Synthesis of Rh(I) complexes bearing N-p-toluenesulfonyl-substituted N-heterocyclic carbenes

Article abstract of DOI:10.1246/cl.130979

We have synthesized Rh(I) complexes bearing a 1,2,4-triazol-3-ylidene or imidazol-2-ylidene substituted N-p-toluene-sulfonyl (N-Ts) group. Structural characterizations of the Rh(I) complexes revealed that, because of its mesomeric and inductive electron-w

Full text of DOI:10.1246/cl.130979

Received: October 21, 2013 | Accepted: November 7, 2013 | Web Released: November 13, 2013
CL-130979
Synthesis of Rh(I) Complexes Bearing N-p-Toluenesulfonyl-substituted
N-Heterocyclic Carbenes
Tetsuo Sato,*1,2 Daisuke Yoshioka,1 Yoichi Hirose,1 and Shuichi Oi*1,2
1Department of Applied Chemistry, Graduate School of Engineering, Tohoku University,
6-6-11 Aramaki-Aoba, Aoba-ku, Sendai, Miyagi 980-8579
2Environment Conservation Research Institute, Tohoku University,
6-6-11 Aramaki-Aoba, Aoba-ku, Sendai, Miyagi 980-8579
(E-mail: tetsuo@aporg.che.tohoku.ac.jp)
We have synthesized Rh(I) complexes bearing a 1,2,4-
triazol-3-ylidene or imidazol-2-ylidene substituted N-p-toluene-
sulfonyl (N-Ts) group. Structural characterizations of the Rh(I)
complexes revealed that, because of its mesomeric and inductive
electron-withdrawing effects, the N-Ts substituent increased
π-accepting ability and decreased σ-donating ability of carbenes
compared to N-mesityl substituents.
in quantitative yields by bubbling CO gas through their solutions
in CH2Cl2 or CDCl3.
To evaluate the electronic properties of N-Ts-substituted
NHCs 2a and 2b, we compared the properties of the Rh(I)
complexes 3a and 3b, and 4a and 4b with those of correspond-
ing Rh(I) complexes bearing 1,3-bis(2,4,6-trimethylphenyl)imi-
dazol-2-ylidene (IMes) 3c and 4c. First, we investigated σ-
donating ability of the NHCs from the infrared (IR) spectra of
complexes 4a-4c (Table 1 and Figure 2). Average CO stretching
frequencies of 4a-4c (¯av(CO)) increased in the order: 4c
(2037.0 cm¹1) < 4b (2046.1 cm¹1) < 4a (2050.5 cm¹1); these
¯av(CO) could be converted to Tolman electronic parameter
N-Heterocyclic carbenes (NHCs) have attracted great
attention as organometallic ligands1 and organocatalysts2 owing
to their strong σ-donating ability and high flexibility in the
molecular design. At the same time, electron-deficient NHCs
with increased π-accepting ability have also been developed in
the last decade.3 Recently, we have reported N-2,4-dinitrophenyl
(N-DNP)-substituted 1,2,4-triazol-3-ylidene A4 and imidazol-2-
ylidene B5 with stronger π-accepting ability than typical NHCs
(Figure 1). However, the electronic properties of the DNP
substituent arise exclusively from inductive electron-withdraw-
ing effect, because it can hardly conjugate with the NHC
framework. Herein, we describe the synthesis and structural
characterizations of Rh(I) complexes bearing N-p-toluenesul-
fonyl (N-Ts)-substituted 1,2,4-triazol-3-ylidene and imidazol-2-
ylidene. Because of its mesomeric effect, the N-Ts group is
expected to conjugate with the NHC framework and act as an
effective electron-withdrawing group.
¹1
(TEP) values as described in the literature, i.e., 2050 cm for
¹1
¹1
4c, 2057 cm for 4b, and 2061 cm for 4a. Thus, NHC 2a
showed the weakest σ-donating ability among the examined
NHCs.
The incorporation of electronegative elements to the NHC
framework decreases the σ-donating ability of carbenes. Meth-
ylation of 1-(p-toluenesulfonyl)-1,2,4-triazole (1a) and 1-(p-
toluenesulfonyl)imidazole (1b) with methyl triflate at 0 °C (2 h
for 1a and 30 min for 1b) afforded azolium salts 2a¢HOTf in
24% yield and 2b¢HOTf in 95% yield (Scheme 1). Reaction of
[{RhCl(cod)}2] (cod: 1,5-cyclooctadiene) with 2a¢HOTf and
2b¢HOTf in the presence of sodium bis(trimethylsilyl)amide
(NaHMDS) afforded Rh(I)-cod complex [RhCl(2a)(cod)] (3a)
in 14% and [RhCl(2b)(cod)] (3b) in 81% yield, respectively,
with in situ generation of free NHC 2a and 2b. Furthermore,
3a and 3b were converted to Rh(I)-carbonyl complexes
[RhCl(2a)(CO)2] (4a) and [RhCl(2b)(CO)2] (4b), respectively,
Scheme 1. Syntheses of Rh(I)-NHC complexes; reaction
conditions: (i) MeOTf, CH2Cl2, 0 °C; (ii) [{RhCl(cod)}2],
NaHMDS, ¹78 °C to rt, 12 h; (iii) CO gas (bubbling), CH2Cl2
or CD3Cl, rt, 30 min (NaHMDS: sodium bis(trimethylsilyl)-
amide; NHC: N-heterocyclic carbene; N-Ts: N-p-toluenesulfo-
nyl).
Table 1. IR ¯(CO) stretching frequencies of [RhCl(L)(CO)2]
(4a-4c) in CH2Cl2 and TEP values
Complex
¯ (CO)a
¯av(CO)b
TEPc
4a (L = 2a)
4b (L = 2b)
4c (L = IMes)
2012.4, 2088.5
2008.5, 2083.7
1995.0, 2078.9
2050.5
2046.1
2037.0
2061
2057
2050
aIR CO stretching frequencies in cm¹1 (CH2Cl2). bThe average
¹1
of ¯(CO) in cm¹1. cTolman electronic parameter (TEP) in cm
calculated using the equation TEP (cm¹1) = 0.8001¯av(CO)
(cm¹1) + 420.0 (cm¹1).1b
Figure 1. N-DNP-substituted N-heterocyclic carbene.
Chem. Lett. 2014, 43, 325–327 | doi:10.1246/cl.130979
© 2014 The Chemical Society of Japan | 325

Figure 2. Comparison of the ¯av(CO) values of [RhCl-
(NHC)(CO)2] 4a-4c.
Table 2. 13C NMR chemical shifts of carbenic carbons and
carbonyls in [RhCl(L)(cod)] 3a-3c in CDCl3
Figure 3. ORTEP representation of 3a with thermal ellipsoids
at 50% probability. Hydrogen atoms are omitted for clarity. One
of the two crystallographically independent molecules is shown.
Complex
13CN2/ppm
trans-13CHCOD/ppm
3a (L = 2a)
3b (L = 2b)
3c (L = IMes)6
197.6
191.2
183.6
99.3, 99.7
98.0, 98.3
96.3
Next, the 13C NMR spectra of Rh(I)-cod complexes 3a-3c
were examined to support the IR spectral analysis (Table 2). As
the TEP values of NHCs increased, both the signals of carbenic
carbons and alkene moieties of cod trans to the NHC ligands
shifted downfield. This deshielding can be explained by (i) the
decrease in the N ¼ Ccarbene π-overlaps caused by the N-Ts
substituent and incorporated N atoms into the NHC frameworks
and (ii) the decrease in the π-backbonding interactions from the
electron-deficient metal centers to the alkene moieties of cod.3b,7
Moreover, single-crystal X-ray diffraction analyses of 3a-3c
also supported the aforementioned trend between the TEP values
of the NHCs and their π-accepting ability. ORTEP representa-
tions of 3a and 3b are shown in Figures 38 and 4,9 respectively;
Figure 3 shows one of the two crystallographically independent
molecules. Based on the increase in the TEP values, it can be
observed that the Rh-Ccarbene crystallographic distance shortened
in the order: 3c (2.0494(16) ¡)10 > 3b (2.010(2) ¡) > 3a (aver-
age 1.997 ¡). It can be construed that the contraction of Rh-
Figure 4. ORTEP representation of 3b with thermal ellipsoids
at 50% probability. Hydrogen atoms are omitted for clarity.
Table 3. Summary of computational analysis of NHCs
c
NHC
Eσ/eVa
Eπ/eVb
¤
C
¹8.19
(HOMO¹4)
¹7.86
(HOMO¹4)
¹7.47
¹1.68
(LUMO)
¹1.41
(LUMO)
¹1.17
2a
0.147
0.131
0.122
2b
C
carbene bond distance is caused by the increase in the bond order
because of the Rh ¼ Ccarbene π-backbonding interaction. Nota-
bly, the NHC frameworks could be conjugated with one of the
S=O bonds in the sulfonyl group; the dihedral angle between
the NHC framework and the S=O bond for 3a is either
12.29(16) or 11.42(17)°, whereas that for 3b is 15.8(3)°.
Incidentally, the dihedral angle between the NHC framework
and DNP ring for [RhCl(B)(cod)] is 136.78(19)°.5 Therefore, it
is assumed that the N-Ts substituent acts as electron-withdraw-
ing group because of both inductive and mesomeric effects.
To evaluate the electronic properties of NHCs, the energies
of σ-donor orbital (Eσ) and π-acceptor orbital (Eπ) and natural
atomic charges of the carbenic carbon atoms (¤C) were
calculated (Table 3).11 Eσ is the energy of an occupied carbene
lone pair and Eπ is the energy of a vacant p orbital on the carbene
carbon atom. Both the values of Eσ and Eπ decreased with
increasing the TEP values, indicating that the σ-donating ability
of the NHCs decreased, whereas the π-accepting ability increas-
ed. Furthermore, the ¤C values positively increased with
increasing downfield shifts of the 13C NMR signals.
IMes
(HOMO¹8)
(LUMO)
b
aEnergies of σ-donor orbital. Energies of π-acceptor orbital.
cNatural atomic charges of carbenic carbon atoms.
ses of [RhCl(2)(cod)] 3 and [RhCl(2)(CO)2] 4 revealed that the
σ-donating ability of the NHCs decreased and the π-accepting
ability increased in the order IMes, 2b, and 2a. Furthermore, X-
ray diffraction analysis made it obvious that the N-Ts substituent
could utilize its mesomeric electron-withdrawing effect.12
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).
In conclusion, we synthesized novel Rh(I) complexes
coordinated to N-Ts-substituted 1,2,4-triazol-3-ylidene 2a and
imidazol-2-ylidene 2b. Experimental and computational analy-
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8
Selected crystallographic data of 3a. C18H23ClN3O2RhS,
Mr = 483.81, T = 120(2) K, triclinic, P1, a = 11.629(14) ¡,
ꢀ
b = 13.037(15) ¡, c = 13.720(16) ¡, ¡ = 67.309(12)°, ¢ =
81.675(18)°, £ = 87.45(3)°, V = 1899(4) ¡3, Z = 4, Dcalcd
=
1.693 g cm¹3, F000 = 984, ®(Mo Kα) = 1.168 mm¹1, 21168
reflections collected, 8493 unique (Rint = 0.0199), GOF =
1.063, R1 = 0.0206 (I > 2σ(I)) and wR2 = 0.0530 (all data).
CCDC 943330 contains the supplementary crystallographic
data for this paper. This data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via http://
www.ccdc.cam.ac.uk/data_request/cif.
9
Selected crystallographic data of 3b. C19H24ClN2O2RhS,
Mr = 482.82, T = 100(2) K, monoclinic, Cc, a = 10.105(2)
¡, b = 17.149(4) ¡, c = 11.381(3) ¡, ¡ = 90°, ¢ =
96.918(3)°, £ = 90°, V = 1957.9(8) ¡3, Z = 4, Dcalcd
=
1.638 g cm¹3, F000 = 984, ®(Mo Kα) = 1.131 mm¹1, 5354
reflections collected, 3019 unique (Rint = 0.0170), GOF =
1.057, R1 = 0.0183 (I > 2σ(I)) and wR2 = 0.0481 (all data).
CCDC 943329.
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Commun. 2005, 63.
11 All calculations were performed at the M06/cc-pVTZ level
using the Gaussian 09 program package and NBO analysis
was carried out with the NBO version 3.1 program. For
details of the calculations see the Supporting Information.
12 Supporting Information is available electronically on the
CSJ-Journal Web site, http://www.csj.jp/journals/chem-lett/
index.html.
Chem. Lett. 2014, 43, 325–327 | doi:10.1246/cl.130979
© 2014 The Chemical Society of Japan | 327