N-heterocyclic carbenes with a N-2,4-dinitrophenyl substituent: Comparison with PPh3 and IPr

Article abstract of DOI:10.1021/om300814f

Synthesis and characterization of N-heterocyclic carbenes (NHCs) bearing a 2,4-dinitrophenyl (DNP) substituent on an NHC framework were performed. The treatment of 1-(2,4-dinitrophenyl)-1H-imidazole (1) with 1-bromo-2,4- dinitrobenzene or methyl triflate afforded imidazolium salts 2a?HBr or 2b?HOTf, respectively, which were corresponding precursors of NHC ligands 1,3-bis(2,4-dinitrophenyl)-1H-imidazol-2-ylidene (2a) and 1-(2,4-dinitrophenyl)- 3-methyl-1H-imidazol-2-ylidene (2b). Rh and Au complexes-RhCl(2a)(cod) [3a (cod = 1,5-cyclooctadiene)], RhCl(2b)(cod) (3b), RhCl(2a)(CO)₂ (4a), RhCl(2b)(CO)₂ (4b), and AuCl(2a) (5a)-were synthesized using 2a?HBr and 2b?HOTf. IR, NMR, and crystallographic analysis of the Rh complexes demonstrated that the DNP substituent remarkably decreased the σ-donating ability of the carbenic carbon and increased the π-accepting ability. In addition, IR spectroscopy of Rh dicarbonyl complexes revealed that the average CO stretching frequency of 2a was equal to that of PPh₃. Au complex 5a exhibited a significantly higher catalytic activity than AuCl(PPh₃) 5d in the Au(I)-catalyzed hydroalkoxylation of cyclohexene. The computational analysis of the Au(I) complexes supported the experimental data, and the results suggested that the Au-C_carbene π-back-bonding interaction energy of 5a was 17-20% larger than that of 5d.

Full text of DOI:10.1021/om300814f

Article
pubs.acs.org/Organometallics
N‑Heterocyclic Carbenes with a N‑2,4-Dinitrophenyl Substituent:
Comparison with PPh3 and IPr
Tetsuo Sato,*,†,‡ Yoichi Hirose,† Daisuke Yoshioka,† and Shuichi Oi*,†,‡
†Department of Applied Chemistry, Graduate School of Engineering, Tohoku University, 6-6-11 Aramaki-Aoba, Aoba-ku, Sendai
980-8579, Japan
‡Environment Conservation Research Institute, Tohoku University, 6-6-11 Aramaki-Aoba, Aoba-ku, Sendai 980-8579, Japan
S
* Supporting Information
ABSTRACT: Synthesis and characterization of N-heterocyclic
carbenes (NHCs) bearing a 2,4-dinitrophenyl (DNP) substituent
on an NHC framework were performed. The treatment of 1-(2,4-
dinitrophenyl)-1H-imidazole (1) with 1-bromo-2,4-dinitroben-
zene or methyl triflate afforded imidazolium salts 2a·HBr or
2b·HOTf, respectively, which were corresponding precursors of
NHC ligands 1,3-bis(2,4-dinitrophenyl)-1H-imidazol-2-ylidene
(2a) and 1-(2,4-dinitrophenyl)-3-methyl-1H-imidazol-2-ylidene
(2b). Rh and Au complexesRhCl(2a)(cod) [3a (cod = 1,5-
cyclooctadiene)], RhCl(2b)(cod) (3b), RhCl(2a)(CO)2 (4a), RhCl(2b)(CO)2 (4b), and AuCl(2a) (5a)were synthesized
using 2a·HBr and 2b·HOTf. IR, NMR, and crystallographic analysis of the Rh complexes demonstrated that the DNP substituent
remarkably decreased the σ-donating ability of the carbenic carbon and increased the π-accepting ability. In addition, IR
spectroscopy of Rh dicarbonyl complexes revealed that the average CO stretching frequency of 2a was equal to that of PPh3. Au
complex 5a exhibited a significantly higher catalytic activity than AuCl(PPh3) 5d in the Au(I)-catalyzed hydroalkoxylation of
cyclohexene. The computational analysis of the Au(I) complexes supported the experimental data, and the results suggested that
the Au−Ccarbene π-back-bonding interaction energy of 5a was 17−20% larger than that of 5d.
INTRODUCTION
■
In a complex, a ligand is an important component that can
control the steric environment around the metal and the
electronic state of the metal center. It is known that N-
heterocyclic carbenes (NHCs), such as 1,3-bis(2,6-diisopropyl-
phenyl)-1H-imidazol-2-ylidene (IPr), commonly have strong σ-
donating properties, and because of this characteristic, NHCs,
which are comparable to phosphines, have mainly been
employed as ligands in transition-metal complex catalysts.
Opportunities to study the chemistry of NHCs were
Figure 1. Examples of N-electron-withdrawing-group-substituted
NHCs.
1
2
̈
activity of the entire complex, including the NHC ligand.
However, the DNP-substituted NHC and its complex have not
been reported, although the azolium salts that could be the
precursors of the NHC have been synthesized.7
significantly enhanced by Wanzlick's and Ofele’s pioneering
studies and their isolation by Arduengo.3 Most investigations
have focused on the σ-donating property; very little attention
has been paid to the π-accepting property. Recently, however,
the synthesis and characterization of electron-deficient NHCs
have been reported.4 The π-accepting properties were
improved by modifying backbone structures and N-substitu-
ents. With respect to the latter, the introduced electron-
withdrawing groups were limited to a few types, such as
nitrated phenyl (Figure 1, A)5 and fluorinated phenyl (B).4m,6
It seems that the 2,4-dinitrophenyl (DNP) group, which is
characterized by strong electron-withdrawing properties, is
suitable as an N-substituent for electron-deficient NHC because
it can lead to a significant decrease of the π-overlap between the
vacant p-orbital of a carbene carbon atom and a lone pair of
contiguous nitrogen atoms (C). Moreover, it is thought that the
steric bulk of the DNP substituent will affect the catalytic
Generally, the electronic properties of NHCs have been
commonly compared with those of phosphines, using Tolman
electronic parameter (TEP) values,8 which were converted
from an average of IR CO stretching frequencies (νav(CO)) of
the corresponding Rh or IrCl(NHC)(CO)2 complexes together
with the 13C NMR spectroscopic chemical shift values of the
carbenic carbons (Ccarbene)9 and the crystallographic Ccarbene
metal bond lengths in the Rh or Ir complexes. Crabtree showed
that the electron-donating abilities of NHCs were comparable
on an identical scale with those of phosphines.10 Nolan and
−
Received: August 22, 2012
Published: September 18, 2012
© 2012 American Chemical Society
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Plenio expanded this understanding, and the comparison of the
TEP value has been established as a standard method for
evaluating the electron-donating abilities of NHCs and
phosphines.11
Scheme 2. Synthesis of 2a-Coordinated Rh Complexes 3a
and 4a
In addition to instrumental analysis, computational studies
have also been performed to evaluate the electronic properties
of NHCs and phosphines. The theoretical estimation of NHC
π-back-bonding interactions based on energy decomposition
analysis,12 the extended transition state methods,13 and related
methods such as the constrained space orbital variation,14
natural orbitals for chemical valence,15 and natural bond orbital
(NBO) analysis16 have revealed that a significant degree of π-
back-bonding contributes to the overall bond energy.17
Recently, Harvey calculated the π-back-bonding interaction
energies of 4.4−31.5 kcal/mol and 3.1−16.9 kcal/mol for
complexes containing NHC and PPh3, respectively, by using an
NBO second-order perturbative energy analysis.18
Scheme 3. Synthesis of 2b-Coordinated Rh Complexes 3b
and 4b
Here we report the electronic and steric properties of
electron-deficient-type NHCs bearing a DNP substituent. Their
Rh and Au complexes were analyzed by experimental and
computational methods. Furthermore, their characteristics were
compared with those of PPh3 and IPr.
converted into Rh dicarbonyl complexes 4a and 4b in
quantitative yield by bubbling CO through CH2Cl2 solutions
for 20 min. The characterization of complexes 4a and 4b was
carried out in a CH2Cl2 or CDCl3 solution. Because of
decarbonylation, those complexes were not sufficiently stable to
grow single crystals suitable for X-ray diffraction.
IR Analysis of Complexes 4a−d. To estimate the donor
properties of the NHC ligands 2a and 2b and compare them
with those of existing NHC ligands IPr and PPh3, the IR
spectra of Rh dicarbonyl complexes 4a−d were recorded in
CH2Cl2. The IR spectra of IPr and PPh3 complexes 4c and 4d
were remeasured in CH2Cl2. The resulting symmetric and
antisymmetric CO stretching frequencies ν(CO) for complexes
4a−d are summarized in Table 1. Significant increases of the
RESULTS AND DISCUSSION
■
2,4-Dinitrophenylation of 1-(2,4-dinitrophenyl)-1H-imidazole
(1) with 1-bromo-2,4-dinitrobenzene in CH3CN or methyl-
ation with excess methyl triflate in CH2Cl2 afforded the
imidazolium salt 2a·HBr in 68% yield or 2b·HOTf in 90% yield,
respectively (Scheme 1).
Scheme 1. Preparation of NHC Precursors 2a·HBr and
2b·HOTf
Table 1. IR ν(CO) Stretching Frequencies of RhCl(L)(CO)2
a
4a−d
complex
L
ν(CO) (cm−1
)
νav(CO) (cm−1
)
4a
4b
4c
4d
2a
2091.4, 2014.3
2086.6, 2007.5
2078.9, 1996.0
2093.4, 2012.4
2052.9
2047.1
2037.5
2052.9
2b
IPr
An attempt to prepare of 2a·HBr by the condensation of the
DNP-substituted diazabutadiene and formaldehyde or its
equivalent was not successful. Although free NHC 2a (1,3-
bis(2,4-dinitrophenyl)-1H-imidazol-2-ylidene, IDNP) and 2b
were not isolated (Figure 2), Rh complexes containing 2a or 2b
PPh3
a
IR spectra were recorded in CH2Cl2.
νav(CO) values were observed for complexes 4b (2047.1 cm−1)
and 4c (2037.5 cm−1), which indicate that the DNP group on
an NHC framework decreases the electron density on the Rh
center as expected. In addition, the metal center in
disubstituted NHC complex 4a, with a νav(CO) value of
2052.9 cm−1, is more electron deficient than that in the
monosubstituted NHC complex 4b. The difference between
the νav(CO) values of 4a and 4c increases to over 15 cm−1.
Representative electron-deficient NHCs D−G are shown in
Figure 3.4b,c,j,l,q The νav(CO) value of the complex 4a is located
in the wavenumber region between those of the Rh complexes
coordinated with the NHCs E and F. In fact, a comparison of
the νav(CO) values of 4a and 4d, which was coordinated with
PPh3, showed complete agreement; that is, the σ-donating
ability of NHC 2a is equal to that of PPh3. This result suggests
that both 2a in complex 4a and PPh3 in complex 4d have
similar electron-donating abilities. Other methods of comparing
electronic properties of 2a with PPh3 will be described below.
NMR Analysis of Complexes 3a−c and 4a−c. NMR
spectroscopy has also been used to evaluate the electronic
Figure 2. Structures of NHC 2a (IDNP) and 2b.
were prepared by the reaction of in situ generated free carbenes
with [RhCl(cod)]2 using a base. The Rh complex RhCl(2a)-
(cod) (3a) was obtained in 75% yield via a ligand exchange of
Br to BF4 for 2a·HBr and subsequent reaction with
[RhCl(cod)]2 in the presence of K2CO3 as the base (Scheme
2).
A reaction of 2a·HBr with [RhCl(cod)]2 without the anion
exchange gave Rh−Br complex RhBr(2a)(cod). Rh complex 3b
coordinated with 2b was also produced in the reaction of
2b·HOTf with [RhCl(cod)]2 in 29% yield (Scheme 3). Rh-1,5-
cyclooctadiene complexes 3a and 3b were respectively
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Figure 3. Examples of electron-donating abilities of NHCs. The
νav(CO) values of RhCl(NHC)(CO)2 are shown in parentheses. IR
spectra were recorded in CHCl3 for D, in CH2Cl2 for E and G, and in
CDCl3 for F.
Figure 4. ORTEP representation of 3a with thermal ellipsoids at 50%
probability. Hydrogen atoms have been omitted for clarity. Selected
distances (Å) and angles (deg): Rh1−C1 1.9993(18), Rh1−C16
2.1100(18), Rh1−C17 2.1273(19), Rh1−C20 2.2211(19), Rh1−C21
2.2309(17), Rh1−Cl1 2.4028(5), C1−N1 1.363(2), C1−N2 1.358(2),
N1−C1−N2 103.03(14), C1−Rh1−Cl1 89.09(5), Cl1−Rh1−C1−N1
96.33(15), C1−N1−C4−C5 131.46(19), O1−N3−C5−C4 −42.6(2),
O3−N4−C7−C6 13.1(2), C1−N2−C10−C11 −116.04(19), O5−
N5−C11−C10 22.9(2), O7−N6−C13−C12 5.9(2).
properties of ligands in transition-metal complexes. In alkene
transition-metal complexes, the alkene coordinated with the
electron-deficient metal center approaches the free alkene and
acts as a conventional two-electron donor ligand; hence, the
13C NMR chemical shifts of the CC double bonds in alkenes
are observed more downfield toward free alkenes. In addition,
the 13C NMR chemical shifts of the carbenic carbons in the
NHC ligands tend to be observed more downfield since the π-
accepting abilities of the NHC ligands increase and the σ-
donating abilities decrease.4c,9 The 13C NMR chemical shifts of
the alkene moiety of 1,5-cyclooctadiene trans to the NHC
ligand and the carbenic carbons in RhCl(NHC)(cod) 3a−c are
summarized in Table 2. With respect to each of the chemical
shifts of alkenes and carbenic carbons, downfield shifts were
observed in the order of 3c, 3b, and 3a. These results indicate
that the DNP group on the nitrogen atom in the NHC ligands
significantly increases the π-accepting abilities, and the electron
density on the metal center for complexes 3a−c decreases. In
RhCl(NHC)(CO)2 4a−c, although the chemical shifts of the
carbenic carbons do not correlate with the π-accepting abilities
of the NHC ligands, upfield shifts for both carbons of the
carbonyls cis and trans to the NHC ligand were observed as the
π-accepting abilities of NHC ligands increased.
X-ray Diffraction Analysis of Complexes 3a and 3b.
Single crystals suitable for X-ray diffraction analysis were
obtained by the slow diffusion of hexane in an acetone solution
of 3a and by the slow diffusion of diethyl ether in a CH2Cl2
solution of 3b. ORTEP representations of 3a and 3b are shown
in Figures 4 and 5, respectively. The structural studies revealed
that both complexes had pseudo-square-planar metal centers
coordinated with the NHC ligand 2a or 2b, a chloride anion,
and 1,5-cyclooctadiene and that the dihedral angles between
the NHC ligand framework and the metal−ligand plane were
approximately orthogonal (96.33(15)° for 3a and 94.40(16)°
for 3b). In both 3a and 3b complexes, the NHC frameworks
could barely conjugate with the DNP groups in the solid-state
structures because they were twisted in such a way that the
respective 2-nitro groups on the DNP groups were oriented
Figure 5. ORTEP representation of 3b with thermal ellipsoids at 50%
probability. Hydrogen atoms have been omitted for clarity. Selected
distances (Å) and angles (deg): Rh1−C1 2.017(2), Rh1−C11
2.113(2), Rh1−C12 2.100(2), Rh1−C15 2.217(2), Rh1−C16
2.197(2), Rh1−Cl1 2.3922(6), C1−N1 1.373(2), C1−N2 1.348(2),
N1−C1−N2 103.44(16), C1−Rh1−Cl1 89.84(5), Cl1−Rh1−C1−N1
94.40(16), C1−N1−C4−C5 136.78(19), O1−N3−C5−C4 −37.8(3),
O3−N4−C7−C6 −12.0(3).
away from the chloride ligand; the Ccarbene−N−CAr‑1−CAr‑2
torsion angles were 131.46(19)° and 116.01(19)° in 3a and
136.78(19)° in 3b. These facts suggest that the DNP group acts
as an electron-withdrawing group by approximating the
inductive effect. With regard to the π-back-donation of the
Table 2. Selected 13C NMR Data for RhCl(NHC)(cod) 3a−c and RhCl(NHC)(CO)2 4a−c
complex
trans-CHCOD (ppm)
CN2 (ppm)
CO (ppm)
CO (ppm)
a
RhCl(2a)(cod) 3a
98.9 (1JRhC = 6.0 Hz)
97.8 (1JRhC = 6.9 Hz)
97.4 (1JRhC = 6.9 Hz)
93.9 (1JRhC = 7.3 Hz)
187.5 (1JRhC = 51.5 Hz)
184.1 (1JRhC = 52.5 Hz)
a
RhCl(2b)(cod) 3b
a
RhCl(IPr)(cod) 3c
RhCl(2a)(CO)2 4a
183.6 (1JRhC = 51.5 Hz)
181.2 (1JRhC = 44.3 Hz)
177.9 (1JRhC = 44.0 Hz)
180.2 (1JRhC = 45.1 Hz)
b
183.8 (1JRhC = 55.6 Hz)
184.7 (1JRhC = 54.7 Hz)
184.9 (1JRhC = 53.8 Hz)
180.4 (1JRhC = 73.9 Hz)
181.4 (1JRhC = 70.4 Hz)
182.7(1JRhC = 73.9 Hz)
b
b
RhCl(2b)(CO)2 4b
RhCl(IPr)(CO)2 4c
a
b
NMR spectra were recorded in DMSO-d6. NMR spectra were recorded in CDCl3.
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NHC ligand, Bielawski has shown that, in the RhCl(NHC)-
(cod) complex, the Ccarbene−Rh bond length contracts, and the
average Ccarbene−N bond length and the length of the bond
between the Rh atom and the alkene moiety of 1,5-
Scheme 4. Synthesis of Au(I) Complex 5a
cyclooctadiene trans to the NHC ligand (Rh−Ctrans‑COD
)
elongate as the π-back-bonding ability of the NHC ligand
increases.4c To elucidate the degree of π-back-bonding abilities
of the NHC ligands 2a and 2b, selected bond lengths in
complexes 3a and 3b were compared. Although the average
Single crystals suitable for X-ray diffraction analysis were
obtained by the slow diffusion of hexane in an acetone solution
of 5a. The ORTEP representation of 5a is shown in Figure 6.
Ccarbene−N bond lengths were the same (1.361 Å), the Ccarbene
−
Rh bond length in 3a (1.9993(18) Å) was shorter than that in
3b (2.017(2) Å), and the Rh−Ctrans‑COD bond lengths in 3a
(2.2211(19) and 2.2309(17) Å) were longer than those in 3b
(2.217(2) and 2.197(2) Å). These changes coincide with the
increasing π-back-bonding ability of 2a compared with 2b and
are compatible with the results of IR and NMR analyses. The
Ccarbene−Rh bond length in 3a was comparable to that in the Rh
complex coordinated with the NHC ligand F (1.998(10) Å).4p
The steric bulk of NHCs is a crucial factor for determining
the characteristics of NHCs. To assess the steric bulk of NHCs
2a and 2b, the percent buried volume (%Vbur) values that were
proposed by Nolan19 were calculated from the crystallographic
data of the corresponding complexes 3a and 3b using the
SambVca program developed by Cavallo20 (Table 3). From the
Figure 6. ORTEP representation of 5a with thermal ellipsoids at 50%
probability. Hydrogen atoms and solvent molecule have been omitted
for clarity. Selected distances (Å) and angles (deg): Au1−C1 1.975(4),
Au1−Cl1 2.3000(9), C1−N1 1.358(5), C1−N2 1.352(5), N1−C1−
N2 103.8(3), C1−Au1−Cl1 176.35(10), C1−N1−C4−C5 103.6(5),
C1−N2−C10−C11 −122.5(4), O1−N3−C5−C4 30.3(6), O3−N4−
C7−C6 7.3(6), O5−N5−C11−C10 30.5(6), O7−N6−C13−C12
11.3(6).
Table 3. Comparison of Percent Buried Volume (%Vbur
)
Values in RhCl(L)(cod) Complexes 3a−c and AuCl(L) 5a
a
and 5d
%Vbur for M−NHC length at
complex
L
2.00 Å
2.10 Å
crystallographic distance
3a
2a
33.2
29.4
34.0
31.1
34.8
31.6
28.0
32.2
29.4
33.0
33.2
29.1
33.0
31.5
30.7
The Ccarbene−Au length in 5a (1.975(4) Å) was shorter than
that in AuCl(IPr) (5c) (1.998(4) Å).23 On the other hand, the
Au−Cl length in 5a (2.3000(9) Å) was longer than those in 5c
(2.2718(11) Å) and AuCl(PPh3) (5d) (2.2903(4) Å).22 With
reference to a previously reported procedure, the reactions were
carried out as follows.24 In the presence of Au(I) complex 5 (5
mol %) and AgOTf (5 mol %) a mixture of 2-methoxyethanol
(7) (0.50 mmol) and cyclohexene (8) (5.0 mmol) in toluene
(0.5 mL) was heated at 120 °C for 24 h (Table 4). The yields
of ether 9 were determined by GLC using n-dodecane as an
internal standard. As shown in Table 4, the use of complex 5a
afforded ether 9 in the best yield (entry 1). The yield of 9 in the
reaction using the complex 5b was not equal to that using 5a
(entry 2). Interestingly, in spite of the result that the νav(CO)
3b
2b
3c21
5a
IPr
2a
5d22
PPh3
a
%Vbur values were calculated with the Bondi radii scaled by 1.17, a
sphere of radius 3.5 Å, and a mesh size of 0.05 Å. All hydrogen atoms
were omitted.
molecular structure, the %Vbur value of 2a was clearly higher
than that of 2b. This result suggests that the π-back-donating
capability of 2a is remarkably larger than that of 2b, given that
the Ccarbene−Rh bond lengths in 3a were shorter than those in
3b. On the other hand, although the %Vbur value of 2a was
slightly lower than that of IPr, when the calculations were
performed for Ccarbene−Rh bond distances of 2.00 and 2.10 Å,
the steric demands of 2a and IPr were approximately the same
in actual solid-state Rh complexes 3a and 3c.
Table 4. Au(I)-Catalyzed Intermolecular Hydroalkoxylation
a
of Cyclohexene
Investigation of Catalytic Activities of Au(I) Com-
plexes Coordinated with 2a, IPr, or PPh3. To this point,
the investigation has focused on the electronic property
differences among the ligands 2a, 2b, IPr, and PPh3. We
focused on the IR analysis result that showed that the νav(CO)
value of 2a was equal to that of PPh3, and the %Vbur value of 2a
was almost the same as that of IPr in the RhCl(cod) complexes.
Then, we attempted to reveal the differences among the
catalytic activities of complexes involving the electron-deficient
NHC 2a, the common NHC IPr, and PPh3 for the Au(I)-
catalyzed hydroalkoxylation of cyclohexene. The Au(I) complex
AuCl(2a) (5a) was synthesized by treating 2a·HBr with Ag2O
and subsequent transmetalation with AuCl(DMS) (DMS =
dimethyl sulfide) in 40% yield as an air-stable solid (Scheme 4).
b
entry
AuCl(L) 5
yield (%)
1
2
3
5a (L = 2a)
5c (L = IPr)
5d (L = PPh3)
78
70
29
a
Reaction conditions: 7 (0.50 mmol), 8 (5.0 mmol), AuCl(L) (5.0
mol %), AgOTf (5 mol %) in toluene (0.5 mL) at 120 °C for 24 h.
Yield was determined by GLC analysis. n-Dodecane was used as
b
internal standard.
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values of 2a and PPh3 were equal, complex 5c led to a low yield
of 9 (entry 3). It is presumed that there is a considerable
difference in the electronic properties of 2a and PPh3, even if
due consideration is given to the difference of %Vbur values of
2a and PPh3 in the AuCl complexes 5 calculated on the basis of
crystallographic data (31.5 for 5a versus 30.7 for 5d) (Table 3).
This suggests that an average of IR CO stretching frequencies
represents only one aspect of the electronic properties of
NHCs. A detailed examination of the hydroalkoxylation is now
in progress.
Computational Analysis. Computational analysis was
performed to complement the experimental results. The
calculated energy levels of σ-donor and π-acceptor orbitals of
NHCs are summarized in Table 5. The σ-donor orbital is an sp2
those of IPr and PPh3, which were employed as ligands in the
Au(I)-catalyzed hydroalkoxylation of cyclohexene. The calcu-
lations were performed using the catalyst precursor AuCl(L) 5
and the possible active species AuOTf(L) 6. The energy
contributions of π-back-bonding interactions (ΔEbb) were
calculated by the summation of all stabilization energies. The
donor orbitals were filled metal d orbitals, while the acceptor
orbital was the vacant p orbital on the carbene carbon atom in
an NHC, which were described as three-center four-electron
antibonding orbitals, or σ* P−C orbitals on the phosphorus
atom in PPh3. The results of the calculations are summarized in
Table 6. The π-back-bonding interaction value and the
Table 6. Summary of NBO Analysis of AuCl(L) 5 and
AuOTf(L) 6
Table 5. Energy Levels of σ-Donor and π-Acceptor Orbitals
and N−Ccarbene π-Overlap Interaction Energies for NHC
Ligands by NBO Analysis
a
b
c
d
e
complex
δAu
WAuL
BDE
ΔEbb
ΔEbb in BDE
5a (L = 2a)
5c (L = IPr)
5d (L = PPh3)
6a (L = 2a)
6c (L = IPr)
6d (L = PPh3)
0.281
0.294
0.214
0.463
0.401
0.389
0.661
0.603
0.658
0.695
0.668
0.703
67.99
78.89
60.70
84.08
95.39
74.01
15.22
13.63
12.59
14.76
13.80
11.72
22.4
17.3
20.7
17.6
14.5
15.8
a
a
b
ligand
σ-donor
π-acceptor
π-overlap
2a
−8.98 (HOMO−12)
−8.30 (HOMO−4)
−7.64 (HOMO−8)
−2.57 (LUMO+4)
−1.86 (LUMO+2)
−1.38 (LUMO)
92.61
94.18
94.91
2b
IPr
a
b
a
b
Natural atomic charge (δ) of the Au atom. Wiberg bond index (W)
between the Au atom and ligand (L). BDE in kcal/mol. π-Back-
bonding interaction energy (ΔEbb) in kcal/mol. Contribution from
Energy levels of σ-donor and π-acceptor orbitals in eV. Average of
c
d
two N−Ccarbene π-overlap interaction energies in kcal/mol.
e
ΔEbb in BDE in %.
carbene lone pair, while the π-acceptor orbital is a vacant p
orbital on a carbene carbon atom. The DNP substituent
lowered the energy levels of both σ-donor and π-acceptor
orbitals. As a result of this effect, 2a has the lowest energy levels
for both orbitals among the investigated ligands, which
indicates that 2a behaves as the weakest σ-donor and the
strongest π-acceptor. In comparison with an NHC bearing an
oxalamide backbone,25 the σ-donor orbital−π-acceptor orbital
gaps of 2a and IPr are almost the same (6.41 eV for 2a versus
6.26 eV for IPr), which suggests that π-delocalization has not
extended even to the DNP group, and accordingly, the DNP
group has almost acted as an electron-withdrawing group
through an inductive effect.
contribution from the π-back-bonding interaction energy in
the bond dissociation energy (BDE) for AuCl(IPr) 5c (13.63
kcal/mol, 17.3%) were similar to those for AuCl(NHC)
reported by Comas-Vives and Harvey18 (12.9 kcal/mol,
16.2%). For both complexes 5 and 6, the largest π-back-
bonding interaction value and contribution were obtained for
the 2a complex, as expected, and the PPh3 complex showed the
smallest π-back-bonding value and ca. 2% smaller contribution
than the 2a complex. Table 6 also includes the natural atomic
charge (δ) of a Au atom. In particular, in the gold(I) triflate
complexes 6, the Au atom in complex 6a coordinated with 2a
has the most positive charge, and there is a noticeable
difference between the charges on Au in complexes 6a and
6d involving PPh3. It seems that different Au atom positivities
have a considerable influence on the catalytic activities of Au(I)
complexes 6 as Lewis acid catalysts. As a result of the
calculation of the Wiberg bond indexes (W) between the Au
atom and ligand (L), it was observed that the trend of the
Wiberg bond index almost corresponded with that of the π-
back-bonding contribution. Although the average of IR CO
stretching frequencies of 2a and PPh3 were equal, computa-
tional analysis revealed that the π-back-bonding interaction
energies of 5a and 6a were 17−20% larger than those of 5d and
6d. A satisfactory correlation was admitted between the natural
atomic charges on the Au atom and the π-back-bonding
interaction energies. These calculation results are consistent
with those reported by Tokunaga24 that AuCl[P(C6F5)3]
exhibited higher catalytic activity than 5d for the hydro-
alkoxylation of alkenes and successfully supported our
experimental data.
In addition, to quantitatively evaluate the degree of π-overlap
interactions between the lone pair of the nitrogen atoms in the
NHC framework and the vacant p orbital on the carbene
carbon, which reflected the π-back-bonding capability of
carbenes in the complexes, we performed an NBO analysis
for 2a, 2b, and IPr.16 The NBO second-order perturbation
theory of the stabilization energies (ΔE(2)i→j) is represented by
the delocalization between a Lewis-type donor (i) and a non-
Lewis-type acceptor (j). In this interaction, the donor orbitals
are the lone pairs of N p orbitals and the acceptor orbitals are
unoccupied p orbitals of Ccarbene. The stabilization energy
ΔE(2) is estimated as follows:
i→j
2
ΔE(2) = −q F /(εj − εi)
i→j
i,j
i
where qi is the occupancy of donor NBO, εj − εi is the orbital
energy difference, and Fi,j is a Fock matrix element. On the basis
of the calculation results, the degree of π-overlap interactions
decreased according to an increase of the DNP substituent
(Table 5). This result suggests that the DNP substitution
enhances the π-back-bonding capability of carbenes.
The magnitude of the π-back-bonding interactions of 2a was
also estimated by NBO analysis in accordance with a method
reported by Comas-Vives and Harvey18 and compared with
CONCLUSION
■
The first examples of DNP-substituted NHCs 2a and 2b have
been synthesized and characterized. The spectroscopic and
structural analyses of Rh complexes coordinated with NHCs
6999
dx.doi.org/10.1021/om300814f | Organometallics 2012, 31, 6995−7003

Organometallics
Article
analysis were obtained by the slow cooling of a hot CH3OH solution
revealed that the DNP substituent acts as a strong electron-
withdrawing group by an approximate inductive effect and
remarkably decreases the σ-donating ability of the carbenic
carbon and increases the π-accepting ability. The N,N′-bis(2,4-
dinitrophenyl)-substituted NHC 2a provided the same steric
demand to IPr in RhCl(NHC) in the solid state. In addition,
we focused on the result that the average of IR CO stretching
frequencies of 2a was equal to that of PPh3. To characterize
various aspects of the electronic properties of 2a, the
hydroalkoxylation of cyclohexene was carried out using Au(I)
complex 5a or 5d coordinated with 2a or PPh3, respectively, as
the catalyst. Complex 5a exhibited high catalytic activity, and a
significant difference was observed between 5a and 5d.
Furthermore, to support the experimental data, the computa-
tional analysis of Au(I) complexes demonstrated that
complexes involving 2a had ca. 20% larger π-back-bonding
interaction energies than those including PPh3, although the
average of IR CO stretching frequencies of 2a and PPh3 were
equal. Because the effectiveness of the DNP substituent to
increase the π-back-bonding capability of NHCs was verified by
experimental and computational analysis, further syntheses of
electron-deficient-type DNP-substituted NHCs bearing other
frameworks and relationships to catalytic systems are now in
progress.
1
4
of 2a·HBr. H NMR (400.03 MHz, DMSO-d6): δ 10.33 (t, 2H, J =
1.5 Hz, NCHN), 9.09 (d, 2H, 4J = 2.5 Hz, CH3‑Ar), 8.97 (dd, 2H, 3J =
4
4
8.7 Hz, J = 2.5 Hz, CH5‑Ar), 8.48 (d, 2H, J = 1.5 Hz, NCHCHN),
8.44 (d, 2H, 3J = 8.7 Hz, CH6‑Ar). 13C{1H} NMR (100.60 MHz,
DMSO-d6): δ 184.8, 143.9, 139.8, 132.1, 132.0, 129.8, 124.6, 121.6. IR
(KBr): 3102.9, 1610.2, 1538.9, 1345.1, 1268.9, 1077.1, 911.2, 836.0,
741.5 cm−1. HRMS (FAB): [M − Br]+ calcd for C15H9N6O8 401.0482;
found 401.0487. Anal. Calcd (%) for C15H9BrN6O8: C 37.44, H 1.89,
N 17.47. Found: C 37.25, H 2.08, N 17.31. Mp: 246 °C (dec).
1-(2,4-Dinitrophenyl)-3-methylimidazolium triflate
(2b·HOTf). In a vial tube (5 mL), 1-(2,4-dinitrophenyl)-1H-imidazole
(1) (351 mg, 1.50 mmol) was dissolved in dry CH2Cl2 (3 mL). To the
solution was added methyl triflate (0.40 mL, 3.5 mmol), and the
reaction mixture was stirred at room temperature for 16 h. After the
reaction, the solvent was removed, and the residue was dissolved in
acetone (0.3 mL). To the solution was added diethyl ether (2 mL),
and the resulting precipitate was collected and washed with diethyl
ether (2 × 5 mL). The solid was dried under vacuum to give 2b·HOTf
as a pale yellow powder (538 mg, 1.35 mmol, 90%). Single crystals
suitable for X-ray diffraction analysis were obtained by the slow
diffusion of hexane in an acetone solution of 2b·HOTf. 1H NMR
(400.03 MHz, acetone-d6): δ 9.60 (br, 1H, NCHN), 9.13 (d, 1H, 4J =
3
4
2.7 Hz, CH3‑Ar), 8.89 (dd, 1H, J = 8.7 Hz, J = 2.7 Hz, CH5‑Ar), 8.38
(d, 1H, 3J = 8.7 Hz, CH6‑Ar), 8.15 (t, 1H, 3J = 1.8 Hz,
ArNCHCHNCH3), 8.05 (t, 1H, 3J = 1.8 Hz, ArNCHCHNCH3),
4.26 (s, 3H, CH3);. 13C{1H} NMR (100.60 MHz, acetone-d6): δ
150.1, 145.4, 139.4, 133.7, 133.2, 130.4, 125.4, 125.3, 122.6, 122.1 (q,
1JCF = 320.9 Hz, CF3SO3), 37.4. IR (KBr): 3119.3, 2883.1, 2710.5,
EXPERIMENTAL SECTION
General Information. All reactions were carried out under a N2
atmosphere using a standard vacuum line, a glovebox, Schlenk tube
techniques, and dried and deoxygenated solvents. H (400.03 MHz)
■
1618.0, 1588.1, 1538.0, 1346.1, 1280.5, 1218.8, 1151.3, 1087.7, 1033.7,
834.1, 749.2 cm−1. HRMS (FAB): [M − CF3SO3]+ calcd for
C10H9N4O4 249.0624; found 249.0618. Anal. Calcd (%) for
C11H9F3N4O7S: C 33.17, H 2.28, N 14.07. Found: C 33.07, H 2.44,
N 13.91. Mp: 116−117 °C.
1
and 13C (100.60 MHz) nuclear magnetic resonance (NMR) spectra
were recorded on a Bruker BioSpin AV400 NMR spectrometer. The
1H NMR in ppm (δ) is relative to the chemical shift of
tetramethylsilane at 0.00 ppm, multiplicities (s = singlet, d = doublet,
t = triplet, q = quartet, quint = quintet, m = multiplet, and br = broad),
coupling constants (Hz), and integration. The 13C{1H} NMR spectra
reported in ppm (δ) are relative to the chemical shifts of solvent
residual signals for CDCl3 at 77.00 ppm, acetone-d6 at 29.84 ppm
(CD3), and DMSO-d6 at 39.52 ppm. High-resolution mass spectra
were obtained on a JEOL JMS-700 spectrometer at the Department of
Instrumental Analysis, of the Technical Division, School of Engineer-
ing, Tohoku University. IR spectra were recorded on a JASCO FT/IR-
350 Fourier-transform infrared spectrophotometer; absorptions are
reported in cm−1. X-ray analysis was carried out using a Bruker AXS
APEX II CCD diffractometer. The GC yield determined by Shimadzu
GC-2010 with a GL Sciences InertCap 5MS/Sil column (L: 30 m ×
i.d.: 0.25 mm, DF = 0.25 μm) equipped with a flame ionization
detector using n-dodecane as an internal standard.
Chloro(η4-cycloocta-1,5-diene)[1,3-bis(2,4-dinitrophenyl)-
1H-imidazol-2-ylidene]rhodium(I) (3a). In a Schlenk tube (10
mL), 2a·HBr (192 mg, 0.400 mmol) and (CH3)3O·BF4 (60.7 mg,
0.410 mmol) were suspended in dry CH2Cl2 (3 mL). The reaction
mixture was stirred at room temperature for 3 h, and then the solvent
was removed under reduced pressure. The Schlenk tube was wrapped
in aluminum foil, and K2CO3 (553 mg, 4.00 mmol), [RhCl(cod)]2
(98.4 mg, 0.200 mmol), and dry toluene (5 mL) were added to the
residue. The mixture was stirred at room temperature for 3 days in the
dark. After the reaction, the resulting salt and excess K2CO3 were
filtered off, and the filtrate was concentrated under reduced pressure.
The resulting solid was purified by recrystallization from CH2Cl2/
diethyl ether to give 3a as a reddish-brown crystal (179 mg, 0.301
mmol, 75%). Single crystals suitable for X-ray diffraction analysis were
obtained by the slow diffusion of hexane in an acetone solution of 3a.
1H NMR (400.03 MHz, DMSO-d6): δ 9.08−9.00 (m, 4H, CH3‑Ar
,
3
4
CH6‑Ar), 8.92 (dd, 2H, J = 8.7 Hz, J = 2.6 Hz, CH5‑Ar), 7.89 (s, 2H,
NCHCHN), 4.83−4.66 (m, 2H, CHCOD), 3.02−2.93 (m, 2H,
CHCOD), 1.89−1.70 (m, 2H, CH2 COD), 1.70−1.46 (m, 6H, CH2 COD).
13C{1H} NMR (100.60 MHz, DMSO-d6): δ 187.5 (d, 1JRhC = 51.5 Hz,
Materials. CH3CN (dehydrated), CH3OH (dehydrated), CH2Cl2
(dehydrated) (Kanto Chemical), diethyl ether (super-dehydrated),
acetone (super-dehydrated), hexane (super-dehydrated), toluene
(super-dehydrated), 1-bromo-2,4-dinitrobenzene (Wako Pure Chem-
ical), CH3OTf, (CH3)3O·BF4 (TCI), Ag2O, n-dodecane (Nacalai
Tesque), AgOTf (Aldrich), and CDCl3 (Acros) were purchased and
used as received. Anhydrous K2CO3 (Kanto Chemical) was dried prior
to use using a heat gun under vacuum. Cyclohexene and 2-
methoxyethanol (TCI) were purified by atmospheric distillation
prior to use. 1-(2,4-Dinitrophenyl)-1H-imidazole (1),26 [RhCl-
(cod)]2,27 AuCl(DMS),28 AuCl(IPr) (5c),29 and AuCl(PPh3) (5d)30
were prepared as described in the literature.
1,3-Bis(2,4-dinitrophenyl)imidazolium Bromide (2a·HBr). In
a sealed vial tube (5 mL), a mixture of 1-(2,4-dinitrophenyl)-1H-
imidazole (1) (937 mg, 4.00 mmol) and 1-bromo-2,4-dinitrobenzene
(988 mg, 4.00 mmol) was dissolved in dry CH3CN (4 mL). The
solution was stirred at 120 °C for 21 h. After the reaction, the resulting
precipitate was collected and washed with CH3CN (3 × 5 mL). The
solid was dried under vacuum to give 2a·HBr as a yellow powder (1.32
g, 2.74 mmol, 68%). Single crystals suitable for X-ray diffraction
NCN), 147.4, 144.4, 137.1, 133.1, 127.9, 124.5, 121.1, 98.9 (d, 1JRh−C
=
1
6.0 Hz, CHCOD), 69.6 (d, JRhC = 13.9 Hz, CHCOD), 31.8, 27.9. IR
(KBr): 3080.7, 2877.2, 1605.5, 1538.9, 1493.6, 1403.0, 1345.1, 1269.9,
907.3, 834.1, 740.5 cm−1. HRMS (FAB): [M]+ calcd for
C23H20ClN6O8Rh 646.0086; found 646.0090. Anal. Calcd (%) for
C23H20ClN6O8Rh: C 42.71, H 3.12, N 12.99. Found: C 42.38, H 3.02,
N 12.56. Mp: 214 °C (dec).
Chloro(η4-cycloocta-1,5-diene)[1-(2,4-dinitrophenyl)-3-
methyl-1H-imidazol-2-ylidene]rhodium(I) (3b). A Schlenk tube
(10 mL) wrapped in aluminum foil was charged with 2b·HOTf (120
mg, 0.300 mmol), [RhCl(cod)]2 (74.0 mg, 0.150 mmol), K2CO3 (415
mg, 3.00 mmol), and dry toluene (3 mL). The reaction mixture was
stirred at room temperature for 3 days in the dark. After the reaction,
the resulting salt and excess K2CO3 were filtered off, and the filtrate
was concentrated under reduced pressure. The resulting solid was
purified by column chromatography (SiO2, first CH2Cl2 to remove
7000
dx.doi.org/10.1021/om300814f | Organometallics 2012, 31, 6995−7003

Organometallics
Article
impurities and then CH2Cl2/acetone, 30:1) and recrystallization from
CH2Cl2/diethyl ether to give 3b as brown crystals (46.0 mg, 0.0875
mmol, 29%). Single crystals suitable for X-ray diffraction analysis were
obtained by the slow diffusion of diethyl ether in a CH2Cl2 solution of
CDCl3): δ 7.50 (t, 2H, 3J = 7.8 Hz, CH4‑Ar), 7.32 (d, 4H, 3J = 7.8 Hz,
CH3‑Ar, CH5‑Ar), 7.18 (s, 2H, NCHCHN), 2.90 (sept, 4H, 3J = 6.8 Hz,
3
3
CH(CH3)2), 1.38 (d, 12H, J = 6.8 Hz, CH3), 1.12 (d, 12H, J = 6.8
1
Hz, CH3). 13C{1H} NMR (100.60 MHz, CDCl3): δ 184.9 (d, JRhC
=
1
3
1
1
3b. H NMR (400.03 MHz, CDCl3): δ 9.38 (d, 1H, J = 8.8 Hz,
53.8 Hz, CO), 182.7 (d, JRhC = 73.2 Hz, CO), 180.2 (d, JRhC = 45.1
Hz, NCN), 146.0, 134.9, 130.4, 128.6, 124.7, 124.0, 28.8, 28.0, 26.4,
22.7. IR (CH2Cl2): 2078.9(ν(CO)), 1996.0 (ν(CO)) cm−1.
CH6‑Ar), 8.85 (d, 1H, 4J = 2.5 Hz, CH3‑Ar), 8.71 (dd, 1H, 3J = 8.8 Hz, 4J
3
= 2.5 Hz, CH5‑Ar), 7.04 (d, 1H, J = 2.0 Hz, ArNCHCHNCH3), 6.90
3
Chlorodicarbonyl(triphenylphosphine)rhodium(I) (4d).
RhCl(PPh3)(CO)2 (4d) was prepared as described in the literature.33a
IR (CH2Cl2): 2093.4 (ν(CO)), 2012.4 (ν(CO)) cm−1 (lit. IR
(CH2Cl2): 2095 (ν(CO)), 2013 (ν(CO)) cm−1).33b
(d, 1H, J = 2.0 Hz, ArNCHCHNCH3), 5.08−4.97 (m, 2H, CHCOD),
4.23 (s, 3H, CH3), 3.47−3.40 (m, 1H, CHCOD), 2.80−2.70 (m, 1H,
CHCOD), 2.50−2.29 (m, 2H, CH2 COD), 2.01−1.76 (m, 4H, CH2 COD),
1.73−1.62 (m, 1H, CH2 COD), 1.60−1.46 (m, 1H, CH2 COD). 13C{1H}
1
Chloro[1,3-bis(2,4-dinitrophenyl)-1H-imidazol-2-ylidene]Au-
(I) (5a). A Schlenk tube (10 mL) wrapped in aluminum foil was
charged with 2a·HBr (241 mg, 0.500 mmol), Ag2O (58.0 mg, 0.250
mmol), dry CH2Cl2 (2.5 mL), and dry CH3OH (2.5 mL). The mixture
was stirred at room temperature for 16 h in the dark. To the red
reaction mixture was added AuCl(DMS) (147.3 mg, 0.500 mmol), and
the mixture was stirred at room temperature for 4 h in the dark. The
resulting salt was filtered off, and the filtrate was concentrated under
reduced pressure. The resulting solid was purified by recrystallization
from acetone/hexane two times to give 5a as a pale yellow crystal (126
mg, 0.199 mmol, 40%). Single crystals suitable for X-ray diffraction
analysis were obtained by the slow diffusion of hexane in an acetone
solution of 5a. 1H NMR (400.03 MHz, DMSO-d6): δ 9.02 (d, 2H, 4J =
2.5 Hz, CH3‑Ar), 8.84 (d, 2H, 3J = 8.7 Hz, 4J = 2.5 Hz, CH5‑Ar), 8.27 (d,
NMR (100.60 MHz, CDCl3): δ 187.2 (d, JRhC = 52.4 Hz, NCN),
147.2, 144.4, 138.2, 134.1, 127.2, 124.2, 121.5, 120.2, 100.4 (d, 1JRhC
=
=
1
1
6.3 Hz, CHCOD), 99.9 (d, JRhC = 7.3 Hz, CHCOD), 69.1 (d, JRhC
1
14.1 Hz, CHCOD), 68.6 (d, JRhC = 14.3 Hz, CHCOD), 38.6, 33.2, 32.4,
29.0, 28.6. 13C{1H} NMR (100.60 MHz, DMSO-d6): δ 184.1 (d, 1JRhC
= 52.5 Hz, NCN), 146.8, 143.9, 137.7, 132.1, 127.5, 124.5, 122.2,
1
1
120.9, 97.8 (d, JRhC = 6.9 Hz, CHCOD), 97.4 (d, JRhC = 6.9 Hz,
1
1
CHCOD), 68.0 (d, JRhC = 13.6 Hz, CHCOD), 67.2 (d, JRhC = 14.5 Hz,
CHCOD), 37.7, 33.0, 31.4, 28.7, 27.8. IR (KBr): 3140.5 3040.2, 2877.3,
2832.0, 1608.3, 1535.1, 1444.4, 1350.9, 1278.6, 1102.1, 963.3, 911.2,
842.7, 741.5 cm−1. HRMS (FAB): [M]+ calcd for C18H20ClN4O4Rh
494.0228; found 494.0223. Anal. Calcd (%) for C18H20ClN4O4Rh: C
43.70, H 4.07, N 11.32. Found: C 43.25, H 4.10, N 11.19. Mp: 182 °C
(dec).
3
2H, J = 8.7 Hz, CH6‑Ar), 8.25 (s, 2H, NCHCHN). 13C{1H} NMR
Chloro(η4-cycloocta-1,5-diene)[1,3-bis(2,6-diisopropylphen-
yl)-1H-imidazol-2-ylidene]rhodium(I) (3c). RhCl(IPr)(cod) (3c)
was prepared as described in the literature.31 1H NMR (400.03 MHz,
DMSO-d6): δ 7.60 (s, 2H, NCHCHN), 7.59−7.49 (m, 2H, CH 4‑Ar),
7.47−7.23 (m, 4H, CH3‑Ar, CH5‑Ar), 4.32 (br, 2H, CHCOD), 3.50 (br,
2H, CH(CH3)2), 3.22 (br, 2H, CHCOD), 2.32 (br, 2H, CH(CH3)2),
1.81−1.65 (m, 2H, CH2 COD), 1.65−1.50 (m, 2H, CH2 COD), 1.50−
1.38 (m, 4H, CH2 COD), 1.38−1.17 (m, 12H, CH3), 1.05 (d, 12H, 3J =
6.8 Hz, CH3). 13C{1H} NMR (100.60 MHz, DMSO-d6): δ 183.6 (d,
1JRhC = 51.5 Hz, NCN), 145.6, 136.2, 129.6, 125.7, 123.5, 93.9 (d, 1JRhC
(100.60 MHz, DMSO-d6): δ 171.7, 148.2, 144.9, 135.7, 132.3, 129.3,
124.3, 121.6. IR (KBr): 3114.5, 1610.3, 1536.0, 1495.5, 1428.0, 1347.0,
1291.1, 1105.0, 908.3, 835.0, 740.5 cm−1. HRMS (FAB): [M − Cl]+
calcd for C15H8N6O8Au 597.0069; found 597.0049. Anal. Calcd (%)
for C15H8AuClN6O8: C 28.48, H 1.27, N 13.28. Found: C 28.44, H
1.38, N 13.21. Mp: 248 °C (dec).
General Procedure for the Au(I)-Catalyzed Intermolecular
Hydroalkoxylation of Cyclohexene. A septum cap vial tube (2
mL) wrapped in aluminum foil was charged with Au(I) complex 5a,
5c, or 5d (0.025 mmol), AgOTf (6.4 mg, 0.025 mmol), and dry
toluene (0.5 mL), and the mixture was stirred for 5 min. Cyclohexene
(411 mg, 5.00 mmol) and 2-methoxyethanol (38.0 mg, 0.500 mmol)
were added to the mixture, and the vial tube was sealed and stirred at
120 °C for 24 h. After the reaction, the reaction mixture was cooled to
room temperature, and n-tetradecane (99 mg, 0.50 mmol, internal
standard) was added. The yield of the resulting ether was determined
by GLC analysis relative to an internal standard.
= 7.3 Hz, CHCOD), 66.9 (d, 1JRhC = 13.9 Hz, CHCOD), 32.2, 30.4, 27.8,
26.3.
Chlorodicarbonyl[1,3-bis(2,4-dinitrophenyl)-1H-imidazol-2-
ylidene]rhodium(I) (4a). CO gas was bubbled through a solution of
3a (59.4 mg, 0.100 mmol) in dry CH2Cl2 (5 mL) for IR analysis or in
CDCl3 (2 mL) for NMR analyses at room temperature for 20 min.
The initial brown solution became a yellow solution of 4a
1
(quantitative yield). The yield was determined by H NMR based
on COD. 1H NMR (400.03 MHz, CDCl3): δ 9.08 (d, 2H, 4J = 2.5 Hz,
CH3‑Ar), 8.70 (dd, 2H, 3J = 8.7 Hz, 4J = 2.5 Hz, CH5‑Ar), 8.28 (d, 2H, 3J
= 8.7 Hz, CH6‑Ar), 7.47 (s, 2H, NCHCHN). 13C{1H} NMR (100.60
MHz, CDCl3): δ 183.8 (d, 1JRhC = 55.6 Hz, CO), 181.2 (d, 1JRhC = 44.3
Hz, NCN), 180.4 (d, 1JRhC = 73.9 Hz, CO), 148.5, 145.4, 136.4, 134.0,
128.4, 124.4, 121.4. IR (CH2Cl2): 2091.4 (ν(CO)), 2014.3 (ν(CO))
cm−1. HRMS (FAB): [M − Cl]+ calcd for C17H8N6O10Rh: 558.9357;
found 558.9365.
X-ray Diffraction Analysis. Single crystals coated with liquid
paraffin were mounted on a thin nylon loop and fixed in a cold
nitrogen stream. X-ray data were collected with a Bruker AXS APEX II
CCD diffractometer using graphite-monochromated Mo Kα radiation
(λ = 0.71073 Å). The data were corrected for Lorentz and polarization
effects. Data integration and reduction were performed with SAINT
and XPREP software.34 Multiscan absorption corrections were applied
using the semiempirical method with SADABS.35 The structures were
solved by direct methods using SHELXS-9736 and refined using full-
matrix least-squares methods on F2 with SHELXL-9736 and Yadokari-
XG 2009.37 Selected data and structure refinement for compounds
2a·HBr, 2b·HOTf, 3a, 3b, and 5a are summarized in Table S1 and
Table S2 in the Supporting Information.
Computational Details. All calculations were performed with the
Gaussian 09 program package.38 Geometry optimizations, frequency
calculations, and NBO analyses of all compounds were carried out
using a hybrid meta-GGA M0639 functional of DFT. The LANL2TZ-
(f), which is the Hay−Wadt relativistic effective core potentials40 and
the valence triple-ζ basis set, was chosen to describe the gold atom,
and the valence triple-ζ basis set cc-pVTZ41 was used for all other
atoms. For the BDE calculations at 298.15 K and 1.00 atm, zero-point
energy corrections were determined at the same level. NBO analysis
was employed to evaluate bond indexes and donor−acceptor
interactions using the NBO version 3.1 program42 included in
Gaussian 09. The NNHC−Ccarbene interaction energies in NHC were
estimated by the second-order perturbation theory of NBO donor−
acceptor interactions (E(2) values). The donor orbitals are the
Chlorodicarbonyl[1-(2,4-dinitrophenyl)-3-methyl-1H-imida-
zol-2-ylidene]rhodium(I) (4b). CO gas was bubbled through a
solution of 3b (10.5 mg, 0.0200 mmol) in dry CH2Cl2 (1 mL) for IR
analysis or in CDCl3 (0.5 mL) for NMR analyses at room temperature
for 30 min. The initial brown solution became a yellow solution of 4b
1
(quantitative yield). The yield was determined by H NMR based on
1
4
COD. H NMR (400.03 MHz, CDCl3): δ 8.96 (d, 1H, J = 2.4 Hz,
CH3‑Ar), 8.63 (dd, 1H, 3J = 8.6 Hz, 4J = 2.4 Hz, CH5‑Ar), 8.23 (d, 1H, 3J
3
= 8.6 Hz, CH6‑Ar), 7.22 (d, 1H, J = 2.0 Hz, ArNCHCHNCH3), 7.17
(d, 1H, J = 2.0 Hz, ArNCHCHNCH3), 4.08 (s, 3H, CH3). 13C{1H}
3
NMR (100.60 MHz, CDCl3): δ 184.7 (d, 1JRhC = 54.7 Hz, CO), 181.4
1
1
(d, JRhC = 70.4 Hz, CO), 177.9 (d, JRhC = 44.0 Hz, NCN), 148.1,
145.5, 137.4, 134.3, 128.8, 128.2, 124.4, 123.2, 121.1, 39.1. IR
(CH2Cl2): 2886.9, 2086.6 (ν(CO)), 2007.5 (ν(CO)), 1609.3, 1544.7
cm−1. HRMS (FAB): [M − CO − Cl]+ calcd for C11H8N4O5Rh
378.9550; found 378.9548.
Chlorodicarbonyl[1,3-bis(2,6-diisopropylphenyl)-1H-imida-
zol-2-ylidene]rhodium(I) (4c) (ref 32). RhCl(IPr)(CO)2 (4c) was
prepared as described in the literature.10 1H NMR (400.03 MHz,
7001
dx.doi.org/10.1021/om300814f | Organometallics 2012, 31, 6995−7003

Organometallics
Article
occupied NNHC p orbitals (n(N)), and the acceptor orbitals are the
unoccupied p orbital (n*(N)) of Ccarbene. The metal−ligand π-back-
bonding interaction energies (ΔEbb) were evaluated by the summation
of all NBO E(2) values. The donor orbitals are the occupied Au d
orbitals (d(Au)), and the acceptor orbitals are the corresponding
antibonding orbitals π*(Ccarbene−NNHC) for NHCs and three σ*(P−
C) for PPh3.
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ASSOCIATED CONTENT
* Supporting Information
■
S
Computational details and crystallographic data for 2a·HBr,
2b·HOTf, 3a, 3b, and 5a (CIF files). This material is available
free of charge via the Internet at http://pubs.acs.org.
K. A.; Miller, S. A. Org. Lett. 2008, 10, 3677. (f) Leuthaußer, S.;
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AUTHOR INFORMATION
Corresponding Author
*E-mail: tetsuo@aporg.che.tohoku.ac.jp (T.S.); oishu@aporg.
che.tohoku.ac.jp (S.O.).
■
A.; Kuhn, F. E. Dalton Trans. 2009, 7055. (k) Harding, D. A.; Hope, E.
̈
G.; Singh, K.; Solan, G. A. Organometallics 2012, 31, 1518.
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Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENTS
Trans. 1 1972, 2927. (d) Franke, H.; Grauhoff, H.; Scherowsky, G.
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Chem. Ber. 1979, 112, 3623. (e) Claramunt, R. M.; Elguero, J.; Meco,
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This work was supported in part by a Grant-in-Aid for Scientific
Research on Innovative Areas “Molecular Activation Directed
toward Straightforward Synthesis” from MEXT, Japan, and by a
commissioned project conducted by New Energy and Industrial
Technology Development Organization (NEDO). We thank
Prof. K. Itaya (Tohoku University) for courteous permission to
use the X-ray diffractometer and Dr. C. Kabuto (Tohoku
University) for her kind advice in X-ray analysis. A part of the
computational results were obtained using supercomputing
resources at the Cyberscience Center, Tohoku University.
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