Selective Alkane C-H Bond Oxidation Catalyzed by a Non-Heme Iron Complex Featuring a Robust Tetradentate Ligand

Article abstract of DOI:10.1021/acs.organomet.8b00611

An iron complex, [Fe^II(BpyPY2Me)(CH₃CN)₂](OTf)₂ (1-Fe, where BpyPY2Me is 6-(1,1-di(pyridin-2-yl)ethyl)-2,2′-bipyridine and OTf is triflate), supported by an oxidatively rugged tetradentate ligand is reported for the catalytic oxidation of unactivated C-H bonds in cyclohexane and adamantane. With the use of m-chloroperbenzoic acid (mCPBA) as the terminal oxidant, the iron catalyst shows an alcohol-to-ketone (A/K) ratio of 7.5 for cyclohexane oxidation with conversion percentages as high as 90% with respect to oxidant. Moreover, catalysis toward adamantane oxidation shows high regioselectivity (3°/2° = 45) favoring tertiary C-H bonds with yields up to 87%. Results, including electrospray ionization mass spectrometry and UV-vis spectroscopy, indicate that a molecular non-heme iron(IV)-oxo intermediate is the catalytically active species.

Full text of DOI:10.1021/acs.organomet.8b00611

Communication
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Selective Alkane C−H Bond Oxidation Catalyzed by a Non-Heme
Iron Complex Featuring a Robust Tetradentate Ligand
Lizhu Chen, Xiao-Jun Su, and Jonah W. Jurss*
Department of Chemistry and Biochemistry, University of Mississippi, University, Mississippi 38677, United States
*
S Supporting Information
II
ABSTRACT: An iron complex, [Fe (BpyPY2Me)(CH CN) ]-
3
2
(
2
OTf) (1-Fe, where BpyPY2Me is 6-(1,1-di(pyridin-2-yl)ethyl)-
,2′-bipyridine and OTf is triflate), supported by an oxidatively
2
rugged tetradentate ligand is reported for the catalytic oxidation
of unactivated C−H bonds in cyclohexane and adamantane. With
the use of m-chloroperbenzoic acid (mCPBA) as the terminal
oxidant, the iron catalyst shows an alcohol-to-ketone (A/K) ratio
of 7.5 for cyclohexane oxidation with conversion percentages as
high as 90% with respect to oxidant. Moreover, catalysis toward
adamantane oxidation shows high regioselectivity (3°/2° = 45)
favoring tertiary C−H bonds with yields up to 87%. Results,
including electrospray ionization mass spectrometry and UV−vis
spectroscopy, indicate that a molecular non-heme iron(IV)-oxo intermediate is the catalytically active species.
aw chemical feedstocks, such as petroleum and natural
R
gas, are important sources of inexpensive hydrocarbons
for the chemical and pharmaceutical industries. However, they
have thermodynamically stable, kinetically inert C−H bonds
that are not often viewed as chemical handles for further
manipulation. A challenge lies in converting these readily
available feedstocks into versatile organic building blocks in a
1
mild and atom economical manner. In this context, synthetic
iron-oxo catalysts have been developed for reactions such as
2
−4
olefin epoxidation and the hydroxylation of unactivated C−
5
−7
H bonds.
Circuitous synthetic routes engrossed in the
maintenance and interconversion of functional groups
throughout a reaction sequence can be avoided with catalysts
7
Figure 1. Representative non-heme iron complexes for C−H bond
capable of selective C−H bond functionalization. Despite the
oxidation bearing alkylamine-type N-donor ligands.
remarkable progress that has been made to demonstrate the
scope of chemistry available to synthetic non-heme iron-oxo
5
catalysts, significant improvements in catalyst selectivity and
14
tion sites. The increased activity and selectivity observed
with these catalysts is thought to arise from cooperativity
between the adjacent sites as proposed in water- or acetic-acid-
stability are needed to realize the full potential of these
8
systems.
Notably, iron-oxo complexes are capable of catalyzing N-
7
,15
9
assisted mechanisms for alkane oxidation. It is worth noting
that non-heme iron oxygenases also possess this structural
dealkylation reactions, which has particular relevance to
catalyst stability as the majority of non-heme Fe-oxo catalysts
employ alkylamine-type ligands. Prominent iron-based cata-
lysts of this ilk are shown in Figure 1. Amine-based ligands are
15
motif. While the protein environment around metalloenzyme
active sites serves, in part, to protect the active site from
degradation, synthetic catalysts do not have the luxury of a
tightly controlled surrounding and must be designed to
withstand the harsh oxidizing conditions of the reaction
medium. Thus, we have turned our attention toward more
rigid, pre-organized frameworks devoid of weak C−H bonds
and alkylamine-type donors as a starting point to enforcing
convenient to synthesize by S 2 reactions but are vulnerable to
N
9
,10
oxidative decomposition.
ligand can also be oxidized through both inter- and
intramolecular pathways.
Likewise, C−H bonds of the
10,11
Demetalation and individual
donor dissociation from iron have also been observed with
12,13
the flexible, polydentate ligands that are commonly used.
Beyond stability, metal coordination geometry is another
important factor as many of the champion iron catalysts
applied to hydrocarbon oxidation feature cis-labile coordina-
Received: August 25, 2018
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.8b00611
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Communication
desired active site configurations and improved catalyst
to solutions of 1-Fe did not generate the near-infrared
stability.
absorption band, which suggests that H O is not a strong
2
2
II
Herein, we report an iron(II) complex, [Fe (BpyPY2Me)-
enough oxidant to form an iron-oxo species from 1-Fe.
(
CH CN) ](OTf) (1-Fe, where BpyPY2Me is 6-(1,1-di-
pyridin-2-yl)ethyl)-2,2′-bipyridine and OTf is triflate),
Cyclic voltammetry (CV) of 1-Fe was performed in CH CN
3
2
2
3
(
solutions containing 0.1 M LiClO4 as the supporting
electrolyte (Figure S7). All reported potentials are referenced
to the ferrocenium/ferrocene couple (Fc+ ). In anhydrous
CH CN, 1-Fe undergoes a reversible (ΔE = 69 mV) one-
supported by an oxidatively rugged tetradentate scaffold that
aims to minimize oxidative ligand decomposition while
providing cis-labile coordination sites at the iron center.
/0
3
p
16
Literature procedures were followed to prepare BpyPY2Me.
electron redox process at E = 0.89 V, consistent with an
1/2
III
II
Complex 1-Fe was prepared by reacting BpyPY2Me with 1
equiv of Fe(OTf)2 in methanol. The complex was
subsequently crystallized from acetonitrile by diethyl ether
diffusion and structurally characterized (Figure 2) by single-
Fe (CH CN) /Fe (CH CN) couple. No additional waves
3 2 3 2
were observed when scanning to more positive voltages. At
negative potentials, two quasi-reversible, overlapping reduc-
tions are found at Ep,c = −1.66 and −1.83 V, which we assign
to ligand-based reductions based on the CV of a reported
Zn(II) complex bearing a very similar ligand that exhibits two
18
closely spaced reductions at similar potentials.
Because 1-Fe can be oxidized with mCPBA to generate an
iron-oxo complex, its application as a catalyst for cyclohexane
C−H bond oxidation was investigated (Figure S8). The ratio
of cyclohexanol-to-cyclohexanone, i.e., the alcohol-to-ketone
ratio (A/K), has been used to differentiate between a likely
radical-based mechanism or a metal-centered oxidation
1
process. The so-called radical mechanism typically proceeds
through a long-lived radical formed via Fenton-type chemistry
in which the oxidant, i.e., peroxide, is decomposed into radicals
that oxidize the substrate directly. In this case, the cyclohexyl
Figure 2. Crystal structure of the cation of [1-Fe](OTf)2 with
thermal ellipsoids rendered at the 70% probability level. Hydrogen
atoms have been omitted for clarity. Selected bond distances: Fe−
N(1), 1.9948(19); Fe−N(2), 1.9054(19); Fe−N(3), 1.9691(19);
Fe−N(4), 1.9509(19); Fe−N(5), 1.951(2); Fe−N(6), 1.950(2) Å.
radical is susceptible to reactivity with O to form cyclohexyl
2
peroxide, which exhibits low selectivity in decomposing to
equivalent amounts of cyclohexanol and cyclohexanone (with
an A/K ratio of ∼1). However, metal-centered C−H bond
oxidations are often characterized by short radical lifetimes that
crystal X-ray crystallography. Selected bond lengths, provided
1
13
in the caption, and the diamagnetic H and C nuclear
magnetic resonance (NMR) spectra of 1-Fe are consistent
with a low-spin iron(II) complex (Figures S1 and S2).
With the addition of one equivalent (equiv) of m-
1
allow higher A/K product ratios and greater selectivity. Here,
a transient substrate radical is formed by H atom abstraction
by the Fe-oxo catalyst and rebounds with the resulting Fe−OH
species to form the C−O bond of the product.
chloroperbenzoic acid (mCPBA) into the CH CN solution
3
containing 1 mM 1-Fe, the initial absorption at λ = 486 nm (ε
All reactions were conducted in CH CN at room temper-
3
−
1
−1
=
2700 M cm ) gives way to a new broad absorption band
ature with a large excess of substrate to avoid overoxidation of
cyclohexanol (A) to cyclohexanone (K). Catalytic reactions
were repeated in triplicate, and products were quantified by gas
chromatography. The conversion percentages with respect to
oxidant, taking the reaction stoichiometry into account (A +
−
1
−1
centered at 739 nm (ε ≈ 100 M cm ), a characteristic
absorption feature of known Fe-oxo complexes (Figure 3).
17
2
K), are presented in Table 1. The results show that
Table 1. Cyclohexane Oxidation with mCPBA Catalyzed by
a
1
-Fe
Figure 3. UV−vis spectra of (A) 1-Fe (black) in anhydrous
4
−1
−1
acetonitrile [λ
= 250 nm (ε = 1.6 × 10 M cm ), 302 nm
b
c
max
entry
1-Fe/mCPBA/substrate
A/K
overall yield (%)
4
−1
−1
3
−1
−1
(
(
1.8 × 10 M cm ), 395 nm (3.9 × 10 M cm ), and 486 nm
3
−1
−1
1
2
3
4
5
6
7
8
1:10:100
1:10:500
1:10:800
1:10:1000
1:20:100
1:20:500
1:20:800
1:20:1000
2.5
5.6
5.8
7.5
1.4
3.8
3.9
4.8
48
79
80
90
48
78
80
84
2.7 × 10 M cm )] and (B) the iron(IV)-oxo intermediate (red)
formed by adding 1 equiv of mCPBA.
High-resolution electrospray ionization mass spectrometry
(
ESI-MS) confirmed its identity as a high-valent, molecular
1
iron(IV)-oxo species (Figure S3). A paramagnetic H NMR
spectrum was also obtained of the iron-oxo complex with
proton signals spanning a range of 54 ppm (Figure S4). This
intermediate is relatively stable (t1/2 ∼ 30 min at 23 °C;
Figures S5, S6) when monitoring self-degradation by the
decrease in absorbance at 739 nm. We note that H O addition
a
b
Conditions: 1 mM 1-Fe, 3 mL total volume. Ratio of moles of
c
cyclohexanol to moles of cyclohexanone. Yield with respect to
mCPBA.
2
2
B
DOI: 10.1021/acs.organomet.8b00611
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Communication
cyclohexanol is the main product for all entries. When 10 equiv
of mCPBA and 1000 equiv of cyclohexane are used, the highest
conversion yield of 90% and largest A/K ratio (7.5) are
observed, consistent with a metal-centered oxidation mecha-
Encouraged by these results, the catalytic oxidation of
adamantane (Figure S14) was investigated to assess the ability
of 1-Fe to select between its tertiary and secondary C−H
bonds, which have reported bond dissociation energies of 96.2
1
19
nism. A small increase in the A/K ratio to 9 was observed in
and 100.2 kcal/mol, respectively. Adding 10 equiv of
experiments conducted in the absence of O . In addition, a
mCPBA into CH CN solutions containing 1 mM 1-Fe and
2
3
series of reactions were quenched with triphenylphosphine at
different time points following the addition of mCPBA to
measure the A/K ratio as a function of time. The A/K ratios
are largely unaffected by the reaction time (Figure S9).
Reaction kinetics for cyclohexane oxidation were also
analyzed by monitoring the decay of the preformed Fe(IV)-
oxo intermediate upon addition of different amounts of
substrate (Figure S10). Pseudo-first-order rate constants
10 equiv of adamantane gives an 87% conversion yield with a
regioselectivity ratio of 45 for tertiary sites (3°) to secondary
sites (2°). Compared with other molecular iron-oxo catalysts,
1-Fe exhibits high selectivity for tertiary C−H bonds in
adamantane oxidation with high conversion (Table 2).
Table 2. Comparison of Selected Iron Catalysts for
Adamantane Oxidation
(
kobs) were determined by fitting the kinetic traces and were
a
b
c
catalysts
oxidant
3°/2°
yield (%)
ref
found to increase linearly with increasing cyclohexane
concentration (Figure S11). From the slope of this plot, a
second-order rate constant (k ) of 1.4 × 10
was obtained. Using various ratios of cyclohexane to d -
1
-Fe
mCPBA
PhIO
45
48
110
30
25
28
87
14
29
19
this work
−
2
−1 −1
III
Fe TPPCl
20
21
22
23
24
14a
25
M
s
at 23 °C
2
2
3
4
5
6
7
mCPBA
H O
1
2
cyclohexane (Figure S12), a kinetic isotope effect (KIE) of 3.2
was calculated (Figure S13) in experiments carried out under
the same conditions as entry 4 of Table 1. The KIE value as
well as the A/K ratios measured with 1-Fe are similar to
2
2
2
2
2
d
H O
2
31
H O
2
−
−
H O
2
33
17
d
14a
mCPBA
50
reported values for known non-heme iron-oxo complexes.
a
b
To address one of the primary aims of this work, the
catalytic stability of 1-Fe was also investigated by analyzing the
amount of oxidized product relative to the amount of added
mCPBA. Here, mCPBA (10 equiv) was added every 10 min
Catalyst structures are shown in Figure S16. 3°/2° = 3 × [1-
c
adamantanol/(2-adamantanol + 2-adamantanone)]. Yield with
respect to oxidant. Calculated from data provided in the cited work.
d
into a CH CN solution containing 1 mM 1-Fe and 1000 equiv
of cyclohexane. As shown in Figure 4, the total amount of
3
Moreover, with increasing amounts of mCPBA added to 500
equiv of adamantane and 1 mM 1-Fe, selectivity remains
unchanged even as overall conversion begins to plateau around
5
0% at higher oxidant-to-substrate ratios (Figure S15).
Importantly, control experiments for both cyclohexane and
adamantane oxidation were conducted under the same
conditions but in the absence of catalyst; no conversion to
products was observed after 2 h from direct substrate oxidation
by mCPBA.
The selected iron complexes, summarized in Table 2 and
shown in Figure S16, have some of the highest reported 3°/2°
values for the regioselective oxidation of adamantane. Until
recently, the highest selectivity for the tertiary C−H bond sites
of adamantane was observed with an iron porphyrin-based
III
20
catalyst (Fe TPPCl). In 2017, Gupta and co-workers
developed a more robust, nitro-substituted biuret-modified
tetra-amido macrocycle (bTAML) to obtain iron catalyst 2
with remarkable regioselectivity (3°/2° of 110), albeit with a
Figure 4. Plot of turnover number (TON), moles of product/moles
of catalyst, for each product versus equivalents of added mCPBA
oxidant to 1 mM 1-Fe and 1000 equivalents of cyclohexane in
CH CN. The theoretical maximum TON for cyclohexanol is shown
3
21
2
9% yield. Notably, the unsubstituted Fe-bTAML catalyst
with the dashed line, and the A+2K values reflect total consumption
of mCBPA to generate products as two equivalents of mCPBA are
required to form cyclohexanone from cyclohexane.
produced a 3°/2° product ratio of 69 and a conversion yield of
2
1
just ∼2%, highlighting the importance of catalyst stability.
III
Indeed, 1-Fe has comparable selectivity to Fe TPPCl but with
-fold higher conversion at 87%. In contrast to the macrocyclic
examples, catalysts 3 through 6 of Table 2 and 1-Fe possess cis-
6
oxidized product (A + 2K) is close to the theoretical value up
to 30 equiv of mCPBA. At 100 total equivalents of mCPBA,
the yield of oxidized products approaches 40%. A comparable
conversion of 48% is observed with the same ratio of mCPBA
to cyclohexane where the oxidant was added all at once (Table
14a,22−24
labile coordination sites.
Relative to these systems, 1-
Fe exhibits superior selectivity for adamantane tertiary C−H
bonds and, to the best of our knowledge, affords the highest
yield with respect to oxidant of all reported iron complexes for
adamantane oxidation. The high regioselectivities obtained
1
, entry 1). These results are consistent with slow deactivation
III
of 1-Fe. When the added oxidant totals 50 equiv, conversion to
both cyclohexanol and cyclohexanone approaches a plateau,
indicating almost complete deactivation of the catalyst after 5
cycles of added mCPBA (10 equiv/cycle). After 5 cycles,
overall conversion is 70% with an alcohol-to-ketone ratio of 2.7
and a total turnover number (TON) of 35.
with 1-Fe and macrocycles Fe TPPCl and 2 also suggest that
the relative orientation of open coordination sites is less
important than other factors in this reaction. Catalyst 7 is a
dinuclear iron complex with two metal center active sites,
which may account for its relatively high conversion yield of
25
50%.
C
DOI: 10.1021/acs.organomet.8b00611
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Communication
Next, the oxidation of cis-dimethylcyclohexane was exam-
ined to assess the ability of 1-Fe to catalyze the stereospecific
hydroxylation of its tertiary C−H bonds. Results summarizing
the reactivity and stereoselectivity of 1-Fe are given in Table 3.
selectivity ratio of 45 for adamantane oxidation is among the
highest reported for molecular iron-oxo catalysts. Moreover,
high conversion yields with respect to oxidant attest to the
improved stability of this iron catalyst in hydrocarbon
oxygenation.
Table 3. Regioselective Oxidation of cis-
Dimethylcyclohexane
ASSOCIATED CONTENT
■
*
S
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.8b00611.
1
-Fe/substrate/mCPBA/
conversion
RC
selectivity
a
b
c
entry
AcOH
(%)
(%)
(%)
Materials and methods, synthetic procedures, character-
ization, X-ray crystallographic data, A/K ratios vs time,
reaction kinetics, KIE plot, and representative gas
chromatograms (PDF)
1
2
3
4
1:25:10
1:25:20
1:120:100
1:120:100:0.5
72
54
25
21
100
100
90
66
77
72
71
90
a
b
Accession Codes
Conversion efficiency with respect to mCPBA. Retention of
configuration, RC = [(trans-OH − cis-OH)/(trans-OH + cis-OH)]
100. Overall selectivity for trans-OH = [trans-OH/(trans-OH + cis-
CCDC 1863860 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
c
×
OH + 2,3-OH + 3,4-OH)] × 100.
The degree to which stereoinformation is retained in the
oxidation products also reports on the lifetime of alkyl radicals
2
6
formed during the reaction. Hydroxylation of the tertiary C−
H bonds can occur with retention or inversion of
configuration, which reflects the competition between C−O
bond formation and epimerization of the tertiary alkyl radical
intermediate. Very short radical lifetimes result in high
stereospecificity, in which C−O bond formation is fast and
the original configuration is preserved to yield trans-1,2-
dimethylcyclohexanol (in which the methyl groups are cis to
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: jwjurss@olemiss.edu.
ORCID
Lizhu Chen: 0000-0003-4645-6161
Xiao-Jun Su: 0000-0002-4060-0962
Jonah W. Jurss: 0000-0002-2780-3415
1
4a
one another). In contrast, long-lived radicals should result in
nearly equivalent amounts of cis and trans tertiary alcohol
Notes
2
6
products. With 1-Fe, 100% retention of configuration (RC)
is observed following the addition of 10 or 20 equiv of mCPBA
to 25 equiv of cis-dimethylcyclohexane substrate (entries 1 and
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
2
3
, Figure S17). Accounting for the small amount of 2,3- and
,4-dimethylcyclohexanol produced (2,3-OH and 3,4-OH,
Acknowledgment is made to the donors of The American
Chemical Society Petroleum Research Fund for support of this
research (grant no. 58707-DNI3).
respectively), the regioselectivity for trans-OH is also as high
as 77%. Only a minor loss in stereoretention is observed at
lower catalyst loading (entry 3, Figure S18), albeit with a drop
in overall conversion. The addition of 0.5 equiv of acetic acid
has previously been shown to improve conversion and
selectivity for C−H bond oxidation with a non-heme iron
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DOI: 10.1021/acs.organomet.8b00611
Organometallics XXXX, XXX, XXX−XXX