Elusive free bisimino-N-heterocyclic carbene and its rearrangement by C-C coupling. Characterization of relevant iridium(I) and chromium(II) complexes

Article abstract of DOI:10.1021/om400599v

The potentially pincer-type N-heterocyclic (NHC) precursor salt 1,3-bis((E)-1-((2,6-diisopropylphenyl)imino)ethyl)-1H-imidazol-3-ium chloride, 3(Cl^-), was prepared by reaction of N-(2,6-diisopropylphenyl) acetimidoyl chloride (1) with N-(1-(1H-imidazol-1-yl)ethylidene)-2,6- diisopropylaniline (2). Attempts to crystallize 3(Cl^-) at different temperatures afforded single crystals of 3(Cl^-)·MeCN from MeCN and the mixed salt 4 from toluene, in which 2H⁺(Cl^-) and 3(Cl^-) have cocrystallized. Deprotonation of 3(Cl^-) yielded 1,3-bis[1-(2,6-diisopropylphenylimino)ethyl]imidazol-2-ylidene (5), the first bis(imino)-N-heterocyclic free carbene, and a novel product, 6, resulting formally from the insertion of the carbene carbon atom of 5 into the C-H bond of the N=CHN moiety of 2 and formation of a new C-C bond. The structures and NMR spectra of 4 and 6 indicate that loss of one N-substituent from the bis(imino) NHC core by exocyclic imidazole N-C bond cleavage is relatively easy. Reaction of 5 with [Ir(μ-Cl)(cod)]₂ in pentane afforded in good yield the mononuclear, 16e Ir(I) NHC complex, [IrCl(cod)(5)] (7). The reaction of 6 with [CrCl₂(THF)₂] led to the isolation of the paramagnetic complex trans-[CrCl₂(6)₂] (8), in which the coordination geometry at the chromium is slightly distorted square planar. This complex is the first structurally characterized Cr(II) imidazole complex. The compounds 3(Cl^-), 4, 6, 7, and 8 were identified by X-ray diffraction methods.

Full text of DOI:10.1021/om400599v

Article
pubs.acs.org/Organometallics
Elusive Free Bisimino-N-heterocyclic Carbene and Its Rearrangement
by C−C Coupling. Characterization of Relevant Iridium(I) and
Chromium(II) Complexes
Ping Liu,† Marcel Wesolek, Andreas A. Danopoulos,* and Pierre Braunstein*
Laboratoire de Chimie de Coordination, Institut de Chimie (UMR 7177 CNRS), Universite
90032, 67081 Strasbourg, France
́
de Strasbourg, 4 Rue Blaise Pascal, CS
S
* Supporting Information
ABSTRACT: The potentially pincer-type N-heterocyclic
(NHC) precursor salt 1,3-bis((E)-1-((2,6-diisopropylphenyl)-
imino)ethyl)-1H-imidazol-3-ium chloride, 3(Cl−), was pre-
pared by reaction of N-(2,6-diisopropylphenyl)acetimidoyl
chloride (1) with N-(1-(1H-imidazol-1-yl)ethylidene)-2,6-
diisopropylaniline (2). Attempts to crystallize 3(Cl−) at
different temperatures afforded single crystals of 3(Cl−)·
MeCN from MeCN and the mixed salt 4 from toluene, in
which 2H+(Cl−) and 3(Cl−) have cocrystallized. Deprotona-
tion of 3(Cl−) yielded 1,3-bis[1-(2,6-diisopropylphenylimino)-
ethyl]imidazol-2-ylidene (5), the first bis(imino)-N-hetero-
cyclic free carbene, and a novel product, 6, resulting formally
from the insertion of the carbene carbon atom of 5 into the C−H bond of the NCHN moiety of 2 and formation of a new C−
C bond. The structures and NMR spectra of 4 and 6 indicate that loss of one N-substituent from the bis(imino) NHC core by
exocyclic imidazole N−C bond cleavage is relatively easy. Reaction of 5 with [Ir(μ-Cl)(cod)]2 in pentane afforded in good yield
the mononuclear, 16e Ir(I) NHC complex, [IrCl(cod)(5)] (7). The reaction of 6 with [CrCl2(THF)2] led to the isolation of the
paramagnetic complex trans-[CrCl2(6)2] (8), in which the coordination geometry at the chromium is slightly distorted square
planar. This complex is the first structurally characterized Cr(II) imidazole complex. The compounds 3(Cl−), 4, 6, 7, and 8 were
identified by X-ray diffraction methods.
imino functional groups as N-substituents could potentially
lead to NHC diimine-type complexes, reminiscent of the α-
diimine and pyridine diimine complexes well known for their
remarkable catalytic properties.18−20 The attractive features of
the imino-NHCs as hybrid donor ligands are based on the
association of a σ-donor/π-acceptor imine with a strong σ-
donor/poor π-acceptor NHC functionality. Coleman, Green, et
al. have isolated Ag(I), Pd(II), and Rh(I) complexes with
imino-NHC ligands,21−23 and Tilset et al. reported recently a
chelated imine-functionalized NHC Rh(I) complex displaying a
remarkable high reactivity and cis-selectivity in the cyclo-
propanation of alkenes.24 Early transition metal complexes were
isolated by Lavoie et al.25,26
Difficulties associated with extending the ligand design to
pincer tridentate N,N′-bis(imino)azolium proligands or to the
corresponding NHC ligands were reported by Bildstein et al. in
2004.27 Recently, Lavoie’s group reported 1,3-bis(N-
arylimino)imidazolium salts and obtained NHC metal com-
plexes with only one or no imino group coordinated to the
metal. The clean preparation and isolation of a bis(imino)-N-
heterocyclic free carbene (N(imine)CN(imine)) has not yet been
INTRODUCTION
■
Pincer-based iridium complexes exhibit unique reactivity as
catalysts in important organometallic transformations, including
the dehydrogenation of alkanes to olefins,1,2 of primary amines
to nitriles,1−3 and of borane-amine complexes,1,2,4 the isomer-
ization of olefins,5 and the intermolecular C−H activation of
substituted aromatic compounds.6,7 The further development
of new pincer-based iridium complexes with improved activity
and selectivity is an attractive goal, which is also justified by the
beneficial effects brought about by the thermal stability and
structural rigidity of pincer ligands, their potential non-
innocence,8 and the metal−ligand cooperativity9 in catalytic
transformations. The design of new ligands and their controlled
modifications have been used to fine-tune the behavior of metal
complexes toward their successful use in catalysis.
In recent years, tailor-made N-substituted imidazol-2-
ylidenes have been incorporated in various tridentate or
pincer-type ligands, which include NCN,10 CCC,11,12
PCP,13,14 CNN,15 and CNC donor sets (C denotes a NHC-
based donor).16,17 In addition to the N-heterocyclic carbene
(NHC) donor functionality, the N-substituents that are part of
the pincer-type system play a key role for the modification of
the stability, electronic properties, coordination behavior, and
catalytic properties of the resulting complexes. The choice of
Received: June 24, 2013
Published: October 24, 2013
© 2013 American Chemical Society
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achieved.28,29 The versatility of such bis(imino)-NHCs as new
potential pincer ligands could reside in the diversity of
coordination modes they could adopt: monodentate, bidentate,
or tridentate through the carbene carbon and one or both
iminic fragments (Scheme 1). In addition, plausible tautomeric
forms of the ligand may be envisaged when an enolizable imine
is present.
Scheme 2. Synthesis of the Imidazolium Salt 3(Cl−)
Scheme 1. Plausible Bonding Modes of Bis(imino)-NHC
Ligands
the corresponding imidazolium backbone carbons appear at δ
138.9 and 118.1 ppm, respectively.29
Attempts were made to obtain single crystals of 3(Cl−) by
varying crystallization solvents and temperatures. Whereas
single crystals of 3(Cl−)·MeCN suitable for X-ray diffraction
were grown from a saturated acetonitrile solution at −30 °C,
dissolution of 3(Cl−) in hot, dry toluene, followed by slow
diffusion of pentane into a saturated solution at room
temperature, afforded single crystals of the mixed salt 4. The
identity of 3(Cl−) and 4 was unambiguously established by X-
ray diffraction and spectroscopic methods. An ORTEP drawing
of the imidazolium salt 3(Cl−)·MeCN is shown in Figure 1,
with selected bond distances and angles.
Although a number of stable carbene complexes of iridium
are known, those with such bis(imino)-NHC ligands have not
been investigated, to the best of our knowledge. The emerging
interest in iridium NHC complexes associated with their
catalytic properties2,5,11,14 led us to attempt their synthesis with
the bis(imino)-NHC ligands. Here we report a detailed study
on the synthesis, isolation, and characterization of the elusive
free carbene 1,3-bis[1-(2,6-diisopropylphenylimino)ethyl]-
imidazol-2-ylidene (5), in the course of which an unexpected
product resulting from C−C coupling of two imidazole-
containing fragments was identified. The latter was used as a
ligand toward Cr(II), and we also report one iridium complex
obtained from 5.
Figure 1. ORTEP of the crystal structure of 3(Cl−) in 3(Cl−)·MeCN.
Hydrogen atoms, except the imidazolium proton, and one acetonitrile
molecule of crystallization are omitted for clarity. Thermal ellipsoids
are set at 30% probability. Selected bond distances (Å) and angles
(deg): C1−N1 1.328(5), N1−C2 1.387(5), C2−C3 1.339(6), C3−N2
1.394(5), C1−N2 1.334(5), N1−C4 1.444(5), C4−C5 1.489(6), C4−
N3 1.244(5), N2−C18 1.448(5), C18−N4 1.260(5), C18−C19
1.493(6); N1−C1−N2 108.1(4), N3−C4−N1 114.3(4), N1−C4−
C5 114.0(3), N4−C18−N2 113.0(4), N2−C18−C19 114.9(4).
RESULTS AND DISCUSSION
■
1. Synthesis of the Bis-imino-imidazolium Salt 3(Cl−)
and Its Stability toward Imidazole N−Cimino Cleavage.
The salt 1,3-bis((E)-1-((2,6-diisopropylphenyl)imino)ethyl)-
1H-imidazol-3-ium chloride, 3(Cl−), was prepared by estab-
lished methods30 as outlined in Scheme 2. Neat N-(2,6-
diisopropylphenyl)acetimidoyl chloride, 1, a colorless and air-
sensitive oil, was reacted under nitrogen with a CH2Cl2 solution
of imidazole to give the corresponding white, air-stable N-(1-
(1H-imidazol-1-yl)ethylidene)-2,6-diisopropylaniline, 2, in 80%
yield. Reaction of the latter with a second equivalent of 1 in
toluene afforded the desired 1,3-bis((E)-1-((2,6-
diisopropylphenyl)imino)ethyl)-1H-imidazol-3-ium chloride,
3(Cl−), in 83% yield as an insoluble, air-sensitive, white solid.
The NMR data of 3(Cl−) in CDCl3 indicate a symmetrical
The salt 3(Cl−) crystallizes with one molecule of acetonitrile
in the monoclinic space group C2, and both imine groups are in
the E conformation. The C4−N3 and C18−N4 bond distances
of 1.244(5) and 1.260(5) Å, respectively, are in the usual range
for double bonds, and both imines are slightly tilted off the
mean plane of the imidazolium ring, as indicated by the N3−
C4−N1−C1 and N4−C18−N2−C1 torsion angles of
170.8(4)° and 177.7(4)°, respectively. The angles between
the planes of the imidazole and the 2,6-diisopropylphenyl rings
are 66.5° and 106.7°. Further discussion on the noncovalent
interactions in the crystal structure will be presented below.
1
structure in solution. The H NMR spectrum contains the
deshielded signal of the NCHN-imidazolium proton at δ 12.43
ppm and the resonance of the imidazole backbone protons
(NCHCHN) at δ 8.29 ppm, while the 13C NMR resonances of
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The single crystals of the mixed salt 4 belong to the P1 space
solution was obtained that contained a major component that
gave rise to signals at δ 7.93, 7.52, 7.20, 2.69, 1.41, and 1.10−
1.03 ppm, consistent with the formation of the neutral
imidazole 2. However, carrying out the recrystallization of
3(Cl−) from hot toluene in a glovebox resulted in the isolation
of crystalline 3(Cl−)·toluene, as established by 1H NMR
̅
group and the asymmetric unit contains two cations, 3 and
2H+, two chlorides, and one toluene molecule, thus
corresponding to the cocrystallization of 2H+(Cl−) and
3(Cl−). The conditions for the cleavage of the imidazole N−
Cimino bond in 3(Cl−) that led to 2H+ will be discussed below.
Similarly to 3, all the imine groups in the constituents of 4
adopt the E configuration, and their C−N bond lengths are
very similar to those in 3 (C4−N3 = 1.259(3) Å, C18−N4 =
1.260(3) Å, and C35−N7 = 1.252(3) Å), consistent with a
significant double-bond character.24,29,31 Further evidence for
the constitution of 4 was obtained from 1H NMR spectroscopy
in CD2Cl2. For 2H+, the NCHN proton (δ 10.31) and the
backbone protons (NCHCHN) (δ 7.97 and 7.37) are upfield
shifted (Δδ = 2.20 ppm and Δδ = 0.34 and nearly 0.29 ppm,
respectively) with respect to 2, owing to the protonation of the
heterocycle. The corresponding 13C NMR resonances are
found at 136.3, 121.7, and 117.5 ppm, respectively. For the
imidazolium cation 3, the NCHN proton is upfield-shifted to δ
12.27, but the backbone protons (NCHCHN) at δ 8.26 are not
affected compared to those in 3(Cl−). In addition, the 1H NMR
spectrum of 4 exhibits two overlapping septets in the range δ
2.69−2.53 (cf. the septet at δ 2.57 in 3(Cl−)), assigned to the
CH(CH3)2 protons and four doublets at δ 1.07, 1.02 ppm for
2H+ and 1.11, 1.05 for 3, respectively, for the CH(CH3)2
protons.
1
spectroscopy and X-ray diffraction analysis (SI). When the H
NMR spectrum of 3(Cl−) in d8-THF was monitored at room
temperature as a function of time, progressive formation of 2
was observed, which did not reach completion (see SI). We
suggest therefore that nucleophilic attack of the chloride anion
in 3(Cl−) on the Cimino carbon, with liberation of 2 and N-(2,6-
diisopropylphenyl)acetimidoyl chloride (1), can occur in THF
at room temperature, where 3(Cl−) is well soluble, but it
requires thermal activation in toluene, also to solubilize 3(Cl−)
(this N−C bond-breaking reaction corresponds to the reverse
of the synthesis of 3(Cl−), the latter being driven by the poor
solubility of the product in the reaction medium) (Scheme 3).
The possibility of a nucleophilic attack of the chloride anion in
3(Cl−) on the Cimino carbon is further supported by the
−
stability, under similar conditions, of 3(BF4 ), obviously due to
the non-nucleophilic nature of the anion. Indeed, when
−
3(BF4 ) was dissolved in hot C6D6, no formation of 2 was
observed by 1H NMR spectroscopy (see SI). If traces of
adventitious water are present when handling 3(Cl−),
hydrolysis of the liberated 1 will readily produce HCl, which
will form 2H+(Cl−) upon reaction with 2, as found in 4.
Previous reports have pointed to the fragmentation of N,N′-
bis(imino)azolium species as being either responsible for their
inaccessibility27 or the result of their hydrolysis.29
2. C−H···Cl Hydrogen Bonding in the Solid-State
Structures of 3(Cl−)·MeCN and 4. Further to hydrogen-
bonding interactions of the type X−H···Y (X, Y = O, N, S),
which have been much investigated, in particular because of
their importance in supramolecular chemistry, C−H···Cl
interactions attract increasing attention.32 The C−H···Cl
hydrogen bond was first reported by Taylor and Kennard in
198233 and is defined by the distance between the proton and
Cl being shorter than the sum of the van der Waals radii of H
and Cl (3.0 Å) and by a donor−proton−Cl angle of at least
90°. H-bonding interactions involving imidazolium salts have
also been shown of relevance to their reactivity and subsequent
metalation.34
Figure 2. ORTEP of the structure of 4, corresponding to the
cocrystallization of 2H+(Cl−) and 3(Cl−). H atoms, except the
imidazolium protons, and one toluene molecule of crystallization are
omitted for clarity. Thermal ellipsoids are set at 30% probability.
Selected bond distances (Å) and angles (deg): C1−N1 1.334(3), C1−
N2 1.327(3), C2−N1 1.398(3), C2−C3 1.335(4), C3−N2 1.389(3),
C4−N1 1.443(3), C4−N3 1.259(3), C18−N2 1.440(3), C18−N4
1.260(3), C32−N5 1.349(3), C32−N6 1.305(3), C33−N5 1.382(3),
C34−N6 1.376(4), C33−C34 1.333(4), C35−N7 1.252(3), C35−N5
1.437(3); N2−C1−N1 108.0(2), N3−C4−N1 114.5(2), N1−C4−C5
114.5(2), N4−C18−N2 113.7(2), N2−C18−C19 15.54(2), N6−
C32−N5 108.3(2), N7−C35−N5 115.3(2), N5−C35−C36 115.3(2).
The chemical shift of 12.43 ppm (in CD2Cl2) for the
imidazolium proton of 3(Cl−) contrasts with that of 3(BF4 ) at
−
10.10 ppm but is very similar to that found for 1,3-bis[1-(2,6-
dimethylphenylimino)ethyl]imidazolium chloride (at 12.54
ppm in CDCl3 and 12.85 ppm in THF-d8),29 here too being
consistent with the occurrence of C−H···Cl− hydrogen-
bonding interactions.34 A careful inspection of the structures
of 3(Cl−)·MeCN and 4 showed that they indeed provide a
good opportunity to examine C−H···Cl− interactions involving
imidazole compounds. A C−H···Cl− hydrogen bond is
apparent in the single-crystal X-ray structure analysis of
3(Cl−)·MeCN. Every Cl− interacts with two neighboring
imidazolium cations through three C−H···Cl− hydrogen-
bonding interactions (Cl···H1#1= 2.636 Å, Cl···H19B =
2.863 Å, and Cl···H19C#1 = 2.839 Å, respectively). The
strongest C−H···Cl− interaction (associated with the shortest
H···Cl− distance) involves the chloride interacting with proton
H1 attached to crystallographic C1 (Figure 3a). This results in
the formation of a one-dimensional left-handed helical
supramolecular chain involving C−H···Cl− hydrogen bonding
The successful isolation of 3(Cl−) from toluene (Scheme 2)
is based on its poor solubility in this solvent and its rapid
precipitation after formation. It was unexpected, however, that
recrystallization of 3(Cl−) from anhydrous hot toluene would
in part result in N−C bond cleavage and formation of
1
2H+(Cl−), as found in 4. Careful H NMR experiments in a
NMR Young’s tube were conducted with a d6-benzene
suspension of 3(Cl−). After heating to 60 °C for ca. 1 h, a
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Scheme 3. Reversible Formation and Cleavage of the Imidazole−Cimino Bond
(Figure 3b). The helix has a calculated pitch of 10.131 Å, and it
contains three Cl− ions per turn. Every Cl− also interacts with
an acetonitrile molecule through C−H···Cl− hydrogen bonds
(Cl···H33B = 2.777 Å). In addition, C2−H···N3 hydrogen-
bonding interactions (3.255 Å) occur between supramolecular
chiral helical chains, which form a 2D layer (Figure 3c).
The intermolecular N−H···Cl− and C−H···Cl− H-bonds in
the crystals of 4 are depicted in Figure 4. The chloride anion
Cl2 forms an intermolecular N−H···Cl− bond (the distance
between Cl2 and H6 is 2.049 Å, with a bond angle of 172.27°).
The chloride Cl1 bridges between two neighboring imidazo-
lium cations 3 and one cation 2H+ through four C−H···Cl−
hydrogen-bonding interactions. The distances between Cl1 and
the proton of the adjacent C−H group are in the range 2.398−
2.767 Å, with bond angles ranging from 138.72° to 172.51°,
thus indicating that this chloride takes part in the formation of
intermolecular N−H···Cl− or C−H···Cl− H-bonds
spectrum of 6 did not allow its straightforward identification.
However, single crystals suitable for X-ray diffraction analysis
were obtained (see below). After removal of the crystalline
material initially formed from the pentane solution (i.e., 6), the
mother liquor was shown by 1H NMR to contain a mixture of 5
and 6. After the solution was taken to dryness, the resulting
solid was washed with pentane several times to eliminate 5,
which afforded an additional amount of pure 6. Compound 6
was obtained as the major product when the deprotonation of
3(Cl−) was performed at room temperature; the minor
product, compound A, is discussed below.
The solid-state structure of 6 is displayed in Figure 5. It
features a 1,3-bis(imino)-2,3-dihydroimidazole moiety linked
via the crystallographic C1 carbon to the C32 carbon of an N-
imino-substituted imidazole ring. The structure of 6 may be
understood as arising from the unexpected C−C bond
formation formally resulting from insertion of the carbene
carbon of 5 into the C−H bond of the NCH-N moiety of 2,
the latter having been produced by cleavage of the exocyclic
imidazole N−Cimino bond of the bis(imino) NHC backbone
(see above, Scheme 3).
3. Deprotonation of the Imidazolium Salt 3(Cl−):
Isolation of the Free Carbene 5 and of the C−C
Coupling Product 6. Although past attempts to isolate free
carbenes from 1,3-bis(imino)imidazolium salts were unsuccess-
ful,28,29 we felt that the nature of the solvent may be critical for
the isolation of the desired product. Therefore, we reacted a
suspension of 3(Cl−) in pentane with a suspension of
potassium bis(trimethylsilyl)amide in the same solvent while
maintaining careful temperature control (from −78 °C to room
temperature over 12 h, not exceeding 1 h at room temperature;
see Experimental Section and Scheme 4). The pure carbene 5
was isolated as a colorless, air- and moisture-sensitive
microcrystalline solid in 85% yield. Its identity was established
The 13C NMR spectrum (C6D6, 298 K) of 6 shows a
resonance at δ 72.0, characteristic for the C2 carbon in the 1,3-
bis(imino)-2,3-dihydro-imidazole moiety, with the correspond-
ing proton resonance at δ 7.79, while the C2 carbon in the
(monoimino)imidazole moiety resonates at δ 149.0, a value
similar to that found in the C−C coupling product obtained
from N,N′-diethyl-3,4-dihydroimidazolium (δ = 66 and 173
ppm).35 One set of 1H NMR signals corresponds to the
backbone protons of the 1,3-bis(imino)-2,3-dihydroimidazole
moiety (at δ 5.78 ppm), and another set corresponds to the
backbone protons of the (monoimino)imidazole group (at δ
1
by H and 13C NMR techniques.
1
The H NMR spectral features of 5 provide evidence for a
3
symmetrically substituted heterocycle, and the disappearance of
the resonance at δ 12.43 in the spectrum of 3(Cl−) supports
formation of a free carbene. In the 13C NMR spectrum, the
8.17 ppm with J(HH) = 1.5 Hz and another masked in the
range 7.12−6.97 ppm); the latter are low-field shifted when
compared to imidazole 2.
C
NHC was observed at δ 222.9 ppm. This free carbene is stable
The crystal structure of 6 is unprecedented. There are
obvious questions to address regarding the sequence of C−H
deprotonation, N−Cimino bond cleavage, and C−C bond
formation as well as the driving forces for these events. Since
at room temperature in the solid state in a glovebox for at least
3 months. Its solutions in C6D6 may be stored at low
temperature (frozen state, −40 °C) for weeks without
noticeable decomposition but can be kept at room temperature
1
we observed by H NMR that mild thermolysis of 5 led to
1
for only ca. 1 h, as indicated by H NMR monitoring, which
formation of 2, we first envisaged the possibility that cleavage of
the N−Cimino bond of 5 could occur and be followed by formal
insertion of the carbene carbon of 5 into the C−H bond of the
resulting imidazole 2 to give 6 (Scheme 5). The base present
during the synthesis of 5 could also act as a catalyst.35 However,
the independent reaction of 5 with 2 in C6D6 (NMR
revealed inter alia formation of 2 (see below Scheme 5).
When the synthesis of 5 was repeated in more concentrated
solutions, new product 6 was formed slowly and was isolated by
fractional crystallization from the light yellow pentane solution
(see Experimental Section). The complicated 1H NMR
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Figure 4. View of C−H···Cl hydrogen-bonding interactions involving
a chloride ion in 4. H atoms, except those involved in hydrogen-
bonding interactions, and i-Pr groups are omitted for clarity. For 4, #1:
−1+x, y, z; #2: −x, 1−y, 1−z.
5), but in none of these experiments was the formation of 6
evident, except when the reaction of 3(Cl−) with KN(SiMe3)2
was performed in d8-toluene at room temperature. This rules
out that for the formation of 6 the initial formation of the C1−
C32 bond between these components is necessary, as discussed
in related systems.36 To examine the possible role of the
chloride anion of 3(Cl−) in the process leading to 6 and A, we
−
reacted 3(BF4 ) with KN(SiMe3)2 in d6-benzene at room
1
temperature. Slow formation of 6 and A was monitored by H
NMR spectroscopy. These data suggest that the counterion
associated with 3 does not play an essential role in the
transformations considered.
Only small amounts of compound A were obtained, and
crystallization by slow evaporation of its pentane solution
afforded single crystals that readily lost pentane. Although the
quality of the X-ray data was not satisfactory, they were
sufficient to establish unambiguously the atom connectivity
within the molecule (Scheme 5). The formation of A results
from a complex sequence of reactions involving formally C−C
bond formation between the carbene carbon atom of 5 and the
carbon atom of the methyl group of an imine fragment, leading
to a six-membered heterocycle fused with the five-membered
ring of the NHC moiety. Activation of C−H bonds from
methyl substituents has led to the CCH2 and CH2−CH
moieties. At this stage, the reaction mechanisms for the
formation of 6 and A remain unclear and would require further
thorough investigations.
Figure 3. (a) C−H···Cl hydrogen bonds in 3(Cl−)·MeCN (H atoms,
except those involved in hydrogen-bonding interactions, and i-Pr
groups are omitted for clarity) (for 3, #1: 1.5−x, 0.5+y, 1−z); (b) one-
dimensional left-handed helical supramolecular chain formed by C−
H···Cl− hydrogen-bonding interactions (H atoms, except those
involved in hydrogen bonding interactions, i-Pr groups, and the
acetonitrile molecule are omitted for clarity); (c) 2D supramolecular
layer constituted by the chiral helices of 3(Cl−)·MeCN.
4. Synthesis of Iridium Complexes. Initial attempts to
prepare NHC iridium(I) complexes by reaction of [Ir(μ-
Cl)(cod)]2 or [Ir(μ-Cl)(coe)2]2 with the free carbene 5,
generated in situ by deprotonation of the imidazolium salt with
a slight excess of base, were unsuccessful, probably owing to
competitive decomposition of the free carbene. We then
decided to use isolated 5 and react it with 0.5 equiv of [Ir(μ-
Cl)(coe)2]2 in pentane. This gave intractable reaction products.
Fortunately, the reaction of isolated 5 with [Ir(μ-Cl)(cod)]2
afforded [IrCl(cod)(5)] (7), which was isolated as a yellow
solid in good yield (Scheme 6).
experiment), in the presence or not of KN(SiMe3)2, did not
lead to the formation of 6, even after a few hours at room
temperature (Scheme 5). In order to gain further insight into
these observations, we monitored by 1H NMR spectroscopy in
C6D6, d8-toluene, or d8-THF various reactions between
different precursors containing potential building blocks of 6,
i.e., 2, 3(Cl−), and 5, in the presence of KN(SiMe3)2.
Transformations of 3(Cl−) or 5 to 2 were observed (Scheme
1
The H NMR data of 7 in solution were consistent with the
presence of a mirror plane passing through the Ir, Cl, and CNHC
atoms. The coordination of the CNHC donor to the metal center
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Scheme 4. Synthesis of the Free NHC Ligand 5 in Pentane
Scheme 5. Experiments Related to the Formation of Compound 6 (see text)
was evidenced by a low-field resonance at δ 184.8 ppm in the
13C{1H} NMR spectrum. The NHC backbone protons
(NCHCHN) were observed at δ 8.00 ppm, and the 13C
NMR signal of the corresponding C atoms appears at δ 120.6
ppm. The vinylic protons of the cod ligand appear at δ 4.63 and
2.86, and the corresponding olefinic carbons at δ 85.0 and 52.6
ppm. As observed in other Ir(I)−cod−NHC complexes,37 the
upfield resonance corresponds to the CH protons trans to
the NHC carbon. Single crystals of 7 suitable for X-ray
diffraction were grown by slow diffusion of a pentane solution
of 5 into a saturated THF solution of [Ir(μ-Cl)(cod)]2 at
ambient temperature under nitrogen. The molecular structure
of 7 and significant bond distances and angles are shown in
Figure 6.
The coordination sphere around the iridium center is defined
by the two olefinic bonds of the cyclooctadiene ligand, the C1
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Scheme 6. Synthesis of the Ir(I) NHC Complex 7
Figure 5. ORTEP of the molecular structure of 6. H atoms are
omitted for clarity, except the imidazolium proton. Three of the
isopropyl groups (C15, C17, C27, C28, C44, C45) were found
disordered in two positions having the tertiary atom in common and
with equal occupancy factors. Thermal ellipsoids are set at 30%
probability. Selected bond distances (Å) and angles (deg): C1−C32
1.511(3), C1−N1 1.474(3), C1−N2 1.473(3), C2−C3 1.318(3), C2−
N1 1.404(3), C3−N2 1.403(3), C4−N3 1.292(3), C4−N1 1.366(3),
C18−N4 1.282(3), C18−N2 1.370(3), C32−N6 1.314(3), C32−N5
1.391(3), C34−N5 1.393(3), C33−N6 1.373(3), C33−C34 1.345(4),
C35−N7 1.260(3), C35−N5 1.434(3); N2−C1−N1 100.60(18),
N2−C1−C32 111.14(18), N1−C1−C32 111.53(19), N3−C4−N1
116.7(2), N1−C4−C5 115.9(2), N4−C18−N2 117.6(2), N2−C18−
C19 115.3(2), N6−C32−N5 111.1(2), N6−C32−C1 119.5(2), N5−
C32−C1 129.4(2), N7−C35−N5 114.9(2), N5−C35−C36 118.1(2).
Figure 6. ORTEP of the molecular structure of 7. H atoms are
omitted for clarity. Thermal ellipsoids are set at 30% probability.
Selected bond distances (Å) and angles (deg): Ir1−C1 2.019(4), Ir1−
C32 2.113(5), Ir1−C33 2.114(4), Ir1−C36 2.166(5), Ir1−C37
2.169(5), Ir1−Cl1 2.3630(1), C32−C33 1.415(6), C36−C37
1.394(7), C1−N1 1.374(5), N1−C2 1.405(5), N1−C4 1.443(5),
C4−N4 1.266(5), C2−C3 1.332(6), C3−N2 1.401(5), N2−C18
1.436(5), C18−N3 1.261(6), C1−N2 1.386(5); C1−Ir1−C32
90.22(2), C1−Ir1−C33 90.92(2), C1−Ir1−C36 161.3(2), C1−Ir1−
C37 160.8(2), C1−Ir1−Cl1 92.64(1), C36−Ir1−Cl1 89.57(2), C37−
Ir1−Cl1 90.43(2), C32−Ir1−Cl1 160.15(1), C33−Ir1−Cl1
160.21(1), C32−Ir1−C33 39.12(2), C36−Ir1−C37 37.5(2), N1−
C1−N2 102.9(3), C1−N1−C4 128.4(3), N1−C4−N4 113.4(4), N1−
C4−C5 117.3(4), N4−C4−C5 129.2(4), C1−N2−C18 128.6(3),
N2−C18−N3 113.5(4), N2−C18−C19 117.7(4), N3−C18−C19
128.8(4).
atom of the NHC ligand, and a terminal chloride atom, leading
to a slightly distorted square-planar geometry, as evidenced by
the trans bond angles Cl1−Ir1−C32/C33 160.15(1)°/
160.21(1)° and C1−Ir1−C36/C37 161.3(2)°/160.8(2)°, con-
sistent with the metal 16-valence-electron configuration. The
molecule is bisected by a plane of symmetry passing through
C1, Ir1, and Cl1 that renders the two imine ends of ligand 5
equivalent, in agreement with the NMR data in solution. The
heterocycle of ligand 5 forms angles of 45.2° and 48.9° with the
bulky 2,6-i-Pr2C6H3 (diisopropylphenyl, dipp) rings.
In 7, the NHC ligand binds to the metal in a monodentate
κ1-fashion through the carbene carbon, and the two imine
groups assume an E configuration, directing the lone pairs away
from the metal. Consistent with the NMR data in solution, the
coordination of the cod ligand exhibits one double bond
disposed trans to the carbene and the other trans to the
chloride, which has implications in the Ir−C and C−C bond
distances owing to the different trans influences of NHC and
Cl. Thus, the Ir1−C36/C37 distances are longer than Ir1−
C32/C33 as a consequence of the larger trans influence of the
NHC ligand, and consistently, the C36−C37 double bond is
slightly shorter than C32−C33 owing to reduced back-bonding
from the metal. The Ir−carbene bond length of 2.019(4) Å is
similar to those found in other Ir(I) carbene complexes.11,12,16
The imino groups are less twisted away from the mean plane of
the azole ring (the torsion angles are N3−C18−N2−C1
(153.3°) and N4−C4−N1−C1 (151.6°), respectively) than in
3(Cl−) (170.8(4)° and 177.7(4)°). As expected, the C1−N1
and C1−N2 bonds in 7 (1.374(5) and 1.386(5) Å) are longer
than in 3(Cl−) (1.328(5) and 1.334(5) Å) owing to the
decreased π-delocalization over the azole ring. As a result, the
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N1−C1−N2 bond angle of 102.9(3)° is also significantly less
obtuse than in 3(Cl−) (108.1(4)°).
The FTIR spectrum of 7 shows an absorption band at 1672
cm−1 for the C−N stretching vibration, a value consistent with
an uncoordinated function. This value is slightly lower than or
similar to those in 3(Cl−) (1695 cm−1) and 5 (1673 cm−1).
5. Synthesis of the Chromium Complex trans-
[CrCl2(6)2] (8) from the C−C Coupling Product 6. The
unusual donor arrangement and steric features of 6 prompted
us to initiate studies on its coordination chemistry. In view of
our interest in the use of chromium complexes for the
oligomerization of alkenes, we reacted 6 with [CrCl2(THF)2].
This led to the isolation of complex 8 after crystallization from
THF/pentane (Scheme 7). Paramagnetic 8 was characterized
by 1H NMR spectroscopy and elemental analysis, and its
structure was fully established by X-ray diffraction.
Scheme 7
Figure 7. ORTEP of the centrosymmetric chromium complex 8 in 8·
4THF. H atoms and THF molecules of crystallization in the
asymmetric unit are omitted for clarity. Thermal ellipsoids are set at
30% probability. Selected bond distances (Å) and angles (deg): Cr1−
Cl1 2.3154(8), Cr1−N1 2.124(3), C1−N1 1.317(5), C2−N1
1.377(4), C2−C3 1.345(4), N1−Cr1−Cl1 90.72(7), N1−Cr1−Cl1′
89.28(7).
found in other adducts of CrCl2 with two bulky trans N-
heterocyclic carbenes39 (e.g., in trans-[CrCl2(IPr)2] [Cr(1)−
Cl(1)/Cl(2) = 2.3140(5)/2.3108(5) Å (IPr = 1,3-bis(2,6-
diisopropylphenyl)-1H-imidazol-2-ylidene, :C[N{C6H3(i-Pr)2-
2,6}CH]2) and are shorter than in analogues with less
congested NHCs [e.g., in trans-[CrCl2(IPrMe)2], [Cr(1)−
Cl(1)/Cl(2) 2.3477(8)/2.3461(9) Å (IPrMe = :C[N(i-Pr)C-
(Me)]2)]. Surprisingly, there are less than 10 Cr imidazole
complexes in the CSD and none of them with Cr(II).40
Compared to the free ligand 6, the bond lengths in complex 8
were only slightly affected by the coordination to chromium [cf.
C32−N6 1.314(3), C33−N6 1.373(3), C33−C34 1.345(4) Å
and C1−N1 1.317(5), C2−N1 1.377(4), C2−C3 1.345(4) Å,
respectively]. Therefore, the emerging coordination behavior of
6 is relevant to that of a bulky imidazole without interference
from the dangling imine functions. Ongoing studies are under
way in our group to further study the coordination properties
of 6 as well as the catalytic reactivity of 8.
1
CONCLUSIONS
The H NMR data showed signals assignable to the protons
■
of the isopropyl groups from the aromatic wingtips of the 1,3-
bis(imino)-2,3-dihydroimidazole moiety that are furthest from
the chromium center; these were only broadened but not
shifted (see SI). In contrast, the signals associated with the
protons of the directly coordinated (monoimino)imidazole
moiety and of the imidazole and the methyl of the 1,3-
bis(imino)-2,3-dihydroimidazole moiety were too broad to be
observed at room temperature.
Single crystals of 8 suitable for X-ray diffraction were grown
slowly from the THF solution. The molecular structure of 8
and significant bond distances and angles are shown in Figure
7.
The molecule is centrosymmetric, with the coordination
geometry at the chromium being slightly distorted square
planar with trans imidazole donors originating from 6. The N−
Cr− Cl angle is close to 90° (90.72°). Although square planar
chromium(II) complexes are not rare,38 the observed Cr−Cl
bond lengths (2.3154(8) Å) are best compared with those
The synthesis and study of the new N,N′-bis(imino)azolium
salt 3(Cl−), its corresponding bis(imino)-N-heterocyclic free
carbene 5, and iridium complex 7 demonstrate an unusual and
complex chemistry of all the species (i.e., the ligand precursors,
the free NHC, and the metal complexes) relevant to this
potentially tridentate ligand framework. Deprotonation of the
imidazolium salt 3(Cl−) yielded the first bis(imino)-N-
heterocyclic free carbene 5 and, importantly, the unexpected
C−C coupling product 6, which turns out to be an interesting
new ligand, as shown with the Cr(II) complex 8. The
underlying features of the observed chemistry point to facile
N(imidazole)−C(imine) bond cleavage under different con-
ditions, e.g., thermolysis, nucleophilic attack by Cl−, and in one
case (i.e., formation of 6) accompanied by C−C bond
formation, leading to a new imidazole ligand framework.
Coordination of 5 to Ir(I) occurs via the CNHC only
(monodentate bonding mode κ1), although κ2 complexes
with other metals have recently been reported.25,26,28,29,31 The
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C3N2H3(imidazole), 100%]+. Anal. Calcd for C17H23N3: (%): C, 75.34; H,
8.64; N, 15.23. Found (%): C, 75.51; H, 8.95; N, 15.54.
novel bulky imidazole 6 offers interesting ways to tune the
metal environment. Further work on this and related chemistry
is currently under way.
Preparation of 1,3-Bis[1-(2,6-diisopropylphenylimino)ethyl]-
imidazolium Chloride (3·Cl−). N-(2,6-Diisopropylphenylimino)-
acetimidoyl chloride (5.60 g, 23.5 mmol) was added to a toluene
(50 mL) solution of N-(1-(1H-imidazol-1-yl)ethylidene)-2,6-diisopro-
pylaniline (2) (6.30 g, 23.4 mmol) at room temperature. The reaction
mixture was stirred for 48 h, and a light yellow precipitate formed. The
solid was filtered, washed with benzene and pentane, respectively,
under nitrogen, and dried under vacuum (9.84 g, 19.40 mmol, 83%).
1H NMR (300 MHz, CDCl3): δ 12.43 (br t, 1H, NCHN), 8.29 (d, 4J =
1.4 Hz, 2H, NCHCHN), 7.20−7.06 (m, 6H, CH(dipp)), 2.83 (s, 6H,
CH3(imine)), 2.57 (sept, 3J = 6.9 Hz, 4H, CH(CH3)2(dipp)), 1.11 (d, 3J =
6.9 Hz, 12H, CH(CH3)2(dipp)), 1.06 (d, 3J = 6.9 Hz, 12H,
CH(CH3)2(dipp)). 13C{1H} NMR (CDCl3): δ149.8 (CN), 140.6
(ipso-C(dipp)), 138.9 (NCHN), 136.5 (o-C(dipp)), 128.4 (C6H6), 125.8
(p-CH(dipp)), 123.6 (m-CH(dipp)), 118.1 (NCHCHN), 29.0 (CH-
(CH3)2(dipp)), 23.3 (CH(CH3)2(dipp)), 23.0 (CH(CH3)2(dipp)), 17.9
(CH3(imine)). IR (pure, orbit diamond): νCN 1695 cm−1. ESI-MS
(CH2Cl2, 50 V, m/z): 489.36 [M + H2O − Cl, 100%]+, 471.35 [M −
Cl, 57%]+. Anal. Calcd for C31H43N4Cl (%): C, 73.27; H, 8.73; N,
11.03. Found (%): C, 72.30; H, 8.53; N, 9.73.
EXPERIMENTAL SECTION
■
General Procedures. All operations were carried out using
standard Schlenk techniques under an inert atmosphere. Solvents were
dried, degassed, and freshly distilled prior to use. THF and Et2O were
dried over sodium/benzophenone, and CH2Cl2, toluene, pentane, and
acetonitrile were dried and freshly distilled. CD3CN was degassed and
stored over 4 Å molecular sieves. d6-Benzene and d8-toluene were
vacuum distilled from KH. CDCl3 and CD2Cl2 were dried over 4 Å
molecular sieves, degassed by freeze−pump−thaw cycles, and stored
under argon. NMR spectra were recorded at room temperature on a
Bruker AVANCE 300 spectrometer or Bruker AVANCE 400
spectrometer (1H, 300 MHz; 13C, 75.47 MHz or H, 400 MHz; 13C,
1
100.62 MHz) and referenced using the residual proton solvent (1H) or
solvent (13C) resonance. Assignments are based on 1H, 1H-COSY, 1H-
1
1
NOESY, H/13C-HSQC, and H/13C-HMBC experiments. IR spectra
were recorded in the region 4000−100 cm−1 on a Nicolet 6700 FT-IR
spectrometer (ATR mode, diamond crystal). Elemental analyses were
Preparation of 1,3-Bis[1-(2,6-diisopropylphenylimino)ethyl]-
́
performed by the “Service de Microanalyses”, Universite de
−
imidazolium Tetrafluoroborate, 3(BF4 ). A silver tetrafluoroborate
Strasbourg. Electrospray mass spectra (ESI-MS) were recorded on a
micro-TOF (Bruker Daltonics, Bremen, Germany) instrument using
nitrogen as drying agent and nebulizing gas. N-(2,6-
Diisopropylphenyl)acetamide was prepared according to the methods
described in the literature,41 and N-(2,6-diisopropylphenyl)-
acetimidoyl chloride (1) was obtained by a modification of a
previously reported procedure.
(0.192 g, 0.98 mmol) solution in THF (10 mL) was added to a
solution of 1,3-bis[1-(2,6-diisopropylphenylimino)ethyl]imidazolium
chloride, 3(Cl−) (0.500 g, 0.98 mmol), in THF (50 mL) at room
temperature. A white precipitate was formed within minutes, and the
reaction mixture was allowed to stir at room temperature for 1 h
protected from light. The precipitate was removed from the solution
by filtration over Celite, and the solvent was removed under vacuum.
An amorphous, white solid was obtained in almost quantitative yield
(0.52 g, 9.89 mmol, 95%). 1H NMR (300 MHz, CDCl3): δ 10.10 (t, 4J
Preparation of N-(2,6-Diisopropylphenyl)acetimidoyl Chloride
(1). N-(2,6-Diisopropylphenyl)acetamide (15.5 g, 70.7 mmol) was
dissolved in dry benzene (150 mL), PCl5 (14.80 g, 71.1 mmol) was
added gradually, and the mixture was stirred under N2. Then the
temperature of the oil-bath was raised to 60 °C, and the mixture was
refluxed for 40 min, until evolution of HCl gas ceased. After the
benzene was distilled off (oil-bath temperature 75 °C, bp 40 °C at 4.7
mbar), the oil-bath temperature was raised to 110 °C and a colorless
oil of 1 distilled at 80−82 °C at 0.3 mbar (14.2 g, 59.7 mmol, 84%
yield). 1H NMR (400 MHz, CD2Cl2): δ 7.28−7.22 (m, 3H, p-CH(dipp)
4
= 1.5 Hz, 1H, NCHN), 8.39 (d, J = 1.5 Hz, 2H, NCHCHN), 7.28−
3
7.16 (m, 6H, CH(dipp)), 2.65 (sept, J = 6.9 Hz, 4H, CH(CH3)2(dipp)),
3
2.55 (s, 6H, CH3(imine)), 1.16 (d, J = 6.9 Hz, 12H, CH(CH3)2(dipp)),
3
1.13 (d, J = 6.9 Hz, 12H, CH(CH3)2(dipp)). 13C{1H} NMR (CDCl3):
δ 149.1 (CN), 140.6 (ipso-C(dipp)), 136.7 (o-C(dipp)), 135.7 (NCHN),
126.0 (p-CH(dipp)), 123.8 (m-CH(dipp)), 118.7 (NCHCHN), 28.7
(CH(CH3)2(dipp)), 23.5 (CH(CH3)2(dipp)), 23.0 (CH(CH3)2(dipp)), 15.9
−
(CH3(imine)). IR (pure, orbit diamond): νCN 1695 cm−1. 3(BF4 ) can
3
+ m-CH(dipp)), 2.91 (sept., J = 6.9 Hz, 2H, CH(CH3)2(dipp)), 2.70 (s,
be recrystallized from toluene to give colorless needles of formula (3·
3H, CH3(imine)), 1.30 (br s, 12H, CH(CH3)2(dipp)). 13C{1H} NMR
(CD2Cl2): δ 143.7 (CN), 143.4 (ipso-C(dipp)), 137.5 (o-C(dipp)),
125.4 (p-CH(dipp)), 123.7 (m-CH(dipp)), 29.6 (CH3(amide)), 29.0
(CH(CH3)2(dipp)), 23.5 (CH(CH3)2(dipp)). IR (pure, orbit diamond):
νCN 1705 cm−1. ESI-MS (CH2Cl2, 50 V, m/z): 461.31 [2 M − HCl +
Na, 27%]+, 242.15 [M − HCl + Na + H2O, 100%]+, 202.16 [M − Cl,
19%]+. Anal. Calcd for C14H20NCl (%): C, 70.72; H, 8.48; N, 5.89.
Found (%): C, 70.52; H, 8.53; N, 5.66.
−
BF4 )·toluene. IR (pure, orbit diamond): νCN 1700 cm−1. Anal.
Calcd for C38H51BF4N4 (%): C, 70.15; H, 7.90; N, 8.61. Found (%):
C, 69.84; H, 7.88; N, 9.02.
Thermal Treatment of 1,3-Bis((E)-1-((2,6-diisopropylphenyl)-
−
imino)ethyl)-1H-imidazol-3-ium Tetrafluoroborate−, 3(BF4 ), in d6-
Benzene. C6D6 (0.8 mL) was added to solid 3(BF4 ) (0.012 g, 0.02
mmol) in a NMR tube, and the suspension was heated at 80 °C until
1
complete dissolution of the salt. The H NMR spectrum was then
1
Preparation of N-(1-(1H-Imidazol-1-yl)ethylidene)-2,6-diisopro-
pylaniline (2) (ref 29). Neat N-(2,6-diisopropylphenyl)acetimidoyl
chloride (7.00 g, 29.4 mmol) was gradually added to a dichloro-
methane (80 mL) solution of imidazole (4.10 g, 59.6 mmol) at room
temperature under N2. A white precipitate formed immediately, and
the reaction mixture was stirred overnight. Water was added to the
mixture, and the product was extracted with dichloromethane. The
combined dichloromethane layers were washed with brine and dried
over Na2SO4. The solvent was removed under vacuum to obtain 2 as a
recorded after cooling to room temperature. H NMR (400 MHz,
4
4
C6D6): δ 10.10 (br t, J = 1.4 Hz, 1H, NCHN), 7.49 (d, J = 1.4 Hz,
2H, NCHCHN), 7.13−7.05 (m, 6H, CH(dipp)), 2.89 (sept, 3J = 6.9 Hz,
4H, CH(CH3)2(dipp)), 2.59 (s, 6H, CH3(imine)), 1.19 (d, J = 6.9 Hz,
12H, CH(CH3)2(dipp)), 1.17 (d, J = 6.9 Hz, 12H, CH(CH3)2(dipp)).
3
3
The salt crystallizes as colorless needles from the solution at room
temperature after about 1 h.
Thermal Treatment of 1,3-Bis((E)-1-((2,6-diisopropylphenyl)-
imino)ethyl)-1H-imidazol-3-ium Chloride, 3(Cl−), in C6D6. C6D6
(0.8 mL) was added to solid 3(Cl−) (0.015 g, 0.03 mmol) in a
NMR tube, and the suspension mixture was heated at 80 °C until
complete dissolution (5−10 min). 1H NMR (400 MHz, C6D6) shows
a mixture of three products (see also SI): 10% of 3(Cl−): δ 12.88 (br s,
1H, NCHN), 7.40 (s, 2H, NCHCHN), 7.17−7.09 (m, aromatic H and
1
white solid (6.40 g, 23.76 mmol, 80.8%). H NMR (CD2Cl2): δ 8.11
3
(br s, 1H, NCHN), 7.63 (t, J = 1.5 Hz, 1H, NCHCHN(near imine)),
3
7.08−6.98 (m, 4H, NCHCHN and CH(dipp)), 2.68 (sept, J = 6.9 Hz,
3
2H, CH(CH3)2), 2.08 (s, 3H, CH3(imine)), 1.07 (d, J = 6.9 Hz, 6H,
3
CH(CH3)2), 1.03 (d, J = 6.9 Hz, 6H, CH(CH3)2). 13C{1H} NMR
3
(CD2Cl2): δ 149.6 (CN), 143.0 (ipso-C(dipp)), 137.5 (o-C(dipp)),
135.9 (NCHN), 130.5 (NCHCHN), 124.6 (p-CH(dipp)), 123.6 (m-
CH(dipp)), 116.7 (NCHCHN(near imine)), 28.7 (CH(CH3)2(dipp)), 23.4
(CH(CH3)2(dipp)), 23.0 (CH(CH3)2(dipp)), 16.6 (CH3(imine)). IR (pure,
orbit diamond): νCN 1677 cm−1. ESI-MS (CH2Cl2, 50 V, m/z):
561.37 [2 M + Na, 26%]+, 292.18 [M + Na, 92%]+, 202.16 [M −
C6D6), 3.19 (s, 6H, CH3(imine)), 2.91 (overlapping sept, J = 6.9 Hz,
4H, CH(CH3)2(dipp)), 1.20 (d, 3J = 6.9 Hz, 12H, CH(CH3)2(dipp)), 1.17
3
(d, J = 6.9 Hz, 12H, CH(CH3)2(dipp)); 45% of 2: 8.13 (v br s, 1H,
NCHN), 7.51 (br t, 3J = 1.3 Hz, 1H, NCHCHN(near imine)), 7.17−7.09
3
(m, aromatic H and C6D6), 2.69 (sept, J = 6.9 Hz, 2H, CH(CH3)2),
2.15 (s, 3H, CH3(imine)), 1.09 (d, 3J = 6.9 Hz, 6H, CH(CH3)2), 1.05 (d,
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3J = 6.9 Hz, 6H, CH(CH3)2); 45% of 1: 7.17−7.09 (m, aromatic H
100%]+, 471.35 [M + H, 13%]+. Anal. Calcd for C31H42N4 (%): C,
79.01; H, 8.99; N, 11.90. Found (%): C, 77.74; H, 9.18; N, 11.33.
Compound 6. Compound 6 can be obtained in different ways: (a)
as a byproduct in the preparation of compound 5 at lower
temperatures especially in larger scales when the concentration of
the reactant is higher (Scheme 5); (b) by the reaction of 3(Cl−) with
KN(SiMe3)2 at room temperature.
a. Formation of 6 as a Byproduct in the Preparation of 5. The
reaction described above for the synthesis of 5 was repeated with the
reagents used in higher concentration as follows: A suspension of
KN(SiMe3)2 (1.920 g, 9.65 mmol) in pentane (20 mL) was added
slowly to a pentane (50 mL) suspension of 3(Cl−) (4.84 g, 9.53
mmol) at −78 °C. The suspension was stirred at −78 °C for 3 h and
then was allowed to warm slowly with stirring overnight in the cooling
bath. The reaction mixture was filtered and taken to dryness under
reduced pressure.
3
and C6D6), 2.95 (sept, J = 6.9 Hz, 2H, CH(CH3)2), 1.45 (s, 3H,
CH3(imine)), 1.26, (v br, CH(CH3)2), 1.19 (vbr, CH(CH3)2). 13C{1H}
NMR (C6D6) (corresponding to 2): δ 149.7 (CN), 143.4 (ipso-
C(dipp)), 137.6 (o-C (dipp)), 136.2 (NCHN), 131.2 (NCHCHN), 125.1
(p-CH(dipp)), 124.0 (m-CH(dipp)), 116.5 (NCHCHN(near imine)), 29.1
(CH(CH3)2(dipp)), 23.7 (CH(CH3)2(dipp)), 23.2 (CH(CH3)2(dipp)), 15.9
(CH3(imine)); attributed to 1: δ 144.1 (CN), 143.0 (ipso-C(dipp)),
137.4 (o-C (dipp)), 126.1 (p-CH(dipp)), 125.8 (m-CH(dipp)), 29.3
(CH(CH3)2(dipp)), 29.1 (CH3(amide)), 23.9−23.5 large (CH-
(CH3)2(dipp)). The signals due to 3(Cl−) are too small to be
determined.
1
Spectrum of 2: H NMR (500 MHz, C6D6): 7.93 (s, 1H, NCHN),
7.53 (br t, 3J = 1.4 Hz, 1H, NCHCHN(near imine)), 7.20 (s, 1H,
NCHCHN), 7.12−7.07 (m, aromatic H), 2.69 (sept, 3J = 6.9 Hz, 2H,
CH(CH3)2), 1.39 (s, 3H, CH3(imine)), 1.09 (d, 3J = 6.9 Hz, 6H,
3
CH(CH3)2), 1.04 (d, J = 6.9 Hz, 6H, CH(CH3)2). 13C{1H} NMR
b. Preparation of 6 at Room Temperature. A suspension of
KN(SiMe3)2 (0.786 g, 3.94 mmol) in pentane (20 mL) was added
slowly to a pentane (30 mL) suspension of 3(Cl−) (2.000 g, 3.94
mmol) at room temperature. The suspension was allowed to stir at
room temperature for 48 h. The suspension was then filtered and the
solvent evaporated under vacuum; 0.650 g of a first crop could be
(C6D6): δ 149.6 (CN), 143.5 (ipso-C(dipp)), 137.6 (o-C (dipp)), 136.1
(NCHN), 131.6 (NCHCHN), 125.1 (p-CH(dipp)), 123.9 (m-CH(dipp)),
116.5 (NCHCHN(near imine)), 29.0 (CH(CH3)2(dipp)), 23.7 (CH-
(CH3)2(dipp)), 23.2 (CH(CH3)2(dipp)), 15.8 (CH3(imine)).
Preparation of [2·HCl + 3(Cl−) + PhMe] (4). Crystals of 4 suitable
for X-ray diffraction studies were grown at room temperature by slow
1
obtained that contained ca. 30% of 6 (determined by H NMR) and
1
diffusion of pentane into a saturated toluene solution of 3(Cl−). H
20% of compound A of similar solubility. The solid obtained from the
filtration was triturated for several hours with pentane (3 × 75 mL) to
extract 6, which could be obtained in higher purity, around 95%. The
combined yield of 6 is 55−62%. Isolated crystalline 6 can be obtained
in 44% yield (0.70 g, 0.86 mmol). Crystals of 6 suitable for X-ray
diffraction were grown from a saturated pentane solution at room
temperature.
4
NMR (300 MHz, CD2Cl2): δ 12.27 (t, J = 1.5 Hz, 1H, NCHN, for
3(Cl−)), 10.31 (dd, 4J = 1.2, 1.5 Hz, 1H, NCHN, for 2·HCl), 8.26 (d,
3J = 1.5 Hz, 2H, NCHCHN, for 3(Cl−)), 7.97 (t, 3J = 4J = 1.7 Hz, 1H,
NCHCHN(near imine), for 2·HCl), 7.37 (dd, J = 1.5, 1.7 Hz, 1H,
NCHCHN, for 2·HCl), 7.15−7.04 (m, 9H for CH(dipp) + 4H for
CH(residual toluene)), 2.81 (s, 6H, CH3(imine), for 3(Cl−), 2.69−2.53 (two
overlapping sept, 3J = 6.9 Hz, 6H, CH(CH3)2(dipp)), 2.35 (s, 3H,
1H NMR (300 MHz, C6D6): δ 8.21 (d, 3J = 1.5 Hz, 1H,
3
NCHCHN), 7.83 (s, 1H, NCHCN), 7.10−6.98 (m, 9H for CH(dipp)
+
CH3(imine), for 2·HCl), 2.25 (s, 2.4H for CH3(residual toluene)), 1.11 (d, J
= 6.9 Hz, 12H, CH(CH3)2(dipp), for 3(Cl−)), 1.07 (d, 3J = 6.9 Hz, 6H,
1H for NCHCHN + 2H for C6H6(residual benzene)), 5.81 (s, 2H,
3
CH(CH3)2(dipp), for 2·HCl), 1.05 (d, 3J = 6.9 Hz, 12H, CH-
NCHCHN), 3.01 (s, 3H, CH3(imine)), 2.84 (sept., J = 6.8 Hz, 2H,
3
3
(CH3)2(dipp), for 3(Cl−)), 1.02 (d, J = 6.9 Hz, 6H, CH(CH3)2(dipp)
,
CH(CH3) (dipp)), 2.56 (sept., J = 6.8 Hz, 2H, CH(CH3)(dipp)), 2.12
3
for 2·HCl). 13C{1H} NMR (CD2Cl2): δ 150.4 (CN, for 3(Cl−)),
149.2 (CN, for 2·HCl), 141.6 (ipso-C(dipp), for 2·HCl), 141.4 (ipso-
C(dipp), for 3(Cl−)), 139.9 (NCHN, for 3(Cl−)), 138.3 (ipso-
C(residual toluene)), 137.0 (o-C(dipp) for both 3(Cl−) and 2·HCl), 136.3
(NCHN, for 2·HCl), 129.3 (o-CH(residual toluene)), 128.5 (m-
(sept., J = 6.8 Hz, 2H, CH(CH3)(dipp)), 1.34 (s, 6H, CH3(imine)), 1.12
(d, 3J = 6.8 Hz, 6H, CH(CH3)2(dipp)), 1.06 (d, 3J = 6.8 Hz, 6H,
CH(CH3)2(dipp)), 0.99 (d, 3J = 6.8 Hz, 18H, CH(CH3)2(dipp)), 0.59 (d,
3J = 6.8 Hz, 6H, CH(CH3)2(dipp)). 13C{1H} NMR (75.5 MHz, C6D6):
δ 153.3(CN), 149.3(CN), 149.0(NCHCN), 145.5, 143.7, 140.1,
139.0, 138.1, 128.6, 127.7 (NCHCHN), 124.6, 123.8, 123.7, 123.5,
118.8 (NCHCHN), 112.1 (NCHCHN), 72.0 (NCHCN), 28.6, 28.3,
27.8, 24.8, 24.3, 24.2, 24.0, 23.2, 19.6, 15.4. IR (pure, orbit diamond):
νCN, CC 1665, 1629, 1609, 1586 cm−1. ESI-MS (PhMe, 50 V, m/z):
740.54 [M + H]+. Anal. Calcd for C48H65N7 (%): C, 77.79; H, 8.98; N,
13.23. Found (%): C, 77.24; H, 8.94; N, 12.63.
CH(residual toluene)), 125.8 (p-CH(dipp), for 3(Cl−)), 125.6 (p-CH(dipp)
,
for 2·HCl), 125.1 (p-CH(toluene)), 123.8 (m-CH(dipp), for both 3·Cl− and
2·HCl), 121.7 (NCHCHN(near imine), for 2·HCl), 118.3 (NCHCHN,
for 3(Cl−)), 117.5 (NCHCHN, for 2·HCl), 28.9 (CH(CH3)2(dipp), for
3·Cl−), 28.8 (CH(CH3)2(dipp), for 2·HCl), 23.4 (CH(CH3)2(dipp), for
3(Cl−)), 23.4 (CH(CH3)2(dipp), for 2·HCl), 23.0 (CH(CH3)2(dipp), for
3(Cl−)), 22.9 (CH(CH3)2(dipp), for 2·HCl), 21.5 (CH3(residual toluene)),
18.2 (CH3(imine), for 3(Cl−), 17.1 (CH3(imine), for 2·HCl). IR (pure,
orbit diamond): νCN 1688 cm−1. ESI-MS (CH2Cl2, 50 V, m/z):
471.35 [M2 − Cl, 18%]+, 292.19 [M1 + Na, 7%]+, 202.16 {[M1 −
C3N2H3(imidazole)]+ + [M2 − C14N1H20(imine) − C3N2H3(imidazole) − Cl]+,
100%}. Anal. Calcd for C54H75N7Cl2 (%): C, 72.62; H, 8.46; N, 10.98.
Found (%): C, 72.52; H, 8.43; N, 10.61.
Compound A could not be isolated pure, but some crystals were
identified in a mixture with 6. X-ray diffraction analysis established the
atom connectivity within the molecule, but the quality of the data did
not allow metrical data of high accuracy for further discussion (see
text). 1H NMR (300 MHz, C6D6): δ 7.42−6.80 (m, 9H for CH(dipp)),
3
3
3
6.37 (dd, J = 10.2 Hz, J = 2.6 Hz, 1H, NCHCH2), 5.77 (d, J = 2.9
Hz, 1H, NCHCHN), 5.46 (d, 3J = 2.9 Hz, 1H, NCHCHN), 3.91 (dd,
2J = 14.9 Hz, J = 2.6 Hz, 1H, CCHHCH), 3.48 (d, J = 2.4 Hz, 1H,
3
2
Preparation of 1,3-Bis[1-(2,6-diisopropylphenylimino)ethyl]-
imidazol-2-ylidene (5). A suspension of KN[Si(CH3)3]2 (0.392 g,
1.97 mmol) in pentane (10 mL) was added slowly to a pentane (50
mL) suspension of 3(Cl−) (0.970 g, 1.91 mmol) at −78 ◦C. The
suspension was stirred at −78 °C for 3 h and then was allowed to
warm slowly with stirring overnight in the cooling bath in order to
avoid decomposition of 5. The reaction mixture was filtered, and 5 was
obtained as a light yellow solid after removal of the solvent in vacuo
(0.79 g, 1.68 mmol, 85%). 1H NMR (300 MHz, C6D6): δ 8.06 (s, 2H,
NCHCHN), 7.19−7.01 (m, 6H, CH(dipp)), 2.94 (sept., 3J = 6.9 Hz, 4H,
3
NCCHH), 3.38 and 3.36 (two closely spaced sept, J = 6.8 Hz, 2H,
CH(CH3)2(dipp)), 3.18 (d, 2J = 2.4 Hz, 1H, NCCHH), 3.10−2.96 (two
overlapping sept, 3J = 6.8 Hz, 2H, CH(CH3)2(dipp)), 2.80 (sept, 3J = 6.8
2
3
Hz, 1H, CH(CH3)2(dipp)), 2.69 (dd, J = 14.9 Hz, J = 10.2 Hz,1H,
NCCHH), 2.45 (sept, 3J = 6.8 Hz, 1H, CH(CH3)2(dipp)), 1.43 (d, 3J =
6.8 Hz, 3H, CH(CH3)2(dipp)), 1.40 (d, 3J = 6.8 Hz, 3H,
CH(CH3)2(dipp)), 1.39 (d, 3J = 6.8 Hz, 3H, CH(CH3)2(dipp)), 1.34
(d, 3J = 6.8 Hz, 3H, CH(CH3)2(dipp)), 1.21 (s, 3H, CH3(imine)), 1.18 (d,
3J = 6.8 Hz, 3H, CH(CH3)2(dipp)), 1.17 (d, 3J = 6.8 Hz, 3H,
CH(CH3)2(dipp)), 1.13−1.11 (two overlapping d, 3J = 6.8 Hz, 6H,
CH(CH3)2(dipp)), 1.08 (d, 3J = 6.8 Hz, 6H, CH(CH3)2(dipp)), 0.87 (d, 3J
= 6.8 Hz, 3H, CH(CH3)2(dipp)), 0.76 (d, 3J = 6.8 Hz, 3H,
CH(CH3)2(dipp)). 13C{1H} NMR (125.77 MHz, C6D6): δ 151.4,
150.1, 148.5, 146.1, 145.7, 144.5 (two peaks), 139.5, 139.4, 138.6,
138.5, 136.5 (quaternary carbon), 129.4, 125.0, 124.8, 124.1, 124.0,
123.9, 123.7, 123.4, 123.3, 115.4, 111.3 (NCHCHN), 71.1 (NCCH2),
3
CH(CH3)2(dipp)), 2.47 (s, 6H, CH3(imine)), 1.17 (d, J = 6.9 Hz, 12H,
CH(CH3)2(dipp)), 1.13 (d, 3J = 6.9 Hz, 12H, CH(CH3)2(dipp)). 13C{1H}
NMR (C6D6): δ 225.9 (NCN), 156.1 (CN), 144.4 (ipso-C(dipp)),
137.6 (o-C(dipp)), 124.9 (p-CH(dipp)), 124.0 (m-CH(dipp)), 117.8
(NCHCHN), 29.2 (CH(CH3)2(dipp)), 23.8 (CH(CH3)2(dipp)), 23.3
(CH(CH3)2(dipp)), 17.6 (CH3(imine)). IR (pure, orbit diamond): νCN
1673 cm−1. ESI-MS (PhMe, 50 V, m/z): 489.36 [M + H2O + H,
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Organometallics
Article
72.1 (NCHCH2), 33.5 (NCHCH2), 29.9, 29.8, 28.9, 28.7, 28.0, 26.2,
25.8, 25.7, 24.5, 24.2, 23.9 (two peaks), 23.4, 23.4, 23.2, 23.1, 15.1
(CH3(imine)) (tertiary and primary carbon unless stated otherwise).
ESI-MS (50 V, m/z): 672.50 (100%), 673.51 (49.4%), 674.51
(10.1%), pattern [M + H]+ [theoretical pattern for [M]+: 671.49
(100.0%), 672.50 (49.4%), 673.50 (11.9%)].
in two positions having the tertiary atom in common and with equal
occupancy factors. These methyl C atoms were refined anisotropically
with constrained geometry and thermal parameters. A pentane
molecule was found badly disordered on multiple positions. Attempts
to locate its atomic coordinates failed and, instead, a SQUEEZE
procedure was applied on the complete anisotropic model. The result
gave missing electron density accounting for 69e per asymmetric unit,
well consistent with the 72 demanded for a pentane molecule. The
quality of the data for A is not sufficient for deposition with the
CCDC, but the following unit cell data were obtained: space group
orthorhombic, Pbca, a = 16.5497(10) Å, b = 16.6413(10) Å, c =
35.077(2) Å, α = β = γ = 90.00°, V = 9660.5(10) Å3, Z = 8. CCDC
945109−945113 and 945115 contain the supplementary crystallo-
graphic data for this paper, which can be obtained free of charge from
the Cambridge Crystallographic Data Center via www.ccdc.cam.ac.uk/
data_request/cif.
Preparation of [IrCl(cod)(5)] (7). The carbene ligand 5 (0.094 g,
0.200 mmol) was dissolved in a minimal amount of pentane (5 mL),
and the mixture was added to a suspension of dark orange crystals of
[Ir(μ-Cl)(cod)]2 (0.067 g, 0.100 mmol) in pentane (10 mL). A light
yellow precipitate formed immediately, and the mixture was stirred at
room temperature for 3 h. The solid was filtered, washed with pentane,
and dried under vacuum to yield spectroscopically pure 7 as a yellow
powder (0.058 g, 0.072 mmol, 72%). Crystals suitable for X-ray
diffraction were grown at room temperature under nitrogen by slow
diffusion of a pentane solution of the free carbene 5 into a saturated
THF solution of [Ir(μ-Cl)(cod)]2. 1H NMR (500 MHz, C6D6): δ 7.65
(s, 2H, NCHCHN), 7.20−7.00 (m, 6H, CH(dipp)), 5.03 (m, 2H,
CH(cod)), 3.32 (s, 6H, CH3(imine)), 3.06 (sept., 3J = 6.9 Hz, 2H,
ASSOCIATED CONTENT
■
3
S
* Supporting Information
CH(CH3)2(dipp)), 2.83 (m, 2H, CH(cod)), 2.76 (sept, J = 6.9 Hz, 2H,
Crystal data and structure refinement for 3(Cl−)·MeCN, 4, 6,
7, and 8 (Table S1). Crystallographic information files (CIF) of
the compounds 3(Cl−)·MeCN, 3(Cl−)·toluene, 4, 6, 7, and 8
have been deposited with the CCDC, 12 Union Road,
Cambridge, CB2 1EZ, U.K., and can be obtained on request
free of charge, by quoting the publication citation and
deposition numbers 945109−945113 and 945115. This ma-
terial is also available free of charge via the Internet at http://
pubs.acs.org.
CH(CH3)2(dipp)), 2.15−2.01 (m, 4H, CH2(cod)), 1.61−1.55 (m, 2H,
3
CH2(cod)), 1.48−1.43 (m, 2H, CH2(cod)), 1.30 (d, J = 6.9 Hz, 6H,
CH(CH3)2(dipp)), 1.21 (d, 3J = 6.9 Hz, 6H, CH(CH3)2(dipp)), 1.16 (d, 3J
= 6.9 Hz, 6H, CH(CH3)2(dipp)) 1.12 (d, 3J = 6.9 Hz, 6H,
CH(CH3)2(dipp)). 13C{1H} NMR (125.7 MHz, C6D6): δ 185.2 (CIr),
155.5 (CN), 143.0 (ipso-C(dipp)), 136.6 (o-C(dipp)), 136.2 (o-C(dipp)),
124.8 (p-CH(dipp)), 123.5 (m-CH(dipp)), 123.3 (m-CH(dipp)), 119.9
(NCHCHN), 84.6 (CH(cod)), 51.7 (CH(cod)), 33.4 (CH2(cod)), 29.3
(CH2(cod)), 28.5 (CH(CH3)2(dipp)), 28.4 (CH(CH3)2(dipp)), 23.3
(CH3(imine) + CH(CH3)2(dipp)), 23.1 (CH(CH3)2(dipp)), 22.9 (CH-
(CH3)2(dipp)), 22.3 (CH(CH3)2(dipp)). IR (pure, orbit diamond): νCN
1672 cm−1. ESI-MS (CH2Cl2, 50 V, m/z): 805.40 [M − H, 100%]+,
771.40 [M − Cl, 57%]+. Anal. Calcd for C39H55N4ClIr (%): C, 58.00;
H, 6.86; N, 6.94. Found (%): C, 57.08; H, 6.96; N, 5.97.
AUTHOR INFORMATION
■
Corresponding Author
Preparation of [CrCl2(6)2] (8). A solution of compound 6 (0.100 g,
0.135 mmol) in 3 mL of THF was added under magnetic stirring to a
solution of [CrCl2(THF)2] (0.018 g, 0.067 mmol, 0.5 equiv) in THF
(4 mL). Stirring was maintained for 2 h at room temperature. Light
green crystals suitable for X-ray diffraction were formed slowly at room
temperature after concentrating the solution under reduced pressure
to ca. 4 mL. Following collection of the crystalline material, pentane
was added to the green THF solution, and additional crystalline
product separated and was collected (overall yield 75%, 0.080 g, 0.050
mmol). The crystals readily lose THF molecules of crystallization;
therefore prior to elemental analysis they were maintained under
vacuum for several hours. Anal. Calcd for C96H130Cl2CrN4 (%): C,
*E-mail: braunstein@unistra.fr.
Present Address
†Key Laboratory of Synthetic and Natural Functional Molecule
Chemistry of the Ministry of Education, Shaanxi Key
Laboratory of Physico-Inorganic Chemistry, College of
Chemistry & Materials Science, Northwest University, Xi’an
710069, People’s Republic of China.
Notes
The authors declare no competing financial interest.
1
71.93; H, 8.17; N, 12.23. Found (%): C, 71.60; H, 8.15; N, 12.24. H
ACKNOWLEDGMENTS
■
NMR (400 MHz, THF-d8): see text above for discussion and ESI. IR
(CsI, Nujol mull): νCN 1664 cm−1, νCr−Cl 375 cm−1.
We thank the CNRS, the Universite
́
de Strasbourg (UdS), the
ieur et de la Recherche
X-ray Data Collection, Structure Solution, and Refinement
for All Compounds. Suitable crystals for the X-ray analysis of all
compounds were obtained as described above. The intensity data for
3(Cl−)·CH3CN, 4, 6, and 8 were collected on a Kappa CCD
diffractometer (graphite-monochromated Mo Kα radiation, λ =
0.71073 Å) at 173(2) K.42 Data for 3(Cl−)·toluene were collected
on an APEX-II CCD (graphite-monochromated Mo Kα radiation, λ =
0.71073 Å). Crystallographic and experimental details for the
structures are summarized in Table S1 (see SI). The structures were
solved by direct methods (SHELXS-97) and refined by full-matrix
least-squares procedures (based on F2, SHELXL-97)43 with aniso-
tropic thermal parameters for all the non-hydrogen atoms. The
hydrogen atoms were introduced into the geometrically calculated
positions (SHELXS-97 procedures) and refined riding on the
corresponding parent atoms. For all compounds, a MULTISCAN
absorption correction was applied.44 The structure of 3(Cl−) was
treated as a racemic twin solved within the C2 space group with a
BASF of 0.62709. The SQUEEZE instruction in PLATON was applied
for 6 and 8.44 The residual electron density was assigned to one
molecule of pentane and two molecules of THF for 6 and 8,
respectively. In 6, three of the isopropyl groups were found disordered
Minister
̀
e de l′Enseignement Super
́
(Paris), the International Center for Frontier Research in
Chemistry, Strasbourg (icFRC, www.icfrc.fr), and the LabEx
“Chemistry of Complex Systems”. Johnson Matthey PLC is
also gratefully acknowledged for a generous loan of iridium
trichloride. We also thank the Service de Radiocristallographie
(Institut de Chimie, Strasbourg) and Dr. R. Pattacini
(Strasbourg) for assistance with the crystallography. We
thank the referees for their helpful comments. P.L. acknowl-
edges financial support from the China Scholarship Council
(No. 2010697512) and the National Natural Science
Foundation of China (Nos. 21143010 and 20931005) and
the Northwest University, Xi’an, for a leave of absence. A.A.D.
is also grateful to the CNRS for a “Chercheur Associe”
fellowship to, to the Region Alsace, the Departement du Bas-
Rhin, and the Communaute Urbaine de Strasbourg for the
́
́
́
́
award of a Gutenberg Excellence Chair (2010−2011), and to
the University of Strasbourg Institute for Advanced Study for a
Fellowship (2013−2014).
6296
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Article
(34) See for example: Kovacevic, A.; Grundemann, S.;
̈
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