Javier Cepeda

Lanthanide(III)/Pyrimidine-4,6-dicarboxylate/Oxalate Extended Frameworks: A Detailed Study Based on the Lanthanide Contraction and Temperature Effects

Detailed structural, magnetic, and luminescence studies of six different crystalline phases obtained in the lanthanide/pyrimidine-4,6-dicarboxylate/oxalate system have been afforded: {[Ln(μ-pmdc)(μ-ox)(0.5)(H(2)O)(2)]·3H(2)O}(n) (1-Ln), {[Ln(μ-pmdc)(μ-ox)(0.5)(H(2)O)(3)]·2H(2)O}(n) (2-Ln), {[Ln(μ(3)-pmdc)(μ-ox)(0.5)(H(2)O)(2)]·~2.33H(2)O}(n) (3-Ln), {[Ln(2)(μ(3)-pmdc)(μ(4)-pmdc)(μ-ox)(H(2)O)(3)]·5H(2)O}(n) (4-Ln), {[Ln(μ(3)-pmdc)(μ-ox)(0.5)(H(2)O)(2)]·H(2)O}(n) (5-Ln), and [Ln(pmdc)(1.5)(H(2)O)(2.5)] (6-Ln). The slow generation of the oxalate (ox) anion, obtained from the in situ partial hydrothermal decomposition of the pyrimidine-4,6-dicarboxylate (pmdc) ligand, allows us to obtain good shaped single crystals, while direct addition of potassium oxalate provides the same compounds but as polycrystalline samples. The crystal structures of all compounds are based on the double chelation established by the pmdc and ox ligands to provide distorted 2D honeycomb layers that, in some cases, are fused together, leading to 3D systems, by replacing some of the coordinated water molecules that complete the coordination sphere of the lanthanide by uncoordinated carboxylate oxygen atoms of the pmdc. The presence of channels occupied by crystallization water molecules is also a common feature with the exception of compounds 5-Ln. It is worth noting that compounds 3-Ln present a commensurate crystal structure related to the partial occupancy of the crystallization water molecules placed within the channels. Topological analyses have been carried out, showing a previously nonregistered topology for compounds 4-Ln, named as jcr1. The crystal structures are strongly dependent on the lanthanide ion size and the temperature employed during the hydrothermal synthesis. The lanthanide contraction favors crystal structures involving sterically less hindranced coordination environments for the final members of the lanthanide series. Additionally, reinforcement of the entropic effects at high temperatures directs the crystallization process toward less hydrated crystal structures. The magnetic data of these compounds indicate that the exchange coupling between the lanthanide atoms is almost negligible, so the magnetic behavior is dominated by the spin-orbit coupling and the ligand field perturbation. The luminescence properties that exhibit the compounds containing Nd(III), Eu(III), and Tb(III) have been also characterized.

Synthetic control to achieve lanthanide(III)/pyrimidine-4,6-dicarboxylate compounds by preventing oxalate formation: structural, magnetic, and luminescent properties.

Control over the synthetic conditions in many metal/diazinedicarboxylato systems is crucial to prevent oxalate formation, since dicarboxylato ligands easily undergo degradation in the presence of metal salts. We report here an efficient route to obtain oxalato-free compounds for the lanthanide/pyrimidine-4,6-dicarboxylato (pmdc) system on the basis of the reaction temperature and nonacidic pH or oxygen free atmosphere. Two different crystal architectures have been obtained: {[Ln(μ-pmdc)(1.5)(H(2)O)(3)]·xH(2)O}(n) (1-Ln) and {[Ln(2)(μ(4)-pmdc)(2)(μ-pmdc)(H(2)O)(2)]·H(2)O}(n) (2-Ln) with Ln(III) = La-Yb, except Pm. Both crystal structures are built from distorted two-dimensional honeycomb networks based on the recurrent double chelating mode established by the pmdc. In compounds 1-Ln, the tricapped trigonal prismatic coordination environment of the lanthanides is completed by three water molecules, precluding a further increase in the dimensionality. Crystallization water molecules are arranged in the interlamellar space, giving rise to highly flexible supramolecular clusters that are responsible for the modulation found in compound 1-Gd. Two of the coordinated water molecules are replaced by nonchelating carboxylate oxygen atoms of pmdc ligands in compounds 2-Ln, joining the metal-organic layers together and thus providing a compact three-dimensional network. The crystal structure of the compounds is governed by the competition between two opposing factors: the ionic size and the reaction temperature. The lanthanide contraction rejects the sterically hindered coordination geometries whereas high-temperature entropy driven desolvation pathway favors the release of solvent molecules leading to more compact frameworks. The characteristic luminescence of the Nd, Eu, and Tb centers is improved when moving from 1-Ln to 2-Ln compounds as a consequence of the decrease of the O-H oscillators. The magnetic properties of the compounds are dominated by the spin-orbit coupling and the ligand field perturbation, the exchange coupling being almost negligible.

Influence of the synthetic conditions on the structural diversity of extended manganese-oxalato-1,2-bis(4-pyridyl)ethylene systems.

We report herein the synthesis and physicochemical characterization of eight new manganese-oxalato compounds with 1,2-bis(4-pyridyl)ethylene (bpe): {(Hbpe)(2)[Mn(2)(μ-ox)(3)]·∼0.8(C(2)H(5)OH)·∼0.4(H(2)O)}(n) (1), {[Mn(μ-ox)(μ-bpe)]·xH(2)O}(n) (2), [Mn(2)(μ-ox)(2)(μ-bpe)(bpe)(2)](n) (3), [Mn(μ-ox)(μ-bpe)](n) (4a and 4b), and {[Mn(4)(μ-ox)(3)(μ-bpe)(4)(H(2)O)(4)]·(X)(2)·mY}(n) with X = NO(3)(-) (5a), Br(-) (5b), and ClO(4)(-) (5c) and Y = solvation molecules. The appropriate selection of the synthetic conditions allowed us to control the crystal structure and to design extended 2D and 3D frameworks. Compound 1 is obtained at acid pH values and its crystal structure consists of stacked [Mn(2)(μ-ox)(3)](2-) layers with cationic Hbpe(+) molecules intercalated among them. Compound 2 was obtained at basic pH values with a manganese/bpe ratio of 1:1, and the resulting 3D structure consists of an interpenetrating framework in which metal-oxalato chains are bridged by bpe ligands, leading to a microporous network that hosts a variable number of water molecules (between 0 and 1) depending on the synthetic conditions. Compound 3, synthesized with a manganese/bpe ratio of 1:3, shows a 2D framework in which linear metal-oxalato chains are joined by bis-monodentate 1,2-bis(4-pyridyl)ethylene ligands. The thermal treatment of compound 3 permits the release of one of the bpe molecules, giving rise to two new 2D crystalline phases of formula [Mn(μ-ox)(μ-bpe)](n) (4a and 4b) depending on the heating rate. The open structures of 5a-5c were synthesized in a medium with a high concentration of nitrate, perchlorate, or bromide salts (potassium or sodium as cations). These anions behave as templating agents directing the crystal growing toward a cationic porous network, in which the anions placed in the voids and channels of the structure present high mobility, as inferred from the ionic exchange experiments. Variable-temperature magnetic susceptibility measurements show an overall antiferromagnetic behavior for all compounds, which are discussed in detail.

Tuning the luminescence performance of metal–organic frameworks based on d10 metal ions: from an inherent versatile behaviour to their response to external stimuli

Metal–organic frameworks (MOFs) are an interesting class of porous crystalline solids assembled from organic ligands and metal clusters (or simply metal cations), which have shown a rich variety of promising applications. In recent years, these interesting materials have become one of the most actively studied areas in the field of new materials development. One of the most interesting aspects of MOFs is their optical properties and porosity, which may be smartly coupled with the aim of creating multifunctional materials. This work gives an overview of the most significant achievements on this particular theme by bringing an up-to-date collection of work focused on tuning the luminescent emission of MOFs, according to structural changes derived from their porous nature.

Porous supramolecular compound based on paddle-wheel shaped copper(II)–adenine dinuclear entities

The reaction between CuCl2 and adenine in a non-aqueous solvent provides a 3D porous structure based on paddle-wheel [Cu2(µ-adenine)4Cl2]2+ cations and Cl− anions that are held together by a robust supramolecular hydrogen bonding network. The desolvated compound is able to host different guest molecules within the ∼6 A diameter 1D channels.

Porous MII/Pyrimidine‐4,6‐Dicarboxylato Neutral Frameworks: Synthetic Influence on the Adsorption Capacity and Evaluation of CO2‐Adsorbent Interactions

The understanding of the factors that affect the real pore-network structure for a given bulk material due to different synthetic procedures is essential to develop the material with the best adsorption properties. In this work, we have deeply studied the influence of the crystallinity degree over the adsorption capacity on three new isostructural MOFs with the formula {[CdM(μ4-pmdc)2(H2O)2]⋅solv}n (in which, pmdc = pyrimidine-4,6-dicarboxylate; solv = corresponding solvent; M(II) = Cd (1), Mn (2), Zn (3)). Compared with other methods, the solvent-free synthesis stands as the most effective route because, apart from enabling the preparation of the heterometallic compounds 2 and 3, it also renders the adsorbents with the highest performance, which is indeed close to the expected one derived from Grand Canonical Monte Carlo (GCMC) calculations. The structural analysis of the as-synthesised and evacuated frameworks reveals the existence of a metal atom exposed to the pore. The accessibility of this site is limited due to its atomic environment, which is why it is considered as a pseudo-open-metal site. The chemical and physical characterisation confirms that this site can be modified as the metal atom is replaced in compounds 2 and 3. To assess the effect of the metal replacement on the adsorption behaviour, an exhaustive study of CO2 experimental isotherms has been performed. The affinity of the pseudo-open metal sites towards CO2 and the distribution of the preferred adsorption sites are discussed on the basis of DFT and GCMC calculations.

Controlling interpenetration for tuning porosity and luminescence properties of flexible MOFs based on biphenyl-4,4′-dicarboxylic acid

Four new compounds based on zinc(II) or cadmium(II) metal ions and elongated dicarboxylate and bipyridine ligands, namely, {[Cd3(μ4-bpdc)3(H2O)2]·DMF}n (1), {[Zn3(μ4-bpdc)3(μ-bpdb)]·5DMF}n (2), {[Zn2(μ4-bpdc)2(μ-bpdb)]·7DMF}n (3), and {[Zn4(μ4-bpdc)3(DMF)(μ4-O)(H2O)]·7DMF·3H2O}n (4), (where bpdc = biphenyl-4,4′-dicarboxylate, bpdb = 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene, DMF = dimethylformamide) have been synthesised under solvothermal conditions and structurally characterised by single crystal X-ray diffraction. The crystal structures range from 2D (in 1) to 3D (2, 3, and 4) systems according to the coordination mode acquired by the bpdc ligand and the presence of an ancillary linker. Compound 1 consists of stacked Cd-bpdc neutral layers containing isolated small voids. The coordination of the bpdb ligand (2 and 3) or the formation of a tetrahedral Zn4O cluster (in 4) generates highly open 3D architectures that share the structural feature of being doubly interpenetrated. A careful computational analysis on the crystal structures permits unravelling their void systems. Moreover, characterising the photoluminescence emission of the compounds at variable excitation wavelengths provides an opportunity to couple the luminescence response with their porosity, which could signify the potential utility of these materials as photofluorescent sensors for small adsorbates.

Two appealing alternatives for MOFs synthesis: solvent-free oven heating vs. microwave heating†

Herein we present a solvent-free process to afford the synthesis of imidazolate and carboxylate based MOFs performed under moderate oven-heating or fast microwave irradiation, paying special attention to the yield and adsorption performance of the products. The measured adsorption surface area values of the resulting samples compare well, and in several cases surpass, the maximum surface area previously reported for the same compounds prepared using a solvent based synthesis.

Slow relaxation of magnetization in 3D-MOFs based on dysprosium dinuclear entities bridged by dicarboxylic linkers

Three novel metal–organic-frameworks (MOFs) based on dysprosium as the metal and dicarboxylic ligands have been solvothermally synthesized with the aim of studying and modulating their magnetic properties according to the variation of the distances between metal centers. These materials display intense photoluminescence properties in the solid state at room temperature. In addition, a very interesting property of compound 1 is that it exhibits slow relaxation of magnetization with an activation energy barrier of 32 K. Magneto-structural correlations have been analyzed.

Experimental and Theoretical Study of a Cadmium Coordination Polymer Based on Aminonicotinate with Second-Timescale Blue/Green Photoluminescent Emission

A new cadmium/6-aminonicotinate-based coordination polymer (CP) with an unprecedented multicolored and long-lasting emission is reported. This material shows a blue fluorescence which rapidly turns to green persistent phosphorescence with a lifetime of nearly 1 s. Time-dependent density functional theory calculations revealed that electronic transitions arising from both first excited singlet and triplet states involving ligand-centered and ligand-to-metal charge-transfer mechanisms are responsible for such behavior.