Cooperative Spin-State Switching and Vapochromism of Mononuclear Ni(II) Complexes by Pyridine Coordination/Decoordination.

Two mononuclear Ni(II) complexes ( and ) have been found to display color changes upon coordination/decoordination of pyridine, resulting in their structural transformation between square-planar and octahedral geometries as well as a change in their spin state. Compound changes between red () and yellow () upon exposure to or elimination of pyridine, while undergoes a two-step transformation, changing orange ( = 0) ⇄ gray ( = 1) → yellow ( = 1) depending on the reaction time. The first step ( → ) takes less than 45 min, which is significantly faster than the previously reported reaction time of 1 day for a Ni(II) complex/pyridine vapor system. Compound reacting with pyridine can be easily prepared by dispersing in methanol instead of annealing at high temperatures (130 °C), which can be applied to develop chemical sensors for pyridine utilizing color changes and/or magnetic switching.