Ligand-induced and thermally-induced orthometalation of the bis(ylide) ligand [Ph3P=C(H)](2)CO. Generation of the C,C-chelating group C6H4-2-PPh2C((H)COCH2PPh3

The dinuclear complex [Pd(mu-Cl){[C(H)PPh3](2)CO}](2)(ClO4)(2) (2c) undergo es thermal rearrangement in refluxing NCMe, giving the dinuclear orthometal ated derivative [Pd(mu-Cl)(C6H4-2-PPh2C(H)COCH2PPh3)](2)(ClO4)(2) (4c) as a mixture of two diastereoisomers (RR ISS and RS/SR). The orthometalation pr oceeds through an electrophilic substitution pathway, and the formation of the C,C-chelating ligand (C6H4-2-PPh2C(H)COCH2PPh3) results from an intramo lecular acid-base reaction in which the proton-generated in the orthometala tion reaction is captured by an ylide group. A decrease in the cone angle o f the phosphonium group dramatically reduces the conversion of the bis(ylid e) ligand into the orthometalated ligand. The orthometalation reaction can also be induced by Ligand addition to the dimer [Pd(mu-Cl){[C(H)PPh3](2)CO} ](2)(ClO4)(2) (2c) under very mild conditions. For instance, complex 2c rea cts with PPh3 or PPhMe2 in CH2Cl2 at room temperature to give [PdCl(C6H4-2P Ph(2)C(H)COCH2PPh3)(PR3)](ClO4) (PR3 = PPh3, PPhMe2 9). Less sterically hin dered ligands such as pyridine or 3,5-lutidine react with 2c to give in a f irst step the bis(ylide) complexes [PdCl{[C(H)PPh3](2)CO}(L)](ClO4) (L = py ; 3,5-lut), which are transformed into the corresponding orthometalated der ivatives [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 [P d{[C(H)PPh3](2) CO}(NCMe)(2)](ClO4)(2) (3c). 3c reacts with py or dppm givi ng [Pd{[C(H)PPh3](2)CO}(L)(2)](ClO4)(2) (L = py 10, L-2 = dppm 11), which i s transformed into [Pd(C6H4-2-PPh2C(H)COCH2PPh3)(L-2)](ClO4) (L = py 12, L- 2 = 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)(L-2)](ClO4) (L-2 = PPh3, NCMe 14,dppe 15, phen 16). Comp lex 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 orthome talation reaction of the bis(ylide) ligand can even occur spontaneously. Th e acetate-bridged dimer [Pd(mu-OOCCH3){[C(H)PPh3](2)CO}](2)(ClO4)(2) (18) t ransforms spontaneously at room temperature into the mixed orthometalated b is(ylide)complex [(C6H4-2-PPh2C(H)COCH2PPh3)Pd(mu-OOCCH3)(2)Pd{[C(H)PPh3](2 )CO}](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 C6 H4-2-PPh2 unit, a C-linked ylide Pd-C(H), and a phosphonium fragment CH2PPh 3. The phosphine group is coordinated cis to the orthometalated carbon atom .