M. Salavati-Niasari et al. / Inorganica Chimica Acta 375 (2011) 11–19
13
tion by UV–Vis and possible leaching of the complex was investi-
gated by UV–Vis in the reaction solution after filtration of the het-
erogeneous catalyst. The amounts of cobalt complex grafted in
MWNTs matrix were determined by the elemental analysis and
by subtracting the amount of cobalt complex left in the solutions
from the amount taken for the synthesis after the synthesis of
the catalysts as determined by UV–Vis spectroscopy. Atomic
absorption spectra (AAS) were recorded on a Perkin-Elmer 4100-
1319 Spectrophotometer using a flame approach, after acid (HF)
dissolution of known amounts of the nanotubes. Diffuse reflec-
tance spectra (DRS) were registered on a Shimadzu UV/3101 PC
spectrophotometer the range 1500–200 nm, using MgO as
reference.
that the carbon nanotubes exhibited a multi-wall structure and
their diameters were about 20–40 nm.
2.6. Chlorination of MWNTs
The purified MWNTs (100 mg) previously dried under vacuum
were suspended in a solution of SOCl2 (25 ml) and DMF (1 ml). The
suspensionwasstirredat 65 °C for24 h. Thesolid wasthenseparated
by filtration and washed with anhydrous THF, and dried in vacuum.
2.7. Anchoring the cobalt(II) complex to the MWNTs; [Co((OH)2-
salophen)]@MWNTs
MWNTs was added (50 mg) to a solution of [Co((OH)2-salophen)]
(100 mg) in degassed CHCl3 (8 ml), and the suspension was stirred
for 20 h under N2 atmosphere at 70 °C. The solid was then separated
by filtration and exhaustively washed with THF and CH2Cl2 and
dried in vacuum. The loading of [Co((OH)2-salophen)] complex in
2.3. Synthesis of N,N-bis(4-hydroxysalycilidene)-phenylene-1,2-
diamine, H2[(OH)2-salophen]
The stoichiometric amount of 4-hydroxysalicylaldehyde (2 mol)
dissolved in ethanol (25 ml) was added drop by drop to phenylene-
1,2-diamine solution (1 mol in 25 ml ethanol). The contents were
refluxed for 4 h and a bright yellow precipitate of H2[(OH)2-salo-
phen] was obtained. The yellow precipitate was separated by filtra-
tion, washed and dried in vacuum. It was then recrystallized from
ethanol to yield H2[(OH)2-salophen]. Elemental and spectroscopic
analysis of neat and MWNTs covalently anchored complex con-
firmed the molecular composition of ligand. The schematic repre-
sentation of the method of preparation of ligands and complexes
is shown in Scheme 1. Anal. Calc. for Schiff-base ligand: C, 68.96;
H, 4.63; N, 8.04; C/N, 8.58. Found: C, 68.80; H, 4.51; N, 8.17; C/N,
the MWNTs determined by elemental (N) analysis was 82 l .
mol gꢀ1
The cobalt content for the cobalt(II) complexes on MWNTs was be-
low the detection limit of quantitative absorption spectroscopy.
Anal. Calc. for [Co((OH)2-salophen)]@MWNTs: Co/N, 2.05%; IR
(KBr, cmꢀ1
) mC@N 1631. Elemental and spectroscopic analysis of
[Co((OH)2-salophen)] and [Co((OH)2-salophen)]@MWNTs con-
firmed the molecular composition of ligand.
2.8. Catalytic epoxidation
The epoxidation of cyclohexene with air was carried out with a
glass round-bottomed flask reactor at 35 °C. In a typical run, a
100 ml round bottomed flask equipped with an efficient water con-
denser was kept in a constant temperature oil bath at 35 °C. Then
cyclohexene (20 mmol) dissolved in acetonitrile (60 ml) and cata-
lysts (1.0 ꢂ 10ꢀ5 mol) were added. The reaction mixture was stir-
red by bubbling air at atmospheric pressure into the reaction
mixture at the rate of 10 ml minꢀ1. The liquid organic products
were quantified using a gas chromatograph. The liquid organic
products were identified by GC–MS analysis. n-Heptane was used
as an internal standard. The conversion was calculated on the basis
of molar percent of cyclohexene. The initial molar percent of cyclo-
hexene was divided by initial area percent (cyclohexene peak area
from GC) to get the response factor.
8.42%. IR (KBr,
m
cmꢀ1): 2970 (O–H); 2917 (C–H); 1618); 1531
(C@C); 1085 (C–O); 1638 (C@N). 1H NMR (DMSO-d6, ppm) d 13.4
(s, 2H, Oꢁ ꢁ ꢁHꢁ ꢁ ꢁN); 10.3 (s, 2H, O–H); 8.73 (s, 2H, CH@N); 7.3–7.4
(m, 3H, aldehyde); 6.3–6.4 (m, 4H, diamine).
2.4. Preparation of [Co((OH)2-salophen)]
The Schiff-base ligand; H2[(OH)2-salophen] (3.30 g, 0.011 mol)
was dissolved in 100 ml of refluxing ethanol and a stream of nitro-
gen was purged for 4 h to eliminate the oxygen. A solution contain-
ing 2.74 g (0.011 mol) of cobalt(II) acetate tetrahydrate in water
was added dropwise to the deoxygenated ligand solution. The
resulting mixture was agitated and refluxed under nitrogen with
5 ml of ethanol followed by 5 ml of water. The mixture was then
cooled and filtered under reduced pressure. The collected solid
was washed with diethyl ether and dried in air to give brown crys-
talline [Co((OH)2-salophen)] which purified by recrystallization
from chloroform (Yield: 73). UV (MeOH) 253, 317, 447 nm. Ele-
mental and spectroscopic analysis confirmed the composition of
these metal complexes and the schematic reaction is as shown in
Scheme 1. Anal. Calc. for [Co((OH)2-salophen)]: C, 59.27; H, 3.48;
N, 6.91; C/N, 8.58; Co, 14.54; Co/N, 2.10. Found: C, 59.14; H,
3.35; N, 7.02; C/N, 8.42; Co, 14.42; Co/N, 2.05%. IR (KBr, cmꢀ1):
3444 (br), 2980, 1625, 1541, 1458, 1385, 1092 (br), 959, 803.
2.9. Leaching experiment
To a mixture of cyclohexene (20 mmol) and butyraldehyde
(40 mmol) in 60 ml of acetonitrile immobilized [Co((OH)2-salo-
phen)] catalyst (1.0 ꢂ 10ꢀ5 mol) was added and stirred under air
for 8 h. After this period, 5 ml of the reaction mixture was filtered
to a second flask and stirred under air without any solid catalyst.
The remaining reaction mixture was allowed to react under air with
the solid catalyst. Samples (0.5 ml) were withdrawn from the two
reaction mixtures at regular 4 h intervals and analyzed by 1H
NMR after removal of the solvent. First reaction mixture with the
solid catalyst showed progress in the reaction with time. Whereas
the samples from the second flask without the solid catalyst
showed no further increase in the product concentrations showing
that no leaching of the cobalt catalytic centers has occurred.
2.5. Multi-wall carbon nanotubes (MWNTs) preparation and
purification
Multi-wall carbon nanotubes (MWNTs) used in this study were
prepared by chemical vapor deposition (provided by Jiang Youg
Trade Co. (China)) with diameters ranging from 20 to 40 nm,
lengths varying from 1 to 10 mm and purity of 85%. The raw prod-
uct was first immersed in an aqueous solution of HF to remove
SiO2, then filtered and washed with distilled water, and refluxed
in diluted HNO3 for 4 h to remove the metals and amorphous car-
bon. The resulting solid was then thoroughly washed with deion-
ized water and THF and dried in vacuum. TEM analysis showed
3. Results and discussion
3.1. Synthesis and characterization
Preparation of the cobalt(II) complex was involved heating and
stirring of stoichiometric amounts of H2[(OH)2-salophen] and co-