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 Ni0 and Pd0–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 Pd0–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 Pd0–hydroquinone
complex (4-H) was prepared using a one-pot metalation
with PdII 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 η2-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 AgI 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.