Synthesis of structurally enhanced magnetite cored poly(propyleneimine) dendrimer nanohybrid material and evaluation of its functionality in sustainable catalysis of condensation reactions

Article abstract of DOI:10.1016/j.reactfunctpolym.2020.104579

The conventional method of dendrimer synthesis especially poly(propyleneimine) dendrimer (PPI) involves complicated reaction workup and tedious separation strategies. Since it has greater number of nitrogen atom in their structure, its applications are in

Full text of DOI:10.1016/j.reactfunctpolym.2020.104579

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Synthesis of structurally enhanced magnetite cored
poly(propyleneimine) dendrimer nanohybrid material and
evaluation of its functionality in sustainable catalysis of
condensation reactions
Kannappan Lakshmi, Rajmohan Rangasamy
PII:
S1381-5148(19)31393-8
DOI:
https://doi.org/10.1016/j.reactfunctpolym.2020.104579
REACT 104579
Reference:
To appear in:
Reactive and Functional Polymers
Received date:
Revised date:
Accepted date:
26 December 2019
16 March 2020
30 March 2020
Please cite this article as: K. Lakshmi and R. Rangasamy, Synthesis of structurally
enhanced magnetite cored poly(propyleneimine) dendrimer nanohybrid material and
evaluation of its functionality in sustainable catalysis of condensation reactions, Reactive
and Functional Polymers (2019), https://doi.org/10.1016/j.reactfunctpolym.2020.104579
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Synthesis of structurally enhanced magnetite cored poly(propyleneimine)
dendrimer nanohybrid material and evaluation of its functionality in
sustainable catalysis of condensation reactions
a
a *
Kannappan Lakshmi , Rajmohan Rangasamy
a Department of Chemistry, Guru Nanak College(autonomous) , Affiliated to University of
Madras, Velachery, Tamil Nadu, India
Abstract: The conventional method of dendrimer synthesis, especially poly(propyleneimine)
dendrimer (PPI) involves complicated reaction workup and tedious separation strategies.
Since it has greater number of nitrogen atom in the structure, it has numerable applications in
different fields. But these dendrimers cannot be recycled for subsequent applications. In order
to overcome these issues, for the first time, divergent synthesis of poly(propyleneimine)
dendrimer G3 on silica coated magnetite has been achieved with structurally enhanced PPI
dendrimer on core-shell type magnetic silica nanostructure. This nanohybrid material was
characterized by using HRTEM, FTIR, XRD, TGA, XPS and VSM studies. Further, nitrogen
concentration of PPI dendrimer was quantified by conductometric titration and Prussian blue
complex method. The probing of catalytic efficacy of structurally enhanced magnetic cored
poly(propyleneimine) dendrimer G3 as magnetically recyclable heterogeneous organic base
catalyst was employed for representative reactions such as Knoevenagel condensation,
Knoevenagel- Michael cyclocondensation and Henry reaction at ambient condition. The
catalyst was reused for minimum of eight cycles without significant loss of its catalytic
activity.
Keywords: magnetic core, PPI dendrimer, nanohybrid, sustainable catalyst, magnetically
recoverable
1. Introduction
Dendrimers are globular macromolecules with highly branched structure and rich peripheral
functional groups which has unique applications in medicine [1], catalysis [2], sensors [3]
etc. Though dendrimers serve as catalyst in homogenous state with prominent selectivity and
catalytic activity, its recoverability is still a challenging task for the chemist [4]. Thus, efforts
had been made to improve the recoverability of the dendrimer catalyst using solid supports

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namely mesoporous silica [5], polymer [6,7], carbon [8], metals [9], metal oxides [10,11] and
magnetic nanoparticles [12] etc. Among the various solid support, superparamagnetic iron
oxide (Fe3O4) i.e. magnetite nanoparticle is widely used as a support material due to its
specific features like low toxicity, negligible magnetic remanence, high magnetic saturation,
minimal aggregation and above all excellent magnetic behavior only in the presence of
external magnetic field [13–15]. These magnetic nanoparticle supported dendrimer make a
juncture between homogenous and heterogenous catalyst and serve as semiheterogeneous
catalyst [16–22]. In recent years, dendrimers anchored on magnetic iron oxide nanoparticle
had drawn greater attention due to facile greener and sustainable recovery by magnetic
separation method [23]. Alper et al., were the first to report the synthesis of magnetic iron
oxide nanoparticle
cored poly(amidoamine) (PAMAM) dendrimer with noble metal
nanoparticles for selective hydroformylation and hydrogenation catalysis [24]. There are
numerous reports available for the synthesis of PAMAM dendrimer on magnetic iron oxide
nanoparticle through covalent bonding, but these dendrimers remains unsuitable for catalysis
under certain conditions [10,25–27].
Indeed, PPI dendrimer is superior to PAMAM dendrimer due to the availability of
more functional nitrogen even in lower generations, greater void space, smaller size, good
biocompatibility and more loading capacity [28] etc. PPI dendrimer is exclusively chosen for
this study as it contains a greater number of amine groups, which can be utilized for base
catalysis, better void space to host the guest molecules and easily tunable amine groups. So
far intrinsically PPI dendrimers grafted onto magnetite nanoparticle and utilized for various
applications [29,30]. Herein for the first time we report the divergent synthesis of PPI
dendrimer G3 on silica coated magnetite core with enhanced dendritic structure with
retainment of magnetic behavior for heterogeneous catalysis. Base catalysed condensation
reactions using active methylene compounds are most useful due to the formation of new
carbon bond. The materials such as pyridine, piperidine, NaOH, K2CO3, triethylamine, amino
functionalized mesoporous silica, Fe3O4@SiO2- pyridine, polyvinylamine coated magnetic
microspheres, Nano DAB-PPI G1 etc., have been used so far in the catalysis of various
condensation reactions [31–36]. Unfortunately all are having their own demerits. As far as
PPI dendrimer G3 on silica coated magnetite core material is concerned, it may overcome the
above demerits associated with the reported bases.
The formation of PPI dendritic structure with magnetic core is well established in this
work and also the structural functions of the nanohybrid material is proved by the probe

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reactions namely Knoevenagel condensation, Knoevenagel- Michael reaction and Henry
reaction even in aqueous phase under ambient condition.
2. Experimental Section
2.1. Reagents and Materials
All the reagents and solvents were purchased from commercial sources. Ferrous sulphate
heptahydrate (98%), anhydrous ferric chloride (98%), acrylonitrile (99%), sodium metal was
purchased from Spectrochem. Oleic acid, Polyvinylpyrrolidone (Mol. Wt. 40,000 Da) were
purchased from LOBA. Tetraethoxysilane (TEOS, 98%), 3-aminopropyltriethoxysilane
(APTES, 98%) were purchased from Alfa Aesar. Lithium aluminum hydride (LAH, powder
reagent, 95%) was purchased from Sigma Aldrich. Solvents such as methanol, ethanol, THF,
toluene (HPLC grade), DCM, DMF were purchased from Fischer, THF and DCM were
distilled and used. Deionized water has been used for the reactions wherever is applicable.
All the reactions were carried out using Remi digital overhead stirrer with digital rpm
controller.
2.2. Synthesis of Oleated Magnetite Nanoparticles
Iron oxide nanoparticles viz., magnetite were prepared via co-precipitation method. The
precursors FeSO4 .7H2O (4.7 g, 16.9 mmol) and FeCl3 (5.5 g, 33.8 mmol) in the ratio 2:1
were dissolved in deionized water and stirred vigorously for 20 min. Then 12.5 mL of 25 %
ammonium hydroxide was added rapidly with continuous stirring at room temperature under
nitrogen atmosphere. The black Fe3O4 nanoparticles was obtained, to this black suspension, 2
o
mL of oleic acid was added dropwise for 1 h with continuous stirring at 80 C. After complete
addition, oleated iron oxide nanoparticles were extracted into toluene by adding sodium
chloride. The magnetite nanoparticles were separated by magnetic attraction using external
magnet and the supernatant solution was decanted. Magnetically separated nanoparticles were
washed thrice with deionized water, acetone and dried in vacuum for 24 h. The dried oleated
Fe3O4 nanoparticles was suspended in toluene for storage.

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2.3. Synthesis of Silica Coated Fe3O4 Nanoparticles
About 75 mL (4 g) of oleated Fe3O4 nanoparticles suspended in toluene was taken in a 1 L
three-necked RB flask. 10 g of PVP dissolved in a mixture of 200 mL DCM and 200 mL
o
DMF was added to the iron oxide suspension and then heated at 95 C for about 15 h. Then,
PVP stabilized iron oxide nanoparticles were precipitated out by dropwise addition of
resultant suspension into diethyl ether, followed by magnetic isolation of PVP coated
magnetite nanoparticles. Finally, it was redispersed in ethanol and 1 mL of the suspension
contains about 0.1 g of PVP coated Fe3O4 nanoparticle.
3 g PVP coated iron oxide nanoparticles were dispersed in 2 L of isopropanol
containing 100 mL of 28 % of ammonium hydroxide solution. Then it was sonicated for 1 h
followed by dropwise addition of TEOS for a period of 3 h under vigorous stirring. The
stirring was continued for another 20 h at room temperature. Finally, silica coated iron oxide
nanoparticles were magnetically separated, washed five times with deionized water and dried
in vacuum for 24 h. 4.5 g of dry dark yellow colored silica coated iron oxide powder was
obtained.
2.4. Synthesis of Amino Silanated Silica Coated Fe3O4Nanoparticles
4 g of silica coated iron oxide powder was suspended in 150 mL of dry toluene and sonicated
for 45 min. To this suspension, 1.2 mL of APTES was added and stirred continuously for 20
0
h at 100 C. The reaction mixture was cooled and the amino silane coated iron oxide
nanoparticles were separated using an external magnet, washed thrice with methanol and
dried in vacuum overnight. The silanated silica coated iron oxide is labelled as
Fe3O4/SiO2/PPI-G0 nanoparticles, which contain approximately 1 mmol/g of amino groups
(determined by pre-validated conductometric titration and pre validated Lassaigne’s test
methods).
2.5. General Procedure for the Synthesis of Magnetite Cored Cyano group Terminated
Dendrimer
3 g of Fe3O4/SiO2/PPI-G0 nanoparticle was dispersed in 100 mL of methanol and sonicated
for 30 min. To this suspensions, 16 mL of acrylonitrile (100 mol equivalent per amino group),
2 mL of acetic acid (10 mol equivalent per amino group) were added and mechanically stirred
0
for 40 h at 50 C. The product Fe3O4/SiO2/PPI-G0.5 was isolated by using an external magnet,

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washed thrice with deionized water, methanol and dried in vacuum for 24 h. Similarly,
Fe3O4/SiO2/PPI-G1.5, Fe3O4/SiO2/PPI-G2.5 were prepared from Fe3O4/SiO2/PPI-G1,
Fe3O4/SiO2/PPI-G2 respectively, using the same procedure.
Fe3O4/SiO2/PPI-G0.5: 3 g of Fe3O4/SiO2/ PPI-G0 was dispersed in 100 mL of methanol, 16 mL
of acrylonitrile and 2 mL of acetic acid
Fe3O4/SiO2/PPI-G1.5: 3 g of Fe3O4/SiO2/ PPI-G1 was dispersed in 100 mL of methanol, 32 mL
of acrylonitrile and 4 mL of acetic acid
Fe3O4/SiO2/PPI-G2.5: 3 g of Fe3O4/SiO2/ PPI-G2 was dispersed in 100 mL of methanol, 64 mL
of acrylonitrile and 4 mL of acetic acid
2.6. General Procedure for the Synthesis of Magnetite Cored Amino Group Terminated
Dendrimer
3 g of magnetite cored cyano group terminated nanohybrid material Fe3O4/SiO2/PPI-G0.5 were
dispersed using sonication bath in 100 mL THF taken in a three necked RB flask fitted with
water condenser and dropping funnel. 114 mg of LAH in dry THF (twice the mole
0
equivalence per amino group) was added dropwise for 1 h under continuous stirring at 0 C.
0
After complete addition of LAH, stirring was continued for 15 h at 60 C. The resulting
nanohybrid material Fe3O4/SiO2/PPI-G1 was separated by an external magnet, excess LAH
was decomposed by the addition of ethyl acetate, washed thrice with THF, deionized water,
dil. HCl, deionized water, dil. NaOH, deionized water, methanol, acetone and dried in
vacuum for 24 h. Fe3O4/SiO2/PPI-G2, Fe3O4/SiO2/PPI-G3 were prepared from Fe3O4/SiO2/PPI-
G1.5 and Fe3O4/SiO2/PPI-G2.5 respectively, using the same procedure.
Fe3O4/SiO2/PPI-G1:3 g of Fe3O4/SiO2/PPI-G0.5 was dispersed in 100 mL of THF and 114 mg
of LAH in THF was added dropwise.
Fe3O4/SiO2/PPI-G2: 3 g of Fe3O4/SiO2/PPI-G1.5 was dispersed in 100 mL of THF and 230 mg
of LAH in THF was added dropwise.
Fe3O4/SiO2/PPI-G3: 3 g of Fe3O4/SiO2/PPI-G2.5 was dispersed in 100 mL of THF and 460 mg
of LAH in THF was added drop wise.

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2.7. General Procedure for Quantitative Analysis of Amino Group
2.7.1. Conductometric Titration Method
Exactly 10 mg of the Fe3O4/SiO2/PPI-G3 nanohybrid material was dispersed in 20 mL of
standardized hydrochloric acid (1 mmol) and stirred mechanically for 30 min. After stirring,
the nanomaterial was separated by magnet and 5 mL of supernatant solution was titrated
against standardized sodium hydroxide solution, conductance was measured for each 0.2 mL
addition of NaOH solution. The HCl consumed by the magnetite cored PPI dendrimer
nanomaterial is equivalent to the concentration of amine groups present in the dendritic
moiety. This experiment was carried for all the three nanohybrid materials namely
Fe3O4/SiO2/PPI-G0, G1, G2 and the amount of nitrogen present in the dendrimers have been
estimated.
2.7.2. Lassaigne’s Prussian Blue Complex Method
25 mg of the dried Fe3O4/SiO2/PPI-G3 nanohybrid material was added to fused sodium metal
and heated to red hot condition, the mixture was cooled and 5 mL of deionized water was
added and then boiled. The sodium fusion extract was cooled and freshly prepared ferrous
sulphate solution was added in excess, boiled and cooled. To this solution conc. sulphuric
acid was added, resulting Prussian blue complex was diluted to 50 mL in standard volumetric
flask and the absorbance was recorded through UV-Vis spectrophotometer. This experiment
has been carried out individually for all the three magnetite cored dendrimers
2.8. General Procedure for Catalysis
10 mg of Fe3O4/SiO2/PPI-G3 catalyst was dispersed in 5 mL ethanol and sonicated for 30 s, 1
mmol of aldehyde, 1 mmol of active methylene compound were added to the above
suspension and stirred at room temperature under nitrogen atmosphere. The completion of
condensation reaction was monitored using TLC with hexane and ethyl acetate as eluent
(4:1). After completion of the reaction, the product was decanted by arresting the catalyst
with external magnet. The obtained product was recrystallised and the yields were noted.
Similarly, the condensation reactions have also been established in aqueous medium and the
yields were compared.
3. Results and Discussion

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Rational design for divergent synthesis of PPI G3 dendrimer on silica coated
superparamagnetic iron oxide nanoparticles is discussed (scheme 1). Initially Fe3O4 was
prepared by conventional coprecipitation method, subsequently controlled silica coating was
carried out on Fe3O4 nanoparticle for better protection and to achieve spherical morphology
with sustainable magnetic retentivity. It was further functionalized with APTES to develop
dendrimer on it. The amino silanated material was labeled as Fe3O4/SiO2/PPI-G0 nanohybrid
material with propylamine surface groups. Subsequently, Fe3O4/SiO2/PPI-G1 was synthesized
in two steps namely Michael addition of acrylonitrile to the amine group using acetic acid as
catalyst, followed by reduction of the nitrile group in to amine group with LAH [37]. The
amount of amine group present in Fe3O4/SiO2/PPI-G0, G1, G2, G3 were determined by
conductometric titration method and Lassaigne’s Prussian blue complex test.

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Scheme 1. Synthesis of magnetite cored poly(propylene imine) dendrimer.
HRTEM images of the Fe3O4/SiO2/PPI-G3 dendrimer nanoparticles at different
resolutions clearly indicates the formation of spherical shaped single core shell type silica
coated iron oxide nanoparticles (Fig.1A and 1B). The particle size obtained from HRTEM
images of magnetic cored PPI-G3 dendrimer shows the size distribution of 10 ± 4 nm for
Fe3O4 core and 25±5 nm for silica coated Fe3O4 nanoparticles. The Dynamic Light Scattering
analysis in the Zeta analyzer was carried out in ethanol, the average particle size of the
materials was found to be 71 nm and 105 nm for Fe3O4/SiO2 and Fe3O4/SiO2/PPI-G3
respectively (Fig. S1.). The hydrodynamic size of Fe3O4/SiO2/PPI-G3 was found to be greater
than Fe3O4/SiO2, which attributes the formation of dendritic entities on silica coated iron
oxide nanoparticles.

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Fig. 1. (A) HRTEM images of Fe3O4/SiO2/PPI-G3 nanohybrid material (B) Core- shell type
silica coated iron oxide nanoparticle.
X-ray Photoelectron spectroscopy for the Fe3O4/SiO2/PPI-G3 confirms the presence of
elements viz., iron, silicon, carbon and nitrogen through the peaks obtained from the wide
spectra with binding energies 751eV, 105 eV, 397 eV and 285 eV for respective elements.
The ratio of peak area for carbon to nitrogen was observed to be 3:7:1 which is closer to the
theoretical value of PPI dendrimer. Si 2p, C1s, N 2p3/2 spectra were also confirmed
(Fig. S2 and S3). The crystal structure and phase composition of the synthesized
nanoparticles were investigated through X-Ray diffraction method (Fig. 2), the XRD patterns
for the oleated superparamagnetic Fe3O4 nanoparticles matched well with the patterns of
magnetite (inverse Spinel cubic structure) obtained from JCPDS data. XRD data showed
sharp peaks at 2 values of 30.250, 35.660, 45.340, 57.370, 62.70 which corresponds to the
lattice planes (220), (311), (400), (511), (440) respectively for magnetite nanoparticle. The
lattice parameters of the superparamagnetic iron oxide nanoparticle remained unaltered
throughout the divergent synthesis of PPI dendrimer on to it. The crystallite size of Fe3O4
nanoparticles has been determined using Scherrer equation and it was found to be 12±1 nm
which shows good agreement with the size obtained from HRTEM images.

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Fig. 2. X-Ray diffraction pattern of (a) oleated iron oxide (b) Fe3O4/SiO2 (c) Fe3O4/SiO2/PPI-
G0 (d) Fe3O4/SiO2/PPI-G1 (e) Fe3O4/SiO2/PPI-G3
Superparamagnetic behavior of nanohybid material studied at room temperature using VSM
analysis proves that iron oxide nanoparticle and the dendrimer grafted silica coated iron oxide
nanomaterials behaves as super paramagnets. They exist with negligible magnetic retention
and attains magnetic saturation when high magnetic field was applied in the order of 14 KOe.
The saturation magnetization for oleated iron oxide, Fe3O4/SiO2 and Fe3O4/SiO2/PPI-G3 were
53.292 emu/g, 15.932 emu/g and 12.797 emu/g respectively. The decrease in the
magnetization values from 53.292 emu/g to 15.932 emu/g (Fig. 3a and 3b) incorporates the
occurrence of silica shell with sufficient thickness and subsequently slight decrease from
15.932 emu/g to 12.797 emu/g (Fig. 3b and 3c) confirms the presence of PPI dendritic layer
on the magnetic cored silica surface; these observation authenticates the presence of silica
shell as well as PPI dendrimer layer on the Fe3O4 nanoparticle.

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Fig. 3.VSM curve at RT for (a) oleated Fe3O4 (b) Fe3O4/SiO2 (c) Fe3O4/SiO2/PPI-G3
Thermogravimetric analysis strongly proves the stepwise growth of dendrimer on the silica
0
coated iron oxide nanoparticles (Fig. 4 and S4). The weight loss around 100 C is due to the
0
0
release of the adsorbed water, weight loss between 100 C - 500 C is attributed to
decomposition of the organic matrix namely PPI dendrimer, interstitial water molecule
0
present in the nanohybrid material and above 500 C should be due to the decomposition of
silica shell. Quantitative weight loss was observed for Fe3O4/SiO2/PPI-G0, Fe3O4/SiO2/PPI-G1,
0
0
Fe3O4/SiO2/PPI-G2 and Fe3O4/SiO2/PPI-G3 between 100 C -500 C were 6.7%, 9.5%, 11.3%
and 16.9% respectively. These observations conclude the growth of poly (propylene imine)
onto the magnetite cored silica particles. The difference in weight loss indicated by curve A,
B, C, D (Fig. 4) corresponds to the stepwise formation of dendritic entity onto Fe3O4/SiO2
nanoparticles

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Fig. 4. TGA curves of (A) Fe3O4/SiO2/PPI-G0 (B) Fe3O4/SiO2/PPI-G1 (C) Fe3O4/SiO2/PPI-G2
(D) Fe3O4/SiO2/PPI-G3
FTIR spectral analysis results are presented in Fig. 5, where the spectrum a designates iron
-1
oxide nanoparticle with very strong band at 580 cm for Fe-O-Fe and absorption band at
-1
3450 cm corresponds to the peripheral -OH stretching of iron oxide nanoparticle. Further,
-1
the intensity of Fe-O-Fe band at 580 cm decreases with respect to the stepwise modification
of Fe3O4 with PVP, silica and PPI dendrimer (5b-5i). Similarly, appearance of new
-1
absorption band at 1080 cm confirms the formation of silica shell around the magnetite core
and a broad intensive band at 3450 cm-1 indicates -OH bond stretching due to peripheral -OH
on the silica shell. A small decrease in the band intensity of Si-O- Si from 5c to 5i represents
the growth of the dendrimer which is shielding the silica coating. Absorption peak around
-1
1640 cm increase from 5c to 5i because of C-N bond stretching frequencies. A shift of the
-1
broad absorption band from 3450 cm-1 to 3420 cm was observed from 5d to 5i, which
confirms the growth of dendrimer with enriched peripheral amino groups.

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Fig. 5. FTIR spectra of (a) Oleated Fe3O4 (b) PVP coated Fe3O4 (c) Fe3O4/SiO2 (d)
Fe3O4/SiO2/PPI-G0 (e) Fe3O4/SiO2/PPI-G0.5 (f) Fe3O4/SiO2/PPI-G1 (g) Fe3O4/SiO2/PPI-G1.5
(h) Fe3O4/SiO2/PPI-G2 (i) Fe3O4/SiO2/PPI-G3
The persistence of dendrimer onto the silica coated magnetite has been proved by
conductance measurement and Prussian blue complex formation. Prussian blue complex
formation method was adapted to justify the generation wise increase of nitrogen from PPI-G1
to PPI-G3. Prussian blue complex was prepared from Fe3O4/SiO2/PPI-G1, Fe3O4/SiO2/PPI-G2,
Fe3O4/SiO2/PPI-G3 individually by taking 25 mg of each material and prepared under
identical experimental conditions. UV-Vis absorption spectra were recorded for each solution
and the observed data proves the increase in amount of nitrogen from PPI-G1 to PPI- G3. The
absorption intensity at 735 nm for Prussian blue complex obtained from Fe3O4/SiO2/PPI-G1,

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Fe3O4/SiO2/PPI-G2, Fe3O4/SiO2/PPI-G3 manifestly shows the increase in nitrogen content
with respect to generation (Fig. 6, Table 1)
Similarly, the formation of dendrimer on the silica coated iron oxide
nanoparticle has also been proved by conductometric titrations for stepwise increase in amino
group concentration (Table 1). As described in the experimental part, fixed concentration of
HCl was mixed with 10 mg of Fe3O4/SiO2/PPI-G0, Fe3O4/SiO2/PPI-G1, Fe3O4/SiO2/PPI-G2 and
Fe3O4/SiO2/PPI-G3 individually under mechanical stirring for 30 min and finally the
nanohybrid materials were arrested by using external magnet. Subsequently, the
representative volume of supernatant solutions was titrated against standardized sodium
hydroxide solution using conductometric method. The result evidences the increasing
concentration of nitrogen from PPI-G0 to PPI-G3 with respect to dendritic growth.
Fig. 6. UV-Vis absorption spectra of Prussian blue complex obtained for (A) Fe3O4/SiO2/PPI-
G1 (B) Fe3O4/SiO2/PPI-G2 (C) Fe3O4/SiO2/PPI-G3

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Table 1. Amount of amine groups determined by conductometric titrations and Lassaigne’s
Tests
Amount of amine group (mmol/g)
Magnetic cored PPI dendritic
Conductometric
titration method
Lassaigne’s
Prussian
Nanohybrid material
blue complex test
Fe3O4/SiO2/PPI-G0
1.32
2.14
3.11
4.86
1.28
1.92
2.85
4.63
Fe3O4/SiO2/PPI-G1
Fe3O4/SiO2/PPI-G2
Fe3O4/SiO2/PPI-G3
The current approach (divergent strategy) comprises the synthesis of dendrimer from
the magnetic core to which the arms (dendron) are built by step-wise manner. The above
process lead the magnetic cored dendrimer which is structurally perfect, symmetrical, well
defined, thermally stable, highly dispersed with stochiometric functional atoms as designed.
The magnetic cored stepwise synthesis of PPI dendrimer eliminates the complicated reaction
workup and tedious isolation and purification process. Since, the core is superparamagnetic
material, the reaction intermediates and the products are isolated by using external magnet.
Similarly we can remove the absorbed impurities by washing with suitable solvents and the
final material can be recovered magnetically without any loss.
In the case of regular synthesis of PPI dendrimer in homogeneous medium require
tedious work set up coupled with complicated purification techniques at each step. The
separation and purification techniques normally adopted are nanofiltration, gel permeation,
column chromatography, centrifugation etc that makes the material (PPI dendrimer) to higher
cost. This dendrimer is used to attach with magnetic support. The conventional attachment of
commercial PPI dendrimer on magnetic core may not yield symmetrical product and also the
steric factor will decide the extent of grafting and ultimately we may not get desired magnetic
nanoparticle supported PPI dendrimer material as expected. These factors will also increase
the cost of material to higher side. If we adopt the method reported in this work, it will
resolve all the issues pertaining with conventional grafting of dendrimer on magnetic

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material. Divergent synthesis approach of magnetic cored PPI dendrimer is generally a
commercial viable method for industrial application via simple method of preparation,
consumes less work and economic.
3.1. Catalysis of condensation reacting using nanohybrid materials
The catalytic efficiency of the synthesized Fe3O4/SiO2/PPI-G3 nanohybrid material was
examined for Knoevenagel condensation between benzaldehyde and malononitrile in ethanol
at room temperature to yield the desired product 2-benzylidene malononitrile as a model
reaction (Scheme 2). It was observed that 10 mg (5 mol%) of Fe3O4/SiO2/PPI-G3 catalyst is
sufficient to accelerate the Knoevenagel condensation reaction (Table 2). Knoevenagel
condensation reaction was also employed to ascertain the comparative efficiency of other
nanohybrid materials viz., Fe3O4/SiO2/PPI-G0 (10 mg, i.e. 1 mol%; calculated from
conductometric measurement and Prussian blue complex method), Fe3O4/SiO2/PPI-G1 (10
mg, 2 mol%) & Fe3O4/SiO2/PPI-G2 (10 mg, 3 mol%), in ethanol at room temperature to yield
the desired product with variations in reaction time (Table 3, entry 1). Results shows that
Fe3O4/SiO2/PPI-G3 with more amino group accelerates the reaction to rapidly and enhances
the product yield in shorter time compared with other nanohybrid material catalyst. The better
void space available at the dendritic entity to host the guest molecules, presence of a greater
number of nitrogen atom in the single molecular entity leads multivalent interaction with the
substrate and fine dispersion of dendritic nanohybrid material in the reaction medium
enhanced the catalytic activity drastically. The condensation reactions were also carried out
with different aldehydes such as aromatic, aliphatic, heterocyclic etc., which favorably yields
the corresponding products (entry 1-6). Knoevenagel condensation in aqueous medium also
gave appreciable amount of the product with quite increase in reaction time.
Scheme 2. Knoevenagel Condensation
Table 2. Optimization of catalyst concentration in Knoeveagel condensationa
S.No. Amount of Catalyst (mg) Time (min) Yieldb %
1
Without catalyst
360
NR

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2
3
4
5
6
3
5
120
60
45
30
45
40
55
75
95
95
7
10
10
7
8
17
17
30
45
95
95
a benzaldehyde (1 mmol), malononitrile (1 mmol), ethanol (5 mL), Catalyst: Fe3O4/SiO2/PPI-G3
b Isolated yield of the product 2-benzylidenemalononitrile after recrystallisation
From entry 1 (table 1), without catalyst no condensation reaction has been taken place.
At the same time, amount of catalyst is influencing the reaction and it has been optimized as
given in the Table 2. It was observed that 10 mg (5 mol% with respect to amine
concentration) of Fe3O4/SiO2/PPI-G3 catalyst is sufficient to accelerate the Knoevenagel
condensation reaction at ambient condition (entry 5).
a
Table 3 Catalytic efficiency of the magnetic cored PPI for various base catalyzed reactions
Time
min
Yieldc
Substrate
Product
Catalyst
Entry
%
A
B
G0
G1
G2
G3
G3
150
90
90
93
95
95
82*
90
1
45
30
60*
30
G3
G3
45*
35
82*
94
2
3

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45*
84*
60
94
4
G3
G3
G3
75*
73*
45
92b
5
6
25*
95*
G3
120
450
89b
7
8
G3
86
a
*
b
Substrates (1 mmol); solvent (5 mL) ethanol, water; Catalyst (10 mg) at room temperature
0
c
temperature at 50 C; Isolated yield by recrystallisation; Catalyst G0-Fe3O4/SiO2/PPI-G0, G1-
Fe3O4/SiO2/PPI-G1 and G2- Fe3O4/SiO2/PPI-G2, G3- Fe3O4/SiO2/PPI-G3
All the products were purified by recrystallisation process and products were
1
confirmed by H and 13C NMR spectroscopic studies using the solvent CDCl3 (see supporting
information).

The feasibility of chemical conversions such as Knoevenagel- Michael
cyclocondensation (entry 7) and Henry reaction (entry 8) were also evaluated by using
Fe3O4/SiO2/PPI-G3. The holistic evaluation of catalyst for all the three different
condensations reactions inevitably proves that magnetite core PPI dendritic nanohybrid
material is a promising catalyst material when compared to the magnetite cored base catalysts
reported by others (Table 4) [38–41]. The reason behind the efficiency is the multiple number
of nitrogen in single molecular entity, fine accessible voids to host the guest molecules, the

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multivalent interaction exerted by dendritic molecules towards the substrates and the enhanced
dispersibility in the reaction medium.
Table 4. Comparison of the catalyst for Knoevenagel Condensation
S.No. Catalyst
Catalyst
weight
Reaction
conditions
Time Product
yield
1
2
3
4
piperidine, pyridine
0.3, 4 mol eq
10 mol%
water/reflux
Water/RT
2 h
2 h
2 h
2h
88[36]
55[35]
44[35]
90[33]
NaOH
K2CO3
10 mol %
1.3 mol eq
Water/RT
Triethylamine
Toulene/ reflux
5
6
Amine functionalized
mesoporous silica
20 mg
Ethanol/RT
100 oC
6 h
4h
90[42]
94[34]
Fe3O4@SiO2-Py
100 mg
6
Fe3O4@SiO2- PVA
50 mg
Ethanol/RT
0.5 h 95[32]
7
Fe3O4/SiO2/PPI-G3
10 mg
Ethanol/RT
0.5 h 93(this work)
Recycle efficacy of the catalyst holds good up to eight cycles with minimal loss in
their activity for the Knoevenagel condensation reaction (Fig. 7). The use of solvent for
isolation and regeneration of catalyst material is because of the magnetic separation method
compared to other solid supported catalyst.

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Fig. 7. Recycle efficiency of Fe3O4/SiO2/PPI-G3 for Knoevenagel condensation reactions
HRTEM image (Fig. 8) of the recycled Fe3O4/SiO2/PPI-G3 catalyst demonstrates the
spherical morphology with core- shell and size of the nanohybrid material seems to be alike
with fresh catalyst. The HRTEM image is prominently supporting the stability of magnetic
PPI dendrimer. Additionally, FTIR spectrum (Fig. S6), VSM data (Fig.S7) and XRD pattern
(Fig. S8) obtained for recycled Fe3O4/SiO2/PPI-G3 catalyst evidences that no significant
changes occurs when compared with fresh Fe3O4/SiO2/PPI-G3 catalyst. The FTIR spectrum
reveals either the PPI dendrimer or the silica coated magnetite nanoparticles is unaltered even
after eighth cycle of catalysis. The XRD spectrum of the recycled Fe3O4/SiO2/PPI-G3 catalyst
proves the lattice planes of the magnetite nanoparticles is retained, even after exposing the
catalyst to diverse chemical environment. VSM curves at room temperature for the fresh and
recycled catalyst confirms the paramagnetic behavior of the catalyst remains unaffected
Fig. 8. HRTEM image of Fe3O4/SiO2/PPI-G3 catalyst after eight cycles
A leaching test was carried out to examine the leaching and stability of magnetic
supported heterogeneous catalyst [43]. For this purpose, the catalyst Fe3O4/SiO2/PPI-G3 was
magnetically separated from the model Knoevenagel condensation reaction medium (scheme
2) at different time intervals such as 10 min, 20 min and 30 min and the catalytic activity of
filtrate was monitored every 5 min (Fig. 9).. The product yield during this continuous
assessment after the removal of catalyst from the reaction mixture seems to be unaffected.

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These observations suggest that the catalyst remains stable and no significant destruction of
organic functionalities present on the magnetite core was observed.
Figure 9. Test for the leaching study of Fe3O4/SiO2/PPI-G3 catalyst
4. Conclusions
Based on the rationale, the structurally enhanced magnetite cored PPI core shell nanohybrid
material has been developed successfully to serve as multivalent organic base catalyst with
enriced activity and facile magnetic recyclability. Though it is magnetic supported synthesis,
the analytical data strongly supports the expected nano architecture as much as possible. The
catalytic activity of magnetite cored PPI dendrimer was studied for Knoevenagel
condensation, Knoevenagel- Michael cyclocondensation and Henry reactions at ambient
conditions and found to be superior to the conventional magnetic cored base catalyst of
previous report. The condensation reaction was also accomplished even in aqueous medium
with good yield. The catalyst was reused for eight times without any
significant loss in their activity. As a preliminary study we have examined the utility of the
new nanohybrid material for various condensations reactions. The further application of this
material is explored with numerous modifications to green and sustainable organic

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transformation for the perspectives of industrial purposes, this material may exploit other
fields such as biomedical, sensor, environmental etc.
Acknowledgements
RR thank SERB, India for financial support (EEQ/2017/000374). We acknowledge HRTEM
and XRD facility at SRMIST set up with support from MNRE (Project No. 31/03/2014-
15/PVSE-R&D), GoI. Analytical facilities rendered from IIT Kanpur, IIT Guwahati, MNIT
Jaipur are gratefully acknowledged.
Keywords: magnetic core, PPI dendrimer, nanohybrid, sustainable catalyst, magnetically
recoverable
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Graphical Abstract

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Rajmohan Rangasamy: Conceptualization; Funding acquisition; Methodology; Project
administration; Supervision; Roles/Writing – original draft; Writing – review & editing.
Kannappan Lakshmi: Data curation; Formal analysis Resources; Software Validation;
Visualization original draft; Investigation; Writing – review & editing.

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Declaration of interests
☐The authors declare that they have no known competing financial interestsor personal
relationships that could have appeared to influence the work reported in this paper.

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Synthesis of structurally enhanced magnetite cored poly(propyleneimine)
dendrimer nanohybrid material and evaluation of its functionality in
sustainable catalysis of condensation reactions
a
a *
Kannappan Lakshmi , Rajmohan Rangasamy
a Department of Chemistry, Guru Nanak College(autonomous) , Affiliated to University of
Madras, Velachery, Tamil Nadu, India