Metal-assisted conversion of an N-ylide mesomeric betaine into its carbenic tautomer: Generation of N-(fluoren-9-yl)imidazol-2-ylidene complexes

Article abstract of DOI:10.1039/c3dt53089b

Whereas the N-ylide mesomeric betaine 2, consisting of a fluorenyl anion directly attached to an imidazolium ring, is not in equilibrium with its putative free N-(fluoren-9-yl)imidazol-2-ylidene tautomer, its reaction with a metallic centre induces its interconversion to yield the corresponding monoligated N-heterocyclic carbene complex (Au(i) and Rh(i)). Deprotonation of 2 and coordination to the Rh^I(COD) fragment allows the isolation of complex 7 displaying a rarely observed four-membered NHC-containing metallacycle and an enforced η¹-fluorenyl ligand. Upon reaction with CpFe(CO)₂I precursor, insertion of a carbonyl ligand into the Fe-fluorenyl bond occurs and yields the acyl-Fe complex 8.

Full text of DOI:10.1039/c3dt53089b

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Metal-assisted conversion of an N-ylide
mesomeric betaine into its carbenic tautomer:
generation of N-(fluoren-9-yl)imidazol-2-ylidene
complexes†
Cite this: Dalton Trans., 2014, 43,
4474
Laure Benhamou,a,b Stéphanie Bastin,a,b Noël Lugan,a,b Guy Lavigne*a,b and
Vincent César*a,b
Whereas the N-ylide mesomeric betaine 2, consisting of a fluorenyl anion directly attached to an imid-
azolium ring, is not in equilibrium with its putative free N-(fluoren-9-yl)imidazol-2-ylidene tautomer, its
reaction with a metallic centre induces its interconversion to yield the corresponding monoligated
N-heterocyclic carbene complex (Au(I) and Rh(I)). Deprotonation of 2 and coordination to the RhI(COD)
fragment allows the isolation of complex 7 displaying a rarely observed four-membered NHC-containing
metallacycle and an enforced η1-fluorenyl ligand. Upon reaction with CpFe(CO)2I precursor, insertion of
a carbonyl ligand into the Fe–fluorenyl bond occurs and yields the acyl–Fe complex 8.
Received 31st October 2013,
Accepted 3rd December 2013
DOI: 10.1039/c3dt53089b
www.rsc.org/dalton
betaines, defined long ago as neutral compounds which can
exclusively be represented by dipolar canonical formulas, in
Introduction
The growing success story of N-heterocyclic carbenes (NHCs)1,2 which the positive and negative charges are delocalized within
during the past two decades is certainly due to the breadth of a common π-electron system.14,15 In particular, our previously
their applicability to various domains of chemistry. Indeed, reported mesoionic compounds A and B both belong to the
while being extensively developed as highly donating and steri- subclass of conjugated mesomeric betaines (CMB), and we
cally protecting ligands in organometallic chemistry and cata- have shown that the imidazolium-4-aminide B is in tautomeric
lysis,3 they are also of high interest in their own right as equilibrium with the free 4-(isopropylamino)imidazol-2-
organocatalysts4 and as stabilizing agents for elusive poly- ylidene B′ (Scheme 1).16 Schmidt reported very recently an
atomic main group allotropes.5 The advent of well established
synthetic methods for these NHCs, allowing
a facile
functionalization of their heterocyclic framework,6 has been
beneficial to all these developments, giving access to, inter
alia, chiral,7 electronically tunable,8 or polydentate/polytopic9
carbenic architectures.
In this respect, we are interested in the conceptual design
of backbone-functionalized NHCs combining
carbene unit with a functional organic backbone such as malo-
nate,10 imidate,11 enolate12 or enamine.13
recurrent
a diamino-
A
characteristic of these carbenic structures, potentially useful in
a retrosynthetic approach, is their relationship with mesomeric
aCNRS, LCC (Laboratoire de Chimie de Coordination), 205 route de Narbonne,
F-31077 Toulouse Cedex 4, France. E-mail: vincent.cesar@lcc-toulouse.fr,
guy.lavigne@lcc-toulouse.fr
bUniversité de Toulouse, UPS, INPT, 31077 Toulouse, France
†Electronic supplementary information (ESI) available: Crystallographic data
and NMR spectra of all new compounds. CCDC 969666–969669. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c3dt53089b
Scheme 1 Conjugated mesomeric betaines A and B, in equilibrium
with the free NHC tautomer B’ and relevant system C/C’ considered
here.
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(imidazolium)indolate, existing in equilibrium with its free or KOtBu and/or operating at higher temperature were less
NHC tautomer and belonging to the subclass of N-ylides efficient, leading to the desired compound, albeit with con-
related to the CMB.14a With the aim of expanding this growing siderable amounts of impurities. Under our optimized con-
family of compounds, we thus became interested in studying ditions, the mesomeric betaine 2 was isolated as an intensively
the chemistry of the N-ylide C, constituted by a fluorenide orange-coloured powder in excellent yield (91%), and was
anion directly linked (through the C9 position) to a nitrogen of found to be extremely air-sensitive and well-soluble only in
the imidazolium ring, and its possible interconversion with its CH2Cl2. Its formulation as an imidazolium–fluorenide betaine
free NHC tautomeric form C′. A closely related system, albeit was inferred from the disappearance of the signal corres-
with an ethylene linker, has been previously reported by Dano- ponding to the proton on position 9 of the fluorenyl group in
poulos and colleagues and shown to involve an equilibrium the 1H NMR spectrum and from the occurrence of a set of
between the zwitterionic imidazolium–fluorenide form and three signals at δ = 8.12, 7.94 and 7.22 ppm, being character-
the free neutral (fluorenylethyl)imidazol-2-ylidene.17
istic of the three protons of the imidazolium ring. In the 13C
Herein, we describe the synthesis and reactivity of an arche- NMR spectrum, the quaternary sp2-type C9 carbon atom of the
type of 9-(imidazolium-3-yl)fluorenide, belonging to the fluorenide unit appeared to be relatively deshielded at δ =
N-ylide subclass of mesomeric betaines. We also show that, 92.5 ppm with respect to the sp3-hybridized CH carbon atom
even though it does not interconvert into its free NHC tauto- in 1 (δ = 63.3 ppm). Most importantly, we performed a VT
mer, it constitutes a suitable precursor to a complexed form of NMR experiment to determine if the betaine 2 might be in
1
the latter with various transition metals. In addition, the equilibrium with its free NHC tautomer. Clearly, the H NMR
coordination behaviour of the fully-deprotonated anionic spectra recorded in the −70 °C–25 °C range, revealed no signal
N-(fluorenide)imidazol-2-ylidene is reported and leads to the broadening, thereby indicating that no equilibrium with the
observation of a chelating mode for this new anionic bidentate free NHC form was taking place.
ligand.
Further evidence for the total absence of a free NHC was
provided by reacting compound 2 with sulphur or carbon di-
sulfide – two classical trapping reagents of a free NHC – both
reactions yielding only intractable mixtures and decompo-
sition products. Moreover, the pronounced ylidic character of
the betaine 2 could be illustrated by its reaction with methyl
iodide, leading to rapid and selective methylation at the fluore-
nyl position to yield cleanly the imidazolium iodide 3.
We next turned our attention toward a possible use of the
betaine 2 as a precursor for the synthesis of N-heterocyclic
carbene complexes, and started our investigations with a
gold(I) precursor (Scheme 3). To our delight, addition of 1 equi-
valent of AuCl(tht) to a solution of 2 in CH2Cl2 at −60 °C fol-
lowed by warming to room temperature yielded the desired
NHC–gold(I) complex 4 in good yield (67%). Its identity was
established by the 1H and 13C NMR spectra, revealing the pres-
ence, respectively, of a singlet at δ = 7.02 ppm corresponding
to the fluorenyl-proton, and of a signal at δ = 173.2 ppm
typical of an AuCl-ligated imidazol-2-ylidene.18 The connec-
tivity in 4 was also firmly confirmed by the analysis of its
solid-state molecular structure (Fig. 1), in which the Au–C dis-
tance [1.987(4) Å] falls into the normal range of Au–imidazoly-
lidene bond lengths.
Results and discussion
The N-(fluoren-9-yl)imidazolium bromide 1 was prepared in
80% yield by quaternization of 1-mesitylimidazole with 9
bromofluorene upon heating the two reagents in toluene
(Scheme 2). The ionic compound 1 was seen to precipitate
along the reaction course and was subsequently purified by
simple washings with a THF–Et2O mixture. Mono-deprotona-
tion of 1 cleanly took place with 1 equiv. of potassium bis(tri-
methylsilyl)amide (KHMDS) in THF at low temperature.
Noticeably, alternate attempts to use other bases such as DBU
In order to gain more insight into the mechanism of for-
mation of complex 4, a low temperature NMR experiment was
carried out aiming at detecting any intermediate. Thus,
AuCl(tht) was added at low temperature to a solution of 2 in
CD2Cl2 and the 1H and 13C NMR spectra of the reaction
mixture were directly recorded at −60 °C (see the ESI†). They
were both found to be fully consistent with the clean and
almost quantitative formation of the zwitterionic complex 4′,
where the AuCl unit is linked to the fluorenyl carbon atom
forming an anionic alkyl–gold(I) complex, placed in close
proximity of the cationic imidazolium ring. Diagnostic signals
for such a formulation consist of a singlet at δ = 9.36 ppm and
Scheme 2 Synthesis and reactivity of the mesomeric betaine 2.
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Fig. 2 Molecular structure of complex 5 (ellipsoids drawn at 30% prob-
ability level). Solvent molecule and hydrogen atoms except on C21 have
been omitted for clarity. Selected bond length [Å]: Rh1–C1 2.043(3).
Selected bond angles [°]: N2–C21–C22 114.3(2), N2–C21–C33 111.6(2),
C22–C21–C33 102.4(2). Selected torsion angle [°]: N1–C1–Rh1–Cl1
115.07.
fully characterized spectroscopically and analytically, including
by an X-ray diffraction analysis (Fig. 2). In order to establish
the transition through the intermediate alkyl–rhodium(I)
complex 5′, the same low-temperature NMR experiment as for
4′ was carried out. Despite being stable for several hours at
−80 °C, complex 5′ was seen to interconvert rapidly into its
NHC–rhodium(I) tautomer 5 at slightly higher temperatures,
since one half of complex 5′ was transformed into 5 during the
time of introduction of the NMR tube in the spectrometer (see
the ESI†). Nevertheless, its nature was firmly confirmed by a
complete NMR analysis and especially by the presence of
broad signals at δ = 9.18 ppm and δ = 137.4 ppm in the 1H and
13C NMR spectra respectively corresponding to the N2CH
proton and N2CH carbon of the imidazolium ring and by the
appearance of a doublet at δ = 73.8 ppm (1JRh–C = 20.1 Hz)
characteristic of the coordination of the fluorenyl C9 onto the
Rh(I) center. Although not proven, two plausible alternative
mechanistic pathways can be reasonably proposed for this
interconversion. The first one may proceed via a two-step
sequence consisting of (i) a protonolysis of the alkyl–metal
bond by the acidic proton of the imidazolium,19 in situ gener-
ating the carbene, and (ii) the immediate trapping of the latter
by the neighbouring metal centre. The alternate possibility
would involve an intramolecular proton-transfer between the
imidazolyl and the fluorenyl carbon atoms, in a manner
similar to that operating in the exchange between sp2- and sp3-
hybridized carbon atoms on Pd(II) and Pt(II) centers.20 This
process may occur through the σ-CAM mechanism proposed
by Perutz and Sabo-Etienne, although the intermediacy of a
Rh(III) intermediate through an oxidative addition–reductive
elimination sequence cannot be totally excluded in the
rhodium case.21
Scheme 3 Complexation of the betaine 2 with Au(I) and Rh(I) centres
leading to NHC-complexes
tetrahydrothiophene).
4
and
5
respectively (tht
=
Fig. 1 Molecular structure of 4 (ellipsoids drawn at 30% probability
level). All hydrogen atoms except on C21 have been omitted for clarity.
Selected bond lengths [Å]: Au1–C1 1.987(4), Au1–Cl1 2.2794(10).
Selected bond angle [°]: C1–Au1–Cl1 177.17(12).
a signal at δ = 138.3 ppm in the 1H and 13C NMR spectra
respectively, corresponding to the pre-carbenic position of the
imidazolium ring. In addition, we were able to detect in the
solution the presence of free tetrahydrothiophene, resulting
from its displacement by the fluorenide ligand. Complex 4′
was found to isomerise cleanly into the NHC-complex 4 upon
warming the NMR tube up to room temperature.
More classically, the imidazolylidene–silver(I) complex 6
could be obtained directly from the imidazolium salt 1 using
the well-known procedure first described by Lin,22 employing
silver oxide (Scheme 4). Its molecular structure is depicted in
The reaction between the betaine 2 and half an equivalent
of [RhCl(COD)]2 was found to be very rapid at room tempera-
ture and gave the expected imidazolylidene–rhodium(I)
complex 5 in excellent yield (96%) (Scheme 3). The latter was
4476 | Dalton Trans., 2014, 43, 4474–4482
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Scheme 4 Synthesis of the silver(I) complex 6.
Scheme 5 Coordination behaviour of the anionic N-(fluorenide)imid-
azol-2-ylidene ligand toward Rh(I) and Fe(II) centres.
Fig. 3 Molecular structure of complex
6
(molecule A,23 ellipsoids
drawn at 30% probability level). All hydrogen atoms except on C21a have
been omitted for clarity. Selected bond length [Å]: Ag1a–C1a 2.103(4).
Selected bond angles [°]: C1a–Ag1a–Br1a 172.76(9), N2a–C21a–C22a
111.5(3), N2a–C21a–C33a 114.9(3), C22a–C21a–C33a 102.7(3).
Fig. 3. As expected, we observed that complex 6 functions as a
carbene-transfer agent with [RhCl(COD)]2 to form complex 5
albeit with ca. 5% contamination by its bromide-analogue.
The coordination properties of the anionic N-(fluorenide)-
imidazol-2-ylidene was then investigated, as the strongly
binding NHC ligand might influence the coordination mode
(η1, η3 or η5) of the fluorenyl anion,24 as previously observed by
Bourissou and colleagues with a phosphazene side arm on the
fluorenyl.25 Hence, after a double deprotonation, the imid-
azolium bromide 1 was reacted with 0.5 equivalent of [RhCl-
(COD)]2 at low temperature (Scheme 5). The resulting complex
7 was isolated in 23% yield after separation of the inorganic
salts and crystallisation. The low isolated yield can be ascribed
to the high solubility of complex 7 in organic solvents such as
toluene or Et2O, making less efficient the purification process.
Fig. 4 Molecular structure of the rhodium(I) complex
7 (ellipsoids
drawn at 30% probability level). Hydrogen atoms have been omitted for
clarity. Selected bond lengths [Å]: Rh1–C1 2.037(2), Rh1–C21 2.2148(18).
Selected bond angles [°]: C1–Rh1–C21 65.26(7), N1–C1–Rh1 156.70(14),
N2–C1–Rh1 97.60(13), N2–C21–C22 123.74(16), N2–C21–C33
118.14(17), C22–C21–C33 105.65(16).
metallacycle.26 The rhodium(I) centre adopts a distorted
square-planar geometry, with a (κ-C,κ-C) metal coordination
and an extraordinarily small bite angle [C1–Rh1–C21
65.26(7)°]. The fluorenyl moiety effectively coordinates the
Rh(I) centre in an η1-fashion, substantiated by the Rh1–C21
bond length [2.2148(18) Å] comparable to the one previously
reported by Bourissou (Rh–C 2.188(3) Å), by the significant pyr-
amidalization of the C21 atom (∑C21α = 347.53°) and by the
remoteness of the other carbons (C22 and C33 in particular)
from the Rh centre (>3 Å).
An interesting reactivity was observed when this anionic
N-(fluorenide)imidazol-2-ylidene was reacted with the carbonyl
complex CpFe(CO)2I (Scheme 5). Indeed, the presence of only
one νCuO stretching band at 1923 cm−1 in the IR spectrum of
the crude product (recorded in THF) clearly suggested that
only one carbonyl ligand remains coordinated to the iron
centre. The appearance of a band at 1669 cm−1 typical of a
νCvO stretching band provided conclusive evidence that the
1
The H NMR spectrum of 7 featuring a symmetry-plane in the
molecule was in agreement with the occurrence of a bidentate
coordination mode for the anionic NHC ligand, and the latter
was definitely established from the 13C NMR spectrum, in
which the N2C carbene and the fluorenyl C9 carbon atoms
resonate as doublets at δ = 160.8 ppm (1JRh–C = 44.3 Hz) and
δ = 63.5 ppm (1JRh–C = 10.7 Hz) respectively. This spectrum pro-
vided also a first evidence for the η1-coordination mode of the
fluorenyl ligand as the other quaternary carbon atoms appear
as singlets. This was unambiguously confirmed by the solid-
state molecular structure of 7 (Fig. 4). Complex 7 represents
a
rare example of an NHC-containing four-membered
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second carbonyl had been inserted into the transient Fe–fluor-
1-Mesityl-3-(fluoren-9-yl)imidazolium bromide (1). A solu-
enyl bond. This was firmly confirmed in particular by the 13C tion of 1-mesitylimidazole (2.16 g, 11.6 mmol) and 9-bromo-
NMR spectrum revealing a highly-deshielded signal at δ = fluorene (3.13 g, 12.8 mmol, 1.1 eq.) in toluene (50 mL) was
257.8 ppm corresponding to the carbon atom of an iron- stirred at 100 °C for 3 days. A white solid precipitated along
bound acyl group.
the reaction course. After cooling to room temperature, the
reaction mixture was poured into Et2O (70 mL), filtered
through a fritted funnel and the solid was washed several
times with a THF–Et2O mixture (1/5 v/v, 30–40 mL) until the
solution became clear (usually 3 times). After drying, the pure
product was isolated as an off-white powder (4.0 g, 80%). mp =
267–268 °C; 1H NMR (400 MHz, CDCl3): δ = 11.43 (s, 1H,
N2CH), 7.83 (s, 1H, CHFlu-9), 7.77 (d, J = 7.7 Hz, 2H, CHFlu),
7.64 (d, J = 7.6 Hz, 2H, CHFlu), 7.51–7.47 (m, 2H, CHFlu),
7.37–7.33 (m, 2H, CHFlu), 7.03 (br, 3H, CHMes + CHIm), 6.84 (t,
J = 1.8 Hz, 1H, CHIm), 2.36 (s, 3H, CH3 para), 2.14 (s, 6H, CH3
ortho); 13C{1H} NMR (105.5 MHz, CDCl3): δ = 141.5, 140.9, 140.0
Conclusions
The existence of known cases where a conjugated mesomeric
betaine can be visualized as a masked carbene through a
simple tautomeric H-shift is an interesting source of inspi-
ration for the conceptual design of new carbenic architectures.
In our initial attempts to apply this to 9-(imidazolium-3-yl)-
fluorenide, readily available in high yield by simply grafting a
fluorenyl arm to 1-mesitylimidazole, we were first slightly fru-
strated by the fact that such a compound exists only in the
form of the N-ylide mesomeric betaine. Gratifyingly, however,
we were led to observe that it reacts spontaneously at low temp-
erature with Au(I) and Rh(I) precursors to afford primarily new
alkyl complexes in which the N-ylide is first bound through
the fluorenide C9, and subsequently undergoes a “metal-
assisted” isomerisation leading to an N-(fluoren-9-yl)imidazol-
2-ylidene complex. Further de-protonation of the fluorenyl side
arm can be made to occur in specific cases, as illustrated by
the generation of a rhodium complex in which the anionic
N-(fluoren-9-ide)imidazol-2-ylidene is forming an uncommon
four-membered chelate through its coordination via the carbe-
nic center and the alkyl center. Not surprisingly, CO insertion
in the metal–alkyl bond was also observed when the ligand
was reacted with carbonyl derivatives such as CpFe(CO)2I. On
account of these observations, it may be anticipated that the
N-(fluoren-9-ide)imidazol-2-ylidene ligand will have a promis-
ing future for the design of a variety of new transition-metal
pre-catalysts.
(CAr), 139.5 (N2CH), 134.1 (CAr), 130.5, 130.0, 128.9 (2 CHFlu
+
CHMes), 125.7 (CHFlu), 123.3 (CHIm), 120.7 (CHFlu), 120.0
(CHIm), 63.2 (CHFlu-9), 21.1 (CH3 para), 17.7 (CH3 ortho); IR (ATR):
ν = 3024, 2947, 2897, 1606, 1559, 1537, 1482, 1448, 1378, 1275,
1199, 1149, 1058, 1030, 850, 747 cm−1; MS (ESI): m/z (%): 351
(100) [M − Br]+, 165 (30) [fluorenyl]+; elemental analysis calcd
(%) for C25H23BrN2 (MW = 431.37): C 69.61, H 5.37, N 6.49;
found: C 69.17, H 5.23, N 6.35.
1-Mesityl-3-(fluorenide-9-yl)imidazolium (2). A solution of
KHMDS (0.5 M in toluene, 4.86 mL, 2.44 mmol, 1.05 eq.) was
added dropwise to a suspension of 1 (1.0 g, 2.32 mmol) in
THF (20 mL) at −50 °C. After 1 hour, the cooling bath was
removed and the mixture was allowed to warm up to room
temperature. Volatiles were removed under vacuum and the
orange crude product was taken up in CH2Cl2 (15 mL) and fil-
tered through Celite. After evaporation, the orange solid was
washed with pentane (2 × 5 mL) and Et2O (5 mL) to yield an
orange powder after drying (740 mg, 91%). 1H NMR (400 MHz,
CD2Cl2): δ = 8.28 (s, 1H, N2CH), 8.12 (d, J = 7.8 Hz, 2H, CHFlu),
7.94 (s, 1H, CHIm), 7.50 (d, J = 8.2 Hz, 2H, CHFlu), 7.25–7.20 (m,
2H, CHFlu), 7.22 (s, 1H, CHIm), 7.13 (s, 2H, CHMes), 6.87–6.83
(m, 2H, CHFlu), 2.42 (s, 3H, CH3 para), 2.19 (s, 6H, CH3 ortho);
13C{1H} NMR (105.5 MHz, CD2Cl2): δ = 141.4, 135.5, 132.3,
131.8 (CAr), 130.2 (CHMes), 129.4 (N2CH), 124.4 (CHFlu), 122.8
(CHIm), 122.6 (CHFlu), 122.0 (CAr), 120.1 (CHIm), 112.9 (CHFlu),
Experimental section
General considerations
All manipulations were performed under an inert atmosphere 111.0 (CHFlu), 92.5 (C−Flu-9), 21.5 (CH3 para), 17.8 (CH3 ortho); IR
of dry nitrogen using a standard vacuum line and Schlenk (ATR): ν = 3155, 3111, 3034, 2972, 2916, 1608, 1574, 1528,
tube techniques. Glassware was dried at 120 °C in an oven for 1474, 1441, 1322, 1222, 1140, 1113, 1093, 1059, 1028, 986, 850,
at least three hours. THF and diethyl ether were distilled from 742, 721, 666 cm−1
sodium/benzophenone, toluene from sodium. Pentane,
.
1-Mesityl-3-(9-methylfluoren-9-yl)imidazolium iodide (3). At
dichloromethane and chloroform were dried over CaH2 and room temperature, methyl iodide (33 μL, 0.52 mmol, 1.5 eq.)
subsequently distilled. NMR spectra were recorded on Bruker was added to a solution of the zwitterion 2 (123 mg,
ARX250, AV300 or AV400 spectrometers. Chemical shifts are 0.350 mmol) in DMF (2 mL). The reaction was monitored by
reported in ppm (δ) compared to TMS (1H and 13C) using the 1H NMR and quantitative conversion was observed after 5 min
residual peak of a deuterated solvent as the internal stan- of reaction. All volatiles were then evaporated and the residue
dard.27 Infrared spectra were obtained on a Perkin-Elmer Spec- was washed with Et2O (3 × 5 mL) in air and using an ultrasonic
trum 100 FT-IR spectrometer. MS spectra were performed by bath. Further purification by flash chromatography (SiO2,
the mass spectrometry service of the “Institut de Chimie de CH2Cl2–MeOH: 95/5) furnished the product in pure analytical
Toulouse”. CpFe(CO)2I28 and N-mesitylimidazole29 were syn- form as a very pale beige powder (151 mg, 88%). 1H NMR
thesized following literature procedures.
(300 MHz, CDCl3): δ = 10.47 (s, 1H, N2CH), 7.79–7.75 (m, 4H,
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3 CHFlu + CHIm), 7.54–7.49 (m, 2H, CHFlu), 7.44–7.39 (m, 2H, at room temperature. The reaction was complete after less
CHFlu), 7.10 (br, 1H, CHIm), 7.09–7.08 (m, 1H, CHFlu), 7.00 (br, than 30 min as judged by TLC and all volatiles were removed
2H, CHMes), 2.61 (s, 3H, CH3 Flu-9), 2.32 (s, 3H, CH3 para), 2.12 under vacuum. The residue was purified by flash chromato-
(s, 6H, CH3 ortho); 13C{1H} NMR (75.5 MHz, CDCl3): δ = 145.0, graphy (neutral Al2O3, Brockmann’s type 3, hexane–CH2Cl2:
141.6, 139.3 (CAr), 137.3 (N2CH), 134.2 (CAr), 130.8, 130.1, 129.6 2/1 then 1/1) to yield a yellow powder (235 mg, 96%). X-Ray-
(2 CHFlu + CHMes), 124.3 (CHFlu), 124.0, 121.5 (CHIm), 121.1 quality crystals were grown by slow diffusion of pentane into a
(CHFlu), 71.8 (C(CH3)Flu-9), 25.6 (CH3 Flu-9), 21.3 (CH3 para), 18.2 CHCl3 solution of 5. 1H NMR (400 MHz, CDCl3): δ = 8.27 (d, J =
(CH3 ortho); IR (ATR): ν = 3168, 3118, 3073, 2971, 1606, 1534, 8.2 Hz, 1H, CHFlu), 8.12 (s, 1H, CHFlu-9), 7.82 (d, J = 7.5 Hz, 1H,
1478, 1444, 1216, 1186, 1154, 1087, 1031, 994, 878, 845, 770, CHFlu), 7.76 (d, J = 7.5 Hz, 1H, CHFlu), 7.50–7.42 (m, 3H, CHFlu),
744, 735 cm−1; MS (ESI): m/z (%): 365 (100) [M − I]+, 187 (56) 7.36–7.33 (m, 2H, CHFlu), 7.14 (s, 1H, CHMes), 6.96 (s, 1H,
[Mes − ImH]+; elemental analysis calcd (%) for C26H25IN2 (MW
492.40): C 63.42, H 5.12, N 5.69; found: C 63.20, H 5.08, N 5.55.
=
CHMes), 6.64 (d, J = 1.9 Hz, 1H, CHIm), 6.30 (d, J = 1.9 Hz, 1H,
CHIm), 5.00–4.96 (m, 1H, CHCOD), 4.91–4.87 (m, 1H, CHCOD),
(1-Mesityl-3-(fluoren-9-yl)imidazol-2-ylidene)gold(I) chloride 3.86–3.82 (m, 1H, CHCOD), 3.27–3.24 (m, 1H, CHCOD), 2.54 (s,
(4). Solid AuCl(tht) (63 mg, 0.196 mmol) was added all at once 3H, CH3 Mes), 2.45–2.36 (m, 1H, CH2COD), 2.41 (s, 3H, CH3 Mes),
into a solution of 2 (75 mg, 0.214 mmol, 1.1 eq.) in CH2Cl2 2.18–1.95 (m, 3H, CH2COD), 1.90 (s, 3H, CH3 Mes), 1.85–1.58 (m,
(5 mL) at −60 °C. The solution was allowed to warm up to 3H, CH2COD), 1.33–1.24 (m, 3H, CH2COD); 13C{1H} NMR
room temperature overnight. Volatiles were removed under (105.5 MHz, CDCl3): δ = 184.0 (d, JRh–C = 51.2 Hz, N2C), 144.1,
vacuum and the yellow residue was purified by flash chromato- 143.4, 141.3, 140.3, 138.9, 138.8, 137.4, 136.2, 134.5 (CAr), 129.8
graphy (SiO2, hexane–CH2Cl2: 1/1) to yield the product as a (CHMes), 129.3, 129.2, 128.6, 128.3 (CHFlu), 127.9 (CHMes), 127.8
white powder (76 mg, 67%). X-Ray-quality crystals were grown (CHFlu), 124.2 (CHIm), 124.1, 120.5, 119.7 (CHFlu), 118.8 (CHIm),
by slow diffusion of pentane into a CH2Cl2 solution of 4. 1H 98.1 (d, JRh–C = 7.5 Hz, CHCOD), 97.6 (d, JRh–C = 6.9 Hz, CHCOD),
NMR (400 MHz, CD2Cl2): δ = 7.84 (d, J = 7.6 Hz, 2H, CHFlu), 68.6 (d, JRh–C = 14.3 Hz, CHCOD), 68.3 (d, JRh–C = 14.3 Hz,
7.59–7.56 (m, 2H, CHFlu), 7.55–7.51 (m, 2H, CHFlu), 7.38 (td, J = CHCOD), 66.0 (CHFlu-9), 33.9, 32.0, 29.0, 28.3 (CH2COD), 21.5,
7.5 Hz, J = 1.1 Hz, 2H, CHFlu), 7.08 (s, 2H, CHMes), 7.02 (s, 1H, 19.9, 17.9 (CH3 Mes); IR (ATR): ν = 2953, 2916, 2869, 2828, 1607,
CHFlu-9), 6.86 (d, J = 2.0 Hz, 1H, CHIm), 6.54 (d, J = 2.0 Hz, 1H, 1489, 1450, 1381, 1301, 1233, 1220, 955, 860, 824, 732,
CHIm), 2.39 (s, 3H, CH3 para), 2.13 (s, 6H, CH3 ortho); 13C{1H} 697 cm−1; MS (ESI): m/z (%): 561 (100) [M − Cl]+; elemental ana-
NMR (105.5 MHz, CD2Cl2): δ = 173.2 (N2C), 141.9, 141.1, 140.4, lysis calcd (%) for C33H34ClN2Rh (MW = 597.00) + 0.25 CH2Cl2:
135.4, 135.2 (CAr), 130.3, 129.7, 128.7 (2 CHFlu + CHMes), 125.3 C 64.60, H 5.62, N 4.53; found: C 64.40, H 5.92, N 4.33.
(CHFlu), 123.5 (CHIm), 121.0 (CHFlu), 118.6 (CHIm), 65.4 (CHFlu-9),
21.3 (CH3 para), 18.0 (CH3 ortho); IR (ATR): ν = 3159, 3133, 3039, fluoren-9-yl)rhodium(I) (5′). A solution of compound
Chloro-(η4-cycloocta-1,5-diene)-(9-(1-mesitylimidazolium-3-yl)-
2
2916, 1722, 1606, 1553, 1486, 1446, 1418, 1407, 1377, 1304, (30.5 mg, 87 μmol) in CD2Cl2 (0.8 mL) pre-cooled at −80 °C
1224, 1211, 1031, 950, 856, 824, 739, 734, 696 cm−1; MS (ESI): was slowly transferred via a cannula into an NMR tube con-
m/z (%): 588 (100) [M − Cl + MeCN]+; elemental analysis calcd taining solid [RhCl(COD)]2 (21.5 mg, 43 μmol, 1.0 eq.) placed
(%) for C25H22AuCl (MW = 582.87) + 0.75 CH2Cl2: C 47.83, H in a cold bath at −80 °C. The NMR tube was rapidly shaken
3.66, N 4.33; found: C 47.80, H 3.65, N 4.18.
and kept at this temperature until its introduction into the
(9-(1-Mesitylimidazolium-3-yl)fluoren-9-yl)gold(I)
chloride NMR spectrometer with the probe temperature set up and
(4′). Solid AuCl(tht) (25 mg, 0.078 mmol) was added all at stabilized at 193 K. The complete NMR data were recorded at
once into a solution of 2 (29 mg, 0.083 mmol, 1.05 eq.) in this temperature and indicated a mixture consisting of com-
CD2Cl2 (0.8 mL) at −80 °C. The reaction mixture was then plexes 5′ and 5 in a ∼1/1 ratio along with excess [RhCl(COD)]2
transferred to a NMR tube placed in a cold bath at −80 °C. The (due to the incomplete transfer of the solution of 2). Warming
NMR tube was kept at this temperature until its introduction up the solution to room temperature gave a quantitative and
into the NMR spectrometer with the probe temperature set up clean conversion into complex 5. NMR data for complex 5′ are
1
and stabilized at 214 K. The following NMR data were recorded as follows: H NMR (500 MHz, CD2Cl2, 193 K): δ = 9.18 (br s,
at this temperature. 1H NMR (400 MHz, CD2Cl2, 214 K): δ = 1H, N2CH), 7.85–7.79 (m, 3H, CHFlu), 7.38–7.31 (m, 1H, CHFlu),
9.36 (s, 1H, N2CH), 7.82–7.76 (m, 2H, CHFlu), 7.41–7.37 (m, 2H, 7.31–7.22 (m, 4H, CHFlu), 7.05 (s, 2H, CHMes), 6.93 (s, 1H,
CHFlu), 7.35–7.28 (m, 4H, CHFlu), 7.06 (s, 1H, CHIm), 7.05 (s, CHIm), 4.16 (br s, 3H, CHCOD), 3.88 (br s, 1H, CHCOD), 2.33 (s,
2H, CHMes), 6.60 (s, 1H, CHIm), 2.34 (s, 3H, CH3 para), 2.05 (s, 3H, CH3 para), 2.16 (br s, 6H, CH3 ortho), 2.41–1.23 (m, 8H,
6H, CH3 ortho); 13C{1H} NMR (105.5 MHz, CD2Cl2, 214 K): δ = CH2 COD); 13C{1H} NMR (125.8 MHz, CD2Cl2, 193 K): δ = 142.9,
148.7, 140.9 (CAr), 138.3 (N2CH), 136.0, 134.3, 130.3 (CAr), 129.2 141.0, 140.0, 138.7, 136.3, (CAr), 137.4 (br, N2CH), 135.2, 134.7
(CHMes), 126.6 (CHFlu), 126.1 (CHFlu), 123.7 (CHIm), 123.2 (CAr), 129.0 (CHMes), 127.8, 127.7, 125.2, 123.6 (CHFlu), 120.4
(CHFlu), 121.9 (CHIm), 120.1(CHFlu), 71.1 (Au-CFlu-9), 20.9 (CHIm), 119.6 (CHFlu), 86.1 (br, CHCOD), 77.1 (d, JRh–C = 10.1
(CH3 para), 17.3 (CH3 ortho).
Hz, CHCOD), 73.8 (d, JRh–C = 21.1 Hz, Rh-CFlu-9), 31.4, 30.5, 29.3
Chloro-(η4-cycloocta-1,5-diene)-(1-mesityl-3-(fluoren-9-yl)- (CH2 COD), 21.0 (CH3 para), 15.2 (CH3 ortho).
imidazol-2-ylidene) rhodium(I) (5). Solid [RhCl(1,5-COD)]2
(1-Mesityl-3-(fluoren-9-yl)imidazol-2-ylidene)silver(I) bromide
(102 mg, 0.206 mmol) was added all at once into a solution of (6). CH2Cl2 (10 mL) was syringed into solid imidazolium
mesoionic 2 (159 mg, 0.453 mmol, 2.2 eq.) in CH2Cl2 (12 mL) bromide 1 (249 mg, 0.577 mmol), silver(I) oxide (80 mg,
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0.346 mmol, 0.6 eq.) and 4 Å molecular sieves and the mixture 0.589 mmol) in THF (15 mL) at −50 °C. After the addition, the
was stirred under exclusion of light for 19 h. After filtration cooling bath was removed and the reaction mixture was
over Celite, the orange solution was evaporated to dryness and allowed to warm up to room temperature. After 40 min, the
the crude product was washed with Et2O (3 × 5 mL) to yield solution was again cooled down to −50 °C and solid CpFe-
after drying a beige powder (235 mg, 76%). Single crystals suit- (CO)2I (179 mg, 0.589 mmol, 1.0 eq.) was added. After
able for an X-ray diffraction experiment were grown by slow warming to room temperature, the stirring was continued for
1
diffusion of pentane into a solution of 6 in CH2Cl2. H NMR 1.5 h and all volatiles were removed under vacuum. The
(400 MHz, CDCl3): δ = 7.80 (d, J = 7.6 Hz, 2H, CHFlu), 7.51 (t, J = residue was dissolved in toluene (10 mL), filtered through
7.6 Hz, 2H, CHFlu), 7.43 (d, J = 7.6 Hz, 2H, CHFlu), 7.37–7.33 Celite and again evaporated to dryness leaving a dark crude
(m, 2H, CHFlu), 7.00 (s, 2H, CHMes), 6.86 (t, J = 1.8 Hz, 1H, product. This was washed with Et2O (2 × 5 mL) to leave a yel-
CHIm), 6.65 (s, 1H, CHFlu-9), 6.57 (s, 1H, CHIm), 2.35 (s, 3H, lowish-brown powder (217 mg, 70%). 1H NMR (250 MHz,
CH3 para), 2.06 (s, 6H, CH3 ortho); 13C{1H} NMR (105.5 MHz, C6D6): δ = 7.61–7.47 (m, 3H, CHFlu), 7.28–7.03 (m, 5H, CHFlu),
CDCl3): δ = 141.8, 140.8, 139.9, 135.4, 134.2 (CAr), 130.2, 129.7, 6.74 (s, 1H, CHMes), 6.65 (s, 1H, CHMes), 6.12 (d, J = 1.9 Hz, 1H,
128.5 (2 CHFlu + CHMes), 124.9 (CHFlu), 123.6 (CHIm), 120.8 CHIm), 5.88 (d, J = 1.9 Hz, 1H, CHIm), 4.21 (s, 5H, CHCp), 2.32
(CHFlu), 118.9 (CHIm), 66.0 (CHFlu-9), 21.2 (CH3 para), 17.9 (s, 3H, CH3 Mes), 2.08 (s, 3H, CH3 Mes), 1.84 (s, 3H, CH3 Mes);
(CH3 ortho); IR (ATR): ν = 3013, 2971, 2916, 1609, 1489, 1448, 13C{1H} NMR (105.5 MHz, C6D6): δ = 257.8 (Fe–C(vO)CFlu),
1411, 1389, 1300, 1232, 1222, 1163, 1104, 1082, 1026, 938, 849, 221.8 (Fe–CO), 199.3 (N2C), 146.2, 145.7, 141.8, 141.5, 138.9,
824, 742 cm−1; MS (ESI): m/z (%): 459 (100) [M − Br]+; elemen- 136.8, 136.6, 135.2 (CAr), 129.6, 129.0, 128.5, 128.4, 128.0,
tal analysis calcd (%) for C25H22AgBrN2 (MW = 538.23): C 127.5 (CHFlu + CHMes), 124.4 (CHIm), 123.3, 121.5, 120.7, 120.6
55.79, H 4.12, N 5.20; found: C 55.83, H 4.17, N 5.05.
(CHFlu), 118.3 (CHIm), 92.2 (CFlu-9), 83.3 (CHCp), 21.0, 18.5, 17.8
(η4-Cycloocta-1,5-diene)(1-mesityl-3-(fluoren-9-yl-κC9)imid- (CH3 Mes); IR (ATR): ν = 3017, 2915, 1911, 1648, 1482, 1446,
azol-2-ylidene-κC2)rhodium(I) (7). KHMDS (0.5 M in toluene, 1410, 1340, 1278, 1181, 1103, 1003, 938, 917, 885, 847, 812,
2.1 mL, 1.06 mmol, 2.1 eq.) was added dropwise to a suspen- 778, 733, 699, 666 cm−1; IR (THF): ν = 1923 (CuO), 1669
sion of imidazolium bromide 1 (218 mg, 0.505 mmol, 2.0 eq.) (CvO) cm−1; MS (ESI): m/z (%): 549 (27) [M + Na]+, 527 (15)
in THF (20 mL) at −50 °C. After the addition, the cooling bath [M + H]+, 493 (53) [M − 2CO + Na]+, 471 (100) [M − 2CO + H]+;
was removed. After 40 min, the solution was again cooled elemental analysis calcd (%) for C32H26FeN2O2 (MW = 526.42):
down to −70 °C and solid [RhCl(COD)]2 (124 mg, 0.252 mmol) C 73.01, H 4.98, N 5.32; found: C 72.21, H 5.49, N 5.10.
was added. The reaction mixture was stirred overnight in the
X-ray diffraction studies
cooling bath (final temperature = 10 °C) and all volatiles were
removed in vacuo. Toluene (10 mL) was syringed into the Schlenk
tube and the mixture was filtered through Celite. After evapor-
ation of toluene, the red residue was washed with pentane (2 ×
5 mL) and dried. Addition of a small volume of Et2O (1 mL)
induced the formation of small red-orange crystals of the complex
Data were collected on a Bruker D8/APEX II/Incoatec IµS
Microsource diffractometer (4, 7), or a Bruker D8/APEX II
diffractometer (5, 6). All calculations were performed on a PC-
compatible computer using the WinGX system.30 Full crystallo-
graphic data are given in Table S1 (see the ESI†). The struc-
tures were solved using the SIR92 program,31 which revealed
in each instance the position of most of the non-hydrogen
atoms. All the remaining non-hydrogen atoms were located by
the usual combination of full matrix least-squares refinement
and difference electron density syntheses using the SHELX
program.32 Atomic scattering factors were taken from the
usual tabulations. Anomalous dispersion terms for Ag, Au, Cl,
and Rh atoms were included in Fc. All non-hydrogen atoms
were allowed to vibrate anisotropically. All the hydrogen atoms
were set in idealized positions (R3CH, C–H = 0.96 Å; R2CH2, C–
H = 0.97 Å; RCH3, C–H = 0.98 Å; C(sp2)–H = 0.93 Å; Uiso 1.2 or
1.5 times greater than the Ueq of the carbon atom to which the
hydrogen atom is attached) and their positions were refined as
“riding” atoms. The four structures have been deposited at the
CCDC under reference numbers CCDC 969666 (4), CCDC
969667 (5), CCDC 969668 (6), and CCDC 969669 (7).
1
(65 mg, 23%), suitable for an X-ray diffraction experiment. H
NMR (400 MHz, C6D6): δ = 8.11 (d, J = 7.6 Hz, 2H, CHFlu), 7.79
(d, J = 7.6 Hz, 2H, CHFlu), 7.51 (d, J = 7.8 Hz, 2H, CHFlu), 7.33
(d, J = 7.4 Hz, 2H, CHFlu), 6.67 (s, 2H, CHMes), 6.07 (d, J = 1.8
Hz, 1H, CHIm), 5.76 (d, J = 1.8 Hz, 1H, CHIm), 4.05–4.03 (m,
2H, CHCOD), 2.82–2.80 (m, 2H, CHCOD), 2.09 (s, 6H, CH3 ortho),
2.04 (s, 3H, CH3 para), 2.00–1.92 (m, 2H, CH2COD), 1.76–1.68 (m,
2H, CH2COD), 1.49–1.42 (m, 2H, CH2COD), 1.30–1.23 (m, 2H,
CH2COD); 13C{1H} NMR (105.5 MHz, CDCl3): δ = 160.8 (d, JRh–C
= 44.3 Hz, N2C), 148.8, 138.7, 135.2, 132.2 (CAr), 129.2 (CHMes),
125.0 (CHFlu), 121.9 (CHIm), 121.5, 120.3 (CHFlu), 119.4 (CHIm),
119.3 (CHFlu), 88.9 (d, JRh–C = 8.2 Hz, CHCOD), 74.3 (d, JRh–C
=
10.8 Hz, CHCOD), 63.5 (d, JRh–C = 10.7 Hz, Rh-CFlu-9), 32.1, 29.7
(CH2COD), 21.0 (CH3 para), 17.5 (CH3 ortho); IR (ATR): ν = 3002,
2913, 2869, 2826, 1609, 1486, 1474, 1436, 1413, 1326, 1304,
1212, 1179, 1142, 1091, 1033, 1014, 970, 935, 849, 758, 725,
678 cm−1; MS (ESI): m/z (%): 561 (100) [M + H]+; HR-MS (ESI):
m/z: calcd for C33H34N2Rh: 561.1777, found: 561.1781.
(η5-Cyclopentadienyl)(carbonyl)(1-mesityl-3-(9-carboxylato-κC-
Acknowledgements
fluoren-9-yl)imidazol-2-ylidene-κC2)iron(II) (8). KHMDS (0.5 M
in toluene, 2.5 mL, 1.25 mmol, 2.1 eq.) was added dropwise Financial support by the CNRS is gratefully acknowledged. L.B.
into a suspension of imidazolium bromide 1 (254 mg, thanks the French MESR for a Ph.D. fellowship.
4480 | Dalton Trans., 2014, 43, 4474–4482
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25 (a) P. Oulié, C. Freund, N. Saffon, B. Martin-Vaca, L. Maron 27 Data taken from: G. R. Fulmer, A. J. M. Miller,
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4482 | Dalton Trans., 2014, 43, 4474–4482
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