The development and study of molecular nanomagnets has witnessed tremendous progress in recent years, with potential applications on the horizon [1]. Of particular interest are so-called single-molecule magnets (SMMs) that display slow magnetic relaxation below a characteristic blocking temperature, TB, due to the combination of a large magnetic moment and appreciable spin-orbit (SO) anisotropy. Early efforts aimed at increasing TB focused primarily on maximizing the molecular spin ground state, S, associated with polynuclear clusters. However, this becomes challenging for large clusters whilst simultaneously maintaining the molecular anisotropy. Thus, a more direct route to increasing TB involves optimization of the magnetic anisotropy, albeit for simpler molecules in which one can exert considerable synthetic control over the ligand field (LF). This approach resulted in the first SMM comprising a single lanthanide ion in 2003 [2]. Since then, much effort has been directed towards increasing the magnetic anisotropy of mononuclear species. Here, we discuss two series of hexa-coordinate VIII complexes: one consisting of trans-dicyanide VIII building blocks with equatorial chelating ligands; the other involving a tridentate scorpionate ligand, with three halides completing the coordination sphere. Multi-high-field/frequency (up to 35 T and 450 GHz) EPR was employed to determine the magnetic anisotropy of these complexes. Members of the Jahn-Teller elongated cyanovanadate series can be described by an effective spin-only Hamiltonian with positive D parameters in the +3 to +6 cm‑1 range, and negligible rhombicity. On the other hand, one of the scorpionate complexes was found to have a very large easy-axis-type zero-field-splitting on the order of ‑40 cm‑1; this complex also possesses a non-zero, albeit small E value of 1.1 cm-1. The large magnetic anisotropy is thought to arise from the trigonal distortion imposed by the rigid scorpionate ligand, which also partially suppresses the Jahn-Teller physics, thereby giving rise to a considerable orbital contribution to the magnetic ground state. [1] Molecular Magnets - Physics and Applications, Springer Series on Nanoscience and Technology, eds J. Bartolomé, F. Luis, J. F. Fernández (Springer, 2014). [2] N. Ishikawa et al., J. Am. Chem. Soc. 125, 8694 (2003).

High-field electron paramagnetic resonance determination of the magnetic anisotropy in pseudo-octahedral mononuclear VIII complexes

249th National Meeting (2015)

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