A Switchable Gold Catalyst by Encapsulation in a Self-Assembled Cage

Gold-Catalyzed Homo/Heterodimerization of Terminal Alkynes Facilitated by Metal-Ligand Cooperation

Gold-catalyzed dimerization of terminal alkynes is achieved under mild reaction conditions and in excellent yields. In addition to homodimerizations, heterodimerizations between TBS acetylene and a range of terminal alkynes were realized using the syringe pump technique. The reaction tolerates various functional groups. The rate acceleration experienced in the reactions is enabled by metal-ligand cooperation. A binaphthyl-2-ylphosphine ligand featuring a 3'-diisopropylphosphoryl group plays a pivotal role in facilitating alkyne attack by accommodating and/or deprotonating its terminal proton.

Interconversion between multicomponent slider-on-deck and palladium capsule: regulation of catalysis and encapsulation

When the slider-on-deck [Cu3(1)(2)]3+ and guest G were treated with palladium(II) ions, the biped 2 was released from [Cu3(1)(2)]3+ generating the nanocage [Pd2(2)4(G)]4+ with guest G being encapsulated (NetState-II). This transformation that was reversed by the addition of DMAP enabled modulation of both the overall fluorescence and the activity of copper(I) catalyzing an aza Hopf cyclization.

A catalytic system based on π-π stacking interactions between a pyrene substituted gold NHC catalyst and amphiphilic polymers for alkyne hydration reactions

Pyrene-substituted amphiphilic ATRP polymers (P1) were used to stabilize the pyrene-substituted gold(I) NHC complex (Au-1) within the hydrophobic compartment of micellar structures in a methanol/water mixture through non-covalent π-π stacking interactions. The stacking interactions between pyrene groups of the polymer and the catalyst were investigated by means of fluorescence spectroscopy by comparing excimer and monomer emission signals of the pyrene moiety. The supported catalyst (Au-1@P1) formed spherical micellar structures in a water/methanol mixture with an average size of 55.6 (±13.1) nm as confirmed from TEM analysis. The performance of the catalytic system was tested on alkyne hydration reactions of alkynes. When compared to the unsupported analog, the supported catalyst showed an improved performance as a result of stabilization through π-stacking interactions within the hydrophobic compartment in the micellar structure.

A photoresponsive gold catalyst based on azobenzene-functionalized NHC ligands

An azobenzene-bearing N-heterocyclic carbene-based gold catalyst is reported of which the reactivity in a cyclization reaction depends on the isomeric state of the azobenzene. The configurations of the catalyst can be reversibly switched by light and are stable during the reaction, effectively leading to a switchable catalyst system.

Hydration Reactions Catalyzed by Transition Metal-NHC (NHC = N-Heterocyclic Carbene) Complexes

The catalytic addition of water to unsaturated C-C or C-N π bonds represent one of the most important and environmentally sustainable methods to form C-O bonds for the production of synthetic intermediates, medicinal agents and natural products. The traditional acid-catalyzed hydration of unsaturated compounds typically requires strong acids or toxic mercury salts, which limits practical applications and presents safety and environmental concerns. Today, transition-metal-catalyzed hydration supported by NHC (NHC = N-heterocyclic carbene) ligands has attracted major attention. By rational design of ligands, choice of metals and counterions as well as mechanistic studies and the development of heterogeneous systems, major progress has been achieved for a broad range of hydration processes. In particular, the combination of NHC ligands with gold shows excellent reactivity compared with other catalytic systems; however, other systems based on silver, ruthenium, osmium, platinum, rhodium and nickel have also been discovered. Ancillary NHC ligands provide stabilization of transition metals and ensure high catalytic activity in hydration owing to their unique electronic and steric properties. NHC-Au(I) complexes are particularly favored for hydration of unsaturated hydrocarbons due to soft and carbophilic properties of gold. In this review, we present a comprehensive overview of hydration reactions catalyzed by transition metal-NHC complexes and their applications in catalytic hydration of different classes of π-substrates with a focus on the role of NHC ligands, types of metals and counterions.

Bioinspired Framework Catalysts: From Enzyme Immobilization to Biomimetic Catalysis

Enzymatic catalysis has fueled considerable interest from chemists due to its high efficiency and selectivity. However, the structural complexity and vulnerability hamper the application potentials of enzymes. Driven by the practical demand for chemical conversion, there is a long-sought quest for bioinspired catalysts reproducing and even surpassing the functions of natural enzymes. As nanoporous materials with high surface areas and crystallinity, metal-organic frameworks (MOFs) represent an exquisite case of how natural enzymes and their active sites are integrated into porous solids, affording bioinspired heterogeneous catalysts with superior stability and customizable structures. In this review, we comprehensively summarize the advances of bioinspired MOFs for catalysis, discuss the design principle of various MOF-based catalysts, such as MOF-enzyme composites and MOFs embedded with active sites, and explore the utility of these catalysts in different reactions. The advantages of MOFs as enzyme mimetics are also highlighted, including confinement, templating effects, and functionality, in comparison with homogeneous supramolecular catalysts. A perspective is provided to discuss potential solutions addressing current challenges in MOF catalysis.

Noncovalent tailoring of coordination complexes by resorcin[4]arene-based supramolecular hosts

Molecular recognition of guest molecules in a confined cavity is one of the important phenomena in biological and artificial molecular systems. When the guest is trapped within an artificial nano-space, its conformation is fixed in an unusual fashion by noncovalent interactions with host frameworks, and also the guest is kept away from the bulk solvent by the steric effect of the host. Therefore, host-guest formations lead to the effective modulation of the chemical and physical properties of guests via noncovalent interactions. In contrast to the many examples of organic guests, the examples of host-guest formation using coordination complex guests have been less explored. This is simply due to the size and shape complementarity problem between small hosts and large coordination complex guests. Resorcin[4]arene-based supramolecular hosts have been shown to provide internal cavities that are large enough to fully accommodate coordination complexes within the internal spaces via effective molecular interactions. In this article, we focus on supramolecular strategies to control the chemical and physical properties of the coordination complex guests within resorcin[4]arene-based supramolecular hosts. By the careful selection of the host and guest complexes, these combinations can produce a new supramolecular system, showing unusual structures, redox, catalytic, and photophysical properties derived from the entrapped coordination complexes.

Reactivity of non-organometallic ruthenium(II) polypyridyl complexes and their application as catalysts for hydride transfer reactions

Recently, we investigated the substitution behavior of a series of ruthenium(II) complexes of the general formula [RuII(terpy)(N∧N)Cl]Cl, where terpy = 2,2′:6′,2″-terpyridine, N∧N = bidentate ligand, in aqueous solutions. We have shown that the most and least reactive complexes of the series are [RuII(terpy)(en)Cl]Cl (en = ethylenediamine) and [RuII(terpy)(phen)Cl]Cl (phen = 1, 10-phenantroline), respectively, as a result of different electronic effects provided by the bidentate spectator chelates. Polypyridyl amine Ru(II) complex, viz. [Ru(terpy)(en)Cl]Cl and [Ru(terpy)(ampy)Cl]Cl (where ampy = 2-(aminomethyl)pyridine), in which the terpy chelate labilizes the metal center, are able to catalyze the conversion of NAD+ to 1,4-NADH using sodium formate as a source of hydride. We showed that this complex can control the [NAD+]/[NADH] ratio and potentially induce reductive stress in living cells, which is accepted as an effective method to kill cancer cells. Polypyridyl Ru(II) complexes, characterized in terms of the behavior in aqueous solutions, can be used as model systems to monitor heterogeneous multiphase ligand substitution reactions at the solid-liquid interface. Colloidal coordination compounds in the submicron range were synthesized from Ru(II)-aqua derivatives of starting chlorido complexes via the anti-solvent procedure and stabilized by a surfactant shell layer.

Just Add Water: Modulating the Structure-Derived Acidity of Catalytic Hexameric Resorcinarene Capsules

The hexameric undecyl-resorcin[4]arene capsule (C11R6) features eight discrete structural water molecules located at the vertices of its cubic suprastructure. Combining NMR spectroscopy with classical molecular dynamics (MD), we identified and characterized two distinct species of this capsule, C11R6-A and C11R6-B, respectively featuring 8 and 15 water molecules incorporated into their respective hydrogen-bonded networks. Furthermore, we found that the ratio of the C11R6-A and C11R6-B found in solution can be modulated by controlling the water content of the sample. The importance of this supramolecular modulation in C11R6 capsules is highlighted by its ability to perform acid-catalyzed transformations, which is an emergent property arising from the hydrogen bonding within the suprastructure. We show that the conversion of C11R6-A to C11R6-B enhances the catalytic rate of a model Diels-Alder cyclization by 10-fold, demonstrating the cofactor-derived control of a supramolecular catalytic process that emulates natural enzymatic systems.

Tuneable Control of Organocatalytic Activity through Host-Guest Chemistry

Dynamic regulation of chemical reactivity is important in many complex chemical reaction networks, such as cascade reactions and signal transduction processes. Signal responsive catalysts could play a crucial role in regulating these reaction pathways. Recently, supramolecular encapsulation was reported to regulate the activities of artificial catalysts. We present a host-guest chemistry strategy to modulate the activity of commercially available synthetic organocatalysts. The molecular container cucurbit[7]uril was successfully applied to change the activity of four different organocatalysts and one initiator, enabling up- or down-regulation of the reaction rates of four different classes of chemical reactions. In most cases CB[7] encapsulation results in catalyst inhibition, however in one case catalyst activation by binding to CB[7] was observed. The mechanism behind this unexpected behavior was explored by NMR binding studies and pKa measurements. The catalytic activity can be instantaneously switched during operation, by addition of either supramolecular host or competitive binding molecules, and the reaction rate can be predicted with a kinetic model. Overall, this signal responsive system proves a promising tool to control catalytic activity.

Towards Dual-Metal Catalyzed Hydroalkoxylation of Alkynes

Poly (vinyl ethers) are compounds with great value in the coating industry due to exhibiting properties such as high viscosity, soft adhesiveness, resistance to saponification and solubility in water and organic solvents. However, the main challenge in this field is the synthesis of vinyl ether monomers that can be synthetized by methodologies such as vinyl transfer, reduction of vinyl phosphate ether, isomerization, hydrogenation of acetylenic ethers, elimination, addition of alcohols to alkyne species etc. Nevertheless, the most successful strategy to access to vinyl ether derivatives is the addition of alcohols to alkynes catalyzed by transition metals such as molybdenum, tungsten, ruthenium, palladium, platinum, gold, silver, iridium and rhodium, where gold-NHC catalysts have shown the best results in vinyl ether synthesis. Recently, the hydrophenoxylation reaction was found to proceed through a digold-assisted process where the species that determine the rate of the reaction are PhO-[Au(IPr)] and alkyne-[Au(IPr)]. Later, the improvement of the hydrophenoxylation reaction by using a mixed combination of Cu-NHC and Au-NHC catalysts was also reported. DFT studies confirmed a cost-effective method for the hydrophenoxylation reaction and located the rate-determining step, which turned out to be quite sensitive to the sterical hindrance due to the NHC ligands.

Selective aldehyde reductions in neutral water catalysed by encapsulation in a supramolecular cage

The enhancement of reactivity inside supramolecular coordination cages has many analogies to the mode of action of enzymes, and continues to inspire the design of new catalysts for a range of reactions. However, despite being a near-ubiquitous class of reactions in organic chemistry, enhancement of the reduction of carbonyls to their corresponding alcohols remains very much underexplored in supramolecular coordination cages. Herein, we show that encapsulation of small aromatic aldehydes inside a supramolecular coordination cage allows the reduction of these aldehydes with the mild reducing agent sodium cyanoborohydride to proceed with high selectivity (ketones and esters are not reduced) and in good yields. In the absence of the cage, low pH conditions are essential for any appreciable conversion of the aldehydes to the alcohols. In contrast, the specific microenvironment inside the cage allows this reaction to proceed in bulk solution that is pH-neutral, or even basic. We propose that the cage acts to stabilise the protonated oxocarbenium ion reaction intermediates (enhancing aldehyde reactivity) whilst simultaneously favouring the encapsulation and reduction of smaller aldehydes (which fit more easily inside the cage). Such dual action (enhancement of reactivity and size-selectivity) is reminiscent of the mode of operation of natural enzymes and highlights the tremendous promise of cage architectures as selective catalysts.

MOF Encapsulation of Ru Olefin Metathesis Catalysts to Block Catalyst Decomposition

In the present work, a catalyst variation of the second-generation Hoveyda–Grubbs catalyst, particularly the ammonium-tagged Ru-alkylidene metathesis catalyst AquaMetTM, is under study, not simply to increase the efficiency in olefin metathesis but also the solubility in polar solvents. Moreover, this ionic catalyst was combined with the metal organic framework (MOF) (Cr)MIL-101-SO3−(Na·15-crown-5)+. We started from the experimental results by Grela et al., who increased the performance when the ruthenium catalyst was confined inside the cavities of the MOF, achieving non-covalent interactions between both moieties. Here, using density functional theory (DFT) calculations, the role of the ammonium N-heterocyclic carbene (NHC) tagged and the confinement effects are checked. The kinetics are used to compare reaction profiles, whereas SambVca steric maps and NCI plots are used to characterize the role of the MOF structurally and electronically.

Asymmetric Hydroformylation Using a Rhodium Catalyst Encapsulated in a Chiral Capsule

Supramolecular capsules can be used to change the activity and selectivity of a catalyst through the influence of the second coordination sphere, reminiscent of how enzymes control the selectivity of their processes. In enzymes, this approach is used to also control the enantioselectivity of reactions in which the active catalytic site is often not chiral but the second coordination sphere is. We are interested in the possibility to generate a chiral second coordination sphere around an otherwise achiral transition metal complex for asymmetric catalysis. In this paper we show that the ligand template approach can be used to generate a chiral second coordination sphere around a rhodium complex, which is used in asymmetric hydroformylation.

A resorcinarene-based tetrabenzoimidazolylidene complex of rhodium

A tetrabenzoimidazolium-resorcinarene cavitand was used for the preparation of a tetra-benzoimidazolylidene of rhodium, which is unprecedented in the field of poly-NHC metal complexes. Both the experimental and computational analyses of the molecule reveal a distorted vase conformation as the most stable one, although several non-interconverting conformational isomers due to the restricted rotation about the Rh-C(carbene) bond coexist in the product. There is a fluxional behaviour involving the vase-kite interconversion. The main interactions between the arms of the cavitand are mostly concentrated on the terminal organometallic fragments attached to NHC, along with those between -CH₃ and a N-heterocyclic carbene ring from benzoimidazoles.

Anion-mediated encapsulation-induced emission enhancement of an IrIII complex within a resorcin[4]arene hexameric capsule

The anions of the Ir complex salts control the thermodynamic stability and photoluminescence properties of the host–guest complex.

Effector responsive hydroformylation catalysis

Herein, we report a supramolecular rhodium complex that can form dimeric or monomeric Rh-species catalytically active in hydroformylation, depending on the binding of effectors within the integrated DIM-receptor. X-ray crystal structures, in situ (high-pressure (HP)) spectroscopy studies, and molecular modelling studies show that in the absence of effectors, the preferred Rh-species formed is the dimer, of which two ligands coordinate to two rhodium metals. Importantly, upon binding guest molecules, -effectors-, to the DIM-receptor under hydroformylation conditions, the monomeric Rh-active species is formed, as evidenced by a combination of in situ HP NMR and IR spectroscopy studies and molecular modelling. As the monomeric complex has different catalytic properties from the dimeric complex, we effectively generate a catalytic system of which the properties respond to the presence of effectors, reminiscent of how the properties of proteins are regulated in nature. Indeed, catalytic and kinetic experiments show that both the selectivity and activity of this supramolecular catalytic system can be influenced in the hydroformylation of 1-octene using acetate as an effector that shift the equilibrium from the dimeric to monomeric species.

Catalysis inside Supramolecular Capsules: Recent Developments

In the last decades, supramolecular chemists have developed new molecular receptors able to include a wide range of guests. In addition, they have designed synthetic hosts able to form capsules having an internal volume of thousands of Å3. This inner space shows different features from the bulk solution. In particular, this environment has recently been employed to perform chemical reactions, obtaining reaction products different from the “normal” conditions. These supramolecular capsules act as nanoreactors, catalyzing many chemical transformations. This review collects the recent developments (since 2015) in this field, focusing on supramolecular capsules based on resorcinarene hexameric capsules and metal-cage capsules.

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.

Cofactor Controlled Encapsulation of a Rhodium Hydroformylation Catalyst

Supramolecular approaches in transition-metal catalysis, including catalyst encapsulation, have attracted considerable attention. Compared to enzymes, supramolecular catalysts in general are less complex. Enzyme activity is often controlled by the use of smaller cofactor molecules, which is important in order to control reactivity in complex mixtures of molecules. Interested in increasing complexity and allowing control over supramolecular catalyst formation in response to external stimuli, we designed a catalytic system that only forms an efficient supramolecular complex when a small cofactor molecule is added to the solution. This in turn affects both the activity and selectivity when applied in a hydroformylation reaction. This contribution shows that catalyst encapsulation can be controlled by the addition of a cofactor, which affects crucial catalyst properties.

Supramolecular Host-Selective Activation of Iodoarenes by Encapsulated Organometallics

Supramolecular hosts offer defined microenvironments that facilitate selective host-guest interactions, enabling reactivity that would otherwise be challenging in bulk solution. While impressive rate enhancements and selectivities have been reported, similar reactivity can often be accessed through modifications of reaction conditions even in the absence of the host. We report here an oxidative addition of aryl halides across the metal centers in Cu(I) and Pd(II) organometallics that is assisted by the presence of a supramolecular host, realized via electrostatic stabilization and increased local substrate concentrations. When reaction conditions were screened to assess background reactivity, alternative reactivity (typically decomposition) resulted, indicating that encapsulation led to host-selective reaction trajectories.

Activation of Primary and Secondary Benzylic and Tertiary Alkyl (sp 3 )C-F Bonds Inside a Self-Assembled Molecular Container

Alkyl fluorides are generally regarded as chemically inert. However, several literature examples describe the activation of alkyl (sp3)C-F bonds via strong Brønsted or Lewis acids under harsh conditions. We here report that catalytic amounts of the self-assembled resorcinarene capsule are able to activate alkyl (sp3)C-F bonds under mild conditions (40°C, no strong Brønsted or Lewis acid present). Kinetic measurements display a sigmoidal reaction progress after an initial induction period. Control experiments indicate that the presence of the supramolecular capsule is required for an efficient reaction acceleration.

Gold Catalysis in (Supra)Molecular Cages to Control Reactivity and Selectivity

Gold catalysis has experienced a tremendous development over the past decades, and is nowadays widely used in organic synthesis to perform chemical transformations of π-bond-containing molecules. Catalyst development has been based mostly on ligand development and counter-ion strategies. More recently, the encapsulation of gold catalysts in (supra)molecular cages was explored as a new way to control selectivity and reactivity of gold catalysts. In this review, we describe the cages that have been employed as hosts for gold complexes, along with their impact on the catalytic performance. Covalent and supramolecular approaches to encapsulate single metal complexes will be described and the impact on the catalytic performance will be discussed. Also, recent strategies to pre-organize multiple metal centers will be discussed.

Reversible multi-electron storage in dual-site redox-active supramolecular cages

Redox-active M₆L₄ cages display multiple reversible redox-events, enabling switching from overall +12 to −4 charged species with reversible storage of 16 electrons.

4-Halo-1,2,3-triazolylidenes: stable carbenes featuring halogen bonding

Synthesis and coordination of 4-halo-1,2,3-triazolylidenes have been developed. These novel ligands featured the character with σ-donation at carbon and a σ-hole at the halogen. Halogen bonding was observed by single crystal X-ray diffraction in their coinage metal complexes. The electronic properties of 4-iodo-1,2,3-triazolylidene were studied by both Ir-CO frequencies of the Tolman electronic parameter (TEP) and Huynh's electronic parameter (HEP) method, which suggested similar electronic properties to those of imidazolylidenes. During HEP tests, an interesting tunability was observed when different electron donors were employed.

Confinement Effects in Catalysis Using Well-Defined Materials and Cages

This review focuses on the effects that confinement of molecular and heterogeneous catalysts with well-defined structure has on the selectivity and activity of these systems. A general introduction about catalysis and how the working principles of enzymes can be used as a source of inspiration for the preparation of catalysts with enhanced performance is provided. Subsequently, relevant studies demonstrate the importance of second coordination sphere effects in a broad sense (in homogeneous and heterogeneous catalysis). Firstly, we discuss examples involving zeolites, MOFs and COFs as heterogeneous catalysts with well-defined structures where confinement influences catalytic performance. Then, specific cases of homogeneous catalysts where non-covalent interactions determine the selectivity and activity are treated in detail. This includes examples based on cyclodextrins, calix[n]arenes, cucurbit[n]urils, and self-assembled container molecules. Throughout the review, the impact of confined spaces is emphasized and put into context, in order to get a better understanding of the effects of confinement on catalyst performance. In addition, this analysis intends to showcase the similarities between homogeneous and heterogeneous catalysts, which may aid the development of novel strategies.

Synthesis and Characterization of Self-Assembled Chiral FeII 2 L3 Cages

We present here the synthesis of chiral BINOL-derived (BINOL=1,1'-bi-2-naphthol) bisamine and bispyridine-aldehyde building blocks that can be used for the self-assembly of novel chiral FeII 2 L3 cages when mixed with an iron(II) precursor. The properties of a series of chiral cages were studied by NMR and circular dichroism (CD) spectroscopy, cold-spray ionization MS, and molecular modeling. Upon formation of the M2 L3 cages, the iron corners can adopt various isomeric forms: mer, fac-Δ, or fac-Λ. We found that the coordination geometry around the metal centers in R-Cages 1 and 2 were influenced by the chiral BINOL backbone only to a limited extent, as a mixture of cages was formed with fac and mer configurations at the iron corners. However, single cage species (fac-RR-Cage and fac-RS-Cage) that are enantiopure and highly symmetric were obtained by generating these chiral M2 L3 cages by using the bispyridine-aldehyde building blocks in combination with chiral amine moieties to form pyridylimine ligands for coordination to iron. Next to consistent NMR spectra, the CD spectra confirm the configurations fac-(Λ,Λ) and fac-(Δ,Δ) corresponding to RR- and RS-Cage, respectively.

Coordination-driven self-assembly of discrete Ru6-Pt6 prismatic cages

The coordination-driven self-assembly of two new Ru6-Pt6 hexanuclear trigonal prismatic cages comprising arene-ruthenium(II) clips (1a(NO3)2 and 1b(NO3)2 ) and a tritopic platinum(II) metalloligand 2 has been performed in methanol at room temperature. The [3 + 2] hexanuclear cages 3a and 3b were isolated in good yields and characterized by well-known spectroscopic techniques including multinuclear NMR, mass spectrometry, UV-vis and infrared studies. Geometry optimization revealed the shapes and sizes of these hexanuclear prismatic cages. The combination of ruthenium and platinum metal center in a one-pot self-assembly reaction showcases the construction of aesthetically elegant heterometallic structures in supramolecular chemistry leading to the formation of a single major product.