Hydroalkoxylation Catalyzed by a Gold(I) Complex Encapsulated in a Supramolecular Host

Self-Assembled Iron(III) Coordination Cage as an MRI-Active Carrier for a Gold(I) Drug

A T1 MRI probe based on a self-assembled coordination cage with four iron(III) centers acts as a host for the hydrolysis product of the gold(I) anticancer drug, Au(PEt3)Cl. 1H NMR characterization of the gold complex encapsulated within the diamagnetic Ga(III) analog of the coordination cage is consistent with loss of chloride to give aquated gold complex, most likely [Au(PEt3)(OH2)]+ within the cage. The gold complex undergoes pH-dependent speciation changes in the Ga(III) cage and is released at mildly acidic pH from both the Ga(III) and Fe(III) cages. NMR spectroscopy studies of the encapsulated gold complex in the presence of human serum albumin (HSA) show that the gold complex remains inside of the Ga(III) cage for several hours, resisting release and binding to cysteine residues of HSA. The Fe(III) cage with encapsulated gold complex shows enhanced contrast of the vasculature and uptake into CT26 tumors in BALB/c mice as shown by MRI. The gold complex is solubilized by the iron(III) cage for intravenous injection, whereas the free complex must be injected intraperitoneally. Gold complex accumulates in the tumor for both caged and free complex over 1-48 h as measured by ex-vivo analysis. Encapsulation in the Fe(III) cage modulates the biodistribution of the gold complex in mice in comparison to the free complex, consistent with the function of the cage as a carrier.

β-Cyclodextrin-NHC-Au(I)-Catalyzed Enantioconvergent 1,5-Enyne Cycloisomerizations

Enantioconvergent transformations from racemic mixtures are attractive since they allow the generation of optically active products with full conversion despite the possibly adverse kinetic resolution process. When dealing with gold(I)-catalyzed cycloisomerizations, chirality transfer from the precursor is another possible diverting pathway, which renders enantioconvergence challenging. Not surprisingly, enantioconvergent Au(I)-catalyzed processes have remained extremely rare. Herein we show that cavity-driven catalysis using β-cyclodextrin-NHC-Au(I) complexes brings opportunities to conduct highly enantioconvergent cycloisomerizations of 1,5-enynes, -enynols, and, -enynyl esters.

Hydration of Propargyl Esters Catalyzed by Gold(I) Complexes with Phosphoramidite Calix[4]pyrrole Cavitands as Ligands

We report the synthesis and characterization of two diastereomeric phosphoramidite calix[4]pyrrole cavitands and their corresponding gold(I) complexes, 2in•Au(I)•Cl and 2out•Au(I)•Cl, featuring the metal center directed inward and outward with respect to their aromatic cavity. We studied the catalytic activity of the complexes in the hydration of a series of propargyl esters as the benchmarking reaction. All substrates were equipped with a six-membered ring substituent either lacking or including a polar group featuring different hydrogen bond acceptor (HBA) capabilities. We designed the substrates with the polar group to form 1:1 inclusion complexes of different stabilities with the catalysts. In the case of 2in•Au(I)•OTf, the 1:1 complex placed the alkynyl group of the bound substrate close to the metal center. We compared the obtained results with those of a model phosphoramidite gold(I) complex lacking a calix[4]pyrrole cavity. We found that for all catalysts, the presence of an increasingly polar HBA group in the substrate provoked a decrease in the hydration rate constants. We attributed this result to the competing coordination of the HBA group of the substrate for the Au(I) metal center of the catalysts.

Transition Metal Catalysis in Living Cells: Progress, Challenges, and Novel Supramolecular Solutions

The importance of transition metal catalysis is exemplified by its wide range of applications, for example in the synthesis of chemicals, natural products, and pharmaceuticals. However, one relatively new application is for carrying out new-to-nature reactions inside living cells. The complex environment of a living cell is not welcoming to transition metal catalysts, as a diverse range of biological components have the potential to inhibit or deactivate the catalyst. Here we review the current progress in the field of transition metal catalysis, and evaluation of catalysis efficiency in living cells and under biological (relevant) conditions. Catalyst poisoning is a ubiquitous problem in this field, and we propose that future research into the development of physical and kinetic protection strategies may provide a route to improve the reactivity of catalysts in cells.

Reactivity and Selectivity of the Diels-Alder Reaction of Anthracene in [Pd6L4]12+ Supramolecular Cages: A Computational Study

The field of supramolecular metal-organic cage catalysis has grown rapidly in recent years. However, theoretical studies regarding the reaction mechanism and reactivity and selectivity controlling factors for supramolecular catalysis are still underdeveloped. Herein, we demonstrate a detailed density functional theory study on the mechanism, catalytic efficiency, and regioselectivity of the Diels-Alder reaction in bulk solution and within two [Pd₆L₄]¹²⁺ supramolecular cages. Our calculations are consistent with experiments. The origins of the catalytic efficiency of the bowl-shaped cage 1 have been elucidated to be the host-guest stabilization of the transition states and the favorable entropy effect. The reasons for the switch of the regioselectivity from 9,10-addition to 1,4-addition within the octahedral cage 2 were attributed to the confinement effect and the noncovalent interactions. This work would shed light on the understanding of [Pd₆L₄]¹²⁺ metallocage-catalyzed reactions and provide a detailed mechanistic profile otherwise difficult to obtain from experiments. The findings of this study could also aid to the improvement and development of more efficient and selective supramolecular catalysis.

Catalysis by [Ga4 L6 ]12- Metallocage on the Nazarov Cyclization: The Basicity of Complexed Alcohol is Key

The Nazarov cyclization is investigated in solution and within K12 [Ga4 L6 ] supramolecular organometallic cage by means of computational methods. The reaction needs acidic condition in solution but works at neutral pH in the presence of the metallocage. The reaction steps for the process are analogous in both media: (a) protonation of the alcohol group, (b) water loss and (c) cyclization. The relative Gibbs energies of all the steps are affected by changing the environment from solvent to the metallocage. The first step in the mechanism, the alcohol protonation, turns out to be the most critical one for the acceleration of the reaction inside the metallocage. In order to calculate the relative stability of protonated alcohol inside the cavity, we propose a computational scheme for the calculation of basicity for species inside cavities and can be of general use. These results are in excellent agreement with the experiments, identifying key steps of catalysis and providing an in-depth understanding of the impact of the metallocage on all the reaction steps.

Heteroleptic metallosupramolecular aggregates/complexation for supramolecular catalysis

Supramolecular catalysis is reviewed with an eye on heteroleptic aggregates/complexation. Since most of the current metallosupramolecular catalytic systems are homoleptic in nature, the idea of breaking/reducing symmetry has ignited a vivid search for heteroleptic aggregates that are made up by different components. Their higher degree of functional diversity and structural heterogeneity allows, as demonstrated by Nature by the multicomponent ATP synthase motor, a more detailed and refined configuration of purposeful machinery. Furthermore, (metallo)supramolecular catalysis is shown to extend beyond the single "supramolecular unit" and to reach far into the field and concepts of systems chemistry and information science.

Origin of the Rate Acceleration in the C-C Reductive Elimination from Pt(IV)-complex in a [Ga4 L6 ]12- Supramolecular Metallocage

The reductive elimination on [(Me3 P)2 Pt(MeOH)(CH3 )3 ]+ , 2P, complex performed in MeOH solution and inside a [Ga4 L6 ]12- metallocage are computationally analysed by mean of QM and MD simulations and compared with the mechanism of gold parent systems previously reported [Et3 PAu(MeOH)(CH3 )2 ]+ , 2Au. The comparative analysis between the encapsulated Au(III) and Pt(IV)-counterparts shows that there are no additional solvent MeOH molecules inside the cavity of the metallocage for both systems. The Gibbs energy barriers for the 2P reductive elimination calculated at DFT level are in good agreement with the experimental values for both environments. The effect of microsolvation and encapsulation on the rate acceleration are evaluated and shows that the latter is far more relevant, conversely to 2Au. Energy decomposition analysis indicates that the encapsulation is the main responsible for most of the energy barrier reduction. Microsolvation and encapsulation effects are not equally contributing for both metal systems and consequently, the reasons of the rate acceleration are not the same for both metallic systems despite the similarity between them.

Size- and Shape-Selective Catalysis with a Functionalized Self-Assembled Cage Host

A self-assembled Fe₄L₆ cage with internally oriented carboxylic acid functions was shown to catalyze a variety of dissociative nucleophilic substitution reactions that proceed via oxocarbenium ion or carbocation intermediates. The catalytic behavior of the cage was compared to that of other small acid catalysts, which illustrated large differences in reactivity of the cage-catalyzed reactions, dependent on the structure of the substrate. For example, only a 5% cage confers a 1000-fold rate acceleration of the thioetherification of vinyldiphenylmethanol when compared to the rate with free carboxylic acid surrogates but only a 52-fold acceleration in the formation of small thioacetals. Multiple factors control the variable reactivity in the host, including substrate inhibition, binding affinity, and accessibility of reactive groups once bound. Simple effective concentration increases or the overall charge of the cage does not explain the variations in reactivity shown by highly similar reactants in the host: small differences in structure can have large effects on reactivity. Reaction of large spherical guests is highly dependent on substitution, whereas flat guests are almost unaffected by size and shape differences. The cage is a promiscuous catalyst but has strong selectivity for particular substrate shapes, reminiscent of enzymatic activity.

Main Avenues in Gold Coordination Chemistry

In this contribution, we provide an overview of the main avenues that have emerged in gold coordination chemistry during the last years. The unique properties of gold have motivated research in gold chemistry, and especially regarding the properties and applications of gold compounds in catalysis, medicine, and materials chemistry. The advances in the synthesis and knowledge of gold coordination compounds have been possible with the design of novel ligands becoming relevant motifs that have allowed the preparation of elusive complexes in this area of research. Strong donor ligands with easily modulable electronic and steric properties, such as stable singlet carbenes or cyclometalated ligands, have been decisive in the stabilization of gold(0) species, gold fluoride complexes, gold hydrides, unprecedented π complexes, or cluster derivatives. These new ligands have been important not only from the fundamental structure and bonding studies but also for the synthesis of sophisticated catalysts to improve activity and selectivity of organic transformations. Moreover, they have enabled the facile oxidative addition from gold(I) to gold(III) and the design of a plethora of complexes with specific properties.

A Novel M8 L6 Cubic Cage That Binds Tetrapyridyl Porphyrins: Cage and Solvent Effects in Cobalt-Porphyrin-Catalyzed Cyclopropanation Reactions

Confinement of a catalyst can have a significant impact on catalytic performance and can lead to otherwise difficult to achieve catalyst properties. Herein, we report the design and synthesis of a novel caged catalyst system Co-G@Fe₈ (Zn-L ⋅ 1)₆ , which is soluble in both polar and apolar solvents without the necessity of any post-functionalization. This is a rare example of a metal-coordination cage able to bind catalytically active porphyrins that is soluble in solvents spanning a wide variety of polarity. This system was used to investigate the combined effects of the solvent and the cage on the catalytic performance in the cobalt catalyzed cyclopropanation of styrene, which involves radical intermediates. Kinetic studies show that DMF has a protective influence on the catalyst, slowing down deactivation of both [Co(TPP)] and Co-G@Fe₈ (Zn-L ⋅ 1)₆ , leading to higher TONs in this solvent. Moreover, DFT studies on the [Co(TPP)] catalyst show that the rate determining energy barrier of this radical-type transformation is not influenced by the coordination of DMF. As such, the increased TONs obtained experimentally stem from the stabilizing effect of DMF and are not due to an intrinsic higher activity caused by axial ligand binding to the cobalt center ([Co(TPP)(L)]). Remarkably, encapsulation of Co-G led to a three times more active catalyst than [Co(TPP)] (TOF_ini ) and a substantially increased TON compared to both [Co(TPP)] and free Co-G. The increased local concentration of the substrates in the hydrophobic cage compared to the bulk explains the observed higher catalytic activities.

Increasing structural and functional complexity in self-assembled coordination cages

Progress in metallo-supramolecular chemistry creates potential to synthesize functional nano systems and intelligent materials of increasing complexity. In the past four decades, metal-mediated self-assembly has produced a wide range of structural motifs such as helicates, grids, links, knots, spheres and cages, with particularly the latter ones catching growing attention, owing to their nano-scale cavities. Assemblies serving as hosts allow application as selective receptors, confined reaction environments and more. Recently, the field has made big steps forward by implementing dedicated functionality, e.g. catalytic centres or photoswitches to allow stimuli control. Besides incorporation in homoleptic systems, composed of one type of ligand, desire arose to include more than one function within the same assembly. Inspiration comes from natural enzymes that congregate, for example, a substrate recognition site, an allosteric regulator element and a reaction centre. Combining several functionalities without creating statistical mixtures, however, requires a toolbox of sophisticated assembly strategies. This review showcases the implementation of function into self-assembled cages and devises strategies to selectively form heteroleptic structures. We discuss first examples resulting from a combination of both principles, namely multicomponent multifunctional host-guest complexes, and their potential in application in areas such as sensing, catalysis, and photo-redox systems.

Deciphering the Chameleonic Chemistry of Allenols: Breaking the Taboo of a Onetime Esoteric Functionality

The allene functionality has participated in one of the most exciting voyages in organic chemistry, from chemical curiosities to a recurring building block in modern organic chemistry. In the last decades, a special kind of allene, namely, allenol, has emerged. Allenols, formed by an allene moiety and a hydroxyl functional group with diverse connectivity, have become common building blocks for the synthesis of a wide range of structures and frequent motif in naturally occurring systems. The synergistic effect of the allene and hydroxyl functional groups enables allenols to be considered as a unique and sole functionality exhibiting a special reactivity. This Review summarizes the most significant contributions to the chemistry of allenols that appeared during the past decade, with emphasis on their synthesis, reactivity, and occurrence in natural products.

A Promising Approach for Controlling the Second Coordination Sphere of Biomimetic Metal Complexes: Encapsulation in a Dynamic Hydrogen-Bonded Capsule

The design of biomimetic models of metalloenzymes needs to take into account many factors and is therefore a challenging task. We propose in this work an original strategy to control the second coordination sphere of a metal centre and its distal environment. A biomimetic complex, reproducing the first coordination sphere, is encapsulated in a self-assembled hydrogen-bonded capsule. The cationic complex is co-encapsulated with its counter-anion or with solvent molecules. The capsule is dynamic, allowing a fast in/out exchange of the co-encapsulated species. It also provides both a hydrogen-bonding site in the second coordination sphere and a source of proton as it can be deprotonated in the presence of the complex, providing a globally neutral host-guest assembly. This simple and broad scope strategy is unprecedented in biomimetic studies. The approach appears to be a very promising method for the stabilisation of reactive species and for the study of their reactivity.

Incorporation of homogeneous organometallic catalysts into metal–organic frameworks for advanced heterogenization: a review

The heterogenization of homogeneous organometallic catalysts by incorporation into MOFs using different strategies, MOF selection, OMC selection, and the use of hybrid heterogeneous catalysts OMC@MOFs in catalytic applications are summarized and discussed.

Design and Applications of Water-Soluble Coordination Cages

Compartmentalization of the aqueous space within a cell is necessary for life. In similar fashion to the nanometer-scale compartments in living systems, synthetic water-soluble coordination cages (WSCCs) can isolate guest molecules and host chemical transformations. Such cages thus show promise in biological, medical, environmental, and industrial domains. This review highlights examples of three-dimensional synthetic WSCCs, offering perspectives so as to enhance their design and applications. Strategies are presented that address key challenges for the preparation of coordination cages that are soluble and stable in water. The peculiarities of guest binding in aqueous media are examined, highlighting amplified binding in water, changing guest properties, and the recognition of specific molecular targets. The properties of WSCC hosts associated with biomedical applications, and their use as vessels to carry out chemical reactions in water, are also presented. These examples sketch a blueprint for the preparation of new metal-organic containers for use in aqueous solution, as well as guidelines for the engineering of new applications in water.

Intrinsic and Extrinsic Control of the pKa of Thiol Guests inside Yoctoliter Containers

Despite decades of research, there are still many open questions surrounding the mechanisms by which enzymes catalyze reactions. Understanding all the noncovalent forces involved has the potential to allow de novo catalysis design, and as a step toward this, understanding how to control the charge state of ionizable groups represents a powerful yet straightforward approach to probing complex systems. Here we utilize supramolecular capsules assembled via the hydrophobic effect to encapsulate guests and control their acidity. We find that the greatest influence on the acidity of bound guests is the location of the acidic group within the yoctoliter space. However, the nature of the electrostatic field generated by the (remote) charged solubilizing groups also plays a significant role in acidity, as does counterion complexation to the outer surfaces of the capsules. Taken together, these results suggest new ways by which to affect reactions in confined spaces.

Unravelling mechanistic insights in the platinum-catalysed dihydroalkoxylation of allenes

The mechanism of the platinum-catalysed dihydroalkoxylation of allenes to give acetals has been studied experimentally and by computational methods. Our findings further explain divergent reactivity encountered for platinum- and gold-vinyl intermediates after the first nucleophilic attack onto the coordinated allene, as well as provide new details on the catalytic cycle with platinum, uncovering enol ethers as resting states of the catalytic cycle, a SEOx process via Pt(IV)–H as the final protodemetallation step after the second nucleophilic attack when neutral platinum complexes are used, and a fast acid promoted addition of methanol to enol ethers when cationic platinum complexes are employed.

Gold-Catalyzed Cross-Coupling Reactions: An Overview of Design Strategies, Mechanistic Studies, and Applications

Transition-metal-catalyzed cross-coupling reactions are central to many organic synthesis methodologies. Traditionally, Pd, Ni, Cu, and Fe catalysts are used to promote these reactions. Recently, many studies have showed that both homogeneous and heterogeneous Au catalysts can be used for activating selective cross-coupling reactions. Here, an overview of the past studies, current trends, and future directions in the field of gold-catalyzed coupling reactions is presented. Design strategies to accomplish selective homocoupling and cross-coupling reactions under both homogeneous and heterogeneous conditions, computational and experimental mechanistic studies, and their applications in diverse fields are critically reviewed. Specific topics covered are: oxidant-assisted and oxidant-free reactions; strain-assisted reactions; dual Au and photoredox catalysis; bimetallic synergistic reactions; mechanisms of reductive elimination processes; enzyme-mimicking Au chemistry; cluster and surface reactions; and plasmonic catalysis. In the relevant sections, theoretical and computational studies of Au^I /Au^III chemistry are discussed and the predictions from the calculations are compared with the experimental observations to derive useful design strategies.

Cofactor-Mediated Nucleophilic Substitution Catalyzed by a Self-Assembled Holoenzyme Mimic

A self-assembled Fe₄L₆ cage is capable of co-encapsulating multiple carboxylic acid containing guests in its cavity, and these acids can act as cofactors for cage-catalyzed nucleophilic substitutions. The kinetics of the substitution reaction depend on the size, shape, and binding affinity of each of the components, and small structural changes in guest size can have large effects on the reaction. The host is quite promiscuous and is capable of binding multiple guests with micromolar binding affinities while retaining the ability to effect turnover and catalysis. Substrate binding modes vary widely, from simple 1:1 complexes to 1:2 complexes that can show either negative or positive cooperativity, depending on the guest. The molecularity of the dissociative substitution reaction varies, depending on the electrophile leaving group, acid cofactor, and nucleophile size: small changes in the nature of substrate can have large effects on reaction kinetics, all controlled by selective molecular recognition in the cage interior.

Microsolvation and Encapsulation Effects on Supramolecular Catalysis: C-C Reductive Elimination inside [Ga4L6]12- Metallocage

The host effect of the supramolecular [Ga₄L₆]^12- tetrahedral metallocage on reductive elimination of substrate by encapsulated Au(III) complexes is investigated by means of computational methods. The behavior of the reactants in solution and within the metallocage is initially evaluated by means of classical molecular dynamics simulations. These results guided the selection of proper computational models to describe the reaction in solution and inside the metallocage at the DFT level. The calculated Gibbs energy barriers are in very good agreement with experiment both in solution and inside the metallocage. The analysis in solution revealed that microsolvation around the Au(III) complex increases the Gibbs energy barrier. The analysis within the metallocage shows that its encapsulation favors the reaction. The process can be formally described as removing explicit microsolvation around the gold complex and encapsulating the metal complex inside the metallocage. Both processes are important for the reaction, but the removal of the solvent molecules surrounding the Au(III) metal complex is fundamental for the reduction of the reaction barrier. The energy decomposition analysis of the barrier among strain, interaction, and thermal terms shows that strain term is very low whereas the contribution of thermal (entropic) effects is moderate. Interestingly, the key term responsible for reducing the Gibbs energy barrier is the interaction. This term can be mainly associated with electrostatic interactions in agreement with previous examples in the literature.

Solvent-assisted coordination driven assembly of a supramolecular architecture featuring two types of connectivity from discrete nanocages

A 3D nanocage architecture with two types of connectivity was successfully assembled from discrete supramolecular nanocages.

The rapid development of supramolecular chemistry provides a powerful bottom-up approach to construct various well-defined nano-architectures with increasing complexity and functionality. Compared to that of small and simple nanometric objects, the self-assembly of larger and more complex nanometric objects, such as nanocages, remains a significant challenge. Herein, we used a discrete nanocage as the monomer to successfully construct a novel three-dimensional (3D) supramolecular architecture, which comprises two types of nanocage building units with different connectivity, using the solvent-assisted coordination-driven assembly approach. The mechanism of this supramolecular assembly process was investigated by electrospray ionization mass spectrometry (ESI-MS) studies, which identified for the first time the formation of a nanocage dimer intermediate during the assembly process. The assembly of discrete nanocages into a 3D supramolecular architecture led to remarkable enhancement of stability and gas adsorption properties.

Gold-catalyzed Cycloisomerization Reactions within Guanidinium M12L24 Nanospheres: the Effect of Local Concentrations

Gold-catalyzed cycloisomerization reactions have been explored using guanidinium functionalized M12L24 nanospheres that strongly encapsulate gold complexes functionalized with a sulfonate group through hydrogen bonds. As the M12L24 nanospheres can bind up to 24 gold complexes, the effect of local catalyst concentration on the reaction outcome can be easily evaluated. Also, the guanidinium groups of the sphere can weakly interact with the carboxylic group of the substrates, facilitating the pre-organization of the substrate near to the catalytic active site. Both effects can influence the selectivity and rate of the gold-catalyzed transformation. Challenging acetate-containing substrates with internal acetylene functional groups can be cyclized efficiently within the M12L24 nanospheres, where the pre-organization of the substrate plays a crucial role. For 2-alkynyl benzoic acids the selectivity of the reaction can be controlled by adjusting the local concentration of gold catalyst in the guanidinium functionalized M12L24 nanosphere.