Metal-organic Networks Research Articles

Hydrophobic, amorphous metal–organic network readily prepared by complexing the aluminum ion with a siloxane spaced dicarboxylic acid in aqueous medium

ABSTRACTA less common dicarboxylic acid with siloxane spacer, 1,3‐bis(carboxypropyl)tetramethyldisiloxane, whose structure is for the first time determined crystallographically, is used to coordinate aluminum through an environment‐friendly preparation process, resulting in a polymeric structure of amorphous metal–organic framework (aMOF) type. The high flexibility of the spacer induced by the siloxane bond gives the polymeric product a glass transition slightly below room temperature, while the long length of the former creates the premises for collapse of the network and consequently reduced porosity. At the same time, the high hydrophobicity of the tetramethyldisiloxane fragment and its low surface energy, which causes it to migrate to the air interface, gives the network low moisture sorption capacity guaranteeing the stability of its properties in a wet environment. The particulate complex, of the order of 1–200 nm, as generated by the synthesis, proves to be suitable as filler for silicone with outstanding reinforcement effect, without significantly affecting their transparency. The presence of the dimethylsiloxane units in the structure of both the matrix and the filler ensures good incorporation of the latter without the need for special compatibility treatments. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47144.

Redox Isomeric Surface Structures Are Preferred over Odd‐Electron Pt1+

The formation of metal-ligand coordination networks on surfaces that contain redox isomers is a topic of considerable interest and is important for bifunctional metallochemistry, including heterogeneous catalysis. Towards this end, a tetrazine with two electron withdrawing pyrimidinyl substituents was co-deposited with platinum metal on the Au(100) surface. In a 2:1 metal:ligand ratio, only half of the platinum is oxidized to the +2 oxidation state, with the remainder coordinating to the ligand without charge transfer, as Pt0 . The resultant Pt0 /PtII mixed valence structure is thought to form due to the aversion of the ligand towards a four-electron reduction and the strong preference of Pt towards 0 and +2 oxidation states. These results were confirmed through a series of experiments varying the on-surface metal:ligand stoichiometry in the redox assembly formed: added oxidant does not oxidize the already complexed Pt0 . Scanning tunneling microscopy reveals irregular chain structures that are attributed to the mixture of Pt valence states, each with distinct local coordination geometries. Density functional theory calculations give further detail about these local geometries. These results demonstrate the formation of a mixture of valence states in on-surface redox assembly of metal-organic networks that extends the library of single-site metal structures for surface chemistry and catalysis. Redox-isomeric Pt0 versus Pt2+ surface structures can coexist in this ligand environment.

Bonding Motifs in Metal-Organic Compounds on Surfaces.

The bonds in metal organic networks on surfaces govern the resulting geometry as well as the electronic properties. Here, we study the nature of these bonds by forming phenazine-copper complexes on a copper surface by means of atomic manipulation. The structures are characterized by a combination of scanning probe microscopy and density functional theory calculations. We observed an increase of the molecule-substrate distance upon covalent bond formation and an out-of-plane geometry that is in direct contradiction with the common expectation that these networks are steered by coordination bonds. Instead, we find that a complex energy balance of hybridization with the substrate, inhomogeneous Pauli repulsion, and elastic deformation drives the phenazine-copper interaction. Most remarkably, this attractive interaction is not driven by electron acceptor properties of copper but is of completely different donation/back-donation mechanism between molecular π-like orbitals and sp-like metal states. Our findings show that the nature of bonds between constituents adsorbed on surfaces does not have to follow the common categories.

Self-assembled coordination polymers of Zn(II): Syntheses, structural diversity, luminescence properties and base catalysis

Two new Zn(II) metal–organic networks, {[Zn(µ1-muco)(4bpdh)(H2O)]·H2O} (1) and {[Zn2(µ3-muco)2(µ-3bpdb)2(H2O)]·6H2O}n (2) (where, muco = trans,trans-muconate dianion, 4bpdh = 2,5-bis(4-pyridyl)-3,4-diaza-2,4-hexadiene and 3bpdb = 1,4-bis(3-pyridyl)-2,3-diaza-1,3-butadiene) were synthesized at room temperature. Single-crystal X-ray structural determination revealed that compound 1 possesses a 1D coordination polymeric chain extended to a 3D supramolecular framework via (HO⋯H and OH⋯N) hydrogen bonding interactions and compound 2 has a 3D pillar-layered framework with 7-connected, {33·47·58·63} net topology. Photoluminescence measurements of 1 and 2 showed ligand based emissions owing to intra ligand and/or ligand to ligand charge transfer {(π∗ → n and π∗ → π)} transitions. The thermal stabilities of the compounds 1 and 2 were also examined. Application of the compounds as heterogeneous catalysts for the Knoevenagel reaction showed higher catalytic activity of 2 over 1 which has been ascribed to the presence of exposed basic azine (CN) groups in the 1D channels of 2. The catalyst can be easily separated from the reaction mixture and reused for five cycles without significant loss in the catalytic activity. Herein, the influence of ancillary ligands on the structural and catalytic properties of two new Zn(II)–organic networks 1 and 2 has been presented.

A Hybrid Organic Linker Forms an Efficient Non-Precious-Metal-Based Metal–Organic Coordination Network Porous Material for Oxygen Electroreduction Reaction

Proton exchange membrane fuel cells (PEMFCs) are one of a few technological solutions that address both alternative energy demands and sustainability (by virtually converting dihydrogen and O2 to water).1 With the prices of precious metals (e.g. Pt, Pd, Au) remaining high, the electrodes’ materials for this type of devices have to be cost-efficient in order to drive the technology into the marketplace. While the state–of–the–art solutions remain being platinum-based and carbon-supported, the alternatives are highly sought after. One of the potential solutions recognized by the community is to use earth-abundant metal (Ni, Co, Fe)-based systems. The design of efficient functional materials often requires exploration of new modes of connectivity. In porous coordination materials, the nature of an organic linker plays a crucial role. Metal–organic frameworks have recently shown a well-received attention in addressing the need for new solutions in energy conversion and storage.2-4 While symmetric linkers are widely employed, we show that a non-symmetric, electron-rich merged scaffold allows for fabrication of crystalline and amorphous metal-organic coordination networks. We have further utilized them to fabricate a non-supported material that shows high electrochemical oxygen reduction performance in alkaline medium with the onset and half-wave potential higher than that of the state-of-the-art 20 wt% Pt/C catalyst. References M. K. Debe, Electrocatalyst approaches and challenges for automotive fuel cells, Nature, 486 , 43 (2012).H. B. Wu and X. W. Lou, Metal-organic frameworks and their derived materials for electrochemical energy storage and conversion: Promises and challenges, Sci. Adv., 3 , eaap9252 (2017).P.-Q. Liao, J-Q. Shen, and J.-P. Zhang, Metal–organic frameworks for electrocatalysis, Coord. Chem. Rev., (2018) in press, DOI:10.1016/j.ccr.2017.09.001.M. Shao, Q. Chang, J.-P. Dodelet, and R. Chenitz, Recent advances in electrocatalysts for oxygen reduction reaction, Chem. Rev., 116, 3594 (2016).

TCNQ Physisorption on the Topological Insulator Bi2 Se3.

Topological insulators are promising candidates for spintronic applications due to their topologically protected, spin-momentum locked and gapless surface states. The breaking of the time-reversal symmetry after the introduction of magnetic impurities, such as 3d transition metal atoms embedded in two-dimensional molecular networks, could lead to several phenomena interesting for device fabrication. The first step towards the fabrication of metal-organic coordination networks on the surface of a topological insulator is to investigate the adsorption of the pure molecular layer, which is the aim of this study. Here, the effect of the deposition of the electron acceptor 7,7,8,8-tetracyanoquinodimethane (TCNQ) molecules on the surface of a prototypical topological insulator, bismuth selenide (Bi2 Se3 ), is investigated. Scanning tunneling microscope images at low-temperature reveal the formation of a highly ordered two-dimensional molecular network. The essentially unperturbed electronic structure of the topological insulator observed by photoemission spectroscopy measurements demonstrates a negligible charge transfer between the molecular layer and the substrate. Density functional theory calculations confirm the picture of a weakly interacting adsorbed molecular layer. These results reveal significant potential of TCNQ for the realization of metal-organic coordination networks on the topological insulator surface.

Structural Transformation of Guanine Coordination Motifs in Water Induced by Metal Ions and Temperature.

The transformation effects of metal ions and temperature on the DNA base guanine (G) metal-organic coordination motifs in water have been investigated by scanning tunneling microcopy (STM). The G molecules form an ordered hydrogen-bonded structure at the water-highly oriented pyrolytic graphite interface. The STM observations reveal that the canonical G/9H form can be transformed into the G/(3H,7H) tautomer by increasing the temperature of the G solution to 38.6 °C. Moreover, metal ions bind with G molecules to form G4Fe13+, G3Fe32+, and the heterochiral intermixed G4Na1+ metal-organic networks after the introduction of alkali-metal ions in cellular environment.

A novel cluster-organic framework built by the threefold interpenetrating networks and the polyoxometalate cluster units: Synthesis, structure and properties

A new polyoxometalate (POM)-based compound, {Cu5L4[H3W12O40]} [L = 1,6-bis(1,2,4-triazol-1-yl)hexane], has been hydrothermally synthesized and characterized by elemental analysis, IR spectroscopy, UV spectroscopy, X-ray powder diffraction and single crystal X-ray diffraction. In 1, there is a threefold interpenetrating metal-organic network. Furthermore, via sharing Cu cations, the metal-organic networks are fused together by [H3W12O40]5− (W12) polyoxoanions to yield a novel 3D POM cluster-organic framework. Additionally, in contrast to free L ligand, compound 1 exhibits an enhanced luminescent property, attributed to an internal heavy metal effect.

Estudio DFT de las Propiedades Electrónicas y Reactividad Química en el Cluster de Cobre Hexaquis (mu2-Benzoato- O,O')-hexa-cobre(I)

Los MOFs (Metal-Organic Frameworks), son una nueva clase de materiales híbridos microporosos definidos como redes extendidas, conformados por iones metálicos unidos mediante enlaces covalentes coordinados con ligandos orgánicos polifuncionales, formando estructuras en 1D, 2D y 3D. Este estudio computacional evalúa las propiedades electrónicas y reactividad química de una sección de la red metal-orgánico (MOFs) para el cobre con ligandos carboxilato (benzoato) reportado en la Cambridge Structural Database (CSD) denominado clúster de cobre hexaquis (m2-Benzoato-O,O')-hexacobre(I), con el fin de explicar el comportamiento de los sitios activos presentes en su estructura. El cálculo se realizó con el programa Q-Chem 4.0, el cual es utilizado para predicciones precisas de estructuras moleculares, reactividades y espectros vibratorios, electrónicos y de RMN, utilizando la interfaz gráfica de Spartan 14 versión 1.1.4 y el funcional de densidad hibrido B3LYP y la función base 6-31G*.

Structural diversity of alkali metal coordination polymers driven by flexible biphenyl-4,4′-dioxydiacetic acid

A new series of four coordination polymers of alkali metals (Li, Na, and K) was assembled from a poorly explored building block, biphenyl-4,4′-dioxydiacetic acid (H2L), and characterized by FTIR-ATR, TG-DSC, elemental analysis, and single crystal X-ray diffraction methods. The obtained structures range from 2D coordination polymers [Li2(µ-L)(µ-H2O)2(H2O)2] (1) and [Na2(µ4-L)(µ-H2O)2(H2O)2] (2) to 3D metal-organic frameworks [Na2(µ6-L)(µ-H2O)2] (3) and [K2(µ10-L)] (4). The influence of metal ionic radii on the crystal structures of the coordination polymers and conformation of the flexible biphenyl-4,4′-dioxydiacetate ligand was analyzed. Structural complexity increases within the series of products 1–4 following the Li<Na<K trend, accompanied by the evolution of the coordination modes of biphenyl-4,4′-dioxydiacetate ligand from the µ2-L2- (1) and µ4-L2- (2) to µ5-L2- (3) and µ10-L2- (4). Topological classification of the simplified underlying metal-organic networks in 1–4 was performed, disclosing the hcb (1), 3,4L13 (2), and fit (4) topological networks, whereas a topologically unique framework was identified in 3. The obtained coordination polymers are thermally stable and decompose in one (4) or several stages (1–3) forming metal carbonates and/or oxides as final decomposition products. The obtained products 1–4 represent the first examples of alkali metal coordination compounds derived from biphenyl-4,4′-dioxydiacetic acid.

Stabilizing and Organizing Bi3Cu4 and Bi7Cu12 Nanoclusters in Two‐Dimensional Metal–Organic Networks

AbstractMultinuclear heterometallic nanoclusters with controllable stoichiometry and structure are anticipated to possess promising catalytic, magnetic, and optical properties. Heterometallic nanoclusters with precise stoichiometry of Bi3Cu4 and Bi7Cu12 can be stabilized in the scaffold of two‐dimensional metal–organic networks on a Cu(111) surface through on‐surface metallosupramolecular self‐assembly processes. The atomic structures of the nanoclusters were resolved using scanning tunneling microscopy and density functional theory calculations. The nanoclusters feature highly symmetric planar hexagonal shapes and core–shell charge modulation. The clusters are arranged as triangular lattices with a periodicity that can be tuned by choosing molecules of different size. This work shows that on‐surface metallosupramolecular self‐assembly creates unique possibilities for the design and synthesis of multinuclear heterometallic nanoclusters.

Stabilizing and Organizing Bi3 Cu4 and Bi7 Cu12 Nanoclusters in Two-Dimensional Metal-Organic Networks.

Multinuclear heterometallic nanoclusters with controllable stoichiometry and structure are anticipated to possess promising catalytic, magnetic, and optical properties. Heterometallic nanoclusters with precise stoichiometry of Bi3 Cu4 and Bi7 Cu12 can be stabilized in the scaffold of two-dimensional metal-organic networks on a Cu(111) surface through on-surface metallosupramolecular self-assembly processes. The atomic structures of the nanoclusters were resolved using scanning tunneling microscopy and density functional theory calculations. The nanoclusters feature highly symmetric planar hexagonal shapes and core-shell charge modulation. The clusters are arranged as triangular lattices with a periodicity that can be tuned by choosing molecules of different size. This work shows that on-surface metallosupramolecular self-assembly creates unique possibilities for the design and synthesis of multinuclear heterometallic nanoclusters.

Assembly, Fluorescent and Adsorption Properties of Two Cadmium(II)/Cobalt(II) Coordination Complexes Functionalized by the Flexible Bis(Pyridyl)-bis(Amide) Ligand

A new cadmium(II) and a new cobalt(II) coordination complexes [Cd5(BTB)4(HL)2]·2H2O (1) and [Co(HBTB)(L)]·2H2O (2) [H3BTB = 1,3,5-tri(4-carboxylphenyl)benzene, L = N,N′-bis(3-pyridyl)octandiamide] were prepared under the solvothermal conditions by the assembly of cadmium(II)/cobalt(II) chloride, the flexible bis(pyridyl)-bis(amide) ligand L and the aromatic tricarboxylic acid H3BTB. The structures of complexes 1–2 have been characterized by X-ray single crystal diffraction analyses, infrared spectroscopy (IR), powder X-ray diffraction (PXRD) and thermogravimetric (TG) analyses. Complex 1 is a 3D metal–organic network with the penta-nodal (2,4,4,5,6)-connected {4·63·8·10}2{42·65·83}2{43·63}2{46·89}{6}2 topology constructed from the 3D [Cd5(BTB)4]n2n− coordiantion framework and the protonated HL ligands. Complex 2 displays a binodal (4,4)-connected {64·8·10}{6} topological 2D layered architecture based on the 1D [Co(HBTB)]n chains and the 1D [CoL]n chains, which are further linked by hydrogen bonding interactions to form a 3D supramolecular network. Moreover, the fluorescent properties of the cadmium(II) complex and the dyes adsorption performances of complexes 1–2 have been studied.

Lanthanide-Directed Assembly of Interfacial Coordination Architectures-From Complex Networks to Functional Nanosystems.

Metallo-supramolecular engineering on surfaces provides a powerful strategy toward low-dimensional coordination architectures with prospects for several application fields. To date, most efforts have relied on transition metal centers, and only recently did we pioneer lanthanide-directed assembly. Coordination spheres and motifs with rare-earth elements generally display distinct properties and structural features. The size of the cations and shielding role of the 4f orbitals induces high coordination numbers, frequently entailing flexible coordination geometries. Following Pearson's hard and soft acid-base theory, lanthanide cations are hard Lewis acids and thus feature strong affinity for nitrile, terpyridine, and carboxylate donor moieties. The prevailing oxidation state is +3, although in certain compounds stable +2 or +4 cations occur. The chemistry of rare-earth elements is currently receiving widespread attention, as they are key ingredients for established and emerging 21st century science and technology with relevance for energy conversion, sensing, catalysis, magnetism, photonics, telecommunications, superconductivity, biomedicine, and quantum engineering. In this Account, we review recent advances toward the design of interfacial supramolecular nanoarchitectures incorporating lanthanide centers. We apply controlled ultrahigh vacuum conditions whereby atomistically clean substrates are prepared and exposed to ultrapure atomic and molecular beams of the chosen sublimable constituents. We focus on direct molecular-level investigations and in situ assembly operative close to equilibrium conditions. Our scanning probe microscopy techniques provide atomistic insights regarding the formation, stability, and manipulability of metal-organic compounds and networks. In order to gain deeper insights into the experimental findings, complementary computational analysis of bond characteristics, electronic properties, and coordination motifs has been performed for several case studies. Exemplary elements under consideration include cerium, gadolinium, dysprosium, and europium. By the use of ditopic molecular linkers equipped with carbonitrile moieties, adaptive coordination spheres are unveiled, yielding vertices with two- to sixfold symmetry. The respective coordination nodes underlie the expression of complex networks, such as semiregular Archimedean tessellations for cerium- or gadolinium-directed assemblies and random-tiling quasicrystalline characteristics for europium. Tunability via constituent stoichiometry regulation is revealed for bimolecular arrangements embedding europium centers, simultaneously connecting to carbonitrile and terypyridine ligands. Ditopic carboxylate linkers yield robust reticular networks based on a lateral coordination number of 8 for either gadolinium or dysprosium complexation, featuring a prevalent ionic nature of the coordination bond. Orthogonal insertion protocols give rise to d-f reticular architectures exploiting macrocyclic tetradentate cobalt complexation and peripheral carbonitrile-gadolinium coordination, respectively. Furthermore, lanthanides may afford metalation of adsorbed free-base tetrapyrrole species and can be engaged for interfacial synthesis of sandwich compounds, thus providing prospects for columnar design of coordination architectures. Finally, direct manipulation experiments achieved lateral displacement of single supramolecules and molecular rotation of sandwich or other molecular units. These findings evidence prospects for advancing molecular machinery components. The presented accomplishements herald further advancements in metallo-supramolecular design on surfaces, with versatile nanosystems and architectures emanating from the flexible coordination spheres. The embedding and systematic rationalization of lanthanide centers in tailored interfacial environments are keys to establishing relations between structure and physicochemical characteristics toward the generation of novel functionalities with technological significance.

Robust Nanowrapping of Reduced Graphene Oxide by Metal-Organic Network Films between Fe Ions and Tetra(Catechol-Substituted) Porphyrin.

We found the utilization of porphyrin-based metal-organic network films composed of tetra(catechol-substituted)porphyrin (cPor) and Fe ions for robust wrapping materials of graphene oxide (GO), which can keep the dispersion state under the chemical reduction of GO to reduced graphene oxide (rGO) in water. The tetra(catechol-substituted)porphyrin (cPor) was designed for soft-wrapping methods because the aromatic porphyrin moieties can be strongly adsorbed onto the surface of GO or rGO via both π-π interactions and the catechol-Fe coordination network formation. The GO sheets covered with the cPor-Fe films were reduced chemically in water under retention of the wrapped nanostructure of the cPor-Fe/GO sheets. The obtained rGO composites after chemical reduction are characterized by using UV-vis absorption, Raman, and X-ray photoelectron spectroscopy (XPS) spectra, as well as thermogravimetric analysis and energy-dispersive X-ray spectroscopy (EDX). XPS and EDX spectra showed the presence of Fe species, which originates from the coordinated Fe-catechol nodes in the wrapped cPor-Fe films. The wrapped rGO sheets could be easily handled in water because of their high solubility therein and exhibits electric conductivity. In dynamic light scattering analysis, the average diameter of rGO composites before and after reduction changed slightly from 419 ± 309 to 663 ± 697 nm, indicating that the chemical reduction is not significantly influenced by the size of the rGO composite or the solubility. It is expected that the soft wrapping cPor-Fe/rGO should employ the applications to prepare functional materials such as modified electrodes, catalysts, energy-storage materials, and electronic devices.

A multi-functional 3D polymolybdate-based copper(II) complex based on an asymmetric pyridyl-amide ligand: synthesis, structure and properties

A polymolybdate-based metal–organic compound {H2[Cu2(L)2(Mo8O27)(H2O)4]} (1) [L = N-(pyridin-3-yl)isonicotinamide] has been synthesized under solvothermal (methanol–water mixed solvent) conditions and structurally characterized by single-crystal X-ray diffraction. Compound 1 exhibits a 3D metal–organic network containing unusual 1D [Mo8O27] 6− inorganic chains, with adjacent 1D chains connected by Cu(II) atoms and L ligands to form a 3D framework with a trinodal 4,4,6-connected {42.63.8}2{44.68.83} topology. The electrocatalytic activities of compound 1 and its photocatalytic properties for the degradation of organic dyes have also been investigated.

Possibility of a ferromagnetic and conducting metal-organic network

In this paper, we present first principles calculations based on the spin-polarized generalized gradient approximation with on-site Coulomb repulsion term (SGGA + U), to explore the electronic and magnetic properties of the novel planar metal-organic networks TM-Pc and TM-TCNB (where TM means a transition metal of the 3d series: Ti, V, Cr, …, or Zn, Pc - Phthalocyanine, and TCNB - Tetracyanobenzene) as free-standing sheets. This work is an extension of two earlier research works dealing with the Mn (Mabrouk et al., 2015) and Fe (Mabrouk et al., 2017) cases. Our theoretical investigations demonstrate that TM-Pc are more stable than TM-TCNB. Our results unveil that all the TM-Pc frameworks have an insulating behavior with the exception of Mn-Pc which is half-metallic and favor antiferromagnetic order in the case of our magnetic systems except for V-Pc which is ferromagnetic. In contrast, the TM-TCNB networks are metallic at least in one spin direction and exhibit long-range ferromagnetic coupling in case for magnetic structures, which represent ideal candidates and an interesting prospect of unprecedented applications in spintronics. In addition, these results may shed light to achieve a new pathway on further experimental research in molecular spintronics.

Polymorphism and metal-induced structural transformation in 5,5'-bis(4-pyridyl)(2,2'-bispyrimidine) adlayers on Au(111).

Metal-organic coordination networks self-assembled on surfaces have emerged as functional low-dimensional architectures with potential applications ranging from the fabrication of functional nanodevices to electrocatalysis. Among them, bis-pyridyl-bispyrimidine (PBP) and Fe-PBP on noble metal surfaces appear as interesting systems in revealing the details of the molecular self-assembly and the effect of metal incorporation on the organic network arrangement. Herein, we report a combined STM, XPS, and DFT study revealing polymorphism in bis-pyridyl-bispyrimidine adsorbed adlayers on the reconstructed Au(111) surface. The polymorphic structures are converted by the addition of Fe adatoms into one unique Fe-PBP surface structure. DFT calculations show that while all PBP phases exhibit a similar thermodynamic stability, metal incorporation selects the PBP structure that maximizes the number of metal-N close contacts. Charge transfer from the Fe adatoms to the Au substrate and N-Fe interactions stabilize the Fe-PBP adlayer. The increased thermodynamic stability of the metal-stabilized structure leads to its sole expression on the surface.

Frameworks with metal nodes improve organic solar cell power conversion efficiency

First principle calculations assess and optimize the physical and optoelectronic properties of a 2-D metal organic network.

A long-range ordered array of copper tetrameric units embedded in an on-surface metal organic framework.

We report on the assembly of a highly ordered array of copper tetrameric clusters, coordinated into a metal-organic network. The ordered cluster array has been achieved by the deposition of tetrahydroxyquinone molecules on the Cu(111) surface at room temperature, and subsequent thermally activated dehydrogenation with the formation of tetraoxyquinone tetra-anions with a 4 × 4 periodicity. The supramolecular organic network acts as a spacer for the highly ordered two-dimensional network of copper tetramers at the very surface.