Bistability Of Magnetization Without Spin-transition In A High-spin Cobalt(ii) Complex Due To Angular Momentum Quenching

摘要

In this communication we describe a new mechanism that causes an abrupt change in the magnetic susceptibility of transition metal complexes through orbital quenching of the angular momentum, instead of controlling their spin state. Functional materials showing a sudden change in magnetic susceptibility, even hysteresis, associated to temperature change or other external perturbation have attracted great attention. Such materials, e.g., spin-crossover complexes and valence tautomeric complexes, are potentially useful as molecular memories, switches, or spintronics. The magnetization change is generally obtained by triggering a transition between different spin states, so the change of spin contribution to magnetic susceptibility is responsible for the control of magnetic properties. However, the magnetic susceptibility of transition metal compounds can also contain a nonvanishing contribution of orbital multiplicity. This contribution can be quenched by distorting the ligand field, which offers an alternative opportunity to alter magnetic properties. To demonstrate this approach, we studied various Co(II) complexes with nitrate ligands. High-spin cobalt(II) compounds often show a large orbital momentum partially quenched by spin-orbit coupling and the distortion of the ligand field. Nitrate ions can adopt different coordination modes: symmetric bidentate, asymmetric bidentate with a difference between the M-O(N) bond distances of 0.4-0.7 A, and monodentate coordination. Isomer-ization between these modes induced by external perturbation has also been reported; therefore nitrates can form a very flexible ligand environment. We found that [Co(NO_3)_2(L)] (L: 2,6-di(pyrazol-1-yl)pyrazine), 1, exhibits bistability connected to a phase transition that distorts the environment of the cobalt and alters the angular contribution to magnetic susceptibility.