Jorge Pasán

Structural versatility of the malonate ligand as a tool for crystal engineering in the design of molecular magnets

The synthesis of ferro- and ferri-magnetic systems with a tunable Tc and three-dimensional (3-D) ordering from molecular precursors implying transition metal ions is one of the active branches of molecular inorganic chemistry. The nature of the interactions between the transition metal ions (or transition metal ions and radicals) is not so easy to grasp by synthetic chemists working in this field since it may be either electrostatic (orbital) or magnetic (mainly dipolar). Therefore, the systems fulfilling the necessary requirements to present the expected magnetic properties are not so easy to design on paper and realize in the beaker. In this work we show how the design of one-, two- and three-dimensional materials can strongly benefit from the use of crystal engineering techniques, which can give rise to structures of different shapes, and how these differences can give rise to different properties. We will focus on the networks constructed by assembling malonate ligands and metal centres. The idea of using malonate (dianion of propanedioic acid, H2mal) is that it can give rise to different coordination modes with the metal ions it binds. Extended magnetic networks of dimensionalities one (1-D), two (2-D) and three (3-D) can be chemically constructed from malonate-bridged metallic complexes. These coordination polymers behave as ferro-, ferri- or canted antiferro-magnets. We are currently trying to obtain analogous compounds using magnetically anisotropic ions, such as cobalt(II), in order to explore how structural differences influence the magnetic properties. In this case the control of the spatial arrangement of the magnetic building blocks is of paramount importance in determining the strength of the magnetic interaction. The possibility of controlling the shape of the networks depends on the coordination bond between the metal ion and the ligands and on supramolecular interactions such as stacking interactions or hydrogen bonding.

Malonate-based copper(II) coordination compounds: ferromagnetic coupling controlled by dicarboxylates

Studies on structural and magnetic properties of polynuclear transition metal complexes, aimed at understanding the structural and chemical factors governing electronic exchange coupling mediated by multiatom bridging ligands, are of continuing interest to design new molecular materials exhibiting unusual magnetic, optical and electrical properties, bound to their molecular nature. Looking at potentially flexible bridging ligands, the malonate group seems a suitable candidate. The occurrence of two carboxylate groups in the 1,3 positions allows this ligand to adopt simultaneously chelating bidentate and different carboxylato bridging modes (syn–syn, anti–anti and syn–anti trough one or two carboxylate groups) In the course of our research we have structurally and magnetically characterized several carboxylato bridged copper(II) complexes. In the present study we start describing briefly the structure and the magnetic behaviour of the compounds, subsequently we analyze the magneto-structural correlations concluding that the parameter that governs, in first order, the magnetic interaction between metal centres is the relative position of the carboxylato bridge of the malonate respect to the copper(II) ions: equatorial–equatorial (strong interaction), equatorial–apical (weak interaction) and apical–apical (negligible interaction). Inside this division another parameters become important such as β (angle between copper(II) basal planes) in the equatorial–equatorial, or the distortion t in the equatorial–apical.

Highly Selective Chemical Sensing in a Luminescent Nanoporous Magnet

Among the wide variety of properties of interest that a given material can exhibit, luminescence is attracting an increasing attention due to its potential application in optical devices for lighting equipment and optical storage, [ 1a − c] optical switching, [ 1d ,e] and sensing. [ 1f − i ] At this respect, many scientists, working in the multidisciplinary fi eld of the materials science, have directed their efforts to the obtention of luminescent materials with potential sensing applications. For instance, sensitive and selective detection of gas and vapor phase analytes can result specially interesting because of the variety of applications that can be found in many different fi elds. A key principle concerning the luminescent chemosensors [ 2 ] is that they must be able to detect differences between small molecules, [ 2 , 3 ] and sequentially implement a recognition– transduction protocol. [ 2b ] In this sense, the remarkable shape selectivity of a class of highly porous materials, the so-called metal-organic frameworks (MOFs) [ 4 ] which have already shown applications in different fi elds (gas storage and separation, molecular recognition and catalysis, molecular electronics and spintronics, molecular photonics, etc) [ 4–6 ] has converted them in excellent candidates for the fabrication of chemical sensors. [ 2 , 3 ] The key point responsible for the high potential success of MOFs as chemo-sensors is the exceptional tunability of their structures and properties.

Crystal engineering of 3-D coordination polymers by pillaring ferromagnetic copper(II)-methylmalonate layers

Three new copper(II) complexes of formula [Cu(Memal)(H2O)]n (1), [Cu2(pyz)(Memal)2] (2) and [Cu2(4,4′-bpy)(Memal)2(H2O)2] (3) (Memal = methylmalonate, pyz = pyrazine and 4,4′-bpy = 4,4′-bipyridine) were obtained and structurally characterized by X-ray diffraction. Complex 1 is a square grid of aquacopper(II) units which are linked by carboxylate-methylmalonate groups in the anti–syn (equatorial–equatorial) coordination mode. The crystal structures of 2 and 3 consist of corrugated layers of copper(II) (2) and aquacopper(II) (3) units with intralayer carboxylate-methylmalonate bridges in the anti–syn (equatorial–apical) coordination mode which are linked through pyrazine (2) and 4,4′-bipyridine (3) ligands; to build up a 3-D network. Magnetic susceptibility measurements of complexes 1–3 in the temperature range 2–300 K show the occurrence of an overall ferromagnetic behaviour with a weak intralayer ferromagnetic coupling (J = +1.61(1) cm−1) in 1 whereas two opposite magnetic interactions occur in 2 and 3, one ferromagnetic through the anti–syn carboxylate (2 and 3) and the other antiferromagnetic through pyz (2) and 4,4′-bpy) (3).

Malonic acid: a multi-modal bridging ligand for new architectures and properties on molecule-based magnets

Abstract In this work, we show how the design of one-, two- and three-dimensional materials can strongly benefit from the use of crystal engineering techniques, which can give rise to structures of different shapes, and how these differences can give rise to different properties. We will focus on the networks constructed by assembling malonate ligands and metal centres. The idea of using malonate (dianion of propanedioic acid, H 2 mal) is that they can give rise to different coordination modes with the metal ions bind. Extended magnetic networks of dimensionalities 1 (1D), 2 (2D) and 3 (3D) can be chemically constructed from malonato-bridged metallic complexes. These coordination polymers behave as ferro-, ferri- or canted antiferromagnets. The control of the spatial arrangement of the magnetic building blocks is of paramount importance in determining the strength of the magnetic interaction. It depends on the coordination bond between the metal ion and the ligands, and on supramolecular interactions such as stacking interactions or hydrogen bonds.

Crystal structure and magnetic properties of two isomeric three-dimensional pyromellitate-containing cobalt(II) complexes.

The hydrothermal preparation, crystal structure determination, and magnetic study of two isomers made up of 1,2,4,5-benzenetetracarboxylate and high-spin Co(II) ions of formula [Co2(bta)(H2O)4]n x 2n H2O (1 and 2; H4bta = 1,2,4,5-benzenetetracarboxylic acid) are reported. 1 and 2 are three-dimensional compounds whose structures can be described as (4,4) rectangular layers of trans-diaquacobalt(II) units with the bta(4-) anion acting as tetrakis-monodentate ligand through the four carboxylate groups, which are further connected through other trans-[Co(H2O)2](2+) (1) and planar [Co(H2O)4](2+) (2) entities, with the bridging units being a carboxylate group in either the anti-syn (1) or syn-syn (2) conformations and a water molecule (2). The study of the magnetic properties of 1 and 2 in the temperature range 1.9-300 K shows the occurrence of weak antiferromagnetic interactions between the high-spin Co(II) ions, with the strong decrease of chi(M)T upon cooling being mainly due to the depopulation of the higher energy Kramers doublets of the six-coordinated Co(II) ions. The computed values of the exchange coupling between the Co(II) ions across anti-syn carboxylate (1) and syn-syn carboxylate/water (2) bridges are J = -0.060 (1) and -1.90 (2) cm(-1) (with the Hamiltonian being defined as H = -Jsigma(i,j)S(i) x S(j)). These values follow the different conformations of the carboxylate bridge in 1 (anti-syn) and 2 (syn-syn) with the occurrence of a double bridge in 2 (water/carboxylate).

The metal-organic framework HKUST-1 as efficient sorbent in a vortex-assisted dispersive micro solid-phase extraction of parabens from environmental waters, cosmetic creams, and human urine.

Three metal-organic frameworks (MOFs), specifically HKUST-1, MOF-5, and MIL-53(Al), have been synthetized, characterized, studied and compared in a vortex-assisted dispersive micro-solid-phase extraction (VA-D-µ-SPE) procedure in combination with high-performance liquid chromatography (HPLC) with diode-array detection (DAD) for determining seven parabens in environmental waters (tap water, swimming pool water, and water coming from a spa pool), human urine (from two volunteers), and cosmetic creams (two commercial brands). Experimental parameters, such as nature and amount of MOF, sample volume, nature of elution solvent and its amount, vortex and centrifugation time, among others, were properly optimized. HKUST-1 was the most adequate MOF to work with. Detection limits for the overall method down to 0.1 μgL(-1) for butylparaben (BPB) and benzylparaben (BzPB) were obtained, with determination coefficients (R(2)) higher than 0.9966 for a range of 0.5-147 μgL(-1) (depending on the paraben), average relative recoveries (RR, in %) of 80.3% at the low spiked level (7 μgL(-1)), and relative standard deviation (RSD) values below 10% also at the low spiked level. The strength of the affinity between HKUST-1 and parabens was evaluated, and it ranged from 33.5% for isopropylparaben (iPPB) to 77.0% for isobutylparaben (iBPB). When analyzing complex environmental waters, RR values of 78%, inter-day precision values (as RSD) lower than 15%, and intra-day precision values lower than 7.8% were obtained, despite the observed matrix effect. When analyzing cosmetic creams, parabens were detected, with contents ranging from 0.14 ± 0.01 μgg(-1) for EPB in the healing cream analyzed to 1.12 ± 0.07 mgg(-1) for MPB in the mask cream analyzed, with precision values (RSD) lower than 12% and RR values from 63.7% for propylparaben (PPB) to 121% for iPPB. When analyzing human urine, no parabens were detected but the method could be performed with RSD values lower than 19%. These results show the adequateness of MOFs as sorbents in VA-D-µ-SPE procedures despite sample complexity.

Homochiral lanthanoid(III) mesoxalate metal–organic frameworks: synthesis, crystal growth, chirality, magnetic and luminescent properties

The achiral chelating and bridging mesoxalato ligand (H2mesox2−), the conjugate base of mesoxalic or dihydroxymalonic acid (H4mesox), is a new enantiopurity enforcer in extended structures by yielding the Λ/Δ-metal configured homochiral MOFs 2D-[Ln2(μ-H2mesox)3(H2O)6], [with Ln(III) = La (1), Ce (2), Pr (3), Nd (4), Sm (5), Eu (6), Gd (7), Tb (8), Dy (9), Er (10) and Yb (11)]; through self-resolution during crystallization. Single crystals of the compounds have been grown in agarose gel. All the compounds obtained are isostructural as deduced by means of single crystal and powder X-ray diffraction analysis and exhibit the Ln(III) ions covalently connected by the mesoxalato ligands into a corrugated grey arsenic-type (6,3)-net (or layer) with chair-shaped six-membered rings. Luminescence measurements reveal that the Eu(III) compound (6) exhibits several strong characteristic emission bands for isolated europium(III) ions in the visible region when excited between 350 and 420 nm; similarly the terbium(III) compound (8) displays the characteristic emission bands for isolated terbium(III) ions. Magnetic susceptibility measurements show deviations from the Curie law mainly owing to the split of the ground term due to the ligand field and spin–orbit coupling in the case of Sm(III) (4) and Eu(III) (6) compounds.

Structural versatility in cobalt(II) complexes with 1,2,4,5-benzenetetracarboxylic acid (H4bta) and 4,4′-bipyridine-N,N′-dioxide (dpo)

Four new high-spin cobalt(II) complexes of formula [Co(H2O)6](H2bta)·dpo·4H2O (1), [{Co(H2O)4(dpo)}2(bta)]·4H2O·(2), [Co(H2O)2)(dpo)2(H2bta)]n (3) and [Co(H2O)3(dpo)(bta)1/2]n (4) (H4bta = 1,2,4,5-benzenetetracarboxylic acid and dpo = 4,4′-bipyridine-N,N′-dioxide) have been synthesized and their structures solved by single crystal X-ray diffraction methods. Compound 1 is an ionic salt whose structure is made up of [Co(H2O)6]2+ cations, H2bta2− anions, uncoordinated dpo groups and crystallization water molecules, which are linked by extensive hydrogen bonds to afford a three-dimensional network. The structure of 2 consists of bta-bridged dinuclear cobalt(II) complexes where four coordinated water molecules and a terminally bound dpo ligand complete the six coordination around each cobalt atom. The bta ligand in 2 adopts the bis-monodentate bridging mode through two trans carboxylate-oxygen atoms, the intramolecular cobalt–cobalt distance being 11.46(2) A. The structure of 3 is constituted by uniform chains of cobalt(II) ions bridged by the dideprotonated H2bta2− species through two trans carboxylate-oxygen atoms. Two coordinated water molecules and two terminally bound dpo ligands achieve the six coordination around each cobalt atom. The intrachain cobalt–cobalt separation is 11.387(1) A. The structure of 4 consists of corrugated layers of cobalt(II) ions bridged by bis-monodentate dpo and bta4− ligands, (through two trans carboxylate-oxygen atoms), three coordinated water molecules in a fac arrangement achieving the six-coordination around each cobalt atom. The investigation of the magnetic behaviour of 2–4 in the temperature range 1.9–300 K shows the occurrence of very weak antiferromagnetic interactions between the high-spin cobalt(II) ions, the strong decrease of χMT upon cooling which is observed for this family of complexes ligands being mainly due to the depopulation of the higher energy Kramers doublets of the high-spin octahedral cobalt(II) centers.

The flexibility of molecular components as a suitable tool in designing extended magnetic systems

In this work we show how the design of n-dimensional magnetic compounds (nD with n = 1–3) can strongly benefit from the use crystal engineering techniques, which can give rive to structures of different shapes with different properties. We focus on the networks built by assembling the malonato-bridged tetranuclear copper(II) units Cu4(mal)4 (mal2− is the dianion of propanedioic acid, H2mal) through the potentially bridging 2,4′-bipyridine (2,4′-bpy), 4,4′-bipyridine (4,4′-bpy) and pyrazine (pyz). The magneto-structural study of the complexes of formula [Cu4(mal)4(2,4′-bpy)4(H2O)4]·8H2O (1), [Cu4(mal)4(H2O)4(4,4′-bpy)2] (2) (this compound was the subject of a previous report but it is included here for comparison) and [Cu4(mal)4(pyz)2]·4H2O (3) reveals that the ferromagnetically coupled Cu4(mal)4 unit which occurs in 1–3 is propagated into two- (2) and three-dimensions (3) by using 4,4′-bpy and pyz as linkers, respectively. Whereas in the case of complex 1, this tetranuclear unit is magnetically isolated, significant antiferromagnetic interactions between these units mediated by the bridges 4,4′-bpy and pyz occur in 2 and 3.