Synthesis of New Cationic Donor-Stabilized Phosphenium Adducts and Their Unexpected P-Substituent Exchange Reactions

The reaction between two 1,3-dialkylimidazolium-2-carboxylates 1a and 1b and two different dichlorophosphines (RPCl2, with R = Ph and NEt2) led to new donor-stabilized phosphenium adducts. When the reaction was performed with the 1,3-dimethylimidazolium-2-carboxylate 1a and PhPCl2 in a 2:1 ratio, the phosphine 4a, bearing two imidazolium moieties, was obtained and led to 5a, after an anion exchange reaction with KPF6, 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 PhPCl2 to the 1-dodecyl-3-methylimidazolium-2-carboxylate 1b. However, by the use of dichloro(diethylamino)phosphine, (Et2N)PCl2, instead of PhPCl2, the reaction with la 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 PhPCl2 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.