Efficient transfer hydrogenation using iridium and rhodium complexes of benzannulated N-heterocyclic carbenes

Article abstract of DOI:10.1002/ejic.200800715

Benzimidazol-2-ylidene(NHC) derivatives of iridium(I) and rhodium(I) bearing electron-rich and/or functional substituents R and R′ at the nitrogen atoms of the heterocycle [IrBr(cod)(NHC)] (a) or [RhBr(cod)(NHC)] (b) (R = 2,3,5,6-tetramethylbenzyl; R′ = m

Full text of DOI:10.1002/ejic.200800715

FULL PAPER
DOI: 10.1002/ejic.200800715
Efficient Transfer Hydrogenation Using Iridium and Rhodium Complexes of
Benzannulated N-Heterocyclic Carbenes
Hayati Türkmen,*[a] Tania Pape,[b] F. Ekkehardt Hahn,*[b] and Bekir Çetinkaya[a]
Keywords: Carbenes / Rhodium / Iridium / Hydrogenation / Homogeneous catalysis
Benzimidazol-2-ylidene(NHC) derivatives of iridium(I) and
rhodium(I) bearing electron-rich and/or functional substitu-
ents R and RЈ at the nitrogen atoms of the heterocycle
[IrBr(cod)(NHC)] (a) or [RhBr(cod)(NHC)] (b) (R = 2,3,5,6-tet-
ramethylbenzyl; RЈ = methoxyethyl, 1a,b; R = RЈ = 2,3,5,6-
mental analysis. In addition, the molecular structures of 1a,
2a, and 3a were determined by single-crystal X-ray diffrac-
tion studies. All of the new benzimidazol-2-ylidene iridium(I)
and rhodium(I) complexes were found to be effective cata-
lysts for transfer hydrogenation with iridium(I) derivative 1a
showing the highest catalytic activity.
tetramethylbenzyl, 2a,b;
R = pentamethylbenzyl, RЈ =
methoxyethyl, 3a,b; R = RЈ = pentamethylbenzyl, 4a,b) were
prepared. All compounds were fully characterized by 1H and
13C NMR spectroscopy, mass spectrometry (MALDI), and ele-
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2009)
These complexes show high activities in the transfer hydro-
genation reactions of C=C, C=O, and N=O bonds.[7] In ad-
dition, neutral iridium(III) complexes with bidentate chelat-
ing bis(NHC) ligands are capable of transfer hydrogenation
of aldehydes under basic conditions.[8]
In previous work, we introduced iridium(I) complexes
with N-allyl-substituted benzimidazol-2-ylidene[9] and an-
nulated saturated NHC ligands[10] and studied their activity
in catalytic transfer hydrogenation reactions. Here we report
the preparation of some neutral rhodium(I) and iridium(I)
complexes [MBr(cod)(NHC)] (M = RhI, IrI) with sterically
demanding N,NЈ-substituted benzimidazol-2-ylidene li-
gands. We also report their activity in catalytic transfer hy-
drogenation reactions.
Introduction
Asymmetric catalytic transfer hydrogenation of C=O or
C=N bonds by using 2-propanol as a source of hydrogen is
an attractive method for the preparation of chiral alcohols
and amines.[1] In particular, transfer hydrogenation of
ketones or aldehydes represents a potent strategy for the
generation of alcohols because of high atom efficiency, no
need for elevated pressure, and economic as well as environ-
mental advantages.[2] Today, a broad scope of alcohols have
become accessible by transfer hydrogenation by using non-
toxic hydrogen donors under mild reaction conditions in
the presence of various homogeneous Ir, Rh, or Ru cata-
lysts.[3]
Homogeneous catalysis offers the possibility of finetun-
ing the properties of the catalyst to enhance its selectivity,
because the activity of homogeneous transition-metal cata-
lysts can be controlled by steric and electronic effects ex-
erted by the ligands. Thus, in the past, phosphane com-
plexes have been used as transfer hydrogenation cata-
lysts,[2,3] where the electronic and steric parameters of the
phosphane ligands are easily modified.[4]
Derived from Crabtree’s catalyst [Ir(cod)(py)(PCy3)]PF6
(py = pyridine, cod = η2:η2-1,5-cyclooctadiene),[5] iridium
complexes [Ir(cod)(py)(NHC)]PF6 with N-heterocyclic
carbene (NHC) ligands[6] have recently been introduced.[7]
Results and Discussion
Synthesis and Characterization of Iridium and Rhodium
Carbene Complexes
Complexes with benzimidazol-2-ylidene ligands can be
prepared from the free carbene ligands.[11] However, the free
carbene ligands are normally highly reactive species that are
difficult to handle. Alternative methods for the synthesis of
NHC complexes are the cleavage of electron-rich enetetra-
amine by transition-metal complexes[12] and the template-
controlled intramolecular cyclization of metal-coordinated
β-functionalized isocyanides.[13] The most convenient meth-
ods, however, are the carbene transfer reactions from easily
accessible silver carbene complexes to other transition
metals[14] and the in situ deprotonation of azolium salts by
complexes with basic ligands or counterions like OAc–[15]
or OR–.[9,10]
[a] Department of Chemistry, Ege University,
35100 Bornova-Izmir, Turkey
Fax: +90-232-3888264
E-mail: hayatiturkmen@hotmail.com
[b] Institut für Anorganische und Analytische Chemie, Westfälis-
che Wilhelms-Universität Münster,
Corrensstraße 36, 48149 Münster, Germany
Fax: +49-251-8333108
E-mail: fehahn@uni-muenster.de
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Efficient Transfer Hydrogenation Using Iridium and Rhodium Complexes
This latter method was used for the preparation of com- 2.117(5) Å for 2a]. Similar observations were made for 1a
plexes 1–4 from dinuclear complexes of the type [M(OMe)- and 3a and for additional complexes of the type [IrBr(cod)-
(cod)]2, (M = Rh, Ir) and a benzimidazolium salt (2 equiv.; (benzimidazolin-2-ylidene)].[9]
Scheme 1). All reactions were carried out in toluene, and
the complexes could be isolated in good yields (73–88%) by
precipitation with diethyl ether from the reaction mixture.
Solutions of 1–4 in chlorinated solvents were observed to
be stable towards air and moisture. The identities of the
compounds were confirmed by 1H and 13C NMR spec-
troscopy, MALDI mass spectrometry, and elemental analy-
sis.
Figure 1. Molecular structure of complex 1a (hydrogen atoms have
been omitted). Selected bond lengths [Å] and angles [°]: Ir–Br
2.4920(6), Ir–C1 2.008(4), Ir–C8 2.103(5), Ir–C9 2.114(5), Ir–C12
2.213(5), Ir–C13 2.193(4), N1–C1 1.351(5), N2–C1 1.362(6), C8–
C9 1.392(7), C12–C13 1.396(7); Br–Ir–C1, 87.81(12), Br–Ir–C8
162.54(14), Br–Ir–C9 158.9(2), Br–Ir–C12 93.63(14), Br–Ir–C13
91.46(13), C1–Ir–C8 93.1(2), C1–Ir–C9 92.4(2), C1–Ir–C12
163.4(2), C1–Ir–C13 159.7(2), C8–Ir–C9 38.6(2), C8–Ir–C12
90.4(2), C8–Ir–C13 81.6(2), C9–Ir–C12 80.4(2), C9–Ir–C13 95.5(2),
C12–Ir–C13 36.9(2), N1–C1–N2 106.4(4).
Scheme 1. Preparation of RhIand IrI complexes 1–4.
1
The H NMR spectra of complexes 1a, 1b, 3a, and 3b
Figure 2. Molecular structure of complex 2a in 2a·CH2Cl2 (hydro-
gen atoms have been omitted). Selected bond lengths [Å] and angles
[°]: Ir–Br 2.4803(6), Ir–C1 2.010(4), Ir–C30 2.109(5), Ir–C31
2.117(5), Ir–C34 2.195(5), Ir–C35 2.192(5), N1–C1 1.363(6), N2–
C1 1.361(6), C30–C31 1.406(7), C34–C35 1.390(7); Br–Ir–C1,
90.75(13), Br–Ir–C30 161.58(14), Br–Ir–C31 159.42(14), Br–Ir–C34
93.12(14), Br–Ir–C35 89.61(14), C1–Ir–C30 93.3(2), C1–Ir–C31
88.1(2), C1–Ir–C34 158.4(2), C1–Ir–C35 164.5(2), C30–Ir–C31
38.9(2), C30–Ir–C34 89.7(2), C30–Ir–C35 81.7(2), C31–Ir–C34
80.9(2), C31–Ir–C35 96.9(2), C34–Ir–C35 36.9(2), N1–C1–N2
105.9(4).
exhibit resonances for the O–CH3 groups at δ = 3.36, 3.27,
3.39, and 3.32 ppm, respectively. The 13C NMR spectra
show the characteristic resonances for the benzimidazol-2-
ylidene carbene carbon atom in the range δ = 192.7–
193.5 ppm (for iridium complexes 1a–4a) and δ = 196.2–
198.0 ppm (for rhodium complexes 1b–4b). These values,
as well as the Rh,Ccarbene coupling constants observed for
rhodium complexes 1b–4b [1J(Rh,C) = 49.6–50.2 Hz], fall
in the range observed previously for iridium[9] and rhodi-
um[12e] complexes with benzimidazol-2-ylidene ligands.
The molecular structures of iridium complexes 1a (Fig-
The Ir–Ccarbene bond lengths are identical within experi-
ure 1), 2a (Figure 2), and 3a (Figure 3) were determined by mental error for 1a, 2a, and 3a and fall in the range pre-
single-crystal X-ray diffraction. The iridium atom in the viously observed for square-planar IrI complexes with
complexes is coordinated in a distorted square-planar fash- benzimidazol-2-ylidene ligands.[9] The different N1,N2-sub-
ion by taking the midpoints of the C=C bonds as vertices. stitution pattern of the benzimidazol-2-ylidene ligands in
The Ir–C bond lengths to the C=C bond trans to the car- 1a, 2a, and 3a does not lead to significantly different geo-
bene carbon atom [Ir–C34 2.195(5) Å, Ir–C35 2.192(5) Å metric parameters for the three complexes. No interaction
for 2a], are clearly longer than those to the C=C bond trans of the ether donor in 1a and 3a with the metal center was
to the bromide ligand [Ir–C30 2.109(5) Å, Ir–C31 observed.
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FULL PAPER
Figure 3. Molecular structure of complex 3a (hydrogen atoms have
been omitted). Selected bond lengths [Å] and angles [°]: Ir–Br
2.4907(5), Ir–C1 1.999(4), Ir–C23 2.110(4), Ir–C23 2.118(4), Ir–C27
2.195(4), Ir–C28 2.197(4), N1–C1 1.367(5), N2–C1 1.361(4), C23–
C24 1.422(6), C27–C28 1.385(6); Br–Ir–C1, 87.46(10), Br–Ir–C23
158.70(11), Br–Ir–C24 161.96(12), Br–Ir–C27 91.12(12), Br–Ir–C28
93.48(11), C1–Ir–C23 90.78(14), C1–Ir–C24 94.85(15), C1–Ir–C27
163.4(2), C1–Ir–C28 159.9(2), C23–Ir–C24 39.33(15), C23–Ir–C27
96.4(2), C23–Ir–C28 81.07(15), C24–Ir–C27 81.6(2), C24–Ir–C28
90.5(2), C27–Ir–C28 36.8(2), N1–C1–N2 105.7(3).
Figure 5. Time dependence of the catalytic transfer hydrogenation
of cyclohexanone; catalyst (1a–4b; 0.02 mmol), KOH (0.2 mmol),
cyclohexanone (4 mmol), and 2-propanol (20 mL), T = 353 K.
In order to estimate the differences in the catalytic activi-
ties of complexes 1–4 we applied a low catalyst loading
(0.2 mmol, at 353 K) in the transfer hydrogenation of aceto-
phenone and cyclohexanone (Figures 4 and 5). After
90 min, a 99% yield was achieved for the preparation of 1-
phenyethanol by using complex 1a. A difference in catalytic
activity was, however, observed in the case of the symmetri-
cal ligands (N,NЈ-disubstituted by identical benzylic groups:
4a Ͻ 2a; 4b Ͻ 2b). Our results indicate that complexes 1a
and 3a are highly efficient in the transfer hydrogenation of
acetophenone to 1-phenyethanol and cyclohexanone to cy-
clohexanol. By far the most efficient catalyst is unsymmetri-
cally N,NЈ-substituted complex 1a, which transfer hydroge-
nates benzophenone to Ͼ68% conversion of 13600 h–1 in
only 30 min. This activity is significantly higher than those
of related iridium(I) monocarbene complexes[7] and com-
pares well with those of the most active transfer hydrogena-
tion catalysts previously described.[17] The data indicate
clearly the superiority of complexes 1 and 3 containing a 2-
methoxyethyl substituent on the N3 atom. This may be due
to the hemilabile functionality of the OMe group.[16a,18]
In order to improve the reactivity of our catalytic system,
we examined the influence of base. The reduction of aceto-
phenone, taken as a model ketone, was carried out in 2-
propanol by using iridium complex 1a. The reactions were
studied at 353 K over 90 min. Typically, to solutions con-
taining acetophenone and catalyst 1a, different bases were
added and the yields obtained are shown in Figure 6. With
the organic bases triethylamine and pyridine poor yields
were observed. By using the inorganic bases Cs2CO3,
K2CO3, KOH, NaOH and tBuOK the conversion was
strongly dependent upon the base strength with the
stronger bases giving higher yields (K2CO3 Ͻ Cs2CO3 Ͻ
tBuOK Ͻ NaOH Ͻ KOH). As in the previous study,[10]
the best results were obtained with KOH and other bases
required longer reaction times.
Catalytic Studies
We reported that the activities of NHC complexes are
influenced by the steric bulk and electronegativities of para
substituents.[10,16] So we have devoted efforts to the study
of the sterically similar N-benzylic substituents on the com-
mon benzimidazolylidene framework. Both rhodium– and
iridium–NHC complexes 1–4 are efficient precursors for the
catalytic transfer hydrogenation of ketones to alcohols by
using 2-propanol as a hydrogen donor in the presence of a
suitable base. The yields for transfer hydrogenation of ace-
tophenone and cyclohexanone (reaction time 180 min) were
studied (Figures 4 and 5). The highest catalytic activity was
observed with iridium complex 1a bearing the sterically le-
ast encumbered, unsymmetrically N,NЈ-substituted benz-
imidazol-2-ylidene ligand.
Figure 4. Time dependence of the catalytic transfer hydrogenation
of acetophenone; catalyst (1a–4b, 0.02 mmol), KOH (0.2 mmol),
acetophenone (4 mmol), and 2-propanol (20 mL), T = 353 K.
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Efficient Transfer Hydrogenation Using Iridium and Rhodium Complexes
Bromo(η4-1,5-cylooctadiene)[1-(2,3,5,6-tetramethylbenzyl)-3-(2-
methoxyethyl)benzimidazol-2-ylidene]rhodium(I) (1b): Yield: 80%.
1H NMR (400 MHz, CDCl3): δ = 7.34 (d, J = 2.1 Hz, 1 H, Ar-H
NHC), 7.01 [s, 1 H, Ar-H C6H(CH3)4], 6.97 (t, J = 2.1 Hz, 1 H,
Ar-H NHC), 6.71 (t, J = 2.1 Hz, 1 H, Ar-H NHC), 6.27 [d, J =
4.1 Hz, 1 H, N-CHH-C6H(CH3)4], 5.9 (d, J = 2.1 Hz, 1 H, Ar-H
NHC), 5.88 [d, J = 4.1 Hz, 1 H, N-CHH-C6H(CH3)4], 5.17 (m, 3
H, cod-CH=CH and NCH 2 CH2 OCH3 ), 4. 84 (m, 2 H,
NCH2CH2OCH3), 4.01, 3.51, 3.43 (s, 3 H, cod-CH=CH), 3.32 (s,
3 H, NCH2CH2OCH3), 2.33 (m, 4 H, cod-CH2), 2.25 [s, 6 H,
C6H(CH3)4-o-CH3], 2.19 [s, 6 H, C6H(CH3)4-m-CH3], 1.94 (m, 4
H, cod-CH2) ppm. 13C NMR (100 MHz, CDCl3): δ = 196.2 (d, J
= 50.2 Hz, Ccarbene), 134.5, 134.2, 133.9, 131.4, 129.9, 121.2, 120.7,
109.5, 109.3 (Ar-C), 99.5, (d, J = 6.1 Hz, cod-CH), 98.4 (d, J =
4.2 Hz, cod-CH), 70.4 (NCH2CH2OCH3), 69.4 (d, J = 14.5 Hz,
cod-CH), 68.2 (d, J = 13.7 Hz, cod-CH), 58.1 (NCH2CH2OCH3),
50.0 [NCH2C6H(CH3)4], 47.6 (NCH2CH2OCH3), 31.9, 31.5, 28.1,
27.9 (cod-CH2), 19.5 [C6H(CH3)4-m-CH3], 15.4 [C6H(CH3)4-o-
CH3] ppm. C29H38BrN2ORh (613.43): calcd. C 56.78, H 6.24, N
4.57; found C 56.77, H 6.32, N 4.71. MS (MALDI): m/z = 533
[M – Br]+.
Figure 6. Influence of the base on the transfer hydrogenation of
acetophenone by using catalyst 1a (0.02 mmol), acetophenone
(4 mmol), and 2-propanol (20 mL), T = 353 K, t = 90 min.
Conclusions
Iridium(I) and rhodium(I) complexes bearing electron-
rich benzimidazol-2-ylidene ligands were prepared by the
reaction of suitable benzimidazolium salts with [M(OMe)-
(cod)]2 (M = Ir, Rh). The complexes show high activity in
the catalytic transfer hydrogenation of acetophenone and
cyclohexanone in 2-propanol. The iridium complex with the
sterically least encumbered unsymmetrically N,NЈ-substi-
tuted benzimidazol-2-ylidene ligands shows the highest
catalytic activity, which is further influenced by the type of
base used.
Bromo(η4-1,5-cylooctadiene)[1,3-bis(2,3,5,6-tetramethylbenzyl)-
benzimidazol-2-ylidene]iridium(I) (2a): Yield: 87 %. 1H NMR
(400 MHz, CDCl3): δ = 7.05 [s, 2 H, Ar-H C6H(CH3)4], 6.70 (m, 2
H, Ar-H NHC), 6.52 [d, J = 4.0 Hz, 2 H, N-CHH-C6H(CH3)4],
6.10 (m, 2 H, Ar-H NHC), 5.84 [d, J = 4.0 Hz, 2 H, N-CHH-
C6H(CH3)4], 4.94 (br., 2 H, cod-CH=CH), 3.24 (br., 2 H, cod-
CH=CH), 2.26 [s, 12 H, C6H(CH3)4-o-CH3], 2.24 [s, 12 H,
C6H(CH3)4-m-CH3], 1.87 (m, 4 H, cod-CH2), 1.61 (m, 4 H, cod-
CH2) ppm. 13C NMR (100 MHz, CDCl3): δ = 193.5 (Ccarbene),
135.3, 134.6, 134.0, 132.3, 130.7, 121.7, 110.8 (Ar-C), 85.6, 53.0
(cod-CH), 50.8 [NCH2C6H(CH3)4], 33.2, 29.4 (cod-CH2), 20.5
[C6H(CH3)4-m-CH3], 16.5 [C6H(CH3)4-o-CH3] ppm. C37H46BrIrN2
(790.89): calcd. C 56.19, H 5.86, N 3.54; found C 56.23, H 5.94, N
3.57. MS (MALDI): m/z = 710 [M – Br]+.
Experimental Section
General Procedure for the Preparation of Complexes [IrBr(η4-cod)-
(NHC)] (complex type a) and [RhBr(η4-cod)(NHC)] (complex type
Bromo(η4-1,5-cylooctadiene)[1,3-bis(2,3,5,6-tetramethylbenzyl)-
benzimidazol-2-ylidene]rhodium(I) (2b): Yield: 73 %. 1H NMR
(400 MHz, CDCl3): δ = 7.06 [s, 2 H, Ar-H C6H(CH3)4], 6.64 (m, 2
H, Ar-H NHC), 6.08 (m, 2 H, Ar-H NHC), 6.03, 5.99 [d, J =
4.1 Hz, 4 H, N-CHH-C6H(CH3)4], 5.27 (s, 2 H, cod-CH=CH), 3.66
(s, 2 H, cod-CH=CH), 2.44 (m, 4 H, cod-CH2), 2.30 [s, 12 H,
C6H(CH3)4-o-CH3], 2.24 [s, 12 H, C6H(CH3)4-m-CH3], 2.00 (m, 4
H, cod-CH2) ppm. 13C NMR (100 MHz, CDCl3): δ = 197.9 (d, J
= 49.6 Hz, Ccarbene), 135.6, 135.0, 134.4, 132.6, 131.3, 121.8, 110.8
(Ar-C), 99.8 (d, J = 6.1 Hz, cod-CH), 70.0 (d, J = 14.5 Hz, cod-
CH), 51.5 [NCH2C6H(CH3)4], 29.2, 33.0 (cod-CH2), 20.8
[ C6 H ( CH3 )4 - m- CH3 ] , 1 6. 4 [ C6 H(CH 3 )4 - o- CH3 ] pp m.
C37H46BrN2Rh (701.58): calcd. C 63.34, H 6.61, N 3.99; found C
63.51, H 6.73, N 4.01. MS (MALDI): m/z = 622 [M – Br]+.
b): The salts were prepared according to known methods.[9,16]
A
sample of 1,3-alkylbenzimidazolium bromide (0.64 mmol) and
[Ir(µ-OMe)cod]2 or [Rh(µ-OMe)cod]2 (0.32 mmol) were suspended
in toluene (8 mL) in a Schlenk tube. The reaction mixture was
heated under reflux overnight. Insoluble reaction products were
separated by filtration and diethyl ether was added to the solution
causing precipitation of the metal complexes. They were separated
by filtration and dried under reduced pressure. The solid complexes
can be recrystallized from CH2Cl2/EtOH.
Bromo(η4-1,5-cylooctadiene)[1-(2,3,5,6-tetramethylbenzyl)-3-(2-
methoxyethyl)benzimidazol-2-ylidene]iridium(I) (1a): Yield: 83%. 1H
NMR (400 MHz, CDCl3): δ = 7.42 (d, J = 2.2 Hz, 2 H, Ar-H
NHC), 7.06 [s, 1 H, Ar-H C6H(CH3)4], 6.26 [d, J = 2.2 Hz, 1 H,
N-CHH-C6H(CH3)4], 6.03 (d, J = 2.2 Hz, 2 H, Ar-H NHC), 5.79
[d, J = 2.2 Hz, 1 H, N-CHH-C6H(CH3)4], 5.12 (m, 1 H, cod-
Bromo(η4-1,5-cylooctadiene)[(1-pentamethylbenzyl)-3-(2-methoxy-
CH=CH), 4.76 (m, 3 H, cod-CH=CH and NCH2CH2OCH3), 3.96 ethyl)benzimidazol-2-ylidene]iridium(I) (3a): Yield: 88%. 1H NMR
(m, 2 H, NCH2CH2OCH3), 3.36 (s, 3 H, NCH2CH2OCH3), 3.16 (400 MHz, CDCl3): δ = 7.43 (d, J = 2.0 Hz, 1 H, Ar-H NHC), 7.06
(m, 1 H, cod-CH=CH), 3.03 (m, 1 H, cod-CH=CH), 2.25 [s, 6 H,
C6H(CH3)4-o-CH3], 2.20 [s, 6 H, C6H(CH3)4-m-CH3], 1.82 (m, 4 NHC), 6.26 [d, J = 4.1 Hz, 1 H, N-CHH-C6(CH3)5], 6.09 (d, J =
H, cod-CH2), 1.59 (m, 4 H, cod-CH2) ppm. 13C NMR (100 MHz,
2.1 Hz, 1 H, Ar-H NHC), 5.78 [d, J = 4.1 Hz, 1 H, N-CHH-
CDCl3): δ = 192.9 (Ccarbene), 135.6, 135.0, 134.8, 134.0, 132.3, C6(CH3)5], 5.12 (m, 1 H, cod-CH=CH), 4.78 (m, 3 H, cod-CH=CH
(t, J = 2.0 Hz, 1 H, Ar-H NHC), 6.80 (t, J = 2.0 Hz, 1 H, Ar-H
122.3, 121.7, 110.9, 110.7, 110.5 (Ar-C), 85.9, 85.8 (cod-CH), 71.4
(NCH2CH2OCH3), 59.1 (NCH2CH2OCH3), 53.7, 52.7 (cod-CH),
and NCH2CH2OCH3), 3.99 (m, 2 H, NCH2CH2OCH3), 3.39 (s, 3
H, NCH2CH2OCH3), 3.21 (m, 1 H, cod-CH=CH), 3.06 (m, 1 H,
50.6 [NCH2C6H(CH3)4], 47.9 (NCH2CH2OCH3), 33.3, 33.1, 29.6, cod-CH=CH), 2.32 [s, 3 H, C6(CH3)5-p-CH3], 2.27 [s, 6 H, C6-
29.4 (cod-CH2), 20.5 [C6H(CH3)4-m-CH3], 16.3 [C6H(CH3)4-o- (CH3)5-m-CH3], 2.25 (s, 6 H, C6(CH3)5-o-CH3), 1.84 (m, 4 H, cod-
CH3] ppm. C29H38BrIrN2O (702.74): calcd. C 49.56, H 5.45, N
CH2), 1.60 (m, 4 H, cod-CH2) ppm. 13C NMR (100 MHz, CDCl3):
3.99; found C 49.57, H 5.42, N 4.01.
δ = 192.7 (Ccarbene), 135.5, 135.1, 134.3, 132.8, 127.9, 122.2, 121.6,
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111.1, 110.4 (Ar-C), 85.7, 85.6 (cod-CH), 71.3 (NCH2CH2OCH3),
59.0 (NCH2CH2OCH3), 53.6, 52.6 (cod-CH), 51.1 [NCH2C6-
(CH3)5], 47.9 (NCH2CH2OCH3), 33.2, 33.0, 29.5, 29.4 (cod-CH2),
17.9 [C6(CH3)5-o-CH3], 17.5 [C6(CH3)5-p-CH3], 17.0 [C6(CH3)5-m-
CH3] ppm. C30H40BrIrN2O (716.77): calcd. C 50.27, H 5.62, N
3.91; found C 50.33, H 5.54, N 3.89. MS (MALDI): m/z = 637
[M – Br]+.
(4 mmol) was added with an Eppendorf pipet. The reaction pro-
gress was monitored by GC analysis.
X-ray Diffraction Studies: Diffraction data for 1a, 2a·CH2Cl2, and
3a were collected with a Bruker AXS APEX CCD diffractometer
equipped with a rotation anode at 153(2) K by using graphite mo-
nochromated Mo-Kα radiation (λ = 0.71073 Å). Diffraction data
were collected over the full sphere and were corrected for absorp-
tion. The data reduction was performed with the Bruker
SMART[19] program package. Structure solutions were found with
Bromo(η4-1,5-cylooctadiene)[(1-pentamethylbenzyl)-3-(2-methoxy-
ethyl)benzimidazol-2-ylidene]rhodium(I) (3b): Yield: 79%. 1H NMR
(400 MHz, CDCl3): δ = 7.33 (d, J = 2.2 Hz, 1 H, Ar-H NHC), 6.96 the SHELXS-97[20] package by using the heavy-atom method and
(m, 1 H, Ar-H NHC), 6.70 (m, 1 H, Ar-H NHC), 6.27 [d, J = were refined with SHELXL-97[21] against |F2| by using first iso-
4.0 Hz, 1 H, N-CHH-C6(CH3)5], 5.99 (d, J = 2.2 Hz, 1 H, Ar-H tropic and later anisotropic thermal parameters for all non-hydro-
NHC), 5.88 [d, J = 4.0 Hz, 1 H, N-CHH-C6(CH3)5], 5.16 (m, 3 H, gen atoms. Hydrogen atoms were added to the structure models on
cod-CH=CH and NCH2CH2OCH3), 5.27 (s, 1 H, cod-CH=CH), calculated positions. CCDC-695451 (for 1a), -691392 (for
4.02 (m, 1 H, NCH2CH2OCH3), 3.94 (m, 1 H, NCH2CH2OCH3), 2a·CH2Cl2), and -691393 (for 3a) contain the supplementary crys-
3.52, 3.43 (s, 2 H, cod-CH=CH), 3.32 (s, 3 H, NCH2CH2OCH3),
2.35 (m, 4 H, cod-CH2), 2.30 [s, 6 H, C6(CH3)5-o-CH3], 2.19 [s, 6
tallographic data for this paper. These data can be obtained free
of charge from The Cambridge Crystallographic Data Centre via
H, C6(CH3)5-m-CH3], 2.18 [s, 3 H, C6(CH3)5-p-CH3], 1.92 (m, 4 H, www.ccdc.cam.ac.uk/data_request/cif.
cod-CH2) ppm. 13C NMR (100 MHz, CDCl3): δ = 197.3 (d, J =
Crystal Data for 1a: C29H38N2BrOIr, M = 702.72, µ = 6.521 mm–1,
49.6 Hz, Ccarbene), 136.1, 135.7, 135.6, 134.7, 133.2, 128.5, 122.5,
121.8, 111.2, 110.6 (Ar-C), 99.7 (d, J = 6.1 Hz, cod-CH), 99.5 (d,
J = 6.1 Hz, cod-CH), 71.7 (NCH2CH2OCH3), 70.5 (d, J = 14.5 Hz,
cod-CH), 69.4 (d, J = 14.0 Hz, cod-CH), 59.3 (NCH2CH2OCH3),
51.8 [NCH2C6(CH3)5], 48.7 (NCH2CH2OCH3), 29.1, 29.3, 32.7,
33.08 (cod-CH2), 17.7 [C6(CH3)5-p-CH3], 17.5 [C6(CH3)5-m-CH3],
17.1 [C6(CH3)5-o-CH3] ppm. C30H40BrN2ORh (627.46): calcd. C
57.43, H 6.73, N 4.46; found C 57.57, H 6.82, N 4.61. MS
(MALDI): m/z = 548 [M – Br]+.
ρ = 1.748 gcm–3, monoclinic, P21/n, Z = 4, a = 15.618(3) Å, b =
7.4645(14) Å, c = 23.833(5) Å, β = 106.062(4)°, V = 2670.0(9) Å3,
29904 measured reflections, 7773 unique reflections (Rint = 0.0539),
R = 0.0392, wR = 0.0832 for 5907 contributing reflections
[IՆ2σ(I)], refinement against |F2| with anisotropic thermal param-
eters for all non-hydrogen atoms and hydrogen atoms on calculated
positions. The asymmetric unit contains one molecule of 1a.
Crystal Data for 2a·CH2Cl2: C38H48N2BrCl2Ir, M = 875.79, µ =
–1
–3
¯
5.124 mm , ρ = 1.657 gcm , triclinic, P1, Z = 2, a = 9.0824(14) Å,
b = 11.968(2) Å, c = 17.802(3) Å, α = 83.499(3)°, β = 78.275(3)°, γ
= 67.980(3)°, V = 1755.0(4) Å3, 20486 measured reflections, 10123
unique reflections (Rint = 0.0417), R = 0.0438, wR = 0.0988 for
8496 contributing reflections [IՆ2σ(I)], refinement against |F2|
with anisotropic thermal parameters for all non-hydrogen atoms
and hydrogen atoms on calculated positions. The asymmetric unit
contains one molecule of 2a and one molecule of CH2Cl2.
Bromo(η4-1,5-cylooctadiene)[1,3-bis(pentamethylbenzyl)benz-
imidazol-2-ylidene]iridium(I) (4a): Yield: 85%. 1H NMR (400 MHz,
CDCl3): δ = 6.66 (m, 2 H, Ar-H NHC), 6.49, 6.41 [d, J = 4.0 Hz,
2 H, N-CHH-C6H(CH3)4], 6.14 (m, 2 H, Ar-H NHC), 5.86, 5.78
[d, J = 4.1 Hz, 2 H, N-CHH-C6H(CH3)4], 4.83 (br., 2 H, cod-
CH=CH), 3.27 (m, 2 H, cod-CH=CH), 2.31 [s, 12 H, C6(CH3)5-p-
CH3], 2.29 [s,12 H, C6(CH3)5-m-CH3], 2.25 [s, 6 H, C6(CH3)5-o-
CH3], 1.84 (m, 4 H, cod-CH2), 1.62 (m, 4 H, cod-CH2) ppm. 13C
NMR (100 MHz, CDCl3): δ = 193.3 (Ccarbene), 135.6, 135.4, 134.2,
132.8, 128.1, 121.5, 110.8 (Ar-C), 85.4, 52.9 (cod-CH), 51.3
[NCH2C6(CH3)5], 33.2, 29.5, (cod-CH2), 17.4 [C6(CH3)5-o-CH3],
17.2 [C6 (CH3 )5 -p-CH3 ], 16.8 [C6 (CH3 )5 -m-CH3 ] ppm.
C39H50BrIrN2 (818.94): calcd. C 57.20, H 6.15, N 3.42; found C
57.27, H 6.22, N 3.39. MS (MALDI): m/z = 739 [M – Br]+.
Crystal Data for 3a: C30H40N2BrIrO, M = 716.75, µ = 6.250 mm–1,
ρ = 1.709 gcm–3, monoclinic, P21/c, Z = 4, a = 15.460(3) Å, b =
7.4701(13) Å, c = 24.613(4) Å, β = 101.406(4)°, V = 2786.4(8) Å3,
30953 measured reflections, 8110 unique reflections (Rint = 0.0530),
R = 0.0333, wR = 0.0711 for 6716 contributing reflections
[IՆ2σ(I)], refinement against |F2| with anisotropic thermal param-
eters for all non-hydrogen atoms and hydrogen atoms on calculated
positions. The asymmetric unit contains one molecule of 3a.
Bromo(η4-1,5-cylooctadiene)[1,3-bis(pentamethylbenzyl)benz-
imidazol-2-ylidene]rhodium(I) (4b): Yield: 73%. 1H NMR
(400 MHz, CDCl3): δ = 6.64 (m, 2 H, Ar-H NHC), 6.57 [d, J =
4.0 Hz, 2 H, NCHH-C6(CH3)5], 6.31 (m, 2 H, Ar-H NHC), 6.01
[d, J = 4.1 Hz, 2 H, NCHH-C6(CH3)5], 5.24 (s, 2 H, cod-CH=CH),
3.60 (s, 2 H, cod-CH=CH), 2.38 (m, 4 H, cod-CH2), 2.27 [s, 12 H,
C6(CH3)5-o-CH3], 2.20 [s, 12 H, C6(CH3)5-m-CH3], 2.11 [s, 6 H,
C6(CH3)5-p-CH3], 2.01 (m, 4 H, cod-CH2) ppm. 13C NMR
(100 MHz, CDCl3): δ = 198.0 (d, J = 50.0 Hz, Ccarbene), 135.8,
135.3, 134.1, 132.2, 131.0, 120.8, 111.7 (Ar-C), 99.4 (d, J = 6.1 Hz,
cod-CH), 70.0 (d, J = 14.5 Hz, cod-CH), 51.2 [NCH2C6(CH3)5],
33.1, 29.1 (cod-CH2), 21.3 [C6(CH3)5-p-CH3], 20.8 [C6(CH3)5-m-
CH3], 16.4 [C6(CH3)5-o-CH3] ppm. C39H50BrN2Rh (729.63): calcd.
C 64.20, H 6.91, N 3.84; found C 64.21, H 6.99, N 3.94. MS
(MALDI): m/z = 649 [M – Br]+.
Acknowledgments
The authors thank the Deutsche Forschungsgemeinschaft and the
Fonds der Chemischen Industrie for financial support.
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(0.02 mmol) was dissolved in a solution of KOH (0.2 mmol), dieth-
yleneglycol-di-n-butyl ether (0.3 mmol, internal standard), and 2-
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© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2008, 5418–5423

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CH2CH2OH) was prepared in a similar manner and charac-
terized by NMR spectroscopy. Yield: 82%. 1H NMR
(400 MHz, CDCl3): δ = 7.30 (d, J = 2.1 Hz, 1 H, Ar-H NHC),
7.26 (s, 1 H, Ar-H NHC), 7.17 (t, J = 2.1 Hz, 1 H, Ar-H NHC),
7.09–7.04 [m, 2 H, Ar-H NHC, C6H(CH3)4], 6.80 (t, J =
2.1 Hz, 1 H, Ar-H NHC), 6.34 [d, J = 4.1 Hz, 1 H, N-CHH-
C6H(CH3)4], 6.04–6.00 [m, 2 H, N-CHH-C6H(CH3)4 and
NCH2CH2OH confirmed with D2O exchange experiment],
5.31–5.25 (m, 3 H, cod-CH=CH and NCH2CH2OH), 4.68 (s,
2 H, NCH2CH2OCH3), 4.22, 3.58, 3.50 (s, 3 H, cod-CH=CH),
2.51–2.42 (m, 4 H, cod-CH2), 2.27 [s, 6 H, C6H(CH3)4-o-CH3],
2.22 [s, 6 H, C6H(CH3)4-m-CH3], 2.06–1.99 (m, 4 H, cod-CH2)
ppm. 13C NMR (100 MHz, CDCl3): δ = 196.4 (d, J = 50.1 Hz,
Ccarbene), 135.6, 135.2, 134.4, 131.1, 129.2, 128.9, 125.5, 122.7,
122.1, 111.3, 109.9, 109.3 (Ar-C), 99.5, (d, J = 6.1 Hz, cod-
CH), 96.4 (d, J = 4.2 Hz, cod-CH), 70.7 (d, J = 14.5 Hz, cod-
CH), 69.8 (d, J = 13.7 Hz, cod-CH), 60.9 (NCH2CH2OH), 51.6
[NCH2C6H(CH3)4], 50.7 (NCH2CH2OH), 33.1, 32.7, 29.3, 28.2
(cod-CH2), 20.7 [C6H(CH3)4-m-CH3], 16.7 [C6H(CH3)4-o-CH3]
ppm. The above complex was tested as a catalyst in the transfer
hydrogenation of acetophenone, but in comparison to 1b (val-
ues in parentheses) only less efficiency was observed under the
same conditions: 17 (56), 30 (61), 48 (73), 60 (82), and 68 (93)%
after 30, 60, 90, 120, and 180 min, respectively.
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Published Online: October 29, 2008
Eur. J. Inorg. Chem. 2008, 5418–5423
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
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