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. 2019 Jan 24:485:155-161.
doi: 10.1016/j.ica.2018.10.015. Epub 2018 Oct 10.

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

David A Quist  1 , Melanie A Ehudin  1 , Kenneth D Karlin  1
Affiliations

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

David A Quist et al. Inorganica Chim Acta. .

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 = 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)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-μ-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 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-μ-oxo dicopper(III) species in copper(I)-dioxygen chemistry.

Keywords: bis-μ-oxo; copper; cupric superoxide; dioxygen activation.

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Figures

Figure 1.
Figure 1.
(A) Depiction of [(OCH3TMPA)CuII(OH2)](ClO4)2, emphasizing the 6-pyridyl methoxy substituent. (B) UV-vis spectra of [(OCH3TMPA)CuII(OH2)](ClO4)2 (0.13 mM) in butyronitrile at room temperature (RT). Inset: UV-vis spectra of [(OCH3TMPA)CuII(OH2)](ClO4)2 (1.3 mM) in butyronitrile at RT. (C) EPR spectrum of [(OCH3TMPA)CuII(OH2)](ClO4)2 (1 mM) in butyronitrile at 64 K. (D) Cyclic voltammetry of [(OCH3TMPA)CuII(OH2)](ClO4)2 (1.4 mM) in a 0.1 M electrolyte solution of tetrabutylammonium hexafluorophosphate (TBAPF6) in CH3CN at RT.
Figure 2.
Figure 2.
TMPA and TMPA-based ligands bearing substituents in their 6-pyridyl positions.
Figure 3.
Figure 3.
UV-vis spectra showing the thermal decomposition of [{(OCH3TMPA)CuIII}2(O2−)2]2+ (blue spectrum) in MeTHF at −80 °C to give a new species (red spectrum) (Inset) Monitoring the decomposition of the bis-μ-oxo intermediate (λmax = 383 nm) over time.
Figure 4.
Figure 4.
(Left) UV-vis monitoring of the conversion of [(OCH3TMPA)CuII(O2•−)]+ (green spectrum) to [{(OCH3TMPA)CuIII}2(O2−)2]2+ (blue spectrum) over time. (Right) Change in absorbance at 433 nm (green) and 383 nm (blue) to show decay of [(OCH3TMPA)CuII(O2•−)]+ and formation of [{(OCH3TMPA)CuIII}2(O2−)2]2+, respectively.
Figure 5.
Figure 5.
(A) Monitoring of the reaction of [{(OCH3TMPA)CuIII}2(O2−)2]2+ (blue spectrum) with p-OMe-2,6-DTBP (final spectrum in magenta) with isosbestic points at λ = 320 and 353 nm. (B) EPR spectra of [{(OCH3TMPA)CuIII}2(O2−)2]2+ before (black, silent EPR) and after (magenta) its reaction with p-OMe-2,6-DTBP. Note: The sharp peak at 408 nm UV-vis feature in (A) and the sharp g = 2.0 signal in (B) are both indicative of formation of the stable p-OMe-2,6-DTBP radical.
Scheme 1.
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.
Scheme 2.
Scheme 2.
Low-temperature oxygenation of [(OCH3TMPA)CuI]+ results in the initial formation of a cupric superoxide species, which then slowly converts to a bis-μ-oxo dicopper(III) complex. The latter forms directly if oxygenation is carried out at temperatures greater than ~ −115 °C. See text.
Scheme 3.
Scheme 3.
Proposed reaction of [{(OCH3TMPA)CuIII}2(O2−)2]2+ with p-OMe-2,6-DTBP and DTBP.
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