Electronic structure of mononuclear and radical-bridged dinuclear cobalt(II) single-molecule magnets
David Hunger, Julia Netz, Simon Suhr, Komalavalli Thirunavukkuarasu, Hans Engelkamp, B. Fåk, Uta Albold, Julia Beerhues, Wolfgang Frey, Ingo Hartenbach, Michael Schulze, Wolfgang Wernsdorfer, Biprajit Sarkar, Andreas Köhn, Joris van Slageren
Abstract
Abstract Metal-organic compounds that feature magnetic bistability have been proposed as bits for magnetic storage, but progress has been slow. Four-coordinate cobalt(II) complexes feature high inversion barriers of the magnetic moment, but they lack magnetic bistability. Developing radical-bridged polynuclear systems is a promising strategy to encounter this; however detailed investigations of such species are scarce. We report an air-stable radical-bridged dinuclear cobalt(II) complex, studied by a combination of magnetometry and spectroscopy. Fits of the data give D = −113 cm −1 for the zero-field splitting (ZFS) and J = 390 cm −1 for the metal–radical exchange. Ab initio investigations reveal first-order spin–orbit coupling of the quasi-degenerate $${{{{\rm{d}}}}}_{{x}^{2}-{y}^{2}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mrow> <mml:mi>d</mml:mi> </mml:mrow> <mml:mrow> <mml:msup> <mml:mrow> <mml:mi>x</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msup> <mml:mo>−</mml:mo> <mml:msup> <mml:mrow> <mml:mi>y</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:msub> </mml:math> and d x y orbitals to be at the heart of the large ZFS. The corresponding transitions are spectroscopically observed, as are transitions related to the exchange coupling. Finally, signatures of spin-phonon coupling are observed and theoretically analyzed. Furthermore, we demonstrate that the spectral features are not predominantly spin excitations, but largely vibrational in character.