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The kinetics and mechanism of the reaction of H2O2 with K[RuIII(PDTA-H)Cl]·2H2O and K[RuIII(HEDTA-H)Cl]
Journal of Molecular Catalysis, 54 (1989)
139
139 - 146
THE KINETICS AND MECHANISM OF THE REACTION OF H,O, WITH K [ Ru”‘(PDTA-H)Cl] - ZH,O AND K[ Ru”‘(HEDTA-H)Cl] M. M. TAQUI KHAN* and R. M. NAIK Discipline of Coordination Chemistry and Homogeneous Catalysis, Central Salt and Marine Chemicals Research Institute, Bhavnagar 364 002 (India) (Received September 20,1988;
accepted January 9,1989)
Summary The kinetics and mechanism of the reaction of HzOz with K[Ru”‘1 (L = propylenediaminetetraacetic acid) and (PDTA-H)Cl] * 2H,O K [ Ru”‘( HEDTA-H)Cl] 2 (L = hydroxyethylethylenediaminetriacetic acid) was studied by stopped flow spectrophotometry at pH = 3.4 and p = 0.1 M (KCl) at various concentrations of HzOz and at different temperatures. The p-peroxo complex [Ru’~L]~O~ (L = PDTA 3, HEDTA 4) was found to be the final product in both cases. The formation of E.c-peroxocomplexes 3 and 4 was first order in HzOz and Ru”‘L complex concentrations. The values of thermodynamic as well as activation parameters for the interaction of HzOz with Ru”‘L complexes are reported.
Introduction Metal complexes containing a coordinated peroxide or superoxide groups are of biochemical significance as potential model systems for enzymes [l - 41. They also catalyze the oxidations of various organic substrates [5 - 81. We have recently reported [ 9, lo] the reversible binding of dioxygen to Ru(III)-aminopolycarboxylic acids to form p-peroxo-_Erhydroxo Ru(IV) complexes. The kinetics and mechanism of the interaction of H,Oz with Ru(III)-EDTA was also reported [ 111. In the present paper we report the kinetics and mechanism of the formation of p-peroxo complexes by the interaction of K[Ru(PDTA-H)Cl] - 2H,O 1 and K[Ru(HEDTA-H)Cl] 2 with Hz02.
Experimental The complexes K[ Ru”I(PDTA-H)Cl] * 2Hz0 1 and K[ Ru( HEDTAH)Cl] 2 were synthesized by a known procedure [ 121. The stock solution of *Author to whom correspondence 0304-5102/89/$3.50
should be addressed. @ Elsevier Sequoiaprinted
in The Netherlands
140
H,Oz was standardised against standard KMn04 solution. The ionic strength of the experimental solution was kept constant at 0.1 M using KC1 solution. The temperature was kept constant, within *O.l “C of the desired value in all the experiments. The kinetic measurements were done on a stopped flow spectrophotometer, Model SF-51 (HiTech), equipped with an Apple data processor. All kinetic measurements were done under pseudo-first-order conditions by using a large excess of HzOz with respect to the complex concentration.
Results Complex 1 israpidly aquated in solution to form the aquo complex [Ru(PDTA-H)H,O] 10. The aquation of 2, however, is extremely slow and the compound reacts with HzOz as the chloro species 2. Complexes la and 2 react with H,Oz under pseudo-first order conditions of HzOz concentratration to form ultimately the Ru(IV)-p-peroxo complexes [ Ruiv(PDTA-H)] 202 3 and [ RdV( HEDTA-H)] 202 4 respectively. Complexes 3 and 4 exhibit LMCT bands at 393 and 390 nm, respectively. A peak at 393 nm in the case of the reaction of la with H,O, and at 390 and 431 nm in the reaction of 2 with HzOz appear immediately after mixing the reactants (Figs. 1 and 2). The rates of formation of the complexes 3 and 4 were followed by the appearance of the characteristic absorption peaks of the p-peroxo species at 393 and 390 nm, respectively, where complexes la and 2 do not have significant absorption. In the case of complex 4, the reaction proceeds through an intermediate [Run’(HEDTA-H)(HOl)Cl] (species 2b in Fig. 2) which loses the chloride ion in a fast step and is converted to 4. The plots of kobsd us. [H,O,] give a straight line at each temperature. These lines pass through the origin, satisfying the condition:
200
300
LOO
500
601
Wavelength (nml
Fig. 1. The absorption spectra at 26 “C of (a) K[Ru(PDTA-H)CI]*2HzO where [ML] = 5.0 x 10” M; (b) [(Ru’VHLhO,] where [MHL] = 1.0 x 10m3 M, [HzOz] = 5 x 10” M, pH = 3.40 and /J= 0.1 M (KCl).
141
200
300
boo
500
600
Wavelength (nm)
Fig. 2. The absorption spectra at 25 “C of (a) K[Ru(HEDTA-H)Cl] where [ML] = 5.0 x lo4 M; (b) [RuW(L)(HO,-)Cl] where [ML] = 1.0 x 10e3, [H202] = 0.2 M; (c) [RuL]202, conditions same as (b).
k obsd =
k’PWz1
(1)
where k’ = k llitos for both systems (Figs. 3 and 4, where K,, = outer sphere formation constant). Thus the rates of formation of E.c-peroxo complex 3 and 4 are first order in [H202] as well as first order each in [Ru(PDTA-H)(H,O)] la and [Ru(HEDTA)Cl]- 2. Based on the kinetic observations on the formation of the complexes 3 and 4, a plausible mechanism is proposed for the reaction of [Ru”‘HL(X)] (L = PDTA, HEDTA) with HzOz.
6
0
2
L
6
6
10
12
lf,
16
1
~202]x102,M
Fig. 3. The dependence of observed pseudo-first order rate constants (k,,bsd) for the reaction of complex 1 with Hz02 at different temperatures and HzOz concentrations (reaction conditions as stated in Table 1).
142
Fig. 4. The dependence of observed pseudo-first order rate constants (k&,d) for the resction of complex 2 with Hz02 at different temperatures and Hz02 concentrations (reaction conditions given in Table 1).
2[Ru=‘HL(X)] [RuIv(L(X)]
+ HzOz z + H,Oz %?
2 [ RuIvL(X)]
(2)
[ RufVL(X)H,02]
tRu’VL(XF120,1 & [Ru’~HL(X)(HO~-)]
+ 2Hz0
(3) (4)
[Ru’~HL(X)(HOJ] fast [ RuIV(HL)( HOl)]
+ X-
(5)
With 1, X = H20; with 2, X = Cl-.
O-O [ RuIVHL(H02-) + [ RuIV( L)H20] fast [ (HL)Ru’ A
‘Ru( HL)]
(6)
Ir
X = H,O (PDTA); X = Cl- (HEDTA) Scheme 1.
In the first step of the above scheme, H202 rapidly oxidises [RI?“HL(X)] to RuTv(L)(X)] . The second step involves the formation of an
143
outer sphere complex [ RuIVL( X) - - - H,O,] by the reaction of [RuIVL(X)H,O] with H,02 in an associative manner. The complex [RurvL(X)*** Hz01 rearranges in a rate-determining step (eqn. 4) to form a hydroperoxide ion complex [ Ru’~HL(X)(HO~-)]. The complex [ Ru’~HL(X)(HO~-)] dissociates X- in a fast step to form [Ru’~HL(HO~-)] species, which reacts with [ RurVL(H,O)] in another fast step to form the final product, the Cc-peroxo complex. The following general rate law can be deduced from the above proposed mechanistic scheme. d[ peroxo complex] /dt = kobsd[ RuL] T where k,bsd =
(7)
klKosW,O,I 1 + &W,W
(8)
L = HEDTA or PDTA K,, = outer sphere formation constant (eqn. 3) k, = rate constant for rate-determining step (eqn. 4). Equation (8) can be rearranged to eqn. (9): 1 -= k obsd
(9)
A plot Of l/kobsd us. 1/[H202] yields a straight line with an intercept of l/k1 and slope l/kIK. This relationship is found to be true over a wide range of HzOz concentrations and temperatures. The values of kI and K can be determined from the intercept and slope of the above plot. The values of K,, and k, have been determined at four different temperatures and are given in Tables 1 and 2, respectively. The plots of eqn. (9) are given for the reaction la with HzOz in Fig. 5; similar plots were obtained for the reaction of 2 with HzOz. Thermodynamic parameters LvI” for the formation of the outer sphere complex were calculated from the slopes of log K,, us. l/T plots. TABLE 1 The equilibrium constants K for the formation of outer sphere [Ru(HEDTA)Cl]---Hz02 and [ Ru(PDTA)( H20)]-Hz02 complexes, /J = 0.1 M (KCl) Temp. (K)
[ Ru(HEDTA)Cl]-HzOz K
288 293 298 308
2.05 1.58 1.27 0.95
*
[Ru(PDTA)(H~O)]-H~O,~ K 6.94 4.95 4.01 2.61
Walculated kinetically according to equilibrium step 2 (Scheme 1).
144 TABLE 2 The V&WEaf rate constants at different temperaturesfor the rearrangementof the outer sphere complex in the rate-determiningstep for [ Ru( L)(HzO) J (L = EDTA, WEDTA) end [Ru(HEDTA)Cl] complexes Temp. (K)
[ Ru(EDTA)H201a (kl s-l)
[ Ru(PDTA)H20] (kl @I
[ Ru( HEDTA)Cl] (kl s--l)
288 293 298 308
17.8 24.2 31.6 56.6
6.5 7.4 Il.*3 16.7
0.40 0.90 2.00 7.14
aData from [II],
Fig. 5. The plot of l/k,bsa vs. l/f&&] to determine formation cozI&u&s (&,) for the outer sphere complex in the reaction of complex 1 with Hz02 (reaction conditions given in Table 1).
AH0 2.30312 The values of A@ and ASo for this step were cakulated using the following slope = -
equations: A@ = 2.303.M’ log K,,
@I)
The values of these parameters are given in Table 3. The values of the energy of activation E, were calculated using the Arrhenius equation: kl = A exp(---E,/RT)
(131
The vahzes of A@ and AS* were also calculated at 25 “Cusing the following equations:
145 TABLE 3 Thermodynamic parameters corresponding to equilibrium constants X at p = 0.1 M (KC]) for ~Ru(HEDTA)Cl]-Ha02 and [Ru(PDTA)&O J-Ha02 reaction systems,
-_- ._-. Reaction system
AHQ (kcal moT1)
AG”s (kcal mol-*)
[Ru(HEDTA)C1]--H&la [ Ru(PDTA)H20]--HzO,
-7.09 -8.51
-6.14 -0.82
-23.30 -26.0
aCab.dated at 298 K. TABLE 4 Activation parameters corresponding to the rata constant kl for the mactious of (Ru(HEDTA)Cl] and [Ru(L)HaO] (L = EDTA, PDTA) with Ha& at I = 0.1 M (KC1) Reaction Systems
Ea (kcal mol-r )
A&a (cal deg-” m&r)
[ Ru(HEDTA)Cl]-H& [Ru(PDTA)H@]-HaOa ~Ru(EDTA)~~~]-~*O~
5.40 12.0 10.4
-33.65 -21.00 -19.00
b
aCslcuiated at 298 K, Wats from [ll].
A@=EE,--RT
(14)
RT AS* A.l@ 2.303 log kl = 2.303 log% + 7 - T
05)
The valuesare listed in Table 4. Discussion Prom Table 1, the vahzes of K for the formation of the outer sphere complex in step 2 jscheme I) decrease in the order PDTA > HEDTA. The values of K for the HEDTA complex at 25 “C is almost equal to those for the EDTA complex reported earlier (Table 1) [ll]. The enthalpy of outer sphere complex formation K is most exothermic for PDTA complex (Table 3). As expected, the entropy AL!J” associatedwith K is highly negative, and increasesin the order PDTA < HEDTA. The value of the rate constants corresponding to the rate determining formation of hydroperoxo species in step 4 (Scheme lf at different temperatures are given in Table 2. The rates decreasein the order EDTA > PDTA > HEDTA. The activation parameterscorrespondingto R, &e given in Table 4, The table also contains the data for the EDTA complex. The entbalpy of
146
activation becomes more exothermic in the order PDTA > EDTA > HEDTA. The entropy of activation AS0 decreases in the order EDTA > PDTA > HEDTA. The AS0 for HEDTA is about l$ times more negative than those of EDTA and ,PDTA complexes. This reflects on the association of Cl- ion in the slow step k, before the formation of the final product. The maximum rate of formation for EDTA complex is thus due to the more exothermic enthalpy and the most positive value of entropy, as compared to other complexes. Though the HEDTA complex has the most exothermic enthalpy, the rate is slowest because of the highly negative entropy of activation. The rate constant kI thus seems to be controlled by the entropy of activation, and follows the same order as the decrease of entropy: EDTA > PDTA S HEDTA. The large negative entropy of activation is characteristic [13 15 ] of the H,02 unimolecular associative reaction and is ascribed to a highly oriented transition state.
References 1 J. T. Grovesand R. S. Meyers, J. Am. Chem. Sot., 105 (1983) 5791. 2 T. C. Bruice and Yuan Lung&i, J, Am. Chem. Sot., 107 (1985) 512. 3 T. G. Traylor, J. C. Marsters Jr., T. Nakano and B. E. Dunlop, J. Am. Chem. Sot., IO7 (1935) 5537. 4 J. P. Colhnan, J. I. Brauman, B. Meunier, T. Hayashi, T. Kodadek and S. A. Raybuck, J. Am. Chem. SW., 107 (1985) 2800. 5 M. M. Taqui Khan, M. R. H. Siddiqui, Amjad Hussain and M. A. Moiz, Inorg. Chem., 25 (1986) 2765. 6 M. M. Taqui Khan, S. A. Mirza and H. C. Bajaj, J. Mol. Catal, 39 (1987) 323. 7 M. M. Taqui Khan and R. S. Shukla, d. Mol. Catal., 44 (1988) 85. 8 M. M. Taqui Khan, S. A. Mhza, A, Prakash Rao and C. Sreelatha, J. Mol. Cutal., 44 (1988) 107. 9 M. M. Taqui Khan, A. Hussain, K. Venlcatasubramanian, G. Ramachandraiah and V. Gommen, J. Mol. Catal, 44 (1988) 117. 10 M. M. Taqui Khan, A. Hussain, G. Ftamachandraiah and M. A. Moiz, Inorg. Chem., 25 (1986) 3023. 11 M. M. Taqui Khan, H. C. Bajaj and M. R. H. Siddiqui, J. &fo1. Cat& 44 (1988) 279. 12 M. M. Taqui Khan, A. Kumar and 2. Shirin, 6. Chem. Res. (S), (1986) 130. 13 S. Funahashi, K. Ishihara and M. Tanaka, Inorg Chem., 20 (1981) 1142. 14 S. Funahashi, Y. Ito and M. Tanaka, J. Coord. Chem., 3 (1973) 125. 15 S. Funahashi, K. Haraguchi and M. Tanaka, Inorg. C&em., 16 (1977) 1349.
139
139 - 146
THE KINETICS AND MECHANISM OF THE REACTION OF H,O, WITH K [ Ru”‘(PDTA-H)Cl] - ZH,O AND K[ Ru”‘(HEDTA-H)Cl] M. M. TAQUI KHAN* and R. M. NAIK Discipline of Coordination Chemistry and Homogeneous Catalysis, Central Salt and Marine Chemicals Research Institute, Bhavnagar 364 002 (India) (Received September 20,1988;
accepted January 9,1989)
Summary The kinetics and mechanism of the reaction of HzOz with K[Ru”‘1 (L = propylenediaminetetraacetic acid) and (PDTA-H)Cl] * 2H,O K [ Ru”‘( HEDTA-H)Cl] 2 (L = hydroxyethylethylenediaminetriacetic acid) was studied by stopped flow spectrophotometry at pH = 3.4 and p = 0.1 M (KCl) at various concentrations of HzOz and at different temperatures. The p-peroxo complex [Ru’~L]~O~ (L = PDTA 3, HEDTA 4) was found to be the final product in both cases. The formation of E.c-peroxocomplexes 3 and 4 was first order in HzOz and Ru”‘L complex concentrations. The values of thermodynamic as well as activation parameters for the interaction of HzOz with Ru”‘L complexes are reported.
Introduction Metal complexes containing a coordinated peroxide or superoxide groups are of biochemical significance as potential model systems for enzymes [l - 41. They also catalyze the oxidations of various organic substrates [5 - 81. We have recently reported [ 9, lo] the reversible binding of dioxygen to Ru(III)-aminopolycarboxylic acids to form p-peroxo-_Erhydroxo Ru(IV) complexes. The kinetics and mechanism of the interaction of H,Oz with Ru(III)-EDTA was also reported [ 111. In the present paper we report the kinetics and mechanism of the formation of p-peroxo complexes by the interaction of K[Ru(PDTA-H)Cl] - 2H,O 1 and K[Ru(HEDTA-H)Cl] 2 with Hz02.
Experimental The complexes K[ Ru”I(PDTA-H)Cl] * 2Hz0 1 and K[ Ru( HEDTAH)Cl] 2 were synthesized by a known procedure [ 121. The stock solution of *Author to whom correspondence 0304-5102/89/$3.50
should be addressed. @ Elsevier Sequoiaprinted
in The Netherlands
140
H,Oz was standardised against standard KMn04 solution. The ionic strength of the experimental solution was kept constant at 0.1 M using KC1 solution. The temperature was kept constant, within *O.l “C of the desired value in all the experiments. The kinetic measurements were done on a stopped flow spectrophotometer, Model SF-51 (HiTech), equipped with an Apple data processor. All kinetic measurements were done under pseudo-first-order conditions by using a large excess of HzOz with respect to the complex concentration.
Results Complex 1 israpidly aquated in solution to form the aquo complex [Ru(PDTA-H)H,O] 10. The aquation of 2, however, is extremely slow and the compound reacts with HzOz as the chloro species 2. Complexes la and 2 react with H,Oz under pseudo-first order conditions of HzOz concentratration to form ultimately the Ru(IV)-p-peroxo complexes [ Ruiv(PDTA-H)] 202 3 and [ RdV( HEDTA-H)] 202 4 respectively. Complexes 3 and 4 exhibit LMCT bands at 393 and 390 nm, respectively. A peak at 393 nm in the case of the reaction of la with H,O, and at 390 and 431 nm in the reaction of 2 with HzOz appear immediately after mixing the reactants (Figs. 1 and 2). The rates of formation of the complexes 3 and 4 were followed by the appearance of the characteristic absorption peaks of the p-peroxo species at 393 and 390 nm, respectively, where complexes la and 2 do not have significant absorption. In the case of complex 4, the reaction proceeds through an intermediate [Run’(HEDTA-H)(HOl)Cl] (species 2b in Fig. 2) which loses the chloride ion in a fast step and is converted to 4. The plots of kobsd us. [H,O,] give a straight line at each temperature. These lines pass through the origin, satisfying the condition:
200
300
LOO
500
601
Wavelength (nml
Fig. 1. The absorption spectra at 26 “C of (a) K[Ru(PDTA-H)CI]*2HzO where [ML] = 5.0 x 10” M; (b) [(Ru’VHLhO,] where [MHL] = 1.0 x 10m3 M, [HzOz] = 5 x 10” M, pH = 3.40 and /J= 0.1 M (KCl).
141
200
300
boo
500
600
Wavelength (nm)
Fig. 2. The absorption spectra at 25 “C of (a) K[Ru(HEDTA-H)Cl] where [ML] = 5.0 x lo4 M; (b) [RuW(L)(HO,-)Cl] where [ML] = 1.0 x 10e3, [H202] = 0.2 M; (c) [RuL]202, conditions same as (b).
k obsd =
k’PWz1
(1)
where k’ = k llitos for both systems (Figs. 3 and 4, where K,, = outer sphere formation constant). Thus the rates of formation of E.c-peroxo complex 3 and 4 are first order in [H202] as well as first order each in [Ru(PDTA-H)(H,O)] la and [Ru(HEDTA)Cl]- 2. Based on the kinetic observations on the formation of the complexes 3 and 4, a plausible mechanism is proposed for the reaction of [Ru”‘HL(X)] (L = PDTA, HEDTA) with HzOz.
6
0
2
L
6
6
10
12
lf,
16
1
~202]x102,M
Fig. 3. The dependence of observed pseudo-first order rate constants (k,,bsd) for the reaction of complex 1 with Hz02 at different temperatures and HzOz concentrations (reaction conditions as stated in Table 1).
142
Fig. 4. The dependence of observed pseudo-first order rate constants (k&,d) for the resction of complex 2 with Hz02 at different temperatures and Hz02 concentrations (reaction conditions given in Table 1).
2[Ru=‘HL(X)] [RuIv(L(X)]
+ HzOz z + H,Oz %?
2 [ RuIvL(X)]
(2)
[ RufVL(X)H,02]
tRu’VL(XF120,1 & [Ru’~HL(X)(HO~-)]
+ 2Hz0
(3) (4)
[Ru’~HL(X)(HOJ] fast [ RuIV(HL)( HOl)]
+ X-
(5)
With 1, X = H20; with 2, X = Cl-.
O-O [ RuIVHL(H02-) + [ RuIV( L)H20] fast [ (HL)Ru’ A
‘Ru( HL)]
(6)
Ir
X = H,O (PDTA); X = Cl- (HEDTA) Scheme 1.
In the first step of the above scheme, H202 rapidly oxidises [RI?“HL(X)] to RuTv(L)(X)] . The second step involves the formation of an
143
outer sphere complex [ RuIVL( X) - - - H,O,] by the reaction of [RuIVL(X)H,O] with H,02 in an associative manner. The complex [RurvL(X)*** Hz01 rearranges in a rate-determining step (eqn. 4) to form a hydroperoxide ion complex [ Ru’~HL(X)(HO~-)]. The complex [ Ru’~HL(X)(HO~-)] dissociates X- in a fast step to form [Ru’~HL(HO~-)] species, which reacts with [ RurVL(H,O)] in another fast step to form the final product, the Cc-peroxo complex. The following general rate law can be deduced from the above proposed mechanistic scheme. d[ peroxo complex] /dt = kobsd[ RuL] T where k,bsd =
(7)
klKosW,O,I 1 + &W,W
(8)
L = HEDTA or PDTA K,, = outer sphere formation constant (eqn. 3) k, = rate constant for rate-determining step (eqn. 4). Equation (8) can be rearranged to eqn. (9): 1 -= k obsd
(9)
A plot Of l/kobsd us. 1/[H202] yields a straight line with an intercept of l/k1 and slope l/kIK. This relationship is found to be true over a wide range of HzOz concentrations and temperatures. The values of kI and K can be determined from the intercept and slope of the above plot. The values of K,, and k, have been determined at four different temperatures and are given in Tables 1 and 2, respectively. The plots of eqn. (9) are given for the reaction la with HzOz in Fig. 5; similar plots were obtained for the reaction of 2 with HzOz. Thermodynamic parameters LvI” for the formation of the outer sphere complex were calculated from the slopes of log K,, us. l/T plots. TABLE 1 The equilibrium constants K for the formation of outer sphere [Ru(HEDTA)Cl]---Hz02 and [ Ru(PDTA)( H20)]-Hz02 complexes, /J = 0.1 M (KCl) Temp. (K)
[ Ru(HEDTA)Cl]-HzOz K
288 293 298 308
2.05 1.58 1.27 0.95
*
[Ru(PDTA)(H~O)]-H~O,~ K 6.94 4.95 4.01 2.61
Walculated kinetically according to equilibrium step 2 (Scheme 1).
144 TABLE 2 The V&WEaf rate constants at different temperaturesfor the rearrangementof the outer sphere complex in the rate-determiningstep for [ Ru( L)(HzO) J (L = EDTA, WEDTA) end [Ru(HEDTA)Cl] complexes Temp. (K)
[ Ru(EDTA)H201a (kl s-l)
[ Ru(PDTA)H20] (kl @I
[ Ru( HEDTA)Cl] (kl s--l)
288 293 298 308
17.8 24.2 31.6 56.6
6.5 7.4 Il.*3 16.7
0.40 0.90 2.00 7.14
aData from [II],
Fig. 5. The plot of l/k,bsa vs. l/f&&] to determine formation cozI&u&s (&,) for the outer sphere complex in the reaction of complex 1 with Hz02 (reaction conditions given in Table 1).
AH0 2.30312 The values of A@ and ASo for this step were cakulated using the following slope = -
equations: A@ = 2.303.M’ log K,,
@I)
The values of these parameters are given in Table 3. The values of the energy of activation E, were calculated using the Arrhenius equation: kl = A exp(---E,/RT)
(131
The vahzes of A@ and AS* were also calculated at 25 “Cusing the following equations:
145 TABLE 3 Thermodynamic parameters corresponding to equilibrium constants X at p = 0.1 M (KC]) for ~Ru(HEDTA)Cl]-Ha02 and [Ru(PDTA)&O J-Ha02 reaction systems,
-_- ._-. Reaction system
AHQ (kcal moT1)
AG”s (kcal mol-*)
[Ru(HEDTA)C1]--H&la [ Ru(PDTA)H20]--HzO,
-7.09 -8.51
-6.14 -0.82
-23.30 -26.0
aCab.dated at 298 K. TABLE 4 Activation parameters corresponding to the rata constant kl for the mactious of (Ru(HEDTA)Cl] and [Ru(L)HaO] (L = EDTA, PDTA) with Ha& at I = 0.1 M (KC1) Reaction Systems
Ea (kcal mol-r )
A&a (cal deg-” m&r)
[ Ru(HEDTA)Cl]-H& [Ru(PDTA)H@]-HaOa ~Ru(EDTA)~~~]-~*O~
5.40 12.0 10.4
-33.65 -21.00 -19.00
b
aCslcuiated at 298 K, Wats from [ll].
A@=EE,--RT
(14)
RT AS* A.l@ 2.303 log kl = 2.303 log% + 7 - T
05)
The valuesare listed in Table 4. Discussion Prom Table 1, the vahzes of K for the formation of the outer sphere complex in step 2 jscheme I) decrease in the order PDTA > HEDTA. The values of K for the HEDTA complex at 25 “C is almost equal to those for the EDTA complex reported earlier (Table 1) [ll]. The enthalpy of outer sphere complex formation K is most exothermic for PDTA complex (Table 3). As expected, the entropy AL!J” associatedwith K is highly negative, and increasesin the order PDTA < HEDTA. The value of the rate constants corresponding to the rate determining formation of hydroperoxo species in step 4 (Scheme lf at different temperatures are given in Table 2. The rates decreasein the order EDTA > PDTA > HEDTA. The activation parameterscorrespondingto R, &e given in Table 4, The table also contains the data for the EDTA complex. The entbalpy of
146
activation becomes more exothermic in the order PDTA > EDTA > HEDTA. The entropy of activation AS0 decreases in the order EDTA > PDTA > HEDTA. The AS0 for HEDTA is about l$ times more negative than those of EDTA and ,PDTA complexes. This reflects on the association of Cl- ion in the slow step k, before the formation of the final product. The maximum rate of formation for EDTA complex is thus due to the more exothermic enthalpy and the most positive value of entropy, as compared to other complexes. Though the HEDTA complex has the most exothermic enthalpy, the rate is slowest because of the highly negative entropy of activation. The rate constant kI thus seems to be controlled by the entropy of activation, and follows the same order as the decrease of entropy: EDTA > PDTA S HEDTA. The large negative entropy of activation is characteristic [13 15 ] of the H,02 unimolecular associative reaction and is ascribed to a highly oriented transition state.
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