Electrocatalytic O2 Reduction by an Organometallic Pd(III) Complex via a Binuclear Pd(III) Intermediate

The development of electrocatalysts for the selective O₂-to-H₂O conversion, the O₂ reduction reaction (ORR), is of great interest for improving the performance of fuel cells. In this context, molecular catalysts that are known to mediate the 4H⁺/4e^– reduction of O₂ to H₂O tend to be marred by limited stability and selectivity in controlling the multi-proton and multi-electron transfer steps. Thus, evaluation of new transition metal complexes, including organometallic species, for ORR reactivity could uncover new molecular catalysts with improved properties. We have previously reported the synthesis and characterization of various organometallic Pd^III complexes stabilized by the tetradentate ligand N,N′-di-tert-butyl-2,11-diaza[3.3](2,6)pyridinophane (^tBuN4). These complexes were shown to react with O₂ and undergo oxidatively-induced C–C and C–heteroatom bond formation reactions in the presence of O₂. These O₂-induced oxidative transformations prompted us to evaluate the ORR reactivity of such organometallic Pd complexes, which to the best of our knowledge has never been studied before for any molecular Pd catalyst. Herein, we report the ORR reactivity of the [(^tBuN4)Pd^IIIMeCl]⁺ complex, under both homogeneous and heterogeneous conditions in a non-aqueous and acidic aqueous electrolyte, respectively. Cyclic voltammetry and hydrodynamic electrochemical studies for [(^tBuN4)Pd^IIIMeCl]⁺ revealed the electrocatalytic reduction of O₂ to H₂O proceeds with Faradaic efficiencies (FE) of 50-70% in the presence of acetic acid (AcOH) in MeCN. The selectivity toward H₂O production further improved to a FE of 80-90% in an acidic aqueous medium (pH 0), upon immobilization of the molecular catalyst onto edge plane graphite (EPG) electrodes. Analysis of electrochemical data suggests the formation of a binuclear Pd^III intermediate in solution, likely a Pd^III-peroxo-Pd^III species, which dictates the thermochemistry of the ORR process for [(^tBuN4)Pd^IIIMeCl]⁺ in MeCN, and thus being a rare example of a bimolecular ORR process. The maximum second-order turnover frequency TOF_max⁽²⁾ = 2.76 x 10⁸ M^–1 sec^–1 was determined for 0.32 mM of [(^tBuN4)Pd^IIIMeCl]⁺ in the presence of 1 M AcOH in O₂-saturated MeCN with an overpotential of 0.32 V. By comparison, a comparatively lower TOF_max⁽²⁾ = 1.25 x 10⁵ M^–1 sec^–1 at a higher overpotential of 0.8 V was observed for [(^tBuN4)Pd^IIIMeCl]PF₆ adsorbed onto EPG electrodes in O₂-saturated 1 M H₂SO₄ aqueous solution. Overall, reported herein is a detailed ORR reactivity study using a novel Pd^III organometallic complex and benchmark its selectivity and energetics toward O₂ reduction in MeCN and acidic aqueous solutions.