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Received 14 October 1998; accepted 22 December 1998. Abstract. New Schiff base ligands derived from vanillin (HL1), 4-dimethylaminobenzaldehyde (HL2) ...
525

Transition Metal Chemistry 24: 525±532, 1999.

Ó 1999 Kluwer Academic Publishers. Printed in the Netherlands.

Transition metal complexes of bidentate Schi€ base ligands Mehmet TuÈmer*, Cumali CËelik, HuÈseyin KoÈksal and Selahattin Serin Department of Chemistry, K. MarasË SuÈtcËuÈimam University, K. MarasË 546100, Turkey Received 14 October 1998; accepted 22 December 1998

Abstract New Schi€ base ligands derived from vanillin (HL1), 4-dimethylaminobenzaldehyde (HL2) and 3,5-di-t-butyl-4hydroxybenzaldehyde (HL3) with N-(pyridyl)-3-methoxy-4-hydroxy-5-aminobenzylamine (2) and their copper(II), cobalt(II), nickel(II), oxovanadium(IV) and zinc(II) transition metal complexes have been synthesized and characterized by elemental analyses, electronic and i.r. spectra, molar conductance data and by 1H and 13C n.m.r. spectra. The results indicate that the ligands coordinate through azomethine nitrogen and phenolic oxygen to the metal ions. In like manner, it was found that the pyridine and amine nitrogen atoms are not coordinated to the metal ions. The 1H and 13C n.m.r. spectral data con®rmed the suggested structure for the Schi€ base ligands, and the mass spectra results con®rmed the proposed structure of the ligands. The antimicrobial activity properties of the ligands and their metal complexes have been studied. Introduction Transition metal complexes derived from Schi€ base (SB) ligands have been among the most widely studied coordination compounds in recent years [1±7], since they are becoming increasingly important as biochemical, analytical and antimicrobial reagents. These complexes containing certain metal ions are active in many biological processes. The fact that copper, magnesium, molybdenum, calcium, iron, zinc, chromium and vanadium are essential metallic elements and exhibit great biological activity when associated with certain metal-protein complexes, participating in oxygen transport, electronic transfer reactions or the storage of ions [8], has created enormous interest in the study of systems containing these metals [9]. In some SB-metal chelates, it has been shown that minor changes in the structure of the ligands containing hard soft donor atoms, e.g., nitrogen, sulfur and/or oxygen markedly a€ected the activity of the compounds [10±12]. In the present paper, the preparation, characterization and antimicrobial activity studies of the Schi€ base ligands and their metal complexes have been described. The snythetic process for the preparation of the amine and its Schi€ base ligands are shown in Scheme 1.

4-hydroxybenzaldehyde, 4-dimethylaminobenzaldehyde and Pd/C (10%) were obtained from Fluka. Solvents were of analytical grade and were puri®ed by standard methods. Physical measurements

Experimental

Elemental analyses (C, H, N) were performed on a Carlo Erba 1106 elemental analyser. The i.r. spectra were recorded using KBr discs (4000±400 cm)1) on a Shimadzu 8300 FT-IR spectrophotometer. Electronic spectra in the 200±900 nm range were obtained on a Shimadzu UV160A spectrophotometer. Magnetic measurements were carried out by the Gouy method using Hg[Co(SCN)4] as calibrant. Molar conductances of the Schi€ base ligands and their transition metal complexes were determined in DMSO (ca. 10)3 M) at room temperature using a Toa CM 405 conductivity meter. Metal analysis was carried out by titration with EDTA [13]. 1H and 13C n.m.r. spectra were recorded on a Varian XL-200 n.m.r. instrument. Mass spectra of the ligands were recorded on a VG ZabSpec GC-MS spectrometer with fast atom bombardment. TMS was used as internal standard and deuteriated DMSO as solvent. Thermal analyses were performed on Shimadzu DTA 50 and TG 50 H models using 10 mg samples. The d.t.a. and t.g. curves were obtained at heating rate of 10 °C min)1 in dry N2 atmosphere in the 25±750 °C range.

Materials

N-(pyridyl)-3-methoxy-4-hydroxy-5-nitrobenzaldimine (1)

Metal salts were purchased from E. Merck and were used as received. 5-Nitrovanillin, vanillin, 3,5-di-t-butyl-

A solution of the 5-nitrovanillin (3 mmol, 0.591 g) in absolute EtOH (20 cm3) was added to the solution of 2-aminopyridine (3 mmol, 0.282 g) in absolute EtOH (20 cm3) at 80 °C and the reaction mixture was stirred

* Author for correspondence

526

Scheme 1.

for 3±4 h. It was then cooled to room temperature and solvent was removed in vacuo. The precipitated orange product was ®ltered o€ and washed with C6H14. Yield: (87%), m.p.: 128 °C. Electronic spectra (EtOH, kmax/ nm): 402, 288, 231. I.r. (KBr pellets, cm)1): 3344 (OAH), 1635 (C@N), 1596, 1560 and 1463 (pyridine ring). N-(pyridyl)-3-methoxy-4-hydroxy-5-aminobenzylamine (2) Ligand (1) (2 mmol, 0.556 g) was dissolved in absolute EtOH (100 cm3) and the solution was heated to 80 °C. Pd/C (10%) (0.70 g) was then added to this solution at the same temperature and N2H4 á H2O (20 cm3) (100%) was added dropwise; the mixture was then stirred and boiled under re¯ux for 50 min. After cooling to room temperature, the mixture was ®ltered and the ®ltrate evaporated until no EtOH could be detected. The dirty yellow residue was extracted with CHCl3. The extract was reduced in vacuo and cooled in a refrigerator at )10 °C. The resulting light yellow crystals were ®ltered o€, washed with cold C6H14. Yield: (85.4%), m.p.: 145 °C (dec.). Electronic spectra (EtOH, kmax/nm): 380, 294, 235, 225. I.r. (KBr pellets, cm)1): 3388 (OAH), 3309 (NH2), 3095 (NH), 2910 (CH2), 1600, 1515, 1461 and 1450 (pyridine ring).

Schi€ Base ligands The Schi€ base ligands HL1, HL2 and HL3 were prepared by similar methods. The derivative of the benzaldehyde (5 mmol) (vanillin for HL1, 4-dimethylaminobenzaldehyde for HL2 and 3,5-di-t-butyl-4-hydroxybenzaldehyde for HL3) in absolute EtOH (20 cm3) was carefully added, with stirring, to a solution of freshly prepared N-(pyridyl)-3-methoxy-4-hydroxy-5aminobenzylamine (5 mmol, 1.125 g) in absolute EtOH (30 cm3). The resulting mixture was boiled under re¯ux for 2±3 h, then left for 2±3 h at room temperature. The solvent was removed and the resulting solid was ®ltered o€ and then recrystallized from EtOH, and dried in a vacuum dessicator. Copper(II) complexes A solution of Cu(AcO)2 á H2O (0.5 mmol, 0.100 g) in MeOH (10 cm3) was added to a solution of each ligand (1 mmol) [HL1 (0.379 g), HL2 (0.376 g) or HL3 (0.461 g)] in absolute EtOH (40 cm3). The mixture was stirred and heated to 80 °C and turned brown. The precipitated complexes were ®ltered o€, washed with cold EtOH and then dried in vacuo over P4O10.

527 Cobalt(II) complexes A solution of Co(AcO)2 á 4H2O (0.5 mmol, 0.125 g) in absolute EtOH (20 cm3) was added to a solution of the ligand (1 mmol) [HL1 (0.379 g), HL2 (0.376 g) or HL3 (0.449 g)] in absolute EtOH (40 cm3). The mixture was re¯uxed for 2±4 h while heating at 80 °C. The precipitated compound was ®ltered o€, washed with cold EtOH and dried in vacuo over P4O10. Nickel(II) complexes Nickel(II) complexes of the ligands HL1, HL2 and HL3 were prepared by a procedure analogous to the other complexes, using Ni(AcO)2 á 4H2O as metal salt and absolute EtOH as solvent. Oxovanadium(IV) complexes Oxovanadium(IV) complexes of the ligands HL1, HL2 and HL3 were prepared by a procedure analogous to the other complexes, using VO(SO4) á 5H2O as metal salt and absolute EtOH as solvent. Zinc(II) complexes Zinc(II) complexes of the ligands HL1, HL2 and HL3 were prepared by a procedure analogous to the other complexes, using Zn(AcO)2 á 2H2O as metal salt and absolute EtOH as solvent. Preparation of microbial cultures Bacillus subtilis IMG 22 (bacterium), Saccharomyces cerevisiae WET 136 (yeast), Eschericha coli DM (bacterium), Klebsiella pneumoniea DIG 1319 (bacterium) and Micrococcus luteus LA 2971 (bacterium) were used as the test organisms in an antimicrobial study. The bacteria and yeast strain were inoculated into nutrient broth (Difco) and malt extract broth (Difco) and incubated for 24 and 48 h, respectively. In the Disc Di€usion method, the sterile Mueller Hinton Agar (Oxoid) for bacteria and Sabouraud Dextrose Agar for yeast were separately inoculated with the test microorganisms [14]. The compounds dissolved in DMF as 100 lg/disc solutions and absorbed on the sterile paper antibiotic discs were placed in wells (6 mm diameter) cut in the agar media, and the plates were incubated at 32 °C for bacteria (18±24 h) and at 25 °C for yeast (72 h). The resulting inhibition zones on the plates were measured after 48 h (Table 6). The control samples were only absorbed in DMF. The data reported in Table 6 are the average of three experiments. Results and discussion The analytical data for the HL1, HL2 and HL3 ligands and their metal complexes are listed in Table 1. The

Schi€ base ligands N-(pyridyl)-3-methoxy-4-hydroxy-5nitrobenzaldimine (1), synthesized from 5-nitrovanillin and 2-aminopyridine in absolute EtOH solution, was obtained in near quantitative yield as a microcrystalline solid. The compound is stable at room temperature and soluble in common organic solvents, such as EtOH, CHCl3 and MeOH, but partially soluble in C6H14, PhMe and C7H16. The azomethine and nitro groups of this Schi€ base are reduced to amines using EtOH as solvent and Pd/C (10%) as catalyst with N2H4 á H2O (100%) as reducing reagent. The compound N-(pyridyl)-3-methoxy-4-hydroxy-5-aminobenzylamine (2), obtained in low yield, is unstable at room temperature and decomposes on standing. The new amine compound is soluble in CHCl3, EtOH and MeOH but insoluble in common apolar organic solvents, such as C6H14, PhMe and C7H16. In order to synthesize the bidentate Schi€ base ligands, three derivatives of the benzaldehyde were used. The Schi€ base ligand derived from 3,5-di-t-butyl-4hydroxybenzaldehyde (HL3) has the highest solubility. However, all ligands are soluble in common polar organic solvents, such as EtOH, CHCl3, MeOH, Me2CO, DMF and DMSO, but partially soluble in nonpolar organic solvents such as, C6H14, PhMe and C7H16 and C6H6. All ligands are stable at room temperature and are non-hygroscopic. The yields of the HL1 and HL2 ligands are higher than that for the HL3 ligand. In like manner, the yields of the complexes also are di€erent. In the complexes of the HL3 ligand, the yield is lower than the complexes of the HL1 and HL2 ligands, suggesting that the steric e€ect of the t-Bu groups on the benzeneoid ring reduces the yield of the complexes. All complexes are stable at room temperature. The copper(II) and cobalt (II) complexes of the HL1 and HL3 ligands have the highest solubility in EtOH. However, all complexes are soluble in DMF, DMSO, THF and dioxane. Molar conductivities of the HL1, HL2 and HL3 ligands and their coordination compounds in DMSO were in the 2.5±10.4 W)1 cm2 mol)1 range and are given in Table 2. These results show that the ligand coordinates to the metal ions to form neutral coordination compounds, accompanied by the release of the anion. Their molar conductances are much smaller than those of 1:1 electrolytes [15], thus they are all considered to be non-electrolytes. The i.r. spectral data of the ligands and their metal complexes are given in Table 3. The HL1 and HL3 ligands show strong bands at 3580 and 3602 cm)1 which may be assigned to the vibration of the free OH group. In the complexes, this band does not shift to the lower or higher wave numbers, suggesting that this free OH group is not coordinated to the metal ions. In the spectra of the HL1, HL2 and HL3 ligands, the bands observed in the range 3138±3010 and 2925± 2912 cm)1 range may be assigned to the vibrations of the NH and CH2 groups, respectively. In the complexes, these bands occur in approximately the same ranges, showing that the nitrogen atom of the NH group is not coordinated to the metal ions. The HL1 and HL3 ligands

528 Table 1. Analytical and physical data of compounds Compound

Colour

M.p. (°C)

Yield (%)

Found (Calcd.) (%) C H

N

HL1 [Cu(L1)2] á H2O Co(L1)2 [Ni(L1)2] á 2H2O [VO(L1)2] á H2O Zn(L1)2] á 2H2O HL2 [Cu(L2)2] á 2H2O [Co(L2)2] á H2O [Ni(L2)2] á H2O [VO(L2)2] á 2H2O [Zn(L2)2] á H2O HL3 [Cu(L3)2] á H2O [Co(L3)2] á 2H2O [Ni(L3)2] á H2O [VO(L3)2] á 2H2O [Zn(L3)2] á 2H2O

light yellow light brown red brown green yellow brown yellow yellow dark red dark red orange orange yellow yellow red light brown light brown dark orange light yellow

194 249 >250 >250 240d >250 192 >250 >250 >250 >250 >250 187 >250 >250 >250 >250 >250

75.4 57.9 63.5 57.6 78.7 57.6 80.1 62.3 59.6 55.4 66.3 59.7 68.7 50.3 53.1 50.2 59.4 48.2

66.1 60.2 61.9 59.3 59.9 58.8 70.0 62.2 63.8 63.6 61.9 63.3 68.0 67.1 66.2 67.7 65.7 65.7

10.9 10.1 10.3 9.9 10.0 9.7 14.7 13.2 13.6 13.6 13.1 13.5 15.8 8.4 8.3 9.9 8.2 8.3

d

(66.5) (60.2) (61.8) (59.2) (59.9) (58.8) (70.2) (62.1) (63.9) (63.6) (61.9) (63.4) (68.1) (67.1) (66.2) (67.6) (65.7) (65.8)

5.3 5.0 4.9 5.1 5.1 5.1 6.0 6.0 5.8 5.7 5.9 5.7 5.6 7.0 7.1 7.1 7.1 7.1

(5.5) (5.0) (4.9) (5.2) (5.4) (5.1) (6.4) (5.9) (5.8) (5.8) (5.9) (5.8) (5.7) (7.0) (7.1) (7.0) (7.1) (7.0)

M (11.1) (10.0) (10.3) (9.9) (10.8) (9.8) (14.9) (13.2) (13.5) (13.5) (13.1) (13.4) (15.9) (8.4) (8.3) (9.9) (8.2) (8.2)

± 7.7 7.2 6.9 8.1 7.7 ± 7.7 7.2 7.2 7.9 7.9 ± 6.4 5.9 5.8 6.6 6.4

(7.6) (7.2) (6.8) (8.0) (7.6) (7.5) (7.1) (7.1) (7.8) (7.8) (6.3) (5.8) (5.9) (6.5) (6.4)

dec.

show broad bands in the 2640±2550 and 2650± 2500 cm)1 range, respectively, which can be attributed to the stretching vibration of the OAH group associated intramolecularly with the nitrogen atom of the CH@N group (OAH¼N) (Figure 1). These broad bands disappear in the complexes as a result of proton replacement by cations coordinated to oxygen. In addition, in the spectra of the ligands, bands in the 1625±1593 cm)1 range are due to the vibration of the azomethine group. These are shifted in the complexes towards lower or higher values as a result of coordination of the azomethine nitrogen atom to metal ion. A similar e€ect is observed for the stretching vibration of the Schi€ base phenolic (CAOH) group with respect to the same group in the complex, indicating oxygen

coordination to the metal ion [16]. The ligands show strong bands in the 1517±1425 and 1050±1015 cm)1 ranges which can be attributed to the pyridine ring vibration [17]. In the complexes, these bands are not shifted. From these results, we suggest that the nitrogen atom of the pyridine ring is not coordinated to the metal ion. In the spectra of the complexes containing hydrated water molecules, the di€used character and broadness of the bands in the 3427±3320 cm)1 region are obviously due to the hydrated water molecules [18]. The oxovanadium(IV) complexes show strong bands in the 998± 976 cm)1 range that can be attributed to the stretching vibration of the (V@O) group [19]. In the complexes, the bands in the 617±461 and 459±410 cm)1 range can be attributed to the m(MAN) and m(MAO) stretching.

Table 2. Magnetic moment, molar conductance and electronic spectra of the Schi€ base ligands and their complexes Compound

le€ (B.M.)a

LM (W)1 cm2 mol)1)

kmax/nm (EtOH)

HL1 [Cu(L1)2] á H2O Co(L1)2 [Ni(L1)2] á 2H2O [VO(L1)2] á H2O Zn(L1)2] á 2H2O HL2 [Cu(L2)2] á 2H2O [Co(L2)2] á H2O [Ni(L2)2] á H2O [VO(L2)2] á 2H2O [Zn(L2)2] á H2O HL3 [Cu(L3)2] á H2O [Co(L3)2] á 2H2O [Ni(L3)2] á H2O [VO(L3)2] á 2H2O [Zn(L3)2] á 2H2O

± 1.83 4.13 diamag. 1.66 diamag. ± 1.90 4.23 diamag. 1.68 diamag. ± 1.97 4.50 diamag. 1.73 diamag.

2.8 8.3 9.2 10.4 9.5 7.3 2.5 5.7 5.4 4.7 4.2 3.9 2.7 8.0 8.7 9.1 9.8 5.8

356(sh), 308, 282, 217 621, 354(sh), 347, 238(sh), 219(sh), 209 608(sh), 348, 283(sh), 219(sh), 210 381(sh), 339, 280(sh), 233(sh), 211 713, 480(sh), 346, 233(sh), 212 376(sh), 348, 307, 284, 226(sh), 215 390, 307, 236, 211 625(sh), 393, 316(sh), 244, 205(sh) 638(sh), 393, 316(sh), 244, 205 474(sh), 387, 322(sh), 238, 207 710(sh), 456(sh), 346, 236, 209 392, 325, 240, 206 454(sh), 335, 308(sh), 235, 213 630(sh), 354(sh), 344, 211 613(sh), 354, 344, 235, 207 580(sh), 489, 458, 341(sh), 291, 225 741(sh), 502(sh), 326(sh), 240, 206 452(sh), 344, 289(sh), 232(sh), 210

a

Magnetic moment per metal atom.

529 Table 3. I.r. spectral data of the ligands and their complexes Compound 1

HL [Cu(L1)2] á H2O Co(L1)2 [Ni(L1)2] á 2H2O [VO(L1)2] á H2O Zn(L1)2] á 2H2O HL2 [Cu(L2)2] á 2H2O [Co(L2)2] á H2O [Ni(L2)2] á H2O [VO(L2)2] á 2H2O [Zn(L2)2] á H2O HL3 [Cu(L3)2] á H2O [Co(L3)2] á 2H2O [Ni(L3)2] á H2O [VO(L3)2] á 2H2O [Zn(L3)2] á 2H2O a

m(H2O/OH)

m(NH)

m(CH2)

m(C@N)

m(CAO)a

m(Py)

± 3380 ± 3400 3320 3350 ± 3427 3427 3425 3420 3423 ± 3400 3423 3400 3370 3425

3010 3010 3010 3010 3010 3010 3130 3168 3136 3140 3142 3135 3138 3145 3120 3120 3125 3110

2925 2904 2937 2937 2939 2937 2916 2925 2956 2825 2958 2918 2912 2935 2956 2924 2914 2922

1593 1600 1601 1601 1597 1596 1625 1599 1625 1604 1625 1605 1601 1615 1650 1666 1602 1606

1344 1285 1283 1278 1284 1282 1364 1320 1315 1350 1319 1305 1344 1319 1316 1305 1313 1312

1512, 1555, 1512, 1514, 1518, 1510, 1510, 1529, 1564, 1523, 1596, 1558, 1517, 1507, 1552, 1530, 1521, 1517,

1461 1440 1427 1427 1427 1458 1425 1434 1423 1429 1425 1431 1431 1470 1427 1460 1431 1431

m(V@O)

m(MAN)

m(MAO)

± ± ± ± 988 ± ± ± ± ± 976 ± ± ± ± ± 998 ±

± 520 574 510 492 518 ± 534 540 538 461 495 ± 461 605 589 617 610

± 418 457 459 455 443 ± 455 440 425 410 420 ± 434 426 426 420 424

The (CAO) group coordinated to the metal ions.

In order to investigate the ketoimine-enolimine tautomeric forms (Figure 2) of the Schi€ base ligands HL1, HL2 and HL3, the electronic spectra were recorded in EtOH, CHCl3, C6H14, PhMe and C7H16 as solvent. As seen in Table 4, the n ® p* and p ® p* transitions of the ligands in a polar solvents such as C6H14, PhMe and C7H16 are observed in the 408±302 and 289±214 nm ranges, respectively. The same transitions in polar solvents as CHCl3 and EtOH are observed in the 492± 303 and 283±211 nm ranges. In apolar solvents, the ligands prefer the ketoimine tautomeric form. In other

Fig. 1. Intramolecular H-bonding for the ligands HL1 and HL3.

Fig. 2. Keto-enol tautomeric forms of the Schi€ base ligands.

words, in polar solvents, the dominant tautomeric form is the enolimine [20]. The electronic spectra of the ligands and their complexes were recorded in EtOH solvent (Table 4). In the complexes, the n ® p* transitions due to the azomethine group are shifted to the lower energy. From these results, the imine group nitrogen atom appears to be coordinated to he metal ion [21]. In addition, in the spectra of some of the complexes of the HL1, HL2 and HL3 ligands, the new bands observed in the 489±381 nm range can be attributed to the charge transfer bands

530 Table 4. Electronic spectral data for the ligands in organic solvents (kmax/nm) Ligands

EtOH

CHCl3

C6H14

C7H16

PhMe

HL1

356a, 308, 282, 217

357a, 319a, 304, 283

390, 307, 236 211

492a, 389, 317a, 236a

HL3

454a, 335, 308a, 235, 213

365a, 346a, 329, 303a

357a, 339, 317a, 304, 277 390a, 372, 349, 308a, 235a, 214a 354a, 334, 323, 230

363a, 319a, 305, 286

HL2

358a, 339a, 318a, 302, 263, 224 388a, 349, 312a, 247a, 233, 219 355a, 334, 322, 230

a

408a, 388, 367a, 312a 363a, 342a,289a

sh.

(ligand to metal or metal to ligand centre) [22]. The bands in the 638±608 nm range in the cobalt(II) complexes come from the d±d transitions of the cobalt(II) ion. For the other complexes, the d±d transition is observed in the 741±580 nm range. The magnetic susceptibilities of all complexes (Figure 3) were measured at room temperature. The copper(II) complexes exhibit the 1.83, 1.90 and 1.97 B.M. values, suggesting that the copper(II) ion has square-planar geometry [23]. The nickel(II) complexes are diamagnetic and have square-planar geometry around the metal centre. The values for the Co(L1)2, [Co(L2)2] á H2O and [Co(L3)2] á 2H2O complexes are 4.13, 4.23 and 4.50 B.M., respectively, consistent with tetrahedral structures. The magnetic moments of the oxovanadium(IV) complexes are 1.66, 1.68 and 1.73 B.M., con®rming that the oxovanadium(IV) ions are squarepyramidal [24]. All zinc(II) complexes are diamagnetic and tetrahedral. The room temperature 1H and 13C n.m.r. spectral data for the HL1, HL2 and HL3 ligands, dissolved in DMSO-d6, are given in Table 5 and exhibit all the expected signals. The 1H n.m.r. spectra of the ligands exhibit singlet signals in the 10.8±12.0 ppm range which

may be attributed to the OH group proton. The NH and OH protons can be identi®ed easily, because they disappear upon D2O exchange. In like manner, all ligands show peaks due to the pyridine and benzenoid rings, azomethine, methoxy, methylene and secondary amine groups approximately in the expected regions [25±27]. The 1H n.m.r. spectrum of the HL2 ligand exhibits a strong singlet at 3.41 ppm attributable to the ANMe2 group protons. Also, in the HL3 ligand, the singlet observed at 1.4 ppm can be assigned to the protons of the t-Bu groups. The 13C n.m.r. spectra of the ligands exhibit signals in the 162.7±164.5 ppm range can be assigned to the azomethine group carbon atoms (Table 5). The signals due to the carbon atoms of the benzenoid and pyridine rings are observed in the 104±158.3 ppm range. The carbon atoms of the methoxy and methylene groups lie in the 55.2±56.1 and 58.5±60.7 ppm ranges, respectively. The signals at 30.4 and 32.0 ppm are due to the carbon atoms of the ANMe2 and t-Bu groups. The quarternary carbon atom in the t-Bu group is observed at 36.0 ppm. In the mass spectra, the molecular ion peaks of the ligands appeared at m/z 377 [M+1]+, 380 [M+1]+

Fig. 3. Suggested structure of the complexes M: CuII, CoII, NiII, VOIV and ZnII; n: 0, 1 or 2.

531 Table 5. The 1H(13C) n.m.r. spectral data of the Schi€ base ligandsa Compound

Chemical shift (d, ppm) CH@N Ph Py

HL1

8.6 (162.7) 8.5 (164.5) 8.8 (164.0)

HL2 HL3 a

6.2±7.2 (a) 7.6 (a) 7.2 (a)

7.4±8.0 (a) 7.4±7.9 (a) 7.3±8.2 (a)

OH

NH

OMe

CH2

NMe2

11.5, 10.8

5.6

11.4

5.3

11.2, 12.0

5.2

3.8 (55.5, 56.0) 3.7 (56.1) 3.7 (55.2)

2.5 (58.7) 2.6 (60.7) 2.4 (58.5)

3.41 (30.4)

t-Bu Me

quarternary C

1.4 (32.0)

(36.0)

13

The C n.m.r. data of the pyridine and benzenoid rings carbon atoms of the ligands are observed at 104±152.5 ppm range for HL1, 110± 155.7 ppm range for HL2 and 104.7±158.3 ppm range for HL3. Solvent: CDCl3.

and 462 [M+1]+. The most intense peaks at m/z 283, 286 and 368 correspond to the [C17H19N2O2]+, [C16H16NO4]+ and [C23H30NO3]+ species which result from loss of the C5H5N2 fragment from the parent ligands. The mass spectrum of the HL2 ligand shows a peak at m/z 239 which may be assigned to the [C15H13NO2]+2 fragment, resulting from loss of the ANMe2 group. In addition, in the spectrum of the HL3 ligand, the peak appeared at m/z 254 can be attributed to the [C15H12NO3]+3 ion. Thermal degradation of the complexes proceed in two steps. The ®rst, corresponding to the removal of the hydrated water molecules, lies within the 30±75 °C range. The second step, within the 470±680 °C range, represents the ®nal decomposition of the complexes to the metal oxide [28]. Antimicrobial activities of the Schi€ base ligands and their metal complexes against bacteria and yeast are recorded in Table 6. In this study, one of the striking features is that while the HL1 and HL3 ligands and the copper(II) complex of the HL2 ligand have activity against Saccharomyces cerevisiae, the HL1 ligand and other compounds have no such activity against the same organism. It is apparent that the HL1 and HL3 ligands,

containing two free hydroxy groups, are more active than the HL2 ligand containing one hydroxy group. Since, the hydroxy group substituent is known to increase the activity of compounds. We have shown that the activity decreased on undergoing complexation. Since, chelation reduces that polarity of the central ion mainly because of the partial sharing of its positive charge with the oxygen atom of the free hydroxy group and nitrogen atom of the azomethine group [29]. Conclusions Three Schi€ base ligands and their copper(II), cobalt(II), nickel(II), oxovanadium(IV) and zinc(II) metal complexes were synthesized and characterized by elemental analyses, conductivity and magnetic susceptibility measurements, i.r, electronic absorption and 1H and 13C n.m.r. spectroscopy. In order to evaluate the hydrated water content of the complexes, t.g. and d.t.a instruments were employed. It was found that all complexes are monomers. Mass spectral data con®rmed the proposed structure of the free Schi€ base ligands. Molar conductivities of the HL1, HL2 and HL3 ligands and

Table 6. Antimicrobial e€ects of the ligands and their complexesa Compound 1

HL [Cu(L1)2] á H2O Co(L1)2 [Ni(L1)2] á 2H2O [VO(L1)2] á 2H2O Zn(L1)2] á 2H2O HL2 [Cu(L2)2] á 2H2O [Co(L2)2] á H2O [Ni(L2)2] á H2O [VO(L2)2] á 2H2O [Zn2(L22)] á H2O HL3 [Cu(L3)2] á H2O [Co(L3)2] á 2H2O [Ni(L3)2] á H2O [VO(L3)2] á 2H2O [Zn(L3)2] á 2H2O a

Microorganisms (Inhibition zoneb) S. cerevisiae B. subtilis

E. Coli

K. pneumoniea

M. luteus

Control

18 ± ± ± ± ± ± 10 ± ± ± ± 19 ± ± ± ± ±

22 ± ± 14 11 11 13 8 ± 10 ± ± 24 14 9 11 8 10

27 9 8 18 11 13 11 10 ± 9 ± 10 21 14 10 8 ± 10

21 10 8 16 9 14 ± 11 ± 8 ± 9 19 11 8 10 7 8

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

32 11 9 16 13 17 15 11 ± 12 ± 13 22 9 11 10 8 12

concn. = 100 lg/disc. b Including disc diameter (6 mm). The symbol `±' reveal that the compounds have not any activity against to the microoganisms.

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