Vapochromism and Magnetochemical Switching of a Nickel(II) Paddlewheel Complex by Reversible NH₃ Uptake and Release

Reaction of [NiCl₂ (PnH)₄ ] (1) (PnH=6-tert-butyl-pyridazine-3-thione) with NiCl₂ affords the binuclear paddlewheel (PW) complex [Ni₂ (Pn)₄ ] (2). Diamagnetic complex 2 is the first example of a PW complex capable of reversibly binding and releasing NH₃ . The NH₃ ligand in [Ni₂ (Pn)₄ (NH₃ )] (2⋅NH₃ ) enforces major spectroscopic and magnetic susceptibility changes, thus displaying vapochromic properties (λ_max (2)=532 nm, λ_max (2⋅NH₃ )=518 nm) and magnetochemical switching (2: S=0; 2⋅NH₃ : S=1). Upon repeated adsorption/desorption cycles of NH₃ the PW core remains intact. Compound 2 can be embedded into thin polyurethane films (2^P ) under retention of its sensing abilities. Therefore, 2 qualifies as reversible optical probe for ammonia. The magnetochemical switching of 2 and 2⋅NH₃ was studied in detail by SQUID measurements showing that in 2⋅NH₃ , solely the Ni atom coordinated the NH₃ molecule is responsible for the paramagnetic behavior.