Ambulkar, R. S.

Munshi, K. K.

English

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Indian Journal of ChemistryVol. 1'", June 1976, pp. 414-417Complexes of Ga(lll) with Thiomalic, Thiolactic, Thioglycollic, MalicGlycollic, Lactic & Aspartic Acids, & =-Alanine & GlycineR. s. AMBULKAR & K. K. MUNSHIDepartment of Chemistry, Nagpur University, NagpurReceived 25 April 1975; accepted 11 August 1975Stepwise fonnation constants have been evaluated ustng Irvtng-Rossottt pH-titrationtechnique of some gallium (Ill) complexes with thiomalic, thiolactic, thio~lycollic, malic,IllycoUic,lactic, aspartic acids and a-alanine and ~lycine. The Iog K values have been computedby alternative methods. The complexes of mercapto compounds are more stable as comparedto hydroxy or amino compounds. Stoichiometry of the various complexes have also been dis-cussed.INcontinuation of our earlier work on In(III)l andGa(III)2 complexes of the title acids we reportin this paper the proton ligand formationconstants of thiomalic acid (TMA) , thiolactic acid(TLA) , thioglycollic acid (TGA) , malic acid (MA) ,glycollic acid (GA), lactic acid (LA), aspartic acid(AA) , ce-alanine (AL) and glycine (GL) and thestepwise stability constants of their chelates withGa(III) employing Irving and Rossetti" titrationtechnique.Materials and MethodsA stock solution of Ga(N03h8H20 (Fluka) wasprepared in a calculated quantity of HCI04 toprevent hydrolysis. The metal content was esti-mated by titration with EDTA4. For stoichiometricexperiments, freshly prepared Ga(~03h.8Hp solutionwas prepared without adding any acid. Extrapureligands were used for preparing the stock solutions,viz 0·02211 of TGA (E. Merck), TLA (Schuchardt,Munchen}, TMA (Schuchardt, Munchen), LA, (Riedel),and GA (Riedel); 0·05211 of MA (E. Merck); 0'015Mof AA (BDH) and 0·025211 AL (BDH) and GL (E.Merck) in doubly distilled water. These werestandardized potentiometrically, GA and AL werestandardized by adding an equal amount ofsolution of formaldehyde (35%) to the aqueoussolution. All reagents used were 99·9% pure exceptTGA which was found to be only 80% pure. Suit-able volume of this solution was added to maintainthe desired [TGAJ. Carbonate-free NaOH (AR)solution was prepared. Stock solution of NaCl04(neutral) and HCI04 (AR grade) were prepared indoubly distilled water.Expanded pH-meter (accuracy ± 0·02 PH units)of (Electronics Corporation of India) with glass-calomel electrode assembly (Cambridge type GHSN23) was used. The calomel electrode :vas connectedexternally by means of an agar-agar bndge saturatedwith K~03 to prevent the formation of chloro com-plexes. The pH-meter was calibrated with suitablebuffers. Thermostat (U-IO model, Germany) wasused to maintain the constant temperatures (± 0·1 "C424accuracy). Measurements were carried out at 25°,35°, 45° ± 0·1 ° under nitrogen atmosphere.Results and DiscussionThree mixtures were prepared as follows: (A}2·0 X 10-2M HCi04, (B) 2·0 X 10-2M HCI04 +5·0 X10-3211ligand and (C) 2·0 X 10-2211HCIO-i+5·0 X 10-3Mligand +1'Ox 10-3211 metal solution. In each casean appropriate amount of NaCI04 was added tomaintain [J. = 0·2./1,£and total volume of mixtureswas made 100 ml, and titrated against standardNaOH (l·004M). The proton ligand and metalligand stability constants were determined fromthe formation curves using methods (i) Interpolationat half riA (n) values, (ii) Interpolation at variousnA (1"i) values and (iii) least squares method. As allthe complexes are formed in short range of PH, toavoid the overlap of stepwise complexes, the valueby method (ii) were calculated in narrow range ofn values.Stoichiometry - For studying the stoichiometry,following four mixtures were titrated against NaOH(1'152M): (A) 5·0xI0-3M ligand; (B) 5'OxI0-3Mligand +5·0x 10-3M metal solution, (C) 5'Ox 10-3Mligand +2·5 X 10-3M metal solution and (D) 5·0 X10-3M ligand +1-66xlO-3M metal solution.In the case of TLA and TGA, the appearance ofan inflection at m~ 1·0 (m = moles of NaOH/moleligand after the addition of 1 mole of base per moleof ligand) correspon.ded to the complete neutraliza-tion of single carboxyl hydrogen and the equilibriummay be represented as:H2TLA+OH-~HTLA-+H20 ... (1)This also indicated that second proton is nott itrable ~nder experimental conditions. Thus onlytwo species H2TLA and HTLA- exist in PH rangecorresponding to the value of m from 0 to LAddition of equimolar concentration of Ga(IU) ions,greatly altered the shape of free ligand titration.curve due to complex formation which was followedby hydrolysis. The large decrease in PH value ofligand titration and lowered buffer region betweenAMBULKAR & MU~SHI: COMPLEXES OF Ga(III) WITH SOME SUBSTITUTED ORGANIC ACIDSm = 0 and 2 clearly showed the displacement ofproton from -SH group due to complexation. Whenthe ratio of Ga(UI)-TLA (or TGA) was 1:2, an in-flection at m:::::: 2. was obtained indicating the for-mation of Ga(TLA)2. The reaction equilibria maybe expressed by Eq. (2)Ga3++2H2TL.A+40H-?:Ga(TLA)2 +4-H20 ... (2)However, titration could not be continued becauseof immediate precipitation at pH --4·5. Whenthe ratio of Ga(UI)- TLA was 1:3 the curve showedonly one inflection at m:::::: 2 indicating the formationof Ga(TLA)~-. The overall reaction equilibriummay be represented by Eq. (3)Ga3++3H2TLA+60H-?:Ga(TLA)~-+6H20 ... (3)TGA-Ga(IU) system also exhibited a similarbehaviour and showed the formation of 1 :1, 1:2 and1:3 complexes.In the case of TMA only one inflection was ob-tained, after the addition of two moles of base permole of ligand, which corresponds to the completeneutralization of two carboxyl hydrogens in a singlestep, Third proton (SH) of TMA was not titrableunder experimental conditions. When the ratioof Ga(IIl)-TMA was kept 1:1, considerable decreasein the initial pH value and a lower buffer regionbetween m = 0 and 3 is observed indicating thedisplacement of proton due to complexation; noinflection, however was obtained before the precipi-tation of Ga(III) complex. In 1:2 ratio theinflection at m:::::: 3 indicated the formation ofGa(TMA)~-. The reaction equilibrium may berepresented by an equation similar to Eq. (3).The formation of only two complexes wasconfirmed by an inflection at m ::::::2·66 when Ga(III)-TMA were taken in 1:3 ratio. In the case of MAand AA, the appearance of only one inflection afterthe addition of two moles of base per mole of ligandcorresponded to complete neutralization of twocarboxyl hydrogens. As all the three complexesare formed below pH 6·0, only lower region isconsidered for discussion. In the ratio 1:1 ofGa(UI)-MA in lower region, the complex forma-tion is followed by slow hydrolysis of complex.The equilibria involved are:Ga3++H3MA+20H-~Ga(HMA)++2H20 (4)Ga(HMA)++H20~Ga(HMA)(OH)+H+ (5)When the ratio of Ga(III)-MA was kept as 1:2and 1:3 an inflection at m = 2 was observed,showing the formation of Ga(HMAt and Ga(HMA)g-respectively. This showed that no proton wasliberated during complexation from the -OH of MAand NHt (protonated form of -NH2) of AA. In thecase of LA and GA, though there was a fall in thePH value showing the displacement of proton dueto complexation, no inflection was observed beforethe precipitation took place for all the ratios.Hence, it may be concluded that no proton wasliberated during complexation from -OH of GA orLA. In all the ratios precipitation took place atPH 4·5.Proton-ligand stability constants - The formationcurves were extended from 0 to 1 for GA, LA, GLTABLE 1 - STEPWISE PROTON-LIGANDSTABILITYCONSTANTSOF THE LIGANDS ATVARIOUSTEMPERATURES (fL = 0·2M)Constant+ Values at25° 35° 45°TMAlog K{f 10·31 10·28 9·98log KIf 4·51 4·44 4·44log KIf 2·96 2·98 3·00TGAlog K{f 10·05 9·85 9·79~og K~ 3-45 3-46 3-47TLAlog K[l 10·38 10·29 10·24log Kif 3·56 3·64 3·68MAlog K{f 4·72 4·72 4;71log KIf 3-16 3·14 3·13AAlog K{f 3·77 3-68 3·60log KIf 1·90 1·88 1·86GAlog KH 3·30 3·34 3·49LAlog KH 3·54 3-48 3-48ALlog KH 2·43 2·45 2-45GLlog KH 2·37 2-40 2-43*Uncertainty limit ± 0·05and AL whereas for AA, MA, TLA and TGA thecurves were extended from 0 to 2 and in the case ofTMA from 0 to 3, in the nA scale. The values oflog K~, log K~ and log K~ as obtained by interpola-tion at various nA values have been summarized inTable 1.Metal-ligand stability constants - In all the cases(except TMA) the. formation curves are extendedfrom n = 0 to n ~ 2·6. Typical plots between nand pL for Ga(III)-TLA system at 25°, 35° and 45°are shown in Fig. 1. The existence of three stepequilibria is thus indicated. In Fig. 1, the forma-tioncurve rises sharply and flattens at n = 1·5-1·8which then again rises in all the cases, and goes ton = 2·6 and then again it remains constant. Thissecond sharp rise and the extension of n values be-yond 2·5 is a clear indication of the formation of the425INDIAN J. CHEM., VOL. 14A, JUNE 19767.0Fig. 1 - Formation curves of Ga(III)-TLA complex atfl.=0·2Mthird complex which was further confirmed bystoichiometric experiment described earlier in thispaper. The formation of first and second complexesin most of the cases have been found to occur in anarrow range of pH. The K; and K2 values werealso calculated by interpolation at various n valuestaking into account only those values where theoverlapping of two complexes appeared to benegligible. In case of GA and LA, the metal curvedid not become parallel with ligand curve after theformation of three complexes indicating the libera-tion of extra protons due to hydrolysis of metal ion.Precipitation in both the cases was observed at PH4·5. Hence only portions of tit rations curves inlower PH region were used for calculations. In thecase of TMA the formation curves were extendedfrom n = 0 to 2. The existence of two stepequilibria is thus indicated. In the case of AL andGL the n value rises abruptly from the very begin-ning and, therefore, metal-ligand stability constantscould not be calculated. The values obtained byinterpolation at various n values are given inTable 2.The complexes of mercapto analogues show higherstability as compared to those of hydroxy and aminoanalogues. The complexes of hydroxy analogue ofsuccinic acid are more stable than that of propionicand acetic acids. From exoerimental data it isclear that Ga(III)-thiomalat~ complexes are muchmore stable than malate and aspartate complexes.This can be attributed to the fact that -SH groupin thiomalic acid undergoes complex formationwhereas protons are not liberated from -OH of malicand -NH2 (present in protonated form) of asparticacid and also due to the fact that thiornalic acid acts426Consta.ntlog K~log K~log ~log ~log K~log K~log ~c log K~Blog K~Alog Kilog ~log K~log K~log Kilog ~log K~log K~log K~log ~log K~log K~log Kilog f3log Kilog K~log I(~log f3TABLE 2 - STEPWISE METAL-LIGANDSTABILITY CONSTANTSOF Ga(III) COMPLEXESATVARIOUS TEMPERATURES (fl. = 0'2M)Values at25° 35c 45°Ga(III)-TMA13'08 13'70 14-417'52 8'02 8'52-20'60 21'72 22'93Ga(I1I)-TGA11·75 11·52 11-4811'08 10'86 10-74-7'18 7·38 7-4830'01 29'76 29'70Ga(I1I)-TLA11'60 11'80 12'0210'99 11'20 11-486·47 6'95 7'2529·06 29·95 30,75Ga(III)-MA5'87 5·93 6·194'07 4·23 4'363'04 2·96 2·96-12·98 13·12 13·51Ga(III)-GA3'39 3042 3·453-14 3-17 3-202·67 2'8.5 2,989'20 9·44 9·63Ga(III)-LA3-28 HI 3·513'09 3'15 3'26-2'92 2·9·1- 2'979·29 9·50 9·74-Ga(III)-AA3·28 3·41 3·552·90 3·03 3'232-84 2'98 3·039'02 9'42 9'81a = ±0·08, b = ±0·15, c = ±0·20.as. tridentate ligand whereas malic and asparticacids act only as bidentate ligands. Comparisonof. stability constants of the thiomalate complexwith those of corresponding analogues of acetic andpropionic acids show large differences (approximately2·0 log units in K, values). This difference is toolarge to be attributed solely to the inductive effectof an additional carboxylate ion in either a five orsix membered bidentate, Hence it is reasonableAMBULKAR & MUNSHI: COMPLEXES OF Ga(III) WITH SOME SUBSTITUTED ORGANIC ACIDSto assume that thiomaIic acid acts as tridentateligand.AcknowledgementThanks are due to IProf R. H. Sahasrabudhey,Head, Chemistry Department, Nagpur University,Nagpur, for providing necessary research facilities..~-References1. SARIN, R. & MUNSHI, K. N., J. inorg, nucl. Chem., 35(1973), 201.2. AWBULKAR, R. s. & MUNSHI, K. N., J. I1Cdia'/l. chem.Soc., 52 (1975), 315.3. IRVING, H. M. & ROSSOTTI, H. S., J. chem, Soc., (19.54),2904.4. MILNER, G. W. C., Analyst, 80 (t 955), 77 .427

Complexes of Ga(III) with Thiomalic, Thiolactic, Thioglycollic, Malic Glycollic, Lactic & Aspartic Acids, & ɑ-Alanine & Glycine

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Complexes of Ga(III) with Thiomalic, Thiolactic, Thioglycollic, Malic Glycollic, Lactic & Aspartic Acids, & ɑ-Alanine & Glycine

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