Synthesis, crystal structures, magnetic properties and antimicrobial screening of octahedral nickel(II) complexes with substituted quinolin-8-olates and pyridine ligands

Article abstract of DOI:10.1016/j.molstruc.2019.127106

New nickel(II) compounds, viz. trans-[Ni(L^HAQ)₂(Py)₂]·C₆H₆ (1), cis-[Ni(L^HAQ)₂(3-MePy)₂] (2), cis-[Ni(L^2?MeAQ)₂(3-MePy)₂] (3), trans-[

Full text of DOI:10.1016/j.molstruc.2019.127106

Journal of Molecular Structure 1200 (2020) 127106
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: http://www.elsevier.com/locate/molstruc
Synthesis, crystal structures, magnetic properties and antimicrobial
screening of octahedral nickel(II) complexes with substituted
quinolin-8-olates and pyridine ligands
,
Tushar S. Basu Baul a *, Khrawborlang Nongsiej a, Augustine Lamin Ka-Ot b,
b
c
c, **
ꢀ
Santa Ram Joshi , Bruno G.M. Rocha , M. Fatima C. Guedes da Silva
a Centre for Advanced Studies in Chemistry, North-Eastern Hill University, NEHU Permanent Campus, Umshing, Shillong, 793 022, India
b Department of Biotechnology & Bioinformatics, North-Eastern Hill University, NEHU Permanent Campus, Umshing, Shillong, 793 022, India
c
ꢀ
Centro de Química Estrutural, Complexo I, Instituto Superior Tecnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001, Lisboa, Portugal
a r t i c l e i n f o
a b s t r a c t
New nickel(II) compounds, viz. trans-[Ni(LHAQ)2(Py)2]$C6H6 (1), cis-[Ni(LHAQ)2(3-MePy)2] (2), cis-
[Ni(L2ꢀMeAQ)2(3-MePy)2] (3), trans-[Ni(L4ꢀMeAQ)2(Py)2]$C6H6 (4), trans-[Ni(L4ꢀOMeAQ)2(Py)2]$C6H6 (5),
trans-[Ni(L4ꢀOEtAQ)2(Py)2]$C6H6 (6), trans-[Ni(L4ꢀBrAQ)2(Py)2]$C6H6 (7) and trans-[Ni(L4ꢀClAQ)2(3-MePy)2]$
2(3-MePy) (8) (primary ligands: LXAQ ¼ substituted 5-[(E)-2-(aryl)-1-diazenyl]quinolin-8-olate; second-
ary ligands: Py ¼ pyridine or 3-MePy ¼ 3-methylpyridine) have been synthesized and characterized by
elemental analysis, IR, UVevis spectroscopy. The magnetic susceptibilities of the compounds were also
measured. Single-crystal X-ray diffraction analysis of the compounds revealed octahedral geometries
with trans configurations for 1, 4e8 and cis configurations for 2 and 3. The occurrence of intramolecular
Ni,,,Hpyridine interactions in 2 and in 3, may account for additional stabilization of their assemblies at
least in the solid state. The effective magnetic moment (meff) in the range 3.02e3.40 BM agree with the
expected values for octahedral Ni(II) cations with two unpaired electrons. Ni(II) compounds were also
screened for their antimicrobial activity.
Article history:
Received 20 June 2019
Received in revised form
27 August 2019
Accepted 20 September 2019
Available online 24 September 2019
Keywords:
Substituted quinolin-8-olate
Nickel
Pyridine derivative
Magnetic moment
Structure elucidation
Antimicrobial activity
© 2019 Elsevier B.V. All rights reserved.
1. Introduction
Nanosized [Co(LXAQ)2] thin films with energy gaps of 4.01 eV [42],
HLXAQ and its M(II) complexes as dyes in polyester fabrics [43],
Quinolin-8-ols (HLQ) are good metal ion chelators [1e9] and
their derivatives 2-(quinolin-8-yloxy)propionates, 5-aryl-8-
hydroxyquinolines [10e12] and 8-aminoquinoline [13e16] were
widely studied. Metal compounds of HLQ were extensively used in
photochemical processes [17e21], optical sensing [22e24] or as
organic light-emitting diodes [25e29]. Other applications of HLQ
encompass thin film formation of nano-sized metal quinolin-8-
olates [30] and nanobelt structures [31] involving 3d metals. The
coordination behavior of 5-[(E)-2-(aryl)-1-diazenyl]-quinolin-8-ol
(HLXAQ) towards M(II)/M(III) ions has been developed [32e39]
with recent focus on their use as colorimetric sensors [40] as well as
in the extraction of Cu(II) and Ni(II) at low concentrations [41].
HLXAQ-Zn(II) complexes as zinc source for nano-structured mate-
rials [44], and promising non-linear optical behavior of some HLXAQ
with 3d transition metal complexes [32d], are recent research
developments.
Despite such progresses crystal structures of the HLXAQ com-
plexes are scarce. The N- and O- atoms of HLXAQ as bidentate ligand
can bind both medium and hard metal ions and give rise to
different geometries. Two molecules of HLXAQ can give rise to
square-planar (e.g., with Cu(II) ions) and tetrahedral complexes
(e.g., with Cu(I) or Co(II) ions). Thus, for obtaining stable six coor-
dinate octahedral metals complexes derived from HLXAQ, additional
ancillary ligands such as pyridines are needed. Pyridine derivatives
not only enhance solubility therefore ensuring the homogeneity of
the reaction mixture, but also modulate the acidity of the metal
center. On the basis of this hypothesis, the pro-ligands HLXAQ have
been utilized for designing various types of discrete compounds,
frequently involving Co(II) and Zn(II) cations with different
* Corresponding author.
** Corresponding author.
E-mail addresses: basubaulchem@gmail.com, basubaul@nehu.ac.in (T.S. Basu
Baul), fatima.guedes@tecnico.ulisboa.pt (M.F.C. Guedes da Silva).
https://doi.org/10.1016/j.molstruc.2019.127106
0022-2860/© 2019 Elsevier B.V. All rights reserved.

2
T.S. Basu Baul et al. / Journal of Molecular Structure 1200 (2020) 127106
coordination numbers and geometries, giving rise to a large variety
of structural motifs in the solid state with promising applications
[44,45].
(CDH) were purified by crystallization and aniline (sd fine) was
distilled prior to use. The solvents used in the reactions were of AR
grade and dried using standard procedures.
Nickel is an essential trace metal for animal species, plants and
micro-organisms; it is also present in alloys and nickel-plated
materials. However, this metal has shown adverse effects on hu-
man health through chronic exposure such as dermatitis, lung
fibrosis, cardiovascular diseases, carcinogenic hazards, etc. [46,47].
The present work focuses on the synthesis, characterization and
structure elucidation of a series of Ni(II) compounds based on
HLXAQ pro-ligands in combination with pyridine derivatives as
ancillary ligands. The chemical structure of the compounds, viz.
trans-[Ni(LHAQ)2(Py)2]$C6H6 (1), cis-[Ni(LHAQ)2(3-MePy)2] (2), cis-
[Ni(L2ꢀMeAQ)2(3-MePy)2] (3), trans-[Ni(L4ꢀMeAQ)2(Py)2]$C6H6 (4),
trans-[Ni(L4ꢀOMeAQ)2(Py)2]$C6H6 (5), trans-[Ni(L4ꢀOEtAQ)2(Py)2]$
C6H6 (6), trans-[Ni(L4ꢀBrAQ)2(Py)2]$C6H6 (7) and trans-[Ni(L4ꢀ-
ClAQ)2(3-MePy)2]$2(3-MePy) (8) (LXAQ ¼ substituted 5-[(E)-2-(aryl)-
Melting points were measured using a Büchi apparatus (M-560)
and are uncorrected. Carbon, hydrogen and nitrogen analyses were
performed with a PerkinElmer 2400 series II instrument. IR spectra
in the range 4000-400 cmꢀ1 were obtained on a PerkinElmer
Spectrum BX series FT-IR spectrophotometer with samples inves-
tigated as KBr discs. Absorption measurements were carried out on
a PerkinElmer Lambda 25 spectrophotometer at ambient temper-
ature in freshly prepared DMSO solutions (Table S1; Fig. S1). The
effective magnetic moments (meff) were measured in solution by
NMR on
a Bruker Advance 300 spectrometer operating at
300.13 MHz (for 1e3) or on a Bruker Advance 400 spectrometer
operating at 400.13 MHz (for the remaining compounds), at room
temperature in an open-air system. The Evans’ method [48] was
applied with measurements performed in standard 5 mm NMR
tubes containing the paramagnetic sample dissolved in DMSO‑d6
and comprising a coaxial insert filled only with the same deuter-
ated solvent. The 1H chemical shifts were referred to the residual
signals from this solvent as reference. The chemical shifts were
measured in ppm. For the data treatment of NMR signals the
MestReNova version 9.0.1 program was used.
1-diazenyl]quinolin-8-olate;
Py ¼ pyridine
or
3-MePy ¼ 3-
methylpyridine) are outlined in Scheme 1. They were character-
ized by spectroscopic methods, including single crystal X-ray
diffraction analysis. Magnetic susceptibility determinations in so-
lution disclosed the octahedral geometry also in solution. The
in vitro antimicrobial screening of compounds 1e8 is also reported.
For the in vitro antimicrobial assays, Mueller-Hinton agar
(HiMedia) was used as nutrient liquid medium and standard anti-
biotic discs of chloramphenicol (C)30 and fluconazole (FLC)25
(HiMedia) were used as positive controls for bacteria and fungus,
respectively.
2. Experimental
2.1. General considerations
Ni(OAc)2,4H2O (S.d. fine), 8-hydroxyquinoline (Loba Chemie),
o-toluidine (SRL), p-ethoxyaniline, p-chloroaniline, p-bromoaniline
(Himedia), pyridine (Merck) and 3-methylpyridine (Spectrochem)
were used without further purification. p-toluidine and p-anisidine
2.2. Synthesis of pro-ligands and Ni(II) compounds
Pro-ligands 5-[(E)-2-(aryl)-1-diazenyl]quinolin-8-ol (HLXAQ
)
Scheme 1. Synthesis of pro-ligand HLXAQ and chemical structures of the nickel(II) compounds 1e8.

T.S. Basu Baul et al. / Journal of Molecular Structure 1200 (2020) 127106
3
viz., 5-[(E)-2-(phenyl)-1-diazenyl]quinolin-8-ol (HLHAQ), 5-[(E)-2-
(2-methylphenyl)-1-diazenyl]quinolin-8-ol (HL2ꢀMeAQ), 5-[(E)-2-
(4-methylphenyl)-1-diazenyl]quinolin-8-ol (HL4ꢀMeAQ), 5-[(E)-2-
(4-ethoxyphenyl)-1-diazenyl]quinolin-8-ol (HL4ꢀOEtAQ), 5-[(E)-2-
(4-bromophenyl)-1-diazenyl]quinolin-8-ol (HL4ꢀBrAQ) [35e] and 5-
1.89 mmol) were used in the reaction. The dried solid was dissolved
by heating the chloroform solution (10 mL) containing pyridine
(0.8 mL, 10.11 mmol) and filtered to remove any suspended parti-
cles. The filtrate was diluted with benzene (1 mL), which upon slow
evaporation afforded reddish-brown crystals. Yield: 0.48 g (61%;
relative to nickel acetate). M.p. > 300 ꢂC. Anal. Found: C, 70.30; H,
5.28; N, 14.02. Calc. for C42H34N8NiO2 (MW 819.58 gmol-1): C,
70.34; H, 4.92; N, 13.67%. IR (cmꢀ1): 1597 m, 1574 m, 1551 m, 1498s,
1465s, 1392 m, 1329s, 1246s, 1188w, 1169w, 1100w, 793w, 760w,
704w, 463w. Electronic absorption data (DMSO) lmax, nm; (ε [Mꢀ1
cmꢀ1]): 440 sh, 499 (14616).
[(E)-2-(4-methoxyphenyl)-1-diazenyl]quinolin-8-ol (HL4ꢀOMeAQ
and 5-[(E)-2-(4-chlorophenyl)-1-diazenyl]quinolin-8-ol (HL4ꢀClAQ
)
)
[35d] were prepared starting from 8-hydroxyquinoline and the
corresponding aniline using conventional diazonium salt chemis-
try, in accordance with literature procedures. In view of the similar
preparation methods employed for 1e8, only the preparation for
[Ni(LHAQ)2(Py)2]$C6H6 (1) is described in detail, as a representative
example.
2.2.5. Synthesis of trans-bis{5-[(E)-2-(4-methoxyphenyl)-1-
diazenyl]quinolin-8-olato-k2N,O}-bis(pyridine-kN) nickel(II)
benzene solvate (5)
2.2.1. Synthesis of trans-bis{5-[(E)-2-(phenyl)-1-diazenyl]quinolin-
8-olato-k2N,O}-bis(pyridine-kN) nickel(II) benzene solvate (1)
Ni(OAc)2,4H2O (0.249 g, 1.00 mmol) in methanol (10 mL) was
added drop-wise to a stirred solution of HLHAQ (0.5 g, 2.00 mmol) in
benzene (50 mL) at room temperature, which resulted in the im-
mediate formation of a reddish-orange precipitate. The reaction
mixture was heated to reflux for 3 h and then filtered while hot. The
residue was washed with hot methanol (3 ꢁ 5 mL) to remove un-
desired materials and dried in vacuo. The dried solid was dissolved
in hot benzene (10 mL) containing pyridine (0.8 mL, 10.11 mmol)
and filtered while hot. The filtrate was diluted with toluene (1 mL)
and upon slow evaporation reddish-brown crystals of 1 were ob-
tained. Yield: 0.48 g (60%; relative to nickel acetate). M.p. > 300 ꢂC.
Anal. Found: C, 70.10; H, 4.88; N, 14.08%. Calc. for C46H36N8NiO2
(MW ¼ 791.52 gmol-1): C, 69.80; H, 4.58; N, 14.16%. IR (cmꢀ1):
1597 m, 1573 m, 1552 m, 1500s, 1465s, 1393s, 1331s, 1249s, 1187 m,
1131 m, 1100 m, 764w, 702 m, 684w, 464w. Electronic absorption
data (DMSO) lmax, nm; (ε [Mꢀ1 cmꢀ1]): 440 sh, 499 (11602).
Ni(OAc)2,4H2O (0.22 g, 0.89 mmol) and HL4ꢀOMeAQ (0.5 g,
1.79 mmol) were used in the reaction. The dried solid was dissolved
by heating the benzene solution (15 mL) containing pyridine
(0.8 mL, 10.11 mmol) and filtered while hot. The filtrate upon slow
evaporation yielded reddish-brown crystals. Yield: 0.47 g (61%;
relative to nickel acetate). M.p. > 300 ꢂC. Anal. Found: C, 67.37; H,
5.12; N, 13.23. Calc. for C42H34N8NiO4 (MW 851.58 gmol-1): C, 67.70;
H, 4.73; N, 13.16%. IR (cmꢀ1): 1595 m, 1573 m, 1552 m, 1496s, 1463s,
1391 m, 1328s, 1244s, 1190 m, 1170 m, 1099w, 832w, 759w, 703w,
472w. Electronic absorption data (DMSO) lmax, nm; (ε [Mꢀ1 cmꢀ1]):
437 sh, 503 (13531).
2.2.6. Synthesis of trans-bis{5-[(E)-2-(4-ethoxyphenyl)-1-diazenyl]
quinolin-8-olato-k2N,O}-bis(pyridine-kN) nickel(II) benzene solvate
(6)
Ni(OAc)2,4H2O (0.212 g, 0.85 mmol) and HL4ꢀOEtAQ (0.50 g,
1.70 mmol) were used in the reaction. The dried solid was dissolved
by heating the benzene solution (10 mL) containing pyridine
(0.8 mL, 10.11 mmol) and filtered while hot. The filtrate upon slow
evaporation yielded reddish-brown crystals. Yield: 0.45 g (60%;
relative to nickel acetate). M.p. > 300 ꢂC. Anal. Found: C, 68.52; H,
5.01; N, 12.58. Calc. for C44H38N8NiO4 (MW 879.63 gmol-1): C,
68.27; H, 5.04; N, 12.74%. IR (cmꢀ1): 1638w, 1596 m, 1571s, 1495s,
1468s, 1402 m, 1322s, 1235s, 1191 m, 1098 m, 1041 m, 920w, 838 m,
767w, 697w, 476w. Electronic absorption data (DMSO) lmax, nm; (ε
[Mꢀ1 cmꢀ1]): 439 sh, 503 (16173).
2.2.2. Synthesis of cis-bis{5-[(E)-2-(phenyl)-1-diazenyl]quinolin-8-
olato-k2N,O}-bis(3-methyl pyridine-kN) nickel(II) (2)
Ni(OAc)2,4H2O (0.249 g, 1.00 mmol) and HLHAQ (0.5g,
2.00 mmol) were used in the reaction. The dried solid was dissolved
by heating the benzene solution (10 mL) containing 3-
methylpyridine (1 mL, 10.73 mmol) and filtered while hot. The
filtrate upon slow evaporation yielded reddish-brown crystals.
Yield: 0.48 g (64%; relative to nickel acetate). M.p. > 300 ꢂC. Anal.
Found: C, 68.32; H, 4.68; N, 14.88. Calc. for C42H34N8NiO2 (MW
741.47 gmol-1): C, 68.03; H, 4.62; N, 15.11%. IR (cmꢀ1): 1596 m,
1570 m, 1551 m, 1498s, 1469s, 1400s, 1330s, 1247s, 1188 m, 1167 m,
1132 m, 831w, 787 m, 764w, 690w, 507w. Electronic absorption
data (DMSO) lmax, nm; (ε [Mꢀ1 cmꢀ1]): 439 sh, 499 (16802).
2.2.7. Synthesis of trans-bis{5-[(E)-2-(4-bromophenyl)-1-diazenyl]
quinolin-8-olato-k2N,O}-bis(pyridine-kN) nickel(II) benzene solvate
(7)
Ni(OAc)2,4H2O (0.189 g, 0.76 mmol) and HL4ꢀBrAQ (0.5 g,
1.523 mmol) were used in the reaction. The dried solid was dis-
solved by heating the chloroform solution (20 mL) containing
pyridine (0.8 mL, 10.113 mmol) and filtered to remove any sus-
pended particles. The filtrate was diluted with benzene (1 mL)
which upon slow evaporation yielded reddish-brown crystals.
Yield: 0.46 g (63%; relative to nickel acetate). M.p. > 300 ꢂC. Anal.
Found: C, 57.80; H, 3.66; N, 12.28. Calc. for C40H28Br2N8NiO2 (MW
949.32 gmol-1): C, 58.20; H, 3.61; N, 11.80%. IR (cmꢀ1): 1597 m,
1573 m, 1549 m, 1500 m, 1464s, 1407 m, 1380 m, 1327s, 1297 m,
1248s, 1187 m, 1167 m, 1131 m, 1100w, 828w, 791w, 702w, 465w.
Electronic absorption data (DMSO) lmax, nm; (ε [Mꢀ1 cmꢀ1]): 440
sh, 510 (11396).
2.2.3. Synthesis of cis-bis{5-[(E)-2-(2-methylphenyl)-1-diazenyl]
quinolin-8-olato-k2N,O}-bis(3-methylpyridine-kN) nickel(II) (3)
Ni(OAc)2,4H2O (0.236 g, 0.95 mmol) and HL2ꢀMeAQ (0.5 g,
1.90 mmol) were used in the reaction. The dried solid was dissolved
by heating the benzene solution (10 mL) containing 3-
methylpyridine (1 mL, 10.74 mmol) and filtered while hot. The
filtrate upon slow evaporation yielded reddish-brown crystals.
Yield: 0.46 g (63%; relative to nickel acetate). M.p. > 300 ꢂC. Anal.
Found: C, 68.72; H, 4.76; N, 14.50. Calc. for C44H38N8NiO2 (MW
769.52 gmol-1): C, 68.68; H, 4.98; N, 14.56%. IR (cmꢀ1): 1595 m,
1571 m, 1554 m, 1498s, 1470s, 1402s, 1329s, 1245s, 1181 m, 1168 m,
1109w, 791 m, 762 m, 704w, 459w. Electronic absorption data
(DMSO) lmax, nm; (ε [Mꢀ1 cmꢀ1]): 439 sh, 501 (9388).
2.2.8. Synthesis of trans-bis{5-[(E)-2-(4-chlorophenyl)-1-diazenyl]
quinolin-8-olato-k2N,O}-bis(3-methylpyridine-kN) nickel(II) 3-
methylpyridine disolvates (8)
2.2.4. Synthesis of trans-bis{5-[(E)-2-(4-methylphenyl)-1-diazenyl]
quinolin-8-olato-k2N,O}-bis(pyridine-kN) nickel(II) benzene solvate
(4)
Ni(OAc)2,4H2O (0.219 g, 0.88 mmol) and HL4ꢀClAQ (0.5 g,
1.76 mmol) were used in the reaction. The dried solid was dissolved
by heating the benzene solution (10 mL) containing 3-
Ni(OAc)2,4H2O (0.236 g, 0.95 mmol) and HL4ꢀMeAQ (0.5 g,

4
T.S. Basu Baul et al. / Journal of Molecular Structure 1200 (2020) 127106
Table 1
Crystal data, data collection parameters and convergence results for compounds 1e8.
1
2
3
4
5
6
7
8
Empirical formula
Formula weight
Crystal system
Space group
a (Å)
C46H36N8NiO2
791.54
triclinic
P ꢀ1
C42H34N8NiO2
741.48
monoclinic
I 2/a
17.2498(19)
14.9060(13)
16.5869(15)
90
101.594(11)
90
C44H38N8NiO2
769.53
triclinic
P ꢀ1
C48H40N8NiO2
819.59
triclinic
P ꢀ1
C48H40N8NiO4
851.59
triclinic
P ꢀ1
C50H44N8NiO4
879.64
triclinic
P ꢀ1
C46H34Br2N8NiO2
949.34
triclinic
P ꢀ1
C54H46Cl2N10NiO2
996.62
triclinic
P ꢀ1
8.5734(4)
10.4854(5)
10.8215(5)
93.587(4)
92.700(4)
92.693(4)
968.60(8)
1
7.9744(8)
16.2187(10)
17.6853(8)
98.049(5)
99.704(6)
97.282(7)
2205.7(3)
2
8.3564(7)
10.9537(5)
11.3606(6)
98.660(4)
91.250(5)
95.885(5)
1021.87(11)
1
8.4401(6)
11.2127(8)
11.3145(8)
100.102(6)
93.302(7)
93.514(6)
1049.63(13)
1
9.2322(10)
10.5635(11)
12.1003(12)
74.593(9)
72.044(8)
84.193(7)
1082.0(2)
1
8.3320(4)
11.0173(6)
11.3430(6)
99.082(4)
91.473(3)
95.433(4)
1022.70(9)
1
9.5962(8)
11.2381(9)
12.4662(10)
105.768(7)
94.026(6)
106.939(7)
1221.27(18)
1
b (Å)
c (Å)
a (ꢂ)
b
(ꢂ)
(ꢂ)
g
V (Å3)
4177.9(7)
4
1.179
Z
Dcalc (g/cm3)
1.357
1.159
1.332
1.347
1.350
1.541
1.355
F000
m
412
0.552
1544
0.507
804
0.483
428
0.525
444
0.518
460
0.505
480
2.478
518
0.560
(mmꢀ1
)
Reflections measured
Obs/Unique reflections
Number of parameters
Rint
7476
4359/3412
287
8505
4852/3053
240
17080
9738, 5493
500
7894
4599, 3937
284
4562
3696/2981
293
7786
4900/3242
302
7961
4631, 3217
268
9048
5478, 3405
315
0.0244
0.0454
0.1121
0.0308
0.0173
0.0446
0.0308
0.0414
R(F) (I ꢃ 2
s
)
0.0465
0.1196
0.842
0.335, ꢀ0.175
0.0636
0.1871
0.786
0.861, ꢀ0.323
0.0836
0.2234
0.926
1.151, ꢀ0.824
0.0470
0.1132
1.057
0.372, ꢀ0.421
0.0635
0.1435
1.331
0.989, ꢀ0.455
0.0704
0.1587
1.038
0.755, ꢀ0.509
0.0533
0.1158
1.166
0.879, ꢀ0.646
0.0673
0.1209
1.020
0.342, ꢀ0.251
wR (F2) (all data)
GOF (F2)
max., min. Dr (e/Å3)
methylpyridine (1 mL, 10.74 mmol) and filtered. The filtrate upon
slow evaporation yielded reddish-brown crystals. Yield: 0.46 g
(52%; relative to nickel acetate). M.p. > 300 ꢂC. Anal. Found: C,
64.82; H, 4.60; N, 14.22. Calc. for C42H32Cl2N8NiO2 (MW 996.61
gmol-1): C, 65.08; H, 4.65; N, 14.05%. IR (cmꢀ1): 1596s, 1570s,
1552 m, 1500s,1469s, 1410 m, 1384 m, 1327s,1298 m, 1247s,1188 m,
1170 m, 1132 m, 1084 m, 835 m, 792 m, 706 m, 518w, 467 m. Elec-
tronic absorption data (DMSO) lmax, nm; (ε [Mꢀ1 cmꢀ1]): 440 sh,
508 (10663).
atoms, and 1.5Ueq of the parent carbon atoms for methyl. Least
square refinements with anisotropic thermal motion parameters
for all the non-hydrogen atoms were employed. In 5, the low
quality of the crystals gave rise to weak diffraction, particularly at
high angles, leading to a relatively low data completeness. How-
ever, the observed data proves the structure and hence it is
included for comparison purposes. Compounds 4 and 7 are
isomorphs.
In compound 2 Platon/Squeeze routine [54] was used to account
for disordered molecules in voids; two void contents of 424 Å3 and
86 electrons each (total of 856 Å3 with 171 electrons) were found,
that fit well for two molecules of benzene in each. The model did
not fit so well for 3-methylpyridine (1.7 molecules per void) but a
mixture of both solvents is not to be disregarded. Platon/Squeeze
[54] was also applied to compound 3 to deal with two void contents
of 200 Å3 with 56 electrons in each (total of 400 Å3 with 113 elec-
trons), that fit well for one molecule of 3-methylpyridine in each;
the possibility of having a mixture of both benzene and 3-
methylpyridine was also favorable. Upon using this procedure,
the final R1 values improved from 0.0905 to 0.0636 (in 2) and from
0.1111 to 0.0836 (in 3).
2.3. X-ray crystallography
The measurements were performed on an Agilent Technologies
four-circle Gemini diffractometer [49] using Mo K
a radiation
(
l
¼ 0.71073 Å) from a fine-focus X-ray source. Full spheres of data
were collected using omega scans of 0.5ꢂ per frame and data
reduction was performed with CrysAlisPro [50]. Intensities were
corrected for Lorentz and polarization effects and empirical ab-
sorption corrections using spherical harmonics were applied [50].
The structures were solved by direct methods using SIR97 package
[51] and refined with SHELXL-2018/1 [52]. Calculations were per-
formed using the WinGX System-Version 2014-1 [53]. Coordinates
of hydrogen atoms, all bonded to carbon atoms, were included in
the refinement using the riding-model approximation with the
Uiso(H) defined as 1.2 Ueq of the parent aromatic and methylene
In compounds 4e6 only half of the benzene molecule was
located. In view of the unreasonable arrangement of the benzene
molecule caused by the proximity to an inversion centre, the
symmetry related carbon ring atoms were generated, the site
Table 2
Selected bond distances (Å) and angles (ꢂ) for 1e8.
1
2
3
4
5
6
7
8
2
Geometric parameters for octahedral structurea 1.008 25.45 1.009 30.39 1.016 50.26
1.008 23.44 1.008 24.07 1.007 23.90 1.008 24.09 1.008 24.78ꢂ
NN
1.258(3)
2.060(2)
2.148(2)
2.0500(15) 2.030(2)
81.65(7)
5.25
1.247(4)
2.087(3)
2.112(3)
1.265(7), 1.280(7)
2.190(4), 2.225(4)
2.153(4), 2.168(4)
2.066(4), 2.069(4)
1.257(3)
1.257(5)
1.259(4)
2.063(3)
2.134(3)
2.039(2)
82.03(10)
25.12
1.251(4)
2.049(3)
2.145(3)
2.050(2)
81.87(10)
6.02
1.265(3)
2.059(3)
2.152(2)
2.0353(19)
81.78(9)
23.85
Ni-Nquinoline
Ni-Npyridine
NieO
2.0517(17) 2.056(3)
2.1536(18) 2.151(3)
2.0473(14) 2.045(2)
: OeNi-Nquinoline
80.55(10)
27.94
39.28
78.02(15) 78.39(15) 82.02(6)
81.92(10)
5.77
88.23
P angleb
10.61 31.81
15.53 27.22
7.974
5.42
85.57
8.356
Q anglec
89.98
8.573
82.34
9.232
86.39
8.332
86.13
9.596
Shortest Ni,,,Ni
8.907
8.440
a
Values refer to quadratic elongation and angle variance (in 2), in this order, according to K. Robinson et al. [56].
Angle between the least-square planes of quinoline and phenyl moieties in the azo-ligand.
Angle between the least-square planes of quinoline and pyridine ligands.
ꢂ
b
c

T.S. Basu Baul et al. / Journal of Molecular Structure 1200 (2020) 127106
5
Table 3
structure finalized normally, through the application of geometric
restraints (e.g., AFIX 66). Crystallographic data, selected bond
lengths and angles and hydrogen-bond interactions are given in
Tables 1e3, respectively. The ellipsoid plots of all compounds with
partial numbering schemes are shown in Figs. 1 and 2. Non-
covalent interactions and packing diagrams are exemplified in
Figs. S2eS6.
Non-classical H-bond interactions in 2e8.
H,,,A (Å) D,,,A (Å) DꢀH,,,A (ꢂ) Symmetry operation
1
C19eH19,,,O1 2.70
2
C16eH16,,,Ni1 2.99
C7eH7,,,O1
3.617(3)
171.3
1 þ x,y,z
3.00
3.407(6)
2.963(5)
80.13
147
114
intra
2.59
-x,-1/2 þ y,1/2-z
C16eH16,,,O1 2.46
intra
3
2.4. In vitro antimicrobial assay
C39eH39,,,Ni1 2.99
C14eH14,,,N2 2.50
C37eH37,,,O1 2.45
C43eH43,,,O2 2.54
4
C18eH18,,,O1 2.59
5
C17eH17,,,Ni1 3.02
C17eH17,,,O1 2.58
6
3.02
78.99
100
118
intra
intra
intra
intra
2.810(8)
3.009(6)
3.090(6)
The antimicrobial activity of pro-ligands HLXAQ and the corre-
sponding Ni(II) compounds 1e8 were tested against three indicator
bacterial strains, i.e., Bacillus 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 [55].
118
3.510(3)
173
-x,1-y,-z
3.04
3.068(5)
80.20
113
intra
intra
The stock solutions for pro-ligands HLXAQ and compounds 1e8
were freshly prepared in DMSO prior to use. The stock concentra-
tions of the tested samples were ~10,000 mg/mL. Mueller-Hilton
agar plates were swabbed with bacterial cell suspensions of the
indicator bacterial strains adjusted to 1.5 ꢁ 108 colony forming
units (CFU)/mL. Then, the agar surface was bored by using a ster-
ilized cork borer to generate wells of 5 mm diameter, which were
C2eH2,,,O2
2.65
3.571(5)
3.045(5)
3.030(5)
173
117
116
x,y,1 þ z
intra
intra
C18eH18,,,O1 2.51
C22eH22,,,O1 2.50
7
C16eH16,,,O1 2.58
C19eH19,,,O1 2.59
C17eH17,,,N3 2.72
8
3.055(5)
3.508(5)
3.639(5)
112
171
170
intra
-x,1-y,-z
x,y,-1þz
filled with a 100 mL aliquot of the corresponding test compound
C16eH16,,,O1 2.56
3.061(4)
114
intra
solution. A disc with the broad-spectrum antibiotic chloramphen-
icol (30 mcg) was used as positive control and a DMSO solution as
negative control. Plates were incubated at 37 ꢂC 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 antibacterial activity was evaluated
occupancies of all the carbon atoms of benzene changed to 0.5
affording, because of this strategy, the whole molecule with occu-
pancy of 0.5 as defined by the multiplicity of the special position.
The atoms were then flanked by PART -1 and PART 0 and the
Fig. 1. Ellipsoid plot (drawn at 30% probability level) of the trans-nickel compounds 1, 4e8 with partial atom labeling schemes. The solvent molecules are omitted for clarity.
Symmetry operation to generate equivalent atoms: 2-x,2-y,-z (1); 1-x,1-y,-z (4, 7); 2-x,-y,2-z (5); -x,1-y,2-z (6); 2-x,1-y,2-z (8).

6
T.S. Basu Baul et al. / Journal of Molecular Structure 1200 (2020) 127106
by measuring the growth inhibition zone diameter.
3. Results and discussion
quinoline and the phenyl moieties (angle P, see Table 2); while in
the remaining compounds, however, it is markedly twisted. As
expected, in view of their cis geometries in 2 and 3, the angles
between the least-squares planes of quinoline and pyridine (angle
Q, see Table 2) are much shorter than in the other cases.
3.1. Synthesis and spectroscopic characterization
An interesting observation in the structures of 2 and 3 is the
occurrence of intramolecular Ni,,,Hpyridine interactions. In these
compounds, the pyridine ligand is clearly facing the metal,
conceivably due to stereochemical effects from the 3-
methylpyridine ligand (Figs. S3 and S4). Such twisting results in
distances of 2.99 Å between the metal cation and the 6-H pyridine
atom of both (in 2) or just one (in 3) 3-methylpyridine ligands,
reasonably shorter than the lengths involving the 2-H pyridine
atom which assume values between 3.02 and 3.12 Å in compounds
of this work. According to the literature [58], these contacts are
typical in metal-coordinated ligands interacting via lone pair dative
bonds and sustain (C)H,,,Ni longer than C,,,Ni, together with
CeH,,,Ni angles shorter than 100ꢂ and may account for additional
stabilization of the assemblies in 2 and 3 at least in the solid state.
The complex molecules in 1 are gathered through CeH/O
Nickel(II) compounds were prepared by reacting a solution of
Ni(OAc)2,4H2O in methanol with HLXAQ dissolved in benzene in
1:2 mole ratios. In all cases a precipitate immediately formed. After
suitable work up procedures, characterization of the isolated pre-
cipitates suggested compounds of general formula [Ni(LXAQ)2]. They
were insoluble in methanol, ethanol, chloroform, methylene
dichloride, benzene, toluene and acetone even after prolonged
heating, but soluble in DMSO, pyridine and pyridine derivatives.
Crystallization experiments of [Ni(LXAQ)2] using sole DMSO, pyri-
dine or pyridine derivatives afforded pasty materials which could
not be worked up. However, when such crystallization experiments
were attempted with a large excess of pyridine or pyridine de-
rivatives in combination with other common solvents, reddish-
brown crystalline materials were invariably obtained. Compounds
1, 4e7 crystallized as benzene solvates, 8 as 3-methylpyridine
disolvates, 2 and 3 were unsolvated (see Scheme 1). It should be
mentioned that crystallization experiments were carried out under
identical conditions with pyridine and 2-/3-/4-methylpyridine, but
only a specific N-base provided a crystalline product, as detailed in
Scheme 1. In all the compounds, the adduct formation of {Ni(L-
XAQ)2} generated the six-coordinate complexes 1e8 with the
participation of 2 mol equivalents of pyridine or 3-methylpyridine,
as ascertained by elemental analyses and by X-ray diffraction. The
characteristic IR and electronic absorption data are given in the
Experimental section.
3.2. Description of the X-ray crystal structures
Crystals of the nickel(II) compounds, 1 (pyridine/benzene/
toluene), 2, 3 and 8 (3-methylpyridine/benzene), 4 and 7 (pyridine/
benzene/chloroform), 5 and 6 (pyridine/benzene), suitable for
single-crystal X-ray structure determination were obtained by slow
evaporation of solutions of the respective compounds.
The asymmetric unit of the compounds include half of the
complex molecule (3 is the exception) and half a benzene (in 1,
4e7) or a full 3-methylpyridine (in 8) crystallization molecule.
Symmetry expansions reveal the cis geometry for compounds 2 and
3 and the trans configuration for the remaining ones. The nickel
cations assume slightly distorted octahedral geometries (Table 2:
angle variances [56] ranging from 23.44 to 50.26ꢂ2) constructed
from the Npyridine, and the N and the O-atoms from oxoquinoline.
The NN lengths are in the expected double bond range. The
NiꢀNquinoline distances vary from 2.049(3) to 2.087(3) Å (Table 2),
being shorter than the NiꢀNpyridine which fluctuate between
2.112(3) and 2.1536(18) Å; complex 3 is the exception, with the
latter dimension being larger than the former (values of 2.190(4)
and 2.225(4) Å, against 2.153(4) and 2.168(4) Å, See Table 2) which
is conceivably related to the geometry of the complex resulting in
stereochemical effects from the 3-methylpyridine ligand. For the
same reasons, the NieO distances in 3 are longer than those in the
other compounds and the OeNiꢀNquinoline angles, as well as the
nearby metal,,,metal distances, are markedly shorter (Table 2).
The NieO dimensions measured in the present study are in the
range of those already reported for quinolinate derivatives in Ni(II)
octahedral environments [57], additionally with the NieNquinoline
length being longer than the NieO ones for the particular cases of
trans-OO/cis-NN N4O2 nickel compounds [57b,57c] as found in 2
and 3. The arylazo ligand in 1, 4, 5 and 7 is roughly planar as
measured by the angle between the least-squares planes of the
Fig. 2. Ellipsoid plot (drawn at 30% probability level) of the cis compounds 2 and 3,
with partial atom labelling schemes. Symmetry operation to generate equivalent
atoms: 1.5-x,y,-z (2).

T.S. Basu Baul et al. / Journal of Molecular Structure 1200 (2020) 127106
7
Fig. 3. 1H NMR spectrum of DMSO‑d6 in the absence (top) and in the presence (bottom) of compound 8.
contacts (Table 3) that give rise to infinite chains along the c axis
(H,,,O and CeH/ O of 2.70 Å, 171.3ꢂ) (Fig. S2). These chains create
channels along the crystallographic a axis, which are filled with
benzene solvent molecules. These trapped benzene molecules
evaluate the solution behavior of these compounds. 1H NMR
spectra of 1e8 could not be obtained as anticipated in view of the
paramagnetic character of the Ni(II). Recently [44], the effective
magnetic moment (meff) of Co(II) compounds of related system was
determined using Evans method [48], which relates the difference
in the chemical shift of an inert reference compound in the pres-
ence and in the absence of a paramagnetic species. The same
methodology was adopted for determining meff of the Ni(II) com-
pounds 1e8 since the samples are highly soluble in DMSO whose
sample preparation is straightforward and requires little material.
With DMSO‑d6 both as solvent and as internal standard its
typical quintet was clearly shifted by the magnetic field of the Ni(II)
paramagnetic compounds. An example of the obtained NMR
spectra is shown in Fig. 3. The calculated magnetic moments, pre-
sented in Table 4 (refer to Table S2 for calculations), assume values
between 3.02 and 3.40 BM which are in agreement with the ex-
pected values for Ni(II) cations in octahedral geometries with two
unpaired electrons [59].
interact with four complex molecules by means of CeH$$$
p con-
tacts, donating to the pyridine ring and the phenyl moiety of
quinoline from two adjacent molecules (H,,,centroid and CeH$$$
centroid of 2.77 Å, 148ꢂ and 2.71 Å, 146ꢂ, in this order), and
accepting from the pyridine moiety of quinoline from other two
(H,,,centroid and CeH$$$ centroid of 2.66 Å and 145ꢂ).
In complex 2, the molecules are gathered by means of inter-
molecular contacts involving the pendent phenyl rings of azo group
and the O-atoms giving rise to infinite 2D sheets (Fig. S3) owing to
the cis geometry. Every molecule is additionally stabilized by means
of intramolecular contacts between one of the pyridine H-atoms
and the oxygen (H,,,O and CeH/ O of 2.46 Å and 114ꢂ). In com-
plex 3, also with the cis geometry, the relevant interactions are of
the intramolecular level (Fig. S4) with methyl pyridines donating to
the O-atoms and b-quinoline to the Nazo.
The molecules of 7 are assembled in 2D sheets (Fig. S5) by
means of the short contacts between a CH groups from pyridine
and the O atoms from vicinal molecules. The halogen substituents
(Br in 7; Cl in 8) also play a role in the stabilization of the structures:
Table 4
Mass susceptibility, molar susceptibility and effective magnetic moment of com-
pounds 1e8.
in 7 the Br establishes a
p-interaction with the pyridine ring of a
vicinal molecule (Br,,,centroid distance of 3.423 Å), while the Cl in
8 contacts with the methyl group of non-coordinated 3-methyl
pyridine (Fig. S6).
Compound
cmass (cm3/g)
cmol (cm3/mol)
meff (BM)
1
2
3
4
5
6
7
8
5.82 ꢁ 10ꢀ6
5.87 ꢁ 10ꢀ6
5.03 ꢁ 10ꢀ6
4.68 ꢁ 10ꢀ6
5.68 ꢁ 10ꢀ6
5.16 ꢁ 10ꢀ6
5.07 ꢁ 10ꢀ6
4.88 ꢁ 10ꢀ6
4.61 ꢁ 10ꢀ3
4.36 ꢁ 10ꢀ3
3.87 ꢁ 10ꢀ3
3.83 ꢁ 10ꢀ3
4.84 ꢁ 10ꢀ3
4.54 ꢁ 10ꢀ3
4.82 ꢁ 10ꢀ3
4.86 ꢁ 10ꢀ3
3.31
3.22
3.04
3.02
3.40
3.29
3.39
3.40
3.3. Magnetic studies
Having now the availability of well-characterized series of
octahedral Ni(II) compounds (1e8), it was thought of interest to

8
T.S. Basu Baul et al. / Journal of Molecular Structure 1200 (2020) 127106
Table 5
Antimicrobial activity of the pro-ligands (HLXAQ) and the nickel compounds (1e8) against representative bacterial and fungal strains.
Pro-ligand/Compound
Inhibition zones (in mm) observed for the tested bacterial and fungal species
B. subtilis
S. aureus MTCC 96
K. pneumoniae MTCC 109
C. albicans MTCC 183
MTCC 441
HLHAQ
17
18
22
12
0
13
12
11
0
0
0
0
0
0
0
0
0
29
e
15
17
12
0
0
0
11
0
0
0
0
0
0
0
0
0
0
28
e
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
24
e
12
10
14
10
0
10
10
13
12
0
HL2-MeAQ
HL4-MeAQ
HL4-OMeAQ
HL4-OEtAQ
HL4-BrAQ
HL4-ClAQ
1
2
3
4
5
6
7
8
0
11
13
11
13
0
Ni(OAc)2,4H2O
Solvent (10% DMSO)
Chloramphenicol (C)30
Fluconazole (FLC)25
0
e
26
3.4. Antibacterial activity results
Conflicts of interest
The stability of compounds 1e8 in dimethyl sulfoxide (DMSO)
solutions was monitored for two days (Fig. S7) and the observed
unchanged pattern of the UV/Vis absorbance spectra indicated that
The authors declare that they have no conflicts of interest with
the contents of this article.
the compounds are stable. The pro-ligands (except HL4ꢀOEtAQ
)
Acknowledgments
generally displayed less activity towards B. subtilis and S. aureus
than bacterial control chloramphenicol (C)30 while no activity was
recorded for K. pneumonia. However, the pro-ligands (except
HL4ꢀOEtAQ) and compounds 1, 2, 5e8 demonstrated moderate ac-
tivity against a fungal species C. albicans when compared with
fluconazole (FLC)25 (Table 5, Figs. S8 and S9). No systematic com-
parisons/correlations could be made when these results were
compared with the analogous series of [Zn(LXAQ)2(yPy)2]
(yPy ¼ pyridine or pyridine derivatives) complexes [44] since
different bacterial stains were utilized. First-row transition metal
compounds of 5-chloro-quinolin-8-olate were tested against bac-
teria, clinical isolates and probiotic bacteria and the results indi-
cated that the zinc, cobalt and nickel compounds performed better
activity than the positive control and the pro-ligand [60]. Thus, it
may be inferred that the microbial activities of 1e8 can be related to
several factors such as the type and number of donor atoms and
their relative positions within the ligand [61].
The financial support of the University Grants Commission, In-
dia (Grant No. 42e396/2013 (SR) 2013, TSBB), the Council of Sci-
entific and Industrial Research, India (Grant No. 01 (2734)/13/EMR-
II, 2013, TSBB) and the University Grants Commission, India,
through SAP-CAS, Phase-I is gratefully acknowledged. KN thanks
the University Grants Commission (UGC), India, for UGC-RGNF
research fellowship. Authors (TSBB, KN) acknowledge DST-PURSE
for the diffractometer facility. MFCGS and BGMR are grateful to
the Foundation for Science and Technology (FCT), Portugal (UID/
QUI/00100/2019).
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.molstruc.2019.127106.
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