Magnetic Relaxation Dynamics and Processes in Mono- and Dinuclear Lanthanide Single-Molecule Magnets
Creators: name: Harriman, Katie Lois Marie
Publisher: name: Université d'Ottawa / University of Ottawa
Year: 2021
Subjects: Single-Molecule Magnets
Lanthanides
Magnetic Relaxation

description: Single-molecule magnets (SMMs) have been lauded for their application in next generation devices for their enhanced information storage capabilities, increased processing speeds, and increased storage densities compared to bulk magnets. However, the success of SMMs in such applications and their technological readiness is hindered by their operation temperatures and memory lifetimes. SMMs are molecular species that possess a bistable ground state and magnetic anisotropy, which together result in an energy barrier to the reorientation of the magnetic moment. The magnetic memory response relies on its ability to retain magnetization in the absence of an external field. To this end, lanthanide ions with their large inherent magnetic anisotropy combined with well-defined crystal field microstates are attractive candidates for eliciting higher operation temperatures and lifetimes. This dissertation focuses on the use of lanthanide ions in the development of high barrier SMMs with a close emphasis on the magnetic anisotropy and crystal field manipulation through geometry, design, and modification. In the pursuit of lanthanide (Ln)-based SMMs, two cyclooctatetraenyl (COT2-) complexes of the non-Kramers ion, TmIII, [TmIII(η8-COT)I(THF)2] and [K(18-C-6)(THF)2][TmIII(η8-COT)2], were isolated. As an ion that possess an integer angular momentum projection (J = 6), it was vital that a highly symmetric local environment was utilized to observe field-induced slow magnetic relaxation. The static and dynamic properties of TmIII(η8-COT)I(THF)2] and [K(18-C-6)(THF)2][TmIII(η8-COT)2] were characterized revealing Ueff of 7.93 K and 53.3 K, respectively. More importantly, the effect of increased symmetry was observed on the rate of quantum tunneling of the magnetization (QTM), where the rate was two orders of magnitude faster in the heteroleptic complex. This emphasized the importance of local symmetry for non-Kramers ions and contributed to the rare class of TmIII SMMs. Due to the prevalent role of QTM in Ln-based SMMs, a common strategy is to induce magnetic communication between Ln ions to overcome its detrimental effects. To this end, bridging units should be sufficiently small enough to bring the Ln ions close in proximity, yet the surrounding environment of the metal center should still promote uniaxial magnetic anisotropy. We compared the effect of ancillary ligands on the magnetic properties of two dinuclear DyIII compounds with the same {μ-Cl}2 core bridge. The complexes [DyIII{N(SiMe3)2}2(μ-Cl)(THF)]2 and [DyIII(η8-COT)(μ-Cl)(THF)]2 were characterized with static and dynamic magnetic measurements. The well-matched ligand field of the silyl amide ligands with the DyIII ion, precluded the observation of zero field tunneling. While both complexes are characterized by antiferromagnetic coupling, it is evident that peripheral ligands also play a vital role in determining the performance of multinuclear SMMs. Magnetic coupling between 4f centers is classically weak; however, the use of ligands with diffuse electron clouds may penetrate the shielded 4f orbitals to effectively promote communication. One such ligand that had not previously been investigated for its ability to couple the magnetic moment of Ln ions was the trianionic cycloheptatrienyl. Utilizing Ln silyl amides, in situ deprotonation afforded the dinuclear complexes [KLnIII2(η7-C7H7){N(SiMe3)2}4] (Ln = GdIII, DyIII, ErIII). The static and dynamic magnetic characterization revealed rare and highly sought-after ferromagnetic coupling in a Ln-based system. The ancillary silyl amide ligands were a necessity for the isolation of these dinuclear species yet did not provide a synergistic ligand field for the Ln ions when combined with the cycloheptatrienyl bridge, ultimately preventing the observation of slow relaxation in some of the variants studied. Pseudo-linear complexes, those molecules with strong axial donors have shown immense promise in the design of highly efficient SMMs. Our work has shown that amides are effective in directing the anisotropy of the Ln ions, thus the removal of the central organometallic bridge from the previous compounds would effectively create a highly anisotropic complex. This was achieved in our study of a formally five-coordinate complex of a ferrocene diamide ligated DyIII ion, [(NNTBS)DyIIII(THF)2]. The static and dynamic magnetic properties were characterized, yielding Ueff = 771 K with open magnetization hysteresis loops at zero-field, due in part to the axial disposition of the nitrogen atoms of the diamide ligand. Computational analysis of the parent compound and its fragments was completed. Our results indicated that the presence of equatorially coordinated solvent molecules such as THF, influence the axiality in the crystal field microstates more significantly than the coordinated halide. The removal of coordinated solvent such as THF, is imperative to improve the performance of DyIII SMMs. By way of a bulky bisanilide ligand that precludes the approach of solvent to the metal center, combined with a large bite angle, [K(DME)n][LArDyIII(X)2], a formally four coordinate complex, was investigated. In contrast to the complex of the ferrocene diamide ligand, retention of the magnetic moment was not observed at zero-field, despite the fact that the slow relaxation dynamics occurred over a greater temperature range for which Ueff = 1278-1334 K. In addition, variants of the bound halide (X = Cl, I) were examined for their effect on the static and dynamic magnetic properties, revealing zero field relaxation times that were on average 5.6x longer for the heavier congener. The collective results of the findings presented herein are being utilized to synthesize new low-coordinate Ln-based SMMs. Combining divalent and redox chemistries with bulky amido ligands will ideally elicit even larger energy barriers to spin reversal and higher blocking temperatures, supporting the push towards Ln-based SMMs with increased technological readiness.