Can\nDonor Ligands Make Pd(OAc)<sub>2</sub> a Stronger\nOxidant? Access to Elusive Palladium(II) Reduction Potentials and\nEffects of Ancillary Ligands via Palladium(II)/Hydroquinone Redox Equilibria

Palladium­(II)-catalyzed oxidation\nreactions represent an important\nclass of methods for selective modification and functionalization\nof organic molecules. This field has benefitted greatly from the discovery\nof ancillary ligands that expand the scope, reactivity, and selectivity\nin these reactions; however, ancillary ligands also commonly poison\nthese reactions. The different influences of ligands in these reactions\nremain poorly understood. For example, over the 60-year history of\nthis field, the Pd^II/0 redox potentials for catalytically\nrelevant Pd complexes have never been determined. Here, we report\nthe unexpected discovery of (L)­Pd^II(OAc)₂-mediated\noxidation of hydroquinones, the microscopic reverse of quinone-mediated\noxidation of Pd⁰ commonly employed in Pd^II-catalyzed\noxidation reactions. Analysis of redox equilibria arising from the\nreaction of (L)­Pd­(OAc)₂ and hydroquinones (L = bathocuproine,\n4,5-diazafluoren-9-one), generating reduced (L)Pd species and benzoquinones,\nprovides the basis for determination of (L)­Pd^II(OAc)₂ reduction potentials. Experimental results are complemented\nby density functional theory calculations to show how a series of\nnitrogen-based ligands modulate the (L)­Pd^II(OAc)₂ reduction potential, thereby tuning the ability of Pd^II to serve as an effective oxidant of organic molecules in catalytic\nreactions.