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The Synthesis of Rare Earth Complexes for the Optimization of Single-Molecule Magnets

  • Author(s): Corbey, Jordan
  • Advisor(s): Evans, William J
  • et al.

This dissertation describes the synthesis and physical characterization of a variety of rare earth-containing molecules that have been targeted for the purpose of better understanding the roles that rare earth elements play in molecular magnetism. The complexes described here also contribute to fundamental understanding of rare earth coordination chemistry. Many of these results and conclusions have been developed in collaboration with the group of Professor Jeffrey R. Long at the University of California, Berkeley.

Chapters 1 through 4 of this dissertation focus on modifying different aspects of the known (N2)3−-bridged bimetallic single-molecule magnet (SMM) system {[(Me3Si)2N]2(THF)Ln}2(μ-η2:η2-N2)[K(18-crown-6)(THF)2], whose Tb analog had the highest blocking temperature ever observed for a SMM when it was published in 2011. Variations in the ancillary ligands, the metals, and the bridging unit were investigated to determine their effects on magnetic properties. In Chapter 1, the variation in the Lewis base L in the precursor to the (N2)3− complexes, the (N=N)2− complexes, {[(Me3Si)2N]2(L)Ln}2(μ-η2:η2-N2), was explored. Previously, only L = THF was known. It was found with the diamagnetic Ln = Y analog that L = pyridines, nitriles, and triphenylphosphine oxide can also support this reduced dinitrogen (N=N)2− system. However, further reduction to obtain a radical (N2)3− complex was not achieved. Density functional theory (DFT) calculations suggest this is due to the presence of lower lying orbitals based on the new neutral donors.

Chapter 2 analyzes how structural modifications in the previously reported SMMs {[(Me3Si)2N]2(THF)Ln}2(μ-η2:η2-N2)[K(18-crown-6)(THF)2] (Ln = Tb, Dy) can affect the molecular magnetism when the K+ counter cation is incorporated into the inner sphere of the magnetic core. The new series of SMMs that resulted, {[(Me3Si)2N]2(THF)Ln}2(μ3-η2:η2:η2-N2)K (Ln = Tb, Dy), displays blocking temperatures lower than the outer sphere K+ analogs. This result is thought to be due in part to the crystallographically-observed bending of the previously planar Ln2N2 core. In Chapter 3, a more drastic modification in the bridging unit of these bimetallic rare earth complexes was achieved: the (N2)2− anion in {[(Me3Si)2N]2(THF)Y}2(μ-η2:η2-N2) can reduce elemental sulfur and selenium to generate bridging E2− and (E2)2− chalcogenide complexes, where E = S, Se. Finally, Chapter 4 investigates the Tb analog of the first reported molecular example of an (NO)2−-containing complex, the radical bridged {[(Me3Si)2N]2(THF)Y}2(μ-η2:η2-NO), and demonstrates the importance of obtaining additional spectral characterization data to support X-ray crystallographic findings.

Chapter 5 describes synthetic aspects of complexes containing the [(C5Me5)2Ln]1+ moiety, which is an important component in rare earth starting materials, and Chapter 6 shows how this is used to design phenazine radical (phz)1−-containing SMMs. Although the targeted complexes, {[(C5Me5)2Ln]2(phz)}{BPh4} (Ln = Tb, Dy; phz = phenazine), could be synthesized and crystallographically characterized, magnetic data suggest the presence of multiple magnetic products in the crystalline sample. In Chapter 7, (C5Me5)1− metallocenes are investigated from another perspective. Attempts were made to obtain linear monometallic trivalent rare earth metallocene cations, as previously demonstrated for U4+, to determine their potential as single-ion magnets (SIMs).

The origin of the research presented in Chapter 8 was the recent discovery that the +2 oxidation state is accessible for all the lanthanides in molecular species with formula [K(2.2.2-cryptand)][Cp′3Ln] (Cp′ = C5H4SiMe3) and that the Ho2+ analog of these complexes possesses the highest magnetic moment ever measured for a single metal ion, 11.4 μB. The synthesis and isolation of more molecular examples of these highly reactive divalent ions were pursued with the indenide ligand, (C9H7)1−. However, reduction of CpIn3Ln (CpIn = C9H7) resulted in C–H bond activation of an indenyl ligand and the first example of the indenyl dianion, (C9H6)2−.

Appendix A is a collection of results on miscellaneous projects not covered in the previous eight chapters. The results in this appendix are presented in chronological order and span a collection of ventures from dinitrogen reduction using lithium to inelastic neutron scattering (INS) experiments and finally a collaborative project with the group of Professor Alan F. Heyduk at the University of California, Irvine, presenting the mixed metal lanthanide/transition metal species (C5Me5)2Dy[M(SNS)2] (M = Mo, W). These latter complexes were synthesized to probe whether magnetic coupling of the mixed metal centers could occur. They truly exemplify the purpose of the research presented in this dissertation which is to explore the frontiers of molecular magnet design.

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