Diaminocarbene and Phosphonium Ylide Ligands: A Systematic Comparison of their Donor Character

The coordinating properties of the diaminocarbene (A) and phosphonium ylide (B) ligand types have been investigated systematically through a test family of C,C-chelating ligands containing two moieties of either kind. The overall character of o-C₆H₄A_aB_b ligands (a + b = 2) has been analyzed from the IR CO stretching frequencies of isostructural complexes [(η²-C₆H₄A_aB_b)Rh(CO)₂][TfO]. The test moieties A = NC₂H₂N⁺(Me)C⁻ and B = Ph₂P⁺CH₂⁻ were first considered. While the ligands bearing at least one diaminocarbene end (AA, a = 2 and AB, a = 1) could be generated (and trapped by complexation), the bis-ylide case BB (a = 0) proved to be awkward: treatment of the dication C₆H₄(P⁺Ph₂Me)₂ with n-BuLi indeed lead to the Schmidbaur’s carbodiphosphorane Ph₃PCPPh₂Me, through an unprecendented ylido-pentacoordinated phosphorane which could be fully characterized by NMR techniques. The bis-ylide ligand type C₆H₄B₂ could however be generated by bridging the phosphonium methyl groups by a methylene link (B₂ = (P⁺Ph₂CH⁻)₂CH₂), preventing the formation of the analogous highly strained carbodiphosphorane. The three complexes [(η²-C₆H₄A_aB_b)Rh(CO)₂][TfO] were fully characterized, including by X-ray diffraction analysis and ¹⁰³Rh NMR spectroscopy. Comparison of their IR spectra indicated that the A₂ type bis-NHC ligand is less donating than the hybrid AB type, which is itself less donating than the B₂ type bis-ylide ligand. The excellent linear variation of the ν_CO frequencies vs a (= 0, 1, 2) shows that the coordinating moieties act in a pseudoindependent way. This was confirmed by DFT calculations at the B3PW91/6-31G**/LANL2DZ*(Rh) level. It is therefore demonstrated that a phosphonium ylide ligand is a stronger donor than a diaminocarbene ligand.