Assessing the potential of zwitterionic NHOCS2 adducts for probing the stereoelectronic parameters of N-heterocyclic carbenes

Article abstract of DOI:10.1002/ejic.200801110

Five imidazol(in)ium-2-dithiocarboxylates bearing cyclo-hexyl, mesityl, or 2,6-diisopropylphenyl substituents on their nitrogen atoms were prepared from the corresponding N-heterocyclic carbenes (NHCs) by reaction with carbon disul-fide. They were charact

Full text of DOI:10.1002/ejic.200801110

FULL PAPER
DOI: 10.1002/ejic.200801110
Assessing the Potential of Zwitterionic NHC·CS2 Adducts for Probing the
Stereoelectronic Parameters of N-Heterocyclic Carbenes
Lionel Delaude,*[a] Albert Demonceau,[a] and Johan Wouters[b]
Keywords: Betaines / Carbenes / Ligand effects / Nitrogen heterocycles / Zwitterions
Five imidazol(in)ium-2-dithiocarboxylates bearing cyclo-
hexyl, mesityl, or 2,6-diisopropylphenyl substituents on their
nitrogen atoms were prepared from the corresponding N-
heterocyclic carbenes (NHCs) by reaction with carbon disul-
fide. They were characterized by IR, UV/Vis, and NMR spec-
troscopy, and by thermogravimetric analysis. Their molecular
structures were determined by X-ray diffraction. For the sake
of comparison, tricyclohexylphosphonium dithiocarboxylate
was also examined. The data acquired were scrutinized to
tronic properties of NHC ligands. Because of their outstand-
ing ability to crystallize, the five NHC·CS2 betaines were
found to be highly suitable for probing the steric influence of
nitrogen atom substituents on imidazolylidene-based ligand
precursors via XRD analysis, while the corresponding
NHC·CO2 adducts were deemed more appropriate for evalu-
ating the σ-donating properties of carbene ligands.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
evaluate their usefulness for assessing the steric and elec- Germany, 2009)
Although most NHCs prepared so far (and/or immediate
Introduction
precursors or complexes derived thereof) were characterized
by various analytical techniques,[8] general and efficient
methods that would allow a detailed comparison of their
steric and electronic properties are still missing.[9] This is in
sharp contrast with the related class of phosphane ligands.
Thanks to the pioneering work of Tolman,[10] cone angles
(θ) and carbonyl stretching vibration frequencies (ν) are
now routinely used to quantitatively assess the impact of
substituents on phosphane ligands.[11] The usefulness of
these parameters to predict structure–activity relationships
is firmly established and has proven highly valuable in the
rational design of new and improved phosphorus-based li-
gands for catalytic applications.[12]
In the case of NHC ligands, most efforts toward the defi-
nition of experimental parameters that would help compare
different structures and permit a better assessment of the
underlying steric and electronic factors originated from the
group of Nolan. Initial work focused on calorimetric mea-
surements during the reaction of [Cp*RuCl]4 with carbene
ligands and X-ray diffraction studies of the resulting
Cp*Ru(NHC)Cl adducts. The two sets of data allowed,
respectively, evaluation of the electron donor properties and
the steric requirements of common imidazolin-2-ylidene li-
gands and their saturated analogues.[13] In 2005, Nolan et
al. proposed another approach to obtain a more thorough
understanding of the steric and electronic factors character-
izing the NHC ligand class and to ease their comparison
with the large array of data available for phosphanes. They
investigated the reaction of Ni(CO)4 with a range of σ-do-
nor ligands. The IR ν(CO) stretching vibration frequencies
and the crystallographic structures of the Ni(CO)x(NHC)
Since they were first isolated and characterized by
Arduengo and co-workers in 1991,[1] stable N-heterocyclic
carbenes (NHCs) have been extensively studied.[2] Over the
past 17 years, they have already afforded a whole new gen-
eration of nucleophilic reagents,[3] organocatalysts,[4] and
organometallic catalysts,[5] including chiral ones,[6] which
have revolutionized key areas of organic synthesis and poly-
mer chemistry. Most NHCs investigated so far derive from
1H-imidazole. This electron-rich heterocycle provides a
suitable framework that stabilizes the carbene center located
between two nitrogen atoms.[7] Depending on the presence
or the absence of a double bond between C4 and C5, imid-
azolin-2-ylidene and imidazolidin-2-ylidene species are ob-
tained (Scheme 1).
Scheme 1. Stable NHCs derived from 1H-imidazole.
[a] Center for Education and Research on Macromolecules
(CERM), Institut de Chimie (B6a), Université de Liège,
Sart-Tilman par, 4000 Liège, Belgium
Fax: +32-4-3663497
E-mail: l.delaude@ulg.ac.be
[b] Laboratoire de Chimie Biologique Structurale, Facultés Uni-
versitaires N.-D. de la Paix,
61 Rue de Bruxelles, 5000 Namur, Belgium
1882
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2009, 1882–1891

Zwitterionic NHC·CS2 Adducts
adducts were determined. In combination with DFT calcu- tert-butoxide, or potassium bis(trimethylsilyl)amide were
lations, they provided valuable information on the steric also tested and found equally suitable. The time needed to
and electronic properties of NHCs.[14]
complete this step varied greatly from one experiment to
A closely related strategy was adopted by Herrmann and another. In some cases, addition of a few drops of DMSO
co-workers in 2006 to examine the ligand properties of or tBuOH helped solubilize the reaction partners and has-
prototypical carbenes based on various nitrogen ring sys- tened the release of free carbenes in solution. Recourse to
tems. The Rh(COD)X(NHC) complexes (COD = 1,5-cyclo- imidazol(in)ium tetrafluoroborates instead of chlorides usu-
octadiene, X = Cl, I) were characterized by X-ray diffrac- ally slowed down the reaction. The nature of the heterocycle
tion analysis, while the corresponding Rh(CO)2I(NHC) (saturated imidazolinium salts are slightly less acidic than
species served for IR measurements.[15] Other authors have their unsaturated imidazolium counterparts),[19b,30] the ste-
relied on IR data of iron,[16] rhodium,[17] or iridium[18] car- ric bulk of the nitrogen substituents (2,6-diisopropylphenyl
bonyl complexes to assess the overall donor strength of a groups may restrain access of the base to the acidic C2–H
carbene ligand. Pentacarbonyl complexes with the generic center),[26d] and the influence of the inorganic counterion
formula M(CO)5(NHC) based on group 6 transition metals [imidazol(in)ium tetrafluoroborates often display a higher
(M = Cr, Mo, W) were also employed to determine the crystallinity and a lower solubility than analogous chlo-
stretching frequency of the CO ligand trans to the NHC.[19] rides] must all intervene in a complex manner to explain
Unfortunately, changes in the metal nature and coordina- the different rates of deprotonation observed.
tion sphere hinder comparison between the various data
sets and no unified scale is presently available to evaluate
the donor ability of NHCs.
Recent investigations from our laboratory[20] and from
other groups[21,22] have shown that imidazol(in)ium-2-
carboxylates were labile zwitterionic adducts that could
serve as carbene precursors for the preparation of late tran-
sition metal–NHC complexes, either preformed or gener-
ated in situ. In this contribution, we report on the synthesis
and characterization of five imidazol(in)ium-2-dithiocarb-
oxylates obtained by reacting free carbenes with CS2 in-
stead of CO2, a synthetic path first explored by Kuhn et al.
in the late 1990s.[23,24] The influence of the nitrogen atom
substituents and of the endocyclic C4–C5 double bond on
their spectroscopic features was probed and we compare
our findings with earlier results from the literature.
Results and Discussion
Synthesis of Imidazol(in)ium-2-dithiocarboxylates
Currently, deprotonation of imidazol(in)ium chlorides or
tetrafluoroborates with a strong base provides the most
convenient and general access to NHCs, whether it is for
preparative purposes[1,25] or for in situ catalytic applica-
tions.[18c,26] This is due, in part, to the availability of ef-
Scheme 2. Synthesis of imidazol(in)ium-2-dithiocarboxylates used
ficient and flexible synthetic procedures that allow the in this work.
straightforward preparation of a wide range of imidazol-
(in)ium salts from readily available acyclic precursors.[27]
Once the deprotonation step was completed, the suspen-
Thus, we chose this approach to release free carbenes in sions were allowed to settle down and the inorganic byprod-
solution prior to their trapping with carbon disulfide. Alter- uct (NaCl) was filtered off, along with any unreacted start-
native methods for generating NHC·CS2 adducts include ing materials. A small excess of carbon disulfide was added
the cleavage of an enetetramine with carbon disulfide[28] or at once to the free carbene solutions. It led to an instan-
the displacement of chloroform by carbon disulfide upon taneous color change and to the rapid formation of zwitter-
thermolysis of a NHC·CHCl3 adduct,[28c,29] but they both ionic adducts. 1,3-Dimesitylimidazolium-2-dithiocarboxyl-
lack generality.
ate (nicknamed IMes·CS2) and its saturated heterocycle an-
In practice, a set of five representative imidazol(in)ium alogue (nicknamed SIMes·CS2) precipitated from the reac-
chlorides was elected as starting materials (Scheme 2). tion medium, whereas betaines bearing 2,6-diisopro-
These NHC precursors were suspended in dry THF and pylphenyl (IDip·CS2 and SIDip·CS2) or cyclohexyl substit-
deprotonated with sodium hydride at room temperature. uents (ICy·CS2) on their nitrogen atoms remained soluble
Other strong bases such as potassium hydride, potassium in THF. The solvent was evaporated under reduced pressure
Eur. J. Inorg. Chem. 2009, 1882–1891
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
1883

L. Delaude, A. Demonceau, J. Wouters
FULL PAPER
to afford the zwitterionic products in solid form. Analyti- quired the use of more polar [D6]DMSO instead. PCy3·CS2,
cally pure samples suitable for X-ray diffraction analysis on the other hand, was highly labile in solution and switch-
and characterization by various experimental techniques ing to less polar CD2Cl2 containing a few drops of CS2
were further recrystallized from MeCN or EtOH (see Exp. allowed one to prevent the release of free phosphane.
Sect.).
Monitoring the betaine formations by 1H NMR spec-
Overall yields for the preparation of dithiocarboxylate troscopy revealed mainly the disappearance of the strongly
adducts from imidazol(in)ium chlorides according to the in deshielded (9–11 ppm) imidazol(in)ium H2 signals, while
situ method described above were highly variable 13C NMR spectra showed the emergence of new resonances
(Scheme 2). Nevertheless, the reaction between carbon di- at ca. 220–226 ppm for the dithiocarboxylate groups. The
sulfide and a free carbene always occurred rapidly and heterocyclic C2 carbon atom remained unaffected by
quantitatively at room temperature. Because of the remark- changes in the substituents on the vicinal nitrogen atoms.
able tendency of the zwitterionic adducts to form crystalline Its signal was located at ca. 149 ppm in imidazolium-2-di-
materials, only minor losses were encountered during the thiocarboxylates and 165 ppm in their saturated imidazolin-
final purification if recrystallization solvents are properly ium counterparts (Table 1). Contrary to what was observed
selected. The main cause for an unsatisfactory mass balance in imidazolium chlorides and free imidazolin-2-ylidenes, re-
is a recalcitrant deprotonation step. This was evidenced by placement of N-aryl substituents by better donor N-alkyl
a control experiment in which the 1,3-dimesitylimidazolin- groups did not result in any significant upfield shift for C2
2-ylidene free carbene (IMes) was isolated and recrys- in imidazolium-2-dithiocarboxylates. This observation is
tallized prior to its reaction with carbon disulfide in THF. not limited to cyclohexyl groups, since identical δC2 values
Under these conditions, the corresponding dithiocarboxyl- of 149–150 ppm were reported by Kuhn et al. for 1,3-di-
ate was isolated in 91% yield, whereas the in situ deproton- methyl, diethyl, diisopropyl, bis(2-methoxyethyl), and bis(3-
ation of IMes·HCl with NaH followed by filtration and di- methoxypropyl)-4,5-dimethylimidazolium-2-dithiocarboxy-
rect addition of CS2 afforded only a 55% isolated yield of lates.[23] In 31P NMR the signal for PCy3·CS2 was found at
IMes·CS2.
δ = 19 ppm, intermediate between those recorded for the
For the sake of comparison, the carbon disulfide adduct free phosphane (δ = 11 ppm) and its oxide (δ = 51 ppm),
of tricylohexylphosphane was also prepared. It precipitated the two decomposition products that appeared when the
rapidly upon addition of CS2 to an ethanolic solution of NMR tube was exposed to air for a few hours.
PCy3 at room temperature and was isolated in high yield
Table 1. 13C NMR chemical shifts of C2 in various imidazol(in)-
and purity by simple filtration and washing [Equation (1)].
ium chlorides, NHCs, and imidazol(in)ium-2-dithiocarboxylates.
This compound and other homologous trialkylphosphon-
NHC·HCl δC2 [ppm] NHC
δC2 [ppm]
NHC·CS2 δC2 [ppm]
ium dithiocarboxylates such as Et3P·CS2 or nBu3P·CS2 have
been known in the literature for more than a hundred
years.[31] Their zwitterionic nature is well established and
they serve as versatile ligands in coordination chemistry.[32]
It should be pointed out that less basic triarylphosphanes
do not afford stable adducts with CS2 alone, although in-
corporation of a Ph3P·CS2 ligand coordinated as a κ2-S,SЈ
chelate was evidenced in cationic complexes of iridium[33]
and iron.[34]
ICy·HCl
IMes·HCl
IDip·HCl
133.9[a]
139.1[a]
139.8[a]
ICy
210.1[b,e]
219.7[b,f]
220.6[c,g]
243.8[c,g]
244.0[c,g]
ICy·CS2
149.4[d]
IMes
IDip
SIMes
SIDip
IMes·CS2 146.7[a]
IDip·CS2 149.1[d]
SIMes·CS2 165.0[d]
SIDip·CS2 164.2[d]
SIMes·HCl 160.2[a]
SIDip·HCl 160.0[a]
[a] Solvent: [D6]DMSO. [b] Solvent: [D8]THF. [c] Solvent: C6D6.
[d] Solvent: CDCl3. [e] Data from ref.[51]. [f] Data from ref.[1b]. [g]
Data from ref.[52]
.
The FT-IR spectra of the five NHC·CS2 and PCy3·CS2
adducts were recorded in KBr pellets. Apart from the vari-
ous C–H stretching vibration bands located between 2840
and 3100 cm–1, the two most intense absorptions were
caused by the asymmetric stretching vibrations of the N2C+
and CS2– groups (Table 2). The former endocyclic unit gave
rise to a strong band in the 1470–1530 cm–1 region, in ac-
cordance with previous experimental observations[35] and
(1)
Characterization of Imidazol(in)ium-2-dithiocarboxylates
Various analytical techniques were employed to investi- theoretical calculations.[36] This position is indicative of a
gate the structural features of the five NHC·CS2 and double bond character within the N2C+ group that can be
PCy3·CS2 adducts under study. NMR spectroscopy unam- easily rationalized by contribution of the canonical forms
biguously confirmed the identity of the hydrocarbon back- +N=C–N and N–C=N+. A second intense absorption in
–
bones but was of limited use to help rank the different car- the 1050–1080 cm–1 region was assigned to the CS2 group.
bene moieties in terms of stereoelectronic properties. This Indeed, ab initio calculations for 1,3-dimethylimidazolin-
was due in part to the difficulty in finding a universal sol- ium-2-dithiocarboxylate predicted a 1041 cm–1 wavenumber
–
vent that would dissolve all samples and allow a straightfor- for νasym(CS2 ), in good agreement with the 1047 cm–1 ex-
ward comparison of chemical shifts between compounds. perimental value recorded.[36] Computations also showed
CDCl3 was a solvent of choice in many cases. However, that the corresponding symmetric stretching vibration was
IMes·CS2 was only poorly soluble in this medium and re- very weak and located below 900 cm–1, thereby precluding
1884
www.eurjic.org
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2009, 1882–1891

Zwitterionic NHC·CS2 Adducts
Table 2. IR stretching vibrations for various imidazol(in)ium-2-carboxylates and -dithiocarboxylates.
–
–
NHC·CS2
νasym(N2C+) [cm–1]
νasym(CS2 ) [cm–1]
NHC·CO2
νasym(CO2 ) [cm–1][a]
ICy·CS2
1474
1488
1469
1531
1524
1058
1052
1058
1064
1080
ICy·CO2
1663
1675
1679
1683
1684
IMes·CS2
IDip·CS2
SIMes·CS2
SIDip·CS2
IMes·CO2
IDip·CO2
SIMes·CO2
SIDip·CO2
[a] Data from ref.[20a]
.
its identification with certainty. For the SIDip·CS2 adduct, cedure is very easy to implement and does not require the
three distinct lines were clearly visible at 1045, 1062, and use of highly toxic reagents such as carbon monoxide or
–
1080 cm–1. We tentatively assigned the CS2 asymmetric nickel tetracarbonyl.
stretching vibration to the most intense one at 1080 cm–1,
The UV/Vis spectra of the five NHC·CS2 adducts were
although this attribution remains questionable. A similar recorded in acetonitrile. They displayed absorption maxima
fine structure was also observed for PCy3·CS2 at 1032, in the 210–270 nm region that most likely originated from
1044, and 1053 cm–1. In this case, however, the three bands πǞπ* transitions within the (hetero)aromatic rings. They
displayed similar intensities. This pattern was already re- also showed a medium absorption above 300 nm ascribable
–
ported in the literature[37] and is not unprecedented for tri- to a πǞπ* transition in the CS2 group.[36] The maximum
alkylphosphane–carbon disulfide adducts.[38] Comparison wavelength for this band was located at 356 nm for
with the IR spectrum of free PCy3 that displayed weak ab- ICy·CS2, 362 nm for SIMes·CS2 and SIDip·CS2, and
sorptions at 1030 and 1044 cm–1 with a shoulder at 364 nm for IMes·CS2 and IDip·CS2, down from 366 nm in
1049 cm–1 did not help us locate unambiguously the band PCy3·CS2. Hence, substitution of the phosphane by a NHC
responsible for the asymmetric stretching vibration of the induced only a minor hypsochromic shift. Yet it had a bene-
dithiocarboxylate anion in this inner salt.
ficial influence on the betaine lifetime in solution. Indeed,
–
All in all, neither the νasym(N2C+) nor the νasym(CS2 ) IR the NHC·CS2 adducts remained stable for at least a week
stretching vibrations are satisfactory probes to compare the in acetonitrile, whereas PCy3·CS2 fully decomposed within
σ-donor strengths of NHCs. A much more convenient and one hour. This result contrasts with previous spectrophoto-
reliable indirect measurement of the electron-donating metric measurements on the PEt3/CS2 system that showed
properties of imidazolylidene ligands was achieved by mon- a complete displacement of the equilibrium depicted in
–
itoring the νasym(CO2 ) stretching vibration bands in imid- Equation (2) toward adduct formation in acetonitrile.[41]
azol(in)ium-2-carboxylates.[20a] In this case, strong absorp-
tion bands located in the 1660–1690 cm–1 region could be
unambiguously assigned to the carboxylate group. ICy ex-
(2)
pected to be the most basic ligand due to its alkyl-substi-
tuted nitrogen atoms led to the lowest stretching wave-
To probe the thermal stability of the five NHC·CS2 and
number among the five NHC·CO2 adducts investigated
PCy3·CS2 adducts in the solid state, we carried out thermo-
(Table 2). Aryl-substituted imidazolin-2-ylidene species
gravimetric analysis (TGA). The results are depicted in Fig-
IMes and IDip came next, closely followed by their imid-
ure 1. The phosphane betaine was the least stable of the
azolidin-2-ylidene analogues SIMes and SIDip. Previous
series and started losing weight at 58 °C, whereas imid-
experimental results obtained when comparing the ν(CO)
stretching wavenumbers in Ni(CO)3(NHC) complexes also
suggested that saturated NHC ligands were slightly less
electron-donating than their unsaturated counterparts,[14]
although examination of Rh(CO)2I(NHC) complexes[15]
and determination of the relative bond dissociation en-
thalpies (BDE) in Cp*Ru(NHC)Cl adducts[13] led to the op-
posite conclusion. Further theoretical,[39] spectroscopic,[40]
and electrochemical[16,18d] studies have stressed the inter-
vention of π-type contributions in addition to σ-donation
in the bonding of NHCs to transition metals. Thus, several
types of stereoelectronic metal–ligand interactions must in-
tervene in a complex manner to determine the relative rank-
ing of saturated and unsaturated carbene ligands. Measur-
–
ing the value of νasym(CO2 ) in NHC·CO2 betaines provides
a new, efficient approach to evaluate selectively the σ-donat-
ing properties of carbene ligands, since it does not involve
any atom with d orbitals. Moreover, the experimental pro- used in this work.
Figure 1. TGA curves of the five NHC·CS2 and PCy3·CS2 adducts
Eur. J. Inorg. Chem. 2009, 1882–1891
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
1885

L. Delaude, A. Demonceau, J. Wouters
FULL PAPER
azol(in)ium-2-dithiocarboxylates resisted decomposition
until temperatures high above 100 °C. Degradation began
at 164 °C for IMes·CS2, 177 °C for SIDip·CS2, 202 °C for
ICy·CS2, 211 °C for IDip·CS2, and 225 °C for SIMes·CS2.
Evidence for a clean loss of carbon disulfide was detected
only with PCy3·CS2 (theoretical weight loss: 21.35%, found
21.33%). This behavior reflects both the lability of the
Cy3P+–CS2– bond and the relative stability of tricyclohexyl-
phosphane. Nucleophilic carbenes, on the other hand, form
much more stable zwitterionic adducts with CS2, but once
released they undergo rapid thermolysis.
Figure 2. Crystals of the five NHC·CS2 adducts used in this work.
Clockwise from top left corner: ICy·CS2, IMes·CS2, IDip·CS2,
SIMes·CS2, SIDip·CS2, euro coin (23.25 mm diameter).
X-ray Diffraction Studies
Whereas the recrystallization of NHC·CO2 compounds
is hampered by the labile and sometimes hygroscopic nature
of these adducts,[21,24,42] crystals of the five NHC·CS2 beta-
ines under investigation were easily grown by dissolving
crude samples into a hot polar solvent, such as ethanol or
acetonitrile, followed by slow evaporation of the saturated
solutions in an open vessel at room temperature. Beautiful
ruby-like specimens with dimensions reaching a few milli-
meters were obtained in most cases (Figure 2). Their molec-
ular structures were determined by X-ray diffraction analy-
sis (Figure 3). The crystal structure of PCy3·CS2 had al-
ready been solved by Vittal and Dean in 1997.[43] Both
ICy·CS2 and IMes·CS2 crystallized with two molecules in
the asymmetric unit. For the sake of clarity, only one of
them is depicted in Figure 3 that also shows the common
atom numbering system adopted to ease comparison be-
tween the various structures. Selected bond lengths and
angles are listed in Table 3.
Within each molecule, the C–S distances were similar, in-
dicating that the negative charge was equally spread over
the two sulfur atoms. This equivalence is best illustrated by
the C2 symmetry observed for all compounds. In the case
of IMes·CS2 and SIMes·CS2, only half of the molecules
Figure 3. (a) Atom numbering system and ORTEP drawings of (b)
ICy·CS2, (c) IMes·CS2, (d) SIMes·CS2, (e) IDip·CS2, (f) SIDip·CS2
in the crystal. Thermal ellipsoids were set at the 50% probability
level.
formed the asymmetric unit, the other half being generated bon atom of the dithiocarboxylate group lay on a special
by twofold axis (non-)crystallographic symmetry. The car- crystallographic position and the second sulfur atom was
Table 3. Selected bond lengths (Å) and angles (°) derived from crystal structures of the five NHC·CS2 adducts used in this work.[a]
NHC·CS2
ICy·CS2
C6–S1
C6–S2
N1–C2
N1–C5
C4–C5
N1–C1a
1.656(3)
1.652(3)
1.667(3)
1.669(3)
1.676(3)
1.662(2)
1.665(2)
1.641(2)
1.657(3)
1.667(3)
1.669(3)
1.659(5)
1.662(2)
1.651(2)
1.335(3)
1.335(3)
1.374(6)
1.387(6)
1.341(2)
1.315(4)
1.326(2)
1.379(3)
1.377(3)
1.374(6)
1.387(6)
1.387(1)
1.480(5)
1.471(1)
1.329(4)
1.333(3)
1.341(10)
1.318(11)
1.333(2)
1.521(8)
1.510(2)
1.479(3)
1.478(3)
1.456(6)
1.461(6)
1.452(2)
1.446(4)
1.447(2)
IMes·CS2
IDip·CS2
SIMes·CS2
SIDip·CS2
NHC·CS2
ICy·CS2
C2–C6
N1–C2–N3
S1–C6–S2
N1–C2–C6–S1
N1–C2–C6–S2
C1b–C1a–N1–C2
1.489(3)
1.483(3)
1.489(7)
1.483(8)
1.487(2)
1.502(6)
1.495(2)
108.1(2)
107.9(2)
122.2(4)
106.4(5)
107.5(1)
112.0(4)
111.6(1)
130.3(1)
130.2(1)
129.1(4)
129.1(4)
128.5(2)
130.3(3)
131.5(2)
–95.2(3)
–91.6(3)
–65.3(2)
–62.1(2)
85.9(2)
92.4(2)
92.0(2)
84.2(3)
87.0(3)
118.0(5)
114.7(5)
93.2(2)
92.8(5)
–88.0(2)
–144.4(3)
–110.6(3)
104.9(6)
106.6(6)
95.9(2)
94.4(5)
84.6(2)
IMes·CS2
IDip·CS2
SIMes·CS2
SIDip·CS2
[a] See Figure 3 (a) for atom numbering.
1886
www.eurjic.org
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2009, 1882–1891

Zwitterionic NHC·CS2 Adducts
generated by symmetry from the coordinates of the first
The rather long C2–C6 distances [between 1.483(3) and
one, leading to identical C–S bonds. Furthermore, the 1.502(6) Å] confirmed the poor electronic communication
lengths of the two C–S bonds were much closer to the dis- between the delocalized CS2 moiety and the imidazol(in)-
–
tances observed for common C=S double bonds (1.67 Å) ium ring. The perpendicular arrangement between the CS2
than for single C–S bonds (1.75 Å).[44] For all five crystal and N2C+ parts is probably retained in solution, as pro-
structures, the N1–C2 and N3–C2 bond lengths were also posed by Nakayma et al. for 1,3-dimethylimidazolinium di-
(nearly) equal and in the range 1.32–1.49 Å (Table 3). This thio-, diseleno-, and thioselenocarboxylates on the grounds
indicates a significant C=N double-bond character, consis- of 1H NMR spectroscopic data and ab initio calcula-
tent with electronic conjugation within the N2C+ motif. tions.[36] Through-space attractive Coulombic interactions
Such a delocalization is in agreement with previous studies between the carbenium ion carbon and the lone pair of elec-
on related crystal structures[36] and correlates well with the trons on the sulfur atoms were held responsible for this situ-
strong absorption band observed in the 1470–1530 cm–1 re- ation. In addition, incorporation of bulky aryl substituents,
gion of the FT-IR spectra (vide supra).
such as 1,3-bis(2,4,6-trimethylphenyl) or 1,3-bis(2,6-diiso-
Within the heterocyclic rings, the C4–C5 distance corre- propylphenyl) groups on the nitrogen atoms should further
sponded to a single C–C bond in the imidazolinium rings stabilize a perpendicular disposition of the carbenium or
[SIMes·CS2: 1.521(8) Å, SIDip·CS2: 1.510(2) Å], whereas it imidazol(in)ium and dithiocarboxylate ions by steric effects.
was significantly shorter in the imidazolium rings [ICy·CS2:
1.329(4) and 1.333(3) Å, IMes·CS2: 1.341(10) and
1.318(11) Å, IDip·CS2: 1.333(2) Å]. For these aromatic
compounds, delocalization in the whole five-membered ring
Computational Analysis
was also underlined by shorter N1–C5 and N3–C4 bonds
(Table 3). Influence of the various nitrogen substituents on
the electronic delocalization of the imidazol(in)ium hetero-
cycles was limited. Indeed, similar geometries were ob-
served for ICy·CS2, IMes·CS2, and IDip·CS2. Thus, elec-
tronic effects of the substituted phenyl rings are not trans-
ferred to the central imidazole moiety. This was underlined
by the rather long N–C inter-ring distances N1–C1a and
N3–C3a (see Table 3), closer to a single N–C bond (1.47 Å)
than to a N=C double bond (1.34 Å).[44] Lack of delocaliza-
tion is a consequence of the perpendicularity between the
N-substituting groups and the imidazol(in)ium rings (see
torsion angle C1b–C1a–N1–C2 in Table 3).
In order to quantify the steric demand of NHC ligands,
Cavallo and Nolan have defined a %VBur parameter that
corresponds to the portion of a 3-Å radius coordination
sphere centered on a metal atom, buried by overlap with
atoms of the ligand (Figure 4).[14,47] Because of the highly
asymmetric geometry of imidazole-based carbenes that usu-
ally display a much larger wingspan along the axis defined
by the two nitrogen exocyclic substituents than in the direc-
tion perpendicular to the central heterocyclic ring, this
model allows for a more realistic comparison with other
ligands, particularly tertiary phosphanes, than the Tolman
cone angle (θ).[10] The bulkier a specific ligand is, the greater
its %VBur
.
In all five crystal structures, the imidazol(in)ium rings
and the dithiocarboxylate unit were nearly orthogonal with
torsion angles N1–C2–C6–S1 close to 90° (Table 3). Similar
conformations were also observed in acyclic carbeniun di-
thiocarboxylates[45] and in all but one of the other imid-
azol(in)ium-2-carboxylates[21,24] and their dithio[23,28,36,46]
or diseleno[36] analogues investigated so far by XRD analy-
sis (Scheme 3). Only in 1,3-dimethylimidazolium-2-carbox-
ylate were the imidazolium and the carboxylate groups al-
most coplanar, with a twist angle of 29°. In this case, steric
requirements from both the nitrogen substituents and the
CX2 moiety were minimized, allowing the formation of hy-
Figure 4. Schematic representation of the sphere dimensions used
for the determination of the %VBur steric parameter.
The values of %VBur extracted from the X-ray crystal
drogen-bonded sheets with a dense columnar π-π stacking structures of the five NHC·CS2 adducts used in this work
in the crystal structure.[42]
are listed in Table 4 along with the data obtained from the
molecular structure of PCy3·CS2. The dummy metal center
defining the origin of the sphere was placed at 2.0 Å from
the coordinating C or P atom. This arbitrary distance is
typical for a metal–NHC bond, such as those recorded pre-
viously in Cp*Ru(NHC)Cl,[5b] Rh(COD)X(NHC) (X = Cl,
I),[15] or Ni(CO)x(NHC) complexes.[14] Putting the reference
atom at 1.5 Å from the ligand, which is the average C2–
C6 bond length observed between the imidazol(in)ium and
dithiocarboxylate units of NHC·CS2 betaines (cf. Table 3)
afforded higher values of %VBur, but the evolution within
the series was not drastically altered.
Scheme 3. Orthogonal disposition of the N2C+ and CX2 units in
various zwitterionic compounds.
–
Eur. J. Inorg. Chem. 2009, 1882–1891
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
1887

L. Delaude, A. Demonceau, J. Wouters
FULL PAPER
Table 4. Steric parameter %VBur for the five NHC ligands and PCy3 spectra of NHC·CO2 adducts provides a more convenient
used in this work.
and reliable approach.
As a follow-up to this study, we are currently synthesiz-
ing other NHC·CS2 adducts from known or new imid-
azol(in)ium salts in order to determine their structure by
X-ray crystallography. This would enlarge the set of data
available for comparison and ranking of NHC precursors
in terms of steric requirements. Additionally, we envision to
take advantage of the fast and irreversible trapping of free
carbenes by carbon disulfide to form stable, colored, well-
defined adducts for analytical or preparative purposes.
Thus, we believe that CS2 could serve as a nonprotic
quenching agent for catalytic systems involving free carb-
enes or that it could help bring to light the involvement of
such highly active species during mechanistic investigations.
Ligand (L)
%VBur computed from
L·CS2
%VBur computed
from Ni(CO)3(L)[a]
ICy
24
27
26
26
27
34
23
26
27
29
30
32
IMes
SIMes
IDip
SIDip
PCy3
[a] Data from ref.[47]
Because the different figures gathered in Table 4 are close
to each other, great care should be taken in their interpret-
ation. Yet, recent investigations by Cavallo and co-workers
showed that %VBur values could be reasonably trusted to
rationalize NHC dimerization energies[48] or binding ener-
gies of NHC ligands in Cp*Ru(NHC)Cl complexes.[49] The
data obtained in the present study are in rather good agree-
ment with those previously computed from Ni(CO)x(NHC)
complexes (Table 4).[47] However, the %VBur values derived
from NHC·CS2 adducts suggest that saturated or unsatu-
rated carbenes bearing mesityl or 2,6-diisopropylphenyl
substituents possess rather similar footprints, whereas the
IDip and SIDip ligands were always found to be slightly
more sterically demanding than IMes and SIMes when
metal–NHC complexes served as templates. This small dis-
crepancy is due to minor changes in the orientation of the
aryl substituents relative to the central imidazole ring in
order to accommodate the steric pressure exerted by the
CS2 unit on the NHC moiety. Variations of packing forces
in the different types of crystals should also be taken into
consideration. As a matter of fact, several structural investi-
gations have evidenced the high flexibility of the nitrogen
exocyclic substituents within NHC ligands in response to
subtle changes of environment.[18d,50]
Experimental Section
General: All syntheses were carried out under a dry argon atmo-
sphere using standard Schlenk techniques. Solvents were distilled
from appropriate drying agents and deoxygenated prior to use.
Imidazol(in)ium salts ICy·HCl,[51] IMes·HCl,[52] IDip·HCl,[52]
SIMes·HCl,[52] and SIDip·HCl[52] were synthesized according to
1
published procedures. H and 13C NMR spectra were recorded at
298 K with a Bruker DRX 400 spectrometer operating at 400.13
and 100.62 MHz, respectively. Chemical shifts are listed in parts
per million downfield from TMS and are referenced from the sol-
vent peaks or TMS. 31P NMR spectra were recorded at 298 K with
a Bruker Avance 250 spectrometer operating at 101.25 MHz with
H3PO4 as the external reference. Infrared spectra were recorded
with a Perkin–Elmer Spectrum One FT-IR spectrometer. UV/Vis
spectra were recorded with a Hewlett–Packard HP 8453 spectro-
photometer. Thermogravimetric analyses were performed with a
TA Q500 instrument using a 5 °C/min ramp. Melting points were
recorded with an Electrothermal 9100 apparatus and are not cor-
rected. Elemental analyses were carried out in the Laboratory of
Pharmaceutical Chemistry at the University of Liège.
Preparation of Imidazol(in)ium-2-dithiocarboxylates: An oven-dried
100-mL round-bottomed flask equipped with a magnetic stirring
bar and capped with a three-way stopcock was charged with imid-
azol(in)ium chloride (5 mmol) and 95% sodium hydride (0.1516 g,
6 mmol). The reactor was purged of air by applying three vacuum/
argon cycles before dry THF (50 mL) was added. The mixture was
Conclusion and Perspectives
A range of imidazol(in)ium chlorides were converted into
the corresponding 2-dithiocarboxylates by a two-step pro-
cedure involving in situ generation of free carbenes with a
strong base followed by trapping with carbon disulfide. The stirred at room temperature until the deprotonation step seemed to
be complete (20 min to overnight). In some cases, a few drops of
tBuOH or DMSO were added to induce the process. After the solid
had settled down, the supernatant solution was filtered trough Ce-
lite and transferred with a cannula under an inert atmosphere into
a two-neck 100-mL round-bottomed flask equipped with a mag-
netic stirring bar and capped with a three-way stopcock. Carbon
disulfide (0.4 mL, 6.7 mmol) was added with a syringe. The color
of the solution changed instantaneously. After 30 min of stirring at
room temperature, the solvent was evaporated under vacuum. The
residue was brought back to air and washed with n-pentane
resulting zwitterionic products were stable, crystalline sol-
ids. They were characterized by various analytical tech-
niques, including IR, UV/Vis, and NMR spectroscopy, and
their molecular structures were determined by X-ray dif-
fraction analysis. The data acquired were scrutinized to
evaluate their usefulness for assessing the steric and elec-
tronic properties of NHC ligands. Because of their out-
standing ability to crystallize, NHC·CS2 betaines are prom-
ising candidates to probe the steric influence of nitrogen
atom substituents on imidazolylidene-based ligand (10 mL). It was dried under high vacuum and recrystallized.
precursors via XRD analysis. Difficulties in locating the
1,3-Dicyclohexylimidazolium-2-dithiocarboxylate (ICy·CS2): Dark
–
νasym(CS2 ) stretching vibration band in IR spectroscopy
red crystals (from EtOH). Yield 61% (0.65 g); m.p. 230–232 °C
1
complicates, however, their use to quantify the electron-do-
(dec.). H NMR (400 MHz, CDCl3): δ = 1.14–1.26 (m, 2 H, Cy),
1.34–1.41 (m, 4 H, Cy), 1.47–1.57 (m, 4 H, Cy), 1.71–1.75 (br. d,
nating properties of N-heterocyclic carbenes. For this pur-
–
pose, measuring the analogous νasym(CO2 ) band in the IR 2 H, Cy), 1.85–1.88 (br. d, 4 H, Cy), 2.24–2.27 (br. d, 4 H, Cy),
1888
www.eurjic.org
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2009, 1882–1891

Zwitterionic NHC·CS2 Adducts
4.51 (m, 2 H, CHN), 6.99 (s, 2 H, =CHN) ppm. 13C NMR (11200), 362 nm (9000 –1 cm–1). C28H38N2S2 (466.74): calcd. C
(100 MHz, CDCl3): δ = 25.0 (CH2), 25.2 (CH2), 32.3 (CH2), 57.5 72.05, H 8.21, N 6.00, S 13.74; found C 72.05, H 8.08, N 6.79, S
(CHN), 115.1 (Im-C4,5), 149.4 (Im-C2), 226.0 (CS2) ppm. IR
14.29.
(KBr): ν = 3090 (m), 2934 (s), 2855 (s), 1570 (m), 1474 (m), 1447
˜
Preparation of Tricyclohexylphosphonium Dithiocarboxylate
(PCy3·CS2): An oven-dried 100-mL round-bottomed flask
equipped with a magnetic stirring bar and capped with a three-way
stopcock was charged with tricyclohexylphosphane (1.40 g,
5 mmol). The reactor was purged of air by applying three vacuum/
argon cycles before degassed EtOH (25 mL) was added. Carbon
disulfide (0.4 mL, 6.7 mmol) was syringed into the clear solution
and it was stirred at room temperature. A precipitate appeared
within a few seconds. After 15 min, the resulting suspension was
brought back to air and filtered with suction. The precipitate was
(m), 1249 (m), 1200 (m), 1058 (s), 901 (m) cm–1. UV/Vis (CH3CN):
λmax (ε) = 217 (12500), 264 (6200), 356 nm (12800 –1 cm–1).
C16H24N2S2 (308.51): calcd. C 62.29, H 7.84, N 9.08, S 20.79;
found C 62.38, H 8.07, N 9.22, S 20.86.
1,3-Bis(2,4,6-trimethylphenyl)imidazolium-2-dithiocarboxylate
(IMes·CS2): Dark purple crystals (from MeCN). Yield 55 %
1
(1.06 g); m.p. 318 °C (dec.). H NMR (400 MHz, [D6]DMSO): δ =
2.23 (s, 12 H, ortho-CH3), 2.28 (s, 6 H, para-CH3), 7.03 (s, 4 H,
meta-CH), 7.84 (s, 2 H, =CHN) ppm. 13C NMR (100 MHz, [D6]-
DMSO): δ = 18.1 (ortho-CH3), 20.5 (para-CH3), 121.2 (Im-C4,5), washed with EtOH (5 mL) and dried under high vacuum to afford
129.0 (meta-CH), 131.1 (Car), 135.3 (Car), 139.8 (Car), 146.7 (Im- the title compound as a brick powder (1.43 g, 83%); m.p. 119 °C
C2), 221.6 (CS ) ppm. IR (KBr): ν = 3178 (m), 3154 (m), 2947 (w),
(dec.), ref.[53] 118 °C. 1H NMR (400 MHz, CD2Cl2 + CS2): δ =
˜
2
2918 (m), 1637 (w), 1615 (m), 1563 (m), 1488 (s), 1460 (s), 1379 1.12–1.33 (m, 9 H, Cy), 1.54–1.64 (m, 6 H, Cy), 1.68–1.76 (m, 3
(m), 1223 (s), 1166 (m), 1105 (m), 1073 (m), 1052 (s), 931 (m) cm–1. H, Cy), 1.78–1.86 (m, 6 H, Cy), 1.88–1.97 (m, 6 H, Cy), 2.84–2.94
UV/Vis (CH3CN): λmax (ε) = 213 (31500), 255 (8000), 364 nm
(8300 –1 cm–1). C22H24N2S2 (380.58): calcd. C 69.43, H 6.36, N
7.36, S 16.85; found C 69.21, H 6.41, N 7.50, S 16.79.
(q, 3 H, CHP) ppm. 13C NMR (100 MHz, CD2Cl2 + CS2): δ = 26.1
(Cy-C4), 26.9 (Cy-C3), 27.4 (d, JCP = 11.10 Hz, Cy-C2), 32.6 (d,
JCP = 38.7 Hz, Cy-C1), 225.9 (d, JCP = 31.10 Hz, CS2) ppm. 31P
NMR (101 MHz, CD Cl + CS ): δ = 19 ppm. IR (KBr): ν = 2934
˜
2
2
2
1,3-Bis(2,6-diisopropylphenyl)imidazolium-2-dithiocarboxylate
(IDip·CS2): Orange red crystals (from MeCN). Yield 89% (2.02 g);
m.p. 294 °C (dec.). 1H NMR (400 MHz, CDCl3): δ = 1.15 [d, 3JHH
(s), 2850 (s), 1637 (m), 1446 (m), 1297 (m), 1178 (m), 1053 (s), 1044
(m), 1031 (s), 1002 (m), 918 (w) cm–1. UV/Vis (CH3CN): λmax (ε)
= 221 (6400), 366 nm (2000 –1 cm–1).
3
= 8.0 Hz, 12 H, CH(CH3)2], 1.33 [d, JHH = 8.0 Hz, 12 H,
3
CH(CH3)2], 3.00 [sept, JHH = 8.0 Hz, 4 H, CH(CH3)2], 7.01 (s, 2
X-ray Crystal Structure Determinations: Data were collected with
an Enraf–Nonius CAD-4 diffractometer at room temperature. Cell
parameters were determined from 25 well-centered reflections.
Data collection program: CAD4-Mach3, data reduction: Helena,
structure solution: SHELXS, structure refinement: SHELXL-97
(on F2),[54] data analysis: PLATON.[55] Reflections were corrected
for Lorentz and polarization effects. Analytical correction was ap-
plied to correct for absorption effects. The quality of structure for
IMes·CS2 was poor despite efforts to improve its quality. It was,
however, included for comparison.
3
3
H, =CHN), 7.23 (d, JHH = 8.0 Hz, 4 H, meta-CH), 7.43 (t, JHH
= 8.0 Hz, 2 H, para-CH) ppm. 13C NMR (100 MHz, CDCl3): δ =
22.8 (CH3), 26.0 (CH3), 29.5 [(CH3)2CH], 120.6 (Im-C4,5), 124.6
(meta-CH), 130.8 (ipso-C), 131.4 (para-CH), 146.5 (ortho-C), 149.1
(Im-C2), 219.7 ppm. (CS ). IR (KBr): ν = 3163 (w), 3059 (m), 2965
˜
2
(s), 2928 (m), 2866 (m), 1557 (m), 1469 (s), 1383 (m), 1363 (m),
1331 (m), 1210 (m), 1180 (m), 1058 (s), 947 (m) cm–1. UV/Vis
(CH3 CN): λm a x (ε) = 213 (25200), 261 (7900), 364 nm
(8400 –1 cm–1). C28H36N2S2 (464.74): calcd. C 72.36, H 7.81, N
6.03, S 13.80; found C 72.13, H 8.18, N 6.30, S 14.18.
CCDC-645144 (for ICy·CS2), -645145 (for IMes·CS2), -645146 (for
SIMes·CS2), -645147 (for IDip·CS2), -645148 (for SIDip·CS2), con-
tain the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
1,3-Bis(2,4,6-trimethylphenyl)imidazolinium-2-dithiocarboxylate
(SIMes·CS2): Orange crystals (from MeCN). Yield 77% (1.47 g);
1
m.p. 296 °C (dec.). H NMR (400 MHz, CDCl3): δ = 2.19 (s, 6 H,
para-CH3), 2.54 (s, 12 H, ortho-CH3), 4.20 (s, 4 H, CH2), 6.87 (s, 4
H, meta-CH) ppm. 13C NMR (100 MHz, CDCl3): δ = 19.7 (ortho-
CH3), 21.9 (para-CH3), 49.9 (CH2N), 130.7 (meta-CH), 131.6 (ipso-
C), 137.4 (ortho-C), 140.8 (para-C), 165.0 (Im-C2), 222.7
ICy·CS2: Dark red crystals (from EtOH) with dimensions
¯
0.22ϫ0.12ϫ0.05 mm, triclinic, P1, a = 9.699(2), b = 13.440(2), c
= 14.367(4) Å, α = 109.87(2), β = 94.23(1), γ = 100.31(3)°, V =
1714.4(7) Å3, Z = 4, ρcalcd = 1.195 gcm–3, F000 = 664, λ Cu-Kα
= 1.54179 Å, θmax = 75.01°, ω/2θ scan mode, 7043 independent
reflections (Rint = 0.0153), 4263 observed reflections [I Ͼ 2σ(I)], µ
= 2.739 mm–1, Tmin = 0.5840, Tmax = 0.8752, 361 parameters, R1
(all data) = 0.1076, R1 (observed data) = 0.0491, S = GooF = 1.025,
∆/s.u. = 0.000, residual ρmax = 0.302 eÅ–3.
(CS ) ppm. IR (KBr): ν = 2947 (w), 2915 (m), 2855 (w), 1609 (m),
˜
2
1531 (s), 1465 (m), 1377 (m), 1351 (w), 1267 (s), 1211 (w), 1173
(w), 1064 (s), 856 (m) cm–1. UV/Vis (CH3CN): λmax (ε) = 216
(32000), 262 (8400), 362 nm (8200 –1 cm–1). C22H26N2S2 (382.59):
calcd. C 69.07, H 6.85, N 7.32, S 16.76; found C 69.53, H 7.07, N
7.58, S 16.98.
1,3-Bis(2,6-diisopropylphenyl)imidazolinium-2-dithiocarboxylate
IMes·CS2: Dark purple crystals (from MeCN) with dimensions
0.22ϫ0.40ϫ0.42 mm, monoclinic, P2/c, a = 14.854(10), b =
7.810(10), c = 17.565(10) Å, β = 92.3(4)°, V = 2036(3) Å3, Z = 4,
(SIDip·CS2): Orange red crystals (from MeCN). Yield 0.79 g
1
(34%); m.p. 268–269 °C (dec.). H NMR (400 MHz, CDCl3): δ =
3
3
1.28 [d, JHH = 6.4 Hz, 12 H, CH(CH3)2], 1.38 [d, JHH = 6.4 Hz,
12 H, CH(CH3)2], 3.47 [sept, 3JHH = 6.4 Hz, 4 H, CH(CH3)2], 4.41
(s, 4 H, CH2), 7.16 (d, 3JHH = 8.0 Hz, 4 H, meta-CH), 7.31 (t, 3JHH
= 7.2 Hz, 2 H, para-CH) ppm. 13C NMR (100 MHz, CDCl3): δ =
23.8 (CH3), 26.6 (CH3), 29.4 [(CH3)2CH], 51.4 (CH2N), 125.0
(meta-CH), 130.6 (para-CH), 130.7 (ipso-C), 147.5 (ortho-C), 164.2
ρcalcd = 1.241 gcm–3, F000 = 808, λ Cu-Kα = 1.54179 Å, θmax
75.10°, ω/2θ scan mode, 4183 independent reflections (Rint
=
=
0.0405), 3672 observed reflections [I Ͼ 2σ(I)], µ = 2.412 mm–1,
Tmin = 0.4306, Tmax = 0.6189, 243 parameters, R1 (all data) =
0.1197, R1 (observed data) = 0.1103, S = GooF = 2.410, ∆/s.u. =
0.00, residual ρmax = 1.076 eÅ–3.
(Im-C2), 219.8 (CS ) ppm. IR (KBr): ν = 2965 (s), 2923 (m), 2865
˜
2
(m), 1590 (w), 1524 (s), 1502 (sh), 1461 (m), 1441 (m), 1381 (m), IDip·CS2: Orange-red crystals (from MeCN) with dimensions
1356 (m), 1328 (m), 1281 (m), 1191 (m), 1080 (s), 1062 (m), 1045 0.20ϫ0.26ϫ0.26 mm, monoclinic, P21/c, a = 13.222(2), b =
(m), 937 (w) cm–1. UV/Vis (CH3CN): λmax (ε) = 216 (31300), 266
14.944(2), c = 15.124(3) Å, β = 110.8(1)°, V = 2793.6(9) Å3, Z = 4,
Eur. J. Inorg. Chem. 2009, 1882–1891
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
1889

L. Delaude, A. Demonceau, J. Wouters
FULL PAPER
ρcalcd = 1.105 gcm–3, F000 = 1000, λ Cu-Kα = 1.54179 Å, θmax
75.04°, ω/2θ scan mode, 5756 independent reflections (Rint
=
=
[6]
[7]
For reviews, see: a) M. C. Perry, K. Burgess, Tetrahedron:
Asymmetry 2003, 14, 951–961; b) D. Enders, T. Balensiefer,
Acc. Chem. Res. 2004, 37, 534–541; c) V. César, S. Bellemin-
Laponnaz, L. H. Gade, Chem. Soc. Rev. 2004, 33, 619–636.
D. Bourrisou, O. Guerret, F. P. Gabbaï, G. Bertrand, Chem.
Rev. 2000, 100, 39–91.
N. Kuhn, A. Al-Sheikh, Coord. Chem. Rev. 2005, 249, 829–857.
S. Díez-González, S. P. Nolan, Coord. Chem. Rev. 2007, 251,
874–883.
0.0232), 5197 observed reflections [I Ͼ 2σ(I)], µ = 1.837 mm–1,
Tmin = 0.6467, Tmax = 0.7102, 315 parameters, R1 (all data) =
0.0441, R1 (observed data) = 0.0396, S = GooF = 1.035, ∆/s.u. =
0.002, residual ρmax = 0.187 eÅ–3.
[8]
[9]
SIMes·CS2: Orange crystals (from MeCN) with dimensions
0.17ϫ0.20ϫ0.30 mm, orthorhombic, C2221, a = 7.626(1), b =
16.761(2), c = 16.312(2) Å, V = 2085.0(4) Å3, Z = 4, ρcalcd
=
[10]
C. A. Tolman, Chem. Rev. 1977, 77, 313–348.
[11] a) M. M. Rahman, H. Y. Liu, A. Prock, W. P. Giering, Organo-
metallics 1987, 6, 650–658; b) M. M. Rahman, H. Y. Liu, K.
Eriks, A. Prock, W. P. Giering, Organometallics 1989, 8, 1–7; c)
D. White, N. J. Coville, Adv. Organomet. Chem. 1994, 36, 95–
158.
1.219 gcm–3, F000 = 816, λ Cu-Kα = 1.54179 Å, θmax = 74.92°,
ω/2θ scan mode, 1236 independent reflections (Rint = 0.0155), 1106
observed reflections [I Ͼ 2σ(I)], µ = 1.837 mm–1, Tmin = 0.600,
Tmax = 0.743, 122 parameters, R1 (all data) = 0.0586, R1 (observed
data) = 0.0509, S = GooF = 1.102, ∆/s.u. = 0.006, residual ρmax
=
[12]
See for example: a) D. C. Woska, M. Wilson, J. Bartholomew,
K. Eriks, A. Prock, W. P. Giering, Organometallics 1992, 11,
3343–3352; b) A. Demonceau, A. W. Stumpf, E. Saive, A. F.
Noels, Macromolecules 1997, 30, 3127–3136.
0.348 eÅ–3.
SIDip·CS2: Orange-red crystals (from MeCN) with dimensions
0.45ϫ0.35ϫ0.30 mm, orthorhombic, Pnma, a = 12.510(6), b =
[13]
a) J. Huang, E. D. Stevens, S. P. Nolan, J. L. Petersen, J. Am.
Chem. Soc. 1999, 121, 2674–2678; b) J. Huang, H.-J. Schanz,
E. D. Stevens, S. P. Nolan, Organometallics 1999, 18, 2370–
2375; c) L. Jafarpour, E. D. Stevens, S. P. Nolan, J. Organomet.
Chem. 2000, 606, 49–54; d) J. Huang, L. Jafarpour, A. C. Hill-
ier, E. D. Stevens, S. P. Nolan, Organometallics 2001, 20, 2878–
2882; e) L. Jafarpour, S. P. Nolan, J. Organomet. Chem. 2001,
617–618, 17–27; f) A. C. Hillier, W. J. Sommer, B. S. Yong, J. L.
Petersen, L. Cavallo, S. P. Nolan, Organometallics 2003, 22,
4322–4326.
20.498(8), c = 10.544(3) Å, V = 2703.8(18) Å3, Z = 4, ρcalcd
=
1.147 gcm–3, F000 = 1008, λ Cu-Kα = 1.54179 Å, θmax = 74.89°,
ω/2θ scan mode, 2861 independent reflections (Rint = 0.0121), 2596
observed reflections [I Ͼ 2σ(I)], µ = 1.898 mm–1, Tmin = 0.549,
Tmax = 0.731, 122 parameters, R1 (all data) = 0. 0485, R1 (observed
data) = 0.0437, S = GooF = 1.095, ∆/s.u. = 0.001, residual ρmax
=
0.181 eÅ–3.
[14]
R. Dorta, E. D. Stevens, N. M. Scott, C. Costabille, L. Cavallo,
C. D. Hoff, S. P. Nolan, J. Am. Chem. Soc. 2005, 127, 2485–
2495.
W. A. Herrmann, J. Schütz, G. D. Frey, E. Herdtweck, Organo-
metallics 2006, 25, 2437–2448.
L. Mercs, G. N. Labat, A. A. Ehlers, M. Albrecht, Organome-
tallics 2006, 25, 5648–5656.
A. Fürstner, M. Alcarazo, H. Krause, C. W. Lehmann, J. Am.
Chem. Soc. 2007, 129, 12676–12677.
Acknowledgments
Financial support from the European Union through contracts
HPRN-CT-2000-00010 “Polycat” and G5RD-CT-2001-00554
“Dentalopt” is gratefully acknowledged. We thank Mrs. B. Nor-
berg (Facultés Universitaires de Namur, Belgium) for the XRD
analyses and Prof. L. Cavallo (University of Salerno, Italy) for the
[15]
[16]
[17]
[18]
computations of %VBur and a preprint of ref.[49]
.
a) A. R. Chianese, X. Li, M. C. Janzen, J. W. Faller, R. H.
Crabtree, Organometallics 2003, 22, 1663–1667; b) A. R. Chi-
anese, A. Kovacevic, B. M. Zeglis, J. W. Faller, R. H. Crabtree,
Organometallics 2004, 23, 2461–2468; c) G. Altenhoff, R. God-
dard, C. W. Lehmann, F. Glorius, J. Am. Chem. Soc. 2004, 126,
15195–15201; d) S. Leuthäusser, D. Schwarz, H. Plenio, Chem.
Eur. J. 2007, 13, 7195–7203.
a) K. Öfele, W. A. Herrmann, D. Mihalios, M. Elison, E.
Herdtweck, W. Scherer, J. Mink, J. Organomet. Chem. 1993,
459, 177–184; b) M.-T. Lee, C.-H. Hu, Organometallics 2004,
23, 976–983; c) M. Nonnenmacher, D. Kunz, F. Rominger, T.
Oeser, J. Organomet. Chem. 2005, 690, 5647–5653; d) F. E.
Hahn, M. Paas, D. Le Van, R. Fröhlich, Chem. Eur. J. 2005,
11, 5080–5085; e) K. E. Krahulic, G. D. Enright, M. Parvez,
R. Roesler, J. Am. Chem. Soc. 2005, 127, 4142–4143.
a) A. Tudose, A. Demonceau, L. Delaude, J. Organomet.
Chem. 2006, 691, 5356–5365; b) A. Tudose, L. Delaude, B.
André, A. Demonceau, Tetrahedron Lett. 2006, 47, 8529–8533.
H. A. Duong, T. N. Tekavec, A. M. Arif, J. Louie, Chem. Com-
mun. 2004, 112–113.
a) A. M. Voutchkova, L. N. Appelhans, A. R. Chianese, R. H.
Crabtree, J. Am. Chem. Soc. 2005, 127, 17624–17625; b) A. M.
Voutchkova, M. Feliz, E. Clot, O. Eisenstein, R. H. Crabtree,
J. Am. Chem. Soc. 2007, 129, 12834–12846.
a) N. Kuhn, H. Bohnen, G. Henkel, Z. Naturforsch., Teil B
1994, 49, 1473–1480; b) N. Kuhn, E. Niquet, M. Steimann, I.
Walker, Z. Naturforsch., Teil B 1999, 54, 1181–1187.
N. Kuhn, M. Steimann, G. Weyers, Z. Naturforsch., Teil B
1999, 54, 427–433.
[1] a) A. J. Arduengo III, R. L. Harlow, M. Kline, J. Am. Chem.
Soc. 1991, 113, 361–363; b) A. J. Arduengo III, H. V. Ra-
sika Dias, R. L. Harlow, M. Kline, J. Am. Chem. Soc. 1992,
114, 5530–5534; c) A. J. Arduengo III, J. R. Goerlich, R. Kraf-
czyk, W. J. Marshall, Angew. Chem. Int. Ed. 1998, 37, 1963–
1965; d) A. J. Arduengo III, Acc. Chem. Res. 1999, 32, 913–
921.
[19]
[2] For monographs, see: a) G. Bertrand (Ed.), Carbene Chemistry:
From Fleeting Intermediates to Powerful Reagents, Marcel
Dekker, New York, 2002; b) S. P. Nolan (Ed.), N-Heterocyclic
Carbenes in Synthesis, Wiley-VCH, Weinheim, 2006; c) F. Glor-
ius (Ed.), N-Heterocyclic Carbenes in Transition Metal Cataly-
sis, Topics in Organometallic Chemistry, vol. 21, Springer,
Berlin, 2007.
[20]
[3] V. Nair, S. Bindu, V. Sreekumar, Angew. Chem. Int. Ed. 2004,
43, 5130–5135.
[4] D. Enders, O. Niemeier, A. Henseler, Chem. Rev. 2007, 107,
5606–5655; N. Marion, S. Díez-González, S. P. Nolan, Angew.
Chem. Int. Ed. 2007, 46, 2988–3000.
[21]
[22]
[5] For reviews, see: a) W. A. Herrmann, C. Köcher, Angew. Chem.
Int. Ed. Engl. 1997, 36, 2162–2187; b) L. Jafarpour, S. P. Nolan,
Adv. Organomet. Chem. 2001, 46, 181–222; c) W. A. Herrmann,
Angew. Chem. Int. Ed. 2002, 41, 1290–1309; d) W. A. Herrm-
ann, T. Weskamp, V. P. W. Böhm, Adv. Organomet. Chem. 2002,
48, 1–69; e) C. M. Crudden, D. P. Allen, Coord. Chem. Rev.
2004, 248, 2247–2273; f) N. M. Scott, S. P. Nolan, Eur. J. Inorg.
Chem. 2005, 1815–1828; g) I. Dragutan, V. Dragutan, L. De-
laude, A. Demonceau, Arkivoc 2005, 10, 206–253; h) J. C. Gar-
rison, W. J. Youngs, Chem. Rev. 2005, 105, 3978–4008; i) F. E.
Hahn, M. C. Jahnke, Angew. Chem. Int. Ed. 2008, 47, 3122–
3172.
[23]
[24]
[25]
a) L. Jafarpour, A. C. Hillier, S. P. Nolan, Organometallics
2002, 21, 442–444; b) M. Alcarazo, S. J. Roseblade, E. Alonso,
R. Fernandez, E. Alvarez, F. J. Lahoz, J. M. Lassaletta, J. Am.
1890
www.eurjic.org
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2009, 1882–1891

Zwitterionic NHC·CS2 Adducts
Chem. Soc. 2004, 126, 13242–13243; c) K. Vehlow, S. Maech-
ling, S. Blechert, Organometallics 2006, 25, 25–28.
[39] a) H. Jacobsen, A. Correa, C. Costabille, L. Cavallo, J. Or-
ganomet. Chem. 2006, 691, 4350–4358; b) R. Tonner, G. Heyd-
enrych, G. Frenking, Chem. Asian J. 2007, 2, 1555–1567.
[40] a) S. Fantasia, J. L. Petersen, H. Jacobsen, L. Cavallo, S. P. No-
lan, Organometallics 2007, 26, 5580–5889; b) D. M. Khramov,
V. M. Lynch, C. W. Bialewski, Organometallics 2007, 26, 6042–
6049; c) R. A. Kelly III, H. Clavier, S. Giudice, N. M. Scott,
E. D. Stevens, J. Bordner, I. Samardjiev, C. D. Hoff, L. Cavallo,
S. P. Nolan, Organometallics 2008, 27, 202–210.
[41] a) F. Ibáñez, J. G. Santos, J. Chem. Soc. Perkin Trans. 2 1984,
1323–1326; b) T. Campino, J. G. Santos, F. Ibáñez, J. Chem.
Soc. Perkin Trans. 2 1986, 1021–1024.
[42] J. D. Holbrey, W. M. Reichert, I. Tkatchenko, E. Bouajila, O.
Walter, I. Tommasi, R. D. Rogers, Chem. Commun. 2003, 28–
29.
[26]
a) S. Lee, J. F. Hartwig, J. Org. Chem. 2001, 66, 3402–3415; b)
M. S. Viciu, G. A. Grasa, S. P. Nolan, Organometallics 2001,
20, 3607–3612; c) G. A. Grasa, M. S. Viciu, J. Huang, S. P. No-
lan, J. Org. Chem. 2001, 66, 7729–7737; d) L. Delaude, M.
Szypa, A. Demonceau, A. F. Noels, Adv. Synth. Catal. 2002,
344, 749–756; e) Y. Ma, C. Song, W. Jiang, Q. Wu, Y. Wang,
X. Liu, M. B. Andrus, Org. Lett. 2003, 5, 3317–3319; f) O. Nav-
arro, H. Kaur, P. Mahjoor, S. P. Nolan, J. Org. Chem. 2004, 69,
3173–3180.
a) A. Aidouni, A. Demonceau, L. Delaude, Synlett 2006, 493–
495; b) A. Fürstner, M. Alcarazo, V. César, C. W. Lehmann,
Chem. Commun. 2006, 2176–2178; c) E. A. Mistryukov, Mend-
eleev Commun. 2006, 16, 258–259; d) L. Hintermann, Beilstein
J. Org. Chem. 2007, 3, 22; e) K. M. Kuhn, R. H. Grubbs, Org.
Lett. 2008, 10, 2075–2077; f) A. Aidouni, S. Bendahou, A. De-
monceau, L. Delaude, J. Comb. Chem. 2008, 10, 886–892.
a) H. E. Winberg, D. D. Coffman, J. Am. Chem. Soc. 1965, 87,
2776–2777; b) W. S. Sheldrick, A. Schönberg, E. Singer, P. Eck-
ert, Chem. Ber. 1980, 113, 3605–3609; c) W. Krasuski, D. Niko-
laus, M. Regitz, Liebigs Ann. Chem. 1982, 1451–1465.
G. W. Nyce, S. Csihony, R. M. Waymouth, J. L. Hedrick,
Chem. Eur. J. 2004, 10, 4073–4079.
a) R. W. Alder, P. R. Allen, S. J. Williams, J. Chem. Soc., Chem.
Commun. 1995, 1267–1268; b) Y.-J. Kim, A. Streitwieser, J. Am.
Chem. Soc. 2002, 124, 5757–5761; c) T. L. Amyes, S. T. Diver,
J. P. Richard, F. M. Rivas, K. Toth, J. Am. Chem. Soc. 2004,
126, 4366–4374; d) A. M. Magill, K. J. Cavell, B. F. Yates, J.
Am. Chem. Soc. 2004, 126, 8717–8724; e) H. Chen, D. R.
Justes, R. G. Cooks, Org. Lett. 2005, 7, 3949–3952.
[27]
[28]
[43] J. J. Vittal, P. A. W. Dean, Acta Crystallogr., Sect. C 1997, 53,
1879–1881.
[44] F. H. Allen, D. G. Watson, L. Brammer, A. G. Orpen, R. Tay-
lor in International Tables for Crystallography (Ed.: E. Prince),
Springer, Berlin, 2006, vol. C, pp. 790–811.
[45] a) M. L. Ziegler, H. Weber, B. Nuber, O. Serhadle, Z. Natur-
forsch., Teil B 1987, 42, 1411–1418; b) A. Nagasawa, I. Aki-
yama, S. Mashima, J. Nakayama, Heteroat. Chem. 1995, 6, 45–
49; c) T. Fujihara, T. Ohba, A. Nagasawa, J. Nakayama, K.
Yoza, Acta Crystallogr., Sect. C 2002, 58, o558–o559.
[46] a) L. L. Borer, J. V. Kong, D. E. Oram, Acta Crystallogr., Sect.
C 1989, 45, 1169–1170; b) M. Akkurt, S. Öztürk, H. Küçükbay,
E. Orhan, O. Büyülgüngör, Acta Crystallogr., Sect. E 2004, 60,
o219–o221.
[47] L. Cavallo, A. Correa, C. Costabille, H. Jacobsen, J. Or-
ganomet. Chem. 2005, 690, 5407–5413.
[48] A. Poater, F. Ragone, S. Giudice, C. Costabile, R. Dorta, S. P.
Nolan, L. Cavallo, Organometallics 2008, 27, 2679–2681.
[49] A. Poater, B. Cosenza, A. Correa, S. Giudice, F. Ragone, V.
Scarano, L. Cavallo, Eur. J. Inorg. Chem. 2009, 1759–1766; this
issue.
[50] a) A. Fürstner, L. Ackermann, B. Gabor, R. Goddard, C. W.
Lehmann, R. Mynott, F. Stelzer, O. R. Thiel, Chem. Eur. J.
2001, 7, 3236–3253; b) M. Barbasiewicz, M. Bieniek, A. Mich-
rowska, A. Szadkowska, A. Makal, K. Wozniak, K. Grela,
Adv. Synth. Catal. 2007, 349, 193–203.
[51] W. A. Herrmann, C. Köcher, L. J. Goossen, G. R. J. Artus,
Chem. Eur. J. 1996, 2, 1627–1636.
[52] A. J. Arduengo III, R. Krafczyk, R. Schmutzler, H. A. Craig,
J. R. Goerlich, W. J. Marshall, M. Unverzagt, Tetrahedron
1999, 55, 14523–14534.
[29]
[30]
[31]
a) A. W. Hofmann, Liebigs Ann. Chem. Suppl. 1 1861, 1–61; b)
A. W. Hofmann, Ber. Dtsch. Chem. Ges. 1880, 13, 1732–1737.
[32] A. Galindo, D. Miguel, J. Perez, Coord. Chem. Rev. 1999, 193–
195, 643–690.
[33] a) G. R. Clark, T. J. Collins, S. M. James, W. R. Roper, K. G.
Town, J. Chem. Soc., Chem. Commun. 1976, 475–476; b) S. M.
Boniface, G. R. Clark, J. Organomet. Chem. 1980, 188, 263–
275.
[34] P. K. Baker, K. Broadley, N. G. Connelly, J. Chem. Soc., Dalton
Trans. 1982, 471–475.
[35] W. Krasuski, D. Nikolaus, M. Regitz, Liebigs Ann. Chem. 1982,
1451–1465.
[36] J. Nakayama, T. Kitahara, Y. Sugihara, A. Sakamoto, A. Ishii,
J. Am. Chem. Soc. 2000, 122, 9120–9126.
[37] I. S. Butler, J. Svedman, Spectrochim. Acta A 1979, 35, 425–
426.
[53] K. Issleib, A. Brack, Z. Anorg. Allg. Chem. 1954, 277, 258–270.
[54] G. M. Sheldrick, SHELXL-97, Program for the Refinement of
Crystal Structures, University of Göttingen, Germany, 1997.
[55] A. L. Speck, PLATON, A Multipurpose Crystallographic Tool,
Utrecht University, The Netherlands, 2005.
[38] K. A. Jensen, P. H. Nielsen, Acta Chem. Scand. 1963, 17, 547–
548.
Received: November 13, 2008
Published Online: February 17, 2009
Eur. J. Inorg. Chem. 2009, 1882–1891
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
1891