Indian Journal of ChemistryVol. 32A, June 1993,pp. 502-505

Studies on speciation of aluminium complexes with malic acid byhigh field 27 Al NMR spectroscopy

S B Karweer, S N Mhatre, B P Pillai, R K Iyer* & P N MoorthyApplied Chemistry Division, Bhabha Atomic Research Centre, Trombay, Bombay 400 085

Received 8 September 1992; revised and accepted 30 November 1992

High field variable pH 27 Al NMR spectral studies of AtClj-malic acid system show the formation of asmany as four complex species in the pH range 2-8. The binding of malate to aluminium takes place through thehydroxyl and carboxyl groups resulting in five- and six-membered chelate rings with deprotonation of thehydroxyl group. Malate functions either as a bidentate or a tridentate ligand. The I: I AI-malate chelate hasbeen isolated and characterised by elemental analysis, potentiometric titration and IR spectroscopy. Theresults indicate deprotonation of hydroxyl group and bidentate nature of carboxylate groups. A bridgedpolymeric structure has been proposed for the chelate.

The coordination chemistry of aluminium hasattracted considerable attention in recent years dueto its possible involvement in several toxicprocesses \-3. There is growing evidence to suggestthat the specific toxic effects of aluminium onterrestrial and aquatic organisms are related to itschemical speciation". Malic acid is a biologicallyimportant aliphatic acid.':". Mohanty and Patnaik?have studied the AI-malic acid system potentio-metrically, while Greenaway" has shown theexistence of three complex species in the pH range 2-8on the basis of 21 Al NMR studies.

As part of a detailed study on speciation ofaluminium chelates with biologically andenvironmentally important organic ligands, wereport in this paper high field 27 AI NMR andpotentiometric studies of aluminium chelates withmalic acid. Co-existence of as many as four complexspecies has been observed. Isolation andcharacterisation of I: I AI-malate are also reported.

Materials and MethodsAlCf solution was prepared from reagent grade

AlC13.6H20 and standardised by EDT A titrationusing xylenol orange as indicator. DL-Malic acid,(LR grade, SO Fine Chemicals) was used. Stocksolutions of the complexes were prepared by mixingAICI3 solution and malic acid in the required molarratios. The pH was adjusted with either NH40H orHCI.

Potentiometric titrations were carried out against0.1 M KOH atan ionic strength 0.1 Mwith KCI as thesupporting electrolyte. The measurements weremade on an Elico Model U-120 pH meter equipped

with glass and saturated calomel electrodes. IRspectra were recorded in flurolube and nujol mull ona FTIR spectrophotometer (model Mattson Sygnus100) with caesium iodide optics.

Preparation of 1:1 AI-malateTo AICb.6H20 (6.04 g, 25 mmol) in 50 ml of water,

was added malic acid (3.35 g, 25 mmol) in smallquantities with stirring. The pH of the solution wasadjusted to 2.5-2.6 by addition of KOH solutiondropwise with stirring. On heating the solution to70-80°C, silky white turbidity appeared. On standingovernight the solution deposited a solid complexwhich was filtered, washed free of chloride and driedover P20S [Found: C, 27.07; H, 2.93; AI, 15.38, Calc.for C4Hs06AI: C, 27.27; H, 2.84; AI, 15.34]. Yield:409/0; the complex could be obtained even at pH 2, butin lesser yield.

27 At NMR spectraThe 27 AI{\H} NMR spectra were recorded at

130.32 MHz on a Bruker AM-500 spectrometerequipped with an Aspect-3000 computer at 297Kwith [Al(H20)6J3+ as the reference. Samples were takenin 10 mm tubes with a concentric 5 mm tubecontaining D20 for a lock. The FT-NMRmeasurement conditions were as follows: pulsewidth, 54 usee; flip angle, 90°; acquisition time, 0.79see; spectral width, 1302 Hz; number of transients200-500; pulse repetition time, I see; number of datapoints, 2K; digital resolution, 1.27 Hz per point;resolution enhancement, nil.

Results and DiscussionMalic acid has three coordinatin , -sites and IS

KARWEER et al.: SPECIATION OF AWMINIUM COMPlEXES WInI MAllC ACID 503

CO~ T coo -

"J~::>cooI AI I +:tHCO

H~O) +I , +CH2 ~H2 AI CH2 CH2 AII I ICOO COO COO COO(0) (b) (e) (d)

capable of binding metal ions in several ways(Structure I). Among these, the possibility ofchelation only through the carboxyl groups (Ie and Id)giving rise to a seven-membered ring is ratherremote. Chelation through the hydroxyl group canoccur with or without deprotonation. Potentio-metric and NMR studies of aluminium complexeswith a number of hydroxycarboxylic acids haveshown that aluminium has a very strong tendencyto displace protons from the hydroxyl groups ofhydroxyacids9-11. pH titration studies of a 1: Imixture of AICI3 and malic acid and aqueous solutionof solid I: I chelate have indicated deprotonation ofthe alcoholic hydroxyl group. The 27Al chemical shiftis dependent on the coordination geometry and theligand atoms involved in complex formation whilethe linewidth depends on the symmetry around thealuminium nucleus and the rate of chemicalexchange. (The more symmetric the complex, thesmaller is the linewidth). In cases where complexformation involves chelation, the observed chemicalshifts for chelates with five-membered ringsB.12-14are much higher than those for chelates containingsix-membered rings 15.16. Detailed studies of chelatesof aluminium with aminopolycarboxylic acids haveshown that replacement of coordinate water by OH-in octahedral geometry results in deshieldmg byabout 2 ppm 17.The 27AI NMR data for AI-malic acidsystem are summarised in Table I. No NMR spectraof I :I mixture could be recorded due to separation ofthe solid chelate. However, reasonably good spectracould be obtained for 1:2 system. In the case of 1:3mixture, the base lines in the spectra were not clearexcept at pH 3. The results show the coexistence of asmany as four complex species under certainconditions. The maximum chemical shift observedwas only 20 ppm compared to 24 ppm for lactate and33 ppm for tartrate. .

A remarkable feature in the spectra of Al-malatesystem is the existence of a resonance at 5 ppm (W 1/2

200-300 Hz) at pH 6-8 (Fig. 1). In analogy with theobserved peaks at 8 4.3 and 5.5 ppm in the case of 1.3Al-salicylate and Al-sulphosalicylate complexesrespectively!", this peak may be assigned to areasonably symmetric 1:3chelate in which the malategroup functions as a bidentate ligand leading to theformation of six-membered chelate ring (Ib). In this

Table 1_27 AI NMR data for aluminium-malic acid complexes[AICI3] 0.5 mol drn?

CL/C".! pH I) 27Ala Wli2 Probable assignment(ppm) (Hz)

2 3 0.25 100 [AI(HzO)6)3 +

2 3 9.1 [AIR, HL(H20)4t + [AIR, L(H2Oh]2 3 19.4 [AIRzLzl'-2 6 -1.72 6 5.0 330 [AIR3L3]6j[AIH_zLJP-2 6 12.2 600 [AIR,L(OH) (HzOH2 6 19.7 420 [AIH_zLzl'-2 8 5.1 220 [AIR3L))6-/[AIH_2L3P-2 8 13.9 750 [AIR, L(OHh(HzO)J2-2 8 20.0 440 [AIH_zL2l'-2 9 64.6 280 [AIH, L(OHhlz-3 3 0.23 165 [AI(H20)6)3 +3 3 7.7 570 [A!R,HL(H2O)4]+3 3 20.0 675 [AIRzL21'-3 5 -1.6 4103 5 11.4 1330 [AIRI L(OH) (H2Oh]"3 5 19.4 900 [AIRzL21'-3 7 -1.03 7 12.3 1200 [AIR1L(OH) (HzOM3 7 19.9 920 [AIH_zLzl'-3 8 5.0 .[AIR,L3l"/[AIRzL,P·3 8 11.9 1840 [AIR1L(OH) (HzOM3 8 19.3 1200 [AIR2Lzl'-

a Relative to external [AI(HzO)6)3+ in H20 (AICI), I mol drn')containing 20% D20. Positive shifts represent deshielding.

30 20 10ppm

o -10

Fig. I-Variable pH 27AI NMR spectra of the 0.5 M AIClrmalicacid (1:2) system [pH: a, 3.0; b, 6.0; c, 8.0]

504 INDIAN J CHEM, SEC A, JUNE 1993

chelate, coordination by one of the three malates mayor may not be with deprotonation of the hydroxylgroup.

The spectrum of a 1:2mixture at pH 3 (Fig. 1,curvea) shows a sharp signal at 0.25 ppm due to[AI(H20)6P+' The major resonance due to the complexspecies is observed at 9.1 ppm with a shoulder at 10-11ppm. In the case ofthe 1:3mixture, the major complexspecies at pH 3 is 0bserved at 7.7 ppm. This resonancemay be assigned to the I: I chelate with structure Ia bycomparison with the 8 8 ppm peak observed for I: IAI-lactate atpH 2.5 (ref. 14). Increase inpH results indownfield shift of this resonance with increase inlinewidth.

In AI-glycollic acid and AI-tartrate systems, theshift from 9 to 14 ppm has been attributed todeprotonation of the hydroxyl group 10.11. In thepresent case the resonance at about 12-14 ppm isobserved in the spectra of both 1:2 and 1:3mixtures inthe pH range 6-8. Earlier studies onAI-nitriolotriacetic acid system have shown theformation of hydroxo complexes at pH> 4.5, eventhough four of the six coordination sites wereoccupied by ligand atoms!". Considering the aboveresults, the resonance at 12 ppm may be tentativelyassigned to a monohydroxo I: 1AI-malate chelate inwhich malate functions as a tridentate ligand.

In the pH range 3-8, the spectra of 1:2 and 1:3mixtures show a broad resonance at 8 19.4-20 ppm.The stability of the complex species with the peak at 820 ppm over a widepH range (up to pH 8) suggests thatall the coordination sites in the chelate are occupiedby ligand atoms. However, the chemical shift ismuch lower than that observed for 1:3 chelates withlactate, glycollate and tartrate (24, 34, 33 ppmrespectively), in which the ligand functions in abidentate manner as shown in la. Since formation of1:3 chelate containing six-membered rings results inlow chemical shift (3-5 ppm), the resonance at 20 ppmmay be assigned to a 1:2chelate in which malate acts asa tridentate ligand (Ie), contribution from eachsix-membered ring being 1.5-2 ppm. Since theresonance is broad (Wlj2 450 Hz), this complexspecies has a distorted symmetry.

At pH 9, the spectrum shows a sharp resonance at65 ppm with Will 280 Hz. The resonance due to[AIu(OHb04r + is observed at 62.5 ppm, the signalbeing sharp!". Hence, the peak at 65 ppm may be dueto a tetrahedral chelate, [AIH-I L(OHhF', in whichmalate functions as a bidentate.

Potentiometric studiesThe pH titration curves for I: I, 1:2and 1:3mixtures

of Alel3 and malic acid show sloping inflections at m= 3.25,5.75 and 7.8 at pH values of 5.4,6.0 and 6.5respectively (m = number of moles of KOH addedper mole of AICl3). In the case of the 1:2 mixture, asecond sloping inflection was observed at m = 6.7, thepH of the solution at the inflection being 7.5. The 1:3titration curve was nearly superimposable on thecurve obtained by linear combination of the m valuesfor titration of 1:2 mixture and malic acid. Thenon-integral values of m may be attributed to thecoexistence of two or more complex species as shownby 27 Al NMR studies.

In the case of the 1:I titration curve, three protonsare liberated, two from the two carboxylic acidgroups and the third from the deprotonation ofalcoholic hydroxyl group. Hence the inflections at m= 3.25 and 5.75 are attributed mainly to theformation of I: I and 1:2 complex species withdeprotonation of hydroxyl group.

Characterisation of 1: I AI-malate chelateThe solubility ofthe 1:1chelate in water being very

low (0.29 g/lOO ml at 298 K), attempts to get acrystalline product were not successful. The pH of a0.001 mol dm-3 solution of the complex in waterprepared by boiling powdered complex with waterwas 5.1. The value of m at pH 5.1 as per the I: IAIClrmalic acid titration curve was 3.15. Hence, itmay be concluded that formation of the complex isaccompanied by the liberation of3 protons. Since thecomplex was isolated in the pH range 2.0-2.7 whereinaluminium complexes do not undergo hydrolysis, thethird proton can arise only by the displacement fromthe alcoholic hydroxyl group of malic acid.

The infrared spectrum of the complex shows thebands due to anti symmetric and symmetricstretching frequencies of carboxylates at 1592 em - J

(with a shoulder at 1650cm-l) and 1403cm -I (with ashoulder at 1460 ern -I) respectively, the separationbetween the two frequencies (6.v) being 189cm - I. Ingeneral, the bonding in aluminium complexes ispredominantly covalent. In cases where thecarboxylate groups coordinate in a unidentatemanner, with increase in the degree of covalency ofthe metal-carboxylate bond, 6. v increases-"-". Valuesas high as 276 and 280 em - 1 have been reported forCr(NH2CH2COOh and protonated aluminiumchelate of 1,2-propylenediaminetetraacetic acid-",However, in the case of I: I AI-malate chelate, thevalue is only 189 em - 1 which suggests that thebonding is either ionic or bridged. Since the formercan be ruled out it may be concluded that thecarboxylates act as bidentate donors. In the resulting

KARWEER et al.: SPECIATION OF ALUMINIUM COMPlEXES WTIH MALlC ACID 505

bridged polymeric structure (II) malate acts as atridentate ligand for one aluminium ion, but thecarboxyl groups are bonded with two additionalaluminium ions. The very low solubility of the chelatein water and its amorphous nature may be attributedto its polymeric structure.

A symmetrical bridged structure has been reportedearlier for Cr2 (acetatek2H20 (6 v, 155em -1)20.23.

Tridentate malate in which the metal ion is bridged bythe ~-carboxyl group and ionised hydroxyl group ispresent in the molybdate(VI) chelate"'. However, nocase has been reported so far in which both thecarboxyl groups of malate coordinate in a bidentatemanner. Further work is needed to confirm theproposed structure.

AcknowledgementThe authors thank the National NMR facility,

TIFR, Bombay, for recording the 27Al NMR spectra.Thanks are also due to the DAE for the award ofSRFto SNM.

;'., -

ReferencesI The environmental chemistry of aluminium, edited by G

Sposilo (CRC Press Inc, Florida, New York), 1989.2 Fatemi S J A, Kadir FHA, Williamson D J & Moore G R, in

Advances in inorganic chemistry, edited by A G Sykes(Academic Press Inc, California), 1991, p 409.

3 Martin R B, in Metal ions in biological systems, edited by HSigel & A Sigel (Dekker, New York), 1988, pi-57.

4 Gostomski F, Environ Geochem Health, 12 (1990) 51.5 The metabolic roles of citrate, edited by T W Goodwin

(Academic Press, New York), 1968.6 Comprehensive coordination chemistry, edited by Sir G

Wilkinson (Pergamon Press, Oxford, London), 2 (1987) 962.7 Mohanty N K & Patnaik R K, Indian J Chem, 14A (1976) 712.8 Greenaway K T, Inorg Chim Acta, 116 (1986) L21.9 Motekaitis R J & Martell A E, Inorg Chem, 23 (1984) 18.

10 Venema F R, Peters J A & Van Bekkum H, J chem Soc DaltonTrans, (1990) 2137.

11 Venema F R, Peters J A & Van Bekkum H, Inorg Chim Acta,191 (1992) 261.

12 Mhatre S N, Iyer R K & Moorthy P N, Magn Res Chem, 31(1993) 169.

13 Llinas M& De Marco A, JAm chem Soc, 102 (1980) 2226.14 Karlik S J, Tarien E, Elgavish G A & Eichhorn G L, Inor Chem,

22 (1983) 525.15 Lutz G, Clevette D J, Rettig S J & Orvig C, Innorg Chem, 28

(1989) 715.16 Haraguchi H & Fujiwara S, J phys Chem, 73 (1969) 3467.17 Iyer R K, KarweerS B&Jain V K, Magn Res Chem, 27 (1989)

328.18 Mhatre S N, Iyer R K & Moorthy P N (Unpublished).19 AkittJ W, Greenwood N N, Khandelwal B L & Lester G D, J

chem Soc Dalton, (1971) 604.20 Nakamoto K, Infrared spectra of inorganic and coordination

compounds (John Wiley, New York), 1963, p.198. 203.21 Nakamoto K, Morimoto Y & Martell A E, JAm chem Soc, 83

(1961) 4528.22 KarweerS B, Pillai B P'& Iyer R K, Indian J Chem, 30A (1991)

1064.23 Van Niekerk & Schoening, Acta Cryst . 6 (1953) 227.24 Tapscott R E. in Transition metal chemistry,Vol8. edited by G

A Melson and B N Figgis(Marcel Dekker. New York). 1982.