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 °C), which can be applied to develop chemical sensors for pyridine utilizing color changes and/or magnetic switching.