Itziar Oyarzabal

Two Polymorphic Forms of a Six-Coordinate Mononuclear Cobalt(II) Complex with Easy-Plane Anisotropy: Structural Features, Theoretical Calculations, and Field-Induced Slow Relaxation of the Magnetization

A mononuclear cobalt(II) complex [Co(3,5-dnb)2(py)2(H2O)2] {3,5-Hdnb = 3,5-dinitrobenzoic acid; py = pyridine} was isolated in two polymorphs, in space groups C2/c (1) and P21/c (2). Single-crystal X-ray diffraction analyses reveal that 1 and 2 are not isostructural in spite of having equal formulas and ligand connectivity. In both structures, the Co(II) centers adopt octahedral {CoN2O4} geometries filled by pairs of mutually trans terminal 3,5-dnb, py, and water ligands. However, the structures of 1 and 2 disclose distinct packing patterns driven by strong intermolecular O-H···O hydrogen bonds, leading to their 0D→2D (1) or 0D→1D (2) extension. The resulting two-dimensional layers and one-dimensional chains were topologically classified as the sql and 2C1 underlying nets, respectively. By means of DFT theoretical calculations, the energy variations between the polymorphs were estimated, and the binding energies associated with the noncovalent interactions observed in the crystal structures were also evaluated. The study of the direct-current magnetic properties, as well as ab initio calculations, reveal that both 1 and 2 present a strong easy-plane magnetic anisotropy (D > 0), which is larger for the latter polymorph (D is found to exhibit values between +58 and 117 cm(-1) depending on the method). Alternating current dynamic susceptibility measurements show that these polymorphs exhibit field-induced slow relaxation of the magnetization with Ueff values of 19.5 and 21.1 cm(-1) for 1 and 2, respectively. The analysis of the whole magnetic data allows the conclusion that the magnetization relaxation in these polymorphs mainly takes place through a virtual excited state (Raman process). It is worth noting that despite the notable difference between the supramolecular networks of 1 and 2, they exhibit almost identical magnetization dynamics. This fact suggests that the relaxation process is intramolecular in nature and that the virtual state involved in the two-phonon Raman process lies at a similar energy in polymorphs 1 and 2 (∼20 cm(-1)). Interestingly, this value is recurrent in Co(II) single-ion magnets, even for those displaying different coordination number and geometry.

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.

Multifunctional applications of a dysprosium-based metal–organic chain with single-ion magnet behaviour

A novel metal–organic chain constructed of dysprosium and sodium ions based on 5-aminopyridine-2-carboxylic acid has been synthesized and characterized. The formation of this 1D-MOF is achieved using a soft solvothermal route with dimethylformamide as solvent. This material displays intense photoluminescence properties in the solid state at room temperature and exhibits SIM behavior with frequency dependence of the out-of-phase susceptibility at zero dc field. More interesting, we report here the magnetic field dependent cytotoxicity and multidrug-resistant properties of this 1D-MOF. This multidisciplinary preliminary study of the applications of this metal complex opens up the possibility of further investigations in the fields of metal-based drugs, luminescence and magnetism.

Rational design of triple-bridged dinuclear ZnIILnIII-based complexes: a structural, magnetic and luminescence study

Twelve novel dinuclear ZnII–LnIII complexes of general formula [Zn(μ-L)(μ-OAc)Ln(NO3)2] (LnIII = Y (1), Pr (2), Nd (3), Sm (4), Eu (5), Gd (6), Tb (7), Dy (8), Ho (9), Er (10), Tm (11), Yb (12)) have been prepared from the symmetric compartmental Mannich base ligand N,N′-dimethyl-N,N′-bis(2-hydroxy-3-methoxy-5-methylbenzyl)-ethylenediamine (H2L). Structural determinations indicate that in all these complexes, ZnII ions are pentacoordinated in the inner position of the ligand, linked to two nitrogen and two oxygen atoms from L2− and a bridging syn–syn acetate group. The LnIII ions are nonacoordinated in the outer position, linked to four oxygen atoms from L2−, two bidentate nitrate anions and the bridging acetate. Complexes 3, 6, 7, 8, 10 and 12 exhibit field induced slow relaxation of the magnetization. It should be remarked that this behaviour is very infrequent in GdIII and NdIII coordination compounds. For compounds 8 and 12, it was possible to extract the effective energy barrier (Ueff) for the reversal of the magnetization, with a value of 27.5 K (52.6 K considering the Raman relaxation process in addition to the Orbach mode) and 13.2 K for DyIII and YbIII compounds, respectively. Moreover, the photophysical properties of these complexes have been studied at different temperatures and show that L2− is able to sensitize luminescence emission of many lanthanide ions. SmIII, EuIII, TbIII, DyIII and TmIII-based compounds show fluorescence in the visible region, whereas the YbIII-based one is sensitized in the near-infrared (NIR) region. Therefore, complexes 7, 8 and 12 may be considered as dual magneto-luminescent materials.

Photoluminescence and magnetic analysis of a family of lanthanide(III) complexes based on diclofenac

A family of ten novel isostructural LnIII complexes based on the nonsteroidal anti-inflammatory drug diclofenac, with the general formula [Ln2(dicl)6(MeOH)2(H2O)2] [LnIII = Pr (1), Nd (2), Sm(3), Eu (4), Tb (5), Dy (6), Ho (7), Er (8), Tm (9), Yb (10)], has been prepared. These complexes consist of centrosymmetric dimers in which each metallic centre is coordinated to three diclofenac ligands as well as water and methanol molecules. Alternating current magnetic measurements of complex 6 show a slight SMM behaviour. The evaluation of solid-state photophysical properties reveals that the diclofenac ligand is able to sensitize the luminescence emission of compounds 2, 3, 4, 5, and 6 both in the visible and near-infrared (NIR) regions at 10 K. Measurements of the decay curves show persistent fluorescence, with lifetimes of nearly 1.5 ms, and temperature stable lifetimes in the case of complex 5. Moreover, the NIR emission of compound 2 may open the way to applications in the field of fluoroimmunoassays and in vivo detection, given the bioactive nature of the diclofenac molecule.

Dinuclear Coordination Compounds Based on a 5-Nitropicolinic Carboxylate Ligand with Single-Molecule Magnet Behavior

Isostructural dinuclear dysprosium and yttrium coordination compounds based on the 5-nitropicolinic carboxylate ligand were synthesized and characterized. The formation of these air-stable complexes is achieved via solvothermal routes employing water as the reaction solvent. The dysprosium-based complex exhibits single-molecule magnet behavior with frequency dependence of the out-of-phase susceptibility at zero direct-current field. High-resolution mass spectrometry (electrospray ionization) experiments and advanced NMR methods including diffusion NMR techniques were applied on the diamagnetic yttrium analogue and established that these species retained their solid-state structure in solution with hydrodynamic radii of 6.5 Å. Full 1H, 13C, 15N, 89Y, Δ1Hcoord, Δ13Ccoord, and Δ15Ncoord NMR data are given, and through the analysis of the Ramsey equation, the first electronic insights of these derivatives are provided.

Mononuclear Lanthanide Complexes: Energy-Barrier Enhancement by Ligand Substitution in Field-Induced DyIII SIMs

The sequential reaction of 2-((6-(hydroxymethyl)pyridin-2-yl)-methyleneamino)phenol (LH2), LnCl3·6H2O, and 1,1,1-trifluoroacetylacetone (Htfa) in the presence of Et3N afforded [Ln(LH) (tfa)2] [Ln = Dy3+ (1), Ln = Tb3+ (2), and Ln = Gd3+ (3)], while under the same reaction conditions, but in the absence of the coligand, another series of mononuclear complexes, namely, [Ln(LH)2]·Cl·2MeOH] [Ln = Dy3+ (4) and Tb3+ (5)] are obtained. Single-crystal X-ray diffraction analysis revealed that the former set contains a mono-deprotonated [LH]- and two tfa ligands, while the latter set comprises of two mono-deprotonated [LH]- ligands that are nearly perpendicular to each other at an angle of 86.9°. Among these complexes, 2 exhibited a ligand-sensitized lanthanide-characteristic emission. Analyses of the alternating current susceptibility measurements reveal the presence of single-molecule magnet behavior for 1 and 4, in the presence of direct-current field, with effective energy barriers of 4.6 and 44.4 K, respectively. The enhancement of the effective energy barrier of the latter can be attributed to the presence of a large energy gap between the ground and first excited Kramers doublets, triggered by the change in coordination environments around the lanthanide centers.

Luminescence and Magnetic Properties of Two Three-Dimensional Terbium and Dysprosium MOFs Based on Azobenzene-4,4′-Dicarboxylic Linker

We report the in situ formation of two novel metal-organic frameworks based on terbium and dysprosium ions using azobenzene-4,4′-dicarboxylic acid (H2abd) as ligand, synthesized by soft hydrothermal routes. Both materials show isostructural three-dimensional networks with channels along a axis and display intense photoluminescence properties in the solid state at room temperature. Textural properties of the metal-organic frameworks (MOFs) have been fully characterized although no appreciable porosity was obtained. Magnetic properties of these materials were studied, highlighting the dysprosium material displays slightly frequency-dependent out of phase signals when measured under zero external field and under an applied field of 1000 Oe.

Slow relaxation of magnetization and luminescence properties of a novel dysprosium and pyrene-1,3,6,8-tetrasulfonate based MOF

We report the formation of a novel multifunctional metal–organic framework (MOF) based on dysprosium(III) ions using pyrene-1,3,6,8-tetrasulfonate (pytet)4− as a ligand, synthesized using a soft hydrothermal route. This material shows a two-dimensional network with small channels along a crystallographic axis and displays intense photoluminescence properties in the solid state at room temperature due to the pyrene derivative ligand. The magnetic properties of this 2D-MOF are also accomplished, confirming slightly frequency-dependent out of phase signals under an applied field of 1000 Oe.