Self-assembly of four new extended architectures based on reduced polyoxometalate clusters and cadmium complexes

Four new [P₄Mo₆] cluster-based extended structures containing cadmium complexes, [Cd₃(4,4′-Hbpy)₂(4,4′-bpy)₂(H₂O)₈][Cd(H₂PO₄)₂(HPO₄)₄(PO₄)₂(MoO₂)₁₂(OH)₆]·7H₂O 1, (4,4′-Hbpy)₂[Cd(4,4′-bpy)₃(H₂O)₃][Cd(4,4′-bpy)(H₂O)]₂[Cd(H₂PO₄)₂(HPO₄)₄ (PO₄)₂(MoO₂)₁₂(OH)₆]·H₂O 2, [Cd₄(phen)₂(H₂O)₄][Cd(phen)(H₂O)]₂[Cd(HPO₄)₄ (HPO₄)₄(MoO₂)₁₂(OH)₆]·5H₂O 3 and [Cd₄(2,2′-bpy)₂(H₂O)₄][Cd(2,2′-bpy)(H₂O)]₂ [Cd(HPO₄)₄(HPO₄)₄(MoO₂)₁₂(OH)₆]·3H₂O 4 (4,4′-bpy=4,4′-bpyridine, phen=1,10-phenanthroline, 2,2′-bpy=2,2′-bpyridine), have been synthesized and characterized by elemental analysis, IR, TG and single crystal X-ray diffraction. The structure of compound 1 is constructed from the Cd[P₄Mo₆]₂ dimers linked by [Cd₃(4,4′-Hbpy)₂(4,4′-bpy)₂(H₂O)₈] subunits to generate a plane layer. Compound 2 consists of the positive 2D sheets that constructed from Cd[P₄Mo₆]₂ dimers linked by [Cd(4,4′-bpy)(H₂O)] complexes, then the 2D sheets are further linked up together to form a 3D supramolecular framework via extensive hydrogen-bonding interactions among the [P₄Mo₆] clusters, free 4,4′-bpy molecules, dissociated [Cd(4,4′-bpy)₃(H₂O)₃]²⁺ complexes and water molecules. Compounds 3 and 4 show new 2D layered structure, with Cd[P₄Mo₆]₂ building blocks connected by tetra-nuclear [Cd₄{(phen)₂/(2,2′-bpy)₂}(H₂O)₄] clusters and [Cd(phen/2,2′-bpy)(H₂O)] complexes. The fluorescent activities of compounds 3 and 4 are reported.     

Self-assembly and characterization of copper 3,4-pyridinedicarboxylate complexes based on a variety of polynuclear hydroxo clusters

Self-assembly of copper(ii) ion, 3,4-pyridinedicarboxylate (PDC), and 1,10-phenanthroline (phen) under basic conditions at 100 °C affords four PDC linked copper(ii) complexes, [Cu(4)(μ(2)-OH)(3)(μ(3)-OH)(PDC)(phen)(4)](n)·n(PDC)·11.5 nH2O (1), [Cu(4)(μ(2)-OH)(2)(μ(3)-OH)(2)(PDC)(phen)(4)](n)·n(PDC)· 11.5 nH(2)O (2), [Cu(8)(μ(2)-OH)(2)(μ(3)-OH)(6)(PDC)(2)(phen)(8)]·2(PDC)·23 H(2)O (3), and [Cu(3.5)(μ(2)-OH)(3) (PDC)(2)(phen)](n) (4). 1-4 are copper hydroxo complexes, and 1, 2 and 3 co-crystallized from the one-pot reaction. X-ray single crystal diffraction analyses indicate that complexes 1 and 2 are linkage isomers and contain tetranuclear copper cluster cores with different geometry, and that PDC links the cluster core to form a one-dimensional chain. Complex 3 is a discrete step-like octanuclear copper hydroxo cluster complex. The involvement of hydroxo and phen in the coordination makes some coordination sites of PDC idle, which leads to rich hydrogen bonds and π-π interactions in complexes 1, 2 and 3. Complex 4 contains two types of copper hydroxo cluster cores: chair-like tetranuclear and linear trinuclear units, and the cluster cores are linked by PDC to a double-layer metal-organic framework. Magnetic properties of 1, 3 and 4 were investigated. The results reveal that complexes 3 and 4 exhibit strong antiferromagnetic interactions whereas ferromagnetic coupling is predominant for complex 1. The magnetic properties are analyzed in connection with their structures.     

Supramolecular architectures and magnetic properties of coordination polymers based on pyrazinedicarboxylato ligands showing embedded water clusters

The synthesis, crystal structure, and magnetic behavior of nine transition-metal complexes based on pyrazine-2,5-dicarboxylato (pz25dc) and pyrazine-2,3-dicarboxylato (pz23dc) ligands are reported. The pz25dc ligand displays a bis-bidentate coordination mode, with the carboxylate groups almost coplanar with the pyrazine ring, to afford polymeric 1-D chains [Mn(1), Fe(2), Zn(3), and Cu(4 and 5)] and discrete dimeric entities [Mn(6)] when the 1,10-phenanthroline (phen) blocking ligand is used to avoid further polymerization. The nonplanar pz23dc ligand chelates to a unique copper center, while it bridges another one or two metal centers via the remaining carboxylate group, leading to 1-D polymeric chains (7), ladder chains (8), and sheets (9). The crystal packing of the metal-organic frameworks of compounds 4-9 generates voids which are occupied by assembled water molecules. The different water cluster patterns (tapes, four-membered discrete rings, and chains for compounds 6, 8, and 9, respectively) and their role in the cohesiveness of supramolecular architectures are analyzed. Thermogravimetric and variable-temperature X-ray powder diffraction studies have revealed the occurrence of reversible dehydration processes in compounds 6, 8, and 9. Furthermore, the magnetic behavior of these compounds has been studied in order to analyze the capability of the pyrazine ring to transmit magnetic interactions.     

Heterometallic architectures based on the combination of heteroleptic copper and cobalt complexes with silver salts

A strategy for the formation of heterometallic coordination polymers based on novel copper(II) and cobalt(III) heteroleptic complexes (acacCN)Cu(dpm) and (acacCN)Co(dpm)(2) (acacCN = 3-cyanoacetylacetonate; dpm = dipyrrin) is presented. Using dipyrrins appended with a p- or m-pyridyl group, dpm-4py and dpm-3py, four novel copper and cobalt complexes were prepared and characterized both in solution and in the solid state. These two classes of complexes show different electrochemical properties upon investigation by cyclic voltammetry in CH(2)Cl(2). While the copper complexes show only irreversible reduction processes, the voltammogram of the cobalt species reveals the presence of two quasi-reversible reductions. In the solid state, the copper(II) compounds self-assemble to form one-dimensional architectures upon coordination of the peripheral pyridyl group to the copper center, as characterized by single-crystal X-ray diffraction. Owing to the filled coordination sphere of the octahedral cobalt centers, the (acacCN)Co(dpm-py)(2) compounds crystallize as isolated molecules. Upon reaction with silver salts, these complexes form crystalline heterometallic architectures with different organization and dimensionality, depending on the nature of the metal center and the position of the nitrogen atom in the pyridyl group. The two copper complexes lead to the formation of trinuclear species, {[(acacCN)Cu(dpm-py)](2)Ag}(+), resulting from coordination of the pyridyl groups to the silver cations. However, while meta-functionalized complexes self-assemble into an extended architecture via weak interaction of the peripheral nitrile of the acacCN ligand to the Ag(+) cation, this interaction is not present in the para-functionalized analogue. In both networks based on the Ag(BF(4)) salt, coordination of the tetrafluoroborate anion to the silver center in the rather rare chelate mode is observed. Upon assembly of the cobalt metallatectons with silver salts, two-dimensional (2D) coordination polymers are obtained in crystalline form, resulting, however, from different sets of interactions. Indeed, no coordination of the peripheral nitrile of the acacCN ligand is observed in the network incorporating the m-pyridyl-appended dpm; coordination of the pyridyl groups to the silver center and d(10)-d(10) interactions lead to a 2D architecture. In the case of the para analogue, a 2D honeycomb network is observed owing to coordination of the Ag(I) ion to two pyridyl nitrogen atoms and to one peripheral nitrile group of a acacCN ligand. This latter polymer represents a geometrical hybrid of the networks reported in the literature based on homoleptic Co(dpm-4py)(3) and Cr(acacCN)(3) complexes.     

Self-organized microporous structures based on surfactant-encapsulated polyoxometalate complexes

Self-organized microporous structures based on a series of surfactant-encapsulated polyoxometalate complexes (SECs) have been prepared by using ordered condensed droplets as a template. Among these structures, ordered honeycomb structures were obtained and characterized in detail by taking (DODA)(12)H[Eu(SiW(11)O(39))2] (SEC-1) as an example. Optical microscope, atomic force microscopic, and scanning electron microscopic measurements confirmed the formation of three-dimensional microporous structure, in which the top surface shows a highly ordered honeycomb structure. As compared to common solvent-casting films, the corresponding honeycomb films are more hydrophobic and possess more ordered lamellar structures. Both the wettability and the size of SECs exert significant influence on the formation of microporous structures. The proper hydrophobicity of SECs was proposed to be an essential factor for the formation of honeycomb films, and large-sized SECs are favorable for the fabrication of highly ordered honeycomb structures. The conditions for the formation of different surface morphologies have been discussed in terms of the contact angle of SECs at the interface between water and chloroform, and a contact angle slightly greater than 90 degrees is found to be a prerequisite for the formation of honeycomb structures. The results reported in this paper not only help to further comprehend the mechanism of the formation of honeycomb structures, but also provide some guidance for the design of ordered microporous films based on organic/inorganic hybrid materials, exemplified by the organic/nanoparticle complexes.     

Self-assembly of supramolecular complexes based on hydrogen bonding

Self-assembly of hydrogen-bonding recognition complexes—2.5-bis(alkylamino)-1,4-benzoquinones were studied, and thermotropic liquid crystalline behavior of 2.5-bis(dodecylamino)-1,4-benzoquinone was further investigated. The obtained results showed that a ribbon-like backbone in this system plays an important role in keeping the layered supramolecular structure.     

Anion directed synthesis of paddlane and trisilver tweezer complexes based upon silver coordination chemistry

Addition of AgBF4 or Ag(O3SCF3) to the tripodal ligand, Me3tacnRh(CCPh)3, yields a paddlane complex as well as a novel trinuclear silver "tweezer" complex based upon silver acetylene coordination chemistry. The paddlane is composed of two Me3tacnRh(CCPh)3 moieties held together by silver acetylene interactions. The tweezer complex is composed of one tripodal moiety with three silver atoms coordinated to each acetylene-Rh-acetylene face. The tweezer complex is stabilized by additional interactions, including a triflate anion which serves to cap the complex.     

Synthesis and characterization of a new dimeric polyoxometalate based on Anderson-type polyoxoanions and a trinuclear lanthanide coordination complex

An unusual compound, H₅[(C₆NO₂H₄)₄(H₂O)₁₅Nd₃][IMo₆O₂₄]₂·13.5H₂O (1) has been synthesized and characterized by elemental analysis, IR spectroscopy, thermogravimetric analysis (TGA), and single crystal X-ray diffraction. Compound 1 consists of two [IMo₆O₂₄]^5? units joined by a trinuclear neodymium-pyridine-4-carboxylic acid complex, resulting in a dimeric polyoxometalate, which further interact via weak intermolecular interactions to form a three-dimensional supramolecular framework with channels. To the best of our knowledge, no analogous dimeric polyoxometalate structure composed of an Anderson-type polyoxometalate has been reported in the literature.     

New coordination cyano-bridged polymers based on tetranuclear rhenium chalcocyanide clusters and copper(ii) ammonia complexes

The reactions of aqueous solutions of the tetranuclear chalcocyanide cluster anions [Re₄Q₄(CN)₁₂]^4–, where Q = S, Se, or Te, with an ammonia solution of copper(ii) chloride at room temperature afforded a series of polymeric cyano-bridged compounds [{Cu(NH₃)₃}₂{Re₄Q₄(CN)₁₂}]·nH₂O (Q = S (1), Se (2)) and [{Cu(NH₃)₃}_1.7{Cu(NH₃)₄}_0.3Re₄Te₄(CN)₁₂] (3) having chain structures. The structures of the compounds were established by X-ray diffraction analysis. Compounds 1 and 2 are isostructural and have ordered structures. In structurally similar compound 3, the copper atoms are disordered over two positions in such a way that the structure contains the bridging {Cu(NH₃)₃} fragments along with the terminal {Cu(NH₃)₄} fragments.     

Construction of three 2-D silver(I) coordination architectures with a flexible pyrazine-based ligand

To investigate the impact of weak intermolecular interactions in construction of metal–organic frameworks, three silver(I) coordination complexes with the flexible N-heterocyclic ligand 1-((2-pyrazinyl)-1H-benzoimidazol-1-yl)methyl)-1H-benzotriazole (PBMBT), {Ag(C₁₈H₁₃N₇)NO₃} _n (1), {Ag(C₁₈H₁₃N₇)ClO₄} _n (2), and {Ag(C₁₈H₁₃N₇)SO₃CF₃} _n (3), were prepared under solvothermal conditions and structurally characterized by single-crystal X-ray diffraction, IR spectrum, and elemental analyses. Complexes 1–3 exhibit 2-D reticulate structures, and these 2-D layers are further connected into 3-D supramolecular motifs by π···π interactions and hydrogen bonds. Luminescence indicates that 1–3 show analogous fluorescent emissions compared with the PBMBT in the solid state at room temperature.     

Self-assembly of organic-inorganic hybrid materials constructed from eight-connected coordination polymer hosts with nanotube channels and polyoxometalate guests as templates

Two polyoxometalate-templated organic-inorganic hybrid porous frameworks, namely, [Cu2(H2O)2(bpp)2Cl][PM12O40].approximately 20H2O (for 1, M = W; for 2, M = Mo; bpp = 1,3-bis(4-pyridyl)propane), were self-assembly obtained and structurally determined by elemental analyses, inductively coupled plasma analyses, infrared spectroscopy, and single-crystal X-ray diffraction analyses. Single-crystal X-ray analysis of these crystals revealed that both of the structures are constructed from eight-connected three-dimensional coordination polymer hosts [Cu2(H2O)2(bpp)2Cl]n(3n+) and ball-shaped Keggin-type guests [PM12O40]n(3n-) as templates. The polymer hosts resulted from a bcc-type framework with nanotubes, and the nanotubes can be regarded as a tetra-stranded helix structure. Furthermore, compounds 1 and 2 exhibit photoluminescent properties at ambient temperature, and the compound 2 bulk-modified carbon paste electrode ( 2-CPE) displays good electrocatalytic activity toward the reduction of nitrite.     

Self-assembly of a cavitand-based heteronuclear coordination cage

A new self-assembly protocol leading to the formation of heteronuclear coordination cage 10 is reported. Reaction of tetradentate cavitand ligand 1, bearing one ethynylpyridine and three benzonitriles at the apical positions, with Pt(dppp)OTf₂ and Pd(dppp)OTf₂ in a 1:3 ratio yields 10 as the thermodynamic product. Under the same conditions, the self-assembly of 1 with either Pt or Pd metal precursors gives a mixture of isomeric homonuclear cages 8a-c or 9a-c, respectively.     

A unique three-dimensional coordination cluster based on a silver carbene complex

In an attempt to perform a simple anion-exchange reaction on a pincer-carbene-ligated nickel complex using AgNO(3), we instead obtained an unexpected three-dimensional (3D) Ag(7) cluster containing a [Ag(6)] core in a twisted-bowtie geometry. The reverse-transmetalation reaction by which the carbene is transferred from nickel to silver is virtually unprecedented. The CNC pincer-carbene ligands exhibit unusual bridging modes of ligand bonding for all three donor atoms. Another unique feature is that the final structure exhibits a 3D structure brought about by the connection of two-dimensional layers of the [Ag(6)] core via a seventh Ag ion.     

Self-assembly as a route to dinuclear lanthanide complexes with rare coordination pattern of salen-type ligand

The self-assembled formation of dinuclear lanthanide salicylaldimines is proved by the X-ray diffraction analysis of europium and gadolinium nitrate complexes containing N,N′-bis(salicylidene)-4-methyl-1,3-phenylenediamine (H₂L). The [Eu₂(H₂L)₂(μ-H₂L)₂(NO₃)₆] complex, isostructural with the gadolinium complex, displays nine-coordinate distorted tricapped trigonal prism geometry with a different coordination mode of four undeprotonated salicylaldimines, which act as terminal monodentate and μ-bridging ditopic ligands using exclusively the oxygens as donor atoms with the nitrogen atoms not being involved in the coordination environment. These complexes along with similar lanthanum, erbium, thulium, and lutetium complexes were prepared in situ in a one-step metal promoted condensation reaction between salicylaldehyde and 4-methyl-1,3-phenylenediamine in the presence of lanthanide nitrates. They were isolated and characterized by microanalysis and spectroscopic (IR, ESI–MS, UV–Vis, and ¹H NMR) data with reference to the preformed N,N′-bis(salicylidene)-4-methyl-1,3-phenylenediamine, which was obtained separately and structurally determined by single crystal X-ray analysis.     

A series of pure inorganic eight-connected self-catenated network based on Silverton-type polyoxometalate

In this paper, three pure inorganic eight-connected self-catenated networks based on the Silverton-type polyoxometalate [CeMo₁₂O₄₂]⁸⁻ with lanthanide, transition metal and alkali metal cations as linkers: [Li(H₂O)₄]₂Co(H₂O)₄Ce(H₂O)₃[CeMo₁₂O₄₂]·3H₂O (1), H_0.5[Li(H₂O)₄]_2.5[Ni(H₂O)₄]_0.5Ce(H₂O)₃[CeMo₁₂O₄₂]·3H₂O (2) and H[Li(H₂O)₄]₃Ce(H₂O)₃[CeMo₁₂O₄₂]·3H₂O (3) have been successfully synthesized and characterized by elemental analysis, IR spectroscopy, thermal gravimetric analysis, X-ray photoelectron spectroscopy, electrochemical analyses and single crystal X-ray diffraction. The single crystal X-ray diffraction analyses reveal that compounds 1-3 are isostructural. The [Ce^IVMo₁₂O₄₂]⁸⁻ polyoxoanions are connected by Ce⁴⁺ to form the infinite 1D chains. And then the parallel stacking chains linked by transition metal cations and lithium ions construct to an eight-connected self-catenated 4²⁴56³ topology framework.     

Molecular assemblies based on the template-directing effect of anionic polyoxometalate clusters and organic cationic flexibility

Molecular assembly processes by utilizing the template-directing effect of anionic polyoxometalate clusters and the flexible organic cation have achieved three hybrids: (H₂bpp)(Hbpp)[PMo₁₂O₄₀]·3DMF (1), (H₂bpp)(Hbpp)(bpp)₂ [PMo₉V₃O₄₀(VO)₂]₂ (2), and (H₂bpp)₂[β-Mo₈O₂₆] (3) (bpp = 1,3-bis(4-pyridyl)propane). Three compounds were characterized using single crystal X-ray crystallography, elemental analysis, IR, XPS, EPR, voltammetric behavior and TGA. Crystal structural analyses revealed that compounds 1–3 were all constructed from protonated organic bpp cations with different POM clusters: isolated α-Keggin P–Mo cluster in 1; dimer of bi-capped α-Keggin P–Mo–V anions linked through a {V₂O₂} unit in 2; β-octamolybdate isopolyanion in 3, respectively. All three assemblies demonstrated a higher thermal stability. The protonated bpp cations lost at temperature higher than 300 °C that the strong intermolecular interactions may account for the high initial temperature of weight loss. The electrochemistry property of compound 2 modified carbon paste electrode was also studied in 1 M H₂SO₄ aqueous.     

Self-assembly and alterable relaxivity of an organic cation-encapsulated gadolinium-containing polyoxometalate

A new amphiphilic molecule bearing poly(ethylene oxide) (PEO) and quaternary ammonium group, was designed and synthesized to encapsulate paramagnetic Gd(III)-containing polyoxometalate (Gd-POM) through electrostatic interaction for obtaining a water-soluble organic-inorganic hybrid building block based on POM. The yielding organic cation-encapsulated Gd-POM (OCEP-Gd) complex exhibited water-solubility and amphiphility, leading to the spontaneous self-assembly into a regular vesicular structure with PEO chains towards water phase and POM units locating at the middle. The vesicular aggregate which has a regular monolayer structure, was further studied by means of dynamic light scattering, transmission electron microscopy, and X-ray diffraction. Due to the synergy of different building units, the self-assembly of the complexes was demonstrated to be efficient to adjust the ability of Gd-POM to accelerate relaxation of water-proton, which results from the paramagnetic property of Gd-POM, to a large extent. The present work provides a new methodology to obtain water-soluble hybrid building blocks based on POM, which may generate more hybrid self-assembly structures in aqueous solution and further direct POM-based materials towards biomedical applications.