Cationic imidazolium‐2‐phosphanes were obtained by the addition of a chlorophosphane (R2PCl, R = Ph, iPr or Cy) to 1,3‐dimethylimidazolium‐2‐carboxylate without the need for a purification step. An additional anion exchange reaction with KPF6 led to the corresponding halide‐free ligands in excellent yields. The molecular structure of one of them was examined both in the solid state and in solution. The lone pair of electrons on the phosphorus atom is not delocalised to the imidazolium fragment and thus remains available for further metal coordination. As such compounds can be described as phosphenium cations stabilised by a N‐heterocarbene donor base, the electronic properties of the Lewis acceptor phosphorus centre was evaluated upon nickel tricarbonyl coordination, and the properties are surprisingly similar to those found for phosphites. The stronger π‐acceptor character of these imidazolium‐2‐phosphanes relative to that of neutral tertiary phosphanes can thus explain the higher catalytic activities observed with the corresponding rhodium complexes in styrene hydroformylation reactions. Additionally, a preliminary study of platinum coordination chemistry indicated that the steric demand of 1,3‐dimethylimidazolium‐2‐phosphanes is comparable to mixed arylalkylphosphanes. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

The reaction between two 1,3-dialkylimidazolium-2-carboxylates 1a and 1b and two different dichlorophosphines (RPCl(2), with R = Ph and NEt(2)) led to new donor-stabilized phosphenium adducts. When the reaction was performed with the 1,3-dimethylimidazolium-2-carboxylate 1a and PhPCl(2) in a 2:1 ratio, the phosphine 4a, bearing two imidazolium moieties, was obtained and led to 5a, after an anion exchange reaction with KPF(6), the latter being fully characterized by an X-ray structure analysis. In similar conditions, the bis-imidazolium phosphine or phosphene-di-ium, 4b, which is analogous to 4a, has been obtained by the addition of PhPCl(2) to the 1-dodecyl-3-methylimidazolium-2-carboxylate 1b. However, by the use of dichloro(diethylamino)phosphine, (Et(2)N)PCl(2), instead of PhPCl(2), the reaction with 1a did not afford the biscationic phosphorus product 6a, an analogue to 4a, but, instead, the water-soluble mixed mono-imidazolium chlorophosphine 7a. Subsequently, additional kinetic experiments have been investigated to rationalize the different reactivities observed with imidazolium-2-carboxylates and the phosphorus halide derivatives. We, thus, found that the bis-imidazolium phosphine 4b was very rapidly formed in the above-mentioned reaction and was slowly converted, thereafter, back to the mixed mono-imidazolium chlorophosphine 8b in the presence of the residual starting dichlorophosphine. Additionally, the addition of PhPCl(2) to the phosphene-di-ium 4b represents, to our knowledge, the first example of a P-substituent exchange reaction involving a P-C bond formation in imidazolium phosphines. On the other hand, the air stability and the solubility of these new cationic functional phosphines in different media render such ligands very appealing in coordination chemistry for catalysis in mono- or biphasic media.

Abstract:The straightforward synthesis of a new donor-stabilized phosphenium ligand 3d by addition of bromodifurylphosphine to 1,3-dimethylimidazolium-2-carboxylate 1 is described. The obtained ligand exhibits a very strong p-acceptor character, comparable to that of triphenyl phosphite [PA C H T U N G T R E N N U N G (OPh) 3 ] or of tris-halogenophosphines, with a n CO (A 1 ) at 2087 cm À1for its nickel tricarbonyl complex. This ligand, as well as the related 3a which was obtained from chloroA C H T U N G T R E N N U N G diphenylphosphine, were tested in palladiumcatalyzed aryl alkynylation and in the platinum-catalyzed selective hydrogenation of chloronitrobenzenes, both in an ionic liquid phase. In C À C bond cross-coupling we observed that the increase of the p-acceptor character in ligand 3d, due to the introduction of an additional electron-withdrawing group, provides a very efficient catalyst in the alkynylation reaction of aryl bromides with phenylacetylene, including the deactivated 4-bromoanisole or the sterically hindered 2-bromonaphthalene. The catalytic activity decreases with recycling due to the sensitiveness of ligands to protonation in the ionic phase.Conversely, a multiple recycling of the metal/ligand system in non-acidic media was achieved from platinum-catalyzed hydrogenation of m-chloronitrobenzene. The catalytic results obtained by employing the complex of platiA C H T U N G T R E N N U N G num(II) chloride with 3a [transPtCl 2 (3a) 2 ] in comparison with the non-ionic related trans-tris(triphenylphosphine)platinum dichloride [trans-PtCl 2 A C H T U N G T R E N N U N G (PPh 3 ) 2 ] complex clearly indicate that the simultaneous existence of a strong p-acceptor character and a positive charge within the ligand 3a significantly increases the life-time of the platinum catalyst. The selectivity of the reaction is also improved by decreasing the undesirable formation of dehalogenation products. This cationic platinum complex trans-PtCl 2 (3a) 2 is the first example of a highly selective catalyst for hydrogenation of chloronitroarenes immobilized in an ionic liquid phase. The system was recycled six times without noticeable metal leaching in the organic phase, and no loss of activity.

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The cover picture shows one of donor‐stabilized phosphenium compounds which have been obtained by direct addition of chlorophosphanes to the 1,3‐dimethylimidazolium‐2‐carboxylate without any further purification step. Their stronger κ‐acceptor character similar to phosphites and their ionic nature render these ligands very promising in the development of new continuous‐flow catalytic processes. Details of phosphorylation mechanisms of imidazolium‐2‐carboxylate as well as the electronic and steric properties of these adducts are discussed in the article by J. Andrieu et al. on p. 4877 ff.

Zinc oxide (ZnO) nanoparticles (NPs) were synthesized using the coprecipitation method. The resulting NPs were characterized by X‐ray diffraction, transmission electron microscopy, and Fourier transform infrared spectroscopy. The surface morphology of the coatings has been characterized by scanning electron microscopy (SEM). SEM images show a uniform and homogeneous distribution of ZnO‐NPs in polyurethane (PU) coatings while aggregates appear in epoxy coatings. Using tensile stress–strain test, the effect of the addition of ZnO‐NPs on the mechanical properties of PU and epoxy coatings was examined. The addition of ZnO‐NPs has enhanced the Young's modulus and the tensile strength of PU and epoxy coatings. Through the open‐circuit potential and electrochemical impedance spectroscopy measurements, the anticorrosive properties of PU‐ or epoxy‐coated mild steel panels containing ZnO‐NPs have been investigated in 0.5 M NaCl solution. Nyquist impedance plots for PU and epoxy coatings in PU‐ or epoxy‐coated mild steel panels containing ZnO‐NPs were tested in the absence and presence of ZnO‐NPs have different responses. It has been shown that the dispersion of ZnO‐NPs significantly improves the anticorrosive performance of PU coatings. A PU coating containing 700 mg/kg ZnO‐NPs is recommended to achieve better mechanical properties and corrosion resistance.

Catalysis O 0020Donor-Stabilized Phosphenium Adducts as New Efficient and Immobilizing Ligands in Palladium-Catalyzed Alkynylation and Platinum-Catalyzed Hydrogenation in Ionic Liquids. -Donor-stabilized phosphenium salts are successfully applied as ligands in the Pd-catalyzed Heck arylation of terminal alkyne (II) with aryl bromides. Best results are achieved for the ligand (DFIP) bearing two electron-withdrawing furyl groups. The recyclability, however, is limited due to protonation of the C-P bond of the ligand under acidic conditions. In contrast, the complex formed from PtCl2 and the diphenylphosphenium ligand (DPIP) shows excellent stability and chemoselectivity in the hydrogenation of m-chloronitrobenzene (IV). It can be recycled and reused for at least seven runs without significant loss of activity or chemoselectivity. -(SALEH, S.; FAYAD, E.; AZOURI, M.; HIERSO*, J.-C.; ANDRIEU, J.; PICQUET, M.; Adv. Synth. Catal. 351 (2009) 10, 1621-1628; Inst. Chim. Mol., Univ. Bourgogne, F-21078 Dijon, Fr.; Eng.) -Mischke 49-025