Magnetic Relaxation of Lanthanide-Based Molecular Magnets

Regulating the magnetic properties of seven-coordinated Dy(III) single-ion magnets through the effect of positional isomers on axial crystal-field

Six Dy(III) single-ion magnets (SIMs) [Dy(n-OMe-bbpen)X] were synthesized by a solvothermal reaction with three positional isomers (ortho, meta, and para) of ligands n-OMe-H₂bbpen and dysprosium halides DyX₃, (n-OMe-H₂bbpen = N,N'-bis(2-hydroxy-n-methoxybenzyl)-N,N'-bis(2-methylpyridyl)ethylenediamine; n = 3, X = Cl, 1; n = 3, X = Br, 2; n = 4, X = Cl, 3; n = 4, X = Br, 4; n = 5, X = Cl, 5; n = 5, X = Br, 6). Dynamic magnetic measurements revealed that the six complexes possess notably different effective barriers of magnetic reversal: 872.0 K (1), 1210.1 K (2), 137.9 K (3), 602.6 K (4), 907.0 K (5) and 1216.7 K (6). 6 showed the best performance as SIMs among the six Dy(III) complexes. Moreover, the magnetic hysteresis loops of 6 remained open at 21 K. The crystal structures indicate the switching of local symmetry around Dy(III) ion, aroused by the variation in intermolecular interactions and steric effects. This switch is primarily correlated with the distinction of magnetic properties. In addition, ab initio calculations confirmed that the different electrostatic potential around Dy(III) ion stemming from the electronic effect of the OMe-substituted group is another factor leading to the distinction in magnetic properties. This work warns us that when designing ligands for Dy-SIMs, the effect of positional isomerism on magnetic performance must be considered, which is one of the factors that can easily be overlooked.

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.

Building-up host-guest helicate motifs and chains: a magneto-structural study of new field-induced cobalt-based single-ion magnets

In this work, we present the synthetic pathway, a refined structural description, complete solid-state characterization and the magnetic properties of four new cobalt(ii) compounds of formulas [Co(H₂O)₆][Co₂(H₂mpba)₃]·2H₂O·0.5dmso (1), [Co(H₂O)₆][Co₂(H₂mpba)₃]·3H₂O·0.5dpss (2), [Co₂(H₂mpba)₂(H₂O)₄]_n·4nH₂O (3), and [Co₂(H₂mpba)₂(CH₃OH)₂(H₂O)₂]_n·0.5nH₂O·2ndpss (4) [dpss = 2,2'-dipyridyldisulfide and H₄mpba = 1,3-phenylenebis(oxamic) acid], where 2 and 4 were obtained from [Co(dpss)Cl₂] (Pre-I) as the source of cobalt(ii). All four compounds are air-stable and were prepared under ambient conditions. 1 and 2 were obtained from a slow diffusion method [cobalt(ii) : H₂mpba^2- molar ratio used 1 : 1] and their structures are made up of [Co₂(H₂mpba)₃]^2- anionic helicate units and [Co(H₂O)₆]²⁺ cations, exhibiting supramolecular three-dimensional structures. Interestingly, a supramolecular honeycomb network between the helicate units interacting with each other through R₂²(10) type hydrogen bonds occurs in 2 hosting one co-crystallized dpss molecule. On the other hand, for the first time, linear (3) and zigzag (4) cobalt(ii) chains were isolated by slow evaporation of stirred solutions of mixed solvents with cobalt(ii) : H₂mpba^2- in 1 : 2 molar ratio at room temperature. Magnetic measurements of Pre-I revealed a quasi magnetically isolated S = 3/2 spin state with a significant second-order spin-orbit contribution as expected for tetrahedrally coordinated cobalt(ii) ions. The analysis of the variable temperature static (dc) magnetic susceptibility data through first- (1 and 3) and second-order spin-orbit coupling models (2 and 4) reveals the presence of magnetically non-interacting high-spin cobalt(ii) ions with easy-axis (1 and 4)/easy-plane magnetic anisotropies (2 and 4) with low rhombic distortions. Dynamic (ac) magnetic measurements for Pre-I and 1-4 below 8.0 K show that they are examples of field-induced Single-Ion Magnets (SIMs).

Control of magnetic anisotropy by macrocyclic ligand distortion in a family of DyIII and ErIII single molecule magnets

A family of hexaazamacrocyclic lanthanide complexes, [Ln(Ln)(NCS)3] (LnIII = Dy, Er; n = 1-3) has been synthesized and characterized by single-crystal X-ray diffraction, magnetic measurements and ab initio calculations. Macrocyclic ligands (Ln) differ in the lateral spacers, which are aliphatic chains with two and three carbons (for Ln, n = 1 and 2, respectively), and an aromatic ring for Ln = 3. Modification of the macrocycle spacer tunes planarity and rigidity of the equatorial coordination for both oblate (Dy) and prolate (Er) lanthanide ions. Ac-susceptibility studies showed that four of the six complexes are field induced single molecule magnets (SMMs). Trends in magnetic relaxation properties are rationalized with the aid of ab initio multireference calculations, highlighting the combined influence of macrocycle planarity, lanthanide electronic density distribution and intermolecular interactions for the achievement of slow demagnetization.

Pentagonal Bipyramidal Ln(III) Complexes Containing an Axial Phosphine Oxide Ligand: Field-induced Single-ion Magnetism Behavior of the Dy(III) Analogues

A series of neutral homologous complexes [(L)Ln(Cy₃PO)Cl] {where Ln = Gd (1), Tb (2), Dy (3), and Er (5)} and [(L)Dy(Ph₃PO)Cl] (4) [H₂L = 2,6-diacetylpyridine bis-benzoylhydrazone] were isolated. In these complexes, the central lanthanide ion possesses a pentagonal bipyramidal geometry with an overall pseudo D_5h symmetry. The coordination environment around the lanthanide ion comprises of three nitrogen and two oxygen donors in an equatorial plane. The axial positions are taken up by a phosphine oxide (O donor) and a chloride ion. Among these compounds, the Dy(III) (3 and 4) analogues were found to be field-induced single-ion magnets.