Advanced Molecular Design for Heterometallic Single Molecule Magnets: Elucidating the Magnetic Properties of Lanthanide-[1]metallocyclophane Complexes

Abstract

Modern advancements in computing have inspired a profound interest in molecular magnetism. Single-molecule magnets show hysteresis of molecular origin, which renders these complexes optimal for next generation computing technologies. Until recently, the functionality of molecular magnets has been hindered by low operating temperatures and by the presence of significant quantum tunneling of the magnetization (QTM), which increases the rate of spin state reorientation. New approaches toward improving practicality involve utilization of magnetic interactions between multiple spin sites within each discrete molecule to increase blocking temperatures and shut down extraneous QTM relaxation processes.

Herein, a detailed study of non-ferrocene tris[1]metallocyclophane complexes with unique architectural changes to optimize magnetic functionality will be presented. While only few examples of [1]ruthenocenophane complexes are known, the first reported lanthanide-[1]ruthenocenophane complexes of the type [LnRc3Li2(thf)2][Li(thf)4] (Ln = Dy3+ (1), Tb3+ (2), Y3+ (3) have been synthesized and magnetically characterized. Complexes 1 and 2 show zero field SMM behavior and structural studies towards optimization of ligand field were discussed.

This work was be expanded to include a new class of tris[1]metalloarenophanes (M = V, Cr) for systematic comparison of first and second row d5- and d6- transition metals. The tris[1]vanadarenophane architecture provides a unique framework for investigation of spin frustration and antisymmetric exchange, as changes in the central templating ion have tremendous effects on spin canting and coupling within the system. Spin triangles of this type have been explored as optimal candidates for quantum computing applications. Ln-tris[1]vanadarenophane complexes of Y3+ (4) and Lu3+ (5) allow for investigation of a V3 spin triangle that shows direct and super exchange pathways that result in diagnostic EPR signatures. Use of a paramagnetic lanthanides, Tb3+ (6), Dy3+ (7), and Yb3+ (8) reveals coupling between the V3 triangle and lanthanide centers to further optimize magnetic properties, as supported by ab initio calculations. Especially for 6, coupling with a non-Kramers ion result in Ueff = 381 cm–1.

Finally, though many analogies have been drawn between ferrocene and bis(benzene)chromium, the variation in electronic structure results in the introduction of temperature-independent paramagnetic properties due to second order Zeeman effects. Ferromagnetic coupling in Gd3+ (9) and Dy3+ (10) tris[1]chromarenophane complexes is observed. Magnetic blocking up to 8 K and Ueff = 180 cm–1 is observed for 10, which displays minimal influence from QTM and increased axiality of the ligand field compared to the ferrocene analogue. Multireference CASSCF calculations were conducted to better understand the origin of temperature-independent magnetic behavior.