Ferrocene-containing sterically hindered phosphonium salts

Article abstract of DOI:10.3390/molecules23112773

The synthesis and physical properties of the series of the ferrocenyl-containing sterically hindered phosphonium salts based on di(tert-butyl)ferrocenylphosphine is reported. Analysis of voltamogramms of the obtained compounds revealed some correlations b

Full text of DOI:10.3390/molecules23112773

molecules
Article
Ferrocene-Containing Sterically Hindered
Phosphonium Salts
Vadim Ermolaev , Tatiana Gerasimova, Liliya Kadyrgulova, Ruslan Shekurov,
Egor Dolengovski, Aleksandr Kononov, Vasily Miluykov , Oleg Sinyashin,
Sergei Katsyuba , Yulia Budnikova and Mikhail Khrizanforov *
Arbuzov Institute of Organic and Physical Chemistry, Kazan Scientific Center, Russian Academy of Sciences,
8 Arbuzov St., Kazan 420088, Russia; ermolaewadim@gmail.com (V.E.); tatyanagr@gmail.com (T.G.);
liliya-kadyrgulova@mail.ru (L.K.); shekurovruslan@gmail.com (R.S.); cat_space@mail.ru (E.D.);
kononovsnz97@gmail.com (A.K.); vasili.miluykov@mail.ru (V.M.); oleg@iopc.ru (O.S.);
katsyuba@iopc.ru (S.K.); yulia@iopc.ru (Y.B.)
* Correspondence: khrizanforov@iopc.ru; Tel.: +7-962-5612-219; Fax: +7-843-2732-253
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀꢁꢂꢃꢄꢅꢆ
Received: 10 September 2018; Accepted: 22 October 2018; Published: 25 October 2018
Abstract: The synthesis and physical properties of the series of the ferrocenyl-containing sterically
hindered phosphonium salts based on di(tert-butyl)ferrocenylphosphine is reported. Analysis of
voltamogramms of the obtained compounds revealed some correlations between their structures
and electrochemical properties. The elongation of the alkyl chain at the P atom as well as replacement
of the Br− anion by [BF4]− shifts the ferrocene/ferrocenium transition of the resulting salts
into the positive region. DFT results shows that in the former case, the Br− anion destabilizes
the corresponding ion pair, making its oxidation easier due to increased highest occupied molecular
−
orbital (HOMO) energy. Increased HOMO energy for ion pairs with the Br− ion compared to BF4
are caused by contribution of bromide atomic orbitals to the HOMO. The observed correlations
can be used for fine-tuning the properties of the salts making them attractive for applications in
multicomponent batteries and capacitors.
Keywords: ferrocene; phosphonium salts; ferrocene-containing compounds; DFT calculation;
cyclic voltammetry; ionic liquids
1. Introduction
Materials that enable selective ion transport under a wide range of conditions (temperature, pH)
play a key role for chemical separation processes and electrochemical batteries [
ways to design such materials is development of synthetic strategies for producing sterically hindered
phosphonium salts [ ]. Phosphonium-based ionic liquids (ILs) are more thermally stable compared
1,2]. One of the important
2
to nitrogen-containing analogues, which provides a wider range of applications (extended reaction
temperature range, distillation of products, etc.) and decreases the amount of decomposition products
in the reaction mixture [
makes the ILs more stable under the reaction conditions [
fragment into ILs leads to unique electrochemical properties [ ] (Figure 1), that allows the Fc-containing
phosphonium salts to be used for modification of the platinum electrodes surfaces [ 10]. The latter also
3]. In addition, the absence of an acidic proton, like in imidazolium salts,
4]. Introduction of the ferrocenyl (Fc)
5
6–
opens perspectives for using the Fc-based ILs as redox agents in electrolytes for lithium ion batteries [11].
This application requires careful choice of both cationic and anionic components. In this sense, [BF4]− is
attractive because it is one of the smallest and most available (from other anions, such as [PF6]−, [NTf]−,
[SbF6]−, with comparable properties) weakly coordinating anion [12]. Moreover, potential batteries
Molecules 2018, 23, 2773; doi:10.3390/molecules23112773
www.mdpi.com/journal/molecules

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based on the [BF4]− anion have excellent low-temperature characteristics [13]. In addition, this anion is
electrochemically inert in a wide range of potentials [14].
Figure 1. Ferrocenyl (Fc)-containing onium compounds [5–10].
Among Fc-containing ILs, the best known and well-studied are the Fc-containing imidazolium
salts (Figure 2). Inclusion of the Fc fragment to the imidazolium compounds increases melting
points and viscosities. In these redox-active salts, Fc acts as a donor and an imidazolium cation
as an acceptor [15]. The inclusion of an organometallic functional groups to the IL structure allows to
change the physico-chemical properties of the salt, that enables their use in photoinduced electron
transfer in solutions [9], and in the esterification reaction of aldehyde oxidation [16].
Figure 2.
1-Ferrocenyl-3-alkylimidazolium
(
a);
1-ferrocenyl-4-methyltriazolium
(b),
and 1-(ferrocenylbutyl)-3-methylimidazolium (c) salts.
One of the main methods to study the redox properties of various organic compounds is
voltammetry with chemically modified electrodes. Modified carbon-paste electrodes (CPE) are widely
used due to their availability and simple manufacturing [17]. Nevertheless, the preparation
of an optimal CPE with high conductivity, wide electrochemical window, and high stability
and reproducibility, is still a challenge. Design of CPE requires using chemically inert compounds
with high viscosity and ionic conductivity. Such electrodes can significantly increase the selectivity
and sensitivity of voltammetric methods [18–20] and at the same time help to obtain important
information about energy data [21–23].
A combination of the required properties makes phosphonium salts attractive for design of CPE.
However, in the literature there are only a few examples of phosphonium salts used as a binder
component for paste electrodes [17]. Thus, the aim of the work is to develop a synthetic strategy for

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obtaining the new Fc-containing phosphonium salts with a sterically hindered cation and to study
their physico-chemical properties.
2. Results and Discussion
Di(tert-butyl)ferrocenylphosphine has been prepared by the most common way, namely by lithiation
of ferrocene with tert-butyllithium and subsequent reaction with di(tert-butyl)chlorophosphine
1 [23].
The starting di(tert-butyl)chlorophosphine has been obtained by the procedure described in [24], i.e.,
by the reaction of an excess of tert-butylmagnesium chloride with phosphorus trichloride with following
separation of the product mixture by distillation under reduced pressure (Scheme 1).
Scheme 1. Preparation of di(tert-butyl)chlorophosphine.
Lithiation with tert-butyllithium usually results in a mixture of mono- and 1,1-dilithiated
derivatives. To avoid the formation of dilithiated compounds and obtain only monolithioferrocene,
the reaction has been carried out in diethyl ether (Scheme 2).
Scheme 2. Preparation of di(tert-butyl)ferrocenylphosphine, 3.
Addition of tert-butyllithium to a solution of ferrocene and t-BuOK [25] in diethyl ether results
in a color change of the solution from brown to orange, indicating that ferrocene was lithiated.
Subsequent slow dropwise addition of di(tert-butyl)chlorophosphine
di(tert-butyl)ferrocenylphosphine 3.
1 leads to precipitation of orange
Phosphonium salts have been obtained by the reaction of di(tert-butyl)ferrocenylphosphine
3
with alkyl halides in an inert atmosphere (Table 1).
The temperature and time of the reaction are affected by the length of the alkyl group in
the haloalkane. Reactions involving haloalkanes with a long alkyl chains proceeded at high temperatures
and longer times. For example, the synthesis of methyl(di-tert-butyl)ferrocenylphosphonium iodide 4a
◦
has been carried out at
−
70 C, because of the strong exothermic effect, being completed in a few minutes.
At the same time, the reaction of di(tert-butyl)ferrocenylphosphine with tetradecyl bromide was carried
out at 100 C for 6 h. Compared to well-known Fc-containing phosphines with n-alkyl substituents,
◦
sterically hindered analogs require slightly higher temperatures in the quaternization reaction with alkyl
halides [5]. We performed the synthesis of phosphonium salts without any solvent. Under the reaction
conditions, the reaction mixture is a viscous liquid. In the case of 4a, the double excess of iodomethane
acting as a solvent was used.

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Table 1. Preparation of ferrocene-containing phosphonium salts. Details of the reaction conditions are
in the supplementary material.
◦
No.
RX
T ( C)
Reaction Time (h)
Yield (%)
4a
4b
4c
4d
4e
CH3I
−70
0.5
5
5
6
8
62
78
71
69
59
n-C3H7Br
n-C6H13Br
n-C10H21Br
n-C14H29Br
70
75
90
100
Synthesized Fc-containing phosphonium salts 4a–e are soluble in dimethylsulfoxide (DMSO)
and in methanol and a lower solubility is observed in chloroform. Non-polar solvents such as toluene,
benzene, as well as esters and aliphatic solvents do not dissolve 4a–e.
Fc-containing phosphonium salts 4a
however compound 4e is hygroscopic. With the temperature increase, compounds 4a
–
e
are air-stable yellow-brown amorphous substances,
gradually
–
e
soften, become viscous, until they completely turn into liquids. The thermogravimetric/differential
scanning calorimetry (TG-DSC) method shows that melting takes place in a rather wide temperature
range for compounds 4a–d
(Table 1). The second peak is registered at a temperature of 165 ◦C (4a),
125 ◦C (4b), 141 ◦C (4c), 150 ◦C (4d), and 155 ◦C (4e), with a loss of mass. Most probably, this value
corresponds to the decomposition temperature, as during preliminarily measurements of melting
point at the Stuart smp30, the blackening of the samples was observed in this temperature range.
Characteristic NMR shifts of synthesized salts are presented in Table 2.
1
Table 2. H and 31P chemical shifts of Fc-containing phosphonium salts 4a–e a.
δP–CH2 in 1H
NMR Spectra
δ
31P NMR
Spectra
Temperature Range
of Melting ( C)
Decomposition
Temperature ( C)
No.
Phosphonium Salts
◦
◦
−
4a
4b
4c
4d
4e
Fc-P+(t-Bu)2CH3I
2.43
2.91
2.47
2.43
2.58
a
48.34
48.59
47.85
47.45
47.76
69–80
60–70
57–68
58–70
61–71
165
125
141
150
155
−
Fc-P+(t-Bu)2C3H7Br
Fc-P+(t-Bu)2C6H13Br
−
−
−
Fc-P+(t-Bu)2C10H21Br
Fc-P+(t-Bu)2C14H29Br
Fc = Fe(η5-C5H5)(η5-C5H4).
The electrochemical properties of 4a–e in acetonitrile were studied by cyclic voltammetry
(Figure 3).
In all cases, reversible transition of ferrocene
]
ferrocenium was observed in the anodic region. It should
−
be noted, that for 4b–4d with the same Br anion, increase of the length of the alkyl chain at the phosphorus
atom is accompanied by the shift of the potential E1/2 (or Esemidiff) to the anodic region (Table 3). This trend
can be characterized by an increase in thermodynamic stability due to a decrease of HOMO energy.
−
Compound 4a with the I anion is oxidized more positively. It should be noted that the ferrocene fragment
is HOMO energy probe 1EHOMO = −(E[semidif,ox vs. ferrocenium/ferrocene] + 4.8 V) [21].
At second oxidation potentials, irreversible oxidation of anionic fragments is observed [26].
In the cathode region (Table 3), an irreversible reduction of the phosphonium fragment is observed for
all salts.

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To expand the field of possible application of the obtained salts, the halide anion was replaced
by a non-coordinating bulk tetrafluoroborate anion in the reaction of halides with a twofold excess of
sodium tetrafluoroborate in ethyl alcohol (Table 4). The oxidation potential of the halide ions is close
to 1 V, and the electrochemical window of the salts 4a–e is 2.8–3.2 V (Table 3). Replacing the anion
with a tetrafluoroborate ion was expected to increase these values.
The obtained phosphonium salts with tetrafluoroborate anion are dark brown substances, 5a
,
5c, and 5e are viscous, amorphous, and 5b and 5d are glassy. The substitution of the halide anion by
the tetrafluoroborate anion results in a slight shift of the chemical shift (
to the weak fields (Table 5).
δ
P–CH2 in 1H NMR Spectra)
Figure 3. Cyclic voltammograms of 4a (red), 4b (green), 4c (blue), 4d (violet), and 4e (black) oxidation
and re-reduction (1 mM) in CH3CN, Bu4NBF4 (0.1M), RE: Ag/AgCl, scan rate 100 mV/s.
Table 3. Electrochemical properties of Fc-containing phosphonium salts 4a
–
4e.
Conditions:
Room temperature, Working Electrode: Glass-carbon, vs. Ag/AgCl. Solvent CH3CN.
1
ox
2
ox
1
red
No.
Phosphonium Salts
Esemidiff (V)
Eonset (V)
Eonset
(V)
4a
4b
4c
4d
4e
Fc-P+(t-Bu)2CH3I
0.50
0.43
0.44
0.45
0.46
1.10
0.80
0.80
0.84
1.16
−1.70
−
Fc-P+(t-Bu)2C3H7Br
−2.09
−2.07
−2.06
−2.05
Fc-P+(t-Bu)2C6H13Br
−
Fc-P+(t-Bu)2C10H21Br
Fc-P+(t-Bu)2C14H29Br
−
−
a
The table shows the averaged shifts made after 20 measurements. Fc = Fe(
at the same conditions is equal to 0.42 V.
η
5-C5H5)(
η
5-C5H4). E1/2 of ferrocene
For all salts 5a, 5c–e electrospray ionization mass spectrometry (ESI) shows a peak at m/z = 87.2
in the region of negatively charged particles, that indicates a complete replacement of the bromide
anion by a tetrafluoroborate anion.
For 5b and 5c, phase transitions have been registered by TG-DSC, decomposition was observed
◦
◦
at 268 C and 233 C respectively. For compounds 5d and 5e, melting occurs in the temperature range
40–50 ◦C and 60–70 ◦C, respectively.

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Table 4. Reaction of substitution of anions in Fc-containing phosphonium salts.
No.
RX
Yield (%)
5a
5b
5c
5d
5e
CH3I
86
82
81
89
81
n-C3H7Br
n-C6H13Br
n-C10H21Br
n-C14H29Br
1
Table 5. H and 31P chemical shifts of Fc-containing phosphonium salts 5a–e a.
δP–CH2 in 1H
NMR Spectra
δ
31P NMR
Spectra
Temperature Range
of Melting ( C)
Decomposition
Temperature ( C)
No.
Phosphonium Salts
◦
◦
−
5a
5b
5c
5d
5e
Fc-P+(t-Bu)2CH3BF4
2.18
2.51
2.52
2.56
2.57
a
49.54
47.44
48.11
48.32
48.54
69–80
39–45
51–52
34–38
57–60
164
275
257
227
245
−
−
Fc-P+(t-Bu)2C3H7BF4
Fc-P+(t-Bu)2C6H13BF4
Fc-P+(t-Bu)2C10H21BF4
Fc-P+(t-Bu)2C14H29BF4
−
−
Fc = Fe(η5-C5H5)(η5-C5H4).
Analysis of cyclic voltammograms of 5a–e in acetonitrile shows that the phosphonium salts with
[BF4]− anion (Figure 4, Table 6) are oxidized more positively compared to the Br− salts (Figure 3,
Table 3), which is also characterized by an increase in thermodynamic stability with an increase in
the length of the alkyl tail at the phosphorus atom, potential E1/2 (or Esemidiff).
Figure 4. Cyclic voltammogram of 5a (red), 5b (blue), 5c (green), 5d (violet), 5e (black)
oxidation and re-reduction (1 mM) in CH3CN, Bu4NBF4 (0.1 M), reference electrode (RE) Ag/AgCl,
scan rate 100 mV/s.

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Table 6.
Electrochemical properties of ferrocene-containing phosphonium salts.
Conditions:
room temperature, working electrode: glass-carbon, RE: Ag/AgCl, solvent: CH3CN a.
1
ox
2
ox
1
red
No.
Phosphonium Salts
Esemidiff (V)
Eonset (V)
Eonset
(V)
−
5a
5b
5c
5d
5e
a
Fc-P+(t-Bu)2CH3BF4
0.89
0.89
0.89
0.90
0.91
2.95
2.95
2.95
2.96
2.97
a
−1.82
−1.94
−2.05
−2.20
−2.40
−
Fc-P+(t-Bu)2C3H7BF4
−
Fc-P+(t-Bu)2C6H13BF4
Fc-P+(t-Bu)2C10H21BF4
−
−
Fc-P+(t-Bu)2C14H29BF4
The table shows the averaged shifts made after 20 measurements. Fc = Fe(η5-C5H5)(η5-C5H4).
The series of ILs with different lengths of alkyl fragment at phosphorus atoms were considered
quantum chemically (Figure 5). To simplify the computations, only the most stable all-trans
conformations of alkyl moieties of [Fc-P(t-Bu)2Alk]+ cations were regarded. This planar zigzag
conformation is preferred for the nearest to the phosphorus atom part of the alkyl chain in
crystals of closely related phosphonium salts and is the most energetically stable form of isolated
tetraalkylphosphonium cations according to quantum chemical computations [27]. For m1
and m9 models with the shortest, medium, and the longest alkyl chains, respectively, three possible
conformations with different torsion (Cp)CCPC(Alk) angles were optimized (Figure 5). In and
cases, quantum chemical optimization leads to structure with eclipsed Fc conformation, whereas in
conformer , the Fc moiety adopts a staggered conformation. The computed energy differences
between and
conformers with the alkyl moiety R turned towards the Fe2+ ion do not exceed 1
kcal/mol. The conformer is the most stable form for the model m1, whereas for the models m5
and m9, conformation is energetically preferred. In all cases, the conformer was predicted to be less
energetically stable as compared to a and b by ~4 kcal/mol.
, m5,
a
c
b
a
b
b
a
c
Figure 5. Considered conformations (
a–c) of cation part of IL models, R = Me (m1), Et (m2), Pr (m3),
Bu (m4), C6H13 (m5) C7H15 (m6), C10H21 (m7), C13H27 (m8), C14H29 (m9).
Energies of frontier molecular orbitals (FMOs) computed for a conformation of m1–m9 models
are collected in Table 7. According to our computations, elongation of the alkyl chain shifts both
the highest occupied (HOMO) and the lowest unoccupied molecular orbitals (LUMO). This effect is
slightly more pronounced for LUMO resulting in a weak increase of the HOMO–LUMO gap.

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Table 7. Absolute HOMO and LUMO energies and HOMO–LUMO gaps (in eV) calculated for
the models m1–m9 a.
Compound
HOMO
LUMO
LUMO–HOMO Gap
Fc-P+(t-Bu)2CH3 (m1)
−9.24
−6.61
−4.93
−9.24
−9.22
−9.20
−9.18
−9.18
−9.17
−9.16
−9.16
−6.67
−5.01
−3.58
−0.91
−0.74
−3.56
−3.53
−3.51
−3.49
−3.49
−3.48
−3.48
−3.47
−0.96
−0.73
5.67
5.70
4.19
5.68
5.69
5.69
5.69
5.69
5.69
5.69
5.69
5.71
4.28
−
Fc-P+(t-Bu)2CH3BF4
−
Fc-P+(t-Bu)2CH3Br
Fc-P+(t-Bu)2C2H5 (m2)
Fc-P+(t-Bu)2C3H7 (m3)
Fc-P+(t-Bu)2C4H8 (m4)
Fc-P+(t-Bu)2C6H13 (m5)
Fc-P+(t-Bu)2C7H15 (m6)
Fc-P+(t-Bu)2C10H21 (m7)
Fc-P+(t-Bu)2C13H27 (m8)
Fc-P+(t-Bu)2C14H29 (m9)
−
−
Fc-P+(t-Bu)2C14H29BF4
Fc-P+(t-Bu)2C14H29Br
a
Fc = Fe(η5-C5H5)(η5-C5H4).
The computations showed similar composition of FMOs for m1 and m9. In both cases, HOMO is
predicted to be localized on the Fc moiety of the cation, whereas LUMO is contributed also by P–C
bond and atomic orbitals (AOs) of t-Bu and nearest carbon atom of alkyl group (Figure 6).
Figure 6. HOMO (left) and LUMO (right) of m1 (top) and m9 (bottom).
For m1 and m9, ion pairs with Br− and [BF4]− counterions were optimized (Figure 7).
According to our computations, the energy of ion pairing is higher by 6–8 kcal/mol for Br− anion
compared to [BF4]− (84 vs. 78 kcal/mol for Fc-P+(t-Bu)2CH3 and 85 vs. 77 for Fc-P+(t-Bu)2C14H29
kcal/mol).

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Figure 7. Structures of considered ion pairs of m1
and corresponding HOMO (b) and LUMO (c).
(a ( (bottom) anions
) with Br− top) and [BF4]−
The ion pairing increases the energies of both HOMO and LUMO; in the case of [BF4]− anion,
the shift is almost parallel, being slightly more pronounced for LUMO, resulting in quite modest
increasing of the HOMO–LUMO gap compared to cations. In contrast, Br− much more essentially
affects the HOMO, making the oxidation of bromide salts easier. The resulting HOMO–LUMO gap
is significantly decreased compared to cati−ons and bromides (Table 7). Such pronounced effect in
the latter case is caused by contribution of Br AOs to HOMO (Figure 5). Similar localization of HOMO
−
−
and LUMO is predicted also for m9 with [BF4] and Br anions. In both cases, elongation of alkyl chain
−
of cationic moiety leads to reduced HOMO energy (from
to
Indeed, according to the abovementioned equation EHOMO
−
4.93 to
−
5.01 in the Br case and from
−
6.61
−
6.67 for [BF4]−), that is in accordance with the experimental trends observed electrochemically.
1
= (E[semidif,ox vs. ferrocenium/ferrocene]
−
+ 4.8 V) the latter is equal to
−
5.23 for Fc-P+(t-Bu)2C3H7Br− and
−
5.−26 for Fc-P+(t-Bu)2C14H29Br−;
−5.69 for Fc-P+(t-Bu)2CH3BF4 and −5.71 for Fc-P+(t-Bu)2C14H29BF4
.
−
3. Materials and Methods
3.1. Electrochemistry
Cyclic voltammograms were recorded with a BASi Epsilon E2P (Bioanalytical Systems, Inc.,
West Lafayette, IN, USA) potentiostat. The device comprises a measuring unit, PC Dell Optiplex
320 (Bioanalytical Systems, Inc.) with the Epsilon-EC-USB-V200 software (Bioanalytical Systems,
Inc.). Tetrabutylammonium tetrafluoroborate (C4H8)4NBF4 was used as background electrolyte.
The working electrode was a stationary disc glassy-carbon electrode (the surface area of 6 mm2).
Ag/AgCl (0.01 M KCl) (Bioanalytical Systems, Inc.) was used as a reference electrode. The reference
electrode was connected with the cell solution by a modified Luggin capillary filled with the supporting
electrolyte solution (0.1 M Bu4NBF4 in CH3CN). Thus, the reference electrode assembly had two
compartments, each terminated with an ultra-fine glass frit to separate the AgCl from the analyte.
A platinum wire was used as an auxiliary electrode. The scan rate was 100 mV/s. The measurements
were performed in a temperature-controlled electrochemical cell (volume from 1 mL to 5 mL)
in an inert gas atmosphere (N2). Between measurements or prior to a registration of a voltammetry
wave, the solution was actively stirred with a magnetic stirrer under constant inflow of an inert
gas that was run through a dehydrating system, and then through a nickel-based purification system
BI-GAS cleaner (OOO Modern Laboratory Equipment, Novosibirsk, Russia) to remove trace quantities
of oxygen.

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3.2. NMR Experiments
The NMR spectra were registered on the equipment of Assigned Spectral-Analytical Center of
FRC Kazan Scientific Center of RAS, namely on multi-nuclear spectrometer Bruker AVANCE-400
(BRUKER BioSpin, Rheinstetten, Germany) (400.1 MHz (1H), 100.6 MHz (13C) and 162.0 MHz (31P)).
Chemical shifts are given in parts per million relative to SiMe4 (1H, internal) and 85% H3PO4
(31P, external).
3.3. Thermogravimetric/Differential Scanning Calorimetry (TG-DSC)
Thermogravimetric analysis was performed on the NETZSCH STA 449F3 (NETZSCH-Gerätebau
GmbH, Selb, Germany) with a heating rate of 10 K per minute up to 400 ◦C under argon atmosphere.
3.4. Calculations
All calculations were performed with the Gaussian 16 suite of programs [28]. The hybrid PBE0
functional [29] and the Ahlrichs’ triple-ζ def-TZVP AO basis set [30] were used for optimization
of all structures. In all geometry optimizations, the D3 approach [31] to describe the London
dispersion interactions together with the Becke–Johnson (BJ) damping function [32
as implemented in the Gaussian 16 program.
–34] were employed
3.5. Mass-Spectra
Mass Electrospray ionization mass spectrometry (ESI-MS) was performed on the AmazonX
mass spectrometer (Bruker Daltonics, Bremen, Germany). The measurements were carried out in
the positive/negative ion detection mode in the m/z range from 100 to 1000. The voltage on the capillary
was 140 V. Data was processed using the DataAnalysis 4.0 program (Bruker Daltonics).
3.6. Materials and Reagents
All the work related to the preparation of the initial reagents, the synthesis and the release of
products was carried out in an inert atmosphere using standard Schlenk apparatus. All solvents
and purchased reagents were absolute by the appropriate methods, mainly by distillation in
an inert atmosphere.
t-BuLi (1.6 M solution) (Sigma Aldrich, St.
Louis, MO, USA) was used without
a preliminary purification.
3.7. Synthesis (General Procedure)
3.7.1. Synthesis of Di(tert-Butyl)Ferrocenylphosphine
In a two-necked Schlenk vessel equipped with a magnetic stirrer, ferrocene (10.3 mol, 1.923 g)
and t-BuOK (1.55 mmol, 0.173 g) were placed, the mixture was dissolved in 150 mL of diethyl
ether. The solution was cooled to
10–15 min. Stirring of the reaction mixture was carried out at
−
78 ◦C. A solution of t-BuLi (10.3 mmol) was added over
−
70 ◦C for 1 h. An orange precipitate
of di(tert-butyl)ferrocenylphosphine was observed to form. The reaction mixture then was warmed
◦
to 0 C and (t-Bu)2PCl (5.15 mmol, 0.98 mL) was added dropwise. After the completion of the dropping,
the cooling bath was removed and the reaction mixture was warmed to room temperature. After Et2O
was removed in vacuum theresidue was dissolved in 200 mL of petroleum ether and filtered
through a short silica column. Removal of the solvents yield a dark-brown solid 2.431 g (71%).
31P (Petroleum ether, δ, ppm) NMR: 28.5 (s). [5]
3.7.2. Method for the Synthesis of Methyl (Di-tert-butyl)Ferrocenylphosphonium Iodide (4a)
Iodomethane (0.191 mL, 3.06 mmol) was added to di(tert-butyl)ferrocenylphosphine (0.507 g,
1.53 mmol) with stirring. The precipitation of a brown solid was observed. The product is then washed

Molecules 2018, 23, 2773
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2 times with 20 mL of petroleum ether and 2 times with 20 mL of diethyl ether. The precipitate is
filtered out, the solvent residues are removed in vacuo. Yield 0.449 g (62%), m.p. = 74.1 ◦C.
3.7.3. Synthesis of Propyl-, Hexyl-, Decyl-, Tetradecyl (Di-tert-butyl)ferrocenylphosphonium
Bromides (4b–e)
An equivalent amount of alkyl bromi◦de was added to di(tert-butyl)ferrocenylphosphine.
◦
◦
◦
The reaction mass was stirred at 70 C (4b), 75 C (4c), 90 C (4d), and 100 C (4e) for 5 h. The product
was then washed with 2 portions of 20 mL of petroleum ether and 2 portions of 20 mL of diethyl ether.
The precipitate was filtered out and the solvent residues were removed in vacuo.
3.7.4. Synthesis of Methyl-, Propyl-, Hexyl-, Decyl-, Tetradecyl-(di-tert-butyl)ferrocenylphosphonium
Tetrafluoroborates (5a–e)
Salts (5a–e) were dissolved in 10 mL of ethyl alcohol and a twofold excess of sodium
tetrafluoroborate solution in ethyl alcohol was added to them. The reaction mixture was stirred for
12 h. The solvent was evaporated in vacuo, and the salt was dissolved in 20 mL of methylene chloride
and washed with distilled water. The organic phase was separated and dried over magnesium sulfate.
◦
The solvent is evaporated, and the salt is dried under vacuum at 50 C for 8 h. For other details,
see the Supplementary Material.
4. Conclusions
A synthetic methodology for the preparation of new Fc-containing sterically hindered phosphonium
ILs based on di(tert-butyl)ferrocenylphosphine is described. Electrochemical measurements showed
that with increasing length of the alkyl fragment at the phosphorus atom, the potential shifts to the anode
region. The replacement of the bromide anion by [BF4]− significantly shifts the ferrocene/ferrocenium
transition into the positive region of the resulting salts. Such a pronounced effect is caused by a stronger
ion pairing with a Br− anion. According to our computations, in the ion pairs with Br−, the HOMOs
are contributed by the counter-ion. For both Br− and [BF4]− cases, elongation of the alkyl chain with
a cationic moiety leads to a reduction of HOMO energy that is in accordance with the experimental trends
observed electrochemically.
The electrochemical properties of the newly synthesized Fc-containing phosphonium salts
mean that they will represent attractive alternative components for batteries and capacitors. This opens
additional potential for using Fc-based ILs as redox agents in electrolytes for lithium ion batteries.
Supplementary Materials: The following are available online: Experimental procedures; NMR spectra of
synthesized compounds; ESI-MS spectra of synthesized compounds and TG-DSC curves of
synthesized compounds.
Author Contributions: V.E., L.K. developed synthetic procedure of the phosphonium salts; T.G. and S.K.
performed quantum-chemical studies, conducted discussion of the results; R.S., E.D. and A.K., performed
voltammetric studies; V.M., O.S., Y.B. and M.K. generated the basic idea of the study, summarized and discussed
the results of individual studies, preparation and writing of the article.
Funding: This work was supported by the Russian Science Foundation No. 18-73-10139.
Acknowledgments: Authors are grateful to A. Khamatgalimov for TG-DSC analysis.
Conflicts of Interest: The authors declare no conflict of interest.
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Sample Availability: Samples of the compounds are available from the authors.
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