in aqueous solution - ACS Publications - American Chemical Society

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Reduction of Co(bpy);+

and Co(phen);+

The Journal of Physical Chemistry, Vol. 83, No. 4, 7979 439

in Solution

(16) P. D. Burrow, private communication. (17) J. R. Frazier, L. G. Ghristophorou, J. G. Carter, and H. C. Schweinler, J . Chem. Pbys., 69, 3807 (1978). (18) S.Lin, Ph.D. Thesis, IJniversity of Houston, 1973. University Microfilms, Internatioiial, Ann Arbor, Mich.

(19) P. D. Burrow, J. A. Michejda, and K. D. Jordan, J. Am. Cbem. Soc., 98, 6392 (1976). (20) J. M. Warman M. P. de Haas, and A. Hummel in "Conduction and Breakdown in Dielectric Liquids", J. M. Goldschwartz, Ed., Delft University Press, 1975, p 70.

One-Electron Reduction of Tris(2,2'-bipyridine) and Tris( 1;IO-phenanthroline) Complexes of Cobalt(II1) iin aqueous Solution' M. G. Simic," M. 2. Hoffman,*2b R. P. Cheney,2band Q. G. Mulazzani*2c3d Food Engineering Laboratory, US. Army Natick Research and Development Command, Natick, Massachusetts 0 1760, Department of Chemistry, Boston University, Boston, Massachusetts 022 15, and Laboratorio di Fotochirnica e Radiazioni d' Alia Energia, Cons@/ioNazionale Delle Ricerche, 40 726 Bologna, Italy (Received August 16, 19;'8) Publication costs assisted by the National Science Foundation

The reaction of Co(bpy)QB+with radiation-generated reducing radicals (ea;, .C02-, (CH3)&0H,and CH,OH) in aqueous solution quantitatively and rapidly ( h = 108-1010M-' s-') yields high-spin (ti, e:) C ~ ( b p y ) (A,~, ~ + 300 nm, e,,, 4.2 X IO4 M-' cm-l) which slowly ( k = 3.4 s-' at pH 0.5-10.5; 8.0 s-l at pH 0.3) equilibrates with the loss of bpy; C ~ ( b p y ) ~does ' + not transfer an electron to O2 or p-benzoquinone. Equilibrium mixturles of Co(bpy):+ react with ea; (h > 1O1O M-l s- ) to produce Co(bpy),+ (Amc 620 nm, emax 5.1 X IO3M-' cm-'). Reduction of Co(phen)QB+yields C ~ ( p h e n ) (A~, ~ + 270 nm, e,, 5.6 X lo4 M-l cm-I); conversion to the equilibrium mixture (Co(phen)2+)occurs in a time frame (>I s) too long to be detected by pulse radiolysis. The relationship of these results to those of Waltz and Pearson and Baxendale and Fiti is discussed.

Introductioin In a paper that has become a classic in the literature of the radiation chemistry of transition metal coordination complexes, Waltz and Pearson3 reported the result of the pulse radiolyrsis of methanolic aqueous solutions containing C ~ ( b p . y ) (bpy ~ ~ + = :!,2'-bipyridine). The reaction of ea; produced a weak tramient absorption a t h >400 nm that was attributed to law-spin (t,",eg) Co(bpy):+; high-spin (ti, e:) C ~ ( b p y )was ~ ~ +the ultimate product. The rate of spin relaxation was estimated as 1 5 x lo3 s?. The transient intermediate reacted with C ~ ( b p y ) , ~(k+ = 8 X lo8 M-l s-l), with Fe(CN):-, Co(CNP:-, Cr(CN)63-,and O2 (k > lo9M-l s-l), and with Zn2+,Mn2+,Mg2+,and Ba2+(k = 106-107 ~ - s-1). 1 Baxendale and Fiti4 reexamined the system and also observed a broad, weak transient absorption in the 4001100-nm region with a pronounced maximum at -620 nm. They confirmed the rate constants of the reaction of that species with Co(bpy)l+,Oz, and Cr(CN),3- but were unable to observe any reaction with Zn2+. The effect of Mg2+and Ba2+was attributed to kinetic salt effects or to the presence of reducing impurities in the solutes. They rejected the low-spin Co(I1) assignment and suggested that the transient precurslor to the Co(I1) product in the reaction of ea; with Co(bpy)ipbe dlescribed as an electron adduct to the ligand (Co(~,py)z(bl~y-.)2+) Coordinated to the Co(1II) center. They admitted that the mechanism whereby this coordinated radical species is converted to the Co(I1) product is not clear, requiring reaction with the substrate and not ligand-to-metal intramolecular electron transfer. In a preliminary communication5 we reported that the reaction of ea; with C ~ ( b p y ) , ~produces + an intense long-lived transient absorption a t 300 nrn which was attributed to ai reduced ligand radical coordinated to the Co(1IIP center and assumed to decay via intrarnolecular electron transfer. A:, part of our continuing investigation of the interaction of radiation-generated free radicals with 0022-3654/79/2083-0439$0 1.OO/O

polypyridines and their coordination we have reexamined the reaction of Co(bpy)gB+and its analogue, Co(phen)?+ (phen = 1,lO-phenanthroline),with reducing radicals and eaq- using pulse and continuous radiolysis techniques. Experimental Section Materials. Co(bpy),(C104),.2H20 was prepared and purified according to described procedures;1° Co(phen)3(C104)3.1/2H20was provided by Professor L. Moggi from his photochemical study.ll Aqueous solutions of these complexes are stable in acidic, neutral, and alkaline media; no changes in their absorption spectra are detectable as a function of pH. Methanol and 2-propanol were Baker Analyzed reagents and were used witlhout further purification or were further purified by the method of Baxendale and Wardman." tert-Butyl alcohol (Mallinckrodt) was used as received. Sodium formate was recrystallized tvvice from water. NzO was either used directly or was passed through a column of NaOH pellets. Free bpy (Aldrich) was recrystallized three times from methanol. All other chemicals were of the highest grade available and were used without further purification. Solutions werie made up in high purity triply distilled water with the plH adjusted with NaOH, phosphate buffer, HzS04, or HC1014. Radiation Techniques. The pulse radiolysis equiprnent a t the U.S. Army Natick Research and Development Command and the C.N.R. Laboratory (Bologna) have been described in detai1.13J4 Transient absorption spectra were obtained by optical spectrophotometry with a time resolution of -0.5 ys. The radiation dose per pulse was established by the use of SCN- d 0 ~ i m e t r y . lIn ~ order to avoid or minimize photodecomposition induced by the analyzing light, solutions were exposed to as little UV Light as possible. Continuous radiolyses were conducted in @Co y sources with dose rates of 4-18 X 10l6 eV mL-l min-l 0 1979 American Chemical Society

440

The Journal of Physical Chemistry, Vol. 83, No. 4, 1979

Simic et al.

I

L 250

0 I

300

250

300

A , nm

5:

(0.64-2.9 krd m i d ) ; the exact dose received by the solutions was determined by use of the Fricke chemical dosimeter.16 The solutions were prepared and deaerated with Ar or N2, or saturated (2.5 X M) with N 2 0 . Analyses for Co(I1) were performed using the SCN'methyl isobutyl ketone extraction method., Analyses for free bpy or phen were made spectrophotometrically on n-heptane extracts of the irradiated solutions. Evaluation of the yields of products were also made directly on the irradiated solutions by spectrophotometric analysis. From a knowledge of the radiation dose delivered to the solutions and the rate of formation of product and/or consumption of substrate, the G values (number of molecules formed or destroyed per 100 ev of energy absorbed by the solution) of the reaction were calculated. Generation of Reducing Radicals. The radiolysis of aqueous solutions generates e,; OH radicals, and H atoms e,; (2.8),OH according to the overall reaction H 2 0 (2.8),H (0.55) where the numbers in parentheses represent the G values of the species. By the use of selected scavengers, a particular reductive radical can be chosen to be the principal reactant in the solution. Thus, .CO2-, (CH,),COH, and .CH20H can be generated conveniently from solutions containing HC02-, (CH,),CHOH, or CH30H by action of OH and H.17918 In N20-saturated solution, e, - is converted to OH;19in acidic solution, e, is convertej to H.19 In the presence of tert-butyl alcohol, OH radicals are effectively ~cavenged;'~ the resultant .CH2C(CH3)20Hradical has a weak optical absorption a t X C280 nm13 which can be easily taken into account when transient absorption spectra are determined.

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Results Co (bpy)?+. The pulse radiolysis of aqueous solutions of Co(bpy),,+ under a wide range of conditions yields the transient absorption spectrum shown in Figure 1with ,A, 300 nm and emax 4.2 x lo4 M-l cm-l; in comparison, absorption a t X >400 nm was of negligible intensity. The transient absorption spectrum is independent of pH (0.3-10.5) and the nature of the reducing radical. The

I

I

0.2

0.3

1

Time, sec

350

Figure 1. Absorption spectra of the transient intermediate and the final product roduced in the one-electron reduction of 4.0 X M C o ( b p ~ ) ~in aqueous solution by eaq- (0)and C0,- (A) at pH 6.9; (CHJ,COH at pH 7.0 (0)and pH 0.5 (B). The conditions for these experiments were 0.1 M ten'-butyl alcohol and Ar-purged solution, 0.1 M HC0,- and N,O-saturated solution, and 0.1 M (CH3),CHOH and N,O-saturated solution, respectively. The absorbance of the transient was read after complete formation had taken place (120 ps). The 6 values were obtained after correction for depletion of the substrate (dose/pulse = 1.6 krd). The solid line in the product section is the spectrum of a 3 1 mixture of bpy and Coaq2+.

0.I

Figure 2. Kinetics of the decay of the transient produced in the pulse radioiysis of 4.0 X M Co(bpy):+ in N,O-saturated solution containing 0.1 M HC0,- at pH 7.0; dose/pulse = 1.3 krd; monitoring wavelength = 310 nm.

TABLE I: Rate Constants for the Reactions of C ~ ( b p y ) ,and ~ + C ~ ( p h e n ) ,with ~ + Reducing Radicals

k. M-I radical eaq-

c0,(cH,),COH .CH,OH

Cofbp~),~' 8.3 i 0.7 x 7.8 i 0.5 X logb 2.5 i 0.3 x 109c 2 x load

s-'

C~(phen),~' 7.5 x 4.6 x 1 0 9 f

a 4.0 X M C ~ ( b p y ) , ~0.1 + , M tert-butyl alcohol, Arpurged, pH 6.9, monitored at 700 nm. b 4.0 x lo" M Co( b p ~ ) , ~0.1 + , M HCO,-, N,O-saturated, pH 6.9, monitored at 330 nm. 4.0 x M C ~ ( b p y ) , ~0.1 +, M (CH,),CHOH, N,O-saturated, pH 0.5, 7.8, monitored at 330 nm. 5 x lo5M C ~ ( b p y ) , ~0.25 + , M CH,OH, N,Osaturated, pH 1, 7, monitored at 326 nm. e Reference 3. 3.8 X M C ~ ( p h e n ) , ~0.5 ' , M (CH,),CHOH, N,Osaturated, pH 7.0, monitored at 360 nm.

transient decays slowly into a permanent product which has the identical absorption spectrum of a solution consisting of a mixture of 3.0 X M M bpy and 1.0 X Co.:,' In the pH range 0.5-10.5, the decay follows first-order kinetics (Figure 2) with k = 3.4 f 0.4 (std dev) s-l independent of [Co(bpy),3+](1-5 X 10" M), dose/pulse (0.5-2.9 krd), monitoring wavelength (270-340 nm), and the conditions used to generate the transient. At pH 0.3, h = 8.0 s-l. From the pseudo-first-order kinetics of formation of the 300-nm transient absorption, the second-order rate constants for the reactions of the various reducing radicals with C ~ ( b p y ) , ~can + be evaluated and are given in Table I. The intermediate from the reduction of Co(bpy)?+ shows no reaction with p-benzoquinone (BQ) and does not yield the characteristic intense absorption spectrum of the reduced semiquinone (BQ-.) (A, 430 nm; emax 7.3 X lo3 M-l cm-1)20under the following conditions: 2.0 X M C ~ ( b p y ) ~2.0 ~ +x, M BQ, 0.5 M tert-butyl alcohol, N2-purged solution, pH 7.0, dose/pulse = 0.5 krd. Similarly, no electron transfer was observed from reduced C ~ ( b p y ) , ~to+O2 as deduced indirectly from the absence M Co(bpy)2+, of 02-by use of the BQ method:20 2.0 X 2.0 X M BQ, 1 M tert-butyl alcohol, 0,-saturated solution (1.3 x lo-, M), pH 7.0, dose/pulse = 0.5 krd. The G values for the reduction of C ~ ( b p y ) ~ 'upon + continuous radiolysis were determined from the direct analysis of Co(I1) or from the spectral changes that result upon irradiation. Free bpy, albeit in less than stoichiometric amounts, was extracted from the solutions. In all

The Journal oii Physlccll Cheml6try, Vol. 85, No. 4, 1870 441

Reductlon of C#a(bpy),3' and Co(phen)33t In Solutlon

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300

SO0

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600

700

800

A , nm Flgure 3, Absorptlon spectrum from the pulse radlotysls of an Ar-purged M C o a t f , 3.0 X lombM bpy, and 0.3 solutlon contalrilng 1.0 X M tert-hutyl alcnhol at pH 7.0; dose/pulse = 1.6 krd.

cases the loss of the substrate or the formation of Co(I1) was a linear function of absorbed radiation dose (time of irradiation), The G values were 6.6 f 0.3 for the radiolyses of 0.5-1.0 X M C ~ ( b p y ) a~t~pH + 7.0 in N2-purged solutions Containing 0.1 M (CH3)&HOH or CH30H and in N20-purgt:d solutions containing 0.1 M HC02-; G = 6.2 a t pH I in 0.2 M (CH[3)2CHOHunder deaerated conditions. In air-saturated solutions ([O,] = 2.5 X M) containing 0.1 M CH,OH, G = 4.9. No reactions were observed pulse radiolytically between the reducing radicals (except e,,) and a 3:l mixture of bpy and Co,? at pH 7; e;! reacts with k > 1O1O M-l s-l to yield the transient absorption spectrum shown in Figure 3. This species decays rapidly (tlj2 100 ps); its kinetica were not characterized. C ~ ( p l z e n ) ~ ~Thle + . pulse radiolyBis (1.7 krd/pulse) of Ar-purged solutions containing 4.2 X lom6M Co(phen)t+ and 0.3 M (CHS)zCllOHa t pH 6.1 yields an absorption spectrum wiith A, 270 nm (emax 5.Ei X lo4 M-l cm-l) and very little albsorption a t X >400 rnm. This albsorption spectrum was stable within the long time scale of the instrumentation ( 1 s), Continuous radiolysis of Co(phen)aat produces the spectral changes shown in Figure 4 with clean isosbestic points at 299, 292, 275, and 253 nm. Exhaustive radiolysis results in a product absorbing a t 227 and 26!3 nm; for quantitative conversion of the substrate, these product bands have c values of 7.8 X lo4,7.9 X lo4,and 7.6 X lo4 Msl cnn-l, respectively, From the 'linear changes in absorbance as a function of irradiation time, a G value of 6,0 for the reduction of Co(phen) 3+ a t pH 3 or 7 is obtained, Unlike the case of the Co(bpy))+, reduction of Co(phen)l+ yielded no extractable Co(I1) at pH 7 and free phen in only very low yields (less than 10% of one phen released per equivalent of C ~ ( p h e n ) ~reduced)~. ~,+ N

N

Discussion In contrast to the previous reports3f4on the reaction of e,; with C ~ ( b p y ) where ~ ~ + a weak transient absorption in the visible rogion was observed, our examination of the reaction of the complex with a wide range of reducing radicals (including )e;, shows a very intense transient absorption in the 300-nm region. Only by the use of concentrations of C ~ o ( b p y ) ~(C5 ~ + >( M) lower than those used previously M) could we examine the spectral region in which the substrate absorbs strongly (A, 307, 318 nm; elnax 3.25 X lo4, 2.88 X lo4 M-l cm-l). The t values for the transient given in Figure 1 are calculated with the assumption that the transient is formed quantitatively; if its formation were less than qwntitative, the E values would be even higher. 1t appears reasonable that the intense transient absorption that we observe with

-200

250

300

350

A , nm Figure 4. Spectral changes obtained In the contlnuous radlolyisis of Ar-purged solutlon of 1.0 X lom4M Co(phen)2' contalnlng 0.2 M (CH3),CHOH at pH 7.0; dose rate = 2.62 krd mln-'. The spectrum of the unradlolyzed solirtlon Is Indicated by I; the other spectra were laken after successive I-mln Irradiation perlods; optical path = 1 mrn.

, , A 300 nm represents the major, if not the only, product is ,clear that of the radiolytic reduction of C ~ ( b p y ) ~It~ + the behavior of this intermediate is completely different than that of the weak transient absorption observed previously;3+' it if3 long-lived and it shows no reaction with O2 or the substrate. The intermediate cannot be attributed to low-spin CO(II);~ it is know known21that the low-spin high-spin equilibrium for Co(I1) is established within lo-' s. The assignment of the intermediate as a reduced ligand radical coordinated to the Co(II1) center415and its decay a13 intramolecular electron transfer5lZ2must also be rejected. It is now apparent that the spectra of free ligand radicals and those coordinated to Co(1II) have virtually identical values of A,, and very similar values of emax;26-30 the tranaient spectra from the reduction of free bpy and bpyH+ has, , A 2,8 X lo4 M-l cm-l) and 375 nm (emax 4.5 X 365 nm lo4 M-l cm-I), respectively.8 In contrast, the reduction of R ~ ( b p y ) ~to~ I%u(bpy)sc + in aqueous solution yields a transient absorption with A,, 360 ( t 2.1 X lo4 M-l c ~ n - 9 ~ which supports the assignment4 of Ru(bpyI3+ a$ a coordinated reduced bpy radical-Ru(I1) species. Therefore, whereas Ru(bpy),+ reacts with O2( k = 7.4 X loBMI' ~ - 9 , ~ the intermediate from the reduction of C ~ ( b p y ) , ~(does + not. Further, whereas reduced free bpy transfere an electron to HQ ( k = 3.4 X lo0 M-l s - ~ )our , ~ intermediate does not. We attribute the intermediate as arising from the one-electron reduction of C ~ I " ( b p y ) ~to ~ +CoI'(bpy)t+ (reaction 1); any low-spin Co(I1) or Coordinated radical

-

+

C o ( b p ~ ) ~ e~ +---* C O ( ~ P Y ) ~ ~ "

(1)

species must have lifetimes