4311
Inorg. Chem. 1991, 30,4311-4315
Contribution from the Department of Chemistry, University of California at Santa Barbara, Santa Barbara, California 93106
Electrochemical Potentials and Associated pKa Values for the Various Oxidation States of a Water-Soluble, Non p o x 0 Dimer Forming Chromium Tetraphenylporphyrin in Aqueous Solution Seungwon Jeon and Thomas C . Bruice* Received April 18, 1991
Electrochemical studies of the water soluble, non p-oxo dimer forming [5,10,15,20-tetrakis(2,6-dimethyl-3-sulfonatophenyl)porphinato]chromium(lll) hydrate {(l)Cri11(Y)2, Y = H 2 0 or HO-) have been carried out in aqueous solutions (p = 0.2 with NaC10,). With an increase in pH, the proton dissociation of the ligated water molecules of (1)Cri11(H20)2 provides (1)Cr1Ii(OH)(H,O) and then (1)Crii1(OH)2.Nernst-Clark plots of the electrode potentials (E,) vs pH for each of the stepwise leoxidations {Le.,
provide the formal potentials (EO') for the interconversion of the various oxidation states of (1)Cr(H20)2,(1)Cr(OH)(H20),and (I)Cr(OH),. Also obtained from these Nernst-Clark plots are the pK, values for the acid dissociations of (1)Cr(H,Oh
(I)Cr(OH)(H20)
e -n+
(1)Cr(OH),
at each oxidation state. The best first and second pK, values for the dissociation of protons from the ligated water molecules are pK,, 9.4 and pKn212.4;(1)CriV(H20)2,pK,, 7.6 and as follows: (l'-)Cr11(H20)2and (1)Crl1(H2O),,pK,, 10.6;(l)Cr111(H20)2, pK,, 10.2;and (I)CrV(H20),,pK,, 7.1. The electrode potentials for the le- reductions of the chromium(II1) porphyrin 7r cation radical to chromium(Il1) porphyrin and (1)Cri1(Y)2-. (l'-)Cr11(Y)2are pH independent, due to the near equality of the pK, values of water ligated to reduced and oxidized species. The near equality of the pK, values of these two pairs results from the fact that the porphyrin ligand rather than the metal moiety is undergoing oxidation and reduction. Average values of Eo' for 0.61 V; the interconversion of chromium(lI1)and chromium(1V) species are as follows: e- + (1)Cr1V(H20)2 (l)Cr111(H20)2, e- + (l)CrrV(H20)(HO) (I)Cr1*1(H20)(HO), 0.50 V; e- + (I)Cr1V(H0)2 (1)Cr111(H0)2, 0.39 V (SCE). Average values of Eo' for the interconversion of chromium(I1) and chromium(II1) species and chromium(1V) and chromium(V) species could only be determined in the neutral and acidic pH range. The values of Eo' are as follows: e- + (1)Cr111(H20)2 (1)Cr"(H20),, -1.04 V; e- + (1)Cr111(H20)(HO) (1)Cr11(H20)(HO),-1.13 V; e- + (1)CrV(H20),-. (1)Cr1V(H20)2, 1.01 V; and e- + (l)CrV(H20)(HO) (1)CrlV(H20)(HO),0.98 V.
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Introduction In a previous electrochemical investigation in aqueous solution between p H 2 and 12, we showed' that H,O and HO- serve as the axial ligands for iron(II1) and iron(1V) tetraphenylporphyrins, iron( IV) tetraphenylporphyrin *-cation radical and manganese(III), manganese(1V) and manganese(V) tetraphenylporphyrins. The tetraphenylporphyrins employed were water soluble and did not form c(-oxo dimers. Further, we established that oxo ligation (Le., M-O) can only occur in water at high pH.' The mechanisms of oxidation of chromium(II1) porphyrins2 and the use of Crv-ligated porphyrins in studies of the stoichiometric epoxidation of alkenes3*' are of continuing interest. In order to understand these processes, a knowledge of the interconversion of the various oxidation states of chromium porphyrin is crucial. Most electrochemical studies on chromium porphyrins have been carried out in aprotic media.3d*5 There is little information on the (1)
Kaaret. T. W.; Zhang, G.-H.; Bruice, T. C. J . Am. Chem. Soc. 1991,
113,4652. (2) (a) Yuan, L.-C.; Calderwood, T. S.; Bruice, T. C. J . Am. Chem. SOC. 1985, 107, 8273. (b) Murray, R. 1.; Sligar, S.0 . J . Am. Chem. SOC. 1985, 107, 2186. (c) Yuan, L.-C.; Bruice, T. C. J. Am. Chem. SOC. 1985, 107, 512. (d) Schmidt, E. S.; Bruice, T. C.; Brown, R. S.; Wilkins, C. L. Inorg. Chem. 1986,25, 4799. (e) Lee, W. A.; Yuan, L.-C.; Bruice. T. C. J . Am. Chem. Soc. 1988. 110,4277. (f) Budge, J. R.;Gatehouse, B. M.K.; Nesbit, M. C.; West, B. 0.J . Chem. Soc., Chem. Commun. 1981, 370. (g) Groves, J. T.; Haushalter, R. C. J. Chem. Soc., Chem. Commun.1981,1165. (h) Buchler, J. W.; Lay, K. L.; Castle, L.; Ullrich, V. Inorg. Chem. 1982, 21, 842. (i) Liston, D. J.; West, B. 0.Inorg. Chem. 1985.21,1568. (j)Groves,J. T.; Kruper, W. J.; Haushalter, R.C.; Butler, W. M. Inorg. Chem. 1982,21, 1363. (3) (a) Garrison, J. M.; Bruice, T. C. J . Am. Chem. Soc. 1989, 1 1 1 , 191. (b) Garrison, J. M.; Ostovic, D.; Bruice, T. C. J . Am. Chem. Soc. 1989, 111,4960.(c) Groves, J. T.; Kruper, W. J., Jr. J. Am. Chem. Soc. 1979, 101,7613. (d) Creager, S.E.; Murray, R. W. Inorg. Chem. 1985. 24, 3824. (4) Garrison, J. M.; Lee. R.W.; Bruice, T. C. Inorg. Chem. 1990,29,2019.
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electrochemistry of water-soluble chromium porphyrins in aqueous solution. Electrode reactions that involve reactants or intermediates with ionizable protons and/or processes that involve proton transfer are best studied in aqueous solutions where pH and ionic strength are easily controlled. In aqueous media, and in the absence of strongly ligating external bases, the axial ligands are represented by either H 2 0 or HO-, depending on the pH of the solution. From the pH dependence of the electrode potential (E,) (i) acid dissociation constants of reactants and products may be determined, (ii) the direction and course of redox reactions may be predicted, (iii) the magnitude of the rate constants (k) for redox reactions occurring in different pH regions may be explained, and (iv) choices may be made between kinetically equivalent mechanisms. From log k vs pH profiles for the oxidation of metal(II1) porphyrins with oxidants and a knowledge of the ionization constants of reactants, one can identify the reacting species at various acidities. We now report the results of an investigation of the pH dependence of the various redox potentials of chromium ligated by 5,10,15,20-tetrakis( 2,6-dimethyl-3-sulfonatophenyl)porphyrin (1H2) and the pK, values for the dissociation of (1)CI-''(Y)~(n = oxidation state and Y = H 2 0or HO-) in aqueous solutions. Experimental Section Materials. 5, IO,15,20-Tetrakis(2,6-dimethylphenyl)porphyrin was synthesized by the method of Lindsey et a1.6 and converted to ( 5 ) (a) Fuhrhop, J.-H.; Kadish, K. M.;Davis, D. G. J. Am. Chem. Soc. 1973,95,5140. (b) Kadish, K. M.; Davis, D. G.; Fuhrhop, J.-H. Angew. Chem., Inr. Ed. Engl. 1972, 1 1 , 1014. (c) Cheung, S.K.; Grimes, C. J.; Wong, J.; Reed, C. A. J. Am. Chem. Soc. 1976, 98, 5028. (d) Murakami, Y.; Matsuda, Y.; Yamada, S.J. Chem. Soc., Dalton Trans. 1981,855. ( e ) Bottomley, L. A.; Kadish, K. M. Inorg. Chem. 1983, 22, 342. (0 Bottomley, L.A.; Kadish, K. M. J. Chem. Soc., Chem. Commun. 1981, 1212. (g) Kelly, S. L.;Kadish, K. M. Inorg. Chem. 1984, 23, 679.
0020-1669/91/1330-4311$02.50/00 1991 American Chemical Society
4312 Inorganic Chemistry, Vol. 30, No. 23, 1991
Jeon and Bruice
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Potential (V) vs. SCE Figure 1. Cyclic voltammograms for the le- oxidation of 4.12 mM (l)Cr"'(Y)2 at scan rates of 0.025,0.05,0.1, and 0.2 V s-l in a deaerated
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aqueous solution (pH 5,0.2 N NaCI04). The inset shows a plot of the anodic peak current as a function of the square root of the scan rate. (1H20)by 5,10,1 5.2O-tetrakis(2,6-dimethyl-3-sulfonatophenyl)porphyrin the procedure of Zipplies and Bruice.' [5,10,15,20-Tetrakis(2,6-dimethyl-3-sulfonatophenyl)porphinato]chromium(llI) hydrate ((1)CTIII(H~O)~) was prepared by refluxing the free base with a 40-fold excess of chromium(l1) chloride in DMF for 2 h.4 The water-soluble ( 1)Cr11'(H20)2was purified by ion-exchange chromatography, ultrafiltration, and size-exclusion chromatography. Electrochemical Measurements. All aqueous solutions used for electrochemistry were prepared from distilled-deionized water which was boiled for at least h and stored under argon scrubbed free of 02,CO, and C02. Preparations of all solutions were carried out under an argon atmospherc scrubbed free of 02,CO, and C02. Metalloporphyrin concentrations ranged from 3.0 to 10.0 mM and contained 0.2 N NaC104 as electrolyte. Dilute solutions of carbonate-free HN03and NaOH were used for the adjustment of pH. The pH values of solutions were measured before and after electrochemical experiments. The measured pH values were within 10.05 pH units. All reported potentials are with respect to saturated calomel electrode (SCE). All experiments were performed at room temperature. The following buffers were employed in spectroelectrochemical experiments: (pH 3. I ) CICH2C00-/C1CH2COOH, (pH 5.2) CH,COO-/CH,COOH, (PH 6.7) H2P03-/HP0J2-, (PH 9.3) HCO
