13
Mn(III) Schiff base complexes have been heterogenized by their
such as zeolites, MCM-41 and clays [17,18]. Preparation of hetero-
complexes on silica surface represents one of the most usable
approaches [19]. Many heterogeneous catalysts contain metal ions
and chemically modified silica gels with active organic compo-
nents [20]. Modified silica exhibits some advantages over modified
resins such as high surface area, high thermal and chemical sta-
bility. Since the early 1990s and the advent of mesoporous silicas,
the development of new solid catalysts and the heterogenation of
homogeneous systems for oxidation processes have become very
attractive topics. A major drawback of transition-metal catalysts
coordinated to monodentate ligands is that they usually undergo
leaching of the active species into solution, especially in the pres-
ence of protic agents such as alcohols or organic peroxides [21]. A
simple way to minimize the leaching of the active ingredient is to
anchor polydentate ligands onto the support surface, as this type of
chelating system offers high coordinative stability for the catalytic
ingredient.
in methanol (10 mL) was added to a solution of salicylaldehyde
(1.22 g, 10.0 mmol) in methanol (10 mL) and refluxed for 2 h. The
colorless solution immediately turned yellow. The reaction was
maintained in reflux for 2 h and then the solvent was removed
under reduced pressure. The resulting viscous yellow oil was dis-
solved in dichloromethane and washed repeatedly with water. The
organic layer was separated and dried over anhydrous magnesium
sulfate followed by solvent evaporation and drying at room tem-
perature for several hours. Yield 2.30 g, 71.0%. IR (KBr, cm−1): 3449
(w) (O–H), 3065 (w), 2977 (vs), 2930 (s), 2894 (s), 2744 (w), 1634
(vs) (C N), 1585 (m), 1498 (m), 1462 (m), 1391 (m), 1344 (w), 1281
(s), 1191 (w), 1167 (s), 1107 (vs), 1084 (vs), 959 (s), 899 (w), 871 (w),
795 (m), 758 (s), 641 (w), 568 (w), 526 (w), 465 (m). 1H NMR (CDCl3)
ı = 0.604 (m, 2H, SiCH2), 1.154 (t, 3J = 7.0 Hz, 9H, SiOCH2CH3), 1.742
(m, 2H, NCH2CH2), 3.473 (t, 3J = 6.75 Hz, NCH2), 3.748 (q, 3J = 7.0 Hz,
6H, SiOCH2), 6.722–7.212 (m, 4H, Ar), 8.190 (s, 1H, N CH), 13.418
(s, 1H, Ar–OH) ppm.
2.3. Functionalization of silica gel to (silica gel)–O2(EtO)Si–L1H
(1)
Against this background, we investigate here the anchoring of
a manganese(II) hydrazide Schiff base moiety onto a silica sub-
has some similarities to non-symmetrical salen-type ligands. These
types of hydrazone Schiff base complexes show considerable cat-
alytic activities. The efficiency of homogeneous hydrazone based
catalysts in alkene epoxidations [22,23] led us to the design of
their heterogeneous analogues using silica as a support. This article
describes the efficient and highly selective epoxidation of alkenes
with 30% aqueous H2O2 in the presence of silica–Mn(II)–hydrazide
Schiff base as a reusable solid catalyst.
The
N-(triethoxysilylpropyl)salicylaldimine
linker
was
anchored onto the silica surface via a grafting method. The
procedure of synthesis was similar to the reported procedure
with some modifications [25]. The amorphous silica gel was
preliminary heated in an oven at 500 ◦C for 24 h to remove
adsorbed water. A 5.0 g sample of the pretreated silica gel powder
was suspended in 50 mL of dry toluene in a round bottomed
flask (100 mL) which was flushed with N2. (EtO)3Si–L1H (1.2 g,
3.7 mmol) was then added and the suspension stirred for 24 h
under reflux at N2 atmosphere. The resulting suspension was
filtered and washed with Et2O (3 × 15 mL). The recovered pow-
der was extracted in a Soxhlet using refluxing CH3OH (750 mL)
for 24 h. The functionalized silica gel, designated as (silica
gel)–O2(EtO)Si–L1H, was dried under vacuum at room tempera-
ture for 18 h. A yellow powder was recovered. IR (KBr, cm−1): 3444
(m, br), 2935 (w), 2864 (w), 1647 (m), 1101 (vs, br), 813 (m), 473
(s).
2. Experimental
2.1. Materials and equipments
Silica gel (0.063–0.200 mm) and all other reagents were
obtained from commercial sources (Merck or Fluka) and were
used as received without further purification. Aqueous 30% hydro-
gen peroxide (8.12 mol/L) was used and its exact concentration
was determined before use by titration with standard KMnO4.
(E)-Nꢀ-(2-Hydroxy-3-methoxybenzylidene)benzohydrazide, H2L2,
was synthesized according to a reported procedure [22]. 1H NMR
spectra were recorded by use of a Bruker 250 MHz, spectrome-
ter. Elemental analyses were determined on a CHN Perkin-Elmer
2400 analyzer. IR spectra were recorded on a Perkin-Elmer 597
spectrometer. The solution and diffuse reflectance DR UV–Vis spec-
tra in the range of 300–800 nm were measured on a CARY 5E
spectrophotometer, with the spectra referenced to BaSO4 (disk
diameter 13 mm, thickness 2 mm). The manganese content of the
final material was determined by ICP model ARL 3410, or alterna-
tively, by a Varian spectrometer AAS-110. The reaction products
of the oxidation were determined and analyzed by an HP Agilent
6890 gas chromatograph equipped with a HP-5 capillary column
(phenyl methyl siloxane 30 m × 320 m × 0.25 m) with flame-
ionization detector and gas chromatograph–mass spectrometry
(Hewlett-Packard 5973 Series MS-HP gas chromatograph with a
mass-selective detector).
2.4. Immobilization of the manganese complex to (silica
gel)–O2(EtO)Si–L1–Mn(HL2) (2)
Mn(O2CCH3)2·4H2O (20 mol, 0.005 g) and H2L2 (19 mol,
0.005 g) were dissolved in 30 mL of dichloromethane. The function-
alized silica gel (silica gel)–O2(EtO)Si–L1H (2.0 g) was then added to
the solution and the mixture stirred under reflux for 24 h followed
by Soxhlet extraction with CH3OH for 24 h. The green material was
dried under vacuum overnight, yielding 1.840 g. Most of the (silica
gel)–O2(EtO)Si–L1–Mn(HL2) was found lost in the Soxhlet extrac-
tion since the cleaning of the Soxhlet timble was difficult. CHN
analyses showed the presence of both L1 and L2. In order to con-
firm that the Mn is indeed bound as L1–Mn–L2 and not (at least to
some extent) as L1–Mn–L1 from two neighboring L1-groups, the
UV–vis spectra of the compounds (silica gel)–O2(EtO)Si–L1H (1)
and (silica gel)–O2(EtO)Si–L1–Mn(HL2) (2) were compared with
its homogeneous analogues (not shown). IR (KBr, cm−1): 3443 (br,
w), 3217 (w), 3075 (w), 2935 (w), 2858 (w), 1617 (m), 1574 (m),
1442 (m), 1216 (s), 1085 (vbr, vs), 808 (s), 741 (w), 712 9 w), 740
(vs).
2.5. General procedure for the epoxidation of alkenes
[(EtO)3Si–L1H]
The epoxidation of alkenes with hydrogen peroxide was per-
formed in a 25-mL round-bottom flask. In a typical experiment,
the flask was charged with 3.0 mL of CH3CN, 1.0 mmol alkene,
1.0 mmol NaHCO3, 0.1 g chlorobenzene as internal standard and
The linker was synthesized according to a reported procedure
[24]. A solution of aminopropyltriethoxysilane (2.21 g, 10.0 mmol)