The electrooxidation of the tetraphenylborate ion revisited

Article abstract of DOI:10.1039/b106356c

The electrochemistry of the tetraphenylborate ion, BPh₄^-, has been studied by cyclic voltammetry and coulometry in water, methanol, ethanol, acetonitrile, acetone, dimethylformamide and dichloromethane under an N₂ atmosphere. While a one-electron and somewhat irreversible oxidation (with an E_1/2 of 0.87 V vs. SCE at a glassy carbon electrode) is observed in dichloromethane, eqn. (i), BPh₄^- ? ^·BPh₄ + e^- the oxidation is somewhat complicated in all other solvents by the occurrence of several consecutive reactions. E_pa, the anodic peak potential in cyclic voltammetry, changes from 0.41 V vs. SCE in water to 0.94 V vs. SCE in dimethylformamide at a glassy carbon electrode. The variation in E_pa with solvent (S) is explained by invoking reaction (ii). ^·BPh₄ + S → S-BPh₃ + ^·Ph. The coulometric results in solvents other than dichloromethane indicate a disproportionation of S-BPh₃, eqn. (iii). 2S-BPh₃ → S-BPh₂⁺ + BPh₄^- + S.

Full text of DOI:10.1039/b106356c

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The electrooxidation of the tetraphenylborate ion revisited
Pankaj K. Pal,a Shubhamoy Chowdhury,a Michael G. B. Drewb and Dipankar Datta*a
a
Department of Inorganic Chemistry, Indian Association for the Cultivation of Science,
Calcutta 700 032, India. E-mail: icdd@mahendra.iacs.res.in
Department of Chemistry, University of Reading, Whiteknights, Reading RG6 6AD, UK
b
Received (in Montpellier, France) 17th July 2001, Accepted 12th November 2001
First published as an Advance Article on the web
The electrochemistry of the tetraphenylborate ion, BPh4ꢀ, has been studied by cyclic voltammetry and
coulometry in water, methanol, ethanol, acetonitrile, acetone, dimethylformamide and dichloromethane under
an N2 atmosphere. While a one-electron and somewhat irreversible oxidation (with an E1=2 of 0.87 V vs. SCE at
a glassy carbon electrode) is observed in dichloromethane, eqn. (i),
ꢁ
ꢀ
BPh4 Ð BPh4 þ eꢀ
the oxidation is somewhat complicated in all other solvents by the occurrence of several consecutive reactions.
ðiÞ
Epa , the anodic peak potential in cyclic voltammetry, changes from 0.41 V vs. SCE in water to 0.94 V vs. SCE in
dimethylformamide at a glassy carbon electrode. The variation in Epa with solvent (S) is explained by invoking
reaction (ii).
ꢁ
ꢁ BPh4 þ S ! SꢀBPh3 þ Ph
ðiiÞ
The coulometric results in solvents other than dichloromethane indicate a disproportionation of S–BPh3 , eqn.
(iii).
2SꢀBPh3 ! SꢀBPh2þ þ BPh4ꢀ þ S
ðiiiÞ
Geske1 has previously examined the electrochemical behaviour
of the tetraphenylborate ion (BPh4ꢀ) in acetonitrile by both
voltammetry and coulometry. From voltammetry experiments
in acetonitrile using a rotating platinum microelectrode, he
found that BPh4 undergoes an irreversible two-electron oxi-
dation according to eqn. (1) around 0.8 V vs. a Ag=AgNO3
electrode.
electrochemistry by cyclic voltammetry and coulometry in
several common solvents. The results are reported here.
ꢀ
Results and discussion
We have performed cyclic voltammetry measuꢀrements on the
tetrabutylammonium (Bu4Nþ) salt of BPh4 in dichloro-
methane at a Pt electrode as well as at a glassy carbon (GC)
electrode under an N2 atmosphere using Bu4NClO4 as the
supporting electrolyte. At a scan rate v of 50 mV sꢀ1, only an
anodic peak is observed on the positive side of the saturated
calomel electrode (SCE) (Fig. 1); however, at higher scan rates,
ꢀ
þ
BPh4 ! BPh2 þ PhꢀPh þ 2eꢀ
ð1Þ
From coulometry experiments in acetonitrile he observed that
the number of electrons transferred depends on the con-
ꢀ
centration of BPh4ꢀ; as the concentration of BPh4 is
decreased, then the number of electrons being transferred
becomes closer to two. A full two-electron count could only be
obtained in the presence of a buffer.1 From these observations
he suggested that electrode process (1) is followed by a sec-
ondary chemical reaction involving a proton, eqn. (2), which
ꢀ
consumes BPh4
.
ꢀ
BPh4 þ Hþ ! BPh3 þ PhH
ð2Þ
The proþton involved in eqn. (2) is generated by the interaction
of BPh2 and the solvent [eqn. (3)].
BPh2þ þ CH3CN ! Ph2BCH3CNþ ! Ph2BCH2CN þ Hþ
ð3Þ
However, prior to Geske, Wittig and Raff2 observed that in
diethyl ether reaction (4)
2LiBPh4 þ 2CuCl2 ! 2CuCl þ 2LiCl þ 2BPh3 þ PhꢀPh ð4Þ
Fig. 1 Cyclic voltammograms of NaBPh4 in water (full line; con-
centration c ¼ 1.08 mmol dmꢀ3) and of Bu4NBPh4 in CH2Cl2 (broken
line; c ¼ 1.04 mmol dmꢀ3) at a glassy carbon electrode under N2 at-
mosphere at v ¼ 50 mV sꢀ1. Supporting electrolyte: 0.1 mol dmꢀ3
NaClO4 in water and 0.1 mol dmꢀ3 Bu4NClO4 in CH2Cl2 .
ꢀ
occurs where BPh4 acts as a one-electron reductant. Witꢀh
the hope of observing a one-electron redoxprocess for BPh
in a solvent of low dielectric constant, we have studied its
4
DOI: 10.1039/b106356c
New J. Chem., 2002, 26, 367–371
367
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the electrode process observed in dichloromethane is given by
eqn. (5).
ꢁ
ꢀ
BPh4 þ eꢀ Ð BPh4
ð5Þ
The cyclic voltammetry study of the sodium salt of BPh4ꢀ in
water, methanol, ethanol, acetone, acetonitrile and dimethyl-
formamide (DMF) was performed also at a GC electrode
under an N2 atmosphere using NaClO4 as the supporting
ꢀ
electrolyte. In all these solvents BPh4 uniformly displays an
irreversible voltammogram at the GC electrode on the positive
side of SCE with complete absence of the cathodic peak; the
anodic peak potential Epa depends on the solvent (Table 2). At
a v of 50 mV sꢀ1, Epa is 0.94 V vs. SCE in DMF (Table 2) as
compared to only 0.41 V vs. SCE in water (Fig. 1). Even at a v
of 5000 mV sꢀ1, it was not possible to observe the cathodic
peaks. A comparison of peak currents with those of the fer-
rocene–ferrocenium couple under the same experimental con-
ditions indicates that the electrode process involves only one
electron.
Fig. 2 Cyclic voltammograms of Bu4NBPh4 in CH2Cl2 at v ¼ 5000
mV sꢀ1 at a glassy carbon electrode (full line) and at a platinum
electrode (broken line) under N2 atmosphere. (c ¼ 0.87 mmol dmꢀ3
.
Supporting electrolyte: 0.1 mol dmꢀ3 Bu4NClO4 .)
ꢀ
The one-electron oxidation of BPh4 produces the neutral
ꢀ
the cathodic peak becomes visible (Fig. 2). The electrode
process is less irreversible at a Pt electrode than at a GC
electrode (Fig. 2); the peak-to-peak separation (DEp) at
v ¼ 5000 mV sꢀ1 at a Pt electrode is 0.75 V with a half-wave
potential E1=2 of 0.82 V vs. SCE and at a GC electrode the DEp
at v ¼ 5000 mV sꢀ1 is 1.04 V with an E1=2 of 0.87 V vs. SCE. In
coulometry at a Pt wire gauge electrode in CH2Cl2 under N2
atmosphere only one electron is transferred (Table 1). Thus,
ꢁ
species BPh4 . Consequently, onlꢀy solvation of BPh4 is per-
tinent here. The volume of BPh4 in the solid state has been
+
previously estimated3 to be 306.6 A3. Assuming a sphere, this
+
ionic volume yields a radius of 4.18 A. By use of the Born
equation4,5 this radius indicates that a change of solvent from
water (dielectric constant e at 25 ꢂC ¼ 78.54)4 to DMF (e at
25 ꢂC ¼ 37.25)6 should lead to a decrease of only 0.024 V in
Epa . Experimentally, however, an increase of 0.53 V in Epa is
observed when going from water to DMF (Table 2).
ꢀ
The variation in Epa of BPh4 with solvent can be rationa-
Table 1 Coulometric data for BPh4ꢀ in some solvents (S) at a Pt wire
lised if we assume that the primary electrode process (5) is
followed by a disintegration of ꢁ BPh4 into BPh3 and ꢁ Ph,
generating a solvato species S–BPh3 (S: solvent) [eqn. (6)].
ꢁ
gauge electrodea
S
c
n
ꢁ BPh4 þ S ! SꢀBPh3 þ Ph
ð6Þ
CH2Cl2
Acetone
Acetonitrile
DMF
0.426
1.040
0.434
1.054
0.441
1.052
0.437
0.991
0.97
1.04
1.68
1.56
1.60
1.28
1.69
1.40
If DG0 is the free energy change due to reaction (6), E1=2 of
couple (5) will decrease as reaction (6) becomes thermo-
dynamically more feasible.7 We have calculated the heat of
reaction DH0 for reaction (6) in the gas phase for the various
solvents in the present study by the AM1 method using the
standard MOPAC package (version 1.1);8 the DH0 values are
presented in Table 2. It is interesting to note that in the gas
phase DMF appears to bind to BPh3 by the oxygen end rather
than by the nitrogen atom. As the number of molecules on
both sides of reaction (6) in the gas phase are equal, the
entropy factor is expected to be similar for all solvents.9 Since
the cathodic peak for couple (5) is only observed in CH2Cl2 ,
we have investigated the possible correlation between ꢀDH0
and Epa , assuming that DEp in all solvents remains comparable
a
NaBPh4 was electrolysed in acetone, acetonitrile and DMF while
Bu4NBPh4 was electrolysed in CH2Cl2 ; in acetone, acetonitrile and
DMF NaClO4 was used as the supporting electrolyte and in CH2Cl2
the supporting electrolyte was Bu4NClO4 . c is the concentration of
BPh4 in the electrolysed solution in mmol dmꢀ3 (total volume of the
ꢀ
solution in each case was 25 ml) and n is the number of electrons
transferred in coulometry.
ꢀ
Table 2 Anodic peak potential (Epa) data for BPh4 in various solvents (S) at a glassy carbon (GC) electrode and at a Pt electrode; some pro-
perties of the solvents, calculated DH0 values for reaction (6) in the gas phase and association constant (Ka) for Bu4NBPh4 in some solventsa
Epa
S
e
AN
DN
GC
Pt
DH0
Ka
b
Water
78.54
32.63
24.30
20.70
35.72e
37.52e
8.92e
54.8
–
ꢃ18.0
19.0
20
17.0
14.1
26.6
<10
0.41
0.67
0.66
0.79
0.78
0.94
0.89
–
–
–
ꢀ57.36
ꢀ27.03
ꢀ33.76
ꢀ16.48
ꢀ21.84
ꢀ11.42
ꢀ13.68
–
b
Methanol
Ethanol
Acetone
Acetonitrile
DMF
37 4c
–
b
37.9
12.5
18.9
16.0
20.4
0.84
<10d
14f
0.85
0.98
0.87
<10g
3300h
CH2Cl2
a
Dielectric constants (e) at 25 ꢂC are taken from ref. 4 unless otherwise specified. Gutmann solvent acceptor numbers (AN) and donor numbers
(DN) are taken from ref. 10. For cyclic voltammmetry Bu4NBPh4 was used in CH2Cl2 and NaBPh4 in all other solvents; Bu4NClO4 was used as the
supporting electrolyte in CH2Cl2 and NaClO4 as the supporting electrolyte in all other solvents. The Epa values were obtained at v ¼ 50 mV sꢀ1 and
b
are given in V vs. SCE. DH0 values are given in kJ molꢀ1 and Ka in molꢀ1 dm3. No proper cyclic voltammetric response could be
c
obtained. From ref. 17. From ref. 18. From ref. 6. From ref. 19. From ref. 20. From ref. 21.
d
e
f
g
h
368
New J. Chem., 2002, 26, 367–371

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disappears from the electrode surface by forming biphenyl,
eqn. (7).
ꢁ
ꢁ
Ph þ Ph ! PhꢀPh
ð7Þ
The cyclic voltammetric behaviour of NaBPh4 in all the sol-
vents except CH2Cl2 has also been examined at a Pt electrode
under N2 atmosphere using NaClO4 as the supporting electro-
lyte. We have not been able to observe the oxidation of BPh4ꢀ in
water, methanol or ethanol at a Pt electrode. For other solvents,
however, an irreversible oxidation has been observed with
complete absence of the cathodic peak, even at a v of 5000 mV
s
ꢀ1. The Epa data are given in Table 2. While current height
considerations indicate a one-electron process like couple (5)
the coulometric results (under N2 atmosphere at a Pt wire gauge
electrode) are essentially similar to thoseꢀobtained by Geske1 in
CH3CN: as the concentration of BPh4 is lowered then the
number of electrons transferred approaches two (Table 1). It
should be mentioned that, as reported by Geske,1 the electro-
lysis of NaBPh4 in acetone, acetonitrile and DMF is very slow.
To reconcile our cyclic voltammetric and coulometric results in
the various solvents with the observations by Geske1 we pro-
ꢀ
Fig. 3 Correlation of the anodic peak potential Epa of BPh4 ob-
tained at a glassy carbon electrode in a variety of solvents with DH0
calculated for reaction (6) in the gas phase. Correlation coefficient
r ¼ 0.978. For data, see Table 2.
pose that in coulometry the species S–BPh3 disproportionates in
ꢀ
a manner shown in eqn. (8) to regenerate BPh4
.
to that observed in CH2Cl2 . Fig. 3 shows that there exists a
satisfactory linear relationship between Epa and ꢀDH0; the
2SꢀBPh3 ! SꢀBPh2þ þ BPh4ꢀ þ S
ð8Þ
ꢀ
lower the value of DH0, the more difficult it is to oxidise BPh4
.
The slope of the line in Fig. 3 is found to be 0.0108, which is
very close to the ideal value of 0.0103, the required factor when
converting kJ molꢀ1 to eV.
This disproportionation, which obviously depends on the nat-
ure of the solvent, does not occur in the weakly coordinating
dichloromethane. Hence, the one-electron redoxprocess for
ꢀ
We have also examined whether the Epa values in various
solvents can be explained in terms of Gutmann10 solvent donor
numbers (DN) and acceptor numbers (AN). The experimen-
tally determined parameters DN and AN are respectively the
measure of the basicity and acidity of a solvent; the larger the
value of DN or AN, the more electron-donating or -accepting
is the solvent. Earlier it has been found that the redoxpoten-
tials of an anionic electroactive species [e.g., Fe(CN)63ꢀ] in
various solvents can be correlated with AN while those of a
cationic electroactive species [e.g., Naþ] with DN.11 We have
found that the Epa values of our anionic electroactive species
BPh4ꢀ show some sort of linear correlation with AN (Table 2;
BPh4 is only observed in CH2Cl2 .
Finally, we have found that in the copper(I) complex
[CuL]BPh4 (1), where L is a 2 : 1 condensate of 2-acetylpyr-
idine and benzil dihydrazone, couple (5) is much less irrever-
sible than for Bu4NBPh4 in dichloromethane in cyclic
voltammetry at a GC electrode; even at v ¼ 50 mV sꢀ1 the
cathodic peak is quite visible, cf. Fig. 5 with Fig. 1. At v ¼ 50
mV sꢀ1 for 1, couple (5) in dichloromethane has an E1=2 of 0.82
V vs. SCE with a DEp of 0.12 V. The quasireversible response
aroundꢀ0.5 V vs. SCE in Fig. 5 is due to the electrode process
II
I
Cu þ e Ð Cu . Incidentally, the perchlorate analogue of 1,
=
[CuL]ClO4 , displays only the Cu couple.12
II
I
ꢀ
Fig. 4). Roughly speaking, the anion BPh4 is stabilised in
solvents of lower AN values, that is in less electron-accepting
solvents. This is in line with the earlier observations that
anions are more stabilised in less electron-accepting solvents.10
Apparently, reaction (6) is so fast that even at a v as high as
5000 mV sꢀ1 no trace of the cathodic peak can be observed in
cyclic voltammetry in any of the solvents studied except
CH2Cl2 . The phenyl radical that is generated by reaction (6)
For the sake of completeness of our study we have also
carried out an X-ray crystallographic study of [CuL]BPh4 .
Direct diffusion of hexane into a moderately concentrated
ꢀ
Fig. 4 Correlation of Epa of BPh4 obtained at a glassy carbon
electrode in a variety of solvents with Gutmann solvent acceptor
Fig. 5 Cyclic voltammogram of 1 in CH2Cl2 at v ¼ 50 mV sꢀ1 at a
Supporting electrolyte: 0.1 mol dmꢀ3 Bu4NClO4 .)
glassy carbon electrode under N2 atmosphere. (c ¼ 1.46 mmol dmꢀ3
.
number AN. r ¼ 0.918. For data, see Table 2.
New J. Chem., 2002, 26, 367–371
369

ꢀ
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The results clearly demonstrate that in most solvents BPh4
is not an innocent anion and may act as a one-electron
reductant. Earlier it has been observed by Drew et al.23 that
attempts to crystallise [CuL0]BPh4 from methanol led to the
formation of the tetraphenylborate of the corresponding cop-
ꢀ
per(I) species, [CuL0]BPh4 . In view of the Epa value of BPh4
=
I
II
in methanol (Table 2), it appears as if the potential of the Cu
couple of the CuN2S3 chromophore in [CuL0]2þ is suffi-
ciently high to oxidise BPh4ꢀ. The Epa values in Table 2 also
show that in a solvent like methanol, BPh4ꢀ can undergo aerial
oxidation to produce a phenyl radical and S–BPh3 . Appar-
2þ
ꢀ
ently, this is why BPh4 may sometimes act as a phenylating
agent24 or may lead to the substitution25 of a proton by the
electron-deficient species BPh3 .
Experimental
Fig. 6 The structure of [CuL]BPh4Á(CH3)2CO (1Áacetone) with ellip-
General
+
soids of 20% probability. Selected bond lengths (A) and angles (ꢂ):
Cu1–N1 1.965(5), Cu1–N8 2.094(5), Cu1–N13 2.077(4), Cu1–N20
1.987(4), N1–Cu1–N8 80.0(2), N13–Cu1–N20 79.3(2), N1–Cu1–N20
139.3(2), N1–Cu1–N13 135.8(2), N20–Cu1–N8 85.9(2), N20–Cu1–N8
130.9(2).
Sodium perchlorate was purchased from Aldrich, USA. The
ligand L was synthesised as reported elsewhere.26 All solvents
used for electrochemical experiments were purified by standard
procedures.27 Cyclic voltammetry and coulometry were per-
formed using an EG&G PARC electrochemical analysis
system, model 250=5=0, in conventional three-electrode con-
figurations. An ECDA-Pt02 platinum disk electrode obtained
from Con-Serv Enterprises, India, and a planar EG&G PARC
G0229 glassy carbon milli electrode were used as the working
electrodes in cyclic voltammetry.
solution of 1 in acetone yields single crystals of [CuL]BPh4Á
(CH3)2CO, the acetone solvate of 1, which slowly loses
the solvent molecule. By the X-ray crystallographic study,
[CuL]BPh4Á(CH3)2CO (1Áacetꢀone) is found to consist of dis-
crete [CuL]þ cations, BPh4 anions and acetone solvent
molecules (Fig. 6).13 Thus one may conclude that the reversi-
bility of couple (5) in dichloromethane depends on the nature
(possibly shape and size) of the cation. This is presumably due
to ion pair formation in CH2Cl2 caused by the low value of e
Syntheses
Bu4NBPh4 . This salt was synthesised by modifying a repor-
ted procedure28 in the following manner. NaBPh4 (0.34 g, 1
mmol), dissolved in 10 ml of water, was added to Bu4NBr
(0.322 g, 1 mmol), dissolved in 15 ml of water, with stirring.
The resulting white colloid-like solution was stirred for 30 min
and left in air overnight. The white precipitate, collected by
filtration, was washed thoroughly with 150 ml of water and
dried in vacuo over fused CaCl2 . Yield: 0.38 g (68%).
6
ꢂ
(8.92 at 25 C ). Though interactions in ion pairs are largely
electrostatic in nature, specific interactions are possible.10,14–16
For [CuL]BPh4 , ion pair formation in CH2Cl2 may be facili-
tated by aromatic ring stacking.
In this context, a knowledge of the ion pair association of
Bu4NBPh4 in solution may be imperative. The association
constants (Ka) of Bu4NBPh4 in some of the solvents used in
the present study are given in Table 2.17–21 In general, log Ka of
an electrolyte varies inversely with e. Consequently, it is found
that only in dichloromethane (Table 2) is ion pair formation
significant. It should be noted that Ka values 4 10 molꢀ1 dm3
have no physical meaning.22
[CuL]BPh4 (1). L (0.44 g, 1 mmol) and NaBPh4 (1.369 g, 4
mmol), dissolved in 30 ml of acetone, were added to a solution
of [Cu(CH3CN)4]ClO4 (0.328 g, 1 mmol) in 10 ml of acetone
under an N2 atmosphere at room temperature. The resulting
deep red reaction mixture was stirred for 2 h. A shiny dark red
compound precipitated, which was collected by filtration,
washed with 5 ml of acetone and dried in vacuo over fused
CaCl2 . The compound was recrystallised from an acetone–
hexane (1 : 2) mixture. Yield: 0.54 g (70%). Anal. found (calcd):
C, 75.52 (75.48); H, 5.30 (5.36); N, 10.10 (10.16%). UV=VIS
(CH2Cl2) lmax=nm (e=dm3 molꢀ1 cmꢀ1): 230 (49 600), 255sh
(41 000), 288 (37 200), 425 (8 800).
Concluding remarks
ꢀ
We have shown that BPh4 undergoes a one-electron oxida-
tion [couple (5)] in dichloromethane, a solvent of fairly low
dielectric constant. The procꢀess, however, is not fully rever-
sible. The oxidation of BPh4 in solvents of higher dielectric
constant is somewhat complicated as several secondary che-
mical reactions follow the primary electrode process (5).
ꢀ
Though the oxidation of BPh4 in such solvents on the cyclic
X-Ray crystallography
voltammetric time scale appears to still be a one-electron
process, coulometry indicates a two-electron process. This is
because the intermediate solvato species S–BPꢀh3 undergoes a
disproportionation reaction regenerating BPh4 [reaction (8)].
The data on 1Áacetone were collected with Mo-Ka radiation
using the MARresearch Image Plate System at 293(2) K. The
crystal was positioned at 70 mm from the image plate. One
hundred frames were measured at 2ꢂ intervals with a counting
time of 4 min. Data analysis was carried out with the XDS
program.29 The structures were solved using direct methods
with the SHELXS-86 program.30 The non-hydrogen atoms
were refined anisotropically and remaining atoms isotropically.
The hydrogen atoms bonded to carbon were included in geo-
metric positions and given thermal parameters equivalent
to 1.2 times those of the atom to which they were attached.
An empirical absorption correction was carried out using
370
New J. Chem., 2002, 26, 367–371

DIFABS.31 The structures were refined on F2 using SHELXL-
9332 to R1 ¼ 0.0758 and wR2 ¼ 0.2032 for 4112 reflections with
I > 2s(I).
View Article Online
12 S. Chowdhury, P. B. Iveson, M. G. B. Drew and D. Datta,
New J. Chem., submitted.
11 V. Gutmann, Coord. Chem. Rev., 1976, 18, 225.
13 The metal coordination sphere in [CuL]þ is a very distorted tet-
rahedron. The void space around the metal atom might allow for
further coordination but no significant interactions between the
cation and the anion less than the sum of van der Waals radii
could be observed. However, the acetone molecule forms a weak
Crystal data. C55H50BCuN6O (1Áacetone): Mw ¼ 885.36,
triclinic, spacegroup P-1, a ¼ 10.779(17), b ¼ 13.887(17), c ¼
+
16.42(2) A, a ¼ 87.56(1)ꢂ, b ¼ 80.70(1)ꢂ, g ¼ 83.01(1)ꢂ, U ¼
hydrogen bond to the hydrogen atom on C3. The anion BPh4ꢀ is
+
2408(6) A3, Z ¼ 2, m ¼ 0.498 mmꢀ1, Dc ¼ 1.221 g cmꢀ3, 7997
+
found to have B–C distances of 1.641(7)–1.657(6) A, Cipso–B–
Cipso angles in the range 106.6(4)–111.6(4)ꢂ and C–Cipso–C angles
of 113.6(4)–114.9(5)ꢂ. We have investigated the dimensions of the
BPh4 anion in the Cambridge Crystallographic Database and
unique data were collected.
CCDC reference number 179190. See http:==www.rsc.org=
suppdata=nj=b1=b106356c= for crystallographic data in CIF
or other electronic format.
ꢀ
found that the anion is susceptible to packing effects and in some
cases is significantly distorted from tetrahedral. Thus, from 1625
observations of the anion, the average range of the C–B–C angle
(defined as the difference between the maximum and minimum
angles) is 7.2ꢂ, somewhat greater than that observed in the present
structure, but 34 examples have a range from 15–32ꢂ. The di-
Acknowledgements
mensions in the present structure are consistent with a mean B–C
M. G. B. D. thanks EPSRC and University of Reading for
funds for the Image Plate System. D. D. thanks the Depart-
ment of Science and Technology, New Delhi, India, for
financial support. Useful advice received from a reviewer is
gratefully acknowledged.
+
distance of 1.659 A and a C–Cipso–C angle of 114.7ꢂ. The average
sum of the three angles at Cipso is 359.8ꢂ in our case, confirming
the trigonal nature of these atoms.
14 D. Datta, H. A. O. Hill and H. Nakayama, J. Electroanal. Chem.,
1992, 324, 307 and references therein.
15 C. J. James and R. M. Fuoss, J. Solution Chem., 1975, 4, 91.
16 A. D’Aprano and R. M. Fuoss, J. Solution Chem., 1975, 4, 175.
17 R. L. Kay, C. Zawoyski and D. F. Evans, J. Phys. Chem., 1965,
69, 4208.
References and notes
18 D. F. Evans, J. Thomas and J. A. Nadas, J. Phys. Chem., 1971,
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19 P. C. Carman, J. Solution Chem., 1978, 7, 845.
20 D. S. Gill, A. Sharma, M. S. Chauhan, A. N. Sharma and J. S.
Cheema, Electrochim. Acta, 1985, 30, 151.
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4
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7
When reactions (5) and (6) are considered together, reaction (6)
has to be rearranged so that the net reaction is:
ꢁ
ꢀ
BPh4 þ eꢀ Ð BPh4
SꢀBPh3 þ Ph ¼ ꢁ BPh4 þ S
ꢁ
ꢁ
SꢀBPh3 þ Ph þ eꢀ ¼ BPh4ꢀ þ S
The total free energy change for the net reaction is then given by
ꢀnFE1=2 ꢀ DG0 where E1=2 is the half-wave potential for couple
(5) in V, n the number of electrons involved in couple (5), F the
faraday constant and DG0 the free energy change for reaction (6).
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371