Bistability of magnetization without spin-transition in a high-spin cobalt(II) complex due to angular momentum quenching.

[Co(NO(3))(2)L] (L: 2,6-di(pyrazol-1-yl)pyrazine) (1) exhibits an abrupt transition with hysteresis in magnetic susceptibility between 228 and 240 K. The results of spectroscopic and XRD measurements showed that 1 is in the high spin state in the whole temperature range. Therefore the observed hysteresis is not due to a spin transition but corresponds to a partial quenching of the angular momentum contribution to the magnetic susceptibility. Crystallographic measurements on the low- and high-temperature form of 1, combined with DFT calculations, showed that a symmetric twisting of the coordinating nitrate ions upon the transition is the most important factor in the orbital quenching mechanism. Utilizing such quenching to control magnetic properties can be a new approach to engineer transition metal complexes with magnetic functionalities without changing their spin or oxidation state.