Catalysis by Polyelectrolytes of the Ag+, Hg2+, and T13+Induced Aquations of Co(NHJ5Br2+BY NORIO ISE AND YOSHINOBU MATSUDADept. of Polymer Chemistry, Kyoto University, Kyoto, JapanReceived 19th June, 1972The aquation reaction of Co(NH3)5Br2+ was studied kinetically with Ag+, HgZ+, and T13+ asinducing cations. Addition of polyethylene sulphonate, polystyrene sulphonate and polyphosphatemarkedly enhanced the reaction. The acceleration factor, ranging from 10 to lo4, was dependenton the inducing cations, polyelectrolytes, their concentrations and the concentrations of foreignsalts. The enthalpy of activation (AH*) and the entropy of activation (AS*) were decreased byaddition of polyelectrolytes in the case of Ag+ and Hg2+, as has been observed previously, whereasAH* was not influenced and AS* increased in the case of T13+.The behaviour of T13+ was ascribedto the strong hydration of this cation.Previously, we have shown that reactions between similarly charged ionicspecies are accelerated by oppositely charged polyions, and this effect is interpretableusing the Brarnsted relation in terms of the marked decrease of the activity coefficientof the activated complex in the strong electrostatic field of the polyions. Thisinterpretation was also applied to the polybase catalysis on an E2 reaction betweenchloromaleic acid or chlorofumaric acid and OH-.lC The validity of this interpreta-tion was clearly shown for a series of ionic reactions between oppositely charged ionicspecies such as the ammonium cyanate-urea conversion.I d In these investigations,relatively low polyelectrolyte concentrations were employed, to see whether the poly-electrolyte catalysis can be mainly accounted for in terms of the activity coefficientin combination with Brnrnsted theory, which is valid for systems deviating only slightlyfrom ideal behaviour. In this paper, we extend our measurements to much higherpolyelectrolyte concentrations, using Ag+ and TI3+, in addition to Hg2+ previouslystudied, as inducing agents for the aquation of Co(NH3),Br2+,Co(NH,),Br2+ + M"++ H20 + Co(NH3),H203+ + MBr("-l)+ (Mn+ : Ag+, Hg2+or TI3+)and discuss the influence of the valency of the inducing ions on the catalysis by poly-electrolytes.EXPERIMENTALMATERIALSThe complex [CO(NH~)~B~](NO,)~ is as used in a previous paper.'* The perchlorate[CO(NH&B~](C~O~)~ was prepared in a similar manner, using 70 % HC10, instead ofconcentrated HN03.(Found: H, 3.88; N, 16.05; C1, 16.10; Br, 18.14. Calc. for[CO(NH~)~B~](C~O~)~ : H, 3.58 ; N, 16.6; C1, 16.77 ; Br, 18.90 %.)The perchlorate salt had, in agreement with the literature, an value of 1 . 6 7 ~ lo4 at253 nm. Different anions in the cobalt complex had no influence on the rates under ourexperimental conditions. A solution containing Hg(C104)2 and HC104 was prepared bydissolving HgO (reagent grade) in an excess of HC104 (to suppress hydrolysis of Hg2+).9100 POLYELECTROLYTE CATALYSISThe acid dissociation constant for the Hg aquo ion pK, is 3.7. A Ag+ solution was preparedeither by dissolving AgN03 (Merck, reagent grade) in pure water or from newly precipitatedAg2O.A stock solution of T1(c104)3 was prepared by oxidizing TINOS (reagent grade)with bromine. T1203 was precipitated by treating the resulting solution with NaOH.The oxide was thoroughly washed and dissolved in HC104 solution. The Agf and Hg2+solutions were titrated with a 0.1 N NH4SCN solution with NH4Fe(S04)2 as internalindicator. The concentration of acid in the stock solutions was determined in the presenceof a large excess of NaBr by titration with NaOH with a mixed indicator of BromothimolBlue and Phenol Red. A known volume of the T P stock solution was diluted to givea solution in HCIO4 of < 0.5 M and added to a 2 % solution of KI. The liberated iodinewas titrated against thiosulphate.Dilution was necessary because concentrated solutionsof HC1O4 liberate iodine from a KI solution. To obtain a NaC104 solution, HC104 wasneutralized with NaHC03. Deionized water having a conductivity of 5 x 0-l cmdlor below was used in the present work. Sodium polyethylene sulphonate (NaPES) wasfrom the Hercules Powder Co., Wilmington, Del. The degree of polymerization was re-ported to be 770. Sodium polystyrene sulphonate (NaPSS) was given by the Dow ChemicalCO., Midland, Mich. The degree of polymerization was claimed to be 2500. Sodiumpolyphosphate (NaPP) was given by the Monsanto Co., St. Louis, Mo. The degree ofpolymerization was believed to be about 5000. Solutions of these polyelectrolytes werepurified by using cation- and anion-exchange resins.The polymer concentration wasdetermined by a potentiometric titration.KINETICSIn most cases, the reactions were followed by the stopped-flow method, using a rapidscan U.V. spectrophotometer (Hitachi Manufacturing Co., Tokyo, model RSP-2). TheU.V. photometer consisted of a Gibson type four-jet mixer and an observation cell with 1 cmoptical path length and a volume of 0.07 ml. The apparatus was used for reactions withhalf-times of 50ms or more. Equal volumes of a 10-4N HC1O4 solution containingCo(NH&Br2+ and a polyelectrolyte solution containing inducing cations were forcedthrough the mixing jet into the stop syringe. Just before the plunger hits a mechanical stop,it made contact with a trigger switch which actuates the horizontal time-basis sweep for theoscilloscope display.The display on the storage oscilloscope, which indicates the opticaldensity against time, usually started just before the reaction began and continued during theexpected period according to the preselected time-base setting. Slower reactions withhalf-times of 30min such as Ag+-induced aquation without polyions were followed by anordinary U.V. spectrophotometer (Hitachi Manufacturing Co., Tokyo, model EPS-3T).Although solutions containing polyions and reactant ions did not give precipitates for a fewhours after preparation under our experimental conditions, a slow increase of the opticaldensity was observed after the reaction was complete for the highest polyion concentrations.This might be due to an interaction between polyions and the tervalent products, namelyCO(NH~)~H~O~+, which introduced a small uncertainty (about 5 %) in the '' infinity "reading of the optical density (Dm).In such cases, the D , values at low concentration wereused for calculation. The disappearance of Co(NH3)5Br2f was followed by changes in theoptical density at 254 nm. Almost all rate measurements were carried out under pseudo-first-order conditions with an initial concentration of inducing reagents in tenfold excessover C O ( N H ~ ) ~ B ~ ~ + . The stopped-flow measurements were repeated three or four timesfor each set of solutions, the average deviation of the rate constant from the mean valuebeing usually f 5 %. The pseudo-first-order rate constant, k , was calculated from the slopeof the In (D- D)03 against t plot, where D is the optical density at time t.The second-orderrate constant, k2, was determined by k2 = k/[inducing ion]. The solutions in a water-jacketed cell were thermostated to +O.I"C.RESULTS AND DISCUSSIONThe value of k/ko (the ratio of the pseudo-first-order rate constants obtained inthe presence and absence of polyelectrolyte) is plotted against concentration (mol l.-INOR10 ISE AND YOSHINOBU MATSUDA 101of the polyelectrolyte in fig. 1-3. In all cases, the addition of a small amount ofpolyelectrolyte led to an enormous acceleration in the reaction rate. The followingpoints are specifically noted. First, generally speaking, the reaction rate increasedwith the polyion concentration when this concentration is low, passes through amaximum, and decreases on further addition of polyion.This behaviour is interpret-able by the Brarnsted equation in terms of the activity coefficients of the reactants andof the critical complex. According to previous measurements of the mean activitycoefficients of the component electrolytes in the ternary system H20 +sodiumpolyacrylate + NaC1,3 the mean activity coefficient of NaCl (y3) decreases at first atlow concentrations, passes through a minimum and then increases with increasingpolyelectrolyte concentration. If this concentration dependence is generally true,and should be more pronounced for higher valence electrolytes, it would give rise tothe above-mentioned concentration dependence of k/ko.(It should be noted thatsuch a concentration dependence was interpreted by Morawetz using the concepts of[NaPSS]FIG. 1 .-Dependence of the acceleration factor for the Ag+-induced aquation on PSS concentrationat 25°C. [Co(NH3),Br2+] = 5.5 x lo-, M, [Ag+] = 7.5 x M (kz0 = 0.0833 M-' s-I).I IrPOher1FIG. 2.-Dependence of the acceleration factor for the Hg*+-induced aquation on polyion con-centration at .25"C. [Co("H3),Br2+] = 6x lo-' M, [Hg"+] = 5 x M (& = 8.6, M-' s-l)102 POLY ELECTROLYTE CATALYSISeffective concentratio~i.~) Secondly, the three kinds of polyelectrolytes showeddifferent acceleration abilities. The order was PP > PES > PSS (see fig. 2). Thismay be ascribed to the difference in the spacial charge density of the polyions.Withcloser spacing of the charged groups, the activity coefficients of the reactants andcomplex would be lowered more. Thus PP which has the highest charge densitywas most effective and PSS least.In order to make a comparison of the inducing cations a set of rate measurementswas performed under the same conditions. Results are given in fig. 4. The orderof the acceleration factors is T13+ z Hg2+ 9 Ag+. If the polyelectrolyte catalysison ionic reactions could be interpreted only in term of the electrostatic interaction0 s -YFIG. 3.-Dependence of the acceleration factor for the TI3+-induced aquation on PES concentrationat 25°C. [Co(NH&Br*+] = 6x M, ITl3+] = M, [HC104] = 0.1 N (kZ0 = 1.63 M-I s-l).4.0 c1 1 1 - 5.0 -40 -30 -20log [PES]FIG.4,-Comparison of the polyion effect on the induced aquations at 20°C. [CO(NH~)~B~~+] =6 x M, [inducing cation] = 1 x equiv. L-l, [HC104] = 0.05 NNOR10 ISE AND YOSHINOBU MATSUDA 103between polyions and oppositely charged reagent ions, the T13+-induced aquationshould be more strongly accelerated than the Hg2+-induced one, as Gould observedfor electron-transfer reaction^.^ Since the acid dissociation constant of the TI aquoion pK, is 1.1: at least half of the TI ions exist in the form of Tl(H20)50H2+under our experimental conditions even in 0.1 N HCL04. It is reasonable, therefore,that on the T13+ system, polyions are as effective as on the Hg2+ system.The influence of inert salts was examined for the Hg2+ system, using NaC104.The result is shown in fig.5. The effect of acidity on the acceleration factor is givenin fig. 6. From these figures, it is clear that increases in NaC104 concentration andacidity result in a sharp decrease in the acceleration factor. Since it has beenknown that the Hg2+-induced aquation rate is independent of the pH of the solution4.0 -3.0-n s5 M 2.0-0 .-.11.0--3.0 -2.0 -1.0log INaC1041FIG. 5.-Dependence a the polyion catalysis on simple salt concentration at 30°C. [Co(NH3), "+I= 6.7 x M, [Hg2+] = 1.32 x M, [HC104] = 1.88 x M, [PSS] = 3.75 x equiv. l.-'.at pH below 3,2 the acidity increase in fig. 6 can be regarded as an increase in ionicstrength. Therefore, the results shown in fig. 5 and 6 may reflect the shielding effectby inert salt ions on the polyions.In this respect, it should be mentioned that theincrease in [ClOJ (fig. 5) complicates the situation because Hg2+ and ClO; form anassociated complex HgCIOz at higher concentrations. The dissociation constantof this ionic complex K is reported to be 1.0 x M.'The temperature dependence of the present reaction systems is interesting. Inprevious papers, we have pointed out that polyelectrolyte cataIysis in substitutionreactions between similarly charged ionic species is due to decreases in AH* and AS*(enthalpy and entropy of activation) whereas simple electrolyte catalysis (primarysalt effect) is due to increases in AH* and AS*. The rate enhancement in the poly-electrolyte case thus originates from the decrease in AH* and that in the simpleelectrolyte case from the increase in AS*.In the present reaction systems with Ag+and Hg2+, AH* and AS* decreased with addition of the polyelectrolytes as is shownin table 1. This is in accord with previous observations mentioned above. However,for the T13+-induced reactions, A H * was not influenced by polyelectrolyte additionwhereas AS* increased, as is shown in table 1104 POLYELECTROLYTE CATALYSIS[PSSIFIG. 6.-Influence of [HC1041 on the polyion catalysis at 30°C. [Co(NH,),Br2+] = 6.7 x M,[Hg2+l = 1.32 x M, [HC104] from top to bottom ; 1.88 x 9.6 x 9.6 x and1.88 x 10-1 N.TABLE 1 .-THERMODYNAMIC QUANTITIES FOR THE INDUCED AQUATION OF Co(NH&Br2+ WITHAND WWHOUT ADDED ELECTROLYTE *added conc.x 1041 A H * / AS+/ AG " 1electrolyte equiv. 1. - 1 kcal mol- cal deg- Irnol- 1 kcal mol-Ag77.5 x M)none 14.4 - 15NaPSS 0.300 12.3 - 193.00 3.3 - 4130.0 12.6 - 13NaN03 90.0 20.8 +8Hg2+(5x M in HC1O4, 0.01 N)none bnoneNaPES 0.1851.85NaPP 4.00NaPSS 4.00NaCIO, 20018.512.014.19.56.34.55.09.214.5- 16-7- 12- 18 - 23- 20-11-5T13+(lW3 M in HC104, 0.10 N)none 10.2 - 23NaPES 1.85 8.9 - 157.40 9.7 -1114.8 9.9 -918.5 9.9 -9none 11.4 - 1818.918.115.516.618.416.716.213.011.711.411.112.416.017.113.412.812.612.516.70 25°C ; [Co(NH3),Br2+] = 6 x M ; "0,) = 6.7 x N ; c [HC1O4] = 0.56 NNOR10 ISE A N D YOSHINOBU MATSUDA 105This “exceptional” behaviour of the T13+ systems invites comment.If wediscuss AS* in terms of solvation-desolvation of the activated complex, the AS*increase for the T13+ case may suggest that the polyelectrolyte addition results in(1) a stronger hydration of the initial state, or (2) a weaker hydration of the activatedcomplex than in the Hg2+ (or Ag+)-containing systems. In other words, reductionin the hydration number of T13+ caused by the polyelectrolyte addition is smaller thanthat for Hg2+ or A@ systems. We suggest that T13+ is more strongly hydrated thanthe other two cations. This interpretation is favoured by the half-life period (7) forthe exchange process of water molecules bound to the metal cations. Accordingto Eigeq8 the z value is s for Hg2+ at 25°C. No measure-ments have been reported for Ag+, but it would be reasonable to assume a value ofs for 2.The difference in the strengths of the interactions of water with T13+and Hg2+ (or Ag+) would be responsible for the liberation of less water moleculesfrom T13+ by polyelectrolyte addition than from Hg2+. This should cause the strongerhydration of the initial state for T13+-containing systems, as mentioned above. Inthis connection, it is interesting to note an experimental observation reported byPosey and Taube : in the case of the T13+-induced aquation an important source ofthe water molecule in the final product, [CO(NH~)~H~O]~+, is the hydration spheres for T13+ andof ~13+.9This work was supported by a Grant-in-Aid from the Ministry of Education.(a) N. Ise and F. Matsui, J. Amer. Chem. SOC., 1968,90,4242.(b) N. Ise, Nature, 1970,225, 66.(c) T. Ueda, S . Harada and N. Ise, Chem. Comm., 1971,99.(d) T. Okubo and N. Ise, Proc, Roy. SOC. A, 1972,327,413.(e) T. Okubo and N. Ise, in preparation.J. N. Brnrnsted and R. Livingston, J. Amer. Chem. SOC., 1927,49,435.T. Okubo, N. Ise and F. Matsui, J. Amer. Chem. SOC., 1967, SS, 3697.(a) H. Morawetz and B. Vogel, J. Amer. Chem. SOC., 1969,91,563;(6) H. Morawetz and G. Gordimer, J. Amer. Chem. Soc., 1970,92,7532;(c) J. R. Cho and H. Morawetz, J. Amer. Chem. SOC., 1972,!M, 375.E. S. Gould, J. Amer. Chem. SOC., 1970,92,6797.F. Bas010 and R. G. Pearson, Mechanism oflnorganic Reacrwnr, 2nd edn. (Wiley, New York,1967), chap. 3. ’ C. W. Davies, Progr. Reaction Kinetics, 1961,1, 161.M. Eigen, Pure Appl. Chem., 1963,6,105.F. A. Posey and H. Taube, J. Amer. Chem. SOC., 1957,79,255