L. Cuesta-Aluja et al. / Journal of Molecular Catalysis A: Chemical 381 (2014) 161–170
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2.2. Synthesis of [Cr(1)Cl] (1a)
A brown suspension was obtained. The autoclave was closed and
purged three times with 5 bar of CO2 gas, and then pressurized to
15 bar of CO2 under stirring at room temperature. As a pressure
decrease was observed, pressure was then adjusted to 10 bar before
heating to 100 ◦C (pressure raised up to 20 bar). After stirring at
100 ◦C for 18 h, the mixture was cooled down to room temperature
using an ice bath. The unreacted gas was released and the mixture
was analyzed by gas chromatography (GC) and IR spectroscopy.
Sodium hydride (31 mg, 1.291 mmol) was washed with hexane
and added slowly as a suspension with dry THF (3 mL) to a solu-
tion of 1 (100 mg, 0.312 mmol) in dry THF (2 mL), and the resulting
mixture was stirred under nitrogen for 30 min. A gas formation
was observed (H2). After this time CrCl2 (43.6 mg, 0.354 mmol)
was added to the yellow solution and the brown resulting mix-
ture was stirred under nitrogen at room temperature for 3 h. The
mixture was further stirred overnight under air and, then, diluted
with tert-butyl methyl ether (25 mL). The organic layer was washed
with brine (3 × 25 mL) and then dried with anhydrous sodium sul-
fate. After filtration, solvent was removed in vacuum to give a
dark brown solid, 1a. Yield: 48%. Calc. for C20H22ClN4Cr·2OC4H8:
C, 61.14; H, 6.98; N 10.19. Found: C, 61.16; H, 7.18; N 10.22.
HR ESI calculated for C20H18CrClN4 m/z: 424.0523, found m/z:
2.7. Standard catalytic reaction in scCO2
In a typical experiment, styrene oxide (0.24 g, 2 mmol) and
mesitylene (0.23 g, 2 mmol) were introduced into a 40 mL auto-
clave. A 20 L sample of this reaction was diluted with CH2Cl2 up
to 100 L and was analyzed by gas chromatography (GC). The cat-
alyst (10 mg of complex, 0.026 mmol) and tetrabutylammonium
iodide (9.6 mg, 0.026 mmol) were introduced. The autoclave was
closed and purged three times with 5 bar of CO2 gas and weighed.
Then, liquid CO2 was rapidly introduced and the autoclave weighed
again. The temperature was increased up to 100 ◦C (pressure up to
170 bar). After stirring at 100 ◦C for 3 h (or the specified time), the
mixture was cooled down to room temperature using an ice bath.
The autoclave was slowly depressurized, the mixture was analyzed
by GC (20 L of sample diluted with CH2Cl2 up to 100 L) or, with-
out dilution, by NMR (when indicated) and IR spectroscopy. To
purify the product the content of the autoclave was dissolved in
CH2Cl2, the solvent was evaporated and the product was purified
by flash chromatography with a silica gel column using a mixture
of CH2Cl2/EtOAc (1:1) as eluent.
424.2514 [M−4H+Na]+. IR KBr pellet: 1611 cm−1 ( N). Raman
C
(250–800 cm−1): 331, 353, 447, 508, 558, 631, 683 cm−1. Conduc-
tivity (distilled H2O, 0.001 M): 39.3 ꢀ−1 cm2 mol−1
.
2.3. Synthesis of [Cr(2)Cl] (2a)
Similar procedure than for 1a was followed to obtain a dark
brown solid 2a. Yield: 44%. Calc. for C16H16ClN4Cr·3H2O: C,
47.35; H, 5.46; Found: C, 47.51; H, 5.36. HR MALDI-TOF (␣-
Cyano-4-hydroxycinnamic acid) calculated for C16H17CrN4 m/z:
317.0858, found m/z: 317.9471 [MH−Cl]+. IR KBr pellet: 1611 cm−1
( N). Raman (250–800 cm−1): 335, 384, 422, 454, 486, 502,
C
562, 594, 615, 639, 739, 785 cm−1. Conductivity (MeOH, 0.001 M):
50.1 ꢀ−1 cm2 mol−1
.
2.4. Synthesis of [Cr(1)Cl]Cl2 (1b)
3. Results and discussion
To a stirred solution of 1 (200 mg, 0.624 mmol) in THF (15 mL)
was added CrCl2 (69.7 mg, 0.567 mmol). The resulting mixture was
stirred under nitrogen at room temperature for 3 h, and then LiCl
(24.0 mg, 0.566 mmol) was added. The mixture solution was stirred
under air for additional 6 h. Cold THF was added to the brown
mixture. The suspension was filtered off, the solid was separated
by filtration, washed with cold THF and vacuum dried to afford
a brown solid 1b. Yield: 97%. Calc. for C20H24Cl3N4Cr·2H2O: C,
46.66; H, 5.48; N, 10.88. Found: C, 46.42; H, 5.44; N, 10.52. HR
MASS MALDI-TOF (dithranol) calculated for C20H21Cl3CrN4 m/z:
474.0237, found m/z: 474.3072 [M−3H]+ (10%); calculated for
C20H23CrN4 m/z: 371.1328, found m/z: 371.30.29 [M−HCl−2Cl]+
3.1. Synthesis of Cr(III) complexes with N4 Schiff bases ligands 1
and 2
The N4 Schiff bases (1 and 2) were prepared according to lit-
erature method [9]. The synthesis is based on the coupling of
2-nitrobenzaldehyde with 1,2-diaminocyclohexane or ethylenedi-
amine (Scheme 2). The dinitro compound formed is then carefully
hydrogenated to 1 and 2 using Pd/C at room temperature.
anionic ligands in anhydrous tetrahydrofuran (THF) under inert
atmosphere. Then, this solution was reacted with chromium
dichloride (CrCl2) and, subsequently, chromium(II) was oxidized
to chromium(III) under air stream (Scheme 3).
The 1a and 2a complexes were isolated both as a dark brown
solid with 48 and 44% yield respectively. In the high resolution
mass spectra the peaks corresponding to the mononuclear frag-
solids agreed to solvated forms. The infrared and Raman spec-
band at 1611 cm−1 attributed to the azomethine (C N) stretching
vibration, which was shifted to lower frequencies (1630 cm−1 and
1632 cm−1 for ligands 1 and 2 respectively) [20] due to backbonding
with the metal [21]. Two new bands in the region 560–680 cm−1
of two types of Cr N bonds [22]. The values of molar conductivi-
ties for these complexes (39.3 ꢀ−1 cm2 mol−1 in water for 1a and
50.1 ꢀ−1 cm2 mol−1 in methanol for 2a) confirmed that they corre-
sponded to neutral species [23] although partial ionization of the
chloride ligand may take place in solution.
(100%). IR KBr pellet: 1615 cm−1 ( N). Raman (250–800 cm−1):
C
368, 463, 502, 557, 571, 640, 750 cm−1. Conductivity (distilled H2O,
0.001 M): 150.6 ꢀ−1 cm2 mol−1. ꢁeff (21 ◦C) = 3.73 ꢁB.
2.5. Synthesis of [Cr(2)Cl2]Cl (2b)
Similar procedure than for 1b was followed to obtain a brown
solid 2b. Yield: 88%. Calc. for C16H16Cl3N4Cr·5H2O: C, 37.33; H, 5.48;
N, 10.88. Found: C, 37.20; H, 5.07; N, 10.40. HR MASS MALDI-TOF
(dithranol) calculated for C16H17CrN4 m/z: 317.0858, found m/z:
317.1530 [M−H−3Cl]+. IR KBr pellet: 1615 cm−1 ( N). Raman
C
(250–800 cm−1): 364, 449, 588, 676 cm−1. Conductivity (MeOH,
0.001 M): 105.7 ꢀ−1 cm2 mol−1. ꢁeff (21 ◦C) = 4.10 ꢁB.
2.6. Standard catalytic reaction in CH2Cl2
In a typical experiment, styrene oxide (0.24 g, 2 mmol), mesity-
lene (0.23 g, 2 mmol) and 5 mL of CH2Cl2 were introduced into
a 40 mL autoclave. 0.5 L of this solution was analyzed by gas
chromatography (GC), before adding the catalyst complex (23 mg,
0.06 mmol) and tetrabutylammonium iodide (21 mg, 0.06 mmol).
To obtain the cationic complexes, the neutral N4 Schiff bases
were reacted with chromium dichloride (CrCl2) in anhydrous THF
and after the oxidation with air stream in the presence of 1eq. of