Electrochemical instability of highly fluorinated tetraphenyl borates and syntheses of their respective biphenyls

Article abstract of DOI:10.1039/c8cc02996b

Highly fluorinated tetraphenyl borate anions are of importance as weakly coordinating anions in metalorganic reactions. However, at high positive potentials their electrochemical stability in organic solvents is not sufficient. This was investigated by a comprehensive cyclic voltammetry study and can be used synthetically to generate highly fluorinated biphenyls.

Full text of DOI:10.1039/c8cc02996b

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DOI: 10.1039/C8CC02996B
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Electrochemical Instability of Highly Fluorinated Tetraphenyl
Borates and the Syntheses of their Respective Biphenyls
Sebastian B. Beil,a,b,† Sabine Möhle,a,† Patrick Enders, and Siegfried R. Waldvogel a,b
a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Highly fluorinated tetraphenyl borate anions are of importance as mechanism. For an intermolecular mechanism, the presence of
weakly coordinating anions in metalorganic reactions. However, at free phenyl radicals in solution is anticipated. Whereas a
high positive potentials their electrochemical stability in organic intramolecular mechanism can either follow a radical or an ionic
+
7,9
solvents is not sufficient. This was investigated by a comprehensive pathway, forming cationic B(Ph)
2
species.
cyclic voltammetry study and can be used synthetically to generate When using the highly fluorinated BArF salts
highly fluorinated biphenyls.
2
and
ting electrolyte, we observed by serendipity the anodic genera-
tion of the corresponding biphenyls decafluorobiphenyl ( ) and
,3',5,5'-tetrakis(trifluoromethyl)biphenyl ( (Scheme 1).
These types of highly fluorinated structures are usually
3 as suppor-
4
The application of weakly coordinating anions plays an impor-
tant role in various field of chemistry.1,2 Due to their low nucleo-
philicity such salts exhibit a strong influence on redox processes
3
5)
10
11
obtained using palladium or gold catalysts, as well as by
oxidative coupling of borate or halide precursors again
catalyzed by palladium, iron, rhodium, or copper. Strong
Lewis acids were also reported to degrade borate anions, but
never described the formation of the respective biphenyls.
3,4
of various organometallic complexes. A prominent example
of this type of anions is the tetraphenyl borate (
1
2
) and its fluori-
12
13
14
15
-
-
nated congeners [B(C
6
F
5
)
4
] (
2
) and [B(C
6
H
3
(CF
3
)
) ] (3
4
), known
16
2
as BArF anions (Figure 1). Their electrochemical properties
have been extensively studied by Geiger and Barrière in terms
of providing a benign electrolyte system for the generation of
4,5
organometallic radical cations. Due to their comparably high
solubility in less polar solvents, even perfluorinated solvents
become accessible for electrochemical applications.5
,6
Scheme 1: Direct electrochemical formation of highly fluorinated biphenyls 4 and 5 by
electrolysis of BArF anions 2 and 3.
This unusual anodic conversion prompted us to further inves-
tigate the electrochemical stability of fluorinated tetraphenyl
Figure 1: Examples of commonly applied tetraphenyl borate anions.
borates
nated biphenyls
molybdenum anode in 1,1,1,3,3,3-hexafluoroisopropanol
2
and
3
as well as the electrochemical access to fluori-
4
and . Initially, our recently reported active
5
First studies about the electrochemical stability of
ported by Geske in the 1950s, who found the formation of the
respective biphenyl as a product of degradation. Despite
intense mechanistic investigation it is still unclear, whether the
biphenyl formation follows an inter- or intramolecular
1 were re-
1
7
7,8
(HFIP) was applied. The three biphenyls could be isolated in
moderate yields ranging from 15% to 46% (Table 1, Entries 1-3).
Nevertheless, a disadvantage of this protocol was the significant
2
drop of the initial current density of 8.3 mA/cm during electro-
lysis due to low conductivity and fouling at the molybdenum
anode, resulting in high voltages. Furthermore, solubility of Na-
a.Institute of Organic Chemistry, Johannes Gutenberg University Mainz,
Duesbergweg 10-14, 55128 Mainz, Germany, waldvogel@uni-mainz.de.
Graduate School of Excellence Material Science in Mainz (MAINZ), Staudingerweg
3
in HFIP was not sufficient. To overcome these issues, the elec-
b.
9
, 55128 Mainz, Germany.
These authors contributed equally to this work.
Electronic Supplementary Information
†
(ESI)
available:
XXX
See
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DOI: 10.1039/x0xx00000x
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Journal Name
trolysis conditions were optimized with regards to Na-
strate (for details see SI). Different anode materials like pla-
3 as sub- additives (Table 2, Entry 1). The addition of 20 vol% of either
View Article Online
DOI: 10.1039/C8CC02996B
,

,
-trifluorotoluene, fluorobenzene or benzene had no sig-
tinum, boron-doped diamond (BDD), glassy carbon, and isosta- nificant effect onto the yield (Table 2, Entries 2-4). Only for pyri-
tic graphite were investigated. The best result, with a yield of dine, a decreased yield of 41% was obtained (Table 2, Entry 5).
5
2% of
anodes gave only traces of
of Na- different solvents and co-solvents were tested. At a BDD free-radical species in the solution. However, the high surplus
anode, the best result was obtained in acetonitrile with the of additive had limited effect on the yield of and only traces of
5
, was obtained using a BDD anode, whereas graphite Traces of cross-coupling products
6 could be clearly identified
5
in HFIP. To increase the solubility by GC-MS, which proves the presence of limited amounts of
3
5
addition of 2vol% of tert-butanol leading to a yield of 63% of
However, the highest yield of (73%, Table 1, Entry 5) was intramolecular mechanism.
observed in acetonitrile at an isostatic graphite anode, whereas
in HFIP only traces of were identified. Shifting from molyb-
5
.
cross-coupling product
6
were found, which indicates an
5
Table 2: Trapping experiments during the electrolysis of sodium tetrakis(3,5-
bis(trifluoromethyl)phenyl borate) (Na-3).
5
denum/HFIP to BDD or isostatic graphite/acetonitrile the con-
ductivity was drastically increased, resulting in a lower applied
voltage according to instrumentation and the electrolysis could
2
be performed at a constant current density of 8.3 mA/cm . The
isolated yield of 73% could not be further increased by varying
applied charge, current density or electrolyte concentration
(
see SI). With this optimized reaction protocol in hand, biphenyl
could be obtained in an isolated yield of 61% from Bu N- (Table
, Entry 4). However, when applying this electrolysis conditions
to Li- only traces of the perfluorinated biphenyl where found
4
1
1
Entry
Additive (vol%)

,,-trifluorotoluene (20)
fluorobenzene (20)
benzene (20)
Yield of 5 [%]a
2
4
by GC-MS technique (Table 1, Entry 6). Despite fouling of the
1
2
3
4
5
57
58
57
60
41
molybdenum anode and a high applied voltage according to
-
instrumentation, for [B(C
1
6
F
5 4
)
] (2
) the highest yield (20%, Table
was obtained at
, Entry 7) of the perfluorinated biphenyl
4
pyridine (20)
molybdenum in HFIP + 1vol% methanol. Traces of methanol,
identified in the recovered HFIP (see SI), were found to be
Electrolytic conditions: 0.5 mmol Na[B(C
(c(Na[B(C (CF
6
H
3
(CF
3
)
2
)
4
], 4 mL MeCN, 1 mL additive,
1
8
advantageous for the formation of biphenyls
4
and
5
in HFIP.
6
H
3
) )
3 2 4
]) = 0.1 M), undivided cell, BDD anode, platinum cathode,
2
8
.3 mA/cm , 2.57 F, 22 °C; a: isolated yield.
Table 1: Isolated yield of biphenyls obtained at various electrolytic conditions.
Further evidence of an intramolecular mechanism was obtained
by electrolyzing a 1:1 mixture of different tetraphenyl borates.
When applying Li-
only observed and isolated homo-coupled biphenyls
yields of 25% and 43% (Scheme 2a). Electrolyzing a 1:1 mixture
of Bu N- and Na- at isostatic graphite in MeCN, a mixture of
biphenyl (82%) and biphenyl (35%) was obtained (Scheme 2b).
2
and Na-
3
as electrolytes at a Mo anode, we
4
and in
5
Entry
1b
2e
3
4
5
Anode
Moc
Solvent
Anion
[BPh
]-
[B(C
Yield [%]a
HFIPd
20
15
46
61
73
tracesf
20
4
4
1
3
Moc
HFIPd
]-
6
F
5
)
4
5
Moc
HFIPd
[B(C
H
[BPh
(CF
)
2
)
)
]-
]-
6
3
3
4
No mixed biphenyls, expected in an intermolecular process,
were identified or traced. Finally, we subjected a non-symme-
graphite
graphite
graphite
Moc
MeCN
MeCN
MeCN
HFIP + 1%
MeOH
4
]-
[B(C
6
H
3
(CF
3
)
2
4
trical BArF salt
electrolysis at an isostatic graphite anode. We anticipated the
formation of the unsymmetrical biphenyl , which was clearly
identified as the sole product of electrolysis by GC-MS (Scheme
c). This finding also supports our hypothesis of an intramole-
7, which exhibits a non-fluorinated phenyl, to
6
7g
[B(C
[B(C
6
F
F
5
)
4
]-
]-
6
)
5 4
8
Electrolytic conditions: 0.5 mmol tetraphenylborate salt, 5 mL solvent (c(BAr
.1 M), undivided cell, platinum cathode, 8.3 mA/cm , 2.57 F, 22 °C; a: isolated
yield; b: Q = 0.68 F; c: initial current density of 8.3 mA/cm decreased during
electrolysis; d: water and methanol impurities were identified by H NMR
4
-) =
2
2
0
cular reaction mechanism, since a first oxidation at the borate
will be potentially localized at the phenyl moiety and a subse-
quent elimination with one fluorinated arene is most likely.
An intense cyclic voltammetry (CV) study was conducted to gain
insight into the electrochemical stability of highly fluorinated
tetraphenylborates (CV data see Figures S2-S33). To determine
2
1
spectroscopy (see SI); e: Q = 1.86 F, f: observed by GC-MS; g: Q = 2.2 F.
After the optimization of the formation of highly fluorinated
biphenyls
4 and 5, we focused on the investigation of the
reaction mechanism to find evidence for either an inter- or
intramolecular proceeding. Through addition of quenching
the anodic limit potential, CVs of 0.1
and Li- in MeCN were recorded at various working electrodes
Figures S2-S5). As expected, the anodic limit potentials of the
fluorinated salts and were, due to different electron-
withdrawing substituents, significantly higher, than that of
Bu N- (Table 3).
4
M solutions of Bu N-1, Na-
3
(
2
agents (
pyridine) we envisioned to trap free radical species in solution.
As a test reaction, we chose the conversion of Na- on a BDD
anode in MeCN. The isolated yield of was 57% without any
,,-trifluorotoluene, fluorobenzene, benzene or
2
3
3
5
4
1
2
| J. Name., 2012, 00, 1-3
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solutions of Bu
4
N-
1
, Li-
2
or Na-
3
is similar to the preparative
View Article Online
DOI: 10.1039/C8CC02996B
electrolyses, the obtained anodic limit potentials are not easily
Ox
comparable to the oxidation potentials (E ) reported by Geiger
5
9
et al. and Datta et al. For comparable results, the oxidation
Ox
potentials (E ) of tetraphenylborates
1
-
3
were determined
N-1, Na- and Li- in
/MeCN, thus adapting
the reported conditions (Figures S8-S33, Table 4, Table S9 and
measuring CVs of 3 m
.1 Bu NClO /CH Cl
M
solutions of Bu
4
3
2
0
M
4
4
2
2
or 0.1 NaClO
M
4
5,9
S10). As working electrodes BDD, glassy carbon, and platinum
were used. At molybdenum anodes oxidation of was not
1-3
observed (Figures S11/S18) and isostatic graphite underwent
fast corrosion.
Table 4: Oxidation potentials (EOx) of tetraphenylborate salts 1-3 in 0.1 M
NaClO /MeCN at different working electrodes.
4
Entry
1
Substrate
Bu N-1
BDD
glassy carbon
.48 V
Lit.: 0.40 V
platinum
0
0.57 V
[
a]
[a]
0.93 V
Lit.: 0.47 V
4
0
.78 V vs. SCE9 0.85 V vs. SCE9
Scheme 2: Control experiments to investigate the reaction mechanism; a/b) electrolysis
of a 1:1 mixture of two different tetraphenylborate salts; c) electrolysis of an
unsymmetrical BArF electrolyte.
shoulder: 1.9
V
2
3
Na-3
Li-2
1.56 V
2.02 V
2
shoulder: 2.0
V
Electrolytic conditions: undivided cell, platinum cathode, 8.3 mA/cm , 22 °C;
a) 0.25 mmol Li[B(C ], 0.25 mmol Na[B(C (CF ], 5 mL HFIP, molybdenum
anode, 1.82 F, decreasing current density; b) 0.25 mmol Bu NBPh , 0.25 mmol
Na[B(C (CF ], 5 mL MeCN, isostatic graphite anode, 2.57 F; c) 0.03 mmol
Bu NBPh(C , 5 mL MeCN, isostatic graphite anode, 2.57 F; d) NMR-yield.
shoulder: 2.1
V
1.83 V
6
F
5
)
4
H
6 3
3 2 4
) )
4
4
Potentials given vs. FcH/FcH ; [a] conversion constant from SCE to FcH/FcH+
according to Pavlishchuk and Addison: 380 mV.20
+
H
6 3
3 2 4
) )
4
6 5 3
F )
Ox
The oxidation potentials (E ) of Na-
3
strongly depend on the
4
For Bu N-1 and Na-3 the lowest anodic limit potential was
Ox
applied electrolyte (Figures S22-S27). In CH
was observed at BDD (shoulder at 2.0 V) and the lowest E at
platinum (1.57 V), whereas in MeCN the picture was inversed.
2
Cl
2
the highest E
observed at isostatic graphite, whereby also the highest yields
of biphenyls were observed in the preparative electrolyses (Tab-
Ox
Ox
le 1). The anodic limit potentials (and oxidation potentials E )
Ox
+
Geiger et al. report an E
(3
) of 1.55 V vs. FcH/FcH in CH
2
Cl
2
.
2
vary depending on the electrode material, due to different over-
potentials for the oxidation of BArF salts.
5
However, the working electrode is not clearly stated. For Li-
Figures S28-S33) oxidation potentials (E ) could only be
Ox
(
2 2
determined in MeCN. In CH Cl oxidation of 2 was in the range
Table 3: Anodic limit potential of tetraphenylborate salts 1-3 in acetonitrile at
different working electrode materials.
of electrochemical decomposition of the electrolyte.
Ox
2 2
Interestingly Geiger et al. report an E (2) of 1.84 V in CH Cl ,
Entry
Working
Bu
4
NBPh
4
Na[B(C
6
H
3
(CF
3
)
2
)
4
]
Li[B(C
(Li-2)
6
F ) ]
5
4
5
again without reporting the material of working electrode.
Electrode
(Bu
4
N-1)
(Na-3)
1.64 V
1.52 V
1.51 V
Ox
4
As expected, the lowest E were obtained for Bu N-1 and the
1
2
3
4
BDD
glassy carbon
platinum
isostatic
0.49 V
0.28 V
0.37 V
1.63 V
1.78 V
1.68 V
values at platinum and glassy carbon are comparable to those
9
reported by Datta et al. Interestingly, the CVs of Bu
4
N-
1
(e.g.
nd
Figure 2, red line) show a 2 oxidation peak in a range from
.5 V to 1.6 V. This 2 peak has also been observed by Geske et
al. at a rotating platinum disk electrode and is most likely
0
.17 V
1.25 V
-
nd
1
graphite
7
Potentials given vs. FcH/FcH+.
associated with the oxidation of biphenyl, generated via
oxidation of
1
(Figure 2, blue line). This observation, together
In agreement with the EOx of
the highest anodic limit was determined for Li-
graphite, no anodic limit potential could be assigned, due to
side reactions among the whole potential range. As traces of
methanol were found to be advantageous for the formation of
2
and
3
reported by Geiger et al.,5
. At isostatic
st
with the disappearance/decrease and shifting of the 1 peak, as
well as the shape of the 2 peak (fast decline of current density
2
nd
after the peak maximum) indicate film formation at the working
21
electrode. Film formation seems to be fast e.g. at BDD in
CH Cl (Figure S8) and less pronounced at platinum in MeCN
Figure S17). Film formation was also visually observed for Na-
and Li- on the electrode and via disappearance/decrease of the
oxidation peak (e.g. Figure 3, red line). Oxidation of biphenyls
or occurs beyond the electrochemical window of the
2
2
4
(and
5) in HFIP, the influence of 1vol% of MeOH on the
(
3
19
oxidation of Li-
2
at molybdenum was studied. In contrast to
2
Ox
pure HFIP, an oxidation peak (E = 2.79 V vs. Ag/AgCl) was
observed, when 1vol% of MeOH was added (Figure S6). This
oxidation is most likely the oxidation of Li-
MeOH was observed, employing Bu NClO
electrolyte (Figure S7). Although recording CVs of 0.1
4
5
2
, as no oxidation of
as supporting
employed electrolyte. However, no film formation was
observed in the electrolyses at BDD or isostatic graphite.
4
4
M
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Figure 2: Cyclic voltammograms of 0.1 M NaClO /MeCN (black line), 3.0 mM Bu NBPh
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fluorinated biphenyls from the respective BArF anions and
provide a comprehensive picture on the reaction mechanism of
this unexpected degradation process. An intense CV study was
performed to gain further insight into the electrochemical
stability of BArF salts. Even though, high oxidation potentials
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tetraphenylborates is most likely the main reaction path and in
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The authors declare no conflict of interest.
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DOI: 10.1039/C8CC02996B
The electrochemical instability of highly fluorinated tetraphenyl borates was studied by cyclic
voltammetry and anodic formation of the respective biphenyls.