Paddle-wheel Units Research Articles

Metal–organic aerogels based on dinuclear rhodium paddle-wheel units: design, synthesis and catalysis

A metal–organic aerogel (MOA-Rh-1d) with Rh2+–Rh2+ bonds has been obtained, which can efficiently promote CO2 conversion and C–H amination reactions.

Density functional calculations reveal a flexible version of the copper paddlewheel unit: implications for metal organic frameworks.

Density functional theory calculations on [Cu2(O2CR)4L2] systems reveal a change in ground state with increasing Cu-L bond strength. For L = N-heterocyclic carbene (NHC), the Jahn-Teller axis switches from parallel to orthogonal to the Cu-Cu vector and the copper coordination geometry becomes highly flexible. While the calculated dimer/monomer equilibrium for isolated complexes slightly favours monomers, the preformed paddlewheel units embedded in many metal organic frameworks are potential targets for developing novel materials.

Hetero-bimetallic paddlewheel clusters in coordination polymers formed by a water-induced single-crystal-to-single-crystal transformation.

Herein we report a water-induced single-crystal to single-crystal transformation that involves the formation of hetero-bimetallic paddlewheel clusters in coordination polymers. Through this transformation, which involves the cleavage and formation of different coordination bonds, two different Cu(ii)-Zn(ii) and Cu(ii)-Ni(ii) paddlewheel units exhibiting a 1 : 1 metal ratio were created.

Adsorption and Desorption of HD on the Metal–Organic Framework Cu2.97Zn0.03(Btc)2Studied by Three-Pulse ESEEM Spectroscopy

Cu2.97Zn0.03(btc)2 is a structural analogue of the well-known HKUST-1 metal–organic framework. In this compound 1% of the Cu2+ ions in the paddle-wheel units are substituted by Zn2+, resulting in the formation of Cu/Zn paddle-wheel units in low concentration. The paramagnetic Cu2+ ions of these mixed Cu/Zn pairs allow to perform pulsed electron paramagnetic resonance experiments at low temperatures. Here we report on the three-pulse electron spin echo envelope modulation (3p ESEEM) study of the deuterated hydrogen gas HD adsorption and desorption in Cu2.97Zn0.03(btc)2. The HD adsorption sites in this modified compound were identified by precisely simulating experimentally observed 3p ESEEM time domain pattern. To elucidate the HD desorption process, the 3p ESEEM experiments were performed at different temperatures. Employing this method, the detachment of HD from the Cu2+ binding sites is found to already occur slightly above 6 K temperature. Hereby 3p ESEEM spectroscopy reveals to be a powerful method to...

Green Synthesis of a Microporous, Partially Fluorinated ZnII Paddlewheel Metal–Organic Framework: H2/CO2 Adsorption Behavior and Solid‐State Conversion to a ZnO–C Nanocomposite

AbstractA microporous, partially fluorinated metal–organic framework consisting of ZnII dimeric paddlewheel units, formulated as [Zn(hfipbba)(4bpdb)0.5]·H2O (1) [hfipbba = 4,4′‐(hexaflouroisopropylene)bis(benzoic acid); 4bpdb = 1,4‐bis(4‐pyridyl)‐2,3‐diaza‐1,3‐butadiene], was synthesized by room‐temperature self‐assembly and characterized structurally by single‐crystal XRD. Compound 1 adopts a 3D microporous framework structure constituted by six‐connected ZnII dimeric paddlewheel nodes with {44.610.8}‐net topology. Further, rapid synthesis of phase‐pure 1 by green synthetic approaches such as mechanochemical and sonochemical routes was achieved. The 3D framework of 1 houses 1D helical channels with narrow pore dimensions of about 3.11 × 4.17 Å and exhibits interesting gas‐uptake (H2/CO2) properties. The isosteric heats of adsorption Qst for H2 and CO2 were estimated to be 8.8 and 36.4 kJ mol–1, respectively. Interestingly, solid‐state conversion of 1 to phase‐pure hexagonal ZnO nanocrystals of 6.1 ± 0.63 nm in size, embedded in carbonaceous layers to form a ZnO–C nanocomposite (NC), occurred on heating of 1 to 600 °C under vacuum for 2 h. The ZnO–C NC was characterized by XRD, UV/Vis, energy‐dispersive X‐ray, and TEM analyses. The as‐synthesized ZnO–C NC exhibits good photocatalytic activity for the degradation of methylene blue.

A Series of Metal–Organic Frameworks Built of Triazolate-Trinuclear and Paddlewheel Units: Solid-Solution Framework Approach for Optimizing CO2 Adsorption and Separation

Metal–organic frameworks with tunable functional groups like solid-solution have received considerable interest. In this paper, a crs-type structure built of [Zn3(OH)(dmtrz)3] triazolate-trinuclear and [Zn2(COO)4] paddlewheel units, linked by 3,5-dimethyl-1,2,4-triazolate (dmtrz) and isophthalate (ipa) ligands (named as MAC-4-A), has been used as a prototype framework. Four types of functional groups (B: 5-hydroxyisophthalate, C: 5-aminoisophthalate, D: 5-ethoxyisophthalate, and E: 5-acetamidoisophthalate) have been integrated into the framework, giving isostructures of MAC-4-B to E, respectively. Then, by the solid-solution framework approach, four group samples are prepared, which are MAC-4-AB-x, MAC-4-AC-x, MAC-4-AD-x, and MAC-4-AE-x (x denoted as the ratio of functional ligands in the framework, x = 0.3, 0.5, 0.7), and their CO2 and N2 adsorptions have been studied. Our results revealed that the CO2 capacity is enhanced with the increase of functional groups and then decreases, showing a maximum uptak...

Asymmetric and Symmetric Dicopper(II) Paddle-Wheel Units with Modified Nucleobases

New copper(II) paddle-wheel complexes with different modified nucleobases and simple molecules in the axial positions have been prepared by direct reactions between copper(II) salts and the corresponding uracil- or thymine-1-acetic acids under inert atmosphere to produce the two homoleptic complexes, [Cu2(μ-OOCCH2-T)4(G)2] and [Cu2(μ-OOCCH2-U)4(G)2], and the heteroleptic one [Cu2(μ-OOCCH2-T)2(μ-OOCCH2-U)2(G)2] (where OOCCH2-T = thymine-1-acetate, OOCCH2-U = uracil-1-acetate, and G = dimethylformamide, water, dimethylacetamide, or dimethyl sulfoxide). Interestingly, the crystal structures of this family of closely related molecules present significant differences in their supramolecular arrangements affecting the crystal networks mainly due to the interactions defined between the different apical ligands and the coordinated modified nucleobases. Thus, depending upon the dimetallic complex several H-bonding interactions have been found in these series: (i) base pairing through Watson–Crick faces of adjacent dimetallic units, (ii) between a coordinated nucleobase and an oxygen atom from a carboxylic group of a neighboring molecule, and (iii) between a nucleobase and a solvent molecule. All these together with weaker π–π staking interactions define different 1D and 2D supramolecular assemblies. Interestingly, the magnetic studies show strong antiferromagnetic Cu···Cu interactions in agreement with the presence of four μ-κO:κO′ carboxylates bridging the metallic centers in each dinuclear complex. Finally, the crystal-to-crystal exchange reaction involving the ligands coordinated in the axial positions of the dimetallic [Cu2(OOCCH2-T)4(DMF)2]3[Cu2(OOCCH2-T)4(DMF)(H2O)]2 to the related 1D coordination polymer [Cu(OOCCH2-T)2(H2O)2]n has been confirmed in the presence of water vapor. This transformation implies a chemical stimuli magnetic response going from strong antiferromagnetic to almost paramagnetic, respectively.

Three Metal–Organic Frameworks Based on Binodal Inorganic Building Units and Hetero-O, N Donor Ligand: Solvothermal Syntheses, Structures, and Gas Sorption Properties

Three new metal–organic frameworks (MOFs) based on hetero-O, N donor ligands, (Cu4I4)[Cu2(PDC)2DMF2]2·2DMF·H2O (JLU-Liu15), (Cu4I4)[Cu3(CPNA)3(Dabco)0.5DMF3]·4DMF·4H2O (JLU-Liu16), and (Cu4I4)[Cu3(CPNA)3(H2O)0.5DMF3]·4DMF·4H2O (JLU-Liu17), (H2PDC = pyridine-3,5-dicarboxylic acid, H2CPNA = 5-(4′-carboxylphenyl) nicotinic acid, Dabco = 1,4-diazabicyclo[2.2.2]octane), have been solvothermally synthesized and structurally characterized by single-crystal X-ray diffraction analyses. JLU-Liu15 features a (3,4,4)-c sqc5588 net, whereas JLU-Liu16 and JLU-Liu17 are isostructural and possess a new (3,4,4)-c topology. All of these compounds are composed of the binodal inorganic Cu2(CO2)4 paddlewheel and Cu4I4 cluster secondary building units (SBUs). It is worthy to note that the linkage between two nearby Cu4I4 clusters in JLU-Liu16 is μ2-Dabco, which changed to a μ2-water molecule in JLU-Liu17. Furthermore, JLU-Liu15 exhibits high CO2 adsorption behaviors as well as I2 uptake and release features.

Structural flexibilities and gas adsorption properties of one-dimensional copper(II) polymers with paddle-wheel units by modification of benzoate ligands.

CO2 and N2 gas adsorption/desorption properties of one-dimensional copper(II) polymers with paddle-wheel units [Cu(II)2(p-XBA)4(pyrazine)]∞ were successfully controlled through the tuning of interchain interactions by modification of para-substituent X groups on the benzoate (BA) ligands (X = Cl, Br, I, and OCH3). Although none of the four crystals had sufficient void space to integrate the crystallization solvents, gate-opening gas adsorption and desorption behaviors coupled with structural phase transitions were observed for CO2 (T = 195 K) and N2 (T = 77 K), with differences depending on the precise substituent. van der Waals interchain interactions, specifically π···π, halogen···π, and C-H···π contacts, were dominant in forming the crystal lattice; their magnitude was associated with gate-opening pressure and hysteresis behaviors. Both the type and magnitude of the interactions were evaluated by Hirshfeld surface analysis, which indicated that structural flexibility decreased as larger halogen atoms were included. Overall, weak interchain interaction and structural flexibility generated new void spaces to adsorb CO2 and N2 gases.

Versatile Tailoring of Paddle-Wheel Zn(II) Metal-Organic Frameworks through Single-Crystal-to-Single-Crystal Transformations.

A new tetracarboxylate ligand having short and long arms formed 2D layer Zn(II) coordination polymer 1 with paddle-wheel secondary building units under solvothermal conditions. The framework undergoes solvent-specific single crystal-to-single crystal (SC-SC) transmetalation to produce 1Cu . With a sterically encumbered dipyridyl linker, the same ligand forms non-interpenetrated, 3D, pillared-layer Zn(II) metal-organic framework (MOF) 2, which takes part in SC-SC linker-exchange reactions to produce three daughter frameworks. The parent MOF 2 shows preferential incorporation of the longest linker in competitive linker-exchange experiments. All the 3D MOFs undergo complete SC-SC transmetalation with Cu(II) , whereby metal exchange in different solvents and monitoring of X-ray structures revealed that bulky solvated metal ions lead to ordering of the shortest linker in the framework, which confirms that the solvated metal ions enter through the pores along the linker axis.

Syntheses and Crystal Structures of Three Copper(II) Compounds with 2‐Furoic Acid: A Dinuclear Paddle‐wheel Unit and Two Coordination Polymers Supported by Pyridyl Donor Ligands

AbstractThree copper(II) complexes, [Cu2(O2CC4H3O)4(H2O)2]·H2O (1), {[Cu2(O2CC4H3O)4(bipy)2(H2O)2]·2H2O}n(2), and {[Cu(bpp)2(H2O)](O2CC4H3O)2·7H2O} (3) [O2CC4H3O = 2‐furanocarboxylate anion, bipy = 4, 4′‐bipyridine, and bpp = 1, 3‐bis(4‐pyridyl)propane] were synthesized and their crystal and molecular structures were determined by single‐crystal X‐ray diffraction analysis. Compound 1 consists on bimetallic tetracarboxylate units exhibiting a common paddle wheel structure. The paddle wheel units are connected by hydrogen bonding along the crystallographic b axis and π–π stacking interactions between the furano rings along the crystallographic c axis, giving rise to a 2D supramolecular array. Compounds 2 and 3 were made up in the presence of 4, 4′‐bipyridine and 1, 3‐bis(4‐pyridyl)propane ligands, respectively, to produce 1D polymeric chains by metal‐ligand interactions. The presence of other weak non‐covalent interactions in these compounds extends the systems dimensionality to form 3D supramolecular networks.

Single Crystal Electron Paramagnetic Resonance with Dielectric Resonators of Mononuclear Cu2+ Ions in a Metal–Organic Framework Containing Cu2 Paddle Wheel Units

Dielectric resonator aided sensitivity-enhancing electron paramagnetic resonance was successfully applied to small single crystals of the previously reported metal–organic framework compound ∞3[Cu2ICu2II(H2O)2L2Cl2] in a conventional X-band EPR spectrometer at 7 K sample temperature to reveal the nature of mononuclear Cu2+ ion defect species. We found that these paramagnetic defects are not related to an impurity phase or extraframework species of the parent metal–organic framework material but are formed within the framework. Novel angular resolved single crystal continuous wave electron paramagnetic resonance supported by powder measurements and single crystal X-ray diffraction on this metal–organic framework compound identified defective copper paddle wheel units with one missing Cu2+ ion as the observed mononuclear paramagnetic species in this compound. The sensitivity enhancement by an estimated factor of 8.6 for the single crystal electron paramagnetic resonance spectroscopy is required to efficient...

New Advances in the One-Dimensional Coordination Polymer Copper(II) Alkanoates Series: Monotropic Polymorphism and Mesomorphism

The polymorphism in the copper(II) alkanoates, recently discovered for one member, has been thoroughly studied for the whole series, from 3 to 16 C atoms. Three polymorphic phases have been found, all of them sharing the same molecular unit, the paddle-wheel, which grows forming a 1D coordination polymer or catena. The three polymorphs are defined by a different packing of these catenae and a specific arrangement of the alkyl chains. Ten new crystal structures of those compounds have been solved by high resolution powder diffraction and presented in this paper. The polymorphism in this series has been found to be monotropic and is responsible for the complex thermal behavior observed. The most characteristic feature, the endothermic–exothermic effect, has been explained for the first time in these compounds by a combination of data from differential scanning calorimetry (in normal and modulated modes), powder X-ray diffraction and Fourier transform infrared spectroscopy. These techniques, together with small-angle X-ray scattering and optical microscopy, were used to analyze the hexagonal columnar discotic liquid crystal phase of copper(II) alkanoates. Thus, new information has been found in the packing and stacking of the discs formed by the paddle-wheel units, also maintained in the mesophase.

A one-dimensional zinc(II) coordination polymer based on mixed multidentate N- and O-donor ligands.

In the title coordination polymer, catena-poly[[bis[{1-[(1H-benzimidazol-2-yl-κN(3))methyl]-1H-tetrazole}zinc(II)]-bis(μ4-pentane-1,5-dioato-1:2:1':2'κ(4)O(1):O(1'):O(5):O(5'))] methanol disolvate], {[Zn(C5H6O4)(C9H8N6)]·CH3OH}n, each Zn(II) ion is five-coordinated by four O atoms from four glutarate ligands and by one N atom from a 1-[(1H-benzimidazol-2-yl)methyl]-1H-tetrazole (bimt) ligand, leading to a slightly distorted square-pyramidal coordination environment. Two Zn(II) ions are linked by four bridging glutarate carboxylate groups to generate a dinuclear [Zn2(CO2)4] paddle-wheel unit. The dinuclear units are further connected into a one-dimensional chain via the glutarate ligands. The bimt ligands coordinate to the Zn(II) ions in a monodentate mode and are pendant on both sides of the main chain. In the crystal, the chains are linked by O-H···O and N-H···O hydrogen bonds into a two-dimensional layered structure. Adjacent layers are further packed into a three-dimensional network through van der Waals forces. A thermogravimetric analysis was carried out and the photoluminescent behaviour of the polymer was investigated.

Synthesis, Structure, and Electron Paramagnetic Resonance Study of a Mixed Valent Metal–Organic Framework Containing Cu2 Paddle-Wheel Units

We report synthesis and composite study of a novel metal–organic framework (MOF) compound of chemical formula ∞3[Cu2ICu2II{H2O}2{(Me–trz–mba)2thio}2]Cl2, where (Me–trz–mba)2thio2– stands for 3,3-(5,5-(thiophene-2,5-diyl)bis(3-methyl-4H-1,2,4-triazole-5,4-diyl))dibenzoate. This coordination polymer was synthesized by solvothermal synthesis. The crystal structure was determined using single crystal X-ray diffraction. The main building block of this compound is a so-called Cu2 paddle-wheel (PW) unit, which contains two Cu2+ ions connected via four carboxylate groups. Magnetic properties of the investigated MOF were studied by continuous-wave electron paramagnetic resonance (EPR) spectroscopy at X- and Q-band frequencies in a wide temperature range. Mononuclear Cu2+ ions were observed in the EPR spectra and characterized by spectral simulations. In addition, the obtained EPR data allowed us to detect and investigate three distinct magnetic interactions related to the Cu2+ pairs. At higher temperatures the fine structure pattern was observed in the EPR spectra and the spin–spin interaction tensor D was determined. The origin of this pattern was assigned to the thermally populated excited triplet states of the Cu2+ pairs. It was found that two Cu2+ ions within a single PW unit couple antiferromagnetically with the exchange coupling constant J = −258 cm–1. Moreover, the EPR spectra of dehydrated MOF samples show a broad, poorly resolved spectral feature, the origin of which is an exchange of the spin triplets between neighboring Cu2 PW units. By simulating the powder pattern of this interdinuclear exchange line, we estimated the exchange coupling between neighboring PW units (|J′| = 4.9 cm–1). It was also found that the interdinuclear exchange gradually disappears, if the dehydrated samples are allowed to interact with air, demonstrating that this exchange can be rather easily manipulated in the investigated MOF.

Syntheses, structures, luminescent and magnetic properties of two coordination polymers based on a flexible multidentate carboxylate ligand

Two coordination polymers, {[Cu3(tci)2(DMAc)3]·6DMAc·2H2O}n (1) and {[Cu3(tci)2(tpt)2(H2O)2]·2DMAc·2H2O}n (2) (H3tci = tris(2-carboxyethyl)isocyanurate, tpt = 2,4,6-tris(4-pyridyl)-1,3,5-triazine, DMAc = N,N-dimethylacetamide), have been constructed under solvothermal conditions. Both polymers were structurally characterized by single crystal X-ray diffraction, elemental analyses, IR spectra, thermogravimetric (TG) analyses and powder X-ray diffraction (RXPD). 1 shows a (3,4)-connected 2D layer structure comprising Cu2(CO2)4 paddle-wheel units, which are further bridged by CH⋯O interactions to give a 3D supramolecular network. The introduction of tpt produces different framework for 2 that comprises a dinuclear and a mononuclear Cu(II) building units, which are further bridged together by tci3− and tpt ligands to give a 4-connected 2D topological net. Adjacent 2D layers are packed together via CH⋯O interactions and π⋯π stacking interactions to form a 3D supramolecular structure. In addition, the luminescent properties and the solid-state UV–vis spectra of 1 and 2 were explored. Furthermore, antiferromagnetic exchange interactions were unveiled in the Cu2(COO)4 units of 1.

Assembly of a Three-Dimensional Metal–Organic Framework with Copper(I) Iodide and 4-(Pyrimidin-5-yl) Benzoic Acid: Controlled Uptake and Release of Iodine

A novel three-dimensional metal–organic framework, [Cu2I2]0.5[Cu2(CO2)4(C10H7N2)4]0.5·2DMF (JLU-Liu14), has been synthesized under solvothermal conditions, which is constructed by ternary secondary building units (SBUs): classical Cu2(CO2)4 paddle-wheel unit, Cu2I2 dimer unit, and organic SBU. In JLU-Liu14, the pyrimidine groups of 4-PmBC (4-(pyrimidin-5-yl) benzoic acid) ligands coordinated to copper paddle-wheel and Cu2I2 dimer units to form a zigzag chain, and the zigzag chains then connected together by 4-PmBC ligands to generate a three-dimensional framework with (3,4,6)-connected topology. There are two types of channels along the c axis with the dimensions of 14 × 14 A and 5 × 5 A, respectively, in which the big hexagonal channel is surrounded by six small trigonal channels. In addition, JLU-Liu14 displays the interesting ability of the uptake and release of iodine.

Syntheses, Crystal Structure, and Luminescence Properties of Three Metal–Organic Compounds Bearing Diverse Dimensionalities Based on Mixed N- and O-Donor Organic Ligands

Three new MOFs, namely [Ni(pip)(H2O)2] (1), [Ba(pip)(H2O)] (2), [Zn(pip)] (3), based on 6-(3-pyridyl)isophthalic acid (H2pip) were obtained and characterized by single-crystal X-ray diffraction, elemental analyses, PXRD, IR and TGA. By adjusting the reaction solvent, temperature and selecting different metal centers, compounds 1 and 2 possess 1D and 2D network, respectively, compound 3 show a 3D network containing a paddlewheel secondary building unit [SBU, Zn2(COO)4] with the Schlafli symbol (4·62)2(42·610·83). The structural differences among compounds 1–3 exhibit the backbone of such mixed N- and O-donor atoms organic ligands can be used to construct diverse dimensional MOFs by adjusting the reaction environment. Furthermore, the luminescence properties of compounds 2 and 3 were investigated, and both of them exhibit intense fluorescent emissions in the solid state at room temperature.

Two Homochiral Bimetallic Metal–Organic Frameworks Composed of a Paramagnetic Metalloligand and Chiral Camphorates: Multifunctional Properties of Sorption, Magnetism, and Enantioselective Separation

Two porous metal–organic frameworks [Co(Tt)2][Cu4(D-cam)4]·5H2O·DMF (1; Tt = tris(triazolyl)borate, D-H2cam = d-(+)-camphoric acid or (1R,3S)-1,2,2-trimethyl-1,3-cyclopentanedicarboxylic acid) and [Co(Tt)2][Cu4(L-cam)4]·5H2O·2DMF (2; L-H2cam = l-(−)-camphoric acid or (1S,3R)-1,2,2-trimethyl-1,3-cyclopentanedicarboxylic acid) were prepared by mixing Cu2+, Co(Tt), and camphoric acid under solvothermal conditions. The structures of 1 and 2 reveal that the two-dimensional layers composed of chiral ligands and Cu-Cu paddlewheel units are connected through the metalloligands to form three-dimensional networks. It is noted that these solids show multifunctional properties such as gas adsorption onto the pores of the frameworks, antiferromagnetic coupling between spin carriers, and a small enantioselective separation of racemic alcohols.

Improving the porosity and catalytic capacity of a zinc paddlewheel metal-organic framework (MOF) through metal-ion metathesis in a single-crystal-to-single-crystal fashion.

Zinc paddlewheel metal-organic frameworks (MOFs) frequently exhibit low stability or complete collapse upon the removal of axial ligands. Hence, there are very few reports on gas adsorption of zinc paddlewheel MOFs. In this work, the N2 and H2 adsorption measurements were carried out for a zinc MOF (namely, SDU-1) based on two types of paddlewheel secondary building units (SBUs): [Zn2(COO)3] and [Zn2(COO)4]. Because of the existence of inherent surface instability upon removal of solvates in zinc paddlewheel SBU, SDU-1 possesses a very low surface area. Through metal-ion metathesis in a single-crystal-to-single-crystal fashion, the Zn(2+) ions in SDU-1 were exchanged by Cu(2+) ions to generate Cu-SDU-1. Through the measurements of gas adsorption and catalytic test, the porosity and catalytic capacity of Cu-SDU-1 have been improved significantly, compared to SDU-1.