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

Substrate scope driven optimization of an encapsulated hydroformylation catalyst

Caged complexes can provide impressive selective catalysts. Due to the complex shapes of such caged catalysts, however, the level of selectivity control of a single substrate cannot be extrapolated to other substrates. Herein, the substrate scope using 41 terminal alkene substrates is investigated in the hydroformylation reaction with an encapsulated rhodium catalyst [Rh(H)(CO)3(P(mPy3(ZnTPP)3))] (CAT1). For all substrates, the amount of branched products formed was higher with CAT1 than with the unencapsulated reference catalyst [Rh(H)(CO)2(P(mPy3))2] (CAT2) (linear/branched ratio between 2.14 and 0.12 for CAT1 and linear/branched ratio between 6.22 and 0.59 for CAT2). Interestingly, the level of cage induced selectivity depends strongly on the substrate structure that is converted. Analysis of the substrate scope combined with DFT calculations suggests that noncovalent interactions between the substrate moieties and cage walls play a key role in controlling the regioselectivity. Consequently, these supramolecular interactions were further optimized by replacing the ZnTPP building block with a zinc porphyrin analog that contained OiPr substituents on the meta position of the aryl rings. The resulting caged catalyst, CAT4, converted substrates with even higher branched selectivity.

Boosting the activity of Mizoroki-Heck cross-coupling reactions with a supramolecular palladium catalyst favouring remote Zn⋯pyridine interactions

Transition metal catalysis benefitting from supramolecular interactions in the secondary coordination sphere in order to pre-organize substrates around the active site and reach a specific selectivity typically occurs under long reaction times and mild reaction temperatures with the aim to maximize such subtle effects. Herein, we demonstrate that the kinetically labile Zn⋯N interaction between a pyridine substrate and a zinc-porphyrin site serving for substrate binding is a unique type of weak interaction that enables identification of supramolecular effects in transition metal catalysis after one hour at a high reaction temperature of 130 °C. Under carefully selected reaction conditions, supramolecularly-regulated palladium-catalyzed Mizoroki-Heck reactions between 3-bromopyridine and terminal olefins (acrylates or styrenes) proceeded in a more efficient manner compared to the non-supramolecular version. The supramolecular catalysis developed here also displayed interesting substrate-selectivity patterns.

Molecular Assemblies Offering Hydrogen-Bonding Cavities: Influence of Macrocyclic Cavity and Hydrogen Bonding on Dye Adsorption

This work presents a set of Hg macrocycles of amide-phosphine-based ligands offering H-bonding cavities of different dimensions. Such macrocycles are shown to selectively adsorb anionic dyes followed by neutral dyes as well as Prontosil, a biologically relevant antibiotic, within their cavities with the aid of H-bonding-assisted encapsulation. Kinetic experiments supported by spectroscopic and docking studies illustrate the importance of the cavity structure as well as H-bonds for the selective adsorption of dyes.

Mimicking Enzymes: Taking Advantage of the Substrate-Recognition Properties of Metalloporphyrins in Supramolecular Catalysis

The present review describes the most relevant advances dealing with supramolecular catalysis in which metalloporphyrins are employed as substrate-recognition sites in the second coordination sphere of the catalyst. The kinetically labile interaction between metallo­porphyrins (typically, those derived from zinc) and nitrogen- or oxygen-containing substrates is energetically comparable to the non-covalent interactions (i.e., hydrogen bonding) found in enzymes enabling substrate preorganization. Much inspired from host–guest phenomena, the catalytic systems described in this account display unique activities, selectivities and action modes that are difficult to reach by applying purely covalent strategies.

Surface-mediated ordering of pristine Salen molecules on coinage metals

The conformational isomers of Salen molecules and their self-assembled structures on coinage metal surfaces.

Architecture and synthesis of P,N-heterocyclic phosphine ligands

Diverse P,N-phosphine ligands reported to date have performed exceptionally well as auxiliary ligands in organometallic catalysis. Phosphines bearing 2-pyridyl moieties prominently feature in literature as compared to phosphines with five-membered N-heterocycles. This discussion seeks to paint a broad picture and consolidate different synthetic protocols and techniques for N-heterocyclic phosphine motifs. The introduction provides an account of P,N-phosphine ligands, and their structural and coordination benefits from combining heteroatoms with different basicity in one ligand. The body discusses the synthetic protocols which focus on P–C, P–N-bond formation, substrate and nucleophile types and different N-heterocycle construction strategies. Selected references are given in relation to the applications of the ligands.

Catalytic Hydrolysis of Phosphate Monoester by Supramolecular Complexes Formed by the Self-Assembly of a Hydrophobic Bis(Zn2+-cyclen) Complex, Copper, and Barbital Units That Are Functionalized with Amino Acids in a Two-Phase Solvent System

We previously reported on the preparation of supramolecular complexes by the 2:2:2 assembly of a dinuclear Zn²⁺-cyclen (cyclen = 1,4,7,10-tetraazacyclododecane) complex having a 2,2′-bipyridyl linker equipped with 0~2 long alkyl chains (Zn₂L¹~Zn₂L³), 5,5-diethylbarbituric acid (Bar) derivatives, and a copper(II) ion (Cu²⁺) in aqueous solution and two-phase solvent systems and their phosphatase activities for the hydrolysis of mono(4-nitrophenyl) phosphate (MNP). These supermolecules contain Cu₂(μ-OH)₂ core that mimics the active site of alkaline phosphatase (AP), and one of the ethyl groups of the barbital moiety is located in close proximity to the Cu₂(μ-OH)₂ core. The generally accepted knowledge that the amino acids around the metal center in the active site of AP play important roles in its hydrolytic activity inspired us to modify the side chain of Bar with various functional groups in an attempt to mimic the active site of AP in the artificial system, especially in two-phase solvent system. In this paper, we report on the design and synthesis of new supramolecular complexes that are prepared by the combined use of bis(Zn²⁺-cyclen) complexes (Zn₂L¹, Zn₂L², and Zn₂L³), Cu²⁺, and Bar derivatives containing amino acid residues. We present successful formation of these artificial AP mimics with respect to the kinetics of the MNP hydrolysis obeying Michaelis–Menten scheme in aqueous solution and a two-phase solvent system and to the mode of the product inhibition by inorganic phosphate.

Catalytic Hydrolysis of Phosphate Monoester by Supramolecular Phosphatases Formed from a Monoalkylated Dizinc(II) Complex, Cyclic Diimide Units, and Copper(II) in Two-Phase Solvent System

Design and synthesis of enzyme mimic with programmed molecular interaction among several building blocks including metal complexes and metal chelators is of intellectual and practical significance. The preparation of artificial enzymes that mimic the natural enzymes such as hydrolases, phosphatases, etc. remains a great challenge in the field of supramolecular chemistry. Herein we report on the design and synthesis of asymmetric (nonsymmetric) supermolecules by the 2:2:2 self-assembly of an amphiphilic zinc(II)-cyclen complex containing a 2,2'-bipyridyl linker and one long alkyl chain (Zn₂L³), barbital analogues, and Cu²⁺ as model compounds of an enzyme alkaline phosphatase that catalyzes the hydrolysis of phosphate monoesters such as mono(4-nitrophenyl)phosphate at neutral pH in two-phase solvent system (H₂O/CHCl₃) in pH 7.4 and 37 °C. Hydrolytic activity of these complexes was found to be catalytic, and their catalytic turnover numbers are 3-4. The mechanistic studies based on the UV/vis and emission spectra of the H₂O and CHCl₃ phases of the reaction mixtures suggest that the hydrophilicity/hydrophobicity balance of the supramolecular catalysts is an important factor for catalytic activity.

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.

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.

Multifunctional Nb–Cu nanostructured materials as potential adsorbents and oxidation catalysts for real wastewater decontamination

Hybrid oxidation catalysts and adsorbent materials containing niobium(v)–copper(ii) for the decontamination of real wastewater containing organic pollutants are reported herein.

Ligand Template Strategies for Catalyst Encapsulation

Binding of molecules in molecular cages based on self-assembled concave building blocks has been of great interest to scientists for decades. The binding of static molecular fragments inside cage-like molecular structures is generally based on complementarity of host and guest in terms of shape and interactions. The encapsulation of homogeneous catalysts in molecular cages is of interest as activity, selectivity, and stability can be controlled by the cage as second coordination sphere, reminiscent of how enzymes control chemical reactivity. Homogeneous catalysts, however, are not static guest molecules as catalysts change in shape, charge, and polarity during the catalytic cycle, representing the challenges involved in cage controlled catalysis. To address these issues, we developed a new strategy that we coined the "ligand template approach for catalyst encapsulation". This strategy relies on ligand building blocks that contain multiple orthogonal binding sites: the central ligand (mostly phosphorus) is bound to the transition metal required for catalysis, while other binding sites are used to construct a cage structure around the transition metal atom through self-assembly. By design, the catalyst is inside the capsule during the catalytic cycle, as the central ligand is coordinated to the catalyst. As the approach is based on a self-assembly process of building blocks, the catalyst properties can be easily modulated by modification of building blocks involved. In this Account, we elaborate on template ligand strategies for single catalyst encapsulation, based on divergent ligand templates and the extension to nanospheres with multiple metal complexes, which are formed by assembly of convergent ligand templates. Using the mononuclear approach, a variety of encapsulated catalysts can be generated, which have led to highly (enantio)selective hydroformylation reactions for encapsulated rhodium atoms. Besides the successes of encapsulated rhodium catalysts in hydroformylation, mononuclear ligand template capsules have been applied in asymmetric hydrogenation, the Heck reaction, copolymerization, gold catalyzed cyclization reactions, and hydrosilylation reactions. By changing the capsule building blocks the electronic and steric properties around the transition metal atom have successfully been modified, which translates to changes in catalyst properties. Using the convergent ligand templates, nanospheres have been generated with up to 24 complexes inside the sphere, leading to very high local concentrations of the transition metal. The effect of local concentrations was explored in gold catalyzed cyclization reactions and ruthenium catalyzed water oxidation, and for both reactions, spectacular reaction rate enhancements have been observed. This Account shows that the template ligand approach to provide catalyst in well-defined specific environments is very versatile and leads to catalyst properties that are not achievable with traditional approaches.

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.

Nanoreactors for green catalysis

Sustainable and environmentally benign production are key drivers for developments in the chemical industrial sector, as protecting our planet has become a significant element that should be considered for every industrial breakthrough or technological advancement. As a result, the concept of green chemistry has been recently defined to guide chemists towards minimizing any harmful outcome of chemical processes in either industry or research. Towards greener reactions, scientists have developed various approaches in order to decrease environmental risks while attaining chemical sustainability and elegancy. Utilizing catalytic nanoreactors for greener reactions, for facilitating multistep synthetic pathways in one-pot procedures, is imperative with far-reaching implications in the field. This review is focused on the applications of some of the most used nanoreactors in catalysis, namely: (polymer) vesicles, micelles, dendrimers and nanogels. The ability and efficiency of catalytic nanoreactors to carry out organic reactions in water, to perform cascade reaction and their ability to be recycled will be discussed.

Molecular Motion and Conformational Interconversion of Ir(I)·COD Included in Rebek's Self-Folding Octaamide Cavitand

We report experimental and theoretical evidence of restrained axial rotation for heteroleptic L2·Ir(I)·1,5-cyclooctadiene (COD) complexes included in the aromatic cavity of Rebek's self-folding octaamide cavitand. At 298 K, the axial spinning motion of the included organometallic guests was slow on the (1)H NMR time scale and produced a proton spectrum for the bound host indicative of C2 symmetry. Signals corresponding to aromatic protons of the bound host coalesced at 323 K, indicating that the spinning process of the included guest became fast on the (1)H NMR time scale and that the complex approached C4 symmetry. Surprisingly, lowering the temperature of the solution to 193 K induced an additional splitting of the proton signals observed at room temperature for both the bound host and the included guest. We propose the emergence of a new element of chirality in the complexes, which was associated with a slow interconversion, on the (1)H NMR time scale between the two chiral twisted-boat conformers of the chelated COD included in the already chiral cavity of the container. This leads to the inclusion complexes existing in solution as pairs of two racemic diastereomers. We estimated that the racemization barrier for the two cyclochiral conformers of the Ir(I) chelated COD was 5 kcal mol(-1) higher as an included organometallic complex than as free in solution. Furthermore, we performed a van't Hoff plot and determined that the inclusion of the organometallic complex in the cavitand was endothermic and exclusively driven by entropy (ΔH = 5.9 kcal mol(-1) and ΔS = 33.9 cal mol(-1) K(-1)).

Transition metal catalysis in confined spaces

Transition metal catalysis plays an important role in both industry and in academia where selectivity, activity and stability are crucial parameters to control. Next to changing the structure of the ligand, introducing a confined space as a second coordination sphere around a metal catalyst has recently been shown to be a viable method to induce new selectivity and activity in transition metal catalysis. In this review we focus on supramolecular strategies to encapsulate transition metal complexes with the aim of controlling the selectivity via the second coordination sphere. As we will discuss, catalyst confinement can result in selective processes that are impossible or difficult to achieve by traditional methods. We will describe the template-ligand approach as well as the host-guest approach to arrive at such supramolecular systems and discuss how the performance of the catalyst is enhanced by confining it in a molecular container.

Carbon dioxide fixation and sulfate sequestration by a supramolecular trigonal bipyramid

The subcomponent self-assembly of a bent dialdehyde ligand and different cationic and anionic templates led to the formation of two new metallosupramolecular architectures: a Fe(II) 4 L6 molecular rectangle was isolated following reaction of the ligand with iron(II) tetrafluoroborate, and a M5 L6 trigonal bipyramidal structure was constructed from either zinc(II) tetrafluoroborate or cadmium(II) trifluoromethanesulfonate. The spatially constrained arrangement of the three equatorial metal ions in the M5 L6 structures was found to induce small-molecule transformations. Atmospheric carbon dioxide was fixed as carbonate and bound to the equatorial metal centers in both the Zn5 L6 and Cd5 L6 assemblies, and sulfur dioxide was hydrated and bound as the sulfite dianion in the Zn5 L6 structure. Subsequent in situ oxidation of the sulfite dianion resulted in a sulfate dianion bound within the supramolecular pocket.

Enantioselective hydroformylation by a Rh-catalyst entrapped in a supramolecular metallocage

Regio- and enantioselective hydroformylation of styrenes is attained upon embedding a chiral Rh complex in a nonchiral supramolecular cage formed from coordination-driven self-assembly of macrocyclic dipalladium complexes and tetracarboxylate zinc porphyrins. The resulting supramolecular catalyst converts styrene derivatives into aldehyde products with much higher chiral induction in comparison to the nonencapsulated Rh catalyst. Spectroscopic analysis shows that encapsulation does not change the electronic properties of the catalyst nor its first coordination sphere. Instead, enhanced enantioselectivity is rationalized by the modification of the second coordination sphere occurring upon catalyst inclusion inside the cage, being one of the few examples in achieving an enantioselective outcome via indirect through-space control of the chirality around the catalyst center. This effect resembles those taking place in enzymatic sites, where structural constraints imposed by the enzyme cavity can impart stereoselectivities that cannot be attained in bulk. These results are a showcase for the future development of asymmetric catalysis by using size-tunable supramolecular capsules.

Metal-organic proximity in a synthetic pocket

Proximity between a noninteractive organic substrate and a transition metal (trans-MCl2(PEt3)2; M = Pd or Pt) is achieved by their co-encapsulation within a synthetic cage, as revealed by X-ray crystallographic analysis and NOESY experiments. Through co-encapsulation with a Pd(II) complex, a terminal alkyne was activated within the cage to give a σ-alkynylpalladium complex.

Structure versatility of coordination polymers constructed from a semirigid ligand and polynuclear metal clusters

Presented here are five coordination polymers based on semirigid polycarboxylate ligand and polynuclear metal clusters of particular interest is that the five compounds feature different topological networks.

Supramolecular control of selectivity in transition-metal catalysis through substrate preorganization

The Perspective highlights possibilities to use supramolecular interactions between a substrate molecule and a (bifunctional) catalyst as a powerful tool to control the selectivity in transition-metal catalysis.

A phosphoramidite-based [FeFe]H2ase functional mimic displaying fast electrocatalytic proton reduction

The work reports rapid electrocatalytic proton reduction by a diiron dithiolate complex bearing the 3-pyridylphosphoramidite ligand as a proton relay.