Uncommon lanthanide ions in purely 4f Single Molecule Magnets

TL;DR: The use of lanthanide ions in the design of SMMs exploded with the discovery of the first example of mononuclear TbIII-based complex which displayed a slow magnetic relaxation in 2003 as discussed by the authors.

About: This article is published in Coordination Chemistry Reviews.The article was published on 2017-09-01 and is currently open access. It has received 229 citations till now. The article focuses on the topics: Lanthanide.

Summary

1. Introduction

• Single-Molecule Magnets (SMMs) were discovered more than twenty years ago on a manganese-based polynuclear complex, the so-called Mn12 [1-3].
• For d-block elements, the spin-orbit coupling is partially, or even totally, quenched by the ligand field which minimize or suppress the magnetic anisotropy.
• The authors set aside the Dy III , Tb III and Er III based SMMs and only cover the work performed on SMMs with the less common tripositive lanthanide ions: Ce, Nd, Ho, Tm and finally, Yb.
• This ion is certainly the most popular amongst the minority group.
• A specific attention will be devoted to quantitative analyses of the magnetic data to elucidate the relaxation processes involved as well as the mapping of the energy diagram in the ground-state multiplet.

1. Mononuclear Ytterbium SMMs

• This section describes firstly mononuclear YbIII-based SMMs in which the lanthanide ion is surrounded solely by oxygen atoms.
• (2) compound , the YbIII ion adopts a distorted octahedral geometry which up to now remains the lowest coordination number for a purely Yb III -based SMM.
• 3+. Anions, solvent molecules of crystallization and hydrogen atoms have been omitted for clarity.
• This energy barrier is much lower than the value determined from ab-initio The LF interactions were evaluated using the Stevens method leading to the conclusion that the ground-state is mainly constituted by the doublet MJ=±5/2.

2. Polynuclear Ytterbium SMMs

• Nevertheless, in the last years, several examples of dinuclear complexes as well as mono-dimensional species which display SMM behaviour have been published.
• Attempts to reproduce the static magnetic properties were done using an extended Stevens operators technique taking into account both crystal field effects and possible ferromagnetic interactions ).
• The stabilisation of such high MJ state led to the observation of a SMM behaviour without applied magnetic field.

3. SMMs built from other uncommon lanthanide ions

• The CeIII ion possesses only one single electron in its 4f shell.
• 18 was however not the first example of Nd III -based SMM since the complex 20 of formula Nd(L16)3 (L 16=trispyrazolylborate) was reported a couple of years before by Rinehart et al. [76].
• The nature of the doublet ground-state for 27 allowed the possibility to observe an SMM behaviour at low temperature.

4. Heterobinuclear Zinc-Lanthanide SMMs: Borderline cases of

• B Effective energy barrier determined from luminescence measurements.
• Interestingly, both compounds have the highest MJ value (MJ=±7/2) as ground-state with the first excitedstate very close in energy (<1 cm-1).
• For these present cases, the magnetic relaxation takes place through a combination of QTM and Raman processes .

5. Heterobimetallic 4f4f’ Lanthanide SMMs

• C Effective energy barrier determined from dc measurements.
• The ground state MJ values were not determined in the corresponding article.
• The X-ray structure of the complex [Yb2(L 26 )2Cl4(H2O)(MeCN)]⋅CH3CN (37) features two Yb III ions in a seven-coordination environment due to the four L 26 ligands (two protonated and two deprotonated), two chloride anions and a disordered MeCN/H2O molecules .
• The Dy III analogue (41) behaves also as a SMM with ∆=38 cm-1 with a magnetic relaxation occurring through Orbach process.

6. Conclusion and perspectives

• Since 2003, both chemist and physicist communities worked hand in hand to design lanthanide-based SMMs, understand the mechanisms at the origin of their magnetic properties, and potentially drive the design of new target assemblies.
• The ligands that have been employed up-to-now for their elaboration are of various natures with the presence of carboxylate, hydroxo, oxo, pyridine coordinating groups, macrocyclic ligands etc… illustrating the diversity in the chemical approaches and offering large perspective that may only be limited by chemists imagination.
• Nevertheless this review 46 highlights an efficient synthetic approach, i.e. organometallic synthesis, for all the lanthanide ions which allowed an efficient control of the crystal field around the lanthanide centre.
• On the magnetic point of view, the mechanisms at play in the slow magnetic relaxation seems to be more complicated than for the TbIII and DyIII SMMs since the Orbach process is not the main pathway of relaxation and should be discarded to the advantage of other processes such as Direct, Raman and Quantum pathways.
• This is induced by the mixing of the MI states due to the intense hyperfine coupling and/or dipoledipole interactions.

Figures

Figure 3. (a) Molecular structure of [Yb(tta)3(L 2)] with experimental (orange) and theoretical (green) anisotropy axis. (b) The solid-state emission spectrum is given with an appropriate shift of energy scale. Correlation between the main contributions of the emission spectrum (black sticks) and the energy splitting of the 2 F7/2 multiplet ground-state obtained by MSCASPT2/RASSI-SO calculations (red sticks). Adapted from ref [46].

Table 4. Lanthanide SMMs (Ln=CeIII, NdIII, HoIII and TmIII).

Table 5. Heterobinuclear Zinc-Lanthanide SMMs (Ln=Ce III and Yb III )

Figure 10. Molecular structure of [Yb2(L 10)2(acac)2(H2O)]. Dichloromethane molecules of crystallization and hydrogen atoms are omitted for clarity. Adapted from ref [64].

Figure 25. (a) Molecular structures of ([Zn(L 24 )Yb(H2O)(CO3)]2)2 + and 35. Hydrogen atoms, anions and solvent molecules of crystallisation have been omitted for clarity. (b) Frequency dependences of the magnetic susceptibilities in the 2-5.5 K temperature range and Arrhenius plots (red line, blue line and black line are representative of QTM+Orbach, QTM+Raman and Orbach respectively). Adapted from ref [131].

Figure 19. (a) Molecular structure of [Ho(W5O18)2] 9- . Sodium cations and water molecule of crystallisation are omitted for clarity. (b) Temperature dependent high-frequency EPR spectrum for the Na9[Ho0.25Y0.75(W5O18)2] sample between 2.2 and 10 K. (c) From up to down: Experimental (blue line) and simulated (red line) X-band EPR spectra using the 9.64 GHz perpendicular excitation mode for Na9[Ho0.1Y0.9(W5O18)2], parallel excitation mode for Na9[Ho0.1Y0.9(W5O18)2], and perpendicular excitation mode for 26. Adapted from ref [101].

Figure 28. (a) Molecular structure of [Yb(L 27 )2] - . Hydrogen atoms and cation are omitted for clarity. (b) Arrhenius plots for complexes 40 and 42 under an applied field of 1800 Oe. Adapted from ref [135].

Figure 15. (a) Molecular structure of 20. Hydrogen atoms are omitted for clarity. (b) Arrhenius plots for different doping values of 20 in a diamagnetic matrix of the La III analogue. Dashed line is given for a pure Orbach process. Green lozenges, purple triangles, blue squares and red full circles are given for a doping of 3.8%, 15%, 59% and pure Neodymium compound. Adapted from ref [76].

Figure 23. (a) Molecular structures of [YbZn4(L 22)4(py)4(DMF)4] 3+ (left) and [YbZn4(L 22 )4(iqn)4(DMF)4] 3+ (right). Hydrogen atoms, triflate anions and solvent molecules of crystallisation have been omitted for clarity. (b) Correlations of the different set of data for the energy levels of the 2 F7/2 ground state multiplet obtained from dc fit (i) ac fit (ii) and luminescence spectra (iii) for 31 (left and 32 (right). Adapted from ref [124].

Figure 21. (a) Molecular structure of 28. Hydrogen atoms are omitted for clarity. (b) Temperature dependence of the out-of-phase component of the magnetic susceptibility in an applied field of 5.5 kOe. In inset, Temperature dependence of the in-phase component of the magnetic susceptibility in an applied field of 5.5 kOe. The applied frequencies ranged from 20 to 1200 Hz. Adapted from ref [103].

Figure 18. (a) Molecular structure of [Ho(L 20 )2] - . Hydrogen and TBA + cation are omitted for clarity. (b) Field-dependence of the magnetization at different temperature and field scan rate for 2% of 24 in a diamagnetic matrix of TBA[Y(L 20 )2]. Adapted from ref [96].

Figure 4. Molecular structure of [Yb(L 3 )3] 3- . The tetraethylamonium cations and hydrogen atoms are omitted for clarity. Adapted from ref [50].

Table 6. Heterobinuclear Lanthanide SMMs (Ln=DyIII and YbIII) DyYb-SMM ∆/cm-1 τ0/s H/Oe MJ ground statee ref [Yb2(L 26 )2Cl4(H2O)(MeCN)]⋅CH3CN (37) [Dy0.87Yb1.13(L 26 )2Cl4(H2O)(MeCN)]⋅CH3CN (39)

Figure 24. (a) Molecular structure of [Zn3Yb(L 23 )(NO3)3]. Hydrogen atoms and methanol molecules of crystallisation have been omitted for clarity. (b) Frequency dependence of the magnetic susceptibility between 1.8 and 5.5 K. Adapted from ref [129].

Figure 17. Molecular structures of {[Nd(L 18 )3(H2O)2]}n (a) and 23 (b). Hydrogen atoms and solvent molecules of crystallisation have been omitted for clarity. Adapted from ref [94].

Table 1. Term symbols for selected lanthanide ions.

Frequently asked questions

Q1. What contributions have the authors mentioned in the paper "Uncommon lanthanide ions in purely 4f single molecule magnets" ?

This review presents the research endeavour in the area of these uncommon lanthanide-based SMMs and underlines the different approaches to better understand their physical properties. 

Q2. Why was the possibility to observe slow magnetic relaxation discarded?

The possibility to observe slow magnetic relaxation for the Ce III ion in eight-coordination was discarded due to the low symmetry of the coordination sphere and the isotropic nature of the ligands composing the coordination sphere. 

Q3. What is the reason for the rarity of Ho III -based SMMs?

One can take in his mind that the rarity of Ho III -based SMM can be due to the non-Kramers nature of such lanthanide, which places constraints on the symmetry of the coordination environment needed for SMM behaviour, and to the fact the 4f electron density is not very anisotropic. 

Q4. What is the reason for the absence of SMM behaviour without external field?

The absence of SMM behaviour without external field wasattributed to the significant transversal g factors and the large tunnelling gap in the ground exchange doublet (0.01 cm -1 ) allowing a strong quantum tunnelling of the magnetization. 

Q5. How long is the energy barrier for Yb-based SMM?

Above 3K, the extracted energy barrier of 19.5 K is one of the highest for Yb-based SMM while the relaxation time of 1.0×10-7 s is classical for such systems. 

Q6. What is the molecule that stabilizes a S=10 spin ground-state?

In this molecule the antiferromagnetic interactions between spins of Mn III and Mn IV ions stabilize a S=10 spin ground-state which can be trapped in up (Ms = +S) or down (Ms = -S) orientations. 

Q7. What was the method used to reproduce the static magnetic properties?

Attempts to reproduce the static magnetic properties were done using an extendedStevens operators technique taking into account both crystal field effects and possible ferromagnetic interactions (because of the increase of χMT vs. T curve below 9 K (Figure 9b)). 

Q8. How many Oe field is required to determine the magnetic susceptibility?

As for almost all YbIII-based SMMs, no out-of-phase signal of the magneticsusceptibility is observed without applied magnetic field, whereas an optimal field of 1000 

Q9. What is the first unusual lanthanide ion involved in a SMM?

The lanthanide mononuclear SMM started in 2003 with the association of a Dy III ionand double Decker ligands [95], while the first uncommon lanthanide ion involved in a SMM was HoIII in 2005 [96]. 

Q10. What is the equatorial ligand used for the lanthanide ?

Gao et al. associated an equatorial ligand such as COT ligand (L 21 , Scheme 3) in order to satisfy the Ising limit condition for a lanthanide ion with a prolate electron density. 

Q11. What other strategies were already successful in obtaining SMM with lanthanide ions?

two other strategies to obtain SMM with this kind of lanthanide ions were already successful: i) the insertion of diamagnetic ions such as Zn II which add an electrostatic effect on the active centre and ii) the doping of the active SMM with a second 4f element. 

Q12. How is the magnetic susceptibility of CeIII ion observed without a magnetic field?

As for 17, no out-of-phase signal of the magnetic susceptibility is observed without magnetic field while a weak field leads to its appearance (200 Oe). 

Q13. What is the energy gap between the ground and first excited states?

The energy gap between the ground- and first excited-states was estimated to 130 cm-1 for 4 which is one order of magnitude higher than the energy gap found for the Er III and Dy III analogues. 

Q14. What is the potential for the elaboration of SMM?

the NdIII ion which possesses a 4f3 electronic configuration could be also a potential candidate for the elaboration of SMM. 

Q15. What is the nature of the first neighbouring atoms?

In other words, the nature of the first neighbouring atoms (oxygen vs. nitrogen) induces two different distortion of the D4d coordination polyhedron (compressed vs. elongated).