Tweaking the affinity of aryl-substituted diazosalicylato- and pyridine ligands towards Zn (II) and its neighbors in the periodic system of the elements, Cu (II) and Cd (II), and their antimicrobial activity

Article abstract of DOI:10.1002/aoc.4905

A series of six new Zn (II) compounds, viz., [Zn(HL^ASA)₂(Py)₂] (1), [Zn(HL^MASA)₂(Py)₂] (2), [Zn(HL^MASA)₂(4-MePy)₂] (3), [Zn(HL^CASA)₂(4-MePy)₂] (4), [Zn(HL^BASA)₂(Py)₂] (5), [Zn(HL^BASA)₂(4-MePy)₂] (6) and representative Cu (II) and Cd (II) complexes, viz., [Cu(HL^ASA)₂(Py)₂(H₂O)] (7) and [Cd(HL^BASA)₂(Py)₃] (8) [(HL^XASA)^??=?para-substituted 5-[(E)-2-(aryl)-1-diazenyl]-2-hydroxybenzoate with X?=?H (ASA), Me (MASA), Cl (CASA) or Br (BASA); Py?=?pyridine; 4-MePy?=?4-methylpyridine] have been synthesized and characterized by spectroscopic techniques and single-crystal X-ray diffraction analysis. The structural characterization of the compounds revealed distorted tetrahedral (1–6), square-pyramidal (7) and pentagonal-bipyramidal (8) coordination geometries around the metal atom, in which the aryl-substituted diazosalicylate ligands are coordinated only through the oxygen atoms of carboxylate groups, either in an anisobidentate or isobidentate mode; meanwhile, the 2-hydroxy groups of the monoanionic ligand (HL^XASA)^? are involved only in intramolecular O-H···O hydrogen bonds with the carboxylate function. In the crystal structures of 1–8, the complex molecules are assembled by π-stacking interactions giving mostly infinite 1D strands. The intermolecular binding in the solid state structures is accomplished by diverse additional non-covalent contacts including C-H···O, C-H···N, C-H···π, C-H···Br, O···Br, Br···π and van der Waals contacts. Although the primary and secondary ligands in the Zn (II) complex series 1–6 carry different substituents at the periphery (X?=?H, Me, Cl, Br for (HL^XASA)^? and R?=?H, Me for 4-Py-R), five of the crystal structures were isostructural. Additionally, the antimicrobial activity of the pro-ligands H₂L^XASA and their Zn (II), Cu (II) and Cd (II) compounds were studied in a comparative manner, showing high sensitivity (IZD?≥?20) against Bacillus subtilis.

Full text of DOI:10.1002/aoc.4905

Received: 16 January 2019
DOI: 10.1002/aoc.4905
Revised: 16 February 2019
Accepted: 20 February 2019
F U L L P A P E R
Tweaking the affinity of aryl‐substituted diazosalicylato‐
and pyridine ligands towards Zn (II) and its neighbors in
the periodic system of the elements, Cu (II) and Cd (II), and
their antimicrobial activity
Tushar S. Basu Baul1
Santa Ram Joshi2 | Irán Rojas León3 | Herbert Höpfl3
| Khrawborlang Nongsiej1 | Augustine Lamin Ka‐Ot2 |
1 Centre for Advanced Studies in
A series of six new Zn (II) compounds, viz., [Zn(HLASA)2(Py)2] (1),
Chemistry, North‐Eastern Hill University,
NEHU Permanent Campus, Umshing,
Shillong 793 022, India
2 Department of Biotechnology &
Bioinformatics, North‐Eastern Hill
University, NEHU Permanent Campus,
Umshing, Shillong 793 022, India
[Zn(HLMASA)2(Py)2] (2), [Zn(HLMASA)2(4‐MePy)2] (3), [Zn(HLCASA)2(4‐MePy)2]
(4), [Zn(HLBASA)2(Py)2] (5), [Zn(HLBASA)2(4‐MePy)2] (6) and representative Cu
(II) and Cd (II) complexes, viz., [Cu(HLASA)2(Py)2(H2O)] (7) and
[Cd(HLBASA)2(Py)3] (8) [(HLXASA −
) = para‐substituted 5‐[(E)‐2‐(aryl)‐1‐diaze-
nyl]‐2‐hydroxybenzoate with X = H (ASA), Me (MASA), Cl (CASA) or Br
(BASA); Py = pyridine; 4‐MePy = 4‐methylpyridine] have been synthesized
and characterized by spectroscopic techniques and single‐crystal X‐ray diffrac-
tion analysis. The structural characterization of the compounds revealed
distorted tetrahedral (1–6), square‐pyramidal (7) and pentagonal‐bipyramidal
(8) coordination geometries around the metal atom, in which the aryl‐
substituted diazosalicylate ligands are coordinated only through the oxygen
atoms of carboxylate groups, either in an anisobidentate or isobidentate mode;
3 Centro de Investigaciones Químicas,
Instituto de Investigación en Ciencias
Básicas y Aplicadas, Universidad
Autónoma del Estado de Morelos, Av.
Universidad 1001, Cuernavaca 62209,
Mexico
Correspondence
Tushar S. Basu Baul, Centre for Advanced
Studies in Chemistry, North‐Eastern Hill
University, NEHU Permanent Campus,
Umshing, Shillong 793 022, India.
Email: basubaulchem@gmail.com;
basubaul@nehu.ac.in
meanwhile, the 2‐hydroxy groups of the monoanionic ligand (HLXASA −
) are
involved only in intramolecular O‐H···O hydrogen bonds with the carboxylate
function. In the crystal structures of 1–8, the complex molecules are assembled
by π‐stacking interactions giving mostly infinite 1D strands. The intermolecular
binding in the solid state structures is accomplished by diverse additional non‐
covalent contacts including C‐H···O, C‐H···N, C‐H···π, C‐H···Br, O···Br, Br···π
and van der Waals contacts. Although the primary and secondary ligands in
the Zn (II) complex series 1–6 carry different substituents at the periphery
(X = H, Me, Cl, Br for (HLXASA)− and R = H, Me for 4‐Py‐R), five of the crystal
structures were isostructural. Additionally, the antimicrobial activity of the pro‐
ligands H2LXASA and their Zn (II), Cu (II) and Cd (II) compounds were studied
in a comparative manner, showing high sensitivity (IZD ≥ 20) against Bacillus
subtilis.
Herbert Höpfl, Centro de Investigaciones
Químicas, Instituto de Investigación en
Ciencias Básicas y Aplicadas, Universidad
Autónoma del Estado de Morelos, Av.
Universidad 1001, Cuernavaca 62209,
Mexico.
Email: hhopfl@uaem.mx
Funding information
Council of Scientific and Industrial
Research, India, Grant/Award Number:
01 (2734)/13/ EMR‐II, 2013 01 (2734)/13/
EMR‐II; University Grants Commission,
India, Grant/Award Number: 42‐396/2013
(SR) 2013
KEYWORDS
antimicrobial activity, diazo salicylate and pyridine ligands, SCXRD structure elucidation, zinc/
copper/cadmium
Appl Organometal Chem. 2019;e4905.
wileyonlinelibrary.com/journal/aoc
© 2019 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.4905

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BASU BAUL ET AL.
[Zn3(HLSASA)2(Py)2(H2O)8]n
(HLASA = 5‐[(E)‐2‐phenyl‐
[16]
1 | INTRODUCTION
1‐diazenyl]‐2‐hydroxybenzoate, HLPASA = 5‐[(E)‐2‐(4‐pyri
dyl)‐1‐diazenyl]‐2‐hydroxybenzoate, HLSASA = 5‐[(E)‐
2‐(4‐sulfophenyl)‐1‐diazenyl]‐2‐hydroxybenzote). These
complexes are mononuclear, but with the twofold func-
tionalized pro‐ligand 5‐[(E)‐2‐(3‐carboxy‐4‐hydroxyphenyl)
diazen‐1‐yl]‐2‐hydroxybenzoic acid (H4LABisSA) oligo‐ and
polynuclear clusters are also known, which include
Zinc is a biometal and an important constituent of various
proteins.[1] In its first coordination shell, zinc can incorpo-
rate ligands based on electronegative atoms such as nitro-
gen, oxygen and sulphur. The structural framework and
physicochemical properties of zinc coordination com-
pounds are dependent on the geometry and functionality
of the ligands. Particularly, Zn (II) carboxylates have been
widely studied and find applications as catalysts for carbon
dioxide activation[2] and C‐C bond formation.[3] Further-
more, zinc carboxylates derived from nitrogen donor
ligands are an important class of compounds, which
exhibit diverse coordination modes.[4] Diprotic salicylic
acid (H2SA) and its derivatives are well‐known non‐
steroidal anti‐inflammatory, anti‐pyretic and analgesic
drugs[5] with excellent affinity to transition metal ions.[6]
Transition metal complexes of salicylates in mono‐anionic
(HSA)− and di‐anionic (SA)2− form are well known. In
these complexes, the carboxylate group is generally
deprotonated and can act as monodentate, bidentate or
bridging moiety (Scheme 1).[6] Although the COOH group
in H2SA is more acidic than the OH group, there are also
reports on salicylate complexes, in which the carboxylate
group is protonated and the hydroxyl group is
deprotonated (Scheme 1; III‐V).[7–11]
[Zn4(HLABisSA)3(1,10‐phenanthroline)2(H2O)2]n with
a
rhombic tetrameric Zn4O4 core[17] and [Zn6(LABisSA)3(4,4′‐
bipyridine)3(H2O)4]·4DMF with a Zn6‐aggregate.[18] The
1D coordination polymer [Zn(H2LABisSA)(H2O)4]n has a
mononuclear ZnO6 coordination environment.[19] The
coordination modes of the salicylate‐fragments in these
complexes fall within the options portrayed in Scheme 1
and the complexes display rich intra‐ and non‐covalent
intermolecular interactions including C–H···O, C–H···N
and C–H···π contacts. Efforts were also made to link Zn
(II) porphyrin complexes among each other by introducing
a diazenyl‐2‐hydroxybenzoic acid fragment in the ligand
where the diazenyl group serves as π‐conjugated bridge
and the carboxyl/hydroxyl groups as anchoring groups.
These complexes demonstrated good photoelectric conver-
sion efficiency in solar cells.[20]
Recently, a variety of Zn (II) complexes with N,O‐donor
ligands containing a diazenyl group, i.e. 5‐[(E)‐2‐(aryl)‐1‐
diazenyl]quinolin‐8‐olate (LXAQ) with X = Me, OMe, Et,
OEt, NMe2, Cl and Br, were investigated, in which the
coordination environment of the zinc atom was modified
by water molecules or other secondary ligands such as
pyridine, 3‐methylpyridine and 4‐methylpyridine.[21]
The SCXRD structure analysis designated three variants of
the Zn (II) coordination environment, i.e. [Zn(LXAQ)2(Py/
On the other hand, linking H2SA with a phenyldiazenyl
group affords the diazenyl bridged pro‐ligand 5‐[(E)‐2‐(phe-
nyl)‐1‐diazenyl]‐2‐hydroxybenzoic acid (here after H2LASA
)
that has demonstrated antimicrobial, analgesic and
antioxidant activity.[12] Replacing the phenyl substituent
by para‐substituted aryl or pyridyl groups allows generating
a large number of ligands, H2LXASA, which in combination
with diverse secondary ligands have been used previously
3‐MePy/4‐MePy)2], [Zn(LXAQ)2(H2O)2] and [Zn(LXAQ
(Py)], which had a significant influence on the supramolec-
)
2
for complexation with Zn (II) ions, e.g., [Zn (HLASA
)
2
(3,5‐dimethylpyrazole)2],[13] {[Zn(HLPASA)2(H2O)2]·2D
MF}n,[14] [Zn(HLASA)2(H2O)2]·2(2‐methylquinoline)[15] and
ular organization in the solid state.
Following our research interest to examine the molec-
ular and crystal structures of Zn (II) compounds with
aryldiazenyl‐linked chelating ligands, the present work
is focused on the preparation, characterization and struc-
ture elucidation of a series of Zn (II) compounds based on
the aryldiazenyl linked O,O‐chelating pro‐ligands
H2LXASA in combination with pyridine derivatives as
ancillary ligands. The purpose of using pyridine deriva-
tives is to enhance the solubility of the reaction constitu-
ents and to maintain the homogeneity of the reaction
mixture, which is important for obtaining crystalline
products. Additionally, pyridine can modulate the acidity
of the metal center, which also influences the electronic
properties of the chelators. To investigate the influence
of other metals upon complexation with H2LXASA, coordi-
nation complexes of two metals adjacent to zinc in the
periodic system of the elements were also examined,
SCHEME 1 Coordination modes of (HSA)– and (SA)2– toward
metal ions as characterized through single‐crystal X‐ray diffraction
analysis

BASU BAUL ET AL.
3 of 18
namely Cu (one before Zn) and Cd (one below Zn). It is
expected that the mixed ligand complexes can generate
extended π‐conjugated systems owing to the integration
of the diazenyl chromophore, which can eventually act
as electronic bank with potential applications for the
design and tuning of the properties in optoelectronic
materials.
The chemical structure of the six new Zn (II) com-
pounds, viz., [Zn(HLXASA)2(Py or 4‐MePy)2] (1–6) and two
additional representative complexes [Cu (HLASA)2(Py)2
(H2O)] (7) and [Cd (HLBASA)2(Py)3] (8) (HLXASA = (5‐[(E)‐
2‐(aryl)‐1‐diazenyl]‐2‐hydroxybenzoate) are outlined in
Scheme 2. The compounds were characterized by elemental
analysis, IR, NMR, UV–vis spectroscopy and single crystal
X‐ray diffraction analysis. Finally, the in vitro antimicrobial
activity of compounds 1–8 against three strains of bacteria
and a fungal strain is reported.
(H2LMASA),[22] 5‐[(E)‐2‐(4‐chlorophenyl)‐1‐diazenyl]‐2‐hy-
droxybenzoic acid (H2LCASA [23]
)
and 5‐[(E)‐2‐(4‐bromo-
phenyl)‐1‐diazenyl]‐2‐hydroxybenzoic acid (H2LBASA [22]
)
were prepared starting from salicylic acid and the
corresponding aniline using conventional diazonium salt
chemistry, in accordance with literature procedures.
The method employed for the preparations of 1–8 are
very similar; therefore, only the preparation of 1 is
described in detail, as representative example. Yields of
1–8 have been reported relative to the respective metal
acetates.
2.2.1 | Synthesis of [Zn(HLASA)2(Py)2] (1)
Zn(OAc)2·2H2O (0.068 g, 0.31 mmol) in methanol (5 ml)
was added drop‐wise at room temperature to a stirred
solution of H2LASA (0.150 g, 0.62 mmol) in methanol
(20 ml), which resulted in the immediate formation of a
yellow‐brown precipitate. The reaction mixture was
heated to reflux for 3 hr and then filtered while hot.
The residue was washed with hot methanol (3 x 5 ml)
to remove undesired materials and dried in vacuo. The
dried solid was dissolved in hot acetone (10 ml) contain-
ing pyridine (0.81 ml, 10.11 mmol) and filtered while
hot. Slow evaporation of the filtrate afforded yellow crys-
tals of 1. Yield: 0.12 g (55%). M.p. 190–191°C. Found: C,
61.53; H, 3.82; N, 11.99%. Calcd. for C36H28N6O6Zn
(MW = 706.02 gmol−1): C, 61.24; H, 4.00; N, 11.90%. IR
(cm−1): 1623s, 1610s, 1569s, 1489s, 1451m, 1425m,
1384s, 1298m, 1261m, 1172w, 1072w, 836m, 770m,
2 | EXPERIMENTAL
2.1 | General considerations
Zn(OAc)2·2H2O, Cu(OAc)2·H2O (SD fine), Cd(OAc)2·
2H2O (SRL), salicylic acid (Merck), p‐toluidine, pyridine,
4‐methylpyridine (Spectrochem), p‐chloroaniline and p‐
bromoaniline (Himedia) were used without further puri-
fication, while aniline (SD Fine) was freshly distilled
prior to use. The solvents used in the reactions were of
AR grade and dried using standard procedures.
Melting points were measured using a Büchi melting
point apparatus (M‐560) and are uncorrected. IR spectra
in the range 4000–400 cm−1 were obtained on a Perkin
Elmer Spectrum BX series FT‐IR spectrophotometer with
1
687m, 574w, 524w. H‐NMR (DMSO‐d6); δH: 8.96–8.90
[m, 4H, ArH], 8.72 [s, 2H, ArH], 8.06–7.96 [m, 4H,
ArH], 7.90–7.76 [m, 4H, ArH], 7.60–7.51 [m, 4H, ArH],
7.50–7.40 [m, 6H, ArH], 7.06 [d, 2H, ArH] ppm. The sig-
nal for the phenol group was not observed due to the
presence of water in the solvent. 13C NMR (DMSO‐d6);
δC: 175.5 [CO2], 164.4 [C2], 152.7, 149.5, 145.1, 140.2,
130.3, 129.0, 128.6, 127.3, 125.7, 122.6, 117.8, 116.4
[ArC] ppm.
1
samples investigated as KBr discs. H and 13C{1H} NMR
spectra, measured at 400.13 and 100.62 MHz, respectively,
were recorded on a Bruker AMX 400 spectrometer. The
1H and 13C chemical shifts were referenced to Me4Si set
at δ 0.00 ppm. Absorption measurements were carried
out on a Perkin‐Elmer Lambda25 spectrophotometer at
ambient temperature in freshly prepared DMSO solutions.
For the in vitro antimicrobial assays, Mueller‐Hinton
agar (HiMedia) was used as nutrient liquid medium and
standard antibiotic discs of chloramphenicol (C)30 and
fluconazole (FLC)25 (HiMedia) were used as positive
controls for bacteria and fungus, respectively.
2.2.2 | Synthesis of [Zn(HLMASA)2(Py)2] (2)
An analogous method to that used for the preparation of
1 was followed using Zn(OAc)2·2H2O (0.068 g, 0.31 mmol)
and H2LMASA (0.158 g, 0.62 mmol). The dried solid was
dissolved in hot acetone (10 ml) containing pyridine
(0.81 ml, 10.11 mmol) and filtered while hot. Slow evapo-
ration of the filtrate yielded yellow crystals of 2. Yield:
0.14 g (61%). M.p. 203–205 °C. Found: C, 62.44; H, 4.48;
N, 11.22%. Calcd. for C38H32N6O6Zn (MW = 734.08
gmol−1): C, 62.17; H, 4.39; N, 11.45%. IR (cm−1): 1623m,
1610s, 1567m, 1485s, 1451m, 1426m, 1380s, 1301m,
2.2 | Synthesis of pro‐ligands and their Zn
(II), Cu (II) and Cd (II) compounds
The pro‐ligands H2LXASA
diazenyl]‐2‐hydroxybenzoic acid (H2LASA), 5‐[(E)‐2‐(4‐
methylphenyl)‐1‐diazenyl]‐2‐hydroxybenzoic acid
,
viz., 5‐[(E)‐2‐phenyl‐1‐

4 of 18
BASU BAUL ET AL.
1259s, 1174m, 1071m, 1046w, 837s, 770m, 754m, 692s,
574m, 523w. 1H‐NMR (DMSO‐d6); δH: 12.8 [brs, 2H,
OH], 8.90–8.80 [m, 4H, ArH], 8.62 [s, 2H, ArH], 7.96–
7.82 [m, 4H, ArH], 7.82–7.70 [m, 4H, ArH], 7.25–7.10
[m, 4H, ArH], 7.00–6.90 [m, 4H, ArH], 7.06 [d, 2H,
ArH], 2.33 [s, 6H, CH3] ppm. 13C NMR (DMSO‐d6); δC:
175.6 [CO2], 164.2 [C2], 150.8, 149.5, 145.2, 140.7, 140.1,
129.7, 128.3, 127.2, 125.6, 122.5, 117.7, 116.4 [ArC], 21.5
[CH3] ppm.
[C2], 152.7, 151.1, 148.9, 144.9, 135.9, 129.2, 128.8, 127.7,
126.5, 123.8, 117.8, 116.7 [ArC], 21.5 [CH3] ppm.
2.2.5 | Synthesis of [Zn(HLBASA)2(Py)2] (5)
An analogous method to that used for the preparation of
1 was followed using Zn(OAc)2·2H2O (0.068 g, 0.31 mmol)
and H2LBASA (0.198 g, 0.62 mmol). The dried solid was
dissolved in hot acetone (10 ml) containing pyridine
(0.81 ml, 10.11 mmol) and filtered while hot. Slow evapo-
ration of the filtrate yielded yellow crystals of 5. Yield:
0.17 g (63%). M.p. 177–178 °C. Found: C, 50.02; H, 3.13;
N, 10.04. Calcd. for C36H26Br2N6O6Zn (MW = 863.82
gmol−1): C, 50.06; H, 3.03; N, 9.73%. IR (cm−1): 1625s,
1607s, 1585s, 1573s, 1478s, 1428s, 1386s, 1306m, 1260m,
1172m, 1006w, 840s, 755w, 697m, 601m, 576m, 525w.
1H‐NMR (DMSO‐d6); δH: 13.1 [brs, 2H, OH], 8.90–8.78
[m, 4H, ArH], 8.70–8.63 [m, 2H, ArH], 8.05–7.90 [m,
4H, ArH], 7.87–7.80 [m, 4H, ArH], 7.60–7.50 [m, 4H,
ArH], 7.50–7.40 [m, 4H, ArH], 7.02 [d, 2H, ArH] ppm.
13C NMR (DMSO‐d6); δC: 174.4 [CO2], 164.2 [C2], 150.4,
148.9, 144.2, 138.7, 135.3, 128.7, 128.0, 126.7, 124.7,
123.3, 117.2, 116.0 [ArC] ppm.
2.2.3 | Synthesis of [Zn(HLMASA
(4‐MePy)2] (3)
)
2
An analogous method to that used for the preparation of
1 was followed using Zn(OAc)2·2H2O (0.068 g, 0.31 mmol)
and H2LMASA (0.158 g, 0.62 mmol). The dried solid was
dissolved in hot acetone (10 ml) containing 4‐methyl
pyridine (1.05 ml, 10.74 mmol) and filtered while hot.
Slow evaporation of the filtrate yielded yellow crystals of
3. Yield: 0.14 g (59%). M.p. 199–200 °C. Found: C, 63.43;
H, 4.74; N, 11.01. Calcd. for C40H36N6O6Zn (MW = 762.13
gmol−1): C, 63.04; H, 4.76; N, 11.03%. IR (cm−1): 1621s,
1591m, 1484m, 1430s, 1386m, 1362m, 1307w, 1259s,
1
1174w, 1071w, 836 s, 692w, 489w. H‐NMR (DMSO‐d6);
2.2.6 | Synthesis of [Zn(HLBASA
(4‐MePy)2] (6)
)
2
δH: 12.8 [brs, 2H, OH], 8.72–8.58 [m, 6H, ArH], 7.95–7.87
[m, 2H, ArH], 7.76–7.66 [m, 4H, ArH], 7.30–7.18 [m, 8H,
ArH], 7.00 [d, 2H, ArH], 2.35 and 2.32 [s, 12H, CH3]
ppm. 13C NMR (DMSO‐d6); δC: 175.4 [CO2], 164.2 [C2],
152.6, 150.8, 148.9, 145.2, 140.6, 129.7, 128.3, 127.1, 126.4,
122.5, 117.6, 116.6 [ArC], 21.5 and 21.4 [CH3] ppm.
An analogous method to that used for the preparation of
1 was followed using Zn(OAc)2·2H2O (0.068 g, 0.31 mmol)
and H2LBASA (0.198 g, 0.62 mmol). The dried solid was
dissolved in hot acetone (10 ml;) containing
4‐methylpyridine (1.05 ml, 10.74 mmol) and filtered
while hot. Slow evaporation of the filtrate yielded yellow
crystals of 6. Yield: 0.17 g (61%). M.p. 200–202 °C. Found:
C, 51.20; H, 3.35; N, 9.36. Calcd. for C38H30Br2N6O6Zn
(MW = 891.87 gmol−1): C, 51.17; H, 3.39; N, 9.42%. IR
(cm−1): 1621s, 1477 m, 1456 m, 1430s, 1386 m, 1360 m,
2.2.4 | Synthesis of [Zn(HLCASA
(4‐MePy)2] (4)
)
2
An analogous method to that used for the preparation of
1 was followed using Zn(OAc)2·2H2O (0.068 g, 0.31 mmol)
and H2LCASA (0.171 g, 0.62 mmol). The dried solid was
dissolved in hot acetone (10 ml) containing 4‐
methylpyridine (1.05 ml, 10.74 mmol) and filtered while
hot. Slow evaporation of the filtrate yielded yellow crys-
tals of 4. Yield: 0.15 g (60%). M.p. 209–210 °C. Found:
C, 57.05; H, 3.88; N, 10.40. Calcd. for C38H30Cl2N6O6Zn
(MW = 802.97 gmol−1): C, 56.84; H, 3.77; N, 10.47%. IR
(cm−1): 1622s, 1589m, 1574m, 1479m, 1432s, 1362m,
1306m, 1262s, 1172m, 1091m, 1072m, 1034w, 841s,
724m, 489w. 1H‐NMR (DMSO‐d6); δH: 13.0 [brs, 2H,
OH], 8.76–8.70 [m, 6H, ArH], 8.05–7.93 [m, 2H, ArH],
7.85–7.80 [m, 4H, ArH], 7.46–7.40 [m, 4H, ArH],
7.42–7.38 [m, 4H, ArH], 7.06 [d, 2H, ArH], 2.44 [s, 6H,
CH3] ppm. 13C NMR (DMSO‐d6); δC: 175.3 [CO2], 164.7
1
1263 m, 1174 m, 1062 m, 841 s, 714w, 488w. H‐NMR
(DMSO‐d6); δH: 12.9 [brs, 2H, OH], 8.70–8.55 [m, 6H,
ArH], 8.95–7.85 [m, 2H, ArH], 7.75–7.65 [m, 4H, ArH],
7.40–7.20 [m, 8H, ArH], 7.96 [d, 2H, ArH], 2.37 [s, 6H,
CH3] ppm. 13C NMR (DMSO‐d6); δC: 176.8 [CO2], 166.1
[C2], 154.1, 152.5, 150.3, 146.4, 137.4, 130.6, 130.2, 128.6,
127.9, 125.2, 119.2, 118.1 [ArC], 23.0 [CH3] ppm.
2.2.7 | Synthesis of
[Cu(HLASA)2(Py)2(H2O)] (7)
An analogous method to that used for the preparation of
1 was followed using Cu(OAc)2·H2O (0.061 g, 0.31 mmol)
and H2LASA (0.150 g, 0.62 mmol). The dried solid was

BASU BAUL ET AL.
5 of 18
dissolved in hot chloroform (10 ml) containing pyridine
(0.81 ml, 10.11 mmol) and filtered while hot. Slow evapo-
ration of the filtrate yielded dark green crystals of 7.
Yield: 0.14 g (63%). M.p. 174–175 °C. Found: C, 60.12;
H, 4.25; N, 11.85. Calcd. for C36H30CuN6O7 (MW = 722.21
gmol−1): C, 59.87; H, 4.19; N, 11.64%. IR (cm−1): 1667w,
1607s, 1562s, 1487m, 1430s, 1382s, 1363s, 1301m, 1255s,
1151m, 843m, 797m, 688s, 508w.
and C‐Nazenyl bonds. Compound 7 crystallized in a chiral
space group and refinement of the Flack parameter[29]
yielded the value 0.030(16), which confidently confirms
that the refined model corresponds to the true enantio-
morph. Crystallographic data for compounds 1–8
reported in this paper have been deposited with the
Cambridge Crystallographic Data Centre (CCDC) as
supplementary publication nos. CCDC‐1887089‐CCDC‐
1887096. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/getstructures.
2.2.8 | Synthesis of [Cd(HLBASA)2(Py)3] (8)
An analogous method to that used for the preparation of
1 was followed using Cd(OAc)2·2H2O (0.082 g, 0.31 mmol)
and H2LBASA (0.198 g, 0.62 mmol). The dried solid was
dissolved in hot acetone (10 ml) containing pyridine
(0.81 ml, 10.11 mmol) and filtered while hot. Slow evapo-
ration of the filtrate yielded yellow crystals of 8. Yield:
0.21 g (69%). M.p. 262 °C (dec.). Found: C, 49.70; H,
3.39; N, 9.92%. Calcd. for C41H31Br2CdN7O6 (MW = 989.95
gmol−1): C, 49.74; H, 3.16; N, 9.90%. IR (cm−1): 1583s,
1464s, 1443s, 1388m, 1307m, 1216w, 1130w, 1070m,
1007w, 831s, 719w, 693m, 572w. 1H‐NMR (DMSO‐d6);
δH: 12.9 [brs, 2H, OH], 8.82–8.67 [m, 9H, ArH], 8.05–
7.95 [m, 2H, ArH], 7.85–7.75 [m, 6H, ArH], 7.45–7.20
[m, 10H, ArH], 7.05 [d, 2H, ArH] ppm. 13C NMR
(DMSO‐d6); δC: 175.1 [CO2], 164.2 [C2], 152.7, 150.8,
148.9, 145.7, 140.7, 129.7, 128.4, 127.1, 126.5, 122.5,
117.7, 116.6 [ArC] ppm.
2.3 | In vitro antimicrobial assay
The antimicrobial activity of pro‐ligands HLXASA (i.e.
H2LASA, H2LMASA, H2LCASA and H2LBASA) and the corre-
sponding Zn (II), Cu (II) and Cd (II) compounds 1–8 was
tested against three indicator bacterial strains, i.e., Bacil-
lus subtilis MTCC 441, Staphylococcus aureus MTCC 96
and Klebsiella pneumoniae MTCC 109, and one fungal
strain, Candida albicans MTCC 183, employing the agar
well diffusion method.[30] The stock solutions for pro‐
ligands HLXASA and compounds 1–8 were prepared
freshly in DMSO prior to use. The stock concentrations
of the tested samples were ~10,000 μg/ml. Mueller‐Hilton
agar plates were swabbed with bacterial cell suspensions
of the indicator bacterial strains adjusted to 1.5 x 108
colony forming units (CFU)/ml. Then, the agar surface
was bored by using a sterilized cork borer to generate
wells of 5 mm diameter, which were filled with a 100 μl
aliquot of the corresponding test compound solution. A
disc with the broad‐spectrum antibiotic chloramphenicol
(30 mcg) was used as positive control and a DMSO
solution as negative control. Plates were incubated at
37 °C for 24 hr and the inhibition zones (mm) formed
around the wells were measured using a Vernier caliper.
For each organism, the experiments were conducted in
triplicate and the antibacterial activity was evaluated by
measuring the growth inhibition zone diameter (IZD, in
mm). The activity was classified as high sensitivity for
IZD ≥ 20 mm, medium sensitivity for IZD = 14–20 mm,
low sensitivity for IZD = 7–13 mm, and insignificant for
IZD < 7 mm.
2.2.9 | X‐ray crystallography
Intensity data for compounds 1–8 were collected at ambi-
ent temperature with Mo‐Kα radiation (λ = 0.71073 Å,
monochromator: graphite) on an Agilent Technologies
Xcalibur diffractometer equipped with an EOS CCD area
detector. The measured intensities were reduced to F 2
and corrected for absorption using spherical harmonics
(CryAlisPro).[24] Intensities were corrected for Lorentz
and polarization effects. Structure solution, refinement
and data output were performed with the OLEX2
program package[25] using SHELXT[26] and SHELXL‐
2014.[27] Non‐hydrogen atoms were refined anisotropi-
cally. C‐H and O‐H hydrogen atoms were placed in
geometrically calculated positions using the riding model.
DIAMOND was used for the creation of figures.[28]
3 | RESULTS AND DISCUSSION
The molecular structures of 1–4, 6 and 7 exhibit
crystallographic symmetry (2‐axis), while the remaining
compounds are asymmetric. The DELU instruction
implemented in the SHELXL program was employed for
the refinement of the slightly disordered –N=N‐C6H4‐R
fragments in 3–5. In addition, in the refinement of com-
pound 5 distance restraints were employed for the N=N
3.1 | Synthesis and spectroscopic
characterization
The Zn (II) complexes 1–6 were prepared in methanol by
reacting Zn(OAc)2·2H2O and H2LXASA in 1:2 molar ratio.
Immediate formation of a precipitate was noticed and the
precipitate was found to be soluble only in DMSO,

6 of 18
BASU BAUL ET AL.
pyridine (Py) and pyridine derivatives such as
methylpyridines (MePy). Crystallization experiments
using an excess of Py or 4‐MePy in acetone or chloroform
afforded crystalline samples, in which either Py (1, 2 and
5) or 4‐MePy (3, 4 and 6) was coordinated to the zinc
atoms. The copper (II) and cadmium (II) complexes 7
and 8 were prepared analogously, however, in the case
of 7 additionally a water molecule was incorporated in
the coordination sphere of the copper atoms that possibly
emerged from the starting reagent, Cu(OAc)2·H2O
(Scheme 2). The products were obtained in moderate to
good yield (>55%) in analytically pure form. Compounds
1–8 are colored crystalline solids that are fairly stable at
room temperature outside the mother liquor and melt
or decompose above 170 °C.
The infrared spectra of compounds 1–8 are very simi-
lar and the most pertinent IR frequencies are included
in the experimental section. The pro‐ligands H2LASA
,
H2LMASA, H2LCASA and H2LBASA display an intense IR
absorption due to the asymmetric stretching vibration of
the carboxyl group, νasym (OCO), at 1664, 1658, 1666
and 1667 cm−1, respectively, which is shifted to approxi-
mately 1620 cm−1 in the metal complexes (exception 7
and 8), confirming that the carboxylate groups are
bonded to the metal atoms. The IR spectra of 1–8 also
displayed a band for νsym (OCO) at around 1425 cm−1
SCHEME 2 Synthesis and chemical structures of pro‐ligands H2LXASA and the Zn (II), Cu (II) and Cd (II) compounds 1–8

BASU BAUL ET AL.
7 of 18
with the frequency differences between νasym (OCO) and
νsym (OCO) suggesting a monodentate coordination mode
for the carboxylate ligands.[31] The stretching vibration
band typical for pyridine ligands, ν(C‐N)py, appear in the
range 1485–1600 cm−1, confirming the coordination of the
pyridine nitrogen atom to the metal ions.[32] The bonding
hypotheses were subsequently confirmed by single‐crystal
X‐ray diffraction studies (vide infra). Previously, the 1H
and 13C NMR signals for pro‐ligands H2LXASA were
are closely related. In these compounds, the zinc atoms
are embedded in ZnN2O2 polyhedra exhibiting a crystallo-
graphic 2‐axis. The metal coordination spheres are consti-
tuted by the oxygen atoms from the carboxylate groups of
two mono anionic (HLXASA)− ligands bound in mode I as
shown in Scheme 1 and the nitrogen atoms from two
pyridine ligand molecules. The coordination geometries
are significantly distorted from ideal values for a tetrahe-
dron, due to two weak secondary Zn···OOCO interactions,
thus generating an overall bicapped tetrahedral environ-
ment (Figures 1–4, 6). The distortion is illustrated by com-
parison of the OOCO‐Zn‐Npy bond angles with values in
the range of 101.0(2)‐101.60(10)° and 124.22(8)‐129.35(11)°
(Table 2). The secondary OOCO···Zn interactions are local-
ized between the atoms forming the larger OOCO‐Zn‐Npy
bond angles. The OOCO···Zn contacts can be considered as
weak interactions, since the distances in the range of
2.688(3)‐2.865(2) Å approach the sum of the van der Waals
radii of oxygen and zinc (2.91 Å). As a consequence,
the remaining bond angles in the coordination polyhedra
are significantly smaller than the ideal value of 109.5°
and decrease in all cases (except compound 1) in the
order Npy‐Zn‐Npy [102.5(3)‐106.69(13)°] > OOCO‐Zn‐Npy
1
assigned completely by the use of 2D H‐1H‐correlation
(COSY), heteronuclear single‐quantum correlation
(HSQC), constant time inverse‐detection gradient accor-
dion rescaled (CIGAR) and heteronuclear multiple‐bond
connectivity (HMBC) experiments.[22] Compounds 1–8
1
were characterized by H and 13C NMR spectroscopy in
CDCl3 solutions (Figures S1‐S14, ESI†). In general, com-
1
pounds 1–6 and 8 exhibit broad H NMR signals, which
prevented the measurement of coupling constants. The
signal assignments of the pro‐ligands were subsequently
extrapolated to the spectral analyses of the metal
complexes (except 7), owing to the data similarity. Aside
1
from the signals for metal‐coordinated (HLXASA)−, the H
and 13C{1H} NMR spectra of 1–6 and 8 displayed also the
expected signals for the secondary Py/4‐MePy ligands.
The UV–Vis absorption spectra of H2LXASA, and the cor-
responding Zn (II), Cu (II) and Cd (II) compounds 1–8
in DMSO are depicted in Figure S15, ESI†. Table S1
summarizes the most relevant characteristics of the
UV–Vis data, showing similar absorption spectra of 1–8
with slight changes in the absorption maximum.
The compounds were found to be non‐emissive in
DMSO solution.
[101.0(2)‐101.60(10)°]
>
OOCO‐Zn‐OOCO [96.09(16)‐
99.76(12)°; 103.37(10)º for 1] (Table 2). The Zn‐OOCO and
Zn‐Npy bond distances of the ZnN2O2 core are 1.9488(17)‐
2.011(5) Å and 2.0275(19)‐2.048(3) Å, respectively. Similar
values were observed in related Zn (II) complexes of com-
position [Zn(OOCR)2(PyX)2], e.g., bis(4‐(dimethylamino)
pyridine‐κN)‐bis(2‐hydroxybenzoato‐κO) ‐zinc (II),[33]
bis(3‐hydroxypyridine‐κN)bis(3‐nitrobenzoato‐κO)zinc (II)[34]
and bis(2‐fluorobenzoato‐κO)bis (pyridin‐2‐amine‐κN1)zinc
(II).[35]
Although the constitution of the coordination sphere in
compound 5 is identical to that found for 1–4 and 6
(ZnN2O2 core with two secondary O···Zn interactions),
there are significant differences in the molecular conforma-
tion and the supramolecular organization (Figure 7). In 1–4
and 6, the faces capped by the weakly bound oxygen atoms
are the triangles formed by the two Npy nitrogen atoms and
one of the OOCO oxygen atoms (O1/O1i), giving O···Zn···O
bond angles approaching 180° [173.58(8)‐179.6(1)°]; mean-
3.2 | Description of the X‐ray crystal
structures
Crystals of compounds 1–8 suitable for single‐crystal
X‐ray diffraction (SCXRD) analysis were obtained by
slow evaporation of solutions of the respective com-
pounds in acetone/pyridine (1, 2, 5 and 8), acetone/4‐
methylpyridine (3, 4 and 6) and chloroform/pyridine
(7). Crystal data, data collection parameters and conver-
gence results are listed in Table 1. The asymmetric units
of the crystal structures including displacement ellipsoids
are shown in Figures S16‐S23, ESI†.
Despite the variation of the substituents in the primary
and secondary ligands (Scheme 2, Figures 1–6), five of the
six Zn (II) complexes 1–6 are isostructural, as indicated by
the unit cell parameters given in Table 1 and the common
monoclinic space group (C2/c). Only compound 5 crystal-
lized in the monoclinic space group P21/n. As a result, the
molecular and crystal structures of compounds 1–4 and 6
while, in 5 the secondary bonding occurs via the OOCO
‐
OOCO‐Npy faces with a O···Zn···O bond angle of 83.4(1)°.
As a consequence, in the molecular structure of 5 the
OOCO‐Zn‐OOCO bond angle is significantly enlarged
[128.41(14)°] when compared to the remaining zinc com-
plexes [96.09(16)‐103.37(10)°] and the OOCO‐Zn‐Npy bond
angles are now in the range of 100.27(15)‐110.76(15)°. Nev-
ertheless, the Npy‐Zn‐Npy bond angles are rather constant
for all six complexes [102.5(3)‐106.69(13)°] (Table 2).
Another common aspect of all Zn (II) complexes (1–6) is
the presence of an intramolecular O‐H···O hydrogen bond

8 of 18
BASU BAUL ET AL.

BASU BAUL ET AL.
9 of 18
FIGURE 1 Perspective view of the molecular structure of
compound 1 with partial atom labeling scheme. Symmetry
operator: (i) 1‐x, y, 0.5‐z. For clarity, most of the hydrogen atoms are
omitted
FIGURE 2 Perspective view of the molecular structure of
compound 2 with partial atom labeling scheme. Symmetry
operator: (i) 1‐x, y, 0.5‐z. For clarity, most of the hydrogen atoms are
omitted
in the (HLXASA)− ligand with H···O and O···O distances in
the range of 1.78–1.83 Å and 2.511(3)‐2.551(6) Å, respec-
tively. The O‐H···O angles vary from 145 to 148° (Table S2).
Contrary to the Zn (II) complexes 1–6, the Cu (II) and
Cd (II) analogues adopt higher coordination numbers by
incorporating additional solvent/base molecules in the

10 of 18
BASU BAUL ET AL.
approach 90° [Npy‐Cu‐OOCO = 89.32(13) and 90.47(13)°];
meanwhile, the apical position is occupied by the water
ligand, which is, as expected, weaker bound than the
anionic ligands [Cu‐Ow = 2.214(4) Å ↔ Cu‐OOCO = 1.973(3)
Å]. The Ow‐Cu‐OOCO/Npy bond angles approach 90°
(Table 3), indicating only a small deviation of the metal
ion from the basal plane, which is attributed to two weak
secondary OOCO···Cu interactions of the carboxylate groups
of (HLASA −
with the Cu (II) ions (binding mode I,
)
Scheme 1). Although the OOCO···Cu distance of 3.014(4) Å
exceeds the sum of the van der Waals radii of the atoms
involved (2.92 Å), the close proximity of the metal ion to
the mean plane of the square pyramid base (0.09 Å) is indic-
ative of an attractive interaction. Complexes of similar com-
position, [Cu (OOCR)2(PyX)2(H2O)], were reported
previously by the research groups of Smith (R = ‐CH2O(2‐
FC6H4) and ‐CH2O(2‐Me,4‐ClC6H3); X = H)[36,37] and
Valach (R = Me; X = NH2).[38]
FIGURE 3 Perspective view of the molecular structure of
compound 3 with partial atom labeling scheme. Symmetry
operator: (i) 1‐x, y, 1.5‐z. For clarity, most of the hydrogen atoms are
omitted
Cd (II) ions are significantly larger than Zn (II) and Cu
(II) ions, which explains the seven‐fold coordination found
in compound 8. As can be seen from Figure 9, the
pentagonal‐bipyramidal coordination polyhedron in 8 is
comprised of a CdN3O4 core, in which contrary to com-
pounds 1–7, the carboxylate groups approach an
isobidentate coordination mode (binding mode II,
Scheme 1), as can be realized from the corresponding Cd‐O-
OCO distances [Cd‐O1/O2 = 2.406(3)/2.464(4) Å and Cd‐O4/
O5 = 2.349(3)/2.560(3) Å] (Table 4). The oxygen atoms and
one of the pyridine ligands (N7) comprise the pentagonal
plane of the coordination sphere with OOCO‐Cd‐OOCO/Npy
bond angles in the range of 53.24(10)‐87.31(12)° between
neighboring donor atoms. The deviation from the ideal
value of 72° is due to the formation of two four‐membered
chelate rings with the OOCO oxygen atoms. Distortion from
the ideal polyhedron is also reflected from the Npy‐Cd‐Npy
angle of 166.00(14)° formed between the pyridine ligands
occupying the axial sites (N3 and N6). The Cd‐Npy distances
in the range of 2.325(4)‐2.361(4) Å are as expected, as
observed from the geometric data reported previously for
structurally related Cd (II) dicarboxylates containing pyri-
dine in the coordination sphere.[39–41] As for 1–6, the molec-
ular structures of 7 and 8 are stabilized by intramolecular
O‐H···O hydrogen bonds (Table S2).
FIGURE 4 Perspective view of the molecular structure of
compound 4 with partial atom labeling scheme. Symmetry
operator: (i) 1‐x, y, 1.5‐z. For clarity, most of the hydrogen atoms are
omitted
coordination sphere. The five‐coordinate Cu (II) complex 7
comprises a symmetric (crystallographic 2‐axis) CuN2O3
polyhedron with distorted square‐pyramidal geometry
formed by two (HLASA)−, two pyridine and one water
ligand molecules (Figure 8). The basal plane is constituted
3.3 | Description of the supramolecular
structures of 1–8
3.3.1 | Analysis of the Zn (II) compounds
1–6
by trans‐oriented pairs of (HLASA −
) and pyridine ligands
with OOCO‐Cu‐OOCO and Npy‐Cu‐Npy bond angles of
174.0(2) and 176.0(2)°, respectively. Accordingly, the bond
angles of the cis‐oriented donor atoms in the basal plane
As mentioned in the introduction, π‐stacking interactions
are relevant for the electron transport and optical

BASU BAUL ET AL.
11 of 18
FIGURE 5 Perspective view of the molecular structure of compound 5 with partial atom labeling scheme. For clarity, most of the
hydrogen atoms are omitted
organization is closely related, albeit there are some dif-
ferences in the specific intermolecular interactions. The
supramolecular organization of these complexes is domi-
nated by π‐interactions, giving 1D π‐stacked assemblies
along the [0 1 0] direction. This arrangement is facilitated
by the double‐V shaped conformation of the molecular
structures that orients the terminal ‐C6H4X groups in
syn‐orientation (Figure 1–4, 6) and enables π‐interactions
between the terminal ‐C6H4X moieties and the salicylate
rings of adjacent metal‐coordinated ligands. The intermo-
lecular binding in the 1D strands is further strengthened
by C‐H···O contacts formed between the pyridine ligands
and the salicylate moieties. As representative example,
Figure 10 shows the assembly found in the crystal
structure of 1. The centroid···centroid distance of the
π‐interactions in 1 is 3.97 Å and ranges from 3.84–4.39 Å
for 2, 3, 4 and 6. Within the crystal structure of 1,
adjacent π‐stacks run in opposite directions and are
connected further through additional C‐H···O hydrogen
bonds, relatively weak π‐interactions between adjacent
FIGURE 6 Perspective view of the molecular structure of
compound 6 with partial atom labeling scheme. Symmetry
pyridine ligands (centroid···centroid, 4.75 Å) and van der
Waals contacts (Table S3). In 2, 3, 4 and 6, the primary
operator: (i) 1‐x, y, 1.5‐z. For clarity, most of the hydrogen atoms are
and the secondary ligands are involved in additional inter-
omitted
actions of the π···π (2, 3, 4 and 6), C‐H···π (2, 6), C‐H···Cl/
Br (4, 6) and Cl···Cl (4) type, which are further strengthen-
properties of metal complexes derived from diazenyl
ligands.[20,21] In view of these characteristics and with
the purpose to analyze the influence of the substituents
ing the crystal lattice (Table S3).
In the molecular structure of compound 5, the terminal
–C6H4X groups of the diazenyl ligands are disposed anti
(Figure 5), thus inducing a tubular 1D assembly based
on a complex combination of π···π, Br···π, C‐H···O and
C‐H···π interactions involving the salicylate and –
C6H4Br groups of the (HLBASA)− ligands (Figure 11). The
resulting 1D strands are oriented along [0 0 1] and interact
at the periphery of the (HLXASA −
) ligands and the pyri-
dine coordination to the Zn (II) centers, the supramolec-
ular interactions in the crystal structures of 1–6 were
examined. As expected from the similar unit cell parame-
ters for compounds 1–4 and 6, the overall supramolecular

12 of 18
BASU BAUL ET AL.
TABLE 2 Selected bond distances (Å) and bond angles (°) for compounds 1–6a
1
2
3
4
5
6
Zn‐OOCO
1.9488(17)
2.865(2)
1.971(2)
2.777(3)
2.031(2)
99.76(12)
1.991(2)
2.688(3)
2.037(3)
96.78(14)
177.9(1)
2.002(3)
2.691(4)
2.048(3)
96.09(16)
179.6(1)
1.971(3) / 1.957(3)
2.647(4) / 2.761(4)
2.046(4) / 2.065(4)
2.011(5)
2.704(7)
2.040(5)
98.5(3)
Zn···OOCO
Zn‐Npy
2.0275(19)
103.37(10)
176.43(6)
OOCO‐Zn‐OOCO
128.41(14)
83.4(1)
OOCO···Zn···OOCO
173.58(8)
179.1(2)
OOCO‐Zn‐Npy
101.09(8)
124.22(8)
101.60(10)
124.64(10)
101.38(11)
129.17(10)
101.51(12)
129.35(11)
102.08(14) / 110.76(15)
100.27(15) / 109.77(16)
101.0(2)
128.8(2)
Npy–Zn–Npy
104.99(11)
106.69(13)
102.77(15)
102.86(15)
103.25(14)
102.5(3)
aExcept for 5, the molecular structures exhibit crystallographic symmetry (2‐axis).
TABLE 3 Selected bond distances (Å) and bond angles (°) for
compound 7a
Cu‐Npy
2.003(3)
1.973(3)
3.014(4)
2.214(4)
176.0(2)
174.0(2)
93.01(10)
Cu‐OOCO
Cu···OOCO
Cu‐Ow
Npy‐Cu‐Npy
OOCO‐Cu‐OOCO
OOCO‐Cu‐Ow
Npy‐Cu‐OOCO
FIGURE
7
Comparison of the coordination polyhedra in
compounds 1 and 5. Symmetry operator: (i) 1‐x, y, 0.5‐z
89.32(13) /
90.47(13)
with adjacent strands through a series of C‐H···O,
C‐H···N, C‐H···π, π···π and C‐H···Br contacts (Table S4).
Although the supramolecular organization of
compound 5 seems to be more complex than that of
Npy‐Cu‐Ow
91.99(11)
aThe molecular structure of 7 exhibits crystallographic symmetry (2‐axis).
FIGURE 8 Perspective view of the molecular structure of compound 7 with partial atom labeling scheme. Symmetry operator: (i) 1‐x, 1‐y,
z. For clarity, most of the hydrogen atoms are omitted and disordered hydrogen atoms of the Ow is not shown

BASU BAUL ET AL.
13 of 18
FIGURE 9 Perspective view of the molecular structure of
compound 8 with partial atom labeling scheme. For clarity, most
of the hydrogen atoms are omitted
TABLE 4 Bond distances (Å) and bond angles (°) for compound 8
Cd‐O1
2.406(3)
2.464(4)
Cd‐N3
Cd‐N6
Cd‐N7
2.325(4)
2.336(4)
2.361(4)
Cd‐O2
Cd‐O4
2.349(3)
Cd‐O5
2.560(3)
O1‐Cd‐O2
O1‐Cd‐O4
O1‐Cd‐O5
O2‐Cd‐O4
O2‐Cd‐O5
O4‐Cd‐O5
O1‐Cd‐N3
O1‐Cd‐N6
O1‐Cd‐N7
O2‐Cd‐N3
O2‐Cd‐N6
53.41(11)
87.31(12)
140.37(11)
140.59(12)
164.95(11)
53.24(10)
88.56(13)
89.32(13)
136.09(13)
90.97(14)
98.85(13)
O2‐Cd‐N7
O4‐Cd‐N3
O4‐Cd‐N6
O4‐Cd‐N7
O5‐Cd‐N3
O5‐Cd‐N6
O5‐Cd‐N7
N3‐Cd‐N6
N3‐Cd‐N7
N6‐Cd‐N7
82.99(13)
84.26(13)
81.82(13)
136.42(13)
84.32(13)
88.53(13)
83.54(13)
166.00(14)
98.11(14)
93.00(14)
FIGURE 10 Fragment of the crystal structure of compound 1
showing the 1D strands along [0 1 0] resulting from π−stacking
and C−H···O interactions. Cg1 = centroid (C2−C7); Cg2 = centroid
(C8−C13). Symmetry operator: x, −1+y, z. For clarity, hydrogen
atoms not involved in hydrogen bonding interactions are omitted
3.3.2 | Analysis of the Cu (II) and Cd (II)
compounds 7–8
Metal‐coordinated water molecules are excellent hydrogen
bond donors, which explains the formation of 1D strands
through double‐bridged motifs based on Ow‐H···O hydro-
gen bonds in the crystal structure of the copper complex
7. As illustrated in Figure 12, the (HLASA −
) ligand mole-
complex 6, which apart from the (HLBASA)− ligand carries
4‐methylpyridine instead of pyridine, the melting point
and, therefore, the crystal lattice energy is significantly
lower (5, M.p. 177–178 °C; 6, M.p. 200–202 °C). This
seems to indicate that for complex 5 probably a polymor-
phous form exists that is isostructural with the remaining
members of the Zn (II) series, which is reasonable when
considering that the molecular volume of 5 is smaller
when compared to compound 6 and the chlorine
analogue 4 (M.p. 209–210 °C).
cules in the 1D strands running along [0 0 1] are further
linked through π‐interactions between adjacent salicylate
and –C6H5 rings (centroid···centroid, 4.64 Å). In the third
dimension, adjacent strands are connected only through a
series of C‐H···π and van der Waals contacts (Table S5).
In the Cd (II) complex 8, the molecular units are orga-
nized by π‐interactions involving the salicylate moieties of
the (HLASA)− ligands with centroid···centroid distances of
4.11 Å. As can be seen from Figure 13, these interactions
are accomplished by C‐H···O, C‐H···N and C‐H···π contacts

14 of 18
BASU BAUL ET AL.
FIGURE 11 Fragment of the crystal structure of compound 5 showing the 1D strands along [0 0 1] resulting from π‐stacking, Br···π, C
−H···O and C−H···π interactions. Cg5 = centroid (C21−C26). Symmetry operators: (i) 1‐x, 1‐y, −z; (ii) 1‐x, 1‐y, 1‐z; (iii) x, y, −1+z. For
clarity, hydrogen atoms not involved in hydrogen bonding interactions are omitted
experiments. The UV/Vis absorbance spectra were
monitored for two days (Figure S24; ESI†); the
unchanged basic pattern of the spectra of the test com-
pounds indicates that they are stable in DMSO solution.
The antibacterial potential of the pro‐ligands
H2LXASA and the corresponding Zn (II), Cu (II) and
Cd (II) compounds 1–8 along with the reference com-
pounds
(Zn(OAc)2·2H2O,
Cu(OAc)2·H2O
and
Cd(OAc)2·2H2O) was evaluated according to the inhibi-
tion zone formation against three bacterial strains, i.e.,
Bacillus subtilis MTCC 441, Staphyllococcus aureus
MTCC 96 and Klebsiella pneumoneae MTCC 109, and a
fungal strain, i.e., Candida albicans MTCC 183. The
results were compared with the activity of chloram-
phenicol and fluconazole as positive controls for bacte-
ria and fungus, respectively, and the solvent used for
the experiments (10% DMSO) as negative control, see
Table 5 (Figures S25, S26 and S27 (Panels A‐D); ESI†).
As expected, the solvent did not inhibit the growth of
the tested microorganisms.
The results of the antimicrobial screening revealed
that among the twelve compounds examined (H2LXASA
and 1–8), all of them could inhibit at least one of the
three indicator organisms, with varying degrees of anti-
microbial activity. In general, compounds 1–8 demon-
strated enhanced activity against B. subtilis MTCC 441
when compared to their respective pro‐ligands. Signifi-
cant activities were noted for the zinc (II) compounds
1–3 and 6 and the cadmium (II) compound 8 against
all three bacterial strains, including K. pneumoniae
MTCC 109, which was resistant to seven of the com-
pounds tested, including the copper (II) complex 7. C.
albicans MTCC 183 was susceptible to most of the com-
pounds except for Zn(OAc)2·2H2O. In general,
Cd(OAc)2·2H2O was the most potent agent of all com-
pounds, showing good activity against all test
organisms.
FIGURE 12 Fragment of the crystal structure of compound 7
showing the 1D strands resulting from Ow−H···Ow hydrogen
bonds and π−stacking interactions. Cg1 = centroid (C2−C7);
Cg2 = centroid (C8−C13). Symmetry operator: x, y, −1 + z. For
clarity, hydrogen atoms not involved in hydrogen bonding
interactions and disordered hydrogen atoms of the H2O molecule
are not shown
to give overall 2D layers. In the third dimension, besides C‐
H···O and van der Waals contacts adjacent layers are
linked through two cyclic motifs involving the bromine
atoms attached at the periphery of the (HLASA −
) ligand
molecules, the first based on a C‐H···Br and an O···Br
contact and the second on a Br···π and a C‐H···N interac-
tion (Figure 14, Table S5).
3.4 | Antibacterial activity results
The solution stability of compounds 1–8 was assessed in
dimethyl sulfoxide (DMSO) prior to the antibacterial

BASU BAUL ET AL.
15 of 18
FIGURE 13 Fragment of the crystal structure of compound 8 showing the 2D layers resulting from π−stacking interactions and C−H···O,
C−H···N and C−H···π contacts. Cg1 = centroid (C2−C7); Cg3 = centroid (C15−C20). Symmetry operators: (i) 1 + x, y, z; (ii) x, −1 + y, z; (iii)
x, 1 + y, z. For clarity, hydrogen atoms not involved in hydrogen bonding interactions are omitted
FIGURE 14 Fragments of the crystal structure of compound 8 showing the cyclic motifs based on a C−H···Br and an O···Br contact (a)
and a C−H···N and Br···π contact (b). Symmetry operators: (i) 0.5‐x, 0.5 + y, 1.5‐z; (ii) 1.5‐x, 0.5+y, 1.5‐z. For clarity, hydrogen atoms not
involved in hydrogen bonding interactions are omitted
The antimicrobial activities of the compounds studied
herein are more promising than those of a recently
investigated series of zinc (II) compounds with 5‐[(4‐
(aryl)‐phenyl)‐1‐diazenyl]quinolin‐8‐olates,[20] testifying
that 5‐[(E)‐2‐(aryl)‐1‐diazenyl]‐2‐hydroxybenzoates are
better ligands for bacterial growth inhibitors with zinc
(II). However, a more detailed interpretation of the
structure–property relationships is difficult, since the
microbial activities of the compounds depend on several
factors such as the type and number of donor atoms, their
relative positions within the ligand and the metal coordi-
nation geometry.

16 of 18
BASU BAUL ET AL.
TABLE 5 Antimicrobial activity of the pro‐ligands (H2LASA, H2LMASA, H2LCASA and H2LBASA) and the metal compounds (1–8) against
representative bacterial and fungal strains
Inhibition zones (in mm) observed for the tested bacterial and fungal species
Pro‐ligand/Compound
B. subtilis MTCC 441
S. aureus MTCC 96
K. pneumoniae MTCC 109
C. albicans MTCC 183
H2LASA
H2LMASA
H2LCASA
H2LBASA
11
19
18
18
21
21
26
30
31
27
18
28
10
26
40
0
11
10
16
11
10
10
16
24
21
19
17
23
0
0
0
11
11
12
12
10
13
18
21
20
22
14
22
14
0
0
0
1
10
11
10
0
2
3
4
5
0
6
11
0
7
8
11
0
Cu (OAc)2·H2O
Zn (OAc)2·2H2O
Cd (OAc)2·2H2O
Solvent (10% DMSO)
Chloramphenicol (C)30
Fluconazole (FLC)25
13
28
0
17
16
0
33
0
29
‐
26
‐
22
‐
‐
18
cadmium (II) complexes revealed that metal ion inter-
change not only influences the electronic structure of the
aryl‐substituted diazenyl‐salicylate ligand but also the solid
state organization. This is due to the significant change in
coordination number and geometries between these metal
ions, even though they are adjacent in the periodic system
of the elements.
Turning to the antimicrobial activity, compounds 1–7
and 8 were found to be more potent against Bacillus
subtilis MTCC 441 with IZD ≥ 20 (high sensitivity) and
less active against Klebsiella pneumoniae MTCC 109.
Similar inhibitory effects were noted for the assays with
Staphyllococcus aureus MTCC 96 and Candida albicans
MTCC 183.
4 | CONCLUSIONS
Monomeric zinc complexes with distorted tetrahedral
(1–6), a copper complex with a square‐pyramidal (7) and a
cadmium complex with a pentagonal‐bipyramidal (8)
coordination geometries have been obtained by using
aryldiazenyl linked O,O‐chelating (salicylates) ligands in
combination with pyridine derivatives as ancillary ligands.
The aryl‐substituted diazosalicylate ligands are coordinated
to the metal complexes only through the oxygen atoms of
carboxylate groups, either in an anisobidentate or
isobidentate mode. Despite the variation of the substituents
at the periphery of the primary (H2LXASA; X = H, Me, Cl,
Br) and secondary ligands (4‐Py‐R; R = H, Me), for the Zn
(II) complex series studied herein 1–6, with one exception,
the formation of isostructural solids was achieved. This
illustrates that the π–electron density and energy levels
can be varied without modification of the solid‐state struc-
ture by changing the substituents in the aryldiazenyl salicy-
late ligands, thus enabling fine‐tuning of its
physicochemical properties. This is an important finding
and fundamental for applications in electron conduction
and optics. The structural analysis of the copper (II) and
ACKNOWLEDGEMENTS
Financial support from the University Grants Commis-
sion, India (Grant No. 42‐396/2013 (SR) 2013, TSBB),
the Council of Scientific and Industrial Research, India
(Grant No. 01 (2734)/13/EMR‐II, 2013, TSBB) and the
University Grants Commission, India (SAP‐CAS,

BASU BAUL ET AL.
17 of 18
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