Reactions of N-heterocyclic carbenes (NHCs) with one-electron oxidants: possible formation of a carbene cation radical

One-electron oxidation of N-heterocyclic carbenes (NHCs) has been carried out using oxidising agents such as tetracyanoethylene (TCNE) and ferrocenium [Cp(2)Fe](+); the formation of carbene radical cations is postulated.     

Antibacterial properties of ruthenium(II) benzamide complexes

Reactions of the new acyclic ligand DNBH with RuCl₃ · 3H₂O, followed by addition of a secondary ligand L (L = PPh₃, 1,10-phenanthroline, 2,2-bipyridine, pyridine and 2,4-diaminotoluene), yield six binuclear metal complexes, TR1–TR6. Two different methods were employed: template and a two-step synthesis, both yielding the same complexes. DNBH and its metal complexes were characterised by a combination of spectroscopic, elemental and magnetic susceptibility data. Coordination was found to be through the carbonyl oxygen of amide and phenolic oxygen in the octahedral environment of the metal. DNBH and some of the metal complexes display antibacterial properties.     

Carbon-centered strong bases in phosphonium ionic liquids

Phosphonium ionic liquids (PhosILs), most notably tetradecyl(trihexyl)phosphonium decanoate (PhosIL-C(9)H(1)9COO), are solvents for bases such as Grignard reagents, isocyanides, Wittig reagents (phosphoranes), and N-heterocyclic carbenes (NHCs). The stability of the organometallic species in PhosIL solution is anion dependent. Small bases, such as hydroxide, react with the phosphonium ions and promote C-H exchange as suggested by deuterium-labeling studies. A method to dry and purify the ionic liquids is described and this step is important for the successful use of basic reagents in PhosIL. NHCs have been generated in PhosIL, and these persistent solutions catalyze organic transformations such as the benzoin condensation and the Kumada-Corriu cross-coupling reaction. Phosphoranes were generated in PhosIL, and their reactivity with various organic reagents was also tested. Inter-ion contacts involving tetraalkylphosphonium ions have been assessed, and the crystal structure of [(n-C(4)H(90)(4)P][CH(3)CO(2).CH(3)CO(2)H] has been determined to aid the discussion. Decomposition of organometallic compounds may also proceed through electron-transfer processes that, inter alia, may lead to decomposition of the IL, and hence the electrochemistry of some representative phosphonium and imidazolium ions has been studied. A radical derived from the electrochemical reduction of an imidazolium ion has been characterized by electron paramagnetic resonance spectroscopy.     

Grignard reagents in ionic solvents: electron transfer reactions and evidence for facile Br-Mg exchange

Grignard reagents form persistent solutions in phosphonium ionic liquids possessing O-donor anions and these solutions are excellent reaction media for electron transfer processes and transmetallation reactions.     

From the reactivity of N-heterocyclic carbenes to new chemistry in ionic liquids

N-Heterocyclic carbenes have numerous applications in synthetic chemistry. We detail the reactivity and chemistry of these molecules including investigations into their reactions with small reagents, their use for the preparation of polarised azines and their potential application as NLO materials. The chemistry of imidazolium salts, which are related to NHCs by the addition of a proton, is also discussed. New chemistry for ionic liquids is also revealed.     

Non-classical NHC transfers from the reaction of (IMes)AgCl with osmium carbonyl clusters

N-Heterocyclic carbene transfer reactions were attempted using IMesAgCl and two osmium clusters. The products isolated from these reactions suggest that NHC transfers can be unpredictable.     

A monomeric imidazol-2-ylidene-silver(I) chloride complex: synthesis,structure, and solid state 109Ag and 13C CP/MAS NMR characterization

The structure of 1,3-bis(2,4,6-trimethylphenyl)-imidazol-2-ylidene-silver(I) chloride, 1, has been determined to be a monomer with weak head-to-tail H...Cl interactions in the solid state. A multinuclear NMR study using a (13)C(carbene) labeled derivative, 1((13)C), exhibits (13)C-(107,109)Ag coupling in solution. Further, the solid state CP/MAS NMR parameters, including the principal components of the chemical shift tensors for both the (13)C and (109)Ag centers, have been determined. With the aid of DFT calculations, the orientation of the chemical shift tensors have been assigned.     

Electrochemical reduction of an imidazolium cation: a convenient preparation of imidazol-2-ylidenes and their observation in an ionic liquid

1,3-Bis(2,4,6-trimethylphenyl)imidazolium chloride is reduced electrochemically and chemically to produce a nucleophilic carbene, namely 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene. The carbene was also shown to be compatible with and persistent in the ionic liquid tetradecyl(trihexyl)phosphonium chloride.     

The reactions of imidazol-2-ylidenes with the hydrogen atom: a theoretical study and experimental confirmation with muonium

The possible radicals resulting from hydrogen atom addition to the imidazole rings of 1,3-bis(isopropyl)-4,5-dimethylimidazol-2-ylidene (1) and 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (2) have been studied by means of density functional calculations (B3LYP). The calculations included solvent effects estimated via the polarized continuum model (PCM) and an empirical treatment of vibrational averaging of hyperfine constants. Addition of a hydrogen (or muonium) atom to the carbeneic carbon of 1,3-bis(isopropyl)-4,5-dimethylimidazol-2-ylidene was found to give a radical 60.46 kJ mol(-)(1) more stable than the radical resulting from addition to the double bond. Estimation of the activation barriers for reaction at the two sites shows that addition at the carbeneic carbon is favored. The site of addition was confirmed experimentally using muonium (Mu), which can be considered a light isotope of hydrogen. Muon spin rotation and muon level-crossing spectroscopy were used to determine muon, (13)C, and (14)N hyperfine coupling constants (hfc's) for the radical products of addition to the two carbenes. Good agreement between the experimental and calculated hfc's confirms that Mu (and hence H) adds exclusively to the carbeneic carbon. The radicals that are produced have nonplanar radical centers with most of the unpaired electron spin density localized on the alpha-carbon.     

Polymorphism of Zn[Au(CN)2]2 and its luminescent sensory response to NH3 vapor

Four polymorphic forms of the complex Zn[Au(CN)2]2 have been synthesized and both structurally and spectroscopically characterized. In each of the four polymorphs, a zinc center in a tetrahedral geometry with a Au(CN)2(-) unit at each tetrahedral vertex is observed. All four structures contain three-dimensional networks based on corner-sharing tetrahedra. Because of the long Au(CN)2(-) bridging unit, the extra space not occupied by one network is filled by two to five additional interpenetrated networks. Short gold-gold bonds with lengths ranging from 3.11 to 3.33 A hold the interpenetrated networks together. Three of the four polymorphs are luminescent, having solid-state emissions with wavelengths ranging from 390 to 480 nm. A linear correlation between the emission energy and the gold-gold distance was observed. Upon exposure to ammonia vapor, the polymers altered their structures and emission energies, with the emission wavelength shifting to 500 nm for {Zn(NH3)2[Au(CN)2]2}, which adopts a two-dimensional layer structure with octahedral, trans-oriented NH3 groups. The adsorption route is dependent on the polymorph used, with NH3 detection limits as low as 1 ppb. Desorption of the ammonia occurred over 30 min at room temperature.