Nickel(II) Aqua Complexes with Chelating Ligands: What Happens When Water Is Gone?

A series of hydrated Ni(II) ion complexes with 2-cyano-2-hydroxyimino-acetamide (HACO), hydroxyimino-pyridin-2-yl-acetonitrile (H2PCO), 2-cyano-2-hydroxyiminoacetic acid ethyl ester (HECO), 2-hydroxyimino-3-oxo-3-piperidin-1-yl-propionitrile (HPiPCO), and 2-cyano-2-hydroxyimino-N-pyrrolidin-acetamide (HPyrCO) ligands were synthesized and characterized as precursors for waterless Ni(II) coordination polymers. All these complexes have [NiL₂(H₂O)₂] composition with an axial position of coordinated water molecules and trans-geometry of the Ni(II) center, surrounded by two deprotonated cyanoxime anions forming five-membered chelate rings. Based on thermal analysis profiles, the hydrated complexes were dehydrated, and structural changes were investigated by spectroscopic magnetochemistry methods and X-ray analysis. The measurements suggested a rearrangement of the complexes upon dehydration, leading to the formation of polymeric chains of the oximato-μ²-bridged nickel(II) complexes. Significant color changes were observed during the process. Thus, in all cases, the Ni···O–N···Ni···O–N···motif with antiferromagnetically coupled Ni(II) centers was observed, instead of the formation of columnar 1D “poker chips” stacks with the formation of weak direct Ni···Ni contacts found in other oxime-based nickel compounds. Some of the obtained “staircase-type” anhydrous coordination polymers can rehydrate upon exposure to moisture, restoring again the spectroscopic characteristics of hydrated complexes. This indicates the reversibility of the process and confirms crystal lattice flexibility.