Rhodium and iridium complexes of N-heterocyclic carbenes: Structural investigations and their catalytic properties in the borylation reaction

Article abstract of DOI:10.1016/j.jorganchem.2006.08.099

Bridged and unbridged N-heterocyclic carbene (NHC) ligands are metalated with [Ir/Rh(COD)₂Cl]₂ to give rhodium(I/III) and iridium(I) mono- and biscarbene substituted complexes. All complexes were characterized by spectroscopy, in addition [Ir(COD)(NHC)₂][Cl,I] [COD = 1,5-cyclooctadiene, NHC = 1,3-dimethyl- or 1,3-dicyclohexylimidazolin-2-ylidene] (1, 4), and the biscarbene chelate complexes 12 [(η⁴-1,5-cyclooctadiene)(1,1′-di-n-butyl-3,3′-ethylene-diimidazolin-2,2′-diylidene)iridium(I) bromide] and 14 [(η⁴-1,5-cyclooctadiene)(1,1′-dimethyl-3,3′-o-xylylene-diimidazolin-2,2′-diylidene)iridium(I) bromide] were characterized by single crystal X-ray analysis. The relative σ-donor/π-acceptor qualities of various NHC ligands were examined and classified in monosubstituted NHC-Rh and NHC-Ir dicarbonyl complexes by means of IR spectroscopy. For the first time, bis(carbene) substituted iridium complexes were used as catalysts in the synthesis of arylboronic acids starting from pinacolborane and arene derivatives.

Full text of DOI:10.1016/j.jorganchem.2006.08.099

Journal of Organometallic Chemistry 691 (2006) 5725–5738
www.elsevier.com/locate/jorganchem
Rhodium and iridium complexes of N-heterocyclic carbenes:
Structural investigations and their catalytic properties
q
in the borylation reaction
1
2
Guido D. Frey , Christoph F. Rentzsch, Denise von Preysing , Tobias Scherg,
Michael M u¨ hlhofer, Eberhardt Herdtweck, Wolfgang A. Herrmann *
Department Chemie, Lehrstuhl f u¨ r Anorganische Chemie, Technische Universit a¨ t M u¨ nchen, Lichtenbergstraße 4, D-85747 Garching, Germany
Received 8 June 2006; received in revised form 9 August 2006; accepted 11 August 2006
Available online 19 September 2006
Abstract
Bridged and unbridged N-heterocyclic carbene (NHC) ligands are metalated with [Ir/Rh(COD)
ium(I) mono- and biscarbene substituted complexes. All complexes were characterized by spectroscopy, in addition [Ir(COD)(NHC)2]-
Cl,I] [COD = 1,5-cyclooctadiene, NHC = 1,3-dimethyl- or 1,3-dicyclohexylimidazolin-2-ylidene] (1, 4), and the biscarbene chelate
2 2
Cl] to give rhodium(I/III) and irid-
[
4
0
0
0
0
4
complexes 12 [(g -1,5-cyclooctadiene)(1,1 -di-n-butyl-3,3 -ethylene-diimidazolin-2,2 -diylidene)iridium(I) bromide] and 14 [(g -1,5-cyclo-
0
0
octadiene)(1,1 -dimethyl-3,3 -o-xylylene-diimidazolin-2,2 -diylidene)iridium(I) bromide] were characterized by single crystal X-ray anal-
ysis. The relative r-donor/p-acceptor qualities of various NHC ligands were examined and classified in monosubstituted NHC-Rh and
NHC-Ir dicarbonyl complexes by means of IR spectroscopy. For the first time, bis(carbene) substituted iridium complexes were used as
catalysts in the synthesis of arylboronic acids starting from pinacolborane and arene derivatives.
ꢀ
2006 Elsevier B.V. All rights reserved.
Keywords: Carbene complexes; Catalysis; Coupling; Hydroborations; Iridium; Rhodium
1
. Introduction
lar C–H activation with saturated and unsaturated hydro-
carbons [1,2]. So far, only a few examples of complexes are
known which could accomplish the activation of a C–H
bond of aromatic substrates.
Recently, several research groups investigated the spec-
tacular synthesis of arylboronic acid ester derivatives via
rhodium and iridium catalyzed C–H-borylation of arenes
and heteroarenes, by means of bis(pinacolato)diborane
The selective functionalization of unreactive C–H bonds
is one of the largest challenges in metal-organic chemistry
1]. Meanwhile a set of transition metal complexes, in par-
[
ticular complexes of palladium, platinum, rhodium and
iridium, were described which are capable of intermolecu-
(
Pin B ) or pinacolborane (PinBH) [3–7]. The C–H-boryla-
2 2
q
N-Heterocyclic carbenes, Part 48. For part 47 see: T. Scherg, S.K.
tion reaction was also established for aliphatic hydrocar-
bons and vinylic compounds [8,9]. The widely available
arylboronic acid ester derivatives can easily be transformed
into various functionalized hydrocarbons such as alcohols,
amines or alkenes. Also, their possible employment in cat-
alytic C–C coupling reactions, like the Suzuki–Miyaura
coupling [10], turns these borylated hydrocarbons into
extraordinarily valuable raw materials for organic chemis-
try [11]. The system published by Miyaura, Ishiyama and
Schneider, G.D. Frey, J. Schwarz, E. Herdtweck, and W.A. Herrmann,
Synlett (2006).
*
Corresponding author. Tel.: +49 89 289 13080; fax: +49 89 289 13473.
E-mail addresses: guido.frey@ch.tum.de (G.D. Frey), lit@arthur.
anorg.chemie.tu-muenchen.de (W.A. Herrmann).
1
Present address: UCR-CNRS Joint Research Chemistry Laboratory
(
UMI 2957), Department of Chemistry, University of California, River-
side, CA 92521-0403, USA.
2
Present address: BASF Aktiengesellschaft, GKD/K - B001, D-67056
Ludwigshafen, Germany.
0
022-328X/$ - see front matter ꢀ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2006.08.099

5726
G.D. Frey et al. / Journal of Organometallic Chemistry 691 (2006) 5725–5738
Hartwig possesses particularly high activity, which was fur-
ther optimized in the course of systematic studies. Thus the
2. Results and discussion
4
0
combination of [Ir(g -COD)(OMe)] and 4,4 di-tert-butyl-
In the present paper we focus on investigations of the
activation and functionalization of aromatic C–H bonds
[19], using iridium as the catalytically active metal, given
the recent success in the borylation of aromatic compounds
[3–9]. Therefore, we tried to apply adequate biscarbene
complexes for the direct catalytic synthesis of arylboronic
acid esters. Convenient cationic biscarbene complexes of
rhodium and iridium can be synthesized by in situ deproto-
nation of the imidazolium salts [20–23]. Substituting the
halide bridge in the precursor dimer by an ethoxy bridge,
this ‘‘internal base’’ deprotonates the imidazolium salt
in situ, leading to the desired complexes (Scheme 2a).
The reaction of an alcoholate adduct of the imidazolium
2
0
2
,2 -bipyridine makes the catalytic borylation of aromatics
with stoichiometric quantities of bis(pinacolato)diborane
in an inert solvent at ambient temperature possible
(
Scheme 1) [3,4a]. In addition the reaction is regioselective
and a wide set of functional groups are tolerated in this
reaction [3,6]. Mediated by a transition metal boryl com-
plex, the mechanism of the borylation reaction proceeds
via oxidative addition of the C–H bond [4]. The direct syn-
thesis of boronic acid esters via activation and functional-
ization of an aromatic C–H bond is a clear simplification.
For a long time our group was focused on the employ-
ment of stable nucleophilic carbenes, such as the N-hetero-
cyclic carbenes [12]. They have the advantageous property
of forming strong bonds to metal centers, with little ten-
dency towards dissociation [13–15]. This is particularly
beneficial in their use as ligands for organometallic cata-
lysts [16]. The r-donor strength varies with the constitution
of the respective derivative. A large number of new mono-
and bi-dentate carbene ligands of imidazolin-2-ylidene and
the imidazolidin-2-ylidene types were developed and
applied in the synthesis of transition metal complexes
salt with the [M(COD)Cl] precursor dimer is another pos-
2
sible synthesis route for the formation of the expected
biscarbene complexes (Scheme 2b) [12,14,24,25]. These
complexes are soluble in most organic solvents and stable
towards exposure to air under dry conditions.
Complexes 1–4 (Table 1) were prepared according to the
procedure shown in (Scheme 2a); for the bridged biscar-
bene complexes 5–12 the synthesis route over the imidazo-
lium alcoholate was used (Scheme 2b, Table 1). Complexes
13 [26,27] and 14 were also prepared according to Scheme
[
17,18].
1
3
2
b. The C NMR signals for the carbene carbons of the
characterized biscarbene complexes are in the range of
d = 172–182 ppm, and are in agreement with the literature
[
20,26–31]
O
O
[cat.]
Pin B + 2 H
2
B
+H 2
Suitable crystals of complexes 1 (Fig. 1), 4 (Fig. 2), 12
Fig. 3) and 14 (Fig. 4) for single X-ray diffraction studies
2
2
R
R
(
were obtained from dichloromethane/n-pentane solutions
at ambient temperature. A selection of characteristic bond
angles and bond distances are given in Table 2.
All complexes favour a slightly disorted square-planar
coordination sphere of the iridium center. The iridium–car-
bene bond lengths of complexes 1 and 4 are marginally
longer [(1) 2.03, 2.08; (4) 2.074(4), 2.076(3)] than the bond
lengths of complexes 12 and 14 [(12) 2.02, 2.04; (14)
1/
[Ir(Cl,I)(COD)]2
+
2
O
O
O
cat. =
Pin B =
B
B
2
2
t-Bu
t-Bu
O
N
N
Scheme 1. Borylation of hydrocarbons.
+
X-
a
+
X-
R
N
Et
O
N
+ 1
R
R
2
N
N
/2
M
M
M
R
R
-
EtOH
O
N
Et
H
N
R
M = Rh, Ir
+
X-
b
+
X-
R
N
N
1/2 [M(COD)Cl]2
2
KOt-Bu
2 KX
R
R
2
N
N
N
N
M
R
R
2R
R
-
THF/toluene/Δ
H Ot-Bu -2 HOt-Bu
N
H
N
R
M = Rh, Ir
Scheme 2. Synthesis of biscarbene substituted iridium and rhodium complexes.

G.D. Frey et al. / Journal of Organometallic Chemistry 691 (2006) 5725–5738
5727
Table 1
Biscarbene substituted iridium and rhodium complexes
+
X-
+
X-
R1
R1
+
X-
R1
N
N
N
N
M
N
N
N
N
N
2
N
N
R
R
M
2
M
(CH2)n
R1
N
R1
R1
1
- 4
5 - 12
13 - 15
1
2
ꢀ
X
Complex
M
R
R
n
Yield (%)
dCcarbene (ppm)
ꢀ
1
2
3
4
5
6
7
8
9
Ir
Rh
Ir
Ir
Rh
Ir
Rh
Ir
Rh
Rh
Ir
Ir
Rh
Ir
Me
t-Bu
t-Bu
Cy
Et
Et
Me
t-Bu
t-Bu
Cy
–
–
–
–
–
–
–
–
–
–
–
I
–
–
–
–
1
1
1
1
2
2
2
2
–
–
–
86
70
67
68
48
52
71
75
57
74
79
92
87
71
71
177.0
180.1
174.5
173.9
179.9
173.7
182.0
174.2
178.4
180.0
175.1
175.0
180.5
175.9
175.7
ꢀ
ꢀ
ꢀ
Cl
Cl
Cl
ꢀ
ꢀ
ꢀ
ꢀ
I
I
I
I
Cy
Cy
ꢀ
Me
t-Bu
t-Bu
n-Bu
Me
Me
Me
PF
6
ꢀ
1
1
1
1
1
1
0
Br
Br
Br
PF
Br
ꢀ
ꢀ
1
2
ꢀ
3 [27]
4
5
6
ꢀ
ꢀ
Ir
PF
6
dination of the bischelate. In all complexes the N–C–N
angles of the carbene ligands are in accordance with coordi-
nated azole rings reported in the literature [33,34].
To form biscarbene substituted rhodium(III) complexes,
a synthetic route first described by Peris in 2003 was used
(
Scheme 3) [27]. Starting from the [Rh(COD)Cl] precursor
2
the first step of the reaction is oxidation by passing air
through the reaction solution to increase the yield of the
desired complexes. The addition of KBr at the end of the
reaction is in order to form only the perbrominated com-
1
3
plexes 16 and 17. In the C NMR spectrum, the carbene
signals are observed as doublets at d = 175.8 ppm (16)
and 176.9 ppm (17) with coupling constants of
1
JRh,C = 34.1 Hz (16) and 33.9 Hz (17). The chemical shift
of the carbene is very similar to the n-butyl substituted
complex synthesized by Poyatos et al. in 2003 [27].
Monosubstituted carbene complexes of rhodium and
iridium are known now from nearly every carbene type.
The variety of such complexes is nowadays enormous and
¨
varies from the classical Ofele-Wanzlick-Arduengo carb-
enes [35], (NHCs) [17,21,23,25,28,29,34,36–38], to acyclic
varieties [39,40] and alternative carbene derivatives [41,42].
To form mono carbene rhodium and iridium (COD)
complexes three different synthesis routes are well estab-
lished (Schemes 4 and 6). First the reaction of a free car-
Fig. 1. Ball and stick model of the cationic part of compound 1 in the
solid state (for details see Ref. [32a]).
2
.044(5), 2.044(5)], where the carbene ligand acts as a bische-
late. These bond lengths show no indication for different
donor strength of the carbene ligandsdue to the similarprop-
erties of these ligands. The iridium–carbene bond lengths in
complexes 12 and 14 are on average 0.03 A shorter than the
bond lengths of complexes 1 and 4, caused by the rigid coor-
bene with the dimeric precursor [M(COD)Cl] (M = Rh,
2
Ir) (Scheme 4a). This method is used for bulky substituents
such as adamantyl or tert-butyl groups in 1,3-position of
cyclic azolin-2-ylidene and acyclic carbenes to give mono-
substituted carbene rhodium complexes by cleaving the
˚

5728
G.D. Frey et al. / Journal of Organometallic Chemistry 691 (2006) 5725–5738
Fig. 3. Ball and stick model of the cationic part of compound 12 in the
solid state (for details see Ref. [32b]).
carbene complexes as carbene transfer reagents has proven
to be an effective method for the synthesis of other transi-
tion metal carbene complexes (Scheme 6) [38,45,46].
The imidazolium salts were treated with Ag O at room
2
temperature to form the corresponding silver carbene com-
Fig. 2. ORTEP style plot of the cationic part of compound 4 in the solid
plexes. The reaction of the silver salts with [Ir(COD)Cl] at
room temperature readily gave the desired yellow metal
carbene complexes 22 and 24.
A summary of the prepared mono carbene rhodium and
iridium complexes is shown in Table 3.
The corresponding carbonyl substituted rhodium and
iridium complexes 29–32 were obtained by passing carbon
monoxide through a dichloromethane solution of the COD
substituted complexes 18, 21, 22, 24 at room temperature
2
state. Thermal ellipsoids are drawn at the 50% probability level. Hydrogen
˚
atoms are omitted for clarity. Selected bond lengths (A) and bond angles
(
ꢁ): Ir–C1 2.074(4), Ir–C16 2.076(3), Ir–C31 2.161(3), Ir–C32 2.206(3), Ir–
C35 2.171(3), Ir–C36 2.200(4), N1–C1 1.368(5), N1–C2 1.384(4), N1–C4
1
1
1
.471(5), N2–C1 1.369(4), N2–C3 1.388(5), N2–C10 1.473(5), N3–C16
.366(5), N3–C17 1.382(4), N3–C19 1.468(5), N4–C16 1.363(4), N4–C18
.393(5), N4–C25 1.471(5); C1–Ir–C16 97.9(1), Ir–C1–N1 124.7(2), Ir–C1–
N2 131.1(3), N1–C1–N2 103.5(3), Ir–C16–N3 124.7(2), Ir–C16–N4
31.2(3), N3–C16–N4 103.8(3).
1
(Scheme 7). The products 29–32 were formed within
chloro bridge of the dimeric COD complexes during the
reaction [28,36,37,39]. Applying imidazolium alcoholate
precursors, (Scheme 4b) preparation is very similar to the
synthesis of the analogous biscarbene complexes [25].
According to a synthetic route from Denk and Herr-
mann, where the first metal-complex of rhodium and irid-
ium with an acyclic carbene was synthesized [39], we
attempted to increase the variety of this type of complexes
with a similar acyclic carbene. The deprotonation of 27 to
form the free acyclic carbene 28 was performed according
to a procedure published by Alder et al. where LDA
30 min, evidenced by a color change from light to dark yel-
low. Due to the strong donor capability of the NHC
ligands the cyclooctadiene ligand can be completely substi-
tuted by stronger acceptor ligands such as carbon monox-
ide [28].
The cis-conformation of the carbonyl ligands in com-
plexes 29–32 was consistently indicated by IR- and
NMR-spectra. The IR spectra exhibit two strong m(CO)-
1
3
bands. The
C NMR-spectra contain three signals
between d = 167 and 187 ppm, two for the carbonyl and
one for the carbene-carbons. All carbonyl complexes are
air-stable in the solid state, but decompose in solution
under air within some hours. They are all soluble in polar
solvents such as chloroform, dichloromethane, DMSO,
and THF and less soluble in less polar solvents such as
diethyl ether, n-hexane or n-pentane. The CO-stretching
frequencies of each complex were recorded in dichloro-
methane solution and are listed in Table 4.
(
(
lithiumdiisopropylamide) was used for deprotonation
Scheme 5) [43]. Complex 25 was first published by our
group [40] and was prepared according to a procedure pub-
lished recently [44].
The third preparation pathway for mono carbene substi-
tuted COD complexes was used for the iridium complexes
2
2 and 24 via a silver intermediate [45]. The use of silver

G.D. Frey et al. / Journal of Organometallic Chemistry 691 (2006) 5725–5738
5729
1
N
/ [Rh(COD)Cl] , NEt , KBr
N
2
2
3
N
N
N
N
N
N
+
N
R
+
N
CH CN, reflux 16 h, air
3
Rh
Br-
Br
Br
Br
2
R
R
R
1
1
6: R = Me
7: R = t-Bu
Scheme 3. Synthesis of biscarbene substituted rhodium(III) complexes.
expected range, as mentioned before for such complexes
[
21,25,28,29,36,38a,39,42].
For catalytic experiments we used the complexes 1, 4, 8,
12 and 14 as catalysts [19]. The catalytically active systems
(1a, 4a, 8a, 12a, 14a) were generated by exchange of the
coordinating anions with the non-coordinating trifluoroac-
etate (TFA) (Scheme 8). The aromatic substrate was used
as solvent and for the boron source pinacolborane was
selected for several reasons: when bis(pinacolato)diborane
is used, only one boron equivalent is converted to the
a
Cl
Cl
NHC
Cl
THF
2
NHC +
M
M
2
M
M = Rh, Ir
19, 20, 23, 25, 26
b
R1
N
+
X-
N
Fig. 4. ORTEP style plot of the cationic part of compound 14 in the solid
2 KOt-Bu
[M(COD)Cl]2
2
N
N
R2
2
Rh
R2
state. Thermal ellipsoids are drawn at the 50% probability level. Hydrogen
R1
-
2 KX
THF/toluene
-2 HOt-Bu
X
˚
atoms are omitted for clarity. Selected bond lengths (A) and bond angles
H
(
ꢁ): Ir–C1 2.044(5), Ir–C10 2.190(5), Ir–C11 2.191(4), N1–C1 1.342(6),
N1–C2 1.399(7), N1–C4 1.458(6), N2–C1 1.343(5), N2–C3 1.386(6), N2–
C5 1.469(6); C1–Ir–C1_a 92.1(2), Ir–C1–N1 127.8(3), Ir–C1–N2 127.0(3),
N1–C1–N2 105.1(4). Symmetry operation to equivalent atom positions _a:
18: X = Cl, R , R = i-Pr
1 2
21: X = I, R = Me, R = Mes
1
2
ꢀx,y,z.
Scheme 4. Synthesis of monocarbene substituted iridium and rhodium
complexes.
The relevant IR data differ for each metal only by five
wave numbers and correlate very well with further pub-
lished results for imidazolin-2-ylidene complexes [44]. Also
the chemical shifts of the carbene carbons are in the
LDA, THF
N + N
H
N
N
- 78˚C
PF -
6
2
7
28
Table 2
˚
Scheme 5. Preparation of the free acyclic carbene 28.
Selected bond distances (A) bond and torsion angles (ꢁ) for the complexes
1, 4, 12 and 14
˚
Complex
Ir–Ccarbene (A)
N–Ccarbene–N (ꢁ)
Ccarbene–Ir–Ccarbene (ꢁ)
R
+
Cl-
N
1
2.03
103.5
106.8
93.4
2
.08
2.074(4)
.076(3)
2.02
N
Ag O
[Ir(COD)Cl]2
CH Cl
RT, 1 h
2
2
N
N
2
Ir
R
R
R
4
103.5(3)
103.8(3)
97.9(1)
85.4
CH Cl
2
2
2
2
Cl
2
RT, 2 h
H
1
1
2
4
103.6
104.5
22: R = i-Pr
4: R = Cy
2.04
2
2.044(5)
105.1(4)
105.1(4)
92.1(2)
Scheme 6. Synthesis of monosubstituted NHC–Iridium complexes using
the silver transmetalation route.
2.044(5)

5730
G.D. Frey et al. / Journal of Organometallic Chemistry 691 (2006) 5725–5738
Table 3
O
O
Δ, [cat.]
Monocarbene substituted rhodium and iridium(COD) complexes
PinBH + H
X
B
X + H2
R1
N
N
X = H, Cl, Br, I, CH3
N
R2
N
M
M
+
CF3CO2-
+
CF3CO2-
R
R
R
N
N
X
Cl
N
N
N
N
R
R
Ir
Ir
(B)
1
8-24
25, 26
cat. = 1.5 mol%
;
1
2
Complex
M
R
R
X
Yield (%)
dCcarbene (ppm)
N
N
R
18
19
20
21
22
23
24
25
26
Rh
Rh
Rh
Rh
Ir
Ir
Ir
Rh
Ir
i-Pr
t-Bu
Ad
Me
i-Pr
t-Bu
Cy
–
i-Pr
t-Bu
Ad
Mes
i-Pr
t-Bu
Cy
Cl
Cl
Cl
I
Cl
Cl
Cl
Cl
Cl
73
38
69
71
66
41
73
49
53
179.9
177.9
183.0
182.1
177.9
179.8
178.1
216.2
214.7
8a: R = Cy; B = CH2
2a: R = n-Bu; B = CH CH
4a: R = Me; B = o-xylylene
1
a: R = Me
4a: R = Cy
1
1
2
2
Scheme 8. Borylation of benzene derivatives with biscarbene iridium
complexes.
–
–
–
Table 5
Conversion of aryls with pinacolborane to the corresponding arylboronic
acids
Complex
Entry Aryl
Cat.
T
t
Yield
(
mol%) (ꢁC) (h) (%)
1
a, 12a, 14a
1
2
3
4
5
Benzene
Toluene
Chlorobenzene
Bromobenzene
Iodobenzene
1.5
1.5
1.5
1.5
1.5
40
40
40
40
40
12
12
12
12
12
100
100
100
100
100
R
N
R
N
N
R
N
R
+
CO
OC
OC
M
M
CH Cl
X
2
2
X
RT, 30 min
COD
4
a
6
7
8
9
0
1
2
Benzene
Toluene
Chlorobenzene
Bromobenzene
Iodobenzene
1.0
1.0
1.0
1.0
1.0
40
40
40
40
40
40
40
10
10
10
10
10
10
10
100
100
100
100
100
92
-
M = Rh, Ir
29 - 32
Scheme 7. Synthesis of monocarbene substituted iridium and rhodium
carbonyl complexes.
1
1
1
2-Chlorotoluene 1.0
ortho-Xylene
1.0
93
8
a
13
Benzene
Toluene
Chlorobenzene
Bromobenzene
Iodobenzene
1.5
1.5
1.5
1.5
1.5
45
45
45
45
45
9
9
9
9
9
100
100
100
100
100
Table 4
1
1
4
5
Monocarbene substituted iridium and rhodium carbonyl complexes
R1
N
16
1
7
OC
OC
N
M
R2
14a
18
19
2-Chlorotoluene 1.5
3-Chlorotoluene 1.5
ortho-Xylene
40
40
40
12
12
12
90
89
94
X
1
2
Complex
M
R
R
X
m(CO) m(CO) Yield dCcarbene
2
0
1.5
sym.
Cl 2078
2073
asym. (%)
(ppm)
29
30
31
32
Rh i-Pr i-Pr
Rh Me Mes
i-Pr i-Pr
Cy Cy
1997
2000
1982
1982
95
91
93
94
170.2
174.3
171.0
171.2
I
only product after 10–12 h by full conversion, according
to pinacolborane for all used catalysts (Table 5, Entries
1
Ir
Ir
Cl 2066
Cl 2064
, 6, 13). When using toluene and halogen aromatics of
conversions up to 100% were obtained.
Thus chloro-, bromo- and iodobenzene were also con-
verted in good yields into the corresponding boronic acid
ester (Table 5) [3a,47]. It became evident from the GC-
MS-analysis that exclusive C–H activation took place while
the C–X bonds were not attacked at all. Analogous reac-
tions were performed with the corresponding rhodium
complexes. Under the same conditions which were used
for the iridium complexes, no conversion of pinacolborane
could be detected within 12 h with benzene, toluene,
chloro- or bromobenzene by using the corresponding rho-
dium complexes. However, it should be mentioned that in
favored product, however bis(pinacolato)diborane is much
more expensive than pinacolborane. The latter offers easy
accessibility from the inexpensive reducing agent sodium
borohydride. Identification of the formed products and
the yield rates was possible by GC-MS. The results of
our experiments are presented in Scheme 8 and Table 5.
It soon appeared during the conversion of benzene that
the desired product was obtained in high selectivity; in this
case phenylboronic acid pinacol ester was detected as the

G.D. Frey et al. / Journal of Organometallic Chemistry 691 (2006) 5725–5738
5731
100
in the literature [25,50]. The silver carbene complexes were
prepared according to the literature [51].
All experiments were carried out under dry argon using
standard Schlenk or dry box techniques. Solvents were
dried by standard methods and distilled under nitrogen.
8
6
4
2
0
0
0
0
0
1
13
H and C NMR spectra were recorded on a JEOL-
JMX-GX 400 MHz spectrometer at room temperature
1
13
and referenced to the residual H and C signals of the sol-
vents. NMR multiplicities are abbreviated as follows:
s = singlet, d = doublet, t = triplet, sept. = septet,
m = multiplet, br = broad signal. Coupling constants J
are given in Hz. IR spectra were recorded on a FTS
4
12a
a
575C BIO-RAD or Jasco 460 spectrometer. Elemental
analyses were carried out by the Microanalytical Labora-
tory at the TU M u¨ nchen. Mass spectra were performed
at the TU M u¨ nchen Mass Spectrometry Laboratory on a
Finnigan MAT 90 spectrometer using the CI or FAB tech-
nique. Melting points were measured with a B u¨ chi melting
point apparatus system (Dr. Tottoli).
0
2
4
6
8
10
12
14
16
t [h]
Fig. 5. Catalytic borylation of benzene with pinacolborane at 40 ꢁC
applying iridium complexes 4a and 12a.
0
0
the last two cases again the C–X bonds were not attacked
during the reactions.
4.2. 1,1 -Di-tert-butyl-3,3 -ethylene-diimidazolium dibromide
(33)
In Fig. 5 the catalytic properties of complexes 4a and
1
2a are monitored in the borylation reaction of benzene
with pinacolborane. Compared to complex 12a, complex
a has a much more intense induction period. Both com-
plexes show a full conversion after 12 h.
Tert-butylimidazole (6.25 g, 50 mmol), 1,2-dibromo-
ethane (4.69 g, 25 mmol) and 5 mL THF were heated in
an ACE pressure-tube under reflux conditions for 48 h.
The light yellow precipitate was collected by filtration
and washed three times with 5 mL THF. The hygroscopic
solid can be recrystallized from a dichloromethane solution
to give pure 33. Yield: 9.26 g (85%). mp: 262 ꢁC (dec.). IR
(KBr): 3131, 3092, 2976, 2883, 2083, 1749, 1636, 1552,
1474, 1407, 1378, 1335, 1317, 1288, 1239, 1205, 1132,
4
3
. Conclusion
We were able to show that N-heterocyclic carbene com-
plexes of iridium(I) show significant catalytic performance
in the direct borylation of arenes. This is in accordance
with the previous observation that complexes with strong
donor ligands provide good catalytic activities in this reac-
tion [3]. GC-MS analysis revealed that the reaction occurs
with high chemo-selectivity via activation of a C–H bond
while C–X bonds are not attacked. Furthermore, high reg-
ioselectivity is observed, since only one borylated product
is detected. This is the first example for the application of
N-heterocyclic carbene complexes in the catalytic activa-
tion and functionalization of aromatic C–H-bonds. Cur-
rently, the further use of various N-heterocyclic carbene
complexes and substrates is under investigation in our lab-
oratory. From these initial results it seems possible that
other functionalized arylboronicacid esters will be accessi-
ble via this one-pot-synthesis.
ꢀ
1
1
846, 762, 689, 657, 635, 566, 489, 460 cm . H NMR
(400 MHz, DMSO-d ): d = 8.51 (s, 2H, NCHN), 7.12 (s,
6
2H, NCHCHN), 6.75 (s, 2H, NCHCHN), 3.84 (s, 4H,
1
3
NCH CH N), 1.59 (s, 18H, NC(CH ) ).
C NMR
2
2
3 3
(100.5 MHz, DMSO-d ): d = 135.3 (NCHN), 122.8, 120.5
6
(NCHCHN), 59.8 (NCH CH N), 48.4 (NC(CH ) ), 28.9
2
2
3 3
+
2+
(NC(CH ) ). MS (FAB): m/z = 355.3 [M ], 275.3 [M ].
3
3
Anal. Calc. for C H N Br (436.23): C, 44.05; H, 6.47;
1
6
28
4
2
N, 12.84. Found: C, 43.81; H, 6.45; N, 12.69%.
4.3. Di-N-pyrrolidylmethylidene (28)
A suspension of 1.2 mmol pyrrolidine-1-ylmethylidene-
pyrrolidinium hexafluorophosphate (27) in 10 mL THF
was mixed at ꢀ78 ꢁC with a solution of 1.2 mmol LDA
in 5 mL THF. After removing the cooling bath the solu-
tion was stirred for 45 min at ambient temperature. A
clear light yellow solution was observed. All volatile com-
ponents were removed in high vacuo. Afterwards the
residual solid was extracted three times with 15 mL n-hex-
ane. After removing the solvent in vacuo and sublimation
the free carbene was received as a colorless crystalline
4
. Experimental
4
.1. General comments
The imidazolium salts [48] and the corresponding carb-
enes were prepared according to the reported procedures.
The formamidinium salt 27 was prepared according to
the literature [49]. The metal dimer precursors
1
solid. Yield: 138 mg (75%). H NMR (C D ): d = 3.05
6
6
(s, 4H, NCH ), 2.60 (s, 4H, NCH ), 1.76 (s, 4H, CH ),
2
2
2
1
3
[
M(COD)Cl] (M = Rh, Ir) were synthesized as reported
1.25 (s, 4H, CH2). C NMR (C D ): d = 241.9 (s,
6 6
2

5732
G.D. Frey et al. / Journal of Organometallic Chemistry 691 (2006) 5725–5738
NCN), 49.6 (s, NCH ), 32.9 (s, NCH ), 25.9 (s, CH ),
4.5. Preparation of chelating biscarbene complexes
2
2
2
2
4.0 (s, CH2).
NaH (40 mg, 1.70 mmol) was dissolved in 10 mL etha-
nol and slowly added to a suspension of 0.34 mmol
4.4. General procedure for the preparation of biscarbene
complexes
[M(COD)Cl] (M = Rh, Ir) in 10 mL ethanol. The reaction
2
mixture was stirred for 30 min at room temperature and
0.85 mmol of the bis(imidazolium) salt was added. After
stirring for 48–72 h at room temperature the solvent was
removed in vacuo. The solid was washed twice with
10 mL ice-cold diethyl ether.
NaH (55 mg, 2.29 mmol) was dissolved in 10 mL etha-
nol and slowly added to a suspension of 0.34 mmol
[
M(COD)Cl] (M = Rh, Ir) in 15 mL ethanol. The reaction
2
mixture was stirred for 30 min at room temperature and
.70 mmol of the imidazolium salt was added. After stir-
1
4
0
0
ring between 90 min and 72 h at room temperature the sol-
vent was reduced in vacuo. The solid was washed twice
with 5 mL ice-cold diethyl ether. The complexes can be
recrystallized from a dichloromethane/n-hexane solution.
4.5.1. (g -1,5-cyclooctadiene)(1,1 -diethyl-3,3 -methylene-
diimidazolin-2,2 -diylidene)rhodium(I) iodide (5)
0
1
Yield: 44 mg (48%). H NMR (DMSO-d ): d = 8.12 (s,
6
4H, NCHCHN), 6.23 (br, 2H, NCH N), 4.03 (br, 4H,
2
CODvinyl), 4.32 (d, 4H, NCH ), 3.07 (br, 8H, CODallyl),
2
4
13
4
ylidene)iridium(I) iodide (1)
.4.1. (g -1,5-cyclooctadiene)bis(1,3-dimethylimidazolin-2-
1.41 (s, 6H, CH3). C NMR (DMSO-d ): d = 179.9
6
(NCN), 122.8 (NCHCHN), 75.4 (CODvinyl), 59.7
(NCH N), 44.6 (NCH ), 34.4 (CODallyl), 14.5 (CH3).
1
Yield: 91 mg (86%). H NMR (CDCl ): d = 7.06 (s, 4H,
3
2
2
NCHCHN), 3.94 (s, 12H, NCH ), 3.77 (s, 4H, COD
),
C
3
vinyl
1
3
4
0
0
2
.29 (m, 4H, CODallyl), 1.94 (m, 4H, CODallyl).
4.5.2. (g -1,5-cyclooctadiene)(1,1 -diethyl-3,3 -methylene-
diimidazolin-2,2 -diylidene)iridium(I) iodide (6)
0
NMR (CDCl ): d = 177.0 (NCN), 123.1 (NCHCHN),
3
1
7
6.7 (CODvinyl), 38.3 (NCH ), 30.9 (CODallyl). MS (FAB)
Yield: 55 mg (52%). H NMR (DMSO-d ): d = 7.89 (s,
4H, NCHCHN), 6.43 (br, 2H, NCH N), 4.21 (br, 4H,
3
6
+
m/z = 493 (M ). Anal. Calc. for C H N IIr (619.57): C,
1
8
28
4
2
3
4.89; H, 4.56; N, 9.04. Found: C, 35.39; H, 4.82; N, 8.96%.
CODvinyl), 4.04 (d, 4H, NCH ), 3.12 (br, 8H, CODallyl),
2
1
3
1
.62 (s, 6H, CH3). C NMR (DMSO-d ): d = 173.7
6
4
4
ylidene)rhodium(I) chloride (2)
.4.2. (g -1,5-cyclooctadiene)bis(1,3-tert-butylimidazolin-2-
(NCN), 122.1 (NCHCHN), 75.9 (CODvinyl), 59.5
(NCH N), 44.4 (NCH ), 34.3 (COD ), 13.1 (CH ). Anal.
2
2
allyl
3
1
Yield: 72 mg (70%). H NMR (DMSO-d ): d = 7.73 (s,
Calc. for C H N IIr (631.57): C, 36.13; H, 4.47; N, 8.87.
19 28 4
6
4
H, NCHCHN), 4.26 (s, 4H, CODvinyl), 3.28 (m, 8H,
Found: C, 36.91; H, 4.58; N, 9.37%.
1
3
CODallyl), 1.85 (m, 36H, C(CH )). C NMR (DMSO-d6):
d = 180.1 (NCN), 121.6 (NCHCHN), 60.5 (CODvinyl),
3
4
0
0
4.5.3. (g -1,5-cyclooctadiene)(1,1 -dicyclohexyl-3,3 -
methylene-diimidazolin-2,2 -diylidene)rhodium(I) iodide
(7)
0
5
6.8 (C(CH ) ), 30.0 (COD ), 19.6 (C(CH ) ). Anal.
3
3
allyl
3 3
Calc. for C H N ClRh (607.12): C, 59.35; H, 8.63; N,
3
0
52
4
1
9
.23. Found: C, 60.23; H, 8.92; N, 9.87%.
Yield: 78 mg (71%). H NMR (DMSO-d ): d = 8.32 (s,
6
4
H, NCHCHN), 6.04 (s, 2H, NCH N), 5.72 (br, 2H,
2
4
4
ylidene)iridium(I) chloride (3)
.4.3. (g -1,5-cyclooctadiene)bis(1,3-tert-butylimidazolin-2-
CH ), 4.11 (br, 4H, COD
2.04 (m, 16H, CH2,Cy), 1.14 (m, 4H, CH2,Cy). C NMR
(DMSO-d ): d = 182.0 (NCN), 123.2 (NCHCHN), 74.4
), 3.10 (br, 8H, CODallyl),
vinyl
Cy
1
3
1
Yield: 80 mg (67%). H NMR (DMSO-d ): d = 7.98 (s,
6
6
4
H, NCHCHN), 4.30 (m, 4H, CODvinyl), 3.38 (s, 8H,
(CODvinyl), 60.2 (NCH N), 55.1, 54.2 (NCH ), 34.2
(CODallyl), 25.0, 24.6, 23.8 (CH2,Cy).
2
Cy
1
3
CODallyl), 1.91 (m, 36H, C(CH )). C NMR (DMSO-d6):
3
d = 174.5 (NCN), 120.4 (NCHCHN), 59.6 (CODvinyl),
4
0
0
5
5.9 (C(CH ) ), 29.1 (COD ), 18.6 (C(CH ) ). Anal.
4.5.4. (g -1,5-cyclooctadiene)(1,1 -dicyclohexyl-3,3 -
methylene-diimidazolin-2,2 -diylidene)iridium(I) iodide (8)
3
3
allyl
3 3
0
Calc. for C H N ClIr (696.43): C, 51.74; H, 7.53; N,
3
0
52
4
1
8
.04. Found: C, 51.95; H, 7.82; N, 8.59%.
Yield: 95 mg (75%). H NMR (DMSO-d ): d = 8.53 (s, 4
6
H, NCHCHN), 6.19 (s, 2H, NCH N), 6.03 (br, 2H, CH ),
2
Cy
4
4
.4.4. (g -1,5-cyclooctadiene)bis(1,3-
dicyclohexylimidazolin-2-ylidene)iridium(I) chloride (4)
4.36 (br, 4H, CODvinyl), 3.26 (br, 8H, CODallyl), 2.31 (m,
1
3
16H, CH2,Cy), 1.57 (m, 4H, CH2,Cy). C NMR (DMSO-d6):
d = 174.2 (NCN), 123.7 (NCHCHN), 74.8 (CODvinyl), 60.3
(NCH N), 55.9, 55.8 (NCH ), 34.8 (CODallyl), 25.3, 25.0,
1
Yield: 93 mg (68%). H NMR (CDCl ): d = 7.15 (s, 4H,
NCHCHN), 5.70 (s, 2H, CH ), 5.30 (s, 2H, CH ), 4.89
3
Cy
Cy
2
Cy
(
3
br, 4H, CODvinyl), 3.67 (br, 8H, CODallyl), 2.26–1.94 (m,
2H, CH2,Cy), 1.43 (m, 8H, CH2,Cy). C NMR (CDCl3):
24.8 (CH2,Cy). Anal. Calc. for C H N IIr (739.75): C,
27 40 4
43.84; H, 5.45; N, 7.57. Found: C, 44.12; H, 5.86; N, 7.97%.
1
3
d = 173.9 (NCN), 119.6 (NCHCHN), 73.9 (CODvinyl),
4
0
0
5
2
9.7 (NCH ), 33.7 (CODallyl), 32.3 (COD ), 25.6,
5.3, 24.8 (CH2,Cy). MS (FAB) m/z = 765.6 (M ). Anal.
4.5.5. (g -1,5-cyclooctadiene)(1,1 -dimethyl-3,3 -ethylene-
diimidazolin-2,2 -diylidene)rhodium(I) bromide (9)
Cy
allyl
+
0
1
Calc. for C H N ClIr (800.58): C, 57.01; H, 7.55; N,
Yield: 124 mg (57%). H NMR (DMSO-d ): d = 7.39 (s,
3
8
60
4
6
7
.00. Found: C, 56.62; H, 7.48; N, 6.65%.
2H, NCHCHN), 6.92 (s, 2H, NCHCHN), 5.92 (s, 4H,

G.D. Frey et al. / Journal of Organometallic Chemistry 691 (2006) 5725–5738
5733
NCH CH N), 4.42 (s, 2H, CODvinyl), 4.28 (s, 2H,
(NCH ), 37.2 (NCH ), 30.8, 30.7 (s, CODallyl). MS (FAB)
2 3
m/z = 567.3 (M ). Anal. Calc. for C H N BrIr
24 30 4
(646.65): C, 44.58; H, 4.68; N, 8.66. Found: C, 44.64; H,
5.03; N, 8.79%.
2
2
+
COD
), 3.86 (s, 6H, NCH ), 2.32 (m, 8H, CODallyl).
vinyl
3
1
3
1
C NMR (DMSO-d ): d = 178.4 (d, J = 49.0 Hz,
6
NCN), 124.2, 123.8 (NCHCHN), 66.5, 65.1 (CODvinyl),
3
8.6 (NCH ), 32.6 (NCH CH N), 31.8, 31.5 (CODallyl).
3 2 2
4
0
0
4
.5.10. (g -1,5-cyclooctadiene)(1,1 -dimethyl-3,3 -o-
4
0
0
0
4.5.6. (g -1,5-cyclooctadiene)(1,1 -di-tert-butyl-3,3 -
ethylene-diimidazolin-2,2 -diylidene)rhodium(I) bromide
xylylene-diimidazolin-2,2 -diylidene)iridium(I)
hexafluorophosphate (15)
0
(
10)
The anion was exchanged by addition of 1.4 equiv.
NH PF into the solution of complex 14 in CH Cl . The
1
Yield: 71 mg (74%). H NMR (DMSO-d ): d = 7.29 (s,
6
4
6
2
2
2
H, NCHCHN), 6.86 (s, 2H, NCHCHN), 5.41 (s, 4H,
desired complex 15 was filtered and dried in vacuo.
1
NCH CH N), 4.57 (s, 2H, CODvinyl), 4.34 (s, 2H,
Yield: 86 mg (71%). H NMR (DMSO-d ): d = 6.85 (m,
2
2
6
CODvinyl), 2.48 (m, 8H, CODallyl), 1.03 (s, 18H,
2H, CH), 6.64 (s, 2H, NCHCHN), 6.42 (m, 2H, CH), 6.23
1
3
1
NC(CH ) ). C NMR (DMSO-d ): d = 180.0 (d, J
=
(s, 2H, NCHCHN), 5.31 (s, 2H, NCH ), 5.28 (s, 2H,
3
3
6
Rh–C
2
5
2.0 Hz, NCN), 126.4, 126.1 (NCHCHN), 67.0, 64.9
NCH ), 4.17 (s, 2H, CODvinyl), 4.13 (s, 2H, CODvinyl),
2
(
3
CODvinyl), 55.9 (NC(CH ) ), 31.4 (NCH CH N), 31.2,
2.91 (s, 3H, NCH ), 2.80 (s, 3H, NCH ), 1.51 (m, 4H,
CODallyl), 0.72 (m, 4H, CODallyl). C NMR (DMSO-d6):
d = 175.7 (NCN), 135.4, 131.7, 129.1, 123.3, 120.9
3 3
2
2
3
3
1
3
1.1 (CODallyl), 18.6 (s, NC(CH ) ). Anal. Calc. for
3 3
C H N BrRh (565.39): C, 50.98; H, 6.77; N, 9.91. Found:
2
4
38
4
C, 51.31; H, 6.89; N, 10.12%.
(NCHCHN, C ), 76.4, 74.9 (s, CODvinyl), 49.9 (NCH2),
Ar
3
7.2 (NCH ), 30.8, 30.7 (s, CODallyl). Anal. Calc. for
3
4
0
0
4
ethylene-diimidazolin-2,2 -diylidene)iridium(I) bromide
.5.7. (g -1,5-cyclooctadiene)(1,1 -di-tert-butyl-3,3 -
C H N IrPF (711.70): C, 40.50; H, 4.25; N, 7.87. Found:
24 30 4 6
C, 40.87; H, 4.62; N, 7.93%.
0
(
11)
1
Yield: 88 mg (79%). H NMR (DMSO-d ): d = 7.84 (s,
4.6. Preparation of rhodium(III) complexes 16 and 17
6
2
H, NCHCHN), 6.95 (s, 2H, NCHCHN), 5.62 (s, 2H,
NCH CH N), 5.51 (s, 2H, NCH CH N), 4.58 (s, 2H,
A mixture of [Rh(COD)Cl]2 (100 mg, 0.21 mmol),
2 equiv. of the corresponding imidazolium salt
(0.41 mmol), KBr (150 mg, 1.25 mmol) and NEt3
(0.25 mL, 1.8 mmol) were refluxed in 7.5 mL acetonitrile
for 12 h. The mixture was aerated several times during
the reaction. The reaction mixture was filtered and the sol-
vent of the filtrate was removed in vacuo. The resulting
solid was purified via column chromatography. Elution
with methylenechloride afforded the separation of
2
2
2
2
CODvinyl), 4.12 (s, 2H, COD
), 1.39 (s, 8H, CODallyl),
vinyl
1
3
0
.98 (s, 18H, NC(CH ) ).
C
NMR (DMSO-d6):
3
3
d = 175.1 (s, NCN), 126.1, 125.6 (NCHCHN), 66.4, 64.2
(
s, CODvinyl), 55.6 (NCH CH N), 31.0 (s, NC(CH ) ),
2 2 3 3
3
0.6, 30.1 (CODallyl), 18.2 (s, NC(CH3)3).
4
0
0
4
diimidazolin-2,2 -diylidene)iridium(I) bromide (12)
.5.8. (g -1,5-cyclooctadiene)(1,1 -di-n-butyl-3,3 -ethylene-
0
1
Yield: 102 mg (92%). H NMR (CD Cl ): d = 6.97 (s,
[Rh(COD)Cl] . The product was obtained by further elu-
2
2
2
4
H, NCHCHN), 4.85 (m, 4H, CODvinyl), 4.30 (m, 4H,
tion with a gradient of methylenechloride/acetone (5:1).
NCH CH N), 4.19 (4H, NCH (CH ) CH ), 2.38 (m, 4H,
2
2
2
2 2
3
0
0
0
CODallyl), 2.21 (m, 4H, CODallyl), 1.74 (m, 8H,
4.6.1. (1,1 -Di-methyl-3,3 -pyrimidinyldiimidazolin-2,2 -
diylidene)rhodium(III) bromide (16)
1
3
NCH (CH ) CH ), 1.02 (m, 6H, CH3).
C
NMR
2
2 2
3
1
(
(
(
CD Cl ): d = 175.0 (NCN), 124.1, 123.5, 122.1, 120.4
Yield: 122 mg (51%). H NMR (DMSO-d ): d = 8.48 (t,
JH,H = 8.1 Hz, 1H, pyridine-H), 8.30 (d, J
2H, NCH), 7.98 (d, J
= 2.3 Hz, 2H, NCH), 3.85 (s, 6H, NCH ).
NMR (DMSO-d ): d = 176.9 (d, J
2
2
6
3
3
NCHCHN), 77.2, 73.5 (s, CODvinyl), 50.1, 48.2
NCH CH N), 33.7 (s, NCH (CH ) CH ), 31.9, 31.3 (s,
= 2.3 Hz,
H,H
= 8.1 Hz, 2H, pyridine-H), 7.71
3
2
2
2
2 2
3
H,H
3
13
CODallyl), 20.0, 19.6 (s, NCH (CH ) CH ), 13.4 (s, CH ).
(d, J
C
2
2 2
3
3
H,H
3
1
Anal. Calc. for C H N BrIr (654.72): C, 44.03; H, 5.85;
= 34.1 Hz, NCN),
Rh,C
2
4
38
4
6
N, 8.56. Found: C, 44.34; H, 6.09; N, 8.95%.
150.1 (C-ipso), 146.7 (C-ortho), 126.2 (NCH), 117.7 (C-
meta), 109.2 (NCH), 39.1 (NCCH3).
4
0
0
4
xylylene-diimidazolin-2,2 -diylidene)iridium(I) bromide
.5.9. (g -1,5-cyclooctadiene)(1,1 -dimethyl-3,3 -o-
0
0
0
0
4.6.2. (1,1 -Di-tert-butyl-3,3 -pyrimidinyldiimidazolin-2,2 -
diylidene)rhodium(III) bromide (17)
(
14)
1
1
Yield: 78 mg (71%). H NMR (DMSO-d ): d = 6.83 (m,
Yield: 134 mg (49%). H NMR (DMSO-d ): d = 8.53 (t,
JH,H = 7.9 Hz, 1H, pyridine_H), 8.42 (d, J
2H, NCH), 8.01 (d, JH,H = 7.9 Hz, 2H, pyridine_H),
6
6
3
3
2
H, CH), 6.61 (s, 2H, NCHCHN), 6.40 (m, 2H, CH), 6.22
= 1.8 Hz,
H,H
3
(
s, 2H, NCHCHN), 5.29 (s, 2H, NCH ), 5.26 (s, 2H,
2
3
NCH ), 4.13 (s, 2H, CH, CODvinyl), 4.09 (s, 2H, CH,
7.82 (d,
J
= 1.8 Hz, 2H, NCH), 1.84 (s, 18H,
H,H
2
1
3
CODvinyl), 2.89 (s, 3H, NCH ), 2.79 (s, 3H, NCH ), 1.49
NC(CH ) ).
C
NMR (DMSO-d6): d = 175.8 (d,
3
3
3 3
1
3
1
(
(
m, 4H, CODallyl), 0.89 (m, 4H, CODallyl). C NMR
JRh,C = 33.9 Hz, NCN), 149.9 (C-ipso), 142.9 (C-ortho),
123.3 (NCH), 116.2 (C-meta), 108.1 (im-C), 62.8
(NC(CH ) ), 30.1 (NC(CH ) ).
DMSO-d ): d = 175.9 (NCN), 135.4, 131.7, 129.1, 123.3,
6
1
20.9 (NCHCHN, C ), 76.4, 74.9 (CODvinyl), 49.8
Ar
3 3
3 3

5734
G.D. Frey et al. / Journal of Organometallic Chemistry 691 (2006) 5725–5738
4.7. Preparation of mono carbene substituted rhodium
complexes
4.8. General preparation of iridium and rhodium complexes
via the free carbene route
A solution of KOt-Bu (0.22 mmol, 2.2 equiv.) in THF
20 mL) was added dropwise to a stirred suspension of
the imidazolium halide (0.22 mmol, 2.2 equiv.) in THF
30 mL) at room temperature. The azolium salt was slowly
dissolved and the colour of the reaction mixture turned to
light yellow. After stirring for 1 h at room temperature,
The free carbene (0.22 mmol) was added at ꢀ78 ꢁC to a
(
stirred solution of [M(COD)Cl] (M = Rh, Ir)] (0.1 mmol)
2
in anhydrous THF (15 mL). A colour change was observed
from light to dark yellow for the rhodium complexes (irid-
ium from light red to orange). After the reaction mixture
was stirred for 6 h at room temperature, the solvent was
removed in vacuo. The precipitate was washed twice with
n-hexane and diethyl ether (10 mL). After the solvent was
decanted, the resulting solid was dried in vacuo. The com-
pounds are soluble in THF and DCM.
(
[
Rh(COD)Cl]2 (50 mg, 0.1 mmol, 1.0 equiv.) in toluene
20 mL) was added. The reaction mixture was stirred at
0 ꢁC for 1 h. After removing the volatile compounds in
vacuo the product was extracted with n-hexane
(
8
(
3 · 10 mL) and dried in vacuo.
4
4
.8.1. Chloro(g -1,5-cyclooctadiene)(1,3-di-tert-
4
4.7.1. Chloro(g -1,5-cyclooctadiene)(1,3-di-iso-
propylimidazolin-2-ylidene)rhodium(I) (18)
butylimidazolin-2-ylidene)rhodium(I) (19)
1
Yellow powder; Yield: 32 mg (38%). H NMR (CDCl ):
3
1
Yield: 58 mg (73%). H NMR (CDCl ): d = 6.86 (s, 2H,
d = 7.10 (s, 2H, NCHCHN), 4.90 (br, 2H, CODvinyl), 3.14
(br, 2H, CODvinyl), 2.37 (m, 4H, CODallyl), 1.80 (s, 18H,
C(CH )), 1.71 (m, 4H, CODallyl). C NMR (CDCl3):
3
3
3
NCHCHN), 5.73 (sept., J
= 6.8 Hz, 2H, CH(CH3)2),
H,H
1
3
4
4
6
.98 (br, 2H, CODvinyl), 3.30 (br, 2H, CODvinyl), 2.36 (m,
3
1
H, CODallyl), 1.91 (m, 4H, COD ), 1.47 (t, J
=
d = 177.9 (d, J
= 49.0 Hz, NCN), 119.9 (NCHCHN),
Rh–C
allyl
H,H
1
1
1
.8 Hz, 12H, CH(CH ) ). H NMR (DMSO-d ): d = 7.02
92.7 (d, JRh–C = 7.7 Hz, CODvinyl), 68.1 (d, JRh–C
=
3
2
6
(
s, 2H, NCHCHN), 5.12 (br, 2H, CODvinyl), 4.99 (m, 2H,
15.4 Hz, CODvinyl), 59.3 (C(CH ) ), 32.9 (C(CH ) ), 32.2
3 3
3 3
CH(CH ) ), 4.67 (s, 2H, CODvinyl), 2.16 (m, 4H, CODallyl),
(COD ), 28.7 (CODallyl). MS (FAB) m/z (%) = 425.8
3
2
allyl
+
1
3
+
+
1
.43 (d, 12H, CH(CH ) ). C NMR (CDCl ): d = 179.9 (d,
(42, [M ]), 390.8 (11, [M ꢀCl]), 281.9 (48, [M ꢀCOD]),
3
2
3
1
JRh–C = 50.8 Hz, NCN), 116.8 (s, NCHCHN), 98.0 (d,
181.1 (100, [carbene]).
1
1
JRh–C = 8.8 Hz, CODvinyl), 67.5 (d, JRh–C = 14.6 Hz,
4
CODvinyl), 52.6 (CH(CH ) ), 33.0, 29.8, 28.9 (CODallyl),
4.8.2. Chloro(g -1,5-cyclooctadiene)(1,3-
diadamantylimidazolin-2-ylidene)rhodium(I) (20)
3 2
2
4.3, 23.3 (CH(CH ) ). MS (FAB) m/z (%) = 397.6 (55,
3
+
2
+
+
1
[
M ]), 362.7 (65, [M ꢀCl]), 320.7 (51, [M ꢀ(Cl + C H )]),
Yield: 137 mg (69%). H NMR (DMSO-d ): d = 7.26 (s,
3
7
6
1
53.0 (100, [carbene]). Anal. Calc. for C H N ClRh
2H, NCHCHN), 5.13 (br, 2H, CODvinyl), 4.42 (s, 2H,
CODvinyl), 2.43 (m, 4H, CODallyl), 2.01 (m, 4H, CODallyl),
1.94 (d, 12H, (CH ) ), 1.38 (m, 12H, (CH ) ), 0.95 (m,
1
7
28
2
(
398.78): C, 51.20; H, 7.08; N, 7.02. Found: C, 51.27; H,
7
.00; N, 6.46%.
2
ad
2 ad
1
3
6
H, (CH)ad).
C NMR (DMSO-d ): d = 183.0 (d,
6
1
JRh–C = 50.0 Hz, NCN), 120.2 (s, NCHCHN), 96.1, 72.9
(CODvinyl), 55.9 (N(CR ) ), 44.9, 36.8, 30.9 (C ), 33.2,
4
4
methylimidazolin-2-ylidene)rhodium(I) (21)
.7.2. Iodo(g -1,5-cyclooctadiene)(1-mesityl-3-
3 ad
ad
+
30.1 (CODallyl). MS (FAB) m/z = 547.5 (M ).
1
Yield: 76 mg (71%). H NMR (CDCl ): d = 7.04 (s,
3
3
1
6
H, m-CH), 7.00 (d, JH,H = 2.0 Hz, 1H, NCHCHN),
.89 (s, 1H, m-CH), 6.75 (d, JH,H = 2.0 Hz, 1H,
4
3
4.8.3. Chloro(g -1,5-cyclooctadiene)(1,3-di-tert-
butylimidazolin-2-ylidene)iridium(I) (23)
NCHCHN), 5.10 (m, 1H, CODvinyl), 4.97 (m, 1H,
CODvinyl), 3.65 (m, 1H, CODvinyl), 3.21 (m, 1H,
1
Yellow powder; Yield: 42 mg (41%). H NMR (CDCl ):
3
d = 7.13 (s, 2H, NCHCHN), 4.50 (m, 2H, CODvinyl), 2.72
CODvinyl), 4.19 (s, 3H, NCH ), 2.55–2.32 (m, 2H, CODallyl),
3
(
m, 2H, CODvinyl), 2.18 (m, 4H, CODallyl), 1.96 (s, 12H,
2
.45 (s, 3H, o-CH ), 2.36 (s, 3H, o-CH ), 2.13–1.92 (m, 2H,
3 3
13
C(CH )), 1.53 (m, 4H, CODallyl). C NMR (CDCl3):
3
CODallyl), 1.82–1.61 (m, 2H, COD ), 1.79 (s, 3H, p-
allyl
1
3
d = 179.8 (NCN), 119.5 (NCHCHN), 78.1 (CODvinyl),
CH ), 1.44–1.35 (m, 2H, CODallyl). C NMR (CDCl3):
3
1
59.1 (CODvinyl), 51.3 (C(CH
) ), 33.4 (C(CH ) ), 32.8
3 3 3 3
d = 182.1 (d, JRh–C = 50.0 Hz, NCN), 138.8, 137.1,
+
(CODallyl), 29.1 (CODallyl). MS (FAB) m/z = 515.7 (M ).
1
9
8
7
3
36.1, 134.6, 129.8, 128.2 (C ), 123.9, 122.4 (NCHCHN),
Ar
1
1
5.3 (d, JRh–C = 6.9 Hz, CODvinyl), 95.3 (d, JRh–C
=
1
4
.6 Hz, COD
), 71.1 (d, J
= 13.9 Hz, CODvinyl),
Rh–C
4.8.4. Chloro(g -1,5-cyclooctadiene)(di-N-
pyrrolidylmethylylidene)rhodium(I) (25)
vinyl
1
0.1 (d, J
= 13.9 Hz, CODvinyl), 40.4 (NCH ), 34.3,
Rh–C
3
1
0.5, 30.4, 28.2 (CODallyl), 21.9, 21.1 (o-CH ), 17.9 (p-
Yield: 39 mg (49%). H NMR (CDCl ): d = 4.73 (2H,
3
3
+
CH ). MS (FAB) m/z (%) = 537.5 (6, [M ]), 410.8 (88,
CODvinyl), 3.85 (2H, CODvinyl), 3.59 (br, 4H, CH ), 3.43
3
+
2
+
[
M ꢀI]), 302.8 (19, [M ꢀ(I + COD)]), 201.1 (47, [car-
(br, 4H, CH ), 3.36 (br, 4H, CODallyl), 3.13 (2H, CH2),
2
bene]), 147 (100). Anal. Calc. for C H N IRh (538.27):
2.58 (2H, CH ), 2.39 (br, 4H, CODallyl), 1.76 (br s, 4H,
2
1
28
2
2
13
C, 46.86; H, 5.24; N, 5.20. Found: C, 47.02; H, 5.49; N,
.04%.
CH2). C NMR (CDCl ): d = 216.2 (NCN), 96.1 (CODvinyl),
3
5
89.6 (CODvinyl), 47.5, 45.6 (CH N), 30.8, 24.5 (CODallyl),
2

G.D. Frey et al. / Journal of Organometallic Chemistry 691 (2006) 5725–5738
5735
2
5
5.1 (CH ). Anal. Calc. for C H N ClRh (398.78): C,
1.20; H, 7.08; N, 7.02. Found: C, 51.63; H, 7.25; N, 7.41%.
lightening was observed. The solution was reduced to
5 mL in vacuo and n-hexane was added to precipitate the
carbonyl complexes in high yields.
2
17 28
2
4
4
pyrrolidylmethylylidene)iridium(I) (26)
.8.5. Chloro(g -1,5-cyclooctadiene)(di-N-
4.10.1. Dicarbonylchloro-(1,3-di-iso-propylimidazolin-2-
ylidene)rhodium(I) (29)
1
Yield: 52 mg (53%). H NMR (CDCl ): d = 4.93 (2H,
3
CODvinyl), 3.98 (2H, CODvinyl), 3.86 (br, 4H, CH ), 3.69
ꢀ1
2
Yield: 33 mg (95%). IR (CH Cl ): m = 2078, 1997 cm
.
2
2
(
br, 4H, CH ), 3.62 (br, 4H, CODallyl), 3.47 (2H, CH2),
1
2
H NMR (CDCl ): d = 7.02 (s, 2H, NCHCHN), 5.19
3
2
4
.82 (2H, CH ), 2.61 (br, 4H, CODallyl), 1.88 (br s,
3
3
2
(
6
sept., J
= 6.8 Hz, 2H, CH(CH ) ), 1.47 (d, J
.8 Hz, 6H, CH(CH ) ), 1.39 (d, JH,H = 6.8 Hz, 6H,
CH(CH ) ). C NMR (CDCl ): d = 184.6 (d, J =
Rh–C
3.2 Hz, CO), 181.8 (d, J
=
H,H
H,H
3 2
3
1
3
H, CH2). C NMR (CDCl ): d = 214.7 (NCN), 96.3
3
3
2
(
(
CODvinyl), 88.3 (CODvinyl), 47.7, 45.7 (CH N), 30.9
CODallyl), 25.3 (CH2).
13
1
2
3
2
3
1
5
= 74.8 Hz, CO), 170.2 (d,
Rh–C
1
JRh–C = 43.1 Hz, NCN), 116.6 (NCHCHN), 52.3 (NCH
CH ) ), 22.3, 22.2 (NCH(CH3)2). MS (FAB) m/z
4.9. Preparation of complexes 22 and 24 via the
corresponding silver carbene complexes
(
(
3
2
+
+
%) = 310.7 (10, [M ꢀCl]), 254.9 (35, [M ꢀ(2CO + Cl)]),
153.0 (100, [carbene]).
To a solution of the silver carbene complex (0.20 mmol)
in 20 mL CH Cl , [Rh(COD)Cl] (0.1 mmol) was added,
the mixture was stirred for six hours at room temperature.
The yellow suspension was concentrated in vacuo. The
product was purified by column chromatography (1:2
AcOEt/n-pentane).
2
2
2
4.10.2. Dicarbonyliodo-(1-mesityl-3-methylimidazolin-2-
ylidene)rhodium(I) (30)
ꢀ1
Yield: 44 mg (91%). IR (CH Cl ): m = 2073, 2000 cm
.
2
2
1
3
H NMR (CDCl ): d = 7.15 (d, JH,H = 1.6 Hz, 1H,
3
3
NCHCHN), 6.95 (s, 2H, m-CH), 6.94 (d, J
= 1.6 Hz,
H,H
4
1H, NCHCHN), 3.96 (s, 3H, NCH ), 2.34 (s, 3H, p-
3
4
.9.1. Chloro(g -1,5-cyclooctadiene)(1,3-di-iso-
propylimidazolin-2-ylidene)iridium(I) (22)
1
3
CH ), 2.09 (br s, 6H, o-CH ). C NMR (CDCl ): d =
3
3
3
1
1
1
186.9 (d,
J
Rh–C = 52.3 Hz, CO), 181.6 (d,
J =
Rh–C
Yield: 64 mg (66%). H NMR (CDCl ): d = 6.87 (s, 2H,
NCHCHN), 5.51 (sept., J
.54 (m, 2H, CODvinyl), 2.94 (m, 2H, CODvinyl), 2.19 (m,
H, CODallyl), 1.71 (m, 2H, COD ) 1.59 (m, 2H, CODallyl),
= 6.4 Hz, 6H, CHCH ), 1.41 (d, J
.4 Hz, 6H, CHCH3).
NCN), 116.5 (NCHCHN), 83.7 (CODvinyl), 52.3, 51.1
CODvinyl, NCH(CH ) ), 33.7 (CODallyl), 29.8 (CODallyl),
3
1
3
77.6 Hz, CO), 174.3 (d, JRh–C = 42.3 Hz, NCN), 139.5,
= 6.4 Hz, 2H, NCH(CH3)2),
H,H
1
35.2, 129.4 (C ), 123.9, 123.4 (NCHCHN), 40.1
Ar
4
4
1
6
(
(
(
NCH ), 21.2, 19.0 (Ar–CH ). MS (FAB) m/z (%) =
3 3
+
allyl
+
3
3
457.4 (16, [M ꢀCO]), 429.5 (30, [M ꢀ2CO]), 330.7 (18,
.46 (d, J
=
H,H
H,H
3
+ +
[M ꢀ(CO + I)]), 302.7 (45, [M ꢀ(2CO + I)]), 201.0 (100,
1
3
C NMR (CDCl ): d = 177.9
3
[
carbene]). Anal. Calc. for C H N IO Rh (486.11): C,
15 16 2 2
37.06; H, 3.36; N, 5.76. Found: C, 37.35; H, 3.36; N, 5.62%.
3 2
2
4
4.1 (CHCH ), 23.4 (CHCH ). MS (FAB) m/z (%) =
3 3
+
+
4.10.3. Dicarbonylchloro-(1,3-di-iso-propylimidazolin-2-
ylidene)iridium(I) (31)
87.6 (24, [M ]), 448.7 (53, [M ꢀC H ]), 338.7 (53,
3
5
+
[
M ꢀ(COD + C H )]), 146.9 (100, [carbene]).
3
5
ꢀ
1
Yield: 41 mg (93%). IR (CH Cl ): m = 2066, 1982 cm
.
2
2
1
4
H NMR (CDCl ): d = 7.03 (s, 2H, NCHCHN), 5.27
4.9.2. Chloro(g -1,5-cyclooctadiene)(1,3-
dicyclohexylimidazolin-2-ylidene)iridium(I) (24)
3
3
(
sept.,
JH,H = 6.8 Hz, 2H, NCH(CH ) ), 1.45 (d,
3 2
3
13
1
JH,H = 6.8 Hz, 12H, CHCH3). C NMR (CDCl ): d =
Yield: 83 mg (73%). H NMR (CDCl ): d = 6.83 (s, 2H,
3
3
3
1
5
81.2 (CO), 171.0 (NCN), 167.3 (CO), 116.3 (NCHCHN),
3.1 (NCH(CH ) ), 23.0 (CH(CH ) ). MS (FAB) m/z
NCHCHN), 5.51 (tt, J
4
1
= 12.0, 3.6 Hz, 2H, NCHCy),
H,H
.57 (br, 2H, CODvinyl), 2.93 (br, 2H, CODvinyl), 2.20–
3 2
3 2
+
+
1
3
(
%) = 436.6 (22, [M ]), 400.6 (40, [M ꢀCl]), 372.7 (100,
.24 (m, 8H, CODallyl, 20H, CH2,Cy). C NMR (CDCl3):
+
[
M ꢀ(CO + Cl)]). Anal. Calc. for C H N ClIrO
d = 178.1 (NCN), 116.5 (NCHCHN), 83.4 (CODvinyl),
0.0 (CODvinyl), 50.8 (NCHCy), 34.4(CH2,Cy), 34.3
CH2,Cy), 33.9 (CODallyl), 29.8 (CODallyl), 26.2 (CH2,Cy),
5.9 (CH2,Cy), 25.5 (CH2,Cy).
MS (FAB) m/z (%) = 567.4 (28, [M ]), 528.5 (15,
11 16
2
2
(435.93): C, 30.31; H, 3.70; N, 6.43. Found: C, 30.98; H,
6
(
2
3.53; N, 6.12%.
+
4.10.4. Dicarbonylchloro-(1,3-dicyclohexylimidazolin-2-
ylidene)iridium(I) (32)
+
[
(
M ꢀC H ]), 146.9 (100). Anal. Calc. for C H N ClIr
3
5
23 36
2
ꢀ1
568.22): C, 48.62; H, 6.39; N, 4.93. Found: C, 49.15; H,
Yield: 49 mg (94%). IR (CH Cl ): m = 2064, 1982 cm .
2
2
1
6
.58; N, 4.52%.
H NMR (CDCl ): d = 6.99 (s, 2H, NCHCHN), 4.83 (m,
3
1
3
2
H, NCH ), 2.02–1.11 (m, 20H, CH2,Cy). C NMR
Cy
4
.10. General procedure for carbonyl derivatives
(CDCl ): d = 181.7 (CO), 171.2 (NCN), 168.3 (CO),
3
1
18.0 (NCHCHN), 60.5 (CH ), 34.1 (CH2,Cy), 33.7
Cy
CO gas (1 bar, 15 mL/min) was passed through a solu-
tion of the COD-complexes (0.17 mmol) in 15 mL dichlo-
romethane at room temperature for 15 min. A colour
(CH2,Cy), 25.6 (CH2,Cy), 25.4 (CH2,Cy), 25.3 (CH ). MS
2,Cy
+
+
(FAB) m/z (%) = 515.4 (7, [M ]), 488.5 (4, [M ꢀCO]),
+ +
448 (46, [M ꢀ C H ]), 418.6 (75, [M ꢀ(2CO + C H )]).
3
5
3
5

5736
G.D. Frey et al. / Journal of Organometallic Chemistry 691 (2006) 5725–5738
Anal. Calc. for C H N ClIrO (516.05): C, 39.57; H,
within the H range of 2.10ꢁ < H < 25.31ꢁ. A total of
34023 intensities were integrated. Raw data were corrected
for Lorentz, polarization, and, arising from the scaling pro-
cedure, for latent decay and absorption effects. After merg-
ing (Rint = 0.059), 2735 (all data) and 2700 [I > 2r(I )]
independent reflections remained and all were used to
refine 180 parameters. The structure was solved by a com-
bination of direct methods and difference-Fourier synthe-
ses. All non-hydrogen atoms were refined with
anisotropic displacement parameters. All hydrogen atoms
were placed in calculated positions and refined using a rid-
1
7
24
2
2
4
.69; N, 5.43. Found: C, 39.50; H, 4.53; N, 5.31%.
.11. General procedure for the borylation reaction
The reactions for the borylation catalysis were typically
4
o
o
conducted as follows: 2 mmol pinacolborane, and 1.0–
.5 mol% of the catalyst (1a, 4a, 8a, 12a, and 14a) were dis-
1
solved in 50 mmol aryl substrate. The solution was stirred
and heated at 40–45 ꢁC for 9–12 h. The reaction progress
was monitored by removal of a small aliquot of the reac-
tion mixture, which was analyzed by GC-MS. The solvent
was removed under vacuum at room temperature and the
residue was chromatographed over silica gel, eluting with
CH Cl to yield the product. The products were also ana-
ing model. Full-matrix least-squares refinements were car-
P
2
o
2
2
c
ried out by minimizing
wðF ꢀ F Þ and converged
with R1 = 0.0192 [I > 2r(I )], wR = 0.0467 (all data),
GOF = 1.102, and shift/error < 0.001. The final differ-
o
o
2
2
2
lyzed via NMR [3a,47].
ence-Fourier map shows no striking features (De
=
min/max
ꢀ
3
+0.48/ꢀ0.52 e A˚ ) [52].
4
.12. Single crystal X-ray structure determination of
compounds 4 Æ 2(CH Cl ) and 14 Æ 2(CH Cl )
2
2
2
2
5. Supplementary material
Crystal structure analysis of compound 4 Æ 2(CH Cl ):
2
2
Crystallographic data (excluding structure factors) for
the structures reported in this paper have been deposited
with the Cambridge Crystallographic Data Centre. CCDC
C H Cl IrN , M = 970.42, red fragment (0.31 · 0.36 ·
4
0
64
5
4
r
3
ꢀ
0
1
(
1
.43 mm ), triclinic, P1 (No.: 2), a = 12.8837(1), b =
˚
3.7657(2), c = 14.5709(2) A, a = 70.3607(4)ꢁ, b = 66.4431
618422 and 618423 contain the supplementary crystallo-
3
˚
5)ꢁ, c = 65.7350(9)ꢁ, V = 2113.40(5) A , Z = 2, d
=
calc
graphic data for [4 Æ 2(CH Cl )] and [14 Æ 2(CH Cl )]. These
2
2
2
2
ꢀ3
ꢀ1
.525 g cm , F0 = 988, l = 3.508 mm . Preliminary
00
data can be obtained free of charge via http://www.ccdc.ca-
m.ac.uk/conts/retrieving.html, or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail:
deposit@ccdc.cam.ac.uk.
examination and data collection were carried out on an
area detecting system (NONIUS, MACH3, j-CCD) at the win-
dow of a rotating anode (NONIUS, FR591) and graphite
˚
monochromated Mo Ka radiation (k = 0.71073 A). Data
collection was performed at 143 K within the H range of
1
.56ꢁ < H < 25.37ꢁ. A total of 30570 intensities were inte-
Acknowledgement
grated. Raw data were corrected for Lorentz, polarization,
and, arising from the scaling procedure, for latent decay
and absorption effects. After merging (Rint = 0.044), 7764
¨
We are grateful to Dr. Karl Ofele for suggestions and
fruitful discussions. This work was generously supported
by NanoCat, an International Graduate Program within
the Elitenetzwerk Bayern (Post-Doc fellowship for GDF),
the Deutsche Forschungsgesellschaft DFG (Ph.D fellow-
ship for CFR), the Margarete Ammon-Stiftung (DvP),
and the S u¨ d-Chemie AG (TS).
(
all data) and 7057 [I > 2r(I )] independent reflections
o o
remained and all were used to refine 451 parameters. The
structure was solved by a combination of direct methods
and difference-Fourier syntheses. All non-hydrogen atoms
were refined with anisotropic displacement parameters.
All hydrogen atoms were placed in calculated positions
and refined using a riding model. Full-matrix least-squares
P
2
2
2
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wðF ꢀ F Þ
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1
o
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