Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 144 (2015) 99–106
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Synthesis, characterization, fluorescence and catalytic activity of some new complexes of unsymmetrical Schiff base of 2-pyridinecarboxaldehyde with 2,6-diaminopyridine Omyma A.M. Ali a,⇑, Samir M. El-Medani b, Doaa A. Ahmed a, Doaa A. Nassar a a b
Chemistry Department, Faculty of Women for Arts, Science and Education, Ain Shams University, Cairo, Egypt Chemistry Department, Faculty of Science, El-Fayoum University, El-Fayoum, Egypt
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A new Schiff base complexes were
Effect of reaction time on decomposition of H2O2 by Cu(II), Ni(II), Co(II), La(III) and Sm(III) complexes at 25 °C.
synthesized and characterized. The ligand and its complexes showed antibacterial activities. The complexes exhibited catalytic activity. The ligand and its complexes can potentially serve as photoactive materials.
a r t i c l e
i n f o
Article history: Received 23 October 2014 Received in revised form 18 December 2014 Accepted 18 February 2015 Available online 26 February 2015 Keywords: Spectral Fluorescence 2-Pyridinecarboxaldehyde Catalytic activity
a b s t r a c t The Schiff base, 2-[(pyridin-2-ylmethylidene)amino]-6-aminopyridine (L) was synthesized by 1:1 condensation of 2-pyridinecarboxaldehyde and 2,6-diaminopyridine. The ligand and its complexes were characterized by different physicochemical studies. The analytical and spectroscopic tools indicated that the synthesized complexes have the general formulae: [M(L)Cl2]2H2O (M = Cu(II), Ni(II) and Co(II)), [La(L)3](NO3)33H2O and [Sm(L)(ClO4)3]3H2O. Vibrational spectra indicated the coordination of L to metal ions through its pyridyl and azomethine nitrogen atoms. The presence of water molecules in all reported complexes has been supported by TG/DTA studies. Kinetic and thermodynamic parameters were computed using Coats and Redfern method. The prepared ligand and its complexes exhibited intraligand (p–p⁄) fluorescence and can potentially serve as photoactive materials. The catalytic activity of the complexes toward the decomposition of hydrogen peroxide was investigated. Both the ligand and its complexes have been screened for antibacterial activities. Ó 2015 Elsevier B.V. All rights reserved.
Introduction Schiff base complexes derived from heterocyclic compounds have been found interest in the context of bioinorganic chemistry ⇑ Corresponding author. Tel.: +20 01224024596. E-mail address: [email protected] (O.A.M. Ali). http://dx.doi.org/10.1016/j.saa.2015.02.078 1386-1425/Ó 2015 Elsevier B.V. All rights reserved.
[1–3]. Not only they have played a seminal role in the development of modern coordination chemistry, but they can be also a key point in the development of inorganic biochemistry [4]. Heterocyclic compounds such as pyridine and related molecules are good ligands due to the presence of one or more ring nitrogen atoms with a localized pair of electrons. Schiff bases and their metal complexes have been found to exhibit biological activities [5–8]. In
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addition, complexes of Schiff base ligands derived from pyridine derivatives with some transition metals such as Cu(II), Co(II), Ni(II), Fe(III) and Zn(II) showed significant biological notification including antimicrobial, antibacterial, antifungal and anticancer activities [9–22]. Moreover, luminescent compounds are attracting much current research interest because of their many applications including emitting materials for organic light emitting diodes, light harvesting materials for photocatalysis and fluorescent sensors for organic or inorganic analytes [23–27]. On the other hand, the coordination chemistry of lanthanides has received interest because of its intriguing variety of architecture and potential applications in various areas such as magnetism, optics, electronics and catalysis, sensors in the biomedical field area [28–31]. The decomposition of hydrogen peroxide has been used as a model reaction for the investigation of the catalytic activity of various metal complexes [32]. The catalytic activity of copper (II) complex of chitosan derived from chitosan and 4,6-diacetylresorcinol was investigated on hydrogen peroxide decomposition [33]. The copper chelate showed high efficiency toward the decomposition of hydrogen peroxide as heterogeneous catalyst. Based on the previous considerations, we are initiating a line of investigation on the coordination chemistry of Schiff base compounds. In previous work, we have described the complexes produced from the reactions of [M(CO)6], M = Cr, Mo and W with 2-[(pyridin-2-ylmethylidene)amino]-6-aminopyridine (L) [34]. In the present manuscript, the reactions of Cu(II), Ni(II), Co(II), La(III) and Sm(III) with the Schiff base (L) are reported.
gen (CHN) were performed on a Perkin-Elmer 2400 CHN elemental analyzer. Mass spectrometry measurement of the solid complex was carried out on a JEOL JMS-AX 500 spectrometer. Thermogravimetric analyses (TG and DTG) were carried out under N2 atmosphere with a heating rate of 10 °C/min. using a Shimadzu DT-50 thermal analyzer. All conductivity measurements were performed in DMF (1 103 M) at 25 °C, by using Jenway 4010 conductivity meter. The Photoluminescent properties of all compounds were studied using a Jenway 6270 Fluorimeter. Synthesis of the Schiff base ligand (L) An ethanolic solution of 2-pyridinecarboxaldhyde (1.00 g, 9.33 mmol) was mixed with a hot ethanolic solution of 2,6-diaminopyridine (1.02 g, 9.33 mmol) with constant stirring for 15 h. The color of the reaction mixture was changed from pale yellow to brown color with the formation of a brown precipitate. The reaction mixture was evaporated on a hot plate to its half initial volume, then left to cool. The isolated brown solid Schiff base was washed several times with ethanol. Scheme 1 represents the reaction expresses the synthesis of the Schiff base ligand 2-[(pyridin-2-ylmethylidene)amino]-6-aminopyridine (L). Synthesis of metal complexes
2-Pyridinecarboxaldehyde, 2,6-diaminopyridine and the metal salts; CuCl22H2O, NiCl26H2O, CoCl26H2O, La(NO3)36H2O and Sm(ClO4)36H2O were provided by Aldrich Chemicals. All solvents were of analytical grade and were purified by distillation before use.
All the metal complexes of the Schiff base L were synthesized by template method. To a hot solution of the Schiff base ligand (L) in 20 mL of a mixed solvent (DMF:THF; 1:2), an equimolar amount of a hot aqueous solution of the metal salt (Cu2+, Ni2+, Co2+, La3+ or Sm3+) was added slowly. The resulting mixture was refluxed for 6 h, and then evaporated on a hot plate to its half initial volume. After leaving the mixture to cool overnight, the formed solid complex was separated by filtration then washed with ethanol followed by petroleum ether. Finally, the complex was left to dry at room temperature. Table 1 gives the elemental analysis and physical data of the formed complexes.
Instruments
Catalytic activity
Infrared measurements using (KBr pellets) were carried out on a Unicam-Mattson 1000 FT-IR spectrometer and magnetic measurements of the complexes in the solid state (Gouy method) were recorded on a Sherwood magnetic susceptibility balance. 1H NMR measurements were performed on a Varian-Mercury 300 MHz spectrometer. Elemental analyses for carbon, hydrogen, and nitro-
The catalytic activity of the complexes has been evaluated by recording the rate of decomposition of hydrogen peroxide of known molarity. The metal complex (4.1 103–8.2 103 mmol) was mixed with 50 mL of H2O2 (0.15 N) in a flask under constant stirring at room temperature. The extent of decomposition was estimated by titrating 5 mL aliquots of reaction mixture with
Experimental Materials and reagents
H2N
N
NH2
+ N
N
Ethanol N
CHO
2 -pyridinecarboxaldehyde
2,6-diaminopyridine
NH2
N
C H Schiff base ligand
Scheme 1. Preparation of Schiff base ligand 2-[(pyridin-2-ylmethylidene)amino]-6-aminopyridine (L).
Table 1 Analytical and physical data of the ligand 2-[(pyridin-2-ylmethylidene)amino]-6-aminopyridine (L) and its complexes. Compound
M.Wt.
Color (% yield)
Elemental analysis found (Calc.)
C11H10N4 (L) [Cu(C11H10N4)Cl2]2H2O [Co(C11H10N4)Cl2]2H2O [Ni (C11H10N4)Cl2]2H2O [La(C11H10N4)3](NO3)33H2O [Sm(C11H10N4)(ClO4)3]3H2O
198.22 368.69 364.08 363.85 973.62 700.95
Pale yellow (88) Black (96) Dark green (95) Dark red (69) Brown (80) Black (79)
66.45 35.45 36.48 36.23 40.41 18.65
C%
H% (66.65) (35.83) (36.28) (36.31) (40.71) (18.85)
5.12 3.90 3.96 3.61 3.51 2.41
mp (°C)
A (X1 mol1 cm2)
250 >300 >300 >300 >300 >300
– 2 5 11 360 13
N% (5.08) (3.82) (3.87) (3.87) (3.72) (2.30)
28.75 15.36 15.45 15.24 21.48 08.00
(28.26) (15.19) (15.38) (15.39) (21.58) (07.99)
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Table 2 IR data of the Schiff base ligand (L) and its complexes (cm1). Compound
L [Cu(L)Cl2]2H2O [Co(L)Cl2]2H2O [Ni (L)Cl2]2H2O [La(L)3](NO3)33H2O [Sm(L)(ClO4)3]3H2O a
IR(cm1)a
t(NH2) or t(H2O)
t(C@N)
d(py)
t(M-N)
Other bands
3333(b) 3332(b) 3331(b) 3329(b) 3328(b) 3395(b)
1640(w) 1588(s) 1651(s) 1590(s) 1652(s) 1585(sh) 1652(s) 1590(sh) 1655(s) 1589(s) 1654(m) 1590(sh)
619(w) 658(w) 664(w) 651(w) 665(w) 678(w)
– 430(w) 414(w) 425(w) 430(w) 420(w)
– – – – 1383(s) t(NO3) (ionic) 1146(s), 1112(s), 1086(s), 630(m) t(ClO4) (bidentate)
3280(m) 3387(b) 3279(b) 3288(sh) 3278(b)
s, strong; m, medium; w, weak; sh, shoulder.
0.01 M KMnO4 in the presence of 0.01 M H2SO4 at different intervals of time (each 30 min; from 0 to 4.5 h). The difference in titer values of the KMnO4 solution in each interval was recorded. Antibacterial activity The in vitro antimicrobial activities of the free ligand and its complexes were tested against the bacteria: Staphylococcus aureus (gram +ve) and Escherichia coli (gram ve) in Mueller Hinton-Agar medium. The standard disc diffusion method was followed to determine the antibacterial activity of the synthesized compounds. The minimum inhibitory concentration of the ligand and complexes were ascertained using the different bacteria. The concentration of the drug solution was maintained to be 200 lg/mL in DMSO. The well (8 mm diameter) was filled with the test solution and the plates were inoculated at 37 °C for 48 h. During this period, the growth of the inoculated microorganisms was affected and then the inhibition zones developed on the plates were measured. The effectiveness of an antibacterial agent was assessed by measuring the zones of inhibition around the well. The diameter of the zone is measured to the nearest millimeter (mm). The antibacterial activity of each compound was compared with that of standard antibiotics such as Tetracycline. DMSO was used as a control under the same conditions for each organism and no activity was found. The activity results were calculated as a mean of triplicates.
deformation band of py in free ligand L (619 cm1) was shifted to higher frequencies (651–678 cm1) confirming the coordination of pyridyl nitrogen to metal. The triply split band maxima at 1146, 1112, 1086 cm1 and the medium band at 630 cm1 appeared in the IR spectrum of samarium complex were attributed to the bidentately coordinated perchlorate anion [35–37]. The band at 1383 cm1 in the IR spectrum of La(III) complex indicated the existence of free nitrate group in the coordination sphere [38]. The appearance of new bands in the IR spectra of the complexes in the region 414–430 cm1 were assigned to m (M–N) stretching vibrations. 1
H NMR spectra
The 1H NMR spectrum of the free Schiff base ligand L showed signals at 8.05 and 9.99 ppm, attributed to the azomethine (– CH@N–) and NH2 protons, respectively. The signal at 7.97 ppm in the 1H NMR spectrum of the La(III) complex showed a higher field shift relative to that of ligand L, indicating the coordination of the lanthanum atom to the nitrogen atom of the azomethine group in L. In addition, the 1H NMR spectrum of the La(III) complex displayed a signal at 9.99 ppm without any field shift relative to that of ligand indicating the nonparticipation of NH2 group in bonding with the metal [34]. The chemical shifts of the different types of protons in the ligand and its La(III) complex are listed in Table 3.
Results and discussion Mass spectra analysis All solid metal complexes are colored, and stable in air and moisture at room temperature. The analytical data along with some physical properties of the Schiff base ligand and its complexes are listed in Table 1. The data are consistent with the proposed molecular formula. IR spectra The data of the IR spectra of Schiff base ligand and its metal complexes are listed in Table 2. The IR spectra of the complexes were compared with those of the free ligand in order to determine the involvement of coordination sites in chelation. Characteristic peaks in the spectra of the ligand and the complexes were considered and compared. The IR spectrum of the ligand L showed two bands at 3333 and 3280 cm1 due to mas(NH2) and ms(NH2). The IR spectra of the complexes exhibited a broad band around 3395–3278 cm1 assigned to m (OH) of crystalline or coordinated water molecules associated with the complex. This band may be overlapped with the bands corresponding to the stretching vibrations of m (NH2) group. In addition, the IR spectrum of L displayed two bands at 1640 and 1588 cm1 due to t(C@N) of azomethine and pyridyl moieties, respectively. The shifts of t(C@N) for azomethine group to 1651–1655 cm1 and for pyridyl group to 1585– 1590 cm1 on complexation, suggested the coordination of metal ion to azomethine and pyridyl nitrogen atoms. The in-plane ring
The molecular formulae of the investigated compounds were substantiated by mass spectra. The Mass spectrum of L exhibited a molecular ion peak at m/z 198 (S. 1). The ion peaks at 55, 69, 143 and 180 referred to the ions [C3H5N]+, [C3H5N2]+, [C8H5N3]+ and [C11H6N3]+, respectively. The most prominent mass spectral peaks of the reported complexes are given in Table 4. The mass spectra of Cu(II), Ni(II), Co(II) and Sm(III) complexes are recorded (S. 2 and 3). The mass spectrum of Cu(II) complex (S. 2(a)) revealed a molecular mass peak at m/e = 368 corresponding to the formula [Cu(L)Cl2]2H2O (M.Wt. = 368.69). The existence of the ligand was indicated by the peak at 197 (Calc. 198.22). The spectrum exhibited a peak at 333 corresponding to the formula [Cu(L)Cl2] (M.Wt. = 332.66) after removal of the two water molecules from the complex.
Table 3 H NMR spectral data of the Schiff base (L) and La(III) complex.
1
Compound
Chemical shift, d(ppm)a
L
7.73–7.70 m (4H, pyridine), 7.95–7.92 m (3H, pyridine), 8.05 s (1H, HC = N), 9.99 s (2H, NH2). 7.31–7.05 m (4H, pyridine), 7.529–7.47 m (3H, pyridine), 7.97 s (1H, HC = N), 9.99 s (2H, NH2).
[La(L)3](NO3)33H2O a
s, singlet; m, as,
a a
s, singlet; m, multiplet.
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Table 4 The most relevant mass spectral peaks of Cu(II), Co(II), Ni(II) and Sm(III) complexes. Compound
m/e Found (calcd.)
Relative intensity (%)
Loss moiety
Peak assignment due to loss moiety
[Cu(L)Cl2]2H2O
368 (368.69) 333 (332.66) 264 (261.54) 64 (63.54)
47.79 58.76 62.83 48.67
– 2H2O 2Cl L
– [Cu(L)Cl2] [Cu(L)] Cu
[Co(L)Cl2]2H2O
365 (364.08) 274 (274.86) 58 (58.93)
15.12 13.49 14.16
– 2Cl + H2O H2O + L
– [Co(L)(H2O)] Co
[Ni(L)Cl2]2H2O
364 (363.85) 328 (327.82) 258 (256.91) 59 (58.71)
45.64 38.61 34.90 36.24
– 2H2O 2Cl L
– [Ni(L)Cl2] [Ni(L)] Ni Ni
[Sm(L)(ClO4)3]3H2O
703 645 450 348 150
23.22 29.50 31.12 33.30 29.00
– 3H2O 2ClO4 ClO4 L
– [Sm(L)(ClO4)3] [Sm(L)(ClO4)] [Sm(L)] Sm
(700.95) (646.90) (448.00) (348.55) (150.33)
The mass spectrum of cobalt complex (S. 2(b)) showed a molecular ion peak at 365 (F.Wt. = 364.08) with the formula [Co(L)Cl2]2H2O. The existence of the ligand was indicated by the peak at 199 (Calc. 198.22). The peak at 274 (Calc. 274.86) referred to the formula [Co(L)(H2O)] by removal of a water molecule and two chlorine atoms from the complex. The mass spectrum of nickel complex (S. 2(c)) exhibited a molecular ion peak at 364 (F.Wt. = 363.85) with the formula [Ni(L)Cl2]2H2O. The existence of the ligand in nickel complex was assigned by the peak at 198 (Calc. 198.22) while the peak at 328 (Calc. 327.82) was referred to the removal of the two water molecules from the complex. The mass spectrum of samarium complex (S. 3) exhibited a molecular ion peak at 703 (F.Wt. = 700.95) with the formula [Sm(L)(ClO4)3]3H2O. The existence of the ligand in complex was assigned by the peak at 198 (Calc. 198.22) while the peak at 645 (Calc. 646.90) was referred to the removal of the three water molecules from the complex.
363 nm, the first band was attributed to p–p⁄ transitions and the other two bands were referred to n–p⁄ transitions, respectively. The band appeared at 395 nm was corresponded to charge transfer transition. The diffuse reflectance spectrum of Cu(II) complex showed one broad band d–d absorption band at 612 nm, suggesting a square-planar geometry [40]. The observed magnetic moment of Cu(II) complex is 1.98 B.M. The Ni(II) complex has magnetic moment value of 3.8 B.M. This magnetic moment value is in the normal range observed in tetrahedral Ni(II) complex. The electronic spectrum of Ni(II) complex displayed three bands in the solid reflectance spectrum at m1 (605 nm: 3T1 (F) ? 3T2 (F)); m2 (521 nm: 3T1 (F) ? 3A2 (F)) and m3 (471 nm: 3T1 (F) ? 3T1 (P)) [41] suggesting the tetrahedral structure around Ni(II) ion. The diffuse reflectance spectrum of the Co(II) complex showed three
Molar conductivity measurements The measured Molar conductance values of 103 M metal complexes in DMF are listed in Table 1. These values indicated that the complexes are non-electrolytes except La(III) complex. The molar conductance value of La(III) chelate was found to have a value of 360 ohm–1 mol–1 cm2 indicating the ionic nature of this complex. Accordingly, the La(III) complex can be considered as 3:1 electrolyte [39]. Magnetic measurements and electronic absorption spectra The diffuse reflectance spectra of L and its complexes are listed in Table 5. The ligand L showed three bands at 251, 337 and
Table 5 The diffuse reflectance data of the Schiff base (L) and its complexes. Compound
L [Cu(L)Cl2]2H2O [Co(L)Cl2]2H2O [Ni(L)Cl2]2H2O [La(L)3](NO3)33H2O [Sm(L)(ClO4)3]3H2O
kmax (nm)
p–p⁄
n–p⁄
251 252 252 278 244 244
337, 337 341 348, 347, 345,
363
383 384 417
Fig. 1. Emission spectra of: (1); Ligand (L); (2) [La(L)3](NO3)33H2O; (3) [Cu(L)Cl2]2H2O; (4) [Sm(L)(ClO4)3]3H2O; (5) [Ni(L)Cl2]2H2O and (6) [Co(L)Cl2]2H2O.
Table 6 Fluorescence data of the Schiff base (L) and its complexes.
Charge transfer
d–d transition
Compound
kexcitation
kemission
395 396 396 476 473 480
– 612 471, 521, 615 518, 541, 605 – –
L [Cu(L)Cl2]2H2O [Co(L)Cl2]2H2O [Ni(L)Cl2]2H2O [La(L)3](NO3)33H2O [Sm(L)(ClO4)3]3H2O
320 362 326 330 318 328
358 434 364, 435 384 357 359, 425
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bands at 471, 521 and 615 nm. The observed bands are assigned to the transitions 4A2 ? 4T2 (m3), 4A2 ? 4T1(F) (m2) and 4A2 ? 4T1(P) (m1), respectively, suggesting the tetrahedral structure around Co(II) ion [42]. The magnetic moment value of Co(II) complex has been found to be 4.42 BM within the range of tetrahedral geometry [43]. The diffuse reflectance spectra in both La(III) and Sm(III) complexes showed a band at 244 nm due to p–p⁄ transitions and two bands at 347 and 384 nm corresponded to n–p⁄ transitions for La(III) complex, while the two bands at 345 and 417 nm were corresponded to n–p⁄ transitions for Sm(III) complex. Furthermore, the two bands appeared at 473 and 480 NM were corresponded to ligand to metal charge transfer in the lanthanum and samarium complexes, respectively. As expected for the diamagnetic La(III) (d0) configuration, the ligand field band due to d–d electronic transitions is not expected [44].
spectrum of Cu(II), Ni(II), Co(II), La(III) and Sm(III) complexes exhibited strong fluorescence emission bands in the range 357– 435 nm when excited at 318–362 nm. Significant difference in the position of emission maximum of Schiff base and each complex reinforces the coordination of the metal ion to the ligand. The fluorescence spectral results reveal that the fluorescence emission intensity of Schiff base increases dramatically on complex formation with transition metal ions. Enhancement of fluorescence through complexation is much interesting as it opens up the opportunity for photochemical applications of these complexes [45]. In general, all the synthesized compounds can serve as potential photoactive materials, as indicated from their characteristic fluorescence properties.
Thermal analysis (TGA and DTA) Luminescence spectral study The fluorescence properties of the Schiff base ligand and its complexes in DMSO (Fig. 1) were recorded at room temperature. The excitation spectra of the ligand showed a maximum emission peak at 355 nm when excited at 320 nm. Generally, Schiff base systems exhibited fluorescence due to intraligand p?p⁄ transitions. The fluorescent data are summarized in Table 6. The excitation
The thermal studies of metal complexes were carried out using the thermogravimetric technique (TG) and differential thermogravimetric (DTG). Table 7 gives the detailed thermal decomposition data for complexes. Thermogravimetric analysis is considered as a tool to distinguish between the position of water molecule(s) in the complex; if it is in the inner or outer coordination sphere.
Table 7 Thermogravimetric data of complexes. Complex
TG range (°C)
Mass loss % Calc.
Found
Assignment
Metallic residue Calc. (found)
[Cu(L)Cl2]2H2O
36–171 171–383 383–813
9.77 19.24 53.76
9.75 19.25 53.25
Loss of 2H2O Loss of 2Cl Loss of L
Cu 17.23% (17.75%)
[Co(L)Cl2]2H2O
36–161 161–430 430–820
9.89 19.48 54.44
9.92 19.39 54.04
Loss of 2H2O Loss of 2Cl Loss of L
Co 16.17% (16.65%)
[Ni(L)Cl2]2H2O
35–53 53–244 244–732
4.95 24.44 54.47
4.84 24.03 54.97
Loss of H2O Loss of 2Cl + H2O Loss of L
Ni 16.14% (16.16%)
[La(L)3](NO3)33H2O
35–115 115–335 335–577
5.55 19.11 61.07
5.51 19.27 61.39
Loss of 3H2O Loss of 3NO3 Loss of 3L
La 14.27% (13.83%)
[Sm(L)(ClO4)3]3H2O
42–168 168–435 435–771
7.71 28.39 42.45
7.77 28.00 43.23
Loss of 3H2O Loss of 2ClO4 Loss of ClO4 + L
Sm 21.45 (21.00)
Table 8 The kinetic and thermodynamic data of the thermal decompositions of complexes. Complex
Decomposition range (°C)
E⁄ (kJ mol1)
A (S1)
S⁄ (kJ mol1)
H⁄ (kJ mol1)
G⁄ (kJ mol1)
7
[Cu(L)Cl2]2H2O
36–95 100–370 528–809
51.39 36.87 81.05
1.9 10 3.3 102 9.8 102
91.15 –193.66 –191.50
50.94 35.15 77.08
55.90 75.16 168.39
[Co(L)Cl2]2H2O
36–96 96–351 538–750
36.04 36.97 78.84
2.9 103 3.1 102 9.8 102
136.71 –195.50 –195.10
35.63 34.92 72.42
43.77 83.10 214.36
[Ni(L)Cl2]2H2O
35–115 431–583 584–701
40.04 6.46 255.42
2.1 105 1.3 102 7.5 1011
128.57 –286.52 –23.79
39.59 1.68 220.14
46.51 166.23 235.24
[La(L)3](NO3)33H2O
36–100 185–321 360–432 457–577
72.20 43.04 123.14 160.57
2.2 1010 1.05 103 2.6 108 8.2 109
33.78 –186.54 –86.65 –59.55
71.68 40.72 119.62 156.34
73.80 92.67 156.18 186.63
[Sm(L)(ClO4)3]3H2O
42–98 98–236 476–700
44.32 36.16 23.77
6.79 105 1.02 103 8.9
121.45 –184.15 –250.65
43.73 34.47 19.40
52.45 71.83 150.97
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The TGA curves of Cu(II) (S. 4) showed that the thermal decomposition takes place at a temperature range 36–813 °C. The first decomposition step occurred at 36–171 °C due to weight loss of 9.75% (Calc. 9.77%) corresponded to the elimination of two H2O molecules. The second stage of decomposition was due to the loss of two chlorine atoms, while the third stage of decomposition at 383–813 °C was referred to the loss of a ligand molecule to give finally copper metal as a residue. The TG plot of Co(II) (S. 5) displayed three decomposition steps. The first stage takes place in the 36–161 °C range corresponding to the release of two H2O molecules (Obs. 9.92%, Calc. 9.89%). The second and third decomposition steps involved the removal of two chlorine atoms and an organic ligand molecule, respectively, leaving cobalt metal as a residue. The TG/DTG curves of Ni(II) complex (S. 6) showed three decomposition steps within the temperature ranges 35–53 °C, 53–244 °C and 244–732 °C. The weight loss of first step (Obs. = 4.84%, Calc. = 4.95%), the second step (Obs. = 24.03%, Calc. = 24.44%) and third step (Obs. = 54.97%, Calc. = 54.47%) were assigned to the loss of H2O, (H2O + 2Cl) and L, respectively. The TG plot of [La(L)3](NO3)3.3H2O (S. 7) displayed three decomposition steps. The first step occurred in the range 35– 115 °C is due to release of three molecules of H2O. The second step occurred in the range 115–335 °C with a weight loss of 19.27% (Calc. 19.11%) could be attributed to the loss of the three nitrate groups. The third step occurred in the range 335–577 °C corresponded to the loss of three ligand molecules leaving lanthanum metal as a residue. The TG plot of [Sm(L)(ClO4)3]3H2O (S. 8) displayed three decomposition steps. The first step decomposition step is consistent with loss of three molecules of H2O. The second step occurred in the range 168–435 °C with a weight loss of 28.00% (Calc. 28.39%) could be attributed to the loss of 2(ClO 4 ) groups. The third step occurred in the range 435–771 °C corresponded to the loss of (ClO-4 ) group and organic ligand moiety leaving samarium metal as a residue. Thermal studies of the metal complexes confirmed the molecular formulae of the studied complexes as: [Cu(L)Cl2]2H2O, [Co(L)Cl2]2H2O, [Ni(L)Cl2]2(H2O), [La(L)3](NO3)33H2O and [Sm(L)(ClO4)3]3H2O.
tion (G⁄) were evaluated graphically by employing the Coats-Redfern relation [46].
log
logðW 1 =ðW 1 WÞÞ T2
¼ log
AR 2RT E 1 /E E 2:303RT
ð1Þ
where W1 is the mass loss at the completion of the decomposition reaction, W is the mass loss up to temperature T, R is the gas constant and / is the heating rate. Since 1–2RT/E⁄ ffi 1, the plot of the left-hand side of Eq. (1) against 1/T would give a straight line. E⁄ was then calculated from the slope and the Arrhenius constant, A, was obtained from the intercept. The other kinetic parameters; the entropy of activation (S⁄), enthalpy of activation (H⁄) and the free energy change of activation (G⁄) were calculated using the following equations:
S ¼ 2:303R log
Ah kT
ð2Þ
H ¼ E RT
ð3Þ
G ¼ H TS
ð4Þ
where, (k) and (h) are the Boltzman’s and Planck’s constants, respectively. The kinetic parameters are listed in Table 8. From the obtained results, it is apparent that G⁄ values of the complexes acquire highly positive magnitudes. The activation energies of
H2N N
N
CH N
N
NH2
N
La
HC
(NO3)3 . 3 H O. 2
N N
N C H
N
Kinetic data
NH2
The thermodynamic activation parameters of decomposition processes of complexes namely activation energy (E⁄), enthalpy (H⁄), entropy (S⁄) and Gibbs free energy change of the decomposi-
(a)
N O2Cl
O2Cl
CH
O O O O
Sm
N
N
NH2
3 H2O.
O O ClO2
(b) Scheme 2. Proposed structure of (M = Cu(II), Ni(II) and Co(II)) complexes.
Scheme 3. Proposed structures of (a) La(III) complex and (b) Sm(III) complex.
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decomposition were found to be in the range 6.4–255.4 kJ mol1. The high activation energy values of the complexes revealed the high stability of the investigated complexes due to their covalent character. The negative values of S⁄ for the degradation process indicated more ordered activated complexes than the reactants and assigned to slow decomposition reaction.
position of 38.6–52.2% of H2O2 catalyzed by complexes. In the absence of complexes, the decomposition percentage of hydrogen peroxide was found to be 20% after 24 h. The mechanism of the catalytic decomposition reaction of H2O2 has been proposed in the literature [33] as given in formulas (1)– (3):
Structural interpretation
H2 O2 ! HO2 þ Hþ
The structures of the studied Cu(II), Ni(II), Co(II), La(III) and Sm(III) complexes were characterized by elemental analyses, molar conductance, magnetic, solid reflectance and thermal analysis data. From IR spectra, it could be concluded that L behaves as a bidentate ligand coordinated to the metal ions via nitrogen atoms of both pyridine and azomethine group. From the molar conductance data, it was found that the complexes are non-electrolytes except La(III) complex. Accordingly, the structures of the complexes could be proposed as shown in Schemes 2 and 3. Catalytic activity The decomposition of hydrogen peroxide has been used as a model reaction for the investigation of the catalytic activity of various metal complexes. The decomposition of H2O2 catalyzed by metal complexes has been monitored by titrating the undecomposed H2O2 with a standard KMnO4 solution. Variables such as the time were found to have important roles in the decomposition of H2O2. The effect of time on decomposition of H2O2 at a constant concentration of H2O2 (0.15 N) and constant concentration of complexes (4.1 103–8.2 103 mmol1) at 25 °C was studied (Table 9). The conditions of catalytic decomposition reactions of hydrogen peroxide were chosen according to the similar studies in literatures [34,47]. The results showed that the decomposition percentage of hydrogen peroxide increased with time (Fig. 2). It was concluded that 4.5 h is an adequate reaction time for decom-
Table 9 Concentration of the complexes and % of decomposition of H2O2 at constant concentration of H2O2 (0.15 N) at 25 °C after 4.5 h. Compound
Concentration of complex (mmol1)
% of decomposition of H2O2 (%)
[Cu(L)Cl2]2H2O [Co(L)Cl2]2H2O [Ni(L)Cl2]2H2O [La(L)3](NO3)33H2O [Sm(L)(ClO4)3]3H2O
8.1 103 8.2 103 8.2 103 4.1 103 5.7 103
51.0 52.2 38.6 45.8 45.4
Fig. 2. Effect of reaction time on decomposition of H2O2 with [Cu(L)Cl2]2H2O, [Co(L)Cl2]2H2O, [Ni(L)Cl2]2H2O, [La(L)3](NO3)33H2O and [Sm(L)(ClO4)3]3H2O complexes at 25 °C.
ð1Þ
The M-L complex may interact with mediate complex.
HO 2
ions to form an inter-
M-L þ HO2 ! ½MLðHO2 Þ
ð2Þ
A second molecule of H2O2 may then interact with the intermediate complex to form the following products.
½MLðHO2 Þ þ H2 O2 ! M-L þ H2 O þ OH þ O2
ð3Þ
Catalytic decomposition of H2O2 leads to the formation of intermediate radical species which can bind to the surfaces where H2O2 undergoes decomposition [48,49]. The enhanced catalytic activity of the complex system may be explained on the basis of the formation of the above intermediate peroxo species. When a metal complex is bound to a support, its motion is restricted. Reactions on such catalysts are expected to be more rapid than one on which the catalyst molecules are free [50]. Antibacterial activity Antibacterial activity of Schiff base ligand and its complexes have been tested against one gram positive bacteria; S. aureus and also against one gram negative bacteria; E. coli microorganisms. The antibacterial activities obtained for the prepared compounds are listed in Table 10. All the investigated compounds showed a remarkable biological activity against bacteria (Fig. 3). The obtained results reflect that all complexes showed higher
Table 10 Biological activities of Schiff base ligand (L) and its complexes. Compound
L [Cu(L)Cl2]2H2O [Co(L)Cl2]2H2O [Ni(L)Cl2]2H2O [La(L)3](NO3)33H2O Tetracycline antibacterial agent
Diameter of inhibition zone (mm) Escherichia coli (G)
Staphylococcus aureus (G+)
9 10 15 12 13 32
11 12 15 14 14 30
Fig. 3. Antibacterial activity of compounds under study.
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antibacterial activity than ligand. On chelation, the delocalization of p-electrons over the whole chelate ring will be increased which enhances the penetration of the complexes into lipid membranes and blocking the metal binding sites in the enzymes of microorganisms. Also, the tested complexes may disturb the respiration process of the cell and consequently block the synthesis of proteins leading to no further growth of the organisms [51]. Conclusion The novel complexes of copper(II), cobalt(II), nickel(II), lanthanum(III) and samarium(III) with the bidentate ligand 2-[(pyridin-2-ylmethylidene)amino]-6-aminopyridine (L) were synthesized and characterized using spectroscopic methods. The molar conductivity data for complexes indicated that they have a non electrolytic nature except La(III) complex. Fluorescence studies indicated that the synthesized compounds can serve as potential photoactive materials, as indicated from their characteristic fluorescence properties. The activation thermodynamic parameters, such as activation energy, enthalpy, entropy and Gibbs free energy change of complexes decomposition indicated the their stability. The catalytic activities of the complexes toward hydrogen peroxide decomposition reaction indicated enhanced decomposition of H2O2 by metal complexes. Antibacterial activity studies of the investigated compounds showed a remarkable biological activity against bacteria. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2015.02.078. References [1] A.T. Chaviara, P.J. Cox, K.H. Repana, R.M. Papi, K.T. Papazisis, D. Zambouli, A.H. Kortsaris, D.A. Kyriakidis, C.A. Bolos, J. Inorg. Biochem. 98 (2004) 1271–1283. [2] J.A. Ciller, C. Seoane, J.L. Soto, B. Yruretagoyena, J. Heterocyclic Chem. 23 (2009) 1583–1586. [3] C.R. Bhattacharjee, P. Goswami, S. Neogi, S. Dhibar, Assam Univ. J. Sci. Technol.: Phys. Sci. Technol. 5 (2010) 81–87. [4] A. Prakash, B.K. Singh, N. Bhojak, D. Adhikari, Spectrochim. Acta, Part A 76 (2010) 356–362. [5] V.A. Shelke, S.M. Jadhav, V.R. Patharkar, S.G. Shankarwar, A.S. Munde, T.K. Chondhekar, Arabian J. Chem. 5 (2012) 501–507. [6] B. Rizwana, S.L. Santha, Int. J. Chem. Tech. Res. 4 (2012) 464–473. [7] R. Suganthi, S.L. Santha, K. Geetha, A. Abdul Rahuman, J. Pharm. Res. 4 (2011) 4574–4576. [8] S. Zhang, Y. Zhu, C. Tu, H. Wei, Z. Yang, L. Lin, J. Ding, J. Zhang, Z. Guo, J. Inorg. Biochem. 98 (2004) 2099–2106. [9] P.D. Badma, L.S. Santha, Int. J. Chem. Tech. Res. 6 (2014) 87–94. [10] S. Brooker, S.S. Iremonger, P.G. Plieger, Polyhedron 22 (2003) 665–671. [11] M.S. Refat, I.M. El-Deen, H.K. Ibrahim, S. El-Ghool, Spectrochim. Acta, Part A 65 (2006) 1208–1220.
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Synthesis, characterization, fluorescence and catalytic activity of some new complexes of unsymmetrical Schiff base of 2-pyridinecarboxaldehyde with 2,6-diaminopyridine Omyma A.M. Ali a,⇑, Samir M. El-Medani b, Doaa A. Ahmed a, Doaa A. Nassar a a b
Chemistry Department, Faculty of Women for Arts, Science and Education, Ain Shams University, Cairo, Egypt Chemistry Department, Faculty of Science, El-Fayoum University, El-Fayoum, Egypt
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A new Schiff base complexes were
Effect of reaction time on decomposition of H2O2 by Cu(II), Ni(II), Co(II), La(III) and Sm(III) complexes at 25 °C.
synthesized and characterized. The ligand and its complexes showed antibacterial activities. The complexes exhibited catalytic activity. The ligand and its complexes can potentially serve as photoactive materials.
a r t i c l e
i n f o
Article history: Received 23 October 2014 Received in revised form 18 December 2014 Accepted 18 February 2015 Available online 26 February 2015 Keywords: Spectral Fluorescence 2-Pyridinecarboxaldehyde Catalytic activity
a b s t r a c t The Schiff base, 2-[(pyridin-2-ylmethylidene)amino]-6-aminopyridine (L) was synthesized by 1:1 condensation of 2-pyridinecarboxaldehyde and 2,6-diaminopyridine. The ligand and its complexes were characterized by different physicochemical studies. The analytical and spectroscopic tools indicated that the synthesized complexes have the general formulae: [M(L)Cl2]2H2O (M = Cu(II), Ni(II) and Co(II)), [La(L)3](NO3)33H2O and [Sm(L)(ClO4)3]3H2O. Vibrational spectra indicated the coordination of L to metal ions through its pyridyl and azomethine nitrogen atoms. The presence of water molecules in all reported complexes has been supported by TG/DTA studies. Kinetic and thermodynamic parameters were computed using Coats and Redfern method. The prepared ligand and its complexes exhibited intraligand (p–p⁄) fluorescence and can potentially serve as photoactive materials. The catalytic activity of the complexes toward the decomposition of hydrogen peroxide was investigated. Both the ligand and its complexes have been screened for antibacterial activities. Ó 2015 Elsevier B.V. All rights reserved.
Introduction Schiff base complexes derived from heterocyclic compounds have been found interest in the context of bioinorganic chemistry ⇑ Corresponding author. Tel.: +20 01224024596. E-mail address: [email protected] (O.A.M. Ali). http://dx.doi.org/10.1016/j.saa.2015.02.078 1386-1425/Ó 2015 Elsevier B.V. All rights reserved.
[1–3]. Not only they have played a seminal role in the development of modern coordination chemistry, but they can be also a key point in the development of inorganic biochemistry [4]. Heterocyclic compounds such as pyridine and related molecules are good ligands due to the presence of one or more ring nitrogen atoms with a localized pair of electrons. Schiff bases and their metal complexes have been found to exhibit biological activities [5–8]. In
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addition, complexes of Schiff base ligands derived from pyridine derivatives with some transition metals such as Cu(II), Co(II), Ni(II), Fe(III) and Zn(II) showed significant biological notification including antimicrobial, antibacterial, antifungal and anticancer activities [9–22]. Moreover, luminescent compounds are attracting much current research interest because of their many applications including emitting materials for organic light emitting diodes, light harvesting materials for photocatalysis and fluorescent sensors for organic or inorganic analytes [23–27]. On the other hand, the coordination chemistry of lanthanides has received interest because of its intriguing variety of architecture and potential applications in various areas such as magnetism, optics, electronics and catalysis, sensors in the biomedical field area [28–31]. The decomposition of hydrogen peroxide has been used as a model reaction for the investigation of the catalytic activity of various metal complexes [32]. The catalytic activity of copper (II) complex of chitosan derived from chitosan and 4,6-diacetylresorcinol was investigated on hydrogen peroxide decomposition [33]. The copper chelate showed high efficiency toward the decomposition of hydrogen peroxide as heterogeneous catalyst. Based on the previous considerations, we are initiating a line of investigation on the coordination chemistry of Schiff base compounds. In previous work, we have described the complexes produced from the reactions of [M(CO)6], M = Cr, Mo and W with 2-[(pyridin-2-ylmethylidene)amino]-6-aminopyridine (L) [34]. In the present manuscript, the reactions of Cu(II), Ni(II), Co(II), La(III) and Sm(III) with the Schiff base (L) are reported.
gen (CHN) were performed on a Perkin-Elmer 2400 CHN elemental analyzer. Mass spectrometry measurement of the solid complex was carried out on a JEOL JMS-AX 500 spectrometer. Thermogravimetric analyses (TG and DTG) were carried out under N2 atmosphere with a heating rate of 10 °C/min. using a Shimadzu DT-50 thermal analyzer. All conductivity measurements were performed in DMF (1 103 M) at 25 °C, by using Jenway 4010 conductivity meter. The Photoluminescent properties of all compounds were studied using a Jenway 6270 Fluorimeter. Synthesis of the Schiff base ligand (L) An ethanolic solution of 2-pyridinecarboxaldhyde (1.00 g, 9.33 mmol) was mixed with a hot ethanolic solution of 2,6-diaminopyridine (1.02 g, 9.33 mmol) with constant stirring for 15 h. The color of the reaction mixture was changed from pale yellow to brown color with the formation of a brown precipitate. The reaction mixture was evaporated on a hot plate to its half initial volume, then left to cool. The isolated brown solid Schiff base was washed several times with ethanol. Scheme 1 represents the reaction expresses the synthesis of the Schiff base ligand 2-[(pyridin-2-ylmethylidene)amino]-6-aminopyridine (L). Synthesis of metal complexes
2-Pyridinecarboxaldehyde, 2,6-diaminopyridine and the metal salts; CuCl22H2O, NiCl26H2O, CoCl26H2O, La(NO3)36H2O and Sm(ClO4)36H2O were provided by Aldrich Chemicals. All solvents were of analytical grade and were purified by distillation before use.
All the metal complexes of the Schiff base L were synthesized by template method. To a hot solution of the Schiff base ligand (L) in 20 mL of a mixed solvent (DMF:THF; 1:2), an equimolar amount of a hot aqueous solution of the metal salt (Cu2+, Ni2+, Co2+, La3+ or Sm3+) was added slowly. The resulting mixture was refluxed for 6 h, and then evaporated on a hot plate to its half initial volume. After leaving the mixture to cool overnight, the formed solid complex was separated by filtration then washed with ethanol followed by petroleum ether. Finally, the complex was left to dry at room temperature. Table 1 gives the elemental analysis and physical data of the formed complexes.
Instruments
Catalytic activity
Infrared measurements using (KBr pellets) were carried out on a Unicam-Mattson 1000 FT-IR spectrometer and magnetic measurements of the complexes in the solid state (Gouy method) were recorded on a Sherwood magnetic susceptibility balance. 1H NMR measurements were performed on a Varian-Mercury 300 MHz spectrometer. Elemental analyses for carbon, hydrogen, and nitro-
The catalytic activity of the complexes has been evaluated by recording the rate of decomposition of hydrogen peroxide of known molarity. The metal complex (4.1 103–8.2 103 mmol) was mixed with 50 mL of H2O2 (0.15 N) in a flask under constant stirring at room temperature. The extent of decomposition was estimated by titrating 5 mL aliquots of reaction mixture with
Experimental Materials and reagents
H2N
N
NH2
+ N
N
Ethanol N
CHO
2 -pyridinecarboxaldehyde
2,6-diaminopyridine
NH2
N
C H Schiff base ligand
Scheme 1. Preparation of Schiff base ligand 2-[(pyridin-2-ylmethylidene)amino]-6-aminopyridine (L).
Table 1 Analytical and physical data of the ligand 2-[(pyridin-2-ylmethylidene)amino]-6-aminopyridine (L) and its complexes. Compound
M.Wt.
Color (% yield)
Elemental analysis found (Calc.)
C11H10N4 (L) [Cu(C11H10N4)Cl2]2H2O [Co(C11H10N4)Cl2]2H2O [Ni (C11H10N4)Cl2]2H2O [La(C11H10N4)3](NO3)33H2O [Sm(C11H10N4)(ClO4)3]3H2O
198.22 368.69 364.08 363.85 973.62 700.95
Pale yellow (88) Black (96) Dark green (95) Dark red (69) Brown (80) Black (79)
66.45 35.45 36.48 36.23 40.41 18.65
C%
H% (66.65) (35.83) (36.28) (36.31) (40.71) (18.85)
5.12 3.90 3.96 3.61 3.51 2.41
mp (°C)
A (X1 mol1 cm2)
250 >300 >300 >300 >300 >300
– 2 5 11 360 13
N% (5.08) (3.82) (3.87) (3.87) (3.72) (2.30)
28.75 15.36 15.45 15.24 21.48 08.00
(28.26) (15.19) (15.38) (15.39) (21.58) (07.99)
O.A.M. Ali et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 144 (2015) 99–106
101
Table 2 IR data of the Schiff base ligand (L) and its complexes (cm1). Compound
L [Cu(L)Cl2]2H2O [Co(L)Cl2]2H2O [Ni (L)Cl2]2H2O [La(L)3](NO3)33H2O [Sm(L)(ClO4)3]3H2O a
IR(cm1)a
t(NH2) or t(H2O)
t(C@N)
d(py)
t(M-N)
Other bands
3333(b) 3332(b) 3331(b) 3329(b) 3328(b) 3395(b)
1640(w) 1588(s) 1651(s) 1590(s) 1652(s) 1585(sh) 1652(s) 1590(sh) 1655(s) 1589(s) 1654(m) 1590(sh)
619(w) 658(w) 664(w) 651(w) 665(w) 678(w)
– 430(w) 414(w) 425(w) 430(w) 420(w)
– – – – 1383(s) t(NO3) (ionic) 1146(s), 1112(s), 1086(s), 630(m) t(ClO4) (bidentate)
3280(m) 3387(b) 3279(b) 3288(sh) 3278(b)
s, strong; m, medium; w, weak; sh, shoulder.
0.01 M KMnO4 in the presence of 0.01 M H2SO4 at different intervals of time (each 30 min; from 0 to 4.5 h). The difference in titer values of the KMnO4 solution in each interval was recorded. Antibacterial activity The in vitro antimicrobial activities of the free ligand and its complexes were tested against the bacteria: Staphylococcus aureus (gram +ve) and Escherichia coli (gram ve) in Mueller Hinton-Agar medium. The standard disc diffusion method was followed to determine the antibacterial activity of the synthesized compounds. The minimum inhibitory concentration of the ligand and complexes were ascertained using the different bacteria. The concentration of the drug solution was maintained to be 200 lg/mL in DMSO. The well (8 mm diameter) was filled with the test solution and the plates were inoculated at 37 °C for 48 h. During this period, the growth of the inoculated microorganisms was affected and then the inhibition zones developed on the plates were measured. The effectiveness of an antibacterial agent was assessed by measuring the zones of inhibition around the well. The diameter of the zone is measured to the nearest millimeter (mm). The antibacterial activity of each compound was compared with that of standard antibiotics such as Tetracycline. DMSO was used as a control under the same conditions for each organism and no activity was found. The activity results were calculated as a mean of triplicates.
deformation band of py in free ligand L (619 cm1) was shifted to higher frequencies (651–678 cm1) confirming the coordination of pyridyl nitrogen to metal. The triply split band maxima at 1146, 1112, 1086 cm1 and the medium band at 630 cm1 appeared in the IR spectrum of samarium complex were attributed to the bidentately coordinated perchlorate anion [35–37]. The band at 1383 cm1 in the IR spectrum of La(III) complex indicated the existence of free nitrate group in the coordination sphere [38]. The appearance of new bands in the IR spectra of the complexes in the region 414–430 cm1 were assigned to m (M–N) stretching vibrations. 1
H NMR spectra
The 1H NMR spectrum of the free Schiff base ligand L showed signals at 8.05 and 9.99 ppm, attributed to the azomethine (– CH@N–) and NH2 protons, respectively. The signal at 7.97 ppm in the 1H NMR spectrum of the La(III) complex showed a higher field shift relative to that of ligand L, indicating the coordination of the lanthanum atom to the nitrogen atom of the azomethine group in L. In addition, the 1H NMR spectrum of the La(III) complex displayed a signal at 9.99 ppm without any field shift relative to that of ligand indicating the nonparticipation of NH2 group in bonding with the metal [34]. The chemical shifts of the different types of protons in the ligand and its La(III) complex are listed in Table 3.
Results and discussion Mass spectra analysis All solid metal complexes are colored, and stable in air and moisture at room temperature. The analytical data along with some physical properties of the Schiff base ligand and its complexes are listed in Table 1. The data are consistent with the proposed molecular formula. IR spectra The data of the IR spectra of Schiff base ligand and its metal complexes are listed in Table 2. The IR spectra of the complexes were compared with those of the free ligand in order to determine the involvement of coordination sites in chelation. Characteristic peaks in the spectra of the ligand and the complexes were considered and compared. The IR spectrum of the ligand L showed two bands at 3333 and 3280 cm1 due to mas(NH2) and ms(NH2). The IR spectra of the complexes exhibited a broad band around 3395–3278 cm1 assigned to m (OH) of crystalline or coordinated water molecules associated with the complex. This band may be overlapped with the bands corresponding to the stretching vibrations of m (NH2) group. In addition, the IR spectrum of L displayed two bands at 1640 and 1588 cm1 due to t(C@N) of azomethine and pyridyl moieties, respectively. The shifts of t(C@N) for azomethine group to 1651–1655 cm1 and for pyridyl group to 1585– 1590 cm1 on complexation, suggested the coordination of metal ion to azomethine and pyridyl nitrogen atoms. The in-plane ring
The molecular formulae of the investigated compounds were substantiated by mass spectra. The Mass spectrum of L exhibited a molecular ion peak at m/z 198 (S. 1). The ion peaks at 55, 69, 143 and 180 referred to the ions [C3H5N]+, [C3H5N2]+, [C8H5N3]+ and [C11H6N3]+, respectively. The most prominent mass spectral peaks of the reported complexes are given in Table 4. The mass spectra of Cu(II), Ni(II), Co(II) and Sm(III) complexes are recorded (S. 2 and 3). The mass spectrum of Cu(II) complex (S. 2(a)) revealed a molecular mass peak at m/e = 368 corresponding to the formula [Cu(L)Cl2]2H2O (M.Wt. = 368.69). The existence of the ligand was indicated by the peak at 197 (Calc. 198.22). The spectrum exhibited a peak at 333 corresponding to the formula [Cu(L)Cl2] (M.Wt. = 332.66) after removal of the two water molecules from the complex.
Table 3 H NMR spectral data of the Schiff base (L) and La(III) complex.
1
Compound
Chemical shift, d(ppm)a
L
7.73–7.70 m (4H, pyridine), 7.95–7.92 m (3H, pyridine), 8.05 s (1H, HC = N), 9.99 s (2H, NH2). 7.31–7.05 m (4H, pyridine), 7.529–7.47 m (3H, pyridine), 7.97 s (1H, HC = N), 9.99 s (2H, NH2).
[La(L)3](NO3)33H2O a
s, singlet; m, as,
a a
s, singlet; m, multiplet.
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Table 4 The most relevant mass spectral peaks of Cu(II), Co(II), Ni(II) and Sm(III) complexes. Compound
m/e Found (calcd.)
Relative intensity (%)
Loss moiety
Peak assignment due to loss moiety
[Cu(L)Cl2]2H2O
368 (368.69) 333 (332.66) 264 (261.54) 64 (63.54)
47.79 58.76 62.83 48.67
– 2H2O 2Cl L
– [Cu(L)Cl2] [Cu(L)] Cu
[Co(L)Cl2]2H2O
365 (364.08) 274 (274.86) 58 (58.93)
15.12 13.49 14.16
– 2Cl + H2O H2O + L
– [Co(L)(H2O)] Co
[Ni(L)Cl2]2H2O
364 (363.85) 328 (327.82) 258 (256.91) 59 (58.71)
45.64 38.61 34.90 36.24
– 2H2O 2Cl L
– [Ni(L)Cl2] [Ni(L)] Ni Ni
[Sm(L)(ClO4)3]3H2O
703 645 450 348 150
23.22 29.50 31.12 33.30 29.00
– 3H2O 2ClO4 ClO4 L
– [Sm(L)(ClO4)3] [Sm(L)(ClO4)] [Sm(L)] Sm
(700.95) (646.90) (448.00) (348.55) (150.33)
The mass spectrum of cobalt complex (S. 2(b)) showed a molecular ion peak at 365 (F.Wt. = 364.08) with the formula [Co(L)Cl2]2H2O. The existence of the ligand was indicated by the peak at 199 (Calc. 198.22). The peak at 274 (Calc. 274.86) referred to the formula [Co(L)(H2O)] by removal of a water molecule and two chlorine atoms from the complex. The mass spectrum of nickel complex (S. 2(c)) exhibited a molecular ion peak at 364 (F.Wt. = 363.85) with the formula [Ni(L)Cl2]2H2O. The existence of the ligand in nickel complex was assigned by the peak at 198 (Calc. 198.22) while the peak at 328 (Calc. 327.82) was referred to the removal of the two water molecules from the complex. The mass spectrum of samarium complex (S. 3) exhibited a molecular ion peak at 703 (F.Wt. = 700.95) with the formula [Sm(L)(ClO4)3]3H2O. The existence of the ligand in complex was assigned by the peak at 198 (Calc. 198.22) while the peak at 645 (Calc. 646.90) was referred to the removal of the three water molecules from the complex.
363 nm, the first band was attributed to p–p⁄ transitions and the other two bands were referred to n–p⁄ transitions, respectively. The band appeared at 395 nm was corresponded to charge transfer transition. The diffuse reflectance spectrum of Cu(II) complex showed one broad band d–d absorption band at 612 nm, suggesting a square-planar geometry [40]. The observed magnetic moment of Cu(II) complex is 1.98 B.M. The Ni(II) complex has magnetic moment value of 3.8 B.M. This magnetic moment value is in the normal range observed in tetrahedral Ni(II) complex. The electronic spectrum of Ni(II) complex displayed three bands in the solid reflectance spectrum at m1 (605 nm: 3T1 (F) ? 3T2 (F)); m2 (521 nm: 3T1 (F) ? 3A2 (F)) and m3 (471 nm: 3T1 (F) ? 3T1 (P)) [41] suggesting the tetrahedral structure around Ni(II) ion. The diffuse reflectance spectrum of the Co(II) complex showed three
Molar conductivity measurements The measured Molar conductance values of 103 M metal complexes in DMF are listed in Table 1. These values indicated that the complexes are non-electrolytes except La(III) complex. The molar conductance value of La(III) chelate was found to have a value of 360 ohm–1 mol–1 cm2 indicating the ionic nature of this complex. Accordingly, the La(III) complex can be considered as 3:1 electrolyte [39]. Magnetic measurements and electronic absorption spectra The diffuse reflectance spectra of L and its complexes are listed in Table 5. The ligand L showed three bands at 251, 337 and
Table 5 The diffuse reflectance data of the Schiff base (L) and its complexes. Compound
L [Cu(L)Cl2]2H2O [Co(L)Cl2]2H2O [Ni(L)Cl2]2H2O [La(L)3](NO3)33H2O [Sm(L)(ClO4)3]3H2O
kmax (nm)
p–p⁄
n–p⁄
251 252 252 278 244 244
337, 337 341 348, 347, 345,
363
383 384 417
Fig. 1. Emission spectra of: (1); Ligand (L); (2) [La(L)3](NO3)33H2O; (3) [Cu(L)Cl2]2H2O; (4) [Sm(L)(ClO4)3]3H2O; (5) [Ni(L)Cl2]2H2O and (6) [Co(L)Cl2]2H2O.
Table 6 Fluorescence data of the Schiff base (L) and its complexes.
Charge transfer
d–d transition
Compound
kexcitation
kemission
395 396 396 476 473 480
– 612 471, 521, 615 518, 541, 605 – –
L [Cu(L)Cl2]2H2O [Co(L)Cl2]2H2O [Ni(L)Cl2]2H2O [La(L)3](NO3)33H2O [Sm(L)(ClO4)3]3H2O
320 362 326 330 318 328
358 434 364, 435 384 357 359, 425
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bands at 471, 521 and 615 nm. The observed bands are assigned to the transitions 4A2 ? 4T2 (m3), 4A2 ? 4T1(F) (m2) and 4A2 ? 4T1(P) (m1), respectively, suggesting the tetrahedral structure around Co(II) ion [42]. The magnetic moment value of Co(II) complex has been found to be 4.42 BM within the range of tetrahedral geometry [43]. The diffuse reflectance spectra in both La(III) and Sm(III) complexes showed a band at 244 nm due to p–p⁄ transitions and two bands at 347 and 384 nm corresponded to n–p⁄ transitions for La(III) complex, while the two bands at 345 and 417 nm were corresponded to n–p⁄ transitions for Sm(III) complex. Furthermore, the two bands appeared at 473 and 480 NM were corresponded to ligand to metal charge transfer in the lanthanum and samarium complexes, respectively. As expected for the diamagnetic La(III) (d0) configuration, the ligand field band due to d–d electronic transitions is not expected [44].
spectrum of Cu(II), Ni(II), Co(II), La(III) and Sm(III) complexes exhibited strong fluorescence emission bands in the range 357– 435 nm when excited at 318–362 nm. Significant difference in the position of emission maximum of Schiff base and each complex reinforces the coordination of the metal ion to the ligand. The fluorescence spectral results reveal that the fluorescence emission intensity of Schiff base increases dramatically on complex formation with transition metal ions. Enhancement of fluorescence through complexation is much interesting as it opens up the opportunity for photochemical applications of these complexes [45]. In general, all the synthesized compounds can serve as potential photoactive materials, as indicated from their characteristic fluorescence properties.
Thermal analysis (TGA and DTA) Luminescence spectral study The fluorescence properties of the Schiff base ligand and its complexes in DMSO (Fig. 1) were recorded at room temperature. The excitation spectra of the ligand showed a maximum emission peak at 355 nm when excited at 320 nm. Generally, Schiff base systems exhibited fluorescence due to intraligand p?p⁄ transitions. The fluorescent data are summarized in Table 6. The excitation
The thermal studies of metal complexes were carried out using the thermogravimetric technique (TG) and differential thermogravimetric (DTG). Table 7 gives the detailed thermal decomposition data for complexes. Thermogravimetric analysis is considered as a tool to distinguish between the position of water molecule(s) in the complex; if it is in the inner or outer coordination sphere.
Table 7 Thermogravimetric data of complexes. Complex
TG range (°C)
Mass loss % Calc.
Found
Assignment
Metallic residue Calc. (found)
[Cu(L)Cl2]2H2O
36–171 171–383 383–813
9.77 19.24 53.76
9.75 19.25 53.25
Loss of 2H2O Loss of 2Cl Loss of L
Cu 17.23% (17.75%)
[Co(L)Cl2]2H2O
36–161 161–430 430–820
9.89 19.48 54.44
9.92 19.39 54.04
Loss of 2H2O Loss of 2Cl Loss of L
Co 16.17% (16.65%)
[Ni(L)Cl2]2H2O
35–53 53–244 244–732
4.95 24.44 54.47
4.84 24.03 54.97
Loss of H2O Loss of 2Cl + H2O Loss of L
Ni 16.14% (16.16%)
[La(L)3](NO3)33H2O
35–115 115–335 335–577
5.55 19.11 61.07
5.51 19.27 61.39
Loss of 3H2O Loss of 3NO3 Loss of 3L
La 14.27% (13.83%)
[Sm(L)(ClO4)3]3H2O
42–168 168–435 435–771
7.71 28.39 42.45
7.77 28.00 43.23
Loss of 3H2O Loss of 2ClO4 Loss of ClO4 + L
Sm 21.45 (21.00)
Table 8 The kinetic and thermodynamic data of the thermal decompositions of complexes. Complex
Decomposition range (°C)
E⁄ (kJ mol1)
A (S1)
S⁄ (kJ mol1)
H⁄ (kJ mol1)
G⁄ (kJ mol1)
7
[Cu(L)Cl2]2H2O
36–95 100–370 528–809
51.39 36.87 81.05
1.9 10 3.3 102 9.8 102
91.15 –193.66 –191.50
50.94 35.15 77.08
55.90 75.16 168.39
[Co(L)Cl2]2H2O
36–96 96–351 538–750
36.04 36.97 78.84
2.9 103 3.1 102 9.8 102
136.71 –195.50 –195.10
35.63 34.92 72.42
43.77 83.10 214.36
[Ni(L)Cl2]2H2O
35–115 431–583 584–701
40.04 6.46 255.42
2.1 105 1.3 102 7.5 1011
128.57 –286.52 –23.79
39.59 1.68 220.14
46.51 166.23 235.24
[La(L)3](NO3)33H2O
36–100 185–321 360–432 457–577
72.20 43.04 123.14 160.57
2.2 1010 1.05 103 2.6 108 8.2 109
33.78 –186.54 –86.65 –59.55
71.68 40.72 119.62 156.34
73.80 92.67 156.18 186.63
[Sm(L)(ClO4)3]3H2O
42–98 98–236 476–700
44.32 36.16 23.77
6.79 105 1.02 103 8.9
121.45 –184.15 –250.65
43.73 34.47 19.40
52.45 71.83 150.97
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The TGA curves of Cu(II) (S. 4) showed that the thermal decomposition takes place at a temperature range 36–813 °C. The first decomposition step occurred at 36–171 °C due to weight loss of 9.75% (Calc. 9.77%) corresponded to the elimination of two H2O molecules. The second stage of decomposition was due to the loss of two chlorine atoms, while the third stage of decomposition at 383–813 °C was referred to the loss of a ligand molecule to give finally copper metal as a residue. The TG plot of Co(II) (S. 5) displayed three decomposition steps. The first stage takes place in the 36–161 °C range corresponding to the release of two H2O molecules (Obs. 9.92%, Calc. 9.89%). The second and third decomposition steps involved the removal of two chlorine atoms and an organic ligand molecule, respectively, leaving cobalt metal as a residue. The TG/DTG curves of Ni(II) complex (S. 6) showed three decomposition steps within the temperature ranges 35–53 °C, 53–244 °C and 244–732 °C. The weight loss of first step (Obs. = 4.84%, Calc. = 4.95%), the second step (Obs. = 24.03%, Calc. = 24.44%) and third step (Obs. = 54.97%, Calc. = 54.47%) were assigned to the loss of H2O, (H2O + 2Cl) and L, respectively. The TG plot of [La(L)3](NO3)3.3H2O (S. 7) displayed three decomposition steps. The first step occurred in the range 35– 115 °C is due to release of three molecules of H2O. The second step occurred in the range 115–335 °C with a weight loss of 19.27% (Calc. 19.11%) could be attributed to the loss of the three nitrate groups. The third step occurred in the range 335–577 °C corresponded to the loss of three ligand molecules leaving lanthanum metal as a residue. The TG plot of [Sm(L)(ClO4)3]3H2O (S. 8) displayed three decomposition steps. The first step decomposition step is consistent with loss of three molecules of H2O. The second step occurred in the range 168–435 °C with a weight loss of 28.00% (Calc. 28.39%) could be attributed to the loss of 2(ClO 4 ) groups. The third step occurred in the range 435–771 °C corresponded to the loss of (ClO-4 ) group and organic ligand moiety leaving samarium metal as a residue. Thermal studies of the metal complexes confirmed the molecular formulae of the studied complexes as: [Cu(L)Cl2]2H2O, [Co(L)Cl2]2H2O, [Ni(L)Cl2]2(H2O), [La(L)3](NO3)33H2O and [Sm(L)(ClO4)3]3H2O.
tion (G⁄) were evaluated graphically by employing the Coats-Redfern relation [46].
log
logðW 1 =ðW 1 WÞÞ T2
¼ log
AR 2RT E 1 /E E 2:303RT
ð1Þ
where W1 is the mass loss at the completion of the decomposition reaction, W is the mass loss up to temperature T, R is the gas constant and / is the heating rate. Since 1–2RT/E⁄ ffi 1, the plot of the left-hand side of Eq. (1) against 1/T would give a straight line. E⁄ was then calculated from the slope and the Arrhenius constant, A, was obtained from the intercept. The other kinetic parameters; the entropy of activation (S⁄), enthalpy of activation (H⁄) and the free energy change of activation (G⁄) were calculated using the following equations:
S ¼ 2:303R log
Ah kT
ð2Þ
H ¼ E RT
ð3Þ
G ¼ H TS
ð4Þ
where, (k) and (h) are the Boltzman’s and Planck’s constants, respectively. The kinetic parameters are listed in Table 8. From the obtained results, it is apparent that G⁄ values of the complexes acquire highly positive magnitudes. The activation energies of
H2N N
N
CH N
N
NH2
N
La
HC
(NO3)3 . 3 H O. 2
N N
N C H
N
Kinetic data
NH2
The thermodynamic activation parameters of decomposition processes of complexes namely activation energy (E⁄), enthalpy (H⁄), entropy (S⁄) and Gibbs free energy change of the decomposi-
(a)
N O2Cl
O2Cl
CH
O O O O
Sm
N
N
NH2
3 H2O.
O O ClO2
(b) Scheme 2. Proposed structure of (M = Cu(II), Ni(II) and Co(II)) complexes.
Scheme 3. Proposed structures of (a) La(III) complex and (b) Sm(III) complex.
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decomposition were found to be in the range 6.4–255.4 kJ mol1. The high activation energy values of the complexes revealed the high stability of the investigated complexes due to their covalent character. The negative values of S⁄ for the degradation process indicated more ordered activated complexes than the reactants and assigned to slow decomposition reaction.
position of 38.6–52.2% of H2O2 catalyzed by complexes. In the absence of complexes, the decomposition percentage of hydrogen peroxide was found to be 20% after 24 h. The mechanism of the catalytic decomposition reaction of H2O2 has been proposed in the literature [33] as given in formulas (1)– (3):
Structural interpretation
H2 O2 ! HO2 þ Hþ
The structures of the studied Cu(II), Ni(II), Co(II), La(III) and Sm(III) complexes were characterized by elemental analyses, molar conductance, magnetic, solid reflectance and thermal analysis data. From IR spectra, it could be concluded that L behaves as a bidentate ligand coordinated to the metal ions via nitrogen atoms of both pyridine and azomethine group. From the molar conductance data, it was found that the complexes are non-electrolytes except La(III) complex. Accordingly, the structures of the complexes could be proposed as shown in Schemes 2 and 3. Catalytic activity The decomposition of hydrogen peroxide has been used as a model reaction for the investigation of the catalytic activity of various metal complexes. The decomposition of H2O2 catalyzed by metal complexes has been monitored by titrating the undecomposed H2O2 with a standard KMnO4 solution. Variables such as the time were found to have important roles in the decomposition of H2O2. The effect of time on decomposition of H2O2 at a constant concentration of H2O2 (0.15 N) and constant concentration of complexes (4.1 103–8.2 103 mmol1) at 25 °C was studied (Table 9). The conditions of catalytic decomposition reactions of hydrogen peroxide were chosen according to the similar studies in literatures [34,47]. The results showed that the decomposition percentage of hydrogen peroxide increased with time (Fig. 2). It was concluded that 4.5 h is an adequate reaction time for decom-
Table 9 Concentration of the complexes and % of decomposition of H2O2 at constant concentration of H2O2 (0.15 N) at 25 °C after 4.5 h. Compound
Concentration of complex (mmol1)
% of decomposition of H2O2 (%)
[Cu(L)Cl2]2H2O [Co(L)Cl2]2H2O [Ni(L)Cl2]2H2O [La(L)3](NO3)33H2O [Sm(L)(ClO4)3]3H2O
8.1 103 8.2 103 8.2 103 4.1 103 5.7 103
51.0 52.2 38.6 45.8 45.4
Fig. 2. Effect of reaction time on decomposition of H2O2 with [Cu(L)Cl2]2H2O, [Co(L)Cl2]2H2O, [Ni(L)Cl2]2H2O, [La(L)3](NO3)33H2O and [Sm(L)(ClO4)3]3H2O complexes at 25 °C.
ð1Þ
The M-L complex may interact with mediate complex.
HO 2
ions to form an inter-
M-L þ HO2 ! ½MLðHO2 Þ
ð2Þ
A second molecule of H2O2 may then interact with the intermediate complex to form the following products.
½MLðHO2 Þ þ H2 O2 ! M-L þ H2 O þ OH þ O2
ð3Þ
Catalytic decomposition of H2O2 leads to the formation of intermediate radical species which can bind to the surfaces where H2O2 undergoes decomposition [48,49]. The enhanced catalytic activity of the complex system may be explained on the basis of the formation of the above intermediate peroxo species. When a metal complex is bound to a support, its motion is restricted. Reactions on such catalysts are expected to be more rapid than one on which the catalyst molecules are free [50]. Antibacterial activity Antibacterial activity of Schiff base ligand and its complexes have been tested against one gram positive bacteria; S. aureus and also against one gram negative bacteria; E. coli microorganisms. The antibacterial activities obtained for the prepared compounds are listed in Table 10. All the investigated compounds showed a remarkable biological activity against bacteria (Fig. 3). The obtained results reflect that all complexes showed higher
Table 10 Biological activities of Schiff base ligand (L) and its complexes. Compound
L [Cu(L)Cl2]2H2O [Co(L)Cl2]2H2O [Ni(L)Cl2]2H2O [La(L)3](NO3)33H2O Tetracycline antibacterial agent
Diameter of inhibition zone (mm) Escherichia coli (G)
Staphylococcus aureus (G+)
9 10 15 12 13 32
11 12 15 14 14 30
Fig. 3. Antibacterial activity of compounds under study.
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antibacterial activity than ligand. On chelation, the delocalization of p-electrons over the whole chelate ring will be increased which enhances the penetration of the complexes into lipid membranes and blocking the metal binding sites in the enzymes of microorganisms. Also, the tested complexes may disturb the respiration process of the cell and consequently block the synthesis of proteins leading to no further growth of the organisms [51]. Conclusion The novel complexes of copper(II), cobalt(II), nickel(II), lanthanum(III) and samarium(III) with the bidentate ligand 2-[(pyridin-2-ylmethylidene)amino]-6-aminopyridine (L) were synthesized and characterized using spectroscopic methods. The molar conductivity data for complexes indicated that they have a non electrolytic nature except La(III) complex. Fluorescence studies indicated that the synthesized compounds can serve as potential photoactive materials, as indicated from their characteristic fluorescence properties. The activation thermodynamic parameters, such as activation energy, enthalpy, entropy and Gibbs free energy change of complexes decomposition indicated the their stability. The catalytic activities of the complexes toward hydrogen peroxide decomposition reaction indicated enhanced decomposition of H2O2 by metal complexes. Antibacterial activity studies of the investigated compounds showed a remarkable biological activity against bacteria. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2015.02.078. References [1] A.T. Chaviara, P.J. Cox, K.H. Repana, R.M. Papi, K.T. Papazisis, D. Zambouli, A.H. Kortsaris, D.A. Kyriakidis, C.A. Bolos, J. Inorg. Biochem. 98 (2004) 1271–1283. [2] J.A. Ciller, C. Seoane, J.L. Soto, B. Yruretagoyena, J. Heterocyclic Chem. 23 (2009) 1583–1586. [3] C.R. Bhattacharjee, P. Goswami, S. Neogi, S. Dhibar, Assam Univ. J. Sci. Technol.: Phys. Sci. Technol. 5 (2010) 81–87. [4] A. Prakash, B.K. Singh, N. Bhojak, D. Adhikari, Spectrochim. Acta, Part A 76 (2010) 356–362. [5] V.A. Shelke, S.M. Jadhav, V.R. Patharkar, S.G. Shankarwar, A.S. Munde, T.K. Chondhekar, Arabian J. Chem. 5 (2012) 501–507. [6] B. Rizwana, S.L. Santha, Int. J. Chem. Tech. Res. 4 (2012) 464–473. [7] R. Suganthi, S.L. Santha, K. Geetha, A. Abdul Rahuman, J. Pharm. Res. 4 (2011) 4574–4576. [8] S. Zhang, Y. Zhu, C. Tu, H. Wei, Z. Yang, L. Lin, J. Ding, J. Zhang, Z. Guo, J. Inorg. Biochem. 98 (2004) 2099–2106. [9] P.D. Badma, L.S. Santha, Int. J. Chem. Tech. Res. 6 (2014) 87–94. [10] S. Brooker, S.S. Iremonger, P.G. Plieger, Polyhedron 22 (2003) 665–671. [11] M.S. Refat, I.M. El-Deen, H.K. Ibrahim, S. El-Ghool, Spectrochim. Acta, Part A 65 (2006) 1208–1220.
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