Magnetic anisotropy in two- to eight-coordinated transition–metal complexes: Recent developments in molecular magnetism

Single-ion magnets (SIMs), exhibiting slow magnetization relaxation in the absence of the magnetic field, originate from their single spin-carrier centre. In pursuit of high-performance magnetic properties, such as high spin-reversal barrier and high blocking temperature, various metal centres were investigated to establish SIMs, including 3d and 5d transition metal ions, 4f lanthanide ions, and 5f actinide ions, which possess unique zero-field splitting and magnetic properties. Therefore, proper ligand field is of great importance to different types of metals. In the given great breakthroughs since the first SIM, [Pc2 Tb]- (Pc=dianion of phthalocyanine), was reported, strategies of ligand field design have emerged. In this review, the developments of SIMs with different metal centres are summarized, as well as the possible strategies.

Single Ion Magnets from 3d to 5f: Developments and Strategies.

Polyoxometalates (POMs) represent an important group of metal-oxo nanoclusters, typically comprised of early transition metals in high oxidation states (mainly V, Mo and W). Many plenary POMs exhibit good pH, solvent, thermal and redox stability, which makes them attractive components for the design of covalently integrated hybrid organic-inorganic molecules, herein referred to as hybrid-POMs. Until now, thousands of organic hybrid-POMs have been reported; however, only a small fraction can be further functionalized using other organic molecules or metal cations. This emerging class of 'post-functionalizable' hybrid-POMs constitute a valuable modular platform that permits coupling of POM properties with different organic and metal cation functionalities, thereby expanding the key physicochemical properties that are relevant for application in (photo)catalysis, bioinorganic chemistry and materials science. The post-functionalizable hybrid-POM platforms offer an opportunity to covalently link multi-electron redox responsive POM cores with virtually any (bio)organic molecule or metal cation, generating a wide range of materials with tailored properties. Over the past few years, these materials have been showcased in the preparation of framework materials, functional surfaces, surfactants, homogeneous and heterogeneous catalysts and light harvesting materials, among others. This review article provides an overview on the state of the art in POM post-functionalization and highlights the key design and structural features that permit the discovery of new hybrid-POM platforms. In doing so, we aim to make the subject more comprehensible, both for chemists and for scientists with different materials science backgrounds interested in the applications of hybrid (POM) materials. The review article goes beyond the realms of polyoxometalate chemistry and encompasses emerging research domains such as reticular materials, surfactants, surface functionalization, light harvesting materials, non-linear optics, charge storing materials, and homogeneous acid-base catalysis among others.

Hybrid polyoxometalates as post-functionalization platforms: from fundamentals to emerging applications

Molecular Nanomagnets : Fundamental Understanding

Polyoxometalate functionalized architectures

https://www.rsc.org/suppdata/cc/c4/c4cc09435b/c4cc09435b1.pdf

Field-induced slow magnetic relaxation of octahedrally coordinated mononuclear Fe(III)-, Co(II)-, and Mn(III)-containing polyoxometalates.

The disassembly and reassembly of giant molecules are essential processes in controlling the structure and function of biological and artificial systems. In this work, the disassembly and reassembly of a giant ring-shaped polyoxometalate (POM) without isomerization of the monomeric units is reported. The reaction of a hexavacant lacunary POM that is soluble in organic solvents, [P2 W12 O48 ](14-) , with manganese cations gave the giant ring-shaped POM [{γ-P2 W12 O48 Mn4 (C5 H7 O2 )2 (CH3 CO2 )}6 ](42-) . This POM is a hexamer of manganese-substituted {P2 W12 O48 Mn4 } units, and its inner cavity was larger than any of those previously reported for ring-shaped polyoxotungstates. It was disassembled into monomeric units in acetonitrile, and the removal of the capping organic ligands on the manganese cations led to reassembly into a tetrameric ring-shaped POM, [{γ-P2 W12 O48 Mn4 (H2 O)6 }4 (H2 O)4 ](24-) .

Synthesis and Disassembly/Reassembly of Giant Ring-Shaped Polyoxotungstate Oligomers.

In this work, we have successfully created several unprecedented discrete 3d–3d′–4f heterotrimetallic clusters in between lacunary polyoxometalates (POMs). By the three-step sequential introduction of metal cations into a trivacant lacunary POM TBA4H6[A-α-SiW9O34] (TBA = tetra-n-butylammonium) in organic media, five kinds of sandwich-type POMs with double-diamond-shaped 3d–3d′–4f heptanuclear clusters (IIIFeM4Ln2, TBAnHm[FeM4{Ln(L)2}2O2(A-α-SiW9O34)2], M = Mn3+, Cu2+; Ln = Gd3+, Dy3+, Lu3+; L = acac (acetylacetonate), hfac (hexafluoroacetylacetonate)) were successfully synthesized for the first time. By introduction of two [Ln(L)2]+ units on the ends of pentanuclear clusters [FeMn4O18(OH)2]23– and [FeCu4O18(OH)2]27–, the magnetic interactions between Mn3+–Mn3+ and Cu2+–Cu2+ could be modulated. Among a series of the heterometallic heptanuclear compounds, IIIFeMn4Lu2 exhibited the slow magnetic relaxation characteristic for a single-molecule magnet under the zero applied magnetic fields.

Sequential Synthesis of 3d–3d′–4f Heterometallic Heptanuclear Clusters in between Lacunary Polyoxometalates

Structurally well-defined hetero-pentanuclear manganese-oxide clusters {MMn4} were successfully synthesized in TBA7Hn[MMn4(OH)2(A-α-SiW9O34)2]·2H2O·C2H4Cl2 (IIMMn4, M = Fe(III), Co(II), Ni(II), Cu(II), Ga(III)) by sequential introduction of metal cations into the trivacant lacunary polyoxometalates (POMs). The pentanuclear manganese-oxide cluster {Mn5} showed a small spin ground state and a low energy barrier for magnetization relaxation. In contrast, the magnetic interactions in the hetero-pentanuclear clusters could be controlled by the arrangements of metals, and the clusters showed large magnetic anisotropy and single-molecule magnet behavior. In particular, the cluster {FeMn4} in IIFeMn4 (S = 11/2) showed the slowest relaxation and the highest energy barrier among the previously reported transition metal-containing POMs.

A cascade approach to hetero-pentanuclear manganese-oxide clusters in polyoxometalates and their single-molecule magnet properties

The design of structurally well-defined anionic molecular metal-oxygen clusters, polyoxometalates (POMs), leads to inorganic receptors with unique and tunable properties. Herein, an α-Dawson-type silicotungstate, TBA8[α-Si2W18O62]⋅3 H2O (II) that possesses a -8 charge was successfully synthesized by dimerization of a trivacant lacunary α-Keggin-type silicotungstate TBA4H6[α-SiW9O34]⋅2 H2O (I) in an organic solvent. POM II could be reversibly protonated (in the presence of acid) and deprotonated (in the presence of base) inside the aperture by means of intramolecular hydrogen bonds with retention of the POM structure. In contrast, the aperture of phosphorus-centered POM TBA6[α-P2W18O62]⋅H2O (III) was not protonated inside the aperture. The density functional theory (DFT) calculations revealed that the basicities and charges of internal μ3-oxygen atoms were increased by changing the central heteroatoms from P(5+) to Si(4+), thereby supporting the protonation of II. Additionally, II showed much higher catalytic performance for the Knoevenagel condensation of ethyl cyanoacetate with benzaldehyde than I and III.

Takuo Minato

Papers

Field-induced slow magnetic relaxation of octahedrally coordinated mononuclear Fe(III)-, Co(II)-, and Mn(III)-containing polyoxometalates.

Synthesis and Disassembly/Reassembly of Giant Ring-Shaped Polyoxotungstate Oligomers.

Sequential Synthesis of 3d–3d′–4f Heterometallic Heptanuclear Clusters in between Lacunary Polyoxometalates

A cascade approach to hetero-pentanuclear manganese-oxide clusters in polyoxometalates and their single-molecule magnet properties

Synthesis of α‐Dawson‐Type Silicotungstate [α‐Si2W18O62]8− and Protonation and Deprotonation Inside the Aperture through Intramolecular Hydrogen Bonds