Straightforward Synthesis of Donor‐Stabilised Phosphenium Adducts from Imidazolium‐2‐carboxylate and Their Electronic Properties

Abstract

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)

Donor‐Stabilized Phosphenium Adducts as New Efficient and Immobilizing Ligands in Palladium‐Catalyzed Alkynylation and Platinum‐Catalyzed Hydrogenation in Ionic Liquids

“…The formation of the phosphorus-carbon bond between the N-heterocycle and the phosphorus atom was confirmed by the presence of a doublet at 141.39 ppm in the 13 C NMR spectrum, with a typical coupling constant 1 J P,C of 47 Hz. [10,11] This observation, combined with a 2:1 ratio for furyl versus imidazolium groups in its proton NMR spectrum, unambiguously proved the formation of the expected ligand 3d.…”

“…[10] Thus, after addition of 3d to an equimolar amount of Ni(CO) 4 in a toluene/CH 2 Cl 2 solution, the related n CO (A 1 ) stretching wave number was observed at 2087 cm À1 in the infrared spectrum. This higher value categorizes 3d as being much less electron-donating than the common aryl tertiary phosphines [n CO (A 1 ) = 2069 cm À1 with PPh 3 ] and more precisely in the range of the tris-halogenophosphines, or of the weaker electron-donating phosphites like PA C H T U N G T R E N N U N G (OPh) 3 (2085 cm…”

“…[1,[7][8][9] In this context, we have recently described a simple and straightforward synthesis of imidazolium-2-phosphines 2 and 3 by addition of monochlorophosphines to the dimethylimidazolium-2-carboxylate 1 (see Scheme 1). [10] Besides, this synthesis provides access to new dicationic or mixed halide donor-stabilized phosphenium adducts by extension of the reactivity of 1 towards others chlorophosphines. [11] Moreover, the measurement of the A 1 symmetric stretching frequency of the related Ni(CO) 3 A C H T U N G T R E N N U N G (3a-c) complexes has shown that the ligands 2 and 3 behave as phosphite-like strong p-acceptors.…”

“…Due to the specific features of the donor-stabilized phosphenium adducts [10] the accumulation of a large excess of acidic pyrrolidinium bromide would be conducive to the C!P bond cleavage through the protonation of the carbenic 1,3-dimethylimidazol-2-ylidene of the ligand at the relatively high temperatures employed. The concomitantly formed phosphine halide being oxygen-sensitive, a mixture of the phosphine and its phosphine oxide is detected in the NMR spectra.…”

“…This new class of ligands is therefore of fundamental and applied interest in molecular catalysis with late transition metals. [10,13,14] Herein, we have further investigated their properties in two different catalytic reactions: namely the palladium-catalyzed aryl alkynylation of aryl bromides and the platinum-catalyzed hydrogenation of nitrobenzenes.…”