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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, 00:00 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.