N-heterocyclic carbenes of iridium(I): Ligand effects on the catalytic activity in transfer hydrogenation

Article abstract of DOI:10.1039/b906855d

New Ir-NHC complexes based on different heterocyclic moieties like imidazole, benzimidazole and imidazolidine are presented and tested in transfer hydrogenation catalysis. A broad range of steric and electronic properties of NHC ligands is covered to give an idea for catalyst design from the experimental point of view.

Full text of DOI:10.1039/b906855d

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N-Heterocyclic Carbenes
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www.rsc.org/dalton | Dalton Transactions
N-heterocyclic carbenes of iridium(I): ligand effects on the catalytic activity
in transfer hydrogenation†
a
a
a
a
Sandra C. Zinner, Christoph F. Rentzsch, Eberhardt Herdtweck, Wolfgang A. Herrmann* and
Fritz. E. K u¨ hn*
a,b
Received 6th April 2009, Accepted 15th June 2009
First published as an Advance Article on the web 14th July 2009
DOI: 10.1039/b906855d
New Ir-NHC complexes based on different heterocyclic moieties like imidazole, benzimidazole and
imidazolidine are presented and tested in transfer hydrogenation catalysis. A broad range of steric and
electronic properties of NHC ligands is covered to give an idea for catalyst design from the
experimental point of view.
7
,8
Introduction
and examined with NHC complexes of iridium. However,
several questions concerning the influence of steric and electronic
properties of the NHC-ligands remain still open and deserve a
detailed examination.
The reduction of organic compounds is an important research field
in both academia and industry. It is gaining, however, additional
importance due to the increasing use of renewable resources
as feedstock for chemical industry. Many compounds available
from such renewable resources are obtained in high oxidation
states and need, at least in part, to be reduced. Several catalytic
reactions useful for such processes e.g. hydrosilylation, classical
Many convenient synthetic routes to iridium complexes ligated
9–13
8
either with NHCs,
or, more recently carbocyclic carbenes
above are known. In this work iridium catalysts are synthesized
with a variety of NHC ligands and applied for transfer hydro-
genation reactions (Scheme 2) in order to examine the influence
of electronic, steric and other effects on the catalytic performance
of their compounds.
hydrogenation, and transfer hydrogenation are well established
1
(
Scheme 1). Transfer hydrogenation is a particularly useful and
convenient catalytic process since there is no need for high
dihydrogen pressure or hazardous reducing agents.
2
Scheme 1
Iridium and rhodium complexes have a long history as
effective hydrogenation catalysts. Most prominent among them
3
are Wilkinson’s catalyst (RhCl(PPh
3
)
3
)
and Crabtree’s cata-
lyst ([Ir(COD)(py)(PCy
3
)]PF
6
, COD = 1,5-cyclooctadiene, py =
4
pyridine). Numerous other examples are also known in the
Scheme 2
5
literature.
N-heterocyclic carbenes (NHC) are well established as efficient
6
alternatives to phosphine ligands. Their electronic properties,
Results and discussion
high thermal stability and strong coordination ability render
them interesting as ligands for transition metal complexes. Ac-
cordingly, transfer hydrogenation has been successfully attempted
NHC complexes of iridium(I)
The new iridium complexes 1-C1, 4-I, and 7-I (Scheme 2) are
1
1
a
synthesized by the so-called alkoxide route shown in Scheme 3.
Chair of Inorganic Chemistry, Department of Chemistry, Technische
Universit a¨ t M u¨ nchen, Lichtenbergstraße 4, D-85747, Garching, Germany.
E-mail: wolfgang.herrmann@ch.tum.de
The iridium precursor [Ir(COD)OEt] is prepared in situ from
2
b
Molecular Catalysis, Catalysis Research Center, Technische Universit a¨ t
M u¨ nchen, Lichtenbergstraße 4, D-85747, Garching, Germany.
E-mail: fritz.kuehn@ch.tum.de
†
Electronic supplementary information (ESI) available: Details of the
X-ray crystallographic data and refinement of complexes 1-Cl and 5-Cl.
CCDC reference numbers 726759 and 726760. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/b906855d
Scheme 3
This journal is © The Royal Society of Chemistry 2009
Dalton Trans., 2009, 7055–7062 | 7055

[
Ir(COD)Cl] by reaction with sodium ethanolate in ethanol at
2
ambient temperature. Further reaction with the azolium salt pre-
∑
cursor (NHC HX) results in the formation of the desired carbene
complexes. It is noteworthy that preparation of 1-Cl bearing the
slightly stronger donating imidazolidin-2-ylidene ligands requires
a particularly high excess of 1,3-dimethylimidazolidinium tetraflu-
oroborate (4 eq.). Preparation of 4-TFA (TFA = trifluoroacetate)
is conducted using 4-Cl and AgTFA in a salt metathesis reaction;
AgCl formed during the reaction, can be filtered off (Scheme 4).
Preparation of 6-Cl is carried out starting from [Ir(COD)Cl]
the silver complex 10 as carbene source (Scheme 5).
2
with
Scheme 4
15
Fig. 2 Diamond plot of molecule A of compound 5-Cl in the solid
state. Thermal ellipsoids are drawn at the 50% probability level. Hydrogen
atoms are omitted for clarity. Selected bond lengths [ A˚ ] and bond angles
[
deg]: Ir1–Cl1 2.382(2)/2.385(2), Ir1–C1 2.069(6)/2.063(6), Ir1–Cg1
2
1
.036/2.068, Ir1–Cg2 1.984/1.988, C1–N1 1.376(8)/1.362(8), C1–N2
.375(7)/1.393(9); Cl1–Ir1–C1 92.1(2)/92.5(2), Cl1–Ir1–Cg1 89.8/89.2,
Cl1–Ir1–Cg2 173.5/174.7, C1–Ir1–Cg1 172.4/176.6, C1–Ir1–Cg2
1.8/91.7, Cg1–Ir1–Cg2 87.2/86.7, N1–C1–N2 104.0(5)/104.6(5). The
9
corresponding values for the second crystallographic independent
molecule B are given in italics. Cg1 and Cg2 define the midpoints of the
double bonds in the COD ligand.
Scheme 5
The NHC complexes 1-Cl, 4-I, 4-TFA, 6-Cl, 7-I and 10 were
1
13
on C1 as observed in the H-NMR spectra. The C-NMR
spectra show the carbene signals for the imidazol-2-ylidene ligands
in the area between 176 and 182 ppm, as expected for Ir-C
1
13
characterized by H-NMR, C-NMR and FAB/MS. Complexes
14b
1
-Cl and the previously published compound 5-Cl were ad-
ditionally characterized by X-ray analysis (Fig. 1 and 2). The
crystal structure of the latter compound has been published
14b
carbon atoms. In comparison to the imidazol-2-ylidene ligands,
the carbene signals for the azolinylidene ligands in 1-Cl, 7-I
12i
independently by another group in the meantime. The gener-
ation of the Ir-C bond is accompanied by the loss of a proton
(208.4 ppm, 192.7 ppm) appear significantly down field shifted.
This, however, cannot be ascribed to the donor strengths of the
ligands, as the CO signals in the IR-spectra of benzimidazol-2-
ylidene, imidazol-2-ylidene, and imidazolidin-2-ylidene rhodium
14
complexes (Rh(NHC)I(CO) ) differ only marginally. Moreover,
2
the benzimidazol-2-ylidene has been assumed to be a weaker
donor than the imidazol-2-ylidene. Spin multiplicity for the
1
3
2
1
C-NMR signals at 161.2 ( JCF = 35.9 Hz) and 118.1( JCF
=
2
89 Hz) caused by C-F coupling prove the almost quantitative
formation of 4-TFA via salt metathesis.
The corresponding carbonyl substituted iridium complexes
1CO-Cl and 2CO-Cl are obtained by passing carbon monoxide
through a dichloromethane solution of the COD substituted
complexes 1-Cl and 2-Cl at room temperature. These complexes
were produced in order to compare the donor strengths of satu-
rated imidazolidin-2-ylidene and unsaturated imidazol-2-ylidene
ligands (Scheme 6). The products are formed almost quantitatively
within 30 min due to the strong donor capability of the NHC
14
ligands.
15
Fig. 1 Diamond plot of compound 1-Cl in the solid state. Thermal
ellipsoids are drawn at the 50% probability level. Hydrogen atoms are
omitted for clarity. Selected bond lengths [ A˚ ] and bond angles [deg]: Ir–Cl
The cis-conformation of both of the carbonyl ligands in
complexes 1CO-Cl and 2CO-Cl was confirmed by both IR and
NMR spectroscopy. The IR spectra exhibit two strong n(CO)-
bands. Each C-NMR-spectrum shows two signals between d =
168 and 183 ppm attributed to the CO carbon atom. As observed
for the COD complexes 1-Cl and 2-Cl, the carbene signals of
2
.352(1), Ir–C1 2.018(4), Ir–Cg1 2.080, Ir–Cg2 1.982, C1–N1 1.335(5),
1
3
C1–N2 1.342(6); Cl–Ir–C1 90.1(1), Cl–Ir–Cg1 91.5, Cl–Ir–Cg2 178.1,
C1–Ir–Cg1 177.3, C1–Ir–Cg2 91.2, Cg1–Ir–Cg2 87.3, N1–C1–N2 107.8(4).
Cg1 and Cg2 define the midpoints of the double bonds in the COD ligand.
7
056 | Dalton Trans., 2009, 7055–7062
This journal is © The Royal Society of Chemistry 2009

We did not determine TONs in this work, since the catalyst
was usually not recovered after one catalytic run, but did not
decompose during this time.
Due to different activation periods not all of the catalysts
show high yields after 10 min, however, almost all of them reach
good product yields (68–98%) within 2 h. No deactivation of the
catalyst occurs. Without catalyst no significant product formation
is determined even after 6 h (Entry 13).
In order to get a deeper insight in the catalytic transfer
hydrogenation reaction with complexes of the general formula
Ir(COD)(X)(NHC) several reaction and catalyst parameters were
varied. These variations include catalyst concentrations, the
applied ligands X, the NHC donor strength, the NHC steric bulk
and the substrate influence.
Scheme 6
1
CO-Cl (199.3 ppm) and 2CO-Cl (174.7 ppm) differ significantly.
However, IR data confirm the donor strength of the imidazol-2-
ylidene and the imidazolidin-2-ylidene ligands being quite similar.
Catalyst concentration
Catalytic transfer hydrogenation
Judging from Table 1, complex 4-Cl appears to be a very
active catalyst without a pronounced initiation period. Therefore,
the catalyst concentration was reduced by the factor 100 (to
1000:100:0.01 substrate:base:catalyst). Under the latter conditions
a considerable initiation period (20 min) is detectable, showing
that the comparatively high catalyst concentration in the 100:10:1
(substrate:base:catalyst) case ensures enough catalyst molecules to
be present to start the reaction with an initiation period of only ca.
Complexes 1-Cl–9-I were examined in transfer hydrogenations of
selected ketones with iPrOH as hydrogen donor to get deeper
insights in how to design highly efficient catalysts of formula
Ir(COD)(X)(NHC) for this catalytic reaction (Scheme 7). The
reaction conditions are described in detail in the experimental
part.
5
min (see Fig. 3). With significantly less catalyst molecules present
transport phenomena obviously dominate the initial period until
the catalytic reaction reaches its full potential. After this initiation
period the catalytical activities for both catalyst concentrations
are indistinguishable within the measurement errors (TOFs ~
Scheme 7
-
1
1000 h ).
The product yields of all examined catalysts applying acetophe-
none (R = Ph in Scheme 7), as the substrate are summarized in
Table 1.
It has to be noted that different catalysts display a different
activation period before the actual reaction starts. This has to
be accounted for when determining the activity of the catalysts,
-
1
expressed by the turnover frequency (TOF, given in time ). Giving
a turnover number (TON, no unit) is misleading in this context
because it expresses the maximum number of catalytic cycles a
16
catalyst can perform before its decomposition. TOFs and TONs
have been mixed up in the literature quite frequently.
a
Table 1 Transfer hydrogenation with complexes 1-Cl–9-I
Fig. 3 Varying catalyst concentrations (reaction conditions: S/C/B =
Entry
Catalyst
Yield [%] after 10 min Yield [%] after 120 min
1
000/10/100 (1 mol% 4-Cl) or S/C/B = 1000/0.1/100 (0.01 mol% 4-Cl)
with 0.6 mmol of acetophenone and 0.06 mmol of KOH in 5 mL of iPrOH
1
2
3
4
5
6
7
8
9
4-Cl
4-I
4-TFA
8-I
7-I
94
5
80
4
96
96
96
92
93
96
87
97
73
84
98
68
0(1)
◦
at 80 C; yields determined by GC).
In the case of lower catalyst concentration, however, after a
reaction time of ca. 20 min the reaction velocity slows down in-
creasingly, most likely due to transport phenomena in the substrate
depleted surrounding of the catalyst molecules. Accordingly, it
takes much more time (180 min instead of 10 min) to reach good
yields (>80%) with lower concentration.
3
3-Cl
6-Cl
2-Cl
2-I
91
31
63
4
19
37
3
1
1
1
1
0
1
2
3
9-I
1-Cl
5-Cl
—
b
0
Influence of the ligand X
a
Reaction conditions: S/C/B = 1000/10/100 with 0.6 mmol of acetophe-
Again to be compared with complex 4-Cl the derivatives 4-I and
4-TFA were examined in more detail with respect to their reaction
kinetics (Fig. 4). The most obvious difference is again the length
◦
none and 0.06 mmol of KOH in 5 mL of iPrOH at 80 C; yields determined
by GC. Yield after 360 min.
b
This journal is © The Royal Society of Chemistry 2009
Dalton Trans., 2009, 7055–7062 | 7057

initiation period. While 4-Cl and 4-TFA display a short initiation
period to reach full activity. Compound 4-I takes much longer to
get activated. It is very interesting to note the high activity of all
-
1
-1
three compounds (4-TFA ca. 960 h , 4-Cl ca. 1260 h and 4-I ca.
2
-
1
200 h ). Furthermore, in all three cases conversions of >90% are
reached within 15 min, or within 10 min after the “takeoff” of the
catalytic reaction. The still existing differences in activity might be
due to the close proximity of the former ligand as a counter ion,
exercising still some influence on the (catalytic) reaction.
Fig. 6 Influence of donor strength in transfer hydrogenation (reaction
conditions: S/C/B = 1000/10/100 with 0.6 mmol of acetonaphtone and
◦
0
.06 mmol of KOH in 5 mL of iPrOH at 80 C; yields determined by GC).
glance, the difference in steric bulk between acetophenone and
acetonaphtone. However, it appears that the catalyst (precursor)
complex with the weaker donating carbene (8-I) shows in this case
worse performance than the catalysts ligated with more strongly
donating carbenes. The influence of the substrate obviously
overcompensates the influence of the carbene ligand on the metal.
Acetonaphtone with its larger aromatic moieties might be able to
better stabilize the transition state, thus influencing the reaction
velocity.
Fig. 4 Anion influence of the catalysts (reaction conditions: S/C/B =
1
5
000/10/100 with 0.6 mmol of acetophenone and 0.06 mmol of KOH in
◦
mL of iPrOH at 80 C; yields determined by GC).
Donor strength
In 1982 Graziani et al. published a screening of different
17
phosphine catalysts in transfer hydrogenation. No relationship
between donor strengths of the phosphines and catalyst activity
could be observed in that case. Nolan et al. could not clearly dis-
When comparingthecomplexes 2-I, 7-I, 8-I and 9-I, with the donor
strength of the NHC-ligand increasing in the order 9-I<8-I<7-
14a
I<2-I it can be noted that compound 9-I displays the shortest
initiation period and the highest initial activity. Compound 8-I
has a somewhat longer initiation period than 9-I but also a high
initial activity. Complexes 7-I and 2-I, however, both display a
longer initiation period and a lower activity, despite reaching
similar product yields to 8-I and 9-I after prolonged reaction
time. A weaker donor capability of the NHC-ligand seems to
enhance the complex activity by both shortening the initiation
time and, additionally, accelerating the catalytic reaction when
acetophenone is the substrate. The results are shown in Fig. 5.
18
tinguish between steric and electronic ligand effects. It appears,
however that such a distinction is possible.
Steric properties
In order to examine the steric influence of the carbene ligands,
four complexes 2-Cl (with CH -groups on the NHC nitrogen-
3
atoms), 3-Cl (with iPr-groups), 4-Cl (with Cy-groups) and 5-Cl
with tBu-groups were compared. The highest activity is obtained
-
1
with 3-Cl and 4-Cl, having identical TOFs (>1000 h ) within
the measurement errors. Compound 2-Cl with the least bulky
N-attached substituent displays at somewhat more pronounced
activation time and after reaching a maximum TOF of only ca.
-
1
5
0 h slows down even more.
The most bulky derivative 5-Cl also shows the most pronounced
activation period, reaches a maximum TOF of somewhat less
-
1
than 100 h , trailing off rather quickly. These observations
might be explained by the pronounced similarity of the sterics
of compounds 3-Cl and 4-Cl (see Scheme 8). The carbene ligand
of compound 5-Cl is seemingly bulky enough to slow down the
approach of substrates, while compound 2-Cl with the sterically
least demanding ligands on the NHC-N-atoms might be unable
to stabilize either the active species and/or the transition states
(Fig. 7).
Fig. 5 Influence of donor strength in transfer hydrogenation (reaction
conditions: S/C/B = 1000/10/100 with 0.6 mmol of acetophenone and
◦
0
.06 mmol of KOH in 5 mL of iPrOH at 80 C; yields determined by GC).
The picture changes, however, when acetonaphtone is applied
Variation of the substrates
as substrate (Fig. 6). This influence of the carbene ligand with
acetonaphtone is more pronounced than the effect observed with
acetophenone. A possible explanation might be, at least on first
Since the catalyst 4-Cl displays the highest activity for the
transfer hydrogenation of acetophenone, we tested 4-Cl with
7
058 | Dalton Trans., 2009, 7055–7062
This journal is © The Royal Society of Chemistry 2009

Conclusion
A series of complexes of the general formula Ir(COD)X(NHC)
was synthesized and examined in transfer hydrogenation. Steric
and electronic effects of both ligands and substrate influence the
catalytic reaction considerable. The reactions display an initiation
period which seems to be dominated by the replacement of X
by isopropanolate, indicating a concerted mechanism. The size
of the N-attached ligands of the carbene moiety seems to be
Scheme 8
optimal when a small unit (H) and larger units (CH or cyclohexyl)
3
can rotate and adopt different positions during the course of the
reaction, thereby possibly stabilizing the M-H intermediate with
its small hydrogen ligand.
Theoretical calculations and the collection of additional kinetic
data to fully establish the proposed mechanistic steps are currently
under way in our laboratories. A better understanding of the
different influences on the catalytic cycle will help to synthesize
more active catalysts and pave the way to highly active chiral
derivatives for chiral transfer hydrogenations with closely related
iridium catalysts.
Fig. 7 Steric effects of the NHC ligands in transfer hydrogenation. In
this particular case catalysts 3-Cl and 4-Cl are obviously identical with
respect to the form of the kinetic curve (reaction conditions: S/C/B =
Experimental
1
5
000/10/100 with 0.6 mmol of acetophenone and 0.06 mmol of KOH in
All experiments were carried out under dry argon using standard
Schlenk or glove box techniques. Solvents were dried by standard
methods and distilled under nitrogen. H, and C NMR spectra
were recorded on a JEOL-JMX-GX 400 MHz spectrometer at
◦
mL of iPrOH at 80 C; yields determined by GC).
1
13
several functionalized acetophenones (Table 2) which are more
difficult to hydrogenate. The substrates were selected so as to have
either electron donating or electron withdrawing moieties on the
phenyl ring. According to the literature, no decomposition or side
reactions of these substituted ketones in transfer hydrogenation
1
13
room temperature and referenced to the residual H and
C
signals of the solvents. NMR multiplicities are abbreviated as
follows: s = singlet, d = doublet, t = triplet, sept. = septet,
m = multiplet, br = broad signal. Coupling constants J are given
in Hz. IR spectra were recorded on a Jasco 460 spectrometer.
Elemental analyses were carried out by the Microanalytical
Laboratory at the TU M u¨ nchen. Mass spectra were performed at
the TU M u¨ nchen Mass Spectrometry Laboratory on a Finnigan
MAT 90 spectrometer using FAB technique. GC spectra were
measured on a Varian gas chromatograph CP-3800 (column:
FactorFour VF-5 ms) equipped with a FID detector. 1,3-
19
has been detected.
The reduction of 4-nitroacetophenone to its alcohol turns out
to be particularly slow with 4-Cl (Entry 1). Even after 6 hours
only a moderate yield of 46% of the corresponding alcohol could
be isolated. The yields for the other substituted acetophenones
reach nearly full conversions after 20 minutes (Entries 2–6). The
slightly lower yields compared to the standard reaction (94% of
21
1
-phenylethanol after 10 minutes) can be explained by the
Dimethylbenzimidazolium iodide, 1,3-dicyclohexylimidazolium
22
23
electronic properties of the substrates. In contrast to previously
iodide and 1,3-dimethylimidazolidinium tetrafluoroborate
were prepared according to reported procedures. The metal
20
published Ir-bis-carbene complexes, a relationship between the
electron withdrawing and the electron donating substituents on
the phenyl ring can be seen. The more electron rich the substrates
are, like methoxy or methyl, the faster the reaction with catalyst
dimer precursor [Ir(COD)Cl]
2
was purchased from Strem. NHC
10d
12a
14
complexes 2-I, 2-Cl, 3-Cl, 4-Cl, 5-Cl, 8-I and 9-I were
prepared according to literature procedures.
4
-Cl is.
Table 2 Transfer hydrogenation with varying substrates
Yield [%] after Yield [%] after
1
,3-Di-(4-fluorophenyl)imidazolium chloride
a
Paraformaldehyde (100 mmol) was suspended in 100 mL of
toluene, and 4-fluoroaniline (100 mmol) was added dropwise.
The slurry was cooled to 0 C and 100 mmoL of 4-fluoroaniline
Entry Catalyst Substrate
10 min
20 min
◦
b
and 100 mmol of HCl (3 N) were added. At room temperature,
1
2
3
4
4-Cl
4-Cl
4-Cl
4-Cl
4-nitroacetophenone
4-methoxy acetophenone 86
4-chloro acetophenone 48
4-trifluoromethyl
acetophenone
4-methylacetophenone
acetonaphtone
8
46
1
00 mmol of glyoxal (40% aqueous solution) was added, and the
87
97
99
◦
reaction mixture was stirred for 12 h at 40 C. The product was
filtered, washed with THF and ether, and dried under vacuum.
Yield: 40%. H-NMR (270.2 MHz, 25 C, DMSO-d
5
1
◦
6
): d 10.49 (s,
5
6
4-Cl
4-Cl
87
93
96
95
1H, NCHN), 8.57 (s, 2 H, NCHCHN) 8.04 (m, 4H, H-Ar), 7.57
1
3
1
◦
(
m, 4H, H-Ar); C{ H}-NMR (100.5 MHz, 25 C, DMSO-d
6
):
a
Reaction conditions: S/C/B = 1000/10/100 with 0.6 mmol of substrate
1
◦
b
d 162.96 (d, JCF = 245.7 Hz, p-FC), 135.6 (NCHN), 131.7 (d,
in 5 mL of iPrOH at 80 C; yields determined by GC. Yield after 360 min.
This journal is © The Royal Society of Chemistry 2009
4
3
J
CF = 2.3 Hz, NC-Ar), 125.3 (d, JCF = 9.2 Hz, o-C), 122.7
Dalton Trans., 2009, 7055–7062 | 7059

2
4
(
2
NCHCHN), 117.6 (d, JCF = 23.7 Hz, m-C); MS (FAB): [m/z]:
Trifluoracetato(g -1,5-cyclooctadiene)(1,3-dicyclohexylimidazol-
+
57 [M] .
2-ylidene)iridium(I) (4-TFA)
4-Cl (0.1 mmol) was dissolved in THF and a solution of 0.1 mmol
Bis[1,3-di(4-fluorphenyl)imidazolin-2-yliden]silver(I)
dichloroargenate (10)
of silver trifluoroacetate in 4 mL of THF was added in small
portions at -40 C, while a voluminous precipitate of silver
◦
chloride formed. The reaction mixture was allowed to warm up to
room temperature and stirred for 1 h. The yellow suspension was
filtered and volatiles were removed in vacuo. The yellow product
was extracted three times with 8 mL of hexane. The volatiles were
1
,3-Di-(4-fluorophenyl)imidazolium chloride (0.81 mmol) was
dissolved in dichloromethane and silver(I) oxide (0.5 mmol) was
added. The reaction mixture was heated at reflux for 4 h and the
resulting suspension was filtered over a plug of Celite. The white
product was precipitated with n-pentane and dried under vacuum.
1
again removed under vacuum. Yield: 91%. H-NMR (400 MHz,
◦
3
C
2
2
6
D
6
, 25 C): d 6.29 (d, 2H, NCHCHN), 5.33 (tt, J = 14 Hz,
1
◦
Yield: 61%. H-NMR (270.2 MHz, 25 C, DMSO-d ): d 8.07 (s, 2
13 1
6
H, NCH-Cy), 4.76 (m, 2H, CH-COD), 3.03 (m, 2H, CH-COD),
H, NCHCHN), 7.80 (m, 4H, H-Ar), 7.39 (m, 4H, H-Ar); C{ H}-
3
.34 (d, JHH = 12, 2H, CH
2
-COD), 2.17 (m, 4H, CH
2
-COD), 1.82
-Cy);
): d 176.74 (Carbene-C),
61.2 (q, JCF = 35.9, CO) 118.1(q, JCF = 289.9, CF ), 117.1
NCHCHN), 83.7, 60.5 (CH-COD), 49.5 (CH-Cy), 35.1, 34.1,
9.4, 26.3, 25.9, 25.6 (CH -Cy, CH
◦
NMR (100.5 MHz, 25 C, DMSO-d
6
): d 207.6 (NCHN), 162.4 (d,
3
(
d, JHH = 11.6, 2H, CH
2
-COD), 1.71–1.04 (m, 20H, CH
2
1
3
J
CF = 244.7 Hz, p-FC), 136.58 (NC-Ar), 125.27 (d, JCF = 8.4 Hz,
13
1
◦
C{ H}-NMR (100.5 MHz, 25 C, C
6
D
6
2
o-C), 122.70 (NCH
2
CH
2
N), 117.61 (d, JCF = 22.9 Hz, m-C); MS
1
1
1
(
2
3
+
+
(
FAB): m/z = 618.3 [M - AgCl
2
] , 362.6 [M - NHC - AgCl
2
] .
2
2
-COD); MS (FAB): m/z =
+
4
645.9 [M] .
Chloro(g -1,5-cyclooctadiene)(1,3-dimethylimidazolidin-2-
ylidene)iridium(I) (1-Cl)
4
Chloro(g -1,5-cyclooctadiene)(1,3-di-(4-fluorphenyl)imidazolin-
NaH (0.6 mmol) was dissolved in 3 mL of ethanol and slowly
added to a suspension of 0.15 mmol of [Ir(COD)Cl] in 2 mL
2
-yliden)iridium(I) (6-Cl)
2
of ethanol. The reaction mixture was stirred for 45 min at
room temperature and 1.2 mmol of 1,3-dimethylimidazolidinium
tetrafluoroborate were added. After stirring for 24 h at room
temperature, the solvent was removed in vacuo. The product
was purified by column chromatography (silica, 1:2 AcOEt/
To a solution of the silver carbene complex 10 (0.10 mmol) in
2
0 mL of CH Cl , [IrCl(COD)] (0.1 mmol) was added. The
2
2
2
mixture was stirred for 6 h at room temperature. The yellow
suspension was concentrated in vacuo. The product was purified
by column chromatography (silica, 1:2 AcOEt/n-pentane). Yield:
1
◦
◦
1
n-pentane). Yield: 56%. H-NMR (400 MHz, 25 C, CDCl
3
): d
7
0%. H-NMR (270.2 MHz, 25 C, C
6
6
D ): d 7.87 (m, 4H, H-Ar),
4
6
1
.50 (br s, 2H, CH-COD), 2.96 (br s, 2H, CH-COD), 3.39 (s,
7
.74 (m, 4H, H-Ar), 6.29 (m, 2H, NCHCHN), 4.99 (m, 2H, CH-
H, NCH
.71(m, 2H, CH
3
), 3.54 (d, 4H, NCH
2
), 2.17 (m, 4H, CH -COD),
2
COD), 2.35 (m, CH-COD), 1.81 (m, 2H, CH
2
-COD), 1.65 - 1.18
1
3
1
◦
1
3
1
2
-COD), 1.57 (m, 2H, CH
2
-COD). C{ H}-
(
1
1
m, 6H, CH
-COD); C{ H}-NMR (100.5 MHz, 25 C, C
6
D ): d
2
6
◦
1
NMR (100.5 MHz, 25 C, CDCl
3
): d 208.4 (Carbene-C), 85.4
), 37.6 (NCH ), 34.8, 30.0 (CH
MS (FAB): m/z = 434.1 [M] ; elemental analysis calcd. for
Ir: C 35.98, H 5.09, N 6.31; found: C 35.97, H 5.09,
82.6 (NCHN), 162.4 (d, JCF = 246.2 Hz, p-FC), 136.6 (NC-Ar),
2
(
CH-COD), 51.5, 51.4 (NCH
2
3
2
-
21.5 (NCHCHN), 115.7 (d, JCF = 23 Hz, m-C), 84.7, 51.5 (CH-
+
◦
1
3
1
COD)
C
.
COD), 33.5, 29.7 (CH
CDCl
(
(
2
-COD); C{ H}-NMR (100.5 MHz, 25 C,
1
13
H
22ClN
2
): d 181.8 (NCHN), 162.2 (d, JCF = 246.7 Hz, p-FC), 136.1
3
3
N 6.31.
NC-Ar), 127.5 (d, JCF = 8.3 Hz, o-C), 122.1 (NCHCHN), 115.5
2
d, JCF = 22.2 Hz, m-C), 84.4, 52.0 (CH-COD), 33.0, 29.2 (CH
2
-
+
COD); MS (FAB): m/z = 592.4 [M] ; elemental analysis calcd. for
4
Iodo(g -1,5-cyclooctadiene)(1,3-dicyclohexylimidazolin-2-
C
23
H
22
F
2
ClIrN : C 46.66, H 3.75, N 4.73; found: C 46.66, H 3.77,
2
yliden)iridium(I) (4-I)
N 4.50.
NaH (0.6 mmol) was dissolved in 3 mL of ethanol and slowly
added to a suspension of 0.15 mmol of [Ir(COD)Cl]
2
in 2 mL
4
Iodo(g -1,5-cyclooctadiene)(1,3-dimethylbenzimidazolin-2-
of ethanol. The reaction mixture was stirred for 45 min at room
temperature and 3.2 mmol of 1,3-cyclohexylimidazolium iodide
and 1.2 mmol of NaI were added. After stirring for 24 h at
room temperature, the solvent was removed in vacuo. The product
was purified by column chromatography (silica, 1:2 AcOEt/
ylidene)iridium(I) (7-I)
NaH (0.6 mmol) was dissolved in 3 mL of ethanol and slowly
added to a suspension of 0.15 mmol of [Ir(COD)Cl] in 2 mL
2
of ethanol. The reaction mixture was stirred for 45 min at room
temperature and 3.2 mmol of 1,3-dimethylbenzimidazolium iodide
and 1.2 mmol of NaI were added. After stirring for 24 h at
room temperature, the solvent was removed in vacuo. The product
was purified by column chromatography (silica, 1:2 AcOEt/
n-pentane). Yield: 81%. H-NMR (400 MHz, 25 C, CDCl
7.24 (m, 4H, CH-Ar), 4.88 (m, 2H, CH-COD), 2.99 (m, 2H, CH-
COD), 4.07 (s, 6H, NCH ), 2.19 (m, 4H, CH -COD), 1.86 (m, 2H,
CH -COD), 2.00 (m, 2H, CH
1
◦
n-pentane). Yield: 81%. H-NMR (400 MHz, 25 C, CDCl ): d 6.84
(
3
3
d, 2H, NCHCHN), 5.21 (tt, JHH = 14 Hz, 2H NCH-Cy), 4.73(m,
2
1
H, CH-COD), 3.00 (m, 2H, CH-COD), 2.34 (m, 2H, CH
2
-COD),
3
.82 (d, JHH = 11.6, 2H, CH
2
-COD), 2.16–1.16 (m, 4H, CH
2
-
1
3
1
◦
1
◦
COD, 20H,CH
2
-Cy); C{ H}-NMR (100.5 MHz, 25 C, CDCl
3
):
3
,): d
d 176.7 (Carbene-C), 117.1 (NCHCHN), 81.5, 59.3 (CH-COD),
5
CH
3.4 (CH-Cy), 34.6, 33.5, 33.2, 30.4, 26.1, 25.6, 25.4 (CH
2
-Cy,
-COD), MS (FAB): m/z = 659.9 [M] , 233 [NHC] ; elemental
: C 41.88, H 5.50, N 4.25; found:
3
2
+
+
2
2
2
-COD), 1.43 (m, 2H, CH
2
-COD);
1
3
1
◦
analysis calcd. for C23
H
36IIrN
2
C{ H}-NMR (100.5 MHz, 25 C, CDCl
3
): d 192.7 (Carbene-
C 42.18, H 5.78, N 4.18.
C), 135.4, 122,74, 109.7 (Ar-C), 134.9, 122.7, 110.31 (Ar-C), 85.2,
7
060 | Dalton Trans., 2009, 7055–7062
This journal is © The Royal Society of Chemistry 2009

5
5.6 (CH-COD), 35.0, 33.0, 30.7 (NCH
3
, CH
2
-COD); MS (FAB):
Specific details. 1-Cl: formula: C13
crystal system: orthorhombic; space group P2
H
22ClIrN
2
; M
r
= 434.00;
+
+
+
m/z = 573.8 [M] , 447.0 [M - I] , 147.1 [NHC] .
1
2
1
2
1
(no. 19);
˚
15.7279(1) A;
a
V
=
7.2573(1);
b
=
12.3428(1);
c
=
3
˚
= 1408.83(2) A ; Z = 4; data collected: 34 211; in-
General procedure for carbonyl derivatives
dependent data [I
data/restraints/parameters: 2576/0/157; R1 [I
data]: 0.0149/0.0151; wR2 [I >2s(I )/all data]: 0.0373/0.0374.
o
>2s(I
o
)/all data/Rint]: 2559/2576/0.030;
CO gas (1 bar, 15 mL/min) was passed through a solution of the
COD complex (0.17 mmol) in 15 mL of dichloromethane at room
temperature for 15 min. The colour lightened. The solution was
reduced to 5 mL in vacuo and n-hexane was added to precipitate
the carbonyl complexes in high yields.
o
>2s(I )/all
o
o
o
The correct enantiomer is proved by Flack’s parameter e =
-0.016(8). Small extinction effects were corrected with the
SHELXL-97 procedure and x = 0.0016(1). 5-Cl: formula:
C
19
H
32ClIrN
2
; M
r
= 516.14; crystal system: monoclinic; space
group P2
1
/c (no. 14); a = 11.1962(4); b = 19.4417(6); c =
Dicarbonylchloro(1,3-dimethylimidazolidin-2-ylidene)iridium(I)
3
˚
˚
1
7.7586(6) A; b = 91.4342(15);V = 3864.4(2) A ; Z = 8; data
(
1CO-Cl)
collected: 47 684; independent data [I
6636/6727/0.127; data/restraints/parameters: 6727/0/428; R1
[I >2s(I )/all data]: 0.0425/0.0430; wR2 [I >2s(I )/all data]:
o
>2s(I
o
)/all data/Rint]:
-
1
1
Yield: 91%. IR (CH
2
Cl
2
): n = 2068, 1984 cm ; H-NMR
): d 3.67 (s, 4H, NCH CH N), 3.30
): d
N), 37.4
◦
(400 MHz, 25 C, CDCl
3
2
2
o
o
o
o
1
3
1
◦
(
m, 6H, NCH
3
); C{ H}-NMR (100.5 MHz, 25 C, CDCl
3
0.1116/0.1120. In the unit cell two crystallographic independent
molecules A and B were found. Small extinction effects were
corrected with the SHELXL-97 procedure and x = 0.00057(3).
1
99.3 (Carbene-C), 181.7, 168.1 (CO), 51.9 (NCH
2
CH
2
+
(
NCH
3
)
;
MS (FAB): m/z = 354.7 [M - CO] .
Dicarbonylchloro(1,3-dimethylimidazolin-2-ylidene)iridium(I)
2CO-Cl)
Acknowledgements
(
This work was supported by the Bayerische Elitef o¨ rderung
(scholarship for S. C. Z.), the Deutsche Forschungsgemeinschaft
DFG (C. F. R) and the Bavarian Elite Network project NanoCat
(S. C. Z.). E. H. thanks Dr S. D. Hoffmann for his help during the
data collection of compound 1-Cl.
-
1
1
Yield: 89%. IR (CH
2
Cl
2
): n = 2067, 1983 cm ; H-NMR
◦
(
(
400 MHz, 25 C, Benzol-d
6
): d 5.59 (s, 2H, NCHCHN), 3.10
1
3
1
◦
s, 6H, NCH
3
); C{ H}-NMR (100.5 MHz, 25 C, Benzol-d
6
): d
1
3
82.7 (CO), 174.7 (Carbene-C), 169.5 (CO), 121.74 (NCHCHN),
+
7.57 (NCH
3
); MS (FAB): m/z = 352.7 [M - CO] .
References
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7
062 | Dalton Trans., 2009, 7055–7062
This journal is © The Royal Society of Chemistry 2009