There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

J. Appl. Cryst.の発刊に際して

The purpose of this critical review is to give a representative and comprehensive overview of the arising developments in the field of magnetic metal–organic frameworks, in particular those containing cobalt(II). We examine the diversity of magnetic exchange interactions between nearest-neighbour moment carriers, covering from dimers to oligomers and discuss their implications in infinite chains, layers and networks, having a variety of topologies. We progress to the different forms of short-range magnetic ordering, giving rise to single-molecule-magnets and single-chain-magnets, to long-range ordering of two- and three-dimensional networks (323 references).

Magnetic metal-organic frameworks.

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Lanthanide single-molecule magnets.

Scientists have long employed lanthanide elements in the design of materials with extraordinary magnetic properties, including the strongest magnets known, SmCo5 and Nd2Fe14B. The properties of these materials are largely a product of fine-tuning the interaction between the lanthanide ion and the crystal lattice. Recently, synthetic chemists have begun to utilize f-elements—both lanthanides and actinides—for the construction of single-molecule magnets, resulting in a rapid expansion of the field. The desirable magnetic characteristics of the f-elements are contingent upon the interaction between the single-ion electron density and the crystal field environment in which it is placed. This interaction leads to the single-ion anisotropies requisite for strong single-molecule magnets. Therefore, it is of vital importance to understand the particular crystal field environments that could lead to maximization of the anisotropy for individual f-elements. Here, we summarize a qualitative method for predicting the ligand architectures that will generate magnetic anisotropy for a variety of f-element ions. It is hoped that this simple model will serve to guide the design of stronger single-molecule magnets incorporating the f-elements.

/pdf/exploiting-single-ion-anisotropy-in-the-design-of-f-element-43gapzimvj.pdf

Exploiting single-ion anisotropy in the design of f-element single-molecule magnets

An organometallic single-ion magnet is synthesized with only 19 non-hydrogen atoms featuring an erbium ion sandwiched by two different aromatic ligands. This molecule displays a butterfly-shaped hysteresis loop at 1.8 K up to even 5 K. Alternating-current (ac) susceptibility measurement reveals the existence of two thermally activated magnetic relaxation processes with the energy barriers as high as 197 and 323 K, respectively.

An organometallic single-ion magnet.

Single-molecule magnets (SMMs) have received much attention owing to their quantum tunneling and slow relaxation. These behaviors can be observed when the molecule has large ground spin state with a uniaxial magnetic anisotropy, namely negative zero-field splitting (ZFS) parameter D. Besides a large ground state, to increase the SMMs energy barrier and blocking temperature, it is of fundamental importance to control the magnetic anisotropy. It is also of great complexity to understand the conditions that determine the anisotropy or zero-field splitting properties for a cluster. As a result, SMMs containing only one spin carrier are of great interest because of the simplification in the analysis of local anisotropy and ZFS. Recently, some molecules with isolated 3d or 4f metal ions were observed to show a direct-current (dc) field-induced slow magnetic relaxation. Studies show that this kind of dc-field-dependent relaxation phenomenon is thermally activated, and the quench of fast quantum tunneling by the dc field could give rise to the slow relaxation. However, as commented by Benelli and Gatteschi, it is an unexpected and puzzling behavior, and the underlying mechanism is still unclear. Ishikawa et al. reported that the phthalocyanine (Pc) double-decker anion complexes [Pc2Ln] with a single Tb or Dy ion show slow relaxation without a dc field. A single Er ion in the polyoxometallate system [ErW10O36] 9 showed similar relaxation behavior in zero static magnetic field. The first single-actinide complex [U(Ph2BPz2)3] (containing the U III ion; Ph2BPz2 = diphenylbis(pyrazolylborate)) reported by Rinehart and Long shows a similar slow relaxation. Owing to the single-ion features, these complexes could be called “single-ion magnets”. In these complexes, ligand fields with a high-order single axis Cn (n> 2; n = 4 for Ref. [8, 9] and n = 3 for Ref. [10]) split the (2J + 1) degenerate ground states into new sublevels, which produces a uniaxial anisotropy, thus giving rise to a higher energy barrier for relaxation. The Dy ion, which possesses a Kramers ground state of H15/2, is an appealing paramagnetic source for the construction of SMMs in a suitable ligand-field symmetry and strength. Moreover, among reported SMMs, some of them are clusters containing Dy : Dy2, [11] Dy3, [12] Dy4, [13] Dy5, [14] Dy-containing chain, and tens of Dycontaining 3d–4f clusters. All three reported types of single-ion magnets are found with a high-order single axis defining the local symmetry. Pursuing this clue, we synthesized a series of mononuclear Ln compounds with a local symmetry close to D4d. Crystal analysis shows that the isomorphous complexes consist of a neutral mononuclear [Ln(acac)3(H2O)2] complex (Ln = Dy, Ho, Er, acac = acetylacetonate) together with an uncoordinated water molecule and an uncoordinated ethanol molecule (Figure 1a). In the Dy complex, Dy is coordinated by eight oxygen atoms with Dy O bonds of 2.311–2.434 , six of which come from the acetylacetonate ligand and two from coordinated water molecules. The eight oxygen atoms form an approximately square-antiprismatic coordination polyhedron, and the local symmetry of Dy is nearly D4d (Figure 1b). Actually, a similar lanthanide acac complex was firstly reported in 1968, but the uncoordinated water and

A Mononuclear Dysprosium Complex Featuring Single-Molecule-Magnet Behavior†

ConspectusSingle-molecule magnets (SMMs) can retain their magnetization status preferentially after removal of the magnetic field below a certain temperature. The unique property, magnetic bistable status, enables the molecule-scale SMM to become the next-generation high-density information storage medium. SMMs’ new applications are also involved in high-speed quantum computation and molecular spintronics. The development of coordination chemistry, especially in transition metal (3d) and lanthanide (4f) complexes, diversifies SMMs by introducing new ones. In both 3d and 4f SMMs, the ligands play a fundamental role in determining the SMMs’ magnetic properties. The strategies for rationally designing and synthesizing high-performance SMMs require a comprehensive understanding of the effects of a crystal field.In this Account, we focus mainly on the magneto-structural correlations of 4f or 3d single-ion magnets (SIMs), within which there is only one spin carrier. These one-spin carrier complexes benefit from...

Understanding the Magnetic Anisotropy toward Single-Ion Magnets.

The pursuit of single-molecule magnets (SMMs) with better performance urges new molecular design that can endow SMMs larger magnetic anisotropy. Here we report that two-coordinate cobalt imido complexes featuring highly covalent Co═N cores exhibit slow relaxation of magnetization under zero direct-current field with a high effective relaxation barrier up to 413 cm-1, a new record for transition metal based SMMs. Two theoretical models were carried out to investigate the anisotropy of these complexes: single-ion model and Co-N coupling model. The former indicates that the pseudo linear ligand field helps to preserve the first-order orbital momentum, while the latter suggests that the strong ferromagnetic interaction between Co and N makes the [CoN]+ fragment a pseudo single paramagnetic ion, and that the excellent performance of these cobalt imido SMMs is attributed to the inherent large magnetic anisotropy of the [CoN]+ core with |MJ = ± 7/2⟩ ground Kramers doublet.

Two-Coordinate Co(II) Imido Complexes as Outstanding Single-Molecule Magnets

An air-stable star-shaped CoIICoIII3 complex with only one paramagnetic Co(II) ion in the D3 coordination environment has been synthesized from a chiral Schiff base ligand. Magnetic studies revealed that this complex exhibits slow magnetic relaxation in the absence of an applied dc field, which is one of the main characteristics of single-molecule magnets (SMMs). The relaxation barrier is as high as 109 K, which is quite large among transition-metal ion-based SMMs. The complex represents the first example of zero-field SMM behavior in a mononuclear six oxygen-coordinate Co(II) complex.

Bing-Wu Wang

Papers

An organometallic single-ion magnet.

A Mononuclear Dysprosium Complex Featuring Single-Molecule-Magnet Behavior†

Understanding the Magnetic Anisotropy toward Single-Ion Magnets.

Two-Coordinate Co(II) Imido Complexes as Outstanding Single-Molecule Magnets

Zero-field slow magnetic relaxation from single Co(II) ion: a transition metal single-molecule magnet with high anisotropy barrier