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.