Chiral N-heterocyclic biscarbenes based on 1,2,4-triazole as ligands for metal-catalyzed asymmetric synthesis

A facile and straightforward synthetic procedure for the preparation of new chiral bis-1,2,4-triazolium salts and their corresponding rhodium(I) biscarbene complexes are reported. It is widely applicable for the synthesis of 1,2,4-triazolium salts. The new chiral biscarbenes represent promising ligands for transition metal-catalyzed asymmetric syntheses. Their first successful application is demonstrated by the rhodium-catalyzed hydrogenation of dimethylitaconate and methyl-2-acetamidoacrylate which yielded enantioselectivities of up to 61% ee.     

Interactions and stability of silver nanoparticles in the aqueous phase: Influence of natural organic matter (NOM) and ionic strength

The rapid development of nanotechnology and the related production and application of nanosized materials such as engineered nanoparticles (ENP) inevitably lead to the emission of these products into environmental systems. So far, little is known about the occurrence and the behaviour of ENP in environmental aquatic systems. In this contribution, the influence of natural organic matter (NOM) and ionic strength on the stability and the interactions of silver nanoparticles (n-Ag) in aqueous suspensions was investigated using UV–vis spectroscopy and asymmetrical flow field-flow fractionation (AF⁴) coupled with UV–vis detection and mass spectrometry (ICP-MS). n-Ag particles were synthesized by chemical reduction of AgNO₃ with NaBH₄ in the liquid phase at different NOM concentrations. It could be observed that the destabilization effect of increasing ionic strength on n-Ag suspensions was significantly decreased in the presence of NOM, leading to a more stable n-Ag particle suspension. The results indicate that this behaviour is due to the adsorption of NOM molecules onto the surface of n-Ag particles (“coating”) and the resulting steric stabilization of the particle suspension. The application of AF⁴ coupled with highly sensitive detectors turned out to be a powerful method to follow the aggregation of n-Ag particle suspensions at different physical–chemical conditions and to get meaningful information on their chemical composition and particle size distributions. The method described will also open the door to obtain reliable data on the occurrence and the behaviour of other ENP in environmental aquatic systems.     

Organic-inorganic nanotube hybrids: Organosilica-nanotubes containing ethane, ethylene and acetylene groups

The synthesis and characterization of novel organic-inorganic hybrid nanotubes containing silica and ethane (EtSNT), ethylene (ESNT) and acetylene (ASNT) units are presented. The unsaturated hydrocarbon linkers can be applied for a chemical functionalization of the surface of the silica nanotubes, e.g. for the grafting of organic/inorganic or organometallic molecules. These new materials are synthesized via a template method using V₃O₇·H₂O fibers and characterized by solid state MAS-NMR, FT-IR, TEM and elementary analysis. In a first approach to potential applications of these materials, the grafting of glycine on ESNT by treatment of the CC groups with Br₂ and subsequent substitution by glycine is investigated. The resulting material is characterized by solid state MAS-NMR, FT-IR, XRD, TGA, BET adsorption measurements and elementary analysis.     

Symmetrically bridged bis-N-heterocyclic carbene rhodium (I) complexes and their catalytic application for transfer hydrogenation reaction

Bridged rhodium(I) bis(NHC) complexes of the formula [bis-(NHC)Rh(I)PF₆] (1c-5c) were synthesized and applied as catalysts in the transfer hydrogenation of acetophenone in 2-propanol. The activity of the rhodium(I) complexes largely depends on the nature of the N-substituents and the applied bases. The synthesized compounds were characterized by elemental analysis, ¹H and ¹³C NMR-spectroscopy and mass spectrometry. The structure of complex 2c was exemplary determined by X-ray analysis.     

The electrochemistry of particles, droplets, and vesicles �C the present situation and future tasks

Presently, a plethora of techniques is available to study the electrochemical properties of solid inorganic and organic micro- and nano-particles immobilized on electrode surfaces, provided that they possess a faradaic electroactivity. Similarily, immobilized droplets of liquids and solutions, which are immiscible with the electrolyte solution, give access to the three-phase electrochemistry of redox centers in the droplets, allowing determinations of free energies of ion transfer between the immiscible liquid phases. Possible and necessary future activities in the field of immobilized particles and droplets will be discussed here. The electrochemistry of suspended micro- and nano-particles possessing faradaic electroactivity is much more complex and needs special attention in future research. Finally, the electrochemistry of liposomes and biological vesicles, which do not possess faradaic activity, but the ability to produce capacitive signals upon attachment to electrodes, will be discussed focusing on possible future developments.     

Irreversible electrostatic deposition of Prussian blue from colloidal solutions

Prussian blue (PB) can be deposited from colloidal solutions (5.4?×?10^?3?mol_PB?L^?1, 0.01?mol?L^?1 KNO₃) on glassy carbon, either by potential cycling or potentiostatically, provided that the deposition potential is more positive than ?0.2?V vs. Hg/Hg₂Cl₂. Depending on the deposition potential, the PB particles form either a single layer of Everitt??s salt, of PB, or multilayers of Berlin green. Also depending on the electrode potential, the deposition was accompanied by currents which were either only of capacitive nature, or represent the sum of capacitive and faradaic currents. The currents were always limited by the diffusion of the colloidal particles to the electrode surface, i.e., they obeyed the Cottrell equation. The PB layers were characterized by in situ atomic force microscopy.     

The growth of single crystal silver wires at the nitrobenzene|water interface

Single crystal silver wires can be grown at the nitrobenzene|water interface when silver ions dissolved in the aqueous phase are reduced by decamethyl ferrocene dissolved in the nitrobenzene phase. The successful growth of these wires depends on a number of experimental conditions, most prominently on the concentration ratio of reactants, nucleation rates, shape of formed nuclei, and wettability of nuclei. The size-time dependence can be modeled on the basis of microelectrode behavior of the silver nuclei and wire. AFM, SEM, light microscopy and single crystal X-ray diffraction has been applied to study the morphology of the silver nuclei and wires.     

From molecules to materials: molecular paddle-wheel synthons of macromolecules, cage compounds and metal-organic frameworks

Metal-organic frameworks (MOF) are becoming a more and more important class of functional materials. Yet, very often, the synthesis of MOFs is not easy to control and requires a profound knowledge and experience in solid state chemistry. One of the most frequently used metal connectors is the so-called 'paddle-wheel' (PW) unit, which is a well-known molecular compound type in inorganic coordination chemistry. Depending on the ligands, the geometry of PWs strictly directs the assembly of ordered networks. This review focuses on the question, to what extent ordered network structures can be accessed by typical molecular syntheses in solution, starting from molecular PW complexes to ordered macromolecules, finite cage compounds and finally, three-dimensional superstructures.     

Energy-level tailoring in a series of redox-rich quinonoid-bridged diruthenium complexes containing tris2-pyridylmethyl)amine as a co-ligand

Reactions of [{Ru(tmpa)}₂(μ‐Cl)₂][ClO₄]₂, ( 2 [ClO₄]₂, tmpa=tris(2‐pyridylmethyl)amine) with 2,5‐dihydroxy‐1,4‐benzoquinone ( L¹ ), 2,5‐di‐[2,6‐(dimethyl)‐anilino]‐1,4‐benzoquinone ( L² ), or 2,5‐di‐[2,4,6‐(trimethyl)‐anilino)]‐1,4‐benzoquinone ( L³ ) in the presence of a base led to the formation of the dinuclear complexes [{Ru(tmpa)}₂(μ‐ L¹ _{?2 H})][ClO₄]₂ ( 3 [ClO₄]₂), [{Ru(tmpa)}₂(μ‐ L² _{?2 H})][ClO₄]₂ ( 4 [ClO₄]₂), and [{Ru(tmpa)}₂(μ‐ L³ _{?2 H})][ClO₄]₂ ( 5 [ClO₄]₂). Structural characterization of 5 [ClO₄]₂ showed the localization of the double bonds within the quinonoid ring and a twisting of the mesityl substituents with respect to the quinonoid plane. Cyclic voltammetry of the complexes show two reversible oxidation and quinonoid‐based reduction processes. Results obtained from UV/Vis/NIR and EPR spectroelectrochemistry are invoked to discuss ruthenium‐ versus quinonoid‐ligand‐centered redox activity. The complex 3 [ClO₄]₂ is compared to the reported complex [{Ru(bpy)}₂(μ‐ L¹ _{?2 H})]²⁺ ( 1²⁺ , bpy=2,2′‐bipyridine). The effects of substituting the bidentate and better π‐accepting bpy co‐ligands with tetradentate tmpa ligands [pure σ‐donating (amine) as well as σ‐donating and π‐accepting (pyridines)] on the redox and electronic properties of the complexes are discussed. Comparisons are also made between complexes containing the dianionic forms of the all‐oxygen‐donating L¹ ligand with the L² and L³ ligands containing an [O,N,O,N] donor set. The one‐electron oxidized forms of the complexes show absorption in the NIR region. The position as well as the intensity of this band can be tuned by the substituents on the quinonoid bridge. In addition, this band can be switched on and off by using tunable redox potentials, making such systems attractive candidates for NIR electrochromism.