Nickel-catalyzed propene dimerization reactions in triphenylbismuth- buffered chloroaluminate ionic liquids: High performance with unconventional cations

Article abstract of DOI:10.1002/adsc.201100559

The addition of triphenylbismuth (BiPh₃) efficiently buffers Lewis acidic chloroaluminate ionic liquids over a wide range of compositions. Since the melting points of these ternary mixtures are lower compared to unbuffered chloroaluminate systems, the scope of cations for the formation of buffered room temperature ionic liquids is greatly extended. The buffered ionic liquids were used to activate nickel complexes for selective biphasic propene dimerization reactions. Lifetimes, selectivities and productivities of such dimerization catalysts could be adjusted by the choice of the cation and the composition. At lower reaction temperatures, the selectivities to give dimers increased significantly. Propene dimers were obtained with selectivities of up to 98%. A cation screening with 100 ammonium and phosphonium halide salts was performed. N-Methylpyrrolidine hydrochloride gave the best results in terms of selectivity and lifetime. Propene dimerization reactions in BiPh ₃-buffered chloroaluminate melts were catalyzed by various soluble nickel compounds with similar performances. The introduction of basic, sterically demanding tricyclohexylphosphine ligands led to higher degrees of branching, however, at the expense of lower dimer selectivities. Copyright

Full text of DOI:10.1002/adsc.201100559

FULL PAPERS
DOI: 10.1002/adsc.201100559
Nickel-Catalyzed Propene Dimerization Reactions in
Triphenylbismuth-Buffered Chloroaluminate Ionic Liquids:
High Performance with Unconventional Cations
Matthias Dçtterla and Helmut G. Alta,*
a
Laboratorium fꢀr Anorganische Chemie II, Universitꢁt Bayreuth, Universitꢁtsstraße 30, 95440 Bayreuth, Germany
Fax : (+49)-921-552535; phone: (+49)-921-43864; e-mail: helmut.alt@uni-bayreuth.de
Received: July 14, 2011; Revised: September 18, 2011; Published online: February 9, 2012
Dedicated to Professor Peter Morys on the occasion of his 70th birthday
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201100559.
Abstract: The addition of triphenylbismuth (BiPh3) dimers were obtained with selectivities of up to 98%.
efficiently buffers Lewis acidic chloroaluminate ionic A cation screening with 100 ammonium and phos-
liquids over a wide range of compositions. Since the phonium halide salts was performed. N-Methylpyrro-
melting points of these ternary mixtures are lower lidine hydrochloride gave the best results in terms of
compared to unbuffered chloroaluminate systems, selectivity and lifetime. Propene dimerization reac-
the scope of cations for the formation of buffered tions in BiPh3-buffered chloroaluminate melts were
room temperature ionic liquids is greatly extended. catalyzed by various soluble nickel compounds with
The buffered ionic liquids were used to activate similar performances. The introduction of basic,
nickel complexes for selective biphasic propene di- sterically demanding tricyclohexylphosphine ligands
merization reactions. Lifetimes, selectivities and pro- led to higher degrees of branching, however, at the
ductivities of such dimerization catalysts could be ad- expense of lower dimer selectivities.
justed by the choice of the cation and the composi-
tion. At lower reaction temperatures, the selectivities Keywords: biphasic catalysis; chloroaluminates; di-
to give dimers increased significantly. Propene merization; ionic liquids; nickel; triphenylbismuth
Introduction
tively. Currently, 35 licensed DIMERSOLꢂ units with
typical capacities between 20,000 and 90,000 tons per
Selective dimerization reactions of a-olefins attracted year are in use for selective dimerization reactions of
a lot of attention in the second half of the 20th centu- short chain olefins.[4]
ry, after Ziegler described the “nickel effect” in
About 30 years ago, Wilkes et al. and Osteryoung
1955.[1] Nickel impurities from previous hydrogena- et al. reported the first room temperature ionic liquids
tion reactions were found to catalyze selective ethene based on aluminum chloride.[5] In the 1980s, these
dimerization reactions to give butenes in the presence chloroaluminate ionic liquids were thoroughly investi-
of alkylaluminum compounds. Ziegler was originally gated in terms of their physical and electrochemical
interested in chain growth and polymerization reac- properties.[6] It took about ten more years until
tions. However, his discovery also launched an inten- chloro
ACHTUNGTRENaNUNG luminate ionic liquids started to attract atten-
sive research on selective nickel-catalyzed dimeriza- tion as new media for biphasic catalysis.[7]
tion reactions of short chain olefins in the following
decades.[2]
Initially, Lewis acidic chloroaluminate melts were
used as catalysts for Friedel–Crafts alkylation reac-
The DIMERSOLꢂ process of the Institut FranÅais tions.[8] In the 1990s, Chauvin et al. developed nickel-
du Pꢃtrole ꢄnergies Nouvelles (IFPEN) is the largest catalyzed biphasic dimerization reactions of a-olefins
industrial process based on nickel complexes activat- in organochloroaluminate ionic liquids.[9] Ethylalumi-
ed by alkylaluminum cocatalysts. It was developed by num groups were found to suppress uncontrolled cat-
Chauvin et al. in the 1970s[3] to dimerize propene (DI- ionic olefin oligomerization reactions, which occur in
MERSOLꢂ G) or 1-butene (DIMERSOLꢂ X) selec- solely AlCl3-based ionic liquids.[10] The IFPEN
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brought this so-called DIFASOLꢂ process to industri- upon addition of BiPh3. Thus, it was possible to
al application by retrofitting it to their existing DI- obtain buffered room temperature ionic liquid com-
MERSOLꢂ units.[4a,11]
positions from aluminum chloride and cations which
Besides alkylaluminum groups, the addition of would only yield solids at ambient temperature with-
weak Lewis bases also reduces the “latent acidity”[12] out BiPh3.[14a] Furthermore, the scope of potential cat-
of chloroaluminate melts. While Wasserscheid et al. ions became even larger since low-melting highly
used pyrrole, pyridine, and quinoline derivatives to acidic systems could be used for nickel-catalyzed se-
prevent undesired cationic olefin oligomerization re- lective olefin dimerization reactions. Because no reac-
actions,[13] we recently reported that triphenylphos- tive alkylaluminum groups were present in BiPh3-buf-
phine and triphenylbismuth efficiently buffer even fered chloroaluminate ionic liquids, these systems
highly acidic chloroaluminate ionic liquids.[14] These were not limited to quaternary ammonium cations. In-
buffered chloroaluminate melts were successfully em- stead, cheap halide salts like amine hydrochlorides
ployed to activate nickel complexes for selective pro- could be used, which can be recovered easily upon
pene dimerization reactions.
hydrolysis and a subsequent drying step.
Buffering with Lewis base additives is advantageous
In this paper, we report nickel-catalyzed propene
compared to the commercial DIFASOLꢂ system. dimerization reactions in BiPh3-buffered chloroalumi-
First, buffering with EtAlCl2 is restricted to only nate melts with 100 mostly unconventional cation
slightly acidic compositions. If higher acidities are compositions. Only two out of 100 mixtures did not
chosen, the dimer selectivities decrease. Second, at form liquids at ambient temperature. Furthermore,
higher alkylaluminum contents, leaching of neutral the effects of Lewis acidities, buffer contents and re-
chloroalkylaluminum compounds formed by dispro- action temperatures on the dimer selectivities were
portionation reactions[15] becomes a major prob- investigated. Finally, several nickel complexes were
lem.[11b] Also, alkylaluminum cocatalysts contribute to tested for their performances in BiPh3-buffered chlor-
the deactivation of Ni(II) catalysts by reducing Ni(II) oaluminate melts.
to its zero oxidation state.[16] Usually, the melting
points of aluminum chloride-based ionic liquids dis-
play a minimum for highly acidic melts, while a local Results and Discussion
maximum is observed around equimolar composi-
tions.[6b,17] Therefore, DIFASOLꢂ systems are limited Variation of Buffering and Acidity Levels
to cations intrinsically forming low-melting liquids,
for example, N,N’-alkylmethylimidazolium or N-alkyl- As described in our recent paper, not only slightly
pyridinium salts.[13b]
acidic chloroaluminate melts but also highly acidic
In a recent paper, we described that the melting [cation] CTHNUGTRNENUG
+A[Al2Cl7]À systems could be buffered with
points of acidic chloroaluminate systems are reduced BiPh3.[14a] Therefore, the effects of different acidities
Table 1. Nickel-catalyzed propene dimerization reactions in BiPh3-buffered chloroaluminate melts with different composi-
tions.[a]
No.
[AlCl3]/
E
E
[BMIMCl]
Productivity[b]
C6 [%]
1
2
3a
3b[e]
4
5
6
7
8
1.20
1.20
1.20
1.20
1.20
1.30
1.50
1.50
2.00
2.00
2.00
2.00
2.00
0.05
0.07
0.12
0.12
0.30
0.12
0.12
0.18
0.12
0.18
0.24
0.30
0.60
>15.8[c]
>10.8[c]
>8.0[c]
0
oil[d]
89.2
93.1
–
96.0
81.8
74.1
83.2
74.8
79.5
83.3
85.5
90.6
6.7
>8.0[c]
>10.6[c]
>12.0[c]
>11.4[c]
>12.3[c]
>7.4[c]
>9.3[c]
>11.5[c]
9
10
11
12
[a]
Reaction conditions: 2.5–3.5 g buffered ionic liquid; catalyst precursor A; [cat]=10À5 mol/gionic liquid; T=258C; stirring
rate=1200 minÀ1; t=60 min; 300-mL glass autoclave; 40–70 mL liquid propene.
gproduct/gionic liquid ꢆh.
Complete conversion.
Higher oligomers derived from a cationic oligomerization reaction.
No catalyst precursor A was added.
[b]
[c]
[d]
[e]
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Nickel-Catalyzed Propene Dimerization Reactions
and BiPh3 contents in chloroaluminate ionic liquids Cation Screening
on nickel-catalyzed propene dimerization reactions
were investigated in detail (Table 1). For the screen- Considering these results, we were curious if our
ing experiments, we used our standard nickel complex system could be extended to rather unusual and
A as the catalyst precursor (Figure 1).
cheap cations. In the literature, only a few chloroalu-
minate systems were mentioned which do not rely on
standard imidazolium or pyridinium cations. Trime-
thylammonium heptachlorodialuminate was used for
cationic oligomerization reactions of 1-decene.[18] The
use of triethylamine hydrochloride, dibutylamine hy-
drochloride, ethylamine hydrochloride, and dimethyl-
amine hydrochloride was reported for Friedel–Crafts
alkylation reactions.[7e] Dimethylaniline hydrochlo-
ride,[19] and several quaternary asymmetrical benzyl-
substituted melts were described, too.[20] However,
none of these were used for nickel-catalyzed olefin di-
merization reactions due to the above-mentioned
Figure 1. Catalyst precursor A used for biphasic propene di-
merization reactions.
We chose a system based on the commercially melting point and acidity restrictions. Friedel–Crafts
available 1-butyl-3-methylimidazolium (BMIM) tetra- alkylation reactions or cationic olefin oligomerization
chloroaluminate, to which the corresponding amounts reactions employ the Lewis acidic ionic liquid itself as
of AlCl3 were added. With slightly acidic AlCl3/ catalyst. Thus, in these cases a variation of the cation
BMIMCl=1.20 systems, which were used by Was- should only result in minor changes in the results of
serscheid et al.[13b] and for DIFASOLꢂ systems,[9c] the reactions. In contrast, the performances of nickel-
dimers were obtained starting with 0.07 equivalents of catalyzed olefin dimerization reactions in BiPh3-buf-
BiPh3 (Table 1, No. 2). With increasing buffer content, fered chloroaluminate melts should strongly depend
selectivities increased. Employing 0.30 equivalents of on the cation. The reactions are catalyzed by a nickel
BiPh3, already 96% dimers were obtained (No. 4), complex, not by the ionic liquid itself. It is necessary
however, at the expense of a reduced productivity. that the BiPh3 buffer dissolves well in the ionic liquids
Such highly selective systems almost exclusively pro- to minimize leaching effects. Also, the cation deter-
duced dimers and trimers. Only traces of higher oligo- mines the viscosities and the solubilities of the sub-
mers were formed. For systems with lower dimer se- strate olefin within the ionic liquids. Both the viscosi-
lectivities, the product curve is shifted to higher oligo- ty and the solubility of the substrate are crucial since
mers.
biphasic nickel-catalyzed dimerization reactions in
In the absence of a nickel catalyst, no dimerization chloroaluminate melts suffer from a mass transport
reaction occurred (No. 3b). Instead, this system was limitation of the olefin.[13d] Therefore, we decided to
completely inactive. The buffer fully suppressed un- perform a systematic screening of cations including
controlled cationic oligomerization reactions, which quaternary ammonium salts, hydrochlorides of pri-
are usually initiated by acidic chloroaluminate mary, secondary and tertiary amines, phosphonium
melts.[10]
salts, heterocyclic ammonium salts, and cations with
Due to mass transfer limitations,[13d] the term “ac- more than one heteroatom. One hundred different
tivity” was replaced by “productivity”. Thus, above a halide salts and their AlCl3/halide salt=2.00 composi-
[14a]
catalyst concentration of about 10À6 mol/gionic
,
tions were synthesized. These compositions were buf-
liquid
the product yield per hour is proportional to the fered with 0.30 equivalents of BiPh3 and tested for
amount of ionic liquid and independent from the cat- nickel-catalyzed propene dimerization reactions. The
alyst concentration. As expected, increasing acidities screening reactions were performed in batch experi-
with a constant buffer level reduced the dimer selec- ments. Furthermore, the lifetime of each system was
tivities. However, from an AlCl3/BMIMCl ratio of investigated qualitatively. After each experiment, the
1.50 to 2.00, the selectivities dropped only a few per- product phase was decanted until the dimer selectivity
cent if the buffer level was held constant. Thus, the dropped significantly. In Table 2, 34 selected cations
advantage of low melting points at highly acidic com- and the corresponding C6 selectivities obtained in the
positions did not have to be paid by disproportional first batch experiment are shown. The complete data-
high amounts of buffer. With only 0.12 equivalents of set can be found in the Supporting Information.
buffer, the maximum acidic system still yielded about
75% dimers (No. 8).
Since the reference complex A had no effect on the
branching of the hexene dimers,[14a] all C6 fractions
consisted of (Æ2%) 25% n-hexenes, 69% 2-methyl-
pentenes, and 6% 2,3-dimethylbutenes. This correlat-
ed to the typical product distribution obtained from
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Table 2. Selected results of the cation screening for nickel-catalyzed propene dimerization reactions in BiPh3-buffered chlo-
roaluminate melts (sorted by C6 selectivity within quaternary, tertiary, secondary, primary amines and phosphonium salts).[a]
ACHTUNGTRENNUNG
No.
13
Cation
C6 [%]
92.0
No.
30
Cation
C6 [%]
89.6
14
15
91.2
90.9
31
32
89.6
89.6
16
17
90.1
90.0
33
34
89.4
89.1
18
19
89.7
89.3
35
36
90.4
83.0
20
21
22
89.3
89.2
86.2
37
38
39
73.3
79.0
76.1
23
75.9
40
66.1
24
25
26
27
93.1
92.5
91.6
90.7
41
42
43
44
92.5
91.0
85.4
78.1
28
90.0
45
68.6
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Nickel-Catalyzed Propene Dimerization Reactions
Table 2. (Continued)
No.
Cation
C6 [%]
89.9
No.
46
Cation
C6 [%]
49.5
29
[a]
+A[Al2Cl7]À =0.30; catalyst precursor A;
Reaction conditions: 2.5–4.0 g buffered ionic liquid; composition [BiPh3]/ACTHNUTRGENNG[U cation] CHTUGNTRENNUGN
[cat]=10À5 mol/gionic liquid; T=258C; stirring rate=1200 minÀ1; t=60 min; 300-mL glass autoclave; 40–60 mL liquid pro-
pene.
ligand free nickel salts.[13d] Because the productivities ethyl (No. 17) instead the butyl chain significantly im-
of such buffered systems mainly depended on the buf- proved the selectivity as well as the lifetime of the
fering level,[14a] the productivities of different cations system (see Supporting Information). As already re-
should not be compared directly. Cations yielding ported, the system based on N-methylpyrrolidine hy-
compositions with a good BiPh3 efficiency automati- drochloride (No. 26) displayed the best performance
cally displayed higher dimer selectivities and subse- in terms of selectivity and qualitative lifetime.[14a] In
quently lower productivities. Therefore, the goal was the case of the BiPh3 buffer, there appears to be an
to identify systems, which kept high selectivities over optimum range of carbon atoms for ammonium salts,
many repetitions.
roughly between five and nine. Ionic liquids derived
Surprisingly, 95 of 100 compositions produced from cations with more or less carbon atoms dropped
dimers. Only two cations, formamidine hydrochloride much faster in terms of selectivity than those within
and 1,4-diazabicycloACTHNUTRGNE[UNG 2.2.2]octane hydrochloride, yield- that range.
ed solids at ambient temperature (see Supporting In-
formation). Even the pentavalent triphenylphosphine
dichloride based system (No. 46) successfully dimer- Temperature Effect on Dimer Selectivities
ized propene to give 50% dimers. However, the sys-
tems displayed big differences in their overall per- Furthermore, we investigated the effect of the reac-
formances. The best results were obtained with qua- tion temperature on dimer selectivities with 0.30 and
ternary ammonium salts and hydrochlorides of terti- 0.60 equivalents of BiPh3 (Table 3). The reactions
ary amines. In general, hydrochlorides of primary and were performed either at ambient temperature or at
secondary amines were less selective. However, even 08C.
small changes on the cation could have severe effects
While the selectivity of an ethyldiisopropylamine
on the lifetimes and selectivities of the systems. For (Hꢀnigꢇs base) hydrochloride-based system with
example, the 1-butyl-3-methylimidazolium chloride 0.30 equivalents of BiPh3 increased from 91% at 258C
cation, which was used for the initial acidity screening (No. 47) to 96% at 08C (No. 48) the mixture contain-
experiments, was found to be among the worst quater- ing 0.60 equivalents of BiPh3 solidified at 08C (No.
nary ammonium cations (No. 22). Introduction of an 50). Thus, the series was repeated with tributylamine
Table 3. Nickel-catalyzed propene dimerization reactions in BiPh3-buffered chloroaluminate melts at 08C and 258C.[a]
No.
Cation
[Buffer]/
E
Temp.[8C]
Productivity[b]
C6 [%]
47
48
49
50
51
52
53
54
0.30
0.30
0.60
0.60
0.30
0.30
0.60
0.60
25
0
25
0
25
0
25
0
>10.1[c]
>6.7[c]
4.6
90.7
95.5
93.8
–
89.4
96.0
90.8
98.0
[d]
–
>10.6[c]
>12.9[c]
>7.6[c]
1.8
[a]
+A[Al2Cl7]À =0.30; catalyst precursor A;
Reaction conditions: 3.2–5.2 g buffered ionic liquid; composition [BiPh3]/ACTHNUTRGENNG[U cation] CHTUGNTRENNUGN
[cat]=10À5 mol/gionic liquid; stirring rate=1200 minÀ1; t=60 min for T=258C; t=120 min for T=08C; 300 mL glass auto-
clave; 40–60 mL liquid propene.
Productivity given in gproduct/gionic liquid ꢆh.
Complete conversion.
[b]
[c]
[d]
Composition solid at 08C.
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Matthias Dçtterl and Helmut G. Alt
FULL PAPERS
hydrochloride. By lowering the temperature to 08C, lyst concentration was chosen. In a recent paper, we
the dimer selectivity could be increased by 7% to determined the minimum catalyst concentration
96% (No. 52) and 98% (No. 54), respectively. The necessary to excludeÀ6catalyst concentration restric-
[14a]
same effect was reported by Chauvin et al. for tions to be around 10 molNi/gionic liquid
.
We applied
EtAlCl2-buffered systems.[9c] An explanation for the a concentration of 10À5 molNi/gionic liquid to ensure that
increased selectivities at lower reaction temperatures the catalyst concentration was not the rate-determin-
might be the lower olefin solubility in combination ing factor.
with a slower rate of olefin insertion in the catalytic
First, the four binary nickel halides were tested. All
cycle. Both increase the probability for the elimina- of them yielded very high selectivities of more than
tion step to occur. The experiments also showed that 90%. However, the productivities were quite differ-
above a certain buffer concentration, the addition of ent. Nickel fluoride produced 24.9 g product per gram
more BiPh3 only resulted in minor improvements of buffered ionic liquid (No. 55). The other halides dis-
the dimer selectivity. Thus, a cation screening was played lower productivities. This behaviour might be
mandatory to identify halide salts which display high explained by the low solubility of these ligand free
lifetimes combined with high selectivities.
halide salts. Although 10À5 molNi/gionic liquid were added
to the liquid, the salts did not dissolve completely.
Nickel chloride seemed to be almost insoluble in the
buffered liquid. The bromide and iodide salts did not
dissolve very well, either. Thus, the amount of catalyst
which had actually dissolved in the ionic liquid, might
Effect of Different Nickel Catalyst Precursors on
Dimer Selectivities
So far, only catalyst precursor A was used as a refer- have been below 10À6 molNi/gionic liquid. In this case, not
ence in all screening experiments. Now, a series of only the mass transfer of the olefin but also the cata-
typical nickel(II) halides and complexes was tested lyst concentration determined the productivity.
for their performances in combination with BiPh3-buf-
To prove this theory, the triphenylphosphine ad-
fered chloroaluminate melts (Table 4). Considering ducts of nickel chloride and nickel bromide were
the results of our previous cation screening, N-meth- tested, too (Nos. 59 and 60). Indeed, both complexes
ylpyrrolidine hydrochloride mixed with two equiva- readily dissolved in the ionic liquid and yielded
lents of aluminum chloride and buffered with almost the same productivity of around 28 gproduct/gionic
0.30 equivalents of BiPh3 was chosen as reference liquid ꢆh with a selectivity around 91% to give C6. The
system.
productivities of nickel hexafluoroacetylacetonate
Since propene dimerization reactions in buffered (No. 61) and bis(N-isopropylsalicylaldimine)nickel(II)
chloroaluminate melts are limited by mass transfer of (No. 62) were only slightly lower.
the olefin,[13d] the productivities of all nickel com-
The compositions of all C6 fractions were consistent
pounds should be the same if a sufficiently high cata- with those of ligand-free nickel salts. Hydrogenation
showed that the hexenes consisted of 25% n-hexenes,
69% 2-methylpentenes, and 6% 2,3-dimethylbutenes
Table 4. Catalyst screening for nickel-catalyzed propene di-
(Æ2%). Either the triphenylphosphine ligands were
merization reactions in BiPh3 buffered chloroaluminate
abstracted from the Lewis acidic ionic liquids, or they
melts.[a]
did not influence the degree of branching.
No. Catalyst Precur- C6
Conversion
[%]
Productivity[b]
sor
[%]
Effect of Tricyclohexylphosphine Ligands on the
Dimer Structure
55 NiF2
56 NiCl2
57 NiBr2
58 NiI2
90.9
95.4
92.6
93.0
90.8
91.0
91.8
90.6
69
7
24.9
2.5
43
47
80
81
75
75
14.7
16.1
28.8
28.2
23.1
27.0
Finally, nickel chloride complexed by two sterically
demanding tricyclohexylphosphine ligands was em-
ployed as catalyst precursor for olefin dimerization
reactions in BiPh3-buffered chloroaluminate ionic
liquids (Table 5). Sterically hindered phosphines
bound to a catalytically active nickel center in pro-
pene dimerization reactions are known to favour the
formation of highly branched dimers.[2c,e,f] Especially
2,3-dimethylbutenes are interesting feedstocks for the
production of various fine chemicals.[21] Due to their
high research octane number (RON), they can also
be used as octane booster to increase the RON of low
octane motor fuel.[5]
59
60
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
61 NiACHTUNGTRENNUNG
62 NiL2
[c]
[a]
Reaction conditions: 4.4 g buffered ionic liquid; composi-
tion [BiPh3]/[N-methylpyrrolidinium]
+A[Al2Cl7]À =0.30;
[cat]=10À5 mol/gionic
T=408C; stirring rate=
CTHUNGTRENNUNG
;
liquid
600 minÀ1; t=60 min; stirred 300-mL Parr stainless steel
autoclave; 130–160 g liquid propene; hfacac=hexafluor-
oacetylacetonate.
[b]
[c]
gproduct/gionic liquid ꢆh.
NiL2 =bis(N-isopropylsalicylaldimine)Ni(II).
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Nickel-Catalyzed Propene Dimerization Reactions
Table 5. Effect of tricyclohexylphosphine on the degree of branching in nickel-catalyzed propene dimerization reactions in
BiPh3 buffered chloroaluminate melts.[a]
Catalyst Precursor
[BiPh3]/
E
C6 [%]
HEX [%]
MP [%]
DMB [%]
Productivity[b]
A
A
ACHTUNGTRENNUNG
0.30
0.30
0.60
93.7
78.5
83.9
25
16
13
69
54
48
6
30
39
18.5
26.3
13.3
[a]
Reaction conditions: 3.5–4.4 g buffered ionic liquid; composition [AlCl3]/[N-methylpyrrolidine·HCl]=2.00; [cat]=
10À5 mol/gionic liquid; T=408C; stirring rate=600 minÀ1; t=60 min; stirred 300-mL Parr stainless steel autoclave; 250 mL
liquid propene; HEX=n-hexenes; MP=2-methylpentenes; DMB=2,3-dimethylbutenes.
Productivity given in gproduct/gionic liquid ꢆh.
[b]
column packed with P4O10. Triphenylbismuth was purchased
Indeed, the tricyclohexylphosphine ligands in-
from ABCR and used without further purification. 1-Butyl-
3-methylimidazolium tetrachloroaluminate (BASF), AlCl3
creased the 2,3-dimethylbutene (DMB) content from
6% to 30% when applying the same buffer level of
0.30 equivalents triphenylbismuth. However, the over-
all dimer selectivity dropped from more than 90% to
79% if tricyclohexylphosphine ligands were present.
With 0.60 equivalents of BiPh3, the DMB yield could
be increased to 39%. The higher buffer level also in-
creased the dimer selectivity slightly to 84%.
(ReagentPlusꢂ), Ni
ACTHNUGTRENGNU(hfacac)2, (PPh3)2NiCl2, (PPh3)2NiBr2,
(PCy3)2NiCl2, and all nickel halides were purchased from
Sigma–Aldrich and used as received. Complex A[14a] and
bis(N-isopropylsalicylaldimine)Ni(II) were synthesized ac-
cording to a literature procedure.[22] Details about the used
ammonium and phosphonium salts can be found in the Sup-
porting Information.
Synthesis of Buffered Ionic Liquid Catalyst
Compositions
Conclusions
The ionic liquids were synthesized by directly mixing the
corresponding amount of AlCl3 and halide salt in a cooled
Schlenk tube. In the case of BMIM-based liquids, AlCl3 was
added to commercial 1-butyl-3-methylimidazolium tetra-
chloroaluminate. A few mL of an ionic liquid were filled
into another Schlenk tube, and the corresponding amount of
BiPh3 was dissolved in the liquid by stirring. After the
nickel catalyst precursor had been added, the homogeneous
solution was syringed into the reaction vessel. Its amount
was determined by the weight difference of the syringe.
We have shown that BiPh3 efficiently buffered Lewis
acidic chloroaluminate melts over a wide range of
compositions. The ratio between cation, aluminum
chloride, and BiPh3 can be tuned to obtain tailor-
made dimerization systems. Lifetimes, selectivities,
and productivities can be adjusted by the choice of
the cation and the composition. The addition of tri-
phenylbismuth reduced the melting points of the re-
sulting chloroaluminate mixtures and, thus, allowed
the use of almost any ammonium or phosphonium
halide cation. By performing a cation screening with
100 halide salts, we found that N-methylpyrrolidine
hydrochloride gave the best catalytic results in terms
of selectivity and lifetime. We further showed that
olefin dimerization reactions in buffered chloroalumi-
nate melts can be catalyzed by various nickel com-
pounds with similar performances. The introduction
of basic, sterically demanding tricyclohexylphosphine
ligands led to higher degrees of branching, however,
at the expense of lower dimer selectivities.
Procedure for Biphasic Propene Dimerization
Reactions
For the Lewis acidity, cation, and temperature screening ex-
periments, a 300-mL glass autoclave equipped with a stirring
bar and a magnetic stirrer with a stirring rate of 1200 minÀ1
was used. The glass autoclave was kept in a drying oven at
1508C for several hours before an experiment. After the ad-
dition of the active ionic liquid, propene (40 to 60 mL) was
condensed into the glass autoclave by liquid nitrogen cool-
ing. Then, the autoclave was placed in a metal box for
safety reasons, and the temperature was regulated by an ex-
ternal water bath. After the experiment, the pressure was
slowly released by opening a valve. The product fraction
was filtered through a short plug of silica and analyzed by
gas chromatography.
Experimental Section
For the catalyst screening experiments, a stirred 300-mL
Parr stainless steel autoclave equipped with an internal ther-
mocouple inside a thermowell was used. The steel vessel
was kept in a drying oven at 1508C for several hours before
an experiment, and it was filled with argon. The catalyst-
containing vessel was evacuated, and liquid propene
(200 mL) was soaked into it at 77 K. The reaction was start-
ed by quickly heating the vessel to 408C with boiling water.
General Remarks
All chemical manipulations were carried out using standard
Schlenk techniques under an argon atmosphere. The prod-
ucts of the dimerization experiments were characterized by
gas chromatography (Agilent 6850) and GC-MS (FOCUS
DSQꢈ Thermo Scientific). Propene (99.3%) was purchased
from Riessner Gase, Lichtenfels, and was dried over a
Adv. Synth. Catal. 2012, 354, 399 – 407
ꢅ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
405

Matthias Dçtterl and Helmut G. Alt
FULL PAPERS
Cooling of the highly exothermic reaction was achieved
manually by an external liquid nitrogen cooled acetone
bath. After the experiment, the vessel was cooled to 08C,
and the pressure was slowly released. After reaching ambi-
ent temperature, the weight difference of the vessel was de-
termined, and the product phase was analyzed by gas chro-
matography. To determine the content of n-hexenes, 2-meth-
ylpentenes, and 2,3-dimethylbutenes, the product fraction
was hydrogenated with palladium on activated carbon and
analyzed by gas chromatography.
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Supporting Information
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Supporting Information
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