Enolphosphato−Phosphines:  A New Class of P,O Ligands

The new bifunctional ligands Ph<sub>2</sub>PCHCPh[OP(O)(OR)<sub>2</sub>] (<b>1</b>) (<b>1a</b>, R = Et; <b>1b</b>, R = Ph) represent the first examples of P,O derivatives resulting from the association of a phosphine moiety and an enolphosphate group. The <i>Z</i> 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 <b>1a</b> or <b>1b</b> 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 <b>1a</b> and <b>1b</b> can display a hemilabile behavior, owing to successive dissociation and recoordination of the O atom. Competition experiments revealed that phosphine <b>1a</b> presents a higher chelating ability than <b>1b</b>, a feature ascribed to the more electrodonating properties of the ethoxy groups in <b>1a</b> compared to the phenoxy groups in <b>1b</b>. P,O-Chelation affords seven-membered metallocycles, which is unusual for P,O-chelates. Complexes <i>trans</i>-[PdCl<sub>2</sub>{Ph<sub>2</sub>PCHC(Ph)OP(O)(OPh)<sub>2</sub>}<sub>2</sub>] (<b>2b</b>), [PdCl{Ph<sub>2</sub>PCHC(Ph)OP(O)(OEt)<sub>2</sub>}(μ-Cl)]<sub>2</sub> (<b>3a</b>), (<b>8a</b>‘), and (<b>10a</b>) have been structurally characterized. Interestingly, the seven-membered rings in <b>8a</b>‘ and <b>10a</b> 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.