Remarkable Structural Diversity of N-Pyrrolidine-cyanoxime and its Ni(II) Complexes

Abstract

A novel cyanoxime, 2-oximino-2-cyano-N-pyrrolidine-acetamide (further HPyrCO), was synthesized and characterized by spectroscopic methods, thermal analysis, and X-ray crystallography. The structure revealed eight independent molecules with elaborate intermolecular H-bonding in the asymmetric unit and adoption of the trans-anti configuration of the cyanoxime. Henceforth, a series of Ni(II) coordination compounds, crystallohydrates, with this new cyanoxime was obtained from aqueous solutions, with four complexes isolated from the same system after filtration of the main product and subsequent crystallization of new products from the mother liquor. These were RED, the main productouter-sphere solvent molecules of crystallization and were obtained as the result of direct interaction of starting compounds in narrow tubes. All isolated complexes were characterized using IR, solid-state UV−visible spectroscopy, TG/DSC analysis, and X-ray crystallography. Data showed the formation of five-membered chelate rings in all structures and adoption of the trans-geometry of cyanoxime in all crystallohydrate complexes. Heating of the RED, GREEN, and VIOLET crystallohydrates led to irreversible dehydration and the formation of an anhydrous brown complex, [Ni(PyrCO) 2 ]. Variable temperature magnetochemical studies of RED trihydrate confirmed a lack of interactions between isolated Ni(II) centers, while there was an antiferromagnetic interaction between metal ions in the linear chain in the polymeric anhydrous compound. Full line shape analysis of solid-state electronic spectra allowed calculation of the Racah Repulsion Parameter (B) and Nephelauxetic Constant (β). In three crystallohydrates, there were six-coordinated high-spin Ni 2+ complexes with different degrees of rhombic (with axial elongation) distortions and, therefore, ligands' crystal field strength. The final product of the thermal decomposition of all complexes under inert gas protection was metallic Ni sponge.