J. Phys. Chem. 1993,97, 7786-7791
7786
Radiolytic Studies of Ruthenium Oxo-Acetato Trinuclear Complexes in Acetonitrile Taira Imamura,*J Takashi Sumiyoshi,* Kenta Takahashi,? and Yoichi Sasakif Department of Chemistry 11, Faculty of Science, and Department of Nuclear Engineering, Faculty of Engineering, Hokkaido University, Sapporo 060, Japan Received: April 30, I993
Redox reactions of the ruthenium(III,III,III) and ruthenium(III,III,II) trinuclear cluster complexes, [Ru3(p3o)(p-CH3Coo)a(py)3]PFa and Ru&~s-O)(p-CH3COO)a(py)3, (Ru(333) and Ru(332), respectively) in acetonitrile were studied by pulse radiolysis. Irradiation of deaerated Ru(333) acetonitrile solutions induced one-electron reduction of the trinuclear Ru(333) center by the acetonitrile radical anion, CH3CN'-, to Ru(332). When Ru(332) was used as a parent complex, irradiation afforded Ru(322). The yield of CH3CN'- was evaluated to be 0.21 pmol J-l. In aerated solutions, Ru(333) was competitively reduced by CH3CN'- with a rate constant of 6.1 X 1Olo M-l s-l and the superoxide ion, 0 2 - , with a rate constant of 3.5 X lo9 M-l s-l at 14 OC. Ru(332) once produced decayed to regenerate Ru(333) in 100-300 ps after the electron-pulse irradiation. Oxidation of Ru(332) by the peroxyl radical, 'OZCH~CN,to Ru(333) with a rate constant of 2.7 X lo9 M-1 s-1 was confirmed. The whole reaction scheme for the radiation-induced processes is discussed.
Introduction Acetonitrile is one of the most versatile solvents for laboratory use and is utilized as a starting material for the synthesis of many chemical substancessuch as acetophenone, 1-naphthylaceticacid, thiamine, and acetamidine.' The basic studies of acetonitrile2 and of short-lived transients derived from it have been of great importance and undertaken using various methods such as electr~hemical3*~ and radiolytic techniques.>l1 Especially, extensive radiolytic studies have characterized and clarified the properties and reactivities of short-livedtransients such as a radical anion, CH3CN'-, a neutral radical, 'CHzCN, and a radical cation, CH3CN*+. Electron-pulse irradiation of liquid acetonitrile produces an electron and CH&N*+ which respectively lead to CH3CN'- and 'CH2CN by releasing H+to C H S C N . ~ ~ ~ CH,CN eCH3CN'+
--
e-
+ CH3CN
+ CH,CN
+ CH3CN'+
(1)
CH,CN'-
(2)
-
-
'CH,CN
+ CH3CNH+
benzophenone
tram-stilbene
0.11od
THFe
0.2, O.lOSd
THF, 2-MeTHF, 1.2-
2-propanol-waterg
dimethoxyethanee acetonitrile* a Used to determine molar absorptivities of the reducing form of substrate. Reference 7. Reference 14. Reference 5. e Reference 15. f Reference 6. E Reference 16. This work. Ru(333)
0.21
profitable substrates which can illustrate whole redox reaction processes concerned. In the present work, the ruthenium oxo-acetato trinuclear pyridine complexes, [Ru3(p3-O)(p-CH3COO)6(py)3]PF6 and Ru&-O) (pc-CH3C00)6(py)3,(Ru( 333) and Ru( 332), respec-
(3)
The formation of CH3CN'- has been confirmed in the solid state,g in vapor,1° and in the liquid phase- in which the formation is completed within 2 ns after pulse irradiation." The radical anion exists as a monomeric (A, = 1450 nm) or dimeric form of (CH3CN)2+(Amax=550nm) whichiscomposedofthemonomeric form and CHSCN.~ The absorption around 550 nm was superpositioned with these two species. In the presence of 0 2 , pulse irradiation of acetonitrile solutions also produces other species such as the superoxide ion, 02-,5J2 and the cyanomethylperoxyl radical, '02CH2CN. l 2 ~ 3 However, studies on the reactivities of these species have been still insufficient because of the lack of appropriate reaction substrates which have stable structure under variable oxidation states and can interact with these species. To account for reactivitiesor to characterize these primary species,many organic and inorganic substrates as shown in Table I have been used as subjects, which generally caused only simple electron attachment or detachment reactions. The authors have been seeking more t Department of Chemistry 11. t
TABLE I: Yields of Reducing Species in Acetonitrile substrate G ("01 J-l) solvent' anthracene 0.161b THF' pyrene 0.17b THFc
Department of Nuclear Engineering.
0022-3654/93/2097-7786$04.00/0
f Ru(333)
tively) were treated as target substrates, because the complexes of the general formula [Ru~(~~-O)(~-CH~C~~)~(L),]~ (L = unidentate ligand) have reversibleelectrochemicalmultistep oneelectron redox states.17-20 Especially, the pyridine complexes, [R~3(r3-O)(p-CH&O0)6(py)~]*,with different oxidation states (111,111,111) and (111, 111,11), have been isolated and well studied. The complexes reveal characteristic electronic spectra due to the ruthenium oxidation ~tates.1~.20The half-wave potentials for Ru(333)/Ru(332) and Ru(332)/Ru(322) couples in acetonitrile 0 1993 American Chemical Society
Ruthenium Trinuclear Complexes in Acetonitrile have been determined to be +0.47 and -0.80 V vs NHE, respectively.20
Experimental Section
The Journal of Physical Chemistry, Vol. 97, No. 29, 1993 7787 SCHEME I
pEq
CH&N
Electron Pulse
Apparatus. Electronicspectra for statistic measurementswere recorded on a Jasco Ubest-30 spectrophotometer. Apparatus used for pulse radiolysis was the same with that previously reported.21 Pulse radiolysis was performed at room temperature (14 "C) using an electron pulse (45 MeV) generated from an S-band linear accelerator (Mitsubishi) installed at Hokkaido University. The half-width of the pulse was 10ns. The absorbed dose per pulse was determined by KSCN dosimetry.22 Irradiation with an absorbed dose of 103 Gy/pulse of neat liquid acetonitrile or acetonitrile solutions of the ruthenium complex, Ru(333) or Ru(332),produced 1.65 X l0-5MofreducingspeciesofCH3CN'as described in the text. The optical path length of a quartz cell was 10 mm. The reaction processes were followed by means of the kinetic optical absorption measurement in the ultravioletvisible region using analyzing light from a 1-kw xenon lamp chopped by a rotating disk at 25 Hz. Rate constants were determined by iterativecomputersimulations of theopticaldensity vs time curves obtained at various experimentalconditions, using the RungeKutta methodS23 The errors in the rate constants thus obtained were within about 110%. Materials. The complexes, [Ru3(r3-o)gc-CH,Coo)6(py)3]PFs and Ru3(p3-O)(p-CH3C00)6(py)3,were prepared by modifying the reported method~.~~,20 Purity of the complexes was checked by absorption spectral measurements and chemical analyses. While Ru(332) was very stable in deaerated acetonitrile, it underwent slow oxidation under the air-saturated conditions to form Ru(333) with a half-life of ca. 2 days around 28 OC. The absorption spectral change accompanied by the reaction showed an isosbestic point at 480 nm. Acetonitrile was nonfluorescent spectral grade from Dojin. Argon and oxygen were ultrahigh purity from Hoxan and Nipponsanso, respectively,and dinitrogen monooxide (N2O) was medical use from Nissankagaku. Sample solutionswere bubbled with argon, dinitrogen monoxide,or oxygen and sealed with a Teflon bulb prior to irradiation. Electrochemistry. The half-wave potential of 0 2 / 0 2 - in acetonitrile was determined to be Ell2 = -0.87 V vs Ag/AgCl electrodes (-0.65 V vs NHE) by cyclicvoltammetry. This value is in good agreement with the reported potentials, i.e., thereduction potential of -0.87 V24and the half-wave potential of -0.82 V vs SCE2' (-0.63 V and -0.58 V vs NHE, respectively). Results Radiolysisof Acetonitrile. Pulse irradiation of neat acetonitrile in the absence of 0 2 gave within 50 ns a transient absorption spectrum with a weak maximum at 550 nm in the visible region, which showed the formation of CH3CN'- and/or (CH$N)2*-.' Since thedifferencein the reactivities of these two reducing species have not been known in detai1,26we used CH3CN-as representing the species under rapid equilibrium. The decay of CH3CN'- was followed at wavelengths between 400 and 700 nm.
k,
CH,CN'product (4) The first-order rate constant was determined by repeated measurements to be k4 = (3.0 f 0.2) X 106 s-1. This value is in the range of the reported rate constants, 7.9 X 105 to 1.7 X lo7 s-l.SJ1 The variation in the value of decay rate constants from one batch acetonitrile to another must be due to the impurity content in the solvent as suggested by Bell et al.5 We used the same batch of acetonitrile throughout this work so that the effect of impurity was constant. Effect of the water content in acetonitrile on the stability of these reducing species was also studied since the protonation reactions of the reducing species with protic solvents are known
MUS"
k\(cH3cN
...,.........._...._...._ \........... Ru(333)
\
>
decom.
'CH2CN
klo
decom.
Ru(332)
decom.
Ru (333) to occur.27 In the concentrations of water larger than 10-2 M, the decay rate of CH3CN'- increased with the increase in the water content, but the rate was virtually unaffected in concentrations less than 10-3 M. Radiolysisof Ru(333) and Ru(332)Argon-SaturatedSolutions. The flow chart of the whole reactions observed for the ruthenium complex systems upon pulse irradiation is depicted in Scheme I. The complexes Ru(333) and Ru(332) in acetonitrile have absorption maxima at 691 (molar absorptivity 5513 M-1 cm-I), and 389.5 (11 600 M-1 cm-1) and 909 nm (9700 M-1 cm-*),2* respectively at room temperature as shown in Figure la. The molar absorptivities are fairly higher than those of the transient species of acetonitrile. Upon irradiation with an electron pulse, the argon-saturated Ru(333) solutions showed rapid absorption spectral change. Transient differential absorption spectra were characterized by the increase in absorbancearound 390 nm and thedecrease around 700 nm with an isosbestic point at 480 nm, indicating the formation of Ru(332) (Figure lb). The isosbestic point accorded with the point observed for the spectral change in the aerobic oxidation of Ru(332) to Ru(333) described in the Experimental Section. The result revealed that simple one-electron transfer occurred at the triruthenium center of the Ru(333) complex.29 CH,CN'-
+ Ru(333)
ks
CH,CN
+ Ru(332)
(5)
The k5 value was determined to be (6.1 f 0.6) X 1O1OM-l s-I at 14 OC as described in the Discussion section. When Ru(332) was treated as a parent complex of pulse radiolysis in argon-saturated solutions, the absorbance around 400 nm decreased in 1 ps with increasing absorbance around 500 nm as shown in Figure 2. The product was very unstable and disappeared within 120 ps after pulse irradiation via an intermediate having absorption around 460 nm. Direct electrolytic reduction of Ru(333) at -0.91 V (vs NHE) showed the spectroelectrochemicallyreversible formationof a complex which had an absorption band at 460 nm.32 Spectroelectrochemical study of a similar trinuclear complex in which three isonicotinamides were coordinated to three ruthenium ions in place of pyridine showed that the electrochemically produced Ru(II1,
Imamura et al.
7788 The Journal of Physical Chemistry, Vol. 97, No. 29, 1993 1.5
t
1.o
A
/
I
0.6
0 0.16
0.10
[Ru(333)],/
I
t
Ru(333) initialconcentrationfor thereductionprocess toRu(332) induced by pulse irradiation of air- (0)and oxygen-saturated acetonitrilesolutions ( 0 )at 14 OC.
4a 4.06 O
PI--
800 wnvrlmgth Inm
300
700
600
Figure 1. (a) Visible absorption spectra of Ru(333) (-) and Ru(332) (- -) in acetonitrile at 20 OC. (b) Differential absorption spectrum at 1 ps after pulse irradiation of the 2.70 X 10-4M Ru(333) argon-saturated acetonitrile solution at 14 'C.
-
300
M
Figure 3. Plots of apparent pseudo-first-order rate constants vs the
500 wavelength I nm
400
600
700
Figure 2. Differential absorption spectra at 800 ns (0)and 40 p s (A) after pulse irradiation of the 4.24 X lk5M Ru(332) argon-saturated acetonitrile solution at 14 OC.
I1,II) species quickly loses central oxide ion within a few seconds as verified by rapid-scanningspectrophotometrictechnique.l8 The initial product has an absorption peak at ca. 500 nm, and the latter at ca. 400 nm. If the electrochemical behavior of the pyridine complexis similar to that of the isonicotinamidecomplex, the pyridine complexwith absorption maximum at 460 nm should correspond to the Ru(IIIIIIIII)species without central oxide. It is concluded that upon pulse irradiation of the Ru(332) pyridine complexone-electronreduction takes place to give Ru(322) species with an absorption peak at ca. 500 nm which then decomposes through an intermediate with no central oxide. The strong reducing ability of CH3CN'- can be ascribed to the very negative reduction potential of CH3CN less than -2.6 V vs NHE.33 Air- and Oxygen-SaturatedSolutions. Pulse radiolysis of airor oxygen-saturated solutions gave essentially the same transient differential spectra as those of deaerated systems at the initial stage, indicating that Ru(332) was formed also under these conditions. Apparent pseudo-first-order rate constants of the
formationof Ru(332) inoxygen-saturated (7 X 1&3M)5solutions were smaller than those of air-saturated solutionsand consequently smaller than in argon-saturated solutions at the same initial concentrations of Ru(333) as shown in Figure 3. That is, the formation of Ru(332) was retarded with increasing oxygen concentration. The apparent second-order rate constants for the formation estimated from the slopes in the figure were 1.4 X 1010 and 7.8 X 109 M-' s-I for air- and oxygen-saturated solution systems, respectively. For these systems, a curious phenomenon was observed, i.e., the Ru(332) complex once formed was reoxidized with the isosbestic point at 480 nm to form Ru(333) after the first grow-in reached its maximum. This reoxidation reaction was completed within 300 ps. The rate was much faster than the rate of spontaneousoxidation of Ru(332) in air-saturated solutionsupon nonirradiation. When the Ru(332) complex wasused as a starting substrate, pulse irradiation of aerated solutions gave the oxidized species Ru(333) rather than Ru(322). With successive irradiations, Ru(333) gradually accumulated and the system became the same one started with Ru(333). This result revealed that the oxidizing agent for Ru(332) was generated by the radiolysis in the presence of 02. Dinitrogen Monoxide Saturated Solutions. Although pulse irradiation of dinitrogen monoxide saturated solutions of Ru(333) induced the formation of Ru(332), the amount of Ru(332) produced is only one-tenth of the maximum yield in argon-, air-, or oxygen-saturated solution systems. This result indicates that Ru(333) is not reduced to Ru(332) by CH,C(O)N*- (generated by the reaction of acetonitrile with 0'-formed from NzO).5 If Ru(333) was reduced by CH3C(O)N'-, more than one-tenth of Ru(333) should be reduced at the maximum change as reported for the reaction of TCNB (tetracyanobenzene)with CH3C(O)Nto form TCNB*-. Therefore, a small amount of Ru(333) would be reduced to Ru(332) by remaining traces of CH3CN'- instead of CH&(O)N*-.
Discussion Yield of CH3CN'-. The reduction of Ru(333) to Ru(332) in argon-saturated Ru(333) solutions enabled us to evaluate the yield of CHpCN*-. The amount of Ru(332) formed upon electronpulse irradiation was estimated from the maximum absorbance reached at 400 nm. The maximum absorbance increased with increasing initial concentration of Ru(333) and approached a constant value at higher Ru(333) concentration. The result can be analyzed by considering the competitive reactions, (4) and (S), as expressed by eq I, where [Ru(333)]0, OD, and OD, were the initial concentration of Ru(333), the maximum absorbance of Ru(332) formed, and the maximum absorbanceevaluated
Ruthenium Trinuclear Complexes in Acetonitrile
The Journal of Physical Chemistry, Vol. 97, No. 29, 1993 7789
by extraporating to Ru(333) infinite concentration, respectively. The OD,, value should correspond to the yield of CHsCN*-. The reciprocal plot of OD vs [Ru(333)]0gave a linear relationship as shown in Figure 4. From the value of the intercept, the yield of CH3CN'- was determined to be G = 0.21 pmol J-1 which corresponds to the formation of 1.65 X 10-5 M of CHpCN*- per 103Gy pulse irradiation. The G values, 0.105-0.2, were previously determined with different reactions in acetonitrile as shown in Table I. The present value, G = 0.21, is one of the highest values obtained so far. The reported values had been evaluated using molar absorptivities of the reduced species of various organic substrate, determined in other solventsdifferent from acetonitrile. Since molar absorptivities of organic radical ions are usually affected strongly by solvent properties even though essential spectral profiles are not changed,35 the reported G values are subjects of significant uncertainties. However, the Ru(333) and Ru(332) complexes are relatively stable in acetonitrile, which led us to determine the accurate and reliable yield of G value for CH3CN'-. Reduction with CH$N*-. The competitive reactions (4) and ( 5 ) were completed with 1 ps. The fast reactions made it difficult to determine the rate constant of ks from direct absorbance traces vs time, because many factors such as the Cherenkov radiation, the time resolution (10 ns), and the spectral superposition of CH3CN'- and the ruthenium complexes at low concentrations of the complexes interfered with the kinetic measurements. Then eq I was transformed to eq 11.
= 1 + k 4 / k , X l/[Ru(333)], (11) The plots of the ratio of OD,,/OD vs [Ru(333)]0-' in the complex concentration range from 7.8 X 1V to 2.75 X l ( r M for the argon-saturated solution gave a straight line with an intercept of unity. The rate constant, ks, was evaluated from the slope of the straight line to be (6.1 f 0.6) X 1OloM-l s-l using the obtained k4 value. The ks value was in the range expected for a diffusion-controlled process and is listed in Table I1 with the corresponding values in other systems. Reaction in the Presence of 0 2 . The maximum absorbances, OD,,, of Ru(332) formed in the air- and oxygen-saturated solutions were the same as that in the argon-saturated solution. Inaddition, plots ofOD,,/ODvs [Ru(333)]0-' gavealsostraight lines passing through the intercept of unity but having slopes different from the deaerated system. It is certain that a reducing species other than CH3CN'- participates in the aerated solution systems. The most probable reducing species should be the superoxide ion, 0 2 - , which has been reported to be formed by pulse radiolysis of acetonitrile solutions in the presence of 02.5J2 02-acts as a reducing agent for tetracyan~benzene~ and stilbene cation radicals in a c e t ~ n i t r i l e .In ~ ~practice, when the Ru(333) solution was treated with KO2, Ru(333) was reduced by 0 2 - to form Ru(332). The occurrence of the reduction with 02-can be supported by the half-wave potentials of -0.65 V (vs NHE) for 02/02and of +0.47 V for Ru(333)/Ru(332). The difficulty in proceeding of the successive reduction of Ru(332) to Ru(322) by 02-is also electrochemicallyclear since the half-wave potential of Ru(332)/Ru(322) is -0.80 V (vs NHE).20 Many organic radicals are known to undergo the reactions with 02 to give the corresponding peroxy radicals which act as oxidizing agents of metal ions or metal complexes such as Mn2+, C02+,39,4Oand ferrocene.41The cyanomethylperoxylradical, ' 0 2 CH2CN, is also formed by the reaction of the neutral radical of 'CH2CN with 0 2 upon electron-pulse irradiation of the oxygensaturated acetonitrile.12J3 This species is the most probable candidate for the oxidizing agent of Ru(332) to Ru(333). As described in the Results section, the apparent reduction rates of Ru(333) for air-saturated solutions were twice those for
OD,,/OD
50 40
i
/
0
5
10
[ R ~ ( 3 3 3 ) ] , '/~1 O 4 M
15
-'
Figure 4. Plot of the reciprocal maximum absorbance at 400 nm vs the reciprocal initial concentration of Ru(333) for the Ru(333) reduction to Ru(332) i n d u d by pulse radiolysis of argon-saturated acetonitrile solutions at 14 OC.
TABLE Ik Rate Constants for the Reactions of Substrates with Reducing Species CHsCN*rate constant
substrate
(10'0 M-1 s-1)
substrate
pyren@ biphenyl. benzophenone r-stilben@ tetracyanobenzend
3.9 3.3 5.5 3.3
tetrachloromethane
rate constant
(1010 M-1
trichloroethylene
VCl3 (?-pic)+ TazCls (4Me-py)rc Ru(333)d
6.1
8-1)
6.6 3.3 9 12 6.1
Reference 5 . Reference 36. Reference 37. This work. oxygen-saturated solutions. This result contradicts a mechanism that Ru(333) was reduced only by 0 2 - , since 0 2 - is supposed to be formed with almost the same amount at the initial stages in the both systems. In addition, we recall that the same amounts of Ru(332) were formed at infinite concentrations of Ru(333) in the three systems, argon-, air-, and oxygen-saturated solutions. These results led to the conclusion that CH3CN'- besides 02-still served as reducing species of Ru(333) even in oxygen-saturated solutions. The most comprehensive reactions in which 0 2 participated after the completion of the fast reactions 1-3 are depicted in Scheme 11. The superoxide ion, 02-, produced in reaction 6 should
SCHEME I1
+ + -+ + + - +
Reactions relevant to reduction process of Ru(333) CH3CN'-
+ 0,
CH,CN
0;
0,- Ru(333)
+ 0; CH,CN'- + 0, CH,CN
0, Ru(332)
0,- product
'CH2CN
0;
0,
(7) (8) (9)
0, '02CH2CN
*O,CH,CN
(6)
-O,CH,CN
(10)
(1 1)
Reactions relevant to oxidation process of Ru(332) '02CH2CN
+ Ru(332) --02CH2CN + Ru(333) '02CH2CN
-
product
(12)
(13)
be consumed via reactions 8,9, and 11. Although the formation of 02-by the reaction of 0 2 with e- besides by reaction 6 is possible, the contribution of such reaction should be negligible, because the concentration of 0 2 in oxygen-saturated solutions is 7 X 10-3 M which is 1/10000 of the concentration of the solvent
Imamura et al.
7790 The Journal of Physical Chemistry, Vol. 97, No. 29, 1993
acetonitrile. Reaction 7 is the back-reaction of (6). Reaction 8 is the reduction process of Ru(333) by 02-.Reactions 9 and 11 are the decomposition processes of 0 2 - by some impurities and the reaction process with '02CH2CN to form the moderately stable anion, -02CH2CN, respectively. The '02CH2CN radical formed by reaction 10 should be decayed via reactions 11, 12, and 13. Reaction 12 is the oxidation process of Ru(332) by '02CH3CN to reproduce Ru(333). The process of (13) may comprise the '02CHzCN decomposition reaction to form cyanide ions as observed in aqueous acetonitrile solutions.l3 To simulate contribution of all reaction steps of (4) to (13), the Runge-Kutta method was applied assuming that reactions can be described with following five reaction equations (111)(VII). d[CH,CN'-]/dt = -k,[CH,CN'-] [Ru(333)] - k6[CH,CN'-] [OJ k,[CH,CN][O;]
+
- k,[CH,CN'-]
d[Ru(332)]/dt = k,[CH,CN'-] [Ru(333)]
(111)
+
k,[O,-] [Ru(333)] - k,,[Ru(332)] ['O,CH,CN]
(IV)
= k6[CH3CN'-][O2] - k,[CH,CN][O;] k,P,-l - k,[O;I [Ru(333)1 - k,,P2-1 ['O,CH,CNI (VI
d[O;]/dt
= k1o['CH2CN] [O,] ['02CH2CN] - k13['02CH,CN] -
k,l[O;]
k12[Ru(3 32)] ['O,CH,CN]
(VII)
Iterative calculations initiated by estimating apparent values for these rate constants were made successively until the smallest deviations from experimental absorbance traces vs time were attained. For reactions 10 and 13,13the reported rate constants had been used as initial values to start the calculation. All rate constants thus obtained are summarizedin Table 111. Simulation
-
curve^ using these constants fitted extremely well the experimental curves under different conditions, Le., argon-, air-, and oxygensaturated solutions with different concentration of Ru(333) as shown in Figure 5. The klo value for the reaction of 'CH2CN with 0 2 determined to be (1.1 f 0.1) X 109 M-1 s-1 is similar to the reported value of 1.3 X lo9 M-l s-l derived from aqueousacetonitrile solution systems. The decay rate constant of k13, (2.0 f 0.2) X lo4s-I, obtained by solving as a first-order process is twice the value of 9 X lo3 s-I determined by the conductivity method for pH 11 aqueous-acetonitrile solutions.13 In spite of the significant difference in the solution composition and the different experimental techniques, the rate constants are in satisfactory agreement. The equilibrium constant, K , for [CH3CN] [02-]/[CH3CN*-][OJ, in the equilibrium 14, was determined to be 5 X lo4from the ratio of the rate constants of k6 and k,. The ratio k6
CH,CN'-
+ 0, eki CH,CN + 0;
(14)
of initial concentrations of CH3CN'- and 02-, Le., [CH3CN*-]o/ [ 0 2 - ] 0 , was evaluated to be 1/3.7 and 1/ 18 for air- and oxygen-saturated solutions, respectively, using the values k6/ k, = 5 x 104, [CHsCN] = 19.2 M, and ( 0 2 1 = 1.4 X 10-3 M for former solutions, and [OZ] = 7 X M for latter solutions. Therefore, the contribution rate ratios of reactions 5 and 8 at the initial stages of the reductions of Ru(333) were estimated to be vs/va = k5/(3.7k8) = 5/1 and 1/1 for air- and oxygen-saturated solutions, respectively. This profile indicates that the reaction 5 process is still significant even in the oxygen-saturated solutions. 0.12
0.15
d
**
(VI)
d['CH,CN]/dt = -klo['CH2CN] [O,] d['O,CH,CN]/dt
TABLE Ill: Reaction Rate Constants Determined for the hadiated Ru(333) Acetonitrile Solutions at 14 'C k4 (3.0 i 0.2) X 106 s-1 kg = (6 1) X 105 s-I klo = (1.1 0.1) x 109 ~ - 8-11 ks (6.1 i 0.6) X 10'0 M-I S-I (1.0 0.1) X 10" M-'S-' kll = (2.8 0.2) X 1010 M-1 s-1 k6 kk: (2 0 & 0 2) X lo6 M-I s-l kI2= (2.7 f 0.2) x 109 M-I S-1 (3:5 o:2) M-l s-l kl3 = (2.0 i 0.2) x 104 s-1
0.10
Q
0.05
0
0 0
1
2
3
4
5
6
7
8
9
0
10
1
2
3
4
5
6
7
8
~
1
tfw
UP8
I
0.15 1
I
n .n
V.IV
%4
0.12,
Id
6
3
Q
1
0.06
0.05
0
.-
0 0
20
40
60
80
100
tlw
120 140
160
180 200
0
1
. 2
3
4
5
6
7
8
Q
1
0
tlw
Figure 5. Absorbance changes at 400 nm vs time for the Ru(333) reduction to Ru(332) induccd by pulse radiolysis of acetonitrile solutions at 14 OC under various conditions. Solid lines are the absorption curves simulated using the rate constants in Table 111. (a) 1,2, and 3 are the absorption curves within 10 ps respectively for the argon-, air-, and oxygen-saturated solutions of [Ru(333)]o = 1.38 X l P M. (b) 4, 5, and 6 are the curves within 200 ps for the same systems with 1, 2, and 3, respectively. (c) 7, 8,9, 10, and 1 1 are the curves for the air-saturated solutions of [Ru(333)]0 = 2.75 X l e , 1.38 X l W , 5.50 X 2.75 X lo-$, and 7.84 X 1V M, respectively. (d) 12, 13, 14, 15, and 16 are the curves for the oxygen-saturated solutions with the same [Ru(333)]0 with 7, 8, 9, 10, and 1 1 , respectively.
0
Ruthenium Trinuclear Complexes in Acetonitrile
The Journal of Physical Chemistry, Vol. 97, No. 29, 1993 7791
Conclusion The aspectsof the reactivities, CH3CN*-, *02CH2CNand 02-, formed pulse radiolytically in the acetonitrile solutions were disclosed using the trinuclear ruthenium complexes as follows: (1) The Ru(333) complex was reduced toRu(332) advantageously by CH3CN'- besides by 02-even in the oxygen-saturatedsolutions in which a large amount of reducing agents exist as 0 2 - . The Ru(332) complex once formed was reoxidized to Ru(333) by 'OzCH2CN which was derived from the reaction of 02 with 'CHzCN. (2) The yield of CH3CN'- was evaluated to be 0.21 Mmol J-' as a reliable value. (3) Simulation using the evaluated rate constants reproduced satisfactorily the experimental absorbance changes vs time for the reduction and oxidation processes of the ruthenium complexes under different conditions. (4) The flow chart of the reaction process simulated by a computer iterative analysis provided a comprehensive reaction mechanism that can be applied to other radiolytic reaction systems of acetonitrile solutions.
Acknowledgment. This work was supported by a Grant-inAid for Scientific Research (No. 04215105) on Priority Area of "Molecular Approaches to Non-equilibrium Processes in Solution''. A research grant from the Mitsubishi Foundation is also acknowledged. References and Notes (1) The Merck Index, 1lth ed.; Merck Co.: Rahway, NJ, 1989; p 63. (2) TechdquesofChemistry. y o l . ~ OrgonfcSol"enrs,4thed.;Riddick, ~, J. A,, Bunger, W. B., Sakano, T. K., Eda.; Wiley, 1986; pp 1048-1054. (3) Mann, C. K. In Electroanalytical Chemistry;Bard, A. J., Ed.; Marcel Dekker: New York, 1969; Vol. 3, p 57. (4). (a) Szkalarczyk,M., Sobkowski, J. Electrochim.Acra 1980,25,1597. (b) Billon, J. P. J. Elecrroanal. Chem. 1959, 1, 486. ( 5 ) Bell, I. P.; Rodgers, M. A. J.; Burrows, H. D. J . Chem.Soc.,Faraday Trans. 1 1977, 73, 315. (6) Baptista, J. L.; Burrows, H. D. J. Chem. Soc., Faraday Trans. I 1974, 70,2066. (7) Hayon, E. J. Chem. Phys. 1970,53,2353. (8) (a) Singh, A.; Gesser, H. D.; Scott, A. R. Chem. Phys. Lett. 2,271. (b) Bonin, M. A.; Link, J.; Tsuji, K.; Williams, F. Adu. Chem. Ser. 1968,82, 289. (c) Hollman, L.; Sprague, E. D.; Williams, F. J. Am. Chem. Soc. 1970,92,429. (d) Burrows, H. D.; Kosower, E. M. J. Phys. Chem. 1974, 78, 112. (9) Sprague, E. D.; Takeda, K.; Williams, F. Chem. Phys. Lett. 1971, 10, 299. (10) Stockdale, J. A,; Davis, F. J.; Compton, R. N.; Klots, C. E. J. Chem. Phys. 1973, 95, 7193. (1 1) Nakayama, T.; Ushida, K.; Hamanoue, K.; Washio, M.; Tagawa, S.; Tabata. Y. J. Chem. Soc.. Faradav Trans. 1990.86, 95. (12)' Tran-Thi, T. H.; Koukes-hjo, A. M.;Giiles,L.; Genies, M.; Sutton, J . Radiat. Phys. Chem. 1980, 15, 209. (13) Mosseri. S.;Neta, P.; Meisel, D. Radiat. Phys. Chem. 1990,36,683. (14) Gill, D.; Jagur-Grodzinski,J.; Snvarc, M. Trans.Faraday Soc.1964, 60, 1424.
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