Platinum and rhodium complexes ligated by imidazolium-substituted phosphine as efficient and recyclable catalysts for hydrosilylation

Article abstract of DOI:10.1039/c9ra05948b

Phosphine ligands functionalized with imidazolium salt were prepared and used for the synthesis of two new ionic Pt(0) complexes and four Rh(i) complexes. The catalysts show very good catalytic activity in hydrosilylation reaction of olefins of different polarities (1-octene and allyl glycidyl ether) with 1,1,1,3,5,5,5-heptamethyltrisiloxane. Their insolubility in the reaction medium facilitated their isolation and permitted their multiple use in subsequent catalytic runs. In hydrosilylation of nonpolar olefins, all the catalysts showed similar activity, while in hydrosilylation of polar olefins the catalysts containing the bromide anion showed higher activity. The results permitted identification of the most effective catalysts for hydrosilylation of olefins of different polarities. The most active complexes did not lose their activity even after 10 catalytic runs, thereby providing a very good alternative to the commonly used homogeneous catalysts.

Full text of DOI:10.1039/c9ra05948b

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PAPER
Platinum and rhodium complexes ligated by
imidazolium-substituted phosphine as efficient and
Cite this: RSC Adv., 2019, 9, 29396
recyclable catalysts for hydrosilylation
a
a
a
*
Magdalena Jankowska-Wajda,
Olga Bartlewicz,
Andrea Szpecht,
b
b
ab
Adrian Zajac,
Marcin Smiglak
and Hieronim Maciejewski
Phosphine ligands functionalized with imidazolium salt were prepared and used for the synthesis of two
new ionic Pt(0) complexes and four Rh(I) complexes. The catalysts show very good catalytic activity in
hydrosilylation reaction of olefins of different polarities (1-octene and allyl glycidyl ether) with
1,1,1,3,5,5,5-heptamethyltrisiloxane. Their insolubility in the reaction medium facilitated their isolation and
permitted their multiple use in subsequent catalytic runs. In hydrosilylation of nonpolar olefins, all the
catalysts showed similar activity, while in hydrosilylation of polar olefins the catalysts containing the
bromide anion showed higher activity. The results permitted identification of the most effective catalysts
for hydrosilylation of olefins of different polarities. The most active complexes did not lose their activity
Received 31st July 2019
Accepted 7th September 2019
DOI: 10.1039/c9ra05948b
even after 10 catalytic runs, thereby providing
homogeneous catalysts.
a very good alternative to the commonly used
rsc.li/rsc-advances
processes in biphasic liquid–liquid systems.7,8 The use of ionic
liquids as solvents for chemical reactions is well documented in
literature and has been for a long time the area of interest.9 Due to
their unique properties; mainly high polarity, extremely low vola-
tility, high thermal stability, ionic liquids allow realization of
chemical processes in new pressure and temperature regimes,
oen acting not only as solvents but also catalysts, catalyst immo-
bilizers or initiators.10–13 ILs (ionic liquids) have been employed in
most of catalytic processes in the eld of organic chemistry, but as
yet their use in the synthesis of organosilicon compounds has been
still poorly perceptible.14–19 There are barely a few literature reports
on the application of ILs to processes of hydrosilylation with poly-
hydrosiloxanes.15–17 Studies on the addition of hydrosiloxanes to
functional olens, catalyzed by diverse platinum complexes in the
presence of imidazolium ILs, have been among the rst in this
area.15,18 The contribution of our research group to this eld has
concerned the processes of hydrosilylation of diverse olens cata-
lyzed both by rhodium and platinum complexes in the medium of
imidazolium,20,21 phosphonium,22,23 morpholinium,24 pyrilium25
and ammonium ionic liquids.26,27 In most cases, ILs can well
dissolve and immobilize metal complexes and, at the same time,
they are insoluble in reactants thus enabling performance the
processes in biphasic systems and easy isolation of products (aer
the reaction) offering the possibility of reusing the catalytic system.
Our research has enabled a selection of suitable ionic liquids and
complexes of rhodium or platinum that formed very active catalytic
systems, however, they have not provided us with the information
on the kind of metal complex and ionic liquid interactions and on
the structure of the actual catalyst. It should be mentioned that this
is a formidable task because of the complexity of the systems and
Introduction
The hydrosilylation process is widely applied in the synthesis of
industrially employed organofunctional silicon compounds
(silanes, polysiloxanes and silsesquioxanes) and also in the
synthesis of important reagents for organic synthesis.1–4 It is also
commonly used as a commercially important method of cross-
linking of silicone rubbers as well as organic polymers containing
vinyl or allyl groups with siloxanes and polysiloxanes with Si–H
functionality.5,6 In most cases the catalysts used in the process are
transition metal complexes, particularly compounds and
complexes of platinum and rhodium. Although both rhodium and
platinum complexes are highly active catalysts, they are homoge-
neous. High prices of the above metals and inadmissibility of their
presence (for many applications) even in trace amounts in the nal
product, imply a continuous search for methods of their recycling
and reuse. Classical methods based on distillation (rectication) of
post-reaction mixtures followed by catalyst regeneration are very
costly and oen difficult to perform because of high viscosity and
boiling point of reaction product (particularly the polymeric prod-
ucts). Thus much effort is devoted to the search for new active
catalysts that would combine the advantages of homogeneous and
heterogeneous catalysis. An example of such a solution is the
application of ionic liquids that make it possible to conduct
aLaboratory of Chemistry and Technology of Inorganic Polymers, Faculty of Chemistry,
Adam Mickiewicz University in Poznan, Uniwersytetu Poznanskiego 8, 61-614 Poznan,
Poland. E-mail: magdajw@amu.edu.pl
bPoznan Science and Technology Park of Adam Mickiewicz University Foundation,
Rubiez 46, 61-612 Poznan, Poland
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a large number of ions. An alternative way for obtaining hetero- chloroform (CDCl3) and acetonitrile (CD3CN) were purchased from
genized metal complexes in ILs media is the application of metal Sigma Aldrich.
complexes in which ionic liquids act as ligands. As indicated above
the use of ionic liquids as solvent media for carrying out the cata-
lytic reactions is well documented in literature. Taking into the
account the possible tuning of the physical properties of ionic
liquids through changes in their structure, it is surprising that the
research on the use of ionic liquids as structural components of the
complexing ions has been much less explored. For example,
diphenylphosphine functionalized phosphonium ionic liquids
have been presented by Moores et al., as an excellent example of
using ionic liquids as rhodium complex stabilizer in the hydro-
formylation reaction.28 In a similar fashion Jin et al. have prepared
chiral pyrrolidinodiphosphine ligands attached directly to the
imidazolium cation and used in the catalytic asymmetric hydro-
genation reaction.29 In another example, Chen et al. have presented
the synthesis and use of novel ionic complex, bis-[1-butyl-2-diphe-
nylphosphanyl-3-methylimidazolium]-tetrachloridorhodium(III)
hexauorophosphate, which was used in the hydroformylation
reaction.30 Taking into consideration that the best effects have been
observed for complexes immobilized in imidazolium and phos-
phonium liquids, we have decided to modify the ionic liquid so that
it could be employed as a ligand in the synthesis of complexes.
The aim of the present study was to synthesize and charac-
terize rhodium and platinum complexes containing these kinds
of ligands, as well as to evaluate their catalytic activity for the
process of olen hydrosilylation. For this purpose we synthe-
sized phosphonium ligands functionalized with imidazolium
salt. Similar syntheses have been presented earlier by Consorti31
and Lai.32 At the next stage we applied the obtained phosphines
as ligands for the synthesis of rhodium and platinum
complexes. Since most of hydrosilylation catalysts are based on
platinum and rhodium compounds, we also determined their
catalytic activity and the possibility of multiple use in subse-
quent reaction runs.
General methods
All used reagents and solvents of high purity were commercially
available, purchased and used as received. All reactions for
synthesis of phosphine ligands and respective complexes were
carried out under a dry argon atmosphere using Schlenk
glassware and vacuum-line techniques. Before use the solvents
were dried and degassed by standard methods.
1H and 31P NMR spectra were measured using a Bruker BioSpin
spectrometer operating at 400, 500 and 600 MHz in commercially
available deuterated solvents. Chemical shis for 1H are given
in ppm relative to the residual proton signal of either CDCl3 (d
7.24), CD3CN (d 2.10) and for DMSO-d6 (d 2.49). 31P NMR chemical
shis were specied relative to 85% H3PO4 as an external stan-
dard. The yield of a product of a given reaction was determined
using a Clarus 680 gas chromatograph (Perkin Elmer) equipped
with a 30 m capillary column Agilent VF-5ms and TCDꢁd1etector,
using the temperature program: 60 ꢀC (3 min), 10 ꢀC min , 290 ꢀC
(5 min). Thermogravimetric analysis (TGA) was carried out using
an analyzer TA Instruments model TG Q50 at a linear heating rate
of 10 ꢀC minꢁ1 under synthetic air (50 mL minꢁ1). 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 1 C for temperature. Melting points were
measured on a Melting Point M-565 instrument (Buchi) equipped
with a video camera. Temperature gradient: 10 ꢀC minꢁ1. FT-IR in
situ measurements were performed using a Mettler Toledo ReactIR
15 instrument. For selected samples, the spectra were recorded
with 256 scans for 1 at 30 second intervals with a resolution of
1 cmꢁ1. Intensity change of the band at 903 cmꢁ1, characteristic of
Si–H bond was recorded by using an ATR probe with a diamond
window. ICP-MS analysis of post-reaction mixture was carried out
on a Perkin Nexion 300D inductively coupled mass spectrometer.
The reaction mixture aer the hydrosilylation reaction, was cooled
and removed from the system using a syringe with the needle. The
samples subjected to quantitative analysis, due to the high
concentration of the standard samples, were diluted 10 and 100
times. In order to determine the concentration of rhodium and
platinum in the analysed samples, a four-point calibration curve
was made and the concentration of the analysed elements was
determined on its basis.
Experimental
Materials
All reagents applied in the synthesis of imidazolium salts,
synthesis of phosphine ligands and catalytic measurements and
i.e., dibromobutane, allyl chloride, diphenylphosphine (HPPh2),
2,20-azobis(2-methylpropionitrile) (AIBN), potassium diphenyl-
phosphide solution (KPPh2, 0.5 M in THF), silver nitrate solution
(AgNO3, 0.1 M), 1-octene, allyl glycidyl ether, n-decane and
1,1,1,3,5,5,5-heptamethyltrisiloxane (HMTS) were purchased from
Synthesis of imidazolium salts
3-(4-Bromobutyl)-1,2-dimethylimidazolium bromide (1).
Sigma Aldrich and used as received. Also metal precursors Dibromobutane (30 g, 0.139 mol) has been poured into a round-
[Pt2{(CH2]CHSiMe2)2O}3], [RhCl(PPh3)3], [{Rh(m-Cl)(cod)}2], used bottomed ask equipped with a reux condenser and a syringe,
for the synthesis of new complexes were supplied by Sigma and dissolved in CH3CN (150 mL) under an argon atmosphere.
ꢀ
Aldrich. The ionic liquid 1,2-dimethylimidazole and aqueous The solution was heated to 60 C. A solution of 1,2-dimethyli-
solution of lithium bis(triuoromethanesulfonyl)amide (LiNTf2, midazole (1.92 g, 0.02 mol) in CH3CN (30 mL) was added
80%) was purchased from Iolitec GmbH, Germany. Acetonitrile dropwise through a syringe into the vigorously stirred solution
HPLC grade (CH3CN) was purchased form Sigma Aldrich and used of dibromobutane. The reaction was carried out overnight. A
as received, as well as all other solvents i.e., dichloromethane white precipitate (by-product) was removed by ltration under
(DCM), tetrahydrofuran (THF), ethyl acetate (AcOEt), diethyl ether reduced pressure, and washed with small portions of cold
(Et2O). Deuterated solvents dimethyl sulfoxide (DMSO-d6), CH3CN. The solutions from ltration and washes were
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combined, and the solvent was removed under reduced pres- AIBN (10 mg, 0.06 mmol) were added to the ionic liquid and
sure using a rotary evaporator. The crude product was washed stirred at 70 ꢀC for 72 h, during that time AIBN (40 mg, 0.24
with AcOEt (25 mL ꢂ 3) and Et2O (25 mL ꢂ 3), and then dried mmol) was recharged twice. Volatiles were removed and then
overnight, 60 ꢀC under high vacuum. Subsequently, it was the product was dried under high vacuum. Obtained yellow oil
recrystallized from a mixture of acetonitrile and ethyl acetate was analyzed by NMR spectroscopy. Yield: 80%.
(60ꢀ: 40), and the puried product was dried again overnight at
1H NMR (600 MHz, DMSO-d6): d ¼ 7.79 (m, 4H, Ar–H), 7.63
(d, J ¼ 2.0 Hz, 2H, imidazolium –CH]), 7.58 (m, 6H, Ar–H), 4.23
60 C, under high vacuum. Yield: 85%.
1H NMR (400 MHz, DMSO-d6): d ¼ 7.81–7.46 (m, 2H, imi- (t, J ¼ 7.1 Hz, 2H, –CH2–), 3.72 (s, 3H, CH3), 2.54 (s, 3H, CH3),
dazolium –CH]), 4.16 (t, J ¼ 6.7 Hz, 2H, CH2), 3.74 (s, 3H, CH3), 2.45 (m, 2H, –CH2–), 1.90 (m, 2H, –CH2–).
3.56 (s, 2H), 2.58 (s, 3H, CH3), 1.83 (dd, J ¼ 6.2 Hz, J ¼ 3.1 Hz,
4H, CH2–CH2).
31P NMR (242 MHz, DMSO-d6): d ¼ ꢁ17.53.
3-(4-(Diphenylphosphanyl)butyl)-1,2-dimethylimidazolium brom-
3-Allyl-1,2-dimethylimidazolium chloride (2). A solution of 1- ide (5). The solution of 3-(4-bromobutyl)-1,2-dimethylimidazolium
2-dimethylimidazole (13.92 g, 0.145 mol) in MeCN (150 mL) was bromide (1) (2.33 g, 4.55 mmol) dissolved in THF (12 mL) was
placed in a two-neck round-bottomed ask equipped with placed in a Schlenk ask. Then the Schlenk ask was ushed with
a reux condenser in one neck and septum in the other and argon. The solution was heated up to 50 ꢀC and then a solution of
ꢀ
heated to 60 C upon stirring on a magnetic stirrer. Then allyl KPPh2 in THF (0.5 M, 10 mL) was added dropwise to the vigorously
chloride (11.63 g, 0.152 mol) was added dropwise (with adꢀdition stirred ionic liquid solution and the mixture was le stirred over-
rate 5 mL hꢁ1). The resulting mixture was stirred at 60 C for night. Then, the solvent was removed under high vacuum. The
96 h. Next, the solvent was evaporated using a rotary evaporator, remaining solid was washed with AcOEt (10 mL ꢂ 3), and Et2O
ꢀ
and the obtained product was dried overnight at 60 C, under (10 mL ꢂ 3). Every time the mixture was decanted off, and at the
vacuum. Yield: 98%.
end ltered off through a cannula. White solid was obtained and
3
1H NMR (400 MHz, DMSO-d6): d ¼ 7.84 (d, J ¼ 2.0 Hz, 1H, analyzed by NMR spectroscopy. Yield: 85%.
imidazolium –CH]), 7.77 (d, J ¼ 2.0 Hz, 1H, imidazolium
1H NMR (600 MHz, DMSO-d6): d ¼ 7.60 (d, J ¼ 2.4 Hz, 2H,
–CH]), 5.95 (ddt, J ¼ 17.1 Hz, J ¼ 10.7 Hz, J ¼ 5.2 Hz, 1H, vinyl imidazolium –CH]), 7.38 (m, 6H, Ar–H), 7.20 (m, 4H, Ar–H),
–CH]), 5.25 (dd, J ¼ 10.4 Hz, J ¼ 1.0 Hz, 1H, vinyl –CH–H), 5.14 4.13 (dt, J ¼ 14.6 Hz, J ¼ 7.2 Hz, 2H, CH2), 3.73 (s, 3H, CH3), 2.55
2
(dd, J ¼ 17.2 Hz, J ¼ 1.1 Hz, 1H, vinyl –CH–H), 4.91 (d, J ¼ (d, J ¼ 2.7 Hz, 2H. CH2), 1.85 (dt, J ¼ 14.7 Hz, J ¼ 7.3 Hz, 2H,
5.5 Hz, 2H, –CH2–), 3.81 (s, 3H, CH3), 2.59 (s, 3H, CH3).
3-Allyl-1,2-dimethylimidazolium bis(triuoromethanesulfonyl)
amide (3). 1-Allyl-2,3-dimethylimidazolium chloride (2) (3.00 g,
17.39 mmol) was dissolved in distilled H2O (40 mL) and trans-
ferred into separating funnel. Next, DCM was added (30 mL), fol-
lowed by LiNTf2 (80%, 125 mg, 0.35 mmol). The obtained mixture
CH2), 1.69 (s, 3H, CH3), 1.37 (m, 2H, CH2).
31P NMR (242 MHz, DMSO-d6): d ¼ ꢁ17.54.
Synthesis of transition-metal based complexes
{1,2-Dimethyl-3-(diphenylphosphine)butylimidazoliumbrom
was shaken vigorously for a few minutes. The layers produced were ide} bis(triphenylphosphine)chloridorhodium(I) [RhCl(PPh3)2(-
then separated, and organic layer was washed with distilled H2O P(Ph)2{BMMIM)Br})] Mw ¼ 1080.25 (6). In a Schlenck's tube
(twice) and H2O with a small addition of aqueous solution of equipped with a magnetic stirrer a portion of 0.27 g (0.29 mmol)
LiNTf2 (80%, 0.02 eq.). The washing procedure was repeated as of [Rh(Cl)(PPh3)3] rhodium precursor with phosphine ligand
many times as needed to ensure no Clꢁ ions in the H2O layer 0.12 g (0.29 mmol) (5) was dissolved in 5 mL toluene. The
(checked by addition of aqueous solution of AgNO3 to subsequent mixture was stirred for 24 hours at room temperature and next
H2O layers till no observation of a precipitate AgCl formation). aer ltration through a cannula system the solvent was evap-
Aer that the organic layer ꢀwas evaporated under reduced pressure orated and dried under vacuum to obtain orange-brown solid.
and dried overnight at 60 C, under high vacuum to give pure 3- The product was washed with diethyl ether (3 ꢂ 5 mL) and dried
allyl-1,2-dimethylimidazolium
amide. Yield: 97%.
bis(triuoromethanesulfonyl) under vacuum. Product yield: 91% anal. calcd.
1H NMR (CDCl3) ppm: 9.05 and 8.74 (s, 2H, imidazolium
1H NMR (500 MHz, CDCl3): d ¼ 7.14 (d, J ¼ 2.0 Hz, 1H, –CH]), 7.80–7.18 (m, 40H, Ar–H), 4.15 (dt, 2H, N–CH2), 3.72 (s,
imidazolium –CH]), 7.10 (d, J ¼ 2.0 Hz, 1H, imidazolium 3H, N–CH3), 2.55 (d, J ¼ 3.7 Hz, 2H, CH2), 2.52 (dt, J ¼ 15.7 Hz, J
–CH]), 5.81 (ddt, J ¼ 16.2 Hz, 3J ¼ 10.7 Hz, J ¼ 5.6 Hz, 1H, vinyl ¼ 8.2 Hz, 2H, CH2), 2.3 (s, 3H, CH3), 1.2 (m, 4H, J ¼ 7.5, CH2).
–CH]), 5.28 (d, J ¼ 10.4 Hz, 1H, vinyl –CH–H), 5.13 (d, J ¼
13C NMR (CDCl3) ppm: 138 (NC(CH3)N), 135, 134, 133, 132,
17.1 Hz, 1H, vinyl –CH–H), 4.58 (d, J ¼ 5.6 Hz, 2H, –CH2–), 3.67 131, 130, 129, 128, 127 (Ar–C), 123 (CH]CH), 121 (CH]CH), 49
(s, 3H, CH3), 2.45 (s, 3H, CH3).
(N–CH3), 35 (N–CH2), 32, 28 (CH2), 13 (CH3).
31P(CDCl3) ppm: 28.35 (P–Ar).
{1,2-Dimethyl-3-(diphenylphosphine)butylimidazolium bromide}
(h4-cycloocta-1,5-diene)chloridorhodium(I) [RhCl(cod)(P(Ph)2{(-
Synthesis of phosphine ligands
3-(3-(Diphenylphosphanyl)propyl-1,2-dimethylimidazolium) BMMIM)Br})] Mw ¼ 707.96 (7). In a Schlenck's tube equipped
bis(triuoromethanesulfonyl)amide (4). 3-Allyl-1,2-dimethyli- with a magnetic stirrer, a portion of 0.18 g (0.37 mmol) of
midazoliumbis(triuoromethanesulfonyl)amide (3) (0.51 g, [{Rh(m-Cl)(cod)}2] rhodium precursor with 0.3 g (0.72 mmol)
1.22 mmol) was placed in a Schlenk ask, and then ushed with phosphine ligand (5) was dissolved in 3 mL toluene. The
argon. Next, diphenylphosphine (0.3 g, 1.59 mmol, 0.3 mL) and mixture was stirred for 24 hours at room temperature and next
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aer ltration by a cannula system the solvent was evaporated a portion of 0.11 g (0.22 mmol) of [{Rh(m-Cl)(cod)}2] rhodium
and dried under vacuum to obtain yellow solid. The product was precursor with 0.14 g (0.45 mmol) phosphine ligand (4) was
washed with diethyl ether (3 ꢂ 5 mL) and dried under vacuum. dissolved in 3 mL toluene. The mixture was stirred for 24 hours
Product yield: 89% anal. calcd.
at room temperature and next aer ltration through a cannula
1H NMR (CDCl3) ppm: 8.57 and 8.16 (s, 2H, imidazolium system the solvent was evaporated and dried under vacuum to
–CH]), 7.85–7.18 (m, 10H, Ar–H), 5.45 (m, 2H, J ¼ 7.5, –CH]), obtain yellow oil. The product was washed with diethyl ether (3
4.34 (dt, 2H, N–CH2), 4.24 (m, 4H, ]CH–), 3.71 (s, 3H, N–CH3), ꢂ 5 mL) and dried under vacuum. Product yield: 83% anal.
3.1 (m, 2H, –CH]), 2.6 (d, J ¼ 3.4 Hz, 2H, CH2), 2.2 (dt, J ¼ 13.5, calcd.
2H, CH2), 1.8 (s, 3H, CH3), 1.4 (m, 4H, J ¼ 7.1, CH2).
1H NMR (CDCl3) ppm: 9.05 and 8.74 (s, 2H, imidazolium
13C NMR (CDCl3) ppm: 139 (NC(CH3)N), 135, 132, 131, 129 –CH]), 7.85–7.18 (m, 10H, Ar–H), 4.45 (m, 2H, J ¼ 6.8 Hz,
(Ar–C), 122 (–CH]CH–), 121 (–CH]CH), 101 (cod, ]CH–), –CH]), 4.30 (t, 2H, N–CH2), 4.14 ( m, 4H, ]CH–), 3.70 (s, 3H,
64.8, 64 (cod, –CH]), 47 (N–CH3), 34 (N–CH2), 33, 31 (cod, N–CH3), 3.35 (m, 2H, –CH]), 2.64 (s, 3H CH3), 2.55 (m, 2H,
CH2), 30, 29 (CH2), 14 (CH3).
CH2), 2.1 (m, 4H, CH2), 1.5 (m, 4H, J ¼ 6.7, CH2).
31P(CDCl3) ppm: 26 (P–Ar).
13C NMR (CDCl3) ppm: 140 (NC(CH3)N), 135, 132, 131, 130,
{1,2-Dimethyl-3-(diphenylphosphine)butylimidazolium bro- (Ar–C), 123 (CH]CH), 122 (CH]CH), 90 (cod, ]CH–), 64.9,
mide}(h4-1,3-divinyl-1,1,3,3-tetramethyldisiloxane)platinum(0) 64.3 (cod, –CH]), 46 (N–CH3), 35 (N–CH2), 33, 32 (cod CH2), 30,
[Pt({ViSi(Me)2}O)(P(Ph)2{(BMMIM)Br})] Mw ¼ 828.87 (8). In 28 (CH2), 12 (CH3).
a Schlenck's tube equipped with a magnetic stirrer, a portion of
0.18 g, (0.19 mmol) of Karstedt's catalyst platinum precursor
31P(CDCl3) ppm: 27 (P–Ar).
{1,2-Dimethyl-3-(diphenylphosphine)propylimidazoliumbis(tri-
with 0.08 g (0.19 mmol) phosphine ligand (5) was dissolved in uoromethylsulfonyl)imide}(h4-1,3-divinyl-1,1,3,3-tetramethyldisi-
3 mL toluene. The mixture was stirred for 24 hours at room loxane)platinum(0) [Pt({ViSi(Me)2}2O)(P(Ph)2{(PrMMIM)NTf2})]
temperature and next aer ltration through a cannula system Mw ¼ 1014.08 (11). In a Schlenck's tube equipped with a magnetic
the solvent was evaporated and dried under vacuum to obtain stirrer, a portion of 0.2 g (0.21 mmol) of Karstedt's catalyst plat-
yellow solid. The product was washed with diethyl ether (3 ꢂ 5 inum precursor with 0.07 g (0.21 mmol) phosphine ligand (4) was
mL) and dried under vacuum. Product yield: 86% anal. calcd.
dissolved in 2 mL toluene. The mixture was stirred for 24 hours at
1H NMR (CDCl3) ppm: 8.98 and 8.75 (s, 2H, imidazolium room temperature and aer ltration through a cannula system
–CH]), 7.85–7.18 (m, 10H, Ar–H), 5.9 (d, 2H, CH]CH2), 5.3 (d, the solvent was evaporated and dried under vacuum to obtain
1H, CH]CH2), 4.25 (dt, 2H, N–CH2), 3.71 (s, 3H, N–CH3), 2.6 (d, yellow oil. The product was washed with diethyl ether (3 ꢂ 5 mL)
2H, CH2), 2.5 (dt, J ¼ 13.4, J ¼ 7.8, 2H, CH2), 2.1 (s, 3H, CH3), 1.9 and dried under vacuum. Product yield: 83% anal. calcd.
(m, 4H, J ¼ 6.9, CH2), 0.34 (s, 6H, SiCH3), ꢁ0.25 (s, 6H, SiCH3).
1H NMR (CDCl3) ppm: 9.01 and 8.72 (s, 2H, imidazolium
13C NMR (CDCl3) ppm: 137 (NC(CH3)N), 135, 132, 131, 130 –CH]), 7.85–7.18 (m, 10H, Ar–H), 5.9 (d, 1H, CH]CH2), 5.3 (d,
(Ar–C), 50 (N–CH3), 40.3 (–CH]CH2), 37 (N–CH2), 33.5 (CH] 2H, CH]CH2), 4.25 (t, 2H, N–CH2), 3.70 (s, 3H, N–CH3), 2.5 (s,
CH2), 32, 31 (CH2), 13 (CH3), 1.4 (SiCH3), ꢁ1.9 (SiCH3).
3H, CH3), 2.5 (m, 2H, CH2), 1.91 (m, 2H, J ¼ 7.1, CH2), 0.34 (s,
31P(CDCl3) ppm: 31 (P–Ar).
6H, SiCH3), ꢁ0.25 (s, 6H, SiCH3).
{1,2-Dimethyl-3-(diphenylphosphine)propylimidazoliumbis(tri-
13C NMR (CDCl3) ppm: 138 (NC(CH3)N), 135, 133, 132, 130
uoromethylsulfonyl)imide}bis(triphenylphosphine)chloridorhod (Ar–C), 122 (CH]CH), 121 (CH]CH), 51 (N–CH3), 41 (CH]
ium(I) [RhCl(PPh3)2(P(Ph)2{(PrMMIM)NTf2})] Mw ¼ 1266.47 (9). CH2), 37 (N–CH2), 33.5 (CH]CH2), 32, 30 (CH2), 13 (–CH3), 1.4
In the Schlenck's tube equipped with magnetic stirrer 0.15 g (SiCH3), ꢁ1.9 (SiCH3).
(0.16 mmol) of [Rh(Cl)(PPh3)3] rhodium precursor with 0.05 g
(0.16 mmol) phosphine ligand (4) was dissolved in 5 mL
toluene. The mixture was stirred for 24 hours at room temper-
ature and next aer ltration by a cannula system the solvent
31P(CDCl3) ppm: 32 (P–Ar).
General procedure for catalytic tests
was evaporated and dried under vacuum to obtain brown oil. The catalytic activity of the obtained platinum and rhodium
The product was washed with diethyl ether (3 ꢂ 5 mL) and dried complexes was determined in the reaction of hydrosilylation of
under vacuum. Product yield: 93% anal. calcd.
1-octene or allyl glycidyl ether with 1,1,1,3,5,5,5-heptamethyl-
1H NMR (CDCl3) ppm: 8.89 and 8.88 (s, s, 2H, imidazolium trisiloxane (HMTS). For this purpose, 10ꢁ5 mol of catalyst molar
–CH]), 8.15–7.18 (m, 40H, Ar–H), 4.32 (t, J ¼ 6.4, 2H, N–CH2), mass per 1 mol of Si–H, 3.68 mmol of 1-octene or 4.41 mmol of
3.92 (s, 3H, N–CH3), 2.6 (s, 3H, CH3), 2.52 (m, 2H, CH2), 1.2 (m, allyl glycidyl ether and 3.68 mmol of HMTS were used. In the
4H, J ¼ 7.5, CH2).
system with allyl glycidyl ether, which is more problematic
13C NMR (CDCl3) ppm: 138 (NC(CH3)N), 135, 132, 131, 130, reagent, to avoid formation of side products and obtain full
129, 128, 127 (Ar–C), 123 (–CH]CH–), 121 (CH]CH), 49 (N– conversion of Si–H, 1.2 equivalent of olen to 1 equivalent of
CH3), 35 (N–CH2), 32, 31 (CH2), 12 (CH3).
31P(CDCl3) ppm: 28.34 (P–Ar).
{1,2-Dimethyl-3-(diphenylphosphine)propylimidazoliumbis vessel in the presence of air at 110 C for 1 h, without stirring.
(triuoromethylsulfonyl)imide}(h4-cycloocta-1,5-diene)chlorido- Then the reaction mixture was cooled down and subjected to GC
rhodium(I) [RhCl(cod)(P(Ph)2}(PrMMIM)NTf2})] Mw ¼ 893.16 analysis to determine the reaction yield, which in both cases,
(10). In a Schlenck's tube equipped with a magnetic stirrer, was calculated based on the HMTS area peak. Due to the very
HMTS was added. Moreover, 1 mmol of n-decane as an internal
standard was added. The reaction was carried out in a reaction
ꢀ
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small amount of catalyst the products were taken entirely with
a syringe with the needle. The catalyst remaining in the ask
was not washed or regenerated in any way. Aer complete
removal of the products from the reaction system, another,
same portion of substrates was added, and the reaction was
carried out in the same way. The above operation was repeated
until 10 catalytic cycles were carried out. The product was iso-
lated and subjected to NMR analyses.
1
3-Octyl-1,1,1,3,5,5,5-heptamethyltrisiloxane. H NMR (CDCl3)-
ppm: 0.02 (s, 3H, Si–CH3), 0.11 (m, 18H, Si–(CH3)3), 0.48 (t; 2H;
Scheme 1 Synthesis of 1,2-dimethylimidazolium based ionic liquids 1
Si–CH2), 0.9 (t; 3H; CH2–CH3), 1.36–1.27 (m; 12H; CH2–CH2– and 3.
CH2); 13C NMR (CDCl3) ppm: 0.28 (O–Si–CH3), 1.89 (Si–CH3),
14.10 (C–CH3), 17.63 (C–Si), 22.70 (Si–C–C), 23.07 (C–CH3),
azobis(2-methylpropionitrile)) at 80 ꢀC.31 The reaction was
29.35, 29.27 (C–C–C), 31.95 (C–C–C), 33.25 (C–C–C).
29Si NMR (CDCl3) ppm: ꢁ2.19 (–O–Si–O–), 6.75 (OSi(CH3)3).
3-(3-Glycidyloxypropyl)-1,1,1,3,5,5,5-heptamethyltrisiloxane.
1H NMR (CD3CN) ppm: 0.05 (m, 3H, –SiCH3); 0.13 (m, 18H,
–Si(CH3)3); 0.5 (m, 2H, –Si–CH2); 1.59 (m, J ¼ 11.3 Hz, 2H,–Si–
CH2–CH2–); 2.54 (m, J ¼ 5.1 Hz, 1H, HC–CH2–O); 2.74 (dd, J ¼
5.1 Hz, 1H, HC–CH2–O); 3.08 (m, J ¼ 6.8 Hz, 1H, HC–O–CH2–);
3.27 (dd, J ¼ 11.5 Hz, 1H, –O–CH2–); 3.43 (m, 2H, –CH2–O–CH2–
); 3.69 (m, J ¼ 17.1 Hz, 1H, –O–CH2–).
carried out entirely in the Schlenk ask, without any solvent,
with constant argon ow, in order to avoid oxidation of both
diphenylphosphine and the product. Diphenylphosphine was
used in excess and aer the reaction time, its excess was evap-
orated under vacuum. while heating the mixture for 24 h at
60 ꢀC obtained. Properties of the phosphine ligand obtained by
our group are compatible with those reported in literature.31 3-
(3-(Diphenylphosphanyl)propyl-1,2-dimethylimidazolium)
bis(triuoromethanesulfonyl)amide (4) was obtained in almost
quantitative yield.
The second approach involved nucleophilic substitution of
bromoalkyl-derivative (1) with potassium diphenylphosphide in
THF at 50 ꢀC and led to the formation of desired phosphinated
product. The synthesis was carried out according to literature33
with one main difference, which was to not oxidize the phos-
phine ligand to the phosphine oxide using hydrogen peroxide.
It was imperative to stop at this moment in order to obtain
phosphine ligand with diphenylphosphinyl group as one of the
ligands of the new complexes. 3-(4-(Diphenylphosphanyl)butyl)-
1,2-dimethylimidazolium bromide (5) was obtained in almost
quantitative yield. The product was obtained with the use of
Schlenk line and Schlenk asks, upon continuous argon ow to
also avoid any oxidation. The phosphine ligand obtained in this
method has been reported in literature33 and the properties are
compatible.
13C NMR (CD3CN) ppm: ꢁ1.0 (–Si–CH3), 0.37–1.39
(–Si(CH3)3); 12.77 (–Si–C–); 23.16 (–Si–C–C–); 43.56 (–C–O–C–);
50.71 (–C–O–C–); 71.41 (–O–C–C–); 73.67 (–C–C–O–).
29Si NMR (CD3CN) ppm: ꢁ20.52 (–O–Si–O–), 8.07
(OSi(CH3)3).
Results and discussion
Synthesis of ionic liquids
The rst step was the synthesis of initial nitrogen-based ionic
liquids. 1,2-Dimethylimidazole was treated with two different
alkylating agents, one being 1,4-dibromobutane and the other
one allyl chloride (Scheme 1).
The product of the rst quaternization reaction was 3-(4-
bromobutyl)-1,2-dimethylimidazolium bromide (1), while the
product of the second reaction was 3-allyl-1,2-dimethylimidazolium
chloride (2). These reactions provided us with two different ionic
liquids bearing modiable substituent, in the form of unsaturated
group or bromoalkyl group in quantitative yield. Next, compound
(2) was treated with aqueous solution of lithium bis(tri-
uoromethylsulfonyl)amide (LiNTf2) in two-phase system at room
temperature to furnish pure 1,2-dimethylimidazolium bis(tri-
uoromethylsulfonyl)amide (3) in quantitative yield. Both ionic
liquids have been previously obtained31,33 and their properties have
been conrmed. In the next step, these ionic liquids were used as
substrates in the synthesis of phosphine-based IL's.
Synthesis of rhodium and platinum complexes
The new Rh(I) and Pt(0) complexes, of the general formula 6–11
were synthesized using a diphenylphosphine group connecting
the ionic liquid (4 or 5) as a ligand (Scheme 3 and 4):
Synthesis of phosphine ligands
Previously obtained IL's (1) and (3), allowed us to use two
different synthesis approaches for obtaining phosphine ligands
(Scheme 2).
1,2-Dimethylimidazolium bis(triuoromethylsulfonyl)amide
ionic liquid bearing allyl group (3) underwent the radical chain
addition of diphenylphosphine (HPPh2), assisted by AIBN (2,20- Scheme 2 Synthesis of phosphine ligands 4 and 5.
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Table 1 Melting points of the synthesized phosphine Rh and Pt
complexes and decomposition temperatures of the complexes at 10%
of weight loss
Decomposition
Complex
Melting point [ꢀC]
temperature [ꢀC]
6
7
8
9
10
11
148
141
178
Oil
Oil
Oil
211.16
206.07
222.98
213.17
235.88
272.80
Scheme 3 Synthesis of complexes 6, 7 and 8.
The thermal stability was evaluated by thermogravimetric
analysis (TGA) and the results are shown in Fig. 1. Decompo-
sition temperatures were estimated by taking temperature at
10% of weight loss (Table 1). Such a value of weight loss was
chosen to discriminate between decomposition temperature
and water desorption temperature.
The rhodium(I) complexes were obtained in the reaction of 3-(4-
(diphenylphosphanyl)butyl)-1,2-dimethylimidazolium bromide (5)
or 3-(3-(diphenylphosphanyl)propyl-1,2-dimethylimidazolium)-bis
(triuoromethanesulfonyl)amide (4) with dimer rhodium(I)
complex or Wilkinson's catalyst in toluene. In the same way, the
synthesis of platinum(0) complexes was carried out, using the
Karstedt complex as platinum precursor. All complexes obtained
in the reaction with the ligand containing Brꢁ anion were solid but
The results in Table 1 show that the platinum complexes (8
and 11) have higher decomposition temperatures than the
rhodium complexes, among the complexes with bromide anion
ꢁ
and NTf2
anion. Thus, divinyltetramethyldisiloxane as
ꢁ
those with the ligand containing NTf2 anion were liquids. The
a chelating ligand (present in complexes 8 and 11) can be
supposed to better stabilize the complexes than triphenyl-
phosphine or cyclooctadiene ligands present in the other
complexes. In addition, the complexes with the NTf2ꢁ anion (9–
11) showed higher decomposition temperatures. Nevertheless,
complexes produced were isolated with very high yields in the
range of 83–91%. The obtained complexes were characterized by
1H, 31P and 13C NMR analysis, which conrmed their assumed
structures. In the 31P NMR spectra, a visible difference in the
chemical shi values of the phosphorus atom signal present in the
imidazolium ligand was observed, which indicates the coordina-
tion of the ligand to the rhodium or platinum atom. The signal
shis from ꢁ17 ppm to +26–32 ppm depending on the type of
precursor used. For the solid complexes their melting points were
determined (Table 1).
ꢀ
all complexes have decomposition temperatures above 200 C,
so they should be considered thermally stable. This observation
is important from the point of view of their use in catalytic
processes carried out at elevated temperature and the possi-
bility of their recycle and reuse.
The melting points certainly affects the type of anion,
because as mentioned above only the complexes with thꢁe
bromide anion were solids, while the complexes with the NTf2
anion were liquid at room temperature. When comparing the
melting points of solid complexes, it can be seen that rhodium
complexes have lower melting points than platinum complex.
In addition, the temperatures of both rhodium complexes are
very similar. A similar relationship can be observed for the
thermal stability of the above complexes.
Catalysis
All the synthesized platinum and rhodium complexes were
employed as catalysts for olen hydrosilylation. The reactants
were 1-octene, which is nonpolar and hydrophobic, and allyl
glycidyl ether, which is polar and hydrophilic. Both olens were
subjected to hydrosilylation with 1,1,1,3,5,5,5-heptamethyl-
trisiloxane (HMTS) (Scheme 5).
Fig. 1 Thermogravimetric curves recorded for rhodium and platinum
Scheme 4 Synthesis of complexes 9, 10 and 11.
complexes.
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Scheme 5 Model reaction of hydrosilylation process of 1-octene/allyl
glycidyl ether with 1,1,1,3,5,5,5-heptamethyltrisiloxane.
Chromatographic analysis of post-reaction mixtures showed
that in each case only the product of b-addition was formed. No
other side products, e.g. a-adducts or products of olen isom-
erization were observed. To determine the catalytic activity and
optimize the reaction conditions, the FT-IR in situ analysis was
performed for the reaction of 1-octene (or allyl-glycidyl ether)
hydrosilylation. Real-time monitoring of the reaction catalyzed
by the rhodium and platinum complexes using an in situ FT-IR
probe enabled determination of particular reaction proles and
differences in their activity. Fig. 2 presents the change in the 1-
octene hydrosilylation product yield as a function of time, as
observed for particular reactions catalyzed by phosphine
rhodium and platinum complexes (at the concentration
Fig. 3 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.
platinum complex 11 containing imide anion was poorer,
because the yield dropped to 69% aer the fourth run, however,
the same yield was maintained throughout subsequent runs.
Turnover numbers (TON) for each catalyst aer 10 reaction runs
are shown in Table 2 to easily compare the catalytic activity.
The differences in the activities are not large but one can
notice some advantage of rhodium complexes, in particular
complex 6 and 10 for which the yield in the next 10 cycles was
stable and very high (95 or 96% respectively). It is worth
mentioning that the catalytic measurements were nished aer
10 runs, although these catalysts were still highly active.
The catalyst effectiveness also depends on the kind of olen
used in the reaction of hydrosilylation. In the reaction of allyl
glycidyl ether hydrosilylation, greater differences are visible in
the activity of the studied complexes.
Fig. 4 presenting the change in the product yield as a func-
tion of time, shows that the reaction proceeds with the highest
rate in the presence of platinum and rhodium complexes with
bromide anion, however, the reaction time extends and falls in
the range between 7 and 45 minutes. Moreover, the rhodium
complex with bromide anion (complex 6) is characterized by
one of the longest reaction time. The product yields obtained in
the presence of the studied complexes are about 80%.
Furthermore, complex 10 which in the reaction with octene was
characterized by one of the shorter reaction times, also in this
case showed high activity, albeit in the rst cycle.
10ꢁ5 mol Pt or Rh/mol Si–H), conducted at 80 C.
ꢀ
These results showed that the complexes with bromide
anions (6–8) permitted almost full conversion of reactants
already within the rst minutes of the reaction, whereas the
complexes with amide anion (except 10) were characterized by
a longer reaction time of about 30–35 minutes.
The catalysts are characterized by a high activity and they do
not dissolve in the reagents. For this reason they could be easily
separated from the reaction mixture and reused in a subsequent
reaction run. To verify this possibility, we conducted 10 reaction
runs using this same catalyst portion. Fig. 3 shows the product
yield in subsequent catalytic reaction runs.
Also in this case, the complexes with bromide anion (6–8)
were characterized by higher activities and reproducibilities in
subsequent runs. However, the most effective catalyst was the
rhodium complex 10 which enabled obtaining the product with
the same yield of 96% in each of 10 runs. The performance of
Table 2 Yields of product and TON values for hydrosilylation 1-octene
with HMTS catalyzed by rhodium and platinum complexesa
Catalyst
Yield of product in subsequent cyclesb [%]
Total TON
6
7
8
9
10
11
95 (95..95)
95 000
94 500
92 000
87 600
96 000
74 100
98 (96, 96, 96, 96, 95, 95, 95, 89, 89)
99 (94, 94, 94, 94, 93, 90, 88, 87, 87)
90 (89, 89, 89, 89, 89, 89, 89, 88, 75)
96 (96, 96, 96, 96, 96, 96, 96, 96, 96)
90 (87, 87, 75, 69, 69, 66, 66, 66, 66)
a
[HSi] : [CH]CH] : [cat] ¼ 1 : 1 : 10ꢁ5; T ¼ 110 ꢀC; t ¼ 1 h. b Yields were
determined on the basis of GC analysis with n-decane as internal
standard.
Fig. 2 The change in the product yield as a function of time for the
reaction of 1-octene hydrosilylation catalyzed by phosphine rhodium
and platinum complexes in the FT-IR in situ reaction.
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Table 3 Yields of product and TON values for hydrosilylation allyl Table
4
ICP-MS analysis of post-reaction mixtures of selected
glycidyl ether with HMTS catalyzed by rhodium and platinum catalysts
complexesa
Complex 6
Complex 11
Catalyst
Yield of product in subsequent cyclesb [%]
Total TON
Rh content in the mixture
Pt content in the mixture
aer the reaction with
aer the reaction with
6
7
8
9
10
11
79 (79..79)
81 (81, 81, 81, 81, 81, 81, 79, 79, 79)
77 (77..77)
83 (83, 72, 12, 8, 7, 4, 3, 2, 0)
85 (85, 53, 20, 0)
81 (81, 19, 7, 5, 4, 4, 4, 3, 2)
79 000
80 400
77 000
27 400
24 300
22 100
Allyl glycidyl
Allyl glycidyl
ether
1-Octene
ether
1-Octene
Run no. [ppm] [%]
[ppm] [%]
[ppm] [%]
[ppm] [%]
a
b
[HSi] : [CH]CH] : [cat] ¼ 1 : 1.2 : 10ꢁ5; T ¼ 110 ꢀC; t ¼ 1 h. Yields
were determined on the basis of GC analysis with n-decane as internal
standard.
1
5
0.037 0.80 0.079 1.69 0.039
0.001 0.02 0.004 0.09 0.0047 0.09 0.0007 0.01
0.72 0.21 3.87
The presented catalytic data conrm the high catalytic
activity of all obtained complexes, however in the rst cycle. The
catalytic activity of the complexes containing NTf2ꢁ anion starts
to decrease drastically in subsequent catalytic cycles, which is
related to the nature of the anion present in the complex but
also to the properties of the olen used in the hydrosilylation
reaction. One of the reasons for decreasing yield is the catalyst
leaching by the reagents, which results in a decrease in the
catalysts concentration aer the isolation of reaction products.
The complexes with weak and delocalized charged NTf2ꢁ anion
are more susceptible to leaching by polar compounds. Since
allyl glycidyl ether is such a compound, there in its hydro-
silylation a successive leaching of this type of complexes takes
place, which was conrmed by ICP-MS analysis (Table 4). The
complexes containing the Brꢁ anion, both in the reaction with
1-octene and with allyl glycidyl ether, are much more resistant
to leaching and thus remain catalytically active for longer time
Fig. 4 The change in the product yield as a function of time for the
reaction of hydrosilylation of allylglycidyl ether, catalyzed by phos-
phine rhodium and platinum complexes in the FT-IR in situ reaction.
ꢁ
than the complexes containing NTf2 anion. Comparing the
results obtained for individual olens it can be concluded that
the polarity of olens plays a key role in the above process. In
the system with non-polar 1-octene, the yields were at a similar
level in subsequent catalytic cycles. In turn, in the systems with
polar allyl glycidyl ether, catalyst efficiency drastically decreases
between the rst and the last catalytic cycle.
Catalytic activity in subsequent reaction runs (see Fig. 5) is
even more diversied because the complexes with imide anion
lose their activity aer the third run, while all complexes with
bromide anion maintain constant activity in 10 subsequent
runs and permit obtaining the product with 80% yield aer
each run.
Conclusions
Functionalization of imidazolium ionic liquids with diphenyl
phosphine allowed obtaining the compounds that were used as
ligands in the synthesis of rhodium and platinum complexes. In
reactions with various metal precursors, new ionic complexes
(four Rh(I) complexes and two Pt(0) complexes) were obtained
with high yields (83–96%). All complexes obtained are stable in
the air and thermally stable (decomposition temperature above
ꢀ
200 C). The complexes thus formed were used as catalysts for
the hydrosilylation of olens of different polarity (1-octene and
allyl glycidyl ether) with 1,1,1,3,5,5,5-heptamethyltrisiloxane.
Due to their insolubility in the reaction medium, their easy
isolation and reuse in subsequent catalytic runs were possible.
The most active complexes did not lose their activity even aer
10 catalytic runs. In general, higher activity and repeatability
was demonstrated by rhodium complexes (I). However, the
Fig. 5 Yields of the product of hydrosilylation of allyl glycidyl ether
with 1,1,1,3,5,5,5-heptamethyltrisiloxane in 10 subsequent reaction
runs catalyzed by the same catalyst portion. The yields in the subse-
quent catalytic cycles and turnover numbers after 10 cycles are pre-
sented in Table 3.
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activity of the complexes also depends on the type of olen and
the products formed. In the case of nonpolar products, the
activity of all catalysts was similar. However, in the case of polar
products, higher stability was demonstrated by bromide anion
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Conflicts of interest
There are no conicts of interest to declare.
23 H. Maciejewski, K. Szubert, B. Marciniec and J. Pernak,
Green Chem., 2009, 11, 1045–1051.
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24 M. Jankowska-Wajda, R. Kukawaka, M. Smiglak and
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Financial support from the National Science Center (Poland), grant
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