Organometallics 1995, 14, 5670-5676
5670
Kinetics and Reaction Mechanisms of Copper(1) Complexes with Aliphatic Free Radicals in Aqueous Solutions. A Pulse-Radiolysis Study Nadav Navon, Gilad Golub, Haim Cohen, and Dan Meyerstein" Nuclear Research Centre Negev, R. Bloch Coal Research Center, and Chemistry Department, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel, and College of Judea and Samaria, Ariel, Israel Received June 8, 199P
+
-
The reactions (Cu'L 'R LCuII-R and the mechanism of decomposition of the transient complexes LCuII-R were studied using the pulse-radiolysis technique (L1 = H20, L2 = 2,5,8,11-tetramethyl-2,5,8,ll-tetraazadodecane; 'R = 'CH3, 'CH2COOH, 'CH(CH3)COOH, *CHzCH2COOH.) The kinetics of formation of the transient complexes obey pseudo-firstorder rate laws. The rate constants measured for these reactions are in t h e range (2-4)x lo9 M-l s-l for L' and 6 x lo6-1 x los M-l s-l for L2. The mechanisms of decomposition of t h e transient complexes depend on t h e nature of R and L as follows: 1. For R = CH3 ethane is t h e final product. The kinetics of its formation obey second-order rate laws. Free methyl radicals are not intermediates in these processes. 2. For R = CH2CH2COOH the transient complexes decompose via a n analogous mechanism to that of R = CH3 for L1 and via homolysis of the copper-carbon 0 bond for L2. 3. For R = CH2COOH and CH(CH3)COOH the transient complexes decompose via heterolysis of the copper-carbon 0 bonds forming Cu"L. The effect of the nature of R and L on the kinetics of these reactions is discussed.
Introduction Many organic processes of industrial and synthetic interest which are catalyzed by copper complexes involve free radicals as key intermediates.lBZ These include among others the Meenvein r e a ~ t i o n ,the ~ addition of polyhalides to alkene^,^ and the Ullmann r e a ~ t i o n .Copper(1) ~ complexes are the active species in these catalytic processes. Furthermore it has been prop~sedl,~ that , ~ , transient ~ complexes with coppercarbon u bonds are key intermediates in these catalytic processes. The latter transient complexes are believed to be formed via the reactions of the aliphatic free radicals with the copper complexes present in the system. The interest in the reactivity of Cu(1)complexes toward organic free radicals has also increased due to the role of copper ions in deleterious biological processes initiated by free radicah6 The role of the copper(1) complexes in most systems is dual as follows. a. Forming the free radicals via halogen abstraction or via the modified Fenton ~ - e a g e n t : ~ - ~ * To whom correspondence should be addressed at Ben-Gurion University of the Negev. Abstract published in Advance A C S Abstracts, November 1,1995. (1)Regitz, M., Giese, B., Eds. Houben-Weyl; Thieme: Stuttgart, Germany, 1989; Vol. E19a. (2) (a) Kochi, J. K. Organometallic Mechanisms and Catalysis; Academic Press: New York, 1978. (b) Sheldon, R. A,;Kochi, J. K. Metal Catalyzed Oxidations of Organic Compounds; Academic Press: New York, 1981. (3) Rondestvedt, C. S., Jr. Org. React. 1976,24,225. (4)Bellus, D. Pure Appl. Chem. 1986,57,1827. (5) (a)Weingarten, H. J . Org. Chem. 1964,29,3624. (b) Schwartz, M. A.; Crowell, J. D.; Musser, J. H. J . Am. Chem. SOC.1972,94,4363. (6) Goldstein, S.; Czapski, G. J . Free Rad. Biol. Med. 1986,2,3. (7)Nonhebel, D. C. Essays on Free Radical Chemistry. Chem. SOC. Spec. Publ. 1970,24,409. @
+ RX - CU'I-X' + 'R Cu(1) + H 2 0 2- Cu(I1) + OH- + 'OH %?. Cu(1)
H20
(1)
+ 'R
(2)
b. Affecting the nature of the final products due to the formation of transient complexes with coppercarbon u bonds via CU(I)
+ 'R
Cu(I1)
+
+ 'R
CU"-R~~'~~
-+
CUI'I-R~~
(3)
(4)
The kinetics of reactions 3 and 4 were studied in aqueous solutions by applying the pulse-radiolysis technique, for several free radicals.1°-12 The results point out that reaction 3 is fast and its rate usually approaches the diffusion-controlled limit, Le. k3 > lo9 M-l s-l, whereas the rates of reaction 4 are usually considerably slower, Le. 8 x lo5 < k4 < 8 x lo8 M-l s-l.l2 Thus even when the concentration of the Cu(1) complex is considerably lower than that of the Cu(I1) complex the free radicals will react mainly via reaction 3. The mechanisms of decomposition of the transient complexes CuII-R and CuIII-R clearly determines the composition of the products of these catalytic processes. ( 8 ) Goldstein, S.; Czapski, G.; Meyerstein, D. J. Free Rad. Biol. Med.
1993,15, 435.
(9) Masanva, M.; Cohen, H.; Meyerstein, D.; Hickman, J.; Bakac, A.; Espenson, J. H. J.Am. Chem. SOC.1988,110, 4293. (10) Freiberg, M.; Mulac, W. A,; Schmidt, K. H.; Meyerstein, D. J. Chem. SOC.,Faraday Trans. 1 1980,76, 1838. (11)Cohen, H.; Meyerstein, D. Inorg. Chem. 1986,25,1506. (12) (a)Freiberg, M.; Meyerstein, D. J . Chem. SOC.,Faraday Trans. 1 1980,76, 1825. (b) Masanva, M.; Cohen, H.; Meyerstein, D. Inorg. Chem. 1986,25,4897.
0276-7333/95/2314-5670$09.00/0 0 1995 American Chemical Society
Organometallics, Vol. 14, No. 12, 1995 5671
&(I) Complexes with Aliphatic Free Radicals The following mechanisms of decomposition of the transient complexes of the type Cu"-R have been reported. 1. Homolysis of the copper-carbon u bond.1°J3
+
Cu"-R == CU(I) 'R
Cu(1) complex17 on the reaction mechanism. It was recently shown that the latter ligand, which is a pure u donor, stabilizes Cu(1) in neutral and alkaline aqueous
solution^.'^
(5)
R\
(" "7
2. Heterolysis of the copper-carbon u bond:ll CUII-R
+ HzO
Cu(I1)
+ RH + OH-
(6)
3. /I-Elimination processes when a good leaving group is bound to the /I carbon atom, e$. X = OH, OR, and NRz:13c,d CU'I-CR'R'CR~R~X -, Cu(I1)
+ R1R2C=CR3R4+ X-
(7)
4. Carbon-carbon bond formation processes, e g .
2Cu1'-CH3+,, (CO)CU"-CH~+,,
2Cufaq
+ C2Hs1'
Cu1'-C(0)CH3+,,
-
CH3CH0
(8)
+
CuZfaq
+ OH-
n/ R
l4
(9)
5. Rearrangement of the carbon skeleton of the organic residue:15
R=H,L R = CH3, L2
Experimental Section Materials. All solutions were prepared from AR grade chemicals and from distilled water further purified by passing through a Milli Q Millipore setup, final resistivity '10 M B k m . The ligand L2 was prepared by N-methylation of the unmethylated ligand (L = 1,4,7,10-tetraaza-decane, Aldrich) using formaldehyde and formic acid according to a procedure described in the literature18 and crystallized as its HCl salt. The degree of N-methylation obtained was verified by C, H, and N analyses and by H-NMR and %-NMR measurements. Both analytical techniques indicated that more than 97% of the ligand molecules were hexamethylated. Cu(1) Preparation. Solutions of L2Cu+were prepared via the comproportionation process:
cu,
+ L2CU2++ L2* 2L2CU+
(10)
These examples clearly point out the large variety of mechanisms by which the transient complexes with copper-carbon u bonds might react. To date, due to the unstable nature of CU'aq, it has been prepared in situ by reduction with e-aq, using ionizing radiation, or by the reduction of CU2+aq by Cr2+,,; in these systems the CU2+aqconcentration was high, and thus, the involvement of CU2+aq reactions with the radical, added to the complexity of the reactions studied, and reactions of free radicals, e g . 'CHZCOZfor which K 4 > 5 x lo7 M-l s-l, with CuU) could not be studied. Recently, we have found that solutions containing up M Cufaq,free of at pH I4.0 can be to 2 x prepared by adding [C~(NH3)2]+aql~ into acidic solutions. The C U +thus ~ ~ prepared is stable for several hours. It was therefore decided to measure the mechanisms of reaction of the free radicals 'CH3, 'CH~COZH,'CH(CH3)COzH, and 'CHzCH2C02H with Cu'aq. Furthermore it was decided to study the mechanisms of reaction of the same free radicals with L2Cu+,where L2 = 2,5,8,ll-tetramethyl-2,5,8,1l-tetraazadodecane, with the hope of e l u c i d a t i n g the effect of a relatively sterically hindered ligand which induces a hydrophobic nature to the
C U +was ~ ~ obtained via comproportionation of C U ( N H ~ ) in ~~+ concentrated NH3 (Linde Inc.), aliquots of which were added to acidic solutions. The pH was measured with a Corning 220 pH meter and was adjusted by HClOd and/or NaOH. For the L2Cu+system the pH was adjusted by HCl. Irradiations. Pulse-radiolysis experiments were carried out using the Varian 7715 linear electron accelerator of the Hebrew University of Jerusalem. The pulse duration was 0.11.5 p s with a 200-mA current of 5-MeV electrons. The dose per pulse was 300-3000 rad. Irradiations were carried out in a 4-cm spectrosil optical cell, the analyzing light passing three times through the cell. A 150-W xenon arc produced the analyzing light. The experimental setup was identical with t h a t described earlier in detail.12aJ9 For dosimetry, a N2O-saturated solution containing 1 x M KSCN was used. The yield of (SCN)2'- was measured by taking €475 = 7600 M-I cm-', and the dose per pulse was calculated assuming G(SCN)2'- = 6.019and a n optical path of 12.3 cm. The dose per pulse was set so that the initial free radical concentration was 2-20 pM. The values of the molar extinction coefficients calculated from the dosimetry measurements have a n error limit of 15% due to the scatter in the pulse intensity and due t o uncertainties in G values. The dose delivered to vials, identical to those irradiated for final product analysis, was measured using the Fricke dosimeter taking G(Fe(II1)) = 15.6and €302 = 2197 M-l cm-I.19 A 6oCosource, Noratom, with a dose rate of 1300 r a d m i n was used for low-dose-rate experiments and product analysis. Product Analysis. Yield of Cu(1). The stability constant n complex of C d a qwith fumaric of the formation of the d
(13)(a) Cohen, H.; Meyerstein, D. Inorg. Chem. 1987,26,2342.(b) Meyerstein, M. Pure Appl. Chem. 1989,61, 885. (c) Masanva, M.; Cohen, H.; Saar, J.;Meyerstein, D. Isr. J . Chem. 1990,61,361. (d) Goldstein, S.;Czapski, G.; Cohen, H.; Meyerstein, D. Inorg. Chem. 1992,31,2439. (14) Szulz, A.; Cohen, H.; Meyerstein, D. To be published. (15) Masanva, M.; Cohen, H.; Meyerstein, D. Inorg. Chem. 1991, 30,1849. (16)Parker, A. J. Search 1973,4.
(17)(a) Golub, G.; Cohen, H.; Meyerstein, D. J. Chem. Soc., Chem. Commun. 1992,398. (b) Golub, G.; Cohen, H.; Paoletti, P.; Bencini, A.; Bertini, I.; Messori, L.;Meyerstein, D. J. Am. Chem. SOC.1995, 117,8353. ( c ) Golub, G.; Zilbermann, I.; Cohen, H.; Meyerstein, D. Supramolecular Chem., in press. (18)Icke, R.N.;Wisegarver, B. B.; Alles, G. A,; Snyder, H. R. Org. Synth. Synth. Coll. 1941,3,723. (19)Matheson, M. S.;Dorfman, M. L.Pulse Radiolysis; MIT Press: Cambridge, MA, 1969.
-
5672 Organometallics,
Navon et al.
Vol. 14,No.12, 1995
acid a t pH 2.0 is K = 7500 M-1.20 This complex has a n absorption band with A,, = 356 nm and E = 1000 f 100 M-' cm-l a t pH 2.0.20 The change in the concentration of C d a q caused by the reaction was measured by adding fumaric acid a t pH 2.0 t o the sample so t h a t the final concentration of the fumaric acid was 2.0 x M and measuring the absorbance at 356 nm. For the L2Cu+ system, the change in the concentration was determined by measuring the absorption due to LZCu2+. UV-vis spectra were recorded using a Hewlett Packard 8452A diode array spectrophotometer. GC Analysis. The 10 mL samples in glass vials were irradiated by the linac. The gaseous products were analyzed using a Varian Model 3700 gas chromatograph. Production of Free Radicals. The radiolysis of water in dilute aqueous solutions can be summarized:19
Table 1. Reactions of Primary Free Radicals with the Solutes Used in this Study reaction (CH3)zSO+ 'OH (CH3)2&O)OH followed by (CH&S(O)OH 'CH3 + CH&O)OH (CH&SO + 'H (CH3)2S(OH) (CH&SO + e-aq products e-,, + Cu+,, CuO 'OH + CU+,~ C U ~ + ~ , 'H + Cu+,, CuH+,, (followed by Cu-H+,, Cu2+,, + Hz) 'OH + C U ~ + , CuOH2+ ~ 'H + Cu2+., Cu+,, + H+ e-aq + Cu2+,, Cu+., 'OH + CH~CHZCOOH products 'H + CH3CH2COOH 'CH(CH9)COOH + H, e-. + CH3CH2COOH 6H3CH2COO- + H' 'OH + CH3CHzCOO- products 'H + CH3CHzCOO'CH(CH3)COO- + Hz *OH+ CHsCOOH 'CH2COOH + HzO 'H + CH3COOH CH2COOH H2 e-*, + CH3COOH CH3COO- + H' 'OH + CH3COO- 'CHzCOO- + HzO 'H + CH3COO- 'CH2COO- + Hz e-8q + CH3COO- products
--
4
----
-
k (M-l s-l 6.6 x logz3 1.5 107 S-1 2.7 x 1.6 x 2.7 x 2.0 x 10'0 32 ' 5 . 0 x 109 33 3.1 x c 106 12
--
--
(Gvalue, in parentheses, is defined as the number of molecules of each productI100 eV radiation absorbed by the solution.) In concentrated solutions, the yields of the primary free radicals are somewhat higher and those of HzO2 and Hz are somewhat 10wer.l~ In neutral N2O-saturated solutions ([NzO] = 0.022 M) the hydrated electron is converted into the hydroxyl radical via e-aq
+ N,O -N, + OH- + 'OH
---
+
4
~
PH
23
5.8 5.6 3-6
3.0 x 1O'O 32 6.2 x 5.9 x 10635
6.8 2 1
2.2
3-4
10736
8.2 x 1.7 x 10738
lo7 25
2.3 x 1.2 x 1.8 x 8.5 x 3.5 x c1.0 x
The 'CH~CHZCO~H and 'CH(CH&02H prepared via
9 7
lo7 26
1 1 5.4 10.7
10527 106 37
10
10527
7
free radicals were
H2O
k,, = 8.7
x
lo9 M-'
s-l
22
(12)
CH,CH,CO,H
+ 'OH
'CH(CH,)CO,H
45%
In acidic solutions the reaction of e-aqwith H atoms, becomes important e-aq
+ H'
H
k,, = 2.2 x
H+, which produces
lo1'
M-' s-'
22
(14)
but a t pH > 3 the contribution of H atoms t o the total free radical yield is less than 10%. The methyl free radicals were produced via
(CH,),SO
+ *OH- (CH,),'S(O)OH
-
'CH,
23
(15)
lo7 s-l 23
(16)
+ CH,S(O)OH k,, = 1.5 x
The 'CH2CO2WCH2C02-, pK, = 4.5,24 free radicals were prepared via
CH,CO,H
+ ' O W - 'CH,CO,H + H,O/H, ko, = (2.3-8.5) x
k,
= (1.2-3.5)
x
lo7 M-ls-l
lo5M-'
s-l
25,26
(17)
27
(20) Navon, N.; Cohen, H.; Meyerstein, D. To be published. (The
stability constant is pH dependent; this is the reason for the difference between K at pH 2.0 and K at pH 3.6 in ref 21.) (21) Meyerstein, D. Inorg. Chem. 1975,14, 1716. (22)Anbar, M.; Bambenck, M.; Ross, A. B. Natl. Stand. R e f . Data Ser. ( U S . , Natl. Bur. Stand.) 1973, NSRDS-NBS 43. (23)Veitwisch. D.; Janata, E.; Asmus, K. D. J . Chem. Soc., Perkin Trans. 1980,2, 146.
(24)Neta, P.; Simic, M.; Hayon, E. Aust. J . Chem. 1976, 29, 637. (25) Ebert, M.: Keene. J. P.: Swallow, A. J.;Baxendale, J. H. Pulse Radiolysis; Academic Press: New York, 1965; p 131. (26)Willson, R. L.; Greenstock, C. L.; Adams, G. E.; Wagenman, R.; Dorfman, L. M. Int. J . Radiat. Phys. Chem. 1971, 3, 211. (27) (a)Neta, P.; Holdern, G. R.; Schuler, R. H. J . Phys. Chem. 1971, 75,449. (b)Appleby, A.; Scholes, G.; Simic, M. J . Am. Chem. Soc. 1963, 85, 3891.
+ H,O
(18)
k,, = (3.8-8.2) x 10' M-' sK1 29 The relative yields given are those for pH 9.0; at pH 3.0 the yield of the a radicals is -40% and that of the /? radicals is -60%.24
Results and Discussion
k,, = 6.6 x lo9 M-' s-'
(CH,),'S(O)OH
'CH,CH,CO,H
+ H,O
M NzO-saturated solutions containing (0.3-1) x of C d a q ,pH 3.0, or (1-10) x M of L2Cu+,pH 10.0 (the concentration of L2 was always twice that of the Cu(1) concentration), and 0.1-0.3 M CH3COzH or CH3CHzCOzH or 0.3 M (CH3)zSO were irradiated by a short electron pulse from the linear accelerator. Under these conditions all the primary free radicals are transformed into the desired aliphatic free radicals during the pulse, Table 1.23,25-29,31-38 In all the systems studied two consecutive processes are observed after the formation of the aliphatic free radicals, Figure 1. The system containing CH3CHz(28)Zuberbiihler, A. D. Helu. Chim. Acta 1970, 53, 473. (29)Anbar, M.; Meyerstein, D.; Neta, P. J . Chem. Soc. B 1966, 742. (30) (a)Espenson, J. H. Adu. Inorg. Bioinorg. Mech. 1982,1, 1. (b) Cohen, H.; Meyerstein, D. Inorg. Chem. 1974, 13, 2432. (31) (a) Chardhri, S. A.; Goebl, M.; Freyholdt, T.; Asmus, K. D. J. A m . Chem. Soc. 1984,106,5988. (b) Koulkes-Pujo, A. M.; Michael, B. D.; Hart, E. J. J . Radiat. Phys. Chem. 1971,3, 333. (32) Buxton, G. V.; Greenstock, C. L.; Helman, W. P.; Ross, A. B. J . Phys. Chem. Ref Data 1988, 17, 513. (33) Mulac, W. A.; Meyerstein, D. Inorg. Chem. 1982,21, 1782. (34) Scholes, G.; Willson, R. L. Trans. Faraday Soc. 1967,63,2983. (35) Neta, P.; Fessenden, R. W.; Schuler, R. H. J . Phys. Chem. 1971, 75, 1654. (36) Micic, 0. J.; Markovic, V. Int. J . Radiat. Phys. Chem. 1972,4, 43. (37) Gordon, S.; Hart, E. J.; Matheson, M. S.; Rabani, J.; Thomas, J. K. Discuss Faraday SOC.1963,36, 193. (38) Neta, P.; Schuler, R. H. J . Phys. Chem. 1972, 76, 2673.
&(I) Complexes with Aliphatic Free Radicals
Organometallics, Vol. 14,No. 12, 1995 5673
Table 2. Rates of Reaction of Aliphatic Free Radicals with Cu(1) Complexes
1 I'
I
0 wo2 sec
-y
1
y =lo0
+
k3 (M-' 9-l)'
+ + +
-
3.5 x 109d 2.8 109 d 4.0 x 109d
+ + +
- -
Cufaq 'CH3 (Cu"-CH3)+ C U + ~ ~'CH~COOH (CUII-CH~COOH)+ C U + ~'CH(CH3)COOHPCH2CH2COOH ~ (CU"-CH(CH~)COOH)+/ (CU"-CH~CH~COOH)+ L2Cu+ 'CH3 (L2Cu"-CH3)+ L~cU+ SCH~COO- ( L ~ C ~ I I - C H ~ C O O - ) L2Cu+ 'CH2CH2COO(L2Cu"-CH2CH2CO0-Y
-
1.2 x 1 0 8 e f 2.7 x 107 6.3 x 106eg
I
2 sec
Figure 1. Computer output of light intensity vs time. Solution composition: NzO-saturated, containing 3 x M Cu(I), 6 x M L2, and 0.1 M DMSO at pH 7.0. Insets: Kinetics of formation of the intermediate, a fit to a first-order rate law; kinetics of decomposition of the intermediate, a fit to a second-order rate law. 30000
reaction
2.7e+7x
a All experiments were carried out a t room temperature, 22 f 2 "C. Error limit f15%. The two reactions are experimentally nonseparable. The reaction of 'CH(CH3)COOH is too slow t o be observed; see text. pH 3.0; the results are independent of pH in the range 2-4, Z 5 0.005 M. e pH 8.0; the results are independent of pH in the range 7-10. f Z c 0.001 M. g Z = 0.30 M.
Table 3. Spectra of the Transient Complexes LCu"-RQ transient complex
,Imax(nm)b
(Cd1-CH3)+ (CU"-CHZCOOH)+ (CU"-CH(CH~)COOH)+ (CU"-CH~CH~COOH)+ (L2Cu"-CH3)+ (L2C~"-CH2COO-) (L2Cu"-CH2CH2COO-)
20000
C L I
Y
375 360 410 390 380 405 390
E
(M-l cm-')C 2700 1700 1750 1750 3000 2700 2450
a Experimental conditions as in Table 1. Error limit f 5 nm. Error limit f15%.
0.00000
0.00030
0.00060
0.00090
[LZcu + ]
Figure 2. Dependence of the rate of formation of (L2CuI1CHzCOO-) on [L2Cuf]. Solution composition: NzO-saturated, 0.1 M CH3COO-, pH 8.0.
CO2H is a n exception to this statement; in this system three consecutive processes are observed. The first process, the formation of the intermediates, always obeys a first-order rate law; see for example Figure 1. The observed rates are proportional to the concentration of the Cu(1) complexes; see for example Figure 2. The rates are independent of the concentrations of all other components of the solution, pulse intensity, and wavelength of observation. Therefore it is reasonable to propose that the unstable intermediates formed in this process are LCuII-R and that the reactions observed are those depicted in eq 3a, i.e. Cu'L
+ 'R - LCu"-R
(3a)
complexes with a copper-carbon u bond; the results are summed up in Table 2. Indeed the spectra of the intermediates formed, Table 3, are similar to those of other complexes of this type; see below. A typical spectrum is shown in Figure 3. The rates of reaction 3, Table 2, for Cu'L = C U +all ~~ approach the diffusioncontrolled limit in accord with reported data on the reactions of other free radicals with this complex. The rates of reaction 3 for Cu'L = L2Cu+ are considerably lower. The order of reactivity is 'CH3 > 'CH2C02- > 'CHzCH2C02- =- 'CH(CH3)C02-. The reaction of the latter radical is too slow to be observed; see below. The latter result indicates that the very large effect of this ligand on the rates of reaction stems mainly from the steric hindrance imposed by it.
8
1
0 300
t
400
wavelength
4
1 500
600
(nm.)
Figure 3. Spectrum of (L2C~11-CH3)+aq measured 200 ps &r the pulse: NzO-saturated solution containing 3 x M Cu(I), 6 x ML2,and 0.1 M DMSO at pH 7.0.
The mechanism of decomposition of the transient complexes LCu'I-R depends on both the nature of R and that of L. In the following paragraphs these mechanisms are discussed separately for each R. Mechanism of Decomposition of the Complexes LCuI'-C&+. The reaction CU+.~ 'CH3 was studied previously in the presence of excess CU2+aqas Cu'aq was prepared via the reduction of CU2+aqby C9'aq.l' The results obtained under the present experimental conditions, Table 4, are in accord with the previous ones and indicate that above pH 2 the mechanism of decomposition is
+
-.
+
~ C U I I - C H ~ ' ~BCU',~ ~ C2H6 2k,, = 3.0 x 10' M-' s-l (19) The mechanism does not involve homolysis of the
5674 Organometallics, Vol. 14,N o . 12, 1995
N a v o n et al.
Table 4. Observed Kinetic Parameters 103[Cu+](M) 103[L] (M) 0.2 0.4 1.0 2.0 0.3 0.6 0.3 0.6 0.3 0.6 2.6 0.3 0.3 0.6 0.3 0.6 0.3 0.6 0.3 2.6 0.3 0.6 0.6 1.2 0.2 0.4 0.2 0.4 0.5 1.0 0.8 1.6 0.064 0.064 0.096 0.064 0.096 0.096 0.096 0.096 0.064 0.032 0.096
solute DMSO DMSO DMSO DMSO DMSO DMSO HAC HAC HAC HAC HAC HAC HAC HPr HPr HPr HPr DMSO DMSO HAC HAC HAC HAC HAC HPr HPr HPr HPr
HzO CH3CN HzO H2O HzO HzO HzO HzO
H2O
[solute] (M) pH 0.1 10 0.1 10 0.1 7 0.3 7 0.1 7 0.1 7 0.1
0.3 0.3 0.3 0.3 0.3 0.3 0.1 0.1 0.1 0.1 0.3 0.3 0.1 0.1 0.1 0.5 0.1 0.1 0.1 0.1 0.1
1 (nm) 400 400 380 380 380 380 8 450 8 405 8 405 8 405 10 405 8 405 8 405 8 420 8 400 8 420 8 340 3 380 3 380 3 400 3 400 2.5 400 3 400 3 400 3 400 3 400 3 400 2.5 400
kp (s-l) 3.0 x lo4 1.17 x 105 3.1 x 104 3.6 104 3.6 x 104 3.8 104 8.1 x 103 7.2 x 103 6.7 103 9.5 103 7.3 103 1.94 104 5.5 x 103 4.6 103 7.1 x 103 8.2 x 103 2.8 105 2.8 x 105 3.3 105 2.5 x 105 4.3 x 105 4.8 105 3.9 x 2.4 x 1.3 x 3.9
105 105 105 105
kd2c
hib
128M-'s-ld 158 M-l s-l 124 M-' s-l 131 M-' s-l 130 M-' 141 M-l s-l
comments
0.1 mM Cu2+
11.1s-1
11.9 s-1 12.4 16.5 s-l 11.6 s-l 11.6 11.8 s-l 1.0x 104 M-1 s-1 d 3.2 x 103 M-I s-1 d 1.7 103 M-I s - l d 1.3 x 105 M-1 8-1 d 6.1 103 M-1 s-ld 6.6 x lo3 M-I s-l 2.6 x 104 S-1 2.4 104 s-1 6.6 x l o 4 6.8 x lo4 s-l 2.3 x 104 s-1 2.5 104 8-1 2.0 x 1 0 4 ~ 4
0.1 mM Cu2+
see ref 11
1.6 x 1.6 x 1.7 x 1.5
104 M-' 8-1 d 104 M-1 s-1 d 104 M-1 s-1 d 104 M-' 8-1 d
HzO 8.9 104 s-1 HzO a Observed rate of formation of the transient complex. * Observed rate of the second process. Obsellred rate of the third process. 2kl APL.
copper-carbon (5 bond as the rate of reaction is independent of [Cu+,,I, Table 4. The rate of reaction 19 is also independent of the presence of excess Cu2+aq;l1 however, the observed rate is higher by 1 order of magnitude than that previously reported. The source of this discrepancy is not clear. The decomposition of the transient complex L2Cu"CH3+ also obeys a second-order rate law. The rate of this process is independent of [L2Cu+l,Table 4. The results thus indicate that L1Cu"-CH3+ and L2Cu11CH3+ decompose via an analogous mechanism. Indeed the product analysis corroborates this conclusion because ethane is the only organic product observed, its yield exceeding 90% of that of the methyl radicals. The observed process is therefore attributed to the reaction
-
2L2C~11-CH3+aq 2L2Cu+,,
+ CzH6
2hI9 = 8.6 x
lo6 M-ls-l
(19a)
In principle reaction 19, taking into account that it does not involve homolysis, may proceed via one of the following routes: 1. Methyl transfer from one of the LCu"-CH3+,, complexes to the second followed by reductive elimination, i.e. via
-[
LCU"-CH~+
~LCU'LCH~+
LCU"'(-CH&+
1
LCU'"CH~+
+ CUL+
}-
-
2CUL'
+ C2H6
(19b)
(It should be noted that LCulI1-R transient complexes are formed in many reactions of alkyl radicals with Cu(11)complexes.12) 2. Via a transition state in which the two methyl groups bridge between the two copper ions, i.e. via
3. Via a transition state in which a carbon-carbon bond is formed directly while each methyl is bound to another copper, i.e. via
The observation that K19, is only slower by a factor of -35 than k19, though steric factors slow down K3 by a similar factor for the same ligands, L, seems to rule out transition states in which one of the methyls is bound coherently to the two copper ions, i.e. mechanisms (19b) and ( 1 9 ~ ) .Thus the results indicate that mechanism (19d) describes best the detailed mechanism of the observed process. It is of interest to note that the addition of 0.1 M CH3CN t o solutions containing C d a q has no effect on the kinetics of decomposition of LCu1'-CH3+aq, Table 4.As the Cu(1) under these conditions is present as Cu(CH3one has to conclude that either CN)2+aq,log p2 = 4.35,28 (CH~CN),CU~~-CH~+,, decomposes at the same rate of C U ' ' - C H ~ + ~or~that the CH3CN ligands are lost prior to this reaction. Mechanisms of Decomposition of the Complexes LCun-CH~COOH+/(LCu"-CH&OO-). The process of decomposition of the transient complex CuII-CHzCOOH'aq obeys a first-order rate law. The observed rate constant obeys the equation kd = KO + K[H+l, Figure 4. The reaction is accompanied by a decrease in [Cu+aq]. The yield of this decrease, G(-CU+,~),equals the yield of 'CH2COOH free radicals. These results indicate that the observed process is
Cu(I) Complexes with Aliphatic Free Radicals
Organometallics, Vol. 14, No. 12, 1995 5675
800
400
0.0010
0.0020
0.0030
c
H+ [MI
Figure 4. Dependence of the rate of decomposition of Cu"-CH2C0OHfaq on [H+]. Solution Composition: N20saturated, 1.0 x M Cufaq,and 0.1 M CH3COOH.
+ H30+/H,0 Cu2+,, + CH3COOH + H,O/OHk,, = (7000 + 1.88 x 107[H+1)s-l
Cu"-CH2COOH+,,
(20)
This rate law is in accord with reports on the heterolysis of various transient complexes with metal-carbon 0 bonds30 and with the observation that C U I ' - C H ~ + ~ ~ decomposes heterolytically at pH < l.ll The observation that CuI'-CH&OOH+aq decomposes heterolytically under conditions where C U ' ' - C H ~ + ~decomposes ~ via an alternative mechanism is not surprising as the electronwithdrawing substituent, -COOH, increases the polar nature of the metal-carbon 0 bond. Analogous observations were reported for the (H20)5Cr111-R2+ complexes.30 Also the process of decomposition of the transient complex L2Cu1'-CH2COO-aq obeys a first-order rate law, Table 4. The spectrum of the final product points out that L2Cu2+aqis one of the final products and that its yield equals the yield of the 'CH2COO- free radicals. Thus the results indicate that the observed process is
+
(L2C~11-CH2COO-)aq H20 CH3COO-
+
+ OH-
L2Cu2+,,
+
k,, = 11s-l (21)
! a
2
0 290
390
490
Wavelength (nm.) 0 Spectrum 22 microseconds after pulse 0 Spectrum 200 microseconds after pulse Spectrum 40 miliseconds after pulse A Difference spectrum between the spectra 22 and 200 microseconds after the pulse
Figure 5. Spectra of the transient complexes formed when a mixture of the free radicals CH2CH2C02- and *CH(CH3)C02-are reacted with Cu+aq. Solution composition: N2O-saturated, 1.0 x M CU+aq, and 0.1 M propionic acid, pH 3.0.
which are well separated in time. The first process obeys a first-order rate law. The rate of the latter reaction increases linearly with [H30+], Table 4, with a rate constant which obeys the relation hob = (8300 2.5 x 107[H+])s-l. The spectrum of the transient complex present in the solution after this process is plotted in Figure 5, the spectrum 200 ,us after the pulse. The second process obeys a second-order rate law; the rate of this process is independent of [Cu+aq]and of the concentrations of all the other components of the solution, Table 4. The decrease in [Cu+aq] points out that G(-Cu+aq) equals ca. half the yield of the aliphatic free radicals. These results indicate that the observed processes are as follows. 1. The first process observed is probably
+
+
-
CU~~-CH(CH,)COOH+,, H30+/H20 The observation that the rate of this reaction is pH Cu2',, CH,CH,COOH H,O/OHindependent, in the range 7 < pH < 10 (Table 4), indicates only that the acid-dependent term is smaller k,, = (8300 2.5 x 107[H+])s-l (22) than 3 x 107[H+]s-l. The fact that k21 is considerably smaller than the acid-independent component of k20 is This proposal is based on the following arguments: a. attributed to the steric hindrance imposed by L2 which The rate law of this process is very similar to that slows down the approach of the water molecule to the obtained for the decomposition of Cu1'-CH2COOH+aq. copper-carbon bond. b. G(-Cu+aq) equals the yield of the 'CH(CH3)COOH Mechanisms of Decomposition of the Complexes free radicals. LCU~~-CH~CH~COOH+/(LCU~~-CH~CH~COO-) and 2. The second process observed is probably LCuII-CH(C&)COOH+/(LC@- CH(CHs)COO-) In acidic solutions the reaction of *OH free radicals with ~CU"-CH,CH,COOH+~~ -,2Cufaq propionic acid yields 40% of 'CH(CH3)COOH and 60% (HOOCCH,CH,-), k23 = 1.7 x lo8 M-' s - ~(23) of *CH&H&OOH free radicals.24 The reaction of the mixture of these radicals with CU+aq cannot be resolved into two processes. The spectrum of the product of these This proposal is based on the following arguments: a. processes, which is expected to be a mixture of CuIIThe rate law of this process is very similar to that CH&H2COOH+aq and Cun-CH(CH3)COOH+aq, is s h o w obtained for the decomposition of CUI1-CH3+aq. b. If in Figure 5, the spectrum 22 ,us after the pulse. CU2+aqis indeed the product of the first process, then The mechanism of decomposition of the mixture of G(-Cu+aq) = 0 for this process. It should be pointed these transient complexes consists of two processes out that the dimer could not be detected under our
+
+
+
+
5676 Organometallics, Vol. 14, No. 12, 1995
Navon et al.
Table 5. Mechanisms and Rates of Decomposition of the Transient Complexes LCu"-Ru reaction ~ C U " - C H ~ +~CU-,, ~ ~ + CzH6 CU"-CHZCOOH+ + H&-/HzO CU*+,~ + CH3COOH CU"-CH(CH~)COOH++ H30+/Hz0 CU'+,, + CHsCH?COOH ~CU~~-C'H~CH~COOH',, 2Cu+,, + (HOOCCHzCH2-)2 2LzCu"-CHs+,o L2Cu+,, + CzHfi . . (L~CUII-CH~COO-), +H ~ O L2Cu2+,, + CH3CdO- + OH(LZC~"-CH2CHzCOO-), * ~zcuz-,, + C H ~ C H ~ C ~ O 2CHzCHzCOO-0OCCHzCHz-CHzCHzC00-
-
-..
-
+
+
-
-
kdb
3.0 x loa M-ls-' (7000 + 1.90 x 107[H+1)s-l (8300 -r 2.5 x 107[H+1)s-' 3.4 x loa M-I s-'
The observed rate decreases with the increase in [L2Cu+l, 2k = 4.2 x lo7, 7.8 x lo7, and 2.7 x lo8 M-l s-l 5 x and 2 x M, for [L2Cu+l = 8 x respectively. These results suggest that the mechanism of decomposition involves the homolysis of the coppercarbon u bond: (L2C~11-CH2CH2COO-),, ==
+
L2Cu+aq 'CH2CH2COO- (24)
8.6 x lo6 M-' s-' 11 s-1
1.3
This reaction might be followed by one of the following:
10-4 M-'
1.1 x 109 M-1
5-1 c.d
a Experimental conditions as in Table 1. Error limit f 1 5 % . Value given for 2k. Measured in this study, the value is an average one for all the possible reactions between the free radicals 'CHzCH2COO- and 'CH(CH3)COO-, which are not separated in time.
experimental conditions. c. Acrylic acid, an alternative product of reaction 23, if disproportionation and not dimerization occurs, is not formed in this process, G(acry1ic acid) < 0.5. Therefore the spectrum measured 200 ps after a pulse is attributed to Cu"-CH2CH2COOH+aq and the difference spectrum between the spectra measured 22 and 200 pus after the pulse, Figure 5, is attributed to Cu'ICH2CH2COOH+aq. When N20-saturated solutions containing (1- 10) x M of L2Cu+,pH 7-10 (the concentration of L2 was always twice that of [Cu(I)I) and 0.1-0.3 M CH3CH2C02H are irradiated, the primary free radicals are transformed into a mixture of 55% 'CH(CH&02- and 45% 'CH2CH2C02- free radicals.24 The reaction of this mixture with L2Cu+was studied. The formation of a n unstable intermediate with an absorption band at 390 nm and apparent, assuming that the yield of the intermediate equals the total free radical yield, cmax = (1100 f 150) M-l cm-1 is observed. This intermediate decomposes in a process which obeys a second-order rate law, Table 4. L2Cu2+is not a product of this process. I t is proposed that these results indicate that under our experimental conditions only the 'CH2CH2C02- free radical reacts with L2Cu+. This proposal is based on the following observations: 1. The kinetics of decomposition indicate that only one transient complex is formed. 2. The apparent molar absorption coefficient of this transient, cmax= 1100 M-l cm-l is considerably smaller than that of the other transients of the type L2CuII-R, Table 3. 3. L2Cu2+is not a product of this process suggesting that the transient is not (L2Cu11CH(CH3)C02-). 4. The rates of reaction of free radicals with L2Cuf, Table 2, indicate that steric hindrance affects them considerably. The free radical 'CH( CH3)C02- is considerably more hindered than 'CH2CH2C02-. Therefore the correct molar absorption coefficient of this transient is 2500 f 400 M-l cm-l, as its yield is only 45% of that of 'OH free radicals. The rate of decomposition of the transient complex (L2CuTr-CH2CH2COO-)obeys a second-order rate law.
2'CH2CH2COO-
-
-00CCH2CH2CH2CH,C00-
k,,
= 1.1 x lo9 M-ls-l (25)
+
*CH2CH2COO- (L2Cu"-CH2CH2C0O-),, L2Cu+,,
+.
+ -00CCH,CH2CH2CH2C00-
(26)
As the observed rate of decomposition approaches at low [L2Cu+,,1 k25, it was checked whether the data are in accord with the assumption that the observed process involves reactions 24 and 25. Indeed the experimental data are in accord with K24 = 1.3 x loW4M. As k-24 = 6.3 x lo6 M-l s-l, one calculates k24 = 800 s-l. Concluding Remarks
The results, summed up in Table 5, clearly point out that a variety of mechanisms contribute to the decomposition of transient complexes of the type LCuII-R. Thus even if p-elimination is ruled out, by the absence of appropriate leaving groups, one observes the following mechanisms: 1. Heterolysis of the copper-carbon u bond occurs; this mechanism is prominent mainly when electron-withdrawing groups are bound to the a carbon. This mechanism is acid catalyzed. 2. Homolysis of the copper-carbon u bond occurs; this mechanism is enhanced if the free radical formed is stabilized by r e s ~ n a n c e l ~and J ~by ~ steric hindrance imposed by the ligand, L, or by substituents on the aliphatic residue 3. There is a reaction of two transient complexes with each other to form a carbon-carbon bond between the two aliphatic residues. Finally it is of interest to note that none of the transient complexes decomposes via a mechanism involving a p-hydride shift, though copper is known to oxidize aliphatic free radicals via this mechanism in other solvents.2 Acknowledgment. This study was supported in part by The Israel Science Foundation administered by
The Israel Academy of Sciences and Humanities and by a grant from the Budgeting and Planing Committee of the Council of Higher Education and the Israel Atomic Energy Commission. D.M. wishes to express his thanks to Mrs. Irene Evens for her ongoing interest and support. OM950434G
