Indian Journal of ChemistryVol. 35A, April 1996, pp. 320-323

Studies on the template synthesis, kineticsand mechanism of cadmium{II) complexes

of schiff base derived from glycine andninhydrin

Zaheer Khan, S I Ali*, D Gupta' & A A Khan!Department of Chemistry, Jamia Millia Islamia,

New Delhi 110025

Received 19 December 1994; revised 16 August 1995;accepted 14 November 1995

N-Diketohydrindylideneglycinatocadmium(II) (schiffbase) has been synthesized from glycine-cadmium(II)interaction with ninhydrin and characterized by IRspectra. Kinetics of the reaction has been carried outspectrophotometrically under varying conditions of[ninhydrin], pH and (acetate ion] at different tempera-tures. During the course of reaction, formation at twodifferent coloured products have been observed. Thereaction follows a first order kinetics with respect to[glycine-cadmium(II)] and fractional order kineticswith respect to [ninhydrin] for the formation of bothproducts. The reaction proceeds through the complex-ation of ninhydrin with glycine-cadmium(II) which isthe feature of template mechanism (CLAM reaction).On the basis of observed data, probable structure andtemplate mechanism have been proposed for the for-mation of two coloured products (Ama. = 375 and510 nm). The values of kl' k2' K, and K, calculatedfrom kinetic data are found to be 3.33 x 10- 3S - "

6.25 x 10 -4S -1,13.4 and 14.2 respectively.

The most important 1.2template reaction is the for-mation of imine bonds (schiff base) used as ligandin the formation of metal complex':", The glycineand ninhydrin reaction is an example of nucleo-philic addition" of amino group to carbonyl groupand proceeds through the formation of schiff basein the early stage and is too unstable for isola-tion?". In the case of glycine-metal complexes, thelone pair of electrons of amino group are coordi-nated to the metal ion. Therefore, the coordinatednitrogen donor atom can be involved in chelateforming template reaction by virtue of nucleophil-ic addition to carbonyl group of ninhydrin. Thesereactions are now widely claimed" - 10but seldomproved and detailed mechanistic work has notbeen carried out on ninhydrin-glycine interaction

'Department of Chemistry, Aligarh Muslim University, Ali-garh 202 002

and nothing is known about the schiff base com-plex derived from glycine-ninhydrin reaction withcadmium{II). Therefore, we planned to study thesynthesis of cadmium{II) chelate of schiff basederived from glycine and ninhydrin with specialinterest in the investigation of kinetics and me-chanism of the reaction and are reported in thisnote.

ExperimentalGlycine, ninhydrin and cadmium nitrate (all

BDH) were used as such. Solutions of glycine,ninhydrin and cadmium nitrate were prepared insodium acetate-acetic acid buffer using Elico LI-120 digital pH-meter. IR spectra (KBr) were re-corded on an IR-408 (Shimadzu). All the reagentsused were of AR grade. Doubly distilled waterwas used throughout the studies.

The kinetics were followed at the desired tem-perature maintained at an accuracy of ± 0.1 "C.The reaction was initiated by adding the appro-priate volume of ninhydrin solution in reactionvessel which has the equimolar (1.0 x 10 - 3 moldm-3) solution of glycine and cadmium{II). Eva-poration was prevented by using a double surfacecondenser. Efficient shaking of the reaction mix-ture was done by bubbling oxygen free nitrogengas which also served to keep in inert atmos-phere. Progress of the reaction was followed spec-trophotometrically by pipetting aliquots of thereaction mixture at definite time intervals and theabsorbance were measured at 375 and 510 nm.Rate constants were calculated over 80% comple-tion of the reaction.

\

Preparation of cadmium lh complex of schiff basederived from glycine and ninhydrin

The yellow coloured (schiff base) complex wasprepared by the following general procedure asdescribed elsewhere II - 1.\ for the formation ofother schiff base complexes. Glycine (0.05 mol)was dissolved in a minimum quantity of distilledwater and to this was added cadmium nitrate(0.05 mol) in water (100 ml) at room temperature.Ninhydrin solution (0.05 mol in 150 ml of 95%ethanol) was added dropwise under constant stirr-ing. A yellow colour developed immediately andafter few minutes a heavy precipitate was ob-served. The mixture was allowed to stir at 313Kfor 1h. The yellowish orange (Amax = 375 nm) pro-

duct was then collected on a filter, washed severaltimes with water, 95% ethanol, ether and thendried in vacuo over P20S. The filtrate of the reac-tion mixture was refluxed for l h and the solutionconcentrated. The reddish coloured solid got sep-arated on cooling when the reaction mixture wasset aside for few hours. The chemical analysis foryellowish orange complex gave: [Found N, 3.98;C, 38.lO; H, 2.30; Reqd for C11HsNOsCd N, 4.04;C, 38.15; H, 2.31%] and for reddish complex:[Found N, 3.12; C, 50.00; H, 2.08; Reqd forC1sHION05Cd: N, 3.24; C, 50.00; H, 2.31%] re-spectively.

The yellowish orange complex ()"max = 375 nm)is partially soluble in ethanol, methanol and chlor-oform. The complex is highly soluble in coordi-nating solvent (DMSO) but insoluble in commonorganic solvents. The melting point was found608K and the complex was air stable. The red-dish complex (Amax = 510 nm) is soluble in meth-anol and chloroform. It is very unstable in aque-ous solution which gives purple colour after theaddition of water in methanol containing reddishcomplex. Similar observations were reponed byWieland et al.14 for the complex of cadmium(U)with the final product of glycine-ninhydrin reac-tion (purple colour). Lennard et al.'5 studied thecomplexation of this product with cadmium(II)and reported the structure of the complex hyX-ray method.

Results and discussionThe yellowish orange complex (I) was found to

be non-conducting in DMSO. The composition ofthe complex has been determined by employingthe Job's method of continuous variations and itwas found that one mole of ninhydrin reacts withone mole of glycine-cadmium(II). The mode ofcoordination has been assigned by comparing theIR spectra of 1 with its Jigands (ninhydrin and gly-cine). Ninhydrin shows vOH at 3300 and vC = 0at 17SO and 1720 ern - 1 respectively. These fre-quencies of > C = 0 showed a negative shift withthe complex suggesting its coordination to themetal ion. The IR spectrum of glycine exhibitsvNH2 at 3200-2700 and vCOO (as) at 1600cm - I. The complex shows a band at 1620 ern - 1

assigned to vCOO (as). A positive shift of 10-20cm - 1 is observed in the antisymmetric stretchingfrequencies of the carboxylate group!", the posi-tive shift implying its coordination with cadrni-um(II). The IR frequencies corresponding to the- OH of ninhydrin and - NH2 of glycine are ab-sent in the spectra of complex. A new band is ob-served at 1570 cm - 1 in the spectra of complex,

NOTES 321

,0 CHz--C=O

C 5~::@X)=,{~/U ' ce 'O~--'vo'H2

l(P)

Sc he rne 1---suggesting unequivocally, the coordination of ni-trogen of C = N group. It could therefore bc con-cluded that schiff base complex (I) behaves as atridentate ONO donor involving a carbonyl oxy-gen and an azornethine nitrogen in coordination(scheme I).

Preliminary kinetic studies showed that cadrni-um(Il) formed a red colourcd complex with pur-ple colour-adduct (end product of glycine-ninhy-drin reaction) giving only one peak (Amax = 5 I()nm) at room temperature. 1t is a very fast reac-tion. If a solution containing an excess of ninhy-drin was reacted with glycine-:admium(1l) solu-tion at pH 5.0 and 303K, the appearance of yd-low colour was observed with Am", = 375 nrn, Asthe temperature of the reaction mixture is in-creased, a new peak begins to develop at A""I\ =510 along with the peak at 375 rom. The peak at510 nm was the same as that previously observedfor the interaction of cadmium(IJ) amino acid withninhydrin!". On the basis of these observations, itwas concluded that two coloured products (yellowand red) were formed during the interaction ofninhydrin with glycine-cadmium complex. Thekinetics of the formation of these two colouredproducts was carried out at two different temper-atures.

Kinetics of formation of 1 [N- diketohydrindyliden-eglycinatocadmium; II) 1

The ninhydrin concentration was always main-tained at a large excess over the concentration ofglycine-cadmium(II). The order of reaction withrespect to glycine-cadmium(lI) is found to be oncas evidenced by the linear plot of log [glycine-cad-mium) versus time. The order with respect to[ninhydrin] is fractional (0.54). Kinetic data for theformation of 1 are compiled in Table t. The effectof pH was studied within a range of 3.8 to 5.0 at303K in acetic acid-sodium acetate buffer beyondwhich no measurable reaction was possible due toprecipitation of basic cadmium hydroxide. Thestahle products were formed in the range of jJH

5.0. Therefore, the studies were limited upto pH

322 INDIAN J CHEM, SEC. A, APRIL 1996

Table l+-Effect of varying ninhydrin concentrations on the rateconstants

102 x [ninhydrin] 103 x kl(ob,) 104 X ~(ob')mol dm - 3 S - I S - I

2.0 0.66±0.002(0.62) 1.38 ±0.005(1.37)3.0 0.98 ±0.002(0.95) 1.98 ±0.02(1.93)4.0 1.23 ± 0.004( 1.10) 2.26 ± 0.004( 2.20)6.0 1.53 ± 0.006( 1.50) 2.29 ± 0.003(2.21)8.0 1.73 ± 0.005(1.71) 3.16 ± 0.002(3.00)

10.0 1.93±0.004(1.89) 3.76±0.004(3.50)12.0 2.06±0.003(2.08) 3.90±0.003(3.70)14.0 2.16±0.001(2.18) 4.13±0.OOI(4.00)18.0 2.33 ± 0.002(2.30) 4.50 ± 0.002(4.20)

Calculated values of rate constant are given in parentheses.Conditions for kl(obs):[Glycine-Cd(II)]= 4.0 x 10- 3mol dm - 3,

pH = 5.0, temp. = 303K.Conditions for ~(Ob.I:[Glycine-Cd(II)J = 1.0 x 10 -3mol dm - 3,

pH = 5.0, temp. = 353K.

5.0 (optimum pH for the amino-acid ninhydrinreaction).

Before attempting to propose a mechanism forthe reaction of ninhydrin with glycine cadmi-um(II), it is necessary to discuss the nature of cad-mium(ll) and species of glycine cadmium(II) com-plex existing at pH 5.0. It has been reported'<'?that cadmium(II) has a weak complex formingability with organic ligands compared to otherbivalent metal ions. Therefore, glycine exists inthe form of cadmium(II) complex and free glycine.Under the present experimental conditions, it canbe assumed that the complex is entirely in theform of [cadmiumrllj-glycine]" at pH 5.0. The va-lue of formation constant of this complex is 4.23.In the light of these observations, the mechanis-mis given in scheme 1.

In scheme 1, lone pair of electrons of amino ni-trogen which are essential for the nucleophilic at-tack on the carbonyl carbon are coordinated tothe cadmium(II). Therefore, simple nucleophilicaddition is not possible. The reaction proceedsthrough the inner sphere complexation of ninhy-drin with cadmium(II) of glycine complex. The in-terligand condensation occurs within the coordi-nation sphere of cadmium(II) leading to the for-mation of schiff base (I). This reaction has thefeatures of kinetic template mechanism (CLAMreaction) in which both the ligand (ninhydrin andglycine) are coordinated with the same metal ion(cadmium (II)). The rate equation for scheme 1 isderived as follows:

rate = d[PVdt = kJ[C][C] = KJA][N]d[P]/dt = kJKJA][N]At any time t, the concentrationof [AJ.r

[A]T= [A] + [C][AJ.r=[A]+ KJA][N] .'d[P]/dt = kJKJninhydrin][AJ.r/l + KJrnnhydnn], andk = k KfninhydrinVI + KJninhydrin] ... (1)J(obs) J A

on rearranging Eq. (1 ),

1/ IG(obs)= B/[ninhydrin] + B2

Where BJ = 1/ IGK, and B2= 1/ kJ

... (2)

According to Eq. (2), the plots of 1/ IG(obs)versusl z[ninhydrin] should be linear. This has beenfound to be the case, thus supporting the pro-posed mechanism. B, and B2 are the gradientsand intercepts of the plots. Fractional order in the[ninhydrin] and a definite intercept in the 1/IG(obs)against 1/[ninhydrin] plot, suggest that the con-densation of coordinated amino group to coordi-nated carbonyl group is the rate determining step.From the intercepts and gradients, the values ofk, and K, have been calculated and found to be3.3 x 10 - 3S - J and 13.4 respectively. These valuesfurther indicate that kl is the rate determiningstep and confirms the proposed mechanism. Sub-stiuting the values of k, and K, in the rate Eq. (1),the rate constants have been calculated in variouskinetic runs and compared with the values deter-mined experimentally (Table 1). The close agree-ment between the observed and calculated valuesprovides supporting evidence for the proposedmechanism and also for Eq. (1).

The metal ions catalyzed'? and retarded-' thehydrolysis of schiff bases. The carbon-nitrogendouble bonds of schiff bases are susceptible bothto hydrolytic cleavage and the coordination withmetal ions. Schiff bases capable of forming bicyc-lie chelate rings are known to be stabilized tow-ards hydrolysis in the presence of metal ions un-der mild acidic conditions. However, the schiffbases Which forms monocyclic chelates are com-pletely hydrolyzed in the presence of metal ions.In the interaction of glycine with ninhydrin, thereaction proceeds through the formation of schiffbase, acts as a potential tridentate ligand contain-ing aNa donor atoms and forms the bicyclicchelate ring with cadmium(II). The presence ofcadmium(lI) inhibits the cleavage of - COOgroup by reducing the escaping tendency of- COO group and by enhancing the electrophiliccharacter of >C = a group, whereas the decar-boxylation of uncomplexed schiff bases is veryfast. The role of cadmium(II) in this reaction is tostabilize(l) towards decarboxylation and hydroly-sis.

Kinetics of the formation of red coloured product(II)

The kinetic studies of the formation of thiscomplex was carried out under same conditionsas for (I). The results are given in Table 1. Thereaction is fractional order with respect to [ninhy-drin) and optimum pH of stable product wasfound in the vicinity of pH 5.0. [Acetate ion) hadno significant effect on the rate constants. Forma-tion of II was observed at temperature higherthan 323K along with the peak at 375 nm.

Due to the labile nature of glycine-cadmium(II)complex, glycine reacts with ninhydrin in theusual way to give 2-aminodanedione which fur-ther reacts with cadmium(H) and ninhydrin toform II through the template reaction path(CLAM reaction). The proposed mechanism isgiven in Scheme 2. The following kinetic rateequation has been derived:

kz(Ohs)=kzK'Jninhydrin)/l + K'Jninhydrin) ... (3)

Rearrangement ofEq. (3) gives,

II ~(ohs)= C/[ninhydrin] + C2 ... (4)

where C 1 = 1I k.K', and C2 = 1/ kz

Thus a plot of 1/kz(ohs)against Lz[ninhydrin]should give a straight line. C1 and C2 were calcu-lated from the intercepts and slope of plots. Thevalues of k2 and K', were found' to be6.2 x lO-~s -I and 14.2 respectively. The rate con-

NOTES 323

stant (kcal) have been calculated by substitutingthe values of kz and K'[ in Eq. (4) and confirmsthe validity of rate equation and proposed me-chanism. During the studies of red colour forma-tion, it was also observed that intensity of peak at375 nm was increased with time and it was con-firmed that the reaction of glycine and ninhydrin

. proceeds through the formation of schiff base inthe early stage of the reaction.

AcknowledgementWe are thankful to Prof. M A Beg, Chairman,

Dept. of Chemistry for providing research facrlit-res.

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