Indian Journal of ChemistryVol. 33A, October 1994, pp. 932-936

Study on kinetics and mechanism ofreaction of ironifll) with salicylaldoxime

and ortho-hydroxyacetophenoneoxime inHCI04-NaCI04 media

Rajani K Mohanty*, Asim K Das & Mahua Das

Department of Chemistry, Visva-Bharati, Santiniketan 731 235,India

Received 10 January 1994; revised and accepted 11 May 1994

The kinetics of the reaction of ironrlll) with the titleligands leading to 1:1 chelate formation have been stud-ied at different temperatures (25°-35°C) and ionicstrength /= 1.0 mol dm-J (NaC104 + HCI04). A dualpath mechanism involving both Fe(aq)H andFe(OH)(aq)2+and undissociated ligand (lli2) is consist-ent with the experimental observations where[H+]»TFe»TL (where TFeand TL stand for the to-tal concentration of iron and ligand respectively). Theresults conform to: kob,lB= k\[H+]+ k2Kh whereB=TFe/([H+]+Kh)+l/Q; Kh=hydrolysis constant ofFeiaql!": k, and k2 are the second order forward rateconstants of Fe(aq)3+ and Fe(OH)(aq)2+ respectivelyand Q is the equilibrium constant of the reaction,Fe(aq)3+lli2?Fe(lli)2++H+. At higher acidities, dis-sociation of the complex has been followed and the rateconstants determined from the principle of reversibilityare in good agreement with those obtained from thestudies of formation of the complex at lower acidities.Activation parameters (!l.H" and !l.S") for each of thesteps have been determined. Fe(OH)(aq)2+ appears toreact in a dissociative fashion (Eigen-Tammmechanism)and its characteristic water exchange rate has .been ob-tained as a rough estimate. On the other hand, Fe(aq)3+appears to react through an associative interchange (I.)mechanism.

Although the mechanism of ligand substitutionprocess of different divalent labile metal ions offirst transition series is well understood 1• the me-chanistic interpretation of kinetic behaviour in thecase of trivalent first row transition metal ions, es-pecially iron(III), is not so clear-cut. Because ofthe strong propensity/ of Fe(aq)3+ to Undergo hy-drolysis even in a moderately acidic solution toproduce Fe(OH)(aq)2+ which may undergo dimeri-sation ':", the number of. possible reactive speciesof iron(III) increases, and consequently, the me-chanistic interpretation gets complicated. The hy-drolysed species i.e., Fe(OHXaq)2+ is more labile"than the unhydrolysed species, Fe(aq)3+ and the

hydrolysed species acquires the dissociative char-acter", Because of the enhanced reactivity ofFe(OH)(aq)2+, very often the unhydrolysed speciesloses its kinetic contribution to the overall process.If fact, in many cases"!", only Fe(OHXaq)2+ hasbeen found kinetically active and in a very fewcases such as hydroxamic acids II, salicylates!", ox-ines'! and phenols's", both Fe(aq)3+ andFe(OH)(aq)2+ have been found kinetically active.In fact for' a generalised mechanistic conclusionregarding kinetic behaviour of Fe(aq)' + more in-formation is required.

Under the experimental conditions, if the li-gands under consideration can participate in pro-tolytic equilibria, then the total number of possiblereaction paths involving all the species get signifi-cantly proliferated to complicate the mechanisticinterpretation. The present system involving the ti-tle reaction does not involve any such proton am-biguity, as the ligands chosen do not participate inprotonation-deprotonation equilibria under the ex-perimental conditions.

ExperimentalStandard stock solutions of Fe(CI04h, NaCI04

and HClO 4 were prepared as described earlier 11 •

The ligand salicylaldoxime (A.R., B.D.H.) wasused without any further purification and the otherligand. ortho-hydroxyacetophenoneoxime was pre-pared from ortho-hydroxyacetophenone as usual'"and its purity was checked by elemental analysisand melting point (m.p. 115°C, lit. 16 m.p. 117°C).Doubly distilled water was used to prepare all thesolutions.

The spectra of the freshly prepared complexesin aqueous solution were recorded using a Beck-man DU-6 spectrophotometer and kinetic mea-surements were carried out in a stopped-flowspectrophotometer (SF-3A, Hi-Tech, U.K.) cou-pled with an oscilloscope (Advance Instruments1000A) and a microprocessor with the help ofwhich the pseudo-first order rate constants werecalculated. The flow module of the SF-3A andthermostatting arrangements (± 0.05°C) for thereacting solutions and the observation cell. Ionicstrength was adjusted to 1.0 mol dm - 3 with anadequate amount of NaCI04 in addition to HCI04present in solution.

Results and discussionThe title ligands form bluish-violet complex in

the reaction with iron(III) in weakly acidic solu-tion. Under the experimental conditions, TFe(2.0-5.0) X 10-3 mol dm-3, TL = (2.0-3.5) X

10-4 mol dm ":', TF/TL~ 10, [H+](0.08-0.15) mol dm-3, l= 1.0 mol dm-3, at theAmax (580 nm for salicylaldoxime system and 570nm for ortho-hydroxyacetophenoneoxime system,(cf. Fig. 1) of the corresponding product complex,the oscilloscope traces of the stopped-flow systemshow a rise in absorbance with time indicating for-mation of the complex. From the knowledge of hy-drolysis constant! (Kh) of Feiaq)!" (Kh = 1.65 x

(e)0·3

0·100·2~eo11« 0·1

ou

01~=c=====r~==B=)~(A=)==~==~~7001420 490 560 630Wavelength, nm

0·05III

«

Fig. I-Absorption spectra of aqueous solutions of the com-plexes, Fe(III) and ligands. Temp. 30°C, [H +]= 0.2 mol dm - 3,

1=1.0 mol dm-3 (NaCl04+HCl04); A: TEe= 8.5 X 10-3 moldm - 3; B: Salicylaldoxime = 2.3 x 10- 3 'mol dm - 3; C:TFe= 8.S X 10-3 mol dm -3, Salicylaldoxime = 2.3 x 10-3 moldm - 3; D: T Fe= 8.5 x 10- 3 mol dm - 3, ortho-hydroxyacetophen-oneoxime=3.0xlO-3 mol dm-3, [H+J= 0.2 mol dm+': (0,..tho-hydroxyacetophenoneoxime (not shown here) under theexperimental condition is almost non-absorbing in the studiedspectral range as in the case of salicylaldoxime; "max for eachcomplex under the experimental conditions remains un-

changed for T FiTL.~ 1).

NOTES 933

10-3 and 2.95 x 10-3 at 25°C and 35°C respect-ively at l= 1.0 mol dm-3) and acid dissociationconstants" (salicylaldoxime: pKa(l)= 9.02, pKa(z)= 11.88 at 25°C, l= 0.1 M; ortho-hydroxyaceto-phenoneoxime: pka(l) = 9.10, pK<i.Z)= 11.97 at30°C, /= 0.1 M) of the ligands, LHz, we can rea-sonably consider reaction Scheme 1.

3+ k 1 2+ + kFc(aq) +L~ ~FcLH +H ,CQ=_.l_).Ci)

<-H+)KhJf k-1 L\2+ !c2 2+ Q «2

FcCOHHaq) +lH2 ~FeLt1 ,C--.:: --), Cii)~ k k

~2 h-2

Scheme1

Scheme 1 leads to rate Eq. (1) (when [H +]> > TFe»TL)·

k )k1[H+]+k2Kh)T +~(k [H+]+k K)obs. [H +] + K h Fe Q 1 2 h

... (1)

or, kob/B=k1[H+]+ k2Kh ... (2)

where,

At fixed [H+], plots of kobsversus TFe are linear(cf, Eq. 1) with positive slopes and intercepts fromwhich the values of Q could be evaluated(Table 1) as follows:

Q=. slope x ([H+]+Kh)

intercept... (3)

The plots of kob/B versus [H +] are linear (cf Eq.2) with positive slopes and intercepts from whichk; and kz could be evaluated (Table 2). By usingthese values along with those of Q, the backwardrate constants were calculated.

Table I-Thermodynamic parameters of the equilibrium constant (Q) of the process, Felaq)!" + LH2;=, FeLH+ + H+(LH2 stand forsalicylaldoxirne and ortho-hydroxyacetophenoneoxime)

(Experimental conditions are given in Fig. 2)

Ovalue'"Ligand.

25°C 30T 3SOC

Salicylaldoxime 47.6± 1.0 43.8 ± 1.0 46.8 ± 1.5 0.0 ±0.4 31.9 ± 1.2(46.5±1.2) (45.7±0.9) (46.4±1.0)

Ortho-hydroxyacetophenoneoxime 1l.4±0.3 13.1 ±0.2 15.2 ±0.2 21.3 ± 1.0 91.8 ± 3.2

(a) Values within the parenthesis are obtained from acid catalysed dissociation studies.

934 INDIAN J CHEM, SEe. A, OcrOBER 1994

Table 2-Kinetic parameters of interaction of iron(III) with salicylaldoxime and onho-hydroxyacetophenoneoxime in aqueous perch-loric acid media

(Experimental conditions are given in detail in Figs 2 and 3)

Ligand Temp. lO-'k, L, 10-4 k2 10 k-2°C (dm3mol-ls-l) (dm3mol-ls-l) (dm3mol-1s-l) (5-1)

Salicylaldoxime 25 1O.60±0.50 2.37±0.09 0.95±0.04 3.50±0.11(11.20±0.70) (2.50±0.11) (1.06±0.06) (3.S0±0.OS)

30 21.00±0.SO 4.75±0.15 1.27±0.09 6.20±0.15(21.80 ± 0.90) (4.S6±0.OS) (1.38±0.12) (6.60±0.18)

35 36.10 ± 1.00 8.02±0.15 1.60±0.1O 1O.30±0.25(35.70 ± 1.20) (7.93±0.17) (1.76±0.12) (11.45 ± 0.20)

s n: (kJ mol-I) 88.5 ±2.5 87.8±2.0 3S.6± 1.5 79.9± 1.5ss- (JK-'mol-') 89.S±3.5 55.5 ±2.2 -40.2±2.0 12.6±0.8Ortho-hydroxyacetophenoneoxime 25 4.00±0.20 3.52±0.06 1.3S±0.04 20.00± 1.00

30 6.20±0.50 4.70±0.05 1.61 ±0.04 27.1O±0.8035 9.30±0.SO 6.13±0.07 I.S7±0.06 36.00±0.70

!l.H~ (kl mol t") 61.9 ± 1.5 40.0± I.S 20.9± 1.0 42.9± 1.2ss- (JK-1mol-l) -S.6 ±0.7 -102.1 ±4.2 -97.0±3.5 97.4±2.8

Values within the parenthesis are obtained from formation kinetics.

3 -310 T • mol dm

F~

Fig. 2-Evaluation of equilibrium constants (cf. Eq. 4) fromkinetic study of acid catalysed dissociation of monocomplex.lron(lII)-ortho-hydroxyacetophenoneoxime system:T~e= (8.0-20.0) X 1O-J mol dm-J, T L= 3.0 x 1O-~ mol dm-J,[H+j=0.30 mol dm", 1= 1.0 mol dm-J (NaCl04 + HCl04)·

A(25°C), B(30°C), C(35°C). Iron(lII)-saIicylaidoxime system:TF• = (8.0-20.0) X 10-3 mol dm-3, TL=2.5xlO-4 mol dm":';:[H +]= 0.50 mol dm - 3, 1= 1.0 mol dm - J (NaCiO. + HCIO.).

D(25°C), E(30·C)

'",

'"D",0

0'2 0'6 0'8

Fig. 3-Evaluation of rate constants (cf. Eq. 5) from kineticstudy of acid catalysed dissociation of mono-complex. lr-on(IIl)-salicylaidoxime system: T Fe= 2.5 x 10- J mol dm --'.TL-2.5XIO-~ mol drn ":' [H+]=(0.3-0.9) mol dm >', 1=1.0mol dm - J (NaCl04 + HCIO~), A(25·C), B(30·C), C(35·C). Ir-on(IlI)-onho-hydroxyacetophenoneoxime system:TFe=4.2XIO-·\ mol dm ' ', TL=3.0XlO-4 mol dm--',[H+] =(0.3-0.9) mol dm >', 1=1.0 mol dm"'

(NaCiO. +HCI04), D(25°C), E(30·C), F(35·C)

The mono-complex was prepared in solutionunder equilibrium conditions at low acid concentr-ations, i.e., TFe = 5 X 10-3 mol dm-3, TL= 5 X 10-4mol dm-3, [HCI04]=0.05 mol dm >' at 1= 1.0mol dm - 3. This equilibrium mixture, in one sy-ringe of the stopped flow system was treated withan excess of acid (containing the requisite amountof NaCI04 and Fe(CI04h for variation of TFe) inthe other syringe of the stopped flow system tomaintain the desired concentrations of the react-ants (conditions are given in detail in Fig. 2 and3). Under these conditions, the oscilloscope traceat f... maxof the complex showed a decay absorbancewith time. This suggests a shift of the equilibrium(i) from right hand side to the left hand side, i.e.,dissociation of the complex. In this case, assumingthe principle of reversibility to be true, Scheme 1,leads to rate equation (4 ).

k = (L I [H +]+ L 2) TFe Q + (k [ +]+ k )obs [H + ] + K h - I H - 2

... (4)

or, kob/C= k_1 [H+]+ k-2 ... (5)

where,

According to Eq. 4, the plots kobs versus TFe

should be linear (cf. Fig. 2) with positive slopesand intercepts from which the values of Q couldbe evaluated. Thus the estimated values of Q nice-ly agree (Table 1) with those obtained from kineticstudy of formation of the complex at low acidities.From the plot, kobsIC versus [H+1 (d., Eq. 5 andFig. 3) the values of L I and L2 could be evalu-ated. By using the values. of Q and backward rateconstants, the corresponding forward rate con-stants have been evaluated (Table 2). The valuesobtained from both studies are in good agreement.

From the Q values (Table 1), it is evident thatsalicylaldoxime forms a stabler complex thanortho-hydroxyacetophenoneoxime. It is probablydue to the higher steric hindrance in ortho-hydroxyacetophenoneoxime. In fact, enthalpychange (l:!.H) for ortho-hydroxyacetophenonoeximeis more positive.

According to Eigen mechanism!", the overallformation rate constant is given by k = kex Kos ,where Kos is the outer-sphere association constantand kex is the first order rate constant for waterexchange. Kos has been estimated 19 as ca. 0.2mol- 'dm - 3 for ion-molecular dipole interactions,as in the present investigation, and it can be com-puted with the Fuoss equation-" in the cases where

NOTES 935

both the reactants are charged. For salicylaldoximeand ortho-hydroxyacetophenoneoxime, using thek2 values, the kex values for Fe(OHXaq)2+ become5.3 x 104s- 1 and 6.9 x 104s- 1 respectively at 25°C.These values are within the range (0.1-8.0) x 104s-I at 25°C, estimated? from kinetic datafor a large number of ligands differing widely innature, structure and basicities of the binding sites.This observation suggests the interchange dissocia-tive (Id) mechanism at Fe(OHXaq)2+ centre.

If the same type of dissociative mechanism issupposed to be operative for the kl path (whereKos is assumed 19 to be 0.20 mol- 'dm ' for ion-di-pole interaction) then the water exchange rate forFe(aq)3+ becomes 5.62 x 102s-[ and 2.00 x 102s- 1

at 25°C for salicylaldoxime and ortho-hydroxyacetophenoneoxime. In contrast to the k2path, the estimated kex values in k, path for differ-ent types of other ligands are found to cover awide range!' (9.0-9.5XI05s-l) depending uponthe nature, structure and basicity of the bindingsites. These observations suggest an interchangeassociative mechanism (Ia) for the kl path. Thisdifference in mechanism is in keeping with theearlier conclusions'v+P regarding associative anddissociative reactions of Feiaq)!" andFe(OH)(aq)2+ species respectively. A similar kinet-ic behaviour has been noted with ceriumil'V )" andchromium( III)24.

AcknowledgementThe encouragement and experimental facilities(stopped-flow spectrophotometry) provided byProfessor D. Banerjea, Sir Rashbehari Ghosh Pro-fessor of Chemistry, University of Calcutta, 92,A.P.c. Road, Calcutta 9, India are gratefullyacknowledged.

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936 INDIAN J CHEM, SEC. A, OCTOBER 1994

12 Saini G & Mentasti E, lnorg Chim Acta, 4 (1970) 210,585.

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phenoneoxime values of pK.(1l and pKa(21 have been deter-mined as described in ref. 17a).

18 (a) Eigen M & Tamm K, Z Electrochem, 66 (1962) 93,107; (b) Eigen M, Pure appl Chern, 6 (1963) 97.

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24 Espenson J H,lnorg Chern, 8 (1969) 1554.