Inorganic Chemistry
Article
mode, acetonitrile): m/z = 326.21 (10%. [M + Na]+, 304.20 (100%,
[M + H]+).
coordination sphere. However, in the absence of crystal
structure of complex 1a or E, the exact mode of interaction
between the urea group and the substrate/product cannot be
unambiguously predicted.
[(tBu-LMe)FeIII(4,6-di-tBu-AP)](ClO4) (1a). To a solution of tBu-LMe
ligand (0.16 g, 0.5 mmol) in methanol was added Fe(ClO4)3·6H2O
(0.23 g, 0.5 mmol). To the resulting yellow solution was added a
methanolic solution (5 mL) of 2-amino-4,6-di-tert-butylphenol (0.11 g,
0.5 mmol) and triethylamine (130 μL). The resulting dark violet
solution was stirred at room temperature under nitrogen atmosphere
for 2 h. A violet precipitate was formed, which was filtered and washed
with diethyl ether to isolate a violet crystalline solid. Yield: 0.25 g
(73%). Elemental Anal. Calcd (%) for C32H45ClFeN5O6 (687.03 g/
mol): C, 55.94; H, 6.60; N, 10.19. Found: C, 55.86; H, 6.43; N, 10.17.
IR (KBr): 3367(br), 3093, 3068, 2958(vs), 2908, 2867, 1641, 1593,
1552(vs), 1456(vs), 1417, 1390, 1361, 1234, 1176, 1145, 1105(s),
1043, 999, 902, 831, 804, 765, 748, 659, 628 cm−1. UV−vis in MeCN:
λ, nm (ε, M−1cm−1): 548 (1100), 880 (1400). Magnetic moment (298
K): 5.98 μB.
CONCLUSION
■
In conclusion, we have isolated and characterized two iron(II)-
chloro complexes and two iron(III)-2-amidophenolate com-
plexes of facial tridentate ligands. All the iron complexes are
found to exhibit regiospecific cleavage of the C−C bond
adjacent to the phenolic OH group of 2-amino-4,6-di-tert-
butylphenol to form 4,6-di-tert-butyl-2-picolinic acid. The iron
complexes of the ligand containing a urea group exhibit
catalytic aromatic ring cleavage of 2-aminophenol in acetoni-
trile-acetate buffer mixture (1:1) at pH = 5.5. On the other
hand, the iron complexes of the ligand without a urea group
studied in this work do not show catalytic aromatic ring
cleavage reactivity. The results reported here support the role of
urea group on catalytic turnovers. The strategy of using a ligand
with hydrogen bond donor group has been successfully
employed in developing a nonheme iron catalyst for the
conversion of 2-aminophenol to 2-picolinic acid using O2 as the
oxidant. Detailed structural and mechanistic studies are being
pursued to unravel the role of urea group in the catalytic
pathway.
[(BA-LMe)2FeII Cl4] (2). To a solution of BA-LMe ligand (0.15 g, 0.5
2
mmol) in dichloromethane was added FeCl2 (0.064 g, 0.5 mmol). The
resulting yellow solution was stirred for 1 h to precipitate a yellow
solid. The precipitate was filtered and then washed with dichloro-
methane to isolate a yellow powder. X-ray quality yellow single crystals
were isolated by diffusion of diethyl ether into a mixture of
dichloromethane-methanol solution of the complex. Yield: 0.38 g
(88%). Elemental Anal. Calcd (%) for C40H42Cl4Fe2N6 (860.30 g/
mol): C, 55.84; H, 4.92; N, 9.77. Found: C, 55.74; H, 4.83; N, 9.94. IR
(KBr): 3396(br), 3060, 2923, 2088, 1639, 1604(s), 1571(m),
1460(vs), 1380, 1159, 765, 703 cm−1. Magnetic moment (298 K):
6.90 μB.
[(BA-LMe)FeIII(4,6-di-tBu-AP)](ClO4) (2a). To a solution of BA-LMe
(0.15 g, 0.5 mmol) in methanol was added Fe(ClO4)3·6H2O (0.22 g,
0.5 mmol). The resulting yellow solution was then treated with a
methanolic solution (5 mL) of 2-amino-4,6-di-tert-butylphenol (0.11 g,
0.5 mmol) and triethylamine (130 μL). The resulting dark violet
solution was stirred at room temperature under nitrogen atmosphere
for 2 h. The solvent was removed under vacuum, and the residue was
washed with dichloromethane. The filtrate was concentrated and kept
for layer diffusion with hexane to isolate a crystalline violet complex.
Yield: 0.24 g (70%). Elemental Anal. Calcd (%) for C34H42ClFeN4O5
(678.02 g/mol): C, 60.23; H, 6.24; N, 8.26. Found: C, 60.38; H, 6.02;
N, 8.12. IR (KBr): 3450(br), 3236, 3137, 2929, 1604(s), 1569(m),
1461(s), 1382, 1244, 1143, 1115, 1090 (vs), 1031, 1006, 757, 630
cm−1. UV−vis in MeCN: λ, nm (ε, M−1cm−1): 618 (1040), 1002
(780). Magnetic moment (298 K): 5.96 μB.
EXPERIMENTAL SECTION
■
All chemicals and reagents were obtained from commercial sources
and were used without further purification unless otherwise noted.
Solvents were distilled and dried before use. Preparation and handling
of air-sensitive materials were carried out under an inert atmosphere in
a glovebox. The ligand tBu-LMe and the complex [(tBu-LMe)-
FeIICl2(MeOH)] (1) were prepared according to a procedure
reported in the literature.36
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. Electro-spray 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.
Room temperature magnetic data were collected on Gouy balance
(Sherwood Scientific, Cambridge, U.K.). Diamagnetic contributions
were calculated for each compound using Pascal’s constants. X-band
EPR measurements were performed on a JEOL JES-FA 200
instrument. GC-MS measurements were carried out with a
PerkinElmer Clarus 600 using Elite 5 MS (30 m × 0.25 mm × 0.25
μm) column with a maximum temperature 300 °C. Labeling
experiments were carried out with 18O2 gas (99 atom %) or H2O18
(98 atom %) purchased from Icon Services Inc., U.S.A.
[(tBu-LMe)FeIII(picolinate)](Cl2) (1b). To a solution of tBu-LMe ligand
(0.16 g, 0.5 mmol) in methanol was added FeCl3 (0.08 g, 0.5 mmol).
To the resulting yellow solution was added a methanolic solution (5
mL) of sodium picolinate (0.07 g, 0.5 mmol). The solution slowly
turned orange, and the solution was then stirred at room temperature
under nitrogen atmosphere for 2 h. The solvent was removed under
vacuum and the residue was washed with dichloromethane and
filtered. The filtrate was concentrated and treated with diethyl ether to
isolate an orange crystalline compound, 1b·CH3OH. Yield: 0.24 g
(82%). Elemental Anal. Calcd (%) for C25H32Cl2FeN5O4 (593.3 g/
mol): C, 50.61; H, 5.44; N, 11.80. Found: C, 50.36; H, 5.17; N, 11.59.
IR (KBr): 3360(br), 2974(s), 2972, 2898, 1645(vs), 1595, 1568, 1552,
1454, 1392, 1365, 1269, 1215, 1170, 1089, 1047(vs), 831, 879, 763,
703, 405, 383 cm−1. Magnetic moment (298 K): 5.97 μB.
Synthesis. Ligand BA-LMe. To a solution of bis(6-methylpyridin-2-
yl)methanamine (0.84 g, 4 mmol) in dry methanol (10 mL) was
added benzaldehyde (408 μL, 4 mmol) and stirred for 24 h. To the
resulting solution, sodium cyanoborohydride (0.28 g, 4.5 mmol) was
added and stirred overnight. The yellow solution thus formed was
treated with concentrated hydrochloric acid until the effervescence
ceased. The solvent was then removed and the residue was treated
with a saturated solution of sodium bicarbonate. The organic product
was extracted with dichloromethane. The organic layer was then dried
over anhydrous sodium sulfate, and the solvent was removed to isolate
a pale yellow oil. Yield: 0.90 g (75%). 1H NMR (CDCl3, 500 MHz): δ
(ppm) 7.52 (t, 2H, J = 7.8 Hz), 7.37 (d, 2H, J = 7 Hz), 7.33 (t, 2H, J =
7.5 Hz), 7.22 (d, 1H, J = 7 Hz), 7.19 (d, 2H, J = 8 Hz), 7.01 4(d, 2H, J
= 7 Hz), 5.13 (s, 1H), 3.83 (s, 2H) 2.55 (s, 6H). ESI-MS (positive ion
[(BA-LMe)FeIII(picolinate)](Cl2) (2b). To a solution of BA-LMe (0.15
g, 0.5 mmol) in methanol was added FeCl3 (0.08 g, 0.5 mmol). The
yellow solution was then treated with a methanolic solution (5 mL) of
sodium picolinate (0.07 g, 0.5 mmol). The resulting orange solution
was stirred at room temperature under nitrogen atmosphere for 2 h.
The solvent was removed in vacuum, and the residue was washed with
dichloromethane and filtered. The filtrate was concentrated and
treated with diethyl ether to isolate an orange solid, 2b·CH3OH. Yield:
0.22 g (81%). Elemental Anal. Calcd (%) for C27H29Cl2FeN4O3 (584.3
g/mol): C, 55.50; H, 5.00; N, 9.59. Found: C, 54.90; H, 4.84; N, 9.69.
IR (KBr): 3406(br), 3286, 3256, 3064, 3033, 2923, 2854,1627(vs),
1593(vs), 1568(s), 1460(s), 1377, 1290, 1261, 1240, 1161, 1093 1031,
F
Inorg. Chem. XXXX, XXX, XXX−XXX