Dioxygen Reduction by a Pd(0)-Hydroquinone Diphosphine Complex

A novel p-terphenyl diphosphine ligand was synthesized with a noninnocent hydroquinone moiety as the central arene (1-H). Pseudo-tetrahedral 4-coordinate Ni(0) and Pd(0)-quinone (2 and 3, respectively) complexes proved accessible by metalating 1-H with the corresponding M(OAc)2 precursors. O2 does not react with the Pd(0)-quinone species (3) and protonation occurs at the quinone moiety indicating that the coordinated oxidized quinonoid moiety prevents reactivity at the metal. A 2-coordinate Pd(0)-hydroquinone complex (4-H) was prepared using a one-pot metalation with Pd(II) followed by reduction. The reduced quinonoid moiety in 4-H shows metal-coupled reactivity with small molecules. 4-H was capable of reducing a variety of substrates including dioxygen, nitric oxide, nitrous oxide, 1-azido adamantane, trimethylamine n-oxide, and 1,4-benzoquinone quantitatively producing 3 as the Pd-containing reaction product. Mechanistic investigations of dioxygen reduction revealed that the reaction proceeds through a η(2)-peroxo intermediate (Int1) at low temperatures followed by subsequent ligand oxidation at higher temperatures in a reaction that consumed half an equivalent of O2 and produced water as a final oxygenic byproduct. Control compounds with methyl protected phenolic moieties (4-Me), displaying a Ag(I) center incapable of O2 binding (7-H) or a cationic Pd-H motif (6-H) allowed for the independent examination of potential reaction pathways. The reaction of 4-Me with dioxygen at low temperature produces a species (8-Me) analogous to Int1 demonstrating that initial dioxygen activation is an inner sphere Pd-based process where the hydroquinone moiety only subsequently participates in the reduction of O2, at higher temperatures, by H(+)/e(-) transfers.