center at the eight corners of the pseudo-cube, many different cage diastereomers are possible. Each of the six tetra-aniline subcomponents investigated in this work assembled with zinc(II) and 2-formylpyridine in acetonitrile into a single Zn8 L6 pseudo-cube diastereomer, however. Each product corresponded to one of four diastereomeric configurations, with T, Th , S6 or D3 symmetry. The preferred diastereomer for a given tetra-aniline subcomponent was shown to be dependent on its aspect ratio and conformational flexibility. Analysis of computationally modeled individual faces or whole pseudo-cubes provided insight as to why the observed diastereomers were favored.

Ferrocene-Derived Palladium(II)-Based Metallosupramolecular Structures: Synthesis, Guest Interaction, and Stimulus-Responsiveness Studies

Using ferrocene-based ligand systems, a series of heterobimetallic architectures of the general formula [Pd_mL_n]^x+ were designed with the aim of installing an opening and closing mechanism that would allow the release and binding of guest molecules. Palladium complex formation was achieved through coordination to pyridyl groups, and using 2-, 3-, and 4-pyridyl derivatives provided access to defined PdL, PdL₂, and Pd₂L₄ structures, respectively. The supramolecular complexes were characterized using nuclear magnetic resonance (NMR) and infrared spectroscopy, mass spectrometry, and elemental analysis, and for some examples density functional theory calculations and single-crystal X-ray diffraction analysis. ¹H NMR spectroscopy was used to investigate disassembly and reassembly of the metallosupramolecular structures. The former was induced by cleavage of the relatively labile Pd-N_pyridyl bonds with the introduction of the competing ligands N,N'-dimethylaminopyridine (DMAP) and Cl^- (using tetrabutylammonium chloride) to yield [Pd(DMAP)₄]²⁺ and [PdCl₄]^2-, respectively. The process was found to be reversible for several of the heterodimetallic compounds, with the addition of H⁺ or Ag⁺ triggering complex reassembly. Guest binding studies with several architectures revealed interactions with the anionic guests p-toluenesulfonate and octyl sulfate, but not with neutral molecules. Furthermore, the release of guests was reversibly induced with Cl^- ions as a stimulus.

Effector Regulated Catalytic Cyclization of Alkynoic Acids Using Pt2 L4 Cages

Metabolic pathways are highly regulated by effector molecules that influences the rate of enzymatic reactions. Inspired by the catalytic regulation found in living cells, we report a Pt₂ L₄ cage of which the activity can be controlled by effectors that bind inside the cage. The cage shows catalytic activity in the lactonization of alkynoic acids, with the reaction rates dependent on the effector guest bound in the cage. Some effector guests enhance the rate of the lactonization by up to 19-fold, whereas one decreases it by 5-fold. When mixtures of specific substrates are used, both starting materials and products act as guests for the Pt₂ L₄ cage, enhancing its catalytic activity for one substrate while reducing its activity for the other. The reported regulatory behavior obtained by the addition of effector molecules paves the way to the development of more complex, metabolic-like catalyst systems.

Maximized axial helicity in a Pd2L4 cage: inverse guest size-dependent compression and mesocate isomerism

Helicity is an archetypal structural motif of many biological systems and provides a basis for molecular recognition in DNA. Whilst artificial supramolecular hosts are often helical, the relationship between helicity and guest encapsulation is not well understood. We report a detailed study on a significantly coiled-up Pd2L4 metallohelicate with an unusually wide azimuthal angle (∼176°). Through a combination of NMR spectroscopy, single-crystal X-ray diffraction, trapped ion mobility mass spectrometry and isothermal titration calorimetry we show that the coiled-up cage exhibits extremely tight anion binding (K of up to 106 M-1) by virtue of a pronounced oblate/prolate cavity expansion, whereby the Pd-Pd separation decreases for mono-anionic guests of increasing size. Electronic structure calculations point toward strong dispersion forces contributing to these host-guest interactions. In the absence of a suitable guest, the helical cage exists in equilibrium with a well-defined mesocate isomer that possesses a distinct cavity environment afforded by a doubled Pd-Pd separation distance.

Supramolecular Capsule-Catalyzed Highly β-Selective Furanosylation Independent of the SN1/SN2 Reaction Pathway

The resorcin[4]arene capsule was found to catalyze β-selective furanosylation reactions for a variety of different furanosyl donors: α-d- and α-l-arabinosyl-, α-l-fucosyl-, α-d-ribosyl-, α-d-xylosyl-, and even α-d-lyxosyl fluorides. The scope is only limited by the inherently finite volume inside the closed capsular catalyst. The catalyst is readily available on a multi-100 g scale and can be recycled for at least seven rounds without significant loss in activity, yield, and selectivity. The mechanistic investigations indicated that the furanosylation mechanism is shifted toward an SN1 reaction on the mechanistic continuum between the prototypical SN1 and SN2 substitution types, as compared to the pyranosylation reaction inside the same catalyst. This is especially true for the lyxosyl donor, as indicated by the nucleophile reaction order of 0.26, and supported by metadynamics calculations. The mechanistic shift toward SN1 is of high interest as it indicates that this catalyst not only enables β-selective furanosylations and pyranoslyations independently of the substrate configuration but in addition also independently of the operating mechanism. To our knowledge, there is no alternative catalyst available that displays such properties.

Anion-Driven Programmable Chiral Self-Sorting in Metal-Organic Cages and Structural Transformations between Heterochiral and Homochiral Cages

When a racemic mixture of chiral building blocks self-assembles to form discrete molecular or supramolecular cages, the system can adopt either social or narcissistic chiral self-sorting. However, control over such chiral self-sorting is hard to achieve with a desired choice of outcome. Herein, we report anion templated high-fidelity chiral self-sorting during the coordination-driven self-assembly of [Pd₂ L₄ ] metal-organic cages, with a racemic mixture of an axially chiral ligand. Upon varying the counter-anions, the outcome of the choice of chiral self-sorting, whether social or narcissistic, leading to kinetically favored heterochiral or thermodynamically favored homochiral cages, can be controlled through specific anion encapsulation. Non-encapsulating anion afforded a mixture of all possible diastereomers. Anion exchange enabled structural transformations between the diastereomers and the conversion of the mixture of diastereomers into homochiral diastereomers.

Electrostatically tuned phenols: a scalable organocatalyst for transfer hydrogenation and tandem reductive alkylation of N-heteroarenes

One of the fundamental aims in catalysis research is to understand what makes a certain scaffold perform better as a catalyst than another. For instance, in nature enzymes act as versatile catalysts, providing a starting point for researchers to understand how to achieve superior performance by positioning the substrate close to the catalyst using non-covalent interactions. However, translating this information to a non-biological catalyst is a challenging task. Here, we report a simple and scalable electrostatically tuned phenol (ETP) as an organocatalyst for transfer hydrogenation of N-arenes using the Hantzsch ester as a hydride source. The biomimetic catalyst (1-5 mol%) displays potential catalytic activity to prepare diverse tetrahydroquinoline derivatives with good to excellent conversion under ambient reaction conditions. Kinetic studies reveal that the ETP is 130-fold faster than the uncharged counterpart, towards completion of the reaction. Control experiments and NMR spectroscopic investigations elucidate the role of the charged environment in the catalytic transformation.

Three-Input Logic AND Gate Drives Sequential Three-Step Catalysis by Parallel Activation of H+ and Ag+ as a Catalyst Duo

Boolean operations with multiple catalysts as output are yet unknown using molecular logic. The issue is solved using a two-component ensemble, composed of a receptor and rotaxane, which acts as a three-input AND gate with a dual catalytic output. Actuation of the ensemble gate by the stoichiometric addition of metal ions (Ag⁺ and Cd²⁺) and 2,2,2-trifluoroacetic acid generated in the (1,1,1) truth table state a catalyst duo that synergistically enabled a three-step reaction, furnishing a dihydroisoquinoline as the output of a three-input logic AND gate operation.

Disentangling contributions to guest binding inside a coordination cage host: analysis of a set of isomeric guests with differing polarities

Binding of a set of three isomeric guests (1,2-, 1,3- and 1,4-dicyanobenzene, abbreviated DCB) inside an octanuclear cubic coordination cage host H (bearing different external substitutents according to solvent used) has been studied in water/dmso (98 : 2) and CD₂Cl₂. These guests have essentially identical molecular surfaces, volumes and external functional groups to interact with the cage interior surface; but they differ in polarity with dipole moments of ca. 7, 4 and 0 Debye respectively. In CD₂Cl₂ guest binding is weak but we observe a clear correlation of binding free energy with guest polarity, with 1,4-DCB showing no detectable binding by NMR spectroscopy but 1,2-DCB having −ΔG = 9 kJ mol⁻¹. In water (containing 2% dmso to solubilise the guests) we see the same trend but all binding free energies are much higher due to an additional hydrophobic contribution to binding, with −ΔG varying from 16 kJ mol⁻¹ for 1,4-DCB to 22 kJ mol⁻¹ for 1,4-DCB: again we see an increase associated with guest polarity but the increase in −ΔG per Debye of dipole moment is around half what we observe in CD₂Cl₂ which we ascribe to the fact the more polar guests will be better solvated in the aqueous solvent. A van't Hoff analysis by variable-temperature NMR showed that the improvement in guest binding in water/dmso is entropy-driven, which suggests that the key factor is not direct electrostatic interactions between a polar guest and the cage surface, but the variation in guest desolvation across the series, with the more polar (and hence more highly solvated) guests having a greater favourable entropy change on desolvation.

The three dicyanobenzene isomers have obvious similarities but differ in their dipole moment: effects on binding in a coordination cage host in different solvents are discussed.

Visible-Light-Responsive Self-Assembled Complexes: Improved Photoswitching Properties by Metal Ion Coordination

A photoswitchable ligand based on azobenzene is self-assembled with palladium(II) ions to form a [Pd2 (E-L)4 ]4+ cage. Irradiation with 470 nm light results in the near-quantitative switching to a monomeric species [Pd(Z-L)2 ]2+ , which can be reversed by irradiation with 405 nm light, or heat. The photoswitching selectivity towards the metastable isomer is significantly improved upon self-assembly, and the thermal half-life is extended from 40 days to 850 days, a promising approach for tuning photoswitching properties.

Inside or outside the box? Effect of substrate location on coordination-cage based catalysis

In this work we compare and contrast the hydrolysis of two different aromatic esters using an octanuclear cubic Co8 coordination cage host as the catalyst. Diacetyl fluorescein (DAF) is too large to bind inside the cage cavity, but in aqueous solution it interacts with the exterior surface of the cage via a hydrophobic interaction with K = 1.5(2) × 104 M-1. This is sufficient to bring it into close proximity to the layer of hydroxide ions which also surrounds the 16+ cage surface even at modest pH values, accelerating the hydrolysis of DAF to fluorescein with kcat/kuncat (the rate acceleration for that fraction of DAF in contact with the cage surface in the equilibrium) ≈50. This is far smaller than many known examples of catalysis inside a cage cavity, but at the exterior surface it is potentially general with no cavity-imposed size/shape limitations for guest binding. In contrast 4-nitrophenyl acetate (4NPA) binds inside the cage cavity with K = 3.5(3) × 103 M-1 and as such is surrounded in solution by the hydroxide ions which accumulate around the cage surface. However its hydrolysis is actually inhibited: either because of a geometrically unfavourable geometry of the bound substrate which makes it inaccessible to surface-bound hydroxide, or because the necessary volume expansion/geometry change associated with formation of a tetrahedral intermediate cannot be accommodated inside the cavity. Any 4NPA that is free in solution as part of the equilibrium undergoes catalysed hydrolysis at the cage exterior surface in the same way as DAF, but the effect is limited by the low affinity of 4NPA for the exterior surface. We conclude that exterior-surface catalysis can be effective and potentially general; and that cavity-binding of guests can result in negative, rather than positive, catalysis.

Enantioselective Tail-to-Head Terpene Cyclizations by Optically Active Hexameric Resorcin[4]arene Capsule Derivatives

Molecular capsules enable the conversion of substrates inside a closed cavity, mimicking to some extent enzymatic catalysis. Chirality transfer from the molecular capsule onto the encapsulated substrate has been only studied in a few cases. Here we demonstrate that chirality transfer is possible inside a rather large molecular container of approximately 1400 Å3 . Specifically, we present 1) the first examples of optically active hexameric resorcin[4]arene capsules, 2) their ability to enantioselectively catalyze tail-to-head terpene cyclizations, and 3) the surprisingly high sensitivity of enantioselectivity on the structural modifications.

Computational Modeling of Supramolecular Metallo-organic Cages-Challenges and Opportunities

Self-assembled metallo-organic cages have emerged as promising biomimetic platforms that can encapsulate whole substrates akin to an enzyme active site. Extensive experimental work has enabled access to a variety of structures, with a few notable examples showing catalytic behavior. However, computational investigations of metallo-organic cages are scarce, not least due to the challenges associated with their modeling and the lack of accurate and efficient protocols to evaluate these systems. In this review, we discuss key molecular principles governing the design of functional metallo-organic cages, from the assembly of building blocks through binding and catalysis. For each of these processes, computational protocols will be reviewed, considering their inherent strengths and weaknesses. We will demonstrate that while each approach may have its own specific pitfalls, they can be a powerful tool for rationalizing experimental observables and to guide synthetic efforts. To illustrate this point, we present several examples where modeling has helped to elucidate fundamental principles behind molecular recognition and reactivity. We highlight the importance of combining computational and experimental efforts to speed up supramolecular catalyst design while reducing time and resources.

Directionally aligned crown ethers as superactive organocatalysts for transition metal ion-free arylation of unactivated arenes

When one-dimensionally aligned to the same side, multiple non-covalently associated crown ether groups could act as a whole to yield a higher catalytic activity than an individual poorly active crown ether group, delivering the lowest catalyst loading of 1 - 2 mol% among all hitherto known organocatalysts for catalyzing direct arylation of unactivated arenes with haloarenes.

Utilizing Raman Spectroscopy as a Tool for Solid- and Solution-Phase Analysis of Metalloorganic Cage Host-Guest Complexes

The host-guest chemistry of coordination cages continues to promote significant interest, not least because confinement effects can be exploited for a range of applications, such as drug delivery, sensing, and catalysis. Often a fundamental analysis of noncovalent encapsulation is required to provide the necessary insight into the design of better functional systems. In this paper, we demonstrate the use of various techniques to probe the host-guest chemistry of a novel Pd₂L₄ cage, which we show is preorganized to selectively bind dicyanoarene guests with high affinity through hydrogen-bonding and other weak interactions. In addition, we exemplify the use of Raman spectroscopy as a tool for analyzing coordination cages, exploiting alkyne and nitrile reporter functional groups that are contained within the host and guest, respectively.

Modifying electron injection kinetics for selective photoreduction of nitroarenes into cyclic and asymmetric azo compounds

Modifying the reactivity of substrates by encapsulation is essential for microenvironment catalysts. Herein, we report an alternative strategy that modifies the entry behaviour of reactants into the microenvironment and substrate inclusion thermodynamics related to the capsule to control the electron injection kinetics and the selectivity of products from the nitroarenes photoreduction. The strategy includes the orchestration of capsule openings to control the electron injection kinetics of electron donors, and the capsule’s pocket to encapsulate more than one nitroarene molecules, facilitating a condensation reaction between the in situ formed azanol and nitroso species to produce azo product. The conceptual microenvironment catalyst endows selective conversion of asymmetric azo products from different nitroarenes, wherein, the estimated diameter and inclusion Gibbs free energy of substrates are used to control and predict the selectivity of products. Inhibition experiments confirm a typical enzymatic conversion, paving a new avenue for rational design of photocatalysts toward green chemistry.

Molecular engineering of confined space in metal–organic cages

The host–guest chemistry of metal–organic cages can be modified through tailoring of structural aspects such as size, shape and functionality. In this review, strategies, opportunities and challenges of such molecular engineering are discussed.

Adaptive coordination assemblies based on a flexible tetraazacyclododecane ligand for promoting carbon dioxide fixation

Coordination hosts based on flexible ligands have received increasing attention due to their inherent adaptive cavities that often show induced-fit guest binding and catalysis like enzymes. Herein, we report the controlled self-assembly of a series of homo/heterometallic coordination hosts (Me₄enPd)_2n(ML)_n [n = 2/3; M = Zn(ii)/Co(ii)/Ni(ii)/Cu(ii)/Pd(ii)/Ag(i); Me₄en: N,N,N′,N′-tetramethylethylenediamine] with different shapes (tube/cage) from a flexible tetraazacyclododecane-based pyridinyl ligand (L) and cis-blocking Me₄enPd(ii) units. While the Ag(i)-metalated ligand (AgL) gave rise to the formation of a (Me₄enPd)₄(ML)₂-type cage, all other M(ii) ions led to isostructural (Me₄enPd)₆(ML)₃-type tubular complexes. Structural transformations between cages and tubes could be realized through transmetalation of the ligand. The buffering effect on the ML panels endows the coordination tubes with remarkable acid–base resistance, which makes the (Me₄enPd)₆(ZnL)₃ host an effective catalyst for the CO₂ to CO₃²⁻ conversion. Control experiments suggested that the integration of multiple active Zn(ii) sites on the tubular host and the perfect geometry match between CO₃²⁻ and the cavity synergistically promoted such a conversion. Our results provide an important strategy for the design of adaptive coordination hosts to achieve efficient carbon fixation.

A series of coordination hosts were prepared and their applications in CO₂ fixation were studied.

Guest-induced magnetic exchange in paramagnetic [M2L4]4+ coordination cages

Paramagnetic complexes that show magnetically switchable properties show promise in a number of applications. A significantly underdeveloped approach is the use of metallocages, whose magnetic properties can be modulated through...

Interaction of anions with the surface of a coordination cage in aqueous solution probed by their effect on a cage-catalysed Kemp elimination

An octanuclear M8L12 coordination cage catalyses the Kemp elimination reaction of 5-nitro-1,2-benzisoxazole (NBI) with hydroxide to give 2-cyano-4-nitrophenolate (CNP) as the product. In contrast to the previously-reported very efficient catalysis of the Kemp elimination reaction of unsubstituted benzisoxazole, which involves the substrate binding inside the cage cavity, the catalysed reaction of NBI with hydroxide is slower and occurs at the external surface of the cage, even though NBI can bind inside the cage cavity. The rate of the catalysed reaction is sensitive to the presence of added anions, which bind to the 16+ cage surface, displacing the hydroxide ions from around the cage which are essential reaction partners in the Kemp elimination. Thus we can observe different binding affinities of anions to the surface of the cationic cage in aqueous solution by the extent to which they displace hydroxide and thereby inhibit the catalysed Kemp elimination and slow down the appearance of CNP. For anions with a -1 charge the observed affinity order for binding to the cage surface is consistent with their ease of desolvation and their ordering in the Hofmeister series. With anions that are significantly basic (fluoride, hydrogen carbonate, carboxylates) the accumulation of the anion around the cage surface accelerates the Kemp elimination compared to the background reaction with hydroxide, which we ascribe to the ability of these anions to participate directly in the Kemp elimination. This work provides valuable mechanistic insights into the role of the cage in co-locating the substrate and the anionic reaction partners in a cage-catalysed reaction.

Creating Dynamic Nanospaces in Solution by Cationic Cages as Multirole Catalytic Platform for Unconventional C(sp)-H Activation Beyond Enzyme Mimics

Herein we demonstrate that, based on the creation of dynamic nanospaces in solution by highly charged positive coordination cage of [Pd₆ (RuL₃ )₈ ]²⁸⁺ , multirole and multi-way cage-confined catalysis is accomplishable for versatile functions and anomalous reactivities with the aid of the biomimetic cage effect. The high cationic-host charges drive partial deprotonation of 24 imidazole-NHs on cage sphere alike imidazole-residuals in proteins, generating amphoteric heterogeneity in solution to enforce effective cavity-basicity against solution-acidity. Synergistic actions arisen from cage hydrophobicity, host-guest electrostatic interactions and imidazole-N coordination facilitate C(sp)-H activation and carbanionic intermediate stabilization of terminal alkynes to achieve unusual H/D-exchange and Glaser coupling under acidic conditions, and enable phase transfers of water-insoluble substrates/products/co-catalysts to make immiscible-phase and bi-phase catalysis feasible, thus providing a useful catalytic protocol to combine merits from homogeneous, heterogeneous, enzymatic and phase transfer catalysis.

Photooxidase Mimicking with Adaptive Coordination Molecular Capsules

One important feature of enzyme catalysis is the induced-fit conformational change after binding substrates. Herein, we report a biomimetic water-soluble molecular capsule featuring adaptive structural change toward substrate binding, which offers an ideal platform for efficient photocatalysis. The molecular capsule was coordination-assembled from three anthracene-bridged bis-TPT [TPT = 2,4,6-tris(4-pyridyl)-1,3,5-triazine] ligands and six (bpy)Pd(NO3)2 (bpy = 2,2'-bipyridine). Once substrates bind to its hydrophobic cavity, this capsule would undergo quantitative capsule-to-bowl transformation. Visible-light absorption brought about by both the anthracene units and the charge-transfer absorption on the late-formed quintuple π-π stacked host-guest complex efficiently facilitates aerobic photooxidation for the sulfide guests by visible-light irradiation under mild conditions. Desired turnover numbers and product selectivity (sulfoxide over sulfone) have been achieved by the transformable nature of the catalyst and the hydrophilicity of the sulfoxide product. Such a photocatalytic process enabled by an adaptive coordination capsule and substrates as the allosteric effector paves the way for constructing artificial systems to mimic enzyme catalysis.

High-Throughput Computational Evaluation of Low Symmetry Pd2 L4 Cages to Aid in System Design*

Unsymmetrical ditopic ligands can self-assemble into reduced-symmetry Pd2 L4 metallo-cages with anisotropic cavities, with implications for high specificity and affinity guest-binding. Mixtures of cage isomers can form, however, resulting in undesirable system heterogeneity. It is paramount to be able to design components that preferentially form a single isomer. Previous data suggested that computational methods could predict with reasonable accuracy whether unsymmetrical ligands would preferentially self-assemble into single cage isomers under constraints of geometrical mismatch. We successfully apply a collaborative computational and experimental workflow to mitigate costly trial-and-error synthetic approaches. Our rapid computational workflow constructs unsymmetrical ligands and their Pd2 L4 cage isomers, ranking the likelihood for exclusively forming cis-Pd2 L4 assemblies. From this narrowed search space, we successfully synthesised four new, low-symmetry, cis-Pd2 L4 cages.

Molecular Cage Promoted Aerobic Oxidation or Photo-Induced Rearrangement of Spiroepoxy Naphthalenone

Herein, we report a Pd4L2-type molecular cage (1) and catalyzed reactions of spiroepoxy naphthalenone (2) in water, where selective formation of 2-(hydroxymethyl)naphthalene-1,4-dione (3) via aerobic oxidation, or 1-hydroxy-2-naphthaldehyde (4) via photo-induced rearrangement under N2 have been accomplished. Encapsulation of four molecules of guest 2 within cage 1, i.e., (2)4⊂1, has been confirmed by NMR, and a final host-guest complex of 3⊂1 has also been determined by single crystal X-Ray diffraction study. While the photo-induced ring-opening isomerization from 2 to 4 are known, appearance of charge-transfer absorption on the host-guest complex of (2)4⊂1 allows low-power blue LEDs irradiation to promote this process.

Thermodynamic driving forces of guest confinement in a photoswitchable cage

Photoswitchable cages that confine small guest molecules inside their cavities offer a way to control the binding/unbinding process through irradiation with light of different wavelengths. However, detailed characterization of the structural and thermodynamic consequences of photoswitching is very challenging to achieve by experiments alone. Thus, all-atom molecular dynamics (MD) simulations were carried out to gain insight into the relationship between the structure and binding affinity. Binding free energies of the B₁₂F₁₂^2- guest were obtained for all photochemically accessible forms of a photoswitchable dithienylethene (DTE) based coordination cage. The MD simulations show that successive photo-induced closure of the four individual DTE ligands that form the cage gradually decreases the binding affinity. Closure of the first ligand significantly lowers the unbinding barrier and the binding free energy, and therefore favours guest unbinding both kinetically and thermodynamically. The analysis of different enthalpy contributions to the free energy shows that binding is enthalpically unfavourable and thus is an entropy-driven process, in agreement with the experimental data. Separating the enthalpy into the contributions from electrostatic, van der Waals, and bonded interactions in the force field shows that the unfavourable binding enthalpy is due to the bonded interactions being more favourable in the dissociated state, suggesting the presence of structural strain in the bound complex. Thus, the simulations provide microscopic explanations for the experimental findings and provide a possible route towards the targeted design of switchable nanocontainers with modified binding properties.

Exploiting host-guest chemistry to manipulate magnetic interactions in metallosupramolecular M4L6 tetrahedral cages

Reaction of Ni(OTf)₂ with the bisbidentate quaterpyridine ligand L results in the self-assembly of a tetrahedral, paramagnetic cage [Ni^II₄L₆]⁸⁺. By selectively exchanging the bound triflate from [OTf⊂Ni^II₄L₆](OTf)₇ (1), we have been able to prepare a series of host–guest complexes that feature an encapsulated paramagnetic tetrahalometallate ion inside this paramagnetic host giving [M^IIX₄⊂Ni^II₄L₆](OTf)₆, where M^IIX₄²⁻ = MnCl₄²⁻ (2), CoCl₄²⁻ (5), CoBr₄²⁻ (6), NiCl₄²⁻ (7), and CuBr₄²⁻ (8) or [M^IIIX₄⊂Ni^II₄L₆](OTf)₇, where M^IIIX₄⁻ = FeCl₄⁻ (3) and FeBr₄⁻ (4). Triflate-to-tetrahalometallate exchange occurs in solution and can also be accomplished through single-crystal-to-single-crystal transformations. Host–guest complexes 1–8 all crystallise as homochiral racemates in monoclinic space groups, wherein the four {NiN₆} vertexes within a single Ni₄L₆ unit possess the same Δ or Λ stereochemistry. Magnetic susceptibility and magnetisation data show that the magnetic exchange between metal ions in the host [Ni^II₄] complex, and between the host and the MX₄ⁿ⁻ guest, are of comparable magnitude and antiferromagnetic in nature. Theoretically derived values for the magnetic exchange are in close agreement with experiment, revealing that large spin densities on the electronegative X-atoms of particular MX₄ⁿ⁻ guest molecules lead to stronger host–guest magnetic exchange interactions.

The tetrahedral [Ni^II₄L₆]⁸⁺ cage can reversibly bind paramagnetic MX₄^1/2− guests, inducing magnetic exchange interactions between host and guest.