Exchange coupling and magnetic blocking in bipyrimidyl radical-bridged dilanthanide complexes

Modelling spin Hamiltonian parameters of molecular nanomagnets

Molecular nanomagnets encompass a wide range of coordination complexes possessing several potential applications. A formidable challenge in realizing these potential applications lies in controlling the magnetic properties of these clusters. Microscopic spin Hamiltonian (SH) parameters describe the magnetic properties of these clusters, and viable ways to control these SH parameters are highly desirable. Computational tools play a proactive role in this area, where SH parameters such as isotropic exchange interaction (J), anisotropic exchange interaction (Jx, Jy, Jz), double exchange interaction (B), zero-field splitting parameters (D, E) and g-tensors can be computed reliably using X-ray structures. In this feature article, we have attempted to provide a holistic view of the modelling of these SH parameters of molecular magnets. The determination of J includes various class of molecules, from di- and polynuclear Mn complexes to the {3d-Gd}, {Gd-Gd} and {Gd-2p} class of complexes. The estimation of anisotropic exchange coupling includes the exchange between an isotropic metal ion and an orbitally degenerate 3d/4d/5d metal ion. The double-exchange section contains some illustrative examples of mixed valance systems, and the section on the estimation of zfs parameters covers some mononuclear transition metal complexes possessing very large axial zfs parameters. The section on the computation of g-anisotropy exclusively covers studies on mononuclear Dy(III) and Er(III) single-ion magnets. The examples depicted in this article clearly illustrate that computational tools not only aid in interpreting and rationalizing the observed magnetic properties but possess the potential to predict new generation MNMs.

A linear trinuclear ferrous single molecule magnet

A linear trinuclear ferrous [FeII3] SMM exhibited clear magnetic hysteresis loops and a significant higher effective energy barrier in comparison with its cobalt(ii) analogue.

A family of multi-spin rare-earth complexes based on a triazole nitronyl nitroxide radical: synthesis, structure and magnetic properties

The combination of Ln^III ions (Gd^III, Tb^III or Dy^III) and a triazole nitronyl nitroxide radical (4-Me-3-NITtrz) as spin carriers results in two mononuclear and one binuclear compounds, namely, [Ln(hfac)₃(4-Me-3-NITtrz)(H₂O)] (Ln = Gd(1), Tb(2); hfac = hexafluoroacetylacetone; 4-Me-3-NITtrz = 2-[3-(4-methyl-l,2,4-triazolyl)]-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide) and [Dy(hfac)₂(4-Me-3-NITtrz)₂][Dy(hfac)₄]·CHCl₃. Compounds 1 and 2 are isostructural and crystallize in the P1̄ space group, whereas compound 3 crystallizes in the P2₁/n space group. In 1 and 2, the central Ln^III ions are nine-coordinated (LnNO₈) in a distorted spherical capped square antiprism geometry (C_4v) finished by three bischelate hfac anions and one bidentate triazole radical and one aqua molecule. While 3 is a mixed-coordinated binuclear compound with Dy1 in a triangular dodecahedron (D_2d) coordination sphere and Dy2 in a biaugmented trigonal prism (C_2v) coordination sphere. Magnetic studies show that compound 2 exhibits field-induced single-molecule magnet (SMM) behavior.

Three Ln^III–triazole–radical complexes have been synthesized, which exhibit interesting magnetic properties. The Tb^III complex shows frequency-dependent ac magnetic susceptibilities, which suggests the presence of slow magnetic relaxation.

Versatile synthesis of chiral 6-oxoverdazyl radical ligands – new building blocks for multifunctional molecule-based magnets

A versatile synthetic methodology to access two series of chiral verdazyl N,N′-chelate ligands 1 and 2 is presented and their ability to coordinate 3d metal ions is demonstrated.

Terbocenium: completing a heavy lanthanide metallocenium cation family with an alternative anion abstraction strategy

A series of lanthanide metallocenium cations [Ln(Cp^ttt)₂]⁺ (Ln = Y, Gd, Dy, Ho, Er, Tm, Yb, Lu; Cp^ttt = C₅H₂^tBu₃-1,2,4) were recently prepared, but curiously the Tb analogue was elusive. Here we access [Tb(Cp^ttt)₂]⁺via an alternative strategy, completing the heavy [Ln(Cp^ttt)₂]⁺ family.

A family of lanthanide compounds with reduced nitronyl nitroxide diradical: syntheses, structures and magnetic properties

A new diradical based on the pyrazine ring and a series of Ln₂ compounds with its reduced form have been synthesized and characterized structurally and magnetically.

Single-molecule magnet behaviour in a dysprosium-triradical complex

The first triradical bridged binuclear penta-spin single-molecule magnet based on a nitronyl nitroxide triradical was synthesized and characterized.

Giant coercivity and high magnetic blocking temperatures for N23- radical-bridged dilanthanide complexes upon ligand dissociation

Increasing the operating temperatures of single-molecule magnets—molecules that can retain magnetic polarization in the absence of an applied field—has potential implications toward information storage and computing, and may also inform the development of new bulk magnets. Progress toward these goals relies upon the development of synthetic chemistry enabling enhancement of the thermal barrier to reversal of the magnetic moment, while suppressing alternative relaxation processes. Herein, we show that pairing the axial magnetic anisotropy enforced by tetramethylcyclopentadienyl (Cp^Me4H) capping ligands with strong magnetic exchange coupling provided by an N₂³⁻ radical bridging ligand results in a series of dilanthanide complexes exhibiting exceptionally large magnetic hysteresis loops that persist to high temperatures. Significantly, reducing the coordination number of the metal centers appears to increase axial magnetic anisotropy, giving rise to larger magnetic relaxation barriers and 100-s magnetic blocking temperatures of up to 20 K, as observed for the complex [K(crypt-222)][(Cp^Me4H₂Tb)₂(μ−\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\rm{N}}_2^ \cdot$$\end{document}N2⋅)].

Single-molecule magnets typically only retain information in the presence of an applied magnetic field and at very low temperatures. Here, Demir, Long and co-workers design N₂^3– radical-bridged dilanthanide complexes that exhibit giant coercivities and 100-s magnetic blocking temperatures of up to 20 K.

The 1,6,7,12-Tetraazaperylene Bridging Ligand as an Electron Reservoir and Its Disulfonato Derivative as Redox Mediator in an Enzyme-Electrode Process

The homodinuclear ruthenium(II) complex [{Ru(l-N₄ Me₂ )}₂ (μ-tape)](PF₆ )₄ {[1](PF₆ )₄ } (l-N₄ Me₂ =N,N'-dimethyl-2,11-diaza[3.3](2,6)-pyridinophane, tape=1,6,7,12-tetraazaperylene) can store one or two electrons in the energetically low-lying π* orbital of the bridging ligand tape. The corresponding singly and doubly reduced complexes [{Ru(l-N₄ Me₂ )}₂ (μ-tape^.- )](PF₆ )₃ {[2](PF₆ )₃ } and [{Ru(l-N₄ Me₂ )}₂ (μ-tape^2- )](PF₆ )₂ {[3](PF₆ )₂ }, respectively, were electrochemically generated, successfully isolated and fully characterized by single-crystal X-ray crystallography, spectroscopic methods and magnetic susceptibility measurements. The singly reduced complex [2](PF₆ )₃ contains the π-radical tape^.- and the doubly reduced [3](PF₆ )₂ the diamagnetic dianion tape^2- as bridging ligand, respectively. Nucleophilic aromatic substitution at the bridging tape in [1]⁴⁺ by two sulfite units gave the complex [{Ru(l-N₄ Me₂ )}₂ {μ-tape-(SO₃ )₂ }]²⁺ ([4]²⁺ ). Complex dication [4]²⁺ was exploited as a redox mediator between an anaerobic homogenous reaction solution of an enzyme system (sulfite/sulfite oxidase) and the electrode via participation of the low-energy π*-orbital of the disulfonato-substituted bridging ligand tape-(SO₃ )₂^2- (E_red1 =-0.1 V versus Ag/AgCl/1 m KCl in water).

Elucidation of Dual Magnetic Relaxation Processes in Dinuclear Dysprosium(III) Phthalocyaninato Triple-Decker Single-Molecule Magnets Depending on the Octacoordination Geometry

When applying single-molecule magnets (SMMs) to spintronic devices, control of the quantum tunneling of the magnetization (QTM) as well as a spin-lattice interactions are important. Attempts have been made to use not only coordination geometry but also magnetic interactions between SMMs as an exchange bias. In this manuscript, dinuclear dysprosium(III) (Dy^III ) SMMs with the same octacoordination geometry undergo dual magnetic relaxation processes at low temperature. In the dinuclear Dy^III phthalocyaninato (Pc^2- ) triple-decker type complex [(Pc)Dy(ooPc)Dy(Pc)] (1) (ooPc^2- =2,3,9,10,16,17,23,24-octakis(octyloxy)phthalocyaninato) with a square-antiprismatic (SAP) geometry, the ground state is divided by the Zeeman effect, and level intersection occurs when a magnetic field is applied. Due to the ground state properties of 1, since the Zeeman diagram where the levels intersect in an H_dc of 2500 Oe, two kinds of QTM and direct processes occur. However, dinuclear Dy^III -Pc systems with C₄ geometry, which have a twist angle (ϕ) of less than 45° do not undergo dual magnetic relaxation processes. From magnetic field and temperature dependences, the dual magnetic relaxation processes were clarified.

Strong Ferromagnetic Exchange Coupling Mediated by a Bridging Tetrazine Radical in a Dinuclear Nickel Complex

The radical bridged compound [(Ni(TPMA))₂-μ-bmtz^•-](BF₄)₃·3CH₃CN (bmtz = 3,6-bis(2'-pyrimidyl)-1,2,4,5-tetrazine, TPMA = tris(2-pyridylmethyl)amine) exhibits strong ferromagnetic exchange between the S = 1 Ni^II centers and the bridging S = 1/2 bmtz radical with J = 96 ± 5 cm^-1 (-2J_Ni-radS_NiS_rad). DFT calculations support the existence of strong ferromagnetic exchange.

Syntheses, structures and magnetic properties of the lanthanide complexes of the pyrimidyl-substituted nitronyl nitroxide radical

Four 2p-4f Ln^III-radical complexes, [(NIT-2-Pm)Ln(hfac)₃]·0.5C₇H₁₆ (NIT-2-Pm = 2-pyrimidyl-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxy-3-oxide, hfac = hexafluoroacetylacetonato, Ln = Tb (1), Dy (2), Ho (3) and Er (4)), and four 4f-2p-4f Ln^III-radical-Ln^III complexes, [(μ-NIT-2-Pm)Ln₂(hfac)₆(H₂O)₂]·0.5C₇H₁₆ (Ln = Tb (5), Dy (6), Ho (7) and Er (8)) have been synthesized and characterized structurally and magnetically. These compounds can be selectively obtained by controlling the reaction ratio of Ln(hfac)₃·2H₂O to the radical ligand NIT-2-Pm. The crystal structures show that in the former four complexes 1-4, the NIT-2-Pm radical acts as a terminal bidentate ligand chelating to one Ln^III ion, while in 5-8, the NIT-2-Pm acts as a bridging ligand linking two Ln^III ions to form a binuclear three-spin system. Magnetic studies revealed that complexes 1-4 and 6 show frequency-dependent ac magnetic susceptibilities, suggesting a possible single-molecule magnet behavior. To the best of our knowledge, complexes 3 and 4 are the first Ho-NIT and Er-NIT compounds showing slow magnetic relaxation. Compounds 5-8 represent a rare family of compounds showing the NIT bridged 4f-2p-4f three-spin motif, while complex 6 is a rare NIT bridged multinuclear lanthanide compound possessing SMM-like behaviour. Ab initio calculations were performed on all these complexes. The fitting of the magnetic susceptibilities of these compounds suggests weak antiferromagnetic coupling between the Ln^III and NIT radical in 1-8 and weak ferromagnetic Ln^III-Ln^III interactions in 5-8.

Three series of heterometallic NiII–LnIII Schiff base complexes: synthesis, crystal structures and magnetic characterization

Three series of Ni^II–Ln^III complexes were synthesized using compartmental Schiff base ligands in conjunction with auxiliary ligands. Their magnetic properties have been well studied.

Slow magnetisation relaxation in tetraoxolene-bridged rare earth complexes

Two tetraoxolene-bridged dinuclear Dy(iii) complexes exhibit slow relaxation in ac magnetic susceptibility studies with zero-field quantum tunnelling of the magnetisation that is suppressed by the application of a dc magnetic field.

An air stable radical-bridged dysprosium single molecule magnet and its neutral counterpart: redox switching of magnetic relaxation dynamics

Addition of a radical to the bridging tetrazine ligand of a Dy₂ complex dramatically alters the magnetic properties. The radical complex undergoes magnetic relaxation via a thermal relaxation pathway, whereas the neutral compound relaxes via quantum tunneling of the magnetization.

Complementation and joint contribution of appropriate intramolecular coupling and local ion symmetry to improve magnetic relaxation in a series of dinuclear Dy2 single-molecule magnets

The joint contribution of appropriate intramolecular coupling and local ion symmetry to improve magnetic relaxation in a series of dinuclear Dy2 SMMs.

Strong direct exchange coupling and single-molecule magnetism in indigo-bridged lanthanide dimers

The first lanthanide–indigo complexes are described, including a dysprosium single-molecule magnet and one of the strongest exchange coupling constants measured in a gadolinium compound.

Syntheses, crystal structures, and magnetic properties of cyclic dimer Ln2L2 complexes constructed from (3-pyridinylmethoxy)phenyl-substituted nitronyl nitroxide ligands

Herein, five lanthanide-radical cyclic dimer complexes have been obtained and complex Tb shows the relaxation behavior in zero dc field with an anisotropy barrier Δ/k_B = 21.6 K.

A comparative study of magnetization dynamics in dinuclear dysprosium complexes featuring bridging chloride or trifluoromethanesulfonate ligands

The rigidity of the ligand PyCp₂²⁻ allows for facile access to Dy³⁺ complexes that exhibit improved dynamic magnetic properties.

Construction of lanthanide single-molecule magnets with the “magnetic motif” [Dy(MQ)4]−

[Dy₂(MQ)₄(NO₃)₃]⁺ is SMM-silent (SMM = single-molecule magnet) owing to the unfavourable magnetic interactions. Introducing an additional “magnetic motif” [Dy(MQ)₄]⁻ to diminish such interactions gave a trinuclear [Dy₃(MQ)₈(NO₃)], which behaves as an SMM.

Thermal expansion and magnetic properties of benzoquinone-bridged dinuclear rare-earth complexes

The complexes [BQ(MCl₂·THF₃)₂] (M = Y or Dy) possessing pentagonal bipyramidal environment around metal centers undergo significant thermal expansion as revealed by single-crystal X-ray and powder diffraction experiments.

Coexistence of long-range antiferromagnetic order and slow relaxation of the magnetization in the first lanthanide complex of a 1,2,4-benzotriazinyl radical

We have synthesised and characterised the compound Dy(1)(tbacac)₃ (2), the first lanthanide complex of a new coordinating 1,2,4-benzotriazinyl radical (1).