Proton-Coupled Electron-Transfer Reactions. Mechanisms of Two

+ 2R + 2H+ - trans-[R~~~(TMC)O(H~0)1~+. + 2R+, have been studied. The first step of the reduction is a simple outer-sphere electron-transfer process, ...
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J . Am. Chem. SOC.1990, 112, 5 176-5 18 I

5176

Proton-Coupled Electron-Transfer Reactions. Mechanisms of Two-Electron Reduction of trans-Dioxoruthenium( VI) to trans-Aquooxoruthenium( VI) and Disproportionation of trans-Dioxoruthenium( V) Chi-Ming Che,*-lP Keung Lau,Ia Tai-Chu Lau,la-b and Chung-Kwong P o o n l P Contribution from the Department of Chemistry, University of Hong Kong, Pokfulam Road, Hong Kong. Received January 10, 1989. Revised Manuscript Received February 26. 1990

Abstract: The kinetics and mechanism of the reduction of 1 r a n s - [ R u ~ ~ ( T M C ) ( 0 )to~ ]~~~+~ ~ ~ - [ R U ~ ~ ( T M C ) O(TMC (H,O)]~+ = 1.4.8.1 1-tetramethyl- 1,4,8,11 -tetraazacyclotetradecane) in aqueous solution by outer-sphere one-electron reductants (R), 0 ) 2R+, 1 ~ + have been studied. The first step of the t r a n s - [ R ~ ~ ~ ( T M C ) (+ 0 )2R ~ ]+ ~ +2H+ t r a n s - [ R ~ ~ ~ ( T M C ) O ( H ~ + reduction is a simple outer-sphere electron-transfer process, t r a n s - [ R ~ ~ ' ( T M C ) ( 0 ) ~+]R~ + t r a n ~ - [ R u ~ ( T M C ) ( 0 )+~ ] + R+ with rate = k2[Ru(VI)] [R]. The rate constants ( k 2 )with cls-[Ru"(NH3),bpyl2+ (bpy = 2.2'-bipyridine), [ R ~ ~ ~ ( N H , ) , i s n j ~ ' (isn = isonicotinamide), and [Ru"(NH3),py12+ (py = pyridine) have been measured and can be correlated with the Marcus couple cross relation. The estimated self-exchange rate constant of the trans-[RuV~(TMC)(0)2]2+/trans-[RuV(TMC)(0)2]+ is 1.5 X 10, mol-' dm3 s-l ( I = 0.1 M) at 298 K. When t r a n s - [ R ~ ~ ' ( T M C ) ( 0 )is~ ]in~excess, + the t r ~ n s - [ R u ~ ( T M C ) ( 0 ) ~ ] + ~ ] 2H+ + t r ~ n s - [ R u ~ ' ( T M C ) ( 0 ) , ]+~ + species formed will undergo rapid disproportionation, 2 t r u n s - [ R ~ ~ ( T M C ) ( O ) + t r a n s - [ R ~ ' ~ ( T M C ) 0 ( H ~ 0 )The ] ~ ' .disproportionation follows the rate law: rate = kdi,[Ru(V)12with kdi, = 2ke,Kp[H+]/(1 + K [H+])2. The K , and k, are referred to the equilibrium constant and rate constant for the respective reactions, trans[RU'(TMC)(O)~]~'+ H+ == t r ~ n s - [ R u ~ ( T M C ) 0 ( 0 H )and ] ~ +r r ~ n s - [ R u ~ ( T M C ) 0 ( 0 H )+] ~t r+u n ~ - [ R u ~ ( T M C ) ( 0 ) ~ ] ' truns-[R~~'(TMC)(0)+ ~ ]t~r +a n s - [ R ~ ' ~ ( T M C ) 0 ( 0 H ) ] At + . 299 K and I = 0.1 M, K , and k, have been determined to be 615 f 50 mol-' dm3 and (2.72 0.34) X IO6 mol-' dm3 s-l, respectively. The self-exchange rate constant of the trans-[RuV(TMC)O(OH)]2+/trans-[Ru~v(TMC)O(OH)]~ has been estimated to be 5 X IO3 mol-' dm3 s-I at 298 K and I = 0.1 M.

-

-+

-

-

*

Oxo complexes of ruthenium(VI), ruthenium(V), and ruthenium(V1) have received much attention in recent years because of their remarkable abilities as stoichiometric and catalytic oxi d a n t ~ . ~ Among -~ the various oxoruthenium species, the Ru(1V)-oxo complex [ R ~ ' ~ ( b p y ) , ( p y ) O ](bpy ~ + = 2,2'-bipyridine; py = pyridine) has been extensively studied by Meyer and coworkers3 Oxidation by this complex can proceed through a variety of pathways including oxygen-atom transfer, hydrogen-atom, and ~~~

( I ) (a) Department of Chemistry, University of Hong Kong, Pokfulam

Road, Hong Kong. (b) Present address: Department of Applied Science, City Polytechnic of Hong Kong, Tat Chee Avenue, Kowloon Tong, Hong Kong. (2) (a) Che, C. M.; Wong, K. Y.; Mak, T. C. W. J . Chem. SOC.,Chem. Commun. 1985,988. (b) Che, C. M.; Wong, K. Y . ;Mak, T. C. W. J . Chem. SOC.,Chem. Commun. 1985, 988. (c) Che, C. M.; Wong, K. Y.; Poon, C. K. Inorg. Chem. 1985,24, 1797 (d) Che, C. M.; Wong, K. Y. J . Chem. SOC., Chem. Commun. 1986, 22. (e) Che, C. M.; Wong, K. Y.; Leung, W. H.; Poon, C. K. Inorg. Chem. 1986, 25, 345. (9 Che, C. M.; Lai, T. F.; Wong, K. Y . Inorg. Chem. 1987,22,2289. (9) Che, C. M.; Leung, W. H. J . Chem. SOC.,Chem. Commun. 1987. 1376. (h) Che, C. M.; Tang, W. T.; Wong, W. T.; Lai, T. F. J . Am. Chem. SOC.1989, 111. 9048. (i) Leung, W. H.; Che, C. M. J . Am. Chem. SOC.1989, 111, 8812. ti) Che, C. M.; Tang, W. T.; Wong,W. T.; Lai, T. F. J . Chem. SOC.,Dalton Trans. 1989,201 1. (k) Che, C. M.; Wong, K. Y. J . Chem. Soc., Dalton Trans. 1989, 2065. ( 3 ) Recent works by Meyer and co-worker: (a) Dobson, J. C.; Seok, W. K.; Meyer, T. J . Inorg. Chem. 1986, 25, 1514. (b) Binstead, R. A,; Meyer, T. J. J . Am. Chem. Soc. 1987, 109, 3287. (c) Gilbert, J.; Roecker, L.; Meyer, T. J . Inorg. Chem. 1987, 26, 1126. (d) Roecker, L.; Meyer, T. J. J . Am. Chem. SOC.1987. 109, 746. (e) Seok, W. K.; Dobson, J. C.; Meyer, T. J . Inorg. Chem. 1988, 27, 5. (9 Dobson, J . C.; Meyer, T.J. Inorg. Chem. 1988, 27, 3283. (4) (a) Griffith, W. P.; Pawson, D. J . Chem. SOC.,Dalton Trans. 1973, 1316. (b) Schroder, M.;Griffith, W. P. J . Chem.Soc., Chem. Commun. 1979, 68. (c) Green, G.; Griffith, W. P.; Hollinshead, D. M.; Ley, S. V.;Schroder, M. J . Chem. Soc., Perkin Trans. I 1984, 681. (d) Griffith, W. P.; Ley, S. V.; White, A. D. J . Chem. SOC.,Chem. Commun. 1987, 1625. (e) El-Hendawy, A. M.; Griffith, W. P.; Piggot, B.; Williams, D. J. J . Chem. Soc.,Dalron Trans. 1988, 1983. ( 5 ) (a) Groves, J. T.; Quinn, R. Inorg. Chem. 1984, 23, 3844. (b) Groves, J. T.;Quinn. R. J . Am. Chem. SOC.1985, 107, 5790. (c) Groves, J. T.;Ahn, K . H. Inorg. Chem. 1987, 26, 3833. ( 6 ) (a) Marmion, M. E.; Takeuchi, K. J . J . Am. Chem. SOC.1986, 108, 510. (b) Marmion. M . E.; Takeuchi, K. J . J . Am. Chem. SOC.1988, 110, 1472. ( 7 ) Lau, T. C.; Kochi, J. K. J . Chem. SOC.,Chem. Commun. 1987, 179.

0002-7863/90/ 15 12-5176$02.50/0

hydride transfer. In contrast, although a number of trans-dioxoruthenium(V1) species have been synthesized and shown to be good oxidants for quite some time,2e-J~4~5~7 relatively few mechanistic details are known.a We have thus initiated a program to study the reactions of trans-dioxoruthenium(V1) complexes with various inorganic and organic substrates in order to find out the various mechanistic pathways and the factors determining which pathway would be preferred in a given reaction. Our interest in the redox chemistry of trans-dioxoruthenium(V1) also arises as a result of previous electrochemical s t u d i e ~ ,indicating ~*~ that this class of compounds are potent electrooxidative catalysts and exhibit reversible dioxoruthenium(VI)/oxoaquoruthenium( IV) couple in aqueous solution. The reversible M=O/M-OH2 redox couple, which has not been encountered with the classical metal-oxo oxidants such as Mn0,- and CrOd2-, is commonly observed in the electrochemistry of ruthenium and osmium oxo c o m p l e x e ~ . ~ ~ * ~ J ~ We report here the results of a rate and mechanistic study on ( T M C = 1,4,8,1 1the reaction of trans-[R~~'(TMC)(0)~]~+,~~ tetramethyl- 1,4,8,11 -tetraazacyclotetradecane) with some simple outer-sphere reductants: cis- [Ru"(NH,),bpy] 2+, [ Rut'~~]~'. (NH3)+nI2' (isn = isonicotinamide), and [ R U ~ ~ ( N H ~ ) , The goal of this work is to obtain detailed kinetic, thermodynamic, and mechanistic information about the reduction of Ru(V1) to Ru(1V). The T M C system is particularly attractive for use in

TMC

(8) Che, C. M.; Wong, K. Y.; Anson, F. C. J . Electroanal. Chem. 1987, 226, 21 I . (9) Che, C . M.; Leung, W. H.; Poon, C. K. J . Chem. SOC.,Chem. Commun. 1987, 173. (IO) (a) Pipes, D. W.; Meyer, T. J. J . Am. Chem. SOC.1984, 106, 7653. (b) Che, C. M.; Cheng, W. K . J . Am. Chem. SOC.1986, 108, 4644.

0 1990 American Chemical Society

P r o t o n - Coupled Electron- Transfer Reactions

A m . Chem. S o c . , Vol. 112, No. 13, 1990 5111

this study for the following reasons: ( 1 ) t r a n r - [ R ~ " ' ( T M C ) ( 0 ) ~ ] ~ + is stable t o decomposition a n d (2) possible intermediates a n d

I I

-_

products such a s t r ~ n s - [ R u ~ ( T M C ) ( 0 ) ~a]n+d trans-[Ru'V(TMC)0(OH2)]*+have been well-characterized,2f t h u s making mechanistic study relatively easy.

Experimental Section Materials. trans- [ Ruv'(TMC)(0),] ( PF6)2,2Ccis-[ Ru"( NH,),bpy](PF6)2,11[ R U " ( N H , ) ~ ~ ~ ] ( P Fand ~ ) ~[ ,R' ~U " ( N H , ) ~ ~ S ~ ] ( Pwere F~)~'~ prepared by literature methods. Water used for kinetic experiments was distilled twice from potassium permanganate. Trifluoromethanesulfonic acid (Aldrich), monochloroacetic acid, acetic acid. and sodium hydroxide were used to maintain pH. Sodium trifluoromethanesulfonate (made by neutralizing trifluoromethanesulfonic acid with sodium hydroxide) was used to maintain ionic strength. D 2 0 (99.9 % D, Aldrich), CD,COOD (99.9 % D, Merck), and DC10, (99.8% D, Merck) were used as received. Kinetic Measurements. Routine UV-vis spectra were obtained with a Shimadzu UV-240 spectrophotometer. Kinetic measurements were made with a Hi-Tech SF-51 stopped-flow module coupled with a Hi-Tech SU-40 spectrophotometric unit. The data collection process was controlled by an Apple IIe microcomputer via an ADS-I interface unit, also from Hi-Tech. The reaction between Ru(VI) and Ru(1l) was found to occur in two stages: a rapid reduction of Ru(VI) to Ru(V) followed by a slower disproportionation of Ru(V) (see results). Under the condition that the concentration of Ru(V1) is in IO-fold excess of the Ru(I1) reductant ([Ru(VI)] = 5 X 10-5-5 X lo4 M, [Ru(lI)] = 5 X 104-5 X M), the kinetics of the first step was followed by monitoring the MLCT band of the Ru(1l) complex ( c i s - [ R ~ " ( N H , ) ~ b p y ] ~522 + , nm; [Ru"(NH3)5py]2+,475 nm; [ R U ' ~ ( N H , ) ~ ~ S478 ~ ] nm). ~ + , Pseudo-first-order rate constants, k,,, were obtained by nonlinear least-squares fit^'^,^^ of A, to time t according to the equation A, = A, + (A,, - A,) exp(-k,,t), where Aa and A, are the initial and final absorbances, respectively. Second-order rate constants, k2, were obtained from linear least-square fits of kob to [Ru(VI)]. The second stage of the reaction, i.e., disproportionation of Ru(V), was of Ru(V) in CH,CN).2d In most runs followed at 300 nm (near A,, concentrations of reactants were higher ([Ru(VI)] = 3 X 104-1 X IO-) M, [Ru(Il)] = 5 X IO-'-2 X IO4 M) than that used in studying the first stage of the reaction in order to obtain a large enough absorbance change and to ensure that the first stage is fast enough as not to interfere with the second stage. In other words, separate experiments were performed, in most cases, to follow the two stages of the reaction. The order of the disproportionation was determined by using Wilkinson's equationI4 t

nt 2

- 5 -

P

01

5

P

0.5

0

P

0.0

wavelength /

+ -K1

where p = 1 - C,/Ca, the fraction reacted, n = order of the reaction, and K = kdisCabrl.kdiris the disproportionation rate constant. C, is the initial concentration of Ru(V) ([Ru(V)],) and is proportional to (Aa- A,). C, is the concentration of Ru(V) at time t [(Ru(V)],) and is proportional to ( A , - A,). Equation I can thus be written as 200

300

500

400

v s v e l e n g h (ma)

Equation 2 was used to verify that the reaction is second order and to obtain an approximate value of kcis. Equation 2 can be rearranged to eq 3, by using n = 2 A, = a / ( I + T f ) + P (3) where CY

= A0 - A,

P = A, The value of kdis was refined by a nonlinear least-squares fit" of absorbance A, to time t according to eq 3, taking the concentration of ( I 1 ) Alvarez, V. E.; Allen, R. J.; Matsubara, T.; Ford, P. C. J . Am. Chem. Soc. 1914, 96. 7686. (12) Gaunder, R. G . ; Taube, H . Inorg. Chem. 1970, 9, 2627. ( I 3) The nonlinear least-squares programs used in this study were written in Apple Soft Basic and run on Apple Ile under the PRODOS environment. Lau, K. Ph.D. Thesis, University of Hong Kong, 1988. (14) Moore, J . W.; Pearson, R. G. Kinetics ond Mechonism; Wiley: New York, 1981; p 21.

Figure 1. (a) UV-vis spectral changes for the reaction between trans[ R U ~ ' ( T M C ) ( O ) , ] ~(0.8 + X IO4 M) and ~is-[Ru"(NH,),bpy]~+(1.6 X M) in 0.1 M CF,SO,H: Ru(VI), (-); Ru(II), and final spectrum, (--). (b) UV-vis spectral changes for the reaction between trans-[RuV(TMC)(0),]+ (- 1 X IO4 M) and 5 equiv of CF,COOH in acetonitrile. The final spectrum (-) is identical with that of an acetonitrile solution containing equal concentrations of trans-[RuV'(TMC)(0),12+ and ~ ~ U ~ ~ - [ R U ' ~ ( T M C ) O ( C H ~ C N ) ] * + . (-e-);

Ru(I1) used as [Ru(V)], and using a, P, and T as adjustable parameters. Product and Stoichiometry. The stoichiometry for the cis-[Ru"(NH3),bpyl2+ reduction of t r o n s - [ R ~ ~ ' ( T M C ) ( 0 ) , ] ~in+ 0.1 M was determined by adding various amounts of Ru(1l) to Ru(VI) (initially present in excess) and monitoring absorbance changes at 522 nm (A, of Ru(l1)). A stoichiometry of 2Ru(Il):IRu(VI) was obtained from the plot of absorbance vs [Ru(Il)]/[Ru(VI)] (Figure SI, Supplementary Material). Determination of the reaction stoichiometry under the condition that [Ru(VI)] is in 5-fold excess of [Ru(lI)] has also been atformed was tempted. The amount of tr~ns-[Ru'~(TMC)(0)(H~0)]~+ estimated by measuring the absorbance change at 420 nm where the Ru(VI) species does not have any absorption. The result obtained is similar to that described above but is less accurate because of the low t( I 5 0 mol-' dm3 cm-I) of Ru(1V) at this wavelength. The product resulting from the reduction of Ru(V1) was determined as follows. Typ-

Che et al.

5178 J. Am. Chem. Soc., Vol. 112, No. 13, 1990

-1

'

,-

Time ( S I

Figure 2. Absorbance vs time output of the nonlinear least-squares

program for the reduction of frans-[R~~'(TMC)(0)2]~+ (2 X IO-' M) by ci~-[Ru"(NH,)~bpy]~+ (2 X M) at 522 nm, T = 292.7 K, I = 0.01 M: dotted line, experimental data; solid line, theoretical curve.

1.0

:o. e

ically 1.07

IOd mol of Ru(V1) was mixed with 2.17 X IO" mol of Ru(l1) in 10 mL of 0.05 M CF,SO,H. The resulting solution was loaded onto a Sephadex Spc-25 cation-exchange resin. By eluting with 0.2 M CF,SO,H and examining the UV-vis spectra of the solution 1.1 X IOd mol of rran~-[Rul~(TMC)O(H~0)]~+ was found to be present. X

. *.a1

. O.0,

.

.

I.0'

P.0,

0.11

1.11

Figure 3. Plot of log k 2 vs v ' l / ( v ' I

0.14

o.,,

0.1'

0.20

1.11

4%'1/('1*V

+ 1) for the reduction of trans-

[RU~'(TMC)(O),]~+ by cis-[Ru"(NH3),bpy12+at 292.7 K.

Table 1. Representative Second-Order Rate Constants for the

-.

Reaction trans-[ RU~'(TMC)(O)~] 2+ + cis- [ Ru"(NH,),bpy] 2+ trans-[R U ~ ( T M C ) ( O ) ~+]ci~-[Ru"'(NH~),bpy]~+ ~+ Results T, K 1, M k2, M-' s'' In the presence of excess t r a n s - [ R ~ ~ ' ( T M C ) ( 0 ) in ~ ]aqueous ~+ ~+ rapidly, as acidic medium, cis-[ R ~ " ( N H ~ ) ~ b p yisl oxidized 279.6 0.010 5.1 f 0.3 evidenced by the disappearance of its intense MLCT band at 522 286.4 0.010 5.6 f 0.3 292.7 0.0 10 5.8 f 0.3 nm (Figure 1a). Spectrophotometric titration indicated that 2 0.025 9.1 f 0.5 mol of Ru(1I) were oxidized for each mol of Ru(V1) reduced (see 0.050 13.9 f 0.8 Experimental Section). The reduction of Ru(V1) was accom0.080 18.5 f 1.0 panied by the corresponding stoichiometric appearance of 0.100 22.6 f 1.3 t r ~ n s - [ R u ~ ~ ( T M C ) 0 ( 0 H ~The ) ] ~formulation +. of the Ru(1V) 298.0 0.010 6.1 f 0.3 product as oxo-Ru( IV) rather than dihydroxyruthenium(1V) is 0.100 24.2 f 1.4 also supported by our recent work, in which the structure of 0.010 6.6 f 0.4 304.8 t r a n s - [ R ~ ' ~ ( L ) O ( H ~ 0 )(L ] ~ += 1,12-dimethyl-3,4:9,IO-di3 10.6 0.010 7.1 f 0.4 benzo- I , 12-diaza-5,8-dioxacyclopentadecane)has been determined 320.7 0.010 7.7 f 0.4 by X-ray crystallography.2h Thus the overall reaction can be "pH = 2.2, [Ru(VI)] = 5 X 10-'-5 X IO4 M, [Ru(lI)] = 5 X represented by the equation 10-5-5 X 10" M. b l= ionic strength. trans- [ R U ~ ' ( T M C ) ( O ) ~+] ~2cis+ [ R ~ " ( N H ~ ) ~ b p y+l ~ + A, ( A , - A,) exp(-k,,t). A typical fit is shown in Figure 2. 2H+ ~~u~s-[Ru'~(TMC)O(OH,)]~+ + The pseudefirst-order rate constants were found to depend linearly 2 ~ i s - [ R u " ' ( N H ~ ) ~ b p y(4) ]~+ on [Ru(VI)]. Thus the experimentally determined rate law is -d[Ru(II)] Preliminary mixing of the reactants in the stopped-flow ap= ~ ~ [ R u ( V I ) ] [ R U ( I= I ) k,h[Ru(II)] ] (7) paratus ([Ru(VI)] N 5 X IO4 M, [Ru(II)] = 5 X M, pH dt = I , T = 298 K ) indicated that the reaction occurs in two stages. Second-order rate constants, k2, obtained from linear least-squares At 300 nm, near the, X of t r ~ n s - [ R u ~ ( T M C ) ( 0 ) (Figure ~]+ Ib) fits of koh to [Ru(VI)], are independent of pH from 1.0 to 4.0. and ~ i s - [ R u " ' ( N H ~ ) ~ b p ya] rapid ~ + first-order rise in absorbance The effect of ionic strength was investigated from 0.01 to 0.1 was observed, which is followed by a slower second-order decay. M at 292.7 K. Plot of log k2 vs dI/( 1 dI)is linear as shown At 522 nm, the A,, of ~ i s - [ R u " ( N H ~ ) ~ ( b p ya) ]clean ~ + rapid in Figure 3. The observed slope of 3.98 is close to 4.08, the first-order decay was observed which is in the same time scale theoretical value given by the Debye-Huckel equation for a bias the rise in absorbance at 300 nm. On the basis of these and molecular reaction between two dications. other evidences presented later, the two stages of the reaction can The activation parameters AH* = (4.84 f 0.20) kJ mol-' and be represented by eqs 5 and 6 in Scheme I. aS*= -( 1 17 f 20) J mol-' K-' ( I = 0.01 M) were calculated Scheme I by a least-squares treatment of log k z / T vs 1/T. Representative kinetic data are collected in Table I. The trans- [ Ruv'(TMC)(0),] 2+ cis- [ R ~ " ( N H ~ ) ~ b p y l ~ + kinetics of the reduction of Ru(V1) were also studied by using t r a n s - [ R ~ ~ ( T M C ) ( 0 ) ~+] c+i s - [ R ~ ' " ( N H ~ ) ~ b p y(5) ]~+ [ R U ~ ' ( N H ~ ) ~and ~ S[~R] L~ I+' ~ ( N H ~ )as~ reductants. ~~]~+ Second-order rate constants at 25.0 O C are 3.0 X lo6 M-' s-' ( I = 2trans-[RuV(TMC)(0),]+ + 2H+ X IO6 M-' s-I ( I = 0.01 M),I5 respectively. t r ~ n s - [ R ~ ~ ' ( T M C ) ( 0 )+ 2 ]~~~+u ~ s - [ R u ' ~ ( T M C ) O ( O H ~ 0.1 ) ~Kinetics ~M) + andof3.4Disproportionation of Dioxoruthenium( V). The (6) kinetics were followed at 300 nm corresponding to the disappearance of Ru(V) (which is produced in situ from Ru(V1) and According to this scheme, the first stage of the reaction is the Ru(11)). Visual inspection suggested that the decay curve is a one-electron reduction of dioxoruthenium(V1) to dioxohyperbola rather than an exponential function. A least-squares ruthenium(V), while the second stage involves the disproportreatment of t / p (p = fraction of unreacted Ru(V), t = time) tionation of dioxoruthenium(V) to give dioxoruthenium(V1) and against t yielded a straight line with slope = I , confirming a oxoruthenium(1V). The kinetics of these two steps are described second-order kinetics (see Experimental Section). Thus the exseparately below. perimentally determined rate law is Kinetics of the Reduction of Dioxoruthenium(V1) to Dioxoruthenium(V). The kinetics were followed by measuring the disappearance of c i s - [ R ~ " ( N H ~ ) , b p y ] ~at+ 522 nm. In the presence of at least a IO-fold excess of tr~ns-[Ru~'(TMC)(0)~]~+, clean pseudo-first-order kinetics were observed. The pseudoSecond-order rate constants, kdis, were obtained by nonlinear first-order rate constants, kobs, were obtained by nonlinear least-squares fits of absorbance A, to time t according to eq 3. A typical fit is shown in Figure 4. The pH dependence of the least-square fits of A, to time t according to the equation A, =

-

+

+

-

+

-

J . Am. Chem. Soc., Vol. 112, No. 13, 1990 5179

Proton- Coupled Electron- Transfer Reactions

-

-0

cmhW-

mq-

nW-

Time

( 5 )

-

Figure 4. Typical absorbance vs time output of the nonlinear leastsquares program for the disproportionation of f r a n s - [ R ~ ~ ( T M C ) ( 0 ) ~ ] + (5 X IO4 M) at 300 nm, T = 299 K, pH = 1.0, and I = 0.10 M: dotted line, experimental data; solid line, theoretical curve.

"0

m-

mhID'Ill

'i

Table 11. pH Dependence of the Disproportionation of ~ r a n s - [ R u ~ ( T M C ) ( 0at ) ~ T] += 299 K and Ionic Strength ( I ) =

0.10 M I 04[Ru(N H3),bpy2+], IO3[ Ru(TMC)O,t'], M M 2.07 1.10 0.72 0.96 0.77 0.51 0.35 0.60 0.55 0.57

1.02 0.6 1 0.38 0.50 0.4 I 0.30 0.30 0.33 0.30 0.35

@-

.c-

p.a m0

X

DH 5.50 5.00 4.50 4.00 3.40 3.00 2.50 2.00 1.50 1.00

1 O-) k,,,

N-

M-l s-I

6.2 f 0.3 12.2 f 0.6 58 f 4 188 f IO 529 f 30 540 f 30 720 f 40 341 f 18 96 f 6 39 f 2

rate constant was studied in the range of 1-5.5 at 299 K and at an ionic strength of 0.1 M, and the results are summarized in Table 11. A plot of kdisvs pH gives a bell-shaped curve, as shown in Figure 5. These results are consistent with the following mechanism (Scheme 11).

-0 c

cm-

h-

InIn-

-cnPI-

"0 - I

I

0.00

I

I

.20

,

3-60

2.40

4.80

i

6.00

PH Figure 5. Plot of kdisvs pH for the disproportionation of frans-[RuV(TMC)(O),]+ at 299 K and I = 0.10 M.

Scheme I1 K

trans-[RuV(TMC)(0)21++ H+ 2 t r a n s - [ R ~ ~ ( T M C ) 0 ( 0 H ) ](9) ~+

-

+ +

ke

trans- [ RuV(TMC)O(O H ) ] 2+ trans- [RuV(TMC)(0)4+ trans-[ R U ~ ' ( T M C ) ( O ) ~ ] ~trans-[Rul"(TMC)O(OH)]+ + (10)

+

fast

t r a n s - [ R ~ ' ~ ( T M C ) 0 ( 0 H ) ] + H+ t r a n s - [ R ~ ' ~ ( T M C ) 0 ( H ~ 0 (1 ) ] 1) ~+ On the basis of this reaction scheme, it can readily be shown that kdis

=

2kSIp [ H+l (1 + K,[H+H2

(12)

A nonlinear least-squares analysisI3 of the data in terms of eq 12 leads to k, = (2.72 f 0.34) X IO6 M-' s-I and Kp = 615 f 50 M-' at 299 K. The temperature dependences of k, and K are summarized in Table Ill. The activation parameters AH* = (18.7 f 2.9) kl mo1-I and AS' = -(59.0 f 10.0) J mol-' K-' were calculated from a least squares treatment of In (k,/T) vs 1/T. Several runs were also made by using [ R u ' ~ ( N H , ) , ~ s ~as ]~+ reductant to generate [RuV(TMC)(0),]+. A 299 K and I = 0.1 M, kdis was found to be (37.9 f 1.8) X IO3 and (6.1 f 0.1) X lo3 MQ s-' at pH 1 .O and 5.5, respectively. The values of k, = (2.7 f 0.4) X IO6 M-l/s-l and K p = 71 1 f 114 are in satisfactory agreement with the values k, = (2.7 f O S ) X IO6 M-' s-l a nd K , = 61 5 f 5 0 obtained by using c i ~ - [ R u " ( N H ~ ) ~ b p as y ] re~+ ductant.

Discussion Reduction of Ru(V1) to Ru(V). The visible spectrum of trans-[ R U ~ ' ( T M C ) ( O ) ~remains ] ~ + unchanged in acid solutions

up to 8 M, thus implying that the pK, of trans-[Ru"'(TMC)O(OH)I3+ is 7, the single two-electron wave begins to split into two effect on the equilibrium constant K , is similar to that found in waves which are assigned to half reactions (1 6) and (1 7). other oxo acid systems.23 E I j z = 0.56 V Self-Exchange Rate of tran~-[Ru(TMC)0(0H)]~+/'+.Asr r ~ n s - [ R u ~ ~ ( T M C ) ( O+) ~e l ~ + suming a simple outer-sphere mechanism, the self-exchange rate trans- [RuV(TMC)(O)J+ ( 16) of tr~ns-[Ru(TMC)0(0H)]~+/+ can be estimated by using a value of 1 X IO5 M-l s-I discussed above for the self-exchange rate of E,,2 10.43 V t r a n s - [ R ~ ( T M C ) 0 ~ ] ~ + /The ' + . E , for the trans-[RuV(TMC)at pH = 10.0 r r a n s - [ R ~ ~ ( T M C ) ( 0 )+ ~ ] H+ + +e O(OH)]2+/trans-[Rutv(TMC)O(6H)]+ couple is shown to be +0.80 V ( I = 0.1 M) using eqs 9 and 17. By entering these data ~~u~I.Y-[Ru'~(TMC)O(OH)]+ (17) into the Marcus-Cross relation, the self-exchange rate of The driving force for the disproportionation reaction (eq 6) can tr~ns-[Ru(TMC)0(0H)]~+/~+ is estimated to be around 5 X IO3 be obtained from eqs 15 and 16; at 298 K, pH = 1.0 and I = 0.1 M-' s-,. Like that of Ru(VI)/Ru(V), electron exchange between M, AG is equal to -15.7 kcal mol-' (&is,, = 3.2 X 10"). This Ru(V) [(dXJ2(d,,)' config~ration]~' involves orbitals of n symdriving force decreases with increasing pH until at pH = 7.3, AG metry. The slower exchange rate of the Ru(V)/Ru(IV) couple = 0. is probably due to a larger Frank-Condon barrier associated with In nonaqueous medium t r a n ~ - [ R u ~ ( T M C ) ( O )is~ lmuch + more the conversion of a RuV=OH2' to RuIV-OH bond upon reduction. stable and can be readily isolated by electrochemical reduction Conclusion and Comments of trans- [ R U " ~ ( T M C ) ( O ) ~ ]Study + . ~ ~ of its disproportionation* establishes the stoichiometry shown in eq 6. Direct kinetics study [ R U ~ ' ( T M C ) ( O ) , ] ~is+unique among metal-oxo systems in using the Ru(V) complexZoproved to be difficult since the reaction two respects. First, electrochemically it exhibits a well-defined, is rather rapid even at high pH. The use of 1-e outer-sphere Ru(1I) complexes, however, provides a convenient means of generating (20)The rrans-[R~~(L)(0)~](ClO,)(L = 1, I 2-dimethyl-3,4:9,10-diRu(V) in situ from Ru(V1). The validity of this method is supbenzc- 1,12-diaza-5,8-dioxacyclopentadecane) complex has recently been isolated and characterized by elemental analyses, IR spectroscopy, and magnetic ported by its clean bimolecular kinetics and the independence of susceptibility measurements. This Ru(V) complex is very stable in aqueous the subsequent disproportionation reaction rates to the reducing solutions having pH > 8. Preliminary kinetic studies showed that the reaction agent used. between trans-[Ru"(L)(O similar to that of the disproportionation of Irons-[Ru $ (TMC)(O),]+ 1 and H+ is very described in this work. Tang, W. From the pH dependence of kdisp, the disproportionation is shown to proceed via protonation of Ru(V) prior to electron T. Ph.D. Thesis, University of Hong Kong, 1989. (21) The RuV-OH bond should have partial double bond character and transfer. The pk, of trans-[RuV(TMC)O(OH)]+is found to be hence is expected to be inert to substitution. 2.8 ( K P= 615 at 26.6 "C); thus on going from dioxoruthenium(V1) (22) Weaver, M. J.; Nettles, S.M. Inorg. Chem. 1980, 19, 1641. to dioxoruthenium(V), the pk, of the protonated species increases (23) Albery, W. J. In Proton TronsJer Reactions; Caldin, E. F., Gold, V., by over 4 units. Ed.; London: Chapman and Hall, 1975; p 263.

+

+

+

.

+

-

J . Am. Chem. SOC.1990,112, 5181-5186 reversible Ru(VI)/Ru(IV) couple in acidic solutions as well as reversible Ru(VI)/Ru(V) and Ru(V)/Ru(IV) couples in basic solutions. Thus accurate redox potentials for these couples can be obtained. Second, the stability and rigidity imposed by the T M C ligand ensures that the complex and its reduced products persist in solution, thus giving rise to clean behavior. These factors together account for the detailed kinetics, thermodynamic, and mechanistic information that can be obtained in this study. Such information, apart from improving our understanding of highvalent ruthenium-oxo chemistry, would also help in the designing of efficient and selective ruthenium oxidants. Electrochemical studies on other trans-dioxoruthenium(V1) species such as t r a n s - [ R ~ ~ ~ ( b p y )indicated ~ O ~ ] ~ +that ~ ~ ~their behavior is very similar to that of t r a n s - [ R ~ ~ ~ ( T M C ) 0 It ~]~+. seems that the disproportionation mechanism that we established in this study is a general one for the trans-dioxoruthenium(V) species. Moreover, this work implies that other d3 metal-oxo systems chould also be unstable, and disproportionation of an oxo-manganese( IV) porphyrin system has been postulated re~ently.~~

5181

The observed large self-exchange rates of the Ru(VI)/(V) and Ru(V)/(IV) couples and rate of disproportionation of Ru(V) suggest that the redox interconversion between Ru(V1) and Ru(IV) is rapid. This explains the reversibility of the two-electron Ru(VI)/( IV) couple observed in the electrochemistry of transdioxoruthenium(V1).

Acknowledgment. We thank the Croucher Foundation, the University and Polytechnic Granting Committee, and the University of Hong Kong for financial support. Registry No. t r a n s - [ R ~ ~ ' ( T M C ) ( 0 ) 95978-17-9; ~]~+, [RuII(NH3),bpyJ2+, 54194-87-5; [ R U " ( N H , ) ~ ~ ~19471-53-5; ~]~+, [Ru"(NH3)5py]2+,21 360-09-8; trans-[Ru"(TMC)(O),]+, 103056-17-3.

Supplementary Material Available: Figure S 1 of the spectrophotometric titration of trans- [ R U ~ ~ ( T M C ) ( O )with ~ ] ~ +cis[ R ~ " ( N H ~ ) ~ b p yatl ~522 + nm ( 1 page). Ordering information is given on any current masthead page. (24) Groves, J. T.; Stern, M. K. J. Am. Chem. SOC.1987, 109, 3812.

Polynuclear ((Dipheny1phosphino)methyl)phenylarsine Bridged Complexes of Gold(1). Bent Chains of Gold(1) and a Role for Au( I)-Au( I) Interactions in Guiding a Reaction Alan L. Balch,* Ella Y. Fung, and Marilyn M. Olmstead Contribution from the Department of Chemistry, University of California, Davis, California 95616. Received February 5, 1990

Abstract: The role of weak Au(1)-Au(1) interactions in determining the structure and reactivity of a set of new, ligand-bridged complexes is described. Addition of 2 equivs of Me2SAuC1to dpma [dpma is bis( (diphenylphosphino)methyl)phenylarsine] yields AuzC12(p-dpma)which has its two gold ions widely separated (7.01 1 (1) A) but which packs about a center of symmetry so that there are two close (3.141 ( 1 ) A) Au-Au contacts between molecules. NMR spectra indicate that this molecule self associates at low temperature in solution also. Treatment of Au2C12(pdpma)with further MGAuCI produces Au3CI,(p-dpma) which has a bent Au3 chain (Au-Au distances 3.131 ( I ) , 3.138 ( 1 ) A; Au-Au-Au angle, 110.9 ( 1 ) O ) . The reaction of Au2Cl2(p-dpma)with ammonium hexafluorophosphate or thallium nitrate yields [Au,Cl,(p-dpma),] [PF,], or [Au4C12(pdpma),][N0J2, respectively. These have bent Au, chains (Au-Au distances 2.965 ( I ) , 3.096 ( I ) ; Au-Au-Au angle, 88.0 (I)' in the hexafluorophosphatesalt) with terminal P-Au-CI groups and the dpma ligands aligned so that phosphorus is trans to arsenic on the central two gold ions. Comparisons of the solid state, associated form of Au2C12(p-dpma)and [Au4CI2(pdpma),l2+ suggest that the initial stage of formation of the Au4 chain involves association of the Au,Cl,(p-dpma) units through Au-Au interactions, despite the presence of vacant coordination sites on the gold ions and lone pairs on the arsenic atoms. The role of empty p acceptor orbitals on gold favoring the bent chains is developed.

Recent work on molecular recognition has focused attention on bonding interactions between molecules which are generally weaker than normal covalent bonds.' Among those are hydrogen bonds, electron donor/acceptor interactons and van der Waal's forces. The attractive interaction between Au(1) centers is another such interaction which, while weak, has important effects in determining molecular conformations and packing within gold(1) complexes.2 It has been suggested3 that Au/Au contacts shorter than 3.5 A are otherwise unexpected for such a large ion and it has been known that short Au-Au contacts (as short as 2.776 A)4 are frequently seen in the solid state.3 Only recently, however, ~

~~~

( I ) Lehn, J.-M. Angew. Chem., In?. Ed. Engl. 1988, 27,89. Cram, D. J. Angew. Chem., In?. Ed. Engl. 1986,25, 1039. Host Guest Chemistry; Parts 1-111, Vbgtle, F., Weber, E., Eds.; Top Curr. 1984, 1982, 1983, 121, 101, 98. (2) Schmidbaur, H . Angew. Chem., Int. Ed. Engl. 1976, IS, 728. Schmidbaur, H.; Dash, K.C. Adu. Inorg. Chem. Radiochem. 1982, 25, 239. Puddephatt, R . J . The Chemistry of Gold Elsevier: Amsterdam, 1978. (3) Jones, P. G.Gold Bull. 1981, 14, 102; 1981, 14, 159; 1983, 16, 114; 1986, 19, 46. (4) Inoguchi, Y.;Milewski-Mahrla. B.; Schmidbaur, H.Chem. Ber. 1982, 115, 3085.

0002-7863/90/ 15 12-518 I$02.50/0

has it been possible to estimate the energy associated with such bonds. Schmidbaur and co-workers have estimated that a Au-Au interaction (with a bond distance of 3.000 A) has a bond energy of 7-8 kcal m01-I.~ Most of the observed Au-Au interactons in Au(1) compounds involve pairs of gold ions: but a few examples of extended Au-Au interactions involving three or more gold ions are known.4 Here we present new structural data that show the ability of Au-Au interactions to orient molecules in the solid state in a configuration which is suggestiveof an intermediate that is suitable for a reaction observed in solution. These interactions result in the formation of bent groups of three or four gold(1) ions. For this we used a flexible ligand, bis((dipheny1phosphino)methyl)phenylarsine (dpma), that places minimal constraints on the gold-gold separations. Previous work with this ligand has focused on the use ( 5 ) Schmidbaur, H.; Graf, W.; Miiller, G.Angew. Chem., Int. Ed. Engl. 1988, 27, 417.

(6) Throughout, our discussion is restricted to dlo Au(1) species, and we specifically exclude more reduced gold clusters from consideration. For emphasis, structural drawings use solid lines for the gold-gold interactions.

0 1990 American Chemical Society