Shape controlled synthesis of nano-sized titanium(IV) metal-organic frameworks

Article abstract of DOI:10.1080/15533174.2010.538026

A convenient method to synthesize shape controlled nano-sized metal-organic frameworks (MOFs) has been explored to prepare some novel mixed-ligand complexes of titanium(IV) with the general formula [Ti(acac)Cl _2-n(OOCC₁₅H₃₁)(SB)_n] (where Hacac = acetylacetone, HSB = Schiff bases and n = 1 or 2). They have been synthesized by stepwise substitutions of chloride ions from titanium(IV) chloride. The complexes were characterized by elemental analyses, molecular weight determinations, spectral, and TEM studies. Coordination number 7 and 8 was assigned for titanium(IV) in the complexes. The powder XRD, TEM, and SAED studies indicates that these complexes are nano-sized having poor crystalline nature. Taylor & Francis Group, LLC.

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Synthesis and Reactivity in Inorganic, Metal-Organic,
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Shape Controlled Synthesis of Nano-Sized
Titanium(IV) Metal-Organic Frameworks
Balram P. Baranwal a , Alok K. Singh a , Anand Varma a & Avinash C. Pandey b
a Department of Chemistry , D.D.U. Gorakhpur University , Gorakhpur, Uttar Pradesh,
India
b Nanophosphor Application Center , Allahabad University , Allahabad, Uttar Pradesh,
India
Published online: 19 Feb 2011.
To cite this article: Balram P. Baranwal , Alok K. Singh , Anand Varma & Avinash C. Pandey (2011) Shape Controlled
Synthesis of Nano-Sized Titanium(IV) Metal-Organic Frameworks, Synthesis and Reactivity in Inorganic, Metal-Organic, and
Nano-Metal Chemistry, 41:2, 150-154
To link to this article: http://dx.doi.org/10.1080/15533174.2010.538026
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Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 41:150–154, 2011
Copyright © Taylor & Francis Group, LLC
ISSN: 1553-3174 print / 1553-3182 online
DOI: 10.1080/15533174.2010.538026
Shape Controlled Synthesis of Nano-Sized Titanium(IV)
Metal-Organic Frameworks
Balram P. Baranwal,1 Alok K. Singh,1 Anand Varma,1 and Avinash C. Pandey2
1Department of Chemistry, D.D.U. Gorakhpur University, Gorakhpur, Uttar Pradesh, India
2Nanophosphor Application Center, Allahabad University, Allahabad, Uttar Pradesh, India
We report here an easier method of synthesis of some metal-
organic frameworks of titanium(IV) using multidentate ligands
like acetylacetone, carboxylic acids, and Schiff bases. These lig-
ands were chosen in accordance with the objectives that all of
them act as potential chelating agents having hard oxygen donor
atoms suitable for titanium(IV). These complexes are hydrolyt-
ically stable and have Ti-O-C linkage, a basic requirement for
catalytic action. Their nano-size and spherical shaped particles
have been observed and are being reported here.
A convenient method to synthesize shape controlled nano-sized
metal-organic frameworks (MOFs) has been explored to prepare
some novel mixed–ligand complexes of titanium(IV) with the gen-
eral formula [Ti(acac)Cl2−n(OOCC15H31)(SB)n] (where Hacac =
acetylacetone, HSB = Schiff bases and n = 1 or 2). They have
been synthesized by stepwise substitutions of chloride ions from
titanium(IV) chloride. The complexes were characterized by ele-
mental analyses, molecular weight determinations, spectral, and
TEM studies. Coordination number 7 and 8 was assigned for ti-
tanium(IV) in the complexes. The powder XRD, TEM, and SAED
studies indicates that these complexes are nano-sized having poor
crystalline nature.
EXPERIMENTAL
Materials and Analytical Methods
Keywords mononuclear, multidentate ligands, nano-size, tita-
All the reactions were carried out under anhydrous condi-
tions. Organic solvents (Qualigens) were dried and distilled
before use by standard methods.[11] Sodium palmitate, acety-
lacetone (Aldrich), and TiCl4 (BDH) were used as received.
Titanium and chlorine were estimated gravimetrically as TiO2
and AgCl, respectively.[12]
nium(IV)
INTRODUCTION
Shape-controlled synthesis of titanium(IV) metal-organic
frameworks (MOFs) provides novel nano- or micromaterials
with opportunities in exploring their unique physical and chem-
ical properties. Metal-organic frameworks (MOFs) composed of
metal ions linked together by multidentate ligands have attracted
Physico-chemical Measurements
Infrared spectra were recorded on a Perkin-Elmer RX1 FTIR
considerable attention in recent years.[1−6] A key challenge for spectrophotometer using KBr discs. 1H NMR spectra were
the industrial use of MOFs is to deliver them in a shape suitable recorded at 300 MHz on a Bruker DRX-300 NMR spectrometer
for applications.
in CDCl3. Molecular weights were determined in a semi-micro
Most of the titanium complexes are sensitive to hydroly- ebulliometer (Gallenkamp) with a thermistor sensing device.
sis, which may cause a subsequent reduction in their catalytic Powder XRD data were collected on a PW 1710 diffractometer.
property.[7] Their rate of hydrolysis can be significantly reduced The operating voltage of the instrument was 30 kV and the op-
by using bulky electron-rich ligands.[8,9] In this light, we have erating current was 20 mA. The intensity data were collected at
recently reported some titanium(IV) complexes having Ti-O-C room temperature over a 2θ range of 5.025–79.925◦ with a con-
bond and increased resistance towards hydrolysis.[10]
tinuous scan mode. Transmission electron microscopy (TEM)
images were obtained on a Tecnai 30 G2S – Twin electron mi-
croscope with an accelerating voltage of 300 kV on the surface
of a carbon coated copper grid. Elemental analyses (C, H and
N) were done on a Vario EL III Carlo Erba 1108 elemental an-
alyzer. Molar conductance were measured on century CC-601
digital conductivity meter at 10−2–10−3 molar solutions in ni-
trobenzene. Solid state conductance measurements were carried
out with Keithley 6220 Precision current source and keithley
2182A Nanovoltmeter.
Received 27 May 2010; accepted 18 August 2010.
The authors are thankful to the CSIR [No. 01(2293)/09/EMR-II]
and [UGC- DSA F.4-5/2006 (XI Plan/BSR) dated September 20, 2007],
New Delhi for financial supports. They also thank CDRI, Lucknow for
spectral and microanalysis.
Address correspondence to Balram P. Baranwal, Department of
Chemistry, D.D.U. Gorakhpur University, Gorakhpur 273009, Uttar
Pradesh, India. E-mail: drbpbaranwal@gmail.com
150

SHAPE CONTROLLED SYNTHESIS OF TITANIUM(IV)
151
TABLE 1
Analytical results for the titanium(IV) complexes
Found (calculated) %
Cl
Product
(color) (% yield)
M.p.
(◦C)
Molecular
weight
Reactants (g, mmol)
Ti
C
H
N
[Ti(acac)Cl2(OOCC15H31)] + NaSB-1 [Ti(acac)Cl(OOCC15H31)(SB-1)] 113 7.09 10.57 61.05 7.11 2.08
(2.10, 4.44) (1.13, 4.45) (Dark grey) (88) (1)
661
(7.16) (10.61) (61.08) (7.10) (2.10) (669)
[Ti(acac)Cl2(OOCC15H31)] + NaSB-1 [Ti(acac)(OOCC15H31)(SB-1)2] 119 5.52 8.18 65.36 6.53 3.25
858
(2.62, 5.54) (2.81, 11.08) (Light grey) (83) (2) (5.54) (8.21) (65.34) (6.55) (3.24) (864)
[Ti(acac)Cl2(OOCC15H31)] + NaSB-2 [Ti(acac)Cl(OOCC15H31)(SB-2)] 92 7.54 5.61 64.38 7.61 2.19
643
(2.43, 5.13) (1.13, 5.16) (Dark grey) (89) (3) (7.55) (5.59) (64.40) (7.65) (2.21) (634)
[Ti(acac)Cl2(OOCC15H31)] + NaSB-2 [Ti(acac)(OOCC15H31)(SB-2)2] 94–97 6.01
(2.82, 5.96) (2.62, 11.95) (Light grey) (81) (4) (6.02)
71.02 7.35 3.51
(71.01) (7.37) (3.53) (795)
789
—
RESULTS AND DISCUSSION
Synthesis of [Ti(acac)Cl(OOCC15H31) (SB-1)] (1)
Some metal-organic frameworks of titanium(IV) were syn-
thesized by stepwise substitutions of chloride ions from titanium
tetrachloride:
Schiff bases HSB-1 and HSB-2 were prepared by the lit-
erature procedure.[13,14] Their sodium salts were prepared by
adding the required amount of sodium metal to an excess of
isopropanol (sodium isopropoxide formed in situ). The solution
thus formed was added slowly to the Schiff bases in isopropanol
and the contents were boiled to reflux for 2 h. Removal of ex-
cess solvent under vacuum yielded the desired sodium salts of
Schiff bases. [Ti(acac)Cl2(OOCC15H31)] was prepared by treat-
ing TiCl4 with one equivalent mole of acetylacetone in benzene
which yielded [Ti(acac)Cl3]. To the suspension of [Ti(acac)Cl3]
(2.11 g, 8.33 mmol) in benzene (40 mL), sodium palmitate
(2.32 g, 8.33 mmol) was added. The contents were stirred for 8
h followed by refluxing for 2 h. The contents were filtered using
G 4 sieve to remove insoluble sodium chloride formed during
the reaction. The excess solvent was removed in vacuo and a
yellow colored solid, [Ti(acac)Cl2(OOCC15H31)] was obtained.
This composition was confirmed by the elemental analysis and
infrared.[10] To a solution of [Ti(acac)Cl2(OOCC15H31)] (2.10 g,
4.44 mmol) in benzene (50 mL), NaSB-1 (1.13 g, 4.45 mmol)
was added. The contents were stirred (10 h) followed by re-
fluxing (3 h) and filtration using G 4 sieve to remove insoluble
sodium chloride formed during the reaction, leaving a black
colored solution. The desired product was isolated by evapora-
tion of the solvent under reduced pressure. It was recrystallized
in dichloromethane-n-hexane mixture (1:1 ratio). The substitu-
tions and stepwise use of different types of ligands made shape
controlled synthesis of titanium(IV) complexes during present
investigations. Other three complexes were synthesized analo-
gously and the details of analytical results are given in Table 1.
TiCl4 + Hacac Benz→ene [Ti(acac)Cl3] + HCl ↑
[1]
Reflux
[Ti(acac)Cl3] + C15H31COONa Benz→ene [Ti(acac)
×Cl2(OOCC15H31)] + NaCl ↓Reflux
[2]
[Ti(acac)Cl2(OOCC15H31)] + nNaSB Benz→ene [Ti(acac)Cl2−n
Reflux
×(OOCC15H31)(SB)n] + nNaCl ↓
[3]
(Where Hacac = acetylacetone, NaSB = NaSB-1 or NaSB-2
and n = 1 or 2)
Benzene was chosen a solvent in these substitutions because
sodium chloride formed during the reaction was insoluble and
the titanium(IV) complexes were soluble, which could made
the separation easy. The molar conductance of complexes at
10−2–10−3 molar concentrations in nitrobenzene was obtained
in the range 1– 8 ꢀ−1cm2mol−1, which indicated them to be
non-electrolytes.[15] The value obtained from conductance mea-
surements (1 × 10−8A current, 2.92 × 10−2 V) in the solid form
of complex 2 indicated a high resistance (2.92 × 106 ꢀ at 295
K). This may lead to the conclusion that the complex shows
the property like an insulator. Elemental analyses were in good
agreement with the calculated values. Ebullioscopic method of
molecular weight determinations showed that all the complexes
were monomeric in refluxing benzene (Table 1).
ONa
ONa
Infrared Spectra
Cl
CH = N
CH = N
In infrared spectra of all the complexes O-H stretching vibra-
tions of different ligands were found absent in the region 3600-
3200 cm−1. The bands at 1710 cm−1 (CO stretching) and at 935
cm−1 (OH deformation) of free palmitic acid were also absent in
NaSB-1
NaSB-2

152
B. P. BARANWAL ET AL.
ance of a titanium-nitrogen (Ti–N) band around 530 cm−1
.
TABLE 2
1H NMR spectral data (δ, ppm) for titanium(IV) complexes
The bands around 560 cm−1 could be assigned to Ti–O
vibrations.[17]
Complex
1H
2
0.87 [s, 3H, CH3], 1.24 [s, 26H, 13 x (-CH2)], 1.59
[t, 2H, (α-CH2)], 2.04 [s, 6H, acac], 5.52 [s, 1H,
CH, acac], 8.37 [s, 2H, CH=N], 7.51-6.64 [m,
16H, arom]
0.88 [s, 3H, CH3], 1.26 [s, 26H, 13 x (-CH2)], 1.63
[t, 2H, (α-CH2)], 2.06 [s, 6H, acac], 5.51 [s, 1H,
CH, acac], 8.21 [s, 1H, CH=N], 7.46-6.87 [m, 9H,
arom]
0.88 [s, 3H, CH3], 1.26 [s, 26H, 13 x (-CH2)], 1.61
[t, 2H, (α-CH2)], 2.05 [s, 6H, acac], 5.51 [s, 1H,
CH, acac], 8.29 [s, 2H, CH=N], 7.47-6.55 [m,
18H, arom]
Hydrolytic Stability of the Complexes
The precursor complex [Ti(acac)Cl2(OOCC15H31)] is mois-
ture sensitive.[10] Its sensitivity towards hydrolysis is reduced
by substituting chloride ions by Schiff bases. This stability was
tested by dissolving the complexes in benzene, and 0.1% water
was added and stirred the contents for 12 h open in the air. It
was also observed that the di-substituted products were more
hydrolytically stable than the mono-substituted ones.
3
4
1H NMR Spectra
No signal was observed in the 1H NMR spectra of the com-
plexes in the region δ 15.0 to 10.0 ppm (observed due to –OH
proton), which indicated deprotonation and bonding of acety-
lacetone, carboxylic and Schiff bases to titanium. In the 1H NMR
spectrum of 1 the azomethine proton in Schiff base suffered an
upfield shifting from δ 8.61 to 8.28 as a singlet which indicated
coordination of azomethine nitrogen to titanium. A multiplet
of phenyl protons was appeared in the region δ 7.51–6.94. A
singlet was appeared at δ = 0.88 ppm (3H) corresponding to
methyl protons of palmitate ion [-OOCCH2(CH2)13CH3] and a
singlet at δ = 1.26 ppm due to 26H of 13-CH2groups. A peak
at δ = 1.63 ppm (2H) indicated the α -CH2 protons of palmitate
ion. A singlet at δ = 2.05 ppm (6H) indicated the methyl pro-
tons of acac while the methine proton of acac was observed as
a singlet at δ = 5.51 ppm (1H). An almost similar pattern of 1H
NMR spectra were observed for other complexes and the signal
positions with assignments are listed in Table 2.
the spectra of the complexes. Two strong bands were observed
at ∼1535 cm−1 and ∼1435 cm−1, which could be assigned
due to (νasymOCO) (antisymmetric) and (νsymOCO) (symmet-
ric) vibrations of the palmitate ions, respectively.[16] The dif-
ference, ꢁ[νasymOCO–νsymOCO] was ∼100 cm−1, which indi-
cated bidentate chelating nature of palmitate ion.[17] No band
was found in 1626–1700 cm−1region indicating the CO group of
acetylacetone was not free in the complexes. Two splitted new
bands observed at ∼1615 cm−1 (νC O) and ∼1530 cm
−1
- - -
- - -
(νC C) indicated the chelating nature of acetylacetonate ion
in the complexes.[18] The ν(C–O) (phenolic) bands appeared in
the region 1280–1263 cm−1 in free Schiff bases were shifted
to higher wave number (∼20 cm−1). The infrared spectra of all
the free Schiff bases exhibited a strong and sharp band in the
region 1631–1620 cm−1, which was shifted to lower frequen-
cies (∼15 cm−1) in complexes indicating coordination through
Powder XRD and TEM Studies
The pattern and results of powder XRD suggested that the
azomethine nitrogen. This was further supported by appear- complexes are showing poor crystallinity. Because of this single
TABLE 3
Powder XRD data of complex 1
Angle
[xZi]
d-value
1[ ]
d-value
2[ ]
Peak width
[xZi]
Peak intensity
[counts]
Back. intensity
[counts]
Rel. intensity
[x]
Significance
7.355
12.0096
6.4584
4.1203
3.6973
3.5527
3.2588
2.8221
1.9973
1.6293
1.4116
1.2629
12.0391
6.4743
4.1304
3.7064
3.5614
3.2669
2.8291
2.0022
1.6333
1.4151
1.2660
0.150
0.150
0.200
0.300
0.200
0.200
0.300
0.300
0.250
0.400
0.400
17
40
110
41
100
34
202
121
35
42
29
52
45
41
36
31
27
18
15
14
8.3
19.7
54.7
20.3
49.6
16.7
100.0
60.0
17.3
8.7
1.20
1.59
1.38
1.26
1.97
0.86
4.94
2.92
0.83
0.77
1.77
13.700
21.550
24.050
25.045
27.345
31.680
45.370
56.430
66.145
75.170
18
38
19.1

SHAPE CONTROLLED SYNTHESIS OF TITANIUM(IV)
153
FIG. 1. Powder XRD spectrum of complex 1.
crystal XRD could not be done. Powder XRD were done for all and β is integral peak width, which is converted into radian
the complexes and one spectrum for 1 along with its crystal data while calculation. The calculated values of particle size from
is given in Table 3 (Figure 1), which are comparable with tita- maximum intensity peak for 1, 2, 3, and 4 were obtained as 29,
nium amide chlorate carbonate, both in diffraction intensity and 48, 26, and 43 nm, respectively. These values are indicative of
position (JCPDS No. 76-2255). Particle size of the complexes the nano-range size for the complexes.
was calculated by the standard Scherrer equation.[19]
The morphology of the complex 1 was found to be spherical
(Figure 2a) having 35 nm average diameter of the particles
which is in agreement with the calculated value obtained from
Scherrer’s equation, i.e. 29 nm. It is also concluded that the nano
particles are showing poor crystallinity on the basis of selected
area electron diffraction (SAED) in Figure 2b.
D = Kλ/(β cos θ)
[4]
Where D is the particle size; K is a constant (= 0.94); λ is
˚
X-ray wavelength (λ = 1.5406 A); θ is Bragg diffraction angle
FIG. 2. (a) TEM image, and (b) the SAED pattern for the complex 1.

154
B. P. BARANWAL ET AL.
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In this communication, we report a novel synthetic method
to prepare shape controlled mixed-ligand complexes of tita-
nium(IV). These are metal-organic frameworks of titanium(IV)
having spherical shape and nano-sized particles. Their increased
hydrolytic stability was noticed, and coordination number 7 and
8 have been assigned in them.
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