J . Phys. Chem. 1990, 94, 122-126
722
Photocatalytic Reduction of Phenacyl Halides by 9,lO-Dihydro-l O-methylacridine. Control between the Reductive and Oxidative Quenching Pathways of Tris( bipyrid9ne)ruthenium Complex Utilizing an Acid Catalysis Shunichi Fukuzumi,*,ls Seiji Mochizuki, and Toshio TanakaIb Department of Applied Chemistry, Faculty of Engineering, Osaka University, Suita, Osaka 565, Japan (Received: May 24, 1989)
The photoreductions of phenacyl halide derivatives [YC,H4COCH2X (X = Br, CI; Y = p-CN, p-Br, H, p-Me, p-MeO)] by 9,lO-dihydro-IO-methylacridine (AcrH2) in both the absence and presence of perchloric acid in acetonitrile proceed efficiently under irradiation of visible light of X = 450 nm at 298 K with [R~(bpy)~],+ (bpy = 2,2'-bipyridine) as a photocatalyst to yield 10-methylacridinium ion (AcrH') and the corresponding acetophenone derivatives. The dependences of the quantum yields for the [Ru(bpy),12+-sensitizedphotoreduction of phenacyl halides by AcrH, on the HC104 concentration have been examined. The reductive and oxidative quenching of the excited state [ R ~ ( b p y ) ~ ] by ~ +AcrH, * and phenacyl halides in the absence and presence of HC104 have also been studied, respectively. The results indicate that the reaction in the absence of HCIO., proceeds via the reductive quenching of [Ru(bpy),12+ by AcrH,, followed by the reduction of phenacyl halides by [Ru(bpy),]+, while the reaction in the strongly acidic media proceeds via the oxidative quenching of [Ru(bpy),12+ by phenacyl halides, followed by the oxidation of AcrH, by [ R ~ ( b p y ) ~ ] ~ + .
Introduction As quenching via electron transfer has become well established, it has been frequently found that the same excited-state substrate can be quenched by electron donors and acceptor^.^^^ A number of works discussed here involve reactions initiated by excitation of poly(pyridyl)ruthenium(II) complexes, typically, [ R ~ ( b p y ) ~ ] , + (bpy = 2,2'-bi~yridine).~-*The excited state, [Ru(bpy),12+*, can be efficiently quenched by electron donors such as amines and aromatic e t h e d 6 or electron acceptors such as bipyridyl dications ( e g , methylviologen)' and a series of nitro aromatics.* While these processes can occur very efficiently, they are generally followed by energy-wasting back-electron-transfer reactions that also usually occur more efficiently such that the frequent result is no net chemical reaction^.^.^ Net chemical reactions can be achieved when the reductive quenching of [ R ~ ( b p y ) ~ ] ~by+ * electron donors Drd (eq 1) is followed by the irreversible reduction
5. Thus, both the reductive and oxidative quenching pathways
are possible for the photocatalytic reactions between Dd and pbx. In general, one reaction pathway, either reductive or oxidative quenching, becomes dominant when the reactant pair Drd and A,, is fixed. However, if the operating quenching pathway can be slowed down orland the other pathway can be accelerated by addition of an appropriate catalyst, it seems possible to change the operating reaction pathway (reductive or oxidative quenching) to the other pathway in the same reaction system. In this study, we report that such a change of the reaction pathway is in fact possible. The reaction system we have chosen consists of 9,lO-dihydro- 10-methylacridine (AcrH,), phenacyl halides, and [ R ~ ( b p y ) ~used ] ~ +as an electron donor, acceptors, and a photocatalyst, respectively. The basic idea to change the [Ru(bpY)3l2+* + Dred [Ru(bpy)31f + (l) operating reaction pathway in this system is the use of an acid [ R u ( ~ P Y ) ~+I +Aox [Ru(bpy)312+ + Ard (2) catalyst. The oxidative quenching of [ R ~ ( b p y ) ~ ] by ~ +carbonyl * of electron acceptors A,, by the resulting strong reductant [Rucompounds such as phenacyl halides are known to be catalyzed (bpy),]+ (eq 2 ) or when the oxidative quenching of [ R ~ ( b p y ) ~ ] ~ + * significantly by the presence of an acid in MeCN.9,'o On the by A,, (eq 3) is followed by the irreversible oxidation of Drd by other hand, the reductive quenching of [Ru(bpy),12+* by A C T H , ' ~may . ~ ~be slowed down by the presence of an acid, since [Ru(bpy)312+* + Am [Ru(bpy)313+ + Ared (3) AcrH, is known to be protonated and the protonated species [Ru(bpy)313+ + ---* [Ru(bPY)312++ Dox (4) AcrH,' becomes a much weaker reductant than the unprotonated AcrH2.I3 Thus, we report here the effects of perchloric acid on the resulting strong oxidant [ R ~ ( b p y ) ~ ] ,(eq + 4).2,3In both cases, the [R~(bpy)~]~+-catalyzed photoreduction of phenacyl halides [ R ~ ( b p y ) ~can ] ~ +act as a photocatalyst for the net reaction, eq by AcrH, in MeCN.
-
-+
---+
( I ) (a) To whom correspondence should be addressed at Osaka University. (b) Current address: Department of Applied Physics and Chemistry, Fukui Institute of Technology, Fukui 910, Japan. (2) (a) Whitten, D. G. Acc. Chem. Res. 1980, 13, 83. (b) Kavarnos, G . J.; Turro, N. J. Chem. Reu. 1986, 86, 401. (3) Serpne, N. In Photoinduced Electron Transfer; Fox, M. A., Chanon, M., Eds.: Elsevier: Amsterdam, 1988; Part D, Chapter 5.3. (4) Kalyanasundaram, K. Coord. Chem. Reo. 1982, 46, 159. (5) (a) Ballardini, R.; Varani, G.; Indelli, M. T.; Scandola, F.; Balzani, V. J . A m . Chem. SOC.1978, 100, 7219. (b) Anderson, C. P.; Salmon, D. J.; Meyer, T. J.; Young, R. C. Ibid. 1977, 99, 1980. (c) Creutz, C.; Sutin, N. Ibid. 1976,98,6384. (d) Juris, A.; Gandolfi, M. T.; Manfrin, M. F.: Balzani, V. Ibid. 1976, 98, 1947. (e) Creutz, C.; Sutin, N. Inorg. Chem. 1976, IS,496. (6) (a) Sandrini, D.; Maestri, M.; Belser, P.; Zelewsky, A,; Balzani, V. J . Phys. Chem. 1985.89, 3675. (b) Kitamura, N.; Rajagopal, S.; Tazuke, S. Ibid. 1987, 91, 3767. (7) (a) Bock. C. R.; Meyer, T. J.; Whitten, D. G. J . Am. Chem. SOC.1974, 96, 4710. (b) Creutz, C.; Keller, A. D.: Sutin, N.; Zipp, A. P. Ibid. 1982, 104, 3618. (c) Shi, W.; Gafney, H. D. Ibid. 1987, 109, 1582. (d) Rau, H.; Frank, R.; Greiner, G . J . Phys. Chem. 1986, 90, 2476. (e) Hoffman, M Z . Ibid. 1988, 92, 3458. (8) (a) Bock. C. R.; Connor, J. A,; Gutierrez, A. R.; Meyer, T. J.: Whitten, D. G.;Sullivan, B. P.; Nagle, J . K. J . Am. Chem. SOC.1979, 101. 4815. (b) Kim, H.-B.: Kitamura, N.; Kawanishi, Y.: Tazuke, S. Ibid. 1987, 109. 2506.
0022-3654/90/2094-0122$02.5010
Experimental Section Materials. IO-Methylacridine (AcrH2) was prepared from 10-methylacridinium iodide (AcrH'I-) by the reduction with NaBH4 in methanol and purified by recrystallization from ethan01.I~ The dideuteriated compound, [9,9-2H2]dihydro-10(9) Fukuzumi, S.; Mochizuki, S.; Tanaka, T. J . Am. Chem. Soc. 1989, I l l , 1491. (10) Fukuzumi, S.; Ishikawa, K.; Hironaka, K.; Tanaka, T. J . Chem. SOC., Perkin Trans. 2 1987, 751. (1 1) Fukuzumi, S.; Tanaka, T. In Photoinduced Electron Transfer; Fox, M. A., Chanon, M., Eds.; Elsevier: Amsterdam, 1988; Part C, Chapters 4-10, pp 518-636. (12) Fukuzumi, S ; Koumitsu, S.; Hironaka, K.: Tanaka, T. J . Am. Chem. Soc 1987, 109, 305. (1 3) (a) Fukuzumi, S.; Ishikawa, M.; Tanaka, T. J . Chem. SOC.,Chem. Commun. 1985, 1069. (b) Fukuzumi, S.; Ishikawa, M.; Tanaka, T. Tetrahedron 1986, 42, 1021. (14) Roberts, R. M . G.; OstoviE, D.; Kreevoy, M. M . Faraday Discuss. Chem. SOC 1982, 7 4 , 251.
0 1990 American Chemical Societv
Photocatalytic Reduction of Phenacyl Halides by AcrH,
The Journal of Physical Chemistry, Vol. 94, No. 2, 1990 723
TABLE I: Photoreduction of Phenacyl Halides (0.30 M) by a NADH Analogue, 9,10-Dihydro-10-methylacridine(0.10 M), in the Absence and M) and HCIO, in Acetonitrile at 298 Ke (5.0 X Presence of [R~(bpy)~]*' N A D H analogue substrate HClOa, M time. h Droduct fvield. 7%) ,I AcrHzb PhCOCH2Br 0 20 AcrH' (86); PhCOCH, (84) PhCOCH2CI AcrHt 0 29 AcrH' (86); PhCOCH, (86) AcrHt p-MeOC,H4COCH2Br 0 19 AcrH' (100); p-MeOC6H4COCH, (100) AcrH2 PhCOCH2Br 0 8 AcrH' (75); PhCOCH, (74) AcrH2 PhCOCH2CI 0 8 no reaction AcrH2 PhCOCH2Br 0.2 8 AcrH' (98); PhCOCH, (98) AcrH2 PhCOCHzBr 7 2.0 AcrH' (100); PhCOCHl (100) AcrD2 PhCOCH2Br 2.0 8 AcrD' (100); PhCOCH, (100) AcrH2-CD, PhCOCH2Br 2.0 8 AcrH'-CD, (100); PhCOCH3 (100) AcrHz PhCOCH2CI 2.0 8 AcrH' (100); PhCOCH3 (100) AcrH2 p-CNC6H4COCHzBr 2.0 8 AcrH' (100); p-CNC6H4COCH, (100) AcrH2 p-MeC6H4COCH2Br 2.0 8 AcrH' (100); p-MeC6H4COCH, (100) I
"The irradiation wavelengths in the absence and presence of [ R u ( b p ~ ) ~ ]are ~ ' 320 and 452 nm, respectively. taken from ref 22.
methylacridine (AcrD,), was prepared from 10-methylacridone by the reduction with LiAID,, which was obtained from Aldrich Chemical Co.Is 9,10-Dihydro-10-( [2H3]methyl)acridine (AcrH,-CD,) was prepared by NaBH, reduction of lo-( [,H,]methy1)acridinium iodide, which was obtained by reaction of acridine with CD,I in methanol.I6 Phenacyl bromide, p bromophenacyl bromide, and phenacyl chloride were obtained commercially and purified by the standard methods.I7 p Methylphenacyl bromide, p-methoxyphenacyl bromide, and p cyanophenacyl bromide were prepared by bromination of the corresponding acetophenone derivatives in methanol.'* Perchloric acid (70%) was obtained from Wako Pure Chemicals. Tris(2,2'-bipyridine)ruthenium(II) dichloride hexahydrate, [Ru(bpy),]CI,.6H2O was prepared and purified by the literature procedure.19 Potassium ferrioxalate used as an actinometer was prepared according to the literature20 and purified by recrystallization from hot water. Acetonitrile and acetonitrile-d3 used as a solvent was purified and thoroughly dried with calcium hydride by the standard p r 0 ~ e d u r e . l ~ Reaction Procedure. Typically, AcrH, (60 pmol) was added to an N M R tube that contained an acetonitrile-d, (CD,CN) solution (0.6 cm3) of phenacyl halide (180 wmol) and [R~(bpy)~],+ (3.0 pmol). After the reactant solution in the N M R tube was thoroughly deaerated by bubbling with argon gas, it was irradiated with monochromatized light (A = 452 nm) from a Ushio Model UXL-157 xenon lamp of a Hitachi 650-10s fluorescence spectrophotometer, when only the absorption band due to [R~(bpy)~],' (A,, = 452 nm) was excited. The products were identified by comparing the ' H N M R spectra of the products with those of authentic samples of AcrH' and acetophenone derivatives. The deuterium incorporation into the products was also determined from the 'H NMR spectra. The 'H NMR measurements were carried out with a Japan Electron Optics JNM-PS-100 'HN M R spectrometer (100 MHz). Quantum Yield Determinations. A standard actinometer (potassium ferrioxaIate),O was used for the quantum yield determinations. The acetinometry experiments were carried out using a square quartz cuvette (IO-mm i.d.) under the same conditions as those of the [Ru(bpy),12+-sensitized reduction of phenacyl halides by AcrH,, when both the actinometer and [Ru(bpy),12+adsorbed essentially all the incident light of X = 452 nm. The intensity of monochromatized light of X = 452 nm with the IO-nm slit width was determined as 1.9 X 10" einstein dm-,
I
the absence of [Ru(bpy)J2',
s-l. The incident light intensity at a fixed wavelength was varied by changing the slit width. The quantum yields were determined from the rates of the rise of the absorption band due to AcrH+ (Amx = 358 nm, E = 1.8 X lo4 M-' cm-l) compared with the rates of the incident light intensity absorbed by the photosensitizer [RU(bPY)3I2+. Fluorescence Quenching. Fluorescence measurements were carried out on a Hitachi 650- 10s fluorescence spectrophotometer. In the quenching experiments, the excitation wavelength beyond the quencher absorption was normally 450 nm for deaerated [ Ru(bpy),] Relative emission intensities were measured for acetonitrile (MeCN) with [Ru(bpy),12+ (2.5 X lod M) with a quencher at various concentrations in the absence and presence of HC104. There has been no appreciable change in the emission intensity of [Ru(bpy),12+* in the presence of HClO, (up to 2.0 M) in MeCN. Thus, the emission lifetime T of [Ru(bpy),I2+* may be the same between the absence and presence of HC10, in MeCN. In addition, there has been no change in the shape but in the intensity of the emission spectrum by the addition of a quencher. The effect of the addition of electrolytes such as Bun4NCIO4has not been examined, since the addition of water, which is difficult to be removed completely from the perchlorate salt, is known to result in a significant decrease in the rate of acid-catalyzed electron-transfer reactions.21 The Stern-Volmer relationship (eq 6) was obtained between the ratio of the emission
,+.
Io/I = 1
+ KqrQl
(6)
intensities in the absence and presence of a quencher ( I o / l ) and the quencher concentration [Q]. The rate constants of electron transfer between [Ru(bpy),12+* and quenchers (keJ (=Kq7-I) was obtained from the quenching constant Kq and the emission lifetime T ([Ru(bpy),I2+, 850 nsSa).
Results [Ru(bpy),] ,+-Sensitized Photoreduction of Phenacyl Halides by AcrH,. Irradiation of the absorption band due to AcrH, in the presence of phenacyl halides (PhCOCH,X, X = Br, C1) results in the conversion of AcrH, and PhCOCH2X into AcrH+ and PhCOCH,, respectively (eq 7 ) . , , The product yields are shown
&&
+
PhCOCH2Br
-!%
..
( 1 5 ) Kerrer, P.; Szabo, L.; Krishna, H. J. V.; Schwyzer, R. Helo. Chim.
Acta 1950, 33, 294.
( I 6) Colter, A. K.; Saito, G.; Sharom, F. J. Can. J . Chem. 1977, 55, 2741. (17) Perrin, D. D.; Armarego, W. L. F.; Perrin, D. R. Purification of Laboratory Chemicals; Pergamon Press: New York, 1966. (18) Lutz, R. E.; Allison, R. K.; Ashburn, G.; Bailey, P. S . ; Clark, M. T.; Codington, J . F.; Deinet, A. J.; Freek, J. A,; Jordan, R. H.; Leake, N. H.; Martin, T. A.; Nicodemus, K. S . ; Rowlett, R. J., Jr.; Shearer, N. H., Jr.; Smith, J . D.; Wilson, J. W., 111. J . Org. Chem. 1947, 12, 617. (19) Burstall, F. H. J . Chem. S o t . 1936, 173. (20) (a) Hatchard, C. G.; Parker, C. A. Proc. R. SOC.London, Ser. A 1956. 235, 518. (b) Calvert, J. G.; Pitts, J. N. Photochemistry; Wiley: New York, 1966: p 783.
I
Me A d 2
+
Br-
+
PhCOCH3
N
I Me AcrH'
(21) Fukuzumi, S.; Chiba, M.; Tanaka, T. Chem. Letr. 1989, 3 1 .
(7)
724
Fukuzumi et al.
The Journal of Physical Chemistry, Vol. 94, No. 2, 1990
TABLE 11: Quantum Yields 0 for the [Ru(bpy),]'+ (2.5 X lo4 M) Sensitized Photoreduction of Phenacyl Halides (1.0 X lo-* M) by a NADH Analogue (1.0 X lo-' M) in the Absence and Presence of HCIOAunder Irradiation with Visible Light (A = 452 nm) NADH
analogue .4crH, AcrH, AcrH, AcrD,
AcrH,-CD3 AcrH, AcrH, AcrH,
OM
substrate [HCIO,] 0.37 p-CNC6H,COCH,Br p-BrC6H4COCH2Br 0.39 0.2 1 PhCOCH2Br 0.21 PhCOCH2Br 0.21 PhCOCH2Br 0.22 p-MeC6H4COCH2Br p-MeOC6H4COCH2Br 0.12 0 PhCOCH2CI
2.0 M [HCIO4] 0.18 0.24 0.61 0.61 0.61 0.62 0.53 0.43
"The experimental errors are within +=lO%.
0
in Table I . In this case, the excitation of AcrHz requires UV or near-UV irradiation (A,, = 285 nm).I8 However, the use of a photosensitizer, [Ru(bpy)J2+, may make it possible to utilize visible light to initiate r e a ~ t i o n . ~ ,Thus, , ~ ~ the addition of [Ru(bpy),I2+ to the AcrH2-phenacyl bromide system and selective irradiation of the absorption band due to [Ru(bpy)J2+ (A,, = 452 nm) with visible light of A = 452 nm results in the formation of equimolar amounts of AcrH' and acetophenone (Table I). The concentration of [Ru(bpy),12+ remained unchanged during the photochemical reaction. No photochemical reaction has been observed in the absence of [Ru(bpy),12+ under otherwise the same experimental conditions. Thus, [Ru(bpy),I2+ functions as a photosensitizer in the photoreduction of phenacyl bromide by AcrH, in MeCN, yielding the same products as the case of the direct excitation of the absorption band due to AcrH,, eq 8. In AcrH2 + PhCOCH2Br
0
5
-2
-1
103[HC1041 / M
0
log([AC1041 / X )
Figure 1. Plot of the quantum yield Q vs the HCI04 concentration for the [R~(bpy),]~+-photosensitizedreduction of phenacyl bromide (1 .O X lo-, M) by AcrH, (1.0 X lo-, M) in MeCN under irradiation of the visible light X = 452 nm: [Ru(bpy)J2+, 2.5 X M; light intensity, 1.85 X lod einstein dm') s-'.
/I
[RU(~PY)~~+
AcrH'
+ Br- + PhCOCH,
(8)
the case of phenacyl chloride, however, no [R~(bpy),]~+-sensitized reduction by AcrH, has been observed (Table I). The [Ru(bpy),12+-sensitized reduction of various phenacyl halides by AcrH, proceeds also in the presence of HCIO, in MeCN to give the same products as those in the absence of HCIO, (Table I). The presence of HC104 has made it possible to reduce phenacyl chloride in the [Ru(bpy),12+-sensitized reaction with AcrH,. Neither absorption nor emission spectrum of [ R ~ ( b p y ) ~has ]~+ been changed in the presence of HCIO, in MeCN.25 In addition, the photodegradationZ6of [Ru(bpy),12+ in the presence of HC10, was negligible compared with the efficient [Ru(bpy)J2+-sensitized photoreduction of phenacyl halides by AcrH,. When AcrH, is replaced by the dideuteriated compound, [9,9-2H2]dihydro-10methylacridine (AcrD,) or 9,1O-dihydro-l0-( [*H,]methyl)acridine (AcrHz-CD3), in the [Ru(bpy)J2+-sensitized reduction of phenacyl bromide in the presence of HC104 in MeCN, no deuterium was incorporated into the reduced product (Table I). Effects of HC104 on Quantum Yields. The quantum yields for the [Ru(bpy),12+-sensitized reduction of phenacyl bromide, phenacyl chloride, and variously para-substituted phenacyl bromides by AcrH, and AcrD2 were determined in both the absence and presence of HC104 (2.0 M) in MeCN. The results are listed in Table 11. No kinetic isotope effect is observed in (22) (a) Fukuzumi. S.; Mochizuki, S.;Tanaka, T. Chem. Lett. 1988, 1983. (b) Fukuzumi, S.; Mochizuki, S.; Tanaka, T. J . Chem. SOC.,Perkin Trans. 2, in press. (23) (a) Hedstrand, D. M.; Kruizinga, W. H.; Keilogg, R. M. Tetrahedron Lett. 1978, 1255. (b) van Bergen, T. J.; Hedstrand, D. M.; Kruizinga, W. H.; Kellogg, R. M. J . Org. Chem. 1979, 44, 4953. (c) Hironaka, K.; Fukuzumi, S.: Tanaka, T. J . Chem. SOC.,Perkin Trans. 2 1984, 1705. (d) Nakamura, K.; Fujii, M.; Mekata, H.; Oka, S.; Ohno, A. Chem. Lerr. 1986, 87. (e) Mandler. D.: Willner, I . J . Chem. Soc., Perkin Trans. 2 1986, 805. (24) (a) Pac, C.; Ihama, M.; Yasuda, M.; Miyauchi, Y.: Sakurai, H. J . Am. Chem. SOC.1981, 103.6495. (b) Ishitani, 0.;Yanagida, S.; Takamuku, S.; Pac, C. J . Org. Chem. 1987, 52, 2790. (c) Ishitani, 0.; Ihama, M.: Miyauchi, Y.; Pac, C. J . Chem. SOC.,Perkin Trans. 1 1985, 1527. (25) Crutchley, R J.; Kress, N.; Lever, A . B. P J . Am. Chem. SOC.1983, 105, 1170. (26) Caspar, J . V . ; Meyer, T. J . J . Am. Chem. SOC.1983, 105. 5583.
0
1
2
rBC1041 1 M Figure 2. Dependence of the electron-transferrate constant (keJ on the HCIO4concentration for the oxidative quenching of [ R ~ ( b p y ) ~ ]by ~+* p-methylphenacyl bromide (0)and phenacyl bromide ( 0 )in the presence of HCIO, in MeCN at 298 K.
the [Ru(bpy),12+-sensitized reduction of phenacyl bromide by AcrH2 when AcrH2 is replaced by AcrD, or AcrH2-CD3 as shown in Table 11. The @ values of the [Ru(bpy),12+-sensitizedreduction of phenacyl bromide by AcrH, are plotted as a function of the HC104 concentration in Figure 1 , which exhibits a complex dependence of @ on the HC104 concentration. The @ value increases with an increase in the HC104 concentration but decreases through a maximum with a further increase in the HC10, concentration. In the high HC1O4 concentrations (see the logarithm scale in Figure l ) , the 9 value increases again with an increase of the HC10, Concentration. Photoinduced Electron Transfer. The luminescence of [ Ru= 608 nm) is known to be quenched by electron (bpy),]*+* (A, transfer from [Ru(bpy),]'+* to ketones in MeCN only when they are activated by electron-withdrawing substituents, such as nitroacetophenone.8b In fact, no quenching of [Ru(bpy),12+*has been observed by phenacyl bromide or phenacyl chloride. In the presence of HCIO,, however, the luminescence of [ Ru(bpy)J2+* can be quenched by acid-catalyzed electron transfer from [Ru(bpy),lZf to phenacyl bromide, eq 9. The k,, values of phenacyl
+
-
[Ru(bpy),]*+* + PhCOCH2Br H + [ R ~ ( b p y ) ~+] ~PhC(OH)CH,Br + (9)
Photocatalytic Reduction of Phenacyl Halides by AcrH2 TABLE III: Electron-Transfer Rate Constants (k,) for the Acid-Catalyzed Reductive Quenching of [Ru(bpy)$+* by Pbenacyl Halides in the Presence of HCIO, in MeCN at 298 K k,,: M-l s-l (in the presence of HCIO,) ohenacvl halide 0.30 M IHCIOAl 2.0 M lHClOdl p-CNC6H4COCH2Br 2.6 x 107 1.5 X lo8 p-BrC6H4COCH2Br 8.7 X lo6 1.3 x 107 PhCOCH2Br 6.8 X IO6 7.8 x 107 p-MeC6H,COCH2Br 2.1 x 107 1.2 x I08 p-MeOC6H4COCH2Br 1.9 x 107 3.3 x 108 PhCOCH2CI 2.9 X IO6 I .6 x 107 Determined from the luminescence quenching of [Ru(bpy)3l2+* with use of eq 6 . The experimental errors are within &lo%. a
The Journal of Physical Chemistry, Vol. 94, No. 2, 1990 125 SCHEME I
H' / A d o
]c-+ PhCOCH3 +
X-
AcrH2
+
AcrHn
L +H'
-H'
\
AcrH3*
SCHEME
103[EC104] / M
Figure 3. Dependence of the electron-transferrate constant (kc,)on the HCIO, concentration for the reductive quenching of [Ru(bpy)J2+*by AcrH2 in the presence of HC10, in MeCN at 298 K.
bromide and p-methylphenacyl bromide increase linearly with an increase in the HC104 concentration as shown in Figure 2. The k,, values of phenacyl chloride and variously para-substituted phenacyl bromides in the presence of HC104 (0.30 and 2.0 M) are listed in Table 111. The luminescence of [Ru(bpy),12+* can also be quenched reductively by electron transfer from AcrH, to [Ru(bpy),12+* in MeCN, eq 10. The addition of HC104 to the AcrH,-[Ru( b ~ y ) ~ ] system ~ + * results in the significant decrease of the kd value as shown in Figure 3.
absence of HC104 under otherwise the same experimental cond i t i o n ~ .Thus, ~ the reduction of phenacyl halides by [Ru(bpy),]+ may also be catalyzed by HC10,. In fact, the [Ru(bpy),12+sensitized reduction of phenacyl chloride is made possible by the presence of HC104 (Table I). The reductive quenching pathway to account for the complexed dependence of 9 on the HC104 concentration (Figure 1) is shown in Scheme I. The reductive AcrH2 [Ru(bpy),12+* ---* AcrH2'+ + [Ru(bpy),]+ (IO) quenching of [Ru(bpy),12+* by AcrH, may be followed by the acid-catalyzed electron transfer from [Ru(bpy)$ to PhCOCH2X Discussion to give PhC(OH)CH,X, accompanied by regeneration of [RuReductioe Quenching Pathway. In the absence of HC104, the (bpy),I2+, in competition with the back electron transfer from reductive quenching of [ R ~ ( b p y ) ~ ] by ~ +AcrH2 * occurs efficiently [Ru(bpy),]+ to AcrH,'+. The AcrH2*+radical may be depro(Figure 3) by electron transfer from AcrH, to [Ru(bpy),lz+* to tonated to give AcrH', which can reduce PhC(OH)CH2X to yield produce AcrH;+ and [Ru(bpy),]+ (eq 9). Since the [ R ~ ( b p y ) ~ ] + AcrH+ and PhCOCH, (Scheme I). According to Scheme I, the produced is known to be a strong reductant (the oxidation potential hydrogen in the reduced acetophenone comes from a proton, and is 1.29 V vs SCE),8a phenacyl bromide and para-substituted thereby, replacement of AcrH2 by AcrD2 or AcrH,-CD, results phenacyl bromides may be readily reduced by [Ru(bpy),]+ to yield in no incorporation of deuterium into acetophenone (Table I). No the corresponding acetophenone derivatives (Table I) accompanied kinetic isotope effect is expected, either, since the rate-determining by regeneration of [ Ru(bpy),I2+. However, phenacyl chloride, step involves only an electron-transfer process, the reductive which is weaker oxidant than phenacyl bromide, may not be quenching of [Ru(bpy),12+* by AcrH, or electron transfer from reduced by [Ru(bpy),]+, and thus, no [Ru(bpy),12+-sensitized [Ru(bpy),]+ to PhCOCH,X. The latter may be the rate-deterreduction of phenacyl chloride has occurred in the absence of mining step, since the quantum yield 9 in the absence of HC104 HC104 (Table I). decreases with a decrease in the donor ability of phenacyl halides We have recently reported that the reduction of phenacyl halides (Table 11) and the 9 value of phenacyl bromide in the presence to acetophenone by one-electron reductants such as 1,l'-diof HC104 increases with an increase in the HC104 concentration methylferrocene, Fe(MeC5H4),, occurs efficiently in the presence [(0-5.0) X M in Figure I], when HC10, may accelerate of HC104 in MeCN (eq 1 I ) , although no reaction occurs in the electron transfer from [ R ~ ( b p y ) ~to ] +PhCOCH,X but inhibits electron transfer from AcrH2 to [ R ~ ( b p y ) ~ ] ~(Figure + * 3). The 2Fe(MeC5H4), PhCOCH2X H+ inhibitory effect of HC104 may be ascribed to the protonation 2Fe(MeC5H4),+ + PhCOCH3 X- (1 1) on AcrH, (Scheme I), since the protonated AcrH3+ in MeCN
+
+
+
-
+
J . Phys. Chem. 1990, 94, 726-729
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is known to be a much weaker reductant than the unprotonated AcrH,.’* With an increase in the HC10, concentration, the rate of reductive quenching of [ R ~ ( b p y ) ~ ]by ~ +AcrH, * decreases as shown in Figure 3, while the rate of reduction of PhCOCH2X by [Ru(bpy)J+ may increase. In such a case, the rate-determining step may be changed from the reduction of PhCOCH,X by [Ru(bpy),]+ to the reductive quenching of [Ru(bpy),I2+*by AcrH,. This may be the reason why the quantum yield decreases with an increase in the HCIO, concentration (5.0 X 10-3-4.0 X IO-’ M in Figure 1). The reason why the % value increases again with a further increase in the HC10, concentration (>4.0 X IO-’ M in Figure 1) is discussed next. Oxidative Quenching Pathway. In the high concentrations of HC104, the [Ru(bpy),l2’-sensitized reactions may be initiated by the reductive quenching of [ R ~ ( b p y ) ~ ] * +instead *, of the oxidative quenching by AcrH,, as shown in Scheme 11. No oxidative quenching of [Ru(bpy),12+* by PhCOCH’X occurs in the absence of HCIO, in MeCN. However, the oxidative quenching occurs in the presence of HCIO,, and the rate constant increases linearly with an increase in the HC10, concentration. Since the rate constant of the reductive quenching of [Ru(bpy)J’+* by AcrH’ decreases with an increase in the HClO, concentration (Figure
3), the oxidative quenching of [Ru(bpy),12+* by phenacyl halides (PhCOCH,X) may become a predominant pathway at the high HCIO, concentration. This may be the reason why the value increases again in the high concentrations of HC10, (Figure 1). The reductive quenching of [Ru(bpy),]*+* by PhCOCH2X in the presence of high concentrations of HCIO4 produces PhC(0H)CH2X and [Ru(bpy)J3+, the latter of which is known to be a very strong oxidant (the reduction potential is 1.3 V vs SCE).8a Thus, [ R ~ ( b p y ) ~can ] ~ +oxidize AcrH, even in the presence of HCIO, to produce AcrH,”, accompanied by regeneration of [ Ru(b~y),],+.~’The AcrH2*+may react with PhC(OH)CH,X after deprotonation to yield the same products as the case in Scheme I. In Scheme I1 as well, the electron transfer step, the acid-catby PhCOCH’X, is alyzed oxidative quenching of [R~(bpy)~]’+* rate-determining, and thus no kinetic isotope effect has been observed in the [Ru((bpy),12+-sensitized reduction of phenacyl bromide by AcrH, in the presence of HCIO, (Table 11). (27) The one-electron reduction potential of [ R ~ ( b p y ) ~ ] ’ is + 1.29 V (vs SCE; ref 8a) is more positive than the one-electron oxidation potential of AcrH, = 0.80 V vs SCE; ref 12), and thereby the electron transfer from AcrH, to [Ru(bpy),]’+ is highly exothermic.
(ex
Direct Rate Measurements of the Combination and Disproportionation of Vinyl Radicals Askar Fahr* Chemical Kinetics Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899
and Allan H. Laufer* Chemical Sciences Division, Office of Basic Energy Sciences, US.Department of Energy, Washington, D.C. 20545 (Received: May 26, 1989; In Final Form: August 1 , 1989)
The rates of removal of vinyl radicals (C,H3) by means of combination and disproportionation reactions have been directly measured at room temperature. Vacuum-ultraviolet (vacuum-UV) flash photolysis of divinylmercury was used to generate vinyl radicals, and vacuum-UV absorption kinetic spectroscopy was used to monitor the time history of vinyl radicals through cm3molecule-’ their absorption at 1647.1 A. The absolute rate constant for vinyl radical decay was determined to be 1.0 X s-l. A ratio of 4.7 for the combination/disproportionationrate constants was measured by gas chromatographic analysis of the final reaction products. Absorption coefficients for the vinyl radical absorption in the 1650-Aregion were redetermined. The absolute rate constants for combination and disproportionation are then 8.2 X IO-’’ and 1.8 X IO-” cm3 molecule-’ s-I, respectively.
Introduction Reactions of vinyl radicals and other unsaturated radicals may play an important chemical role in both the low temperatures of planetary atmospheres and the high temperatures involved in hydrocarbon pyrolysis and combustion. They also have been suggested to be important intermediates in processes involved in carbon growth and soot formation. Despite their perceived importance, relatively little is known about the chemistry of vinyl radicals. This has been due, in part, to the difficulties in generating cleanly these radicals and monitoring directly their behavior. Only in recent years has progress in understanding some aspects of the spectroscopy and gas-phase chemistry of vinyl radicals been reported. By use of either mass spectrometric detection or diode laser absorption techniques, gas-phase kinetic parameters for reactions of vinyl radicals with molecular and atomic oxygen, hydrogen, and HCI have been investigated at low temperature^.'-^ ( I ) Slagle, I . R.; Park, J. Y . ;Heaven, M . C.; Gutman, D. J . Am. Chem. Soc. 1984, 106, 4356. (2) Callear, A . B.; Smith, G. B. J . Phys. Chem. 1986, 90, 3229. (3) Rao, V. S.; Skinner, G. D. J . Phys. Chem. 1988, 92, 6313. (4) Heinemann, P.; Hofmann-Sievert, R.; Hoyermann, K. Symp. (Int.) Combust., [Proc.],2lsr 1986, 865. ( 5 ) Krueger. H.: Weitz, E. J . Chem. Phys. 1988, 88, 1608.
In earlier work from our laboratory we reported the observation of two relatively strong vacuum-UV absorption features at 1647.1 and 1683.3 A with extinction coefficients of 1650 and 1120 cm-’ atm-l (base e), respectively, as well as kinetic data for the C2H3 0, reaction.6 There it was suggested that the relatively strong vacuum-UV absorptions could provide an important new probe for the study of the reaction kinetics of vinyl radicals. The first use of these absorption bands in such a kinetic measurement is reported here. In separate work by one of us (A.F.), rates of reactions of vinyl radicals with acetylene, ethylene, and benzene have been measured over the temperature range from 1000 to 1330 K using the very low pressure pyrolysis (VLPP) technique.’ In these studies divinylmercury or phenyl vinyl sulfone were used as thermal radical initiators, and reaction products were monitored by mass spectroscopy. Assuming a vinyl radical combination rate constant of 3.3 X IO-’’ cm3 molecule-’ s-’ at 1100 K, then rate constants of (3.3 f 0.7) X (2.3 f 0.3) X and (7.5 f 2.0) X lo-’, cm3 molecule-’ s-’ were measured for vinylation of acetylene, ethylene, and benzene, respectively.
+
(6) Fahr, A.; Laufer, A. J . Phys. Chem. 1988, 92, 7229. (7) Fahr. A.; Stein, S. Symp. ( I n t . ) Combust., [Proc.],22nd, in press.
This article not subject to U S . Copyright. Published 1990 by the American Chemical Society
