Distorted square-antiprism geometry of new zirconium (IV) Schiff base complexes: Synthesis, spectral characterization, crystal structure and investigation of biological properties

Article abstract of DOI:10.1016/j.molstruc.2017.08.022

A series of zirconium (IV) Schiff-base complexes formulated as [Zr (L^x)₂] and [Zr (L^x′)₂] were synthesized by reaction of tetradentate salicylaldimine Schiff-base ligands with [Zr (acac)₄] in methanol

Full text of DOI:10.1016/j.molstruc.2017.08.022

Accepted Manuscript
Distorted square-antiprism geometry of new zirconium (IV) Schiff base complexes:
Synthesis, spectral characterization, crystal structure and investigation of
biological properties
Atefeh Sahraei, Hadi Kargar, Mohammad Hakimi, Muhammad Nawaz Tahir
PII:
S0022-2860(17)31088-8
10.1016/j.molstruc.2017.08.022
MOLSTR 24160
DOI:
Reference:
To appear in:
Journal of Molecular Structure
Received Date:
Revised Date:
Accepted Date:
09 May 2017
05 August 2017
07 August 2017
Please cite this article as: Atefeh Sahraei, Hadi Kargar, Mohammad Hakimi, Muhammad Nawaz
Tahir, Distorted square-antiprism geometry of new zirconium (IV) Schiff base complexes:
Synthesis, spectral characterization, crystal structure and investigation of biological properties,
Journal of Molecular Structure (2017), doi: 10.1016/j.molstruc.2017.08.022
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ACCEPTED MANUSCRIPT
Distorted square-antiprism geometry of new zirconium (IV) Schiff
base complexes: Synthesis, spectral characterization, crystal structure
and investigation of biological properties
Atefeh Sahraeia, Hadi Kargara,*, Mohammad Hakimia, Muhammad Nawaz Tahirb
a Department of Chemistry, Payame Noor University, 19395-3697 Tehran, I. R. Iran
b Department of Physics, University of Sargodha, Punjab, Pakistan
e-mail*: h.kargar@pnu.ac.ir and hadi_kargar@yahoo.com
Abstract
A series of zirconium(IV) Schiff-base complexes formulated as [Zr(Lx)2] and [Zr(Lx′)2] were
synthesized by reaction of tetradentate salicylaldimine Schiff-base ligands with [Zr(acac)4] in
methanol. These ligands are as follows: H2Lx and H2Lx′ (x=x′=1,2,3,4). The new complexes have
l
been characterized using FT-IR, H NMR, 13C{1H} NMR spectroscopic techniques, elemental
analyses (CHN) and the crystal structure of Zr(L3)2, Zr(L1′)2 and Zr(L3′)2 were determined by X-ray
crystallography. The crystal structure analysis revealed that the coordination environment of the
metal in all complexes is eight-coordinate occupied by N2O2 sets of the coordinated ligands in
distorted square-antiprism geometry. The in vitro biological screening effects of the synthesized
compounds were tested against different microbial kinds and the zirconium(IV) complexes exhibit
antimicrobial activity.
Keywords: N2O2 donor, Tetradentate Schiff-base ligands, Zirconium(IV) complexes, Distorted
square-antiprism, Biological properties
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Introduction
Schiff-base ligands are one of the most prevalent coordinating ligands in coordination chemistry.
Schiff-base ligands such as salen (salicylaldiminatoethylenediamine) and salophen
(salicylaldiminatophenylenediamine) and their metal complexes have a variety of biological,
medicinal and analytical applications in addition to their important roles in catalysis and organic
syntheses [1–4]. Metal complexes of Schiff-base ligands are sometimes more effective than their
free ligands in catalytic activities [5–7]. Schiff-base complexes are also known as antifungal and
anti-bacterial compounds [8, 9]. Therefore, experimental studies and also structural elucidation of
these complexes is of great interest for chemists. In contrast to the considerable growth of literature
on the chemistry of Schiff-base complexes of first row transition metals, the chemistry of the
second and third row metal complexes such as zirconium is less well studied [10–14]. A high
charge-to-size ratio (Z2/r = 22.22 e2 m-10) of the zirconium(IV) ion gives it a marked tendency for
higher coordination numbers including 7 and 8. Among the zirconium salts, [Zr(acac)4] is stable and
low toxicity complex (LD50 {[Zr(acac)4] oral rat}=719 mg⁄kg) [15-17]. This compound can undergo
partial or total exchange of monoanionic acetylacetonate (acac) with other suitable ligands to form
various complexes. The continuing interest in eight-coordinate zirconium complexes is partly due to
their potential application as components of coordination polymers [18–21] and also their
coordination and structural chemistry [10–14].
According to the aforementioned points about the chemistry of zirconium Schiff base complexes,
herein we report two types of zirconium(IV) Schiff-base complexes synthesized based on ligands
derived from 2,2′-dimethyl-1,3-propanediamine and ligands derived from 4,5-dimethyl-1,2-
l
phenylenediamine. These complexes were characterized using FT-IR, H NMR, 13C{1H} NMR
spectroscopic techniques, elemental analyses (CHN) and the crystal structures of Zr(L3)2, Zr(L1′)2
and Zr(L3′)2 were determined by X-ray crystallography. The in vitro biological screening effects of
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the synthesized compounds were tested against different microbial kinds and the zirconium(IV)
complexes exhibit antimicrobial activity.
Experimental
Chemicals and instrumentations
1
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All chemicals are of reagent grade, used without more purification. H and C{1H}NMR spectra
were recorded at ambient temperature with BRUKER AVANCE 400 MHz spectrometer using
CDCl3 as solvent. The chemical shift values (δ) are given in ppm. Infrared spectra were recorded
from KBr pellets in the range from 4000 to 400 cm−1 using a Shimadzu FT-IR Prestige 21
spectrophotometer. Microanalyses (CHN) of the ligands and the complexes were carried out on a
Leco CHNS elemental analyzer. For X-ray data collection light-yellow crystals were obtained by
slow evaporation of a methanol solution of the respective complexes. X-ray data were collected on
a Bruker Smart APEX CCD diffractometer with graphite monochromated Mo Kα (λ = 0.71073 Å)
radiation. Full spheres of reciprocal lattice were scanned by 0.3° steps in omega with a crystal–to–
detector distance of 5 cm. Cell refinement and data reduction were performed with the help of the
SAINT program [22]. Corrections for absorption were carried out with the multi-scan method and
program SADABS [22]. The structures were solved with direct methods using SHELX2014/7 and
structure refinement on F2 was carried out with SHELX2014/7 program [23]. All non-hydrogen
atoms were refined anisotropically. All calculations were done by PLATON [24].
Syntheses of Schiff-base ligands
The ligand H2L1 (3-methoxysalicylidene)-4,5-dimethyl-1,2-phenylenediamine was synthesized by
adding 3-methoxy-salicylaldehyde (0.609 g, 4 mmol) to a solution of 4,5-dimethyl-1,2-
phenylenediamine (0.272 g, 2 mmol) in ethanol (20 ml). After refluxing for 1.5 h the solution was
left to evaporate slowly at room temperature. After 5 days, orange crystals of H2L1 were isolated.
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The compound was characterized by using IR, H NMR spectroscopy and elemental analyses
(CHN). Other ligands were produced by this method. The X-ray structures of some of the ligands
were reported in previous journal papers [25-27]
H2L1: (3-methoxysalicylidene)-4,5-dimethyl-1,2-phenylenediamine, Yield: 93 %, m.p.: 192 °C.
Anal. calcd. for C24H19N2O4 (%): C, 72.17; H, 4.79; N, 7.01. Found: C, 72.55; H, 4.68; N, 7.15.
IR (KBr, cm-1): 1618 (C=N), 1574, 1465 (C=C), 1249 (C-O).
1H NMR (400 MHz, CDCl3): 13.38 (s, 2 H, O-H), 8.60 (s, 2 H, HC=N), 6.85 (t, 3J = 7.85 Hz, 2 H,
Hb), 6.95 (dd, 3J = 8.0, 4J = 1.2 Hz, 2 H, Ha), 6.99 (s, 2 H, Hd), 7.00 (dd, 3J = 8.0 Hz, 4J = 1.2 Hz, 2
H, Hc), 3.89 (s, 6 H, -OCH3), 2.32 (s, 6 H, CH3).
H2L2: (4-methoxysalicylidene)-4,5-dimethyl-1,2-phenylenediamine, Yield: 87 %, m.p.: 193 °C.
Anal. calcd. for C24H19N2O4 (%): C, 72.17; H, 4.79; N, 7.01. Found: C, 72.38; H, 4.87; N, 6.80.
IR (KBr, cm-1): 1612 (C=N), 1591,1494 (C=C), 1292 (C-O).
1H NMR (400 MHz, CDCl3): 13.76 (s, 2 H, O-H), 8.53 (s, 2 H, HC=N), 6.46 (dd, 3J = 8.6 Hz, 4J =
2.4 Hz, 2 H, Hb), 6.55 (d, 4J = 2.4 Hz, 2 H, Ha), 7.00 (s, 2 H, Hd), 7.24 (d, 3J = 8.6 Hz, 2 H, Hc), 3.83
(s, 6 H, -OCH3), 2.31 (s, 6 H, CH3).
H2L3: (5-methoxysalicylidene)-4,5-dimethyl-1,2-phenylenediamine, Yield: 91 %, m.p.: 135 °C.
Anal. calcd. for C24H19N2O4 (%): C, 72.17; H, 4.79; N, 7.01. Found: C, 72.42; H, 4.88; N, 6.93.
IR (KBr, cm-1): 1618 (C=N), 1575,1487 (C=C), 1271 (C-O).
1H NMR (400 MHz, CDCl3): 12.74 (s, 2 H, O-H), 8.60 (s, 2 H, HC=N), 7.04 (s, 2 H, Hc), 6.99 (s, 2
H, Hd), 6.98 (d, 3J = 8.0 Hz, 2 H, Ha), 6.88 (d, 3J = 8.0 Hz, 2 H, Hb), 3.80 (s, 6 H, -OCH3), 2.34 (s, 6
H, CH3).
H2L4: (6-methoxysalicylidene)-4,5-dimethyl-1,2-phenylenediamine, Yield: 84 %, m.p.: 201 °C.
Anal. calcd. for C24H19N2O4 (%): C, 72.17; H, 4.79; N, 7.01. Found: C, 72.53; H, 4.87; N, 6.91.
IR (KBr, cm-1): 1612 (C=N), 1595,1462 (C=C), 1249 (C-O).
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1H NMR (400 MHz, CDCl3): 14.14 (s, 2 H, O-H), 9.11 (s, 2 H, HC=N), 7.22 (t, J = 8.0 Hz, 2 H,
Hb), 7.03 (s, 2 H, Hd), 6.63 (d, 3J = 8.0 Hz, 2 H, Hc), 6.34 (d, 3J = 8.0 Hz, 2 H, Ha), 3.86 (s, 6 H, -
OCH3), 2.33 (s, 6 H, CH3).
The ligand H2L1′ (3-methoxysalicylidene)-2,2-dimethyl-1,3-propandiamine was synthesized by
adding 3-methoxy-salicylaldehyde (0.609 g, 4 mmol) to a solution of 2,2-dimethyl-1,3-
propanediamine (0.204 g, 2 mmol) in ethanol (20 ml). After refluxing for 2 h the solution was left
to evaporate slowly at room temperature. After 7 days, yellow crystals of H2L1′ were isolated. The
l
compound was characterized by using IR, H NMR spectroscopy and elemental analyses (CHN).
Other ligands were produced by this method. The X-ray structures of some of the ligands were
reported in previous journal papers [28-30].
H2L1′: (3-methoxysalicylidene)-2,2-dimethyl-1,3-propanediamine, Yield: 89 %, m.p.: 102 °C.
Anal. calcd. for C21H26N2O4 (%): C, 68.09; H, 7.07; N, 7.56. Found: C, 68.49; H, 7.19; N, 7.37.
IR (KBr, cm-1): 1627 (C=N), 1475 (C=C), 1253 (C-O).
1H NMR (400 MHz, CDCl3): 14.12 (broad s, 2 H, O-H), 8.20 (s, 2 H, HC=N), 7.15 (d, 3J = 8.5 Hz,
2 H, Ha), 6.47 (d, 4J = 2.4 Hz, 2 H, Hc), 6.44 (dd, 3J = 8.5 Hz, 4J = 2.4 Hz, 2 H, Hb), 3.86 (s, 6 H, -
OCH3), 3.45 (s, 4 H, Hd), 1.10 (s, 6 H, CH3).
H2L2′: (4-methoxysalicylidene)-2,2-dimethyl-1,3-propanediamine, Yield: 78 %, m.p.: 103 °C.
Anal. calcd. for C21H26N2O4 (%): C, 68.09; H, 7.07; N, 7.56. Found: C, 67.88; H, 7.19; N, 7.42.
IR (KBr, cm-1): 1627 (C=N), 1442 (C=C), 1222 (C-O).
1H NMR (400 MHz; CDCl3): 13.28 (broad s, 2 H, O-H), 8.46 (s, 2 H, HC=N), 7.61 (d, 3J = 8.0 Hz,
4
2 H, Hc), 6.43 (dd, 3J = 8.0 Hz, J = 2.4, 2 H, Hb), 6.37 (d, 4J = 2.4 Hz, 2 H, Hc), 3.74 (s, 6 H, -
OCH3), 3.31 (s, 4 H, Hd), 0.97 (s, 6 H, CH3).
H2L3′: (5-methoxysalicylidene)-2,2-dimethyl-1,3-propanediamine, Yield: 83 %, m.p.: 89 °C.
Anal. calcd. for C21H26N2O4 (%): C, 68.09; H, 7.07; N, 7.56. Found: C, 68.33; H, 6.95; N, 7.65.
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IR (KBr, cm-1): 1637 (C=N), 1494 (C=C), 1271 (C-O).
1H NMR (400 MHz; CDCl3): 13.03 (broad s, 2 H, O-H), 8.28 (s, 2 H, HC=N), 6.94 (d, 3J = 8.8 Hz,
4
2 H, Ha), 6.92 (dd, 3J = 8.8 Hz, J = 2.7, 2 H, Hb), 6.77 (d, 4J = 2.7 Hz, 2 H, Hc), 3.77 (s, 6 H, -
OCH3), 3.48 (s, 4 H, Hd), 1.07 (s, 6 H, CH3).
H2L4′: (6-methoxysalicylidene)-2,2-dimethyl-1,3-propanediamine, Yield: 75 %, m.p.: 116 °C.
Anal. calcd. for C21H26N2O4 (%): C, 68.09; H, 7.07; N, 7.56. Found: C, 68.34; H, 7.01; N, 7.35.
IR (KBr, cm-1): 1624 (C=N), 1467 (C=C), 1249 (C-O).
1H NMR (400 MHz; CDCl3): 14.68 (broad s, 2 H, O-H), 8.76 (s, 2 H, HC=N), 7.22 (t , 3J = 8.2 Hz,
2 H, Hb), 6.54 (d, 3J = 8.2 Hz, 2 H, Ha), 6.27 (d, 3J = 8.2 Hz, 2 H, Hc), 3.81 (s, 6 H, -OCH3), 3.47 (s,
4 H, Hd), 1.06 (s, 6 H, CH3).
Syntheses of zirconium(IV) Schiff-base complexes
Zr(L1)2: [bis(3-methoxysalicylidene)-4,5-dimethyl-1,2-phenylenediamine] zirconium, Yield: 89%.
The Schiff-base bis(3-methoxysalicylidene)-4,5-dimethyl-1,2-phenylenediamine (0.809 g, 2 mmol)
was dissolved in freshly distilled methanol (30 mL). To this yellow solution was added [Zr(acac)4]
(0.488 g, 1 mmol) in one portion. The reaction mixture was then stirred and maintained at the reflux
temperature for 4 h. After filtration of the suspension with a cannula, the filtrate was concentrated
and a light yellow crystalline solid appeared over a period of several days. The solid was washed
twice with Et2O and dried in vacuo. Yellow crystals suitable for X-ray analysis were grown from a
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methanol solution of complex Zr(L1)2. This complex was characterized by H NMR, C NMR, IR
spectroscopy and elemental analyses (CHN).
Anal. calcd. for C48H44N4O8Zr (%): C, 64.34; H, 4.95; N, 6.25. Found: C, 64.54; H, 5.05; N, 5.97.
IR (KBr, cm-1): 1618, 1593 (C=N), 1550, 1469 (C=C), 1313 (C-O), 559 (Zr-O), 416 (Zr-N).
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1H NMR (400 MHz, CDCl3, 293 K): δ 8.88 (s, 2 H, Hi′-C=N), 8.21 (s, 2 H, Hi-C=N), 7.80 (s, 2 H,
Hd′), 7.04 (dd, 3J = 8 Hz, 4J = 1.6 Hz, 2 H, Hc′), 6.85 (dd, 3J = 8 Hz, 4J = 1.6 Hz, 2 H, Hc), 6.60 (t, J
= 7.6 Hz, 2 H, Hb′), 6.54 (t, J = 7.6 Hz, 2 H, Hb), 6.49 (dd, 3J = 8 Hz, 4J = 1.6 Hz, 2 H, Ha′), 6.39 (s,
2 H, Hd), 6.37 (dd, 3J = 8 Hz, 4J = 1.6 Hz, 2 H, Ha), 3.49 (s, 6 H, -OCH3(o′)), 3.43 (s, 6 H, -OCH3(o)),
2.42 (s, 6 H, -CH3(m′)), 2.14 (s, 6 H, -CH3(m)).
13C{1H} NMR (100.6 MHz, CDCl3, 293 K): δ 161.32, 159.66, 156.67, 154.62, 150.88, 149.72,
143.32, 143.25, 135.69, 134.84, 125.24, 124.03, 122.88, 121.72, 119.15, 117.66, 115.89, 115.18,
112.52, 111.64, 54.47, 54.38, 20.07, 19.63.
Zr(L2)2: [bis(4-methoxysalicylidene)-4,5-dimethyl-1,2-phenylenediamine] zirconium, Yield: 81%.
Anal. calcd. for C48H44N4O8Zr (%): C, 64.34; H, 4.95; N, 6.25. Found: C, 64.52; H, 5.07; N, 6.03.
IR (KBr, cm-1): 1608, 1593 (C=N), 1589, 1438 (C=C), 1311 (C-O), 536 (Zr-O), 433 (Zr-N).
1H NMR (400 MHz, CDCl3, 293 K): δ 8.67 (s, 2 H, Hi′-C=N), 8.14 (s, 2 H, Hi-C=N), 7.62 (s, 2 H,
Hd′), 7.21 (d, 3J = 8.8 Hz, 2 H, Hc′), 7.05 (d, 3J = 8.4 Hz, 2 H, Hc), 6.52 (s, 2 H, Hd), 6.23 (dd, 3J =
8.4 Hz, 4J = 2.4 Hz, 2 H, Hb′), 6.20 (dd, 3J = 8.4 Hz, 4J = 2.4 Hz, 2 H, Hb), 5.72 (d, 4J = 2.4 Hz, 2 H,
Ha′), 5.44 (d, 4J = 2.4 Hz, 2 H, Ha), 3.55 (s, 6 H, -OCH3(o′)), 3.52 (s, 6 H, -OCH3(o)), 2.39 (s, 6 H, -
CH3(m′)), 2.13 (s, 6 H, -CH3(m)).
13C{1H} NMR (100.6 MHz, CDCl3, 293 K): δ 169.59, 166.75, 164.99, 164.80, 159.09, 158.135,
143.95, 143.73, 134.51, 134.41, 133.68, 133.43, 118.37, 117.87, 116.90, 115.99, 107.16, 106.90,
103.75, 102.41, 55.06, 54.72, 19.98, 19.36.
Zr(L3)2: [bis(5-methoxysalicylidene)-4,5-dimethyl-1,2-phenylenediamine] zirconium, Yield: 86%.
The title compound was synthesized like the previous method. Dark red single crystals of the title
compound, that was suitable for X-ray structure analysis, were obtained by the slow evaporation at
RT of a solution in methanol during a period of the days.
Anal. calcd. for C48H44N4O8Zr (%): C, 64.34; H, 4.95; N, 6.25. Found: C, 64.47; H, 5.02; N, 6.34.
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IR (KBr, cm-1): 1620, 1589 (C=N), 1539, 1463 (C=C), 1301 (C-O), 518 (Zr-O), 448 (Zr-N).
1H NMR (400 MHz, CDCl3, 293 K): δ 8.67 (s, 2 H, Hi′-C=N), 8.20 (s, 2 H, Hi-C=N), 7.66 (s, 2 H,
Hd′), 6.77 (d, 4J = 3.2 Hz, 2 H, Hc′), 6.62 (s, 2 H, Hd), 6.60 (d, 4J = 3.2 Hz, 2 H, Hc), 6.55 (dd, 3J =
3
9.2 Hz, 4J = 3.2 Hz, 2 H, Hb′), 6.52 (dd, J = 9.2 Hz, 4J = 3.2 Hz, 2 H, Hb), 6.10 (d, 3J = 9.2, 2 H,
Ha′), 5.77 (d, 3J = 9.2, 2 H, Ha), 3.76 (s, 6 H, -OCH3(o′)), 3.75 (s, 6 H, -OCH3(o)), 2.43 (s, 6 H, -
CH3(m′)), 2.17 (s, 6 H, -CH3(m)).
13C{1H} NMR (100.6 MHz, CDCl3, 293 K): δ 165.09, 164.02, 162.66, 159.79, 158.90, 155.27,
154.99, 149.96, 143.65, 143.55, 135.85, 135.14, 122.95, 122.38, 121.10, 120.35, 118.73, 118.34,
113.60, 113.03, 56.19, 55.86, 19.97, 19.55.
Zr(L4)2 : [bis(6-methoxysalicylidene)-4,5-dimethyl-1,2-phenylenediamine] zirconium, Yield: 78%.
Anal. calcd. for C48H44N4O8Zr (%): C, 64.34; H, 4.95; N, 6.25. Found: C, 64.53; H, 5.12; N, 6.02.
IR (KBr, cm-1): 1610, 1581 (C=N), 1544, 1460 (C=C), 1319 (C-O), 569 (Zr-O), 416 (Zr-N).
1H NMR (400 MHz, CDCl3, 293 K): δ 9.26 (s, 2 H, Hi′-C=N), 8.79 (s, 2 H, Hi-C=N), 7.64 (s, 2 H,
Hd′), 6.80 (t, 3J = 8.4 Hz, 2 H, Hb′), 6.75 (t, 3J = 8.4 Hz, 2 H, Hb), 6.55 (s, 2 H, Hd), 6.01 (d, 3J = 8
Hz, 2 H, Ha′), 6.94 (d, 3J = 8 Hz, 2 H, Ha), 5.87 (d, 3J = 8.4 Hz, 2 H, Hc′), 5.50 (d, 3J = 8.4 Hz, 2 H,
Hc), 3.90 (s, 6 H, -OCH3(o′)), 3.83 (s, 6 H, -OCH3(o)), 2.41 (s, 6 H, -CH3(m′)), 2.14 (s, 6 H, -CH3(m)).
13C{1H} NMR (100.6 MHz, CDCl3, 293 K): δ 167.54, 165.15, 160.28, 160.19, 155.20, 154.82,
144.41, 144.33, 135.20, 134.64, 133.25, 132.92, 118.96, 118.51, 114.96, 114.35, 113.30, 112.32,
96.96, 96.17, 55.80, 55.61, 19.89, 19.46. (For synthetic method to the formation of Zr(Lx)2
complexes see Scheme 1)
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R'
R'
H
H
H3C
2
N
N
OH
O
O
N
N
CH3
CH3
Methanol
Reflux
Zr
+ Zr(acac)4
OH
H3C
H
H
R
R
2
H2Lx, x=1,2,3,4
Zr(Lx)2, x=1,2,3,4
R=R'=3-OMe H2L1
R=R'=4-OMe H2L2
R=R'=5-OMe H2L3
R=R'=6-OMe H2L4
R=R'=3-OMe Zr(L1)2
R=R'=4-OMe Zr(L2)2
R=R'=5-OMe Zr(L3)2
R=R'=6-OMe Zr(L4)2
Scheme 1. Synthetic pathway to Zr(Lx)2 complexes
Zr(L1′)2:[bis(3-methoxysalicylidene)-2,2-dimethyl-1,3-propanediamine] zirconium(IV),Yield: 82%.
By adding bis(3-methoxysalicylidene)-2,2-dimethyl-1,3-propanediamine (0.741 g, 2 mmol) to a
solution of Zr(acac)4 (0.488 g, 1 mmol) in methanol (20 ml) the title compound was synthesized.
The mixture was refluxed with stirring for 5 hours. Then the resultant light yellow solution was
filtered. Colorless single crystals of the title compound, that was suitable for X-ray structure
analysis, were obtained by the slow evaporation at RT of a solution in methanol during several
1
13
days. This complex was characterized by H NMR, C{1H} NMR, IR spectroscopy, elemental
analyses (CHN) and its solid state structure was determined using single crystal X-ray diffraction.
Anal. calcd. for C42H50N4O8Zr (%): C, 60.77; H, 6.07; N, 6.75. Found: C, 60.54; H, 6.19; N, 6.54.
IR (KBr, cm-1): 1629, 1599 (C=N), 1558, 1456 (C=C), 1315 (C-O), 555 (Zr-O), 426 (Zr-N).
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1H NMR (400 MHz, CDCl3, 298 K): δ 8.08 (s, 4 H, HC=N), 6.61 (d, 3J = 7.6 Hz, 4 H, Hc), 6.34 (t,
3J = 7.6 Hz, 4 H, Hb), 6.28 (d, 3J = 7.6 Hz, 4 H, Ha), 5.75 (d, J = 10.8 Hz, 4 H, Hd′), 3.53 (s, 12 H, -
OCH3), 2.92 (d, J = 10.8 Hz, 4 H, Hd), 1.31 (s, 12 H, -CH3).
13C{1H} NMR (100.6 MHz, CDCl3, 298 K): δ 165.31, 154.53, 149.83, 122.52, 121.99, 114.63,
111.93, 68.42, 54.32, 37.47, 25.30.
Zr(L2′)2:[bis (4-methoxysalicylidene)-2,2-dimethyl-1,3-propanediamine] zirconium(IV),Yield: 77%.
Anal. calcd. for C42H50N4O8Zr (%): C, 60.77; H, 6.07; N, 6.75. Found: C, 60.89; H, 6.12; N, 6.63.
IR (KBr, cm-1): 1600, 1612 (C=N), 1523, 1442 (C=C), 1311 (C-O), 536 (Zr-O), 464 (Zr-N).
1H NMR (400 MHz, CDCl3, 292.6 K): δ 8.17 (s, 4 H, HC=N), 7.12 (d, 3J = 8.8 Hz, 4 H, Hc), 6.44
(d, 4J = 2.4 Hz, 4 H, Ha), 6.41 (dd, 3J = 8.8 Hz, 4J =2.4 Hz, 4 H, Hb), 5.08 (d, J = 11.0 Hz, 4 H, Hd′),
3.83 (s, 12 H, -OCH3), 2.98 (d, J = 11.0 Hz, 4 H, Hd), 1.07 (s, 12 H, -CH3).
13C{1H} NMR (100.6 MHz, CDCl3, 293 K): δ 165.77, 164.67, 134.38, 132.68, 106.40, 103.06,
101.22, 66.53, 55.38, 36.25, 24.89.
Zr(L3′)2:[bis (5-methoxysalicylidene)-2,2-dimethyl-1,3-propanediamine] zirconium(IV),Yield: 81%.
The yellow solid complex was prepared in a similar manner described above. Light yellow single
crystals of the title compound, suitable for X-ray structure analysis, were obtained by slow
evaporation at RT of a solution in methanol during several days.
Anal. calcd. for C42H50N4O8Zr (%): C, 60.77; H, 6.07; N, 6.75. Found: C, 60.82; H, 6.15; N, 6.84.
IR (KBr, cm-1): 1633, 1604 (C=N), 1556, 1479 (C=C), 1298 (C-O), 518 (Zr-O), 441 (Zr-N).
1H NMR (400 MHz, CDCl3, 293 K): δ 8.02 (s, 4 H, HC=N), 6.53 (d, 4J = 3.2 Hz, 4 H, Hc), 6.47 (dd,
4
3
3J = 8.8 Hz, J = 3.2 Hz, 4 H, Hb), 6.06 (d, J = 8.8 Hz, 4 H, Ha), 5.19 (d, J = 11.2 Hz, 4 H, Hd′),
3.66 (s, 12 H, -OCH3), 3.00 (d, J = 11.2 Hz, 4 H, Hd), 1.26 (s, 12 H, -CH3).
13C{1H} NMR (100.6 MHz, CDCl3, 293.2 K): δ 164.97, 158.60, 149.74, 121.33, 120.74, 120.05,
114.38, 69.40, 56.25, 36.96, 25.59.
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Zr(L4′)2:[bis (6-methoxysalicylidene)-2,2-dimethyl-1,3-propanediamine] zirconium(IV),Yield: 72%.
Anal. calcd. for C42H50N4O8Zr (%): C, 60.77; H, 6.07; N, 6.75. Found: C, 60.52; H, 6.12; N, 6.88.
IR (KBr, cm-1): 1618, 1600 (C=N), 1560, 1460 (C=C), 1313 (C-O), 542 (Zr-O), 424 (Zr-N).
1H NMR (400 MHz, CDCl3, 293 K): δ 8.50 (s, 4 H, HC=N), 6.75 (t, 3J = 8.2 Hz, 4 H, Hb), 5.90 (d,
3J = 8.2 Hz, 4 H, Hc), 5.84 (d, 3J = 8.2 Hz, 4 H, Ha), 5.18 (d, J = 11.0 Hz, 4 H, Hd′), 3.75 (s, 12 H, -
OCH3), 2.98 (d, J = 11.0 Hz, 4 H, Hd), 1.25 (s, 12 H, -CH3).
13C{1H} NMR (100.6 MHz, CDCl3, 294 K): δ 163.89, 160.12, 133.69, 132.23, 112.69, 110.75,
97.39, 69.64, 55.69, 36.70, 25.31. (For synthetic method to the formation of Zr(Lx′)2 complexes see
Scheme 2)
R'
R'
H
H
O
O
N
N
CH3
CH3
H3C
H3C
N
N
OH
Methanol
Reflux
Zr
+ Zr(acac)4
2
OH
H
H
R
R
2
H2Lx', x'=1,2,3,4
R=R'=3-OMe H2L1'
R=R'=4-OMe H2L2'
R=R'=5-OMe H2L3'
R=R'=6-OMe H2L4'
Zr(Lx')2, x'=1,2,3,4
R=R'=3-OMe Zr(L1')2
R=R'=4-OMe Zr(L2')2
R=R'=5-OMe Zr(L3')2
R=R'=6-OMe Zr(L4')2
Scheme 2. Synthetic pathway to Zr(Lx′)2 complexes
11

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In vitro Antibacterial activity
The antibacterial activities of the Schiff-base ligands and their zirconium(IV) complexes were
tested against the bacterial species Escherichia coli ATCC 25922 (gram negative bacteria) and
Staphylococcus aureus ATCC 25923 (gram positive bacteria) by the disc diffusion method. The
primary screening was performed using agar well diffusion at fixed concentrations of 10 mg/ml by
dissolving in 10% DMSO. The experiments were performed in triplicates. The MIC of the
chemically synthesized compounds were tested against bacterial strains through a broth dilutions
method. The test concentrations of chemically synthesized compounds were made from 128 to 0.25
mg/ml in the sterile.
Results and Discussion
Syntheses
The Schiff-base ligands were synthesized from 4,5-dimethyl-1,2-phenylenediamine and 2,2′-
dimethyl-1,3-propanediamine, respectively, in a Schiff-base condensation reaction with
methoxysalicylaldehyde, in ethanol. A one-pot reaction of Zr(acac)4 and the related tetradentate
Schiff-base ligands H2Lx and H2Lx′ (x=x′=1,2,3,4), in refluxing methanol, afforded the neutral
zirconium(IV) complexes. Both ligands and the related complexes were obtained in high yields
and are stable in the solid state and in solution. The complexes are light-yellow solids and could
be crystallized out from methanol.
3.2. Spectroscopic characterization
FT-IR Spectra
The information about the stretching frequencies of the C=N and C-O bonds of the ligands and
their corresponding Zr(IV) complexes are summarized in Table 1. The strong bands around 1612-
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1637 cm-1 in the FT-IR spectra of ligands are attributed to the azomethine stretching band
which are red shifted to 1581-1633 cm-1 upon coordination to Zr(IV) ion. While the C-O
stretching band of the ligands shift from 1222-1292 cm-1 to 1298-1315 cm-1 upon coordination
to Zr(IV) ion. These observations are in accordance with the data obtained for the C=N and C-O
bonds lengths by X-ray crystallography in which the C=N bond is weakened by coordination to
Zr(IV). In contrast, the C-O bond is strengthened upon coordination to Zr(IV) [31-37]. This is
extremely supported by the disappearance of the broad, (O-H) band in the (3300-3150 cm-1) region
in the complexes. The medium sharp bands at 518-569 and 416-464 cm-1 can be related to Zr-O and
Zr-N bonds, respectively [33, 38, 39].
Table 1. IR spectral data of the ligands and their corresponding Zr(IV) complexes
Compound
H2L1
Zr(L1)2
H2L2
Zr(L2)2
H2L3
Zr(L3)2
H2L4
Zr(L4)2
H2L1′
Zr(L1′)2
H2L2′
Zr(L2′)2
H2L3′
υ(C=C)
υ(C-O)
1249
1313
1292
1311
1271
1301
1249
1319
1253
1315
1222
1311
1271
1298
1249
1313
υ(C=N)
1618
1618, 1593
1612
1608, 1589
1618
1620, 1589
1612
1610, 1581
1612
1629, 1599
1627
1612, 1600
1637
1633, 1604
1624
υ(Zr-O)
υ(Zr-N)
1574, 1465
1550, 1469
1591, 1494
1589, 1438
1575, 1487
1539, 1463
1595,1462
1544, 1460
1525,1463
1558, 1456
1512,1442
1523, 1442
1494,1467
1556, 1479
1577,1467
1560,1460
-
559
-
536
-
518
-
569
-
555
-
536
-
518
-
542
-
416
-
433
-
448
-
416
-
426
-
464
-
441
-
424
Zr(L3′)2
H2L4′
Zr(L4′)2
1618, 1600
lH NMR, 13C{1H} NMR Spectra
The 1H NMR spectra of the Schiff-base ligands and zirconium(IV) complexes were taken in CDCl3
(400 MHz, 298 K). In the 1H NMR spectrum of the Zr(L1)2 complex (Fig. 1), the signals at  = 8.88
13

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and  = 8.21 ppm are assigned to the Hi′-C=N- and Hi-C=N- proton of azomethine, respectively.
1
Appearance of two H NMR peaks for azomethine protons present in the Zr(L1)2 complex are
indicative of the magnetically non-equivalence environment of these protons. The 1H NMR spectra
of the ligands, show the disappearing of phenolic proton signal and downfield shift of the
azomethine H, confirming the coordination of the phenolic oxygen and azomethine nitrogen to Zr
atom. The aromatic protons of the Zr(L1)2 complex appear in the range of  = 6.36–7.80 ppm. Ha
and Ha′ signals observed at 6.37 and 6.49 ppm as a doublet of doublet due to coupling with Hb, Hc
and Hb′, Hc′ [3J = 8.0 Hz and 4J = 1.6 Hz] respectively. Hb and Hb′ signals appeared at 6.54 and 6.60
ppm as a triplet due to the couplings with Ha, Hc and Ha′, Hc′, respectively. The signals due to Hd'
4
and Hc' obsereved as singlet and doublet of doublet [3J = 8.0 Hz and J = 1.6 Hz] at 7.80 and 7.04
ppm, respectively. Hc signal appeared at 6.85 ppm as a doublet of doublet due to the coupling with
Hb [3J = 8.0 Hz] and Ha [4J = 1.6 Hz]. The Hd is observed as a singlet at = 6.39 ppm. The –CH3(o′)
and –CH3(o) are observed as a singlet at  = 3.49 and  = 3.43 ppm, respectively. The –CH3(m′) and –
CH3(m) are observed as a singlet at  = 2.42 and  = 2.14 ppm, respectively.
In the 1H NMR spectrum of the Zr(L1′)2 complex (Fig. 3), the signal  = 8.08 ppm is assigned to the
Hi-C=N- proton of azomethine. The aromatic protons of the Zr(L1′)2 complex appear in the range of
 = 6.27–6.62 ppm. Hc signal appeared at 6.61 ppm as a doublet due to the coupling with Hb [3J =
7.6 Hz]. Hb signal appeared at 6.34 ppm as a triplet due to the couplings with Ha and Hc. Ha signal
appeared at 6.28 ppm as a doublet of doublet due to the coupling with Hb [3J = 7.6 Hz]. Hd′ and Hd
signals observed at 5.75 and 2.92 ppm as a doublet due to coupling with Hd and Hd′, respectively. A
singlet at = 3.53 ppm is assigned to the –OCH3 protons and the –CH3 is observed as a singlet at 
= 1.31 ppm.
13
1
The C{1H}NMR spectrum of the of zirconium(IV) complexes are in agreement with H NMR.
The 13C{1H}NMR spectrum of the Zr(L1)2 complex (Fig. 2) shows the two iminic carbon, C(7) and
14

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C(14), resonances, as functional group signals at 161.3 and 159.6 ppm, respectively. According to
the spectrum, Zr(L1)2 complex has 24 signals, showing that the structures in solution are indicative
13
of the magnetically non-equivalence environment of these carbons but the C{1H}NMR spectrum
of the Zr(L1′)2 complex (Fig. 4) has 11 signals, indicating that the structures in solution are
indicative of the magnetically equivalence environment, therefore the 1H and 13C{1H}NMR, spectra
of complexes in CDCl3 revealed the coordination of the Schiff base ligands to zirconium obviously.
Fig. 1. 1H NMR spectrum of Zr(L1)2 in CDCl3.
15

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Fig. 2. 13C{1H}NMR spectrum of Zr(L1)2 in CDCl3.
16

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Fig. 3. 1H NMR spectrum of Zr(L1′)2 in CDCl3.
17

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Fig. 4. 13C{1H}NMR spectrum of Zr(L1′)2 in CDCl3.
X-Ray crystallography
The solid state structures of Zr(L1′)2, Zr(L3′)2 and Zr(L3)2 complexes were studied by X-ray
diffraction. The crystal data and refinement parameters of the complexes are summarized in Table
2. The ORTEP of Zr(L1′)2, Zr(L3′)2 and Zr(L3)2 are shown in Fig. 5. Selected bond lengths and
angles are summarized in Table 3. The coordination environment of the metal in all complexes is
eight-coordinate occupied by N2O2 sets of the coordinated ligands in distorted square-antiprism
geometry. In Zr(L1′)2 and Zr(L3′)2, each ligand behaves in tetradentate mode in which N2O2 donor
atoms of each ligand occupy top and bottom coordination sites of Zr in the square-antiprism. The
angle of the least square planes of the coordinated ligands with N2O2 atoms in Zr(L1′)2 and
Zr(L3′)2 is 89.23(9) and 89.81(6)°, respectively. The N2O2 cores on the top and bottom of the Zr
center are close to planarity [maximum displacement -0.017(2), -0.054(2), -0.079(1), and -0.045(1)
18

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Å for O1/O8/N1/N4, O4/O5/N2/N3 in Zr(L1′)2, and Zr(L3′)2 respectively] and nearly parallel to
each other [dihedral angle 0.37(16) and 0.25(14)° in the complexes, respectively]. The asymmetric
unit of the Zr(L3)2 complex comprises a mononuclear Zr complex with the tetradentate ligand
H2L3. The N2O2 donor set of the symmetry related ligands occupy the corner of each square on the
top and bottom of the Zr center giving rise to a square-antiprism arrangement of the donor atoms.
The N2O2 cores on the top and bottom of the Zr center are close to planarity [maximum
displacement 0.013(1) Ǻ for O1/O2/N1/N2] and nearly parallel to each other with dihedral angle
1.53(1)°. The Zr–N and Zr–O bond lengths and geometry of the coordinated ligand around Zr in
complex Zr(L3)2 are comparable to the related Zr(salophen)2 complex [13]. The Zr–N and Zr–O
bond lengths in complex Zr(L1′)2, and Zr(L3′)2 are comparable to the previously published
structures [39]. The interesting feature of the crystal packing of Zr(L3′)2 is the intermolecular C5–
H5…O6 interaction forming one dimensional infinite chain of the complexes running along the c-
axis (Fig. 6).
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Figure 5. The ORTEP of Zr(L1′)2 (left), Zr(L3′)2 (right) and Zr(L3)2 (bottom) complexes.
Hydrogen atoms were omitted for clarity.
Figure 6. The packing of Zr(L3′)2 viewed down the a-axis showing one dimensional extended
chain along c-axis.
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Table 2. Crystal data and refinement parameters of Zr(L1′)2, Zr(L3′)2 and Zr(L3)2
Zr(L1′)2
C42H48N4O8Zr
828.06
0.18 × 0.20 × 0.25
light-yellow
monoclinic
P21
Zr(L3′)2
C42H48N4O8Zr
884.7
Zr(L3)2
C48H44N4O8Zr
896.09
Complex
Empirical formula
Formula mass
Crystal size (mm)
Colour
Crystal system
Space group
θmax (°)
0.22 × 0.25 × 0.30 0.16 × 0.18 × 0.26
light-yellow
monoclinic
P21/c
light-yellow
orthorhombic
Pbcn
27.2
26.0
28.3
a (Å)
b (Å)
c (Å)
α (°)
11.0403(6)
14.4722(8)
13.4025(9)
90
21.0777(18)
9.5534(6)
20.2209(17)
90
13.4549(8)
21.6826(13)
13.6122(7)
90
β (°)
112.654(2)
90
1976.2(2)
2
1.392
0.336
99.900(3)
90
4011.1(5)
4
1.371
0.331
90
90
3971.2(4)
4
1.499
γ (°)
V (Å3)
Z
Dcalc (Mg/m3)
μ (mm−1)
F (000)
0.341
1856
864
1728
Index ranges
-14 ≤ h ≤ 11
-17 ≤ k ≤ 18
-17 ≤ l ≤ 17
-25 ≤ h ≤25
-11 ≤ k ≤11
-24 ≤ l ≤24
30679
-17 ≤ h ≤17
-28 ≤ k ≤26
-17 ≤ l ≤16
34499
No. of measured reflns. 17032
No. of independent
reflns./Rint
8446/0.032
7889/0.038
4911/0.074
No. of observed
reflns. I > 2σ(I)
No. of parameters
Goodness-of-fit (GOF)
R1 (observed data)
wR2 (all data)
7096
504
1.00
0.0388
0.0701
5517
504
1.02
0.0426
0.1167
2904
280
1.02
0.0406
0.1017
Table 3. Selected bond lengths (Ǻ) and angles (°) of Zr(L1′)2, Zr(L3′)2 and Zr(L3)2
Zr(L1′)2
2.080(3)
2.074(3)
2.069(3)
2.079(3)
2.453(4)
2.459(4)
2.455(4)
2.455(3)
Zr(L3′)2
2.071(2)
2.057(2)
2.070(2)
2.059(2)
2.484(3)
2.499(3)
2.475(3)
2.453(3)
Zr(L3)2
2.1044(16)
2.1136(18)
2.416(2)
Bond lengths (Ǻ)
Bond lengths (Ǻ)
Zr(1)–O(1)
Zr(1)–O(1)
Zr(1)–O(4)
Zr(1)–O(5)
Zr(1)–O(8)
Zr(1)–N(1)
Zr(1)–N(2)
Zr(1)–N(3)
Zr(1)–N(4)
Zr(1)–O(2)
Zr(1)–N(1)
Zr(1)–N(2)
2.385(2)
Bond angles (°)
O(1)–Zr(1)–O(4)
O(1)–Zr(1)–O(5)
O(1)–Zr(1)–O(8)
O(1)–Zr(1)–N(1)
O(1)–Zr(1)–N(2)
O(1)–Zr(1)–N(3)
O(1)–Zr(1)–N(4)
O(4)–Zr(1)–O(5)
O(4)–Zr(1)–O(8)
Bond angles (°)
O(1)–Zr(1)–O(2)
O(1)–Zr(1)–N(1)
O(1)–Zr(1)–N(2)
O(2)–Zr(1)–N(1)
O(2)–Zr(1)–N(2)
144.56(12)
84.58(11)
107.62(11)
73.41(14)
140.16(14)
77.67(13)
72.93(12)
105.14(12)
84.21(11)
143.62(9)
102.35(8)
88.53(8)
73.28(8)
142.67(9)
73.10(9)
78.09(8)
87.65(8)
105.13(8)
75.25(7)
73.03(7)
113.00(7)
112.26(7)
74.22(7)
21

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Antibacterial activity
The synthesized Schiff-base ligands H2Lx, H2Lx′ and their zirconium(IV) complexes [Zr(Lx)2],
[Zr(Lx′)2] (x=x′=1,2,3,4) were tested for their in vitro antibacterial. The minimum inhibitory
concentrations of the investigated compounds are summarized in Table 4. A comparative study of
the ligands and the complexes indicates that the Zr compounds exhibit more profound antibacterial
activity than free ligands. Such increase activity of the metal complexes can be explained on the
basis of Overtone’s concept [40] and Tweedy’s chelation theory [41]. There is a decrease in the
polarity of the Zr atom significantly after chelation, because of the partial sharing of its positive
charge with the donor groups and also due to π-electron delocalization on the whole chelate ring.
The Zr complexes interaction is preferred with the lipids and polysaccharides which are the
important constituents of the cell wall and membranes. Furthermore, decreasing of polarity led to
increasing the lipophilic character of the chelates and an interaction between the Zr complexes and
the lipid is favoured which may lead to the subsequent breakdown of the permeability barrier of the
cell resulting in obstruction with the normal cell processes [42]. Due to liposolubility, this is an
important factor that controls antimicrobial activity.
The results revealed that the complexes [Zr(Lx)2] are the most effective against Escherichia coli and
Staphylococcus aureus with MIC quantity of (2-4 μg/cm3), while the complexes [Zr(Lx′)2] are less
effective (4-8 μg/cm3) and relative ligands almost have no effectiveness. Because [Zr(Lx)2]
complexes have less polarity than [Zr(Lx′)2] complexes are due to the aromatic ring in the lipophilic
diamine so more antibacterial activity.
Apparently, the Zr complexes are more toxic towards Gram-negative strain (Escherichia coli) than
Gram-positive strain (Staphylococcus aureus). The reason probably lies in the difference between
the structures of the cell walls, due to the peptidoglycan layer thickness (the middle layer of the
22

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bacterial cell membrane) that is thinner in gram-negative bacteria than gram positive and the
complex easier pass this layer.
Compared antibacterial properties of synthesized zirconium complexes with other similar
complexes show that these complexes are more active and the structural features of these complexes
may contribute to the activity of these complexes [43, 44].
Table 4 The inhibition diameter zone values (mm), MIC and MBC of the ligands and their corresponding Zr(IV)
complexes
compound
Inhibition zone (mm)
MIC (mg/ml)
MBC (mg/ml)
E. coli
0.128
0.002
0.128
0.002
0.128
0.001
0.128
0.002
0.128
0.004
0.128
0.004
0.128
S. aureus
E. coli
S. aureus
0.128
0.008
0.128
0.008
0.064
0.004
0.128
0.008
0.128
0.016
0.128
0.016
0.128
E. coli
8
S. aureus
H2L1
Zr(L1)2
H2L2
0.128
0.004
0.128
0.004
0.064
0.002
0.128
0.004
0.128
0.008
0.128
0.008
0.128
0.128
0.002
0.128
0.002
0.128
0.001
0.128
0.002
0.128
0.008
0.128
0.008
0.128
7
11.7
6.5
12
13
7
Zr(L2)2
H2L3
14
8
8
Zr(L3)2
H2L4
15
8.3
13.7
_
13.3
7
Zr(L4)2
H2L1′
11.6
_
Zr(L1′)2
H2L2′
11.2
_
10.5
_
Zr(L2′)2
H2L3′
11.5
_
10.7
_
Zr(L3′)2
H2L4′
0.002
0.128
0.004
0.004
0.128
0.008
0.004
0.128
0.008
0.008
0.128
0.016
12.7
_
11
_
Zr(L4′)2
11.3
10
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Conclusions
We have reported the synthesis of new zirconium(IV) Schiff-base complexes of two potentially
tetradentate ligands. The ligands and their respected zirconium(IV) complexes were
characterized by spectroscopic techniques and elemental analyses (CHN). The single crystal X-
ray diffraction analysis of the complexes revealed that the coordination environment of the
metal in all complexes is eight-coordinate occupied by N2O2 sets of the coordinated ligands in
distorted square-antiprism geometry. The biological activity of the compounds showed the
zirconium(IV) complexes exhibit antimicrobial activity.
Appendix A. Supplementary data
CCDC 1526749-1526751 contain the supplementary crystallographic data for the 5, 7 and 3. These
data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from
the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44)
1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Acknowledgments
The support of this work by Payame Noor University Council of Research is gratefully
acknowledged.
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► Synthesis, characterization and spectral properties of new Zr(IV) Schiff base complexes.
► Selective geometry around Zr(IV) by tuning the steric hindrance of the coordinated ligands.
► Structural and biological studies of the new Zr(IV) complexes.