Langmuir 1993,9, 2210-2214
2210
Kinetics of Mediated Reductions by Cathodically Generated Dimethylpyrrolidinium-Tin Composites. The Case of Phenyl Bromides Vesna SvetliEi&**+ Eric G. Gunderson, and Essie Kariv-Miller Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455
Vera Zutie Rudjer BoJkoviE Institute, P.O.Box 1016, 41001 Zagreb, Croatia Received February 12,1993. In Final Form: May 10,1993 The initial electronation in the mediated reduction of phenyl bromides has been studied wing a novel electroanalyticalmethod. The mediator was dimethylpyrrolidmiutin (DMP-Sn), generated in situ as a thin film at a Sn cathode in DMF containing (DMP)BFd. Electron transfer from the electron-rich DMP-Sn composite to the phenyl bromide leads to oxidative dissolution of the DMP-Sn to DMP+ and Sn(Q). The rate of mediated reduction was experimentally determined from the dissolution rate of the DMP-Sn film. The rate constants for 12 phenyl bromides studied so far were evaluated through the application of the Langmuir-Hinshelwood model for unimolecular surface reaction. The trend in rate constants in relation to the properties of the PhBr molecules was discussed.
Introduction Previous work in our group has shown that several different cathode metals (Hg, Pb, Sn, Bi, Sb), in the presence of tetraalkylammonium cations (R4N+), form organometallic composites (R4N-metal) at potentials specific to each metal.' These materials contain R4N+, electrons, and metal from the cathode. For Hg2and Pb,9 the stoichiometry of the formation process is 1:1:5 in R4N+, electrons, and metal, respectively. The dimethylpyrrolidinium (DMP+)derivatives of H e and Pb6 have been utilized as mediators, generated in situ, for reduction of several difficult-to-reduce organic substrates (org in Scheme I). Of the four metals for which the DMP composite has been electrochemicallycharacterized (Hg, Pb, Bi, and Sn)! tin formed the compositemost readily, Le., a t a less negative potential. The DMP-Sn composite was isolated and characterized by solid-state techniques7 (X-ray powder diffraction, ESR, 13C solid state NMR, and differential scanning calorimetry). It was concluded that the composite consisted of small particles or clusters of negatively charged metal derived from the Sns- units, covered by a layer of DMP+ in such a way as to maintain the 5:l Sn to DMP+ ratio. Preliminary experiments with the preparative-scale reduction of phenyl bromides a t tin cathodes* have + Permanent address: Rudjer Bogkovid Institute, P.O. Box 1016,
41001 Zagreb, Croatia. (1) Kariv-Miller,E.; Lawin, P. B.; Vajtner, Z. J. J. Electroanul. Chem. 1986,195,435. (2) (a) Kariv-Miller,E.;SvetliEiE, V. J. Electroanul. Chem. 1986,205, 319. (b) Ryan,C. R.; SvetliEiC,V.; Kariv-Miller,E. J. Electroanul. Chem. 1987,219,-247. (3) (a) Kariv-Mder, E.;Lawin, P. B. J . Electroanul. Chem. 1988,247, 345. (b)Lawin, P. B.:SvetliEiC,V.; Kariv-Miller,E. J. Electroanul. Chem. 1989,258,357. (4) (a) Kariv-Miller, E.;Mahachi, T. J. J. Org.Chem. 1986,51,1041. (b)Swartz,J. E.;Mahachi, T. J. Kariv-Miller,E. J. Am. Chem. Soc. 1988, 110, 3622. (c) Loffredo, D. M.; Swartz,J. E.; Kariv-Miller, E. J. Om. Chem. 1989,54,5953. (5) Lawin, P. B.:Hutaon, A. C.: Kariv-Miller. E. J. Ora. - Chem. 1989, 64, 526. (6) SvetliEiE, V.; Lawin, P. B.; Kariv-Miller, E. J. Electroanul. Chem. 1990,284,185. (7) Christian, P. D. Ph.D. Thesis, University of Minnesota, 1990. (8) Gunderson, E. G. Ph.D. Thesis, University of Minnesota, 1993.
Scheme I. Mechanism of DMP(M6) Mediated
Reduction of Organic Substrates
a
ON\
+
le-
M(ca*ode)w
(DMP+)
DMP(M,)
+
org
DMP(Ms)
-
orgL
I
+
DMP'
M=Hg,Pb
+
5(M)
(1)
(2)
product
indicated that DMP+ acted as a catalyst, increasing both the current efficiency and the yield of reduction products. To further examine the catalytic effect of DMP+ upon the reduction of organic substrates at Sn cathodes, an electroanalytical method was developed which paralleled Phenyl brothe conditions of preparative electroly~is.~ mides substituted with electron-withdrawing and electrondonating groups were used owing to the fact that the are readily available, their chemistry is well underat00d,10and, with substitution on the ring, they exhibit a wide range of reduction potentials. It was found that all of the phenyl bromide substrates, readily reacted with preformed DMP-Sn composite at low concentrations (0.2-0.8 mM) in the absence of externally applied potential DMP(Sn5),d,d
+ PhBr
-
PhBr*-
+ DMP' + 5%
(3)
1
prodlucl
Interestingly, under the same conditions, chlorobenzene did not show any activity up to a concentration of 15mM. In this paper we shall investigate the mechanism and determine the kinetic parameters for this surface reaction, (9) (a) Gunderson,E.G.; SvetliEib, V.; Kariv-Miller,E. J. Electrochem. SOC.Ext. Abet. 1992,92,647. (b) Gunderson, E.G.; SvetliEib,V.; KarivMiller, E. J. Electrochem. SOC.,in press. (10) (a) Andrieux, C.P.; Saveant,J. M.; Zann, D. Nouv. J. Chim. 1984, 9,107. (b) Ruling, J. F.;Arena, J. V. J. Electroanal. Chem. 198S,ISS, 225. (c) Sucheta, A.;Ruling, J. F. J. Phys. Chem. 1989,93, 5796. (d) Andrieux, C. P.; Saveant,J. M.; Su, K. B. J.Phys. Chem. 1986,90,3816.
0 1993 American Chemical Society Q743-7463/93/24Q9-221Q$Q4.QQ/Q
Cathodically Generated DMPSn Composites
Langmuir, Vol. 9, No.8,1993 2211
Le., the initial electronation of the phenyl bromides by the DMP-Sn composite. 0
Experimental Section Materials. The cathode was Sn foil (Aesar 99%). Dimethylpyrrolidinium tetrafluoroborate, (DMP)BFd, was prepared by a previously reported procedure.ll The phenyl bromides examined were bromobenzene,4-bromochlorobenzene, o-bromoanisole, and ethyl 2-bromobenzoate. These compounds were commercially available (Aldrich, >97%) and used without further purification. N,N-Dimethylformamide, (DMF) (Burdick and Jackson, high purity) was distilled in vacuo (55 %/15 mmH ) and the middle 60 % stored over activated molecular sieves (4 Davison,Fisher Scientific)under an Nz atmosphere in the absence of light. Mediation Experiments. The experiments were performed using a Princeton Applied Research (PAR) 173 potentiostat in combination with a PAR 175 universal programmer. The electrochemical cell was a four-port 25-mL flask. The working electrode was a Sn foil (0.5 mm thick). The geometric area of the Sn foil submerged in solution was 0.2 f 0.06cm2. The counter electrode was a Pt wire and the reference a SCE modified for use in DMF." All preparative and measurement steps were performed in situ. The experiments were carried out in DMF as a solvent containing 0.06 M (DMP)BF, as the supporting electrolyte at room temperature under an argon atmosphere. A scan rate of 50 mV 8-l was used throughout. The Method. A new procedure was developed to follow the kinetics of electron transfer from the organometallicf i i (DMPSn) to the organic substrate? Tliis method consisted of the following three steps, all performed in situ: Step 1. Electrochemical formation of the DMP-Sn composite film by constant potential electrolysis for 5 s without stirring
1,
DMP'
+ 5Sne,de
-
t
z
w w
20
3 0
15
10
v - 18
5 0
-2.0
I -8
E f V vs. SCE
E -2.W tt961
DMP(Snd,,,
--
(4)
Step 2. Interaction of the DMP(Sn5) film with organic substrate under constant stirring during a defiied time t r when no external potential is applied DMP($n,),,,d
+ PhBr
PhBr'-
+ DMP' + 5Sn
(5)
t,
The reaction time, t,, was in the range from 5 to 30 s and is limited bythestabilityofthecomposite. Itwas foundthatstirring the solution did not dislodge the DMP(Sa) from the electrode surface or hasten ita decomposition relative to a nonstirred solution. The stirring intensity was kept constant and selected within a range of intensities where ita variation had no more effect on the surface reaction. Reaction 5 can be viewed as oxidative dissolution of the DMP(Sn& Step 3. Electrochemical oxidation of the unreaded DMP(Sns) f i i after tr has elapsed and stirring discontinued DMP(Sn,)dd - e-
-
Imdic#trlpphg
P o b a m (-22 to -1.8 VI
DMP'
+ 5Sn
(6)
Results In a typical experiment, DMP(Sn5) is generated at the Sn cathode at a potential of -2.5 V in a solution of DMF containing 0.05 M (DMP)BF4 and the organic substrate. The electric circuit was then disconnectedand the organic substrate was allowed to interact with the compositeunder intensive stirring. After the reaction time t,, the stirring is discontinued, and the remaining composite quantitatively reoxidized by application of a positively going potential sweep (anodic stripping) to the Sn cathode. The complete stripping of unreacted film was achieved at 50 mV s-l. The charge Qtr, obtained by graphical integration (11)Kariv-Miller, E.;Andruzzi, R. J. Electroanol. Chem. 1985,187, 175.
-1.8
-2.0
-i 2
E f V vs. SCE Figure 1. Anodic stripping voltammograms of unreaded DMP(Sa) recorded in: (a) 0.2 mM bromobenzene in 0.05 M (DMP)-
BF4, DMF; (b) 0.6 mM bromobenzene in 0.05 M (DMP)BF,, DMF. Reaction time, tr, indicated on the curve. Potentialsweep from -2.2 to -1.8 V, scan rate Y = 50 mV 8-1,
of the reoxidation peak in the ASV (anodic stripping voltammogram), is a direct measure of the quantity of DMP(Sn5) that was not consumed by the reaction with organic substrate. In the absence of added organic substrate to the solution, the reoxidation peak was measurably decreased only for t, 2 30 a. Without external potential applied to the cell (open circuit), the insoluble DMP(S-) film was found to be sufficiently stable for time intervals 130 a. It was verified that the formation of the composite is not affected by the presence of the organic substrate. This can be rationalized by the fact that dimethylpyrrolidinium tetrafluoroborate is present in a concentration of 0.05 M,
2212 Langmuir, Vol. 9, No. 8,1993
SuetliEiC et al. I
I”
7
6
0
E
@ \
1:
1
U
a 0’ 0
2-
10
20
30
tr I seconds
Figure 2. Charge of the anodic stripping peaks in Figure l a plotted aa a function of the reaction time (0). The charge of unreacted DMP(Sn6) in 0.05 M (DMP)BF4, DMF is given for comparison (M). which is 2 orders of magnitude greater than the substrate. The initial amount of DMP(Sn5) generated could be determined from the charge density at t r = 0 s. The charge at tr = 0, 80,was found to be 9.7 0.37 mC cm-2, which corresponds to a surface film containing 1.01 X le7mol DMP(Sn5) cm-2. Since the surface concentration of the monolayer,8 I’ = 5 X 10-10 mol cm-2, the initial DMP(Sn5) film is approximately 200 equivalent monolayers thick. The anodic stripping voltammograms for the unreacted DMP(Sn5) in the presence of 0.2 mM bromobenzene at tr = 0,5,10,15,20,25, and 30 s, measured in independent runs,12are shown in Figure l a and compared with curves for a higher concentration of the same substrate (Figure lb). The ASV is clearly dependent upon the reaction time t, and the concentration of bromobenzene in the solution. The charge of the ASV peaks in the experiment with 0.2 mM bromobenzenewas plotted as a function of the reaction time and compared with values obtained in the absence of substrate (Figure 2). The linear dependence of the amount of unreacted DMP(Sn5) in 0.2 mM bromobenzene means that in the time interval of 5-30 s, a constant rate dissolution of the DMP(Sn5) was attained. This further implies that the solid surface was continuously renewed and that the surface concentration of dissolved substrate, bromobenzene, was constant. Independence of the results on the variation of the stirring rate proves that (under the experimental conditions used in this study) the surface reaction was the limiting step and not the transport of reactant or products in the adjacent solution layer. The slope of the straight line in Figure 2, expressed as flux per unit surface area, represents the rate of oxidative dissolution (rate of reaction) of the DMP(Sn6) in the presence of 0.2 mM bromobenzene.
*
-(dQldt)lJ’ = -(dnDman/dtr) = uob = -(dnpmr/dtr) (7) where F is the Faraday constant, and the number of electrons exchanged in the electrochemical oxidation (6) is 1. The reproducibility of this procedure was determined through the repetition of the experiments under identical conditions and was found to be * 5 % in the slopes of the Q vs tr. Time dependence of the charge Qtr, corresponding to ASV peaks recorded for five different concentrations of bromobenzene in the range of 0.05-0.6 mM is displayed in Figure 3. In all cases a constant slope is obtained,
*
(12)A new DMP(Sn5) f i b (8. = 9.7 0.37 mC cm-2) was formed for each t , investigated.
n ”
0
8
10
30
20
tr I seconds Figure 3. Charge of the anodic stripping peaks of unreacted DMP(Sn5)plotted vs reactiontime (Qvs t,plot) for five different concentrations of bromobenzene in 0.05 M (DMP)BFd, DMF. Concentrationof bromobenzene added in mM: (1)0.05;(2)0.10; (3) 0.20; (4)0.40; (5) 0.60.
Table 1. Experimental Values of the ‘Slopes” (dQ/dtJ for the DMP(Sn6) Composite Film in the Prewnce of Four Phenyl Bromides in the Concentration Range 0.05 t o 0.8 mM -(dQ/dt,)/mC cm-2 0.05 0.1 0.2 0.4 0.6 0.8 compound mM mM m M mM mM mM 0.13 0.18 0.30 0.54 0.56
0.13
0.21
0.32
0.54
0.65
0.10
0.15
0.25
0.44
0.52
0.17
0.37
0.52
CI
0.62
indicating that a constant rate dissolution of the DMP(Sns) film was achieved for each concentration of the organic substrate. Table I contains the experimental values for the slopes, dQldt,, for four different phenyl bromide substrates in the concentration range studied. Therefore it was possibleto compute the rates of the surface reaction, uob, for all concentrations of organic substrates. Parts a to d of Figure 4 show the dependence of the observed rates for bromobenzene, 4-bromochlorobenzene, o-bromoanisole, and ethyl 2-bromobenzoate, respectively.
Discussion The dissolution rate for all four substrates increases with their concentration in dilute solution and approaches a plateau a t higher concentrations. The exception was ethyl 2-bromobenzoate, where the plateau could not be detected in the measurable range (Figure 4d). This behavior is reminiscent of the Langmuir type of adsorption isotherm observed in ligand promoted dissolution of oxides.13 In a recent paper by Hering and Stumml4 ~~
~~
~
(13)Stumm, W. Chemistryofthe Solid-Water Znterface;Wiley: New York, 1992,pp 166-169. (14)Hering, J. G.; Stumm, W. Langmuir 1991, 7, 1667.
Langmuir, Vol. 9, No.8, 1993 2213
Cathodically Generated DMP-Sn Composites
+
PhBr&rM e-- PhBr'(8) and occurs simultaneously with the decomposition of the DMP(Sn5) to DMP+ and Sn metal
lo7 [PhBrVmol cm3
DMP(Sn5),l,d-e--DMP(Sns),lid 4- DMP++ 5Sn (9) In this manner a new active layer of DMP(Sn5) is continuouslyexposed to the solution and the active surface remains constant until the DMP(Sn5) film is completely depleted. A constant concentration of the phenyl bromide is maintained at the solid DMP(Sns)/solution interface by stirring the solution. In order to correlate the surface concentration of the phenyl bromide with the bulk concentration, we have applied the Langmuir-Hinshelwood model for unimolecular surface reactions without strong adsorption of the reaction productslS A-C+D with the corresponding rate law
(10)
for k' = kS bA 1O7[PhBr1/molcm3
where k is the rate constant in s-l, S the number of surface sites per cm2,SAthe number of occupied surface sites in mob cm-2, bA the adsorption coefficient in cm3mol-', and CAthe concentration of organic substrate in the solution in mol cm-S. Equation 12 predicts that at low concentrations of substrate, u is proportional to CA but should reach a limiting rate k S at higher CA. This type of behavior is observed for these phenyl bromides as shown in Figure 4. Equation 12 can be put in a linear form lo7 [PhBrl/mol cm3
1O7[PhBr1/molc m 3 Figure 4. Observed rates of the surface reaction, uOb, vs concentration of phenyl bromide: (a) bromobenzene;(b) 4-bromochlorobenzene; (c) 0-bromoanisole;and (d) ethyl 2-bromobenzoate. Points are the experimental values and the full line is obtained by fitting the LangmubHinshelwood model for a unimolecularsurface reaction without strong adsorption of the products.
concerning the ligand-promoted dissolution of aluminum oxide, it was proven experimentally that the rate of dissolution was directly dependent on the concentration of the surface complex; the solid surface was continuously regenerated while the concentration of the surface complex with the ligand was constant. It follows that electron transfer from the electron-rich DMP(Sn5) should proceed through an adsorbed state to generate an anion radical
= l/kC, 4- bA/k' (13) Values of kS and bA can be determined from l / v vs 1/CA plots. They intercept is equal to llk' and the slope of the linear line equal to bdk'. Plots of l/v,b vs 1/cA were constructed d i n g the experimental data presented in Figure 4 to determine the kinetic parameters. The adsorption coefficients bA of the PhBr at theDMP(Sn5)film/solution interface,the limiting rate of the surface reaction k S , and the rate constant k, are given in Table 11. The value of S, the total number of surface sites of the DMP(Sn5) film was approximated by the monolayer concentration of the DMP(Sn6), which is equal to 5.1 X mol cm-2. Comparison of the model prediction with the experimental values in Figure 4 shows a very good fit for all four of the phenyl bromides. Therefore the following mechanism for the DMP(Sn5) mediated reduction of phenyl bromides is proposed: l/U
DMP(Sn,),,,
+ PhBr
kd.o.b
F= [DMP(Sn5).-PhBrlsda, k k b
-
(14)
k
[DMP(Sns)--PhBrlsd,,
[PhBrl'-
+ DMP' + 5Sn
(15) The kinetic parameters were determined for 12 phenyl bromides (Table 111)for which slopes of the Q vs t , plots (16)Adamson,A. Physical Chemistry of Surfacer; Wiley New York, 1982,pp 631-634.
SvetliEi6 et a1.
2214 Langmuir, Vol. 9,No.8, 1993 Table 11. Adsorption Coefficients (6) at the DMP(Sn6) Film/Solution Interface, the Limiting Rate of the Surface Reaction (k@, and the Rate Constant (k)aDerived via the Application of the Langmuir-Hinshelwood Model 1@br, lPkS, compound cma mol-' mol cm2 8-1 k, 8-1 1.89 1.17 23.4
2.52
1.06
20.8
1.63
1.01
22.0
J""
L
CI QB Jr
0.09
10.0
204
Figure5. Log plot of rata constants for the initial electronation (k)va adsorption coefficients ( b 3 for the 12 phenyl bromides studied.
aThe number of surface sites (S)waa approximated by the monolayer concentration of DMP(S-) i.e., 6 X 10-10 mol cm-2. Table 111. Reduction Peak Potentials (Ep): Adsorption Coefficients ( b A ) , and Rate Conrtants (k)for the Mediated Reduction of 12 Phenyl Bromides by DMP(Sn6) compound Ed V a l@br,cma mol-' k,8-1 o-BrPhNOz -1.16 98.5 0.36 -1.80 42.6 0.83 m-BrPhC(0)Me -1.91 25.1 1.83 o-BrPhC(0)Me -2.01 204 o-BrPhCOOEt 0.09 -2.33 2.52 20.8 p-BrPhC1 -2.50 24.8 o-BrPhC1 1.60 22.0 -2.56 1.63 o-BrPhOMe -2.57 o-BrPhNHz 37.6 0.79 111 -2.58 0.17
adsorption coefficients b A (Figure 5 ) shows an inverse relationship. It seems that the stability of the adsorbed state of the PhBr, [DMP(Sn,+PhBrI plays a role in the rate of the oxidative detachment and thus affects the overall rate of mediated reduction.
Conclusions
obtained from cyclic voltammetry measurements on a mercury electrode. All the rate constants are within a range from 10to 200 s-l, while the span of the adsorption coefficients is from 9 X l@to 4 X lo6. Surprisingly, there was no obvious trend in the rate constant k with the measured reduction potentials or the electronic nature of the phenyl substituent.16 However, the log plot of the rate constants vs
An electroanalytical procedure has been introduced which allows for the evaluation of heterogeneous electron transfer between a new class of organometallic mediators and difficult-to-reduceorganic substrates. The mediator DMP(Sn& belongs to a novel class of electron-rich tetraalkylammonium metals, formed by combination of DMP+,Sn, and an electron. Since the R4N+and the metal can be widely varied, it is possible to design mediators that would transfer electrons in a highly selective and synthetically useful manner. The electroanalytical procedure used gives direct evidence for mediation and a measure for the rate of the surface reaction. The electrochemical technique is advantageous in that it can be used in both the formation and quantitative analysis of the mediator. Electron transfer from the mediator (DMP-Sn composite) to the organic substrate (phenyl bromide) led to oxidative dissolution of the DMP-Sn to DMP+ and Sn metal. The rate of mediated reduction was experimentally determined from the dissolution rate of the DMP-Sn film. It was found that the dissolution rate increases with the concentration of substrate in dilute solution and approaches a plateau at higher concentration. The values for the rate constants of initial electronation and adsorption coefficients of 12 phenyl bromides were determined through the application of the Langmuir-Hinshelwood model for unimolecular surface reactions without strong adsorption of the reaction products.
(16)The reportedrata of cleavage of the don-radical increase with a decrease in the reduction potenW;'a o-bromonitrobenzene(-1.16 V) with k = 103 6-1 YB m-bromoacetophenone(-1.80 V) with k = 1V 8-1.
Acknowledgment. This work was supported by the National Science Foundation.
? o PhBr
fY--
-2.67 -2.68
1.89 3.83
23.4 16.1
kABr
pBrPhNH2 -2.76 2.59 16.2 0 CV's were recorded at a M i l e Hg drop i n 0.1 M (B@)BFJ DMF' at &ImV 8-1. Concentration of phenyl bromide waa in the range of 6 to 10 mM.
have been reported!b The compoundsare listed according to increasing negative value of the peak potentials E,,
