Straightforward Synthesis of Donor‐Stabilised Phosphenium Adducts from Imidazolium‐2‐carboxylate and Their Electronic Properties (Eur. J. Inorg. Chem. 31/2007)

Abstract

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.

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.…”

“…) are now more commonly used (such as [Rh(CO) 2 Cl] 2 ), we opted to prepare the nickel carbonyl adduct in order to compare easily the parameters of the new imidazolium-2-phosphane complexes directly with the large database tabulated by Tolman [12] and with our other nickel tricarbonyl phosphenium complexes [Ni(CO) 3 [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 À1 ) and P(OCH 2 CH 2 CN) 3 (2088 cm À1 ).…”

“…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.…”

“…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.…”

“…[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.…”