A mononuclear nonheme cobalt(III)-hydroperoxide complex with an amphoteric reactivity in electrophilic and nucleophilic oxidative reactions

Article abstract of DOI:10.1039/c6dt01194b

A mononuclear nonheme cobalt(iii)-hydroperoxide complex bearing a tetramethylated cyclam ligand, [(12-TMC)Co^III(OOH)]²⁺, was synthesized and characterized spectroscopically and computationally; the cobalt(iii)-hydroperoxide complex exhibited an amphoteric reactivity in electrophilic and nucleophilic oxidative reactions.

Full text of DOI:10.1039/c6dt01194b

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A mononuclear nonheme cobalt(III)–hydroperoxide
complex with an amphoteric reactivity in
Cite this: DOI: 10.1039/c6dt01194b
electrophilic and nucleophilic oxidative reactions†
Received 28th March 2016,
Accepted 10th May 2016
Woon-Young Tcho,‡a Bin Wang,‡a Yong-Min Lee,a Kyung-Bin Cho,a
DOI: 10.1039/c6dt01194b
Jason Shearer*b and Wonwoo Nam*a
www.rsc.org/dalton
A mononuclear nonheme cobalt(III)–hydroperoxide complex bearing gen atom (H-atom) in DNA strand scission reactions by bleo-
a tetramethylated cyclam ligand, [(12-TMC)CoIII(OOH)]2+, was syn- mycins (BLM),6 iron(III)–hydroperoxide species have widely
thesized and characterized spectroscopically and computationally; been considered to be sluggish oxidants in oxygenation reac-
the cobalt(III)–hydroperoxide complex exhibited an amphoteric tions by heme and nonheme iron enzymes and their model
reactivity in electrophilic and nucleophilic oxidative reactions.
compounds.5,7 Recently, we have provided strong experimental
evidence showing that a high-spin (S = 5/2) iron(III)–hydro-
peroxide complex bearing a macrocyclic N-methylated cyclam
ligand, [(14-TMC)FeIII(OOH)]2+ (14-TMC = 1,4,8,11-tetramethyl-
1,4,8,11-tetraazacyclotetradecane), is a competent oxidant in
both electrophilic and nucleophilic oxidative reactions.8 The
reactivities of nonheme iron(III)–hydroperoxide species have
also been shown to be markedly influenced by their spin states
(i.e., high-spin versus low-spin iron(III)–hydroperoxide species).
Mononuclear nonheme metal-oxygen species, such as metal–
oxide, –hydroperoxide, and –peroxide complexes, have been
invoked as key intermediates in the oxidation of organic sub-
strates, such as in electrophilic and nucleophilic oxidative reac-
tions, by metalloenzymes and their biomimetic compounds.1
Among them, high-valent nonheme metal–oxide complexes
have shown reactivities in electrophilic oxidative reactions,
such as hydrogen atom transfer (HAT) and oxygen atom trans-
fer (OAT) reactions.1,2 One notable class of such compounds is
the mononuclear nonheme iron(IV)–oxide complexes that have
been intensively investigated in various oxidation reactions
over the past decade since the first X-ray crystal structure of a
mononuclear nonheme iron(IV)–oxide complex was reported in
2003.3 In contrast, metal–peroxide complexes have shown
nucleophilic reactivities in both enzymatic and biomimetic
reactions.1,4 Thus, reactivities of the high-valent metal–oxide
and metal–peroxide complexes have been well established in
electrophilic and nucleophilic oxidative reactions, respectively.
Metal–hydroperoxide species have attracted much attention
in the biological, bioinorganic, and synthetic chemistry com-
munities for many years, especially due to the long-standing
controversy concerning the possible involvement of such inter-
mediates in oxidation reactions.5 For example, although a low-
spin (S = 1/2) iron(III)–hydroperoxide intermediate has been
proposed as a reactive species capable of abstracting an hydro-
More recently,
a
manganese(III)–hydroperoxide complex
bearing the 14-TMC ligand has been shown to be a competent
oxidant in OAT reactions, such as the oxidation of sulfides.9
Therefore, the reactivities of metal–hydroperoxide species
have begun to be unveiled in oxidation reactions. In this com-
munication, we report the synthesis and characterization of a
cobalt(III)–hydroperoxide complex bearing a macrocyclic N-methyl-
ated cyclam ligand, [(12-TMC)CoIII(OOH)]2+ (12-TMC = 1,4,7,10-
tetramethyl-1,4,7,10-tetraazacyclododecane; see Scheme 1 for the
Density Functional Theory (DFT) optimized structure). The Co(III)–
hydroperoxide complex was synthesized by the protonation of the
aDepartment of Chemistry and Nano Science, Ewha Womans University,
Seoul 03760, Korea. E-mail: wwnam@ewha.ac.kr
bDepartment of Chemistry, University of Nevada, Reno, NV 89557, USA.
Scheme 1 DFT-optimized structure of [(12-TMC)CoIII(OOH)]2+ and its
reactions in electrophilic and nucleophilic reactions. Hydrogen atoms,
except for the one on the hydroperoxide moiety, are omitted for clarity.
Atom colours are cobalt blue for cobalt, blue for nitrogen, red for
oxygen, black for carbon, and white for hydrogen.
E-mail: shearer@unr.edu
†Electronic supplementary information (ESI) available: Experimental details,
XANES/EXAFS measurements, UV-vis, EPR and NMR spectra, kinetic data, and
DFT calculation details. See DOI: 10.1039/c6dt01194b
‡These authors contributed equally to this work.
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corresponding Co(III)–peroxide complex, [(12-TMC)CoIII(O2)]+. We
also report for the first time that the Co(III)–hydroperoxide
complex is a competent oxidant in HAT and OAT reactions (i.e.,
electrophilic oxidative reaction) as well as in the deformylation of
the aldehyde (i.e., nucleophilic oxidative reaction) (Scheme 1).
The cobalt(III)–peroxide complex, [(12-TMC)CoIII(O2)]+ (1),
was synthesised and isolated as a purple solid according to lit-
erature methods.10 Addition of 10 equiv. of triflic acid (HOTf)
to a solution of 1 in acetone at 0 °C immediately generated a
greenish-blue intermediate (2), which was metastable at 0 °C
(t1/2 ≈ 15 min) and subsequently characterized with various
spectroscopic methods. The UV-vis spectrum of 2 exhibits elec-
tronic absorption bands at 445 (ε ≈ 120 M−1 cm−1) and
620 nm (ε ≈ 240 M−1 cm−1) (Fig. 1). The formation constant
(Kf) of 1.3 × 104 M−1 for 2 was also determined by acid titration
(see Fig. S1, ESI†). It should be noted that the intermediate 2
reverted to 1 upon addition of 10 equiv. of TEA (Fig. S2, ESI†),
suggesting that 1 and 2 are inter-convertible through acid–
base reactions similar to that previously reported for mono-
nuclear nonheme metal(III)–peroxide and metal(III)–hydro-
peroxide complexes [eqn (1)].8a,9,11
ð12-TMCÞCoIIIðO2Þ þaÐcid Âð12-TMCÞCoIIIðOOHÞ
ð1Þ
Â
Ã
Ã
2þ
Fig. 2 (a) The normalized Co K-edge X-ray absorption spectrum of 2.
(b) The non-phase shift corrected magnitude Fourier transform for 2.
Inset shows the corresponding Co K-edge k3 EXAFS spectrum. Black
base
The X-band electron paramagnetic resonance (EPR) spec- lines are original data and red lines are the best fits to 2.
trum recorded at 5 K for 2 is silent (Fig. S3, ESI†). All peaks in
the 1H NMR spectrum of 2 are located in a diamagnetic region
(Fig. S4a, ESI†) and the peaks are completely distinct from
with the Co(III)-centre contained in a noncentrosymmetric
coordination geometry (i.e., a five coordinate Co(III) ion). It is
those of 1 (Fig. S4b, ESI†). The results of the EPR and 1H NMR
experiments together with the observed inter-conversion
worth noting that the pre-edge energy of 7710.1(1) eV and the
between 1 and 2 by acid–base titrations indicate that 2 is a
low-spin (S = 0) Co(III)-OOH species. The oxidation state of 2
was also confirmed by using Co K-edge X-ray absorption spec-
troscopy (XAS) (Fig. 2; see also the Experimental section, ESI†).
Based on the maximum of the first derivative of the cobalt
edge (7720.9(1) eV), we assign a Co(III) oxidation state for 2.
The X-ray absorption near edge structure (XANES) region con-
edge energy of 7720.9(1) eV for 2 are quite similar to those of 1
(7710.1 and 7720.6 eV, respectively), but much higher than
those of the CoII(12-TMC)(OTf)2 (OTf = CF3SO3−) complex
(7709.2 and 7719.3 eV, respectively).12 The X-ray absorption
fine structure (EXAFS) data were best modelled as a five coordi-
nate Co-center with four N ligands at a distance of 2.00 Å and
one O ligand at a distance of 1.86 Å (Table S1, ESI†). In
addition to the inner-sphere ligands, there are significant con-
tributions to the EXAFS data resulting from the outer-sphere
carbon scatterers derived from the ligand backbone. We were
tains a pre-edge feature at 7710.1(1) eV corresponding to a
Co(1s → 3d) transition with an integrated peak intensity of
10.8(1) eV relative to the edge height. This is fully consistent
able to model the data using single-scattering phase and
amplitude functions by including two C shells at 2.80 and
2.93 Å.13 We note that, consistent with the DFT results (vide
infra), we were able to locate a model to the EXAFS data of 2
using 5 Co–N scatterers in the Co–N shell (2.00 Å). Although
the resulting model yielded a fit to the data with an error vir-
tually identical to the 4 Co–N model, the resulting σ2 para-
meter for the Co–N shell was significantly higher than that
obtained for the 4 scatterer model. This is likely the result of
the large range of Co–N/O bond lengths present in 2.
DFT was used to model the putative structure of 2. The cal-
culations yielded an S = 0 ground state (Tables S2–S4, ESI†), in
agreement with the EPR and 1H NMR data. The geometry opti-
mized structure of 2 in the S = 0 state can be best described as a
side-on Co(III)-OOH species that is perturbed by the protona-
Fig. 1 (a) UV-vis spectra of 1 (1.0 mM, black line), 2 (1.0 mM, blue line),
and [CoII(12-TMC)](OTf)2 (1.0 mM, red line) in acetone at 0 °C.
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Table 1 Comparing the distances in 1 and 2 in Å
1a
2 (EXAFS)
2 (DFT)
Co–O (proximal)
Co–O (distal)
O–O
1.86
1.87
1.44
2.00–2.02
(average: 2.01)
2.81–2.98
(average: 2.88)
1.86
—
—
1.86
2.03
1.43
1.99–2.08
(average: 2.04)
2.81–3.13
(average: 3.01)
Co–N
2.00
Co–C
2.80/2.93
a Values taken from the X-ray structure in ref. 10.
tion of one of the peroxide oxygen atoms (Scheme 1). The
shorter Co–O distance is thus calculated to be 1.86 Å (Table S4,
ESI†), in excellent agreement with the EXAFS data above. The
Co–O distance to the protonated distal oxygen is calculated to
be 2.03 Å. This is in close proximity to the calculated average
Co–N distance of 2.04 Å (Table 1), the latter in reasonable
agreement with EXAFS. Hence, the absence of the longer Co–O
distance in EXAFS is explained by the presumed overlap with
the Co–N distances. This supposition is consistent with the
alternate fit to the EXAFS data.13 As shown in Table 1, the
structures of 1 and 2 are very similar to each other, requiring
careful analysis. Table S4, ESI† also lists the distances for the
S = 1 and 2 states, showing that the EXAFS results are most
compatible with the S = 0 state distances. Based on the above
edge position, integrated pre-edge peak intensity, EXAFS
Fig. 3 (a) UV-vis spectral changes observed in the reaction of
(1.0 mM) with CHD (100 mM) in acetone at 0 °C. The inset shows the
time course monitored at 620 nm. (b) Plot of pseudo-first-order rate
2
constants (kobs) against the concentrations of CHD to determine
second-order rate constant (k2) for the oxidation of CHD by 2.
1
derived metric parameters, EPR and H NMR spectra and DFT
calculations, we formulate the cobalt species 2 as an S = 0
[(12-TMC)CoIII(OOH)]2+ complex.
The electrophilic reactivity of 2 was investigated kinetically
in C–H bond activation of hydrocarbons (i.e., HAT), such as
1,4-cyclohexadiene (CHD) and xanthene. Upon addition of
CHD to the solution of 2 in acetone at 0 °C, 2 was converted to
the [CoII(12-TMC)]2+ species, as evidenced by EPR spectroscopy
(Fig. S5a, ESI†), with a pseudo-first-order decay profile (Fig. 3a;
see also Fig. 1, red line for the UV-vis spectrum of [CoII(12-
TMC)]2+ species). The formation of [CoII(12-TMC)]2+ species
was also confirmed by electrospray ionization mass spec-
troscopy (ESI-MS) (Fig. S5b, ESI†); the ESI-MS of the reaction
solution exhibited a prominent ion peak at m/z = 436.2, whose
mass and isotope distribution patterns correspond to [CoII(12-
TMC)(OTf)]+ (calc. m/z = 436.1). First-order rate constants
(kobs), determined by the pseudo-first-order fitting of the
kinetic data for the decay of 2 at 620 nm, increased proportion-
Fig. 4 Plots of pseudo-first-order rate constants (kobs) against the con-
centrations of xanthene-h2 (black circles) and xanthene-d2 (red circles)
to determine second-order rate constants (k2) for the oxidations of
xanthene-h2 and xanthene-d2 by 2 in acetone at 0 °C.
ally with the increase of CHD concentration, leading us to Benzene (82(5)%) and xanthone (48(4)%) were formed as the
determine the second-order rate constant (k2) of 2.5(2) × 10−2 sole organic products in the oxidations of CHD and xanthene
M−1 s−1 at 0 °C (Fig. 3b). In order to determine the KIE value, by 2, respectively (Table S5, ESI†).
xanthene-h2 and xanthene-d2 were used as substrates, and the
The electrophilic reactivity of 2 was also investigated kineti-
second-order rate constants of 3.5(3) × 10−2 M−1 s−1 and cally in the sulfoxidation reaction (i.e., OAT). Upon addition of
8.6(6) × 10−3 M−1 s−1 were obtained, respectively (Fig. 4), which thioanisole to the solution of 2 under pseudo-first order con-
resulted in a KIE value of 4.1(3). The results discussed above ditions in acetone at 0 °C, 2 disappeared with a first-order
indicate that 2 is capable of activating C–H bonds with a rate- decay profile (Fig. S6a, ESI†), and a second-order rate constant
determining hydrogen atom abstraction step. The final reac- (k2) of 4.7(4) × 10−3 M−1 s−1 was determined (Fig. S6b, ESI†).
tion solutions obtained in the oxidations of CHD and Similarly, second-order rate constants of 1.2(1) × 10−2 M−1 s−1
xanthene by 2 were analysed by gas chromatography (GC). and 7.4(5) × 10−3 M−1 s−1 were also determined in the
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oxidation of 4-MeO-thioanisole and 4-Me-thioanisole by 2 in Beamline 2-2 at SSRL was partially supported by the SSRL
acetone at 0 °C, respectively (Fig. S6b, ESI†). A plot of log k2 under the U. S. Department of Energy Contract No. DE-AC02-
+
against a function of σp showed a good Hammett correlation 76SF00515, the National Synchrotron Light Source II, Brookha-
with a ρ value of −0.52 (Fig. S6c, ESI†). The Co product formed ven National Laboratory, under U. S. Department of Energy
in the reaction of 2 and thioanisole was analysed by EPR and Contract No. DE-SC0012704 and the CWRU CSB, funded by
ESI-MS spectroscopy. We found that the Co product formed in NIH grant P30-EB-009998.
this reaction was identical to that obtained in the oxidations of
CHD and xanthene by 2; the X-band EPR spectrum of the Co
product showed signals with g1 = 5.4, g2 = 4.4, and g3 = 2.03
Notes and references
(Fig. S7a, ESI†), indicating that the high-spin (S = 3/2) CoII
species was formed. The ESI-MS of the reaction solution
exhibited a prominent mass peak at m/z = 436.1 as a sole peak
(Fig. S7b, ESI†), which corresponds to [CoII(12-TMC)(OTf)]+
(calc. m/z = 436.1). The organic product analysis of the reaction
solution revealed that methyl phenyl sulfoxide was formed as a
sole product with 83(5)% yield (Table S5, ESI†). Based on the
results presented above, we conclude that 2 acts as an electro-
phile in HAT and OAT reactions.
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0 °C, 2 disappeared with a first-order decay profile (Fig. S8a,
ESI†), and a second-order rate constant (k2) of 1.4(2) × 10−2
M−1 s−1 was determined (Fig. S8b, ESI†). The Co product
formed in the reaction of 2 and 2-PPA was analysed by EPR
and ESI-MS spectroscopy, and we found that the Co product
formed in this reaction was identical to those obtained in elec-
trophilic HAT and OAT reactions by 2; the X-band EPR spec-
trum of the Co product exhibited signals with g1 = 5.4, g2 = 4.4,
and g3 = 2.03 (Fig. S9a, ESI†) that are characteristic of S = 3/2
CoII and the ESI-MS spectrum exhibited a prominent mass
peak at m/z = 436.2 as a sole peak (Fig. S9b, ESI†), which
corresponds to [CoII(12-TMC)(OTf)]+ (calc. m/z = 436.1). GC
analysis of organic product(s) revealed the formation of the
expected deformylated product, acetophenone (80(6)%), which
is frequently observed in other nucleophilic oxidative reactions
by metal–peroxide, –hydroperoxide, and –superoxide
complexes.4b,8a,16–18 These results indicate that 2 acts as a
nucleophile in the aldehyde deformylation reaction.
In conclusion, we have synthesized and characterised a
mononuclear nonheme cobalt(III)–hydroperoxide complex
bearing a macrocyclic N-methylated cyclam ligand, [(12-TMC)
CoIII(OOH)]2+, spectroscopically and computationally. The
[(12-TMC)CoIII(OOH)]2+ intermediate was shown to be an
active oxidant in both electrophilic and nucleophilic reactions.
We thus conclude that the cobalt(III)–hydroperoxide complex pos-
sesses an amphoteric reactivity in electrophilic and nucleophilic
oxidative reactions. Future studies will be focused on understand-
ing the detailed mechanism(s) of the electrophilic and nucleo-
philic oxidative reactions by metal–hydroperoxide complexes.
The authors acknowledge financial support from the NRF
of Korea through the CRI (NRF-2012R1A3A2048842 to W. N.),
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(NRF-2010-00353
to
W.
N.)
and
MSIP 10 J. Cho, R. Sarangi, H. Y. Kang, J. Y. Lee, M. Kubo, T. Ogura,
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