An aryl exchange reaction with full retention of configuration of the complexes:: Mechanism of the aryl exchange between [PdR2L2] complexes in chloroform (R = pentahalophenyl, L = thioether)

The complexes [Pd(3,5-C6Cl2F3)(2)L-2] and [Pd(C6F5)(2)L-2] (L = SC4H8 (tht), or SMe2) react in CCl3D to give a mixture also containing the heteroaryl products [Pd(3,5-C6Cl2F3)(C6F5)L-2], whereas for L = PPh3, AsPh3, 2-picoline, 4-picoline, or 1/2COD, this exchange is not observed. The F-19 NMR kinetic study supports that the scrambling of the aryl groups takes place with retention of configuration at both Pd centers via a triply-bridged binuclear activated complex [L(C6Cl2F3)Pd(mu-C6Cl2F3)(mu-C6F5)(mu-L)Pd(C6F5)L].(double dagger) The aryl double bridge is supported by a bridging S-donor ligand facilitating an otherwise difficult exchange (not observed for other weak L ligands, such as picolines, lacking a second lone pair). This constitutes the first reported example of reversible migration of sigma-C-bonded groups between bis-organo Pd(II) complexes proceeding without cis-trans isomerization. The isomerization process is also observed, occurring at a much slower rate.