Ligand-Induced and Thermally-Induced Orthometalation of the Bis(ylide) Ligand [Ph₃PC(H)]₂CO. Generation of the C,C-Chelating Group C₆H₄-2-PPh₂C((H)COCH₂PPh₃

The dinuclear complex [Pd(μ-Cl){[C(H)PPh₃]₂CO}]₂(ClO₄)₂ (2c) undergoes thermal rearrangement in refluxing NCMe, giving the dinuclear orthometalated derivative [Pd(μ-Cl)(C₆H₄-2-PPh₂C(H)COCH₂PPh₃)]₂(ClO₄)₂ (4c) as a mixture of two diastereoisomers (RR/SS and RS/SR). The orthometalation proceeds through an electrophilic substitution pathway, and the formation of the C,C-chelating ligand (C₆H₄-2-PPh₂C(H)COCH₂PPh₃) results from an intramolecular acid−base reaction in which the proton generated in the orthometalation reaction is captured by an ylide group. A decrease in the cone angle of the phosphonium group dramatically reduces the conversion of the bis(ylide) ligand into the orthometalated ligand. The orthometalation reaction can also be induced by ligand addition to the dimer [Pd(μ-Cl){[C(H)PPh₃]₂CO}]₂(ClO₄)₂ (2c) under very mild conditions. For instance, complex 2c reacts with PPh₃ or PPhMe₂ in CH₂Cl₂ at room temperature to give [PdCl(C₆H₄-2-PPh₂C(H)COCH₂PPh₃)(PR₃)](ClO₄) (PR₃ = PPh₃ 8, PPhMe₂ 9). Less sterically hindered ligands such as pyridine or 3,5-lutidine react with 2c to give in a first step the bis(ylide) complexes [PdCl{[C(H)PPh₃]₂CO}(L)](ClO₄) (L = py; 3,5-lut), which are transformed into the corresponding orthometalated derivatives [PdCl(C₆H₄-2-PPh₂C(H)COCH₂PPh₃)(L)](ClO₄) (L = py 6, 3,5-lut 7) by thermal treatment in refluxing NCMe. This different behavior is explained on the grounds of the different steric requirements of the incoming ligand (phosphine/pyridine). Similar behavior has been observed for the complex [Pd{[C(H)PPh₃]₂CO}(NCMe)₂](ClO₄)₂ (3c). 3c reacts with py or dppm giving [Pd{[C(H)PPh₃]₂CO}(L)₂](ClO₄)₂ (L = py 10, L₂ = dppm 11), which is transformed into [Pd(C₆H₄-2-PPh₂C(H)COCH₂PPh₃)(L₂)](ClO₄) (L = py 12, L₂ = dppm 13a + dppm-O 13b) by refluxing in NCMe. However, complex 3c reacts with PPh₃, dppe, or phen in CH₂Cl₂ at room temperature giving [Pd(C₆H₄-2-PPh₂C(H)COCH₂PPh₃)(L₂)](ClO₄) (L₂ = PPh₃, NCMe 14, dppe 15, phen 16). Complex 3c is not transformed into its corresponding orthometalated derivative [Pd(C₆H₄-2-PPh₂C(H)COCH₂PPh₃)(NCMe)₂](ClO₄)₂ (17) by refluxing in NCMe, but 17 can be obtained by treatment of 4c with TlClO₄ in NCMe. The orthometalation reaction of the bis(ylide) ligand can even occur spontaneously. The acetate-bridged dimer [Pd(μ-OOCCH₃){[C(H)PPh₃]₂CO}]₂(ClO₄)₂ (18) transforms spontaneously at room temperature into the mixed orthometalated bis(ylide)complex [(C₆H₄-2-PPh₂C(H)COCH₂PPh₃)Pd(μ-OOCCH₃)₂Pd{[C(H)PPh₃]₂CO}](ClO₄)₂ (19). The crystal structure of [Pd(C₆H₄-2-PPh₂C(H)COCH₂PPh₃)(PPh₃)(NCMe)](ClO₄)₂ (14) has been determined and reveals the presence of an orthometalated C₆H₄-2-PPh₂ unit, a C-linked ylide Pd−C(H), and a phosphonium fragment CH₂PPh₃. The phosphine group is coordinated cis to the orthometalated carbon atom.