Pyridine aided progression from amorphous to crystalline bis([5-(aryl)-1-diazenyl]quinolin-8-olato)zinc(II) compounds???Solution and solid-state structural characterization, nanoparticle formation and antibacterial activity

Article abstract of DOI:10.1016/j.ica.2018.06.049

Ten new zinc(II) compounds, viz. [Zn(L^Q)₂(4-MePy)₂]·H₂O (1), [Zn(L^MeAQ)₂(Py)₂]·C₆H₆ (2a), [Zn(L^MeAQ)₂(H₂O)₂] (2b

Full text of DOI:10.1016/j.ica.2018.06.049

Accepted Manuscript
Research paper
Pyridine aided progression from amorphous to crystalline bis([5-(aryl)-1-dia-
zenyl]quinolin-8-olato)zinc(II) compounds − Solution and solid-state structural
characterization, nanoparticle formation and antibacterial activity
Tushar S. Basu Baul, Khrawborlang Nongsiej, Koel Biswas, Santa Ram Joshi,
Herbert Höpfl
PII:
S0020-1693(18)30501-2
https://doi.org/10.1016/j.ica.2018.06.049
ICA 18341
DOI:
Reference:
Inorganica Chimica Acta
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Received Date:
Revised Date:
Accepted Date:
4 April 2018
28 June 2018
28 June 2018
Please cite this article as: T.S. Basu Baul, K. Nongsiej, K. Biswas, S.R. Joshi, H. Höpfl, Pyridine aided progression
from amorphous to crystalline bis([5-(aryl)-1-diazenyl]quinolin-8-olato)zinc(II) compounds − Solution and solid-
state structural characterization, nanoparticle formation and antibacterial activity, Inorganica Chimica Acta (2018),
doi: https://doi.org/10.1016/j.ica.2018.06.049
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Pyridine aided progression from amorphous to crystalline bis([5-(aryl)-1-
diazenyl]quinolin-8-olato)zinc(II) compounds  Solution and solid-state
structural characterization, nanoparticle formation and antibacterial activity
Tushar S. Basu Baul,a,* Khrawborlang Nongsiej,a Koel Biswas,b Santa Ram Joshib and
Herbert Höpflc,*
aCentre for Advanced Studies in Chemistry, North-Eastern Hill University, NEHU Permanent
Campus, Umshing, Shillong 793 022, India
bDepartment of Biotechnology & Bioinformatics, North-Eastern Hill University, NEHU
Permanent Campus, Umshing, Shillong 793 022, India
cCentro de Investigaciones Químicas, Instituto de Investigación en Ciencias Básicas y Aplicadas,
Universidad Autónoma del Estado de Morelos, Av. Universidad 1001, Cuernavaca 62209, Mexico
1

Abstract
Ten new zinc(II) compounds, viz. [Zn(LQ)2(4-MePy)2]·H2O (1), [Zn(LMeAQ)2(Py)2]·C6H6 (2a),
[Zn(LMeAQ)2(H2O)2] (2b), [Zn(LOMeAQ)2(H2O)2] (3), [Zn(LEtAQ)2(3-MePy)2] (4), [Zn(LOEtAQ)2(4-
MePy)2]·0.5C6H7N (5a), [Zn(LOEtAQ)2(H2O)2] (5b), [Zn(LNMe2AQ)2(Py)2]·1.5C6H6·H2O (6),
[Zn(LCAQ)2(Py)] (7) and [Zn(LBAQ)2(Py)] (8) (primary ligands: LQ = quinolin-8-olate and LXAQ = 4-
substituted 5-[(E)-2-(aryl)-1-diazenyl]quinolin-8-olate; secondary ligands: Py = pyridine, 3-MePy
= 3-methylpyridine, 4-MePy = 4-methylpyridine) have been synthesized and characterized by
elemental analysis, IR, NMR, UV-vis and fluorescence spectroscopy and single-crystal X-ray
diffraction analysis. The structural characterization revealed a distorted octahedral geometries for
1-6, in which trans-disposed 8-quinolate ligands occupy the equatorial and the secondary ligands
the axial positions. Compounds 7-8 comprise five-coordinate molecular structure with square-
pyramidal coordination polyhedra based on trans-oriented quinolate ligands and a Py molecule in
the apical position. In the crystal structures of 1-8, the molecules are assembled by –stacking
interactions and for the aquo-compounds additionally by OHO hydrogen bonds. As
representative example, compound 2a was used as starting material for the synthesis of ZnO
nanoparticles in an average size range of 70120 nm, as illustrated by PXRD analysis and TEM
images. The antimicrobial activity of the pro-ligands and zinc(II) compounds studied herein was
assessed by inhibition zone test in agar cultures against five indicator bacterial strains, i.e.
Escherichia coli MTCC 730, Streptococcus pyogenes MTCC 1925, Klebsiella pneumoniae MTCC
109, Bacillus cereus MTCC 430 and Salmonella enterica MTCC 735.
Keywords: substituted quinolin-8-olate, zinc, secondary ligands, structure elucidation,
nanoparticles, antimicrobial activity
__________________________
* Corresponding authors. E-mail addresses: basubaulchem@gmail.com; basubaul@nehu.ac.in (T.
S. Basu Baul), hhopfl@uaem.mx (Herbert Höpfl)
2

1. Introduction
Quinolin-8-ol (HLQ), a well-known bidentate chelating ligand, has been utilized for the
qualitative and quantitative analysis of metal ions in gravimetric analysis to selectively bind and
precipitate metal ions [1,2]. In addition, quinolin-8-olate metal compounds are extensively used
in areas such as photochemical processes [3-7] and optical sensing [8-10]. The high thermal
stability, excellent electron transport properties and the unique electroluminescent properties of
tris(quinoline-8-olato)aluminium (III) [Al(LQ)3] provided a pivotal role in the development of
organic light-emitting diodes (OLEDs) that lead to the introduction of the flat-panel display
technology [11]. This innovation has stimulated research groups’ world wide to develop further
compounds based on derivatives of HLQ including derivatives with other metal ions. Of special
interest is the tuning of the emission color of metal-based OLEDs, which is achieved via the
introduction of substituents in the 2-, 4-, 5- and 7-position of the HLQ ligand [12]. A more recent
approach is based on modifications of the supramolecular interaction patterns of HLQ
compounds [13].
In addition, HLQ has emerged as a privileged scaffold for the development of new drug
candidates, such as fungicidal, insecticidal, anti-microbial, anti-oxidant, anti-inflammatory, anti-
neurodegenerative and antitumor/antineoplastic agents [14-25]. In bacteria, zinc performs a
beneficial role in enzymatic processes and the organization of protein structures [26].
Nevertheless, at high concentrations an inhibitory action on the growth of bacterial species such
as Escherichia coli, Staphylococcus aureus, Staphylococcus faecalis, Staphylococcus
epidermidis and Proteus aeruginosa is observed [27,28]. It is also known that the interaction of
zinc with bioactive organic compounds can increase the biological activity [29], while other
metals inhibit their activity [30]. In this context, clioquinol (5-chloro-7-iodoquinolin-8-ol), which
is an antimicrobial agent known for many years [31] has been combined with a series of 3d metal
compounds, showing that the antimicrobial activities vary depending on the microorganism and
metal used for complex formation [32].
Zinc(II) cations can adopt various coordination geometries, giving a versatile
complexation chemistry with HLQ (Chart 1). Usually 2:1 complexes are obtained, but 3:1
complexes can also be formed [33]. The preparation of bis(quinolin-8-olato)zinc(II), [Zn(LQ)2],
if carried out in aqueous medium, the corresponding diaquo compound is formed, i.e.
[Zn(LQ)2(H2O)2], in which two water molecules are bound in trans-fashion to the square-planar
3

Zn(LQ)2 moiety (I) [34]. With the 5-nitro-HLQ analog a related structure was reported, but in this
case the compound crystallized as methanol solvate [35]. Anhydrous zinc(II) quinolin-8-olate
consists of four 2:1 complex units linked to a linear tetramer, [Zn(LQ)2]4 (II), in which the zinc
atoms are connected by bridging quinolin-8-olate oxygen atoms [36]. Similar to [Al(LQ)3],
[Zn(LQ)2] was explored for the fabrication of electroluminescent devices, showing good electron-
transport properties due to -stacking interactions of the metal-coordinated ligands within the
crystal structure [36,37]. Both dimeric [Zn(LQ)2]2 (III) and trimeric [Zn(LQ)2]3 (IV) compounds
are formed when the quinolin-8-olate ligands are substituted in the 7- or 2-position [38,39]. In
trimeric (IV), the cyano-substitution induces strong red-light emission [39]. On the other hand,
lipophilic quinolin-8-ol derivatives featuring substitution at the 2- as well as in the 7-position by
methyl and long alkyl groups revealed compounds of composition [Zn(LQ)2] (V) with a pseudo-
tetrahedral coordination geometry at the zinc(II) center, possibly as a consequence of the steric
bulk [38a]. The supramolecular solid-state assembly can be also modified by introduction of an
amide substituent into the quinolin-8-olate ring, featuring a polymeric network, [Zn(LQ)2(-Q)]n
(VI), with penta-coordinated distorted square-pyramidal zinc centers through the formation of
C=OZn bonds via the amide oxygen atoms [40]. In the presence of strong metal-coordinating
solvents, the product obtained from equimolar reactions of ZnX2 (X = Cl or Br) with HLQ in
methanol, e.g. dimethylformamide, dimethylacetamide and dimethysulphoxide, afforded the
respective solvate of a tetranuclear cluster of composition [Zn4(LQ)6X2]2 solvent (VII). Using
the sterically more hindered 3-methylpyridine ligand, a compound containing mononuclear zinc
ions resulted, [Zn(LQ)2(3-MePy)][Zn(LQ)2(3-MePy)2]3H2O (IX) [41]. In VII, the [Zn4(LQ)6X2]
core displays two different coordination environments for the zinc cations. The zinc sites holding
the halide ions are penta-coordinated, while the remaining zinc sites are hexa-coordinated giving
a distorted octahedral coordination polyhedron. On the other hand, the zinc atoms in the
molecular compounds portrayed in IX adopt a five-coordinate distorted square-pyramidal and a
distorted octahedral geometry, in which the 3-methylpyridine molecules occupies the apical and
axial positions, respectively. Reaction of zinc chloride with HLQ in an equimolar ratio followed
by crystallization from water afforded the tetrachlorozincate salt of [HQH]+, i.e. (H2LQ)2[ZnCl4]
(VIII) (H2LQ is quinolin-1-ium-8-ol) [42,43].
The luminescent properties of HLQ ligands can be tailored by extending the -electron
system with additional functional groups [44]. In line with this strategy, several zinc(II)
4

compounds have been reported. [(2,6-Dichlorophenyl)ethenyl]quinolin-8-olate), here after LSQ,
gave a complex of composition [Zn2(LSQ)2]2 and the structure reveals the same binuclear core
and a trigonal-bipyramidal geometries around Zn atoms as shown in Chart 1 (III) [38b]. In
contrast to [Zn(LSQ)2]2, with [(2,6-difluorophenyl)-, [(2,4-dinitrophenyl)- [44a] and [(2,4,6-
trifluorophenyl)ethenyl]quinolin-8-olates [44c] unique trimeric zinc(II) compound of
formulation [Zn(LSQ)2]3 (X) were obtained. The SCXRD analyses indicated that the terminal zinc
atoms in the trimeric unit have distorted trigonal-bipyramidal geometries with the equatorial
plane occupied by the N2O donor atoms of three different ligands and the apical positions formed
by the phenoxy oxygen atoms of two ligand molecules. On the contrary, the coordination
environment of the central Zn(II) atom is a distorted octahedron, where the equatorial plane is
occupied by the NO3 donors from three ligands, while the apical positions are occupied by one
phenoxy oxygen atom and a quinoline nitrogen atom. Because of the promising properties and
potential applications of luminescent coordination polymers, in a subsequent development of this
ligand type, 3- and 4-pyridyl-substituted quinolin-8-olates were explored, giving 1D coordination
polymers of the type {[Zn(L3-SQ)(X)]2}n (XI) (L3-SQ = [(3-pyridyl)ethenyl]quinolin-8-olate, X =
Cl, Br or I) [44e] and {[Zn(L3-SQ)2(-L3-SQ)2]}n (XII). In XI, the basic building unit comprises a
dinuclear Zn compound, in which two-phenolate oxygen atoms of two ligands bridge the metal
atoms. The five-coordinate Zn(II) centers are bound to the quinoline and pyridyl nitrogen atoms
from two ligands L, two phenolate oxygen atoms and one halide anion, establishing a distorted
trigonal-bipyramidal geometry [44e]. In XII, the building blocks are mononuclear with the
Zn(II) centers adopting a slightly distorted octahedral geometry, in which the equatorial plane is
occupied by two chelating NO donor sets from two ligands and the axial positions are constituted
by two bridging 4-pyridyl nitrogen atoms of two adjacent ligands [44h]. In summary, quinolin-8-
olate-derived ligands have shown to function in a myriad of coordination modes, leading to a
large variation of the structural motifs with different coordination numbers and geometries of the
zinc(II) atoms (I-XII, Chart 1).
5

6

Chart 1. Overview showing the coordination behavior of quinolin-8-olate ligand derivatives towards zinc(II) atoms.
7

Besides the complexation and structural aspects of zinc(II) quinolin-8-olates, the zinc
analog of [Al(LQ)3], i.e. [Zn(LQ)2], has received attention because of its potential to enhance the
electron-transporting properties in OLEDs [35,45]. Consequently, the photophysical properties
of mono- and multinuclear zinc(II) compounds derived from quinolin-8-olates (Chart 1) are
generally analyzed in terms of their molecular structures and the supramolecular solid-state
packing [38b,44a,44c,44e,44h]. However, the above-mentioned compounds exhibit divergent
photophysical properties and, hence, a conclusive analysis to achieve the tuning of the optical
properties has yet not been reached so far. In addition, octahedral [Zn(LQ)2(H2O)2] was
investigated for photolysis reactions in non-aqueous solvents, revealing that the photo-activation
of the water ligand coordinated to zinc(II) is based on the charge shift induced by IL excitation
of the quinolin-8-olate ligand. This phenomenon is also found in thermal hydrolysis reactions
catalyzed by zinc enzymes in nature [46]. In 1965, Takamoto et al. synthesized a series of 5-
(aryl)-1-diazenyl]quinolin-8-ols (HLXAQ) and studied the reactivity in homogeneous media and
biphasic systems [47]. Later, the ligands were explored as analytical reagents for the qualitative
detection of metal ions [48] and successfully employed for the synthesis of organometallic
compounds [49]. More recently, HLXAQ type ligands were grafted on colloidal silica nanoparticles
in order to study the kinetics of the complexation with Ni2+ ions [50] and to tune the optical
properties of structurally characterized rhenium(I) compounds [51]. The complexation chemistry
of transition metal [52,53] and rare earth metal [54] complexes with HLXAQ are also on the record.
Concerning Zn(II) compounds, there is a report on compounds with ligands having peripheral 4-
(X) groups, i.e. 5-[(4-(X)-phenyl)-1-diazenyl]quinolin-8-ol (X = Et, nBu, CMe3, H and F), which
showed interesting luminescent and electroluminescent properties. Particularly, bis(5-[(4-
(dimethylamino)phenyl)-1-diazenyl]quinolin-8-olato)zinc(II) showed substantial luminescent
quantum yields at room temperature and was identified as electron transport material, which may
find applications in photonic devices [52,53], but surprisingly none of these compounds were
characterized by X-ray diffraction methods, possibly due to low solubility.
In recent past, we have been focusing on the rational design of various types of discrete
compounds and coordination polymers using (E)-N-(pyridin-2-ylmethylene)arylamine ligands
and d10 metal ions (Zn, Cd and Hg) [55]. As part of a program enabling the solid-state structural
characterization of Zn(II) compounds derived from pro-ligands quinolin-8-ol (HLQ) and 5-[(4-
8

(X)-phenyl)-1-diazenyl]quinolin-8-ol (HLXAQ) have been reported and, thus providing a series of
ten new compounds [Zn(LQ)2(4-MePy)2]·H2O (1), [Zn(LMeAQ)2(Py)2]·C6H6 (2a),
[Zn(LMeAQ)2(H2O)2] (2b), [Zn(LOMeAQ)2(H2O)2] (3), [Zn(LEtAQ)2(3-MePy)2] (4), [Zn(LOEtAQ)2(4-
MePy)2]·0.5C6H7N (5a), [Zn(LOEtAQ)2(H2O)2] (5b), [Zn(LNMe2AQ)2(Py)2]·1.5C6H6·H2O (6),
[Zn(LCAQ)2(Py)] (7) and [Zn(LBAQ)2(Py)] (8), Scheme 1. The zinc(II) compounds were
1
structurally characterized in solution using H and 13C NMR, UV-Vis and photoluminescence
spectroscopy. Detailed structural information on the molecular and crystal structures of the
Zn(II) compounds 1-8 was obtained by single-crystal X-ray crystallographic analysis. The results
provided three variants of the Zn(II) coordination environment, i.e. [Zn(LXAQ)2(Py/3-MePy/4-
MePy)2], [Zn(LXAQ)2(H2O)2] and [Zn(LXAQ)2(Py)]. Different pyridine ligands were explored as
secondary ligands with the objective to vary the molecular and supramolecular structures,
envisioning the modification of electronic features for future solid-state applications. As
representative case, compound 2a was used as starting material for the synthesis of ZnO
nanoparticles. In addition, the in vitro antibacterial activity of 1-8 against five strains of bacteria
is reported.
9

Scheme 1. Synthesis of pro-ligand HLXAQ and chemical structures of the zinc(II) complexes 1-8
along with the atom-numbering protocol.
2.1. General considerations
Zn(OAc)22H2O (S.d. fine), 8-hydroxyquinoline (Loba Chemie), p-toluidine, p-anisidine
(CDH), p-ethylaniline, p-ethoxyaniline, p-N,N-dimethylphenyldiamine (Spectrochem), p-
chloroaniline, p-bromoaniline (Himedia), pyridine (Merck), 3-methylpyridine and 4-
methylpyridine (Spectrochem) were used without further purification. The solvents used in the
reactions were of AR grade and dried using standard procedures.
g po s r sur us g g po pp r us (M-560) and are
uncorrected. Carbon, hydrogen and nitrogen analyses were performed with a Perkin-Elmer 2400
series II instrument. IR spectra in the range 4000-400 cm-1 were obtained on a Perkin Elmer
10

1
Spectrum BX series FT-IR spectrophotometer with 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. Emission
measurements were carried out on a Hitachi model FL4500 spectrofluorometer with appropriate
background correction at ambient temperature in freshly prepared DMSO solutions.
Fluorescence spectra in DMSO solutions were obtained with the excitation and emission slits
fixed at 10 and 20 nm, respectively, in quartz cuvettes with 10 mm optical path length.
Fluorescence quantum yields ϕf were determined using acridine yellow in ethanol (ϕR = 0.47),
based on the following equation:
AS ODR S2
S  R .
.
.
AR ODS 2R
r “A” r s o gr r u r f uor s urv , “OD” o s
absorbance,  the refractive index of the medium and , the fluorescence quantum yield.
Subs r p s “S” “R” o p r rs for s u s p r f r , r sp v y.
Solid-state fluorescence spectra of compound 1 and HLQ were obtained using a Hitachi
fluorescence spectrophotometer F-7000 (with the excitation and emission slits fixed at 2.5 and 5
nm, respectively), and spectra were corrected for the instrument response function. For the
nanomaterial work, the calcination of 2a was carried out in a Revotek GMP muffle furnace at a
heating rate of 10 °C min-1 from 25 °C to 600 °C. Ultrasound irradiations were accomplished
with a Sonics VCX 750 instrument equipped with a high intensity ultrasonic probe (13 mm tip
diameter; Ti-horn, 20 kHz, 750 Watt) that was immersed directly into the reaction mixture.
Powder X-ray diffraction (PXRD) studies were carried out on a GNR X-ray Explorer using
CuK radiation (1.54 Å). For the analysis of the ZnO nanoparticles, a Field Emission Scanning
Electron Microscope (FESEM) from Carl Zeiss SIGMA and a Transmission Electron
Microscope (TEM), from Jeol (JEM-2100) was used. For the in vitro antimicrobial assays,
Mueller-Hinton agar (HiMedia) was used as nutrient liquid medium and chloramphenicol
(Sigma-Aldrich) as reference antibiotics.
11

2.2. Synthesis of pro-ligands and zinc(II) compounds
Pro-ligands (HLXAQ viz., HLMeAQ, HLOMeAQ, HLOEtAQ, HLCAQ, HLBAQ) [49a,49b], (HLEtAQ
and HLNMe2AQ) [53] were prepared starting from 8-hydroxyquinoline 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
preparations of [Zn(LQ)2(4-MePy)2] (1) and [Zn(LMeAQ)2(Py)2]·C6H6 (2a) are described in detail,
as representative examples.
2.2.1. Synthesis of [Zn(LQ)2(4-MePy)2]·H2O (1). Zn(OAc)22H2O (0.23 g, 1.03 mmol) in
methanol (10 mL) was added drop-wise to a stirred solution of 8-hydroxyquinoline, HLQ (0.30 g,
2.07 mmol) in methanol (10 mL) at room temperature, which resulted in the immediate
formation of straw-colored precipitate. The reaction mixture was heated to reflux for 3 h 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 ethanol (5 mL)
containing 4-methylpyridine (1 mL, 10.74 mmol) and filtered while hot. Upon cooling to room
temperature, the filtrate afforded pale yellow crystals of 1. Yield: 0.33 g (57%). M.p. >300 °C.
Found: C, 64.33; H, 4.82; N, 9.90%. Calc. for C30H28N4O3Zn (MW = 557.93 gmol-1): C, 64.58;
H, 5.06; N, 10.04%. IR (cm–1): 1615w, 1599m, 1570s, 1494s, 1462s, 1420m, 1391s, 1366m,
1
1325s, 1105s, 827m, 796m, 744s, 486w. H-NMR (DMSO-d6): δH; 8.54-6.87 [br m, 20H, ArH],
2.25 [s, 6H, Me] ppm. 13C NMR (DMSO-d6): δC; 162.20, 149.53, 147.88, 145.59, 140.18,
139.13, 130.30, 129.88, 125.30, 121.84, 112.82, 110.17 [ArC], 20.87 [CH3] ppm.
2.2.2. Synthesis of [Zn(LMeAQ)2(Py)2]·C6H6 (2a). A slightly modified method to that used for the
preparation of 1 was followed. Zn(OAc)22H2O (0.21 g, 0.95 mmol) in methanol (10 mL) was
added drop-wise to a stirred solution of HLMeAQ (0.50 g, 1.90 mmol) in benzene (50 mL) at room
temperature, which resulted in the immediate formation of a red-colored precipitate. The reaction
mixture was heated to reflux for 3 h 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 chloroform (15 mL) containing pyridine (0.8 mL, 10 mmol) and filtered to
remove any suspended particles. The filtrate was diluted with benzene (1 mL) and filtered while
12

hot. The filtrate, upon slow evaporation afforded dark red crystals. Yield: 0.49 g (62%). M.p.
>300 °C. Found: C, 69.92; H, 4.75; N, 13.28%. Calcd. for C48H40N8O2Zn (MW = 826.25 gmol-
1): C, 69.77; H, 4.88; N, 13.56%. IR (cm–1): 1598m, 1573m, 1552m, 1497s, 1463s, 1408m,
1
1391m, 1329s, 1246s, 1213m, 1188m, 1169m, 1104s, 818w, 461w. H NMR (DMSO-d6): δH;
9.40 [br d, 2H, H-4], 8.69-8.57 [m, 8H, H-2, H-6, H-(Py)], 8.13 [m, 2H, H-3], 7.90-7.70 [m,
10H, H-(Py), H-(Py), H-2,6], 7.50-7.28 [m, 10H, H-3,5, C6H6], 6.92 [d, 2H, H-7], 2.37 [s,
13
6H, CH3] ppm. C NMR (DMSO-d6): δC; 169.06, 151.63, 150.03, 145.98, 139.24, 138.96,
136.70, 135.12, 132.46, 130.19, 128.77, 124.42, 123.81, 122.21, 119.39, 113.52 [ArC], 21.36
[CH3] ppm.
2.2.3. Synthesis of [Zn(LMeAQ)2(H2O)2] (2b). An analogous method to that used for the
preparation of 2a was followed using Zn(OAc)22H2O (0.21 g, 0.95 mmol) and HLMeAQ (0.50 g,
1.90 mmol). The dried solid was dissolved in hot chloroform (15 mL) containing 3-
methylpyridine (1 ml, 10 mmol) and filtered. The filtrate upon slow evaporation yielded red
orange crystals. Yield: 0.49 g (82%). M.p. >300 °C. Found: C, 61.25; H, 2.63; N, 13.86%. Calcd.
for C32H28N6O4Zn (MW = 625.97 gmol-1): C, 61.40; H, 4.51; N, 13.43%. IR (cm–1): 1600m,
1573m, 1555m, 1500s, 1468s, 1408s, 1390m, 1334s, 1258s, 1193m, 1172m, 1132m, 1103m,
814m, 789m, 462m. 1H NMR (DMSO-d6+CDCl3): δH; 9.40 [br d, 2H, H-4], 8.74 [br d, 2H, H-2],
8.36 [d, 2H, H-6], 8.13 [m, 2H, H-3], 7.75 [m, 4H, H-2,6), 7.28 [m, 4H, H-3,5], 6.90 [d, 2H,
H-7], 2.37 [s, 6H, CH3] ppm. 13C NMR (DMSO-d6+CDCl3): δC; 151.09, 149.59, 146.45, 145.49,
136.55, 134.72, 132.19, 129.48, 123.20, 122.89, 121.66, 118.78, 112.94 [ArC], 20.92 [CH3]
ppm.
2.2.4. Synthesis of [Zn(LOMeAQ)2(H2O)2] (3). An analogous method to that used for the
preparation of 2a was followed using Zn(OAc)22H2O (0.20 g, 0.89 mmol) and HLOMeAQ (0.50 g,
1.79 mmol). The dried solid was dissolved in hot chloroform (15 mL) containing pyridine (0.8
mL, 10 mmol) and filtered to remove any suspended particles. The filtrate was diluted with
hexane (1 mL) and filtered while hot. The filtrate upon slow evaporation afforded red orange
crystals of 3. Yield: 0.51 g (86%). M.p. >300 °C. Found: C, 58.65; H, 4.43; N, 12.60%. Calcd.
for C32H28N6O6Zn (MW = 657.97 gmol-1): C, 58.41; H, 4.29; N, 12.77%. IR (cm–1): 1598m,
1573m, 1555m, 1499s, 1469s, 1431m, 1403m, 1393m, 1335s, 1293m, 1257s, 1192m, 1171m,
13

1
1151m, 1104m, 1024m, 837m, 789w, 464w. H NMR (DMSO-d6 + CDCl3): δH; 9.37 [br d, 2H,
H-4], 8.74 [br d, 2H, H-2], 8.08 [d, 2H, H-6], 7.81 [m, 4H, H-2,6), 7.67 [m, 2H, H-3], 6.98 [m,
4H, H-3,5], 6.92 [d, 2H, H-7], 3.81 [s, 6H, OCH3] ppm. 13C NMR (DMSO-d6+CDCl3): δC;
160.07, 152.37, 147.25, 145.43, 138.47, 136.27, 134.38, 129.56, 128.63, 123.26, 122.51, 113.98,
112.81 [ArC], 55.19 [OMe] ppm.
2.2.5. Synthesis of [Zn(LEtAQ)2(3-MePy)2] (4). An analogous method to that used for the
preparation of 2a was followed using Zn(OAc)22H2O (0.20 g, 0.90 mmol) and HLEtAQ (0.50 g,
1.80 mmol). The dried solid was dissolved in hot benzene (10 mL) containing 3-methylpyridine
(1 ml, 10 mmol) and filtered while hot. The filtrate upon slow evaporation afforded red crystals.
Yield: 0.47 g (65%). M.p. >300 °C. Found: C, 68.85; H, 5.18; N, 14.23%. Calcd. for
C46H42N8O2Zn (MW = 804.24 gmol-1): C, 68.70; H, 5.26; N, 13.93%. IR (cm–1): 1598s, 1568s,
1498s, 1465s, 1403s, 1328s, 1248s, 1214w, 1191m, 1172m, 1135m, 1100m, 840m, 791m, 754w,
1
469w. H NMR (DMSO-d6): δH; 9.40 [br d, 2H, H-4], 8.71 [br d, 2H, H-2], 8.41-8.37 [m, 4H,
H (3-MePy)], 8.13 [d, 2H, H-6], 7.80 [m, 6H, H-3 and H-2,6), 7.60 [d, 2H, H (3-MePy)],
7.34-7.28 [m, 6H, H-3,5, H (3-MePy)], 6.92 [d, 2H, H-7], 2.64 (q, 4H, CH2), 2.26 [s, 6H, CH3
(3-MePy)], 1.19 (t, 6H, CH3) ppm. 13C NMR (DMSO-d6): δC; 168.40, 151.35, 149.66, 146.56,
145.59, 145.03, 138.45, 136.77, 134.66, 133.14, 132.09, 128.82, 128.49, 123.42, 123.31, 121.79,
118.91, 113.02 [ArC], 27.95 [CH2], 17.87 [CH3 (3-MePy)], 15.47 [CH3] ppm.
2.2.6. Synthesis of [Zn(LOEtAQ)2(4-MePy)2]·0.5C6H7N (5a). An analogous method to that used for
the preparation of 2a was followed using Zn(OAc)22H2O (0.19 g, 0.85 mmol) and HLOEtAQ
(0.50 g, 1.70 mmol). The dried solid was dissolved in hot DMF (6 mL) containing 4-
methylpyridine (1 ml, 10 mmol) and filtered while hot. The filtrate was diluted with toluene (1
mL) and filtered, which upon slow evaporation afforded dark red crystals. Yield: 0.51 g (34%).
M.p. >300 °C. Found: C, 67.28; H, 5.26; N, 13.04%. Calcd. for C98H91N17O8Zn2 (MW = 1765.61
gmol-1): C, 66.66; H, 5.19; N, 13.49%. IR (cm–1): 1597s, 1570s, 1496s, 1464s, 1434m, 1400s,
1325s, 1242s, 1190m, 1150m, 1101m, 1046m, 922w, 836m, 791m, 766m, 722w, 463w. 1H NMR
(DMSO-d6): δH; 9.31 [br d, 2H, H-4], 8.63 [br d, 2H, H-(Py)], 8.50-8.60 [m, 7H, H-(Py), H-
2,6], 8.00 [br d, 2H, H-2], 7.76 [d, 2H, H-6], 7.15-7.00 [m, 8H, H-(Py), H-3,5], 6.96 [m, 3H,
H-3, H-(Py)], 6.80 [d, 2H, H-7], 4.00 [q, 4H, CH2], 3.27 [s, 7.5H, CH3 (4-MePy)], 1.25 [t, 6H,
14

CH3] ppm. 13C NMR (DMSO-d6): δC; 167.29, 159.70, 149.23, 147.55, 146.82, 138.22, 134.85,
131.47, 129.14, 128.05, 124.65, 123.77, 123.38, 118.42, 114.75, 112.85 [ArC], 63.37 [CH2],
20.39 [CH3 (4-MePy)], 14.60 [CH3] ppm.
2.2.7. Synthesis of [Zn(LOEtAQ)2(H2O)2] (5b). An analogous method to that used for the
preparation of 2a was followed using Zn(OAc)22H2O (0.19 g, 0.85 mmol and HLOEtAQ (0.50 g,
1.70 mmol). The dried solid was dissolved in hot chloroform (15 mL) containing pyridine (0.8
mL, 10 mmol) and filtered to remove any suspended particles. The filtrate was diluted with
hexane (1 mL) and filtered while hot, which upon slow evaporation afforded red crystals of the
desired product. Yield: 0.51 g (87%). M.p. >300 °C. Found: C, 59.35; H, 4.60; N, 12.50%.
Calcd. for C34H32N6O6Zn (MW = 686.02 gmol-1): C, 59.53; H, 4.70; N, 12.25%. IR (cm–1):
1598m, 1573m, 1555w, 1499s, 1469s, 1431w, 1394m, 1335s, 1255s, 1192w, 1172m, 1150m,
1103m, 1043m, 838w, 466w. 1H NMR (DMSO-d6): δH; 9.41 [br d, 2H, H-4], 8.72 [br d, 2H, H-
2], 8.09 [d, 2H, H-6], 8.00-7.60 [m, 6H, H-3 and H-2,6), 7.06 [m, 4H, H-3,5], 6.88 [d, 2H, H-
7], 4.10 [q, 4H, CH2], 1.35 (t, 6H, CH3) ppm. 13C NMR (DMSO-d6): δC; 167.85, 159.43, 147.16,
145.59, 138.46, 134.74, 131.99, 128.62, 123.40, 123.16, 118.40, 114.75, 112.83 [ArC], 63.37
[CH2], 14.61 [CH3] ppm.
2.2.8. Synthesis of [Zn(LNMe2AQ)2(Py)2]·1.5C6H6·H2O (6). An analogous method to that used for
the preparation of 2a was followed using Zn(OAc)22H2O (0.19 g, 0.85 mmol) and HLNMe2AQ
(0.50 g, 1.71 mmol). The dried solid was dissolved in hot benzene (10 mL) containing pyridine
(0.8 mL, 10 mmol) and filtered to remove any suspended particles. The filtrate was diluted with
chloroform (2 mL) and filtered while hot, which upon slow evaporation afforded dark red
crystals of 6. Yield: 0.47 g (58%). M.p. >300 °C. Found: C, 67.81; H, 5.10; N, 15.06%. Calcd.
for C53H51N10O3Zn (MW = 941.41 gmol-1): C, 67.62; H, 5.46; N, 14.88%. IR (cm–1): 1595s,
1572m, 1565m, 1493s, 1459s, 1443m, 1406m, 1385w, 1364m, 1321s, 1257m, 1227m, 1155m,
1
1095m, 945w, 820m, 792, 700w, 484w. H NMR (DMSO-d6): δH; 9.31 [br d, 2H, H-4], 8.68 [br
d, 2H, H-2], 8.50 [d, 4H, H-(Py)], 7.96 [d, 2H, H-6], 7.80-7.60 [m, 10H, H-(Py), H-(Py), H-
2,6], 7.40-7.20 [m, 11H, H-3,5, C6H6], 6.83 [d, 2H, H-(Py)], 6.73 [m, 4H, H-3,H-7], 2.93 [s,
13
12H, N(CH3)2] ppm. C NMR (DMSO-d6): δC; 166.01, 150.52, 148.99, 145.09, 143.28, 138.03,
15

135.77, 134.31, 127.75, 127.64, 123.43, 122.96, 122.20, 117.01, 112.16, 111.25 [ArC], 54.55
[N(CH3)2] ppm.
2.2.9. Synthesis of [Zn(LCAQ)2(Py)] (7). An analogous method to that used for the preparation of
2a was followed using Zn(OAc)22H2O (0.19 g, 0.88 mmol) and HLCAQ (0.50 g, 1.76 mmol). The
dried solid was dissolved in hot chloroform (10 mL) containing pyridine (0.4 ml, 5 mmol). The
filtrate was diluted with benzene (2 mL) and filtered while hot, which upon slow evaporation
afforded dark red crystals of 7. Yield: 0.51 g (81%). M.p. >300 °C. Found: C, 58.92; H, 3.20; N,
13.99%. Calcd. for C35H23Cl2N7O2Zn (MW = 709.87 gmol-1): C, 59.22; H, 3.27; N, 13.81%. IR
(cm–1): 1598m, 1573m, 1557m, 1498m, 1465s, 1423w, 1408m, 1388m, 1328s, 1298w, 1246m,
1221w, 1188w, 1170w, 837w, 739w, 470w. 1H NMR (DMSO-d6+CDCl3): δH; 9.38 [br d, 2H, H-
4], 8.75 [br d, 2H, H-2], 8.65-8.40 [m, 5H, H-(Py), H-2,6], 8.15 [d, 1H, H-(Py)], 7.82 [d, 2H,
H-6], 7.69 [m, 3H, H-3, H-(Py)], 7.44 [m, 1H, H-(Py)], 7.40-7.20 [m, 5H, H-(Py), H-3,5],
13
6.94 [d, 2H, H-7] ppm. C NMR (DMSO-d6+CDCl3): δC; 151.84, 149.36, 146.72, 145.52,
139.63, 135.90, 134.11, 132.25, 130.26, 128.94, 128.12, 123.66, 123.10, 121.18, 118.81, 112.40
[ArC] ppm.
2.2.10. Synthesis of [Zn(LBAQ)2(Py)] (8). An analogous method to that used for the preparation of
2a was followed using Zn(OAc)22H2O (0.17 g, 0.76 mmol) and HLBAQ (0.50 g, 1.52 mmol). The
dried solid was dissolved in hot chloroform (10 mL) containing pyridine (0.4 ml, 5 mmol). The
filtrate was diluted with benzene (2 mL) and filtered while hot, which upon slow evaporation
afforded dark red crystals of 8. Yield: 0.47 g (78%). M.p. >300 °C. Found: C, 52.76; H, 3.12; N,
12.28%. Calcd. for C35H23Br2N7O2Zn (MW = 798.79 gmol-1): C, 52.63; H, 2.90; N, 12.27%. IR
(cm–1): 1598m, 1572m, 1557m, 1499s, 1465s, 1422w, 1407m, 1327s, 1250s, 1219w, 1188m,
1
1170w, 1134w, 1103w, 1064w, 836w, 738w, 699w, 470w. H NMR (DMSO-d6+CDCl3): δH;
9.38 [br d, 2H, H-4], 8.74 [br d, 2H, H-2], 8.65-8.40 [m, 5H, H-(Py), H-2,6], 8.15 [d, 1H, H-
(Py)], 7.80-7.50 [m, 5H, H-6, H-3, H-(Py)], 7.44 [m, 1H, H-(Py)], 7.50-7.20 [m, 5H, H-
(Py), H-3, 5], 6.94 [d, 2H, H-7] ppm. 13C NMR (DMSO-d6+CDCl3): δH; 151.37, 149.34,
146.52, 145.25, 139.78, 135.87, 134.34, 131.84, 130.40, 128.55, 128.12, 123.63, 123.37, 121.58,
118.71, 112.38 [ArC] ppm.
16

2.3. X-ray crystallography
Intensity data for compounds 1-8 were collected at ambient temperature with Mo-Kα
r o (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 F2 and corrected for absorption using spherical harmonics (CryAlisPro) [56]. Intensities were
corrected for Lorentz and polarization effects. Structure solution, refinement and data output
were performed with the OLEX2 program package [57] using SHELXTL [58a] and SHELXL-
2014 [58b]. Non-hydrogen atoms were refined anisotropically. C-H hydrogen atoms were placed
in geometrically calculated positions using the riding model. The hydrogen atoms of water
molecules were treated with AFIX 6 (compound 1) or a distance restraint (0.8500.001 Å for
compounds 2b, 3, 5b and 6). Uiso factors for Ow-H hydrogen atoms were restrained to 1.5Uiso(O).
DIAMOND was used for the creation of figures [59].
With exception of 6, the molecular structures of the remaining compounds exhibited
crystallographic symmetry (2-axis for 1, 7 and 8; inversion symmetry for 2a, 2b, 3, 4, 5a, and
5b). In addition, the asymmetric unit of compound 5a comprises two crystallographically
independent molecule halves. The crystal structures of compounds 1 (H2O), 2a (benzene), 5a (4-
methylpyridine) and 6 (benzene and H2O) contain solvate molecules. In 2a, benzene molecules
were disordered over crystallographic inversion centers and were refined using the AFIX66,
SIMU and ISOR instructions implemented in the SHELXL program. In the crystal structure of 6
with one and a half benzene molecules in the asymmetric unit, AFIX66, SIMU, DELU and ISOR
commands were used during the refinement. Finally, the disorder of the N-C6H4-R fragments in
4, 7 and 8 and, additionally, the 3-methylpyridine ligands in 4 were refined using AFIX66,
SIMU and EADP. Compound 1 crystallized in a chiral space group and refinement of the
absolute structure parameter yielded a value, which confidently confirms that the refined model
corresponds to the true enantiomorph (Table 1). 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-1811363-CCDC-1811372. These data can be
obtained free of charge via www.ccdc.cam.ac.uk/getstructures.
17

2.4. Synthesis of ZnO nanoparticles (NPs)
The dried zinc precursor compound 2a (0.054 g) was transferred to a platinum crucible
and gradually heated in a muffle furnace at a rate of 10 °C/min until the temperature reached 600
°C. The crucible was heated for 4 h at 600 °C and then cooled gradually to room temperature.
The calcinated ZnO (0.005 g) was suspended in double-distilled water (20 mL) and the mixture
was exposed to high-intensity ultrasound irradiation under ambient air for 30 min., giving a
uniform white suspension. After cooling to room temperature, the particles were separated by
centrifugation at 6000 rpm. The procedure was repeated three times by re-dispersing the sample
in acetone. Acetone was separated by decantation, the ZnO residue was dried in hot air oven for
an hour at 120 °C and stored in vacuum. Dried sample was used directly for the measurement of
PXRD pattern. For imaging studies, the dried ZnO NPs were re-dispersed in ethanol.
2.5. In vitro antimicrobial assay
The antimicrobial activity of pro-ligands HLQ/HLXAQ and the corresponding zinc(II)
compounds 1-8 was tested against five indicator bacterial strains i.e. Escherichia coli MTCC
730, Streptococcus pyogenes MTCC 1925, Klebsiella pneumoniae MTCC 109, Bacillus cereus
MTCC 430 and Salmonella enterica MTCC 735, according to the agar-well diffusion method
[60] .
The stock solutions for pro-ligands HLQ/HLXAQ and the corresponding zinc(II)
compounds 1-8 were prepared freshly in DMSO prior to use. The stock concentration of the
tested sample was 10,000 μg/ L. Muller-Hilton agar plates were swabbed with bacterial cell
suspension 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.
The broad-spectrum antibiotic chloramphenicol (30 mcg) disc was used as positive control and a
DMSO solution as negative control. Plates were incubated at 37 °C for 24 h 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 results were recorded as the mean diameter
of the zones of inhibition.
18

3. Results and discussion
3.1. Synthesis and spectroscopic characterization
The pro-ligands 5-[(4-(X)-phenyl)-1-diazenyl]quinolin-8-ol (HLXAQ) with X = Me, OMe,
Et, OEt, NMe2, Cl and Br were synthesized following literature procedures [49a,49b,53] and
characterized by microanalytical methods, IR and NMR spectroscopy. The respective Zn(II)
compounds were prepared by reacting in 1:2 molar ratio of a solution of Zn(OAc)22H2O in
methanol either with HLQ dissolved in methanol or HLXAQ dissolved in benzene. In all cases, the
immediate formation of a precipitate was noticed. Characterization of the isolated precipitates
revealed compounds of compositions [Zn(LQ)2] and [Zn(LXAQ)2], respectively. The solids were
amorphous and insoluble in most of the common organic solvents even after prolonged heating.
Solubility was detected in DMSO, pyridine and pyridine derivatives such as methylpyridines.
Crystallization experiments with [Zn(LQ)2] and [Zn(LXAQ)2] using pure DMSO or a combination
with other common solvents invariably afforded amorphous materials. However, when a large
excess of pyridine (Py), 3-methylpyridine (3-MePy) or 4-methylpyridine (4-MePy) was added to
a solution of the zinc(II) compounds in benzene, chloroform, ethanol, DMF or a solvent mixture
with either of these solvents, the isolation of crystalline zinc(II) compounds, in which either Py
(2a, 6, 7, 8), 3-MePy (4), 4-MePy (1, 5a) or H2O (2b, 3, 5b) was coordinated to the zinc atoms,
became feasible. Of these, compounds 1, 2a, 5a and 6 crystallized as solvates with H2O (1),
benzene (2a), 4-MePy (5a) and benzene/H2O (6), see Table 1. The presence of water in
compounds 2b, 3, 5b and 6 is attributed to traces of water in the solvents and reagents used for
the crystallization experiments. It should be mentioned that complexation experiments were
carried out under identical conditions with pyridine and all three monomethyl pyridine
derivatives (picolines), i.e. 2-/3-/4-methylpyridine, but for each ligand only a specific N-base or
water provided a crystalline product, as detailed in Scheme 1.
In compounds 1-6, the adduct formation of [Zn(LQ)2] and [Zn(LXAQ)2] generated six-
coordinate structures with the participation of two mole equivalents of pyridine, 3-MePy, 4-
MePy or water. On the contrary, compounds 7 and 8 assume a five-coordinate geometry, even if
the concentration of pyridine employed for the reaction was increased from 5 to 10 mmol,
indicating that the molecular structure variation is influenced by crystal packing effects.
19

The formulations of the compounds were ascertained from the microanalytical data, the
1H/13C NMR and are further supported by the results of single-crystal X-ray diffraction analyses.
The infrared spectra of compounds 1-8 are very similar and the most pertinent IR frequencies are
XAQ
1
included in the experimental section. The H and 13C NMR signals for the pro-ligands HL
1
were assigned by using 2D H-1H-correlation spectroscopy (COSY) as well as heteronuclear
single-quantum correlation (HSQC) and heteronuclear multiple-bond connectivity (HMBC)
1
experiments [49a]. Compounds 1-8 were characterized in solution by H and 13C NMR
spectroscopy, either in DMSO-d6 or a solvent mixture of DMSO-d6 and CDCl3 (1:1, v/v). The
ligand signal assignments of HLQ and HLXAQ were subsequently extrapolated to the zinc(II)
compounds, owing to the data similarity. It is worth mentioning that the Zn(II) compounds 1-8
1
displayed broad H NMR signals and did not show much difference in the chemical shifts when
compared within the series studied herein, indicating that the changes in the magnetic
1
13
environment of the metal ions are insignificant. The H and C NMR spectra of 1-8 displayed
the expected signals for the metal- coordinated LQ/LXAQ ligand skeletons and coordinated
pyridine/3-methylpyridine/4-methylpyridine in 1, 2a, 4, 5a and 6-8.
The photophysical properties of the tetrahedral zinc compounds [Zn(LQ)2 and [Zn(LXAQ)2]
were thoroughly studied in previous reports [46,52,53]. As expected, the zinc compound with
HLQ i.e. [Zn(LQ)2(4-MePy)2]·H2O (1) exhibited different experimental absorption and emission
features in comparison to compounds 2-8 that contain 5-substituted diazenyl ligands. The UV-
Vis absorption spectra of HLQ, HLXAQ and the corresponding zinc(II) compounds 1-8 in DMSO
are depicted in Fig. S1, ESI†. Table S1 summarizes the most relevant characteristics of the UV-
Vis data. The absorption spectra of 2-8 are comparable with those measured for the tetrahedral
bis(5-(aryl)-1-diazenyl]quinolin-8-olato)zinc(II) compounds, [Zn(LXAQ)2], exhibiting only
compound with slight changes in the absorption maximum. This is because the electronic
excitation does not involve transitions with the participation of metal-based molecular orbitals,
since the 3d10 shell is too low and the empty 4s and 4p orbitals are too high in energy [52,53].
Pro-ligands, HLXAQ showed very similar absorption spectra and were non-emissive except for
HLNMe2AQ. Emission experiments with ex = 425 nm for the five and six-coordinate zinc(II)
compounds 2-8 revealed the same result (non-emissive) except for compound 6 (em = 525 nm),
which contain a strong electron donor group -NMe2 and the emission spectrum resembles that
of the pro-ligand HLNMe2AQ (Fig. S2 (a), ESI†). On the contrary, in DMSO solution, octahedral
20

[Zn(LQ)2(4-MePy)2]H2O (1) gave a significantly improved fluorescence quantum yield ( =
0.25), when compared to the tetrahedral analog [Zn(LQ)2], signaling enhanced electron-
transporting properties (Fig. S2 (b), ESI†). The highly fluorescent performance of compound 1 is
also detected in the solid-state fluorescence spectrum with em 500 nm (Fig. S2 (b), ESI†).
3.2. Description of the X-ray crystal structures
Crystals of the zinc(II) compounds, [Zn(LQ)2(4-MePy)2]·H2O
1
(ethanol/4-
methylpyridine), [Zn(LMeAQ)2(Py)2]·C6H6 2a (chloroform/pyridine/benzene), [Zn(LMeAQ)2(H2O)2]
2b (chloroform/3-methylpyridine), [Zn(LOMeAQ)2(H2O)2] 3 (chloroform/pyridine/hexane),
[Zn(LEtAQ)2(3-MePy)2] 4 (benzene/3-methylpyridine), [Zn(LOEtAQ)2(4-MePy)2]·0.5C6H7N 5a
(DMF/4-methylpyridine),
[Zn(LNMe2AQ)2(Py)2]·1.5C6H6·H2O
[Zn(LOEtAQ)2(H2O)2]
(benzene/pyridine/chloroform), [Zn(LCAQ)2(Py)]
5b
(chloroform/pyridine/hexane),
6
7
(chloroform/pyridine/benzene) and [Zn(LBAQ)2(Py)] 8 (chloroform /pyridine/benzene) suitable
for single-crystal X-ray structure determination were obtained by slow evaporation of solutions
of the respective compounds. Crystal data, data collection parameters and convergence results
are listed in Table 1, while a comparison of selected bond distances and angles is shown in
Tables 2 and 3. Perspective views of the molecular structures are shown in Figs. 1-7, while views
of the asymmetric units of the crystal structures including displacement ellipsoids are shown in
Fig. S3, ESI†.
21

Fig. 1. Perspective view of the molecular structure of compound 1 with partial atom labeling
scheme. Symmetry operator: (i) 1-y, 1-x, 0.75-z.
22

Fig. 2. Perspective views of the molecular structures of compounds (a) 2a and (b) 2b with partial
atom labeling schemes. Symmetry operators: (i) 1-x, -y, 2-z; (ii) 1.5-x, 1.5-y, 0.5-z.
Fig. 3. Perspective view of the molecular structure of compound 3 with partial atom labeling
scheme. Symmetry operator: (i) 1-x, 2-y, 1-z.
23

Fig. 4. Perspective view of the molecular structure of compound 4 with partial atom labeling
scheme. Symmetry operator: (i) 1-x, 1-y, 2-z. For clarity, disorder is not shown.
Fig. 5. Perspective views of the molecular structures of compounds (a) 5a and (b) 5b with partial
atom labeling schemes. Symmetry operators: (i) -x, 1-y, 1-z; (ii) 2-x, -y, 1-z.
24

Fig. 6. Perspective view of the molecular structure of compound 6 with partial atom labeling
scheme.
Fig. 7. Perspective views of the molecular structures of compounds (a) 7 and (b) 8 with partial
atom labeling schemes. Symmetry operators: (i) 0.5-x, y, 1-z; (ii) 1-x, 1-y, 2-z; (iii) x, 1-y, 0.5+z.
For clarity, disorder is not shown.
25

The molecular structure analysis of the SCXRD data reveals closely related coordination
geometries for compounds 1-6, in which the zinc atoms are embedded either in a ZnO2N4 (1, 2a,
4, 5a, 6) or a ZnO4N2 (2b, 3, 5b) polyhedron, depending whether the base coordinated to
[ZnLQ)2] or [Zn(LXAQ)]2 is a pyridine derivative or water. In all seven compounds, the quinolin-8-
olate ligands occupy the equatorial positions with mutual trans-orientation of the phenoxide
oxygen and quinoline nitrogen atoms. As a consequence, the Py and H2O ligands are in the axial
position, exhibiting significantly longer ZnN and ZnO distances with differences in the range
of 0.150.22 Å for d(NpyNaquin) and 0.110.12 Å for d(OwOquin). Except for 6, the
molecular structures of 1-5 exhibit crystallographic symmetry (2-axis for 1; inversion symmetry
for 2a, 2b, 3, 4, 5a, and 5b). The presence of symmetry in 1, 2a, 2b, 3, 4, 5a and 5b restraints the
NpyZnNpy and OwZnOw bond angles to 180.0° and for compounds 2a, 2b, 3, 4, 5a and 5b,
additional bond angles NquinZnNquin and OquinZnOquin attain the same value. The
coordination environments are significantly distorted from an ideal octahedral polyhedron owing
to the involvement of LQ/LXAQ ligands in the formation of chelate ring (Table 3). The
OquinZnNquin bond angles within the chelate rings are significantly smaller than 90.0° and
range from 80.27(10) to 81.19(7)° in close relationship to previously reported and related zinc(II)
compounds [61]. As a consequence, the interligand OquinZnNquin bond angles are significantly
increased with values of 98.81(7)99.63(8)° for 1, 2a, 2b, 3, 4, 5a and 5b and
93.33(11)°/105.93(11)° for the asymmetric complex 6. The bond angles with the axial
substituents are close to 90.0° for 1-6 and the deviations from this value do no exceed 5° (Table
3).
The five-coordinate zinc compounds 7 and 8 comprise distorted square-pyramidal
geometries with the pyridine ligands in apical position. The LXAQ ligands are trans-oriented, but in
this case the ZnN distances with the Npy atoms are significantly shorter, 2.013(5) and 2.035(4)
Å when compared with ZnNquin nitrogen atoms, 2.092(3) and 2.102(3) Å. As expected, the
ZnNquin and ZnOquin distances in 7-8 are significantly shorter than for the six-coordinate
analogs (Table 2).
A search of the Cambridge Structural Database (CSD) revealed 17 entries for compounds
of composition [M(LQ/LXAQ)2] (M are first row transition metals) [61]. Interestingly, all have
square-planar molecular structures with trans-coordination of the quinolin-8-olate ligands.
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Among five six-coordinate compounds of composition [M(LXQ)2(H2O)2], cis-coordination of the
two water molecules was observed only in the structure of [Co(LQ)2(H2O)2]. The remaining
compounds comprise two trans-coordinated ligand molecules in the equatorial plane, while the
two water molecules occupy axial positions, as for 2b, 3 and 5b.
3.3. Supramolecular structure analysis
As mentioned in the introduction, stacking interaction are relevant for the electron-
transport and optical properties of metal compounds derived from quinolin-8-olate ligands
including [Zn(LQ)2] [36,37]. In view of this, it was thought worthwhile to analyze the influence
of the substituents at the periphery of the LQ/LXAQ ligands and the base coordinated to the
zinc(II) metal center and hence the supramolecular interactions in the crystal structures of 18
were examined in this section.
Pyridine adduct of tetrahedral [Zn(LQ)2] viz., [Zn(LQ)2(Py)2]H2O (1), did not exhibit stacking
interactions in the solid state structure while the crystal structures of the remaining members of
the series (2-8) are significantly influenced by  contacts, giving mostly 1D or 2D stacked
assemblies. This difference is originated mainly by the significantly enhanced electron system
of LXAQ compared to unsubstituted quinolin-8-olate LQ. From the molecular structure perspective,
compounds 2-8 can be divided into three groups. Group I comprises of four compounds with
approximate octahedral geometry and two pyridine-bases coordinated to the metal center: 2a (X
= Me, base = Py); 4 (X = Me, base = 3-MePy); 5a (X = OEt, base = 4-MePy); 6 (X = NMe2, base
= Py). Group II is composed of three compounds with similar coordination geometry, but
coordination of H2O instead of the pyridine base: 2b (X = Me, base = H2O); 3 (X = OMe, base =
H2O); 5b (X = OEt, base = H2O). Finally, group III is constituted by two compounds with
distorted square-pyramidal geometries and pyridine as base: 7 (X = Cl, base = Py); 8 (X = Br,
base = Py). Therefore, the crystal structures are analyzed according to this classification.
Group I (2a, 4, 5a and 6)
Compounds 2a, 4 and 6 have similar supramolecular characteristics, particularly concerning the
stacking interactions. In this context, the crystal structures of 2a and 4 are structurally closely
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related, albeit compound 2a crystallized as benzene solvate while 4 is solvent-free. The central
motif in 2a and 4 consists of staircase-shaped 1D strands, which are stabilized by 
interactions occurring mainly between the terminal –C6H4X moieties of the metal-coordinated
ligands and the diazo functional groups. As representative example, Fig. 8 shows the assembly
found in 2a with an interplanar distance of 3.56 Å. Neighboring stacked 1D strands are
connected into 2D layers parallel to (-1 1 1) by intermolecular CH contacts involving the
zinc complex molecules and benzene solvate (Table 4). On the other hand, the crystal structure
of 4 consists of the mutual parallel shift of neighboring stacked 1D strands arranged towards
each other (in direction of the arrows given in Fig. 8), owing to the absence of the solvent. This
allows the generation of infinite stacked pillars perpendicular to the direction of the strand
and, in addition, the formation of close  contacts between the 3-MePy rings, giving a more
compact 2D arrangement. This arrangement is also reflected in the crystal structure of compound
6 (Fig. 9a) but in this case the infinite stacking is interrupted due to the significant torsion of
the aryldiazenyl-fragment from planarity. The deviation from planarity can be attributed to
OwHN and OwHO hydrogen bonding interactions shown in Fig. 9b (see also Table 4) that
interconnect the 2D layers in the third dimension. On the contrary, in 2a and 4 there are no
further contacts in the third dimension, but only CHO, CHN or van der Waals contacts.
In contrast to 2a, 4 and 6, in the crystal structure of compound 5a the LXAQ ligands are
not involved in interactions and  contacts are only found between the 4-MePy ligands and
uncoordinated 4-MePy molecules, giving a finite stacked assembly formed between four
zinc(II) complex molecules and one guest solvate.
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Fig. 8. Fragment of the crystal structure of [Zn(LMeAQ)2(Py)2]C6H6 (2a), showing the 2D layers
formed through  and CH interactions. Cg1 = centroid (C4C9); Cg2 = centroid
(C10C15); Cg3 = centroid (N4, C17C21). Symmetry operators: (i) 1-x, -y, 2-z; (ii) -1+x, -1+y,
z; (iii) 1-x, 1-y, 1-z. For clarity, hydrogen atoms not involved in hydrogen bonding interactions,
and the disorder of the benzene solvate molecule are not shown.
29