Hexanuclear Cluster Research Articles

A twelve-connected 3D mixed valence Ni(II)/Ni(III) metal-organic framework based on hexnuclear nickel clusters building unit: Synthesis, structure and magnetic properties

We have successfully synthesized and characterised a metal-organic framework Ni 6 (μ 3 -OH) 2 (H 2 BTB)(HBTB) 3 (BTB) 2 (H 2 O) 5 ]·13H 2 O ( 1 ), which is a twelve-connected 3D framework with (3 2 .4) 2 (3 8 .4 22 .5 24. 6 10 .7 2 ) topology. The magnetic behavior of 1 shows that an antiferromagnetic coupling among the Ni ions is observed. • A twelve-connected 3D metal-organic framework was successfully synthesized and charactered. • The newly synthesized MOF is based on a rare mixed-equilibrium Ni6 metal cluster and a tripod ligands. • There exists predominantly antiferromagnetic interaction between the Ni centers. A novel (3,12)-connected framework [Ni 6 (μ 3 -OH) 2 (H 2 BTB)(HBTB) 3 (BTB) 2 (H 2 O) 5 ]·13H 2 O with (3 2 .4) 2 (3 8 .4 22 .5 24 .6 10 .7 2 ) topology has been prepared by self-assembly of benzene-1,3,5-tribenzoates and two hexanuclear nickel carboxylate clusters under hydrothermal conditions. The magnetic property of this compound was investigated in the temperature range of 2.0–300.0 K under an outer field of 2000 Oe. The result shows the dominant antiferromagnetic interaction between Ni centers.

Accessing 14-Connected Nets: Continuous Breathing, Hydrophobic Rare-Earth Metal Organic Frameworks Based on 14-c Hexanuclear Clusters with High Affinity for Non-Polar Vapors.

Highly connected metal organic frameworks (MOFs) in which at least one building block has connectivity higher than twelve are very rare and much desirable. We report here the first examples of isostructural 14-connected MOFs, RE-frt-MOF-1, constructed from the assembly of 14-c hexanuclear rare-earth clusters, [RE6(μ3-X)8(COO)12]2- (RE: Y3+, Tb3+, Dy3+, Ho3+, Er3+, Yb3+ and X: OH-/F-) with a tritopic carboxylate-based organic linker. This linker serves as a 3-c and 4-c organic node resulting in the formation of a unique, trinodal (3,4,14)-c framework. RE-frt-MOF-1 are stable in air and alkaline aqueous solutions and show an intriguingly continuous, reversible breathing behavior, between a wide and a narrow-pore phase, upon guest removal. Crystallinity is retained during breathing, and single-crystal X-ray diffraction shed light into the associated structural transformation. Vapor sorption studies performed on Y-frt-MOF-1 revealed a high affinity for non-polar vapors such as n-hexane, cyclohexane, and benzene, displaying type I isotherms with high uptake at low relative pressures (<10-3 p/p0), associated with the hydrophobic nature of the 1D channels and also with their rhombic shape. In contrast, polar vapors such as acetonitrile and ethanol show type V isotherms due to favorable vapor-vapor interactions. Notably these vapors, except cyclohexane, trigger the transition from the narrow to the wide pore phase, accompanied by a remarkable increase in uptake, reaching 70.6, 109, 100.4, and 87.7% for n-hexane, benzene, acetonitrile, and ethanol, respectively.

New 3D Coordination Polymer Based on the Tetrapyridyl Derivative of Thiacalix[4]arene in the 1,3-Alternate Configuration and Hexanuclear Clusters of Monovalent Silver: Synthesis and Structure

New 3D Coordination Polymer Based on the Tetrapyridyl Derivative of Thiacalix[4]arene in the 1,3-Alternate Configuration and Hexanuclear Clusters of Monovalent Silver: Synthesis and Structure

Unveiling the nature of boric acid adsorption by metal-organic frameworks with hexanuclear clusters

A growing demand of boric acid has been forecasted with the rapid development of industries while the world is suffering the consequence from boron-containing wastewater. Metal–organic frameworks (MOFs) with exceptional stability and remarkable surface area have been preliminarily identified as effective boron adsorbents, adsorption mechanism has yet been elucidated though. Herein Zr6/Hf6-based NU-1008 and NU-903 were studied for boron removal from water for the first time where around 4.82, 6.27, 3.49 and 6.34 boron were captured per Zr6/Hf6 cluster for Zr-NU-1008, Hf-NU-1008, Zr-NU-903 and Hf-NU-903 through a spontaneous endothermic process with fast kinetics by means of chemisorption. Larger capacity of Hf6-based MOFs than Zr6-based MOFs can be explained by the weaker electronegativity of hafnium than zirconium. The increased binding energy of Zr 3d, Hf 4f and O 1s as well as reduced binding energy of B 1s suggested the interaction between electron-deficient boron and the hexanuclear clusters (M6). Owing to the crystalline nature of NU-1008, the boron binding sites were precisely elucidated by single crystal X-ray diffraction where boric acid was attached to the available coordinative sites on the M6 clusters through the M6-O-B(OH)2 connection.

Characterization of a Hexanuclear Plutonium(IV) Nanostructure in an Acetate Solution via Visible-Near Infrared Absorption Spectroscopy, Extended X-ray Absorption Fine Structure Spectroscopy, and Density Functional Theory.

A new hexanuclear plutonium cluster has been stabilized in aqueous media with acetate ligands. To probe the formation of such a complex structure, visible-near infrared (vis-NIR) absorption spectroscopy, extended X-ray absorption fine structure (EXAFS) spectroscopy, and density functional theory (DFT) were combined. The presence of Pu6O4(OH)4(CH3COO)12 species in solution was first detected by vis-NIR and EXAFS spectroscopy. To confirm unambiguously this structure, EXAFS spectra were simulated from ab initio calculations. Debye-Waller factors and structural parameters were derived from DFT calculations. A large number of 5f electrons were treated as valence or core electrons using small- and large-core relativistic effective pseudopotentials. It is possible to reproduce accurately the EXAFS spectrum of the octahedral hexamer cluster at both levels of calculations. Further DFT and EXAFS calculations were performed on clusters of lower or higher nuclearities and of different geometries using the 5f-core approximation. The result shows that trimer, tetramer, flat hexamer, and even 16-mer clusters exhibit different EXAFS patterns and confirm the very specific octahedral hexanuclear EXAFS signature.

Aquatic arsenic removal with a Zr-MOF constructed via in situ nitroso coupling

Assembly of hexanuclear Zr-oxo clusters via in situ covalent coupling of nitroso groups afforded a metal–organic framework (MOF) with fcu topology. The MOF, namely SUM-8, emerged as an excellent adsorbent for aquatic arsenic removal in terms of adsorptive efficiency, capacity, selectivity, and cyclability, on accounts of its chemical stability and structural parameters suitable for arsenate diffusion/accommodation.

Programmable Photoluminescence via Intrinsic and DNA-Fluorophore Association in a Mixed Cluster Heterometallic MOF.

A rapid and facile design strategy to create a highly complex optical tag with programmable, multimodal photoluminescent properties is described. This was achieved via intrinsic and DNA-fluorophore hidden signatures. As a first covert feature of the tag, an intricate novel heterometallic near-infrared (NIR)-emitting mesoporous metal-organic framework (MOF) was designed and synthesized. The material is constructed from two chemically distinct, homometallic hexanuclear clusters based on Nd and Yb. Uniquely, the Nd-based cluster is observed here for the first time in a MOF and consists of two staggered Nd μ3-oxo trimers. To generate controlled, multimodal, and tailorable emission with difficult to counterfeit features, the NIR-emissive MOF was post-synthetically modified via a fluorescent DNA oligo labeling design strategy. The surface attachment of several distinct fluorophores, including the simultaneous attachment of up to three distinct fluorescently labeled oligos was achieved, with excitation and emission properties across the visible spectrum (480-800 nm). The DNA inclusion as a secondary covert element in the tag was demonstrated via the detection of SYBR Gold dye association. Importantly, the approach implemented here serves as a rapid and tailorable way to encrypt distinct information in a facile and modular fashion and provides an innovative technology in the quest toward complex optical tags.

Solvents and Ligands Matter: Structurally Variable Palladium and Nickel Clusters Assembled by Tridentate Selenium- and Tellurium-Containing Schiff Bases

Structurally variable organochalcogen clusters containing palladium(II) and nickel(II) ions were assembled starting from the salicylidene-substituted dichalcogenides (Y-C6H4-N═CH-C6H4-OH)2 ({HLY}2, where Y = Se or Te), and palladium or nickel acetate. The tetrameric palladium clusters contain reduced chalcogenolato ligands {Y-C6H4-N═CH-C6H4-O)}2- ({L'Y}2-, where Y = Se or Te), while the initially formed trimeric nickel clusters contain the intact, coordinated dichalcogenides. The palladium clusters have a general formula of [Pd4(L'Y)4] and represent the first examples of palladium complexes where both a gyrobifastigial and a pseudocubane arrangement of the central Pd4Y4 unit could be established with the same ligand, only depending on the solvents used for crystallization. Reduced density gradient (RDG) considerations based on density functional theory calculations suggest that the commonly referred to stabilizing chalcogen-palladium or palladium-palladium interactions for the two geometric arrangements are weak van der Waals contacts resulting from the contact of two nonbinding lone pairs. In the case of the pseudocubane arrangement, a repulsive steric effect, which is indicated by RDG analysis, is clearly supported by the cuplike distortions detected in the solid-state structure of the compound. In contrast to the reactions with palladium acetate, where the dichalcogenides were cleaved, during similar reactions with nickel acetate, the dichalcogenides remained intact and trimeric clusters of the composition [Ni-μ2-κ2-(Ni{κ5-LY}2)2-μ2-(OAc)2] (Y = Se, Te) were formed. Air oxidation and hydrolysis of [Ni-μ2-κ2-(Ni{κ5-LTe}2)2-μ2-(OAc)2] gave a rare example of a hexanuclear nickel cluster of the composition [Ni2-κ5-(Ni4-κ6-μ6-{(L'Te2O3)(L'TeO2)2}2)-μ2-(H2O)2], which is composed of a well-defined framework consisting of tellurinic anhydride and tellurinate units, which proves the comparably higher oxidation sensitivity of the trinickel dichalcogenide complexes. Electron spray ionization mass spectrometry spectra of both the palladium and nickel clusters indicate that they show fluctional behavior with varying nuclearity in solution and can adopt multiple charge states especially because of the noninnocence of the chalcogen-based ligands. The complexes were fully characterized by spectroscopic methods, elemental analyses, and X-ray diffraction.

A Microporous Metal-Organic Framework Incorporating Both Primary and Secondary Building Units for Splitting Alkane Isomers.

We demonstrate the assembly of a mononuclear metal center, a hexanuclear cluster, and a V-shaped, trapezoidal tetracarboxylate linker into a microporous metal-organic framework featuring an unprecedented 3-nodal (4,4,8)-c lyu topology. The compound, HIAM-302, represents the first example that incorporates both a primary building unit and a hexanuclear secondary building unit in one structure, which should be attributed to the desymmetrized geometry of the organic linker. HIAM-302 possesses optimal pore dimensions and can separate monobranched and dibranched alkanes through selective molecular sieving, which is of significant value in the petrochemical industry.

Nb6Cl14(pyrazine)4], a Versatile Precursor for Ligand-Supported Hexanuclear Niobium Cluster Compounds: Synthesis, Characterization, Follow-Up Reactions, and Intermolecular Interactions.

The compound [Nb6Cl14(pyrazine)4]·2CH2Cl2 (1) was investigated for its suitability as a starting compound for new ligand-supported hexanuclear niobium cluster compounds. The synthesis, stability to air and increased temperature, solubility and usability for subsequent reactions of 1, and purification and separation of the reaction products are discussed. The compounds with cluster units [Nb6Cl14L4], where L = iso-quinoline N-oxides (2), 1,1-dimethylethylenediamines (3), or thiazoles (4), and [Nb6Cl14(PEt3)3.76(Et3PO)0.24][Nb6Cl14(MeCN)4]·4MeCN (5) are presented as follow-up products. The crystal structures of compounds 1-5 are analyzed, and the structures are discussed with respect to their intra- and intermolecular bonding situations and crystal packing. In addition to hydrogen bonds and π-π interactions, the appearance of chalcogen and halogen bonds and lone pair-π interactions between Nb6 cluster units was observed for the first time.

Hexanuclear Niobium Cluster Iodides with Pyridine Ligands – Temperature Induced Auto‐Oxidation

AbstractThe cluster compounds [Nb6I8Py6] and [Nb6I8Py6]I ⋅ 9Py, both with octahedral Nb6 metal atom cores, were obtained from the reaction of [Nb6I11] with pyridine (Py). Both compounds are rare examples of iodide supported hexanuclear cluster compounds, which are obtained through solvent‐based chemical reactions. The Nb6 cluster core is surrounded by μ3‐bridging iodido and apical‐bonded pyridine ligands. [Nb6I8Py6] contains 22 cluster‐based electrons (CBE's), whereas [Nb6I8Py6]I ⋅ 9Py is the oxidized version with 21 CBE's. The oxidation happens just by lowering the temperature from room temperature to −20 °C of the same starting materials as used for [Nb6I8Py6]. The structures of the two cluster compounds were determined by single‐crystal X‐ray diffraction.

Investigating the Influence of Hexanuclear Clusters in Isostructural Metal-Organic Frameworks on Toxic Gas Adsorption.

The efficient capture of toxic gases, such as ammonia (NH3) and sulfur dioxide (SO2), can protect the general population and mitigate widespread air pollution. Metal-organic frameworks (MOFs) comprise a tunable class of adsorbents with high surface areas that can meet this challenge by selectively capturing these gases at low concentrations. In this work, we explored how modifying the metal ions in the node of an isostructural MOF series from a transition metal to a lanthanide or actinide influences the electronic environment of the node-based active site. Next, we investigated the adsorption properties of each MOF toward the relatively basic NH3 and relatively acidic SO2 gases. Within the NU-907 family of MOFs, we found that Zr6-NU-907 exhibits the best uptake toward NH3 at low pressures, while Th6-NU-907 demonstrates the best low-pressure performance for SO2 adsorption. Tracking the infrared (IR) stretching frequency of the node-based μ3-OH groups provides insights into the electronegativity of the metal ion and suggests that the most electronegative metal ion (Zr) affords the node with the best NH3 uptake at low pressures. In contrast, the Th6 node contains additional coordinated water groups relative to the other M6 nodes, which appears to yield the MOF with the greatest affinity for SO2 uptake that occurs predominately through reversible physisorption interactions. Finally, in situ NH3 IR spectroscopic studies indicate that both NH4+ and Lewis-bound NH3 species form during adsorption. Combined, these results suggest that tuning the electronic properties and structure of the node-based active site in an MOF presents a viable strategy to change the affinity of an MOF toward toxic gases.

Structural transformation of metal oxo species within UiO-66 type metal–organic frameworks

A rare example of phase transitions within Th-based MOFs is reported, relevant for nuclear energy and waste management. Further investigations into phase transitions in isostructural frameworks (Zr, Hf, Ce) provide a comparison of different hexanuclear clusters' stabilities.

Size and structure of hexanuclear plutonium oxo-hydroxo clusters in aqueous solution from synchrotron analysis.

The size and shape of a water-soluble hexanuclear plutonium cluster were probed by combining synchrotron small-angle X-ray scattering (SAXS) and extended X-ray absorption fine structure (EXAFS). A specific setup coupling both techniques and dedicated to radioactive samples on the MARS beamline endstation at Synchrotron SOLEIL is described. The plutonium hexanuclear cores are well stabilized by the 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid ligands and this allows a good evaluation of the setup to probe the very small plutonium core. The results show that, in spite of the constrained conditions required to avoid any risk of sample dispersion, the flux and the sample environment are optimized to obtain a very good signal-to-noise ratio, allowing the detection of small plutonium aggregates in an aqueous phase. The structure of the well defined hexanuclear cluster has been confirmed by EXAFS measurements in solution and correlated with SAXS data processing and modelling. An iterative comparison of classical fit models (Guinier or sphere form factor) with the experimental results allowed a better interpretation of the SAXS signal that will be relevant for future work under environmentally relevant conditions.

A new hexanuclear Fe(III) nanocluster: synthesis, structure, magnetic properties, and efficient activity as a precatalyst in water oxidation.

The oxo-bridged hexanuclear iron cluster formulated [FeIII6(μ4-O)2(edteH)2(piv)4(SCN)4]·2MeCN·2H2O (1) (where edteH = N,N,N',N'-tetrakis(2-hydroxyethyl)ethylenediamine; piv = pivalic acid) has been synthesized by the reaction of FeCl2·4H2O with edteH4 and piv in the presence of KSCN in CH2Cl2/MeCN. The single crystal X-ray measurements indicated that the cluster is centrosymmetric in structure. The magnetic study demonstrated the presence of very strong antiferromagnetic coupling between the iron centers and the Brillouin fitting showed the best fit with S = 5/2 and g = 1.87. In addition, the water oxidation activity of the cluster has been studied by electrochemical techniques. Electrochemical experiments revealed that the electrode modified by 1 has high efficiency for the oxidation of water and needs an overpotential of 484 mV under a constant current density of 15 mA cm-2 with a Tafel slope of 114 mV dec-1 in neutral media. Experiments indicated that in the presence of 1, a yellow solid film was formed on the electrode surface under the applied electrochemical conditions. This yellow material is likely a compound of iron and oxygen and has a crystalline nature. Our findings revealed that along with the cluster, this compound is active in water oxidation reactions.

Porous nickel and cobalt hexanuclear ring-like clusters built from two different kind of calixarene ligands – new molecular traps for small volatile molecules

The formation and structural analysis of porous hexanuclear ring-like cluster complexes built from two different kind of calixarene ligands is presented, together with their stability and vapor solvent sorption properties.

A microporous Zr6@Zr-MOF for the separation of Xe and Kr.

We report here the self-assembly of a she-type zirconium-based metal-organic framework with discrete hexanuclear Zr-oxo clusters residing inside its pore windows. The overall structure features microporosity showing preferential adsorption of Xe over Kr.

First observation of [Pu6(OH)4O4]12+ cluster during the hydrolytic formation of PuO2 nanoparticles using H/D kinetic isotope effect.

New insights are provided about the formation mechanism of PuO2 nanoparticles (NPs) by investigating an unprecedented kinetic isotope effect observed during their hydrolytic synthesis in H2O or D2O and attributed to OH/OD zero point energy difference. The signature of a Pu(IV) oxo-hydroxo hexanuclear cluster, appearing as an important intermediate during the formation of the 2 nm PuO2 NPs (synchrotron SAXS/XAS), is further revealed indicating that their formation is controlled by H-transfer reactions occurring during hydroxo to oxo-bridge conversions.

Synthesis of Coordination Polymers and Discrete Complexes from the Reaction of Copper(II) Carboxylates with Pyrazole: Role of Carboxylates Basicity

The reaction of select copper(II) monocarboxylates Cu(RCOO)2(H2O)x (R = C4H3O, C6H5, H; x = 2, 4) with pyrazole (Hpz), carried out in protic solvents, yields mono- or dinuclear species [Cu(RCOO)2(Hpz)n]m (n = 2, 4; m = 1, 2), always accompanied by tri- or hexanuclear derivatives based on the triangular [Cu3(μ3-OH)(μ-pz)3]2+ fragment. The molecular structures of all isolated compounds have been established through XRD determinations. In some cases, the mono-, or dinuclear species act as secondary building units, self-assembling into 1D or 3D coordination polymers. The hexanuclear benzoate cluster contains coordinated and crystallization MeOH molecules, whose removal generates a crystalline 1D coordination polymer that can be reconverted into the hexanuclear cluster by soaking in MeOH. The relatively high pKb values of carboxylate anions employed in the syntheses are responsible for the preferential formation of mono- or dinuclear species rather than tri- or hexanuclear ones.

Polyanionic Imido-P(V) Ligands: From Transition Metal Complexes to Coordination Driven Self-Assemblies.

The chemistry of the imido-anions of the main group elements has been studied for more than three decades. The imido (NR)- group is isoelectronic to the oxo (=O) group and can coordinate with metal ions through its lone pairs of electrons. The polyimido-P(V) anions are well explored as they resemble the phosphorus oxo moieties such as H3 PO4 , H2 PO4 - , HPO4 2- and PO4 3- species. These imido anions are typically generated using strong main group organometallic reagents such as n BuLi, Et2 Zn, Me3 Al and n Bu2 Mg, etc. As a result, their coordination chemistry has been restricted to reactions in anhydrous aprotic solvents for a few main group metal ions. This account presents our findings on using certain soft transition metal such Ag(I) and Pd (II) for isolating these imido-P(V) anions as their corresponding self-assembled clusters and cages. Using the various salts of Ag(I) ions in reaction with 2-pyridyl (2 Py) functionalized phosphonium salts and phosphoric triamides, we obtained the mono- and dianionic form of these imido ligands {[P(N2 Py)2 (NH2 Py)2 ]- , [P(N2 Py)2 (NH2 Py)]- , [PO(N2 Py)(NH2 Py)2 ]2- } and derived interesting examples of tri, penta, hepta and octanuclear Ag(I) clusters. Interestingly, by using the salts of Pd (II) ions, the elusive imido-phosphate trianions of the type [(RN)3 PO]3- (R=t Bu, c Hex, i Pr) were generated in a facile one pot reaction as their corresponding tri- and hexanuclear clusters of the type {Pd3 [(NR)3 PO](OAc)3 }n (n=1 or 2). These trianions acts as a cis-coordinated hexadentate ligand for a trinuclear Pd (II) cluster and serve as the polyhedral building units for constructing hitherto unknown family of neutral cages in tetrahedral {Pd3 [(Ni Pr)3 PO]4 (L)6 } and cubic {Pd3 [(Ni Pr)3 PO]8 (L)12 } structures in the presence of suitable linker ligands (L2- ). These cages show interesting host-guest chemistry and post-assembly reactions. Remarkably, by employing chiral tris(imido)phosphate trianions, enantiopure chiral cages of the type [(Pd3 X*)4 (L)6 ], ([X*]3- =RRR- or SSS-[PO(N(*CH(CH3 )Ph)3 ]3- ), were synthesized and used for the chiral-recognition and enantio-separation of small racemic guest molecules. Some of these chiral cages were also shown to exhibit polyradical framework structures. In future, these and other similar types of cages are envisioned as potential molecular vessels for performing the reactions in their confined environment. The enantiomeric cages can be probed for asymmetric catalysis and the separation of a range of small chiral molecules.