Hydrogen Bonding is Key Factor in Reversible NH₃ Uptake Causing Vapochromic and Magnetochemical Switching in Ni(II) Paddlewheel Complexes

The bidendate ligands H Pn Me (4‐methyl‐6‐ tert ‐butyl‐3( 2H )‐thiopyridazine) and H Pn t Bu (4,6‐di‐ tert ‐butyl‐3(2 H )‐thiopyridazine) were coordinated to Ni(II) for the formation of paddlewheel (PW) complexes [Ni 2 ( Pn R ) 4 ]. Starting from [NiCl 2 (H Pn Me ) 4 ] ( 1 Me ), the poorly soluble hexanuclear cluster [Ni 6 ( Pn Me ) 12 ] ( 2 Me′ ) forms which is capable of binding NH 3 reversibly. The obtained monomeric PW complex [Ni 2 ( Pn Me ) 4 (NH 3 )] ( 3 Me ) shows vapochromism and magnetochemical switching. The third donor site in the pyridazine heterocycle Pn Me is found to be a key factor for selective NH 3 uptake due to the ideal position of these additional nitrogen atoms in the second coordination sphere to form hydrogen bonds to NH 3 . With excess NH 3 the PW core of 3 Me collapses, and the octahedral bis–ammine complex [Ni( Pn Me ) 2 (NH 3 ) 2 ] ( 4 Me ) is formed. Upon loss of NH 3 , 3 Me is reformed. All three compounds 2 Me ′ , 3 Me and 4 Me have a very different color in solution or suspension, which allows for basic sensing properties. Quantum mechanical calculations on complexes clarify the preference for the N site over sulfur in coordinating small molecules. The influence of hydrogen bonding on this preference is quantitatively emphasized, providing confirmation of the high selectivity of these systems for NH 3 . In contrast to H Pn Me , with the bulkier ligand H Pn t Bu , two H Pn t Bu molecules coordinate to NiCl 2 , giving the tetrahedral complex [NiCl 2 (H Pn t Bu ) 2 ] ( 5 t Bu ) while a PW complex was not observed.