КООРДИНАЦИОННАЯ ХИМИЯ, 2007, том 33, № 2, с. 129-134
УДК 541.49
SYNTHESIS, CHARACTERIZATION, AND THERMAL ANALYSIS OF TRANSITION METAL COMPLEXES OF POLYDENTATE ONO DONOR
SCHIFF BASE LIGAND
© 2007 J. Joseph and B. H. Mehta
Department of Chemistry, University of Mumbai, Vidyanagari, Santacruz (E) Mumbai, 400098 India
Received November 9, 2005
Mn(II), Co(II), Ni(II), Cu(II), Cd(II), and Hg(II) metal complexes with Schiff bases derived from 3-formyl-4-hydroxycoumarin and semicabazone are synthesized and characterized on the basis of elemental analysis, molar conductance, magnetic moment, IR, electronic, XH NMR spectrum, and ESR spectrum, TGA, and X-ray diffraction powder methods. Molar conductance values indicate that the complexes are nonelectrolytic in nature. Magnetic moment and spectral studies suggest either tetrahedral or square-planar geometry around the central metal ions. The analytical data indicate that metal-to-ligand stoichiometry in all complexes is 1 : 1.
The chemistry of transition metal complexes of Schiff bases has played an important role in the development of coordination chemistry as a whole. Multidentate Schiff bases have been widely used as ligands, because they can be easily attached to metal ions due to high stability of coordination compounds. Metal complexes of S-, N-, and O-chelating ligands have attracted considerable attention because of their interesting physico-chemical properties pronounced biological activities, and as models of metalloenzyme active sites [1, 2]. Schiff bases and their transition metal complexes have been used in anticancer, antitubercular, antibacterial, antifungal, hypertensive, and hypothermic reagents [3-5]. Transition metal complexes are used as catalysts for many organic reactions [6, 7]. Schiff bases derived from coumarin and its metal complexes have been found to exhibit antibacterial, antifungal, anticoagulation, and plant regulating activities [8-10]. Literature survey reveals that comparatively less work has been carried out on the transition metal complexes of Schiff bases derived from 3-formyl-4-hydroxycoumarin and semicarbazone.
In this article, we describe the synthesis and characterization of transition metal complexes of Schiff base derived from 3-formyl-4-hydroxycoumarin and semicarbazone with Mn(II), Co(II), Ni(II), Cu(II), Cd(II), and Hg(II). The structure of the ligand is the following:
(HL)
EXPERIMENTAL
All the chemicals used for the synthesis were of AR grade. 4-Hydroxycoumarin was obtained from Fluka
Ltd., triethylorthoformate, semicarbazone, and metal salts were obtained from S.D. Fine chemicals. Distilled solvents were used throughout the experiments.
3-Formyl-4-hydroxycoumarin was synthesized according to the method reported in [11]. The Schiff base was prepared by refluxing 3-formyl-4-hydroxycou-marin (3.8 g, 20 mmol) in 50 cm3 of ethanol with semicarbazone (2.25 g, 20 mmol) in 20 cm3 of a water-etha-nol mixture for 3 h. The resulting solution was concentrated and then cooled. The solid formed was filtered off, washed with eater and then with ethanol and dried in oven. The Schiff bases were recrystallized from ethanol, m.p. being 230°C.
The metal complexes were synthesized by mixing a hot methanolic solution of the ligand with a hot metha-nolic solution of metal salts in a ratio 1 : 1. The resulting mixture was refluxed for 3 h in a water bath and cooled. The precipitate formed was filtered, washed with water and then with hot ethanol, and dried in oven. In the case of the Mn(II) complex, the pH of the solution was adjusted in the range 6.5-8.0 by adding of alcoholic ammonia, and the reaction mixture was digested for 30 min. The obtained metal complex was filtered, washed with water and then with hot ethanol, and dried in oven.
The melting point of all complexes was determined by the open capillary method. Elemental analysis was carried out at the Micro-analytical laboratory (University of Mumbai). The metal content of all the metal complexes was determined by the reported method [12]. The complexes were examined for solubility using various solvents. Molar conductivities of the ligand and complexes were recorded using 1 x 10-3 M solutions in DMF on a Toshniwal TSM-15 Conductivity meter. The electronic absorption spectra of the ligand and complexes were recorded in the UV-Vis region using DMF as solvent on a UV-Vis 2100 spectrophoto-
meter (M/s Shimadzu Corporation). IR spectra were recorded using KBr pellets on a FTIR 4200 spectrophotometer (M/s Shimadzu Corporation). Magnetic susceptibility measurements were made on Gouy's balance using Hg[Co(NCS)4] as reference. Thermogravimetric analyses were carried out on Perkin Elmer instrument, Pyris Diamond TG/DTA analyzer in a static nitrogen atmosphere with a heating rate of 10 K/min. The XH NMR spectrum of
O II
+ H2N—NH-C-NH
CHO
OH
The general equation for the preparation of the complexes is illustrated by the following equation:
M"+ + H2L
ML + 2H+
The Schiff base and its metal complexes are very stable at room temperature in the solid state. The ligand is soluble in common organic solvents. However, its metal complexes are generally soluble in DMF and DMSO. The color, melting point, elemental analysis, and molar conductance of the ligand and its metal complexes are given in Table 1. The analytical data of the
ligand was recorded using a Brucker Spectrospin at 400 MHz. X-ray diffraction spectra were recorded on an X-ray diffractometer (M/s Philips, Holland).
RESULTS AND DISCUSSION
Condensation of 3-formyl-4-hydroxycoumarin with semicarbazone readily gives rise to corresponding Schiff base H2L
O^ ^O
2
O
CH=N—NH— C —NH2
(H2L)
OH
complexes suggest 1 : 1 metal-to-ligand stoichiometry. The molar conductivity of 1 x 10-3 solutions of the metal complexes in DMF falls in the range from 4 x 10-3 to 8 x 10-3 Ohm cm2 mol-1. Such low conductance value indicates their non-electrolytic nature [13].
Table 2 presents the most important IR spectral bands of the ligand and its metal complexes. A comparison of the characteristic IR absorption bands of the ligand with those of the corresponding metal complexes reveals an important features. 4-Hydroxycoumarin is known to exist in tautomeric (a) lactone and (b) chromone forms [14].
The IR band for v(C=O) of the chromone form generally appears in the range from 1650 to 1690 cm-1, while that of v(C=O) of the lactone form appears in the range from 1700 to 1720 cm-1. The IR spectrum of ligand H2L shows an intense band at 1692 cm-1. From this data it is concluded that this ligand exists in the chromone form.
The IR spectrum of the ligand shows a sharp band at 3408 cm-1, which can be due to v(OH). The observed low frequency of this band is due to intermolecular hydrogen bonding between the H atom of the OH group and azomethine nitrogen atom. The bands at 3283 and 3198 cm-1 are due to the asymmetric and symmetric stretching of the NH2 group. On complexation the band due to the v(OH) vibration disappears, which indicates the deprotonation of the phenol group and coordination of the oxygen atom to the metal ions. However, the bands due to the free NH2 group remain at the same position and indicate that the NH2 group is not involved in bond formation. The ligand shows bands at 1692, 1673, and 1643 cm-1, which are assigned to v(C=O) chromone,
v(C=O) semicarbazone, and azomethine groups [15]. The lowering of the v(CH=N) vibration of the azome-thine group to the extent of 15 to 25 cm-1 in all complexes indicates the participation of the azomethine nitrogen in the bond formation. The band due to v(C=O) semicarba-zone disappears in all complexes due to the tautomeriza-tion of semicarbazone from the keto to enol form during the reaction and then is coordinated through the deproto-nated oxygen [16]. This is confirmed by the appearance of a new band at approximately 1660 cm-1 due to the azomethine group. The ligand exhibits the v(C=O) stretching vibration at 1309 cm-1, while on complexation the v(C=O) phenolic absorption band appears by 10-20 cm-1 lower than the corresponding band of the free ligand. This indicates the bonding of the phenolic oxygen to the metal ions [17]. The additional bands at 570 and 460 cm-1 are due to the v(M=N) and v(M=O) mode, respectively. The complexes show a broad band in the region from 3300 to 3500 cm-1 due to the coordinated water molecule.
DMSO-d6 was used as a deuterated solvent to measure the 1H NMR spectrum of the ligand. The Mn(II), Co(II),
SYNTHESIS, CHARACTERIZATION, AND THERMAL ANALYSIS Table 1. Analytical and physical data for the ligand and its metal complexes
Compounds Empirical formula Mol. weight Color M.p., °C Molar conductivity, A x 10-3, Ohm1 cm2 mol1 Magnetic moment, M'eff, Mb Content (found/calcd), %
C H N M
HL C11H9N3O4 247 White 230 4.15 54.02 3.25 17.32
(53.44) (3.64) (17.00)
[Mn(L)H2O] C11H9N3O5Mn 317.94 Brown >300 6.25 4.86 41.65 2.88 13.70 17.64
(41.25) (2.50) (13.13) (17.17)
[Co(L)H2O] C11H9N3O5C0 321.93 Orange >300 5.89 1.94 40.26 2.58 13.18 17.93
(40.74) (2.47) (12.96) (18.20)
[Ni(L)H2O] CnH9N3O5Ni 321.71 Gray 285 7.42 Diamagnetic 0.18 2.34 13.15 18.45
(40.77) (2.47) (12.97) (18.13)
[Cu(L)H2O] C11H9N3O5CU 326.54 Gray 280 6.58 1.82 40.20 2.19 12.43 19.68
(40.17) (2.44) (12.76) (19.34)
Cd(L)H2O] CnH9N3O5Cd 375.41 Yellow >300 4.98 Diamagnetic 35.43 2.55 11.45 30.18
(35.17) (2.39) (11.17) (29.92)
[Hg(L)H2O] C11H9N3O5Hg 463.59 Yellow >300 5.32 Diamagnetic 28.88 2.17 9.45 43.58
(28.47) (1.97) (9.05) (43.26)
Table 2. Salient features of IR spectral data on ligands and metal complexes (cm x)
Comlex v(OH) v(C=O) v(C=N) v(C-O) v(M-N) v(M-O)
4308 1692 1634 1309
[Mn(L)H2O] 3472 1690 1613 1296 596 474
[Co(L)H2O] 3400 1690 1618 1264 573 424
[Ni(L)H2O] 3440 1690 1612 1294 571 463
[Cu(L)H2O] 3440 1692 1612 1294 571 463
[Cd(L)H2O] 3500 1692 1609 1295 528 453
[Hg(L)H2O 3500 1692 1606 1274 553 457
and Cu(II) c
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