Synthesis and Characterization as well as Antimicrobial Studies of Lanthanide (III) Chloride Complexes of P-Hydroxybenzylidene-2-Aminothiazole and P-Hydroxybenzylidene-2-Amino-6-Methylpyridine

 

Harsh Vardhan Pratap Singh and Rajesh Dhakarey

University Department of Chemistry, Dr. BRA University, Khandari, Agra

Corresponding author: harshvp.singh@gmail.com

ABSTRACT:

p-hydroxybenzylidene-2-aminothiazole (HBAT) and p-hydroxybenzylidene-2-amino-6-methylpyridine(HBAMP) complexes of Lanthanide(III) chloride with the general composition LnL₂.X₃ [Ln = La(III), Gd(III) and Nd(III); L = HBAT and HBAMP; X = Chloride] have been prepared on the basis of analytical and molecular weight,  infrared and electronic spectral data. Infrared studies of these complexes reveal that HBAT and HBAMP act as ONS and ONN tridentate ligand and coordinate through oxygen, nitrogen and sulphur. Coordination number of lanthanide (III) ion for La(III), Gd(III) and Nd(III) complexes were found to be six. Atomic absorption spectral data shows the percentage values of the complexes. All the complexes exhibited distorted octahedral geometry. Antimicrobial studies of these complexes against E. coli, K. pneumoniae, Aspergillus niger and Aspergillus fumigatus are also reported.

 

 

 

INTRODUCTION:

In the past, a number of workers were invested to isolate solid complexes of lanthanide(III) with drugs. But literature survey shows that the solid lanthanide(III) complexes of p-hydroxybenzylidene-2-aminothiazole(HBAT) and p-hydroxybenzylidene-2-amino-6-methylpyridie (HBAMP) have not been reported. We report here the synthesis of lanthanide(III) complexes of              p-hydroxybenzylidene-2-aminothiazole and p-hydroxybenzylidene-2-amino-6-methylpyridie, and their characterization. Mannich bases HBAT and HBAMP (ligands) are widely studied subjects because of their applications in pharmaceutical and polymer chemistry¹. Mannich base prepared from heterocycles bearing nitrogen, sulfur and thiazole moieties constitute, the core structure of a number of biologically interesting compounds.

 

EXPERIMENTAL:

All the lanthanide(III) chloride were obtained from M/S Indian Rare Earths Ltd., Kerala (India) and all other chemicals obtained from M/S SIGMA chemicals company, E. Mark, U.S.A. were used as such.

 

PREPARATION AND ANALYSIS OF COMPLEXES:

(1) Preparation of Schiff Bases: For the preparation of p-hydroxybenzylidene-2-imino-thiazole (SB₁), 1.22 gm 4-hydroxybenzaldehyde and 1.0gm 2-aminothiazole were dissolved in 20ml ethanol and reflux for 6-7 hours over water bath using water condenser. The obtained solution was allowed to cool at room temperature. The concentrated solution was cooled in refrigerator and obtained product was filtered, washed with ether and dried under reduced pressure over anhydrous calcium chloride.

 

For the preparation of p-hydroxybenzylidene-2-amino-6-methyl pyridine (SB₂), 1.22gm 4-hydroxybenzaldehyde in ethanol was mixed with an ethanolic solution of 1.08gm of 2-amino-6-methylpyridine (dissolved in 20 ml ethanol). Mixture was refluxed for 6-7 hours over a water bath using water condenser. The obtained solution was allowed to cool at room temperature and the concentrated solution was cooled in refrigeration for 24 hours. The obtained product was filtrated, washed with acetone several times and followed by ether. It was recrystallized with absolute alcohol and dried under reduced pressure over anhydrous calcium chloride.

 

(2) Preparation of Mannich Bases: For the preparation of p-hydroxybenzylidene-2-aminothiazole (MB₁) derived from p-hydroxybenzylidene-2-imino-thiazole (SB₁) (2.04 gm) was stirred with 20 ml of methanol. The product is then cooled to 0^oC and sodium borohydride (1.0 gm) was added over a period of 1 hour in 3 to 4 installments. Slowly the temperature was raised to room temperature. A dark brown solution resulted and the solvent was slowly evaporated. A solid coloured powder was obtained. It was then washed with ethanol and dried in air, a deep brown coloured crystals was obtained. Mass spectra of the ligand exhibits m/z values: 205, 189, 107, 122, 113 and 99 assignable to C₁₀H₁₀N₂OS, C₁₀H₁₀N₂OS, C₇H₇O, C₇H₈NO, C₄H₅N₂S and C₃H₃N₂S molecular ion.

 

And for the preparation of p-hydroxybenzylidene-2-amino-6-methylpyridine (MB₂) derived from p-hydroxybenzylidene-2-imino-6-methylpyridine(SB₂):p-hydroxybenzylidene-2-imino-6-methylpyridine (2.12 gm) was stirred with 20 ml of methanol.  The product was then cooled to 0^oC and then sodium borohydride (1.0 gm) was added to it, in a period of one hour. Slowly the temperature was raised to room temperature. A light brown coloured solution was resulted and then solvent was slowly evaporated. A solid chocolate coloured powder is obtained. It was then washed with alcohol and dried in air. The mass spectra of the ligand exhibits m/z values 214, 198, 176, 121, 107 and 93 assignable to C₁₃H₁₄N₂O, C₁₃H₁₄N₂, C₁₂H₁₂N₂O, C₇H₉N₂, C₆H₇N₂ and C₆H₇N molecular ion.         

 

(3) Preparation of Metal Complexes: The metal complexes of Lanthanide(III) chloride were prepared by refluxing the alcoholic solution of HBAT and HBAMP with Lanthanide(III) chloride for 3-4 hours (M:L ratio 1:2). The precipitated complex was filtered and washed with ethanol, ether and dried in air.

PHYSICAL MEASUREMENTS: The Lanthanide metal ions were estimated by standard method². The percentage of metal of Lanthanide(III) in complexes were determined by AAS method from Dayalbagh Educational Institute, Agra. FTIR spectra of the ligand and their lanthanide(III) complexes were recorded on (KBr matrix) Perkin Elmer 842 gratting, IR spectrometer in the range 4000-200cm^-1. FAB Mass spectra of ligands were determined on Jeal, JMS-D-300 double focusing spectrometer at R.S.I.C., C.D.R.I., Lucknow. The electronic spectrum of the complexes in (DMF/Ethanol) solution were recorded on UV 5704SS (Double Beam UV-Vis Spectrophotometer) in the range of 380-1100 nm at the Department of chemistry, I.B.S., Khandari, Agra. The ¹H-NMR spectra of the ligand was recorded in DMSO at R.S.I.C., C.D.R.I., Lucknow.

 

RESULTS AND DISCUSSION:

The analytical data of the present complexes (Table-1) indicate that all the six complexes show 1:2 (Metal:ligand) ratio. The analytical data, percentage of metal of lanthanide (III) HBAT and HBAMP complexes are represented in Table-1. All these complexes are soluble in ethanol, methanol and DMSO.

 

The ¹H-NMR spectra of the Mannich bases (MB₁ and MB₂) were recorded in DMSO, assigned as a solvent peak in the spectrum at δ 3.41 ppm. A doublet may be due to heterocyclic proton (2H) present in thiazole ring was observed in the spectrum at δ 7.0 ppm³ in MB₁ while a triplet at δ 7.2 ppm equivalent was observed may be due to 3H proton present in pyridine ring in MB₂. A singlet was observed at δ 5.3-5.7 ppm equivalent to 1H, may be assigned as –OH proton on para position while a doublet at δ 6.4-6.68 ppm equivalent to 4H was present in the aromatic benzene ring⁴ in the Mannich bases. A singlet was observed at δ 2.56 ppm equivalent to 2H of –CH₂ group attached with the benzene ring⁵. A singlet at δ 5.56-5.58 ppm equivalent to the 1H proton of secondary –NH group was present in these Mannich bases. These data are in good agreement with other spectrometric results.

 

 

Table:1 Physical Properties and Analytical Data of Metal Complexes of MB₁ and MB₂

S. No.	Ligand/ Complexes	Yield %	M.P. ( oC )	Formula Weight	Colour	Percentage of Elements(Found/Calculated)
						C	H	N	S	M	Cl
1	C10H10N2OS	88	150	206	Dark brown	57.92/58.25	4.80/ 4.85	17.53/ 13.59	15.50/ 15.53	-	-
1.1	La(C10H10N2OS)2 .Cl3	82	215-217	656	Dark brown	36.56/ 36.58	3.00/ 3.04	8.52/ 8.53	9.70/ 9.75	21.08/ 21.18	15.95/ 16.00
1.2	Nd(C10H10N2OS)2 .Cl3	82	180-183	661	Dark brown	32.15/ 36.30	3.00/ 3.02	8.40/ 8.47	9.68/ 9.68	21.68/ 21.78	15.88/ 15.88
1.3	Gd(C10H10N2OS)2 .Cl3	81	210-212	674	Brown	35.45/ 35.60	2.91/ 2.96	8.20/ 8.30	9.48/ 9.49	23.14/ 23.29	15.60/ 15.57
2	C13H14N2O	87	190	214	Light brown	72.80/ 72.98	6.50/6.54	13.56/ 13.08	-	-	-
2.1	La(C13H14N2O)2 .Cl3	79	245-248	672	Light Brown	46.30/ 46.42	4.06/ 4.16	8.23/ 8.33	-	20.50/ 20.68	15.45/ 15.62
2.2	Nd(C13H14N2O)2 .Cl3	80	225-228	677	Light Brown	46.00/ 46.08	4.11/ 4.13	8.17/ 8.27	-	21.17/ 21.27	15.49/ 15.50
2.3	Gd(C13H14N2O)2 .Cl3	80	250-252	690	Yellow	45.11/ 45.21	4.00/ 4.05	8.06/ 8.11	-	22.60/ 22.75	15.20/ 15.21

 

Table:2 Infra-Red Frequencies (cm^-1) of MB₁ and its Metal Complexes

S. No.	Ligand/Complexes	Ligand Modes						Coordination Modes
		υ(OH)	υ(CH2-NH)	υ(C-O) Phenolic	υ(C=N) cyclic	υ(C-N ) cyclic	υ(C-S-C) thiazole	υ(M-O)	υ(M-N)	υ(M-S)
1 1.1 1.2 1.3	C10H10N2OS La(C10H10N2OS)2 Cl3 Nd(C10H10N2OS)2 Cl3 Gd(C10H10N2OS)2 Cl3	3450 - - -	3355 3376 3382 3378	1243 1247 1248 1245	1512 1512 1521 1513	1350 1349 1356 1352	833 836 838 838	- 525 544 514	- 465 472 480	- 341 360 328

                                     

Table:3 Infra-Red Frequencies (cm^-1) of MB₂ and its Metal Complexes

S. No.	Ligand/Complexes	Ligand Modes				Coordination Modes
		υ(OH)	υ(CH2-NH)	υ(C-O) Phenolic	υ(C-N-C) pyridine	υ(M-O)	υ(M-N)	υ(M-N) pyridine
1 1.1 1.2 1.3	C13H14N2O La(C13H14N2O)2 Cl3 Nd(C13H14N2O)2 Cl3 Gd(C13H14N2O)2 Cl3	3518 - - -	3353 3384 3377 3382	1480 1470 1467 1457	1502 1510 1512 1514	- 532 538 542	- 483 484 482	- 507 518 513

 

 

IR Spectra: Infrared spectral data and their tentative assignments are shown in Table-2,3. The IR spectra of SB₁ and SB₂ show the bands at ~1605-1610 cm^-1 due to  azomethine⁶ (υ>C=N) which were disappeared in the spectrum of MB₁ and MB₂ and new bands was observed in the region ~3358-3365 cm^-1 due to secondary amino group. In the IR spectra of all the Lanthanide(III) complexes, a broad band at ~3452-3520cm^-1 due to υ_(-OH) disappeared thus indicating the deprotonation of phenolic OH. The bands observed at ~1230-1480 cm^-1 in the ligands is due to υ_(-C-O-) phenolic mode. These bands are shifted to higher frequency region (~1242-1251 cm^-1) for the complexes of MB₁. The characteristic IR bands in the spectrum of he MB₁ is observed at ~1513 υ_{(C=N cyclic)}, ~1352 υ _{(C-N cyclic)} and ~830 υ _(C-S-C) cm ^-1 of the thiazole moiety. The position of the former two bands remains unaltered in all the complexes ruling out the possibility of coordination of thiazole ring nitrogen. The vibration modes due to υ_(-M-S) coordination were observed at 300-365cm^-1 in all the complexes complexes confirming the participation of thiazole ring S-atom in complexation. The strong absorption band due to pyridine ring occurring at ~1498 cm^-1 (υ_(C-N-C)pyridine) has shifted to higher frequency region by ~10-25 cm^-1 in the complexes showing the participation of nitrogen atom of pyridine in the complexation. This is also supported by the appearance of bond in the far infrared region ~500-520 cm^-1 due to υ_(M-N) vibration. And the above discussion indicates that both the ligands (MB₁ and MB₂) are tridentate in nature with ONS and ONN donor system having distorted octahedral environment around metal ion.

 

Electronic Spectra: The electronic spectral data of the representative complexes are given in Table-4. The electronic spectra of the Nd(III) complexes have been attributed to the transition from ground levels 4I_9/2 to the excited J-levels of 4fⁿ configuration⁷. The electronic spectra of f-f transition for Nd(III) complexes and their assignments are given in Table-4. The nephelauxetic ratio β (υ_complex/υ_aquo), covalency factor (b^1/2) and Sinha’s parameter δ% suggest covalency in the metal-ligand bonding⁸.

 

Thermal Studies: From the TGA analysis it is divided into four stages based on the heating rate, the first stage is evaporation of water vapour⁹ and chloride molecules between 90-130^oC from the complex, the second stage shows 19% mass loss of remaining compounds between 130-317^oC and third stage can be seen above 320-867^oC. The fourth stage can be seen between 867-1400^oC, the oxide of neodymium chloride was remaining. The decomposition temperatures are approximately 130, 320, 867 and 1400^oC for components 1, 2, 3 and 4 respectively. Thermal data are represented in Table-5.

 

X-Ray (Powder Diffraction) analysis: The data were collected on Bruker AXS D8 Advance Diffractometer using lanthanide (III)  radiation over a range of 3-79° 2θ and the values Sin²θ (observed and calculated), interplanar spacings (d), relative intensities and hkl values of different lattice planes have been summarized in tables. In the X-ray powder diffraction pattern the following interesting points were noted¹⁰. X-ray powder diffraction pattern of the lanthanide(III) complexes remained the same indicating that the crystal structure did not change. However, a large decrease in the line intensity of the X-ray powder diffraction was noticed when the ligand was exchanged by La(III) ions. On the basis of electronic spectra studies and x-ray (Powder diffraction) analysis, it was concluded that the La(III) complexes of MB₁ and MB₂ have distorted octahedral geometry. (Table: 5,6)

 

Antibacterial activities:

The antibacterial activities of ligands (MB₁ and MB₂) and the complexes were assayed against the bacteria E. coli and K. pneumoniae using paper-dick method¹¹.  The results of antibacterial activities of the ligand (MB₁) and complexes show that MB₁ has good antibacterial activity against E. coli and K. pneumoniae at all concentrations i.e. 250 ppm, 500 ppm and 750 ppm. But MB₂ was inactive against E. coli and K. pneumoniae at all concentrations i.e. 250 ppm, 500 ppm and 750 ppm. Ln(III) and Nd(III) complexes of MB₁ have good antibacterial activities against E. coli and K.pneumoniae at all the concentrations but Gd(III) is less active. And La(III), Gd(III) and Nd(III)  complexes of MB₂ have less active or inactive at all the concentrations against E. coli and K.pneumoniae.

 

Table:4 Electronic Spectral Data (cm^-1)

S. No.	Electronic Specral Band (In Standard)	Electronic Specral Band (In Complex)	J-Levels (Assignments)	(1-β)	β	b1/2	δ%	η
1	Nd(C10H10N2OS)2Cl3
	19420	19250	4I9/2 → 2G9/2	0.00875	0.99124	0.04677	0.88273	0.00440
	17390	17200	→ 4G5/2, 2G7/2	0.01092	0.98907	0.05224	1.10406	0.00551
	13420	13200	→ 2S3/2, 4F7/2	0.01639	0.98360	0.06401	1.66632	0.00823
	12500	12350	→ 4F5/2, 4H9/2	0.01200	0.98800	0.05477	1.21457	0.00601
2	Nd(C13H14N2O)2Cl3
	19607	19520	4I9/2 → 2G9/2	0.0045	0.9955	0.0335	0.4520	0.0022
	17241	17177	→ 4G5/2, 2G7/2	0.0037	0.9963	0.0304	0.3713	0.0018
	13513	13487	→ 2S3/2, 4F7/2	0.0019	0.9981	0.0217	0.1903	0.0009
	12500	12420	→ 4F5/2, 4H9/2	0.0064	0.9936	0.0400	0.6441	0.0032

 

 

Table:5 Thermal Decomposition Data of NCMB₁

Compound	Decomposition Temp. °C	Weight Loss				Probable Assignments
		Observed Values		Calculated Values
		Weight Loss	Residual Mass	Weight Loss	Residual Mass
Nd(C10H10N2OS)2 Cl3	90-130	125.6	535.4	124.5	536.5	3Cl + H2O
	130-317	101.7	433.7	104	432.5	C6H6.CH2N
	320-867	86.7	347	85	347.5	C3H3NS
	867-1400	11	336	11.5	336	Nd2O3

 

 

Table:5 Lanthanum (III) chloride Complex of MB₁

S. No.	Interplanar Spacing (dA0)	Rel. Int.	Sin2θ (Obs.)	Sin2θ (Calc.)	hkl
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15	26.39091 23.35719 22.18282 21.42934 19.71494 17.75024 16.29418 15.62379 14.67935 14.08525 13.48309 12.84675 12.2934 11.83782 11.3601	70.1 90.8 79.7 65.4 78.6 77.4 83.6 100 85 97 80.9 96.6 81.6 75.6 89.7	0.00085 0.00108 0.00120 0.00129 0.00152 0.00188 0.00223 0.00243 0.00275 0.00299 0.00326 0.00471 0.00392 0.00423 0.00459	0.00084 0.00109 0.00119 0.00121 0.00147 0.00182 0.00222 0.00241 0.00274 0.00291 0.00317 0.00419 0.00390 0.00421 0.00453	100 110 112 111 122 123 103 201 211 202 213 310 311 312 400

 

Fig.:1 X-ray Diffractograph of Lanthanum(III) Chloride of MB₁

 

Table:6 Lanthanum (III) chloride Complex of MB₂

S. No.	Interplanar Spacing (dA0)	Rel. Int.	Sin2θ (Obs.)	Sin2θ (Calc.)	hkl
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15	27.50023 25.24384 23.13189 21.5871 20.33243 18.44238 17.14533 16.22249 15.22544 14.83288 14.0508 13.08545 12.54623 11.797 11.342	60.7 46.3 87.8 71.7 77.8 69.4 88 55.4 73.9 77.6 100 98.7 75.8 66.2 92.5	0.00077 0.00092 0.00109 0.00126 0.00143 0.00173 0.00201 0.00225 0.00255 0.00268 0.00300 0.00345 0.00376 0.00425 0.00460	0.00078 0.00091 0.00108 0.00127 0.00144 0.00174 0.00202 0.00226 0.00256 0.00266 0.00301 0.00344 0.00377 0.00424 0.00461	110 111 112 122 123 108 211 212 222 223 225 206 300 301 311

 

Fig.:2 X-ray Diffractograph of Lanthanum(III) Chloride of MB₂

 

Antifungal activities:

Form the studies it was observed that the ligands (MB₁ and MB₂) and their metal complexes were inactive at all concentration levels against fungi A.niger and A.fumigatus.

 

Stereo chemistry:

Both the present complexes are non-ionic in nature and infra-red data reveal the tridentate nature (ONS, ONN) of the coordinating lignds, resulting in the coordination number six in these complexes. The tentative structure of the complexes may be represented as

 

Proposed structure of LnL₂.X₃ (where Ln = La, Gd and Nd; L=HBAT and HBAMP; X=chloride ions).

 

ACKNOWLEDGMENTS:

The authors are thankful to Head, University Department of Chemistry, Dr. BRA University, Khandari, Agra, for providing laboratory facilities. The authors are also thankful to IIT Chennai, SAIF Cochin and CDRI Lucknow for various instrumental facilities.

 

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Received on 21.11.2011         Modified on 12.12.2011

Accepted on 31.12.2011         © AJRC All right reserved