Iridium(i) N-heterocyclic carbene complexes of benzimidazol-2-ylidene: Effect of electron donating groups on the catalytic transfer hydrogenation reaction

Article abstract of DOI:10.1039/c2dt32482b

Two new [Ir(NHC)(COD)Cl] (NHC = N-heterocyclic carbene; COD = 1,5-cyclooctadiene) iridium complexes (2a,b) have been prepared by the reaction of [Ir(COD)Cl]₂ with in situ prepared NHC-Ag carbene transfer agents in dichloromethane at ambient temperature. They have been fully characterized by ¹H, ¹³C NMR, and elemental analysis. X-ray diffraction studies on single crystals of 2a and 2b confirm the square planar geometry. Complexes of type [Ir(NHC)(CO)₂Cl] [NHC = 1,3-diisopropyl(5,6- dimethyl)benzimidazol-2-ylidene] 3 were also synthesized to compare σ-donor/π-acceptor strength of NHC ligands. Transfer hydrogenation (TH) reactions of various aldehydes and ketones have been studied using complexes 2a and 2b as catalysts. The 5,6-dimethyl substituted iridium complex (2b) showed the highest catalytic activity for the TH reaction.

Full text of DOI:10.1039/c2dt32482b

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Iridium(I) N-heterocyclic carbene complexes of
benzimidazol-2-ylidene: effect of electron donating
groups on the catalytic transfer hydrogenation
reaction†
Cite this: Dalton Trans., 2013, 42, 7305
Süleyman Gülcemal,*a Aytaç Gürhan Gökçeb and Bekir Çetinkayaa
Two new [Ir(NHC)(COD)Cl] (NHC = N-heterocyclic carbene; COD = 1,5-cyclooctadiene) iridium complexes
(2a,b) have been prepared by the reaction of [Ir(COD)Cl]2 with in situ prepared NHC–Ag carbene transfer
agents in dichloromethane at ambient temperature. They have been fully characterized by 1H, 13C NMR,
and elemental analysis. X-ray diffraction studies on single crystals of 2a and 2b confirm the square planar
geometry. Complexes of type [Ir(NHC)(CO)2Cl] [NHC = 1,3-diisopropyl(5,6-dimethyl)benzimidazol-2-
ylidene] 3 were also synthesized to compare σ-donor/π-acceptor strength of NHC ligands. Transfer hydro-
genation (TH) reactions of various aldehydes and ketones have been studied using complexes 2a and 2b
as catalysts. The 5,6-dimethyl substituted iridium complex (2b) showed the highest catalytic activity for
the TH reaction.
Received 17th October 2012,
Accepted 21st November 2012
DOI: 10.1039/c2dt32482b
www.rsc.org/dalton
preferred for industrial use because of the low cost of iridium
when compared to rhodium.
Introduction
The transition metal complexes of N-heterocyclic carbenes
In 2009, Herrmann’s group prepared [Ir(NHC)(COD)(X)]
(NHCs) based on the imidazol, imidazolin and benzimidazole type iridium(I) complexes (X = halogen) and investigated the
framework have been the focus of intense research in organo- effect of different NHC ligands on catalytic activity in the TH
metallic chemistry and homogeneous catalysis.1 The electronic reaction.7 They observed that the weaker donor strength of the
and steric parameters of NHC complexes can be modified NHC ligand seems to increase the complex activity by both
easily, and they have greater stability toward air, moisture and shortening the initiation time and accelerating the catalytic
heating when compared with phosphine analogues.2 NHCs are reaction when acetophenone is the substrate. When iPr or Cy
strongly bound to the metal, which avoid decomposition to groups were attached to the N-atom of the NHC, catalytic
free and inactive metal under catalytic conditions.3 Thus, activity increased considerably, because the introduction of an
iridium, rhodium and ruthenium complexes bearing NHCs α-hydrogen on the nitrogen substituent is enhancing the
have been employed as catalysts for a number of catalytic activity. Another important result obtained from this study was
reduction reactions such as hydrogenation, transfer hydrogen- that the 1,3-dimethylbenzimidazol-2-ylidene ligand shows
ation (TH), and hydrosilylation.4
better activity than the 1,3-dimethylimidazol-2-ylidene ligand.
TH reactions require typically a hydrogen donor such as As a result, it was found that steric and electronic effects influ-
2-propanol together with a strong base and transition metal ence the catalytic activity considerably.
catalyst,1b and are preferred for large-scale industrial use in
However, benzannulated NHCs derived from benzimid-
the hope of developing a greener process by reducing waste azolium precursors exhibit interesting properties due to their
production and energy use and lowering toxicity.5 It has been intermediate position between saturated and unsaturated ana-
found that NHC complexes of iridium(I) show superior activi- logues;8 a few groups have reported the catalytic activities of
ties when compared with their rhodium(I) analogues.6 This is iridium benzimidazol-2-ylidene complexes.6b,7,9 Our previous
studies indicated that the benzimidazole-based rhodium com-
plexes turned out to be superior to the imidazole-based ones
aEge University, Department of Chemistry, 35100 Bornova, Izmir, Turkey. E-mail:
in catalytic hydrosilylation reactions10 and attaching methyl
suleyman.gulcemal@ege.edu.tr; Fax: +90 232 3888264; Tel: +90 232 3111581
groups on the 5,6-position of the benzene ring of benzimid-
azole has also played an important role in rhodium-catalyzed
reduction reactions.10,11 The above facts suggested the
bDokuz Eylül University, Department of Physics, 35160 Buca, Izmir, Turkey
†CCDC 898188 and 898189. For crystallographic data in CIF or other electronic
format see DOI: 10.1039/c2dt32482b
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possibility of achieving high activity with the same benzene
The identity of the complexes 2a,b was confirmed by 1H,
annulated NHC ligands. Therefore, in this study, iridium(I) 13C, NMR analyses, elemental analysis and X-ray diffraction
complexes derived from 1,3-diisopropylbenzimidazol-2-ylidene studies. The characteristic downfield signals for the NCHN+
(2a) and 1,3-diisopropyl-(5,6-dimethyl)benzimidazol-2-ylidene protons of the benzimidazolium salts 1a,b disappeared in the
(2b) ligands are synthesized to apply for TH reactions to 1H NMR spectra of complexes 2a,b. Upon coordination, iso-
examine the influence of the electronic effect of 5,6-dimethyl propyl CH protons of NHC significantly shifted downfield
groups and to compare the effect of the benzimidazole ligand (>1 ppm) relative to the NHC precursors. These downfield
with previously synthesized imidazole derivatives. As a result, shifts were attributed to the orientation of CH groups towards
the iPr group attached to the 5,6-dimethylbenzimidazole the metal center and weak C–H⋯M interactions.12 These com-
ligand was found to be the most active catalyst for the iridium(I) plexes exhibit 13C chemical shifts at 190.0 and 188.4 ppm for
catalyzed TH reaction with a variety of substrates.
the complexes 2a and 2b respectively that are comparable
to those of other reported NHC–iridium(I) complexes.6,7,9 The
13C chemical shifts showed that Ccarbene is substantially
deshielded.
Results and discussion
NHCs exhibit good σ-donor and weak π-acceptor electronic
properties.14 Metal-bound carbonyls are useful spectroscopic
handles for measuring the electron density at ligated metal
centers. Measuring of carbonyl stretching frequencies in
[M(NHC)(CO)2X] (M = Rh or Ir, X = halogen) complexes with
infrared (IR) spectroscopy can be used to compare donor prop-
erties of NHC ligands, which are important for homogeneous
catalysis.15 Thus, [Ir(NHC)(CO)2Cl] complexes 3a,b were pro-
duced in order to compare with the electronic properties of
the corresponding NHC ligands, which could be done by
measuring the carbonyl stretching frequencies of [Ir(NHC)-
(CO)2Cl] complexes by IR. The corresponding carbonyl substi-
tuted iridium(I) complexes, [Ir(NHC)(CO)2Cl], 3a,b were
obtained by passing carbon monoxide through a dichloro-
methane solution of the [Ir(NHC)(COD)Cl] complexes16 2a,b at
room temperature (Scheme 3). These reactions resulted in
almost quantitative replacement of the COD ligand by CO
ligands.
Synthesis and characterization of compounds
The NHC precursors 1,3-diisopropylbenzimidazolium bromide
1a and its 5,6-dimethyl substituted analogue 1,3-diisopropyl-
(5,6-dimethyl)benzimidazolium bromide 1b were synthesized
according to the published procedure12 as colorless powders
in the presence of a weak base such as K2CO3 (Scheme 1).
While the salt 1a was obtained in 82% yield the same con-
ditions gave 1b in 97% yield. In the 1H NMR spectrum the
NCHN+ protons of 1a and 1b appear at 11.03 and 10.82 ppm
while the 13C NMR shifts of NCHN+ appear at 140.2 and
138.9 ppm respectively. The NMR results for the known com-
pound 1a are consistent with the literature.12 The 1H NMR
spectrum of salt 1b shows a doublet at 1.72 ppm and a septet
at 5.04 ppm which are characteristic for the isopropyl group.
The two methyl groups of the benzene backbone give a singlet
at 2.36 ppm. In addition, the downfield signal at 10.82 ppm
indicates the formation of a benzimidazolium salt.
The new [Ir(NHC)(COD)Cl] complexes 2a,b were obtained
by the carbene-transfer reaction of in situ formed NHC–Ag
species13 with [Ir(COD)Cl]2 in dichloromethane at ambient
temperature. The complexes 2a,b were obtained in high yields
as air and moisture stable yellow solids (Scheme 2).
The cis conformation of the CO ligands in complexes 3a,b
was confirmed by IR and NMR spectroscopy. The IR spectra
exhibit two strong νCO bands in each complex. IR data also
confirm the donor strength of the NHC ligands in the com-
plexes being quite different. The average carbonyl stretching
frequencies were found to be νCO = 2028 cm−1 for complex
av
3a, and νCO = 2026 cm−1 for complex 3b. These differences
av
could be explained by the donor strength of the 5,6-dimethyl
substituents on the benzimidazol-2-ylidene ligand. The
average carbonyl stretching frequency for the corresponding
[Ir(1,3-diisopropylimidazol-2-ylidene)(CO)2Cl] complex was found
to be 2024 cm−1 by Herrmann.16e This observation was also sup-
ported by Bielawski’s previous study, where the imidazol-
2-ylidene ligand was found to be more donating than
Scheme 1 Synthesis of (5,6-dimethyl)benzimidazolium salts.
Scheme 2 Synthesis of [(NHC)IrCl(COD)] complexes.
7306 | Dalton Trans., 2013, 42, 7305–7311
Scheme 3 Synthesis of [Ir(NHC)(CO)2Cl] complexes.
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benzimidazol-2-ylidene in [Ir(NHC)(CO)2Cl] type complexes.16c
As a result, annulation decreases the electron density at the
carbene. 13C NMR spectra exhibit three signals at 181.5, 180.5
and 168.2 ppm for the complex 3a, and 181.7, 179.0 and
168.3 ppm for the complex 3b, for the two CO and Ccarbene
ligands, which are consistent with known [Ir(NHC)(CO)2Cl] type
complexes.16c
Table 1 Crystallographic data and structure refinement summary for 2a and
2b
Complex
2a
2b
Formula
Formula weight
Crystal system
Space group
a (Å)
b (Å)
c (Å)
C
21H30ClIrN2
C23H34ClIrN2
566.17
Monoclinic
C 2/m
17.1618(16)
14.1773(7)
12.8587(11)
133.312(16)
2276.5(3)
4
1.65
1120
3.26–29.38
0.0464
538.12
Monoclinic
P 21/n
9.7923(2)
15.4117(3)
13.5711(3)
97.422(2)
2030.94(7)
4
1.76
1056
3.04–29.22
0.0272
Structural studies
β (°)
V (Å3)
In order to get detailed information on the structural para-
meters in complexes 2a and 2b, the molecular structures of
the iridium complexes (Fig. 1 and 2) were determined by
single crystal X-ray diffraction studies. Details of data collec-
tion and refinement are summarized in Table 1.
In both complexes, the coordination geometries around the
iridium centers, formed by the coordination to the metal of
the two olefinic bonds of the cyclooctadiene ligand, the
carbon atom of the NHC ligand and the chlorine atom, are
Z
Dx (g cm−3
F(000)
θ (°)
)
Rint
Data/restraints/parameter
4641/0/226
1.033
2843/0/130
1.043
GOF (F2)
R1, wR2 [I > 2σ(I)]
R1, wR2 (all data)
0.0285, 0.0537
0.0409, 0.0580
0.0368, 0.0888
0.0428, 0.0933
Table 2 Selected bond lengths (Å) and angles (°) for 2a and 2b
Complex 2aa
Ir1–C1
Ir1–C21
Ir1–C25
C1–N1
C1–Ir1–Cl1
M2–Ir1–Cl1
M1–Ir1–Cl1
2.020(4)
2.092(4)
2.155(4)
1.372(5)
89.79(10)
91.30
Ir1–Cl1
Ir1–C22
Ir1–C26
C1–N2
M1–Ir1–C1
M1–Ir1–M2
M2–Ir1–C1
2.3650(11)
2.112(4)
2.194(4)
1.349(5)
91.99
87.29
174.28
176.05
Complex 2bb
Ir1–C1
Ir1–C21
2.028(7)
2.162(5)
1.351(6)
90.37(19)
90.16
Ir1–Cl1
Ir1–C24
2.349(2)
2.092(5)
C1–N1
C1–Ir1–Cl1
M3–Ir1–Cl1
M3–Ir1–C1
M3–Ir1–M4
M4–Ir1–C1
M4–Ir1–Cl1
87.22
92.25
177.39
179.47
a M1 and M2 represent the midpoints of the olefinic bonds C21–C22
and C25–C26, respectively. b M3 and M4 represent the midpoints of
the olefinic bonds C21–C21i and C24–C24i, respectively.
Fig. 1 An ORTEP3 view of 2a with the atom numbering scheme. Displacement
ellipsoids are drawn at the 30% probability level and hydrogen atoms have
been omitted for clarity.
slightly distorted square-planar. The significant bond dis-
tances and angles are tabulated in Table 2.
In 2a, there is one molecule in the asymmetric unit. In 2b,
the asymmetric unit contains a one symmetry independent
half molecule lying on a mirror plane parallel to [010] which
passes through the midpoints of the double bonds of the COD
ligand, the C atom of the NHC ligand, Ir and Cl atoms.
The Ir–C(NHC) distances of 2.020(4) Å (2a) and 2.028(7)
Å (2b) are within the expected range and consistent with pre-
viously characterized [Ir(NHC)(COD)X] complexes.6b,7,9d,h,16a,d,17
The distance between Ir and the midpoint of the double
bond of the COD ligand trans to the NHC ligand is longer than
that trans to chloride with respective distances of 2.063,
1.977 Å for 2a and 2.046, 1.966 Å for 2b. This binding of
cyclooctadiene reflects the larger trans influence of the NHC
ligand as compared to chloride. The dihedral angles between
the least-squares planes of the five- and six-membered ring
Fig. 2 An ORTEP3 view of 2b with the atom numbering scheme. Displacement
ellipsoids are drawn at the 30% probability level and hydrogen atoms have
been omitted for clarity [i: x, 1 − y, z].
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components of the benzimidazole ring systems [maximum was achieved in 1 min. The efficiency difference between cata-
deviations from the least-squares planes of −0.029(4) Å for C1 lyst 2a and 2b could be explained by the electronic effect of
(2a) and 0.029(5) Å for C2 (2b) atoms] are 1.9(2)° and 2.6(3)° benzimidazol-2-ylidene ligands. The methyl groups attached to
for 2a and 2b, respectively. The presence of 5,6-dimethyl the benzene ring of the benzimidazol-2-ylidene ligand in
groups in 2b results in a slight deviation from planarity of the complex 2b give greater donor strength than its nonsubstituted
benzimidazole ring system. There are no classical hydrogen analogue in complex 2a. These results are in accordance with
bonds present in the crystal structures of 2a and 2b. The mole- our previous observations, rhodium(I) complexes with 5,6-
cular packing of crystals is influenced by the van der Waals dimethylbenzimidazol-2-ylidene ligands gave greater activities
forces.
than its nonsubtituted analogue in the catalytic hydrosilyl-
ation10 and TH11 of acetophenone to 1-phenylethanol.
The effect of catalyst concentration was also investigated.
When the catalyst loading of 2b decreased from 0.5% to 0.1%,
only 3% conversion was achieved after 1 min, and after
120 min the conversion was 83%. It could be because of lower
catalyst molecules present “transport phenomena” dominate
the initial period until the catalytic reaction reaches its full
potential, which has been suggested by Herrmann.7
Encouraged by the results obtained with complex 2b, we
decided to determine the catalytic activity of this complex with
a variation of acetophenone and benzaldehyde derivatives
(Table 3). The catalyst was found to be very active for a variety
of substrates. Generally, high conversions, 87 to 100%, were
obtained in 15 to 60 min. It is worth nothing that even the
presence of two ortho substituents in 2,4,6-trimethylbenz-
aldehyde did not lower the conversion (Table 3, entry 7) and total
conversion to the 2,4,6-trimethylbenzylalcohol was achieved
only in 30 min. Within the substrates examined, only the
transformation of 3,4,5-trimethoxybenzaldehyde to 3,4,5-tri-
methoxybenzylalcohol (Table 3, entry 9) was disappointing,
and in this case 56% conversion was observed in 120 min.
Catalytic studies
[Ir(NHC)(COD)Cl] complexes (2a,b) were tested as catalysts for
TH of acetophenone to 1-phenylethanol using 2-propanol as a
hydrogen donor in the presence of KOH (Scheme 4). The cata-
lytic experiments were carried out using 1.0 mmol of aceto-
phenone, 0.005 mmol (0.5 mol%) of 2a,b or 0.001 mmol
(0.1 mol%) of 2b, 0.05 mmol of KOH, 5 mL of 2-propanol, with
a catalyst–base–substrate ratio of 0.5 : 5 : 100 or 0.1 : 5 : 100.
The catalyst was added to a solution of 2-propanol containing
KOH, which was kept at 82 °C for 10 min and acetophenone
was added into this solution. Percentage conversion was calcu-
lated by comparing the methyl proton signals of acetophenone
(s, δ 2.60 ppm) and 1-phenylethanol (d, δ 1.50 ppm, J = 6.8 Hz)
1
in the H NMR spectrum of the crude product in CDCl3 and
the time-dependent conversions were followed (Fig. 3).
It was observed that the activation period was very short for
catalyst 2b with a 0.5% catalyst loading, 63% conversion was
achieved only in 1 min (TOF = 7560 h−1) and for the total con-
version only 10 min was required (TOF = 1200 h−1). These TOF
values could be addressed as one of the notable results for the
NHC-based TH reaction. Under the same reaction conditions,
the tested complex 2a required 60 min (TOF = 200 h−1) for
total conversion, however 50% conversion (TOF = 6000 h−1
)
Conclusions
With respect to TH reactions a comparative study on the influ-
ence of benzene and 5,6-dimethylbenzene annulations at the
4,5-position of 1,3-diisopropylimidazol-2-ylidene iridium(I)
complexes was carried out. It is clear that 4,5-annulation of the
1,3-diisopropylimidazol-2-ylidene ligand with 5,6-dimethyl-
benzene increased the TH performance significantly (TOF values
up to 7560 h−1).
Scheme 4 TH of aldehydes and ketones.
Experimental
General considerations
Unless otherwise noted all manipulations were performed in
air. The solvents were used as received. The reagents were pur-
chased from Sigma-Aldrich, Merck and Alfa Aesar. 1,3-Di-
isopropylbenzimidazolium bromide (1a)12 and [Ir(COD)Cl]2
18
were prepared according to the published procedures. 1H NMR
(400 MHz) and 13C NMR (100 MHz) spectra were measured on
Varian AS 400 Mercury spectrometers with CDCl3 as the
solvent and tetramethylsilane (TMS) as the internal standard.
Chemical shifts (δ) were reported in units (ppm) by assigning
TMS resonance in the 1H spectrum as 0.00 ppm and CDCl3
Fig. 3 Time dependence of the catalytic transfer hydrogenation of aceto-
phenone to 1-phenylethanol.
7308 | Dalton Trans., 2013, 42, 7305–7311
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procedure12 with a small modification. A mixture of 5,6-
dimethylbenzimidazole (1.46 g, 10 mmol) and K2CO3 (1.52 g,
11 mmol) was suspended in CH3CN (10 mL) and stirred at
ambient temperature for 1 h. Isopropyl bromide (2.80 mL,
30 mmol) was then added to the suspension. The reaction
mixture was stirred under reflux conditions for 24 h followed
Table 3 Transfer hydrogenation of aldehydes and ketonesa
Time
(min)
Conv.b
(%)
TOFd
TONc (h−1
Entry Substrate
1
)
15
15
30
100
200
194
186
800
776
372
by
a second addition of isopropyl bromide (2.80 mL,
30 mmol). Stirring under reflux continued for an additional
72 h. After removing the volatiles in vacuo, CH2Cl2 (60 mL) was
added to the residue and the resulting suspension was filtered
over Celite®. The remaining solid was washed with CH2Cl2 (3
× 20 mL) and the solvent of the filtrate was evaporated to give
a colorless solid (3.01 g, 97%). 1H NMR (400 MHz, CDCl3,
TMS, 25 °C, ppm): δ = 10.82 (s, 1 H, NCHN+), 7.48 (s, 2 H, Ar-
H), 5.04 (m, J = 6.8 Hz, 2 H, NCH(CH3)2), 2.36 (s, 6 H, Ar-
(CH3)2), 1.72 (d, J = 6.4 Hz, 12 H, NCH(CH3)2). 13C NMR
(100.6 MHz, CDCl3, TMS, 25 °C, ppm): δ = 138.9 (NCHN+),
137.3 (Ar-C), 129.5 (Ar-C), 113.7
2
3
97
93
4
30
91
182
364
(Ar-C), 52.1 (NCH(CH3)2), 22.4 (Ar-CH3), 20.8 (NCH(CH3)2).
Anal. Calc. for C15H23BrN2: C, 57.88; H, 7.45; N, 9.00. Found:
C, 57.69; H, 7.41; N, 9.06. MS (ESI+): m/z 231.4 [M − Br]+.
Chloro(μ4-1,5-cyclooctadiene)(1,3-diisopropylbenzimidazol-
2-ylidene) iridium(I) (2a). Under an argon atmosphere, a
mixture of 1a (283 mg, 1 mmol) and Ag2O (116 mg, 0.5 mmol)
was suspended in CH2Cl2 (5 mL) and stirred at ambient temp-
erature for 1 h shielded from light. [Ir(COD)Cl]2 (336 mg,
0.5 mmol) was then added to the suspension and the reaction
mixture was stirred at ambient temperature for more than 4 h.
The resulting suspension was filtered over Celite®. The
remaining solid was washed with CH2Cl2 (2 × 2 mL) and the
solvent of the filtrate was evaporated. The residue was purified
by column chromatography on silica gel (eluent: CH2Cl2) to
give pure complex as a yellow solid (511 mg, 95%). 1H NMR
(400 MHz, CDCl3, TMS, 25 °C, ppm): δ = 7.51 (dd, J = 3.0 Hz,
2 H, Ar-H), 7.15 (dd, J = 3.0 Hz, 2 H, Ar-H), 6.19 (m, J = 6.4 Hz,
2 H, NCH(CH3)2), 4.68 (br, 2 H, COD-CH), 3.10 (br, 2 H,
COD-CH), 2.28–2.24 (m, 4 H, COD-CH2), 1.77 (d, J = 7.2 Hz,
6 H, NCH(CH3)2), 1.69–1.64 (m, 4 H, COD-CH2), 1.63 (d, J = 7.2
Hz, 6 H, NCH(CH3)2). 13C NMR (100.6 MHz, CDCl3, TMS,
25 °C, ppm): δ = 190.0 (Ccarbene), 134.0 (Ar-C), 121.9 (Ar-C),
112.6 (Ar-C), 85.5 (COD-CH), 53.8 (NCH(CH3)2), 51.8 (COD-
CH), 33.8 (COD-CH2), 29.6 (COD-CH2), 21.8 (NCH(CH3)2), 20.9
(NCH(CH3)2). Anal. Calc. for C21H30ClIrN2: C, 46.87; H, 5.62;
N, 5.21. Found: C, 46.61; H, 5.52; N, 5.03. MS (ESI+): m/z 502.7
[M − Cl]+.
5
6
30
45
100
96
200
192
400
256
7
8
30
60
100
87
200
174
400
174
9
120
56
112
56
a Substrate (1 mmol), KOH (5 mol%), cat. 2b (0.5 mol%), IPA (5 mL),
82 °C. b Determined by 1H NMR analysis (average of two runs) after the
desired reaction time. c TON: turnover number ((mol of product)/(mol
of catalyst)). d TOF: turnover frequency ([(mol of product)/(mol of
catalyst)]/h).
Chloro(μ4-1,5-cyclooctadiene)(1,3-diisopropyl-(5,6-dimethyl)-
benzimidazol-2-ylidene) iridium(I) (2b). Complex 2b was pre-
pared in analogy to 2a from the salt 1b (311 mg, 1 mmol).
resonance in the 13C spectrum as 77.0 ppm. All coupling con-
stants (J values) were reported in Hertz (Hz). Elemental ana-
lyses were performed on a Perkin-Elmer PE 2400 elemental
analyzer. FTIR spectra were recorded on a Perkin Elmer Spec-
trum 100 series.
1
Yellow solid (498 mg, 88%). H NMR (400 MHz, CDCl3, TMS,
25 °C, ppm): δ = 7.27 (s, 2 H, Ar-H), 6.12 (m, J = 6.8 Hz, 2 H,
NCH(CH3)2), 4.64 (br, 2 H, COD-CH), 3.08 (br, 2 H, COD-CH),
2.34 (s, 6 H, Ar-(CH3)2), 2.29–2.21 (m, 4 H, COD-CH2), 1.75
(d, J = 7.2 Hz, 6 H, NCH(CH3)2), 1.70–1.66 (m, 4 H, COD-CH2),
1.62 (d, J = 7.2 Hz, 6 H, NCH(CH3)2). 13C NMR (100.6 MHz,
Syntheses
1,3-Diisopropyl-(5,6-dimethyl)benzimidazolium
bromide CDCl3, TMS, 25 °C, ppm): δ = 188.4 (Ccarbene), 132.5 (Ar-C),
(1b). This salt was synthesized according to the published 130.7 (Ar-C), 113.1 (Ar-C), 85.0 (COD-CH), 53.6 (NCH(CH3)2),
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51.6 (COD-CH), 33.8 (COD-CH2), 29.6 (COD-CH2), 21.9 (Ar- Et2O and the organic phase separated. The reaction progress
CH3), 20.9 (NCH(CH3)2), 20.5 (NCH(CH3)2). Anal. Calc. for was monitored by 1H-NMR and the results for each experiment
C23H34ClIrN2: C, 48.79; H, 6.05; N, 4.95. Found: C, 48.49; H, are averages over two runs.
5.92; N, 4.86. MS (ESI+): m/z 530.8 [M − Cl]+.
Chloro(dicarbonyl)(1,3-diisopropylbenzimidazol-2-ylidene)
Acknowledgements
iridium(I) (3a). Complex 2a (108 mg, 0.2 mmol) was dissolved
in CH2Cl2 (5 mL) and carbon monoxide was bubbled through
the solution for 30 min. The color of the solution changed
from yellow to pale yellow. The solution was concentrated to
ca. 1 mL and pentane was added. The pale yellow solid that
separated out was filtered and washed with pentane and dried
under reduced pressure (93 mg, 96%). IR (CH2Cl2): νCO = 2069,
We thank Prof. S. T. Astley for helpful discussions. The finan-
cial support from The Turkish Academy of Sciences and Ege
University is gratefully acknowledged. The authors also
acknowledge Dokuz Eylul University for the use of the Agilent
Xcalibur Eos diffractometer (purchased under University
Research Grant No: 2010.KB.FEN.13).
1
1986 cm−1. H NMR (400 MHz, CDCl3, TMS, 25 °C, ppm): δ =
7.57 (dd, J = 3.2 Hz, 2 H, Ar-H), 7.22 (dd, J = 3.2 Hz, 2 H, Ar-H),
5.76 (m, J = 7.2 Hz, 2 H, NCH(CH3)2), 1.63 (t, J = 7.2 Hz, 12 H,
NCH(CH3)2). 13C NMR (100.6 MHz, CDCl3, TMS, 25 °C, ppm):
δ = 181.5 (CO), 180.5 (Ccarbene), 168.2 (CO), 133.4 (Ar-C), 123.5
(Ar-C), 113.7 (Ar-C), 54.9 (NCH(CH3)2), 21.2 (NCH(CH3)2), 20.9
(NCH(CH3)2). C15H18ClIrN2O2: C, 37.07; H, 3.73; N, 5.76.
Found: C, 36.99; H, 3.78; N, 5.82.
Notes and references
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Series No. 6, Royal Society of Chemistry, Cambridge, 2011,
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NHC-bearing complexes, in N-Heterocyclic Carbenes: From
Laboratory Curiosities to Efficient Synthetic Tools, ed. S. Díez-
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Chloro(dicarbonyl)(1,3-diisopropyl-(5,6-dimethyl)benzimid-
azol-2-ylidene) iridium(I) (3b). Complex 3b was prepared in
analogy to 3a from the complex 2b (113 mg, 0.2 mmol).
Pale yellow solid (100 mg, 97%). IR (CH2Cl2): νCO = 2067,
1
1985 cm−1. H NMR (400 MHz, CDCl3, TMS, 25 °C, ppm): δ =
7.31 (s, 2 H, Ar-H), 5.69 (m, J = 7.2 Hz, 2 H, NCH(CH3)2), 2.30
(s, 6 H, Ar-(CH3)2), 1.60 (t, J = 7.2 Hz, 12 H, NCH(CH3)2).
13C NMR (100.6 MHz, CDCl3, TMS, 25 °C, ppm): δ = 181.7
(CO), 179.0 (Ccarbene), 168.3 (CO), 132.7 (Ar-C), 132.0 (Ar-C),
113.8 (Ar-C), 54.6 (NCH(CH3)2), 21.3 (Ar-CH3), 20.9 (NCH
(CH3)2), 20.6 (NCH(CH3)2). C17H22ClIrN2O2: C, 39.72; H, 4.31;
N, 5.45. Found: C, 39.79; H, 4.28; N, 5.39.
X-Ray crystallography
CCDC 898188 and 898189 contain the supplementary crystallo-
graphic data for 2a and 2b, respectively. Intensity data were
collected at room temperature with an Agilent XCalibur X-ray
diffractometer with an EOS CCD detector using Mo-Kα radi-
ation (graphite crystal monochromator λ = 0.7107 Å). The data
collection, cell refinement and data reduction were executed
using the CrysAlisPro19 program. Absorption corrections were
based on multiple scans.19 The crystal structures were solved
by SHELXS-97.20 The refinements (on F2) were carried out by
full-matrix least squares techniques using the SHELXL-97
program.20 All non-hydrogen atoms were refined anisotropi-
cally. All hydrogen atoms were treated as riding atoms.
Thermal ellipsoid plots were generated using the program
ORTEP-3.21
General procedure for the transfer hydrogenation reaction
The tested complex (0.005 mmol; 0.5 mol%) was dissolved in a
solution of KOH (0.5 mmol) and 2-propanol (5 mL) in a two-
necked flask. The solution was heated to 82 °C for 10 min
under reflux conditions. Subsequently, corresponding alde-
hyde or ketone (1 mmol) was added. After the desired reaction
time the reaction was quenched with 1 M HCl, extracted with
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7310 | Dalton Trans., 2013, 42, 7305–7311
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