Reactivity of a 1:1 copper-oxygen complex: Isolation of a Cu(II)-o-iminosemiquinonato species

Article abstract of DOI:10.1039/b418939f

While a 1:1 Cu-O₂ adduct is generally unreactive with organic substrates, phosphines displace O₂ via an associative process and added Cu(I) leads to a novel internal ligand oxidation to yield a Cu(II)-o-iminosemiquinone complex. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b418939f

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Reactivity of a 1:1 copper–oxygen complex: isolation of a
Cu(II)-o-iminosemiquinonato species{
Anne M. Reynolds, Elizabeth A. Lewis, Nermeen W. Aboelella and William B. Tolman*
Received (in Cambridge, UK) 17th December 2004, Accepted 11th February 2005
First published as an Advance Article on the web 24th February 2005
DOI: 10.1039/b418939f
result in changes in the UV-vis spectrum, even after prolonged
While a 1:1 Cu–O2 adduct is generally unreactive with organic
substrates, phosphines displace O2 via an associative process
and added Cu(I) leads to a novel internal ligand oxidation to
yield a Cu(II)-o-iminosemiquinone complex.
reaction times. Addition of PMePh2 to 1a at 280 uC did not result
in appreciable phosphine oxidation; instead, displacement of the
bound O2 to yield LCu(PMePh2) occurred.5 A similar displace-
ment was observed with PPh3, but only upon warming. In both
cases, the identities of the Cu(I)-phosphine adducts were confirmed
by comparison of spectral features to independently prepared
samples that were fully characterized, including by X-ray crystal-
lography.{,{ No reaction was observed with the bulkier trimesityl-
phosphine. Monitoring of the displacement reaction with excess
PMePh2 (15–35-fold) by UV-vis spectroscopy showed pseudo-
first-order kinetics. A plot of kobs vs. [PMePh2] was linear with an
intercept of zero, indicating an overall second-order rate law,
2d[1a]/dt 5 k[1a][PMePh2], k 5 (8.4 ¡ 0.4) 6 1023 M21 s21
(280 uC). These data are consistent with an associative mechan-
ism, implicating accessibility of the copper center in 1a to added
substrates. The lack of reactivity with other molecules thus
appears not to be solely due to the steric bulk imposed by the
b-diketiminate ligand, and points to notably enhanced stabilization
of the bound O2 in the complex.
Dioxygen activation by copper(I) centers is a critical first step in
many fundamentally important biological and catalytic processes.1
Significant mechanistic understanding of these reactions has been
obtained through studies of synthetic Cu/O2 intermediates.2 The
manner by which O2 may bind to a single copper center has
recently been modeled by complexes 1. In addition to having been
characterized by spectroscopy, theory, and X-ray crystallography,
the mechanism of formation of these adducts has been defined3
and their utility as synthons for the construction of asymmetric
bis(m-oxo)dimetal complexes has been demonstrated.3b,4 Herein,
we report results of further investigations of the reactivity of this
class of 1:1 Cu/O2 adducts. We have found that while 1a is
generally unreactive toward exogenous organic substrates, in the
presence of [Cu(MeCN)4]O3SCF3 1a undergoes a novel internal
ligand oxidation process to yield a Cu(II)-o-iminosemiquinone
complex.
Consistent with the demonstrated reactivity of 1a with added
redox-active metal reagents that provides bis(m-oxo)dimetal
complexes,3b,4 addition of 1 equiv. [Cu(MeCN)4]O3SCF3 to a
degassed solution of 1a at 280 uC in THF or acetone resulted in a
rapid color change from pale green to yellow/brown. Interestingly,
however, the product was found not to be a bis(m-oxo) complex,
but instead a new species (2, Fig. 1). An X-ray structure of crystals
Solutions of 1a decompose upon warming to give as-yet
unidentified Cu(II)-containing species, which upon removal of
copper yield a complex mixture of organic products (.25 different
species by GC/MS).{ The few that could be identified by com-
parison to independently synthesized material include ligand (m/z
418), 2,6-diisopropylaniline (m/z 177), and (C6H3i-Pr2)NCMe2
(m/z 217). Some of the other ligand fragments contain an oxygen
atom derived from 1a on the basis of 18O labeling experiments.
Due to the complexity of the product mixture, however, charac-
terization of the decomposition reaction has not been feasible.
No acceleration of the decay of 1a was observed when reagents
were added that typically undergo hydrogen- or oxygen-atom
transfer reactions with dinuclear copper–oxygen adducts at low
temperatures.2 For example, treatment of degassed solutions of
1a with phenols, phenolates, thioanisole, cyclohexene, ferrocene or
acids such as HBF4 at temperatures as high as 260 uC did not
{ Electronic supplementary information (ESI) available: experimental
details, and kinetic and X-ray crystallographic data. See http://
www.rsc.org/suppdata/cc/b4/b418939f/
Fig. 1 Cationic portion of the crystal structure of 2 (the triflate anion has
been omitted for clarity).
*tolman@chem.umn.edu
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and an oxidation step would rationalize generation of 2.
Precedence for this pathway is provided by previous reports of
aromatic hydroxylation of internal ligand substrates by discrete
dicopper-O2 adducts9,10 and of hydroxylation coupled to an NIH
shift in selected cases.11 Also relevant is a report of hydroxylation
of the open position of a fluorinated b-diketiminate ligand, albeit
in the absence of an observable intermediate.12 The reaction we
have discovered is unique in that hydroxylation of the substrate is
followed by oxidation to a semiquinone-type ligand.
Fig. 2 Resonance formulations for the cationic portion of 2. Pz 5 3,5-
diphenylpyrazolyl; Ar 5 2,6-diisopropylphenyl.
We thank the NIH (GM47365), the NSF (pre-doctoral
fellowships to A.M.R. and N.W.A.), and the University of
Minnesota (Louise T. Dosdall Fellowships to A.M.R. and
N.W.A.) for financial support, and Dr Neil Brooks for assistance
with the X-ray crystallography.
isolated in the presence of an added coordinating ligand, 3,5-
diphenylpyrazole, showed that 2 arises from oxo-transfer to one of
the aryl moieties of the b-diketiminate in conjunction with a 2,3-
isopropyl-group shift.{ A triflate counterion is associated with
the complex hydrogen-bonded to the 3,5-diphenylpyrazolyl unit
Anne M. Reynolds, Elizabeth A. Lewis, Nermeen W. Aboelella and
William B. Tolman*
Department of Chemistry and Center for Metals in Biocatalysis,
University of Minnesota, 207 Pleasant Street, SE, Minneapolis, MN
55455, USA. E-mail: tolman@chem.umn.edu; Fax: 612-624-7029;
Tel: 612-625-4061
˚
(N4–O3 5 2.732 A), indicating an overall charge of +1. The low
temperature UV-vis spectrum of 2 features an intense shoulder
at #385 nm (e 5 17 000 M21 cm21); this feature bleached upon
warming above 280 uC, indicating complex decomposition.
Solutions of 2 are EPR silent (X-band, 20 K).
In view of its diamagnetic character and overall charge of +1,
the bonding in 2 may be envisioned in terms of the resonance
formulations shown in Fig. 2. Careful analysis of the ligand bond
lengths in the X-ray crystal structure (Fig. 3) allows the Cu(III)-
o-amidophenolate form 2c to be ruled out.6 The six C–C bonds in
the oxygenated ring are distinctly different, with two alternating
shorter CLC bonds and four longer C–C bonds, indicating a
quinone-type distortion. The two ligand ‘‘aryl’’ C–N bonds are
Notes and references
{ X-Ray data for LCu(PMePh2): C42H54CuN2P, M 5 681.38, monoclinic,
˚
a 5 11.6105(10), b 5 21.0202(12), c 5 15.9832(9) A, b 5 97.837(2),
3
˚
V 5 3864(1) A , T 5 173 K, space group P21/n, Z 5 4, m(Mo Ka) 5
0.636 mm21, 37565 reflections measured, 6828 unique (Rint 5 0.0543),
R1 5 0.0452, wR2 5 0.0907 (F2, all data). X-Ray data for LCu(PPh3):
C47H56CuN2P, M 5 743.45, monoclinic, a 5 23.439(2), b 5 15.9011(15),
˚
c 5 24.217(2) A, b 5 111.850(2), V 5 8377.2(14), T 5 173 K, space group
P21/n, Z 5 8, m(Mo Ka) 5 0.593 mm21, 79358 reflections measured, 14812
˚
also significantly different, and the C–O bond length of 1.284(4) A
unique (Rint 5 0.0497), R1 5 0.0416, wR2 5 0.0978 (F2, all data). X-Ray
data for 2: C49H60CuF3N4O5S, M 5 937.61, monoclinic, a 5 29.967(3),
is in the range of those reported for transition metal o-iminose-
miquinonato(21) species, supporting structure 2b.7 In further
support of this assignment, all metal–ligand bond lengths are typi-
3
˚
˚
b 5 13.6607(12), c 5 23.462(2) A, b 5 92.711(2), V 5 9593.9(15) A ,
T 5 173 K, space group C2/c, Z 5 8, m(Mo Ka) 5 0.559 mm21, 23905
reflections collected, 8490 unique (Rint 5 0.0712), final R1 5 0.0551,
wR2 5 0.1188 (F2, all data). CCDC 258882–258884. See http://
www.rsc.org/suppdata/cc/b4/b418939f/ for crystallographic data in .cif or
other electronic format.
˚
cal of Cu(II), where the three Cu–N distances average 1.94 A. The
results of a bond valence sum analysis are also consistent with a +2
oxidation state for the copper center, and argue against structures
2a8 and 2c.{ Taken together, the available data are thus most
consistent with the Cu(II)-o-iminosemiquinonato assignment 2b.
Little mechanistic information concerning the formation of 2
is currently available. We do know that [Cu(MeCN)4]+ is critical
for the reaction, as the UV-vis spectral features of 1a are
not perturbed by addition of other Lewis acids (e.g. BF3?Et2O
or AgO3SCF3) or redox agents (e.g. ferrocene, Cu(O3SCF3)2
or Fe(MeCN)2(O3SCF3)2). We speculate that [Cu(MeCN)4]+ may
lead to formation of a species in which a bound dioxygen ligand is
activated for attack at the aryl ring of the b-diketiminate, perhaps
as a m-g2:g2-peroxide- or bis(m-oxo)dicopper unit. After electro-
philic attack at the aryl ring, an NIH shift of an isopropyl group
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Fig. 3 Selected bond distances (A) in 2.
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