Novel immobilized hydrosilylation catalysts based on rhodium 1,3-bis(2,4,6-trimethylphenyl)-3,4,5,6-tetrahydropyrimidin-2-ylidenes

Article abstract of DOI:10.1007/s00706-004-0255-3

The reactivity of a well defined Rh (I) complex, i.e. Rh(CF ₃COO)(NHC)(COD) (1, NHC = 1,3-bis(2,4,6-trimethylphenyl)-3,4,5,6- tetrahydropyrimidin-2-ylidene, COD = η⁴-cycloocta-1,5-diene) in the hydrosilylation of 1-alkenes, alkynes,

Full text of DOI:10.1007/s00706-004-0255-3

Monatshefte f€ur Chemie 136, 47–57 (2005)
DOI 10.1007/s00706-004-0255-3
Invited Review
Novel Immobilized Hydrosilylation Catalysts
BasedonRhodium1,3-Bis(2,4,6-trimethylphenyl)-
3,4,5,6-tetrahydropyrimidin-2-ylidenes
Nicolas Imlinger1, Klaus Wurst2, and Michael R. Buchmeiser1;ꢀ
1
€
Arbeitskreis Makromolekulare Chemie, Institut fur Analytische Chemie
und Radiochemie, Leopold-Franzens-Universitat Innsbruck, A-6020 Innsbruck
€
2
€
Institut fur Allgemeine, Anorganische und Theoretische Chemie,
Leopold-Franzens-Universitat Innsbruck, A-6020 Innsbruck
€
Received July 15, 2004; accepted September 23, 2004
Published online December 30, 2004 # Springer-Verlag 2004
Summary. The reactivity of a well defined Rh (I) complex, i.e. Rh(CF3COO)(NHC)(COD)
(1, NHC ¼ 1,3-bis(2,4,6-trimethylphenyl)-3,4,5,6-tetrahydropyrimidin-2-ylidene, COD ¼ ꢀ4-cycloocta-
1,5-diene) in the hydrosilylation of 1-alkenes, alkynes, and ꢁ,ꢂ-unsaturated carbonyl compounds,
respectively, is described. With this complex, excellent reactivity was observed and turn-over
numbers (TONs) up to 1000 were reached. A supported version of 1 was realized by reaction of
RhCl(NHC)(COD) with PS-DVB–CH2–O–CO–CF2–CF2–CF2–COOAg (PS-DVB ¼ poly(styrene-
co-divinylbenzene) to yield PS-DVB–CH2–O–CO–CF2–CF2–CF2–COORh(NHC)(COD). This sup-
ported version of 1 exhibited at least comparable, in some cases increased reactivity compared
to 1 and allowed the rapid removal of the catalyst from the reaction mixture. Due to reduced
catalyst bleeding, the synthesis of target compounds with a Rh-content of less than 130 ppm was
accomplished.
Keywords. N-Heterocyclic carbenes; Heterogeneous catalysis; Homogeneous catalysis;
Hydrosilylation; Rhodium.
Introduction
Hydrosilylation (or hydrosilation) reactions involve the addition of inorganic
or organic silicon hydrides to multiple bonds such as alkyne, alkene, ke-
toxime, and carbonyl groups. Reactions first described by Sommer et al.
proceeded via a radical mechanism and required the presence of peroxides [1].
ꢀ
Corresponding author. E-mail: michael.r.buchmeiser@uibk.ac.at

48
N. Imlinger et al.
Scheme 1
Later, Speier et al. identified hexachloroplatinic acid as a quite efficient cat-
alyst for these types of reactions [2]. Depending on the catalyst used, e.g.
peroxides, tertiary amines, Lewis acids [3], supported metals, transition metal
complexes, Karstedt’s catalyst [4], etc., the reaction mechanism may be based
on free radicals [5] or on oxidative addition-reductive elimination steps (polar
mechanism). The development of various hydrosilylation catalysts has already
been summarized in some excellent reviews [6–9]. For hydrosilylations pro-
moted by transition metal complexes, as is the case in the present report, the
(modified) Chalk-Harrod mechanism [10, 11] (Scheme 1) is still the generally
accepted one, however, other mechanisms have been suggested recently
[9, 12, 13].
The transition metals most commonly employed in hydrosilylation reactions
are Pd [14], Pt, Rh, Ir, Ni, and Cu [9, 15–17]. In terms of transition metal
complexes used for catalysis, two major groups need to be mentioned. The first
comprises of transition metal ions complexed by neutral ligands where the metal
ion is bonded to a heteroatom, e.g. bipyridyls [18], ethylene diamines [19], di-
phosphines [20, 21], (P,O)-ligands such as 2-methoxy-20-diphenylphosphinobi-
naphthyl, as well as (P,N)-ligands [22], (P,S)-ligands [23], etc. Particularly the
latter have successfully been used for asymmetric hydrosilylations [24–26]. The
second group is based on transition metal N-heterocyclic carbene (NHC) com-
plexes [27, 28]. So far, solely Rh-1,3-R2-imidazol-2-ylidenes have been used for
hydrosilylations [29–38]. In this contribution, we describe the reactivity of a
novel Rh(I)-NHC complex, Rh(CF3COO)(1,3-dimesityl-3,4,5,6-tetrahydropyrim-
idin-2-ylidene)(COD) (1, mesityl ¼ 2,4,6-trimethylphenyl, COD ¼ ꢀ4-cycloocta-
1,5-diene) in the hydrosilylation of various alkynes, alkenes, and carbonyl
compounds.

Novel Immobilized Hydrosilylation Catalysts
49
Results and Discussion
Synthesis and Reactivity of Rh(CF3COO)(1,3-dimesityl-3,4,5,6-
tetrahydropyrimidin-2-ylidene) (COD, 1)
The synthesis of compound 1 is described elsewhere [39–41]. Briefly, RhCl(1,3-
dimesityl-3,4,5,6-tetrahydropyrimidin-2-ylidene) (COD) was formed by reaction
of [RhCl(COD)]2 with 2 equiv. of LiO-tertBu and 1,3-dimesityl-3,4,5,6-tetrahydro-
pyrimidinium bromide or tetrafluoroborate. This precursor was subject to reaction
with CF3COOAg to yield the desired target compound 1. It crystallizes in the
monoclinic space group P21=c, a ¼ 1449.58(3), b ¼ 1216.53(4), c ¼ 3451.4(1) pm,
ꢁ ¼ 90, ꢂ ¼ 97.698(2), ꢃ ¼ 90ꢁ, Z ¼ 8 (Fig. 1). Selected X-ray data are summarized
in Table 1, selected bond lengths and angles are given in Table 2.
The Rh-NHC distance (Rh(1)–C(9)) is 206.5(8) pm and comparable to the
distance of Rh to the NHC in RhBr(NHC)(COD) (204.7(3) pm, NHC ¼ 1,3-dime-
sityl-3,4,5,6-tetrahydropyrimidin-2-ylidene) [39, 40]. The same accounts for
the distance Rh-COD (208.2(8), 209.3(8), 216.6(11), and 219.5(11) pm in 1 vs.
210.3(3), 211.5(3), 219.7(3), and 223.4(3) pm in RhBr(1,3-dimesityl-3,4,5,6-tetra-
hydropyrimidin-2-ylidene)(COD)). In a first step, the reactivity of 1 in the hydro-
silylation of both internal and 1-alkynes was investigated. For this purpose,
phenylacetylene, 1-hexyne, trimethylsilylacetylene, and diphenylacetylene, respec-
tively, were reacted with a series of organosilanes, i.e. triethylsilane, dichloro-
methylsilane, triphenylsilane, and trimethoxysilane. Scheme 2 summarizes the
possible products of these hydrosilylation reactions.
The results are summarized in Table 3, entries 1–10. Using 0.08 mol-% of 1,
yields in the range of 17–72% were observed. Whereas virtually quantitative yields
may be achieved using 0.5 mol-% of catalyst, the reduced amounts of catalyst, i.e.
0.08 mol-%, were used in order to determine the maximum turn over numbers
(TONs) that can be achieved. As can be deduced from Table 3, entries 1–10, these
Fig. 1.

50
N. Imlinger et al.
Table 1. Selected crystal data and structure refinement for 1
1
mol formula
C32H40F3N2O2Rh
fw
644.57
crystal system
monoclinic
C2=c (No. 15)
1449.58(3)
1216.53(4)
3451.4(1)
90
space group
a=pm
b=pm
c=pm
ꢁ=ꢁ
ꢂ=ꢁ
97.698(2)
90
ꢃ=ꢁ
V=nm3
6.0315(3)
8
Z
temp=K
233(2)
density (calcd)=Mg=m3
abs coeff=mmꢂ1
color, habit
1.42
0.616
colorless plate
2703
no. of rflns with I>2ꢄ(I)
goodness-of-fit on F2
R indices I>2ꢄ(I)
1.033
R1 ¼ 0.0645
!R2 ¼ 0.1252
Table 2. Selected bond lengths [pm] and angles [ꢁ] for 1
Rh(1)–C(9)
Rh(1)–C(6)
Rh(1)–C(5)
Rh(1)–O(1)
Rh(1)–C(1)
Rh(1)–C(2)
206.5(8)
208.2(8)
209.3(8)
212.4(5)
216.6(11)
219.5(11)
C(9)–Rh(1)–C(6)
C(9)–Rh(1)–C(5)
C(9)–Rh(1)–O(1)
C(9)–Rh(1)–C(1)
C(9)–Rh(1)–C(2)
98.8(3)
98.4(3)
82.3(2)
161.7(6)
162.2(5)
Scheme 2

Novel Immobilized Hydrosilylation Catalysts
51
Table 3. Summary of results for the hydrosilylation of alkynes, alkenes, and carbonyl compounds using 1 and 2
#
Main product
cat. (mol-%)
ꢂ-trans
ꢂ-cis
ꢁa
TONmax
yield=%
1
2
3
4
5
6
7
8
9
1-phenyl-2-(triethylsilyl)ethene
1-phenyl-2-(dichloromethylsilyl)ethene
1-phenyl-2-(trimethoxysilyl)ethene
1-(triphenylsilyl)hex-1-ene
1 (0.08)
54
79
46
73
0
7
38
11
99
0
14
16
16
1
680
810
430
340
900
510
760
210
780
890
100
460
180
1000
310
530
300
830
1210
1270
490
570
1 (0.08)
65
1 (0.08)
34
1 (0.07)
24
1-(dichloromethylsilyl)hex-1-ene
1-(trimethylsilyl)-2-(dichloromethylsilyl)ethene
1-(trimethoxysilyl)hex-1-ene
1 (0.08)
72
100
99
70
77
–
0
1 (0.08)
41
0
1
1 (0.08)
61
0
30
23
–
1-(trimethylsilyl)-2-(trimethoxysilyl)ethene
1-(triethylsilyl)stilbene
1 (0.08)
17
0
1 (0.08)
62
–
10
11
12
13
14
15
16
17
18
19
20
21
22
1-(triethylsilyl)hex-1-ene
1 (0.08)
71
61
–
11
–
27
–
1-(triethylsilyl)octane
1 (0.1)
10
1-phenyl-2-(triethylsilyl)ethane
1-(triethylsilyl)hexane
1 (0.08)
37
–
–
–
1 (0.1)
18
–
–
–
1-(triethylsilyloxy)cyclohexene
1-(triphenylsilyloxy)cyclohexene
triethylsilyloxymethylbenzene
1 (0.08)
80
–
–
–
1 (0.08)
25
–
–
–
1 (0.1)
53
–
–
–
1-(triethylsilyloxy)-1-(4-chlorophenyl)ethane
triethylsilyloxymethyl-4-fluorobenzene
1-phenyl-2-(dichloromethylsilyl)ethene
1-(dichloromethylsilyl)hex-1-ene
1-(trimethylsilyl)-2-(dichloromethylsilyl)ethene
1-(triethylsilyl)hex-1-ene
1 (0.09)
27
–
–
–
1 (0.09)
75
–
–
–
2 (0.07)
85
89
100
100
45
11
0
0
2 (0.07)
89
0
2 (007)
34
0
0
2 (0.06)
34
20
35
(continued)

52
N. Imlinger et al.
Table 3 (continued)
23
24
25
26
27
28
29
30
31
32
33
34
1-phenyl-2-(trimethoxysilyl)ethene
2 (0.07)
46
70
58
0
3
0
27
42
6
660
1320
900
540
740
880
160
730
220
490
310
220
1-(trimethoxysilyl)hex-1-ene
1-(triphenylsilyl)hex-1-ene
2 (0.05)
66
2 (0.05)
45
94
9
1-phenyl-2-(triethylsilyl)ethene
1-(triethylsilyl)stilbene
2 (0.07)
34
51
–
40
–
2 (0.07)
52
–
1-phenyl-2-(triethylsilyl)ethane
1-(triethylsilyl)hexane
2 (0.05)
44
–
–
–
2 (0.07)
11
–
–
–
1-(triethylsilyloxy)cyclohexene
1-(triphenylsilyloxy)cyclohexene
triethylsilyloxymethylbenzene
triethylsilyloxymethyl-4-fluorobenzene
1-(triethylsilyloxy)-1-(4-chlorophenyl)ethane
2 (0.07)
51
–
–
–
2 (0.05)
11
–
–
–
2 (0.09)
44
–
–
–
2 (0.07)
22
–
–
–
2 (0.1)
22
–
–
–
Reactions were run in dimethoxyethane (DME), reaction time: 12 h, T ¼ 80ꢁC; a different positional numbers for
the substituents according to Scheme 2 apply
were in the range of 210–900. With one exception, i.e. 1-(triphenylsilyl)hex-1-ene,
Table 3, entry 4, the trans products were predominantly formed. A somewhat
reduced reactivity was observed for terminal alkenes (1-hexene, 1-octene, styrene),
here TONs were in the range of 100–460 (Table 3, entries 11–13). Finally, and
important enough, the hydrosilylation of aldehydes, aryl ketones, and ꢁ,ꢂ-unsatu-
rated carbonyl compounds (i.e. cyclohex-2-en-1-one, benzaldehyde, 4-fluorobenz-
aldehyde, 4-chloroacetophenone) was investigated. As can be deduced from Table
3, entries 14–18, good reactivity was observed giving raise to TONs in the range
of 300–1000. Cyclohex-2-en-1-one underwent selective 1,4-addition to yield
1-(triethylsilyloxy)cyclohexene and (triphenylsilyloxy)cyclohexene, respectively.
Both the aromatic aldehydes and aryl ketones (Table 3, entries 16–18) were con-
verted into the corresponding silylethers with good efficiency (300 ꢃ TON ꢃ 830).
Synthesis of the Immobilized Version of 1
The immobilization of catalysts is nowadays an intensively investigated area of
research. For various reasons, there is an increasing demand for supported versions
of modern, highly selective, and active catalysts. First, contamination of products
with metal ions and=or ligands needs to be low, particularly in cases relevant to
pharmaceutical chemistry. Second, modern catalysts significantly add to the total

Novel Immobilized Hydrosilylation Catalysts
53
costs of a product, therefore regeneration or reuse is highly desirable. And third,
supported catalysts offer access to high-throughput techniques and continuous flow
reactors, respectively. Key issues associated with supported catalysts are (i) pre-
servation of the high activities, (enantio-)selectivities, and reaction rates observed
with homogeneous catalysts, (ii) ease of catalyst separation, (iii) (multiple) catalyst
recycling, and (iv) metal- and contaminant-free products. So far, immobilization
has been accomplished by simply attaching phosphine- or diaminoethane-based
ligands to a suitable support such as poly(styrene-co-divinylbenzene) or silica
[9, 19, 42]. In addition, pyridine-containing polyamides as well as chiral N,N0-
and N,P-ligand-derivatized zeolithes have been used [43–45]. Poly(styrene)-b-poly
(butadiene) copolymers have been used to immobilize H2PtCl6 and RhCl3,
respectively [46]. Finally, issues mentioned above including recycling may be
accomplished by a fluorous approach [47].
For the immobilization of 1, an approach originally developed for the immobi-
lization of ruthenium-based metathesis catalysts was chosen [48–51]. Thus,
hydroxymethylated poly(styrene-co-divinylbenzene) (hydroxymethyl PS-DVB) was
reacted with hexafluoroglutaric anhydride to yield the corresponding polymer-
immobilized perfluorocarboxylic acid (Scheme 3). Conversion into the silver salt
was accomplished via reaction with NaOH and AgNO3. Finally, this polymer-bound
silver salt was reacted with RhCl(1,3-bis(2,4,6-trimethylphenyl)-3,4,5,6-tetrahydro-
pyrimidin-2-ylidene(COD) to yield 2, the polymer-supported analogue of 1. Follow-
ing this synthetic protocol, a catalyst loading of 0.6% w=w was accomplished.
To retrieve information about the reactivity of 2 in comparison with 1, it was
again used in the hydrosilylation of alkynes, 1-alkenes, aldehydes, and carbonyl
compounds. The results are summarized in Table 3, entries 19–34. Interestingly,
2 displayed an even enhanced reactivity in the hydrosilylation of both alkynes
and 1-alkenes. Thus, TONs in the range of 160–1320 were obtained using
0.05–0.07 mol-% of catalyst (Table 3, entries 19–29). This may be explained
by the heterogeneous nature of the catalysts, effectively reducing bimetallic
decomposition reactions. The influence on regioselectivity turned out to be
non-predictable. Thus, the product distribution of some reactions obtained by
Scheme 3

54
N. Imlinger et al.
the action of 2 remained virtually unchanged (Table 3, entries 20, 21, 23, 25),
other distributions were subject to change in either direction. Finally, a compa-
rable yet somewhat lower reactivity was observed in the hydrosilylation of alde-
hydes, ꢁ,ꢂ-unsaturated ketones and aryl ketones, respectively. Here TONs were
in the range of 220–730 (Table 3, entries 30–34). Again 100% 1,4-selectivity was
observed in the hydrosilylation of cyclohex-2-en-1-one. Leaching of the metal
from 2 into reaction mixture was comparably low, resulting in an average
Rh-content of 130 ppm.
Conclusion
The N-heterocyclic carbene-derived Rh (I) complex, Rh(CF3COO)(1,3-dimesityl-
3,4,5,6-tetrahydropyrimidin-2-ylidene)(ꢀ5-1,5-cyclooctadiene) is an efficient cata-
lyst for the hydrosilylation of both terminal and internal alkynes, 1-olefins,
aromatic aldehydes and ketones, and ꢁ,ꢂ-unsaturated carbonyl compounds, rival-
ing existing Rh-, Pd-, and Pt-based systems. TONs up to 1000 may be obtained. Its
PS-DVB-supported version shows at least similar, in some cases even enhanced
reactivity for the same substrates and allows the synthesis of the target compounds
with a comparably low Rh content.
Experimental
All experiments involving transition metals were performed under a nitrogen atmosphere in a MBraun
glove box or by standard Schlenk techniques. Reagent grade dimethoxyethane (DME), tetrahydrofu-
rane (THF), diethyl ether, toluene, and pentane were distilled from sodium benzophenone ketyl under
argon. Dichloromethane and chloroform were distilled from calcium hydride under argon. All other
reagents were commercially available and used as received. Column chromatography was performed
on silica gel 60 (220–440 mesh, Fluka, Buchs, Switzerland). NMR spectra were recorded at 25ꢁC on a
Bruker Spektrospin 300 at 300.13 MHz for proton and at 75.47 MHz for carbon in the indicated solvent
and referenced using the solvent peaks (CDCl3: ꢅ ¼ 7.24, 77.0 ppm; CD2Cl2: ꢅ ¼ 5.32, 54.0 ppm).
Coupling constants are listed in Hertz. FT-IR spectra were recorded on a Bruker Vector 22 using
ATR technology. Bands are listed in cmꢂ1 and characterized as broad (br), strong (s), medium (m), and
weak (w). GC-MS measurements were carried out on a Shimadzu GCMS QP 5050, using a SPB-5
fused silica column (30 mꢄ0.25 mmꢄ25ꢆm film thickness) and helium as carrier gas. A split ratio of
70 was used throughout. The column flow was 2 cm3=min, the total flow was 146.2cm3=min. The
temperature program was as follows: 70ꢁC (1min and 3 min for low boiling compounds, respectively),
300ꢁC (25ꢁC=min), 300ꢁC (9min), 70ꢁC. Injector temp. 150ꢁC, interface temp. 300ꢁC. A JOBIN
YVON JY 38 plus was used for ICP-OES measurements, a MLS 1200 mega for microwave ex-
periments. Further instrumentation is described elsewhere [51]. RhCl(NHC)(COD) [39, 40] and
Rh(CF3COO)(NHC)(COD) (NHC ¼ 1,3-dimesityl-3,4,5,6-tetrahydropyrimidin-2-ylidene) [41] were
prepared according to published procedures.
Standard Procedure for Hydrosilylation Reactions
The catalyst (for the amount refer to Table 3) was dissolved in DME (2cm3). Tetradecane (100mm3)
was added as an internal standard followed by the silane (0.50 mmol) and the unsaturated compound
(alkyne-, alkene-, or carbonyl compound, 0.50mmol). The mixture was stirred at 80ꢁC until no further
conversion was monitored by GC-MS. Yields and turn over numbers (TONs) were determined by GC-
MS using the conversion of the educts. For the alkyne compounds the ratio of the product isomers was

Novel Immobilized Hydrosilylation Catalysts
55
determined by integration of the GC-MS peaks. These were correlated with the results taken from
NMR measurements.
For NMR-measurements, the solvent of the reaction mixture was removed in vacuo and the residue
was taken up in CDCl3. Integration of the olefinic region provided the ratio of the product isomers of
the reaction of an alkyne compound with a silane. Concerning the reaction of the ꢁꢂ-unsaturated
carbonyl compound with a silane, the 1,4-addition was confirmed by NMR, the lack of olefinic proton-
signals and the appearance of a singlett around 5 ppm.
Generation of Polymer-Supported 1 [Rh(polymer–CH2–O–CO–CF2–CF2–CF2–COO)(1,3-
dimesityl-3,4,5,6-tetrahydropyrimidin-2-ylidene)(COD)]
The modified poly(styrene-co-divinylbenzene) support Ag(polymer–CH2–O–CO–CF2–CF2–CF2–COO)
was synthesized according to a previous disclosure [51]. Ag(polymer–CH2–O–CO–CF2–CF2–CF2–
COO) (500mg) was suspended in THF (10 cm3) and a solution of RhCl(1,3-dimesityl-3,4,5,6-tetra-
hydropyrimidin-2-ylidene)(COD) (40 mg, 0.071mmol) in THF (3cm3) was added dropwise. After
stirring for 90min at room temperature the reaction mixture was filtered, the residue was washed with
THF and dried in vacuo to yield a yellow powder (495mg, 99%). FTIR (ATR mode): ꢇꢀ¼ 3057 (w),
3024 (w), 2919 (m), 2852 (w), 1672 (w), 1596 (m), 1488 (m), 1446 (m), 1362 (m), 1304 (m), 1155 (m),
1058 (m), 1026 (m), 908 (m), 828 (m), 753 (m), 695cmꢂ1 (s). For the determination of the rhodium
content, Rh(polymer–CH2–O–CO–CF2–CF2–CF2–COO)(1,3-dimesityl-3,4,5,6-tetrahydropyrimidin-
2-ylidene)(COD) (30.9mg) was placed inside a high pressure Teflon tube. Nitric acid (1cm3) and
aqueous hydrochloric acid (3cm3) were added and the mixture was exposed to microwave conditions
(50, 600, and 450 W pulsed, t ¼ 32 min). After cooling to room temperature the mixture was filtered
and water was added up to a volume of roughly 10 cm3 (the solution was weighed). The rhodium
content as determined by ICP-OES was 61.4ꢆmol=g polymer, corresponding to 6.32 mg=g polymer
(0.6% w=w catalyst loading).
Determination of Leaching in Heterogeneous Reactions
The reaction mixture was filtered, dried in vacuo, dissolved in dichloromethane (2cm3), and transferred
into a high pressure Teflon tube. The solvent was then removed at ambient conditions. Nitric acid
(1cm3) and aqueous hydrochloric acid (3cm3) were added and the mixture was exposed to microwave
conditions (50, 600, and 450 W pulsed, t ¼ 32 min). After cooling to room temperature, the mixture
was filtered and water was added up to a volume of roughly 10cm3 (the solution was weighed). The
rhodium content was determined by ICP-OES.
Rh Measurements
Rh was measured by ICP-OES (ꢈ ¼ 233.477nm, ion line). The background was measured at
ꢈ ¼ 233.4904 and 233.4602 nm. Standardization was carried out with a Rh standard containing
10ppm of Rh. Leaching of rhodium into the reaction mixture: 130 ppm.
X-Ray Measurement and Structure Determination of 1
Data collection was performed on a Nonius Kappa CCD equipped with graphite-monochromatized
˚
Mo-Kꢁ-radiation (ꢈ ¼ 0.71073A) and a nominal crystal to area detector distance of 36mm. The
structures were solved with direct methods SHELXS86 and refined against F2 using SHELX97
[52]. Hydrogen Atoms were calculated, with isotropic displacement parameters 1.2 and 1.5 times
˚
higher than for the carbon atoms; refined with bond restraints d(C–H)¼ 0.94A at C1, C2, C5, and C6.
Since only small crystals with low diffraction were obtained, 100 reflections were omitted because of

56
N. Imlinger et al.
systematic twinning. CCDC-244481 contains the supplementary crystallographic data for this paper.
These data can be obtained free of charge via www.ccdc.cam.ac.uk=conts=retrieving.html (or from the
Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge, CB2 1EZ, U.K.; fax: (þ44)-
1223-336033; or deposit@ccdc.cam.ac.uk).
Acknowledgement
Our work was supported by the Austrian Science Fund (START Y-158).
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