Experimental and Computational Investigation of the Oxime Bond Stereochemistry in c-Jun N-terminal Kinase 3 Inhibitors 11H-Indeno[1,2-b]quinoxalin-11-one Oxime and Tryptanthrin-6-oxime

11H-Indeno[1,2-b]quinoxalin-11-one oxime (IQ-1) and tryptanthrin-6-oxime are potent c-Jun N-terminal kinase 3 (JNK-3) inhibitors demonstrating neuroprotective, anti-inflammatory and anti-arthritic activity. However, the stereochemical configuration of the oxime carbon-nitrogen double bond (E- or Z-) in these compounds was so far unknown. In this contribution, we report the results of the determination of the double bond configuration in the solid state by single crystal X-ray diffraction and in solution by 1D and 2D NMR techniques and DFT calculations. It was found that both in the solid state and in solution, IQ-1 adopts the E-configuration stabilized by intermolecular hydrogen bonds, in contrast to previously assumed Z-configuration that could be stabilized only by an intramolecular hydrogen bond.

1D polymeric platinum cyanoximate: a strategy toward luminescence in the near-infrared region beyond 1000 nm

We report the synthesis and properties of the first representative of a new class of PtL2 complexes with ambidentate mixed-donor cyanoxime ligands [L = 2-cyano-2-oximino-N,N'-diethylaminoacetamide, DECO (1)]. Three differently colored polymorphs of "Pt(DECO)2" (3-5) were isolated, with the first two being crystallographically characterized. The dark-green complex [Pt(DECO)2]n (5) spontaneously forms in aqueous solution via aggregation of yellow monomeric complex 3 into the red dimer [Pt(DECO)2]2 (4), followed by further oligomerization into coordination polymer 5. A spectroscopic and light-scattering study revealed a "poker-chips"-type 1D polymeric structure of 5 in which units are held by noncovalent metallophilic interactions, forming a Pt---Pt wire. The polymer 5 shows a broad absorption at 400-900 nm and emission at unusually long wavelengths in the range of 1000-1100 nm in the solid state. The near-infrared (NIR) emission of polymer 5 is due to the formation of a small amount of nonstoichiometric mixed-valence Pt(II)/Pt(IV) species during synthesis. A featureless electron paramagnetic resonance spectrum of solid sample 5 recorded at +23 and -193 °C evidences the absence of Pt(III) states, and the compound represents a "solid solution" containing mixed-valence Pt(II)/Pt(IV) centers. Exposure of KBr pellets with 5% 5 to Br2 vapors leads to an immediate ∼30% increase in the intensity of photoluminescence at 1024 nm, which confirms the role and importance of mixed-valence species for the NIR emission. Thus, the emission is further enhanced upon additional oxidation of Pt(II) centers, which improves delocalization of electrons along the Pt---Pt vector. Other polymorph of the "Pt(DECO)2" complex--monomer--did not demonstrate luminescent properties in solutions and the solid state. An excitation scan of 5 embedded in KBr tablets revealed an emission only weakly dependent on the wavelength of excitation. The NIR emission of quasi-1D complex 5 was studied in the range of -193 to +67 °C. Data showed a blue shift of λmax and a simultaneous increase in the emission line intensity with a temperature rise, which is explained by analogy with similar behavior of known quasi-1D K2[Pt(CN)4]-based solids, quantum dots, and quantum wells with delocalized carriers. The presented finding opens a route to a new class of platinum cyanoxime based NIR emissive complexes that could be used in the design of novel NIR emitters and imaging agents.

Synthesis and Characterization of Two Cyanoxime Ligands, Their Precursors, and Light Insensitive Antimicrobial Silver(I) Cyanoximates

High-yield syntheses of N-piperidine-cyanacetamide (1), N-morpholyl-cyanacetamide (4) and their oxime derivatives N-piperidine-2-cyano-2-oximino-acetamide (HPiPCO, 2) and N-morpholyc-2-cyano-2-oximino-acetamide (HMCO, 5) were developed using two-step preparations. At first, the reactions of neat cyanoacetic acid esters and the respective cyclic secondary amines such as piperideine and morpholine afforded pure cyanacetamides, which were converted into cyanoximes at room temperature using the nitrosation reaction with gaseous CH3ONO. The synthesized compounds were investigated by means of IR, 1H, 13C and UV-visible spectroscopy. Crystal structures of two starting substituted cyan-acetamides and two target cyanoximes were determined. Silver(I) complexes of AgL composition (L = PipCO, 3; MCO, 6) were prepared in high yield. Both metal complexes are thermally stable above 100oC, and remarkably stable to high intensity visible light. The stability of dried AgL compounds towards short wavelength UV-radiation (a frequently used germicidal light) was examined using diffusion reflectance spectroscopy. Both complexes demonstrate slow photoreduction within ~3 hrs, observable as a gradual color change and darkening due to the formation of fine (nano-scale) particles of metallic silver. The complex Ag(MCO), 6, is about 2.6 times less stable towards UV-radiation than its more lypophyllic analog Ag(PipCO), 3. Antimicrobial and biofilm growth inhibition properties of the prepared solid acrylate-based polymeric composites containing embedded silver(I) cyanoximates were investigated using three human pathogens: P. aeruginosa PAO1 (wound isolate), S. aureus NRS70 (methicillin resistant respiratory isolate), and S. mutans UA159 (cariogenic dental isolate). Studies showed that both 3 and 6 compounds completely abolished the growth of PAO1 at 0.5 weight % concentration, and the growth of UA159 and NRS70 at 1% concentration. Moreover, data demonstrates that complexes 3 and 6 also inhibit both planktonic and biofilm growth of Gram-positive and Gram-negative bacterial pathogens. The demonstrated thermal stability and pronounced antimicrobial activity of both silver(I) cyanoximates indicates the strong potential for the studied complexes to be used as light insensitive antimicrobial additives to light-curable adhesives that set indwelling devices in place.

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.