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

作者：Horak Kyle T; Agapie Theodor

来源：Journal of the American Chemical Society, 2016, 138(10): 3443-3452.

DOI：10.1021/jacs.5b12928

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. O-2 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 hydroquinone complex (4-H) was prepared using a one-pot metalation with Pd" 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-0-containing reaction product. Mechanistic investigations of dioxygen reduction revealed that the reaction proceeds through a eta(2)-peroxo intermediate (Int1) at low temperatures followed by subsequent ligand oxidation at higher temperatures in a reaction that consumed half an equivalent of O-2 and produced water as a final oxygenic byproduct. Control compounds with methyl protected phenolic moieties (4-Me), displaying a Ag-I center incapable of O-2 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 Intl demonstrating that initial dioxygen activation is an inner sphere Pd-based process where the hydroquinone moiety only subsequently participates in the reduction of O-2, at higher temperatures, by H+/e(-) transfers.

• 出版日期2016-3-16