Paddle-wheel Units Research Articles

Trans Influence across a Metal-Metal Bond of a Paddle-Wheel Unit on Interaction with Gases in a Metal-Organic Framework.

Metal-organic frameworks (MOFs) are known as promising adsorbent materials that can recognize gases specifically. In the frameworks, gases favor interacting with specific binding sites such as open metal sites (OMSs), which can consist of various metals and show characteristic adsorption properties. A recently reported framework possessing OMSs of rhodium paddle-wheel (Rh-PW) showed distinct adsorption properties between NO and CO. We investigated theoretically the reasons for stronger NO binding to the Rh-PW and different adsorption amounts between NO and CO using Rh-PW cluster models, as well as the frequently reported Cu-PW for comparison. We also analyzed the cases of CO2 and N2, which are often used to probe functions of MOFs. We observed an increase in binding energy of NO at the second adduction of NO. On the basis of energy decomposition analysis, we found that Rh-NO bond formation inducing a trans influence is important for the stronger binding than with CO. Furthermore, we proposed a reason for twice the adsorption amount of NO than CO. The results are consistent with experimental observations, giving us insight into design functions of MOFs by selecting metal species.

Scrutinizing the Pore Chemistry and the Importance of Cu(I) Defects in TCNQ-Loaded Cu3(BTC)2 by a Multitechnique Spectroscopic Approach.

Host-guest interactions control the fundamental processes in porous materials for many applications such as gas storage and catalysis. The study of these processes, however, is not trivial, even if the material is crystalline. In particular, metal-organic frameworks (MOFs) represent a complex situation since guest molecules can interact with different parts of the organic linkers and the metal clusters and may alter the details of the pore structure and system properties. A prominent example is the so-called retrofitted MOF material TCNQ@Cu3(BTC)2 that has attracted a lot of attention due to its electronic properties induced by the host-guest interactions. Only recently, structural evidence has been presented for a bridging binding mode of TCNQ to two Cu paddlewheel units; however, many issues regarding the redox chemistry of Cu3(BTC)2 and TCNQ are currently unsolved. Herein, we report a powerful spectroscopic approach to study the host-guest chemistry of this material. Combining IR spectroscopy in the presence of CO and EPR spectroscopy, we found that the intrinsic Cu(I) defects of the host react with the guest, forming TCNQ radical anions. This chemistry has profound implications, in particular, with respect to the performance of TCNQ@Cu3(BTC)2 as an electronic conductor. A decreasing availability of open Cu(II) sites with increasing TCNQ loading proves the coordinative binding of TCNQ to the paddlewheel nodes, and a heterogeneous structure is formed with different TCNQ arrangements and pore environments at low TCNQ loadings. Finally, the combined use of spectroscopic characterization techniques has proven to be, in general, a powerful approach for studying the complex chemistry of host-guest materials.

Dyes encapsulated in a novel flexible metal−organic framework show tunable and stimuli-responsive phosphorescence

A novel flexible metal-organic framework (MOF), [ZnL]∙H2O∙DMA (USTC-1), with interesting topology has been constructed based on a new T-shaped ligand, 5-(2-chloroimidazo[1,2-a]pyridine-3-carboxamido)isophthalic acid (H2L). This MOF was characterized by IR spectroscopy, thermogravimetry, single-crystal, and powder X-ray diffraction methods. The single-crystal analysis reveals four carboxylate groups connect two zinc ions to generate a paddlewheel secondary building unit (SBU). Each paddlewheel SBU is six-connected to six L2− ligands, and each L2− is three-connected to three SBUs, thus, the framework can be described as a binodal (3,6)-connected three-dimensional (3D) flexible network. The low N2 and CO2 gas uptake should be attributed to the flexible vibration of amide chains at ligands that close the pore window, however, the closed pore window can be opened by dye molecules, such as Rhodamine B (RB), Methyl Orange (MO) or Methylene Blue (MB). Two methods for dye encapsulation were investigated: soaking and in situ encapsulation. PXRD data showed that the framework in USTC-1 was unchanged after encapsulation of dyes. Interestingly, RB@USTC-1 simultaneously displayed the characteristic emissions of both the RB dye and the MOF, resulting in red/blue two-color luminescence with intensity ratio of 4.5 and tunable phosphorescence color. Moreover, RB@USTC-1 and MO@USTC-1 could undergo protonation upon exposure to HCl vapor, and reveal reversible phosphorescent color switching in the response to acid-base vapor stimuli. The present work provides a promising approach for synthesizing novel flexible MOFs and a new access to develop the phosphorescent and stimuli-responsive MOF materials via encapsulation of various guests.

A stable pillared metal–organic framework constructed by H4TCPP ligand as luminescent sensor for selective detection of TNP and Fe3+ ions

AbstractA novel luminescent metal–organic framework (Zn‐TCPP/BPY) with pillared structure based on 2,3,5,6‐tetrakis(4‐carboxyphenyl)pyrazine (H4TCPP) and 4,4′‐bipyridine (BPY) has been designed and synthesized through a solvothermal reaction. The [Zn2(COO)4] paddlewheel units are linked by TCPP4− ligands to form two‐dimensional layers and further connected by BPY ligands as pillars to construct the twofold interpenetrating three‐dimensional framework. Interestingly, Zn‐TCPP/BPY possesses outstanding stability in organic solvents and water as well as maintains its structural rigidity in aqueous solutions of different pH values (3–12). After activation, Zn‐TCPP/BPY possesses permanent porosity with Brunauer–Emmett–Teller surface area of 630 m2 g–1. Remarkably, Zn‐TCPP/BPY displays excellent fluorescent property in virtue of the aggregation‐induced emission effect of the H4TCPP ligand, which can be highly active and quenched by small amounts of 2,4,6‐trinitrophenol (TNP) and Fe3+ ions. Furthermore, the detection effect of Zn‐TCPP/BPY remains basically the same even after five cycles. The excellent stability, high sensitivity, and recyclability of Zn‐TCPP/BPY make it an outstanding chemical sensor for detecting TNP and Fe3+ ions.

Selective Gas Adsorption of Alkane/Alkene in a Single-Crystal and Powder Bimetallic Metal–Organic Framework Compound Cu2.97Zn0.03(btc)2 Studied by Electron Paramagnetic Resonance

The metal–organic framework HKUST-1 is an attractive material for gas storage and gas separation applications because of its large surface area and high pore volume, and the presence of coordinatively unsaturated (cus) cupric ion sites. We investigated the interaction of alkenes and alkanes (ethene, 1-butene, ethane, and butane) with Cu2+ ions in their Zn-doped variant Cu2.97Zn0.03(btc)2 by continuous-wave electron paramagnetic resonance spectroscopy. Powder and single-crystal experiments revealed a characteristic change of cupric ion coordination from square planar to square pyramidal for the adsorption of ethylene and 1-butene and confirmed the formation of adsorption complexes at the cus sites of the Cu2+ ion in the mixed metal ion Cu–Zn paddle wheel (PW) units. Quantum chemical calculations based on density functional theory established a weak specific interaction between alkene π-orbitals and Cu2+ ions in a side-on coordination geometry. In contrast, no direct interaction between alkanes and the metal ions was found for this porous material. However, adsorption of ethane and n-butane leads to significant strain effects in the HKUST-1 framework which can be indirectly sensed by the cupric ions in the paramagnetic Cu–Zn PW units.

Mono- and poly-nuclear copper(II) carboxylates with flourous ligands: Synthesis, structure and improved properties

Polynuclear copper(II) complexes [Cu(para-fluorophenyl acetate)2]n (1) and [Cu(para-nitrophenyl acetate)2]n (2) were converted in-situ to mono-nuclear complexes [Cu(para-fluorophenyl acetate)2(1,10-Phenanthroline)(H2O)] (3) and [Cu(para-nitrophenyl acetate)2(2,2́-Bipyridine)(H2O)].3H2O (4). The complexes were isolated in quantitative yield, purified and characterized using FT-IR, absorption, electrochemical, electron paramagnetic resonance and powder XRD techniques yielding results in accordance with structural data. Structures of 1 and 2 consist of directly interlinked paddlewheel units with square pyramidal geometry while those of 3 and 4 are mononuclear with octahedral and square pyramidal geometry around Cu. Fluorinated complexes 1 and 3 were found to consist of extensive intermolecular interactions while 2 and 4 contained no such interactions. Their optical band gap was around 1.4 eV which indicates semiconducting property of the complexes. Moreover, the flourous complex 3 was found to possess significant activity against Bacillus subtilis and Escherichia coli and good activity against Micrococcus luteus. A similar trend was observed in their DNA-binding potency studied through absorption as well as electrochemical solution studies yielding Kb values 1.494 × 104, 1.342 × 104 and 1.411 × 104 M−1 and 7.547 × 104, 2.457 × 104 and 3.667 × 104 M−1 for 1–3, respectively. The electronic structures of the complexes elucidated using density functional theory ab-initio calculations confirmed the Intermediate states which play important role in the enhancement of the optoelectronic properties. The structure and properties of the flourous complexes were found different than their non-flourous analogues which was attributable to fluorine.

A metal organic cage with semi-rigid ligand for heterogeneous alcoholysis of epoxides

A metal organic cage, [Cu6(L)4(DMF)2(H2O)4](1,4-Dioxane)2(DMF)10(H2O)4 (H3L = 4,4′,4″-(benzene-1,3,5-triyltris(oxy))tribenzoic acid) (FJU-18), has been constructed by six Cu2(CO2)4 paddle-wheel building units and eight semi-rigid L. The porous FJU-18 containing large cage of 1.6 nm shows high catalytic activity for the ring opening of styrene oxide in methanol at 65 °C under nitrogen atmosphere.

Radical-Doped Metal-Organic Framework: Route to Nanoscale Defects and Magnetostructural Functionalities.

Nanosized structural defects in metal-organic frameworks (MOFs) attract growing attention and often remarkably enhance functional properties of these materials for various applications. In this work, a series of MOFs [Cu2(TPTA)1- x(BDPBTR) x] (H4TPTA, [1,1':3',1″-terphenyl]-3,3'',5,5''-tetracarboxylic acid; H4BDPBTR, 1,3-bis(3,5-dicarboxyphenyl)-1,2,4-benzotriazin-4-yl radical)) with a new stable radical linker doped into the structure has been synthesized and investigated using Electron Paramagnetic Resonance (EPR). Mixed linkers H4TPTA and H4BDPBTR were used to bridge copper(II) paddle-wheel units into a porous framework, where H4BDPBTR is the close structural analogue of H4TPTA. MOFs with various x = 0-0.4 were investigated. EPR studies indicated that the radical linker binds to the copper(II) units differently compared to diamagnetic linker, resulting in the formation of nanosized structural defects. Moreover, remarkable kinetic phenomena were observed upon cooling of this MOF, where slow structural rearrangements and concomitant changes of magnetic interactions were induced. Thus, our findings demonstrate that doping of structurally mimicking radical linkers into MOFs represents an efficient approach for designing target nanosized defects and introducing new magnetostructural functionalities for a variety of applications.

One-, two- and three-dimensional coordination polymers based on copper paddle-wheel SBUs as selective catalysts for benzyl alcohol oxidation

Copper-based coordination polymers, Cu2(μ2-MeCO2)4(DABCO) (1), [Cu(1,4-BDC-Br)(DABCO)0.5]. xDMF.yH2O (2) and Cu(1,4-BDC-OH)(DMF) (3) (1,4-BDC-Br = 2-bromoterephthalate, DABCO = triethylenediamine, 1,4-BDC-OH = 2-hydroxyterephthalate) were solvothermally synthesized. Two polymorphs of 1 (1a and 1b) were obtained which both contain 1-D chains with similar connectivity and different packing. Compound 2 is a 3-D metal-organic framework (MOF) and 3 exhibits a 2-D structure. Structural analyses of the coordination polymers showed that they all have similar paddle-wheel secondary building units (SBUs). The coordination polymers 1–3 were characterized by FT-IR, thermogravimetric analysis (TGA), powder X-ray diffraction (XRD) and atomic absorption spectroscopy. Compound 2 is further analyzed by nitrogen adsorption/desorption technique. The obtained coordination compounds were employed as heterogeneous catalysts for selective oxidation of benzyl alcohol to benzoic acid using CH2Cl2 as solvent and tert-butyl hydroperoxide (TBHP) as oxidant. The catalysts displayed good activity in the oxidation reaction and could be reused without noticeable loss of activity.

Pore Space Partitioning of Metal-Organic Framework for C2H x Separation from Methane.

To develop efficient adsorbent materials for C2H x separation from methane, herein, we design a new 3-D framework (NbU-5) using the pore space partition strategy: Zn2(O2CR)4 paddlewheel units connect tetra-carboxylic linkers to build a primary framework, and 4,4-bipyridine, which acts as a pore-partitioning agent, has been successfully inserted into the cage of the primary framework. Remarkably, NbU-5 exhibits excellent C2H2/CH4 and C2H6/CH4 selectivity, which is proved by breakthrough experiments.

A Metal-organic Framework with Paddle-wheel Zn2(CO2)4 Secondary Building Units and Cubane-1,4-dicarboxylic Acid Linkers

A new crystal structure of catena-(bis((µ4-cubane-1,4-dicarboxylato)-(N-methyl-2-pyrrolidone)-zinc(II)) N-methyl-2-pyrrolidone solvate) (1) was prepared by solvothermal method. The crystal structure of the compound was analyzed by single-crystal X-ray diffraction. It has a P1 space group, with lattice parameters a = 10.7190(4) Å, b = 10.8245(5) Å, c = 10.8403(8) Å, α = 80.291(5)°, β = 70.0015(5)°, γ = 77.531(4)°, V = 1147.97(12) Å3. The secondary building units of 1 consist of 2 central Zn ions, coordinated by 4 carboxylate groups in a bis-monodentate way, forming a square planar configuration of Zn2(CO2)4, known as paddle-wheel units. The paddle-wheels are connected by cubane-1,4-dicarboxylic acid linkers at the edges, resulting in a two-dimensional coordination polymer with a square lattice (sql) underlying network topology. The axial sites of the zinc atoms are occupied by N-methyl-2-pyrrolidone molecules. In this new crystal structure the two-dimensional polymer planes are interstacked by weak dispersion bonds. The axial N-methyl-2-pyrrolidone solvent molecules determine the distances of planar polymer planes. The thermal properties of this new structure were studied by thermogravimetry/mass spectrometry in inert atmosphere. It was found, that the organic linkers in the framework structure do not decompose below 200 °C. The stochiometry of the activated compound is Zn2[C8H6(COO)2]2(C5H9NO)2, as determined by thermogravimetry in oxidative atmosphere.

Synthesis and Characterization of Cu-Ni Mixed Metal Paddlewheels Occurring in the Metal-Organic Framework DUT-8(Ni0.98Cu0.02) for Monitoring Open-Closed-Pore Phase Transitions by X-Band Continuous Wave Electron Paramagnetic Resonance Spectroscopy.

A Cu2+-doped metal-organic framework (DUT-8(Ni0.98Cu0.02), M2(NDC)2DABCO, M = Ni, Cu, NDC = 2,6-napththalene dicarboxylate, DABCO = 1,4-diazabicyclo[2.2.2]octane, DUT = Dresden University of Technology) was synthesized in the form of large (>1 μm) and small crystals (<1 μm) to analyze their switchability by X-band continuous wave (cw) electron paramagnetic resonance (EPR) spectroscopy. The large crystals are flexible and in a porous open pore (op) phase after solvation in N, N-dimethylformamide (DMF), but in the activated solvent-free form, a nonporous closed pore (cp) phase forms. EPR measurements of the rigid Ni-free DUT-8(Cu) show a characteristic electron spin S = 1 room temperature signal of the antiferromagnetically coupled Cu2+-Cu2+ paddlewheel building unit of this metal-organic framework. None of the mixed metal DUT-8(Ni0.98Cu0.02) materials showed comparable signals, indicating the absence of dimeric Cu2+-Cu2+ paddlewheel units in the materials. Instead, characteristic electron spin S = 3/2 signals are detected for all DUT-8(Ni0.98Cu0.02) samples at temperatures T < 77 K, which can be assigned to ferromagnetically coupled mixed metal Ni2+-Cu2+ paddlewheel units. Those signals differ characteristically for the op and cp phase and enable monitoring the reversible op-cp transition during the de-/adsorption of DMF.

Synthesis, crystal structure and photoluminescence of a three-dimensional zinc coordination compound with NBO-type topology.

The assembly of metal-organic frameworks (MOFs) with metal ions and organic ligands is currently attracting considerable attention in crystal engineering and materials science due to their intriguing architectures and potential applications. A new three-dimensional MOF, namely poly[[diaqua(μ8-para-terphenyl-3,3',5,5'-tetracarboxylato)dizinc(II)] dimethylformamide disolvate monohydrate], {[Zn2(C22H10O8)(H2O)2]·2C3H7NO·H2O}n, was synthesized by the self-assembly of Zn(NO3)2·6H2O and para-terphenyl-3,3',5,5'-tetracarboxylic acid (H4TPTC) under solvothermal conditions. The compound was structurally characterized by FT-IR spectroscopy, elemental analysis and single-crystal X-ray diffraction analysis. Each ZnII ion is located in a square-pyramidal geometry and is coordinated by four carboxylate O atoms from four different TPTC4- ligands. Pairs of adjacent equivalent ZnII ions are bridged by four carboxylate groups, forming [Zn2(O2CR)4] (R = terphenyl) paddle-wheel units. One aqua ligand binds to each ZnII centre along the paddle-wheel axis. Each [Zn2(O2CR)4] paddle wheel is further linked to four terphenyl connectors to give a three-dimensional framework with NBO-type topology. The thermal stability and solid-state photoluminescence properties of the title compound have also been investigated.

Postsynthetic Covalent and Coordination Functionalization of Rhodium(II)-Based Metal-Organic Polyhedra.

Metal-organic polyhedra (MOP) are ultrasmall (typically 1-4 nm) porous coordination cages made from the self-assembly of metal ions and organic linkers and are amenable to the chemical functionalization of its periphery; however, it has been challenging to implement postsynthetic functionalization due to their chemical instability. Herein, we report the use of coordination chemistries and covalent chemistries to postsynthetically functionalize the external surface of ≈2.5 nm stable Rh(II)-based cuboctahedra through their Rh-Rh paddlewheel units or organic linkers, respectively. We demonstrate that 12 N-donor ligands, including amino acids, can be coordinated on the periphery of Rh-MOPs. We used this reactivity to introduce new functionalities (e.g., chirality) to the MOPs and to tune their hydrophilic/hydrophobic characteristics, which allowed us to modulate their solubility in diverse solvents such as dichloromethane and water. We also demonstrate that all 24 organic linkers can be postsynthetically functionalized with esters via covalent chemistry. In addition, we anticipate that these two types of postsynthetic reactions can be combined to yield doubly functionalized Rh-MOPs, in which a total of 36 new functional molecules can be incorporated on their surfaces. Likewise, these chemistries could be synergistically combined to enable covalent functionalization of MOPs through new linkages such as ethers. We believe that both reported postsynthetic pathways can potentially be used to engineer Rh-MOPs as scaffolds for applications in delivery, sorption, and catalysis.

Probing Local Structural Changes at Cu2+ in a Flexible Mixed-Metal Metal-Organic Framework by in Situ Electron Paramagnetic Resonance during CO2 Ad- and Desorption

The flexible mixed-metal pillared-layered metal-organic framework (MOF) Zn1.9Cu0.1(BME-bdc)2(dabco) (BME-bdc2– = 2,5-bis(2-methoxyethoxy)-1,4-benzenedicarboxylate, dabco = diazabicyclo[2.2.2]octane) with Cu2+ dopants as electron paramagnetic resonance (EPR) active probes was synthesized, and the guest-induced phase transition was studied by a newly developed in situ gas sorption EPR setup. Within the structure, the Cu2+ ions do not build any Cu2+–Cu2+ paddlewheels, and only mixed-metal Zn2+–Cu2+ paddlewheels were identified by EPR. Two distinct Zn2+–Cu2+ species (A and B) occur in the structure: Species A occurs in regions with low Cu2+ concentrations and has no adjacent mixed-metal paddlewheel units. Species B is present in domains with higher local copper concentrations, where adjacent mixed-metal paddlewheel units alter the EPR signal. The parent monometallic Zn MOF is known to undergo phase transitions from a narrow pore (np) to a large pore (lp) phase due to host-guest interactions with CO2. In situ ...

Two zinc(ii)-based coordination polymers with flexible dicarboxylate and pyridine mixed ligands: effect of π⋯π interactions on electrical activity

We synthesized two Zn(ii)-based 1D coordination polymers with paddle-wheel units using flexible 1,4-cyclohexanedicarboxylate linkers; these compounds exhibited electrical conductivity and Schottky barrier diode behavior.

Structural diversity of zinc(ii) coordination polymers with octafluorobiphenyl-4,4′-dicarboxylate based on mononuclear, paddle wheel and cuboidal units

Certain modifications of the synthetic conditions lead to 6 novel coordination polymers, thus enriching the coordination chemistry of perfluorinated ligands.

Cobalt substitution in a flexible metal-organic framework: modulating a soft paddle-wheel unit for tunable gate-opening adsorption.

Developing feasible ways to achieve tunable gate-opening pressure (Pgo) while minimizing the side effects on the adsorption capacity and enthalpy is greatly desired for flexible MOFs. In this work, we focused on solving this issue by cobalt substitution. We showed the successful modulation of the energy required for the reversible transformation of a soft paddle-wheel so that the whole framework presented a substitution-dependent Pgo for CO2 adsorption.

Mixed precious-group metal-organic frameworks: a case study of the HKUST-1 analogue [RuxRh3-x(BTC)2

This work presents the first full series of mixed precious-group metal-organic frameworks (MPG-MOFs) using ruthenium and rhodium. The obtained crystalline, highly porous and thermally robust materials were characterized by means of powder X-ray diffraction, N2/CO2 sorption isotherms, thermogravimetry, spectroscopy methods (IR, Raman, UV/VIS-, NMR and XPS) and as well by high resolution transmission electron microscopy (HR-TEM) with elemental mapping (HAADF-EDS). Additionally, the assignment of spectroscopic data is supported by computational (time dependent)-density functional theory methods. The materials turned out to consist of homogeneously dispersed Ru2 and Rh2 paddlewheel units being linked by benzenetricarboxylate (BTC) to yield a framework that is isoreticular to [Cu3(BTC)2] (HKUST-1, Hong Kong University of Science and Technology). However, acetate (OAc) is incorporated as an intrinsic component which compensates for missing BTC-linker defects and some Cl is coordinated to the Ru centre at an apical position. The exact empirical formula of the MPG-MOFs is derived as [RuxRh3-x(BTC)2-a(OAc)b(Cl)c].

Two New MOFs Based on Cu2 Paddlewheel Units and Biphenyltetracarboxylate Ligands with a Different Degree of Fluorination

Two new MOFs (metal‐organic frameworks), namely 3∞[Cu2(H2O)2(L)]·3DMF·H2O [L = 4‐fluoro‐biphenyl‐3,3′,5,5′‐tetracarboxylate (4‐mF‐BPTC4–) and 4,4′‐difluoro‐biphenyl‐3,3′,5,5′‐tetracarboxylate (4,4′‐dF‐BPTC4–)] termed UoC‐1(1F) and UoC‐1(2F) (UoC ≡ University of Cologne), were synthesized by solvothermal reactions in DMF. The crystal structure of UoC‐1(1F) was solved and refined from X‐ray single crystal data (Imma, Z = 4). The X‐ray powder diffraction data of UoC‐1(2F) confirmed that both MOFs are isostructural. In their crystal structures each of the four carboxylate groups of the BPTC4– linker is involved in a Cu2 paddlewheel unit leading to a 3D structure with high porosity, which was confirmed by N2 gas sorption measurements revealing specific surface areas of SBET = 1256 m2·g–1 [UoC‐1(1F)] and 1162 m2·g–1 [UoC‐1(2F)] after activation at 120 °C for 72 h. Higher activation temperatures lead to a decomposition of the framework and significantly decreased specific surface areas.