Hybrid polyoxovanadates: anion-influenced formation of nanoscopic cages and supramolecular assemblies of asymmetric clusters

Two novel hybrid polyoxovanadates that are functionalized by (4-aminophenyl)arsonic acid ligands form upon the reduction of vanadates(V) in aqueous systems, whereby the underlying condensation reactions are influenced by the nature of the employed acid. In the presence of Cl(-) ions that derive from hydrochloric acid, a tetradecanuclear cage structure [V(IV)(14)O(16)(OH)(8)-(O(3)AsC(6)H(4)-4-NH(2))(10)](4-), whose cavity contains stabilizing halide ions and water molecules, is obtained. When nitric acid is used, a decanuclear [V(10)O(18)(O(3)AsC(6)H(4)-4-NH(2))(7)(DMF)(2)](5-) cluster can be isolated. The latter organizes into a hexagonal packing arrangement in the solid state.     

Self-assembly of alkyl-substituted cubic siloxane cages into ordered hybrid materials

Siloxane-organic hybrids with well-ordered mesostructures were synthesized through the self-assembly of novel amphiphilic molecules that consist of cubic siloxane heads and hydrophobic alkyl tails. The monoalkyl precursors functionalized with ethoxy groups (C(n)H(2n+1)Si(8)O(12)(OEt)(7), 1 Cn, n=16, 18, and 20) were hydrolyzed under acidic conditions with the retention of the siloxane cages, leading to the formation of two-dimensional hexagonal phases by evaporation-induced self-assembly processes. Analysis of the solid-state (29)Si MAS NMR spectra of these hybrid mesostructures confirmed that the cubic siloxane units were cross-linked to form siloxane networks. Calcination of these hybrids gave mesoporous silica, the pore diameter of which varied depending on the alkyl-chain length. We also found that the precursors that had two alkyl chains formed lamellar phases, thus confirming that the number of alkyl chains per cage had a strong influence on the mesostructures. These results expand the design possibility of novel nanohybrid and nanoporous materials through the self-assembly of well-defined oligosiloxane-based precursors.     

1,4-Benzene-bridged covalent hybrid of triarylamine and cyclometalated ruthenium: a new type of organic-inorganic mixed-valent system

A 1,4-benzene-bridged covalent hybrid of triarylamine and cyclometalated ruthenium 1(2+) was isolated as a bench-stable open-shell substance. The free spin in this complex is mainly associated with the triarylamine unit, as indicated by EPR and DFT calculations and electrochemical analysis. It exhibits an intense intervalence-charge-transfer transition around 1050 nm that is not present in 1(+) and 1(3+).     

Phosphomolybdate clusters as molecular building blocks in the design of one-, two- and three-dimensional organic-inorganic hybrid materials

An attractive approach to the design of inorganic solids exploits the tethering of inorganic clusters through organic spacers to produce hybrid materials with composite properties. We have recently described a modified strategy in which polyoxometalate clusters are linked through organic subunits to give an anionic hybrid substructure which may be further modified through the introduction of secondary metal-ligand complex (SMLC) cations, serving as a third component building block. In this application, the molybdophosphonate cluster {Mo₅O₁₅(O₃PR)₂}⁴⁻ serves as a secondary building unit (SBU) with alkyl (CH₂)_n or aromatic -(C₆H₄)_n- tethers providing one-dimensional structural expansion. A binucleating ligand such as tetrapyridylpyrazine (tpyprz) is used to bridge secondary metal sites into a binuclear {Cu₂(tpyprz)}⁴⁺ SBU which may link phosphomolybdate clusters into two- or three-dimensional structures. The influence of a variety of structural determinants is discussed, including the tether length of the diphosphonate ligand, the coordination preferences of the secondary metal, expansion of the ligand component of the SMLC, and substitution of As for P in the oxide SBU.     

Self-assembly of organic-inorganic hybrid amphiphilic surfactants with large polyoxometalates as polar head groups

Mn-Anderson-C6 and Mn-Anderson-C16, A type of inorganic-organic hybrid molecules containing a large anionic polyoxometalate (POM) cluster and two C6 and C16 alkyl chains, respectively, demonstrate amphiphilic surfactant behavior in the mixed solvents of acetonitrile and water. The amphiphilic hybrid molecules can slowly assemble into membrane-like vesicles by using the POM clusters as polar head groups, as studied by laser light scattering and TEM techniques. The hollow vesicles have a typical bilayer structure with the hydrophilic Mn-Anderson cluster facing outside and long hydrophobic alkyl chains staying inside to form the solvent-phobic layer. Due to the rigidity of the POM polar heads, the two alkyl tails have to bend significantly for the vesicle formation, which makes the vesicle formation more difficult compared to some conventional surfactants. This is the first example of using hydrophilic POM macroions as polar head groups for a surfactant system.     

Formation of high nuclearity mixed-valent polyoxovanadates in the presence of copper amine complexes

Two new Müller-type clusters, a one-dimensional solid [Cu(en)]_{2 4}[Cl ⊂V₁₅O₃₆]−12H₂O1, and a three-dimensional solid [Cu(pn)]_{2 4}[Cl ⊂V₁₈O₄₂]·12H₂O2, have been synthesised by employing identical hydrothermal conditions e_xcept varying the nature of organic diamine.1 crystallised in a chiral space groupP2₁2₁2₁ witha = 12.757(1),b = 18.927(2) andc = 28.590(3) ?, andZ =4.2 crystallised in a tetragonal system with space groupP4/nnc,a = 15.113(1) andc = 18.542(3) ?, andZ = 2. Mixed-valent vanadium ions in structures1 and2 have been established both by magnetisation and bond-length bond-valence measurements. Chemistry of formation of high nuclearity polyoxovanadate clusters is discussed. Dedicated to Prof J Gopalakrishnan on his 62nd birthday.     

Self-assembly in inorganic and hybrid systems: beyond the molecular scale

The availability of well defined inorganic nanocrystals allows the construction of aligned structures with characteristic architectural elements in the nanometer range, the mesoscale. Contrary to alignment driven by external conditions and fields, we focus on strict "self-effects", where the organization is already encoded in the shape and mutual interaction potentials of the particles. This Perspective discusses the potential of this approach for generating valuable functional inorganic mesostructures.     

Synthesis of molecular imprinted organic-inorganic hybrid polymer binding caffeine

Random copolymers of poly{(methacrylamide)-co-(vinyl trimethoxysilane)} and poly{(methacrylic acid)-co-(vinyl trimethoxysilane)} were synthesized via a free radical polymerization reaction. Acid catalyzed sol-gel process of tetraethylorthosilicate (TEOS) with aforementioned polymers in the presence of methyl xanthine class of alkaloid like caffeine resulted in the formation of highly transparent monoliths. Solvent extraction of the template leaves behind the recognition sites intact with high selectivity towards the print molecule. The ionic and non-specific adsorptions, which are considered to be the main disadvantages of the molecularly imprinted polymers (MIP), are prevented to a considerable extent by the end capping of surface silanol groups. The template binding efficiencies of MIP were determined by HPLC analysis.     

A molecular dynamics simulation of a homogeneous organic-inorganic hybrid silica membrane

A new molecular dynamics simulation method was successfully applied to construct a homogeneous organic-inorganic hybrid silica membrane using the hybrid-pcff (h-pcff) potential function. Analysis suggested that the hybrid BTESE silica membrane provided a looser network and larger cavity size for the enhancement of gas permeability and selectivity.     

Two organic-inorganic poly(pseudo-rotaxane)-like composite solids constructed from polyoxovanadates and silver organonitrogen polymers

Two new composite solids, [Ag(btx)]₄H₂V₁₀O₂₈·2H₂O 1 and [Ag(biim)]₂V₄O₁₁2 (btx=1,4-bis(triazol-1-ylmethyl)benzene, biim=1,1′-(1,4-butanediyl)bis(imidazole)), have been synthesized and characterized by elemental analysis, IR, TGA and single-crystal X-ray diffraction. Compound 1 contains two one-dimensional (1D) polymeric chains, {Ag₂(btx)₂}²⁺ and [Ag₂(btx)₂H₂V₁₀O₂₈]²⁻, that are assembled by supramolecular forces into an intriguing two-dimensional (2D) poly(pseudo-rotaxane) network. Compound 2 comprises cationic {Ag₂(biim)₂}²⁺ three-dimensional (3D) framework penetrated by anionic chains. The electrochemical properties of the two compounds have been studied.     

Self-assembly and anion encapsulation properties of cavitand-based coordination cages

Two novel classes of cavitand-based coordination cages 7a--j and 8a--d have been synthesized via self-assembly procedures. The main factors controlling cage self-assembly (CSA) have been identified in (i) a P--M--P angle close to 90 degrees between the chelating ligand and the metal precursor, (ii) Pd and Pt as metal centers, (iii) a weakly coordinated counterion, and (iv) preorganization of the tetradentate cavitand ligand. Calorimetric measurements and dynamic (1)H and (19)F NMR experiments indicated that CSA is entropy driven. The temperature range of the equilibrium cage-oligomers is determined by the level of preorganization of the cavitand component. The crystal structure of cage 7d revealed the presence of a single triflate anion encapsulated. Guest competition experiments revealed that the encapsulation preference of cages 7b,d follows the order BF(4)(-) > CF(3)SO(3)(-) > PF(6)(-) at 300 K. ES-MS experiments coupled to molecular modeling provided a rationale for the observed encapsulation selectivities. The basic selectivity pattern, which follows the solvation enthalpy of the guests, is altered by size and shape of the cavity, allowing the entrance of an ancillary solvent molecule only in the case of BF(4)(-).     

Perspectives of aerosol-photopolymerization: organic-inorganic hybrid nanoparticles

The technique of aerosol-photopolymerization is employed for the generation of organic-inorganic spherical polymer-matrix nanocomposites (PMNCs). The loading amount of well-distributed ZnO nanoparticles in polymer networks is varied in a broad range up to 40 wt.%. Similar hybrid particles are produced without the addition of a conventional photoinitiator by making use of the UV absorptivity of ZnO nanoparticles only. Highlights of the process are the continuous, aerosol-based setup with a flow-through photoreactor operated at ambient temperature and atmospheric pressure. Aerosol-photopolymerization possesses great potential of incorporating various materials in situ into a polymer matrix, resulting in hybrid materials for diverse applications. Furthermore, the process can be integrated with further unit operations for the design of smart materials.     

An organic-inorganic hybrid material constructed from a three-dimensional coordination complex cationic framework and entrapped hexadecavanadate clusters

A unique organic-inorganic hybrid compound has been separated under hydrothermal condition, which is constructed from a three-dimensional second metal-organic subunit and entrapped hexadecavanadate clusters.     

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.     

From chain to network: design and analysis of novel organic-inorganic assemblies from organically functionalized zinc-substituted polyoxovanadates and zinc organoamine subunits

A series of novel organic-inorganic assemblies, [Zn(Meen)2]2[(4,4'-bipy)Zn2As8V12O40(H2O)] (1), [Zn(en)2(H2O)][Zn(en)2(4,4'-bipy)Zn2As8V12O40(H2O)].3H2O (2), [[Zn(en)3]2[Zn2As8V12O40(H2O)]].4H2O.0.25bipy (3) and [Zn2(en)5][[Zn(en)2][(bpe)HZn2As8V12O40(H2O)]2].7H2O (4) [en = ethylenediamine, Meen = 1,2-diaminopropane, 4,4'-bipy = 4,4'-bipyridine, and bpe = 1,2-bis(4-pyridyl)ethane] constructed from organically modified Zn-substituted polyoxovanadates and zinc organoamine subunits have been synthesized. Each anion cluster of compound 1 is directly linked by the 4,4'-bipy ligand into a one-dimensional (1D) straight chain. The secondary metal complex [Zn(Meen)2]2+ acts as an isolated countercation. The 1D chain structure of 2 is similar to that of 1 but sinuate because of the secondary metal complex [Zn(en)2]2+ decorated on the anion cluster. The en ligands covalently bonding to the surface anion of 3 not only support the secondary metal complex [Zn(en)2]2+ but also coordinate to another anion through the secondary metal complex [Zn(en)2]2+ bridge to form an "eight-shaped" chiral helix. The unprecedented 2D layer of compound 4 with large nanosized inner rectangular cavities [33.669(6) x 14.720(8) A] is successfully achieved through the anion clusters polymerized first into chains by flexible organic ligands and then secondary metal complexes bridged between the chains. The different coordination abilities and geometries of the bidentate organodiamine ligands used in the four-reaction systems play important roles in the formation of the final structures: from straight chains to sinuate chains, to helical chiral chains, and finally to a 2D layer with helices.     

Metal atoms and clusters in fullerene cages

Fullerenes containing metal atoms and clusters can be formed by the arc-vaporization method. The electronic structure of these metallofullerenes can be probed using magnetic resonance techniques. Electron paramagnetic resonance (EPR) spectra of LaC₈₂, YC₈₂, ScC₈₂ and Sc₃C₈₂ have been obtained. Metallofullerenes containing a single metal atom (MC₈₂ with M = La, Y, or Sc), have small hyperfine couplings and g-values close to 2, implying that they can be described as + 3 metal cations within — 3 fullerene radical anion cages. In the La and Y cases, additional EPR active MC₈₂ species have been found. The EPR spectrum of Sc₃C₈₂ shows that the metal atoms are equivalent, suggesting that they may form a triangular molecule. No EPR spectrum is found for Y₂C₈₂ or for Sc₂C_2n species (with 2n = 82,84,86), suggesting that they are diamagnetic. Sc NMR spectra of a solution containing Sc₂C_2n species have been obtained which confirm the diamagnetism of the discandium metallofullerenes.