Disubstituted imidazolium-2-carboxylates as efficient precursors to N-heterocyclic carbene complexes of Rh, Ru, Ir, and Pd

Article abstract of DOI:10.1021/ja056625k

N,N′-Disubstituted imidazolium-2-carboxylates are efficient precursors to NHC complexes of Rh, Ir, Pd, and Ru. Copyright

Full text of DOI:10.1021/ja056625k

Published on Web 11/22/2005
Disubstituted Imidazolium-2-Carboxylates as Efficient Precursors to
N-Heterocyclic Carbene Complexes of Rh, Ru, Ir, and Pd
Adelina M. Voutchkova, Leah N. Appelhans, Anthony R. Chianese, and Robert H. Crabtree*
Chemistry Department, Yale UniVersity, 225 Prospect Street, New HaVen, Connecticut 06520-8107
Received October 4, 2005; E-mail: robert.crabtree@yale.edu
Scheme 1
While N-heterocyclic carbene (NHC) complexes have provided
many of the most recent powerful homogeneous catalysts,1 methods
of synthesizing them have advanced more slowly. Two common
routes are deprotonation of a precursor imidazolium salt (1), either
by a strong base2a or by a basic ligand,2b and oxidative addition of
the C-H bond of 1.2c-d The free carbene intermediate of the first
route necessitates dry, air-free conditions and provides limited
Table 1. NHC Metal Complexes Obtained from Reaction of Metal
tolerance of other functionalities, while direct oxidative addition
is known only for a number of specific cases. In an important
Salts with N,N′-Dimethyl Imidazolium Carboxylate (2)a,b
time
(h)
yield %
(lit.)
advance, Lin et al. showed that Ag2O can be used to form a Ag-
NHC complex from the imidazolium salt which readily transfers
the NHC to palladium and gold.3 Transmetalation to various metal
species can also give a wide variety of NHCs of rhodium, copper,
ruthenium, and iridium.4,5 Failures can still be encountered,6
however, and Ag-induced oxidative degradation of the imidazolium
precursor has also been noted.7 Further methods8 are therefore
eagerly sought.
entry
reactant
product
1
2
3
4
5
6
[Rh(cod)Cl]2
[Ir(cod)Cl]2
[(ArH)RuCl2]2 (5)
[Ir(cod)(PPh3)2]PF6 (7) [Ir(cod)(NHC)2]PF6 (8)
Ir(cod)(py)2]PF6(9)
Pd(OAc)2
Rh(cod)(NHC)Cl(3)
Ir(cod)(NHC)Cl (4)
(ArH)RuCl2(NHC) (6)
0.3 93(91)9
0.3 82(90)9
2
2
2
85(90)9
84
76
[Ir(cod)(NHC)2]PF6 (8)
[Pd(NHC)3(OAc)]OAc (10) 12
71
a Conditions: MeCN, 75 °C. b NHC ) 1,3-dimethylimidazole-2-ylidene;
cod ) 1,5-cyclooctadiene; ArH ) η6-p-cymene; py ) pyridine.
We now report that N,N′-disubstituted imidazolium-2-carbox-
ylates, air- and moisture-stable species, can transfer NHCs to a
variety of metal salts with release of CO2. For example, the N,N′-
dimethyl imidazolium-2-carboxylate (2) reacts rapidly with
[Rh(cod)Cl]2 to produce Rh(cod)(NHC)Cl in 93% isolated yield
(Scheme 1, cod ) 1,5-cyclooctadiene) to give 3. Carboxylate 2
also transfers the NHC to several other metal salts of Rh, Ir, Ru,
and Pd (Table 1). Reactions of entries 1 and 2 are complete in as
little as 20 min, while those of entries 3-6, in 2-12 h, to give
mono-, bis-, or tris-NHC complexes in high yields. In addition to
the cleavage of chloride-bridged dimers (Table 1, entries 1-3),
displacement of neutral ligands such as triphenylphosphine (entry
4) and pyridine (entry 5) as well as anionic ligands such as acetate
(entry 6) was also found. The reaction with Pd(OAc)2 affords an
unusual tris-NHC derivative in 71% yield (Figure 1).
This method affords products, such as 8 and 10, which have not
previously been synthesized. The remaining product complexes in
Table 1 have previously been synthesized by the free carbene route9
or by oxidative addition of 1,5 but the present method is consider-
ably faster and affords comparable or better yields. This method
does not require dry or air-free conditions for the NHC transfer,
provided the substrate metal salt is not itself affected by air or water,
as is the case for entries 2, 4, 5, and 6. The reaction in Scheme 1
was even carried out in 90:10 (v/v) H2O-MeCN and afforded the
same yield with no observable imidazolium salt. Conversion also
even occurs at room temperature but, depending on the solvent, is
limited by the low solubility of 2 under these conditions.
Figure 1. An ORTEP diagram of the cation of 10. Selected bond lengths
(Å) and angles (deg): Pd(1)-C(6), 1.968(3); Pd(1)-C(1), 2.036(3); Pd(1)-
C(11), 2.049(3); Pd(1)-O(1), 2.096(2); C(6)-Pd(1)-C(1), 91.39(13);
C(6)-Pd(1)-C(11), 89.72(13); C(1)-Pd(1)-C(11), 177.84(13); C(6)-
Pd(1)-O(1), 175.81(11).
The utility of this procedure requires a generally applicable
synthesis of the NHC carboxylates. The known10 synthesis of 2 by
alkylation of methyl imidazole with (MeO)2CO (Scheme 2) is of
limited generality. Louie and co-workers11 have reported a synthesis
of the N,N′-di(mesityl)imidazolium-2-carboxylate using KOBut/CO2
(Scheme 3).
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10.1021/ja056625k CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2
Scheme 3
generality of this method remain to be investigated, as does the
mechanism of carbene transfer. By avoiding the strong base needed
in the free carbene route, our method minimizes possible side
reactions. The oxidative degradation seen7 in the Ag2O route should
also be avoided. These conditions are some of the mildest thus far
reported in the literature and offer hope for access to a new range
of NHC metal complexes.
Acknowledgment. We would like to thank Dr. C. Incarvito for
the crystallographic work and Prof. G. C. Micalizio for helpful
discussions.
Supporting Information Available: Crystallographic data of 10
and experimental data are available (PDF). This material is available
free of charge via the Internet at http://pubs.acs.org.
Scheme 4
References
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In an alternate procedure, we now find that the new, easily
synthesized imidazolium ester 11 is also capable of transferring
the corresponding NHC (IMes) to Rh in 68% yield (Scheme 4).
The imidazolium ester is prepared from IMes by deprotonation with
NaH followed by treatment with isobutyl chloroformate. This allows
extension of the method beyond the N,N′-dimethyl case.
Ester 11 provides a storable, convenient NHC transfer agent that
requires less stringent conditions compared to the free NHC route.
Since the transfer reactions described here are all insensitive to air
and water, it seems unlikely that the free NHC is involved. Instead,
an alternative process, such as coordination of the carboxylate via
oxygen, followed by â-elimination, will need to be considered.
Computational studies to resolve this point are in hand. In control
experiments, we verified that reaction of the same metal precursors
with imidazolium salt 1 under the conditions in Table 1 did not
give any of the product obtained from 2.
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Tommasi, I.; Rogers, R. D. Chem. Commun. 2003, 28-29.
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2004, 112.
We have shown that N,N′-dialkylimidazolium-2-carboxylates
offer a mild and highly efficient route to NHC complexes of
rhodium, iridium, ruthenium, and palladium. The scope and
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