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

Two mononuclear Ni(II) complexes (1 and 2) 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 1 changes between red (1r) and yellow (1y) upon exposure to or elimination of pyridine, while 2 undergoes a two-step transformation, changing orange 2o (S = 0) ? gray 2g' (S = 1)-* yellow 2y' (S = 1) depending on the reaction time. The first step (2o-* 2g') 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 2o reacting with pyridine can be easily prepared by dispersing 2g in methanol instead of annealing at high temperatures (130 degrees C), which can be applied to develop chemical sensors for pyridine utilizing color changes and/or magnetic switching.