Enolphosphato-phosphines: A new class of P,O ligands

The new bifunctional ligands Ph2PCH=CPh[OP(O)(OR)(2)] (1) (1a, R = Et; 1b, R = Ph) represent the first examples of P,O derivatives resulting from the association of a phosphine moiety and an enolphosphate group. The Z stereochemistry about the double bond provides a favorable situation for these ligands to act as P,O-chelates. Neutral and cationic Pd(II) complexes have been synthesized and characterized, in which 1a or 1b acts either as a P-monodentate ligand or a P,O-chelate, via coordination of the oxygen atom of the P=O group. In the latter case, it has been observed that phosphines 1a and 1b can display a hemilabile behavior, owing to successive dissociation and recoordination of the O atom. Competition experiments revealed that phosphine la presents a higher chelating ability than 1b, a feature ascribed to the more electrodonating properties of the ethoxy groups in 1a compared to the phenoxy groups in 1b. P,O-Chelation affords seven-membered metallocycles, which is unusual for RO-chelates. Complexes trans-[PdCl2{Ph2PCH=C(Ph)OP(O)(OPh)(2)}(2)] (2b), [PdCl{Ph2PCH=C(Ph)OP(O)(OEt)(2)}(mu-Cl)](2) (3a), [Pd(dmba){Ph2PCH=C(Ph)OP(O)(OEt)(2)}][PF6] (8a'), and cis-[Pd{Ph2PCH=C(Ph)OP(O)(OEt)(2)}][BF4](2) (10a) have been structurally characterized. Interestingly, the seven-membered rings in 8a' and 10a adopt a sofa conformation with the double bond lying almost perpendicular to the plane containing the Pd, the two P, and the two O atoms.