Formation of a doubly C,N-bridged six-membered metallacyclic bis[(2-imidazolyl)zirconocene] dication

Article abstract of DOI:10.1515/znb-2003-0409

Treatment of 1-methylimidazol-2-yl lithium with Cp₂Zr(C1)CH₃ (6) gave the corresponding methyl(1-methyl-2-imidazolyl)zirconocene complex 7. Treatment of 7 with B(C₆F₅)₃ (8) led to selective methyl abstraction. The resulting (imidazolyl)zirconocene cation 9 dimerized under the reaction conditions to yield the dinuclear imidazolyl bridged bis(zirconocene) dication complex 10 that was structurally characterized by X-ray diffraction and by DFT calculations.

Full text of DOI:10.1515/znb-2003-0409

Formation of a Doubly C,N-Bridged Six-Membered Metallacyclic
Bis[(2-imidazolyl)zirconocene] Dication
Dominik Vagedes, Gerhard Erker, Gerald Kehr, Roland Fro¨hlich ∗, Stefan Grimme∗∗,
and Christian Mu¨ck-Lichtenfeld∗∗
Organisch-Chemisches Institut der Universita¨t, Corrensstraße 40, D-48149 Mu¨nster, Germany
Reprint requests to Prof. Dr. G. Erker. Fax: (+49) 251 83 36503. E-mail: erker@uni-muenster.de
Z. Naturforsch. 58b, 305 – 310 (2003); received October 23, 2002
Treatment of 1-methylimidazol-2-yl lithium with Cp2Zr(Cl)CH3 (6) gave the corresponding
methyl(1-methyl-2-imidazolyl)zirconocene complex 7. Treatment of 7 with B(C6F5)3 (8) led to se-
lective methyl abstraction. The resulting (imidazolyl)zirconocene cation 9 dimerized under the reac-
tion conditions to yield the dinuclear imidazolyl bridged bis(zirconocene) dication complex 10 that
was structurally characterized by X-ray diffraction and by DFT calculations.
Key words: Zirconocene Cation, Methyl Abstraction, Arduengo Carbene, Imidazole,
DFT Calculation
Introduction
solution. The resulting precipitate of lithium chloride
was removed by filtration and the neutral (2-imid-
azolyl)zirconium complex 7 was isolated in > 90%
yield from the solution (Scheme 2). The product is
1,3-Dialkyl (or -aryl) imidazole-2-ylidenes [1, 2]
have become important ligands in organometallic
chemistry and catalysis [3, 4]. Charged analogues of
these strong σ-donor ligands could in principle be ob-
tained by a formal replacement of one of the hydrocar-
byl substituents at a nitrogen center of the imidazole
ring by a neutral Lewis acid or metal complex frag-
ment. Such anion equivalents (2) of the neutral “Ar-
duengo carbenes” (1) were prepared using BR3 frag-
ments, and the chemistry of the systems 2 was ex-
plored [5,6]. We here describe a rare example of an
early transition metal variant of such species. The sys-
tem was obtained by spontaneous dimerization of a
(2-imidazolyl)zirconocene cation complex under the
conditions of its generation from a neutral precur-
sor [7].
1
characterized by a H NMR singlet at δ = 0.06 origi-
1
nating from the Zr-CH3 σ-ligand, a Cp H NMR sin-
glet at δ = 5.62 (in d6-benzene) and a set of imidazolyl
proton NMR resonances at δ = 6.79 (4-H), 6.41 (5-H)
and 3.07 (N-CH3). All the 1H NMR signals of the Zr-
bound heterocycle occur markedly shifted to smaller
δ values as compared to the imidazolide starting ma-
terial [5: δ = 7.23 (4-H), 6.89 (5-H), 3.64 (N-CH3)].
A similar effect is observed in the 13C NMR spectrum,
the C-2 resonance of 5 (δ = 201.7) is shifted consid-
erably to low field as compared to the Zr complex 7
(δ = 191.0). This trend probably reflects the electro-
static nature of the Li+[imidazolide]− interaction in 5
as opposed to the pronounced covalent Zr-C σ-bond
in 7.
We then converted complex 7 into a cationic zir-
conocene complex. We chose to employ tris(penta-
fluorophenyl)borane (8) [8] as the reagent to abstract
a carbanion equivalent from zirconium. The abstrac-
tion reaction starting from 7 poses a selectivity prob-
lem as principally either the imidazolide or a methyl
anion could be transferred from the transition metal to
zirconium. There was evidence from the literature that
both cases might be feasible [9,10]. However, the re-
action between 7 and B(C6F5)3 turned out to be very
selective.
Scheme 1.
N-Methylimidazole (4) was deprotonated at C-2 by
treatment with n-butyl lithium to yield 2-lithium-1-
methylimidazolide (5). We have then reacted the carb-
anion equivalent 5 with Cp2Zr(Cl)CH3 (6) in toluene
∗
∗∗
X-ray crystal structure analysis;
istry.
computational chem-
c
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306
D. Vagedes et al. · A C,N-Bridged Metallacyclic Bis[(2-imidazolyl)zirconocene] Dication
C5*
N1*
C6*
C4*
C15-C19
C2* N3*
Z
r*
Z
r
C2
N3
C10
-
C14
C4
C6
Scheme 2.
N1
C5
Treatment of Cp2Zr(CH3)(imidazolyl) (7) with
the strong Lewis acid 8 resulted only in re-
moval of the methyl group from zirconium to give
the [CH3B(C6F5)3]− anion [1H/13C NMR: δ =
0.50/10.2 (B-CH3), 11B NMR: δ = −15.2]. The cation
section of the product 10 showed a Cp singlet (1H/13C
NMR at δ = 6.23/112.9) and the imidazolide reso-
nances [1H NMR: δ = 6.79 (4-H), 7.01 (5-H), and
3.83 (N-CH3)]. The C-2 13C NMR resonance of 10
is observed at δ = 192.7, which is a similar value as
found in the neutral starting material 7 (see above).
Fig. 1. Molecular structure of 10 (dication section only).
˚
The Zr-C2 bond length (2.295(12) A) is in the
typical zirconocene-carbon σ-bond range [11]. Cor-
responding alkenyl-C(sp2)-Zr linkages were described
to typically be around this value [12], and iminoacyl-
C(sp2)-Zr bond distances were found in the same
range [13]. The Zr-N3 bond length in the dication 10
˚
amounts to 2.266(7) A which is slightly longer than
found in other N-coordinated zirconocene complexes
The X-ray crystal structure analysis of 10 (single [11, 14].
crystals were obtained from dichloromethane)revealed
a dimeric dication structure in the solid state. We as-
sume an analogous structure of 10 in solution.
The crystal structure analysis yielded a pronounced
probably arbitrary bond localization inside the imida-
zolide nuclei. The bonding features of these moieties
will, therefore, not be discussed. A DFT calculation
[15, 16] revealed the likely CN bond lengths of the
typical delocalized structure of the imidazolide ring
systems inside the dication (see Fig. 2). The X-ray
crystal structure analysis of the organic molecule 1-
methyl-4,5-diphenylimidazole (12), whose molecular
geometry is depicted in Fig. 3, may serve as a suit-
able reference for comparison. It shows the typical
tendency of aromatic bond delocalization with a se-
In the crystal of 10 separated, non-interacting
cations and anions were found. The [CH3B(C6F5)3]−
anion shows the typical structural features [9]. The
cationic counterpart in 10 is a dimetallic dica-
tion, probably formed by dimerization of the al-
leged intermediate 9 (see Scheme 2). In the dica-
tion we find two symmetry-equivalent mono-nuclear
Cp2Zr(imidazolyl)+ halves, which are related by an in-
tramolecular C2 operation. The (η5-Cp)-Zr distances
are in the usual range. The organometallic core of the
molecule is almost planar. The bonding angle at zir-
conium amounts to 98.8(3)◦ (C2*-Zr-N3). The endo-
cyclic bonding angle at C2 is in the typical C(sp2)
range at 124.4(9)◦ (Zr*-C2-N3) and the angle at the
adjacent nitrogen is rather large at 135.9(9)◦ (C2-N3-
Zr). This increase may be due to geometric reasons to
···
···
quence of typical C — C and C — N bond distances of
˚
˚
˚
1.349(2) A (N1-C2), 1.312(2) A (C2-N3), 1.384(2) A
˚
˚
(N3-C4), 1.378(2) A (C4-C5), and 1.386(2)A (C5-N1),
with only the geminal pair of N(1/3)-C(2) bonds being
slightly shorter as compared to the remaining bonds in-
side the five-membered heterocycle.
Eventually, we have carried out the cation-forming
allow for the formation of a planar central core: the reaction starting from 7 in the presence of THF as a
sum of the six endocyclic bonding angles is 718.2◦, donor ligand and obtained the corresponding adduct
which indicates only a marginal deviation of the cen- 11 (see Scheme 2), which was characterized by NMR.
tral Zr2N2C2 framework from planarity.
It shows a 13C NMR resonance at δ = 194.4 for
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D. Vagedes et al. · A C,N-Bridged Metallacyclic Bis[(2-imidazolyl)zirconocene] Dication
307
First, the abstraction reaction poses a selectivity prob-
lem with the electrophilic borane having to choose
between abstraction of a methyl anion equivalent or
the imidazolide from the Group 4 metal center. Al-
though [Cp2ZrCH3]+ would have been a very favor-
able product, the selective transfer of CH−3 is ob-
served. This is probably due to several reasons, includ-
ing steric effects in the actual abstraction step. How-
ever, the 2-imidazolide ligand is probably a stronger
σ-donor which exceeds the methyl anion in its abil-
ity to stabilize a neighboring zirconocene cation. A
mononuclear [Cp2Zr(2-imidazolyl)]+ cation was in-
deed trapped from the reaction mixture by added THF.
The imidazole nitrogen serves as a very effective σ-
donor ligand toward [Cp2Zr]+. This leads to a very fa-
vorable dimer formation, as we have observed in this
study.
Fig. 2. DFT-calculated structure of the dication 10 (RI-
BP86/TZVP [17], calcd. forC1 symmetry, optimized struc-
˚
ture is Ci). Selected calculated bond lengths (A) and an-
gles (◦): Zr-C1 2.358; Zr*-N2 2.286; C1- N5 1.376; C1-N2
1.370; N5-C4 1.383; C4-C3 1.362; C3-N2 1.396; N5-C6
1.465; C1- Zr-N2* 101.2.
Experimental Section
All reactions with organometallic compounds were car-
ried out under an argon atmosphere using Schlenk-type
glassware or in a glovebox. Solvents, including deuter-
ated solvents used for NMR spectroscopy, were dried and
distilled under argon prior to use. The starting materials
2-lithium-1-methylimidazolide (5) [18], Cp2Zr(Cl)CH3 (6)
[19], B(C6F5)3 (8) [8], and 1-methyl-4,5-diphenylimidazole
(12) [20] were prepared according to published methods, or
were used as commercial products [methylimidazole (4) and
BuLi (1.6 M)] without further purification. The following
instruments were used for physical characterization of the
compounds: NMR: Bruker AC 200 P-FT and Varian UNITY
plus 600 spectrometers were used (δ1H = 7.15 (C6D5H),
5.32 (CDHCl2); δ13C = 128.0 (d6-benzene), 53.8 (CD2Cl2);
δ
δ
11B = 0 for ext. BF3-OEt2 with Ξ (11B) = 32.083971 MHz;
19F = 0 for ext. CCl3F with Ξ (19F) = 94.094033 MHz).
The connectivity was confirmed by 1D and 2D NMR experi-
ments. IR spectra were recorded with a Nicolet 5DXC FT-IR-
spectrometer, melting points were measured with a Differen-
tial Scanning Calorimeter 2010 CE (TA Instruments). For el-
emental analysis a Foss Heraeus CHN-O-Rapid and a Vario
El III Mikro CHN-O-Rapid instrument, for mass spectra a
Finnigan MAT 312 and a micromass-quatro LC-Z-electro-
spray mass spectrometers were used.
Fig. 3. A view of the structure of 12. Selected bond lengths
and angles for 12: N1-C2 1.349(2), C2-N3 1.312(2), N3-
C4 1.384(2), C4-C5 1.378(2), C5-N1 1.386(2), N1-C6
1.459(2), C4-C41 1.468(2), C5-C51 1.473(2); C5-N1-C2
106.8(1), C5-N1-C6 128.5(1), C6-N1-C2 124.7(1), N1-
C2-N3 113.1(1), C2-N3-C4 104.8(1), N3-C4-C5 110.2(1),
N3-C4-C41 119.2(1), C41-C4-C5 130.6(1), C4-C5-N1
105.2(1), C4-C5-C51 131.6(1), C51-C5-N1 122.9(1).
Crystal structure analysis: Data sets were collected with
Enraf-Nonius CAD4 and Nonius KappaCCD diffractome-
ters, the latter one equipped with a rotating anode generator
Nonius FR591. Programs used: data collection EXPRESS
(Nonius B. V., 1994) and COLLECT (Nonius B. V., 1998),
data reduction MolEN (K. Fair, Enraf-Nonius B. V., 1990)
and Denzo-SMN [Z. Otwinowski, W. Minor, Methods in En-
C-2(imidazole), which is intermediate between the
neutral precursor 7 and the dinuclear dication complex
10, respectively, and the anionic starting material 5.
Conclusions
The reaction of the Cp2Zr(CH3)(2-imidazolyl) com-
plex 10 with B(C6F5)3 is remarkable in several aspects. zymology 276, 307 (1997)], absorption correction for CCD
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308
D. Vagedes et al. · A C,N-Bridged Metallacyclic Bis[(2-imidazolyl)zirconocene] Dication
data SORTAV [R. H. Blessing, Acta Crystallogr. A51, 33 3J = 1.0 Hz, 1H, 5-H), 5.62 (s, 10H, Cp), 3.07 (s, 3H,
(1995); R. H. Blessing, J. Appl. Crystallogr. 30, 421 (1997)], NCH3), 0.06 (s, 3H, ZrCH3); δ (CD2Cl2) = 6.96 (d,
structure solution SHELXS-97 [G. M. Sheldrick, Acta Crys- 3J = 1.0 Hz, 1H, 5-H), 6.71 (d, 3J = 1.0 Hz, 1H, 4-H),
tallogr. A46, 467 (1990)], structure refinement SHELXL- 5.71 (s, 10H, Cp), 3.78 (s, 3H, NCH3), 0.02 (s, 3H, ZrCH3).
1
97 (G. M. Sheldrick, Universita¨t Go¨ttingen, 1997), graphics
SCHAKAL (E. Keller, Universita¨t Freiburg, 1997).
Crystallographic data (excluding structure factors) for
the structures reported in this paper have been deposited
with the Cambridge Crystallographic Data Centre as sup-
plementary publication no. CCDC-195057 and 195058.
Copies of the data can be obtained free of charge on ap-
plication to The Director, CCDC, 12 Union Road, Cam-
bridgeCB2 1EZ, UK [fax: int. code +44(1223)336-033,
e-mail: deposit@ccdc.cam.ac.uk].
–
13C{ H} NMR (150.8 MHz, d6-benzene): δ = 191.0
(C-1), 126.9 (C-5), 123.7 (C-4), 107.7 (Cp), 35.1 (NCH3),
17.0 (ZrCH3); (CD2Cl2): δ = 190.9 (C-1), 127.4 (C-5),
123.2 (C-4), 107.8 (Cp), 36.0 (NCH3), 15.8 (ZrCH3). –
C15H18N2Zr (317.5): calcd. C 56.74, H 5.71, N 8.82; found
C 55.32, H 5.36, N 7.56.
Bis{[bis(η5-cyclopentadienyl)(1-methylimidazol-2-yl)zir-
conium]}bis[methyl-tris(pentafluorophenyl)borate] (10)
At 0 ◦C dichloromethane (10 ml) was added to a mixture
of 7 (150 mg, 472 µmol) and tris(pentafluorophenyl)borane
(8, 242 mg, 472 µmol). After 30 minutes at 0 ◦C the formed
yellow suspension was stored for 12 hours at −30 ◦C. Sub-
sequently the resulting precipitate was collected by filtra-
tion, washed with a small amount of pentane and dried
in vacuo. This fractional precipitation was repeated twice.
Single crystals for the X-ray structure analysis were ob-
tained by recrystallization of 10 from dichloromethane.
Technical details of the quantum chemical calculations:
The calculations were performed with the TURBOMOLE
suite of programs [15]. The structure of the dication (10)
was fully optimized without any symmetry restrictions at the
density functional (DFT) level employing the BP86 func-
tional [16], a Gaussian AO basis of valence-triple-zeta qual-
ity including polarization functions (TZVP) [17] and the
resolution-of-the-identity (RI) approximation to represent
the Coulomb operator [21]. For zirconium a [5s3p3d] basis
set and an effective core potential with 28 core electrons [22]
was used.
◦
Yield: 270 mg (69%). – M.p.: 122 C (dec.). – IR (KBr):
˜
ν = 1644 (w), 1512 (vs), 1457 (vs), 1380 (w), 1267 (m),
1087 (vs), 1018 (w), 993 (m), 950 (s), 826 (s), 745 (m)
cm−1. – 1H NMR (599.8 MHz, CD2Cl2): δ = 7.01 (d,
3J = 1.0 Hz, 1H, 5-H), 6.79 (d, 3J = 1.0 Hz, 1H, 4-
H), 6.23 (s, 10H, Cp), 3.83 (s, 3H, NCH3), 0.50 (broad,
2-Lithium-1-methylimidazolide (5)
◦
At −78 C butyl lithium (9.00 ml, 14.4 mmol) in pen-
1
3H, BCH3). – 13C{ H} NMR (150.8 MHz, CD2Cl2): δ =
tane was added to a suspension of 1-methylimidazole (1.18 g,
14.4 mmol) in toluene (20 ml). The reaction mixture was al-
lowed to warm to room temperature and was stirred for an-
other 3 days. The precipitate was collected by filtration and
washed with pentane to obtain a slight green solid. Yield:
1.24 g (99%). – 1H NMR (599.9 MHz, d6-benzene / d8-
THF = 10:1): δ = 7.23 (s, 1H, 4-H), 6.89 (s, 1H, 5-H), 3.64 (s,
192.7 (C-1), 146.6 (dm, 1JCF = 236 Hz, PhoFrtho), 137.9 (dm,
1
1JCF = 243 Hz, PhFpara), 136.8 (dm, JCF = 245 Hz,
PhFmeta), 131.1 (C-5), 129.0 (broad, PhiFpso), 124.1 (C-4),
1
112.9 (Cp), 36.4 (NCH3), 10.2 (broad, BCH3). – 11B{ H}
NMR (64.2 MHz, CD2Cl2): δ = −15.2(ν1/2 = 50 20 Hz).
–
19F NMR (563.6 MHz, CD2Cl2): δ = −133.1 (m, 6F,
1
3H, NCH3). – 13C{ H} NMR (150.8 MHz, d6-benzene / d8-
PhFortho), −164.7 (m, 3F, PhFpara), −167.5 (m, 6F, PhFmeta).
THF = 10:1): δ = 201.7 (C-2), 128.4 (C-4), 117.8 (C-5),
35.7 (NCH3).
[Bis(η5-cyclopentadienyl)(1-methylimidazol-2-yl)(tetra-
hydrofuran)zirconium][methyltris(pentafluorophenyl)
borate] (11)
Bis(η5-cyclopentadienyl)(1-methylimidazol-2-yl)methyl-
zirconium (7)
At 0 ◦C 2-lithium-1-methylimidazolide (5, 97.1 mg,
At room temperature tetrahydrofuran (8.0 µl) was
1.10 mmol) was added to a solution of chlorobis(η5- added to
a solution of 7 (30.0 mg, 94.4 µmol),
cyclopentadienyl)methylzirconium (6, 300 mg, 1.10 mmol) tris(pentafluorophenyl)borane (8, 48.3 mg, 94.4 µmol), and
in toluene (15 ml). The reaction mixture was allowed to d2-dichloromethane (0.7 ml) to give 11, which was di-
warm to room temperature and was stirred for further rectly characterized by NMR spectroscopy. – 1H NMR
2 hours. The suspension was filtered and the filtrate was dried (599.8 MHz, CD2Cl2): δ = 7.01 (d, 3J = 1.2 Hz,
in vacuo to yield a brown solid. Yield: 320 mg (91%). – 1H, 5-H), 6.74 (d, 3J = 1.2 Hz, 1H, 4-H), 6.10 (s,
◦
˜
M.p.: 108 C. – IR (KBr): ν = 1646 (s), 1627 (m), 1519 (vs), 10H, Cp), 3.84 (s, 3H, NCH3), 3.81 (broad, 4H, THF),
1
1465 (vs), 1378 (m), 1282 (s), 1166 (m), 1095 (vs), 981 (vs), 1.91 (broad, 4H, THF), 0.51 (s, 3H, BCH3). – 13C{ H}
800 (s), 764 (m), 702 (m) cm−1. – 1H NMR (599.8 MHz, NMR (150.8 MHz, CD2Cl2): δ = 194.4 (C-1), 148.7 (dm,
d6-benzene): δ = 6.79 (d, J = 1.0 Hz, 1H, 4-H), 6.41 (d, 1JCF = 236 Hz, PhoFrtho), 137.8 (dm, JCF = 245 Hz,
3
1
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D. Vagedes et al. · A C,N-Bridged Metallacyclic Bis[(2-imidazolyl)zirconocene] Dication
309
F
1
−1
Phpara), 136.8 (dm, JCF = 248 Hz, PhFmeta), 130.2 (C-5), reflections collected ( h, k, l), [(sinθ)/λ] = 0.59 A
,
˚
128.7 (broad, PhFipso), 123.9 (C-4), 111.6 (Cp), 70.1 (THF),
5351 independent (Rint = 0.087) and 3365 observed reflec-
tions [I ≥ 2σ(I)], 471 refined parameters, R = 0.089, wR2 =
36.1 (NCH3), 25.9 (THF), 10.3 (broad, BCH3). – 11B{ H}
1
−3
˚
0.252, max. residual electron density 0.64 (−0.61) e A
.
NMR (64.2 MHz, CD2Cl2): δ = −15.3(ν1/2 = 45 20 Hz).
Hydrogen atom positions were calculated and hydrogen
atoms refined as riding atoms. The results of the analysis are
less accurate due to the small crystals of poor quality.
–
19F NMR (563.6 MHz, CD2Cl2): δ = −133.0 (m, 6F,
PhFortho), −165.0 (m, 3F, PhFpara), −167.7 (m, 6F, PhFmeta).
4,5-Diphenyl-1-methylimidazole (12) [20]
X-ray crystal structure analysis of 12
1,1-dimethylhydrazine (3.00 g, 3.79 ml, 23.8 mmol) was
added to benzil (5.00 g, 23.8 mmol) in ethanol (10 ml).
The reaction flask was sealed and heated to 110 – 120 C
Formula C16H14N2, M = 234.29, colourless crystal
◦
0.50 × 0.30 × 0.05 mm. Crystal data: a = 11.563(1),
˚
b = 8.935(1), c = 12.135(1) A,
β = 94.37(1),
for 5 hours. The generated precipitate was collected by fil-
tration and recrystallized in ethanol (5 ml) (white solid).
Yield: 4.52 g (81%). – 1H NMR (200.1 MHz, CDCl3): δ =
7.55 (s, 1H, CH), 7.50 – 7.13 (m, 10H, Ph), 3.46 (s, 3H,
V = 1250.1(2) A , ρcalc = 1.245 g cm−3, µ = 5.74 cm−1
,
3
˚
Z = 4, monoclinic, space group P21/c (No. 14). Data col-
˚
lection and structure refinement: Cu-Kα (λ = 1.54178 A),
CH3). – 13C{ H} NMR (50.3 MHz): δ = 138.3 (CH), 137.4,
1
T = 223 K, ω/2θ scans, empirical absorption correction
via ψ scan data (0.762 ≤ transmission factor ≤ 0.972), 5071
134.6 (each Ph), 130.6 (Cipso), 128.9, 128.5, 128.1, 126.6,
126.3 (Ph), 32.1 (CH3).
−1
˚
reflections collected ( h, −k, l), [(sin θ)/λ] = 0.62 A
,
2540 independent (Rint = 0.028) and 2094 observed reflec-
tions [I ≥ 2σ(I)], 162 refined parameters, R = 0.038,
wR2 = 0.106, max. residual electron density 0.21
X-ray crystal structure analysis of 10
Formula C28H30N4Zr2 · 2CH3B(C6F5)3, M = 1659.05,
light yellow crystal 0.20×0.10×0.03 mm. Crystal data: a =
(−0.25) e A−3. Hydrogen atom positions were calcu-
˚
lated and hydrogen atoms refined as riding atoms.
◦
˚
11.751(1), b = 13.080(1), c = 20.057(1) A, β = 98.85(1) ,
V = 3046.1(4) A , ρcalc = 1.809 g cm−3, µ = 4.83 cm−1
,
3
˚
Acknowledgements
Z = 4, monoclinic, space group P21/n (No. 14). Data col-
˚
Financial support from the Fonds der Chemischen Indus-
trie and the Deutsche Forschungsgemeinschaft is gratefully
acknowledged.
lection and structure refinement: Mo-Kα (λ = 0.71073 A),
T = 198 K, ω and ϕ scans, empirical absorption correction
via SORTAV (0.910 ≤ transmission factor ≤ 0.986), 10398
¨
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