Inorganic Chemistry
Article
1-(4-Nitrophenyl)-3-(pyridin-2-yl)-1H-imidazol-3-ium Bromide
(15). Into a flame-dried pressure vial were added 1-(4-nitrophenyl)-
1H-imidazole (11) (250 mg, 1.32 mmol) and 2-bromopyridine (0.25
mL, 2.64 mmol). The neat mixture was stirred at 175 °C, and the
reaction was monitored using 1H NMR. After 1 day the reaction
mixture was cooled to room temperature to form a solid. The filtered
solid was dissolved in 1% methanol:dichloromethane and reprecipi-
tated with diethyl ether to give a brown microcrystalline solid (96%).
1H NMR (500 MHz, CDCl3): δ 12.73 (s, 1H), 7.35 (d, J = 3.6, 1H),
p-PhCN (4), and Phbis(m-CF3) (5), to study the effects of
these electron-withdrawing groups on electrocatalytic over-
potentials and rates (Figure 2). Catalysts 3 and 4 were selected
8.60−8.48 (m, 6H), 8.17 (t, J = 7.6, 1H), 7.79 (s, 1H), 7.55 (dd, J =
4.7, 3.1 Hz, 1H) ppm. 13C NMR (125 MHz, DMSO): δ 149.9, 148.4,
146.6, 141.2, 139.6, 135.8, 126.2, 126.0, 124.0, 122.7, 120.7, 115.3
ppm. IR (neat, cm−1): 3080, 3015, 1555, 1525, 1290, 854, 780, 745.
HRMS (ESI) m/z calculated for C14H11N4O2+ ([M − Br]+) 267.0882,
found 267.0872.
1-(4-Cyanophenyl)-3-(pyridin-2-yl)-1H-imidazol-3-ium Bromide
(16). Into a flame-dried pressure vial were added 12 (250 mg, 1.32
mmol) and 2-bromopyridine (0.30 mL, 2.96 mmol). The neat mixture
was stirred at 175 °C, and the reaction was monitored using 1H NMR.
After 1 day the reaction mixture was cooled to room temperature to
form a solid. The filtered solid was dissolved in dichloromethane and
reprecipitated with diethyl ether to give a brown microcrystalline solid
1
(83%). H NMR (300 MHz, DMSO-d6): δ 10.76 (s, 1H), 8.80 (s,
Figure 2. Structures of benchmark Re(bpy)(CO)3Br (1) and target
Re(pyNHC-aryl) catalysts (2−5) with variable electron-withdrawing
groups on the pyNHC-aryl ligand (2−5).
1H), 8.73 (d, J = 4.6 Hz, 1H), 8.68 (s, 1H), 8.33−8.18 (m, 6H), 7.72
(dd, J = 5.0, 2.1, 1H) ppm. 13C NMR (125 MHz, DMSO): δ 149.8,
146.6, 141.2, 138.3, 135.5, 134.8, 126.17, 123.6, 122.5, 120.7, 118.3,
115.4, 113.1 ppm. IR (neat, cm−1): 3045, 3010, 2224, 1602, 1545,
1469, 1436, 1262, 841, 782. HRMS (ESI) m/z calculated for
to evaluate the effects of resonance active electron-withdrawing
groups, while catalysts 2 and 5 incorporate strong inductively
withdrawing substituents.
+
C15H11N4 ([M − Br]+) 247.0984, found 247.0943.
1-(3,5-Bis(trifluoromethyl)phenyl)-3-(pyridin-2-yl)-1H-imidazol-3-
ium Bromide (17). Into a flame-dried flask connected to the reflux
condenser were added 13 (200 mg, 0.71 mmol) and 2-bromopyridine
(0.14 mL, 1.43 mmol), DMF (∼1 mL), then it was stirred to reflux,
EXPERIMENTAL SECTION
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1
and the reaction was monitored using H NMR. After 2 days, the
General Information. All commercially obtained reagents were
used as received. Unless otherwise noted, all of the reactions were
conducted under a N2 atmosphere. Thin-layer chromatography (TLC)
was conducted with sigma T-6145 precoated TLC silica gel 60 F254
polyester sheets and visualized with UV and potassium permanganate
staining. Flash column chromatography was performed with
reaction mixture was cooled to rt to form a solid. The solid then was
dissolved in dichloromethane and reprecipitated with ether to give a
1
brown microcrystalline solid (81%). H NMR (500 MHz, CDCl3): δ
12.55 (s, 1H), 9.21 (d, J = 8.3 Hz, 1H), 8.72 (s, 2H), 8.63 (t, J = 1.6
Hz, 1H), 8.57 (dd, J = 3.7 Hz, 1H), 8.15−8.12 (m, 1H), 8.10 (s, 1H),
7.92 (s, 1H), 7.54 (dd, J = 4.5, 2.7 Hz, 1H) ppm. 13C NMR (125 MHz,
CDCl3): δ 149.9, 146.6, 141.3, 136.6, 136.1, 132.4, 132.1, 126.3, 124.6,
124.3, 123.2, 122.1, 120.4, 115.2 ppm. IR (neat, cm−1): 3151, 3010,
2942, 2827, 2249, 1595, 1364, 1279, 1126. HRMS (ESI) m/z
1
SilicaFlash P60, 40−63 μm (230−400 mesh). H NMR spectra were
recorded on a Bruker Avance-300 (300 MHz), Bruker Avance DRX-
500 (500 MHz) spectrometer, and reported in ppm using solvent as
an internal standard (CDCl3 at 7.26 ppm). Data were reported as s =
singlet, d = doublet, t = triplet, q = quartet, p = pentet, m = multiplet,
b = broad, ap = apparent; coupling constants were in hertz.
Compounds 6−11 are commercially available. 4-(1H-Imidazol-1-
yl)benzonitrile (12) is available via a previously reported procedure.26
1-(3,5-Bis(trifluoromethyl)phenyl)-1H-imidazole (13) is available via a
previously reported procedure.27 Syntheses of 3-(pyridin-2-yl)-1-(4-
(trifluoromethyl)phenyl)-1H-imidazol-3-ium bromide (14) and fac-{3-
(4-trifluoromethylphenyl)-1-(2′-pyridyl)imidazolin-2-ylidene}
tricarbonylbromorhenium(I) (2) are available through previously
reported procedures; however, an improved synthetic protocol for 14
is given below.11 Additional experimental information on cyclic
voltammetry experiments, controlled potential electrolysis experi-
ments, additional electrochemical data, and NMR data is available via
NHE (Fc/Fc+ = 0.4 V vs SCE; SCE − 0.24 V = NHE).36
+
calculated for C16H10F6N3 ([M − Br]+) 358.0779, found 358.0727.
fac-{3-(4-Nitrophenyl)-1-(2′-pyridyl)imidazolin-2-ylidene}
Tricarbonylbromorhenium(I) (3). To a flame-dried flask equipped
with a reflux condenser were added 15 (0.1 g, 0.29 mmol), Re(CO)5Br
(0.12 g, 0.29 mmol), triethylamine (0.4 mL, 2.88 mmol), and
anhydrous, degassed toluene (5 mL). The mixture was refluxed under
a N2 atmosphere overnight. The reaction mixture then was cooled to
room temperature, and toluene was removed under reduced pressure.
The crude product was purified through silica gel column by using 2%
acetone:ethyl acetate to give a yellow crystalline solid of 3 (40%). The
isolated compound was found to immediately change color upon
standing to a green-brown color. 1H NMR (300 MHz, CDCl3): δ 8.96
(d, J = 4.6 Hz, 1H), 8.48 (d, J = 8.8 Hz, 2H), 8.13−8.07 (m, 1H), 7.97
(d, J = 8.9 Hz, 2H), 7.82 (d, J = 1.9 Hz, 1H), 7.70−7.68 (m, 1H),
7.42−7.36 (m, 2H) ppm. IR (neat, cm−1): 2956, 2925, 2856, 2362,
2336, 2018, 1920, 1892, 1612, 1526, 1487, 1346. HRMS (ESI) m/z
calculated for C17H10BrN4O5ReCs ([M + Cs]+) 748.8429, found
748.8687.
3-(Pyridin-2-yl)-1-(4-(trifluoromethyl)phenyl)-1H-imidazol-3-ium
Bromide (14). Into a flame-dried flask connected to a reflux condenser
were added 1-(4-(trifluoromethyl)phenyl)-1H-imidazole (10) (200
mg, 0.94 mmol), 2-bromopyridine (0.18 mL, 1.88 mmol), and
anhydrous DMF (1 mL). The mixture then was degassed with N2 for
30 min and stirred with reflux under a N2 atmosphere. After 2 days, the
reaction mixture was cooled to room temperature to form a solid. The
filtered solid was dissolved in dichloromethane and reprecipitated with
fac-{3-(4-Cyanophenyl)-1-(2′-pyridyl)imidazolin-2-ylidene}
Tricarbonylbromorhenium(I) (4). To a flame-dried flask equipped
with a reflux condenser were added 16 (0.04 g, 0.123 mmol),
Re(CO)5Br (0.05 g, 0.123 mmol), triethylamine (0.2 mL, 1.23 mmol),
and anhydrous, degassed toluene (2 mL). The mixture was refluxed
under a N2 atmosphere overnight. The reaction mixture then was
cooled to room temperature, and toluene was removed under reduced
pressure. The crude product was purified through a silica gel plug by
using ethyl acetate → 2% acetone:ethyl acetate to give a pale yellow
1
diethyl ether to give a brown microcrystalline solid (99%). H NMR
(300 MHz, CDCl3): δ 12.47 (s, 1H), 9.33 (d, J = 7.9 Hz, 1H), 8.58−
8.38 (m, 2H), 8.37 (d, J = 8.0, 2H), 8.17−8.13 (m, 1H), 7.95 (d, J =
8.0 Hz, 2H), 7.79 (s, 1H), 7.55−7.51 (m, 1H) ppm.
B
Inorg. Chem. XXXX, XXX, XXX−XXX