Mono-and bimetallic zwitterionic chromium(0) and tungsten(0) allenyls

Article abstract of DOI:10.1021/acs.inorgchem.5b00492

A series of stable chiral (racemic), formally neutral, zwitterionic mono- and bimetallic M(CO)₅[C(OEt)=C=CR(NHC)] (M = Cr, W) σ-allenyls are ready available by the addition of N-heterocyclic carbenes (NHCs) to Cr(0) and W(0) alkynyl Fischer car

Full text of DOI:10.1021/acs.inorgchem.5b00492

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Mono- and Bimetallic Zwitterionic Chromium(0) and Tungsten(0)
Allenyls
Elena A. Giner,†,∥ Alicia Santiago,†,∥ Mar Gomez-Gallego,*,†,∥ Carmen Ramírez de Arellano,‡,∥
́
Rebecca C. Poulten,§ Michael K. Whittlesey,§ and Miguel A. Sierra*,†,∥
†Departamento de Química Organ
́
ica, Facultad de Ciencias Químicas, Universidad Complutense, 28040 Madrid, Spain
ica, Facultad de Química, Universidad de Valencia, 46100-Valencia, Spain
‡Departamento de Química Organ
́
§Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, U.K.
∥Centro de Innovacion
en Química Avanzada, ORFEO−CINQA, Madrid, Spain
́
S
* Supporting Information
ABSTRACT: A series of stable chiral (racemic), formally neutral, zwitterionic
mono- and bimetallic M(CO)5[C(OEt)CCR(NHC)] (M = Cr, W) σ-
allenyls are ready available by the addition of N-heterocyclic carbenes (NHCs) to
Cr(0) and W(0) alkynyl Fischer carbene complexes. Different classes of NHCs,
(e.g., 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene, 1,3-bis(2,4,6-
trimethylphenyl)imidazolin-2-ylidene, and their six- and seven-membered
analogues and 1,3-bis(dimethyl)imidazol-2-ylidene) were employed as nucleo-
philes in these C−C bond-forming reactions yielding the novel complexes in
essentially quantitative yields. A systematic experimental and computational study
of the electronic properties of the Cr- and W-allenyls shows that their UV−vis
spectra are directly influenced by the structure of the heterocyclic moiety derived
from the NHC (ring size, substituents on the N atoms) and by the nature of the metal fragment (Cr/W). The electron-releasing
nature of these complexes allows them to participate in electron-transfer reactions in the ground state, leading to a type of
charged α,β-unsaturated Fischer carbenes that incorporate an NHC fragment in their structure.
a
Scheme 1.
INTRODUCTION
■
The special properties of N-heterocyclic carbenes (NHCs), like
ambiphilicity (σ-donor and π-acceptor character), strong
basicity, and moderate nucleophilicity,1 have found widespread
applications in organic and organometallic chemistry, not just
in the design of new catalysts but also as components for
medicinal, luminescent, and functional materials.2 Their role as
organocatalysts is now well-recognized,3 and their affinity for
metals has converted them into habitual ligands in homoge-
neous metal catalysis,4 in most cases being a superior alternative
to the classical phosphine ligands.5 However, the reactivity of
NHCs as simple nucleophiles in C−C bond-forming reactions
is largely unknown. In this context, our interest has been
directed to a study of the compatibility of NHCs as
nucleophiles with metal complexes having several reactive
centers. In preliminary results,6 we reported the synthesis of
stable chromium and tungsten zwitterionic η1-allenyl complexes
2a−c, by the addition of 1,3-bis(2,4,6-trimethylphenyl)-
imidazol-2-ylidene (IMes) to the corresponding alkynyl Fischer
carbene complex 1, at room temperature and in quantitative
yields (Scheme 1). The efficient synthesis of compounds 2a−c
demonstrates that despite the high affinity of NHCs to bind
metals,7 the thermodynamically favorable CO exchange from
metal carbonyls,8 and their ability to displace carbene ligands in
goup 6 Fischer (M(CO)5L) carbenes,9,10 it is possible to
a
See ref 6.
combine a NHC with an electrophilic group 6 alkynyl Fischer
carbene complex to make a C−C bond.
Compounds 2 constitute a new type of formally neutral
zwitterionic η1-metal allenyls, a class of compounds that, due to
their elusive character, have been scarcely studied.11−13 In this
regard, the stability of metal allenyls 2 offers an excellent
opportunity to obtain more information about their structures,
Received: March 3, 2015
© XXXX American Chemical Society
A
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Table 1. 13C{1H} NMR Chemical Shifts (ppm in CDCl3) and
IR Data (cm−1) of Metal Allenyls 2, 3, 5, and 6
their physicochemical properties, as well as to establish the
main features of their reactivity. In this work, we explore the
scope of the addition of NHCs to α,β-unsaturated Fischer
mono- and biscarbene complexes, undertaken as a study to
establish the influence of the substituents in both reagents and
the nature of the metal fragment on their structures, electronic
properties, and reactivity.
compound
C1
C2
C3
ν(CCC) and ν(CO)
a
2a
201.3
160.1
84.9 2040(17), 1959(20), 1907(100),
1868(56)
a
2b
199.4
199.6
199.5
200.5
200.8
201.0
199.6
209.1
198.6
196.7
207.2
208.4
216.4
176.0
176.0
178.7
182.4
155.8
159.2
160.3
159.6
159.4
158.9
160.7
155.3
170.1
170.5
170.0
167.8
158.5
147.3
147.3
146.6
145.6
88.9 2038(13), 1953(16), 1907(100),
1861(60)
2d
2e
2f
79.2 2040(27), 1958(27), 1907(100),
1857(69)
RESULTS AND DISCUSSION
■
83.2 2041(27), 1958(27), 1912(100),
1867(81)
To study the effect of the substituent in the aromatic ring
attached to the triple bond, complexes 2d−2i were prepared
following our previously reported method.6 Thus, reaction of
IMes with the corresponding Cr(0) and W(0) alkynyl Fischer
carbene complexes 1 in pentane, at room temperature, affords
metal−allenyl complexes 2 in quantitative yields (Scheme 1).
All new complexes were obtained as yellow solids and were
fully characterized by NMR and mass spectrometry (MS)
analysis. As for the previously reported parent complex 2a,
compounds 2d−2i show the characteristic diastereotopic
protons of the ethoxy group in the 1H NMR spectra (multiplets
at ca. 1.97 and 2.50 ppm, respectively), noticeably shielded
compared to the starting carbene (the CH2 protons in the
ethoxy group in alkynyl carbenes 1 appear as a quartet at ca. 4.5
ppm). Also significant are the 13C NMR signals of the M−C
carbon (C1) at ca. 200 ppm, the central allenyl carbon (C2) at
∼155−160 ppm, and the terminal carbon (C3) at 79−88 ppm
(Table 1). The effect of the metal fragment on the chemical
shift of the allene carbons is remarkable. The most deshielded
signal in simple allenes corresponds to the central carbon atom
(at ca. 200−220 ppm),14 whereas in metal allenyls 2, this signal
corresponds to the neighboring position to the metal. The
change from Cr to W in the M(CO)5 moiety (complex 2c)6
also causes the deshielding of the M−C (C1) signal. Results in
Scheme 1 and Table 1 indicate that the reaction is not strongly
influenced by the p-substituent at the aromatic ring and hence
the electrophilicity of the conjugated position (C3) in the
starting alkynyl Fischer carbene. The addition of the NHC
works well regardless of the electronic nature of the substituent
placed in the para position of the phenyl ring in C3 (electron-
donating or electron-withdrawing groups). The incorporation
of an electron-rich heteroaromatic ring (2-thienyl) or a
ferrocenyl group at the conjugated position of the alkynyl
carbene also has a negligible effect on the reaction course,
although an effect is noticeable in terms of the C3 13C NMR
chemical shifts, which are shielded (2-thienyl, 2d) and
deshielded (ferrocenyl, 2b), respectively, compared to the
rest of the compounds of the series (Table 1). Regarding the IR
data, no significant changes were observed in the CCC
and CO stretching bands with the electronic properties of the
aryl substituent at C3.
84.4 2040(25), 1956(25), 1909(100),
1864(67)
2g
2h
2i
84.0 2040(36), 1959(36), 1902(100),
1865(81)
84.6 2046(31), 1919(br)(100),
1865(67)
83.6 2042(43), 1962(31), 1911(100),
1874(81)
a
2c
85.9 2051(23), 1961(39), 1904(100),
1857(78)
a
3a
85.7 2042(16), 1965(22), 191(100),
1853(61)
3b
3c
5a
5b
6a
6b
6c
6d
84.3 2043(75), 1963(80), 1922(100),
1865(95)
85.9 2053(24), 1968(29), 1907(100),
1845(67)
91.4 2041(21), 1965(29), 1904(100),
1849(64)
93.1 2040(31), 1964(31), 1905(100),
1855(64)
106.5 2038(31),1963(35),1903(100),
1859(67)
106.5 2037(23),1948(28), 1906(100),
1854(66)
106.2 2038(27), 1964(36), 1907(100),
1844(87)
105.7 2050(17),1961(22), 1900(100),
1855(51)
a
See ref 6.
Scheme 2
Given the influence of the ring size, N-substituent bulk, and
N−C−N angle on the electronic properties of NHCs,1,3b,15
other types of NHCs were also tested. Saturated NHCs are
more basic than their unsaturated counterparts, and the
increase of the ring size is translated into larger N−C−N
angles and higher steric congestion.1,16 Reaction of Cr(0) and
W(0) alkynyl carbenes 1a, 1e, and 1c and 1,3-bis(2,4,6-
trimethylphenyl)imidazolin-2-ylidene afforded the expected
metal allenyls 3a−c in quantitative yields (Scheme 2, Table
1). The study of the effect of the enlargement in the NHC ring
was achieved by reaction of alkynyl carbene 1a with the
corresponding six- and seven-membered NHC carbenes,
salts 4 and potassium bis(trimethylsilyl)amide (KHMDS) in
tetrahydrofuran (THF), at room temperature and in
quantitative yields. The ring expansion of the heterocyclic
fragment in metal allenyls 5a−b causes significant shielding of
1
the signals of the ethoxy group in H NMR spectra compared
to their five-membered counterparts 3. In fact these signals
appear now at ∼1.7 and 2.1 ppm, respectively, while the
−
generated in situ from their BF4 pyrimidinium/diazepinium
B
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analogous compounds 3 showed signals at ∼2.0 and 2.6 ppm,
respectively. Also, in the 13C NMR spectra, the signals of M−C
carbon (C1) in “enlarged” derivatives 5a−b are noticeably
deshielded (10−18 ppm) compared to their five-membered
counterparts 3 (Table 1). The central allenyl carbon (C2) is
less affected, but the extreme position (C3) moves up to 91.4
and 93.1 ppm, respectively (compare with the signals at 79−88
ppm in complexes 1b). The less effective electron donation
from the N-atoms to stabilize the neighboring cationic center,
as well as the effect of the bulkier ring on the structure of the
allene, could be the origin of these observations.16
to the formation of bis-allenyl metal complexes 9 and 10 as
yellow solids in almost quantitative yields (Scheme 4).
In an alternative approach to bis-metal allenyls, the bis-allenyl
complex 12, having a flexible tether between the nitrogen
atoms, was obtained in 96% yield by reaction of alkynyl carbene
1a and a bis-NHC-carbene generated in situ from the
dibromide salt 11 and KH (Scheme 5).
Scheme 5
The impact of the N-substituents was addressed next.
Addition of 1,3-bis(dimethyl)imidazol-2-ylidene (IMe2, 7) to
alkynyl carbenes 1a, 1d, 1f, and 1j afforded metal allenyls 6a−d
in 81% to quantitative yields and as brownish solids (Scheme
3). Reaction times and yields were unaffected by using either
Scheme 3
All the allenyl metals prepared through this work are chiral
compounds obtained as racemic mixtures. The incorporation of
two allenyl fragments within the same molecule in compounds
9, 10, and 12 should then lead to diastereomeric mixtures, at
least a symmetrical meso form and an enantiomeric pair. The
NMR spectra of the bis-allenyl metals confirm this fact.
Compounds 9b,c, 10b, and 12 having flexible tethers, show
1
well-resolved signals with little splitting in the H and 13C
NMR spectra, the chemical shifts being similar to those of the
monometallic counterparts. The combination of the bulky IMes
and the short, rigid 1,3-substituted phenyl ring as tether in bis-
the free, freshly prepared 7,17 or the carbene generated in situ
from the 1,3-dimethylimidazolium iodide salt and KH in
THF.18 Compounds 6 were characterized by spectroscopic
analysis. The replacement of the bulky mesityl groups by
methyl arms in the heterocyclic moiety allows a higher
conformational freedom of the molecule, which is reflected in
1
allenyl metal complexes 9a and 10a leads to more complex H
NMR spectra, with broad and noticeably split signals, in accord
with structures possessing little conformational mobility.
All compounds prepared through this work (2, 3, 5, 6, 9, 10,
12, 13) are perfectly stable in the solid state at room
temperature, and they do not need to be handled under
special conditions. They do not require Schlenck techniques;
they are not air sensitive and can be stored as solids in perfect
conditions. In solution they are stable in Et2O, THF, and
hexanes but slowly decompose when left to stand in CH2Cl2 or
CHCl3 solution.
Finally, further structural information on the different types
of σ-metal allenyls prepared through this work was gained by a
computational study (B3LYP/def2-SVP method) of 2a, 3a, 5a,
5b, and 6a, as the extent of the coplanarity between the NHC
moiety and the attached sp2 carbon could give information
about the Lewis donor/imidazolium character of the NHC
units. Our calculations indicate a torsion angle of 41.47° in the
1
the signals of the NMR spectra. Thus, the H NMR signals of
the protons of the ethoxy group in compounds 6, although still
diastereotopic, almost overlap and appear noticeably deshielded
(at ∼4 ppm) compared to the IMes derivatives 2 (see above).
Other changes evident in the 13C{1H} NMR spectra were the
shielding of the signals of the allenyl system (C1 ∼20 ppm and
C2 ∼15 ppm relative to compounds 2) and the strong
deshielding (∼24 ppm) of the signals of the allenyl-carbon
(C3). The latter is probably due to the increase of the positive
character of the NCN carbon in Me2Im-compounds 6.
The preparation of zwitterionic bis-metal allenyls was tested
next. Double nucleophilic addition of IMes or SIMes, to the
corresponding chromium bis-alkynyl carbene complexes 8, led
Scheme 4
C
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gas phase for the IMes derivative 2a, a value considerably
higher than that obtained in the crystal structure (16.97°, see
ref 6) and consistent with the expected greater packing of the
molecules in the crystalline form. The replacement of the bulky
mesityl groups by methyl groups does not affect seriously the
calculated torsion angle (40.46° in 6a). Finally, the
incorporation of a saturated NHC of different ring size
provokes changes in the torsion angles from 11.75° in 3a to
42.40° in 5a and 20.44° in 5b.
Electronic Structure of the Allenyl Metal Complexes.
Information about the electronic structure of metal allenyl
complexes prepared through this work was obtained by UV−vis
spectroscopy (Table 2).
Table 2. UV−vis Excitation Energies (λmax in nm) and
Calculated Oscillator Strengths (f, in parentheses) for
Mono- and Bis-Metal Allenyl Complexes
a
a
Figure 1. Room-temperature UV−vis spectra of complexes 2a−i
(Et2O, concentration ca. 1 × 10−4 mol L−1).
entry complex
λ1/nm
λ2/nm
c
1
2
3
4
1a
1b
1c
2a
281
309
290
497
c
487, 544
495
oscillator strengths of the main absorptions) show a good
agreement between the calculated vertical excitation energies
and the wavelengths of the absorption maxima in the
experimental UV−vis spectra. The deviation of the calculated
from the experimental values is in the range of 10−40 nm (the
calculated data are red-shifted compared to the experimental
measurements). The TD-CAM-DFT calculations indicate that
the bands in the range of 430 nm are mainly due to the
promotion of one electron from the metal-centered highest
occupied molecular orbital (HOMO) to the lowest unoccupied
molecular orbital (LUMO). As depicted in Figure 2 for
compound 2a, the HOMO should be viewed as π-molecular
orbital mainly located at the metal fragment, whereas the
LUMO is a π*-orbital mostly distributed on the imidazolium
salt. The observed bands can thus be assigned to metal-to-
ligand charge transfer (MLCT) transitions possessing remark-
able π−π* character. On the other hand, the bands at ∼340 nm
can be assigned, on the basis of the TD-CAM-DFT
calculations, to metal-centered HOMO−1 to LUMO tran-
sitions. Compared to the starting chromium alkynyl carbene
complexes 1a−c (Table 2, entries 1−3) the UV−vis spectra of
the metal−allenyl species 2 show a remarkable blue shift of the
MLCT band (from 490 to 430 nm) and a red shift of the ligand
field (LF) band (from 290 to 340 nm).19a The origin of these
shifts may be due to the loss of the partial double bond
character of the starting complexes and replacement by a M−C
single bond in the allenyl complexes. This effect has been
reported previously by us for Cr(0) and W(0) carbene
complexes having donor or acceptor substituents attached to
the MC moiety.19b
338 (344, 0.123)
424 (404, 0.002)
b
b
338
413
5
2e
2f
334 (345, 0.125)
339 (346, 0.133)
342 (344, 0.125)
354 (345, 0.135)
336 (347, 0.133)
315
429 (407, 0.002)
424 (404, 0.001)
423 (402, 0.003)
416 (404, 0,002)
433 (407, 0.001)
417
6
7
2g
2h
2i
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
2b
2d
2c
3a
3b
3c
5a
5b
336 (331, 0.373)
347
424 (436, 0.020)
403
341 (360, 0.130)
336 (363, 0.134)
347
451 (427, 0.001)
453 (426, 0.001)
408
343 (339, 0.054)
440 (401, 0.007)
350 (broad) (357, 0.155) 472 (467, 0.002)
c
6a
334 (371, 0.017, 333, 0.061)
c
6b
c
335
333
340
418
427
419
430
429
343
6c
6d
9a
9b
9c
c
328
339
338
336
334
b
b
10a
10b
c
12
a
The first value corresponds to experimental data (recorded at room
temperature in Et2O with a concentration of 1 × 10‑4 M unless
specified). The first value in parentheses corresponds to the computed
TD-CAM-B3LYP/def2-SVP gas-phase vertical excitation energies. The
b
second one is the corresponding oscillator strength f. Data recorded
c
at room temperature in THF, with a concentration 1 × 10−4 M. Data
As expected, and taking into account the distribution of the
HOMO and LUMO orbitals involved in the MLCT transition
(Figure 2), the substituents attached to the aryl group in
compounds 2 have very little influence on this absorption band.
However, the data in Table 2 show that the higher energy LF
band is somewhat influenced by electronic effects. Thus, the
presence of π electron donating substituents in the para
position of the phenyl group causes a slight red shift in this
band, which moves from 338 nm in the unsubstituted
compound 2a (R = H entry 4) to 342 nm in 2g (R = OMe,
entry 7) and 354 nm in 2h (R = NMe2, entry 8). Replacement
of the phenyl group at C3 by ferrocenyl (2b) causes a
recorded at room temperature in THF, with a concentration 1 × 10−5
M.
Metal allenyl complexes 2a−i bearing 1,3-bis(2,4,6-
trimethylphenyl)imidazolium moieties show two main absorp-
tions around 340 and 430 nm (data obtained in Et2O in 1 ×
10−4 M solutions, Figure 1). The assignment of the absorption
bands was made by time-dependent (TD) density functional
theory (DFT) calculations using the TD-CAM-B3LYP/def2-
SVP method. Data in Table 2 (which include the calculated
D
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Figure 2. Calculated DFT (B3LYP/def2-SVP level) HOMO−1, HOMO and LUMO orbitals of compound 2a (isosurface value 0.04).
noticeable blue shift to 315 nm (entry 10). These effects may
be explained by taking into account the participation of the C3
position (next to the aryl group) in the description of the
HOMO−1 of the allene (the origin of this UV transition; see
Figure 2). This participation is missing in the HOMO (the
origin of the MLCT absorption). Finally, the nature of the
metal fragment (Cr(CO)5, W(CO)5) has an effect on the
MLCT band, which is blue-shifted in the tungsten complex 2c
(424 cm in 2a cf. 403 nm in 2c, entry 12). This was previously
reported by us19 in M(0) carbene complexes.
derivatives, as would be expected considering that the metallic
moieties are independent of each other.
Replacement of the bulky mesityl by methyl groups in the
imidazolium moiety causes evident changes in the UV spectra
of compounds 6 compared to the structurally related
compounds 2 (Table 2, entries 18−21 and Figure 4). In this
UV−vis absorption spectra of metal−allenyls 3 and 5 bearing
(saturated) 1,3-bis(2,4,6-trimethylphenyl)imidazolinium, pyri-
midinium, and diazepinium moieties are displayed in Figure 3.
Figure 4. UV−vis absorption spectra of compounds 6a−c and 12
(THF, concentration ca. 1 × 10−5 mol L−1). UV−vis spectrum of
compound 2a recorded in the same conditions is included for
comparison.
case for solubility reasons, the spectra were recorded in THF.
The LF and MLCT bands observed in compounds 2 are
replaced by a single absorption at ∼330−340 nm, assigned by
TD-DFT calculations to a combination of HOMO to LUMO
(53%) and HOMO−1 to LUMO (29%) transitions.
Figure 3. UV−vis absorption spectra of compounds 3a−b and 5a−b
(Et2O, concentration ca. 1 × 10−4 mol L−1). Compound 2a is included
for comparison.
The frontier molecular orbitals distribution of 6a (Figure 5)
is similar to that depicted in Figure 2 for 2a, with the HOMO
and HOMO−1 centered on the metal fragment and the LUMO
mostly located on the heterocyclic moiety. As a consequence,
replacement of the mesityl groups by methyl groups in 6a
should affect the LUMO more; this in fact causes a rise in
energy (−1.56 eV in 2a but −1.40 eV in 6a), whereas the
HOMO hardly changes (−4.86 eV in 2a and −4.80 eV in 6a).
The increase in the HOMO−LUMO gap could explain the
strong blue shift observed for the MLCT band in 6a−d
compared to their IMes counterparts 2. The UV−vis spectrum
of bis-allenyl metal complex 12 derived from a bis-NHC
carbene tethered with a flexible (C3H6) alkyl chain (entry 27 in
Table 2) follows a similar pattern.
Although the general pattern is similar to those of the
unsaturated counterparts 2 (see above), there is a remarkable
red shift observed in their MLCT bands. Thus, 3a and 3b show
MLCT absorptions at 451 and 453 nm, respectively, ∼30 nm
red-shifted compared with those of the structurally related 2a
and 2e (compare entries 13−14 and 4−5). The effect is
particularly evident when the heterocyclic ring is enlarged.
Thus, 5b, having a seven-membered ring, shifts the MLCT
band to 472 nm (entry 17). The structural changes made in the
cationic heterocyclic moiety (imidazolinium, pyrimidinium,
diazepinium) undoubtedly influence the energy of the LUMO
[−1.56 eV for 2a but −1.69 eV (3a), −1.76 (3b), −1.66 eV
(5a), and −1.90 eV (5b)] and are the origin of the red shifts
observed in the MLCT bands. Finally, the UV−vis spectra of
the bis-allenyl metal complexes 9 and 10 (entries 22 to 26 in
Table 2, Figure S1 in the Supporting Information) show similar
absorptions to those of the corresponding monoallenyl
Reactivity. In agreement with our preliminary studies6 the
metal allenyls prepared through this work are essentially
unreactive toward nucleophiles/electrophiles in the ground
state. However, their zwitterionic character and the fact that the
E
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Figure 5. Calculated HOMO−1, HOMO, and LUMO orbitals of compound 6a (isosurface value 0.04).
HOMO is mostly localized on the metal fragment makes them
interesting substrates on which to test electron-transfer
reactions. In this context, their electrochemical properties
were addressed first. Thus, cyclic voltammograms of 2a, 2c, and
6a were measured in CH3CN solution with tetrabutylammo-
nium perchlorate as supporting electrolyte, using Ag/AgCl as
reference electrode (data are collected in Table 3 and shown in
mesityl groups by methyl groups in the imidazolium moiety
facilitates the first oxidation of the complex.
Chemical reactivity toward the oxidant [Ph3C]PF6 was tested
next. Reaction with chromium or tungsten alkenyl complexes
2a, 2c, and 2e, in 1,2-dichloroethane at room temperature led
quantitatively to the α,β-unsaturated Fischer carbenes 13 as
mixtures of Z/E isomers (Scheme 6). Consistent with the
a
Table 3
Scheme 6
1
2
3
compound
Epa
Epa
Epa
b
2a
2c
6a
0.51
0.60
0.36
0.88
1.28
0.88
1.42 (sh 1.11)
b
1.43
a
Oxidation potentials in CH3CN/0.1M[nBu4N]ClO4; 298 K, Ag/AgCl
3 M as reference and a Pt wire counter electrode (scan rate 0.1 V/s).
b
E1/2
.
above discussion about the oxidation process, the reactions
could be interpreted as involving initial loss of one electron
from the metal fragment to give the radical cation 14, followed
by capture of a hydrogen radical at the conjugated position
(presumably from the solvent). Similar reactivity was observed
using [Cp2Fe] (PF6) in THF as oxidant, although with lower
yields due to the extensive oxidation of the reaction products.
The structure of compounds 13a−c was established by
spectroscopic analysis. Being Fischer carbenes, the 13C NMR
spectra of the Cr(0) complexes 13a−b show the characteristic
signals for the C−M carbon at ca. 335 ppm together with those
of the CO ligands (COtrans ca. 224 ppm, COcis ca. 215 ppm).
These signals are deshielded, as expected, in the case of the
W(0) complex 13c (305 ppm for the C−W and 202 and 196
ppm for the CO ligands, respectively). Also characteristic are
the signals corresponding to the methylene protons of the
ethoxy group that appear as quartets in the range of 4.3−5.2
ppm. The different solubility of the Z/E isomers allowed the
confirmation of the structure of Z-13a by X-ray diffraction
(Figure 7). A suitable crystal of Z-13a·CHCl3, obtained by
CHCl3/pentane liquid diffusion, was used for X-ray structure
determination. Z-13a·CHCl3 shows two independent mole-
cules in the asymmetric unit. In the crystal, the alkene moiety is
distorted from planarity for both independent cations, with
C(2)−C(6)−C(7)−C(8) and C(21)−C(6)−C(7)−C(8) tor-
sion angles of 12.4(8)/10.0(8)° and 10.9(5)/ 9.0(5)°,
Figure 6. Cyclic voltammograms of 2a, 2c, and 6a in CH3CN/0.1 M
[NBu4]ClO4 using Ag/AgCl 3 M as reference and a Pt wire counter
electrode (scan rate 0.1 V/s).
Figure 6). All show three main oxidation events, accompanied
by other low-intensity waves in the case of the IMes derivatives.
To gain more information about the oxidation process, a DFT
investigation (B3LYP/def2-SVP) was conducted on 2a and 6a.
As expected, the calculations assign the first oxidation wave in
both cases to the loss of one electron from the metal-centered
HOMO, leading to a radical cation in which the electronic
density is still mainly located on the metal fragment (spin
densities 0.98 e− for 2a·+ and 1.03 e− for 6a·+, respectively).
Data in Table 3 also indicate that the change of the metal (Cr/
W) increases the Epa values, whereas the replacement of the
F
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Article
solutions (dry and free of acid traces) containing 0.1 M [NBu4]ClO4
as supporting electrolyte, using an AUTOLAB PGSTAT302N
potentiostat and Ag/AgCl 3 M as reference and a Pt wire counter
electrode (scan rate 0.1 V/s).
Commercially available reagents were used without further
purification. Compounds 1a (M = Cr, R = Ph),20 1b (M = Cr, R1 =
Fc),21 1c (M = W, R = Ph),20b 1f (M = Cr, R = 4-methyl),22 1g (M =
Cr, R = 4-methoxy),22 1h (M = Cr, R = 4-dimethylamino),23 1j (M =
W, R = 4-methyl),24 2a,6 2b,6 2c,6 3a,6 4a,16d 4b,16d 8a,25 8b,26 8c,26
and 1127 were prepared according to reported procedures. For
experimental details of the synthesis and characterization of Fischer
alkynyl carbene complexes 1d (M = Cr, R = 2-thienyl), 1e (M = Cr, R
= 4-Br) and 1i (M = Cr, R = 4-CF3) see the Supporting Information.
General Procedure for the Synthesis of Compounds 2, 3, 9,
and 10. Under argon atmosphere, a solution of the corresponding
alkynyl Fischer carbene 1 or bis-alkynyl Fischer carbene 8 and 1,3-
bis(2,4,6-trimethylphenyl)-1,3-dihydro-2H-imidazol-2-ylidene (IMes)
or 1,3-bis(2,4,6-trimethylphenyl)-imidazolin-2-ylidene (SIMes) in
pentane (previously distilled and degassed) was stirred at room
temperature. The reaction was accompanied by a color change from
red to yellow and the subsequent formation of a yellow precipitate.
The solvent was decanted, and the obtained yellow solid washed with
dry pentane and dried in vacuum. Compounds 2, 3, 9, and 10 are
stable in the solid state at room temperature and in solution in Et2O,
THF, and hexanes but dissociate to some extent to the starting
components when left to stand in CH2Cl2 or CH3CN solution.
Samples in CDCl3 solution are stable for their NMR analysis, although
they slowly revert to some extent to the starting reagents if the
solution is left to stand at room temperature.
Synthesis of 2d. From a solution of alkynyl carbene 1d (50 mg,
0.14 mmol) and 1,3-bis(2,4,6-trimethylphenyl)-1,3-dihydro-2H-imida-
zol-2-ylidene (55 mg, 0.18 mmol) in 4 mL of pentane, 2d (94 mg,
quantitative yield) was obtained after 20 min at room temperature. 1H
NMR (300 MHz, CDCl3) δ 7.06 (s, 2H, NCHCHN), 6.95−6.87
(m, 4H, CHarom), 6.87−6.80 (m, 1H, CHarom), 6.75−6.64 (m, 2H,
CHarom), 2.63−2.50 (m, 1H, OCH2CH3), 2.30 (s, 6H, CH3), 2.24−
2.14 (m, 12H, CH3), 1.96−1.86 (m, 1H, OCH2CH3), 0.76 (t, J = 7.0
Hz, 3H, OCH2CH3). 13C{1H} NMR (176 MHz, CDCl3) δ 226.2
(CO), 220.5 (CO), 199.6 (CCr), 159.2 (C2), 150.9 (Carom), 140.6
(Carom), 137.6 (Carom), 135.2 (Carom), 134.5 (Carom), 134.2 (Carom),
131.9 (Carom), 129.9 (CH), 129.7 (CH), 129.2 (CH), 128.0 (CH),
124.1 (CH), 122.2 (CH), 120.4 (CH), 79.2 (C3), 67.6 (CH2), 21.0
(CH3), 18.5 (CH3), 18.3 (CH3), 15.2 (CH2CH3). IR (ATR): ν cm−1
3442, 2973, 2923, 2041, 1958, 1907, 1857, 1485, 1453, 668.
Electrospray ionization (ESI) HRMS m/z: calcd. for C35H33CrN2O6S
[M + H]+ 661.1464; found 661.1460.
Figure 7. Thermal ellipsoid plot (50% probability level) and labeling
scheme of one of the two independent cations of Z-13a·CHCl3.
Selected bond distances (Å): Cr(1)−C(8) 2.051(5), C(2)−C(6)
1.474(6), C(6)−C(7) 1.360(6), C(6)−C(21) 1.490(6), C(7)−C(8)
1.473(6).
respectively. Compared to the starting allenyl complex 2a the
distance Cr−C distance is reduced (from 2.18 to 2.05 Å in
13a), but the Csp2−NHC distances are almost identical (1.46 Å
in 2a and 1.47 Å in 13a).
CONCLUSIONS
■
In summary, a series of of chiral (racemic), stable, formally
neutral, zwitterionic mono- and bimetallic chromium and
tungsten allenyls can be prepared in quantitative yield by
addition of NHCs to Cr(0) and W(0) alkynyl Fischer carbenes.
The reaction provides an example of the use of different types
of NHCs as simple nucleophiles in C−C bond forming
reactions. A study of the electronic properties of the metal
allenyls prepared through this work shows that their UV−vis
spectra are influenced by the structure of the heterocyclic
moiety derived from the NHC (ring size, substituents on the
N-atoms) and by the nature of the metal fragment (Cr/W).
Although our previous investigations showed that this new class
of compounds was only reactive in the excited (T1) state, their
reductor character allows them to participate in electron-
transfer reactions in the ground state, leading to a new type of
α,β-unsaturated Fischer carbenes.
Synthesis of 2e. From a solution of alkynyl carbene 1e (50 mg, 0.12
mmol) and 1,3-bis(2,4,6-trimethylphenyl)-1,3-dihydro-2H-imidazol-2-
ylidene (42 mg, 0.14 mmol) in 4 mL of pentane, 2e (86 mg,
1
quantitative yield) was obtained after 1 h at room temperature. H
NMR (700 MHz, CDCl3) δ 7.11 (s, 2H, NCHCHN), 7.07 (d, J =
8.2 Hz, 2H, CHarom), 6.91−6.89 (m, 6H, CHarom), 2.57−2.49 (m, 1H,
OCH2CH3), 2.29 (s, 6H, CH3), 2.24 (s, 6H, CH3), 2.20 (s, 6H, CH3),
2.02−1.94 (m, 1H, OCH2CH3), 0.75 (t, J = 7.0 Hz, 3H, OCH2CH3).
13C{1H} NMR (176 MHz, CDCl3) δ 225.9 (CO), 220.3 (CO), 199.5
(CCr), 160.3 (C2), 150.3 (Carom), 140.8 (Carom), 135.8 (Carom), 135.0
(Carom), 134.4 (Carom), 131.8 (Carom), 130.4 (CHarom), 129.8 (CH),
129.3 (CH), 128.4 (Carom), 122.4 (CH), 119.4 (Carom), 83.2 (C3),
67.8 (CH2), 21.0 (CH3), 18.6 (CH3), 18.5 (CH3), 15.2 (CH2CH3). IR
(ATR): ν cm−1 3448, 2925, 2041, 1958, 1912, 1867, 1485, 1450, 667.
ESI-HRMS m/z: calcd for C37H34BrCrN2O6 [M + H]+ 733.1050;
found 733.1028.
Synthesis of 2f. From a solution of alkynyl carbene 1f (50 mg, 0.14
mmol) and 1,3-bis(2,4,6-trimethylphenyl)-1,3-dihydro-2H-imidazol-2-
ylidene (52 mg, 0.17 mmol) in 5 mL of pentane, 2f (94 mg,
quantitative yield) was obtained after 20 min at room temperature. 1H
NMR (300 MHz, CDCl3) δ 7.08 (s, 2H, NCHCHN), 6.94−6.84
(m, 5H, CHarom), 6.79−6.71 (m, 2H, CHarom), 2.60−2.47 (m, 1H,
OCH2CH3), 2.27 (s, 6H, CH3), 2.25−2.21 (2 s, 12H, CH3), 2.16 (s,
3H, CH3), 2.01−1.89 (m, 1H, OCH2CH3), 0.74 (t, J = 7.0 Hz, 3H,
EXPERIMENTAL GENERAL
■
Flame-dried glassware was used for moisture-sensitive reactions, and
anhydrous solvents were taken from a Pure Solvent PS-MD-5
apparatus. In the synthesis of the carbene precursors, silica gel
(Merck: 230−400 mesh) was used as stationary phases for purification
of crude reaction mixtures by flash column chromatography. NMR
spectra were recorded at 25 °C in CDCl3 on a 300 MHz (300 MHz for
1H, 75 MHz for 13C) and 500 MHz (500 MHz for H, 126 MHz for
1
13C) and 700 MHz (700 MHz for 1H, 176 MHz for 13C)
spectrometers. Chemical shifts are given in parts per million relative
to CDCl3 (1H, 7.27 ppm and 13C, 77.00 ppm). IR spectra were taken
on a mid-IR (8000−400 cm−1) spectrometer as solid films by slow
evaporation of the solvent using the attenuated total reflectance
(ATR) technique. High-resolution (HR) MS experiments were
conducted on an Accurate Mass Q-TOF system. UV−vis spectra
were recorded in Et2O or THF in concentrations of 1 × 10−4 M on an
Agilent Technologies Cary 60 spectrometer, using quartz cells. Cyclic
voltammograms were recorded at room temperature from CH3CN
G
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OCH2CH3). 13C{1H} NMR (176 MHz, CDCl3) δ 226.2 (CO), 220.5
(CO), 200.5 (CCr), 159.6 (C2), 151.0 (Carom), 140.5 (Carom), 135.3
(Carom), 135.1 (Carom), 134.5 (Carom), 133.9 (Carom), 132.0 (Carom),
129.6 (CH), 129.1 (CH), 128.0 (CH), 126.8 (CH), 122.2 (CH), 84.4
(C3), 67.4 (CH2), 21.0 (CH3), 20.9 (CH3), 18.6 (CH3), 18.5 (CH3),
15.2 (CH2CH3). IR (ATR): ν cm−1 3442, 2924, 2040, 1956, 1909,
1864, 1486, 1448, 668. ESI-HRMS m/z: calcd for C38H37CrN2O6:
669.2051 [M + H]+; found 669.2047.
1865, 1516, 1490, 1459, 667. ESI-HRMS m/z: calcd for
C37H36BrCrN2O6 [M + H]+ 737.1145; found 735.1150.
Synthesis of 3c. From a solution of alkynyl carbene 1c (50 mg, 0.10
mmol) and 1,3-bis(2,4,6-trimethylphenyl)-imidazolin-2-ylidene (35
mg, 0.11 mmol) in 5 mL of pentane, 3c (87 mg, quantitative yield)
1
was obtained after 30 min at room temperature. H NMR (300 MHz,
CDCl3) δ 6.98 (m, 2H, CHarom), 6.95−6.87 (m, 3H, CHarom), 6.83 (m,
2H, CHarom), 6.75 (m, 2H, CHarom), 4.33 (m, 2H, NCH2), 3.93 (m,
2H, NCH2), 2.46 (s, 6H, CH3), 2.36 (s, 6H, CH3), 2.21 (s, 6H, CH3),
1.85 (dd, J1 = 9.1, J2 = 7.0 Hz, 2H, OCH2CH3), 0.71 (t, J = 7.0 Hz, 3H,
OCH2CH3). 13C{1H} NMR (176 MHz, CDCl3) δ 207.2 (CW), 205.4
(CO), 201.1 (CO), 170.0 (C2), 146.8 (Carom), 139.4 (Carom), 136.2
(Carom), 136.0 (Carom), 134.4 (Carom), 133.1 (Carom), 129.7 (CH),
129.4 (CH), 127.4 (CH), 127.2 (CH), 125.8 (CH), 86.0 (C3), 68.4
(CH2CH3), 50.0 (NCH2), 20.9 (CH3), 18.8 (CH3), 18.7 (CH2CH3),
15.1 (CH3). IR (ATR): ν cm−1 3448, 2925, 2053, 1968, 1907, 1845,
1608, 1520, 1496, 590. ESI-HRMS m/z: calcd for C37H37N2O6W [M +
H]+; 789.2161; found.789.2164.
General Procedure for the Synthesis of Compounds 5a,b.
To a solution of alkynyl carbene 1a and the BF4 salt 4 in 5 mL of dry
pentane was added KHMDS (1 M in THF). The reaction product
precipitates in the medium once it is formed. After 30 min at room
temperature, the solvent was removed, and the precipitated orange
solid was washed with dry pentane. Compounds 5 are perfectly stable
in the solid state at room temperature and in solution in Et2O, THF,
and hexanes but slowly decompose when left to stand in CH2Cl2 or
CHCl3 solution. Samples in CDCl3 solution are stable for their NMR
analysis.
Synthesis of 2g. From a solution of alkynyl carbene 1g (60 mg,
0.16 mmol) and 1,3-bis(2,4,6-trimethylphenyl)-1,3-dihydro-2H-imida-
zol-2-ylidene (56 mg, 0.18 mmol) in 6 mL of pentane, 2g (110 mg,
1
quantitative yield) was obtained after 1 h at room temperature. H
NMR (300 MHz, CDCl3) δ 7.07 (s, 2H, NCHCHN), 6.96−6.84
(m, 6H, CHarom), 6.53−6.44 (m, 2H, CHarom), 3.68 (s, 3H, OCH3),
2.58−2.49 (m, 1H, OCH2CH3), 2.28 (s, 6H, CH3), 2.23 (s, 6H, CH3),
2.22 (s, 6H, CH3), 2.02−1.93 (m, 1H, OCH2CH3), 0.74 (t, J = 7.0 Hz,
3H, OCH2CH3). 13C{1H} NMR (176 MHz, CDCl3) δ 226.1 (CO),
220.5 (CO), 200.8 (CCr), 159.4 (C2), 151.0 (Carom), 140.4 (Carom),
135.2 (Carom), 134.5 (Carom), 132.0 (Carom), 130.0 (Carom), 129.7
(CH), 129.1 (CH), 128.2 (CH), 122.2 (CH), 112.8 (CH), 84.0 (C3),
67.3 (CH2), 55.2 (OCH3), 21.0 (CH3), 18.6 (CH3), 18.5 (CH3), 15.1
(CH2CH3). IR (ATR): ν cm−1 3447, 2966, 2925, 2040, 1959, 1902,
1865, 1506, 1485, 1448, 668. ESI-HRMS m/z: calcd for
C38H37CrN2O7 [M + H]+ 685.2001; found 685.2016.
Synthesis of 2h. From a solution of alkynyl carbene 1h (50 mg,
0.13 mmol) and 1,3-bis(2,4,6-trimethylphenyl)-1,3-dihydro-2H-imida-
zol-2-ylidene (46 mg, 0.15 mmol) in 4 mL of pentane, 2h (91 mg,
quantitative yield) was obtained after 30 min at room temperature. 1H
NMR (700 MHz, CDCl3) δ 7.06 (s, 2H, NCHCHN), 6.91−6.83
(m, 6H, CHarom), 6.33 (d, J = 8.0 Hz, 2H, CHarom), 2.80 (s, 6H,
NCH3), 2.58−2.53 (m, 1H, OCH2CH3), 2.27 (s, 6H, CH3), 2.23 (s,
12H, CH3), 1.95−1.90 (m, 1H, OCH2CH3), 0.74 (t, J = 6.9 Hz, 3H,
OCH2CH3). 13C{1H} NMR (176 MHz, CDCl3) δ 226.3 (CO), 220.6
(CO), 201.0 (CCr), 158.9 (C2), 151.5 (Carom), 148.9 (Carom), 140.2
(Carom), 135.2 (Carom), 134.6 (Carom), 132.1, 129.7 (CH), 129.1 (CH),
127.8 (CH), 125.6 (Carom), 122.1 (CH), 112.0 (CH), 84.6 (C3), 67.2
(CH2), 40.8 (NCH3), 21.0 (CH3), 18.6 (CH3), 18.5 (CH3), 15.2
(CH2CH3). IR (ATR): ν cm−1 3414, 2979, 2923, 2111, 2046, 1919,
1865, 1524, 1484, 1445, 665. ESI-HRMS m/z: calcd for [M + H]+
C39H40CrN3O6 698.2317; found 698.2318.
Synthesis of 2i. From a solution of alkynyl carbene 1i (55 mg, 0.13
mmol) and 1,3-bis(2,4,6-trimethylphenyl)-1,3-dihydro-2H-imidazol-2-
ylidene (48 mg, 0.16 mmol) in 5 mL of pentane, 2i (96 mg,
quantitative yield) was obtained after 40 min at room temperature. 1H
NMR (300 MHz, CDCl3) δ 7.23−7.17 (m, 2H, NCHCHN), 7.16−
7.10 (m, 4H, CHarom), 6.90−6.83 (m, 4H, CHarom), 2.62−2.51 (m, 1H,
OCH2CH3), 2.26 (s, 6H, CH3), 2.25 (s, 6H, CH3), 2.22 (s, 6H, CH3),
2.05−1.93 (m, 1H, OCH2CH3), 0.76 (t, J = 7.0 Hz, 3H, OCH2CH3).
13C{1H} NMR (75 MHz, CDCl3) δ 226.3 (CO), 220.6 (CO), 199.6
(CCr), 160.7 (C2), 150.5 (Carom), 141.3 (Carom), 135.4 (Carom), 134.8
(Carom), 132.1 (Carom), 130.2 (CH), 129.6 (CH), 127.3 (CH), 124.7
(CH), 122.9 (CH), 83.6 (C3), 68.4 (CH2), 21.3 (CH3), 19.0 (CH3),
18.8 (CH3), 15.5 (CH2CH3). IR (ATR): ν cm−1 3441, 2973, 2925,
2042, 1962, 1911, 1874, 1484, 1449, 1325, 665. ESI-HRMS m/z: calcd
for C38H34CrF3N2O6 [M + H]+ 723.1770; found 723.1769.
Synthesis of 3b. From a solution of alkynyl carbene 1e (50 mg, 0.12
mmol) and 1,3-bis(2,4,6-trimethylphenyl)-imidazolin-2-ylidene (43
mg, 0.14 mmol) in 5 mL of pentane, 3b (88 mg, quantitative yield)
was obtained after 30 min at room temperature. 1H NMR (300 MHz,
CDCl3) δ 7.12−6.98 (m, 2H, CHarom), 6.93−6.77 (m, 6H, CHarom),
4.40−4.22 (m, 2H, NCH2), 4.02−3.86 (m, 2H, NCH2), 2.62−2.51 (m,
1H, OCH2CH3), 2.43 (s, 6H, CH3), 2.37 (s, 6H, CH3), 2.25 (s, 6H,
CH3), 2.05−1.94 (m, 1H, OCH2CH3), 0.75 (t, J = 7.0 Hz, 3H,
OCH2CH3). 13C{1H} NMR (75 MHz, CDCl3) δ 226.2 (CO), 220.2
(CO), 196.7 (CCr), 170.5 (C2), 157.4 (Carom), 140.0 (Carom), 136.5
(Carom), 135.7 (Carom), 134.6 (Carom), 133.6 (Carom), 130.6 (CH),
130.2 (CH), 129.9 (Carom), 129.7 (CH), 120.0 (Carom), 84.3 (C3),
69.5 (CH2 CH3), 50.4 (NCH2), 21.3 (CH3), 18.9 (CH3), 15.6
(CH2CH3). IR (ATR): ν cm−1 3423, 2971, 2923, 2043, 1963, 1922,
Synthesis of 5a. From 1a (43 mg, 0.12 mmol), BF4 salt 4a (50 mg,
0.12 mmol), and KHMDS (134 μL, 0.13 mmol, 1 M in THF), 5a (96
1
mg, quantitative yield) was obtained as dark orange solid. H NMR
(300 MHz, CDCl3) δ 6.94−6.58 (m, 9H, CHarom), 4.04−3.84 (m, 2H,
NCH2), 3.61−3.46 (m, 2H, NCH2), 2.57 (s, 6H, CH3), 2.49−2.39 (m,
2H, CH2CH2CH2), 2.24−2.14 (m, 12H, CH3), 2.11−1.95 (m, 1H,
OCH2CH3), 1.69−1.55 (m, 1H, OCH2CH3), 0.52 (t, J = 6.9 Hz, 3H,
OCH2CH3). 13C{1H} NMR (CDCl3, 176 MHz) δ 226.1 (CO), 220.4
(CO), 208.4 (CCr), 167.8 (C2), 159.3 (Carom), 139.0, 138.7, 138.7,
132.9, 130.0, 129.7, 127.1, 127.0, 125.0, (all CHarom), 91.4 (C3), 66.9
(CH2CH3), 50.7 (NCH2), 21.6 (CH2), 20.8 (CH3), 19.5 (CH3), 19.4
(CH3), 15.0 (CH2CH3). IR (ATR): ν cm−1 3447, 2923, 2857, 2041,
1965, 1904, 1849, 1539, 1081, 1037, 668. ESI-HRMS m/z: calcd for
C38H39CrN2O6 [M + H] 671.2208; found 671.2203.
Synthesis of 5b. From 1a (39 mg, 0.11 mmol), BF4 salt 4b (60 mg,
0.14 mmol), and KHDMS (156 μL, 0.16 mmol, 1 M in THF), 5b (92
1
mg, quantitative yield) was obtained as dark orange solid. H NMR
(300 MHz, CDCl3) δ 7.03−6.83(m, 7H, CHarom), 6.65−6.51 (m, 2H,
CHarom), 4.19−4.03 (m, 2H, NCH2), 3.95−3.82 (m, 2H, NCH2), 2.64
(s, 6H, CH3), 2.41 (s, 3H, CH3), 2.24 (s, 9H, CH3), 2.08−1.96 (m,
5H, NCH2CH2CH2CH2N and OCH2CH3), 1.79−1.74 (m, 1H,
OCH2CH3), 0.50 (t, J = 6.9 Hz, 3H, OCH2CH3). 13C{1H} NMR
(176 MHz, CDCl3) δ 226.3 (CO), 220.2 (CO), 216.3 (CCr), 157.6
(C2), 138.9 (Carom), 138.1 (Carom), 133.5 (Carom), 130.8 (Carom), 130.4
(Carom), 128.2 (Carom), 127.4 (CH), 125.2 (CH), 93.1 (C3), 68.1
(CH2CH3), 56.8 (NCH2), 25.3 (CH2CH2), 24.0 (CH2CH2), 20.7
(CH3), 20.4 (CH3), 15.19 (CH2CH3). IR (ATR): ν cm−1 3443, 2971,
2924, 2040, 1964, 1905, 1855, 1519, 1481, 1438, 1209, 1081, 1053,
699.45; ESI-HRMS m/z: calcd for C39H41CrN2O6 [M + H]+
685.2365; found 685.2365.
General Procedure for the Synthesis of Compounds 6.
Method A. Synthesis of 1,3-Dimethylimidazol-2-ylidene (Me2Im).
This compound was obtanied by a modification of the previously
reported method.17 1,3-Di(methyl)imidazolium iodide17 (32.0 g, 0.143
mol) and NaH (3.43 g, 0.143 mol) were dissolved at −78 °C in 200
mL of liquid NH3. The ammonia was previously dried by passing the
gas successively through a flask containing KOH flakes and through
another containing 3 Å molecular sieves. The reaction mixture was
stirred for 1 h at this temperature before the temperature was raised to
room temperature to remove NH3. The last traces of ammonia were
eliminated in vacuo at room temperature. The yellow precipitate
obtained was carefully distilled (avoiding overheating of the oil bath)
H
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at 150 °C and 1 × 10−2 mbar into a flask that was cooled with liquid
nitrogen to give 1.87 g (43%) of a red, crystalline solid (at −30 °C).
1H NMR (C6D6) δ 6.70 (s, 2H, NCHCHN), 3.43 (s, 6 H, CH3).
13C{1H}NMR (C6D6) δ 214.55 (NCN), 120.24 (NCCN), 37.35
(CH3). The free carbene was stored in the fridge as a solution in dry
benzene (1M).
Reaction of (Me2Im) with Alkynyl Carbenes. Under argon
atmosphere, to a solution of the corresponding alkynyl carbene 1 (1
equiv) in pentane (previously distilled and degassed) was added the
1,3-dimethylimidazol-2-ylidene (Me2Im; 2 equiv, 1 M solution in
benzene), and the mixture was stirred at room temperature. The
reaction was accompanied by the formation of a brownish precipitate.
The solvent was decanted, and the obtained solid washed with dry
pentane and dried in vacuum.
Method B. Generation in situ of 1,3-dimethylimidazol-2-ylidene
(Me2Im) was done according to the reported method.18 1,3-
Dimethylimidazolium iodide (2.10 mmol) and KH (13.0 mmol)
were placed in a Schlenk tube, and the suspension was stirred in THF
(3 mL). Slow gas evolution was observed. After 4 h, effervescence had
stopped, and Et2O (10 mL) was added to ensure nearly complete
precipitation of the generated potassium iodide. The THF/Et2O
solution of the 1,3-dimethylimidazol-2-ylidene was filtered and added
dropwise to a 50 mL flask containing the carbene complex (1.00
mmol) dissolved in Et2O (5 mL). The precipitate obtained was
washed with dry pentane. Compounds 6 are perfectly stable in the
solid state at room temperature and in solution in Et2O, THF, and
hexanes but slowly decompose when left to stand in CH2Cl2 or CHCl3
solution. Samples in CDCl3 solution are stable for their NMR analysis.
Synthesis of 6a. Following Method A. From 1a (50 mg, 0.14
mmol) in 5 mL of pentane and 1,3-dimethylimidazol-2-ylidene (1 M
solution in benzene) (0.32 mL, 0.32 mmol). Compound 6a was
obtained as brownish powder (62 mg, quantitative yield).
Following Method B. From 1a (100 mg; 0.29 mmol), potassium
hydride (151 mg, 3.77 mmol), and 1,3 dimethylimidazolium iodide
(134 mg, 0.60 mmol) in 0.85 mL of THF, compound 6a (127 mg,
quantitative yield) was obtained after 15 min at room temperature.
1H NMR (300 MHz, CDCl3) δ 7.26−7.24 (m, 2H, CHarom), 7.20−
7.17 (m, 2H, CHarom), 7.09−7.07 (m, 1H, CHarom), 6.91 (s, 2H,
NCHCHN), 4.01 (q, J = 7.1 Hz, 2H, OCH2CH3), 3.74 (s, 6H,
CH3), 1.39 (t, J = 7.1 Hz, 3H, OCH2CH3). 13C{1H} NMR (176 MHz,
CDCl3) δ 226.4 (CO), 220.7 (CO), 176.0 (CCr), 147.3 (C2), 146.4
(Carom), 143.8 (Carom), 128.0 (CH), 124.7 (CH), 124.4 (CH), 121.5
(CH), 106.5 (C3), 63.8 (CH2CH3), 37.2 (NCH3), 15.2 (CH2CH3). IR
(ATR): ν cm−1 3424, 2973, 2038, 1963, 1903, 1859, 1576, 1515, 1483,
1446, 1237, 661. ESI-HRMS m/z: calcd for C21H18CrN2O6 446.3735;
found 306.2094 [M-5(CO)].
1964, 1907, 1844, 1576, 1114, 1074, 667. ESI-HRMS m/z: calcd for
C19H17CrN2O6S [M + H]+ 453.0207; found 453.0196.
Synthesis of 6d. Method B: From 95 mg (0.42 mmol) of 1,3-
di(methyl)imidazolium iodide, 104 mg (2.69 mmol) of potassium
hydride, and 100 mg (0.20 mmol) of 1j in 1 mL of dry diethyl ether,
compound 6d was obtained as brown powder (117 mg, quantitative
yield). 1H NMR (700 MHz, CDCl3) δ 7.23 (d, J = 8.0 Hz, 2H,
CHarom), 7.09 (d, J = 8.0 Hz, 2H, CHarom), 6.91 (s, 2H, NCHCHN),
4.04−3.95 (m, 2H, OCH2CH3), 3.70 (s, 6H), 2.32 (s, 3H), 1.37 (t, J =
7.0 Hz, 3H, OCH2CH3). 13C{1H} NMR (176 MHz, CDCl3) δ 205.7
(CO), 201.7 (CO), 182.4 (CW), 145.7 (C2), 143.5 (Carom), 134.9
(Carom), 133.5 (Carom), 128.7 (CH), 127.1 (CH), 121.4 (CH), 105.7
(C3), 63.9 (CH2CH3), 37.3 (NCH3), 21.2 (ArCH3), 15.2 (CH2CH3).
IR (ATR): ν cm−1: 3424, 2925, 2050, 1961, 1900, 1855, 1644, 1577,
1508, 1450, 1075, 821, 584. ESI-HRMS m/z: calcd for C22H21N2O6W
[M + H]+ 593.0907; found 593.0900.
Synthesis of Bis-Allenyl Complexes 9 and 10. These were prepared
following the general methodology described for compounds 2 but
using THF as solvent. The solid obtained after the removal of the
solvent under vacuum was washed several times with pentane.
Synthesis of 9a. From a solution of alkynyl carbene 8a (50 mg, 0.08
mmol) and 1,3-bis(2,4,6-trimethylphenyl)-1,3-dihydro-2H-imidazol-2-
ylidene (54 mg, 0.18 mmol) in 5 mL of THF, compound 9a (83 mg,
1
96%) was obtained after 15 min at room temperature. H NMR (300
MHz, CDCl3) δ 7.13−6.63 (m, 13.18H), 6.61−6.20 (m, 1.46H),
6.12−6.11 (m, 0.96H), 6.02−5.80 (m, 0.4H), (NCHCHN, CHarom),
2.69−2.59 (m, 2H, OCH2CH3), 2.33 (s, 7.08H), 2.27 (s, 5.86H), 2.21
(s, 4.83H), 2.17 (s, 1.71H), 2.10 (s, 10.40H), 1.91 (s, 6.12H), (CH3),
1.64−1.59 (m, 2H, OCH2CH3), 0.88 (t, J = 6.9 Hz, 1.87H), 0.74 (t, J
= 6.9 Hz, 3.60H), 0.59 (t, J = 7.0 Hz, 0.32H), (OCH2CH3). 13C{1H}
NMR (CDCl3, 176 MHz) δ 226.2 (CO), 226.0 (CO), 220.4 (CO),
219.8 (CO), 196.7 (CCr), 160.9 (C2), 150.2 (Carom), 140.9 (CH),
136.5 (Carom), 134.6 (Carom), 134.1 (Carom), 132.3 (Carom), 130.0
(CH), 128.8 (CH), 126.4 (CH), 122.4 (CH), 83.2 (C2), 67.2
(CH2CH3), 21.1 (CH3), 17.8 (CH3), 17.7 (CH3), 15.3 (CH2CH3),
15.2 (CH2CH3). IR (ATR): ν cm−1 3442, 2974, 2924, 2041, 1959,
1906, 1867, 1485, 1449, 665. ESI-HRMS m/z: calcd for
C68H63Cr2N4O12 [M + H]+ 1231.3296; found 1231.3252.
Synthesis of 9b. From a solution of alkynyl carbene 8b (50 mg,
0.07 mmol) and 1,3-bis(2,4,6-trimethylphenyl)-1,3-dihydro-2H-imida-
zol-2-ylidene (44 mg, 0.15 mmol) in 4 mL of THF, compound 9b (94
mg, quantitative yield) was obtained after 30 min at room temperature.
1H NMR (700 MHz, CDCl3) δ 7.15−6.82 (m, 20H, NCHCHN,
CHarom), 2.65−2.56 (m, 2H, OCH2CH3), 2.25 (s, 36H, CH3), 2.02−
1.93 (m, 2H, OCH2CH3), 0.78 (t, J = 6.3 Hz, 6H, OCH2CH3).
13C{1H} NMR (176 MHz, CDCl3) δ 226.1 (CO), 220.5 (CO), 200.2
(CCr), 199.9 (C−Cr), 160.0 (C2), 150.6 (Carom), 140.6 (Carom), 135.1
(Carom), 134.5 (Carom), 131.9 (Carom), 129.7 (CH), 129.1 (CH), 127.1
(CH), 125.7 (CH), 122.4 (CH), 84.2 (C3), 68.0 (CH2CH3), 21.0
(CH3), 18.6 (CH3), 18.5 (CH3), 15.2 (CH2CH3). IR (ATR): ν cm−1
3443, 2973, 2922, 2041, 1958, 1906, 1867, 1488, 1451, 664. ESI-
HRMS m/z: calcd for C74H67Cr2N4O12 [M + H]+ 1307.3625; found
1307.3660.
Synthesis of 9c. From a solution of alkynyl carbene 8c (50 mg, 0.07
mmol) and 1,3-bis(2,4,6-trimethylphenyl)-1,3-dihydro-2H-imidazol-2-
ylidene (51 mg, 0.17 mmol) in 5 mL of THF, compound 9c (93 mg,
quantitative yield) was obtained after 20 min at room temperature. 1H
NMR (700 MHz, CDCl3) δ 7.10 (s, 2H, NCHCHN), 7.09 (s, 2H,
NCHCHN), 7.00−6.95 (m, 4H, CHarom), 6.93−6.88 (m, 8H,
CHarom), 6.45−6.40 (m, 4H, CHarom), 2.63−2.56 (m, 2H, OCH2CH3),
2.30 (s, 6H, CH3), 2.28 (s, 6H, CH3), 2.25−2.20 (m, 24H, CH3),
2.04−1.98 (m, 2H, OCH2CH3), 0.81−0.75 (m, 6H, OCH2CH3).
13C{1H} NMR (CDCl3, 176 MHz) δ 226.1 (CO), 220.5 (CO), 220.4
(CO), 201.2 (CCr), 159.9 (Carom), 155.4 (Carom), 155.2 (Carom), 150.0
(Carom), 140.7 (Carom), 140.6 (Carom), 135.3 (Carom), 135.1 (Carom),
134.5 (Carom), 134.5 (Carom), 132.4 (Carom), 132.1 (Carom), 132.0
(Carom), 131.9 (Carom), 130.0 (CH), 129.7 (CH), 129.7 (CH), 129.2
(CH), 129.1 (CH), 129.1 (CH), 128.4 (CH), 128.1 (CH), 122.4
(CH), 122.3 (CH), 117.7 (CH), 117.5 (CH), 84.0 (C3), 67.6 (CH2),
67.4 (CH2CH3), 21.1 (CH3), 21.0 (CH3), 18.6 (CH3), 18.5 (CH3),
Synthesis of 6b. Method A: From 60 mg (0.16 mmol) of 1f in 5
mL of pentane and 1,3-dimethylimidazol-2-ylidene (1 M solution in
benzene; 0.25 mL, 0.25 mmol), compound 6b was obtained as dark
orange powder (60 mg, 81%). 1H NMR (300 MHz, CDCl3) δ 7.13 (d,
J = 8.1 Hz, 2H, CHarom), 7.07 (d, J = 8.1 Hz, 2H, CHarom), 6.90 (s, 2H,
NCHCHN), 4.05−3.95 (m, 2H, OCH2CH3), 3.72 (s, 6H, CH3),
2.30 (s, 3H, CH3), 1.38 (t, J = 7.0 Hz, 3H, OCH2CH3). 13C{1H} NMR
(176 MHz, CDCl3) δ 226.4 (CO), 220.9 (CO), 176.1 (CCr), 146.5
(C2), 143.8 (Carom), 143.3 (Carom), 134.1 (Carom), 128.7 (CH), 125.2
(CH), 121.5 (CH), 106.4 (C3), 63.8 (CH2CH3), 37.2 (NCH3), 21.0
(ArCH3), 15.2 (CH2CH3). IR (ATR): ν cm−1 3424, 3124, 2927, 2037,
1948, 1906, 1854, 1664, 1579, 1453, 1077, 1037, 665. ESI-HRMS m/z:
calcd for C22H21N2O6Cr [M + H]+ 461.0799; found 461.0799.
Synthesis of 6c. Method B. From 106 mg (0.47 mmol) of 1,3-
di(methyl)imidazolium iodide, 115 mg (2.86 mmol) of potassium
hydride, and 80 mg (0.22 mmol) of 1d in 1 mL of dry diethyl ether,
1
compound 6c was obtained as pale yellow powder (96 mg, 96%). H
NMR (300 MHz, CDCl3) δ 7.23−7.22 (m, 2H, CHarom), 7.17−7.16
(m, 1H, CHarom), 6.92 (s, 2H, NCHCHN), 4.03−3.90 (m, 2H,
OCH2CH3), 3.69 (s, 6H, CH3), 1.40−1.35 (m, 3H, OCH2CH3).
13C{1H} NMR (176 MHz, CDCl3) δ 226.0 (CO), 221.0 (CO), 178.7
(CCr), 146.6 (C2), 144.9 (Carom), 135.5 (Carom), 127.8 (CH), 124.5
(CH), 121.5 (CH), 120.9 (CH), 106.2 (C3), 63.8 (CH2CH3), 37.1
(NCH3), 15.2 (CH2CH3). IR (ATR): ν cm−1: 3446, 2974, 2926, 2038,
I
DOI: 10.1021/acs.inorgchem.5b00492
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
15.2 (CH2CH3). IR (ATR): ν cm−1 3444, 2973, 2924, 2041, 1959,
1907, 1868, 1495, 1451, 1233, 665. ESI-HRMS m/z: calcd for
C74H67Cr2N4O13 [M + H]+ 1323.3574; found 1323.3515.
(CH3), 17.8 (CH3), 17.4 (CH3), 14.6 (CH2CH3). IR (ATR): ν cm−1:
3419, 2924, 2038, 1907, 1874, 1667. ESI-HRMS m/z: calcd for
C59H53Cr2N4O12 [M + H]+ 1113.2468; found 1113.2495.
General Procedure for the Synthesis of Compounds 13. The
corresponding metal allenyl (1 mmol) was dissolved in anhydrous 1,2-
dichloroethane (67 mL) in a Schlenk flask; trityl hexafluorophosphate
(1.2 mmol) was added, and the reaction mixture was stirred for 4−5
min. The solvent was removed under reduced pressure, and the solid
formed was washed with dry diethyl ether.
Synthesis of 10a. From a solution of alkynyl carbene 8a (50 mg,
0.08 mmol) and 1,3-bis(2,4,6-trimethylphenyl)-imidazolin-2-ylidene
(49 mg, 0.16 mmol) in 5 mL of THF, compound 10a (70 mg, 89%
1
yield) was obtained after 5 min at room temperature. H NMR (700
MHz, CDCl3) δ 7.03−6.66 (m, 9H), 6.49−6.43 (m, 0.7H), 6.39−6.33
(m, 0.5H), 6.01−5.93 (m, 0.9H), 5.82−5.71 (m, 0.8H), (CHarom),
4.79−4.73 (m, 0.5H), 4.41−4.32 (m, 1H), 4.30−4.18 (m, 2.5H),
4.03−3.95 (m, 0.5), 3.88−3.70 (m, 3.5H), (NCH2), 3.23−3.14 (m,
1H), 2.83−2.74 (m, 1H), (OCH2CH3), 1.61−2.00 (m, 36H, CH3),
1.67−1.53 (m, 2H, OCH2CH3), 1.42−1.38 (m, 0.5H), 1.35−1.22 (m,
1.5H), 0.96−0.81 (m, 4H), (OCH2CH3). 13C{1H} NMR (176 MHz,
CDCl3) δ 226.0 (CO), 220.0 (CO), 219.9 (CO), 216.3 (CO), 193.4
(CCr), 184.2 (CCr), 170.0 (C2), 164.1 (C2), 156.3 (Carom), 139.7
(Carom), 139.3 (Carom), 137.5 (Carom), 137.1 (Carom), 136.4 (Carom),
136.2 (Carom), 134.4 (Carom), 134.3 (Carom), 133.8 (Carom), 133.4
(Carom), 131.9 (Carom), 131.3 (CH), 130.1 (CH), 129.7 (CH), 129.6
(CH), 129.5 (CH), 129.4 (CH), 129.3 (CH), 129.2 (CH), 129.1
(CH), 125.9 (CH), 83.8 (C3), 69.1 (CH2CH3), 68.5 (CH2CH3), 50.0
(NCH2), 47.1 (NCH2), 46.5 (NCH2), 44.2 (NCH2), 20.1 (CH3), 18.6
(CH3), 18.5 (CH3), 18.2 (CH3), 17.9 (CH3), 17.7 (CH3), 15.5
(CH2CH3), 15.2 (CH2CH3), 14.1 (CH2CH3). IR (ATR): ν cm−1
3775, 3699, 3660, 3634, 3576, 3454, 2923, 2042, 1909, 1860, 1829,
1604, 1520, 1493, 1444, 1382, 1356, 1284, 1213, 1067, 1036, 852, 666.
ESI-HRMS m/z: calcd for C58H66N4O2 [M-2(CrCO5)]+ 850.5185;
found 851.5195.
Synthesis of 13a. Following the general procedure, from 2a (60
mg, 0.09 mmol) and trityl hexafluorophosphate (43 mg, 0.11 mmol) in
6 mL of 1,2-dichloroethane, 13a (70 mg, quantitative yield) was
obtained as a Z/E isomeric mixture (1:2). 1H NMR (700 MHz,
CDCl3) δ 7.76 (d, J = 2.8 Hz), 7.68 (s), 7.57 (s), 7.35−7.31 (m),
7.31−7.26 (m), 7.22−7.20 (m), 7.18−7.14 (m), 7.05 (s), 7.04−7.00
(m), 6.96−6.90 (m), 6.89 (s), 6.75 (s), 6.62 (d, J = 7.7 Hz, 12H,
CHarom, CHC and NCHCHN), 5.13 (q, J = 7.0 Hz, 1.33H, E-
isomer), 4.50 (q, J = 7.1 Hz, 0.66H, Z-isomer), (2H, OCH2CH3), 2.36
(s), 2.31−2.22 (m), 2.14−2.10 (m), (18H, CH3), 1.54 (t, J = 7.0 Hz,
2H, E-isomer), 0.83 (t, J = 7.1 Hz, 1H,-Z isomer), (3 H, OCH2CH3).
13C{1H} NMR (176 MHz, CDCl3) δ: 336.4 (CCr), 332.2 (CCr),
224.4 (CO), 222.6 (CO), 215.1 (CO), 214.9 (CO), 147.0 (CHC),
146.8 (Carom), 146.3 (CHC), 142.3 (Carom), 142.0 (CH), 141.7
(Caromn), 141.7 (Carom), 141.1 (Carom), 137.2 (Carom), 136.6 (CH),
134.0 (Carom), 133.6 (Carom), 133.3 (Carom), 133.1 (Carom), 132.8
(Carom), 130.9 (CH), 130.7 (CH), 130.6 (Carom), 130.5 (CH), 130.3
(Carom), 130.2 (Carom), 130.2 (CH), 130.1 (CH), 130.0 (CH), 129.9
(CH), 129.3 (CH), 129.0 (Carom), 128.7 (CH), 128.6 (CH), 128.3
(CH), 127.9 (CH), 127.9 (CH), 127.5 (CH), 127.3 (CH), 126.9
(CH), 126.7 (CH), 125.1 (CH), 116.0 (CHC), 113.4 (CHC),
82.0 (CH2CH3), 79.1 (CH2CH3), 21.2 (CH3), 21.1 (CH3), 21.0
(CH3), 20.8 (CH3), 19.8 (CH3), 17.9 (CH3), 17.8 (CH3), 17.2 (CH3),
15.6 (CH2CH3), 13.7 (CH2CH3). IR (ATR): ν cm−1: 3452, 3183,
2925, 2061, 1949, 1609. ESI-HRMS m/z: calcd for C37H35CrN2O6
655.1895; found 655.1925.
Synthesis of 10b. From a solution of alkynyl carbene 8b (50 mg,
0.072 mmol) and 1,3-bis(2,4,6-trimethylphenyl)-imidazolin-2-ylidene
(44 mg, 0.14 mmol) in 5 mL of THF, compound 10b (88 mg, 94%
1
yield) was obtained after 5 min at room temperature. H NMR (700
MHz, CDCl3) δ 7.04−6.93 (m, 6H, CHarom), 6.85−6.82 (m, 6H,
CHarom), 6.76−6.72 (m, 4H, CHarom), 4.32−4.28 (m, 4H, NCH2),
3.97−3.90 (m, 4H, NCH2), 2.62−2.54 (m, 2H, OCH2CH3), 2.46 (s,
10H, CH3), 2.36 (s, 10H, CH3), 2.18−2.20 (m, 16H, CH3), 2.00−1.94
(m, 2H, OCH2CH3), 0.78−0.76 (m, 6H, OCH2CH3). 13C{1H} NMR
(176 MHz, CDCl3) δ 226.0 (CO), 220.0 (CO), 197.6 (CCr), 170.4
(Carom), 164.1(Carom), 156.8 (Carom), 139.4 (Carom), 138.4 (Carom),
136.2 (Carom), 134.3 (Carom), 133.3 (Carom), 129.8 (Carom), 129.7
(CH), 129.5 (CH), 128.0 (CH), 125.6 (CH), 125.5 (CH), 84.9 (C3),
69.0 (CH2), 68.9 (CH2CH3), 50.0 (CH2CH3), 20.9 (CH3), 18.7
(CH3), 18.6 (CH3), 18.5 (CH3), 18.3 (CH3), 15.3 (CH2CH3), 15.2
(CH2CH3). IR (ATR): ν cm−1 3448, 2924, 2043, 1962, 1910, 1869,
1609, 1517, 1494, 1459, 1358, 1282, 1211, 1066, 1031, 666. ESI-
HRMS m/z: calcd for C74H71Cr2 N4O12 [M + H]+ 1312.3901; found
1312.3854.
Synthesis of 13b. Following the general procedure, from 2c (60
mg, 0.082 mmol) and trityl hexafluorophosphate (48 mg, 0.12 mmol)
in 6 mL of 1,2-dichloroethane 13b (60 mg, quantitative yield) was
1
obtained as a Z/E mixture of isomers (1:2). H NMR (700 MHz,
CDCl3) δ 7.75 (s, 2H), 7.70 (s), 7.68 (s), 7.30−7.28 (m), 7.17(dd, J1 =
8.3, J2 = 2.2 Hz), 7.00 (s), 6.96 (s), 6.92 (s,), 6.83−6.76 (m), 6.49 (d, J
= 8.3 Hz), (12 H, CHarom, CHC and NCHCHN), 5.12 (q, J = 7.0
Hz, 1.33H), 4.55 (d, J = 7.1 Hz, 0.66H), (2H, OCH2CH3), 2.36−2.33
(m), 2.30 (s), 2.26 (s), 2.24 (s), 2.11 (s), 1.96−1.93 (m), (18H, CH3),
1.53 (t, J = 7.1 Hz, 2H), 0.91 (t, J = 7.1 Hz, 1H), (3H, OCH2CH3).
13C{1H} NMR (176 MHz, CDCl3) δ 335.5 (CCr), 331.8 (CCr), 224.3
(CO), 222.4 (CO), 215.1 (CO), 214.8 (CO), 146.8 (CHC), 146.2
(CHC), 142.0 (Carom), 141.7 (Carom), 141.5 (Carom), 141.4 (Carom),
136.0 (Carom), 133.6 (Carom), 133.40 (Carom), 133.1 (Carom), 131.9
(CH), 131.5 (Carom), 131.4 (CH), 130.9 (CH), 130.7 (CH), 130.5
(Carom), 130.3 (Carom), 130.2 (CH), 130.0 (CH), 129.3 (CH), 129.0
(CH), 128.2 (CH), 127.9 (CH), 127.7 (CH), 127.0 (CH), 126.8
(CH), 126.3 (CH), 125.2 (Carom), 123.3 (Carom), 114.4 (CHC),
112.0 (CHC), 79.1 (CH2CH3), 77.3 (CH2CH3), 21.0 (CH3), 20.9
(CH3), 19.8 (CH3), 17.8 (CH3), 15.6 (CH2CH3), 13.5 (CH2CH3). IR
(ATR): ν cm−1: 3446, 2925, 2855, 2061, 1954. ESI-HRMS m/z: calcd
for C37H34BrCrN2O6 735.0989; found 735.1002.
Synthesis of 13c. Following the general procedure, from 2e (50 mg,
0.08 mmol) and trityl hexafluorophosphate (35 mg, 0.09 mmol) in 6
mL of 1,2-dichloroethane 13c (50 mg, quantitative yield) was obtained
as Z/E mixture of isomers (1:1). 1H NMR (700 MHz, CDCl3) δ 7.81
(s), 7.73 (s), 7.59 (d, J = 1.6 Hz), 7.56 (s), 7.33−7.29 (m), 7.19−7.13
(m), 7.08 (s), 7.06−7.03 (m), 6.98−6.96 (m), 6.91 (s), 6.84 (s), 6.77
(s), 6.68−6.64 (m), (11 H, CHarom, CHC and NCHCHN), 4.84
(q, J = 7.1 Hz, 1H), 4.28 (q, J = 7.1 Hz, 1H), (2H, OCH2CH3), 2.38
(s), 2.30 (s), 2.28−2.25 (m), 2.16−2.12 (m), 2.09 (s), (18H, CH3),
1.53 (t, J = 7.1 Hz, 1.5H), 0.80 (t, J = 7.1 Hz, 1.5H), (3H, OCH2CH3).
13C{1H} NMR (176 MHz, CDCl3) δ 307.81 (CW), 304.0 (CW),
202.9 (CO), 202.1 (CO), 196.1 (CO), 195.7 (CO), 152.3 (CHC),
151.4 (CHC), 141.7 (Carom), 141.1 (Carom), 134.0 (Carom), 133.6
Synthesis of Compound 12. The salt 1116d (125 mg, 0.22 mmol)
and KH (229 mg, 5.72 mmol) were placed in a Schlenk tube, and the
suspension was stirred in THF (1 mL). Slow gas evolution was
observed. After 4 h, effervescence had stopped, and Et2O (2 mL) was
added. The THF/Et2O solution of the generated bis-NHC carbene
was filtered and added dropwise via cannula to a 50 mL flask
containing the carbene complex 1a (76.2 mg, 0.22 mmol) dissolved in
Et2O (5 mL). The precipitate obtained was washed with dry pentane,
1
to yield 118 mg (96%) of 12 as brown solid. H NMR (700 MHz,
CDCl3) δ 7.23−7.19 (m, 4H), 7.13−7.06 (m, 6H), 7.02−6.95 (m,
2H), 6.89−6.81 (m, 4H), 6.76−6.72 (m, 2H), (CHarom and NCH
CHN), 3.75−3.60 (m, 4H, NCH2), 2.52−2.43 (m, 2H, CH2), 2.36−
2.33 (m, 2H, CH2), 2.23 (s, 6H, CH3), 2.19−2.16 (m, 2H, CH2),
2.08−2.02 (m, 6H, CH3), 1.99−1.95 (m, 6H, CH3), 1.06−0.96 (m,
6H, OCH2CH3). 13C{1H} NMR (176 MHz, CDCl3) δ 225.6 (CO),
224.7 (CO), 220.5 (CO), 211.4 (CCr), 202.4 (CCr), 184.2 (CCr),
175.9 (Carom), 175.7 (Carom), 146.5(Carom), 146.3 (Carom), 145.0
(Carom), 141.5 (Carom), 141.1 (Carom), 139.9 (Carom), 138.4 (Carom),
137.0 (Carom), 135.2 (Carom), 134.4 (Carom), 133.3 (Carom), 132.6
(Carom), 129.3 (CH), 129.2 (CH), 129.1 (CH), 128.7 (CH), 127.7
(CH), 126.8 (CH), 126.5 (CH), 125.1 (CH), 124.6 (CH), 121.8
(CH), 121.4 (CH), 107.0 (C3), 106.8 (C3), 64.5 (CH2CH3), 46.8
(NCH2), 45.50 (NCH2), 29.7 (CH2), 29.3 (CH2), 21.1 (CH3), 18.4
J
DOI: 10.1021/acs.inorgchem.5b00492
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(Carom), 133.4 (Carom), 133.0 (C), 131.0 (CH), 130.7 (CH), 130.6
(Carom), 130.5 (CH), 130.4 (Carom), 130.3 (Carom), 130.2 (CH), 130.1
(CH), 130.0 (CH), 129.0 (CH), 128.8 (CH), 128.2 (CH), 128.1
(CH), 128.1 (CH), 127.8, 127.7 (CH), 127.5 (CH), 127.0 (CH),
126.9 (Carom), 126.8 (CH), 126.7 (CH), 125.1 (CH), 117.9 (CHC),
116.3 (CHC), 81.7 (CH2CH3), 79.3 (CH2CH3), 21.2 (CH3), 21.1
(CH3), 21.0 (CH3), 20.9 (CH3), 19.9 (CH3), 19.8 (CH3), 17.9 (CH3),
17.2 (CH3), 15.3 (CH2CH3), 13.4 (CH2CH3). IR (ATR): ν cm−1:
3445, 2924, 2071, 1946, 1929, 1635. ESI-HRMS m/z: calcd for
C37H35N2O6W 787.2005; found 787.1997.
Pratt, R. C.; Lohmeijer, B. G. G.; Hedrick, J. L. Chem. Rev. 2007, 107,
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Crsytallographic Data for Z-13a·CHCl3. C38H36Cl3CrF6N2O6, M
= 920.01, triclinic, a = 11.9124(3), b = 16.7703(4), c = 22.7908(10) Å,
α = 76.840(3), β = 80.306(3), γ = 71.624(2)°, V = 4184.3(2) Å3, space
group P1, Z = 4, T = 120(2) K, λ = 0.710 73 Å, Dcalcd =1.460 g cm−3,
̅
μ = 0.576 cm−1, 6162 reflections measured, 24 185 unique (Rint
=
32, 1537. (l) Riener, K.; Haslinger, S.; Raba, A.; Hogerl, M. P.; Cokoja,
̈
0.0788), crystal structure was solved by direct methods, and all non-
hydrogen atoms refined anisotropically on F2 using the programs
SHELXT and SHELXL-2014,28 one of the solvent molecules is
disordered over two positions, both solvent molecules and anions were
refined with appropriate similarity restraints (SAME) and with U value
components restrained to be equal (RIGU, SIMU, ISOR), methyl
group hydrogen atoms were included using a rigid model and other
hydrogen atoms as riding, R (Fo, I > 2σ(I)) = 0.0979, Rw (Fo2, all
data) = 0.2562.
M.; Herrmann, W. A.; Kuhn, F- E. Chem. Rev. 2014, 114, 5215.
̈
(5) Although they are considered to behave like tertiary phosphines
in many respects, NHCs exhibit stronger σ-donor properties, and this
is translated as ligands into tighter binding, greater thermal stability,
and increased basicity, see: (a) Herrmann, W. A.; Reisinger, C.;
Spiegler, M. J. Organomet. Chem. 1998, 557, 93. (b) Trnka, T. M.;
Grubbs, R. H. Acc. Chem. Res. 2001, 34, 18. (c) Nemcsok, D.;
Wichmann, K.; Frenking, G. Organometallics 2004, 23, 3640.
́
(6) Santiago, A.; Gomez-Gallego, M.; Ramírez de Arellano, C.; Sierra,
M. A. Chem. Commun. 2013, 49, 1112.
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ASSOCIATED CONTENT
■
Furstner, A. Chem.Eur. J. 2005, 11, 1833. (b) Lappert, M. F. J.
̈
S
* Supporting Information
Organomet. Chem. 2005, 690, 5467. (c) Lin, I. J. B.; Vasam, C. S.
Coord. Chem. Rev. 2007, 251, 642. (d) Colacino, E.; Martinez, J.;
Details for the synthesis of compounds 1d, 1e, and 1i,
computational details, UV−vis of compounds 9 and 10, a CIF
file containing X-ray crystallographic data for Z-13a·CHCl3,
and proton and carbon NMR spectra for all synthesized
compounds. The Supporting Information is available free of
charge on the ACS Publications website at DOI: 10.1021/
acs.inorgchem.5b00492.
́
Lamaty, F. Coord. Chem. Rev. 2007, 251, 726. (e) Díez-Gonzalez, S.;
Marion, N.; Nolan, S. P. Chem. Rev. 2009, 109, 3612. (f) Gaillard, S.;
Cazin, C. S. J.; Nolan, S. P. Acc. Chem. Res. 2012, 45, 778. (g) Gibard,
C.; Ibrahim, H.; Gautier, A.; Cisnetti, F. Organometallics 2013, 32,
4279 and references therein.
(8) Lee, M.-T.; Hu, C.-H. Organometallics 2004, 23, 976.
(9) (a) Kim, S.; Choi, S. Y.; Lee, Y. T.; Park, K. H.; Sitzmann, H.;
Chung, Y. K. J. Organomet. Chem. 2007, 692, 5390. (b) Barluenga, J.;
AUTHOR INFORMATION
■
Vicente, R.; Lop
5642.
́ ́
ez, L. A.; Tomas, M. J. Organomet. Chem. 2006, 691,
Corresponding Authors
*E-mail: margg@ucm.es. (M.G.G.)
*E-mail: sierraor@ucm.es. (M.A.S.)
Notes
(10) For the preparation of group 6 mixed at the metal Fischer-NHC
carbenes see: (a) Hitchcock, P. B.; Lappert, M. F.; Pye, P. L. J. Chem.
Soc., Dalton Trans. 1977, 2160. (b) Lappert, M. F.; Pye, P. L.;
McLaughlin, G. M. J. Chem. Soc., Dalton Trans. 1977, 1272.
(c) Lappert, M. F.; Pye, P. L. J. Chem. Soc., Dalton Trans. 1977, 1283.
(11) (a) Doherty, S.; Corrigan, J. F.; Carty, A. J.; Sappa, E. Adv.
Organomet. Chem. 1995, 37, 39. (b) Wojcicki, A. Inorg. Chem.
Commun. 2002, 5, 82 and references therein.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Support for this work under grants (CTQ2013-46459-C2-01-P,
CTQ2014-51912-REDC Programa Redes Consolider) from
the MINECO (Spain) is gratefully acknowledged. M.K.W. and
R.C.P. thank the Univ. of Bath for support.
(12) (a) Aumann, R.; Jasper, B.; Lage, M.; Krebs, B. Chem. Ber. 1994,
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127, 2475. (b) Aumann, R.; Nienaber, H. Adv. Organomet. Chem.
1997, 4, 163. (c) Aumann, R.; Althaus, S.; Kruger, C.; Betz, P. Chem.
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Ber. 1989, 122, 357.
(13) (a) For a recent example of a gold(III) σ-metal allenyl see:
Johnson, A.; Laguna, A.; Gimeno, M. C. J. Am. Chem. Soc. 2014, 136,
12812. (b) For a reference about structurally related borata-allenes
see: Bertermann, R.; Braunschweig, H.; Brown, C. K. L.; Damme, A.;
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