Reactivity of a Cobalt(III)-Hydroperoxo Complex in Electrophilic Reactions

Article abstract of DOI:10.1021/acs.inorgchem.6b02288

The reactivity of mononuclear metal-hydroperoxo adducts has fascinated researchers in many areas due to their diverse biological and catalytic processes. In this study, a mononuclear cobalt(III)-peroxo complex bearing a tetradentate macrocyclic ligand, [C

Full text of DOI:10.1021/acs.inorgchem.6b02288

Article
pubs.acs.org/IC
Reactivity of a Cobalt(III)−Hydroperoxo Complex in Electrophilic
Reactions
Bongki Shin,† Kyle D. Sutherlin,‡ Takehiro Ohta,§ Takashi Ogura,§ Edward I. Solomon,*,‡,∥
and Jaeheung Cho*,†
†Department of Emerging Materials Science, DGIST, Daegu 42988, Korea
‡Department of Chemistry, Stanford University, Stanford, California 94305, United States
§Picobiology Institute, Graduate School of Life Science, University of Hyogo, RSC-UH LP Center, Hyogo 679-5148, Japan
∥Stanford Synchrotron Radiation Laboratory, Stanford Linear Accelerator Center, Menlo Park, California 94025, United States
S
* Supporting Information
ABSTRACT: The reactivity of mononuclear metal−hydroperoxo adducts has
fascinated researchers in many areas due to their diverse biological and catalytic
processes. In this study, a mononuclear cobalt(III)−peroxo complex bearing a
tetradentate macrocyclic ligand, [CoIII(Me3-TPADP)(O2)]+ (Me3-TPADP = 3,6,9-
trimethyl-3,6,9-triaza-1(2,6)-pyridinacyclodecaphane), was prepared by reacting
[CoII(Me3-TPADP)(CH3CN)2]2+ with H2O2 in the presence of triethylamine.
Upon protonation, the cobalt(III)−peroxo intermediate was converted into a
cobalt(III)−hydroperoxo complex, [CoIII(Me3-TPADP)(O2H)(CH3CN)]2+. The
mononuclear cobalt(III)−peroxo and −hydroperoxo intermediates were charac-
terized by a variety of physicochemical methods. Results of electrospray ionization
mass spectrometry clearly show the transformation of the intermediates: the peak
at m/z 339.2 assignable to the cobalt(III)−peroxo species disappears with concomitant growth of the peak at m/z 190.7
corresponding to the cobalt(III)−hydroperoxo complex (with bound CH3CN). Isotope labeling experiments further support the
existence of the cobalt(III)−peroxo and −hydroperoxo complexes. In particular, the O−O bond stretching frequency of the
cobalt(III)−hydroperoxo complex was determined to be 851 cm−1 for 16O2H samples (803 cm−1 for 18O2H samples), and its
2
Co−O vibrational energy was observed at 571 cm−1 for 16O2H samples (551 cm−1 for 18O2H samples; 568 cm−1 for 16O2 H
samples) by resonance Raman spectroscopy. Reactivity studies performed with the cobalt(III)−peroxo and −hydroperoxo
complexes in organic functionalizations reveal that the latter is capable of conducting oxygen atom transfer with an electrophilic
character, whereas the former exhibits no oxygen atom transfer reactivity under the same reaction conditions. Alternatively, the
cobalt(III)−hydroperoxo complex does not perform hydrogen atom transfer reactions, while analogous low-spin Fe(III)−
hydroperoxo complexes are capable of this reactivity. Density functional theory calculations indicate that this lack of reactivity is
due to the high free energy cost of O−O bond homolysis that would be required to produce the hypothetical Co(IV)−oxo
product.
cobalt(II) complexes with SALEN ligands.10 In addition, the
INTRODUCTION
■
synthetic complexes have potential applications in dioxygen
Mononuclear metal−oxygen species such as metal−superoxo,
separation and storage.11
−peroxo, −hydroperoxo, and −oxo complexes have important
Recent advances in the characterization and reactivity of
roles as key intermediates involved in catalytic oxygenation
reactions and biological oxidation reactions.1−5 Heme and
nonheme iron−oxygen intermediates have been invoked in the
catalytic cycles of dioxygen activation by metalloenzymes.6
Among the iron−oxygen adducts, iron−hydroperoxo species
have received considerable attention, since the intermediates
have been implicated as reactive species in DNA cleavage
activity of bleomycin, a glycopeptide that is effective as an
antitumor drug in concert with iron and dioxygen.7
In the case of cobalt complexes, a large number of cobalt−O2
species have been synthesized as models of dioxygen carrier
proteins and as oxidants in organic functionalizations.8,9 In
models of biological oxygen carriers, Cavin et al., for example,
have extensively examined the oxygen carrier properties of
cobalt−O2 intermediates reveal that the cobalt−superoxo and
−peroxo species are active oxidants in electrophilic and
nucleophilic reactions.12 A notable example is the formation
of side-on cobalt(III)−peroxo complexes bearing a series of
tetraazamacrocyclic ligands.8a,e The intermediates have nucle-
ophilic character (e.g., aldehyde deformylation) toward organic
substrates. The end-on cobalt(III)−superoxo species has been
proposed as reactive species in electrophilic reactions (e.g.,
hydrogen atom transfer).13 Very recently, the reactivity of an
end-on cobalt(II)−superoxo intermediate with a redox non-
Received: September 22, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.6b02288
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
carried out with a spinning cell (1000 rpm) at −30 °C. Raman shifts
were calibrated with indene, and the accuracy of the peak positions of
the Raman bands was 1 cm−1. The effective magnetic moments were
innocent ligand has been investigated in catalytic deformylation
reactions.14
In contrast, there are few examples of cobalt(III)−hydro-
peroxo intermediates.15 Recently, the interconversion of
cobalt(III)−peroxo and −hydroperoxo species via acid−base
reactions has been reported where the cobalt(III)−hydro-
peroxo complex has shown reactivity in ligand oxidation
(Scheme 1).16 Nevertheless, the reactivity of cobalt(III)−
1
determined using the modified H NMR method of Evans at room
temperature.19 A WILMAD coaxial insert (sealed capillary) tube
containing the blank acetonitrile-d3 solvent (with 1.0% TMS) only was
inserted into the normal NMR tubes containing the complexes
dissolved in acetonitrile-d3 (with 0.03% TMS). The chemical shift of
the TMS peak (and/or solvent peak) in the presence of the
paramagnetic metal complexes was compared to that of the TMS
peak (and/or solvent peak) in the inner coaxial insert tube. The
effective magnetic moment was calculated using the equation μ =
0.0618(ΔνT/2fM)1/2, where f is the oscillator frequency (MHz) of the
superconducting spectrometer, T is the absolute temperature, M is the
molar concentration of the metal ion, and Δv is the difference in
frequency (Hz) between the two reference signals. CW-EPR spectra
were taken at 5 K using an X-band Bruker EMX-plus spectrometer
equipped with a dual mode cavity (ER 4116DM). Low temperatures
were achieved and controlled using an Oxford Instruments ESR900
liquid He quartz cryostat with an Oxford Instruments ITC503
temperature and gas flow controller. Product analysis was performed
on a Thermo Fisher Trace 1310 gas chromatograph (GC) system
equipped with a flame ionization detector (FID) and mass
Scheme 1. Acid−Base Reaction between the Metal−Peroxo
and Metal−Hydroperoxo Complexes
hydroperoxo species has been rarely explored in external
substrate oxidation.17 Herein, we report the synthesis and
characterization of a cobalt(III)−peroxo complex bearing a
tetradentate macrocyclic ligand, 3,6,9-trimethyl-3,6,9-triaza-
1(2,6)-pyridinacyclodecaphane (Me3-TPADP) (Scheme 2).
Upon protonation, the cobalt(III)−peroxo complex,
[CoIII(Me3-TPADP)(O2)]+ (2), was converted into a cobalt-
(III)−hydroperoxo complex, [CoIII(Me3-TPADP)(O2H)-
(CH3CN)]2+ (3). Complex 3 shows high reactivity in
sulfoxidation reactions with electrophilic character, which was
confirmed by a Hammett plot. Further, density functional
theory (DFT) calculations were applied to explain the reactivity
difference between 3 and its hypothetical iron analogue in
electrophilic reactions.
1
spectrometer (GC−MS). H NMR, 13C NMR, and 31P NMR spectra
were measured with Bruker AVANCE III-400 spectrometer at CCRF
in DGIST.
Generation and Characterization of [CoIII(Me3-TPADP)(O2)]+
(2). Treatment of [CoII(Me3-TPADP)(CH3CN)2](ClO4)2 (0.5 mM)
with 10 equiv of H2O2 in the presence of 2 equiv of triethylamine
(TEA) in CH3CN (2 mL) afforded the formation of a blue solution at
low temperature. Physicochemical data, including UV−vis and ESI-
MS, were reported in Figure 2. [CoIII(Me3-TPADP)(18O2)]+ was
prepared by adding 10 equiv of H218O2 (18 μL, 90% 18O-enriched,
2.2% H218O2 in water) to a solution containing [CoII(Me3-TPADP)-
(CH3CN)2](ClO4)2 (0.5 mM) and 2 equiv of TEA in CH3CN (2 mL)
at 25 °C.
Generation and Characterization of [CoIII(Me3-TPADP)(O2H)-
(CH3CN)]2+ (3). Treatment of [CoIII(Me3-TPADP)(O2)]+ (0.5 mM)
with 3 equiv of perchloric acid (HClO4) in CH3CN (2 mL) at −20 °C
afforded a magenta solution. Physicochemical data, including UV−vis,
ESI-MS, and resonance Raman, were reported in Figures 2 and 3.
[CoIII(Me3-TPADP)(18O2H)(CH3CN)]2+ was prepared by adding 3
equiv of HClO4 to a solution containing [CoIII(Me3-TPADP)(18O2)]+
(0.5 mM) in CH3CN (2 mL) at −20 °C. [CoIII(Me3-TPADP)-
EXPERIMENTAL SECTION
■
Materials. All chemicals obtained from Aldrich Chemical Co. were
the best available purity and used without further purification unless
otherwise indicated. Solvents were dried according to published
procedures and distilled under Ar prior to use.18 H218O2 (95% 18O-
enriched, 2.2% H218O2 in water) was purchased from ICON Services
Inc. (Summit, NJ).
Physical Methods. UV−vis spectra were recorded on a Hewlett-
Packard 8454 diode array spectrophotometer equipped with a
UNISOKU Scientific Instruments for low-temperature experiments
or with a circulating water bath. Electrospray ionization mass spectra
(ESI-MS) were collected on a Waters (Milford, MA) Acquity SQD
quadrupole mass instrument, by infusing samples directly into the
source using a manual method. The spray voltage was set at 2.5 kV and
the capillary temperature at 80 °C. Resonance Raman spectra were
obtained using a liquid nitrogen cooled CCD detector (CCD-1024 ×
256-OPEN-1LS, HORIBA Jobin Yvon) attached to a 1 m single
polychromator (MC-100DG, Ritsu Oyo Kogaku) with a 1200
grooves/mm holographic grating. An excitation wavelength of 355
nm was provided by an Nd:YAG laser (Photonic Solutions, SNV-20F),
with 10 mW power at the sample point. All measurements were
2
(16O2 H)(CH3CN)]2+ was prepared by adding 113.8 μL of D2O to a
solution containing 3 (0.5 mM) in CH3CN (2 mL) at −20 °C.
X-ray Crystallography. A single crystal of [CoII(Me3-TPADP)-
(CH3CN)2](ClO4)2 was picked from solutions by a nylon loop
(Hampton Research Co.) on a handmade copper plate mounted inside
a liquid N2 Dewar vessel at ca. −40 °C and mounted on a goniometer
head in a N2 cryostream. Data collections were carried out on a Bruker
SMART APEX II CCD diffractometer equipped with a mono-
chromator in the Mo Kα (λ = 0.71073 Å) incident beam. The CCD
data were integrated and scaled using the Bruker-SAINT software
package, and the structure was solved and refined using SHELXTL V
6.12.20 Hydrogen atoms were located in the calculated positions. All
non-hydrogen atoms were refined with anisotropic thermal parame-
ters. Crystal data for [CoII(Me3-TPADP)(CH3CN)2](ClO4)2 follow:
Scheme 2. Synthetic Procedures for Mononuclear Cobalt Complexes
B
DOI: 10.1021/acs.inorgchem.6b02288
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
C18H30Cl2CoN6O8, monoclinic, P21/n, Z = 8, a = 15.7400(3) Å, b =
19.7237(4) Å, c = 16.3257(3) Å, β = 97.6290(10)°, V = 5023.47(17)
Å3, μ = 0.951 mm−1, ρcalcd = 1.556 g/cm3, R1 = 0.0345, wR2 = 0.0890
for 12456 unique reflections, 641 variables. The crystallographic data
for [CoII(Me3-TPADP)(CH3CN)2](ClO4)2 is listed in Table S1, and
Table S2 lists the selected bond distances and angles. CCDC-1448782
for [CoII(Me3-TPADP)(CH3CN)2](ClO4)2 contains the supplemen-
tary crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif (or from the
Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge
CB2 1EZ, U.K.; fax (+44) 1223-336-033; or e-mail deposit@ccdc.cam.
ac.uk).
Computational Details. DFT calculations were performed using
the Gaussian 09 computational package.21 All calculations used the
B3LYP functional and the TZVP basis, and solvent effects were
included using the polarized continuum model with acetonitrile as the
solvent. Geometry optimizations were carried out using the default
convergence criteria and were confirmed as minima by the absence of
imaginary modes in a frequency calculation. Enthalpies and Gibbs free
energies were calculated at 298.15 K. Mayer bond order analysis was
performed using QMForge,22 and molecular orbital contours were
obtained using LUMO.23
Reactivity Studies. All reactions were run monitoring UV−vis
spectral changes of reaction solutions, and rate constants were
determined by fitting the changes in absorbance at 523 nm for
[CoIII(Me3-TPADP)(O2H)(CH3CN)]2+ (3). Reactions were run at
least in triplicate, and the data reported represent the average of these
reactions. In-situ-generated 3 was used in kinetic studies, such as the
oxidation of triphenylphosphine (PPh3) and thioanisole in CH3CN at
−20 and −40 °C. After the completion of reactions, pseudo-first-order
fitting of the kinetic data allowed us to determine kobs values. Products
formed in the oxidation of PPh3 by 3 in CH3CN at −20 °C were
analyzed by 31P NMR. Products formed in the oxidation of thioanisole
by 3 in CH3CN at −20 °C were analyzed by injecting the reaction
mixture directly into GC and GC−MS. Products were identified by
comparison with authentic samples, and product yields were
determined by comparison against standard curves prepared with
authentic samples.
Figure 1. (a) ORTEP plot of [CoII(Me3-TPADP)(CH3CN)2]2+ (1A)
with thermal ellipsoid drawn at the 30% probability level. Hydrogen
atoms are omitted for clarity. Selected bond lengths (Å): Co1−N1
2.0574(14), Co1−N2 2.1999(14), Co1−N3 2.2108(14), Co1−N4
2.2050(14), Co1−N5 2.0514(15), Co1−N6 2.1151(15). (b) X-band
EPR spectrum of 1 in frozen CH3CN at 5 K. Spectroscopic settings:
frequency = 9.646 GHz, microwave power = 0.998 mW, modulation
frequency = 100 kHz, and modulation amplitude = 1 mT.
moment of 4.2 μB in CD3CN using the 1H NMR Evans method
consistent with the presence of three unpaired electrons,19 and
the X-band electron paramagnetic resonance (EPR) spectrum
of 1 in CH3CN at 5 K shows an axial signal at g = 4.5 and 2.08
(Figure 1b). These results indicate an S = 3/2 ground state for
1.25 In addition, the redox potential of 1 was determined as E1/2
= 0.16 V (vs Fc+/Fc) by cyclic voltammetry (see the SI, Figure
S2).26
RESULTS AND DISCUSSION
■
Synthesis and Characterization. Me3-TPADP was
synthesized by a modification of a previously reported
procedure [see the Experimental Section in the Supporting
Information (SI)].24 The Me3-TPADP was characterized by
The cobalt(III)−peroxo complex, [CoIII(Me3-TPADP)-
(O2)]+ (2), was prepared by adding 10 equiv of hydrogen
peroxide (H2O2) to a reaction solution containing 1 in the
presence of 2 equiv of triethylamine (TEA) in CH3CN at 25 °C
(Scheme 2), where the color of the solution changed from
purple to blue. Complex 2 could not be isolated due to its
thermal instability; however, it was generated and characterized
at low temperature in solution. The UV−vis spectrum of 2 in
CH3CN at −40 °C shows an intense absorption band at 338
nm (ε = 2400 M−1 cm−1) and a weak absorption band at 550
nm (ε = 230 M−1 cm−1) (Figure 2a). The ESI-MS spectrum of
2 exhibits a prominent signal at m/z 339.2 (Figure 2b), whose
mass and isotope distribution pattern correspond to [Co(Me3-
TPADP)(O2)]+ (calcd m/z 339.1) (see the SI, Figure S3).
When the reaction was carried out with isotopically labeled
H218O2, a mass peak corresponding to [Co(Me3-TPADP)-
(18O2)]+ appeared at m/z 343.2 (calcd m/z 343.1) (Figure 2b,
inset). The four mass unit shift upon the substitution of 16O
with 18O indicates that 2 has two oxygen atoms.
1
electrospray ionization mass spectrometry (ESI-MS) and H
and 13C nuclear magnetic resonance (NMR) methods (see the
SI Experimental Section).
Synthetic procedures for Co(II) and Co(III) complexes used
are outlined in Scheme 2. The purple starting Co(II) complex,
[CoII(Me3-TPADP)(CH3CN)2](ClO4)2 (1-(ClO4)2), was syn-
thesized by reacting Co(ClO4)2·6H2O with the Me3-TPADP
ligand in CH3CN. Single crystals of 1-(ClO4)2 contained two
crystallographically independent but virtually identical cations
in the asymmetric unit (denoted “A” and “B”; see the SI, Tables
S1 and S2). Complex 1 has a six-coordinated Co(II) ion with
four nitrogen atoms of the Me3-TPADP ligand and two
nitrogen atoms of CH3CN solvent molecules (Figure 1a). The
Co(II) geometry is best described as a distorted octahedral.
The UV−vis spectrum of 1 in CH3CN shows a broad
absorption band at 498 nm (ε = 80 M−1 cm−1) (Figure 2a).
The ESI-MS spectrum of 1 exhibits three signals at a mass-to-
charge ratio (m/z) of 174.2, 194.7, and 406.2 (see the SI,
Figure S1), corresponding to [Co(Me3-TPADP)(CH3CN)]2+
(calcd m/z 174.1), [Co(Me3-TPADP)(CH3CN)2]2+ (calcd m/
z 194.6), and [Co(Me3-TPADP)(ClO4)]+ (calcd m/z 406.1),
respectively. At room temperature, 1 exhibits a magnetic
The resonance Raman spectrum of 2 was collected using 355
nm excitation in CH3CN at −30 °C. Compound 2 prepared
with H216O2 exhibits an isotope-sensitive band at 888 cm−1,
which shifts to 842 cm−1 when H218O2 is used (Figure 2a, inset,
see the SI, Figure S4). The observed isotopic shift of its 16Δ −
18Δ value of 46 cm−1 is in agreement with the calculated value
C
DOI: 10.1021/acs.inorgchem.6b02288
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 2. (a) UV−vis spectra of [CoII(Me3-TPADP)(CH3CN)2]2+
(1) (black line), [CoIII(Me3-TPADP)(O2)]+ (2) (red line), and
[CoIII(Me3-TPADP)(O2H)(CH3CN)]2+ (3) (blue line) in CH3CN at
−30 °C. Inset shows the resonance Raman spectra of 2 prepared with
H216O2 (red line) and H218O2 (blue line) obtained upon excitation at
355 nm in CH3CN at −30 °C. (b) ESI-MS of 2 in CH3CN at −40 °C.
Insets show the observed isotope distribution patterns for [Co(Me3-
TPADP)(16O2)]+ (lower) and [Co(Me3-TPADP)(18O2)]+ (upper).
Figure 3. (a) ESI-MS spectra of [CoIII(Me3-TPADP)(O2H)-
(CH3CN)]2+ (3) in CH3CN at −40 °C, which show the observed
isotope distribution patterns for [Co(Me3-TPADP)(16O2H)-
(CH3CN)]2+ (red line, left), [Co(Me3-TPADP)(18O2H)
(CH3CN)]2+ (blue line, left), [Co(Me3-TPADP)(16O2H)(ClO4)]+
(red line, right), and [Co(Me3-TPADP)(18O2H)(ClO4)]+ (blue line,
right). (b) Resonance Raman spectra of 3 (16 mM) prepared with 16O
(red line) and 18O (blue line) labeled samples of 2 obtained upon
excitation at 355 nm in CH3CN at −30 °C. Green line shows the
spectrum of 3 prepared with a 16O labeled sample of 2 and HClO4
diluted in D2O.
of 51 cm−1 for the O−O harmonic oscillator. This value is
comparable to those reported for structurally and spectroscopi-
cally characterized side-on cobalt(III)−peroxo complexes, such
as [CoIII(12-TMC)(O2)]+ (902 cm−1) and [CoIII(13-TMC)-
(O2)]+ (902 cm−1).8e In addition, the Co−O2 symmetric
stretch of 2 was observed at 565 cm−1, which shifts to 546 cm−1
labeled H218O2, the mass peaks corresponding to 3 shifted to
m/z 192.7 for [Co(Me3-TPADP)(18O2H)(CH3CN)]2+ (calcd
m/z 192.6) and 443.2 for [Co(Me3-TPADP)(18O2H)(ClO4)]+
(calcd m/z 443.1) (Figure 3a). Intermediate 3 reverted back to
2 on addition of 3 equiv of TEA. The addition of HClO4 to the
resulting solution regenerated 3, suggesting that 2 and 3 can be
readily interconverted by the acid−base reaction depicted in
Scheme 2. Such chemistry is well-known for other metal-based
peroxo and hydroperoxo systems.16,27a,28,29
upon 18O-substitution (16Δ − 18Δ = 19 cm−1; 16Δ − 18Δ(calcd)
=
21 cm−1) (Figure 2a, inset, see the SI, Figure S4).
The X-band EPR spectrum of 2 is silent at 4.3 K, suggesting
either a low-spin (S = 0) or an integer-spin (S = 1 or 2) Co(III)
d6 species. The 1H NMR spectrum of 2 recorded in CD3CN at
−40 °C exhibits sharp features in the 0−10 ppm region (data
not shown), indicating that 2 is in a low-spin (S = 0) state.
Addition of 3 equiv of perchloric acid (HClO4) to a solution
containing 2 in CH3CN at −20 °C immediately produced an
EPR silent magenta intermediate 3 with electronic absorption
bands at ∼330 nm (ε = 3300 M−1 cm−1) and 528 nm (ε = 240
M−1 cm−1) (Figure 2a). Compound 3 is thermally unstable.
Even at −40 °C, the characteristic absorption bands of 3
decayed over the course of hours (18% decay for 1 h). The
decay of 3 obeyed first-order reaction kinetics, and activation
parameters, ΔH⧧ = −115.9 kcal mol−1 and ΔS⧧ = −15.7 cal
mol−1 K−1, are obtained by the Eyring plot between 283 and
313 K (Figure S5). Compound 3 could not be formed via
simple oxidation of 1 with excess H2O2. The ESI-MS spectrum
of the solution at −40 °C suggested the formation of Co(III)−
hydroperoxo species, [Co(Me3-TPADP)(O2H)(CH3CN)]2+ at
m/z 190.7 (calcd m/z 190.6) and [Co(Me3-TPADP)(O2H)-
(ClO4)]+ at m/z 439.2 (calcd m/z 439.1) (Figure 3a, see the SI,
Figure S6), together with some unidentified species due to the
thermal instability of 3 (see the SI, Figure S7). When the
reaction was carried out with 2 prepared with isotopically
The resonance Raman spectrum of 3, obtained upon 355 nm
excitation in CH3CN at −30 °C, exhibits an isotope-sensitive
band at 851 cm−1 which shifted to 803 cm−1 upon 18O-
substitution (Figure 3b). The observed isotopic shift of its 16Δ
−
18Δ value of 48 cm−1 is in good agreement with the
calculated value of 49 cm−1 for the O−O harmonic oscillator.
The observed O−O frequency at 851 cm−1 is comparable to
that of a low-spin bleomycin−Co(III)−hydroperoxo complex
(828 cm−1).7h The Co−O vibrational frequency of 3 was
observed at 571 cm−1, which shifts to 551 cm−1 on 18O-
substitution (16Δ − 18Δ = 20 cm−1; 16Δ − 18 (calcd) = 26 cm−1)
Δ
(Figure 3b). In order to support the formation of a
hydroperoxide ligand in 3, we performed additional isotope
2
labeling experiments. Upon H-substitution in 3, the 571 cm−1
feature exhibited a 3 cm−1 downshift (Figure 3b). The result is
quite similar to those of previously reported Fe(III)−O2H
species.7g,30 This deuterium isotope effect supports the
2
presence of a hydroperoxide ligand. The H-substitution in 3
was further confirmed by ESI-MS (see the SI, Figure S7, inset).
D
DOI: 10.1021/acs.inorgchem.6b02288
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
1
The H NMR spectrum of 3 recorded in CD3CN at −40 °C
shows sharp features in the 0−10 ppm region (data not
shown), indicating that 3 is a low-spin Co(III) d6 species. On
the basis of the spectroscopic data presented above,
intermediate 3 is assigned as a low-spin Co(III)−hydroperoxo
complex.
Reactivity. The reactivity of 2 and 3 has been investigated
in electrophilic reactions: oxygen atom transfer (i.e., the
oxidation of triphenylphosphine (PPh3) and thioanisole) and
hydrogen atom transfer (i.e., the oxidation of xanthene and
cyclohexadiene (CHD)) reactions (Scheme 3). Upon addition
Scheme 3. Overall Reactivity of 2 and 3 in Electrophilic
Reactions (e.g., HAT and OAT)
of the substrates to the solution of 2 in CH3CN at −20 and
−40 °C, the intermediate remained intact and product analysis
of the reaction solution did not show oxygenated products (see
the SI, Figure S8).
Unlike 2, addition of PPh3 to a solution of 3 in CH3CN at
−20 °C shows the disappearance of the characteristic
absorption band of 3 with a pseudo-first-order decay (see the
SI, Figure S9). Product analysis of the reaction solution
revealed the formation of triphenylphosphine oxide in a
quantitative yield. Kinetic studies of 3 with thioanisole under
the same reaction conditions also exhibit a pseudo-first-order
reaction profile (Figure 4a, see the SI, Figure S10), and the
first-order rate constants increased proportionally with the
substrate concentration (k2 = 6.7(8) × 10−1 M−1 s−1) (Figure
4b). The rates were dependent on reaction temperature, where
a linear Eyring plot was obtained between 233 and 263 K to
give the activation parameters of ΔH⧧ = 4.2 kcal mol−1 and
ΔS⧧ = −49.4 cal mol−1 K−1 (Figure 4c). The product analysis
of the reaction solution of the oxidation of thioanisole by 3
revealed that methyl phenyl sulfoxide was produced with a yield
of 75 8%, and the oxygen source of the product was found to
be the hydroperoxo ligand of 3 on the basis of an 18O labeling
experiment performed with 18O labeled 3 (see the SI, Figure
S11). In addition, [CoIII(Me3-TPADP)(OH)(CH3CN)]2+ was
found in the reaction solution as a decomposed product of 3
(see the SI, Figure S12). The FT-IR spectrum of the Co(III)−
OH product obtained via precipitation had a peak at 3500
cm−1, which is consistent with an O−H vibration.31 The
reactivity of 3 was further examined with para-substituted
thioanisoles, para-X-Ph-SCH3 (X = Me, F, H, Cl, Br), to
investigate the electronic effect of para-substituents on the
oxidation of thioanisoles by 3 (Figure 4d). The Hammett plot
Figure 4. Reactions of [CoIII(Me3-TPADP)(O2H)(CH3CN)]2+ (3)
with thioanisole in CH3CN. (a) UV−vis spectral changes of 3 (0.5
mM) upon addition of 25 equiv of thioanisole at −40 °C. Inset shows
the time course of the absorbance at 523 nm and its first-order fitting
(red line). (b) Plot of kobs against thioanisole concentration to
determine a second-order rate constant for 3 at −40 °C. (c) Plot of
pseudo-first-order rate constants against 1/T to determine activation
parameters for the reaction of 3 (0.5 mM) and 50 equiv of thioanisole.
(d) Hammett plot of ln krel against σp+ of para-substituted thioanisoles.
The krel values were calculated by dividing kobs of para-X-Ph-SCH3 (X
= Me, F, H, Cl, Br) by kobs of thioanisole at −40 °C.
of the final reaction mixture revealed the formation of para-
substituted methyl phenyl sulfoxides. It should be noted that
there is a proton effect on the reactivity of 3 in the sulfoxidation
reaction (see the SI, Figure S13). This result is in sharp contrast
with the reactivity of [FeIII(TMC)(O2H)]2+, where no
significant proton effect was observed.27c The origin of such a
proton effect remains to be established in future experiments.
+
of the pseudo-first-order rate constants versus σp gave a ρ
value of −3.6(6). The negative ρ value indicates the
electrophilic character of 3 in OAT reactions. Product analyses
E
DOI: 10.1021/acs.inorgchem.6b02288
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
It is worth noting that 3 is not capable of performing HAT
reactions (data not shown). However, the reactivity of
[FeIII(TMC)(O2H)]2+ in both OAT and HAT reactions has
previously been reported.27b One possible explanation for the
different reactivity of metal(III)−hydroperoxo species is the
inability of cobalt to access its high-valent state. DFT
calculations for thermodynamics of O−O homolysis for 3
relative to a low-spin Fe(III) analogue provide significant
insight into their difference in reactivity (vide supra).
Table 1. Calculated Geometric and Vibrational Parameters
for 3 and Its S = 1/2 FeIII−O2H Analogue
complex
3
S = 1/2 FeIII−O2H
M−O (Å)
O−O (Å)
1.88
1.44
117
2.03
555
928
1.80
1.44
117
2.05
621
888
M−O−O (deg)
M−Lequatorial,av (Å)
ν(M−(O2H)) (cm−1
)
ν(O−O) (cm−1
)
Density Functional Theory Studies. In a previous study
evaluating the HAT reactivity of the low-spin Fe-
(III)−hydroperoxo complex [FeIII(N4Py)(O2H)]2+, it was
found that the barrier for HAT was mostly due to homolysis
of the OO bond.27d In the Me3-TPADP ligand system in this
study, the Fe(III)-hydroperoxo complex is too unstable to be
isolated for experimental evaluation. To calibrate this ligand set
to the [FeIII(N4Py)(O2H)]2+ results for correlation to 3, DFT
calculations were performed on the hypothetical [FeIII(Me3-
TPADP)(O2H)(CH3CN)]2+ complex to determine the
thermodynamics of O−O bond homolysis relative to the
results for the low-spin [FeIII(N4Py)(O2H)]2+ complex, which
were supported by experiment.7g,27d The calculated ΔH of O
O homolysis for the Me3-TPADP on the S = 1/2 surface is 23.2
kcal mol−1, compared to 23.1 kcal mol−1 previously reported
for N4Py, and the ΔG for Me3-TPADP was found to be 13.0
kcal mol−1, compared to 12.7 kcal mol−1 for N4Py. In the Me3-
TPADP case, as in the N4Py case, the final products are an S =
1 FeIVO and an hydroxyl radical. Given the good agreement
between the present set of calculations and those previously
reported,7g these calculations were extended to the Co(III)−
hydroperoxo complex 3.
the additional electron in the Co(III) relative to the low-spin
Fe(III) complex, which occupies a metal d orbital π
antibonding with the hydroperoxide. The calculated ν-
(Co(O2H)) is 555 cm−1, in agreement with the experimental
value of 571 and 66 cm−1 lower in energy than the
corresponding calculated stretch in the iron analogue,
consistent with the longer Co−O bond length. The calculated
ν(OO) is 928 cm−1, which overestimates the experimental
value of 851 cm−1 by 77 cm−1. A similar disagreement between
calculation and experiment was observed in the previous study
on the low-spin [FeIII(N4Py)(O2H)]2+ complex,7g where an
equivalent DFT calculation overestimated the experimental
ν(OO) by 98 cm−1. This was attributed to the calculations
underestimating the donation from hydroperoxy σ and π
bonding orbitals to the metal. Given the similarity between the
present DFT calculations for 3 and the previous experimentally
calibrated calculations for [FeIII(N4Py)(O2H)]2+, these calcu-
lations for 3 were used to evaluate the thermodynamics of its
OO bond homolysis. The enthalpies and Gibbs free energies
of the OO homolysis products of 3 on the S = 0, S = 1, and S
= 2 surfaces are presented on the right-hand side of Figure 5.
The S = 2 products are uphill by ΔG = 39.3 kcal mol−1 and are
discounted. The S = 1 and S = 0 products are at 21.7 and 27.3
kcal mol−1 in Gibbs free energy and correspond to S = 3/2
CoIVO and S = 1/2 CoIVO complexes, respectively,
antiferromagnetically aligned to the S = 1/2 hydroxyl radical.
Figure 5 also presents the energetics of OO homolysis for
the iron analogue of 3, which is ∼9 kcal mol−1 more favorable
than the lowest-energy OO homolysis pathway for 3. Note
that this pathway would further require a crossover from the S
= 0 to the S = 1 surface.
The geometry of 3 was optimized on the S = 0, S = 1, and S
= 2 surfaces. The relative energies of these structures are given
on the left of Figure 5. The lowest-energy structure for 3 was
To understand the difference in O−O homolysis energy for
3 relative to its iron analogue, the electronic structures of the
MIVO reaction products were analyzed, and are summarized
in Figure S14 in the SI. Geometric parameters, as well as Mayer
bond orders, for the products are presented in Table S3 in the
SI. An S = 1 FeIVO has one σ bond between Fe and O from
Figure 5. Thermodynamics of O−O homolysis for 3 on the S = 0
(violet line), S = 1 (red line), and S = 2 (magenta line) surfaces, as well
as for the Fe analogue on the S = 1/2 (green line) surface. Energies are
given as enthalpies, with Gibbs free energy at 298.15 K in parentheses.
Homolysis of the O−O bond is 8.7 kcal mol−1 more unfavorable for 3
relative to the iron complex. Insets show the geometry optimized
structures of 3 (upper) and [CoIV(Me3-TPADP)(O)(CH3CN)]2+
(lower) (black, C; blue, N; cyan, H; red, O; pink, Co).
2
dz interacting with the oxo pz orbital, and one net FeO π
bond from dxz and dyz interacting with the oxo px and py,
respectively. In going from an S = 1 FeIVO to an S = 1/2
CoIVO complex, an MO π bonding interaction is lost
through addition of an electron to the dxz orbital, which has a
π* interaction with the O px orbital (see the SI, Figure S14,
bottom to upper left), leading to a weaker CoO relative to
FeO bond. This additional π* interaction is reflected both in
its longer bond length (1.79 vs 1.64 Å) and lower Mayer bond
order (1.14 vs 1.39). This weaker CoO bond destabilizes the
S = 1/2 CoIVO complex relative to the S = 1 FeIVO. The
homolysis to form this CoIVO product is 14.3 kcal mol−1
higher in energy than the comparable homolysis reaction for Fe
(27.3 vs 13.0 kcal mol−1). For the lower energy S = 3/2 CoIV
O case, the extra electron relative to FeIVO S = 1 is instead
1
found to be S = 0, consistent with the H NMR data (vide
supra); thus, we focus on the S = 0 calculation. Relevant
geometric and vibrational parameters calculated for 3, as well as
those for its iron analogue, are presented in Table 1. The
structures are similar, with the main difference being the
calculated MO bond length, which is 1.88 Å for 3 and 1.80 Å
for the hypothetical iron analogue. This difference arises due to
F
DOI: 10.1021/acs.inorgchem.6b02288
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
2
2
added to the dx −y orbital (see the SI, Figure S14, bottom to
upper right), which is σ antibonding to the equatorial ligands.
In this case, the Co−O bond is not perturbed by the additional
electron, and is in fact stronger than the FeO bond, as
reflected in its shorter bond length (1.62 Å vs 1.64 Å for FeIV
O) and higher Mayer bond order (1.85 vs 1.39). This is due to
the greater Zeff on CoIV relative to FeIV, which stabilizes its d
manifold and allows stronger mixing with the oxygen p orbitals.
However, the equatorial CoL bonds are greatly weakened
relative to those of the iron complex (see the SI, Table S3; the
average MLequatorial bond increases to 2.17 from 2.07 Å, and
the total Mayer bond order for these bonds decreases to 1.74
from 2.16). This overcomes the stronger CoO bonding
interaction and results in an S = 3/2 CoIVO that is less stable
than the S = 1 FeIVO by 9 kcal mol−1. Note that the CoIII
O2H is also less stable than the FeIIIO2H due to weakened
MO2H bonding arising from the additional dπ* electron, but
this involves loss of a relatively weak π bonding interaction
(vide supra; also see differences in calculated MO bond
lengths and stretching frequencies in Table 1). Thus, 3 is much
less reactive than equivalent low-spin Fe(III)−hydroperoxo
complexes due to the higher barrier for OO bond homolysis
resulting from the additional electron in a equatorial σ
antibonding orbital of the CoIVO S = 3/2 product.
*E-mail: jaeheung@dgist.ac.kr.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
J.C. at DGIST acknowledges the financial support from the
NRF (2014R1A1A2056051), the Ministry of Science, ICT and
Future Planning (DGIST R&D Program 16-BD-0403, KCRC
2014M1A8A1049320, and KCGRC 2016M3D3A01913243),
and the Ministry of Oceans and Fisheries (Marine Biotechnol-
ogy Program 20150220) of Korea. T. Og. acknowledges the
support of “Strategic Young Researcher Overseas Visits
Program for Accelerating Brain Circulation” and Grant-in-Aid
for Scientific Research (No. 15H00960) both by JSPS. E.I.S. at
Stanford University acknowledges financial support by the
National Institutes of Health (Grant GM 40392).
REFERENCES
■
(1) (a) Nam, W. Dioxygen Activation by Metalloenzymes and
Models. Acc. Chem. Res. 2007, 40, 465 and review articles in the special
issue. (b) Ray, K.; Pfaff, F. F.; Wang, B.; Nam, W. Status of Reactive
Non-Heme Metal−Oxygen Intermediates in Chemical and Enzymatic
Reactions. J. Am. Chem. Soc. 2014, 136, 13942−13958.
(2) (a) Sheldon, R. A.; Kochi, J. K. Metal-Catalyzed Oxidations of
Organic Compounds; Academic Press: New York, 1981. (b) The
Chemistry of Peroxides, Patai, S., Ed.; Wiley: Chichester, U.K., 1983.
(c) Norman, J. A.; Pez, G. P.; Roberts, D. A. In Oxygen Complexes and
Oxygen Activation by Transition Metals; Martel, A. E., Sawyer, D. T.,
Eds.; Plenum Press: New York, 1988. (d) Organic Peroxides; Ando,
W., Ed.; Wiley: Chichester, U.K., 1992.
(3) (a) Special issue on Metal−Dioxygen Complexes: Chem. Rev.
1994, 94, 567−856. (b) Strukul, G. Transition Metal Catalysis in the
Baeyer−Villiger Oxidation of Ketones. Angew. Chem., Int. Ed. 1998, 37,
1198−1209. (c) Murahashi, S. Synthetic Aspects of Metal-Catalyzed
Oxidations of Amines and Related Reactions. Angew. Chem., Int. Ed.
Engl. 1995, 34, 2443−2465.
(4) Ortiz de Montellano, P. R. Cytochrome P450: Structure,
Mechanism, and Biochemistry, 3rd ed.; Kluwer Academic/Plenum
Publishers: New York, 2005.
(5) (a) Holm, R. H.; Solomon, E. I. Introduction: Bioinorganic
Enzymology II. Chem. Rev. 2014, 114, 3367−3368 and review articles
in the special issue. (b) Kovaleva, E. G.; Lipscomb, J. D. Versatility of
Biological Non-heme Fe(II) Centers in Oxygen Activation Reactions.
Nat. Chem. Biol. 2008, 4, 186−193. (c) Ortiz de Montellano, P. R.
Hydrocarbon Hydroxylation by Cytochrome P450 Enzymes. Chem.
Rev. 2010, 110, 932−948. (d) McQuarters, A. B.; Wolf, M. W.; Hunt,
A. P.; Lehnert, N. 1958−2014: after 56 Years of Research, Cytochrome
P450 Reactivity is Finally Explained. Angew. Chem., Int. Ed. 2014, 53,
4750−4752. (e) Tolman, W. B. Editorial for the Virtual Issue on
Models of Metalloenzymes. Inorg. Chem. 2013, 52, 7307−7310.
(6) (a) Wertz, D. L.; Valentine, J. S. Nucleophilicity of Iron−Peroxo
Porphyrin Complexes. Struct. Bonding (Berlin) 2000, 97, 37−60.
(b) Gibson, D. T.; Parales, R. E. Aromatic Hydrocarbon Dioxygenases
in Environmental Biotechnology. Curr. Opin. Biotechnol. 2000, 11,
236−243. (c) Wertz, D. L.; Sisemore, M. F.; Selke, M.; Driscoll, J.;
Valentine, J. S. Mimicking Cytochrome P-450 2B4 and Aromatase:
Aromatization of a Substrate Analogue by a Peroxo Fe(III) Porphyrin
Complex. J. Am. Chem. Soc. 1998, 120, 5331−5332. (d) Goto, Y.;
Wada, S.; Morishima, I.; Watanabe, Y. Reactivity of Peroxoiron(III)
Porphyrin Complexes: Models for Deformylation Reactions Catalyzed
by Cytochrome P-450. J. Inorg. Biochem. 1998, 69, 241−247.
(e) Hashimoto, K.; Nagatomo, S.; Fujinami, S.; Furutachi, H.; Ogo,
S.; Suzuki, M.; Uehara, A.; Maeda, Y.; Watanabe, Y.; Kitagawa, T. A
New Mononuclear Iron(III) Complex Containing a Peroxocarbonate
Ligand. Angew. Chem., Int. Ed. 2002, 41, 1202−1205. (f) Annaraj, J.;
Suh, Y.; Seo, M. S.; Kim, S. O.; Nam, W. Mononuclear Nonheme
Ferric-Peroxo Complex in Aldehyde Deformylation. Chem. Commun.
CONCLUSIONS
■
The metal−hydroperoxo intermediates in organic functionali-
zations are of current interest in enzymatic processes,
pharmaceutical research, and industrial catalysis. We have
synthesized mononuclear cobalt(III)−peroxo (2) and −hydro-
peroxo (3) complexes bearing a common macrocyclic Me3-
TPADP ligand, where 3 was prepared by protonation of 2. The
consecutive interconversion of 3 to 2 by addition of a base
supports acid−base chemistry. The intermediates were
characterized with a variety of physicochemical methods.
Although the UV−vis spectra of 2 and 3 are not very different,
ESI-MS spectra of 2 clearly exhibit the formation of the
cobalt(III)−peroxo adduct at low temperature, and resonance
Raman spectra of 3 show an O−O stretching vibration at 851
cm−1 for 16O samples (803 cm−1 for 18O samples), which is
assignable to that of hydroperoxo species. The reactivities of 2
and 3 were compared in electrophilic reactions; 3 is capable of
conducting OAT reactions, whereas 2 exhibits no OAT
reactivity. Alternatively, 3 does not perform HAT, whereas
low-spin Fe(III)−hydroperoxo complexes do. DFT calculations
show that this is due to the high O−O bond homolysis energy
relative to that of the iron analogue. This energy difference is
due to the additional electron in an antibonding orbital that
would destabilize a high-valent Co−oxo product.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.6b02288.
Experimental details, Figures S1−S14, and Tables S1−S3
(PDF)
Crystallographic details (CIF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail: edward.solomon@stanford.edu.
■
G
DOI: 10.1021/acs.inorgchem.6b02288
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
2005, 4529−4531. (g) Kovaleva, E. G.; Lipscomb, J. D. Crystal
Structures of Fe2+ Dioxygenase Superoxo, Alkylperoxo, and Bound
Product Intermediates. Science 2007, 316, 453−457.
R.; Masarwa, M.; Stephenson, N. A.; Christoph, G.; Alcock, N. W.
Dioxygen Adducts of Lacunar Cobalt(II) Cyclidene Complexes. Inorg.
Chem. 1994, 33, 910−923. (h) Wang, C.-C.; Chang, H.-C.; Lai, Y.-C.;
Fang, H.; Li, C.-C.; Hsu, H.-K.; Li, Z.-Y.; Lin, T.-S.; Kuo, T.-S.; Neese,
F.; Ye, S.; Chiang, Y.-W.; Tsai, M.-L.; Liaw, W.-F.; Lee, W.-Z. A
structurally characterized Nonheme Cobalt−Hydroperoxo Complex
Derived from Its Superoxo Intermediate via Hydrogen Atom
Abstraction. J. Am. Chem. Soc. 2016, 14186−14189.
(10) (a) Calvin, M.; Bailes, R. H.; Wilmarth, W. K. The Oxygen-
Carrying Synthetic Chelate Compounds.1a I. J. Am. Chem. Soc. 1946,
68, 2254−2256. (b) Barkelew, C. H.; Calvin, M. Oxygen-Carrying
Synthetic Chelate Compounds. II. The Rates of Oxygenation of the
Solid Compounds1. J. Am. Chem. Soc. 1946, 68, 2257−2262. (c) Bailes,
R. H.; Calvin, M. The Oxygen-Carrying Synthetic Chelate
Compounds. VII. Preparation1. J. Am. Chem. Soc. 1947, 69, 1886−
1893.
(7) (a) Chow, M. S.; Liu, L. V.; Solomon, E. I. Further Insights into
the Mechanism of the Reaction of Activated Bleomycin with DNA.
Proc. Natl. Acad. Sci. U. S. A. 2008, 105, 13241−13245. (b) Neese, F.;
Zaleski, J. M.; Loeb-Zaleski, K.; Solomon, E. I. Electronic Structure of
Activated Bleomycin: Oxygen Intermediates in Heme versus Non-
Heme Iron. J. Am. Chem. Soc. 2000, 122, 11703−11724. (c) Sam, J.
W.; Tang, X.-J.; Peisach, J. Electrospray Mass Spectrometry of Iron
Bleomycin: Demonstration that Activated Bleomycin is a Ferric
Peroxide Complex. J. Am. Chem. Soc. 1994, 116, 5250−5256.
(d) Westre, T. E.; Loeb, K. E.; Zaleski, J. M.; Hedman, B.;
Hodgson, K. O.; Solomon, E. I. Determination of the Geometric
and Electronic Structure of Activated Bleomycin Using X-ray
Absorption Spectroscopy. J. Am. Chem. Soc. 1995, 117, 1309−1313.
(e) Burger, R. M.; Peisach, J.; Horwitz, S. B. Activated Bleomycin. A
Transient Complex of Drug, Iron, and Oxygen that Degrades DNA. J.
Biol. Chem. 1981, 256, 11636−11644. (f) Hecht, S. M. The Chemistry
of Activated Bleomycin. Acc. Chem. Res. 1986, 19, 383−391.
(g) Lehnert, N.; Neese, F.; Ho, R. Y. N.; Que, L., Jr.; Solomon, E.
I. Electronic Structure and Reactivity of Low-Spin Fe-
(III)−Hydroperoxo Complexes: Comparison to Activated Bleomycin.
J. Am. Chem. Soc. 2002, 124, 10810−10822. (h) Rajani, C.; Kincaid, J.
R.; Petering, D. H. Resonance Raman Studies of HOO−Co(III)
Bleomycin and Co(III) Bleomycin: Identification of Two Important
Vibrational Modes, ν(Co−OOH) and ν(O−OH). J. Am. Chem. Soc.
2004, 126, 3829−3836.
(8) (a) Jo, Y.; Annaraj, J.; Seo, M. S.; Lee, Y.-M.; Kim, S. Y.; Cho, J.;
Nam, W. Reactivity of a Cobalt(III)-Peroxo Complex in Oxidative
Nucleophilic Reactions. J. Inorg. Biochem. 2008, 102, 2155−2159.
(b) Hikichi, S.; Akita, M.; Moro-oka, Y. New Aspects of the Cobalt-
Dioxygen Complex Chemistry Opened by Hydrotris(pyrazoly)borate
Ligands (TpR): Unique Properties of TpRCo-Dioxygen Complexes.
Coord. Chem. Rev. 2000, 198, 61−87. (c) Rahman, A. F. M. M.;
Jackson, W. G.; Willis, A. C. The First Sideways-Bonded Peroxo
Complex for a Tetraaminecobalt(III) Species. Inorg. Chem. 2004, 43,
7558−7560. (d) Hu, X.; Castro-Rodriguez, I.; Meyer, K. Dioxygen
Activation by a Low-Valent Cobalt Complex Employing a Flexible
Tripodal N-Heterocyclic Carbene Ligand. J. Am. Chem. Soc. 2004, 126,
13464−13473. (e) Cho, J.; Sarangi, R.; Kang, H. Y.; Lee, J. Y.; Kubo,
M.; Ogura, T.; Solomon, E. I.; Nam, W. Synthesis, Structural, and
Spectroscopic Characterization and Reactivities of Mononuclear
Cobalt(III)−Peroxo Complexes. J. Am. Chem. Soc. 2010, 132,
16977−16986. (f) Smith, T. D.; Pilbrow, J. R. Recent Developments
in the Studies of Molecular Oxygen Adducts of Cobalt(II)
Compounds and Related Systems. Coord. Chem. Rev. 1981, 39,
295−383.
(11) Niederhoffer, E. C.; Timmons, J. H.; Martell, A. E.
Thermodynamics of Oxygen Binding in Natural and Synthetic
Dioxygen Complexes. Chem. Rev. 1984, 84, 137−203.
(12) (a) Bailey, C. L.; Drago, R. S. Utilization of O2 for the Specific
Oxidation of Organic Substrates with Cobalt(II) Catalysts. Coord.
Chem. Rev. 1987, 79, 321−332. (b) Cho, J.; Sarangi, S.; Nam, W.
Mononuclear Metal−O2 Complexes Bearing Macrocyclic N-Tetrame-
thylated Cyclam Ligands. Acc. Chem. Res. 2012, 45, 1321−1330.
(13) (a) Zombeck, A.; Drago, R. S.; Corden, B. B.; Gaul, J. H.
Activation of Molecular Oxygen. Kinetic Studies of the Oxidation of
Hindered Phenols with Cobalt−Dioxygen Complexes. J. Am. Chem.
Soc. 1981, 103, 7580−7585. (b) Hamilton, D. E.; Drago, R. S.;
Zombeck, A. Mechanistic Studies on the Cobalt(II) Schiff Base
Catalyzed Oxidation of Olefins by O2. J. Am. Chem. Soc. 1987, 109,
374−379.
(14) Corcos, A. R.; Villanueva, O.; Walroth, R. C.; Sharma, S. K.;
Bacsa, J.; Lancaster, K. M.; MacBeth, C. E.; Berry, J. F. Oxygen
Activation by Co(II) and a Redox Non-Innocent Ligand: Spectro-
scopic Characterization of a Radical−Co(II)−Superoxide Complex
with Divergent Catalytic Reactivity. J. Am. Chem. Soc. 2016, 138,
1796−1799.
(15) (a) Bayston, J. H.; Winfield, M. E. A Mononuclear Hydroperoxo
Complex and Its Significance in Catalytic Oxidation Mechanisms. J.
Catal. 1964, 3, 123−128. (b) Guzei, I. A.; Bakac, A. Macrocyclic
Hydroperoxocobalt(III) Complex: Photochemistry, Spectroscopy, and
Crystal Structure. Inorg. Chem. 2001, 40, 2390−2393. (c) Wang, W.-
D.; Bakac, A.; Espenson, J. H. Oxidation and Reduction Reactions of
Hydroperoxo Cobalt Macrocycles. Inorg. Chem. 1995, 34, 4049−4056.
(16) Kim, D.; Cho, J.; Lee, Y.-M.; Sarangi, R.; Nam, W. Synthesis,
Characterization, and Reactivity of Cobalt(III)−Oxygen Complexes
Bearing a Macrocyclic N-Tetramethylated Cyclam Ligand. Chem. - Eur.
J. 2013, 19, 14112−14118.
(9) (a) Terry, N. W.; Amma, E. L.; Vaska, L. Molecular Oxygen
Binding in a Monomeric Cobalt Complex. Crystal and Molecular
Structure of Dioxygen−Bis[cis-1,2-bis(diphenylphosphino)ethylene]-
cobalt Tetrafluoroborate. J. Am. Chem. Soc. 1972, 94, 653−655.
(b) Egan, J. W., Jr.; Haggerty, B. S.; Rheingold, A. L.; Sendlinger, S. C.;
Theopold, K. H. Crystal Structure of a Side-On Superoxo Complex of
Cobalt and Hydrogen Abstraction by a Reactive Terminal Oxo Ligand.
J. Am. Chem. Soc. 1990, 112, 2445−2446. (c) Rodley, G. A.; Robinson,
W. T. Structure of a Monomeric Oxygen-Carrying Complex. Nature
1972, 235, 438−439. (d) Gall, R. S.; Rogers, J. F.; Schaefer, W. P.;
Christoph, G. G. The Structure of a Monomeric Oxygen Carrying
Cobalt Complex: Dioxygen-N,N′-(1,1,2,2-tetramethyl)ethylenebis(3-
tert-butylsalicylideniminato)(1-benzylimidazole)cobalt(II). J. Am.
(17) (a) Mirza, S. A.; Bocquet, B.; Robyr, C.; Thomi, S.; Williams, A.
F. Reactivity of the Coordinated Hydroperoxo Ligand. Inorg. Chem.
1996, 35, 1332−1337. (b) Tcho, W.-Y.; Wang, B.; Lee, Y.-M.; Cho, K.-
B.; Shearer, J.; Nam, W. A Mononuclear Nonheme Cobalt(III)−
Hydroperoxide Complex with an Amphoteric Reactivity in Electro-
philic and Nucleophilic Oxidative Reactions. Dalton Trans. 2016, 45,
14511−14515.
(18) Purification of Laboratory Chemicals; Armarego, W. L. F., Perrin,
D. D., Eds.; Pergamon Press: Oxford, 1997.
(19) (a) Evans, D. F. 400. The Determination of the Paramagnetic
Susceptibility of Substances in Solution by Nuclear Magnetic
Resonance. J. Chem. Soc. 1959, 2003−2005. (b) Loliger, J.;
̈
Scheffold, R. Paramagnetic Moment Measurements by NMR. A
Micro Technique. J. Chem. Educ. 1972, 49, 646−647. (c) Evans, D. F.;
Jakubovic, D. A. Water-Soluble Hexadentate Schiff-Base Ligands as
Sequestrating Agents for Iron(III) and Gallium(III). J. Chem. Soc.,
Dalton Trans. 1988, 2927−2933.
́
Chem. Soc. 1976, 98, 5135−5144. (e) Tine, M. R. Cobalt Complexes
in Aqueous Solutions as Dioxygen Carriers. Coord. Chem. Rev. 2012,
256, 316−327. (f) Schaefer, W. P.; Huie, B. T.; Kurilla, M. G.; Ealick,
S. E. Oxygen-Carrying Cobalt Complexes. 10. Structures of N,N′-
Ethylenebis(3-tert-butylsalicylideniminato)cobalt(II) and Its Mono-
meric Dioxygen Adduct. Inorg. Chem. 1980, 19, 340−344. (g) Busch,
D. H.; Jackson, P. J.; Kojima, M.; Chmielewski, P.; Matsumoto, N.;
Stevens, J. C.; Wu, W.; Nosco, D.; Herron, N.; Ye, N.; Warburton, P.
(20) Sheldrick, G. M. SHELXTL/PC for Windows XP, Version 6.12;
Bruker AXS Inc.: Madison, WI, 2001.
(21) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci,
H
DOI: 10.1021/acs.inorgchem.6b02288
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H.
P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.;
Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima,
T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.;
Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin,
K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.;
Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega,
N.; Millam, N. J.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.;
Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.;
Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.;
Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.;
Modes. Eur. J. Inorg. Chem. 2000, 2000, 1627−1633. (d) Mairata i
Payeras, A.; Ho, R. Y. N.; Fujita, M.; Que, L., Jr. The Reaction of
[FeII(tpa)] with H2O2 in Acetonitrile and Acetone-Distinct
Intermediates and Yet Similar Catalysis. Chem. - Eur. J. 2004, 10,
4944−4953. (e) Ohta, T.; Liu, J.-G.; Naruta, Y. Resonance Raman
Characterization of Mononuclear Heme-Peroxo Intermediate Models.
Coord. Chem. Rev. 2013, 257, 407−413.
(31) (a) Bergquist, C.; Fillebeen, T.; Morlok, M. M.; Parkin, G.
Protonation and Reactivity towards Carbon Dioxide of the
Mononuclear Tetrahedral Zinc and Cobalt Hydroxide Complexes,
[TpBut,Me]ZnOH and [TpBut,Me]CoOH: Comparison of the Reactivity
of the Metal Hydroxide Function in Synthetic Analogues of Carbonic
Anhydrase. J. Am. Chem. Soc. 2003, 125, 6189−6199. (b) Lucas, R. L.;
Zart, M. K.; Murkerjee, J.; Sorrell, T. N.; Powell, D. R.; Borovik, A. S.
A Modular Approach toward Regulating the Secondary Coordination
Sphere of Metal Ions: Differential Dioxygen Activation Assisted by
Intramolecular Hydrogen Bonds. J. Am. Chem. Soc. 2006, 128, 15476−
15489. (c) Singh, U. P.; Babbar, P.; Tyagi, P.; Weyhermuller, T. A
Mononuclear Cobalt(II) Hydroxo Complex: Synthesis, Molecular
Structure, and Reactivity Studies. Transition Met. Chem. 2008, 33,
931−940.
̈
Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.;
Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09,
Revision D.01; Gaussian, Inc.: Wallingford, CT, 2013.
(22) Tenderholt, A. L. QMForge, Version 2.2; http://qmforge.
sourceforge.net.
(23) Kieber-Emmons, M. T. LUMO, Version 1.0.1.
(24) Wen, J.; Geng, Z.; Yin, Y.; Wang, Z. A Mononuclear Mn2+
Complex Based on a Novel Tris-(ethyl acetate) Pendant-Armed
Tetraazamacrocycle: Effect of Pyridine on Self-Assembly and Weak
Interactions. Inorg. Chem. Commun. 2012, 21, 16−20.
(25) Kang, P. C.; Eaton, G. R.; Eaton, S. S. Pulsed Electron
Paramagnetic Resonance of High-Spin Cobalt(II) Complexes. Inorg.
Chem. 1994, 33, 3660−3665.
(26) Complexes 2 and 3 did not give reversible or quasireversible
behavior in cyclovoltammetry.
(27) (a) Li, F.; Meier, K. K.; Cranswick, M. A.; Chakrabarti, M.; Van
Heuvelen, K. M.; Munck, E.; Que, L., Jr. Characterization of a High-
̈
Spin Non-Heme FeIII−OOH Intermediate and Its Quantitative
Conversion to an FeIVO Complex. J. Am. Chem. Soc. 2011, 133,
7256−7259. (b) Cho, J.; Jeon, S.; Wilson, S. A.; Liu, L. V.; Kang, E. A.;
Braymer, J. J.; Lim, M. H.; Hedman, B.; Hodgson, K. O.; Valentine, J.
S.; Solomon, E. I.; Nam, W. Structure and Reactivity of a Mononuclear
Non-Haem Iron(III)−Peroxo Complex. Nature 2011, 478, 502−505.
(c) Kim, Y. M.; Cho, K.-B.; Cho, J.; Wang, B.; Li, C.; Shaik, S.; Nam,
W. A Mononuclear Non-Heme High-Spin Iron(III)−Hydroperoxo
Complex as an Active Oxidant in Sulfoxidation Reactions. J. Am. Chem.
Soc. 2013, 135, 8838−8841. (d) Liu, L. V.; Hong, S.; Cho, J.; Nam,
W.; Solomon, E. I. Comparison of High-Spin and Low-Spin Nonheme
FeIII−OOH Complexes in O−O Bond Homolysis and H-Atom
Abstraction Reactivities. J. Am. Chem. Soc. 2013, 135, 3286−3299.
(28) So, H.; Park, Y. J.; Cho, K.-B.; Lee, Y.-M.; Seo, M. S.; Cho, J.;
et al. Sarangi, R.; Nam, W. Spectroscopic Characterization and
Reactivity Studies of a Mononuclear Nonheme Mn(III)−Hydroperoxo
Complex. J. Am. Chem. Soc. 2014, 136, 12229−12232.
(29) (a) Jensen, K. B.; McKenzie, C. J.; Nielsen, L. P.; Pedersen, J. Z.;
Svendsen, H. M. Deprotonation of Low-Spin Mononuclear Iron(III)−
Hydroperoxide Complexes Give Transient Blue Species Assigned to
High-Spin Iron(III)−Peroxide Complexes. Chem. Commun. 1999,
1313−1314. (b) Ho, R. Y. N.; Roelfes, G.; Hermant, R.; Hage, R.;
Feringa, B. L.; Que, L., Jr. Resonance Raman Evidence for the
Interconversion between an [FeIII−η1-OOH]2+ and [FeIII−η2-O2]+
Species and Mechanistic Implications Thereof. Chem. Commun.
1999, 2161−2162. (c) Nebe, T.; Beitat, A.; Wurtele, C.; Ducker-
̈
̈
Benfer, C.; van Eldik, R.; McKenzie, C. J.; Schindler, S. Reinvestigation
of the Formation of a Mononuclear Fe(III) Hydroperoxido Complex
Using High Pressure Kinetics. Dalton Trans. 2010, 39, 7768−7773.
(30) (a) Roelfes, G.; Lubben, M.; Chen, K.; Ho, R. Y. N.; Meetsma,
A.; Genseberger, S.; Hermant, R. M.; Hage, R.; Mandal, S. K.; Young,
V. G., Jr.; Zang, Y.; Kooijman, H.; Spek, A. L.; Que, L., Jr.; Feringa, B.
L. Iron Chemistry of a Pentadentate Ligand That Generates a
Metastable FeIII−OOH Intermediate. Inorg. Chem. 1999, 38, 1929−
1936. (b) Ho, R. Y. N.; Roelfes, G.; Feringa, B. L.; Que, L., Jr. Raman
Evidence for a Weakened O−O Bond in Mononuclear Low-Spin
Iron(III)-Hydroperoxides. J. Am. Chem. Soc. 1999, 121, 264−265.
(c) Simaan, A. J.; Dopner, S.; Banse, F.; Bourcier, S.; Bouchoux, G.;
̈
Boussac, A.; Hildebrandt, P.; Girerd, J.-J. FeIII−Hydroperoxo and
Peroxo Complexes with Aminopyridyl Ligands and the Resonance
Raman Spectroscopic Identification of the Fe−O and O−O Stretching
I
DOI: 10.1021/acs.inorgchem.6b02288
Inorg. Chem. XXXX, XXX, XXX−XXX