Imidazolium carboxylates as versatile and selective N-heterocyclic carbene transfer agents: Synthesis, mechanism, and applications

Article abstract of DOI:10.1021/ja0742885

N,N′-Disubstituted imidazolium carboxylates, readily synthetically available, isolable, air- and water-stable reagents, efficiently transfer N-heterocyclic carbene (NHC) groups to Rh, Ir, Ru, Pt, and Pd, to give novel NHC complexes, e.g., [Pd(NHC)₃

Full text of DOI:10.1021/ja0742885

Published on Web 09/27/2007
Imidazolium Carboxylates as Versatile and Selective
N-Heterocyclic Carbene Transfer Agents: Synthesis,
Mechanism, and Applications
†
‡
‡
,‡
Adelina M. Voutchkova, Marta Feliz, Eric Clot, Odile Eisenstein,* and
,
†
Robert H. Crabtree*
Contribution from the Chemistry Department, Yale UniVersity, 225 Prospect Street, New HaVen,
Connecticut 06511-8107, and Institut Charles Gerhardt, CNRS, UniVersit e´ Montpellier 2,
cc 1501, Place Eug e` ne Bataillon, 34095 Montpellier, France
Received June 13, 2007; E-mail: odile.eisenstein@univ-montp2.fr; robert.crabtree@yale.edu
Abstract: N,N′-Disubstituted imidazolium carboxylates, readily synthetically available, isolable, air- and
water-stable reagents, efficiently transfer N-heterocyclic carbene (NHC) groups to Rh, Ir, Ru, Pt, and Pd,
to give novel NHC complexes, e.g., [Pd(NHC)
-ylidene). The NHC esters are also effective. Tuning the reaction conditions for NHC transfer can give
either mono- or bis-NHCs, or bis- and tris-NHCs. A net N to C rearrangement of the N-alkyl imidazole
complex to the corresponding NHC complex was seen with (MeO) CO (DMC). DFT calculations identify
the steps needed to form the carboxylate from imidazole and DMC: S 2 methyl transfer from DMC to
imidazole, followed by proton transfer from the imidazolium CH to the carboxylate counterion, produces
the free NHC H-bonded to MeOH with a weakly associated CO . The nucleophilic NHC attacks CO to
form NHC-CO . NHC transfer to the metal with loss of CO has been calculated for Rh(cod)Cl. A proposed
two-cis-site reactivity model rationalizes the experimental data: two such vacant sites at the metal are
needed to allow coordination of the NHC-CO carboxylate and subsequent CC cleavage with NHC transfer.
3 3
OAc]OAc and [Pt(NHC) Cl]Cl (NHC ) 1,3-dimethyl imidazol-
2
2
N
2
2
2
2
2
Partial cod decoordination or chloride loss is thus required for Rh(cod)Cl. Chloride dissociation, calculated
to be easier in polar solvent, is confirmed experimentally from the retarding effect of excess chloride.
Introduction
N-Heterocyclic carbene (NHC) complexes are now fully
established as an important class of ligands for homogeneous
catalysis. While new catalytic applications are frequently
1
-5
reported,
synthetic routes to NHC complexes still lack full
advance, Lin et al.10,11 showed that Ag O can react with an
generality and new routes are very desirable. Some common
routes include deprotonation of a precursor imidazolium salt
2
imidazolium salt to form a Ag-NHC complex that readily
transferred the NHC group to palladium and gold. This route
has now been extended to give a wide variety of NHC
6
7
(
1), either by a strong base or by a basic ligand, and oxidative
8
,9
addition of the C-H bond of the imidazolium salt. The
intermediacy of any free carbene, as in the first route, neces-
sitates dry, air-free conditions and provides only limited
tolerance of other functionalities, while direct oxidative addition
is only known for a number of specific cases. In an important
1
2-16
complexes of Rh, Cu, Ru, and Ir.
While very general, this
1
7
method can give unexpected products however, such as in the
Ag-induced oxidative C-C bond cleavage of the imidazolium
1
8
19,20
precursor of Scheme 1. Thus further methods
are eagerly
sought.
†
Yale University.
Universit e´ Montpellier 2.
‡
(10) Wang, H. M. J.; Lin, I. J. B. Organometallics 1998, 17, 972-975.
(11) Lin, I. J. B.; Vasam, C. S. Comments Inorg. Chem. 2004, 25, 75-129.
(12) Viciano, M.; Mas-Marza, E.; Poyatos, M.; Sanau, M.; Crabtree, R. H.; Peris,
E. Angew. Chem., Int. Ed. 2005, 44, 444-447.
(
1) Viciu, M. S.; Germaneau, R. F.; Nolan, S. P. Org. Lett. 2002, 4, 4053-
4
056.
(
(
2) Peris, E.; Crabtree, R. H. Coord. Chem. ReV. 2004, 248, 2239-2246.
3) Herrmann, W. A.; K o¨ cher, C. Angew. Chem., Int. Ed. Engl. 1997, 36, 2163-
(13) Hu, X. L.; Castro-Rodriguez, I.; Olsen, K.; Meyer, K. Organometallics
2004, 23, 755-764.
2
187.
(
(
4) Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41, 1290-1309.
5) Bourissou, D.; Guerret, O.; Gabbai, F. P.; Bertrand, G. Chem. ReV. 2000,
(14) Coleman, K. S.; Chamberlayne, H. T.; Turberville, S.; Green, M. L. H.;
Cowley, A. R. Dalton Trans. 2003, 2917-2922.
1
00, 39-91.
(15) Chianese, A. R.; Li, X. W.; Janzen, M. C.; Faller, J. W.; Crabtree, R. H.
Organometallics 2003, 22, 1663-1667.
(
6) Herrmann, W. A.; K o¨ cher, C.; Goossen, L. J.; Artus, G. R. J. Chem.sEur.
J. 1996, 2, 1627-1636.
(16) Arnold, P. L.; Scarisbrick, A. C.; Blake, A. J.; Wilson, C. Chem. Commun.
2001, 2340-2341.
(
(
7) O¨ fele, K. J. Organomet. Chem. 1968, 12, P42-P43.
8) McGuinness, D. S.; Cavell, K. J.; Skelton, B. W.; White, A. H. Organo-
metallics 1999, 18, 1596-1605.
(17) Mata, J. A.; Chianese, A. R.; Miecznikowski, J. R.; Poyatos, M.; Peris, E.;
Faller, J. W.; Crabtree, R. H. Organometallics 2004, 23, 1253-1263.
(18) Chianese, A. R.; Zeglis, B. M.; Crabtree, R. H. Chem. Commun. 2004,
2176-2177.
(
9) Gr u¨ ndemann, S.; Albrecht, M.; Kovacevic, A.; Faller, J. W.; Crabtree, R.
H. J. Chem. Soc., Dalton Trans. 2002, 2163-2167.
12834
9
J. AM. CHEM. SOC. 2007, 129, 12834-12846
10.1021/ja0742885 CCC: $37.00 © 2007 American Chemical Society

Imidazolium Carboxylates as NHC Transfer Agents
A R T I C L E S
Scheme 1
Scheme 4
Scheme 2
2
5
available from the imidazolium salt. Louie and co-workers,
26
23
Tommasi and co-workers, and Delaude and co-workers have
Scheme 3
reported such syntheses, e.g., of N,N′-di(mesityl)imidazolium-
2
-carboxylate, using base and in some cases a high pressure of
CO2 (Scheme 4).
In an unexpected complication, variation of temperature in
the DMC reaction of Scheme 3 alters the isomeric product ratio
of 2- to 4-carboxylate: below 95 °C the 2-carboxylate pre-
dominates (>95%), while above 120 °C the 4-carboxylate is
mainly obtained (>90%). For this reason, we have slightly
modified the Louie conditions in our work by using a temper-
ature of 90 °C to obtain the 2-carboxylate. The symmetrical
diethyl or dibenzyl carbonates have proved unsuccessful alky-
lating agents for N-methylimidazole under the same conditions.
Synthesis of N,N′-Disubstituted Imidazolium Carboxylate
Esters. A wider variety of NHC transfer agents can be prepared
using the related carboxylate esters because these can be reliably
prepared from the corresponding imidazolium salts. As reported
In elucidating our oxidative C-C cleavage pathway of
18
Scheme 1, we found that the acyl imidazoles of type 3 readily
+
transferred the NHC group to Ag . Compounds of type 3 are
not efficient transfer agents to other, nonoxidizing metals,
however. Looking for a more active analogue led us to
investigate the reactivity of N,N′-dimethyl imidazolium-2-
carboxylate (NHC-CO2) (2) as an NHC transfer agent to
transition metals. Our preliminary report2 showed that isolable
compound 2 indeed transfers the NHC to several metal
precursors under mild conditions with release of CO2 without
exclusion of air and water (Scheme 2). This approach has proved
1
21
in our initial communication, we find that NHC esters (NHC-
useful, and others have reported a successful application of the
i
COO Bu) are easily synthesized by the deprotonation of the
method.2
2,23
We now report further examples of this process,
t
appropriate imidazolium precursor by KO Bu followed by
including improved methods for synthesizing a range of
analogues of 2 and their esters. The mechanism of the poorly
understood formation of 2 from methyl imidazole and (MeO)2CO
treatment with freshly distilled isobutyl chloroformate (Scheme
5
). This procedure has been successfully carried out for bis-
mesityl and bis-isopropyl cases. Ester 6 is capable of transferring
the corresponding NHC to Rh and Ir and provides a storable,
convenient NHC transfer agent that, like the imidazolium
carboxylates, does not require exclusion of air and water. This
allows extension of the method beyond the cases discussed
above. Of course, the use of the strong base in the synthesis of
the ester is a disadvantage, but NHC transfer is possible under
the same mild conditions, presumably with concomitant hy-
drolysis and formation of the corresponding carboxylate as an
intermediate.
DFT Study of Formation of NHC-CO2, 2. The reaction
between methyl imidazole and (MeO)2CO (DMC) to form
NHC-CO2, 2, previously unknown, was studied computation-
ally for N-methylimidazole via DFT with the hybrid B3PW91
functional. It has not of course been possible to explore the
entire potential energy surface, but a realistic three-step pathway
has been proposed based on the computational data. In the first
step, rate-determining methyl transfer from DMC to imidazole
gives an ion pair consisting of the cationic alkylated imidazolium
ion and the MeOCO2 counterion. In the second step, a proton
transfer within the imidazolium/carboxylate ion pair induces the
C-O bond cleavage between NHC and CO2 with concomitant
formation of MeOH. Finally the third step consists of the
nucleophilic attack of NHC to CO2 to form the new C-C bond.
(
(
DMC) has now been studied computationally by DFT-
B3PW91) calculations. Calculations on the mechanism of NHC
transfer from 2 to Rh(cod)Cl lead to a two-site reactivity model.
Results
Synthesis of NHC-CO2. The utility of our transfer procedure
depends on the availability of a general synthesis of the NHC
carboxylates. The prototype N,N′-dimethyl imidazolium-2-
carboxylate can be easily synthesized by a variation of a
previously reported procedure24 involving alkylation of methyl
imidazole with (MeO)2CO (DMC, Scheme 3). This procedure
was also found to be general for several monoalkylated
imidazoles such as n-butyl imidazole (5) and isopropyl imida-
zole, allowing access to the corresponding N-methyl, N′-alkyl
analogues of 2 as intermediates for subsequent transfer to Rh.
To escape from the limitation that one N-substituent must be
methyl, other very useful syntheses have recently become
(
(
(
(
(
(
19) Ogata, K.; Yamaguchi, Y.; Kashiwabara, T.; Ito, T. J. Organomet. Chem.
2
005, 690, 5701-5709.
20) Yamaguchi, Y.; Kashiwabara, T.; Ogata, K.; Miura, Y.; Nakamura, Y.;
Kobayashi, K.; Ito, T. Chem. Commun. 2004, 2160-2161.
21) Voutchkova, A. M.; Appelhans, L. N.; Chianese, A. R.; Crabtree, R. H. J.
Am. Chem. Soc. 2005, 127, 17624-17625.
22) Tudose, A.; Delaude, L.; Andre, B.; Demonceau, A. Tetrahedron Lett. 2006,
4
7, 8529-8533.
23) Tudose, A.; Demonceau, A.; Delaude, L. J. Organomet. Chem. 2006, 691,
5
356-5365.
(25) Duong, H. A.; Tekavec, T. N.; Arif, A. M.; Louie, J. Chem. Commun.
2004, 112-113.
(26) Tommasi, I.; Sorrentino, F. Tetrahedron Lett. 2006, 47, 6453-6456.
24) Holbrey, J. D.; Reichert. W. M.; Tkatchenko, I.; Bouajila, E.; Walter, O.;
Tommasi I.; Rogers R. D. Chem. Commun. 2003, 28-29.
J. AM. CHEM. SOC.
9
VOL. 129, NO. 42, 2007 12835

A R T I C L E S
Voutchkova et al.
Figure 1. Optimized geometry for the two transition states for SN2 methyl
transfer from DMC to methyl imidazole and the corresponding products of
O
OMe
Figure 2. Optimized geometries for the transition states and the products
corresponding to the C-O bond cleavage (top) and C-C bond formation
reaction. TS1 is favored over TS1
.
The reaction of Scheme 3 in which reactants and products
are considered as isolated molecules in the gas phase is
(bottom) in the reaction of imidazole with DMC to form NHC-CO and
2
methanol.
-
1
calculated to be endothermic by 9.4 kcal mol . Although
slightly disfavored thermodynamically, the equilibrium is shifted
to the right by precipitation of 2 from the reaction medium.
Furthermore, inclusion of an H-bond between the NHC-CO2
and CH3OH products gives a significant additional stabilization.
the methoxy group of the carboxylate anion with an energy
‡
-1
barrier of ∆E ) 12.2 kcal mol relative to the ion pair (Figure
). This transition state is associated with simultaneous cleavage
2
of the C2-H and O2C-OMe bonds to form the NHC, MeOH,
O
-1
and CO . Proton transfer from Ion-pair would protonate an
The energy of reaction is then calculated to be -2.0 kcal mol .
2
-
oxygen of the CO2 group, an unfavorable path for CO2
formation. Thus Ion-pair , a kinetically accessible intermediate,
needs to rearrange to Ion-pair , an intermediate on the
reaction path and no more than 4 kcal mol higher in energy.
The methyl group transfer from DMC to the imidazole nitrogen
O
O
follows a classic SN2 mechanism. Two transition states, TS1
OMe
OMe
‡
and TS1
, have been located with an energy barrier of ∆E
-
1
-
1
)
32.2 and 37 kcal mol above separated methyl imidazole
+
At TS2, the transferring H is not far from being midway
and DMC, respectively. The transition state geometries are
typical of an SN2 mechanism (Figure 1), with an almost planar
between C2 and O (C2...H ) 1.361 Å and O...H ) 1.219 Å).
CO2 formation is significantly advanced with a distance of 1.802
CH3 group located at similar distances from both nucleophiles
O
Å between the C of the nascent CO and O of the nascent
(
C...O ) 2.027 Å, C...N ) 1.845 Å, TS1 ; C...O ) 2.035 Å,
2
OMe
OMe
methanol (vs 1.419 Å in Ion-pair ). The O-C-O angle is
C...N ) 1.831 Å, TS1 ). At the two transition states, an extra
OMe
1
48°, opened up from 130.5° in Ion-pair . This transition
stabilizing interaction develops between one H atom of CH3
O
state leads to a species, Prod (Figure 2), in which the NHC
and the closest oxygen atom (H...O ) 1.994 Å, TS1 ; H...OMe
2
OMe
and MeOH interact via an H-bond (C2...H ) 1.833 Å, O-H )
)
2.059 Å, TS1
). This H-bonding interaction is stronger
O
1.001 Å) and a CO molecule weakly interacts with the methanol
with the carbonate oxygen in TS1 than with the methoxy
2
OMe
oxygen atom (O...C ) 2.665 Å). The transformation from Ion-
oxygen in TS1
, thus favoring the former.
OMe
-1
+
-
pair
to Prod is slightly endothermic (∆E ) 4.7 kcal mol ).
Several H-bonds are also present in the MeIm /O2COMe
2
ion pair (Figure 1) resulting from the SN2 step (MeIm ) N,N′-
dimethyl imidazolium). A strong H-bond between the acidic
C2-H on MeIm and one carboxylate O is accompanied by a
weaker one between a N-Me CH bond and the second
Breaking the O-H...C2 interaction in Prod2 leads to TS3,
corresponding to the coupling between the NHC and CO2
(Figure 2). The distance between (MeO)-H and C2 is now
2.207 Å, and the MeOH has moved out of the carbene plane.
This results in CO2 oxygen interacting with the N-methyl CH
bond (OCO...HC ) 2.330 Å and 2.266 Å), with the CO2 moving
into the carbene plane as appropriate for subsequent carboxylate
formation (C...C2 ) 2.165 Å). This supramolecular network
leads to an easy coupling process with an energy barrier of only
∆E ) 8.4 kcal mol (Figure 3). The product of the reaction
is NHC-CO2 (2) in an H-bonded adduct with a methanol
molecule (Figure 2). A short O-H...O contact (1.760 Å) is
supplemented by an O...CH3 interaction with a N-methyl CH
bond (2.187 Å).
O
carboxylate oxygen in Ion-pair or the methoxy group in Ion-
OMe
-1
pair . The 3.8 kcal mol lower stability of the latter comes
from the lower basicity of methoxy vs carboxylate oxygen. Short
H-bond distances indicate stronger interactions in Ion-pairO
(
1
C2-H ) 1.155 Å, (C2)H...O ) 1.55 Å, (N-methyl)CH...O )
OMe
‡
-1
.965 Å) than in Ion-pair
(C2-H ) 1.209 Å, (C2)H...O )
1
.403 Å, (N-methyl)CH...O ) 2.277 Å). The carboxylate next
slides along the imidazolium to create the strongest possible
O
O
H-bond with C2-H, thus leading to Ion-pair from TS1 and
OMe
OMe
. The formation of Ion-pairO is
to Ion-Pair
from TS1
marginally exothermic from separated reactants (∆E ) -1.1
The low barrier for this key nucleophilic attack of NHC on
CO2 explains the experimental observation that the yield of
NHC-CO2 is only slightly reduced (to 66% from 82%) when
the DMC/methylimidazole reaction is performed under reflux
with a flow of N2 through the reaction mixture instead of in a
-1
OMe
kcal mol ), and that of Ion-pair
is very slightly endothermic
-
1
(
∆E ) 2.7 kcal mol )
From Ion-pairOMe, the carbonate slides along the imidazolium
to reach TS2 where a proton transfers from the imidazolium to
12836 J. AM. CHEM. SOC.
9
VOL. 129, NO. 42, 2007

Imidazolium Carboxylates as NHC Transfer Agents
A R T I C L E S
Figure 3. Energy diagram (kcal mol-1) for the organic synthesis of NHC-CO2. Separated methyl imidazole and DMC serve as the origin for the energies.
Table 1. NHC Metal Complexes Obtained from Reaction of Metal Salts with N,N′-Dimethyl Imidazolium Carboxylate (2)
entry
reactant
product
time (h)
yield % (lit.)a
b
a
1
2
3
[Rh(cod)Cl]2
[Ir(cod)Cl]2
Rh(cod)(NHC)Cl (4)
0.5
0.5
2
2
2
1
1
12
12
28
6
93 (91)
82 (90)
85 (90)
b
a
Ir(cod)(NHC)Cl (7)
(ArH)RuCl2(NHC) (9)
[Ir(cod)(NHC)2]PF6 (11)
[Ir(cod)(PPh3)(NHC)]PF6 (12)
[Ir(cod)(NHC)2]PF6 (11)
[Ir(cod)(py)(NHC)]PF6 (13)
[Pd(NHC)3(OAc)]OAc (14)
[Pd(NHC)3(Cl)]Cl (15)
c
a
[(ArH)RuCl2]2 (8)
[Ir(cod)(PPh3)2]PF6 (10)
[Ir(cod)(PPh3)2]PF6 (10)
Ir(cod)(py)2]PF6
Ir(cod)(py)2]PF6
Pd(OAc)2
PdCl2(MeCN)2
K2PtCl4
IrCl(CO)(PPh3)2
d
4
4
5
5
6
7
8
9
a
b
a
b
84
89
76
78
71
65
76
79
b
d
b
c
c
c
[Pt(NHC)3Cl]Cl (16)
[Ir(PPh3)2(CO)(NHC)]Cl (17)
c
a
b
c
By the traditional free carbene route. Conditions: MeCN, 25 °C. Conditions: MeCN, 75 °C; NHC ) 1,3-dimethylimidazol-2-ylidene (1 equiv), cod
6
d
)
1,5-cyclooctadiene; ArH ) η -p-cymene; py ) pyridine. Conditions same as those in footnote c but with 2 equiv of NHC-CO2.
closed vial. This suggests that CO2 is efficiently scavenged and
may not even readily escape from the cage in which it is formed.
the chloro bridges (Table 1, entries 1-2). The same bridge
cleavage occurs for the conversion of [(η -ArH)RuCl2]2 to the
6
The transformation (Figure 3) of methyl imidazole and DMC
mono NHC derivative (entry 3). It is also possible to displace
one or two neutral ligands from Ir(I) such as PPh3 (entry 4)
and pyridine (entry 5). Room-temperature conditions using 1
equiv of the carboxylate yield the mono-NHC complexes
(entries 4b and 5b). In contrast, at 75 °C with 2 equiv of
carboxylate, both neutral ligands are displaced to give the bis-
NHC derivatives (entries 4a and 5a). The carboxylate reagent
is therefore extremely selective for the formation of either
-
1
to 2 and MeOH is exothermic by 2 kcal mol in the H-bonded
product pair and slightly endothermic by 9.4 kcal mol- without
this interaction. The polar nature of NHC-CO2 and the anionic
character of the oxygens give rise to a large H-bond stabilization.
The rate-determining step for the formation of NHC-CO2 is
the methyl transfer from DMC to the imidazole nitrogen atom
1
-
1
with an energy barrier of around 35 kcal mol (Figure 3). The
8
subsequent bond cleavage (C2-H, O-C) and bond forming
compound with appropriate conditions. The d square planar
(
C-C, O-H) steps have lower energy barriers of ca. 10 kcal
precursors Pd(OAc)2, PdCl2(MeCN)2, and K2PtCl4 also react
to give the unusual tris-NHC derivatives with displacement of
all but one of the anionic ligands (entries 6 to 8). Entries 1-4
of Table 1, where the precursors and products are air-stable,
-
1
mol . Although it is not possible to quantify the stabilization
associated with all of the supramolecular H-bonding interactions,
they are certainly important in keeping the transition states and
intermediates at a relatively low energy. Consistent with the
experimental failure of diethyl carbonate to react, the barrier
was calculated to be 2.7 kcal/mol higher in this case.
give equally good NHC transfer results in air or N . This makes
2
the procedure particularly convenient. For the less reactive
precursors of entries 6-8, exclusion of water is needed to
prevent slow decomposition of the carboxylates to the imida-
zolium salts. Vaska’s complex (entry 9) gives substitution of
the Cl, perhaps because of the high trans effect of CO. The
imidazolium-4-carboxylates do not transfer efficiently to any
of the precursors shown in Table 1.
NHC Transfer to Metal Centers. Transfer of NHCs from
the NHC-CO2 precursors in refluxing acetonitrile has been
investigated for a variety of metal complexes of the late
transition metals (Table 1). In the case of [M(cod)Cl]2, (M )
Rh, Ir), the reaction yields M(cod)(NHC)Cl with cleavage of
J. AM. CHEM. SOC.
9
VOL. 129, NO. 42, 2007 12837

A R T I C L E S
Scheme 5
Voutchkova et al.
Scheme 6
quent reaction with DMC gives the corresponding NHC
complexes in good yields (Scheme 7). The method gives not
only high yields but also more easily isolated products by
removal of excess DMC and byproducts by treatment with
diethyl ether. Bis-NHC products were also observed for N-
isopropyl imidazole, but monosubstituted [M(cod)Cl(NHC)], for
the bulky N-adamantyl imidazole, presumably due to steric
effects.
Scheme 7
Initial formation of the known N-bound bisimidazole cations
20 was confirmed by NMR and mass spectroscopic studies. The
imidazole probably dissociates, undergoes carboxylation and
methylation as in Scheme 3, and then reacts as in Table 1 entry
5
a.
Transfer of NHC Carboxylate Esters to Metal Precursors.
Scheme 8
To broaden the applicability of the method still further to cover
a wider range of NHCs, we looked at the corresponding NHC
esters. These are readily available from the imidazolium salt,
base, and isobutyl chloroformate as described above. Indeed,
carboxylate esters such as 6 are also able to transfer the NHC
to [Rh(cod)Cl]2 to give [Rh(cod)(NHC)Cl] (Scheme 8), presum-
ably after hydrolysis to the carboxylate in situ. We find that
the presence of K2CO3 in the medium facilitates the transfer,
possibly by assisting the hydrolysis of the ester. Adventitious
water is always present in the solvents and glassware, but
deliberate addition of water does not seem to enhance the yield
and deliberate drying does not completely suppress the reaction.
Even so, the reaction times for the esters are considerably longer
than those for the carboxylates (60 min vs 15 min), consistent
with the need for slow hydrolysis. The bulky mesityl groups
only permit monosubstitution in this case (Scheme 8).
The complexes were characterized by NMR spectroscopy and
elemental analysis as well as, for known compounds, by
comparison with authentic materials prepared according to
standard methods. For [Pd(NHC)3(OAc)]OAc (14), an X-ray
21
structure was also obtained, as reported in the communication.
The addition of NaOAc to the medium modifies the results
of entries 1 and 2 in Table 1. Instead of the mono-NHC products
normally formed, the acetate labilizes the chloride ligands in
[M(cod)Cl]2 to give [(cod)M(NHC)2]OAc within minutes as the
final product in >90% isolated yield (Scheme 6). Once again
high selectivity can be achieved. Likewise, although reactions
with the simple chloride salts RhCl3, IrCl3, and RuCl3 without
any additives did not yield any product, upon addition of a
stoichiometric amount of sodium acetate, a mixture of mono-
and bis-NHC products of these metals is formed. However these
reactions have not so far proved synthetically useful because
mixtures of products are always obtained. The asymmetrically
substituted carboxylates mentioned earlier, such as N-methyl-
N′-n-butylimidazolium-2-carboxylate, can also be efficiently
transferred to the same metals in a similar fashion.
Initial attempts to apply the ester method to chelating NHCs
have so far proved unsatisfactory. The appropriate precursors,
23 (n ) 1, 2, 3; X ) H; m ) 0), gave the diesters (X ) COO -
Bu; m ) 2) in poor yield, and the subsequent transfer reactions
to Rh and Ir were not clean.
i
N to C Rearrangement of N-Alkyl Imidazole to NHC.
Several recent reports of N-donor to C-donor rearrangements
Carboxylate Is the Active Transfer Agent. To eliminate
the possibility that the NHC-CO2 first loses CO2 to give the
free NHC which only then transfers NHC, we showed that the
reaction in entry 1 (Table 1) proceeds normally in H2O/MeCN
(90:10 v/v), which would be expected to protonate any
intermediate free NHC to give the imidazolium salt. To eliminate
the further possibility that the imidazolium salt that would result
from decarboxylation and protonation of 2 was the true transfer
agent, we looked at the reaction of the same metal precursors
with N,N′-dimethyl imidazolium bromide. In all but one case,
no NHC complex was formed, but Pd(OAc)2 did yield PdBr2-
27
of imidazoles to NHCs have attracted attention. We now report
similar chemistry (Scheme 7) that also helps lift the limitation
to N-methyl NHCs. This useful two-step, one-pot route to NHC
+
complexes goes via [M(cod)L2] (L ) N-butyl imidazole),
readily formed from [Rh(cod)Cl]2 or [Ir(cod)Cl]2. The subse-
(
27) (a) Lewis, J. C.; Bergman, R. G.; Ellman, J. A. J. Am. Chem. Soc. 2007,
1
29, 5332-5333. (b) Burling, S.; Mahon, M. F.; Powell, R. E.; Whittlesey,
M. K.; Williams, J. M. J. J. Am. Chem. Soc. 2006, 128, 13702-13703. (c)
Esteruelas, M. A.; Fernandez-Alvarez, F. J.; O n˜ ate, E. J. Am. Chem. Soc.
2
006, 128, 13044-13045. (d) Sini, G.; Eisenstein, O.; Crabtree, R. H. Inorg.
Chem. 2002, 41, 602-604.
12838 J. AM. CHEM. SOC.
9
VOL. 129, NO. 42, 2007

Imidazolium Carboxylates as NHC Transfer Agents
A R T I C L E S
Figure 4. (a) Plot of square root of concentration of [Rh(cod)Cl]2 as a function of time showing half order behavior with respect to Rh dimer (Scheme 2).
(b) Plot showing first-order relationship for decay of 24 on reaction with [Rh(cod)Cl]2.
became about three times as long. Solubility problems prevented
quantitative interpretation in terms of reaction order.
NHC Transfer to the Metal Fragment: Computational
Study for the Rh(cod)Cl System. The mechanism for NHC
transfer from NHC-CO2, 2, to Rh has been studied computa-
tionally for Rh(cod)(Cl), a choice which allows us to consider
2
9
both neutral and cationic cases. The geometry optimizations
were performed at the B3PW91 level for molecules in the gas
phase. The reactions proceed via dissociation of neutral or
anionic ligands from Rh, either cod or chloride. Comparison
between these two processes required us to consider the
influence of the solvent, especially important in the case of
chloride. The solvent (CH3CN) has been considered through
single-point PCM calculations on the gas-phase geometries. In
addition, an empty coordination site formed by departure of a
ligand (cod or Cl) may be occupied by the solvent. For this
reason, the necessary numbers of CH3CN molecules are
introduced in the modeling when needed.
Figure 5. Structures of the acetonitrile-stabilized Rh(cod)Cl complexes.
(
NHC)2, a reaction that has previously been reported.28 This is
Forming Two Empty Coordination Sites at the Metal. The
dissociation of the dinuclear Rh complex, [Rh(cod)Cl]2, in
acetonitrile to form the solvento-stabilized monomer Rh(cod)-
Cl(NCCH3), Rh-cod-Cl (Figure 5), consistent with the kinetics,
is calculated to be essentially thermoneutral after allowing for
acetonitrile coordination and continuum stabilization via the
a different product than is formed from NHC-CO2, however,
so the intermediacy of the imidazolium salt is unlikely even
here.
In the case of less active metal precursors, such as Pd(OAc)2
and K2PtCl4, the reaction takes 6-28 h. Attempts to accelerate
it by reflux (82 °C) were unavailing because the carboxylate
decomposes slowly under these conditions and the resulting
NHC protonates to give the imidazolium salt, which is inef-
fective as an NHC transfer agent, thus significantly decreasing
the yield of NHC complex.
-
1
PCM approach (∆E ) -1.9 kcal mol ). The monomer Rh-
cod-Cl is then used as a reference for the energies of all the
species involved in the reaction. The NHC transfer from 2 to
Rh needs two vacant sites at Rh: one to anchor the substrate
through O-coordination, and one to create the new Rh-C2 bond
with the NHC ligand. No reaction path could be identified from
Rh-cod-Cl which has only one coordination site. The complexes
with two solvent-stabilized vacant sites corresponding to partial
Kinetic Studies. If the dissociation equilibrium of [Rh(cod)-
Cl]2 is assumed to be fast, then the rate law for [Rh(cod)Cl]2
reacting with NHC-CO2 can be written as V ) kobsd[NHC-
1/2
CO2][[Rh(cod)Cl]2] . To check the validity of this rate law,
kinetic studies (see Experimental Section) were conducted on
the reaction of Scheme 2 using [Rh(cod)Cl]2 and the more
soluble 1-methyl-3-butyl imidazolium carboxylate (24). When
2
dissociation of the cod ligand (η -cod) denoted Rh-Cl, or
-
dissociation of Cl denoted Rh-cod, have therefore been
optimized (Figure 5).
2
4 was introduced in excess, the reaction was shown to be half
Opening the cod ligand to allow for a second acetonitrile
-
1
order with respect to [Rh(cod)Cl]2 (Figure 4), and when the Rh
dimer was in excess, data indicated that the reaction is first order
with respect to the carboxylate, as illustrated in Figure 4. The
addition of chloride (NBu4Cl, 0.1 M) strongly inhibited the NHC
transfer. The kinetics of [Rh(cod)Cl]2’s disappearance was
unaffected by the Cl ion, but the half time of NHC transfer
ligand to coordinate and form Rh-Cl is 19.3 kcal mol uphill.
The transition state, Rh-Cl-TS (Figure 5), corresponding to cod
opening has been located on the potential energy surface 47
-
1
kcal mol above Rh-cod-Cl. The structure of the transition
(
29) Computational studies did not address transfer of NHC from the 4-car-
boxylate, because all experimental attempts to obtain the 4-NHC complexes
in this way failed. Transfer from the 4-carboxylate should be less favored
because of the higher C-C bond energy associated with the more
nucleophilic 4-NHC.
(28) Hermann, W. A.; Elison, M.; Fischer, J.; Kocher, C.; Artus, G. R. J. Angew.
Chem., Int. Ed., 1995, 34, 2371-2374.
J. AM. CHEM. SOC.
9
VOL. 129, NO. 42, 2007 12839

A R T I C L E S
Voutchkova et al.
NHC methyl group and the dangling double bond of the η2-
cod. All experimental attempts to securely identify such
intermediates in the reaction failed, although in situ NMR
spectra showed small transient peaks for unidentifiable inter-
mediates.
From this adduct, isomerization to the less stable (3.7 kcal
-1
mol ) Rh-Cl-OC intermediate occurs with a low-energy barrier
-
1
(
10.4 kcal mol ) through the transition state denoted Rh-Cl-
OOTS (Figure 6). The transition for isomerization is associated
with the loss of one RhsO interaction (Rh...O ) 2.856 Å) and
π-coordination of the other CO bond in the coordination plane
(RhsO ) 2.189 Å, RhsC ) 2.217 Å, CdO ) 1.29 Å). The
geometry of the transition state is best described as a slightly
distorted T-shaped ML3 fragment with two double bonds, Cd
C of cod and CO of the carboxylate, coordinated perpendicu-
larly. The C2 carbon of the NHC is well positioned to approach
the empty coordination site of the metal in the crucial transfer
step. The isomerization product, Rh-Cl-OC, is a four-membered
metallacyle with a square planar geometry, O trans to Cl (Rhs
O ) 2.123 Å) and the C2 atom of the NHC group at 2.198 Å
from Rh (Figure 6). The carboxylate carbon atom is no longer
coordinated (Rh...C ) 2.611 Å) but takes the â position in the
metallacycle, and the NHC ring is perpendicular to the
coordination plane. The geometry of Rh-Cl-OC is ideally suited
for CsC bond cleavage.
The transition state structure, Rh-Cl-OCTS (Figure 6),
-
1
associated with CsC bond cleavage, lies 6.9 kcal mol above
Rh-Cl-OC. The RhsO and RhsC2 distances of 2.165 Å and
Figure 6. Optimized geometry of minima and transition states for the
formation of the Rh-NHC bond from Rh-Cl and NHC-CO2.
2
.153 Å, respectively, are still close to the values in Rh-Cl-
state features an agostic interaction between one methylene
group and the Rh center with typical metric parameters (Rh...H
OC. The main difference from Rh-Cl-OC is a decrease in the
distance between Rh and the â carbon of the CO fragment
(2.258 Å in Rh-Cl-OCTS vs 2.611 Å in Rh-Cl-OC) and an
2
)
2.130 Å, C-H ) 1.124 Å). This interaction slightly stabilizes
the transition state. Nevertheless, the energy barrier for cod
opening is relatively high, and heating is required to overcome
it. Coordination of the second MeCN could certainly lower this
barrier, but the transition state for cod decoordination via such
an associative step was not sought. Partial decoordination of
cod, a well-established fact,30 is not the focus of this study.
Forming Rh(cod)Cl(NHC) via Partial Cod Decoordina-
increase of the CC distance between the NHC and the CO2
fragments (1.743 Å at the transition state vs 1.551 Å in Rh-
Cl-OC) indicating the formation of the RhsC bond and the
associated cleavage of the CC bond. The geometry of the
carboxylated NHC ligand in Rh-Cl-OCTS is best described as
a CsCsO allyl-type coordination. The transition state Rh-Cl-
OCTS leads to Rh-Cl-prod, a square planar complex, in which
2
3
tion; Unusual η -NHC-CO2 Coordination Mode. In Rh-Cl,
the CC bond of NHCsCO is fully broken, the NHC is now
substitution of the two acetonitriles by NHC-CO2, 2, affords
the complex Rh-Cl-OO having a highly unusual η structure.
coordinated to the metal and CO is π-bonded to the metal via
2
3
one CdO bond (Figure 6). The complex Rh-Cl-prod is 20.0
-
1
NHC-CO2 is coordinated to Rh via the carbon and the two
oxygen atoms, featuring an allyl-type coordination of the
carboxylate part of NHC-CO2 (Figure 6). The geometry at Rh
is square planar with the coordinated cod CdC bond perpen-
kcal mol more stable than Rh-Cl-OC indicating that the CC
cleavage is an exothermic step. CO2 is not a good ligand, and
substitution by the dangling cod double bond is strongly
-1
exothermic (-25.4 kcal mol ), leading to the product Rh(cod)-
3
dicular to the coordination plane and the η NHCsCO2 in the
(Cl)(NHC), 4 (Figure 6). No attempt was made to optimize a
molecular plane. The RhsO bond nearly trans to Cl is longer
than the RhsO bond trans to the cod CdC bond (2.235 vs 2.150
Å). The carbon atom of the carboxylate lies below the
coordination plane at 2.091 Å from Rh. The substitution of the
transition state for displacement of CO2 by the cod CdC bond.
The overall reaction (see Figure 7) from separated Rh-cod-Cl
and NHC-CO2 to Rh(cod)(Cl)(NHC) and CO2 is exothermic
by -7.5 kcal mol , and the rate-determining step is the opening
of the cod ligand to create a total of two vacant sites.
-1
3
two MeCN groups by η -NHCsCO2 is endothermic by +14.9
-
1
kcal mol suggesting that NHCsCO2 is a weaker ligand than
Forming Rh(cod)Cl(NHC) Bond by Loss of Chloride. In
an alternate pathway, that appears to be distinctly more favored
in a polar solvent such as MeCN, chloride dissociates. The
3
two MeCN groups. The η coordination of NHCsCO2 allows
only for a relatively weak interaction between the oxygen centers
2
and Rh and is far from the usual κ -coordination of carboxylate
-
substitution of Cl with CH3CN leads to a cationic complex
2
groups. The traditional κ -σ coordination with M, O, O and C
-
Rh-cod with two coordinated MeCN ligands and free Cl
(
all coplanar is prevented here by the steric clash between the
-1
Figure 5) calculated to be only 2.4 kcal mol above neutral
Rh-cod-Cl if the stabilizing effect of the solvent MeCN is
included via the continuum model. Coordination of the NHC-
(
30) Hanasaka, F.; Tanabe, Y.; Fujita, K.; Yamaguchi, R. Organometallics 2006,
2
5, 826-831.
12840 J. AM. CHEM. SOC.
9
VOL. 129, NO. 42, 2007

Imidazolium Carboxylates as NHC Transfer Agents
A R T I C L E S
Figure 7. Energy profile (kcal mol-1) for the pathway involving partial decoordination of cod. Dotted lines involve steps with gain or loss of MeCN.
Figure 8. Optimized geometry of minima and transition states for the formation of the Rh-NHC bond from Rh-cod and NHC-CO2.
Figure 9. Energy profile (kcal mol-1) for the chloride dissociation pathway. Dotted lines involve steps with gain or loss of MeCN.
CO2 carboxylate is thermodynamically favored and gives Rh-
the CO bond of the carboxylate group is coordinated to Rh prior
to C-C bond cleavage, as found in the case of Rh-Cl, failed.
Thus Rh-cod-OCTS leads directly to the product of reaction,
-
1
cod-OO, which lies only 3 kcal mol above Rh-cod. The
2
coordination of the NHC-CO2 is the usual κ -type with two
-1
1
identical Rh-O σ bonds (2.223 Å). The carboxylate lies in the
molecular plane putting the carbon 2.52 Å from Rh (Figure 8).
The NHC ring lies in the coordination plane to allow conjugation
Rh-cod-prod, 6 kcal mol above Rh-cod-OO, having an η -O
-
coordinated CO2. Substitution of the labile CO2 ligand by Cl
produces the desired Rh(cod)Cl(NHC) 4.
3
with the carboxylate. This contrasts with the η mode found
Discussion
for NHC-CO2 when cod dissociates.
The transition state for C-C cleavage, Rh-cod-OCTS, has
been located on the potential energy surface (Figure 8) 25.5
kcal mol above Rh-cod-OO (Figure 9). At the transition state,
the Rh-C2 and Rh-O bonds are almost fully formed (2.220
and 2.102 Å, respectively) and the C-C bond is significantly
elongated (1.845 Å). All attempts to locate a minimum in which
Phosphines readily form complexes because they have a
kinetically available lone pair. In contrast, N,N′-disubstituted
imidazolium salts, typical precursors of NHCs, need to be
deprotonated first. The resulting free NHC has a free lone pair
and often readily binds to a metal, but it is itself a strong base.
A strong base is not always compatible with the presence of a
-1
J. AM. CHEM. SOC.
9
VOL. 129, NO. 42, 2007 12841

A R T I C L E S
Voutchkova et al.
sensitive functionality on the NHC or with certain metal salts
or complexes required for specific applications.
relevant form, consistent with the kinetics showing dissociation
of the dimer. Two alternate pathways must be considered, having
the potential energy profiles shown in Figures 7 and 9. In either
case, the reaction requires the presence of two cis empty
coordination sites on the metal. Despite all attempts, no pathway
in which only a single coordination on the metal was available
could be identified, implying the absence of an associative
pathway that might normally have been thought likely for a d8
square planar metal. To create these empty sites, we invoke
To solve this problem, we have used NHC carboxylates,
NHC-CO2, neutral, isolable reagents, capable of transferring
the NHC to a variety of metal centers to give complexes with
a variety of ligand to metal ratios depending on the exact
conditions. We have adapted a known synthesis of N,N′-
dimethyl NHC carboxylates where no base is needed, using
dimethyl carbonate (DMC) as the alkylating and carboxylating
agent. This converts a N-substituted imidazole into the N,N′-
disubstituted NHC carboxylates avoiding the high CO2 pressures
and strong base conditions required in the traditional synthesis
of NHC carboxylates from imidazolium salts.
-
partial decoordination of cod or, alternatively, loss of Cl . The
2
decoordination of cod, which leads to an η -cod (i.e., bound to
the metal only via one of the two double bonds), has a rather
-
1
high-energy barrier (around 45 kcal mol ). After coordination
of NHC-CO2, the individual steps leading to the desired final
product have significantly lower barriers. The coordination mode
of NHC-CO2 in the case of partially coordinated cod is highly
A limitation of the DMC route is that it cannot so far be
extended to other dialkyl carbonates and so at least one
N-substituent of the NHC has to be methyl. To help resolve
this problem, we have shown that the most widely used NHC,
N,N′-dimesityl NHC or IMes, can be introduced using the
2
unusual. The expected κ -coordination mode, bound via lone
pairs on each of the two O atoms, is perhaps prevented by the
i
2
corresponding ester, NHC-COO Bu. This ester is formed from
steric effect of the nearby dangling HCdCH group of the η -
the imidazolium salt with base and isobutyl chloroformate. The
ester reacts more slowly with typical metal precursors than does
NHC-CO2, perhaps because the ester has to hydrolyze with
adventitious water to give the NHC-CO2 form before NHC
transfer can occur.
cod.
3
The NHC-CO ligand is coordinated in an η -allylic manner,
2
but in two possible ways depending on the degree of advance-
ment along the pathway. The first way involves the O-C-O
group, which leaves the C2 carbon of NHC uncoordinated. The
second way involves the O-C-C group, which leaves the
remaining carboxylate oxygen uncoordinated. These two coor-
dination modes are energetically relatively close because of the
Mechanism of Formation of NHC-CO2. For the purely
organic alkylation/carboxylation of N-methyl imidazole by
DMC, the calculations show an endothermicity of 9.4 kcal
-
1
mol , not accounting for the interaction between NHC-CO2
and methanol, but precipitation of the product is expected to
drive the reaction. The overall potential energy surface for the
calculated path, shown in Figure 3, has just one significant
barrier associated with the SN2 methyl transfer from DMC to
the imidazole nitrogen. The barrier is consistent with the need
to heat the reaction mixture. The overall transformation does
not contain any unusual chemical step other than the formation
of the CC bond between the NHC and CO2, which normally
requires a high CO2 pressure,26 but this is not needed here. This
may be because the formation of Prod2 creates both the NHC
and the CO2 at the same time and in close proximity, favoring
rapid formation of the NHC carboxylate via the low-energy
barrier nucleophilic attack on CO2 by the NHC. Even passage
of N2 through the reaction mixture only slightly decreases the
yield of NHC-CO2.
conjugation between the CO and NHC fragments, which adds
2
electron density at C2 of NHC. The cleavage of the CC bond
‡
occurs with a relatively low-energy barrier (∆E ) 6.9 kcal
-
1
mol ) because the associated formation of CO drives the
2
energy down.
The pathway via chloride dissociation is only feasible in a
highly polar solvent, such as the MeCN used experimentally.
Modeling the real solvent by the continuum method, with single-
point calculations on gas-phase optimized geometries, under-
estimates the stabilizing effect of the solvent especially because
the minima and transition states were not reoptimized with the
continuum model. Despite this, the loss of chloride is found to
be energetically very easy. The NHC-CO2 coordinates in the
2
4
usual κ form because η -cod does not approach the N-methyl
substituent of the NHC group. The energy barrier for CC
-1
cleavage is calculated to be rather high (ca. 25 kcal mol ). To
The fact that the SN2 step is rate-determining explains why
the reaction was not possible with any other carbonate than
DMC. Diethyl carbonate, for example, would be expected to
have a higher barrier SN2 step, leading to a higher reaction
temperature. The higher SN2 barrier for the ethyl carbonate is
supported by the general observation that steric effects strongly
retard such reactions31 and computationally for the present
reach this transition state, one needs to bend the NHC-CO2
group to move it out of the plane and approach a geometry
3
similar to the η -form discussed above. Since we start from a
more stable coordination mode, this costs more than by using
the cod decoordination path. Even with this requirement, the
highest point on the potential energy surface is lower than the
transition state for cod dissociation. Provided that the solvent
is sufficiently polar, this path should be favored. Indeed, the
strongly inhibiting effect of excess chloride ion on the NHC
transfer rate is entirely consistent with the chloride dissociation
pathway being followed in the experimental system.
-
1
system by a 2.7 kcal mol increase in the barrier height on
moving to ethyl from methyl. Beyond 90 °C, however, the
2
2
-carboxylate converts to the 4-carboxylate, so the desired
-isomer cannot be obtained at the higher temperature required
for diethyl carbonate.
Two vacant sites are required for NHC transfer. We propose
that only the low barrier chloride dissociation path can occur
in the absence of heating. This generates two sites from the
Alternate NHC Transfer Pathways: Partial Decoordina-
-
tion of cod versus Loss of Cl . Moving to the transfer of the
NHC to the metal, [Rh(cod)Cl] monomer has been taken as the
[
1
Rh(cod)Cl] monomer and [M(cod)(NHC)Cl] is formed in Table
entries 1 and 2. The second NHC cannot be transferred
because two sites are no longer available. Likewise, at room
(
31) Caldwell, D.; Magnera, T. F.; Kebarle, P. J. Am. Chem. Soc. 1984, 106,
9
59-966.
12842 J. AM. CHEM. SOC.
9
VOL. 129, NO. 42, 2007

Imidazolium Carboxylates as NHC Transfer Agents
A R T I C L E S
temperature, only one PPh3 or one pyridine is substituted in
spectra were obtained using a Bruker spectrometer operating at 400
MHz. Chemical shifts are reported in ppm with the residual solvent as
an internal reference. Compounds 1 and 318 were synthesized by
previously described methods. Di-µ-chlorobis(p-cymene)chlororuthe-
nium(II) was obtained from Strem Chemicals (Newburyport, MA 01950
USA) and was used as received.
+
[
M(cod)L2] (L ) PPh3 or pyridine; Table 1 entries 4b and 5b)
+
because once [M(cod)(NHC)L] is formed, only one site is
available by dissociation of the second L and not two sites as
+
in [M(cod)L2] , so the second NHC transfer cannot occur. On
heating to 75 °C with the second equivalent of NHC-CO2, the
Kinetic Studies. Rate Order with Respect to [Rh(cod)Cl]
2
.
2
second pathway, cod dissociation to the η form can now occur
-
5
Trimethoxybenzene (0.0100 g, 6.0 × 10 mol) and [Rh(cod)Cl]
2
as well as dissociation of L, creating two sites and allowing
-5
(
0.0082 g, 1.66 × 10 mol) were added to an airtight NMR tube which
the second NHC transfer. This explains the formation of
was purged with nitrogen. A 1.0 mL aliquot of d -acetonitrile was
3
+
[
M(cod)(NHC)2] in Table 1 (entries 4a and 5a). In the Pd(II)
added, and a t ) 0 NMR spectrum was taken to measure the relative
concentrations of the Rh dimer (4.12 ppm, 4H) to the internal standard
(6.05 ppm, 3H). This was found to be 0.2798, after correcting for the
number of protons. The temperature of the sample was recorded to be
cases of Table 1, entries 6 and 7, the tris-NHC is formed even
though models show no particular steric problem in the
hypothetical tetrakis-NHC species. This fits the two-site reactiv-
2
2.5 °C. Under a nitrogen flow 0.060 g of 1-methyl,3-butyl imidazolium
ity model in that successive NHC transfers occur until, in the
-4
+
carboxylate (24) (3.32 × 10 mol, 10× excess given the 2:1 stoichio-
metric ratio of the reactants) was added quickly, and spectra were
immediately taken at the set times and the relative concentration of
the Rh dimer to the internal standard was recorded.
[
Pd(NHC)3X] species, only one site can be created by
dissociation of X () OAc or Cl), so the reaction goes no further.
The Pt(II) case of entry 8 follows exactly the same logic. The
Ru(II) case, where only one NHC is transferred, may be the
result of dissociation of a single Cl from the [(ArH)RuCl2]
monomer alone being permitted. This would be explained on
Rate Order with Respect to NHC-CO
2
. Trimethoxybenzene
-5
-4
(
6
0.0100 g, 6.0 × 10 mol), [Rh(cod)Cl] (0.0843 g, 1.71 × 10 mol,
2
× excess) and 1-methyl-3-butyl imidazolium carboxylate (24) (0.0052
the plausible basis that a second Cl dissociation from a cationic
-5
g, 2.85 × 10 mol) were added to an airtight NMR tube which was
+
[
(ArH)RuCl] monomer is harder than the first. In the case of
purged with nitrogen. A 1.0 mL aliquot of d -acetonitrile was added.
3
IrCl(CO)(PPh3)2 (entry 9), only one NHC is transferred to give
trans-[Ir(CO)(NHC)(PPh3)2]Cl. If a chloride and a PPh3 can
dissociate, the first transfer is consistent with the two-site model.
Subsequent dissociation of the two mutually trans PPh3 ligands
can only give trans vacant sites, but the two-site model requires
cis sites to be created. The second NHC is therefore not
transferred.
The temperature of the sample was recorded to be 28.3 °C. NMR spectra
were immediately taken at the set times, and the relative concentration
of the carboxylate (7.28 ppm, 2H) to the internal standard (6.05 ppm,
3
H) was recorded and corrected for the number of protons.
N,N′-Dimethylimidazolium-2-carboxylate (2). This known com-
2
4
pound was prepared by a modification of the previously reported
procedure. A screw-top pressure tube (Fischer) was charged with
dimethyl carbonate (3.0 mL), 1-methylimidazole (2 mL), and a stir
bar. It was sealed and heated for 30 h at 90 °C. After approximately 2
h, the initially clear solution became cloudy with white precipitate,
and after 24 h there was a copious amount of the white solid in a brown
supernatant liquid. The solid was filtered and was washed thoroughly
with methylene chloride (3 × 15 mL), acetone (3 × 15 mL), and ether
(2 × 10 mL). The white solid proved to be the 2-carboxylate with less
than 3% of the 4-carboxylate isomer. The solid was isolated with a
Conclusion
N,N′-Disubstituted imidazolium carboxylates, readily avail-
able, isolable, air- and water-stable reagents, efficiently transfer
NHC groups to Rh, Ir, Ru, Pt, and Pd, to give novel NHC
complexes. The NHC esters are also effective. A net N to C
rearrangement of an N-alkyl imidazole rhodium complex to the
corresponding NHC complex was also possible with DMC. DFT
calculations identify the steps needed to form the carboxylate
from imidazole and (MeO)2CO (DMC): an SN2 methyl transfer
from DMC to imidazole, followed by a proton transfer from
the imidazolium CH to the carboxylate counterion, produce the
free NHC H-bonded to a methanol molecule with a weakly
bonded CO2. The nucleophilic NHC attacks CO2 to form NHC-
CO2. NHC transfer to the metal with loss of CO2 has been
calculated for Rh(cod)Cl. A proposed two-cis-site reactivity
model rationalizes the experimental data: two such vacant sites
at the metal are needed to allow coordination of the NHC-
CO2 carboxylate and subsequent CC cleavage with NHC
transfer. Partial decoordination of cod or loss of chloride is thus
required for Rh(cod)Cl. Chloride dissociation, calculated to be
easier in a polar solvent, is confirmed experimentally from the
retarding effect of excess chloride ion. This chemistry provides
an alternative route to NHC complexes that may prove widely
useful.
1
total yield of 2.80 g (82%) and characterized by H NMR (500 MHz,
D
(
2
O) δ 3.82 (s, 6H, N-CH
100 MHz, D
are consistent with literature data. If the reaction is carried out above
00 °C the ratio of 4-carboxylate to 2-carboxylate is greatly increased.
3
), 7.26 (s, 2H, H-4 and H-5). 13C{ H} NMR
1
2
O) δ 36.58, 121.89, 139.25, 157.56. The spectral data
24
1
The mixed 2- and 4-isomeric material can be used directly for transfer
to a transition metal, because only the 2-isomer undergoes transfer.
N,N′-Dimethylimidazolium-4-carboxylate. This known26 com-
pound was synthesized according to the same method used above for
the 2-carboxylate but at 120 °C for 30 h. The white precipitate so
formed was filtered and washed as described above to give a yield of
1
the 4-isomer of 3.06 g (92%). H NMR (500 MHz, D
2
O) δ 3.75 (s,
1
3
3
H, N1-CH
O, 125 MHz): δ 35.50, 36.41, 127.0, 130.51, 137.82, 163.13. The
identification of the 4-carboxylate was confirmed by comparison to
3
3
), 3.82 (s, 3H, N3-CH ), 7.64 (s, 1H, H-5); C NMR
(D
2
26
the literature NMR spectra.
4
Chloro(η -1,5-cyclooctadiene)(1,3-dimethylimidazole-2-ylidene)-
rhodium(I) (4). This known28 compound was prepared from a mixture
2
of [Rh(cod)Cl] (500 mg, 1.01 mmol) and N,N′-dimethyl imidazolium
carboxylate (2) (340 mg, 2.41 mmol) by stirring in acetonitrile (7 mL)
for 40 min at room temperature in a Schlenk flask. Alternatively, this
compound can be prepared by heating the same mixture to 75 °C for
15 min. In either case, the reaction mixture was dried in Vacuo and
Experimental Section
General Methods. All starting materials and reagents were obtained
from commercial sources and used as received unless otherwise noted.
All reactions were performed under an argon atmosphere using standard
Schlenk techniques, and acetonitrile was distilled over calcium hydride.
washed three times with diethyl ether. The yellow solid obtained was
1
analytically pure (638 mg, 93%). H NMR (400 MHz, CDCl
3
) δ 6.72
(s, 2H, NCH), 4.94 (m, 2H, cod CH), 4.02 (s, 6H, CH
2
cod CH), 2.34 (m, 4H, cod CH ), 1.88 (m, 4H, cod CH ). C { H}
3
), 3.22 (m, 2H,
1
13
31
13
1
All glassware was dried overnight prior to use. H, C, and P NMR
2
J. AM. CHEM. SOC.
9
VOL. 129, NO. 42, 2007 12843

A R T I C L E S
Voutchkova et al.
1
NMR (100 MHz, CDCl
(
Hz, cod CH), 37.81 (CH
3
) δ 182.84 (d, JRh-C ) 50.9 Hz, NCN), 122.04
3 2 6
compound was prepared as follows. [Ir(PPh ) (cod)]PF (144.5 mg, 149
1
1
NCH), 98.72 (d, JRh-C ) 6.9 Hz, cod CH), 67.84 (d, JRh-C ) 14.6
µmol) and 2 (21 mg, 149 µmol) were dissolved in acetonitrile (10 mL)
in a 25-mL Schlenk flask. This reaction mixture was stirred for 2 h at
room temperature, after which it was filtered and concentrated to
dryness in Vacuo. The solid was dissolved in 2 mL of methylene
chloride and was purified by column chromatography by elution with
1:1 hexane/ethyl acetate followed by elution of a bright yellow fraction
1
3
2
), 33.13 (d, JRh-C ) 1.0 Hz, cod CH ), 29.05
(
cod CH ). These NMR data are consistent with those reported in the
2
2
8
literature.
4
Chloro(η -1,5-cyclooctadiene)(1,3-dimethylimidazole-2-ylidene)-
iridium(I) (7). This known28 compound was synthesized following the
above procedure but with [Ir(cod)Cl]
2
(60 mg, 89.2 µmol) and N,N′-
6
with a solution of KPF (1 equiv) in acetone, which afforded the bright
yellow product as the hexafluorophosphate salt. Crystallization of this
fraction from 1:4 acetone/diethyl ether yielded compound 12 as orange
dimethyl imidazolium carboxylate (2) (30 mg, 212 µmol) in MeCN (5
mL). After 20 min of heating (oil bath, 75 °C) the color of the solution
changed from red to orange. The reaction mixture was cooled, and the
solvent was removed in Vacuo. The solid was dissolved in 2 mL of
methylene chloride and was purified by column chromatography by
elution with 1:1 hexane/ethyl acetate. The product was obtained by
dissolving the solid in dichloromethane and recrystallizing by addition
1
6
needles (106 mg, 89%). H NMR (400 MHz, d -CDCl
7.51 (m, 15H, Ar), 6.91 (s, 2H, NCH), 4.21 (b s, 2H, cod CH), 3.70 (b
s, 2H, cod CH), 3.49 (s, 6H, CH ), 2.42 (br m, 4H, COD CH ), 2.05
(br m, 4H, cod CH ) δ: 174.86, 132.13,
). 13C NMR (75 MHz, CDCl
133.20, 131.96, 130.05, 129.72, 129.23, 128.81, 124.14, 86.22, 86.02,
3
) δ: 7.05-
3
2
2
3
1
31
of ether to yield an orange microcrystalline solid (63 mg, 82%). H
80.45, 37.11, 31.26, 30.10. P NMR (121 MHz, CD
2
Cl
2
) δ: 18.29
PIr, 659.2;
+
NMR (400 MHz, CDCl
3
) δ 6.97 (s, 2H, NCH), 4.35 (m, 2H cod CH),
), 2.99 (m, 2H, cod CH), 2.20 (m, 4H, cod CH ), 1.60-
) δ 176.60
(PPh
3
), -144.41 (PF
6
). ESI-MS (m/z) M calcd for C31
H
35
N
2
3
1
.89 (s, 6H, CH
.73 (m, 4H, cod CH
3
2
found, 659.7. These NMR data are consistent with those reported in
the literature.4
1
3
1
2
). C { H} NMR (100 MHz, CDCl
3
4
(s, NCN), 122.80 (s, NCH), 83.64, 53.28, 31.02, 31.75 (s, cod), 37.25
(η -1,5-Cyclooctadiene)(1,3-dimethylimidazole-2-ylidene)(pyridi-
27
4
(
NCH
3
). These NMR spectra are consistent with literature data. The
ne)iridium(I) Hexafluorophosphate (13). This known compound was
1
literature H NMR shifts for the cod ligand in CDCl
3
solution were
2 6
prepared from [Ir(py) (cod)]PF (90 mg, 149 µmol) and 2 (21 mg, 149
reported as δ: 5.23, 4.2, 1.89, 1.50. However in our hands we found
different values, reported above. All other spectroscopic data for this
compound are in agreement with the published data.2
µmol) dissolved in acetonitrile (10 mL) in a 25-mL Schlenk flask. The
reaction mixture was stirred for 2 h at room temperature, after which
it was concentrated to dryness in Vacuo. The solid was dissolved in 2
mL of methylene chloride and was purified by column chromatography
by elution with 1:1 hexane/ethyl acetate followed by elution of a bright
8
Dichloro(1,3-dimethylimidazole-2-ylidene)(p-cymene)ruthenium-
2
8
(
II) (9). This known compound was prepared as follows. A dry
Schlenk tube was charged with di-µ-chlorobis[p-cymene)chlororuthe-
nium(II) (8) (60 mg, 98.3 µmol) and 2 (29 mg, 210 µmol) which were
dissolved in 4 mL of acetonitrile. The reaction was stirred for 10 min
at room temperature and then heated in an oil bath at 75 °C for 2 h. It
was allowed to cool, and excess 2 was filtered from the reaction mixture.
The filtrate was then concentrated to dryness in Vacuo and recrystallized
6
yellow fraction with a solution of KPF (1 equiv) in acetone, which
afforded the bright yellow product as the hexafluorophosphate salt.
Crystallization of this fraction from 1:4 acetone/diethyl ether yielded
1
compound 13 as yellow needles (72 mg, 78%). H NMR (400 MHz,
6
d -acetone) δ: 8.97 (d, 2H, pyridine), 7.98 (t, 1H, pyridine), 7.66 (dd,
2H, pyridine), 7.24 (s, 2H, NCH), 4.18 (s, 6H, CH
3
), 4.09 (br m, 2H,
), 1.97 (br
). C { H} NMR (100 MHz, d -acetone) δ: 172.25,
151.70, 139,31, 127.65, 123.90, 83.87, 64.01, 37.50 (s, COD) 32.75,
from 1:3 CH
2
Cl
2
1
/ether to afford compound 9 as orange-red needles
cod CH), 3.79 (br m, 2H, cod CH), 2.44 (br m, 4H, cod CH
2
13
1
6
(
5
66 mg, 85%). H NMR (400 MHz, CDCl
.41 (d, J ) 5.8 Hz, 2H), 5.13 (d, J ) 5.8 Hz, 2H), 2.68 (sept, J )
3
) δ: 6.97 (s, 2H, NCH),
m, 4H, cod CH
2
3
3
3
3
31
6
6
.8 Hz, 1H), 2.02 (s, 3H, methyl) 3.96 (s, 6H, CH
3
), 1.22 (d, J ) 6.8
) δ : 173.14, 123.68,
4.68, 82.20, 39.51, 30.78, 22.46, 18.62. These NMR data are consistent
30.21 (s, NCH
3
). P NMR (162 MHz, d -acetone) δ: -143.12 (PF
6
).
13
1
+
Hz, 6H, methyl). C { H} NMR (100 MHz, CDCl
8
3
ESI-MS (m/z) M calculated for C18
H
25IrN
3
, 476.17; found, 476.87.
Anal. calcd for C18
H
25
F
6
IrN
3
P: C, 34.84%; H, 4.06%; N, 6.77%.
2
8
with the literature.
Found: C, 34.80%, H, 4.01%, N, 6.65%.
(
η4-1,5-Cyclooctadiene)(bis-1,3-dimethylimidazole-2-ylidene)iri-
dium(I) Hexafluorophosphate (11). This compound was prepared from
Ir(cod)(PPh ]PF (10) (114 mg, 117.5 µmol) and 2 (41 mg, 293.8
Acetato(tris-1,3-dimethylimidazole-2-ylidene)palladium(II) Ac-
etate (14). A 10-mL Schlenk flask was charged with Pd(OAc) (109
2
[
3
)
2
6
mg, 486.6 µmol) and 2 (274 mg, 1.94 mmol). Acetonitrile (7 mL) was
added under an inert atmosphere, and the reaction mixture was heated
in a 75 °C oil bath for 12 h. The solution turned yellow and then pale
yellow during the course of the reaction. It was then cooled and filtered
through Celite, after which the filtrate was concentrated to dryness in
Vacuo and washed with ether (2 × 10 mL). The product was
µmol) dissolved in acetonitrile (4 mL) in a 10-mL Schlenk flask. The
reaction mixture was stirred for 10 min at room temperature and was
then heated in an oil bath (75 °C, 2 h), during which it turned to a
bright orange. It was then allowed to cool, and the solution was filtered
to remove excess imidazolium carboxylate, after which the solution
was concentrated to dryness in Vacuo. The solid was dissolved in 2
mL of methylene chloride and was purified by column chromatography
by elution with 4:1 hexane/ethyl acetate. A bright orange fraction was
2 2
recrystallized twice from 1:3 CH Cl /diethyl ether to yield light yellow
1
6
prisms of 14 (177 mg, 71%). H NMR (400 MHz, d -DMSO) δ: 7.37
(s, 2H, central NCH), 7.30 (s, 4H, wing NCH), 3.73 (s, 12H, wing
eluted with a solution of KPF
this fraction from 1:3 CH Cl
orange prisms (64 mg, 84%). H NMR (400 MHz, CDCl
6
(1 equiv) in acetone. Crystallization of
/diethyl ether yielded compound 11 as
CH
CCH
MHz, d -DMSO) δ: 175.64 (s, NCN), 174.69 (s, NCN), 122.45 (s,
NCH), 122.16 (s, NCH′), 36.92 (s, NCH ), 36.70 (s, NCH ′), 34.13 (s,
OAc), 22.88 (s, OAc′). Anal. calcd for for C19 Pd‚3.5H O: C,
3
), 3.55 (s, 6H, central CH
3
), 3.20 (br s, 6H, H
2 2
O) 1.60 (s, 3H, O -
CCH
3
, unbound). 13C { H} NMR (100
1
2
2
1
3
, bound) 1.49 (s, 3H, O
2
6
3
) δ: 7.10 (s,
), 3.72 (br m, 4H, cod CH), 2.10 (br m,
4
4
H, NCH), 3.89 (s, 6H, CH
H, cod CH ), 1.89 (br m, 4H, cod CH
) δ: 178.88 (s, NCN), 124.70 (s, NCH), 77.22, 39.22 (s, cod)
3
3
3
1
3
1
2
2
). C { H} NMR (100 MHz,
H
33
N
O
6 4
2
CDCl
5.87 (s, NCH
Analysis calculated for C20
.79%. Found: C, 34.06%; H, 4.53%, N, 8.84%. Complex 11 was also
made by an analogous procedure using [Ir(cod)(py) ]PF rather than
Ir(cod)(PPh ]PF to obtain a yield of 58 mg, 76%. In both cases the
3
39.43%; H, 6.97%; N, 14.51%. Found: C, 39.26%; H, 6.36%; N,
14.51%.
3
1
3
3 3 6
). P NMR (162 MHz, CDCl ) δ: -143.12 (PF ).
H
36
F
6
IrN : C, 33.91%; H, 4.43%; N,
4
P
2
Chloro(tris-1,3-dimethylimidazole-2-ylidene)palladium(II) Chlo-
8
ride (15). A 10-mL Schlenk flask was charged with PdCl (MeCN)
2
2
2
6
(100 mg, 389 µmol) and 2 (220 mg, 1.55 mmol). Acetonitrile (5 mL)
was added under an inert atmosphere, and the reaction mixture was
heated in a 75 °C oil bath for 12 h. The solution was then cooled and
filtered through Celite, after which the filtrate was concentrated to
dryness in Vacuo and washed with ether (2 × 10 mL). The product
[
3
)
2
6
identity of the products was confirmed by NMR spectroscopic
comparison with our own authentic materials.
4
(
η -1,5-Cyclooctadiene)(1,3-dimethylimidazole-2-ylidene)(triph-
4
enylphosphine)iridium(I) Hexafluorophosphate (12). This known
2 2
was recrystallized twice from 1:3 CH Cl /diethyl ether to yield yellow
1
2844 J. AM. CHEM. SOC. VOL. 129, NO. 42, 2007
9

Imidazolium Carboxylates as NHC Transfer Agents
A R T I C L E S
1
6
prisms of 15 (117 mg, 65%). H NMR (400 MHz, d -DMSO) δ: 7.34
Celite, and the filtrate was concentrated to dryness in Vacuo and washed
with ether (2 × 10 mL). The crude solid was found to be analytically
(
s, 2H, central NCH), 7.21 (s, 4H, wing NCH), 3.84 (s, 12H, wing
13
1
6
3
CH ), 3.42 (s, 6H, central CH
3
). C { H} NMR (100 MHz, d -DMSO)
2
pure without further purification. It was recrystallized from 1:4 CH -
δ: 176.51 (s, NCN), 172.12 (s, NCN), 142.52 (s, NCH), 122.10 (s,
NCH′), 34.45 (s, NCH ), 32.21 (s, NCH ′). The identity of the products
Cl /diethyl ether to yield bright orange needles of 18 (157 mg, 88%).
2
1
3
3
H NMR (400 MHz, CDCl ), see compound 11. The compound was
3
was confirmed by NMR spectroscopic comparison with authentic
material.
converted to the hexafluorophosphate salt, 11, by chromatographic
separation using acetone/KPF , and all characterization data were found
6
Chloro(tris-1,3-dimethylimidazole-2-ylidene)platinum(II) Chlo-
to match those of the authentic material (compound 11).
ride (16). A 10-mL Schlenk flask was charged with K
86.6 µmol) and 2 (274 mg, 1.94 mmol). Acetonitrile (8 mL) was added
under an inert atmosphere, and the reaction mixture was heated in a
5 °C oil bath for 24 h. The solution was cooled and filtered through
2
PtCl
6
(109 mg,
4
(
η -1,5-Cyclooctadiene)(bis-1,3-dimethylimidazole-2-ylidene)-
4
33
rhodium(I) Acetate (19). The known complex 19 was also made by
an analogous procedure using [Rh(cod)Cl]
obtain a yield of 112 mg, 75%. H NMR (400 MHz, CDCl
H, NCH), 4.05 (br m, 4H, cod CH) 3.85 (s, 12H, CH
H, cod CH
confirmed by comparison (NMR) with literature data.
η4-1,5-Cyclooctadiene)(bis-1-n-butyl-3-methylimidazole-2-ylide-
ne)iridium(I) Hexafluorophosphate (21). A screw-top pressure tube
Fischer) was charged with dimethyl carbonate (3.0 mL), 1-n-
butylimidazole (2 mL), [Ir(cod)Cl] (150 mg), and 2 mL of acetonitrile.
The tube was immersed in a 90 °C oil bath and allowed to stir for 12
h. It was then fitted with a condenser and further refluxed for 14 h.
The reaction mixture was then allowed to cool and was treated with
2
rather than [Ir(cod)Cl]
): 6.96 (s,
), 2.22 (br m,
), 2.10 (s, 3H, OAc). The identity of the compound was
2
to
7
1
3
Celite, after which the filtrate was evaporated to dryness in Vacuo and
washed with ether (2 × 10 mL). The product was recrystallized first
4
8
3
2
from 1:3 acetone/diethyl ether and a second time from 1:3:2 CH
2
Cl
2
/
33
diethyl ether/pentane to yield light yellow prisms of 12 (177 mg, 71%).
(
1
H NMR (400 MHz, CD
3
CN) δ: 7.21 (s, 2H, central NCH), 7.12 (s,
), 3.55 (s, 6H, central CH ).
CN) δ: 146.35 (s, NCN), 145.56
s, NCN), 121.26 (s, NCH), 120.89 (s, NCH′), 36.82 (s, NCH ), 35.54
s, NCH ′). Anal. calcd for C15 Pt: C, 32.50%; H, 4.36%; N,
5.16%. Found: C, 32.61%; H, 4.30%, N, 15.28%. ESI-MS (m/e)
4
H, wing NCH), 3.73 (s, 12H, wing CH
3
3
(
13
1
6
C { H} NMR (100 MHz, d -CD
3
2
(
(
1
3
3
2 6
H24Cl N
+
3
calcd: for [Pt(NHC) Cl)] , 518.91; found, 519.85.
NaPF (60 mg) for 5 min, and the solvent was removed in Vacuo. The
6
trans-Bis(triphenylphosphine)(carbonyl)(1,3-dimethylimidazole-
-ylidene)iridium(I) Chloride (17). This new compound was prepared
oil was thoroughly washed with diethyl ether (5 × 10 mL) and was
2
recrystallized from methylene chloride/pentane (1:3) to afford red
3 2
from Ir(PPh ) (CO)Cl (100 mg, 128 µmol) and 2 (36 mg, 256 µmol)
1
crystals of 21 (88%). H NMR (400 MHz, CDCl
.00 (d, 2H), 4.47 (m, 2H), 4.31 (m, 2H), 3.91 (d, 6H), 3.81(m, 4H),
.69 (m, 1H), 3.63 (m. 1H), 2.17 (m, 4H), 1.82 (m, 4H), 1.60 (m, 2H),
.36 (m, 4H), 0.95 (2, three diastereomers, 6H). ESI-MS (m/z) calcd
3
) δ: 7.17 (d, 2H),
in acetonitrile (5 mL) in a 10-mL Schlenk flask. The mixture was stirred
for 6 h at room temperature, after which it was filtered and the filtrate
was concentrated to dryness in Vacuo. Crystallization of this fraction
7
3
1
from 1:4 dichloromethane/diethyl ether yielded the product as yellow
+
for [Ir(NHC)
IrN
N, 7.82%.
2
(cod)] , 576.82; found, 576.24. Anal. calcd for C24
40 6
H F -
1
prisms (88 mg, 79%). H NMR (400 MHz, CD
2
Cl
2
) δ: 7.56-7.47 (m,
3
). C { H} NMR
13
1
4
P: C, 39.94%; H, 5.59%; N, 7.76%. Found: C, 39.71%; H, 5.73%;
3
0H, PPh
3
), 6.46 (s, 2H, CNH), 2.91 (s, 6H, wing CH
(
1
100 MHz, CD
28.55, 36.27. P NMR (161 MHz, CD
2
3
Cl ) δ: 184.36, 175.26, 133.48, 133.43, 131.93, 131.56,
2
4
1
Chloro(η -1,5-cyclooctadiene)(1,3-dimesitylimidazole-2-ylidene)-
rhodium(I) (22). This known compound was synthesized as follows.
2
Cl
(CO)] , 841.21; found, 841.72. Analysis
Ir: C, 57.56%; H, 4.37%; N, 3.20%.
2
) δ: 20.68. ESI-MS (m/e)
3
+
3 2
calculated: for [Ir(NHC)(PPh )
calculated for C42
A 25 mL round-bottom flask was charged with N,N′-2,4,6-trimethyl-
phenyl imidazolium chloride (100 mg, 294.1 µmol) and potassium tert-
butoxide (16 mg, 378 µmol). The flask was then charged with dry
acetonitrile (10 mL) and stirred in an ice bath for 5 min, after which
isobutyl chloroformate (1.20 mL, 880 µmol) was added dropwise
through a syringe over the course of 10 min. The reaction was allowed
to stir in an ice bath under a strictly inert atmosphere for an additional
hour and then at room temperature for 6 h. The reaction mixture was
then filtered and dried in Vacuo to give a light yellow oil, which was
washed with ether three times. To the resulting oil 6 mL of acetone
were added with 1 equiv of NaPF . The resulting precipitate was filtered,
6
and the filtrate was dried in vacuo. The white solid obtained was again
washed with ether to give 59 mg, 78% of 11. H NMR (400 MHz,
d -acetone) δ: 8.23 (s, 2H, NCCN), 7.08 (s, 4H, Ph), 3.77 (d, 2H,
H38ClN OP
2 2
Found: C, 57.42%; H, 4.31%, N, 3.35%.
4
Chloro(η -1,5-cyclooctadiene)(1-butyl-3-methylimidazole-2-ylidene)-
3
2
rhodium(I). This known compound was prepared from a mixture of
4
bis(η -1,5-cyclooctadiene)(di-µ-chloro)dirhodium (500 mg, 1.01 mmol)
and N-methyl-N′-n-butylimidazolium-2-carboxylate (404 mg, 2.2 mmol)
stirred in acetonitrile (10 mL) for 40 min at room temperature in a
Schlenk flask. The reaction mixture was dried in Vacuo and washed
with diethyl ether (3 × 10 mL). The yellow solid was dissolved in
methylene chloride (2 mL) and purified by column chromatography
via elution with 1:1 (v/v) hexanes/ethyl acetate or, better, can
alternatively be recrystallized from methylene chloride/diethyl ether
1
1
to give small yellow prisms (693 mg, 90%). H NMR (400 MHz,
6
CDCl
3
) δ: 6.78 (d, J ) 1.8 Hz, 2H), 4.99 (br m, 2H, COD CH), 4.46
t, 2H, J ) 7.8 Hz), 4.02 (s, 3H, CH ), 3.30 (br m, 1H, COD CH),
.23 (br m, 1H, COD CH), 2.34 (m, 4H, COD CH ), 1.92 (br m 4H
and 2H Bu CH ), 1.48 (m, 2H), 1.01 (t, 3H, J ) 7.4 Hz).
C { H} NMR (75 MHz, CDCl ) δ: 13.71 (s), 28.75 (s), 32.68 (s),
(
3
3
OCH
0.44 (d, 6H, Me). 13C { H} NMR (125 MHz, d -acetone) δ: 153.55,
142.79, 130.96, 126.46, 75.92, 21.49, 18.98, 17.77. [Rh(cod)Cl] (57
2 2 2
), 2.21 (s, 6H, Me) 2.04 (s, 12H, Me), 1.41 (m, 1H, OCH CHMe ),
1
6
2
COD CH
2
2
2
13
1
mg, 114 µmol) was then added in MeCN (2 mL), and the reaction
mixture was refluxed for 2 h. The solution was cooled, and the solvent
was removed in Vacuo. The resulting solid was redissolved in methylene
chloride (2 mL) and subjected to column chromatography with 80/20
ethyl acetate/hexanes to afford a yellow solid after evaporation of
3
1
3
J
3.05 (s), 37.62 (s), 50.23 (s), 67.78 (d, JRh-C ) 14.1 Hz), 98.26 (d,
1
1
Rh-C ) 7.7 Hz), 120.13 (s), 121.99 (s), 181.43 (d, J
C
-Rh ) 48.2
+
Hz). ESI-MS (m/z) calcd for [Rh(NHC)(cod)] , 385.09; found, 385.26.
These data are consistent with those reported in the literature.32
η4-1,5-Cyclooctadiene)(bis-1,3-dimethylimidazole-2-ylidene)iri-
dium(I) Acetate (18). A 10-mL Schlenk flask was charged with [Ir-
solvent. It was recrystallized from methylene chloride/pentane (1:3) to
(
1
afford pure product (0.49 g, 62%). H NMR (400 MHz, CDCl
3
) δ:
(
(
cod)Cl]
2
(162 mg, 162 µmol), 2 (57 mg, 0.4 mmol), and sodium acetate
7.06 (s, 4H Mes), 6.99 (s, 2H NHC), 4.42 (s, 2H cod) 3.30 (s, 2H
0.100 g). Acetonitrile (10 mL) was added under an inert atmosphere,
3 3 3
cod), 2.32 (s, 6H CH ), 2.36 (s, 6H CH ), 2.14 (s, 6H CH ), 1.81 (m,
4H cod), 1.58 (m, 4H cod). 13C { H} NMR (100 MHz, CDCl
1
) δ: 18.5,
and the mixture was heated to reflux for 2 h, over which time it turned
to a bright orange color. The solution was cooled and filtered through
3
20.0, 21.4, 28.9, 33.2, 68.7 (d, JRh-C ) 14.3 Hz), 96.3 (d, 1JRh-C
1
)
(
32) Park, K. H.; Kim, S. Y.; Son, S. U.; Chung, Y. K. Eur. J. Org. Chem.
(33) Herrmann, W. A.; Elison, M.; Fischer, J.; K o¨ cher, C.; Artus, G. R. J.
2
003, 4341-4345.
Chem.sEur. J. 1996, 2, 772-780.
J. AM. CHEM. SOC.
9
VOL. 129, NO. 42, 2007 12845

A R T I C L E S
Voutchkova et al.
7
)
.6 Hz), 124.3, 128.8, 129.8, 135.1, 137.0, 137.9, 139.2, 183.5 (d, 1JRh-C
52.5 Hz). These NMR data are consistent with those reported in the
a minimum or a transition state had been reached. The nature of the
species connected by a given transition state structure was checked by
optimization as minima of slightly altered TS geometries along both
directions of the transition state vector. The energies of all the systems
studied at the B3PW91 level in the gas phase for the mechanism of
NHC transfer to Rh were computed with inclusion of solvent effects
(acetonitrile) according to the PCM scheme as implemented in
Gaussian.42 The geometries obtained in the gas phase were used without
further reoptimization within the PCM methodology, and the united
atom topological model with UAHF radii was used. It should be noted
that although the reactions under studies are associated with a change
of molecularity, we have used PCM energies E and not Gibbs energies
G for characterizing the reaction paths. Computing the entropic
contribution for these reactions, run in solution, by gas-phase modeling
is most likely inadequate, in particular because no geometry optimiza-
tion within the PCM scheme was performed. Due to the difficulties in
calculating entropy in condensed phases, various alternatives have been
suggested in the literature, including even the full neglect of the
translational entropy, which is the greatest contribution to the total
entropy changes.43 Because of these limitations, we have considered
only energies for the discussion but we provide Gibbs free energies in
the Supporting Information for all species. In the cases where the gas-
phase calculations could be used, we have verified that no change of
mechanism results from the consideration of Gibbs free energies.
3
literature.
4
Chloro(η -1,5-cyclooctadiene)(di-n-butylimidazole-2-ylidene)iri-
3
4
dium(I). This known compound was synthesized according to an
analogous procedure to compound 22, but using N,N′-di-n-butyl
imidazolium bromide (76 mg, 294.1 µmol). The ester was used directly
after its synthesis as above. [Ir(cod)Cl] (77 mg, 114 µmol) was then
added in MeCN (2 mL), and the reaction mixture was refluxed for 2
2
h, after which the compound was isolated by column chromatography
with 80:20 hexanes/ethyl acetate and recrystallized in 4:1 CH Cl /ether
2 2
to give orange needles (112 mg, 63%) identical to authentic material.
N-Methyl-N′-n-butylimidazolium-2-carboxylate (23). This known,26
asymmetrically substituted carboxylate was prepared by a modification
of the procedure for 2. A screw-top pressure tube (Fischer) was charged
with dimethyl carbonate (3.0 mL), 1-n-butylimidazole (2 mL), and a
stir bar. It was sealed and heated for 40 h at 95 °C. The solvent was
then removed in Vacuo to give a brown oil, which was washed with
diethyl ether (3 × 15 mL). It was purified by recrystallization in
acetonitrile to give a yellow solid, which was dried in vacuo to give
the product (yield: 2.9 g, 78%). H NMR (500 MHz, D
1
1
2
O) δ 7.41 (d,
-),
), 0.70 (m, C-CH ).
CN) δ 12.52 (s, C-CH ), 18.99 (s, CH ),
) 35.19 (s, N-CH ), 48.56 (s, N-CH -), 121.04 (s, C4),
21.91 (s, C5), 137.61 (s, C2), 160.12 (s, COO). This material was
3
3
H, J ) 1.8 Hz), 7.36 (d, 1H, J ) 1.8 Hz), 4.28 (t, 2H, N-CH
.83 (s, N-Me), 1.58 (m, CH ), 1.05 (m, CH
2
3
2
2
3
13
1
C{ H} NMR (125 MHz, CD
3
3
2
3
1
2.44 (s, CH
2
3
2
Acknowledgment. We thank the U.S. DOE (A.V., R.H.C.),
the CNRS, and the MESR (O.E., E.C.) for funding. M.F. thanks
the Spanish Ministerio de Educaci o´ n y Ciencia for a postdoctoral
fellowship.
2
6
identified by comparison with the literature spectra.
Computational Details. The calculations were performed with the
35
36
Gaussian03 package at the B3PW91 level. Rhodium was represented
by the relativistic effective core potential (RECP) from the Stuttgart
group and the associated basis set, augmented by an f polarization
function (R ) 1.350). Chlorine was represented by the relativistic
effective core potential (RECP) from the Stuttgart group and the
associated basis set, augmented by a d polarization function (R )
0
N, H, O).
3
7
Supporting Information Available: Complete reference for
Gaussian 03 and DFT-computed Cartesian coordinates for the
optimized molecules and their electronic E and Gibbs free
energies G values. For the NHC transfer mechanism, PCM
energy values are also given. This material is available free of
charge via the Internet at http://pubs.acs.org.
3
8
3
9
4
0
.640). A 6-31G(d,p) basis set was used for all the other atoms (C,
4
1
The geometry optimizations were performed without any symmetry
constraint followed by analytical frequency calculations to confirm that
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34) Chianese, A. R.; Li, X. W.; Janzen, M. C.; Faller, J. W.; Crabtree, R. H.
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