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

Abstract

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 + /4ereduction of O2 to H2O tend to be marred by limited stability and selectivity in controlling the multiproton and multielectron 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-2,11diaza[3.3](2,6)pyridinophane ( t BuN4). 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 2 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 [( t BuN4)Pd III MeCl] + complex, under both homogeneous and heterogeneous conditions in a non-aqueous and acidic aqueous electrolyte, respectively. Cyclic voltammetry and hydrodynamic electrochemical studies for [( t BuN4)Pd III MeCl] + 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 [( t BuN4)Pd III MeCl] + in MeCN, and thus being a rare example of a bimolecular ORR process. The maximum second-order turnover frequency TOFmax (2) = 2.76 10 8 M -1 sec -1 was determined for 0.32 mM of [( t BuN4)Pd III MeCl] + in the presence of 1 M AcOH in O2-saturated MeCN with an overpotential of 0.32 V. By comparison, a comparatively lower TOFmax (2) = 1.25 10 5 M -1 sec -1 at a higher overpotential of 0.8 V was observed for [( t BuN4)Pd III MeCl]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.

Recent progress in the chemistry of 12-membered pyridine-containing tetraazamacrocycles: from synthesis to catalysis

“…In this sense, Mirica and Sinha recently reported the first electro-catalytic oxygen reduction reaction (ORR) to H 2 O using a molecular palladium complex (Scheme 26). 92 In this case, robust cationic Pd( iii ) complex Me(Cl)Pd III L41d , 93 was capable of promoting the ORR under low potentials in the presence of moderate to weak acids such as AcOH. Interestingly, the ORR could be accomplished under homo- or heterogeneous conditions.…”

Periodic TableTalks: An Oasis of Science within a Conference Desert

“…These high-valent organometallics are thought to be important intermediates in palladium-mediated C–C bond-forming chemistry. The Mirica group’s high-valent palladium complexes can also effect electrocatalytic O 2 reduction . In particular, [( t BuN4)­Pd III (Me)­Cl]­[PF 6 ] ( t BuN4 = N , N ′-di- tert -butyl-2,11-diaza­[3.3]­(2,6)­pyridinophane) was found to electrocatalyze the reduction of O 2 to H 2 O with good faradaic efficiencies and relatively low overpotentials.…”

Metal-Free Triazine-Based Porous Organic Polymer-Derived N-Doped Porous Carbons as Effective Electrocatalysts for Oxygen Reduction Reaction

“…Thus, non-noble-metal-based electrocatalysts are preferable for many applications in the energy conversion process, specifically fuel cells, to replace the Pt-based electrocatalyst. However, earth-abundant metal catalysts like metal oxide, , metal sulfide, , chalcogenides, , and organometallic complexes , have been developed to speed up the sluggish ORR kinetics, showing superior electrocatalytic activity as well as durability for the ORR process. Notably, one major problem in the ORR process is the generation of H 2 O 2 as an intermediate product instead of producing H 2 O directly …”