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

Abstract

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.

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

“…The presence of the cyano-group lowers the p K a of cyanoximes to the factor of 10 3 –10 5 , as compared to conventional oximes or dioximes, making them very good ligands. Yellow anionic cyanoximes represent excellent polydentate ligands for coordination and organometallic chemistry, − forming numerous complexes with main group − and transition metals. − Chelating coordination most often involves the formation of five- or six-membered metallocycles with N′,N″, N,O , or N,S , donor atoms sets, depending on the particular cyanoxime used. In complexes with transition metals, the bis -chelating function is accompanied by the axial coordination of water molecules which makes these complexes well suited for the formation of 1D coordination polymers of the I or III type, as well as dimeric structures II (Scheme ).…”

1D Polymeric Platinum Cyanoximate: A Strategy toward Luminescence in the Near-Infrared Region beyond 1000 nm

“…Among ambidentate ligands, oximes used in this work take a special place because of their strong affinity for nickel triad metals that favor square-planar geometry. − On the basis of our previous studies, − we selected cyanoximes ,− as ligands for binding Pt II centers. Cyanoximes are particularly flexible mixed-donor ligands that allow modular design and are the platform of choice for developing metal complexes with controlled self-assembly in solutions and the solid state. − …”

“…We also rationalized that the anion’s bulkiness from substituents would control the distance between the metal centers. Therefore, among known cyanoxime ligands, those with amide groups provide a greater variety of electronic and steric properties and solubility. − Thus, novel bivalent palladium and platinum complexes were synthesized using a cyanoxime ligand well soluble in organic solvents, 2-cyano-2-oximino- N , N ′-diethylaminoacetamide [HDECO, 1 ]. As we demonstrate below, the platinum complex self-assembles in solutions into a luminescent 1D polymeric “poker-chips” structure.…”