J. Phys. Chem. 1986, 90, 3815-3823
3815
Kinetics of Dissociative Electron Transfer. Direct and Mediated Electrochemical Reductive Cleavage of the Carbon-Halogen Bond Claude P. Andrieux, Jean-Michel Saveant,* and Kbac Binh Su Laboratoire d'Electrochimie de l'Universit2 Paris 7, Unit2 Associte au C.N.R.S.No. 438, 75251 Paris Cedex 05, France (Received: January 10, 1986)
The kinetics of the heterogeneous and homogeneous reductive cleavage of organic halides is investigatedby direct electrochemical techniques and by the redox catalysis method. In the case of aromatic halides the reduction involves the intermediacy of the anion radical of the starting molecule. There is a rough correlation between the rate constant for the cleavage of the anion radical and the ArX/ArX- standard potential which can be viewed as a Bronsted-Marcus type activation4riving force free energy relationship. The aliphatic halides undergo a true dissociative electron transfer, Le., the breaking of the carbon-halogen bond is concerted with the electron-transfer process. The heterogeneous and homogeneous kinetic data fit reasonably well with an activation-driving force relationship derived from the Marcus quadratic theory. The dependency of the thermodynamics and kinetics of the dissociative electron transfer upon the energy and strength of the aliphatic carbon-halogen bond is discussed.
Electrochemistry appears as a particularly convenient tool for investigating the kinetics of electron-transfer reactions as a function of their thermodynamics. The driving force can indeed be simply and continuously varied by means of the electrode potential. On the other hand, the current flowing through the electrode is a direct measure of the reaction kinetics. The possibility of using simply controlable and measurable electrical variables offers a considerable advantage over the conventional methods of homogeneous kinetics even though it must be recognized that particular limitations and difficulties in the gathering and treatment of the kinetic data result from the heterogeneous character of the electrochemical electron-transfer reactions. This may explain why activationdriving force free energy relationships have very early taken a central place in electrochemistry under the form of the ButlerVolmer' equation:
( i / S is the current density, E is the electrode potential, (CA)oand (CB)oare the reactant concentrations at the electrode surface for the reaction A e F! B, Eo is the standard potential of the A/B couple, kSd-ap is the standard rate constant, Le., rate constant far E = EO, uncorrected from the work terms derived from the existence of the electrochemical double layer.) This is a linear free energy relationship in the sense that the transfer coefficient, a, is regarded as independent from the driving force, E - EO. In spite of its considerable practical usefulness, the Volmer-Butler equation has serious drawbacks which derive from its phenomenological character. The electrical part of the activation free energies are considered as a constant fraction-a for the reduction, 1 - a for the oxidation-of the electrical part of the driving force with no attempt to describe more intimately the mechanism of the electron-transfer reaction. Hence is precluded the establishement of relationships between the kinetics of the electron transfer and the molecular structure of the reactants on one hand and the characteristics of the reaction medium on the other. Marcus theory of outer-sphere electron transfer2 fills this gap by giving a description of the activation process in terms of two main factors: the reorganization of the solvent and the internal
+
(1) (a) Butler, J. A. V. Trans. Faraday SOC.1924, 19,729. (b) Butler, J. A. V. Trans. Faraday Soc. 1924,19,734. (c) Erdey-Gruz, T.; Volmer, M. Z . Phys. Chem. 1930, ZSOA, 203. (d) Delahay, P. Double Layer and Electrode Kinetics; Interscience: New York, 1965;pp 154-159. (2) (a) Marcus, R. A. J. Chem. Phys. 1956,24, 966. (b) Marcus, R.A. Annu. Rev. Phys. Chem. 1964, IS, 155. (c) Marcus, R. A. Faraday Discuss. Chem. SOC.1982, 72, 7.
0022-3654/86/2090-38 15$01.50/0
reorganization of the molecule (changes in bond angles and lengths). On the other hand, since both factors are treated under an harmonic approximation, the transfer coefficient is no longer a constant and varies linearly with the electrode potential, being close to 0.5 at the standard potential. Since electrochemical and homogeneous electron-transfer reactions are described essentially on the same grounds, Marcus theory predicts a simple relationship between the electrochemical standard rate constant and the homogeneous isotopic rate constant pertaining to the same redox couple. Several of these aspects of Marcus theory have been investigated experimentally in the context of organic electrochemistry for outer-sphere electron transfers? effect of solvent p~larization,~ relationship between homogeneous and electrochemical chargetransfer kinetic^,^ variation of the transfer coefficient with potentiaL6 We discuss in the following the case where electron transfer to an organic molecule is accompanied by the cleavage of a bond. The reductive cleavage of the carbon-halogen bond is one of the simplest example of such a process in organic electrochemistry.' Two main behaviors must be distinguish@ according to the nature of the carbon atom. When the functional carbon belongs to an aromatic ring, the anion radical formed upon injection of one electron appears as a true intermediate in most cases, even though it may cleave very rapidly into the corresponding aryl radical and the halide ion. Most of the available kinetic information then concerns the cleavage of the anion radical rather than the initial electron transfer itself. In the case of an alphatic carbon, there is good evidence, as further elaborated below, that the anion radical does not actually exist as an intermediate. Strictly speaking, we thus deal with a dissociative electron transfer in the sense that (3) The term 'outer-sphere" electron transfer was originally coined for coordination compounds as designating a category of electron-transfer reactions where no drastic changes, such as bond breaking or forming, occur within the coordination sphere. We can likewise term "outer-sphere", electron transfers involving organic molecules where no breaking or forming of bonds take place within the time scale within which kinetic data are gathered. (4)(a) Peover, M. E.; Powell, J. S.J. Electroanal. Chem. 1969, 20, 427. (b) Falsig, M.; Lund, H.; Nadjo, L.; Saviant, J. M. N o w . J . Chim. 1980, 4 , 445. (c) Fawcet, W. R.; Jawovski, J. s. J. Phys. Chem. 1983, 87, 2972-6. (5) Kojima, H.; Bard, A. J. J . Am. Chem. SOC.1975, 97,6317. (6)(a) SavCnt, J. M.; Tessier, D. J. Electroawl. Chem. 1975,65, 57. (b) SavCnt, J. M..; Tessier, D. J. Phys. Chem. 1977, 81, 2192. (c) Saviant, J. M; Tessier, D. J . Phys. Chem. 1978,82, 1723. (d) Garreau, D.; Saviant, J. M.; Tessier, D. J . Phys. Chem. 1979, 83, 3003. (e) Savbant, J. M.; Tessier, D. Faraday Discuss. Chem. SOC.1982, 74, 57. (7) (a) Hawley, M. D. In Ewyclopedia of Electrochemistry of the Elements, Organic Section, Vol. XIV,Bard, A. J., Lund, H., EMS.; Dekker: New York, 1980; (b) Becker, J. Y . In The Chemistry of Functional Groups, Supplement D, Patai, S., Rappoport, Z.,Ed.; Wiley: New York, 1983; Chapter 6,pp 203-285.
0 1986 American Chemical Society
3816
The J o u r n a l of P h y s i c a l C h e m i s t r y , Vol. 90, No. 16, 1986
Andrieux et al.
electron transfer and breaking of the carbon-halogen bond are concerted processes.
ArCl 10
Results and Discussion A r o m a t i c H a l i d e s . Mechanistic and kinetic information about electron transfer to these molecules is available from electrochemical* and pulse radiolysis9 investigations. The rate constant for t h e cleavage of t h e initially formed anion r a d i c a l
-
8-
6-
+ e ~i ArX'k Ar' + XArX'-
4-
varies considerably (the half-life ranges from hours to nanoseconds) with the nature of the aromatic residue and of the halogen (I- is a better leaving group than Br-, itself better than C1-). When not too large, Le., smaller than about lo4 s-I, the cleavage rate constants can be measured by using directly electrochemical techniques such as cyclic voltammetry or double potential step chronoamperometry.1° For faster cleavages, an indirect electrochemical method, namely redox catalysis, can be used instead." The principles of the method involve the electrochemical generation of the reduced form Q of a reversible and chemically stable redox couple, P/Q, at a potential positive to the reduction of ArX. Q then transfers an electron to ArX then regenerating P, this step being followed by the cleavage of the ensuing ArX'- radical:
0-
ArX
P
+
log k ( k t s-')
-
0'-
e
-1
ArBr , 1
/.-
(0)
0
C-CH,
E?"
a'-
+
ArX
ArX' -
RI ====
k- I
Ar'
P
+ +
ArX'X-
(1)
(2)
The regeneration of P results in a loss of reversibility and an increase in height of the P / Q wave upon addition of increasing amounts of ArX. This catalytic enhancement of the P / Q wave contains information on the kinetics of the two followup reactions 1 and 2. Provided k is not too large as compared to k-l, the former rate constant can be derived" from experiments carried out with mediators of appropriate standard potentials and in which the
0
1 I
1
1
2
EqVviSC
Figure 1. Correlation between the cleavage rate constant of the aryl halide anion radical and the standard potential of the aryl halide/anion radical couple in dimethylformamide.
(8) (a) Lawless, J. G.; Hawley, M. D. J. Electroanal. Chem. 1969, 21, 365.
(b) Danen, W. C.; Kensler, T. T.; Lawless, J. G.; Marcus, M. F.; Hawley, M. D. J. Phys. Chem. 1969, 73, 4389. (c) Nadjo, L.; SavCnt, J. M. J. Electroanal. Chem. 1971, 30, 41. (d) Nelson, R. F.; Carpenter, A. K.; Seo, E. T. J . Electrochem. SOC.1973, 120, 206. (e) Alwair, K.; Grimshaw, J. J . Chem. SOC.,Perkin Trans. 2 1973, 1150. (f) Alwair, K.; Grimshaw, J. J. Chem. SOC.,Perkin Trans. 2 1973, 1811. (9) Houser, K. J.; Bartak, D. E.; Hawley, M. D. J. Am. Chem. SOC.1973,95,6033. (h) Grimshaw, J.; Trocha-Grimshaw, J. J . Electroanal. Chem. 1974, 56, 443. (i) SavBant, J. M.; Thibbault, A. J . Electroanal. Chem. 1978,89, 335. (j)MHalla, F.; Pinson, J.; SavCant, J. M. J. Electroanal. Chem. 1978, 89, 347. (k) Amatore, C.; Chaussard, J.; Pinson, J.; Savbant, J.. M.; ThiBbault, A. J . Am. Chem. SOC. 1979,101,6012. (1) Gores, G. J.; Koeppe, C. E.; Bartak, D. E. J. Org. Chem. 1979, 44, 380. (m) Amatore, C.; Pinson, J.; Saveant, J. M.; Thiebault, A. J. Electroanal. Chem. 1980, 107, 59. (n) Andrieux, C. P.; Blocman, C.; Dumas-Bouchiat, J. M.; M'Halla, F.; SavCnt, J. M. J. Am. Chem. SOC.1980, 102, 3806. ( 0 ) M'Halla, F.; Pinson, J.; Savtant, J. M. J. Am. Chem. SOC. 1980,102,4120. (p) Heinze, J.; Schwart, J. J. Electroanal. Chem. 1981, 126, 283. (9) Aalstad, J.; Parker, V. D. Acta Chem. Scand., Sect. B 1982, 36, 47. (r) Parker, V. D. Acra Chem. Scand., Sect. B 1981, 35, 595. (s) Parker, V. D. Acta Chem. Scand., Sect. B 1981, 35, 655. (t) Amatore, C.; BadozLambling, J.; Bonnel-Hughes, C.; Pinson, J.; SavBant, J. M.; Thi€bault, A. J. Am. Chem. SOC.1982, 104, 1919. (9) (a) Steelhammer, J. C.; Wentworth, W. E. J . Chem. Phys. 1969,51, 1802. (b) Neta, P.; Behar, D. J . Am. Chem. SOC.1981, 103, 103. (10) (a) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; Wiley: New York, 1980. (b) Andrieux, C. P.; SavCnt, J. M. 'Electrochemical Reactions" in 'Investigation of Rates and Mechanisms of Reactions", Bernasconi, C. F., Ed., Techniques of Chemistry, Weissberger, A., Ed.; Wiley, New York, in press. (11) (a) Andrieux, C. P.; Dumas-Bouchiast, J. M.; Saveant, J. M. J. Electroanal. Chem. 1978, 87, 39. (b) Ibid. 1978, 87, 55. (c) Ibid. 1978, 88 27. (d) Andrieux, C. P.; Blocman, C.; Saveant, J. M. J. Electroanal. Chem. 1979, 79, 413. (e) Andrieux, C. P.; Dumas-Bouchiat, J. M.; Savhnt, J. M. J. Electroanal. Chem. 1970, 113, 1. (0 SavQnt, J. M.; Su,K. B. J. Electroanal. Chem. 1984, 171, 341. (g) Ibid. 1985,196, 1. (h) Nadjo, L.; SavCnt, J. M.; Su, K. B. J. Electroanal. Chem. 1985, 196, 23.
competition between reaction 2 and backward reaction 1 can be varied by changing the mediator concentration. The standard potential of the ArX/ArX- couple can also be derived from this type of experiments. A dramatic extension of the range of cleavage rate constants measurable electrochemically ensues: half-lives down to the nanosecond range are thus attainable. The reason for this huge gain in performances is that the mediated process amounts to an electrochemical process having an extremely small diffusion layer thickness, of the order of molecular dimensions, i.e., a few cm as compared to the 10-3-104-cm diffusion layer thicknesses commonly encountered in direct electrochemistry. Gathering together a number of values of the cleavage rate constants, obtained with aryl chlorides and bromides by direct electrochemistry or by the above indirect redox catalytic method, w e can see (Figure 1) that there is a rough linear correlation12a between log k and the standard potential of the ArX/ArX- redox couple. For each halogen, the more negative the standard potential the faster the cleavage.Izb The cleavage of the ArX- radical into Ar' and X- can be viewed as an electron-transfer reaction. The impaired electron is first located in the ?r* orbital of the aromatic system. As the C-X bond stretches, a three-electron bond is formed which eventually splits, an unpaired electron remaining in the @-HOMOorbital of t h e aryl radical and an electron doublet being carried away in the 2p atomic orbital of the halogen. Extended Huckel molecular orbital (12).(a) The correlation coefficient is 0.88 for ArCl and 0.94 for ArBr. (b) A similar qualitative trend has been found previously for the cleavage of anion radicals of aromatic thio~arbonates.~~
Kinetics of Dissociative Electron Transfer
The Journal of Physical Chemistry, Vol. 90, No. 16, 1986 3817 CHART I
log k
(1) DIFFUSION
CONTROL
PhBr PhCl 2-PyBr 3-PyBr
Figure 2. Variations of the homogeneous forward electron-tranfer rate constant with potential showing activation and diffusion control.
calculationsi3" show that the increase of the C-X distance results in a stabilization of the C-X u* orbital whereas the energy of the A* orbital varies very little. The crossing point or A* and u* energy levels can thus be taken as a likely representation of the transition state.'3b The calculations show that the A* orbital energy varies significantly with the nature of Ar whereas the u* orbital and the Ar' u-HOMO orbital energies are practically independent of Ar. It follows that the activation, as represented by the energy at the crossing point, is correlated with the energy of the A* 0rbita1.l~" Since the latter and the standard potential of the A r X / A r X couple are linearly related in aryl chlorides and aryl bromide~'~" as they are in the case of stable aromatic anion radicals,14 the correlation between the activation energy and the ArX/ArX'standard potential ensues. On the other hand, since the A* orbital energy varies with Ar while the Ar' u-HOMO orbital energy does not depend on Ar, the driving force of the cleavage reaction can be represented by the x* orbital energy and therefore by the ArX/ArX'- standard p0tentia1.I~~The diagrams in Figure 1 can thus be approximately considered as activation-driving force free energy relationships characterizing the cleavage reaction. I t is noteworthy in this connection that their slope are very close to 0.5 in terms of activation free energy vs. driving force free energy, reminiscent of what is predicted by the Marcus theory of electron transfer. The roughness of the observed correlation should, however, be emphasized. The above-described model rationalizes a general trend but is obviously not able to account for the effect of small structural molecular changes on the kinetics of the cleavage reaction. The model is indeed rather crude, neglecting possibly important factors such as entropy variations and solvent effects at the level of the transition state. As it stands, the above-described correlation between the ArX'- cleavage rate constant and the A r X / A r X - standard potential can serve as a useful rule of a thumb for predicting how fragile is the anion radical of an aryl halide. The above-described model of the transition state has also proved useful for interpreting the variations of the reactivity with the molecular structure in the converse reaction, Le., the addition of nucleophiles on aryl radicalsi5 which is the key step for elec(13) (a) Andrieux, C. P.; Savdant, J. M.; Zann, D. Nouu.J. Chim. 1984, 8, 107. (b) The orthogonality constraint of the two orbitals can be circumvented by bending vibrations. (c) The slope of the linear relationship between the r* orbital energy and the ArX/ArX- standard potential is slightly larger than 1 (ml.2). This can be ascribed to the effect of solvation on the free energy of ArX-: the more negative the standard potential, the more localized the negative charge in the anion radical and thus the stronger the solvation. It follows that the standard potential is a even better representation of the driving force of the cleavage reaction than the a* orbital energy in a series with a constant halogen since the solvation term of A r X - is then taken into account. (14) (a) Fukui, K.; Morokuma, K.;Kato, H.; Yonezawa, T . Bull. Chem. SOC.Jpn. 1963, 36, 217. (b) Dewar, M. J. S.; Hashmall, J. A.; Trinajstic, N. J. Am. Chem. SOC.1970,92, 5555. (c) Yamabe, S.; Minato, T.;Arai, T. Bull. Chem. SOC.Jpn. 1979, 52, 2143.
-2.50 -2.83 -2.34 -2.34
-2.46
-2.80 -2.32 -2.31
5.1 1.6 3.8 5.5
X lo-.' 1.0 X lo-* X lo-' 3.2 X lo-' X 10-1 5.4 X lo-' X 10-1 9.4 X lo-'
1.6 X 3.5 X 4.5 X 4.5 X
lo6 lo6 lo7 lo7
3.2 X IO6 7.0 X IO6 6.4 X lo7 7.7 X IO7
tron-transfer-induced aromatic nucleophilic substitutions (SRN1).I6 There is much less available data about the kinetics of the electron transfer to aromatic halides. It concerns fast cleaving molecules such as chlorobenzene, bromobenzene, and 2- and 3-bromopyridine in dimethylformamide. It follows that both the direct electrochemical reaction and the mediated electrochemical process are kinetically controlled by the forward electron-transfer step. The problem, in order to determine the heterogeneous and homogeneous standard rate constants, ks" and ksd, is then to know the standard potential of the A r X / A r X - couple. The redox catalytic approach offers a possibility for determining this quantity as schematically illustrated in Figure 2 (neglecting the variation of the transfer coefficient with the driving force around the standard potential). In the two regions of diffusion control of the electron-transfer reaction, the rate constant is expressed as log k l = log kdif (region 1)
In the region of activation log kl = log kSSOl- d o l
302 E0pq - E o T 0.06
-
(region 2)
where kdlfis the diffusion limit, Eo the standard potential of the ArX/ArX'- couple, Eom that of the mediator couple, and do' the homogeneous transfer coefficient. An appropriate choice of the mediator couples allows one to obtain the two straight lines (3) and (2) and therefore to determine Eo and ksw'. Once Eo is known, ksel can be derived from the electrochemical data. These determinations were carried out for the four above-mentioned aromatic halides. It, however, appears that the above-described procedure for determining the Eo can be somewhat in error due to the fact that, for very fast decaying anion radicals, the cleavage reaction partly occurs within the molecular diffusion layer, which is not taken into account in the equation describing region 3 in Figure 2.17b3cEstimation of the range of these possible errors allows one to bracket the value of the heterogeneous and homogeneous standard rate contant of electron transfer as shown in Chart I.'7b It appears that the standard rate constants are clearly below those that have been determined for stable aromatic radicals of comparable size, thus having comparable solvent free e n e r g i e ~ . ~ This points to the existence of a nonnegligible internal reorganization factors in the kinetics of electron transfer to aryl halides resulting from the stretching of the carbon-halogen bond when passing from the halide to its anion radical. Aliphatic Halides. In contrast with the case of aromatic halides, there is good evidence that aliphatic halides (RX) involving simple (15) (a) Amatore, C.; Chaussard, J.; Pinson, J.; SavCant, J. M.; Thitbault, A. J. Am. Chem. SOC.1979,101, 6012. (b) Balli, C.; Bunnett, J. F. J. Am. Chem. SOC.1981, 103, 7140. (c) Amatore, C.; Oturan, M. A.; Pinson, J.; SavCnt, J. M.; ThiCbault, A. J . Am. Chem. SOC.1985, 107, 3451. (d) Amatore, C.; Combellas, C.; Pinson, J.; Oturan, M. A.; Robvieille, S.; SavCant, J. M.; ThiCbault, A. J . Am. Chem. SOC.1985, 107, 4846. (e) Amatore, C.; Combellas, C.; Robvieille, S.; SavCant, J. M.; Thitbault, A., submitted for publication. (16) (a) Bunnett, J.. F. Acc. Chem. Res. 1978, 1 1 , 413. (b) Savant, J. M. Acc. Chem. Res. 1980, 13, 323. (c) Rossi, R. A.; Rossi, R.H. Aromatic Nucleophilic Substitution by the SRNl Mechanism; ACS Monograph 178; The American Chemical Society, Washington, DC, 1983. (17) (a) Andrieux, C. P.; Blocman, C.; Dumas-Bouchiat, J. M.; Savtant, J. M. J . Am. Chem. SOC.1979, 101, 3431. (b) Andrieux, C. P.; Savtant, J. M. J . Electroanal, Chem., in press. (c) Grimshaw, J.; Thompson, N. J . Electroanal. Chem., in press.
3818
The Journal of Physical Chemistry, Vol. 90, No. 16, 1986
Andrieux et al.
TABLE I: Kinetics of the Heterogeneous and Homogeneous Dissociative Electron Transfer to 9-Chloro-9-mesitylfluorenein Acetonitrile Characteristic Thermodynamic Parameters
E'R+IR? 0.750
EDR,/R"
KRX/R'tX-,
-0.690
EoRX/R+X-a
2.5 x 10-9
0.246 CMS
Electrochemical Kinetics application of Marcus theory
exptlb.'
E," at 1 V s-I
a at 1 V s d
Ld
AGn*.l,eeV
theor ab
-1.970
0.25
84.75
1.11
0.272
Homogeneous Electron Transfer RX (Autocatalysis)
+ R-
2R-
+ X-
responding Rt carbocation, R radical, and R- carbanion are then chemically stable. The Rt/R' and R / R - cyclic voltammetric waves are reversible which allows one to determine the corresponding standard potentials (Table I).23a On the other hand, redox catalysis of the electrochemical reduction of CMS by a series of substituted ferrocenes and quinoid anion radicals has been shown to involve a predissociation mechanism of the "CE" type:23b
application of AGthom,' AG*,,,,R.iR-,g Marcus theory,g EoRX/R.+XeV eV AG*m,Rxp.tx-.eV -0.936 0.506 0.10-0.45 1.38-1.73
P + e - Q
-
k , M-' s-l 8 X lo2
6~ = -G(AC'/F)/aE. 'From the peak width. d [ p = EoRXIR.tX- &) 6, is the potential a t the reaction site assumed to be located at the outer Helmoltz plane. & = -0.075 V from double layer capacitance data.24 'Electrochemical standard free energy of activation, taking X,, = 2 X IO3 cm s-l and D = 5 X 10" cm2 S - I . ~/Activation free energy of the RX R2R. X- reaction, from k = Z,,, exp(-AG'h,,/RT) with Z,,, = 3.5 X IO" M-I S - I . ~ glsotopic activation free energy of the subscript couple. Cathodic peak potential in V.
r
R
"In V vs. SCE.
-(F/RT)(E,
+
-
+
alkyl or unsubstituted benzyl residues do not form discrete anion radicals (RX.-).'* It is thus expected that heterogeneous and homogeneous redutive cleavage of these molecules will proceed, strictly speaking, along a dissociative electron-transfer pathway in the sense that electron transfer and bond breaking are concerted processes. If this is correct, the investigation of the activationdriving force free energy relationships characterizing such reactions should take as the origin of the driving force scale the standard potential, E o R X p . + X - of the concerted reaction RX
(18) Low-temperature y irradiation of simple aliphatic halides coupled with ESR characterization of the ensuing speciesIgashowed that R X - anion radicals are not produced upon addition of one e l e ~ t r o n . 'Polarizable ~ dipole (R)-charge (X-) loose adducts are formed instead. In contrast, CF3X- anion radicals have been detected at low temperature by the same t e ~ h n i q u e . ' ~ Quantum chemical calculations by the ab-initio method led to somewhat confusing conclusions.m In the case of CH3CI,one study predicts the existence of the CH3CI'- anion radical2&whereas another concludes to the formation of a loose CHI', Cl- complex.20b In the case of CF3CI,the conclusion is that no discrete anion radical is predicted to exist,20Eat variance with the lowtemperature experimental results. The results obtained in pulse radiolysis investigations also point to the nonexistence of discrete anion radicals in the case of simple aliphatic halides2' and also in that of unsubstituted benzyl halides2I whereas nitro- and cyano-substituted benzyl halides give rise to discrete anion radical^.^^,^^ (19) (a) Symons, M. C. R. Pure Appl. Chem. 1981,53,223. (b) Sprague, E. D.; Williams, F. J . Chem. Phys. 1971,54, 5425. (c) Mishra, S. P.; Symons, M. C. R. J . Chem. Sor., Perkin Trans. 2 1973, 391. (d) Symons, M. C. R. J . Chem. Soc., Chem. Commun. 1977,403. (e) Symons, M. C. R. J . Chem. Res. (S)1978, 360. (0 Sprague, E. D. J . Phys. Chem. 1979, 83, 849. (20) (a) Canadell, E.; Karofiloglou, P.; Salem, L. J. Am. Chem. Soc. 1980, 102, 855. (b) Clark, T. J . Chem. Soc., Chem. Commun. 19818 515. (c) Peyerimhoff, S. D.; Buenker, R. J. Chem. Phys. Left. 1976, 65, 434. (21) (a) Wentworth, W. E.; Becker, R. S.; Tung, R. J. Phys. Chem. 1967, 71, 1952. (b) Wentworth, W. E.; George, R.; Keith, H. J. Chem. Phys. 1969, 51, 1791. (c) Kojima, T.; Tanaka, Y . ;Satouchi, M. Anal. Chem. 1976, 48,
C_
-
Q
+
Rt
R'
+
X-
P
From the data gathered it was possible to estimate the rate and equilibrium constants of the predissociation reaction. Combining the latter with the standard potential of the R t / R couple provides the value of the standard potential of the R X / R X- couple (Table I) and thus the origin of the driving force scale for the dissociative electron transfer. Two features of the electrochemical reduction of CMSZ3'in acetonitrile are worth noting. The reduction occurs at potentials that are considerably more negative (more than 2 V) than the RX/R' + X- standard potential (Table I). There is thus ample room for redox catalysis of this very irreversible electrochemical process. This is what occurs with a particular catalyst-the carbanion, R-, produced by the electrochemical reaction itself. We thus observe an autocatalytic process:
+
RX
+ +
-
2e
R-
+
+
R-
+ e- s R' + X-
With simple alkyls, the analysis of the electrochemical and redox catalysis data is complicated by the extreme reactivity of the R radicals and of the carbanions R- resulting from their reduction. In this connection, an interesting particular case is the reduction of 9-chloro-9-mesitylfluorene (CMS) in acetonitrile. The cor-
+
Rt
-
x
R'
RX
+
2R'
e
==E
+
XX-
R-
of the "avalanche" type since two molecules of catalyst are regenerated each time one molecule of catalyst reacts with one molecule of substrate. Simulation of the cyclic voltammograms, which exhibit characteristic trace crossings, at several sweep rates allows the determination of the rate constant, k , of the electron transfer between R- and RX (Table I). The effect of the autocatalytic process on the cyclic voltammograms vanishes upon raising the sweep rate which allows kinetic characterization of the direct electrochemical reduction (Table I). It is then seen from the width of the voltammogram that the transfer coefficient is remarkably small ( a = 0.25). This falls in line with the concept that the R X - anion radical is not an intermediate in the reductive cleavage process. If the opposite was true, a should be close to 0.5 or larger than 0.5 since the R X / R X - standard potential is expected to be in the vicinity of the reduction potential or negative to it. Although not originally devised for concerted electron-transfer bond-breaking reaction, Marcus theory can be used to rationalize the above observation. In this context, the particularly low value of the transfer coefficient would be a consequence of the fact that, at the potential where the electrochemical reduction takes place, the driving force is very large, 2.14 eV at a sweep rate of 1 V s-]
(22) (a) Neta, P.; Behar, D. J . Am. Chem. SOC.1980, 102,4798. (b) Bays, J. P.; Blumer, S.T., Baral-Tosh, S.; Behar, D.; Neta, P. J . Am. Chem. SOC.
(23) (a) Merz, A.; Tomahogh, R. Angew. Chem., Znt. Ed. Engl. 1979,18, 938. (b) Andrieux, C. P.; Merz, A,; Savdant, J. M.; Tomahogh, R. J. Am. Chem. SOC.1984,106, 1957. (c) Andrieux, C. P.; Merz, A.; Savdant, J. M. J . Am. Chem. Sor. 1985, 127, 6097. (24) Petrii, 0. A,; Khomchenko, I. G. J . Elecfroonal. Chem. 1980, 106,
1985, 105, 320
277.
1730.
The Journal of Physical Chemistry, Vol. 90, No. 16, 1986 3819
Kinetics of Dissociative Electron Transfer (Table I). Quantitatively, the activation free energy of the electrochemical reaction is related to the driving force through2
TABLE II: Kinetics of the Electrochemical Reduction of Butyl Iodides and Bromides@ ~~
compd
V vs. S C E
peak potential at 0.1 v S-I, V vs. S C E
n-BuI n-BuBr sec-BuI sec-BuBr tert-BuI tert-BuBr
-1.20 -1.22 -0.93 -1.21 -0.91 -1.00
-2.33 -2.85 -2.05 -2.63 -0.91 -2.5 1
EoRX/R.+X-,b
is the standard electrochemical activation free energy. 4f is the potential at the reaction site.) The dimensionless expression of the voltammograms corresponding to the above Marcus equation is obtained by a simple extension of the derivation previously carried out for the Volmer-Butler kinetic^.^
-:(
-1 --J,I + - Ao e x ~ [
at
eV
theof
exptld
0.80
0.33 0.30 0.34 0.32 0.35 0.31
0.30 0.25 0.33 0.25 0.32 0.20
0.98 0.79 0.90 0.74 0.93
+
1- f)2]
(4)
where
"In DMF 0.1 M NBu4BF,, at 10 ' C on a glassy carbon electrode. bSee reference 29. eAccording to Marcus theory. Z,] and D estimated as in ref 5. dFrom the peak width.
CHART I1
J, = i/FSC?'(FuD/Rr)i/2
Ao = Z,i/(F~D/RT)'/2 6
= 4AGo*,i/RT
(Zeiis the electrochemical collision frequency, D is the diffusion coefficient of the reactants, C?' is the bulk concentration of the reactant, and u is the sweep rate). Numerical computation of the above integral equation leads to the results reported in Table 1. There is a good agreement between the theoretical and experimental values of CY. On the other hand, the kinetics of the homogeneous reaction 2 R X- can be treated as follows. According to R X RMarcus theory4
+
av value of a
AGO*,^^
-
+
AGO*homis thus derived from the experimental values of k and (Table I). On the other of the driving force, E'R./R- - E'RXIR.+Xhand, the cyclic voltammetric analysis of the R / R - couple allows one to bracket the value of the corresponding isotopic activation free energy, AG*iso,R./R-, as reported in Table I. Since AG*iw,RX/R*+XAGO*hom
=
+ AG*iso,R./R'
2
The values bracketing AG*h,RX/R.+X- ensue (Table I). Comparison between the electrochemical standard activation free energy and the homogeneous isotopic activation free energy shows that the former is between being equal to the latter and to half of it, Le., in between what is predicted by Hush theory that neglects the image force effect in the electrochemical reaction% and by Marcus theory that estimates this effect under the assumption of a reaction site located at a distance from the electrode equal to the equivalent radius of the reactant.* If it is assumed, as usually done, that the reaction site, i.e., the functional carbon, is located in the outer Helmholtz plane, it follows that the image force effect falls in between the two estimations. We can thus conclude that the kinetics of the reduction of this particular aliphatic halide matches Marcus theory in the context of a concerted electron-transfer-bond-breaking process having, as origin of the driving force scale the standard potential of the RX/R + x- couple. indeed predicts a value of the transfer and shows coefficient that is quite to the (25) Matsuda, H.; Ayabe, Y.2. Elektrochem. 1955, 59, 494. (26) Hush, N. S. J . Chem. Phys. 1958 28, 962.
potential, V vs. S C E experimental a theoretical a
n-BuI -2.10 0.37 0.36
-2.40 0.30 0.31
-2.67 0.22 0.27
potential, V vs. S C E experimental a theoretical a
tert-BuI -1.60 0.38 0.38
-1.95 0.35 0.33
-2.20 0.26 0.28
that the kinetics of the electrochemical reduction and that of the homogeneous reduction of the alkyl halide by the R- carbanion are satisfactorily consistent. It is obviously desirable to extend this type of analysis to the heterogeneous and homogeneous reductive cleavages of other aliphatic halides for testing the generality of the above conclusions. It is also worth investigating more thoroughly the dependency of the transfer coefficient upon potential which was precluded in the case of C M S by the occurrence of the autocatalytic reaction at low sweep rates. The low value of CY observed in the case of CMS could indeed be interpreted as resulting from a dissymmetry of the potential energy curves of the reactant and products in the context of a Volmer-Butler kinetic as well as from a potential variation of CY from the standard potential to the actual reduction potential in the context of Marcus theory. This was attempted by investigating the reduction of a series of simple aliphatic halides in dimethylformamide including n-, sec-, tert-butyl iodides, bromides, and chlorides.*' Both the electrochemical reduction28aand the homogeneous reduction by aromatic anion radicals were investigated. n-BuBr, sec-BuBr, tert-BuBr, and n-Bul exhibit a single twoelectron wave whereas sec-BuI and tert-BuI show two waves, the first of which involves the exchange of one electron per molecule.28b This shows that in the first series the R radical formed upon reductive cleavage is reduced at a potential which is positive or at least equal to the RX reduction potential. In the second series, R is more difficult to reduce than RX. At the level of the first wave it presumably undergoes dimerization and/or H-atom disproportionation. In all cases, the width of the cyclic voltammograms are large featuring a small value of the transfer coefficient (Table 11), indicating, as with CMS, that the R X - anion radical is not an intermediate in the reductive cleavage process. Convolution potential sweep voltammetry30 over an extended range
(27) Andrieux, C. P.; Gallardo, I.; Saveant, J. M.; Su,K. B. J . Am. Chem. sot, 1986, lo8, 638, (28) (a) This was restricted to the iodides and bromides since the reduction of the chlorides is too close to the discharge of the supporting electrolyte. A glassy carbon electrode was used. The data were not significantly dependent upon electrode pretreatments. Similar results were obtained with a gold electrode. (b) The number of electrons per mole was determined by comparison with the reversible wave of anthracene. No change of the waves was observed upon addition of acids, water, or phenol to the solution. (29) (a) From thermodynamic data compilations*' using a previously described m e t h ~ d . ~ ~ , ~ N. s. Z.Elektrochem. 1957, 61, 734. (c) (b)~ Hush, Eberson, L. Acta Chem. Scand., Sect. B 1982, 36, 533.
3820 The Journal of Physical Chemistry, Vol. 90, No. 16, 1986
Andrieux et al.
TABLE III: Kinetics of the Homoneneous Electron Transfer from Electroaenerated Anion Radicals to Butvl Halides in DMF ~~
~~
RX
av a
n-Bur" n-BuBr" n-BuCI" sec-BuIb sec-BuBrb sec-BuClb rert-Burb tert-BuBrb tert-BuClb
0.56 0.48 0.28 0.46 0.40 0.35 0.36 0.37 0.36
0.77 0.93 0.74 0.78 0.94 0.68 0.86 0.97
1.38 1.70 1.31 1.40 1.71 1.21 1.56 1.79
0.71
668 688 541 617 670 533 550 582
0.60 0.64 0.61 0.60
0.25 0.30 0.25 0.25 0.31 0.24 0.29 0.36
20 OC. I O OC. CHomogeneous standard activation free energy using the standard potentials in Table I1 and Zsolvalues calculated as described in ref 5 . dC-X stretching wave numbers. From ref 33. CIncreaseof the C-X distance in the transition state.
of sweep rates showed that the transfer coefficient varies with the electrode potential. The results obtained in this connection with n-BuI (which gives rise to a two-electron wave) and with tert-BuI (which gives rise to a first one-electron wave) are shown in Figure 3. Although there is important scatter in the data points, the log k(E) vs. E plots exhibit a definite downward curvature showing that CY does vary with potential. The kinetic data can be analyzed according to the Marcus activation-driving force free energy relationship by eq 3 and 4 as in the case of CMS. Note that, unlike the case of CMS, the values of EoRX/R.+X- cannot be obtained experimentally. They were derived from previously known thermodynamic data.29 The results are shown in Table 11. There is a good agreement between the experimental and theoretical values of the transfer coefficient. It is noted that the agreement is significantly better for the iodides than for the bromides. It is also observed, by computing the theoretical CY values at several potentials, that the variation of CY with the potential fits satisfactorily what is predicted by the theory (Chart 11). Homogeneous electron transfer to the butyl halides was investigated by the redox catalysis method, Le., the rate was derived from the catalytic enhancement of the aromatic mediatorjanion radical wave upon addition of the butyl halide as described earlier in the case of aromatic halides. The reaction is, however, less simple in the present case P
Q'-
+
e-
+
RX
== I
-
Q' - ( E o p Q )
P
+
R'
+
X-
(0)
(1)
(30) (a) Convolution potential sweep voltammetry consists in computing from the linear sweep current potential curves the convolution integral:
and using it in the data processing.3ob In the case of the electron-transfer reaction A + e s B, the potential dependent rate constant k ( E ) defined as
is derived from the experimental i and I data as K(E) i log -= log Dl/2 I/- I
where I, is the limiting value of I which is reached at potentials beyond the reduction wave and D is the diffusion coefficient of A and B. The same procedure has been extensively used for the analysis of nondissociative electron-transfer kinetics.6 (b) Imbeaux, J. C.; Saveant, J. M. J. Electroanal. Chem. 1973, 4 4 , 169.
since R is not only reduced by Q into R- (reaction 2) but simultaneously couples with it, yielding alkylation products of the aromatic mediator (reaction 3). In addition, R may also be reduced at the electrode surface in cases where kl is large and/or the excess of RX over P is big. An appropriate theory has been recently developed which allows one to define conditions where the latter pathway can be neglected and to extract the values of k, and k2/k3from the current data.11f&27~31 The results concerning the electron-transfer rate constant, kl, are shown in Figure 4 in the form of log k, vs. E o p 9 plots, Le., activation vs. driving force diagrams. (The various points on each plot correspond to a series of aromatic catalysts.) The solid lines in Figure 4 correspond to the best-fit parabola for each of the butyl halides.32 The ensuing values of AGO*hom (from eq 5) are listed in Table I11 together with the values of AG*iso,RX/R.+X- derived from AGOthom by the equation4 Ac*i~,RXIR.+X-
AGO*hom
=
-k AG*iso,P/Q
2
taking 0.159 eV as the average value of AG*b,PIQin the mediator series.' Comparison of AGO*,, and AG*,so,RX~R.+X- shows a satisfactory correlation between the heterogeneous and homogeneous kinetic characteristics. The ratio of these two quantities is about constant in the series, ranging from 0.6 to 0.75, Le., in between what is predicted by Marcus (0.5) and Hush (1) theories which is expected from a realistic estimate of the force effect as discussed earlier. We can thus conclude that Marcus theory provides a reasonably (31) (a) In the case where the addition reaction 3 overtakes the electrontransfer step 2, the P/Q wave becomes irreversible with a height of 2e per molecule, thus showing no catalytic character. The question thus arises of whether the alkylation of the aromatic anion radical then results from a SN2 displacement rather than from the above reaction sequence. A stereochemical investigation, on the example of anthracene anion radical and optically active secondary octyl halides,)Ibhas shown that the interference of the sN2 pathway can be practically neglected in kinetic studies. (b) Hebert, E.; Mazaleyrat, J. P., Nadjo, L.; Savtant, J. M.; Welvart, 2.Nouv. J . Chim. 1985, 9, 75. (3Z.) (a) Kinetic data have also been gathered for the reaction of aliphatic halides with other electron-rich rea ents, mostly low oxidation states of transition-metal complexes, C O ( I I ) : ~Co(I),"'-I ~ Fe(I):& Fe("O")?Z1 In most cases, however, the reaction has been shown to proceed according to atom transfer32b3d or SN2 di~placement'~~*~ mechanisms. The very rough correlation that exists between the rate constant and the driving forcezgcis not therefore very meaningful. It simply reflects the general tendency of the reactivity to increase with the reducing power of the reagent whatever the reaction mechanism. This is clearly seen in the case of iron(1) porphyrins: for the same value of the standard potential they are several orders of magnitude more reactive than aromatic anion radicals showing that the reaction mechanism is not the same in both (b) Chock, P. B.; Halpern, J. J . Am. Chem. SOC.1969, 91, 582. (c) Sandwick, M. G.; Waters, W. A. J . Chem. SOC.B 1971, 1059. (d) Blazer, H. U.; Halpern, J. J. Am. Chem. SOC.1970, 202, 1684. (e) Schrauzer, G. N.; Windgassen, R. I.; Kohnle, J. Chem. Ber. 1965, 98, 3324. ( f ) Schrauzer, G. N.; Doeutsch, E. J . Am. Chem. SOC.1969, 91, 3341. (g) Eckert, H.; Ugi, I. Angew. Chem. 1975,87,847. (h) Clark, D. W.; Hush, N. S.; Woolsey, I. S.Inorg. Chim. Acta 1976, 19, 129. (i) Lexa, D.; SavCnt, J. M.; Soufflet, J. P. J. Electroanal. Chem. 1979, 100, 159. (j) Puxeddu, A,; Costa, G.; Marsich, N. J. J . Chem. SOC.,Dalton Trans. 1980, 1489. (k) Lexa, D.; Mispelter, J.; Savhnt, J. M. J . Am. Chem. SOC.1981, 103, 6806. (I) Lexa, D.; Savtant, J. M.; Wang, D. L. Organometallics, in press. (33) Bellamy, L. J. The Infrared Spectra of Complex Molecules; Chapman and Hall: London, 1975; p 368.
Kinetics of Dissociative Electron Transfer
The Journal of Physical Chemistry, Vol. 90, No. 16. 1986 3821
.
-2
v LV.A**) x
0.1 0.2 0.5
A
1.0
+ -1
0 2.0 # 5.0 0 10 A 20
-0
- -1 a
- -2
. -2.2
-2.4
-2.6
E/V
-2.8
Y IV.b-'1
.
+
0.1 0.2 0 0.5 x 1.0 A
2.0
A 5.0
+0 1020 .L 50
b
-1.6 I
-1.8
-2.0
-2.2
Figure 3. Convolution potential sweep voltammetry of n-BuI (a) and tert-BuI (b) in DMF
satisfactory description of the activation-driving force relationship characterizing both the heterogeneous and homogeneous concerted electron-transfer-bond-breaking reduction of butyl halides. As regards the relation between the thermodynamics and kinetics of the reduction and the structure of the aliphatic halide, it is seen that the differences between the v&-ious isomers are small. In contrast, significant variations are observed when passing from iodides to bromides to chlorides. The fact that the standard potentials are more and more negative in this order essentially reflects the concomittant increase of the carbon-halogen bond energy?' The solvation free energy of the halide ions also becomes more and more negative in the same order but to a much lesser
E/V
-2.4
+ 0.1 M n-Bu4BF4at 10
O C
on glassy carbon.
extent. The standard activation free energy also increased from I to Br to C1. Thus, for what concerns the actual reduction potential, as featured for example by the peak potential in cyclic voltammetry, the kinetics amplifies the thermodynamics. The same has been previously found in another dissociative electrontransfer reaction, viz, the reduction of cobalt(I1) into cobalt(1) in the vitamin B12 series which is accompanied by the expulsion of the axial ligand borne by ~ o b a l t ( I I ) . ~Then, ~ the stronger the (34) (a) Faure, D.; Lexa, D.; Savdant, J. M. J . Electroanal. Chem. 1982, 140,285. (b) Ibid. 1982,140,245. (c) Lexa, D.; Savdant, J. M. Acc. Chem. Res. 1983, 16, 2 5 5 .
3822 The Journal of Physical Chemistry, Vol, 90, No. 16, 1986
Andrieux et al.
4 1
1 -1.3
-1.8
-1.7
-1.6
-2.4-2.3-2.2-2.1-2.0-1.9
-1.5
-2.6
-2.5
-2.4
I
I
I
-2.3
tert- BuBr
3 -
2 1 I
I
I
I
,Eft@
I
I
1
I
I
I
-1.8 -1.7 -1.6 -1.5 -1.4 -1.3 -2.2 -2.1 -2.0 -1.9 -1.8 -2.6 -2.5 -2.1 Figure 4. Kinetics of the reduction of butyl halide by a series of electrogenerated aromatic anion radicals in DMF.
ligand the more negative the standard potential and the lower the electron-transfer rate constant. In the context of a model of the concerted electron-transferbond-breaking reaction where the main coordinate of the RX potential energy surface is assumed to be the stretching of the C-X band, the isotopic activation free energy would be related to the stretching frequency, v, and the reduced mass, p, through where Ad* stands for the difference in carbon-halogen bond distance between the transition and initial states. Ad* is about constant, varying less than f20% in the series (Table 111) which falls in line with the increase of the van der Waals radius from C1 to Br and I being compensated by the increased deformability of the halogen atoms.35 Figure 5 illustrates the fact that a Marcus type model based on harmonic potential energy surfaces having the same aperture for reactants and products is, however, at best a gross approxi(35) This is a more meaningful correlation than previously considered where the variation of the reduced mass was negle~ted.~' (36) It would be interesting to examine if this two-step process is replaced by a concerted electron-transfer-bond-breaking reaction when the driving force decreases as suggested by a simple model of the potential energy surfaces.
POTENTIAL
1
ENERGY
I
E L
-2.3
1
[POTENTIAL ENERGY
,8'
c-x
c-x
Figure 5. Potential energy surfaces. Harmonic approximation (-) and Hatched zone: domain of investigation of the Morse curve (---). heterogeneous and homogeneous electron transfers.
mation of the actual situation. It is indeed seen, on the example of tert-BuI and n-BuC1 (the same is true for the other members of the series) that, owing to the fact that the activation energy
J. Phys. Chem. 1986, 90, 3823-3829 is large, the more realistic Morse curVe21b representing the stretching of the C-X bond in the reactant differs substantially from the approximating parabola not only under standard conditions but also in the range of driving forces where the heterogeneous and homogeneous electron transfers were experimentally investigated. A Morse curve description of the reactant could be used for improving the activation model but the actual difficulty X- potential energy then resides in the description of the R surface. The improvement of the model thus hinges upon the development of an accurate quantum mechanical description of X- system as a function of their distance. the R
+
+
Concluding Remarks The main conclusions that emerge from the preceding discussion can be summarized as follows: In the case of aromatic halides, the kinetic data gathered so far indicate that the reduction cleavage of the carbon-halogen bond involves the intermediacy of the anion radical of the starting molecule.35 There is a rough correlation between the cleavage rate constant of the anion radical and the ArX/ArX- standard potential. Since the latter is an approximate representation of the cleavage free energy, this correlation (which exhibits a transfer coefficient close to 0.5) can be viewed as a Bronsted-Marcus activation-driving force free energy relationship. The transition state corresponds to the unpaired electron passing from the T * orbital to the g* orbital of the C-X bond. In contrast, aliphatic halides undergo a strictly speaking dissociative electron transfer as suggested by the low value of the electrochemical transfer coefficient and confirmed by the analysis of the whole set of heterogeneous and homogeneous kinetic data. The latter are reasonably well fitted by the activation-driving force relationship based on Marcus quadratic theory, even though this is a grossly approximate representation owing to the large magnitude of the activation energies. Upon passing from I to Br to C1 the reduction becomes more difficult from two additive factors: the standard potential shifts negatively essentially because of the increase of the carbon-halogen bond energy; the activation free energy increases essentially because the force required to stretch the bond concomitantly becomes larger and larger.
3823
Acknowledgment. We thank Drs. D. Zann and I. Gallardo for their active contribution to the study of the aromatic and butyl halides, respectively. Collaboration with Prof. A. Merz (University of Regensburg, BDR) in the investigation of 9-chloro-9-mesitylfluorene was essential. Registry No. n-BuI, 542-69-8; n-BuI'-, 101980-38-5;n-BuBr, 10965-9; n-BuBr'-, 102045-61-4;n-BuCI, 109-69-3;n-BuCl'-, 77347-44-5; sec-BuI, 513-48-4; sec-BuI'-, 102045-62-5; sec-BuBr, 78-76-2; secBuBr'-, 102045-63-6; sec-BuCI, 78-86-4; sec-BuCl'-, 102045-64-7; tert-BuI, 558-17-8; tert-BuI'-, 53487-00-6; tert-BuBr, 507-19-7; tertBuBr'-, 57422-69-2;tert-BuCI, 507-20-0; tert-BuCI'-, 102045-65-8;p NOZC~H~CI, 100-00-5;p-N02C6H,CI'-, 34473-09-1;p-CIC&C(O)Ph, 134-85-0;p-C1C6H4C(O)Ph'-, 81439-06-7;p-ClC,H,CN, 623-03-0;p CIC6H4CN'-, 68271-91-0;p-CIC6H4C(O)CH,, 99-91-2;p-CIC6H4C(O)CH3'-, 68225-77-4;m-C1C6H4C(0)CH,,99-02-5;m-CIC&C(O)CH3'-, 68225-76-3; o-NO~C~H~CI, 88-73-3;o-N02C6H4CIS-,34470-274; p-BrC6H4No2,586-78-7;p-BrC6H4N02'-,34470-26-3;p-BrC6H4C(O)Ph, 90-90-4;p-BrC6H4C(0)Ph'-, 57365-05-6;p-BrC6H4C(0)CH,, 99-90-1; p-BrC6H4C(0)CH3'-,34473-43-3; m-BrC6H4C(0)CH3,214263-4; m-BrC6H4C(0)CH3'-,77510-39-5;m-BrC6H4C(0)Ph,1016-77-9; m-BrC6H4C(0)Ph'-, 101980-39-6; 9-chloroanthracene, 716-53-0; 9chloroanthracene radical anion, 74430-88-9;4-chloroquinoline, 6 1 1-35-8; 4-chloroquinoline radical anion, 101980-40-9;2-chloroquinoline, 61 262-4; 2-chlorquinoline radical anion, 7 1803-43-5;2-chloronaphthalene, 9 1-58-7; 2-chloronaphthalene radical anion, 5 1703-41-4; 1-chloronaphthalene, 90-13-1; 1-chloronaphthaleneradical anion, 51703-40-3; 2-chloroanthracene, 17135-78-3; 2-chloroanthracene radical anion, 91503-57-0; (E)-4-[2-@-chlorophenyl)ethenyl]pyridine, 46459-15-8; (E)-4-[2-@-chlorophenyl)ethenyl]pyridine radical anion, 102045-66-9; 1-chloroanthracene, 4985-70-0; 1-chloroanthracene radical anion, 91 503-58-1; l-bromo-2-methyl-4-nitrobenzene, 7149-70-4; I-bromo-2methyl-4-nitrobenzeneradical anion, 34470-34-3;1-bromo-2-isopropyl5-nitrobenzene, 101980-41-0;l-bromo-2-isopropyl-5-nitrobenzene radical 94832-09-4; anion, I O 1980-42-1; 1-bromo-2-isopropyl-4-nitrobenzene, I-bromo-2-isopropyl-4-nitrobenzeneradical anion, 101980-43-2; 9bromoanthracene, 1564-64-3;9-bromoanthraceneradical anion, 549 1 151-2; 1-bromonaphthalene, 90-11-9; 1-bromonaphthaleneradical anion, 51703-42-5; 2-bromo-l,3-dimethyl-5-nitrobenzene,53906-84-6; 2bromo-l,3-dimethyl-5-nitrobenzeneradical anion, 101980-44-3; 3bromo-9H-fluoren-9-one, 2041- 19-2; 3-bromo-9H-fluoren-9-one radical 36804-63-4; l-bromoanion, 101980-45-4;l-bromo-9H-fluoren-9-one, 9H-fluoren-9-oneradical anion, 101980-46-5.
Rate-Structure Dependencies for Intramolecular Electron Transfer via Organic Anchoring Groups at Metal Surfaces Michael J. Weaver* and Tomi T.-T. Li Department of Chemistry, Purdue Unifiersity, West Lafayette, Indiana 47907 (Received: January 21, 1986: In Final Form: April I , 1986)
The electroreduction kinetics of pentaamminecobalt(II1) complexes that are surface-attached to mercury or gold electrodes via extended thioorganic linkages featuring nitrogen Co(II1) coordination are examined and compared to similar systems involving oxygen coordination sites. These comparisons utilize unimolecular rate constants, k,, (SI), and preexpotential factors for the elementary elebtron-transfer step, together with thermodynamic adsorption data and rate constants for the homogeneous outer-sphere reduction of the complexes by Ru(NH,),~+. These data enable the observed rate variations with the bridging ligand structure arising from changes in the activation free energy to be separated from those due to variations in the electronic transmission coefficient, K,,. This analysis indicates that the latter provides the predominant component of the ca. 105-fold observed variations in ket. All nonconjugated, and even some conjugated, organic bridges yield significantly or substantially nonadiabatic pathways (Le., K,,
