untitled Studies o cobalt(II Farah Sami Ghassan Om a Chemistry Depar b Chemistry Depar *Corresponding a fax: +2.034276590 ARTICLE INFO Received: 13 Apr Received in revis Accepted: 05 July Online: 31 Decem KEYWORDS Ion‐pair Aquation Solvent effect Cobalt(III) compl Dicarboxylic acid Tert‐butanol 1. Introductio Transition roles in biolo complexes hav rabbit brain an [1‐2]. Also cob vitamin B12 [3] Different s halogenopenta investigations tial, selective useful in accou in solution. It w some bivalen chromium(III) compared to t pairing effects cobalt(III) per some dicarbo treatment for fairly establish aquated by an The aim o bromopentaam solutions [0.00 Na2CO3 contai to determine extrathermody on ion‐pai I) comple ih Zeitounia, mar Younes rtment, Faculty of S rtment, Faculty of S uthor at: Chemistry 0. E‐mail address: f ORMATION il 2011 ed form: 27 June 2 y 2011 mber 2011 lex d on n metal comple gical systems. ve been the sub nd the inhibitio balt(III) is bene ]. studies had des aamminecobalt include nume and specific so unting for the e was shown [9] t anions and ) cations und the free cation s on the rate o rchlorate in 20% oxylates must r calculating th hed that halop essentially diss of the present w mminecobalt(II 08‐0.040 mol/L ning tert‐butan e the solvent ynamic parame Eu ISSN 2153‐ Europ J iring effec ex in malo , Gehan Mou a and Moham Science, Beirut Arab Science, Alexandria y Department, Facu fawzyamira54@ya 2011 exes play a va The biological bject of studies on of soybean li ficial for human scribed the kin t(III) in differen erous factors, d olute‐solvent in quilibrium beh that the ion‐pa halopentaamm dergo aquation ns. It was found of aquation of b % (v/v) ethylen follow Waytt he ion‐pair rat entaammine co sociative proce work is to study II) perchlorate L malonic acid nol (10%:50%, effects on th eters of activat uropean Journal Europe 2249 (Print) / IS DOI:10.5155 pean Jo Journal home cts of the onate med ustafa El‐Sub mmad Fawz b University, Riad E a University, Ibrahim ulty of Science, Alex ahoo.com (M.F. Ami ABSTRACT The aquation o anion in mixe investigated sp Log (kip) ion‐pa water concentr extrathermody effect on the i systems indicat The extrema fo be attributed to solvent structu stability of the ∆Gip*with the m between ∆Hip* a ariety of impo activities of c on the inhibiti poxygenase enz n since it is a pa netics of aquati nt media [4‐8]. describing prefe nteractions and havior of electro airs formed betw mine cobalt(III) n at a faster d that [10] the bromopentaam ne glycol solutio t and Davies te coefficients. obalt(III) comp ess [12]. y of the aquati in malonate b neutralized by v:v)] in an att hermodynamic tion where the of Chemistry 2 ( ean Journal of Ch SSN 2153‐2257 5/eurjchem.2.4.4 ournal o epage: www.e kinetics o dia bruitib, zi Amirab,* El‐Solh, Beirut‐11‐5 mia, Alexandria‐21 xandria University, ira). of bromopentaa ed solvent med pectrophotometr air rate constan ration and Grun namic analyses ion‐pair aquatio te the existence und in the chan o the change of ure. Application e initial and the mole fraction of and ∆Sip*. ortant cobalt on of zyme art of on of Such eren‐ d are olytes ween and rate e ion‐ mmine ons of [11] It is plexes on of buffer 80% tempt and data can inter 2. Ex 2.1. C malo purc purc purc acid coba meth 2.2. T 10% 30‐6 Heli spec heat The the m (4) (2011) 489‐4 hemistry (Online)  2011 489‐494.440 of Chem eurjchem.com of aquatio 5020, Lebanon 321, Egypt P.O. Box 426, Ibrah mminecobalt(III dia of water w rically at differe nts against the nwald‐Winstein Y of the kinetic on reactions. Th of compensatio ge of ∆Hip* and ∆ the physical pr of a free energ e transition stat f the co‐solvent be linked to o raction. xperimental Reagents Cobalt(II) carb onic acid, so chased from F chased from chased from Ch 60% was purc alt(III) perchlo hod of Hynes [1 Procedure The rate of aq %:50% (v:v) of t 60 oC was follo os Alpha and B ctrophotometer ted by water ci inflow and the mean temperat 494 1 EURJCHEM mistry m on of brom himia, Alexandria‐2 I) ion in the pr with tert‐butano nt temperatures reciprocal of th Y values were f data have been he obtained iso n effect arising ∆Sip*with the mo operties of the gy cycle is perfo te of the compl was found, ind obtain informat bonate, hydrob dium carbona Fluka Chemika Riedel‐de Haë hemical Manag chased from Me orate complex 13]. quation of [Co tert‐butanol wit owed spectrop Beta spectroph r was fitted wi irculating from e outflow temp ture was taken a mopentaa 21321, Egypt. Tel.: esence of ion‐p ol (10%:50%, v s (30‐60 oC). No he dielectric con found. The therm n discussed in te okinetic temper from solute‐solv ole fraction of th solvent‐water m ormed to compa ex. However, sm dicating a comp tion about the bromic acid 4 ate and tert‐ a. Hydrogen p ën. Hydrochlo gement Consult erck. The brom was prepared o(NH3)5Br](ClO4 th malonate bu hotometrically otometer at λ th thermostate a Heto HMT 2 peratures were as that of the ce ammine +2.034276580; airing malonate v:v) have been onlinear plots of nstant D, Log of modynamic and erms of solvent ratures of these vent interaction e co‐solvent can mixture with the are between the mall changes in ensating effects solute‐solvent 8%, ammonia, ‐butanol were peroxide, were oric acid was ting. Perchloric opentaammine d by using the 4)2 complex in uffer solution at using Unicam = 250 nm. The ed cell holders, 00 thermostat. measured and ell. e n f f d t e . n e e n s t , e e s c e e n t m e , . d 490 Zeitouni et al. / European Journal of Chemistry 2 (4) (2011) 489‐494 Table 1. Values of rate constants (kox106) in sec‐1 for the aquation of [Co(NH3)5Br]2+ in tert‐butanol (10‐50%) at different temperatures. T, oC 10% tert‐butanol 20% tert‐butanol 30% tert‐butanol 40% tert‐butanol 50% tert‐butanol 30 2.96 2.68 0.11 0.61 4.45 35 3.32 7.12 0.72 1.09 4.61 40 12.10 17.00 4.83 18.30 9.60 50 92.30 93.90 95.70 44.80 94.20 60 364.00 224.00 227.00 208.00 180.00 Table 2. Values of rate constants (kobsx106) for the aquation of [Co(NH3)5Br]2+ in malonate media (m1) containing tert‐butanol (10‐50%) at different temperatures. m1, mol/L Temperature, oC 30 35 40 50 60 (kobsx106) in 10% 0.008 4.71 14.64 29.17 58.77 473.1 0.016 5.08 11.95 34.06 141.2 394.1 0.024 5.71 17.28 47.44 163.95 469.4 0.032 2.07 26.53 38.78 164.81 546.1 0.04 3.99 20.1 47.34 187.23 546.1 (kobsx106) in 20% 0.008 7.07 10.92 26.31 136.56 439.7 0.016 6.74 11.49 16.17 156.53 416.2 0.024 10.36 15.35 26.67 152.21 391.3 0.032 6.62 14.23 27.28 167.6 530.1 0.04 6.15 12.69 34.400 174.7 437.7 (kobsx106) in 30% 0.008 4.18 7.35 16.75 140.29 422.4 0.016 5.66 8.14 21.39 177.67 382.7 0.024 9.52 5.56 16.66 169.60 522.2 0.032 3.12 19.96 29.11 147.88 420.9 0.04 4.50 26.87 32.01 146.66 471.2 (kobsx106) in 40% 0.008 4.94 15.32 77.16 167.18 617.96 0.016 6.98 10.41 28.44 166.91 506.12 0.024 7.08 24.91 85.99 175.47 548.32 0.032 5.63 23.39 45.65 158.45 450.8 0.04 4.71 13.39 40.42 162.24 436.1 (kobsx106) in 50% 0.008 5.66 11.69 33.095 149.09 414.9 0.016 9.13 18.18 34.61 152.11 383.1 0.024 9.21 19.16 43.76 167.38 405.1 0.032 5.58 24.35 45.77 194.56 442.4 0.04 14.19 24.11 46.3 222.22 428.0 3. Results and discussion Plots of log (At‐A∞) versus time were found to be linear for the aquation of [Co(NH3)5Br](ClO4)2 complex. The observed rate constants are collected in Table 1 and 2. The ion‐pair rate coefficient (kip) was calculated according to the following Wyatt and Davis equation [11]. kobs.m3 = k0 [CpX2+] + kip [CpXL] (1) where, k0 ‐ the observed rate constant in the absence of dicarboxylate ion, kobs ‐ the observed rate constant in the presence of dicarboxylate ion, m3 ‐ the stoichiometric concentration of the complex salt, [CpX2+] ‐ the free complex ion concentration and [CpXL] ‐ the ion‐pair concentration. [CpXL] was calculated with the aid of the following equations: CpXL ⇆ CpX2+ + L2‐ KD (2) NaL‐ ⇆ Na+ + L2‐ KNaL‐ (3) H2L ⇆ HL‐ + H+ K1 (4) HL‐ ⇆ L2‐ + H+ K2 (5) where; KD = [CpX2+][L2‐] / [CpXL] (L2‐ represents the dicarboxylate anion) (6) K1 = [H+][HL‐] /[H2L] (7) K2 = [H+] [L2‐] γ2/ [HL‐] (8) KNaL‐ = [Na+] [L2‐] γ2/ [NaL‐] (9) Log γi = ‐A (I1/2/ (1+1.3 I1/2)‐0.3I) (Debye‐Huckle equation) (10) (Log γ2 =4.Log γ1), (I is the ionic strength γ1 and γ2 are the activity coefficients of the univalent and divalent ions, respectively). I = 0.5 ([H+] + [HL‐] +4[L2‐] +4[CpX2+] +2m3 + [Na+] + [NaL1‐]) (11) m1= [H2L] + [HL‐] + [CpXL] + [NaL‐] (12) m3= [CpX2+] + [CpXL] (13) The principle of calculations performed by computer programs can be summarized as: For the first cycle [H+] = 0, [CpXL] = 0, [NaL‐] = 0, (14) [CpX2+] = m3 – [CpXL], [HL‐] =0.5 m2, [H2L] = 0.3m1, (15) [L2‐] = m1 – [HL‐] – [CpXL] – [NaL‐] – [H2L], (16) [Na+] = 2m2 – [ NaL‐]. (17) where, m2 is the concentration of sodium carbonate. Afterwards, the ionic strength takes its first approximated value and then γ1and γ2 after which the following terms take their new value; Zeitouni et al. / European Journal of Chemistry 2 (4) (2011) 489‐494 491 [H+] = K2 [HL‐] / [L2‐] γ2 (18) [H2L] = [HL‐][H+] / K1 (19) [HL‐] = 2m1 ‐2m2 – 2[H2L] – [H+] (20) [L2‐] = m1 – [HL‐] – [H2L] – [CpXL] – [NaL‐] (21) [NaL‐] = [Na+] [L2‐] γ2/ KNaL‐ (22) [CpXL] = m3/[(KD/ [L2‐]) +1] (23) [CpX2+] = m3 – [CpXL] (24) Then I, γ1 and γ2 were recalculated again. These steps of calculations were repeated many times until the difference between two successive values of [CpXL] becomes equal to or less than 10‐7. The obtained average values of kip in malonate media with different percentage of tert‐butanol are collected in Table 3. Table 3. Calculated values of rate constants (kipx105) in sec‐1 for the aquation of [Co(NH3)5Br]2+ in malonate buffer containing tert‐butanol (10‐50%) at different temperatures. Temp., oC kip x105 in Malonate buffer at different % of tert‐butanol 10 20 30 40 50 25 9.43 0.39 2.39 0.39 0.71 30 16.46 1.04 4.16 0.86 1.36 35 28.22 2.59 7.10 1.83 2.56 40 47.54 6.32 11.92 3.79 4.73 50 128.60 34.56 32.00 15.26 15.19 60 327.60 170.60 80.98 56.42 45.54 3.1. Variation of Ion‐pair coefficients (kip) with solvent parameters The effect of water concentration in aqueous organic solvent mixture was investigated by the plot of the Log kip versus Log [H2O] at 30 °C. This plot was found to be nonlinear as shown in Figure 1. This nonlinearity confirms that the internal structure of the media studied suffered from serious changes on addition of tert‐butanol in the presence of malonate buffer, indicating that the hydrogen bond between the organic solvent and the buffer added to water is stronger than between two water molecules. Thus, hydrogen bonding between neighboring water molecules will be replaced gradually by hydrogen bonding with solvent and buffer molecules where the tetrahedral structure of water is largely broken [14]. Figure 1. Log kip versus log [H2O] at 30 oC at different concentrations of Malanote buffer. The plot of Log kip against the Grunwald‐Winstein Y‐values was found to be nonlinear as shown in Figure 2. This confirms the differential effect of solvent structures between the initial and the transition states. Figure 2. Log kip versus Y at 25 oC at different concentrations of Malonate buffer. Tert‐butanol has special effect on the formation of ion‐pair. This fact was also proven in previous studies [15‐16], where it was found that tert‐butanol has a greater effect on the water structure than the other solvents used earlier due to its branched chain. This may be attributed to the variation of important properties of the medium, such as the structure of the solvent, ionizing power, basicity and dielectric constant, which greatly influence the rate of the reaction. In addition, tert‐butanol is a dipolar protic solvent [17], since the oxygen atom of an alcohol molecule carries one proton and two lone pairs of electrons, it might the expected to form three hydrogen bonds with its neighbors, but all the evidences show that no more than two bonds are formed, each oxygen acting once as proton donor and once as proton acceptors due to steric effects of the alkyl groups. Also Log kip was plotted against reciprocal of the dielectric constant D at 25 oC, where the dielectric constant values of different compositions are obtained from Akerlof data [18]. As shown in Figure(3), the plot was found to be non linear in accordance with the general observations found in the aquation of a large number of other cobalt(III) complexes [19‐20] in water‐cosolvent mixtures, which led to a conclusion, that the contribution of the non electrostatic part of solvent effect, overcomes the electrostatic component part. Also, these parameter measures macroscopic properties, while specific solute‐solvent interactions occur on a microscopic scale are completely neglected. In such cases, the differential solvation of the initial and transition states is the controlling factor for the changes in the rate constant [21]. Figure 3. Log kip versus 1/D at 30 oC at different percentages of tert‐butanol in Malonate buffer media. However, Elsemongy and Amira [22] proposed a general equation for the variation of specific rate constant (k) with the dielectric constant (D) for any reaction, in which the transition state may or may not polarized. This equation was proved to be applicable to usual reactions, as well as those exhibiting minima or maxima with solvent composition variation. Their equation takes the following for Log A/K = E.b/2.302.R.Log C/D (25) Log C = Log A + 293.15b (26) 492 Zeitouni et al. / European Journal of Chemistry 2 (4) (2011) 489‐494 where A is the frequency factor, a and b are Akerlof’s empirical constants [17]. The plot Log A/K versus E.b/Log C/D as shown in Figure 4, gives a good straight line with a slope approximately equals to 0.052, which is equal to 1/2.302.R = 0.052. This result adds a further support of the validity of the above equation in the case of aquation of bromo‐ pentaamminecobalt(III) complex ion in water‐tert‐butanol medium. Figure 4. Correlation between the rate constant and the dielectric constant for Malonate buffer. 3.2. Thermodynamic parameters of the ion‐pair aquation reactions The thermodynamic parameters of the activated complex at 40 oC were collected in Table 4. Table 4. Values of the thermodynamic parameters of the ion‐ pairing aquation of [Co(NH3)5Br]2+ in malonate buffer at different mole fraction tert‐butanol at 40 oC. Temp., oC x2 (Mole fraction) ∆Hip* (kJ/mole) ∆Sip* (J/K.mole) ∆Gip* (kJ/mole) 40 0.02 81.11 ‐49.81 96.69 0.045 140.23 122.51 101.94 0.075 80.50 ‐63.23 100.29 0.112 114.42 35.62 103.27 0.116 95.66 ‐22.49 102.42 A linear plot of ∆Gip* against ∆Gass* (free energy of ion‐pair formed) is obtained as shown in Figure 5, which takes the form, ΔG∗ = ′ΔG +b’ (27)   Figure 5. ∆Gip*versus ∆Gass*for Malonate buffer at different solvent compositions at 40 oC. The relationship between the enthalpy of activation ∆Hip*and the enthalpy of association (ion‐pair formation) ΔH at different solvent compositions is shown in Figure 6. The plot shows a linear correlation which takes the form of Equation (28): ∆Hip* =c∆Hass* + d’ (28) This behavior is due to the interaction between the mixed solvent medium containing the dicarboxylate anions and water. These interactions are attributed to the essentially co‐operative nature of hydrogen bonding; together with the unfavorable steric effected of the organic group for both the tert‐butanol and the malonate buffer. Thus, it restricts the degree of order, which can be established in the liquid state, and precludes the kind of three dimensional associations which is dominant in water. Figure 6. ΔH∗ versus ∆H for Malonate buffer at different solvent compositions at 40 oC. Finally, the plots of ΔS∗ versus ΔS for different solvent compositions are also found to be linear as represented in Figure 7 according to Equation (29): ΔS∗ =e ΔS + f (29) The obtained correspondence suggests that both entropies of activation ΔS∗ and association ΔS are largely charge‐ controlled. Figure 7. ΔS∗ versus ∆S for Malonate buffer at different solvent compositions at 40 oC. 3.3. Variation of activation parameters with solvent composition The variation of the thermodynamic parameters of activation ΔG∗ , ΔH ∗ and ΔS ∗ with the mole fraction of the tert‐ butanol x2 is introduced in Figure 8 at different composition of tert‐butanol. ΔH ∗ and ΔS∗ plots versus mole fraction of the co‐ solvent x2 displayed extrema at different values of x2, indicating that the addition of tert‐butanol in small amounts plays an important role. This was also observed for trans‐[Copy4Cl2]1+ [23] and [Co (NH3)5Br]2+ [24] where the structure formation becomes greater at lower values of x2 . At the lower x2 (between 20% to 30%; v:v of tert‐butanol), the water is rich in the media where the tert‐butanol molecules progressively occupy the cavities in the water network without enhancing the water structure. However, with an increase in the tert‐butanol concentration, the water structure undergoes a gradual disruption and clusters are formed through the formation of hydrogen bonding. By adding more and more tert‐butanol, it Zeitouni et al. / European Journal of Chemistry 2 (4) (2011) 489‐494 493 has characteristics that the alkyl groups cooperative ordering of water molecules by a hydrophobic hydration effect. The changing of solvent composition has little effect on ΔG∗ as shown in Table 4 by which linear plots are observed between ΔG∗ against x2, indicating the presence of compensation effect between ΔH ∗ and ΔS∗ . Figure 8. ΔG∗ , ΔH∗ and ΔS∗ versus x2 for Malonate buffer at different solvent compositions. Figure 9 shows the plots of ΔH ∗ versus ΔS∗ for the ion‐ pair aquation reactions at different compositions of tert‐ butanol with an isokinetic temperature, β=58 oC, which lies within the temperature range studied (30‐60 oC), this emphasizes the compensation effect. The true explanation of compensation effect lies in solute‐solvent interactions, where the interaction between the solute and one of the solvent components, which is tert‐butanol, is particularly strong. This will lower the enthalpy; it will also, by restricting the freedom of vibration and of rotation of the solvent molecules, will lower the entropy.   Figure 9. H∗ versus S∗ for Malonate buffer at different solvent compositions at 60 oC. 3.4. Variation of free energy of transfer with solvent composition It has long been recognized that the solvolysis reactions of Co(III) complexes [25,26], proceed via the Id mechanism [27,28] involving the rate‐determining loss of the halide ion according to the following equations: (30) (31) The extreme extension of Co‐X bond in the transition state is revealed by a comparison of the volumes of activation with the overall volume changes for related Co(III) complexes. The situation affects the ions in the transition state to be independently solvated, which allows the application of a free energy cycle constructed by Wells [29], relating the process initial state into transition state for the aquation of the six co‐ ordinated complex [Co(NH3)5X]+2 to give five co‐ordinated [Co(NH3)5X]+3 intermediate together with the halide ion in the transition state in water (W) and in the mixed solvent (S) [29‐ 30] according to the following scheme: Initial state transition state where ΔG ∗is the activation free energy in the medium i (i = W for water and S for the mixed solvent). Equation (32) results from this cycle, where ΔG (i) is the free energy of transfer of species (i) between water and the mixed solvent. ΔG∗– ΔG∗ = ΔG [Co(NH3)5)]+3 + ΔG (X) –ΔG [Co(NH3)5X]+2 (32) Converting free energies of activation, ΔG*, into rate constant (32) can be rearranged to Equation (33): 2.303.R.T.Log ‐ΔG (X) = ΔG [Co(NH3)5)]+3–ΔG [Co(NH3)5X]+2 (33) which relates the rate constant with non‐electrostatic component of the solvent mixture. While applying the free energy cycle for the ion‐pair effect on the aquation of the bromopentaamminecobalt(III) complex ion in the dicarboxylates media follow the following equations: (34) (35) However, Equation (33) can be converted to Equation (36) 2.303.R.T.Log ‐ΔG (X) = ΔG (L‐2…[Co(NH3)5)]+3–ΔG (L‐2…[Co(NH3)5X]+2 (36) Values of k0 and kip at 25 °C were obtained from the interpolation of Arrhenius plot and values of ΔG (Br‐) were obtained from the literature [29], for tert‐butanol‐water mixtures with a range of co‐solvent. Figure 10 depicts the variation of the values of the left hand side of the Equation (36) at 25 °C as a function of mole fraction of the co‐solvent from which the differential solvating behavior of different malonate buffer for the initial and transition state is clearly evident. All the values on the left hand side of Equation (36) are negative as found for the aquation of a wide range of cobalt(III) complexes in a mixture of water with different co‐solvents [31‐33]. In all these cases, the effect of changing solvent structure on the pentacoordinatedcobalt cation in the transition state dominate the effect on the hexacoordinatedcobalt(III) cation in the initial state. Thus, these negative values in the left hand side of Equation (36) indicate that [Co(NH3)5]+3 is more stable in tert‐ butanol‐water in the presence of malonate buffer than [Co(NH3)5Br]+2. Moreover, the presence of the malonate buffer in tert‐butanol stabilizes the complex more than the free solvent. This is obvious from the more negativity of left hand side of the Equation (36) of malonate media and the free complex. 494 Zeitouni et al. / European Journal of Chemistry 2 (4) (2011) 489‐494 Figure 10. Variation of the value of L.H.S. of equation (36) versus the mole fraction x2 at 25 oC. 4. Conclusion In conclusion from all the above discussion, the aquation reaction was accelerated in the presence of malonate buffer due to the formation of ion‐pair between the complex cation and the added anion. The plot of log kip versus the dielectric constant is non linear which means that the internal structure of the medium is suffered from serious changes on the progress addition of tert‐butanol to water. As well as, the non linearity of log kip versus Log [H2O] and the Grunwald‐Winstein y‐values confirms the differential effect of solvent structures between the initial and the transition states. 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