Mechanisms for Hydrogen-Atom Abstraction by Mononuclear Copper(III) Cores: Hydrogen-Atom Transfer or Concerted Proton-Coupled Electron Transfer?

Mukunda Mandal, Courtney E. Elwell, Caitlin J. Bouchey, Timothy J. Zerk, William B. Tolman, Christopher J. Cramer

Research output: Contribution to journalArticlepeer-review

14 Scopus citations

Abstract

In a possibly biomimetic fashion, formally copper(III)-oxygen complexes LCu(III)-OH (1) and LCu(III)-OOCm (2) (L2- = N,N′-bis(2,6-diisopropylphenyl)-2,6-pyridinedicarboxamide, Cm = α,α-dimethylbenzyl) have been shown to activate X-H bonds (X = C, O). Herein, we demonstrate similar X-H bond activation by a formally Cu(III) complex supported by the same dicarboxamido ligand, LCu(III)-O2CAr1 (3, Ar1 = meta-chlorophenyl), and we compare its reactivity to that of 1 and 2. Kinetic measurements revealed a second order reaction with distinct differences in the rates: 1 reacts the fastest in the presence of O-H or C-H based substrates, followed by 3, which is followed by (unreactive) 2. The difference in reactivity is attributed to both a varying oxidizing ability of the studied complexes and to a variation in X-H bond functionalization mechanisms, which in these cases are characterized as either a hydrogen-atom transfer (HAT) or a concerted proton-coupled electron transfer (cPCET). Select theoretical tools have been employed to distinguish these two cases, both of which generally focus on whether the electron (e-) and proton (H+) travel "together" as a true H atom, (HAT), or whether the H+ and e- are transferred in concert, but travel between different donor/acceptor centers (cPCET). In this work, we reveal that both mechanisms are active for X-H bond activation by 1-3, with interesting variations as a function of substrate and copper functionality.

Original languageEnglish (US)
Pages (from-to)17236-17244
Number of pages9
JournalJournal of the American Chemical Society
Volume141
Issue number43
DOIs
StatePublished - Oct 30 2019

Bibliographical note

Funding Information:
We thank Prof. J.E.M.N. Klein, Prof. M. Srnec, and Dr. B. Dereli for helpful discussions on modeling strategies. MM and CJC acknowledge funding support as part of the Inorganometallic Catalyst Design Center, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Basic Energy Sciences, under Award No. DESC0012702. WBT thanks the NIH (GM47365) for funding. We also thank Dr. V.G. Young, Jr. for assistance with X-ray crystallography. X-ray diffraction data were collected using a crystal diffractometer acquired through NSF-MRI Award No. CHE-1229400.

Funding Information:
We thank Prof. J.E.M.N. Klein, Prof. M. Srnec, and Dr. B. Dereli for helpful discussions on modeling strategies. MM and CJC acknowledge funding support as part of the Inorganometallic Catalyst Design Center, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Basic Energy Sciences, under Award No. DESC0012702. WBT thanks the NIH (GM47365) for funding. We also thank Dr. V.G. Young, Jr. for assistance with X-ray crystallography. X-ray diffraction data were collected using a crystal diffractometer acquired through NSF-MRI Award No. CHE-1229400.

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