A novel dinuclear zirconium (IV) complex derived from [Zr(acac) 4] and a pentadentate Schiff base ligand: Synthesis, characterization and catalytic performance in synthesis of indole derivatives

Article abstract of DOI:10.1016/j.inoche.2013.12.011

[Zr(acac)₄] underwent ligand exchanges with pentadentate Schiff base ligand of N,N′-bis(3-salicylidenaminopropyl)amine. Full characterization of this complex was accomplished with elemental analyses, spectroscopic studies (¹H NMR,

Full text of DOI:10.1016/j.inoche.2013.12.011

Inorganic Chemistry Communications 41 (2014) 14–18
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Feature article
A novel dinuclear zirconium (IV) complex derived from [Zr(acac)4] and a
pentadentate Schiff base ligand: Synthesis, characterization and catalytic
performance in synthesis of indole derivatives
b
N. Tayyebi Sabet Khomami a, F. Heshmatpour a, , B. Neumüller
⁎
a
Department of Chemistry, K.N. Toosi University of Technology, Tehran, Iran
Fachbereich Chemie der Universitäit, Hans-Meerwein-Strasse, D-35032 Marburg, Germany
b
a r t i c l e i n f o
a b s t r a c t
Article history:
[Zr(acac)4] underwent ligand exchanges with pentadentate Schiff base ligand of N,N′-bis(3-
salicylidenaminopropyl)amine. Full characterization of this complex was accomplished with elemental anal-
yses, spectroscopic studies (1H NMR, 13C NMR, FT-IR, UV–vis) and X-ray structure analysis. X-ray crystallography
indicated that this complex had a dinuclear structure in which each seven coordinated zirconium atom adopted a
distorted pentagonal bipyramid geometry that was joined to the second zirconium by a μ-oxo bridge. The cata-
lytic activity of the prepared complex was investigated in the synthesis of indole derivatives under mild condi-
tions. The results revealed that although in this complex Zr (IV) was surrounded by seven other atoms, it
could still catalyze indole condensation reaction and lead to plausible yields of products.
Received 13 October 2013
Accepted 9 December 2013
Available online 31 December 2013
Keywords:
Zirconium
Schiff base
Catalyst
© 2013 Published by Elsevier B.V.
Crystallography
Contents
Acknowledgment
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Appendix A.
Appendix B.
Appendix B.
References
Analytical data
Supplementary material
Supplementary material
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Introduction. The chemistry of zirconium is highly dominated by
compounds in oxidation state (IV) [1]. A high charge-to-size ratio (Z2⁄
r = 22.22 e2m−10) of the zirconium (IV) ion gives it a marked prefer-
ence for higher coordination numbers [2] including 7 and 8. The com-
mon starting materials for zirconium (IV) complexes are ZrCl4,
ZrOCl2·8H2O, and [Zr(acac)4]. ZrCl4 is prone to easy hydrolysis [3], and
ZrOCl2·8H2O has a tetrameric structure which leads to tetrameric com-
plexes with double hydroxo bridges [4]. Among them, [Zr(acac)4] is sta-
ble [1], and low toxic (LD50 {[Zr(acac)4] oral rat} = 719 mg⁄kg). This
compound can undergo partial or total exchange of acac with other suit-
able ligands and produce various complexes.
complexes have potential biological interest and are utilized as almost
successful models of biological compounds [5]. Besides, their use in cat-
alytic reactions has been well regarded [6]. On the other hand, the rela-
tively large size and high charge of the Zirconium (IV) give it strong
coordination ability with oxygen- and nitrogen-containing Schiff base
ligands [2]. Recently, some investigations have been reported on the
salen-type of Schiff bases and Zr (IV) derived from ZrCl4 [7] and
ZrOCl2·8H2O [8], resulting in the synthesis of monoclear metal com-
plexes [9]. However, there are few studies on dinuclear complexes
with Schiff base ligands of NxOy and zirconium (IV) [10].
Indole derivatives have attracted great interest due to ring system
that is the most widely distributed heterocycle found in nature. Indole
core presents in many natural products, pharmaceuticals, and agro-
chemicals [11]. They are also effective in the prevention of cancer due
to their ability to modulate certain cancer causing estrogen metabolites
[12]. Indole can readily undergo electrophilic substitution reaction with
the oxygen atom of carbonyl compounds in the present of a suitable
Schiff base ligands represent one of the most widely utilized and ver-
satile ligands in metal coordination chemistry. Many Schiff base
⁎
Corresponding author. Tel.: +98 21 22853650.
E-mail address: heshmatpour@kntu.ac.ir (F. Heshmatpour).
1387-7003/$ – see front matter © 2013 Published by Elsevier B.V.
http://dx.doi.org/10.1016/j.inoche.2013.12.011

N.T.S. Khomami et al. / Inorganic Chemistry Communications 41 (2014) 14–18
15
catalyst bis and tris (1H-indole-3-yl) methanes [2]. Different com-
pounds are known to catalyze this reaction [13].
at 50 °C for an appropriate time, as monitored by TLC. The isolated
yields were measured by using TLC plates and products were character-
ized by 1H NMR and 13C NMR.
In the present study, synthesis and characterization of a new and
stable Zr (IV) complex 1 obtained by the ligand exchange of Schiff
base ligand of N,N′-bis(3-salicyliden-aminopropyl)amine with
[Zr(acac)4] is reported. A single-crystal X-ray structure of this com-
pound proves that the complex is a centrosymmetric dinuclear unit in
the solid state consisting of two mononuclear zirconium containing li-
gands that are joined by a μ-oxo bridge. Each monomeric unit exhibits
distorted pentagonal bipyramidalgeometry. The catalytic activity of
this complex is investigated in the reaction of indole condensation
with different aldehydes in different solvents.
Experimental. General information. Diethylenetriamine,
salicylaldehyde, zirconium (IV) acetylacetonate, methanol and chloro-
form were purchased from Merck Chemical Company and utilized with-
out any more purification.
Instrumentation. Infrared spectra were recorded as KBr pellets
using Unicam Mattson 1000 FT-IR. Elemental analysis (C, H, N) was per-
formed using a Heraeus Elemental Analyzer CHN-O-Rapid (Elemental-
Analysesysteme, GmbH, West Germany). 1H and 13C NMR spectra
X-ray crystallography. The selected crystals of Zr (IV) were covered
with perfluorinated oil and mounted on the top of a glass capillary
under a flow of cold gaseous nitrogen. The orientation matrix and the
unit cell dimensions were determined from 4000 reflections (Stoe
IPDS 2 T (Zr (IV) complex) graphite-monochromated Mo-Kα radiation
(λ = 71.073 pm); see also Table 5. The intensities were corrected for
Lorentz and polarization effects. In addition, absorption corrections
were applied for Zr (IV) complex (numerical). The structures were
solved by direct methods using SIR-92 and were refined against F2 by
full-matrix least-squares using the program SHELXL-97. (See Table 1.)
Results and discussion. The Schiff base complex crystallized as a
stable light yellow crystal which decomposed at 267 °C. It was soluble
in acetonitrile, dichloromethane and chloroform. Analytical and physi-
cal data for 1·2CHCl3·MeOH complex is given in Table 2.
IR. A sharp band at 1634 cm−1 due to υ (C_N) (azomethine) in free
ligand, shifts to lower wavenumber, and appears at 1628 cm−1. It de-
picts the involvement of azomethine nitrogen in coordination [15]. A
rather sharp band at 1317 cm−1 is assigned to the phenolic C\O
strength of the coordinated ligands [1]. The C\O stretching vibration
in free ligand is observed at 1278 cm−1. This frequency shifts in the
complex towards lower or higher values as a result of coordination of
the oxygen to the metal ion [16]. Besides, lowering a wavenumber relat-
ed to stretching vibration of carbonyl group from 1668 cm−1 [23] to
1665 cm−1, depicts the presence of intramolecular hydrogen bonding
in this compound. Bands at 570–540 and 525–430 cm−1 might be
assigned to Zr\O and Zr\N bonds [17] and a band at 714 cm−1 repre-
sents Zr\O\Zr bond [22a].
were obtained in CDCl3 solutions with
a Bruker T-NMR 250
(250 MHz) spectrometer. A digital melting point measuring device
(Electrotermal 9100) was used. A double beam spectrophotometer
(Shimadzu, UV-240) was used for the UV–vis absorption determination.
Preparations. H2saldien. The pentadentate Schiff base ligand of N,
N′-bis(3-salicyliden-aminopropyl)amine was synthesized similar to
Coleman et al. [14] by 1:3 condensation reaction of diethylenetriamine
(dien) and salicylaldehyde (sal) in absolute ethanol at room tempera-
ture for 15 min. The volume of the solution was reduced until an oil
remained which was identified via NMR and FT-IR [14].
1·2CHCl3·MeOH. Zr (IV) complex 1 was prepared by treatment of
prepared ligand with [Zr(acac)4] in molar ratio 2:1, respectively, in
methanol according to Scheme 1.
A solution of H2saldien (0.311 g, 1 mmol) in methanol:water (95:5)
(10 mL) was added to a suspension of [Zr(acac)4] (0.24 g, 0.5 mmol) in
methanol: water (5 mL). A light yellow compound separated immedi-
ately. Stirring was continued for further 3 h. Then, it was filtered and
washed with methanol and dried at room temperature. Finally, the ob-
tained product was recrystallized in chloroform. Yield: 95%.
General procedure for the condensation of indoles with aldehyde
compounds catalyzed by complex. Indole (0.234 g, 2 mmol) was
added to a solution of the aldehyde compound (1 mmol) and complex
(0.026 g, 0.02 mmol) in acetonitrile (0.5 mL). The mixture was stirred
UV. Table 3 provides electronic spectra of the prepared zirconium
complex along with its assignments. The electronic absorption spec-
trum which was measured in CHCl3 at room temperature exhibits
three bands. Two bands between 257 and 350 nm are due to π → π*
and n → π* ligand transitions. Appearance of a lower intensity band
at 379 nm is due to ligand to metal charge transfer (LMCT) transition
[18]. The presence of a weak shoulder at 288 nm is indicative of an in-
tramolecular hydrogen bonding [19]. In addition, no other absorption
bands correspondent to d-d transitions are seen that is in agreement
with d0 configuration of zirconium (IV).
Table 1
Crystallographic data of 1·2CHCl3·MeOH.
Compound
1·2CHCl3·MeOH
C53H54Cl6N6O10Zr
1330.16
0.22 × 0.19 × 0.15
Tetragonal
I41/a
Empirical formula
Formula mass
Crystal size/mm
Crystal system
Space group
a, b, c/pm
2930.3(1), 2930.3(1), 1260.1(1)
α, β, γ/°
90, 90, 90
10,992(1)
8
Volume/pm3·106
Z
Dcalcd./g·cm−3
Absorp. correction
μ/cm−1 (Mo-Kα)
Temp./K
1.608
Empirical (X-red32/X-area (Stoe))
7.35
100
2θmax/°
51.76
Index range (h, k, l)
Reflections collected
Uniq. reflect.
−35 → 31,−22 → 35,−14 → 15
14,737
5313
Rint
0.0327
3674
362
Reflect. with Fo N 4σ(Fo)
Parameters
R1
0.035
0.0824
0.662
wR2 (all data)
Max.residual electron density/(e.pm−3).10−6
Scheme 1. Preparation of 1.
a) w = 1/[σ2(Fo2) + (0.0492.P)2]; P = [max(Fo2, 0) + 2.Fc2]/3.

16
N.T.S. Khomami et al. / Inorganic Chemistry Communications 41 (2014) 14–18
Table 2
Analytical and physical data for 1.
Compound formula
C53H54Cl6N6O10Zr
Formula weight
Yield/(%)
Color
Decomposition temperature/(°C)
1330.16
%C
95
%H
Light yellow
%N
267
%Zra
Found (calculated)
47.02
(47.85)
3.89
(4.06)
6.30
(6.32)
13.44
(13.71)
a
Zr was estimated by slow decomposition of complex with conc. HNO3 (AR) and then weighing as ZrO2 after igniting to 1000 °C [1].
NMR. The 1H NMR spectrum of the H2saldien ligand exhibits the fol-
lowing signals [19]. 13.02 (s, 2H, OH), 8.50 (s, 2H,\CH_N\), 6.40–7.10
(m, 8H, aromatic), 2.5–4 (complex multiplet, 8H, CH2). The 1H NMR
spectrum of 1, however, shows the disappearing of phenolic signal
and downfield shift (9.66 ppm) of azomethine signal indicates the coor-
dination of phenolic oxygen and azomethine nitrogen to Zr atom. A sig-
nal at 9.99 (s, 2H,\CHO) indicates the presence of aromatic aldehyde.
The signal related to\NH at 5–6 ppm, is not located here probably
due to the coordination of nitrogen of the Schiff base to zirconium,
and producing a tertiary amine. Furthermore, 1H NMR spectrum depicts
no signal correspondent to acac groups [1], which indicates all acac
groups were exchanged with other ligands. Other signals i.e. those due
to aromatic and CH2 protons appear nearly at the same position as the
free ligand.
atom of salicylaldehyde. Another oxygen which is considered as the
μ-oxo bridge is presumably sourced from a water molecule (or hy-
droxide) of solvent [23a], as the solvent of the reaction is methanol
containing 5% water. It can be assumed that the reaction yields to
[(sal)Zr(saldien)OH2]1+ or [(sal)Zr(saldien)OH] intermediates. In the
next step, the remained acac− groups make a basic condition and get in-
termediates to dimerize and form a μ-oxo bridge. To our knowledge
only two examples of Zr compounds with a single Zr\O\Zr bridge
and distorted pentagonal-bipyramidal coordination sphere were de-
scribed [22]. The distance Zr1–O5 of 196.04(3) pm in 1 is in good agree-
ment with the measured values of the two corresponding compounds
(196 pm, mean value) [22]. O5 lies on a center of inversion and occupies
one apical position of the pentagonal bipyramid. On the second apical
position O3 from the corresponding phenolate molecule of the
salicylaldehyde is located. The Zr\N bonds between Zr1 and the neutral
N atoms N1, N2, and N3 show typical long bond lengths between
243.2(3) and 244.8(2) pm. Neutral aza-crownethers bonded to Zr(IV)
exhibit values of 246 pm (average) [22a].
Catalytic activity studies. As it has been mentioned, high charge-to-
size ratio of the zirconium (IV) ion gives it a marked preference for co-
ordination number of 8. Since in our prepared complex, zirconium
(IV) has the coordination number of 7 which two coordinated atoms
are electronegative oxygens; hence, the complex can be considered as
a Lewis acid. Therefore, its catalytic activity was investigated in the reac-
tion of indole condensation with different derivatives of benzaldehyde.
In the first step, the reaction of indole with benzaldehyde was cho-
sen as a model (Scheme 2) in various solvents (Table 4). The reaction
was carried out within 45 min that led to 79% yield. Increasing the
time of the reaction up to 60 min gave 80% yield; therefore, 45 min
was chosen as the optimized time of running the reaction. Moreover,
Based on the optimized solvent, the procedure was applied in acetoni-
trile, to a variety of benzaldehyde derivatives to investigate the reaction
scope. Results are summarized in Table 5.
The 13C NMR spectrum of 1 is in agreement with 1H NMR. In the com-
plex, signals due to azomethine carbon are observed at 165.64 ppm that
are shifted downfield compared to free Schiff base ligand, suggesting the
involvement of the nitrogen lone pair in coordination with metal ion
[20]. It also exhibits the signals corresponding to aromatic aldehyde at
192.77 ppm. Besides, signals related to acac groups are not seen due to
their exchanges with ligand atoms.
Crystal structure analysis of 1·2CHCl3·MeOH. Light yellow crystals
of 1·2CHCl3·MeOH, suitable for X-ray diffraction studies, were obtain-
ed. The compound crystallizes in the tetragonal space group I41/a with
eight molecules in the unit cell. An ORTEP view of 1 is shown in Fig. 1.
It is noteworthy to mention that 1 has a centrosymmetric dinu-
clear structure. The coordination sphere of the Zr1 can be described
as distorted pentagonal-bipyramidal. Pentadentate Schiff base li-
gand coordinates to zirconium from all its five donor atoms. Besides,
as Zr (IV) is characterized by its high affinity for oxygen [21], each Zr
is also bound to two other oxygen atoms. One of them is the oxygen
As shown in Table 5, 4-NO2C6H5 with the electron-withdrawing
group affords higher yield of product (82%) compared to benzaldehyde
(79%). However, 2-NO2C6H5 leads to a low yield (70%) that is probably
due to the strict hindrance of the nitro group in ortho position with
the groups surrounding the zirconium (IV). Besides, 4-MeC6H5 with
the electron-donating group leads to low yield of product (59%).
Table 3
UV–vis data for 1.
1
λ
max (nm) (ε (×105), M−1, cm−1
)
Assignment
257, 2.299
350, 2.429
379, 1.263
π → π*
n → π*
LMCT
Table 4
Table 5
Yields of condensation of indole with benzaldehyde in various solvents.
Condensation of indole with different derivatives of benzaldehyde.
Entry
Solvent
Yield/(%)a
Entry
Benzaldehyde compounds
Isolated yield/(%)a
1
2
3
4
5
6
7
CH3CN
CH3Cl
CH2Cl2
MeOHb
EtOHb
AcOEtb
79
71
70
–
1
2
3
4
5
6
7
X = H
79
82
70
67
64
63
59
X = 4-NO2
X = 2-NO2
X = 4-Br
X = 4-OMe
X = 4-OH
X = 4-Me
–
–
b
n-C6H14
–
The reactions were run at 50 °C, in 45 min, and the molar ratio of aldehyde/indole/catalyst
was 100:200:2 in 0.5 mL solvent.
The reactions were run at 50 °C, in 45 min, and the molar ratio of aldehyde/indole/catalyst
was 100:200:2 in 0.5 mL CH3CN.
All products were identified by their spectroscopic data and their comparison with
known samples [2].
a
a
Isolated yield.
b
Catalyst was not soluble in solvent.

N.T.S. Khomami et al. / Inorganic Chemistry Communications 41 (2014) 14–18
17
Fig. 1. Molecular structure of 1(thermal ellipsoids are at the 30% probability level). Selected bond lengths/pm and angles/°:Zr1-O5 196.04(3), Zr1-O2 205.7(2), Zr1-O3 208.5(2), Zr1-O1
209.4(2), Zr1-N2 243.2(3), Zr1-N3 243.7(3), Zr1-N1 244.8(2); Zr1-O5-Zr1a 180, O1-Zr1-O2 77.09(8),O1-Zr1-N2 150.21(8), O1-Zr1-N1 77.81(8), N1-Zr1-N2 68.33(9), N1-Zr1-N3
136.56(9), N2-Zr1-N3 68.28(9), O3-Zr1-O5 162.30(6), O1-Zr1-O3 92.24(8).
Hence, it can be concluded that Zr complex plays a Lewis acid role; giv-
ing rise to zirconium interaction with the oxygen atom of aldehyde and
promoting the condensation reaction of indoles with aldehydes. It is
noteworthy that on the one hand, the prepared dinuclear complex is
composed of two ions of zirconium (IV) that increases its Lewis acidic
sites. On the other hand, each zirconium (IV) is surrounded by 7 other
groups which decrease its availability for the reaction. Therefore, al-
though the prepared catalyst could catalyze the reaction, the obtained
yields were not higher than 82%.
Appendix A. Analytical data
1H NMR (CDCl3, TMS, ppm): δ = 9.68 (s, 4H, CH_N), 6.45–
7.64 ppm (m, 24H, Ar-H and Ar-CHO), 2.6–3.55 ppm (complex multi-
plet, about 16H, CH2). 13CNMR (CDCl3, TMS, ppm): δ = 192.75 ppm
(Ar-CHO), 165.64 ppm (HC_N), 115.90–122.82 ppm (Aromatic Ar-H
and Ar-CHO), 61.61 ppm (\CH2-N\), 48.59 ppm (\CH2-C_N\). IR
(KBr, cm−1): υ(C_N) 1628, υ (phenolic C\O) 1317, υ (C_O) 1665,
υ(Zr\O and Zr\N) 570–540 and 525–430, υ(Zr\O\Zr) 714 cm−1
.
Conclusions. A new dinuclear complex of Zr (IV) with N3O2
pentadentate Schiff base ligand was prepared and characterized. Crystal
structure of the complex was determined. In the crystal structure, zirco-
nium atoms have distorted pentagonal bipyramidal geometries. This
complex had potential Lewis acid properties, so it was used as the cata-
lyst in the reaction of indole condensation. In this way, the catalyzed re-
action of indole condensation with 4-NO2C6H5 depicted the plausible
yield (82%) in acetonitrile.
UV–vis spectrum in CHCl3 [λmax (nm)]: 257, 350, 379. C53H54Cl6N6O10-
Zr(1330.16): calcd. C 47.85, H 4.06, N 6.32, Zr 13.71% ; found C 47.02, H
3.89, N 6.30, Zr 13.44%.
Appendix B. Supplementary material
The crystallographic data (excluding structure factors) have been
deposited with the Cambridge Crystallographic Data Centre (CCDC) as
supplementary publication number CCDC 945942. Copies of the data
can be obtained, free of charge, by application to CCDC, 12 Union
Road, Cambridge, CB2 1EZ, UK (Fax: +44-1223-336033; Email: data_
request@ccdc.cam.ac.uk or via the internet: http://www.ccdc.cam.ac.
Acknowledgment
This work was supported by the K.N. Toosi University of Technology.
Scheme 2. Condensation of indole with benzaldehyde compounds catalyzed by 1.

18
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