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and D, in which the anion forms part of the respective
chelate ligand. However, C showed comparatively low
activity in the hydrogenation of styrene in nonpolar me-
dia (37% at 0.5 mol% and 1 bar H2 in benzene and 17% at
0.5 mol% and 1 bar H2 in hexanes). In contrast, catalysts
of type D carrying a covalently bound WCA fragment are
able to efficiently catalyze the hydrogenation of trisubsti-
tuted alkenes even in hexane (1 mol%, 50 bar H2), which
could not be accomplished with the previous systems.8
modified this procedure to allow for the preparation of
1
2
3
4
5
6
7
8
the target lithium-carbene complexes as their toluene or
THF solvates in one step from the free NHCs (Scheme 1).
Accordingly, the respective carbene 2 was deprotonated
with n-BuLi in toluene, resulting in a jelly-like mass (for R
= Dipp) or in suspensions of the deprotonated intermedi-
ates, which was followed by addition of the appropriate
borane component. Whereas the toluene solvates 3a and
3d could be directly obtained as white powders by filtra-
tion, the isolation of 3b, 3c and 3e required the addition
of a few drops of THF to precipitate their THF solvates
from toluene solution. This procedure affords the pure
complexes 3a-3e in good to excellent yields (70-99%). All
lithium complexes are highly air- and moisture-sensitive
white solids, which, upon exposure to air, undergo fast
hydrolysis and protonation of the carbene carbon atom as
demonstrated for 3b. The resulting zwitterionic imidazo-
lium borate was characterized by X-ray diffraction analy-
sis, and its molecular structure (see Supporting Infor-
mation) is similar to that previously established for the
zwitterion formally derived from 3e.15a
9
Another important modification was made by Nolan
and coworkers,9 who obtained a thermally more stable
congener E of Crabtree’s catalyst by replacing the phos-
phine in A with an N-heterocyclic carbene (NHC), which
can be expected to bind more strongly to the metal at-
om.10 In the same year, Burgess and coworkers reported
chiral NHC-oxazoline iridium(I) complexes of type F for
asymmetric hydrogenation of alkenes.11 Since then, a
large variety of related phosphine-free NHC12 and also
mixed phosphine-NHC13 iridium(I) complexes have been
introduced as efficient hydrogenation catalysts.
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Recently, we introduced a novel type of anionic N-
heterocyclic carbenes with a weakly coordinating borate
moiety (WCA-NHC, Figure 1),14 which was developed as a
spin-off from our studies on frustrated carbene-borane
Scheme 1. Preparation of the lithium-carbene com-
plexes 3a-3e
,
Lewis pairs.15 16 It was demonstrated that such carbene
ligands give access to zwitterionic gold(I) complexes such
as [(WCA-NHC)Au(THT)] (THT = tetrahydrothiophene),
which serve as catalysts for the skeletal rearrangement of
enynes even in less polar solvents such as toluene.14 Natu-
rally, the coordination of such anionic carbenes to the
iridium(I)-cyclooctadiene fragment would potentially add
a new member to the large family of Crabtree-type cata-
lysts and combine two of the successful strategies out-
lined above, i.e. the use of NHC ligands, as present in
structures E and F, with that of anionic ligands, as real-
ized in the ylidic structures C and D. Furthermore, the
resulting zwitterionic complexes of the type [(WCA-
NHC)Ir(COD)] of type 1 might exhibit an intramolecular
arene-iridium interaction (Figure 1) and thus afford one-
component and neutrally charged catalysts for use in
non-polar solvents. Accordingly, we wish to report herein
the synthesis and characterization of iridium(I) complex-
es of type 1 together with a detailed study of their use as
olefin hydrogenation catalysts. In addition, related cy-
clooctadiene and dicarbonyl rhodium(I) complexes are
presented, with the latter serving as a tool to establish the
electronic character and donor ability of the new carbene
ligands by IR spectroscopy.
The new complexes were characterized by NMR spec-
troscopy in THF-d8; however, the 13C NMR signal of the
carbene carbon atom was only detected for 3c at 218.9
ppm, which is upfield from the resonance of the corre-
sponding free carbene IDipp ( = 220.6 ppm)18 and close
to the chemical shift found for 3b at 217.2 ppm.14 The
introduction of the borate moiety renders the N-
substituents magnetically inequivalent, and two different
sets of signals are detected for the R groups (Mes, Dipp, t-
1
13
Bu) in the H and C NMR spectra. Two of the three new
lithium complexes were characterized by X-ray diffraction
analyses, and selected bond lengths and angles are as-
sembled in Table 1. Complex 3d (Figure 2)has a structure
similar to that previously reported for 3b,14 with the lithi-
um atom residing in a trigonal-planar coordination
sphere created by the carbene and two THF ligands (angle
sum = 359.9°). The angle between the O1-Li-O2 and N1-
We have
previously reported a two-step procedure for the prepara-
tion of the lithium-carbene complexes 3b and 3e.14 The
first step involved deprotonation of the free carbenes 2
with n-BuLi in hexane according to the procedure de-
scribed by Robinson and coworkers.17 The resulting lithi-
um salts were isolated by filtration and subsequently
treated with B(C6F5)3 in toluene solution. We have now
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