Rhodium(iii) and ruthenium(ii) complexes of redox-active, chelating N-heterocyclic carbene/thioether ligands

Article abstract of DOI:10.1039/c1nj20224c

Half-sandwich rhodium(iii) and ruthenium(ii) complexes bearing a new redox-active ferrocenyl NHC-thioether ligand have been prepared. The synthesis of ferrocenyl thioether-imidazolium salts 3a and 3b was carried out via intermediate 2 using an improved pr

Full text of DOI:10.1039/c1nj20224c

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
NJC
Cite this: New J. Chem., 2011, 35, 2162–2168
www.rsc.org/njc
PAPER
Rhodium(III) and ruthenium(II) complexes of redox-active,
chelating N-heterocyclic carbene/thioether ligandswz
Agnes Labande,*ab Jean-Claude Daran,ab Nicholas J. Long,*c Andrew J. P. Whitec
and Rinaldo Poliabd
Received (in Montpellier, France) 10th March 2011, Accepted 17th May 2011
DOI: 10.1039/c1nj20224c
Half-sandwich rhodium(III) and ruthenium(II) complexes bearing a new redox-active ferrocenyl
NHC–thioether ligand have been prepared. The synthesis of ferrocenyl thioether-imidazolium
salts 3a and 3b was carried out via intermediate 2 using an improved procedure. Rhodium(III)
complex 4 and ruthenium(II) complex 5 were obtained in good yields and were fully characterised
by NMR spectroscopy, X-ray diffraction analysis and electrochemistry. Complex 4 shows a
1
complex ABCD system by H NMR, which denotes conformational rigidity due to the presence
of several bulky groups. Electrochemical analysis by cyclic voltammetry reveals reversible redox
behaviour about the iron centre in 4 and 5, and indicates electronic communication between iron
and rhodium or ruthenium.
Fine-tuning of the electronic properties can also be achieved
Introduction
by introduction of redox-active moieties in the ligands.7
This has been demonstrated in the field of small molecule
sensing8,9 and in catalysis.10 In the latter domain, transition
N-Heterocyclic carbenes (NHCs) have become widely used
ligands in many areas from transition metal catalysis,1
organocatalysis,2 to biochemistry and medicine.3 They are
generally described as organophosphine analogues, although
their s-donor character is more pronounced. Unlike phosphines,
they generally lead to air-stable, robust complexes which do
not easily undergo ligand dissociation. However, the M–C
bond stability could be detrimental to the catalytic activity.
Functionalised N-heterocyclic carbene ligands combine the
robust nature of the resulting metal–NHC bond with the
flexibility of the secondary donor function (hemilability,
stereoelectronic control on the active site) and have been
successfully used in many catalytic reactions.4–6 However,
any alteration of steric or electronic properties of a ligand
implies structural modifications and thus more synthetic work.
11
metal complexes containing either substitutionally inert or
hemilabile 12 redox-active ligands have shown charge-dependent
behaviour.
Our group has a strong interest in the chemistry and
catalytic activity of functionalised N-heterocyclic carbene
complexes. We have shown that NHCs bearing thioether or
phosphine donors give very efficient and robust catalysts.13,14
The work reported here describes the preparation of redox-
active NHC–thioether ligands incorporating the ubiquitous
ferrocene group and the study of their coordination chemistry
and electrochemical behaviour.
Results and discussion
a CNRS, LCC (Laboratoire de Chimie de Coordination),
205, route de Narbonne, F-31077 Toulouse, France.
E-mail: agnes.labande@lcc-toulouse.fr; Fax: +33 561553003;
Synthesis
The synthesis of the imidazolium salt 3b, i.e. the NHC–thioether
precursor, was achieved in three steps from bromoferrocene 1
(Scheme 1). Although the preparation of intermediate 2
and its 1,10-disubstituted analogue has been described by
Mirkin et al.,15,16 we decided to follow a different synthetic
route. Mirkin’s reported procedures involved the mono-
lithiation of ferrocene and afforded the products in very low
yields, by reaction with S8 and tosylate TsOCH2CH2Cl
(compound 2, 15% yield),15 or with disulfide (ClCH2CH2S)2
(1,10-disubstituted analogue, 13% yield).16 We decided to
start from bromoferrocene, which could be selectively and
efficiently transformed into lithioferrocene. The reaction of the
latter with bis(2-chloroethyl) disulfide was then carried out at
Tel: +33 561333158
´
Universite de Toulouse, UPS, INPT, LCC, F-31077 Toulouse,
b
France
c Department of Chemistry, Imperial College London, London,
SW7 2AZ, UK. E-mail: n.long@imperial.ac.uk;
Fax: +44 2075945804; Tel: +44 2075945781
d Institut Universitaire de France, 103, bd Saint-Michel, 75005 Paris,
France
w Dedicated to Professor Didier Astruc on the occasion of his 65th
birthday.
z Electronic supplementary information (ESI) available: Square-wave
voltammogram of complex 5, crystal data and structure refinement
and selected bonds and angles for complexes 4 and 5. CCDC reference
numbers 816175 (4) and 816176 (5). For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c1nj20224c
c
2162 New J. Chem., 2011, 35, 2162–2168
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2011

View Article Online
Scheme 1 Synthesis of imidazolium salts 3a and 3b.
the THF reflux temperature for 16 h and allowed us to
dramatically improve the average yield of 2, ranging from 70
to 79%.
Scheme 2 Synthesis of rhodium(III) and ruthenium(II) complexes.
whereas the two ortho methyl groups had the same chemical
shift in 3b. This means that the rotation of the mesityl group
along the C–N bond is blocked, presumably because of close
contact with the Cp* ligand. The CH2S signal shifts upfield on
going from 3b (d 38.1) to 4 (d 35.2). However, the effects of
The intermediate imidazolium salt 3a was prepared by
reaction of 2 with N-mesitylimidazole in the presence of NaI
to facilitate the substitution (54% yield). This compound was
characterised by 1H NMR to confirm the presence of the
acidic proton, characteristic of an imidazolium salt, at 9.87 ppm.
However, due to the potential hygroscopic nature of salts
bearing halogen counter-anions, and to the non-innocent
behaviour of these anions in coordination chemistry, a
metathesis with NaBF4 was carried out to afford 3b with an
overall yield of 52% from 2. 1H NMR of 3b showed the
characteristic imidazolium proton signal at 8.78 ppm, shifted
upfield relative to 3a.
1
rhodium coordination were best evidenced by H NMR. All
the protons of the SCH2CH2Im bridge become non-equivalent
and appear as a complex ABCD system composed of two
doublets of doublets and two doublets of triplets, with the
1
coupling pattern confirmed by a 2D-COSY H–1H experiment.
This specific pattern is set by the conformational rigidity
imposed by the presence of several bulky groups. Indeed, the
measured coupling constants are in good agreement with those
predicted from the measured dihedral angles from the X-ray
structure of complex 4, allowing us to assign these resonances
more precisely as shown in Fig. 1. As in the 13C NMR
spectrum, differentiation of the two aromatic protons (slight)
and ortho, ortho0 methyl substituents (large) is observed in the
1H NMR spectrum.
The 1H NMR spectrum of the ruthenium complex 5 showed
a very different pattern, as all signals were broad even at low
temperatures. The formation of the Ru–NHC bond, however,
was confirmed by 13C NMR with a signal at d 164.5, similar
to other NHC ruthenium(II) complexes bearing a p-cymene
substituent.6 Most signals were assigned by 13C NMR: the
The NHC–thioether rhodium(III) and ruthenium(II) com-
plexes 4 and 5 were then prepared by a one-pot synthesis in the
presence of Ag2O as a transmetallating agent (Scheme 2).6,17
The reaction was carried out with 3b in order to avoid a
potential mixture of Cl and I on the Rh and Ru centres. It was
conducted in the presence of excess Ag2O in dichloromethane
at reflux. Forty hours were generally necessary to drive the
reaction to completion and consume all the imidazolium salt,
as confirmed by the disappearance of the proton signal at
8.78 ppm. The synthesis of silver carbene complexes from
imidazolium salts containing non-coordinating counteranions
is known but usually requires harsher reaction conditions,
such as high temperatures, longer reaction times and the use of
molecular sieves, than for those generated from halide-
containing imidazolium salts.18 At least one example describes
the use of such silver complexes in transmetallation reactions
to prepare Rh(I) complexes.19 Cationic complexes 4 and 5 were
obtained in very good yields (4, 86%; 5, 85%) as deep red and
orange solids respectively. Both complexes are air-stable and
can be purified by column chromatography on neutral
alumina.
Characterisation of complexes 4 and 5
The 1H and 13C NMR spectra of complex 4 confirmed the
coordination of both NHC and thioether moieties. The
coordination of the carbene on rhodium was evidenced by
13C NMR by a doublet at d 166.3, with a coupling constant
JRh–C of 54 Hz, typical of rhodium(III)–NHC complexes.13,14,20
All three methyl groups from the mesityl moiety are now distinct,
Fig. 1 Assignment of the methylene protons in the 1H NMR spectrum
of 4: ABCD system, simulated (top) and experimental (bottom).
c
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2011
New J. Chem., 2011, 35, 2162–2168 2163

View Article Online
o,o0-CH3 substituents of the mesityl group were barely
distinct, whereas both CH(CH3)2 signals from the p-cymene
moiety were well separated at d 20.4 and 24.8. The signals for
the CH2Im carbon appeared in the expected region at d 49.2,
as well as the CH2S signal at d 33.7. The 13C NMR signals for
ferrocene were situated in the expected region between d 72
and 68 but appeared very broad. The 1H NMR spectrum
analysis was more complicated. The proton signals of the
mesityl and p-cymene groups were assigned without ambiguity
and all six methyl groups were differentiated. However, only
one proton of CH2Im could be assigned at d 4.86. The other
one was not found even with the help of 2D 1H–1H or 1H–13C
correlation experiments, as were the CH2S protons. Those
might be overlapping with other signals, or broadened out.
However, the mass spectrometry and X-ray diffraction
analyses confirmed the expected structure for 5.
Table 1 Selected bond lengths (A), angles (1) and torsion angle (1) for
complexes 4 and 5
4 (M = Rh)
5 (M = Ru)
M1–Cg1
M1–Cl1
M1–S1
M1–C5
S1–C2
S1–C18
C2–C3
C3–N4
C7–C8
1.8334(8)
2.3785(4)
2.3622(4)
2.0550(17)
1.822(2)
1.7631(9)
1.524(3)
1.459(2)
1.341(3)
1.7460(15)
2.3944(9)
2.3501(10)
2.091(4)
1.798(4)
1.774(4)
1.510(6)
1.454(6)
1.331(7)
Cg1–M1–Cl1
Cg1–M1–S1
Cg1–M1–C5
Cl1–M1–S1
Cl1–M1–C5
S1–M1–C5
119.92(3)
123.28(3)
133.87(6)
90.407(16)
91.64(5)
125.09(6)
119.72(6)
134.56(12)
91.66(4)
81.79(11)
91.54(12)
85.68(5)
Complexes 4 and 5 were analysed by single crystal X-ray
diffraction.y Both structures consist of isolated cations and
anions with no short interionic contacts. In compound 4, there
is one CH2Cl2 solvent molecule. Comparison of selected bond
distances, angles and torsion angles are given in Table 1 (also
in ESIz) and ORTEP views for each complex are shown in
Fig. 2. The structures are closely related, they both contain the
same thioether–NHC ligand chelating the metal to form a
six-membered ring and a Cl atom but differ by the p-ligand:
Cp* for the rhodium complex and p-cymene for the ruthenium
compound. The Cp* is in Z5-coordination mode whereas the
p-cymene is in Z6-coordination. Considering these two ligands
as a single coordination site, the overall coordination geometry
about each metal centre is pseudo-tetrahedral or typical piano-
stool geometry. However, in the case of ruthenium p-cymene
complexes, it is usual to describe them as pseudo-octahedral,
the Z6-ligand occupying one face of the octahedron. The other
three sites in complex 5 are occupied by the S1, C5 and
Cl1 atoms.
S1–C2–C3–N4 ꢀ50.5(2)
Cg1 defines the centroid of the cycle above the metal.
ꢀ73.3(4)
compared to 85.68(5)1 for S1–Rh1–C5. This is certainly related
to the difference in conformation for the chelate ring, which is
boat for the rhodium complex and half-chair for the ruthenium
one. The difference is reflected in the S1–C2–C3–N4 torsion
angles, ꢀ50.5(2)1 for 4 and ꢀ73.3(4)1 for 5. This difference in
conformation results in a different dihedral angle between the
coordinated Cp of the ferrocene moiety and the imidazol-2-
ylidene ring: 61.03(12)1 for 4 and 39.9(2)1 for 5.
The Rh1–C5 distance of 2.0550(17) A compares well with
related NHC–Cp* rhodium complexes.20,21 The Ru1–C5
distance of 2.091(4) A is slightly long but in the range of other
NHC–ruthenium complexes, which are found between 2.028(4)
and 2.0828(14) A.6,21 The average M1–Cl1 and M1–S1 distances
are in agreement with other structurally characterised cationic
Z5–Cp* rhodium or Z6–p-cymene ruthenium complexes reported
in the Cambridge Structural Database.22 Both ferrocene moieties
have eclipsed conformation and usual geometry.22
Whatever the description of the coordination about the
metal, the M1–S1, M1–C5 and M1–Cl1 bond lengths are
comparable for the two compounds (Table 1). The only
marked differences concern the angles. Indeed, the Cg1–M1–Cl1
angle is much larger for the ruthenium complex 5, 125.09(6)1,
than for the rhodium one 4, 119.92(3)1. Such a difference may be
explained by the bulkier p-cymene ligand with the substituted
methyl roughly above the Cl position. Another difference is the
larger value observed for the S1–Ru1–C5 angle ꢀ91.54(12)1
Electrochemistry
The imidazolium salts 3a, 3b and complexes 4 and 5 were
analysed by cyclic voltammetry in dichloromethane. The
voltammogram of 3a exhibited several waves, as the iodide
anion itself is redox-active.23 The redox potential for the
ferrocene group is observed at E1/2 = +0.64 V/SCE with a
chemically reversible behaviour (DEp is 120 mV, however, and
may indicate slow electron transfer at the surface of the
electrode). The voltammogram of 3b only shows the chemically
reversible wave for the ferrocene unit at +0.64 V/SCE
(DEp = 111 mV), see Fig. 3.24
y Crystal data for 4: [C34H41ClFeN2RhS](BF4)ꢁCH2Cl2, M = 875.69,
%
triclinic, P1 (no. 2), a = 10.6786(2), b = 11.8064(3), c = 15.8586(3) A,
a = 96.8086(18)1, b = 102.4583(19)1, g = 106.009(2)1, V =
1842.52(7) A3, Z = 2, Dc = 1.578 g cmꢀ3, m(Mo-Ka) = 1.164 mmꢀ1
,
T = 173 K, orange tablets, Oxford Diffraction Xcalibur 3 diffracto-
meter; 12 107 independent measured reflections (Rint = 0.0239), F2
refinement, R1(obs) = 0.0331, wR2(all) = 0.0856, 10 327 independent
The FeII/FeIII couple in the rhodium complex 4 (Fig. 4) is
anodically shifted compared to the imidazolium salts with a
potential of +0.77 V/SCE (DEp = 111 mV) and exhibits
chemically reversible behaviour. The anodic shift signals a
weak electron-withdrawing effect of coordination to rhodium
and indicates electronic communication between iron and
rhodium through the sulfur atom. The cyclic voltammogram
of the ruthenium complex 5 also presents a chemically reversible
redox wave at +0.76 V/SCE (DEp = 84 mV) attributed to the
observed absorption-corrected reflections [|Fo| 4 4s(|Fo|), 2ymax
=
661], 484 parameters. CCDC 816175. Crystal data for 5:
[C34H40ClFeN2RuS](BF4)ꢁCH2Cl2, M = 787.92, monoclinic, P21/n
(no. 14), a = 11.8820(3), b = 10.2423(2), c = 27.5100(7) A, b =
94.493(2)1, V = 3337.65(14) A3, Z = 4, Dc = 1.568 g cm–3, m(Mo-Ka) =
1.080 mm–1, T = 180 K, orange tablets, Oxford Diffraction Gemini Eos
diffractometer; 7574 independent measured reflections (Rint = 0.063),
F2 refinement, R1(obs) = 0.0512, wR2(all) = 0.1073, 5590 independent
observed absorption-corrected reflections [|Fo| 4 4s(|Fo|), 2ymax = 551],
412 parameters. CCDC 816176.
c
2164 New J. Chem., 2011, 35, 2162–2168
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2011

View Article Online
Fig. 2 Molecular views of compounds 4 (left) and 5 (right) with the atom-labelling scheme. Displacement ellipsoids are drawn at the 50%
probability level. H atoms have been omitted for clarity.
exhibits a RuII/RuIII redox wave at +1.55 V/SCE.6 The
anodic shift of the oxidation potential for the RuII/RuIII
couple is attributed to the stronger electron-withdrawing effect
of the oxidised form of ferrocene compared to CH3. The
aspect of the voltammogram also suggests that the iron
oxidation has an effect on the redox behaviour of ruthenium,
whose oxidation becomes irreversible.25 The electrochemical
behaviour of both complexes suggests that it should be
possible to alter selectively and reversibly the oxidation state
of iron by a careful choice of oxidizing and reducing agents.23
Conclusions
Two half-sandwich rhodium(III) and ruthenium(II) complexes
Fig. 3 Cyclic voltammograms of imidazolium salts 3a (dashed) and
3b (plain), 1 mM in CH2Cl2 at a scan rate of 0.2 V sꢀ1
bearing a new redox-active NHC–thioether ligand have been
.
prepared and their electrochemical properties investigated.
Cyclic voltammetry experiments have shown that the ferrocenyl
unit can be oxidised and reduced reversibly. The catalytic
properties of these complexes and the influence of the redox
state of iron on the catalytic performances are under investi-
gation and the results will be reported in due course.
Experimental
General procedures
All reactions were performed under an atmosphere of dry
nitrogen or argon using standard Schlenk techniques, unless
otherwise stated. Solvents were dried and distilled by standard
techniques prior to use. NMR spectra were recorded on
Bruker AV300, AV400 or AV500 spectrometers. NMR
chemical shifts were determined by reference to the residual
1H or 13C solvent peaks in the deuterated solvents. The NMR
simulation experiment was done with SpinWorks 3. Electro-
spray (ES) mass spectra were recorded either at Imperial
College on a Micromass LCT Premier spectrometer or at the
Fig. 4 Cyclic voltammogram of complexes 4 and 5, 1 mM in CH2Cl2
at a scan rate of 0.2 V sꢀ1
.
FeII/FeIII couple, see Fig. 4. A very weak oxidation wave was
detected at approximately +1.7 V/SCE and was tentatively
assigned to the RuII/RuIII couple. Analysis by square-wave
voltammetry more clearly revealed this wave at +1.68 V/SCE
(see ESIz), however it is far less intense than the FeII/FeIII
wave. A similar Ru(II) p-cymene complex, bearing a chelating
NHC–thioether ligand with methyl in place of ferrocenyl,
Universite Paul Sabatier by the Service Commun de Spectro-
´
metrie de Masse on a MS/MS API-365 (Perkin Elmer Sciex).
´
Elemental analyses were performed either by Mr Stephen
Boyer at London Metropolitan University or by the Service
d’Analyses du Laboratoire de Chimie de Coordination in
c
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2011
New J. Chem., 2011, 35, 2162–2168 2165

View Article Online
Toulouse. Cyclic voltammetry (CV) and square-wave
voltammetry (SQW) experiments were carried out with a
PGSTAT-302N potentiostat (Metrohm) at 20 1C using a Pt
disk working electrode, a Pt wire counter electrode and
a saturated calomel reference electrode (SCE); a 0.1 M
n-Bu4BF4 solution was used as the supporting electrolyte. All
electrochemical data are referenced versus SCE. Bromoferrocene 1
was prepared by a previously described procedure.26
Et2O, then taken up into CH2Cl2. Evaporation of the solvent
afforded 3b as a yellow solid (216 mg, 52%). 1H NMR (400 MHz,
CDCl3) d 8.78 (s, 1H, NCHN), 7.73 (s, 1H, HCQCH Im+), 7.23
(s, 1H, HCQCH Im+), 7.03 (s, 2H, CH Mes), 4.48 (t, JHH
=
5.9 Hz, 2H, CH2Im+), 4.33 (d, JHH = 1.7 Hz, 2H, Cp), 4.25
(d, JHH = 1.7 Hz, 2H, Cp), 4.18 (s, 5H, Cp0), 3.07 (t, JHH
=
5.9 Hz, 2H, CH2S), 2.36 (s, 3H, p-CH3 Mes), 2.08 (s, 6H, o-CH3
Mes). 13C NMR (100 MHz, CDCl3) d 141.36 (Cquat Mes), 136.94
(NCHN), 134.49 (2 ꢂ Cquat Mes), 130.64 (Cquat Mes), 129.83
(2 ꢂ CH Mes), 123.70 (CQC Im+), 123.52 (CQC Im+), 73.94
(2 ꢂ CH Cp), 69.99 (2 ꢂ CH Cp), 69.61 (5 ꢂ CH Cp0), 49.29
(CH2–Im+), 38.07 (CH2S), 21.14 (p-CH3 Mes), 17.32 (2 ꢂ o-CH3
Mes). Cquat, Cp not found. 19F NMR (282 MHz, CDCl3)
d ꢀ151.6 (BF4ꢀ). MS (ESI+) m/z 431 (M+, 100%). Elem. anal.
C24H27BF4FeN2S (518.03): calc. C, 55.60; H, 5.25; N, 5.41; found
C, 55.48; H, 5.28; N, 5.37%.
Syntheses
2-Chloroethylthioferrocene (2)15,16. A solution of bromo-
ferrocene (300 mg, 1.13 mmol, containing 92% FcBr, 7%
ferrocene and 1% FcBr2) in THF (15 mL) was cooled to
ꢀ781C. n-BuLi (800 mL, 1.24 mmol) was added dropwise and
the solution was stirred for 30 min at ꢀ78 1C. A solution of
2-chloroethyldisulfide (260 mg, 1.36 mmol) in THF (5 mL)
was added dropwise and the mixture was allowed to warm to
room temperature for 2 h. The mixture was then heated at
65 1C for 16 h. It was allowed to cool to room temperature and
H2O (10 mL) was added. The phases were separated and the
aqueous phase was extracted with Et2O. The organic phases
were combined, washed with H2O, dried (MgSO4), filtered and
concentrated in vacuo. The yellow liquid was purified by
column chromatography on neutral alumina (grade I). The
first band contained mainly ferrocene and the desired product
was obtained as a yellow liquid in the second band (250 mg,
Rhodium complex (4). Imidazolium salt 3b (56.4 mg,
0.109 mmol), [RhCp*Cl2]2 (59.3 mg, 0.096 mmol) and Ag2O
(14.8 mg, 0.064 mmol) were dissolved in dry CH2Cl2 (50 mL)
and the Schlenk tube was protected from light. The mixture
was heated at reflux for 40 h then filtered through Celite and
the solvent was evaporated in vacuo. The residue was purified
by column chromatography on neutral alumina (grade I,
deactivated with 6–8% H2O; eluent: CH2Cl2/MeOH 95 : 5)
to give a deep red solid (74 mg, 86%). X-Ray quality crystals
were obtained by slow diffusion of cyclohexane in a saturated
CH2Cl2 solution. 1H NMR (400 MHz, CDCl3) d 7.94
1
79%). H NMR (400 MHz, CDCl3) d 4.34 (t, JHH = 1.8 Hz,
2H, Cp), 4.26 (t, JHH = 1.8 Hz, 2H, Cp), 4.24 (s, 5H, Cp0),
3.60–3.56 (m, 2H, CH2Cl), 2.89–2.85 (m, 2H, CH2S).
3
3
(d, JHH = 1.8 Hz, 1H, HCQCH Im), 6.83 (d, JHH
=
1.8 Hz, 1H, HCQCH Im), 6.82–6.81 (m, 2H, C6H2(CH3)3),
2
3
5.33 (dd, JHH = 14.6 Hz, JHH = 3.0 Hz, 1H, CHCHDIm),
4.35 (app. s, 6H, 5 ꢂ C5H5 + C5H4S), 4.26 (m, 1H, C5H4S),
1-(b-(Ferrocenylthio)-ethyl)-3-(2,4,6-trimethylphenyl) imid-
azolium iodide (3a). A mixture of 2 (250 mg, 0.89 mmol),
N-mesitylimidazole (249 mg, 1.34 mmol) and NaI (267 mg,
1.78 mmol) in degassed acetonitrile (20 mL) was heated at
80 1C for 16 h. After cooling to room temperature, the solvent
was evaporated and the residue taken up into CH2Cl2. The
solution was filtered through Celite and the solvent evaporated
to a minimum. Excess Et2O was added and the yellow
precipitate was filtered on Celite, rinsed with Et2O then taken
up into CH2Cl2. Evaporation of the solvent afforded 3a as a
2
4.20 (m, 1H, C5H4S), 3.91 (dd, JHH = 14.2 Hz, JHH
3
=
2
2.4 Hz, 1H, CHAHBS), 3.78 (td, JHH
=
3JHH = 14.2 Hz,
3JHH = 3.7 Hz, 1H, CHCHDIm), 3.55 (td, JHH = JHH
=
2
3
3
13.5 Hz, JHH = 4.2 Hz, 1H, CHAHBS), 3.34 (app. s, 1H,
C5H4S), 2.29 (s, 3H, C6H2(CH3)3), 1.98 (s, 3H, C6H2(CH3)3),
1.76 (s, 3H, C6H2(CH3)3), 1.63 (s, 15H, C5(CH3)5). 13C NMR
(100 MHz, CDCl3) d 166.29 (d, JCRh = 54.3 Hz, NCN),
139.10, 138.61, 135.44, 133.57 (4 ꢂ Cquat C6H2(CH3)3), 129.16,
127.32 (2 ꢂ CH C6H2(CH3)3), 126.08, 124.75 (2 ꢂ HCQCH Im),
100.03 (d, JCRh = 6.2 Hz, (C5(CH3)5), 80.35 (Cquat C5H4S),
72.92 (CH C5H4S), 70.21 (5 ꢂ CH C5H5), 69.87, 69.02, 66.50
(3 ꢂ CH C5H4S), 50.03 (CH2Im), 35.17 (CH2S), 21.11,
20.08, 18.74 (3 ꢂ C6H2(CH3)3), 9.47 (C5(CH3)5). MS (ESI+)
m/z 703.5 (M+–BF4, 100%). Elem. anal. C34H41BClF4FeN2RhS
(790.50): calc. C, 51.61; H, 5.23; N, 3.54; found C, 51.51; H,
5.16; N, 3.46%.
1
yellow solid (268 mg, 54%). H NMR (400 MHz, CDCl3) d
9.87 (t, JHH = 1.5 Hz, 1H, NCHN), 7.85 (t, JHH = 1.7 Hz,
1H, HCQCH Im+), 7.20 (t, JHH = 1.8 Hz, 1H, HCQCH
Im+), 7.05 (s, 2H, CH Mes), 4.79 (pseudo t, JHH = 5.9 Hz,
2H, CH2Im+), 4.34 (d, JHH = 1.8 Hz, 2H, Cp), 4.27 (d,
JHH = 1.8 Hz, 2H, Cp), 4.20 (s, 5H, Cp0), 3.21 (pseudo t,
JHH = 5.9 Hz, 2H, CH2S), 2.38 (s, 3H, p-CH3 Mes), 2.15
(s, 6H, o-CH3 Mes).
1-(b-(Ferrocenylthio)-ethyl)-3-(2,4,6-trimethylphenyl) imidazolium
tetrafluoroborate (3b). Compound 2 (224 mg) was treated as
described previously and crude 3a was dissolved in degassed
CH2Cl2 (15 mL). A solution of NaBF4 (438 mg, 3.99 mmol) in
degassed H2O (10 mL) was added and the mixture was stirred
vigorously for 16 h. The two phases were separated and the
aqueous phase was extracted with CH2Cl2. The combined
organic phases were washed with H2O, dried (Na2SO4), filtered
and concentrated in vacuo to ca. 2 mL. Excess Et2O was added
and the yellow precipitate was filtered through Celite, rinsed with
Ruthenium complex (5). Imidazolium salt 3b (52.0 mg,
0.1 mmol), [Ru(p-cymene)Cl2]2 (54.0 mg, 0.088 mmol) and
Ag2O (13.6 mg, 0.059 mmol) were dissolved in dry CH2Cl2
(30 mL) and the Schlenk tube was protected from light. The
mixture was heated at reflux for 40 h then filtered through
Celite and the solvent was evaporated to ca. 2 mL. Excess
hexane was added and the precipitate was filtered through
Celite, rinsed with hexane and recovered by dissolving in
CH2Cl2. The solid was further purified by column chromato-
graphy on neutral alumina (grade I, deactivated with 6–8%
c
2166 New J. Chem., 2011, 35, 2162–2168
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2011

View Article Online
H2O; eluent: CH2Cl2/MeOH 95 : 5) to give an orange solid
(67 mg, 85%). X-Ray quality crystals were obtained by
layering a saturated CDCl3 solution with methyl tert-butyl
ether. 1H NMR (500 MHz, CDCl3) d 7.56 (br s, 1H, HCQCH
Im), 7.20 (br s, 1H, C6H2(CH3)3), 7.09 (br s, 1H, C6H2(CH3)3),
6.84 (br s, 1H, HCQCH Im), 5.51, 5.07, 4.97 (br s, 3 ꢂ 1H,
CH p-cym), 4.87 (m, 1H, CH2Im), 4.70–4.30 (m, X ꢂ 1H,
C5H4), 4.47 (br s, 5 ꢂ 1H, C5H5), 3.71 (br s, 1H, CH p-cym),
2.69 (br s, 1H, (CH3)2CH p-cym), 2.44, 2.20, 1.98 (br s, 3 ꢂ
3H, C6H2(CH3)3), 1.81 (br s, 3H, CH3 p-cym), 1.23, 0.95 (br s,
2 ꢂ 3H, (CH3)2CH p-cym). 13C NMR (125.8 MHz, CDCl3)
d 164.53 (NCN), 141.02, 137.90, 137.45, 135.45 (4 ꢂ Cquat
C6H2(CH3)3), 129.74, 129.67 (2 ꢂ CH C6H2(CH3)3), 126.63,
122.67 (2 ꢂ HCQCH Im), 107.77, 103.52 (2 ꢂ Cquat p-cym),
97.68, 93.16, 88.51, 79.78 (4 ꢂ CH p-cym), 71.08 (C5H5),
71.5–68.0 (C5H4), 49.20 (CH2Im), 33.69 (CH2S), 29.51
((CH3)2CH p-cym), 24.77 ((CH3)2CH p-cym), 21.53
(C6H2(CH3)3), 20.41 ((CH3)2CH p-cym), 18.63, 18.62
(C6H2(CH3)3), 17.95 (CH3 p-cym). MS (ESI+) m/z 701
(M+–BF4, 100%).
Chem. Rev., 2009, 109, 3561; P. L. Arnold and I. J. Casely,
Chem. Rev., 2009, 109, 3599.
2 For selected reviews, see: D. Enders, O. Niemeier and A. Henseler,
´ ´
Chem. Rev., 2007, 107, 5606; N. Marion, S. Dıez-Gonzalez and
S. P. Nolan, Angew. Chem., Int. Ed., 2007, 46, 2988.
3 G. Gasser, I. Ott and N. Metzler-Nolte, J. Med. Chem., 2011, 54, 3;
A. Kascatan-Nebioglu, M. J. Panzner, C. A. Tessier, C. L. Cannon
and W. J. Youngs, Coord. Chem. Rev., 2007, 251, 884.
4 H. M. Lee, C. C. Lee and P. Y. Cheng, Curr. Org. Chem., 2007,
11, 1491; O. Kuhl, Chem. Soc. Rev., 2007, 36, 592; A. T. Normand
and K. J. Cavell, Eur. J. Inorg. Chem., 2008, 2781; A. John and
P. Ghosh, Dalton Trans., 2010, 39, 7183.
5 Selected papers on S–NHC ligands: M. Bierenstiel and
E. D. Cross, Coord. Chem. Rev., 2011, 255, 574, and references
therein; C. Fliedel and P. Braunstein, Organometallics, 2010,
29, 5614; C. Fliedel, A. Sabbatini and P. Braunstein, Dalton Trans.,
2010, 39, 8820; C. Fliedel, G. Schnee and P. Braunstein, Dalton
´
Trans., 2009, 2474; A. Ros, M. Alcarazo, D. Monge, E. Alvarez,
R. Fernandez and J. M. Lassaletta, Tetrahedron: Asymmetry, 2010,
´
21, 1557; S. J. Roseblade, A. Ros, D. Monge, M. Alcarazo,
E. Alvarez, J. M. Lassaletta and R. Fernandez, Organometallics,
2007, 26, 2570; A. Ros, D. Monge, M. Alcarazo, E. Alvarez,
J. M. Lassaletta and R. Fernandez, Organometallics, 2006,
25, 6039; H. V. Huynh, D. Yuan and Y. Han, Dalton Trans.,
2009, 7262; H. V. Huynh, C. H. Yeo and G. K. Tan, Chem.
Commun., 2006, 3833.
6 C. Gandolfi, M. Heckenroth, A. Neels, G. Laurenczy and
M. Albrecht, Organometallics, 2009, 28, 5112.
X-Ray crystallography. A single crystal of each compound
was mounted under inert perfluoropolyether at the tip of a
cryoloop and cooled in the cryostream of an Agilent Technologies
OD XCALIBUR 3 CCD diffractometer for 4 or a GEMINI
EOS CCD diffractometer for 5. Data were collected using the
monochromatic MoKa radiation (l = 0.71073). The structures
were solved by direct methods (SIR97) 27 and refined by least-
squares procedures on F2 using SHELXL-97.28 All H atoms
attached to carbon were introduced in idealised positions and
treated as riding on their parent atoms in the calculations. In
compound 4, the BF4 was found to be disordered, and three
orientations of ca. 81, 10 and 9% occupancy were identified.
The geometries of all three orientations were optimised, the
thermal parameters of adjacent equivalent atoms were restrained
to be similar, and only the non-hydrogen atoms of the major
occupancy orientation were refined anisotropically (the others
were refined isotropically). The drawing of the molecules was
realised with the help of ORTEP3.29 Crystal data and refinement
parameters: see footnote y. Selected bond distances and angles are
given in Table 1. CCDC 816175 and 816176
7 For ferrocene-containing NHC ligands, see: B. Bildstein,
M. Malaun, H. Kopacka, K.-H. Ongania and K. Wurst,
J. Organomet. Chem., 1998, 552, 45; H. Seo, H.-J. Park,
B. Y. Kim, J. H. Lee, S. U. Son and Y. K. Chung, Organometallics,
2003, 22, 618; K. S. Coleman, S. Gischig and A. Togni,
Organometallics, 2004, 23, 2479; S. Turberville, S. I. Pascu and
M. L. H. Green, J. Organomet. Chem., 2005, 690, 653;
D. M. Khramov, E. L. Rosen, V. M. Lynch and
C. W. Bielawski, Angew. Chem., Int. Ed., 2008, 47, 2267;
E. L. Rosen, C. D. Varnado, A. G. Tennyson, D. M. Khramov,
J. W. Kamplain, D. H. Sung, P. T. Cresswell, V. M. Lynch and
C. W. Bielawski, Organometallics, 2009, 28, 6695; C. D. Varnado
Jr, V. M. Lynch and C. W. Bielawski, Dalton Trans., 2009, 7253;
U. Siemeling, C. Farber and C. Bruhn, Chem. Commun., 2009, 98.
¨
8 P. D. Beer, P. A. Gale and Z. Chen, Adv. Phys. Org. Chem., 1999,
31, 1; A. Niemz and V. M. Rotello, Acc. Chem. Res., 1998, 32, 44.
9 For selected papers, see: C. Valerio, J.-L. Fillaut, J. Ruiz,
´
J. Guittard, J.-C. Blais and D. Astruc, J. Am. Chem. Soc., 1997,
119, 2588; A. Labande, J. Ruiz and D. Astruc, J. Am. Chem. Soc.,
2002, 124, 1782; D. Astruc, C. Ornelas and J. Ruiz, Acc. Chem.
Res., 2008, 41, 841; D. Astruc, E. Boisselier and C. Ornelas, Chem.
Rev., 2010, 110, 1857.
10 A. M. Allgeier and C. A. Mirkin, Angew. Chem., Int. Ed., 1998,
37, 894; G. Liu, H. He and J. Wang, Adv. Synth. Catal., 2009,
351, 1610.
11 I. M. Lorkovic, R. R. Duff Jr. and M. S. Wrighton, J. Am. Chem.
Soc., 1995, 117, 3617; C. K. A. Gregson, V. C. Gibson, N. J. Long,
E. L. Marshall, P. J. Oxford and A. J. P. White, J. Am. Chem. Soc.,
2006, 128, 7410; M. D. Sanderson, J. W. Kamplain and
C. W. Bielawski, J. Am. Chem. Soc., 2006, 128, 16514;
A. B. Powell, C. W. Bielawski and A. H. Cowley, J. Am. Chem.
Soc., 2009, 131, 18232; A. G. Tennyson, V. M. Lynch and
C. W. Bielawski, J. Am. Chem. Soc., 2010, 132, 9420;
A. B. Powell, C. W. Bielawski and A. H. Cowley, J. Am. Chem.
Soc., 2010, 132, 10184; A. G. Tennyson, R. J. Ono, T. W. Hudnall,
D. M. Khramov, J. A. V. Er, J. W. Kamplain, V. M. Lynch,
J. L. Sessler and C. W. Bielawski, Chem.–Eur. J., 2010, 16, 304.
12 T. B. Higgins and C. A. Mirkin, Inorg. Chim. Acta, 1995, 240, 347;
E. T. Singewald, X. Shi, C. A. Mirkin, S. J. Schofer and
C. L. Stern, Organometallics, 1996, 15, 3062; A. M. Allgeier and
C. A. Mirkin, Organometallics, 1997, 16, 3071; C. S. Slone,
C. A. Mirkin, G. P. A. Yap, I. A. Guzei and A. L. Rheingold,
J. Am. Chem. Soc., 1997, 119, 10743; X. Liu, A. H. Eisenberg,
C. L. Stern and C. A. Mirkin, Inorg. Chem., 2001, 40, 2940;
D. A. Weinberger, T. B. Higgins, C. A. Mirkin, C. L. Stern,
L. M. Liable-Sands and A. L. Rheingold, J. Am. Chem. Soc.,
2001, 123, 2503.
Acknowledgements
The authors thank the CNRS, the Agence Nationale de la
Recherche for a grant to A.L. (ANR-07-JCJC-0041) and the
Institut National Polytechnique de Toulouse for a grant to
R.P. and A.L. (Soutien a la Mobilite Internationale).
´
Notes and references
1 For selected books and recent reviews, see: S. Dıez-Gonzalez,
´ ´
N-Heterocyclic Carbenes: From Laboratory Curiosities to Efficient
Synthetic Tools, RSC Publishing, Cambridge, UK, 2011;
F. Glorius, N-Heterocyclic Carbenes in Transition Metal Catalysis,
Springer-Verlag, Heidelberg, Germany, 2007; V. Cesar,
´
S. Bellemin-Laponnaz and L. H. Gade, Chem. Soc. Rev., 2004,
33, 619; F. E. Hahn and M. C. Jahnke, Angew. Chem., Int. Ed.,
2008, 47, 3122; S. Dıez-Gonzalez, N. Marion and S. P. Nolan,
´ ´
Chem. Rev., 2009, 109, 3612; J. C. Y. Lin, R. T. W. Huang,
C. S. Lee, A. Bhattacharyya, W. S. Hwang and I. J. B. Lin,
c
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2011
New J. Chem., 2011, 35, 2162–2168 2167

View Article Online
13 J. Wolf, A. Labande, J.-C. Daran and R. Poli, Eur. J. Inorg.
Chem., 2007, 5069.
2006, 25, 2126; X. Wang, S. Liu, L.-H. Weng and G.-X. Jin,
Chem.–Eur. J., 2007, 13, 188; Y. Zhang, C. Chen, S. C. Ghosh,
Y. Li and S. H. Hong, Organometallics, 2010, 29, 1374;
14 S. Gulcemal, A. Labande, J.-C. Daran, B. C¸ etinkaya and R. Poli,
¨
Eur. J. Inorg. Chem., 2009, 1806; N. Debono, A. Labande,
E. Manoury, J.-C. Daran and R. Poli, Organometallics, 2010,
29, 1879.
W. Ghattas, H. Muller-Bunz and M. Albrecht, Organometallics,
2010, 29, 6782; W. N. O. Wylie, A. J. Lough and R. H. Morris,
Organometallics, 2011, 30, 1236.
¨
15 I. V. Kourkine, C. S. Slone, E. T. Singewald, C. A. Mirkin,
L. M. Liable-Sands and A. L. Rheingold, Inorg. Chem., 1999, 38, 2758.
16 A. M. Allgeier, C. S. Slone, C. A. Mirkin, L. M. Liable-Sands,
G. P. A. Yap and A. L. Rheingold, J. Am. Chem. Soc., 1997,
119, 550.
17 J. A. Mata, A. R. Chianese, J. R. Miecznikowski, M. Poyatos,
E. Peris, J. W. Faller and R. H. Crabtree, Organometallics, 2004,
23, 1253.
22 F. H. Allen, Acta Crystallogr., Sect. B: Struct. Sci., 2002, B58,
380.
23 For a comprehensive review on chemical redox agents, see:
N. G. Connelly and W. E. Geiger, Chem. Rev., 1996, 96, 877.
24 The weak feature at 0.1 V is probably due to traces of oxygen. A
second experiment carried out on the same sample in the range ꢀ1
V/+1.2 V showed no such feature but only the ferrocene/
ferrocenium redox wave.
18 S. Saravanakumar, M. K. Kindermann, J. Heinicke and
M. Koeckerling, Chem. Commun., 2006, 640; D. M. Khramov,
V. M. Lynch and C. W. Bielawski, Organometallics, 2007, 26,
6042; D. Tapu, C. Owens, D. VanDerveer and K. Gwaltney,
Organometallics, 2009, 28, 270.
25 T. Sixt, M. Sieger, M. J. Krafft, D. Bubrin, J. Fiedler and
W. Kaim, Organometallics, 2010, 29, 5511.
26 A. G. Tennyson, D. M. Khramov, C. D. Varnado, P. T. Creswell,
J. W. Kamplain, V. M. Lynch and C. W. Bielawski, Organo-
metallics, 2009, 28, 5142.
19 A. Kascatan-Nebioglu, M. J. Panzner, J. C. Garrison,
C. A. Tessier and W. J. Youngs, Organometallics, 2004, 23, 1928.
20 A. P. da Costa, M. Sanau´ , E. Peris and B. Royo, Dalton Trans.,
27 A. Altomare, M. C. Burla, M. Camalli, G. L. Cascarano,
C. Giacovazzo, A. Guagliardi, A. G. G. Moliterni, G. Polidori
and R. Spagna, J. Appl. Crystallogr., 1999, 32, 115.
28 G. M. Sheldrick, Acta Crystallogr., Sect. A: Fundam. Crystallogr.,
2008, A64, 112.
2009, 6960; K. Ogata, T. Nagaya and S.-i. Fukuzawa,
J. Organomet. Chem., 2010, 695, 1675.
21 M. Poyatos, A. Maisse-Franc
¸
ois, S. Bellemin-Laponnaz, E. Peris
29 M. N. Burnett and C. K. Johnson, ORTEPIII, Report ORNL-6895,
Oak Ridge National Laboratory, Oak Ridge, Tennessee, US, 1996;
L. J. Farrugia, J. Appl. Crystallogr., 1997, 30, 565.
and L. H. Gade, J. Organomet. Chem., 2006, 691, 2713; R. Cariou,
C. Fischmeister, L. Toupet and P. H. Dixneuf, Organometallics,
c
2168 New J. Chem., 2011, 35, 2162–2168
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2011