Molecular Magnetism

Abstract

Molecular magnetism is an interdisciplinary field that incorporates concepts from physics, chemistry, and material sciences. One of our projects in this area is the synthesis and investigation of the properties of Single Molecule Magnets (SMMs). These molecules possess a non-zero spin and magnetic anisotropy, preferably of the (z-axis) type. At low temperatures, SMMs are reminiscent of classical bulk magnets, exhibiting properties such as magnetic bi-stability and hysteresis, in other words they maintain a magnetization like a 3-D ordered magnet but for very different reasons. The origin of SMM behavior is a thermal barrier (Ueff) which inhibits reversal of the magnetization. Such magnets could revolutionize the world of electronics, with potential applications in data storage, quantum computing, and spintronics. However, the best SMMs exhibit these magnetic properties only at temperatures below 60 K (-213° C). The prevalence of alternative relaxation pathways, such as quantum tunneling of magnetization (QTM) which is tunneling through the barrier rather than going over the barrier, shortcut the thermal relaxation barrier and limit the practicality of using these molecules in cutting-edge technology. In order to make SMMs functional at higher temperatures, our research focuses on strategies to improve the thermal barrier and reduce QTM through the synthesis of novel mononuclear SMMs, cyanide based SMMs, and radical-bridged SMMs.

Content


This multidisciplinary research involves a variety of bench techniques including the use of Schlenk-lines and inert atmosphere dry boxes to carry out inorganic and organic synthesis, crystal growth and general manipulations and to gain experience in advanced experimental techniques in chemistry and physics. Students interested in theory have ample opportunities to hone their skills in DFT and ab initio methods. Characterization tools typically used are X-ray crystallography, infrared, electronic and electron paramagnetic resonance spectroscopies (EPR), electrochemistry, magnetometry as well as others. The students on this project have exciting opportunities to carry out experiments at National Laboratories and to collaborate with scientists from all over the world. Many opportunities are provided for students to broaden their education through travel and interactions with visitors. The Dunbar group hosts numerous collaborators and other experts in molecular materials research in her laboratories and the students present their work in informal discussions and at National and International conferences. Students are actively encouraged to engage in outreach activities with the entire Dunbar group participating in our yearly Chemistry Open House for local school children and their parents and teachers. Other examples of service and outreach are instances of service as judges for Science Fairs and coaches for Science Olympiad, and visits to local elementary schools to perform demonstrations.

Mononuclear Single Molecule Magnets

The study of mononuclear SMMs has come to the forefront of molecular magnetism research in the last several years. Our goal is to design transition metal and lanthanide molecules with highly symmetric, discrete geometries that, by virtue of their inherent electronic properties, are predicted to lead to SMMs with large barriers to reversal of the spin. These geometries are rare and, in some cases, predicted to lead to large negative axial zero-field splitting resulting from orbitally degenerate ground states and unquenched spin-orbit coupling. They also lead to strong axial symmetry, which reduces the prevalence of quantum tunneling. We approach these molecules by using a variety of bulky ligands to enforce the desired geometry.

For example, we recently synthesized a series of compounds of the formula [Li(THF)MII(N3N)] (MII = MnII, FeII, CoII, NiII; N3N = tris(N-trimethylsilyl-2-amidoethyl)amine).1 In these compounds, the N3N ligand enforces a pseudo-trigonal monopyramidal geometry on the MII ion. This series of compounds was magnetically characterized, validating theoretical predictions that properly controlling coordination geometry and electronic structure of the 3d metal ion can lead to magnetic anisotropy.


Building off this work, we wanted to explore a more pure geometry, as well as expand into new geometries. We isolated a series of six compounds with trigonal monopyramidal and bipyramidal geometries, (Me4N)[M(MST)] and (Me4N)[M(MST)(OH2)] (M = FeII, CoII, NiII, MST = N,N′,N″-[2,2′,2″-nitrilotris-(ethane-2,1-diyl)]tris(2,4,6-trimethylbenzenesulfonamido) respectively.2 We performed a systematic comparison of the two complexes experimentally and computationally. In each case, we found that trigonal monopyramidal complexes exhibit a larger axial zero field splitting parameter, D, than their trigonal bipyramidal counterparts. Remarkably, the NiII analogue displays an extraordinarily large axial zero field splitting parameter, D, of -434 cm-1 due to extremely low lying excited states. In the case of the iron complexes, coordination of a single water molecule in the axial position was observed to completely quench the slow magnetic relaxation observed in the trigonal monopyramidal complex. Both CoII exhibit slow magnetic relaxation, although coordination of water results in a reduction of the thermal barrier.



We have also been exploring rare geometries in lanthanide based molecules. Namely, we isolated complexes of the formula [CoIII(Tp)2]1.3[M(NO3)2(dbm)2](NO3)0.3 (M = Dy, Er, Tb, Y; Tp = tris(pyrazolyl)borate; dbm = 1,3-diphenyl-1,3-propanedionate), which exhibit a rare cubic geometry.3 This is one of the only examples of lanthanide compounds with this geometry in spite of the vast library of 4f complexes. We explored the magnetic behavior of the complexes experimentally and computationally. The Dy analogue exhibits a thermal barrier of 96 K under a small applied field of 200 Oe. We are currently exploring how alternative anions and ligands affect the cubic geometry and the magnetic behavior of these complexes.


In summary, the systematic study of highly symmetric geometries is yielding fascinating insights into the mechanisms of SMM behavior and improving predictions about which geometries are promising to pursue in the future.


References

1.      Pinkowicz, D.; Birk, F. J.; Magott, M.; Schulte, K.; Dunbar, K. R., Systematic Study of OpenShell Trigonal Pyramidal TransitionMetal          Complexes with a RigidLigand Scaffold. Chem. Eur. J. 2017, 23, 3548-3552.

2.      Schulte, K. A.; Vignesh, K. R.; Dunbar, K. R., Effects of coordination sphere on unusually large zero field splitting and slow magneti           relaxation in trigonally symmetric molecules. Chem. Sci. 2018, DOI: 10.1039/c8sc02820f.

3.      Alexandropoulos, D. I.; Schulte, K. A.; Vignesh, K. R.; Dunbar, K. R., Slow magnetic dynamics in a family of mononuclear lanthanide          complexes exhibiting the rare cubic coordination geometry. Chem. Commun. 2018, 54, 10136-10139.


Cyanide-Based Molecular Magnets

In the cyanide-based SMM project, we are focusing on using small, molecular cyanide anions to connect metal centers to each other. Exchange coupling between metal-based spins can significantly enhance magnetic properties and is strongly influenced by the geometry of the ligand field. Since the discovery of impressive magnetic properties in extended solids known as Prussian Blue analogues, cyanide has been a staple in the field of molecular magnetism due to its ability to mediate strong coupling between metal centers. There is also potential for interesting multi-functional effects, such as photomagnetic responses and charge-transfer induced spin transitions.1-5

The magnetic properties of polynuclear SMMs, however, are not as well understood as those of mononuclear SMMs. We are taking a theory-based approach to making new polynuclear SMMs that will help us understand the properties of these fascinating molecules. The building block approach6 is a critical strategy for controlling synthesis of these molcules, as it allows us to design metal building blocks that we can then test in a variety of molecules.7-10

Our work focuses on 4d and 5d transition metals, namely Mo, W, and Re, because they have stronger spin-orbit coupling and the potential for exhibiting stronger exchange interactions than their 3d cousins. The building block approach4 has become an important part of that strategy, as it allows us to design heavy metal building blocks that we can then test in a variety of molecules. Our group exemplified this strategy through the synthesis of [Mn(LN5Me)(H2O)]2[Mo(CN)76H2O (LN5Me =  2,6-bis[1-(2-(N-methylamino)-ethylimino) ethyl]pyridine), in which two MnII ions are linked to a central MoIII ion via the axial cyanides of [Mo(CN)7]4-.5 This complex exhibits an effective thermal
barrier of 55 K and hysteresis up to 3.2 K, both of which are records for cyanide-bridged SMMs. Further work with this anion shows similarly impressive results.12

We also synthesize new building blocks for cyanide chemistry. One such example is [WIV(CN)7]4-. 13 The incorporation of tungsten into a homoleptic, 7-coordinate cyanometallate has been a focus in the field for many years, largely because of the impressive results that have been obtained by our group with [MoIII(CN)7]4-. Now that we have this anion, we are able to make new molecular magnetic materials that incorporate the previously unknown seven-coordinate tungsten center.

Another heavy metal building blocs is [(triphos)ReII(CN)3]-. 14-16 This molecule was a key component of the first 5d metal SMM, [{MnCl}4{Re(triphos)(CN)3}4], which was synthesized by our research group in 2004.17-18 New approaches in our group have shown that smaller compounds can be synthesized by using blocking ligands on the 3d metal center. Recent compounds include [(VII(tmphen)2)2Re(CN)3(triphos)](CF3SO3)2 and [{VII(tren)}(μ-CN)Re(triphos)}]2(μ-O) (shown below), which are new geometries that incorporate VII into structures with [(triphos)ReII(CN)3]-.

In summary, there are many exciting discoveries that are still waiting for new students to work in the Dunbar laboratories in the area of polynuclear transition metal SMMs.


References

1.        Berlinguette, C. P.; Dragulescu-Andrasi, A.; Sieber, A.; Galán-Mascarós, J. R.; Güdel, H.-U.; Achim, C.; Dunbar, K. R., A
           Charge-Transfer-Induced Spin Transition in the Discrete Cyanide-Bridged Complex {[Co(tmphen)2]3[Fe(CN)6]2}. J. Am. Chem.              Soc. 2004, 126 (20), 6222-6223.

2.        Berlinguette, C. P.; Dragulescu-Andrasi, A.; Sieber, A.; Güdel, H.-U.; Achim, C.; Dunbar, K. R., A Charge-Transfer-Induced Spin            Transition in a Discrete Complex:  The Role of Extrinsic Factors in Stabilizing Three Electronic Isomeric Forms of a           
           Cyanide-Bridged Co/Fe Cluster. J. Am. Chem. Soc. 2005, 127 (18), 6766-6779.

3.         Hilfiger, M. G.; Shatruk, M.; Prosvirin, A.; Dunbar, K. R., Hexacyanoosmate(iii) chemistry: preparation and magnetic properties of a             pentanuclear cluster and a Prussian blue analogue with Ni(ii). Chem. Commun. 2008,  (44), 5752-5754.

4.         Wang, X.-Y.; Hilfiger, M. G.; Prosvirin, A.; Dunbar, K. R., Trigonal bipyramidal magnetic molecules based on [MoIII(CN)6]3. Chem.             Commun. 2010, 46 (25), 4484-4486.

5.         Wang, X.-Y.; Avendaño, C.; Dunbar, K. R., Molecular magnetic materials based on 4d and 5d transition metals. Chem. Soc. Rev.             2011, 40 (6), 3213-3238.

6.         Shatruk, M.; Avendano, C.; Dunbar, K. R., Cyanide-Bridged Complexes of Transition Metals: A Molecular Magnetism Perspective.             In Progress in Inorganic Chemistry, John Wiley & Sons, Inc.: 2009; pp 155-334.

7.         Palii, A. V.; Ostrovsky, S. M.; Klokishner, S. I.; Tsukerblat, B. S.; Berlinguette, C. P.; Dunbar, K. R.; Galán-Mascarós, J. R., Role of             the Orbitally Degenerate Mn(III) Ions in the Single-Molecule Magnet Behavior of the Cyanide Cluster             {[MnII(tmphen)2]3[MnIII(CN)6]2} (tmphen = 3,4,7,8-tetramethyl-1,10-phenanthroline). J. Am. Chem. Soc. 2004, 126 (51),             16860-16867.

8.         Karadas, F.; Schelter, E. J.; Prosvirin, A. V.; Bacsa, J.; Dunbar, K. R., A high spin molecular square based on square pyramidal             CoII and tetrahedral MnII centers: [{MnIICl2}2{CoII(triphos)(CN)2}2]. Chem. Commun. 2005,  (11), 1414-1416.

9.         Shatruk, M.; Dragulescu-Andrasi, A.; Chambers, K. E.; Stoian, S. A.; Bominaar, E. L.; Achim, C.; Dunbar, K. R., Properties of             Prussian Blue Materials Manifested in Molecular Complexes:  Observation of Cyanide Linkage Isomerism and Spin-Crossover             Behavior in Pentanuclear Cyanide Clusters. J. Am. Chem. Soc. 2007, 129 (19), 6104-6116.

10.       Dunbar, K. R.; Heintz, R. A., Chemistry of Transition Metal Cyanide Compounds: Modern Perspectives. Progress in Inorganic             Chemistry 2007.

11.       Qian, K.; Huang, X.-C.; Zhou, C.; You, X.-Z.; Wang, X.-Y.; Dunbar, K. R., A Single-Molecule Magnet Based on             Heptacyanomolybdate with the Highest Energy Barrier for a Cyanide Compound. J. Am. Chem. Soc. 2013, 135 (36), 13302-13305.

12.       Wu, D.-Q.; Shao, D.; Wei, X.-Q.; Shen, F.-X.; Shi, L.; Kempe, D.; Zhang, Y.-Z.; Dunbar, K. R.; Wang, X.-Y., Reversible On–Off             Switching of a Single-Molecule Magnet via a Crystal-to-Crystal Chemical Transformation. J. Am. Chem. Soc. 2017, 139 (34),             11714-11717.

13.       Birk, F. J.; Pinkowicz, D.; Dunbar, K. R., The Heptacyanotungstate(IV) Anion: A New 5 d Transition-Metal Member of the Rare             Heptacyanometallate Family of Anions. Angew. Chem. Int. Ed. 2016, 55 (38), 11368-11371.

14.       Dunbar, K. R.; Schelter, E. J.; Palii, A. V.; Ostrovsky, S. M.; Mirovitskii, V. Y.; Hudson, J. M.; Omary, M. A.; Klokishner, S. I.;             Tsukerblat, B. S., Unusual Magnetic Behavior of Six-Coordinate, Mixed-Ligand Re(II) Complexes:  Origin of a Strong             Temperature- Independent Paramagnetism†. J. Phys. Chem. A. 2003, 107 (50), 11102-11111.

15.       Schelter, E. J.; Bera, J. K.; Bacsa, J.; Galán-Mascarós, J. R.; Dunbar, K. R., New Paramagnetic Re(II) Compounds with Nitrile and             Cyanide Ligands Prepared by Homolytic Scission of Dirhenium Complexes. Inorg. Chem. 2003, 42 (14), 4256-4258.

16.       Schelter, E. J.; Prosvirin, A. V.; Reiff, W. M.; Dunbar, K. R., Unusual Magnetic Metal–Cyanide Cubes of ReII with Alternating             Octahedral and Tetrahedral Corners. Angew. Chem. Int. Ed. 2004, 43 (37), 4912-4915.

17.       Schelter, E. J.; Karadas, F.; Avendano, C.; Prosvirin, A. V.; Wernsdorfer, W.; Dunbar, K. R., A Family of Mixed-Metal Cyanide             Cubes with Alternating Octahedral and Tetrahedral Corners Exhibiting a Variety of Magnetic Behaviors Including Single Molecule             Magnetism. J. Am. Chem. Soc. 2007, 129 (26), 8139-8149.

18.       Palii, A. V.; Ostrovsky, S. M.; Klokishner, S. I.; Tsukerblat, B. S.; Schelter, E. J.; Prosvirin, A. V.; Dunbar, K. R., Magnetic anisotropy             in the octanuclear cluster exhibiting Single-Molecule Magnet behavior: Quantum-spin and classical-spin approaches. Inorg. Chim.             Acta. 2007, 360 (13), 3915-3924.