[IrCl2Cp*(NHC)] complexes as highly versatile efficient catalysts for the cross-coupling of alcohols and amines

Article abstract of DOI:10.1002/chem.200801580

A comparative study on the catalytic activity of a series of [IrCl ₂Cp*(NHC)] complexes in several C-O and C-N coupling processes implying hydrogen-borrowing mechanisms has been performed. The compound [IrCl₂Cp*(I^nBu)] (Cp* = pentamethyl cyclopentadiene; I^nBu = 1,3-di-n-butylimidazolylidene) showed to be highly effective in the cross-coupling reactions of amines and alcohols, providing high yields in the production of unsymmetrical ethers and N-alkylated amines. A remarkable feature is that the processes were carried out in the absence of base, phosphine, or any other external additive. A comparative study with other known catalysts, such as Shvo's catalyst, is also reported.

Full text of DOI:10.1002/chem.200801580

DOI: 10.1002/chem.200801580
ACHTUNGTRENNUNG[IrCl2Cp*ACHTUNGTRENNUNG(NHC)] Complexes as Highly Versatile Efficient Catalysts for the
Cross-Coupling of Alcohols and Amines
Amparo Prades, Rosa Corberꢀn, Macarena Poyatos, and Eduardo Peris*[a]
Abstract: A comparative study on the
catalytic activity of series of
[IrCl2Cp*(NHC)] complexes in several
butylimidazolylidene) showed to be
highly effective in the cross-coupling
reactions of amines and alcohols, pro-
viding high yields in the production of
unsymmetrical ethers and N-alkylated
amines. A remarkable feature is that
the processes were carried out in the
absence of base, phosphine, or any
other external additive. A comparative
study with other known catalysts, such
as Shvoꢀs catalyst, is also reported.
a
ACHTUNGTRENNUNG
C–O and C–N coupling processes im-
plying hydrogen-borrowing mecha-
nisms has been performed. The com-
Keywords: alcohols · alkylation ·
amines · etherification · iridium ·
N-heterocyclic carbenes
pound [IrCl2Cp*
ACHTUNGTRENNUNG
methyl cyclopentadiene; InBu =1,3-di-n-
ACHTUNGTRENNUNG
(InBu)] (Cp*=penta-
Introduction
IrIIICp* species was recently described by Fujita and Yama-
guchi.[10] The dehydrogenation of alcohols is an interesting
process, not only because it provides the more reactive car-
bonylated species, but because it also affords an easy way
for the generation of hydrogen.[5,11]
We now report a wide set of reactions that are effectively
catalyzed by 1 and widens the scope of this versatile
AHCTUNGTRENNcGUN atalyst.
The search for efficient catalysts for “hydrogen-borrow-
ing”[1] strategies is very challenging, since it can provide
easy access to a wide variety of highly valuable organic mol-
ecules introducing environmental benefits. Although the
principles governing hydrogen-transfer (or hydrogen-bor-
rowing) processes are common to amines and alcohols, ver-
satile catalysts capable of reacting with both types of sub-
strates are not yet available, the only exception being Shvoꢀs
catalyst,[2–5] the exceptional features of which make it one of
the most versatile catalysts described so far.
Results and Discussion
We recently described the base-free-catalyzed reduction
In the presence of a weak base, 1 is capable of dehydrogen-
ating aromatic alcohols to the corresponding carbonylated
species. The results for this reaction are shown in Table 1.
Although the yields were moderate (the reactions were not
optimized), this process establishes the basis for a full set of
“hydrogen-borrowing” reactions.
of C=O and C=NR bonds with the complex [IrCl2Cp*
(1; Cp*=pentamethyl cyclopentadiene; InBu =1,3-di-n-bu
imidazolylidene).[6] The same complex has been used for a
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG
[7,8]
À
wide range of C H activation processes,
including the
catalytic deuteration of organic molecules.[8] A related Ir
complex with a Cp*–NHC functionalized ligand was also
very active in the alkylation of amines with primary alco-
hols.[9] This latter compound was capable of dehydrogenat-
ing secondary alcohols to ketones in the presence of a base,
although we did not quantify the process.[9] An example of a
“ligand-promoted” dehydrogenation of alcohols using a
Under base-free conditions, and with the addition of an
excess of AgOTf, the same catalyst affords the etherification
Table 1. Dehydrogenation of aromatic alcohols.[a]
[a] A. Prades, Dr. R. Corberꢁn, Dr. M. Poyatos, Prof. E. Peris
Departamento de Quꢂmica Inorgꢁnica y Orgꢁnica
Universitat Jaume I, 12071-Castellꢃn (Spain)
Fax : (+34)964-728-214
Entry
Alcohol
t [h]
Yield [%]
1
2
benzyl alcohol
1-phenylethanol
24
24
50
70
E-mail: eperis@qio.uji.es
[a] Reaction conditions: alcohol (0.4 mmol), catalyst
1
(0.02 mmol;
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200801580.
5 mol%) and base (0.08 mmol; 20 mol%). Yields determined by
1H NMR spectroscopy.
11474
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Chem. Eur. J. 2008, 14, 11474 – 11479

FULL PAPER
Table 3. Comparison of the different catalysts towards cross-coupling of
alcohols and amines.[a]
of aliphatic alcohols. The results for this reaction are shown
in Table 2.
Table 2. Homo-coupling of alcohols.[a]
Entry
Reaction
Catalyst
t [h]
Yield [%]
1
2
3
4
5
6
7
8
9
A
A
A
A
B
B
B
B
C
C
C
C
1
2
3
2
3
2
>95
91
>95
90
>95
75
70
70
>95
75
83
80
Entry
Alcohol
t [h]
Yield [%]
1
2
3
n-butanol
n-hexanol
n-dodecanol
12
12
12
95
71
87
A
3
1
2
3
24
24
24
24
7
7
7
7
[a] Reaction conditions: alcohol (0.4 mmol), catalyst
1 mol%) and AgOTf (0.012 mmol). Yields determined by H NMR spec-
troscopy.
1
(0.004 mmol;
1
A
1
2
3
10
11
12
Based on these two preliminary results and on our previ-
ous research on hydrogen-transfer processes, we thought
that {IrCp*ACHTUNGTRENNUNG(NHC)} complexes may be good candidates for
reactions implying the cross-coupling of alcohols and
amines, in all three possible combinations. For the studies
reported herein we used the [IrCl2Cp*ACTHNUTRGNEUNG(NHC)] catalysts 1–3.
A
[a] Reaction conditions: entries 1–4: benzyl alcohol (0.4 mmol), n-butanol
(2 mmol), catalyst (0.004 mmol; 1 mol%) and AgOTf (0.012 mmol); en-
tries 5–8: aniline (0.2 mmol), n-hexylamine (0.4 mmol), catalyst
(0.01 mmol; 5 mol%) and AgOTf (0.03 mmol) in [D8]toluene; entries 9–
12: amine (0.2 mmol), alcohol (0.24 mmol), catalyst (0.01 mmol;
5 mol%) and AgOTf (0.03 mmol). Yields determined by 1H NMR spec-
troscopy.
of great interest, specially if we take into account all the
synthetic applications that can be obtained by combination
of these three model reactions. It should be mentioned that
the catalytic activity displayed by [{IrCl2Cp*}2] in the pres-
ence of AgOTf in these reactions, although not as good as
that shown by 1, is remarkable, and encourages the use of
this simple combination of reagents in other catalytic reac-
tions for which [{IrCl2Cp*}2] has proved to be active.
To study the scope of catalyst 1, we decided to perform a
series of reactions using a wider set of substrates for each in-
dividual cross-coupling reaction. Table 4 shows the catalytic
results for the cross-coupling of benzyl alcohol with primary
and secondary alcohols to provide unsymmetrical ethers. As
can be seen from the results shown, the reaction is quantita-
Complexes 1[8] and 2[12] have been previously reported, and
the preparation of compound 3 is described in the Experi-
mental Section. For comparison, we also tested the catalytic
activity of [{IrCl2Cp*}2]. For a rapid screening of the catalyt-
ic activities of these four compounds we performed a set of
three experiments implying the cross-coupling of: 1) benzyl
alcohol with n-butanol, 2) aniline with n-hexylamine, and 3)
benzyl alcohol with tert-butyl-
ACHTUNGTRENNUNGamine (Table 3). Addition of
Table 4. Etherification of benzyl alcohol with primary and secondary alcohols with catalyst 1.[a]
AgOTf was needed in order to
activate the catalysts. However,
the isolation of the triflate com-
pounds, [IrCp*ACHTUNGTRENNUNG(NHC)ACHUTGNTREN(NUNG OTf)2],
Entry
Alcohol
T [8C]
t [h]
Conv. [%]
A [%][b]
B [%][b]
C [%][c]
and their use under the same
catalytic conditions did not
show any differences in the ac-
tivities obtained. All four cata-
lysts showed good activities in
the three model reactions, al-
though catalyst 1 achieved the
best catalytic performances
(Table 3, entries 1, 5, and 9).
The fact that 1 behaves as a
very active catalyst in such a
1
2
3
4
5
6
7
8
9
methanol
ethanol
n-butanol
n-hexanol
n-dodecanol
benzyl alcohol
allyl alcohol
isopropyl alcohol
cyclohexanol
110
110
130
130
130
130
110
110
110
12
12
2
6
3
12
12
12
10
94
>95
>95
>95
>95
>95[e]
93
88
>95
>95(90)[d]
94
6
0
0
5
2
–
8
16
11
0
6
8
10
14
–
15
10
–
95(93)[d]
10
85
80
70
>95
81
[a] Reaction conditions: benzyl alcohol (0.4 mmol), primary or secondary alcohol (2 mmol), catalyst
(0.004 mmol; 1 mol%) and AgOTf (0.012 mmol). Conversion and yields determined by H NMR spectroscopy.
[b] Yield based on the amount of benzyl alcohol. [c] Yield based on the amount of primary or secondary alco-
1
wide set of catalytic reactions is hol. [d] Isolated yields [e] Benzaldehyde was formed in 80% yield.
Chem. Eur. J. 2008, 14, 11474 – 11479
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11475

E. Peris et al.
tive or almost quantitative for all the experiments per-
formed, showing a high selectivity to the unsymmetrical
ethers. Only for the homocoupling of benzyl alcohol did we
obtain a very low yield in the formation of benzyl ether
(entry 6), but this is because the dehydrogenation to form
benzaldehyde is a highly favorable process for this alcohol.
The results compare well and in some cases are even an im-
provement in comparison with recently reported results in
which the same or similar substrates were used.[13–15] In par-
ticular, we observed that 1 provided higher yields in the for-
mation of the unsymmetrical ethers than the highly active
catalyst [ReBr(CO)5],[15] a result that is even more interest-
ing if we consider that 1 needed lower temperatures and
shorter reaction times. It is also important to point out the
high performances obtained when allyl alcohol is used (85%
yield, entry 7). Allyl ethers are important starting materials
for a wide variety of organic reactions, and our result is
clearly improving recent reports for the same process.[14]
Brønsted acids are known to catalyze the formation of
ethers through the protonation of the hydroxy group that is
converted into a better leaving group. For example, many
studies in the homogeneous phases have been carried out
with sulfuric acid,[16] although this procedure is known to
give low yields when secondary and tertiary alcohols are
+
used. In this case, the leaving groups are ROH2 or
ROSO2H+.[17] To rule out catalysis simply by AgOTf or
even HOTf, we carried out the reaction of benzyl alcohol
and n-butanol in the presence of each of these reactants
without adding compound 1. These experiments showed
that addition of AgOTf did not provide any conversion to
the desired ether, while addition of HOTf provided negligi-
ble conversions (22% after 4 h) to the desired unsymmetri-
cal ether. This result confirms that the cross-coupling of al-
cohols to provide the unsymmetrical ethers (data in Table 4)
is an IrIII-catalyzed process. Also, the activation of 1 with
AgPF6 (instead of AgOTf) afforded much lower activities,
probably because the generation of a dicationic species pro-
vides a compound with lower electron density than the neu-
tral bistriflate adduct generated by addition of AgOTf, as
we previously discussed in a recent paper.[6]
Scheme 1. Proposed mechanism for the activation of 1 with base and
AgOTf.
subsequent coupling of the alkyl group to the coordinated
alkoxide fragment and loss of H2O. This latter mechanism is
very interesting, since it combines a typical organometallic
Although we do not have a clear explanation for the dif-
ferent behavior of 1 in the presence of alcohols depending
on the addition of AgOTf or a base, we believe that
Scheme 1 shows the two plausible mechanisms that may jus-
tify this difference.
catalytic cycle, with an acid-catalyzed process in which the
V
À
cationic H Ir intermediate acts as the protic acid. We are
aware that this mechanism is merely speculative and needs a
detailed study for its complete elucidation. Mechanistic
studies based on DFT analysis of the process are currently
underway.
Both processes rely on the oxidative addition of the alco-
hol (RCH2OH) to one {IrCp*
G
The arylation of aliphatic amines with anilines has recent-
ly been described as a convenient method for the prepara-
tion of N-alkylated anilines.[2,3] For this process, Beller and
co-workers elegantly showed that, from a series of Ru-based
catalysts, Shvoꢀs catalyst proved to be the most active.[2,3]
We have recently observed that compound 1 resembles
Shvoꢀs catalyst in terms of reactivity and catalytic activity.
For example, like Shvoꢀs catalyst, 1 is able to catalyze the
hydrogen-transfer reactions from alcohols to ketones at
room temperature in the absence of an external base.[6]
These similarities made us think that 1 would probably be
have previously proposed to justify our findings on the base-
free reduction of ketones with iPrOH.[6] The key step of
both processes is the formation of the corresponding H Ir
species. In the presence of the base, this hydride is expected
to be “deprotonated” to generate a neutral complex with a
vacant coordination site, that is, the first active species in
the oxidation of alcohols to ketones. On the other hand, for
the AgOTf activation process, the cationic H Ir species
acts as a Brønsted acid, and activates a second molecule of
alcohol by electrophilic attack to the oxygen atom, with the
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Chem. Eur. J. 2008, 14, 11474 – 11479

N-Heterocyclic Carbenes
FULL PAPER
Table 5. N-alkylation of anilines with aliphatic amines with catalyst 1.[a]
active in some other processes for which Shvoꢀs catalyst has
shown good catalytic performances. After proving that 1 is a
good catalyst for the alkylation of aniline with n-hexylamine
(Table 3, entry 5) we decided to study the generality of this
reaction by using a wider set of substituted anilines and dif-
ferent alkyl amines. This process is known to proceed
through a hydrogen-borrowing mechanism in which the
alkyl amine is the hydrogen-transfer agent, although this
strategy has only been proved once in the recent and pio-
neering work by Beller and co-workers.[3] The results that
we obtained are summarized in Table 5. The catalytic activi-
ty of 1 was very high in the N-alkylation of a wide set of
substituted anilines with several alkyl amines. For compara-
tive reasons, we also performed a series of experiments
using Shvoꢀs catalyst under our reaction conditions (Table 1
in the Supporting Information). Both 1 and Shvoꢀs catalyst
showed similar catalytic performances, which in turn vali-
dates that the arylation of aliphatic amines with anilines
under hydrogen-transfer conditions may be considered as a
general and valuable method for the preparation of the cor-
responding aryl amines.
Entry Aryl amine
Alkyl amine
Conv. [%] Yield [%]
1
2
3
4
5
6
7
8
9
10
11
12
13
aniline
aniline
aniline
aniline
o-toluidine
o-toluidine
o-toluidine
o-toluidine
p-toluidine
p-fluoroaniline
p-chloroaniline
p-methoxyaniline
n-hexylamine
benzylamine
cyclohexylamine >95
n-dodecylamine
n-hexylamine
benzylamine
>95
>95
>95 (85)[b]
94
>95
>95
80
80
>95
70
>95
>95
90
cyclohexylamine >95
n-dodecylamine
n-hexylamine
n-hexylamine
n-hexylamine
n-hexylamine
>95
90
>95
>95
50
70
>95
>95 (80)[b]
50
2,4,6-methylaniline n-hexylamine
90
90
[a] Reaction conditions: alkyl amine (0.2 mmol), aryl amine (0.4 mmol),
catalyst (0.01 mmol; 5 mol%) and AgOTf (0.03 mmol) in [D8]toluene,
24 h, 1508C. Conversions and yields were determined by 1H NMR spec-
troscopy using 1,3,5-trimethoxybenzene (0.02 mmol) as internal standard.
[b] Isolated yields.
Finally, we studied the N-alkylation of primary amines
with primary and secondary al-
Table 6. N-alkylation of primary amines with primary and secondary alcohols with catalyst 1.[a]
cohols. This reaction is known
to be catalyzed by Ir[18,19] and
Ru[4,20] compounds.
A recent
computational study on the re-
action mechanism for this pro-
cess has been reported, showing
Entry
Alcohol
Amine
t [h]
Conv. [%]
A [%]
B [%]
1
2
3
4
5
6
7
8
9
10
benzyl alcohol
aniline
7
7
7
>95
>95
>95
>95
>95
>95
>95
>95
>95
>95
>95
0
50
35
51
0
benzylamine
benzylamine
benzylamine
n-hexylamine
n-hexylamine
benzylamine
benzylamine
cyclohexylamine
cyclohexylamine
>95(81)[b]
n-butanol
n-butanol
1-phenylethanol
50
65
19
30
0
0
13
10
that
a three-step mechanism
24[c]
24
operates, implying: 1) metal-
catalyzed oxidation of the alco-
hol, 2) nucleophilic addition of
the amine to the aldehyde, and
3) metal-catalyzed reduction of
the imine to the final secondary
amine.[21] As observed from the
data that we obtained (Table 6),
all combinations of amines and
alcohols provided high conver-
sions to the corresponding N-al-
kylated amines. The selectivity of the process highly depend-
ed on the combination of amine and alcohol used. Full con-
versions to the corresponding secondary amines were ob-
served for the reactions of aniline and benzyl alcohol
(entry 1, Table 6) and benzylamine and 1-phenylethanol
(entry 7, Table 6). Interestingly, the reaction afforded a high
selectivity toward the tertiary amine in the reaction of ben-
zylamine and benzyl alcohol (entry 2, Table 6), a result that
clearly contrasts with the results previously described by
Fujita and co-workers, who obtained full conversion to the
secondary amine for the same process.[19] For comparative
purposes we also tested Shvoꢀs catalyst under our reaction
conditions and observed that it provided lower catalytic ac-
tivities (Table 2 in the Supporting Information). For exam-
ple, the reaction of 1-phenylethanol with n-hexylamine
yielded 30% of the secondary amine using Shvoꢀs catalyst
24[c]
24
70
1-phenylethanol
1-phenylethanol
>95
>95
52
24[c]
24
24[c]
45
[a] Reaction conditions: amine (0.2 mmol), alcohol (1.0 mmol), catalyst (0.01 mmol; 5 mol%) and AgOTf
(0.03 mmol), 1108C. Conversions and yields were determined by 1H NMR spectroscopy using 1,3,5-trimethoxy-
benzene (0.02 mmol) as internal standard. [b] Isolated yield. [c] Amine/alcohol=1:10.
(39% yield using previously described conditions[4]), while
we obtained 70% of the secondary amine (entry 6, Table 6)
and 30% of the tertiary amine.
Interestingly, 1 was also very active in the alkylation of
the secondary amine N-methylaniline with benzyl alcohol af-
fording full conversion to the corresponding tertiary amine
(benzylmethylaniline), as shown in Scheme 2. We also tried
the alkylation of other secondary amines (namely, dibutyl-
Scheme 2.
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11477

E. Peris et al.
was refluxed in toluene (1 mL) for 24 h. The reaction mixture was ana-
lyzed by H NMR spectroscopy and the products were identified by com-
parison with the commercially available products benzaldehyde and ace-
tophenone.
ACHTUNGTRENNUNGamine, diethylamine and diallylamine), but unfortunately
the reactions gave mixtures that we were unable to identify.
1
Homo-coupling of alcohols: A mixture of alcohol (0.4 mmol), catalyst
(1 mol%), silver triflate (3 mol%) and [D8]toluene (200 mL) was heated
at 1108C in a thick-walled glass tube fitted with a Teflon cap. The reac-
tion mixture was analyzed by H NMR spectroscopy. Products were iden-
tified according to commercially available samples: dihexyl ether, dibutyl
ether, and dodecyl ether.
Conclusion
1
In summary, after performing a comparative study in which
the catalytic activities of a series of [IrCl2Cp*
plexes and [{IrCl2Cp*}2] were evaluated, we observed that
[IrCl2Cp*
(InBu)] (1) provided the best catalytic performan-
ACHTUNGTRENUN(NG NHC)] com-
Etherification of benzyl alcohol with primary and secondary alcohols: A
mixture of benzyl alcohol (0.4 mmol), alkylating alcohol (2.0 mmol), cata-
lyst (1 mol%), and silver triflate (3 mol%) was heated at 1108C or
1308C in a thick-walled glass tube fitted with a Teflon cap. The reaction
ACHTUNGTRENNUNG
ces. A more detailed study showed that 1 provided excellent
results in all three possible cross-coupling combinations be-
tween amines and alcohols. All the reactions constitute val-
uable processes for the preparation of biologically active
species and industrial chemicals. The catalyst proved to be
highly active in all the reactions tested, in most cases im-
proving the catalytic performances of the most active cata-
lysts recently described for the same processes. This feature
confirms the extraordinary versatility and potential synthetic
applications of this unique compound. Only Shvoꢀs catalyst
shows a similar activity for the case of the N-alkylation of
anilines with aliphatic amines. The catalytic reactions that
we studied were carried out in the absence of base, phos-
phine, or any other additive (for the use of the triflate ad-
ducts there is no need to add any extra amount of AgOTf),
which in fact is not only simplifies the reaction workup pro-
cesses (the products are more easily separated from the re-
action mixtures), but also provides a more environmentally
benign processes.
1
mixture was analyzed by H NMR spectroscopy. Products were identified
according to commercially available samples (benzyl methyl ether,
benzyl ethyl ether, benzyl butyl ether, benzyl hexyl ether, benzyl dodecyl
ether, dibenzyl ether, allyl benzyl ether, and benzyl isopropyl ether) or
previously reported spectroscopic data (benzyl cyclohexyl ether).[23] For
the isolation of the products, the crude was extracted with dichlorome-
thane and filtrated through a pad of Celite. The solvent was removed
under vacuum, and the crude oil purified by column chromatography on
silica gel using hexanes as eluent.
N-alkylation of aromatic amines with aliphatic amines: A mixture of aro-
matic amine (0.4 mmol), the corresponding aliphatic amine (0.2 mmol),
catalyst (5 or 2 mol%), silver triflate (15 or 6 mol%), and [D8]toluene
(200 mL) was heated at 1508C in a thick-walled glass tube fitted with a
1
Teflon cap. The reaction mixture was analyzed by H NMR spectroscopy.
1,3,5-trimethoxybenzene (10 mol%) was used in all cases as an internal
standard in order to determine conversions and yields. Products were
identified according to previously reported spectroscopic data: N-hexyl-
AHCTUNGTRENNUNG
aniline,[24] N-hexyl-4-methylaniline,[3] N-hexyl-2-methylaniline,[3] N-hexyl-
2,4,6-trimethylaniline,[3] 4-fluoro-N-hexylaniline,[3] 4-chloro-N-hexylani-
line,[3] N-hexyl-4-methoxyaniline,[3] N-benzylaniline,[19] N-cyclohexylani-
line,[19] N-dodecylaniline,[25] N-cyclohexyl-2-methylaniline,[25] N-dodecyl-2-
methylaniline,[25] and N-benzyl-2-methylaniline.[26] For the isolation of the
products, the crude was filtered through a pad of Celite and the solvent
removed under vacuum. The crude alkyl aryl amine product was purified
by column chromatography on silica gel using hexanes as eluent.
Experimental Section
N-alkylation of primary amines with alcohols: A mixture of alcohol (0.24,
1.00 or 2.00 mmol), primary amine (0.20 mmol), catalyst (5 mol%), and
silver triflate (15 mol%) was heated at 1108C in a thick-walled glass tube
fitted with a Teflon cap. The reaction mixture was analyzed by 1H NMR
spectroscopy. 1,3,5-trimethoxybenzene (10 mol%) was used in all cases
as an internal standard in order to determine conversions and yields.
Products were identified according to commercially available samples
(N-benzyl-tert-butylamine and dibenzylamine) or previously reported
spectroscopic data (N-benzylidenebenzylamine,[27] N-benzyl-n-butyl-
amine,[28] N,N’-di-n-butylbenzylamine,[28] N-hexyl-1-phenethylamine,[29] N-
benzyl-1-phenethylamine,[27] and N-cyclohexyl-1-phenetylamine).[30] For
the isolation of the products, the crude was extracted with dichlorome-
thane and filtrated through a pad of Celite. The solvent was removed
under vacuum, and the crude solid purified by flash chromatography on
silica gel using hexanes as eluent.
Synthesis and characterization of the compounds
General procedures: [{IrCl2Cp*}2][22] and compounds 1[8] and 2[12] were
prepared according to literature procedures. All other reagents were
used as received from commercial suppliers and used without further pu-
rification. NMR spectra were recorded on a Varian Innova 300 MHz and
500 MHz, using CDCl3 as solvent. Electrospray mass spectra (ESI-MS)
were recorded on a Micromass Quatro LC instrument; nitrogen was em-
ployed as drying and nebulizing gas. Elemental analyses were carried out
on a EuroEA3000 Eurovector Analyser.
Synthesis of 3: A suspension of 1,2-dimethylpyrazolium iodide (74 mg,
0.33 mmol) and silver oxide (176 mg, 0.50 mmol) in acetonitrile (20 mL)
was stirred at room temperature for 2 h. The mixture was filtered
through Celite and [{IrCl2Cp*}2] (120 mg, 0.15 mmol) was added. The
mixture was refluxed for 3 h, the suspension was filtered through Celite
and the solvent was evaporated under reduce pressure. The crude solid
was purified by column chromatography. Elution with a mixture of
CH2Cl2/acetone (1:1) afforded a yellow band that contained compound 3.
The pure compound was precipitated from a mixture of CH2Cl2/Et2O as
Acknowledgements
1
a yellow solid. Yield: 60 mg (40%). H NMR (CDCl3, 300 MHz): d=7.36
3
3
(d, JH-H =2.70 Hz, 1H; CHpyrazole), 6.49 (d, JH-H =2.70 Hz, 1H; CHpyrazole),
4.11 (s, 3H; NCH3), 3.92 (s, 3H; NCH3), 1.60 ppm (s, 15H; CH3Cp*);
13C{1H} NMR (CDCl3, 75 MHz): d=133.7, 117.4 (CHpyrazole), 87.9 (C5-
We gratefully acknowledge financial support from the MEC of Spain
(CTQ2007-31175) and Bancaixa (P1.1B2007-04). We would also like to
thank the Spanish MEC for a fellowship (R.C.), and to the Juan de la
Cierva program (M.P.).
ACHTUNGTRENNUNG(CH3)5), 37.1, 36.9 (NCH3), 8.9 ppm (C5AHCTUNGTRENN(UNG CH3)5); electrospray MS (30 V):
m/z: 459.2 [MÀCl]+; elemental analysis calcd (%) for C15N2IrCl2H23
(494.48): C 36.43, H 4.69, N 5.67; found: C 36.43, H 4.89, N 5.66.
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Received: August 1, 2008
Published online: November 19, 2008
Chem. Eur. J. 2008, 14, 11474 – 11479
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www.chemeurj.org
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