Two mononuclear nickel(II) complexes with Schiff base ligands: Synthesis, crystal structures, and antibacterial activities

Article abstract of DOI:10.1080/24701556.2017.1284126

Two new mononuclear nickel(II) complexes, [NiL¹(NCS)] (1) and [Ni(L²)₂]·2NO₃ (2), where L¹ is the deprotonated form of 1-[(2-ethylaminoethylimino)methyl]-naphthalen-2-ol, and L² is the zwit

Full text of DOI:10.1080/24701556.2017.1284126

Inorganic and Nano-Metal Chemistry
ISSN: 2470-1556 (Print) 2470-1564 (Online) Journal homepage: http://www.tandfonline.com/loi/lsrt21
Two Mononuclear Nickel(II) Complexes with Schiff
Base Ligands: Synthesis, Crystal Structures and
Antibacterial Activities
Wei-Hang Sun, Lu-Lu Wu, Lei Ye, Yu Xin, Yan Zhang, Hui Liu, Kai-Wen Lv,
Chao-Nan Shang, Zhonglu You & Wei Li
To cite this article: Wei-Hang Sun, Lu-Lu Wu, Lei Ye, Yu Xin, Yan Zhang, Hui Liu, Kai-Wen Lv,
Chao-Nan Shang, Zhonglu You & Wei Li (2017): Two Mononuclear Nickel(II) Complexes with Schiff
Base Ligands: Synthesis, Crystal Structures and Antibacterial Activities, Inorganic and Nano-Metal
Chemistry, DOI: 10.1080/24701556.2017.1284126
To link to this article: http://dx.doi.org/10.1080/24701556.2017.1284126
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Two Mononuclear Nickel(II) Complexes with Schiff Base Ligands: Synthesis, Crystal Structures
and Antibacterial Activities
WEI-HANG SUN1, LU-LU WU1, LEI YE2, YU XIN2, YAN ZHANG2, HUI LIU2, KAI-WEN
LV2, CHAO-NAN SHANG2, ZHONGLU YOU2, WEI LI1,3,*
1Department of Radiology, The Second Hospital of Dalian Medical University, Dalian 116023,
P. R. China
2Department of Chemistry and Chemical Engineering, Liaoning Normal University, Dalian
116029, P. R. China
3Department of Hepatobiliary and Pancreatic Surgery, The Second Hospital of Dalian Medical
University, Dalian 116023, P. R. China
*Corresponding author. Email: liwei_dlmu@126.com
Abstract
Two new mononuclear nickel(II) complexes, [NiL1(NCS)] (1) and [Ni(L2)2]·2NO3 (2), where L1
is the deprotonated form of 1-[(2-ethylaminoethylimino)methyl]-naphthalen-2-ol, and L2 is the
zwitterionic form of 2,4-dibromo-6-[(2-ethylaminoethylimino)methyl]phenol, have been
prepared and characterized by physico-chemical methods and single crystal X-ray diffraction.
The Ni atoms in both complexes are in square planar coordination. The Ni atom in 1 is
coordinated by one phenolate O, one imine N and one amine N atoms of L1 ligand and one
thiocyanate N atom. The Ni atom in 2 is coordinated by two phenolate O and two imine N atoms
of two L2 ligands. Crystals of the complexes are stabilized by hydrogen bonds. Thermal stability
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of the complexes has been studied. The Schiff bases and the complexes showed potent
antibacterial activities.
Keywords
nickel complex, Schiff base, crystal structure, X-ray diffraction, antibacterial activity
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Introduction
Schiff bases and their transition metal complexes are of great interest in coordination chemistry
due to their facile synthesis, versatile structures and various properties.[1-7] Schiff bases and their
complexes, bearing azomethine functional groups, C = N, are of particular interest for their
biological activities, such as antibacterial[8--10], antifungal[11, 12], anticancer[13, 14], and other
biological applications[15-17]. With the increasing incidence of deep mycosis, there has been
increasing emphasis on the screening of new and more effective antimicrobial drugs with low
toxicity. Nickel is an important element in biological systems, functions as the active site of
hydrolytic enzymes, such as urease.[18] Nickel complexes with Schiff bases have proved to
possess interesting biological activities, including antibacterial aspects.[19-22] This prompted us to
synthesize new nickel complexes with Schiff bases. We report herein the syntheses, structures,
and thermal stability of two new mononuclear Schiff base nickel(II) complexes, [NiL1(NCS)] (1)
and [Ni(L2)2]·2NO3 (2), where L1 is the deprotonated form of
1-[(2-ethylaminoethylimino)methyl]-naphthalen-2-ol, and L2 is the zwitterionic form of
2,4-dibromo-6-[(2-ethylaminoethylimino)methyl]phenol. The preliminary antibacterial activities
have been investigated.
Experimental
Materials and Measurements
Commercially available 2-hydroxy-1-naphthaldehyde, 3,5-dibromosalicylaldehyde, and
N-ethylethane-1,2-diamine were purchased from Aldrich and used without further purification.
Other solvents and reagents were made in China and used as received. C, H and N elemental
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analyses were performed with a Perkin-Elmer elemental analyser. Infrared spectra were recorded
on a Nicolet AVATAR 360 spectrometer as KBr pellets in the 4000--400 cm--1 region. Thermal
stability analysis was performed on a Perkin-Elmer Pyris Diamond TG-DTA thermal analyses
system. Molar conductance was measured with a Shanghai DDS-11A conductometer.
Syntheses of HL1 and HL2
HL1 was synthesized by reaction of 2-hydroxy-1-naphthaldehyde (1.0 mmol, 0.172 g) and
N-ethylethane-1,2-diamine (1.0 mmol, 0.088 g) in methanol (20 mL). The mixture was stirred for
about 30 min at room temperature to give a yellow solution. The solvent was evaporated to give
a gummy product of HL1.
HL2 was prepared by the same method as that described for HL1, but with
2-hydroxy-1-naphthaldehyde replaced by 3,5-dibromosalicylaldehyde (1.0 mmol, 0.280 g).
Syntheses of [NiL1(NCS)](1) and [Ni(L2)2]·2NO3 (2)
Complex 1 was synthesized by reaction of a methanolic solution (10 mL) of nickel acetate (0.1
mmol, 24.9 mg) with a methanolic solution (10 mL) of L1 (0.1 mmol, 24.2 mg) and ammonium
thiocyanate (0.1 mmol, 7.6 mg). The mixture was stirred for 20 min to give a red solution. The
resulting solution was allowed to stand in air for a few days. Red block-shaped crystals suitable
for X-ray single crystal analysis were formed at the bottom of the vessel. The isolated product
was washed three times with cold methanol, and dried in a vacuum over anhydrous CaCl2. Yield,
51%. For C16H17N3NiOS: anal. calcd., %: C, 53.7; H, 4.8; N, 11.7. Found, %: C, 53.5; H, 4.9; N,
11.8. ΛM(CH3CN) = 15 Ω--1·cm2·mol--1. IR data (cm--1, KBr pellet): 3170w, 2112s, 1614s,
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1537m, 1443m, 1405m, 1346m, 1300w, 1257w, 1195w, 1133w, 1082w, 1040w, 985w, 942w,
831m, 746m, 619w, 525w, 470w. UV-Vis (MeCN, λ/nm): 290, 340, 415.
Complex 2 was synthesized by reaction of a methanolic solution (10 mL) of nickel nitrate (0.1
mmol, 29.1 mg) with a methanolic solution (10 mL) of L2 (0.1 mmol, 35.1 mg). The mixture was
stirred for 20 min to give a red solution. The resulting solution was allowed to stand in air for a
few days. Red block-shaped crystals suitable for X-ray single crystal analysis were formed at the
bottom of the vessel. The isolated product was washed three times with cold methanol, and dried
in a vacuum over anhydrous CaCl2. Yield, 45%. For C22H28Br4N6NiO8: anal. calcd., %: C, 29.9;
H, 3.2; N, 9.5. Found, %: C, 29.8; H, 3.1; N, 9.5. ΛM(CH3CN) = 232 Ω--1·cm2·mol--1. IR data
(cm--1, KBr pellet): 3213w, 1608s, 1511w, 1448s, 1388m, 1325s, 1218w, 1176w, 1031w, 959w,
865w, 780w, 712m, 550w, 508w. UV-Vis (MeCN, λ/nm): 270, 385.
Data Collection, Structural Determination and Refinement
Diffraction intensities for the complexes were collected at 298(2) K using a Bruker D8
VENTURE PHOTON diffractometer with MoK radiation ( = 0.71073 Å). The collected data
were reduced using the SAINT program,[23] and multi-scan absorption corrections were
performed using the SADABS program.[24] The structures were solved by direct methods and
refined against F2 by full-matrix least-squares methods using the SHELXTL.[25] All of the
non-hydrogen atoms were refined anisotropically. All H atoms were placed in idealized positions
and constrained to ride on their parent atoms. The crystallographic data for the complexes are
summarized in Table 1. Selected bond lengths and angles are given in Table 2.
Antibacterial Tests
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1.5 cm3 of a 24-hour broth culture containing approximately 106 CFU/cm3 was placed in sterile
petri-dishes. Molten nutrient agar (20 cm3) kept at ca. 45°C was then poured in the petri-dishes
and allowed to solidify. Then 2 holes of 6 mm diameter were punched carefully using a sterile
cork borer and these were completely filled with the test solutions (5000 μg cm3 in DMSO). The
mean value obtained for the two holes was used to calculate the zone of growth inhibition of
each sample.
Determination of MIC (Minimum Inhibitation Concentration) was done using the serial
dilutions in liquid broth method. The materials used were 96-well plates, suspensions of
microorganism (0.5 McFarland), Muller-Hinton broth (Merck) and stock solutions of each
substance to be tested (5000 μg cm3 in DMSO). The following concentrations of the substances
to be tested were obtained in the 96-well plates: 2500, 1250, 625, 312, 156, 78, 39, 20, 10 and 5
μg cm3. After incubation at 37°C for 18--24 h, the MIC for each tested substance was
determined by microscopic observation of microbial growth. It corresponds to the well with the
lowest concentration of the tested substance where microbial growth was clearly inhibited.
Results and Discussion
Chemistry
The Schiff bases HL1 and HL2 were synthesized by reaction of equimolar quantities of
N-ethylethane-1,2-diamine with 2-hydroxy-1-naphthaldehyde and 3,5-dibromosalicylaldehyde,
respectively (Scheme 1). The Schiff bases were used to prepare the complexes without
purification.
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The complexes were synthesized by similar method, as shown by Scheme 2. Complex 1 was
prepared by reaction of equimolar quantities of HL1, nickel acetate and ammonium thiocyanate.
Complex 2 was prepared by reaction of equimolar quantities of HL2 and nickel nitrate. It should
be noted that for the preparation of complex 2, even though ammonium thiocyanate was added, it
was not participate in coordination.
Structure Description of 1
The molecular structure and atom numbering scheme of complex 1 is shown in Figure 1. The
asymmetric unit of the complex contains two independent molecules. The Ni atom in each of the
molecules is coordinated by one phenolate O, one imine N, and one amine N atoms of L1, and
one thiocyanate N atom, forming a square planar geometry. The square planar coordination is
slightly distorted, as evidenced from the bond angles, which are range from 86.8(3) to 93.5(3)°
for the perpendicular angles, and from 174.4(3) to 178.1(3)° for the trans angles. The Schiff base
ligands act in planar tridentate manner, forming one five- and one six-membered chelate rings
with the Ni atoms. The distortion of the square planar coordination is mainly caused by the strain
created by the five-membered chelate rings Ni1-N1-C12-C13-N2 and Ni2-N4-C28-C29-N5. The
Ni-O and Ni-N bond lengths are comparable to those observed in similar nickel complexes with
Schiff bases.[26-30]
In the crystal structure of 1, adjacent two molecules are linked through intermolecular
N--H···O hydrogen bonds (Table 3), to form a dimer. The dimers are further linked through
intermolecular N--H···S hydrogen bonds, as shown in Figure 2.
Structure Description of 2
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The molecular structure and atom numbering scheme of complex 2 is shown in Figure 3. The
complex is a centrosymmetric mononuclear nickel(II) compound, with the Ni atom located at the
inversion center. The asymmetric unit of the complex contains one [Ni(L2)2]2+ cation and two
nitrate anions. The Ni atom in the cation is coordinated by two phenolate O and two imine N
atoms from two L2 ligands, forming a square planar geometry. The square planar coordination is
slightly distorted, as evidenced from the bond lengths, which are range from 1.844(3) to 1.940(3)
Å. The Ni-O and Ni-N bond lengths in this complex are comparable to those observed in similar
nickel complexes with Schiff bases.[31, 32]
In the crystal structure of 2, the nickel anions are linked by nitrate anions through
intermolecular N--H···O and N--H···N hydrogen bonds (Table 3), to form a 3D network, as
shown in Figure 4.
IR Spectra
The IR spectra of the free Schiff bases and the complexes provide information about the
metal-ligand bonding. The sharp bands located at about 3170--3300 cm--1 for the free ligands and
the complexes can be attributed to the ν(N--H). The bands in the range of 2850--2980 cm--1 are
characteristic of aliphatic ν(C--H) vibrations for the complexes and the bands observed at about
3070--3090 cm--1 are attributed to the aromatic ν(C--H) vibrations. The strong absorption bands
at 1645 cm--1 for L1 and 1637 cm--1 for L2 are assigned to the azomethine groups, ν(C = N). The
bands shift to lower wave numbers of 1614 cm--1 for 1 and 1608 cm--1 for 2, respectively, which
can be attributed to the coordination of the nitrogen atoms of the azomethine groups to the Ni
atoms. The phenolic ν(Ar-O) in the free Schiff base ligands exhibits strong bands at about
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1200--1210 cm--1. However, the bands appear at 1195 cm--1 for 1 and 1176 cm--1 for 2, which
may be assigned to the skeletal vibrations related to the phenolic oxygen of the Schiff base
ligands. The typical intense absorption at 2112 cm--1 is assigned to the vibration of the
thiocyanate ligands. The asymmetric and symmetric vibration of the nitrate anions in complex 2
are observed at 1388 and 865 cm--1, respectively. The weak bands located at the low wave
numbers, from 450--650 cm--1, may be assigned to the ν(Ni--O) and ν(Ni--N).
Thermal Stability of the Complexes
Differential thermal (DT) and thermal gravimetric analyses (TGA) were conducted to examine
the stability of the complexes. For 1 (Figure 5), the first step started at about 266°C and was
completed at about 378°C, corresponding to the loss of the N1-C12-C13-N2-C14-C15 moiety
and the thiocyanate ligand. The observed weight loss of 40.0% is very close to the calculated
value of 40.3%. The second step, from 378°C to 722°C, corresponds to the loss of the remaining
contents of the Schiff base ligand, and the formation of the final product (NiO). The observed
weight loss of 39.5% is close to the calculated value of 38.8%. The total weight loss of 79.5% is
close to the calculated value of 79.2%. For 2 (Figure 6), the first step started at about 217°C and
was completed at about 363°C, corresponding to the loss of the N1-C8-C9-N2-C10-C11 moieties
and the nitrate anions. The observed weight loss of 33.8% is very close to the calculated value of
33.5%. The second step, from 363°C to 621°C, corresponds to the loss of the remaining contents
of the Schiff base ligand, and the formation of the final product (NiO). The observed weight loss
of 58.1% is close to the calculated value of 57.7%. The total weight loss of 91.7% is close to the
calculated value of 91.5%.
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Anti-Bacterial Screening
The zone of inhibition for the 5000 μg cm3 test solutions (DMSO as the solvent) on the four
bacteria, Escherichia coli, Pseudomonas aeruginosa, Salmonella typhi and Staphylococcus
aureus is given in Table 4. The MIC values are given in Table 5. The results indicated that there
was no obvious activity observed by the Schiff bases against E. coli and S. aureus, except for the
weak inhibition to P. aeruginosa and S. typhi, whereas the two nickel complexes had obvious
inhibition on the culture of the four bacteria. It is noteworthy that the zones of inhibition areas
are somewhat larger for the complexes than the ligands. The trend in this work is accord with
those reported earlier,[33, 34] which have shown that metal complexes are more potent bactericidal
than that of the corresponding Schiff bases. The antibacterial activities of the complexes are also
comparable to those observed in nickel complexes.[33-36]
Conclusion
Two mononuclear nickel(II) complexes with Schiff base ligands have been prepared and
structurally characterized by single crystal X-ray diffraction method, as well as elemental
analysis and FT-IR spectra. Thermal stability of the complexes has also been studied. The two
nickel(II) complexes exhibited higher antibacterial activities than that of the corresponding
Schiff base ligands, which is in accordance with those reported in literature.
Acknowledgment
This work was financially supported by the Open Laboratory Program of Liaoning Province
(Project No. cx20160102).
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Table 1. Crystallographic data and refinement parameters for the complexes
1
2
Chemical formula
Mr
C16H17N3NiOS
358.1
C22H28Br4N6NiO8
882.8
Crystal color, habit
Crystal size (mm3)
Crystal system
Space group
Unit cell parameters
a (Å)
Red, block
0.17 × 0.15 × 0.15
Monoclinic
P21/c
Red, block
0.17 × 0.13 × 0.12
Triclinic
P-1
10.244(1)
17.234(2)
19.394(2)
6.724(1)
8.810(1)
12.591(1)
76.411(4)
88.279(3)
84.817(4)
722.0(1)
1
b (Å)
c (Å)
α (°)
β (°)
108.807(2)
γ (°)
V (Å3)
3241.3(6)
8
Z
Dcalc (g cm3)
Temperature (K)
1.468
298(2)
2.030
298(2)
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μ (mm1)
1.330
6.263
434
F(000)
1488
6035
4328
Number of unique data
2585
2250
Number of observed data
[I > 2σ(I)]
Number of parameters
Number of restraints
R1, wR2 [I > 2σ(I)]
R1, wR2 (all data)
399
188
12
0
0.0880, 0.1641
0.1222, 0.1753
1.182
0.0374, 0.0965
0.0447, 0.1012
1.056
Goodness of fit on F2
Max and min electron
density (e Å3)
0.899, -0.574
0.996, -0.739
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Table 2. Selected bond distances (Å) and angles (°) for the complexes
1
Ni1-O1
1.823(5)
1.929(7)
1.821(5)
1.925(8)
93.5(3)
Ni1-N1
1.833(6)
1.867(7)
1.831(6)
1.874(7)
87.7(3)
Ni1-N2
Ni1-N3
Ni2-O2
Ni2-N4
Ni2-N5
Ni2-N6
O1-Ni1-N1
N1-Ni1-N3
N1-Ni1-N2
O2-Ni2-N4
N4-Ni2-N6
N4-Ni2-N5
2
O1-Ni1-N3
O1-Ni1-N2
N3-Ni1-N2
O2-Ni2-N6
O2-Ni2-N5
N6-Ni2-N5
178.1(3)
86.8(3)
174.4(3)
92.1(3)
93.2(3)
87.6(3)
178.0(4)
86.9(3)
177.6(5)
92.4(3)
Ni1-O1
1.844(3)
180
Ni1-N1
1.940(3)
87.58(13)
180
O1-Ni1-O1A
O1-Ni1-N1
O1-Ni1-N1A
N1-Ni1-N1A
92.42(13)
Symmetry code for A: -- x, 1 -- y, 1 -- z.
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Table 3. Geometrical parameters for hydrogen bonds
Hydrogen bonds
1
D−H (Å)
H∙∙∙A (Å)
D∙∙∙A (Å)
D−H∙∙∙A (o)
N5−H5A∙∙∙S1#1
N2−H2A∙∙∙O1#2
2
0.91
0.91
2.90
2.38
3.781(14)
3.251(9)
163
160
N2−H2B∙∙∙O3
N2−H2B∙∙∙O3#3
N2−H2A∙∙∙O4#4
N2−H2A∙∙∙O2#4
N2−H2A∙∙∙N3#4
0.90
0.90
0.90
0.90
0.90
2.19
2.22
2.10
2.32
2.56
2.919(5)
3.001(5)
2.910(6)
3.132(6)
3.440(6)
138
145
149
150
165
#1 1 -- x, 1/2 + y, 3/2 -- z; #2 -- x, -- y, 1 -- z; #3 -- x, 1 -- y, -- z; #4 --1 + x, y, z.
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Table 4. Antibacterial screening results
Compound
Zone of inhibition (mm)
E. coli
--a
P. aeruginosa
S. typhi
3.5
S. aureus
HL1
HL2
1
1.7
3.1
7.8
12.1
--
--
5.4
--
6.2
4.5
13.2
17.5
3.8
5.3
2
a “--” indicates that the bacteria are resistant to the compound.
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Table 5. Antibacterial activities as MIC values (μg cm3)
Compound
E. coli
--
P. aeruginosa
S. typhi
625
312
39
S. aureus
HL1
HL2
1
1250
312
156
78
--
--
--
312
156
312
312
39
2
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Figure 1. ORTEP plot of the crystal structure of 1. Displacement ellipsoids of non-hydrogen
atoms are drawn at the 30% probability level.
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Figure 2. Molecular packing arrangement of 1 displayed in the unit cell. Hydrogen bonds are
shown as dashed lines.
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Figure 3. ORTEP plot of the crystal structure of 2. Displacement ellipsoids of non-hydrogen
atoms are drawn at the 30% probability level.
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Figure 4. Molecular packing arrangement of 2 displayed in the unit cell. Hydrogen bonds are
shown as dashed lines.
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40
30
20
10
0
100
80
60
40
20
-10
-20
-30
200
400
600
800
1000
Temperature (oC)
Figure 5. DT-TGA curve of 1.
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0
100
80
60
40
20
0
-5
-10
-15
-20
-25
-30
-35
200
400
600
800
1000
Temperature (oC)
Figure 6. DT-TGA curve of 2.
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Scheme 1. Synthetic procedure for the Schiff bases.
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