Mont-K10 Supported Fe(II) Schiff-Base Complex as an Efficient Catalyst for Hydrogenation of Ketones

Article abstract of DOI:10.1007/s10562-019-02810-x

Abstract: A new Fe(II) Schiff base complex anchored on mont-K10 (Fe@imine-mont-K10) was synthesized and extensively characterized by FTIR, powder X-ray diffraction, SEM–EDX, TEM, ESR, X-ray photoelectron spectroscopy (XPS), BET surface area measurement, solid state ²⁹Si NMR and ICP-AES analysis. The catalytic activity of the complex was investigated for hydrogenation of ketones. The results indicated that it exhibited good catalytic activity for hydrogenation of aromatic as well as aliphatic ketones in i-PrOH/CH₃CN (1:1) using Na-i-OPr as base at 80?°C resulting in moderate to excellent isolated yields (51–99%) of their corresponding products. The catalyst shows good reusability. Graphical Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-019-02810-x

Catalysis Letters (2019) 149:2142–2157
https://doi.org/10.1007/s10562-019-02810-x
Mont‑K10 Supported Fe(II) Schif‑Base Complex as an Efcient Catalyst
for Hydrogenation of Ketones
Samim Sultana1 · Geetika Borah1 · Pradip. K. Gogoi1
Received: 17 December 2018 / Accepted: 4 May 2019 / Published online: 17 May 2019
© Springer Science+Business Media, LLC, part of Springer Nature 2019
Abstract
A new Fe(II) Schif base complex anchored on mont-K10 (Fe@imine-mont-K10) was synthesized and extensively charac-
terized by FTIR, powder X-ray difraction, SEM–EDX, TEM, ESR, X-ray photoelectron spectroscopy (XPS), BET surface
area measurement, solid state 29Si NMR and ICP-AES analysis. The catalytic activity of the complex was investigated for
hydrogenation of ketones. The results indicated that it exhibited good catalytic activity for hydrogenation of aromatic as
well as aliphatic ketones in i-PrOH/CH3CN (1:1) using Na-i-OPr as base at 80 °C resulting in moderate to excellent isolated
yields (51–99%) of their corresponding products. The catalyst shows good reusability.
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s10562-019-02810-x) contains
supplementary material, which is available to authorized users.
* Geetika Borah
geetikachem@yahoo.co.in
* Pradip. K. Gogoi
dr.pradip54@gmail.com
Samim Sultana
samimsultana786@gmail.com
1
Department of Chemistry, Dibrugarh University, Dibrugarh,
Assam, India
Vol:.(1234567890)
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Mont‑K10 Supported Fe(II) Schif‑Base Complex as an Efcient Catalyst for Hydrogenation…
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Graphical Abstract
Keywords Fe@imine-mont-K10 · Hydrogenation · Aromatic · Aliphatic
pharmaceuticals, pesticides, fragrances, favor ingredients
and fne chemicals [7, 8]. Hydrogenation reactions have
often been performed using precious and toxic metal based
catalysts such as Ru [9–12], Rh [13, 14], Ir [15–17], Au
[18, 19], Pt [20, 21], Pd [22–24] etc. at high temperatures
(100 °C, 82 °C) [8, 10, 11, 13, 25] or under refuxing con-
ditions [8, 14, 25] and the separation of traces of these
toxic metals from organic products becomes problematic.
In most of the hydrogenation reactions inert atmosphere of
N2 [26–28] or Ar [29] have also been applied. Therefore, it
has become a major challenge for the chemists to develop
mild and environmentally benign synthetic routes which
can substitute these precious metals by cheaper, more abun-
dant and less toxic metals such as Fe [30–33], Cu [34], Mn
[35, 36], Co [37], Ni [38] etc. In recent years, Fe, Cu, Mn
and Co complexes are becoming more challenging cata-
lysts because of their efectiveness and solubility in various
organic reactions. Of all these transition metals, iron (Fe) is
one of the most abundant and inexpensive on earth and is
1 Introduction
Transition metal catalyzed hydrogenation of ketones is
one of the well-known sustainable methodologies in syn-
thetic organic chemistry. Due to its operational simplic-
ity, cost efectiveness, eco-friendly nature, selectivity and
high atom efciency it has attracted the chemists much
more compared to direct reduction using molecular hydro-
gen [1–4]. The hydrogen sources such as cyclohexene,
cyclohexadiene, alcohols, formic acids or inorganic rea-
gents like hydrazine are used to avoid molecular hydro-
gen gas which requires high pressure equipment and extra
safety precautions [5]. Because of low cost, 2-propanol or
formate are generally used as hydrogen source in catalytic
hydrogenation reactions leading to greener alternative to
conventional processes [6]. In recent years, research in the
area of asymmetric hydrogenation of ketones is acquiring
much more attention as their products i.e., optically active
alcohols, serve as the building blocks for the synthesis of
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S. Sultana et al.
environmentally benign and safe for humans [39]. Moreo-
ver, in industrial applications heterogeneous metal cata-
lysts are more preferable than the homogeneous ones due to
their high activity, easy separation and recycling capability
[40–42]. The development of solid surface with defnite,
active metal site provides a suitable path to materials that
improve catalytic activity as well as helps in understand-
ing of the molecular basis of heterogeneous catalysis [43].
As the nature of support plays a signifcant role in cata-
lytic activity by stabilizing interaction between supported
metal particles, it is important to choose a suitable support
for catalytic efciency and stability of the catalyst [44]. In
recent years, insertion of organic or organometallic species
into inorganic solids have also been studied in which com-
plementary properties of the two components are expressed
in a single material [45]. Various heterogeneous transition
metal catalysts for hydrogenation reactions have been
reported, such as zirconium (IV) Schif base complexes
on mesoporous silica [46], iron-phthalocyanine catalyst
[47, 48], polymer supported Ru(III) complex [49], ionic
tagged ferrocene-ruthenium catalyst [50], immobilized iron
metal containing ionic liquid (ImmFe-IL) [51], magnetic
ruthenium-coated iron nanoparticles [52], silica-supported
iridium catalysts [53], iridium and ruthenium sulfonate-
functionalized N-heterocyclic carbene catalysts [54] etc.
But very few hydrogenation reactions have been carried out
using catalysts supported on mont-K10 clay [55]. Because
of low cost, large surface area, high cation exchange capac-
ity, non-toxic and non-corrosive nature, economical and
recyclable, clays are often used as support in many organic
reactions [56, 57]. Many commercially available clay based
catalysts are known such as claycopTM, clayzincTM, clay-
fenTM, envirocatTM, but K-10 and KSF montmorillonites
are the most common modifed clays which are applied in
organic synthesis. Both the synthetic clays have similar
physicochemical properties but mont-K10 has a higher sur-
face area (about 250 m2g−1) compared to that of mont-KSF
(10 m2g−1) [58]. Mont-K10, with its unique swelling prop-
erties, non-corrosive nature, low cost and easy regeneration
makes it economically feasible and environmentally benign
solid support [59–61]. It is capable of interchanging cations
through intercalation and expansion [62]. Thus, in continu-
ation of our previous work on catalytic activity of mont-
K10 [63] and rice husk derived nanosilica immobilized
[64] transition metal compounds, herein we wish to report
the development of an environmentally benign Fe(II) Schif
base catalyst immobilized on mont-K10 which catalyze
the hydrogenation of ketones efectively under mild reac-
tion condition. The novelty of the present work is that our
catalytic system is quite efective towards hydrogenation of
aliphatic and aromatic ketones as well, and to the best of
our knowledge such a cost-efective mont-K10 supported
Fe(II) Schif-base complex have not reported till days for
the said reaction.
2 Experimental
2.1 General Information
Mont-K10 and APTES (3-Aminopropyl triethoxysilane)
were purchased from Sigma-Aldrich, FeSO4.7H2O was pur-
chased from SRL (Sisco Research Laboratories Pvt. Ltd),
2-propanol, acetonitrile and sodium hydroxide were procured
from MERCK. 2-quinoline carboxaldehyde, Benzophenone,
acetophenone, 4-nitro acetophenone, 4-bromo acetophenone,
4-chloro acetophenone, 4-methyl-2-pentanone, 2-pentanone,
3-pentanone, 2-methyl cyclohexanone, cyclohexanone, cyclo-
heptanone, p-hydroxy acetophenone and p-bromo benzophe-
none were obtained from Tokyo Chemical Industry. All rea-
gents were of analytical grade and used as received without
further purifcation.
FTIR spectra of mont-K10, APTES@mont-K10, imine@
mont-K10 and Fe@imine-mont-K10 were recorded in KBr
pellets on a Shimadzu IR prestige-21 FTIR spectrophotom-
eter (500–4000 cm−1). ESR spectrum of Fe@imine-mont-
K10 at liquid N2 temperature was recorded on JES—FA200
ESR Spectrometer. The 29Si-NMR spectroscopic analyses
of APTES@mont-K10, imine@mont-K10 and Fe@imine-
mont-K10 in solid state were recorded using JEOL, Model-
ECX400 operating at 400 MHz. The SEM–EDX images of
imine@mont-K10, Fe@imine-mont-K10 and the recycled
catalyst were recorded using JEOL, JSM IT-300 operating
at 20 kV. Powdered X-ray difraction patterns of APTES@
mont-K10, imine@mont-K10 and Fe@imine-mont-K10
were recorded using Rikagu Ultima IV difractometer with
Cu-Kα (1.541Å radiation). The Inductively Coupled Plasma
Atomic Emission Spectrometric (ICP-AES) analyses of Fe@
imine-mont-K10 and the reused catalyst were recorded on
ARCOS, Simultaneous ICP Spectrometer. TEM images of
APTES@mont-K10, imine@mont-K10 and Fe@imine-
mont-K10 were recorded using JEM-100 CX II. GC–MS
of the products were performed on Agilent Technologies
GC system 7820 coupled with a mass detector 5975 and
SHRXI-5MS column. 1H NMR spectra of the products were
recorded in CDCl3 and D2O using TMS as an internal stand-
ard using Bruker Avance II and III, 400 MHz spectrometer.
X-ray photoelectron spectrum (XPS) of Fe@imine-mont-
K10 was performed on an XPS-AES Module, Model: PHI
5000Versa Prob II, FEI Inc. The Brunauer–Emmett–Teller
(BET) surface area measurement of APTES@mont-K10,
imine@mont-K10 and Fe@imine-mont-K10 at liquid
nitrogen temperature were carried out using Quantachrome
Novae 2200.
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Mont‑K10 Supported Fe(II) Schif‑Base Complex as an Efcient Catalyst for Hydrogenation…
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for 24 h. It was fltered and then washed with acetone
using Soxhlet extraction which was designated as Fe@
imine-mont-K10.
2.2 Methods
2.2.1 Synthesis of APTES Functionalized Montmorillonite
The procedure for synthesis of the catalyst has been
shown in Scheme 1.
(APTES@mont‑K10)
APTES@mont-K10, imine@mont-K10 and Fe@imine-
mont-K10 were prepared following reported procedure [65]:
Montmorillonite (500 mg) was suspended in dry toluene
(100 mL) to which 0.11 g APTES (0.5 mmol) was added and
refuxed under nitrogen atmosphere for 6 h. It was cooled,
fltered and then washed repeatedly with toluene using Sox-
hlet extraction. The resulting mixture was dried which was
designated as APTES@mont-K10.
2.2.4 General Procedure for Hydrogenation of Ketones
Using Fe@imine‑mont‑K10 as the Catalyst
The hydrogenation of ketones was carried out by taking
15 mg of Fe@imine-mont-K10 catalyst (19.9 mol %),
1 mmol of ketone, 5 mL sodium iso-propoxide (Na-i-OPr),
5 mL acetonitrile and i-PrOH (5 mL) in a round bottomed
fask and stirring at 80 °C for required time (Table 5). The
progress of the reaction was monitored from time to time
by thin layer chromatography (TLC) using 5% ethylace-
tate-hexane as the eluent. After completion of the reac-
tion (monitored by GC–MS), the catalyst was separated
by fltration. The fltrate was diluted with water (10 mL)
and then extracted with ethyl acetate (10 mL) followed
by washing with brine and dehydrated using anhydrous
Na2SO4. The resulting residue was purifed by silica gel
column chromatography using ethylacetate-hexane (5:95)
as the eluent to get the pure products. In order to recycle
the catalyst, it was separated from the reaction mixture by
centrifugation and then washed several times with water
and ethyl acetate after each cycle. After drying at 110 °C
in an oven for overnight, the recovered catalyst was sub-
jected to subsequent runs under same reaction conditions.
2.2.2 Synthesis of Imine Functionalized APTES@mont‑K10
(imine@mont‑K10)
APTES@mont-K10 (400 mg) was suspended in acetone
(60 mL) to which 0.027 g of 2-quinoline carboxaldehyde
(0.17 mmol) was added, followed by refuxing (4 h). The
resulting mixture was filtered, washed repeatedly with
dichloromethane using Soxhlet extraction, dried and then
kept in a desiccator. It was designated as imine@mont-K10.
2.2.3 Synthesis of the Metal Functionalized imine@
mont‑K10 (Fe@imine‑mont‑K10)
FeSO4.7H2O (0.01725 g, 0.062055 mmol) was dissolved
in methanol and acetone (1:1) to which imine@ mont-
K10 (100 mg) was added and stirred at room temperature
Scheme 1 Procedure for the synthesis of Fe@imine-mont-K10
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S. Sultana et al.
3 Results and Discussion
3.1 Characterization
3.1.1 FTIR Study
In the FT-IR spectrum of mont-K10, the peaks at 3344 cm−1
(broad) and 1641 cm−1 (sharp) are assigned to OH stretch-
ing and OH bending, respectively (Fig. S1). The sharp peak
at 1100 cm−1 is due to asymmetric stretching frequency of
siloxane (Si–O–Si) framework. The FT-IR spectrum of
APTES@mont-K10 shows a new band at 1571 cm−1 (γNH2)
indicating successful grafting of APTES onto the surface of
mont-K10. In the FTIR spectrum of imine@mont-K10, a new
band observed at 1640 cm−1 is assigned to νC=N stretching
of imine group which indicates the formation of Schif base
by condensation of amine group of APTES@mont-K10 and
C=O group of 2-quinoline carboxaldehyde. The FT-IR spec-
trum of Fe@imine-mont-K10 shows new bands at 512 cm−1
and 608 cm−1 which are assigned to bending vibrations of
Fe–O and Fe–N respectively, confrming that Fe is coordi-
nated through oxygen [66] and nitrogen [67] respectively
indicating anchoring of Fe(II) on imine@mont-K10 through
chemical bonding.
Fig. 1 XRD patterns of mont-K10 and Fe@imine-mont-K10
3.1.2 XRD Study
The powdered X-ray difraction pattern of mont-K10 and Fe@
imine-mont-K10 in the range 2θ=5° to 80° have shown in
Fig. 1. The variation in the peak of Fe@imine-mont-K10 from
that of mont-K10 indicates successful immobilization of Fe
on the mont-K10 surface. The sharp peaks at 26.26°, 34.74°
and 37.05° corresponds to the {110},{130} and {111} crystal
planes of Fe (+2) loading (19.9 mol %) in Fe@imine-mont-
K10 [68].
Table 1 BET surface area measurements of APTES@mont-K10,
imine@mont-K10 and Fe@imine-mont-K10
Serial No. Materials
Surface
area
Pore
Pore
volume
(cc/g)
diameter
(nm)
(m2/g)
1
2
3
APTES@mont-K10 21.448
imine@mont-K10 56.134
Fe@imine-mont-K10 63.953
0.059
0.225
0.249
1.756
11.418
11.748
3.2 Surface Area Measurement
From the ICP-AES analysis of the catalyst it was found that
it contains 19.9 mol % of Fe per 25 mg of the catalyst.
The SEM–EDX analysis of APTES@mont-K10 (Fig. 2a)
shows the signals for C, O, N and Si elements while that of
Fe@imine-mont-K10 (Fig. 2b) records signals of Fe con-
tent along with C, O, N and Si proportions suggesting that
the metal complex have been successfully grafted with the
anchored ligand.
The BET specific surface area and pore diameter of the
samples are shown in Table 1. The reported surface area of
mont-K10 is 19.0 m2/g. It has been observed that grafting of
APTES on the surface of mont-K10 increases the surface area
to 21.448 m2/g indicating increase in porosity of the frame-
work [69]. On further immobilization of imine on the surface
of APTES@mont-K10, its surface area increases making it a
good candidate for complexation of metal ion. Immobiliza-
tion of Fe on imine@mont-K10 further increases surface area
indicating enhanced catalytic activity [70].
3.3 X‑ray Photoelectron Spectroscopy (XPS)
Analysis
3.2.1 ESR, ICP‑AES and SEM–EDX Analysis
The XPS analysis for Fe 2p core of Fe@imine-mont-K10
is demonstrated in Fig. 3. It produces a broad doublet with
distinct peaks at binding energy 711.33 and 724.91 eV
The ESR spectrum of Fe@imine-mont-K10 shows only one
peak with g-value of 2.04 which indicates paramagnetism.
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Fig. 2 a SEM–EDX of APTES@mont-K10. b SEM–EDX of Fe@imine-mont-K10
corresponding to Fe 2p3/2 and Fe 2p1/2, respectively. Satel-
lite peaks at 713.9 and 729.15 eV are also observed. This
indicates the presence of Fe (+2) in the complex (C 1 s
with binding energy at 283.24 eV was used as standard)
[71].
The selected area electron diffraction (SAED) patterns
of imine@mont-K10 and Fe@imine-mont-K10 confirm
their crystalline nature.
3.5 29Si NMR Analysis
The 29Si NMR spectrum of APTES@mont-K10 (Fig. 5a) in
solid state showed three resonance peaks at about −111.403,
−99.806 and −93.214 ppm which can be assigned to 29Si
nuclei with four Si–O–Si linkages (Q4), three Si–O–Si
linkages and one –OH (Q3) and two Si–O–Si linkages and
two –OH (Q2), respectively. Moreover, two other weak
3.4 TEM Analysis
The morphology and size distribution patterns of
APTES@mont-K10, imine@mont-K10 and Fe@imine-
mont-K10 have been studied by TEM analyzer (Fig. 4).
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S. Sultana et al.
when catalysts such as CoCl2.6H2O:2- quinoline carbox-
aldehyde (1:1), NiCl2.6H2O: 2- quinoline carboxalde-
hyde (1:1) or FeSO4.7H2O:2-quinoline-carboxaldehyde
(1:1) were used (Table 2, Entry 10–12), only
FeSO4.7H2O:2-quinoline-carboxaldehyde (1:1) leads to
30% product formation (Table 2, Entry 12). This shows
that Fe is the best metal for this reaction instead of Co or
Ni. Therefore, in order to investigate the catalytic activity
of Fe@imine-mont-K10 for the hydrogenation of benzo-
phenone, solvents such as i-PrOH, HCOOH, THF, CH3CN
or mixtures of diferent solvents have been examined. Our
investigation showed that the catalyst is not active at room
temperature but at higher temperature (i.e., 80 °C and
90 °C) it gives best results (Table 2, Entry 17, 20 and 23).
When compared it is observed that the reaction condition
(Table 2, Entry 17) is greener than the other two (Table 2,
Entry 20 and 23) in terms of quantity of the catalyst and
temperature required. Hence 15 mg of the catalyst at 80 °C
is chosen as the best amount of catalyst for this reaction.
Since base plays an important role in hydrogenation of
benzophenone, we have examined the reaction with various
bases such as KOH, K-i-OPr, NaOH, Na-i-OPr, Na2CO3,
Et3N, NaHCO3 and K2CO3 (Table 3). On investigation it is
observed that 2 mmol of Na-i-OPr gives best result at 80º C
in 5 h (Table 3, Entry 5).
Fig. 3 XPS of Fe@imine-mont-K10
resonances at about −55.211 and −67.457 ppm which can
be attributed to RSi(OSi)(OH)2 and RSi(OSi)3, respectively
were also observed indicating successful grafting of APTES
onto the surface of mont-K10 through covalent bonding [65,
72]. In the 29Si NMR spectra of imine@mont-K10 (Fig. 5b)
and Fe@imine-mont-K10 (Fig. 5c) there are slight shift
in the peaks from − 111.403 to − 99.806 ppm (Fig. 5a)
to − 111.159 and − 100.539 (Fig. 5b), and − 112.258
and − 101.141 ppm (Fig. 5c) which indicates grafting of
imine on APTES@mont-K10 and metal complexed to the
anchored ligand, respectively.
On examining the reaction in various solvents such as
CH3CN, THF, CHCl3, DMF, DCM, DMSO, H2O, C2H5OH,
CH3OH, CH3COOH and CCl4 (Table 4), it has been found
that acetonitrile gives the best result in product formation
(Table 4, Entry 1).
In order to investigate the scope of this reaction we
have applied these optimized reaction conditions (i.e.
15 mg Fe@imine-mont-K10 as catalyst, 5 mL i-PrOH,
5 mL Na-i-OPr as the base and 5 mL acetonitrile as the
solvent at 80º C for 5 h) to various aliphatic and aromatic
ketones with electron-withdrawing or electron-donating
groups. The results are summarized in Table 5. It has
been observed that benzophenone and its derivative with
electron with-drawing group such as Br (Table 5, Entry 1
and 2), both cyclic and acyclic aliphatic ketones (Table 5,
Entry 3–7), acetophenone (Table 5, Entry 8) and its deriv-
atives with both electron withdrawing groups Cl and Br
(Table 5, Entry 9 and 10) and electron donating OH group
(Table 5, Entry 12) undergoes the reaction with highest
isolated yield (99%). But p-nitro acetophenone and 3-pen-
tanone gives only moderate isolated yield (Table 5, Entry
11 and 13). Hydrogenation of benzophenone is slower
than that of acetophenone (Table 5, Entry 1 and 8), which
may be due to increased steric hindrance to reduction due
3.6 Hydrogenation of Carbonyl Compounds
Catalyzed by Fe@imine‑mont‑K10
In order to determine the suitable reaction condition for
hydrogenation of carbonyl compounds we have chosen
benzophenone (1 mmol) as the starting substrate and
carried out the reaction under various conditions which
are summarized in Table 2. Initially, we have taken ben-
zophenone (1 mmol), i-PrOH (5 mL) as the H-donor,
KOH (2 mmol) as the base in absence of any catalyst
and stirred for 24 h which results no product formation
(Table 2, Entry 1). In the next reaction, we have intro-
duced the catalyst (Fe@imine-mont-K10, 19.9 mol %)
but in absence of base and refuxed for 24 h (Table 2,
Entry 2) which also results no product formation. This
indicates that both catalyst and base are essential in order
to carry out this reaction. It has been observed that in
presence of CH3CN/i-PrOH (1:1) and base (K-i-OPr)
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Fig. 4 TEM images of a APTES@mont-K10, b imine@mont-K10 ▸
and c Fe@imine-mont-K10 with SAED patterns inset
to two bulky phenyl rings in the benzophenone substrate
[73]. Introduction of electron withdrawing substituent
(Br) at the p-position of benzophenone (Table 5, Entry 2)
decreases the electron density of the C = O bond improv-
ing its activity making it easier for hydrogenation and
hence undergoes hydrogenation faster than that of benzo-
phenone. The catalyst exhibited higher activity to hydro-
genation of aliphatic ketones (Table 5, Entry 3–7) than
that of aromatic ketones except for 3-pentanone (Table 5,
Entry 13) [7, 73]. o-methyl cyclohexanone (Table 5, Entry
4) inspite of bearing a methyl substituent at o-position
requires same amount of time for hydrogenation as that
of cyclohexanone (Table 5, Entry 3) which was some-
what different from data reported earlier [73]. In case
of acetophenone with electron withdrawing substituents
such as Cl, Br and NO2 (Table 5, Entry 9–11) results
were slightly different; with Cl as substituent there is
no difference with acetophenone, however, in case of Br
and NO2, results were different, requiring longer reaction
time. Acetophenone with electron donating substituent
such as OH (Table 5, Entry 12) hydrogenation requires
more time than that of acetophenone (Table 5, Entry 10
and 11) [23].
After a comparative study on various Fe based catalysts
for hydrogenation of carbonyl compounds (Table 6) it was
observed that most of them require either high temperature
or inert atmosphere and aforded 99% highest isolated yield
and could be recycled up to 3–6 times. However, our catalyst
Fe@imine-mont-K10 gave 51–99% isolated yield at 80 °C
with at least 5 times recyclability. Hence, Table 6 shows
the superior efcacy of our catalyst over the other Fe based
catalysts till date.
3.7 Probable Mechanism
The plausible mechanism for hydrogenation of ketones [37,
52] using Fe@imine-mont-K10 catalyst is demonstrated
in the Scheme 2. Initially, isopropanolate ion and acetoni-
trile bind to the Fe center of the catalyst to form a six co-
ordinate Fe complex (I), followed by β-elimination giving
Fe hydride species (II). The addition of ketone (substrate)
undergoes 1,2-insertion giving the intermediate (III) which
results exchange of isopropanol yielding alcohol as the end
product and the intermediate (I), completing the catalytic
cycle.
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S. Sultana et al.
Fig. 5 29Si NMR spectra of a
APTES@mont-K10, b imine@
mont-K10 and c Fe@imine-
mont-K10
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Table 2 Optimization of the reaction conditions for hydrogenation of benzophenone using Fe@imine-mont-K10 catalyst
Entry
Catalyst (Amount in mg)
Solvent/H-donor
Base
Time (h)
Isolated yield (%)
1
2
3
4
5
6
7
8
–
i-PrOH
i-PrOH
KOH
–
KOH
K-i-OPr
NaOH
24
24
24
6
24
5
5
5
5
12
12
7
12
12
5
5
5
5
–
–
–
Fe@imine-mont-K10 (20)
Fe@imine-mont-K10 (15)
Fe@imine-mont-K10 (15)
Fe@imine-mont-K10 (15)
Fe@imine-mont-K10 (15)
Fe@imine-mont-K10 (10)
Fe@imine-mont-K10 (5)
Fe@imine-mont-K10 (20)
CoCl2.6H2O:2-quinoline-carboxaldehyde (1:1)
NiCl2.6H2O:2-quinoline-carboxaldehyde (1:1)
FeSO4. 7H2O:2-quinoline-carboxaldehyde (1:1)
Fe@imine-mont-K10 (20)
Fe@imine-mont-K10 (20)
Fe@imine-mont-K10 (15)
Fe@imine-mont-K10 (15)
Fe@imine-mont-K10 (15)
Fe@imine-mont-K10 (10)
Fe@imine-mont-K10 (5)
Fe@imine-mont-K10 (20)
Fe@imine-mont-K10 (20)
Fe@imine-mont-K10 (20)
Fe@imine-mont-K10 (20)
Mont-K10 (20)
a
CH3CN/i-PrOH (1:1)
CH3CN/i-PrOH (1:1)
CH3CN/i-PrOH (1:1)
CH3CN/i-PrOH (1:1)
CH3CN/i-PrOH (1:1)
CH3CN/i-PrOH (1:1)
CH3CN/i-PrOH (1:1)
CH3CN/i-PrOH (1:1)
CH3CN/i-PrOH (1:1)
CH3CN/i-PrOH (1:1)
CH3CN/i-PrOH (1:1)
CH3CN/i-PrOH (1:1)
THF/i-PrOH (1:1)
CH3CN/i-PrOH (1:1)
CH3CN/i-PrOH (1:1)
CH3CN/i-PrOH (1:1)
CH3CN/i-PrOH (1:1)
CH3CN/i-PrOH (1:1)
CH3CN/HCOOH (1:1)
CH3CN/HCOOH (1:1)
THF/i-PrOH (1:1)
Trace amounta
a
–
Na-i-OPr
Na-i-OPr
Na-i-OPr
Na-i-OPr
K-i-OPr
K-i-OPr
K-i-OPr
K-i-OPr
Na-i-OPr
K-i-OPr
K-i-OPr
Na-i-OPr
Na-i-OPr
Na-i-OPr
Na-i-OPr
KOH
90a
55a
30a
92a
9
b
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
–
b
–
30b
–
–
40b
45b
99b
83b
32b
99b
5
5
5
5
5
12
7
b
–
b
NaOH
–
K-i-OPr
Na-i-OPr
Na-i-OPr
99c
c
CH3CN/i-PrOH (1:1)
CH3CN/i-PrOH (1:1)
–
Fe@mont-K10
33b
Reaction condition: Benzophenone (1 mmol), H-donor (5 mL), solvent (5 mL), base (2 mmol or 5 mL Na-i-OPr/K-i-OPr which were prepared by
refuxing 2 mmol of base in 20 mL i-PrOH), catalyst (Fe@imine-mont-K10, 19.9 mol % of Fe per 25 mg), r.t=24 °C in air unless otherwise noted
aAt 70 °C
bAt 80 °C
cAt 90 °C
mixture was stirred at 80º C. After 1 h, the reaction was
stopped and the catalyst was fltered of and the product was
analyzed by GC–MS (40% conversion). The fltrate was
allowed to continue the reaction under same condition with-
out the solid catalyst for more 4 h. Substrate conversion in
the fltrate was studied (GC–MS analysis) and found a little
increase in the reaction products which shows the possibility
of semi-heterogeneous catalysis (Fig.S6). From ICP-AES
3.8 Catalyst Leaching and Reusability
3.8.1 Hot Filtration Test
In order to check the heterogeneity of the catalyst, we
have carried out a hot fltration test using benzophenone
(1 mmol), Na-i-OPr (5 mL), Fe@imine-mont-K10 (15 mg)
in acetonitrile (5 mL), i-PrOH (5 mL) and the reaction
1 3

2152
S. Sultana et al.
Table 3 Optimization of base for hydrogenation of benzophenone
Entry
Base
Isolated
yield
(%)
1
2
3
4
5
6
7
8
9
KOH
K-i-OPr
NaOH
Na-i-OPr
Na-i-OPr
Na2CO3
Et3N
NaHCO3
K2CO3
–
50a
–
92
99a
–
–
–
–
Reaction condition: Benzophenone (1 mmol), H-donor (i-PrOH, 5 mL), solvent (CH3CN, 5 mL), base (1 mmol)
a(2 mmol), catalyst (Fe@imine-mont-K10, 19.9 mol% of Fe per 25 mg, 15 mg), Temperature (80 °C)
Table 4 Solvent screening for hydrogenation of benzophenone
Entry
Solvent
Isolated
yield
(%)
1
CH3CN
THF
CHCl3
DMF
99
70
–
2
3
4
–
5
6
7
DCM
DMSO
H2O
50
–
–
8
9
10
11
C2H5OH
CH3OH
CH3COOH
CCl4
–
–
–
–
Reaction condition: Benzophenone (1 mmol), i-PrOH (5 mL), Na-i-OPr (5 mL), solvent (5 mL), catalyst (Fe@imine-mont-K10, 19.9 mol% of
Fe per 25 mg, 15 mg), Temperature (80 °C)
1 3

Mont‑K10 Supported Fe(II) Schif‑Base Complex as an Efcient Catalyst for Hydrogenation…
Table 5 Hydrogenation of various substrates using Fe@imine-mont-K10 catalyst
2153
Entry
1
Substrate
Time (h)
5
Isolated yield (%)
99
TON
8.29
2
2
99
8.29
3
4
5
1
99
99
99
8.29
8.29
8.29
1
0.5
6
7
8
1
2
3
99
99
99
8.29
8.29
8.29
9
3
99
8.29
1 3

2154
S. Sultana et al.
Table 5 (continued)
Entry
10
Substrate
Time (h)
4
Isolated yield (%)
99
TON
8.29
11
5
57
4.77
12
13
5
5
99
51
8.29
4.27
Reaction condition: Ketone (1 mmol), i-PrOH (5 mL), solvent (CH3CN, 5 mL), Na-i-OPr (5 mL), Fe@imine-mont-K10 (15 mg, 19.9 mol % of
Fe per 25 mg) at 80 °C in air. Formation of products were confrmed by FTIR, 1H-NMR and GC–MS analysis and then compared authentically
TON Turn over number
analysis of the fltrate, a little amount of iron (331.24 ppm)
was detected which suggests some leaching of Fe content
during the reaction process confrming the semi-heteroge-
neous nature of the catalyst.
from the catalyst during catalysis (Fig. S7). Moreover, ICP-
AES analysis of the fltrate after the 5th cycle shows 0.91 ppm
Fe content in the fltrate indicating semi-heterogeneous nature
of the catalyst.
3.8.2 Recycling
4 Conclusion
Though for heterogeneous catalyst high catalytic activity is
important but its stability is one of the most important con-
cerns. In order to study the stability, recyclability of the cata-
lyst was investigated by using benzophenone as the model sub-
strate. It was found that the catalyst could be reused up to 5th
cycle without major loss in activity. As the reaction completes
the catalyst was separated from the reaction mixture by centrif-
ugation. After washing with water and drying, the catalyst was
used for the next cycle and so on. The slight decrease in prod-
uct yield on the next run may be due to some metal leaching
In summary, we have successfully synthesized a new semi-
heterogeneous Fe(II)-Schif base complex by immobilizing
on mont-K10 as support. It was extensively characterized
by various spectroscopic techniques and found to be crys-
talline and paramagnetic. It acts as an efcient, reusable
and semi-heterogeneous catalyst for hydrogenation of
aromatic as well as aliphatic ketones under mild reaction
condition in i-PrOH/CH3CN.
1 3

Mont‑K10 Supported Fe(II) Schif‑Base Complex as an Efcient Catalyst for Hydrogenation…
2155
1 3

2156
S. Sultana et al.
Scheme 2 Plausible mechanism
for hydrogenation of ketones
Acknowledgements The authors thank SAIF, IIT Bombay for ESR
Compliance with Ethical Standards
1
and ICP-AES facilities, SAIF, NEHU, Shillong for TEM and H-
NMR facilities, SAIF, IISC Bangalore for 29Si NMR facilities, SAIF,
Conflict of interest The authors declare that there is no confict of in-
terests regarding the publication of this article.
1
STIC, Kochi University, Kochi for H-NMR facilities, ACMS, IIT
Kanpur for X-ray photoelectron spectroscopic facilities and BIT,
Bangalore for BET surface area measurement facilities. S. Sul-
tana gratefully thanks UGC-SAP-DRS-I programme (2016-2021)
and UGC-Maulana Azad National Fellowship, Delhi for fnancial
support.
1 3

Mont‑K10 Supported Fe(II) Schif‑Base Complex as an Efcient Catalyst for Hydrogenation…
2157
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