1,2,4-triazol-3-ylidenes with an N-2,4-dinitrophenyl substituent as strongly π-accepting N-heterocyclic carbenes

Article abstract of DOI:10.1002/chem.201302567

The synthesis and characterisation of a series of new Rh and Au complexes bearing 1,2,4-triazol-3-ylidenes with a N-2,4-dinitrophenyl (N-DNP) substituent are described. IR, NMR, single-crystal X-ray diffraction and computational analyses of the Rh complexes revealed that the N-heterocyclic carbenes (NHCs) behaved as strong π acceptors and weak σ donors. In particular, a natural bond orbital (NBO) analysis revealed that the contributions of the Rh→C_carbene π backbonding interaction energies (ΔE _bb) to the bond dissociation energies (BDE) of the Rh-C _carbene bond for [RhCl(NHC)(cod)] (cod=1,5-cyclooctadiene) reached up to 63 %. The Au complex exhibited superior catalytic activity in the intermolecular hydroalkoxylation of cyclohexene with 2-methoxyethanol. The NBO analysis suggested that the high catalytic activity of the Au^I complex resulted from the enhanced π acidity of the Au atom. π-Acidic carbene complexes: A series of Rh and Au complexes bearing 1,2,4-triazol-3- ylidenes with a N-2,4-dinitrophenyl (DNP) substituent were synthesised. Experimental and theoretical analyses revealed that these N-heterocyclic carbenes (NHCs) behave as strong π acceptors and weak σ donors, and the metal centers exibit enhanced π acidity compared to complexes with traditional σ-donating NHCs (see figure, EWG=electron withdrawing group). Copyright

Full text of DOI:10.1002/chem.201302567

DOI: 10.1002/chem.201302567
1,2,4-Triazol-3-ylidenes with an N-2,4-Dinitrophenyl Substituent
as Strongly p-Accepting N-Heterocyclic Carbenes
Tetsuo Sato,*[a, b] Yoichi Hirose,[a] Daisuke Yoshioka,[a] Tsubasa Shimojo,[a] and
Shuichi Oi*[a, b]
ꢀ
Abstract: The synthesis and characteri-
sation of a series of new Rh and Au
complexes bearing 1,2,4-triazol-3-yli-
denes with a N-2,4-dinitrophenyl (N-
DNP) substituent are described. IR,
NMR, single-crystal X-ray diffraction
and computational analyses of the Rh
complexes revealed that the N-hetero-
cyclic carbenes (NHCs) behaved as
strong p acceptors and weak s donors.
In particular, a natural bond orbital
(NBO) analysis revealed that the con-
tributions of the Rh!Ccarbene p back-
bonding interaction energies (DEbb) to
the bond dissociation energies (BDE)
of the Rh Ccarbene bond for [RhCl-
(NHC)
E
(cod=1,5-cycloocta-
diene) reached up to 63%. The Au
complex exhibited superior catalytic
activity in the intermolecular hydroal-
koxylation of cyclohexene with 2-me-
thoxyethanol. The NBO analysis sug-
gested that the high catalytic activity of
the AuI complex resulted from the en-
hanced p acidity of the Au atom.
Keywords: acidity
·
carbene li-
gands · gold · ligand effects · rhodi-
um
Introduction
There has been great interest in N-heterocyclic carbenes
(NHCs) as ligands for transition metal complex catalysts
and organocatalysts because of the high flexibility in the
molecular design of their electronic and structural proper-
ties.[1] Although research on metal NHC complexes has fo-
cused primarily on their excellent carbene carbon-to-metal
s-donating properties, which make the metal atom highly
nucleophilic, some electron-deficient NHCs, which were
shown to be weak s donors from the IR spectra of [RhCl-
Figure 1. Comparison of s-donating abilities of selected NHCs. The aver-
age IR stretching frequencies navðCOÞ [cmꢀ1
] of CO in [RhCl-
~
AHCTUNGTREG(NNNU NHC)(CO)2] are shown in parentheses. IR spectra were recorded in
CH2Cl2 for A, C, IDNP, and IPr, in KBr for B, and as a film for D.
DNP=2,4-dinitrophenyl. Dipp=2,6-diisopropylphenyl.
ACHTUNGTRENNUNG(NHC)(CO)2], have been developed in the last decade
(Figure 1).[2] Experimental investigations and theoretical
studies on this new class of NHCs have revealed improved
metal-to-carbene carbon p-accepting abilities. Electron-defi-
cient NHCs have been prepared by various methods, such as
modification of the backbone (A),[2e,f] using electron-with-
drawing N substituents (IDNP and C),[3] and incorporation
of electronegative heteroelements in the basic framework of
imidazol-2-ylidenes (B and D). Furthermore, various 1,2,4-
triazol-3-ylidenes have been synthesised.[1n–r,2m–o,4] However,
there have been very few reports on the effects of p-accept-
ing NHC ligands on transition-metal catalysis.[2p,4e–g,5] More-
over, most ligands studied, such as D,[5] have stronger s-do-
nating than p-accepting capabilities. Recently, we synthes-
ised N-2,4-dinitrophenyl (DNP)-substituted imidazol-2-yli-
denes IDNP and C, and showed that the DNP group effec-
tively increased the p-accepting abilities of carbenes.[3] Here
we report the synthesis of a series of RhI and AuI complexes
of 1,2,4-triazol-3-ylidenes bearing the N-DNP substituent
and evaluate the electronic properties of these NHCs
through experimental and theoretical analyses.
[a] Dr. T. Sato, Y. Hirose, D. Yoshioka, T. Shimojo, Prof. Dr. S. Oi
Department of Applied Chemistry
Graduate School of Engineering, Tohoku University
6-6-11 Aramaki-Aoba, Aoba-ku, Sendai 980-8579 (Japan)
Fax : (+81)22-795-5874
Results and Discussion
Synthesis of RhI and AuI complexes: To compare the differ-
ences in the electronic properties of the carbenes, we chose
three triazoles 1a–c as the starting materials, substituted
with an H, cyano, or nitro group at the 5-position
(Scheme 1). 2,4-Dinitrophenylation of 1a–c afforded 2a–c,
and subsequent methylation with an excess of methyl triflate
E-mail: tetsuo@aporg.che.tohoku.ac.jp
oishu@aporg.che.tohoku.ac.jp
[b] Dr. T. Sato, Prof. Dr. S. Oi
Environment Conservation Research Institute, Tohoku University
6-6-11 Aramaki-Aoba, Aoba-ku, Sendai 980-8579 (Japan)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201302567.
15710
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Chem. Eur. J. 2013, 19, 15710 – 15718

FULL PAPER
Table 1. Summary of the IR and 13C NMR spectroscopic analyses of
[RhCl(L)(CO)2] (5a–e).
[a]
[b]
Complex
TEP[c]
dACHTUNGTRENNUNG
(13CN2)[d]
~
n(CO)
~
nav(CO)
5a
5b
5c
5d
5e
N
2093.4, 2015.3
2099.1, 2021.0
2100.1, 2022.0
2086.6, 2007.5[3]
2078.9, 1996.0[3]
2054.4
2060.1
2064
2068
2069
2058
2050
182.7
186.9
190.6
–
2061.1
2047.1[3]
2037.5[3]
179.8
[a] IR CO stretching frequencies [cmꢀ1
~
] in CH2Cl2. [b] Average of
n(CO) [cmꢀ1]. [c] Tolman electronic parameter (TEP) [cmꢀ1] calculated
with the equation TEP=0.8001nav(CO)+420.0. [d] 13C NMR chemical
[1b]
~
shift [ppm] of the carbenic carbon atom in [D6]acetone.
~
of IPr, calculated from navðCOÞ of 5e. Thus, 3a–c exhibit ex-
Scheme 1. Synthesis of RhI complexes 4a–c and 5a–c.
tremely poor s-donating properties (Table 1). Since the TEP
value of 3a (2064 cmꢀ1) was larger than that of
C
provided triazolium salts 3a–c·HOTf, the structures of
which were confirmed by NMR spectroscopy, elemental
analysis, and single crystal X-ray diffraction.[6] The genera-
tion of carbenes and complexation was then examined. Al-
though the free NHCs 3a–c could not be isolated, treatment
of triazolium salts 3a–c·HOTf with [{RhClACTHNURTGNENG(U cod)}2] (cod=
1,5-cyclooctadiene) in the presence of K2CO3 as base in tol-
uene at room temperature afforded the corresponding
(2058 cmꢀ1), it appears that the incorporation of the N atom
into the NHC framework reduced the s-donating ability of
the NHC. Notably, the TEP values of 3c and 3b (2069 and
2068 cmꢀ1, respectively) are the highest reported so far,
even higher than those of A[2e,f] and B[2m] (Figure 1). In this
work, as the TEP value increased, we observed a downfield
shift in the 13C NMR signal of the carbenic carbon atom in
the RhI dicarbonyl complexes (Table 1).[11] These results sug-
gest that an increase in the deshielding of the carbon atom
corresponds to a decrease in its s-donating ability.
[RhClACHTUNGTRENNUNG(NHC)ACHTUNGTRENNUNG(cod)] (4a–c) in good yields through in situ
generation of NHCs 3a–c, respectively. To estimate the s-
donating properties of NHCs 3a–c, Rh complexes 4a–
c were converted to [RhCl
tive yield by bubbling CO gas through their solutions in
CH2Cl2 or [D6]acetone. [RhCl(IPr)
(cod)] (4e)[7] and [RhCl-
(IPr)(CO)2] (5e)[8] were also prepared for comparison of the
electronic properties of the NHCs. In addition, [AuCl-
(NHC)] complexes 6a and 6c were synthesised by the reac-
tions of triazolium salts 3a·HOTf or 3c·HOTf with AuCl-
G
Characterisation of [RhClACTHNGURTENNU(G NHC)ACHTUNGRTEN(NUNG cod)] (4): Next, we char-
acterised the RhI complexes 4a–c by performing crystallo-
graphic, 13C NMR spectroscopic, and theoretical analyses.
Figures 2–4 show ORTEPs of 4a–c, respectively. Single crys-
tals for X-ray diffraction analysis were obtained by slow
cooling of a hot CH3CN solution of 4a and slow diffusion of
diethyl ether into CH2Cl2 solutions of 4b and 4c. All com-
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG(SMe2) and K2CO3 as base (Scheme 2). The structures of the
RhI complexes 4a–c and AuI complexes 6a and 6c were de-
termined by NMR spectroscopy, elemental analysis, and
single-crystal X-ray diffraction.
Characterisation of [RhClACTHNUTRGNE(UNG NHC)(CO)2] (5): First, we char-
acterised the electronic properties of the new NHCs 3a–
c by performing IR and 13C NMR spectroscopic analyses of
RhI complexes 5a–c. They were compared to the corre-
sponding RhI complexes bearing imidazol-2-ylidene ana-
logues such as C and IPr. The Tolman electronic parameters
(TEP)[9] of 3a–c, determined from the averages of the IR
I
~
CO stretching frequencies navðCOÞ of Rh dicarbonyl com-
plexes 5a–c,[1b,10] were significantly higher compared to that
Figure 2. ORTEP of 4a with thermal ellipsoids at 50% probability. Hy-
drogen atoms and an acetonitrile solvent molecule have been omitted for
ꢀ
ꢀ
clarity. Selected distances [ꢁ] and angles [8]: Rh1 C1 2.003(3), Rh1 C10
ꢀ
ꢀ
ꢀ
ꢀ
2.112(3), Rh1 C11 2.131(3), Rh1 C14 2.218(2), Rh1 C15 2.209(3), Rh1
ꢀ
ꢀ
C11 2.3776(7), C1 N1 1.356(3), C1 N3 1.359(3); N1-C1-N3 102.0(2), C1-
Rh1-Cl1 88.22(8), N1-C1-Rh1-Cl1 ꢀ98.1(2), C1-N1-C3-C4 ꢀ141.7(3), C3-
C4-N4-O1 46.4(3), C5-C6-N5-O3 ꢀ6.2(4).
Scheme 2. Synthesis of AuI complexes 6a and 6c.
Chem. Eur. J. 2013, 19, 15710 – 15718
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T. Sato, S. Oi et al.
In all complexes, the NHC frameworks scarcely conjugate
with the DNP group in the solid-state structure, and this in-
dicates that the DNP group acts as an electron-withdrawing
group through an inductive effect. The dihedral angles be-
tween the NHC and DNP frameworks are ꢀ141.7(3)8 for
4a, ꢀ118.9(3)8 for 4b, and ꢀ128.3(2)8 for 4c. However, the
NHC framework is on almost the same plane as the nitro
group at the 5-position in complex 4c (O1-N4-C2-N2
ꢀ3.3(4)8). As expected from the TEP values of 3a–c, C, and
IPr, the Rh Ccarbene crystallographic distance in the RhI com-
ꢀ
plexes 4a–e (Table 2) decrease in the order 4e
Table 2. Summary of the crystallographic and 13C NMR spectroscopic
analyses of RhCl(L)ACTHUNRGTNE(NUG cod) 4a–e.
[a]
[b]
[d]
Complex
%Vbur
(13CN2)[c] d(trans-13CHCOD
dACHTUNGRTNEGNU )
ꢀ
Rh Ccarbene
4a
4b
4c
4d
4e
N
2.003(3)
1.993(2)
1.989(2)
2.017(2)[3]
29.0
28.7
190.3
194.2
99.5, 99.3
100.9, 100.7
101.3, 101.1
97.8, 97.4[3]
93.9[3]
29.6
198.1
29.4[3]
184.1[3]
183.6[3]
34.0[12]
Figure 3. ORTEP of 4b with thermal ellipsoids at 50% probability. Hy-
drogen atoms have been omitted for clarity. Selected distances [ꢁ] and
[a] The crystallographic distance [ꢁ] between the Rh atom and the car-
benic carbon atom. [b] The%Vbur values were calculated with the Bondi
radii scaled by 1.17, a sphere of radius 3.5 ꢁ, a mesh size of 0.05 ꢁ, and
ꢀ
ꢀ
ꢀ
ꢀ
angles [8]: Rh1 C1 1.993(2), Rh1 C11 2.119(2), Rh1 C12 2.140(2), Rh1
ꢀ
ꢀ
ꢀ
C15 2.199(2), Rh1 C16 2.226(2), Rh1 C11 2.3808(7), C1 N1 1.351(3),
ꢀ
C1 N3 1.358(3); N1-C1-N3 102.1(2), C1-Rh1-Cl1 85.04(7), Cl1-Rh1-C1-
ꢀ
an Rh NHC distance of 2.00 ꢁ with omission of all hydrogen atoms.
N1 ꢀ90.9(2), C1-N1-C4-C5 ꢀ118.9(3), O1-N5-C5-C4 22.9(3), O3-N6-C7-
C6 ꢀ6.0(3).
[c] 13C NMR chemical shift [ppm] of the carbenic carbon atom in
[D6]DMSO. [d] 13C NMR chemical shift [ppm] of the alkene moiety of
COD trans to the ligand (L) in [D6]DMSO.
(2.054(4) ꢁ[12]), 4d (2.017(2) ꢁ[3]), 4a (2.003(3) ꢁ), 4b
(1.993(2) ꢁ), and 4c (1.989(2) ꢁ). There is not much differ-
ence in the steric bulk of the NHCs 3a, 3b, 3c, and C in
their respective complexes 4a–d. The percent buried volume
(%Vbur) values were estimated to be 29.0, 28.7, 29.6, and
29.4%,[3] respectively (Table 2).[13] Furthermore, the
13C NMR chemical shifts of both of the carbenic carbon
atom and the alkene moiety of COD trans to the NHC
ligand of 4a–e shifted to the downfield with decreasing s-
donating ability of the ligands, which is in agreement with
previous results (Table 2).[2b,14] These results indicate that
the p-accepting abilities of the NHC ligands increased in the
order of 4e, 4d, 4a, 4b, and 4c.
Theoretical analysis of [RhClACHTUNTGRENNUG(NHC)ACHTUNGTNER(NUGN cod)] (4): To evaluate
the electronic properties of the NHC ligands for 4a–e, we
performed a natural bond orbital (NBO) analysis (Table 3).
The calculation showed that the natural atomic charges d of
the carbenic carbon atoms in 4a–e increased positively cor-
responding to a downfield shift of the 13C NMR signals, and
carbenic carbon atom in 4c had the most positive charge.
The Wiberg bond indexes W between the Rh atom and the
carbenic carbon atom in 4a–e increased with decreasing
Figure 4. ORTEP of 4c with thermal ellipsoids at 50% probability. Hy-
drogen atoms have been omitted for clarity. Selected distances [ꢁ] and
ꢀ
ꢀ
ꢀ
ꢀ
angles [8]: Rh1 C1 1.989(2), Rh1 C10 2.124(3), Rh1 C11 2.119(2), Rh1
ꢀ
ꢀ
ꢀ
C14 2.230(3), Rh1 C15 2.218(2), Rh1 C11 2.3801(7), C1 N1 1.350(3),
ꢀ
C1 N3 1.362(3); N1-C1-N3 102.7(2), C1-Rh1-Cl1 87.68(7), Cl1-Rh1-C1-
N1 ꢀ98.9(2), O1-N4-C2-N2 ꢀ3.3(4), C1-N1-C3-C4 ꢀ128.3(2), O3-N5-C4-
C3 39.4(3), O5-N6-C6-C5 15.1(3).
ꢀ
Rh Ccarbene crystallographic distance. In addition, an NBO
second-order perturbation analysis revealed that the in-
plexes have pseudo-square-planar metal centres coordinated
by NHC ligands 3a, 3b, or 3c, a chloride anion, and COD
ligand. The dihedral angles between the NHC framework
and the metal–ligand plane are approximately orthogonal
(ꢀ98.1(2)8 for 4a, ꢀ90.9(2)8 for 4b, and ꢀ98.9(2)8 for 4c).
crease in the p backbonding interaction energies (DEbb) was
ꢀ
accompanied by a decrease in the Rh Ccarbene bond length
and an increase in the TEP value. The DEbb value of 4a
with triazol-3-ylidene 3a (27.64 kcalmolꢀ1) is larger than
that of 4d bearing imidazol-2-yliden analogue C (24.68 kcal
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Chem. Eur. J. 2013, 19, 15710 – 15718

Strongly p-Accepting N-Heterocyclic Carbenes
FULL PAPER
Table 3. Summary of the computational analysis of [RhCl(L)ACTHNUGRTNEUNG(cod)] (4a–
e).
[a]
[b]
[d]
Complex
dC
WRhC
BDE[c]
DEbb
DEbb in BDE[e]
4a
4b
4c
4d
4e
N
0.400
0.407
0.416
0.402
0.370
0.723
0.728
0.737
0.709
0.662
50.40
48.93
49.15
51.95
53.90
27.64
30.86
30.92
24.68
20.08
54.8
63.1
62.9
47.5
37.3
[a] Natural atomic charge d of the carbenic carbon atom. [b] Wiberg
bond index W between the Rh atom and the carbenic carbon atom in the
NHC ligand. [c] Bond dissociation energy [kcalmolꢀ1]. [d] p backbonding
interaction energy [kcalmolꢀ1]. [e] Percentage contribution of DEbb to
BDE.
molꢀ1). Furthermore, the nitro and cyano substituents on
the NHC framework effectively enhanced the p backbond-
ing interactions. As a result, the DEbb values of 4b and 4c
(30.86 and 30.92 kcalmolꢀ1, respectively) were about 12%
larger than that of 4a. Consequently, the contribution of
Figure 5. ORTEP of 6a with thermal ellipsoids at 50% probability. Hy-
drogen atoms and an acetone solvent molecule have been omitted for
ꢀ
ꢀ
clarity. Selected distances [ꢁ] and angles [8]: Au1 C1 1.976(3), Au1 Cl1
ꢀ
ꢀ
2.2741(8), C1 N1 1.339(3), C1 N3 1.348(3); N1-C1-N3 103.5(2), C1-Au1-
Cl1 177.45(7), C1-N1-C3-C4 121.4(3), O1-N4-C4-C3 ꢀ30.7(4), O3-N5-C6-
C5 9.9(4).
ꢀ
DEbb to the bond dissociation energy (BDE) of the Rh
Ccarbene bond for 4b and 4c was 63%, whereas that for 4e
was 37%. Interestingly, the BDE did not decrease consider-
ably, even though the electron-donating ability of the coor-
dinated NHC decreased. This suggests that p backdonation
of the NHC compensated for the decrease in s-donation.
Energies of the s-donor and p-acceptor orbitals of NHCs:
The energies of the s-donor orbital (Es) and p-acceptor or-
bital (Ep) for 3a–c, C, and IPr were calculated to evaluate
their s-donating and p-accepting abilities. The s-donor orbi-
tal is an occupied carbene lone pair and the p-accepting or-
bital is a vacant p orbital on the carbene carbon atom
(Table 4). The calculations support the above results. Substi-
Table 4. Energies of the s-donor orbital (Es) and p-acceptor orbital (Ep)
for NHCs 3a–c, C, and IPr.
NHC
Es [eV]
(HOMOꢀ7)
(HOMOꢀ10)
(HOMOꢀ12)
(HOMOꢀ4)
(HOMOꢀ8)
Ep [eV]
(LUMO+2)
(LUMO+2)
(LUMO+3)
(LUMO+2)
(LUMO)
3a
3b
ꢀ8.70
ꢀ9.14
ꢀ9.24
ꢀ8.30
ꢀ7.64
N
ꢀ2.15
ꢀ2.63
ꢀ2.71
ꢀ1.86
ꢀ1.38
ACHTUNGTRENNUNG
Figure 6. ORTEP of 6c with thermal ellipsoids at 50% probability. Hy-
drogen atoms have been omitted for clarity. Selected distances [ꢁ] and
ACHTUNGTRENNUNG
3c
ACHTUNGTRENNUNG
ꢀ
ꢀ
ꢀ
ꢀ
angles [8]: Au1 C1 1.966(4), Au1 Cl1 2.2665(12), C1 N1 1.352(5), C1
N3 1.433(5); N1-C1-N3 103.5(4), C1-Au1-Cl1 177.88(13), O1-N4-C2-N2
1.0(6), C1-N1-C3-C4 ꢀ64.7(6), O3-N5-C4-C3 152.9(4), O5-N6-C6-C5
ꢀ16.1(5).
C[3]
IPr[3]
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
tution of the NHC framework with a cyano or nitro group
in decreased both the Es and the Ep values. The Es values
decreased from ꢀ8.70 eV in 3a to ꢀ9.14 and ꢀ9.24 eV in 3b
and 3c, respectively. The Ep values decrease from ꢀ2.15 eV
in 3a to ꢀ2.63 and ꢀ2.71 eV in 3b and 3c, respectively. As
is evident from the substantial decrease in the values of
both of Es and Ep, the s-donating abilities of N-DNP-substi-
tuted triazol-3-ylidenes 3a–c significantly deteriorated,
whereas the p-accepting capabilities improved, compared to
C and IPr.
6c (1.966(4) ꢁ) is shorter than that in 6a (1.976(3) ꢁ). As
with the RhI complexes 4a and 4c, the NHC frameworks
scarcely conjugate with the DNP group, but are on the same
plane as the nitro group at the 5-position in complex 6c.
The C1-N1-C3-C4 dihedral angles are 121.4(3)8 for 6a and
ꢀ64.7(6)8 for 6c, and the O1-N4-C2-N2 dihedral angle of 6c
is 1.0(6)8.
Au-catalysed hydroalkoxylation of alkenes: We investigated
the new p-accepting NHCs as ligands for p-acidic Au cata-
lysts. It was demonstrated recently that IDNP is a more ef-
fective ligand than IPr and PPh3 in Au-catalysed hydroal-
koxylation of cyclohexene with 2-ethoxyethanol.[3,15] Elec-
tronic activation of the alkene to form the alkyl Au inter-
Characterisation of [AuClACTHNURGTNE(UNG NHC)] (6): Figures 5 and 6 show
ORTEPs of 6a and 6c, respectively. Single crystals for X-
ray diffraction analysis were obtained by slowly cooling ace-
ꢀ
tone solutions of 6a and 6c. The Au Ccarbene bond length in
Chem. Eur. J. 2013, 19, 15710 – 15718
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T. Sato, S. Oi et al.
mediate is believed to be the rate-determining step in this
reaction. Since electron-deficient ligands accelerate the
overall rate of a reaction,[16] we evaluated the catalytic activ-
ity of the new AuI complexes in the intermolecular hydroal-
koxylation of alkenes. To elucidate the difference in the cat-
alytic activities, we first chose a combination of cyclohexene
(7) and 2-ethoxyethanol (8), which has also been examined
previously. The results of the hydroalkoxylation of 8 with 7
are summarised in Table 5. Under optimised reaction condi-
tions (see Supporting Information, Table S2), the catalytic
though the conversion of 7 reached 93% (Table 5, entry 7).
When the AuI complex was not added, the intermolecular
hydroalkoxylation did not proceed at all (Table 5, entry 8).
In all cases, by-products derived from 7 were not detected
by GC-MS analysis. To examine in more detail the catalytic
properties of the AuI complexes we performed the reactions
at 908C (see Supporting Information, Table S2). The yields
decreased in all reactions, but a similar trend of catalytic ac-
tivities to when the reactions were carried out at 1008C was
obtained. A conversion versus time plot for the addition of
7 to 8 at 1008C with various AuI precatalysts is shown in
Figure 7. With increasing TEP value of the NHC ligand
(IPr<IDNP<6a<6c), the reaction rate increased. Al-
Table 5. Au-catalysed hydroalkoxylation of cyclohexene (8) with 2-me-
thoxyethanol (7).[a]
though the initial rate of reaction with the AuI–P
ACHTUNGTRENNUNG(OPh)3
complex was slightly faster than that with the AuI–6c com-
plex, the AuI–6c complex afforded 9 in 10% higher yield
than the AuI–P
ACTHUNTRGNE(UGN OPh)3 complex under the optimised reaction
conditions (Table 5, entries 1 and 7).
Entry
Au complex
[AuCl(3c)] (6c)
[AuCl(3a)] (6a)
[AuCl(IDNP)]
[AuCl(IMes)]
[AuCl(IPr)]
[AuCl(PPh3)]
[AuCl[P(OPh)3]
none
T [8C]
Conversion [%][b]
Yield [%][b]
1
2
3
4
5
6
7
8
G
N
100
100
100
100
100
100
100
100
95
93
83
75
20
77
93
15
81
74
66
59
7
63
71
0
Theoretical analysis of AuI complexes: To understand the
difference between the intermolecular hydroalkoxylation of
the alkenes catalysed by AuI complexes bearing p-accepting
NHC 3c and s-donating imidazol-2-ylidenes on an electron-
ic level, we carried out an NBO analysis of Au
AHCTUNGTREG(NNUN ethylene) cationic complexes 10a and 10b, which are
R
E
E
G
N
ACHTUNGTRENNUNG(NHC)-
A
ACHTUNGTRENNUNG
model complexes of plausible intermediates. The results are
summarised in Table 6.[18] The calculation of Au!Ccarbene
and Au!ethylene p backbonding interaction energies for
10a (coordinated by 3a) and 10b (bearing an imidazol-2-yli-
dene analogue) revealed that the same d orbital of Au was
involved in the p backbonding interactions in both com-
plexes. The Au!Ccarbene interaction of 10a (9.57 kcalmolꢀ1)
was larger than that of 10b (8.68 kcalmolꢀ1). In conjunction
with this interaction, the magnitude of the Au!ethylene p
backbonding interactions of 10a and 10b were reversed
(24.08 and 27.13 kcalmolꢀ1, respectively), and the average
natural atomic charge dav on the carbon atoms of ethylene
in 10a was more positive than that in 10b. This indicates
[a] Reactions were carried out with 7 (0.25 mmol), 8 (2.5 mmol), Au com-
plex (0.0125 mmol), and AgNTf2 (0.0125 mmol) in PhCl (0.25 mL) for
20 h. [b] Conversions and yields were determined by GLC analysis.
activities of the AuI complexes bearing NHC ligands, PPh3,
and PACHTUNGTRENNUNG(OPh)3 were compared. The combination of 6c, which
contains the weakest s-donating NHC 3c, and AgNTf2 af-
forded adduct 9 in the highest yield (Table 5, entry 1); 9 was
formed in 81% yield with 95% conversion of 7 by using ten
equivalents of cyclohexene with 5 mol% 6c/AgNTf2 in
chlorobenzene at 1008C for 20 h. The yields of the target
adduct 9 followed a similar order to the TEP values
(Table 5, entries 1–5). The AuI
complex bearing PPh3 gave
adduct 9 in moderate yield and
conversion (Table 5, entry 6). It
is known that PACHTUNGTRENNUNG(OPh)3 is more
electron-deficient ligand than
PPh3; their TEP values are
2085.3 and 2068.9 cmꢀ1, respec-
tively.[9] Moreover, it was re-
ported that the initial turnover
frequency for addition of meth-
anol to propyne catalysed by
the AuI complex bearing P-
ACHTUNGTRENNUNG(OPh)3 was 2.46 times higher
than that of the reaction with
the AuI–PPh3 complex.[17] In the
hydroalkoxylation of 8 with 7
Figure 7. Time course of Au-catalysed hydroalkoxylation of cyclohexene 8 with 2-methoxyethanol 7. Reactions
were carried out with 7 (0.50 mmol), 8 (5.0 mmol), Au complex (0.025 mmol), and AgNTf2 (0.025 mmol) in
using [AuCl{PACHTUNGTRENNUNG(OPh)3}] (9) was
only obtained in 71% yield, al- PhCl (0.5 mL) at 1008C for 20 h. Conversions were determined by GLC analysis.
15714
www.chemeurj.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 15710 – 15718

Strongly p-Accepting N-Heterocyclic Carbenes
FULL PAPER
Table 6. NBO analysis of [Au
C
G
30 on Uniport B 60/80 mesh, L: 3 mꢂi.d.: 3 mm) equipped with a thermal
conductivity detector.
1-(2,4-Dinitrophenyl)-1,2,4-triazole (2a):[19] A Schlenk flask (50 mL) was
charged with 1H-1,2,4-triazole (1a, 207 mg, 3.00 mmol), K2CO3 (622 mg,
4.50 mmol), 1-fluoro-2,4-dinitrobenzene (558 mg, 3.00 mmol), and dry
CH3CN (5 mL). The mixture was stirred at 608C for 3 h. Then, the result-
ing salt and excess K2CO3 were filtered off, and the filtrate was concen-
trated under reduced pressure. Water (10 mL) was added to the resulting
oil, and the obtained precipitate was collected by filtration. The solid was
washed with water (3ꢂ5 mL) and dried under vacuum to give 2a as
Complex
DE
G
DEbb(Au!ethylene)[b]
davACTHUNGETRNNUG
(CH2)[c]
10a
10b
24.08
27.13
ꢀ0.395
ꢀ0.403
[a] p backbonding interaction energy [kcalmolꢀ1] between the Au atom
and the carbenic carbon atom. [b] p backbonding interaction energy [kcal
molꢀ1] between the Au atom and ethylene. [c] Average of the natural
atomic charges d on the carbon atoms in ethylene.
a
yellow-green powder (540 mg, 2.30 mmol, 77%). 1H NMR
(400.03 MHz, [D6]DMSO): d=9.33 (s, 1H; ArNCHN), 8.95 (d, 4J=
2.5 Hz, 1H; CH3-Ar), 8.72 (dd, 3J=8.8, 4J=2.5 Hz, 1H; CH5-Ar), 8.37 (s,
1H; ArNNCHN), 8.25 ppm (d, 1H, 3J=8.8 Hz; CH6-Ar); 13C{1H} NMR
(100.60 MHz, [D6]DMSO): d=153.6, 146.9, 145.5, 143.0, 133.2, 128.5,
~
127.4, 121.3 ppm; IR (KBr): n=3126.0, 3082.7, 3061.4, 1610.3, 1546.6,
1346.1, 1030.8, 978.7, 918.9, 881.3, 853.4, 833.1, 743.4 cmꢀ1
3-Cyano-1-(2,4-dinitrophenyl)-1,2,4-triazole (2b):
.
that 3c enhances the p acidity of an Au atom owing to its p-
accepting ability. Hence, it seems that 10a is more favoura-
ble for the nucleophilic attack of alcohols.
A
Schlenk flask
(50 mL) was charged with 3-cyano-1H-1,2,4-triazole (1b, 282 mg,
3.00 mmol), K2CO3 (622 mg, 4.50 mmol), 1-fluoro-2,4-dinitrobenzene
(558 mg, 3.00 mmol), and dry CH3CN (5 mL). The mixture was stirred at
608C for 4 h. Then, the resulting salt and excess K2CO3 were filtered off,
and the filtrate was concentrated under reduced pressure. CH2Cl2
(15 mL) was added to the resulting solid, and the insoluble solid was fil-
tered off. The filtrate was concentrated under reduced pressure to give
2b as an orange powder (473 mg, 1.82 mmol, 61%). M.p. 106–1078C;
1H NMR (400.03 MHz, [D6]acetone): d=9.40 (s, 1H; NCHN), 9.05 (d,
4J=2.5 Hz, 1H; CH3-Ar), 8.88 (dd, 3J=8.8, 4J=2.5 Hz, 1H; CH5-Ar),
8.40 ppm (d, 3J=8.8 Hz, 1H; CH6-Ar); 13C{1H} NMR (100.60 MHz,
[D6]acetone): d=162.0, 149.5, 148.4, 141.4, 134.0, 130.5, 129.8, 122.5,
Conclusion
We have synthesised a series of RhI complexes bearing
novel N-DNP-substituted 1,2,4-triazol-3-ylidenes 3a–c. The
characterisation of the RhI complexes by spectroscopic, crys-
tallographic, and theoretical analyses revealed that NHC 3c
with a nitro group on the NHC framework exhibited notably
strong p-accepting and weak s-donating properties. In addi-
tion, we conclude that the AuI complex 6c bearing the
strongly p-accepting NHC 3c is a superior precatalyst in the
hydroalkoxylation of cyclohexene with 2-methoxyethanol on
the basis of the yields of adducts, the conversion of alcohol,
and reaction rates. An NBO analysis suggested that the high
catalytic activity of the AuI complex bearing 3c was due to
the enhanced p acidity of the Au atom resulting from re-
markably large p backdonation of 3c.
~
112.2 ppm; IR (KBr): n=3129.9, 2260.2, 1611.2, 1542.8, 1507.1, 1345.1,
1316.2, 1087.7, 983.5, 912.2, 741.5 cmꢀ1
HRMS (EI): m/z calcd for
C9H4N6O4+: 260.0294 [M+]; found: 260.0294.
1-(2,4-Dinitrophenyl)-3-nitro-1,2,4-triazole (2c):[20]
;
A
Schlenk flask
(50 mL) was charged with 3-nitro-1H-1,2,4-triazole (1c, 570 mg,
5.00 mmol), K2CO3 (1037 mg, 7.50 mmol), 1-fluoro-2,4-dinitrobenzene
(931 mg, 5.00 mmol), and dry CH3CN (10 mL). The mixture was stirred
at 608C for 6 h. Then, the resulting salt and excess K2CO3 were filtered
off, and the filtrate was concentrated under reduced pressure. CH2Cl2
(20 mL) was added to the resulting solid, and the insoluble solid was fil-
tered off. The filtrate was concentrated under reduced pressure to give
2c as a pale yellow powder (1308 mg, 4.67 mmol, 93%). M.p. 101–1028C;
1H NMR (400.03 MHz, [D6]acetone): d=9.32 (s, 1H; NCHN), 9.07 (d,
4J=2.5 Hz, 1H; CH3-Ar), 8.90 (dd, 3J=8.8, 4J=2.5 Hz, 1H; CH5-Ar),
8.44 ppm (d, 3J=8.8 Hz, 1H; CH6-Ar); 13C{1H} NMR (100.60 MHz,
[D6]acetone): d=164.6, 149.6, 148.6, 144.9, 134.1, 131.0, 130.0, 122.5 ppm;
Experimental Section
~
IR (KBr): n=1609.3, 1560.1, 1465.6, 1432.9, 1379.8, 1335.5, 1252.5,
1141.7, 836.0, 744.4 cmꢀ1
.
General information: All reactions were carried out under an N2 atmos-
1-(2,4-Dinitrophenyl)-4-methyl-1,2,4-triazol-4-ium triflate (3a·HOTf): In
a vial tube (5 mL), a mixture of 2a (470 mg, 2.00 mmol) and methyl tri-
flate (0.30 mL, 2.6 mmol) was stirred at room temperature for 15 h. To
the resulting mixture, diethyl ether (3 mL) was added, and the precipitate
was collected and washed with diethyl ether (2ꢂ5 mL). The solid was
dried under vacuum to give 3a·HOTf as a yellow powder (733 mg,
1.84 mmol, 92%). Single crystals suitable for X-ray diffraction analysis
were obtained by slow diffusion of hexane into an acetone solution of
3a·HOTf. M.p. 189–1908C; 1H NMR (400.03 MHz, [D6]DMSO): d=
10.86 (s, 1H; ArNCHN), 9.49 (s, 1H; ArNNCHN), 9.06 (d, 4J=2.4 Hz,
1H; CH3-Ar), 8.91 (dd, 3J=8.8, 4J=2.4 Hz, 1H; CH5-Ar), 8.25 (d, 3J=
8.8 Hz, 1H; CH6-Ar), 4.08 ppm (s, 3H; CH3); 13C{1H} NMR (100.60 MHz,
[D6]DMSO): d=148.9, 146.7, 145.8, 143.3, 131.3, 129.8, 129.5, 121.9, 120.7
~
phere by using a standard vacuum line, Schlenk tube techniques, and
1
dried and deoxygenated solvents. H (400.03 MHz) and 13C (100.60 MHz)
NMR spectra were recorded on a Bruker BioSpin AV400 NMR spec-
trometer. The 1H NMR chemical shifts are given in parts per million
(ppm) relative to tetramethylsilane at (d=0.00 ppm). The 13C{1H} NMR
spectra reported in ppm relative to the chemical shifts of residual undeu-
terated solvent signals for CDCl3 at 77.00 ppm, [D6]acetone at 29.84 ppm
(CD3), and [D6]DMSO at 39.52 ppm. High-resolution mass spectra were
obtained on a JEOL JMS-700 spectrometer at the Department of Instru-
mental Analysis of the Technical Division, School of Engineering,
Tohoku University and a Bruker Daltonics solariX 9.4T at the Research
and Analytical Center for Giant Molecules, Graduate School of Science,
Tohoku University. IR spectra were recorded on a JASCO FT/IR-350
Fourier-transform infrared spectrophotometer. X-ray analysis was carried
out on a Bruker AXS APEX II CCD diffractometer. GLC yields were
determined on a Shimadzu GC-2010 with a GL Sciences InertCap 5MS/
Sil column (L: 30 mꢂi.d.: 0.25 mm, df =0.25 mm) equipped with a flame
ionization detector and n-tetradecane as internal standard. GC-MS analy-
sis was performed on a Shimadzu PARVUM2. Preparative fractionations
were performed on a Shimadzu GC-8A with a packed column (20% SE-
(q, 1J
ACTHNUTRGNEUNG(C,F)=322.0 Hz), 34.9 ppm; IR (KBr): n=3106.8, 3055.7, 1614,
1591.0, 1550.5, 1356.7, 1260.3, 1226.5, 1156.1, 1029.8, 637.4 cmꢀ1; HRMS
+
(FAB): m/z calcd for C9H8N5O4
:
250.0576 [M+ꢀCF3SO3]; found:
250.0576; elemental analysis calcd (%) for C10H8F3N5O7S: C 30.08, H
2.02, N 17.54; found: C 29.98, H 2.01, N 17.45.
3-Cyano-1-(2,4-dinitrophenyl)-4-methyl-1,2,4-triazol-4-ium
triflate
(3b·HOTf): In a vial tube (5 mL), a mixture of 2b (130 mg, 0.500 mmol)
Chem. Eur. J. 2013, 19, 15710 – 15718
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
15715

T. Sato, S. Oi et al.
and methyl triflate (1.0 mL, 9.1 mmol) was treated with ultrasonic irradi-
ation for 1.5 h. The resulting precipitate was collected and washed with
CH2Cl2 (2ꢂ3 mL). The solid was dried under vacuum to give 3b·HOTf
as an off-white powder (116 mg, 0.274 mmol, 55%). Single crystals suita-
ble for X-ray diffraction analysis were obtained by slow cooling of a hot
CH3CN solution of 3b·HOTf. M.p. 191–1928C; 1H NMR (400.03 MHz,
[D6]acetone): d=11.02 (s, 1H; ArNCHN), 9.19 (d, 4J=2.4 Hz, 1H; CH3-
Ar), 9.01 (dd, 3J=8.7, 4J=2.4 Hz, 1H; CH5-Ar), 8.47 (d, 3J=8.7 Hz, 1H;
CH6-Ar), 4.57 ppm (s, 3H; CH3); 13C{1H} NMR (100.60 MHz,
[D6]acetone): d=151.1, 148.9, 144.8, 135.4, 132.7, 132.5, 130.8, 123.0,
~
a CH2Cl2 solution of 4b. M.p. 2018C (decomp); 1H NMR (400.03 MHz,
[D6]DMSO): d=9.27 (d, J=9.4 Hz, 1H; CH6-Ar), 9.03–8.97 (m, 2H; CH3-
3
Ar, CH5-Ar), 5.12–5.04 (m, 1H; CHCOD), 5.01–4.93 (m, 1H; CHCOD), 4.27
(s, 3H; CH3), 3.60–3.53 (m, 1H; CHCOD), 2.92–2.85 (m,1H; CHCOD),
2.47–2.26 (m, 2H; CH2COD), 2.14–1.78 (m, 4H; CH2COD), 1.74–1.62 ppm
(m, 2H; CH2COD); 13C{1H} NMR (100.60 MHz, [D6]DMSO): d=194.2 (d,
1J
ACTHNUTRGNE(UNG Rh,C)=52.2 Hz; NCN), 147.9, 143.7, 134.9, 133.8, 129.9, 128.1, 121.5,
106.9, 100.9 (d, 1J=7.0 Hz; CHCOD), 100.7 (d, 1J=6.8 Hz; CHCOD), 69.9
(d, 1J=13.7 Hz; CHCOD), 69.8 (d, 1J=13.3 Hz; CHCOD), 36.5, 32.4, 31.6,
~
28.5, 28.0 ppm; IR (KBr): n=2963.1, 2253.4, 1703.8, 1611.2, 1541.8,
121.9 (q, 1J
(C,F)=322.1 Hz), 106.0, 37.2 ppm; IR (KBr): n=3098.1,
1349.0, 1261.2, 1096.3, 801.3 cmꢀ1
; HRMS (FAB): m/z calcd for
2268.8, 1544.7, 1348.0, 1284.4, 1169.6, 1028.8, 635.4 cmꢀ1; HRMS (FAB):
m/z calcd for C10H7N6O4+: 275.0529 [M+ꢀCF3SO3]; found: 275.0532; ele-
mental analysis calcd (%) for C11H7F3N6O7S: C 31.14, H 1.66, N 19.81;
found: C 30.95, H 1.69, N 19.90.
C18H18ClN6O4Rh+: 520.0133 [M+]; found: 520.0130; elemental analysis
calcd (%) for C18H18ClN6O4Rh: C 41.52, H 3.48, N 16.14; found: C 41.19,
H 3.87, N 16.18.
Chloro(h4-cycloocta-1,5-diene)[2-(2,4-dinitrophenyl)-4-methyl-5-nitro-
1-(2,4-Dinitrophenyl)-4-methyl-3-nitro-1,2,4-triazol-4-ium
triflate
3H-1,2,4-triazol-3-ylidene]rhodium(I) (4c):
wrapped in aluminium foil was charged with 3c·HOTf (223 mg,
0.500 mmol), [{RhCl(cod)}2] (123 mg, 0.250 mmol), K2CO3 (691 g,
5.00 mmol), and dry toluene (5 mL). The reaction mixture was stirred at
room temperature for 3 d 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 chro-
matography (SiO2, CH2Cl2/acetone 50/1) and recrystallization from
A Schlenk tube (10 mL)
(3c·HOTf): In a vial tube (5 mL), a mixture of 2c (323 mg, 1.15 mmol)
and methyl triflate (1.00 mL, 9.1 mmol) was treated with ultrasonic irra-
diation for 3 h. The resulting precipitate was collected and washed with
CH2Cl2 (2ꢂ3 mL). The solid was dried under vacuum to give 3c·HOTf as
an orange powder (434 mg, 0.973 mmol, 84%). Single crystals suitable
for X-ray diffraction analysis were obtained by the slow cooling of a hot
CH3OH solution of 3c·HOTf. M.p. 158–1598C; 1H NMR (400.03 MHz,
[D6]acetone): d=10.96 (s, 1H; ArNCHN), 9.21 (d, 4J=2.4 Hz, 1H; CH3-
Ar), 9.01 (dd, 3J=8.7, 4J=2.4 Hz, 1H; CH5-Ar), 8.49 (d, 3J=8.7 Hz, 1H;
CH6-Ar), 4.65 ppm (s, 3H; CH3); 13C{1H} NMR (100.60 MHz,
[D6]acetone): d=153.1, 151.2, 150.4, 144.8, 132.8, 132.4, 130.9, 123.0,
~
AHCTUNGTRENNUNG
CH2Cl2/diethyl ether to give [RhClACTHNUTRGENN(UG 3c)ACHTUTGNREN(NUGN cod) (4c)] as an orange crystal
(228 mg, 0.422 mmol, 84%). Single crystals suitable for X-ray diffraction
analysis were obtained by slow diffusion of diethyl ether into a CH2Cl2
solution of 4c. M.p. 2078C (decomp); 1H NMR (400.03 MHz, CDCl3):
d=9.46 (d, 3J=8.7 Hz, 1H; CH6-Ar), 8.95 (d, 4J=2.5 Hz, 1H; CH3-Ar),
8.79 (dd, 3J=8.7, 4J=2.5 Hz, 1H; CH5-Ar), 5.34–5.20 (m, 2H; CHCOD),
4.66 (s, 3H; CH3), 3.57–3.50 (m, 1H; CHCOD), 3.03–2.96 (m,1H; CHCOD),
2.52–2.37 (m, 2H; CH2COD), 2.14–1.77 (m, 5H; CH2COD), 1.72–1.58 ppm
(m, 1H; CH2COD); 13C{1H} NMR (100.60 MHz, [D6]DMSO): d=198.1 (d,
121.7 (q, 1J
ACHTUNGTRENNUNG(C,F)=320.2 Hz), 39.4 ppm; IR (KBr): n=3089.4, 3014.2,
1616.1, 1573.6, 1464.7, 1350.9, 1292.1, 1252.5, 1226.5, 1150.3, 1029.8, 918.0,
841.8 cmꢀ1; HRMS (FAB): m/z calcd for C9H7N6O6+: 295.0427 [M+
ꢀCF3SO3]; found: 295.0425; elemental analysis calcd (%) for
C10H7F3N8O9S: C 27.04, H 1.59, N 18.92; found: C 26.69, H 1.81, N 19.28.
1J
101.3 (d, 1J
CHCOD), 70.0 (d, 1J
13.6 Hz; CHCOD), 38.7, 32.5, 31.5, 28.6, 27.9 ppm; IR (KBr): n=3053,
ACHTUNGTRENNUNG
Chloro(h4-cycloocta-1,5-diene)[2-(2,4-dinitrophenyl)-4-methyl-3H-1,2,4-
triazol-3-ylidene]rhodium(I) (4a): A Schlenk tube (10 mL) wrapped in
aluminium foil was charged with 3a·HOTf (399 mg, 1.00 mmol), [{RhCl-
G
ACHTUNGTRENNUNG
E
ACHTUNGTRENNUNG
~
ACHTUNGTRENNUNG(cod)}2] (247 mg, 0.500 mmol), K2CO3 (1.38 g, 10.0 mmol), and dry tolu-
2877, 1608, 1578, 1544, 1345 cmꢀ1
; HRMS (FAB): m/z calcd for
ene (5 mL). The reaction mixture was stirred at room temperature for
3 d in the dark. After the reaction, the resulting salt and excess K2CO3
were filtered off, and the filtrate was concentrated under reduced pres-
C17H18ClN6O6Rh+: 540.0031 [M+]; found: 540.0019; elemental analysis
calcd (%) for C17H18ClN6O6Rh: C 37.76, H 3.36, N 15.54; found: C 37.54,
H 3.32, N 15.55.
sure. The solid was dried under vacuum to give [RhClACTHNUTRGENN(GU 3a)ACHUTNGTREN(NNGU cod)] (4a) as
an orange powder (495 mg, 0.999 mmol, 99.9%). Single crystals suitable
for X-ray diffraction analysis were obtained by slow cooling of a hot
CH3CN solution of 4a. M.p. 2048C (decomp); 1H NMR (400.03 MHz,
Chlorodicarbonyl[2-(2,4-dinitrophenyl)-4-methyl-3H-1,2,4-triazol-3-ylide-
ne]rhodium(I) (5a): CO gas was bubbled through a solution of 4a
(24.8 mg, 0.0500 mmol) in dry CH2Cl2 (5 mL) for IR analysis or in
[D6]acetone (1 mL) for NMR analysis at room temperature for 30 min.
The initial brown solution became a light brown solution of [RhCl-
3
[D6]DMSO): d=9.34 (d, J=8.6 Hz, 1H; CH6-Ar), 9.00–8.93 (m, 3H; CH3-
,
Ar
CH5-Ar, NCHN), 5.10–5.02 (m, 1H; CHCOD), 4.96–4.87 (m, 1H;
CHCOD), 4.13 (s, 3H; CH3), 3.48–3.42 (m, 1H; CHCOD), 2.89–2.80 (m,1H;
CHCOD), 2.44–2.26 (m, 2H; CH2COD), 2.15–2.03 (m, 1H; CH2COD), 1.96–
ACHTUNGTRNE(NUGN 3a)(CO)2] (5a) (quantitative yield). The yield was determined by
1H NMR spectroscopy based on COD. When the solution of 5a was con-
centrated, 5a decomposed. 1H NMR (400.03 MHz, [D6]acetone): d=9.01
(s, 1H; NCHN), 8.99 (d, 4J=2.5 Hz, 1H; CH3-Ar), 8.87 (dd, 3J=8.7, 4J=
2.5 Hz, 1H; CH5-Ar), 8.54 (d, 3J=8.7 Hz, 1H; CH6-Ar), 4.19 ppm (s, 3H;
1.78 (m, 3H; CH2COD), 1.74–1.60 ppm (m, 2H; CH2COD); 13C{1H} NMR
1
(100.60 MHz, [D6]DMSO): d=190.3 (d, J
A
146.5, 143.9, 136.0, 129.2, 127.6, 121.1, 99.5 (d, 1J
CHCOD), 99.3 (d, 1J(Rh,C)=6.2 Hz; CHCOD), 69.2 (d, 1J
CHCOD), 68.8 (d, 1J
ACHTUNGTRENNUNG
G
E
CH3); 13C{1H} NMR (100.60 MHz, [D6]acetone): d=186.2 (d, 1J
55.6 Hz; CO), 182.7 (d, 1J(Rh,C)=44.2 Hz; NCN), 182.2 (d, 1J
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
N
G
~
27.9 ppm; IR (KBr): n=3121.2, 2987.2, 2874.4, 2825.2, 1609.3, 1543.7,
1505.2, 1425.1, 1349.0, 1262.2, 746.3 cmꢀ1; HRMS (FAB): m/z calcd for
C17H19ClN5O4Rh+: 495.0181 [M+]; found: 495.0183; elemental analysis
calcd (%) for C17H19ClN5O4Rh: C 41.19, H 3.86, N 14.13; found: C 40.86,
H 3.96, N 14.20.
76.0 Hz; CO), 149.3, 147.5, 146.0, 137.2, 132.8, 129.1, 122.1, 36.8 ppm; IR
(CH2Cl2): n=2093.4 (CO), 2015.3 (CO), 1701.9 cmꢀ1; HRMS (ESI): m/z
~
calcd for C11H7ClN5NaO6Rh+Na+: 465.90321 [M+Na+]; found:
465.90325.
Chlorodicarbonyl[5-cyano-2-(2,4-dinitrophenyl)-4-methyl-3H-1,2,4-tria-
zol-3-ylidene]rhodium(I) (5b): CO gas was bubbled through a solution of
4a (26.1 mg, 0.0500 mmol) in dry CH2Cl2 (5 mL) for IR analysis or in
[D6]acetone (1 mL) for NMR analysis at room temperature for 30 min.
Chloro(h4-cycloocta-1,5-diene)[5-cyano-2-(2,4-dinitrophenyl)-4-methyl-
3H-1,2,4-triazol-3-ylidene]rhodium(I) (4b):
wrapped in aluminium foil was charged with 3b·HOTf (106 mg,
0.250 mmol), [{RhCl(cod)}2] (61.6 mg, 0.125 mmol), K2CO3 (346 g,
2.50 mmol), and dry toluene (3 mL). The reaction mixture was stirred at
room temperature for 3 d in the dark. Then, 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
A Schlenk tube (10 mL)
G
The initial orange solution became a yellow solution of [RhClACHTNGUTERNNU(G 3b)(CO)2]
1
(5b) (quantitative yield). The yield was determined by H NMR spectros-
copy based on COD. When the solution of 5b was concentrated, 5b de-
composed. 1H NMR (400.03 MHz, [D6]acetone): d=9.08 (d, 4J=2.5 Hz,
1H; CH3-Ar), 8.92 (dd, 3J=8.7, 4J=2.5 Hz, 1H; CH5-Ar), 8.47 (d, 3J=
8.7 Hz, 1H; CH6-Ar), 4.39 ppm (s, 3H; CH3); 13C{1H} NMR (100.60 MHz,
(SiO2, CH2Cl2/acetone 30/1) to give [RhClACTHNUTRGNENGU(3b)HCATUNGTRENN(UGN cod)] (4b) as a brown
powder (85.5 mg, 0.164 mmol, 66%). Single crystals suitable for X-ray
diffraction analysis were obtained by slow diffusion of diethyl ether into
[D6]acetone): d=186.9 (d, 1J
AHCTUNGTRENNUNG
(Rh,C)=45.2 Hz; NCN), 185.6 (d, 1J-
ACHTUNGTRENNUNG
(Rh,C)=56.4 Hz; CO), 181.7 (d, 1J=71.7 Hz; CO), 150.1, 145.8, 136.2,
15716
www.chemeurj.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 15710 – 15718

Strongly p-Accepting N-Heterocyclic Carbenes
FULL PAPER
~
135.3, 133.5, 129.7, 122.5, 107.3, 37.8 ppm; IR (CH2Cl2): n=2099.1 (CO),
2021.0 cmꢀ1 (CO); HRMS (ESI): m/z calcd for C12H6ClN6NaO6Rh+Na+
: 490.89846 [M+Na+]; found: 490.89848.
added to the mixture. The vial tube was sealed and stirred at 1008C for
20 h. After the reaction, the mixture was cooled to room temperature.
Then, n-tetradecane was added to the mixture as an internal standard.
The yield of the resulting (2-methoxyethoxy)cyclohexane (9)[15] was de-
termined by GLC analysis.
Chlorodicarbonyl[2-(2,4-dinitrophenyl)-4-methyl-5-nitro-3H-1,2,4-triazol-
3-ylidene]rhodium(I) (5c): CO gas was bubbled through a solution of 4c
(27.0 mg, 0.0500 mmol) in dry CH2Cl2 (5 mL) for IR analysis or in
[D6]acetone (1 mL) for NMR analysis at room temperature for 30 min.
General procedure for Au-catalysed intermolecular hydroalkoxylation of
1-octene with 1-octanol: A septum-cap vial tube (2 mL) wrapped in alu-
minium foil was charged with Au complex (0.0250 mmol), AgOTf
(6.4 mg, 0.025 mmol), and benzotrifluoride (0.5 mL), and the mixture was
stirred for 5 min. 1-Octene (561 mg, 5.00 mmol) and 1-octanol (65.1 mg,
0.500 mmol) were added to the mixture. The vial tube was sealed and
stirred at 1108C for 20 h. Then, the mixture was cooled to room tempera-
ture, and n-tetradecane was added to the mixture as an internal standard.
The yields of the resulting 1-(octan-2-yloxy)octane[21] and 1-(octyloxy)oc-
tane[21] and molar ratios of products were determined by GLC analysis.
The initial brown solution became a yellow solution of [RhClACHTNUTRGNEUNG(3c)(CO)2]
1
(5c) (quantitative yield). The yield was determined by H NMR spectros-
copy based on COD. When the solution of 5c was concentrated, 5c de-
composed. 1H NMR (400.03 MHz, [D6]acetone): d=9.11 (d, 4J=2.5 Hz,
1H; CH3-Ar), 8.94 (dd, 3J=8.7, 4J=2.5 Hz, 1H; CH5-Ar), 8.47 (d, 3J=
8.7 Hz, 1H; CH6-Ar), 4.56 ppm (s, 3H; CH3); 13C{1H} NMR (100.60 MHz,
[D6]acetone): d=190.6 (d, 1J
(Rh,C)=56.1 Hz; CO), 181.7 (d, 1J
145.8, 136.4, 133.8, 129.8, 122.6, 40.0 ppm; IR (CH2Cl2): n=3053.7,
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
~
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 MoKa radiation (l=0.71073 ꢁ). The data
were corrected for Lorentzian and polarization effects. Data integration
and reduction were performed with SAINT and XPREP software.[22]
Multiscan absorption corrections were applied by using the semiempirical
method with SADABS.[23] The structures were solved by direct methods
with SHELXS-97[24] and refined by full-matrix, least-squares methods on
F2 with SHELXL-97[24] and Yadokari-XG 2009.[25] Selected data and
structure refinement for all relevant compounds are summarised in the
Supporting Information (Tables S3–S5). CCDC 886836 (3a·HOTf),
886835 (3b·HOTf), 886837 (3c·HOTf), 886834 (4a·acetonitrile), 886832
(4b), 886833 (4c), 927996 (6a·acetone), and 895568 (6c) contain the sup-
plementary crystallographic data for this paper. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif.
2985.3, 2100.1 (CO), 2022.0 (CO), 1548.6, 1422.2, 1346.1, 895.8 cmꢀ1
;
HRMS (ESI): m/z calcd for C11H6ClN6NaO8Rh+Na+: 510.88829
[M+Na+]; found: 510.88830.
Chloro[2-(2,4-dinitrophenyl)-4-methyl-3H-1,2,4-triazol-3-ylidene]gold(I)
(6a): A Schlenk tube (10 mL) wrapped in aluminium foil was charged
with 3a·HOTf (1.20 g, 3.00 mmol), AuClACTHNUTRGNEUNG(SMe2) (884 mg, 3.00 mmol),
K2CO3 (4.56 mg, 33.0 mmol), and dry toluene (30 mL). The reaction mix-
ture was stirred at room temperature for 44 h in the dark. Then the re-
sulting salt and excess K2CO3 were filtered off, and the solid was washed
with hexane. The solid was suspended in acetone (30 mL). The solid was
filtered off, and the filtrate was concentrated under reduced pressure.
The resulting solid was washed with methanol (10 mLꢂ3) and the solid
was dried in vacuo to give [AuClACHTNUTRGNE(UNG 3a)] (6a) as a white powder (453 mg,
0.941 mmol, 31%). Single crystals suitable for X-ray diffraction analysis
were obtained by slow cooling of an acetone solution of 6a. M.p. 2018C
(decomp); 1H NMR (400.03 MHz, [D6]DMSO): d=9.23 (s, 1H; NCHN),
8.97 (d, 4J=2.5 Hz, 1H; CH3-Ar), 8.82 (dd, 3J=8.7, 4J=2.5 Hz, 1H; CH5-
Ar), 8.40 (d, 3J=8.7 Hz, 1H; CH6-Ar), 3.95 ppm (s, 3H; CH3); 13C{1H}
NMR (100.60 MHz, [D6]DMSO): d=175.1, 148.2, 146.5, 144.5, 135.0,
Computational analysis: All calculations were performed at the M06[26]
/
cc-pVTZ[27]/LanL2TZ(f)[28](Rh) level with the Gaussian 09 program pack-
age[29] and NBO analysis was carried out with the NBO version 3.1 pro-
gram.[30] For details of the calculations, see the Supporting Information.
~
130.5, 129.1, 121.7, 35.9 ppm; IR (KBr): n=3144.4, 3094.2, 1613.2, 1544.7,
1349.9, 977.7, 854.3, 740.5 cmꢀ1
;
HRMS (ESI): m/z calcd for
C9H7N5O4AuCl+Na+: 503.9750 [M+Na+]; found: 503.9745; elemental
analysis calcd (%) for C9H7AuClN5O4: C 22.45, H 1.47, N 14.54; found:
C 22.73, H 1.76, N 14.28.
Chloro[2-(2,4-dinitrophenyl)-4-methyl-5-nitro-3H-1,2,4-triazol-3-ylidene]-
gold(I) (6c): A Schlenk tube (10 mL) wrapped in aluminium foil was
Acknowledgements
charged with 3c·HOTf (199 mg, 0.446 mmol), AuClACTHNUGTRENUNG(SMe2) (132 mg,
This work was supported in part by a Grant-in-Aid for Scientific Re-
search on Innovative Areas “Molecular Activation Directed toward
Straightforward Synthesis” from MEXT, Japan and by a commissioned
project conducted by New Energy and Industrial Technology Develop-
ment Organization (NEDO). We thank Prof. K. Itaya (Tohoku Universi-
ty) 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 resour-
ces at the Cyberscience Center, Tohoku University (Japan).
0.446 mmol), K2CO3 (617 mg, 4.46 mmol), and dry toluene (10 mL). The
reaction mixture was stirred at room temperature for 70 h in the dark.
After the reaction, the resulting salt and excess K2CO3 were filtered off,
and the solid was washed with hexane. Then, the solid was suspended in
acetone (15 mL). The insoluble solid was filtered off, and the filtrate was
concentrated under reduced pressure. The yellow solid was washed with
methanol (5 mLꢂ3) and dried in vacuo. The resulting solid was purified
by recrystallization from acetone/hexane to give [AuClACTHNUTRGNEUG(N 3c)] (6c) as
a pale yellow crystal (114 mg, 0.216 mmol, 48%). Single crystals suitable
for X-ray diffraction analysis were obtained by slow cooling of an ace-
tone solution of 6c. M.p. 2058C (dec); 1H NMR (400.03 MHz,
[D6]DMSO): d=9.04 (d, 4J=2.5 Hz, 1H; CH3-Ar), 8.86 (dd, 3J=8.7, 4J=
2.5 Hz, 1H; CH5-Ar), 8.34 (d, 3J=8.7 Hz, 1H; CH6-Ar), 4.22 ppm (s, 3H;
CH3); 13C{1H} NMR (100.60 MHz, [D6]DMSO): d=181.5, 153.0, 148.9,
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144.2, 133.9, 131.4, 129.8, 122.1, 30.7 ppm; IR (KBr): n=3115.4, 1611.2,
1582.3, 1540.8, 1458.9, 1441.5, 1347.0, 1331.6, 915.1, 848.5, 742.5 cmꢀ1
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General procedure for Au-catalysed intermolecular hydroalkoxylation of
cyclohexene (8) with 2-methoxyethanol (7): A septum-cap vial tube
(2 mL) wrapped in aluminium foil was charged with Au complex
(0.0125 mmol), AgNTf2 (4.9 mg, 0.0125 mmol), and chlorobenzene
(0.25 mL), and the mixture was stirred for 5 min. Cyclohexene (8,
205 mg, 2.50 mmol) and 2-methoxyethanol (7, 19.0 mg, 0.250 mmol) were
Chem. Eur. J. 2013, 19, 15710 – 15718
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
15717

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Chem. Eur. J. 2013, 19, 15710 – 15718