Inorganic Chemistry
Article
collected by filtration. X-ray quality single-crystals were obtained by
slow evaporation of solvent from a solution of the complex in
dichloromethane and hexane mixture. Yield: 0.386 g (75%). Anal.
Calcd for C26H22ClFeN3O3 (515.77 g/mol): C, 60.55; H, 4.30; N,
8.15. Found: C, 60.43; H, 4.09; N, 8.17%. IR (cm−1): 3433(br), 3068
(w), 2924(w), 2854(w), 1664(vs), 1630(m), 1601(s), 1493(w),
1443(m), 1236(m), 762(m), 692(m). UV−vis in CH3CN λmax, nm
cyclohexane by 2 in the presence of varying amounts of alkyl
radical trap CCl3Br (1−5 equiv) (Table 4). With 5 equiv of
CCl3Br, the yield of chlorocyclohexane remained the same
(6%), whereas the yields of cyclohexanol and cyclohexanone
decreased to 8−9% along with 12% bromocyclohexane. The
cage escape alkyl radical reacted with O2 and with CCl3Br
resulting in oxygenated or brominated products, respectively.
That observation supported a low alcohol-to-ketone ratio in
the oxidation of cyclohexane. In the absence of any substrate,
the putative iron-oxo oxidant decayed to some unidentified
iron(III) product as observed in the X-band EPR spectrum
1
(ε, M−1 cm−1): 386 (1315), 580 (240), 645 (240). H NMR (500
MHz, CDCl3): δ 52.0, 47.1, 29.4, 24.7, 17.2, 14.4, 11.8, 9.2, 7.9, 5.7,
5.5, 3.9, 3.2, 2.9, 2.0, −0.7, −13.6 ppm.
II
[(1,4-tpbd)Fe2 (BF)2Cl2] (2). Anhydrous FeCl2 (0.254 g, 2.0 mmol)
was added to a solution of 1,4-tpbd ligand (0.473 g, 1.0 mmol) in 5
mL of dichloromethane. The mixture was vigorously stirred for 12 h
in a glovebox. A methanolic solution (1 mL) of sodium
benzoylformate (0.344 g, 2.0 mmol) was then added dropwise to
the reaction mixture. The resulting green solution was allowed to stir
for 12 h and then dried. The residue obtained was dissolved in
dichloromethane (10 mL) and filtered. The filtrate was then
concentrated, and the green solid was collected by filtration. X-ray
quality single-crystals were obtained by slow evaporation of solvent
from the solution of the complex in a mixture of dichloromethane and
hexane. Yield: 0.638 g (67%). Anal. Calcd for C46H38Cl2Fe2N6O6
(953.43 g/mol): C, 57.95; H, 4.02; N, 8.81. Found: C, 57.31; H, 3.85;
N, 8.70%. IR (cm−1): 3433(br), 3063 (w), 2924(w), 1661(s),
1628(m), 1603(m), 1514(w), 1479(w), 1438(w), 1315(w), 1238(s),
774(m), 690(w). UV−vis in CH3CN λmax, nm (ε, M−1 cm−1): 390
(2490), 592 (700), 650 (710). 1H NMR (500 MHz, CDCl3): δ 52.9,
49.0, 36.1, 24.3, 17.3, 9.9, 9.3, 6.5, 5.7, 5.4, 4.1, 3.9, 3.8, 3.7, 3.1, 2.8,
1.9, −7.9 ppm.
CONCLUSIONS
■
We have developed two nonheme iron(II)-α-keto acid
complexes that activated the C−H bonds of aliphatic
substrates with O2 to form the corresponding halogenated
product. The dinuclear complex exhibited enhanced reactivity
compared to the mononuclear complex. Unlike enzyme
systems, the formation of hydroxylated/oxidized products
could not be avoided and the halogenated products were
detected in low yields. Yet the complexes presented here
represent the only examples of nonheme iron(II)-α-keto acid
complexes exhibiting bioinspired C−H bond halogenation.
The strong iron-chloro bond and slow rebound of chloride
radical to substrate-derived alkyl radical resulted in low yield
and selectivity in halogenation reactions. Design of supporting
ligands incorporating electronic/structural features for enhanc-
ing the rate of chloride rebound step and deactivating the
hydroxide rebound step might allow achievement of selective
halogenation of aliphatic C−H bonds with nonheme iron
complexes and dioxygen.
Reactivity with Dioxygen. A solution of the complex (1 or 2)
(0.02 mmol) in 10 mL of dioxygen-saturated CH3CN was allowed to
stir at room temperature. After the reaction, the solvent was removed
under reduced pressure, and the residue was treated with 10 mL of 3
M HCl. The organic products were extracted with diethyl ether (3 ×
20 mL), and the organic phases were washed with brine solution. The
combined organic part was dried over Na2SO4 and filtered. The
solvent was then evaporated to dryness. The organic products were
1
EXPERIMENTAL SECTION
analyzed by H NMR spectroscopy or by GC−MS.
■
Reactions of Iron(II) Complexes with Thioanisole. Each of the
iron(II)-BF-Cl complexes (0.02 mmol) was dissolved in 10 mL of dry
acetonitrile. To the resulting solution was added thioanisole (20
equiv). Pure dry dioxygen was purged through the solution for 5 min
and was allowed to stir at room temperature under oxygen
atmosphere. The color of the solution slowly changed from green
to yellowish-green. After the reaction, the solution was concentrated
under reduced pressure and treated with 10 mL of 3 M HCl. The
organic products were extracted with diethyl ether (3 × 20 mL) and
dried over anhydrous sodium sulfate. The organic layer was filtered
Materials and Methods. All chemicals and reagents were
purchased from commercial sources and were used without further
purification unless otherwise mentioned. Solvents were distilled and
dried before use. Preparation and handling of air-sensitive materials
were carried out under inert atmosphere in a glovebox. The ligands,
N,N-bis(2-pyridylmethyl)aniline (phdpa) and N,N,N′,N′-tetrakis(2-
pyridylmethyl)benzene-1,4-diamine (1,4-tpbd), were prepared follow-
ing reported procedures.24
Fourier transform infrared spectroscopy on KBr pellets was
performed on a Shimadzu FT-IR 8400S instrument. Elemental
analyses were performed on a PerkinElmer 2400 series II CHN
analyzer. Electrospray ionization (ESI) mass spectra were recorded
with a Waters QTOF Micro YA263 instrument. Solution electronic
spectra (single and time-dependent) were measured on an Agilent
8453 diode array spectrophotometer. All room temperature NMR
spectra were collected on a Bruker Avance 500 MHz spectrometer. X-
band EPR spectra were recorded on a JEOL JES-FA 200 instrument
with 100 kHz magnetic modulation, a microwave power of 2.00 mW,
and a microwave frequency of 9.1195 GHz. GC−MS measurements
were carried out with a PerkinElmer Clarus 600 using an Elite 5 MS
(30 m × 0.25 mm × 0.25 μm) column with a maximum temperature
of 300 °C. Labeling experiments were carried out with 18O2 gas (99
atom %) or H218O (98 atom %) purchased from Icon Services Inc.
Synthesis of Metal Complexes. [(phdpa)FeII(BF)(Cl)] (1).
Anhydrous FeCl2 (0.127 g, 1.0 mmol) was added to a solution of
phdpa ligand (0.275 g, 1.0 mmol) in 5 mL of dichloromethane, and
the mixture was vigorously stirred for 12 h in a glovebox. To the
mixture was added a methanolic solution (1 mL) of sodium
benzoylformate (0.172 g, 1.0 mmol) dropwise. The resulting green
solution was allowed to stir for 12 h and then dried. The residue
obtained was dissolved in dichloromethane (10 mL) and filtered. The
filtrate was then concentrated, and the resulting green solid was
1
and evaporated to dryness. The products were analyzed by H NMR
and GC−MS. Quantification of the oxidized organic substrates were
performed by comparing the peak area associated with two ortho-
protons of benzoic acid (δ 8.11 ppm) with the peak area for the
protons of the oxidized substrate.
1H NMR (500 MHz, CDCl3, 298 K) data of organic products:
benzoic acid δ (ppm) 8.11 (d. 2H), 7.63 (t, 1H), 7.48 (t, 2H);
benzoylformic acid δ (ppm) 8.31 (d, 2H), 7.70 (t, 1H), 7.57 (t, 2H);
thioanisole oxide: δ (ppm) 7.66 (m, 2H), 7.52 (m, 3H), 2.73 (s, 3H).
Reactions of Iron(II)-BF-Cl Complexes with Alkanes. The
iron(II)-BF-Cl complex (0.02 mmol) was dissolved in 2 mL of dry
acetonitrile. To the resulting solution was added an excess amount of
substrate. Pure dry oxygen was purged through the solution and was
allowed to stir at room temperature. After the oxidation, the iron
complex was decomposed by addition of 1 mL of 1 M H2SO4
solution. Organic products were extracted by diethyl ether, and the
organic layer was dried over anhydrous sodium sulfate. The organic
products were then analyzed by GC−MS. The oxidized products were
quantified by comparison of their GC retention times and GC−MS
with those of authentic compounds. The halogenated products were
quantified by comparison of the relative peak area of the halogenated
products with that of naphthalene.
G
Inorg. Chem. XXXX, XXX, XXX−XXX