The Conformational Flexibility of the Tetradentate Ligand ^tBuN4 is Essential for the Stabilization of (^tBuN4)Pd^III Complexes

The conformationally flexible tetradentate pyridinophane ligand ^tBuN4 effectively lowers the oxidation potential of (^tBuN4)­Pd^II complexes and promotes their facile chemical and electrochemical oxidation, including unpredecented aerobic oxidation reactivity. While the low potential of a number of Pd^II (and Pt^II) complexes supported by various fac-chelating polydentate ligands is often attributed to the presence of a coordinating group in the axial position of the metal center, no detailed electrochemical studies have been reported for such systems. Described herein is the detailed electrochemical investigation of the effect of ligand conformation on the redox properties of the corresponding Pd^II complexes. These Pd complexes adopt different conformations in solution, as supported by studies using variable scan rate, variable-temperature cyclic voltammetry (CV), differential pulse voltammety, and digital CV simulations at variable scan rates. The effect of the axial amine protonation on the spectroscopic and electrochemical properties of the complexes was also investigated. A number of new Pd^III complexes were characterized by electron paramagnetic resonance, UV–vis spectroscopy, and X-ray diffraction including [(^tBuN4)­Pd^IIICl₂]­ClO₄, a dicationic [(^tBuN4)­Pd^IIIMe­(MeCN)]­(OTf)₂, and an unstable tricationic [(^tBuN4)­Pd^III­(EtCN)₂]³⁺ species. Although the electron-rich neutral complexes (^tBuN4)­PdMeCl and (^tBuN4)­PdMe₂ are present in solution as a single isomer with the axial amines not interacting with the metal center, their low oxidation potentials are due to the presence of a minor conformer in which the ^tBuN4 ligand adopts a tridentade (κ³) conformation. In addition, the redox properties of the (^tBuN4)Pd complexes show a significant temperature dependence, as the low-temperature behavior is mainly due to the contribution from the major, most stable conformer, while the room-temperature redox properties are due to the formation of the minor, more easily oxidized conformer(s) with the ^tBuN4 ligand acting as a tridentate (κ³) or tetradentate (κ⁴) ligand. Overall, the coordination to the metal center of each axial amine donor of the ^tBuN4 ligand leads to a lowering of the Pd^II/III oxidation potential by ∼0.6 V. These detailed electrochemical studies can thus provide important insights into the design of new ligands that can promote Pd-catalyzed oxidation reactions employing mild oxidants such as O₂.