DOI: 10.17344/acsi.2016.2895
Acta Chim. Slov. 2016, 63, 891-898
891
Scientific paper
Synthesis, Structure Evaluation, Spectroscopic and Antibacterial Investigation of Metal Complexes with 2-(Pyridin-4-yl)quinoline-4-carboxylic Acid
Long Zhang,t Zhong-Wei Man,t Yan Zhang, Jing Hong, Meng-Ran Guo
and Jie Qin*
School of Life Sciences, Shandong University of Technology, Zibo 255049, P. R. China
* Corresponding author: E-mail: qinjietutu@ 163.com Tel.: 0086-533-2780271; Fax: 0086-533-2781329.
Received: 07-09-2016
t These authors contributed equally to this work.
Abstract
Four metal complexes based on quinoline carboxylate ligand from 2-(pyridin-4-yl)quinoline-4-carboxylic acid (HL), {[ML2(H2O)2] • 2H2O}n (M = Mn11, 1; M = Co11, 2; M = Cd11, 3) and {[Ag2L2(H2O)2] • 3H2O}n (4) have been synthesized under hydrothermal conditions. Their structures were determined by elemental analyses, IR spectra, and further characterized by single-crystal X-ray diffraction analysis. Complexes 1-3 feature a 1D chain structure which is further linked together to construct the 3D supramolecular network through hydrogen bonds. Complex 4 exhibits a 3D configuration. The fluorescent behavior and antibacterial activities of these compounds have been investigated.
Keywords: Quinoline derivative; X-ray crystal structure; fluorescent property; antibacterial activity
1. Introduction
Since Pt-based drugs play a significant role in antitumor chemotherapy, development of metal complexes possessing biological activities has been an active research field.1,2 The biological properties of metal complexes depend on the organic ligands and the nature of metal ions. Thereby, the combination of suitable metal ion as well as ligand is an important prerequisite for the construction of bioactive metal complex.3,4
Quinoline scaffold has been known for wide spectrum of biological and pharmaceutical activities, such as antimalarial,5,6 antitumor,7,8 antibacterial,9,10 and antiallergic.11 Carboxyl modified heterocycle compounds are also excellent metal-binding compounds, which exhibit versatile coordination modes depending on reaction conditions and the nature of central metal ion.1214
In our earlier investigations, we have shown that di-nuclear complexes of Mn(II), Co(II), Cd(II), and Zn(II) based on 2-phenylquinoline carboxylic derivatives HL1 and HL2 (Scheme 1) possess broad and effective antibacterial activity.14,15 These results prompted us to examine
the controlling factors on the antibacterial activity of metal complex based on quinoline derivatives. In continuation of our work, this paper deals with the synthesis and characterization of four metal complexes, {[ML2(H2O)2] • 2H2O} (M = Mn11, 1; M = Co11, 2; M = Cd11, 3) and {[Ag2L2(H2O)2] • 4H2O}n (4), derived from 2-pyridylino-line derivative 2-(pyridin-4-yl)quinoline-4-carboxylic acid (HL). The photoluminescent property and antibacterial assays over gram-positive and gram-negative bacterial strains are also presented in this manuscript.
COOH	COOH
Scheme 1. Structure of ligands HL1, HL2 and HL.
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2. Experimental Section
2. 1. Materials and Instruments
All the reactions were carried out under air atmosphere. All chemicals and solvents used in the synthesis were reagent grade without further purification.
The IR spectra were taken on a Vector22 Bruker spectrophotometer (400-4000 cm-1) with KBr pellets. NMR spectra were measured on a Bruker AM 500 spectrometer. Elemental analyses for C, H and N were performed on a Perkin-Elmer 240C analyzer. Fluorescence spectra were recorded on a Hitachi F-4500 fluorescence spectrophotometer.
2. 2. Synthesis of 2-(Pyridin-4-yl)quinoline-4-carboxylic Acid (HL)
A mixture of isatin (1.18 g, 8.00 mmol), 4-acetylp-yridine (0.24 g, 2.00 mmol), and potassium hydroxide (2.24 g, 40.00 mmol) in 2 mL of ethanol and 18 mL of water was mixed and refluxed for 10 hours. When the reaction was finished, the orange solution was cooled to room temperature and adjusted to pH 5 with 1 M HCl. The synthetic yellow precipitate was filtered, washed several times with water and dried. Yield: 1.16 g, 58%. IR (KBr, cm-1): 3431, 3064, 2759, 2445, 1942, 1720, 1609, 1588, 1541, 1500, 1456, 1416, 1348, 1308, 1253, 1152, 1060, 1016, 864, 835, 796, 778, 763, 735, 716, 652. 1H-NMR (500Hz, DMSO, 5): 8.79 (d, J = 5Hz, 2H), 8.67 (d, J = 10Hz, 1H), 8.46 (s, 1H), 8.25 (d, J = 10Hz, 2H), 8.19 (d, J = 10Hz, 1H), 7.87 (t, 1H), 7.73 (t, 1H). Anal. Calcd for C15H10N2O2: C, 71.99; H, 4.03; N, 11.19. Found: C, 72.21; H, 4.02; N, 11.22%.
2. 3. General Procedure for the Synthesis of Complexes 1-4
HL (0.10 mmol) and nitrates (Mn(NO3)2, Co(NO3)2, Cd(NO3)2, or AgNO3) (0.10 mmol) in 10 mL mixed solvent of DMF/CH3OH/H2O (v/v/v = 2/4/4) were sealed in 25 mL Teflon cup and heated at 90 °C for 2-3 days and cooled slowly to room temperature over a period of 20 h. Then crystals suitable for X-ray diffraction analysis were obtained.
{[MnL2(H2O)2] ■ 2H2O}n (1) Yield: 0.026 g (42% on the basis of HL). IR (KBr, cm1): 3214, 1608, 1581, 1501, 1421, 1396, 1327, 1308, 1239, 1067, 1013, 837, 813, 771, 655, 572, 546. Anal. Calcd. for C30H26N4O8Mn: C, 57.61; H, 4.19; N: 8.96. Found: C, 57.78; H, 4.18; N, 8.99%.
{[CoL2(H2O)2] ■ 2H2O}n (2) Yield: 0.035 g (56% on the basis of HL). IR (KBr2 cm-1): 3217, 1953, 1611, 1582, 1502, 1458, 1422, 1398, 1329, 1309, 1239, 1068, 1018, 870, 838, 814, 772, 721, 684, 572, 555. Anal. Calcd. for C30H26N4O8Co: C, 57.24; H, 4.16; N, 8.90. Found: C, 57.41;H, 4.15; N, 8.93%.
{[CdL2(H2O)2] ■ 2H2O}n (3) Yield: 0.031 g (46% on the basis of HL). IR (KBr2 cm-1): 3434, 3105, 1951, 1611,
1552, 1537, 1498, 1453, 1425, 1390, 1324, 1305, 1239, 1064, 1020, 866, 838, 804, 785, 660, 573. Anal. Calcd. for C30H26N4O8Cd: C, 52.76; H, 3.84; N, 8.20. Found: C, 523.92; H, 3.83; N, 8.22%.
{[Ag2L2(H2O)2] ■ 4H2O}n (4) Yield: 0.035 g (43% on the basis of HL). IR (KBr, cm1): 3401, 3080, 3036, 1948, 1599, 1574, 1492, 1455, 1417, 1380, 1321, 1152, 1066, 999, 864, 853, 814, 776, 720, 684, 571. Anal. Calcd. for C30 H29 N4 O10Ag2: C, 43.87; H, 3.56; N, 6.82. Found: C, 44°00; H, 3.550; N, 6.84%.
2. 4. X-Ray Data Collection and Structure Refinement
Structure measurements of all the complexes were performed on Bruker Smart Apex CCD diffractometer equipped with graphite-monochromated Mo Ka with radiation wavelength of 0.71073 À by using a co-2d scan mode. The collected data were reduced using the SAINT pro-gram.16 Absorption corrections were applied using SAD-ABS program.17 These structures were solved by direct methods and refined with the full-matrix least-squares technique using SHELXS-97.18 Anisotropic thermal parameters were assigned to all non-hydrogen atoms. The solvent molecules O8 and O9 in complex 4 were disordered over two positions (50: 50 occupancy). The hydrogen atoms bonded to C were generated geometrically; while the hydrogen atoms of the water molecules were located from difference maps and fixed at their ideal positions with O-H = 0.85(2) À, H-H = 1.44(2) À and Uiso(H) = 1.5Ueq(O).
2. 5. Bioassay Conditions
Four referenced bacterial strains, B. subtilis, E. coli, P. aeruginosa and S. aureus-were selected. Streptomycin was used as a positive control. The IC50 (half minimum inhibitory concentrations) of the test compounds were determined by a colorimetric method using the dye MTT (3-(4,5-dimethylth-iazol-2-yl)-2,5-diphenyl tetrazolium-bromide). Stock solutions of the synthesized compounds (100 |^g/mL) were prepared in DMSO, and sequentially diluted with Mueller-Hinton medium. The antibacterial activities were evaluated by the method reported befo-re.14,15,19 The procedure of antimicrobial activity was given in detail in Supporting Information.
3. Results and Discussion
3. 1. Synthesis and General Characterization
Complexes 1-4 were constructed from HL and the related metal salt under hydrothermal conditions. All of complexes are soluble in high polarity solvents.
HL was prepared via the Pfitzinger reaction.14 Characterization of the ligand has been accomplished by IR, 1H NMR, and elemental analysis. The FT-IR spectrum for
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Table 1. Crystallographic data for 1-4.
	1	2	3	4
Empirical formula	C30H26N4°8Mn	C3oH26N408Co	C30H26N4O8Cd	C30H28N4°9A§2
Mr	625.49	629.48	682.95	804.30
Crystal system	triclinic	triclinic	triclinic	monoclinic
Space group	P-1	P-1	P-1	Cc
a (Ä)	8.5296(14)	8.5511(4)	8.6281(5)	14.2981(7)
b (Ä)	8.8986(14)	8.7942(5)	8.9010(5)	17.5489(8)
c (Ä)	10.5757(15)	10.4459(5)	10.6287(7)	13.4398(7)
a(°)	71.300(4)	70.748(2)	72.133(2)	90.00
ß(°)	78.698(4)	78.9820(10)	78.500(2)	114.6570(10)
X°) V (Ä3)	74.264(5)	74.252(2)	74.6320(10)	90.00
	726.59(19)	709.29(6)	742.90(8)	3064.8(3)
Z	1	1	1	4
Pc (gcm-3)	1.429	1.474	1.527	1.743
F(000)	323	325	346	1608
T / K	298(2)	298(2)	298(2)	298(2)
jU(Mo-Ka)/ mm1	0.512	0.664	0.791	1.338
Data/param./restr.	2748 / 196 / 0	2607 / 196 / 0	2877 / 196 / 0	6509 / 410 / 2
GOF	1.023	1.108	1.131	1.065
R1, wR2 (I >2a(I))	0.0592 / 0.1414	0.0279 / 0.0720	0.0218 / 0.0561	0.0361 / 0.0913
R1, wR2 (all data)	0.0792 / 0.1543	0.0300 / 0.0733	0.0223 / 0.0569	0.0399 / 0.0939
Large diff. peak / hole (e Ä-3)	0.974 / -0.689	0.282 / -0.256	0.506 / -0.401	1.593 / -0.964
ligand HL shows characteristic C=O stretching vibration at 1720 cm-1. The absorption bonds around 3400 cm-1 corresponds to the presence of hydroxyl group. The infrared spectral peaks around 3064 cm-1 are due to the aromatic C-H vibrations. The compounds 1-4 show similar types of vibration frequencies. An intense and broad band at about 3200 cm-1 suggests the presence of water molecule. The vas(COO") band is observed at around 1607 cm-1, while the vs(COO") vibration is observed at about 1396 cm-1. The separation value dv (vas(COO~) -vs(COO") is about 211 cm-1 which is in good agreement with the monodentate coordination mode features shown by the results of crystal structures.20
3. 2. Crystal Structures of Complexes 1-4
The solid structures of complexes 1-4 were determined by single-crystal X-ray diffraction. The crystallograp-
Table 2. Selected Bond Distance (Â) and Angles (deg) for 1.
Mn1-O1	2.146(2)	Mn1-03	2.173(2)
Mn1-N2i	2.292(2)	01-C10	1.240(3)
02-C10	1.239(4)		
01-Mn1-01iii	180.0	01-Mn1-03	89.95(8)
01-Mn1-03iii	90.05(8)	01-Mn1-N2i	88.40(8)
01-Mn1-N2ii	91.60(9)	03iii-Mn1-N2i	89.79(8)
03-Mn1-N2i	90.21(8)		
Symmetry codes: (i) -x + 1, -y + 1, -z + 1; (ii) x, y + 1, z - 1; (iii) -x + 1, -y + 2, -z.
Figure 1. The coordination environment of Mn(II) in 1 at 50% probability displacement. Symmetry codes: (i) —x + 1, —y + 1, —z + 1; (ii) x, y + 1, z — 1; (iii) —x + 1, —y + 2, —z.
hic and data collection parameters for complexes 1-4 are given in Table 1; selected bond lengths and angles are listed in Tables 2, 3 and S1, S2.
The result of single crystal analysis indicates that complexes 1-3 are isomorphous. They all crystallize in triclinic system, space group Pl, therefore, the crystal structure of 1 is described in detail herein as the example. The ORTEP plots of 1-3 are shown in Figures 1, S1 and S2.
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There are half of the Mn(II) center, one deprotona-ted ligand L1-, one coordinated water molecule, and one solvent water molecule in the asymmetric unit of 1. As depicted in Fig. 1, the Mn(II) center is six-coordinated in pseudo-octahedral coordination geometry. The equatorial plane was defined by two coordination water molecules (O3 and O3m), and two N atoms (N21 and N2U) from two different pyridine rings. The axial positions are occupied by the monodentate carboxylic O atoms (O1 and O1111). The sum of the equatorial bond angles are 360.0(8)°, which ensures the planarity of the equatorial plane. The axial O1-Mn1-O1m bond angle of 180.0° clearly indicates the linear configuration. As shown in Table 2, the equatorial bond length distances between the Mn atom and the N, O atoms, 2.293(2) and 2.173(2) À, are longer than the axial Mn1-O1 distance, 2.146(2) À, showing the squashed octahedron configuration.
Compared with the metal complexes based on HL1 and HL2,14,15 HL in 1 is more planer, as evidenced by the dihedral angle between the pyridine ring and the quinoline heterocycle being 21.01(1)°.The carboxylate group of HL in complex 1 adopts a monodentate coordination mode, while in previous reported dinuclear complex [Mn2(L1)4(MeOH)4], the carboxylate group adopts a syn-syn bidentate coordination mode.14
As can be seen from Figure 2, the Mn(II) cations are bridged by the carboxylate and pyridine group to form a double chain structure extending along the crystallograp-hic [0-1 1] direction with the nearest intrachain Mn - Mn distance of 11.431(2) À. In complex 1, the water molecules, acting as both hydrogen-donors and hydrogen-acceptor, play a crucial role in determining the supramolecular structure. First of all, the lattice water molecules are bond to the chain via O4-H4A—O2, meanwhile the intrachain
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Figure 4. The 3D hydrogen-bonded network of 1.
hydrogen bonding interactions between the coordinated water molecules and the uncoordinated carboxylic O atoms are also observed (O3-H3B—O2). Such 1D chains are aligned side by side in the bc plane, and the adjacent chains are held together through interchain hydrogen bonds between the coordinated water molecules and the latticed water molecules (O3-H3A—O4) into a supramo-lecular layer (Figure 3).
These supramolecular layers are stacked along the crystallography a axis as a -AA- fashion. The further O4-H4A-N2 interactions involving the latticed water molecules and the uncoordinated quinoline N atoms link these sheets into a three dimensional network (Figure 4).
Contrary to the formation of 1D infinite single-chains in complexes 1-3, a 3D polymeric architecture of complex 4 was built when AgNO3 is used for the self-as-
Table 3 Selected Bond Distance (Â) and Angles (deg) for 4.
Ag1-N3	2.357(3)	Ag1-O6	2.437(5)
Ag1-O5	2.440(4)	Ag1-N1	2.494(3)
Ag2-N4i	2.177(3)	Ag2-N2	2.215(4)
Ag2-O4ii	2.354(3)	O1-C10	1.221(6)
O2-C10	1.236(6)	O3-C25	1.250(5)
O4-C25	1.246(6)		
N3-Ag1-O6	136.74(17)	N4i-Ag2-N2	148.03(15)
N3-Ag1-O5	119.33(13)	O6-Ag1-O5	77.77(16)
N3-Ag1-N1	98.09(11)	O6-Ag1-N1	109.50(15)
O5-Ag1-N1	115.95(12)	N2-Ag2-O4ii	88.81(13)
N4i-Ag2-O4a	122.98(13)		
Symmetry codes: (i) x + /2, —y + 3/2, z - /2; (ii) x, -y + 1, z - '/2.
Figure 5. The coordination environment of Ag(I) in 4 at 50% probability displacement. Symmetry codes: (i) x + '/2, —y + 3/2, z - '/2; (ii) x, -y + 1, z - /2.
semble reaction. Crystal structure analysis data reveal that 4 crystallizes in monoclinic system, space group Cc.
Two crystallographically independent Ag+ ions, two deprotonated ligands L1-, two coordinated water molecules and three solvent water molecules compose the asymmetric unit of 4. As illustrated in Figure 5, the Ag1 and Ag2 coordination environments are distinct from each other. Ag1 is four-coordinated by two water molecules (O5 and O6), and two N atoms (N3 and N1) from two quinoline rings, forming a distorted tetrahedral geometry. However, Ag2 is three-coordinated with triangle coordination geometry surrounded by two pyridine N atoms (N2 and N41), and one monodentate carboxyl oxygen atom (O411). The Ag-O bond lengths are in the range of 2.354(3)-2.441(4) A, which all fall in the normal range
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Figure 6. The one-dimensional chain structure in complex 4.
and are in agreement with those values in the previous re-
21
port.21
The deprotonated ligand L1- acts as bridging biden-tate ligand in complexes 1-3 using the pyridine N and car-boxylate O atoms. While L1- displays two different coordination modes in complex 4. One kind of L1- anion presents terminal bidentate coordination mode using the pyridine N2 and quinoline N1 atoms, keeping the carboxyl group uncoordinated. The coordination mode of the other kind of L1- can be described as tridentate bridge, the pyridine N4, quinoline N3 and carboxyl O4 atoms are all involved in coordination.
In the structure of 4, the Ag2 cations are connected with three L1- ligands to form 1D chain architecture, and the shortest Ag ••• Ag distance is 11.318(2) A (Figure 6). These one-dimensional chains are aligned in an -AB- fas-
hion in the crystallographic ac plane, and are further connected together via Ag1-N1 and Ag1-N3 linkages to form the 3D microporous network with circular channels along c axis, in which the lattice H2O molecules are present (Figure 7).
3. 3. Photoluminescence Properties of HL, 3 and 4
Photoluminiscence properties of the d10 metal complexes 3 and 4 as well as the ligand HL in dimethylsulfo-xide solution were measured. Their emission spectrum is shown in Figure 8.
Upon excitation at 345 nm at room temperature, the ligand emits strong fluorescence centered at 402 nm, resulting from the ligand-centered (LC) n-n and n-n
Figure 7. The 3D network of complex 4, with the water solvat molecules shown in space-filling format.
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Figure 8. Photoluminescence spectra of HL, 3 and 4 in DMSO solution at room temperature.(Xex = 345 nm)
relaxations.15 Compared with HL, in complex 3, a blue shift of 16 nm in the emission maxima and much weaker fluorescence intensity is observed. This is probably caused by the reduced n-n conjugated effect upon coordination to the Cd(II) and the non-coplanar arrangement in 3. The emission of complex 4 is blue shifted to 392 nm and the emission intensity is stronger than the ligand which may be ascribed to the increased rigidity of the frame-work.15,22
3. 4. Antibacterial Activities
In the present study, the in vitro antimicrobial properties of HL and its complexes 1-4 expressed as IC50 are presented in Table 4. Known antibiotic like streptomycin was used as control drug.
As shown in Table 4, against the all tested bacteria, free ligand HL and complex 1 were inactive under the tested conditions. For 2-4, the introduction of metal ions on ligand is endowed with improved inhibition activity. Complex 2 shows its moderate activity towards gram-positive strain. Complex 3 exhibited an enhancement of antibacterial level against gram-positive strains among all the test compounds. The action of the test compounds 2
and 3 on gram-positive bacteria is better than that on gram-negative bacteria, which is the same as Co or Cd complexes based on HL1 and HL2.14,15 While the antibacterial activities observed for 1-3 are all poorer than the corresponding previously reported metal complexes derived from HLX and HL2,14,15 which can be ascribed to the lack of chelate effect of the carboxylic ligand in 1-3. Ag(I) involved complex 4 exhibits activity towards gramnegative strain P. aeruginosa with IC50 value of 6.74 Ug/mL. This selective activity can be attributed to the nature of the Ag ion.21,23
4. Conclusions
Chain like Mn(II), Co(II) and Cd(II) complexes and 3D network Ag(I) complex with the quinoline carboxylic ligand 2-(pyridin-4-yl)quinoline-4-carboxylic acid were synthesized and their crystal structures were determined. The ligand and its Cd(II) and Ag(I) complexes are luminescent with maximum emission wavelength at 402 nm, 386 nm and 382 nm, respectively. The compounds were screened for their antibacterial activity. The bioassay results indicate that Co(II) and Cd(II) complexes exhibit antibacterial activity against gram-positive bacteria B. subti-lis and S. aureus, and Ag(I) complex exhibits antibacterial activity against P. aeruginosa. The present observations demonstrate that the modulation of antibacterial activity can be achieved by the coordination geometrical shape and the nature of the central atoms.
5. Supplementary Material
Crystallographic data (excluding structure factors) for the structural analysis have been deposited with the Cambridge Crystallographic Data Center as supplementary publication Nos. CCDC 1456423 (1), 1456422 (2), 1456421 (3) and 1496618 (4). Copies of the data can be obtained free of charge via www.ccdc.ac.uk/conts/retrie-ving.html (or from The Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK, Fax: +44-1223-336-033. Email: deposit@ccdc.cam.ac.uk).
Table 4. Antimicrobial activity of the tested compounds.
Half maximal inhibitory concentrations(pg/mL) Compounds	Gram-negative	Gram-positive
E.coli	P.aeruginosa	B.subtilis	S.aureus
HL	- -	-	-
1	- -	-	-
2	--	18.21	20.59
3	--	11.24	1.87
4	- 6.74	-	-
Streptomycin	3.81 -	3.42	4.62
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http://dx.doi.org/10.1080/00958972.2014.971405
Povzetek
Štiri kovinske komplekse s kinolinkarboksilatnim ligand iz 2-(piridin-4-il)kinolin-4-karboksilne kisline (HL), {[ML2(H2O)2] • 2H2O}n (M = Mn11, 1; M = Co11, 2; M= Cd11, 3) in {[Ag2L2(H2O)2] v 3H2O}n (4) smo sintetizirali z uporabo hidrotermalnih pogojev. Spojine smo okarakterizirali s pomočjo elementne analize, IR spektroskopije ter določili strukture z monokristalno rentgensko difrakcijo. Kompleksi 1-3 imajo 1D verižno strukturo, ki je nadalje preko vodikovih vezi medsebojno povezana, da tvori 3D supramolekularno mrežo. Kompleks 4 ima 3D strukturo. Raziskali smo tudi fluorescenčne lastnosti in antibakterijsko aktivnost teh spojin.
Zhang et al.: Synthesis, Structure Evaluation, Spectroscopic