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

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posted on 02.07.2008 by Yves Canac, Christine Lepetit, Mohammed Abdalilah, Carine Duhayon, Remi Chauvin
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-C6H4AaBb ligands (a + b = 2) has been analyzed from the IR CO stretching frequencies of isostructural complexes [(η2-C6H4AaBb)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-C6H4AaBb)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 νCO 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.