Electronic structure of mononuclear and radical-bridged dinuclear cobalt(II) single-molecule magnets

Metal-organic compounds that feature magnetic bistability (single-molecule magnets) have been proposed as data bits for magnetic storage, but progress has been slow. Four-coordinate cobalt(II) complexes have been shown to be a robust, air-stable class of complexes with high energy barriers toward inversion of the magnetic moment, but they do not display sizable magnetic bistability. Developing radical-bridged polynuclear systems is a promising but challenging strategy to improve the magnetic properties. Here we report a novel air-stable radicalbridged dinuclear cobalt(II) complex, studied by a combination of magnetometry and variablefield spectroscopy (Far-Infrared and Raman). Fits of the data gave D = −113 cm−1 for the zero-field splitting (ZFS) and J = 390 cm−1 for the metal–radical exchange interaction. Detailed ab initio investigations revealed first order spin–orbit coupling of the quasi-degenerate dx2−y2 and dxy orbitals to be at the heart of the large ZFS in the dinuclear as well as in the corresponding mononuclear complex. The corresponding transitions were spectroscopically observed, as were transitions related to the exchange coupling in the dinuclear complex, ensuring robustness of the electronic structure analysis. Finally, clear signatures of spin–phonon coupling were observed and theoretically analyzed in detail, revealing modulation of off-diagonal elements of the dynamic ZFS to be the main coupling elements, corresponding to local rotations of the ZFS tensors. Furthermore, we conclusively demonstrate that the spectral features are not predominantly spin excitations, but largely vibrational in character.