Langmuir 1989,5, 645-650
645
Symposium on Electrocatalysis: Preface Electrochemistry is that unique component of heterogeneous oxidation-reduction chemistry in which the interconversion of electrical and chemical energy occurs with electron transfer across the interface between a solvent phase (the electrolytic solution) and an electronically conductive phase (the electrode). Electrocatalysis is that part of electrochemical science devoted to the study of those interfacial parameters, other than potential and reactant concentration, which influence or control the rate of electron-transfer reactions. On the basis of a keyword search in CAS, the first use of the term electrocatalysis was in 1965 by A. Damjanovic, A. Dey, and J. O’M. Bockris (J. Catal. 1965,4,721-4) in the intercomparison of Tafel data obtained for oxygen evolution and reduction at a Pt-Rh alloy electrode and the pure Pt and Rh electrodes. Recent use of the term electrocatalysis has been applied to the manipulation of virtually any factor which caws beneficial alteration of electron-transfer reaction rates through manipulation of the interfacial structure (i.e., interfacial engineering). This includes not only the choice of electrode material but also the chemical modification of the electrode surface to attach functional groups which mediate the electron-transfer process and/or any coupled chemical steps required within the mechanism of the desired electrochemical reaction. Appropriate goals of electrocatalytic research now are seen to include the prevention of undesired electron-transfer reactions (“decatalysis”) as well as the enhancement of desired electron-transfer rates to achieve selectivity (if not specificity) of electrochemical interconversions. The organization of this symposium was based on the broadest possible perspective of electrocatalysis, including the study and application of electrocatalytic phenomena, as well as developments in methodology for studying interfacial composition and structure. I express my sincere thanks to the ACS Divisions of Colloid and Surface Science and Analytical Chemistry for their cosponsorship of this symposium. I also am grateful to the authors and participanta for their valuable contributions to the success of the symposium and the editors of Langmuir for offering this opportunity to present a portion of the manuscripts within this forum. Lastly, I offer thanks to my coorganizers, Patricia A. Thiel and Marc D. Porter, for their helpful advice, especially during the process of speaker selection.
Dennis C.Johnson Iowa State University Symposium Organizer
Electrochemical Studies of Cobalt-Carbon Bond Formation. A Kinetic Investigation of the Reaction between (Tetrapheny1porphinato)cobalt (I) and Alkyl Halides? G. B. Maiya, B. C. Han, and K. M. Kadish* Department of Chemistry, University of Houston, Houston, Texas 77204-5641 Received December 13, 1988. In Final Form: January 19, 1989 The reactions between electrochemically generated [ (TPP)Co’]- (where TPP is the dianion of tetraphenylporphyrin)and 17 different alkyl halides (RX, where X = I, Br, C1, or F) were monitored in THF by cyclic voltammetry. (TPP)Co was reduced to [(TPP)Co’]-, which reacted with RX to form (TPP)Co(R) and X-. Rate constants for this oxidative addition reaction were evaluated as a function of the specific X and R group of the alkyl halide. On the basis of analysis of the kinetic data, an s N 2 mechanism was suggested to occur for all but one of the reactions between [(TPP)Co’]- and RX. The only exception was for CHg, where the readion involved features of both a s N 2 and an outer-sphere electron-transfermechanism. The electrosynthesis and characterization of four (TPP)Co(R) complexes was also reported. Introduction Square-planar cobalt(I), iron(I), and rhodium(I1) porphyrins undergo nucleophilic substitution reactions with
alkyl halides, olefins, or acetylenes to generate trivalent metal complexes with metal-carbon bonds.’-16 These reactions have been discussed in the literature from the
Presented at the symposium entitled “Electrocatalysis”, 196th National Meeting of the America1 Chemical Society, Loa Angeles, CA, Sept 27-29,1988.
(1) Clarke, D. A.; Dolphin, D.; Grigg, R.; Johnson, A. W.; Pinnock, H. A. J. Chem. SOC.1968,881.
0743-7463/89/2405-0645$01.50/0
0 1989 American Chemical Society
646 Langmuir, Vol. 5, No. 3, 1989
Maiya et al.
standpoint of synthetic c h e m i ~ t r y , ~ structural p~J~ organometallic chemistry,16 and e l e c t r o ~ h e m i s t r y . ~ ~ ~ ~ ~ - ~ ~ Mechanistic aspects of electrochemically initiated metal-carbon bond formation reactions are important in view of the utility of metal-carbon bonded species as precursors in the electrocatalytic reduction of alkyl halides. In this regard, the use of (TPP)Co (where TPP is the dianion of tetraphenylporphyrin) in catalyzing the electrochemical reductive cleavage of butyl bromide has been e~plored.~ The second-orderrate constant for the reaction between n-C4HgBrand [ (TPP)Co']- to give (TPP)Co(nC4Hg)is approximately 30 M-l s-l in DMF. This is the only kinetic measurement of Cc-C bond formation in porphyrin systems that has been made using electrochemical methods. In the present paper, the reactions of electrochemically generated [(TPP)Co']- with 17 different alkyl halides (RX) are monitored. The halide and the R group in RX were systematically varied, and the effects of R and X on the reaction rate constants were examined. The electrochemically initiated synthesis, characterization, and electrochemical reactivity of four (TPP)Co(R)complexes are also reported.
Experimental Section Materials. Spectroscopic grade T H F was purchased from Aldrich Co. and purified by distillation under nitrogen, first from CaHz and then from sodiumjbenzophenone just prior t o use. Spectroscopic grade CHzClz (Fischer Scientific Co.) was purified according to standard methods.'e Spectroscopic grade benzene (Aldrich Co.) was used without further purification. Tetra-nbutylammonium perchlorate (TBAP; Kodak Chemical Co.) was twice recrystallized from absolute ethyl alcohol and dried in a vacuum oven at 40 OC. All organic reagents (Aldrich) were purchased a t the highest available level of purity and used as received. Deuterated benzene ((&De) purchased from Aldrich co. was used as received. (TPP)Co was synthesized according t o a published p r 0 ~ e d u r e . l ~ Instrumentation and Methods. All electrochemical measurements were performed in cells modified for Schlenk techniques. A platinum disk electrode (area = 0.8 mm2) was used as the working electrode, and a Pt wire served as the counter electrode. The reference electrode (SCE) was separated from the bulk of the solution by means of a fritted glass bridge. The bulk electrolysis cell was modified for Schlenk techniques and consisted of platinum gauze working and counter electrodes. A platinumtipped glass frit was used to separate the two compartments for vacuum operation. (2) Momenteau, M.; Fournier, M.; Rougee, M. J. Chim. Phys. 1970, 67, 926.
( 3 ) Perree-Fauvet, M.; Gaudemer, A.; Boucly, P.; Devynck, J. J. Organomet. Chem. 1976, 120, 439. (4) Ogoshi, H.; Watanabe, E.; Koketsu, N.; Yoshida, 2.Bull. Chem. SOC.Jpn. 1976,49, 2529. (5) Lexa, D.; SavBant, J.-M.; Soufflet, J. P. J. Electroanal. Chem. 1979, 100, 159. (6) Callot, H. J.; Metz, F.; Cromer, R. Nouu.J. Chim. 1984, 8, 759. (7) Lexa, D.; Mispelbr, J.;SavBant, J.-M. J.Am. Chem. SOC.1981,103, 6806. (8) Lexa, D.; SavBant, J.-M.; Wang, D. L. Organometallics 1986, 5, 1428. (9) Anderson, J. E.; Yao, C.-L.; Kadish, K. M. Inorg. Chem. 1986,25, 718. (10) Anderson, J. E.; Yao, C.-L.; Kadish, K. M. J. Am. Chem. SOC. 1987,109, 1106. (11) Anderson, J. E.; Yao, C.-L.; Kadish, K. M. Organometallics 1987, 6, 706. (12) Anderson, J. E.; Liu, Y. H.; Kadish, K. M. Inorg. Chem. 1987,26, 4174. (13) Kadish, K. M.; Guilard, R. Chem. Reu. 1988,88, 1121-1146. (14) Guilard, R.; Lecomte, C.; Kadish, K. M. Struct. Bonding 1987,64, 205-268. (15) Brothers, P. J.; Collman, J. P. Acc. Chem. Res. 1986, 19, 209. (16) Perrin, D. D.; Armarego, W. L. F.; Perrin, D. R. Purification of Laboratory Chemicals, 2nd ed.; Pergamon Press: New York, 1980. (17) Adler, A. D.; Longo, F. R.; Finarelli, J.; Goldmacher, J.; Assour, J.; Korsakoff, L. J. Org. Chem. 1967, 32, 476.
TPPCo
n
-tCH3(CHz)zBr In THF
YA
(b) 3.2 eq
/
-
' r-n
v
c Z
w
0
U
a
0
I
I
I
I
I
I
0.00
-0.40
-0.00
-1.20
-1.60
-2.OC
POTENTIAL, V vs SCE
Figure 1. Cyclic voltammograms of 8.67 X lo-' M (TPP)Co in T H F containing 0.2 M TBAP and the following equivalents of CH3(CH&Br: (a) 0, (b) 3.2, (c) 12.8, and (d) 70.5 equiv. Scan rate = 100 mVjs. An IBM EC 225 voltammetric analyzer coupled with a Houston Instruments Model 2000 X-Y recorder was used for cyclic voltammetric measurements. Bulk controlled potential coulometry was performed with an EG&G PAR Model 174 potentiostat/179 coulometer system which was coupled with an EG&G PAR RE0074 time base X-Y recorder. UV-vis spectra were recorded with a Tracor-Northern 1710 spectrometer/multichannel analyzer or with an IBM 9430 spectrophotometer using cells adapted for inert atmosphere measurements. 'H NMR spectra were taken on a QE-300 FT NMR spectrometer, and tetramethylsilane was used as an internal reference.
Synthesis of (TPP)Co(R) where R = CHzCl,CH,, CPHS, or C3HVThe electrosynthesis of (TPP)Co(R) where R = CH,, CzHs, or C3H7was performed under an inert atmosphere and in the dark by using the following procedure: (TPP)Co (30-40mg) and 50-100 equiv of CH31, CHSCH21,or CH3(CH2),I were dissolved in T H F containing 0.2 M TBAP. Bulk electrolysis was performed a t -1.2 V versus SCE, and the current-time curve was monitored until completion of electrolysis. The solution then was transferred from the bulk electrolysis cell to another flask and the solvent evaporated t o dryness by using a stream of dry nitrogen. The residue was dissolved in benzene and twice chromatographed on alumina (neutral, activity =' 1). All operations were performed in a Vacuum Atmospheres glovebox. The yields of (TPP)Co(R) where R = CH3, C2Hs, or C3H7were between 70% and 80%. The electrosynthesis of (TPP)Co(CHzCl)was performed by reduction of (TPP)Co in CH2C12,which was used as a solvent.'* The workup procedure of the compound was similar t o the one described above and gave yields greater than 70%.
Results and Discussion Electrochemistry of (TPP)Coin Solutions Containing RX. The reactions between electrogenerated [(TPP)Co]- and 17 different alkyl halides in THF were monitored by cyclic voltammetry. An example of the resulting current-voltage curves is given in Figure 1,which illustrates cyclic voltammograms for the reduction of (TPP)Co in THF containing 0.2 M TBAP and various concentrations of CH3(CHJ2Br. (18) Kadish, K. M.; Lin, X. Q.;Han,B. C. Inorg. Chem. 1987,26,4161.
Electrochemistry of Cobalt-Carbon Bond Formation
I
Langmuir, Vol. 5, No. 3, 1989 647
(a)
c z
kK
100 200 300 500
CH3CH21
0.9
s c
I
I
-3.5
I
-2.
-3.0
2 W
a a 3
0 I 0.00 -0.20 1
I -0.40
1
-0.60
I -0.80
I -1.00
I -1.20
I
-1.40
I -1.60
POTENTIAL, V vs SCE
0.6
Figure 2. Cyclic voltammogramsof (TPP)Coin THF containing 0.2 M TBAP and (a) 5.2 equiv of CH3CH21and (b) 0.3 equiv of CH31. Scan rate = 100 mV/s.
0.8
(TPP)Co undergoes two, well-defined one-electron reversible reductions in THF containing 0.2 M TBAP.18 These reductions are illustrated by the cyclic voltammogram in Figure la, where the reduction processes I and I1 correspond to the reactions given by eq 1 and 2 below: (TPP)Co + e[(TPP)Co']-
F?
[(TPP)Co']-
+ e- F?
[(TPP)Co1I2-
(1) (2)
Parts b-d of Figure 1show the changea that occur in the cyclic voltammograms of (TPP)Co as CH3(CH2)2Bris added to the THF solution. After addition of -3 equiv of CH3(CH2)2Br,a new reversible set of reduction/oxidation peaks (labeled process 111) begins to appear at El12 = -1.35 V. The cathodic and anodic peak currents for this reaction increase with increasing concentration of CH3(CH&Br, as shown in Figure lb-ld. There is no change in magnitude of the reduction current (idfor process I with concentration, but the reverse increase in the CH3(CH2)2Br reoxidation peak current for this process (i,) decreases in magnitude, consistent with a loss of the electrogenerated [ (TPP)Co']- from solution. The ratio of anodic to cathodic peak current, i l i p , for peak I increases with increase in scan rate for &PP)Co solutions containing CH3(CH2)2Brand reaches a plateau at scan rates higher than 500 mV/s. The peak potential, Ep,for peak I also shifts with increase in scan rate by -35 f 5 mV for each 10-fold increase in scan. Furthermore, peak I becomes reversible and peak I11 disappears when the cyclic voltammetry is carried out at low temperatures. All of these characteristics are typical of a reversible one-electron-transfer process followed by an irreversible chemical reaction (an electrochemical EC type mechanism).lg The data in Figure 1are also consistent with formation of (TPP)Co(CH2CH2CHJas a product in the reaction with electrochemically generated [(TPP)Co']-, as shown in eq 3, where RX = CH3CH2CH2Br [(TPP)Co']-
+ RX
kf
(TPP)Co(R) + X-
(3)
The formation of (TPP)Co(CH2CH2CH3)as a final species in eq 3 is consistent with data in the literature for the electrochemically initiated formation of (TPP)Co(CH3)18 (19)Nicholson, R.S.; Shain, I. Anal. Chem. 1964, 36,706.
0 3 0 0 mv/s e 5 0 0 mv/s
-4.5
-4.0
-3.5
-3.0
-2.5
Log[RXl
Figure 3. Scan rate dependence on the ratio iw/iw for the reduction of (TPP)Coin THF containing different concentrations of (a) CH3CHzIand (b) CH31.
and ( T P P ) C O ( ~ - C ~and, H ~ in ) ~the present case, was also verified by 'H NMR analysis of the isolated porphyrin product. Current-voltage curves similar to those in Figure 1were obtained for the reduction of (TPP)Co in THF solutions containing other alkyl halides, but different concentrations of RX were needed in order for the reaction with [(TPP)Co]-to occur on the cyclic voltammetric time scale. This is demonstrated in Figure 2, which shows that the reduction of (TPP)Co in solutions containing 5.2 equiv of CH3CH21or 0.3 equiv of CH31 has similar voltammetric properties. The first reaction (process I) involves an EC mechanism, and the electroreduction of (TPP)Co leads to formation of (TPP)Co(R), where R = CH2CH3or CH3. These latter complexes are both reversibly reduced to [(TPP)Co(R)]- in THF, and this reaction (process 111) occurs at El12= -1.36 V (R = CH2CH3)and -1.38 V (R = CH3). The formation of (TPP)Co(CH2CH3)and (TPP)Co(CH3) depends upon both the concentration of alkyl halide in solution and the scan rate, as shown in Figure 3. At all scan rates, the anodic-to-cathodic peak current ratio decreases as log [RX] is increased, consistent with an increase in the rate of the oxidative addition reaction at higher concentrations of alkyl halide. A decrease in the peak-current ratio is also observed as the scan rate is decreased, and this is consistent with a more complete formation of (TPP)Co(R) on the electrochemical time scale. The data in Figure 3a for CH3CH21imply that the system is under kinetic control on the time scale of the cyclic voltammetric measurements. A similar dependence of ip./i on scan rate was observed for reduction of (TPP)Co in f H F containing each investigated alkyl halide. However, a reduction of the porphyrin in the presence of CH31led to a peak current ratio i,/ip, which was essentially independent of scan rate between 100 and 500 mV/s (see Figure 3b). Under these conditions, the system was under thermodynamic control. The first-order rate constants (123 for the formation of (TPP)Co(R)from [(TPP)Co']- were calculated according to the method of Nicholson and Shain.lg The potential was scanned from 0.0 to -1.25 V, and the ratio ip,/iPcfor
Maiya et al.
648 Langmuir, Vol. 5, No. 3, 1989 Table I. Second-Order Rate Constants for t h e Reaction between [(TPP)Co']- and RX" RX k2, M-I s-l RX kz, M-' s-' CH31 CH3(CH&Br 1.1 1.9 X lo3 CHSCHZI 2.2 X 10' CH3(CH2)4Cl 1.6 X CHS(CH2)dF 4.3 x 10-2 CH3(CH2)2I 4.3 X 10' PhCHzCl 2.8 X 10' CHS(CH2)J 7.9 X 10' C2H&H(Cl)CH3 no reaction 1.0 X 10' CH,q(CH2)41 n-heptyne 8.5 X 1.1 X lo2 CH3(CH2)J n-heptene no reaction CH3CH2Br 1.3 1.4 X 10' CHzBrz CH3(CH2)2Br 2.3 CHgClz 8.8 X lo-' CH3(CH2)3Br 2.7 no reaction l-bromoadamantane CHS(CH2),Br 0.6 'Reactions were carried out in THF, 0.2 M TBAP, at 22 The uncertainty in k2 is f5%.
1 OC.
Solvent: CH2C1,. *El for reduction to generate [(TPP)Co(R)]- in THF, 0.2 M T B d . The electrogenerated species is only stable on short time scales and decomposes to give [(TPP)Co']- as the porphyrin product. Solvent: CsDs. s = singlet, t = triplet, q = quartet, m = multiplet.
(a)
0
1
2
3
4
5
I I
I
0.00
1 -0.20
I -0.40
1 -0.60
I
-0.80
I -1.00
I -1.20
POTENTIAL, V vs SCE
Figure 4. (a) Cyclic voltammograms of 9.84 X lo-' M (TPP)Co in THF, containing 0.2 M TBAP and 7.38 X M CH3(CH&Br. Scan rates: 50 (-), 100 (-), 300 and 500 mV/s (-). (b) Plot of first-order rate constant, kf (s-l), vs concentration of CH&(e-),
HhBr.
peak I was measured as a function of scan rate. This was done for solutions containing a constant (TPP)Co concentration and different concentrations of RX. The second-order rate constants could then be calculated from the slope of kfversus concentration of RX at a fixed scan rate, which in this case was 50 mV/s. An example of the resulting data is shown in Figure 4 for CH3(CH2)J3r.The cyclic voltammograms for reduction of (TPP)Co in THF solutions containing CH3(CH2)5Brat different scan rates are shown in Figure 4a, and the plots of kf vs [RX] are shown in Figure 4b. For the example given in this figure, the calculated second-order rate constant was 1.1M-ls-l. The reduction of (TPP)Co in the presence of each reactive RX compound exhibited similar behavior, and Table I lists the measured second-orderrate constants for the reactions between [(TPP)Co']- and these alkyl halides. An inspection of the kinetic data in Table I reveals that, within a series of alkyl halides with the same R group, the values of k2 decrease with X in the following order: I > Br > F 2 C1. The same trend in reactivity of RX was also observed for the reactions of (TPP)Rh with similar RX compounds in THF'O and can be attributed to an increase in the R-X bond strength as X is changed from I to F.,O (20) Kerr, J. A. Chem. Reu. 1966, 66, 465.
Table 11. Room Temperature UV-vis, Electrochemical, and 'H NMR Data on Electrosynthesized (TPP)Co(R) Complexes 'H NMR datac (chemical E1,2,* V Rgroup X-t(lnm vsSCE shift), ppm CHZCl 406, 525 -1.39 8.98 ( 8 , 8 H), 8.05 (m, H); 7.43 (m, 12 H), -1.41 (s, 2 H) CH3 407, 529 -1.38 8.96 ( 8 , 8 H), 8.03 (m, 8 H), 7.42 (m, 12 H), -3.99 (s, 3 H) CHzCH3 407, 524 -1.35 8.91 ( 8 , 8 H), 8.12 (m, 8 H), 7.46 (m, 12 H), -4.66 (t, 3 H), -3.13 (9,2 H) CHzCHzCH3 407, 526 -1.36 8.96 ( 8 , 8 H), 8.07 (m, 8 H), 7.43 (m, 12 H), -1.61 (t, 3 H), -3.19 (m, 2 H), -4.15 (m, 2 H)
The rate constant k2 for the reaction between [ (TPP)Co]- and RX complexes having the same X atom and different R groups appears to first increase with the number of carbon atoms in CH3(CH2),X from n = 1to n = 3 and then to decrease. Similar trends are noted with alkyl iodides (except for CH31), where the value of k2 is approximately 1order of magnitude larger than for reaction with any of the other alkyl halides. Thus, the values of k, are sensitive to the size of the alkyl halide. The importance of the size of the alkyl halide is most dramatically demonstrated for the reactions of RX where the carbon chain is branched. No reaction between [(TPP)Co]- and RX was electrochemically detectable for these alkyl halides even at the slower time scale of spectroelectrochemical measurements (scan rate = 2 mV/s). The aryl halides also do not react with [(TPP)Co']- on the electrochemical time scale, but this is not the case for the benzyl halides or for compounds with acetylinic bonds (n-heptyne). The fact that the aryl halides do not react with [ (TPP)Co']- may seem surprising since (TPP)Co(arene) is known.' However, there are no reports in the literature on the formation of (TPP)Co(C6H5)by the reaction of chemically or electrochemically generated [ (TPP)Co(I)]and CBH5X. In contrast, (TPP)Co(arene) can be synthesized from a halo or halo-pyridine cobalt(1II) porphyrin via reaction with the corresponding aryllithium.176 Under the conditions employed in this study, the values of kz increase in the following order with respect to changes in the R group: benzyl halide > alkyl halide > n-heptyne > aryl halide. CHzBrz and CH2C12 also react with [ (TPP)Co*]-to form (TPP)Co(RX),18and a similar reaction occurs with (TPP)Rh to give (TPP)Rh(RX).1°J2 Finally, n-heptene and other compounds possessing a CIC moeity, as well as l-bromoadamanthane, do not react with [ (TPP)Co']- on the electrochemical time scale. Electrochemical Synthesis of (TPP)Co(R). The electrosynthesis of (TPP)Co(R),where R = CHZCl, CH3, C2Hs, or C3H7, was carried out as described in the Experimental Section. Each complex was identified by UVvis, lH NMR, and electrochemicalmethods. The UV-vis and 'H NMR data of the four electrosynthesized compounds are given in Table I1 and generally agree with values reported in the literature for the chemically synthesized c 0 m p l e x e s . 3 ~However, ~ ~ ~ ~ ~ ~it~is important to note that a 7 0 4 0 % yield of electrochemically prepared (TPP)Co(R) was obtained while chemical methods for
Electrochemistry of Cobalt-Carbon Bond Formation
Langmuir, Vol. 5, No. 3, 1989 649
*
n (b) c
rai (b) TEA1
In M F
e WAVELENGTH,
nm
1
.60
'
0.40
$
0.20
0.00
-0.20
-0.40
-0.60 -0.80
-1.00
-1.20
8
,
-1.40 -1.60
POTENTIAL, V va SCE
Figure 6. Cyclic voltammograms of (a) (TPP)Coin THF containing 0.1 M TBAP and 1.5 equiv of CHJ and (b) 2 x lo-' M TBAI in THF containing 0.1 M TBAP. Scan rate = 100 mV/s.
preparlng me same complexes gave a yeia or WJYO or iemAyw The W-vis and 'H NMR data for (TPP)Co(CH2C1)are shown in Figure 5 and are very similar to those of the other (TPP)Co(R) complexes. The CH2 protons of CH2Cl in (TPP)Co(CH2Cl)appear as a singlet at -1.41 ppm due to the influence of the porphyrin ring current. Mechanism for the Reaction of [(TPP)Co']- with RX. The reaction of [(TPP)Co']- with RX can proceed by one of three different mechanisms. These are (i) an outer-sphere electron-transfer mechanism, (ii) an innersphere mechanism involving halogen atom abstraction by Co(1) followed by R'-Co(1) coupling, and (iii) an s N 2 substitution of the halogen ion by the Co(1) complex. A halogen atom abstraction mechanism seems unlikely to occur in view of the unfavorable charge on Co(I), which would work against an orientation of the R-X dipole in the proper direction. Evidence against such a mechanism is also suggested by the low affinity of [(TPP)Co']- toward the binding of halide ions.18 The fact that the reactivity of alkyl halides with [ (TPP)Co']- is strongly dependent upon both the nature of the carbon-halogen bond and the alkyl chain length supports the s N 2 mechanism shown in eq 3. In this regard, it is noted that the alkyl halides with branched carbon skeletons as well as l-bromoadamantane, which contains a bridgehead carbon, do not react with the Co(1) nucleophile. These observations are also consistent with the proposed s N 2 mechanism. Figure 6a shows cyclic voltammograms for the reduction of (TPP)Co in THF containing 1.5 equiv of CH31. Initial positive scans from 0.00 to +0.60 V do not show any oxidations. However, a new oxidation peak is clearly observed at +0.35 V after negatively scanning past potentials for reduction of (TPP)Co. This new peak at 0.35 V can thus be attributed to the oxidation of I-, which is generated from the reaction between [(TPP)Co]-and CH,I. This was verified by a comparison of this cyclic voltammogram with a voltammogram of 2 X lo4 M tetrabutylammonium iodide (TBAI) (Figure 6b). This latter solution shows an oxidation peak at E p = 0.35 V, which is not present when TBAP alone was present in solution. The oxidation of I- was observed to occur after the reaction of each RI complex with [(TPP)Co]-. However, the reaction between [(TPP)Co]- and RBr does not provide electrochemical evidence for the generation of Br-. The
electrooxidation of Br- occurs at 0.73 V,l0 and it is possible that, in THF, this peak is overlapped with the Co(II)/ Co(II1) oxidation peak of the metalloporphyrin. Also, any Br- produced during the reaction between [ (TPP)Col]-and RBr will diffuse away from the electrode surface during the time needed to scan between -1.20 V (where Br- is generated) and +0.73 V (where Br- would be consumed). The oxidation of Cl- and F both should occur at potentials positive of 0.90 V. These electrode reactions were not observed in THF due to the oxidation of the metalloporphyrin and the small solvent window for oxidation in this solvent. Reaction of CH31 with Reduced Porphyrins. The kinetic data in Table I indicate that the second-order rate constant for the reaction of [(TPP)Co']- with CH31 is 1 order of magnitude higher than rate constants measured for reaction with other alkyl iodides. Similarly, the reactions of (P)Rh" and [(P)Fe']- with CH31 are faster than expected on the "chain length" basis of reactivity pattern~.'.'~ It should also be noted that the plots of i,,/i,, versus log [CH31]at different scan rates (Figure 3b) suggest that the reaction is thermodynamically controlled. A similar observation was made for the reaction between electrochemically generated (TPP)Rh and CH31.10 This feature, along with the knowledge that the reduction potential of CH31is much more anodic than that of the other alkyl iodides,21prompts us to invoke an electron-transfer mechanism for the reaction between [ (TPP)Co']- and CH31.22 A precedent already exists in the literature for such a "change-over"mechanism in the reactions of alkyl halides with various organic nucleophile^.^^ It had been established that an sN2 pathway results when a substrate is difficult to reduce (i.e., has very negative reduction potentials). In contrast, when the substrate is easy to reduce, an electron-transfer mechanism is often favored. Rate constants for the reaction of CH31with three different porphyrin-based nucleophiles, [(TPP)Fe']-, [(TPP)Co']-, and [(TpivPP)Ni]- (where TpivPP is the dianion of the meso-tetrakis(a,a,a,a-o-pivalamidopheny1)porphyrin) in THF were also measured by cyclic voltammetry. The second-order rate constants (k2) for these reactions along with the formal M(II)/M(I) potentials are given in Table III. The rate constank3 for reaction (21) Rifi, M. R. in Organic Electrochemistry; Baizer, M. M., Ed.; Marcel1 Dekker: New York, 1973; Chapter VI. (22) Bulk electrolysis of (TPP)Co in THF containing CHJ coupled with on-line EPR detection did not provide evidence for the possible generation of a radical species. This is due, perhaps, in part to the presence of strong signals of the paramagnetic starting compound (TPP)Co. (23) Ashby, E. C.; Pham, T. N. Tetrahedron Lett. 1987, 28, 3183.
650
Langmuir 1989,5,650-660
Table 111. Second-Order Rate Constants for the Reaction of CHJ with Electroreduced Iron, Cobalt, and Nickel Porohurins"
[(TPP)Coi][ (TpivPP)Ni"]-
1.9 x 103 1.4 X 10'
-0.87 -0.92b
"Reactions were carried out in THF, 0.2 M TBAP, at 22 f 1 O C . bThe actual reaction involves formation of a Ni(I1) anion radical.= Values given were measured as V vs SCE.
with the low-valent iron and cobalt porphyrins were calculated as described in ref 19, while the method of Sav6ant and VianelloZ4was used to calculate rate constants for the reaction between [ (TpivPP)Ni]- and CH31. As seen in Table 111, [(TPP)Co]- reacts much faster with CH31 than with either [(TpivPP)Ni]- or [(TPP)Fe]-. A comparison of the M(I1) porphyrin reduction potentials with the corresponding values of k 2 indicates a trend opposite to the one which would be predicted by an outersphere electron-transfer mechanism. This trend thus contradicts the mechanism proposed above for the reaction of [(TPP)Co']- with CH31, and thus the reaction has features of both a sN2 and an electron-transfer mechanism. Similar reaction mechanisms involving both SN2 and electron-transfer pathways have been reported in the lite r a t ~ r e . ~ ~ For , ~ ~example, -~' the extent of a single-electron-transfer pathway in a formally SN2-type reaction of
alkyl halides with LWH4 or AH3 is found to be a function of the solvent, the substrate, the leaving group, and the hydride reagent.25
Acknowledgment. The support of the National Science Foundation (Grant CHE-8515411) is gratefully acknowledged. Registry No. CHJ, 74-88-4;CH3CH21,75-03-6;CH3(CH2)21, 107-084;CH&CH,)J,542-69-8;CHJCHdJ, 62817-1;CH,(CH&,I, 638-45-9;CH3CH2Br,74-96-4; CH3(CH2)2Br,106-94-5;CH3(CH2)3Br,109-65-9;CH3(CHd4Br,110-53-2;CH3(CH2)@r,111-25-1; CH&CH2)4Cl,543-59-9;CH&CHd,F, 592-50-7;PhCH2C1,100-44-7; C,H5CH(Cl)CH,, 7886-4; HC*(CH2)4CHS, 628-71-7; H2C-CH(CH2),CH3,592-76-7;CH2Br2,74-95-3;CH2C12,75-09-2;[ (TPP)Co']-, 31886-96-1;(TPP)Co,14172-90-8;[(TPP)Fe']-,54547-68-1; (TPP)Fe, 16591-56-3;[ (TPP)Ni']-, 88669-50-5; (TPP)Ni, 1417292-0; (TPP)Co(CH,),65856-25-9;(TPP)Co(CH&l),29130-60-7; (TPP)CO(C~HS), 61730-43-6;(TPP)Co(CsH,),61730-44-7; [(TPP)Co(CH,)]-, 119679-60-6; [(TPP)Co(CH&l)]-, 109123-06-0; [(TPP)CO(C~H~)]-, 119679-61-7; [(TPP)Co(CSH,)]-,119679-62-8; 1-bromoadamantane,768-90-1;tetrahydrofuran, 109-99-9. (24) SavCant, J.-M.; Vianello, E. Electrochim. Acta 1965, 10, 905. (25) khby, E. C.; DePriest, R. N.; Goel, A. G.; Wenderoth, B.; Pham, T. N. J. Org. Chem. 1984,49, 3545. (26) Lund, T.; Lund, H. Tetrahedron Lett. 1986,27,95. (27) Lund, T.; Lund, H. Acta Chem. Scand. Ser. E 1986, 40, 470. (28) Kadieh, K. M.; Sazou,D.; Liu, Y.M.; Maiya, G. B.; Han,B. C.; Saoiabi, M.; Ferhat, M.; Guilard, R. Inorg. Chem. 1989, in press.
Electrocatalytic Reactions in Organized Assemblies. 6. Electrochemical and Spectroscopic Studies of Catalytic Clay/Micelle Electrodes? Chunnian Shi,* James F. Rusling,* Zhenghoa Wang,s William S. Willis, Ann M. Winiecki, and Steven L. Suib* Department of Chemistry (U-60) and Institute of Materials Science, University of Connecticut, Storrs, Connecticut 06269-3060 Received December 13, 1988. I n Final Form: January 25,1989 Compared to bare electrodes, enhanced catalytic efficiencies were found for eledrocatalyticdehalogenations of 4,4'-dibromobiphenyl and allyl chloride by cobalt bipyridyl complexes on clay-modified electrodes (CMEs) in solutions 0.1 M or more in cetyltrimethylammoniumbromide (CTAB). UV absorption, X-ray fluorescence, surface spectroscopic,and X-ray powder diffraction studies indicated that reactants and surfactant were adsorbed to the CME. Adsorbed CTAB most likely facilitates adsorption of the catalyst and nonpolar organohalide substrates on the external surface of the bentonite clay colloid films, where the reactions take place. The CME-micelle surface provides a microenvironment featuring high local concentrations of reactants on the electrode causing enhancement of the rates of second-order catalytic processes.
Introduction In principle, electrocatalytic reduction of nonpolar organohalides on clay modified electrodes (CMEs) in cationic surfactant solutions can combine the cation-binding ability of the clay with the solubilizing properties of the surfadant to provide high local concentrations of reactants on the electrode, thereby enhancing rates of reaction. We recently 'Presented at the symposium entitled 'Electrocatalysis", 196th National Meeting of the American Chemical Society, Los Angeles, CA, Sept 27-29, 1988. *Present address: Department of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA. *On leave from Beijing Normal University, Beijing, China.
0743-7463/89/2405-0650$01.50/0
reported preliminary studies' of the stepwise catalytic dehalogenation of 4,4'-dibromobiphenyl (4,4'-DBB) to biphenyl a t CMEs prepared on pyrolytic graphite and graphite felt in aqueous solutions of hexadecyltrimethylammonium bromide (cetyltrimethylammonium bromide (CTAB)). The reaction was mediated by the two-electron reduction of tris(bipyridyl)cobalt(I) bound to the clay. Catalytic efficiencies (indicative of reaction rates) monitored by the ratio of voltammetric current in the presence of 4,4'-DBB to that of the catalyst in the absence of 4,4'-DBB were somewhat higher on CMEs compared to (1)Rusling, J. F.; Shi, C.-N.; Suib, S. L. J. Electroanol. Chem. 1988, 245, 331.
0 1989 American Chemical Society
