- Email: [email protected]
Depolymerization kinetics of Ru4(OH)48+ and characterization of the reaction product: Dimeric Ru(III) ion
Pdyhedm Vol. 2, No. II. pi. tinted in Great Britain.
11074110,
ozn-m7p3 Rrlpmon
1983
53.00 + .oo Press Ltd.
DEPOLYMERIZATION KINETICS OF Ru,(O#+ AND CHARACTERIZATION OF THE REACTION PRODUCT: DIMERIC RI@) ION
Laboratorium
,W. D’OLIESLAGER*, L. HEERMAN and M. CLARYSSE voor Radiochemie, Katholieke Universiteit Leuven, Celestijnenlaan 2OOF, 3030 Heverlee, Belgium (Received 13 September 1982; accepted 5 February 1983)
Ah&act-Perchloric acid solutions of Ru,(O# + were prepared by the controlledpotential reduction of Ru(IV) at platinum electrodes in a high-speed coulometric cell. The solution spectrum of Ru.,(OH)i+ is reported. Tetrameric Ru(II1) ion decomposes in a reaction which is first-order with respect to its concentration, but independent of the acidity, to another species of the same valence state which is believed to be a dimeric ion (Ru,(OH)‘:+ or similar). Electrochemical reduction of this species yields Ru2+ which can be reoxidized only to Ru3+ , the most stable of the Ru(III)-aquo ions.
Ruthenium(IV) exists in non-complexing media (e.g. acid perchlorate solutions) as a tetrameric ion, formulated as Ru,(OH);‘c (or Ru40i+, etc.).‘.’ On platinum rotating disk electrodes, two reversible waves are observed for the reduction steps Ru(IV)-,Ru(3.5)+Ru(III), each wave being the sum of two one-electron steps:)
products possibly are formed in less acidic solutions. The further reduction of tetrameric Ru (III)-ion to Ru(II) occurs at more cathodic potentials and can be observed only on mercury electrodes.66 According to this scheme, tetrameric Ru(III)-ion slowly decomposes into another more stable species of the s’ame valence state which was
1st wave
,Ru,(OH);,+ (IV)
0.653 V 0.650 V
Ru,(OH):,+ (3.75)
Ru,(OH;+(3.5)
O.493
Ru,(OH)‘,+(3.25)
Ru.,(OH):+(III)
5
2Ru,(OH): + (III).
0.656 V _
1 Ru,(OH:+(3.5)
2hd wave
o.469v 1
I
Potentials (vs. nhe) and species are given for 1.0 M HClO, solutions (the high charge on these ions most probably is reduced by perchlorate complex or ion-pair formation); more hydrolized
0.455 v
Ru,(OH:+ (III) (I)
hypothesized to be a dimeric species (vi& infa).3 The existence of such a follow-up chemical reactions was already indicated by Wehner and Hindman who noticed profound changes in the UV absorption spectrum of Ru(II1) solutions prepared by electrolytic reduction of Ru(IV) at plat*Author to whom correspondence should be ad- inum electrodes, when stored at 0°C under an inert dressed. atmosphere (see also Refs. 1 and 3). _-
1108
W. D’OLIESLAGER
et al.
This paper reports on the depolymerization kinetics of Ru,(OH)i+ and on the electrochemical reduction of its decomposition product (dimeric Ru(III)-ion). EXPERIMENTAL
Acid perchlorate solutions of Ru(IV) were prepared and analyzed as described previously.3 Solutions of tetrameric Ru(III)-ion for spectrophotometric studies were prepared, in an air thermostat under inert atmosphere, by electrolysis at a platinum gauze electrode in a high-speed coulometric cell (p N 3.5 x lop2 set-‘; for a definition of p, see Bard’); complete electrolysis was achieved in about 180-200 sec. The solution was then transferred immediately to a spectrophotometer cell and the spectral changes were recorded as a function of time (Beckman Acta M IV spectrophotometer). The spectrum of Ru,(OH)j + was obtained by recording a spectrum immediately after the preparative electrolysis and correcting it for the small amount of dimer (less than 5% at O’C) already formed during the electrolysis time. The concentration of the dimer was estimated by treating the mass transfer and the follow-up chemical reaction during the electrolysis as a system of consecutive first-order reactions using experimental values of p and k, (vide infra). Electrochemical experiments were carried out using a PAR Model 303 SMDE (static mercury drop electrode) and a PAR Model 174A polarographic analyzer; other experimental details have been reported elsewhere.3
zso
300
400
500
Fig. 1. Absorption spectrum of (1) Ru.,(OH),8+and (2) Ru,(OH); + in 1.0 M HCIO, solution. Inset. Decomposition kinetics of Ru,(OH)i + : plot of log(chNC,, - Azw) vs time (O’C).
lives, as expected for a reaction that is first order with respect to the concentration of Ru,(OH)i+. The rate constant k, was found to be independent of the solution acidity over the range 0 I pcH I 2. Values of ki are 2.37 (kO.03) x lop4 sec’(O”C), 9.82( k 0.20) x 10e4 set-’ (IS’C) and 2.47 (fO.06) x 10m3set-‘(25°C); the energy of activaRESULTS AND DISCUSSION tion is calculated as 63 kJ/mole. Furthermore, it The spectra of tetrameric and dimeric Ru (using (III)-ions are shown in Fig. 1. The spectrum of was established by direct potentiometry dimeric Ru(II1) contains a single peak at 290 nm glass electrodes) that protons are neither consumed (tg” = 2360 + 50), in agreement with the work of nor liberated in the reaction. These facts seem to Wallace and-Props? (Wehner and Hindman’ re- suggest that the stoichiometry of the follow-up chemical reaction is adequately represented by (1); ported a peak at 300 nm). The spectrum of tetramit must be stressed however that an unquestionable eric Ru(III)-ion, not previously reported in the proof of the existence of a dimeric Ru(II1) aquoion literature, exhibits plateaus at 285-310 nm and has not been given thus far. 390-405nm (6$“‘= 730 _+ 10) (the formal molar Methods such as ion exchange, solvent extracabsorptivities were calculated using the analytical tion and potentiometry cannot be used, at least in concentration of ruthenium, C,,). perchloric acid solutions, to investigate the ionic The depolymerization kinetics of tetrameric nature of this Ru(II1) species since it is slowly Ru(III)-ion was studied by recording the spectral oxidized by perchlorate ion (Wehner and changes at 290 nm and plotting log($C,, - A290) Hindman’ already implicitly dismissed the possivs time, as is shown in Fig. 1, inset; from the mass bility that the species is a chloro-complex, chloride balance and the postulate of additive absorbances being produced in the oxidation reaction). it follows that (chic& - A290)is proportional to Some indirect evidence for the existence of a the concentration of Ru,(OH)i+ (this is true, irredimeric Ru(II1) species was obtained from the spective of whether the product of the follow-up following experiment. chemical reaction is a dimeric ion or not). Such A solution of 2,2’-bipyridine in toluene plots were linear over at least two to three half-
1109
Depolymerization kinetics of Ru,(OH): + (10 - 3 M) was added to a solution of “stable” Ru(II1) (absorption maximum at 290mp). The organic ligand was slowly extracted into the aqueous phase with formation of an intensely green coloured complex. The aqueous phase was then 50-fold diluted with acetonitrile and the spectrum was recorded; the complex absorbs at 410 nm and 640 nm. According to Weaver et aL9 a high intt?fJZQ, km PTmzgj bflfd u: ,zTkmt +%%h?TJk &iEacteristic for oxo-bridged dimers such as [(bipy)r(H,0)RuORu(H,0)(bipy)J4+ (these authors preferred the diaquo-p-oxo formulation over the di-,u-hydroxy formuIation proposed by Dwyer et al.” for similar 1, lo-phenanthroline complexes). The electrochemical reduction of dimeric Ru (III) has been studied by poIarography (sampIed DC) and cyclic voltammetry. Over the acidity range 0 I pcH I 2, a single polarographic wave is cbserued with Fs= - Q.290 V rs SCE (for 1.OM HClO, solutions and t = 1 s). The electrode reaction involves one electron per ruthenium atom and, therefore, corresponds with the reduction step dimeric Ru(IIIj + Ru(II). The number of electrons involved in the reduction was estimated by comparing the diffusion current with the diffusion cuTrent for Q&er‘&YL&X%steps of UYQV()UT&s identical conditions of concentration, drop time, etc, a X%X<< &+3m+i&&JTl JL%.J$ G,c,r&& I%.?&&%~
coulometry is impossible since Ru(I1) is oxidized by perchlorate ion during the experiment. The half-wave potential is independent of the ruthenium con=nQation but shifts to more negative values if the solution acidity is decreased (dEi/dlog cu + = - 0.044 V/decade). Furthermore, the half-wave potential shifts to more negative times values shorter drop for @~&XI~< = - &ZZ? ?+?l3ZZZl$, &Xi*. G?L&iZ&& the irreversibility of the electron transfer process. The cyclic voltammogram obtained for a solution of dimeric Ru(II1) at the HMDE, shown in Fig. 2, exhibits a single reduction peak A on the first scan but the corresponding anodic peak is absent on the reverse sweep (as expected for an irreversible reaction); instead, a new reversible couple B/C is produced which can be identified with the Ru’+/Ru’+ couple (E4 = 0.002 V vs SCE) in good agreement with the measurements of Meccer and Buckley.” During successive scans, the height of peak A decreases while the height of peaks B/C increases; similarly, the ratio of the peak heights A/B(C) increases at slower scan rates. These observations are in agreement with a reaction scheme where the irreversible charge transfer step is followed by 8 &em&51 reaGt&n, s5& zis H (Ru-0 + Rur+ 3 LH+ k2
A
(Ru-&Ru)’
+
2Ru2+.
Obviously, the overall reaction is the reduction of dimeric Ru(II1) to Ru2+ which can be reoxidized to Ru3 + , the most stable of the Ru(III)-aquo-ions.
Acknowledgement-The authors thank the Institute for Nuclear Research (I.I.K.W., Brussels) for financial support of this research.
REFERENCES
+0.2
0
-0.2
-0.4
-0.6
Fig. 2. Cyclic voltammogram at the HMDE of Ru,(OH)‘:+ solution in 1.0 M HClO, (25°C). Scan rate: 0.1 V/s. Numbers between brackets indicate successive scans.
1. R. M. Wallace and R. C. Props& J. Am. Chem. Sot. 1969, 91, 3779. 2. C. Brkmard, G. Nowogrocki and G. Tridot, Bull. Sot. Chim. Fr., 1974, 392. 3. J. Schauwers, F. Meuris, L. Heerman and W. D’Olieslager, Electrochim. Acta 1981, 26, 1065. 4. L. W. Niedrach and A. D. Tevebaugh, J. Am. Chem. Sot. 1951, 73, 2835. 5. D. K. Atwood and T. De Vries, J. Am. Chem. Sot. 1962, 84, 2659. 6. L. N. Lazarev and J. S. Khvorostin, Russ. J. Inorg. Chem. 1968, 13, 1297. I. P. Wehner and J. C. Hindman, J. Am. Chem. Sot. 1950, 72, 3911.
1110
W. D’OLIESLAGER
8. A. J. Bard and K. S. V. Santhanam, Electroanalytical Chemistry (Edited by A. J. Bard), Vol. 4, p. 215. Marcel Dekker, New York (1970). 9. T. R. Weaver, T. J. Meyer, S. Ajao Adeyemi, G. M. Brown, R. P. Eckberg, W. E. Hatfield, E. C. Johnson, R. W. Murray and D. Untereker, J. Am. Chem. Sot. 1975, 97, 3039.
et al.
10. F. P. Dwyer, H. A. Goodwin and E. C. Gyarfas, At&. J. Chem. 1963, 16, 544. 11. R. R. Buckley and E. E. Mercer, J. Phys. Chem. 1966, 70, 3103.
11074110,
ozn-m7p3 Rrlpmon
1983
53.00 + .oo Press Ltd.
DEPOLYMERIZATION KINETICS OF Ru,(O#+ AND CHARACTERIZATION OF THE REACTION PRODUCT: DIMERIC RI@) ION
Laboratorium
,W. D’OLIESLAGER*, L. HEERMAN and M. CLARYSSE voor Radiochemie, Katholieke Universiteit Leuven, Celestijnenlaan 2OOF, 3030 Heverlee, Belgium (Received 13 September 1982; accepted 5 February 1983)
Ah&act-Perchloric acid solutions of Ru,(O# + were prepared by the controlledpotential reduction of Ru(IV) at platinum electrodes in a high-speed coulometric cell. The solution spectrum of Ru.,(OH)i+ is reported. Tetrameric Ru(II1) ion decomposes in a reaction which is first-order with respect to its concentration, but independent of the acidity, to another species of the same valence state which is believed to be a dimeric ion (Ru,(OH)‘:+ or similar). Electrochemical reduction of this species yields Ru2+ which can be reoxidized only to Ru3+ , the most stable of the Ru(III)-aquo ions.
Ruthenium(IV) exists in non-complexing media (e.g. acid perchlorate solutions) as a tetrameric ion, formulated as Ru,(OH);‘c (or Ru40i+, etc.).‘.’ On platinum rotating disk electrodes, two reversible waves are observed for the reduction steps Ru(IV)-,Ru(3.5)+Ru(III), each wave being the sum of two one-electron steps:)
products possibly are formed in less acidic solutions. The further reduction of tetrameric Ru (III)-ion to Ru(II) occurs at more cathodic potentials and can be observed only on mercury electrodes.66 According to this scheme, tetrameric Ru(III)-ion slowly decomposes into another more stable species of the s’ame valence state which was
1st wave
,Ru,(OH);,+ (IV)
0.653 V 0.650 V
Ru,(OH):,+ (3.75)
Ru,(OH;+(3.5)
O.493
Ru,(OH)‘,+(3.25)
Ru.,(OH):+(III)
5
2Ru,(OH): + (III).
0.656 V _
1 Ru,(OH:+(3.5)
2hd wave
o.469v 1
I
Potentials (vs. nhe) and species are given for 1.0 M HClO, solutions (the high charge on these ions most probably is reduced by perchlorate complex or ion-pair formation); more hydrolized
0.455 v
Ru,(OH:+ (III) (I)
hypothesized to be a dimeric species (vi& infa).3 The existence of such a follow-up chemical reactions was already indicated by Wehner and Hindman who noticed profound changes in the UV absorption spectrum of Ru(II1) solutions prepared by electrolytic reduction of Ru(IV) at plat*Author to whom correspondence should be ad- inum electrodes, when stored at 0°C under an inert dressed. atmosphere (see also Refs. 1 and 3). _-
1108
W. D’OLIESLAGER
et al.
This paper reports on the depolymerization kinetics of Ru,(OH)i+ and on the electrochemical reduction of its decomposition product (dimeric Ru(III)-ion). EXPERIMENTAL
Acid perchlorate solutions of Ru(IV) were prepared and analyzed as described previously.3 Solutions of tetrameric Ru(III)-ion for spectrophotometric studies were prepared, in an air thermostat under inert atmosphere, by electrolysis at a platinum gauze electrode in a high-speed coulometric cell (p N 3.5 x lop2 set-‘; for a definition of p, see Bard’); complete electrolysis was achieved in about 180-200 sec. The solution was then transferred immediately to a spectrophotometer cell and the spectral changes were recorded as a function of time (Beckman Acta M IV spectrophotometer). The spectrum of Ru,(OH)j + was obtained by recording a spectrum immediately after the preparative electrolysis and correcting it for the small amount of dimer (less than 5% at O’C) already formed during the electrolysis time. The concentration of the dimer was estimated by treating the mass transfer and the follow-up chemical reaction during the electrolysis as a system of consecutive first-order reactions using experimental values of p and k, (vide infra). Electrochemical experiments were carried out using a PAR Model 303 SMDE (static mercury drop electrode) and a PAR Model 174A polarographic analyzer; other experimental details have been reported elsewhere.3
zso
300
400
500
Fig. 1. Absorption spectrum of (1) Ru.,(OH),8+and (2) Ru,(OH); + in 1.0 M HCIO, solution. Inset. Decomposition kinetics of Ru,(OH)i + : plot of log(chNC,, - Azw) vs time (O’C).
lives, as expected for a reaction that is first order with respect to the concentration of Ru,(OH)i+. The rate constant k, was found to be independent of the solution acidity over the range 0 I pcH I 2. Values of ki are 2.37 (kO.03) x lop4 sec’(O”C), 9.82( k 0.20) x 10e4 set-’ (IS’C) and 2.47 (fO.06) x 10m3set-‘(25°C); the energy of activaRESULTS AND DISCUSSION tion is calculated as 63 kJ/mole. Furthermore, it The spectra of tetrameric and dimeric Ru (using (III)-ions are shown in Fig. 1. The spectrum of was established by direct potentiometry dimeric Ru(II1) contains a single peak at 290 nm glass electrodes) that protons are neither consumed (tg” = 2360 + 50), in agreement with the work of nor liberated in the reaction. These facts seem to Wallace and-Props? (Wehner and Hindman’ re- suggest that the stoichiometry of the follow-up chemical reaction is adequately represented by (1); ported a peak at 300 nm). The spectrum of tetramit must be stressed however that an unquestionable eric Ru(III)-ion, not previously reported in the proof of the existence of a dimeric Ru(II1) aquoion literature, exhibits plateaus at 285-310 nm and has not been given thus far. 390-405nm (6$“‘= 730 _+ 10) (the formal molar Methods such as ion exchange, solvent extracabsorptivities were calculated using the analytical tion and potentiometry cannot be used, at least in concentration of ruthenium, C,,). perchloric acid solutions, to investigate the ionic The depolymerization kinetics of tetrameric nature of this Ru(II1) species since it is slowly Ru(III)-ion was studied by recording the spectral oxidized by perchlorate ion (Wehner and changes at 290 nm and plotting log($C,, - A290) Hindman’ already implicitly dismissed the possivs time, as is shown in Fig. 1, inset; from the mass bility that the species is a chloro-complex, chloride balance and the postulate of additive absorbances being produced in the oxidation reaction). it follows that (chic& - A290)is proportional to Some indirect evidence for the existence of a the concentration of Ru,(OH)i+ (this is true, irredimeric Ru(II1) species was obtained from the spective of whether the product of the follow-up following experiment. chemical reaction is a dimeric ion or not). Such A solution of 2,2’-bipyridine in toluene plots were linear over at least two to three half-
1109
Depolymerization kinetics of Ru,(OH): + (10 - 3 M) was added to a solution of “stable” Ru(II1) (absorption maximum at 290mp). The organic ligand was slowly extracted into the aqueous phase with formation of an intensely green coloured complex. The aqueous phase was then 50-fold diluted with acetonitrile and the spectrum was recorded; the complex absorbs at 410 nm and 640 nm. According to Weaver et aL9 a high intt?fJZQ, km PTmzgj bflfd u: ,zTkmt +%%h?TJk &iEacteristic for oxo-bridged dimers such as [(bipy)r(H,0)RuORu(H,0)(bipy)J4+ (these authors preferred the diaquo-p-oxo formulation over the di-,u-hydroxy formuIation proposed by Dwyer et al.” for similar 1, lo-phenanthroline complexes). The electrochemical reduction of dimeric Ru (III) has been studied by poIarography (sampIed DC) and cyclic voltammetry. Over the acidity range 0 I pcH I 2, a single polarographic wave is cbserued with Fs= - Q.290 V rs SCE (for 1.OM HClO, solutions and t = 1 s). The electrode reaction involves one electron per ruthenium atom and, therefore, corresponds with the reduction step dimeric Ru(IIIj + Ru(II). The number of electrons involved in the reduction was estimated by comparing the diffusion current with the diffusion cuTrent for Q&er‘&YL&X%steps of UYQV()UT&s identical conditions of concentration, drop time, etc, a X%X<< &+3m+i&&JTl JL%.J$ G,c,r&& I%.?&&%~
coulometry is impossible since Ru(I1) is oxidized by perchlorate ion during the experiment. The half-wave potential is independent of the ruthenium con=nQation but shifts to more negative values if the solution acidity is decreased (dEi/dlog cu + = - 0.044 V/decade). Furthermore, the half-wave potential shifts to more negative times values shorter drop for @~&XI~< = - &ZZ? ?+?l3ZZZl$, &Xi*. G?L&iZ&& the irreversibility of the electron transfer process. The cyclic voltammogram obtained for a solution of dimeric Ru(II1) at the HMDE, shown in Fig. 2, exhibits a single reduction peak A on the first scan but the corresponding anodic peak is absent on the reverse sweep (as expected for an irreversible reaction); instead, a new reversible couple B/C is produced which can be identified with the Ru’+/Ru’+ couple (E4 = 0.002 V vs SCE) in good agreement with the measurements of Meccer and Buckley.” During successive scans, the height of peak A decreases while the height of peaks B/C increases; similarly, the ratio of the peak heights A/B(C) increases at slower scan rates. These observations are in agreement with a reaction scheme where the irreversible charge transfer step is followed by 8 &em&51 reaGt&n, s5& zis H (Ru-0 + Rur+ 3 LH+ k2
A
(Ru-&Ru)’
+
2Ru2+.
Obviously, the overall reaction is the reduction of dimeric Ru(II1) to Ru2+ which can be reoxidized to Ru3 + , the most stable of the Ru(III)-aquo-ions.
Acknowledgement-The authors thank the Institute for Nuclear Research (I.I.K.W., Brussels) for financial support of this research.
REFERENCES
+0.2
0
-0.2
-0.4
-0.6
Fig. 2. Cyclic voltammogram at the HMDE of Ru,(OH)‘:+ solution in 1.0 M HClO, (25°C). Scan rate: 0.1 V/s. Numbers between brackets indicate successive scans.
1. R. M. Wallace and R. C. Props& J. Am. Chem. Sot. 1969, 91, 3779. 2. C. Brkmard, G. Nowogrocki and G. Tridot, Bull. Sot. Chim. Fr., 1974, 392. 3. J. Schauwers, F. Meuris, L. Heerman and W. D’Olieslager, Electrochim. Acta 1981, 26, 1065. 4. L. W. Niedrach and A. D. Tevebaugh, J. Am. Chem. Sot. 1951, 73, 2835. 5. D. K. Atwood and T. De Vries, J. Am. Chem. Sot. 1962, 84, 2659. 6. L. N. Lazarev and J. S. Khvorostin, Russ. J. Inorg. Chem. 1968, 13, 1297. I. P. Wehner and J. C. Hindman, J. Am. Chem. Sot. 1950, 72, 3911.
1110
W. D’OLIESLAGER
8. A. J. Bard and K. S. V. Santhanam, Electroanalytical Chemistry (Edited by A. J. Bard), Vol. 4, p. 215. Marcel Dekker, New York (1970). 9. T. R. Weaver, T. J. Meyer, S. Ajao Adeyemi, G. M. Brown, R. P. Eckberg, W. E. Hatfield, E. C. Johnson, R. W. Murray and D. Untereker, J. Am. Chem. Sot. 1975, 97, 3039.
et al.
10. F. P. Dwyer, H. A. Goodwin and E. C. Gyarfas, At&. J. Chem. 1963, 16, 544. 11. R. R. Buckley and E. E. Mercer, J. Phys. Chem. 1966, 70, 3103.