Electron transfer between benzenediols and aquapentachloro-, diaquatetrachloro-, and hexabromo-iridate(IV)

摘要

J.C.S. Perkin I1 Electron Transfer between Benzenediols and Aquapentachloro-, Diaqua- tetrachloro-, and Hexabromo-iridate( iv) By Ezio Pelizzetti," Edoardo Mentasti, and Edmondo Pramauro, lstituto di Chimica Analitica, Via P.Giuria 5, 10125 Torino, Italy The agreement of the kinetic data concerning the oxidation of benzenediols by aquapentachloro-, diaquatetra- chloro-, and hexabromo-iridate(iv) with a model proposed in the light of the Marcus theory, suggests that an outer- sphere electron transfer is the operative mechanism. The same reorganizational parameter h (26 kcal mol-l) satisfies the reactions involving the chloro-complexes, whereas a lower value is valid for hexabromoiridate(iv) (21kcal mol-l). An estimation of the intrinsic parameters for the self-exchange reactions involving these lrrv-klll pairs has been attempted. HEXACHLOROIRIDATE(IV) has been extensively investi- gated as oxidizing agent both with inorganic ions as well as with organic and organometallic compound^.^ However, little attention has been devoted to the kine- tics and mechanism of reaction involving related species, like aquapentachloro-,2 diaquatetrachloro-,2 and hexa- bromo-iridate(1v).294 In previous papers the kinetics and mechanisms of oxidation of quinols and catechols by means of hexachloroiridate(1v) were investigated and an electron transfer mechanism was suggested.Also a phenomenological model, developed in the light of the Marcus theory,' was proposed in order to account for the dependence of the observed reaction rates on the free energies of reaction.We now examine the reaction mechanism of related 11-1" species and evaluate the characteristic intrinsic parameters for these compounds. EXPERIMENTAL Reagents.-Sodium hexachloroiridate(xv) was supplied by Merck and the spectrum of fresh solution agreed with literature data. Sodium hexachloroiridate(II1) solutions were prepared by cathodic reduction of IrClG2- at a platinum electrode or by dissolution of K31rC16 (hlfa). Sodium aquapentachloroiridate(111) solutions were prepared by aquating a sample of IrCle3- in acid solution at 40-50" for ca. 2 h,8 and these solutions were anodically oxidized in order to obtain the aquapentachloroiridate(1v). Sodium diaquatetrachloroiridate( 111) was prepared by operating the aquation of IrCl,3- solutions for 2 min; 9 the corresponding Ir'T compound was obtained by anodic oxidation.The spectra of IF species agreed with literature data.*g Perchloric acid (Merck) was used to bring the solution to the desired acidity. The following benzenediols (Merck, Aldrich, K amp; I) were investigated : Z-methylbenzene- 1,4- diol (I), benzene- 1,4-diol (11), 2,5-dihydroxybenzoic acid (111), 2,5-dihydroxybenzenesulphonic acid (IV), 2,3-di- cyanobenzene- 1,4-diol (V), 4-methylbenzene- 1,2-diol (VI) , benzene-1,Z-diol (VIl), (-)-4-l-hydroxy-2-(methylamino)-ethyllbenzene- 1,2-diol (adrenalin) (VIII), 3,4-dihydroxy- A. G. Sykes and R. N. F. Thorneley, J. Chem. Soc. (A),1970, 232, 1036; J.P. Birkand J. W. Gasiewski, Inorg. Chem., 1971,10, 1586; I3. Grossman and A. Haim, J. Amev. Chem. SOC.,1970, 92, 4836. R. Cecil and J. S. Littler, J. Chem. Soc. (B),1968, 1420; 1970, 626, 632. 3 H. C. Gardner and 3. K. Kochi, J. Amer. Chem. SOC.,1975,97, 1855; J. Y. Chen, H. C. Gardner, and J. I. Kochi, ibid.,1976, 98, 6150. benzoic acid (IX), 4- l-oxo-2-(methylamino)ethylbenzene-1,2-diol (adrenalone) (X),4-cyanobenzene- 1,2-diol (XI), and 4,5-dihydroxybenzene- 1,3-disulphonic acid (XII) . Procedure.-The reactions were followed with a Durrum- Gibson stopped-flow spectrophotometer at hmxs for IrIV species, i.e. 445 (E 2 920 1 mol-l cm-l) for Ir(H,O),Cl,,* 450 (3 320) for Ir(H20)C15-,7 and 585 nm (3 800) for IrBr,2-.4 Kinetic runs were performed with IrIv 1-2 x 10-5~ and an excess of organic substrate (up to 2 x 10-3~for less reactive compounds).Measurements were carried out at HClO, 1.00~and p 1.0~.Kinetic runs were performed at two different temperatures in order to estimate the activ- ation parameters. A series of runs was carried out in the presence of the corresponding IrIII species: no effect was observed. The rate constants were evaluated with a weighted least squares method (based on the deviation of the single points of each run) and the other kinetic para- meters were derived by assigning the weights on the basis of standard deviations. RESULTS Stoicheiometry.-By means of spectrophotometric measurements with IrTV species in excess, the overall equation (1)was derived where HBQrepresents the benzene- 2 IrIv + H2Q+2 IrIII + Q + 2 H+ (1) diols and Q the corresponding quinones.The values of the potentials for the pairs IrIv-IrIII and Q-H2Q show that all the investigated reactions go to completion. Experiments with parent quinol as reductant were carried out in order to detect the inorganic products. When Ir(H20)C15- was the oxidant, Ir(H20)C1,2- was formed quantitatively,8 and the same behaviour was observed for 1r(H2O)amp;l4 and for IrBre3- which form Ir(H20) amp;la- and 1rB1-,~- respectively .@ Kinetics.-When operating with a large excess of organic substrates, plots of ln(A, -.Am), where At and A, are the absorbance at time t and at equilibrium respectively, against time, were linear for at least two half-lives; the observed rate constants were also linearly dependent on the concentration of the organic substrates.Hence equation (2) P. Hurwitz and K. Kustin, Inow. Chem., 1964.8.823. E. Pelizzetti, E. Mentasti, and c.Baiocchi, j.Phys. Chem., 1976, 80, 2979. E. Mentasti, E. Pelizzetti, and C. Baiocchi, J.C.S. Dalton, 1977, 132. R. A. Marcus, J. Phys. Chem., 1968, 72, 891 and references therein. J. C. Chang and C. S. Garner, Inorg. Chem., 1965,4,209. A. A. El-Awady, E. J. Bounsall, and C. S. Garner, Inorg.Chem., 1967, 6, 79. 1978 621 obtains. For the more reactive compounds, second-order k, at different temperatures were collected in Tables conditions were adopted and the corresponding second-1-3.order plots were also linear to at least 75 completion. The comparison of the activation parameters of the reactions investigated with those for displacement of -dIrIv/dt = hoIrIVH2Q (2) ligands from the co-ordination sphere of IrIV complexes, DISCUSSION and the detection of the IrIII end species, suggest that The reaction scheme can be represented by equations an electron transfer mechanism is operative, as pre-(3) and (4) where SQ represents the semiquinone radical. viously established for oxidation of benzenediol with TABLE1 Kinetic and thermodynamic parameters for the oxidation of benzenediols by IrCl,(H,O) -Substrate k (7.0 "C) k (25.0 "C) AH: c~d AS AG*,,, c*f E" g E"' ' AGO '1' AG*oalo.'9' (11) 5.0 x 105 8.8 x lo5 4.6 -16 6.9 0.699 1.14 1.2 7.1 (111) 2.9 x 104 5.4 x 104 5.2 8.5, 0.769 1.23 3.3 8.2, (IV) 2.6 x 104 4.6 x 104 4.7 -"-22 8.6, 0.787 1.25 3.7 8.5 (V) 4.3 x 102 7.0 x lo2 3.9 -32 11.1 0.910 1.41 7.4 10.7 3.8 x 104 6.2 x 104 3.9 -23 8.4, 0.792 1.25 3.7 8.5 ('I1)(VIII) 6.8 x lo4 8.9 x 104 1.9 -30 8.2, 0.812 1.28 4.4 8.9(1x1 1.25 x 103 2.7 x 103 6.5 -21 10.3 0.885 1.38 6.7 10.3 (X) 1.15 x lo3 2.3 x 103 5.8 -24 10.4 0.910 1.41 7.4 10.7 (XI) 3.2 x lo2 7.2 x lo2 6.9 -22 11.1 0.924 1.43 7.9 11.0, (XI:) 6.2 x 10' 1.4 x lo2 7.0 -25 12.1 0.955 1.47 8.8 11.7 HClO, = 1.00~,p.= 1.0~. 1 mol-l s-l (error is amp;3-5). kcal mol-l. Error -amp;0.7-1.2 kcal mol-l. cal mol-l K-l (error f2.4-4.0 cal mol-l K-l). f At 25.0 "C, calculated from K = 2 exp (-AG*/RT).Reduction potential (V) for Q + 2H+ + 2e +H2Qcouples. Reduction potential (V) for H,Q.-+-+ e HaQpairs calculated as reported in refs. 5 and 6, iCalculated by assuming E" 1.088 V for the IrCl, (HZ0)-+ pair (ref. 2). I Calculated from equation (6) with h 26 kcal mol-1. TABLE2a Kinetic and thermodynamic parameters for the oxidation of benzenediols by IrCI,(H,O) k (8.0 "C) k (25.0 "C) AH$ AS AG*exp AG AG*CtblC 3.1 x 105 6.6 x lo5 6.8 -9 7.0, 0.6 6.8 2.7 x 105 5.4 x 105 6.2 -11 7.6 1.1 7.0, 5.2 x 103 9.0 x 103 4.8 -24 9.6 4.8 9.1 3.6 x 105 6.4 x 105 5.1 -15 7.1 1.1 7.0, 5.5 x 105 7.8 x 105 2.8 -22 6.9, 1.8 7.4 1.6 x 104 3.2 x 104 6.2 -17 8.85 4.1 8.7 1.15 x 104 2.2 x 104 5.8 -. 19 9.1 4.8 9.1 3.8 x 103 8.6 x 103 7.0 -17 9.6, 5.2 9.4 1.85 x lo3 3.6 x 103 6.0 -22 10.1, 6.2 9.9, Footnotes as for Table 1; E" for IrC14(H20)2-IrC14(H,0)2-is taken as 1.023 V (ref.2). TABLE3a Kinetic and thermodynamic parameters for the oxidant of benzenediols by IrBr,2-Substrate k (7.0 "C) K (25.0 "C) AHt AS AG*,X, AGO AG*ca~c (1) 2.5 x 105 4.1 x 105 4.0 -20 7.3, 4.2 7.5, (11) 4.2 x 104 7.4 x 104 4.7 -21 8.3, 5.9, 8.65 (111) 1.0 x 103 1.75 x 103 4.6 -28 10.6 8.0 10.0 (IV) 8.0 x lo2 1.45 x 103 4.9 -28 10.7 8.5 10.4 (VIt 2.0 x 104 3.8 x 104 5.4 -20 8.7, 7.1 9.4 (VII) d 1.25 x 103 2.7 x 103 6.5 -21 10.3 8.5 10.4 (VIII) 1.8 x 103 3.6 x 103 5.8 -22 10.1, 9.2 10.8 0 Footnotes as for Tables 1 and 2; E" for the IrBr,2-'3-pair is taken as 0.882 V (ref.2). Calculated from equation (6) with h 21 kcal mol-l. E" 0.644, E"' 1.065 V. E" 0.739, E"' 1.19 V. The application of the steady-state treatment to SQ 1rCle2- (with H2Q*+radical species formation) .596 The leads to the rate equation (5). The observed rate law Marcus theory predicts, for this class of reaction, a relationship between the free energy of activation and IrIV + H2Q k IrIII + SQ (3) the free energy change of the form (6) where k = 2 k-3 kIrIV + SQ "c IrIII + Q (4) AG*12 = w12 + A(l + AG012'(A)2/4 (6) -d IrIV /dt = exp(-AG*12/RT), 2 being the collision frequency in 2k,k, IrIV H2Q/ (k-, Irl**-1h, Ir IV ) (5) solution (loll 1 mot1 s-l),is defined as 2(AG*ll -wll + AG*22 -w,~),where AG*ll and AG*22 refer to the self- and the absence of IrrT1 species effect suggest that k,- exchange reactions of the reagents and wll and 'wZ2 IrIV k-3Ir111, hence k, = 2k3.The values of represent the work terms involved in the self-exchange reactions; AGO,,' = AGOl2 + w21 -w,,, where w2,and w12are the work terms required to bring the products or reactants together at the separation distance in the activated complex, and AGO,, is the free energy of reaction for the prevailing medium and temperature. At our high ionic strength and with one of the reactants uncharged, the work terms can be neglected. For the reaction of benzenediols with hexachloro- iridate(Iv), equation (6) was found to be satisfied by a.dopting A 26 kcal mol-l and by calculating AGO,, on the assumption of the dependence of deprotonation con-stants of H2Q' species on the redox potentials of Q-H2Q + pairs5y6 For the IrIV species investigated, the experi- mental points concerning aquapentachloro-and di-aquatetrachloro-iridate(1v) lie on the same curve for AG:2 I kcal mol-' Plot of AG*,, as a function of AGO,, for the oxidation of benzene-diols by IrIv species: A, 1r(Hamp;))Cl5-; .,1r(H2O).amp;l4; 4,IrC1,Z-(taken from refs. 5 and 6);0,1rBra2-; upper curve calculated from equation (6) with A 26 and lower curve with A 21 kcal mol-l hexachloroiridate(Iv), drawn with A 26 kcal molkl (see Figure); the values of were calculated as for the AGOl2 IrClG2- reaction, that is assuming H,Q'+ as the product of the rate-determining ~tep.~?~ The observed agreement implies that the substitution of chloride with water into the co-ordination sphere of IrIV does not change the reaction mechanism.Also the oxidation of cyclo-hexanone by the same oxidizing species suggested that the same intrinsic parameter AG** holds for this family of chloro-complexes of However, the oxidation rates of benzenediols by IrBrG2- were higher than those for chloro-derivatives, when the same free energy of reaction is involved (similarly, the cyclohexanone oxidation rate was higher for IrBr,,- than for IrC162f, although the free energy of reaction is more favourable for this last complex).2 Satisfactory agreement with the experimental data is obtained for 1, 21 kcal mol-l (see Figure).These two A values imply that AG**lrCp-p--lo P. Hurwitz and K. Kustin, Trans. Furaday SOC.,1966, 62, 247. l1 A. Haim and N. Sutin, Inorg. Chem., 1976, 15, 476. l2 D. Meisel, Chem. Phys. Letters, 1975, 54, 263; B. A. Kowert, L. Marcoux, and A. J. Bard, J. Amer. Chem. SOC.,1972, 94, 5538; D. Meisel and R. W. Fessenden, ibid., 1976, 98, 7505. J.C.S. Perkin I1 AG+*IrBr,a-la-= 2.5 kcal mol-1 (where AG** = AG* -w). Consequently, taking into account, by means of equation (6),the contribution of AGOl2 (due to the differ- ent reduction potentials of IrC162-/3- and IrBr2-P-pairs), it can be expected that the reaction rates of IrCl2-and IrBr,2- toward the same reducing agent should be of the same order of magnitude {namely the reaction rate of IrBr,,- toward a substrate should be twice that of the same substrate with IrC162-).It is worth mentioning that the the reaction rates for cyclo- hexanone (2.57 x lo-, and 9.4 x lo-, 1 mol-l s-l at 25.0' for IrC1,2-and IrBrz+, respectively) are in moderate agreement with these estimates and there are siniilar results for the reactions with tris(5,g-dimethyl- 1,lO-phenanthroline)iron(rI) (2.2 x lo8 and 1.8 x lo8 1 mol-l s-l) and with tris(4,4'-dimethyl-2,2'-bipyridine)-iron(I1) (9.6 x lo8 and 6.8 x lo8 1 mol-l s-~).~ In order to estimate the values of AG** for self- exchange reactions of IrIV-IrlII systems, the data reported by Hurwitz and Kustin can be taken into account.1deg; The main problem arises from the estim- ation of the work terms involved in reactions between charged ions.Formula (7), where z1 and x2 are the charge on the reactants, e the electron charge, D the static dielectric constant, Y the radius of the activated complex, and the exponential coefficient is the Debye- Hiickel term, where x is the reciprocal Debye radius, can be adopted for estimating the work terms. The import- ance of the correct evaluation of the work terms was recently pointed out by Haim and Sutin,ll and without any doubt our AG** values are affected by some un- certain ties. If the value of 2.3 x lo5 1 mol-1 s-1 is assumed for the rate of exchange for IrCl,2-/3-,10 a value of wll 1.20 (the radius is assumed to be 4.3A) leads to AG** 6.5 kcal mol-l. Consequently a value of AG** of 4.0 kcal mol-l should pertain to the IrBr,2-/3- self-exchange reaction.It is noteworthy that these data assign a value of ca. 6.5 kcal mol-l to AG** for the H,Q'+-H,Q exchange reaction (corresponding to a rate of ca. 2 x lo, 1 mol-l s-l, a value which seems slightly low in respect to other radical- parent molecule exchange rates).,, Anyway, it must be also taken into account that the A values estimated for the benzenediol oxidations are the upper values since the deprotonation constant of H2Q*+ (for pa.rent quinol) was chosen as the lower probable value O OM).^ It follows that AG** (H,Q*+-H,Q) 6.5 kcal mol-l (that is 22 x 106 1 mol-l s-l for the self-exchange rate). Analogous comparison with benzenediol oxidations by -AG**(FeL33+ I,+17eI*IL3,13leads to AG** (IrC162+/3+) = 5 kcal mol-l, hence AG**(FeLQ3+I2+ = 1.5 kcal mol-l).A value of 3 x 108 1 moi-1 s-i was reported for self- l3 E. Mentasti and E. Pelizzetti, Internat. J. Chem. Kinetics, 1977, 9, 215; E. Pelizzetti and E. Mentasti, 2. Phys. Chem. (Frankfurt),1977, 105, 21. exchange reaction rate for FeL33+/2+ in 1.84~-sodium sulphate; l4 this means AG* = 3.4 kcal mol-l, but the evaluation of the work terms is rather uncertain at this ionic strength. A test for these values of AG** can be performed by calculating the cross-reaction rates with the aid of equation (6) and by comparing the results with the experimental data. For IrCb2- -IrBre3-a value of 6.4 x lo6 1 mol-1 s-l is obtained from equations (6) and (7) (AErdquo; 0.075 V)z to be compared with 1.2 x lo7 1 mol-1 s-l, experimentally determined; lo for IrCl,ldquo;+ -tris(4,4rsquo;-dimethyl-2 ,Zrsquo;-bipyridineiron)2+, calculated 6.5 x lo8, experimental 9.6 x loa; l5 for IrBr62--tris(4,4rsquo;-dimethyl-2,2rsquo;-bipyridine)iron 2+, calculated 1.6 x lo9,experimental 6.8 x 108.15 Unfortunately, no experimental data on electron-transfer reactions are available on mixed aquachloro-complexes of IrIY Tak-ing into account the difficulties in the estimation of the parameters involved, the agreement can be considered satisfactory. It can be concluded that the proposed phenomenolo- gical model describes the presently investigated systems and that an outer-sphere mechanism operates in the oxidation of benzenediols by these related IrIV species; moreover the same intrinsic parameter can be adopted for the IrC162-/3-, Ir(H20)C15-/2-, Ir(H,0)2C1,0/- pairs (AG** 6.5 kcal mol-l), while a lower value pertains to the IrBr62-/3-pair (AG** 4 kcal mol-l 7/1005 Received, 131h June, 19771 l4 I.Ruff and M. Zimonyi, Electrochim. Acta, 1973, 18, 515. P. Hurwitz and K. Kustin, Inorg. Chem., 1964,8, 823.