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

Horak, Kyle T. and Agapie, Theodor (2016) Dioxygen Reduction by a Pd(0)–Hydroquinone Diphosphine Complex. Journal of the American Chemical Society, 138 (10). pp. 3443-3452. ISSN 0002-7863. doi:10.1021/jacs.5b12928.

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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^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 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 Int1 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.

Item Type:Article
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URLURL TypeDescription Information
Agapie, Theodor0000-0002-9692-7614
Additional Information:© 2016 American Chemical Society. Received: December 10, 2015; Publication Date (Web): March 7, 2016. We thank Lawrence M. Henling and Mike Takase for crystallographic assistance. We thank Caltech and NSF (CHE-1151918) for funding. T.A. is grateful for Sloan, Cottrell, and Dreyfus fellowships. The Bruker KAPPA APEXII X-ray diffractometer was purchased via an NSF CRIF:MU award to Caltech, CHE0639094. The authors declare no competing financial interest.
Funding AgencyGrant Number
Alfred P. Sloan FoundationUNSPECIFIED
Cottrell Scholar of Research CorporationUNSPECIFIED
Camille and Henry Dreyfus FoundationUNSPECIFIED
Issue or Number:10
Record Number:CaltechAUTHORS:20160315-101431160
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Official Citation:Dioxygen Reduction by a Pd(0)–Hydroquinone Diphosphine Complex Kyle T. Horak and Theodor Agapie Journal of the American Chemical Society 2016 138 (10), 3443-3452 DOI: 10.1021/jacs.5b12928
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:65357
Deposited By: Tony Diaz
Deposited On:15 Mar 2016 19:02
Last Modified:10 Nov 2021 23:44

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