Indian Journal of ChemistryVol. 25A, March 1986, pp. 230-233

Studies on Dissociation of a Seven-membered Chelate Complex:Aquation of Succinatobisethylenediamineco~alt(III) Complex Ion

BISWANATH CHAKRAVARTY· & (Mrs) INDRANI GHOSHDepartment of Chemistry, University of Kalyani, Kalyani 741235

Received 22 April 1985; revised 6 August 1985; accepted 16 September 1985

Rates of dissociation of a seven-membered chelating ligand succinate ion from succinatobisethylenediaminecobalt (III)complex ion have been determined spectrophotometrically in aqueous solution at four different temperatures. The reactions atdifferent temperatures exhibit simple first order kinetics. The rate of dissociation of succinate ion isalmost 10' times faster thanthat of the corresponding five-membered chelating ligand oxalate ion. The rates of dissociation are higher in methanol-waterand ethanol-water media as compared to that in pure aqueous medium. Grunwald-Weinstein plots yield slopes of 0.27 ±0.06and 0.24 ± 0.04 respectively, in the above mixed solvents. A dissociative mechanism has been proposed for the aquation of thecomplex ion.

As the number of chelate rings increases in a chelatecomplex, both its thermodynamic and kinetic stabilityalso increase. But as the chelate ring sizes, from five-membered ring onwards, increase the stability of thecomplex decreases 1. However, chelate rings greaterthan seven-membered have also been found in some ofthe macrocyclic complexes", which owe their stabilityto macrocyclic effect". Because of the reduced stabilityof seven-membered ring complexes, only a few of suchcomplexes of cobalt (III) have been synthesized so far.These are: succinatobisethylenediaminecobalt (111)4,succinatobisphenanthrolinecobalt (111)5: succinate-bisbipyridylcobalt (III) 5; and chloro-I, 4-butane-diaminediethylenetriaminecobalt (II1)6. Kinetics ofdissociation of a seven-membered chelate ring complexhave not been attempted so far, hence, we describeherein the kinetics of aquation of succinatobisethylenediaminecobalt (III) ion, a seven-memberedchelate ring complex, in water and also in methylalcohol-water and ethyl alcohol-water mixtures with aview to elucidating the mechanism of aquation.

Materials and MethodsSuccin:ttobisethyl.:nediaminecobait (III) nitrate

dihydrate was prepared by following a method similarto Duff's method". Carbonatobromide complex (1equiv) was boiled with succinic anhydride (1 equiv) inwater (100 ml) for 5 min and concentrated on a rotaryevaporator. On cooling pink crystals of thesuccinatobromide deposited at the bottom of thevessel. The complex nitrate was prepared by themetathesis of the complex bromide with silver nitratein solution and then concentrating on a rotaryevaporator. (Found: Co, 15.26; N, 17.72.

230

[CoeniSuccinato)] NO).2 H20 requires Co, 15.01;N,17.85'YJ.

Analar grade methanol and ethanol and doublydistilled water were used.

In a preliminary observation it was noticed that inacidic aqueous solution the complex ion reacted toorapidly. However, in the absence of acid the complexspecies still underwent a dissociative reaction but witha measurable rate. The spectrum of the complex ion inaqueous as well as in methanol-water and ethanol-water mixtures (PH -- 5.6 to 5.9)changed with time andafter 2-3 hr became constant (Fig. 1). The visiblespectra were compared with the known spectra of cis-,,~.J Irans-Coen2(H zoH + and their equilibrium.iydroxoaqua products 7. The spectra presentlyrecorded indicated that the products did not belong totrans-series and the spectra did not match with those ofcis-diaqua or the cis-hydroxoaqua complexes, whichhave absorption maxima at 492 and 520 nm withdifferent extinction coefficients". The present spectraalso indicated that these belong to an equilibriummixture of products with cis-diaqua product as themajor product. The pH of complex solution wasbetween 4.7 and 4.9, which was lower than that of thesolvent probably due to protonation-deprotonationequilibria of the coordinated solvent 'aqua' moleculesand thus forming a self-buffered solution. At this pH ofthe solution, diaqua product predominated. Isosbesticpoints were retained throughout the course of thereaction (Fig. I). This fact along with the nature ofproducts indicated that no other reaction was takingplace simultaneously.

Kinetic runs were carried out in a thermostaticwater-bath. Weighed amounts of the complex were

CHAKRAVARTY & GHOSH: AQUATION OF COBALT(III) COMPLEX ION

0200

340 360

/

0.150

C»oc0.0

'- 01000

'"D<{

0.050

Wave length> nm

Fig. I-Spectra of a 2.0 x 10 -3 moldm -3 solution of the complex ion in water at room temperature [(i) Fresh solution; (ii) after 24 hr;(iii) after 3 days and; (iv) after seven days. Curve (v) shows the spectrum of ccis-Coen2(H20)~ + (equilibrium mixture, same concenrrationj]

o 10 20 30 40 50 60 70 80 90 100 110 120TIME IN MINUTES

Fig. 2 --First order rate plot for the complex at 50" (I), 56° (II). 63" (III) and 70° (IV)

added to temperature-equilibrated water (50 ml), fromwhich aliquots were withdrawn at regular timeintervals, cooled in ice-cold water and absorbanceswere measured. No buffer could be used formaintaining pH, as phosphate buffer, which isgenerally used for maintaining pH around 7.0,interfered with the reaction. The reaction followedsimple first order kinetics and rates were evaluated by

the usual plot of log (Ao - A -x,)/(A1 - A "Xl) versus time. Atypical rate plot is shown in Fig. 2.

Results and DiscussionThe observed rates of dissociation of

[Coen2(Succ)] + (2.0 x 10 -3 moldm -3) in water at I= 2.0 x to -3 moldm -3 and 50°, 56°, 63° as 70°C are:104 k(s -1)= 1.07 ±0.09, 1.44±O.07, 2.21 ±O.13 and

231

INDIAN J. CHEM., VOL. 25A, MARCH 1986

3.2 ±0.28, respectively; the rate values are the averageof three individual runs. Unimolecular rate plots arelinear throughout the course of the reaction, indicatingthat the dissociation of the succinato ligand takes placein a single step without ring-opening. Activationparameters have been determined in usual mannerfrom Eyring's equation. The values are: !1Ht = 50± 4klrnol :"; and !1st= -160± 11 JK -lmol-l. Themechanism of the dissociation of succinato chelatering may be explained as follows. By the concertedattack of solvent molecules rupture of one end of thechelate ring occurs with the formation of amonodentate succinato complex (Eq. 1). Suchmonodentate complexes of bidentate chelating Iigandsare not uncommon". The monodentate succinatoligand is then replaced by the attack of another solventmolecule with the formation of the diaqua species(Eq.2).

kCoen, (Succ)" +H20 -.; Coen, (-Succ) (H20) +k2 (1)

Coen, (-Succ) (H20)+ +H20 - Coen, (H20)~++Succ" -

... (2)

As observed from Fig. 1 the entire reaction takesplace in a single step or both kl and k2 have samevalues. So, one of the reactions, expressed by either Eq.(I) or Eq. (2) must be fast. If step (1) is fast,concentration of ring-opened complex in solutionshould increase with the progress of the reaction.However, we could not detect the formation of anysuch ring-opened complex. This has been ascertainedby passing the reaction mixture through cationexchange resin (Dowex AG-50W-X2; 200-400 mesh)column (20 ern in length) and eluting successively with0.5, 1.0 and 3.0 moldm -3 of KN03 solution. No suchmonodentate succinato complex has ever beenobtained by any means. So, step (1)represents the rate-determining step. This is followed by the fastdissociation of the other end of the chelate ring, thevacant coordination site thereby being filled up by asolvent molecule.

This one-step dissociation may also be explained bythe fact that both the points of attachment of thechelate ring rupture simultaneously by the concertedattack of two solvent 'aqua' molecules. As there isoverwhelming excess of solvent molecules, such amodel cannot be ruled out. In such a case the transitionstate would necessitate freezing of a large number ofsolvent molecules and a low entropy factor might beobserved. Some other cobalt (III) carboxylatocomplexes also show highly negative entropy values!".These are known to aquate by Id or D process.

In the formation of the intermediate complex, if thesolvent molecules approach the metal ion from the

232

same side of the displaced chelating ligand with theformation of a square pyramidal intermediate, thencis-diaqua complex would result. However, when theapproach of the incoming solvent molecules is fromopposite side, a trigonal bipyramidal complex resultsin the transition state with the formation of trans-diaqua complex. Nevertheless, the formation of thecis-diaqua complex as the major product in the presentcase indicates that the solvent molecules approach themetal ion from the same side of the displaced chelatingligand.

Seven-membered 'succinato' chelate forms a lessstable complex. It is the chelate formation or in otherwords "the chelate effect" that imparts stability.However, as the chelate-metal bond cleavage occurs,the complex looses its extra stability obtained by'chelate effect' and dissociates spontaneously. Therates of aquation of the oxalato and the succinatocomplexes in water are: 10-10 S-1 at 25cc (see ref. II)and 1.07 x 10 -4S -1 at 5!)OC (present work) re-spectively. The rate data indicate that as the size ofchelate ring increases, its kinetic stability decreases inaccordance with a dissociative mechanism .

For a further clarification of mechanism, aquationof the complex species has been investigated inmethanol-water and ethanol-water media containingupto 40%(vjv) methanol or ethanol. During the kineticinvestigations no spectral evidence for methylation orethylation of the complex is observed as the nature ofabsorption curves changes in an identical manner inwater, methyl alcohol-water or ethyl alcohol-watermedium. In these mixed solvents aquation proceeds ina similar manner as in water giving the diaquacomplex. Good first-order kinetics have been observedagain in all these media throughout the entire course ofthe reaction. The rates thus obtained are reported inTable 1. These are average of two individual runswhich are reproducible to within 5%.

As the percentage of alcohol in the mixtureincreases, the rate decreases. The rates in pure aqueousmedium are smaller than those in alcohol-watermixtures. In aqueous medium extensive hydrogenbond formation takes place between the succinatoligand and the solvent water molecules, and as a resultthe dissociation of a succinato ligand is entropy-controlled. Such hydrogen bonding is less extensive inalcohol-water media in which the succinato ligand isrelatively free. In the latter case dissociation of asuccinato ligand takes place with little disturbance ofsecondary sphere of hydrogen-bonded solventmolecules, as in this case, a smaller number of solventmolecules are involved in the formation of thesecondary sphere, leading to a higher rate of aquationin alcohol-water mixtures. Apart from the entropyfactor, greater solvation of these 'organic' chelate

CHAKRAVARTI & GHOSH: AQUATION OF COBALT(IIO COMPLEXES ION

Table I-Rate Constants of Aquation of [Coenz(succ)]+

in MeOH-Water and EtOH-Water at 500

[Complex) = I = 2.0 x 10-l moldm :"

.os k (see -1) 5 + log kAlcohol(%, v/v)in water

y

Methanol-water

0 10.7 1.03 3.4910 16.5 1.20 3.2820 12.6 1.10 2.9030 10.4 1.02 2.3940 7.42 0.87 1.97

Ethanol-water

0 10.7 1.03 3.4910 15.0 1.17 3.3120 13.0 1.11 3.0530 11.2 1.05 2.7240 8.6 0.94 2.20

complexes in the 'more organic' alcohol-water mediummay also be responsible for the higher rate. However,as the percentage of alcohol increases in the medium,the rate decreases steadily. The same trend has alsobeen observed with some other complexes of cobalt(IIW2•13. The decrease in rate with the change indielectric constant (as the percentage of alcoholincreases, the dielectric constant of the mediumdecreases) of the medium is indicative of a'dissociative' mechanism. In water-methanol or water-ethanol medium, the relation between rate constantsand Grunwald- Winstein solvent parameters ( Y-values)14 gives a much better insight into themechanism of the reactions. The Y-values exhibit littlechange with temperature' and the values at 25° havebeen used as such in the present work. Plots of Y-values versus log (rate constants) in methanol-waterand ethanol-water media for the complex are shown inFig. 3. These plots are linear with slopes (m) equal to0.27 ± 0.06 and 0.24 ± 0.04 in methanol-water andethanol-water media respectively.

It is now fairly well established that cobalt (III)complexes which aquate by a dissociative process,generally have a gradient of about 0.3 for Y-valuesversus rate plots in MeOH-water solvent and little lessin EtOH-water solvent 16. The present observedgradients of 0.27 and 0.24 for MeOH-water andEtOH-water media respectively indicate that thepresent complex aquates by a dissociative process.

It'

3.5

I 2 IzI'

//

/~

/1.1/

I

.s:a>o+ 1.0

o

/if

/'I

~

t;j/,

0.9 II

I.I

y

Fig. 3-Grunwald-Weinstein plots in MeOH-HzO and EtOH-H20(500

)

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(Wiley-Eastern, New Delhi), 1973, pp. 223-226.2 Bembi R, Bhardwarj V K, Singh R, Singh R, Taneja K & Aftab

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II Andrade C & Taube H. J Am chem Soc. 86 (1964) 13211.12 Burgess J & Price M G. J chem Soc (A). (1971) 3108.13 Kane-Maguire LAP & Thomas G. J chem Soc Dalton Trans.

(1975) 1324.14 Grunwald E & Winstein S. J Am chem Soc. 70 (1948) 846.15 Fainberg A H & Winstein S. J Am chem Soc. 78 (1956) 2770.16 Langford C H. Inorg Chern. 3 (1964) 228: Burgess J. J chem Soc

(A). (1970) 2703.

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