Nonheme iron mediated oxidation of light alkanes with oxone: Characterization of reactive oxoiron(IV) ligand cation radical intermediates by spectroscopic studies and DFT calculations

Article abstract of DOI:10.1002/anie.201305153

The oxidation of light alkanes that is catalyzed by heme and nonheme iron enzymes is widely proposed to involve highly reactive {Fe^ViO} species or {Fe^IViO} ligand cation radicals. The identification of these high-valent iron species and the development of an iron-catalyzed oxidation of light alkanes under mild conditions are of vital importance. Herein, a combination of tridentate and bidentate ligands was used for the generation of highly reactive nonheme {FeiO} species. A method that employs [Fe ^III(Me₃tacn)(Cl-acac)Cl]⁺ as a catalyst in the presence of oxone was developed for the oxidation of hydrocarbons, including cyclohexane, propane, and ethane (Me₃tacn=1,4,7-trimethyl-1,4,7- triazacyclononane; Cl-acac=3-chloro-acetylacetonate). The complex [Fe ^III(Tp)₂]⁺ and oxone enabled stoichiometric oxidation of propane and ethane. ESI-MS, EPR and UV/Vis spectroscopy, ¹⁸O labeling experiments, and DFT studies point to [Fe ^IV(Me₃tacn)({Cl-acac}^.+)(O)]²⁺ as the catalytically active species. Highly reactive {FeiO} intermediates, such as [Fe(Tp)₂(O)]⁺ or complex I (see Scheme), are likely to be involved in the oxidation of propane and ethane with oxone that is either mediated by [Fe^III(Tp)₂]⁺ (1) or catalyzed by iron complex 2. The cationic intermediate I features an {FeiO} moiety and is stabilized by a combination of tridentate and bidentate ligands. Copyright

Full text of DOI:10.1002/anie.201305153

.
Angewandte
Communications
DOI: 10.1002/anie.201305153
À
C H Oxidation Very Important Paper
Nonheme Iron Mediated Oxidation of Light Alkanes with Oxone:
Characterization of Reactive Oxoiron(IV) Ligand Cation Radical
Intermediates by Spectroscopic Studies and DFT Calculations**
Chun-Wai Tse, Toby Wai-Shan Chow, Zhen Guo, Hung Kay Lee, Jie-Sheng Huang, and Chi-
Ming Che*
Abstract: The oxidation of light alkanes that is catalyzed by
heme and nonheme iron enzymes is widely proposed to involve
catalyzed by synthetic nonheme iron complexes.[1–3] These
III
=
reactive {Fe O} species are usually generated from an Fe
V
IV
highly reactive {Fe O} species or {Fe O} ligand cation
radicals. The identification of these high-valent iron species
and the development of an iron-catalyzed oxidation of light
alkanes under mild conditions are of vital importance. Herein,
a combination of tridentate and bidentate ligands was used for
complex and an oxygen-donor ligand[4a] or by two-electron
=
=
III
oxidative deprotonation of {FeIII OH} or {Fe OH2} species.
À
À
Although a considerable number of nonheme {FeIV O}
=
complexes have recently been characterized,[3] there are
V
[3]
=
only a few studies on nonheme {Fe O} species and
the generation of highly reactive nonheme {Fe O} species. A
heme[4] or heme-like[5] {FeIV O} p-cation radicals. Notable
=
=
method that employs [FeIII(Me3tacn)(Cl-acac)Cl]+ as a catalyst
in the presence of oxone was developed for the oxidation of
hydrocarbons, including cyclohexane, propane, and ethane
(Me3tacn = 1,4,7-trimethyl-1,4,7-triazacyclononane; Cl-acac =
3-chloro-acetylacetonate). The complex [FeIII(Tp)2]+ and
oxone enabled stoichiometric oxidation of propane and
ethane. ESI-MS, EPR and UV/Vis spectroscopy, 18O labeling
experiments, and DFT studies point to [FeIV(Me3tacn)({Cl-
recent examples include cis-{X-FeV O} (I; X = solvent or
[2i]
=
OH[6]) and {FeV O} (II,[7] III[2k]) complexes that are supported
=
by polydentate ligands, or the cationic radical [FeIV(sa-
lenC+)(O)(X)] (IV; X = Cl), where salen is a tetradentate
Schiff base ligand[8] (Figure 1). In the literature, experimental
evidence can be found that supports participation of {FeV O}
=
IV
+
=
or {Fe (LC ) O} species in the iron-catalyzed oxidation of
[9,10]
À
strong C H bonds of light alkanes (ꢀ C4).
+
acac}C )(O)]2+ as the catalytically active species.
R
eactive high-valent iron–mono(oxo) intermediates beyond
+ +
V
IV
{FeIV O}, such as {Fe O} or {Fe (LC ) O} (LC = ligand
cation radical), are among the species that are proposed to be
directly responsible for the oxidation of hydrocarbons in
processes that are catalyzed by heme and nonheme iron
enzymes and in biomimetic alkane hydroxylations that are
=
=
=
[*] C.-W. Tse,[+] Dr. T. W.-S. Chow,[+] Dr. Z. Guo, Dr. J.-S. Huang,
Prof. Dr. C.-M. Che
State Key Laboratory of Synthetic Chemistry
Institute of Molecular Functional Materials and Department of
Chemistry, The University of Hong Kong
Pokfulam Road, Hong Kong (China)
Figure 1. Chelating ligand systems that were either previously reported
=
(I–IV) or used in this work (V) for supporting nonheme {Fe O}
species with oxidation states beyond FeIV (charges of complexes, if
any, are omitted), along with schematic representations of the catalysts
[FeIII(Tp)2]+ (1) and [FeIII(Me3tacn)(R-acac)Cl]+ (2).
E-mail: cmche@hku.hk
Prof. Dr. C.-M. Che
HKU Shenzhen Institute of Research and Innovation
Shenzhen 518053 (China)
Prof. Dr. H. K. Lee
Department of Chemistry
The Chinese University of Hong Kong
Shatin, New Territories, Hong Kong (China)
We were interested in developing the oxidation chemistry
III
=
of reactive {Fe O} species by reacting Fe precursors with
a two-electron oxygen atom donor.[11a] Herein, we used
a combination of tridentate and bidentate ligands for the
preparation of FeIII precursors. By coordination of one neutral
tridentate ligand and one monoanionic bidentate ligand to the
octahedral FeIII center, a single labile coordination site is
[+] These authors contributed equally to this work.
[**] This work was supported by the Hong Kong Research Grants
Council (HKU 700810 and HKU 700813), the National Key Basic
Research Program of China (2013CB834802), the University Grants
Committee of the Hong Kong Special Administrative Region (AoE/
P-03/08), and the CAS-Croucher Foundation Funding Scheme for
Joint Laboratories.
obtained, so that these complexes are suitable for the
V
=
formation of highly oxidizing cationic {Fe O} or
IV
+
=
{Fe (LC ) O} (V; Figure 1) species. The neutral tridentate
ligand employed is the strongly s-donating Me3tacn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201305153.
798
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 798 –803

Angewandte
Chemie
Table 2: Oxidation of cyclohexane with oxone catalyzed by 1 and 2.[a]
(Me3tacn = 1,4,7-trimethyl-1,4,7-triazacyclononane), which is
[12]
=
Product TON[b]
Alcohol Ketone
Total
TON
known to support highly oxidizing {Ru O} complexes.
Entry Catalyst Substrate
In initial studies, we found that that propane and ethane
could be oxidized within five minutes at room temperature in
the presence of [FeIII(Tp)2]+ (1; Tp = hydrotris(1-pyrazolyl)-
borate)) and oxone (potassium peroxymonosulfate,
2 KHSO5·KHSO4·K2SO4), albeit in a stoichiometric manner.
We subsequently turned our attention to the [FeIII(Me3tacn)-
(R-acac)Cl]+ system (2, R-acac = 3-R-acetylacetonate; R =
H: 2a, R = Cl: 2b; Figure 1). Herein, we report the oxidation
of light alkanes that was mediated or catalyzed by the iron
complexes 1 and 2. Remarkably, the oxidation of propane
with oxone that was catalyzed by 2 afforded a total turnover
number (TON) of approximately 14 for a five-minute
reaction at room temperature.
1
1
2a
2b
6.4
13.0
17.5
6.3
28.1
17.6
12.7
41.1
35.1
2[c]
3[c]
[a] Reaction conditions: cyclohexane (5 mmol), catalyst (0.03 mmol),
oxone (1.5 mmol), NaHCO3 (4.5 mmol), acetone (5 mL), H2O (5 mL),
RT, 5 min. [b] Number of micromoles of product (determined by GC)
divided by number of micromoles of catalyst. [c] Cyclohexane (7 mmol),
catalyst (0.004 mmol), oxone (0.4 mmol), NaHCO3 (1.2 mmol).
a mixture of ethanol and acetic acid; under the same
conditions, propane was oxidized to a mixture of isopropanol,
acetone, and propanoic acid (Scheme 1).[16] These reactions,
Complexes 1[13] and 2[14] were prepared as their ClO4
À
salts; the structures of 1 and 2a were determined by X-ray
crystallography (Supporting Information, Figure S1 and
S2).[15] The cyclic voltammograms of aqueous solutions of
1 (pH 7) and 2c ([FeIII(Me3tacn)(acac)(CH3CN)]2+; pH 1)
gave FeIII/II reduction potentials at Epa/Epc = 0.27/0.17 V (1)
and 0.54/0.1 V (2c), and a large catalytic oxidation current,
which is indicative of an electrochemical oxidation reaction,
at approximately 1.5 V relative to the saturated calomel
electrode (SCE); the latter is tentatively assigned to oxidation
of the reaction medium, presumably water (Figure S3 and
S4). 2c also gave an irreversible two-electron oxidation wave
at 1.46 V (vs. SCE).
The catalytic oxidation of styrene, cyclohexene, cyclo-
octene, and 1-decene in the presence of 1 or 2 (4 mol%) and
oxone in CH3CN/H2O or acetone/H2O at room temperature
yielded the corresponding epoxides in up to 96% yield within
five minutes (alkene as the limiting reagent; Table 1).
Importantly, cyclohexane was oxidized to cyclohexanol and
cyclohexanone within five minutes with a total TON of up to
approximately 41 (Table 2; catalyst loading: 0.6 or
0.06 mol%).
Scheme 1. Oxidation of ethane and propane with oxone mediated by
1 or catalyzed by 2.
however, each gave a total TON of 1.0 and were thus not
catalytic; presumably, the Fe complex decomposed during
oxidation. The oxidation was assisted by H2O, as a control
experiment for the oxidation of propane in the absence of
H2O, but in the presence of 1 and tetrabutylammonium
oxone, with a reaction time of five minutes gave very little
acetone (10% yield based on 1), and no other oxidation
products were detected. Using 2a or 2b (1 mol%) as the
catalyst, the oxidation of propane with oxone in CH3CN/H2O
Treatment of ethane (6.9 bar) with 1 and oxone in
CH3CN/H2O at room temperature for five minutes afforded
Table 1: Epoxidation of alkenes with oxone catalyzed by 1 and 2.[a]
Entry Catalyst Substrate Conv.[b] [%]
Product
Yield[b,c] [%]
À
(1:1, v/v) at room temperature gave the C H oxidized
products with a total TON of 13.7 after five minutes
(Table S1); the product distribution (isopropanol/acetone/
propanoic acid ꢁ 10:23:1) was different from that of the
previously reported FeCl3-catalyzed oxidation of propane
with H2O2 (258C, 1 h; isopropylhydroperoxide as the major
product).[9b] Overoxidation likely results in the formation of
acetone and propanoic acid, as oxidation of isopropanol and
propanol in the presence of 2 and oxone under similar
conditions gave acetone and propanoic acid in 63% and 52%
yield, respectively. Oxidation of ethane and propane with iron
complex 2 and oxone gave the oxidized products with total
TONs of up to 3.4 and 20.4, respectively after 30 min
(Scheme 1).
1
2
1
1
85
81
72[d] (85[e])
75 (93[e])
3
4
5
1
2a
2b
97
99
97
96 (99[e])
79 (80[e])
87 (90[e])
6
1
77[f]
67 (87[e])
[a] Reaction conditions: substrate (0.5 mmol) in CH3CN (3 mL) for 1 or
in acetone (3 mL) for 2, aqueous solution of oxone (0.75 mmol, 3 mL),
NaHCO3 (2.25 mmol), catalyst (0.02 mmol), RT, 5 min. [b] Determined
by GC analysis. [c] Based on the starting substrate. [d] Benzaldehyde was
also formed (9.4% yield). [e] Selectivity (yield of epoxide product divided
by substrate conversion). [f] Reaction time: 15 min.
=
The involvement of {Fe O} intermediates V (Figure 1) in
the hydrocarbon oxidation with oxone catalyzed by 1 or 2 is
supported by the following observations: First, high-resolu-
Angew. Chem. Int. Ed. 2014, 53, 798 –803
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
799

.
Angewandte
Communications
tion ESI-MS analysis of a mixture of 1 with oxone (10 equiv)
in CH3CN/H2O (reaction time: 15 seconds) revealed a species
with m/z 498.1463, which gave an isotope pattern and
a collision-induced dissociation pattern that both matched
the corresponding patterns of [Fe(Tp)2(O)]+ (calcd m/z
498.1426); this species was transformed into a mixture,
possibly of [Fe(Tp)2(OH)]+ and [Fe(Tp)2(H2O)]+, within
one minute (Figure S5 and S6). Second, for a mixture of 2b
with oxone (8 equiv) in CH3CN/H2O, high-resolution ESI-MS
analysis at a reaction time of 30 seconds revealed a cluster
peak at m/z 411.0787, which may be attributed to {[Fe-
(Me3tacn)(Cl-acac)(O)]Cl}+ (calcd m/z 411.0780) based on
the isotope pattern and collision-induced dissociation
(Figure 2). Changing the solvent to CH3CN/H218O led to an
with 2b and oxone in CH3CN/H218O (reaction time: 5 min),
and no significant change in the product yields was observed
when this reaction (in CH3CN/H2O) was conducted under an
atmosphere of argon or O2. With 1 and oxone in CH3CN/
H218O or acetone/H218O, the oxidation of styrene or cyclo-
hexane led to no significant 18O incorporation into styrene
=
oxide or cyclohexanol. Probably, the {Fe O} reaction inter-
mediate underwent oxygen transfer to these two hydrocarbon
substrates at higher rates than oxygen exchange with H2O.
Indeed, a stoichiometric reaction of 2b (0.2 mmol) with
oxone in CH3CN/H218O at room temperature for 30 seconds
to allow oxygen exchange between {Fe O} and H218O,
=
followed by treatment with excess cyclohexane (3.5 mmol)
for 5 min, resulted in an 18O incorporation into cyclohexanol
of 32%. Fourth, the oxidation of para-substituted styrenes
with 1 or 2b and oxone revealed linear Hammett plots with
1+ = À0.87 (1), À0.63 (2b; Figure S7 and S8), which is in
accordance with attack on the alkenes by an electrophilic
=
{Fe O} species. Competitive oxidation of several hydro-
carbons with 2b and oxone revealed a linear relationship
À
between logk’rel and the C H bond dissociation energy
(Figure S9), which is in agreement with rate-limiting H-
atom abstraction by the {Fe O} species.[2h] Fifth, the oxidation
=
of cyclohexane catalyzed by 1 or 2 with oxone exhibited
a primary kinetic isotope effect (KIE) of kH/kD = 4.7 (1), 3.6
(2a), or 3.8 (2b); these values are comparable to those
reported for the cyclohexane oxidation with Fe–tpa com-
plexes and H2O2 (tpa = tris(2-pyridylmethyl)amine; kH/kD =
3–4).[2a]
The DFT-optimized structure of [Fe(Tp)2(O)]+ (Va, Fig-
ure S10) features a six-coordinate iron, while one of the Tp
pyrazolyl groups remains uncoordinated (Figure 3), and is
Figure 3. DFT-optimized structures of [Fe(Tp)2(O)]+ (Va) and
[Fe(Me3tacn)(Cl-acac)(O)]2+ (Vb).
reminiscent of the crystal structures of [TiIV(Tp)2(Se)][17] and
[RuIV(H+tpa)(bpy)(O)]3+ [18] (Figure S11). Based on the com-
puted frontier molecular orbitals for the ground states of Va
(Figure S10) and Vb (Figure 4) and the spin distributions that
were determined using B3LYP and M06L functionals
(Table S2), Va can be described to have approximately 40%
[FeV(Tp)2(O)]+ character and about 60% [FeIV(Tp)-
Figure 2. a) Isotope patterns simulated for {[Fe(Me3tacn)(Cl-acac)-
(O)]Cl}+ and observed for the reactive species with m/z 411.0787 in
high-resolution ESI-MS analysis of the reaction mixture of 2b with
oxone (8 equiv) at 30 s. b) Collision-induced dissociation of this
reactive species (collision energy: 20 eV).
+
(TpC )(O)]+ character, whereas Vb has insignificant [FeV-
18O incorporation into the {[Fe(Me3tacn)(Cl-acac)(O)]Cl}+
complex of approximately 60%, which suggests that oxygen
(Me3tacn)(Cl-acac)(O)]2+ character and could be described as
IV
an {FeIV O} cation radical complex [Fe (Me3tacn)({Cl-
=
exchange between {Fe O} and H218O has occurred.[1a,c] The
acac}C )(O)]2+ (Figure 3 and Figure S12). Changing the
+
=
peak at m/z 411.0787 was suppressed in the presence of
cyclohexane (40 equiv). Third, neither H2O nor O2 served as
the major oxygen source of the oxidation products. Under
catalytic conditions, an 18O incorporation of approximately
6% into cyclohexanol was found for cyclohexane oxidation
ligand system from two anionic Tp ligands to one neutral
Me3tacn ligand and one anionic R-acac ligand might account
for the smaller amount of electronic-charge injection into the
3d shell of the iron center. Therefore, oxidation of the metal
center of the latter dicationic complex is conceived to occur at
800
www.angewandte.org
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 798 –803

Angewandte
Chemie
reaction barriers because of the similar electron configura-
tion/occupancy, and a two-state reactivity is thus also
anticipated.
Consistent with the formulation of [FeIV(Me3tacn)({Cl-
+
acac}C )(O)]2+ (Vb, S = 1/2) by DFT calculations, the X-band
EPR spectrum of a reaction mixture of 2b with oxone
(10 equiv) in CH3CN/H2O at 4 K exhibited a new signal with
g = 1.97, 1.93, 1.91 (gave = 1.94) that is attributable to Vb
(Figure 5). This new signal accounted for 35% of the total
iron content in the sample (Figure S15). It resembles the
Figure 5. X-band EPR spectrum of the reaction mixture of 2b with
oxone (10 equiv) in CH3CN/H2O (1:1, v/v; c) at 4 K compared with
the spectrum of the reaction mixture in the presence of cyclohexane
(12.5 equiv; b) and with the spectrum of 2b (c).
Figure 4. MO diagram for Vb (ground state, doublet) obtained by DFT
calculations (B3LYP/def2-TZVP).
+
signal reported for the [FeIV({TBP8Cz}C )(O)] (S = 1/2, g =
2.09, 2.05, 2.02; TBP8Cz = a corrolazine ligand),[5a] but is
distinct from the signal of 2b (S = 5/2, g = 7.0, 4.8, 4.1;
Figure S16) with meff = 6.0 mB (measured by the Evans
method) close to 5.9 mB reported for [FeIII(Me3tacn)-
(aacac)Cl]ClO4 (S = 5/2; aacac = 3-(9-anthryl)acetylaceto-
nate).[14] Its value of gave is similar to the gave value of 1.94
a higher potential than that of the former monocationic
=
complex. The Fe O distances that were computed at the
B3LYP/def2-TZVP level of theory are 1.631 ꢀ (Va) and
1.625 ꢀ (Vb); these values are comparable to that for
[FeV(taml)(O)]À (1.60 ꢀ; taml = tetraamido macrocyclic
that was found for
a Cytochrome P450 Compound I
ligand), which was computed with
a
similar method
(CYP119-I),[22b] and is comparable to gave = 1.90 for [FeV-
(B3LYP/6-311G).[7] Our initial calculations on the oxidation
of ethane and propane by Va and Va·H2O (the latter features
hydrogen bonding between Va and H2O, see Figure S10) in
the gas phase with the B3LYP functional showed that these
reactions involve rate-limiting H-atom abstraction by the
(taml)(O)]À.[7] CYP119-I and also CPO-I[22a] (another Cyto-
chromeP450 Compound I) are {FeIV O} porphyrin cation
=
radicals; their gave < 2 values are rationalized by a spin-
coupling model that exhibits substantial dependence of the
g values on the J/D ratio (J: exchange coupling; D: zero-field
splitting).[22] A similar rationalization could be provided for
the gave value of < 2 that is attributed to Vb in this work.
Analysis of a reaction mixture of 2b with oxone (10 equiv) in
CH3CN/H2O in the presence of cyclohexane (2, 5, or
12.5 equiv) by X-band EPR spectroscopy at 4 K revealed
that the signal that is attributed to Vb had a lower intensity at
higher cyclohexane concentrations and was almost com-
pletely suppressed in the presence of 12.5 equiv of cyclohex-
ane (Figure S17); this finding corroborates the idea that Vb is
a key reaction intermediate that is responsible for alkane
oxidation in the presence of 2b and oxone. For 1 and its
reaction mixture with oxone (10 equiv) in CH3CN/H2O, well-
defined signals could not be detected by X-band EPR at 4 K
(Figure S18), which is presumably due to quick relaxation or
the short lifetime of Va.
=
{Fe O} species, as was previously reported for the oxiation of
light alkanes by nonheme {FeIV O}
or {FeV O} spe-
=
[19a,c–e]
=
cies,[19b] which was studied by DFT calculations. The calcu-
lated reaction barriers are low (see Figure S13 and S14; the
reaction barrier is lower for Va·H2O than for Va because
oxidation is assisted by water), which is in line with the
experimentally observed oxidation of ethane and propane
that occurred in the presence of 1 and oxone at room
temperature within five minutes. After Wigner tunneling
correction, the calculated KIE[20] for the cyclohexane oxida-
tion by the ground state of Va·H2O is 4.0 (Table S3), which is
comparable to the experimental value of 4.7. At this stage, we
cannot conclude whether the doublet state or both the
doublet and quartet states are responsible for alkane oxida-
tion. In the literature, a two-state reactivity has commonly
been proposed for the hydroxylation of hydrocarbons by
TD-DFT calculations of the absorption spectrum of Vb at
the B3LYP/def2-TZVP level of theory with the PCM model
in water gave a simulated spectrum (Figure S19 and Table S4)
[21]
=
reactive heme or nonheme {Fe O} species. The doublet
and quartet states of Vb are conceived to have similar
Angew. Chem. Int. Ed. 2014, 53, 798 –803
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
801

.
Angewandte
Communications
that features a low-energy absorption at 737 nm. This
calculated absorption band is comparable to a new absorption
at approximately 750 nm that developed in the UV/Vis
absorption spectrum of a reaction mixture that was obtained
immediately after mixing 2b with oxone at room temperature.
The intensity of the new absorption at approximately 750 nm
decreased with an increase in cyclohexane concentration
(Figure S20), which is consistent with the findings by EPR
spectroscopy.
To demonstrate the generality of the approach that is
based on tridentate and bidentate ligands for the design of
nonheme iron oxidation catalysts, we prepared 2c, 2d, and 3
and examined their catalytic activities (Figure 6). Complexes
Keywords: alkanes · iron · nonheme iron complexes · oxidation ·
oxoiron complexes
.
[1] For selected reviews, see: a) X. Shan, L. Que, Jr., J. Inorg.
Biochem. 2006, 100, 421; b) E. G. Kovaleva, M. B. Neibergall, S.
Chakrabarty, J. D. Lipscomb, Acc. Chem. Res. 2007, 40, 475;
c) P. C. A. Bruijnincx, G. van Koten, R. J. M. Klein Gebbink,
Chem. Soc. Rev. 2008, 37, 2716; d) P. R. Ortiz de Montellano,
Chem. Rev. 2010, 110, 932; e) K. Schrçder, K. Junge, B.
Bitterlich, M. Beller in Iron Catalysis: Fundamentals and
Applications (Ed.: B. Plietker), Springer, Heidelberg, 2011,
p. 83; f) E. P. Talsi, K. P. Bryliakov, Coord. Chem. Rev. 2012, 256,
1418; g) M. Bordeaux, A. Galarneau, J. Drone, Angew. Chem.
2012, 124, 10870; Angew. Chem. Int. Ed. 2012, 51, 10712.
[2] For examples, see: a) K. Chen, L. Que, Jr., J. Am. Chem. Soc.
2001, 123, 6327; b) K. Chen, M. Costas, J. Kim, A. K. Tipton, L.
Que, Jr., J. Am. Chem. Soc. 2002, 124, 3026; c) A. Bassan,
M. R. A. Blomberg, P. E. M. Siegbahn, L. Que, Jr., J. Am. Chem.
Soc. 2002, 124, 11056; d) A. Nielsen, F. B. Larsen, A. D. Bond,
C. J. McKenzie, Angew. Chem. 2006, 118, 1632; Angew. Chem.
Int. Ed. 2006, 45, 1602; e) D. Kumar, H. Hirao, S. Shaik, P. M.
Kozlowski, J. Am. Chem. Soc. 2006, 128, 16148; f) S. H. Lee, J. H.
Han, H. Kwak, S. J. Lee, E. Y. Lee, H. J. Kim, J. H. Lee, C. Bae,
S. N. Lee, Y. Kim, C. Kim, Chem. Eur. J. 2007, 13, 9393; g) R.
Mas-Ballestꢁ, L. Que, Jr., J. Am. Chem. Soc. 2007, 129, 15964;
h) J. Yoon, S. A. Wilson, Y. K. Jang, M. S. Seo, K. Nehru, B.
Hedman, K. O. Hodgson, E. Bill, E. I. Solomon, W. Nam,
Angew. Chem. 2009, 121, 1283; Angew. Chem. Int. Ed. 2009, 48,
1257; i) O. Y. Lyakin, K. P. Bryliakov, G. J. P. Britovsek, E. P.
Talsi, J. Am. Chem. Soc. 2009, 131, 10798; j) G. Xue, R. De Hont,
E. Mꢂnck, L. Que, Jr., Nat. Chem. 2010, 2, 400; k) Y. Hitomi, K.
Arakawa, T. Funabiki, M. Kodera, Angew. Chem. 2012, 124,
3504; Angew. Chem. Int. Ed. 2012, 51, 3448.
[3] A. R. McDonald, L. Que, Jr., Coord. Chem. Rev. 2013, 257, 414.
[4] a) J. T. Groves, R. C. Haushalter, M. Nakamura, T. E. Nemo,
B. J. Evans, J. Am. Chem. Soc. 1981, 103, 2884; b) W. Nam, Acc.
Chem. Res. 2007, 40, 522; c) Z. Cong, T. Kurahashi, H. Fujii,
Angew. Chem. 2011, 123, 10109; Angew. Chem. Int. Ed. 2011, 50,
9935; d) E. V. Kudrik, P. Afanasiev, L. X. Alvarez, P. Dubour-
deaux, M. Clꢁmancey, J.-M. Latour, G. Blondin, D. Bouchu, F.
Albrieux, S. E. Nefedov, A. B. Sorokin, Nat. Chem. 2012, 4, 1024.
[5] a) A. J. McGown, W. D. Kerber, H. Fujii, D. P. Goldberg, J. Am.
Chem. Soc. 2009, 131, 8040; b) K. Cho, P. Leeladee, A. J.
McGown, S. DeBeer, D. P. Goldberg, J. Am. Chem. Soc. 2012,
134, 7392; c) P. Afanasiev, E. V. Kudrik, F. Albrieux, V. Briois,
O. I. Koifman, A. B. Sorokin, Chem. Commun. 2012, 48, 6088.
[6] I. Prat, J. S. Mathieson, M. Gꢂell, X. Ribas, J. M. Luis, L. Cronin,
M. Costas, Nat. Chem. 2011, 3, 788.
Figure 6. Nonheme iron oxidation catalysts 2c, 2d, and 3.
2c and 2d catalyzed the oxidation of cyclohexane with oxone
to give cyclohexanol and cyclohexanone with total TONs of
39.5 (2c) and 30.8 (2d) for a 5 min reaction at room
temperature (Table S5). Under the same conditions, the
oxidation of cyclooctane catalyzed by 2a–2d afforded cyclo-
octanol and cyclooctanone with total TONs of 34.2–36.9.
Replacing the catalyst for [FeIII(Me3tacn)(aacac)Cl]+ (2e)[14]
led to a slight decrease of the total TON to 29.9 and 32.2 for
the oxidations of cyclohexane and cyclooctane, respectively
(Tables S5 and S6). The applicability of the method that is
based on the use of 2 as the catalyst and oxone was examined
for the oxidation of a limiting amount of alkane substrate
using adamantane as an example (Table S7); the selectivity of
À
this process with respect to tertiary and secondary C H bonds
was also studied. The reactions that were catalyzed by 2a–2e
at room temperature with a reaction time of five minutes
afforded adamantan-1-ol, adamantan-2-ol, and adamantan-2-
one with moderate substrate conversion (43.9–52.4%), high
mass balance (73.9–85.8%), and with a selectivity for the
[11b]
À
À
tertiary/secondary C H bonds of 3.9–5.1 (per C H bond).
Complexes 2c and 2d effectively catalyzed propane oxidation
in the presence of oxone at room temperature, although 3
exhibited a lower catalytic activity (total TON: up to 7.8) than
2 (total TON: up to 11.9; Table S8).
[7] F. T. 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.
[8] a) V. K. Sivasubramanian, M. Ganesan, S. Rajagopal, R. Ram-
araj, J. Org. Chem. 2002, 67, 1506; b) N. S. Venkataramanan, G.
Kuppuraj, S. Rajagopal, Coord. Chem. Rev. 2005, 249, 1249.
[9] A few nonheme diiron or tetrairon catalysts have been reported
for the oxidation of methane, ethane, and propane with H2O2 at
RT or 50–808C; for examples, see: a) G. V. Nizova, B. Krebs, G.
Sꢂss-Fink, S. Schindler, L. Westerheide, L. G. Cuervo, G. B.
Shulꢃpin, Tetrahedron 2002, 58, 9231; b) G. B. Shulꢃpin, G. V.
Nizova, Y. N. Kozlov, L. G. Cuervo, G. Sꢂss-Fink, Adv. Synth.
Catal. 2004, 346, 317; c) V. B. Romakh, B. Therrien, G. Sꢂss-Fink,
G. B. Shulꢃpin, Inorg. Chem. 2007, 46, 3166.
In summary, mononuclear nonheme iron complexes 2
exhibit high catalytic activity towards the oxidation of light
alkanes, including propane and ethane, by oxone at room
temperature. These reactions proceed through reactive
{FeIV O} ligand cation radical intermediates. It is conceived
=
that the judicious choice of a robust neutral tripodal nitrogen-
donor ligand and an anionic bidentate ligand can give
nonheme iron catalysts for the oxidation of light alkanes
with synthetic interest.
=
[10] For the involvement of heme or heme-like {Fe O} intermediates
Received: June 15, 2013
Revised: October 8, 2013
Published online: November 27, 2013
in the oxidation of methane with meta-ClC6H4COOOH or H2O2
catalyzed by nitrido-bridged iron porphyrin or phthalocyanine
complexes, see Ref. [4d] and references therein.
802
www.angewandte.org
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 798 –803

Angewandte
Chemie
[11] a) T. W.-S. Chow, E. L.-M. Wong, Z. Guo, Y. Liu, J.-S. Huang, C.-
M. Che, J. Am. Chem. Soc. 2010, 132, 13229; b) P. Liu, Y. Liu,
E. L.-M. Wong, S. Xiang, C.-M. Che, Chem. Sci. 2011, 2, 2187.
[12] S. L.-F. Chan, Y.-H. Kan, K.-L. Yip, J.-S. Huang, C.-M. Che,
Coord. Chem. Rev. 2011, 255, 899.
[13] C.-G. Hong, A.-J. Zhou, M.-L. Tong, Acta Crystallogr. Sect. E
2005, 61, m1774.
[14] M. Mꢂller, T. Weyhermꢂller, E. Bill, K. Wieghardt, J. Biol. Inorg.
Chem. 1998, 3, 96.
[15] CCDC 870633 (1·ClO4) and 943377 (2a·ClO4) contain the
supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
[16] In control experiments under the same conditions, the reaction
of propane with oxone in the absence of 1 gave trace amounts of
acetone; neither isopropanol nor propanoic acid was detected.
[17] A. Kayal, J. Kuncheria, S. C. Lee, Chem. Commun. 2001, 2482.
[18] a) T. Kojima, K. Nakayama, K. Ikemura, T. Ogura, S. Fukuzumi,
J. Am. Chem. Soc. 2011, 133, 11692; b) T. Kojima, K. Nakayama,
M. Sakaguchi, T. Ogura, K. Ohkubo, S. Fukuzumi, J. Am. Chem.
Soc. 2011, 133, 17901.
[19] For selected examples, see: a) B. Ensing, F. Buda, M. C. M.
Gribnau, E. J. Baerends, J. Am. Chem. Soc. 2004, 126, 4355;
b) A. Bassan, M. R. A. Blomberg, P. E. M. Siegbahn, L. Que, Jr.,
Chem. Eur. J. 2005, 11, 692; c) S. P. de Visser, Angew. Chem.
2006, 118, 1822; Angew. Chem. Int. Ed. 2006, 45, 1790; d) C.
Geng, S. Ye, F. Neese, Angew. Chem. 2010, 122, 5853; Angew.
Chem. Int. Ed. 2010, 49, 5717; e) D. Usharani, D. Janardanan, S.
Shaik, J. Am. Chem. Soc. 2011, 133, 176.
[20] KIEs are sensitive probes of spin-state reactivity: for examples,
see: a) S. P. de Visser, F. Ogliaro, P. K. Sharma, S. Shaik, J. Am.
Chem. Soc. 2002, 124, 11809; b) D. Kumar, S. P. de Visser, S.
Shaik, J. Am. Chem. Soc. 2003, 125, 13024; c) K. Yoshizawa, Y.
Shiota, J. Am. Chem. Soc. 2006, 128, 9873; d) C. Li, W. Wu, K.-B.
Cho, S. Shaik, Chem. Eur. J. 2009, 15, 8492.
[21] For examples, see: S. Shaik, H. Hirao, D. Kumar, Acc. Chem.
Res. 2007, 40, 532, and references therein.
[22] a) R. Rutter, L. P. Hager, H. Dhonau, M. Hendrich, M.
Valentine, P. Debrunner, Biochemistry 1984, 23, 6809; b) J.
Rittle, M. T. Green, Science 2010, 330, 933.
Angew. Chem. Int. Ed. 2014, 53, 798 –803
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
803