Structure and supramolecular organization of tri-n-butyl(octadecyl)phosphonium tetrafluoroborate and its influence on catalytic activity of stabilized palladium nanoparticles

Article abstract of DOI:10.1007/s11172-015-1181-2

A crystal structure and aggregation properties of tri-n-butyl(octadecyl)phosphonium tetrafluoroborate were studied. Palladium nanoparticles stabilized by this phosphonium salt exhibit different catalytic activity in the Suzuki reaction, depending on the concentration of the stabilizing agent, which is related to the supramolecular organization of the Pd–phosphonium salt system.

Full text of DOI:10.1007/s11172-015-1181-2

Russian Chemical Bulletin, International Edition, Vol. 64, No. 10, pp. 2486—2492, October, 2015
2486
Structure and supramolecular organization
of triꢀnꢀbutyl(octadecyl)phosphonium tetrafluoroborate and its influence
on catalytic activity of stabilized palladium nanoparticles*
D. M. Arkhipova,a V. V. Ermolaev,a V. A. Miluykov,a G. A. Gaynanova,a F. G. Valeeva,a L. Ya. Zakharova,a
A. I. Konovalov,a D. R. Islamov,b O. N. Kataeva,a and O. G. Sinyashina
aA. E. Arbuzov Institute of Organic and Physical Chemistry,
Kazan Scientific Center of the Russian Academy of Sciences,
8 ul. Akad. Arbuzova, 420088 Kazan, Russian Federation.
Eꢀmail: arkhipovaꢀdaria@mail.ru
bA. M. Butlerov Chemical Institute of Kazan (Volga region) Federal University,
18 ul. Kremlevskaya, 420008 Kazan, Russian Federation
A crystal structure and aggregation properties of triꢀnꢀbutyl(octadecyl)phosphonium tetꢀ
rafluoroborate were studied. Palladium nanoparticles stabilized by this phosphonium salt exꢀ
hibit different catalytic activity in the Suzuki reaction, depending on the concentration of the
stabilizing agent, which is related to the supramolecular organization of the Pd—phosphonium
salt system.
Key words: phosphonium salts, palladium nanoparticles, stabilization, catalysis, Suzuki
reaction, aggregation.
Phosphonium salts possess a unique combination of
um salts can lead to the change in the type of stabilization
of palladium nanoparticles and, consequently, to the
change in the properties of the nanoparticle—phosphoniꢀ
um salt system. We suggested that a phosphonium salt
containing an octadecyl substituent at the phosphorus
atom could provide the steric stabilization of palladium
nanoparticles due to the long hydrophobic "tail".
In the present work, we report the results of our studies
devoted to the synthesis and examination of properties of
a new phosphonium salt, triꢀnꢀbutyl(octadecyl)phosphoꢀ
nium tetrafluoroborate (1), which was used for the stabiliꢀ
zation of palladium nanoparticles. We also studied a posꢀ
sibility of their subsequent application as the catalyst in
the crossꢀcoupling reaction. The Suzuki reaction was choꢀ
sen because it is the most important method for the synꢀ
thesis of biaryls13—17 and is a convenient model reaction.
Special attention was paid to the study of the aggregation
properties of phosphonium salt 1 as one of the factors
determining the catalytic activity of palladium nanopartiꢀ
cles stabilized by this salt.
properties such as nonꢀflammability, nonꢀvolatility, elecꢀ
troconductivity, thermal stability, a wide temperature
range of liquid state, which make them very useful1 as
electrolytes, solvents, and organic catalysts. One of the
most important areas of application of phosphonium salts
is the stabilization of transition metal nanoparticles, which
can be used in organic reaction. There are several types of
nanoparticle stabilization: electrostatic, steric, and elecꢀ
trosteric. The type of stabilization largely determines their
catalytic activity. By now, several models for the binding
of metal nanoparticles with organic salts have been sugꢀ
gested.2,3 However, the mechanism of stabilization of
nanoparticles with phosphonium salts and its influence on
the catalytic activity are still in dispute. In order to deveꢀ
lop a versatile approach to the design of most efficient
catalytic system, it is necessary to model the process of
metal nanoparticles stabilization.
Phosphonium salts based on R1 P+R2 cations (R1 =
3
= Bu, C6H13; R2 = Bu, C8H17, C14H29) are among the
most studied such compounds.4—8 It is known that the
properties of compounds are considerably affected by the
length of the alkyl substituents in organic salts.9—12 The
elongation of one of the alkyl substituents in phosphoniꢀ
Results and Discussion
In previous works, we have shown that palladium
nanoparticles stabilized by phosphonium salts exhibit
high activity as catalysts of Suzuki reaction involving
bromoꢀ and chloroarenes under mild conditions,18 we
* Based on the Materials of the XXVI International Chugaev
Conference on Coordination Chemistry (October 6—10, 2014,
Kazan, Russia).
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2486—2492, October, 2015.
1066ꢀ5285/15/6410ꢀ2486 © 2015 Springer Science+Business Media, Inc.

Tributyl(octadecyl)phosphonium tetrafluoroborate
Russ.Chem.Bull., Int.Ed., Vol. 64, No. 10, October, 2015
2487
also studied the influence of the aggregation properties
of the medium on the catalytic properties of nanoꢀ
particles.19 In the next step of our work, we used phosꢀ
phonium salt as the stabilizing agent which increases
the contribution of the steric factor in the type of stabiꢀ
lization of palladium nanoparticles, we also analyzed
supramolecular organization and efficiency of this
catalytic system.
Phosphonium salt 1 was obtained according to the
known procedure20,21 by the reaction of triꢀnꢀbutylphosꢀ
phine with octadecyl bromide and subsequent exchange
of the bromide anion with the tetrafluoroborate anion.
Compound 1 was studied by a number of physicochemical
methods: NMR spectroscopy, elemental analysis, TGA,
IR spectroscopy, and MS, its structure was confirmed by
Xꢀray crystallography.
The crystals of phosphonium salt 1 are composed of
triꢀnꢀbutyl(octadecyl)phosphonium cations and tetraꢀ
fluoroborate anions. The asymmetric part of the cell conꢀ
tains two independent cations and anions. Tetrafluoroboꢀ
rate anions are disordered over two positions.
In the cations, all the units of the aliphatic substituents
have the transꢀconfiguration (Fig. 1), the phosphorus atom
has a virtually ideal tetrahedral coordination. Within the
experimental error, the P—C bonds have the same length
(1.795(6) Å) and are considerably shorter than the P—C
bonds in the Pri4P+ (1.830(2) Å)22 and But4P+ cations
(1.924(4) Å),23 which is attributed to the steric effects.
The multiple dispersion interactions between the octaꢀ
decyl substituents determine the layered structure of the
crystal with the alternation of hydrophobic and hydroꢀ
philic layers (Fig. 2).
Fig. 1. The structure of compound 1 in crystal.
Fig. 2. Molecular packing in crystals of compound 1.

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Arkhipova et al.
χ/mS cm–1
70
The structure of the phosphonium salt with the long
hydrophobic substituent and its packing in crystal suggest
its possible surface activity in solutions.
60
50
40
30
20
10
Since the salt 1 is insoluble in water at 25 °C, the
studies of its aggregation properties were carried out in
a mixture of H2O—DMF (30 vol.% DMF) at the elevated
temperature of 50 °C. Tensiometry and conductometry
are the most frequently used methods for determination
of critical micelle concentration (CMC) of surfaceꢀacꢀ
tive compounds (surfactants). The isotherm of surface tenꢀ
sion of compound 1 shown in Fig. 3 has two inflection
points at the concentrations of 4•10–4 (CMC1) and
7.7•10–4 mol L–1 (CMC2). Small aggregates are formed
when the first CMC threshold is reached, whereas an
increase in the concentration of the phosphonium salt is
accompanied by structural rearrangements in the system,
most likely, with the formation of vesicles. The latter sugꢀ
gestion was confirmed by the dynamic light scattering: the
hydrodynamic diameter of aggregates (D) at the concentraꢀ
tions of 0.001 and 0.003 mol L–1 (considerably higher then
CMC2) is equal to 146 and 191 nm, respectively (Fig. 4).
The plot of specific electroconductivity against conꢀ
centration of the phosphonium salt (Fig. 5) is characterꢀ
ized by the decrease in the straight line slope, that reflects
a decrease in the amount of chargeꢀbearing particles and
indicates the proceeding of aggregation processes in the
CMC2
CMC1
0.001 0.002 0.003 0.004 0.005 C/mol L–1
Fig. 5. Specific electroconductivity (χ) versus concentration of
phosphonium salt 1 (C) in H2O—DMF (30 vol.% DMF);
T = 50 °C. CMC1 and CMC2 are the critical micelle concentraꢀ
tions.
H2O—DMF (30 vol.% DMF) solvent system. The CMC
values determined by conductometry were found to be
2•10–4 (CMC1) and 1•10–3 mol L–1 (CMC2).
Palladium nanoparticles were synthesized and stabiꢀ
lized according to the following procedure:19 palladium
acetate was dissolved in ethanol in the presence of phosꢀ
phonium salt 1 without addition of reducing agents
(Scheme 1). Upon the dissolution of the palladium salt,
the solution became colored in light orange. After 20 min,
the solution acquired a grey brown shade, which indirectly
indicated the reduction of palladium to Pd0 and the forꢀ
mation of nanoparticles.5 As of now, there is no consensus
in the world scientific literature on the mechanism of the
reduction of palladium,7 the formation of nanoparticles in
different systems can take different pathways. The reducꢀ
tion of palladium by the solvent molecules seems the most
probable.
σ/mN m–1
50
46
CMC1
42
CMC2
38
Scheme 1
1•10–5
1•10–4
1•10–3
C/mol L–1
Fig. 3. Surface tension (σ) versus concentration of phosphonium
salt 1 (C) in H2O—DMF (30 vol.% DMF); T = 50 °C. CMC1
and CMC2 are the critical micelle concentrations.
The formation of palladium nanoparticles was conꢀ
firmed by transmission electron microscopy (Fig. 6), which
showed the presence of both the isolated nanoparticles
and their aggregates.
N (%)
50
A freshly obtained colloidal solution of palladium
was immediately used in the catalytic reaction. The Suzuꢀ
ki crossꢀcoupling of 1,3,5ꢀtribromobenzene (2) and pheꢀ
nylboronic acid was chosen as a model reaction
(Scheme 2). The reaction was carried out under mild conꢀ
ditions using a 0.36 mol.% catalyst and predominantly led
to 1,3,5ꢀtriphenylbenzene (3). The intermediate products 4
and 5 were present in the trace amounts. No products of
2
1
0
2
8
33
142
615 D/nm
Fig. 4. Size distribution of aggregates of phosphonium salt 1 in
H2O—DMF averaged on the amount of particles (30 vol.%
DMF) at 0.001 (1) and 0.003 mol L–1 concentration (2); T = 50 °C.

Tributyl(octadecyl)phosphonium tetrafluoroborate
Russ.Chem.Bull., Int.Ed., Vol. 64, No. 10, October, 2015
a
2489
b
200 nm
50 nm
Fig. 6. Isolated palladium nanoparticles (a) and their aggregates (b).
Scheme 2
Reagents and conditions: i. PhB(OH)2, 0.36 mol.% Pd, KOH, EtOH, 30 °C, 16 h.
homocoupling or hydrodebromination of the substrate
were detected in the chromatographic patterns.
reaction carried out in the absence of the stabilizing agent.
Further increase in the 1 : Pd ratio leads to the deactivaꢀ
tion of the catalytic system. The moderate catalytic actiꢀ
vity can be explained by the high degree of steric stabilizaꢀ
tion secured by the long alkyl substituent in the phosphoꢀ
nium cation. Most likely, the stabilizer molecules restrict
the migration of the substrate to the catalytically active
centers of the particles and prevents the leaching of sepaꢀ
According to the literature data,24 the yield of the reꢀ
action product can be affected by the amount of the orꢀ
ganic salt used. Therefore, we studied the dependence of
the conversion 2 → 3 on the 1 : Pd ratio (Figs 7 and 8).
The use of the phosphonium salt 1 in small amounts
leads to the increase in the conversion as compared to the
D/nm
С (%)
30
ξ/mV
С (%)
30
1200
1000
800
15
10
25
20
15
10
5
25
20
15
10
5
1
1
5
0
600
–10
–15
–20
2
2
400
200
100 200 300 400 500 600 700 1 : Pd
100 200 300 400 500 600 700 1 : Pd
Fig. 8. Zeta potential of aggregates (1) and conversion of the
reaction (2) versus the 1 : Pd ratio.
Fig. 7. Hydrodynamic diameter (D) of aggregates (1) and conꢀ
version of the reaction (C) versus the 1 : Pd ratio (2).

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Russ.Chem.Bull., Int.Ed., Vol. 64, No. 10, October, 2015
Arkhipova et al.
equipped with an electrospray ionizer. Elemental analysis of
compounds was carried out on an EuroVector Euro3028ꢀHTꢀ
OM analyzer.
rate palladium atoms and small clusters into the solution,
which is reflected in the decrease of the conversion of
compound 2 to the reaction products.
The IR spectrum of the compound in the 4000—400 cm–1
range was recorded on a Tensor 27 IR Fourierꢀtransform specꢀ
trometer (Bruker) at room temperature with the optical resoluꢀ
tion of 4 cm–1. The sample was prepared in a KBr pellet.
The Raman spectrum of the powdered compound placed
into an aluminum cell was recorded in the 4000—100 cm–1 range
on a Vertexꢀ70 Fourierꢀtransform spectrometer with a RAM II
console (Bruker) at room temperature with optical resolution of
2 cm–1. A Nd:YAG laser with a 1064 nm wavelength was used as
a a source of excitation. The sample was prepared in a KBr
pellet.
It is probable that the ability of phosphonium salt 1 to
form supramolecular aggregates due to the hydrophobic
"tail" affects the stabilization type of palladium nanopartiꢀ
cles, which is reflected in the different degree of the cataꢀ
lytic effect. To confirm this, we determined the hydroꢀ
dynamic diameter and the electrokinetic potential of the
aggregates in the binary system phosphonium salt 1 : Pd
in the same solvent, which was used for the catalytic reacꢀ
tion, i.e., ethanol (see Fig. 7, 8). It was found that the
increase in the concentration of the phosphonium salt was
accompanied by the increase in the aggregate sizes and the
zeta potential values, with subsequent coming out of the
dependence on a plateau (see Fig. 7, 8). In this case, the
conversion passes a maximum, which is a typical profile
for the catalysis of bimolecular reaction in the organized
systems.25 When the concentrations of surfactants are close
to the CMC, local concentrations of reagents in aggreꢀ
gates, playing the role of nanoꢀ or microreactors, greatly
increases, which leads to a sharp increase in the catalytic
effect. The further increase in the concentration of the
amphiphilic compound leads to the increase in the amount
of aggregates and, therefore, to the dilution of the reꢀ
agents, which is observed as a decrease in the catalytic
effect. The data obtained indicate that the sizes of the
aggregates close to 1 μm, and the aggregate charges equal
to 5 mV are the optimal conditions for the exertion of the
maximal catalytic effect in the crossꢀcoupling reaction for
this stabilizing agent.
Xꢀray diffraction analysis of compound 1 was carried out on
a Bruker KappaApex DUO diffractometer (λ(CuꢀKα) = 1.54178 Å).
A semiempirical allowance for absorption was made, using the
SADABS program.26 The structure was solved by the direct
method using the SHELXꢀ97 program.27 The positions and temꢀ
perature parameters of nonhydrogen atoms were refined in anisoꢀ
tropic approximation using the SHELXꢀ97 program.27 Hydroꢀ
gen atoms were placed in the geometrically calculated positions
and refined using a riding model. All the fluorine atoms in
anions are disordered over two positions. All the calculations
were performed using the WinGX28 and APEX229 software. The
figures of the molecule and the packing were performed using
the Mercury 3.1 program.30
Solutions were prepared using the water purified on a Directꢀ
Q 5 UV apparatus (water resistance 18.2 MΩ cm at 25 °C). The
surface tension of solutions was determined by the "ring detachꢀ
ment" (du Nouy) method. The measurements were carried out
manually on a K6 tensiometer (KRUSS, Germany), which deꢀ
termines the surface tension using an ideally wettable measureꢀ
ment ring hanged to a precision balance. Every reported value
was obtained at least from five readings.
In conclusion, the phosphonium salt 1 containing
a long hydrophobic substituent at the phosphorus atom
was shown to have a clearly ordered structure in crystal
and tend to form aggregates in solution, including that in
the presence of a second component, palladium nanoparꢀ
ticles. The strong steric stabilization of palladium nanoꢀ
particles by compound 1 did not allow one to achieve high
yields in the catalytic reaction. The 1 : Pd ratio exerts
a considerable influence on the supramolecular organizaꢀ
tion of the catalytic system and, consequently, on its acꢀ
tivity in the Suzuki crossꢀcoupling reaction.
The data on specific electroconductivity (χ, μS cm–1) of the
system were obtained on an InoLabCond 7110 conductometer.
The CMC was determined based from the inflection point on
the plot of specific electroconductivity against surfactant conꢀ
centration. Electroconductivity of water was taken as zero.
Zeta potential of the systems was determined by electroꢀ
phoretic light scattering on a Zetasizer Nano instrument. A He—
Ne gas laser (4 mW) with a 633 nm wavelength served as a source
of laser radiation.
The images of palladium nanoparticles were obtained on
a Hitachi HT7700 transmission electron microscope (Japan) at
accelerating potential of 100 kV.
Conversion of 1,3,5ꢀtribromobenzene was evaluated by
GCꢀMS on a DFS Thermo Electron Corporation instrument
(Germany), ionization by electron impact (70 eV), the temperaꢀ
ture of the source of ions 250 °C, a IDꢀBP5Х capillary column
(analogue of DBꢀ5MS, 50 m in length, 0.32 mm in diameter, the
phase layer thickness of 0.25 μm), carrier gas helium. The mass
spectral data were processed using the Xcalibur program. A samꢀ
ple of a compound under study before injection was dissolved in
Experimental
All the manipulations related to the preparation of starting
reagents, carrying out synthesis and isolation of products were
performed under argon, using standard Schlenk glassware.
1H, 31P, and 13C NMR spectra were obtained on a Bruker
MSLꢀ400 spectrometer (400.1, 161.9, and 100.6 MHz, respecꢀ
tively). Tetramethylsilane was used as an internal standard (1H
and 13C) and 85% aqueous H3PO4 (31P) as an external standard.
Thermogravimetric analysis was carried out on a NETZSCHSTA
449 F3 instrument at the rate of heating 10 deg min–1 to 450 °C
under argon. High resolution mass spectra were obtained on an
Amazon X spectrometer (Bruker Daltonik GmbH, Germany)
ethanol (HPLC grade) in the concentration of ∼10–3 g μL–1
,
amount of injected sample 0.1 μL. The following chromatoꢀ
graphic regime was used: injector temperature 250 °C, flow seꢀ
paration 1 : 10; programmed temperature of the column: initial
temperature 120 °C (1 min), then heating at the rate 20 °C min–1
to 280 °C, final temperature 280 °C (15 min); the carrier gas rate

Tributyl(octadecyl)phosphonium tetrafluoroborate
Russ.Chem.Bull., Int.Ed., Vol. 64, No. 10, October, 2015
2491
through the column 2 mL min–1. Temperature of communicaꢀ
tion device with mass spectrometer 280 °C. The basic product
1,3,5ꢀtriphenylbenzene was used for calibration.
Preparation of samples of palladium nanoparticles for transꢀ
mission electron microscopy. A drop of a solution with reduced
palladium was placed on a copper grid 200 mesh with carbon
coating. The sample was dried in air for several minutes and
placed in the transmission electron microscope. The most effiꢀ
cient ratio Pd : 1 = 1/100 was chosen for the analysis (the amount
of salt 1 0.0964 g, or 0.178 mmol).
Suzuki reaction crossꢀcoupling. 1,3,5ꢀTribromobenzene
(0.16 g, 0.52 mmol), phenylboronic acid (0.28 g, 2.33 mmol),
and potassium hydroxide (0.13 g, 2.33 mmol) were added to
a colloid solution of palladium obtained as above (9 mL). The
reaction mixture was stirred for 16 h at 30 °C. The components
of the reaction were extracted with toluene and analyzed withꢀ
out isolation of the reaction products.
Triꢀnꢀbutylphosphine (SigmaꢀAldrich), nꢀoctadecyl bromide
(SigmaꢀAldrich), phenylboronic acid (Alfa Aesar), 1,3,5ꢀtribroꢀ
mobenzene (Alfa Aesar), 1,3,5ꢀtriphenylbenzene (SigmaꢀAldrich),
Pd(OAc)2 (Alfa Aesar), KOH, and NaBF4 (both of analytical
grade, without additional purification) were used in the work.
Triꢀnꢀbutyl(octadecyl)phosphonium tetrafluoroborate (1).
A mixture of triꢀnꢀbutylphosphine (3.7 mL, 14.8 mmol) and
nꢀoctadecyl bromide (4.94 g, 14.8 mmol) was stirred for 8 h at
60 °C. After cooling, the mixture was washed with light petroꢀ
leum ether, the residual solvent was evaporated in vacuo to obꢀ
tain triꢀnꢀbutyl(octadecyl)phosphonium bromide (6.89 g, 87%)
as an amorphous white a powder, m.p. 70.9 °C. 31P NMR
(CDCl3), δ: 32.8 (s). 1H NMR (CDCl3), δ: 2.54—2.42 (m, 8 H,
P—CH2); 1.62—1.47 (m, 16 H, P—CH2—CH2—CH2); 1.38—
1.20 (m, 28 H, CH2); 1.00 (t, 9 H, CH3, J = 6.82 Hz); 0.89 (t, 3 H,
CH3, J = 6.97 Hz). MS, m/z (Irel (%)): 455.6 [M+] (100), 81.1 [A–]
(100). Found (%): C, 67.35; H, 12.03; P, 5.90. C30H64BrP. Calꢀ
culated (%): C, 67.26; H, 12.04; Br, 14.92; P, 5.78.
The microscopic measurements were carried out using
facilities of the Transmission Electron Microscopy Laboꢀ
ratory of the Community User Center of Scientific Equipꢀ
ment of the Kazan National Research Technological Uniꢀ
versity.
This work was financially supported by the Russian
Foundation for Basic Research (Project Nos 13ꢀ03ꢀ12170
and 14ꢀ03ꢀ90409_Ukr_a) and the State Program for the
Increase of Competitiveness of Kazan Federal University.
Then, NaBF4 (0.92 g, 8.4 mmol) and ethanol (5 mL) were
added to the triꢀnꢀbutyl(octadecyl)phosphonium bromide (3 g,
5.6 mmol), the mixture was stirred at room temperature for 4 h.
The solvent was evaporated in vacuo, the residue was washed
with a mixture of CH2Cl2—H2O, the organic layer was separatꢀ
ed, dried with anhydrous MgSO4, and filtered, the solvent was
evaporated in vacuo. The yield was 2.7 g (89%), a white crystalꢀ
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C
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1
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C
30H64BF4P. Calculated (%): C, 66.41; H, 11.89; B, 1.99; F, 14.01;
P, 5.71.
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system; space group Pꢀ1, a = 9.4592(7) Å; b = 10.8561(7) Å; c =
= 33.263(2) Å; α = 95.816(4)°; β = 91.385(4)°; γ = 90.117(4)°;
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3
V = 3397.2(4) Å ; Z = 4; dcalc = 1.061 g cm–3; μ = 1.019 mm–1
;
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=
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formation for this structure is available in the Cambridge Crysꢀ
tallographic Data Centre, CCDC 1043578).
Preparation of palladium nanoparticles. Palladium acetate
(0.4 mg, 0.00178 mmol) and the corresponding amount of phosꢀ
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Received January 15, 2015;
in revised form March 24, 2015