Hydrocarbon oxidation catalyzed by a cheap nonheme imine-based iron(II) complex

Article abstract of DOI:10.1039/c4cy00626g

Nonheme iron complex 1 is easily obtained by one-pot assembly of cheap and commercially available starting materials. This complex effectively catalyzes the oxidation of a number of non-activated C-H bonds by H₂O ₂ with high turnover numbers and good selectivity. the Partner Organisations 2014.

Full text of DOI:10.1039/c4cy00626g

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Hydrocarbon oxidation catalyzed by a cheap
nonheme imine-based iron(II) complex†
Cite this: Catal. Sci. Technol., 2014,
, 2900
4
Giorgio Olivo, Giorgio Arancio, Luigi Mandolini, Osvaldo Lanzalunga and
Stefano Di Stefano*
Received 13th May 2014,
Accepted 22nd June 2014
DOI: 10.1039/c4cy00626g
www.rsc.org/catalysis
Nonheme iron complex 1 is easily obtained by one-pot assembly
of cheap and commercially available starting materials. This
complex effectively catalyzes the oxidation of a number of non-
Fe(II)-catalyzed oxidation using imine-based ligands with a
very simple structure.
In this communication, we report on the in situ one-pot
preparation of complex 1 from cheap and commercially
available precursors and show that this complex catalyzes the
2 2
activated C–H bonds by H O with high turnover numbers and
good selectivity.
2 2
oxidation of hydrocarbon by H O with high efficiency.
The past decade has witnessed the disclosure of a promising
path towards non-activated C–H bond oxidation. A great
1
impulse to explore this research topic came from several
studies on nonheme iron oxygenase models, which were
2
demonstrated to exhibit enzyme-like activity. Subsequently,
some iron complexes with abiotic ligands have been prepared
and shown to be good catalysts for the hydroxylation of non-
3
activated C–H bonds with high yields and selectivities.
These catalysts use cheap and abundant iron(II) as the
metal center and environmentally friendly H O as the oxi-
The formation of imine ligand 4 is complete within
0 min upon addition of equimolar amounts of pyridine-2-
2
2
4
dant. Efficient ligand design is crucial for both activity and
selectivity. The most active complexes prepared so far consist
of tetra- and pentadentate amine and/or pyridine-based
ligands arranged in a cis-alpha topology around the iron
center. The ligand should be quite rigid and sterically encum-
carbaldehyde (2) and 2-aminomethylpyridine (3) (33 mM) in
1
3
CD CN solution at room temperature. The aldehydic H NMR
signal of 2 (9.98 ppm) disappears at the expense of the imine
signal of 4 at 8.50 ppm (see Fig. S1, ESI†). When Fe(CF SO )
3
3 2
3
,4
is added to the solution, the latter instantaneously becomes
bered in order to obtain high selectivities. Effective ligands
developed so far are either commercially available but expen-
sive or require multistep synthesis.
purple. UV–Vis titration of 0.5 mM Fe(CF SO ) with
3
3 2
preformed ligand 4 in CH CN and the corresponding Job
3
plot (Fig. 1 and 2) definitely demonstrate that complex 1 has
a 1 : 2 iron(II) to 4 stoichiometry, as previously suggested in
the literature for a similar complex having a perchlorate
Despite the large use of imine ligands in organometallic
5
chemistry, the number of examples in which imine ligands
6
,7
are employed as nonheme iron catalysts is scanty. Since
imines are very easily prepared by simple mixing of the par-
ent primary amine and the carbonyl compound in a suitable
9
counterion. Furthermore, UV–Vis spectra demonstrate that
when compounds 2, 3 and Fe(CF SO ) are added at the
3
3 2
8
same time in solution at concentrations as low as 0.50, 0.50
and 0.25 mM, respectively, complex 1 is instantaneously
formed, showing that the rate of imine formation is signifi-
cantly enhanced by the iron(II) template (compare Fig. S3a
with S3b, ESI†).
solvent, we became interested in exploring the possibility of
Dipartimento di Chimica, Sapienza Università di Roma and Istituto CNR
di Metodologie Chimiche (IMC-CNR), Sezione Meccanismi di Reazione,
c/o Dipartimento di Chimica, Sapienza Università di Roma, P.le A. Moro 5,
I-00185 Rome, Italy. E-mail: stefano.distefano@uniroma1.it;
Complex 1 was tested as a catalyst in the oxidation of a
series of hydrocarbon substrates by H O . The first series of
Fax: +39 06 49913629; Tel: +39 06 49913057
1
2
2
†
Electronic supplementary information (ESI) available: H-NMR and UV–Vis
reactions was carried out on cyclohexane (Scheme 1) with the
aim of choosing the optimal ligand/Fe(II) molar ratio for effi-
cient catalysis (see entries 1–5 of Table 1 and Fig. 3). In these
spectra, details of oxidation procedures, and results related to oxidation
reactions carried out in the presence of Fe(CF
c4cy00626g
3 3 2
SO ) alone. See DOI: 10.1039/
2900 | Catal. Sci. Technol., 2014, 4, 2900–2903
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Table 1 Oxidation of cyclohexane to cyclohexanol (A) and cyclohexanone
(
K) by H
2
O
2
in CH
3
CN at 30 °C in the presence of Fe(CF
SO )
3 3 2
and imine
a
ligand 4 unless otherwise stated
b
2
b
c
c
d
Entry
3 3
Fe(CF SO )
Ligand 4
A
K
A + K
1
2
3
4
5
6
7
8
9
2.5
2.5
2.5
2.5
2.5
2.5
2.5
0.5
1.0
5.0
1.0
—
1.4 ± 0.2
4.2 ± 0.2
14 ± 1
13 ± 1
11 ± 1
13 ± 1
14 ± 1
8.3 ± 0.3
18 ± 1
13 ± 1
1.7 ± 0.2
9.3 ± 0.3
9.1 ± 0.3
17 ± 1
6.2 ± 0.2
17 ± 1
14 ± 1
5.4 ± 0.2
15 ± 1
3 (2.0)
13 (8.8)
23 (13)
30 (19)
17 (9.2)
30 (19)
28 (17)
13 (36)
33 (48)
27 (8.2)
3 (4.3)
1.2
2.5
5.0
10
5.0
5.0
1.0
2.0
h10.0
e
f
3 3 2
Fig. 1 The titration curve of 0.5 mM Fe(CF SO ) with preformed 4 in
g
CH
3
CN at 25 °C found at 506 nm. Saturation is reached after addition
of 2 mol equiv. of ligand.
10
1
14 ± 1
1.5 ± 0.2
1
1.3 ± 0.2
a
Cyclohexane (0.51 mmol), hydrogen peroxide (0.61 mmol,
b
1
20 mol%), acetic acid (0.25 mmol, 50 mol%). mol% refers to the
c
substrate amount. %GC yields are based on the initial amount
of cyclohexane and the average of two or three independent
determinations. Turnover number (TON) in brackets. Hydrogen
d
e
f
g
peroxide (1.02 mmol, 200 mol%). In the absence of acetic acid. An
additional loading of the same amounts of H , Fe(CF SO and 4
into the reaction mixture gave the following results: A, 11%; K, 32%.
2
O
2
3
3 2
)
h
2-Picolinic acid added as a ligand (4 mol%) instead of 4.
Fig. 2 The Job plot for complexation between 4 and Fe(CF
CH CN at 25 °C found at 567 nm. Maximum is reached for a 1 : 2
Fe(CF SO to 4 stoichiometry.
3 3 2
SO ) in
3
3
3 2
)
Fig. 3 Yields for oxidation of cyclohexane to cyclohexanol (grey) and
Scheme 1
3 3 2
cyclohexanone (black) as a function of the ligand 4/Fe(CF SO ) molar
ratio. Reaction conditions as reported in Table 1.
experiments, only the concentration of ligand 4 was varied.‡
When only Fe(CF SO ) was present in solution, similar
3
3 2
amounts of cyclohexanol and cyclohexanone were obtained
in very low yields (entry 1). The best results were observed for
a 2 : 1 ligand 4/Fe(II) molar ratio (entry 4).
This is in accordance with complex 1 being the pre-active
species in the catalytic event. The reaction of this octahedral
III
complex with H O would lead to a Fe –OOH species, which
2
2
is eventually transformed into an active oxo-complex able to
1
0
carry out cyclohexane oxidation. It is likely that complex 1
temporarily loses one of its six coordination legs to host
hydrogen peroxide. Alternatively, the latter could occupy a
seventh site in the inner metal ion coordination sphere, as
suggested by Bauer et al., in the activation of the analogous
Fig. 4 Time evolution for production of cyclohexanol (triangles) and
cyclohexanone (squares) as a function of time (data points related to
entry 4 of Table 1). Total yield is given by circles.
1
1
saturated amine-based iron complex.
Fig. 4 shows the time evolution of cyclohexane oxidation
products (conditions of entry 4 in Table 1). Cyclohexanone is
clearly formed by further oxidation of the initial cyclohexanol
product. Thus, a Russell-type termination mechanism involv-
ing free hydroxyl radicals should be confidently ruled out.
1
2
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Further experiments were carried out at varying amounts
activity and selectivity of 1 with other nonheme iron(II) com-
plexes and iron(II) triflate salts (Fenton chemistry).
With all of the investigated substrates, conversions and
selectivities turn out to be much higher in the presence of
complex 1 than with the free iron(II) salt (compare Table 2
with Table S1, ESI†).
of the oxidant, catalyst and additive, as reported in Table 1
3
(
entries 6–11). The presence of acetic acid as an additive is
not determinant for the catalytic efficiency (compare entry
with entry 7), although in its presence, a slightly higher
4
selectivity towards cyclohexanone was observed. No improve-
ment was obtained on increasing the catalyst (entry 10) or
oxidant (entry 6) loading, but enhanced catalytic efficiency in
terms of turnover number (TON) was observed when a
smaller catalytic amount (1%) was added (entry 9). Whereas
a further decrease in catalyst loading definitely led to lower
conversions (entry 8), the total yield of oxidation products
somewhat increased as a consequence of the subsequent
Adamantane was converted into oxidation products,
adamantanols and adamantanone, in 29% yield with a total
TON of 29 (Table 2, entry 1). The latter value is significantly
higher than the TONs reported in the literature, ranging
1
4,15
from 4.9 to 8.4, for other nonheme iron complexes.
The
3°/2° selectivity observed (9.4) is in agreement with the
7
,14,15
results obtained with other nonheme iron catalysts,
3
a,f
addition of the catalyst and the oxidant
(see footnote g to
even though it is significantly lower than that provided by
Table 1). The marked dependence of catalytic efficiency on
catalyst concentration may be the result of auto-oxidative
wasting processes, which become important at high catalyst
concentration.
the very selective Py–TACN complex developed by Costas and
coworkers (30).
1
6
In the oxidation of (d)-menthyl acetate, the main product
was the tertiary alcohol derived from the oxidation of the
most reactive C–H bond with retention of configuration, as
Some pyridin-2-yl-based iron and manganese complexes
1
4
have been recently reported to be oxidized by H
2
O
2
to give
previously observed in the oxidation with White's complex.
1
3
2
-picolinic acid, which then becomes the effective ligand in
However, the selectivity was lower, with the main oxidation
product accounting for about half of total product yields
(entry 2).
Finally, we investigated the efficiency of catalyst 1 in the
oxidation of the benzylic position of tetraline (entry 3). As
expected on the basis of the presence of the activated benzylic
C–H bonds, the products 1-tetralol and 1-tetralone were
formed with a higher conversion with respect to the oxida-
tion of aliphatic hydrocarbons. The efficiency of our catalyst
the catalytically active species. The absence of a time lag in
the formation of cyclohexanol (Fig. 4) and, more importantly,
the negligible catalytic activity observed when 2-picolinic acid
was added as a ligand instead of 4 (compare entry 11 with
entry 9 in Table 1) definitely rule out any significant role of
2
-picolinic acid in our catalytic system.
Next, we turned our attention to the oxidation of other
benchmark hydrocarbons in order to compare the catalytic
2 2 3
Table 2 Oxidation of hydrocarbons by H O catalyzed by the imine complex 1 in CH CN at 30 °C. Yields are the average of two or three independent
runs. TON are reported in brackets
Entry
Substrate
Products
Total yield
29 (29)
a,b
1
2
8
2 ± 2
± 1
7 ± 2
c,d
2
15 (15)
b,e
3
43 (28)
5 (2.0)
15 ± 1 (6.0)
28 ± 1 (22)
b,e
4
5
± 1 (4.0)
a
Conditions: Fe(CF
3
SO
3
)
2
(1.5 μmol, 1 mol%), 2 (3.0 μmol, 2 mol%), 3 (3.0 μmol, 2 mol%), adamantane (150 μmol), H
2
O
2
(180 μmol, 120 mol%),
b
c
AcOH (75 μmol, 50 mol%). GC yields. Conditions: Fe(CF
acetate (508 μmol), AcOH (254 μmol, 50 mol%), H (610 μmol, 120 mol%).
(13 μmol, 2.5 mol%), 2 (26 μmol, 5 mol%), 3 (26 μmol, 5 mol%), substrate (508 μmol), H
AcOH (254 μmol, 50 mol%).
3 3 2
SO ) (5.1 μmol, 1 mol%), 2 (10.2 μmol, 2 mol%), 3 (10.2 μmol, 2 mol%), (d)-menthyl
d 1
2
O
2
H-NMR yield. Only yield of the main product is reported.
(1.16 mmol, 200 mol%),
e
Conditions: Fe(CF
3
SO
3
)
2
2 2
O
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was found to be very sensitive to electronic effects, as shown
by the drop in catalytic activity in the oxidation of α-tetralone
due to the presence of the deactivating electron-withdrawing
carbonyl group (entry 4).
Trans., 2009, 5319; (c) Y. Feng, J. England and L. Que Jr.,
ACS Catal., 2011, 1, 1035.
5 P. G. Cozzi, Chem. Soc. Rev., 2004, 33, 410.
6 M. Lenze, E. T. Martin, N. P. Rath and E. B. Bauer,
ChemPlusChem, 2013, 78, 101.
7
(a) B. Retcher, J. S. Costa, J. Tang, R. Hage, P. Gamez and
J. Reedijk, J. Mol. Catal. A: Chem., 2008, 286, 1; (b) S. Tanase,
J. Reedijk, R. Hage and G. Rothenberg, Top. Catal., 2010, 53,
Conclusions
To sum up, we have shown that nonheme imine-based iron(II)
complex 1, easily prepared in situ from cheap and commer-
cially available starting materials, is a promising catalyst for
1
039; (c) P. Shejwalkar, N. P. Rath and E. B. Bauer, Dalton
Trans., 2011, 40, 7617.
the oxidation of hydrocarbons by H O with a good activity,
2
2
8 (a) M. Ciaccia, P. Mencarelli, R. Cacciapaglia, L. Mandolini
and S. Di Stefano, Chem. Sci., 2013, 4, 2253; (b) M. Ciaccia,
S. Pilati, R. Cacciapaglia, L. Mandolini and S. Di Stefano,
Org. Biomol. Chem., 2014, 12, 3282.
even at low catalyst loadings (as low as 1%). In terms of
the turnover number, the catalytic efficiency of complex 1
compares well with that of the majority of non-heme iron
2
b,c,6,14,15,17
complexes reported so far in the literature.
9
(a) F. Lions and K. V. Martin, J. Am. Chem. Soc., 1957, 79,
733; (b) B. A. Frazier, E. R. Bartholomew, E. R. Wolczanski,
The study of the catalytic activity in the oxidation of
hydrocarbons promoted by other imine-based iron(II) com-
plexes is currently under way in our laboratory.
2
T. Peter, S. DeBeer, M. Santiago-Berrios, H. D. Abruna,
E. B. Loblovsky, S. C. Bart, S. Bart, K. Meyer and T. R. Kundari,
Inorg. Chem., 2011, 50, 12414.
Notes and references
10 (a) W. Nam, Acc. Chem. Res., 2007, 40, 522; (b) J. Yoon,
S. A. Wilson, K. J. Yu, M. S. Seo, K. Nehru, K. O. Hedma
Hodgson, E. Bill, E. I. Solomon and W. Nam, Angew. Chem.,
Int. Ed., 2009, 48, 1257; (c) W. N. Oloo, A. J. Fielding and
L. Que Jr., J. Am. Chem. Soc., 2013, 135, 6438; (d) I. Prat,
A. Company, V. Postils, X. Ribas, L. Que Jr., J. M. Luis and
M. Costas, Chem. – Eur. J., 2013, 19, 6724.
11 M. Lenze and E. B. Bauer, Chem. Commun., 2013, 49, 5889.
12 M. Mitra, J. Lloret-Fillol, M. Haukka, M. Costas and
E. Nordlander, Chem. Commun., 2014, 50, 1408.
13 D. Pijper, P. Saisaha, J. W. De Boer, R. Hoen, C. Smit,
A. Meetsma, R. Hage, R. P. van Summeren, P. L. Alsters,
B. L. Feringa and W. R. Browne, Dalton Trans., 2010, 39, 10375.
14 (a) P. Liu, Y. Liu, E. L.-M. Wong, S. Xiang and C.-M. Che,
Chem. Sci., 2011, 2, 2187; (b) S. Cheng, J. Li, X. Yu, C. Chen,
H. Ji, W. Ma and J. Zhao, New J. Chem., 2013, 37, 3267.
15 G. Olivo, O. Lanzalunga, L. Mandolini and S. Di Stefano,
J. Org. Chem., 2013, 78, 11508.
‡
The ligand, Fe(CF
3
SO
3
)
2
and the additive were one-shot added to the acetoni-
was added
trile substrate solution at the beginning of the reaction while H
2 2
O
over a period of 15 min using a syringe pump. The reaction mixture was then
left under stirring at 30 °C for additional 75 min.
1
2
(a) L. Que Jr. and W. B. Tolman, Nature, 2008, 455, 333; (b)
C.-L. Sun, B.-J. Li and Z.-J. Shi, Chem. Rev., 2011, 111, 1293;
(
c) M. C. White, Science, 2012, 335, 807.
(a) M. Costas, J.-U. Rohde, A. Stubna, R. Y. N. Ho,
L. Quaroni, E. Münck and L. Que Jr., J. Am. Chem. Soc., 2001,
1
2
23, 12931; (b) K. Chen and L. Que Jr, J. Am. Chem. Soc.,
001, 123, 6327; (c) J. Kaizer, E. J. Klinker, J.-U. Oh,
W. J. Song, A. Stubna, J. Kim, E. Munck, W. Nam and L. Que
Jr., J. Am. Chem. Soc., 2004, 126, 472; (d) M. Costas,
M. P. Mehn, M. P. Jensen and L. Que Jr., Chem. Rev., 2004,
1
1
04, 939; (e) X. Shan and L. Que Jr., J. Inorg. Biochem., 2006,
00, 421.
3
(a) M. S. Chen and M. C. White, Science, 2007, 318, 783; (b)
L. Gómez, I. Garcia-Bosch, A. Company, J. Benet-Buchholz,
A. Polo, X. Sala, X. Ribas and M. Costas, Angew. Chem., Int.
Ed., 2009, 48, 5720; (c) S. Chen and M. C. White, Science,
16 A. Company, L. Gómez, X. Fontrodona, X. Ribas and
M. Costas, Chem. – Eur. J., 2008, 14, 5727.
17 (a) J. England, G. J. P. Britovsek, N. Rabadia and A. J. P. White,
Inorg. Chem., 2007, 46, 3752; (b) A. Company, L. Gómez,
M. Güell, X. Ribas, J. M. Luis, L. Que Jr. and M. Costas,
J. Am. Chem. Soc., 2007, 129, 15766; (c) Y. Hitomi,
S. Furukawa, M. Higuchi, T. Shishido and T. Tanaka, J. Mol.
Catal. A: Chem., 2008, 288, 83; (d) A. Thibon, J.-F. Barioli,
G. Bourcier and F. Banse, Dalton Trans., 2009, 9587; (e)
Y. He, J. D. Gorden and C. R. Goldsmith, Inorg. Chem., 2011,
50, 12651; ( f ) Q. Zhang, J. D. Gorden and C. R. Goldsmith,
Inorg. Chem., 2013, 52, 13546.
2
010, 327, 566; (d) Y. Hitomi, K. Arakawa, T. Funabiki and
M. Kodera, Angew. Chem., Int. Ed., 2012, 51, 3448; (e)
P. E. Gorminsky and M. C. White, J. Am. Chem. Soc., 2013,
1
35, 14052; ( f ) M. Cantà, D. Font, L. Gòmez, X. Ribas and
M. Costas, Adv. Synth. Catal., 2014, 356, 818.
(a) M. Costas and L. Que Jr., Angew. Chem., Int. Ed., 2002,
4
4
1, 2179; (b) J. England, R. Gondhia, L. Bigorra-Lopez,
A. R. Petersen, A. J. P. White and G. J. P. Britovsek, Dalton
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