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171
stored in a lightly closed bottle and used without reactiva-
tion. The concentration of vanadium determined by AAS was
0.014 mmol g−1
2.2.6. Preparation of [VO(salen)]-Al2O3 (7-alum) and
[VO(salophen)]-Al2O3 (8-alum)
.
The Schiff-base was obtained by condensation of the
appropriate diamine and salicylaldehyde [12–16]. Ligands
were recrystallized from ethanol at least once before met-
alation. The vanadyl Schiff-base complexes were prepared
under ambient condition. To a hot solution of Schiff-base lig-
and (2 mmol) in a mixed solvents (chloroform 15 ml/ethanol
15 ml), a hot solution of VO(acac)2 (2 mmol) in methanol
(15 ml) was added. The mixture was heated and a few drops
of triethylamine were added. The reaction mixture was re-
fluxed for 30 min. The colored solution concentrated and
cooled to yield crystals. Analytical data and physical prop-
erties for vanadyl complexes are similar to their complexes
were prepared by literature methods [17–19]. Acidic alumina
was stirred with [VO(salen)] or [VO(salophen)] in CH3Cl and
after 40 ◦C for 8 h, the solid colour changed to light green.
It was then washed with CH3Cl and dried at 60 ◦C under
vacuum.
2.2.2. Preparation of [VO(en)2](SO4)-Al2O3 (2-alum)
Ethylenediamine (4.5 mmol) was dissolved in 50 ml of
ethanol, and the solution was stirred. To this solution was
added vanadyl sulfate pentahydrate (2.25 mmol) dissolved
in 25 ml of ethanol. The color of the solution changed to
green. The mixture was then heated at 40 ◦C for 4 h. Upon
cooling the solution, a green solid was obtained which was
filtered, washed with chloroform, and dried in vacuum and
purified by recrystallization from ethanol. Anal. Calcd for
C4H16N4O5SV: V, 17.98; C, 16.96; H, 5.69; N, 19.78%.
Found: V, 17.88; C, 16.82, H, 5.59; N, 19.83%. A solution
of the [VO(en)2](SO4) in CH3OH was added to a suspension
of Al2O3 in CH3OH. The resulting suspension was stirred at
40 ◦C for 8 h. The green solid was filtered, and washed with
CH3OH. The [VO(en)2](SO4)-Al2O3 catalyst was dried at
60 ◦C under vacuum overnight prior to use.
2.2.7. Oxidation of cyclohexene; general procedure
A mixture of 1.0 g catalyst, 25 ml solvent and 10 mmol cy-
clohexene was stirred under nitrogen atmosphere in a 50 ml
round bottom flask equipped with a condenser and a drop-
ping funnel at room temperature for 30 min. Then 16 mmol
of TBHP (solution 80% in di-tert-butylperoxide) was added.
The resulting mixture was then refluxed for 8 h under N2
atmosphere. After filtration and washing with solvent, the
filtrate was concentrated and then subjected to GC analysis.
2.2.3. Preparation of [VO(TMED)2](SO4)-Al2O3
(3-alum)
This compound was prepared by a method similar to
that for [VO(en)2]SO4] except that tetramethylethylenedi-
amine was used instead of ethylenediamine. Anal. Calcd for
C12H32N4O5SV: V, 12.88; C, 36.45; H, 8.16; N, 14.17%.
Found: V, 12.76; C, 36.35, H, 8.07; N, 14.22%. The
[VO(TMED)2](SO4) in CH3OH was added to a suspen-
sion of acidic alumina in CH3OH. The suspension was
heated at 40 ◦C for 8 h. The green solid was filtered, washed
with methanol. The [VO(TMED)2](SO4)-Al2O3 catalyst was
dried at 60 ◦C under vacuum overnight prior to use.
3. Results and discussion
3.1. Synthesis of neat complexes
Elemental analyses indicate that all of the vanadyl com-
plexes are monomeric species formed by coordination of
1 mol of the vanadyl ion (VO2+) and 2 mol of bidentate di-
amine (en, TMED, bpy, phen) or 1 mol of tetradentate Schiff-
base (salen, slaophen) ligands. The vanadyl Schiff-base com-
plexes are insoluble in water but soluble in most organic sol-
vents. Infrared and UV–vis spectral analyses indicate that
less electrophilic metal-oxo center (Table 1). Green vanadyl
2.2.4. Preparation of [VO(bpy)2](SO4)-Al2O3 (4-alum)
and [VO(phen)2](SO4)-Al2O3 (5-alum)
This compound was prepared by a method similar to
that for [VO(en)2]SO4] except that bipyridine or phenantro-
line was used instead of ethylenediamine. Anal. Calcd for
C20H16N4O5SV: V, 10.72; C, 50.53; H, 3.39; N, 11.79%.
Found: V, 10.68; C, 50.42, H, 3.34; N, 11.82%. Anal. Calcd
for C24H16N4O5SV: V, 9.73; C, 55.07; H, 3.08; N, 10.70%.
Found: V, 9.67; C, 54.12, H, 2.98; N, 10.81%. Acidic alumina
was stirred with [VO(bpy)2](SO4) or [VO(phen)2](SO4)-
Al2O3 in CH3OH and after 40 ◦C for 8 h, the solid color
changed to light green. It was then washed with CH3OH and
dried at 60 ◦C under vacuum.
=
complexes have V O stretching frequencies in the range
976–993 cm−1, which are characteristic of monomeric five
coordinate structures [20].
3.2. Heterogenisation of complexes
2.2.5. Preparation of [VO(acac)2]-Al2O3 (6-alum)
Acidic alumina was added to a well stirred solution of
[VO(acac)2] (from Merck) in methanol. The mixture was
then heated at 40 ◦C for 8 h. The light green solid was fil-
tered, washed with methanol and dried at 60 ◦C under vacuum
overnight prior to use.
The heterogenisation of homogenous catalysts is a field of
continuing interest. Although some of homogeneous transi-
tion metal complexes exhibit remarkable catalytic properties
(activities and selectivity), they are unsuitable to separate in-
tact, from the reaction medium making their reuse difficult
and contaminating the reaction products. Thus, the hetero-