Slow Magnetic Relaxation Properties Research Articles

Fabrication of a series of isostructural water-stable lanthanide metal-organic frameworks: Tunable luminescence, sensing for antibiotics and magnetic properties

A series of new 1D isomorphic lanthanide metal-organic frameworks (Ln-MOFs), {[Ln(L)(H2O)5]·2H2O}n (1-Ln, Ln ​= ​Eu, Tb, Dy and EuxTb1-x), were synthesized via a rigid bifunctional N,O-containing 2-(1H-tetrazol-5-yl)terephthalic acid (H2L) and Ln(III) ions as raw materials. It is worth noting that different ratios of Eu(III) and Tb(III) can be combined into the same Ln-MOFs to generate a series of new differently doped 1-EuxTb1-x MOFs. Under the same excitation wavelength, the double emission peaks of Eu(III) and Tb(III) are produced, and the emission color gradually changes between yellow-green, yellow, orange, and red. The sensing studies have shown that 1-Tb is an excellent reusable luminescence probe, suitable for simultaneous detection of dimetridazole (DMZ), metronidazole (MDZ) and tetracyclinehydrate (TET) in the aqueous solutions. Furthermore, magnetic measurements show that 1-Dy has slow magnetic relaxation properties.

Mononuclear Lanthanide(III)-Salicylideneaniline Complexes: Synthetic, Structural, Spectroscopic, and Magnetic Studies

The reactions of hydrated lanthanide(III) [Ln(III)] nitrates and salicylideneaniline (salanH) have provided access to two families of mononuclear complexes depending on the reaction solvent used. In MeCN, the products are [Ln(NO3)3(salanH)2(H2O)]·MeCN, and, in MeOH, the products are [Ln(NO3)3(salanH)2(MeOH)]·(salanH). The complexes within each family are proven to be isomorphous. The structures of complexes [Ln(NO3)3(salanH)2(H2O)]·MeCN (Ln = Eu, 4·MeCN_Eu, Ln = Dy, 7·MeCN_Dy; Ln = Yb, 10·MeCN_Yb) and [Ln(NO3)3(salanH)2(MeOH)]·(salanH) (Ln = Tb, 17_Tb; Ln = Dy, 18_Dy) have been solved by single-crystal X-ray crystallography. In the five complexes, the LnIII center is bound to six oxygen atoms from the three bidentate chelating nitrato groups, two oxygen atoms from the two monodentate zwitterionic salanH ligands, and one oxygen atom from the coordinated H2O or MeOH group. The salanH ligands are mutually “cis” in 4·MeCN_Eu, 7·MeCN_Dy and 10·MeCN_Yb while they are “trans” in 17_Tb and 18_Dy. The lattice salanH molecule in 17_Tb and 18_Dy is also in its zwitterionic form with the acidic H atom being clearly located on the imine nitrogen atom. The coordination polyhedra defined by the nine oxygen donor atoms can be described as spherical tricapped trigonal prisms in 4·MeCN_Eu, 7·MeCN_Dy, and 10·MeCN_Yb and as spherical capped square antiprisms in 17_Tb and 18_Dy. Various intermolecular interactions build the crystal structures, which are completely different in the members of the two families. Solid-state IR data of the complexes are discussed in terms of their structural features. 1H NMR data for the diamagnetic Y(III) complexes provide strong evidence that the compounds decompose in DMSO by releasing the coordinated salanH ligands. The solid complexes emit green light upon excitation at 360 nm (room temperature) or 405 nm (room temperature). The emission is ligand-based. The solid Pr(III), Nd(III), Sm(III), Er(III), and Yb(III) complexes of both families exhibit LnIII-centered emission in the near-IR region of the electromagnetic spectrum, but there is probably no efficient salanH→LnIII energy transfer responsible for this emission. Detailed magnetic studies reveal that complexes 7·MeCN_Dy, 17_Tb and 18_Dy show field-induced slow magnetic relaxation while complex [Tb(NO3)3(salanH)2(H2O)]·MeCN (6·MeCN_Tb) does not display such properties. The values of the effective energy barrier for magnetization reversal are 13.1 cm−1 for 7·MeCN_Dy, 14.8 cm−1 for 17_Tb, and 31.0 cm−1 for 18_Dy. The enhanced/improved properties of 17_Tb and 18_Dy, compared to those of 6_Tb and 7_Dy, have been correlated with the different supramolecular structural features of the two families. The molecules [Ln(NO3)3(salanH)2(MeOH)] of complexes 17_Tb and 18_Dy are by far better isolated (allowing for better slow magnetic relaxation properties) than the molecules [Ln(NO3)3(salanH)2(H2O)] in 6·MeCN_Tb and 7·MeCN_Dy. The perspectives of the present initial studies in the Ln(III)/salanH chemistry are discussed.

A 2-D dysprosium glutarate exhibiting slow magnetic relaxation and luminescent properties

A 2-D dysprosium glutarate [Dy2(glu)3(phen)2]n (1, glu = glutarate, phen =1,10-phenanthroline) has been synthesized from the hydrothermal reaction of DyCl3, glutaric acid, and phen ligand. Compound 1 displays a classical 2-D (4,4) grid structure constructed by the linkages of Dy3+ ions and three types of glutarate ligands. Compound 1 shows slow magnetic relaxation and luminescent properties. Although some lanthanide glutarates have been reported, they do not exhibit slow magnetic relaxation properties. Compound 1 offers the only example of a dysprosium coordination polymer with field-induced single-molecule magnets (SMMs) based on aliphatic dicarboxylic acids in the absence of ancillary aromatic carboxylic acids.

Cyclopentadienyl Ligands in Lanthanide Single-Molecule Magnets: One Ring To Rule Them All?

The discovery of materials capable of storing magnetic information at the level of single molecules and even single atoms has fueled renewed interest in the slow magnetic relaxation properties of single-molecule magnets (SMMs). The lanthanide elements, especially dysprosium, continue to play a pivotal role in the development of potential nanoscale applications of SMMs, including, for example, in molecular spintronics and quantum computing. Aside from their fundamentally fascinating physics, the realization of functional materials based on SMMs requires significant scientific and technical challenges to be overcome. In particular, extremely low temperatures are needed to observe slow magnetic relaxation, and while many SMMs possess a measurable energy barrier to reversal of the magnetization ( Ueff), very few such materials display the important properties of magnetic hysteresis with remanence and coercivity. Werner-type coordination chemistry has been the dominant method used in the synthesis of lanthanide SMMs, and most of our knowledge and understanding of these materials is built on the many important contributions based on this approach. In contrast, lanthanide organometallic chemistry and lanthanide magnetochemistry have effectively evolved along separate lines, hence our goal was to promote a new direction in single-molecule magnetism by uniting the nonclassical organometallic synthetic approach with the traditionally distinct field of molecular magnetism. Over the last several years, our work on SMMs has focused on obtaining a detailed understanding of why magnetic materials based on the dysprosium metallocene cation building block {Cp2Dy}+ display slow magnetic relaxation. Specifically, we aspired to control the SMM properties using novel coordination chemistry in a way that hinges on key considerations, such as the strength and the symmetry of the crystal field. In establishing that the two cyclopentadienyl ligands combine to provide a strongly axial crystal field, we were able to propose a robust magneto-structural correlation for understanding the properties of dysprosium metallocene SMMs. In doing so, a blueprint was established that allows Ueff and the magnetic blocking temperature ( TB) to be improved in a well-defined way. Although experimental discoveries with SMMs occur more rapidly than quantitative theory can (currently) process and explain, a clear message emanating from the literature is that a combination of the two approaches is most effective. In this Account, we summarize the main findings from our own work on dysprosium metallocene SMMs, and consider them in the light of related experimental studies and theoretical interpretations of related materials reported by other protagonists. In doing so, we aim to contribute to the nascent and healthy debate on the nature of spin dynamics in SMMs and allied molecular nanomagnets, which will be crucial for the further advancement of this vibrant research field.

A family of related Co(II) terpyridine compounds exhibiting field induced single-molecule magnet properties

We describe the syntheses, crystal structures and magnetic properties of several Co(II) complexes containing 4′-pyrrolidine-2,2′:6′,2″-terpyridine (pyrtpy) or 4′-triflouromethylsulfonate-2,2′:6′,2″-terpyridine (OTftpy) ligands: [(OTftpy)CoCl2] (1), [Co(pyrtpy)2](BPh4)2 (2b), and [(OTftpy)Co(κ2-O2NO)2] (3). The compounds display different coordination numbers, where 1, 2b and 3 are distorted square pyramidal (pentacoordinate), octahedral (hexacoordinate) and pentagonal bipyramidal (heptacoordinate) about the cobalt center, respectively. Field-induced slow magnetic relaxation properties are observed for all compounds; however, compounds 1 and 3 display two relaxation events, whereas 2b shows only one relaxation event. The origin of the varied dynamic magnetic properties for these complexes arises from the change in coordination environment as well as dipolar interactions between neighboring complexes. These experimental results are supported by computational investigations for each species, implicating lower-lying excited states for complex 2b, which increase the impact of spin–orbit coupling, in turn leading to a larger axial anisotropy value.

From a Piano Stool to a Sandwich: A Stepwise Route for Improving the Slow Magnetic Relaxation Properties of Thulium

Two mononuclear TmIII complexes, [Tm(COT)I(THF)2] (1-Tm) and [K(18-crown-6)(THF)2][Tm(COT)2] (2-Tm), display slow relaxation of the magnetization, making these compounds rare examples of non-Kramers TmIII single-molecule magnets (SMMs). Utilizing a stepwise synthetic approach for the installation of cyclooctatetraenide (COT) ligands, we can observe the effect of symmetry optimization at the metal center. This method results in an 85% increase in the energy barrier to magnetization reversal (Ueff) for 2-Tm (Ueff = 53.3 K) over 1-Tm (Ueff = 7.93 K). The increased local symmetry of 2-Tm reduces the need for large static fields, eliciting SMM behavior under a small field of 200 Oe. This illustrates the power of fine-tuning the ligand environment to enhance the magnetic relaxation properties of non-Kramers ions.

2p-4f MOFs based on naphthalene-1,4,5,8-tetracarboxylate with magnetocaloric effect and slow magnetic relaxation properties

A family of new three-dimensional (3D) 2p-4f metal-organic frameworks (MOFs), namely [Na2Ln(L)(OH)(H2O)3]n (H4L=naphthalene-1,4,5,8-tetracarboxylic acid, Ln=Gd (1), Tb (2) and Dy (3)) have been synthesized by using hydrothermal method and structurally characterized by single-crystal X-ray diffraction, powder X-ray diffraction and IR spectra. Complexes 1–3 are isostructural and crystallize in the space group Cmca of orthorhombic system. Their crystal structures can be described as a kind of homeomorphism configurations with 3D framework. Magnetic measurements indicated that complex 1 exhibits antiferromagnetic behavior and magnetocaloric effect with −ΔSmmax=28.8Jkg−1K−1 (ΔH=7T and T=2K), and complex 2 exhibits field-induced slow magnetic relaxation behaviors.

Magnetic Properties and Photoluminescence of Lanthanide Coordination Polymers Constructed with Conformation-Flexible Cyclohexane-Tetracarboxylate Ligands

To investigate the effects on the physical properties of complexes with different conformations of flexible ligands, four lanthanide (Ln) coordination polymers with flexible ligand cyclohexane-1e,2a,4a,5e-tetracarboxylic acid (H4LI), [Ln(HLII)(H2O)2]·1.5H2O (Ln = Dy, 1; Tb, 2) and [Ln2(H2LIV)(LIV)(H2O)4]·1.5H2O (Ln = Dy, 3; Tb, 4), have been synthesized and characterized. Complexes 1 and 2 are 2D layers composed of LII ligands and [Ln2O2] dinuclear units whereas complexes 3 and 4 are 3D coordination frameworks constructed with LIV ligands and two kinds of dinuclear units, [Ln2O2] and [Ln2(COO)2]. Notably, the conformations of ligands in two structures, which were different and changed from the original LI conformation to LII (e,a,e,e) and LIV (e,e,e,e) conformations, impact the construction of different structures and the magnetic properties. Complex 1 shows antiferromagnetic interactions whereas complex 3 exhibits weak ferromagnetic interactions and slow magnetic relaxation properties. Additionally, the photoluminescence of complexes 2 and 4 have been investigated.

Controlling Molecular Alignment on Highly Ordered Pyrolitic Graphite of Porphyrin-Tb(III) Double-Decker Single Molecular Magnets

Single molecular magnets (SMMs) that show slow magnetic relaxation properties at low temperatures have great potential for use as nanoscale memory devices or quantum computers. In our study, we focused on porphyrin-Tb(III) double-decker complexes and their magnetic properties.1,2 Our goal was to control the molecular alignment of SMMs on highly ordered pyrolitic graphite (HOPG) by a self-assembling process. We synthesized protonated, anionic, and radical forms of octaethylporphyrin (OEP)-Tb(III) double-decker complexes and observed the molecular alignment of each molecule on HOPG using scanning tunneling microscopy (STM). The STM observations demonstrated that all OEP-Tb(III) double-decker complexes had similar pseudo-hexagonal packing structure stability on HOPG, regardless of differences in the electronic structure (Figure 1). In particular, the lattice parameters of protonated form 1 (a = 1.41 ± 0.03 nm, b = 1.46 ± 0.05 nm) showed almost the same values as anionic form 2 (a = 1.34 ± 0.01 nm, b = 1.42 ± 0.01 nm) and radical form 3 (a = 1.39 ± 0.04 nm, b= 1.45 ± 0.06 nm).We also prepared and observed the molecular alignment of a protonated 5,15-bisdodecylporphyrin (BDP)-Tb(III) double-decker complex. This complex showed a lamellar packing structure on HOPG (Figure 2). Herein, we succeeded in isolating stable 2D supramolecular structures of all the OEP-Tb(III) double-decker complexes on HOPG. Control of molecular alignment was also realized by changing the substituents.

Assembling 1D magnetic chain based on octacyanotungstate(v) and [Cu2L2Ln] sub-building units (Ln = Eu, Gd, Tb and Dy)

Reaction of (NBu3H)3[W(CN)8]·H2O and linear trinuclear complexes [Cu(II)2L2Ln(III)(NO3)3] (L = 2,6-di(acetoacetyl)pyridine, Ln = Eu (1), Gd (2), Tb (3) and Dy (4)) leads to the formation of four one-dimensional (1D) complexes. Single crystal X-ray analysis reveals that [W(CN)8](3-) bridges between the [Cu(II)2L2Ln(III)] sub-building units to construct the alternative chains. Magnetic investigation indicates that ferromagnetic interaction occurred between all paramagnetic ions and the slow magnetic relaxation properties were observed in complexes 3 and 4.