Oxidation of [Co(en)2(SOCH2CO2)]+ by S2O

2ÿ8 and by IOÿ4 in

binary aqueous solvent mixtures at pressures up to 1.25 kbar

John Burgess

Department of Chemistry of Leicester University, Leicester LE1 7RH, UK

Olga Volla rova and Ja n Benko*,

Department of Physical Chemistry, Comenius University, 842 15 Bratislava, Slovakia

Summary

Second order rate constants and activation parametersDHà, DSà, and DVà have been measured for the oxida-tion of [Co(en)2(SOCH2CO2)]

+ by S2O2ÿ8 and by IOÿ4 in

highly aqueous H2O ± t-BuOH mixtures. The changes insolvation on going from the initial to the transition stateare discussed on the basis of the transfer functions DGo

t ,DHo

t and DSot . Whereas DGzt changes smoothly as theproportion of t-BuOH increases, the plots of DHzt andTDSzt exhibit mirror behaviour and pass through ext-rema located around x2(t-BuOH) � 0.038. Informationon the role of solvation is complemented by the deter-mination of activation volumes. These are discussed interms of intrinsic and solvational contributions. It isproposed that changes in hydrophobic hydration are ofprincipal importance in determining the response ofDHà, DSà, and DVà to changes in solvent composition inH2O ± t-BuOH mixtures.

Introduction

The ®rst step in the oxidation of [Co(en)2(SCH2CO2)]+

by various oxidants has been the subject of severalstudies(1±5). Kinetic results are consistent with operationof an SN2 mechanism connected with oxygen transfer tothe nucleophilic sulphur of the thioglycolate ligand. Wehave now extended our study to the second step, oxi-dation of [Co(en)2{S(O)CH2CO2}]

+ by S2O2ÿ8 and by

IOÿ4 . The role of the medium in determining reactivitiesin these reactions has been probed through examinationof plots of activation parameters DHà and DSà againstsolvent composition. In order to gain more informationabout solvent e�ects we have established the activationvolumes for both reactions in H2O ± t-BuOH mixtures.To a more limited extent methanol (MeOH) and acetone(Me2CO) were used as cosolvents. The activation vol-umes for both reactions in aqueous solution have pre-viously been reported(6). In the present paper we havemade an analysis of the reactivity trends in binaryaqueous mixtures using the initial state-transition stateanalysis, and contributions arising from solvent e�ectshave been treated by combining results of the initialstate-transition state analysis with the results of activa-tion volume determinations in aqueous organic mix-tures.

Experimental

[Co(en)2(SCH2CO2)]ClO4 was prepared by a methodanalogous to that used for [Co(en)2{SCH2CH(CO2)-NH2}] ClO4

(7). All chemicals used were p.a. grade.

Kinetic measurements were made using the apparatusand procedures described earlier(8). In every case a highpressure run and an atmospheric pressure run werecarried out concurrently on two aliquots from the samereaction mixture. The ratios of rate constants soobtained were used in the derivation of activation vol-umes. All runs were carried out at 298 K. Other condi-tions are speci®ed in Tables 1 and 2. The standard errorin the rate constants is 3%; the uncertainties in DVà are inthe region of �1 cm3 mol)1. The DHà and DSà valueswere determined from the temperature dependences ofthe rate constants over the region 298 to 308 K; theirerrors are �1.5 kJ mol)1 and �8 J K)1 mol)1 respec-tively. The solubilities of the complex salts and of K2S2O8

and KIO4 in water and in aqueous mixtures of acetonex2(Me2CO) � 0.144 and t-butanol x2(t-BuOH) � 0.161were determined using procedures described earlier(1,9).

Results

The kinetic results reported in Table 1 and 2 show thevariation of rate constant and activation parametersDHà, DSà, and DVà with solvent composition for boththe reactions studied.

The change in thermodynamic activation parameterson transfer from water into an aqueous organic solventis given by expression (1)

DXzt � DXot �ts� ÿ DXo

t �is�� DXo

t �ts� ÿ DXot �M�� ÿ DXo

t �ox�: �1�

DXot �ts� and DXo

t �is� are the transfer functions of theactivated complex and of the initial state, respectively.DXo

t �M�� represents the transfer function of the com-plex cation [Co(en)2{S(O)CH2CO2}]

+, assumed equal tothat for [Co(en)2(SCH2CO2)]

+. DXot �ox� is the transfer

function of the oxidant, S2O2ÿ8 or IOÿ4 . The DHo

t datafor M+ and ox for H2O ± t-BuOH mixtures were takenfrom the literature(2,4). DGo

t values for M+ and bothanions in aqueous methanol and t-BuOH up to x2(t-BuOH)� 0.113 are published(10). DGo

t values for M+,S2O

2ÿ8 and IOÿ4 in aqueous mixtures of acetone

x2(Me2CO) � 0.144 and of t-butanol x2(t-BuOH) �0.161 are derived from measured solubilities (Table 3).All thermodynamic transfer functions have been calcu-lated using the extrathermodynamic assumption basedon equality of values for the transfer functions for thelarge unipositive and uninegative ions Ph4As+ (orPh4P

+ for MeOH) and BPhÿ4 . Because data for thecalculation of ionic DGo

t in the aqueous mixture x2(t-BuOH) � 0.161 were not available, we measured thenecessary solubilities of KBPh4, Ph4Ppic, and Kpic(pic � picrate) (Table 3).

0340±4285 Ó 1998 Kluwer Academic Publishers

Transition Met. Chem., 23, 677±681 (1998) Oxidation of [Co(en)2(SOCH2CO2)]+ 677

*Author to whom all correspondence should be directed.

Discussion

The oxidation of coordinated thioglycollate in thecomplex [Co(en)2(SOCH2CO2)]

+, by S2O2ÿ8 as well as

by IOÿ4 , takes place in two kinetically distinguishablestages (Scheme 1).

The ®rst step is inconveniently fast for us to follow inour high pressure apparatus, so we have establishedpressure e�ects on reactivity for addition of the secondoxygen atom to coordinated sulphur. The second-order

kinetics, negative DSà and DVà values (Tables 1 and 2)®rmly indicate an associative mechanism for both reac-tions studied. For reactions of established mechanism thechange of rate constant and activation parameters DHà,DSà and DVà with added cosolvent can provide the in-formation about solvent e�ects. The solvent propertymost often invoked in explanations of reactivity trends isrelative permittivity. Protic cosolvents such as t-BuOHaccelerated the oxidation of complex ion by S2O

2ÿ8 but

Table 1. Second-order rate constants, ko, activation enthalpies, DHà, activation entropies, DSà, activation volumes, DVà, and ratios, kp/ko, of rateconstants at high pressure, kp, to those at atmospheric pressure, ko, for oxidation of [Co(en)2{S(O)CH2CO2}]

+ by S2O2ÿ8 at 298.2 K. Initial

concentration of complex 2.17 ´ 10)4 mol dm)3, of S2O2ÿ8 3.16 ´ 10)3 mol dm)3; k = 371 nm

Solvent x2(%)

ko(dm3 mol)1 s)1)

DHà

(kJ mol)1)DSà

(J mol)1 K)1)DVà

(cm3 mol)1)kp/ko at p/kbar

H2O ± 0.277 52.8 )78 )10.5 p 0.50 0.75 1.00 1.25kp/ko 1.20 1.39 1.62 1.63

t-BuOH 3.8 0.245 45.3 )105 )12.2 p 0.50 0.75 1.00 1.25kp/ko 1.27 1.43 1.56 1.90

t-BuOH 10.5 0.301 56.5 )65 )7.5 p 0.50 0.75 1.00 1.25kp/ko 1.19 1.25 1.34 1.48

t-BuOH 16.1 0.405 58.2 )57 )6.2 p 0.50 0.75 1.00 1.25kp/ko 1.11 1.24 1.24 1.38

MeOH 22.9 0.211 49.2 )93 )10.9 p 0.50 0.75 1.00 1.25kp/ko 1.25 1.46 1.58 1.71

Me2CO 14.0 0.105 57.2 )72 )9.0 p 0.50 0.75 1.00 1.25kp/ko 1.18 1.32 1.49 1.54

Table 2. Second-order rate constants, ko, activation enthalpies, DHà, activation entropies, DSà, activation volumes, DVà, and ratios, kp/ko, of rateconstants at high pressure, kp, to those at atmospheric pressure, ko, for oxidation of [Co(en)2{S(O)CH2CO2}]

+ by IOÿ4 at 298.2 K. Initialconcentration of complex 2.17 ´ 10)4 mol dm)3, of iodate 2 ´ 10)3 mol dm)3; k = 371 nm

Solvent x2 ko DHà DSà DVà kp/ko at p/kbar(%) (dm3 mol)1 s)1) (kJ mol)1) (J mol)1 K)1) (cm3 mol)1)

H2O ± 0.431 56.2 )63 )4.6 p 0.50 0.75 1.00 1.25kp/ko 1.10 1.18 1.19 1.26

t-BuOH 3.8 0.411 51.3 )80 )6.1 p 0.50 0.75 1.00 1.25kp/ko 1.13 1.17 1.27 1.37

t-BuOH 10.5 0.247 54.5 )73 )5.0 p 0.50 0.75 1.00 1.25kp/ko 1.16 1.15 1.21 1.32

t-BuOH 16.1 0.206 60.3 )56 )4.4 p 0.50 0.75 1.00 1.25kp/ko 1.06 1.14 1.19 1.23

MeOH 22.9 0.285 50.2 )87 )7.7 p 0.50 0.75 1.00 1.25kp/ko 1.14 1.26 1.35 1.47

Me2CO 14.0 0.091 53.0 )87 )11.3 p 0.50 0.75 1.00 1.25kp/ko 1.34 1.43 1.61 1.79

Table 3. Solubilities, S, of salts investigated and Gibbs free energy of transfer, DGot , for [Co(en)2(SCH2CO2)]

+, IOÿ4 and S2O2ÿ8 in H2O ± t-BuOH

and H2O ± Me2CO mixtures at 298.2 K

Solvent S ´ 104/mol dm)3 DGot /kJ mol)1

x2 (%) MClO4 KIO4 K2S2O8 KBPh4 Ph4Ppic Kpic M+a IOÿ4 S2O2ÿ8

H2O 340 226 2440 1.74 0.447 247 ± ± ±t-BuOH 16.1 176 114 292 32.4 28.0 397 0.3 1.6 12.2(Me)2CO 14.0 410 238 102 )3.3 6.4 13.0

a M+ = [Co(en)2(SCH2CO2)]+.

Scheme 1.

678 Benko et al. Transition Met. Chem., 23, 677±681 (1998)

moderated the oxidation by IOÿ4 . These di�erent trendsshow an important role for the speci®c solvent e�ects.

Transfer functions of reactants provide key informa-tion about the initial states of the reactions studied. Thetransfer of S2O

2ÿ8 and IOÿ4 ions from water into aqueous

mixtures with t-BuOH is not favourable, as indicated bythe positive values of DGo

t (Figure 1). The positive DGot

values have been interpreted in terms of stronger inter-action of anions with water than with cosolvent. Pref-erential solvation by water is also expected to be a�ectedby the interaction of the water molecules with organiccosolvent. The destabilisation observed (Figure 1) isprobably connected with particular di�culties in for-mation of the solvation shells of anions in the hydro-phobically ordered solvent. Dinegative charged S2O

2ÿ8 is

more destabilized on adding t-BuOH than uninegativeIOÿ4 . The DGo

t variations in H2O ± t-BuOH mixtures arecommonly simpler than those of DHo

t and DSot . Thechange in DGo

t for S2O2ÿ8 is governed by the entropic

term at higher cosolvent concentration, but for thecomplex and IOÿ4 ions compensation of enthalpic andentropic terms leads to low DGo

t values (Figure 1). Theextrema in DSot observed in these H2O-t-BuOH mixturesresult from large di�erences in the solvent compositionof the solvation spheres of the ions and of the bulksolvent. For large solutes such as the complex cation,cavity formation is the main contribution to the en-thalpic term (Figure 1), and the maximum endothermice�ect is at x2(t-BuOH) � 0.038, where the waterstructure near the hydrophobic groups of t-BuOH ismost enhanced(11,12). The di�erent sign of DHo

t for thecomplex and S2O

2ÿ8 ions may be attributed to the

opposite e�ect of these ions on the hydrogen-bondstructure of water.

Both the initial state (is) and the transition state (ts)are destabilized on going from water to water ± t-BuOHmixtures for both reactions studied [Figure 2(a) and2(b)]. The decrease in rate constant for IOÿ4 oxidationwith increase in cosolvent concentration is the result ofgreater destabilisation of the uncharged ts comparedwith the is (Figure 2(a) and 2(b)). For S2O

2ÿ8 oxidation

[Figure 2(a)], at higher cosolvent concentration the re-verse e�ect was observed. The decrease in rate constantwith added MeOH and with added Me2CO is the resultof greater destabilisation of the ts than of the is (see thediscussion of Figure 4 below).

The activation parameters for both oxidations aresigni®cantly in¯uenced by the addition of t-BuOH(Figure 3). The negative DVà presumably arises from theresultant of the decrease in intrinsic volume and inelectrostriction on going from is to ts. The di�erence inDVà between two reactions studied can be presumed toarise from di�erent solvation changes corresponding notonly to di�erently charged oxidants but to di�erentlycharged ts too. The singly charged IOÿ4 will need to loseless electrostricted water of hydration and will be lessconstrained in volume on entering into the ts thandinegative peroxodisulphate. In the case of the S2O

2ÿ8

oxidations, the ts has the charge )1, but for the IOÿ4oxidations the ts is uncharged, so there is a negligibleelectrostriction e�ect. The packing geometry of solventaround an ion or a neutral particle is di�erent. That isprobably the reason why IOÿ4 oxidation has a rathersmaller negative DVà than the corresponding S2O

2ÿ8

Figure 1. ±± Gibbs free energy, enthalpy and - - - entropy of transfer to H2O ± t-BuOH mixtures at 298.2 K: h [Co(en)2SCH2CO2]+, n IOÿ4 ,

s S2O2ÿ8 .

Transition Met. Chem., 23, 677±681 (1998) Oxidation of [Co(en)2(SOCH2CO2)]+ 679

oxidation. Minima in the plots of DVz vs x2(t-BuOH)at x2(t-BuOH) � 0.038 indicate that solvent-solventinteractions take place in these reaction systems(Figure 3). According to Figure 1, preferential solvationof S2O

2ÿ8 in comparison to IOÿ4 is more reduced as the

proportion of t-BuOH rises. This di�erence is re¯ectedin considerably less negative activation volumes in H2O± t-BuOH mixtures compared with water, especially forS2O

2ÿ8 oxidation. Destabilisation of both oxidants at

x2(t-BuOH) � 0.160 can be explained by a decrease indi�erential loosening of electrostricted water; theDVz�IOÿ4 � and DVz�S2O2ÿ

8 � values approach each other(Tables 1 and 2). A signi®cant change in DVà on goingfrom water to aqueous mixtures with MeOH andMe2CO was observed only for IOÿ4 oxidation. Theincreasely negative DVà values in H2O ± Me2CO mix-tures (Table 2) are probably connected with stabilisationof the complex ion, which in contrast to S2O

2ÿ8 is only

partly compensated by destabilisation of IOÿ4[Figure 4(b) and (c)]. The stabilisation of the complexcation is probably due to predominant ion±dipole anddispersion interactions between it and the solvent. Thesolvent e�ect on the rate constant for the reactionsstudied in aqueous mixtures with MeOH, Me2CO andMe3COH (t-BuOH) does not correlate with the numberof hydrophobic Me groups, but the presence of thehydrophilic groups ±OH or >C@O is e�ective in thesesystems(13). The behaviour of MeOH as the organiccomponent of solutions suggests a balance between thee�ects of its hydrophilic and hydrophobic groups. In thewater-Me2CO system the interaction of the hydrophilicelectron donor group of the cosolvent with water is amore important factor than the hydrocarbon ± waterinteractions. Cosolvents such as MeOH, t-BuOH, andMe2CO give ``typically aqueous''(14) mixtures in whichentropy e�ects dominate, jTSEj > jHEj , GE > 0.

Figure 2. Initial state ± transition state analysis for oxidation of [Co(en)2(SOCH2CO2)]+ by S2O

2ÿ8 (a) and IOÿ4 (b) at 298.2 K. ±± DGo

t , DHot ,

- - - TDSot ; initial state: a (s), b (n), transition state: a (d), b (m) in H2O ± t-BuOH mixtures.

Figure 3. Transfer activation parameters DHzt , - - - TDSzt , ±± DVzt for oxidation of [Co(en)2{S(O)CH2CO2}]+ by S2O

2ÿ8 (s) and IOÿ4 (n) in H2O ±

t-BuOH mixtures; DXzt � DXà(x2) ) DXà(H2O).

680 Benko et al. Transition Met. Chem., 23, 677±681 (1998)

A similar pattern was observed for DSà vs x2(t-BuOH)as for DVà (Figure 3). There are many contributions toDSà ± translational, rotational, and vibrational contri-butions from the reactants together with a contributionfrom solvent ordering. It is their sum which determinesobserved DSà values. The mechanism of these reactionsis taken to be nucleophilic attack by coordinated sul-phur on the OAO peroxide bond of S2O

2ÿ8 , but for IOÿ4

oxidation attack is by one of the four equivalent non-peroxidic oxygens. The rotation barrier may be pre-sumed to be higher in S2O

2ÿ8 than in IOÿ4 . There is no

doubt that these details wil a�ect ordering in the tran-sition state and thus the DSà value. The magnitudes ofDSà (Tables 1 and 2) in water and in 0.038 mole fractiont-BuOH are in accordance with the qualitative expec-tation of a lower DSà value for reaction with S2O

2ÿ8 .

That DHtà, DStà and DVt

à all exhibit extrema at the sameposition (Figure 3) as those for excess enthalpy, HE,and excess volume, VE, of water ± t-BuOH(15) shows acommon origin for the solvent structural e�ects onadding cosolvent. The decrease in mobility and the in-creased structural order of water in these H2O ± t-BuOHmixtures is attributed to hydrophobic hydration(16).According to a molecular dynamics study, the hydro-phobic e�ects are entropy-driven(17). It is proposed thatchanges in hydrophobic hydration are of prime impor-tance in determining the response of DHà, DSà, and DVà

to changing solvent composition in H2O ± t-BuOHsystems.

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(Received 20 April 1998;Accepted 27 April 1998) TMC 4219

Figure 4. Initial state ± transition state analysis for oxidation of [Co(en)2{S(O)CH2CO2}]+ by S2O

2ÿ8 (a,b) and IOÿ4 (c,d) at 298.2 K in H2O ±

MeOH and H2O ± Me2CO mixtures; initial state � IS, transition state � TS, M+ º [Co(en)2(SCH2CO2)]+.

Transition Met. Chem., 23, 677±681 (1998) Oxidation of [Co(en)2(SOCH2CO2)]+ 681