Catalytic reduction of ?±,??-dihaloalkanes, X(CH2)mX (X = Cl, Br, or I and m = 2-8), by electrochemically generated C70n- (n = 2 or 3) in benzonitrile solutions

Article abstract of DOI:10.1021/jp980713s

The first report on the homogeneous electrocatalytic reduction of halogenated alkanes by C₇₀ anions is presented. Cyclic voltammetry (CV) measurements indicate that C₇₀^n- (n = 2 or 3), electrochemically generated in 0.1 M (TBA)PF₆, benzonitrile, catalyzes the dehalogenation of ?±,??-dihaloalkanes, X(CH₂)_mX (X = Cl, Br, or I and m = 2-8). Values of the second-order rate constant, k, for the electrocatalytic dehalogenation of the ?±,??-dihaloalkanes by the C₇₀^n- anions were determined by using the rotating-disk electrode (RDE) voltammetry under pseudo-first-order conditions with respect to the ?±,??-dihaloalkanes. In accordance with Saveì?ant's theory of dissociative reduction, the k values increase in the order Cl < Br < I for the investigated 1,2-dihaloethanes. The k values also decrease with the number of carbon atoms of the alkyl chain for the investigated series of ?±,??-diiodoalkanes. The calculated k values for the C₇₀^n- (n = 2 or 3) catalysts are generally smaller than those for the C₆₀^n- (n = 2 or 3) catalysts. Unlike the C₆₀^n- electrocatalysis, reported earlier to be accompanied by a chemical reaction between C₆₀^n- and certain ?±,??-diiodoalkanes yielding alkyl adducts of C₆₀, no reaction between C₇₀^n- (n = 2 or 3) and the ?±,??-diiodoalkanes leading to alkylated C₇₀ was observed on the voltammetry time scale.

Full text of DOI:10.1021/jp980713s

J. Phys. Chem. B 1998, 102, 4247-4252
4247
Catalytic Reduction of r,ω-Dihaloalkanes, X(CH2)mX (X ) Cl, Br, or I and m ) 2-8), by
n-
Electrochemically Generated C70 (n ) 2 or 3) in Benzonitrile Solutions
,†
†
,‡
Francis D’Souza,* Jai-pil Choi, and Wlodzimierz Kutner*
Department of Chemistry, Wichita State UniVersity, Wichita, Kansas 67260-0051, and Institute of Physical
Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, PL-01-224 Warsaw, Poland
ReceiVed: January 7, 1998; In Final Form: March 16, 1998
The first report on the homogeneous electrocatalytic reduction of halogenated alkanes by C70 anions is presented.
n-
Cyclic voltammetry (CV) measurements indicate that C70 (n ) 2 or 3), electrochemically generated in 0.1
M (TBA)PF
6
2 m
, benzonitrile, catalyzes the dehalogenation of R,ω-dihaloalkanes, X(CH ) X (X ) Cl, Br, or I
and m ) 2-8). Values of the second-order rate constant, k, for the electrocatalytic dehalogenation of the
R,ω-dihaloalkanes by the C70 anions were determined by using the rotating-disk electrode (RDE) voltammetry
n-
under pseudo-first-order conditions with respect to the R,ω-dihaloalkanes. In accordance with Sav e´ ant’s
theory of dissociative reduction, the k values increase in the order Cl < Br < I for the investigated 1,2-
dihaloethanes. The k values also decrease with the number of carbon atoms of the alkyl chain for the
n-
investigated series of R,ω-diiodoalkanes. The calculated k values for the C70 (n ) 2 or 3) catalysts are
n-
n-
generally smaller than those for the C60 (n ) 2 or 3) catalysts. Unlike the C60 electrocatalysis, reported
n-
earlier to be accompanied by a chemical reaction between C60 and certain R,ω-diiodoalkanes yielding alkyl
n-
adducts of C60, no reaction between C70 (n ) 2 or 3) and the R,ω-diiodoalkanes leading to alkylated C70
was observed on the voltammetry time scale.
Introduction
against the alkyl chain length of the R,ω-diiodoalkanes.17
n-
Importantly, C60 (n ) 2 or 3) reacts with n-iodoalkanes,
Since the first report on electrochemical behavior of C60 in
1
8-21
yielding a range of alkyl adducts of C60.
1
1
990, several laboratories have contributed to the research on
Although the electrocatalytic properties of C60 are now largely
fundamental and application aspects of the fullerene and
fullerene derivative electrochemistry. Now, it is well docu-
1
5-17
revealed,
no electrocatalytic activity of higher fullerenes
2
has been reported to date. Study of electrocatalytic behavior
of higher fullerenes, in comparison to that of C60, is important
for the understanding of fullerene reactivity. In the present
paper, we report on our cyclic voltammetry (CV) and rotating-
disk electrode (RDE) voltammetry study involving electrocata-
lytic dehalogenation of R,ω-dihaloalkanes in a benzonitrile
solution by electrochemically generated di- and trianions of C70.
Also, we examine the dependence of the apparent second-order
rate constant values, determined by the RDE voltammetry, on
the nature of the halogen of the 1,2-dihaloethane as well as on
the alkyl chain length of the employed R,ω-diiodoalkanes.
mented that both C60 and C70 are electroreduced by six one-
3
-6
electron reversible steps in nonaqueous solvent solutions, in
agreement with quantum chemistry calculations.7
-10
Owing to
the high stability of the resulting C60 and C70 anions electro-
chemically generated in deaerated aprotic solvent solutions as
well as a large and nearly equal spacing of the formal redox
11-14
potentials of the consecutive electrode redox processes,
the
anions of both fullerenes are promising as selective inter-
molecular electron-transfer catalysts. Accordingly, Huang and
Wayner1 reported on the electrocatalytic debromination of alkyl
bromides and vicinal dibromides by mono-, di-, and trianions
of C60. Also, preparative-scale electrolysis, followed by product
analysis, of Vic-dihalides and perfluoroalkyl halides performed
5
Experimental Section
n-
16
by using the C60 catalysts was reported by Fuchigami et al.
Chemicals. C70 (+99.8% purity by HPLC) was from
BuckyUSA (Bellaire, TX). All R,ω-diiodoalkanes, i.e., 1,2-
diiodoethane, 1,3-diiodopropane, 1,4-diiodobutane, 1,5-diiodo-
pentane, 1,6-diiodohexane, and 1,8-diiodooctane, as well as 1,2-
dichloroethane, 1,2-dibromoethane, tetra-n-butylammonium
hexafluorophosphate, (TBA)PF6 (98% purity), and anhydrous
benzonitrile (+99% purity, water content < 0.005%) were from
Aldrich Chemicals (Milwaukee, WI). All chemicals were used
without further purification.
Instrumentation. The CV experiments were performed by
using a model 263A potentiostat/galvanostat of EG&G Princeton
Applied Research (Princeton, NJ) and a typical three-electrode
electrochemical cell. A 1.6 mm diameter disk platinum
electrode of Bioanalytical Systems (West Lafayette, IN) was
used as the working electrode, while a platinum wire and sodium
Recently, in our systematic study of electrocatalytic reduction
of R,ω-diiodoalkanes, I(CH2)mI (m ) 2-8), by the C60 anions
electrochemically generated in nonaqueous solvent solutions as
well as in the C60 film-modified electrodes, we demonstrated
that the C60 anions electrocatalytically deiodinate R,ω-diiodo-
alkanes.17 This dehalogenation results in the formation of a
range of alkanes, alkenes, and monoiodoalkane derivatives as
the reaction products. A competing alkylation involving C60
anions and certain R,ω-diiodoalkanes was also observed.
Presumably, this is why no systematic trend of changes was
found in the determined apparent second-order rate constants
*
To whom correspondence should be addressed.
Wichita State University.
Polish Academy of Sciences.
†
‡
S1089-5647(98)00713-5 CCC: $15.00 © 1998 American Chemical Society
Published on Web 04/29/1998

4248 J. Phys. Chem. B, Vol. 102, No. 21, 1998
D’Souza et al.
chloride saturated Ag/AgCl electrode served as the auxiliary
and reference electrode, respectively. The RDE voltammetry
experiments were carried out by using a model AFCB1
bipotentiostat, MSRX speed control unit, and AFMSRX modu-
lated speed rotator of Pine Instrument Co. (Grove City, PA).
The diameter of the rotating platinum electrode was 4.6 mm.
Bulk electrolyses of about 0.2 mM C70, 0.1 M (TBA)PF6 in
benzonitrile solutions containing R,ω-diiodoalkanes were carried
out by using an EG&G model 377A coulometry cell. All
potentials cited are against the Ag/AgCl reference electrode.
All experiments were performed at ambient temperature, 20 (
1
°C, in the dark in order to avoid decomposition of the R,ω-
diiodoalkanes. Solutions were deaerated by N2 purge prior to
experiments.
The GC analysis of the bulk electrolysis products was
performed by using a model 5890A gas chromatograph equipped
with a model 7673A automatic injector and an FID detector of
Hewlett-Packard on a SPB-5 nonpolar phase fused silica
capillary column (30 m × 0.53 mm) of Supelco (Bellofonte,
PA). The GC-MS analysis was carried out by using a model
Q-Mass 910 of Perkin Elmer Co. (Norwalk, CT) on a DB-1
nonpolar phase fused silica capillary column (30 m × 0.253
mm) operating in an electron ionization mode.
Figure 1. Cyclic voltammograms for 0.1 mM C70 (curve 1), 0.1 mM
C
1
70 and 0.4 M 1,2-dichloroethane (curve 2), 0.1 mM C70 and 0.4 M
,2-dibromoethane (curve 3), as well as 0.1 mM C70 and 1.4 mM 1,2-
diiodoethane (curve 4) in 0.1 M (TBA)PF in benzonitrile, at a Pt
The HPLC analysis of the bulk electrolysis products, per-
formed in order to check if any alkyl adducts of C70 were
formed, was carried out by using an isocratic chromatograph
of Perkin-Elmer consisting of a model 250 binary pump and a
LC-290 UV-visible spectrophotometric detector set at 340 nm.
An analytical Cosmosil Buckyprep column (250 mm × 4.6 mm)
of Nacalai Tesque (Kyoto, Japan) was used with toluene as a
6
-
1
electrode. Potential scan rate is 0.1 V s
.
new electro-oxidation peaks are observed (curves 2 and 3 in
Figure 1) in the presence of 1,2-dibromoethane and 1,2-
-
dichloroethane that correspond to the irreversible Br /Br2 and
-
22
Cl /Cl2 electro-oxidation, respectively.
These CV results
-
1
indicate that 1,2-dihaloethanes are reduced and halogens are
mobile phase with a flow rate of 1 mL min .
released in the solution during the initial C70-
/2-
electro-
reduction. Importantly, no additional peaks were observed if
reversal of the cathodic scan of potential was set at a value less
negative than that of the formal redox potential of the C70
Results and Discussion
Electrocatalytic properties of the C70n-/(n+1)- (n ) 0-2) redox
couples with respect to R,ω-diiodoalkanes were qualitatively
examined by using CV. Quantitative results, however, were
obtained by using the RDE voltammetry because (i) the
background currents of the irreversible electroreduction of the
R,ω-dihaloalkanes observed in the absence of C70 are better
discriminated than those measured with CV, and (ii) in contrast
-/2-
couple. Similar voltammetric behavior is observed for all other
R,ω-diiodoalkanes studied.
By holding for about 30 s the potential at -1.2 V, i.e., at a
-
/2-
value more negative than the C70
electroreduction peak
n-/(n+1)-
potential and subsequent potential cycling over the C70
redox couples, we found no new peaks in the voltammograms.
Hence, we conclude that no alkyl adducts of C are formed on
to CV, the RDE voltammetry current of the electrocatalytic
process corresponding to each C70
7
0
n-/(n+1)-
20
couple is virtually
the CV experiment time scale.
free from the influence of any electrochemical process that
would proceed at more positive potentials, since fresh solution
containing C70 and the substrate is supplied from the bulk
solution by forced convection to the electrode surface during
the potential scan.
Importantly, a step rather than a peak is observed for the
C70-
/2-
electroreduction in excess of the 1,2-diiodoalkane
(Figure 2) and the limiting current of this step is proportional
to the square root of the 1,2-diiodoalkane concentration (inset
in Figure 2). Moreover, the limiting current of this step is
virtually independent of the potential scan rate (Figure 3). This
Cyclic Voltammetry. In 0.1 M (TBA)PF6, benzonitrile, there
n-/(n+1)-
0/-
are four reversible C70
electroreduction/electro-oxidation
is in contrast to the behavior of the C70 electroreduction peak
peak pairs in the cyclic voltammogram of C70 within the
current, which is proportional to the square root of the potential
scan rate, as expected for a peak whose current is controlled by
accessible potential window of the solvent.12 Addition of the
0
/-
1
,2-dihaloethanes to the solution has no effect on the C70
diffusion (inset in Figure 3). Collectively, the CV results
-
/2-
-/2-
electroreduction peak (Figure 1). However, the C70
elec-
indicate that the C70
redox couple electrocatalytically reduces
0/-
troreduction peak is increased and the corresponding electro-
R,ω-dihaloalkanes and that the C70 couple is electrocatalyti-
cally inactive. Similar but more pronounced catalytic currents
(not shown) are observed for the C70
-/2-
oxidation peak is gone. Noticeably, the C70
peak current markedly increases in the order Cl < Br < I for
,2-dihaloethanes of the same concentrations. Moreover, there
electroreduction
2
-/3-
redox couple,
1
indicating that the catalytic efficiency increases with an increase
of the negative charge of the C70 anion.
Rotating-Disk Electrode Voltammetry. Four well-devel-
are new CV peaks in the positive potential range produced
during the anodic scan that follows the initial cathodic one.
Accordingly, in the presence of 1,2-diiodoethane, two new pairs
of electro-oxidation and electroreduction peaks are seen during
subsequent cycling over the positive potential range (curve 4
oped cathodic steps corresponding to the first four one-electron
n-/(n+1)-
C70
electroreductions are present in the RDE voltam-
mograms of C70 in 0.1 M (TBA)PF6, benzonitrile, for different
rotation rates. Limiting currents, iL, of these steps are controlled
by convectional diffusion, as verified by the Levich analysis
-
-
in Figure 1). They correspond to the irreversible I /I3 and
-
22
quasi-reversible I3 /I2 redox couples, respectively. Similarly,

Reduction of R,ω-Dihaloalkanes
J. Phys. Chem. B, Vol. 102, No. 21, 1998 4249
Figure 2. Cyclic voltammograms for 0.1 mM C70 in 0.1 M (TBA)PF
6
in benzonitrile and different concentrations of 1,2-diiodoethane indicated
Figure 4. Rotating disk electrode voltammetry curves for 0.05 mM
-
1
at each curve. Potential scan rate is 0.1 V s . Inset is the dependence
C
70, 0.05 M 1,6-diiodohexane in 0.1 M (TBA)PF
different rotation rates indicated at each curve. Inset is the dependence
of the limiting current, i , on the square root of rotation rate for the
(n ) 0-2) electroreductions.
6
in benzonitrile for
-
/2-
of the C70
electroreduction peak current on the square root of the
1
,2-diiodoethane concentration.
L
n-/(n+1)-
first three C70
be calculated from the determined limiting catalytic currents
2
4
by using the equation
2
icat
k )
(1)
2
(
nFAC ) D C
R R S
where DR and CR are the diffusion coefficient and concentration
of C70, respectively, CS is the concentration of the substrate,
i.e., R,ω-diiodoalkane, F is the Faraday constant, n is the number
of electrons transferred, and A is the electrode surface area. The
determined icat values are plotted against the square root of the
-
/2-
concentration of each R,ω-diiodoalkane both for the C70
and
2-/3-
C70
redox couple in parts a and b of Figure 5, respectively.
Noticeably, straight lines with the zero plot intercept are
obtained, as eq 1 predicts. The k values calculated from the
slopes of these lines by using eq 1 are presented in Table 1.
Because of the dynamic nature of both CV and RDE
voltammetry, the R,ω-dihaloalkanes are reduced in the reaction
layer by di- and trianions of C70 generated at the electrode
surface. Accordingly, one would expect that concomitant
(
n+2)-
Figure 3. Cyclic voltammograms for 0.1 mM C70, 0.3 mM 1,2-
diiodoethane in 0.1 M (TBA)PF in benzonitrile for different potential
scan rates indicated at each curve. Inset is the dependence of peak
comproportionation of C70
(n ) 0 or 1), generated at the
n -
6
electrode and C70 present in bulk solution according to eq 2,
0
/-
-/2-
currents of the C70 and C70
of the potential scan rate.
electroreductions on the square root
C70n- + C70(n+2)- h 2C70(n+1)-
(2)
2
3
(
not shown). However, this is not the case for the second,
could affect the electrocatalysis. The comproportionation could
C70-/2-, and third, C702-/3-, electroreduction in excess of the
R,ω-diiodoalkane (Figure 4). That is, there are positive devia-
tions at smaller rotation rates for the Levich plots of the limiting
current versus the square root of the rotation rate, f 1 (inset in
Figure 4), indicating that the R,ω-diiodoalkane is electrocata-
lytically reduced. Values of catalytic limiting current, icat, were
determined by extrapolating the limiting currents to the zero
value of the square root of the rotation rate (inset in Figure 4).
Under pseudo-first-order conditions with respect to the R,ω-
diiodoalkanes, the second-order catalytic rate constant, k, can
deplete the C70
(n+2)-
catalyst in the reaction layer, which might
result in overestimation of the determined k values. Obviously,
the effect of reaction 2 is more important for slow electroca-
talysis, observed for R,ω-dihaloalkanes of a long alkyl chain
(m g 5), than for fast electrocatalysis for R,ω-dihaloalkanes of
a short alkyl chain (m < 5). Noticeably, the rate of the
comproportionation is twice as sensitive to the change of the
C70 concentration than that of the electrocatalysis because the
former reaction is second order and the latter is first order against
C70. Therefore, apparent catalytic rate constants for different
/2

4250 J. Phys. Chem. B, Vol. 102, No. 21, 1998
D’Souza et al.
tionation reaction 2 is negligibly small. In any case, the
(n+2)-
determined rate constants for dilute solutions of the C70
catalysts summarized in Table 1 are not overestimated more
than by 10-15%.
3
-
The calculated k values for the C70 catalyst are from about
2
(for 1,2-diiodopentane, 1,2-diiodohexane, and 1,2-diiodo-
octane) to almost 10 (for 1,2-diiodopropane) times larger than
2
-
2-
those for the C70 catalyst. The k values both for the C70
3
-
2-
3-
and C70 catalysts as well as for the C60 and C60 catalysts
reported previously largely decrease with the increase of heat
of formation of the carbon-halogen bond.
2
5,26
This behavior
was demonstrated in Figure 6 for 1,2-dihaloethanes for which
the k values decrease in the order Cl < Br < I. This result
may indicate that the free energy change of the carbon-halogen
bond rupture largely contributes to the activation energy change
2-
of the 1,2-dihaloethane reduction. Apparently, the C60 and
3
-
2-
3-
C60
catalysts are more efficient than the C70 and C70
catalysts, though the former are known to form alkyl adducts
see below). Moreover, the k values for the electrocatalyses
(
2
-
3-
involving C70 and C70 , determined in the present study,
largely decrease with the increase of the number of carbon atoms
in the alkyl chain of the R,ω-diiodoalkanes for m < 5 and are
virtually independent of m for m g 5 (Figure 7a). Importantly,
this behavior is different from that displayed by either the C60
or C60 catalyst (Figure 7b) reported previously. That is,
2-
3
-
17
the k value decreases for m e 5 but there is a maximum for m
2
-
3-
)
6 in the log k vs m plot for both the C60 and C60 catalyst.
The proposed mechanism of catalytic reduction of the R,ω-
dihaloethane substrates by the C70 (n ) 2 or 3) catalysts is
n-
summarized in Scheme 1.
SCHEME 1
-
•
-
X(CH ) X + e f X(CH ) + X
(3)
2
m
2 m
+
H+sol
X(CH2)m• + e f X(CH )
-
-
8 f haloalkane (4a)
Figure 5. Dependence of the rotation-rate-independent RDE voltam-
metry catalytic currents, icat, on the square root of concentration of (1)
2 m
X(CH2)m• + e- -X-8 f alkene + cycloalkane (4b)
1
,2-diiodoethane, (2) 1,3-diiodopropane, (3) 1,4-diiodobutane, (4) 1,5-
diiodopentane, (5) 1,6-diiodohexane, and (6) 1,8-diiodooctane for the
-
/2-
2-/3-
(
(
a) C70
and (b) C70
in benzonitrile.
electroreduction for 0.05 mM C70 in 0.1 M
+
H+sol
TBA)PF
6
-
-
haloalkane + 2e
8 f n-alkane + X
(4c)
(5a)
TABLE 1: Second-Order Rate Constant, k, for
Electrocatalytic Reduction of r,ω-Dihaloalkanes by C70 (n
2X(CH2)m• f X(CH ) -(CH ) X
n-
2 m
2 m
)
2 or 3) Anions, Determined by Voltammetry at the
Rotating Pt Disk Electrode in 0.1 M (TBA)PF
6
, Benzonitrile
+2e- + H+sol
+2e-
-
X(CH ) -(CH ) X
8 f haloalkane + X (5b)
k, M-1 s-1
2 m
2 m
-
2-
3-
C
70
R,ω-dihaloalkane
C
70
C
70
X(CH ) -(CH ) X
8 f alkene + cycloalkane (5c)
2
m
2 m
-2X-
1
,2-dichloroethane
,2-dibromoethane
,2-diiodoethane
,3-diiodopropane
,4-diiodobutane
,5-diiodopentane
,6-diiodohexane
,8-diiodooctane
-
-
-
-
-
-
-
-
2.2 ( 0.1
33.0 ( 1.0
(1.1 ( 0.1) × 10
40.4 ( 0.3
18.7 ( 0.9
10.2 ( 0.3
10.5 ( 0.2
11.7 ( 0.1
3.3 ( 0.3
2
5
2
1
1
1
1
1
1
1
(1.3 ( 0.3) × 10
(5.1 ( 0.4) × 10
(3.6 ( 0.1) × 10
65.1 ( 0.2
+4e-
+ 2H+sol
5
-
X(CH ) -(CH ) X
8 f n-alkane + 2X (5d)
2 m
2 m
22.7 ( 0.1
where X ) Cl, Br, or I and m ) 2-8.
21.1 ( 0.2
Once the R,ω-dihaloalkane, X(CH2)mX, is reduced, the di-
or trianion of C70, which acts as a catalyst capable of transferring
up to two or three electrons, respectively, repeatedly regenerates
its oxidation state in the electrode vicinity provided that
sufficiently negative potential is applied. This regeneration is
responsible for the observed catalytic currents. The initial
electron transfer to X(CH2)mX, assumed to be the rate-
19.4 ( 0.8
C70 concentrations should be determined in order to evaluate
the effect of comproportionation on electrocatalysis. Accord-
ingly, we compared the k values for 0.05 mM C70, presented in
Table 1, with those for 0.5 mM C70 for the R,ω-dihaloalkanes
of a short and long alkyl chain. The rate constants determined
for 0.5 mM C70 were 10-15% larger than those for 0.05 mM
C70. Interestingly, the CV limiting catalytic currents observed
in the presence of the R,ω-dihaloalkanes are virtually indepen-
26
determining step governed by an outer-sphere electron-transfer
mechanism that was originally proposed for reduction of vicinal
2
6-28
dibromoalkanes with aromatic anion radicals,
yields a
dent of the potential scan rate (inset in Figure 3), indicating
1-halogenated alkane radical and liberates halide in the solution
(eq 3). Possibly, a series of reactions can follow the formation
that consumption of the C70(
n+2)-
catalyst in the compropor-

Reduction of R,ω-Dihaloalkanes
J. Phys. Chem. B, Vol. 102, No. 21, 1998 4251
that these radical addition reactions are less favored than the
radical reduction within the voltammetry time scale.
For better understanding of the different possible reaction
routes, we analyzed the products of bulk electrolyses performed
at constant potentials in solutions containing C70 and one of
the R,ω-dihaloalkanes by both GC-MS and HPLC. That is,
in independent bulk electrolysis experiments, either the dianion
or trianion of C70 was electrochemically generated in a ben-
zonitrile solution of the R,ω-dihaloalkane. For the 1,2-diiodo-
ethane substrate, ethane and ethene were the major electrolysis
products. The yield of the former increased with the increase
of the water content in the solvent needed as a proton supply.
However, in the presence of the 1,2-dibromoethane or 1,2-
dichloroethane substrate, only bromoethane or chloroethane was
the major reaction product. In the case of R,ω-diiodoalkanes
of the long alkyl chains (m > 3), the major reaction products
were 1-iodoalkanes and trace amounts of alkenes. An HPLC
chromatogram of the reaction product mixture obtained after
bulk electrolysis revealed no new peaks that would correspond
to the formation of alkyl adducts of C70. This result is in
contrast to our former results on the C60 electrocatalytic
reduction of R,ω-diiodoalkanes, where substantial amounts of
alkyl adducts of C60 were found both for the 1,3-diiodopropane
and 1,5-diiodopentane substrates.
Figure 6. Semilogarithmic dependence of the second-order catalytic
rate constants for the reduction of 1,2-haloethanes in 0.1 M (TBA)-
2
-
3-
2-
3-
6
PF , benzonitrile, by (9) C60 , (0) C60 , (b) C70 , and (O) C70 on
heat of formation of the C-X bond.
n-
The results presented above indicate that the C70 (n ) 2 or
n-
3
) catalysts are more stable than the C60 (n ) 2 or 3) catalysts
with respect to the alkyl adduct formation that would deplete
the effective catalyst concentration in the electrode vicinity. This
lack of the adduct formation could be also a reason why the
determined log k decays monotonically with the number of
n-
carbon atoms of the R,ω-diiodoalkane for the C70 catalysts
(Figure 7a) and could be accounted for by only the inductive
effects of the halogen substituents. Apparently, this effect is
the less pronounced the longer the alkyl chain. This result is
1
7
n-
in contrast to that obtained previously for the C60 catalysts
where maximum in plots is seen (Figure 7b). This maximum
n-
could be due to the competing alkylation of C60 observed for
I(CH2)mI (m ) 3 and 5), which would decrease the effective
n-
C60 concentration in the reaction layer at the electrode and,
hence, decrease the apparent rate of catalysis.
Conclusions
Electrocatalytic reduction of a homologous series of R,ω-
dihaloalkanes, X(CH2)mX (X ) Cl, Br, I), is catalyzed by both
di- and trianions of C70 in 0.1 M (TBA)PF6, benzonitrile, at a
platinum working electrode. By use of the RDE voltammetry,
the second-order rate constants for electrocatalytic dehaloge-
Figure 7. Semilogarithmic dependence of the second-order catalytic
rate constant, k, for reduction of R,ω-diiodoalkanes on the number of
2
-
3-
carbon atoms in their alkyl chains for (a) C70 and C70 as well as
2
-
3-
for (b) C60 and C60 catalysts.
n-
nation of the R,ω-dihaloalkanes by the C70 (n ) 2 or 3)
of this radical. These are presented in eqs 4 and 5 on the basis
of results of the product analysis (see below). These reactions
include (i) further electrocatalytic reduction of the radical species
to form 1-halogenated alkyl anions followed by a proton
abstraction from solvent to yield 1-haloalkanes (eq 4a), (ii)
radical pairing to form higher R,ω-dihaloalkanes (eq 5a), (iii)
formation of cycloalkanes or alkenes as a result of further
reduction of mono-halogenated alkyl radicals in the absence of
a proton supply (eq 4b), and (iv) further catalytic reduction of
higher R,ω-dihaloalkanes formed in eq 4a to yield 1-haloalkanes
catalysts are determined under pseudo-first-order conditions with
respect to the R,ω-dihaloalkanes. The k values increase in the
order Cl < Br < I for the investigated 1,2-dihaloethanes. The
n-
calculated k values for C70 (n ) 2 or 3) catalysts are generally
n-
smaller than those for the C60 (n ) 2 or 3) catalysts. For the
n-
C70 catalysts, there are monotonic decays of log k with the
number of carbon atoms of alkyl chains of the R,ω-diiodo-
alkanes. The GC-MS and HPLC analyses of the reaction
products, obtained by the constant potential bulk electrolyses,
revealed a mixture of alkanes and alkenes as major reaction
products for 1,2-diiodoethane and 1,3-diiodopropane substrates
while monohalogenated alkanes were major reaction products
(
eq 5b), alkenes and cycloalkanes (eq 5c), or alkanes (eq 5d).
A radical addition to the catalyst that would result in the C70
alkyl derivatives could also occur besides the radical pairing
that would lead to higher R,ω-dihaloalkanes (eq 5a). However,
the C70 alkyl derivatives were not detected by the HPLC analysis
of the reaction product mixture (see below). This result suggests
n-
for other R,ω-dihaloalkanes. No reaction between C70 (n )
2 or 3) and the R,ω-diiodoalkanes leading to alkyl adducts of
C70 was observed neither on the CV or RDE voltammetry time
scale nor under the bulk electrolysis conditions. This is unlike

4252 J. Phys. Chem. B, Vol. 102, No. 21, 1998
D’Souza et al.
the results for the C60n- catalysts reported earlier,17 where
corresponding electrocatalytic reductions are accompanied by
chemical reaction between C60 and certain R,ω-diiodoalkanes
yielding alkyl adducts of C60.
(12) Krishnan, V.; Moninot, G.; Dubois, D.; Kutner, W.; Kadish, K.
M. J. Electroanal. Chem. 1993, 356, 93.
(13) Soucaze-Guillous, B.; Jones, M. T.; Kutner, W.; Kadish, K. M. J.
Electrochem. Soc. 1996, 143, 550.
n-
(14) Noviandri, I.; Bolskar, R. D.; Lay, P. A.; Reed, C. A. J. Phys. Chem.
B 1997, 101, 6350.
Acknowledgment. Dr. A. Kapturkiewicz is acknowledged
for helpful comments. We thank BG Products, Wichita, for
rendering its GC-MS instrumentation available for the present
investigations. Financial support from the U.S.-Polish Maria
Sklodowska-Curie Joint Fund II, through Grant No. PAN/NSF-
(
(
15) Huang, Y.; Wayner, D. D. M. J. Am. Chem. Soc. 1993, 115, 367.
16) Fuchigami, T.; Kasuga, M.; Konno, A. J. Electroanal. Chem. 1996,
411, 115.
(17) D’Souza, F.; Choi, J.-p.; Hsieh, Y.-Y.; Shriever, K.; Kutner, W. J.
Phys. Chem. B 1998, 102, 212.
18) Caron, C.; Subramanian, R.; D’Souza, F.; Kim, J.; Kutner, W.;
Jones, M. T.; Kadish, K. M. J. Am. Chem. Soc. 1993, 115, 8505.
19) D’Souza, F.; Caron, C.; Subramanian, R.; Kutner, W.; Jones, M.
(
9
6-275 to W.K. and F.D., as well as from the donors of the
Petroleum Research Fund, administered by the American
Chemical Society, is gratefully acknowledged.
(
T.; Kadish, K. M. In Recent AdVances in the Chemistry and Physics of
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