A 1:1 copper-dioxygen adduct is an end-on bound superoxo copper(II) complex which undergoes oxygenation reactions with phenols

Article abstract of DOI:10.1021/ja067411l

Employing a strongly electron-donating tripodal tetradentate ligand along with a reaction strategy designed to suppress binuclear peroxo complex formation, an end-on bound superoxo-copper(II) complex [Cu^II(NMe₂-TMPA)(O₂^-)]⁺ (1) has been generated in solution [UV-vis (THF, -85 °C): λmax = 418 nm (ε, 4300 M-1 cm-1), 615 nm (ε, 1100), 767 nm (ε, 840)]. Resonance Raman spectroscopy employing isotopically substituted dioxygen (including mixed isotope ^16/18O₂) proves the end-on superoxo Cu^II(O₂-) structural formulation, ν(O-O) = 1121 cm-1; ν(Cu-O) = 472 cm^-1. The first demonstration of Cu^II(O₂-) oxidative reactivity with exogenous substrates, likely involving H-atom abstraction chemistry, comes with the finding that 1 effects the oxygenation and hydroperoxylation of substituted phenols. Copyright

Full text of DOI:10.1021/ja067411l

Published on Web 12/15/2006
A 1:1 Copper-Dioxygen Adduct is an End-on Bound Superoxo Copper(II)
Complex which Undergoes Oxygenation Reactions with Phenols
Debabrata Maiti,† H. Christopher Fry,† Julia S. Woertink,‡ Michael A. Vance,‡
Edward I. Solomon,‡ and Kenneth D. Karlin*,†
Department of Chemistry, The Johns Hopkins UniVersity, Baltimore, Maryland 21218, and
Department of Chemistry, Stanford UniVersity, Stanford, California 94305
Received October 16, 2006; E-mail: karlin@jhu.edu
Reactive intermediates derived from binding and activation of
dioxygen by a single copper ion site play a central role in the mech-
anism of biologically significant metalloenzymes, including amine
oxidases, peptidylglycine-R-hydroxylating monooxygenase (PHM),
and dopamine-â monooxygenases (DâM).1-3 This enzyme chem-
istry, in particular, for PHM/DâM, has spurred continued emphasis
on the development of mononuclear Cu-oxygen-derived chemis-
- 3-5
try,1 for example, CuII-O2
,
CuII-OOH,1,6 and CuII-O•
species,1,7-9 along with interrogation of their structures (e.g., end-
on vs side-on), electronic nature/spectroscopy and reactivity.
Herein, we conclusively identify and prove end-on η1-O2- liga-
tion for a low-temperature stable cupric superoxide CuII(O2-) spe-
cies and for the first time describe exogenous substrate oxygenative
reactivity. In a recent X-ray structure of PHM,10 this 1:1 CuI/O2
adduct has been observed and it has been suggested by some that
such an entity would effect H-atom abstraction.3-5 The end-on
bound CuII(O2-) moiety has been previously suggested to occur in
a number of synthetically derived copper complex systems,11-16
typically stopped-flow kinetic transients or low-temperature and
sterically or electronically stabilized species. With a highly basic
tripodal tetradentate N4 ligand, Schindler and Sundermeyer re-
cently obtained the first X-ray structure for a complex in this
CuII(O2-) class ( Cu-O-O ) 123.5°, Cu-O ) 1.927 Å, O-O
) 1.280 Å).16
Figure 1. UV-visible spectra of [CuI(NMe2-TMPA)(CO)]+ (2) in CO-
saturated THF at -85 °C, λmax ) 350 nm (blue) and after addition of O2,
producing [CuII(NMe2-TMPA)(O2-)]+ (1), λmax ) 418, 615, 767 nm
(green).
The generation of superoxo-Cu(II) complex 1 starts with the
Figure 2. Solvent-subtracted rR spectra of THF solutions of [CuII(NMe2-
TMPA)(O2-)]+ (1) (λex ) 413.1 nm). (A) Detail of the νCu-O region. (B)
Detail of the νO-O region. red, 16O2; blue, 18O2; green, 16-18O2.24
colorless carbonyl complex [CuI(NMe2-TMPA)(CO)]+ (2),17 in
CO-saturated THF at -85 °C [λmax) 350 nm (ꢀ ) 2500 M-1 cm-1
,
Figure 1)]. Bubbling O2 directly through the solution for 1 min
(slowly displacing CO)18 results in a color change to a brilliant
green, giving an EPR silent complex formulated as [CuII(NMe2-
TMPA)(O2-)]+ (1), λmax ) 418 nm (ꢀ ) 4300 M-1 cm-1), 615 nm
(ꢀ) 1100 M-1 cm-1), and 767 nm (ꢀ ) 840 M-1 cm-1) (Figure 1),
features which are identical to those observed in a stopped-flow
spectroscopic [CuI(NMe2-TMPA)]+/O2 reactivity study.17 The
UV-vis features also match the data extracted for the parent
compound CuII(O2-) species, [CuII(TMPA)(O2-)]+ [λmax ) 410 nm
(ꢀ ) 4000 M-1 cm-1), 747 nm (ꢀ ) 1000 M-1 cm-1)].11 The
superoxide complex decays very slowly (t1/2 g 4 h) and without
formation of the bridging binuclear peroxo species. The electron-
rich nature of the NMe2-TMPA ligand and the procedures/
conditions used here lead to severely diminished (and not detectable)
formation of the binuclear µ-1,2-bridged peroxo species [{CuII-
The resonance Raman (rR) spectrum (λex) 413.1 nm, 77 K) of
[CuII(NMe2-TMPA)(O2-)]+ (1) in THF shows an O-O stretch at
1121 cm-1 which shifts to 1058 cm-1 upon 18O2 isotopic substitution
(Figure 2B). This stretching frequency is consistent with a bound
superoxo species.1,15,25 A Cu-O stretch was observed for the com-
plex at 472 cm-1 and shifted to 452 cm-1 upon 18O2 isotopic substi-
tution. Upon substitution with a mixed isotope gas containing a
1:2:1 stoichiometric mixture of 18O2/16/18O2/18O2 (16/18O2), a new
intermediate O-O stretch at 1091 cm-1 appears (Figure 2B) as
expected. For this mixed isotope sample, two equal intensity Cu-O
stretches were resolved at 472 and 452 cm-1, overlaying the pure
isotope stretches (Figure 2A). This Cu-O splitting pattern is only
consistent with an end-on, η1 superoxo-Cu binding mode, as a
symmetrically bound η2 superoxo-Cu complex would be expected
to show an additional ν(Cu-O) vibration at an averaged frequency
position.26
The reactivity of [CuII(NMe2-TMPA)(O2-)]+ (1) toward sub-
stituted phenols as potential H-atom donors was probed; there is
considerable practical interest in such oxidations (with Cu),27 and
there is relevant literature with Co superoxo complexes,28-34 along
with examples involving Fe,34,35 Mn,34 as well as Cr and Rh.36
When p-MeO-2,6-DTBP37 is added to a -85 °C THF solution
of [CuII(NMe2-TMPA)(O2-)]+ (1) (excess O2 removed), decom-
(NMe2-TMPA)}2(O22-)]2+ 17
The X-ray structure and properties
.
of the parent TMPA peroxo-dicopper(II) complex have been
previously established.19-22 Attack of the primary CuI/O2 1:1 adduct
by a second copper(I) ion is generally favorable in this and many
other systems, making it normally difficult to characterize initial
1:1 CuI/O2 (i.e., CuII-O2-) adducts.1,21,23
† The Johns Hopkins University.
‡ Stanford University.
9
264
J. AM. CHEM. SOC. 2007, 129, 264-265
10.1021/ja067411l CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
Supporting Information Available: Synthetic details, descriptions
of reactions, and product analyses. This material is available free of
charge via the Internet at http://pubs.acs.org.
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Acknowledgment. This work was supported by grants from
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