Lanthanide and transition metal complexes as molecular magnets

Molecular magnetism has been a topic of considerable interest to chemists and physicists alike for a long time.¹ The discovery that [Mn₁₂O₁₂(CH₃COO)₁₆(H₂O)₄] showed slow relaxation of magnetization and magnetic hysteresis at low temperatures² spawned the interdisciplinary field of molecular magnets involving coordination complexes of transition metal and lanthanide metal ions.^3–10 These systems which were called single molecule magnets (SMMs), once magnetized, show an effective energy barrier to magnetization reversal. Currently many types of coordination complexes are being investigated. These include (a) polynuclear transition metal complexes (b) heterometallic 3d(4d)/4f complexes (c) lanthanide complexes. Apart from complexes containing multiple metal ions, in recent years there has also been a realization that control of magnetic anisotropy is better accomplished in mononuclear complexes. This has resulted in the development of mononuclear single-molecule magnets (MSMMs) or single ion magnets (SIMs). The many facets of research in this topic have been embraced by synthetic coordination chemists, theoretical chemists, and those with expertise in magnetism. While many issues have been understood many more remain to be solved. These include the control of spin, magnetic anisotropy and the relaxation mechanisms that undercut the energy barrier to magnetisation reversal. Finally, although many potential applications of these exotic molecules have been proposed, ranging from qubits to molecular refrigerants, realizing these needs a lot of effort to solve many practical challenges. Meanwhile research in this area continues to flourish and it is in this context that this virtual themed collection on Lanthanide and transition metal complexes as molecular magnets has been brought about. In this collection we have 28 articles including perspectives that deal with this rapidly evolving subject. It is gratifying that authors from 22 countries contributed to this virtual collection: Australia, Austria, Belgium, China, Czech Republic, Egypt, France, Germany, India, Iran, Ireland, Italy, Japan, Poland, Portugal, Romania, Russia, Slovakia, Spain, Taiwan, UK and USA. The following is the gist of the various contributions to this virtual collection.

Aromi et al. present a perspective that discusses the use of dissymmetric organic ligands for the assembly of heterometallic lanthanide complexes which can be used as logical quantum gates in information processing (doi.org/10.1039/D1DT01862K).

Yamashita et al. has reviewed multi-functional molecular materials. The focus of this review is on SMMs that also have conductivity properties. The potential applications of such materials in electronic devices are presented (doi.org/10.1039/C8DT01015C).

Cornia et al. reported chloride/bromide capped linear Fe(II) complexes which exhibit SMM behavior. It has been found from theoretical work that the main magnetic axis of the four iron centers is close to the molecular axis containing them (doi.org/10.1039/D1DT01007G).

Garcia et al. reported spin cross-over behavior in a series of iron complexes containing a mixed ligand system. The length of the alkyl chains on the bipyridyl ligands have been shown to influence the spin-crossover behavior (doi.org/10.1039/D1DT01787J).

Trávníček et al. investigated pentacoordinated Co(II) complexes in a 5 N coordination environment containing distorted square-pyramidal/trigonal bipyramidal geometries. Both complexes revealed a negative D and revealed a field-dependent slow relaxation behavior (doi.org/10.1039/D0DT02338H).

Konar et al. trapped [Co(H₂O)₆]²⁺ in a decavanadate matrix and studied its magnetic behavior. This revealed a field-dependent slow relaxation (doi.org/10.1039/D0DT04339G).

Song et al. examined two [Co(Imidazole)₆]²⁺ complexes that crystallized in two different space groups viz., C_i and D_3d. While both complexes were found to be field-induced SIMs, the complex with the higher symmetry revealed a lower energy barrier to magnetisation reversal (doi.org/10.1039/C7DT04651K).

Lescouëzec et al. reported the use of bis(1-methylimidazol-2-yl)ketone ligand for the preparation of mononuclear Co(II) and 1D mixed valent Co(II)/Co(III) complexes both of which revealed field-induced SIM behavior (doi.org/10.1039/D1DT02441H).

Chen et al. prepared an azide-linked Co(II) coordination polymer which revealed field-induced slow relaxation behavior (doi.org/10.1039/C8DT02335B).

Kiskin et al. reported molecular and 1D polymers containing a butterfly-shaped heterometallic tetranuclear Cu₂Ln₂ motif. The Dy(III) analogues and the Yb(III) analogue of the 1D polymeric system were shown to be field-induced SMMs (doi.org/10.1039/D1DT01161H).

In an interesting study, Pointillart et al. examined the role of ¹⁶²Dy and ¹⁶³Dy isotopes on slow relaxation in a heterometallic Zn₂Dy complex. The former nuclear spin-free complex was found to exhibit a slower magnetic relaxation than the latter. This has been attributed to the suppression of hyperfine interactions emanating from the metal centre in the spin-free complex (doi.org/10.1039/D1DT01608C).

Visinescu et al. reported cyanide bridged Fe(III)₂Ln(III)₂ complexes in a square-shaped geometry. Weak intra- and intermolecular anti-ferromagnetic interactions were observed. These were shown to exhibit SMM behavior (doi.org/10.1039/D1DT02512K).

Heterometallic Ln(III)/V(IV) complexes were studied by Herchel et al.. The Dy(III) and the Tb(III) analogues were found to be field-induced SMMs (doi.org/10.1039/D1DT01944A).

Multifunctional molecular magnets possessing photoluminescence behavior were studied by Ohkoshi et al.. Z-shaped Dy₂Pt₃ complexes possessing cyanide bridging ligands were prepared. In one of these complexes where Dy(III) is eight coordinate, zero-field SMM behavior was observed (doi.org/10.1039/D1DT03071J).

Murray et al. studied 4d/4f Ru₂Ln₂ complexes possessing a butterfly topology. Local structural modification around the Dy(III) centres has been shown to be important in modulating the magnetic properties (doi.org/10.1039/D1DT01770E).

Peng et al. reported a heptanuclear Ni₃Ru₂Ni₂complex. This contains a mixed-valent [Ru₂]⁵⁺ motif and showed magnetic interactions between the metal centres leading to a high magnetic moment (doi.org/10.1039/D0DT00156B).

Yamashita et al. investigated sandwich complexes of Tb(III) containing phthalocyaninato ligands. Both cationic and anionic forms were investigated. The cationic form showed longer relaxation time and a higher energy barrier for magnetisation reversal (doi.org/10.1039/D1DT00775K).

Utilizing guanidine based ligands, Tang et al. reported Dy(III) complexes. It was found that in three of these complexes where Dy(III) had a coordination number of 8, field-induced SIM behavior was observed (doi.org/10.1039/D1DT00260K).

Chen et al. reported two eight coordinated dinuclear Dy(III) complexes in a 2N,O6 coordination environment. In one of these complexes the Dy(III) had a trigonal dodecahedron geometry while in the other a square antiprismatic configuration was found. Both these complexes were field-induced SMMs (doi.org/10.1039/C8DT02361A).

Meyer et al. utilized pyrazolate based macrocyclic ligands and prepared dinuclear Dy(III) complexes. The Dy(III) centers were found to be in a pentagonal bipyramidal geometry in a D_5h symmetry. These were shown to be field-induced SMMs (doi.org/10.1039/D1DT02815D).

Shanmugam et al. reported six-coordinated dinuclear Ln(III) complexes using a bulky 2,6-diisopropyl-N-(trimethylsilyl)anilide ligand. The Dy(III) analogue was shown to be a zero-field SMM. This has been attributed to the high charge density of the amide ligand (doi.org/10.1039/D1DT03708K).

3-Methoxysalicylhydrazone-based ligands were used by Liu et al. to construct chiral trinuclear (La(III)) and pentanuclear (Dy(III)) complexes. An interesting aspect was the transfer of chirality from the ligand to the complexes and a metal ion radii-dependent modulation of the nuclearity (doi.org/10.1039/D0DT01711F).

Cui et al. used 8-hydroxy quinoline based-Schiff base ligands to prepare tetranuclear Ln(III) assemblies. While the Gd(III) derivative showed a magnetocaloric effect, the Dy(III) analogue was a field-dependent SMM (doi.org/10.1039/C8DT00063H).

Mirzaei et al. reported the use of polyoxometalates for preparing 2D cationic coordination polymers containing tetrameric Ho(III) or Tb(III) motifs. Both complexes were shown to be field-induced SMMs (doi.org/10.1039/D1DT01708J).

3D coordination networks containing Ln(III) were assembled by Wang et al. using a nicotinic acid-based multifunctional ligand. While the Dy(III) analogue revealed field-induced SMM behavior, these frameworks also showed luminescence-based detection of Fe(III) (doi.org/10.1039/C8DT01034J).

Walisinghe and Chilton calculated the Slater–Condon parameters and the spin–orbit (SO) coupling constants for various oxidation states of transition metal ions (3d/4d/5d) and trivalent f-block ions using theoretical methods (doi.org/10.1039/D1DT02346B).

Rajaraman et al. used theoretical studies to answer the question of whether a direct 3d metal ion-lanthanide ion bond could be beneficial for creating SMM behavior. Although the axial anisotropy is hindered in such systems, strong ferromagnetic exchange interaction between the lanthanide and transition metal ions is believed to yield high energy barriers for magnetisation reversal (doi.org/10.1039/D1DT02256C).

Briganti and Totti in a theoretical study, examined the influence of pressure on controlling the anisotropy tensor on a model Dy(III) complex containing an axial water molecule (https://doi.org/10.1039/D1DT01719E).

This virtual collection represents a cross-section of the wide range of current research in this topic and hopefully would be of interest not only to practitioners of this field but also to a more general readership.

References

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4. J. Tang and P. Zheng, Lanthanide Single Molecule Magnets, Springer-Verlag, Berlin, 2015.
5. Lanthanides and Actinides in Molecular Magnetism, ed. R. A. Layfield and M. Murugesu, Wiley-VCH, Weinheim, 2015.
6. Organometallic Magnets, ed. V. Chandrasekhar and F. Pointillart, Springer-Verlag, Heidelberg, 2019.
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10. A. Zabala-Lekuona, J. M. Seco and E. Colacio, Coord. Chem. Rev., 2021, 441, 213984.

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