ЖУРНАЛ ФИЗИЧЕСКОЙ ХИМИИ, 2007, том 81, № 10, с. 1880-1886
БИОФИЗИЧЕСКАЯ ХИМИЯ =
УДК 541.64
A POTENTIOMETRIC INVESTIGATION OF COMPLEX FORMATION BETWEEN SOME METAL IONS AND BIOLOGICALLY ACTIVE QUINAZOLIN-4-3(H)-ONE SCHIFF'S BASE © 2007 r. K. Shivakumar*, S. Shashidhar**, M. B. Halli*
*Department of Chemistry, Gulbarga University, India **Materials Research Center, Indian Institute of Science, Bangalore, India E-mail: mbhalli@rediffmail.com Received December 29, 2005
Abstract - The proton dissociation constant of the ligand and the stability of the complexes with Co(II), Ni(II), Zn(II), Cd(II), Hg(II), Pb(II), Cu(II), Ba(II), Mg(II), Mn(II), Th(IV), and UO2(II) metal ions with 2-phenyl-3-(2'-hydroxy-5'-benzylidine)-quinazolin-4-(3H)-one [PBQ] have been determined potentiometrically at 30 ± 0.1°C temperature and at different ionic strengths viz. 0.025, 0.05, 0.10, 0.15 and 0.20 M NaNO3 in 60 : 40% (v/v) ethanol-water medium. The proton-ligand and metal-ligand stability constants of complexes were determined pH metrically by Calvin-Bjerrum titration technique. The order of stability constants obeys the Irving-Rossotti order. The negative values of AG° suggest that the reactions are spontaneous one.
The quinazolines and their derivatives are well known for their biological activities such as anti-inflammatory [1], analgesic [2], diuretic, hypersensitive, anticonvulsant [3, 4] antiviral [5], CNS depressant [6] anti-HIV [7, 8], anticancer [9] etc. Besides biological activity these compounds have O and N donor atoms, so they can act as good chelating agents. Survey of literature shows that large amount of work has been done on synthesis of quinazoline derivatives and their phar macological activity studies [10]. However no quanti -tative work has been done on the Schiff's bases derived from these quinazolines. The Schiff's bases are also good chelating agents and they can also be used as bio chemical and anti microbial agents [11, 12].
In the light of biological importance of quinazolines and Schiff's bases, it was thought worthwhile to synthesize the Schiff's base by the reaction between 3-ami -no-2-phenyl-3H-quinazolin-4-one and salicylaldehyde. In the present study we report determination of formation constants of proton-ligand and metal-ligand sys tems, using Calvin-Bjerrum pH-titration technique at 30 ± 0.1°C temperature and at different ionic strengths of0.025, 0.050, 0.10, 0.15 and 0.20 M NaNO3 in 60 : 40% (v/v) alcohol-water medium. The metal ions used were Co(II), Ni(II), Zn(II), Cd(II), Hg(II), Pb(II), Cu(II), Ba(II), Mg(II), Mn(II), Th(IV), and UO2(II).
THEORETICAL CONSIDERATION
This method is mainly employed in the systems, in which the ligand possesses neither acidity nor basicity by means of which its concentration can be followed conveniently. The use of pH measurements and the concept of the degree of formation introduced by Bjer-rum is of fundamental importance. The degree of formation n is defined as an "average number of ligands bounds per metal ion in any of its several forms", i.e.
- = [ ML ] + 2 [ ML2 ] [ ML3 ] + ... + N [ ML N ] n [ M ] + [ ML ] + [ ML2 ] + ... + [MLn] ' ()
Overall stability constants are
n = P 1 [ L ] + 2 P 2 [ L2 ] 2 + 3 P 3 [L ] 3 + ... + NpN [ L ] N " 1 + p 1 [ L ] + P2 [ L2 ] 2 + P3 [ L ] 3 + ... + pN [ L ] N ' (2)
N
n = X i Pi [ L ] V Pi[ L ](3)
i = 0
where P0 = 1. The values n, the average number of ligand attached per metal ion were calculated using this equation (3).
The function n A may be defined in a similar man -ner for the proton-ligand equilibria:
(4)
kH [ H ] + 2 KH [ H ] 2 + ... + JKH KH + ... + Kj [ H ]1
1 + KH [ H ] + 2 KH[ H ]2 + ... + JKf KH + ... + KH [ H ]1
nA = ¿i PH [H r/pH [H ]
i = 0
1880
where p0 = 1.
The values of n A have been calculated from the ac -id and ligand titration curves. The free ligand exponent pL has been calculated using equation
pL = log
Pf ( 1 / antilogpH )j ( V0 + V" )
I [ T L - nTM IV0 .
(6)
where P, is the overall proton-ligand stability constant, V0 is initial volume, V" is the volume of alkali required for the same pH in metal titration, TL is the final concentration of reagent in the solution, TM is the final
concentration of metal ion in the solution and n is the average number of ligand bond per metal ion. The formation curves obtained by plotting n vs. pL were used to evaluate the stepwise formation constants.
EXPERIMENTAL
All the chemicals used were of analytical grade and the compounds 3-amino-2-phenyl-(3H)-quinazolin-4-one was prepared by the literature method [13].
Preparation of Schff s base. The SchifFs base used in the present study was prepared by the reaction between 3-amino-2-phenyl-(3H)-quinazoline-4-one (0.10 mole) in ethanol (25 ml) and salicylaldehyde (0.10 mole) in ethanol (10 ml). The reaction mixture was refluxed on water bath for about three hours. The light yellowish Schiff s base was separated out on partial removal of the solvent. The solution was cooled to room tempera ture, solid mass was filtered out, washed with alcohol and recrystallized from aqueous ethanol. The synthesis of ligand is as shown in Fig. 1. Molecular formula: C21H15N3O2, Yield: 80%, M.P: 220°C.
Preparation of reagent solution. The solution of the Schiffs base (PBQ) is prepared by dissolving requisite quantity of the compounds in double distilled alcohol just before use.
Preparation of HNO3, NaOH andNaNO3 solutions. The stock solution of HNO3 (0.01 M), NaOH (0.1052 N) and NaNO3 (1 M) were prepared by dissolving appro -priate quantity of the AR grade respective compounds
in CO2 free double distilled water. The same solutions were used for all titration.
Metal ion solutions. The stock solutions of metal ions (0.002 M) were prepared in CO2 free double dis -tilled water using metal nitrates. The metal contents were determined using standard methods.
Apparatus. An Elico digital pH meter model LI-127 equipped with combined glass electrode type CL-51B (an accuracy of ±0.1 unit having pH range 0-14 and the temperature range of 10-100°C) was used for the pH measurements. The pH meter was calibrated with the standard buffers (pH 4.00 and 9.18) before taking readings. The pH meter reading were corrected for the nonaqueous medium [14].
All the titration were carried out in double walled glass cell in an inert atmosphere of nitrogen at different ionic strengths, i.e. 0.025, 0.05, 0.10, 0.15, and 0.20 M NaNO3 and at 30 ± 0.1°C temperature. The following solutions were titrated pH metrically against standard (0.1052 N) NaOH solution and total volume being 50 ml in each case.
a) Acid titration: 5 ml of (0.010 N) HNO3 + 1.22 ml of (1.0 M) NaNO3 + 30 ml of alcohol + 13.78 ml water.
b) Ligand titration: 5 ml of (0.010 N) HNO3 + 1.22 ml of (1.0 M) NaNO3 + 25 ml of alcohol + 13.78 ml water + + 5.00 ml of (0.002 M) ligand.
c) Metal titration: 5 ml of (0.010 N) HNO3 + 1.22 ml of (1.0 M) NaNO3 + 25 ml of alcohol + 11.28 ml water + + 5.00 ml of (0.002 M) ligand + 2.5 ml of (0.002 M) metal ions solution.
In other sets required amount of NaNO3 was added to maintain the ionic strengths of 0.05, 0.10, 0.15 and 0.20 M. The ratio between metal and ligand was kept at 1 : 1 in all systems, all titrations were carried out in 60 : 40% (v/v) ethanol-water medium at constant temperature 30 ± 0.1°C.
Infrared frequencies, NMR and mass spectral data's of the quinazolin-4-(3H)-one Schiff's base are represented in Table 1.
RESULTS AND DISCUSSION
The proton-ligand stability constant (pKa) values for PBQ were determined pH metrically for the first time. In the present study the titration curve for the ligand shows (Fig. 2) only one buffer regions pH 6-10. This shows that the protonation of ring nitrogen does
Fig. 1. Structure of the ligand. ЖУРНАЛ ФИЗИЧЕСКОЙ ХИМИИ том 81 < 10 2007
Table 1. Infrared frequencies, NMR and mass spectral data's of the quinazolin-4-(3H)-one Schiff's base
IR (cm-1)
"C=O
"C=N
1690 1610
Vc=n (ring) 1580 Vn-h 3210
1H NMR
5 6.92-8.8 (14 H, m, Ar-H) 5 8.68 (1H, s, HN = C) 5 12.4 (1H, s, N-H)
HL — H + L, KHH = [H][L]/[HL].
mass-spectrum
Molecular ion The values of n A vary in the regions for M+ (m/z = 341)
is equivalent I = 0.025 M 0.375 < n A < 0.602, to
this molecular | = 0.050 M 0.432 < n A < 0.773, weight
| = 0.100 M 0.455 < n A < 0.625,
not take place under the present experimental condition or if the protonation takes place the proton from nitrogen atom dissociate at much lower pH. The dissociation equilibrium can be represented as
pH
—*—Cd Hg
acid
ligand H
Cu
Co
Ni
Zn
acid
ligand H
Mn
Mg
UO
Th
Ba
Pb
0.8
V, l
Fig. 2. Plot of pH vs. volume of NaOH added at ц = 0.25 mole/l of NaNO3.
| = 0.150 M 0.341 < n A < 0.628, | = 0.200 M 0.351 < n A < 0.579.
These values shows that there is only one proton per molecule of ligand which is liberated, i.e. phenolic OH. The value of proton-ligand stability constants was determined by the following methods:
1) by half integral methods, from graphs ri A vs. pH at nA = 0.5;
2) by graphical methods, from graph log[ n A/(1 - n A)] vs. pH;
3) by point wise calculation methods, p KJH = pH + + log[ n A/(1 - nA)].
The p Kobtained by all the three methods are in good agreement and the average values are given in Table 2. It is observed that there is a decrease in p kOh values as ionic strength is increased. The same trend is observed by various workers and is in agreement with Debye-Huckel equation [15].
The metal-ligand stability constants (logK) were determined by using Calvin-Bjerrum titration technique as modified by Irving-Rossotti from the three titration curves. The n values of the metal complexes were determined at various pH values. From these data the corresponding pL values were calculated. The n values were then plotted against the corresponding pL values to get the formation curves of the metal complex equilibria. The comparison between ligand and metal titration curves indicates that the metal curves are well separated from the
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