Rhodium(III) complexes containing C4-bound N-heterocyclic carbenes: Synthesis, coordination chemistry, and catalytic activity in transfer hydrogenation

Article abstract of DOI:10.1021/om800110c

Direct metalation of C2-protected diimidozolium salts with RhCl₃ or [RhCl(cod)]₂ and KI afforded a series of new rhodium(III) complexes with abnormally C4-bound, cis-chelating NHC ligands. The complexes were isolated as dimetallic species containing two (μ²-I) ₃-bridged rhodium(III) centers. In the presence of coordinating ligands such as CH₃CN, PPh₃, or dppe, the dimeric structure is readily cleaved and yields monometallic complexes. Crystallographic analysis of representative structures indicates a higher trans influence of abnormally C4-bound carbenes as compared to normal NHCs. The exceptionally strong donor ability of carbenes in such a C4 coordination mode increases the catalytic activity of the rhodium center and allows for efficient transfer hydrogenation of ketones in iPrOH/KOH.

Full text of DOI:10.1021/om800110c

Organometallics 2008, 27, 3161–3171
3161
Rhodium(III) Complexes Containing C4-Bound N-Heterocyclic
Carbenes: Synthesis, Coordination Chemistry, and Catalytic Activity
in Transfer Hydrogenation
Liangru Yang,†,§ Anneke Krüger,† Antonia Neels,‡ and Martin Albrecht*,†
Department of Chemistry, UniVersity of Fribourg, Chemin du Muse´e 9, 1700 Fribourg,
Switzerland, and Institute of Microtechnology, UniVersity of Neuchaˆtel, Rue Jaquet-Droz 1,
2002 Neuchaˆtel, Switzerland
ReceiVed February 7, 2008
Direct metalation of C2-protected diimidozolium salts with RhCl3 or [RhCl(cod)]2 and KI afforded
a series of new rhodium(III) complexes with abnormally C4-bound, cis-chelating NHC ligands. The
complexes were isolated as dimetallic species containing two (µ2-I)3-bridged rhodium(III) centers.
In the presence of coordinating ligands such as CH3CN, PPh3, or dppe, the dimeric structure is
readily cleaved and yields monometallic complexes. Crystallographic analysis of representative
structures indicates a higher trans influence of abnormally C4-bound carbenes as compared to normal
NHCs. The exceptionally strong donor ability of carbenes in such a C4 coordination mode increases
the catalytic activity of the rhodium center and allows for efficient transfer hydrogenation of ketones
in iPrOH/KOH.
Chart 1. Normally C2-Bound Imidazolylidene Ligand (A)
and Abnormally C4-Bound Carbene Ligand (B)
Introduction
N-Heterocyclic carbenes (NHCs) have emerged as a very
powerful class of ligands in organometallic chemistry and
homogeneous catalysis.1 Most of the reported NHCs are derived
from imidazolium salts and bind the metal via the C2 carbon
(Chart 1) and hence comprise a classical carbene bonding mode.2
Recently, it has been reported that, under specific conditions,
metalation of imidazolium salts takes place at the C4 or C5
* To whom correspondence should be addressed. E-mail: martin.albrecht@
unifr.ch. Fax: +41-263009738.
position (Chart 1).3 These so-called abnormal carbenes are
exceptionally basic ligands with a higher donor power than their
C2-bound analogues.4,5 The first evidence has been provided
for the beneficial impact of such C4-bound carbenes on the
catalytic activity of the coordinated metal center.5,6
† University of Fribourg.
‡ University of Neuchaˆtel.
§ Current address: School of Chemistry and Chemical Engineering, Henan
University of Techology, P. R. China.
(1) (a) Arduengo, A. J. Acc. Chem. Res. 1999, 32, 913. (b) Bourissou,
D.; Guerret, O.; Gabbai, F. P.; Bertrand, G. Chem. ReV. 2000, 100, 39. (c)
Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41, 1290. (d) Hahn, F. E.
Angew. Chem., Int. Ed. 2008, 47, 3122. (e) Topics in Organometallic
Chemistry; Glorius F., Ed.; 2007; Vol. 21. (f) Coord. Chem. ReV. (Crabtree,
R. H. Ed.) 2005, 251, 5-6. (g) J. Organomet. Chem. (Bertrand, G., Ed.)
2005, 690, 23-25. (h) N-heterocyclic carbenes in synthesis; Nolan, S. P.,
Ed.; Wiley-VCH: Weinheim, 2006.
Theoretical investigations on the reactivity of imidazolium
salts have indicated that formation of the free C4-carbene is
less favored than the C2-carbene by approximately 80 kJ mol-1 7
.
Similarly, the pKa of the C4-bound hydrogen has been calculated
to be about 5-8 pKa units higher8 than that of the hydrogen
attached to C2.9 Despite these drawbacks, various methods have
been established for installing the metal selectively at the C4
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from imidazolium salts, see:(a) Fraser, P. J.; Roper, W. R.; Stone, F. G. A.
J. Chem. Soc., Dalton Trans. 1974, 760. (b) Crociani, B.; Di Bianca, F.;
Giovenco, A.; Berton, A. J. Organomet. Chem. 1987, 323, 123. (c) Cave,
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Ed. 1998, 37, 3270. (d) Koizumi, T.; Tomon, T.; Tanaka, K. Organome-
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Lavallo, V.; Canac, Y.; DeHope, A.; Donnadieu, B.; Bertrand, G. Angew.
Chem., Int. Ed. 2005, 44, 7236. (h) Schneider, S. K.; Roembke, P.; Julius,
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Int. Ed. 2007, 46, 6293.
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Ehlers, A. Personal communication.
10.1021/om800110c CCC: $40.75
2008 American Chemical Society
Publication on Web 05/28/2008

3162 Organometallics, Vol. 27, No. 13, 2008
Scheme 1. Synthesis of C2- and C4-Bound Dicarbene Rhodium(III) Complexes 3a-ga
Yang et al.
a
Reagents and conditions: (i) CH2I2, toluene, reflux; (ii) [RhCl(cod)]2, NaOAc, KI, MeCN, reflux, or RhCl3, NaOAc, MeCN, reflux, then NaI,
acetone; (iii) [RhCl(cod)]2, NaOAc, KI, MeCN, 60 °C; (iv) MeCN, RT.
position.10 Some approaches are highly reagent-specific such
as oxidative C-H bond activation,11 rearrangement of C2-
imidazolylidenes,12 and anion-triggered differentiation between
the C2-H and C4-H positions.3,13 A more general access to
C4-bound NHC complexes consists of quaternizing an anionic
azolyl ligand14 or using halide-functionalized imidazolium salts
for oxidative addition.15 A particularly useful route was
introduced by Crabtree and co-workers and relies on the
protection of the imidazolium C2 position,4 typically by an alkyl
or aryl substituent.16 On the basis of this approach, we have
recently prepared chelating C4-bound dicarbene palladium
complexes.5 Owing to the rigid cis-chelation of the dicarbene
and the strong donor power of the ligand, these complexes are
significantly better catalysts for olefin hydrogenation than their
C2-bound analogues. These results stimulated our interest in
extending the abnormal carbene concept to different catalytic
processes where bond activation requires electron-rich metal
centers.17 Here we report on the synthesis and (catalytic)
reactivity of the first rhodium(III) complexes bound to C4-
coordinating N-heterocyclic dicarbenes. Most strikingly, the
catalytic activity of the rhodium center in transfer hydrogenation
is markedly increased when bound to C4- rather than C2-
coordinating carbenes. These results underline the high potential
of this class of carbene ligands in catalysis.
Results and Discussion
Synthesis of the Ligands and Rhodium(III) Complexes.
The C2-protected diimidazolium salts 1a-e were prepared from
the corresponding 1,2-disubstituted imidazoles18 by condensation
with CH2I2 in an apolar solvent (Scheme 1).19 Complexation
of these ligand precursors was accomplished according to a
procedure similar to the one established for the metalation of
C2-bonding dicarbene precursors.20 Reaction of the C2-protected
diimidazolium salts 1a-e with [RhCl(cod)]2 (cod ) 1,5-
cyclooctadiene) in the presence of iodide ions and acetate as a
mild base induced C-H bond activation and metalation at
the C4 position (Scheme 1).21 Purification by column chroma-
tography and subsequent recrystallization afforded the rhod-
ium(III) complexes 2a-e as dimeric tris-(µ2-iodo)-bridged
species. Notably, no methyl C-H bond activation was detected
when using ligand 1a, 1c, or 1e. This contrasts with previous
metalation results using Ag(I) and Ir(III) precursors, where
(9) (a) Amyes, T. L.; Diver, S. T.; Richard, J. P.; Rivas, F. M.; Toth,
K. J. Am. Chem. Soc. 2004, 126, 4366. (b) Magill, A. M.; Cavell, K. J.;
Yates, B. F. J. Am. Chem. Soc. 2004, 126, 8717. (c) Kim, Y.-J.; Streitwieser,
A. J. Am. Chem. Soc. 2002, 124, 5757. (d) Alder, R. W.; Allen, P. R.;
Williams, S. J. J. Chem. Soc., Chem. Commun. 1995, 1267.
(10) Arnold, P. L.; Pearson, S. Coord. Chem. ReV. 2007, 251, 596.
(11) Bacciu, D.; Cavell, K. J.; Fallis, I. A.; Ooi, L. Angew. Chem., Int.
Ed. 2005, 44, 5282.
(12) (a) Hu, X.; Castro-Rodriguez, I.; Meyer, K. Organometallics 2003,
22, 3016. (b) Danopoulos, A. A.; Tsoureas, N.; Wright, J. A.; Light, M. E.
Organometallics 2004, 23, 166. (c) Stylianides, N.; Danopoulos, A. A.;
Tsoureas, N. J. Organomet. Chem. 2005, 690, 5948. (d) Ellul, C. E.; Mahon,
M. F.; Saker, O.; Whittlesey, M. K. Angew. Chem., Int. Ed. 2007, 46, 6343.
(13) Baya, M.; Eguillor, B.; Esteruelas, M. A.; Olivan, M.; Onate, E.
Organometallics 2007, 26, 6556.
(17) (a) Cornils B.; Herrmann, W. A. Applied Homogeneous Catalysis
with Organometallic Compounds; VCH: New York, 2002. (b) Labinger,
J. A.; Bercaw, J. E. Nature 2002, 417, 507. (c) ActiVation and Function-
alization of C-H Bonds; Goldberg, K. I., Goldman, A. S., Eds.; ACS
Symposium. Series 885; American Chemical Society: Washington DC,
2004.
(14) (a) Raubenheimer, H. G.; Desmet, M.; Olivier, P.; Kruger, G. J.
J. Chem. Soc., Dalton Trans. 1996, 4431. (b) Desmet, M.; Raubenheimer,
H. G.; Kruger, G. J. Organometallics 1997, 16, 3324.
(18) Gridnev, A. A.; Mihaltseva, I. M. Synth. Commun. 1994, 24, 1547.
(19) Williams, D. J.; Vanderveer, D.; Jones, R. L.; Menaldino, D. S.
Inorg. Chim. Acta 1989, 165, 173.
(15) Kluser, E.; Neels, A.; Albrecht, M. Chem. Commun. 2006, 4495.
(16) (a) Chianese, A. R.; Zeglis, B. M.; Crabtree, R. H. Chem. Commun.
2004, 2176. (b) Viciano, M.; Feliz, M.; Corberan, R.; Mata, J. A.; Clot, E.;
Peris, E. Organometallics 2007, 26, 5304. (c) Alcarazo, M.; Roseblade,
S. J.; Cowley, A. R.; Fernandez, R.; Brown, J. M.; Lassaletta, J. M. J. Am.
Chem. Soc. 2005, 127, 3290. (d) Arnold, P. L.; Liddle, S. T. Organometallics
2006, 25, 1485.
(20) (a) Albrecht, M.; Crabtree, R. H.; Mata, J.; Peris, E. Chem. Commun.
2002, 32. (b) Albrecht, M.; Miecznikowski, J. R.; Samuel, A.; Faller, J. W.;
Crabtree, R. H. Organometallics 2002, 21, 3596.
(21) Formally, metalation of the diimidazolium precursors 1 occurs at
the C5 position. For reasons of consistency with the literature, the atom
numbering as shown in Chart 1 is adopted here.

Rhodium(III) Complexes Containing C4-Bound NHCs
Organometallics, Vol. 27, No. 13, 2008 3163
Calkyl-H activation and carbene formation are competitive
processes.16a,b
While some of the metalation yields were rather low, they
were consistently higher when the complexation was performed
in air rather than under rigorous argon atmosphere. This suggests
a cascade-type mechanism, in which iodide is oxidized to iodine,
perhaps mediated by oxygen, followed by I2 oxidation of the
metal center to the rhodium(III) species. Indeed, performing the
metalation with commercially available RhCl3 as rhodium(III)
precursor induced straightforward complexation and afforded
the dicarbene complex 4, which was converted into complex
2b by halide metathesis (eq 1). Starting directly from rhod-
ium(III) precursors provides a simplified metalation procedure,
and in addition, the product is formed in higher purity and yield
as compared to metalation with [RhCl(cod)]2. These results
suggest that the cascade redox process proposed in the formation
of 2 from rhodium(I) salts occurs prior to metal-carbene bond
formation. This hypothesis is further supported by the fact that
metalation in the absence of KI proceeded sluggishly, while
replacing KI by molecular I2 had no negative effect on the yield.
Figure 1. ORTEP representation of the cationic portion of complex
2b (50% probability level, cocrystallized solvent molecules and the
disordered iodide anion omitted for clarity).
Table 1. Selected Bond Lengths (Å) and Angles (deg) for Complex 2b
Bond Lengths
Rh1-C1
Rh1-C7
Rh1-I1
Rh1-I2
Rh1-I3
Rh1-I5
C1-C2
C6-C7
1.979(11)
1.998(12)
2.8130(11)
2.7578(11)
2.6524(11)
2.6585(11)
1.369(17)
1.371(17)
Rh2-C28
Rh2-C34
Rh2-I1
2.003(11)
1.991(11)
2.8140(10)
2.7664(11)
2.6463(11)
2.6792(12)
1.373(16)
1.368(15)
Rh2-I2
Rh2-I3
Rh2-I4
C28-C29
C33-C34
Bond Angles
All dimeric complexes 2 are stable toward air and moisture,
and they are moderately soluble in chlorinated solvents and THF.
The 1H NMR spectrum of 2b in CD2Cl2 is illustrative and shows
the presence of three signals for the methylene group in a 10:
1:1 ratio as well-separated AB doublets between 5.5 and 6.5
ppm. This points to the presence of different isomers, perhaps
due to the different conformations that can be adopted by the
dimer 2, including a syn and an eclipsed orientation of the
nonbridging iodides. In the 13C{1H} NMR spectrum of 2b only
the signals of the major isomer are well resolved. Two sets of
signals are observed in a 1:1 ratio, thus suggesting a dissym-
metric arrangement of the two heterocycles of the dicarbene
ligand. The largest shift difference has been noted for the
rhodium-bound carbene nuclei (δC 139.4 and 137.2). These
signals appear as doublets, each with a characteristic coupling
C1-Rh1-C7
C1-Rh1-I1
C1-Rh1-I2
C1-Rh1-I3
C1-Rh1-I5
I1-Rh1-I2
I1-Rh1-I3
I2-Rh1-I3
88.8(5)
173.4(4)
92.0(4)
90.3(4)
91.1(4)
85.60(3)
83.40(3)
84.48(3)
C28-Rh2-C34
C28-Rh2-I1
C28-Rh2-I2
C28-Rh2-I3
C28-Rh2-I4
I1-Rh2-I2
88.0(4)
173.9(3)
92.9(3)
90.6(3)
91.5(3)
85.41(3)
83.49(3)
84.42(3)
I1-Rh2-I3
I2-Rh2-I3
Complexes 2a-e are unstable in coordinating solvents. In
the presence of even weak ligands such as MeCN, instantaneous
cleavage of the dimeric structure gave the monometallic
complexes 3a-e. The H and 13C NMR spectra of complexes
1
3a-e in CD3CN are significantly simpler than those of the
corresponding dimers 2 and reveal a single set of signals. The
1H NMR spectrum of 3a, for example, shows a sharp singlet
for the protons of the heterocycle (δΗ 6.90) and also for the
methylene group (δΗ 6.19). This is in good agreement with a
C2V-symmetric structure comprising a flexible metallacycle that
undergoes rapid inversion of its boat-type conformation. In the
13C{1H} NMR spectrum, a doublet for the metal-bound carbon
is observed at δC 130.0 (1JCRh ) 38.0 Hz). The related
rhodium(III) complexes 3f and 3g comprising C2-bound dicar-
bene ligands have been prepared for comparative purposes and
according to an established procedure (Scheme 1).22
1
constant, JCRh ) 42.0 Hz.
While NMR spectroscopic analyses did not allow us to
conclusively determine the structure of complexes 2, elemental
analyses were in good agreement with a compound of general
formula [RhI3(dicarbene)]n. A crystal structure analysis of 2b
provided unambiguous evidence for the anticipated dimeric
arrangement (Figure 1). The two rhodium centers are both
coordinated to a chelating C4-bound dicarbene ligand. The
distorted octahedral geometry around rhodium is completed by
one µ1-coordinating and three µ2-bridging iodides. The µ1-bound
iodides are in mutual cis position, thus implying an apparent
Cs symmetry in the solid state. As observed in related palladium
chemistry,5 the metal-carbon bond lengths are not significantly
different from those of C2-bound imidazolylidene complexes.
The Rh-I bonds that are trans to carbenes (Rh-I1 and Rh-I2)
are significantly longer than the Rh-I3 bond trans to the
nonbridging iodide, thus reflecting the high trans influence of
C4-bound carbene ligands.
Solid State Structures. Unambiguous structural information
on complexes 3a-f was obtained from X-ray analyses (Figure
2). Selected bond lengths and angles are listed in Table 2. In
all structures, the rhodium atom is in a slightly distorted
octahedral geometry that is defined by the cis-chelating dicar-
bene, two MeCN molecules, and two iodides in mutual trans
position. Such an arrangement may be surmised from the
(22) Poyatos, M.; Sanau´, M.; Peris, E. Inorg. Chem. 2003, 42, 2572.

3164 Organometallics, Vol. 27, No. 13, 2008
Yang et al.
Figure 2. ORTEP representation of the cations of the C4-bound carbene complexs 3a (a), 3b (b), 3c (c), and 3d (d) and of the C2-bound
dicarbene complex 3f (e). Ellipsoids are drawn at the 50% probability level (3c at 30%); hydrogen atoms and cocrystallized solvent molecules
and I2 (for 3c) have been omitted for clarity.
different trans influence of the ligands (carbene > I- > MeCN)
and indicates the formation of the thermodynamically most
stable products. The average Rh-C bond distance in the C4-
bound dicarbene complexes is 1.985(4) Å and does not differ
significantly from the Rh-C bond length in similar C2-bound
rhodium(III) dicarbenes such as complex 3f. The Rh-NMeCN
distances of complexes 3a-d are in the range 2.10-2.12 Å
and hence only slightly longer than those in the C2-bound
systems (Rh-N 2.07-2.11 Å; cf. 3f and ref 22). This similarity
may be a superimposition of the higher trans influence of the
C4-bound carbene and the smaller steric shielding due to the
more remote location of the wingtip substituents (cf. the different
bond angles around Rh). Notably, the heterocyclic C-C bond
in 3a-d is predominantly conjugated (average C-C 1.360(5)
Å), while in the C2-bound analogues, this bond resembles rather
a localized olefinic CdC bond (1.33 Å). Similarly long C-C
bonds were also observed in related C4-bound carbene pal-
ladium complexes.5 This may be due to a C-C-Rh three-center
four-electron interaction as the principal contribution for metal
bonding, thus pointing to a weak π-electron delocalization within
the heterocycle.7b Interestingly, complex 3c cocrystallized with
one molecule of I2, which provides additional support for the
proposed cascade mechanism proposed for the formation of
these rhodium(III) complexes from [RhCl(cod)]2 (vide supra).
Ligand Substitution Reactions. The dimer 2 is a useful
starting material for investigating the coordination properties
of the rhodium center in a C4-bound dicarbene ligand environ-
ment. For example, treatment of complex 2b with PPh3 afforded

Rhodium(III) Complexes Containing C4-Bound NHCs
Organometallics, Vol. 27, No. 13, 2008 3165
Table 2. Selected Bond Lengths (Å) and Angles (deg) for Complexes 3a-d and 3f
3a
3b
3c
3d
3f
Rh1-C1
Rh1-C7
Rh1-N5
Rh1-N6
Rh1-I1
Rh1-I2
C1-C2
C6-C7
1.988(2)
1.987(2)
2.115(2)
2.103(2)
2.6809(3)
2.6647(3)
1.359(3)
1.357(3)
1.983(3)
1.981(3)
2.109(3)
2.117(3)
2.6510(3)
2.6714(3)
1.358(5)
1.364(4)
1.976(3)
1.988(3)
2.102(3)
2.113(3)
2.6671(3)
2.6822(3)
1.367(4)
1.350(4)
1.989(2)
1.985(2)
2.118(2)
2.124(2)
2.6874(3)
2.6523(3)
1.361(3)
1.361(4)
1.994(4)
1.988(4)
2.109(4)
2.092(4)
2.6665(5)
2.6746(5)
C1-Rh1-C7
C1-Rh1-N6
N6-Rh1-N5
N5-Rh1-C7
I1-Rh1-I2
88.60(9)
89.93(9)
91.84(8)
89.62(8)
176.828(9)
88.19(12)
91.38(11)
88.14(10)
92.27(11)
177.613(12)
87.41(12)
89.86(12)
92.82(10)
89.92(11)
175.593(12)
90.03(10)
92.09(10)
85.43(9)
92.35(10)
177.390(9)
87.15(17)
95.55(16)
82.96(14)
94.24(16)
177.20(2)
Scheme 2. Reactivity of Dinuclear Rhodium(III) Complexes
toward Phosphinesa
equation gave a reaction enthalpy of ∆H° ) +24.5 ((0.1) kJ
mol-1 and an entropy change of ∆S° ) -84.2 ((0.3) J K-1
mol-1 within the 263-333 K temperature range. The large
entropy term makes an intramolecular process unlikely and puts
forward a dissociation of iodide from neutral 6 and formation
of the ionic complex 6′ at low temperature (eq 2).
a
Reagents and conditions: (i) excess PPh3, CH2Cl2; (ii) SiO2, CH2Cl2;
In such a model, the negative entropy term reflects the
increased ionic strength in 6′, which is accompanied by a more
pronounced solvatization of the two ions in 6′ as compared to
the neutral species 6.23 The assignment of ionic 6′ as the
exothermic product is further supported by the results obtained
from adding KPF6 to a solution of 6. Only the signals of 6′
(albeit with a different counterion) were observed, while the
resonances attributed to 6 disappeared completely. The equi-
librium between 6 and 6′ probably arises as a consequence of
the steric congestion imposed by the boat-type metallacycle and
the bulky PPh3 ligand. This is in agreement with the large P-Rh
coupling constant (1JPRh ) 112.8 Hz) and hence a weak bonding
of the trans-located ligand.24 In addition, the strong donor
properties of the C4-bound dicarbene ligand facilitate anion
dissociation in 6.
(iii) dppe, CH2Cl2.
initially the bis(phosphine) complex 5 (Scheme 2). Complex 5
is stable as solid in air, while in solution slow decomposition
was observed. The H and 31P NMR spectra of 5 in CD2Cl2
1
showed one major and two minor sets of signals, implying the
existence of three isomers in CD2Cl2 solution. Again the CH2
group between the two heterocycles is diagnostic for determining
the ratio of these isomers (12:1:1). A similar product distribution
is also indicated by 31P NMR, showing a resonance for the major
isomer at δP 12.5 (1JPRh ) 89.7 Hz) and overlapping resonances
for the minor species at δP 15.3 (1JPRh ) 93.5 Hz). Integration
and chemical shift values are in agreement with two phosphines
coordinated to the metal center in a symmetric arrangement.
While electronic arguments would favor a trans conformation
of the phosphines (trans effect carbene > phosphine > iodide),
steric repulsion with the twisted carbene heterocycles may force
the phosphines into an arrangement in which the phosphines
are coplanar with the carbenes and, therefore, the iodides in
mutual trans position.
Coordination of two phosphines to the Rh(dicarbene) frag-
ment is more stable when a chelating diphosphine is used rather
than an excess of PPh3. For example, the monomeric complex
7 was obtained in good yields upon treating complex 2b with
diphenylphosphinoethane (dppe) in CH2Cl2. The 1H NMR
indicates symmetry-related heterocycles. Similarly, a single
In contrast to the measurements in CD2Cl2, the NMR spectra
in CD3CN solution showed several sets of signals and in addition
also the presence of free PPh3. This indicates relatively easy
solvolysis of complex 5 and concomitant release of PPh3.
Purification of the diphosphine complex 5 by silica gel chro-
matography indeed induced the clean dissociation of one PPh3
1
resonance is seen in the 31P NMR (δP 21.4, JRhP ) 75.1 Hz),
thus implying a C2V-symmetric structure in solution. In the solid
state, however, C1 symmetry was established with an all-trans
ligand coordination (Figure 3). In the crystallized Λ-form, the
Rh-I bond trans to phosphine is unexpectedly longer (Rh-I2
2.721(1) Å) than the one trans to the C4-bound carbene (Rh-I1
2.636(1) Å).
1
ligand and gave the monophosphine complex 6. The H NMR
spectrum indicates the presence of two isomers due to the
appearance of two AX sets centered at δH 6.22 and 5.39 and at
δH 5.94 and 5.07, respectively (2JHH ) 13.2 Hz). Temperature-
dependent measurements revealed an equilibrium between the
two isomers with the low-field pattern belonging to the
exothermic product. Analyses according to the van’t Hoff
(23) (a) Marcus Y. Ion SolVation; Wiley: Chichester; 1985. This concept
is also known in biological systems, see for example:(b) Dill, K. A.; Truskett,
T. M.; Vlachy, V.; Hribar-Lee, B. Annu. ReV. Biophys. Biomol. Struct. 2005,
34, 173.
(24) Meek, D. W.; Mazanec, T. J. Acc. Chem. Res. 1981, 14, 266.

3166 Organometallics, Vol. 27, No. 13, 2008
Yang et al.
Table 3. Catalytic Transfer Hydrogenation of Benzophenone with
Rhodium(III) Dicarbene Complexesa
(3)
entry
catalyst
mol %
time (h)
conversion (%)b
TON
1
2
3
4
5
6
7
8
9
10
11
12
13
14
0.5/16
0.5/2
0.5/2
0.5/2
0.5/2
0.5/2
0.5/2
0.5/2
0.5/2
0.5/2
0.5/2
0.5/2
0.5/2
0.5/40
0.6/11
54/97
47/94
36/73
5/14
45/82
44/84
48/87
60/91
53/91
5/17
2b
2b
2b
2b
2a
2c
2d
2e
3a
3f
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
1
97
94
73
14
82
84
87
91
91
17
6
c
d
e
3g
4
4
1
1
0.1
3/6
73/97
10/33
97
330
a General conditions: 1 mmol of benzophenone, 5 mL of iPrOH, 100
µmol of base (substrate/base 10:1), reflux temperature. b Average of at
least two runs. c KOtBu used as base. d Addition of H2O (0.2 mL).
e Addition of MeCN (0.2 mL).
Figure 3. ORTEP representation of complex 7 (30% probability
level, hydrogen atoms, the cocrystallized ether molecules, and the
I- anion omitted for clarity; only one of the two disordered positions
of the CH2CH2PPh2 moiety of the dppe ligand is shown). Selected
bond lengths (Å): Rh1-C1 2.031(9), Rh1-C7 2.050(10), Rh1-I1
2.6359(12), Rh1-I2 2.7209(10), Rh1-P1 2.357(2), Rh1-P2
2.237(13), C1-C2 1.370(12), C6-C7 1.352(13). Selected bond
angles(deg):C1-Rh1-C788.3(4),C1-Rh1-I1175.9(2),C1-Rh1-I2
92.4(2), C1-Rh1-P1 92.6(2), C1-Rh1-P2 94.9(3), C7-Rh1-P1
176.4(2), C7-Rh1-P2 90.1(4), P1-Rh1-P2 86.7(3), I1-Rh1-I2
89.85(4).
Table 4. Catalytic Transfer Hydrogenation of Ketones and Imines
with the Dicarbene Complex 2ba
(4)
entry
ketone
cat 2b (mol %) time (h) conversion (%)
1
2
3
4
5
6
7
benzophenone
acetophenone
propiophenone
3,3,5,5-Me4-cydO
2-octanone
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5/2
0.5/2
0.5/2
0.5/2
0.5/2
0.5/2
2
54/97
66/93
32/74
18/40
17/57
12/40
<2
Catalytic Transfer Hydrogenation. The versatility of com-
plexes 2 in coordinating various ligands was further exploited
in hydrogen transfer catalysis.25 The reduction of benzophenone
to diphenyl methanol in iPrOH was used as a model reaction
for probing the catalytic activity of the rhodium(III) complexes
(eq 3 and Table 3). A first screening aimed at optimizing
reaction conditions was carried out with the dimeric complex
2b as catalyst. In a typical reaction, the active catalyst was
formed by heating a mixture of 2b in basic iPrOH for 10 min
prior to substrate addition. When the substrate was added first,
the catalytic activity was low. Catalyst manipulation does not
need particular precautions, and transfer hydrogenations were
generally performed in air and without solvent pretreatment.
The base was added as a concentrated aqueous solution, which
underlines the stability of the rhodium catalyst system toward
moisture. Identical conversions were obtained in a comparative
experiment using KOtBu dissolved in iPrOH as base, thus
avoiding the addition of traces of water (entries 2, 3). A Karl-
Fischer titration of the iPrOH solvent indicated a water content
of 0.5%, which corresponds to 25 µL in the 5 mL volume used
for standard reactions. Notably, addition of deliberate amounts
of water (0.2 mL) did slow down the reaction slightly (entry
4). Hence, water at low concentration (0.5-1%) does not affect
the catalyst, while higher concentrations (>5%) reduce its
activity. Much more detrimental is, however, the presence of
MeCN (entry 5). Apparently, formation of complexes 3 is faster
3-octanone
Ph-CHdNMe
a General conditions: 1 mmol of ketone, 5 mL of iPrOH, 100 µmol
of base (substrate/base 10:1), reflux temperature.
and competitive to alkoxide/ketone coordination to 2. An excess
of MeCN effectively poisons the catalyst, while equimolar
concentrations as in complex 3a do not reduce the catalytic
activity (entry 10).
The substitution pattern on the dicarbene ligand has a
relatively small influence on the catalytic activity. From our
limited variation in catalyst design, no clear trend can be
deduced and the differences in activity are small (entries 2,
6-9). The turnover frequency at 50% conversion (TOF50) is
100 ( 20 h-1 for all catalyst precursors 2a-e. Dimer cleavage
in 2 seems to be fast also during catalysis, and conversions are
identical when starting from the dimer 2 or the corresponding
monometallic complexes (e.g., 3a, entry 10). However, rhodium
coordination via the C4 position is critical for inducing catalytic
activity. The rhodium complexes 3f and 3g comprising a C2-
bound dicarbene ligand are significantly less active catalysts
than the C4-bound complexes and give only poor conversions
(entries 11, 12). Apparently, the stronger donor properties of
the C4-bound carbene ligands produce a more electron-rich
rhodium center, which is supposed to accelerate in particular
the product release step in the catalytic cycle.25
(25) (a) Klomp, D.; Hanefeld, U.; Peters, J. A. In The Handbook of
Homogeneous Hydrogenation; de Vries, J. G., Elsevier, C. J., Eds.; Wiley-
VCH: Weinheim, 2007, p 585. (b) Samec, J. S. M.; Ba¨ckvall, J-E.;
Andersson, P. G.; Brandt, P. Chem. Soc. ReV. 2006, 35, 237. (c) Gladiali,
S.; Alberico, E. Chem. Soc. ReV. 2006, 35, 226. (d) Clapham, S. E.;
Hadzovic, A.; Morris, R. H. Coord. Chem. ReV. 2004, 248, 2201.
The relevance of high electron density at the metal center is
further strengthened by the catalytic activity of the chloro
complex 4. This complex is the most active catalyst of the series.

Rhodium(III) Complexes Containing C4-Bound NHCs
Organometallics, Vol. 27, No. 13, 2008 3167
operating at 400 or 500 MHz (1H NMR), 100 or 125 MHz (13C
NMR), and 162 MHz (31P NMR), respectively. Chemical shifts (δ
in ppm, coupling constants J in Hz) were referenced to external
SiMe4 (1H, 13C) or H3PO4 (31P). Assignments are based on homo-
and heteronuclear shift correlation spectroscopy. Elemental analyses
were performed by the Microanalytical Laboratory of Ilse Beetz
(Kronach, Germany).
Transfer hydrogenation is essentially complete in 1 h, and the
TOF50 is raised to 300 h-1 at 1 mol % catalyst loading (entry
13).26 Lowering the catalyst loading gave only incomplete
conversions, and the noncatalyzed background reduction be-
comes a competitive process (entry 14). Perhaps, catalyst
deactivation due to the higher relative water concentration may
become relevant at such low rhodium levels.
General Procedure for the Synthesis of Diimidazolium
Ligand Precursors 1a-e. A solution of the appropriate 1,2-
disubstituted imidazole (20 mmol) and CH2I2 (2.67 g, 10 mmol)
was stirred in toluene (20 mL) at reflux for 24 h. The formed
precipitate was collected by filtration and recrystallized from MeOH/
Et2O at -20 °C.
Different ketones are efficiently hydrogenated with these new
dicarbene rhodium catalysts. Acetophenone reduction takes place
at slightly faster rates than benzophenone hydrogenation (Table
4, entries 1, 2). Larger alkyl groups are also tolerated. Pro-
piophenone, for example, is converted only a little slower than
acetophenone and transfer hydrogenation is complete within 4 h
(entry 3). Aliphatic methylketones require longer reaction times
than their aryl analogues. This effect is even more pronounced
for sterically demanding substrates such as 3-octanone and
3,3,5,5-tetramethylcyclohexanone, for which hydrogenation does
not reach completion even after 16 h (entries 4-6). Imines
appear to poison the catalytically active species. When using
N-methylbenzylimine, no amine formation was observed (entry
7). Instead, traces of benzyl alcohol were initially detected,
probably originating from hydrolysis of the imine and subse-
quent transfer hydrogenation of the corresponding aldehyde.27
However, this reaction does not progress, and even after
prolonged reaction time, more than 95% of imine was present
in the reaction mixture.
N,N-Methylenedi(N′-n-butyl-2-methyl)imidazolium diiodide
1
(1a):. white solid (3.8 g, 70%). H NMR (DMSO-d6, 400 MHz):
3
3
δ 7.92 (d, 2H, JHH ) 2.2 Hz, Himi), 7.83 (d, 2H, JHH ) 2.2 Hz,
3
H
imi), 6.57 (s, 2H, NCH2N), 4.13 (t, 4H, JHH ) 7.4 Hz,
NCH2CH2CH2CH3), 2.77 (s, 6H, CH3), 1.71 (quintett, 4H, 3JHH
)
3
7.4 Hz, NCH2CH2CH2CH3), 1.30 (sextet, 4H, JHH ) 7.4 Hz,
3
NCH2CH2CH2CH3), 0.90 (t, 6H, JHH ) 7.4 Hz, NCH2CH2CH2-
CH3). 13C{1H} NMR (DMSO-d6, 100 MHz): δ 145.9 (Cimi), 122.0
(Cimi), 121.3 (Cimi), 56.8 (NCH2N), 47.7 (NCH2CH2CH2CH3), 30.8
(NCH2CH2CH2CH3), 18.9 (NCH2CH2CH2CH3), 13.4 (NCH2CH2CH2-
CH3), 10.3 (CH3). Anal. Calcd for C17H30I2N4 (544.26): C, 37.52;
H, 5.56; N, 10.29. Found: C, 37.41; H, 5.44; N, 10.17.
N,N-Methylenedi(N′-n-butyl-2-phenyl)imidazolium diiodide
1
(1b):. light yellow solid (5.0 g, 75%). H NMR (DMSO-d6, 400
3
3
MHz): δ 8.06 (d, 2H, JHH ) 2.2 Hz, Himi), 7.92 (d, 2H, JHH
)
2.2 Hz, Himi), 7.72 (m, 2H, Haryl), 7.58 (m, 8H, Haryl), 6.29 (s, 2H,
Conclusions
3
NCH2N), 3.92 (t, 4H, JHH ) 7.4 Hz, NCH2CH2CH2CH3), 1.57
(quintet, 4H, 3JHH ) 7.4 Hz, NCH2CH2CH2CH3), 1.07 (sextett, 4H,
Direct metalation of C2-substituted diimidazolium salts
provides access to chelating rhodium dicarbene complexes
in which the heterocyclic carbenes are bound unusually via
the C4 carbon. This bonding mode increases the electron
density at the metal center, thus invoking new reactivity
patterns. The exceptionally strong donor properties of such
C4-bound carbenes have been utilized for the development
of new rhodium catalysts for the transfer hydrogenation of
ketones. The catalytic activity of the rhodium center critically
depends on the carbene bonding mode and requires C4-bound
carbenes, whereas C2-bound analogues are essentially inac-
tive. These results are another indication for the high impact
of C4-bound carbene ligands in inducing new (catalytic)
properties of the coordinated metal center. Further investiga-
tions along these lines are currently in progress in our
laboratories, in particular with the aim of catalytically
activating otherwise unreactive bonds.
3JHH ) 7.4 Hz, NCH2CH2CH2CH3), 0.71 (t, 6H, JHH ) 7.4 Hz,
3
NCH2CH2CH2CH3). 13C{1H} NMR (DMSO-d6, 100 MHz): δ 144.7
(Cimi), 132.9 (Caryl), 130.4 (Caryl), 129.7 (Caryl), 122.8 (Cimi), 122.2
(Cimi), 119.6 (Caryl), 57.9 (NCH2N), 48.1 (NCH2CH2CH2CH3), 30.7
(NCH2CH2CH2CH3), 18.5 (NCH2CH2CH2CH3), 13.0 (NCH2CH2CH2-
CH3). Anal. Calcd for C27H34I2N4 (668.39): C, 48.52; H, 5.13; N,
8.38. Found: C, 48.33; H, 5.16; N, 8.38.
N,N-Methylenedi(N′-isopropyl-2-phenyl)imidazolium diiodide
1
(1d):. light yellow solid (1.5 g, 23%). H NMR (DMSO-d6, 400
3
3
MHz): δ 8.17 (d, 2H, JHH ) 2.2 Hz, Himi), 7.91 (d, 2H, JHH
)
2.2 Hz, Himi), 7.72 (m, 2H, Haryl), 7.60 (m, 8H, Haryl), 6.29 (s, 2H,
3
NCH2N), 4.15 (septet, 2H, JHH ) 6.6 Hz, NCH(CH3)2), 1.38 (d,
12H, 3JHH ) 6.6 Hz, NCH(CH3)2). 13C{1H} NMR (DMSO-d6, 100
MHz): δ 143.7 (Cimi), 132.9 (Caryl), 130.3 (Caryl), 129.8 (Caryl), 122.9
(Cimi), 119.7(Cimi), 119.4 (Caryl), 57.9 (NCH2N), 51.6 (NCH(CH3)2),
22.0 (NCH(CH3)2). Anal. Calcd for C25H30I2N4 (640.06) × H2O:
C, 45.61; H, 4.90; N, 8.51. Found: C, 45.73; H, 5.06; N, 8.52.
General Procedure for the Synthesis of [(µ-I)3-
{RhI(dicarbene)}2]I (2a-e). Method A: A mixture of the corre-
sponding diimidazolium salt 1 (1 molar equiv), [RhCl(cod)]2 (0.5
molar equiv), KI (4 mol equiv), and NaOAc (8 molar equiv) was
stirred in MeCN at reflux temperature for 24 h. After cooling, all
volatiles were removed under reduced pressure and the residue was
purified by gradient column chromatography. Method B: A mixture
of the diimidazolium salt (1 molar equiv), [RhCl3(H2O)x] (1 molar
equiv), and NaOAc (8 molar equiv) was refluxed in MeCN for
24 h. After cooling, all volatiles were removed under reduced
pressure and the residue was purified by gradient column chroma-
tography (SiO2; CH2Cl2/acetone, 5:1). The orange fraction was
dissolved in a solution of NaI (10 molar equiv) in acetone and stirred
for 24 h. After filtration of the mixture, the volatiles of the filtrate
were removed under reduced pressure and subsequently extracted
into CH2Cl2. Evaporation of this solution to dryness gave com-
plex 2.
Experimental Section
General Comments. The N-alkylated imidazoles,18 the ligand
precursors 1c and 1e,5 and [RhCl(cod)]228 were prepared according
to literature procedures. All other reagents are commercially
available and were used as received. Unless specified otherwise,
NMR spectra were recorded at 25 °C on Bruker spectrometers
(26) Notably, this catalyst activity is still 1 to 2 orders of magnitude
lower than that of the most active systems known today. See for examples
ref 18b and:(a) Mestroni, G.; Zassinovich, G.; Camus, A.; Martinelli, F. J.
Organomet. Chem. 1980, 198, 87. (b) Thoumazet, C.; Melaimi, M.; Ricard,
L.; Mathey, F.; Le Floch, P. Organometallics 2003, 22, 1580. (c) Dani, P.;
Karlen, T.; Gossage, R. A.; Gladiali, S.; van Koten, G. Angew. Chem., Int.
Ed. 2000, 39, 743. (d) Yang, H.; Alvarez, M.; Lugan, N.; Mathieu, R.
J. Chem. Soc., Chem. Commun. 1995, 1721. (e) For a reverse reaction,
see:
(27) Fujita, K.; Li, Z.; Ozeki, N.; Yamaguchi, R. Tetrahedron Lett. 2003,
44, 2687.
Synthesis of 2a. According to method A, starting from 1a (272
mg, 0.5 mmol), [RhCl(cod)]2 (123 mg, 0.25 mmol), KI (332 mg,
(28) Crabtree, R. H.; Morehouse, S. M.; Quirk, J. M. Inorg. Synth. 1986,
24, 173.

3168 Organometallics, Vol. 27, No. 13, 2008
Yang et al.
Hz, Haryl), 7.48 (t, 4H, 3JHH ) 7.6 Hz, Haryl), 7.36 (d, 4H, 3JHH
) 7.6 Hz, Haryl), 7.20 (s, 2H, Himi), 6.00 (s, 2H, NCH2N), 3.91
2.0 mmol), and NaOAc (328 mg, 4.0 mmol). Gradient column
chromatography (SiO2; first CH2Cl2 then CH2Cl2/acetone, 3:1) gave
2a as an orange solid (104 mg, 27%). Addition of Et2O to a
suspension of 2a in CH2Cl2 afforded analytically pure material.
Anal. Calcd for C34H56I6N8Rh2 (1544.10): C, 26.45; H, 3.66; N,
7.26. Found: C, 26.36; H, 3.66; N, 7.34.
3
(t, 4H, JHH ) 7.2 Hz, CH2CH2CH2CH3), 1.68 (quintet, 4H,
3JHH ) 7.2 Hz, CH2CH2CH2CH3), 1.20 (sextet, 4H, 3JHH ) 7.2
3
Hz, CH2CH2CH2CH3), 0.78 (t, 6H, JHH
) 7.2 Hz,
CH2CH2CH2CH3). 13C{1H} NMR (CD3CN, 100 MHz): δ 143.8
(Cimi), 133.0 (Caryl), 132.2 (C-Rh, JCRh ) 38.0 Hz), 131.7
1
Synthesis of 2b. According to method A, starting from 1b (334
mg, 0.5 mmol), [RhCl(cod)]2 (123 mg, 0.25 mmol), KI (332 mg,
2.0 mmol), and NaOAc (328 mg, 4.0 mmol). Gradient column
chromatography (SiO2; first CH2Cl2 then CH2Cl2/acetone, 5:1) gave
2b as an orange solid (140 mg, 31%), which was then crystallized
from CH2Cl2/hexane. 1H NMR (CD2Cl2, 400 MHz): δ 7.63-7.23
(Caryl), 130.5 (Caryl), 128.0 (Cimi), 122.9 (Caryl), 62.8 (NCH2N),
48.8 (CH2CH2CH2CH3), 32.7 (CH2CH2CH2CH3), 20.0
(CH2CH2CH2CH3), 13.7 (CH2CH2CH2CH3).
Synthesis of 3c. Slow diffusion of Et2O into a solution of 2c
1
2
in CH3CN yielded crystals of complex 3c. H NMR (CD3CN,
(m, 24H, Haryl, Himi), 6.47 (d, 2H, JHH ) 12.6 Hz, lowfield AX
2
400 MHz): δ 6.97 (s, 2H, Himi), 6.25 (s, 2H, NCH2N), 4.52
part of NCH2N), 5.86 (d, 2H, JHH ) 12.6 Hz, high-field AX part
3
(septet, 2H, JHH ) 6.6 Hz, CHMe2), 2.70 (s, 6H, CH3), 1.46
of NCH2N), 3.98-3.91 (m, 8H, CH2CH2CH2CH3), 1.84-1.75 (m,
8H, CH2CH2CH2CH3), 1.32-1.27 (m, 8H, CH2CH2CH2CH3),
0.89-0.84 (m, 12H, CH2CH2CH2CH3). 13C{1H} NMR (CD2Cl2,
100 MHz): δ 142.1, 141.8 (Cimi-2), 139.4 (d, 1JCRh ) 42.0 Hz, Cimi-
3
(d, 12H, JHH ) 6.6 Hz, CH(CH3)2). 13C{1H} NMR (CD3CN,
1
100 MHz): δ 141.7 (Cimi), 129.8 (C-Rh, JCRh ) 38.0 Hz),
122.4 (Cimi), 61.5 (NCH2N), 50.8 (CHMe2), 22.6 (CH(CH3)2),
1
5), 137.2 (d, JCRh ) 42.0 Hz, Cimi-5), 132.1, 131.9 (Caryl),
10.8 (CH3).
130.0-129.5 (m, Cimi-4, Caryl), 61.1 (NCH2N), 48.0 (CH2CH2CH2-
CH3), 32.0, 31.9 (CH2CH2CH2CH3), 19.4, 19.3 (CH2CH2CH2CH3),
12.9 (CH2CH2CH2CH3). The signals of the two minor species are
overlapping with the major product in the 1H NMR spectrum except
for the AX-type signal of the NCH2N group; minor species A: δ
6.31 (d, 2JHH ) 12.6 Hz), 5.90 (d, 2JHH ) 12.6 Hz); minor species
B: δ 6.59 (d, JHH ) 12.6 Hz), 5.69 (d, JHH ) 12.6 Hz). Anal.
Calcd for C54H64I6N8Rh2 (1792.38): C, 36.19; H, 3.60; N, 6.25.
Found: C, 36.09; H, 3.62; N, 5.91.
Synthesis of 2c. According to method A, starting from 1c (260
mg, 0.5 mmol), [RhCl(cod)]2 (123 mg, 0.25 mmol), KI (332 mg,
2.0 mmol), and NaOAc (328 mg, 4.0 mmol). Gradient column
chromatography (SiO2; first CH2Cl2 then CH2Cl2/acetone, 5:1) gave
2c as an orange solid (50 mg, 13%). Analytically pure material
was obtained by slow diffusion of Et2O into a solution of 2c in
CH2Cl2/CH3CN (8:1). Anal. Calcd for C30H48I6N8O0.5Rh2 (1487.99)
× 0.5 Et2O: C, 25.20; H, 3.50; N, 7.35. Found: C, 25.20; H, 3.78;
N, 7.58.
Synthesis of 2d. According to method A, starting from 1d (320
mg, 0.5 mmol), [RhCl(cod)]2 (123 mg, 0.25 mmol), KI (332 mg,
2.0 mmol), and NaOAc (328 mg, 4.0 mmol). Gradient column
chromatography (SiO2; first CH2Cl2 then CH2Cl2/acetone, 5:1) gave
2d as an orange solid (100 mg, 23%), which was crystallized from
CH2Cl2/hexane. Anal. Calcd for C50H56I6N8Rh2 (1736.27): C, 34.59;
H, 3.25; N, 6.45. Found: C, 34.21; H, 3.31; N, 6.77.
Synthesis of 3d. Saturation of a solution of 2d in CH2Cl2/
1
CH3CN (1:1) with Et2O gave complex 3d. H NMR (CD3CN,
3
400 MHz): δ 7.59 (t, 2H, JHH ) 7.6 Hz, Haryl), 7.48 (t, 4H,
3
3JHH ) 7.6 Hz, Haryl), 7.36 (d, 4H, JHH ) 7.6 Hz, Haryl), 7.28
3
(s, 2H, Himi), 5.99 (s, 2H, NCH2N), 4.25 (septet, 2H, JHH
)
3
6.6 Hz, CHMe2), 1.44 (d, 12H, JHH ) 6.6 Hz, CH(CH3)2).
2
2
13C{1H} NMR (CD3CN, 100 MHz): δ 142.9 (Cimi), 133.0 (Caryl),
1
132.6 (C-Rh, JCRh ) 38.0 Hz), 131.6 (Caryl), 130.5 (Caryl),
124.2 (Cimi), 123.0 (Caryl), 62.4 (NCH2N), 51.8 (CHMe2), 23.0
(CH(CH3)2.
Synthesis of 3e. Dissolution of 2e in CH3CN and subsequent
precipitation with Et2O gave the monometallic complex 3e. 1H
NMR (CD3CN, 500 MHz): δ 7.13 (s, 4H, HMes), 6.88 (s, 2H,
H
imi), 6.35 (s, 2H, NCH2N), 2.46 (s, 6H, CH3), 2.36 (s, 6H,
CH3), 2.03 (s, 12H, CH3). 13C{1H} NMR (CD3CN, 125 MHz):
δ 143.4 (Cimi), 141.5 (CMes), 136.0 (CMes), 132.2 (CMes), 130.6
1
(C-Rh, JCRh ) 37.9 Hz), 130.3 (CMes), 126.6 (Cimi), 62.3
(NCH2N), 21.1 (Mes-CH3), 17.6 (Mes-CH3), 10.5 (Imi-CH3).
Synthesis of 3f. A mixture of [RhCl(cod)]2 (100 mg, 0.20
mmol), methylene-1,1′-di(3,3′-n-butyl)imidazolium diiodide (212
mg, 0.41 mmol), KI (220 mg, 1.3 mmol), and NEt3 (0.25 g, 2.5
mmol) was stirred in CH3CN (12 mL) at 50 °C. The color of
the mixture gradually changed from yellow to dark red. After
1 h, the solids were filtered off and the filtrate was evaporated
under reduced pressure. The residue was purified by gradient
column chromatography (SiO2, CH2Cl2 to CH2Cl2/acetone (3:
1) as eluents). The red fraction was collected and afforded after
evaporation crude 3f (208 mg, 61%). Recrystallization from
CH3CN/Et2O/hexanes gave an analytically pure sample of 3f.
Synthesis of 2e. According to method B, starting from 1e
(201 mg, 0.3 mmol), [RhCl3(H2O)x] (79 mg, 0.3 mmol), and
NaOAc (197 mg, 2.4 mmol) afforded 2e as an orange solid
(110 mg, 41%). Analytically pure material was obtained by slow
diffusion of Et2O into a solution of 2e in CH2Cl2/acetone (1:1).
Anal. Calcd for C54H64I6N8Rh2 (1792.38): C, 36.19; H, 3.60;
N, 6.25. Found: C, 36.23; H, 3.71; N, 6.16.
3
1H NMR (CD3CN, 400 MHz): δ 7.43 (d, 2H, JHH ) 1.9 Hz,
H
imi), 7.37 (d, 2H, 3JHH ) 1.9 Hz, Himi), 6.38 (s, 2H, NCH2N),
Synthesis of 3a. Crystallization of 2a from CH3CN/Et2O
afforded the monomeric bis-acetonitrile complex 3a. 1H NMR
(CD3CN, 400 MHz): δ 6.90 (s, 2H, Himi), 6.19 (s, 2H, NCH2N),
4.01 (t, 4H, 3JHH ) 7.2 Hz, CH2CH2CH2CH3), 2.67 (s, 6H, CH3),
4.40 (m, 4H, CH2CH2CH2CH3), 1.98 (s, 6H, NCCH3), 1.83 (m,
3
4H, CH2CH2CH2CH3), 1.45 (sextet, 4H, JHH ) 7.4 Hz,
3
CH2CH2CH2CH3), 1.00 (t, 6H, JHH ) 7.4 Hz, CH2CH2-
CH2CH3). 13C{1H} NMR (CD3CN, 100 MHz): δ 149.0 (C-Rh,
1JCRh ) 42.7 Hz), 124.2 (Cimi), 123.8 (Cimi), 65.7 (NCH2N),
53.3 (CH2CH2CH2CH3), 33.8 (CH2CH2CH2CH3), 20.6
(CH2CH2CH2CH3), 14.1 (CH2CH2CH2CH3). Anal. Calcd for
C19H30I3N6Rh (825.87): C, 27.62; H, 3.66; N, 10.17. Found:
C, 27.69; H, 3.76; N, 9.64.
3
1.74 (quintet, 4H, JHH ) 7.2 Hz, CH2CH2CH2CH3), 1.34
(sextet, 4H, 3JHH ) 7.2 Hz, CH2CH2CH2CH3), 0.94 (t, 6H, 3JHH
) 7.2 Hz, CH2CH2CH2CH3). 13C{1H} NMR (CD3CN, 100
1
MHz): δ 143.0 (Cimi), 130.0 (C-Rh, JCRh ) 38.0 Hz), 127.2
(Cimi),62.4(NCH2N),48.9(CH2CH2CH2CH3),33.3(CH2CH2CH2-
CH3), 20.8 (CH2CH2CH2CH3), 14.4 (CH2CH2CH2CH3), 11.2
(CH3). Anal. Calcd for C21H34I3N6Rh (845.15) × CH3CN: C,
30.86; H, 4.17; N, 10.95. Found: C, 30.96; H, 4.13; N, 10.89.
Synthesis of 3g. Following a procedure as described for 3f,
the title product was obtained from a mixture of [RhCl(cod)]2
(243 mg, 0.50 mmol), N,N-methylenedi(N′-mesityl)imidazolium
diiodide (640 mg, 1.0 mmol), NaI (500 mg, 3.3 mmol), and
NEt3 (0.45 g, 4.5 mmol) in CH3CN (20 mL) as a red powder
(yield 647 mg, 70%). Recrystallization from CH3CN/Et2O gave
Synthesis of 3b. Slow diffusion of Et2O into a solution of
2b in CH2Cl2/CH3CN (1:1) induced crystallization of complex
1
3
3b. H NMR (CD3CN, 400 MHz): δ 7.59 (t, 2H, JHH ) 7.6

Rhodium(III) Complexes Containing C4-Bound NHCs
Organometallics, Vol. 27, No. 13, 2008 3169
an analytically pure sample of 3g. 1H NMR (CD3CN, 500 MHz):
Calcd for C63H62I3N4P2Rh (1420.76): C, 53.26; H, 4.40; N, 3.94.
Found: C, 53.33; H, 4.53; N, 3.76.
3
3
δ 7.58 (d, 2H, JHH ) 2.2 Hz, Himi), 7.13 (d, 2H, JHH ) 2.2
Hz, Himi), 6.99 (s, 4H, HMes), 6.45 (s, 2H, NCH2N), 2.30 (s,
6H, CH3), 2.21 (s, 12H, CH3). 13C{1H} NMR (CD3CN, 125
[RhI3(dicarbene)PPh3] (6). A mixture of 2b (40 mg, 0.02
mmol) and PPh3 (26 mg, 0.1 mmol) in CH2Cl2 (5 mL) was
stirred at rt for 2 h. After solvent evaporation, the residue was
purified by gradient column chromatography (SiO2; first CH2Cl2
then CH2Cl2/acetone, 1:1), thus affording complex 6a (37 mg,
1
MHz): δ 150.87 (C-Rh, JCRh ) 43.2 Hz), 140.69 (CMes),
138.23 (CMes), 136.51 (CMes), 129.64 (CMes), 127.83 (Cimi),
123.78 (Cimi), 65.61 (NCH2N), 21.03 (CH3 ortho), 20.91 (CH3
para). Anal. Calcd for C29H34I3N6Rh (950.24): C, 36.66; H, 3.61;
N, 8.84. Found: C, 36.86; H, 3.67; N, 8.82.
1
95%). H NMR (CD3CN, 400 MHz, 333 K): δ 7.7-7.15 (m,
2
27H, Haryl, Himi), 5.91 (d, 1H, JHH ) 13.2 Hz, low-field AX
2
part of NCH2N), 5.03 (d, 1H, JHH ) 13.2 Hz, high-field AX
Synthesis of [RhCl2(dicarbene)(MeCN)2]BF4 (4). A suspen-
sion of 1b (460 mg, 0.69 mmol) and AgBF4 (296 mg, 1.5 mmol)
in CH2Cl2 (100 mL) was stirred at room temperature and in the
dark for 24 h. The reaction mixture was filtered and the filtrate
concentrated under reduced pressure to ca. 5 mL. Upon addition
of Et2O, an off-white precipitate formed, which analyzed as N,N-
methylenedi(N′-n-butyl-2-phenyl)imidazolium (BF4)2 (1b′). Yield:
293 mg (72%). 1H NMR (DMSO-d6, 360 MHz): δ 8.03 (s, 2H,
part of NCH2N), 3.88-3.73 (m, 4H, CH2CH2CH2CH3),
1.68-1.27 (m, 4H, CH2CH2CH2CH3), 1.14-0.96 (m, 4H,
CH2CH2CH2CH3), 0.77 (t, 6H, 3JHH
)
7.3 Hz,
CH2CH2CH2CH3). 13C{1H} NMR (CD3CN, 100 MHz, 333 K):
δ 143.9 (Cimi), 135.8 (Caryl-P, 3JCP ) 8.2 Hz), 133.5 (Cimi), 132.1
2
(Caryl-imi), 131.6 (Caryl-imi), 131.1 (Caryl-P, JCP ) 17 Hz), 131.0
1
(Caryl-P), 129.2 (Caryl-P, JCP ) 10.0 Hz), 122.7 (Caryl-imi), 61.9
(NCH2N), 49.2 (CH2CH2CH2CH3), 33.0 (CH2CH2CH2CH3),
20.3 (CH2CH2CH2CH3), 13.7 (CH2CH2CH2CH3), C-Rh not
H
imi), 7.89 (s, 2H, Himi), 7.70 (m, 2H, Haryl), 7.60 (m, 4H, Haryl),
7.55 (m, 4H, Haryl), 6.27 (s, 2H, NCH2N), 3.90 (m, 4H, 3JHH
)
1
resolved. 31P{1H} NMR (CD2Cl2, 162 MHz): δ 19.4 (d, JPRh
7.1 Hz, NCH2CH2CH2CH3), 1.56 (m, 4H, NCH2CH2CH2CH3),
1.07 (m, 4H, NCH2CH2CH2CH3), 0.71 (t, 6H, JHH ) 7.2 Hz,
NCH2CH2CH2CH3).
) 112.8 Hz). Anal. Calcd for C45H47I3N4PRh (1158.47): C,
46.65; H, 4.09; N, 4.84. Found: C, 46.55; H, 3.98; N, 4.75.
3
[RhI2(dicarbene)(dppe)]I (7). Following a similar procedure
as for 6, reaction of 2b (40 mg, 0.02 mmol) with dppe (21 mg,
0.05 mmol) yielded complex 7 as a yellow powder (50 mg,
This product 1b′ (293 mg, 0.50 mmol), [RhCl3(H2O)x] (131
mg, 0.50 mmol), and NaOAc (327 mg, 4.0 mmol) were
suspended in MeCN (40 mL) and stirred at reflux temperature
for 24 h. After cooling, all volatiles were removed under reduced
pressure, and the residue was purified by gradient column
chromatography (SiO2; first CH2Cl2 then CH2Cl2/acetone, 10:
1). The yellow fraction was collected and evaporated to dryness
to give 4 as a yellow, crystalline solid (yield: 177 mg, 55%).
Analytically pure crystals of the title product were obtained by
1
95%). H NMR (CD2Cl2, 500 MHz, 273 K): δ 7.70-7.38 (m,
28H, Haryl), 7.23 (m, 2H, Haryl), 7.2 (br, 2H, NCH2N), 6.74 (s,
2H, Himi), 3.80 (m, 4H, CH2CH2CH2CH3), 3.21 (m, 4H, PCH2),
1.58 (m, 4H, CH2CH2CH2CH3), 1.18 (m, 4H, CH2CH2CH2CH3),
0.81 (m, 6H, CH2CH2CH2CH3). 13C{1H} NMR (CD2Cl2, 125
1
MHz, 273 K): δ 142.5 (Cimi), 136.8 (C-Rh, JCRh ) 32.7 Hz,
2
2JCPcis ) 10 Hz, JCPtrans ) 147 Hz), 133.2 (br, Caryl-P), 131.9
1
1
slow diffusion of hexane into a solution of 4 in CH2Cl2. H
(Caryl-imi), 129.8 (Caryl-imi), 129.7 (Caryl-P, JCP ) 17 Hz), 129.7
NMR (CD3CN, 500 MHz): δ 7.58 (d, 2H, 3JHH ) 7.9 Hz, Haryl),
(Cimi), 129.5 (Caryl-P), 127.3 (Caryl-imi), 127.2 (Caryl-P, 2JCP ) 4.4
Hz), 120.8 (Caryl-imi), 60.3 (NCH2N), 47.3 (CH2CH2CH2CH3),
31.6 (CH2CH2CH2CH3), 26.9 (m, PC2H4P), 18.9 (CH2CH2CH2-
CH3), 12.8 (CH2CH2CH2CH3). 31P{1H} NMR (CD2Cl2, 162
7.46 (t, 4H, 3JHH ) 7.9 Hz, Haryl), 7.37 (d, 4H, 3JHH ) 7.9 Hz,
H
aryl), 7.19 (s, 2H, Himi), 6.03 (s, 2H, NCH2N), 3.93 (t, 4H,
3
3JHH ) 7.5 Hz, CH2CH2CH2CH3), 1.69 (quintet, 4H, JHH
)
3
1
7.5 Hz, CH2CH2CH2CH3), 1.20 (sextet, 4H, JHH ) 7.5 Hz,
MHz, 273 K): δ 38.9 (d, JRhP ) 60.5). Anal. Calcd for
3
CH2CH2CH2CH3), 0.79 (t, 6H, JHH ) 7.5 Hz, CH2CH2-
C53H56I3N4P2Rh (1294.60): C, 49.17; H, 4.36; N, 4.33. Found:
C, 49.00; H, 4.20; N, 4.31.
CH2CH3). 13C{1H} NMR (CD3CN, 125 MHz): δ 143.6 (Cimi),
1
134.3 (C-Rh, JCRh ) 39.5 Hz), 132.9 (Caryl), 131.5 (Caryl),
Typical Procedure for Catalytic Transfer Hydrogenation.
The catalyst was either used as a solution (0.6 mL, 4 mM in
iPrOH, 5 µmol Rh) or weighed off directly into the reaction
flask. It was stirred, together with KOH (0.05 mL of 2 M
solution in H2O, 0.1 mmol) and iPrOH (5.0 mL) at reflux for
10 min. Then the ketone (1.0 mmol) was added at once. Aliquots
(0.2 mL) were taken at fixed times, quenched in hexane (2 mL),
and filtered through a short path of silica, and the silica was
washed with diethyl ether or tert-butyl methyl ether. The
combined organic filtrates were evaporated and analyzed by 1H
NMR spectroscopy.
Structure Determination and Refinement of the Com-
plexes. A suitable single crystal was mounted on a Stoe Mark
II-imaging plate diffractometer system (Stoe & Cie, 2002)
equipped with a graphite monochromator. Data collection was
performed at -100 °C (-80 °C for 3f) using Mo KR radiation
(λ ) 0.71073 Å) with a nominal crystal to detector distance of
70 mm (for 3f), 100 mm (for 3a, 3c, and 3d), and 135 mm (for
2b, 3b, and 7), respectively. All structures were solved by direct
methods using the program SHELXS-97 and refined by full
matrix least-squares on F2 with SHELXL-97.29 The hydrogen
atoms were included in calculated positions and treated as riding
130.3 (Caryl), 124.6 (Cimi), 122.9 (Caryl), 59.9 (NCH2N), 48.7
(CH2CH2CH2CH3), 32.7 (CH2CH2CH2CH3), 20.1 (CH2CH2CH2-
CH3), 13.5 (CH2CH2CH2CH3). Anal. Calcd for C27H32BCl2-
F4N4Rh (673.19): C, 48.17; H 4.79; N 8.32. Found: C, 48.50;
H, 5.20; N, 8.57.
[RhI2(dicarbene)(PPh3)2]I (5). To a solution of 2b (40 mg,
0.02 mmol) in CH2Cl2 (5 mL) was added PPh3 (26 mg, 0.1
mmol). The mixture was stirred at rt for 2 h and subsequently
added dropwise to Et2O (100 mL). The formed precipitate was
separated by centrifugation and dried under vacuum (40 mg,
70%). Analytically pure material was obtained by crystallization
from ClC2H4Cl/Et2O. The three species show similar 1H NMR
1
spectra except the Himi and bridged CH2. Major species: H
NMR (CD2Cl2, 400 MHz): δ 7.67-7.26 (m, 40H, Haryl), 6.65
(s, 2H,
Himi), 5.02 (s, 2H, NCH2N), 3.20 (m, 4H,
CH2CH2CH2CH3), 1.41 (m, 4H, CH2CH2CH2CH3), 1.15 (m, 4H,
CH2CH2CH2CH3), 0.83 (m, 6H, CH2CH2CH2CH3). 31P{1H}
NMR (CD2Cl2, 162 MHz): δ 12.3 (d, 1JPRh ) 89.7 Hz). Minor
species A: 1H NMR (CD2Cl2, 400 MHz): δ 6.57 (s, 2H, Himi),
5.06 (s, 2H, NCH2N). 31P{1H} NMR (CD2Cl2, 162 MHz): δ
15.2 (d, 1JPRh ) 93.5 Hz). Minor species B: 1H NMR (CD2Cl2,
400 MHz): δ 6.39 (s, 2H, Himi), 5.06 (s, 2H, NCH2N). 31P{1H}
1
NMR (CD2Cl2, 162 MHz): δ 15.2 (d, JPRh ) 93.5 Hz). Anal.
(29) Sheldrick, G. M. Acta Crystallogr. A 2008, 64, 112.

3170 Organometallics, Vol. 27, No. 13, 2008
Yang et al.

Rhodium(III) Complexes Containing C4-Bound NHCs
Organometallics, Vol. 27, No. 13, 2008 3171
atoms using SHELXL-97 default parameters. All non-hydrogen
atoms were refined anisotropically. A semiempirical absorption
correction was applied using MULscanABS as implemented in
PLATON03.30
were included in all further calculations. Further details on
data collection and refinement parameters are collected in
Table 5. Crystallographic data (excluding structure factors)
for the structures 2b, 3a-d, 3f, and 7 have been deposited
with the Cambridge Crystallographic Data Centre as supple-
mentary publication nos. CCDC 688121-688127. Copies of
the data can be obtained free of charge on application to
CCDS, 12 Union Road, Cambridge CB2 1EZ, UK [fax: (int.)
+44-1223-336-033; e-mail: deposit@ccds.cam.ac.uk].
The binuclear complex 2b crystallizes with five CHCl3
molecules per asymmetric unit (six positions, of which two
are half-occupied and two are disordered). In compound 3c,
one of the two cocrystallized acetonitrile molecules and the
I2 molecule are disordered. Complex 3d crystallizes with three
acetonitrile molecules per asymmetric unit; one of them is
half-occupied. The cocrystallized hexane molecule in 3f is
on a special position and half-occupied. It is strongly
disordered, and therefore, the SQUEEZE option in PLATON
has been used to remove the electron density corresponding
to the hexane molecule. Nevertheless it has been included
in all further calculations. In the structure of 7, two strongly
disordered ether molecules are cocrystallized. It was not
possible to find a reasonable model defining the disorder.
The SQUEEZE instruction in PLATON0330 was used to
calculate the potential solvent-accessible area in the unit cell;
926 Å3 was calculated containing about 260 electrons.
Therefore, 6 ether molecules (6 × 42 electrons) per unit cell
Acknowledgment. We thank Mr. F. Fehr for technical
assistance. This research was supported by the Swiss National
Science Foundation and Sasol. M.A. is very grateful for an
Alfred Werner Assistant Professorship.
Supporting Information Available: Crystallographic data for
all complexes in CIF format. This material is available free of
charge via the Internet at http://pubs.acs.org.
OM800110C
(30) Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7.