Palladium Alkyl Complexes Containing β-Phosphonato−Phosphine P,O Ligands and Their Reactivity toward CO-Ethylene or -Acrylate Insertion

Neutral and cationic square-planar Pd(II) complexes containing the β-phosphonato-phosphine ligand rac-Ph2PCH(Ph)P(O)(OEt)2 (1) or the new α-silyl-phosphonato-phosphine ligand Ph2PCH(SiMe3)P(O)(OMe)2 (4) have been prepared. Reaction of 4 with [Pd(μ-Cl)(dmba)]2 (dmba = o-C6H4CH2NMe2) afforded an equilibrium mixture of [Pd(dmba){Ph2PCH(SiMe3)P(O)(OMe)2}]Cl (5) and [PdCl(dmba){Ph2PCH(SiMe3)P(O)(OMe)2}] (5′), which illustrates the hemilabile behavior of the ligand in this system. From the stepwise insertion reaction of CO and ethylene into the Pd−C bond of [PdCl(Me){Ph2PCH(Ph)P(O)(OEt)2}] (2a), the metallacyclic complex [Pd{CH2CH2C(O)Me{Ph2PCH(Ph)P(O)(OEt)2][BF4] (10) was obtained which features the first acetyl−ethylene coupling product containing a phosphine−phosphonate ligand. Chloride abstraction from 2a led to the cationic complex [PdMe(NCMe){Ph2PCH(Ph)P(O)(OEt)2}][BF4] (8), which reacted sequentially with CO and methyl acrylate to afford [Pd{CH[C(O)OMe][CH2C(O)Me]}{Ph2PCH(Ph)P(O)(OEt)2}][BF4] (11). The reaction between 8 and the P,O-ligand Ph2PNHC(O)Me resulted in an equilibrated ligand exchange where the heteroleptic system is preferred over the mixture of the two homoleptic complexes. The complexes 2a and 10·1/2CHCl3 have been characterized by X-ray diffraction and represent rare examples of crystal structures showing a (diethyl)methylphosphonate ligand coordinated to a metal center.