Facile derivatization of a chemo-active NHC incorporating an enolate backbone and relevant tuning of its electronic properties

Article abstract of DOI:10.1021/om1003607

The present report discloses a modular synthetic route to a new type of anionic N-heterocyclic carbene ligand incorporating an enolate group as a reactive backbone component of its heterocyclic framework. The presence of such a reactive unit facilitates further tailoring of the ligand, even after its complexation to transition metals, with concomitant tuning of the electronic donor properties of the carbene center. Simple acylation of the formamidine Ar-NH-CH=NAr (1a,b) (a: Ar = mesityl (Mes); b: Ar = 2,6-diisopropylphenyl (DIPP)) by chloroacetyl chloride gives an acylated formamidine (2a,b), which undergoes a thermally induced cyclization to afford the 4-hydroxyimidazolium salt 3a,b. Single deprotonation by NEt₃ gives the zwitterionic imidazolium-4-olate 4, whereas double deprotonation by 2 equiv of LiHMDS gives the imidazolin-2-ylidene-4-olate ligand 5^-, which reacts with 0.5 equiv of [RhCl(1,5-COD)]₂ to give, after acidification with HCl, the complex [(5_H)RhCl(COD)] (7), in which the acidified carbene ligand noted 5_H adopts the keto form. By contrast, treatment of 4 with electrophilic reagents E-X (E₁ = tBuCO; E₂ = Me; E₃ = Tf) results in the O-functionalized imidazolium derivatives [4^E1-3]⁺X^-, which can be subsequently complexed under the form of the carbene 5^E1-3 to give [(5^E1-3)RhCl(COD)] (7^E1-3). Alternatively, the carbene ligand 5 can be post-functionalized from its complex [(5_H)RhCl(COD)] (7) by a simple sequence involving (i) base-induced deprotonation and (ii) addition of E-X, exemplified by the generation of [(5^E4-5)RhCl(COD)] (7^E4-5) (with E₄ = Ph₂PO; E₅ = tBuSiMe₂ (TBDMS)). In parallel, C-functionalization of the metal-bound ligand can be achieved from 7 by deprotonation with LiHMDS, leading to [(5^-)RhCl(COD)]Li(thf)_n⁺ followed by a classical aldolization/elimination sequence leading to [(5_CH2) RhCl(COD)] (8), in which the C5 carbon of the NHC bears a methylene group. Parallel direct transformations can be achieved after ligand complexation to copper, as exemplified by the generation of [(5_H)CuCl] (9) or [(5^E5)CuCl] (9^E5) (E₅ = TBDMS). The donor properties of all the above NHC ligands were evaluated from the standard complexes (NHC)RhCl(CO)₂ against the classical IR ν(CO) scale. The resulting order of nucleophilicities for the modulated carbenes is 5 _CH2 < 5_H ? 5^E4 < 5^E1 < 5^E2 < 5^E5 ? 5^-. The X-ray structures analyses of 7^E2 and 7^E4 are reported.

Full text of DOI:10.1021/om1003607

2616 Organometallics 2010, 29, 2616–2630
DOI: 10.1021/om1003607
Facile Derivatization of a “Chemo-active” NHC Incorporating an Enolate
Backbone and Relevant Tuning of Its Electronic Properties
ꢀ
€
Laure Benhamou, Nadia Vujkovic, Vincent Cesar,* Heinz Gornitzka, Noel Lugan, and
Guy Lavigne*
LCC (Laboratoire de Chimie de Coordination), CNRS, 205 Route de Narbonne, F-31077 Toulouse, France,
ꢀ
and Universite de Toulouse, UPS, INPT, 31077 Toulouse, France
Received April 28, 2010
The present report discloses a modular synthetic route to a new type of anionic N-heterocyclic
carbene ligand incorporating an enolate group as a reactive backbone component of its heterocyclic
framework. The presence of such a reactive unit facilitates further tailoring of the ligand, even after its
complexation to transition metals, with concomitant tuning of the electronic donor properties of the
carbene center. Simple acylation of the formamidine Ar-NH-CHdNAr (1a,b) (a: Ar = mesityl
(Mes); b: Ar = 2,6-diisopropylphenyl (DIPP)) by chloroacetyl chloride gives an acylated formami-
dine (2a,b), which undergoes a thermally induced cyclization to afford the 4-hydroxyimidazolium salt
3a,b. Single deprotonation by NEt3 gives the zwitterionic imidazolium-4-olate 4, whereas double
deprotonation by 2 equiv of LiHMDS gives the imidazolin-2-ylidene-4-olate ligand 5-, which reacts
with 0.5 equiv of [RhCl(1,5-COD)]2 to give, after acidification with HCl, the complex
[(5H)RhCl(COD)] (7), in which the “acidified” carbene ligand noted “5H” adopts the keto form.
By contrast, treatment of 4 with electrophilic reagents E-X (E1 = tBuCO; E2 = Me; E3 = Tf) results
in the O-functionalized imidazolium derivatives [4E1-3]þX-, which can be subsequently complexed
under the form of the carbene 5E1-3 to give [(5E1-3)RhCl(COD)] (7E1-3). Alternatively, the carbene
ligand 5 can be post-functionalized from its complex [(5H)RhCl(COD)] (7) by a simple sequence
involving (i) base-induced deprotonation and (ii) addition of E-X, exemplified by the generation of
[(5E4-5)RhCl(COD)] (7E4-5) (with E4 = Ph2PO; E5 = tBuSiMe2 (TBDMS)). In parallel, C-functio-
nalization of the metal-bound ligand can be achieved from 7 by deprotonation with LiHMDS,
leading to [(5-)RhCl(COD)]Li(thf)n followed by a classical aldolization/elimination sequence
þ
leading to [(5CH2)RhCl(COD)] (8), in which the C5 carbon of the NHC bears a methylene group.
Parallel direct transformations can be achieved after ligand complexation to copper, as exemplified
by the generation of [(5H)CuCl] (9) or [(5E5)CuCl] (9E5) (E5 = TBDMS). The donor properties of all
the above NHC ligands were evaluated from the standard complexes (NHC)RhCl(CO)2 against the
classical IR ν(CO) scale. The resulting order of nucleophilicities for the modulated carbenes is 5CH2
<
5H , 5E4 < 5E1 < 5E2 < 5E5 , 5- . The X-ray structures analyses of 7E2 and 7E4 are reported.
Introduction
in the synthetic approaches that have been recently devised for
such compounds,3 offering attractive possibilities to modify
distinctly their steric and electronic properties.
Whereas the steric properties, quantifiable in terms of
“percentage of buried volume”,4 are generally adjusted by
Synthetic versatility and structural modularity are desirable
criteria in the selection of ancillary ligands for transition metal
complex catalyzed reactions. Indeed, a ligand whose func-
tional “modules” can be easily interchanged will be ideally
suited for a rapid precatalyst screening, ultimately allowing a
fast optimization of the catalyst’s performance. In the specific
case of N-heterocyclic carbenes,1,2 there is a growing diversity
(3) For synthetic methods of NHCs, see: (a) Arduengo, A. J., III.
U.S. Patent 5 077 414, 1992. (b) Glorius, F.; Altenhoff, G.; Goddard, R.;
Lehmann, C. W. Chem. Commun. 2002, 2704. (c) Arduengo, A. J., III;
Krafczyk, R.; Schmultzer, R.; Craig, H. A.; Goerlich, J. R.; Marshall, W. J.;
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109 348, 2006. (e) Jafarpour, L.; Stevens, E. D.; Nolan, S. P. J. Organomet.
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Inorg. Chem. 2005, 347. (g) Hirano, K.; Urban, S.; Wang, C.; Glorius, F.
*Corresponding authors. E-mail: vincent.cesar@lcc-toulouse.fr; guy.
lavigne@lcc-toulouse.fr.
(1) For monographs, see: (a) N-Heterocyclic Carbenes in Transition
Metal Catalysis (Topics in Organometallic Chemistry 2007; Vol. 21);
Glorius, F., Ed.; Springer: Berlin, 2007; (b) N-Heterocyclic Carbenes in
Synthesis; Nolan, S. P., Ed.; Wiley-VCH: Weinheim, 2006.
(2) For general reviews on NHCs, see: (a) Schuster, O.; Yang, L.;
Raubenheimer, H. G.; Albrecht, M. Chem. Rev. 2009, 109, 3445. (b) Hahn,
F. E.; Jahnke, M. C. Angew. Chem., Int. Ed. 2008, 47, 3122. (c) Bourissou, D.;
Guerret, O.; Gabbaï, F.; Bertrand, G. Chem. Rev. 2000, 100, 39.
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Org. Lett. 2009, 11, 1019. (h) F€urstner, A.; Alcarazo, M.; Cesar, V.;
Lehmann, C. W. Chem. Commun. 2006, 2176.
(4) (a) For a general review, see: Diez-Gonzalez, S.; Nolan, S. P.
Coord. Chem. Rev. 2007, 251, 874. (b) For a leading reference on the
calculation of the buried volume of monodentate NHCs, see: Poater, A.;
Cosenza, B.; Correa, A.; Giudice, S.; Ragone, F.; Scarano, V.; Cavallo, L.
Eur. J. Inorg. Chem. 2009, 1759.
r
pubs.acs.org/Organometallics
Published on Web 05/19/2010
2010 American Chemical Society

Article
Organometallics, Vol. 29, No. 11, 2010 2617
playing with the ring size or the nature of the nitrogen
substituents,5 the electronic properties may be varied by
diverse means including (i) the grafting of various electron-
withdrawing or electron-releasing elements, either on the
nitrogen substituents6 or, even more efficiently, on the back-
bone atoms of the heterocycle (in some cases fused with
nitrogen substituents),7 (ii) the introduction of heteroele-
ments within the backbone,8 or (iii) the direct replacement of
a nitrogen atom of the diaminocarbene unit by a quaternary
or ylidic carbon.9
Chart 1. Literature Example of Post-functionalization of
a Coordinated Carbene Induced by Reversible
UV-Visible Excitation13
In practice, however, the structural modularity of these
systems is not always readily exploitable for a precatalyst
screening, since most ligand modifications have to be planned
in advance and introduced in the early stages of the synthesis,
even often requiring a total reconstruction of the heterocyclic
framework. So, there is still a need for the development of
N-heterocyclic carbenes incorporating a functional backbone
susceptible to allowing further chemical modifications allow-
ing a real-time tuning of the electronic properties.
In this context, the recent transposition of the concept of
“redox-active” ligands10 to the case of N-heterocyclic car-
benes, by Bielawski11 and Siemelink,12 has proven to be
particularly useful, with the spectacular example of a metal-
lacyclic N-heterocyclic carbene possessing an electro-active
1,10-ferrocenediyl backbone. Indeed, the electronic proper-
ties of the latter could be remotely switched by oxidation or
reduction of the iron center, albeit offering the possibility to
obtain only two stable states.
In the same vein, a remarkable example of ligand post-
functionalization can be found in the recent report, by Yam
and co-workers, of a photoswitchable NHC incorporating a
di(thiophenyl)ethene backbone, which was seen to undergo a
reversible skeletal reorganization via a C-C bond activation
triggered by UV-visible irradiation at specific wavelengths
(Chart 1).13
(5) For a comparison of the catalytic performances of a series of
electronically identical NHCs exhibiting different steric constraints, see:
€
(a) Wurtz, S.; Glorius, F. Acc. Chem. Res. 2008, 41, 1523. (b) Altenhoff,
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€
With these results in mind, we became interested in the
search for simple routes to chemo-active NHC ligands. Our
first investigation in this area led to the disclosure of a series
of anionic six-membered N-heterocyclic carbenes incorpor-
ating a malonate backbone (Chart 2).14 Later on, we demon-
strated that they are effectively chemo-active, in the sense
that they are sensitive to an external chemical modification
applicable even after their coordination to a transition metal.15
It was found that their electronic properties can be smoothly
modified by addition of a variety of electrophilic reagents
interacting directly with the oxygen of the backbone malonate
unit in the outer coordination sphere of the complex, and a
logical correlation could be established between the ligand’s
donor properties and the catalytic performances of its com-
plexes. In parallel, it was also noted that formal addition of the
electrophile to the central carbon of the malonate group also
results in an important decrease of the ligand’s donicity.16,17
Beyond this, we were also interested in a transposition of
this concept to the case of five-membered heterocycles. While
thinking of a representative chemically reactive function
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2006, 640. (h) Kascatan-Nebioglu, A.; Panzner, M. J.; Garrison, J. C.; Tessier,
C. A.; Youngs, W. J. Organometallics 2004, 23, 1928. (i) Sch€utz, J.;
Herrmann, W. A. J. Organomet. Chem. 2004, 689, 2995. (j) Herrmann,
W. A.; Sch€utz, J.; Frey, G. D.; Herdtweck, E. Organometallics 2006, 25,
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tallics 2009, 28, 270. (l) Ullah, F.; Kindermann, M. K.; Jones, P. G.; Heinicke,
J. Organometallics 2009, 28, 2441. (m) Nonnenmacher, M.; Kunz, D.;
Rominger, F.; Oeser, T. Chem. Commun. 2006, 1378. (n) Nonnenmacher,
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Mendosa-Espinosa, D.; Donnadieu, B.; Bertrand, G. J. Am. Chem. Soc.
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(11) (a) Tennyson, A. G.; Ono, R. J.; Hudnall, T. W.; Khramov,
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ꢀ

2618 Organometallics, Vol. 29, No. 11, 2010
Benhamou et al.
Chart 2. Post-functionalization of an Anionic N-Heterocyclic Carbene after Its Coordination to Rhodium15
Scheme 1. Synthesis of the 4-Hydroxyimidazolium Ligand
Scheme 2. In Situ Generation of the Imidazol-2-ylidene-4-olate
and Subsequent Trapping with a Transition Metal Complex
susceptible to serve as a backbone for the targeted hetero-
cycle, we came to the idea that an enolate group, possessing a
readily exploitable intrinsic chemical reactivity, could be an
attractive candidate, provided one could find a simple syn-
thetic strategy to incorporate it.
Results and Discussion
As previously disclosed in a preliminary communication
of the present work,18,19 the target 4-hydroxyimidazolium
cation 3 (Scheme 1) was easily prepared on a gram scale by an
original modular synthetic route involving a simple acylation
of the formamidine 1 by chloroacetyl chloride in air, giving
an acylformamidine (2), which, upon heating, was subse-
quently cyclized via an intramolecular nucleophilic substitu-
tion leading to the precipitation of the salt 3. The elusive
intermediate 4-oxo-1,3-diarylimidazolinium salt was not
detected, due to the existence of a tautomeric keto/enol
equilibrium being strongly favorable to the formation of
the 4-hydroxyimidazolium cation, representing the preferred
enol form.
equivalent of the classical rhodium dimer [RhCl(COD)]2 to
give, after acidification by HCl, the new complex [(5H)RhCl-
(COD)] (7), representing the principal starting compound of
the present investigation. The analogous NHC species incor-
porating DIPP as nitrogen substituent is also accessible, but
we will deliberately restrict our comments here to the case of
the mesityl derivative.
The method effectively proved to work with a good effi-
ciency. The cationic precursor 3, possessing two acidic func-
tions differentiable by their pKa values, namely, a phenol-
type OH group (pKa ∼9-10) and an acidic imidazolium
proton on the C2 center (pKa ∼22-24),20 could be engaged
in single or double deprotonation depending on the pKa of
the added base. Typically, deprotonation of the OH group
was made to occur by reaction with a weak base such as tri-
ethylamine (pKa = 10.75), generating the zwitterionic imi-
dazolium-4-olate 4 (Scheme 2). Besides, a double deproto-
nation was observed in the presence of a stronger base such
as LiHMDS, giving directly the anionic imidazol-2-yliden-
Whereas our preliminary communication18 focused mainly
on the synthetic procedure and showed that the ligand can be
post-functionalized either at the oxygen or at the carbon of
the enolate group, the present report gives a more detailed
analysis of such modifications and examines their respective
consequences in terms of electronic modulation of the car-
bene center.
Two main strategies are available for functionalizing the
reactive enolate backbone of the imidazol-2-yliden-4-olate
ligand disclosed in this work. We first briefly describe the
classical one, consisting in modifications introduced directly
onto the preligand, and then focus more deeply on those
implemented after the coordination of the NHC to transition
metal complexes, exemplified below by Rh and Cu species.
A. Functionalization of the Imidazolium-4-olate by Reac-
tion with Electrophiles and Subsequent Complexation with
Rhodium. Our first experiments (Scheme 3) indicated that the
4-olate 5- Liþ, which was identified as a carbene through
3
its reaction with sulfur to give the thiourea-type ligand 6
(not represented here).18 It could be trapped with half an
ꢀ
(18) Benhamou, L.; Cesar, V.; Gornitzka, H.; Lugan, N.; Lavigne, G.
Chem. Commun. 2009, 4720.
(19) See also the following related work developed by the Glorius’
€
group: Biju, A. T.; Hirano, K.; Frohlich, R.; Glorius, F. Chem. Asian J.
2009, 4, 1786.
(20) (a) Kim, Y.-J.; Streitwieser, A. J. Am. Chem. Soc. 2002, 124,
5757. (b) Alder, R. W.; Allen, P. R.; Williams, S. J. J. Chem. Soc., Chem.
Commun. 1995, 1267.

Article
Scheme 3. O-Functionalization of 4 by Electrophiles
Organometallics, Vol. 29, No. 11, 2010 2619
Scheme 4. Synthesis of the Rhodium(I) Complexes 7aE1-2 from
the Salts 4aE1-2
Figure 1. Perspective view of complex 7E2. Selected interatomic
˚
distances [A] and bond angles [deg]: Rh-C(1) 2.044(4), Rh-Cl-
(1) 2.3653(15), Rh-C(5) 2.097(5), Rh-C(6) 2.101(5), Rh-C(9)
2.186(4), Rh-C(10) 2.195(5), C(2)-C(3) 1.328(6), C(1)-Rh-
Cl(1) 88.03(10), C(1)-Rh-C(6) 94.03(15), C(1)-Rh-C(9)
92.44(15), N(1)-C(1)-N(2) 102.8(3).
Both complexes were isolated in analytically pure form
and were fully characterized by 1H and 13C NMR. The char-
acteristic signals of the carbenic carbon are observed at
δ(N2C)piv = 179.6 ppm (d, JRhC = 53.1 Hz) ppm for 7E1
zwitterionic preligand 4 readily reacts with electrophiles
(E1-3-X: E1-X = pivaloyl chloride; E2-X = methyl triflate;
E3-X = triflic anhydride) at 0 °C in ethyl acetate to give
precipitation of the corresponding O-functionalized imida-
zolium salts [4E]þX-, recovered in good yields as white
powders, which were fully characterized both spectroscopi-
cally and analytically.
and at δ(N2C)Me = 178.9 ppm (d, JRhC = 53.1 Hz) for 7E2
,
consistent with literature data for imidazol-2-ylidene com-
plexes of Rh(I).24
Yellow single crystals of 7E2 were obtained by slow eva-
poration of a dichloromethane/pentane (1/10) mixture and
were subjected to an X-ray structure analysis. The molecular
structure is depicted in Figure 1, also giving a selection of
Rhodium complexes incorporating an NHC derived from
the above precursors were then prepared by the method
originally proposed by Hermann,21 which is generally used in
the case of unstable carbenes.22 It consists in the generation
of a rhodium precursor incorporating a basic ligand suscep-
tible to deprotonating the imidazolium salt, in such a way
that the carbene, generated in situ in the vicinity of the metal
center, can be more efficiently trapped. Although the pub-
lished procedure generally uses potassium tert-butoxide,
such a base was not found suitable in the present case since
it might undergo nucleophilic attack at the previously intro-
duced functional group. So, we opted for a less nucleophilic
base such as potassium bis(trimethylsilyl)amide (KHMDS).
In a typical procedure, the amido Rh(I) complex [Rh(μ-
N(SiMe3)2)(COD)]2 was generated in situ by treatment of
[RhCl(COD)]2 with KHMDS (0.5 M solution in toluene) in
THF at room temperature (Scheme 4, eq 1).23 The desired
imidazolium salt [4E]þX- was then added as a solid at room
temperature, and the solution was stirred for 1 h (Scheme 4,
eq 2). The resulting Rh(I) complexes 7E1-2 were purified by
chromatography on neutral alumina in dichloromethane
and isolated in satisfactory yields (7E1: 57%; 7E2: 55%) as
yellow, air-stable solids.
˚
interatomic distances (A) and bond angles (deg). The com-
plex is tetracoordinated and adopts a classical square-planar
˚
geometry. The Rh-C1 bond length, 2.044(4) A, lies within
the normal range of those observed for comparable NHC
complexes of Rh(I).25 The mesityl cycles are roughly ortho-
gonal to the plane of the heterocycle (dihedral angles
C1-N1-C21-C22 = 89.4(5)° and C1-N2-C31-C32 =
82.5(5)°). It is noteworthy that the interatomic Rh-CCOD
bond distances corresponding to the olefinic carbon atoms
trans to the carbene center are slightly longer than those trans
to the chloride, which indicates that the trans influence of the
NHC is higher than that of the chloride, in agreement with
relevant data on related complexes.
B. Alternate Route to Rh Complexes from the Anionic
Imidazol-2-yliden-4-olate. The 4-hydroxyimidazolium cation
3 was deprotonated by two equivalents of LiHMDS, and the
incipient carbene 5- thus generated in situ was treated with
half an equivalent of [RhCl(COD)]2 in THF at 0 °C to
produce, after neutralization, the Rh(I) complex [(5H)RhCl-
(COD)] (7H), isolated in 90% yield after purification through
an alumina column using dichloromethane as eluent (Scheme 5).
Very characteristically, whereas the imidazolium precursor 3
exists exclusively in the enol form, the complexed carbene in
€
(21) Kocher, C.; Herrmann, W. A. J. Organomet. Chem. 1997, 532,
261.
(22) Peris, E. In N-Heterocyclic Carbenes in Transition Metal Cata-
lysis (Topics in Organometallic Chemistry, 2007; Vol. 21); Glorius, F.,
Ed.; Springer: Berlin, 2007; p 83.
(23) Alcarazo, M.; Roseblade, S. J.; Alonso, E.; Fernandez, R.;
Alvarez, E.; Lahoz, F. J.; Lassaletta, J. M. J. Am. Chem. Soc. 2004,
126, 13242.
(24) See for instance: (a) Tapu, D.; Dixon, A.; Roe, C. Chem. Rev.
2009, 109, 3385, and references therein. (b) Wolf, S.; Plenio, H.
J. Organomet. Chem. 2009, 694, 1487. (c) Xiao-Yan, Y.; Brian, O. P.; Brian,
R. J. Organometallics 2006, 25, 2359.
(25) Evans, P. A.; Baum, E. W.; Fazal, A. N.; Pink, M. Chem.
Commun. 2005, 63.

2620 Organometallics, Vol. 29, No. 11, 2010
Benhamou et al.
Scheme 5. Synthesis of Complex 7H
Scheme 6. Direct O-Functionalization of the Carbene from 7H
7H adopts the keto form. This is consistent with theoretical
studies indicating that the aromatic character of imidazol-2-
ylidenes is less pronounced than that of their imidazolium
precursors.26
1. O-Functionalization. In a series of typical experiments,
complex 7H was readily deprotonated upon reaction with
lithium bis(trimethylsilyl)amide (LiHMDS) in THF at -78 °C
for 1.5 h, giving the anionic intermediate species [(5-)RhCl-
(COD)]Li(thf)nþ (Scheme 6), which was subsequently treated
with electrophilic reagents E-X to give the O-functionalized
species [(5E)RhCl(COD)] (7E). Such a reaction was tested with
two new electrophiles, diphenylphosphinic chloride (Ph2P-
(dO)Cl) and chloro-tert-butyldimethylsilane (TBDMSCl),
leading to the complexes 7E4 and 7E5, respectively.
The formulation of 7H was established by classical spectro-
scopic NMR analyses and analytical data. The peak at 531.2
au (100%) appearing in the electrospray mass spectrum
corresponds to the cationic fragment [M - Cl]þ. In the 13
C
NMR spectrum, the ketonic carbon resonates at δ(CO) =
171.2 ppm. Besides, the characteristic signal of the carbene
arises as a doublet δ(N2C) = 229.7 ppm due to its coupling
with the metal (JRhC = 51.5 Hz). Such a value is significantly
higher than that observed in the reference complex [(SIMes)-
1
The success of the reaction was confirmed by H NMR
data revealing the disappearance of the AB type signal of the
methylene group on the C5 carbon and the concomitant
appearance of a singlet at δCHIm-5 = 6.22 ppm for 7E4 and at
δCHIm-5 = 6.18 ppm for 7E5, corresponding to the aromatic
proton at the C5 position. The presence of the carbene was
confirmed by 13C NMR, showing the carbene signals arising
respectively at δN2C = 179.4 ppm (d, JRhC = 52.7 Hz) for
RhCl(COD)], in which δ(N2C) = 212.0 ppm, and JRhC
=
48.1 Hz.24a Such a deshielding of the carbenic carbon signal
in 7H reflects the fact that the ketonic group exerts a with-
drawing effect on the electronic density of the heterocycle. It
is noteworthy that all spectroscopic analyses are consistent
with the keto form. Very characteristically, the rotation
around the Rh-NHC bond appears to be hindered by the
steric crowding of mesityl groups. So, the molecule adopts C1
symmetry, and the two signals of the methylene group at
position 5 of the heterocycle are becoming diastereotopic
and are strongly coupled (2JHH = 20.7 Hz). Single crystals of
the complex were obtained by slow diffusion of pentane into
a solution of 7H in CH2Cl2. The complex exists under the
form of a racemate, with the two enantiomers coexisting in
the centrosymmetric unit cell. Since the molecular structure
was already presented in our preliminary communication,18
it will not be discussed here.
7E4 and δN2C = 176.9 ppm (d, JRhC = 52.7 Hz) for 7E5
.
These values are indeed consistent with those reported in the
literature for Rh(I)-imidazol-2-ylidene complexes.25
Single crystals of 7E4 were obtained by slow diffusion of
pentane in a concentrated solution of the complex in dichloro-
methane and were subjected to an X-ray structure analysis.
The molecular structure of 7E4 is depicted in Figure 2 along
˚
with a selection of interatomic distances (A) and bond
angles (deg).
Compex 7E4 exhibits a slightly distorded square-planar
˚
geometry. The interatomic distances Rh-C1 (2.042(3) A)
˚
and Rh-Cl1 (2.3611(9) A) are similar to those found for the
C. Post-functionalization of the Carbene Starting from Its
Rh Complex 7H. In subsequent work, we were able to demon-
strate that complex 7H can be used as a starting material for
further modifications of the NHC, first by O-functionaliza-
tion and, second, by C-functionalization.
methylated derivative 7E2 described above. The C2-C3
distance 1.343(5) A corresponds to a double-bond length.
˚
The mesityl cycles are roughly orthogonal to the plane of the
heterocycle (C1-N1-C4-C9 = 101.5° and C1-N2-C4-
C9 = -73.9°). As previously noted for related complexes, we
still observe the characteristic trans influence of the carbene,
with Rh-CCOD bonds trans to the carbene (2.214(2) and
(26) (a) Boehme, C.; Frenking, G. J. Am. Chem. Soc. 1996, 118, 2039.
˚
2.174(2) A) being significantly longer than those trans to the
€
(b) Tafipolsky, M.; Scherer, W.; Ofele, K.; Artus, G.; Pedersen, B.; Herrmann,
˚
chloride (2.096(4) and 2.114(4) A). Such a phenomenon,
W. A.; McGrady, G. S. J. Am. Chem. Soc. 2002, 124, 5865.

Article
Organometallics, Vol. 29, No. 11, 2010 2621
Scheme 7. C-Functionalization of the Carbene from Complex 7H
IR spectrum, the stretching ν(CdO) vibration appears at
1752 cm-1
.
Given that both O- and C-functionalized complexes des-
cribed above are synthesized from the preformed complex
7H, we also tested the possibility of carrying out a direct one-
pot reaction starting from the 4-hydroxyimidazolium 3,
without attempting to isolate the intermediates, but such a
simplified synthetic protocol did not give satisfactory pro-
duct yields. By contrast, however, we found that the one-pot
synthetic strategy applies perfectly well to the case of copper,
as illustrated below.
D. Digression into the Case of Copper Complexes. One-
Pot Synthesis of Copper(I) Complexes. In a typical experi-
ment, the 4-hydroxyimidazolium cation 3 was doubly depro-
tonated by reaction with LiHMDS in THF at 0 °C. The
incipient anionic carbene thus generated was then reacted
in situ with 1 equiv of CuCl at ambient temperature to givþe
the intermediate anionic complex [(5-)CuCl]Li(thf)n
(Scheme 8), which was simply neutralized with HCl at 0 °C
to give, after addition of degassed water, the complex 8H in
satisfactory yield (68%). Such a complex is air sensitive and
should be kept under inert atmosphere.
Figure 2. Perspective view of complex 7E4. Selected interatomic
distances [A] and bond angles [deg]: Rh-C(1) 2.042(3), Rh-Cl(1)
˚
2.3611(9), Rh-C(35) 2.114(4), Rh-C(36) 2.096(4), Rh-C(39)
2.214(4), Rh-C(40) 2.174(4), C(2)-C(3) 1.343(5), C(2)-O(1)
1.364(4), O(1)-P(1) 1.613(2), P(1)-O(2) 1.470(2), C(35)-C(36)
1.401(8), C(39)-C(40) 1.344(6) C(1)-Rh-Cl(1) 89.50(9), Cl(1)-
Rh-C(39) 172.78(17), Cl(1)-Rh-C(35) 171.5(2), C(1)-Rh-
C(35) 96.97(16), C(1)-Rh-C(39) 172.78(15), O(1)-C(2)-C(3)
134.2(3), N(1)-C(1)-N(2) 103.1(3).
even more pronounced in the present case, is also correlated
with differences in the double-bond lengths within the COD
ligand. Typically, the length of the double bond that is trans
In practice, the addition of distilled water after the acid-
ification step (Scheme 8, right equation) is necessary to
obtain the final product in a clean form. Indeed, in the
absence of water, the product 8H appears to be contaminated
by an additional silylated derivative 8SiMe3 resulting from the
competing reaction of the intermediate enolate copper com-
plex [(5-)CuCl]Li(thf)nþ with the silylated base HMDS gene-
rated through the deprotonation of the carbene by KHMDS
(Scheme 9). The existence of a mixture of both 8H and 8SiMe3
in a ratio of 0.6/1 (evaluated by integration of NMR signals)
is confirmed by the 1H NMR spectrum exhibiting both
the δCH2 signal at 4.29 ppm, belonging to 8H, and the
δCHIm-5 signal at 6.31 ppm, corresponding to the silylated
˚
to the carbene, 1.344(6) A, is significantly shorter than the
˚
length (1.401(8) A) of the double bond trans to the halogen.
2. C-Functionalization. In parallel to the above experi-
ments, we also performed the C-functionalization of the
complex by a classical aldolization/crotonization sequence.
As shown in Scþheme 7, the anionic complex [(5-)RhCl-
(COD)]-Li(thf)n was generated in situ by deprotonation
of 7H and reacted with paraformaldehyde in THF at 0 °C for
1.5 h, producing the new complex [(5CH2)RhCl(COD)]
(7CH2), in which the carbon C5 is now functionalized by a
methylene group.
The complex was purified by flash chromatography on
neutral alumina (type III), using dichloromethane as eluent.
The product, which was obtained as a red-orange solid
isolated in good yield (73%), was fully characterized by
spectroscopic and analytical methods.
derivative 8SiMe3
.
Attempts to prevent this side reaction by using lithium
diisopropylamide as a base effectively led to compound 8H,
albeit contaminated by impurities, which we were unable to
eliminate. So, the treatment with water, as depicted in
Scheme 8, remains the best procedure to obtain 8H in
analytically pure form.
1
Examination of the H NMR spectrum of 7CH2 revealed
that the olefinic protons arise as two doublets at δ(CH2) =
5.68 ppm and δ(CH2) = 4.87 ppm coupled with a JHH
=
Just like in the rhodium complex 7H described above, the
functionalized backbone of the NHC ligand in 8H (Scheme 8)
exhibits the preferred keto form, reflecting a lack of aroma-
ticity of the heterocycle. This can be inferred from the
occurrence of the IR ν(CO) band at 1750 cm-1 and from
the presence of the characteristic 13C NMR signal δ(CO) =
172.0 ppm. The resonance of the carbene δ(N2C) = 213.5
ppm is considerably deshielded as compared with that of the
corresponding carbene center in [(SIMes)CuCl], δ(N2C) =
202.8 ppm.24a Such a difference is ascribed to the fact that the
ketonic function releases part of the electron density from the
1.8 Hz characteristic of the two geminated protons of an
olefin. The proton spectrum also shows that rotation
around the Rh-C bond is hindered on the NMR time scale.
Indeed, the signals corresponding to the methyl groups of
the mesityl substituents appear as six singlets, each inte-
grating for three protons. Similarly, the dCHCOD signals,
affected by the same phenomenon, arise as four multiplets.
In the 13C NMR, the carbene arises as a doublet, δ(N2C) =
227.0 ppm (d, JCRh = 52.6 Hz). The ketonic carbon arises
at δ(CO) = 161.0 ppm (d, JCRh = 1.6 Hz), whereas, in the

2622 Organometallics, Vol. 29, No. 11, 2010
Benhamou et al.
Scheme 8. Straightforward One-Pot Generation of Copper Complexes 8H and 8E5 from 4-Hydroxyimidazolium Salt 3
Scheme 9. Parasite Side Reaction of the in Situ Generated Anionic Intermediate [(5-)CuCl] with HMDS
Scheme 10. Generation of Carbonyl Derivatives [(5)RhCl(CO)2]
heterocycle, thereby reducing the overall nucleophilicity of
the carbene. Such a phenomenon was in fact corroborated by
complementary investigations reported below. Further evi-
dence for the existence of the ligand in the keto form here
comes from the observation of the CH2 signal at δ(CH2) =
4.29 ppm arising as a singlet, since the complex now exhibits
Cs symmetry.
Quite logically, these observations led us to think of
another silylated electrophile, tert-butyldimethylsilyl chlo-
ride (TBDMSCl) (Scheme 8, left equation), the addition of
which was found to proceed cleanly at room temperature to
give 8E5 in high yield (82%). The formation of the latter
was inferred from the existence of a 13C NMR signal δN2C
at 173.1 ppm. Let us note that the corresponding chemical
shift for the closely related IMes compound [(IMes)CuCl]
was detected at δ(N2C) = 178.7 ppm (CDCl3).24a The obser-
ved difference clearly indicates that the presence of the
silylated electrophilic group enriches the electron density
on the heterocycle, thereby rendering the carbene more
nucleophilic. Finally, the existence of a proton signal at
[(5E)RhCl(CO)2] (9E), and [(5CH2)RhCl(CO)2] (9CH2), upon
bubbling CO for a few minutes in dichloromethane solution
(Scheme 10). Apart from the complex 9CH2, which was found
to spontaneously decarbonylate in the absence of CO,27 all
such compounds are stable square-planar dicarbonyl Rh(I)
derivatives. The presence of two carbonyl ligands and a
carbene is inferred from 13C NMR data showing three doub-
lets coupled with the Rh center, between 170 and 225 ppm
(Table 1). Clearly, the chemical shifts of the carbon of car-
bonyl groups are not influenced by the nature of the carbene.
For the unsaturated carbenes 5E, the chemical shifts of the
carbenic carbon (171 ppm < δ(N2C) < 173 ppm) appear to
be insensitive to the nature of the functional group and are
within the range of those reported in the literature for com-
parable imidazol-2-ylidene ligands (δ(N2C) = 177.7 ppm in
[(IMes)RhCl(CO)2]).24a By contrast, the signals of the car-
benic carbon center for 9H (δ = 220.7 ppm) and 9CH2 (δ =
218.0 ppm) are significantly higher than that published for
the reference compound [(SIMes)RhCl(CO)2], for which
δ(N2C) = 205.7 ppm.24a Such a deshielding can be explained
by the fact that the ketone function acts as an electron-
withdrawing group relative to the heterocycle. Such an effect
was already observed for the cyclooctadiene derivatives 7H
1
δCHIm-5 = 6.27 ppm (CDCl3) in the H NMR spectrum
corroborates the fact that the heterocycle is unsaturated and
O-functionalized.
E. Examination of the Effects of Various Types of Func-
tionalization on the Donor Properties of the Carbene. Simple
transition metal carbonyl complexes incorporating an
N-heterocyclic carbene, such as Ni(NHC)(CO)3 or Rh(NHC)-
(CO)2Cl, are currently used as standard derivatives for eval-
uating the donicity of a given NHC ligand relative to the
IMes ligand, taken as a reference. Indeed, due to the retro-
donation of electrons from the metal to the antibonding π*
of the carbon, IR ν(CO) stretching vibrations of carbonyl
ligands constitute a very sensitive probe to estimate the
ligand donicity.
and 7CH2
.
In the present work, the cyclooctadiene complexes 7H, 7E,
and 7CH2 were respectively converted into the corresponding
carbonyl chloro Rh(I) derivatives [(5H)RhCl(CO)2] (9H),
(27) Related decarbonylation processes have already been observed
in Rh(I)-carbonyl complexes incorporating poor electron donors; see:
€
(a) Bittermann, A.; Herdtweck, E.; Harter, P.; Herrmann, W. A. Orga-
nometallics 2009, 28, 6963. (b) See also ref 11e.

Article
Organometallics, Vol. 29, No. 11, 2010 2623
Table 1. 13C NMR Data for the Series of Carbonylated Species 9
compound
δN2C (ppm)
δCO
δCO
9E1
9E2
9E4
9E5
9H
173.2 (JRhC = 44.3 Hz)
172.8 (JRhC = 44.8 Hz)
173.4 (JRhC = 45.6 Hz)
171.1 (JRhC = 45.1 Hz)
220.7 (JRhC = 44.2 Hz)
218.0 (JRhC = 45.0 Hz)
182.7 (JRhC = 74.5 Hz)
182.7 (JRhC = 74.2 Hz)
182.6 (JRhC = 74.6 Hz)
182.9 (JRhC = 74.6 Hz)
182.3 (JRhC = 73.9 Hz)
183.9 (JRhC = 58.0 Hz)
184.9 (JRhC = 54.7 Hz)
184.9 (JRhC = 54.1 Hz)
184.1 (JRhC = 54.0 Hz)
185.1 (d, JRhC = 54.1 Hz)
184.3 (JRhC = 53.6 Hz)
9CH2
Scheme 11. Spontaneous Decarbonylation of 9CH2 and
Concomitant Condensation into the Dimeric Species 10
Scheme 12. Preparation of the Complex
[(5-)RhCl(CO)2][Li(thf)n]+, [9-][Li(thf)n]+
Table 2. Recorded IR ν(CO) Stretching Vibrations for the Whole
Series of Chloro-Carbonyl NHC Rh(I) Complexes Resulting from
the Modulation of the Anionic Imidazol-2-yliden-4-olate
The cis conformation of the two carbonyls was verified in
all cases by IR spectroscopy, systematically giving two intense
ν(CO) bands. As noted in Scheme 10, the complex 9CH2 could
not be isolated in pure form, due to its observed transforma-
tion via spontaneous loss of CO, but was investigated in
solution by IR and NMR. Its IR spectrum in the ν(CO) shows
two bands of equal intensityat2090and 2007cm-1, consistent
with the presence of two mutually cis carbonyl ligands CO.
In the 13C NMR spectrum of complex 9CH2 (Table 1), the
signals of the corresponding carbons are fortuitously over-
lapped, giving a doublet δ(CO) at 183.9 ppm. The presence of
the carbene is corroborated by the occurrence of a character-
entry
NHC ligand
CH2 in 9CH2
5H in 9H
ν(CO) (cm-1 a
)
ν
av(CO) (cm-1
)
1
2
3
4
5
6
7
8
9
5
2090, 2007
2090, 2005
2084, 1997
2080, 1997
2080, 1997
2081, 1996
2079, 1995
2077, 1994
2071, 1988
2048.5
2047.5
2040.5
2038.5
2038.5
2038.5
2037
SIMesb
5
5
E4 in 9E4
E1 in 9E1
IMesb
5
5
5
E2 in 9E2
E5 in 9E5
- in 9-
2035.5
2029.5
a IR spectra recorded in CH2Cl2. b Values obtained from ref 23b.
istic signal δ(N2C) at 218.0 ppm arising as a doublet (JRhC =
45.0 Hz). The complex was found to slowly decarbonylate,
even at low temperature (Scheme 11).
anionic imidazol-2-yliden-4-olate, we still had to prepare the
corresponding chloro-carbonyl Rh(I) derivative, which was
simply done by deprotonation of 9H, giving the anion 9-, the
IR spectrum of which was recorded in dichloromethane.
The IR ν(CO) frequencies of the complexes recorded in
this work are reported in Table 2 and in Figure 3, the latter
also showing relevant literature data on comparable NHCs,
recorded under the same conditions.
Although the resulting product was not fully characteri-
zed, it could be reasonably formulated as the dimeric species
10. Indeed, when complex 9CH2 was heated at 80 °C under
reduced pressure, monitoring by IR spectroscopy indicated
the progressive disappearance of the two ν(CO) stretching
bands and their replacement by a unique absorption band
arising at 1988 cm-1. Such a transformation could also be
followed by 1H NMR spectroscopy. After 24 h, the product
10 was isolated in pure form after purification through a
chromatographic column (SiO2), using dichloromethane
as eluent. Its 13C NMR spectrum reveals the occurrence of
the carbenic carbon center arising as a doublet δ(N2C) at
213.5 ppm (JRhC = 54 Hz). The same spectrum shows a
doublet ascribed to the unique CO, arising as a doublet at
δ(CO) = 183.1 ppm (JRhC = 88 Hz), whereas the singlet
appearing at δ = 161.4 ppm corresponds to the ketonic
carbon center. Complex 10 still exhibits the characteristic
signals of the functionalized carbene 5CH2. In order to corro-
borate the formation of the above dimer, we carried out a
DOSY NMR experiment aimed at determining the respec-
tive hydrodynamic radius of both 9CH2 and 10. The results
revealed that the volume occupied by the molecular unit
of 10 is 1.4 times larger than that of 9CH2, which is roughly
consistent with the expected value based on the above
formulation.
The results summarized in Figure 3 deserve the following
comments:
(1) Our prototype of anionic carbene, the imidazol-2-
ylidene-4-olate 5-, appearing on the extreme right side of
the IR ν(CO) scale, is thus the more nucleophilic carbene of
the whole series of five-membered-ring NHCs shown here,
including those directly comparable independently reported
by other authors.7k,o,22,23b,28
(2) The donor properties of such a ligand are effectively
modulated in a simple way and after its complexation to a
transition metal, by post-functionalization of its reactive
enolate backbone, either at the oxygen or at the carbon.
Such a modulation, quantified against the IR ν(CO) scale
based on the standard chloro-carbonyl Rh(I) complex
[(NHC)RhCl(CO)2], covers 19 cm-1, one of the largest ones
reported so far for modulable carbenes.
(3) Interestingly, Figure 3 reveals a distribution of the
modified ligands over three different zones. The middle zone
includes the O-functionalized carbenes, namely, those result-
ing from the selective addition of electrophiles to the oxygen.
F. Evaluation of the Donor Properties of the Aforemen-
tioned NHCs. In order to evaluate the donor properties of the

2624 Organometallics, Vol. 29, No. 11, 2010
Benhamou et al.
Figure 3. Classification of the carbenes according to their donor properties, determined from the IR ν(CO) frequencies of their
respective chloro-carbonyl rhodium complexes of the type [(NHC)RhCl(CO)2].7k,o,22,23b,28
Here, the modulation is indeed correlated with the electron-
withdrawing properties of the electrophilic group, as repre-
sented in the following chart:
but the preferred tautomer, namely, the keto form. It is
noteworthy that in the latter cases we no longer have a
diaminocarbene, but a mixed amino-amido-carbene, and
we now know from earlier studies that diamidocarbenes
are among the weakest electron donor carbenes reported
so far.16,17
Conclusion
As part of a current research program directed to the
design of N-heterocyclic carbenes as functional molecules,
we have presented here a simple and modulable synthetic
strategy for the construction of a new type of NHC incor-
porating an enolate group in the remote part of its hetero-
cyclic framework. Due to its intrinsic chemical reactivity,
such a function confers interesting properties to the N-het-
erocyclic carbene, which is then prone to be derivatized in a
simple way by elementary chemical reactions. Whereas the
anionic enolate-based NHC proved to be better electron
donor than the closest conventional NHC, IMes, its electro-
nic properties can be modulated in a very simple way and
over a relatively broad range, either by simple addition of
various electrophiles interacting selectively with the oxygen
of the enolate (O-functionalization) or by exploiting the
However, such a domain is relatively narrow as compared
with the one we previously obtained with the six-membered-
ring NHCs incorporating a malonate backbone.15 The extreme
left zone corresponds to the less nucleophilic carbenes. One of
them results from C-functionalization, whereas the second one
represents the case where the addition of Hþ as the incoming
electrophile does not give an enol-functionalized-type carbene,

Article
Organometallics, Vol. 29, No. 11, 2010 2625
reactivity of the enolate-type carbon atom (C-functiona-
lization). A beneficial interest of such transformations is that
they can be achieved after the ligand’s coordination to
transition metals, as illustrated here by the cases of rhodium
and copper complexes. Since the ligands presented here
exhibit the same steric hindrance as IMes, currently used in
many catalytic systems, they represent a potentially useful
alternative to such a widely used prototype, with the addi-
tional advantage of offering readily modulable electronic
properties.
conformer 1), 1647 (m, νCdN conformer 2), 1608, 1482, 1439,
1406, 1360, 1315, 1279, 1235, 1190, 1143, 1088, 1032, 849, 826,
790, 743, 719 cm-1; MS (ESI) m/z (%): 735.3 (45) [2 M þ Na]þ,
379.2 (100) [M þ Na]þ, 357.2 (34) [M þ H]þ. Anal. Calcd (%) for
C21H25ClN2O (356.89): C 70.67, H 7.06, N 7.85. Found: C 69.48,
H 7.05, N 7.57.
1-(1-Oxo-2-chloroeth-1-yl)-1,3-(2,6-diisopropylphenyl)formamidine
(2b). This reaction was carried out without any special precaution to
exclude air and moisture. Solvent and reagents were used as received.
1,3-(2,6-Diisopropylphenyl)formamidine (1.0 g, 2.76 mmol) was
suspended in ethyl acetate (20 mL). Triethylamine (0.58 mL, 4.1
mmol) and chloroacetyl chloride (0.27 mL, 3.3 mmol) were added at
€
0 °C. After reacting 1 h, the mixture was filtered through a Buchner
Experimental Section
funnel and the filtrate was evaporated. The sticky residue was purified
by flash chromatography (SiO2, hexane/CH2Cl2, 1/2) to afford a
white solid (1.0 g, 82%): mp 116 °C; 1H NMR (300 MHz, CDCl3) δ
1.15 (d, 12H, JHH = 6.9 Hz, CH3 iPr), 1.26 (d, 6H, JHH = 6.8 Hz,
CH3 iPr), 1.29 (d, 6H, JHH = 6.9 Hz, CH3 iPr), 2.95 (m, 4H, CHiPr),
3.97 (br s, 2H, CH2), 7.09 (m, 3H, CHDIPP), 7.35 (m, 2H, CHDIPP),
7.49 (m, 1H, CHDIPP), 8.80 (br s, 1H, N2CH); 13C{1H} NMR
(75.5 MHz, CDCl3) δ 24.0 (CH3 iPr), 24.6 (CH3 iPr), 27.5 (C iPr),
28.6 (CHiPr), 42.6 (CH2), 123.0 (CHDIPP), 124.3 (CHDIPP), 124.9
(CHDIPP), 130.5 (CHDIPP), 130.9 (CDIPP), 138.8 (CDIPP), 145.3
(CDIPP), 146.3 (CDIPP), 148.3 (N2CH), 168.3 (CdO); IR (ATR) ν~
2963, 2929, 2868, 1712 (m, νCO), 1647 (m, νCdN), 1590, 1572, 1465,
1456, 1441, 108, 1383, 1361, 1313, 1283, 1221, 1180, 1169, 1124, 1098,
General Considerations. All manipulations were carried out
under an inert atmosphere of dry nitrogen by using standard
vacuum line and Schlenk tube techniques. Glassware was dried
at 120 °C in an oven for at least three hours. THF and diethyl
ether were freshly distilled from sodium/benzophenone, and
toluene was distilled from molten sodium, prior to use. Pentane,
ethyl acetate, and dichloromethane were dried over CaH2 and
subsequently distilled. DMF was dried over CaH2 and distilled
under reduced pressure. NMR spectra were recorded on Bruker
ARX250, AV300 AV400, and AV500 spectrometers, in the
solvents indicated; chemical shifts (δ) are reported in ppm
compared to TMS using the residual peak of deuterated solvent
as internal standard; coupling constants (J) are in Hz. Infrared
spectra were recorded on a Perkin-Elmer Spectrum 100 FT-IR
spectrometer. Microanalyses were performed by the Labora-
toire de Chimie de Coordination Microanalytical Service and
MS spectra by the mass spectrometry service of Paul Sabatier
University. Melting points were obtained with a Stuart Scientific
SMP1 melting point apparatus and were not corrected.
1081, 1058, 1042, 934, 923, 849, 796, 764, 756, 711, 694 cm-1
;
MS (ESI) m/z (%) 463.4 (10) [M þ Na]þ, 441.5 (8) [M]þ, 427.5
(26) [M þ Na - Cl]þ, 405.5 (16) [M - Cl]þ, 365.5 (100) [M þ H -
(CO - CH2 - Cl)]þ. Anal. Calcd (%) for C27H37ClN2O (441.04): C
73.53, H 8.46, N 6.35. Found: C 73.46, H 8.09, N 6.18.
4-Hydroxy-1,3-dimesitylimidazolium Chloride (3a). 1-(1-Oxo-
2-chloroethyl)-1,3-dimesitylformamidine (2a) (1.078 g, 3.0
mmol) was solubilized in DMF (7.5 mL, 0.4 mol L-1), and
Starting Materials and Reagents. N,N0-Dimesitylformami-
3
dine, N,N0-di(2,6-diisopropylphenyl)formamidine,29 and [RhCl-
(1,5-COD)]2 were synthesized according to literature proce-
the mixture was heated at 100 °C for 1 h. During this period, the
product started to precipitate. After cooling at room tempera-
ture, Et2O (40 mL) was added and the supernatant solution was
evacuated through a filter canula. The white to off-white solid
residue was washed with Et2O (2 ꢀ 10 mL) and dried in vacuo
to yield the imidazolium salt as a white to off-white powder
(810 mg, 75%): mp 277 °C (dec); 1H NMR (300 MHz, CDCl3) δ
2.13 (s, 12H, CH3 ortho), 2.35 (s, 6H, CH3 para), 7.02 (s, 2H,
CHMes), 7.04 (s, 2H, CHMes), 7.22 (d, JHH = 2.1 Hz, 1H, CHIm-5),
7.54 (d, 1H, JHH = 2.1 Hz, N2CH); 13C{1H} NMR (75.5 MHz,
CDCl3) δ 17.1 (CH3 ortho), 17.6 (CH3 ortho), 21.1 (CH3 para), 21.1
(CH3 para), 102.6 (CHIm-5), 124.8 (N2CH), 127.3 (Cipso), 129.7
(CHMes), 129.8 (CHMes), 131.1 (Cipso), 134.0 (Cortho), 135.0
(Cortho), 141.4 (Cpara), 148.3 (C-OH); IR (ATR) ν~ 2970, 2298
(br m, νOH), 1595 (vs, νCdN), 1543, 1483, 1463, 1375, 1283, 1244,
1223, 1065, 1039, 861, 832, 776, 726, 683, 673 cm-1; MS (ESI) m/z
(%) 343.2 (93) [M - Cl þ Na]þ, 321.3 (100) [M - Cl]þ. Anal.
Calcd (%) for C21H25ClN2O (356.17): C 70.67, H 7.06, N 7.85.
Found: C 70.89, H 7.05, N 7.79.
30
dures. Lithium bis(trimethylsilyl)amide (LiHMDS) was prepared
just before use and was obtained by reacting nBuLi (1.6 M in
hexane) with an excess of distilled hexamethyldisilazane (1.1
equiv) in pentane and subsequent evaporation of all volatiles in
vacuo. It was then dissolved in THF to obtain a solution of
approximately 1 M.
Synthetic Procedures. 1-(1-Oxo-2-chloroeth-1-yl)-1,3-dime-
sitylformamidine (2a). This reaction was carried out without any
special precaution to exclude air and moisture. Solvent and
reagents were used as received. 1,3-Dimesitylformamidine (3.0 g,
10.7 mmol) was suspended in ethyl acetate (75 mL), and the round-
bottomed flask was placed in an ice bath. Triethylamine (2.3 mL,
16.1 mmol) was added, and chloroacetyl chloride (1.0 mL, 12.8
mmol) was dropwise added at 0 °C. After reacting 40 min (TLC
€
control), the mixture was filtered through a Buchner funnel to
remove triethylammonium chloride. The solvent was evaporated
and the sticky residue was purified by flash chromatography (SiO2,
hexane/EtOAc, 2/1) to afford a white solid (3.6 g, 95%): mp
123-124 °C; 1H NMR (250 MHz, CDCl3) δ 2.03 (s, 6H,
CH3 ortho), 2.22 (s, 3H, CH3 para), 2.27 (s, 6H, CH3 ortho), 2.33 (s,
3H, CH3 para), 3.82 (br s, 2H, CH2), 6.79 (s, 2H, CHMes), 7.04 (s,
2H, CHMes), 8.77 (br s, 1H, N2CH); 13C{1H} NMR (62.9 MHz,
CDCl3) δ 17.9 (CH3 ortho), 18.6 (CH3 ortho), 20.7 (CH3 para), 21.2
(CH3 para), 42.5 (CH2), 127.6 (CMes), 128.7 (CHMes), 130.1
(CHMes), 131.3 (CMes), 132.9 (CMes), 135.7 (CMes), 139.8 (CMes),
145.3 (CMes), 147.9 (N2CH); IR (ATR) ν~ 2942, 2914, 2854, 1706
(m, νCO conformer 1), 1697 (m, νCO conformer 2), 1660 (m, νCdN
4-Hydroxy-1,3-(2,6-diisopropylphenyl)imidazolium chloride
(3b). 1-(1-Oxo-2-chloroethyl)-1,3-(2,6-diisopropylphenyl)form-
amidine (2b) (294 mg, 0.7 mmol) was solubilized in toluene and
heated at 100 °C for 22 h. The product precipitated along the
reactioncourse. After cooling atroomtemperature, Et2O (10mL)
was added and the supernatant liquid was evacuated through a
canula. The white solid residue was washed with Et2O (2 ꢀ 5 mL)
and dried in vacuo to yield the imidazolium salt as a white powder
(200 mg, 69%): mp 250 °C; 1H NMR (300 MHz, CDCl3) δ 1.18
(d, 6H, JHH = 6.9 Hz, CH3), 1.18 (d, 6H, JHH = 6.9 Hz, CH3),
1.32 (d, 6H, JHH = 6.7 Hz, CH3), 1.32 (d, 6H, JHH = 6.7 Hz,
CH3), 2.59(m,4H, CH iPr), 7.32-7.37 (m, 5H, CHDIPP þ CHIm-5),
7.53-7.59(m, 3H, CHDIPP þ CHIm-2);13C{1H} NMR (75.5 MHz,
CDCl3) δ 23.6 (CH3), 23.8 (CH3), 24.4 (CH3), 25.2 (CH3), 28.8
(CHiPr), 29.2 (CHiPr), 103.3 (CHIm-5), 124.5 (CHm-DIPP), 124.6
(CHm-DIPP), 125.9 (CHIm-2), 130.5 (Cq), 131.9 (Cq), 132.0 (Cq),
145.3 (CHp-DIPP), 146.0 (CHp-DIPP); IR (ATR) ν~ 2962, 2869, 2324
(28) The known NHC ligands in the figure were also taken from: (a)
€
Bittermann, A.; Harter, P.; Herdtweck, E.; Hoffmann, S. D.; Herrmann,
W. A. J. Organomet. Chem. 2008, 693, 2079. (b) Neveling, A.; Julius, G. R.;
Cronje, S.; Esterhuisen, C.; Raubenheimer, H. G. Dalton Trans. 2005, 181.
(29) Krahulic, K. E.; Enright, G. D.; Parvez, M.; Roesler, R. J. Am.
Chem. Soc. 2005, 127, 4142.
(30) Giordiano, G.; Crabtree, R. H. Inorg. Synth. 1990, 28, 88.

2626 Organometallics, Vol. 29, No. 11, 2010
Benhamou et al.
(br m, ν OH), 1620 (s, ν CdN), 1589, 1541, 1496, 1461, 1384, 1364,
1331, 1285, 1271, 1258, 1210, 1181, 1106, 1062, 940, 876, 806, 755,
734, 709, 697, 672 cm-1; MS (ESI) m/z (%) 427.5 (100) [M - Cl]þ,
405.5 (79) [M - Cl þ Na]þ. Anal. Calcd (%) for C27H37ClN2O
(441.04): C 73.53, H 8.46, N 6.35. Found: C 72.73, H 8.30, N 6.28.
1,3-Dimesitylimidazolium-4-olate (4). Triethylamine (185 μL,
1.32 mmol) was added at room temperature to a solution of
4-hydroxy-1,3-dimesitylimidazolium chloride (3a) (429 mg, 1.20
mmol) in CH2Cl2 (25 mL). After 15 min, volatiles were removed
to dryness in vacuo. EtOAc was added (30 mL), and the white
suspension was filtered through a pad of Celite. EtOAc was
then evaporated, and the residue was washed with pentane (2 ꢀ
10 mL) to give a pale yellow powder (355 mg, 92%): mp 104 °C;
1H NMR (300 MHz, CDCl3) δ 2.21 (s, 6H, CH3 ortho), 2.22
(s, 6H, CH3 ortho), 2.32 (s, 3H, CH3 para), 2.35 (s, 3H, CH3 para),
(MeO-C); IR (ATR) ν~ 1625, 1549, 1482, 1445, 1406, 1276, 1256,
1223, 1149, 1031, 990, 853, 752, 727, 667 cm-1; MS (ESI) m/z
(%) 335 (100) [M]þ. Anal. Calcd (%) for C23H27F3N2O4S
(484.2): C 57.01, H 5.62, N 5.78. Found: C 56.44, H 5.82, N 5.54.
4-(Trifluoromethanesulfonyloxy)-1,3-dimesitylimidazolium Tri-
fluoromethanesulfonate (4E3). Triethylamine (23 μL, 0.15 mmol,
1.1 equiv) was added to a suspension of 3a (50 mg, 0.14 mmol), in
CH2Cl2 (2 mL). The solution immediately became pale yellow.
After 5 min of stirring, volatiles were removed to dryness. The
residue was dissolved in EtOAc, filtered through a pad of Celite,
and concentrated until the volume was about half of the initial
volume. The solution was then cooled in an ice bath, and triflic
anhydride (26 μL, 0.15 mmol, 1.1 equiv) was slowly added. The
mixture was then allowed to warm at room temperature. The
product precipitated during the reaction course. After 1 h vola-
tiles were removed in vacuo, and the residue was washed with
Et2O (2 ꢀ 5 mL) to afford the imidazolium salt as a white powder
(55 mg, 65%);: mp 77 °C; 1H NMR (300 MHz, CDCl3) δ 2.16 (s,
6H, CH3 ortho), 2.19 (s, 6H, CH3 ortho), 2.38 (s, 3H, CH3 para), 2.40
(s, 3H, CH3 para), 7.09 (s, 2H, CHMes), 7.12 (s, 2H, CHMes), 7.44
(d, 1H, JHH = 1.2 Hz, CHIm-5), 9.90 (d, 1H, JHH = 1.2 Hz,
N2CH); 13C{1H} NMR (75.5 MHz, CDCl3) δ 17.2 (CH3 ortho),
17.4 (CH3 ortho), 21.2 (CH3 para), 21.3 (CH3 para), 112.6 (CHIm-5),
130.2 (CHMes), 130.4 (CHMes), 133.8 (Cq), 134.8 (Cq), 138.3
(N2CH), 142.4 (Cq), 142.8 (Cq); IR (ATR) ν~ 1605, 1541, 1444,
1432, 1294, 1238, 1217, 1155, 1126, 1028, 1006, 845, 811, 735, 724,
707, 665 cm-1; MS (ESI) m/z (%) 453 (100) [M - TfO]þ; 320
(43) [M - TfO - TfO]þ. Anal. Calcd (%) for C23H24F6N2O6S2
(602.6): C 45.84, H 4.01, N 4.65. Found: C 45.01, H 3.79, N 4.48.
Chloro(η4-cycloocta-1,5-diene)(1,3-dimesitylimidazolin-4-on-2-
ylidene)rhodium(I) (7H). A solution of LiHMDS (2.36 mL, 1 M
in THF, 2.36 mmol, 2.1 equiv) was added to a solution of 3a
(400 mg, 1.12 mmol) in THF (15 mL) at 0 °C, and the mixture was
stirred for 30 min. To this solution was added [RhCl(1,5-COD)]2
(277 mg, 0.56 mmol, 0.5 equiv), and the resulting orange mixture was
stirred 2 h at room temperature before addition of HCl (1.24 mL,
1 Min Et2O, 1.24 mmol, 1.1 equiv) at 0 °C. The solution was stirred
5 min and then was allowed to warm to room temperature. After
20 min, THF was removed in vacuo, the yellow foam obtained was
purified by flash chromatography (SiO2, CH2Cl2/MeOH, 98/2) to
afford a yellow powder (575 mg, 90%): mp 194 °C (dec); 1H NMR
(250 MHz, CD2Cl2) δ 1.59-1.88 (m, 8H, CH2 COD), 2.26 (s, 3H,
CH3 Mes), 2.27 (s, 3H, CH3 Mes), 2.43 (s, 3H, CH3 Mes), 2.44 (s, 3H,
CH3 Mes), 2.47 (s, 3H, CH3 Mes), 2.60 (s, 3H, CH3 Mes), 3.42 (m, 1H,
CHCOD), 3.58 (m, 1H, CHCOD), 4.20 (d, 1H, JHH = 20.7 Hz,
CH2), 4.30 (d, 1H, JHH = 20.7 Hz, CH2), 4.57 (m, 1H, CHCOD),
4.70 (m, 1H, CHCOD), 7.08 (br s, 1H, CHMes), 7.12 (m, 3H,
CHMes); 13C{1H} NMR (62.9 MHz, CD2Cl2) δ 18.0 (CH3 Mes),
18.5 (CH3 Mes), 19.5 (CH3 Mes), 19.7 (CH3 Mes), 20.8 (CH3 Mes),
20.9 (CH3 Mes), 27.2 (CH2 COD), 28.6 (CH2 COD), 31.6 (CH2 COD),
33.4 (CH2 COD), 54.8 (CH2), 67.9 (d, JRhC = 13.9 Hz, CHCOD),
69.9 (d, JRhC = 14.1 Hz, CHCOD), 100.6 (d, JRhC = 6.4 Hz,
CHCOD), 101.6 (d, JRhC = 6.6 Hz, CHCOD), 128.5 (CHMes), 128.8
(CHMes), 129.6 (CHMes), 130.1 (CHMes), 130.9 (CMes), 131.2
(CMes), 134.5 (CMes), 135.1 (CMes), 135.3 (CMes), 137.3 (CMes),
138.3 (CMes), 139.4 (CMes), 171.2 (CdO), 229.7 (d, JRhC = 51.5
Hz, N2C); IR (ATR) ν~ 2953, 2915, 2868, 2824, 1748, 1737, 1606,
1480, 1456, 1400, 1376, 1359, 1333, 1300, 1271, 1250, 1167, 1147,
1074, 1033, 951, 846, 782, 734, 692 cm-1; MS (ESI) m/z (%) 572
(16) [M - Cl þ CH3CN]þ, 531.2 (100) [M - Cl]þ. Anal. Calcd (%)
for C29H36ClN2ORh (566.96): C 61.43, H 6.40, N 4.94. Found: C
61.19, H 6.67, N 4.85.
5.87 (d, 1H, JHH = 2.0 Hz, CHIm-5), 7.00 (br s, 5H, CHMes
þ
N2CH); 13C{1H} NMR (75.4 MHz, CDCl3) δ 16.8 (CH3 ortho),
17.9 (CH3 ortho), 21.1 (CH3 para), 95.0 (CHIm-5), 118.6 (N2CH),
129.3 (CHMes), 129.9 (Cipso), 133.1 (Cipso), 134.7 (Cortho), 135.7
(Cortho), 139.3 (Cpara), 139.9 (Cpara), 156.2 (C-O-); IR (ATR) ν~
2962, 2919, 2864, 1742, 1726, 1676, 1636, 1609, 1547, 1484, 1439,
1400, 1378, 1253, 1223, 1167, 1140, 1034, 1014, 850, 785, 713 cm-1
.
4-(Pivaloyloxy)-1,3-dimesitylimidazolium Chloride (4E1). Tri-
ethylamine (30 μL, 0.22 mmol, 1.1 equiv) was added to a
suspension of 3a (70 mg, 0.2 mmol) in CH2Cl2 (2 mL). The
solution immediately became pale yellow. After 5 min of stir-
ring, volatiles were removed to dryness. The residue was dis-
solved in EtOAc, filtered through a pad of Celite, and concen-
trated until the volume was about half of the initial volume. The
solution was then cooled in an ice bath, and pivaloyl chloride
(27 μL, 0.22 mmol, 1.1 equiv) was slowly added. The mixture
was then allowed to warm at room temperature. The product
precipitated during the reaction course. After 1 h, volatiles were
removed in vacuo and the residue was washed with Et2O (2 ꢀ
5 mL) to afford the imidazolium salt as a white powder (75 mg,
1
87%): mp 245 °C (dec); H NMR (300 MHz, CDCl3) δ 1.11
(s, 9H, CH3 tBu), 2.22 (s, 6H, CH3 ortho), 2.28 (s, 6H, CH3 ortho),
2.36 (s, 6H, CH3 para), 7.04 (s, 4H, CHMes), 7.23 (d, 1H, JHH
=
1.5 Hz, CHIm-5), 11.73 (d, 1H, JHH = 1.5 Hz, N2CH); 13C{1H}
NMR (62.90 MHz, CDCl3) δ 17.7 (CH3 tBu), 21.2 (CH3 para),
26.4 (CH3 ortho), 110.1 (CHIm-5), 129.8 (CHMes), 129.1 (CHMes),
134.2 (Cq) 134.9 (Cq), 137.1 (N2CH), 141.6 (Cq), CdO and CtBu
are not observed on this spectrum; IR (ATR) ν~ 2976, 2918,
2867, 2754, 1785 (νCO), 1621, 1610, 1534, 1479, 1460, 1245, 1065,
1023, 855, 771, 670 cm-1; MS (ESI) m/z (%) 663 (21) [2(M -
Cl - tBuCO) - 2H þ Na]þ, 405 (30) [M - Cl]þ, 343 (100) [M -
Cl - tBuCO þ Na]þ, 321 (48) [M - Cl - tBuCO]þ. Anal. Calcd
(%) for C26H33ClN2O2 (441.0): C 70.81, H 7.54, N 6.35. Found:
C 69.28, H 7.73, N 6.17.
4-(Methoxy)-1,3-dimesitylimidazolium Trifluoromethanesul-
fonate (4E2). Triethylamine (23 μL, 0.15 mmol, 1.1 equiv) was
added to a suspension of 3a (50 mg, 0.14 mmol) in CH2Cl2
(2 mL). The solution immediately became pale yellow. After
5 min of stirring, volatiles were removed to dryness. The residue
was dissolved in EtOAc, filtered through a pad of Celite, and
concentrated until the volume was about half of the initial
volume. The solution was then cooled in an ice bath, and methyl
triflate (18 μL, 0.15 mmol, 1.1 equiv) was slowly added. The
mixture was then allowed to warm at room temperature. The
product precipitated during the reaction course. After 1 h,
volatiles were removed in vacuo, and the residue was washed
with Et2O (2 ꢀ 5 mL) to afford the imidazolium salt as a white
powder (52 mg, 76%): mp 89 °C; 1H NMR (300 MHz, CDCl3) δ
2.15 (s, 6H, CH3 ortho), 2.21 (s, 6H, CH3 ortho), 2.38 (s, 3H,
CH3 para), 2.39 (s, 3H, CH3 para), 4.10 (s, 3H, O-CH3), 7.06
Chloro(η4-cycloocta-1,5-diene)(4-pivaloyloxy-1,3-dimesitylimi-
dazol-2-ylidene)rhodium(I) (7E1). A solution of KHMDS (335 μL,
0.5 M in toluene, 0.17 mmol, 2.1 equiv) was added to a solution of
[RhCl(COD)]2 (39 mg, 0.08 mmol) in THF (2 mL) at room
temperature. The mixture was stirred 15 min and became dark
orange. To this solution was added 4-(pivaloyloxy)-1,3-dimesity-
limidazolium chloride (4E1) (70 mg, 0.16 mmol), and the resulting
orange mixture was stirred 10 h at room temperature. Then THF
(s, 2H, CHMes), 7.07 (s, 3H, CHMes, CHIm-5), 8.74 (d, 1H, JHH
=
2.1 Hz, N2CH); 13C{1H} NMR (75.5 MHz, CDCl3) δ 17.2
(CH3 ortho), 17.4 (CH3 ortho), 21.1 (CH3 para), 21.2 (CH3 para),
60.6 (O-CH3), 101.1 (CHIm-5), 129.8 (CHMes), 131.7 (N2CH),
134.1 (Cortho), 134.9 (Cortho), 141.5 (Cpara), 141.7 (Cpara), 148.6

Article
Organometallics, Vol. 29, No. 11, 2010 2627
was removed in vacuo, and the orange residue obtained was
purified by flash chromatography (neutral Al2O3 type III, CH2Cl2)
to afford a bright yellow powder (59 mg, 57%): mp 105 °C (dec);
1H NMR (300 MHz, CDCl3) δ 1.00 (s, 9H, CH3 tBu), 1.55 (m, 4H,
CH2 COD), 1.85 (m, 4H, CH2 COD), 2.11 (s, 3H, CH3 ortho), 2.13
(s, 3H, CH3 ortho), 2.35 (br s, 6H, CH3 ortho), 2.38 (s, 3H, CH3 para),
2.46 (s, 3H, CH3 para), 3.31 (br s, 2H, dCHCOD), 4.52 (br s, 2H,
dCHCOD), 6.89 (s, 1H, CHIm-5), 7.02 (m, 4H, CHMes); 13C{1H}
NMR (75.5 MHz, CDCl3) δ 26.5 (CH3 tBu), 28.2 (CH2 COD), 28.6
(CH2 COD), 32.5 (CH2 COD), 32.9 (CH2 COD), 18.1 (CH3 ortho), 18.2
(CH3 ortho), 19.6 (CH3 para), 19.7 (CH3 para), 21.1 (CH3 ortho), 21.2
(CH3 ortho), 39.0 (CtBu), 67.6 (d, JRhC = 14.1 Hz, dCHCOD), 96.4
(d, JRhC = 7.9 Hz, dCHCOD), 110.1 (CHIm-5), 127.9 (CHMes),
128.1 (CHMes), 129.6 (CHMes), 129.7 (CHMes), 131.9 (Cq), 134.5
(Cq), 135.2 (Cq), 136.5 (Cq), 137.6 (Cq), 138.4 (Cq), 138.7 (Cq),
138.9 (Cq), 139.0 (Cq), 173.1 (CdO), 179.6 (d, JRhC = 53.1 Hz,
N2C); IR (ATR) ν~ 2915, 2973, 2827, 1774, 1760, 1626, 1609, 1478,
1461, 1381, 1315, 1267, 1098, 1074, 1028, 851, 816, 729 cm-1; MS
(EI) m/z (%) 615 (100) [M - Cl]þ, 531 (12) [M - Cl - tBuCO]þ;
HR-MS (ESI) m/z calcd for C34H44N2O2Rh 615.2458, found
615.2460.
95.9 (d, JRhC = 6.7 Hz, CHCOD), 96.0 (d, JRhC = 7.4 Hz, CHCOD),
107.9 (d, JCP = 3.0 Hz, CHIm-5), 128.3 (br s, CHMes), 128.7 (d,
JCP = 13.4 Hz, CHPh), 129.2 (CHMes), 129.4 (CHMes), 131.4 (d,
JCP = 10.7 Hz, CHPh), 131.7 (d, JCP = 11.2 Hz, CHPh), 133.1 (d,
JCP = 4.1 Hz, CHPh), 133.2 (d, JCP = 3.4 Hz, CHPh), 136.5 (Cq),
138.6 (Cq), 139.2 (Cq), 179.4 (d, JRhC = 52.7 Hz, N2C); 31P{1H}
NMR (161.9 MHz, CD2Cl2) δ 33.4; IR (ATR) ν~ 2917, 2873, 2827,
1631, 1609, 1591, 1482, 1438, 1379, 1311, 1282, 1243, 1205, 1155,
1129, 1111, 1071, 1036, 1018, 997, 973, 853, 873, 851, 749, 731, 693
cm-1; MS (ESI) m/z (%) 772 (14) [M - Cl þ CH3CN]þ, 731 (100)
[M - Cl]þ, 531 (10) [M - Cl - Ph2PO þ H ]þ; HR-MS (ESI) m/z
calcd for C41H45N2ORh 731.2274, found 731.2267.
Chloro(η4-cycloocta-1,5-diene)(1,3-dimesitylimidazol-4-(dimethyl-
tert-butylsilyloxy)-2-ylidene)rhodium(I) (7E5).AsolutionofLiHMDS
(0.19 mL, 1 M in THF, 0.2 mmol, 1.1 equiv) was added to a solution
of 7H (100 mg, 0.18 mmol) in THF (5 mL) at -78 °C, and the mixture
was stirred for 1.5 h. To this solution was added chlorodimethyl-tert-
butylsilane (28 mg, 0.19 mmol), and the solution was allowed to
warm to room temperature and to stir for 1.5 h. THF was removed in
vacuo, and the yellow foam obtained was purified by flash chromato-
graphy (SiO2, hexane/EtOAc, 4/1) to afford a yellow powder (68 mg,
67%): mp 94 °C (dec); 1H NMR (400 MHz, CDCl3) δ 0.04 (s, 3H,
Si-CH3), 0.09 (s, 3H, Si-CH3), 0.69 (s, 9H, CH3 tBu), 1.52 (br, 4H,
CH2 COD), 1.85 (br, 4H, CH2 COD),2.07(s,3H,CH3 Mes),2.18(s, 3H,
CH3 Mes),2.36(m,6H,CH3 Mes),2.41(m,6H,CH3 Mes), 3.28 (br, 2H,
CHCOD), 4.49 (br, 2H, CHCOD), 6.18 (s, 1H, CHIm-5), 6.99 (m, 4H,
CHMes); 13C{1H} NMR (100.6 MHz, CDCl3) δ -5.7 (Si-CH3), 5.9
(Si-CH3), 17.2 (CtBu), 17.7 (CH3 Mes),17.9(CH3 Mes), 19.4 (CH3 Mes),
19.7 (CH3 mes), 20.8 (CH3 Mes), 24.5 (CH3 tBu), 27.9 (CH2 COD), 28.1
(CH2 COD), 32.4 (CH2 COD), 67.1 (d, JRhC = 14.5 Hz, CHCOD), 67.6
(d,JRhC =14.5Hz,CHCOD), 95.3 (d, JRhC =7.5Hz,CHCOD), 102.4
(CHIm-5), 127.3 (CHMes), 127.7 (CHMes), 128.9 (CHMes), 129.2
(CHMes), 132.2 (Cq), 134.9 (Cq), 135.1 (Cq), 136.4 (Cq), 137.6 (Cq),
Chloro(η4-cycloocta-1,5-diene)(4-methoxy-1,3-dimesitylimidazol-
2-ylidene)rhodium(I) (7E2). A solution of KHMDS (389 μL, 0.5 M
in toluene, 0.20 mmol, 2.4 equiv) was added to a solution of
[RhCl(COD)]2 (40 mg, 0.08 mmol) in THF (2 mL) at room
temperature. The mixture was stirred 15 min and became dark
orange. To this solution was added 4-(methoxy)-1,3-dimesitylimi-
dazolium chloride (4E2) (78 mg, 0.16 mmol), and the resulting
orange mixture was stirred 1 h at room temperature. Then THF was
removed in vacuo, and the yellow residue obtained was purified by
flash chromatography (neutral Al2O3, CH2Cl2) to afford a bright
1
yellow powder (51 mg, 55%): mp 229 °C (dec); H NMR (300
MHz, CDCl3) δ 1.55 (m, 4H, CH2 COD), 1.82 (m, 4H, CH2 COD),
2.22 (s, 6H, CH3 ortho), 2.40 (m, 3H, CH3 ortho, 6H, CH3 para) 2.47
(s, 3H, CH3 ortho), 3.31 (m, 2H, dCHCOD), 3.75 (s, 3H, O-CH3),
4.50 (m, 2H, dCHCOD), 6.25 (s, 1H, CHIm-5), 7.03 (m, 4H, CHMes);
13C{1H} NMR (75.5 MHz, CDCl3) δ 18.1 (CH3 ortho), 18.2
(CH3 ortho), 19.8 (s, CH3 ortho), 21.2 (CH3 para), 21.3 (CH3 para),
28.2 (CH2 COD), 28.6 (CH2 COD), 32.4 (CH2 COD), 33.0 (CH2 COD),
137.7 (Cq), 137.9 (Cq), 138.0 (Cq), 143.6 (Cq), 176.9 (d, JRhC
=
52.7 Hz, N2C); IR (ATR) ν~ 2927, 2859, 2827, 1637, 1610, 1472, 1431,
1376, 1304, 1286, 1254, 1210, 1166, 1076, 1034, 993, 972, 953, 939,
878, 844, 785, 750, 733, 714, 695, 663 cm-1; MS (ESI) m/z (%) 645
(100) [M - Cl]þ, 531 (40) [M - Cl - TBDMS þ H]þ. Anal. Calcd
(%) forC35H50ClN2ORhSi: C 61.71, H 7.40, N 4.11. Found: C 61.42,
H 7.79, N 3.94.
58.7 (O-CH3), 67.4 (d, JRhC = 14.5 Hz, dCHCOD), 68.0 (d, JRhC
=
Chloro(η4-cycloocta-1,5-diene)(1,3-dimesityl-5-(methylenyl)-
imidazolin-4-on-2-ylidene)rhodium(I) (7CH2).A solution of LiHMDS
(0.29 mL, 1 M in THF, 0.29 mmol, 1.1 equiv) was added to a
solution of 7H (150 mg, 0.26 mmol) in THF (10 mL) at -78 °C,
and the mixture was stirred for 1.5 h. To this solution was added
paraformaldehyde (20 mg, 0.66 mmol), and the mixture was
allowed to warm to room temperature. The resulting orange
mixture was stirred for 1.5 h at room temperature. THF was
removed in vacuo, and the orange foam obtained was purified by
flash chromatography (neutral Al2O3 type III, CH2Cl2) to afford
14.7 Hz, dCHCOD), 95.7 (d, JRhC = 7.5 Hz, dCHCOD), 96.2
(d, JRhC = 7.9 Hz, dCHCOD), 99.4 CHIm-5), 127.9 (CHMes), 128.0
(CHMes), 129.6 (CHMes), 129.7 (CHMes), 132.3 (Cq), 134.4 (Cq),
136.7 (Cq), 138.5 (Cq), 138.8 (Cq), 150.2 (Cq), 178.9 (d, JRhC
=
53.1 Hz, N2C); IR (ATR) ν~ 2912, 2872, 2826, 1637, 1609, 1481,
1453, 1428, 1389, 1311, 1285, 1216, 1046; 998, 956, 866, 848, 767, 752,
727, 710, 691, 654 cm-1; MS (EI) m/z (%) 615 (100) [M - Cl]þ, 335
(14) [Imidazolium]þ. Anal. Calcd (%) for C30H38ClN2ORh þ
0.25CH2Cl2: C 60.33, H 6.44, N 4.65. Found: C 60.04, H 6.88, N 4.45.
Chloro(η4-cycloocta-1,5-diene)(1,3-dimesityl-4-(diphenylphos-
phinato)imidazol-2-ylidene)rhodium(I) (7E4).A solution of LiHMDS
(0.20 mL, 1 M in THF, 0.20 mmol, 1.1 equiv) was added to a
solution of 7H (100 mg, 0.18 mmol) in THF (5 mL) at -78 °C, and
the mixture was stirred for 1.5 h. To this solution was added
diphenylphosphinic chloride (41 μL, 0.21 mmol). The resulting
orange mixture was allowed to warm to room temperature and
was stirred for 1.5 h. THF was removed in vacuo. The orange foam
obtained was dissolved in toluene and filtered through a pad of
Celite. After evaporation, the residue was washed with CH2Cl2/
pentane (1.5 mL/20 mL) and pentane (10 mL) to afford a yellow
1
an orange powder (112 mg, 73%): mp 183 °C (dec); H NMR
(300 MHz, CDCl3) δ 1.58-1.88 (m, 8H, CH2 COD), 2.13 (s, 3H,
CH3 Mes), 2.16 (s, 3H, CH3 Mes), 2.38 (s, 3H, CH3 Mes), 2.39 (s, 3H,
CH3 Mes), 2.47 (s, 3H, CH3 Mes), 2.49 (s, 3H, CH3 Mes), 3.44 (m,
1H, CHCOD), 3.52 (m, 1H, CHCOD), 4.74 (m, 1H, CHCOD), 4.82
(m, 1H, CHCOD), 4.86 (d, 1H, JHH = 1.9 Hz, dCH2), 5.66 (d, 1H,
JHH = 1.9 Hz, dCH2), 7.04 (s, 2H, CHMes), 7.08 (s, 1H, CHMes),
7.11 (s, 1H, CHMes); 13C{1H} NMR (100.6 MHz, CDCl3) δ 17.7
(CH3 Mes), 18.3 (CH3 Mes), 19.3 (CH3 Mes), 19.6 (CH3 Mes), 20.7
(CH3 Mes), 20.8 (CH3 Mes), 27.2 (CH2 COD), 28.0 (CH2 COD), 31.7
(CH2 COD), 32.7 (CH2 COD), 68.6 (d, JRhC = 13.9 Hz, CHCOD),
69.4 (d, JRhC = 14.2 Hz, CHCOD), 102.7 (d, JRhC = 6.1 Hz,
CHCOD), 103.1 (dCH2), 103.3 (d, JRhC = 6.1 Hz, CHCOD), 128.0
(CHMes), 128.3 (CHMes), 129.7 (CHMes), 130.1 (CHMes), 130.8
1
powder (110 mg, 82%): mp 132 °C (dec); H NMR (300 MHz,
CD2Cl2) δ 1.58 (m, 4H, CH2 COD), 1.88 (m, 4H, CH2 COD), 2.10 (m,
6H, CH3 Mes),2.22(brs,6H,CH3 Mes),2.40(s,3H,CH3 Mes),2.49(s,
3H, CH3 Mes), 3.28 (m, 2H, CHCOD), 4.45 (m, 2H, CHCOD), 6.71 (d,
1H, JHP = 1.0 Hz, CHIm-5), 7.03 (s, 2H, CHMes), 7.11 (br s, 2H,
CHMes), 7.54-7.56 (m, 10H, CHPh); 13C{1H} NMR (75.5 MHz,
CD2Cl2) δ 17.9 (CH3 Mes), 18.2 (CH3 Mes), 19.3 (CH3 Mes), 19.7
(CH3 Mes), 20.8 (CH3 Mes), 20.9 (CH3 Mes), 28.2 (CH2 COD),
28.3 (CH2 COD), 32.4 (CH2 COD), 32.6 (CH2 COD), 68.4 (d,
(Cipso), 131.9 (Cipso), 134.6 (Cortho), 134.7 (Cortho), 135.4 (d, JRhC
=
1.0 Hz, dCIm-5), 137.9 (Cortho), 139.0 (Cpara), 139.3 (Cpara), 161.0
(d, JRhC = 1.6 Hz, CdO), 227.0 (d, JRhC = 52.6 Hz, N2C); IR
(ATR) ν~ 2917, 2872, 2826, 1752 (s, νCO), 1648, 1608, 1478, 1427,
1362, 1306, 1278, 1210, 1188, 1137, 1087, 1031, 991, 959, 902, 840,
793, 762, 745, 694 cm-1; MS (ESI) m/z (%) 543(100) [M - Cl]þ,
J
RhC = 14.7 Hz, CHCOD), 68.6 (d, JRhC = 13.9 Hz, CHCOD),

2628 Organometallics, Vol. 29, No. 11, 2010
Benhamou et al.
584 (30) [M - Cl þ CH3CN]þ; HR-MS (ESI) m/z calcd for
C30H36N2ORh 543.1883, found 543.1870.
18.5 (CH3 Mes), 18.1 (CH3 Mes), 18.9 (CH3 Mes), 21.2 (CH3 Mes), 21.2
(CH3 Mes), 54.3 (CH2), 129.5 (CHMes), 129.6 (Cq), 129.7 (CHMes),
134.0 (CMes), 135.0 (CMes), 135.9 (CMes), 140.0 (CMes), 140.2 (CMes),
171.0 (d, JRhC = 1.7 Hz, CdO), 182.2 (d, JRhC = 73.8 Hz, CO),
184.3 (d, JRhC = 54.5 Hz, CO), 220.7 (d, JRhC = 43.8 Hz, N2C);
IR(CH2Cl2) ν~ 2090 (νCO), 2005 (νCO), 1769 (νCdO) cm-1;MS(ESI)
m/z (%) 492 (100) [M - CO - Cl þ CH3CN]þ, 464 (10) [M -
2CO - Cl þ CH3CN]þ, 451 (37) [M - CO - Cl]þ, 423 (12) [M -
2CO - Cl]þ. Anal. Calcd (%) for C23H24ClN2O3Rh þ 0.45C5H12:
C 55.42, H 5.41, N 5.12. Found: C 55.61, H 5.03, N 5.06.
Chlorodicarbonyl(1,3-dimesityl-4-(tert-butylcarbonyloxy)imidazol-
2-ylidene)rhodium(I) (9E1). The general procedure was followed
using complex 7E1 (23 mg, 0.04 mmol), CO(g), and CH2Cl2 (2mL).
The initial orange solution became pale yellow after 15 min. After
washing with pentane (5 mL), the complex was isolated as a
yellow powder (20 mg, 94%): mp 184 °C; 1H NMR (400 MHz,
CDCl3) δ 1.06 (s, 9H, CH3 tBu), 2.23 (s, 6H, CH3 ortho), 2.28 (s, 6H,
CH3 ortho), 2.38 (s, 3H, CH3 para), 2.39 (s, 3H, CH3 para), 7.01 (s,
2H, CHMes), 7.03 (s, 2H, CHMes), 7.06 (s, 1H, CHIm-5); 13C{1H}
NMR (100.6 MHz, CDCl3) δ 18.0 (CH3 ortho), 21.0 (CH3 para),
26.5 (CH3 tBu), 39.1 (CtBu), 110.4 (CHIm-5), 29.2 (CHMes), 129.3
(CHMes), 130.8 (Cq), 135.4 (Cortho), 135.5 (Cq), 136.0 (Cortho),
139.4 (Cpara), 139.6(Cpara), 173.2 (d, JRhC = 44.3 Hz, N2C), 182.7
(d, JRhC = 74.5 Hz, CO), 184.85 (d, JRhC = 54.7 Hz, CO);
IR (CH2Cl2) ν~ 2080 (νCO), 1997 (νCO) cm-1; MS (ESI) m/z
(%) 621 (100) [M þ Na]þ, 567 (68) [M - Cl - CO þ MeOH]þ,
563 (74) [M - Cl ]þ, 360 (35). Anal. Calcd (%) for C28H32-
ClN2O4Rh (598.9): C 56.15, H 5.39, N 4.68. Found: C 56.70, H
5.54, N 4.40.
Chloro(1,3-dimesitylimidazolin-4-on-2-ylidene)copper(I) (8H).
A solution of LiHMDS (1.18 mL, 1 M in THF, 1.18 mmol, 2.1
equiv) was added to a solution of 3a (200 mg, 0.56 mmol) in THF
(10 mL) at 0 °C, and the mixture was stirred for 30 min. To this
solution was added CuCl (56 mg, 0.56 mmol), and the resulting
mixture was stirred 45 min at room temperature before addition
of HCl (618 μL, 1 M in Et2O, 0.62 mmol, 1.1 equiv) at 0 °C. The
solution was stirred 20 min and then was allowed to warm to
room temperature before volatiles were removed in vacuo. Then,
the residue was dissolved in toluene, filtered through a pad of
Celite, and washed with pentane (2 ꢀ 10 mL) to afford a white
powder. This residue was dissolved in CH2Cl2, distilled water
(10 μL, 1 equiv) was added, and the mixture was stirred for
10 min. Then the solution was dried on Na2SO4. After filtration
through a canula, volatiles were removed in vacuo, and the
residue was washed with pentane to afford an off-white powder
(159 mg, 68%): mp 115 °C; 1H NMR (300 MHz, CDCl3) δ 2.18
(s, 6H, CH3 ortho), 2.31 (s, 6H, CH3 ortho), 2.35 (s, 6H, CH3 para),
4.30 (s, 2H, CH2), 7.04 (s, 4H, CHMes); 13C{1H} NMR (75.5 MHz,
CDCl3) δ 17.7 (CH3 ortho), 18.3 (CH3 ortho), 21.1 (CH3 para), 21.2
(CH3 para), 52.9 (CH2), 129.9 (CHMes), 130.2 (CHMes), 134.1 (Cq),
1344 (Cortho), 135.0 (Cortho), 140.2 (Cpara), 140.5 (Cpara), 172.0
(CdO), 213.5 (N2C); IR (ATR) ν~ 2948, 2916, 2859, 1750, 1608,
1547, 1485, 1444, 1404, 1368, 1315, 1299, 1287, 1248, 1202, 1170,
1154, 1034, 970, 957, 936, 920, 851, 778, 735, 720 cm-1; MS (ESI)
m/z (%) 424 (30) [M - Cl þ CH3CN]þ, 321 (100) [IMes - OH]þ.
Anal. Calcd (%) for C21H24ClCuN2O (419.43): C 60.14, H 5.77, N
6.68. Found: C 60.96, H 5.91, N 6.56.
Chloro(4-tert-butyldimethylsilyloxy-1,3-dimesitylimidazol-2-
ylidene)copper(I) (8E5). A solution of LiHMDS (0.63 mL, 1 M
in THF, 0.63 mmol, 2.1 equiv) was added to a solution of 3a
(106 mg, 0.26 mmol) in THF (10 mL) at 0 °C, and the mixture
was stirred for 25 min. To this solution was added CuCl (29.5 mg,
0.26 mmol), and the resulting mixture was stirred 1 h at room
temperature before addition of tert-butyldimethylsilyl chloride
(53 mg, 0.31 mmol, 1.2 equiv) at this temperature. The solution
was stirred 1 h before volatiles were removed in vacuo. The
product was then dissolved in CH2Cl2 and filtered through Celite.
Solvents were evaporated, and the residue was washed with
pentane (2 ꢀ 5 mL) to obtain a white solid (130 mg, 82%): mp
80 °C; 1H NMR (300 MHz, CDCl3) δ 0.11 (s, 6H, Si-CH3), 0.73
(s, 9H, CH3 tBu), 2.11 (s, 6H, CH3 ortho), 2.14 (s, 6H, CH3 ortho),
2.32 (s, 3H, CH3 para), 2.33 (s, 3H, CH3 para), 6.27 (s, 1H, CHIm-5),
6.95 (s, 2H, CHMes),6.97(s,2H,CHMes);13C{1H} NMR (62.9 MHz,
CDCl3) δ -5.4 (Si-CH3), 17.6 (CH3 ortho), 17.7 (CtBu), 18.0
(CH3 ortho), 21.0 (CH3 para), 21.1 (CH3 para), 24.8 (CH3 tBu), 101.2
(CHIm-5), 129.2 (CHMes), 129.4 (CHMes), 131.5 (Cipso), 134.1
(Cortho), 135.1 (Cortho), 135.9 (Cipso), 139.1 (Cpara), 139.2 (Cpara),
143.3 (C-O), 173.1 (N2C); IR (ATR) ν~ 2952, 2928, 2857, 1767,1623,
1487, 1472, 1440, 1376, 1303, 1251, 1211, 1170, 1157, 1132, 1034,
1011, 954, 938, 920, 876, 847, 788 732, 695, 665 cm-1; MS (ESI) m/z
(%) 931 (100) [Cu(5E5)2]þ, 817 (6) [Cu(5E5)(5H)]þ, 435 (13) [4E5]þ,
384 (4) [Cu(5H) þ H]þ. Anal. Calcd (%) for C27H38ClCuN2OSi þ
0.1CH2Cl2: C 60.03, H 7.10, N 5.17. Found: C 59.71, H 7.60, N 5.50.
General Procedure for the Preparation of [(NHC)RhCl(CO)2]
(9). CO gas was bubbled into a solution of 7 in CH2Cl2 (or
CD2Cl2 for 7CH2) at room temperature for 15 min. Then CH2Cl2
was removed in vacuo, and the yellow residue was washed with
pentane to remove 1,5-cyclooctadiene and dried to give the
product.
Chlorodicarbonyl(1,3-dimesityl-4-(methoxy)imidazol-2-ylidene)-
rhodium(I) (9E2). The general procedure was followed using com-
plex 7E2 (70 mg, 0.12 mmol) in CH2Cl2 (4 mL). The initial orange
solution became pale yellow after 15 min. After washing with
pentane (2 ꢀ 5 mL), the complex was isolated as a yellow powder
1
(50 mg, 79%): mp 180 °C (dec); H NMR (400 MHz, CDCl3)
δ 2.23 (s, 6H, CH3 ortho), 2.28 (s, 6H, CH3 ortho), 2.38 (s, 3H,
CH3 para), 2.39 (s, 3H, CH3 para), 3.83 (s, 3H, O-CH3), 6.41 (s, 1H,
CHIm-5), 7.01 (s, 2H, CHMes), 7.03 (s, 2H, CHMes); 13C{1H} NMR
(100.6 MHz, CDCl3) δ 18.4 (CH3 ortho), 18.5 (CH3 ortho), 21.2
(CH3 para), 21.3 (CH3 para), 59.1 (O-CH3), 99.6 (CHIm-5), 129.2
(CHMes), 129.3 (CHMes), 131.2 (CMes), 135.4 (Cortho), 135.8
(CMes), 136.1 (Cortho), 139.2 (Cpara), 139.5 (Cpara), 150.1 (C-O),
172.5 (d, JRhC = 45.0 Hz, N2C), 182.7 (d, JRhC = 74.2 Hz, CO),
184.9 (d, JRhC = 54.1 Hz, CO); IR (CH2Cl2) ν~ 2079 (νCO), 1995
(νCO) cm-1; MS (ESI) m/z (%) 547 (100) [M - CO - Cl þ
2CH3CN]þ, 533 (22) [M - Cl þ CH3CN]þ, 519 (12) [M - 2CO -
Cl þ 2CH3CN]þ, 506 (28) [M - CO - Cl þ CH3CN]þ. Anal.
Calcd (%) for C24H26ClN2O3Rh (528.8): C 54.51, H 4.96, N 5.30.
Found: C 52.99, H 5.02, N 5.08.
Chlorodicarbonyl(1,3-dimesityl-4-diphenylphosphinatoimidazol-
2-ylidene)rhodium(I) (9E4). The general procedure was followed
using complex 7E4 (40 mg, 0.052 mmol). The initial orange solu-
tion became pale yellow after 15 min. After washing with a mix-
tureofCH2Cl2/pentane(0.5mL/10mL), the complex wasisolated
as a yellow powder (30 mg, 80%): mp 175 °C; 1H NMR (300 MHz,
CDCl3) δ 2.10 (s, 6H, CH3 ortho), 2.12 (s, 6H, CH3 ortho), 2.36 (s,
3H, CH3 para), 2.42 (s, 3H, CH3 para), 6.90 (d, 1H, JPH = 1.2 Hz,
CHIm-5), 6.98 (s, 2H, CHMes), 7.02 (s, 2H, CHMes), 7.40-7.62
(m, 10H, CHPh); 31P{1H} NMR (121.5 MHz, CDCl3) δ 34.6
(s, P(dO)Ph2); 13C{1H} NMR (100.6 MHz, CDCl3) δ 18.2
(CH3 ortho), 18.6 (CH3 ortho), 21.2 (CH3 para), 21.3 (CH3 para),
108.0 (d, JPC =3.1Hz, CHIm-5), 128.8 (d, JPC =13.8Hz, CHortho),
129.2 (s, CHMes), 131.7 (d, JPC = 10.9 Hz, CHmeta), 133.6 (d,
JPC = 2.8 Hz, CHpara), 125.2 (Cortho), 136.2 (Cortho), 139.4 (Cpara),
Chlorodicarbonyl(1,3-dimesitylimidazolin-4-on-2-ylidene)rhodium-
(I) (9H). The general procedure was followed using complex 7H
(50 mg, 0.09 mmol) in CH2Cl2 (3 mL). The initial orange solution
became pale yellow after 15 min. After washing with pentane (2 ꢀ
5 mL), the complex was isolated as a yellow powder (28 mg, 61%):
139.6 (Cpara), 173.4 (d, JRhC = 45.6 Hz, N2C), 182.6 (d, JRhC
=
74.6 Hz, CO), 184.1 (d, JRhC = 54.0 Hz, CO); IR (CH2Cl2) ν~
2080 (νCO), 1997 (νCO) cm-1; MS (ESI) m/z (%) 733 (100)
[M- CO - Cl þ 2CH3CN]þ, 692 (34) [M - CO - Cl þ CH3CN]þ,
651 (6) [M - CO - Cl]þ, 533 (42) [M - CO - Cl - P(dO)Ph2 þ
2CH3CN]þ, 492 (21) [M - CO - Cl - P(dO)Ph2 þ CH3CN]þ,
1
mp 168 °C (dec); H NMR (300 MHz, CDCl3) δ 2.34 (s, 6H,
CH3 Mes), 2.38 (s, 6H, CH3 Mes), 2.41 (s, 6H, CH3 Mes), 4.42 (s, 2H,
CH2), 7.04 (s, 4H, CHMes); 13C{1H} NMR (75.5 MHz, CDCl3) δ

Article
Organometallics, Vol. 29, No. 11, 2010 2629
Table 3. X-ray Experimental Data for Complexes 7E2 and 7E4
7E2
7E4
formula
C30H38ClN2ORh
580.98
monoclinic
P21/n
13.200(5)
11.630(5)
18.518(5)
90
100.288(5)
90
C41H45ClN2O2PRh
767.12
monoclinic
P21/n
18.6377(5)
11.2954(4)
19.2758(6)
90
107.192(2)
90
molecular weight
cryst syst
space group
˚
a (A)
˚
b (A)
˚
c (A)
R (deg)
β (deg)
γ (deg)
3
˚
V (A )
Z
cryst dimens (mm)
2797.1(18)
4
3876.6(2)
4
0.15 ꢀ 0.15 ꢀ 0.15
0.40 ꢀ 0.30 ꢀ 0.10
F
calc (g cm-3
)
F000
1.380
1208
1.314
1592
μ (mm-1
temp (K)
)
0.731
180(2)
0.71073
Mo KR
78 899
4471
361
0.0385
0.1028
0.586
110(2)
0.71073
Mo KR
68 541
6446
439
0.0386
0.0935
˚
wavelength (A)
radiation
no. of data measd
no. of data with I > 2σ(I)
no. of variables
R
Rw
goodness-of-fit on F2
largest peak in final difference (e A )
1.21
0.87 and -0.53
1.047
0.924 and -0.381
3
˚
321 (38) [ImOH]þ. Anal. Calcd (%) for C35H33ClN2O4PRh þ
0.7CH2Cl2: C 55.37, H 4.48, N 3.62. Found: C 55.36, H 4.78,
N 3.72.
(quantitative yield): 1H NMR (300 MHz, CDCl3) δ 2.19 (s,
6H, CH3 ortho-mes), 2.20 (s, 6H, CH3 ortho-mes), 2.36 (s, 3H,
CH3 para-mes), 2.37 (s, 3H, CH3 para-mes), 4.87 (d, JHH = 1.2 Hz,
1H, dCH2) 5.74 (d, JHH = 1.2 Hz, 1H, dCH2), 6.97 (s, 2H,
CHMes), 6.99 (s, 2H, CHMes); 13C{1H} NMR (75.5 MHz, CDCl3)
δ 18.7 (CH3 ortho-mes), 19.0 (CH3 ortho-mes), 21.2 (CH3 para-mes),
103.7 (dCH2), 129.4 (CHMes), 129.8 (CHMes), 130.8 (Cipso), 132.2
(Cipso), 135.8 (dC), 136.0 (Cortho), 139.1 (Cpara), 139.5 (Cpara),
Chlorodicarbonyl(1,3-dimesityl-4-tert-butyldimethylsilyloxyi-
midazol-2-ylidene)rhodium(I) (9E5). The general procedure was
followed using complex 7E5 (40 mg, 0.07 mmol), CO(g), and
CH2Cl2 (5 mL). After washing with pentane (2 ꢀ 5 mL), the com-
plex was isolated as a yellow powder (38mg, 86%):mp172 °C; 1H
NMR (400 MHz, CDCl3) δ 0.14 (s, 6H, Si-CH3), 0.75 (s, 9H,
CH3 tBu), 2.24 (s, 6H, CH3 ortho), 2.28 (s, 6H, CH3 ortho), 2.37 (s,
3H, CH3 para), 2.38 (s, 3H, CH3 para), 6.35 (s, 1H, CHIm-5), 6.99 (s,
2H, CHMes), 7.01 (s, 2H, CHMes); 13C{1H} NMR (100.6 MHz,
CDCl3) δ -5.4 (Si-CH3), 17.6 (CtBu), 18.4 (CH3 ortho), 18.7
(CH3 ortho), 21.1 (CH3 para), 21.2 (CH3 para), 24.8 (CH3 tBu),
102.8 (CHIm-5), 128.9 (CHMes), 129.2 (CHMes), 131.5 (Cq), 135.9
(Cortho), 136.0 (Cortho), 139.0 (Cpara), 139.1 (Cpara), 144.0 (C-O),
171.1 (d, JRhC = 45.1 Hz, N2C), 182.9 (d, JRhC = 74.6 Hz, CO),
185.1 (d, JRhC = 54.1 Hz, CO); IR (CH2Cl2) ν~ 2077 (νCO), 1994
(νCO) cm-1; MS (ESI) m/z (%) 621 (100) [M - Cl þ MeOH þ
Na]þ, 567 (38) [M - Cl - CO þ CH3CN]þ. Anal. Calcd (%) for
C29H38ClN2O3RhSi (629.1): C 55.37, H 6.09, N 4.45. Found: C
55.28, H 6.49, N 4.22.
161.4 (CdO), 183.2 (JRhC = 86.4 Hz, CO), 213.5 (JRhC
=
59.8 Hz, N2C); IR (ATR) ν~ 1988 (νCO) cm-1; MS (ESI) m/z
(%) 504 (100) [M/2 - Cl þ CH3CN]þ, 463 (69) [M/2 - Cl]þ, 435
(50) [M/2 - CO - Cl]þ.
X-ray Diffraction Studies. Single crystals of 7E2 and 7E4
suitable for X-ray diffraction were obtained by slow evapora-
tion of a dichloromethane/pentane mixture, and the data were
collected on a Bruker D8 APEX II diffractometer. Crystal and
intensity data are summarized in Table 3. All calculations were
performed on a PC-compatible computer using the WinGX
system.31 The structures were solved using the SIR92 program,32
which revealed in each instance the position of most of the non-
hydrogen atoms. All remaining non-hydrogen atoms were lo-
cated by the usual combination of full matrix least-squares
refinement and difference electron density syntheses by using
the SHELXL97 program.33 For 7E2, carbon atoms C7 and C12
belonging to the cyclo-octadiene ring were found to be disordered
over two positions. Concomitantly, the methoxy group attached
to the NHC backbone was found to be distributed over two
different sites, being attached to either C2 or C3. Each couple of
atoms C7a and C7b, C12a and C12b, O1a and O1b, and C4a and
C4b were refined with structure occupancy factors of 0.68 and
0.32, respectively. Atomic scattering factors were taken from the
usual tabulations. Anomalous dispersion terms for Rh, Cl, and P
atoms were included in Fc. All non-hydrogen atoms were allowed
to vibrate anisotropically. All the hydrogen atoms were set in
Chlorodicarbonyl(1,3-dimesityl-5-(methylenyl)imidazolin-4-on-
2-ylidene)rhodium(I) (9CH2). The general procedure was followed
using complex 7CH2 (23mg,0.04mmol),CO(g),andCDCl3 (2mL).
The initial orange solution became pale yellow after 15 min. The
solution was poured into a NMR tube, and NMR analyses
1
were performed: H NMR (300 MHz, CDCl3) δ 2.31 (s, 12H,
CH3 ortho-mes), 2.39-2.41 (CH3 para-mes þ CH2 COD), 5.19 (d, 1H,
JHH = 2.4 Hz, dCH2), 5.60 (br s, 4H, dCHCOD), 5.03 (d, 1H,
J
HH = 2.4 Hz, dCH2), 7.05 (s, 2H, CHMes); 7.08 (s, 2H, CHMes);
13C{1H} NMR (75.5 MHz, CDCl3) δ 18.4 (CH3 ortho-mes), 18.7
(CH3 ortho-mes), 21.2 (CH3 para-mes), 21.2 (CH3 para-mes), 28.0
(CH2 COD), 108.8 (dCH2), 128.6 (dCHCOD), 129.6 (CHMes),
129.8 (CHMes), 131.2 (Cq), 135.2 (Cq), 135.8 (Cq), 140.0 (Cq),
140.5 (Cq), 162.0 (CdO), 183.9 (d, JRhC = 58 Hz, CO), 218.0 (d,
˚
idealized position (R3CH, C-H = 0.96 A; R2CH2, C-H =
2
˚
˚
˚
0.97 A; RCH3, C-H = 0.98 A; C(sp )-H = 0.93 A; Uiso 1.2 or
1.5 time greater than the Ueq of the carbon atom to which the
J
RhC = 45 Hz, N2C); IR(CH2Cl2) ν~ 2090 (νCO), 2007 (νCO) cm-1
;
MS(FAB, MNBAmatrix) m/z (%)527(29) [M]þ, 498 (100) [M -
CO]þ, 471 (42) [M - 2CO]þ, 440 (43).
(31) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837.
(32) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A.
J. Appl. Crystallogr. 1993, 26, 343.
Generation of the Dimeric Species 10. Compound 9CH2 was
heated in vacuo at 80 °C for 8 h. The red residue was washed with
pentane (2.5 mL) and dried in vacuo to afford an orange solid
(33) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112.

2630 Organometallics, Vol. 29, No. 11, 2010
Benhamou et al.
hydrogen atom is attached), and their positions were refined as
“riding” atoms.
de la Recherche (ANR) for financial support of the
proposal ANR-08-BLAN-0137-01.
Acknowledgment. This research was supported by the
ꢁ
Supporting Information Available: CIF files giving crystal-
lographic data and including a full list of interatomic bond lengths
and angles for the reported complexes 7E2 and 7E4. This material is
available free of charge via the Internet at http://pubs.acs.org
CNRS. We thank the Ministere de l’Education Nationale
for a Ph.D. Fellowship to L.B., the RDR2 network of the
CNRS for additional funding, and the Agence Nationale