Hydroxylation of alkanes by molecular oxygen with dinuclear FeII macrocyclic complexes as catalysts

Article abstract of DOI:10.1039/a804258f

The iron(II) complexes of two new macrocyclic ligands: [24]RBPyBC (L24), a 24-membered macrocycle containing phenol, pyridine and amino donor groups, and [30]RBBPyBC (L30), a 30-membered macrocycle containing phenol, bipyridine, and amino donor groups, were found to be effective for the hydroxylation of alkanes, including cyclohexane and adamantane, with H₂S as a two-electron reductant.

Full text of DOI:10.1039/a804258f

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Hydroxylation of alkanes by molecular oxygen with dinuclear FeII macrocyclic
complexes as catalysts
Zheng Wang, Arthur E. Martell*† and R. J. Motekaitis
Department of Chemistry, Texas A&M University, College Station, Texas TX77843-3255, USA
OH
The iron(II) complexes of two new macrocyclic ligands:
OH
[24]RBPyBC (L24), a 24-membered macrocycle containing
Fe2L + H2S
phenol, pyridine and amino donor groups, and
+
+
+ O2
CH3CN + Py
[30]RBBPyBC (L30), a 30-membered macrocycle containing
phenol, bipyridine, and amino donor groups, were found to
be effective for the hydroxylation of alkanes, including
cyclohexane and adamantane, with H2S as a two-electron
reductant.
C3-OH
OH
C2-OH
O
Fe2L + H2S
CH3CN + Py
+ O2
The synthesis, characterization, and metal-binding properties of
two new macrocyclic ligands [24]RBPyBC (L24) and
[30]RBBPyBC (L30) have been reported,1–3 and the stabilities
CyOH
CyO
of their mixed-valence dinuclear iron(II
described. It has now been discovered that the dinuclear iron(II
,
III) chelates were
Scheme 1 Oxidation of adamantane and cyclohexane by molecular
dioxygen with diiron complexes as catalysts
)
complexes of these ligands catalyze the hydroxylation of the
hydrocarbons cyclohexane and adamantane in the presence of a
two-electron reductant (H2S) and thus may serve as functional
models of methane monooxygenase.
Methane monooxygenase is known to contain a binuclear
iron active center4 and has been mimicked by a number of
dinuclear model complexes,5 but in most cases an oxidant such
as H2O2 or ROOH was used. Descriptions of model systems
containing porphyrin ligands in which the oxidant was molec-
ular oxygen have been published.6 Of the model systems that
have molecular oxygen as an oxidant and do not contain
porphyrin ligands are the GIF systems of Barton et al.,7 an
O2/Zn/HOAc system of Christou et al.5 and of Kitajima et al.,8
and the present work.
The new dinuclear iron(II) complexes of the macrocyclic
ligands designated above, L24 and L30, are indicated below by
1 and 2, respectively. These dinuclear FeII complexes are
believed to react with oxygen to form a diiron–peroxide
intermediate. The active species may decay to a dinuclear FeIII
complex with an oxo bridge, releasing an oxygen atom for
insertion (hydroxylation) of a hydrocarbon. In the presence of a
two-electron reductant the dinuclear FeIII species may revert
back to the dinuclear FeII complex to complete the catalytic
cycle.
The overall oxidation reactions are shown in Scheme 1,
which shows the organic substrates and reaction products. The
results obtained for the hydroxylation of adamantane and
cyclohexane are summarized in Tables 1 and 2. Typical
oxidation reaction procedures were as follows: at room
temperature and atmospheric pressure, 0.10 mmol of free
macrocyclic ligand L24 or L30 was dissolved in 40 ml of
CH3CN. FeCl2·4H2O (0.20 mmol) was added to initiate iron
complex formation. After the mixture was stirred for 10 min, the
solution turned dark violet signifying the formation of the
dinuclear iron complexes Fe2L24, 1, or Fe2L30, 2. While
stirring was continued, 20 mmol cyclohexane or adamantane
was added, then 1.0 ml pyridine was added. Hydrogen sulfide (2
ml min21) and dioxygen (20 ml min21) were simultaneously
passed through the solution. After successive 2 h periods, the
reaction mixture was filtered from the deposited sulfur and the
solution was analyzed.
An aliquot (1.0 ml) was taken from the reaction solution and
added to an aqueous NaOH solution (5%, 2 ml) at 0 °C. The
products were extracted with diethyl ether (3 3 5 ml) and dried
over MgSO4. A naphthalene solution (1.0 ml, 0.080
M
in diethyl
ether) was added as an internal standard. The organic products
were quantitatively analyzed by gas chromatography.
CH3
CH3
NH
HN
O–
NH
NH
HN
Fe2+
HN
O–
O–
N
N
N
N
Fe2+
Fe2+
HN
N
Fe2+
N
O–
NH
CH3
CH3
1
2
Chem. Commun., 1998
1523

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Table 1 Oxidation of adamantane by molecular oxygen with diiron
tronic spectra of the reaction mixture before and after the
reactions show the same characteristic absorption bands that are
attributed to hydroxo dinuclear iron complexes. Thus, we
suggest that the active center is a diiron species and the reaction
mechanism probably resembles that of MMO.
complexes as catalysts and hydrogen sulfide as reductanta
C3-OH/ C2-OH/ C3-OH/
Catalyst
Fe2L24
Time/h
mmol
mmol
C2-OH
Turnover
H2S serves as both an electron and a proton donor, the
equivalent of NADH in MMO systems. The diiron(II) complex
can be regenerated from the oxidized diiron(III) complex with
H2S as a two electron reductant to produce a sulfur precipitate
which was analyzed quantitatively after the experiment.
In the control experiments for the oxidation of cyclohexane,
the turnover is 1.1 in 8 h with FeII as a catalyst and is 1.7 in 8
h with pyridine as a ligand. Significantly, the turnover is 28.4 in
8 h with the use of the macrocyclic ligand. Autoxidation is
clearly ruled out by comparison with these turnover numbers.
It is interesting that the oxidation of adamantane produces
only hydroxylation products. The ratio of tertiary adamantanol
to secondary adamantanol is around 1.2 for both catalysts. No
ketonization product is observed even when the turnover is 45.1
after 20 h. A similar result was obtained by Kitajima et al.,8b
who reported only a trace of ketonization product. This result
seems to imply a difference in the mechanisms of oxidation of
cyclohexane and adamantane.
2
4
6
0.099
0.867
1.13
1.27
1.51
0.29
0.37
1.16
1.28
1.21
1.24
1.12
1.01
1.26
1.26
1.36
1.24
1.18
3.7
16.2
20.1
23.2
27.3
45.1
7.2
0.749
0.882
1.05
1.22
2.13
0.355
0.400
0.492
0.574
0.642
0.719
8
10
20
2
4
6
8
10
12
2.38
Fe2L30
0.360
0.502
0.621
0.782
0.79
9.0
11.1
13.6
14.4
15.7
0.850
a Reaction conditions: FeCl2 (0.20 mmol), ligand (0.10 mmol), adamantane
(20 mmol), pyridine (1.0 ml), and CH3CN (40 ml) at 25 °C. Oxygen and
hydrogen sulfide were purged through the solution. Turnover is based on
mmols of the products per mmol of the catalyst used.
Table 2 Oxidation of cyclohexane by molecular oxygen with diiron
complexes as catalysts and hydrogen sulfide as reductanta
The turnover numbers show that the iron complex of the
24-membered macrocycle is a better catalyst for these oxidation
reactions than that of the 30-membered macrocycle.
This research was supported by The Robert A. Welch
Foundation under Grant No. A-0259. The authors thank Dr Li
Tingsheng for assistance with gas chromatography.
CyOH/
mmol
CyO/
mmol
CyOH/
CyO
Catalyst
Fe2L24
Time/h
Turnover
2
4
6
0.96
1.38
1.61
1.85
2.06
2.33
0.64
0.89
1.10
0.50
0.70
0.73
0.99
1.04
1.26
0.60
0.83
1.04
1.92
1.97
1.94
1.87
1.98
1.85
1.07
1.07
1.06
14.6
20.8
24.4
28.4
31.0
35.9
12.4
17.2
21.4
8
Notes and References
10
12
2
4
6
† E-mail: martell@chemvx.tamu.edu
Fe2L30
1 Z. Wang, J. H. Reibenspies and A. E. Martell, Inorg. Chem., 1997, 36,
629.
2 Z. Wang, Ph.D. Dissertation, Texas A&M University, 1997.
3 Z. Wang and A. E. Martell, Inorg. Chem., submitted.
a Reaction conditions: FeCl2 (0.20 mmol), ligand (0.10 mmol), cyclohexane
(20 mmol), pyridine (1.0 ml), and CH3CN (40 ml) at 25 °C. Oxygen and
hydrogen sulfide were purged through the solution. Turnover is based on
mmols of the products per mmol of the catalyst used.
4 M. P. Woodland, D. S. Patil, R. Cammack and H. Dalton, Biochem.
Biophys. Acta,1986, 873, 237; A. Ericson, B. Hedman, K. O. Hodgson, J.
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J. D. Lipscomb, J. Biol. Chem., 1989, 263, 10023.
The mmols of products = Pproduct/Pnaphthalene 3 40 ml 3
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0.080 . The turnovers = the sum of mmols of products/mmol
M
of catalyst. The turnover numbers show that these macrocyclic
iron complexes are several times more effective as hydroxylat-
ing catalysts for cyclohexane than are the µ-oxo dinuclear iron
complexes containing a tris(pyrazol-1-yl)borate ligand de-
scribed by Kitajima et al.8b The effectiveness in the catalytic
oxidation of adamantane is approximately at the same level as
Kitajima’s catalyst.8b Thus the dinuclear macrocyclic iron(II
)
complexes described in this paper are among the most effective
functional models of methane monooxygenase reported thus
far.
The oxidation of cyclohexane gives cyclohexanol and
cyclohexanone. The turnovers are 24.4 after 6 h for the
24-membered macrocyclic diiron complex as a catalyst, or 21.4
after 6 h with the 30-membered macrocyclic complex. The
ratios of cyclohexanol to cyclohexanone are 1.9 and 1.1 for 1
and 2, respectively, during the whole period of time of the
oxidation reactions. This result shows that the mechanisms of
oxidation are similar but slightly different for the two
catalysts.
8 (a) N. Kitajima, H. Fukui and Y. Moro-oka, J. Chem. Soc., Chem.
Commun., 1988, 485; (b) N. Kitajima, M. Ito, H. Fukui and Y. Moro-oka,
J. Chem. Soc., Chem. Commun., 1991, 102.
The recent stability studies of dinuclear and mononuclear FeII
and FeIII complexes of the ligand indicate the dinuclearity of the
complexes under reaction conditions. Accordingly, the elec-
Received in Cambridge, UK, 6th February 1998; revised manuscript
received 29th May 1998; 8/04258F
1524
Chem. Commun., 1998