On the P‐Coordinating Limit of NHC–Phosphenium Cations toward Rh^I Centers

Abstract

Abstract 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 N 2 C⋅⋅⋅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)<PF 3 <(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.