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On the P-coordinating limit of NHC-phosphenium cations toward Rh(I) 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 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.