From Radical-Bridged to Polymetallic Lanthanide Metallocenes: Designing Single-Molecule Magnets Across Nuclearities

Abstract

Single-molecule magnets (SMMs), particularly those based on lanthanide (Ln) ions, have emerged as promising candidates for next-generation molecular materials due to their potential applications in high-density data storage, quantum computing, and spintronic devices. The inherent large magnetic anisotropy and strong spin-orbit coupling of Ln ions have enabled mononuclear organometallic complexes to achieve record figure of merit for slow magnetic relaxation including high energy barriers and elevated blocking temperatures. However, extending this success to polynuclear systems has proven challenging, largely due to the weak magnetic exchange interactions between Ln centers. To overcome this limitation, radical ligands capable of mediating stronger coupling through their delocalized unpaired electrons have been employed as bridging units. This dissertation explores the design, synthesis, and magnetic properties of polynuclear Ln SMMs, with a focus on radical-bridging strategies to enhance magnetic exchange.

The investigation began with tetrazine-based radical bridges, focusing on the 3,6-bis(2-pyrimidyl)-1,2,4,5-tetrazine (bmtz) ligand. Two new dinuclear complexes of the formula [(Cp*₂Ln)₂(bmtz^•-)](BPh₄)·Solvent (Ln = Gd (2-Gd₂), Solvent = THF; Dy (2-Dy₂), Solvent = toluene) were synthesized and fully characterized. The bmtz ligand was selected for its rigid planar structure and ability to coordinate both in bridging and chelating modes, providing a strong platform for magnetic communication. Magnetic measurements combined with broken-symmetry density functional theory (DFT) calculations revealed a remarkably strong antiferromagnetic exchange coupling in 2-Gd₂ with J_Gd-rad = -12.6 cm⁻¹, among the highest reported for tetrazine-bridged systems. Moreover, 2-Dy₂ shows slow magnetic relaxation behavior under zero applied field, with an effective energy barrier (U_eff) of 125 cm⁻¹, which is one of the highest reported for tetrazine-based radical-bridged SMMs. These results establish bmtz^•- as a bridging ligand capable of enhancing the magnetic exchange coupling between lanthanide ions, thereby expanding the toolkit for designing high-performance lanthanide SMMs.

Building on these findings, the focus shifted to pyrazine (pyz) , specifically the use of pyrazinyl (pyz^•-) as a radical bridge, which offer several practical advantages, including synthetic accessibility, commercial availability, rigid and planar geometry that facilitates predictable coordination behavior. A series of dinuclear and tetranuclear pyz^•- -bridged complexes were synthesized: dinuclear [(Cp*₂Ln)₂(pyz^•-)(THF)₂][BPh₄]‧Et₂O (Ln = Gd (3-Gd₂), Dy (3-Dy₂) ; THF = tetrahydrofuran) and tetranuclear [(Cp*₂Ln)₄(pyz^•-)₄]‧10THF (Ln = Gd (3-Gd₄), Dy (3-Dy₄)) complexes. The dinuclear species can also serve as precursors to the tetranuclear complexes, acting as a building block. The magnetic coupling in the 3-Dy₂ complex was determined to be J_Gd-pyz = -22.2 cm⁻¹, setting a new benchmark for organic radical bridges in lanthanide SMMs and surpassing previous tetrazine and dinitrogen-bridged complexes. This strong exchange coupling translates into robust single-molecule magnet behavior under zero-field conditions, resulting in open magnetic hysteresis loops for the Dy complexes. Remarkably, 3-Dy₄ exhibited a giant coercive field of 65 kOe at 1.8 K, the largest coercivity reported to date for Dy radical-bridged SMMs.

The final stage of this dissertation involves extending the nuclearity to unprecedented levels via radical-radical cross-coupling reaction. A novel decanuclear dysprosium complex [(Cp*)₂₀Dy₁₀(L1)₁₀]·12(C₇H₈), where L1 is the anion of 2-prop-2-enylenyl-2𝐻-pyrazine, was synthesized. This complex forms a highly symmetric nanoscale wheel through in situ radical coupling, incorporating both S- and R-enantiomers of L1. Although L1 is not a radical species and thus does not mediate Dy-radical exchange, the large nuclearity and strong axial anisotropy of Dy ions lead to slow magnetic relaxation under an applied field. This synthetic advancement demonstrates the potential of using well-studied organic radical to form high-nuclearity species.

Collectively, this dissertation provides new insights into the role of redox-active organic ligands, coordination geometry, and nuclearity in governing the magnetic properties of lanthanide SMMs. The strong exchange interactions and slow magnetic relaxation observed in pyrazine- and tetrazine-bridged systems establish these ligands as valuable platforms for next-generation molecular magnets. Moreover, the successful isolation of high nuclearity complexes via controlled radical coupling highlights the potential for constructing multifunctional molecular architectures with tunable magnetic properties. This work lays a foundation for future exploration of radical-bridged lanthanide SMMs toward applications in information storage, quantum computing, and molecular spintronics.