Thioether- and selenoether-carboxylates in palladium chemistry: conclusive proof of hemilabile properties of O–Se ligands

Reaction of trans-[PdCl(Ph)(PPh3)2] with the thallium salts R–X–(CH2)n–COOTl (n=1, X=S, R=Me, Et, i-Pr, t-Bu, Ph; n=1, X=Se, R=Me, Ph; n=2, X=S, R=Et, Ph) yields the compounds trans-[Pd(OOC–(CH2)n–X–R–κ1-O)Ph(PPh3)2] (1a–1i). In solution, complexes 1a–1i participate in hemilabile equilibria in which one PPh3 ligand is replaced by a sulphur/selenium atom of the ligand to afford the O–S/Se chelates under certain circumstances. Solid state structures of the monodentate complex 1h as well as its corresponding chelate (2h) together with the NMR observation of both species in solution, conclusively prove the hemilabile properties of this O–Se ligand. Depending on the size of the substituent on the potential sulphur or selenium donor atom, however, a PPh3 ligand is not always displaced upon donor atom coordination. In such equilibria no free PPh3 is detected and a five-coordinate square pyramidal structure is proposed for the O–S/Se chelate. The position of the relevant equilibrium depends on both the basicity of the potential sulphur or selenium donor atom as well as on the polarity of the solvent.