Unprecedented direct cupric-superoxo conversion to a bis-μ-oxo dicopper(III) complex and resulting oxidative activity

Article abstract of DOI:10.1016/j.ica.2018.10.015

Investigations of small molecule copper-dioxygen chemistry can and have provided fundamental insights into enzymatic processes (e.g., copper metalloenzyme dioxygen binding geometries and their associated spectroscopy and substrate reactivity). Strategically designing copper-binding ligands has allowed for insight into properties that favor specific (di)copper-dioxygen species. Herein, the tetradentate tripodal TMPA-based ligand (TMPA = tris((2-pyridyl)methyl)amine) possessing a methoxy moiety in the 6-pyridyl position on one arm (^OCH3TMPA) was investigated. This system allows for a trigonal bipyramidal copper(II) geometry as shown by the UV–vis and EPR spectra of the cupric complex [(^OCH3TMPA)Cu^II(OH₂)](ClO₄)₂. Cyclic voltammetry experiments determined the reduction potential of this copper(II) species to be ?0.35 V vs. Fc^+/0 in acetonitrile, similar to other TMPA-derivatives bearing sterically bulky 6-pyridyl substituents. The copper-dioxygen reactivity is also analogous to these TMPA-derivatives, affording a bis-μ-oxo dicopper(III) complex, [{(^OCH3TMPA)Cu^III}₂(O^2?)₂]²⁺, upon oxygenation of the copper(I) complex [(^OCH3TMPA)Cu^I](B(C₆F₅)₄) at cryogenic temperatures in 2-methyltetrahydrofuran. This highly reactive intermediate is capable of oxidizing phenolic substrates through a net hydrogen atom abstraction. However, after bubbling of the precursor copper(I) complex with dioxygen at very low temperatures (?135 °C), a cupric superoxide species, [(^OCH3TMPA)Cu^II(O₂^[rad]?)]⁺, is initially formed before slowly converting to [{(^OCH3TMPA)Cu^III}₂(O^2?)₂]²⁺. This appears to be the first instance of the direct conversion of a cupric superoxide to a bis-μ-oxo dicopper(III) species in copper(I)-dioxygen chemistry.

Full text of DOI:10.1016/j.ica.2018.10.015

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Accepted Manuscript
Research paper
Unprecedented direct cupric-superoxo conversion to a bis-μ-oxo dicopper(III)
complex and resulting oxidative activity
David A. Quist, Melanie A. Ehudin, Kenneth D. Karlin
PII:
S0020-1693(18)31292-1
https://doi.org/10.1016/j.ica.2018.10.015
ICA 18561
DOI:
Reference:
Inorganica Chimica Acta
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Revised Date:
Accepted Date:
20 August 2018
9 October 2018
9 October 2018
Please cite this article as: D.A. Quist, M.A. Ehudin, K.D. Karlin, Unprecedented direct cupric-superoxo conversion
to a bis-μ-oxo dicopper(III) complex and resulting oxidative activity, Inorganica Chimica Acta (2018), doi: https://
doi.org/10.1016/j.ica.2018.10.015
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Unprecedented direct cupric-superoxo conversion to a bis--oxo
dicopper(III) complex and resulting oxidative activity
David A. Quist, Melanie A. Ehudin, Kenneth D. Karlin*
Department of Chemistry, Johns Hopkins University, Baltimore, Maryland 21218, United States
*Corresponding author. E-mail address: karlin@jhu.edu
Keywords: cupric superoxide; dioxygen activation; bis--oxo; copper
Abstract
Investigations of small molecule copper-dioxygen chemistry can and have provided fundamental
insights into enzymatic processes (e.g., copper metalloenzyme dioxygen binding geometries
and their associated spectroscopy and substrate reactivity). Strategically designing copper-
binding ligands has allowed for insight into properties that favor specific (di)copper-dioxygen
species. Herein, the tetradentate tripodal TMPA-based ligand (TMPA
pyridyl)methyl)amine) possessing a methoxy moiety in the 6-pyridyl position on one arm
OCH3TMPA) was investigated. This system allows for a trigonal bipyramidal copper(II) geometry
=
tris((2-
(
as shown by the UV-vis and EPR spectra of the cupric complex [(OCH3TMPA)CuII(OH2)](ClO4)2.
Cyclic voltammetry experiments determined the reduction potential of this copper(II) species to
be –0.35 V vs. Fc+/0 in acetonitrile, similar to other TMPA-derivatives bearing sterically bulky 6-
pyridyl substituents. The copper-dioxygen reactivity is also analogous to these TMPA-
derivatives, affording a bis--oxo dicopper(III) complex, [{(OCH3TMPA)CuIII}2(O2–)2]2+, upon
oxygenation of the copper(I) complex [(OCH3TMPA)CuI](B(C6F5)4) at cryogenic temperatures in 2-
methyltetrahydrofuran. This highly reactive intermediate is capable of oxidizing phenolic
substrates through a net hydrogen atom abstraction. However, after bubbling of the precursor
o
copper(I) complex with dioxygen at very low temperatures (–135 C), a cupric superoxide
•–
species, [(OCH3TMPA)CuII(O2 )]+, is initially formed before slowly converting to
[{(OCH3TMPA)CuIII}2(O2–)2]2+. This appears to be the first instance of the direct conversion of a
cupric superoxide to a bis--oxo dicopper(III) species in copper(I)-dioxygen chemistry.
1

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1. Introduction
The reaction of copper(I) with dioxygen is of key importance in catalytic and enzymatic
processes. For instance, copper-containing metalloenzymes such as peptidylglycine -
hydroxylating monooxygenase (PHM) and dopamine -hydroxylating monooxygenase (DM)
•–
reductively activate dioxygen, forming a cupric superoxide species (CuII-O2 ) to hydroxylate
organic substrates during pro-hormone and neurotransmitter biosynthesis [1–3]. Copper-
catalyzed organic reactions cover a wide range of transformations [4], including alcohol
oxidation and hydroxylation of C–H containing substrates (one important example being the
conversion of methane to methanol) [5–7]. Insight from synthetic model complexes has
expanded the scope of copper-dioxygen chemistry and allowed for direct and thorough
examination into the nature of various (di)copper-dioxygen adducts, including their oxidizing
capabilities [8,9]. These approaches are commonly carried out in organic solvents at cryogenic
temperatures in order to stabilize these reactive intermediates.
Initial oxygenation of cuprous complexes may result in the formation of a cupric superoxide
S
complex binding in either an end-on or side-on geometry (ES or S, respectively, Scheme 1)
[1,8,9]. Although these intermediates are not always observed, due to their instability, they are
often implicated. Further reaction of a cupric superoxide with a second equivalent of copper(I)
results in various dicopper-dioxygen adducts that have been reported to be in equilibrium with
each other in many cases [8–11]. These species can be peroxodicopper(II) (binding trans--1,2
T
S
or -2:2 to the copper ions, P or P, respectively, Scheme 1) or a bis--oxo dicopper(III)
entity (O, Scheme 1) [8,9].
2

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Scheme 1. Bubbling dioxygen into a copper(I) solution results in the formation of different
CunO2 (n = 1 or 2) isomers, dependent on a variety of factors. The conversions shown with
black solid arrows have been previously demonstrated. The new conversion (ES to O, blue
dashed arrow) is reported herein for the first time.
Copper complexes bearing TMPA-derivatized ligands (TMPA = tris((2-pyridyl)methyl)amine)
have been extensively studied. In particular, copper-dioxygen chemistry has greatly benefited
from thoughtfully targeted ligand modifications, incorporating electron-donating, H-bonding, or
steric moieties on the pyridyl donors to examine electronic and structural influences on reactivity
[12–20]. Studies have shown that functionalizing the 6-pyridyl position of TMPA can significantly
alter the observed copper-dioxygen reactivity, resulting in the formation of different dicopper-
dioxygen intermediates [18].
Herein, novel copper complexes employing a TMPA-based ligand with a methoxy group in
the 6-pyridyl position (OCH3TMPA, Scheme 2) were synthesized and their fundamental properties
and/or reactivity with O2 were investigated. The cupric complex [(OCH3TMPA)CuII(OH2)](ClO4)2
was synthesized to examine its electrochemical behavior (i.e., its CuII/I E° value) through cyclic
voltammetry (CV) and geometry preferences (trigonal bipyramidal vs. square pyramidal)
enforced by this methoxy ligand system. A copper(I) dioxygen (CuI/O2) study of
–
[(OCH3TMPA)CuI]BArF (BArF = B(C6F5)4 ) is also explored at cryogenic temperatures in 2-
o
methyltetrahydrofuran (MeTHF) as solvent. At higher temperatures (i.e., greater than –115 C),
3