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

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.

Carbeniophosphines versus Phosphoniocarbenes: The Role of the Positive Charge

“…The N‐functionalization of neutral imidazolophosphines 7 was also reported to be an effective method for the preparation of imidazoliophosphines 8 , although the nature of the N substituent introduced has so far been limited to alkyl groups (route C) . By analogy with the condensation method, routes A–C could be extended to the preparation of diimidazoliophosphines …”

“…Based on the calculated ν (CO) values in the [RhCl(CO) 2 L] complexes 16 , the donor properties of a series of imidazoliophosphines were also evaluated with regard to the number of cationic substituents at the P atom (Scheme ) . As anticipated, the introduction of cationic P substituents causes a significant decrease in the donating character of the ligand (ν av(CO) =2071.6 ( 17 ) vs. 2102.2 cm −1 ( 20 )).…”

“…As anticipated, the introduction of cationic P substituents causes a significant decrease in the donating character of the ligand (ν av(CO) =2071.6 ( 17 ) vs. 2102.2 cm −1 ( 20 )). The substitution of a phenyl group for an imidazolium substituent was found to be more efficient at weakening the donating character of the ligand than substitution of a phenyl group for an ethoxy group (ν av(CO) =2072.7 ( 18 ) vs. 2102.2 cm −1 ( 20 )) . The electronic scale shows also that the phosphinite 21 (ν av(CO) =2102.5 cm −1 ) featuring two imidazolium and an ethoxy substituents is the least‐electron‐donating ligand known so far.…”

α‐Cationic Phosphines: Synthesis and Applications

“…This generally makes the phosphorus–metal bond weaker and, as consequence, the corresponding metal complexes are more labile than their non‐charged counterparts. In extreme cases, for example, when the phosphine is polycationic, this circumstance may even prevent the formation of coordination complexes 9…”

“…It is of note that the predicted donor endowment of bisimidazolium‐substituted phosphines 18 is characterised by a calculated TEP of 2115 cm −1 and, hence, it is expected to be the weakest donor along the complete series. Unfortunately, despite of its potential, no metal complex bearing this ancillary ligand has been prepared to date 9. The weakness of the hypothetical metal–phosphorus bond to be formed probably inhibits its coordination chemistry.…”