Structural effects of potentially hexadentate Schiff base ligands involving pyrrolic, etheric or thioetheric donors towards zinc(II) cation: Synthesis, characterization and crystal structures

Article abstract of DOI:10.1016/j.ica.2013.02.035

The three new zinc (II) complexes (ZnL^m, m = 1-3) of potentially hexadentate Schiff base ligands containing N₄O₂ and N₄S₂ donors with pyrrole terminal binding groups H ₂L¹:[(1Z)-1H-pyrrole-2-ylmethylene]{2-[2-(2-{[(1Z)-1H- pyrrole-2-ylmethylene]amino}phenoxy)ethoxy]phenyl}amine, H₂L ²:[(1Z)-1H-pyrrole-2-ylmethylene]{2-[4-(2-{[(1Z)-1H-pyrrole-2- ylmethylene]amino}phenoxy)butoxy]phenyl}amine and H₂L ³:[1H-pyrrole-2-ylmethylene][2-({2-[(2-{[1H-pyrrole-2-ylmethylene] amino}phenyl)thio]ethyl}thio)phenyl]amine, were synthesized and physicochemically characterized. Cyclic voltammetry data indicate that the complexes are electrochemically inactive. The molecular structures of the complexes were determined by single crystal X-ray diffraction. The Zn(II) is five coordinated by N₄O donor set of the ligands in the ZnL ¹ and ZnL² and six coordinated by N₄S ₂ donor set in the ZnL³.

Full text of DOI:10.1016/j.ica.2013.02.035

Inorganica Chimica Acta 400 (2013) 203–209
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Inorganica Chimica Acta
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Structural effects of potentially hexadentate Schiff base ligands involving
pyrrolic, etheric or thioetheric donors towards zinc(II) cation: Synthesis,
characterization and crystal structures
b
a
a
Ali Akbar Khandar a, , Jonathan White , Tahereh Taghvaee-Yazdeli , Seyed Abolfazl Hosseini-Yazdi ,
⇑
Patrick McArdle c
a Department of Inorganic Chemistry, Faculty of Chemistry, University of Tabriz, Tabriz 51666-14766, Iran
b School of Chemistry, University of Melbourne, Parkville, Vic. 3010, Australia
c Department of Chemistry, NUI Galway, Galway, Ireland
a r t i c l e i n f o
a b s t r a c t
The three new zinc (II) complexes (ZnLm, m = 1–3) of potentially hexadentate Schiff base ligands containing
N4O2 and N4S2 donors with pyrrole terminal binding groups H2L1:[(1Z)-1H-pyrrole-2-ylmethylene]
{2-[2-(2-{[(1Z)-1H-pyrrole-2-ylmethylene]amino}phenoxy)ethoxy]phenyl}amine, H2L2:[(1Z)-1H-pyrrole-
Article history:
Received 24 July 2012
Received in revised form 9 February 2013
Accepted 26 February 2013
Available online 7 March 2013
2-ylmethylene]{2-[4-(2-{[(1Z)-1H-pyrrole-2-ylmethylene]amino}phenoxy)butoxy]phenyl}amine
and
H2L3:[1H-pyrrole-2-ylmethylene][2-({2-[(2-{[1H-pyrrole-2-ylmethylene] amino}phenyl)thio]ethyl}thio)
phenyl]amine, were synthesized and physicochemically characterized. Cyclic voltammetry data indicate
that the complexes are electrochemically inactive. The molecular structures of the complexes were deter-
mined by single crystal X-ray diffraction. The Zn(II) is five coordinated by N4O donor set of the ligands in
the ZnL1 and ZnL2 and six coordinated by N4S2 donor set in the ZnL3.
Keywords:
N4O2 and N4S2 Schiff base complexes
Zn(II) complexes
Crystal structures of Zn(II) complexes
Pyrrole-2-carboxaldehyde
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
deformylase. Binuclear cores are present at the active sites of many
metalloenzymes and play an essential role in many biological sys-
Schiff bases form an interesting class of chelating ligands that
have enjoyed popular use in the coordination chemistry of transi-
tion, inner-transition and main group elements [1–5]. The chemis-
try of metal complexes with chelate ligands containing nitrogen,
sulfur or oxygen donors has been extensively studied in order to
gain an understanding of the following processes: (i) the redox
function of various metalloenzymes in living systems; (ii) the for-
mation and reactivity of dioxygen in synthetic, industrial and bio-
logical processes. In enzymes metal ions have several functions: (i)
redox as in superoxide dismutase-like activity [6–10], (ii) struc-
tural and catalytical functions [11–15]. The complexation sites of
these proteins are N, S or O donors coming from histidine, tyrosine,
aspartic or glutamic acids and cysteine[16–20].
tems. The zinc(II) ion is known to have a high affinity towards
nitrogen and sulfur donors. Dowling and Parkin investigated Zn(II)
complexes with mixed N, O and S coordination to understand the
reactivity of the pseudotetrahedral zinc center in proteins [22]. In
order to elucidate the effects of the distinctive structural features
of the ligands on the properties of their complexes, we recently
described [23] the coordination of copper and nickel atoms with
a series of potentially hexadentate Schiff base ligands containing
N4O2 and N4S2 donors with pyrrole terminal binding groups,
H2Lm (m = 1–4) (Scheme 1).
As a continuation of our interest to provide a better understand-
ing of the physicochemical and coordination properties of com-
plexes, and as models for the active sites in metalloproteins, we
present herein the synthesis, spectroscopic characterization and
electrochemical behavior of the three zinc(II) complexes of H2Lm
(m = 1–3) ligands. The complexes were also characterized by single
crystal X-ray crystallography.
Zinc atom has either a structural or catalytic role in several pro-
teins. It has also been recognized as an important cofactor in bio-
logical molecules, either as a structural template in protein
folding or as a Lewis acid catalyst that can readily adopt 4-, 5- or
6-coordination [21].
Mononuclear zinc complexes may serve as model compounds
for zinc enzymes such as phospholipase C, bovine lens leucineami-
nopeptidase, ATPases, carbonic anhydrases and peptide
2. Experimental
2.1. Materials
⇑
The solvents and reagents used in these studies were obtained
from commercial sources and were used as received.
Corresponding author. Tel.: +98 411 3346494; fax: +98 411 3340191.
E-mail addresses: khandar@tabrizu.ac.ir, akhandar@yahoo.com (A.A. Khandar).
0020-1693/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ica.2013.02.035

204
A.A. Khandar et al. / Inorganica Chimica Acta 400 (2013) 203–209
(CH2)n
X
N
X
(CH2)n
O
H
N
X
X
N
H
2
H2N
NH2
HN
NH
X= O, S
n = 2, 4
X= O, n=2, H2L1
X= O, n=4, H2L2
X= S, n=2, H2L3
X= S, n=4, H2L4
Scheme 1. Synthesis and structure representation of Schiff base ligands (H2Lm, m = 1–4).
2.2. Physical measurements and methods
cedures [23,26,27] but with an extension of the reflux time to
48 h in the case of H2L3.
FT-IR spectra were recorded using KBr discs on a Bruker Tensor
27 instrument. The electronic spectra in the 220–500 nm range
were obtained on a Shimadzu UV-1650 PC spectrophotometer
using 1.0 cm quartz cells and solutions with the concentration of
5 ꢀ 10ꢁ5 M in CH2Cl2. Elemental analyses were carried out using
an Elementar Vario EL III instrument. Melting points were taken
using an electrothermal IA 9100 apparatus in open capillary tubes.
Cyclic voltammograms were obtained using 1 ꢀ 10ꢁ3 M solutions
of the complexes in DMF using an Auto lab potentiostat PGSTAT
302 ECO CHEMIE. All solutions were deoxygenated by passing a
stream of argon into the solution for at least 10 min prior to
recording the voltammogram. All potentials were measured at
room temperature and referenced to the saturated Ag/AgCl elec-
trode with ferrocene as an internal standard. A glassy carbon disc
with a diameter of 3 mm was used as the working electrode and
a platinum wire was used as the counter electrode. Before each
experiment the working electrode was polished with alumina
and rinsed thoroughly with distilled water and acetone. The elec-
trolytic medium consisted of 0.1 M LiClO4 in DMF. Conductivity
data were measured in DMF on a Metrohem 712 conductometer
instrument.
2.4.2. Zinc(II) complexes
All zinc (II) complexes of the ligands were prepared by addition
of a solution of zinc acetate dihydrate (0.5 mmol, 0.1097 g) in eth-
anol (20 ml) to a solution of H2L1, H2L2 and H2L3 (0.5 mmol) in
absolute ethanol. In each case, the reaction mixtures were refluxed
for 4 h, and the precipitate was filtered and recrystallized from
CH3CH2OH/CH2Cl2 (1:4, v/v)
ZnL1: color: brown, Yield: 47.63% (0.11 g), M.P > 280 °C (dec),
Selected FT-IR data
(CHaliph), 1562 s (C@N), 1296s (C–O–C)asym, 1028 s (C–O–C)sym
741 m (d CHaromatic). Anal. Calc. for C24H20ZnN4O2: C, 62.41; H,
4.36; N, 12.13. Found: C, 62.37; H, 4.261; N, 12.04%. Km = 1.6 Xꢁ1
cm2 molꢁ1 in DMF. UV–Vis [kmax/nm ( /Mꢁ1 cmꢁ1)]: 396(28620),
299(4640), 248(6720), 224(7720) in CH2Cl2
ZnL2: color: yellow, Yield: 57.15% (0.14 g), M.P > 215 °C (dec),
Selected FT-IR data
(cmꢁ1): 3066 w (CHarom), 2933 w (CHaliph),
m
(cmꢁ1); 3066 w (CHarom), 2925–2857 w
,
-
e
m
1561 s (C@N), 1285 s (C–O–C)asym, 1030 s (C–O–C)sym, 745 m (d
CHaromatic). Anal. Calc. for C28H27ZnN5O2: C, 63.74; H, 4.94; N,
11.44. Found: C, 63.69; H, 4.846; N, 11.44%. Km = 1.8
molꢁ1 in DMF. UV–Vis [kmax/nm /Mꢁ1 cmꢁ1)]: 388(30420),
X
ꢁ1 cm2 -
(
e
299(4640), 247(6740), 227(6880) in CH2Cl2
ZnL3: color: yellow, Yield: 52.63% (0.13 g), M.P > 285 °C (dec),
2.3. X-ray crystallography
Selected FT-IR data
(CHaliph), 1547 s (C@N), 1179 s (C–S–C)asym, 1034 s (C–S–C)asym
744 m (d CHaromatic). Anal. Calc. for C24H20ZnN4S2: C, 58.35; H,
4.08; N, 11.34. Found: C, 58.32; H, 3.986; N, 11.29%. Km = 2.1 Xꢁ1
cm2 molꢁ1 /Mꢁ1 cmꢁ1)]:
in DMF. UV–Vis [kmax/nm
m
(cmꢁ1): 3067 w (CHarom), 2909–2845 w
Single crystals of the complex ZnL1 were obtained from ethanol/
dichloromethane (1:4, v/v), and those of ZnL2 and ZnL3 from aceto-
nitrile/dichloromethane (1:4, v/v) by slow evaporation at room
temperature. The data sets for ZnL1 and ZnL3 were collected on
an Oxford Diffraction Super Nova diffractometer, using Enhance
,
-
(e
427(19440)(sh), 407(28140), 308(4540), 259(4760), 225(9260) in
CH2Cl2.
(Mo) X-ray structure, mirror-mono chromatized Mo K
a radiation
k = 0.7107 Å at 130 K. The data for ZnL2 were collected on an Ox-
ford Diffraction Xalibur, Sapphire 3 diffractometer with graphite-
3. Results and discussion
monochromatized Mo Ka at 293 K. Data were reduced and cor-
rected for absorption using the CrysAlisPro software [24]. The
structures were solved using direct methods and refined on F2 by
full-matrix least-squares procedures using the SHELXL-97 program
package [25]. Non-hydrogen atoms were refined anisotropically
and hydrogen atoms were located to carbon in geometric positions
refined by using a riding model. A summary of crystallographic
data for the complexes is given in Table 1.
The ZnLm (m = 1–3) complexes were prepared by the reaction of
the ligands, H2Lm, with zinc acetate dihydrate in a 1:1 ratio in eth-
anol (Scheme 1). Unfortunately all attempts using a range of differ-
ent procedures failed to synthesize ZnL4. All elemental analyses are
consistent with the proposed molecular formulae which had a ratio
of 1:1 metal:ligand in all cases. The low molar conductivity of the
complexes in ca. 10ꢁ3 M solutions in DMF at room temperature
[28], on the one hand, and the absence of pyrrole N–H stretches
in the FT-IR spectra of the complexes, on the other hand, indicate
that the complexes are all neutral and that the ligands act as dou-
bly negatively charged anions in complexation by deprotonation of
the pyrrole groups, prior to complexation. The zinc ions are also
bound to the ligands through the azomethine nitrogens. This can
2.4. Synthesis
2.4.1. Diamines and ligands
The Schiff base ligands, H2L1, H2L2 and H2L3 (Scheme 1) and
their related diamines were prepared according to published pro-

A.A. Khandar et al. / Inorganica Chimica Acta 400 (2013) 203–209
205
Table 1
Crystallographic data for ZnL1, ZnL2 and ZnL3.
ZnL1
ZnL2
ZnL3
Formula
Formula weight
T (K)
C24H20N4O2Zn
461.81
130.0
C28H27N5O2Zn
530.92
293(2)
C24H20N4S2Zn
493.93
130.0
Crystal color
Crystal size (mm)
Crystal system
Space group
a (Å)
brown
yellow
yellow
0.34 ꢀ 0.23 ꢀ 0.20
monoclinic
C2
20.7753(4)
10.6096(2)
20.0017(4)
109.329(2)
4160.22(14)
8
1.210
1.475
0.7107
5652/5371
1904
0.54 ꢀ 0.19 ꢀ 0.10
monoclinic
P21/n
14.0530(10)
22.5538(13)
17.2518(13)
110.235(8)
5130.5(6)
0.53 ꢀ 0.36 ꢀ 0.21
monoclinic
P21/c
14.2936(3)
10.6233(2)
15.3449(3)
109.423(2)
2197.44(8)
4
1.327
1.493
0.7107
3865/3618
1016
b (Å)
c (Å)
b (°)
V (Å3)
Z
l
8
(mmꢁ1
)
0.992
1.375
0.7107
9371/4850
2208
Dcalc (Mg/m3)
Radiation (k, Å)
Reflections collected/unique
F(000)
h (°)
Index ranges
3.2105–28.0752
ꢁ24 6 h 6 18, ꢁ12 6 k 6 11,
ꢁ23 6 l 6 20
5652/1/559
0.957
2.9228–29.2060
ꢁ18 6 h 6 18, ꢁ31 6 k 6 23,
ꢁ22 6 l 6 20
9371/0/651
1.025
3.0162–29.2362
ꢁ16 6 h 6 16, ꢁ12 6 k 6 11,
ꢁ18 6 l 6 18
3865/0/281
1.052
Data/restraints/parameters
Goodness-of-fit (GOF) on F2
Final R indices [I > 2
R indices (all data)
r
(I)]
R1 = 0.0272, wR2 = 0.0516
R1 = 0.0295, wR2 = 0.0493
0.293 and ꢁ0.291
R1 = 0.0648, wR2 = 0.1792
R1 = 0.1288, wR2 = 0.1621
0.536 and ꢁ0.590
R1 = 0.0222, wR2 = 0.0571
R1 = 0.0244, wR2 = 0.0560
0.276 and ꢁ0.247
Largest difference in peak and hole (e/
Å3)
P
P
P
P
R1 = [ ꢁ||Fo| ꢁ |Fc||]/ |Fo| (based on F), wR2 = {[ w(|Fo ꢁ Fc2|)2]/[ w(Fo2)2]}1/2 (based on F2).
2
Fig. 1. UV–Vis spectra of the free ligands and their corrosponding zinc complexes.
be deduced from the observed decreases of 55, 62 and 74 cmꢁ1 in
one broad and intense transition in 330–450 nm region, centered
at 396, 388 and 407 nm for ZnL1, ZnL2 and ZnL3, respectively, are
C@N stretching frequencies in comparison with those of corre-
sponding free ligands, H2Lm (m = 1–3, 1617, 1623 and 1621 cmꢁ1
,
corresponding to p ?
p⁄ and n ? p⁄ transitions [23]. In contrast
respectively). It seems that the C@N frequency in the complex is af-
fected by the coordination geometry of the complex but the length
of the aliphatic linkage has relatively little impact. ZnL1 and ZnL2
the electronic transitions of the free ligands are intense and blue
shifted with respect to the complexes, Fig. 1. It seems that the di-
pole moments in the complexes are quite different from the free
ligands.
with five coordinated X-ray structures have equivalent mC
of 1562 and 1561 cmꢁ1. In contrast, ZnL3 with a six-coordinated X-
ray structure has a quite different
C@N, of 1547 cmꢁ1. Compared
@N values
m
4. Structural studies
to these complexes, NiL3 and CuL2 analogs [23] with six- and
four-coordination geometries, respectively, show comparable
C@N stretching frequencies of 1540 cmꢁ1 for NiL3 and 1558 cmꢁ1
CuL2. The UV–Vis spectra of the complexes were studied in dichlo-
romethane at 5 ꢀ 10ꢁ5 M concentration in the region of 220–
500 nm. All the complexes have similar spectral features with only
X-ray analyses reveal that the compounds ZnL1, ZnL2 and ZnL3
crystalize in the monoclinic space groups C2, P21/n and P21/c,
respectively. The ORTEP diagrams of the compounds along with
atomic numbering schemes are in Figs. 2,3 and 4 and selected bond
lengths and angles are given in Tables 2,3 and 4, respectively.

206
A.A. Khandar et al. / Inorganica Chimica Acta 400 (2013) 203–209
Fig. 2. Crystal structure of the ZnL1. Hydrogen atoms are omitted for clarity.
Table 2
Selected bond lengths (Å) and bond angles (°) for ZnL1.
Bond lengths
Zn–N(1)
Zn–N(2)
Zn–N(3)
Zn–N(4)
Zn–O(2)
2.062(3)
2.049(3)
2.007(3)
2.027(3)
2.559(2)
Zn0–N0(1)
Zn0–N0(2)
Zn0–N0(3)
Zn0–N0(4)
Zn0–O0(2)
2.073(3)
2.050(3)
1.996(3)
2.028(3)
2.574(2)
Bond angles
N(1)–Zn–N(2)
N(2)–Zn–N(3)
N(3)–Zn–N(1)
O(2)–Zn–N(4)
N(4)–Zn–N(1)
N(4)–Zn–N(2)
N(4)–Zn–N(3)
O(2)–Zn–N(1)
O(2)–Zn–N(2)
O(2)–Zn–N(3)
145.27(10)
120.47(11)
82.72(11)
149.25(9)
111.82(11)
82.12(11)
116.75(10)
86.27(8)
N0(1)–Zn0–N0(2)
N0(2)–Zn0–N0(3)
N0(3)–Zn0–N0(1)
O0(2)–Zn0–N0(4)
N0(4)–Zn0–N0(1)
N0(4)–Zn0–N0(2)
N0(4)–Zn0–N0(3)
O0(2)–Zn0–N0(1)
O0(2)–Zn0–N0(2)
O0(2)–Zn0–N0(3)
146.00(11)
122.01(11)
82.97(11)
149.10(9)
110.29(11)
81.93(11)
114.07(10)
88.91(8)
69.95(9)
89.21(9)
69.12(9)
91.15(9)
Fig. 3. Crystal structure of the ZnL2. Hydrogen atoms are omitted for clarity.
Table 3
Selected bond lengths (Å) and bond angles (°) for ZnL2.
Bond lengths
Zn(1)–N(1)
Zn(1)–N(2)
Zn(1)–N(3)
Zn(1)–N(4)
Zn(1)–O(1)
2.052(4)
2.060(4)
2.007(4)
2.009(4)
2.603(3)
Zn(2)–N(6)
Zn(2)–N(5)
Zn(2)–N(8)
Zn(2)–N(7)
Zn(2)–O(3)
2.050(4)
2.056(4)
2.010(4)
2.010(4)
2.633(4)
Bond angles
N(1)–Zn(1)–N(3)
N(3)–Zn(1)–N(2)
N(2)–Zn(1)–N(4)
N(4)–Zn(1)–N(1)
N(1)–Zn(1)–N(2)
N(4)–Zn(1)–N(3)
O(1)–Zn(1)–N(1)
O(1)–Zn(1)–N(2)
O(1)–Zn(1)–N(3)
O(1)–Zn(1)–N(4)
82.73(19)
110.51(18)
82.50(17)
109.96(17)
153.57(15)
123.24(17)
66.89(14)
91.07(14)
142.89(15)
88.22(13)
N(6)–Zn(2)–N(8)
N(8)–Zn(2)–N(5)
N(5)–Zn(2)–N(7)
N(7)–Zn(2)–N(6)
N(6)–Zn(2)–N(5)
N(7)–Zn(2)–N(8)
O(3)–Zn(2)–N(5)
O(3)–Zn(2)–N(6)
O(3)–Zn(2)–N(7)
O(3)–Zn(2)–N(8)
83.14(19)
103.53(18)
81.60(16)
110.96(16)
159.51(15)
124.27(17)
66.63(14)
94.26(14)
137.98(13)
90.69(17)
other are presented in brackets. The crystal structure of ZnL2 also
contains two acetonitrile molecules in each asymmetric unit.
In the ZnL1and ZnL2 compounds, Zn(II) is five-coordinated hav-
ing strong interactions with two imine and two pyrrole nitrogens
and a weaker interaction with an etheric oxygen atom. These inter-
actions are characterized by the following bond distances (Å); Zn–
N(1) 2.062(3)[2.073(3)], Zn–N(2) 2.049(3)[2.050(3)], Zn–N(3)
Fig. 4. Crystal structure of the ZnL3. Hydrogen atoms are omitted for clarity.
The asymmetric units of the crystals of ZnL1 and ZnL2 contain
two independent molecules, which are essentially identical; there-
fore herein after the values for one molecule the values for the

A.A. Khandar et al. / Inorganica Chimica Acta 400 (2013) 203–209
207
Table 4
normal, when compared with the values of 2.550–2.711 Å cited
in the literatures [33–41].
Selected bond lengths (Å) and bond angles (°) for ZnL3.
These long Zn–O distances indicate a relatively weak interac-
tion, which can be considered to be secondary coordination. The
distance between Zn and uncoordinated oxygen atoms O(1) and
O(1)0 in ZnL1 and O(2) and O(4) in ZnL2 (2.845 Å [2.755 Å] and
2.965 Å [2.771 Å], respectively) are distinctly longer than the
bonded Zn–O distances.
Bond lengths
Zn–N(1)
Zn–N(2)
Zn–N(3)
2.124(1)
2.031(1)
2.133(1)
Zn–S(1)
Zn–N(4)
Zn–S(2)
2.731(1)
2.046(1)
2.717(1)
Bond angles
S(1)–Zn–N(1)
S(1)–Zn–N(2)
N(1)–Zn–N(2)
N(1)–Zn–N(3)
N(2)–Zn–N(3)
N(3)–Zn–S(1)
S(2)–Zn–N(4)
S(2)–Zn–S(1)
75.24(4)
154.49(4)
81.61(5)
156.36(5)
118.25(5)
82.51(4)
153.27(4)
78.84(1)
S(2)–Zn–N(1)
S(2)–Zn–N(2)
S(2)–Zn–N(3)
N(4)–Zn–S(1)
N(4)–Zn–N(1)
N(4)–Zn–N(2)
N(4)–Zn–N(3)
92.19(4)
91.63(4)
75.53(4)
86.09(4)
105.24(5)
110.58(5)
80.75(6)
Ignoring the weak Zn–O bonds in the ZnL1 and ZnL2, both com-
plexes become four coordinated by two imine and two pyrrole
nitrogen atoms as CuL2 analog [23]. This assumption probably
could be supported by the C@N frequencies of these three com-
plexes 1562(ZnL1), 1561(ZnL2) and 1558 cmꢁ1 (CuL2). The four
ZnꢁN primary coordinate bonds have seesaw geometry according
to the geometrical parameters
s
4, defined as [360ꢁ(
a
+ b)]/141,
4 Val-
where and b are the two largest coordination angles [42].
a
s
2.007(3)[1.997(3)] and Zn–N(4) 2.027(3)[2.028(3)] in the ZnL1 and
the corresponding mean values (Å) in the ZnL2 are Zn(1)–Nimine
2.056[2.054], Zn(1)–Npyrrole 2.009[2.010] and Zn(1)–O(1)
2.602[2.633].
ues are zero and unity for perfect square planar and tetrahedral
geometry, respectively. The calculated s4 indexes are 0.67[0.65]
and 0.59[.054] for ZnL1 and ZnL2, respectively. The angle between
the
Nimine–Zn–Nimine and Npyrrole–Zn–Npyrrole mean planes is
70.35° [70.94°] in ZnL1 and 73.91° [76.79°] in ZnL2. It seems that
the lengths of aliphatic linkage do not affect the coordination mode
in ZnL1 and ZnL2 complexes.
The bond angles around the metal ions in those two complexes
are in the range 145.27(10)–159.51(15)° for Nimine–Zn–Nimine
114.07(10)–124.27(17)° for pyrrole–Zn–Npyrrole 81.60(16)ꢁ
83(14)° for cis Nimine–Zn–Npyrrole, 103.53(18)ꢁ122.01(11)° for trans
imine–Zn–Npyrrole, 88.22(13)ꢁ149.25(9)° for O–Zn–Npyrrole, and
66.63(14)–94.26(14)° for O–Zn–Nimine
The coordination geometry around Zn(II) is better described as a
distorted tetragonal pyramidal geometry, according to Addison
and Reedijk’s angular structural parameter,
ZnL1 and 0.18[0.36] for ZnL2. In five-coordinated geometry, s5 is
defined as (b– )/60, where and b are the two largest coordination
,
N
,
In the complex ZnL3, Zn(II) is located in a very distorted octahe-
dral environment, judged from the spread in its observed cis and
trans angles of [75.24(4)–118.25(5)°] and [153.27(4)–156.36(5)°],
respectively. The Zn(II) ion is bound through the N4S2 atoms where
both imine nitrogens are disposed trans to each other with a bond
angle of 156.36(5)° and both thioether sulfur atoms and the two
pyrrole nitrogen atoms occupy cis coordination sites with bond an-
gles of 78.84(1)° and 110.58(5)°, respectively. The Zn–Npyrrole bond
distances [Zn–N(2) 2.031(1) Å and Zn–N(4) 2.046(1) Å] are compa-
rable to corresponding distances in ZnL1 and ZnL2, but the Zn–
Nimine distances [Zn–N(1)2.124(1) Å and Zn–N(3)2.133(1) Å] are
slightly longer than corresponding distances in ZnL1 and ZnL2.
In all of these complexes, the Zn–Npyrrole bond distances are
shorter than the Zn–Nimine bond lengths, leading to rather strong
Zn–Npyrrole bonding, because the Npyrrole’s participate in bonding
with metal center as anionic nitrogens and could donate more
electron density to the metal ion [23].
N
.
s5 = 0.07[0.05] for
a
a
angles [29]. The values of s5 are zero and unity for perfect tetrag-
onal pyramidal and trigonal bipyramidal geometry, respectively.
In the present case b = N(4)–Zn–O(2) = 149.25° [149.09°] and
a
= N(1)–Zn–N(2) = 145.27° [146.00°] in the ZnL1 and b and
a in
ZnL2 are 153.57° [159.51°] and 142.89° [137.98°], respectively.
The Zn–O bond distances in ZnL1 and ZnL2 (2.559(2) Å
[2.574(2) Å] and (2.602(3) Å [2.633(4) Å], respectively) are signifi-
cantly longer than normal Zn–O bond distances, which are gener-
ally around 2 Å [30–32], but nevertheless, it may be considered
Fig. 5. The 1D coordination structure of the ZnL2 complex.
Fig. 6. Cyclic voltammogram of ZnL2 in DMF at scan rates of 50 and 100 mV sꢁ1
.

208
A.A. Khandar et al. / Inorganica Chimica Acta 400 (2013) 203–209
The Zn–S bond distances (average 2.724 Å) are comparable to
Acknowledgements
those in references [43–46], but however seem rather longer than
to those of comparable complex (average 2.617 Å) [47]. According
to Brand and Vahrenkamp, the octahedral zinc complexes with Zn–
S coordination are rare and that Zn–S bonding may be weak in such
complexes [48].
We are grateful to University of Tabriz Research Council for the
financial support of this research.
The Zn–O bond distances in ZnL1 and ZnL2 (2.559–2.965 Å) are
comparable with Zn–OGlu or Zn–OTyr bond distances (2.5 and 3.0 Å
respectively) reported for bacillolysin [36] and protease enzymes
[37,38]. Thus it seems that the H2L1, H2L2 and H2L3 ligands are good
platforms with which to model the structure or reactivity of zinc
metalloproteins [49]. Although, the Zn–S bond distances in ZnL3
(2.717, 2.731 Å) are longer than those reported for zinc proteins
such as cobalamin with Zn–S bond distances of 2.32 Å [21].
The crystal structures of ZnL1 and ZnL3 show no significant
interactions between the adjacent molecules. But in the ZnL2 crys-
tals each molecule has three H-bonding interactions with two dif-
ferent adjacent molecules (Table S1). One of the interactions is
between the adjacent molecules in the same asymmetric unit
(C(10)pyrrole ringꢂ ꢂ ꢂHC(28)phenol ring) and two others are between
the adjacent molecules from adjacent asymmetric units (CH(21)
phenol ringꢂ ꢂ ꢂC(39) pyrrole ring) and CH(23) aliphatic link-
ageꢂ ꢂ ꢂC(41) pyrrole ring) and build up a one dimensional polymer
along the b axis (see Fig. 5). Acetonitrile molecules in the ZnL2 crys-
tal are also H-bonded to different adjacent complex molecules. One
is involved with two adjacent complex molecules (N(10)ꢂ ꢂ ꢂHC(16),
CH(55)Bꢂ ꢂ ꢂC(31)) and the other one is engaged with three mole-
Appendix A. Supplementary material
CCDC 880929, 880930, and 880931 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif. Supplementary data
associated with this article can be found, in the online version, at
http://dx.doi.org/10.1016/j.ica.2013.02.035.
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