Surface-grafted lanthanoid complexes of the tungstosilicate polyanion [SiW12O40]4−: a synthetic, structural and computational investigation

Single-molecule magnets within polyoxometalate-based frameworks

As an extension of our interest in polyoxometalates (POMs) and lanthanoids, we report the design and synthesis of two polyoxometalate-based frameworks under hydrothermal conditions; [Ho₄(PDA)₄(H₂O)₁₁][(SiO₄)@W₁₂O₃₆]·8H₂O (1) and [Tb₄(PDA)₄(H₂O)₁₂][(SiO₄)@W₁₂O₃₆]·4H₂O (2) (H₂PDA = 1,10-phenanthroline-2,9-dicarboxylic acid). Both hybrids have been characterized by elemental analysis, Fourier transform infrared spectroscopy, thermogravimetric analysis, and powder/single-crystal X-ray diffraction. According to the structural analysis, 1 and 2 consist of 2D-cationic coordination polymers based on the respective lanthanoids and PDA^2- as well as Keggin anions that reside in the interspaces between two adjacent layers as discrete counterions connected by extensive hydrogen bonding. Although the overall structures of 1 and 2 are composed of cationic and anionic layers, there are many differences in the cationic layers such as various coordination modes of PDA^2-, different void shapes, and the existence of dinuclear Tb(III) units only in 2. Frameworks 1 and 2 were further characterized by dc and ac magnetic measurements and both exhibit slow relaxation of magnetization at low temperatures under an applied dc field. Their single-molecule magnet (SMM) properties are investigated, where weak field-induced SMM behaviour is observed at low temperatures in dynamic magnetic studies.

Supramolecular network of a framework material supported by the anion-π linkage of Keggin-type heteropolyoxotungstates: experimental and theoretical insights

A novel inorganic-organic hybrid, {Na[Tb(L)(H2O)4]3(SiW12O40)}·4H2O, in which L = 4-hydroxypyridine-2,6-dicarboxylic acid (chelidamic acid), was prepared and characterized by elemental analyses, Fourier transform infrared spectroscopy, and single-crystal X-ray diffraction. It consists of a trinuclear terbium cluster bonded to a Keggin-type polyoxotungstate. In the terbium cluster three crystallographically equivalent Tb3+ ions are bridged by carboxylate oxygen atoms belonging to three chelidamic acid units to form an equilateral triangle. These triangle units connect to each other through hydrogen-bonding and anion-π interactions. A CSD survey was carried out focusing on the cation and the ligand roles in the structure formation of this type of compounds. Finally, DFT calculations were used to analyse the role of the anion-π interactions that are important in the formation of 1D supramolecular assemblies in the solid state. The noncovalent interaction plot (NCIplot) computational tool has been used to characterize the anion-π interaction.

Supramolecular networks supported by the anion...π linkage of Keggin-type heteropolyoxotungstates

Anion...π interactions are newly recognized weak supramolecular forces which are relevant to many types of electron-deficient aromatic substrates. Being less competitive with respect to conventional hydrogen bonding, anion...π interactions are only rarely considered as a crystal-structure-defining factor. Their significance dramatically increases for polyoxometalate (POM) species, which offer extended oxide surfaces for maintaining dense aromatic/inorganic stacks. The structures of tetrakis(caffeinium) μ₁₂-silicato-tetracosa-μ₂-oxido-dodecaoxidododecatungsten trihydrate, (C₈H₁₁N₄O₂)₄[SiW₁₂O₄₀]·3H₂O, (1), and tris(theobrominium) μ₁₂-phosphato-tetracosa-μ₂-oxido-dodecaoxidododecatungsten ethanol sesquisolvate, (C₇H₉N₄O₂)₃[PW₁₂O₄₀]·1.5C₂H₅OH, (2), support the utility of anion...π interactions as a special kind of supramolecular synthon controlling the structures of ionic lattices. Both caffeinium [(HCaf)⁺ in (1)] and theobrominium cations [(HTbr)⁺ in (2)] reveal double stacking patterns at both axial sides of the aromatic frameworks, leading to the generation of anion...π...anion bridges. The latter provide the rare face-to-face linkage of the anions. In (1), every square face of the metal-oxide cuboctahedra accepts the interaction and the above bridges yield flat square nets, i.e. {(HCaf⁺)₂[SiW₁₂O₄₀]^4-}_n. Two additional cations afford single stacks only and they terminate the connectivity. Salt (2) retains a two-dimensional (2D) motif of square nets, with anion...π...anion bridges involving two of the three (HTbr)⁺ cations. The remaining cations complete a fivefold anion...π environment of [PW₁₂O₄₀]^3-, acting as terminal groups. This single anion...π interaction is influenced by the specific pairing of (HTbr)⁺ cations by double amide-to-amide hydrogen bonding. Nevertheless, invariable 2D patterns in (1) and (2) suggest the dominant role of anion...π interactions as the structure-governing factor, which is applicable to the construction of noncovalent linkages involving Keggin-type oxometalates.

An inorganic–organic hybrid supramolecular framework based on the γ-[Mo8O26]4− cluster and cobalt complex of aspartic acid: X-ray structure and DFT study

A new inorganic–organic hybrid based on an aspartate functionalized polyoxomolybdate, [pentaaquacobalt(II)]-μ-aspartate-[γ-octamolybdate]-μ-aspartate-[pentaaquacobalt(II)] tetrahydrate, [Co2(C4H6NO4)2(γ-Mo8O26)(H2O)10]·4H2O (1), has been synthesized under hydrothermal conditions from the reaction of an Evans–Showell-type polyoxometalate, (NH4)6[Co2Mo10H4O38], and L-aspartic acid. The complex exhibits a supramolecular three-dimensional framework structure in the crystal lattice. Compound 1 was structurally characterized by elemental analyses, IR and UV–Vis (diffuse reflectance) spectroscopy and single-crystal X-ray diffraction. In this compound, aspartic acid acts as a bridge between the two Co atoms and the Mo centres, with the –CH2COOH side chain directly linked to the Mo centre in γ-[Mo8O26]4− and the α-carboxylate side chain bound to the Co centre. Commonly, the binding of transition-metal complexes to POMs involves coordination of the metal to a terminal O atom of the POM so that 1, with a bridging ligand between Mo and Co atoms, belongs to a separate class of hybrid materials. While the starting materials are both chiral and one might expect them to form a chiral hybrid, the decomposition of the chiral Evans–Showell-type POM and its conversion to the centrosymmetric γ-octamolybdate POM, plus the presence of two aspartate ligands centrosymmetrically placed on either side of the POM, leads to the formation of an achiral hybrid. We have studied energetically by means of density functional theory (DFT) calculations and using the Bader's `atoms-in-molecules' analysis the electrostatically enhanced hydrogen bonds (EEHBs) observed in the solid state of 1, which are crucial for the formation of one-dimensional supramolecular assemblies.

On the preferences of five-membered chelate rings in coordination chemistry: insights from the Cambridge Structural Database and theoretical calculations

The purpose of this review is to give an overview of three important N-bidentate ligands: 1,10-phenanthroline (phen), 2,2'-bipyridine (bpy), and ethylenediamine (en). We have not attempted to be comprehensive because of the huge amount of activity being done in coordination chemistry using these ligands. Instead we present a full structural and geometrical study by using the Cambridge Structural Database (CSD) combined with theoretical calculations that allow us to parameterize their coordinating properties and ability to coordinate to transition and non-transition metals. More importantly, we illustrate that upon coordination and formation of the five-membered chelate ring, these ligands are able to adapt themselves to the requirements of the different metals by changing the MN distances and NMN angles. Therefore, a redefinition of the preferences of these ligands to metals with large ionic radii is needed. Finally, we will present some facts about the participation of these ligands in inorganic-organic hybrids (IOHs) based on Keggin polyoxometalates (POMs).