Synthesis and Bond Activation Chemistry of Palladium(II) Pincer Complexes with a Weakly Coordinating Side Arm

A series of Pd(II) aryl complexes of a PNO-type pincer ligand bearing a weakly coordinated benzofuran side arm has been prepared. Crystallographic studies showed very long Pd-O distances of more than 2.3 Å, which is significantly longer than the Pd-O distances in structurally similar PNO-Pd complexes with an exocylic oxygen donor. Crystallographic and 19F NMR solution studies of complexes containing electron-donating (OMe) and electron-withdrawing (CF3) substituents in the position para to the benzofuran oxygen atom revealed the dependence of the Pd-O interactions on the nature of the aromatic group at the Pd center. The ability to influence these interactions by changing the electron density at the metal was demonstrated in the stoichiometric Sonogashira-type cross-coupling reactions between the Pd complexes and phenylacetylene, which proceed via a reversible aromatization/dearomatization of the pincer ligand. Complexes with the electron-poor CF3 group showed higher reactivity in comparison to their electron-neutral or -rich analogues. DFT studies of these systems provided further mechanistic insight into the origin of the observed reactivity patterns.