π-Lewis Base Activation of Carbonyls and Hexafluorobenzene

We report hitherto elusive side-on η2-bonded palladium(0) carbonyl (anthraquinone, benzaldehyde) and arene (benzene, hexa-fluorobenzene) palladium(0) complexes and present the catalytic hydrodefluorination of hexafluorobenzene by cyclohexene. The comparison with respective cyclohexene, pyridine and tetrahydrofuran complexes reveals that the experimental ligand binding strengths follow the order THF < C6H6 < C6F6 < cyclohexene < pyridine < benzaldehyde < anthraquinone. To understand this surprising order, the complexes' electronic structures were elucidated by nuclear magnetic resonance (NMR), single crystal X-Ray diffraction (sc-XRD), ultraviolet/visible (UV/Vis) electronic absorption, infrared (IR) vibrational, Pd L3-edge X-ray absorption (XAS), and X-ray photoelectron (XP) spectroscopic techniques, complemented by Density Functional Theory (DFT) calculations including energy decomposition (EDA-NOCV) and effective oxidation state (EOS) analyses. For benzene, pyridine and cyclohexene, bonding follows the donor/acceptor picture of the Dewar-Chatt-Duncanson model. In stark contrast, hexafluorobenzene, benzaldehyde and anthraquinone bind via essentially the π-channel only and thus as π-analogues of Z-acceptor ligands. This contribution elucidates the control of functional-group selectivity in palladium(0) catalysis and delineates a novel strategy to activate electron-deficient π-systems.