Pushing the Limits of Magnetic Anisotropy in a Mononuclear Ni(II) Single-Molecule Magnet: a High-Field EPR Study

Single-Molecule Magnets (SMMs) are potential candidates for nanoscale magnetic information storage, where slow magnetization dynamics (bistability) is realized at low temperatures due to a magnetic anisotropy barrier separating the ``spin-up'' and ``spin-down'' states of the SMMs. Here, we report spectroscopic evidence for a huge easy-axis anisotropy in a trigonal bipyramidal (TBP) [Ni$^{\mathrm{II}}$Cl$_{3}$(Me-dabco)$_{2}$] complex with an orbitally degenerate ground state. Single-crystal EPR studies were carried out in a 35T resistive magnet at the NHMFL. A very strong angle-dependence of the spectrum was observed within a few degrees of the hard plane, suggesting a huge zero-field-splitting (zfs) parameter, \textbar D\textbar \textgreater 300 cm$^{-1}$, associated with first order spin-orbit coupling. This value is considerably larger than previously reported for a Ni$^{\mathrm{II}}$ TBP complex [1], and is thought to be due to the rigidity of the ligand that prevents Jahn-Teller type effects that can reduce D [2]. This is confirmed by the small value of the rhombic parameter, \textbar E\textbar $=$ 0.66 cm$^{-1}$.\\[4pt] [1] R. Ruamps et al., JACS, 135, 3017-3026 (2013).\\[0pt] [2] M. Gruden-Pavlovic et al., Chem. Sci., 5, 1453-1462 (2014)