Enantiodifferentiation of a silane and the analogous hydrocarbon by the dirhodium method-silane...dirhodium complex interaction

Article abstract of DOI:10.1016/j.tetasy.2006.05.030

Chiral silane 1 was enantiodifferentiated by recording NMR spectra in the presence of the chiral dirhodium complex Rh₂^(II) [(R) - (+) - MTPA₄] (Rh^*). This is the first report of direct chiral recognition by spec

Full text of DOI:10.1016/j.tetasy.2006.05.030

Tetrahedron: Asymmetry 17 (2006) 1743–1748
Enantiodifferentiation of a silane and the analogous hydrocarbon
by the dirhodium method—silaneꢀ ꢀ ꢀdirhodium complex interaction
a
a
a
a,
*
´
´
Edison Dıaz Gomez, Dieter Albert, Jens Mattiza, Helmut Duddeck,
Julian Chojnowskib and Marek Cyprykb
aInstitut fu¨r Organische Chemie der Universita¨t Hannover, Schneiderberg 1B, D-30167 Hannover, Germany
bCentre of Molecular and Macromolecular Studies of Polish Academy of Sciences, Department of Heteroorganic Polymers,
Sienkiewicza 112, PL-90-363 Ło´dz´, Poland
Received 18 May 2006; accepted 30 May 2006
Abstract—Chiral silane 1 was enantiodifferentiated by recording NMR spectra in the presence of the chiral dirhodium complex
Rhð2IIÞ ½ðRÞ-ðþÞ-MTPA4ꢁ (Rh*). This is the first report of direct chiral recognition by spectroscopic methods with this class of compounds.
A comparison with the carba-analogue 2 shows that 1 is a soft Lewis-base ligand and that the Si–H hydrogen is the binding site, pre-
sumably by forming an adduct with a (3c-2e) silicon–hydrogen–rhodium contact (end-on), which exists in equilibrium with the free
ligands.
Ó 2006 Elsevier Ltd. All rights reserved.
5´
8´
4´
1. Introduction
10´
9´
6´
7´
3´
2´
R*
O
Chiral silanes play an important role in modern synthetic
organic chemistry, for example, in intramolecular Si!C
chirality transfer1 or in catalyzed hydrosilylation reactions.2
Thus, a easy spectroscopic method for direct observation of
a diastereomeric interaction between enantiomeric silane
molecules and an enantiopure NMR auxiliary is desirable.
R*
O
O
O
1´
2
1
Rh
Rh
H3C
M
H
OCH3
O
O
1"
C
R* =
O
O
2"
3"
R*
Ph
CF3
1: M = Si
2: M = C
R*
Rh*
4"
The application of chiral lanthanide shift reagents (CLSR)
was introduced several decades ago but this method is
restricted to hard Lewis-base ligands, such as carbonyls,
amines, alcohols as well as some others.3 Over the last dec-
ade however, we introduced the dirhodium method of chi-
rality recognition in which a chiral enantiopure dirhodium
complex Rhð2IIÞ ½ðRÞ-ðþÞ-MTPA4ꢁ (Rh*: MTPA–H = meth-
oxytrifluoromethylphenylacetic acid ꢂ Mosher’s acid; see
Scheme 1) is added to a CDCl3 solution followed by
Scheme 1. Structures of the dirhodium complex Rh*, the chiral silane 1
and its carba-analogue 2.
act with the dirhodium complex. For comparison, we
prepared the corresponding carba-analogue 2 (Scheme 1)
in order to see how this hydrocarbon behaves under the
same NMR experimental conditions.
1
recording a H NMR spectrum.4 It turned out during the
course of this project5 that soft-base functionalities are par-
ticularly suitable for forming relatively stable adducts to
Rh*. As a result, we decided to subject chiral silane 1
(Scheme 1) to the dirhodium experiment in order to deter-
mine whether or not this molecule is basic enough to inter-
2. Results and discussion
2.1. Synthesis, properties and NMR spectroscopy of methyl-
1-naphthylphenylsilane 1 and 1-(1-phenylethyl)naphthalene 2
The chiral silane used, (+)-methyl-1-naphthalenylphenyl-
silane 1, has been synthesized before; ½aꢁD ¼ þ33:4
*
Corresponding author. Tel.: +49 511 762 4615; fax: +49 511 762 4616;
e-mail: duddeck@mbox.oci.uni-hannover.de
20
0957-4166/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2006.05.030

1744
E. Dı´az Go´mez et al. / Tetrahedron: Asymmetry 17 (2006) 1743–1748
corresponding to the (R)-configuration at the silicon
atom.6 Alternative pathways to prepare non-racemic 1
have been described,7 and further studies have confirmed
this stereochemical assignment.8 The sample of 1 used in
the dirhodium experiments (see below) had an ee of 22%.
H
H
H
H
H
H3C
H3C
C
H
Si
H
The carba-analogue 1-(1-phenylethyl)naphthalene 2 was
prepared by a new route described in the Experimental
starting from 1-acetylnaphthalene via a-methyl-a-phenyl-
naphthalenemethanol and 1-(1-phenylethenyl)naphthalene.
All compounds were identified by IR, NMR and MS and
by comparison with reported data.9,10
1
2
Scheme 2. Strong (red circles) and weak (blue circles) complexation shifts
Dd in 1 and 2; unmarked atoms show no significant Dd-values (for details,
see text and Table 1).
1
All H and 13C NMR signals of 1 and 2 were assigned
unequivocally by applying two-dimensional NMR correla-
tion spectroscopy (COSY, HMQC and HMBC).11 Some
severe signal overlap occurred in the presence of Rh*, par-
ticularly for the phenyl signal, so that their exact position
could not be identified safely. These nuclei were excluded
from the discussion in the following sections.
It is obvious that the two molecules have a completely dif-
ferent binding mode to Rh*. In 1, only the silicon nucleus
and the hydrogen directly attached (H-1) show strong sig-
nal shifts, proving that H-1 is the favourable binding site.
All other effects are small, if not even negligible so that a
noticeable contribution from the aromatic moieties must
be excluded. In contrast, all Dd-effects are extremely small
in the case of the hydrocarbon 2; only some weak effects
can be identified at the naphthalene atoms.
2.2. Complex formation of 1 and 2 with Rh*
Ligand molecules can form 1:1- and/or 2:1-adducts with
the enantiopure dirhodium complex (Rh*) depending on
the relative molar ratio of L versus Rh* used.5 All ad-
ducts—except those of phosphane ligands (category I)—
are kinetically labile so that room-temperature NMR sig-
nals are averaged.5 The equilibria of complex formation
are strongly dependent upon the binding energy so that
three further categories, II–IV, can be discerned depending
on the stabilization of the ligand molecules by adduct for-
mation. In category-II ligands, the binding energy is high,
and equilibria are strongly shifted towards the adducts.
The equilibria are approximately balanced for category-
III-ligands and driven towards free ligands in category-
IV-ligands (low or even zero binding energy).5
It is interesting to note that the changes in the relevant cou-
pling constants (see captions of Tables 1 and 2) point into
the same direction. In the ligated 1, there is a considerable
reduction of the one-bond coupling involving the binding
hydrogen H-1 and silicon [1J(29Si,1H)] by adduct formation
from 194.2 to 184.7/183.5 Hz. A corresponding change of
the one-bond 13C,1H coupling constant [1J(13C,1H)] in 2
does not exist; the values are 126.6 and 126.3 Hz,
respectively.
Table 1. 1H, 13C and 29Si chemical shifts (d, in ppm) of the silane 1 and its
carba-analogue 2, recorded in CDCl3; in ppm relative to internal
tetramethylsilane
The time-averaged NMR signals of those ligands (catego-
ries II–IV)5 are shifted to some extent when compared to
those of the free ligands. In general, such complexation
shifts Dd are small or even negligible. Only if the complex-
ation site is close-by, can noticeable deshielding Dd-values
may be observed because the inductive through-bond effect
of the ligand’s functional group is enhanced by complexa-
tion. Thus, Dd-values are good indicators for the preferred
complexation site. Negative, that is shielding, complexa-
tion effects are often observed as well. This can be attrib-
uted to through-space anisotropy effects of the various
aromatic rings in the adduct.5
1
2
d(1H)
d(M)a
d(1H)
d(13C)
1
5.354b
0.753
—
7.734
7.465
7.898
7.851
7.444
7.417
8.051
—
—
—
7.567
7.337
7.370
ꢃ19.8c
4.919d
1.761
—
7.425
7.440
7.735
7.834
7.459
7.408
8.032
—
—
—
7.234
7.219
7.148
40.5
22.5
2
ꢃ4.5
10
133.3
135.2
125.2
130.5
128.9
125.6
126.0
128.0
137.1
133.2
135.4
134.9
128.0
129.5
141.5
124.3
125.4
127.0
128.8
125.3
125.8
124.0
131.7
134.0
146.6
127.6
128.4
126.0
20
30
40
50
60
70
Complexation shifts obtained for 1 and 2 are presented in
Table 2. It was expected that the silane 1 and its carba-ana-
logue 2 are category-IV ligands because they are very weak
Lewis bases. Correspondingly, most Dd-values are small,
with only a few nuclei displaying noticeable complexation
shifts. Nonetheless, there are some trends, which are visu-
80
90
1000 0
100
2 /600
300/500
400
1
alized in Scheme 2: H; dashed circles: Dd >0.1 ppm, open
a M = 13C, except atom 1 (29Si).
circles: Dd = 0.06–0.09 ppm; no circles Dd 60.05 ppm
(insignificant). 29Si/13C; dashed circles: Dd >0.5 ppm, open
circles: Dd = 0.1–0.5 ppm; no circles Dd 60.1 ppm
(insignificant).
b 1J(29Si,1H) = 194.2 Hz; 3J(1H,1H) = 3.9 Hz.
c 1J(29Si,13CH3) = 53.5 Hz.
d 1J(13C,1H) = 126.6 Hz; 3J(1H,1H) = 7.1 Hz.

E. Dı´az Go´mez et al. / Tetrahedron: Asymmetry 17 (2006) 1743–1748
1745
Table 2. 1H, 13C and 29Si complexation shifts (Dd, in ppm) of the silane 1
and its carba-analogue 2, recorded in CDCl3
Interactions between metal atoms and Si–H bonds have
been investigated intensively and were reviewed recently.13
It has been shown that the 1J(29Si,1H) coupling constant in
particular is a very sensitive probe for this interaction be-
cause it is reduced enormously from more than 190 Hz in
the free silane to 30–70 Hz in a kinetically stable silane
r-complex. Based on those findings,13 two different kinds
of Si–H–Rh contacts are conceivable for the present case:
a
Atom
1
2
Dd(1H)
Dd(M)b
Dd(1H)
Dd(M)b
1
0.31/0.26c
0.04/0.03
—
0.06
0.08
0.07
0.09
0.05
0.02
0.05
—
—
—
0.04
0.00
0.01
1.87/1.70d
0.00
ꢃ0.01/ꢃ0.02e
ꢃ0.02
—
0.03
0.06/0.05
0.04
0.04
0.02
0.02
0.02
—
—
—
n.d.f
0.02
ꢃ0.01
2
ꢃ0.03/ꢃ0.04
0.12/0.10
0.08/0.07
ꢃ0.09/ꢃ0.11
ꢃ0.10/ꢃ0.15
ꢃ0.04/ꢃ0.09
ꢃ0.11/ꢃ0.15
0.01/0.00
0.11/0.09
0.10/0.08
0.07
10
0.68
20
ca. ꢃ0.1
ca. ꢃ0.1
ꢃ0.04
30
(a) The two adduct components Rh* and 1 exist in a fast-
exchange equilibrium with species A (side-on; Scheme
4) while the observed reduction of the 1J(29Si,1H) cou-
pling constant of ca. 10 Hz indicates a predominance
of the free components (6–10% for the r-complex), a
behaviour typical of category-IV ligands (as stated
above).
40
50
ꢃ0.48/ꢃ0.49
ca. ꢃ0.5
0.21/0.17
0.49
60
70
80
90
0.18/0.10
0.07/0.05
0.07/0.05
0.05
0.00
0.05
1000 0
1
0.04/0.03
0.04
(b) The interaction is essentially a three-centre-two-elec-
tron binding (3c-2e) as represented by species B
(end-on; Scheme 4). Thus, the Si–H bond order is
somewhat reduced, again time averaged, leading to a
200/600
300/500
400
ꢃ0.15
0.00
ꢃ0.03
a Two entries if the signal dispersion (Table 3) exceeds 0.01 ppm.
b M = 13C, except atom 1 (29Si).
1
decrease of the J(29Si,1H)-value.
c D1J(29Si,1H) = ꢃ9.5/ꢃ10.7 Hz.
d D1J(29Si,13CH3) = ꢄ0 Hz.
e D1J(13C,1H) = ꢃ0.3 Hz; D3J(1H,1H) = ꢃ0.1 Hz.
f Not detectable.
+
Rh Rh
H Si
H
These divergences in the adduct formation behaviour of 1
and 2 originate from the different electron distribution
within the Si–H versus the C–H bond (Scheme 3). In the
Si–H bond of 1, the electrostatic (Mulliken) charges, as cal-
culated by density-functional methods, are +0.386 (Si) and
ꢃ0.101 (H) whereas the corresponding values for the C–H
bond of 2 are +0.054 (C) and +0.091 (H). Moreover, the
dipole moment of 1 (0.78 D) is nearly parallel to the
Si–H bond whereas that of 2 (0.16 D) is much lower and,
furthermore, points into the cavity opened by the two
aromatic ring systems, thereby, having a strong component
antiparallel to the C–H bond (Scheme 3).
Rh Rh
Rh Rh
H
Si
Si
A
B
(side-on)
(end-on)
Scheme 4. Conceivable interaction mechanisms between the rhodium
atoms in Rh* and the silicon–hydrogen bond; Rh* is represented by ‘Rh–
Rh’.
An evaluation of all the arguments and an inspection of all
the other data in Table 2 led to the following interpreta-
tion: the hydridic nature of H-1 in the silane 1 makes this
atom a soft Lewis base and enables it to act as a weak,
but significant, binding site (category-III or -IV ligand).
The ligand approach to Rh* is supported by the electro-
static properties of the Si–H bond and the favourable ori-
entation of the dipole moment. The change in the
1J(29Si,1H)-value suggests that, at least to some extent, an
additional orbital interaction exists. Species B is more rea-
sonable than species A. The silane r-complex A involves a
much closer contact between the aromatic residues of both
components, 1 and Rh*, than species B and this should re-
sult in significant 1H and 13C shieldings due to mutual
anisotropic effects. However, none such effects are visible
in the data body of Table 2.
CH3
CH3
-0.101
+0.397
+0.054 +0.091
H
H
C
Si
Ph
Ph
0.78 D
0.16 D
Naph
Naph
Scheme 3. Electrostatic (Mulliken) charges of the atoms in the Si–H bond
of 1 (left) and the respective C–H bond of 2 (right) as well as dipole
moments (in Debye), as calculated by density-functional calculations
(B3LYP, 6-31G*).
It has been shown in a theoretical study12 that the binding
between a dirhodium tetracarboxylate complex and a
ligand is accomplished primarily by two interactions: (a)
electrostatic attraction and (b) HOMO–LUMO interac-
tion; the electrostatic attraction prevails. In essence, this
factorization of interaction contributions has been qualita-
tively confirmed by our earlier dirhodium experiments
although we found that there is, in addition, a steric repul-
sion from the Mosher acid residues if the binding site in the
ligand molecule is sterically congested.5
Apparently, the C–H bond of the hydrocarbon 2, which
corresponds to the Si–H bond of 1, does not contribute
at all to any binding. This is not surprising because interac-
tion models as outlined in Scheme 4 are not valid here.13
The C–H bond in 2 shows no Lewis basicity and its electro-
static properties favour an approach of the naphthalene p-
electron system, which, according to a density-functional
calculation, bears the largest electron density. Altogether,
2 is an extremely weak ligand (category IV); however, as

1746
E. Dı´az Go´mez et al. / Tetrahedron: Asymmetry 17 (2006) 1743–1748
Table 3. 1H, 13C and 29Si dispersion effects (Dm, in Hz) of the silane 1 and
its carba-analogue 2 at 9.4 T (400.1 MHz 1H, 100.6 MHz 13C, and
79.5 MHz 29Si), recorded in CDCl3. Signs of Dm-values (for 1 only) are
defined according to the equation: Dm = Dm[(+)-RSi] ꢃ Dm[(ꢃ)-SSi]
will be shown in the next section it is still strong enough to
profit from diastereomeric interactions with Rh* enabling
chiral recognition.
Atom
1
2
2.3. Diastereomeric dispersions and chiral recognition
Dm(1H)
Dm(M)a
Dm(1H)
Dm(M)a
Beside the complexation shift Dd, the second parameter
extractable from the dirhodium experiments is the diaste-
reomeric dispersion effect Dm, which provides chiral recog-
nition; in principle, all nuclei in the free chiral ligand
molecules become diastereotopic in the adducts. Hence,
the signal of a given ligand nucleus is split into two aniso-
chronous signals, while the chemical shift difference is the
dispersion Dm. The relative signal areas (integrals) represent
the ratio of the enantiomers. Since such values are mostly
in the range of 1–100 Hz, we prefer to calibrate them in
Hertz. The reader should be aware, however, that the mag-
nitudes of these values depend linearly on the external field
B0, here 9.4 T (Table 2).
1
ꢃ17
ꢃ4
—
0
n.d.
0
ꢃ14
0
0
n.d.b
n.d.
0
+1
n.d.
ꢃ4
n.d.
ꢃ3
+3
ꢃ2
0
3
0
—
n.d.
3
0
0
n.d.
n.d.
0
0
1
2
1
2
5
5
4
1
1
1
0
1
0
0
0
2
10
20
30
40
50
0
60
n.d.
n.d.
0
—
—
—
0
n.d.
n.d.
70
80
90
ꢃ
1000 0
100
2 /600
300/500
400
—
—
n.d.
n.d.
n.d.
0
ꢃ1
Whereas the sample of 2 was racemic, that of 1 used in the
20
dirhodium experiments had an ½aꢁD ¼ þ13:1 correspond-
a M = 13C, except atom 1 in 1 (29Si).
b n.d.: not detectable.
ing to an ee of 22% [(+)-RSi:(ꢃ)-SSi = 61:39].6–8 This is well
reflected by the relative intensities in the 1H and 29Si NMR
signal sets (see Fig. 1).
All diastereomeric dispersion effects (Dm) on the NMR sig-
nals of 1 and 2 are compiled in Table 3. For some signals,
Dm-values could not be identified safely due to signal over-
lap, and the corresponding entry in Table 3 was marked by
‘n.d.’ (not detectable). On the other hand, a number of
signals were not split; their dispersion is too low to be
resolved. As a result, their Dm-values were notified by ‘0’.
For those signals of 1, however, where both signals and
their differential intensities could be identified, the signs
of the Dm-values were determined according to the follow-
ing definition: Dm = Dm[(+)-RSi] ꢃ Dm[(ꢃ)-SSi].
et al. described the use of HPLC with chiral stationary
phases for this purpose.14
It is worth mentioning that another method of chiral recog-
nition has been reported for aromatic hydrocarbons
involving mononuclear rhodium complexes.15 In addition,
the use of a CLSR-related method for differentiating aro-
matic hydrocarbon enantiomers has been reported more
than 20 years ago (addition of Ag+ salts).16 However, it
should be kept in mind that the latter are three-compo-
nent-experiments and it is not easy to find a proper experi-
mental set-up.
It can be seen, from both Figure 1 and Table 3, that an
enantiodifferentiation of both ligands 1 and 2 is easily pos-
sible by the direct measurement of their NMR signals and
by their integration. Enough nuclei are available in both
cases. To the best of our knowledge, this is the first report
of direct spectroscopic enantiodifferentiation of silanes.
However, it should be noted that, recently, Oestreich
At this stage, it is not yet clear whether a correlation rule
for the determination of the absolute configuration may
emerge, a rule analogous to those described by us previ-
ously for some spirochalcogenuranes17 and phospholene
chalcogenides.18 More chiral silane compounds are
required to answer this question.
Si
H
Si
H
5.75
5.65
5.55
5.45
5.35
-17.5
-18.5
-19.5
-20.5
Figure 1. 1H (left) and 29Si NMR spectral sections (right) of the silane 1; bottom: normal spectra of the free silane, top: spectra in the presence of an
equimolar amount of Rh*.

E. Dı´az Go´mez et al. / Tetrahedron: Asymmetry 17 (2006) 1743–1748
1747
3. Conclusion
3054, 1660, 1592, 1491, 1252, 1024, 907, 774, 694. EI-MS
m/z (rel. int.) 232 (M+, 85), 217 (100), 215 (69), 202 (62),
189 (11), 155 (20), 143 (15), 128 (16), 115 (19), 108 (39), 91
(24), 77 (17), 51 (9). HRMS 232.1252; calcd for C18H16
232.1252. For the NMR spectra (see Table 1).
It has been shown that chiral silane 1 can be enantiodiffer-
entiated easily by recording the NMR spectra in the pres-
ence of an equimolar amount of the chiral dirhodium
complex Rh* observing changes in the chemical shifts (com-
plexation shifts, Dd) and coupling constants, as well as sig-
nal splittings (diastereomeric dispersions, Dm). Due to its
hydridic nature, the hydrogen atom attached to the silicon
atom acts as a weak Lewis base, presumably, by interacting
in a three-centre-two-electron (3c-2e) binding to one rho-
dium atom of Rh* in the axial position. This interpretation
is supported by the completely different behaviour of the
carba-analogue 2 in an analogous dirhodium experiment.
4.2. NMR spectroscopy
1H (400.1 MHz) and 13C NMR spectra (100.6 MHz) were
performed on a Bruker Avance DPX-400 spectrometer,
the 29Si NMR spectra were recorded at 79.5 MHz in the
direct mode on the same instrument. Chemical shift stan-
1
dards were internal tetramethylsilane (d = 0 ppm) for H,
1
13C and 29Si. H and 13C signal assignments are based on
DEPT and two-dimensional COSY, HMQC and HMBC
spectra (standard Bruker software). Digital resolutions
were 0.14 Hz/point in the 1H, 0.24 Hz/point in the 13C
and 0.49 Hz/point in the 29Si NMR spectra.
This is the first report of direct spectroscopic chiral recog-
nition of silanes lacking any further Lewis-basic hetero-
atom functionality.
In the standard dirhodium experiment, Rh* and an
equimolar amount of the ligands 1 or 2, respectively, were
dissolved in 0.7 ml CDCl3. Typically, 48.6 mg of Rh*
(0.043 mM concentration) were employed. No acetone–d6
was added for assisting Rh* solubility19 in order to avoid
competition of the acetone molecules with ligands 1 and
2 in the adduct formation.5 Instead, the dissolution process
was accelerated by exposing the NMR sample tubes to an
ultrasonic bath.
4. Experimental
4.1. Substances
4
The syntheses of Rh* and 16 [ee = 22%; (+)-RSi:(ꢃ)-
SSi = 61:39] have been described before.
4.1.1. 1-(1-Phenylethenyl)naphthalene. 1-Acetylnaphthal-
ene (1 g, 5.9 mmol) was dissolved in 25 ml dried THF
and cooled to ꢃ78 °C. Phenyllitihium (3 ml) was added
dropwise with stirring in an inert gas atmosphere. The solu-
tion was further stirred at ꢃ78 °C for 30 min and then
warmed to room temperature. Ice-water was added, the
organic phase separated, dried over Na2SO4, filtered and
the solvent evaporated in vacuo. The resulting a-methyl-
a-phenyl-naphthalenemethanol, a yellowish oil, contained
4.3. Calculations
Density-functional calculations (B3LYP, 6-31G*) of the
free ligands were performed using Spartan ‘04 (Wavefunc-
tionÒ software package).
a small amount of the starting material (1-acetylnaphthal-
Acknowledgements
ꢃ1
~
ene), as proven by the carbonyl band ðm ¼ 1681 cm Þ in
the IR spectrum of the crude product. Nevertheless, the
crude a-methyl-a-phenyl-naphthalenemethanol was sub-
jected to water elimination without further purification
by dissolving it in 25 ml of methanol with a catalytic
amount of p-toluenesulfonic acid and refluxing for two
hours. After evaporation of the solvent, a yellow viscous
liquid was obtained, which was chromatographed on silica
gel with n-hexane and ethylacetate (16:1). After drying,
1-(1-phenylethenyl)naphthalene (195 mg, 0.9 mmol, 15%
yield relative to 1-acetylnaphthalene) was isolated and
identified by comparison of its spectral properties with lit-
erature data.9 This reaction sequence was not optimized for
yield.
This work has been performed within the project ‘Biologi-
cally Active Natural Products: Synthetic Diversity’
(Department of Chemistry, Hannover University) and
was supported by the Deutsche Forschungsgemeinschaft.
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ꢃ1
tained as a crystalline solid.10 IR (solid state) ðm; cm Þ
~

1748
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