E–Z Isomerization of Phosphine-Olefin (PEWO‑F4) Ligands Revealed upon PdCl2 Capture: Facts and Mechanism

The PEWO phosphines R2P­(o-C6H4CHCHC­(O)­Ph), R2P­(o-C6H2F2CHCHC­(O)­Ph), and R2P­(o-C6F4CHCHC­(O)­Ph) and their P-monodentate complexes trans-[PdCl2(P-monodentate)2] show, in solution and (when available) in the X-ray diffraction structures, an E configuration of the double bond. In contrast, the structures of [PdCl2(P-chelate)] display E and Z configurations. The E/Z isomerization of the latter requires first decoordination of the double bond, which then allows for easy rotation about the electron-deficient double bond. Thus, the E/Z equilibria exist for the free and the P-monodentate complexes as well but are not observed because they are extremely displaced toward the E isomer. Their capture in the form of [PdCl2(P-chelate)], with equilibrium constants on the order K eq ≈ 1–3, allows the two configurations to be observed and isolated. Evaluation of their ability to couple Pf–Pf from cis-[PdPf2(THF)2] (Pf = C6F5) affords values of their ΔG ⧧(Pf–Pf)Pd parameters confirming that higher substitution of H by F produces lower coupling barriers and a double bond that is more electron deficient when it is free and more electron withdrawing when it is coordinated.