The area of 3d-metal coordination clusters that behave as Single-Molecule Magnets (SMMs) is now quite mature within the interdisciplinary field of Molecular Magnetism. This area has created a renaissance in Inorganic Chemistry. From the synthetic Inorganic Chemistry viewpoint, the early years of “try and see” exercises (1993–2000) have been followed by the development of strategies and strict approaches. Our review will first summarize the early synthetic efforts and routes for the preparation of polynuclear 3d-metal SMMs, and it will be then concentrated on the description of the existing strategies. The former involve the combination of appropriate 3d-metal-containing starting materials (simple salts with inorganic anions, metal cardoxylates, and pre-formed carboxylate clusters, metal phosphonates) and one or two primary organic ligands; the importance of the end-on azido group as a ferromagnetic coupler in 3d-metal SMM chemistry will be discussed. The utility of comproportionation reactions and the reductive aggregation route for the construction of manganese SMMs will also be described. Most of the existing strategies for the synthesis of SMMs concern manganese. These involve substitution of carboxylate ligands in pre-formed SMMs by other carboxylate or non-carboxylate groups, reduction procedures for the {Mn8IIIMn4IV} SMMs, spin “tweaking,” “switching on” SMM properties upon conversion of low-spin clusters into high-spin ones, ground-state spin switching and enhancing SMM properties via targeted structural distortions, the use of radical bridging ligands and supramolecular approaches. A very useful strategy is also the “switching on” of SMM behavior through replacement of bridging hydroxide groups by end-on azido or isocyanato ligands in clusters. Selected examples will be mentioned and critically discussed. Particular emphasis will be given on the criteria for the choice of ligands.

The initial use of anils, i.e. bidentate Schiff bases derived from the condensation of anilines with salicylaldehyde or its derivatives, in 4f-metal chemistry is described. The 1 : 1 reactions between Ln(NO3)3·xH2O (Ln = lanthanide) or Y(NO3)3·6H2O and N-(5-bromosalicylidene)aniline (5BrsalanH) in MeCN has provided access to complexes [Ln(NO3)3(5BrsalanH)2(H2O)]·MeCN (Ln = Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb) and [Y(NO3)3(5BrsalanH)2(H2O)]·MeCN, respectively, in good yields. The structures of the isomorphous complexes with Ln = Pr(1·MeCN), Sm(3·MeCN), Gd(5·MeCN), Dy(7·MeCN) and Er(9·MeCN) have been determined by single-crystal X-ray crystallography. The other complexes were proven to be isostructural with the fully structurally characterized compounds based on elemental analyses, IR spectra, unit cell determinations and powder X-ray patterns. The 9-coordinate LnIII centre in the [Ln(NO3)3(5BrsalanH)2(H2O)] molecules is bound to six oxygen atoms from the three bidentate chelating nitrato groups, two oxygen atoms that belong to the organic ligands and one oxygen atom from the aquo ligand. The 5BrsalanH molecules behave as monodentate O-donors; the acidic H atom is clearly located on the imino N atom and thus the formally neutral ligands adopt an extremely rare coordination mode participating in the zwitterionic form. The coordination polyhedra defined by the nine donor atoms around the LnIII centres are best described as spherical capped square antiprisms. Various intermolecular interactions build the crystal structures and Hirshfeld surface analysis was applied to evaluate the magnitude of interactions between the molecules. Solid-state IR and UV/VIS data are discussed in terms of structural features. 1H NMR data prove that the diamagnetic [Y(NO3)3(5BrsalanH)2(H2O)] complex decomposes in DMSO. Combined dc and ac magnetic susceptibility, as well as magnetization data for 7 suggest that this complex shows field-induced slow magnetic relaxation. Two magnetization relaxation processes are evident. The fit to the Arrhenius law has been performed using the 6.5-8.5 K ac data, affording an effective barrier for the magnetization reversal of 27 cm-1. Cole-Cole plot analysis in the temperature range in which the Orbach relaxation process is assumed, reveals a narrow distribution of relaxation times. The solid Dy(iii) complex 7 emits green light at 338 nm, the emission being ligand-centered. The perspectives of the present, first results in the lanthanide(iii)-anil chemistry are critically discussed.

There has been a renaissance in the interdisciplinary field of Molecular Magnetism since ~2000, due to the discovery of the impressive properties and potential applications of d- and f-metal Single-Molecule Magnets (SMMs) and Single-Ion Magnets (SIMs) or Monometallic Single-Molecule Magnets. One of the consequences of this discovery has been an explosive growth in synthetic molecular inorganic and organometallic chemistry. In SMM and SIM chemistry, inorganic and organic ligands play a decisive role, sometimes equally important to that of the magnetic metal ion(s). In SMM chemistry, bridging ligands that propagate strong ferromagnetic exchange interactions between the metal ions resulting in large spin ground states, well isolated from excited states, are preferable; however, antiferromagnetic coupling can also lead to SMM behavior. In SIM chemistry, ligands that create a strong axial crystal field are highly desirable for metal ions with oblate electron density, e.g., TbIII and DyIII, whereas equatorial crystal fields lead to SMM behavior in complexes based on metal ions with prolate electron density, e.g., ErIII. In this review, we have attempted to highlight the use of few, efficient ligands in the chemistry of transition-metal SMMs and SIMs, through selected examples. The content of the review is purely chemical and it is assumed that the reader has a good knowledge of synthetic, structural and physical inorganic chemistry, as well as of the properties of SIMs and SMMs and the techniques of their study. The ligands that will be discussed are the azide ion, the cyanido group, the tris(trimethylsilyl)methanide, the cyclopentanienido group, soft (based on the Hard-Soft Acid-Base model) ligands, metallacrowns combined with click chemistry, deprotonated aliphatic diols, and the family of 2-pyridyl ketoximes, including some of its elaborate derivatives. The rationale behind the selection of the ligands will be emphasized.

The reactions of hydrated lanthanide(III) [Ln(III)] nitrates and salicylideneaniline (salanH) have provided access to two families of mononuclear complexes depending on the reaction solvent used. In MeCN, the products are [Ln(NO3)3(salanH)2(H2O)]·MeCN, and, in MeOH, the products are [Ln(NO3)3(salanH)2(MeOH)]·(salanH). The complexes within each family are proven to be isomorphous. The structures of complexes [Ln(NO3)3(salanH)2(H2O)]·MeCN (Ln = Eu, 4·MeCN_Eu, Ln = Dy, 7·MeCN_Dy; Ln = Yb, 10·MeCN_Yb) and [Ln(NO3)3(salanH)2(MeOH)]·(salanH) (Ln = Tb, 17_Tb; Ln = Dy, 18_Dy) have been solved by single-crystal X-ray crystallography. In the five complexes, the LnIII center is bound to six oxygen atoms from the three bidentate chelating nitrato groups, two oxygen atoms from the two monodentate zwitterionic salanH ligands, and one oxygen atom from the coordinated H2O or MeOH group. The salanH ligands are mutually “cis” in 4·MeCN_Eu, 7·MeCN_Dy and 10·MeCN_Yb while they are “trans” in 17_Tb and 18_Dy. The lattice salanH molecule in 17_Tb and 18_Dy is also in its zwitterionic form with the acidic H atom being clearly located on the imine nitrogen atom. The coordination polyhedra defined by the nine oxygen donor atoms can be described as spherical tricapped trigonal prisms in 4·MeCN_Eu, 7·MeCN_Dy, and 10·MeCN_Yb and as spherical capped square antiprisms in 17_Tb and 18_Dy. Various intermolecular interactions build the crystal structures, which are completely different in the members of the two families. Solid-state IR data of the complexes are discussed in terms of their structural features. 1H NMR data for the diamagnetic Y(III) complexes provide strong evidence that the compounds decompose in DMSO by releasing the coordinated salanH ligands. The solid complexes emit green light upon excitation at 360 nm (room temperature) or 405 nm (room temperature). The emission is ligand-based. The solid Pr(III), Nd(III), Sm(III), Er(III), and Yb(III) complexes of both families exhibit LnIII-centered emission in the near-IR region of the electromagnetic spectrum, but there is probably no efficient salanH→LnIII energy transfer responsible for this emission. Detailed magnetic studies reveal that complexes 7·MeCN_Dy, 17_Tb and 18_Dy show field-induced slow magnetic relaxation while complex [Tb(NO3)3(salanH)2(H2O)]·MeCN (6·MeCN_Tb) does not display such properties. The values of the effective energy barrier for magnetization reversal are 13.1 cm−1 for 7·MeCN_Dy, 14.8 cm−1 for 17_Tb, and 31.0 cm−1 for 18_Dy. The enhanced/improved properties of 17_Tb and 18_Dy, compared to those of 6_Tb and 7_Dy, have been correlated with the different supramolecular structural features of the two families. The molecules [Ln(NO3)3(salanH)2(MeOH)] of complexes 17_Tb and 18_Dy are by far better isolated (allowing for better slow magnetic relaxation properties) than the molecules [Ln(NO3)3(salanH)2(H2O)] in 6·MeCN_Tb and 7·MeCN_Dy. The perspectives of the present initial studies in the Ln(III)/salanH chemistry are discussed.

Heterometallic lanthanide [LnLn’] coordination complexes accessible thermodynamically are very scarce because the metals of this series have very similar chemical behaviour. Trinuclear systems of this category are virtually non-existent. A...