Silver-Exchanged Zeolite Y Catalyzes a Selective Insertion of Carbenes into C-H and O-H Bonds.

Commercially available zeolite Y modulates the catalytic activity and selectivity of ultrasmall silver species during the Buchner reaction and the carbene addition to methylene and hydroxyl bonds, by simply exchanging the counter cations of the zeolite framework. The zeolite acts as a macroligand to tune the silver catalytic site, enabling the use of this cheap and recyclable solid catalyst for the in situ formation of carbenes from diazoacetate and selective insertion in different C-H (i.e., cyclohexane) and C-O (i.e., water) bonds. The amount of catalyst in the reaction can be as low as ≤0.1 mol % silver. Besides, this reactivity allows deeply drying the HY zeolite framework by making the strongly adsorbed water molecules react with the in situ formed carbenes.

MOF-Triggered Synthesis of Subnanometer Ag02 Clusters and Fe3+ Single Atoms: Heterogenization Led to Efficient and Synergetic One-Pot Catalytic Reactions.

The combination of well-defined Fe3+ isolated single-metal atoms and Ag2 subnanometer metal clusters within the channels of a metal-organic framework (MOF) is reported and characterized by single-crystal X-ray diffraction for the first time. The resulting hybrid material, with the formula [Ag02(Ag0)1.34FeIII0.66]@NaI2{NiII4[CuII2(Me3mpba)2]3}·63H2O (Fe3+Ag02@MOF), is capable of catalyzing the unprecedented direct conversion of styrene to phenylacetylene in one pot. In particular, Fe3+Ag02@MOF─which can easily be obtained in a gram scale─exhibits superior catalytic activity for the TEMPO-free oxidative cross-coupling of styrenes with phenyl sulfone to give vinyl sulfones in yields up to >99%, which are ultimately transformed, in situ, to the corresponding phenylacetylene product. The results presented here constitute a paradigmatic example of how the synthesis of different metal species in well-defined solid catalysts, combined with speciation of the true metal catalyst of an organic reaction in solution, allows the design of a new challenging reaction.

Evidence for Ruthenium(II) Peralkene Complexes as Catalytic Species during the Isomerization of Terminal Alkenes in Solution.

The isomerization (chain-walking) reaction of terminal to internal alkenes is catalyzed by part-per-million amounts of practically any Ru source when the reaction is carried out with a neat terminal alkene. Here, we provide evidence that the soluble starting Ru sources evolve to catalytically active peralkene Ru(II) species under reaction conditions. These species may also explain the isomerization products found during other Ru-catalyzed alkene processes, i.e., alkene metathesis reactions. A Finke-Watzky mechanism for catalyst formation is consistent with the evidence obtained.

Diastereoisomeric enrichment of 1,4-enediols and H2-splitting inhibition on Pd-supported catalysts.

Pd-supported catalysts are fundamental tools in organic reactions involving H2 splitting. Here we show that 1,4-enediols enriched in one diastereoisomer are produced from the classical Pd-catalyzed semi-hydrogenation reaction with H2, starting from the corresponding, widely available 1,4-diacetylenic diols. The semi-hydrogenation reaction proceeds concomitantly with the desymmetrization of the meso/racemic form of the enediol. We also show that these products, if added in advance to H2, completely inactivate the Pd catalyst (only when added before H2). These results provide a simple way not only to produce 1,4-enediols enriched in one diastereoisomer by a classical catalytic method but also to stop H2 dissociation on Pd nanoparticles.

Highly Efficient MOF-Driven Silver Subnanometer Clusters for the Catalytic Buchner Ring Expansion Reaction.

The preparation of novel efficient catalysts─that could be applicable in industrially important chemical processes─has attracted great interest. Small subnanometer metal clusters can exhibit outstanding catalytic capabilities, and thus, research efforts have been devoted, recently, to synthesize novel catalysts bearing such active sites. Here, we report the gram-scale preparation of Ag20 subnanometer clusters within the channels of a highly crystalline three-dimensional anionic metal-organic framework, with the formula [Ag20]@AgI2NaI2{NiII4[CuII2(Me3mpba)2]3}·48H2O [Me3mpba4- = N,N'-2,4,6-trimethyl-1,3-phenylenebis(oxamate)]. The resulting crystalline solid catalyst─fully characterized with the help of single-crystal X-ray diffraction─exhibits high catalytic activity for the catalytic Buchner ring expansion reaction.

Soluble/MOF-Supported Palladium Single Atoms Catalyze the Ligand-, Additive-, and Solvent-Free Aerobic Oxidation of Benzyl Alcohols to Benzoic Acids.

Metal single-atom catalysts (SACs) promise great rewards in terms of metal atom efficiency. However, the requirement of particular conditions and supports for their synthesis, together with the need of solvents and additives for catalytic implementation, often precludes their use under industrially viable conditions. Here, we show that palladium single atoms are spontaneously formed after dissolving tiny amounts of palladium salts in neat benzyl alcohols, to catalyze their direct aerobic oxidation to benzoic acids without ligands, additives, or solvents. With this result in hand, the gram-scale preparation and stabilization of Pd SACs within the functional channels of a novel methyl-cysteine-based metal-organic framework (MOF) was accomplished, to give a robust and crystalline solid catalyst fully characterized with the help of single-crystal X-ray diffraction (SCXRD). These results illustrate the advantages of metal speciation in ligand-free homogeneous organic reactions and the translation into solid catalysts for potential industrial implementation.

Metal-Organic Frameworks as Chemical Nanoreactors: Synthesis and Stabilization of Catalytically Active Metal Species in Confined Spaces.

Since the advent of the first metal-organic frameworks (MOFs), we have witnessed an explosion of captivating architectures with exciting physicochemical properties and applications in a wide range of fields. This, in part, can be understood under the light of their rich host-guest chemistry and the possibility to use single-crystal X-ray diffraction (SC-XRD) as a basic characterization tool. Moreover, chemistry on preformed MOFs, applying recent developments in template-directed synthesis and postsynthetic methodologies (PSMs), has shown to be a powerful synthetic tool to (i) tailor MOFs channels of known topology via single-crystal to single-crystal (SC-SC) processes, (ii) impart higher degrees of complexity and heterogeneity within them, and most importantly, (iii) improve their capabilities toward applications with respect to the parent MOFs. However, the unique properties of MOFs have been, somehow, limited and underestimated. This is clearly reflected on the use of MOFs as chemical nanoreactors, which has been barely uncovered. In this Account, we bring together our recent advances on the construction of MOFs with appealing properties to act as chemical nanoreactors and be used to synthesize and stabilize, within their channels, catalytically active species that otherwise could be hardly accessible. First, through two relevant examples, we present the potential of the metalloligand approach to build highly robust and crystalline oxamato- and oxamidato-MOFs with tailored channels, in terms of size, charge and functionality. These are initial requisites to have a playground where we can develop and fully take advantage of singular properties of MOFs as well as visualize/understand the processes that take place within MOFs pores and somehow make structure-functionalities correlations and develop more performant MOFs nanoreactors. Then, we describe how to exploit the unique and singular features that offer each of these MOFs confined space for (i) the incorporation and stabilization of metals salts and complexes, (ii) the in situ stepwise synthesis of subnanometric metal clusters (SNMCs), and (iii) the confined-space self-assembly of supramolecular coordination complexes (SCCs), by means of PSMs and underpinned by SC-XRD. The strategy outlined here has led to unique rewards such as the highly challenging gram-scale preparation of stable and well-defined ligand-free SNMCs, exhibiting outstanding catalytic activities, and the preparation of unique SCCs, different to those assembled in solution, with enhanced stabilities, catalytic activities, recyclabilities, and selectivities. The results presented in this Accounts are just a few recent examples, but highly encouraging, of the large potential way of MOFs acting as chemical nanoreactors. More work is needed to found the boundaries and fully understand the chemistry in the confined space. In this sense, mastering the synthetic chemistry of discrete organic molecules and inorganic complexes has basically changed our way of live. Thus, achieving the same degree of control on extended hybrid networks will open new frontiers of knowledge with unforeseen possibilities. We aim to stimulate the interest of researchers working in broadly different fields to fully unleash the host-guest chemistry in MOFs as chemical nanoreactors with exclusive functional species.

A Biocompatible Aspartic-Decorated Metal-Organic Framework with Tubular Motif Degradable under Physiological Conditions.

Achieving a precise control of the final structure of metal-organic frameworks (MOFs) is necessary to obtain desired physical properties. Here, we describe how the use of a metalloligand design strategy and a judicious choice of ligands inspired from nature is a versatile approach to succeed in this challenging task. We report a new porous chiral MOF, with the formula Ca5II{CuII10[(S,S)-aspartamox]5}·160H2O (1), constructed from Cu2+ and Ca2+ ions and aspartic acid-decorated ligands, where biometal Cu2+ ions are bridged by the carboxylate groups of aspartic acid moieties. The structure of MOF 1 reveals an infinite network of basket-like cages, built by 10 crystallographically distinct Cu(II) metal ions and five aspartamox ligands acting as bricks of a tubular motif, composed of seven basket-like cages each. The pillared hepta-packed cages generate pseudo-rhombohedral nanosized channels of ca. 0.7 and 0.4 nm along the b and a crystallographic axes. This intricate porous 3D network is anionic and chiral, each cage displaying receptor properties toward three-nuclear [Ca3(µ-H2O)4(H2O)17]6+ entities. 1 represents the first example of an extended porous structure based on essential biometals Cu2+ and Ca2+ ions together with aspartic acid as amino acid. 1 shows good biocompatibility, making it a good candidate to be used as a drug carrier, and hydrolyzes in acid water. The hypothesis has been further supported by an adsorption experiment here reported, as a proof-of-principle study, using dopamine hydrochloride as a model drug to follow the encapsulation process. Results validate the potential ability of 1 to act as a drug carrier. Thus, these make this MOF one of the few examples of biocompatible and degradable porous solid carriers for eventual release of drugs in the stomach stimulated by gastric low pH.

Bio-metal-organic frameworks for molecular recognition and sorbent extraction of hydrophilic vitamins followed by their determination using HPLC-UV.

A bio-metal-organic framework (bio-MOF) derived from the amino acid L-serine has been prepared in bulk form and evaluated as sorbent for the molecular recognition and extraction of B-vitamins. The functional pores of bio-MOF exhibit high amounts of hydroxyl groups jointly directing other supramolecular host-guest interactions thus providing the recognition of B-vitamins in fruit juices and energy drinks. Single-crystal X-ray diffraction studies reveal the specific B-vitamin binding sites and the existence of multiple hydrogen bonds between these target molecules and the framework. It offered unique snapshots to accomplish an efficient capture of these solutes in complex aqueous matrices. Four B-vitamins (thiamin, nicotinic acid, nicotinamide, and pyridoxine) were investigated. They were eluted from the sorbent with phosphate buffer at pH 7 and analyzed by HPLC with UV detection. The sorbent was compared with commercial C18 cartridges. Following the procedure, acceptable reproducibility (RSD values < 14%) was achieved, and the detection limits were in the range 0.4 to 1.4 ng mL-1. The method was applied to the analysis of energy drink and juice samples and the recoveries were between 75 and 123% in spiked beverage samples. Graphical abstractA bio-MOF as SPE sorbent was prepared and applied to the extraction of B-vitamins in fruit juices and energy drinks.

Multivariate Metal-Organic Frameworks for the Simultaneous Capture of Organic and Inorganic Contaminants from Water.

We report a new water-stable multivariate (MTV) metal-organic framework (MOF) prepared by combining two different oxamide-based metalloligands derived from the natural amino acids l-serine and l-methionine. This unique material features hexagonal channels decorated with two types of flexible and functional "arms" (-CH2OH and -CH2CH2SCH3) capable of enabling, synergistically, the simultaneous and efficient removal of both inorganic (heavy metals such as Hg2+, Pb2+, and Tl+) and organic (dyes such as Pyronin Y, Auramine O, Brilliant green, and Methylene blue) contaminants, and, in addition, this MTV-MOF is completely reusable. Single-crystal X-ray diffraction measurements allowed solving the crystal structure of a host-guest adsorbate, containing both HgCl2 and Methylene blue, and offered unprecedented snapshots of this unique dual capture process. This is the very first time that a MOF can be used for the removal of all sorts of pollutants from water resources, thus opening new perspectives for this emerging type of MTV-MOF.

Self-Assembly of Catalytically Active Supramolecular Coordination Compounds within Metal-Organic Frameworks.

Supramolecular coordination compounds (SCCs) represent the power of coordination chemistry methodologies to self-assemble discrete architectures with targeted properties. SCCs are generally synthesized in solution, with isolated fully coordinated metal atoms as structural nodes, thus severely limited as metal-based catalysts. Metal-organic frameworks (MOFs) show unique features to act as chemical nanoreactors for the in situ synthesis and stabilization of otherwise not accessible functional species. Here, we present the self-assembly of PdII SCCs within the confined space of a pre-formed MOF (SCCs@MOF) and its post-assembly metalation to give a PdII-AuIII supramolecular assembly, crystallography underpinned. These SCCs@MOFs catalyze the coupling of boronic acids and/or alkynes, representative multi-site metal-catalyzed reactions in which traditional SCCs tend to decompose, and retain their structural integrity as a consequence of the synergetic hybridization between SCCs and MOFs. These results open new avenues in both the synthesis of novel SCCs and their use in heterogeneous metal-based supramolecular catalysis.

Metal-Organic Frameworks as Playgrounds for Reticulate Single-Molecule Magnets.

Achieving fine control on the structure of metal-organic frameworks (MOFs) is mandatory to obtain target physical properties. Herein, we present how the combination of a metalloligand approach and a postsynthetic method is a suitable and highly useful synthetic strategy to success on this extremely difficult task. First, a novel oxamato-based tetranuclear cobalt(III) compound with a tetrahedron-shaped geometry is used, for the first time, as the metalloligand toward calcium(II) metal ions to lead to a diamagnetic CaII-CoIII three-dimensional (3D) MOF (1). In a second stage, in a single-crystal-to-single-crystal manner, the calcium(II) ions are replaced by terbium(III), dysprosium(III), holmium(III), and erbium(III) ions to yield four isostructural novel LnIII-CoIII [Ln = Tb (2), Dy (3), Ho (4), and Er (5)] 3D MOFs. Direct-current magnetic properties for 2-5 show typical performances for the ground-state terms of the lanthanoid cations [7F6 (TbIII), 6H15/2 (DyIII), 5I8 (HoIII), and 4I15/2 (ErIII)]. Analysis of the χMT data indicates that the ground state is the lowest MJ value, that is, MJ = 0 (2 and 4) and ±1/2 (3 and 5). Kramers' ions (3 and 5) exhibit field-induced emergent frequency-dependent alternating-current magnetic susceptibility signals, which is indicative of the presence of slow magnetic relaxation typical of single-molecule magnets.

Isolated Fe(III)-O Sites Catalyze the Hydrogenation of Acetylene in Ethylene Flows under Front-End Industrial Conditions.

The search for simple, earth-abundant, cheap, and nontoxic metal catalysts able to perform industrial hydrogenations is a topic of interest, transversal to many catalytic processes. Here, we show that isolated FeIII-O sites on solids are able to dissociate and chemoselectively transfer H2 to acetylene in an industrial process. For that, a novel, robust, and highly crystalline metal-organic framework (MOF), embedding FeIII-OH2 single sites within its pores, was prepared in multigram scale and used as an efficient catalyst for the hydrogenation of 1% acetylene in ethylene streams under front-end conditions. Cutting-edge X-ray crystallography allowed the resolution of the crystal structure and snapshotted the single-atom nature of the catalytic FeIII-O site. Translation of the active site concept to even more robust and inexpensive titania and zirconia supports enabled the industrially relevant hydrogenation of acetylene with similar activity to the Pd-catalyzed process.

The MOF-driven synthesis of supported palladium clusters with catalytic activity for carbene-mediated chemistry.

The development of catalysts able to assist industrially important chemical processes is a topic of high importance. In view of the catalytic capabilities of small metal clusters, research efforts are being focused on the synthesis of novel catalysts bearing such active sites. Here we report a heterogeneous catalyst consisting of Pd4 clusters with mixed-valence 0/+1 oxidation states, stabilized and homogeneously organized within the walls of a metal-organic framework (MOF). The resulting solid catalyst outperforms state-of-the-art metal catalysts in carbene-mediated reactions of diazoacetates, with high yields (>90%) and turnover numbers (up to 100,000). In addition, the MOF-supported Pd4 clusters retain their catalytic activity in repeated batch and flow reactions (>20 cycles). Our findings demonstrate how this synthetic approach may now instruct the future design of heterogeneous catalysts with advantageous reaction capabilities for other important processes.

Efficient Capture of Organic Dyes and Crystallographic Snapshots by a Highly Crystalline Amino-Acid-Derived Metal-Organic Framework.

The presence of residual organic dyes in water resources or wastewater treatment systems, derived mainly from effluents of different industries, is a major environmental problem with no easy solution. Herein, an ecofriendly, water-stable metal-organic framework was prepared from a derivative of the natural amino acid l-serine. Its functional channels are densely decorated with highly flexible l-serine residues bearing hydroxyl groups. The presence of such a flexible and functional environment within the confined environment of the MOF leads to efficient removal of different organic dyes from water: Pyronin Y, Auramine O, Methylene Blue and Brilliant Green, as unveiled by unprecedented snapshots offered by single-crystal X-ray diffraction. This MOF enables highly efficient water remediation by capturing more than 90 % of dye content, even at very low concentrations such as 10 ppm, which is similar to those usually found in industrial wastewaters. Remarkably, the removal efficiency is improved in simulated contaminated mineral water with multiple dyes.

Lanthanide Discrimination with Hydroxyl-Decorated Flexible Metal-Organic Frameworks.

We report two new highly crystalline metal-organic frameworks (MOFs), derived from the natural amino acids serine (1) and threonine (2), featuring hexagonal channels densely decorated with hydroxyl groups belonging to the amino acid residues. Both 1 and 2 are capable of discriminating, via solid-phase extraction, a mixture of selected chloride salts of lanthanides on the basis of their size, chemical affinity, and/or the flexibility of the network. In addition, this discrimination follows a completely different trend for 1 and 2 because of the different locations of the hydroxyl groups in each compound, which is evocative of steric complementarity between the substrate and receptor. Last but not least, the crystal structures of selected adsorbates could be resolved, offering unprecedented snapshots on the capture process and enabling structural correlations with the separation mechanism.

Confined Pt1 1+ Water Clusters in a MOF Catalyze the Low-Temperature Water-Gas Shift Reaction with both CO2 Oxygen Atoms Coming from Water.

The synthesis and reactivity of single metal atoms in a low-valence state bound to just water, rather than to organic ligands or surfaces, is a major experimental challenge. Herein, we show a gram-scale wet synthesis of Pt1 1+ stabilized in a confined space by a crystallographically well-defined first water sphere, and with a second coordination sphere linked to a metal-organic framework (MOF) through electrostatic and H-bonding interactions. The role of the water cluster is not only isolating and stabilizing the Pt atoms, but also regulating the charge of the metal and the adsorption of reactants. This is shown for the low-temperature water-gas shift reaction (WGSR: CO + H2 O → CO2 + H2 ), where both metal coordinated and H-bonded water molecules trigger a double water attack mechanism to CO and give CO2 with both oxygen atoms coming from water. The stabilized Pt1+ single sites allow performing the WGSR at temperatures as low as 50 °C.

Synthesis of Densely Packaged, Ultrasmall Pt02 Clusters within a Thioether-Functionalized MOF: Catalytic Activity in Industrial Reactions at Low Temperature.

The gram-scale synthesis, stabilization, and characterization of well-defined ultrasmall subnanometric catalytic clusters on solids is a challenge. The chemical synthesis and X-ray snapshots of Pt02 clusters, homogenously distributed and densely packaged within the channels of a metal-organic framework, is presented. This hybrid material catalyzes efficiently, and even more importantly from an economic and environmental viewpoint, at low temperature (25 to 140 °C), energetically costly industrial reactions in the gas phase such as HCN production, CO2 methanation, and alkene hydrogenations. These results open the way for the design of precisely defined catalytically active ultrasmall metal clusters in solids for technically easier, cheaper, and dramatically less-dangerous industrial reactions.

Postsynthetic Approach for the Rational Design of Chiral Ferroelectric Metal-Organic Frameworks.

Ferroelectrics (FEs) are materials of paramount importance with a wide diversity of applications. Herein, we propose a postsynthetic methodology for the smart implementation of ferroelectricity in chiral metal-organic frameworks (MOFs): following a single-crystal to single-crystal cation metathesis, the Ca2+ counterions of a preformed chiral MOF of formula Ca6II{CuII24[(S,S)-hismox]12(OH2)3}·212H2O (1), where hismox is a chiral ligand derived from the natural amino acid l-histidine, are replaced by CH3NH3+. The resulting compound, (CH3NH3)12{CuII24[(S,S)-hismox]12(OH2)3}·178H2O (2), retains the polar space group of 1 and is ferroelectric below 260 K. These results open a new synthetic avenue to enlarge the limited number of FE MOFs.

Selective Gold Recovery and Catalysis in a Highly Flexible Methionine-Decorated Metal-Organic Framework.

A novel chiral 3D bioMOF exhibiting functional channels with thio-alkyl chains derived from the natural amino acid l-methionine (1) has been rationally prepared. The well-known strong affinity of gold for sulfur derivatives, together with the extremely high flexibility of the thioether "arms" decorating the channels, account for a selective capture of gold(III) and gold(I) salts in the presence of other metal cations typically found in electronic wastes. The X-ray single-crystal structures of the different gold adsorbates Au(III)@1 and Au(I)@1 suggest that the selective metal capture occurs in a metal ion recognition process somehow mimicking what happens in biological systems and protein receptors. Both Au(III)@1 and Au(I)@1 display high activity as heterogeneous catalyst for the hydroalkoxylation of alkynes, further expanding the application of these novel hybrid materials.