Rhodium(I) N-heterocyclic carbene complexes as highly selective catalysts for 1-hexene hydroformylation

Article abstract of DOI:10.1021/om800143m

Rhodium(I) carbene complexes of the type [Rh(NHC)(cod)X] (where NHC = N-heterocyclic carbene obtained from dialkylimidazolium cation; cod = 1,5-cyclooctadiene; X = Cl, Br, I, SCN) with P(OPh)₃ as a modifying ligand have been found to be very active catalysts for 1-hexene hydroformylation under mild conditions (80 °C, 10 atm of H₂/CO, 4 h). Even at a low rhodium concentration (1-hexene]/[Rh] = 2000), aldehydes have been obtained in yields of up to 90% with a high n/iso ratio of ca. 7. During the catalytic process, rhodium(1) carbene complexes, [Rh(NHC)(P(OPh)₃) ₂X], reacted with H₂/CO, giving a catalytically active rhodium(I) hydrido complex containing an N-heterocyclic carbene ligand. The presence of [HRh(CO)(P(OPh)₃)₃] and [Rh(NHC)(P(OPh) ₃)(CO)X] complexes in the catalytic system has been confirmed. When P(OCH₂CF₃)₃ was used as modifying ligand with [Rh(NHC)(cod)Br] as catalyst precursor, formation of [HRh(CO)(NHC)(P(OCH ₂CF₃)₃)₂] with a square-pyramidal structure was evidenced by ¹H and ³¹P NMR.

Full text of DOI:10.1021/om800143m

Organometallics 2008, 27, 4131–4138
4131
Rhodium(I) N-Heterocyclic Carbene Complexes as Highly Selective
Catalysts for 1-Hexene Hydroformylation
Wojciech Gil, Anna M. Trzeciak,* and Jo´zef J. Zio´łkowski
Faculty of Chemistry, UniVersity of Wrocław, 14 F. Joliot-Curie Street, 50-383 Wrocław, Poland
ReceiVed February 17, 2008
Rhodium(I) carbene complexes of the type [Rh(NHC)(cod)X] (where NHC ) N-heterocyclic carbene
obtained from dialkylimidazolium cation; cod ) 1,5-cyclooctadiene; X ) Cl, Br, I, SCN) with P(OPh)3
as a modifying ligand have been found to be very active catalysts for 1-hexene hydroformylation under
mild conditions (80 °C, 10 atm of H2/CO, 4 h). Even at a low rhodium concentration ([1-hexene]/[Rh]
) 2000), aldehydes have been obtained in yields of up to 90% with a high n/iso ratio of ca. 7. During
the catalytic process, rhodium(I) carbene complexes, [Rh(NHC)(P(OPh)3)2X], reacted with H2/CO, giving
a catalytically active rhodium(I) hydrido complex containing an N-heterocyclic carbene ligand. The
presence of [HRh(CO)(P(OPh)3)3] and [Rh(NHC)(P(OPh)3)(CO)X] complexes in the catalytic system
has been confirmed. When P(OCH2CF3)3 was used as modifying ligand with [Rh(NHC)(cod)Br] as catalyst
precursor, formation of [HRh(CO)(NHC)(P(OCH2CF3)3)2] with a square-pyramidal structure was evidenced
1
by H and 31P NMR.
conversion and selectivity to branched aldehydes were higher
than those for [RhCl(CO)(PPh3)2] used as a catalyst precursor.
For rhodium carbene complexes with acetate instead of halogen
ligands, even higher regioselectivity has been obtained in the
hydroformylation of vinylarenes.8 Different Rh(I) carbene
complexes with imidazole, xanthin, and tetrazole derivatives
as ligands have been used as catalysts in the hydroformylation
of 1-octene at 50 bar of pressure.9 Complexes with imidazol-
2-ylidene ligands showed high activity, expressed by TOF values
of 1150-1785 h-1. In all cases the n/iso ratio dropped from
2.6 at the beginning of the reaction to 0.7 at ca. 50% conversion.
The authors concluded that electron-poor carbene ligands led
to a higher hydroformylation activity.9 Rhodium complexes with
carbenes obtained from substituted pyrimidines have been tested
in 1-octene hydroformylation at 50 bar, and an influence of
substituents on the reaction rate was found.10
Introduction
In the hydroformylation reaction, which is one of the most
important industrial processes, soluble rhodium(I) complexes
are typically used as catalyst precursors together with a high
excess of phosphorus ligands.1–3 Such modifying ligands usually
make it possible to create sufficient electron density on the
rhodium(I) center to facilitate maximum conversion of terminal
olefins to linear aldehydes, the desired hydroformylation reaction
products.1–3 The appropriate selection of modifying ligand is
based on a compromise between steric and electronic properties.
Among monodentate phosphorus ligands, those having stronger
π-acceptor properties often produce aldehydes with higher n/iso
ratios. In the case of bidentate ligands, an essential role is played
by both the P-Rh-P angle (bite angle) and the geometry of
the active rhodium complex.1–4
N-heterocyclic carbenes are comparable to phosphorus ligands
and therefore are of great interest as potential non-phosphorus
modifying ligands for rhodium catalysts for olefin hydroformy-
lation. The results of theoretical calculations have suggested
that mixed-ligand systems, composed of NHC and phosphorus
ligands, can also exhibit high catalytic activity in hydroformy-
lation.6
A rhodium carbene complex immobilized on a water-soluble
amphiphilic block copolymer was an active catalyst for 1-octene
hydroformylation at 50 bar, giving TOF values of up to 2360
h .
-1 11 The n/iso ratio changed in successive cycles from 2.6 in
the first cycle to 1.22 in the third and fourth cycles.11
A
dirhodium bisimidazolium--carbene complex showed good
selectivity for the branched aldehyde in styrene hydroformyla-
tion at 30-80 °C.12 High-pressure NMR spectroscopy provides
evidence that the dinuclear unit is maintained under catalytic
conditions.12 A rhodium complex with a noncyclic imino-NHC
ligand was used in 1-octene hydroformylation at 8-55 bar, and
aldehydes were obtained with yields of 10-99% and n/iso ratios
In recent years only a few papers dealing with the application
of Rh(I) carbene complexes as hydroformylation catalysts have
been published. The first one presented the use of
[Rh(NHC)(PPh3)2Cl] and [Rh(NHC)(CO)(PPh3)Cl] complexes
in the hydroformylation of styrene.7 In this reaction, both
(1) Cornils, B., Herrmann, W. A., Eds. Applied Homogeneous Catalysis
with Organometallic Compounds: A ComprehensiVe Handbook in Two
Volumes; VCH: Weinheim, Germany, 1996.
(2) Pruett, R. L. AdV. Organomet. Chem. 1979, 17, 1.
(3) Trzeciak, A. M.; Zioˇłkowski, J. J. Coord. Chem. ReV., 1999, 883,
190–192.
(4) Kranenburg, M.; van der Burgt, Y. E. M.; Kamer, P. C. J.; van
Leeuwen, P. W. N. M. Organometallics 1995, 14, 3081.
(5) Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41, 1290.
(6) Sparta, M.; Borve, K. J.; Jensen, V. R. J. Am. Chem. Soc. 2007,
129, 8487.
(8) Praetorius, J. M.; Kotyk, M. W.; Webb, J. D.; Wang, R.; Crudden,
C. M. Organometallics 2007, 26, 1057.
(9) Bortenschlager, M.; Schu¨tz, J.; Preysing, D.; Nuyken, O.; Herrmann,
W. A.; Weberskirch, R. J. Organomet. Chem. 2005, 690, 6233.
(10) Bortenschlager, M.; Mayr, M.; Nuyken, O.; Buchmeiser, M. R. J.
Mol. Catal. 2005, 233, 67.
(11) Zarka, M. T.; Bortenschlager, M.; Wurst, K.; Nuyken, O.; Weber-
skirch, R. Organometallics 2004, 23, 4817.
(7) Chen, A. C.; Ren, L.; Decken, A.; Crudden, C. M. Organometallics
2000, 19, 3459.
(12) Poyatos, M.; Uriz, P.; Mata, J. A.; Claver, C.; Fernandez, E.; Peris,
E. Organometallics 2003, 22, 440.
10.1021/om800143m CCC: $40.75
2008 American Chemical Society
Publication on Web 07/29/2008

4132 Organometallics, Vol. 27, No. 16, 2008
Gil et al.
Scheme 1
of 0.82-1.9.13 Thione complexes were found to be better
catalysts for 1-hexene hydroformylation than carbene complexes,
although rather low selectivity (n/iso ) ca. 1) was noted for
both types of precursors.14
The aim of the studies presented in this paper was to test
Rh-NHC bond containing complexes as catalyst precursors
for 1-hexene hydroformylation under mild conditions (10 atm,
80 °C). To achieve this goal, we selected a catalytic system
in which Rh-NHC complexes of the general formula
[Rh(NHC)(cod)X] were modified with an additional phos-
phorus ligand, in this case P(OPh)3, and formed [Rh-
(NHC)(P(OPh)3)2X] precursors.
Figure 1. Crystal structure of [Rh(bmim-y)(η4-1,5-cod)SCN], (4).
Selected bond distances (Å) and angles (deg): Rh-C11, 2.046(2);
Rh-Cg1, 2.020(2); Rh-Cg2, 2.086(2); Rh-S31, 2.3638(8);
C11-Rh-Cg1, 91.28(6); C11-Rh-S31, 84.48(4); Cg1-Rh-Cg2,
86.63(6); Cg2-Rh-S31, 97.57(6). Cg1 and Cg2 denote the
midpoints of the C21-C22 and C25-C26 double bonds, respectively.
Results and Discussion
Five Rh(I) complexes with N-heterocyclic carbene ligands
were synthesized and tested as catalyst precursors in 1-hexene
hydroformylation: [Rh(bmim-y)(cod)Cl] (1), [Rh(bmim-y)(cod)-
Br] (2), [Rh(bmim-y)(cod)I] (3), [Rh(bmim-y)(cod)SCN] (4),
and [Rh(demim-y)(cod)Cl] (5), where bmim-y ) 1-butyl-3-
methylimidazolin-2-ylidene and demim-y ) 1,3-diethoxymeth-
ylimidazolin-2-ylidene (Scheme 1).
Complexes 1-3 have been previously tested by us in the
polymerization of phenylacetylene.15 Complexes 4 and 5 were
obtained for the first time, and the molecular structure of 4 was
determined by X-ray diffraction (Figure 1). The most charac-
teristic Rh-C11 distance in 4, amounting to 2.046(2) Å, is
similar to that in other carbene rhodium complexes.8,16,17
[Rh(NHC)(cod)X] complexes 1-5, used without any phos-
phorus modifying ligand added, did not produce aldehydes or
2-hexene under the applied reaction conditions (80 °C, 10 atm).
However, after P(OPh)3 addition, a catalytic system was formed
in situ and aldehydes were obtained with a yield of ca. 80%
together with ca. 20% of 2-hexene. The n/iso ratio is clearly
dependent on the amount of P(OPh)3 and increased from 3.6 at
[P(OPh)3]/[Rh] ) 1 to 7.8 at [P(OPh)3]/[Rh] ) 3. It is interesting
to note that the amount of phosphite impacted on the value of
the n/iso ratio but not on the total yield of aldehydes, as
illustrated by the data in Table 1.
Table 1. Influence of P(OPh)3 Concentration on the Yield of
Aldehydes Obtained in the Hydroformylation of 1-Hexene Catalyzed
by 1a
[P(OPh)3]/[1]
aldehyde (%)
n-heptanal (%)
2-hexene
n/iso
1
2
3
4
79.9
81.7
79.7
76.0
62.5
71.1
70.3
67.4
2.9
5.2
19.6
17.9
3.6
6.7
7.5
7.9
a Reaction conditions: [Rh] ) 1.38 × 10-5 mol, [1-hexene]/[Rh] )
1000, 80 °C, 10 atm of H2/CO, 4 h.
from 500 to 2000, both the yield of aldehydes and the n/iso
ratio decreased only slightly. Only in the case of 3 did the yield
of aldehydes drop to 32% at the [1-hexene]/[Rh] ratio of 2000
(Table 2). Two parameters were used for characterization of
the catalytic activity of the systems under studies (Table 2).
The first one was a rate of pressure drop in time and the second,
catalyst turnover frequency, TOF, that is usually used for
description of the catalytic systems. Both parameters indicated
the similar and reasonably high catalytic activity of the systems
containing complexes 1-5 and P(OPh)3.
Under the same reaction conditions, the complex [HRh(CO)-
(P(OPh)3)3], known as a catalytically active form of rhodium
in all systems with P(OPh)3 as a modifying ligand,20 was tested
for comparison. Its very high activity not only in hydroformy-
lation but also in isomerization of 1-hexene to 2-hexene may
explain the low n/iso ratio (1.3) obtained in hydroformylation
with that complex alone (Table 3). Another reaction was
conducted in the presence of imidazolium ionic liquid, using a
[HRh(CO)(P(OPh)3)3] + [bmim]Br mixture as a catalyst. First,
the mixture was heated in 1-hexene for 20 min; next, H2/CO
was added and the reaction was carried out for 4 h. In this
All of the tested rhodium(I) complexes with NHC ligands
used with the addition of P(OPh)3 produced aldehydes with a
high n/iso ratio, ca. 7-8, remarkably higher than that obtained
with other precursors under similar conditions.18,19 The yield
of aldehydes, 80-85%, only slightly depended on the catalysts
concentration. When the molar ratio [1-hexene]/[Rh] increased
(13) Dastgir, S.; Coleman, K. S.; Cowley, A. R.; Green, M. L. H.
Organometallics 2006, 25, 300.
(14) Neveling, A.; Julius, G. R.; Cronje, S.; Estehuysen, C.; Rauben-
heimer, H. G. Dalton Trans. 2005, 181.
(15) Gil, W.; Lis, T.; Trzeciak, A. M.; Zio´łkowski, J. J. Inorg. Chim.
Acta 2006, 359, 2835.
(16) Herrmann, W. A.; Schutz, J.; Frey, G. D.; Herdtweck, E. Orga-
nometallics 2006, 25, 2437.
(18) Trzeciak, A. M.; Zio´łkowski, J. J. J. Mol. Catal. 1987, 43, 15.
(19) Trzeciak, A. M.; Zio´łkowski, J. J.; Choukroun, R. J. Organomet.
Chem. 1991, 420, 353.
¨
(17) Herrmann, W. A.; Fischer, J.; Ofele, K.; Artus, G. R. J. J.
Organomet. Chem. 1997, 530, 259.
(20) Trzeciak, A. M. J. Organomet. Chem. 1990, 390, 105.

Rhodium(I) N-Heterocyclic Carbene Complexes
Organometallics, Vol. 27, No. 16, 2008 4133
Table 2. Influence of [1-Hexene]/[Rh] Ratio on the Yield of Aldehydes and n/iso Ratioa
amt (%)
cat.
[1-hexene]/[Rh]
aldehydes
n-heptanal
2-hexene
n/iso
activityb (atm/min)
0.088
TOFc (h-1
350
)
1
1
1
500
1000
2000
81.5
81.7
79.9
72.2
71.1
68.7
16.4
5.2
4.2
7.8
6.7
6.2
2
2
2
500
1000
2000
80.4
80.8
76.1
71.3
71.1
66.4
5.5
5.6
17.2
7.8
7.3
6.9
0.098
404
3
3
3
500
1000
2000
79.1
76.9
32.0
70.1
67.5
27.7
17.4
18.2
4.9
7.8
7.2
6.4
0.054
0.108
0.087
231
407
485
4
4
1000
2000
81.4
73.7
70.7
64.7
5.0
15.2
6.6
7.2
5
5
1000
2000
80.8
78.5
70.3
67.2
5.3
19.6
6.7
5.9
a Reaction conditions: [P(OPh)3]/[Rh] ) 2, 80 °C, 10 atm of H2/CO, 4 h. b The activity (rate of pressure drop in time) was determined as tan R taken
from the linear part of the plot pressure vs time (after an induction period). c TOF in units of (mol of aldehyde) (mol of Rh)-1 h-1
.
Table 3. Results of 1-Hexene Hydroformylation Catalyzed by
[HRh(CO)(P(OPh)3)3] and [Rh4(CO)8(P(OPh)3)4]a
Table 4. Effect of [bmim]Br in the Isomerization Reaction of
a
1-Hexene Catalyzed by HRh(CO)(P(OPh)3)3
amt (%)
amt (%)
entry
cat.
HRh(CO)P3
aldehyde n-heptanal 2-hexene n/iso
entry
cat.
HRh(CO)P3
HRh(CO)P3 + [bmim]Br
1-hexene
2-hexene
1
2
3
4
88.8
88.0
82.1
86.3
80.0
82.0
85.9
85.7
50.9
50.6
49.4
53.7
47.1
54.1
48.8
50.6
11.0
11.9
17.8
11.0
19.7
17.8
14.1
14.3
1.3
1.4
1.5
1.6
1.4
1.9
1.3
1.4
1
2
81.5
100
18.5
0
HRh(CO)P3 + [bmim]Br
HRh(CO)P3 + 2
a Reaction conditions: [Rh] ) 7.0 × 10-6 mol, [hexene]/[Rh] )
HRh(CO)P3 + 2 + 2 P
2000, 80 °C, 20 min.
5b Rh4(CO)8P4
6b Rh4(CO)8P4 + 4 [bmim]Br
Table 5. Effect of [bmim]Br in the Hydroformylation Reaction
Catalyzed by [Rh(acac)(CO)2]a
7
8
Rh(acac)P2
Rh(acac)P2 + [bmim]Br
amt (%)
a Reaction conditions (unless otherwise noted) and abbreviation: [Rh]
) 7.0 × 10-6 mol, [1-hexene]/[Rh] ) 2000, 80 °C, 10 atm of H2/CO,
4 h; P ) P(OPh)3 b [Rh] ) 1.38 × 10-5 mol, [1-hexene]/[Rh] ) 1000.
entry
cat.
Rh(acac)(CO)2
Rh(acac)(CO)2 + [bmim]Br
2-hexene n-heptanal iso aldehyde n/iso
1
2
90.4
51.5
3.7
0.4
2.5
1.4
experiment the yield (88%) and the n/iso ratio (1.4) obtained
were similar to those with [HRh(CO)(P(OPh)3)3] alone (Table
3). Additional experiments were performed with the equimolar
mixture of 2 and [HRh(CO)(P(OPh)3)3] to see whether or not
this composition of catalyst precursor changes the selectivity
and whether or not the induction period remains. In both
reactions (Table 3) the induction period was not observed and
the reaction was complete in ca. 2 h. The final n/iso ratio was
equal to 1.5 or 1.6, almost the same as with [HRh(CO)(P(OPh)3)3]
only. The presence of 2 had practically no influence on the reaction
course catalyzed by carbene-free hydride. This may suggest that
the less selective [HRh(CO)(P(OPh)3)3] is more active than the
rhodium hydrido carbene complex, but in a real catalytic system
it exists in a very low concentration. Probably the carbene-free
hydride is formed after the induction period necessary to produce
the rhodium hydrido carbene, a less active but more selective
species. This conclusion is to some extent supported by the analysis
of the n/iso ratio over time in the reaction catalyzed by 2 + 2
P(OPh)3. The n/iso ratio changed only slightly and was equal to
5.1, 6.6, and 6.3 after 30, 60, and 120 min, respectively.
a Reaction conditions: [Rh] ) 7.0 × 10-6 mol, [hexene]/[Rh] )
2000, 80 °C, 10 atm of H2/CO, 4 h.
[HRh(CO)(P(OPh)3)3] alone was warmed with 1-hexene for 20
min under an N2 atmosphere, 18.5% of 2-hexene was produced.
Isomerization was completely restrained in the presence of
[bmim]Br (Table 4). Similar inhibition of isomerization was
also observed for [Rh(acac)(CO)2] used as a catalyst precursor,
which was very active in the isomerization of 1-hexene under
an H2/CO atmosphere and produced 90.4% of 2-hexene. The
addition of an equimolar amount of [bmim]Br caused a decrease
in the yield of 2-hexene to 51.5% (Table 5).
Unexpectedly, catalytic systems composed with [Rh(NHC)-
(cod)X] complexes and stoichiometric amounts of P(OPh)3 do not
lose their catalytic activity after the first cycle and the recycled
catalyst can be successfully used again. The organic products and
unreacted substrates can be removed easily by the vacuum transfer
technique, and a new portion of 1-hexene can be added to the solid
residue containing the rhodium species. In most such experiments
an increase in aldehyde yield and a small decrease in the n/iso
ratio were observed (Table 6, Figure 2).
The measure of the pressure drop during the hydroformylation
reaction made it possible to note the induction period for all
catalyst precursors. During the induction time, lasting ca. 50
min, the pressure in the autoclave was constant at ca. 12 atm,
after increasing from 10 atm as a result of the temperature
increase to 80 °C. Only in the case of catalyst 5 did the reaction
start more quickly. It is also characteristic that recycled reactions
started practically without any induction time, which may
When[Rh4(CO)8(P(OPh)3)4] was used as a hydroformylation
catalyst precursor, aldehydes were formed with a yield of ca.
80% and the n/iso ratio was 1.4. In the same reaction carried
out with the addition of [bmim]Br, 82% of aldehydes with an
n/iso ratio of 1.9 was obtained (Table 3). Similarly, the presence
of [bmim]Br had practically no effect on the composition of
products formed in hydroformylation catalyzed by
[Rh(acac)(P(OPh)3)2] (Table 3).
It was found that the presence of [bmim]Br caused inhibition
of the isomerization of 1-hexene to 2-hexene. When the complex

4134 Organometallics, Vol. 27, No. 16, 2008
Gil et al.
Table 6. Results of 1-Hexene Hydroformylation in Recycling Experiments with Catalysts 1-5a
amt (%)
[1-hexene]/[Rh]
recycled cat.a
aldehydes
n-heptanal
2-hexene
n/iso
activityc (atm/min)
0.165
TOFc (h-1
)
1000
2000b
1000
2000b
1000
2000b
1000
2000b
1000
2000b
1
1
2
2
3
3
4
4
5
5
92.4
88.6
86.9
75.9
83.2
65.7
83.2
77.8
84.8
70.8
73.1
65.0
70.9
62.8
68.7
55.4
71.4
65.7
69.5
57.4
6.1
1.3
1.7
6.3
2.8
13.0
2.9
19.1
2.3
3.8
2.8
4.4
4.8
4.7
5.4
6.0
5.4
4.5
4.3
792
852
713
624
509
0.148
0.168
0.108
0.112
26.3
a The catalysts 1-5 have been obtained from experiments collected in Table3. Reaction conditions (unless otherwise noted): [Rh] ) 1.38 × 10-5
mol, [P]/[Rh] ) 2, 80 °C, 10 atm of H2/CO, 4 h. b [Rh] ) 7.0 × 10-6 mol. c See Table2.
Table 7. Results of 1-Hexene Hydroformylation Catalyzed by 2 in
the Presence of Different Phosphines or Phosphites as Modifying
Ligandsa
amt (%)
entry phosphine/phosphite aldehydes n-heptanal 2-hexene n/iso
1b
2b
3b
4
PPh3
1.5
3.3
3.7
1.1
2.4
2.6
0.6
0.7
0.7
30. 5
33.0
31.5
0.6
38.2
5.5
2.4
2.5
2.4
2.2
4.7
7.1
2.7
1.9
4.0
16.6
7.8
27.7
3.2
P(p-CH3-C6H4)3
P(p-CH3O-C6H4)3
P(C6F5)3
P(NC4H4)3
P(NC4H4)3
P(2-CH3O-C6H4)(Ph)2
P(O-C6H3-2,4-tBu2)3
P(OC6H4-m-CH3)3
P(OCH2CF3)3
1.5
1.0
5
66.5
67.8
2.5
57.9
25.1
18.7
69.1
26.2
83.0
54.7
59.5
1.8
38.2
20.1
17.6
61.3
25.2
63.4
6c
7c
8
9
10
5.2
30.4
5.1
11d P(OCH2CF3)3
Figure 2. Yield of aldehydes obtained in two cycles of 1-hexene
hydroformylation at different [1-hexene] to [Rh] ratios.
12c P(OCH2CF3)3
13
P(OCH2CH3)3
15.7
a Reaction conditions (unless otherwise noted): [Rh] ) 7.0 × 10-6
mol, [hexene]/[Rh] ) 2000, [P]/[Rh] ) 2, 80 °C, 10 atm of H2/CO, 4 h.
b Phosphines dissolved in 0.5 cm3 of toluene. c [P]/[Rh] ) 3. d Time
8 h.
confirm the presence of catalytically active forms in a postre-
action mixture. In all recycling experiments TOF values were
remarkably higher than in the first entries; only for the catalyst
5 were similar results observed in both cases (Table 6).
It was expected that not only P(OPh)3 but also other
phosphorus ligands can form attractive catalytic systems with
Rh(I) N-heterocyclic carbene precursors. Thus, hydroformylation
experiments were performed under standard conditions, and
surprisingly, only for P(NC4H4)3 and P(OCH2CF3)3 did the yield
of n-heptanal exceed 55% with n/iso ratios of 7.1 and 7.8,
respectively. Very low yields of aldehydes were obtained in
the presence of all other phosphines tested. The application of
triethyl phosphite led to 83% of aldehydes; however, the n/iso
ratio was only 3.2 (Table 7).
Mechanistic Studies. The results obtained for the hydro-
formylation reaction suggested a very positive cooperatiVe effect
of carbene and P(OPh)3 ligands in the system. Using NMR, we
were able to identify some complexes participating in the
catalytic process. In the first stage of the reaction, the cod ligand
in the precursor [Rh(NHC)(cod)X] is replaced by P(OPh)3,
leading to the formation of cis-[Rh(NHC)(P(OPh)3)2X] (Scheme
2).15
Analysis of the 31P NMR spectrum also revealed that
[Rh(NHC)(P(OPh)3)(CO)X] did not react with H2/CO.
In contrast to [HRh(CO)(P(OPh)3)3], the Rh(0) complex
[Rh4(CO)8(P(OPh)3)4] reacted with [bmim]Br at 80 °C under
10 atm of H2/CO, giving a mixture of [HRh(CO)(P(OPh)3)3]
and [Rh(NHC)(P(OPh)3)(CO)Br] after 1 h.
On the basis of the performed experiments, we propose that
the main hydroformylation cycle is based on the activity of the
carbene hydrido complex [HRh(CO)(NHC)(P(OPh)3)2], which
is responsible for an increase in reaction selectivity in com-
parison with a carbene-free system. In Scheme 3 we propose
heterolytic splitting of H2 coordinated to rhodium with elimina-
tion of HX bonded next to P(OPh)3. As a result, phosphonium
halide, [HP(OPh)3]X, is formed, as evidenced by the presence
of resonances at ca. 0-5 ppm. The other active form, [HRh(CO)-
(P(OPh)3)3], appeared in the reaction course as a result of
Rh-C(carbene) bond splitting and the formation of imidazolium
halide, [HNHC]+X- (Scheme 3).
[Rh(NHC)(P(OPh)3)2X] reacts with the H2/CO mixture at 80
°C, giving two compounds, [Rh(NHC)(P(OPh)3)(CO)X] and
[HRh(CO)(P(OPh)3)3],20 both identified by 31P NMR (Scheme
3, Figure 3). To confirm the identification of [Rh(NHC)-
(P(OPh)3)(CO)Cl], this complex was independently obtained in
the reaction of [Rh(NHC)(CO)2Cl] with 1 equiv of P(OPh)3.
It is interesting to note that it is possible to obtain a mixture
of [Rh(NHC)(P(OPh)3)(CO)X] and [Rh(NHC)(P(OPh)3)2X]
compounds in reactions between [HRh(CO)(P(OPh)3)3] and
[bmim]X, but only in the absence of H2/CO. Under 10 atm of
H2/CO the only product observed was [HRh(CO)2(P(OPh)3)2].
To confirm the presence of a rhodium hydrido carbene
intermediate, a series of NMR experiments were performed for
a system containing 2 + 3 P(OPh)3 + H2/CO. In addition to
the previously identified complexes [HRh(CO)(P(OPh)3)3] and
[Rh(bmim-y)(P(OPh)3)(CO)Br], two new signals appeared, as
a broad singlet at 123.3 ppm and a doublet at 119.7 ppm (JRh-P
) 240.6 Hz). The doublet was observed only at -60 °C,
pointing to the phosphito ligand exchange (Figure 3).
When instead of H2/CO only H2 was used, in 31P NMR
spectra another new compound was found, characterized by a
doublet of doublets at 106.1 ppm (JRh-P ) 240.2 Hz) observed

Rhodium(I) N-Heterocyclic Carbene Complexes
Organometallics, Vol. 27, No. 16, 2008 4135
Scheme 2. Reaction of [Rh(NHC)(cod)X] Complexes with P(OPh)3
Scheme 3. Reactions of [Rh(NHC)(P(OPh)3)2X] Complexes
with H2/CO Mixtures
explained by the exchange of CO and phosphorus ligands. In
addition, the intensity of signals originating from the complex
1
[HRh(CO)(P(OCH2CF3)3)3] in H and 31P NMR spectra de-
creased ove time, in contrast to the signals of [Rh(bmim-y)-
(P(OCH2CF3)3)2Br] in 31P NMR. This observation in our opinion
is important for explanation of the stability of the catalytic
system in which less selective [HRh(CO)(P(OCH2-
CF3)3)3] (or [HRh(CO)(P(OPh)3)3]) catalyst is converted into
the carbene complex [Rh(bmim-y)(P(OCH2CF3)3)2Br] (or [Rh-
(bmim-y)(P(OPh)3)2Br] as the precursor of the most selective
rhodium hydrido carbene species (Scheme 3).
only at -30 °C (Figure 4). It is interesting to note that in contrast
to all the familiar systems containing rhodium complexes with
P(OPh)3 ligands, the system under consideration presents
dynamic behavior in 31P NMR. This is the reason that under
this conditions the precise detection of hydrido species was not
successful.
Good evidence for the presence of a rhodium hydrido carbene
intermediate was found for a system containing 2 + 3
P(OCH2CF3)3 + H2/CO. In the hydrido region of the 1H NMR
spectrum three different hydrido species were identified. A
multiplet (doublet of quartets) at -11.52 ppm originating from
[HRh(CO)(P(OCH2CF3)3)3], a quintet of doublets at -11.76
ppm assigned to [HRh(P(OCH2CF3)3)4], and four partially
screened doublets at -11.83 ppm from the rhodium carbene
hydride [HRh(bmim-y)(CO)(P(OCH2CF3)3)2] (Figure 5) were
detected and assigned. The shape of the last spectrum indicates
that two nonequivalent phosphorus ligands are occupying cis
positions in the plane of rhodium. The difference of 37.8 Hz
between two JP-H values clearly confirms that the hydrido ligand
is located trans to phosphorus. The rather unexpected geometry
of the rhodium hydrido carbene complex was additionally
corroborated by the presence of two double doublets at 157.3
and 142.4 ppm (Figure 6). It was interesting to find that the
system with P(OCH2CF3)3 as modifying ligand also presents
dynamic behavior in 31P NMR, similarly to that observed for
P(OPh)3. Immediately after transfer of the solution from the
autoclave to an NMR tube, two broadened lines were observed
in the 31P NMR spectrum; however, after 240 min these lines
changed into narrow multiplets. This dynamic process can be
Figure 3. 31P NMR (202.5 MHz, in toluene-d8) of a [Rh(bmim-
y)(cod)Br] + 3 P(OPh)3 mixture after reaction with H2/CO (1 h,
80 °C, 10 atm): (1) [HRh(CO)(P(OPh)3)3] (141.8 ppm, JRh-P
)
239.0 Hz); (2) [Rh(bmim-y)(CO)(P(OPh)3)Br] (126.8 ppm, JRh-P
) 196.9 Hz); (3) [HRh(bmim-y)(CO)(P(OPh)3)2] (119.7 ppm, JRh-P
) 240.6 Hz); (*) P(OPh)3.

4136 Organometallics, Vol. 27, No. 16, 2008
Gil et al.
Figure 5. 1H NMR (500 MHz, in toluene-d8) of a [Rh(bmim-
y)(cod)Br] + 3 P(OCH2CF3)3 mixture after reaction with H2/CO
(1 h, 80 °C, 10 atm): (1) [HRh(CO)(P(OCH2CF3)3)3] (-11.52 ppm,
JP-H ) 10.2 Hz, JRh-H ) 6.3 Hz); (2) [HRh(P(OCH2CF3)3)4]
(-11.76 ppm, JP-H ) 39.0 Hz, JRh-H ) 9.2 Hz); (3) [HRh(bmim-
y)(CO)(P(OCH2CF3)3)2] (-11.83 ppm, JP1-H ) 66.1 Hz, JP2-H
28.3 Hz, JRh-H ) 9.4 Hz).
)
Table 8. Crystal Data and Structure Refinement Details for
Compound 4
formula
Mr
cryst syst
space group
a/Å
b/Å
c/Å
C17H26N3SRh
530.58
monoclinic
C21/n
11.936(3)
8.874(2)
16.722(5)
97.10(3)
1757.6(8)
4
1.540
1.09
100(2)
yellow
0.54 × 0.48 × 0.16
analytical
0.602/0.855
23 526
5092
4210
Figure 4. 31P NMR (202.5 MHz, in toluene-d8) of a [Rh(bmim-
y)(cod)Br] + 3 P(OPh)3 mixture after reaction with H2 (1 h, 80
°C, 10 atm): (1) [Rh(bmim-y)(P(OPh)3)2Br] (127.6 ppm, JRh-P
)
204.3 Hz, JP-P ) 63.2 Hz; 115.3 ppm, JRh-P ) 336.2 Hz); (2)
[RhBr(P(OPh)3)3] (118.6 ppm, JRh-P ) 289.0 Hz, JP-P ) 50.3 Hz;
114.1 ppm, JRh-P ) 219.2 Hz); (3) unidentified complex; (*)
P(OPh)3.
ꢀ/deg
V/Å3
Z
Conclusion
Dc/g cm-3
We have shown that a catalytic system based on Rh(I) carbene
complexes of the general formula [Rh(NHC)(cod)X], modified
by a small amount of P(OPh)3, presents attractive activity and
selectivity in the hydroformylation of 1-hexene under mild
conditions. The results of hydroformylation are better than in
analogous carbene-free systems with [HRh(CO)(P(OPh)3)3] and
[Rh(acac)(P(OPh)3)2] as catalyst precursors. In addition, the new
system can be easily reused, which is often complicated with
homogeneous catalysts. We expect that even better results could
be obtained after optimization of the recycling procedure.
Spectroscopic (1H and 31P NMR) studies allowed us to
identify the rhodium hydrido carbene intermediate in the model
system containing 2 + 3 P(OCH2CF3)3 + H2/CO. In contrast
to the case for many known rhodium hydrido complexes, this
new complex exists in a square-pyramidal structure, not in a
trigonal-bipyramidal one (like [HRh(CO)(P(OPh)3)3] or
[HRh(CO)(P(OCH2CF3)3)]). This is undoubtely the influence
of the carbene ligand, which is probably also important in the
high selectivity of the hydroformylation reaction.
µ/mm-1
T/K
color
cryst size/mm
abs cor
Tmin/Tmax
no. of measd rflns
no. of unique rflns
no. of rflns with I > 2σ(I)
Rint
0.033
θ range/deg
3.0-30.0
5092/0/205
1.20
0.030, 0.071
0.024, 0.066
1.04/-0.32
no. of data/restraints/params
Goodness of fit on F2
R1, wR2 (all data)
R1, wR2 (I > 2σ(I))
max/min residual density/e Å-3
The ionic liquids [bmim]Cl and [bmim]Br were used as
purchased from Fluka. The ionic liquid [bmim]I was obtained by
the reaction of 2-methylimidazole with BuI. The ionic liquid
[bmim]SCN was prepared by anion exchange between [bmim]Cl
and [NH4]SCN in 2-propanol, and [demim]Cl was prepared by
Professor Juliusz Pernak of the Technical University of Poznan´.
Experimental Section
GC-FID measurements were performed on an HP 5890 II gas
chromatograph. GC-MS measurements were performed on an HP
5890 II gas chromatograph with an HP 5971 mass detector and
HP5 column.
General Considerations. All manipulations were performed
under an atmosphere of nitrogen by use of standard Schlenk
techniques. Solvents were dried and purified by standard methods.
1-Hexene was distilled prior to use.

Rhodium(I) N-Heterocyclic Carbene Complexes
Organometallics, Vol. 27, No. 16, 2008 4137
cod, 2H), 3.67 (pseudo multiplet, JH-H ) 7.2 Hz, 2 × CH2, 4H),
5.02 (s, cod, 2H), 5.82 (d, JH-H ) 9.6 Hz, CH2, 2H), 5.97 (d, JH-H
) 9.6 Hz, CH2, 2H), 7.06 (s, 2 × CH, 2H).
[Rh(bmim-y)(P(OPh)3)(CO)Br] was prepared by stirring a solu-
tion of [Rh(bmim-y)(cod)Br] in benzene for 10 min under a CO
atmosphere. Next the solvent was removed in vacuo and the residue
was redissolved in benzene. One equivalent of P(OPh)3 was
subsequently added, and after the mixture was stirred for 10 min,
the solvent was evaporated and the residue was dried in vacuo. 1H
NMR (300 MHz, C6D6, δ, ppm): 0.79 (t, JH-H ) 6.96 Hz, CH3,
3H), 1.07 (m, JH-H ) 6.99 Hz, CH2, 2H), 1.51 (t, 6.97 Hz, CH2,
2H), 3.19 (s, CH3, 3H), 3.83 (double multiplet, CH2, 2H), 5.9 (s,
CH, 1H), 6.03 (s, CH, 1H), 6.8-7.7 (multiplet, 3 × C6H5, 15H).
31P NMR (121 MHz, C6D6, δ, ppm): 126.42 (d, JRh-P ) 199.75
Hz). IR (KBr): 1984 cm-1 (νCO).
[HRh(P(OCH2CF3)3)4] was prepared by dissolving 0.031 g of
Rh(acac)(CO)2 in benzene (1 cm3); then 0.120 g of P(OCH2CF3)3
was added and the resulting solution was stirred for 3 h under a
CO/H2 atmosphere. Next the solution was condenced in vacuo, and
the white precipitate that formed was filtered off and analyzed by
1H and 31P NMR without purification. 1H NMR (500 MHz, toluene-
d8, δ, ppm): -11.76 (q of d, JP-H ) 39.0, JRh-H ) 9.2 Hz). 31P
NMR (121 MHz, toluene-d8, δ, ppm): 155.5 (d, JRh-P ) 216.4 Hz).
[Rh(bmim-y)(P(OCH2CF3)3)2Br] was prepared only in toluene-
d8 solution by addition of 0.0076 g of P(OCH2CF3)3 to 0.005 g of
1
[Rh(bmim-y)(cod)Br]. H NMR (500 MHz, toluene-d8, δ, ppm):
0.9 (t, 3H, JH-H ) 7.3 Hz), 1.4 (m, 2H, JH-H ) 7.3 Hz), 1.9 (m,
2H, JH-H ) 7.3 Hz), 3.8 (s, 3H), 4.4 (t, 2H, JH-H ) 7.3 Hz), 4.6
(m, 4H, JH-H ) 7.7 Hz), 6.84 (s, 1H), 6.85 (s, 1H). 31P NMR (202.5
MHz, toluene-d8, δ, ppm): 148.9 (dd, JRh-P ) 197.8, JP-P ) 60.6,
142.0, JRh-P ) 316.0 Hz).
Hydroformylation Reaction Procedure. Catalytic reactions
were carried out in neat 1-hexene (1.75 cm3) without extra solvent,
in a 55 cm3 steel autoclave, which was charged with 7.0 × 10-6
or 1.38 × 10-5 mol of the catalyst precursor (complexes 1-5)
and phosphite (P(OPh)3; [P(OPh)3]/[Rh] ) 1-3) under a dinitrogen
atmosphere. Next, the autoclave was filled with an equimolar
mixture of H2 and CO to a pressure of 10 atm and heated to 80 °C.
During the reaction the mixture was magnetically stirred. After the
reaction was finished, the autoclave was cooled and then opened,
and the organic products were separated from the catalyst by
vacuum transfer and analyzed using a GC-FID HP 5890 II Hewlett-
Packard instrument with toluene as an internal standard. In recycling
experiments the residue left after vacuum transfer was again
dissolved in 1.75 cm3 of 1-hexene and used for the next reaction
without phosphite addition.
Crystallographic Data collection and Refinement. A single
crystal of 4 suitable for X-ray measurements was mounted on a
glass fiber in silicone grease and cooled to -173 °C under a nitrogen
gas stream, and the diffraction data were collected on a Kuma KM-4
CCD diffractometer with graphite-monochromated Mo KR radiation
(λ ) 0.710 73 Å). The structure was subsequently solved using
direct methods and developed by full least-squares refinement on
F2. Structural solution and refinement was carried out using the
SHELX suite of programs.21 Analytical absorption corrections
(performed with CrysAlis RED22) were applied. C, N, S, and Rh
atoms were refined anisotropically. The carbon-bonded H atoms
were positioned geometrically and refined isotropically using a
riding model.
Figure 6. 31P NMR (202.5 MHz, in toluene-d8) of a [Rh(bmim-
y)(cod)Br] + 3 P(OCH2CF3)3 mixture after reaction with H2/CO
(1 h, 80 °C, 10 atm): (1) [HRh(CO)(P(OCH2CF3)3)3] (160.4 ppm,
JRh-P ) 223.6 Hz); (2) [HRh(P(OCH2CF3)3)4] (155.2 ppm, JRh-P
) 216.4 Hz); (3) [Rh(bmim-y)(CO)(P(OCH2CF3)3)Br] (143.0 ppm,
JRh-P ) 197.0 Hz); (4) [HRh(bmim-y)(CO)(P(OCH2CF3)3)2] (157.3
ppm, JRh-P ) 220.8 Hz, JP-P ) 54.5 Hz; 142.4 ppm, JRh-P ) 241
Hz); (5) [Rh(bmim-y)(P(OCH2CF3)3)2Br] (148.9 ppm, JRh-P
197.8 Hz, JP-P ) 60.6 Hz; 142.0 ppm, JRh-P ) 316.0 Hz).
)
1H and 31P NMR spectra were recorded on Bruker Avance III
300 and Bruker Avance 500 spectrometers with chemical shifts
(δ) referenced to internal solvent resonances and reported relative
to Me4Si and H3PO4, respectively. IR spectra were recorded on a
Nicolet Impact 400 spectrometer.
Preparation of Rhodium Complexes. Rh(bmim-y)(η4-1,5-
cod)Cl (1), Rh(bmim-y)(η4-1,5-cod)Br (2), and Rh(bmim-y)(η4-
1,5-cod)I (3) (bmim-y ) 1-butyl-3-methylimidazolin-2-ylidene)
were prepared according to the literature.15
Rh(bmim-y)(η4-1,5-cod)SCN (4) was prepared similarly to 1,
with [bmim]SCN used instead of [bmim]Cl. Anal. Calcd for
C17H26N3SRh: C, 50.12; H, 6.43; N, 10.31; S, 7.87. Found: C, 48.96;
1
H, 6.59; N, 9.94; S, 7.96. H NMR (300 MHz, CDCl3; δ, ppm):
1.02 (t, JH-H ) 7.35 Hz, CH3, 3H), 1.46 (m, JH-H ) 7.35 Hz, CH2,
2H), 2.02 (bm, CH2, cod, 6H), 2.37 (m, cod, 4H), 3.49 (br, cod,
1H), 3.59 (br, cod, 1H), 3.99 (s, CH3, 3H), 4.4 (double multiplet,
CH2, 2H), 4.73 (br, cod, 2H), 6.87 (s, 2 × CH, 2H).
A crystal of 4 suitable for an X-ray structure determination has
been crystallized from CH2Cl2. Cell parameters are given in Table
8, and selected structural parameters are given in Figure 1.
Rh(demim-y)(η4-1,5-cod)Cl (5) (demim-y ) 1,3-diethoxymeth-
ylimidazolin-2-ylidene) was prepared similarly to 1 with [demim]Cl
used instead of [bmim]Cl. Anal. Calcd for C17H28N2O2ClRh: C,
47.4; H, 6.55; N, 6.50. Found: C, 46.26; H, 5.99; N, 6.04. 1H NMR
(500 MHz, CDCl3, δ, ppm): 1.24 (t, JH-H ) 6.95 Hz, 2 × CH3,
6H), 1.94 (d, JH-H ) 1.99 Hz, cod, 4H), 2.34 (s, cod, 4H), 3.32 (s,
Crystal data and structure refinement details of 4 are summarized
in Table 8. The molecular structure plot (Figure 1) was prepared
using the ORTEP-323 program.
(21) Sheldrick, G. M. SHELXS97 and SHELXL97; University of
Go¨ttingen, Go¨ttingen, Germany, 1997.
¨
(22) CrysAlis CCD and CrysAlis RED, Version 1.171; Oxford Diffrac-
tion Ltd, Wrocław, Poland, 2003.

4138 Organometallics, Vol. 27, No. 16, 2008
Gil et al.
Supporting Information Available: A CIF file giving X-ray
crystal data for 4. This material is available free of charge via the
Internet at http://pubs.acs.org.
Acknowledgment. The financial support from Grant No.
PBZ-KBN-118/T09/2004 is gratefully acknowledged. We
thank Professor Juliusz Pernak (Technical University of
Poznan´) for a generous gift of the ionic liquid [demim]Cl
and Mr. Marek Hojniak (Faculty of Chemistry, University
of Wrocław) for performing of GC-MS analyses.
OM800143M
(23) Farrugia, L. J. J. Appl. Crystallogr. 1997, 30, 565.