Symmetric and dissymmetric N-heterocyclic carbene rhodium(I) complexes: A comparative study of their catalytic activities in transfer hydrogenation reaction

Article abstract of DOI:10.1002/aoc.2857

Six new [RhBr(NHC)(cod)] (NHC=N-heterocyclic carbene; cod=1,5- cyclooctadiene) type rhodium complexes (4-6) have been prepared by the reaction of [Rh(μ-OMe)(cod)]₂ with a series of corresponding imidazoli(in)ium bromides (1-3) bearing mesityl (Mes) or 2,4,6-trimethylbenzyl (CH₂Mes) substituents at N¹ and N³ positions. They have been fully characterized by ¹H, ¹³C and heteronuclear multiple quantum correlation NMR analyses, elemental analysis and mass spectroscopy. Complexes of type [(NHC)RhBr(CO)₂] (NHC=imidazol-2-ylidene) (7b-9b) were also synthesized to compare σ-donor/π-acceptor strength of NHC ligands. Transfer hydrogenation (TH) reaction of acetophenone has been comparatively studied by using complexes 4-6 as catalysts. The symmetrically CH₂Mes-substituted rhodium complex bearing a saturated NHC ligand (5a) showed the highest catalytic activity for TH reaction.

Full text of DOI:10.1002/aoc.2857

Full Paper
Received: 20 January 2012
Revised: 1 March 2012
Accepted: 10 March 2012
Published online in Wiley Online Library: 12 April 2012
(wileyonlinelibrary.com) DOI 10.1002/aoc.2857
Symmetric and dissymmetric N-heterocyclic
carbene rhodium(I) complexes: a comparative
study of their catalytic activities in transfer
hydrogenation reaction
Süleyman Gülcemal*
Six new [RhBr(NHC)(cod)] (NHC = N-heterocyclic carbene; cod = 1,5-cyclooctadiene) type rhodium complexes (4–6) have been
prepared by the reaction of [Rh(m-OMe)(cod)]2 with a series of corresponding imidazoli(in)ium bromides (1–3) bearing mesityl
(Mes) or 2,4,6-trimethylbenzyl (CH2Mes) substituents at N1 and N3 positions. They have been fully characterized by 1 H, 13 C
and heteronuclear multiple quantum correlation NMR analyses, elemental analysis and mass spectroscopy. Complexes of type
[(NHC)RhBr(CO)2] (NHC = imidazol-2-ylidene) (7b–9b) were also synthesized to compare s-donor/p-acceptor strength of NHC
ligands. Transfer hydrogenation (TH) reaction of acetophenone has been comparatively studied by using complexes 4–6 as
catalysts. The symmetrically CH2Mes-substituted rhodium complex bearing a saturated NHC ligand (5a) showed the highest
catalytic activity for TH reaction. Copyright © 2012 John Wiley & Sons, Ltd.
Keywords: N-heterocyclic carbene; rhodium; transfer hydrogenation
was found that steric and electronic effects influence the catalytic
Introduction
activity considerably.
Nolan’s group reported that whereas [(IMes)Ir(py)(cod)]+PF6À
Rhodium complexes of N-heterocyclic carbene (NHC) ligands have
been the focus of intense research in organometallic chemistry
and homogeneous catalysis,[1] after the first efficient catalytic appli-
cations of these complexes reported by Lappert and Maskell.[2] The
electronic and steric parameters of NHC complexes can be
modified easily, and they have greater stability toward air, moisture
and heating when compared with phosphine analogues.[3] NHCs
are strongly bound to the metal, which avoids decomposition to
free and inactive metal under catalytic conditions in many cases.[4]
Thus NHC–rhodium complexes have been employed as catalysts
for number of catalytic applications such as hydrogenation, transfer
hydrogenation (TH), hydrosilylation and hydroformylation.[5]
NHCs exhibit good s-donor and weak p-acceptor electronic
properties.[6] Measurement of carbonyl stretching frequencies
in [(NHC)Rh(X)(CO)2] (X = halogen) complexes with infrared
(IR) spectroscopy can be used to compare donor properties
of NHC ligands, which are important for homogeneous
catalysis.[7] These electronic properties can be modified by
changing the substituents at the 4,5-position of imidazol(in)e
backbone or by adding different substituents on nitrogen
atoms of heterocycle.[8]
In 2009, Herrmann’s group prepared [(NHC)Ir(X)(cod)] (cod= 1,
5-cyclooctadiene) type iridium(I) complexes and investigated the
effect of different NHC ligands on catalytic activity in the TH reac-
tion.[9] They observed that weaker donor strength of the NHC
ligand seems to increase the complex activity by both shortening
the initiation time and accelerating the catalytic reaction when
acetophenone is the substrate. However, no significant difference
was observed when the NHC was 1,3-dimethylimidazol-2-ylidene
or 1,3-dimethylimidazolin-2-ylidene (IR data confirm the donor
strength of these two ligands being quite similar). As a result, it
complex requires 4.5 h for total conversion of cyclohexanone to
cyclohexyl alcohol, saturated [(SIMes)Ir(py)(cod)]+PF6À complex
requires a longer reaction time (6 h).[10]
Practically almost all catalytic studies have been devoted to satu-
rated imidazolin-2-ylidene and unsaturated imidazol-2-ylidene
bearing symmetric aryl or benzyl substituents, while related
dissymmetrically substituted imidazol(in)-2-ylidenes have not
received sufficient attention.[11] Therefore, a comparative experi-
mental study of NHCs with Mes and CH2Mes substituents on the
N atoms is presented.
Results and Discussion
The NHC ligand precursors used in this study fall into three
general types, featuring different substituents: symmetric N1,
N3-dimesityl (Scheme 1), N1,N3-dibenzyl and dissymmetric
N1-benzyl-N3-mesityl (Scheme 2) on the N1 and N3 atoms.
The salt 2a was synthesized using a published procedure with
slight modification (Scheme 2, route i).[12] Its unsaturated analogue
2b was prepared by addition of 2 equiv. of 2,4,6-trimethylbenzyl
bromide to a solution of imidazole in DMF without additional base
*
Correspondence to: Süleyman Gülcemal, Department of Chemistry, Ege Univer-
sity, İzmir, Turkey. E-mail: suleyman.gulcemal@ege.edu.tr
Dedicated to Prof. Bekir Çetinkaya for his great contribution in the field of
N-heterocyclic carbene chemistry since 1971.
Department of Chemistry, Ege University, İzmir, Turkey
Appl. Organometal. Chem. 2012, 26, 246–251
Copyright © 2012 John Wiley & Sons, Ltd.

NHC-Rh-catalyzed transfer hydrogenation
Scheme 1. Saturated (a) and unsaturated (b) dimesitylimidzol(in)ium
salts.
(Scheme 2, route ii). In the 1 H NMR spectra the NCHN+ protons of
2a and 2b appear at 9.30 and 9.70 ppm respectively. 13 C NMR shifts
of NCHN+ appear at 157.7 and 140.0 ppm respectively.
Dissymmetric imidazol(in)ium bromide salts (3a, b)
were obtained in almost quantitative yield by quaternization
of 1-mesitylimidazoline[13] or 1-mesitylimidazole[14] in toluene
with 2,4,6-trimethylbenzyl bromide (Scheme 2). These salts
are air stable and colorless solids. The 1 H and 13 C NMR
spectra of these salts exhibit characteristic downfield signals.
In the 1 H NMR spectrum, the NCHN+ protons of 3a and 3b
appear at 9.13 and 10.16 ppm, while the 13 C NMR shifts of
NCHN+ appear at 158.2 and 141.2 ppm respectively.
All the new [(NHC)RhBr(cod)] complexes were obtained
through reaction of two equivalents imidazol(in)ium salts (1–3)
with [Rh(m-OMe)(cod)]2 in refluxing dichloromethane. Complexes
4–6 were obtained in high yield as air-stable orange solids
(Scheme 3).
Scheme 3. Synthesis of [(NHC)RhBr(cod)] complexes: (i) [Rh(OMe)(cod)]2,
CH2Cl2, reflux, 24 h.
differently (JRh–C = ~6.8 and ~14.5 Hz), which is consistent with
their position trans to NHC and the Br atom, respectively.
The corresponding carbonyl substituted NHC–rhodium
complexes, [(NHC)RhBr(CO)2], 7b, 8b and 9b are obtained by
passing carbon monoxide through a dichloromethane solution
of the [(NHC)RhBr(cod)] complexes, 4b, 5b, and 6b at r.t.
(Scheme 4). These reactions resulted in almost quantitative
replacement of cod ligand by CO ligands. These complexes were
produced in order to compare electronic properties of
corresponding NHC ligand, which could be done by measuring
the carbonyl stretching frequencies of [(NHC)RhBr(CO)2]
complexes with IR.
The cis conformation of the CO ligands in complexes 7b, 8b,
and 9b was confirmed by IR and NMR spectroscopy. The IR
spectra exhibit two strong nCO bands in each complex. IR data
confirm the donor strength of the NHC ligands in the complexes
being quite similar. 13 C NMR spectra exhibit three doublets
around 175, 183 and 186 ppm with the coupling constant of 43,
76 and 53 Hz for the two CO and Ccarbene ligands respectively.
The relevant data for the characterization of all ligand pre-
cursors (1–3), cod complexes (4–6) and carbonyl complexes
(7b–9b) are complied in Table 1 for the sake of comparison.
TH reactions require typically a hydrogen donor such as
2-propanol together with a strong base and an Ru, Rh or Ir
as catalyst,[16] and is preferred for large-scale industrial use
The identity of complexes 4–6 was confirmed by 1 H, 13 C,
heteronuclear multiple quantum correlation (HMQC) NMR,
elemental analysis and mass spectroscopy. The characteristic
downfield signals for the NCHN+ protons of the imidazol(in)ium
salts disappeared in the 1 H NMR spectra of complexes 4–6. These
complexes exhibit 13 C chemical shifts and coupling constants
that are comparable to those of other reported NHC–rhodium(I)
complexes.[8,11,15] 13 C chemical shifts showed that Ccarbene is
substantially deshielded. The carbon atoms in the two cycloocta-
diene double bonds are coupled with the rhodium centre
Scheme 2. Synthesis of imidazol(in)ium salts: (i) PhMe, r.t., 4 h; MeOH,
NaBH4, r.t., overnight; CH(OEt)3, NH4Br, 100 ꢀC, 6 h. (ii) DMF, 100 ꢀC, over-
night. (iii) PhMe, 80 ꢀC, 1 h.
Scheme 4. Synthesis of [(NHC)RhBr(CO)2] complexes.
Appl. Organometal. Chem. 2012, 26, 246–251
Copyright © 2012 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

S. Gülcemal
100
80
60
40
20
0
Table 1. Spectroscopic data for compounds 1–6 and 7b–9b
4a
4b
5a
5b
6a
6b
Compound
d C2-H
d C2-Rh
n
av-CO (cmÀ1
)
1a
1b
2a
2b
3a
3b
4a
4b
5a
5b
6a
6b
7b
8b
9b
9.18
9.20
9.30
9.70
9.13
10.16
—
—
—
—
—
—
—
—
—
30
60
90
120
—
—
Time (min)
—
—
212.7
183.7
214.5
182.3
212.3
180.3
185.3
186.5
186.1
—
Figure 1. Time dependence of the catalytic transfer hydrogenation of
acetophenone.
—
—
—
—
—
—
position of the ring. But for the complexes 6a and 6b bearing
dissymmetrically substituted NHC ligand, the catalytic activity
differs significantly during the first 90 min. Saturated NHC-
bearing complex 6a shows a better initiation time than unsat-
urated NHC-bearing complex 6b.
—
—
—
—
—
2038.2
2039.6
2039.3
—
—
Conclusions
in the hope of developing a greener process by reducing
waste production and energy use, and lowering toxicity.[17]
Complexes 4–6 were tested as catalysts for transfer hydro-
genation of acetophenone to 1-phenylethanol using 2-propa-
nol as hydrogen donor in the presence of KOH (Scheme 5).
The catalytic experiments were carried out using 4.0 mmol
of acetophenone, 0.02 mmol (0.5 mol%) of NHC–rhodium
complexes (4–6), 0.2 mmol KOH and 20 ml 2-propanol, with
a catalyst/base/substrate ratio of 0.5:5:100. The catalyst was
added to a solution of 2-propanol containing KOH, which
was kept at 82 ꢀC for 30 min. and acetophenone was added
to this solution. Percentage conversion was calculated
by comparing the methyl proton signals of acetophenone
(s, d 2.60 ppm) and 1-phenylethanol (d, d 1.50 ppm, J = 6.8Hz)
in the 1 H NMR spectrum of the crude product in CDCl3 and
the time-dependent conversions were followed (Fig. 1).
The activity of complexes 4–6 largely depends on the
nature of N-substituents and decreases in the order 5a > 5b ~
6a > 6b ~ 4a ~ 4b, indicating that the most bulky 4a and 4b
show the most noticeable activation period and reach a
maximum yield of ~50% after 2 h. As Herrmann and cowor-
kers suggested, the introduction of an a-hydrogen on the
nitrogen substituent enhances the activity.[9] The flexibility
of the benzyl substituent may also contribute to the catalytic
performance of the complexes.
In this report, a comparative study on the efficiencies of
imidazol(in)-2-ylidenes bearing 2,4,6-trimethylphenyl (Mes) or
2,4,6-trimethylbenzyl (CH2Mes) substituents on nitrogen
atoms was investigated. It is clear that the introduction of
the CH2Mes group to the nitrogen atoms increased TH perfor-
mance. The 4,5-position of the imidazole ring did not show a
dramatic effect.
Experimental
All manipulations were performed in air. The solvents
were used as received. The reagents were purchased
from Sigma-Aldrich, Merck, Alfa Aesar and Acros Organics.
1-Mesitylimidazoline,[13] 1-mesitylimidazole,[14] N1,N3-(dimesi-
tyl)imidazolinium bromide (SIMes.HBr)[18], N1,N3-(dimesitylim)
[20]
idazolium bromide (IMes.HBr)[19] and [Rh(m-OMe)(cod)]2
were prepared according to the published procedures. 1 H,
13 C and HMQC NMR spectra were recorded with a Varian
AS 400 Mercury instrument. As solvent, CDCl3 was employed.
Chemical shifts (d) are given in ppm and coupling constants
(J) in Hz. FT-IR spectra were recorded on a PerkinElmer
Spectrum 100 series. Elemental analyses were performed on
a PerkinElmer PE 2400 elemental analyzer. Mass spectra
experiments were conducted on Bruker HCT Ultra ion trap
mass spectrometer.
In this series of compounds, it has been observed that
the greatest efficiency in rate was achieved with dibenzyl
substituent on N atoms of imidazolin-2-ylidene. This feature
of the reaction was attributed to the flexibility of the CH2Mes
substituents versus rigidity of Mes substituents. Within these
frameworks it has been demonstrated that the catalytic
efficiency is influenced by the type of substituents attached
to the nitrogen atoms and to a lesser extent by the 4,5-
Preparation of Imidazol(in)ium Salts
Synthesis of 2a
A mixture of ethylenediamine (0.600 g, 10 mmol) and mesitylal-
dehyde (2.96 g, 20 mmol) was stirred in toluene (15 ml) at r.t. for
4 h. Solvent was then removed in vacuo and MeOH (15 ml) was
added to the resulting white solid. NaBH4 (1.76 g, 40 mmol) was
added with portions over 1 h at r.t. and the mixture was stirred
overnight. MeOH was then removed in vacuo, and the residue
was dissolved in Et2O (20 ml) and extracted with H2O
(3 Â 10 ml). The organic layer was separated and dried with
Na2SO4. The solvent was removed and to the resulting diimine
triethylorthoformate (4.45 g, 30 mmol) and NH4Br (0.980 g,
10 mmol) were added, and stirred for 6 h. The excess of
Scheme 5. TH of acetophenone
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Copyright © 2012 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2012, 26, 246–251

NHC-Rh-catalyzed transfer hydrogenation
triethylorthoformate was removed in vacuo; the resulting white
solid was dissolved in CH2Cl2 (5 ml) and precipitated with Et2O
(25 ml). The white solid was filtered and dried. Yield: 3.90 g
General Procedure for Preparation of the [(NHC)RhBr(cod)]
Complexes
Imidazol(in)ium salt (1.0 mmol) was dissolved in 10 ml CH2Cl2 and
[Rh(m-OMe)(cod)]2 (0.5 mmol, 0.242 g) was added to the solution.
The mixture was refluxed for 24 h. The solvent was removed in
vacuo. The residue was purified by column chromatography on
silica gel (eluent: CH2Cl2) to give pure complex as an orange solid.
The following complexes (4–6) were synthesized according to
this procedure.
1
(86%). H NMR (400 MHz, CDCl3, TMS, 25 ꢀC, ppm): d = 9.30 (s,
1 H, NCHN+), 6.77 (s, 4 H, CHarom), 4.78 (s, 4 H, Mes-CH2-N-), 3.73
(s, 4 H, -N-CH2CH2-N-), 2.25 (s, 6 H, Ar-CH3), 2.16 (s, 3 H, Ar-CH3).
13 C NMR (100.6 MHz, CDCl3, TMS, 25 ꢀC, ppm): d = 157.7 (NCHN+),
139.2 (Ar-C), 138.0 (Ar-C), 129.7 (Ar-C), 125.5 (Ar-C), 48.1 (Mes-CH2-
N-), 46.6 (-N-CH2CH2-N-), 21.1 (Ar-CH3), 20.3 (Ar-CH3. Anal. Calcd
for C23H31BrN2: C, 66.50; H, 7.52; N, 6.74. Found: C, 66.21; H,
7.57; N, 6.72. MS (ESI+): m/z 335.4 [M À Br]+, calcd 335.5.
Complex 4a
1
Yield: 0.480 g (80%). H NMR (400 MHz, CDCl3, TMS, 25 ꢀC, ppm):
Synthesis of 2b
d = 7.01 (s, 2 H, CHarom), 6.97 (s, 2 H, CHarom), 4.59 (br, 2 H, COD-
CH), 389–3.86 and 3.83–3.81 (m, 4 H, -N-CH2CH2-N-), 3.46 (br,
2 H, COD-CH), 2.61 (s, 6 H, Ar-CH3), 2.34 (s, 12 H, Ar-CH3), 1.80–1.75
(m, 4 H, COD-CH2), 1.56–1.48 (m, 4 H, COD-CH2). 13 C NMR
(100.6 MHz, CDCl3, TMS, 25 ꢀC, ppm): d = 212.7 (d, JRh–Carbene = 48.4
Hz, Ccarbene), 138.4 (Ar-C), 138.1 (Ar-C), 136.6 (Ar-C), 135.5 (Ar-C),
130.2 (Ar-C), 128.7 (Ar-C), 96.9 (d, JRh-C = 6.9 Hz, COD-CH), 68.8
(d, JRh–C = 14.6Hz, COD-CH), 32.7 (COD-CH2), 28.7 (COD-CH2), 21.3
(Ar-CH3), 21.0 (Ar-CH3), 18.7 (Ar-CH3). Anal. Calcd for C29H38BrN2Rh:
C, 58.30; H, 6.41; N, 4.69. Found: C, 58.22; H, 6.46; N, 4.72. MS (ESI+):
m/z 517.2 [M À Br]+, calcd 517.5.
A mixture of 2,4,6-trimethylbenzyl bromide (2.14 g, 10 mmol) and
imidazole (0.340 g, 5 mmol) was stirred in DMF (10 ml) at 100 ꢀC
overnight. It was then cooled to r.t. and the solvent was removed
in vacuo. The resulting white solid was dissolved in CH2Cl2 (3 ml)
and precipitated with Et2O (15 ml). The white solid was filtered
and dried. Yield: 1.95 mg (92%). 1 H NMR (400 MHz, CDCl3,
TMS, 25 ꢀC, ppm): d = 9.70 (s, 1 H, NCHN+), 6.85 (s, 2 H, -N-CHCH-
N-), 6.75 (s, 4 H, CHarom), 5.42 (s, 4 H, Mes-CH2-N-), 2.12 (s, 12 H,
Ar-CH3), 2.11 (s, 6 H, Ar-CH3). 13 C NMR (100.6 MHz, CDCl3, TMS,
25 ꢀC, ppm): d = 140.0 (NCHN+), 138.2 (Ar-C), 136.2 (Ar-C), 130.0
(Ar-C), 125.5 (Ar-C), 121.3 (-N-CHCH-N-), 48.3 (Mes-CH2-N-), 21.3
(Ar-CH3), 20.0 (Ar-CH3). Anal. Calcd for C23H29BrN2: C, 66.82; H,
7.07; N, 6.78. Found: C, 66.85; H, 7.04; N, 6.82. MS (ESI+): m/z
333.2 [M À Br]+, calcd 333.5.
Complex 4b
1
Yield: 0.549 g (92%). H NMR (400 MHz, CDCl3, TMS, 25 ꢀC, ppm):
d = 7.03 (s, 2 H, CHarom), 7.00 (s, 2 H, CHarom), 6.94 (m, 4 H, -N-
CHCH-N-), 4.50 (br, 2 H, COD-CH), 3.29 (br, 2 H, COD-CH), 2.39
(s, 6 H, Ar-CH3), 2.37 (s, 6 H, Ar-CH3), 2.13 (s, 6 H, Ar-CH3), 1.84–1.82
(m, 4 H, COD-CH2), 1.54–1.52 (m, 4 H, COD-CH2). 13 C NMR
(100.6 MHz, CDCl3, TMS, 25 ꢀC, ppm): d = 183.7 (d, JRh–Carbene = 52.9
Hz, Ccarbene), 138.8 (Ar-C), 137.7 (Ar-C), 136.5 (Ar-C), 134.6 (Ar-C),
129.9 (-N-CHCH-N-), 128.4 (-N-CHCH-N-), 96.2 (d, JRh–C = 7.8Hz,
COD-CH), 68.1 (d, JRh–C = 14.6 Hz, COD-CH), 32.9 (COD-CH2), 28.6
(COD-CH2), 21.4 (Ar-CH3), 20.0 (Ar-CH3), 18.4 (Ar-CH3). Anal. calcd
for C29H36BrN2Rh: C, 58.50; H, 6.09; N, 4.70. Found: C, 58.56; H,
6.13; N, 4.67. MS (ESI+): m/z 515.1 [M À Br]+, calcd 515.5.
Synthesis of 3a
To a solution of 1-mesitylimidazoline (5 mmol, 0.942 g) in toluene
(10 ml), 2,4,6-trimethylbenzyl bromide (5 mmol, 1.07 g) was
added. The solution stirred at 80 ꢀC for 1 h. The white solid that
separated out after cooling to r.t. was filtered off and washed
with diethyl ether (20 ml). The product was recrystallized from
CH2Cl2/Et2O. Yield: 1.85 g (92%). 1 H NMR (400 MHz, CDCl3, TMS,
25 ꢀC, ppm): d = 9.13 (s, 1 H, NCHN+), 6.78 (s, 2 H, CHarom), 6.77
(s, 2 H, CHarom), 5.05 (s, 2 H, Mes-CH2-N-), 4.05–4.11 (m, 4 H, -N-
CH2CH2-N-), 2.28 (s, 6 H, Ar-CH3 ortho), 2.18 (s, 6 H, Ar-CH3 ortho),
2.17 (s, 3 H, Ar-CH3 para), 2.15 (s, 3 H, Ar-CH3 para). 13 C NMR
(100.6 MHz, CDCl3, TMS, 25 ꢀC, ppm): d = 158.2 (NCHN+), 140.3
(Ar-C), 139.2 (Ar-C), 138.2 (Ar-C), 135.4 (Ar-C), 130.8 (Ar-C), 130.1
(Ar-C), 130.0 (Ar-C), 125.6 (Ar-C), 51.4 (Mes-CH2-N-), 48.7 (-N-
CH2CH2-N-), 46.9 (-N-CH2CH2-N-), 21.1 (Ar-CH3), 20.4 (Ar-CH3),
18.3 (Ar-CH3). Anal. Calcd for C22H29BrN2: C, 65.83; H, 7.28;
N, 6.98. Found: C, 65.86; H, 7.24; N, 6.99. MS (ESI+): m/z 321.3
[M-Br]+, calcd 321.5.
Complex 5a
1
Yield: 0.506 g (81%). H NMR (400 MHz, CDCl3, TMS, 25 ꢀC, ppm):
d = 6.85 (s, 4 H, CHarom), 5.57 (d, J = 14.0 Hz, 2 H, Mes-CH2-N-),
5.10 (d, J = 14.0 Hz, 2 H, Mes-CH2-N-), 5.06 (br, 2 H, COD-CH), 3.67
(br, 2 H, COD-CH), 2.90 (s, 4 H, -N-CH2CH2-N-), 2.45–2.41 (m, 4 H,
COD-CH2), 2.42 (s, 12 H, Ar-CH3), 2.25 (s, 6 H, Ar-CH3), 2.00–1.94
(m, 4 H, COD-CH2). 13 C NMR (100.6 MHz, CDCl3, TMS, 25 ꢀC,
ppm): d = 214.5 (d, JRh–Carbene = 46.9 Hz, Ccarbene), 138.5 (Ar-C),
137.8 (Ar-C), 129.5 (Ar-C), 129.2 (Ar-C), 99.2 (d, JRh–C = 6.1 Hz,
COD-CH), 68.0 (d, JRh–C = 15.4 Hz, COD-CH), 48.9 (Mes-CH2-N-),
47.5 (-N-CH2CH2-N-), 33.2 (COD-CH2), 28.9 (COD-CH2), 21.2
(Ar-CH3), 20.9 (Ar-CH3). Anal. Calcd for C31H42BrN2Rh: C, 59.53; H,
6.77; N, 4.48. Found: C, 59.57; H, 6.81; N, 4.46. MS (ESI+): m/z
545.3 [M À Br]+, calcd 545.6.
Synthesis of 3b
The salt was synthesized with 1-mesitylimidazole (5 mmol,
0.931 g) by a similar method to that used for 2a. Yield: 1.90 g
1
(95%). H NMR (400 MHz, CDCl3, TMS, 25 ꢀC, ppm): d = 10.16
(s, 1 H, NCHN+), 7.32 (s, 1 H, -N-CHCH-N-), 7.06 (s, 1 H, -N-CHCH-
N-), 6.79 (s, 2 H, CHarom), 6.75 (s, 2 H, CHarom), 5.74 (s, 2 H, Mes-
CH2-N-), 2.15 (s, 9 H, Ar-CH3), 2.12 (s, 3 H, Ar-CH3), 1.87 (s, 6 H,
Complex 5b
13
1
Ar-CH3). C NMR (100.6 MHz, CDCl3, TMS, 25 ꢀC, ppm): d = 141.2
Yield: 0.545 g (87%). H NMR (400 MHz, CDCl3, TMS, 25 ꢀC, ppm):
(NCHN+), 139.8 (Ar-C), 138.1 (Ar-C), 137.3 (Ar-C), 134.2 (Ar-C),
130.8 (Ar-C), 130.0 (Ar-C), 129.9 (Ar-C), 125.8 (Ar-C), 124.4 (-N-
CHCH-N-), 121.9 (-N-CHCH-N-), 48.5 (Mes-CH2-N-), 21.1 (Ar-CH3),
20.0 (Ar-CH3), 17.7 (Ar-CH3). Anal. Calcd for C22H27BrN2: C, 66.16;
H, 6.81; N, 7.01. Found: C, 66.14; H, 6.76; N, 6.97. MS (ESI+): m/z
319.2 [M À Br]+, calculated 319.5.
d = 6.89 (s, 4 H, CHarom), 6.07 (s, 2 H, -N-CHCH-N-), 5.86 (d,
J = 14.0 Hz, 2 H, Mes-CH2-N-), 5.45 (d, J = 14.0Hz, 2 H, Mes-CH2-N-),
5.18 (br, 2 H, COD-CH), 3.65 (br, 2 H, COD-CH), 2.49–2.40 (m, 4 H,
COD-CH2), 2.29 (s, 12 H, Ar-CH3), 2.28 (s, 6 H, Ar-CH3), 2.03–1.95
(m, 4 H, COD-CH2). 13 C NMR (100.6MHz, CDCl3, TMS, 25 ꢀC, ppm):
d = 182.3 (d, JRh–Carbene = 49.1Hz, Ccarbene), 138.7 (Ar-C), 138.6 (Ar-C),
Appl. Organometal. Chem. 2012, 26, 246–251
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S. Gülcemal
Complex 7b
129.6 (Ar-C), 128.4 (Ar-C), 118.4 (-N-CHCH-N-), 98.1 (d, JRh–C = 6.9 Hz,
COD-CH), 68.8 (d, JRh–C = 14.6 Hz, COD-CH), 49.3 (Mes-CH2-N-), 33.1
(COD-CH2), 29.4 (COD-CH2), 21.3 (Ar-CH3), 20.4 (Ar-CH3). Anal. Calcd
for C29H36BrN2Rh: C, 59.72; H, 6.47; N, 4.49. Found: C, 59.66; H, 6.42;
N, 4.53. MS (ESI+): m/z 543.5 [M À Br]+, calcd 543.7.
Yield: 0.102 g (94%). IR (CH2Cl2): nCO = 2079.7, 1996.7 cmÀ1 1 H
.
NMR (400 MHz, CDCl3, TMS, 25 ꢀC, ppm): d = 7.12 (s, 2 H, -N-
CHCH-N-), 7.02 (s, 4 H, CHarom), 2.37 (s, 6 H, Ar-CH3), 2.23 (s, 12 H,
13
Ar-CH3). C NMR (100.6 MHz, CDCl3, TMS, 25 ꢀC, ppm): d = 185.3
(d, JRh–Carbene = 54.5 Hz, Ccarbene), 183.0 (d, JRh–C = 74.3 Hz, CO),
177.6 (d, JRh–C = 44.5Hz, CO), 139.6 (Ar-C), 135.5 (Ar-C), 135.4 (Ar-C),
129.5 (Ar-C), 124.1 (-N-CHCH-N-), 21.4 (Ar-CH3), 18.7 (Ar-CH3). Anal.
Calcd for C23H24BrN2O2Rh: C, 50.85; H, 4.45; N, 5.16. Found: C,
50.76; H, 4.42; N, 5.20.
Complex 6a
1
Yield: 0.538 g (88%). H NMR (400 MHz, CDCl3, TMS, 25 ꢀC, ppm):
d = 6.95 (s, 1 H, CHarom), 6.82 (s, 2 H, CHarom), 6.80 (s, 1 H, CHarom),
5.62 (d, J = 14.4 Hz, 1 H, Mes-CH2-N-), 5.20 (d, J = 14.4 Hz, 1 H,
Mes-CH2-N-), 4.97–4.92 (m, 1 H, COD-CH), 4.77–4.72 (m, 1 H,
COD-CH), 3.75–3.71 (m, 1 H, COD-CH), 3.55–3.41 (m, 2 H, -N-
CH2CH2-N-), 3.20–3.01 (m, 3 H, COD-CH + -N-CH2CH2-N-), 2.57
(s, 3 H, Ar-CH3), 2.36 (s, 6 H, Ar-CH3), 2.34–2.28 (m, 1 H, COD-CH2),
2.25 (s, 3 H, Ar-CH3), 2.21 (s, 3 H, Ar-CH3), 2.13–2.04 (m, 1 H,
COD-CH2), 1.94 (s, 3 H, Ar-CH3), 1.89–1.75 (m, 2 H, COD-CH2),
1.68–1.58 (m, 2 H, COD-CH2), 1.45–1.38 (m, 2 H, COD-CH2).
13 C NMR (100.6 MHz, CDCl3, TMS, 25 ꢀC, ppm): d = 212.3 (d,
Complex 8b
Yield: 0.104 g (91%). IR (CH2Cl2): nCO = 2078.9, 2000.3 cmÀ1 1 H
.
NMR (400 MHz, CDCl3, TMS, 25 ꢀC, ppm): d = 6.92 (s, 4 H, CHarom),
6.31 (s, 2 H, -N-CHCH-N-), 5.40 (s, 4 H, Mes-CH2-N-), 2.30 (s, 6 H,
Ar-CH3), 2.27 (s, 12 H, Ar-CH3). 13 C NMR (100.6 MHz, CDCl3, TMS,
25 ꢀC, ppm): d = 186.5 (d, JRh–Carbene = 53.7 Hz, Ccarbene), 182.8 (d,
JRh–C = 76.0 Hz, CO), 173.4 (d, JRh–C = 43.0 Hz, CO), 139.0 (Ar-C),
138.4 (Ar-C), 129.8 (Ar-C), 128.1 (Ar-C), 119.8 (-N-CHCH-N-), 50.1
(Mes-CH2-N-), 21.2 (Ar-CH3), 20.3 (Ar-CH3). Anal. Calcd for
C25H28BrN2O2Rh: C, 52.56; H, 4.94; N, 4.90. Found: C, 52.64; H,
5.01; N, 4.93.
J
Rh–Carbene = 46.1 Hz, Ccarbene), 138.5 (Ar-C), 138.3 (Ar-C), 138.0
(Ar-C), 137.7 (Ar-C), 136.8 (Ar-C), 135.5 (Ar-C), 130.0 (Ar-C),
129.7 (Ar-C), 129.5 (Ar-C), 128.6 (Ar-C), 97.5 (d, JRh–C = 6.7 Hz,
COD-CH), 97.2 (d, JRh–C = 6.7Hz, COD-CH), 69.5 (d, JRh–C = 14.5 Hz,
COD-CH), 67.8 (d, JRh–C = 14.5 Hz, COD-CH), 51.2 (-N-CH2CH2-N-),
50.4 (Mes-CH2-N-), 47.7 (-N-CH2CH2-N-), 33.8 (COD-CH2), 31.9
(COD-CH2), 29.2 (COD-CH2), 28.5 (COD-CH2), 21.2 (Ar-CH3), 21.1
(Ar-CH3), 20.7 (Ar-CH3), 18.0 (Ar-CH3). Anal. Calcd for C30H40BrN2Rh:
C, 58.93; H, 6.59; N, 4.58. Found: C, 58.86; H, 6.62; N, 4.57. MS (ESI+):
m/z 531.4 [M À Br]+, calcd 531.6.
complex 9b
Yield: 0.108 g (97%). IR (CH2Cl2): nCO = 2078.5, 2000.1 cmÀ1 1 H
.
NMR (400 MHz, CDCl3, TMS, 25 ꢀC, ppm): d = 6.99 (s, 2 H, CHarom),
6.95 (s, 2 H, CHarom), 6.83 (d, J = 2.0Hz, 1 H, -N-CHCH-N-), 6.63 (d,
J = 2.0 Hz, 1 H, -N-CHCH-N-), 5.58 (s, 2 H, Mes-CH2-N-), 2.37 (s, 3 H,
Ar-CH3), 2.34 (s, 6 H, Ar-CH3), 2.32 (s, 3 H, Ar-CH3), 2.11 (s, 6 H,
13
Ar-CH3). C NMR (100.6 MHz, CDCl3, TMS, 25 ꢀC, ppm): d = 186.1
Complex 6b
(d, JRh–Carbene = 52.9 Hz, Ccarbene), 182.6 (d, JRh–C = 76.0 Hz, CO),
175.2 (d, JRh–C = 43.0 Hz, CO), 139.6 (Ar-C), 138.9 (Ar-C), 138.3
(Ar-C), 135.6 (Ar-C), 135.4 (Ar-C), 129.8 (Ar-C), 129.5 (Ar-C), 128.6
(Ar-C), 123.4 (-N-CHCH-N-), 120.8 (-N-CHCH-N-), 50.9 (Mes-CH2-N-),
21.4 (Ar-CH3), 21.3 (Ar-CH3), 20.3 (Ar-CH3), 18.7 (Ar-CH3). Anal. calcd
for C24H26BrN2O2Rh: C, 51.73; H, 4.70; N, 5.03. Found: C, 51.67; H,
4.73; N, 4.98.
1
Yield: 0.527 g (86%). H NMR (400 MHz, CDCl3, TMS, 25 ꢀC, ppm):
d = 7.00 (s, 1 H, CHarom), 6.86 (s, 2 H, CHarom), 6.82 (s, 1 H, CHarom),
6.55 (d, J = 2.0 Hz, 1 H, -N-CHCH-N-), 6.32 (s, 1 H, -N-CHCH-N-),
5.83 (s, 2 H, Mes-CH2-N-), 4.98–4.94 (m, 1 H, COD-CH), 4.86–4.81
(m, 1 H, COD-CH), 3.59–3.55 (m, 1 H, COD-CH), 3.08–3.03 (m, 1 H,
COD-CH), 2.42 (s, 3 H, Ar-CH3), 2.39–2.21 (m, 2 H, COD-CH2), 2.29
(s, 3 H, Ar-CH3), 2.26 (s, 6 H, Ar-CH3), 2.23 (s, 3 H, Ar-CH3), 2.15–2.10
(m, 1 H, COD-CH2), 1.98–1.92 (m, 1 H, COD-CH2), 1.80–1.62 (m, 2 H,
COD-CH2), 1.48 1.37 (m, 2 H, COD-CH2). 13 C NMR (100.6 MHz, CDCl3,
TMS, 25ꢀC, ppm): d = 180.3 (d, JRh–Carbene = 50.7 Hz, Ccarbene), 137.6
(Ar-C), 137.4 (Ar-C), 137.3 (Ar-C), 136.1 (Ar-C), 135.3 (Ar-C),
133.5 (Ar-C), 128.6 (Ar-C), 128.4 (Ar-C), 127.8 (Ar-C), 127.0 (Ar-C),
121.6 (-N-CHCH-N-), 118.3 (-N-CHCH-N-), 95.6 (d, JRh–C = 6.7Hz,
COD-CH), 95.4 (d, JRh–C = 6.7Hz, COD-CH), 67.5 (d, JRh–C = 14.3 Hz,
COD-CH), 67.3 (d, JRh–C = 14.3 Hz, COD-CH), 49.9 (Mes-CH2-N-), 33.0
(COD-CH2), 30.4 (COD-CH2), 28.4 (COD-CH2), 27.3 (COD-CH2), 20.1
(Ar-CH3), 20.0 (Ar-CH3), 19.5 (Ar-CH3), 18.9 (Ar-CH3), 16.6 (Ar-CH3).
Anal. Calcd for C30H38BrN2Rh: C, 59.12; H, 6.28; N, 4.60. Found: C,
59.05; H, 6.23; N, 4.64. MS (ESI+): m/z 529.2 [MÀ Br]+, calcd 529.5.
General Procedure for the Transfer Hydrogenation Reaction
The tested complex (0.02 mmol; 0.5 mol%) was dissolved in a
solution of KOH (0.2 mmol) and 2-propanol (20 ml) in a two-
necked flask. The solution was heated to 82 ꢀC for 30 min.
Subsequently, acetophenone (4 mmol) was added. After the
desired reaction time the solution was allowed to cool and
quenched with 1 M HCl, extracted with CH2Cl2 and the organic
phase separated. The reaction progress was monitored by
1 H NMR and the results for each experiment are averages over
two runs.
Acknowledgments
Financial support from TUBITAK and EBILTEM is gratefully
acknowledged. I also thank Mr Salih Günnaz for NMR analyses
and Mr Umut Şahar for mass analyses.
General Procedure for Preparation of the [(NHC)RhBr(CO)2]
Complexes
[(NHC)RhBr(cod)] (4b–6b) (0.2 mmol) was dissolved in CH2Cl2
(5 ml) and carbon monoxide was bubbled through the solution
for 1 h. The color of the solution changed from orange to
pale yellow. The solution was concentrated to ~2 ml and pentane
was added. The pale-yellow solid that separated out was filtered
and washed with pentane. The following complexes (7b–9b)
were synthesized according to this procedure.
References
[1] a) V. César, L. H. Gade, S. Bellemin-Laponnaz in N-Heterocyclic
Carbenes: From Laboratory Curiosities to Efficient Synthetic Tools.
RSC Catalysis Series No. 6 (Ed.: S. Díez-González) Ed.:, Royal Society
of Chemistry, Cambridge, UK, 2011, pp. 228. b) F. E. Hahn, M. C.
Jahnke, Angew. Chem. Int. Ed. 2008, 47, 3122. c) F. E. Hahn, Angew.
wileyonlinelibrary.com/journal/aoc
Copyright © 2012 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2012, 26, 246–251

NHC-Rh-catalyzed transfer hydrogenation
Chem. Int. Ed. 2006, 45, 1348. d) K. J. Cavell, D. S. McGuinnes, Coord.
Chem. Rev. 2004, 248, 671. e) V. César, S. Bellemin-Laponnaz, L. H.
Gade, Chem. Soc. Rev. 2004, 33, 619. f) C. M. Crudden, D. P. Allen,
Coord. Chem. Rev. 2004, 248, 2247. g) W. A. Herrmann, Angew. Chem.
Int. Ed. 2002, 41, 1290. h) W. A. Herrmann, Adv. Organomet. Chem.
2002, 48, 1. i) A. C. Hillier, G. A. Grasa, M. S. Viciu, H. M. Lee, C. Yang,
S. P. Nolan, J. Organomet. Chem. 2002, 653, 69. j) D. Bourissou, O.
Guerret, F. P. Gabbai, G. Bertrand, Chem. Rev. 2009, 100, 39. k) W. A.
Herrmann, C. Köcher, Angew. Chem. Int. Ed. 1997, 36, 2162.
[2] M. F. Lappert, R. K. Maskell, J. Organomet. Chem. 1984, 264, 218.
[3] D. Tapu, D. A. Dixon, C. Roe, Chem. Rev. 2009, 109, 3385.
[4] a) R. Dorta, E. D. Stevens, C. D. Hoff, S. P. Nolan, J. Am. Chem. Soc.
2003, 125, 10490. b) J. K. Huang, H. J. Schanz, E. D. Stevens, S. P.
Nolan, Organometallics 1999, 18, 2370.
[11] a) A. Bittermann, D. Baskakov, W. A. Herrmann, Organometallics
2009, 28, 5107. b) A. Bittermann, P. Härter, E. Herdtweck, S. D. Hoffmann,
W. A. Herrmann, J. Organomet. Chem. 2079, 2008, 693. c) H. M. Peng,
R. D. Webster, X. Li, Organometallics 2008, 27, 4484. d) H. Ohta, T. Fujihara,
Y. Tsuji, Dalton Trans. 2008, 379. e) M. Yiğit, I. Özdemir, E. Çetinkaya,
B. Çetinkaya, Transition Met. Chem. 2007, 32, 536. f) M. Yiğit, B. Yiğit,
I. Özdemir, E. Çetinkaya, B. Çetinkaya, Appl. Organometal. Chem. 2006,
20, 322. g) I. Özdemir, S. Yaşar, B. Çetinkaya, Transition Met. Chem.
2005, 30, 831. h) I. Özdemir, S. Demir, B. Çetinkaya, J. Mol. Catal. A
2004, 215, 45.
[12] H. V. Huynh, Y. Han, R. Jothibasu, J. A. Yang, Organometallics 2009,
28, 5395.
[13] M. Bessel, F. Rominger, B. F. Straub, Synthesis 2010, 9, 1459.
[14] A. J. Arduengo III, F. P. Gentry Jr, P. K. Taverkere, H. E. Simmons III, US
Patent 6177575, 2001.
[15] a) N. B. Jokić, M. Zhang-Presse, S. L. M. Goh, C. S. Straubinger, B.
Bechlars, W. A. Herrmann, F. E. Kühn, J. Organomet. Chem. 2011,
696, 3900. b) S. K. U. Riederer, P. Gigler, M. P. Högerl, E. Herdtweck,
B. Bechlars, W. A. Herrmann, F. E. Kühn, Organometallics 2010, 29,
5681. c) M. Rubio, E. Jellema, M. A. Siegler, A. L. Spek, J. N. H. Reek,
B. de Bruin, Dalton Trans. 2009, 8970. d) A. R. Chianese, R. H. Crabtree,
Organometallics 2005, 24, 4432. e) A. R. Chianese, X. Li, M. C. Janzen,
J. W. Faller, R. H. Crabtree, Organometallics 2003, 22, 1663.
[16] B. Çetinkaya in N-Heterocyclic Carbenes: From Laboratory Curiosities
to Efficient Synthetic Tools. RSC Catalysis Series No. 6 (Ed.: S. Díez-
González)Ed.:, Royal Society of Chemistry, Cambridge, UK, 2011,
pp. 366.
[17] a) S. Gladiali, E. Alberico, Chem. Soc. Rev. 2006, 35, 226. b) H. U. Blaser,
C. Malan, B. Pugin, F. Spindler, M. Studer, Adv. Synth. Catal. 2003, 345,
103. c) G. Zassinovich, G. Mestroni, S. Gladiali, Chem. Rev. 1992, 51,
1051. d) R. Noyori, S. Hashiguchi, Acc. Chem. Res. 1997, 30, 97.
[18] S. P. Roche, M.-L. Teyssot, A. Gautier, Tetrahedron Lett. 2010, 51, 1265.
[19] S. Li, C. W. Kee, K.-W. Huang, T. S. A. Hor, J. Zhao, Organometallics
2010, 29, 1924.
[5] a) S. Díez-González, N. Marion, S. P. Nolan, Chem. Rev. 2009, 109,
3612. b) X. Bantreil, J. Broggi, S. P. Nolan, Annu. Rep. Prog. Chem. B
2009, 105, 232. c) J. M. Praetorius, C. M. Crudden, Dalton Trans.
2008, 4079. d) W. Gil, A. M. Trzeciak, Coord. Chem. Rev. 2011,
255, 473.
[6] O. Schuster, L. Yang, H. G. Raubenheimer, M. Albrecht, Chem. Rev.
2009, 109, 3445.
[7] a)T. Dröge, F. Glorius, Angew. Chem. Int. Ed. 2010, 49, 6940. b) J.
Kobayashi, S. Y. Nakafuji, A. Yatabe, T. Kawashima, Chem. Commun.
2008, 6233. c) D. M. Khramov, E. L. Rosen, V. M. Lynch, C. W. Bielawski,
Angew. Chem. Int. Ed. 2008, 47, 2267. d) H. Türkmen, T. Pape, F. E.
Hahn, B. Çetinkaya, Organometallics 2008, 27, 571. e) A. Fürstner, M.
Alcarazo, H. Krause, C. W. Lehmann, J. Am. Chem. Soc. 2007,
129, 12676.
[8] a) H. Türkmen, B. Çetinkaya, Appl. Organometal. Chem. 2011, 25, 226.
b) S. Wolf, H. Plenio, J. Organomet. Chem. 2009, 694, 1487. c) K. Hirano,
S. Urban, C. Wang, F. Glorius, Org. Lett. 2009, 11, 1019.
[9] S. C. Zinner, C. F. Rentzsch, E. Herdtweck, W. A. Herrmann, F. E. Kühn,
Dalton Trans. 2009, 7055.
[10] A. C. Hillier, H. M. Lee, E. D. Stevens, S. P. Nolan, Organometallics
2001, 20, 4246.
[20] R. Uson, L. A. Oro, J. A. Cabeza, Inorg. Synth. 1985, 23, 126.
Appl. Organometal. Chem. 2012, 26, 246–251
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