3430
J. Phys. Chem. 1982, 86, 3436-3439
Cation-Assisted Ligand Photosubstitution in Transition-Metal Complexes. Photoreactions of Ru(bpy):+ with Ag+ in Acetonitrile T. K. Foreman, J. B. S. Bonllha, and D. 0. WhHten' hpadment of Chemistry, UnhmW of North Cardlna, Chapel HIII, North Carolina 27514 (Recehed:March I , 1982; In Fhal Form: May 6, 1982)
Irradiation of R u ( b p ~ ) in ~ ~the + presence of Ag+ in acetonitrile leads to the photosubstitution product Ru(bpy)Z(CH3CN)Z2+. The reaction does not occur in the absence of Ag+ or acetonitrile; although Ag+ quenches the luminescent MLCT state of Ru(bpy)?+, a kinetic analysis indicates the photosubstitution does not originate from this excited state. The most reasonable mechanism for the process involves decay of the MLCT state via a d-d excited state to a ligand-labilized intermediate which is intercepted by Ag+ in a process which assists the substitution by removal of a bpy ligand. This mechanism is thus parallel to anion-induced substitution reactions which evidently proceed via competitive anion-ligand capture of the same or similar intermediates.
The complex tris(2,2'-bipyridine)r~thenium(II)~+and related derivatives have been widely used as sensitizers or light-absorbing catalysts in a number of diverse reactions due to their light-absorbing properties and apparent ph~tostability.'~~ Nonetheless, it has been demonstrated that Ru(bpy);+ and several structurally similar complexes can undergo a number of irreversible photoreaction^,"'^ especially several involving ligand photosubstitution as well as a photoracemization process when optically active Ru( b ~ y )is~ irradiated ~+ in aqueous s01ution.l~ The ligand substitution reactions have been observed in both aqueous and nonaqueous solvents. In most cmes examined to date these reactions have involved the substitution of anions for a bipyridine ligand as indicated in eq 1. Several studies R ~ ( b p y ) , ~++2X- -!!L Ru(bpy),X2 + bpy
(1)
have included attempts to determine the mechanism or mechanisms for these reactions. The apparent insensitivity of luminescent yields from the prominent metal-ligand charge transfer (MLCT) excited state on potential ligands or anions suggest that this state is not likely the direct reactant in these processes." Siece a number of studies with structurally similar complexes suggest that nonemissive metal-localized d-d excited states lie very close in energy to the luminescent MLCT states,l5,l6it has been suggested that these s t a h may be precursors for the ligand (1)Sutin, N.J. Photochem. 1979, 10,19. (2)Whitten, D. G. Acc. Chem. Res. 1980, 13, 83. (3)Ballardini, R.;Varani, G.; Indelli, M. T.; Scandola, F.; Balzani, V. J. Am. Chem. SOC.1978, 100, 7219. (4)Van Houten, J.; Watts, R. J. J. Am. Chem. SOC.1976, 98,4853. (5)Watts, R.J.; Harrington, J. S.; Van Houten, J. Adu. Chem. Ser. 1978, No. 168, 57. (6) Van Houten, 3.; Watts, R. J. Imrg. Chem. 1978, 17, 3381. (7)Durham, B.; Wilson, S. R.; Hodgson, D. J.; Meyer, T. J. J . Am. Chem. SOC.1980,102,600. (8) Hoggard, P. E.; Porter, G. B. J. Am. Chem. SOC.1978,100, 1457. (9)Wallace, K.M.; Hoggard, P. E. Znorg. Chem. 1980,19,2141. (10)Wallace, K. M.; Hoggard, P. E. Znorg. Chem. 1979, 18, 2934. (11)Durham, B.; Caspar, J. V.; Nagle, J. K.; Meyer, T. J. J.Am. Chem. SOC.In press. We thank F'rofeeaor Meyer for providing UB with a preprint of this manuscript. (12)Gleria, M.; Minto, F.; Beggiato, G.; Bortulus, D. J . Chem. Soc., Chem. Common. 1978, 285. (13)Durham, B.; Walsh, J. L.; Carter, C. C.; Meyer, T. J. Znorg. Chem. 1980, 19,860. (14)Porter, G. B.; Sparks, R. H. J.Photochem. 1980,13, 123. (15)Malouf, G.;Ford, P. C. J. Am. Chem. SOC.1977,99, 7213. (16)Figard, J. E.;Petersen, J. D. Znorg. Chem. 1978, 17, 1059. 0022-3654/82/2086-3436$01.25/0
substitution reactions.6Jl Since d-d states are frequently reactive toward ligand loss, it is not unreasonable to expect that these excited states could labilize a bipyridine ligand leading to a five-coordinate intermediate (eq 2). Indeed, Ru(bpy),2+' (d-d)
+
R~(bpy)2bpy'~+
(2)
Porter and Sparks14have suggested that such a process can explain the photoracemization reaction and Meyer and co-workers have shown that a monodentate bipyridine intermediate Ru(bpy)2(bpy)(NCS)+occurs in substitution by SCN- (eq 1, X = SCN-).13 We have recently investigated a number of photoreactions involving various ruthenium(I1) complexes and cations such as Ag+."-19 Among the prominent photoreactions of Ru(bpy)QB+with Ag+ in both aqueous and nonaqueous solutions is electron-transfer quenching of the MLCT excited state. Although this process has relatively low rate constants, the electron-transfer quenching process is relatively efficient. However, there is normally no net photoreaction via electron-transfer quenching in this case since the ensuing back-electron-transfer reaction is highly efficient.17J9 In the course of our investigations with Ru(bpy)?+ and Ag+ in acetonitrile we have observed a novel photosubstitution reaction which involves both the solvent and Ag+. In the present paper we report results of this cation-induced photosubstitution. These results, taken together with results of the previous studies with anions, provide a clearer picture of the mechanisms for ligand substitution and the rich array of intermediates and pathways possible following photoexcitation of transition-metal complexes. Experimental Section
Materials. For most of these studies the Ru(bpy)?+ was the hexafluorophosphate salt, prepared by standard methods from RuC13.2H20 (Alfa), 2,2'-bipyridine, and NH,PF6 in ethanol. The product was purified by chromatography over alumina (Fisher) with 50:50 (v/v) acetonitri1e:benzene as solvent and its purity confirmed by satisfactory elemental analysis. An authentic sample of the photosubstitution product Ru(bpy),(CH3CN),(PF& (17)Kinnaird, M. G.;Whitten, D. G. Chem. Phys. Lett. In press. (18)Chandrasekaran, K.; Foreman, T. K.; Whitten, D. G. Nouu. J . Chim. 1981.5. 275. (19)Fore&, T. K.; Giannotti, C.; Whitten, D. G. J.Am. Chem. SOC. 1980, 102, 1170.
0 1982 American Chemical Society
The Journal of Physical Chemistty, Vol. 86, No. 17, 1982 3437
Photosubstkution in Transklon-Metal Complexes
0 495
I
0 130
/---
1
0 165
I
430
450
500
550
Flgure 1. Spectral changes produced when Ru(bpy):+ is irradiated in 0.50 M AgCiOI in freeze-pump-thaw degassed acetonitrile with a 450-W mercury lamp at 436 nm. Abscissa is wavelength in nm, ordinate is absorbance (-) lnitlal spectrum before irradiation, (- - -1 0.5-h irradiation, ( - e - a ) 4841 irradiation.
TABLE I: Quantum Yield Data for the Disappearance of Ru(bpy)32+Irradiated in the Presence of A@ in Acetonitrilea
7.00 1.005 14.28 7.03 0.05 20.0 5.58 5.62 0.08 12.5 1.008 17.78 5.08 5.03 0.10 10.0 1.010 19.68 4.81 4.75 1.012 20.81 0.12 8.33 4.56 4.63 0.14 7.14 1.014 21.61 4.55 4.48 0.16 6.25 1.016 21.99 4.16 1.018 23.59 4.24 0.18 5.55 4.14 5.00 1.020 23.70 4.22 0.20 3.95 4.04 0.25 4.00 1.025 24.73 3.76 3.87 0.30 3.33 1.030 25.84 3.83 3.65 0.50 2.00 1.050 26.08 3.36 3.63 0.80 1.25 1.080 27.54 M, dry degassed acetonitrile. a (Ru(bpy)32+) = 5.0X Decrease in absorbCalculated from Ksv = 0.1M - I . ance at 452 nm, monitored light intensity was 10.67 X lo-' mol photon/min at 436 nm. 1/@lr = [@r(@O/@)]-'.
["*'
Flgure 2. Doublereciprocal plot of l / 4 r (c$r is the quantum yield for Ru(bpy),(CH3CN):+ formation) vs. l/[Ag+].
occurs with high chemical yield (near 100%; isosbestic points are maintained throughout the reaction) but with very low quantum efficiency (Table I). The quantum yield increases monotonically with increase in [Ag+]reaching a maximum near $C = 0.0003 for 0.8 M Ag+. Comparison of the product formed in this process to an authentic sample confirmed it to be the photosubstitution product R u ( b ~ y ) , ( c H ~ c Nformed ) ~ ~ + via photolysis as indicated in eq 3. The reaction does not occur at measurable rates
+
Ru(bpy)g2+ 2CH3CN
hv
Ag+
R ~ ( ~ P Y ) ~ ( C H ~+Cbpy N ) ~(3) ~+
when Ru(bpy),2+is irradiated in acetonitrile without Ag+. If Ru(bpy),2+ and Ag+ (0.6 M) are kept in acetonitrile in the dark for 20 days there is a slow conversion (90% The irradiating wavelength for the photosubstitution reof the R ~ ( b p y ) ~formed, ~ + * we found that no substitution action was the 436-nm line of a medium-pressure mercury product could be detected. This indicates that Ru(bpy)QB+ lamp mounted in a merry-go-round apparatus. is probably not an intermediate in reaction 3. Results Discussion As previously reported, Ag+ quenches the luminescence The observation that Ag+ quenches the MLCT excited of Ru(bpy)gP+in several s~lvents;'*~'~ the rate constant for state of Ru(bpy)gP+and that a concurrent photosubstituthe quenching in acetonitrile is 1.1 X lo5 M-l s-l. In adtion reaction mediated by Ag+ occurs could be most simply dition to the luminescence quenching there is a slow explained by a mechanism described by eq 4-7. Considchange in the absorption spectrum (Figure 1)which involves a shift in the k- from 460 to 425 nm. The reaction Ru(bpy)2+-!% R ~ ( b p y ) ~(MLCT) ~+' (4) (20) Brown, G. M.; Callahan, R. W.; Meyer, T. J. Znorg. Chem. 1976, 14, 1915.
Ru(bpy)32+*(MLCT)
+ Ag+
ak,
R ~ ( b p y ) , ~++Ago (5)
3438
The Journal of physical Chemistry, Vol. 86, No. 17, 1982
Foreman et al.
Bk,
Ru(~PY)~’+ (MLCT) + Ag+ CH,CNR ~ ( ~ P Y ) ~ ( C H ~(6) CN)~~+
R~(bpy),~+’ (MLCT)
k,j=l/r
R ~ ( b p y ) , ~ + (7)
eration of this mechanism leads to the prediction of the usual linear Stern-Volmer relationship for quenching of the MLCT luminescence with a slope/intercept value kSv = k,r. The quantum yield of the photosubstition process for this mechanism is given by eq 8. From eq 8 a linear
relationship between l/4pgand l/[Ag+] can be predicted (eq 9) with an intercept/slope ratio also equal to kq7. In
-1- -4PS
l
1
+
~MLC&,T[A~+]
4MLCTP
(9)
fact good linear plots for both processes are obtained; the luminescence quenching plot gives Ksv = 0.1 M-’ l9 while the reciprocal plot corresponding to eq 9 (Figure 2) gives an intercept/slope ratio of 18 M-l. The difference of greater than two orders of magnitude between these values is clearly inconsistent with the MLCT being the key intermediate for the observed substitution and suggests that a more complex mechanism must be operative. Since experiments with methyl viologen appear to rule out the involvement of Ru(bpy)t+ in the photosubstitution it appears most reasonable that intermediates or excited states formed subsequent to the MLCT state must be the immediate precursors for photosubstitution. If this is true, a slightly modified scheme consisting of eq 4 and 10-14 can be developed. This scheme leads to an expression for R ~ ( b p y ) -~!!L ~ +R ~ ( b p y ) ~(MLCT) ~+* R ~ ( b p y ) ~(MLCT) ~+* + Ag+
(4)
5 Ru(bpy)l+ + Ago (10)
R ~ ( b p y ) ~(MLCT) ~ + * -% R~(bpy),~+’ (d-d)
kI
I (11)
R ~ ( b p y ) ~(MLCT) ~ + * -% R ~ ( b p y ) , ~ + (12) R ~ ( b p y ) ~(d-d) ~+* I
kh
R~(bpy),~+
Ru(~PY)~~+
(13) (14)
I
I
IO
I5
Flgue 3. “Corrected”double-reciprocal plot. For the definition of see text.
4PS
4 ’,
is predicted which has an intercept/slope ratio of kr/k 2. Such a plot (Figure 3) shows good linearity; the intercept/slope is 17 M-l and the limiting quantum yield exIt is worthwhile to note that tracted is 4IPs = 3.9 X the plots in Figures 2 and 3 do not yield dramatically different values since the correction applied via eq 16 is not very large, even at the highest [Ag+] employed. At this point it is useful to compare the present results with other studies of Ru(bpy);+ photosubstitution in order to derive a specific mechanism. Meyer and co-workers as well as others have suggested that an important channel for decay of the MLCT state is via a nonemissive d-d ~ t a t e . ~It” has recently been estimated that this state is populated with efficiencies ranging from 0.2 in water to 0.57 in methylene chloride.” Although precise data are not available, it would seem reasonable that a value near that for methylene chloride should apply for acetonitrile.21 Thus if we compare our extrapolated “limiting” quantum yield of 3.9 X lo4 with the much higher anticipated efficiency of population of the d-d state of ca. 0.5, it appears that the reaction precursor for the Ag+-inducedsubstitution is not the d-d excited state but rather an intermediate formed subsequent to it in a reasonably inefficient process. A reasonable alternative is that the d-d state is formed from the MLCT in high efficiency and subsequently decays with lower efficiency to form either a pentacoordinate intermediate Ru(bpy)2bpy’in which one pyridine is free or the analogous acetonitrile complex (eq 18 and 19).
2R ~ ( b p y ) ~ b p y ’ (18) R ~ ( b p y ) , ~ (d-d) +* 5 CH&N R ~ ( ~ P Y ) ~ ~ P ~ ’ ( C (19) H~CN) R ~ ( b p y ) , ~(d-d) +*
the photosubstitution efficiency given by eq 16, where the
23 ll[A9*,
2+
Either of these species could then decay back to groundstate starting material or react with Ag+ to permanently lose a bipyridine ligand (eq 20 and 21). The observation
=
2+ ~+PAg+ R~(~PY)~ Y ’ CH,CN-
(16)
term # J ~is included to allow for inefficiency in reaching the reactive state (I) following decay of the MLCT state via = k I / [ k I kI3]. If the measured quantum reaction 11 [c#J~ yield is corrected for quenching of the MLCT state by eq 17, a linear relationship between 1/4’pS and l/[Ag+]
+
4’PS
=
R u ( ~ P Y ) ~ ( C H ~ C+NA) ~d ~ b y+ ) + (20) R u ( ~ P Y ) ~ ~ P Y ’ ( C H ~+CAg+ N ) ~CH,CN+ R ~ ( ~ P Y ) ~ ( C H &+ NAg(bpy)+ )~~+ (21) of a good linear relationship between 1/VPand l/[Ag+] suggests that the silver is involved in only one product-
~OLUM
4PS
~LUM
(21) Caspar, J. V.; Meyer, T. J., unpublished results.
J. Phys. Chem. 1982,86,3439-3446
determining step subsequent to the MLCT state. The overall low efficiency of the reaction probably occurs in the formation of the key intermediate (&) rather than in the silver-assisted product-forming step since it appears unlikely that a Ag+-enhanced decay process leads back to the starting material from either of the intermediates reaction in eq 20/ or 21. The kinetic treatment indicates that the capture/decay ratio k,/k$ = 17 M-' is relatively favorable; however, since it gives only a ratio, it is not possible to determine either the lifetime of the intermediate or the rate of capture from these experiments. Jonah, Matheson and Meiselsn have performed pulse radiolysis studies on Ru(bpy),3+ in aqueous solution where the key reaction observed is reduction of the trication by ea;. In these studies formation of Ru(bpy)t+ in the ground state as well as the MLCT state was observed as well as an additional process giving rise somewhat more slowly (k = lo4 s-1)22to R ~ ( b p y ) , ~by+ an as yet unidentified intermediate. If we assume, as has been suggested," that this intermediate is R~(bpy),bpy'(H~O)~+ and that this is the species trapped by Ag+ via eq 20 we can estimate that k, = km = 2 X lo5M-' s-l. However, it could be argued that the use of a decay constant determined for aqueous solution is not appropriate since no Ag+-assisted photosubstitution occurs in water. (22) Jonah, C. D.;Matheson, M. S.; Meisels, D. J. Am. Chem. SOC. 1978,100, 1449.
Kinetics of the Reaction 0
+ HOP
-
OH
3439
A perhaps more plausible explanation for the observed results is that the Ag+-assisted photosubstition involves reaction of the partially solvated intermediate R ~ ( b p y ) ~ (bpy')(CH,CN) (eq 21). Acetonitrile should be a reasonable ligand for the labilized metal center and it is likely that the aforementioned species might have an appreciable lifetime such that even relatively slow interception by Ag+ could compete with decay. The metal-cation-assisted photosubstitution described here is a potentially useful process which could prove to be quite general. Thus, even though the present process occurs with a low quantum efficiency its chemical yield is quite high. We plan to extend these studies to other solvent-metal ion systems aa well as other transition-metal complexes.
Acknowledgment. We are grateful to the National Science Foundation (Grant No. CHE7823126) for support of this research. Joiio B. S. Bonilha (Visiting Scholar from Department of Chemistry, Faculdade de Filosofia Ciencias e Letras de Ribeirlo Preto da Universidade de Sao Paulo, 14100-RibeirZoPreto S. P.-Brasil) acknowledges research fellowship support from the Fundaqao de Amparo 5 Pesquisa do Estado de Sao Paulo (FAPESP 16-quimica 80/ 0292) and research fellowship support from the Conselho Nacional de Desenvolvimento Cientifico e Tecnologico (CNPq 200.555-81-QU). We thank Professor T. J. Meyer for helpful discussions.
+ O2 from 229 to 372 K
Leon F. Keyser Molecular physlcs and Chemism Section, Jet Propulsion Laboratory, Califomla Institute of Techndogy. Pasadena, Callfornia 9 1109 (Received: February 16, 1982; I n Final Form: April 23, 1982)
The discharge-flow resonance fluorescence technique has been used to obtain absolute rate data for the 0 + H 0 2 reaction from 229 to 372 K at a total pressure of 1torr. Pseudo-first-order conditions were used with HOz concentrations in large excess over initial atomic oxygen in order to minimize interference from secondary reactions. The results are independent of the method used to generate H 0 2 and atomic oxygen. A t 299 K, the result is (6.1 f 0.4)X lo-'' cm3molecule-' s-'. The temperature dependence expressed in Arrhenius form is (3.1 i 0.3) X lo-" exp[(+200 i 28)/TI. The error limits given are twice the standard deviation; overall experimental error is estimated to be *25%.
centrations of OH and HOP Reaction 1 is also an imIntroduction The reaction of atomic oxygen with the ~ y ~ r o p e r o x y ~ portant chain-breaking step in combustion chemistry. There have been several earlier determinations of the radical (eq 1)plays an important role in the chemistry of rate constant, kl, at 298 K with values ranging from 2.5 0 + HO2'OH +02 (1) x 10-1' to 7 x 10-11 cm3 molecule-' s-1.5-7 NO Drevious study of the temperature dependence of kl has been rethe mesosphere and upper ~tratosphere.'-~ Reaction 1 ported. along with reaction 2 are the major paths by which odd In the present study absolute measurements of kl have 0 + OH 02 + H been made from 229 to 372 K by using resonance (2) fluorescence detection of radical and atomic species. oxygen (O,O,)is converted to O2in these regions of the Pseudo-first-order conditions were used with [HO,] >> atmosphere. In addition, the rate constant ratio, kl/k2, [O]. The rate constant was determined directly from the is an important factor in determining the relative conslopes of [O] vs. time plots. A t 299 K the result is (6.1 f +
(1) Nicolet, M. Rev. Geophys. Space Phys. 1975, 13, 593. (2) Allen, M.; Yung, Y. L.; Waters, J. W. J . Geophys. Res. 1981,86, 3617. (3) Prather, M. J. J. Geophys. Res. 1981,86, 5325. (4) Logan, J. A.;Prather, M. J.; Wofsy, S. C.; McElroy, M. B. Phil. Trans. R. SOC.,Ser. A 1978, 290, 187.
(5) Hack, W.; Preuss, A. W.; Temps, F.; Wagner, H. Gg. Ber. Bunsenges. Phys. Chem. 1979,83, 1275. (6) Burrows,J. P.; Cliff,D. I.; Harris, G. N.; Thrush, B. A.; Wilkinson, J. P. T.Proc. R. SOC.London, Ser. A 1979,368, 463. (7) Lii, R. R. Sauer, M. C., Jr.; Gordon, S. J. Phys. Chem. 1980, 84, 817.
0022-385418212086-3439$QI.25/0 0 1982 American Chemical Society
