Selective molecular recognition by nanoscale environments in a supported iridium cluster catalyst

Remotely Bonded Bridging Dioxygen Ligands Enhance Hydrogen Transfer in a Silica-Supported Tetrairidium Cluster Catalyst

A longstanding challenge in catalysis by noble metals has been to understand the origin of enhancements of rates of hydrogen transfer that result from the bonding of oxygen near metal sites. We investigated structurally well-defined catalysts consisting of supported tetrairidium carbonyl clusters with single-atom (apical iridium) catalytic sites for ethylene hydrogenation. Reaction of the clusters with ethylene and H2 followed by O2 led to the onset of catalytic activity as a terminal CO ligand at each apical Ir atom was removed and bridging dioxygen ligands replaced CO ligands at neighboring (basal-plane) sites. The presence of the dioxygen ligands caused a 6-fold increase in the catalytic reaction rate, which is explained by the electron-withdrawing capability induced by the bridging dioxygen ligands, consistent with the inference that reductive elimination is rate-determining. Electronic-structure calculations demonstrate an additional role of the dioxygen ligands, changing the mechanism of hydrogen transfer from one involving equatorial hydride ligands to that involving bridging hydride ligands. This mechanism is made evident by an inverse kinetic isotope effect observed in ethylene hydrogenation reactions with H2 and, alternatively, with D2 on the cluster incorporating the dioxygen ligands and is a consequence of quasi-equilibrated hydrogen transfer in this catalyst. The same mechanism accounts for rate enhancements induced by the bridging dioxygen ligands for the catalytic reaction of H2 with D2 to give HD. We posit that the mechanism involving bridging hydride ligands facilitated by oxygen ligands remote from the catalytic site may have some generality in catalysis by oxide-supported noble metals.

Electronic Tuning of Gold Nanoparticle Active Sites for Reduction Catalysis

Electronic tuning of active sites in heterogeneous catalysis with organic ligands remains challenging since the ligands are often bound to the most active sites on the catalysts' surfaces. In this work, gold nanoparticles, which are on average less than 2 nm in diameter, are synthesized with strongly binding thiol and phosphine ligands and have measurable quantities of accessible sites on their surfaces in both cases. Triphenylphosphine (TPP) is used as the phosphine ligand, while triphenylmethyl mercaptan (TPMT) serves as the thiol ligand. Phosphines are chosen because they are electron-donating ligands when bound to Au, and thiols are selected because they are electron-withdrawing on the Au surface. X-ray photoelectron spectroscopy (XPS) results show differences in the Au 4f binding energies between the TPP- and TPMT-bound Au nanoparticles. Fourier transform infrared spectroscopy (FTIR) measurements of bound CO indicate that the TPP-bound Au nanoparticles are more electron-rich than the TPMT-bound Au nanoparticles. The number of binding sites on the surface is quantified using 2-naphthalenethiol titration experiments. It is observed that the number of binding sites on the thiol and phosphine-bound Au nanoparticles is similar in both cases. The Au nanoparticles are used for three different reactions: resazurin reduction, CO oxidation, and benzyl alcohol oxidation. For both CO oxidation and benzyl alcohol oxidation, which are performed with the ligands attached, TPP- and TPMT-bound nanoparticles are both catalytically active. However, for resazurin reduction, the TPMT-bound Au nanoparticles are not active, while the TPP-bound Au nanoparticles are catalytically active. These results illustrate that the catalytic activity can be tuned using bound organic ligands with different electronic properties for reduction reactions using Au nanoparticle catalysts.

Surface and Interface Coordination Chemistry Learned from Model Heterogeneous Metal Nanocatalysts: From Atomically Dispersed Catalysts to Atomically Precise Clusters

The surface and interface coordination structures of heterogeneous metal catalysts are crucial to their catalytic performance. However, the complicated surface and interface structures of heterogeneous catalysts make it challenging to identify the molecular-level structure of their active sites and thus precisely control their performance. To address this challenge, atomically dispersed metal catalysts (ADMCs) and ligand-protected atomically precise metal clusters (APMCs) have been emerging as two important classes of model heterogeneous catalysts in recent years, helping to build bridge between homogeneous and heterogeneous catalysis. This review illustrates how the surface and interface coordination chemistry of these two types of model catalysts determines the catalytic performance from multiple dimensions. The section of ADMCs starts with the local coordination structure of metal sites at the metal-support interface, and then focuses on the effects of coordinating atoms, including their basicity and hardness/softness. Studies are also summarized to discuss the cooperativity achieved by dual metal sites and remote effects. In the section of APMCs, the roles of surface ligands and supports in determining the catalytic activity, selectivity, and stability of APMCs are illustrated. Finally, some personal perspectives on the further development of surface coordination and interface chemistry for model heterogeneous metal catalysts are presented.

Cytotoxic sub-nanometer aqueous platinum clusters as potential antitumoral agents

Ligand-free sub-nanometer metal clusters (MCs) of Pt, Ir, Rh, Au and Cu, are prepared here in neat water and used as extremely active (nM) antitumoral agents for HeLa and A2870 cells. The preparation just consists of adding the biocompatible polymer ethylene-vinyl alcohol (EVOH) to an aqueous solution of the corresponding metal salt, to give liters of a MC solution after filtration of the polymer. Since the MC solution is composed of just neat metal atoms and water, the intrinsic antitumoral activity of the different sub-nanometer metal clusters can now fairly be evaluated. Pt clusters show an IC50 of 0.48 μM for HeLa and A2870 cancer cells, 23 times higher than that of cisplatin and 1000 times higher than that of Pt NPs, and this extremely high cytotoxicity also occurs for cisplatin-resistant (A2870 cis) cells, with a resistance factor of 1.4 (IC50 = 0.68 μM). Rh and Ir clusters showed an IC50 ∼ 1 μM. Combined experimental and computational studies support an enhanced internalization and cytotoxic activation.

Microenvironment engineering of supported metal nanoparticles for chemoselective hydrogenation

Selective hydrogenation with supported metal catalysts widely used in the production of fine chemicals and pharmaceuticals often faces a trade-off between activity and selectivity, mainly due to the inability to adjust one factor of the active sites without affecting other factors. In order to solve this bottleneck problem, the modulation of the microenvironment of active sites has attracted more and more attention, inspired by the collaborative catalytic mode of enzymes. In this perspective, we aim to summarize recent advances in the regulation of the microenvironment surrounding supported metal nanoparticles (NPs) using porous materials enriched with organic functional groups. Insights on how the microenvironment induces the enrichment, oriented adsorption and activation of substrates through non-covalent interaction and thus determines the hydrogenation activity and selectivity will be particularly discussed. Finally, a brief summary will be provided, and challenges together with a perspective in microenvironment engineering will be proposed.

Second-Sphere Interaction Promoted Turn-On Fluorescence for Selective Sensing of Organic Amines in a TbIII -based Macrocyclic Framework

Guided by a second-sphere interaction strategy, we fabricated a Tb(III)-based metal-organic framework (MMCF-4) for turn-on sensing of methyl amine with ultra-low detection limit and high turn-on efficiency. MMCF-4 features lanthanide nodes shielded in a nonacoordinate geometry along with secondary coordination spheres that are densely populated with H-bond interacting sites. Nonradiative routes were inhibited by binding-induced rigidification of the ligand on the second coordination sphere, resulting in luminescence amplification. Such remote interacting mechanism involved in the turn-on sensing event was confirmed by single-crystal X-ray diffraction and molecular dynamic simulation studies. The design of both primary and secondary coordination spheres of Tb(III) enabled the first turn-on sensing of organic amines in aqueous conditions. Our work suggests a promising strategy for high-performance turn-on sensing for Ln-MOFs and luminous materials driven by other metal chromophores.

Stability Changes in Iridium Nanoclusters via Monoxide Adsorption: A DFT Study within the van der Waals Corrections

Small iridium nanoclusters are prominent subnanometric systems for catalysis-related applications, mainly because of a large surface-to-volume ratio, noncoalescence feature, and tunable properties, which are completely influenced by the number of atoms, geometry, and molecular interaction with the chemical environment. Herein, we investigate the interaction between Ir_n nanoclusters (n = 2-7) and polluting molecules, CO, NO, and SO, using van der Waals D3 corrected density functional theory calculations. Starting from a representative structural set, we determine the growth pattern of the lowest energy unprotected Ir_n nanoclusters, which is based on open structural motifs, and from the adsorption of a XO (X = C, N, and S) molecule, the preferred high-symmetric adsorption sites were determined, dominated by the onefold top site. For protected systems, 4XO/Ir₄ and 6XO/Ir₆, we found a reduction in the total magnetic moment, while the equilibrium bonds of the nanoclusters expanded (contracted) due to mCO and mNO (mSO) adsorption, with exceptions for systems with large structural distortions (4SO/Ir₄ and 6NO/Ir₆). Meanwhile, the C-O and N-O (S-O) bond strength decreases (increases) following an increase (decrease) in the C-O and N-O (S-O) distances upon adsorption. We show, through energetic analysis, that for the different chemical environments, relative stability changes occur from the most stable unprotected nanoclusters, planar square (Ir₄), and prism (Ir₆) to higher energy isomers. The change in the stability order between the two competing protected systems is feasible if the balance between the interaction energy (additive term) and distortion energies (nonadditive terms) compensates for the relative total energies of the unprotected configurations. For all systems, the interaction energy is the main reason responsible for stability alterations, except for 4SO/Ir₄, where the main contribution is from a small penalty due to Ir₄ distortions upon adsorption, and for 4NO/Ir₄, where the energetic effects from the adsorption do not overcome the difference between the binding energies of the unprotected nanoclusters. Finally, from energy decomposition and Hirshfeld charge analysis, we find a predominant covalent nature of the physical contributions in mOX···Ir_n interactions with a cationic core (Ir_n) and an anionic shell (XO coverage).

Dual-atom Pt heterogeneous catalyst with excellent catalytic performances for the selective hydrogenation and epoxidation

Atomically monodispersed heterogeneous catalysts with uniform active sites and high atom utilization efficiency are ideal heterogeneous catalytic materials. Designing such type of catalysts, however, remains a formidable challenge. Herein, using a wet-chemical method, we successfully achieved a mesoporous graphitic carbon nitride (mpg-C3N4) supported dual-atom Pt2 catalyst, which exhibited excellent catalytic performance for the highly selective hydrogenation of nitrobenzene to aniline. The conversion of ˃99% is significantly superior to the corresponding values of mpg-C3N4-supported single Pt atoms and ultra-small Pt nanoparticles (~2 nm). First-principles calculations revealed that the excellent and unique catalytic performance of the Pt2 species originates from the facile H2 dissociation induced by the diatomic characteristics of Pt and the easy desorption of the aniline product. The produced Pt2/mpg-C3N4 samples are versatile and can be applied in catalyzing other important reactions, such as the selective hydrogenation of benzaldehyde and the epoxidation of styrene.

Synthesis and Catalytic Application of Silver Nanoparticles Supported on Lactobacillus kefiri S-Layer Proteins

Research on nanoparticles obtained on biological supports is a topic of growing interest in nanoscience, especially regarding catalytic applications. Silver nanoparticles (AgNPs) have been studied due to their low toxicity, but they tend to aggregation, oxidation, and low stability. In this work, we synthesized and characterized AgNPs supported on S-layer proteins (SLPs) as bidimensional regularly arranged biotemplates. By different reduction strategies, six AgNPs of variable sizes were obtained on two different SLPs. Transmission electron microscopy (TEM) images showed that SLPs are mostly decorated by evenly distributed AgNPs; however, a drastic reduction by NaBH4 led to large AgNPs whereas a smooth reduction with H2 or H2/NaBH4 at low concentration leads to smaller AgNPs, regardless of the SLP used as support. All the nanosystems showed conversion values between 75-80% of p-nitrophenol to p-aminophenol, however, the increment in the AgNPs size led to a great decrease in Kapp showing the influence of reduction strategy in the performance of the catalysts. Density functional theory (DFT) calculations indicated that the adsorption of p-nitrophenolate species through the nitro group is the most favored mechanism, leading to p-aminophenol as the only feasible product of the reaction, which was corroborated experimentally.

Atomically-precise dopant-controlled single cluster catalysis for electrochemical nitrogen reduction

The ability to precisely engineer the doping of sub-nanometer bimetallic clusters offers exciting opportunities for tailoring their catalytic performance with atomic accuracy. However, the fabrication of singly dispersed bimetallic cluster catalysts with atomic-level control of dopants has been a long-standing challenge. Herein, we report a strategy for the controllable synthesis of a precisely doped single cluster catalyst consisting of partially ligand-enveloped Au₄Pt₂ clusters supported on defective graphene. This creates a bimetal single cluster catalyst (Au₄Pt₂/G) with exceptional activity for electrochemical nitrogen (N₂) reduction. Our mechanistic study reveals that each N₂ molecule is activated in the confined region between cluster and graphene. The heteroatom dopant plays an indispensable role in the activation of N₂ via an enhanced back donation of electrons to the N₂ LUMO. Moreover, besides the heteroatom Pt, the catalytic performance of single cluster catalyst can be further tuned by using Pd in place of Pt as the dopant.

The fabrication of singly dispersed metal cluster catalysts with atomic-level control of dopants is a long-standing challenge. Here, the authors report a strategy for the synthesis of a precisely doped single cluster catalyst which shows exceptional activity for electrochemical dinitrogen reduction.

Intercalation chemistry and thermal characteristics of layered double hydroxides possessing organic phosphonates and sulfonates

The distinct roles of organic sulfonates that enable delamination in water and formation of microporous structures via thermal activation are elucidated.

Well-Defined Materials for Heterogeneous Catalysis: From Nanoparticles to Isolated Single-Atom Sites

The use of well-defined materials in heterogeneous catalysis will open up numerous new opportunities for the development of advanced catalysts to address the global challenges in energy and the environment. This review surveys the roles of nanoparticles and isolated single atom sites in catalytic reactions. In the second section, the effects of size, shape, and metal-support interactions are discussed for nanostructured catalysts. Case studies are summarized to illustrate the dynamics of structure evolution of well-defined nanoparticles under certain reaction conditions. In the third section, we review the syntheses and catalytic applications of isolated single atomic sites anchored on different types of supports. In the final part, we conclude by highlighting the challenges and opportunities of well-defined materials for catalyst development and gaining a fundamental understanding of their active sites.

Ultrasmall Metal Nanoparticles Confined within Crystalline Nanoporous Materials: A Fascinating Class of Nanocatalysts

Crystalline nanoporous materials with uniform porous structures, such as zeolites and metal-organic frameworks (MOFs), have proven to be ideal supports to encapsulate ultrasmall metal nanoparticles (MNPs) inside their void nanospaces to generate high-efficiency nanocatalysts. The nanopore-encaged metal catalysts exhibit superior catalytic performance as well as high stability and catalytic shape selectivity endowed by the nanoporous matrix. In addition, the synergistic effect of confined MNPs and nanoporous frameworks with active sites can further promote the catalytic activities of the composite catalysts. Herein, recent progress in nanopore-encaged metal nanocatalysts is reviewed, with a special focus on advances in synthetic strategies for ultrasmall MNPs (<5 nm), clusters, and even single atoms confined within zeolites and MOFs for various heterogeneous catalytic reactions. In addition, some advanced characterization methods to elucidate the atomic-scale structures of the nanocatalysts are presented, and the current limitations of and future opportunities for these fantastic nanocatalysts are also highlighted and discussed. The aim is to provide some guidance for the rational synthesis of nanopore-encaged metal catalysts and to inspire their further applications to meet the emerging demands in catalytic fields.

Control of Molecular Recognition via Modulation of the Nanoenvironment

Many biological processes are driven by the interaction of a host with a guest molecule. We show such interactions can be modulated by carefully defining the local molecular environment to give a specific chemical outcome. Particularly, the selectivity of a host toward two different ions (Ca²⁺ and Al³⁺) is defined by it being in solution or the physisorbed state. In solution, the host displays greater selectivity toward Ca²⁺. When physisorbed, the selectivity profile of the host is reversed with enhanced binding of Al³⁺. This demonstrates a single host molecule can be tailored to selectively bind multiple guests by altering its nanoenvironment.

Weakly interacting solvation spheres surrounding a calixarene-protected tetrairidium carbonyl cluster: contrasting effects on reactivity of alkane solvent and silica support

The tetrairidium carbonyl cluster Ir4L3(CO)9 (L = tert-butyl-calix[4]arene(OPr)3(OCH2PPh2) (Ph = phenyl; Pr = propyl)) on a partially dehydroxylated silica support undergoes hydrogen activation at a rate and with a mechanism different from those pertaining to the cluster in alkane solution. These results are unobvious in view of the sterically bulky ligands protecting the cluster and the nearly identical CO band frequencies in the infrared spectra characterizing the supported and dissolved Ir4L3(CO)9, both before reaction and during reaction involving decarbonylation in the presence of either helium or H2 (and H2 reacted with the clusters to form hydrides with the same Ir-H band frequencies for clusters in alkane solvent and supported on silica). The initial rates of CO loss from the supported clusters in the presence of helium were the same as those in the presence of H2. The comparison demonstrates that the rate-determining step for hydride formation on the silica-supported cluster is CO dissociation. In contrast, the comparable dissociation of CO from the cluster in n-decane solution requires a higher temperature, 343 K, and is at least an order of magnitude slower than when the clusters were supported on silica. CO dissociation is not the rate-determining step for hydrogen activation on the cluster in n-decane, as the rate is influenced by reactant H2 as well.

Metal Catalysts for Heterogeneous Catalysis: From Single Atoms to Nanoclusters and Nanoparticles

Metal species with different size (single atoms, nanoclusters, and nanoparticles) show different catalytic behavior for various heterogeneous catalytic reactions. It has been shown in the literature that many factors including the particle size, shape, chemical composition, metal-support interaction, and metal-reactant/solvent interaction can have significant influences on the catalytic properties of metal catalysts. The recent developments of well-controlled synthesis methodologies and advanced characterization tools allow one to correlate the relationships at the molecular level. In this Review, the electronic and geometric structures of single atoms, nanoclusters, and nanoparticles will be discussed. Furthermore, we will summarize the catalytic applications of single atoms, nanoclusters, and nanoparticles for different types of reactions, including CO oxidation, selective oxidation, selective hydrogenation, organic reactions, electrocatalytic, and photocatalytic reactions. We will compare the results obtained from different systems and try to give a picture on how different types of metal species work in different reactions and give perspectives on the future directions toward better understanding of the catalytic behavior of different metal entities (single atoms, nanoclusters, and nanoparticles) in a unifying manner.

Bareversusprotected tetrairidium clusters by density functional theory

The lowest energy configuration of the tetrairidium cluster is a square planar isomer in bare case, while the tetrahedral configuration is assumed in different chemical environments.

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.

Dialing in single-site reactivity of a supported calixarene-protected tetrairidium cluster catalyst

A closed Ir₄ carbonyl cluster, 1, comprising a tetrahedral metal frame and three sterically bulky tert-butyl-calix[4]arene(OPr)₃(OCH₂PPh₂) (Ph = phenyl; Pr = propyl) ligands at the basal plane, was characterized with variable-temperature ¹³C NMR spectroscopy, which show the absence of scrambling of the CO ligands at temperatures up to 313 K. This demonstration of distinct sites for the CO ligands was found to extend to the reactivity and catalytic properties, as shown by selective decarbonylation in a reaction with trimethylamine N-oxide (TMAO) as an oxidant, which, reacting in the presence of ethylene, leads to the selective bonding of an ethyl ligand at the apical Ir site. These clusters were supported intact on porous silica and found to catalyze ethylene hydrogenation, and a comparison of the kinetics of the single-hydrogenation reaction and steady-state hydrogenation catalysis demonstrates a unique single-site catalyst-with each site having the same catalytic activity. Reaction orders in the catalytic ethylene hydrogenation reaction of approximately 1/2 and 0 for H₂ and C₂H₄, respectively, nearly match those for conventional noble-metal catalysts. In contrast to oxidative decarbonylation, thermal desorption of CO from silica-supported cluster 1 occurred exclusively at the basal plane, giving rise to sites that do not react with ethylene and are catalytically inactive for ethylene hydrogenation. The evidence of distinctive sites on the cluster catalyst leads to a model that links to hydrogen-transfer catalysis on metals-involving some surface sites that bond to both hydrocarbon and hydrogen and are catalytically engaged (so-called "*" sites) and others, at the basal plane, which bond hydrogen and CO but not hydrocarbon and are reservoir sites (so-called "S" sites).

Atomically dispersed supported metal catalysts: perspectives and suggestions for future research

Catalysts consisting of metal atoms that are atomically dispersed on supports are gaining wide attention because of the rapidly developing understanding of their structures and functions and the discovery of new, stable catalysts with new properties.

Generation of subnanometric platinum with high stability during transformation of a 2D zeolite into 3D

Single metal atoms and metal clusters have attracted much attention thanks to their advantageous capabilities as heterogeneous catalysts. However, the generation of stable single atoms and clusters on a solid support is still challenging. Herein, we report a new strategy for the generation of single Pt atoms and Pt clusters with exceptionally high thermal stability, formed within purely siliceous MCM-22 during the growth of a two-dimensional zeolite into three dimensions. These subnanometric Pt species are stabilized by MCM-22, even after treatment in air up to 540 °C. Furthermore, these stable Pt species confined within internal framework cavities show size-selective catalysis for the hydrogenation of alkenes. High-temperature oxidation-reduction treatments result in the growth of encapsulated Pt species to small nanoparticles in the approximate size range of 1 to 2 nm. The stability and catalytic activity of encapsulated Pt species is also reflected in the dehydrogenation of propane to propylene.

A DFT Study of CO2 Hydrogenation on Faujasite-Supported Ir4 Clusters: on the Role of Water for Selectivity Control

Reaction mechanisms for the catalytic hydrogenation of CO₂ by faujasite‐supported Ir₄ clusters were studied by periodic DFT calculations. The reaction can proceed through two alternative paths. The thermodynamically favoured path results in the reduction of CO₂ to CO, whereas the other, kinetically preferred channel involves CO₂ hydrogenation to formic acid under water‐free conditions. Both paths are promoted by catalytic amounts of water confined inside the zeolite micropores with a stronger promotion effect for the reduction path. Co‐adsorbed water facilitates the cooperation between the zeolite Brønsted acid sites and Ir₄ cluster by opening low‐energy reaction channels for CO₂ conversion.

Partially oxidized iridium clusters within dendrimers: size-controlled synthesis and selective hydrogenation of 2-nitrobenzaldehyde

Iridium clusters nominally composed of 15, 30 or 60 atoms were size-selectively synthesized within OH-terminated poly(amidoamine) dendrimers of generation 6. Spectroscopic characterization revealed that the Ir clusters were partially oxidized. All the Ir clusters efficiently converted 2-nitrobenzaldehyde to anthranil and 2-aminobenzaldehyde under atmospheric hydrogen at room temperature in toluene via selective hydrogenation of the NO2 group. The selectivity toward 2-aminobenzaldehyde over anthranil was improved with the reduction of the cluster size. The improved selectivity is ascribed to more efficient reduction than intramolecular heterocyclization of a hydroxylamine intermediate on smaller clusters that have a higher Ir(0)-phase population on the surface.

C3-symmetry Mo3S4 aminophosphino clusters combining three sources of stereogenicity: stereocontrol directed by hydrogen bond interactions and ligand configuration

A diastereoselective synthesis of proline containing aminophosphino cubane-type Mo3S4 clusters, (P)-[Mo3S4Cl3((1S,2R)-PPro)3]Cl (Cl) and (P)-[Mo3S4Cl3((1S,2S)-PPro)3]Cl (Cl), has been achieved in high yields by reacting the corresponding enantiomerically pure PPro ((R)- and (S)-2-[(diphenylphosphino)methyl]pyrrolidine) ligands with the Mo3S4Cl4(PPh3)3(H2O)2 complex. Circular dichroism, nuclear magnetic resonance and X-ray techniques confirm that the Cl and Cl cluster cations are diastereoisomers which combine three sources of stereogenicity provided by the cluster framework, one carbon atom of the aminophosphine ligand and the nitrogen stereogenic center. The higher stability of the (+) cation is due to stabilizing vicinal ClHN interactions as well as due to the cis-fused conformation of the bicyclic system formed upon coordination of the aminophosphine ligand.

High Catalytic Activity and Chemoselectivity of Sub-nanometric Pd Clusters on Porous Nanorods of CeO2 for Hydrogenation of Nitroarenes

Sub-nanometric Pd clusters on porous nanorods of CeO2 (PN-CeO2) with a high Pd dispersion of 73.6% exhibit the highest catalytic activity and best chemoselectivity for hydrogenation of nitroarenes to date. For hydrogenation of 4-nitrophenol, the catalysts yield a TOF of ∼44059 h(-1) and a chemoselectivity to 4-aminophenol of >99.9%. The superior catalytic performance can be attributed to a cooperative effect between the highly dispersed sub-nanometric Pd clusters for hydrogen activation and unique surface sites of PN-CeO2 with a high concentration of oxygen vacancy for an energetically and geometrically preferential adsorption of nitroarenes via nitro group. The high concentration of surface defects of PN-CeO2 and large Pd dispersion contribute to the enhanced catalytic activity for the hydrogenation reactions. The high chemoselectivity is mainly governed by the high Pd dispersion on the support. The catalysts also deliver high catalytic activity and selectivity for nitroaromatics with various reducible substituents into the corresponding aminoarenes.

Understanding of multimetallic cluster growth

The elucidation of formation mechanisms is mandatory for understanding and planning of synthetic routes. For (bio-)organic and organometallic compounds, this has long been realized even for very complicated molecules, whereas the formation of ligand-free inorganic molecules has widely remained a black box to date. This is due to poor structural relationships between reactants and products and the lack of structurally related intermediates—due to the comparably high coordination flexibility of involved atoms. Here we report on investigations of the stepwise formation of multimetallic clusters, based on a series of crystal structures and complementary quantum-chemical studies of (Ge₂As₂)²⁻, (Ge₇As₂)²⁻, [Ta@Ge₆As₄]³⁻, [Ta@Ge₈As₄]³⁻ and [Ta@Ge₈As₆]³⁻. The study makes use of efficient quantum-chemical tools, enabling the first detailed screening of the energy hypersurface along the formation of ligand-free inorganic species for a semi-quantitative picture. The results can be generalized for an entire family of multimetallic clusters.

Elucidation of formation mechanisms of inorganic cluster compounds is challenging due to the high coordination flexibility of the atoms involved. Here, the authors combine crystallographic and quantum-chemical studies to probe the energy hypersurface of a series of multimetallic clusters.

A propeller-shaped μ4-carbonate hexanuclear dysprosium complex with a high energetic barrier to magnetisation relaxation

A Dy₆ complex composed of two Dy₃ triangular units was isolated and found to exhibit slow relaxation of the magnetisation under zero applied dc field, resulting in a high energetic barrier to relaxation.

Self-Assembly of Silver Metal Clusters of Small Atomicity on Cyclic Peptide Nanotubes

Subnanometric noble metal clusters, composed by only a few atoms, behave like molecular entities and display magnetic, luminescent and catalytic activities. However, noncovalent interactions of molecular metal clusters, lacking of any ligand or surfactant, have not been seen at work. Theoretically attractive and experimentally discernible, van der Waals forces and noncovalent interactions at the metal/organic interfaces will be crucial to understand and develop the next generation of hybrid nanomaterials. Here, we present experimental and theoretical evidence of noncovalent interactions between subnanometric metal (0) silver clusters and aromatic rings and their application in the preparation of 1D self-assembled hybrid architectures with ditopic peptide nanotubes. Atomic force microscopy, fluorescence experiments, circular dichroism and computational simulations verified the occurrence of these interactions in the clean and mild formation of a novel peptide nanotube and metal cluster hybrid material. The findings reported here confirmed the sensitivity of silver metal clusters of small atomicity toward noncovalent interactions, a concept that could find multiple applications in nanotechnology. We conclude that induced supramolecular forces are optimal candidates for the precise spatial positioning and properties modulation of molecular metal clusters. The reported results herein outline and generalize the possibilities that noncovalent interactions will have in this emerging field.

The Birmingham parallel genetic algorithm and its application to the direct DFT global optimisation of Ir(N) (N = 10-20) clusters

A new open-source parallel genetic algorithm, the Birmingham parallel genetic algorithm, is introduced for the direct density functional theory global optimisation of metallic nanoparticles. The program utilises a pool genetic algorithm methodology for the efficient use of massively parallel computational resources. The scaling capability of the Birmingham parallel genetic algorithm is demonstrated through its application to the global optimisation of iridium clusters with 10 to 20 atoms, a catalytically important system with interesting size-specific effects. This is the first study of its type on Iridium clusters of this size and the parallel algorithm is shown to be capable of scaling beyond previous size restrictions and accurately characterising the structures of these larger system sizes. By globally optimising the system directly at the density functional level of theory, the code captures the cubic structures commonly found in sub-nanometre sized Ir clusters.

Probing the energetics of organic-nanoparticle interactions of ethanol on calcite

Knowing the nature of interactions between small organic molecules and surfaces of nanoparticles (NP) is crucial for fundamental understanding of natural phenomena and engineering processes. Herein, we report direct adsorption enthalpy measurement of ethanol on a series of calcite nanocrystals, with the aim of mimicking organic-NP interactions in various environments. The energetics suggests a spectrum of adsorption events as a function of coverage: strongest initial chemisorption on active sites on fresh calcite surfaces, followed by major chemical binding to form an ethanol monolayer and, subsequently, very weak, near-zero energy, physisorption. These thermochemical observations directly support a structure where the ethanol monolayer is bonded to the calcite surface through its polar hydroxyl group, leaving the hydrophobic ends of the ethanol molecules to interact only weakly with the next layer of adsorbing ethanol and resulting in a spatial gap with low ethanol density between the monolayer and subsequent added ethanol molecules, as predicted by molecular dynamics and density functional calculations. Such an ordered assembly of ethanol on calcite NP is analogous to, although less strongly bonded than, a capping layer of organics intentionally introduced during NP synthesis, and suggests a continuous variation of surface structure depending on molecular chemistry, ranging from largely disordered surface layers to ordered layers that nevertheless are mobile and can rearrange or be displaced by other molecules to strongly bonded immobile organic capping layers. These differences in surface structure will affect chemical reactions, including the further nucleation and growth of nanocrystals on organic ligand-capped surfaces.

Molecular metal catalysts on supports: organometallic chemistry meets surface science

Recent advances in the synthesis and characterization of small, essentially molecular metal complexes and metal clusters on support surfaces have brought new insights to catalysis and point the way to systematic catalyst design. We summarize recent work unraveling effects of key design variables of site-isolated catalysts: the metal, metal nuclearity, support, and other ligands on the metals, also considering catalysts with separate, complementary functions on supports. The catalysts were synthesized with the goal of structural simplicity and uniformity to facilitate incisive characterization. Thus, they are essentially molecular species bonded to porous supports chosen for their high degree of uniformity; the supports are crystalline aluminosilicates (zeolites) and MgO. The catalytic species are synthesized in reactions of organometallic precursors with the support surfaces; the precursors include M(L)2(acetylacetonate)1-2, with M = Ru, Rh, Ir, or Au and the ligands L = C2H4, CO, or CH3. Os3(CO)12 and Ir4(CO)12 are used as precursors of supported metal clusters, and some such catalysts are made by ship-in-a-bottle syntheses to trap the clusters in zeolite cages. The simplicity and uniformity of the supported catalysts facilitate precise structure determinations, even in reactive atmospheres and during catalysis. The methods of characterizing catalysts in reactive atmospheres include infrared (IR), extended X-ray absorption fine structure (EXAFS), X-ray absorption near edge structure (XANES), and nuclear magnetic resonance (NMR) spectroscopies, and complementary methods include density functional theory and atomic-resolution aberration-corrected scanning transmission electron microscopy for imaging of individual metal atoms. IR, NMR, XANES, and microscopy data demonstrate the high degrees of uniformity of well-prepared supported species. The characterizations determine the compositions of surface metal complexes and clusters, including the ligands and the metal-support bonding and structure, which identify the supports as ligands with electron-donor properties that influence reactivity and catalysis. Each of the catalyst design variables has been varied independently, illustrated by mononuclear and tetranuclear iridium on zeolite HY and on MgO and by isostructural rhodium and iridium (diethylene or dicarbonyl) complexes on these supports. The data provide examples resolving the roles of the catalyst design variables and place the catalysis science on a firm foundation of organometallic chemistry linked with surface science. Supported molecular catalysts offer the advantages of characterization in the absence of solvents and with surface-science methods that do not require ultrahigh vacuum. Families of supported metal complexes have been made by replacement of ligands with others from the gas phase. Spectroscopically identified catalytic reaction intermediates help to elucidate catalyst performance and guide design. The methods are illustrated for supported complexes and clusters of rhodium, iridium, osmium, and gold used to catalyze reactions of small molecules that facilitate identification of the ligands present during catalysis: alkene dimerization and hydrogenation, H-D exchange in the reaction of H2 with D2, and CO oxidation. The approach is illustrated with the discovery of a highly active and selective MgO-supported rhodium carbonyl dimer catalyst for hydrogenation of 1,3-butadiene to give butenes.