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 1a 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 P,O-chelates. Complexes trans-[PdCl2{Ph2PCHC(Ph)OP(O)(OPh)2}2] (2b), [PdCl{Ph2PCHC(Ph)OP(O)(OEt)2}(μ-Cl)]2 (3a), (8a‘), and
(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.