Different hydrogen-bonding modes in two closely related oximes

The first bis-cyanoxime: synthesis and properties of a new versatile and accessible polydentate bifunctional building block for coordination and supramolecular chemistry

A new multidentate bifunctional organic ligand – di-N,N′-(2-cyano-2-oximinoacetyl)piperazine – was synthesized in high yield using a two-step procedure carried out under ambient conditions. At first, the reaction of piperazine and neat methylcyanoacetate led to the di-N,N′-(cyanoacetyl)piperazine (1), which then was converted into bis-cyanoxime, di-N,N′-(2-cyano-2-oximinoacetyl)piperazine (HL, 2) using a room temperature nitrosation reaction with gaseous methylnitrite. Synthesized bis-cyanoxime was characterized by 1H, 13C NMR, UV-visible, IR spectroscopy and the X-ray analysis. The ligand 2 exists as a mixture of three diastereomers arising from the syn- and anti-geometry of the cyanoxime group. The prolonged crystallization of 2 from an ethanol–water mixture leads to the formation of: (a) colorless crystals that according to the X-ray analysis contain a 51.2:48.8% co-crystallized mixture of both isomers that have the same H-bonding motif (minority), and (b) a white amorphous material that represents an almost pure anti-isomer (majority). The deprotonation of 2 leads to the formation of a yellow dianion that demonstrated pronounced solvatochromism of its n → π* transition in the nitroso-chromophore. The disodium salt Na2L·4H2O (3) was obtained from 2 using NaOC2H5 in ethanol. The new bis-cyanoxime 2 reacts with Tl2CO3 and AgNO3 in aqueous solutions with the formation of light-stable, sparingly soluble yellow precipitates of M′2L·xH2O composition (M′ = Tl, Ag; Tl = 4, x = 0; Ag = 5, x = 2). The reaction of 3 with Ni2+ or K2M′′Cl4 (M′′ = Pd, Pt) in aqueous solutions leads to NiL·4H2O (6), PdL·4H2O (7) and PtL·5H2O (8). The crystal structure of 4 was determined and revealed the formation of a 3D-coordination polymeric complex in which the bis-cyanoxime acts as a dianionic, bridging, formally decadentate ligand. Each Tl(I) center has two bonds (2.655, 2.769 Å), shorter than the sum of ionic radii Tl–O (oxime group), and three longer, >2.89 Å, mostly electrostatic Tl···O contacts, involving oxygen atoms of the amide-group and the oxime-group of neighboring units. Among several possible binding modes, the coordination of the bis-cyanoxime dianion of 2 adopted in complex 4 is unusual, and evidenced its great potential as a versatile building block for coordination and supramolecular chemistry.