Lanthanide Single-Molecule Magnets Woodruff, Daniel N; Winpenny, Richard E. P; Layfield, Richard A
Chemical reviews,
07/2013, Volume:
113, Issue:
7
Journal Article
Peer reviewed
Open access
Woodruff et al examine studies on lanthanide single-molecule magnets.
We report a monometallic dysprosium complex, Dy(OtBu)2(py)5BPh4 (5), that shows the largest effective energy barrier to magnetic relaxation of Ueff=1815(1) K. The massive magnetic anisotropy is due ...to bis‐trans‐disposed tert‐butoxide ligands with weak equatorial pyridine donors, approaching proposed schemes for high‐temperature single‐molecule magnets (SMMs). The blocking temperature, TB , is 14 K, defined by zero‐field‐cooled magnetization experiments, and is the largest for any monometallic complex and equal with the current record for Tb2N2{N(SiMe3)2}4(THF)2.
Record‐breaking: A monometallic dysprosium complex, Dy(OtBu)2(py)5BPh4, displaying near‐perfect pentagonal bipyramid geometry defined by two strong axial tert‐butoxide ligands and five weak equatorial pyridine donors is reported. This complex displays massive magnetic anisotropy, approaching the limit of a two‐coordinate complex, with an energy barrier to magnetic relaxation of Ueff=1815(1) K and a blocking temperature of TB=14 K.
The synthesis, structures, and magnetic properties of six families of cobalt–lanthanide mixed-metal phosphonate complexes are reported in this Article. These six families can be divided into two ...structural types: grids, where the metal centers lie in a single plane, and cages. The grids include 4 × 3 {Co8Ln4}, 3 × 3 {Co4Ln6}, and 2 × 2 {Co4Ln2} families and a 4 × 4 {Co8Ln8} family where the central 2 × 2 square is rotated with respect to the external square. The cages include {Co6Ln8} and {Co8Ln2} families. Magnetic studies have been performed for these compounds, and for each family, the maximum magnetocaloric effect (MCE) has been observed for the Ln = Gd derivative, with a smaller MCE for the compounds containing magnetically anisotropic 4f-ions. The resulting entropy changes of the gadolinium derivatives are (for 3 K and 7 T) 11.8 J kg–1 K–1 for {Co8Gd2}; 20.0 J kg–1 K–1 for {Co4Gd2}; 21.1 J kg–1 K–1 for {Co8Gd4}; 21.4 J kg–1 K–1 for {Co8Gd8}; 23.6 J kg–1 K–1 for {Co4Gd6}; and 28.6 J kg–1 K–1 for {Co6Gd8}, from which we can see these values are proportional to the percentage of the gadolinium in the core.
Recent studies have shown that mononuclear lanthanide (Ln) complexes can be high‐performing single‐molecule magnets (SMMs). Recently, there has been an influx of mononuclear Ln alkoxide and aryloxide ...SMMs, which have provided the necessary geometrical control to improve SMM properties and to allow the intricate relaxation dynamics of Ln SMMs to be studied in detail. Here non‐aqueous Ln alkoxide and aryloxide chemistry applied to the synthesis of low‐coordinate mononuclear Ln SMMs are reviewed. The focus is on mononuclear DyIII alkoxide and aryloxide SMMs with coordination numbers up to eight, covering synthesis, solid‐state structures and magnetic attributes. Brief overviews are also provided of mononuclear TbIII, HoIII, ErIII and YbIII alkoxide and aryloxide SMMs.
Mononuclear lanthanide single‐molecule magnets (SMMs) have progressively evolved in recent years. Lanthanide alkoxide and aryloxide chemistry offers a modular design approach to study the relaxation dynamics of high‐performing SMMs. Alkoxide and aryloxide ligands on a single axis at a DyIII ion enhances the magnetic anisotropy and the barrier to magnetic reversal (Ueff). Leading examples of DyIII alkoxide and aryloxide SMMs are discussed in this review.
Although the development of single‐molecule magnets (SMMs) is rapid, there are only two families of high energy barrier (Ueff) dysprosium(III) SMMs known so far: the cyclopentadienyl (Cp) family with ...a sandwich structure and the pentagonal‐bipyramidal (PB) family with D5h symmetry. These high‐barrier SMMs, which usually possess Ueff>500 cm−1 allow the separate study of the four magnetic relaxation paths, namely, direct, quantum tunnelling, Raman and Orbach processes, in detail. Whereas the first family is chemically more challenging to modify the Cp rings, it is shown herein that the latter family, with the common formulae DyX1X2(Leq)5+, such as X1/X2=−OCMe3, −OSiMe3, −OPh, Cl− or Br−; Leq=THF/pyridine/4‐methylpyridine, can be readily fine‐tuned with a range of axial and equatorial ligands by simple substitution reactions. This allows unambiguous confirmation that the Ueff mainly depends on the identity of X1 and X2, rather than on Leq. More importantly, the fitted parameters are barrier dependent. If X1 is an O donor and X2 is a halide, 500<Ueff<600 cm−1, log τ0avg (s)=−10.66, log Cavg (s−1 K−n)= −5.05, navg=4.1 and TH=9 K (in which τ0 is the pre‐exponential factor for the Orbach relaxation process, C and n are parameters used to describe Raman relaxation, and TH is the highest temperature at which magnetic hysteresis is observed). For cases in which both X1 and X2 are O donors, 900<Ueff<1300 cm−1, log τ0avg (s)=−11.63, log Cavg (s−1 K−n)= −6.03, navg=4.1 and 18<TH<25 K. Based on these results, it can be further concluded that Ueff not only has a linear correlation to the axial Dy−X bond lengths, but also to TH for these PB SMMs. This represents the first systematic study of a family of lanthanide SMMs and derives the first magneto‐structural correlation in Dy SMMs.
Pyramid scheme: An extended and systematic study of a family of lanthanide single‐molecule magnets (SMMs) with pentagonal‐bipyramidal geometry reveals the relaxation parameters and correlations. The energy barriers correlate with structure, mainly the electronegativity of donor atoms on the axial sites (the axial Dy−X bond lengths), and the energy barriers correlate with the blocking and hysteresis temperatures.
Single-molecule magnets are compounds that exhibit magnetic bistability caused by an energy barrier for the reversal of magnetization (relaxation). Lanthanide compounds are proving promising as ...single-molecule magnets: recent studies show that terbium phthalocyanine complexes possess large energy barriers, and dysprosium and terbium complexes bridged by an N2(3-) radical ligand exhibit magnetic hysteresis up to 13 K. Magnetic relaxation is typically controlled by single-ion factors rather than magnetic exchange (whether one or more 4f ions are present) and proceeds through thermal relaxation of the lowest excited states. Here we report polylanthanide alkoxide cage complexes, and their doped diamagnetic yttrium analogues, in which competing relaxation pathways are observed and relaxation through the first excited state can be quenched. This leads to energy barriers for relaxation of magnetization that exceed 800 K. We investigated the factors at the lanthanide sites that govern this behaviour.
Supramolecular chemistry has grown rapidly over the past three decades, yet synthetic supramolecular chemists still face several challenges when it comes to characterising their compounds. In this ...review, we present an introduction to structural characterisation techniques commonly used for non-crystalline supramolecular molecules,
nuclear magnetic and electron paramagnetic resonance spectroscopy (NMR and EPR), mass spectrometry (MS), ion mobility mass spectrometry (IM-MS), small-angle neutron and X-ray scattering (SANS and SAXS) as well as cryogenic transmission electron microscopy (cryo-TEM). We provide an overview of their fundamental concepts based on case studies from different fields of supramolecular chemistry,
interlocked structures, molecular self-assembly and host-guest chemistry, while focussing on particular strengths and weaknesses of the discussed methods. Additionally, three multi-technique case studies are examined in detail to illustrate the benefits of using complementary techniques simultaneously.
Quantum leap: Recent observations using pulsed EPR spectroscopy suggest that it is possible magnetic molecules such as the {V15} polyoxometalate cage (see picture; V red, As blue, O white) could be ...used in quantum information processing (QIP). Controlled interactions of cage complexes with an S=1/2 ground state, acting as qubits, could allow QIP.
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