Biphasic hydroformylation in new molten salts - Analogies and differences to organic solvents

Article abstract of DOI:10.1039/b107720a

New ionic compounds 1,2,3-trimethylimidazolium triflate (1) and 1-ethyl-2,3-dimethylimidazolium triflate (2) and (3-butylimidazole)triphenylboron (4) have been formed by alkylation and condensation reactions. An ion exchange reaction yielded the ionic com

Full text of DOI:10.1039/b107720a

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Biphasic hydroformylation in new molten salts—analogies and
differences to organic solvents†
Oleg Stenzel, Helgard G. Raubenheimer* and Catharine Esterhuysen
Department of Chemistry, University of Stellenbosch, Private Bag X1, 7602 Matieland,
South Africa. E-mail: hgr@sun.ac.za
Received 29th August 2001, Accepted 18th December 2001
First published as an Advance Article on the web 20th February 2002
New ionic compounds 1,2,3-trimethylimidazolium triflate (1) and 1-ethyl-2,3-dimethylimidazolium triflate (2) and
(
3-butylimidazole)triphenylboron (4) have been formed by alkylation and condensation reactions. An ion exchange
reaction yielded the ionic compound 1-butyl-3-methylimidazolium tetraphenylborate (3). Molecular structures of
all four compounds were determined by single crystal X-ray diffractometry. Utilizing two known rhodium complexes
as catalyst precursors, compounds 1, 2 and 4 were used as solvents for biphasic hydroformylation reactions of two
long chain 1-olefins. The results were compared to reactivities in the conventional solvent toluene in which similar
turnover numbers but a higher tendency towards isomerization and hydrogenation occurred.
Introduction
heating/cooling rate of 10 ЊC per minute. Thermogravimetric
analyses were performed on a Perkin Elmer Thermogravimetric
Analyzer TGA 7. NMR spectra were recorded on a Varian
Within the last 15 years ionic liquids have experienced a renais-
sance, especially since these liquids, which consist of ion pairs,
have found application as solvents in biphasic catalysis. With
melting points above 100 ЊC, similar compounds are known as
molten, fused or liquid organic salts. The first ionic liquid was
1
13
1
VXR 300 spectrometer ( H, 300 MHz; C{ H}, 75.48 MHz;
19
1
F{ H}, 283.65 MHz) or Varian INOVA 600 spectrometer
1
13
1
(
H, 600 MHz; C{ H}, 150.87 MHz) at 25 ЊC. Chemical shifts
1 13
are reported in ppm relative to the H and C residue of the
deuterated solvents. Chemical shifts for F{ H} measurements
1
synthesized in 1914, however, it was only once the first reports
of their use as reaction media for organic synthesis appeared
19
1
are given relative to CFCl in acetone. The IR spectra were
3
that their importance became apparent to the broader scientific
recorded on a Perkin Elmer 1600 Series FTIR spectrometer.
Mass spectra (FAB in 3-nitrobenzyl alcohol) were obtained
using a Micromass (VG) instrument with 70SE magnet
sector. Only characteristic fragments containing the isotopes
of the highest abundance are listed. Elemental analyses were
performed on a Fisons CHNS 1108 elemental analyser. 1-
2
community. Ionic liquids show a unique combination of
properties including high polarity, low viscosity, high liquid
temperature range, high thermal stability, immiscibility with
certain organic solvents, wide tunability and no effective
3
vapour pressure. Typical components are 1-alkylpyridinium,
1
,3-dialkylimidazolium, ammonium or phosphonium cations
16
4
Butyl-3-methylimidazolium chloride and (Ϫ)-(η -cycloocta-
1
Ϫ
4
Ϫ
6
Ϫ
6
Ϫ
and halide, AlCl , PF , SbF or BF4 anions. Ionic liquids
5
17
,5-diene)(η -2-menthyl-4,7-dimethylindenyl)rhodium() were
2,4
have been utilized in Friedel–Crafts reactions, aromatic
prepared according to published procedures. 1-Butylimidazole
and 1,2-dimethylimidazole (Aldrich) were dried over P O
10
5
6
7
substitution, acylative cleavage, Heck reactions, hydroesterifi-
8
9
10
11
4
cation, carbonylation, dimerization, polymerization, and
and distilled prior to use. Ammonium tetraphenylborate, ethyl
trifluoromethanesulfonate, methyl trifluoromethanesulfonate,
silver() oxide, and tris(triphenylphosphine)rhodium() chloride
were used as purchased (Aldrich).
1
2
hydrogenation in which often novel and unusual chemical
reactivities were found.
Rhodium complexes show high catalytic activities for organic
transformations and are widely used in combination with chiral
phosphine, sulfide or cyclopentadienyl ligand systems in asym-
1
3,14
Syntheses
metric catalysis.
We report here how our newly synthesized
molten salts were used as solvents in catalytic reactions utilizing
chiral and achiral catalyst precursors, thus improving the
scope and potential enantiomeric excess in biphasic catalytic
1,2,3-Trimethylimidazolium trifluoromethanesulfonate (1). To
a solution of 1,2-dimethylimidazole (34.4 g, 357.8 mmol) in
1,1,1-trichloroethane (140 mL) was slowly added methyl tri-
fluoromethanesulfonate (50.0 g, 304.7 mmol) at 0 ЊC. The white
suspension was heated to 75 ЊC for 3 h, cooled and stirred for
15
hydroformylation.
8
h at 25 ЊC. After filtration through a d4-frit the residue was
Experimental
washed twice with 1,1,1-trichloroethane (80 mL). The solvent
was removed under vacuum (10 mbar), leaving 69.5 g (88%)
of a white solid, mp 129 ЊC (Found: C, 32.54; H, 4.13; N, 10.56;
Ϫ2
All reactions and manipulations were carried out under a dry
argon atmosphere using standard Schlenk, vacuum-line and
glovebox techniques. All solvents were dried and purified by
conventional methods and were freshly distilled under argon
before use. Melting points were measured by differential scan-
S, 12.06%. C H N O SF requires C, 32.31; H, 4.26; N, 10.77;
7
11
2
3
3
Ϫ1
¯
S, 12.32%; M 260.23); ν/cm 3150m, 1598m, 1555m, 1466m,
1
5
3
1
418m, 1277s, 1161s, 1029s, 775s, 757s, 738s, 724w, 659s, 638s,
4,5
74s, 518s (KBr); δ (acetone-d , 300 MHz) 7.60 (s, 2 H, H ),
nd
H
6
ning calorimetry (2 run) on a Du Pont 2100 apparatus at a
6,8
7
.95 (s, 6 H, H ), 2.75 (s, 3 H, H ); δ (acetone-d , 75.41 MHz)
37.6 (C ), 114.4 (C ), 113.7 (q, J = 323.3 Hz, CF ), 26.5
C
6
2
4,5
1
CF
3
6,8
7
(
C ), 0.6 (C ); δ (acetone-d , 283.65 MHz) Ϫ78.1; m/z (FAB,
F 6
ϩ Ϫ
†
Electronic supplementary information (ESI) available: rotatable 3-D
positive ion) 111 (M , 100%); m/z (FAB, negative ion) 149 (M ,
crystal structure diagrams in CHIME format. See http://www.rsc.org/
suppdata/dt/b1/b107720a/
100%).
1
132
J. Chem. Soc., Dalton Trans., 2002, 1132–1138
This journal is © The Royal Society of Chemistry 2002
DOI: 10.1039/b107720a

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8
3
9
1
-Ethyl-2,3-dimethylimidazolium trifluoromethanesulfonate
2 H, H ), 0.93 (t, J = 7.4 Hz, 3 H, H ); δ (acetone-d , 150.87
C 6
ipso 2 ortho
(
2). Similar to the preparation of 1, reaction of 1,2-dimethyl-
MHz) 155.4 (m, C(phenyl) ), 139.2 (C ), 135.9 (C(phenyl) ),
4 meta para
imidazole (15.5 g, 161.2 mmol) with ethyl trifluoromethane-
sulfonate (25.0 g, 140.3 mmol) in 1,1,1-trichloroethane (60 mL)
gave after workup 30.9 g (80%) of a white solid, mp 113 ЊC
128.2 (C ), 128.0 (C(phenyl) ), 125.9 (C(phenyl) ), 121.5
5 6 7 8 9
(C ), 49.5 (C ), 33.9 (C ), 20.7 (C ), 14.3 (C ); m/z (FAB, positive
ϩ
ion) 366 (M , 5%), 289 (M Ϫ phenyl, 100%).
(
Found: C, 35.23; H, 4.90; N, 10.15; S, 11.47%. C H N O SF
8 13 2 3 3
requires C, 35.04; H, 4.78; N, 10.21; S, 11.69%; M 274.26);
ν/cm 3146s, 2983m, 2296w, 1641w, 1594m, 1548m, 1456w,
General procedure for hydroformylation experiments
Ϫ1
¯
In a nitrogen glovebox the catalyst precursor (0.15 mmol), a
magnetic stir bar and the solid reaction medium (5 g) or toluene
1
7
426m, 1392m, 1266s, 1227s, 1153s, 1088w, 1033s, 954m, 806m,
73s, 757m, 727m, 712w, 671m, 639s, 573s, 518s (KBr);
(
20 mL) were placed in a glass lined 300 ml stainless steel auto-
3
4/5
δ (acetone-d , 300 MHz) 7.68 (d, J = 2.1 Hz, 1 H, H ), 7.63
H
6
clave. The apparatus was sealed and 30 mmol of the freshly
distilled substrate was added. The vessel was placed in a
thermostatically controlled oil bath at the chosen temperature
and charged with a 1 : 1 hydrogen/carbon monoxide mixture
3
4/5
3
6
(
(
d, J = 2.1 Hz, 1 H, H ), 4.33 (q, J = 7.4 Hz, 2 H, H ), 3.94
9 8 3 7
s, 3 H, H ), 2.77 (s, 3 H, H ), 1.47 (t, J = 7.4 Hz, 3 H, H );
2
4/5
δC (acetone-d , 75.41 MHz) 136.7 (C ), 114.5 (C ), 113.3
6
1
4/5
6
9
(
(
q, J = 323.4 Hz, CF ), 112.4 (C ), 35.0 (C ), 26.3 (C ), 6.1
CF 3
7 8
(
Afrox) at 80 bar. After a reaction time of 6 h the apparatus was
C ), 0.4 (C ); δ (acetone-d , 283.65 MHz) Ϫ81.0; m/z (FAB,
F
6
cooled, opened, and the organic layer decanted off. The now
solid reaction medium was further extracted with chloroform
ϩ
Ϫ
positive ion) 125 (M , 100%); m/z (FAB, negative ion) 149 (M ,
00%).
1
(
10 mL). A sample of the combined organic fractions was
taken to determine the conversion and the linear : branched
ratio by H NMR. The reported results represent the average of
experiments carried out in duplicate.
1
-Butyl-3-methylimidazolium tetraphenylborate (3). To
1
ammonium tetraphenylborate (17.8 g, 52.8 mmol) was slowly
added at 25 ЊC a suspension of silver() oxide (6.1 g, 26.4 mmol)
in degassed H O (50 mL) under light protection. The grey
mixture was stirred for 1 h and a solution of 1-butyl-3-methyl-
2
X-Ray structure determination
imidazolium chloride (10.0 g, 57.3 mmol) in degassed H O
X-Ray structures were obtained for 1, 2, 3 and 4, however,
2
18
(
100 mL) was added. The suspension was stirred for 30 h and
recently the structure of 3 was published by Dupont et al. The
details of the data collection and structure solution and refine-
ment for 3 are, therefore, not included in this paper, although
they have been deposited with the Cambridge Crystallographic
Data Centre (CCDC deposition number 169679). However, in
the previous publication the authors were not able to determine
absolute structure, possibly as a result of twinning, since this
was also found to be present in the current determination.
In addition, and in contrast to the current determination, no
disorder was observed. As shown in the discussion, disorder
observed in a crystal structure is related to the melting point of
that crystal, therefore it is of great importance. Details of the
modelling of the disorder, as well as of the twinning in 3 are
thus included in this paper, and the discussion refers to the
current structural determination.
filtered using a d4-frit. After washing twice with degassed H O
2
Ϫ2
(
50 mL), the solvent was removed under vacuum (10 mbar)
and the remaining off-white solid was extracted with dichloro-
methane (50 mL). The organic fraction was dried with calcium
Ϫ2
chloride and the solvent was removed under vacuum (10
mbar) leaving 15.4 g (63%) of a white solid, mp 131 ЊC
(
decomp.) (Found: C, 83.75; H, 7.27; N, 6.00%. C H N B
32 35 2
Ϫ1
¯
requires C, 83.84; H, 7.69; N, 6.11%; M 458.45); ν/cm 3438m,
3
1
1
1
162m, 3133m, 3100s, 3082s, 3051s, 2994m, 2954s, 2869m,
941w, 1888w, 1824w, 1766w, 1678w, 1608w, 1579m, 1561s,
479s, 1449m, 1426s, 1379m, 1338w, 1267m, 1248s, 1159s,
143s, 1032m, 841s, 741s, 708s, 646m, 622m, 611s, 484m, 472w
(
KBr); δ (chloroform-d , 300 MHz) 7.53 (m, 8 H, CH(phen-
H 1
ortho meta
yl) ), 6.97 (m, 8 H, CH(phenyl) ), 6.78 (m, 4 H, CH(phen-
yl) ), 5.94 (t, J = 2.0 Hz, 1 H, H ), 5.76 (t, J = 2.0 Hz, 1 H,
H ), 4.67 (s, 1 H, H ), 3.15 (t, J = 7.6 Hz, 2 H, H ), 2.74 (s, 3H,
H ), 1.32 (m, 2 H, H ), 1.14 (m, 2 H, H ), 0.89 (t, J = 7.2 Hz,
para
3
5
3
Data were collected on a Nonius Kappa CCD diffract-
4
2
3
6
19
ometer using monochomated Mo-Kα radiation (λ = 0.71073
10
7
8
3
2
Å). Refinement was based on F . All non-hydrogen atoms were
9
1
3
H, H ); δ (chloroform-d , 75.41 MHz) 164.4 (q, J = 49.4
refined anisotropically. H atoms could in many cases be identi-
fied from Fourier difference maps, but were placed in calculated
positions using a riding model. Disorder was observed in the
structures of 2, 3 (this disorder had not been modelled in a
C
1
CB
2
ipso
ortho
Hz, C(phenyl) ), 135.9 (C(phenyl) ), 134.9 (C ), 126.1
meta
5
para
4
(
(
C(phenyl) ), 122.6 (C ), 122.2 (C(phenyl) ), 120.5 (C ), 48.9
6 10 7 8 9
C ), 35.3 (C ), 31.5 (C ), 19.2 (C ), 13.2 (C ); m/z (FAB, posi-
ϩ
Ϫ
18
tive ion) 139 (M , 100%); m/z (FAB, negative ion) 319 (M ,
00%).
previous structure determination ) and 4. Each structure was
1
dealt with in the same way: the positions of the disordered
atoms were identified from the Fourier difference map; all
atoms in the same disordered group were refined anisotropically
with common site occupancies and the total site occupancy for
the two disordered groups was constrained to 1; if necessary the
ISOR, SADI and/or SIMU restraint commands were used to
obtain sensible structures and displacement parameters. In
addition to the large amount of static disorder there is also a
considerable amount of dynamic disorder that is evidenced by
large anisotropic displacement ellipsoids. Structures for both 2
and 3 were found to be twinned. The space group in 3 is non-
(
3-Butylimidazole)triphenylboron (4). To a suspension of
ammonium tetraphenylborate (17.6 g, 52.2 mmol) in
acetonitrile (50 mL) was added 1-butylimidazole (15.0 mL,
1
14.1 mmol) at 20 ЊC. The white suspension was heated for 70 h
to 81 ЊC while the system was continuously purged with argon.
The mixture was filtered using a d4-frit and the residue was
washed twice with acetonitrile (10 mL). The solvent was
Ϫ2
removed under vacuum (10 mbar) and the remaining solid
was recrystallized from ethylacetate yielding 16.5 g (86%) of a
white solid, mp 152 ЊC (Found: C, 81.60; H, 7.75; N, 7.49%.
C H N B requires C, 81.97; H, 7.43; N, 7.65%; M 366.31);
20
centrosymmetric, and the Flack test suggested the presence of
a racemic twin. Refinement revealed a racemic twin ratio of
0.86 : 0.14. Identification of twinning in 2 was more compli-
cated—systematic absences observed were not consistent with
any space group, although the unit cell was found to have all
angles approximately equal to 90Њ, and thus data were collected
in P1. Careful inspection of the systematic absences and atomic
positions identified from an initial direct methods solution
2
5
27
Ϫ1
2
¯
ν/cm 3426m, 3145m, 3125s, 3062s, 3000s, 2966s, 2930s,
2
1
1
8
877m, 1961w, 1889w, 1829w, 1775w, 1725w, 1644w, 1614m,
586m, 1537s, 1484m, 1455s, 1428s, 1386w, 1361m, 1309w,
261m, 1241m, 1162s, 1108s, 1069m, 1027m, 998m, 882w,
65m, 834s, 799m, 751s, 727s, 706s, 672s, 648s, 620m, 608m
4
2
(
KBr); δ (acetone-d , 600 MHz) 8.01 (t, J = 1.5 Hz, 1 H, H ),
H
6
5
ortho
7
.39 (m, 1 H, H ), 7.20 (m, 6 H, CH(phenyl) ), 7.14 (m, 6 H,
revealed that the space group was P2 /c with twinning across a
1
meta
para
CH(phenyl) ), 7.07 (m, 3 H, CH(phenyl) ), 7.02 (m, 1 H,
H ), 4.19 (t, J = 7.6 Hz, 2 H, H ), 1.84 (m, 2 H, H ), 1.33 (m,
mirror plane perpendicular to the c-axis. In addition it was
found that the asymmetric unit consisted of two chemically
4
3
6
7
J. Chem. Soc., Dalton Trans., 2002, 1132–1138
1133

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Table 1 Crystal data and refinement for [C H N ؒCF SO ] (1), [C H N ؒCF SO ] (2), [C H N ؒBC H ] (3) and [C H N B] (4)
6
11
2
3
3
7
13
2
3
3
2
8
15
2
24 20
25 27
2
2
1
2
4
Formula
M
T /K
C H N F SO
260.23
298(2)
C H N F S O
548.52
172(2)
C H N B
50 54 4 2
732.62
150(2)
7
11
2
3
3
16 26
4
6
2
6
Crystal system
Space group
Monoclinic
P2 /n
1
Monoclinic
P2 /c
1
Triclinic
¯
P1
a/Å
b/Å
c/Å
α/Њ
β/Њ
γ/Њ
U/Å
Z
µ/mm
6.8313(2)
14.4530(5)
11.3689(4)
90
96.503(2)
90
1115.26(6)
4
0.327
15.722(1)
8.615(1)
18.081(1)
90
89.979(11)
90
2449.0(4)
4
0.302
11.3171(10)
12.7981(10)
15.6049(10)
73.70(1)
74.38(1)
77.22(1)
2063.1(3)
2
3
Ϫ1
0.068
Reflections collected/unique
Data/restraints/parameters
Reflections with I > 2σ(I )
R1(I > 2σ(I ))
5124/2524 [Rint = 0.020]
2524/0/153
1977
0.043
0.116
8798/4027 [Rint = 0.019]
4027/12/428
3411
0.039
0.103
11327/9087 [Rint = 0.016]
9087/36/534
6746
0.050
0.133
wR2 (all data)
Table 2 Selected bond lengths (Å) and angles (Њ) for compounds 1, 2 and 4
1
2
4
Imidazole ring
a
a
N(1)–C(2)
N(2)–C(2)
N(1)–C(5)
N(2)–C(4)
N(1)–C(6)
1.326(3)
1.324(3)
1.392(3)
1.391(3)
1.463(3)
N(1)–C(2)a
N(2)–C(2)
1.34(1)
1.32(3)
1.39(2)
1.39(2)
1.45(3)
N(1)–C(1)a
1.326(3)
1.381(2)
1.337(2)
1.373(3)
1.615(3)
1.632(3)
1.474(2)
N(1)–C(2)
a
a
N(1)–C(5)a
N(2)–C(1)a
N(2)–C(4)a
N(1)–C(3)
N(2)–C(3)
a
B–Na
B–C
a
a
N(2)–C(3)
1.465(3)
N(2)–C(6)
1.46(3)
N(2)–C(4)
a
a
C(2)–N(1)–C(5)
C(2)–N(2)–C(4)
N(2)–C(2)–N(1)
108.47(18)
108.53(18)
108.40(19)
C(2)–N(1)–C(5)
108.5(6)
107.9(5)
109.1(6)
C(1)–N(1)–C(2)a
C(1)–N(2)–C(3)
106.4(1)
107.8(2)
110.4(2)
129.8(1)
123.7(1)
111(3)
a
N(2)–C(2)–N(1)a
a
C(2)–N(2)–C(4)
N(1)–C(1)–N(2)
a
C(1)–N(1)–Ba
C(2)–N(1)–B
a
C–B–C
Anion
a
a
S–O
1.437(5)
1.821(2)
1.329(3)
S–O
S–C
C–F
1.426(9)
1.801(6)
1.32(1)
a
S(1)–C(7)
a
a
C–F
a
a
O–S–O
O–S–C
F–C–F
F–C–S
115.0(6)
103.20(1)
107.5(3)
111.3(1)
O–S–Oa
114.8(16)
103.3(4)
107.0(8)
111.8(7)
a
O–S–C
a
a
F–C–Fa
a
F–C–S
a
Average value.
equivalent but crystallographically independent ion pairs. All
cations can be deprotonated under basic conditions to form the
corresponding carbene compounds. To inhibit this degrad-
21
24
calculations were performed using SHELXL 97 within the
22
WINGX package. Details of the data collection and structure
refinement are included in Table 1. Selected bond lengths and
angles are given in Table 2. When a bond length or angle occurs
more than once (i.e. for two chemically equivalent but crystal-
lographically different molecules in an asymmetric unit), the
average value is given. Average values for similar bonds (e.g.
B–C bonds in B(C H ) anion) are also given. Figures were
ation we synthesized two new compounds with alkyl substitu-
ents in the 2 position. Depending on the triflate used, com-
pounds 1 and 2 were obtained by alkylation of 1,2-
dimethylimidazole at 0 ЊC in analogy to a previously published
25
procedure. After work up the products were isolated as white
solids in 88 and 80% yield (Scheme 1).
6
5 4
generated using Ortep3 for Windows; displacement ellipsoids
23
are at the 50% probability level.
CCDC reference numbers 169677–169680.
See http://www.rsc.org/suppdata/dt/b1/b107720a/ for crystal-
lographic data in CIF or other electronic format.
Scheme 1
Results and discussion
The highly symmetrical compound 1 melts at 129 ЊC, while
the unsymmetrical substituted compound, 2, melts at 113 ЊC.
Furthermore, 1 shows a liquid crystal phase formation at 48
Syntheses of ionic compounds
The commonly used ionic liquids with 1,3-dialkylimidazolium
1
134
J. Chem. Soc., Dalton Trans., 2002, 1132–1138

View Article Online
2
6
13
1
ЊC. Their melting points are significantly higher than for
the unsubstituted compounds 1,3-dimethylimidazolium triflate
(
202 ppm in the C{ H} NMR in acetone-d at 75.48 MHz).
6
Pure crystalline compounds 1–4 although fairly stable in air,
were handled under an inert atmosphere at all times. The high
thermal stability of 1–3 was confirmed by mass spectrometry
(FAB) as the molecular ions form the peaks of highest intensity.
In the case of the covalent compound 4, however, easy loss of a
phenyl ring was detected.
39 ЊC) and 1-ethyl-3-methylimidazolium triflate (Ϫ9 ЊC).
However, the melts decompose only at very high temperatures
of 495 ЊC (1) and 450 ЊC (2) as shown by DSC and TGA meas-
urements. The molten salts are, therefore, suitable reaction
media for catalytic applications over a wide temperature range.
A different synthesis of compound 3 that contains the
sterically demanding tetraphenylborate anion, was published
Catalytic hydroformylation
18
after we finished this work. Salt 3 was obtained by ion
exchange between ammonium tetraphenylborate and 1-butyl-3-
methylimidazolium chloride in the presence of silver() oxide in
water. Formation of insoluble silver() chloride and volatile
ammonia drives the reaction (Scheme 2). After work up the
compound was isolated in 63% yield as a white solid, melting
at 131 ЊC with degradation (DSC) and exhibiting an actual
decomposition temperature of 270 ЊC (TGA).
The newly synthesized ionic compounds 1 and 2 as well as
the new coordination compound 4 were used as solvents for
biphasic hydroformylation reactions. The prochiral substrates
1
-hexene and 1-dodecene were reacted with the chiral, stereo-
4
merically pure catalyst precursor (Ϫ)-(η -cycloocta-1,5-diene)-
17,30
(
2-menthyl-4,7-dimethylindenyl)rhodium(),
5, and the
achiral Wilkinson complex [(PPh ) RhCl], 6 (Scheme 4) at
3
3
1
40 or 155 ЊC. No Rh() deposition, that could have led to
heterogeneous reactions, was observed. The results are com-
piled in Table 3. High olefin conversions of 68 to 100% were
achieved at 80 bar CO/H pressure after reaction times of 6 h.
2
The turnover numbers are in the typical range for rhodium
31
catalysts. The Wilkinson catalyst, however, shows higher con-
versions than 5 and reactions in the melts of 1, 2 and 4 occurred
in general slower than in toluene. This is probably due to
lower solubility and diffusion coefficients of the gases and the
substrates in the molten reaction media.
Scheme 2
Much lower melting points (<50 ЊC) were reported for
molten tetraalkylammonium tetraalkylborates (linear alkyls
with 2, 4, 6, and 8 carbon atoms) which, however, were only
27
stable at temperatures up to 100 ЊC. Decomposition of 3 pre-
sumably occurs because the tetraphenylborate anion is not
chemically inert and easily fragments by generating benzene.
Furthermore, this fragmentation was observed during our
attempted synthesis of 1-butylimidazolium tetraphenylborate.
This compound was expected to have a lower melting point
Scheme 4
The actual yields of the corresponding linear and branched
aldehydes are often also lower and vary between 38 and 100%.
This is due to competing hydrogenation which transforms the
olefins into the corresponding unreactive alkane in 0 to 60%
yield. This hydrogenation rate is significantly higher than in the
conventional solvent toluene in which less than 10% hydrogen-
ation occurs. In previous articles on the use of ionic liquids in
hydroformylation reactions, this increase was not mentioned.
26
than 3. Reaction of ammonium tetraphenylborate and excess
1
-butylimidazole gave the desired compound in solution. How-
ever, as coordinated acetonitrile had to be removed at high
temperatures under vacuum, the salt decomposed and formed
the coordination compound (3-butylimidazole)triphenylboron,
32
4
, in 86% yield (Scheme 3). Volatile benzene and ammonia were
Only 1–2% byproducts were reported even though hydrogen-
also produced.
ation and hydroformylation were catalyzed simultaneously by a
3
3
rhodium catalyst in an ionic liquid.
The observed linear/branched ratios varied between 1 : 0.4
and 1 : 2.2 and are unaffected by the nature of the solvent.
Typical linear/branched ratios of 1 : 1 to 1 : 3 in toluene are
34
mentioned in the literature. In molten salts 1, 2 and in toluene
the tendency of catalyst precursor 6 to form the linear aldehyde
is higher than that of 5, which favours the branched products.
In the covalent solvent (4) at increased temperature, however,
both catalyst precursors favour the branched products as a
dramatic increase in isomerization activity occurs.
Scheme 3
Similar bond cleavages have been reported for alkylammon-
ium salts of N-triphenylboron-NЈ-methylpiperazinium and
during the accidental synthesis of (3-methylimidazole)-
2
8
In the case of 1-dodecene, for example, all six possible alde-
hydes are detectable and the 3- and higher aldehydes are formed
in up to 44% yield. No essential reactivity change occurred
when the covalent compound 4 was substituted for the molten
salts 1 and 2. However, only from the latter could the catalyst be
recovered successfully in multicyclic reactions. In general, the
tendency towards isomerization of the olefinic double bond to
the thermodynamically more stable isomers is higher than in
toluene, as ionic liquids are known to stabilize the necessary
ϩ
triphenylboron starting from the cation [ReO (1-mim) ] (mim
2
4
2
9
=
methylimidazole) and Na[BPh4]. Compound 4 melts with-
out decomposition at 152 ЊC (DSC) with an eventual decom-
position temperature of 280 ЊC (TGA). The melt is, therefore,
suitable as a reaction medium. Imidazolium compound 3 with
no substituent in the 2-position, can indeed be deprotonated
under basic conditions by a methyllithium solution in diethyl
ether to form the corresponding free carbene (carbene signal at
33
cationic intermediates. The isomerizations under the current
J. Chem. Soc., Dalton Trans., 2002, 1132–1138 1135

View Article Online
a
Table 3 Hydroformylation in the presence of rhodium catalyst precursors
Yield of
aldehyde(%)
Yield of
alkane(%)
b
c
d
Solvent
Catalyst
Substrate
T /ЊC
Conversion(%)
n : i
Isomerisation (%)
n
1
5
5
6
6
5
5
6
6
5
5
6
6
5
5
6
6
hexene
dodecene
hexene
dodecene
hexene
dodecene
hexene
dodecene
hexene
dodecene
hexene
dodecene
hexene
dodecene
hexene
140
140
140
140
140
140
140
140
155
155
155
155
140
140
140
140
81
82
91
100
68
89
53
60
66
59
40
51
82
73
57
38
58
68
92
94
90
100
28
22
25
41
28
38
14
27
43
60
41
32
8
1 : 1.0
1 : 1.4
1 : 0.6
1 : 0.4
1 : 0.9
1 : 2.1
1 : 0.5
1 : 0.4
1 : 2.2
1 : 1.3
1 : 1.8
1 : 1.5
1 : 1.3
1 : 0.9
1 : 0.6
1 : 0.5
9
23
5
3
4
3
4
3
3
3
4
3
4
3
6
3
4
3
4
1
1
1
4
2
10
35
2
2
2
96
2
100
100
98
3
4
14
44
28
29
15
9
4
4
99
4
100
100
100
100
100
toluene
toluene
toluene
toluene
6
10
0
5
3
dodecene
a
b
c
Reaction conditions as described in the Experimental section (6 h, 80 bar CO/H ). n : i = n-aldehyde : all iso-aldehydes. Amount of 3- and higher
2
d
aldehydes/amount of all aldehydes × 100. Number of different aldehydes formed.
conditions are still less predominant than in the most successful
synthetically applied isomerizations in toluene with up to 92%
35,36
conversion.
Isomerized 1-dodecene can give rise to six different aldehydes
of which five are chiral. Isomerized 1-hexene can be converted
into three different aldehydes of which two are chiral. Due to
the high reaction temperatures, kinetic discrimination of the
diastereomorphic catalytic transition states is low, resulting in
low selectivity and enantiomeric excess. Therefore, no attempts
were made to separate the different isomers or to determine
enantiomeric purity of the products.
Structural studies
Colourless crystals of 1, 2, 3, and 4 suitable for single crystal
X-ray diffraction analysis were obtained by recrystallization
from acetonitrile (1) or ethylacetate (2–4) at room temperature
(
2
1–3) or Ϫ78 ЊC (4). The molecular structures of molten salts 1,
, and 3 (Figs. 1–3) consist of ion pairs with the cation being an
Fig. 2 Molecular structure of complex 2 showing the numbering
scheme and disordered conformations. Hydrogen atoms have been
omitted for clarity.
imidazole with varying substituents, while the anion is either
triflate (1, 2) or tetraphenylborate (3). The covalent compound
4
(Fig. 4) consists of a triphenylboron functionalized imidazole.
Fig. 1 Molecular structure of complex 1 showing the numbering
scheme.
Conformational disorder was observed in the butyl chains of
(ratio 0.82 : 0.18) and 4 (ratio 0.62 : 0.38), while in 2 the entire
3
Fig. 3 Molecular structure of complex 3 showing the numbering
scheme and disordered conformation. Hydrogen atoms have been
omitted for clarity.
imidazole ring is disordered (ratio 0.57 : 0.43). Very low peaks
of electron density indicating a very small amount of disorder
in the imidazole ring were also observed in the Fourier differ-
ence map of 1, although the disorder could not be successfully
modelled. Selected bond lengths and angles (Table 2) compare
well with values found in the structures of (3-methylimid-
39
1-imidazolediphenylboranes and the previously published
18
structure of 3.
Crystal packing of 1 consists of tightly packed channels of
alternating cation–anion pairs connected by hydrogen bonding
between a triflate oxygen and a hydrogen of an imidazole
2
9
37
azole)triphenylboron,
3-borane-1,4,5-trimethylimidazole,
38
lithium triethyl(1,4,5-trimethylimidazolyl)borate, polymeric
1
136
J. Chem. Soc., Dalton Trans., 2002, 1132–1138

View Article Online
of 1-butylimidazolium tetraphenylborate, yielding selectively
3-butylimidazole)triphenylboron.
X-Ray diffraction analyses revealed disorder in the molecular
(
structures and hydrogen-bonding (or other intermolecular
interaction) in the crystal structures of 1, 2, 3, and 4. These
opposing structural influences accounted for the variation in
measured melting temperatures.
The molten salts 1, 2 and the covalent compound 4 were
successfully used as reaction media for biphasic rhodium
catalysed hydroformylation reactions of 1-olefins. High conver-
sions with varying linear/branched ratios were observed. In
comparison to the conventional solvent toluene, similar turn-
over numbers but a higher tendency towards isomerization and
hydrogenation occurred.
Acknowledgements
This work was supported by the Alexander von Humboldt-
Stiftung (Feodor-Lynen-Fellowship for O.S.), the National
Research Foundation, and the partnership program University
of Stellenbosch-Technische Universität Berlin (Germany).
Fig. 4 Molecular structure of complex 4 showing the numbering
scheme and disordered conformation. Hydrogen atoms have been
omitted for clarity.
carbon. Although alternating rows of cations and anions are
also observed in 2 the channel structure is not as clear as in 1 as
a result of the different orientations of the two crystallographic-
ally independent molecules in the asymmetric unit. Hydrogen-
bonding occurs between methyl hydrogens and triflate oxygens
as well as between hydrogens on the terminal carbon of the
ethyl chain and triflate fluorines. Packing in 3 consists of
an interconnecting network of cations and anions linked by
C–H ؒ ؒ ؒ π interactions from imidazole hydrogens to phenyl
rings and vice-versa as well as hydrogens on the butyl chains to
phenyl rings. Similar C–H ؒ ؒ ؒ π interactions are also observed
in 4, although the resulting interconnecting network of
molecules is more tightly packed than in 3.
Two opposing effects are observed in the crystal and molecu-
lar structures of 1–4 that have an influence on their melting
points, namely hydrogen bonding (or similar non-bonded inter-
actions such as C–H ؒ ؒ ؒ π interactions) and disorder. Inter-
molecular interactions increase the order of the system by
organising the crystal framework and thus raise the melting
point, while disorder is associated with a lowering of the
melting point.
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