Magnetic properties and magnetocaloric effect of Ln = Dy, Tb carborane-based metal-organic frameworks

We present the synthesis and magneto-thermal properties of carborane-based lanthanide metal-organic frameworks (MOFs) with the formula {[(Ln)3(mCB-L)4(NO3)(DMF)n]·Solv}, where Ln = Dy or Tb, characterized by dc and ac susceptibility, X-ray absorption spectroscopy (XAS), X-ray magnetic circular dichroism (XMCD) and heat capacity measurements. The MOF structure is formed by polymeric 1D chains of Ln ions with three different coordination environments (Ln1, Ln2, Ln3) running along the b-axis, linked by carborane-based linkers thus to provide a 3D structure. Static magnetic measurements reveal that these MOFs behave at low temperature as a system of S* = 1/2 Ising spins, weakly interacting ferromagnetically along the 1D polymeric chain (J*/kB = +0.45 K (+0.5 K) interaction constant estimated for Dy-MOF (Tb-MOF)) and coupled to Ln ions in adjacent chains through dipolar antiferromagnetic interactions. The Dy MOF exhibits slow relaxation of magnetization through a thermally activated process, transitioning to quantum tunneling of the magnetization at low temperatures, while both compounds exhibit field-induced relaxation through a very slow, direct process. The maximum magnetic entropy changes (-ΔSmaxm) for an applied magnetic field change of 2-0 T are 5.71 J kg-1 K-1 and 4.78 J kg-1 K-1, for Dy and Tb MOFs, respectively, while the magnetocaloric effect (MCE) peak for both occurs at T ∼ 1.6 K, approximately double that for the Gd counterpart.

Combined experimental and theoretical investigation on the magnetic properties derived from the coordination of 6-methyl-2-oxonicotinate to 3d-metal ions

Five new compounds are reported herein starting from 2-hydroxy-6-methylnicotinic acid (H2h6mnic) and first-row transition metal ions, although H2h6mnic shows a prototropy in solution to lead to the 6-methyl-2-oxonicotinate (6m2onic) ligand that is the molecule eventually present in the compounds. The structural and chemical characterization reveals the following chemical formulae: {[MnNa(μ₃-6m2onic)₂(μ-6m2onic)(MeOH)]·H₂O·MeOH}_n (1Mn), {[M₂Na₂(μ₃-6m2onic)₂(μ-6m2onic)₂(μ-H₂O)(H₂O)₆](NO₃)₂}_n [M^II = Co (2Co) and Ni (3Ni)], 2[Cu₂(6m2onic)₃(μ-6m2onic)(MeOH)]·[Cu₂(6m2onic)₂(μ-6m2onic)₂]·2[Cu(6m2onic)₂(MeOH)]·32H₂O (4Cu) and {[Cu(μ-6m2onic)₂]·6H₂O}_n (5Cu) (where 6m2onic = 6-methyl-2-oxonicotinate). An unusual structural diversity is observed for the compounds, ranging from isolated complexes (in 4Cu), 1D arrays (in 1Mn and 5Cu) and 3D frameworks (in 2Co and 3Ni). Magnetic properties have been studied for all compounds. Analysis of the magnetic dc susceptibility and magnetization data for 4Cu and 5Cu suggests the occurrence of ferromagnetic exchange, which is well explained by broken-symmetry and CASSCF calculations. The sizeable easy-plane magnetic anisotropy present in compound 2Co allows for a field-induced magnet behaviour with an experimental effective energy barrier of 16.2 cm^-1, although the slow relaxation seems to be best described through Raman and direct processes in agreement with the results of ab initio calculations.

Sensing Capacity in Dysprosium Metal-Organic Frameworks Based on 5-Aminoisophthalic Acid Ligand

Two novel metal-organic frameworks (MOFs), based on dysprosium as the metal and the 5-aminoisophthalic acid (5aip) ligand, have been solvothermally synthesized, with the aim of studying and modulating their luminescence properties according to the variation of solvent in the structure. These materials display intense photo-luminescence properties in the solid state at room temperature. Interestingly, one fascinating sensory capacity of compound 2 regards obtaining a variation of the signal, depending on the solvent to which it is exposed. These results pave the way for a new generation of sensitive chemical sensors.

An experimental and theoretical study of the magnetic relaxation in heterometallic coordination polymers based on 6-methyl-2-oxonicotinate and lanthanide(iii) ions with square antriprismatic environment

Two isostructural lanthanide(iii)-based coordination polymers with square antiprismatic environment are described. Magnetic properties are studied from experimental and theoretical viewpoints to analyze their SIM behavior.

Slow Magnetic Relaxation in {[CoCxAPy)] 2.15 H2O}n MOF Built from Ladder-Structured 2D Layers with Dimeric SMM Rungs

We present the magnetic properties of the metal-organic framework {[CoCxAPy]·2.15 H2O}n (Cx = bis(carboxypropyl)tetramethyldisiloxane; APy = 4,4`-azopyridine) (1) that builds up from the stacking of 2D coordination polymers. The 2D-coordination polymer in the bc plane is formed by the adjacent bonding of [CoCxAPy] 1D two-leg ladders with Co dimer rungs, running parallel to the c-axis. The crystal packing of 2D layers shows the presence of infinite channels running along the c crystallographic axis, which accommodate the disordered solvate molecules. The Co(II) is six-coordinated in a distorted octahedral geometry, where the equatorial plane is occupied by four carboxylate oxygen atoms. Two nitrogen atoms from APy ligands are coordinated in apical positions. The single-ion magnetic anisotropy has been determined by low temperature EPR and magnetization measurements on an isostructural compound {[Zn0.8Co0.2CxAPy]·1.5 CH3OH}n (2). The results show that the Co(II) ion has orthorhombic anisotropy with the hard-axis direction in the C2V main axis, lying the easy axis in the distorted octahedron equatorial plane, as predicted by the ab initio calculations of the g-tensor. Magnetic and heat capacity properties at very low temperatures are rationalized within a S* = 1/2 magnetic dimer model with anisotropic antiferromagnetic interaction. The magnetic dimer exhibits slow relaxation of the magnetization (SMM) below 6 K in applied field, with a tlf ≈ 2 s direct process at low frequencies, and an Orbach process at higher frequencies with U/kB = 6.7 ± 0.5 K. This compound represents a singular SMM MOF built-up of Co-dimers with an anisotropic exchange interaction.

5-Aminopyridine-2-carboxylic acid as appropriate ligand for constructing coordination polymers with luminescence, slow magnetic relaxation and anti-cancer properties

Five new coordination polymers (CPs) constructed of aminopyridine-2-carboxylate (ampy) ligand have been synthesized and fully characterized. Three of them correspond to metal-organic chains built from the coordination of ampy to sodium and lanthanides with formulae [MNa(ampy)₄]_n (M = terbium (2), erbium (1) and ytterbium (3)) resembling a previously reported dysprosium material which shows anticancer activity. On another level, the reaction of Hampy with cobalt and copper ions ({[CoK(ampy)₃(H₂O)₃](H₂O)₃}_n (4) and [Cu(ampy)₂]_n (5)) lead to CPs with variable dimensionalities, which gives the opportunity of analyzing the structural properties of this new family. Lanthanide materials display solid state intense photoluminescent emissions in both the visible and near-infrared region and exhibit slow relaxation of magnetization with frequency dependence of the out-of-phase susceptibility. More interestingly, in our search for multifunctional materials, we have carried out antitumor measurements of these compounds. These multidisciplinary studies of metal complexes open up the possibility for further exploring the applications in the fields of metal-based drugs.

Eu@PMMA and Sm@PMMA nanofibers prepared by electrospinning: structure, morphology, luminescence and magnetic properties

Eu³⁺ and Sm³⁺ coordination polymers and a series of Eu@PMMA and Sm@PMMA nanofibers were prepared. Color tunability was achieved with the luminescence changing from reddish orange in the bulk state to near-white light in the nanofiber stage.

Three chiral one-dimensional lanthanide-ditoluoyl-tartrate bifunctional polymers exhibiting luminescence and magnetic behaviors

Since Ln-CPs have excellent optical properties (higher color purity, longer fluorescence lifetime and higher quantum yield) and magnetic properties, it is of great significance to prepare dual magneto-optical materials based on Ln(iii). Herein, we obtained three versatile Ln-CPs, [Ln(HDTTA)3(CH3OH)3] n , derived from reactions of lanthanide salts (Ln = Tb 1, Dy 2, Ho 3) and a chiral and flexible ligand, namely, (+)-di-p-toluoyl-d-tartaric acid (d-H2DTTA) in a methanol-water solution, at room temperature and pressure. The structures of these compounds have been characterized by single crystal and powder X-ray diffraction, infrared spectroscopy, elemental analyses and thermogravimetric analyses. Complexes 1-3 are isomorphic, crystallizing in the chiral trigonal R3 space group with the linkage of Ln3+ ions and featuring 1D propeller chain structures. The circular dichroism spectra confirm that the complexes maintain the chirality from the ligands. Furthermore, the luminescent and magnetic properties have been investigated, relying on intrinsic properties of the lanthanide ions. The photoluminescence measurements indicate that 1, 2 and 3 show strong green, white and blue emission bands with CIE chromaticity coordinates of (0.32, 0.56), (0.29, 0.26) and (0.21, 0.12), respectively. The decay lifetime curve of 1 shows the exponential decay with long lifetime of 1.169 ms and the relative quantum yield for 1 was 19.31%. In addition, the magnetic properties of complexes 1-3 have been investigated by measuring the magnetic susceptibility in the temperature range of 2-300 K. They are all dominated by spin-orbit coupling and ligand field perturbation, and the exchange coupling between Ln3+ ions is almost negligible. Therefore, complexes 1-3 are promising chiral, optical and magnetic multifunctional materials.

Electron Transfer and Electron Excitation Processes in 2,5-Diaminoterephthalate Derivatives with Broad Scope for Functionalization

Derivatives of 2,5-diaminoterephthalate (DAT) are efficient fluorescence dyes that are also redox-active, thus allowing for the electrochemical manipulation of spectral properties. The electrochemical behaviour of seven DAT derivatives was studied by cyclic voltammetry in dichloromethane. In the absence of a proton donor, DATs should be oxidized in two one-electron steps. The first step is usually quasi-reversible while the second step is either quasi-reversible or irreversible. Some electrochemical properties such as the formal potentials and the ratio between the anodic and the cathodic current were determined from the cyclic voltammograms. Correlation between the formal potential of first oxidation and the absorption or the fluorescence emission wavelengths are established for this specific type of dyes. These correlations were confirmed with density functional theory calculations.

Lanthanide anthracene complexes: slow magnetic relaxation and luminescence in DyIII, ErIII and YbIII based materials

Four mononuclear lanthanide complexes, [Ln(depma)2(H2O)6]Cl3·xH2O·yCH3OH [Ln = Dy (1Dy), x = 6, y = 0; Gd (2Gd), x = 3, y = 1; Er (3Er), x = 6, y = 0; Yb (4Yb), x = 3, y = 1; depma = 9-diethylphosphono-methylanthracene], are reported and structurally characterized. The octa-coordinated environment of LnIII is composed of two oxygen atoms from two different depma ligands and six oxygen atoms from each bound water molecule. All four complexes show photoluminescence in the solid state at room temperature, originating from the ligand-centered emissions of depma. Furthermore, slow magnetization relaxation is found in 1Dy, 3Er and 4Yb with the energy barriers (Ueff) of 40.5 K (1 kOe), 9.8 K (1 kOe) and 28.9 K (0.75 kOe), respectively.

Effect of the change of the ancillary carboxylate bridging ligand on the SMM and luminescence properties of a series of carboxylate-diphenoxido triply bridged dinuclear ZnLn and tetranuclear Zn2Ln2 complexes (Ln = Dy, Er)

Eleven new dinuclear and tetranuclear compounds of general formulae [Zn(μ-L)(μ-X)Ln(NO3)2]·nS, [Zn2Dy2(μ3-L')2(μ-sal)2(NO3)(CH3OH)](NO3)·5CH3OH and [Zn2Er2(μ3-L')2(μ-sal)2(CH3OH)2](NO3)2·4CH3OH (X = benzoate, anthracenate, diclofenac, salicylate, 2,6-dihydroxybenzoate; Ln = Dy, Er; S = water, acetonitrile, methanol) were prepared from the N,N'-dimethyl-N,N'-bis(2-hydroxy-3-formyl-5-bromobenzyl)ethylenediamine compartmental ligand (H2L). Complexes 1-6 and 9-11 consist of diphenoxido-carboxylate triply bridged compounds, which differ mainly in the carboxylate bridging ligand. It should be noted that the acidic character of the salicylic acid promotes, in the presence of methanol, the methoxylation of the H2L ligand thereby yielding a hemiacetal H3L', which is able to connect the Ln(iii) ions of two ZnLn dinuclear units forming the Zn2Ln2 tetranuclear complexes 7 and 8. All compounds display SMM behaviour in the presence of an external field with effective energy barriers (Ueff) as high as 61 K. Magneto-structural data for these complexes reveal that their SMM behaviour is not only significantly affected by the type of Ln(iii) ion but also by the carboxylate bridging ligand connecting the Zn(ii) and Ln(iii) ions. Photoluminescence properties have also been accomplished, showing that the ligands are able to sensitize lanthanide centred emissions in the visible and near-infrared regions with variable capacity. Moreover, the analysis of the luminescence decay curves reveals emission lifetimes in the range of few microsecond or hundreds of nanoseconds for Dy(iii)-based or Er(iii)-based luminophores, respectively.

Magnetic and luminescent coordination networks based on imidazolium salts and lanthanides for sensitive ratiometric thermometry

The synthesis and characterization of six new lanthanide networks [Ln(L)(ox)(H₂O)] with Ln = Eu³⁺, Gd³⁺, Tb³⁺, Dy³⁺, Ho³⁺ and Yb³⁺ is reported. They were synthesized by solvo-ionothermal reaction of lanthanide nitrate Ln(NO₃)₃·xH₂O with the 1,3-bis(carboxymethyl)imidazolium [HL] ligand and oxalic acid (H₂ox) in a water/ethanol solution. The crystal structure of these compounds has been solved on single crystals and the magnetic and luminescent properties have been investigated relying on intrinsic properties of the lanthanide ions. The synthetic strategy has been extended to mixed lanthanide networks leading to four isostructural networks of formula [Tb_{1- x} Eu _x (L)(ox)(H₂O)] with x = 0.01, 0.03, 0.05 and 0.10. These materials were assessed as luminescent ratiometric thermometers based on the emission intensities of ligand, Tb³⁺ and Eu³⁺. The best sensitivities were obtained using the ratio between the emission intensities of Eu³⁺ (⁵D₀→⁷F₂ transition) and of the ligand as the thermometric parameter. [Tb_0.97Eu_0.03(L)(ox)(H₂O)] was found to be one of the best thermometers among lanthanide-bearing coordination polymers and metal-organic frameworks, operative in the physiological range with a maximum sensitivity of 1.38%·K^-1 at 340 K.

Design and synthesis of a family of 1D-lanthanide-coordination polymers showing luminescence and slow relaxation of the magnetization

We have designed and synthesized eight isostructural 1D coordination polymers (CPs) with the general formula {[Ln(aapc)3(DMF)]}n [where Ln(iii) = Y (2), La (3), Nd (4), Eu (5), Gd (6), Tb (7), Dy (8), Er (9); and aapc = 3-((anthraquinone-1-yl)amino)propanoate]. These CPs consist of Ln-carboxylate infinite rods in which the bulky anthraquinone scaffolds arise from it in such a way that the resulting supramolecular packing exhibits isolated 1D chains. Solution structures have been corroborated through NMR methods including PGSE and EXSY NMR studies and, due to the presence of lanthanide ions, magnetic and luminescence properties have been studied. Alternating current magnetic measurements of compound 8 show slow relaxation of the magnetization, a characteristic of single molecule magnets (SMMs). The evaluation of solid-state photophysical properties reveals that the aapc scaffold sensitizes lanthanide(iii) based emission of compounds 4-9 both in the visible and near-infrared (NIR) regions at 10 K.

Magnetic functionalities in MOFs: from the framework to the pore

In this review, we show the different approaches developed so far to prepare metal-organic frameworks (MOFs) presenting electronic functionalities, with particular attention to magnetic properties. We will cover the chemical design of frameworks necessary for the incorporation of different magnetic phenomena, as well as the encapsulation of functional species in their pores leading to hybrid multifunctional MOFs combining an extended lattice with a molecular lattice.

Implementation of slow magnetic relaxation in a SIM-MOF through a structural rearrangement

Dehydration of a Dy-based MOF causes the appearance of slow-magnetic relaxation due to the changes of the charge density of the coordination environment.

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

A novel multifunctional Dy(iii) and pyrene-1,3,6,8-tetrasulfonate based MOF displays intense PL/ SIM behaviour.

Field-Induced Relaxation of Magnetization in a Three-Dimensional LnMOF with the Second Bridging Ligand Squarate

A three-dimensional (3D) dysprosium(III) metal-organic framework with nicotinate N-oxide (NNO^-) and squarate (C₄O₄ ^2-) mixed bridging ligands, [Dy(NNO)(C₄O₄)(H₂O)] _n (1), has been hydrothermally synthesized. The dysprosium(III) ions are linked to each other by the squarate anions to form a unique dysprosium(III) squarate double-layered network; the NNO^- anions then bridge such layers to complete the 3D framework. Complex 1 exhibits a two-step relaxation of magnetization under a dc field of 1000 Oe, with effective energy barrier values of 8.5 and 14.3 K, respectively.