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Following a novel synthetic strategy where the strong uniaxial ligand field generated by the Ph3SiO− (Ph3SiO−=anion of triphenylsilanol) and the 2,4‐di‐tBu‐PhO− (2,4‐di‐tBu‐PhO−=anion of 2,4‐di‐tertbutylphenol) ligands combined with the weak equatorial field of the ligand LN6, leads to [DyIII(LN6)(2,4‐di‐tBu‐PhO)2](PF6) (1), [DyIII(LN6)(Ph3SiO)2](PF6) (2) and [DyIII(LN6)(Ph3SiO)2](BPh4) (3) hexagonal bipyramidal dysprosium(III) single‐molecule magnets (SMMs) with high anisotropy barriers of Ueff=973 K for 1, Ueff=1080 K for 2 and Ueff=1124 K for 3 under zero applied dc field. Ab initio calculations predict that the dominant magnetization reversal barrier of these complexes expands up to the 3rd Kramers doublet, thus revealing for the first time the exceptional uniaxial magnetic anisotropy that even the six equatorial donor atoms fail to negate, opening up the possibility to other higher‐order symmetry SMMs.

Since the last decade, the focus in the area of single‐molecule magnets (SMMs) has been shifting constructively towards the development of single‐ion magnets (SIMs) based on transition metals and lanthanides. Although ground‐breaking results have been witnessed for DyIII‐based SIMs, significant results have also been obtained for some mononuclear transition metal SIMs. Among others, studies based on CoII ion are very prominent as they often exhibit high magnetic anisotropy or zero‐field splitting parameters and offer a large barrier height for magnetisation reversal. Although CoII possibly holds the record for having the largest number of zero‐field SIMs known for any transition metal ion, controlling the magnetic anisotropy in these systems are is still a challenge. In addition to the modern spectroscopic techniques, theoretical studies, especially ab initio CASSCF/NEVPT2 approaches, have been used to uncover the electronic structure of various CoII SIMs. In this article, with some selected examples, the aim is to showcase how varying the coordination number from two to eight, and the geometry around the CoII centre alters the magnetic anisotropy. This offers some design principles for the experimentalists to target new generation SIMs based on the CoII ion. Additionally, some important FeII/FeIII and NiII complexes exhibiting large magnetic anisotropy and SIM properties are also discussed.

Three dysprosium(III) single-molecule magnets (SMMs) with the rare hexagonal bipyramidal geometry have been isolated for the first time. Following a novel synthetic strategy where the strong uniaxial ligand field generated by the Ph₃SiO^- (Ph₃SiO^- = anion of triphenylsilanol) and the 2,4-di-^tBu-PhO^- (2,4-di-^tBu-PhO^- = anion of 2,4-di-tertbutylphenol) ligands combined with the weak equatorial field of the ligand L^N6, leads to [Dy^III(L^N6)(2,4-di-^tBu-PhO)₂](PF₆) (<b>1</b>), [Dy^III(L^N6)(Ph₃SiO)₂](PF₆) (<b>2</b>) and [Dy^III(L^N6)(Ph₃SiO)₂](BPh₄) (<b>3</b>) hexagonal bipyramidal complexes with high anisotropy barriers of U_eff = 973 K for <b>1</b>, U_eff = 1080 K for <b>2</b> and U_eff = 1124 K for <b>3 </b>under zero applied dc field. <i>Ab initio</i> calculations predict that the dominant magnetization reversal barrier of these complexes expands up to the 3rd Kramers doublet, thus revealing for the first time the exceptional uniaxial magnetic anisotropy that even the six equatorial donor atoms fail to negate, opening up the possibility to other higher-order symmetry SMMs. <br>

We generate a new air-stable pseudo-D5h Dy(III) Single-Molecule Magnet (Ueff = 1108 K, TB = 14 K) by combining a weak equatorial ligand field from a macrocyclic LN5 ligand with...

A series of neutral homologous complexes [(L)Ln(Cy 3 PO)Cl] {where Ln = Gd (1), Tb (2), Dy (3) and Er (5)} and [(L)Dy(Ph 3 PO)Cl] (4) [H 2 L = 2,6-diacetylpyridine bisbenzoylhydrazone] have been isolated. In these complexes, the central lanthanide ion possesses a pentagonal bipyramidal (PBP) geometry with an overall pseudo D 5h symmetry. The coordination environment around the lanthanide ion comprises of three nitrogen and two oxygen donors in an equatorial plane. The axial positions are taken up by a phosphine oxide (O donor) and a chloride ion. Among these compounds, the Dy(III) (3 and 4) analogues were found to be field-induced single-ion magnets.

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A family of MnLn strictly dinuclear complexes of general formula [Mn(μ-L)(μ-OMe)(NO)Ln(NO)(MeOH)] (Ln = Gd, Dy, Er, Ho) has been assembled in a one pot synthesis from a polydentate, multipocket aminobis(phenol)ligand [6,6'-{(2-(1-morpholyl)ethylazanediyl)bis(methylene)}bis(2-methoxy-4-methylphenol)], Mn(NO)·4HO, Ln(NO)· nHO, and NEt in MeOH. These compounds represent the first examples of fully structurally and magnetically characterized dinuclear MnLn complexes. Single X-ray diffraction studies reveal that all complexes are isostructural, consisting of neutral dinuclear molecules where the Mn and Ln metal ions, which exhibit distorted octahedral MnNO and distorted LnO coordination spheres, are linked by phenoxide/methoxide double bridging groups. Static magnetic studies show that the MnGd derivative exhibits a weak antiferromagnetic interaction between the metal ions, with a negative axial zero-field splitting D parameter. The MnGd complex shows a notable magnetocaloric effect with magnetic entropy change at 5 T and 3 K of -Δ S = 16.8 J kg K. Theoretical studies were performed to support the sign and magnitude of the magnetic anisotropy of the Mn ion ( ab initio), to predict the value and nature of J, to disclose the mechanism of magnetic coupling, and to establish magneto-structural correlations (DFT calculations). The results of these calculations are corroborated by quantum theory of atoms in molecule analysis (QTAIM). Finally, Mn-Dy and Mn-Er complexes show field-induced slow relaxation of the magnetization but without reaching a maximum above 2 K in the out-of-phase ac susceptibility. Ab initio calculations were also performed on Mn-Dy/Ho systems to unravel the origin behind the weak SMM characteristics of the molecules possessing two strongly anisotropic ions. The mechanism of magnetic relaxation was developed, revealing a large QTM/tunnel splitting at the single-ion level. Furthermore, the anisotropy axes of the Mn and Ln ions were calculated to be noncollinear, leading to reduction of the overall anisotropy in the molecules. Hence, the herein reported complexes demonstrate that a combination of two anisotropic metal ions does not guarantee SMM behavior.