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-C(6)H(4)A(a)B(b) ligands (a + b = 2) has been analyzed from the IR CO stretching frequencies of isostructural complexes [(eta(2)-C(6)H(4)A(a)B(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 (B-2 = (P+Ph2CH-)(2)CH2), preventing the formation of the analogous highly strained carbodiphosphorane. The three complexes [(eta(2)-C(6)H(4)A(a)B(b))Rh(CO)(2)][TfO] were fully characterized, including by X-ray diffraction analysis and Rh-103 NMR spectroscopy. Comparison of their IR spectra indicated that the A(2) type bis-NHC ligand is less donating than the hybrid AB type, which is itself less donating than the B-2 type bis-ylide ligand. The excellent linear variation of the nu(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(star star)/LANL2DZ(star)(Rh) level. It is therefore demonstrated that a phosphonium ylide ligand is a stronger donor than a diaminocarbene ligand.