High magnetization reversal barriers in luminescent dysprosium octahedral and pentagonal bipyramidal single-molecule magnets based on fluorinated alkoxide ligands

Structures and Single-Molecule Magnet Behavior of Dy3 and Dy4 Clusters Constructed by Different Dysprosium(III) Salts

Using the Schiff base ligand H2L-pyra (N'-(2-hydroxybenzoyl)pyrazine-2-carbohydrazonamide) with multiple dentate sites, the trinuclear DyIII-based complex [Dy3(HL-pyra)2(L-pyra)2(CH3COO)3]·2H2O (1) was synthesized. By analyzing the fragmented assembly process and fine-tuning the bridging anions, complex [Dy4(HL-pyra)2(L-pyra)4(NO3)2(H2O)2]·8H2O (2) with different nuclear numbers was successfully synthesized. Magnetic studies demonstrated that 1 did not exhibit magnetic relaxation behavior under the external field; however, 2 exhibited zero-field single-molecule magnetic relaxation behavior with an effective energy barrier (Ueff) of 197.44 K. This is attributed to the improved anisotropy of the single ion after the normalization of the crystal structure, thus realizing the molecular magnetic switching. Moreover, magnetic dilution analysis of 2 demonstrated that the weak magnetic interaction between metal ions inhibited the occurrence of quantum tunneling of magnetization (QTM), resulting in high-performance single-molecule magnet (SMM) behavior. The reasons for the magnetic difference between these two complexes were analyzed using ab initio calculation and magneto-structural correlations. This study provides a reasonable prediction of the ideal configuration of the approximately parallelogram DyIII-based SMMs, thus offering an effective approach for synthesizing Dy4 complexes with excellent properties.

Trityl-Based Lanthanide-Supramolecular Assemblies Exhibiting Slow Magnetic Relaxation

The triphenylmethane (trityl) group has been recognized as a supramolecular synthon in crystal engineering, molecular machine rotors and stereochemical chirality inductors in materials science. Herein we demonstrate for the first time how it can be utilized in the domain of molecular magnetic materials through shaping of single molecule magnet (SMM) properties within the lanthanide complexes in tandem with other non-covalent interactions. Trityl-appended mono- (HL1 ) and bis-compartmental (HL2 ) hydrazone ligands were synthesized and complexated with Dy(III) and Er(III) triflate and nitrate salts to generate four monometallic (1-4) and two bimetallic (5, 6) complexes. The static and dynamic magnetic properties of 1-6 were investigated, revealing that only ligand HL1 induces assemblies (1-4) capable of showing SMM behaviour, with Dy(III) congeners (1, 2) able to exhibit the phenomenon also under zero field conditions. Theoretical ab initio studies helped in determination of Dy(III) energetic levels, magnetic anisotropic axes and corroborated magnetic relaxation mechanisms to be a combination of Raman and quantum tunnelling in zero dc field, the latter being cancelled in the optimum non-zero dc field. Our work represents the first study of magneto-structural correlations within the trityl Ln-SMMs, leading to generation of slowly relaxing zero-field dysprosium complexes within the hydrogen-bonded assemblies.

Ligand Fluorination to Mitigate the Raman Relaxation of DyIII Single-Molecule Magnets: A Combined Terahertz, Far-IR and Vibronic Barrier Model Study

The fast Raman relaxation process via a virtual energy level has become a puzzle for how to chemically engineer single-molecule magnets (SMMs) with better performance. Here, we use the trifluoromethyl group to systematically substitute the methyl groups in the axial position of the parent bis-butoxide pentapyridyl dysprosium(III) SMM. The resulting complexes-[Dy(OL^A )₂ py₅ ][BPh₄ ] (L^A =CH(CF₃ )₂ ^- 1, CH₂ CF₃ ^- 2, CMe₂ CF₃ ^- 3)-show progressively enhanced T_B ^hys (@100 Oe s^-1 ) from 17 K (for 3), 20 K (for 2) to 23 K (for 1). By experimentally identifying the varied under barrier relaxation energy in the 5-500 cm^-1 regime, we are able to identify that the C-F bond related vibration energy of the axial ligand ranging from 200 to 350 cm^-1 is the key variant for this improvement. Thus, this finding not only reveals a correlation between the structure and the Raman process but also provides a paradigm for how to apply the vibronic barrier model to analyze multi-phonon relaxation processes in lanthanide SMMs.

Magnetic anisotropy of transition metal and lanthanide ions in pentagonal bipyramidal geometry

The magnetic anisotropy associated with a pentagonal bipyramidal (PBP) coordination sphere is examined on the basis of experimental and theoretical investigations. The origin and the characteristics of this anisotropy are discussed in relation to the electronic configuration of the metal ions. The effects of crystal field, structural distortion, and a second-coordination sphere on the observed anisotropies for transition meal and lanthanide ions are outlined. For the Ln derivatives, we focus on compounds showing SMM-like behavior (i.e. slow relaxation of their magnetization) in order to highlight the essential chemical and structural parameters for achieving strong axial anisotropy. The use of PBP complexes to impart controlled magnetic anisotropy in polynuclear species such as SMMs or SCMs is also addressed. This review of the magnetic anisotropies associated with a pentagonal bipyramidal coordination sphere for transition metal and lanthanide ions is intended to highlight some general trends that can guide chemists towards designing a compound with specific properties.

Using N-Heterocyclic Carbenes as Weak Equatorial Ligands to Design Single-Molecule Magnets: Zero-Field Slow Relaxation in Two Octahedral Dysprosium(III) Complexes

We report the synthesis, structures, and magnetic investigations of two new octahedral dysprosium complexes, based on the original N-heterocyclic carbene (NHC) tridentate bis(phenoxide) ligand, of the respective formulas mer-[DyL(THF)₂Cl] (1) and mer-[DyL(THF)₃][BPh₄] (2), where L = 1,3-bis(3,5-di-tert-butyl-2-oxidophenyl)-5,5-dimethyl-3,4,5,6-tetrahydropyrimidin-1-ium chloride and THF = tetrahydrofuran. The short Dy-O distances in the axial direction in association with the weak donor ability of the NHC moiety provide a suitable environment for slow relaxation of magnetization, overcoming the previous single-molecule magnets based on NHC ligands.

Effect on the geometry over the slow relaxation of the magnetization in a series of erbium(iii) complexes based on halogenated ligands

Erbium(iii) complexes based on halogenated ligands.

Effects of strong coordination bonds at the axial or equatorial positions on magnetic relaxation for pentagonal bipyramidal dysprosium(iii) single-ion magnets

Pentagonal bipyramidal dysprosium(iii) complexes with different strong coordination bonds at axial or equatorial positions show a significant variation in their magnetic relaxation behavior.

Six-coordinated dinuclear lanthanide(III) amide complexes: investigation of magnetization relaxation dynamics and their electronic structures

A series of rare six-coordinated dinuclear Ln(III) complexes [Ln₂(μ-Cl)₂Cl₄Li₂(L)₂(THF)₆] were structurally characterized using a bulky amide ligand (L; Ln = Gd(1), Dy(2) and Y(3)). Detailed magnetic studies disclose that a weak antiferromagnetic coupling exists within 1 (-0.09 cm^-1) and 2 (-0.07 cm^-1; -2J Hamiltonian). Additionally, this study unveils the importance of the amide ligand at the coordination site of Dy(III), which manifests a slow relaxation of magnetization in the absence of an external magnetic field. This has been rationalized by detailed ab initio calculations as well as the electronic structure determination of 1 and 2.