On the P‐Coordinating Limit of NHC–Phosphenium Cations toward RhI Centers

Two types of imidazoliophosphane with additional electron‐withdrawing substituents, such as alkoxy or imidazolio groups, are experimentally described and theoretically studied. Diethyl N,N′‐2,4,6‐methyl(phenyl)imidazoliophosphonite is shown to retain a P‐coordinating ability toward a {RhCl(cod)} (cod=cycloocta‐1,5‐diene) center, thus competing with the cleavage of the labile C? P bond. Derivatives of N,N′‐phenylene‐bridged diimidazolylphenylphosphane were isolated in good yield. Whereas the dicationic phosphane proved to be inert in the presence of {RhCl(cod)}₂], the monocationic counterpart was shown to retain the P‐coordinating ability toward a {RhCl(cod)} center, thus competing with the N‐coordinating ability of the nonmethylated imidazolyl substituent. The ethyl phosphinite version of the dication, thus possessing an extremely electron‐poor P^III center, was also characterized. According to the difference between the calculated homolytic and heterolytic dissociation energies, the N₂C???P bond of imidazoliophosphanes with aryl, amino, or alkoxy substituents on the P atom is shown to be of dative nature. The P‐coordinating properties of imidazoliophosphanes with various combinations of phenyl or ethoxy substituents on the P atom and those of six diimidazolophosphane derivatives with zero, one, or two methylium substituents on the N atom, were analyzed by comparison of the corresponding HOMOs and LUMOs and by calculation of the IR C?O stretching frequencies of their RhCl(CO)₂] complexes. Comparison of the ν_CO values allows the family of the electron‐poor Im⁺PRR′ (Im=imidazolyl) potential ligands to be ranked in the following order versus (R,R′): P(OEt)₃<(Ph,Ph)<(Ph,OEt)<(OEt,OEt)<PF₃<(Ph,Im)<(Ph,Im⁺)<(OEt,Im⁺). The (Ph,Im) representative is therefore the least electron‐donating phosphane for which coordinating behavior toward a Rh^I center has been experimentally evidenced to date. Ultimate applications in catalysis could be envisaged.