M.D. Bala, M.I. Ikhile / Journal of Molecular Catalysis A: Chemical 385 (2014) 98–105
99
metal acetates, alkoxides, or amides with imidazolium salts [10].
Hence, a few successful cases of iron-NHC complexes have been
recorded as reviewed by Ingleson and Layfield [4]. More specifi-
cally, by combining the advantages of [CpFe(CO)2]2 as a precursor
and the strong -donor properties of NHC ligands, Buchgraber et al.
in length), a flame ionization detector and nitrogen gas was used
as carrier gas at a flow rate of 2 mL/min. Reagents were purchased
from Aldrich or Merck and were used as received.
2.2. General procedure for the synthesis of ligands 1–9
[
11] and Mercs et al. [12] have studied the coordination chemistry
of piano-stool Cp-iron-NHC complexes similar to those isolated in
this study.
The ligands (except 3) were all synthesized by adaptation of
the methods of Starikova et al. [20]. A typical and generic proce-
dure is described. Spectroscopic and analyses data are presented.
N-monosubstituted azole (0.1 mmol) and dry toluene were placed
in a two-neck flask and stirred until a homogeneous solution was
formed; then alkyl halide (0.3 mmol) was added drop wise with
continuous stirring. After addition of the alkyl halide, the mixture
It is important to note that iron is not new to catalysis espe-
cially heterogeneously and as a component of biological systems.
Prominent amongst its use are in Harber-Bosch ammonia synthe-
sis [13] and Fischer-Tropsch process [14]. Examples of well-defined
NHC or Cp containing homogeneous systems include work by Tat-
sumi and co-workers who utilized unsaturated half-sandwich NHC
iron complexes to activate C H bonds of thiophenes, furans and
pyridine [15]. The application of organometallic and coordination
complexes of iron as catalysts for the reduction of carbonyl groups
has been explored by several groups. For example, the group of
Morris and co-workers has developed very efficient systems for
asymmetric TH of ketones based on tetradentate PNNP ligands [16].
Casey and co-workers, inspired by bifunctional, ionic hydrogena-
tion catalysis established for ruthenium, have disclosed catalytic
activity of related Cp-iron counterparts [17]. Their catalysts showed
high activity in hydrogenation of several ketones, aldehydes, and
imines using molecular H2 as a reducing agent and also displayed
activity in transfer hydrogenation reaction by use of 2-propanol
as source of hydrogen. In addition, Kandepi and co-workers have
developed new Fe(II) complexes containing functionalized-Cp NHC
ligands that showed good catalytic activity in hydrosilylation of
aldehydes and transfer hydrogenation of ketones [18].
The examples mentioned above reveal a reemergence in the
last decade of interest in well-defined iron complexes as credi-
ble alternatives to PGMs in some homogeneous catalysis projects.
Hence, in continuation with our interest in the study of imidazolium
family of compounds as ionic liquids and ligands in organometallic
chemistry [19], the current work was aimed at the evaluation of
NHC-Fe systems as simple, active, in situ generated one pot cata-
lysts. The study reports combination of the simplicity of TH, in situ
one pot catalyst generation and use of abundant, cheap and envi-
ronmentally friendlier iron catalyst in one process.
◦
was stirred while heating at 40 C for 24 h. The solvent was removed
and the ligand was dried under vacuum.
2.2.1. 1,3-Dimethylimidazolium iodide (1)
Brown solid. Yield (4.80 g, 98%) IR (ATR cm−1): 3433, 3152,
3094, 2953, 1619, 1572, 1341, 1170, 1084, 1020, 826, 748, 617;
ıH (400 MHz, CDCl ): 4.07 (6H, s, NCH ), 7.35 (2H, s, NCH) and
3
3
9.97 ppm (1H, s, CH); ıC (100 MHz, CDCl ): 37.12 (NCH ),123.36
3
3
+
−
(NCH) and 137.76 ppm.; m/z (ESI) 96.7 (M −I ). HRMS (ESI) calcd
+
−
+
−
for C5H IN , 97.07657 (M −I ); found, 97.07628 (M −I ).
9
2
2.2.2. 1-Methyl-3-ethylimidazolium bromide (2)
White solid. Yield (4.70 g, 98%). IR (ATR cm−1): 3065, 2975, 1670,
1571, 1467, 1172, 1101, 856, 789, 649, 789, 621, 417; ıH (400 MHz,
CDCl ): 1.47 (3H, t, J 7.3 Hz, CH ), 3.97 (3H, s, NCH ), 4.32 (2H, q,
3
3
3
NCH ), 7.54 (2H, s, NCH) and 10.07 ppm (1H, s, CH); ı (100 MHz,
2
C
CDCl ): 15.64 (CH ), 36.63 (NCH ), 45.18 (NCH ), 122.01 (NCH),
3
3
3
2
+
−
123.71 (NCH) and 136.73 ppm.; m/z (ESI) 111.5 (M −Br ). HRMS
+
−
(ESI) calcd for C H BrN , 111.09222 (M −Br ); found, 111.09196
6
11
2
+
−
(M −Br ).
2.2.3. 1-Methyl-3-butylimidazolium bromide (4)
Colorless oil. Yield (1.98 g, 91%). IR (ATR cm−1): 3077, 2959,
1626, 1570, 1463, 1166, 1109, 752, 619, 460; ıH (400 MHz, CDCl ):
3
0.74 (3H, t, J 7.4 Hz, CH ), 1.18 (2H, m, CH ), 1.70 (2H, m, CH ), 3.92
3
2
2
(3H, s, NCH ), 4.14 (2H, t, J 6.7 Hz, NCH ), 7.42 (1H, s, NCH), 7.53 (1H,
3
2
s, NCH) and 10.03 ppm (1H, s, CH), ı (100 MHz, CDCl ): 13.41 (CH ),
C
3
3
1
1
9.37 (CH ), 32.11 (CH ), 36.65 (NCH ), 49.72 (NCH ), 122.29,
2 2 3 2
+
−
23.83, 136.99 ppm; m/z (ESI) 139.4 (M −Br ) HRMS (ESI) calcd
+
−
+
−
2
. Experimental
for C H BrN , 139.12352 (M −Br ); found, 139.12327 (M −Br ).
6
11
2
2.1. General procedures
2.2.4. 1-Methyl-3-pentylimidazolium chloride (5)
Light yellowish oil. Yield (1.62 g, 86%). IR (ATR cm−1): 2929,
All manipulations were performed using standard Schlenk tech-
2859, 1520, 1466, 1123, 1108, 731, 662; ıH (400 MHz, CDCl ): 0.82
3
niques under an atmosphere of dry nitrogen. All solvents were
dried and purified by standard procedures prior to use. Glassware
was oven dried at 110 C. All NMR experiments were done using
(3H, t, J 7.4 Hz, CH ), 1.24 (4H, m, CH ), 1.48 (2H, q, CH ), 3.52 (2H, t, J
3
2
2
6.7 Hz, NCH ), 3.58 (3H, s, NCH ), 6.78 (1H, s, NCH), 6.93 (1H, s, NCH)
2
3
◦
and 7.33 ppm (1H, s, CH), ıC (100 MHz, CDCl ): 14.05 (CH ), 22.52
3 3
a 400 MHz Bruker Ultrashield spectrometer and samples were dis-
solved in deuterated chloroform. Infrared spectra for the ligands
were recorded neat using a Perkin Elmer universal ATR Spectrum
(CH ), 28.04, 32.51, 33.29, 62.36, 125.29, 128.21, 137.73 ppm; m/z
2
+
−
(ESI) 153.0 (M −Cl ) HRMS (ESI) calcd for C H ClN , 153.13917
9
17
2
+
−
+
−
(M −Cl ); found, 153.13877 (M −Cl ).
1
00 FT-IR spectrophotometer, while the solution IR data for the
complexes were recorded in CH Cl on a Perkin Elmer FT-IR spec-
2.2.5. 1,3-Diethylimidazolium bromide (6)
2
2
trophotometer; model RX 1. Low resolution MS samples were run
on a Thermo Finnigan Linear ion trap mass spectrometer using
electrospray ionization in positive mode. Accurate mass data was
obtained on a Thermo Electron DFS Dual focusing magnetic sector
instrument using ESI in positive mode; polyethylenimine was used
as reference solution. Ligand 3 was purchased from Aldrich, while
other ligands were synthesized according to a literature method
Colorless oil. Yield (1.34 g, 94%). IR (ATR cm−1): 3426, 3066,
2977, 1562, 1448, 1350, 1229, 1164, 1083, 1032, 956, 908, 803, 753,
643; ıH (400 MHz, CDCl ): 1.40 (6H, t, J 7.4 Hz, CH ), 4.38 (4H, q,
3
3
NCH ), 7.45 (2H, s, NCH) and 10.38 ppm (1H, s, CH), ı (100 MHz,
2
C
CDCl ): 16.48 (CH ), 41.96 (NCH ), 129.46 (NCH), 136.72 ppm; m/z
3
3
2
+
−
(ESI) 124 (M −Br ) HRMS (ESI) calcd for C7H BrN , 125.10787
13
2
+
−
+
−
(M −Br ), found, 125.10700 (M −Br ).
[
20]. Preparation of CpFe(CO) I was based on our published proce-
2
dure [21]. Transfer hydrogenation reaction was monitored by gas
chromatography (GC) with an Agilent capillary GC model 6820 fit-
ted with a DB wax polyethylene column (0.25 mm in diameter, 30 m
2.2.6. 1,3-Dibutylimidazolium bromide (7)
Colorless oil. Yield (1.45 g, 80%). IR (ATR cm−1): 3401, 2959,
2874, 1649, 1510, 1462, 1280, 1107, 1080, 1025, 951, 734, 664; ıH