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

The bidendate ligands HPnMe (4-methyl-6-tert-butyl-3(2H)-thio-pyridazine) and HPntBu (4,6-di-tert-butyl-3(2H)-thiopyridazine)were coordinated to Ni(II) for the formation of paddlewheel(PW) complexes [Ni2 (Pn R) 4 ]. Starting from [NiCl 2(HPn Me) 4] (1Me), the poorly soluble hexanuclear cluster [Ni6 (PnMe) 12] (2 Me 0) forms, which is capable of binding NH 3 reversibly. The obtained monomeric PW complex [Ni2 (Pn Me) 4 (NH3 )] (3 Me) shows vapochromism and magnetochemical switching. The third donor site in the pyridazine heterocycle PnMe is found to be a key factor for selective NH3 uptake due to the ideal position of these additional nitrogen atoms in the second coordination sphere to form hydrogen bonds to NH3. With excess NH3, the PW core of 3Me collapses, and the octahedral bis–ammine complex [Ni(PnMe) 2 (NH3) 2 ](4 Me) is formed. Upon loss of NH3 , 3Me is reformed. All three compounds 2Me 0, 3 Me and 4Me 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 emphasised, providing confirmation of the high selectivity of these systems for NH 3 . In contrast to HPn Me, with the bulkier ligand HPntBu, two HPntBu molecules coordinate to NiCl 2, giving the tetrahedral complex [NiCl 2(HPntBu) 2 ] (5tBu) while a PW com-plex was not observed.