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S. Haslinger et al. / Journal of Catalysis 331 (2015) 147–153
both a large turnover number and a high A/K ratio still poses a
challenge in the field of iron-catalyzed oxidation of cyclohexane.
Recently, the Fe(II) complex 1 bearing a tetradentate bis
(pyridyl-N-heterocyclic carbene) ligand (NCCN) in the equatorial
plane (Fig. 1) has been reported by our group as a highly active
catalyst for olefin epoxidation and aromatic hydroxylation,
including investigations on electronic fine-tuning [36–39].
The intense yellow compound crystallizes in the monoclinic crystal
system in space group P21/c (No. 14) with the cell parameters
a = 19.4664(8) Å, b = 10.7818(4) Å, c = 15.7930(6) Å, b = 108.113(2)°.
2.3. Synthesis of [Fe(NCCN)(MeCN)(CNtBu)](PF6)2 (3)
Complex 1 (1.37 mmol, 1.00 g) was dissolved in 70 mL acetoni-
In this work, compound 1 is applied as catalyst to the oxidation of
unreactiveCAH bonds (e.g., cyclohexane). Bio-inspired modification
of the axial ligands in analogy to cyt-P450 is a potentially powerful
tool for influencing the catalyst performance, as seen in the crucial
role of the apical thiolate in cyt-P450 for the OAO bond splitting of
dioxygen [1,40–43]. Encouraged by this, two irreversibly axially
monosubstituted derivatives of 1 are introduced as active catalysts
for CAH bond oxidation in this article. The influence of the substitu-
tion on the catalyst performance is investigated with a main focus on
catalyst stability and product selectivity. Crucial parameters such as
the amount of oxidant, the relative catalyst concentration, and the
reaction temperature are varied in order to increase the turnovers
while studying the selectivity of product formation.
trile. tert-Butyl isocyanide (2.05 mmol, 232 lL) was added under
vigorous stirring and the resulting mixture was stirred at room
temperature for 30 min. Diethyl ether (600 mL) was added, giving
a yellow suspension. Filtration yielded a yellow powder, which was
washed three times with diethyl ether and dried under high
vacuum (0.85 g of 3, 80% yield). 1H NMR (400.13 MHz, MeCN-d3):
d 9.26 (d, J = 5.3 Hz, 2H, o-Hpy), 8.31 (t, J = 7.4 Hz, 2H, Hpy), 8.21
(d, J = 2.1 Hz, 2H, HNHC), 8.00 (d, J = 8.2 Hz, 2H, Hpy), 7.76 (d,
J = 2.1 Hz, 2H, HNHC), 7.69 (t, J = 6.3 Hz, 2H, Hpy), 6.77 (dd, J = 12.4,
45.5 Hz, 2H, CH2), 0.87 (s, 9H, C(CH3)3). 13C{1H} NMR
(125.83 MHz, MeCN-d3): d 209.1 (NCNHCN), 154.3, 153.4 (o-Cpy),
142.2 (Cpy), 131.1, 125.7 (CNHC), 124.3 (Cpy), 119.7 (CNHC), 113.3
(Cpy), 64.7 (NCH2N), 59.5 (C(CH3)3), 30.2 (C(CH3)3). IR: 2139 cmÀ1
(C„NtBu). MS-ESI (m/z): [3 – PF6]+ calcd., 627.13; found, 626.38;
[3 – MeCN – PF6]+ calcd., 586.10; found, 585.48. Anal. calcd. for
2. Experimental section
C
24H26F12FeN8P2: C, 37.33; H, 3.39; N, 14.51. Found: C, 36.95; H,
2.1. General remarks
3.13; N, 14.21.
Caution: Hydrogen peroxide as well as organic peroxides are
potentially explosive if highly concentrated and exposed to heat
or mechanical impact. All chemicals were purchased from
commercial suppliers and used without further purification with
the exception of the iron source FeBr2, which was purified by
extraction with THF under standard Schlenk conditions to give
[FeBr2(THF)2]. Complexes 1 and 2 were synthesized according to
the literature [38,39,44]. Liquid NMR spectra were recorded on a
Bruker Avance DPX 400 and a Bruker Ultrashield 500 Plus with
cryo unit. Chemical shifts are given in parts per million (ppm)
and the spectra were referenced by using the residual solvent
shifts as internal standards (MeCN-d3, 1H NMR d 1.94, 13C NMR d
1.32). A Thermo Scientific LCQ/Fleet spectrometer by Thermo
Fisher Scientific was used to collect MS-ESI data and elemental
analyses were obtained from the microanalytical laboratory of
TUM. IR spectra were acquired on a Bruker Vertex-70 FT-IR
spectrometer with a Platinum ATR unit at room temperature using
a solid sample of bulk material. GC-FID measurements were
2.4. Experimental procedure for the catalytic oxidation of cyclohexane
For the catalytic oxidation of cyclohexane under standard
conditions (0.50 mol% relative catalyst concentration), 1.00 mL of
a 2.80 mM stock solution of 1, 2, or 3 in acetonitrile was added
to a mixture of 61.5
acetonitrile under air. The catalytic reaction was started by the
addition of 272 L of an acetonitrile solution containing the
lL (569 lmol) of cyclohexane and 2.00 mL
l
respective amount of hydrogen peroxide (50% aqueous solution)
or the respective organic peroxide used as oxidant. For other cata-
lyst concentrations, the amount taken from the stock solution and
the amount of acetonitrile used for dilution were adjusted accord-
ingly, giving the same reaction volume for each reaction. For each
data point, after the respective reaction time an aliquot of 1.00 mL
from the reaction solution was taken and added to 1.00 mL of a sat-
urated solution of triphenylphosphine in acetonitrile. The resulting
mixture was filtered through a short plug of silica. For GC analysis,
performed on
a Varian CP-3800 equipped with an Optima
two individual samples were prepared by combining 400
lL of the
5-Amin capillary column by Macherey-Nagel (1.50 lm;
30 m  0.32 mm), using p-xylene as external standard.
filtered solution with 400 L of the solution containing the exter-
l
nal standard (p-xylene in acetonitrile). Double injections before
and after the reduction with triphenylphosphine were performed
for selected data points to identify cyclohexyl hydroperoxide, as
it has been introduced originally by Shul’pin et al. [18,45]. In case
of substrates other than cyclohexane, 1H NMR was used for
quantification of the respective products and the catalytic reaction
was carried out in acetonitrile-d3. Nitromethane was added as
external standard after the reaction was finished (d 4.30, 3H,
CH3). The following signals were used for the quantification of
the respective substrates: 9,10-dihydroanthracene d 3.90 (4H,
2 Â CH2), xanthene d 4.06 (2H, CH2), triphenylmethane d 5.61
(1H, Ph3CH), 2,3-dimethyl-2-butanol d 1.07 (1H, C(CH3)2OH).
2.2. Single crystal X-ray diffraction
Single crystals of 3 suitable for X-ray diffraction were obtained
by slow diffusion of diethyl ether into an acetonitrile solution of 3.
2+
NCMe
N
N
N
N
N
-
Fe
2 PF6
N
3. Results and discussion
NCMe
3.1. Preparation of the catalysts
1
The syntheses of the iron(II) complexes 1 and 2 have been
reported previously by our group [38,39,44]. Here, the mono(iso-
cyanide) derivative 3, obtained by addition of tert-butyl isocyanide
Fig. 1. Fe(II) catalyst 1 bearing an bis(pyridyl-N-heterocyclic carbene) ligand in its
equatorial plane and two axial acetonitrile ligands [38].