Heterogeneous catalysts based on supported Rh-NHC complexes: Synthesis of high molecular weight poly(silyl ether)s by catalytic hydrosilylation

Article abstract of DOI:10.1039/c3cy00598d

The new rhodium(i) complexes [Rh(Cl)(COD)(R-NHC-(CH₂) ₃Si(OⁱPr₃)₃)] (R = 2,6-diisopropylphenyl (2a); n-butyl (2b)) have been synthesised and fully characterised. The study of their application as ketone hydrosilylation catalysts showed a clear N-substituent effect, 2a being the most active catalyst precursor. Complex 2a has been immobilised in the mesoporous materials MCM-41 and KIT-6. The new hybrid materials have been fully characterised and used as catalyst precursors for the preparation of poly(silyl ether)s by catalytic hydrosilylation. The heterogeneous catalytic systems based on the materials 2a-MCM-41 and 2a-KIT-6 afford polymers with high average molecular weight (M_w) M_w = 2.61 × 10⁶ g mol^-1 (2a-MCM-41) and M_w = 4.43 × 10⁵ g mol^-1 (2a-KIT-6).

Full text of DOI:10.1039/c3cy00598d

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DOI: 10.1039/C3CY00598D.
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DOI: 10.1039/C3CY00598D
Cite this: DOI: 10.1039/c0xx00000x
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ARTICLE TYPE
Heterogeneous Catalysts Based on Supported Rh-NHC Complexes:
Synthesis of High Molecular Weight Poly(silyl ether)s by Catalytic
Hydrosilylation
Guillermo Lázaro,a Francisco J. Fernández-Alvarez,*a Manuel Iglesias,a Cristina Horna,a Eugenio Vispe,a
5
Rodrigo Sancho,a Fernando J. Lahoz,a Marta Iglesias,b Jesús J. Pérez-Torrentea and Luis Oro*a
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
The new rhodium(I) complexes [Rh(Cl)(COD)(R-NHC-(CH2)3Si(OiPr3)3)] (R = 2,6-diisopropylphenyl (2a); n-butyl (2b)) have been
synthesised and fully characterised. The study of their application as ketone hydrosilylation catalysts showed a clear N-substituent effect,
10 2a being the most active catalyst precursor. Complex 2a has been immobilised in the mesoporous materials MCM-41 and KIT-6. The
new hybrid materials have been fully characterised and used as catalyst precursors for the preparation of poly(silyl ether)s by catalytic
hydrosilylation. The heterogeneous catalytic systems based on the materials 2a-MCM-41 and 2a-KIT-6 afford polymers with high
average molecular weight (Mw) Mw = 2.61 106 gmol-1 (2a-MCM-41) and Mw = 4.43 105 gmol-1 (2a-KIT-6).
.
45 2. Experimental
15 1. Introduction
General information. All manipulations were performed with
Transition-metal catalysed hydrosilylation has demonstrated to be
rigorous exclusion of air at an argon/vacuum manifold using
a
convenient route for the synthesis of new functional
standard Schlenk-tube techniques or in
a dry-box (MB-
materials.1,2 In this context, it is remarkable that the most efficient
hydrosilylation catalysts are based on platinum or rhodium
20 complexes.1 Such metals are particularly expensive and scarce.
Consequently, there is an increasing interest in the development
of heterogeneous and therefore easily recyclable hydrosilylation
catalytic systems. The immobilisation of homogenous catalyst
onto supports has been a methodology commonly used to
25 produce heterogeneous catalysts maintaining or improving the
catalytic performance.3-9
50 UNILAB). Solvents were dried by the usual procedures and
distilled under argon prior to use or taken under argon from a
Solvent Purification System (SPS). The reagents 1,1,1,3,5,5,5-
heptamethyltrisiloxane,
1,1,3,3,5,5-hexamethyltrisiloxane,
terephthalaldehyde and the solids MCM-41 and KIT-6 were
55 purchased from commercial sources and used without further
purifications. The rhodium(I) [Rh(µ-Cl)(COD)]2 complex18 and
19
I(CH2)3(iPrO)3 were prepared according to methods reported in
the literature. NMR spectra were recorded on a Varian Gemini
2000, Bruker ARX 300, Bruker Avance 300 MHz or Bruker
60 Avance 400 MHz instrument. Chemical shifts (expressed in parts
per million) are referenced to residual solvent peaks (1H,
13C{1H}). Coupling constants, J, are given in hertz. C, H, and N
analyses were carried out in a Perkin-Elmer 2400 CHNS/O
analyzer. Mass spectrometry was measured on an Esquire 3000+
65 with Ion trap detector interfaced to an Agilent 1100 series HPLC
system. FT-IR spectra were recorded on a Nicolet Nexus 5700 FT
spectrophotometer equipped with a Nicolet Smart Collector
diffuse reflectance accessory. TEM microscopy images were
collected with an INCA 200 X-Sight from Oxford Instruments
70 with a resolution in energy between 136 eV and 5.9 KeV. Gas
chromatography analyses were performed using an Agilent
6890N with FID detector equipped with a HP-1 column from
J&W Scientific (30m, 0.25 mm i.d.). Parameters were as follows:
Initial temperature 50 ºC, initial time 5 min, ramp 30 ºC/min,
75 final temperature 250 ºC, final time 5 min, injector temperature
250 ºC, detector temperature 300 ºC. Isotherms were obtained on
During the last decades N-heterocyclic carbenes (NHCs) have
been widely used as ligands in homogeneous catalysis.10,11 This
could be attributable to the interesting electronic properties of
30 NHC ligands. Indeed, they are strong σ-donor ligands with a very
poor π-acceptor character.12 On the other hand, Si(OR)3-
functionalised transition metal-NHC complexes are species of
great interest since they have the potential for immobilisation on
mineral supports by reaction between the Si-OR functionalities
35 and the Si-OH active sites present on the supports surface.13-17
Here we report on the synthesis and characterisation of new
Rh(I)-NHC complexes functionalized with a triisopropoxysilyl
tail group for their immobilisation on silica-based inorganic
supports. Fine tuning of the homogeneous catalyst by
40 modification of the N-substituent on the NHC ligand have
allowed for the synthesis of active ketone hydrosilylation
catalysts. Moreover, the supported catalysts have found
application in the synthesis of high molecular weight poly(silyl
ether)s by rhodium catalysed hydrosilylation.
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a Quantachrome AUTOSORB by measuring the volume of N2
51.7 (CH2N(1)), 49.8 (CH2N(3)), 32.1 (CH2CH2CH2), 25.5 (CH3-
absorbed at relative pressures between 0.05 and 0.99 at 77.3 K 60 iPrO), 24.6 (CH2CH3), 19.4 (CH2CH2CH2Si), 13.4 (CH2CH3), 8.5
after drying the sample at 120 ºC in vacuo. Absolute Molecular
weights of polymers were determined by GPC-MALS. Before
injection all samples were filtered through 0.45 mm PTFE
membranes. The column oven temperature was maintained at 35
(CH2Si). 29Si NMR (64.52 MHz, CDCl3, 293 K): δ -50.0 (CH2Si).
Mass Spectrometry (ESI+): m/z 371.3 (M+-I).
5
Preparation of 2a: Dicholromethane (15 mL) was added to a
ºC during all the experiments. The measurements were carried 65 mixture of the imidazolium salt 1a (602 mg, 1.00 mmol) and
out on a Waters 2695 autosampler equipped with three in line
PLGel Mixed C (7.8x300 mm) columns and a Wyatt three
Ag2O (232 mg, 1.00 mmol). The reaction mixture was stirred in
absence of light at room temperature for 4 h. The resulting
mixture was filtered and the solvent was removed in vacuo to
yield a brown residue of the silver carbene. A THF (15 mL)
10 detector setup (Minidaw TREOS® (MALS), Optilab Rex® (DRI)
and VIscostarII® Viscometer). The samples were eluted with
THF at a rate of 1mL/min. Tg of the polymer was determined by 70 solution of [RhCl(COD)]2 (247 mg, 0.500 mmol) was added to
DSC on a TA instruments DSC Q1000 with liquid nitrogen
cooling system. The temperature program for the analysis was
15 begun at -120 ⁰C and the temperature was increased at a rate of
10 ⁰C/ min to 25 ºC.
the residue and the mixture was stirred at room temperature for
16 h. The solvent was removed in vacuo and the residue thus
obtained was extracted with hexane (3 x 20 mL) to afford a
yellow crystalline solid. Yield 635 mg (88 %). Anal. Calcd for
75 C35H58ClN2O3RhSi: C, 58.30, H, 8.11, N, 3.88. Found: C, 58.09,
H, 8.40, N, 4.07. 1H NMR plus COSY (300 MHz, C6D6, 293 K):
Preparation
of
1-(3-triisopropoxysilylpropyl)-3-(2,6-
diisopropylphenyl)-Imidazolium iodide (1a): To an acetonitrile
20 (30 mL) solution of 1-(2,6-diisopropylphenyl)imidazole (465 mg,
2.00 mmol), I(CH2)3Si(iPrO)3 (1.12 g, 3.00 mmol) was added.
δ 7.31 (dd, 1H, JH-H = 8 Hz, JH-H = 2 Hz, CH), 7.25 (t, 1H, JH-H =
8 Hz, CH), 6.97 (dd, 1H, JH-H = 8 Hz, JH-H = 2 Hz, CH), 6.43 (d,
1H, JH-H = 2 Hz, CHimd), 6.36 (d, 1H, JH-H = 2 Hz, CHimd), 5.38
The reaction mixture was stirred at 90 °C for 24 h. The resulting 80 (m, 2H, CHCOD, 1H, CH2N), 4.34 (m, 1H, CH2N), 4.25 (spt, 3H,
solution was filtered through Celite and the solvent was removed
in vacuo. The residue thus obtained was washed with hexane (2 x
25 20 mL) to give a brown solid. Yield 1.07 g (89 %). Anal. Calcd.
for C27H47IN2O3Si: C, 53.81, H, 7.86, N, 4.65. Found: C, 54.03,
JH-H = 6 Hz, CH-iPrO), 4.06 (spt, 1H, JH-H = 7 Hz, CH-iPr), 3.60
(m, 1H, CHCOD), 3.04 (m, 1H, CHCOD), 2.51 (m, 1H, CH2COD),
2.32 (m, 1H, -CH2-), 2.23-2.05 (m, 1H, -CH2-, 2H, CH2COD), 2.00
(spt, 1H, JH-H = 7 Hz, CH-iPr), 1.89 (m, 1H, CH2COD), 1.83 (d,
H, 7.92, N, 4.53. 1H NMR (300 MHz, CDCl3, 293 K) plus 85 3H, JH-H = 7 Hz, CH3-iPr), 1.71 (m, 1H, CH2COD), 1.58 (m, 2H,
COSY: δ 9.95 (s, 1H, NCHN), 7.91 (t, 1H, JH-H= 2 Hz, CHimd),
7.54 (m, 1H, CH), 7.30 (m, 2H, CH), 7.23 (t, 1H, JH-H = 2 Hz,
30 CHimd), 4.79 (t, 2H, JH-H = 7 Hz, CH2N), 4.20 (spt, 3H, JH-H = 6
Hz, CH-iPrO), 2.30 (spt, 2H, JH-H = 7 Hz, CH-iPr), 2.11 (m, 2H, -
CH2COD), 1.45 (s, 1H, CH2COD), 1.22 (d, 18H, JH-H = 6 Hz, CH3-
iPrO), 1.06 (d, 3H, JH-H = 7 Hz, CH3-iPr), 1.03 (d, 3H, JH-H = 7
Hz, CH3-iPr), 0.92 (d, 3H, JH-H = 7 Hz, CH3-iPr), 0.76 (m, 2H,
CH2Si). 13C {1H} NMR plus HSQC (75 MHz, C6D6, 293 K): δ
CH2-), 1.23 (d, 6H, JH-H = 7 Hz, CH3-iPr), 1.17 (d, 18H, JH-H = 6 90 183.5 (d, JRh-C = 52 Hz, RhCcarbene), 148.3 (Cipso), 145.1 (Cipso),
Hz, CH3-iPrO), 1.14 (d, 6H, JH-H = 7 Hz, CH3-iPr), 0.58 (m, 2H,
CH2Si). 13C{1H} NMR plus HSQC (75.46 MHz, CDCl3, 293 K):
35 δ 145.5 (Cipso, 2C), 138.0 (NCHN), 132.2 (CH), 130.1 (Cipso),
124.9 (CH, 2C), 124.3 (CHimd), 123.5 (CHimd), 65.5 (CH-iPrO),
136.2 (Cipso), 129.6 (CH), 124.6 (CH), 123.8 (CHimd), 122.8 (CH),
120.1 (CHimd), 96.9 (d, JRh-C = 7 Hz, CHCOD), 96.5 (d, JRh-C = 7
Hz, CHCOD), 67.1 (d, JRh-C = 14 Hz, CHCOD), 66.9 (d, JRh-C = 14
Hz, CHCOD), 64.9 (CH-iPrO), 54.4 (CH2N), 34.5 (CH2COD), 31.5
52.5 (CH2N), 28.9 (CH-iPr), 25.7 (CH3-iPrO), 24.9 (-CH2-), 24.6 95 (CH2COD), 29.4 (CH2COD), 28.4 (CH-iPr), 28.0 (CH-iPr), 27.9
(CH3-iPr), 24.3 (CH3-iPr), 8.4 (CH2Si). 29Si NMR (64.52, CDCl3,
293 K): δ -51.6 (CH2Si). Mass Spectrometry (ESI+): m/z 475.3
40 (M+-I).
(CH2COD), 26.0 (CH3-iPr), 25.8 (CH3-iPr), 25.5 (CH3-iPrO), 24.9
(-CH2-), 23.8 (CH3-iPr), 22.6 (CH3-iPr), 9.6 (CH2Si). 29Si NMR
(64.52 MHz, CDCl3, 293 K): δ -49.9 (CH2Si). Mass spectrometry
(ESI+): m/z 685.2 (M+-Cl).
Preparation of 1-(3-triisopropoxysilylpropyl)-3-(n-butyl)- 100
Imidazolium iodide (1b): To an acetonitrile (30 mL) solution of
1-(butyl)imidazole (124 mg, 1.00 mmol), I(CH2)3Si(iPrO)3 (486
45 mg, 1.30 mmol) was added. The reaction mixture was stirred at
90 °C for 24 h. The resulting solution was filtered through Celite
Preparation of 2b: CH2Cl2 (15 mL) was added to a mixture of
the imidazolium salt 1b (498 mg, 1.00 mmol) and Ag2O (232 mg,
1.00 mmol). The reaction mixture was stirred in absence of light
at room temperature for 4 h. The resulting mixture was filtered
and the solvent was removed in vacuo. The residue thus obtained 105 and the solvent was removed in vacuo to yield an off-white
was washed with hexane (2 x 20 mL) to give a white solid. Yield
459 mg (92 %). Anal. Calcd. for C19H39IN2O3Si: C, 45.78, H,
50 7.89, N, 5.62. Found: C, 45.96, H, 7.51, N, 5.81. H NMR (300
residue of the silver carbene. A THF (15 mL) solution of
[RhCl(COD)]2 (247 mg, 0.500 mmol) was added to the residue
and the mixture was stirred at room temperature for 16 h. The
solvent was removed in vacuo and the residue thus obtained was
1
MHz, CDCl3, 293 K) plus COSY: δ 10.0 (s, 1H, NCHN), 7.62 (t,
1H, JH-H= 2 Hz, CHimd), 7.36 (t, 1H, JH-H = 2 Hz, CHimd), 4.33 (m, 110 extracted with hexane (3 x 20 mL) to afford a yellow crystalline
2H, CH2N(3)), 4.29 (m, 2H, CH2N(1)) 4.12 (spt, 3H, JH-H = 6 Hz,
CH-iPrO), 1.94 (m, 2H, CH2CH2CH2Si), 1.85 (m, 2H,
55 CH2CH2CH2but), 1.32 (m, 2H, CH3CH2), 1.10 (d, 18H, JH-H = 6
Hz, CH3-iPrO), 0.89 (t, 3H, JH-H = 7 Hz, CH3CH2), 0.47 (m, 2H,
solid. Yield 0.55 g (89%). Anal. Calcd for C27H50ClN2O3RhSi: C,
52.55, H, 8.17, N, 4.54. Found: C, 51.97, H, 8.40, N, 4.27. H
1
NMR plus COSY (300 MHz, C6D6, 293 K): δ 6.29 (d, 1H, JH-H
=
2 Hz, CHimd), 6.12 (d, 1H, JH-H = 2 Hz, CHimd), 5.45 (m, 2H,
CH2Si). 13C{1H} NMR plus HSQC (75.46 MHz, CDCl3, 293 K): 115 CHCOD), 4.60 (m, 1H, CH2N(3)), 4.43 (m, 1H, CH2N(1)), 4.28 (spt,
δ 136.0 (NCHN), 122.6 (CHimd), 122.0 (CHimd), 65.2 (CH-iPrO),
3H, JH-H = 6 Hz, CH-iPrO; m, 1H, CH2N(3) ), 4.08 (m, 1H,
2
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CH2N(1)), 3.32 (m, 1H, CHCOD), 3.25 (m, 1H, CHCOD), 2.42 (m,
2H, CH2COD), 2.26 (m, 3H, CH2COD; 1H, CH2CH3), 2.11 (m, 1H, 60 mg/g). FT-IR: 1629 cm-1 (br.), ν(C=N) and ν(C=C), 1241 cm-1,
2a-MCM-41: Yield 0.465 g (93.0 %). Found: Rh, 0.96 % (9.57
CH2CH3), 1.80 (m, 3H, CH2COD), 1.57 (m, 2H, CH2CH2CH2But),
1.31 (m, 2H, CH2CH2CH2But), 1.23 (dvd, 18H, JH-H = 6 Hz, N =
6.5 Hz, CH3-iPrO), 0.88 (t, 3H, CH2CH3), 0.78 (m, 2H, CH2Si).
13C {1H} NMR plus HSQC (75 MHz, C6D6, 293 K): δ 183.3 (d,
1043 cm-1, 800 cm-1 ν(Si-O-Si). 13C-NMR CP-MAS: δ 147.5
(CH), 137.2 (CH), 130.8 (CH), 124.8 (CHimd), 69.4 (CHCOD),
53.7 (CH2N), 32.4-19.4 (CH2COD and -CH2-), 8.5 (CH2Si). 29Si-
NMR CP-MAS: δ -92.4, -101.1, -110.7 (MCM-41), -70.2 to -51.4
5
JRh-C = 51 Hz, RhCcarbene), 120.2 (CHimd), 120.0 (CHimd), 98.2 (d, 65 (br, CH2Si).
Rh-C = 7 Hz, CHCOD), 97.9 (d, JRh-C = 7 Hz, CHCOD), 67.4 (d, JRh-C
J
2a-KIT-6: Yield 0.484 g (96.8 %). Found: Rh, 0.55 % (5.53
mg/g). FT-IR: 1636 cm-1 (br.), ν(C=N) and ν(C=C), 1241 cm-1,
1048 cm-1, 800 cm-1 ν(Si-O-Si). 13C-NMR CP-MAS: δ 147.5
(CH), 136.9 (CH), 130.4 (CH), 123.1 (CHimd), 96.2 (CHCOD),
= 14 Hz, CHCOD), 67.0 (d, JRh-C = 14 Hz, CHCOD), 65.3 (CH-
10 iPrO), 53.6 (CH2N(3)), 50.7 (CH2N(1)), 33.7 (CH2COD), 33.3
(CH2COD), 33.1 (CH2CH2CH2But) 29.6 (CH2COD), 29.2 (CH2COD),
25.9 (CH3-iPrO), 25.4 (CH2CH3), 20.3 (CH2CH2CH2Si), 14.0 70 67.6 (CHCOD), 53.9 (CH2N), 28.1-22.8 (CH2COD and -CH2-), 8.4
(CH2CH3), 9.9 (CH2Si). CH2Si). 29Si NMR (64.52 MHz, CDCl3,
293 K): δ -49.9 (CH2Si). Mass spectrometry (ESI+): m/z 581.7
15 (M+-Cl).
(CH2Si). 29Si-NMR CP-MAS: δ -92.5, -100.9, -110.0 (KIT-6), -
66.3 to -52.6 (br, CH2Si).
Hydrosilylation of acetophenone: A mixture of 1,1,1,3,5,5,5-
Crystal structure determination of compound 2a: Crystals of 75 heptamethylsiloxane (2.0 mL), the corresponding catalyst (14.4
2a suitable for the X-ray analysis were obtained by slow
evaporation of a solution of the complex in hexane. X-ray
20 diffraction data were collected at 100(2) K with graphite-
monochromated Mo Kα radiation (λ=0.71073 Å), using narrow
ω rotation (0.3 º) on a Bruker APEX DUO diffractometer.
Intensities were integrated with SAINT+ program and corrected
for absorption effect with SADABS, integrated in APEX2
25 package. The structures were solved by direct methods with
SHELXS-97.20 Refinement, by full-matrix least-squares on F2,
mg (2a), 12.3 mg (2b), 0.020 mmol), acetophenone (0.234 mL,
2.00 mmol) and mesitylene (0.234 mL, 2.00 mmol, internal
standard) was stirred at T = 80 ºC. Samples were taken at regular
time intervals and analysed by gas chromatography.
80
Co-polimerization by homogeneous catalytic hydrosilyation
(2.0 mol % cat): 1,4-Dioxane (2.0 mL) was added to a mixture
of 1,1,3,3,5,5-hexamethylsiloxane (0.254 mL, 1.00 mmol,)
terephthalaldehyde (134 mg, 1.00 mmol) and 2a (14.4 mg, 0.020
was performed with SHELXL-97.20 Hydrogen atoms were 85 mmol). The reaction mixture was stirred at 110 ºC for 4 days. The
included in calculated positions and defined with displacement
and positional riding parameters. A typical disorder for the
30 oxygen atoms of the alkoxy groups was observed and modelled
with two oxygens having complimentary occupancy factors
solvent was removed in vacuo and the residue was extracted with
THF (5 mL) to afford orange oil.
Co-polimerization by heterogeneous catalytic hydrosilylation
(0.839 and 0.161(6)). Half a molecule of hexane, highly 90 (2.0 mol % catalyst based on Rh content): 1,4-Dioxane (2.0
disordered, was also observed in the asymmetric unit of the
crystal; six residual peaks were introduced in the final refinement
35 to take account of this electron density. Crystal data for 2a:
C35H58ClN2O3RhSi 0.5 C6H14, M = 764.37; yellow prism, 0.211 ×
mL) was added to a mixture of 1,1,3,3,5,5-hexamethylsiloxane
(0.025 mL, 0.100 mmol,), terephthalaldehyde (13.4 mg, 0.100
mmol) and the corresponding heterogeneous catalysts (0.002
mmol). The reaction mixture was stirred at 110 ºC for 4 days. The
0.211 × 0.141 mm3; triclinic, P-1; a = 10.2529(12), b = 95 heterogeneous catalyst was removed by decantation. The
12.6020(15), c = 16.390(2) Å, α = 73.082(2), β = 87.303(2), γ =
89.419(2)º; Z = 2; V = 2023.8(4) ų; Dc = 1.254 g/cm³; µ = 0.553
resulting mixture was dried in vacuo and the residue was
extracted with THF (5 mL) to afford orange oil.
40 mm-1, min. and max. correction factors 0.675 and 0.926; 2θmax
=
61.0º; 19266 collected reflections, 10794 unique [Rint=0.0440];
Co-polimerization by heterogeneous catalytic hydrosilylation
number of data/restraints/parameters 10794/7/442; final GoF 100 (0.2 mol % catalyst based on Rh content): 1,4-Dioxane (2.0
1.012; R1 = 0.0545 [8226 reflections, I > 2σ(I)]; wR2 = 0.1460
for all data; largest difference peak: 1.71 e/ų close to the metal
45 atom. CCDC-945443 (2a) contains the supplementary
crystallographic data for this paper. These data can be obtained
mL) was added to a mixture of 1,1,3,3,5,5-hexamethylsiloxane
(0.254 mL, 1.00 mmol), terephthalaldehyde (134 mg, 1.00 mmol)
and the corresponding heterogeneous catalysts (0.002 mmol). The
reaction mixture was stirred at 110 ºC for 1 day. The
free of charge from the Cambridge Crystallographic Data Centre 105 heterogeneous catalyst was removed by decantation. The
via www.ccdc.cam.ac.uk/data_request/cif.
resulting mixture was dried in vacuo and the residue was
extracted with THF (5 mL) to afford amber gelatine.
50 Immobilisation of 2a on inorganic supports: To a suspension
of the corresponding inorganic support (0.50 g) in wet toluene
Recycling of the heterogeneous catalyst: The reaction mixture
(10.0 mL) a solution of 2a (50 mg) in toluene (5.0 mL) was 110 was centrifuged and the corresponding heterogeneous catalyst
added. The reaction slurry was stirred for one hour at r.t. and after
that refluxed for 24 h. The resulting mixture was cooled down to
55 r.t. The solid was separated by decantation and washed with
toluene (10 mL), dichloromethane (2 x 10 mL) and diethylether
(2 x 10 mL) and dried in vacuo at 60 ºC to afford an off-white 115
solid.
was separated from the product by decantation, washed with
CH2Cl2 (3 x 3.0 mL), Et2O (3 x 3.0 mL), dried in vacuo and
reused without further purifications.
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Catalysis Science & Technology
Page 4 of 10
chloride, making the two Rh-olefin bond distances clearly
different (Rh-M(1) 1.989(3) Å trans to Cl, and Rh-M(2) 2.084(3)
Å trans to NHC ligand; see Figure 1 caption).
3. Results and Discussion
3.1. Synthesis and characterisation of the catalyst precursors.
[Rh(X)(COD)(NHC)] (X = Cl, Br) complexes have revealed as
efficient precatalysts in homogeneously catalysed hydrosilylation
processes.10 In particular, Si(OR)3-functionalized rhodium-NHC
complexes are species of great interest due to their potential for
immobilisation on mineral supports.13,21,22,23
5
The preparation of Si(OR)3-functionalised rhodium-NHC
complexes can be accomplished using imidazolium salts as NHC
10 ligand precursors in order to obtain silver(I)-NHC compounds,
which are widely used as reagents for transmetallation
reactions.24 The imidazolium iodide salts used as ligand
precursors in this work were synthesized in good yield by
reaction of the corresponding 1-substituted imidazole and
15 I(CH2)3Si(iPrO)3 in acetonitrile at 90 ºC (Scheme 1). The
imidazolium salts 1a and 1b have been fully characterised by
1
elemental analysis, mass spectrometry (ESI+), H, 13C{1H} and
29Si NMR spectroscopy (see Experimental).
55
Scheme2
20
Scheme1
The rhodium complexes 2a and 2b were synthesised by Lin’s
method of transmetallation from intermediate silver(I) complexes
prepared in situ by reaction of the corresponding imidazolium salt
with Ag2O (Scheme 2).24 Complexes 2a and 2b were isolated,
25 after recrystallisation, as pure yellow crystalline solids in high
yield and characterised by elemental analysis, mass spectrometry
(ESI+) and H and 13C{1H} NMR spectroscopy. The H NMR
spectra of complexes 2a and 2b evidence the absence of the
NCHN resonance of the parent imidazolium salts and show the
30 characteristic set of resonances for both the 3-
triisopropoxysilylpropyl, and the 2,6-diisopropylphenyl (2a) and
n-butyl (2b) substituents on the NHC ligands. The 13C{1H} NMR
spectra exhibit a doublet resonance at δ 183.5 ppm (1JRh-C = 52
Hz) (2a) and 183.3 ppm (1JRh-C = 51 Hz) (2b), which confirms the
35 coordination of the carbene ligand to the rhodium centre.23,25,26
The structure of complex 2a was determined by X-ray
crystallography. A view of the molecular geometry of this species
is shown in Figure 1. The rhodium atom displays a slightly
distorted square planar coordination. The COD ligand bonds to
40 the metal through its two olefinic bonds in a chelate mode,
showing a typical tub conformation; the carbon atom C(9) of the
NHC ligand and the chlorido ligand complete the metal
coordination environment. The NHC ligand contains two
different N-substituents, namely 2,6-diisopropylphenyl at N(1)
45 and triisopropoxysilylpropyl at N(2). The geometry of the
fragment [Rh(Cl)(COD)(NHC)] compares well with those
reported for complex [Rh(Cl)(COD)(2-methoxyethyl-NHC-
(CH2)3Si(OiPr3)3)] (2c)23 and related compounds.25,26 The most
striking feature of the structure concerns the high structural trans
50 effect showed by the NHC ligand compared to that of the
1
1
Fig. 1. Molecular diagram of complex 2a. Selected bond lengths (Å) and
60 angles (°): Rh-Cl 2.3913(9), Rh-C(1) 2.111(3), Rh-C(2) 2.105(3), Rh-C(5)
2.203(3), Rh-C(6) 2.188(3), Rh-C(9) 2.043(3), C(1)-C(2) 1.400(5), C(5)-C(6)
1.380(5); Cl-Rh-C(9) 87.79(9), Cl-Rh-M(1) 175.82(8), Cl-Rh-M(2) 90.53(8),
C(9)-Rh-M(1) 94.39(11), C(9)-Rh-M(2) 87.36(10, N(1)-C(9)-N(2) 104.6(2)
(M(1) and M(2) represent the midpoints of the olefinic bonds C(1)-C(2)
65 and C(5)-C(6).
3.2. Homogeneous catalytic hydrosilylation of acetophenone:
influence of the N-substituent. The rhodium-catalyzed
hydrosilylation
of
acetophenone
with
1,1,1,3,5,5,5-
70 heptamethyltrisiloxane (HepTMS) to produce PhMeCH-O-
SiMe(OSiMe3)2 (Scheme 3) has demonstrated to be an accurate
activity test bench, whose results could be extrapolated to the
synthetic methodology established for the preparation of
poly(silyl ether)s by catalytic hydrosilylation.23 The new
75 compounds 2a and 2b and the previously reported 2c, having a 2-
methoxyethyl substituent of hemilabile character, were used as
4
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Catalysis Science & Technology
catalyst precursors for the catalytic hydrosilylation of 40 of a mixture of not identified rhodium hydride complexes. On the
acetophenone with HepTMS (Scheme 3).
other hand, solutions of 2a in C6D6 and wet C6D6 (5 % H2O) were
stable for 24 h at 80 ºC and no traces of any decomposition
i
product nor PrOH were observed. These observations suggest a
hydride-mediated mechanism resulting from the silane activation
45 in full agreement with the established rhodium-catalysed ketones
hydrosilylation mechanisms.27-30
Scheme3
3.3. Immobilisation of 2a on inorganic supports. The above
described results clearly evidenced the superior catalytic
50 performance of precursor 2a. These results prompted us to select
complex 2a as precursor for the preparation of different
heterogeneous catalysts. Thus, 2a-MCM-41 and 2a-KIT-6 were
prepared by refluxing a mixture of 2a and the corresponding
inorganic support in wet toluene for 24h (Scheme 4).
5
The catalytic reactions of acetophenone with HeptMTS using
the rhodium(I)-catalyst 2a, 2b or 2c were monitored by GC and
1H NMR. As can be seen in Figure 2 the N-substituent on the
NHC ligand strongly influences the catalytic activity. Complex
2a, which contains the bulky 2,6-diisopropylphenyl substituent,
10 exhibited the highest catalytic activity with conversion of 95 % in
less than 2h and a TOF1/2 of 73 h-1. Complexes 2b and 2c, which
contain a straight-chain flexible wingtip group of approximately
the same length, were considerably less active exhibiting TOF1/2
values of 36 and 24 h-1, respectively. Nevertheless, full
15 acetophenone conversion was observed at longer reaction times
with both catalyst precursors.
55
Scheme4
The new materials were isolated as off-white solids that were
characterised by ICP-MS, FT-IR, 13C and 29Si CP-MAS NMR
and TEM. We have determined a rhodium loading of 9.57 mg/g
60 and 5.53 mg/g for 2a-MCM-41 and 2a-KIT-6, respectively. The
FT-IR spectra of the new materials show the stretching vibration
modes of the mesoporous framework (Si-O-Si) at around 1241
cm-1, 1043 cm-1 and 800 cm-1.13 The FT-IR spectra of the grafted
materials exhibit an additional broad absorption corresponding to
65 the ν(C=N) and ν(C=C) stretching modes of the ligand at around
1630 cm-1.21-22
On the other hand, the inferior catalytic activity of 2c
compared to 2b points to a negative influence of the potentially
hemilabile 2-methoxyethyl substituent. This observation is in
20 sharp contrast with the positive effect of this group on the
hydrogen transfer catalytic activity of related complex
[Ir(Cl)(COD)(Me-NHC-(CH2)2OCH3],
which
has
been
substantiated by DFT calculations.26 The superior catalytic
activity of 2a could be ascribed to the steric protection of the
25 catalytic active species introduced by the bulky substituent on the
NHC ligand.
The resonances observed in the 13C CP-MAS solid state NMR
spectra of 2a-MCM-41 and 2a-KIT-6 compare well with those
observed in the 13C{1H} NMR spectra of the parent complex 2a.
70 The most prominent resonances in the 13C CP-MAS solid state
NMR spectra of the solids are those due to the CHimd [δ 124.8 -
123.1 ppm], CH2N [δ 53.9 - 52.6 ppm] and CH2Si [δ 8.5 - 8.3
ppm] carbon atoms. The 29Si CP-MAS solid state NMR spectra
of these new materials exhibit resonances of great intensity
75 corresponding to the silicon atoms of the silica (at around -92.0, -
100 and -110 ppm) and less intense broad resonances in between
δ –51.4 and –70.2 ppm (2a-MCM-41) and δ –52.6 and –66.3
ppm (2a-KIT-6) assigned to T1, T2 and T3 environments of the
CH2Si silicon atoms.31
Fig 2. Conversion (%) versus time (min) for catalytic hydrosilylation of
acetophenone with HeptMTS using 2a, 2b or 2c (1.0 mol %). Turnover
30 frequency (TOF1/2) recorded at 50 % of conversion.
1H NMR studies of the reaction of acetophenone with
HepMTS in C6D6, using 2a (10 mol %) as catalyst confirm that a
pre-activation step is required as no reaction was observed below
60 ºC. Initially, we observed the appearance of the resonances
35 corresponding to the hydrosilylation product, PhMeCH-O-
SiMe(OSiMe3)2,23 together with a doublet resonance at δ – 20.62
ppm (1JRh-H = 40 Hz) due to a Rh-H intermediate species. After
1
30 minutes at 60 ºC the H NMR spectra showed an increase of
the resonances due to PhMeCH-O-SiMe(OSiMe3)2 and formation
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Page 6 of 10
25 number of poly(silyl ether)s with an average Mw between 10000
and 150000 gmol-1.40-41 Following our interest on the on the
chemistry and catalytic applications of Rh(I)-NHC (NHC = N-
heterocyclic carbene) species,25,26,42-44 we have recently found
that both homogeneous and heterogeneous catalytic systems,
30 based on the species 2c, which contain a potentially hemilabile 2-
methoxyethyl substituent, are also effective for the synthesis of
poly(silyl ether)s.23 However, such catalytic systems afford
polymers with a relatively low average Mw up to 94000 gmol-1
and the recycled heterogeneous catalyst showed a drastic
35 decrease of the average Mw. Thus, the design and development of
effective synthetic methodologies that permit to obtain high
molecular weight poly(silyl ether)s and recycling of the
heterogeneous catalyst still remain a challenge.
In this work, we have studied the reaction of
40 terephthalaldehyde
with
1,1,3,3,5,5-hexamethyltrisiloxane
(HexMTS) in 1,4-dioxane at 110 ºC using the rhodium chlorido
species 2a, or the materials 2a-MCM-41 and 2a-KIT-6 as
catalyst precursors. These catalytic reactions proceed
quantitatively affording orange oils or amber gelatines
45 (depending on the Mw) which have been characterised as the
poly(silyl ether) 3 by comparison of their 1H, 13C{1H} and
29Si{1H} NMR spectra with reported data (Scheme 5).23 It is
worth to note that GPC analyses of 3 showed UV absorption at
254 nm, due to the aromatic rings, all along the polymer peaks in
50 agreement with the copolymeric nature of the samples.
Fig 3. Transmission electron microscopy (TEM) of 2a-MCM-41 (a-left 100
nm; a-right 20 nm) and 2a-KIT-6 (b-left 200 nm; b-right-50 nm).
The results of N2-adsorption/desorption studies of MCM-41,
2a-MCM-41, KIT-6 and 2a-KIT-6, including BET surface area,
total pore volume, and Barret-Joyner-Halenda (BJH) pore size are
shown in Table 1. These results evidenced that the surface area,
pore volume and pore diameter of 2a-MCM-41 and 2a-KIT-6
decrease, which suggests the inclusion of the rhodium species 2a
5
10 inside the channels of the mesoporous materials MCM-41 and
KIT-6. As a whole, the above data confirm that complex 2a has
been effectively immobilised on MCM-41 and KIT-6.
Scheme5
Table 1. Results of the N2-adsortion/desorption studies.
55
It is notable that glass transition temperature (Tg) and
temperature of thermic destruction (Td) values are related with the
average Mw (Table 2). The Tg of these polymers are in the range
of –86.3 ºC and –89.4 ºC. The polymers with a high Mw (Table 2,
entry 2) having a higher Tg value. These Tg values compare well
Material
Rh
SBET
Vp
Dp
[mg g-1][a]
[m2 g-1][b]
[cm3 g-1][c]
(Å)[d]
MCM-41
2a-MCM-41
KIT-6
-
1731
1379
665
1.41
1.05
0.77
0.64
27.6
23.8
54.2
53.2
60 with those reported for analogous poly(silyl ether)s23 and are
lower than those reported for polymers obtained with the
ruthenium catalytic system.39-40 The thermogravimetric analysis
(TGA) studies showed the complete destruction of the polymers
in the range of 456 ºC and 510 ºC with a weight loss of around 96
65 %. In this case the average Mw also influences the temperature of
thermic destruction which is higher for polymers with high
average Mw (Table 2). Thus, an increasing of the average Mw
produces polymers with higher thermic resistance.
9.6
-
2a-KIT-6
5.5
492
[a] Inductively coupled plasma mass spectrometry (ICP-MS). [b] Surface
15 area. [c] Pore volume. [d] Pore diameter.
70
3.4. Synthesis of high molecular weight poly(silyl ether)s.
Poly(silyl ether)s represent a family of promising polymeric
materials.32-34 The backbones of poly(silyl ether)s contain
hydrolytically reactive Si-O-C bonds,35 which makes them
20 attractive in many applications, such as, biodegradable materials
or controlled release of drugs.36-38 Weber´s group reported in
1998 the first example of synthesis of poly(silyl ether)s by
75
transition
metal
catalysed
hydrosilylation
using
[Ru(CO)H2(PPh3)3] as precatalyst.39 This method allowed a
6
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Catalysis Science & Technology
Table 2. Glass transition temperature (Tg) and Temperature of thermic
destruction (Td).
time (0.2 mol % catalyst and 1 day of reaction at 110 ºC) and
using the heterogeneous catalysts 2a-MCM-41 and 2a-KIT-6 we
observed an extraordinary enhancement of the average Mw of the
obtained polymers. This is remarkable in the case of the catalytic
40 systems based on the material 2a-MCM-41 which allow for the
preparation of polymers with an average Mw = 2.61 106 gmol-1
(Table 4, entry 2). Additionally, we have proved that increasing
the reaction time from 1 day to 2 days did not produce a
noticeable change (Table 4, entries 3 and 6). This is an exciting
45 result, and represents the first synthetic route to poly(silyl ether)s
with high average molecular weight.
Entry
Mw (gmol-1)
34000
Tg (ºC)
Td(ºC)
1
2
-89.4
456 (95.7%)b
510 (96.4%)b
2.61106/(35%)a
1.79105/(65%)a
-86.3
[a] Bimodal polymer. [b] (weight loss %).
The above described results evidence that the inorganic
support strongly influences the average Mw of the polymers.
Thus, polymers with higher average Mw were obtained using 2a-
50 MCM-41 and 2a-KIT-6 as catalysts (Table 4). The best catalytic
performance, in terms of the average Mw, was achieved with 2a-
MCM-41. The polymers obtained using the hybrid heterogeneous
catalysts have a bimodal distribution This fact could be attributed
to a confinement effect exerted by the support.
The homogenous catalyst precursor 2a gave a polymer with an
average Mw = 34000 gmol-1, much higher than that obtained with
2c as catalyst precursor (Mw = 5200 gmol-1) under the same
reaction conditions (2.0 mol % of catalyst and 110 ºC in 1,4-
dioxane).23 The solvent has an impact on the molecular weight of
the polymer (Table 3). Interestingly, the copolymerization
5
10 reaction also occurs under solvent free conditions (Table 3, entry
3). However, polymers with a higher average Mw were always 55 Table 4. Heterogeneously catalysed synthesis of 3.[a]
observed when 1,4-dioxane was used as solvent (Table 3, entry
2).
Entry
Catalyst
Catalyst
loading
(mol %)
Mw (gmol-1)[e]
PDI
Table 3. Solvent effect on the homogeneously catalysed synthesis of 3.[a]
1
2
3
4
2a-MCM-41[b]
2a-MCM-41[c]
2a-MCM-41[d]
2a-KIT-6[b]
Mw (gmol-1)
2300
PDI
2.4
2.2
1.4
2.0
0.2
0.2
2.0
3.78 105/(19 %)
2.81 104/(81 %)
1.9
1.2
Entry
Solvent
Toluene
2.61 106/(35%)
1.79 105/(65%)
2.1
1.2
1
2
3
1,4-Dioxane
HexMTS
34000
2.78 106/(34%)
6.48 105/(66%)
1.6
1.2
7000
1.34 105/(13 %)
3.8 103/(87 %)
1.8
1.4
15 [a] 4 days at 110 ºC using 2.0 mol % of 2a as catalyst precursor, 100 % of
conversion.
5
6
2a-KIT-6[c]
2a-KIT-6[d]
0.2
0.2
4.43 105/(21%)
2.37 104/(79%)
2.3
1.2
The hybrid catalyst 2a-MCM-41 (2.0 mol % of catalyst and 4
days at 110 ºC in 1,4-dioxane) efficiently catalysed the co-
polymerisation of terephthalaldehyde with 1,1,3,3,5,5-
20 hexamethyltrisiloxane (HexMTS) affording polymers also with a
bimodal distribution. However, the polymer with the lowest
average molecular weight fraction, Mw = 28100 gmol-1 and PDI =
1.2 represents 81 % of the wt of the total mass whereas the high
average molecular weight, Mw = 378000 gmol-1 and PDI = 1.9, is
25 the 19 % (Table 4, Entry 1).
5.30 105/(19%)
2.66 104/(81%)
2.1
1.2
[a] 100 % conversion. [b] 4 days at 110ºC. [c] 1 day at 110ºC. [d] 2 days at
110ºC. [e] Polymers with a bimodal distribution (mass % wt).
GC monitoring of the copolymerisation reactions of
terephthalaldehyde
with
1,1,3,3,5,5-hexamethyltrisiloxane
60 (HexMTS) revealed the total consumption of the co-monomers
after 5 min, using both the homogeneous catalyst precursor 2a
and the heterogeneous catalyst 2a-MCM-41. Additionally,
studies on the variation of the Mw with the reaction time evidence
that in both, homo- and heterogeneously catalysed processes the
65 polymerisation proceeds in a stepwise manner with the Mw of the
polymer continuously increasing with time. These results are in
agreement with a step-growth polymerisation.45
On the other hand, the heterogeneous catalyst 2a-KIT-6
produced polymers of lower average molecular weight exhibiting
also a bimodal distribution (Table 4, entry 4). The low molecular
weight polymer represent the most abundant fraction (≈ 90 %)
30 with average Mw lower than 4000 gmol-1 and PDI values in the
range of 1.3-1.4. However, the average Mw of the high molecular
weight fraction obtained using the heterogeneous catalyst 2a-
KIT-6 (Table 4, entry 4), Mw = 134000 gmol-1 is considerably
bigger.
Recycling studies using 2a-MCM-41 and 2a-KIT-6 as catalyst
showed that in both cases and independently of the catalyst
70 loading the average molecular weight of the obtained polymers
35
Interestingly, reducing the catalyst loading and the reaction
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Page 8 of 10
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41. We have studied the possibility of leaching of active species
from the heterogeneous material, however, the solutions obtained
after decantation of the heterogeneous catalysts showed no-
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(CH2)3Si(OiPr3)3)] (R = 2,6-diisopropylphenyl (2a); n-butyl (2b))
20 have been synthesised and fully characterised. The study of their
application as ketone hydrosilylation catalysts showed a clear N-
substituent effect, 2a being the most active catalyst precursor.
Complex 2a has been immobilised in the mesoporous materials
MCM-41 and KIT-6. The new hybrid materials have been fully
25 characterised and used as catalyst precursors for the preparation
of poly(silyl ether)s. The heterogeneous catalytic systems based
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with higher average molecular weight (Mw) than the
homogeneous catalyst 2a. Remarkably, reducing the catalyst
30 loading from 2.0 mol % to 0.2 mol % and using the most active
heterogeneous catalytic systems it was possible to obtain
polymers with high average molecular weight Mw = 2.61 106
gmol-1 (2a-MCM-41) and Mw = 4.43 105 gmol-1 (2a-KIT-6).
In summary, it has been demonstrated that by using bulky N-
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40 Financial support from the Spanish “Ministerio de Economia y
Competividad” projects, CONSOLIDER INGENIO CSD2006-
0015 and CSD2009-00050, CTQ2011-27593 and CTQ2012-
35665, “Juan de la Cierva” program (M. I.) and “Diputación
General de Aragón” group E07 is acknowledged.
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Picture for Table of Contents
Abstract for Table of Contents
Poly(silyl ether)s with a high average Mw (2.6 106 gmol-1) were obtained by Rhodium-catalyzed
hydrosilylation.