Assembly of Encapsulated Transition Metal Catalysts

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.

Peptide cyclisation promoted by supramolecular complex formation

Phenol ester activated dipeptides that are reluctant to ring-close have been cyclised with the aid of sterically shielding metallo-porphyrins avoiding unwanted intermolecular reactions. The binding of ZnTPP to the dipyridine-functionalised activating phenolic ester was studied by NMR titrations and modelling. Staudinger-mediated cyclisations in the presence of ZnTPP increased the yield of the cyclic dipeptide from 16% to 40%.

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.

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.

Self-assembled M12L24 nanospheres as a reaction vessel to facilitate a dinuclear Cu(i) catalyzed cyclization reaction

The application of large M12L24 nanospheres allows the pre-concentration of catalysts to reach high local concentrations, facilitating reactions that proceed through dinuclear mechanisms. The mechanism of the copper(i)-catalyzed cyclization of 4-pentynoic acid has been elucidated by means of a detailed mechanistic study. The kinetics of the reaction show a higher order in copper, indicating the formation of a bis-Cu intermediate as the key rate determining step of the reaction. This intermediate was further identified during catalysis by CIS-HRMS analysis of the reaction mixture. Based on the mechanistic findings, an M12L24 nanosphere was applied that can bind up to 12 copper catalysts by hydrogen bonding. This pre-organization of copper catalysts in the nanosphere results in a high local concentration of copper leading to higher reaction rates and turnover numbers as the dinuclear pathway is favored.

Size-Selective Hydroformylation by a Rhodium Catalyst Confined in a Supramolecular Cage

Size-selective hydroformylation of terminal alkenes was attained upon embedding a rhodium bisphosphine complex in a supramolecular metal-organic cage that was formed by subcomponent self-assembly. The catalyst was bound in the cage by a ligand-template approach, in which pyridyl-zinc(II) porphyrin interactions led to high association constants (>105  m-1 ) for the binding of the ligands and the corresponding rhodium complex. DFT calculations confirm that the second coordination sphere forces the encapsulated active species to adopt the ee coordination geometry (i.e., both phosphine ligands in equatorial positions), in line with in situ high-pressure IR studies of the host-guest complex. The window aperture of the cage decreases slightly upon binding the catalyst. As a result, the diffusion of larger substrates into the cage is slower compared to that of smaller substrates. Consequently, the encapsulated rhodium catalyst displays substrate selectivity, converting smaller substrates faster to the corresponding aldehydes. This selectivity bears a resemblance to an effect observed in nature, where enzymes are able to discriminate between substrates based on shape and size by embedding the active site deep inside the hydrophobic pocket of a bulky protein structure.

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.

Supramolecular Approaches To Control Activity and Selectivity in Hydroformylation Catalysis

The hydroformylation reaction is one of the most intensively explored reactions in the field of homogeneous transition metal catalysis, and many industrial applications are known. However, this atom economical reaction has not been used to its full potential, as many selectivity issues have not been solved. Traditionally, the selectivity is controlled by the ligand that is coordinated to the active metal center. Recently, supramolecular strategies have been demonstrated to provide powerful complementary tools to control activity and selectivity in hydroformylation reactions. In this review, we will highlight these supramolecular strategies. We have organized this paper in sections in which we describe the use of supramolecular bidentate ligands, substrate preorganization by interactions between the substrate and functional groups of the ligands, and hydroformylation catalysis in molecular cages.

Tuning the Porphyrin Building Block in Self-Assembled Cages for Branched-Selective Hydroformylation of Propene

Unprecedented regioselectivity to the branched aldehyde product in the hydroformylation of propene was attained on embedding a rhodium complex in supramolecular assembly L2, formed by coordination-driven self-assembly of tris(meta-pyridyl)phosphine and zinc(II) porpholactone. The design of cage L2 is based on the ligand-template approach, in which the ligand acts as a template for cage formation. Previously, first-generation cage L1, in which zinc(II) porphyrin units were utilized instead of porpholactones, was reported. Binding studies demonstrate that the association constant for the formation of second-generation cage L2 is nearly an order of magnitude higher than that of L1. This strengthened binding allows cage L2 to remain intact in polar and industrially relevant solvents. As a consequence, the unprecedented regioselectivity for branched aldehyde products can be maintained in polar and coordinating solvents by using the second-generation assembly.

Selective Co-Encapsulation Inside an M6 L4 Cage

There is broad interest in molecular encapsulation as such systems can be utilized to stabilize guests, facilitate reactions inside a cavity, or give rise to energy-transfer processes in a confined space. Detailed understanding of encapsulation events is required to facilitate functional molecular encapsulation. In this contribution, it is demonstrated that Ir and Rh-Cp-type metal complexes can be encapsulated inside a self-assembled M6 L4 metallocage only in the presence of an aromatic compound as a second guest. The individual guests are not encapsulated, suggesting that only the pair of guests can fill the void of the cage. Hence, selective co-encapsulation is observed. This principle is demonstrated by co-encapsulation of a variety of combinations of metal complexes and aromatic guests, leading to several ternary complexes. These experiments demonstrate that the efficiency of formation of the ternary complexes depends on the individual components. Moreover, selective exchange of the components is possible, leading to formation of the most favorable complex. Besides the obvious size effect, a charge-transfer interaction may also contribute to this effect. Charge-transfer bands are clearly observed by UV/Vis spectrophotometry. A change in the oxidation potential of the encapsulated electron donor also leads to a shift in the charge-transfer energy bands. As expected, metal complexes with a higher oxidation potential give rise to a higher charge-transfer energy and a larger hypsochromic shift in the UV/Vis spectrum. These subtle energy differences may potentially be used to control the binding and reactivity of the complexes bound in a confined space.

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

Dinuclear gold complexes have the ability to interact with one or more substrates in a dual-activation mode, leading to different reactivity and selectivity than their mononuclear relatives. In this contribution, this difference was used to control the catalytic properties of a gold-based catalytic system by site-isolation of mononuclear gold complexes by selective encapsulation. The typical dual-activation mode is prohibited by this catalyst encapsulation, leading to typical behavior as a result of mononuclear activation. This strategy can be used as a switch (on/off) for a catalytic reaction and also permits reversible control over the product distribution during the course of a reaction.

Encapsulation of metalloporphyrins in a self-assembled cubic M8L6 cage: a new molecular flask for cobalt-porphyrin-catalysed radical-type reactions

The synthesis of a new, cubic M8L6 cage is described. This new assembly was characterised by using NMR spectroscopy, DOSY, TGA, MS, and molecular modelling techniques. Interestingly, the enlarged cavity size of this new supramolecular assembly allows the selective encapsulation of tetra(4-pyridyl)metalloporphyrins (M(II)(TPyP), M = Zn, Co). The obtained encapsulated cobalt-porphyrin embedded in the cubic zinc-porphyrin assembly is the first example of a catalytically active encapsulated transition-metal complex in a cubic M8L6 cage. The substrate accessibility of this system was demonstrated through radical-trapping experiments, and its catalytic activity was demonstrated in two different radical-type transformations. The reactivity of the encapsulated Co(II)(TPyP) complex is significantly increased compared to free Co(II)(TPyP) and other cobalt-porphyrin complexes. The reactions catalysed by this system are the first examples of cobalt-porphyrin-catalysed radical-type transformations involving diazo compounds which occur inside a supramolecular cage.

Coordination studies on supramolecular chiral ligands and application in asymmetric hydroformylation

In this study we introduce a series of monodentate pyridine-based ligands for which the phosphorus coordination mode to rhodium can be controlled by the binding of Zn(II)-templates to the pyridyl group. A series of monodentate phosphoroamidite and phosphite ligands have been prepared and studied under hydroformylation conditions by in situ high-pressure NMR and IR techniques. These studies reveal the exclusive formation of rhodium hydride complexes in which the phosphorus atom of the ligand resides in an axial position, trans to the hydride, but only after addition of Zn(II)-template. In the absence of these templates the usual mono-coordinated rhodium hydrido complexes are formed, with the phosphorus ligated in the equatorial plane, cis to the hydride. The catalytic performance of these complexes is evaluated in asymmetric hydroformylation of unfunctionalised internal alkenes in which the supramolecular change is reflected in higher activity and selectivity.

Diphosphine capsules for transition-metal encapsulation

Self-assembly and characterization of novel heterodimeric diphosphine capsules formed by multiple ionic interactions and composed of one tetracationic diphosphine ligand and one complementary tetraanionic calix[4]arene are described. Encapsulation of a palladium atom within a diphosphine capsule is achieved successfully by using the metal complex of the tetracationic diphosphine ligand for the assembly process. In this templated approach to metal encapsulation, the transition-metal complex is an integrated part of the capsule with the transition metal located inside the capsule and is not involved in the assembly process. We present two approaches for capsule assembly by mixing solutions of the precharged building blocks in methanol and mixing solutions of the neutral building blocks in methanol. The scope of the diphosphine capsules and the metallodiphosphine capsules is easily extended by applying tetracationic diphosphine ligands with different backbones (ethylene, diphenyl ether, and xanthene) and cationic binding motifs (p-C(6)H(4)-CH(2)-ammonium, m-C(6)H(4)-ammonium, and m-C(6)H(4)-guanidinium). These tetracationic building blocks with different flexibilities and shapes readily associate into capsules with the proper capsular structure, as is indicated by (1)H NMR spectroscopy, 1D NOESY, ESI-MS, and modeling studies.

Ligand-Template Directed Assembly: An Efficient Approach for the Supramolecular Encapsulation of Transition-Metal Catalysts

Supramolecular encapsulation of small guest molecules inside well-defined cavities of molecular capsules has witnessed broad attention because of the unusual behaviour of these systems. The molecular capsules generally consist of rigid complementary building blocks that are held together by multiple, complementary non-covalent interactions. Interestingly, it has been shown that chemical transformations can take place inside these capsules and in some examples the reaction is accelerated, while in other cases otherwise instable intermediates could be isolated in the capsulated form. Many reactions of interest require a transition-metal (TM) catalyst, and the creation of new capsules in which such catalysts are implemented within the structure is thus required for the development of resourceful type of catalyst systems for these processes. In this concept article we will discuss new strategies to arrive at such systems, with a focus on a ligand-templated approach. In this approach, multifunctional ligands are used as templates for the encapsulation process by supramolecular building blocks and concomitantly for the formation of TM complexes that are active in catalytic processes. The obtained encapsulated transition-metal catalysts show unusual reactivity and selectivity behaviour that will be discussed in detail.