Highly efficient hydrosilylation catalysts based on chloroplatinate “ionic liquids”

Article abstract of DOI:10.1016/j.jcat.2019.05.005

The reaction between ionic liquid [Cat]⁺Cl^? (where Cat stands for 1-butyl-3-methylimidazolium, 1-butyl-2,3-dimethylimidazolium or 1-butyl-4-methylpyridinium)and the metal precursor ([PtCl₂(cod)], PtCl₄, K₂[PtCl₄]or K₂[PtCl₆])yielded two groups of derivatives: [Cat]⁺[PtCl₄]^? and [Cat]⁺[PtCl₆]^?, which formally are counted among halometallate ionic liquids, however, due to their high melting points they should be classified into anionic platinum complexes rather than into ionic liquids. All the derivatives were isolated and characterized spectroscopically (NMR, ESI-MS)and crystallographic structures were determined for three derivatives: ([BMPy]₂[PtCl₄], [BMIM]₂[PtCl₆]and [BMMIM]₂[PtCl₆]. Moreover, their melting points were measured and thermal stability was assessed. The above derivatives were employed as catalysts for hydrosilylation of olefins with diverse properties. All the studied catalysts showed high activity and their insolubility in the reaction medium made easy their isolation and multiple use in subsequent catalytic runs. The most effective catalysts did not lose their activity even after ten runs, thereby they make a very good alternative to commonly used homogeneous catalysts. Their simple synthesis and stability make them interesting both for economic and ecological reasons.

Full text of DOI:10.1016/j.jcat.2019.05.005

Journal of Catalysis 374 (2019) 266–275
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Highly efficient hydrosilylation catalysts based on chloroplatinate ‘‘ionic
liquids”
Magdalena Jankowska-Wajda , Olga Bartlewicz , Anna Walczak a,b, Artur R. Stefankiewicz a,b
Hieronim Maciejewski
a
a
a
,
a,c,
⇑
Faculty of Chemistry, Adam Mickiewicz University in Pozna n´ , Umultowska 89b, 61-614 Pozna n´ , Poland
Center for Advanced Technologies, Adam Mickiewicz University in Pozna n´ , Umultowska 89c, 61-614 Pozna n´ , Poland
Pozna n´ Science and Technology Park of Adam Mickiewicz University Foundation, Rubie z_ 46, 61-612 Pozna n´ , Poland
b
c
a r t i c l e i n f o
a b s t r a c t
+
À
Article history:
The reaction between ionic liquid [Cat] Cl (where Cat stands for 1-butyl-3-methylimidazolium, 1-butyl-
Received 27 February 2019
Revised 29 April 2019
Accepted 3 May 2019
2
,3-dimethylimidazolium or 1-butyl-4-methylpyridinium) and the metal precursor ([PtCl
2
(cod)], PtCl
4
,
+
À
+
À
6
K
2
[PtCl
4
] or K
2
[PtCl
6
]) yielded two groups of derivatives: [Cat] [PtCl
4
]
and [Cat] [PtCl
] , which formally
are counted among halometallate ionic liquids, however, due to their high melting points they should be
classified into anionic platinum complexes rather than into ionic liquids. All the derivatives were isolated
and characterized spectroscopically (NMR, ESI-MS) and crystallographic structures were determined for
Keywords:
2 4 2 6 2 6
three derivatives: ([BMPy] [PtCl ], [BMIM] [PtCl ] and [BMMIM] [PtCl ]. Moreover, their melting points
Biphasic catalysis
Hydrosilylation
Ionic liquids
Platinum complexes
X-ray diffraction
were measured and thermal stability was assessed. The above derivatives were employed as catalysts for
hydrosilylation of olefins with diverse properties. All the studied catalysts showed high activity and their
insolubility in the reaction medium made easy their isolation and multiple use in subsequent catalytic
runs. The most effective catalysts did not lose their activity even after ten runs, thereby they make a very
good alternative to commonly used homogeneous catalysts. Their simple synthesis and stability make
them interesting both for economic and ecological reasons.
Ó 2019 Elsevier Inc. All rights reserved.
1
. Introduction
The reaction of hydrosilylation is unquestionably one of the
chemistry”. One of examples of catalyst immobilizing agents are
ionic liquids which have been employed with a considerable suc-
cess for over 30 years in a wide range of laboratory scale reactions
[7–9]. A number of excellent review papers as well as books were
devoted to the role played by ionic liquids in catalysis [10–12].
Over 100 types of chemical reactions catalyzed by transition metal
complexes were performed in the presence of ionic liquids [11–
13]. Among them are also reactions of hydrosilylation, however,
the number of literature reports on this aspect of their use is still
small enough [14–19]. Our contribution to this field was the devel-
opment of a number of effective catalytic systems based on diverse
platinum and rhodium complexes immobilized in the most impor-
tant kinds of ionic liquids, namely imidazolium [20,21], phospho-
nium [22,23], morpholinium [24], pyrylium [25] and ammonium
[26] derivatives. All the systems developed by us are insoluble in
the reaction medium, therefore they can be easily isolated from
it and used many times without losing their catalytic activity.
Our research has enabled to select suitable ionic liquids and com-
plexes of rhodium or platinum that formed very active catalytic
systems, however, they have not provided us with information
on the kind of metal complex – ionic liquid interactions and on
the structure of the actual catalyst. This is why we have recently
most important methods for laboratory and industrial syntheses
of various organosilicon compounds [1–4]. It is also extensively
used in polymer chemistry and materials science [5,6].
The hydrosilylation reaction is usually performed in a single-
phase homogeneous system. Because platinum and rhodium com-
plexes are commonly employed as catalysts, therefore the recovery
and recycling of the expensive catalysts make a challenging aspect
of the process. Moreover, sometimes the presence of metals in the
reaction products, even in trace amounts, is unacceptable. Unfortu-
nately, the separation of the catalyst from the post-reaction mix-
ture, particularly in the case of polymeric systems of high
viscosity, is a serious problem. This is why efforts are made at
applying heterogeneous catalysts or immobilized metal complexes
to obtain both high catalytic activity in many recycled runs and
easy product isolation, which is of crucial importance to ‘‘green
⇑
Corresponding author.
E-mail address: maciejm@amu.edu.pl (H. Maciejewski).
https://doi.org/10.1016/j.jcat.2019.05.005
021-9517/Ó 2019 Elsevier Inc. All rights reserved.
0

M. Jankowska-Wajda et al. / Journal of Catalysis 374 (2019) 266–275
267
made attempts to isolate and identify them. However, it has to be
mentioned that this is a formidable task because of the complexity
of the systems and large number of ions. There are, however, con-
siderably simpler systems that have recently attracted our interest.
These are halometallate ionic liquids which can be defined as ionic
liquids formed by the reaction of metal halide with an organic
halide salt. The first representatives of this group of compounds
were chloroaluminate ionic liquids [27], however, at present,
examples are known of the compounds that contain transition
metals such as cobalt, nickel, iridium, gold, palladium as well as
platinum [28–32]. Nevertheless, the number of theses complexes
is still low. In the case of platinum compounds, there are only a few
reports. One of the methods of the synthesis of latter compounds
was based on the reaction of imidazolium chloroaluminate ionic
Point M-565 instrument (Buchi) equipped with a video camera.
Temperature gradient: 10 °C/min. FT-IR in situ measurements were
performed using a Mettler Toledo ReactIR 15 instrument. For
selected samples, spectra were recorded with 256 scans for 1 or
À1
2 h at 30 s intervals with the resolution of 1 cm . Intensity change
À1
of the band at 903 cm , characteristic of Si-H bond, was recorded
using an ATR probe with diamond window. The ICP-MS analysis of
post-reaction samples was carried out on a PerkinElmer Nexion
300D inductively coupled mass spectrometer. The powder XRD
(pXRD) analysis was conducted by using a Brucker D8 Advance
diffractometer equipped with a Johansson monochromator (kCu
Ka1 = 1.5406 Å) and silicon strip detector LynxEye. Minimum mea-
surement angle is 0.6 2 deg.
H
liquids [EMIM]Cl/AlCl
of which [EMIM] [PtCl
3
with PtCl
4
and PtCl at 150 °C as a result
4
[PtCl ] were obtained with
2
2.3. X-ray crystallography
2
6
] and [EMIM]
2
the yields of 80% and 71%, respectively [33]. Additionally, the single
crystal structures of these compounds were determined by X-ray
Data were collected on a New Xcalibur EosS2 diffractometer
using graphite-monochromated Mo radiation
(k = 0.71073 Å). For the data reduction, UB-matrix determination
K
a
diffraction. Another compound, [BMIM]
the yield of 55% using a standard method based on the reaction
between [BMIM]Cl and PtCl in acetonitrile medium under reflux
33]. The crystallographic structure of this compound has also been
determined [34]. Also known is [BMIM] [PtCl ] which was synthe-
sized from K [PtCl ] and [BMIM]Cl (in a large excess) and the pro-
2
[PtCl
4
], was obtained with
a
and absorption correction CrysAlisPro [35] software was used.
Using Olex2 [36], the structures were solved by direct methods
using ShelXT [37] and refined by full-matrix least-squares against
F2 using the program SHELXL [37] refinement package. Non-
hydrogen atoms were refined anisotropically. Hydrogen atoms
were located in idealized positions by molecular geometry and
refined as rigid groups. The positions of Uiso of hydrogen atoms
were set as 1.2 (for C-carries) times Ueq of the corresponding car-
rier atom. Selected structural parameters are presented in
Table S2 in the Supplementary Material.
The data have been deposited with the Cambridge Crystallo-
graphic Data Collection (CCDC), deposition numbers CCDC
1558450, 1,558,451 and 1860948. These data can be obtained free
of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12, Union Road, Cambridge CB2.
2
[
2
6
2
6
duct yield was 57% [33].
The aim of the present study was to synthesize and characterize
chloroplatinate ionic liquids containing different cations and plat-
inum at different oxidation states, as well as to evaluate their cat-
alytic activity for the process of olefin hydrosilylation. Compounds
of this type were never applied as catalysts for the above process,
and since most of hydrosilylation catalysts are based on platinum
compounds, it was advisable to determine their catalytic activity
and possibility of multiple use in subsequent reaction runs. The
present report describes our first efforts to explore this hypothesis.
2
. Experimental
2.4. Synthesis of complexes bis(1-butyl-2,3-dimethylimidazolium)
tetrachloroplatinate(II), [BMMIM] [PtCl
2
4
]
2.1. Materials
Method 1. To a 25 ml Schlenck tube equipped with a magnetic
stirring bar 0.45 g (2.4 mmol) of [BMMIM]Cl and 0.50 g (1.2 mmol)
of [K PtCl ] in hot CH CN (2 ml) were added. The mixture was stir-
2 4 3
red for 3 h under reflux and then, after filtration by a cannula sys-
tem, the solvent was evaporated and the product was dried under
vacuum to yield orange crystals. Product yield: 78% anal. calcd.
Method 2. To a 25 ml Schlenck tube equipped with a magnetic
stirring bar 0.51 g (2.7 mmol) of [BMMIM]Cl and 0.51 g
All reagents applied in catalytic measurements, i.e., 1-octene,
allyl glycidyl ether, n-decane and 1,1,1,3,5,5,5-heptamethyltrisilox
ane were purchased from Sigma Aldrich and used as received. Also
metal precursors: [PtCl
PtCl (for platinum(IV)) were supplied by Sigma Aldrich. The ionic
liquids: 1-butyl-4-methylpyridinium chloride [BMPy]Cl, 1-butyl-
-methylimidazolium chloride [BMIM]Cl, and 1-butyl-2,3-
2 2 4 2
(cod)], K PtCl (for platinum(II)) and K -
6
3
dimethylimidazolium chloride [BMMIM]Cl were purchased from
Iolitec GmbH, Germany.
(
2 3
1.35 mmol) of [PtCl (cod)] in hot CH CN (2 ml) were added. The
mixture was stirred for 3 h under reflux. Then the solution was
cooled to room temperature and the solvent was evaporated. The
orange product was washed with diethyl ether (3 Â 5 ml) and
dried under vacuum. Product yield: 92% anal. calcd.
2
.2. Techniques
The yield of a product of a given reaction was determined using
a Clarus 680 gas chromatograph (Perkin Elmer) equipped with a
2.5. Bis(1-butyl-2,3-dimethylimidazolium) hexachloroplatinate(IV),
3
0 m capillary column Agilent VF-5 ms and TCD detector, using
2 6
[BMMIM] [PtCl ]
À1
the temperature program: 60 °C (3 min.), 10 °C min , 290 °C
(
5 min.). For the products obtained, NMR spectra were made with
Method 1. To a 25 ml Schlenck tube equipped with a magnetic
Bruker BioSpin (400 MHz) spectrometer using acetonitrile-d as a
3
stirring bar 0.56 g (2.9 mmol) of [BMMIM]Cl and 0.5 g (1.5 mmol)
solvent; chemical shifts are given in ppm. ESI-MS spectra were
recorded using a QTOF-type mass spectrometer (Impact HD, Bru-
ker). Thermogravimetric analysis (TGA) was carried out using a
TA Instruments TG Q50 analyzer at a linear heating rate of 10 °C/
min under synthetic air (50 ml/min). The tested samples were
placed in a platinum pan and the weight of the samples was kept
within 9–10 mg. The experimental error was 0.5% for weight and
4 3
of PtCl in hot CH CN (2 ml) were added. The mixture was stirred
for 3 h under reflux. The solution was cooled to room temperature
and the solvent was evaporated. The orange product was washed
with diethyl ether (3 Â 5 ml) and dried under vacuum. Product
yield: 96% anal. calcd.
Method 2. To a 25 ml Schlenck tube equipped with a magnetic
stirring bar 0.4 g (2 mmol) of [BMMIM]Cl and 0.52 g (1.1 mmol)
1
°C for temperature. Melting points were measured on Melting
2 6 3
of K PtCl in hot CH CN (2 ml) were added. The mixture was stirred

2
68
M. Jankowska-Wajda et al. / Journal of Catalysis 374 (2019) 266–275
for 3 h under reflux followed by filtration by a cannula system. The
solvent was evaporated and the product was dried under vacuum
to yield orange crystals. Product yield: 91% anal. calcd.
Method 2. To a 25 ml Schlenck tube equipped with a magnetic
stirring bar 0.39 g (2 mmol) of [BMPy]Cl and 0.56 g (1.1 mmol) of
K PtCl in hot CH CN (2 ml) were added. The mixture was stirred
2 6 3
for 3 h under reflux followed by filtration by a cannula system.
The solvent was evaporated and the product was dried under vac-
uum to yield orange crystals. Product yield: 94% anal. calcd.
2
.6. Bis(1-butyl-3-methylimidazolium) tetrachloroplatinate(II),
[BMIM] [PtCl
2
4
]
Method 1. To a 25 ml Schlenck tube equipped with a magnetic
stirring bar 0.42 g (2.4 mmol) of [BMIM]Cl and 0.50 g (1.2 mmol)
of [K PtCl ] in hot CH CN (2 ml) were added. The mixture was stir-
2
.10. General procedure for catalytic tests
2
4
3
The catalytic activity of the obtained anionic platinumcomplexes
red for 3 h under reflux followed by filtration by a cannula system.
The solvent was evaporated and the product was dried under vac-
uum to yield orange crystals. Product yield: 74% anal. calcd.
Method 2. To a 25 ml Schlenck tube equipped with a magnetic
stirring bar 0.54 g (3 mmol) of [BMIM]Cl and 0.58 g (1.5 mmol) of
was determined in the reaction of hydrosilylation of 1-octeneor allyl
glycidyl ether with 1,1,1,3,5,5,5-heptamethyltrisiloxane (HMTS).
For this purpose, 10 mol of catalyst per 1 mol of Si-H, 3.68 mmol
À4
of 1-octene or 4.41 mmol of allyl glycidyl ether and 3.68 mmol of
HMTS were used. Moreover, 1 mmol of n-decane as an internal stan-
dard was added. The reaction was carried out in a reaction vessel in
the presence of air at 110 °C for 1 h without stirring. The reaction
mixture was then cooled down and subjected to GC analysis to
determine the reaction yield. The product was isolated and sub-
jected to NMR analyses. Due to very small amount of catalyst and
its total insolubility, the post-reaction mixture could be fully
removed using a syringe with needle after the reaction completion.
The catalyst remaining in the reaction vessel was not subjected to
any washing and regeneration. After the complete removal of prod-
ucts, a new portion of the reaction substrates was added to the reac-
tion vessel and the reaction was carried out in the same way as
described above. The above operation was repeated 10 times and
no further reaction runs were performed even if catalyst was still
active. After each catalytic cycle, the post-reaction mixture was ana-
lyzed chromatographically. In each case, the catalysts were insol-
uble and formed a biphasic system with reagents. The color of
products of the reaction with allyl glycidyl ether slightly changed
from colorless to light orange as the reaction progressed and the
most intense change was observed after the first run. In the case of
the reaction with octene, no color change occurred and the mixture
remained colorless.
[
2 3
PtCl (cod)] in hot CH CN (2 ml) were added. The mixture was stir-
red for 3 h under reflux. The solution was cooled to room temper-
ature and the solvent was evaporated. The obtained orange crystals
were washed with diethyl ether (3 Â 5 ml) and dried under vac-
uum. Product yield: 87% anal. calcd.
2.7. Bis(1-butyl-2,3-methylimidazolium) hexachloroplatinate(IV),
[
BMIM] [PtCl
2
6
]
Method 1. To a 25 ml Schlenck tube equipped with a magnetic
stirring bar 0.5 g (2.9 mmol) of [BMIM]Cl and 0.5 g (1.5 mmol) of
PtCl in hot CH CN (2 ml) were added. The mixture was stirred
4
3
for 3 h under reflux. The solution was cooled to room temperature
and the solvent was evaporated. The orange product was washed
with diethyl ether (3 Â 5 ml) and dried under vacuum. Product
yield: 95% anal. calcd.
Method 2. To a 25 ml Schlenck tube equipped with a magnetic
stirring bar 0.36 g (2 mmol) of [BMIM]Cl and 0.5 g (1 mmol) of K
PtCl in hot CH CN (2 ml) were added. The mixture was stirred for
h under reflux followed by filtration by a cannula system. The
2
-
6
3
3
solvent was evaporated and the product was dried under vacuum
to yield orange crystals. Product yield: 93% anal. calcd.
3
. Results and discussion
2.8. Bis(1-butyl-4-methylpyridinium) tetrachloroplatinate(II),
[
BMPy] [PtCl
2
4
]
In our research we have employed dialkylimidazolium (or tri-
alkylimidazolium) and N-alkylpyridinium-based salts as they are
among the most frequently studied ionic liquids due to their easy
synthesis. As metal precursors we have applied compounds of plat-
inum at different oxidation states. In this way we have obtained
two series of tetrachloroplatinate and hexachloroplatinate com-
pounds differing in cations as shown in Scheme 1. Each of the com-
pounds was obtained by two methods.
The first series (tetrachloroplatinates(II)) has been prepared by
two methods based on the use of potassium tetrachloroplatinate or
dichloro(cyclooctadiene)platinum(II). In both cases the reactions
yields were high, however, those obtained in the latter reaction
appeared to be higher because cyclooctadiene released in the reac-
tion can be easily removed by evaporation, contrary to potassium
chloride produced in the former reaction. The second series of
compounds (hexachloroplatinates(IV)) was obtained using potas-
sium tetrachloroplatinate or platinum(IV) chloride. Also in this
case, yields were very high, but those obtained in the reactions
Method 1. To a 25 ml Schlenck tube equipped with a magnetic
stirring bar 0.45 g (2.4 mmol) of [BMPy]Cl and 0.50 g (1.2 mmol)
of [K PtCl ] in hot CH CN (2 ml) were added. The mixture was stir-
red for 3 h under reflux followed by filtration by a cannula system.
The solvent was evaporated and the product was dried under vac-
uum to yield orange crystals. Product yield: 79% anal. calcd.
Method 2. To a 25 ml Schlenck tube equipped with a magnetic
stirring bar 0.54 g (3 mmol) of [BMPy]Cl and 0.58 g (1.5 mmol) of
2
4
3
[
2 3
PtCl (cod)] in hot CH CN (2 ml) were added. The mixture was stir-
red for 3 h under reflux. The solution was cooled to room temper-
ature and the solvent was evaporated. The orange product was
washed with diethyl ether (3 Â 5 ml) and dried under vacuum.
Product yield: 97% anal. calcd.
2.9. Bis(1-butyl-4-methylpyridinium) hexachloroplatinate(IV),
[
BMPy] [PtCl
2
6
]
2 6
with K [PtCl ] were slightly lower. However, the differences were
Method 1. To a 25 ml Schlenck tube equipped with a magnetic
stirring bar 0.55 g (2.9 mmol) of [BMPy]Cl and 0.5 g (1.5 mmol)
of PtCl in hot CH CN (2 ml) were added. The mixture was stirred
for 3 h under reflux. The solution was cooled to room temperature
and the solvent was evaporated. The orange product was washed
with diethyl ether (3 Â 5 ml) and dried under vacuum. Product
yield: 98% anal. calcd.
small compared to those observed in the case of the first series
of compounds. It is important that all the methods make it possible
to obtain, in a very simple way and with high yields, the com-
pounds that are easy to isolate and stable in air. Moreover, it is
4
3
worth mentioning that the synthesis of [BMIM]
use of K [PtCl ] resulted in the yield of 74%, despite the opinion
expressed in the literature [33] that the above product is not
2 4
[PtCl ] with the
2
4

M. Jankowska-Wajda et al. / Journal of Catalysis 374 (2019) 266–275
269
The best evidence that the obtained compounds have the
assumed structure comes from X-ray structural analysis. In the
case of four of the above compounds we were able to grow crystals
and determine their structures. However, because the structure of
[
2 4
BMIM] [PtCl ] has been previously reported in the literature [34],
we will discuss below the three remaining structures.
3.1. Crystal structures of salts [BMPy]
2 4 2 6
[PtCl ], [BMIM] [PtCl ] and
[
BMMIM] [PtCl
2
6
]
Selected bond lengths and angles are given in Table S1 in the
Supplementary Material.
3
.1.1. Crystal structures of salts [BMPy]
2 4
[PtCl ]
X-ray quality crystal of [BMPy] [PtCl ] was grown by slow
2
4
evaporation from acetonitrile. The complex crystallizes as trape-
zoid orange crystals in a monoclinic system with the space group
C2.
Scheme 1. Methods of the synthesis of chloroplatinate ionic liquids applied in the
study.
The asymmetric unit contains one molecule of 4-butyl-1-
2
À
methyl-pyridinium and one-half molecule of PtCl
half generated by an inversion center, which lies at the midpoint of
4
with the other
2
À
the Pt(II) atom. As expected, PtCl
4
ions show square planar geom-
formed in such a reaction. All the obtained compounds are orange
solids that are soluble in chloroform, acetonitrile and water and
insoluble in benzene and ether.
etry with an average Pt-Cl bond length of 2.296 Å. Eight molecules
of cationic 4-butyl-1-methyl-pyridinium surround the anionic
2
À
PtCl in a distorted cubic fashion (Fig. 1a). In turn, each of the
4
2
À
Due to ionic character of the complexes, their identification was
cationic units is surrounded by four PtCl anions (Fig. 1b). Several
4
1
13
based on the comparison of H and C NMR spectra of synthesized
complexes and relevant ionic liquids used for their preparation, as
well as ESI-MS spectra. A clear difference was observed in chemical
shifts of the signal originating from hydrogen atom in position C2,
i.e. that linked to carbon atom located between two nitrogen atoms
in imidazolium salts or between nitrogen atom and carbon atom
C3 in pyridinium salt. We attribute this shift to the formation of
weak hydrogen bonds CAHÁÁÁCl between proton and chlorine atom
close interionic CAHÁÁÁCl type interactions involving both aromatic
and aliphatic protons of the pyridinium moieties (the average
2
À
length 2.841 Å) and chlorine atoms from PtCl were found. Com-
4
parable CAHÁÁÁCl distances are observed for aromatic and aliphatic
protons (from 2.725 to 2.942 Å and from 2.805 to 2.870 Å,
respectively).
In addition, close contacts between cations are observed in the
complex [BMPy] [PtCl ]. The CAHÁÁÁ
2
4
p type interactions between
methyl hydrogen of the butyl moiety and the pyridinium ring sys-
2À
2À
6
originating from either [PtCl
4
]
or [PtCl
]
anion. The difference
in chemical shifts in the newly formed anionic platinum complexes
and ionic liquids depends on the kind of cation and ranges from 0.4
to 0.6 ppm. The presence of hydrogen bond has no significant effect
tem of another cation (CAHÁÁÁC N ring centroid of 2.644 Å) lead to
5
1D arrays of cations in [BMPy] [PtCl ] (Fig. 2).
2
4
13
on chemical shift values in C NMR spectra. The chemical shifts in
2 6 2 6
3.1.2. Crystal structures of salts [BMIM] [PtCl ] and [BMMIM] [PtCl ]
1
H NMR of NCHN on imidazolium rings in [BMIM]
at 9.13–9.11 ppm. In the case of [BMIM] [PtCl ], the chemical shifts
moved to 9.13 ppm. In NMR spectra reported in the literature for
BMIM] [PtCl ], the chemical shifts of NCHN on imidazolium rings
appeared at 9.42 ppm [34] À 8.71 ppm [33], whereas for [BMIM] [-
PtCl ] – at 8.74 ppm [2]. The results obtained by us are convergent
with literature data [33,34].
2
[PtCl
6
] appeared
2 6 2 6
X-ray quality crystals of [BMIM] [PtCl ] and [BMMIM] [PtCl ],
suitable for X-ray crystallographic measurements, were obtained
by slow evaporation of their acetonitrile solutions. Both complexes
2
4
[
2
4
crystallize in the monoclinic system with the space group P2 /c as
1
2
a shiny orange rectangular block. The Pt nucleus lies on a center of
6
symmetry. The coordination sphere of the Pt atom can be
described as a typical octahedral geometry. The structure of both
complexes consists of octahedral hexachloroplatinate(IV) anions
and either 1-butyl-3-methylimidazolium in [BMIM] [PtCl ]
In ESI-MS spectra, visible are MS(+) signals originating from
cations and MS(–) ones coming from anions of the new platinum
complexes. These signals have characteristic shape of a multiplet
due to the presence of three isotopes of platinum. Each signal can
be ascribed to a given isotope on the basis of the intensity corre-
sponding to its percentage in the sample. For all the tested com-
pounds, the most intensive peak in the positive mode
corresponded to the molecular peak of the cations: m/z 139.12
2
6
(Fig. 3a and b) or 1-butyl-2,3-dimethylimidazolium in [BMMIM] [-
2
PtCl ] (Fig. 3c and d) cations, respectively. In both complexes ionic
6
pairs are joined in chains by CAHÁÁÁCl type interactions (Fig. 3b and
d) (relevant bond distances and angles are marked and given in
Fig. 3). The Pt-Cl bond lengths range from 2.318 to 2.322 Å and
are comparable to those found in other hexachloroplatinate(IV)
structures [41,42] (relevant bond distances and angles are given
in Table S1 in the Supplementary material). The imidazolium rings
are essentially planar, the CAC and CAN bond distances are consis-
tent with the values of the known imidazolium salts [43]. The ionic
packing for the [BMIM] [PtCl ] and [BMMIM] [PtCl ] crystal struc-
+
+
+
[
BMIM] , m/z 153 [BMMIM] and m/z 150.13 [BMPy] . In the case
of tetrachloroplatinate complexes, the MS(–) spectra showed the
2
À
2À
4
presence of ions with two negative charges, [PtCl
2
]
and [PtCl ] ,
À
as well as with one negative charge [PtCl
3
] . The most pronounced
signal in the negative spectra of Pt(II) was m/z 335.84 correspond-
2
6
2
6
2À
2À
À
3
ing to [PtCl
4
]
, but the anions [PtCl
2
]
(m/z 265.15) and [PtCl
]
tures can be described approximately as that of an anti-fluorite lat-
tice. Eight cations surround the anionic unit in a distorted cubic
fashion and the cation unit is surrounded tetrahedrally by four
(
m/z 300.89) were also detected, however, they were always far less
2À
4
intense than those corresponding to [PtCl ] . Di Marco et al.
2
À
[
38,39] and Kebarle [40] postulated that the ions observed in the
PtCl
anions in both ionic liquids. The crystal packing in
6
gas phase can be different from those present in the solution owing
to processes which occur at the threshold between solution and gas
phase and ion-molecule reactions in the gas phase.
[BMIM] [PtCl ] and [BMMIM] [PtCl ] complexes, besides coulom-
bic attractions between positive and negative charges, is signifi-
cantly determined by CAHÁÁÁCl interactions involving CAH atoms
2
6
2
6

2
70
M. Jankowska-Wajda et al. / Journal of Catalysis 374 (2019) 266–275
2
À
Fig. 1. X-ray structures of [BMPy]
b) top view of the cationic unit surrounded by four PtCl
2
[PtCl ] complex: (a) side view of PtCl anion surrounded by cationic 4-butyl-1-methyl-pyridinium moieties in a distorted cubic fashion,
4
4
2À
(
4
anions.
Fig. 2. X-ray structures of [BMPy]
2
[PtCl
4
] complex: (a) CAHÁÁÁp type interactions between methyl hydrogen of the butyl moiety and the pyridinium ring of adjacent cation; (b)
top view of CAHÁÁÁ type interactions which lead to 1D arrays, (c) side view of 1D arrays; anions omitted for clarity.
p
of methyl, butyl and imidazolium moieties of the cation and a chlo-
for H4/5 (ACH@CHA) aromatic protons. The lengths of bonds for
their aliphatic counterparts are ranging from 2.880 to 3.414 Å,
respectively.
2À
rine atom of PtCl
6
anion, which results in the formation of a
supramolecular assembly. Close interionic CAHÁÁÁCl distances are
observed in [BMIM]
2
[PtCl
6
] for aromatic protons (from 2.699 to
We have also performed pXRD on two selected compounds, i.e.
2
2
.815 Å) compared to their aliphatic counterparts (from 2.880 to
2 6 2 6
[BMMIM] [PtCl ] and [BMIM] [PtCl ], and compared the obtained
data with those simulated from the single crystal structure (results
are given in the Supplementary material). The experimental pXRD
.910 Å). In [BMIM]
2
[PtCl
6
] ionic liquid, both H2 (ANACH@NA)
4
/5
and H
(ACH@CHA) protons are involved in hydrogen bonding
interactions. The bond lengths observed between the atoms
involved in these H-bonds can suggest stronger interactions with
2 6
pattern for [BMMIM] [PtCl ] complex exactly corresponds with
that simulated from the single-crystal data which undoubtedly
identifies the species in the bulk. The second studied complex,
2
4/5
2
4/5
H than H (2.699 Å and 2.769 to 2.815 Å for H and H , respec-
tively). The hydrogen bonds between H and Cl ions are shorter than
expected from the sum of the van der Waals radii of H and Cl
atoms, which is 2.95 Å, while weaker interactions CAHÁÁÁCl occur
2 6
i.e. [BMIM] [PtCl ], showed phase compatibility in around 80%.
The difference between the patterns obtained from the powder
XRD and those generated from a single crystal is quite common
and can result from the presence of e.g. more than one phase (poly-
2 6
in [BMMIM] [PtCl ], where lengths are from 2.741 and 2.835 Å

M. Jankowska-Wajda et al. / Journal of Catalysis 374 (2019) 266–275
271
À
Fig. 3. X-ray structures of [BMIM]
view of the cationic unit surrounded by four [PtCl
the cationic unit surrounded by four PtCl
2
[PtCl
6
] and [BMMIM]
2
[PtCl
6
] complexes: (a) side view of PtCl2
6
anion surrounded by eight 1-butyl-3-methylimidazolium cations; (b) top
2À
2À
6
6
]
anions; (c) side view of [PtCl
]
anion surrounded by four 1-butyl-2,3-dimethylimidazolium cations; (d) top view of
anions with marked lengths of CAHÁÁÁCl type interactions.
2À
6
morph) of this complex in the bulk material. In such cases each
polymorph has its own characteristic arrangement of atoms within
the material examined that strongly influences the resulting pat-
tern. On the other hand, the difference in reflection intensities
between the simulated and experimental patterns comes from
variation in preferred orientation of the powder samples during
collection of the experimental pXRD data.
increase in the series corresponding to the cations: [BMIM] <
[BMPy] < [BMMIM]. All the above compounds are formally classified
into the group of halometallate ionic liquids, however, their melting
points which exceed 128 °C make it difficult to classify them among
ionic liquids, whose melting points should be below 100 °C. This is
why it would be better to call them anionic platinum complexes.
The thermal stability of the above compounds was evaluated by
thermogravimetric analysis (TGA) and the results are shown in
Fig. 4. Decomposition temperatures were estimated by taking tem-
perature at 10% of weight loss (Table 2). Such a value of weight loss
was chosen to discriminate between decomposition temperature
3.2. Melting points and thermal stability of the obtained compounds
Melting points of all the synthesized compounds were deter-
mined and the results of the measurements were presented in
Table 1.
According to literature data, melting points of such compounds
depend on many factors, among others on the symmetry of cations
Table 1
Melting points of the synthesized chloroplatinate ionic liquids.
o
[
44] and anions [34]. The higher symmetry, the higher melting point.
Compound
BMIM] [PtCl
Melting point [ C]
A comparison of melting points (Table 1) of compounds containing
the same cation show that melting points of hexachloroplatinates
are higher than those of tetrachloroplatinates because the symme-
[
2
4
]
128
170
165
185
130
180
[BMIM] [ PtCl ]
[BMMIM]
[BMMIM] [PtCl ]
2
6
2
4
[PtCl ]
2
À
2À
try of [PtCl
4h symmetry). On the other hand, when taking into consideration
compounds with the same anion, one can notice that melting points
6
]
anion is higher (O
h
) than that of [PtCl
4
]
anion (with
2
6
[
[
BMPy]
BMPy]
2
[PtCl
4
]
]
D
2
[PtCl
6

2
72
M. Jankowska-Wajda et al. / Journal of Catalysis 374 (2019) 266–275
Fig. 4. Thermogravimetric curves of anionic platinum complexes.
Table 2
Decomposition temperatures of compounds at 10% of weight loss.
3.3. Catalysis
All the synthesized anionic platinum complexes were employed
as catalysts for olefin hydrosilylation. The reactants were 1-octene,
which is nonpolar and hydrophobic, and allyl glycidyl ether, which
is polar and hydrophilic. Both olefins were subjected to hydrosilyla-
tion with 1,1,1,3,5,5,5-heptamethyltrisiloxane (HMTS) (Scheme 2).
Chromatographic analysis of post-reaction mixtures showed
that in each case only the product of b-addition was formed. No
Compound Decomposition temperature [°C]
[
[
[
[
[
[
[
[
[
BMMIM]Cl
277
208
249
244
218
222
234
247
230
BMMIM]
BMMIM]
BMIM]Cl
2
[PtCl
[PtCl
4
]
]
2
6
BMIM]
BMIM]
2
[PtCl
[PtCl
4
]
]
2
6
BMPy]Cl
other side products, e.g.
tion were observed.
a-adducts or products of olefin isomeriza-
BMPy]
BMPy]
2
[PtCl
[PtCl
4
]
]
2
6
To determine catalytic activity and optimize reaction condi-
tions, the FT-IR in situ analysis was performed for the reaction of
octene hydrosilylation. Real-time monitoring of the reaction cat-
alyzed by the platinum complexes using an in situ FT-IR probe
enabled to determine respective reaction profiles and show differ-
ences in their activity. Fig. 5 presents the change in the hydrosily-
lation product yield as a function of time as observed for respective
reactions catalyzed by anionic platinum complexes (at the concen-
and water desorption temperature. An example illustrating the
problem of water desorption can be the TGA curve of [BMMIM] [-
PtCl ] (Fig. 4), where weight loss is observed already at a relatively
low temperature (above 70 °C), however, when the criterion of 10%
weight loss is applied, then the temperature is 208 °C and this is in
agreement with literature data [34].
2
4
À5
tration 10 mol Pt/mol SiAH), conducted at 110 °C.
The obtained results show that in the case of imidazolium
derivatives the decomposition temperatures of chloroplatinates
The conducted research unequivocally shows that the activity
of respective catalysts differs, but mainly due to the induction per-
iod, whereas most of the catalysts at the moment of the activation
enable a very fast reaction course (vertical courses of the curves).
Differences in the induction periods can be associated with diffu-
sion processes (characteristic of heterogeneous processes), particu-
larly in the absence of the reagent stirring. Taking into account that
the reaction times for respective systems differed so much, we took
one hour as the time interval enabling a proper comparison of cat-
alytic activity.
(
both tetra- and hexachloroplatinates) are lower than those of
starting chlorides, albeit decomposition temperatures of chloro-
platinates are higher compared to tetrachloroplatinates. On the
other hand, in the case of pyridinium derivatives, the decomposi-
tion temperatures of both chloroplatinates and starting chlorides
are close to each other and range between 230 and 247 °C. Never-
theless, one can state that all the obtained platinum complexes are
stable up to 200 °C.
Scheme 2. Hydrosilylation of olefins with 1,1,1,3,5,5,5-heptamethyltrisiloxane.

M. Jankowska-Wajda et al. / Journal of Catalysis 374 (2019) 266–275
273
Fig. 5. The change in the hydrosilylation product yield as a function of time for the reaction catalyzed by anionic platinum complexes.
The catalysts studied by us are characterized by a high activity.
For the sake of comparison, we conducted the reactions of hydrosi-
lylation of octene and allyl glycidyl ether in the presence of the
concentration because of an easier separation of the catalyst (how-
ever, even in the latter case, the amount of catalyst was still very
small compared to the reagents).
metal precursor alone (i.e. K
uids (in the amount of 10% relative to the amount of all substrates
of hydrosilylation reaction). The reactions catalyzed by K PtCl
2
PtCl
4
) and in the presence of ionic liq-
The product yields in subsequent runs of 1-octene hydrosilyla-
tion are presented in Fig. 6.
2
4
2 4
In the first run, all catalysts, except [BMPy] [PtCl ], enabled to
proceeded with very low yields, 13% with octene and 17% with allyl
glycidyl ether, respectively. On the other hand, the platinum com-
plex dissolved in respective ionic liquids showed relatively high
activity only in the liquid [BMPy]Cl, whereas in the remaining liq-
uids the yields were considerably lower compared to the com-
plexes studied. The results are given in the Supplementary
material.
The catalysts do not dissolve in the reagents. For this reason
easy separation from the reaction mixture and reuse in a subse-
quent reaction run were possible. In this way 10 reaction runs
obtain the product with the yield above 90%. In the subsequent
runs, the yields changed but to a different extent. For the catalysts
[BMIM] [PtCl ] and [BMPy] [PtCl ] the yield was almost constant
2 6 2 6
and after 10 runs exceeded 90%. It is worth mentioning that the
catalytic measurements were finished after 10 runs, although most
of the catalysts were still highly active. Turnover numbers (TON)
for each catalyst after 10 reaction runs are shown in Table 3 to easy
compare the catalytic activity.
When comparing complexes containing the same cation, one
can notice that in each case the complexes with hexachloroplati-
nate anion are more active. Moreover, all complexes containing
the hexachloroplatinate anion show similar high activity, whereas
tetrachloroplatinate-containing complexes are more diverse in this
respect. On the other hand, when comparing complexes that con-
tain the same anion (particularly in the case of imidazolium deriva-
(
using the same catalyst portion in the successive runs) were car-
ried out. Albeit it should be mentioned that the FT-IR in situ mea-
À5
surements were carried out for the catalyst concentration of 10
mol per 1 mol SiAH, whereas further measurements with the use
of the same portion of catalyst were performed for 10 times greater
Fig. 6. Yields of the product of hydrosilylation of 1-octene with 1,1,1,3,5,5,5-heptamethyltrisiloxane as determined for 10 subsequent reaction runs catalyzed by the same
catalyst portion.

2
74
M. Jankowska-Wajda et al. / Journal of Catalysis 374 (2019) 266–275
Table 3
Table 4
Yields of the product and TON values for hydrosilylation 1-octene with HMTS,
catalyzed by anionic platinum complexes.
Yields of the product of hydrosilylation of allyl glycidyl ether with HMTS catalyzed by
anionic platinum complexes.
Catalyst
Yield [%]
Total TON
Catalyst
Yield [%]
Total TON
+
2À
2À
+
2À
2À
[
[
[
[
[
[
BMIM]
2
[PtCl
[PtCl
4
]
]
]
92 (92, 90, 86, 86, 82, 71, 71, 64, 63)
97 (96, 95, 95, 95, 95, 95, 94, 92, 90)
78 (78, 78, 78, 76, 58, 16, 13, 9, 7)
97 (96, 96, 96, 95, 95, 95, 94, 94, 94)
94 (94, 92, 90, 83, 80, 77, 77, 77)
95 (95, 95, 95, 95, 95, 95, 95, 90, 87)
79 700
94 400
49 100
95 200
85 400
93 700
[BMIM]
[BMIM]
[BMPy]
[BMPy]
2
[PtCl
[PtCl
4
]
]
]
73 (73, 73, 73, 73, 73, 73, 73, 73, 73)
97 (87, 87, 87, 82, 71, 8, 2, 0)
62 (45, 45, 43, 43, 43, 43, 43, 39, 37)
97 (93, 88, 88, 81, 66, 57, 33, 0)
98 (91, 91, 91, 90, 21, 6, 0, 0, 0)
98 (98, 97, 80, 59, 12, 4, 0, 0, 0)
73 000
52 100
44 300
60 300
48 800
44 800
+
+
BMIM]
2
6
2
6
BMPy]+
[PtCl
2À
2À
+
[PtCl
[PtCl
2À
2À
2
4
2
4
BMPy]+
]
+
]
2
[PtCl
6
2
6
+
2À
2À
+
2À
2À
BMMIM]
BMMIM]
2
[PtCl
[PtCl
4
]
]
[BMMIM]
[BMMIM]
2
[PtCl
[PtCl
4
]
]
+
+
2
6
2
6
À4
À4
Reaction conditions: [HASi]:[CH@CH]:[Pt] = 1:1:10 ; T = 110 °C, t = 1 h.
Reaction conditions: [HASi]:[CH@CH]:[Pt] = 1:1.2:10 , T = 110 °C, t = 1 h.
tives), one can state that such significant differences in the cat-
alytic activity do not occur. In particular, [PtCl ] anion-containing
6
complexes have shown a very similar activity. A potential possibil-
ity of the formation of NHC carbene complex in the reaction with
run. The ICP-MS analysis has shown (for results see Supplementary
material) that after the first run of the reaction with octene 0.32%
of the initial platinum content was leached to the post-reaction
mixture, whereas after the fifth run the amount of leached plat-
inum was non-measurable. In the case of the reaction with allyl
glycidyl ether as much as 2.5% of the initial platinum was leached
after the first run, whereas after the fifth run it was a bit smaller,
namely 1.9%. Assuming that each run of the reaction with allyl gly-
cidyl ether resulted in leaching the same amount of platinum, one
can conclude that platinum content after the fifth run was too low
to effectively catalyze the reaction.
[
BMIM] does not occur in this case (due to conditions at which
hydrosilylation reaction was carried out), therefore one can say
that blocking of the C2 position in [BMMIM] does not affect the
activity of the produced catalysts.
The most stable and reproducible catalyst of the studied reac-
tion is [BMPy]
taining the same cation [BMPy]
2
[PtCl
6
], however, tetrachloroplatinate complex con-
[PtCl ] has the lowest activity. One
2
4
of the reasons for decreasing yield is catalyst leaching by reagents
which results in a decrease in the catalyst concentration after the
isolation of reaction products. Among factors influencing catalyst
leaching are its hydrophobic/hydrophilic properties. In the case
of octyltrisiloxane, which is hydrophobic, the leaching of ionic cat-
alyst is small. The higher platinum oxidation state (and thereby
stronger ionic effect), the lower catalyst solubility which is
reflected by better reproducibility of the catalytic activity of hex-
achloroplatinate anion-containing complexes. Some confirmation
of the above comes from results obtained in the reaction of
hydrosilylation of allyl glycidyl ether which has polar character
Tetrachloroplatinate complexes are characterized by higher sta-
bility and reproducibility (despite a lower activity in subsequent
runs). This is why the highest activity, when comparing TON val-
2 4
ues, was shown by the complex [BMIM] [PtCl ].
The conducted studies made it possible to select catalysts which
are characterized by very high activity. The aforementioned cata-
lysts, due to the possibility of their isolation and multiple use,
make a very good alternative to homogeneous catalysts applied
hitherto.
(Fig. 7 and Table 4). In the first runs of the latter reaction, higher
4
. Conclusion
activity was shown by hexachloroplatinate complexes, however,
after the fifth run their activity decreased considerably as a result
of easier leaching caused by a greater compatibility of polar pro-
duct with ionic complex having a higher partial charge. To verify
this observation, platinum content in the post-reaction mixture
was determined by ICP-MS both for the reaction of hydrosilylation
of octene and allyl glycidyl ether catalyzed by [BMIM][PtCl ] after
6
the 1-st and 5-th run of the reactions. In both reactions studied, the
latter catalyst was characterized by the highest yield in the first
Reactions of imidazolium and pyridinium ionic liquids with
simple salts or complexes of platinum yielded six compounds,
which formally could be classified into halometallate ionic liquids.
However, their melting points that exceed 100 °C do not meet the
definition of ionic liquids and therefore they should be regarded
rather as anionic complexes of platinum. Nevertheless, taking into
consideration their similarity to halometallate ionic liquids (as
concerns synthesis) we put the term ‘‘ionic liquids” (although in
Fig. 7. Yields of the product of hydrosilylation of allyl glycidyl ether with 1,1,1,3,5,5,5-heptamethyltrisiloxane during 10 subsequent reaction runs catalyzed by the same
catalyst portion.

M. Jankowska-Wajda et al. / Journal of Catalysis 374 (2019) 266–275
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Financial support from the National Science Center (Poland), grant
OPUS UMO-2014/15/B/ST5/04257, is gratefully acknowledged.
[
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Declaration of interest
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