Synthesis and spectral studies of some new dimeric [(μ-Cl) 2M2(L)2 · xTHF] [M = Zn(II) and Hg(II)] complexes containing Schiff-base ligands

Article abstract of DOI:10.1080/00958972.2011.596934

A series of binuclear Schiff-base complexes of zinc(II) and mercury(II) containing bidentate ligands (HL) [HL = salicylidene-2-methyl-1-aminobenzene (HL¹), salicylidene-2-aminopyridine (HL²), and salicylidene-3-nitro-1-aminobenzene (

Full text of DOI:10.1080/00958972.2011.596934

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: http://www.tandfonline.com/loi/gcoo20
Synthesis and spectral studies of some new
dimeric [(µ-Cl)2M2(L)2ꢀ·ꢀxTHF] [Mꢀ=ꢀZn(II) and Hg(II)]
complexes containing Schiff-base ligands
R.K. Dubey , P. Baranwal , S.K. Dwivedi & U.N. Tripathi
To cite this article: R.K. Dubey , P. Baranwal , S.K. Dwivedi & U.N. Tripathi (2011) Synthesis and
spectral studies of some new dimeric [(µ-Cl)2M2(L)2ꢀ·ꢀxTHF] [Mꢀ=ꢀZn(II) and Hg(II)] complexes
containing Schiff-base ligands, Journal of Coordination Chemistry, 64:15, 2649-2657, DOI:
10.1080/00958972.2011.596934
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Date: 25 March 2016, At: 02:14

Journal of Coordination Chemistry
Vol. 64, No. 15, 10 August 2011, 2649–2657
Synthesis and spectral studies of some new dimeric
[(l-Cl)2M2(L)2 E xTHF] [M ^ Zn(II) and Hg(II)] complexes
containing Schiff-base ligands
R.K. DUBEY*y, P. BARANWAL*y, S.K. DWIVEDI*z* and U.N. TRIPATHI*z*
ySynthetic Inorganic and Metallo-organic Research Laboratory, Department of Chemistry,
University of Allahabad, Allahabad-211 002, India
zDepartment of Chemistry, DDU Gorakhpur University, Gorakhpur-273001, India
(Received 24 February 2011; in final form 25 May 2011)
A series of binuclear Schiff-base complexes of zinc(II) and mercury(II) containing bidentate
ligands (HL) [HL ¼ salicylidene-2-methyl-1-aminobenzene (HL1), salicylidene-2-aminopyridine
(HL2), and salicylidene-3-nitro-1-aminobenzene (HL3)] with ‘‘N’’ and ‘‘O’’ donors have been
synthesized by simple metathetic reactions of anhydrous metal chlorides with sodium salts of
Schiff bases (in tetrahydrofuran (THF)/MeOH) in equimolar ratio to produce
[(m-Cl)2M2(L)2 ꢀ xTHF] [where M ¼ Zn(II) and Hg(II); L ¼ HL1, HL2, and HL3; x ¼ 0 for (1),
(4), (6) and x ¼ 2 for (2), (3), (5)]. The main emphasis on the complexes [(m-Cl)2M2(L)2 ꢀ 2THF]
(2), (3), and (5) is given due to their five-coordinate environment around metal ions. The
complexes have been characterized by elemental analyses (M, Cl, C, H, N), melting point, and
spectral (FT-IR, 1H-NMR, and 13C-NMR) studies. The structural composition of the
complexes has been determined by FAB-MS spectral studies. FAB-MS showed the isotopic
molecular ion peak [Mþ] and fragments supporting the formulation. Powder X-ray diffraction
˚
study of 6 is also reported showing the crystallite size (404.5 A) of the complex.
Keywords: Synthesis, Schiff bases; Metal complexes; IR; NMR; FAB-MS; PXRD
1. Introduction
The chemistry of Schiff bases has developed because of their facile syntheses, electronic
properties, and fair solubility in common organic solvents [1]. Metal complexes of Schiff
base are of interest in biomedical research [2–5] and chelation therapy for the treatment
of heavy-metal poisoning [6]. Schiff bases containing oxygen and nitrogen donors have
a variety of ways in which they are bonded to metal ions [7, 8]. Zinc can function as the
active site of hydrolytic enzymes and is an important co-factor in biological molecules,
either as a structural template in protein folding or as a Lewis acid catalyst that can
readily adopt coordination numbers of four, five, or six [9–11]. The variety of possible
Schiff-base metal complexes has prompted us to undertake research in this area.
*Corresponding author. Email: rajalkoxy@yahoo.com
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ß 2011 Taylor & Francis
DOI: 10.1080/00958972.2011.596934

2650
R.K. Dubey et al.
In this work, we synthesize and study the mode of coordination of chelates of
salicylidene-2-methyl-1-aminobenzene (HL1), salicylidene-2-aminopyridine (HL2), and
salicylidene-3-nitro-1-aminobenzene (HL3) Schiff bases with Zn(II) and Hg(II). This
article focuses, for the first time, on binuclear chloro-bridged (m-Cl) Zn(II) and Hg(II)
complexes of Schiff bases.
2. Experimental
Reagent grade (BDH) precursors to ligands and solvents were purified by standard
procedures [12]. Mercury(II) chloride was purchased from E. Merck and used without
purification. All other chemicals were purchased from commercial sources and used as
received. Elemental analyses (C, H, and N) were performed on a Heraceous Carlo Erba
1108 elemental analyzer. Mercury was estimated by standard method [13]. Infrared (IR)
spectra of ligands and complexes from 4000 to 200 cmꢁ1 were recorded in KBr pellets
on a Perkin-Elmer 1000 FT-IR spectrophotometer. NMR spectra of the ligands and
complexes were recorded in CDCI3 and DMSO-d6, respectively, on a Bruker DRX-300
spectrometer at the sweep width of 300 MHz and a sweep time of 300 s. FAB-MS
spectra were recorded on JEOLSX 102/DA-6000 mass spectrometer/data system using
argon/xenon (6 kV, 10 mA) as the FAB-gas. Powder X-ray diffraction (PXRD) was
˚
recorded on a Rigaku model D/Max-2200 PC using Cu-Ka1 radiation (ꢀ ¼ 1.5406 A).
2.1. Syntheses
2.1.1. Synthesis of ligands. Salicylidene-2-methyl-1-aminobenzene (HL1) was pre-
pared by refluxing (ꢂ4–5 h) equimolar salicylaldehyde (5.73 g, 47.0 mmol) and
o-toluidine (5.06 g, 47.0 mmol) in methanol (ꢂ30 cm3). After recrystallization bright
yellow solid product was obtained. [Yield (7.73 g, 78%); Anal. Calcd for C14H13NO
(%): C, 79.58; H, 6.20; N, 6.61. Found (%): C, 79.64; H, 6.25; N, 6.67].
Similar procedure was adopted for the preparation of salicylidene-2-aminopyridine
[14] (HL2) and salicylidene-3-nitro-1-aminobenzene [14] (HL3) using equimolar
amounts of salicylaldehyde and 2-aminopyridine/3-nitro-1-aminobenzene in methanol.
Sodium salts, Na(L1), Na(L2), and Na(L3), were prepared by dissolving equimolar
amounts of sodium metal and salicylidene-2-methyl-1-aminobenzene, salicylidene-2-
aminopyridine, and salicylidene-3-nitro-1-aminobenzene in methanol, respectively.
2.1.2. Synthesis of metal complexes
2.1.2.1. [(ꢁ-Cl)2Zn2(L1)2] (1). To a suspension/solution of zinc(II) chloride
(0.646 g, 4.74 mmol) in tetrahydrofuran (THF; ꢂ20 mL) was added a methanolic
solution of Na(L1) (1.105 g, 4.74 mmol) dropwise with constant stirring. The reaction
mixture was allowed to reflux for ꢂ4 h. NaCl was removed by filtration and filtrate was
concentrated by distillation. The product was dried under reduced pressure and purified
by recrystallization from THF–MeOH mixture (purity was further checked by TLC) to
afford greenish yellow powdered solid. [Yield (1.112 g, 75.4%); Anal. Calcd for

Dimeric Schiff-base complexes
2651
C28H24N2O2Cl2Zn2 (%): Zn, 21.02; Cl, 11.41; C, 54.01; H, 3.86; N, 4.50. Found (%):
Zn, 21.08; Cl, 11.43; C, 54.02; H, 3.81; N, 4.49].
Similar procedure was adopted for the synthesis of [(m-Cl)2Hg2(L1)2] (4) and
[(m-Cl)2Hg2(L3)2] (6).
2.1.2.2. [(ꢁ-Cl)2M2(L)2 ꢀ 2THF] (2), (3) and (5). Zinc(II) and mercury(II) com-
plexes [(m-Cl)2Zn2(L2)2 ꢀ 2THF] (2), [(m-Cl)2Zn2(L3)2 ꢀ 2THF] (3), and [(m-Cl)2Hg2(L2)2 ꢀ
2THF] (5) have been prepared by adopting similar procedure as described above for the
preparation of mono(chloro) complexes of the type [(m-Cl)2M2(L)2]. For the sake of
brevity, all details are collected in table 1.
3. Results and discussion
The Schiff-base complexes, [(Cl)M(L)] and [(Cl)M(L) ꢀ 2THF], have been prepared by
the reaction of metal chlorides with sodium salts of corresponding Schiff bases in
MeOH and THF:
MCl2 þ NaðLÞ ! ½ðCl ÞMðLÞꢃ þ NaCl #
1,4,and 6
THF=MeOH
MCl2 þ NaðLÞ ꢁꢁꢁꢁꢁꢁ! ½ðCl ÞMðLÞ ꢀ 2THFꢃ þ NaCl #
stir=reflux
2,3,and 5
where M ¼ Zn(II), Hg(II); L ¼ HL1, HL2, and HL3; x ¼ 2.
All these metal complexes are soluble in polar solvents such as THF, DMF, and
DMSO but insoluble in water, methanol, ethanol, pyridine, etc. The physical and
analytical details are given in table 1.
The tentative structures of the Schiff-base ligand and the metal complexes have been
suggested by IR, 1H-NMR and 13C-NMR, and elemental analysis, and structural
composition determined by FAB-MS spectral studies.
3.1. IR spectra
The IR spectra of the ligand showed bands at 1620 to 1622 cmꢁ1 attributed to ꢂ(C¼N)
[15], which is shifted to lower frequency 1598 to 1610 cmꢁ1 in the zinc(II) and
mercury(II) complexes (table 2), indicating that bonding takes place through the
azomethine nitrogen. The ꢂ(C–O) (phenolic) stretching frequencies of the ligands were
observed at 1269–1277 cmꢁ1 and shift to 1274–1289 cmꢁ1 in complexes, indicating
bonding through phenolic oxygen [14, 16].
In [(m-Cl)2Zn2(L2)2 ꢀ 2THF] (2), [(m-Cl)2Zn2(L3)2 ꢀ 2THF] (3), and [(m-
Cl)2Hg2(L2)2 ꢀ 2THF] (5), asymmetric and symmetric C–O–C stretching frequencies,
due to the presence of THF, were observed at ꢂ897, 905, and 891 cmꢁ1 (THF
asymmetric), and 761, 770, and 765 cmꢁ1 (THF symmetric) [17] as compared to ꢂ1025–
1015 cmꢁ1 (THF asymmetric) and 860–850 cmꢁ1 (THF symmetric) in free THF. Such
shifting to lower frequency in the complexes suggests coordination by oxygen of THF.
Zinc(II) and mercury(II) form dimeric four/five-coordinated [18] complexes where the

2652
R.K. Dubey et al.

Dimeric Schiff-base complexes
2653
Table 2. IR spectral data (cmꢁ1) of monochloro complexes of 1–6.
ꢂ(M–Cl)
(bridging)
ꢂ(C–O)
(phenolic)
Compound
ꢂ(M–N)
ꢂ(M–O)
ꢂ(C¼N)
ꢂNO2
ꢂ(C–O–C)
1
2
236
220
455
459
376
365
1282
1289
1604
1598
–
–
–
897(as)
761(sy)
905(as)
770(sy)
–
891(as)
756(sy)
–
3
240
457
359
1277
1602
1419(as)
1385(sy)
4
5
5200
5200
421
415
310
305
1280
1274
1610
1599
–
–
6
5200
419
302
1279
1609
1417(as)
1380(sy)
Cl
Cl
chloride forms bridges between two metal centers [
]. This is supported by
M
M
FAB-MS spectral studies showing dimeric nature of the complexes.
The bands [19, 20] at 366–315 and 464–407 cmꢁ1 are attributed to ꢂ(M–O) and ꢂ(M–N)
respectively. An intense band at ꢂ220–240 cmꢁ1 has been assigned for ꢂ(Zn–Cl)
,
,
characteristic of bridging chloride. In mercury(II) chloro complexes, ꢂ(Hg–Cl) for
bridging mercury chloride occurred [21] at 5200 cmꢁ1 [22].
3.2. Nuclear magnetic resonance (NMR) spectra
3.2.1. 1H-NMR spectra. The complexes exhibited signals at 9.50–8.77 ppm because of
deshielding on complex formation, which suggest the involvement of azomethine
nitrogen in coordination. In the complexes, the absence of a signal due to phenolic
proton (–OH) at 12.65–12.40 ppm further supports the IR spectral observations. NMR
spectra of all the ligands exhibited signals due to CH aromatic protons at 7.84–6.35 ppm
(a multiplet range), shifted downfield in comparison to free ligands due to distortion of
1
the aromatic rings as a result of complex formation [23] (tables 3 and 4). In H-NMR
spectra of 2, 3, and 5 the signal due to coordinated THF shows small upfield [24] shifts
compared to free THF (3.10 and 1.53 ppm).
3.2.2. 13C-NMR spectra. The 13C-NMR spectra provide direct information about the
skeleton of the ligand and complexes [25] (table 5). In 13C-NMR spectra of ligands
signals [26] at 150.38–149.30 and 149.31–148.59(d) ppm are assigned to azomethine,
signals at 170.65–167.69 ppm suggest phenolic oxygen coordination.
The 13C-NMR spectra of 2, 3, and 5 indicate coordination of THF to zinc. Signals at
67.06, 65.46, 64.74(t) ppm are assigned to C1 of THF; C2 of THF are at 26.77, 25.40,
24.27(t) ppm.
3.3. FAB-MS spectra
The FAB-MS spectrum [27–29] of HL2 shows a characteristic molecular ion (Mþ) peak
at m/z 199. In the zinc(II) complex, [(m-Cl)2Zn2(L2)2 ꢀ 2THF] (2), molecular ion peaks

2654
R.K. Dubey et al.
Table 3. 1H-NMR chemical shifts (ꢃ ppm) for the Schiff base and 1, 4, and 6 in DMSO-d6.
4
3
3
4
a
a
2
CH=N
X
2
CH=N
X
1
OH
1
O
5
6
M
Cl
5
6
7
complex 1,4; M=Zn; X=
H3C
1
7
Cl
HL ; X =
8
M
O
8
H3C
5
6
5
6
HL³; X =
7
7
complex 6; M=Hg; X=
X
N CH
b
b
NO2
NO2
Schiff
base/complex
ꢃ Hb
(3H, t, Hb)
ꢃ Hc (s)
(1H, s, Hc)
ꢃ OH (s)
ꢃ Ha (s)
ꢃ H (m)
HL1
HL3
1
4
6
12.40
12.62
–
–
–
8.47
8.51
8.97
8.77
8.93
2.32
–
2.43
2.41
–
–
7.98
–
–
8.38
7.33–6.79(8H, m, H1–H8)
7.84–6.35(7H, m, H1–H7)
7.77–6.60(8H, m, H1–H8)
7.80–6.72(8H, m, H1–H8)
7.89–6.57(7H, m, H1–H7)
Table 4. 1H-NMR chemical shifts (ꢃ ppm) for the Schiff base and 2, 3, and 5 in DMSO-d6.
4
3
3
4
a
a
2
CH=N
X
2
CH=N
X
1
OH
1
O
5
6
M
Cl
O
5
6
7
complex 2,5; M=Zn,Hg; X=
complex 3; M=Zn; X=
d
c
7
Cl
HL²; X =
HL³; X =
N
8
N
M
O
O
8
c
d
5
6
5
6
7
7
X
N CH
b
b
NO2
NO2
Schiff
base/complex
D Hb (s)
(1H, s, Hb)
ꢃ OH (s)
ꢃ Ha (s)
ꢃ H (THF)
ꢃ H (m)
HL2
HL3
2
12.65
12.62
–
8.59
8.51
9.37
–
7.98
–
–
–
7.52–6.86(8H, m, H1–H8)
7.84–6.35(7H, m, H1–H7)
8.09–6.46(8H, m, H1–H8)
3.81–3.48(Hc)
1.91–1.83(Hd)
3.84–3.51(Hc)
1.95–1.87(Hd)
3.98–3.57(Hc)
1.96–1.89(Hd)
3
5
–
–
9.10
9.50
8.72
–
8.32–7.03(7H, m, H1–H7)
7.94–6.48(8H, m, H1–H8)

Dimeric Schiff-base complexes
2655
Table 5. 13C-NMR data (ꢃ ppm) of Schiff bases and complexes.
Schiff
bases/complexes
HC¼N
C–O
(phenolic)
Aromatic
carbons
THF
(C1–C2) (t)
(d)
CH3
HL1
HL2
HL3
1
150.20, 149.31
149.30, 148.59
150.38, 149.23
149.13, 146.89
148.79, 146.66
161.12
160.74
160.19
170.65
167.69
133.05–117.12
133.80–115.21
134.13–114.89
149.13–109.47
139.00–136.27(py)
123.03–109.18
150.77–109.18
18.17
–
–
17.83
–
–
–
–
–
2
67.06, 65.46, 64.74(C1)
25.16, 24.60, 23.09(C2)
66.87, 66.06, 65.73(C1)
26.77, 25.40, 24.27(C2)
3
148.39, 146.53
168.37
–
Figure 1. FAB-MS spectrum of [(m-Cl)2Zn2(L2)2 ꢀ 2THF] (2).
observed at m/z 742 and m/z 740 correspond to the molecular mass
[(C32H34N4O4Cl2Zn2); calculated mass ¼ 740.04] due to isotopic pattern including
zinc and chlorine [30]. Therefore, the position of molecular ion peak in the mass
spectrum (figure 1) is consistent with the empirical molecular formula [31]. Peaks at m/z
596 and 298 are indicative of release of two THF molecules and fragmentation from
dimer to monomer species, respectively. After fragmentation of the complex, several
other peaks (Supplementary material) were also observed at m/z 670, 521, 493, 385, 321,
261, 221, 197 186, 106. The base peak with m/z 413 represents C13H9NO2Zn2Cl2 as
residue.
[(m-Cl)2Zn2(L3)2 ꢀ 2THF] (3) showed molecular ion peak at m/z 828 (Supplementary
material), confirming the proposed formula [32]. Peaks at m/z 684 and 342 observed due
to the release of two THF and dimer to monomer species, respectively [33]. The
spectrum exhibited prominent molecular ion peaks at m/z 756, 638, 563, 534, 518, 458,
426, 397, 321, 294, 242, 221, 207, 123, 97, 94 (Supplementary material). The base peak
[34] m/z 413 represents C13H10NO2Zn2Cl2 as residue.
Mercury(II) complex [(m-Cl)2Hg2(L3)2 ꢀ 2THF] (5) exhibited molecular ion peaks
(Supplementary material) at m/z 1010 and m/z 1029, which are consistent with the

2656
R.K. Dubey et al.
combination of mercury and chlorine isotopic distribution [35]. Two important peaks
(m/z 938 and 866) have also been observed, corresponding to release of two THFs and
dimer (m/z 860) to monomer (m/z 433) species. The spectrum showed prominent peaks
(Supplementary material) at m/z 788, 685, 669, 505, 398, 355, 328, 105. The base peak
with m/z 198 corresponds to HL2 (C12H9N2O).
3.4. PXRD studies
PXRD [36] is widely used to characterize a crystalline solid. XRD spectrum of 6 clearly
showed crystalline material (Supplementary material). Diffraction pattern comprises a
number of sharp Bragg’s reflections corresponding to different orientation of
crystallites. Non-uniform broadening of lines is attributed to anisotropic crystallites,
used here to calculate the crystallite size using Debye Scherrer formula [37]. The
˚
crystallite size of prepared materials is 404.5 A by using Rigaku JADE 6.0 software.
4. Conclusion
Six Schiff-base complexes, [(Cl)M(L)] and [(Cl)M(L) ꢀ 2THF] [where M ¼ Zn(II) and
Hg(II); L ¼ salicylidene-2-methyl-1-aminobenzene (HL1), salicylidene-2-aminopyridine
(HL2), and salicylidene-3-nitro-1-aminobenzene (HL3); x ¼ 0 for (1), (4), (6) and x ¼ 2
for (2), (3), (5)], have been synthesized by the reactions of metal chlorides with sodium
salts of corresponding Schiff bases in the mixture of MeOH and THF in equimolar
ratio. These chloro-bridged dimeric complexes have been characterized by elemental
1
analyses (M, Cl, C, H, N), melting point, and spectroscopic (FT-IR, H-NMR, and
13C-NMR) data as well as PXRD study. The complexes have also been proposed by
FAB-MS spectral studies.
The formulae are [(m-Cl)2Zn2(L1)2] (1), [(m-Cl)2Hg2(L1)2] (4), and [(m-Cl)2Hg2(L3)2] (6)
confirmed by FAB-mass spectra whereas 2, 3, and 5 are dimers with two THF
molecules, [(m-Cl)2Zn2(L2)2 ꢀ 2THF], [(m-Cl)2Zn2(L3)2 ꢀ 2THF], and [(m-Cl)2Hg2(L2)2 ꢀ
2THF], which have been clearly indicated in the FAB-mass spectra (figure 1 and
Supplementary material) containing five-coordinate Zn(II)/Hg(II).
Acknowledgments
The authors are grateful to the Director, C.D.R.I., Lucknow for providing spectral and
analytical data. Nanophosphor Application Center, University of Allahabad, is also
acknowledged for helping with PXRD studies of one of the compounds. One of the
authors (P.B.) thanks UGC for providing financial support.

Dimeric Schiff-base complexes
2657
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