Muralee Murugesu

A Polynuclear Lanthanide Single‐Molecule Magnet with a Record Anisotropic Barrier

Single-molecule magnets (SMMs) continue to be an attractive research field because of their unique and intriguing properties and potential applications in high-density data storage technologies and molecular spintronics. The anisotropic barrier (U) of an SMM is derived from a combination of an appreciable spin ground state (S) and uniaxial Ising-like magneto-anisotropy (D). The magnet-like behavior can be observed by slow relaxation of the magnetization below the blocking temperature. Since the discovery of SMMs in the early 1990s, this assumption has formed the basis for the understanding of the origin of the anisotropic barrier. However, in recent years the development of novel lanthanide-only SMMs that challenge and defy this theory pose a number of questions: How can slow relaxation of the magnetization be observed in a nonmagnetic state complex? Why are large energy barriers seen for mononuclear lanthanide(III) complexes? To answer such important questions, it is vital to investigate novel SMMs with high magnetoanisotropy for which the influence of the large negative D value could result in higher anisotropic barriers. Clearly lanthanide-based polynuclear systems are an important avenue to explore in the pursuit of SMMs with higher anisotropic barriers, because of the strong spin–orbit coupling commonly observed in 4f systems. However, lanthanide-only SMMs are rare. The majority of reported SMMs have been prepared with transition-metal ions, although the recent application of a mixed transition-metal/ lanthanide strategy also yielded many structurally and magnetically interesting systems. The scarcity of lanthanide-only SMMs results from the difficulty in promoting magnetic interactions between the lanthanide ions. The interactions can, however, be enhanced by overlapping bridging ligand orbitals. In addition, fast quantum tunneling of the magnetization (QTM), which is common for lanthanide systems, generally prevents the isolation of SMMs with high anisotropic energy barriers. Our recent work suggests that dysprosium(III) ions may hold the key to obtaining high-blocking-temperature lanthanide-only SMMs. When an appropriate ligand system is employed, it is possible to exploit the large intrinsic magnetoanisotropy, high spin, and reduced QTM that dysprosium(III) ions offer. Recently, we have focused our attention towards the synthesis of dysprosium(III) cluster complexes with 1,2bis(2-hydroxy-3-methoxybenzylidene) hydrazone (H2bmh) and 3-methoxysalicylaldehyde hydrazone (Hmsh) as chelating agents (see Figure S1 in the Supporting Information). This strategy has proven to be successful and has led to a polynuclear lanthanide SMM with a record anisotropic barrier. Herein, we report the synthesis, structure, and magnetism of a tetranuclear dysprosium(III) SMM that exhibits the largest relaxation barrier seen for any polynuclear SMM to date. A suspension of DyCl3·6H2O and o-vanillin (2:1 ratio) in DMF/CH2Cl2 (1:5 ratio) was treated with 4 equivalents of Et3N. The solution was stirred for 1 minute, and then 4 equivalents of N2H4·H2O was added. The resulting yellow solution yielded rectangular, orange-yellow crystals of the tetranuclear complex [Dy4(m3-OH)2(bmh)2(msh)4Cl2] (1) in 19.1% yield after 2 days. The msh and bmh ligands were formed in situ by the reaction of o-vanillin and hydrazine. The slight excess of hydrazine is essential for the formation of both ligands; when an excess of o-vanillin was used instead, no product was isolated. The basic conditions promote the deprotonation of the ligands and the formation of bridging hydroxide anions. Single-crystal X-ray analysis revealed the centrosymmetric complex 1 (Figure 1), which has a defect-dicubane central core. The four coplanar Dy ions are bridged by two m3-OH ligands displaced above and below (0.922 ) the Dy4 plane with Dy O bond lengths of 2.362(6), 2.302(6), and 2.447(6) andDy O Dy angles of 106.5(2), 107.7(2), and 105.7(2)8, and also by a combination of four phenoxide oxygen atoms [Dy O 2.312(2), 2.298(6), 2.448(6), 2.345(6) ] and two diaza bridging groups [Dy N 2.508(8), 2.564(8) ]. Close inspection of the packing arrangement reveals stacking of the [*] P.-H. Lin, Dr. T. J. Burchell, Dr. M. Murugesu Chemistry Department, University of Ottawa and Centre for Catalysis Research and Innovation D’Iorio Hall, 10 Marie Curie, Ottawa, ON, K1N6N5 (Canada) Fax: (+1)613-562-5170 E-mail: m.murugesu@uottawa.ca Homepage: http://www.science.uottawa.ca/~mmuruges/

Dinuclear Dysprosium(III) Single-Molecule Magnets with a Large Anisotropic Barrier†

Due to the large intrinsic magnetic anisotropy of the lanthanide ions, rare-earth metal systems, and in particular dysprosium (Dy) based materials, have sparked increasing interest in the area of molecular magnetism. In a molecular complex, when such a unique property is combined with a high-spin ground state (S), slow relaxation of the magnetization can be obtained as seen for single-molecule magnets (SMMs). Although, a number of mixed transition-metal/ lanthanide SMMs have been reported, pure lanthanide SMMs are relatively scarce. The latter molecules are rare owing to the difficulty in promoting magnetic interactions in these systems. These interactions are attained by the overlap of bridging ligand orbitals with the 4f orbitals of the lanthanide ions. Thus, ligand design is one of the key components for achieving such interactions in pure lanthanide-based systems. To induce significant magnetic interaction between the lanthanide ions and synthesize high-energy-barrier SMMs, we have been investigating the use of (2-hydroxy-3-methoxyphenyl)methylene (isonicotino)hydrazine (H2hmi) as a rigid chelate in lanthanide chemistry. Such a linear ligand provides O,N,O,O-based multichelating sites that are especially favorable for lanthanide ion complex formation. They can form dinuclear systems using the bridging phenoxide oxygen atom, and the pyridine group promotes the formation of extended networks that can control the organization of the SMM units in the three-dimensional structure. Herein we report the use of the H2hmi ligand to design materials based on ferromagnetically coupled dinuclear dysprosium(III) SMMs with large relaxation barriers. [Dy2(hmi)2(NO3)2(MeOH)2] (1) and [Dy2(hmi)2(NO3)2 (MeOH)2]1·MeCN (2·MeCN) were obtained from a suspension of Dy(NO3)3·5H2O / H2hmi in methanol (treated with triethylamine) and in a 3:1 mixture of acetonitrile and methanol (treated with pyridine), respectively. After two days, pale orange single crystals were obtained, which were kept in contact with the mother liquor to prevent deterioration. Complexes 1 (Figure 1) and 2 (Figure 2) crystallize in monoclinic P21/c and orthorhombic Pbca space groups, respectively. Both complexes have similar dinuclear

Single-Molecule Magnet Behavior for an Antiferromagnetically Superexchange-Coupled Dinuclear Dysprosium(III) Complex

A family of five dinuclear lanthanide complexes has been synthesized with general formula [Ln(III)(2)(valdien)(2)(NO(3))(2)] where (H(2)valdien = N1,N3-bis(3-methoxysalicylidene)diethylenetriamine) and Ln(III) = Eu(III)1, Gd(III)2, Tb(III)3, Dy(III)4, and Ho(III)5. The magnetic investigations reveal that 4 exhibits single-molecule magnet (SMM) behavior with an anisotropic barrier U(eff) = 76 K. The step-like features in the hysteresis loops observed for 4 reveal an antiferromagnetic exchange coupling between the two dysprosium ions. Ab initio calculations confirm the weak antiferromagnetic interaction with an exchange constant J(Dy-Dy) = -0.21 cm(-1). The observed steps in the hysteresis loops correspond to a weakly coupled system similar to exchange-biased SMMs. The Dy(2) complex is an ideal candidate for the elucidation of slow relaxation of the magnetization mechanism seen in lanthanide systems.

Lessons learned from dinuclear lanthanide nano-magnets

The quest for higher density information storage has led to the investigation of Single-Molecule Magnets (SMMs) as potential molecules to be applied in materials such as hard discs. In order for this to occur, one must first design metal complexes which can retain magnetic information at temperatures where these applications become possible. This can only be achieved through answering and understanding fundamental questions regarding the observed physical properties of SMMs. While mononuclear lanthanide complexes have shown promise in obtaining high energy barriers for the reversal of the magnetisation they are limited to Single-Ion Magnet behaviour intrinsic to one metal centre with a limited number of unpaired electrons. As a way of increasing the effective anisotropic barrier, systems with higher nuclearity have been sought to increase the spin ground state of the molecule. Dinuclear complexes are presented as key compounds in studying and understanding the nature of magnetic interactions between metal ions. This tutorial review will span a number of dinuclear 4f complexes which have been critical in our understanding of the way in which lanthanide centres in a complex interact magnetically. It will examine key bridging moieties from the more common oxygen-based groups to newly discovered radical-based bridges and draw conclusions regarding the most effective superexchange pathways allowing the most efficient intracomplex interactions.

The use of magnetic dilution to elucidate the slow magnetic relaxation effects of a Dy2 single-molecule magnet.

The magnetic dilution method was employed in order to elucidate the origin of the slow relaxation of the magnetization in a Dy(2) single-molecule magnet (SMM). The doping effect was studied using SQUID and micro-SQUID measurements on a Dy(2) SMM diluted in a diamagnetic Y(2) matrix. The quantum tunneling of the magnetization that can occur was suppressed by applying optimum dc fields. The dominant single-ion relaxation was found to be entangled with the neighboring Dy(III) ion relaxation within the molecule, greatly influencing the quantum tunneling of the magnetization in this complex.

Fine-tuning the Local Symmetry to Attain Record Blocking Temperature and Magnetic Remanence in a Single-Ion Magnet**

Remanence and coercivity are the basic characteristics of permanent magnets. They are also tightly correlated with the existence of long relaxation times of magnetization in a number of molecular complexes, called accordingly single-molecule magnets (SMMs). Up to now, hysteresis loops with large coercive fields have only been observed in polynuclear metal complexes and metal-radical SMMs. On the contrary, mononuclear complexes, called single-ion magnets (SIM), have shown hysteresis loops of butterfly/phonon bottleneck type, with negligible coercivity, and therefore with much shorter relaxation times of magnetization. A mononuclear Er(III) complex is presented with hysteresis loops having large coercive fields, achieving 7000 Oe at T=1.8 K and field variation as slow as 1 h for the entire cycle. The coercivity persists up to about 5 K, while the hysteresis loops persist to 12 K. Our finding shows that SIMs can be as efficient as polynuclear SMMs, thus opening new perspectives for their applications.

Single-Molecule Magnet Behavior with a Single Metal Center Enhanced through Peripheral Ligand Modifications

Bis(imino)pyridine pincer ligands in conjunction with two isothiocyanate ligands have been used to prepare two mononuclear Co(II) complexes. Both complexes have a distorted square-pyramidal geometry with the Co(II) centers lying above the basal plane. This leads to significant spin-orbit coupling for the d(7) Co(II) ions and consequently to slow relaxation of the magnetization that is characteristic of Single-Molecule Magnet (SMM) behavior.

An Organometallic Sandwich Lanthanide Single-Ion Magnet with an Unusual Multiple Relaxation Mechanism

A dysprosium(III) sandwich complex, [Dy(III)(COT″)(2)Li(THF)(DME)], was synthesized using 1,4-bis(trimethylsilyl)cyclooctatetraenyl dianion (COT″). The complex behaves as a single-ion magnet and demonstrates unusual multiple relaxation modes. The observed relaxation pathways strongly depend on the applied static dc fields.

Significant Enhancement of Energy Barriers in Dinuclear Dysprosium Single-Molecule Magnets Through Electron-Withdrawing Effects

The effect of electron-withdrawing ligands on the energy barriers of Single-Molecule Magnets (SMMs) is investigated. By introducing highly electron-withdrawing atoms on targeted ligands, the energy barrier was significantly enhanced. The structural and magnetic properties of five novel SMMs based on a dinuclear {Dy2} phenoxo-bridged motif are explored and compared with a previously studied {Dy2} SMM (1). All complexes share the formula [Dy2(valdien)2(L)2]·solvent, where H2valdien = N1,N3-bis(3-methoxysalicylidene) diethylenetriamine, the terminal ligand L = NO3(-) (1), CH3COO(-) (2), ClCH2COO(-) (3), Cl2CHCOO(-) (4), CH3COCHCOCH3(-) (5), CF3COCHCOCF3(-) (6), and solvent = 0.5 MeOH (4), 2 CH2Cl2 (5). Systematic increase of the barrier was observed for all complexes with the most drastic increase seen in 6 when the acac ligand of 5 was fluorinated resulting in a 7-fold enhancement of the anisotropic barrier. Ab initio calculations reveal more axial g tensors as well as higher energy first excited Kramers doublets in 4 and 6 leading to higher energy barriers for those complexes.

Linking high anisotropy Dy-3 triangles to create a Dy-6 single-molecule magnet

A unique Dy(III)(6) complex is created by linking of two Dy(III)(3) triangles, in which intramolecular ferromagnetic interactions and single-molecule magnetic behaviour have been observed.