Spectroscopic capture and reactivity of a low-spin cobalt(IV)-oxo complex stabilized by binding redox-inactive metal ions

Article abstract of DOI:10.1002/anie.201405874

High-valent cobalt-oxo intermediates are proposed as reactive intermediates in a number of cobalt-complex-mediated oxidation reactions. Herein we report the spectroscopic capture of low-spin (S=1/2) Co^IV-oxo species in the presence of redox-inactive metal ions, such as Sc³⁺, Ce³⁺, Y³⁺, and Zn²⁺, and the investigation of their reactivity in C-H bond activation and sulfoxidation reactions. Theoretical calculations predict that the binding of Lewis acidic metal ions to the cobalt-oxo core increases the electrophilicity of the oxygen atom, resulting in the redox tautomerism of a highly unstable [(TAML)Co^III(O^.)]^2- species to a more stable [(TAML)Co^IV(O)(Mⁿ⁺)] core. The present report supports the proposed role of the redox-inactive metal ions in facilitating the formation of high-valent metal-oxo cores as a necessary step for oxygen evolution in chemistry and biology. What is the metal's role? Cobalt(IV)-oxo complexes binding redox-inactive metal ions, such as Sc³⁺, Ce³⁺, Y³⁺, and Zn²⁺, are investigated in oxygenation reactions. Theory predicts that the binding of metal ions to the cobalt-oxo core increases the electrophilicity of the oxygen atom. This result supports the role of redox-inactive metal ions in facilitating the formation of high-valent metal-oxo cores as a necessary step for oxygen evolution in chemistry and biology. CAN=cerium ammonium nitrate.

Full text of DOI:10.1002/anie.201405874

Angewandte
Chemie
DOI: 10.1002/anie.201405874
Cobalt-Oxo Complexes
Spectroscopic Capture and Reactivity of a Low-Spin Cobalt(IV)-Oxo
Complex Stabilized by Binding Redox-Inactive Metal Ions**
Seungwoo Hong, Florian F. Pfaff, Eunji Kwon, Yong Wang, Mi-Sook Seo, Eckhard Bill,
Kallol Ray,* and Wonwoo Nam*
Abstract: High-valent cobalt-oxo intermediates are proposed
as reactive intermediates in a number of cobalt-complex-
mediated oxidation reactions. Herein we report the spectro-
metal–oxo p* orbitals, such that their electronic structure is
(n+1)+
best described as intermediate between [M
ÀO·] and
(
n+2)+
[7]
[M
=O]. It is argued that the radical character resulting
IV
scopic capture of low-spin (S = 1/2) Co -oxo species in the
from significant spin density on the oxygen atom would make
3
+
3+
3+
n+
presence of redox-inactive metal ions, such as Sc , Ce , Y ,
the [M(O)] species particularly powerful hydrogen atom
2
+
and Zn , and the investigation of their reactivity in CÀH bond
activation and sulfoxidation reactions. Theoretical calculations
predict that the binding of Lewis acidic metal ions to the cobalt-
oxo core increases the electrophilicity of the oxygen atom,
resulting in the redox tautomerism of a highly unstable
abstracting agents and therefore extremely difficult to trap.
Some of us recently reported the stabilization of an S = 3/2
cobalt(IV)-oxo species in the presence of a redox-inactive
3
+
[8]
Lewis acidic metal ion (e.g., Sc ion); however, others have
III
[9]
claimed it to be a Co ÀOH species instead. In this study, we
III
2À
[
[
(TAML)Co (OC)]
species
to
a
more
stable
report the extension of the same methodology in stabilizing
low-spin (S = 1/2) [Co (O)(M )]
Sc , Ce , Y , and Zn ). Notably, a similar strategy was
used recently to stabilize a [Cu (NTsC)(Sc)] core, with
IV
n+
IV
n+ (n+2)+
n+
(TAML)Co (O)(M )] core. The present report supports
intermediates (M
=
3
+
3+
3+
2+
the proposed role of the redox-inactive metal ions in facilitating
the formation of high-valent metal–oxo cores as a necessary
step for oxygen evolution in chemistry and biology.
II
4+
[10]
copper in an unusual formal oxidation state of + 3. Thus,
although the oxo wall still holds (as terminal cobalt(IV)-oxo
species by itself are still elusive), the present study firmly
establishes the wider application of the strategy of utilizing
redox-innocent Lewis acids in trapping short-lived intermedi-
ates responsible for the oxo- and nitrene-transfer reactions
mediated by mid-to-late transition metals (Scheme 1).
We have employed a tetraamido macrocyclic ligand
(TAML), which has found application in the stabilization of
a variety of high-valent iron(IVor V)-oxo and manganese(V)-
H
igh-valent metal-oxo complexes have been implicated as
active oxidants in oxygenation and water oxidation reactions
[1]
with a number of biological and chemical catalysts.
Although high-valent terminal metal-oxo complexes of
Groups 3–8 are ubiquitous, isolation of metal-oxo species to
the right of Group 8 (also known as the “oxo wall” for C4v
symmetry) has only been achieved for heavy transition metals
[2]
[3]
like iridium and platinum. For example, high-valent
terminal metal-oxo complexes of the lighter analogues like
cobalt, nickel, and copper still remain elusive, although they
are often invoked as highly reactive transient intermediates in
metal-complex-catalyzed CÀH bond activation and OÀO
[
1e,g,4,5]
bond formation reactions.
Theory suggests that they
should be powerful oxidants, perhaps even more reactive than
[
1,6]
the related iron-oxo species.
The predicted high reactivity
n+
of the [M(O)] (M = Co, Ni, and Cu) species is attributed to
n+
a relatively weak metal–oxo bond due to the population of the
Scheme 1. Oxidation of complex 1 to 2-M by CAN/H O or PhIO/M
.
2
[
+]
[+]
+
[
*] S. Hong, E. Kwon, M.-S. Seo, Prof. Dr. W. Nam
Department of Chemistry and Nano Science
Ewha Womans University
[ ] These authors contributed equally to this work.
[
**] This work was supported by the NRF of Korea through CRI (2-2012-
794-001-1 to W.N.), GRL (2013-4213-001-1 to W.N.), and NFSC
grants 21173211 and 21233008 to Y.W.). K.R. acknowledges
1
(
Seoul 120-750 (Korea)
E-mail: wwnam@ewha.ac.kr
financial support from the Cluster of Excellence “Unifying Concepts
in Catalysis” (EXC 314/1), Berlin and Cost Action CM1305
ECOSTBio. XAS data were obtained on NSLS beamline X3B
(Brookhaven National Laboratory), with support from NIH Grant
P30-EB-009998 and the U.S. Department of Energy. We thank Dr.
Erik R. Farquhar for help with XAS data collection and analysis.
[
+]
F. F. Pfaff, Dr. K. Ray
Humboldt-Universitꢀt zu Berlin, Department of Chemistry
Brook Taylor Strasse 2, 12489 Berlin (Germany)
E-mail: kallol.ray@chemie.hu-berlin.de
Y. Wang
State Key Laboratory of Molecular Reaction Dynamics
Dalian Institute of Chemical Physics
Chinese Academy of Science, Dalian 116023 (China)
Supporting information for this article (including experimental
details, Tables S1–S8, and Figures S1–S17) is available on the WWW
under http://dx.doi.org/10.1002/anie.201405874.
E. Bill
Max-Plank-Institut fꢁr chemische Energiekonversion
Mꢁlheim an der Ruhr (Germany)
Angew. Chem. Int. Ed. 2014, 53, 10403 –10407
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
10403

.
Angewandte
Communications
[
11]
oxo complexes.
In contrast to the previously employed
(tris[2-(N-tetramethylguanidyl)ethyl]amine)
Similarly, treatment of 1 with PhIO in the presence of other
+
3
2+
TMG tren
redox-inactive metal ions, such as Y and Zn , results in the
generation of the characteristic absorption band at 600 nm
corresponding to the formation of 2-Y and 2-Zn, respectively.
It is notable that reactions of 1 with PhIO in the absence of
redox-inactive metal ions or in the presence of weaker Lewis
3
[8]
ligand, which enforced a trigonal-bipyramidal geometry at
the Co center stabilizing a S = 3/2 ground state, TAML can
provide access to a square-pyramidal cobalt(IV)-oxo center
that would be a prerequisite for the targeted S = 1/2 state.
Reactions of equimolar amounts of deprotonated TAML and
cobalt(III) acetylacetonate in tetrahydrofuran afforded the
previously reported purple [(TAML)Co ] anion, which is
obtained as a lithium salt Li[(TAML)Co ]·3(H O) (1)
containing three cocrystallized molecules of water (see the
Supporting Information (SI), Tables S1 and S2 and Figures S1
and S2). Complex 1 is paramagnetic with S = 1 (meff = 3.1 Bohr
magnetons) ground state, as expected for a Co ion in the
observed square-planar geometry. Cyclic voltammetry (CV)
measurements with 1 at 258C in CH CN give a reversible
oxidation wave centered at 1.00 V vs. saturated calomel
electrode (SI, Figure S3); coulometric measurements show
that the oxidation corresponds to a 1e process. The
reversibility of this oxidation wave at room temperature
suggests that a formal Co state is thermally and kinetically
IV
[13]
acidic metal ions, such as Mg(CF SO ) , Ca(CF SO ) , and
3
3
2
3
3 2
Sr(CF SO ) , does not form the 2-M species, thereby estab-
3
3 2
III
À
[12]
lishing the importance of the Lewis acidity of the redox-
III
inactive metal ions in trapping the 2-M intermediates. Never-
2
3
+
3+
theless, the absorption features of the 2-M (M = Sc , Ce ,
3
+
2+
Y , and Zn ) complexes are all identical irrespective of the
nature of the redox-inactive metal ions.
III
The nature of these blue species can be established by
a variety of spectroscopic techniques; characterizations were
performed mainly on 2-Sc, because it was the most stable
species among the trapped 2-M complexes. Thus, cold-spray
ionization time-of-flight mass spectrometry (CSI-TOF MS) of
2-Sc reveals two high-resolution ion peaks at a mass-to-charge
(m/z) ratio of 785.1010 and 803.1042, whose mass and isotope
distribution patterns correspond to respective formulations of
3
À
IV
1
6
accessible.
[(TAML)Co( O)(Sc)(CH CN) (CH OH) (CF SO )] (calcu-
3 2 3 2 3 3
1
6
In agreement with the electrochemical data, 1 is readily
oxidized in the presence of cerium ammonium nitrate (CAN)
in acetone at 58C to form a metastable blue species (2-Ce)
with a half-life (t ) of 20 min (SI, Figure S4, inset). The blue
color is associated with a band at 600 nm (e = 7200m cm )
with a shoulder near 730 nm (SI, Figure S4). We tentatively
assign these bands to be ligand-to-metal charge transfer
lated m/z = 785.1057) and [(TAML)Co( O)(Sc)(CH CN) -
3 4
(CF SO )] (calculated m/z = 803.1064) (SI, Figure S5). Upon
3
3
1
8
18
introduction of O into 2-Sc using PhI O, mass shifts from
785.1057 to 787.1015 and from 803.1042 to 805.1021 occur (SI,
Figure S5 inset), thereby demonstrating that an oxygen atom
from PhIO is incorporated into 2-Sc.
1
/2
À1
À1
In contrast to 1, which is EPR silent, the X-band EPR
(
LMCT) in origin, which presumably arise from transitions
spectrum of 2-Sc at 5.0 K shows a highly rhombic signal at g =
x
IV
from the amide nitrogens of TAML to the Co center in 2-
Ce; as expected, these bands are significantly blue-shifted in
the corresponding Co
4
treatment of 1 with iodosylbenzene (PhIO) in the presence
of Sc(CF SO ) also generates a blue species (2-Sc) with
2.57, g = 2.16, and g = 2.03 (Figure 1, inset). Additionally,
y
z
5
9
a Co hyperfine pattern is observed at g according to an A
y y
component of the hyperfine coupling tensor of 52 ꢀ 10 cm .
III
À4
À1
complex 1 [l = 510 nm (e =
max
À1
À1
À1
À1
000m cm ) and 650 (e = 1200m cm )]. Interestingly,
The observed large g-anisotropy (Dg = g Àg ) of 0.54 is
z
x
consistent with a cobalt-centered radical (S = 1/2) resulting
IV
[14]
from a low-spin Co configuration in the ground state of 2-
Sc; this is further corroborated by X-ray absorption and DFT
studies (see below). Spin quantification of the EPR signal can
account for 70% of the total cobalt spins present in solution,
thereby showing that 2-Sc represents the major product of the
3
3 3
absorption features indistinguishable from those of 2-Ce, but
-Sc is slightly more stable with a t1/2 of 35 min (Figure 1).
2
3
+
reaction of 1 with PhIO in the presence of Sc ion. EPR
spectra of other 2-M complexes were also measured and
[
15]
found to be identical to 2-Sc.
All our attempts to collect resonance Raman spectra
proved unsuccessful owing to the photodecomposition of 2-Sc
under laser irradiation. Hence, structural evidence for the
nature of 2-Sc was derived from X-ray absorption spectros-
copy (XAS) studies at the Co K-edge (Figure 2; SI, Figures S6
and S7). A frozen acetone solution of 2-Sc exhibits a K-edge
at 7721.5 eV, an energy value 0.9 eV higher than that
associated with its cobalt(III) precursor 1, thereby supporting
IV
metal-centered oxidation to the Co state. The principal
feature of the inner-sphere scattering peaks in the extended
X-ray absorption fine structure (EXAFS) spectra of 2-Sc at R’
ꢀ
1.40 ꢁ can be best fitted (SI, Table S3, fit 11) by considering
Figure 1. UV/Vis spectra of 2-Sc (gray line) observed in the reaction of
(black line, 0.25 mm) and PhIO (3.0 equiv) in the presence of
Sc(CF SO ) (20 equiv) in acetone at 58C. Inset shows the experimen-
two shells: an 0.7 O/N scatterer at 1.67 ꢁ (assigned to the
1
IV
Co ÀO unit) and a further shell of 4.3 O/N scatterers at
3
3 3
1
.86 ꢁ (corresponding to the N-donors of TAML). Fitting was
tal (gray line) and simulated (black line) X-band EPR spectrum of 2-Sc
0.5 mm) at 10 K. For simulation parameters see text.
(
also performed by considering only one shell of 5.0 O/N
1
0404 www.angewandte.org
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 10403 –10407

Angewandte
Chemie
[17]
(
CF SO ) (OH)].
Furthermore, although
a
doublet
3
3 4
ground state is predicted for both 2 and 2-Sc, analysis of the
Mulliken spin densities reveals significant differences in their
electronic structures (SI, Table S5). While the electronic
III
2À
structure of 2 can be best represented as [(TAML)Co (OC)]
with 70% of the unpaired-electron spin being localized on the
oxygen and only 24% on Co, 2-Sc possesses a [(TAML)-
IV
+
Co (O)(Sc)] ground state with the unpaired spin being
predominantly (78%) localized on the cobalt center. Thus,
3
+
III
2À
the binding of Sc ion to the [(TAML)Co (OC)] core
initiates a redox tautomerization event, whereby one electron
III
is transferred from the Co center to the “oxo” ligand
IV
+
thereby stabilizing a [(TAML)Co (O)(Sc)] core in 2-Sc.
Such a Lewis acid induced valence tautomerism is not
2
+
unprecedented in the literature; for example, Zn was
V
recently shown to bind to a Mn -oxo corrolazine complex to
induce interconversion to a valence tautomer characterized as
IV
[18]
a Mn -oxo corrolazine p-cation-radical.
The reactivity of 2-Sc was then investigated in hydrogen
atom (H atom) abstraction and oxygen atom transfer (OAT)
reactions. In the H atom abstraction reaction, addition of
xanthene to the solution of 2-Sc at 58C regenerated 1 with an
isosbestic point at 530 nm (Figure 3a; SI, Figure S10). The
first-order rate constant, determined by pseudo-first-order
fitting of the kinetic data for the decay of 2-Sc, increases
Figure 2. X-ray absorption spectroscopy (XAS) at the cobalt K-edge.
Left: XANES spectra of 1 (black line) and 2-Sc (red line). Top right:
EXAFS data of 2-Sc on a wave vector scale as back-Fourier trans-
formation (black solid line) and simulation (dotted blue line). Bottom
right: Fourier-transformed EXAFS spectrum of 2-Sc (black solid line)
and simulation (blue dotted line). For details, see SI.
linearly with increasing xanthene concentration, thereby
À1 À1 À1
scatterers at 1.86 ꢁ (SI, Table S3, fit 3); the quality of the fit
was significantly improved by incorporating an additional O/
N shell (SI, Table S3, fit 4). Whereas the outer-shell features
can be satisfactorily accounted for by several single-scattering
paths involving 9C at 2.80 ꢁ, 1Sc at 3.38 ꢁ, 7C at 3.10 ꢁ, and
giving a second-order rate constant of 2.3 ꢀ 10 m
s
at
58C (SI, Figure S11). A kinetic isotope effect (KIE) value of
13 was determined when [D ]-xanthene was used in the
reaction (SI, Figure S11). Such a large KIE value suggests that
the H atom abstraction by 2-Sc occurs through tunnelling,
which is typical for H atom abstraction reactions mediated by
2
4
O at 4.17 ꢁ, the fit could be satisfactorily improved by
[
1]
introducing multiscattering paths involving elements of the
supporting TAML ligand. Based on the 0.9 eV blue shift of
the K-edge of 2-Sc relative to 1 and the presence of oxygen
and scandium scatterers at 1.67 ꢁ and 3.38 ꢁ, 2-Sc is
metal-oxo species. The second-order rate constants were
also determined for the oxidation of other substrates by 2-Sc,
and a linear correlation between the reaction rates and the CÀ
[
19]
H bond dissociation energies (BDE) of the substrates was
obtained, leading us to propose that the H atom abstraction
from the CÀH bonds of substrates by 2-Sc is the rate-
determining step in the CÀH bond activation reaction (Fig-
3
+
proposed to have one Sc ion directly bound to the S = 1/2
IV
2À
[
(TAML)Co (O)] core. Notably, the determined Co–O
distance of 1.67 ꢁ for 2-Sc in EXAFS is significantly shorter
than the Co–O distances of 1.79 ꢁ and 1.85 ꢁ determined for
ure 3b; SI, Table S6 and Figure S12). Product analysis of the
reaction mixture revealed the formation of xanthone as
a major product (30 Æ 5)% yield based on the amount of 2-Sc
used) (SI, Table S7). Also, the source of oxygen in the
xanthone product turned out to be the cobalt-oxo species,
IV
14À
[16]
[
[
(a -P W O Co ) O]
(by X-ray diffraction)
and
2
2
17 61
2
IV
2+
[8]
(TMG tren)Co (O)(Sc)(CF SO ) )] (by EXAFS) com-
3
3
3 3
plexes, respectively, in which the formal oxidation state of the
cobalt centers is also + 4. This may reflect a higher single-
1
4À
18
18
bond character of the CoÀO bonds in [(a -P W O Co) O]
which was shown by an O-labeling experiment using O-
labeled 2-Sc (SI, Figure S13).
2
2
17 61
2
IV
2+
and [(TMG tren)Co (O)(Sc(CF SO ) )] than in 2-Sc.
3
3
3 3
In our effort to understand the role of redox-inactive
metal ions in the stabilization of the S = 1/2 Co –oxo core,
Complex 2-Sc also showed a high reactivity in OAT
reaction with thioanisole at 58C, thereby producing thioani-
sole oxide quantitatively and regenerating 1 with an isosbestic
point at 530 nm (SI, Table S7 and Figure S14). The source of
oxygen in the sulfoxide product was found to be 2-Sc on the
basis of the result of O-labeling experiment (SI, Figure S15);
this is consistent with the cobalt-oxo assignment of 2-Sc.
Moreover, as observed in the oxidation of sulfides by metal-
IV
density functional theory (DFT) calculations were performed
IV
2À
on 2-Sc as well as on the putative [(TAML)Co (O)] (2)
3
+
species in the absence of Sc ion (SI, Tables S4 and S5 and
Figures S8 and S9). The calculated bond lengths of Co–O
1
8
1
.700 ꢁ, Co–N 1.855 ꢁ, and Co–Sc 3.683 ꢁ for 2-Sc in the
doublet ground state are in good agreement with the EXAFS-
derived distances of 1.67, 1.86, and 3.38 ꢁ, respectively, which
validates our theoretical methods. Notably, a six-coordinate
scandium geometry is assumed in the calculation (with three
[
20]
3+
oxo species, the electrophilic character of the CoÀOÀSc
core in 2-Sc was established by a large negative 1 value of
À2.8 in the oxidation of para-substituted thioanisoles (Fig-
ure 3c; SI, Table S8). Finally, the effect of redox-inactive
metal ions on the reactivities of 2-M was investigated in the
bound triflates and two hydroxides; SI, Figure S8) based on
IV
the
crystal
structure
of
[(TMC)Fe (O)(Sc)-
Angew. Chem. Int. Ed. 2014, 53, 10403 –10407
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 10405

.
Angewandte
Communications
III
2À
[
[
(TAML)Co (OC)]
species
to
a
more
stable
IV
n+ (nÀ2)+
(TAML)Co (O)(M )]
core. Our particular interest in
low-spin cobalt(IV)-oxo species pertains to their proven
potential as active oxidants in CÀH bond activation and water
[
4]
oxidation reactions. Very recently, a heterogeneous cobalt
oxide catalyst for water oxidation has been proposed to
contain alkaline metals (albeit not detected by X-ray absorp-
[4f]
[4c,e]
tion spectroscopy) as part of Co O cubane moieties.
3
4
The alkaline metal ion in this species could facilitate access to
cobalt species with high oxidation states, similar to the Lewis
acidic metal ions described in this manuscript. Similarly, the
2
+
potential role of Ca in biological oxygen evolution by
facilitating high oxidation states at manganese has been
suggested by structural and electrochemical comparison
6
+
between a [Mn CaO ]
cubane and a related Mn O
4 4
3
4
[
22]
cubane. The present work supports the above hypotheses,
and therefore, provides an opportunity to elucidate elemen-
tary steps in cobalt-complex-mediated alkane and water
oxidation reactions, using the high purity of the Lewis acid
bound low-spin Co(IV)-oxo species, with its various prom-
inent spectroscopic signatures, as a stoichiometric reagent.
Received: June 3, 2014
Published online: July 31, 2014
Keywords: cobalt · Lewis acids · oxo ligands · oxygenation ·
.
redox tautomerization
[
1] a) W. Nam, Acc. Chem. Res. 2007, 40, 465, and references
therein; b) M. M. Abu-Omar, A. Loaiza, N. Hontzeas, Chem.
Rev. 2005, 105, 2227; c) C. Krebs, D. G. Fujimori, C. T. Walsh,
J. M. Bollinger, Jr., Acc. Chem. Res. 2007, 40, 484; d) A. Gunay,
K. H. Theopold, Chem. Rev. 2010, 110, 1060; e) J. Hohenberger,
K. Ray, K. Meyer, Nat. Commun. 2012, 3, 720; f) A. S. Borovik,
Chem. Soc. Rev. 2011, 40, 1870; g) K. Ray, F. Heims, F. F. Pfaff,
Eur. J. Inorg. Chem. 2013, 3784; h) S. P. de Visser, J.-U. Rohde,
Y.-M. Lee, J. Cho, W. Nam, Coord. Chem. Rev. 2013, 257, 381;
i) I. Hammarstrçm, S. Hammes-Schiffer, Acc. Chem. Res. 2009,
Figure 3. a) UV/Vis spectral changes observed in the reaction of 2-Sc
(
0.25 mm) and xanthene (20 equiv) in acetone at 58C. Inset shows the
ÀH
time course monitored at 600 nm. b) Plot of log k ’ against the C
BDE of substrates for the reactions of 2-Sc. Second-order rate
2
4
2, 1859; j) I. Rivalta, G. W. Brudvig, V. S. Batista, Curr. Opin.
constants, k , were determined and then adjusted for reaction stoichi-
2
ometry to yield k ’ based on the number of equivalent target C
bonds of substrates (see data in SI, Table S6 and Figure S12). c) Plots
ÀH
Chem. Biol. 2012, 16, 11.
2
[
2] R. S. Hay-Motherwell, G. Wilkinson, B. Hussain-Bates, M. B.
Hursthouse, Polyhedron 1993, 12, 2009.
3] E. Poverenov, I. Efremenko, A. I. Frenkel, Y. Ben-David,
L. J. W. Shimon, G. Leitus, L. Konstantinovski, J. M. L. Martin,
D. Milstein, Nature 2008, 455, 1093.
of log (k ) against Hammett s constants for para-substituted
rel
p
[
thioanisoles in acetone at 58C. (see data in SI, Table S8, and Fig-
ure S14).
[
4] a) C. J. Chang, Z.-H. Loh, C. Shi, F. C. Anson, D. G. Nocera, J.
Am. Chem. Soc. 2004, 126, 10013; b) M. W. Kanan, D. G.
Nocera, Science 2008, 321, 1072; c) W. Nam, I. Kim, Y. Kim, C.
Kim, Chem. Commun. 2001, 1262; d) S. Fukuzumi, S. Mandal, K.
Mase, K. Ohkubo, H. Park, J. Benet-Buchholz, W. Nam, A.
Llobet, J. Am. Chem. Soc. 2012, 134, 9906; e) M. D. Symes, Y.
Surendranath, D. A. Lutterman, D. G. Nocera, J. Am. Chem.
Soc. 2011, 133, 5174; f) M. W. Kanan, J. Yano, Y. Surendranath,
M. Dinc a˘ , V. K. Yachandra, D. G. Nocera, J. Am. Chem. Soc.
2010, 132, 13692; g) M. Zhang, M. de Respinis, H. Frei, Nat.
Chem. 2014, 6, 362; h) A. I. Nguyen, R. G. Hadt, E. I. Solomon,
T. D. Tilley, Chem. Sci. 2014, 5, 2874.
oxidation of thioanisole. As we have shown in the case of
nonheme iron(IV)-oxo and manganese(IV)-oxo com-
[21]
plexes, the OAT reactivity of 2-M becomes greater with
[
13]
increasing Lewis acidity of the redox-inactive metal ions in
2-M (SI, Figure S16).
In summary, we report the trapping of reactive low-spin
IV
n+ (n-2)+
n+
3+
3+
3+
S = 1/2 [(TAML)Co (O)(M )]
(M = Sc , Ce , Y ,
2
+
III
À
and Zn ) complexes in the reaction of [(TAML)Co ] with
CAN/H O or with PhIO/M mixtures. In the absence of M
n+
n+
2
ions, no high-valent cobalt-oxo cores were accessible. The
binding of Lewis acidic metal ions presumably increases the
electrophilicity of the oxygen atom of the cobalt-oxo core,
resulting in the redox tautomerism of a highly unstable
[
5] a) F. F. Pfaff, F. Heims, S. Kundu, S. Mebs, K. Ray, Chem.
Commun. 2012, 48, 3730; b) T. Nagataki, Y. Tachi, S. Itoh, Chem.
Commun. 2006, 4016; c) S. Yao, M. Driess, Acc. Chem. Res. 2012,
45, 276.
1
0406 www.angewandte.org
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 10403 –10407

Angewandte
Chemie
[
6] a) D. Schrçder, M. C. Holthausen, H. Schwarz, J. Phys. Chem. B
004, 108, 14407; b) B. F. Gherman, W. B. Tolman, C. J. Cramer,
[15] The EPR spectrum (Figure 1 inset) of 2-Sc also shows a minor
2
sharp signal (< 5% yield) at g = 2.00, which corresponds to the
product formed as a result of the thermal decay of 2-Sc (SI,
Figure S17).
J. Comput. Chem. 2006, 27, 1950; c) A. W. Pierpont, T. R.
Cundari, Inorg. Chem. 2010, 49, 2038.
[
7] C. Limberg, Angew. Chem. 2009, 121, 2305; Angew. Chem. Int.
Ed. 2009, 48, 2270.
8] F. F. Pfaff, S. Kundu, M. Risch, S. Pandian, F. Heims, I.
Pryjomska-Ray, P. Haack, R. Metzinger, E. Bill, H. Dau, P.
Comba, K. Ray, Angew. Chem. 2011, 123, 1749; Angew. Chem.
Int. Ed. 2011, 50, 1711.
[16] D. B. Damatoy, L. J. W. Shimon, L. Weiner, R. E. Schreiber, P.
Jimꢃnez-Lozano, J. M. Poblet, C. de Graaf, R. Neumann, Inorg.
Chem. 2014, 53, 1779.
[17] S. Fukuzumi, Y. Morimoto, H. Kotani, P. Naumov, Y.-M. Lee, W.
Nam, Nat. Chem. 2010, 2, 756.
[18] P. Leeladee, R. A. Baglia, K. A. Prokop, R. Latifi, S. P. de Visser,
D. P. Goldberg, J. Am. Chem. Soc. 2012, 134, 10397.
[19] Y.-R. Luo, Comprehensive Handbook of Chemical Bond Ener-
gies, CRC, Boca Raton, 2007.
[
[
9] D. C. Lacy, Y. J. Park, J. W. Ziller, J. Yano, A. S. Borovik, J. Am.
Chem. Soc. 2012, 134, 17526.
[
10] S. Kundu, E. Miceli, E. Farquhar, F. F. Pfaff, U. Kuhlmann, P.
Hildebrandt, B. Braun, C. Greco, K. Ray, J. Am. Chem. Soc.
[20] a) M. Taki, S. Itoh, S. Fukuzumi, J. Am. Chem. Soc. 2002, 124,
998; b) J. Arias, C. R. Newlands, M. M. Abu-Omar, Inorg. Chem.
2001, 40, 2185; c) L. D. McPherson, M. Drees, S. I. Khan, T.
Strassner, M. M. Abu-Omar, Inorg. Chem. 2004, 43, 4036; d) A.
Kumar, I. Goldberg, M. Botoshansky, Y. Buchman, Z. Gross, J.
Am. Chem. Soc. 2010, 132, 15233.
[21] a) J. Chen, Y.-M. Lee, K. M. Davis, X. Wu, M. S. Seo, K.-B. Cho,
H. Yoon, Y. J. Park, S. Fukuzumi, Y. N. Pushkar, W. Nam, J. Am.
Chem. Soc. 2013, 135, 6388; b) J. Park, Y. Morimoto, Y.-M. Lee,
W. Nam, S. Fukuzumi, J. Am. Chem. Soc. 2011, 133, 5236.
[22] a) J. S. Kanady, E. Y. Tsui, M. W. Day, T. Agapie, Science 2011,
333, 733; b) N. Cox, L. Rapatskiy, J.-H. Su, D. A. Pantazis, M.
Sugiura, L. Kulik, P. Dorlet, A. W. Rutherford, F. Neese, A.
Boussac, W. Lubitz, J. Messinger, J. Am. Chem. Soc. 2011, 133,
3635.
2012, 134, 14710.
[
11] a) F. Tiago de Oliveira, A. Chanda, D. Banerjee, X. Shan, S.
Mondal, L. Que, Jr., E. L. Bominaar, E. Mꢂnck, T. J. Collins,
Science 2007, 315, 835; b) A. Chanda, X. Shan, M. Chakrabarti,
W. C. Ellis, D. L. Popescu, F. Tiago de Oliveira, D. Wang, L.
Que, Jr., T. J. Collins, E. Mꢂnck, E. L. Bominaar, Inorg. Chem.
2008, 47, 3669; c) T. J. Collins, S. W. Gordon-Wylie, J. Am. Chem.
Soc. 1989, 111, 4511.
[
12] A. Ghosh, P. Ramidi, S. Pulla, S. Z. Sullivan, S. L. Collom, Y.
Gartia, P. Munshi, A. S. Biris, B. C. Noll, B. C. Berry, Catal. Lett.
2010, 137, 1.
[
13] a) S. Fukuzumi, K. Ohkubo, Chem. Eur. J. 2000, 6, 4532; b) S.
Fukuzumi, K. Ohkubo, J. Am. Chem. Soc. 2002, 124, 10270.
14] J. G. McAlpin, Y. Surendranath, M. Dinc a˘ , T. A. Stich, S. A.
Stoian, W. H. Casey, D. G. Nocera, R. D. Britt, J. Am. Chem. Soc.
[
2010, 132, 6882.
Angew. Chem. Int. Ed. 2014, 53, 10403 –10407
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 10407