Indian Journal of ChemistryVol. 18A, September 1979, pp. 252-214

Potentiometric Investigation of Complex Equilibria InvolvingS(3) ..Methylpyrazole-3(s)-carboxylic Acid with Some Metal

Ions in Aqueous Solution.

S. N. PODDAR*, S. GHOSH, B. SURt, S. ROY CHAUDHURJ, S. M. BHATIACHARYYA & A. K. DASDepartment of Inorganic Chemistry, Indian Association for the Cultivation of Science, Jadavpur,

Calcutta 700032

Received 18 September 1978 : revised 1 March 1979; accepted 6 April 1979

The acid dissociation constants of 3(5)-methylpyratole-5(3)-carboxylic acid and the stability constants of itschelates with bivalent metals have been evaluated potentiometrically at 25° ±O. r in aqueous solution with perchlorate assupporting electrolyte. The pK values of the ligand obtained are 0.815 and 3.322 and the log K.v of its metalcomplexes are as follows: Cu(II) 4.76, Ni (II) 4.04, Co (II) 3.77, Zn (II) 3.86, Pb rm 3.89, Cd (II) 3.59, Fe(II) 3.49and Mn (II) 3.28. The order of stability is in accord with the observation made by Albert. The calculated values of8H and Er (Mn-Zn) indicate establishment of metal-ligand bonds through oxygen and nitrogen atoms of the ligand.A parallelism has been obtained between the logarithm of the stability constants and the second ionization potential(I.P.) of the metal ions.

ALTHOUGH investigations have been carriedout recently on the preparative aspects of themetal complexes of 3(S)-methylpyrazole-S(3)-

carboxylic acid (I) in some details by Poddar andHalder+", the stability of metal complexes of the ligandhas not been reported so far. Thepresent investigation

~COOH COOH

,\c~.J~ ">H c~H 3

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is intended to assess in a quantitative manner, thecoordinating ability of the ligand in solution towardsCu (II), Ni (II), Co (II), Zn (II), Cd (II), Pb (II), Fe (II)and Mn (II). All the measurements were done at2So ± O.I°C and the stability constants were com-puted by the method of Irving and Rossetti".

Materials and MethodsS (3)-Methylpyrazole-3 (S)-caboxylic acid was

prepared by standard procedure' and its puritychecked. All the reagents used were of A R grade.The metal perchlorates were prepared by reactingexcess carbonates of the metals with HCl04 and thesolutions standardised by usual methods. Lead saltwas used as nitrate.

Throughout the course of titration, standard per-chloric acid was used along with the standard sodiumperchlorate, the latter being used to maintain theionic strength of the solution. Standard carbonate-free NaOH solution was used for titrating the acid.

Jadavpur UniversitytDepartment of Chemistry,Calcutta 700032.

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All solutions were made in oxygen-free doubly-distilled water.

Acid dissociationconstants- The acid dissociationconstants of the ligand were determined from pHtitration curves obtained by titrating following twosets of solutions against carbonate-free NaOH : (i)O.ISM perchloric acid(lO ml)+O.SM sodium perchlo-rate (12 mlj-l-water (8 ml); and (ii) O.ISM perchloricacid (11 ml)+O. SM sodium perchlorate (12 ml) +O.OSM ligand (3 ml) -l-water (4 ml). The total ionicstrength was maintained atO.2 Mwith NaCI04•

Stability constants- The stability constants of themetal chelates were determined in aqueous solutionby performing, in addition to above two titrations,one more titration of a solution containing O. ISM perchloric acid (11 ml) + O.S M sodium perchlo-rate (12 ml)+O.OS M ligand (3 ml) + O.OIM respectivemetal perchlorate (3 mlj-l-water (1 ml). The ligand tometal ratio was S : 1 and the ionic strength of thesolution was maintained at 0.2 M (NaCI04).

Both the acid-dissociation constants of the ligand andstability constants of its metal complexes were cal-culated from the data by the least squares method.

Results and DiscussionnH values at different pH meter readings were

calculated and these provided the derived formationcurve which gave a maximum nH value of around 2.This indicated that two protons from carboxylicgroup and pyridine nitrogen are dissociated. UsingBjerrum's equation and applying the method of leastsquares, the stepwise protonation constants and hencethe acid dissociation constants of the ligand weredetermined and the values obtained at 25° are:

pKF{ = 0.81S ± 0.022 and pKl£ = 3.322 ± 0.031

rPODDAR et al.: BIVALENT METAL COMPLEXES WITH A PYRAZOLE DERIVATIVE.

These values are sufficiently far apart to precludesignificant overlapping of equilibria controlling thedissociation of these two protons.

The degrees of formation n of the metal-ligandsystem corresponding to the different pH meterreadings along with corresponding pL values werecalculated using Irving and Rossotti method". Theformation curves thus obtained for the metal com-plexes under study indicate maximum n~2.0 for allthe metal ions and hence provide data for the evalug-tion of stability constants of I :1 and 1: 2 complexes.The values of the stepwise stability constants havebeen calculated by the method of least squares as hasbeen suggested by Irving and Rossetti" and arepresented in Table 1.

Copper (II) forms complexes of high stability.The formation curve could only be obtained downto n~ I.4. During titration of the mixture of mineralacid and ligand in presence of the Cu(II) ions it hasbeen observed that the complex gets precipitated atpH~ 3.5. We thus restricted our calculation uptoa pH value of 2.75 and computed stability constantvalues of Cu (II) complexes. Results indicate thatK, ~ K2 with a log Kav = 4.76.

With nickel (II), complexes of stability comparableto that of Cu(II) complexes are formed. Although Iiwas only obtained down to 0.8 with nickel(II) complex,in the case of Co (II) complexes, the correspondingvalue is 0.4. It is intriguing that log K2 of Co (II)complex is slightly greater than that of Ni(II) complex.

During the titration of a solution containing lead(II) ions slight precipitation started above pH 5. O.Thus calculations for the stability of lead (II)complexes were restricted upto pH 4. O. The for-

TABLE1 - STEPWISESTABILITYCONSTANTS*OFMETALCHELATESOF 5(3)-METHYLPYRAZOLE-3(5)-CARBOXYLICACID IN AQUEOUS

SOLUTION.

[Temp. = 25° ± 0.1°; 14 = 0.2 M (NaCIOc)]

Metal logKl logK2 logKav log~J No. of Iipoints range

Cu (II) 4.81 4.71 4.76 9,'52 10 1.4-2.0Ni (II) 4.74 3.34 4.04 8.08 12 0.8-2.0Co (II) 4.11 3.42 3.77 7.53 12 0.4-2.0Mn(H) 3.93 2.63 3.28 6.56 12 0.3 -1.6Fe (II) 3.98 2.99 3.49, 6.97 11 0.4-1.7Zn (II) 4.32 3.39 3.86 7.71 12 0.6-2.0Cd(IJ) 4.08 3.09 3.59 7.17 12 0.4-1.65Pb (II) 4.59 3.19 3.89 7.78 10 0.7-2.0

*Estimated errors in the values are within 2 %.

mation constant values fit the data between Ii =0.7and 1 .97 which lie within this pH range. With othermetal ions like Zn (II), Cd (II). Fe (II), Mn (II)no such observation has been made. With thesemetal ions the calculated set of formation constantsfit data between a wide range of Ii values viz. 0.2 to2.0.

The present investigation further shows that inaqueous medium with sodium perchlorate (0. 2M)as the supporting electrolyte the first step as well asthe overall stability of the metal complexes with5(3)-methylpyrazole-3(5)-carboxylic acid followed theorder Cu > Ni > Pb > Zn > Co > Cd> Fe >Mn,in accord with the observation made by Albert? forvarious amino acids as well as with Mellor-Maleyt=series which holds for a large number of oxygen andnitrogen donors. It has been observed that thecomplexes of non-transition metals cadmium, leadand zinc form a group of comparable stability,different from the more stable complexes of the tran-sition metals nickel and copper.

As the maxiumji has been found to be 2, in all thecases it is expected that formation of 1:2 metalcomplexes having the structure (II) occurs.

II

Table 2 gives the stability values of a few similarcomplexes of some heterocyclic acids along with thoseobtained in the present investigations. Althoughall these values have not been determined underexactly identical conditions a rough comparisonof our data can be made with the corresponding dataof the heterocyclic acids". In spite of the fact thatthe same heterocyclic rings are not present in the twoligands, the results show similar trend in the associa-tion behaviour of the ligands towards metal ionsas well as towards proton and as pointed out byIrving and Rossetti'? the values of pK2 for the ligandand log Kl for the complex are related to each other.

The values of E, and stabilization energies ~H (ref.11) observed for the bis-complexes of Fe (II), Ni(II)and Cu(II) are presented in Table 3. Both E; and ~Hvalues are little less than those for the bonding of

TABLE2 - STABILITYDATA FORSIMILARCOMPLEXESOF SOMEHETEROCYCLICCARBOXYLICACIDS

Acid dissocia-Ligand tion constant Cu(II) Ni (II) Co (II)

1st 2nd logKl logK2 logKl logK. logKl logK.

Picolinic acids 1.5 5.43 6.0 5.9 5.4 6.0 4.86-Methyl picolinic acids 0.9 5.77 6.3 5.6 5.1 4.1 4.5 3.3Quinoline 2-carboxylic acids 1.9 4.92 4.6 4.3Quinoline 8-carboxylic acids 2.0 6.71 4.4 3.55(3)-Methylpyrazole carbo- 0.815 3.322 4.81 4.71 4.74 3.34 4.11 3.42xylic acid (this work)

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INDIAN J. CHEM., VOL. 18A, SEPTEMBER 1979.

TABLE 3 - VALUES OP l::,.HL RELATIVE TO THE VALUE OF MnH,FOR BIS-COMPLEXES OF 5(3)-METHYLPYRAZOLE-3(5)-CARBOXYLlCACID ANO VALUES OF THE STABILlsATION ENERGIES aH

Mn"J+ Fe2+ Co 2+ Ni2+ Cu2+ Zn2+

log (stability const.) 6.56 6.97 7.53 8.08 9.52 7.71l::,.F, kcal/mol 8.96 9.52 10.28 11.04 13.00 10.53l::,.Frelative to the value 0.56 1.32 2.08 4.04 1.57

for Mn2+l::,.HH relative to value 26 43 62 63 47

for Mn2+l::,.HL relative to the

value for Mn2+ 27 44 64 67 49[(11- 5)/5] Er 10 20 29 39aH 17 24 35 28

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0/,/\\'17~

/ c'" C'J

",/15

Mlt Fl· c;t N?t ci+ 2n2f'

dS d6 d7 dB d9 d>O

Atomic number

r-.C .11-0

;:c:ou 9.0

~~ 7·0~0-.Q

5'0

Fig. 2 - Plots of log (stability constant) and second ionizationpotential (LP.) versus atomic number.

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two nitrogen atoms'! with ligands like amino mono-carboxylic acids. This may be a result of co-ordinationvia tertiary nitrogen atom of the ligand which isexpected to form weaker complexes. However, thevalues obtained confirm that the metal ions arebonded through one nitrogen and one oxygen atom ofthe ligand molecule as cited earlier. This issupportedby the slight shift, to higher atomic numbers in thecurve, of 6.Ih (heat of complexation) against atomicnumber (Fig. 1, curve b) as compared to the curveof 6.HH (heat of hydration of metal ion) againstatomic number (Fig. 1, curve a). The straightline joining Mn and Zn represent the situation in theabsence of crystal field stabilization (Er).

A parallelism between log (stability constant) andsecond ionization potential (IP) (Fig. 2) is also ob-served in the present case as has been suggested byIrving and Williams'", Except for Zn(II), this potentialmeasures the energies required for the removal of ad-electron suggesting the involvement of d-orbitalsin the complex formation.

AcknowledgeinentThanks are due to Prof. N. N. Ghosh, Depart-

ment of Chemistry, Calcutta University, for providingvaluable suggestions during the course of the work.

References1. HALDER, J. B., Ph.D. Dissertation, Univ. of Calcutta (1970).2. PODDAR, S. N. & HALDER, J. B., Indian J. chem., (commu-

nicated).3. IRVING, H. M. & ROSSOTTI, H. S., J. chem. Soc., (1954),

2904.4. KNORR, D. P. A. & McDoNALD, M. C, Ann. cliem., 279

(1894), 217.5. IRVING, H. M. & ROSSOTT!, H. S., J. chem, Soc., (1953),

3397.6. ALBERT, A., Bioclietn. J., 47 (1950), 531; 54 (1953), 646.7. MALEY, L. E. & MELLOR, D. P., AilS/I'. J. Sci. Res., 2A

(1949), 579.8. MALEY, L. E. & MELLOR, D. P., NO/lire, Lond., 165 (1950),

453.9. HOLMES, F. & CRIMMIN, W. R. C; J. diem. Soc., (1955),

1175.10. IRVING, H. M. & ROSSOTT!, H. S., J. chem. Soc., (1954),

2910.11. GEORGE, P. & MCCLURE, D. S., "Progress in inorganic

chemistry", Vol. 1 (Tnterscience, New York), 1959, 38l.12. IRVING, H. & WILLIAMS, R. J. P., Nature, Land., 162 (1948),

746; J. diem. Soc., (1953), 3192.

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