Bimetallic nanozymes synergize to regulate the behavior of oxygen intermediates and substrate HMF adsorption

We have constructed a bimetallic (CoNiP) nanozyme, leveraging the synergistic effect of cobalt and nickel, which efficiently catalyzes the oxidation of TMB from colorless to ox-TMB (blue). Density functional theory (DFT) calculations further highlight the pivotal role of this synergistic effect in improving the adsorption energy of oxygen intermediates, accelerating the catalytic process.

Can the Rate of a Catalytic Turnover Be Altered by Ligands in the Absence of Direct Binding Interactions?

Second sphere coordination effects ubiquitous in enzymatic catalysis occur through direct interactions, either covalent or non-covalent, with reaction intermediates and transition states. We present herein evidence of indirect second sphere coordination effects in which ligation of water/alkanols far removed from the primary coordination sphere of the active site nevertheless alter energetic landscapes within catalytic redox cycles in the absence of direct physicochemical interactions with surface species mediating catalytic turnovers. Density functional theory, in situ X-ray absorption and infrared spectroscopy, and a wide array of steady-state and transient CO oxidation rate data suggest that the presence of peripheral water renders oxidation half-cycles within two-electron redox cycles over μ3-oxo-bridged trimers in MIL-100(M) more kinetically demanding. Communication between ligated water and the active site appears to occur through the Fe-O-Fe backbone, as inferred from spin density variations on the central μ3-oxygen 'junction'. Evidence is provided for the generality of these second sphere effects in that they influence different types of redox half-cycles or metals, and can be amplified or attenuated through choice of coordinating ligand. Specifically in the case of MIL-100(M) materials, the Cr isostructure can be made to kinetically mimic the Fe variant by disproportionately hindering oxidation half-cycles relative to the reduction half-cycles. Kinetic and spectroscopic inferences presented here significantly expand both the conceptual definition of second sphere effects as well as the palette of synthetic levers available for tuning catalytic redox performance through chemical ligation.

Photoinduced Site-Selective Aryl C-H Borylation with Electron-Donor-Acceptor Complex Derived from B2Pin2 and Isoquinoline

Due to boron's metalloid properties, aromatic boron reagents are prevalent synthetic intermediates. The direct borylation of aryl C-H bonds for producing aromatic boron compounds offers an appealing, one-step solution. Despite significant advances in this field, achieving regioselective aryl C-H bond borylation using simple and readily available starting materials still remains a challenge. In this work, we attempted to enhance the reactivity of the electron-donor-acceptor (EDA) complex by selecting different bases to replace the organic base (NEt3) used in our previous research. To our delight, when using NH4HCO3 as the base, we have achieved a mild visible-light-mediated aromatic C-H bond borylation reaction with exceptional regioselectivity (rr > 40:1 to single isomers). Compared with our previous borylation methodologies, this protocol provides a more efficient and broader scope for aryl C-H bond borylation through the use of N-Bromosuccinimide. The protocol's good functional-group tolerance and excellent regioselectivity enable the functionalization of a variety of biologically relevant compounds and novel cascade transformations. Mechanistic experiments and theoretical calculations conducted in this study have indicated that, for certain arenes, the aryl C-H bond borylation might proceed through a new reaction mechanism, which involves the formation of a novel transient EDA complex.

Application of sSPhos as a Chiral Ligand for Palladium-Catalyzed Asymmetric Allylic Alkylation

Palladium-catalyzed asymmetric allylic alkylation is a versatile method for C-C bond formation. Many established classes of chiral ligands can perform allylic alkylation reactions enantioselectively, but identification of new ligand classes remains important for future development of the field. We demonstrate that enantiopure sSPhos, a bifunctional chiral monophosphine ligand, when used as its tetrabutyl ammonium salt, is a highly effective ligand for a benchmark Pd-catalyzed allylic alkylation reaction. We explore the scope and limitations and perform experiments to probe the origin of selectivity. In contrast with reactions previously explored using enantiopure sSPhos, it appears that steric bulk around the sulfonate group is responsible for the high enantioselectivity in this case, rather than attractive noncovalent interactions.

Rhodium-Catalyzed Asymmetric Hydrogenation and Transfer Hydrogenation of 1,3-Dipolar Nitrones

Owing to their distinctive 1,3-dipolar structure, the catalytic asymmetric hydrogenation of nitrones to hydroxylamines has been a formidable and longstanding challenge, characterized by intricate enantiocontrol and susceptibility to N-O bond cleavage. In this study, the asymmetric hydrogenation and transfer hydrogenation of nitrones were accomplished with a tethered TsDPEN-derived cyclopentadienyl rhodium(III) catalyst (TsDPEN: p-toluenesulfonyl-1,2-diphenylethylene-1,2-diamine), the reaction proceeds via a novel 7-membered cyclic transition state, producing chiral hydroxylamines with up to 99 % yield and >99 % ee. The practical viability of this methodology was underscored by gram-scale catalytic reactions and subsequent transformations. Furthermore, mechanistic investigations and DFT calculations were also conducted to elucidate the origin of enantioselectivity.

Zwitterionic Aqua Palladacycles with Noncovalent Interactions for meta-Selective Suzuki Coupling of 3,4-Dichlorophenol and 3,4-Dichlorobenzyl Alcohol in Water

The site-selective reaction of substrates with multiple reactive sites has been a focus of the current synthetic chemistry. The use of attractive noncovalent interactions between the catalyst and substrate is emerging as a versatile approach to address site-selectivity challenges. Herein, we designed and synthesized a series of palladacycles, to control meta-selective Suzuki coupling of 3,4-dichlorophenol and 3,4-dichlorobenzyl alcohol. Noncovalent interactions directed zwitterionic aqua palladacycles catalyzed meta-selective Suzuki couplings of 3,4-dichloroarenes bearing hydroxyl in water have been developed. Experiments and density functional theory (DFT) calculations demonstrated that the electrostatic interactions play a critical role in meta-selective coupling of 3,4-dichlorophenol, while meta-selective coupling of 3,4-dichlorobenzyl alcohol arises due to the hydrogen-bonding interactions.

Markovnikov-Selective Cobalt-Catalyzed Wacker-Type Oxidation of Styrenes into Ketones under Ambient Conditions Enabled by Hydrogen Bonding

The replacement of palladium catalysts for Wacker-type oxidation of olefins into ketones by first-row transition metals is a relevant approach for searching more sustainable protocols. Besides highly sophisticated iron catalysts, all the other first-row transition metal complexes have only led to poor activities and selectivities. Herein, we show that the cobalt-tetraphenylporphyrin complex is a competent catalyst for the aerobic oxidation of styrenes into ketones with silanes as the hydrogen sources. Remarkably, under room temperature and air atmosphere, the reactions were exceedingly fast (up to 10 minutes) with a low catalyst loading (1 mol %) while keeping an excellent chemo- and Markovnikov-selectivity (up to 99 % of ketone). Unprecedently high TOF (864 h-1 ) and TON (5,800) were reached for the oxidation of aromatic olefins under these benign conditions. Mechanistic studies suggest a reaction mechanism similar to the Mukaiyama-type hydration of olefins with a change in the last fundamental step, which controls the chemoselectivity, thanks to a unique hydrogen bonding network between the ethanol solvent and the cobalt peroxo intermediate.

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.

Synthesis of Hybrid Ligands with Nitrile and Cage Phosphane Donor Groups and their Applications in Gold-Mediated Reactions

Altering the donor properties of phosphane ligands through substituent variation is an established tool in coordination chemistry and catalysis. This contribution describes the synthesis of two new hybrid donors (L) combining 1,3,5,7-tetramethyl-2,4,6-trioxa-8-phosphaadamantane-8-yl (PCg) and nitrile donor groups at different molecular scaffolds, viz. ferrocene-1,1'-diyl (fc) and 1,2-phenylene. These ligands were used to prepare dimeric Au(I) complexes [Au2 (μ(P,N)-L)2 ][SbF6 ]2 , which were evaluated as silver-free, preformed catalysts in Au-mediated cycloisomerization of (Z)-3-methylpent-2-en-4-yn-1-ol to 2,3-dimethylfuran. The catalyst featuring the ferrocene-based ligand, viz., [Au2 (μ(P,N)-CgPfcCN)2 ][SbF6 ]2 , showed the best catalytic performance at low catalyst loading (0.5 or 0.15 mol.%), which exceeded that of its diphenylphosphanyl analog [Au2 (μ(P,N)-Ph2 PfcCN)2 ][SbF6 ]2 studied earlier and the prototypical Au(I) precatalyst [Au(PPh3 )(MeCN)][SbF6 ].

Reversible Modulation of the Electronic and Spatial Environment around Ni(0) Centers Bearing Multifunctional Carbene Ligands with Triarylaluminum

Designing and modulating the electronic and spatial environments surrounding metal centers is a crucial issue in a wide range of chemistry fields that use organometallic compounds. Herein, we demonstrate a Lewis-acid-mediated reversible expansion, contraction, and transformation of the spatial environment surrounding nickel(0) centers that bear N-phosphine oxide-substituted N-heterocyclic carbenes (henceforth referred to as (S)PoxIms). Reaction between tetrahedral (syn-κ-C,O-(S)PoxIm)Ni(CO)₂ and Al(C₆F₅)₃ smoothly afforded heterobimetallic Ni/Al species such as trigonal-planar {κ-C-Ni(CO)₂}(μ-anti-(S)PoxIm){κ-O-Al(C₆F₅)₃} via a complexation-induced rotation of the N-phosphine oxide moieties, while the addition of 4-dimethylaminopyridine resulted in the quantitative regeneration of the former Ni complexes. The corresponding interconversion also occurred between (SPoxIm)Ni(η²:η²-diphenyldivinylsilane) and {κ-C-Ni(η²:η²-diene)}(μ-anti-SPoxIm){κ-O-Al(C₆F₅)₃} via the coordination and dissociation of Al(C₆F₅)₃. The shape and size of the space around the Ni(0) center was drastically changed through this Lewis-acid-mediated interconversion. Moreover, the multinuclear NMR, IR, and XAS analyses of the aforementioned carbonyl complexes clarified the details of the changes in the electronic states on the Ni centers; i.e., the electron delocalization was effectively enhanced among the Ni atom and CO ligands in the heterobimetallic Ni/Al species. The results presented in this work thus provide a strategy for reversibly modulating both the electronic and spatial environment of organometallic complexes, in addition to the well-accepted Lewis-base-mediated ligand-substitution methods.

Structural and dimensional control of porphyrin capsules using Group 15 tris(3-pyridyl) linkers

While supramolecular chemistry involving organic and metallo-organic host assemblies is a well-established and important field with applications in gas-storage, drug-delivery and the regio- and stereo-control of organic reactions, the use of main group elements in this setting (beyond the second row of the p-block) has been little explored. In this paper we show how periodic trends in the p-block can provide the means for systematic size and structural control in an important class of supramolecular porphyrin-based capsules. The formation of molecular and extended 2D capsule arrangements between the heavier Group 15 tris(3-pyridyl) linkers Sb(3-py)₃ and Bi(3-py)₃ and the metallo-porphyrins MTPP (M = Zn, Mg; TPP = tetraphenylporphyrin, 3-py = 3-pyridyl) is the first study involving heavier Group 15 pyridyl linkers. The increase in C-E bond length in the E(3-py)₃ linkers moving down Group 15 (from E = P, to Sb, to Bi) can be used to alter the dimensions and structural preference of the capsules, as can oxidation of the Group 15 bridgehead atoms themselves. The subtle changes in the dimensions and Lewis acidity of the encapsulates have a dramatic effect on the rate and selectivity of the catalytic oxidative cleavage of organic diols and catalytic oxidation of α-hydroxyketones. By providing simple tools for modulating the chemical and steric properties of the capsules this work should have direct applications for the tuning of the activity and specificity of a range of catalytic systems based on main-group-based capsules of this type.

NiII and CuII complexes of a salen ligand bearing ferrocenes in its secondary coordination sphere

Herein, we report the synthesis, spectroscopic characterization and electrochemical investigation of the NiII and CuII complexes of a novel Sal ligand bearing two ferrocene moieties attached at its diimine linker, M(Sal)Fc. The electronic spectra of M(Sal)Fc are near identical to its phenyl-substituted counterpart, M(Sal)Ph, indicating the ferrocene moieties exist in the secondary coordination sphere of M(Sal)Fc. The cyclic voltammograms of M(Sal)Fc exhibit an additional two-electron wave in comparison to M(Sal)Ph, which is assigned to the sequential oxidation of the two ferrocene moieties. The chemical oxidation of M(Sal)Fc, monitored by low temperature UV-vis spectroscopy, supports the formation of a mixed valent FeIIFeIII species followed by a bis(ferrocenium) species upon sequential addition of one and two equivalents of chemical oxidant. The addition of a third equivalent of oxidant to Ni(Sal)Fc yielded intense near-IR transitions that are indicative of the formation of a fully delocalized Sal-ligand radical (Sal˙), while the same addition to Cu(Sal)Fc yielded a species that is currently under further spectroscopic investigation. These results suggest the oxidation of the ferrocene moieties of M(Sal)Fc does not affect the electronic structure of the M(Sal) core, and these are thus in the secondary coordination sphere of the overall complex.

Strategies That Utilize Ion Pairing Interactions to Exert Selectivity Control in the Functionalization of C-H Bonds

Electrostatic attraction between two groups of opposite charge, typically known as ion-pairing, offers unique opportunities for the design of systems to enable selectivity control in chemical reactions. Catalysis using noncovalent interactions is an established and vibrant research area, but it is noticeable that hydrogen bonding interactions are still the main interaction of choice in system design. Opposite charges experience the powerful force of Coulombic attraction and have the ability to exert fundamental influence on the outcome of reactions that involve charged reagents, intermediates or catalysts. In this Perspective, we will examine how ion-pairing interactions have been used to control selectivity in C-H bond functionalization processes. This broad class of reactions provides an interesting and thought-provoking lens through which to examine the application of ion-pairing design strategies because it is one that encompasses great mechanistic diversity, poses significant selectivity challenges, and perhaps most importantly is of immense interest to synthetic chemists in both industry and academia. We survey reactions that proceed via radical and ionic mechanisms alongside those that involve transition metal catalysis and will deal with control of site-selectivity and enantioselectivity. We anticipate that as this emerging area develops, it will become an ever-more important design strategy for selectivity control.

Unravelling Enzymatic Features in a Supramolecular Iridium Catalyst by Computational Calculations

Non-biological catalysts following the governing principles of enzymes are attractive systems to disclose unprecedented reactivities. Most of those existing catalysts feature an adaptable molecular recognition site for substrate binding that are prone to undergo conformational selection pathways. Herein, we present a non-biological catalyst that is able to bind substrates via the induced fit model according to in-depth computational calculations. The system, which is constituted by an inflexible substrate-recognition site derived from a zinc-porphyrin in the second coordination sphere, features destabilization of ground states as well as stabilization of transition states for the relevant iridium-catalyzed C-H bond borylation of pyridine. In addition, this catalyst appears to be most suited to tightly bind the transition state rather than the substrate. Besides these features, which are reminiscent of the action modes of enzymes, new elementary catalytic steps (i. e. C-B bond formation and catalyst regeneration) have been disclosed owing to the unique distortions encountered in the different intermediates and transition states.

A Borane Lewis Acid in the Secondary Coordination Sphere of a Ni(II) Imido Imparts Distinct C-H Activation Selectivity

Two borane-functionalized bidentate phosphine ligands that vary in tether length have been prepared to examine cooperative metal-substrate interactions. Ni(0) complexes react with aryl azides at low temperatures to form structurally unusual κ2-(N,N)-N3Ar adducts. Warming these adducts affords products of N2 extrusion and in one case, a Ni-imido compound that is capped by the appended borane. Reactions with 1-azidoadamantane (AdN3) provide a distinct outcome, where a proposed nickel imido intermediate activates the sp2 C-H bonds of arenes, even in the presence of benzylic C-H sites. Combined experimental and computational mechanistic studies demonstrate that the unique reactivity is a consequence of Lewis-acid-induced polarization of the Ni-NR bond, potentially providing a synthetic strategy for chemoselective reaction engineering.

Observation of oxygen evolution over a {Ni12}-cluster-based metal-organic framework

The development of efficient electrocatalysts based on non-noble metals for oxygen evolution reaction (OER) remains an important and challenging task. Multinuclear transition-metal clusters with high structural stability are promising OER catalysts but their catalytic role is poorly understood. Here we report the crystallographic observation of OER activity over robust {Ni12}-clusters immobilised in a porous metal-organic framework, NKU-100, by single-crystal X-ray diffraction as a function of external applied potential. We observed the aggregation of confined oxygen species around the {Ni12}-cluster as a function of applied potential during the electrocatalytic process. The refined occupancy of these oxygen species shows a strong correlation with the variation of current density. This study demonstrates that the enrichment of oxygen species in the secondary co-ordination sphere of multinuclear transition-metal clusters can promote the OER activity.

Modifying electron injection kinetics for selective photoreduction of nitroarenes into cyclic and asymmetric azo compounds

Modifying the reactivity of substrates by encapsulation is essential for microenvironment catalysts. Herein, we report an alternative strategy that modifies the entry behaviour of reactants into the microenvironment and substrate inclusion thermodynamics related to the capsule to control the electron injection kinetics and the selectivity of products from the nitroarenes photoreduction. The strategy includes the orchestration of capsule openings to control the electron injection kinetics of electron donors, and the capsule’s pocket to encapsulate more than one nitroarene molecules, facilitating a condensation reaction between the in situ formed azanol and nitroso species to produce azo product. The conceptual microenvironment catalyst endows selective conversion of asymmetric azo products from different nitroarenes, wherein, the estimated diameter and inclusion Gibbs free energy of substrates are used to control and predict the selectivity of products. Inhibition experiments confirm a typical enzymatic conversion, paving a new avenue for rational design of photocatalysts toward green chemistry.

Ag(I)/PPh3-catalyzed diastereoselective syntheses of spiro[indole-3,4'-piperidine] derivatives via cycloisomerizations of tryptamine-ynamides

A Ag(I)/PPh₃-catalyzed chelation-controlled cycloisomerization of tryptamine-ynamide was developed to access the spiro[indole-3,4'-piperidine] scaffold in a racemic and diastereoselective manner. The diastereoselective products were achieved by a chiron approach. Density functional theory (DFT) calculations indicated that strong non-covalent effects between the substrate and catalyst/ligand complex stabilized the spiroindoleninium intermediate via cation-π-π interactions.

Steric Shielding Effects Induced by Intramolecular C-H⋯O Hydrogen Bonding: Remote Borylation Directed by Bpin Groups

Regioselectivities in catalytic C-H borylations (CHBs) have been rationalized using simplistic steric models and correlations with nuclear magnetic resonance (NMR) chemical shifts. However, regioselectivity can be significant for important substrate classes where none would be expected from these arguments. In this study, intramolecular hydrogen bonding (IMHB) can lead to steric shielding effects that can direct Ir-catalyzed CHB regiochemistry. Bpin (Bpin = pinacol boronic ester)/arene IMHB can promote remote borylations of N-borylated anilines, 2-amino-N-alkylpyridine, tetrahydroquinolines, indoles, and 1-borylated naphthalenes. Experimental and computational studies support molecular geometries with the Bpin orientation controlled by a C-H⋯O IMHB. IMHB-directed remote CHB appeared operative in the C6 borylation of 3-aminoindazole (seven-membered IMHB) and C6 borylation of an osimertinib analogue where a pyrimidine IMHB creates the steric shield. This study informs researchers to evaluate not only inter- but also intramolecular noncovalent interactions as potential drivers of remote CHB regioselectivity.

H-Bonded Counterion-Directed Enantioselective Au(I) Catalysis

A new strategy for enantioselective transition-metal catalysis is presented, wherein a H-bond donor placed on the ligand of a cationic complex allows precise positioning of the chiral counteranion responsible for asymmetric induction. The successful implementation of this paradigm is demonstrated in 5-exo-dig and 6-endo-dig cyclizations of 1,6-enynes, combining an achiral phosphinourea Au(I) chloride complex with a BINOL-derived phosphoramidate Ag(I) salt and thus allowing the first general use of chiral anions in Au(I)-catalyzed reactions of challenging alkyne substrates. Experiments with modified complexes and anions, ¹H NMR titrations, kinetic data, and studies of solvent and nonlinear effects substantiate the key H-bonding interaction at the heart of the catalytic system. This conceptually novel approach, which lies at the intersection of metal catalysis, H-bond organocatalysis, and asymmetric counterion-directed catalysis, provides a blueprint for the development of supramolecularly assembled chiral ligands for metal complexes.

Remote Amino Acid Recognition Enables Effective Hydrogen Peroxide Activation at a Manganese Oxidation Catalyst

Precise delivery of a proton plays a key role in O2 activation at iron oxygenases, enabling the crucial O-O cleavage step that generates the oxidizing high-valent metal-oxo species. Such a proton is delivered by acidic residues that may either directly bind the iron center or lie in its second coordination sphere. Herein, a supramolecular strategy for enzyme-like H2 O2 activation at a biologically inspired manganese catalyst, with a nearly stoichiometric amount (1-1.5 equiv) of a carboxylic acid is disclosed. Key for this strategy is the incorporation of an α,ω-amino acid in the second coordination sphere of a chiral catalyst via remote ammonium-crown ether recognition. The properly positioned carboxylic acid function enables effective activation of hydrogen peroxide, leading to catalytic asymmetric epoxidation. Modulation of both amino acid and catalyst structure can tune the efficiency and the enantioselectivity of the reaction, and a study on the oxidative degradation pathway of the system is presented.

Distal Ionic Substrate-Catalyst Interactions Enable Long-Range Stereocontrol: Access to Remote Quaternary Stereocenters through a Desymmetrizing Suzuki-Miyaura Reaction

Spatial distancing of a substrate's reactive group and nonreactive catalyst-binding group from its pro-stereogenic element presents substantial hurdles in asymmetric catalysis. In this context, we report a desymmetrizing Suzuki-Miyaura reaction that establishes chirality at a remote quaternary carbon. The anionic, chiral catalyst exerts stereocontrol through electrostatic steering of substrates, even as the substrate's reactive group and charged catalyst-binding group become increasingly distanced. This study demonstrates that precise long-range stereocontrol is achievable by engaging ionic substrate-ligand interactions at a distal position.

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.

Photoinduced Metal-Free Borylation of Aryl Halides Catalysed by in situ Formed Donor-Acceptor Complex

Organoboron compounds are very important building blocks which can be applied in medicinal, biological and industrial fields. However, direct borylation in a metal free manner has been very rarely reported. Herein, we described the successful direct borylation of haloarenes under mild, operationally simple, catalyst-free conditions, promoted by irradiation with visible light. Mechanistic experiments and computational investigations indicate the formation of an excited donor–acceptor complex with a −3.12 V reduction potential, which is a highly active reductant and can facilitate single-electron-transfer (SET) with aryl halides to produce aryl radical intermediates. A two-step one-pot method was developed for site selective aromatic C–H bond borylation. The protocol's good functional group tolerance enables the functionalization of a variety of biologically relevant compounds, representing a new application of aryl radicals merged with photochemistry.

We reported a facile metal-free conversion of aryl halides to the corresponding boronic esters catalysed by an in situ formed donor–acceptor complex. A two-step one-pot method was also developed for site selective aromatic C–H bond borylation.

Reaction of H2 with polyoxometalate supported Rhodium(0) and Iridium(0) nanoparticles in aqueous suspensions: a kinetic study

The mechanism of the reaction between Rh0 and Ir0 NPs with H2 was measured in the absence of an electrical bias via monitoring the catalytic reduction of PW12O403− and it was compared to the previous results of Pt0 NPs.

Tuning the coordination properties of chiral pseudopeptide bis(2-picolyl)amine and iminodiacetamide ligands in Zn(ii) and Cu(ii) complexes

Modifications of the chiral side chains of bpa and imda ligands lead to different metal ion coordination and hydrogen bonding ability.

Using molten salts to probe outer-coordination sphere effects on lanthanide(III)/(II) electron-transfer reactions

Controlling structure and reactivity by manipulating the outer-coordination sphere around a given reagent represents a longstanding challenge in chemistry. Despite advances toward solving this problem, it remains difficult to experimentally interrogate and characterize outer-coordination sphere impact. This work describes an alternative approach that quantifies outer-coordination sphere effects. It shows how molten salt metal chlorides (MCl_n; M = K, Na, n = 1; M = Ca, n = 2) provided excellent platforms for experimentally characterizing the influence of the outer-coordination sphere cations (Mⁿ⁺) on redox reactions accessible to lanthanide ions; Ln³⁺ + e^1- → Ln²⁺ (Ln = Eu, Yb, Sm; e^1- = electron). As a representative example, X-ray absorption spectroscopy and cyclic voltammetry results showed that Eu²⁺ instantaneously formed when Eu³⁺ dissolved in molten chloride salts that had strongly polarizing cations (like Ca²⁺ from CaCl₂) via the Eu³⁺ + Cl^1- → Eu²⁺ + ½Cl₂ reaction. Conversely, molten salts with less polarizing outer-sphere M¹⁺ cations (e.g., K¹⁺ in KCl) stabilized Ln³⁺. For instance, the Eu³⁺/Eu²⁺ reduction potential was >0.5 V more positive in CaCl₂ than in KCl. In accordance with first-principle molecular dynamics (FPMD) simulations, we postulated that hard Mⁿ⁺ cations (high polarization power) inductively removed electron density from Lnⁿ⁺ across Ln-Cl⋯Mⁿ⁺ networks and stabilized electron-rich and low oxidation state Ln²⁺ ions. Conversely, less polarizing Mⁿ⁺ cations (like K¹⁺) left electron density on Lnⁿ⁺ and stabilized electron-deficient and high-oxidation state Ln³⁺ ions.

Macrocylases as synthetic tools for ligand synthesis: enzymatic synthesis of cyclic peptides containing metal-binding amino acids

Improving the sustainability of synthesis is a major goal in green chemistry, which has been greatly aided by the development of asymmetric transition metal catalysis. Recent advances in asymmetric catalysis show that the ability to control the coordination sphere of substrates can lead to improvements in enantioselectivity and activity, in a manner resembling the operation of enzymes. Peptides can be used to mimic enzyme structures and their secondary interactions and they are easily accessible through solid-phase peptide synthesis. Despite this, cyclic peptides remain underexplored as chiral ligands for catalysis due to synthetic complications upon macrocyclization. Here, we show that the solid-phase synthesis of peptides containing metal-binding amino acids, bipyridylalanine (1), phenyl pyridylalanine (2) and N,N-dimethylhistidine (3) can be combined with peptide macrocylization using peptide cyclase 1 (PCY1) to yield cyclic peptides under mild conditions. High conversions of the linear peptides were observed (approx. 90%) and the Cu-bound cyclo(FSAS(1)SSKP) was shown to be a competent catalyst in the Friedel-Crafts/conjugate addition of indole. This study shows that PCY1 can tolerate peptides containing amino acids with classic inorganic and organometallic ligands as side chains, opening the door to the streamlined and efficient development of cyclic peptides as metal ligands.

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.

Substrate Specific Metal-Ligand Cooperative Binding: Considerations for Weak Intramolecular Lewis Acid/Base Pairs

Metal-ligand cooperative binding modes were interrogated in a series of zinc bis(thiophenoxide) complexes. A weak B-S binding interaction is observed in solution between the weakly Lewis basic thiophenoxide ligands and an appended trialkylborane. The energy of this binding event is dependent upon the strength of the Lewis acid and its proximity to the zinc thiophenoxide.

Cooperativity in Transition Metal Tetrylene Complexes

Cooperative reactivity between transition metals and ligands, or between two metals, has created significant opportunities for the development of new transformations that would be difficult to carry out with a single metal. Here we explore cooperativity between transition metals and divalent heavier group 14 elements (tetrylenes), a less-explored facet of the field of cooperativity. Tetrylenes combine their strong σ-donor properties with an empty p-orbital that can accept electron density. This ambiphilicity has allowed them to form metal tetrylene and metallotetrylene complexes that place a reactive site adjacent to the metal. We have selected examples to demonstrate what has been achieved so far regarding cooperative reactivity, as this already spans metal-, tetrylene- or multi-site-centred bond cleavage, cycloaddition, migration, metathesis, and insertion. We also highlight some challenges that need to be overcome for this cooperativity to make it to catalysis.

Enzyme-like Supramolecular Iridium Catalysis Enabling C-H Bond Borylation of Pyridines with meta-Selectivity

The use of secondary interactions between substrates and catalysts is a promising strategy to discover selective transition metal catalysts for atom-economy C-H bond functionalization. The most powerful catalysts are found via trial-and-error screening due to the low association constants between the substrate and the catalyst in which small stereo-electronic modifications within them can lead to very different reactivities. To circumvent these limitations and to increase the level of reactivity prediction in these important reactions, we report herein a supramolecular catalyst harnessing Zn⋅⋅⋅N interactions that binds to pyridine-like substrates as tight as it can be found in some enzymes. The distance and spatial geometry between the active site and the substrate binding site is ideal to target unprecedented meta-selective iridium-catalyzed C-H bond borylations with enzymatic Michaelis-Menten kinetics, besides unique substrate selectivity and dormant reactivity patterns.

Dehydrohalogenation reactions in second-sphere coordination complexes

The latest advances of solid-state dehydrohalogenation and halogenation reactions of hydrogen bonded halometallate salts from the second sphere coordination perspective are reported. Since the second sphere englobes many different materials, our focus has been limited to outer sphere adducts where protonated organic cations act as outer sphere hydrogen bond donors and transition metal anions act as first sphere hydrogen bond acceptors. This is our attempt to analyze dehydrohalogenation/hydrohalogenation reactions viewed as transformations from the second sphere coordination to first sphere coordination of a complex and vice versa. The examples describe a unique solid-state chemistry and reactivity in outer sphere adducts where C-H, N-H and M-X chemical bonds are cleaved and new M-N and H-X bonds are formed (where M = Cu, Zn, Co, Pt, Pd, Hg and X = Cl, Br). The transformations are induced by external stimuli, mainly by mechanochemical and thermal methods. Different reactivities have been observed depending on the lability of the transition metals, the position of the reacting functional groups in the cations and the relative position of organic cations and metal anions. The reverse hydrohalogenation reactions (i.e., from the first sphere coordination to second sphere coordination) via the gas-solid chemisorption process occur even if the materials are non-porous implying a rather dynamic behaviour of these materials. Moreover, due to the implicit changes in the coordination sphere of transition metal ions, dehydrohalogenation/halogenation reactions allow structure-function correlation to be established, for instance involving optical, sensing and magnetic aspects.

Ultrashort Hδ+Hδ- intermolecular distance in a supramolecular system in the solid state

Herein we report experimental evidence for the shortest intermolecular distance reported for two electronically-different hydrogen atoms in the solid state. The Hδ+Hδ- non-covalent interaction was studied using theoretical calculations indicating that electrostatic and dispersion forces are of paramount importance.

New horizons for catalysis disclosed by supramolecular chemistry

The adoption of a supramolecular approach in catalysis promises to address a number of unmet challenges, ranging from activity (unlocking of novel reaction pathways) to selectivity (alteration of the innate selectivity of a reaction, e.g. selective functionalization of C-H bonds) and regulation (switch ON/OFF, sequential catalysis, etc.). Supramolecular tools such as reversible association and recognition, pre-organization of reactants and stabilization of transition states upon binding offer a unique chance to achieve the above goals disclosing new horizons whose potential is being increasingly recognized and used, sometimes reaching the degree of ripeness for practical use. This review summarizes the main developments that have opened such new frontiers, with the aim of providing a guide to researchers approaching the field. We focus on artificial supramolecular catalysts of defined stoichiometry which, under homogeneous conditions, unlock outcomes that are highly difficult if not impossible to attain otherwise, namely unnatural reactivity or selectivity and catalysis regulation. The different strategies recently explored in supramolecular catalysis are concisely presented, and, for each one, a single or very few examples is/are described (mainly last 10 years, with only milestone older works discussed). The subject is divided into four sections in light of the key design principle: (i) nanoconfinement of reactants, (ii) recognition-driven catalysis, (iii) catalysis regulation by molecular machines and (iv) processive catalysis.