4694
J. Am. Chem. SOC.1992, 114, 4694-4701
Dynamics of Proton Transfer from Cation Radicals. Kinetic and Thermodynamic Acidities of Cation Radicals of NADH Analogues Agnb Anne,'" Philippe Hapiot,l"Jacques Moiroux,l"Pedatsur Neta,lb and Jean-Michel SavBant*Jn Contribution from the Laboratoire d'Electrochimie MolPculaire, Unit8 de Recherche AssociPe au CNRS No. 438. UniversitP Paris 7 , 2 Place Jussieu 75251 Paris Cedex 05, France, and Chemical Kinetics and Thermodynamics Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899. Received October 21, 1991. Revised Manuscript Received February 3, I992
Abstract: Combined application of direct electrochemistry,redox catalysis, and laser flash photolysis has allowed the determination of the deprotonation rate constants of the cation radicals of four synthetic analogues of NADH by an extended series of bases. When sterically encumbered bases are avoided, the diffusion limit is reached in all cases at the upper edge of the driving force range. The intrinsic kinetic acidities,derived from the fitting of the rate data with a quadratic activation4riving force relationship, show no correlation with the thermodynamic acidities. They correlate with the homolytic bond dissociation energies of the "C-H bond suggesting that the proton transfer from the cation radicals is better viewed as a concerted electron-H atom transfer rather than a stricto sensu proton transfer. Application of the dissociative electron-transfer theory shows that the homolytic bond dissociation energy is the main factor governing the reaction dynamics. Previous rate data from the literature pertaining to other cation radicals show the same trends.
Although investigated and discussed for many years,2 the dynamics of proton transfer continue to raise considerable interest from both an experimental3 and a theoretical point of views4 Deprotonation of organic cation radicals has attracted special attention in view of its relevance to the oxidation of organic compounds.s Analysis of the kinetics of proton transfer from this particular class of carbon acids may also provide valuable clues (1) (a) Universitb de Paris 7. (b) N E T . (2)(a) Eigen, M. Angew. Chem. Int. Ed. Engl. 1964,3, 1. (b) Bell, R. P. The Proton in Chemistry, 2nd ed.; Cornell University Press: Ithaca, NY, 1973. (c) Caldin, E.; Gold, V., Eds. Proton Transfer Reactions; Chapman and Hall: London, 1975. (d) Marcus, R. A. J . Phys. Chem. 1968,72,891. (e) Cohen, A. 0.;Marcus, R. A. J. Phys. Chem. 1968,72,4249.( f ) Marcus, R. A. Faraday Symp. Chem. SOC.1975,10,60.(g) Marcus, R. A. Faraday Discuss. Chem. Soc. 1982,74,7.(h) Albery, W. J. Annu. Rev. Phys. Chem. 1980,31,227. (i) Albery, W. J. Faraday Discuss. Chem. Soc. 1982,74,245. 6)Kresge, A. J. Chem. Soc. Rev. 1973,2,475.(k) Kresge, A. J. Acc. Chem. Res. 1975,8,354.(I) Kreevoy, M. M.; Konasewich, D. E. Ado. Chem. Phys. 1972,21,243.(m) Kreevoy, M. M.; Oh, S.W. J. Am. Chem. SOC.1973,95, 4805. (n) Hupe, D.J.; Wu, D. J . Am. Chem. SOC.1977, 99, 7653. (0) Bernasconi, C. F. In Techniques and Applications of Fast Reactions in Solution; Gettins, w. J., Wyn-Jones, E., Eds.; Reidel: Dordrecht, 1979; pp 453-462. (p) Murdoch, J. R. J. Am. Chem. Soc. 1980,102,71.(9)Murdoch, J. R. J . Am. Chem. SOC.1983, 105, 2660. (3)(a) Mostly in nonprotic soIvents,3b~C but see also refs, 3d and 3e. (b) Richtie, C. D.; Lu, S . J . Am. Chem. SOC.1989,111, 8542. (c) Richtie, C. D.; Lu, S . J . Am. Chem. SOC.1990,112,7748. (d) Hibbert, F. Ado. Phys. Org. Chem. 1986,22, 113. (e) Perrin, C. Acc. Chem. Res. 1989,22, 268. (4)(a) Phenomenological analysis of experimental activation vs driving force relationships,4bf adaptation of Marcus equation to gas-phase proton transfer,4ei and ab initio quantum mechanical m~deIling.~fi (b) Jencks, W. P. Chem. Rev. 1985,85,511.(c) Murray, C. J.; Jencks, W. P. J . Am. Chem. Soc. 1990,112,1880. (d) Bunting, J. W.; Stefanidis, D. J. Am. Chem. SOC. 1988, 110,4008. (e) Bunting, J. W.; Stefanidis, D. J. J. Am. Chem. SOC. 1989,111,5834.( f ) Bunting, J. W.; Stefanidis, D. J. J . Am. Chem. Soc. 1990, 112,779. (g) Jasinski, J. M.; Braumam, J. I. J. Am. Chem. SOC.1980,102, 2906. (h) Lim, K. F.; Brauman, J. I. J. Chem. Phys. 1991,94, 7164. (i) Mest-Ner, M.; Smith, S.C. J. Am. Chem. Soc. 1991,113,862.6 ) Scheiner, S.Acc. Chem. Res. 1985,18, 174. (k) Cao, H. Z.; Allavena, M.; Tapia, 0.; Eveleth, E. M. J. Phys. Chem. 1985,89,1581. (I) Gill, P. M. W.; Radom, L. J . Am. Chem. Soc. 1988,110,5311.(m) Swanton, D. J.; Marsden, D. C. J.; Radom, L. Org. Mass Spectrom. 1991, 26,227. (n) Wolfe, S.;Hoz, S.; Kim, C. K.; Yong, K. J. Am. Chem. SOC.1990,112,4186. (5) (a) Schlesener, C. J.; Amatore, C.; Kochi, J. K. J . Am. Chem. SOC. 1984,106,7472. (b) Schlesener, C. J.; Amatore, C.; Kochi, J. K. J . Phys. Chem. 1986,90, 3747. (c) Masnovi, J. M.; Sankararaman, S.; Kochi, J. K. J . Am. Chem. Soc. 1989,111,2263.(d) Manring, L.E.; Peters, K. S . J . Am. Chem. SOC.1985, 107,6452. (e) Dinnocenzo, J. P.; Banach, T. E. J. Am. Chem. Soc. 1989,Ill, 8646. ( f ) Reitstkn, B.; Parker, V. D. J . Am. Chem. Soc. 1990,112,4968. (g) Parker, V. D.; Chao, Y.; Reitstkn, B. J. Am. Chem. SOC.1991, 113, 2336.
Chart I
I CH3 MAH
I
CHpPh BOAH
I
CH,Ph BOCNH
I
CH2Ph BNAH
for a better understanding of the dynamics of proton transfer in general. We have shown recently with the example of N-methylacridan that the comparative use of direct and indirect (redox catalysis) electrochemical techniques and of laser flash photolysis allows one to establish the mechanism of the oxidation of the substrate, AH, into A+ and to measure the rate constants of the deprotonation of the AH'+ cation radical by adding bases up to the diffusion limit.6a,b Determination of the standard potentials, @(AH'++e-/AH) and P ( A + + e - / R ) by electrochemical techniques and of P(A++H++2e-/AH) by a slow equilibration spectrometric method& will also allow the determination of the pK, of the cation radical. At 20 O C O.058pKa(AH''/A'+H+) = 2@(A++H++2e-/AH) P(AH*++e-/AH) - P(A'+e-/A*) (I) (6) (a) Hapiot, P.; Moiroux, J.; Savbant, J-M. J. Am. Chem. SOC.1990, 112, 1337. (b) Anne, A.; Hapiot, P.; Moiroux, J.; Neta, P.; Savbant, J-M. J. Phys. Chem. 1991, 95,2370. (c) Anne, A,; Moiroux, J. J. Org. Chem. 1990, 55, 4608. (d) Andrieux, C. P.; Anne, A.; Moiroux, J.; Savbant, J. M. J. Electroanal. Chem. 1991,307, 17. (7)(a) Amatore, C.; Savbant, J-M. J. Electroanal. Chem. 1977,85, 27. (b) Amatore, C.; Gareil, M.; SavEant, J-M. J. Electroanal. Chem. 1983,147, 1. (c) Andrieux, C. P.; Savbnt, J-M. Electrochemical Reactions. In Investigation of Rates and Mechanisms of Reactions. Bernasconi, C. P., Ed.; Techniques of Chemistry; Wiley-Interscience: New York, 1986;Vol. 6,Part 2,pp 305-3 10.
0002-7863/92/ 1 5 14-4694%03.00/0 0 1992 American Chemical Society
Dynamics of Proton Transfer from Cation Radicals
J . Am. Chem. Soc., Vol. 114, No. 12, 1992 4695
Table I. Deprotonation Rate Constants of the AH" Cation Radicals'
base 2-fluoropyridine 2-chloropyridine 3-cyanopyridine 4-cyanopyridine 3-chloropyridine
PK,2O 4.2
MAH 3.3 f 0.2'
6.3
3-fluoropyridine
9.4
4.2 f 0.1' 4.4 f 0.2' 4.8 f 0.1' 5.2 f 0.2' 5.3 f O.ld 5.3 f 0.26
7.0 8.0 9.0
nicotinamide isonicotinamide pyridine
log k, M-' s-I BQCNH BQAH 6.2 f 0.2b 7.0 f O.Zb 7.2 f 0.2' 7.5 f 0.1'
13.5
3,Sdimethylpyridine
14.5
ammonia benzylamine rert-butylamine
16.5 16.8 18.1
piperidine pyrrolidine 1,3-diaminopropane'
18.9 19.6 19.7
7.3 f O.ld 7.65 f O.ld
8.65 f 0.1'
8.6 f 0.1' 8.75 f O.ld
7.8 f 0.1' 7.8 f O.ld 8.1 f O.ld 8.2 & 0.2' 8.9 & 0.1' 8.9 f O.ld
8.9 f O.lc
8.9 f 0.1'
9.05 f O . l d
9.2 f 0.1'
8.8 f O.ld 9.1 f 0.1' 8.9 f O . l d
9-05 f 0.1' 8.95 f O.ld
8.0 f 0.1'
7.8 f 0.lc 7.9 f O.ld 8.0 f O.ld
6.5 f 0.2' 6.3 f 0.1' 6.6 f O.ld
3-methylpyridine
6.95 f O . l d 7.7 f O.ld
9.8
10.1 12.3
BNAH
6.3 f 0.2' 6.7, f O.ld 6.4 f 0.2' 6.95 f 0.1' 6.95 f O.ld 8.3 f 0.2d 8.4 f 0.1' 8.6 f 0.1' 8.4 f O.ld 8.9 f O , l c 9.0 f 0.2' 8.7 f O.Zc 9.1 f O.ld 9.5 f O.ld
8.9 f O.ld 9.4 f 0.1' 9.4 f 0.1' 9.g5 f O.ld
9.4 f O.ld
9.7 f O . l d
9.7 f O.ld
1,4-diamin~butane~ 20.1 9.9 f 0.ld 9.8 f O.ld acetate 22.3 10.1 f O.ld 10.0~f 0 . 1 ~ 'In acetonitrile at 20 OC. 'Cyclic voltammetry at ultramicroelectrodes. 'Redox catalysis. dLaser flash photolysis. ' k values corrected for the statistical factor (2). We applied these methods to obtain the rate constants of deprotonation of the cation radicals in acetonitrile of three analogues of NADH, BNAH, BQAH, and BQCNH (see Chart I) by an extended series of bases including pyridines and also aliphatic amines and acetate. Since the combination of methods described above allows the determination of the pK, of each of the cation radicals in the series and since the pK, of each base is known, it was possible to investigate, in each case, the variation of the activation energy with the exact driving force of the reaction. We could thus compare the intrinsic kinetic acidities, as derived from the intrinsic barriers (activation free energies at zero driving force), to the thermodynamic acidities. It was thus found that no correlation exists between these two quantities to the point that the cation radical that is thermodynamically the weakest acid is the fastest in terms of intrinsic kinetic acidity. We observed that, on the other hand, a good correlation exists between the intrinsic A+ barrier and the homolytic bond dissociation energy (AH" H') with a slope not far from one-fourth. A model viewing homolytic cleavage of the carbon-hydrogen bond of the cation radical, concerted with electron transfer, as the predominant reorganization factor in the reaction is suggested to account for these observations. It is shown to also fit literature kinetic data pertaining to other cation radicals.
-
+
Results The rate constants for the deprotonation of the AH" cation radicals by a wries of bases encompassing the largest possible range of pKa's were obtained from the combined use of cyclic voltammetry at ultramicroelectrodes, redox catalysis, and laser flash photolysis. There is a lower limit of the pK,'s that can be investigated due to the basic properties of the starting AH molecules, leading to the reaction AH"
+ AH
-
A'
+ AH2+
(1)
Sound results are thus obtainable only when the reaction with the base is faster than the reaction with AH. Thus, even if the base was introduced in excess by a factor of about 1000, a limitation exists as to the possibility of investigating the deprotonation of the AH" cation radicals by weak bases. This is the reason that the lower pK, limit was 4.2 with MAH and BQCNH but 8 for
Table 11. Standard Potentialsa and &'sb MAH BQCNH BOAH BNAH E0(A+H++2e-/AH)' 218 f 24" 257 f 26" 115 f 25" 22 i'92 EO(AH'++e-/AH) 855 i 4d 1144 f 4d 924 f ge 786 i ge Eo (A++e-/A') -465 i lod -520 f lod -720 f 10" -1105 f lod pK,(AH"/A'+H+)' 0.8 f 0.9" -1.9 f 0.9 0.4 i 0.9 4.7f 1.0 In mV vs SCE. In acetonitrile at 20 'C. 'The main uncertainty occurs in the case of MAH." The other determinations are made by reference to MAH with a small additional error. The relative values in the series are thus known with a much better accuracy than the absolute values. The same is consequently true for the pK,'s. dCyclic voltammetry at ultramicroelectrodes. 'From the determination of k,, by redox catalysis and k, by laser flash photolysis. /From eq 1.
BQAH and BNAH. The latter compounds have indeed been observed to be more basic than the former.6c Cyclic Voltammetry at Ultramicroelectrodes. Because of its limitations in the determination of the very high rate constants, the use of cyclic voltammetry at ultramicroelectrodes was restricted to MAH,6ain the pKa range 4.2-14.5, and to BQCNH, in the range 4.2-7.0. For BQCNH, analysis of the current response as a function of the scan rate and substrate concentration led to the conclusion that the electrochemical oxidation follows the same DISPl m e ~ h a n i s m ' ~as* ~observed before with MAH:6a AH - e-
AH*+ AH"
AH'+
+ B A A* + BH+ + A' A+ + AH
-
(3)
(4)
The values of the deprotonation rate constant could thus be determined for the three weakest bases in the series (Table I) as well as the value of the standard potential of the AH/AH'+, E"(AH/AH'+) (Table 11). As seen earlier (eq I), the standard potential of the A+/A' couple, E"(A+/A'), is a useful quantity for estimating the pK, of the AH'+ cation radical. Its values were derived from high scan rate cyclic voltammetry of the A+ cations.' (8)(a) See ref 8b for more details. (b) Anne, A.; Hapiot, P.; Moiroux, J.; Savtant, J-M. J. Electroanal. Chem., in press.
Anne e? al.
4696 J. Am. Chem. SOC..Vol. 114, No. 15, 1992 I
h
r
6
e
20
s
n
a
0 400
500
600
A (nm> Figure 1. Transient absorption spectra recorded upon laser photolysis (351 nm) of an acetonitrile solution of BNAH (0.14 mM)and CHBrp (0.25 M) 1 ps (0)and 5 ps (0)after the pulse. After 400 ps the absorption at 5 M 0 0 nm decays to zero and that at
