Alkene Cross-Metathesis with 2,5-Dimethyl-2,4-Hexadiene Enables Isobutylenyl/Prenyl Functionalizations and Rubber Valorization.

2,5-Dimethyl-2,4-hexadiene is a readily available and easily managable compound, whose symmetric and polymethylated dienic structure should be prone to engage in cross-metathesis reactions with other alkenes, but this has not been apparently exploited so far. Here we show that this reactant enables the easy synthesis of tri- and tetra-susbtituted alkenes (i. e. isobutylenyl and prenyl groups) from simple alkenes under mild reaction conditions, not only with the conventional 2nd generation Grubbs catalyst but also with other Grela-type catalyts such as StickyCat,TM AquaMetTM and GreenCatTM. The use of liquid and low volatile 2,5-dimethyl-2,4-hexadiene avoids the use of gaseous alkene reactants and, besides, showcases the reactivity of polyisoprene (rubber), thus allowing to optimize the reaction conditions for rubber upcycling, after metathesis reaction of the pristine or used polymer with simple alkenes. These results bring low volatile isoprene-type compounds as privileged poly-substituted reactants for alkene cross-metathesis reactions.

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.

Hexagonal Hybrid Bismuthene by Molecular Interface Engineering.

High-quality devices based on layered heterostructures are typically built from materials obtained by complex solid-state physical approaches or laborious mechanical exfoliation and transfer. Meanwhile, wet-chemically synthesized materials commonly suffer from surface residuals and intrinsic defects. Here, we synthesize using an unprecedented colloidal photocatalyzed, one-pot redox reaction a few-layers bismuth hybrid of "electronic grade" structural quality. Intriguingly, the material presents a sulfur-alkyl-functionalized reconstructed surface that prevents it from oxidation and leads to a tuned electronic structure that results from the altered arrangement of the surface. The metallic behavior of the hybrid is supported by ab initio predictions and room temperature transport measurements of individual nanoflakes. Our findings indicate how surface reconstructions in two-dimensional (2D) systems can promote unexpected properties that can pave the way to new functionalities and devices. Moreover, this scalable synthetic process opens new avenues for applications in plasmonics or electronic (and spintronic) device fabrication. Beyond electronics, this 2D hybrid material may be of interest in organic catalysis, biomedicine, or energy storage and conversion.

Degradation of Penicillinic Antibiotics and ß-Lactamase Enzymatic Catalysis in a Biomimetic Zn-Based Metal-Organic Framework.

ß-Lactam antibiotics are one of the most commonly prescribed drugs to treat bacterial infections. However, their use has been somehow limited given the emergence of bacteria with resistance mechanisms, such as ß-lactamases, which inactivate them by degrading their four-membered ß-lactam rings. So, a total knowledge of the mechanisms governing the catalytic activity of ß-lactamases is required. Here, we report a novel Zn-based metal-organic framework (MOF, 1), possessing functional channels capable to accommodate and interact with antibiotics, which catalyze the selective hydrolysis of the penicillinic antibiotics amoxicillin and ceftriaxone. In particular, MOF 1 degrades, very efficiently, the four-membered ß-lactam ring of amoxicillin, acting as a ß-lactamase mimic, and expands the very limited number of MOFs capable to mimic catalytic enzymatic processes. Combined single-crystal X-ray diffraction (SCXRD) studies and density functional (DFT) calculations offer unique snapshots on the host-guest interactions established between amoxicillin and the functional channels of 1. This allows to propose a degradation mechanism based on the activation of a water molecule, promoted by a Zn-bridging hydroxyl group, concertedly to the nucleophilic attack to the carbonyl moiety and the cleaving of C-N bond of the lactam ring.

Degradation of Penicillinic Antibiotics and ß-Lactamase Enzymatic Catalysis in a Biomimetic Zn-Based Metal-Organic Framework.

Invited for the cover of this issue are Jesús Ferrando-Soria, Donatella Armentano, Antonio Leyva-Pérez, Emilio Pardo and co-workers at University of Valencia, Technical University of Valencia and University of Calabria. The image depicts the crystal structure of a novel ZnII biological metal-organic framework that mimics ß-lactamase enzymes. Read the full text of the article at 10.1002/chem.202301325.

Parts-Per-Million of Soluble Pd0 Catalyze the Semi-Hydrogenation Reaction of Alkynes to Alkenes.

The synthesis of cis-alkenes is industrially carried out by selective semi-hydrogenation of alkynes with complex Pd catalysts, which include the Lindlar catalyst (PdPb on CaCO3) and c-Pd/TiS (colloidal ligand-protected Pd nanoparticles), among others. Here, we show that Pd0 atoms are generated from primary Pd salts (PdCl2, PdSO4, Pd(OH)2, PdO) with H2 in alcohol solutions, independently of the alkyne, to catalyze the semi-hydrogenation reaction with extraordinarily high efficiency (up to 735 s-1), yield (up to 99%), and selectivity (up to 99%). The easy-to-prepare Pd0 species hold other potential catalytic applications.

Indomuscone-Based Sterically Encumbered Phosphines as Ligands for Palladium-Catalyzed Reactions.

The fragrance compound indomuscone is used here as a scaffold to prepare two different sterically hindered phosphines, one aromatic and another alkylic, in good yields, after four synthetic steps. The new phosphines show enhanced electronic and steric properties when compared to benchmark commercial phosphine ligands, which is reflected in the catalytic results obtained for representative palladium-catalyzed reactions such as the telomerization reaction, the Buchwald-Hartwig and Suzuki cross-coupling reactions of chloroaromatic rings, and the semi-hydrogenation reaction of an alkyne. In particular, the indomuscone-based aromatic phosphine ligand leads to the highest selectivity for the tail-to-head telomerization product between isoprene and methanol, while the indomuscone-based alkylic phosphine ligand shows extraordinary similarities with the Buchwald-type SPhos phosphine ligand.

Exploring the Role of Amino Acid-Derived Multivariate Metal-Organic Frameworks as Catalysts in Hemiketalization Reactions.

Understanding the host-guest chemistry in MOFs represents a research field with outstanding potential to develop in a rational manner novel porous materials with improved performances in fields such as heterogeneous catalysis. Herein, we report a family of three isoreticular MOFs derived from amino acids and study the influence of the number and nature of functional groups decorating the channels as a catalyst in hemiketalization reactions. In particular, a multivariate (MTV) MOF 3, prepared by using equal percentages of amino acids L-serine and L-mecysteine, in comparison to single-component ("traditional") MOFs, derived from either L-serine or L-mecysteine (MOFs 1 and 2), exhibits the most efficient catalytic conversions for the hemiketalization of different aldehydes and ketalization of cyclohexanone. On the basis of the experimental data reported, the good catalytic performance of MTV-MOF 3 is attributed to the intrinsic heterogeneity of MTV-MOFs. These results highlight the potential of MTV-MOFs as strong candidates to mimic natural nonacidic enzymes, such as glycosidases, and to unveil novel catalytic mechanisms not so easily accessible with other microporous materials.

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.

A Cascade Sonogashira Cross-Coupling-Substitution-Elimination Reaction for the Synthesis of Linear Conjugated Dienynes.

The engagement in tandem of well-known organic reactions such as the Pd-catalyzed Sonogashira cross-coupling reaction, nucleophilic substitution and elimination reactions, enables the synthesis of otherwise difficult to obtain linear dienynes, in moderate to high yields. This retrosynthetic approach opens new ways to prepare highly conjugated alkenes and alkynes. Furthermore, ionic liquids are suitable solvents to perform the cascade reaction and recycle the metal catalysts.

MOF-Stabilized Perfluorinated Palladium Cages Catalyze the Additive-Free Aerobic Oxidation of Aliphatic Alcohols to Acids.

Extremely high electrophilic metal complexes, composed by a metal cation and very electron poor σ-donor ancillary ligands, are expected to be privileged catalysts for oxidation reactions in organic chemistry. However, their low lifetime prevents any use in catalysis. Here we show the synthesis of fluorinated pyridine-Pd2+ coordinate cages within the channels of an anionic tridimensional metal-organic framework (MOF), and their use as efficient metal catalysts for the aerobic oxidation of aliphatic alcohols to carboxylic acids without any additive. Mechanistic studies strongly support that the MOF-stabilized coordination cage with perfluorinated ligands unleashes the full electrophilic potential of Pd2+ to dehydrogenate primary alcohols, without any base, and also to activate O2 for the radical oxidation to the aldehyde intermediate. This study opens the door to design catalytic perfluorinated complexes for challenging organic transformations, where an extremely high electrophilic metal site is required.

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.

Intermolecular Carbonyl-olefin Metathesis with Vinyl Ethers Catalyzed by Homogeneous and Solid Acids in Flow.

The carbonyl-olefin metathesis reaction has experienced significant advances in the last seven years with new catalysts and reaction protocols. However, most of these procedures involve soluble catalysts for intramolecular reactions in batch. Herein, we show that recoverable, inexpensive, easy to handle, non-toxic, and widely available simple solid acids, such as the aluminosilicate montmorillonite, can catalyze the intermolecular carbonyl-olefin metathesis of aromatic ketones and aldehydes with vinyl ethers in-flow, to give alkenes with complete trans stereoselectivity on multi-gram scale and high yields. Experimental and computational data support a mechanism based on a carbocation-induced Grob fragmentation. These results open the way for the industrial implementation of carbonyl-olefin metathesis over solid catalysts in continuous mode, which is still the origin and main application of the parent alkene-alkene cross-metathesis.

Base-Controlled Heck, Suzuki, and Sonogashira Reactions Catalyzed by Ligand-Free Platinum or Palladium Single Atom and Sub-Nanometer Clusters.

The assumption that oxidative addition is the key step during the cross-coupling reaction of aryl halides has led to the development of a plethora of increasingly complex metal catalysts, thereby obviating in many cases the exact influence of the base, which is a simple, inexpensive, and necessary reagent for this paramount transformation. Here, a combined experimental and computational study shows that the oxidative addition is not the single kinetically relevant step in different cross-coupling reactions catalyzed by sub-nanometer Pt or Pd species, since the reactivity control is shifted toward subtle changes in the base. The exposed metal atoms in the cluster cooperate to enable an extremely easy oxidative addition of the aryl halide, even chlorides, and allow the base to bifurcate the coupling. With sub-nanometer Pd species, amines drive to the Heck reaction, carbonate drives to the Sonogahira reaction, and phosphate drives to the Suzuki reaction, while for Pt clusters and single atoms, good conversion is only achieved using acetate as a base. This base-controlled orthogonal reactivity with ligand-free catalysts opens new avenues in the design of cross-coupling reactions in organic synthesis.

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.

Lattice Opening upon Bulk Reductive Covalent Functionalization of Black Phosphorus.

The chemical bulk reductive covalent functionalization of thin-layer black phosphorus (BP) using BP intercalation compounds has been developed. Through effective reductive activation, covalent functionalization of the charged BP by reaction with organic alkyl halides is achieved. Functionalization was extensively demonstrated by means of several spectroscopic techniques and DFT calculations; the products showed higher functionalization degrees than those obtained by neutral routes.