Review
Strategies towards single molecule magnets based on lanthanide ions

https://doi.org/10.1016/j.ccr.2008.12.014Get rights and content

Abstract

We review here the synthetic strategies employed in a concerted effort to obtain new single molecule magnets based on lanthanide ions in the framework of the research program on Molecular Magnetism funded by the Deutsche Forschungsgemeinschaft. The reported systems are grouped in 4f–2p, 4f–3d, and pure 4f materials. While the use of compartmentalized ligands, assisted self assembly, and site-targeted reactions have provided interesting examples of high nuclearity clusters, mostly characterized by large magnetic moments in the ground state, a deeper magnetic characterization of systems with smaller nuclearity has allowed us to gain evidence regarding the role played by weak exchange interactions and geometrical factors on the slow dynamics of the magnetization. In the case of a triangular cluster based on dysprosium the novel phenomenon of spin chirality has been observed.

Introduction

Lanthanides play a special role in magnetism, thanks to their large magnetic moments and in most cases huge magnetic anisotropy [1]. When present in what is usually their most stable trivalent oxidation state, there is the drawback of only a very weak exchange interaction as a result of the efficient shielding of the unpaired electrons in the 4f orbitals. This has resulted in only limited interest in the quest to create 3D molecular-based magnets using 4f ions exclusively and has led to the exploration of systems combining the 4f ions with other paramagnetic species such as organic radicals or 3d ions.

In complete contrast to this, for the case of slow-relaxing molecular units, which have been termed as single molecule magnets or SMMs [2], the fact that the magnetic anisotropy is a major component to see this effect has meant that the use of lanthanides has become of increasing interest. The interest in SMMs based on 4f ions was boosted by the report of Ishikawa et al. that slow relaxation of the magnetization is observable also in the case of mononuclear complexes [3], like the double-decker compound based on phthalocyanine, schematized in Fig. 1.

In the case of mononuclear complexes the difference between single molecule magnet behavior and normal paramagnetic relaxation is rather subtle. In fact slow relaxation of lanthanide ions had been widely investigated through ac susceptibility since the early days of cryogenic investigations at Kammerlingh Omnes laboratory in Leiden [4], [5], [6]. The application of a static magnetic field was necessary to monitor spin-lattice relaxation because in zero magnetic field transitions between opposite spin projections do not require any energy exchange with the lattice and are therefore fast even at low temperature. The difference between a SMM and a normal paramagnet is in the probability of this direct transition being able to cancel any remnant magnetization, which is slow also in zero applied field because of the large S value. In the case of clusters comprising 3d metal ions the term that mainly affects the energy splitting of the spin multiplet, also known as zero field splitting, is given byH=DSz2while higher order terms are often orders of magnitude smaller. This gives rise to the well known double well energy potential and the reversal of the magnetization has a characteristic time that increases exponentially on decreasing the temperature, according to the Arrhenius law:τ=τ0expΔEkBTThe energy barrier to be overcome is of the order of DS2 and D(S2  1/4) for integer and half-integer spins, respectively. Under barrier processes are also important but not to the extent for the case of a simple paramagnet. In fact the states characterized by the largest projection, mS = ±S, are admixed only at the 2Sth level of perturbation by any local transverse field.

In the case of lanthanide double-decker compounds the situation is more complicated. In fact, terms of different orders have comparable amplitude in the crystal field Hamiltonian [7]. Slow relaxation of the magnetization in zero static field has, however, been observed using ac susceptibility measurements at relatively high temperature [3], [8], [9]. This is probably a fortunate consequence of the tetragonal symmetry of the double-decker molecules, which reduces the admixing of sublevels characterized by opposite projection of the total angular momentum J, where J = L + S. On cooling, deviations from the Arrhenius law predicted for SMM behavior become, however, more and more important and hysteresis is only observed at very low temperature.

It is therefore interesting to pursue a rational approach to creating lanthanide-based SMMs to investigate what role the magnetic interaction between lanthanide ions can play in such a complicated scenario. Two different approaches have merged in a collaborative effort within the framework of the research project financed by DFG on “Molekularer Magnetismus”. Small polynuclear complexes have been investigated in detail to elucidate the key role played by the magnetic interaction between lanthanide ions in addition to other geometrical factors. At the same time synthetic skills have been developed to increase the nuclearity of lanthanide-based clusters as well as to arrange them in extended structures and to explore new means of enhancing the SMM characteristics of 3d–4f systems. An overview of the main results achieved as an outcome of the complementarity of the two approaches and skills will be given in the following sections, without the aim of providing an exhaustive review of previous literature, covered by recent reviews [9], [10], [11], [12], [13].

Section snippets

Slow relaxation in 4f–2p systems

Among the several chemical approaches to molecular magnetic materials comprising rare earths a crucial role has been played by the combination of metal ions with paramagnetic ligands like nitronyl-nitroxide radicals [10], [13], [14]. Thanks to the strength of magnetic interaction promoted by these radicals, which have the unpaired spin density delocalized on the coordinating oxygen atoms, it has been possible to overcome the drawback in using weakly interacting trivalent lanthanides.

One of the

4f–3d systems

Practically at the beginning of research into molecular-based magnetism it was predicted that mixing 3d and 4f ions could lead to systems showing the sort of interesting magnetic behavior observed for bulk magnets combining metal ions from these areas of the Periodic Table. However, early forays proved disappointing since, on one hand, the interactions were rather weak and, on the other, the synthesis of such mixed-metal compounds was more complicated than envisaged. In fact, until relatively

Pure 4f polynuclear systems

We have already mentioned the exciting results obtained in phthalocyanine-based single ion SMM comprising Tb, Dy, and Ho. From a chemical point of view the oxidation of [M(Pc)2] to the neutral state has provided a further increase in the blocking temperature [8]. More recently it has been reported that polyoxometalates (POMs) are able to encapsulate lanthanides with coordination geometries similar to those of bis(phthalocyaninato)lanthanide complexes to give the polyanion [ErW10O36]9−.

Conclusions

While not intending to review exhaustively the results obtained in the very active and promising field of single molecule magnets based on lanthanide ions, we hope to have provided here an interesting example of concerted actions in the synthesis and characterization of lanthanide-based molecular nanomagnets.

Notwithstanding the unavoidable drawbacks related to the inherently weak magnetic exchange interactions, lanthanides present appealing features. Isotropic ions, like GdIII, can contribute

Acknowledgments

The financial support of the Deutsche Forschungsgemeinschaft thanks to the Schwerpunktptprogramm Molekularer Magnetismus (SPP1137) has been key for the development of this coordinated work. We mourn the premature passing away of Dr. Karlheinz Schmidt, whose passionate supervision of the program promoted the development and integration of the molecular magnetism community.

The results reviewed here have of course been obtained thanks to the outstanding contribution of other scientists, some of

References (87)

  • L.J.F. Broer et al.

    Physica

    (1943)
  • N. Ishikawa

    Polyhedron

    (2007)
  • D. Luneau et al.

    Coord. Chem. Rev.

    (2005)
  • C. Benelli et al.

    J. Magn. Magn. Mater.

    (1995)
  • F. Cinti et al.

    J. Magn. Magn. Mater.

    (2007)
  • F. Pointillart et al.

    Inorg. Chem. Commun.

    (2007)
  • F. Mori et al.

    Polyhedron

    (2005)
  • S. Ueki et al.

    Polyhedron

    (2007)
  • S. Ueki et al.

    Chem. Phys. Lett.

    (2007)
  • V. Mereacre et al.

    Polyhedron

    (2008)
  • M. Ferbinteanu et al.

    J. Am. Chem. Soc.

    (2006)
  • A. Kamiyama et al.

    Inorg. Chem.

    (2002)
  • A. Kamiyama et al.

    Cryst. Eng. Commun.

    (2003)
  • A.M. Madalan et al.

    Eur. J. Inorg. Chem.

    (2007)
  • A.M. Ako, V. Mereacre, R. Clérac, I.J. Hewitt, Y. Lan, G. Buth, C.E. Anson, A.K. Powell, Inorg. Chem., submitted for...
  • M.A. Aldamen et al.

    J. Am. Chem. Soc.

    (2008)
  • M.T. Gamer et al.

    Inorg. Chem.

    (2008)
  • J.P. Costes et al.

    Inorg. Chem.

    (2001)
  • A. Abragam et al.

    Electron Paramagnetic Resonance of Transition Ions

    (1986)
  • D. Gatteschi et al.

    Molecular Nanomagnets

    (2006)
  • N. Ishikawa et al.

    J. Am. Chem. Soc.

    (2003)
  • C.J. Gorter

    Paramagnetic Relaxation

    (1947)
  • A.H. Morrish

    The Physical Principles of Magnetism

    (1966)
  • N. Ishikawa et al.

    Inorg. Chem.

    (2003)
  • N. Ishikawa et al.

    Inorg. Chem.

    (2004)
  • J.C.G. Bunzli et al.

    Chem. Soc. Rev.

    (2005)
  • J.C.G. Bunzli et al.

    Chem. Rev.

    (2002)
  • C. Benelli et al.

    Chem. Rev.

    (2002)
  • X.H. Zhang et al.

    Prog. Chem.

    (2008)
  • A. Caneschi et al.

    Acc. Chem. Res.

    (1989)
  • C. Benelli et al.

    Inorg. Chem.

    (1989)
  • F. Cinti et al.

    Phys. Rev. Lett.

    (2008)
  • C. Benelli et al.

    J. Appl. Phys.

    (1993)
  • C. Benelli et al.

    Inorg. Chem.

    (1993)
  • C. Benelli et al.

    Adv. Mater.

    (1992)
  • K. Bernot et al.

    J. Am. Chem. Soc.

    (2006)
  • L. Bogani et al.

    Angew. Chem. Int. Ed.

    (2005)
  • A. Caneschi et al.

    Angew. Chem. Int. Ed.

    (2001)
  • R. Clerac et al.

    J. Am. Chem. Soc.

    (2002)
  • C. Coulon et al.

    Struct. Bond

    (2006)
  • L. Bogani et al.

    J. Mater. Chem.

    (2008)
  • G. Poneti et al.

    Chem. Commun.

    (2007)
  • C. Benelli et al.

    Inorg. Chem.

    (1992)
  • Cited by (0)

    View full text