The reaction of (butadiene)tantalocene cation with bulky carbonyl derivatives

Article abstract of DOI:10.1016/S0022-328X(99)00169-2

The (s-trans-η⁴-butadiene)tantalocene cation (1, with CH₃B(C₆F₅)₃^- anion) adds one molar equivalent of adamantanone to yield the metallacyclic seven-membered ring product 2a. The X-ray crystal structure analysis of the tetrahydrotantalaoxepine cation 2a reveals a non-planar envelope shaped structure of the metallacyclic framework. In solution a conformational equilibration of this framework is observed to take place on the NMR time scale with a Gibbs-activation energy of ΔG^≠ (260 K)=12.5±0.3 kcal mol^-1. Treatment of (η⁴-C₄H₆)TaCp₂⁺ (1) with 1-cyano-adamantane at 60°C in bromobenzene gave the analogously structured 2,2-bis(η ⁵-cyclopentadienyl)-3,6-dihydro-7-(1-adamantyl)-2-tantala-2H-azepine cation complex (3a) (topomerization barrier in solution: ΔG^≠ (256 K)=12.1±0.3 kcal mol^-1).

Full text of DOI:10.1016/S0022-328X(99)00169-2

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 584 (1999) 318–322
The reaction of (butadiene)tantalocene cation with bulky carbonyl
derivatives
Hans Christian Strauch, Rolf Roesmann, Silke Pacheiner, Gerhard Erker *,
Roland Fro¨hlich 1, Birgit Wibbeling
Organisch-Chemisches Institut der Uni6ersita¨t Mu¨nster, Corrensstraße 40, D-48149 Mu¨nster, Germany
Received 10 February 1999; received in revised form 26 February 1999
Abstract
The (s-trans-h4-butadiene)tantalocene cation (1, with CH3B(C6F5)3− anion) adds one molar equivalent of adamantanone to yield
the metallacyclic seven-membered ring product 2a. The X-ray crystal structure analysis of the tetrahydrotantalaoxepine cation 2a
reveals a non-planar envelope shaped structure of the metallacyclic framework. In solution a conformational equilibration of this
framework is observed to take place on the NMR time scale with a Gibbs-activation energy of DG" (260 K)=12.590.3 kcal
mol−1. Treatment of (h4-C4H6)TaCp2+ (1) with 1-cyano-adamantane at 60°C in bromobenzene gave the analogously structured
2,2-bis(h5-cyclopentadienyl)-3,6-dihydro-7-(1-adamantyl)-2-tantala-2H-azepine cation complex (3a) (topomerization barrier in
solution: DG" (256 K)=12.190.3 kcal mol−1). © 1999 Published by Elsevier Science S.A. All rights reserved.
Keywords: Group 5 metallocene cation complexes; (s-trans-h4-Butadiene)metallocene; Metallacyclic s-allyl complexes; (Butadiene)metallocene
addition reactions
diffraction, but until recently we were not able to
describe an example of the tetrahydrotantalaoxepine
cations 2 by means of an X-ray crystal structure analy-
sis. We have now obtained single crystals of a prime
example of this class of compounds. The structural and
spectroscopic features of this system (2a) are described
in this article.
1. Introduction
We have recently described the synthesis, structure
and several reactions of the bis(h5-cyclopentadienyl)(s-
trans-h4-butadiene)tantalum cation [1]. It was prepared
by Cp-anion ligand abstraction from (h5-Cp)2(h1-
Cp)(h2-butadiene)Ta with [Cp2ZrCH3+][CH3B(C6F5)3−]
[3]. (h5-Cp)2(h1-Cp)(h2-butadiene)Ta was prepared by
treatment of CpTa(butadiene)Cl2 [2] with two equiva-
lents of sodium cyclopentadienide. The resulting salt
[Cp2Ta(h4-C4H6)+][CH3B(C6F5)3−] (1) was character-
ized by X-ray diffraction. The Cp2Ta(s-trans-h4-butadi-
ene)+ cation is structurally analogous to the neutral
Group 4 (s-trans-h4-C4H6)MCp2 complexes (M=Zr,
Hf) [4], and it undergoes similar addition reactions [5].
Ketones as well as nitriles add readily to 1 to yield the
seven-membered metallacyclic cation systems 2 and 3,
respectively (see Scheme 1). Several examples of the
dihydrotantalaazepines 3 were characterized by X-ray
2. Results and discussion
The (butadiene)TaCp2+ cation (1, with CH3B(C6F5)3−
anion) was treated with an equimolar quantity of
adamantanone in bromobenzene. The reaction required
heating to 60°C for 6 h reaction time to go to comple-
tion. The 1:1 addition product 2a was isolated in \
70% yield as a gray–green solid. At 298 K the
1H-NMR spectrum of 2a (in dichloromethane-d2) is
rather simple. It features a single Cp resonance (10H) at
l 6.25 ppm and broad signals of the 2-tantala-tetrahy-
drooxepine CH2 groups. The corresponding C4HꢀC5H
* Corresponding author. Fax: +49-251-8336503.
E-mail address: erker@uni-muenster.de (G. Erker)
1 X-ray crystal structure analyses by Roland Fro¨hlich and Birgit
Wibbeling.
1
double bond H-NMR signals are observed at l 6.45
and 5.32 ppm. However, complex 2a shows a dynamic
0022-328X/99/$ - see front matter © 1999 Published by Elsevier Science S.A. All rights reserved.
PII: S0022-328X(99)00169-2

H.C. Strauch et al. / Journal of Organometallic Chemistry 584 (1999) 318–322
319
Scheme 2.
For the purpose of a structural comparison we have
added 1-cyano-adamantane to (s-trans-h4-butadi-
ene)tantalocene cation (1). The reaction takes place
smoothly at 60°C. The 1:1 addition product is formed
during 6 h at that temperature in bromobenzene solu-
tion, and it was also isolated in \70% yield. Again the
1H- and 13C-NMR spectra indicate the presence of a
non-planar conformation of the heterocyclic ring sys-
Scheme 1.
NMR behavior, and the CH2 and Cp signals listed
above are averaged resonances at high temperature.
Lowering the temperature rapidly results in broadening
of the respective signals and eventually to monitoring a
tem. At 223
K
(at 150.8/599.8 MHz in
pair of
1
pair of diastereotopic H-NMR Cp singlets at (213 K)
dichloromethane-d2) the features of
a
l 6.26 and 6.20 ppm. The metallacyclic methylene
groups feature H-NMR resonances that are pairwise
diastereotopic Cp ligands at tantalum appear at l
107.4, 105.0 (13C) and l 5.96, 5.87 ppm (1H), respec-
tively. The C6H2 methylene hydrogens are
diastereotopic (l 3.90, 1.72 ppm) as are the C3H2
hydrogen atoms (l 2.10, 1.38 ppm). Increasing the
monitoring temperature rapidly leads to a broadening
and eventually pairwise coalescence of the NMR signals
1
diastereotopic at l 2.78 and 0.91 ppm (6-H, H%, see
Scheme 2) and l 2.58 and 2.49 ppm (3-H, H%). From
1
the Cp coalescence of the H-NMR Cp resonances a
Gibbs activation energy of DG" (260 K)=12.590.3
kcal mol−1 was obtained for the ring topomerization
process [6] of the non-planar 2-tantala-tetrahydroox-
epine framework of complex 2a.
1
listed above (see Fig. 2). From the H-NMR Cp coales-
cence a Gibbs activation energy of DG" (256 K)=
12.190.3 kcal mol−1 was deduced for the
ring-inversion process of the dihydrotantala-2H-
azepine cation complex (3a) (see Scheme 3).
The non-planar framework structure of complex 2a
was also established by X-ray diffraction. Diffusion of
pentane vapor into a solution of 2a in dichloromethane
gave single crystals of the metallacycle that were suited
for an X-ray crystal structure analysis. In the crystal,
complex 2a exhibits an envelope-shaped core of atoms
as it is typical for many early transition metal seven-
membered ring systems [5]. To a first approximation,
the atoms C6, Ta, O1, C2, C3 (atom numbering as in
Fig. 1) are almost coplanarly arranged. A second plane
is comprised of the atoms C3–C6, which originate from
the butadiene building block. These two planes are
arranged relative to each other at an angle of ca. 64°.
Carbon atom C2 has the adamantane framework
spiro-connected with the seven-membered metallacycle.
The corresponding O1–C2–C3 angle is 103.1(2)°. The
most prominent feature of the structure of complex 2a
is the very large Ta–O1–C2 angle (160.76(17)°). This
indicates a pronounced metal–oxygen in-plane p-bond-
ing component of the Cp2Ta+ –O-linkage [5,7]. Conse-
Single crystals of 3a were obtained from
CH2Cl2:pentane by means of the diffusion method. In
,
quently, the Ta–O1 bond is very short at 1.846(2) A.
The adjacent Ta–C6 bond is much longer at 2.247(3) A
Fig. 1. A view of the molecular structure of complex 2a (cation only,
with unsystematical atom numbering scheme). Selected bond lengths
,
,
and the in-plane bond angle at the Cp2Ta bent metal-
locene unit is 90.15(11)° (O1–Ta–C6). The remaining
bond lengths and angles inside the metallacyclic frame-
work of the cationic complex 2a are in the normal
expected range (for details see the legend of Fig. 1).
(A) and angles (°): Ta–O1 1.846(2), O1–C2 1.443(3), C2–C11
1.542(4), C2–C17 1.532(4), C2–C3 1.556(4), C3–C4 1.501(5), C4–C5
1.328(5), C5–C6 1.483(5), Ta–C6 2.247(3); O1–Ta–C6 90.15(11),
Ta–O1–C2 160.76(17), O1–C2–C3 103.1(2), O1–C2–C17 109.6(2),
O1–C2–C11 108.7(2), C2–C3–C4 114.0(2), C3–C4–C5 125.4(3),
C4–C5–C6 126.1(3), C5–C6–Ta 108.5(2).

320
H.C. Strauch et al. / Journal of Organometallic Chemistry 584 (1999) 318–322
Fig. 2. Temperature-dependent dynamic 1H-NMR spectra of complex
3a (in CD2Cl2).
Fig. 3. Molecular structure of 3a (cation only). Selected bond lengths
,
(A) and angles (°): Ta2–N1 1.897(4), Ta2–C3 2.294(5), C3–C4
the crystal complex 3a exhibits a non-planar envelope-
shaped seven-membered metallacyclic ring system. The
idealized C3–C6 and C3, Ta2, N1, C7, C6 planes of
atoms are arranged in an angle of ca. 60°. The bond
angle at nitrogen (Ta2–N1–C7) is very large at
161.4(4)°. Again this probably indicates a substantial
nitrogen lone-pair interaction with the available and
spatially very favorably oriented acceptor orbital at
tantalum in the s-ligand ring plane [7,8]. The corre-
sponding Ta2–N1 bond consequently is very short at
1.462(7), C4–C5 1.331(7), C5–C6 1.477(7), C6–C7 1.555(7), C7–C8
1.520(7), N1–C7 1.266(6); Ta2–N1–C7 161.4(4), N1–Ta2–C3
85.70(19), Ta2–C3–C4 111.9(4), C3–C4–C5 127.8(6), C4–C5–C6
126.6(6), C5–C6–C7 113.4(4), C6–C7–C8 121.8(4), N1–C7–C8
123.5(5), N1–C7–C6 114.6(5).
Cp2Ta+ unit in these complexes behaves quite
analogously as the neutral Cp2M moiety in the respec-
tive zirconocene and hafnocene systems [7]. Experimen-
tal studies are currently carried out in order to reveal
the close structural and chemical similarities of the
neutral [Cp2MIV] and cationic [Cp2MV]+ derived sys-
tems, but also to characterize their differences.
,
,
1.897(4) A. The Ta2–C3 bond length is 2.294(5) A, and
the s-ligand bond angle at the bent metallocene unit is
85.70(19)° (C3–Ta2–N1) [9]. The remaining bond
lengths and angles of the seven-membered metallacycle
are unexceptional and as expected (see Fig. 3).
3. Experimental
This study shows that the (s-trans-h4-butadi-
ene)tantalocene cation (1) exhibits a reaction pattern
that is typical for many (diene)metallocene complexes
of the d-block elements at the left side of the periodic
table. The structural properties of the metallacyclic
insertion products of 1 with organic carbonyl com-
pounds and derivatives fall within the typical range
expected for such early metal heterometallacycles. The
pronounced acceptor properties of the coordinatively
unsaturated bent metallocene unit in the complexes 2
and 3 strongly determine the structural features of the
obtained metallacycles. It appears that the cationic
All reactions were carried out in an inert atmosphere
(argon) using Schlenk-type glassware or in a glove-box.
For the preparation of the [Cp2Ta(butadiene)+]-
[CH3B(C6F5)3−] starting material (1) and general infor-
mation, including a list of the instruments used for
spectroscopic and physical characterization of the com-
pounds, see Ref. [1]. X-ray crystal structure analyses:
data sets were collected with Enraf–Nonius Kap-
paCCD (2a) and MACH3 (3a) diffractometers,
equipped with a rotating anode generator. Programs
used: DENZO-SMN/MolEN (data reduction), SHELXS-86
(structure solution), SHELXL-97 (structure refinement),
and SCHAKAL (graphics). For a compilation of the 1D
and 2D NMR experiments used, see Ref. [10].
3.1. Reaction of (s-trans-p4-butadiene)tantalocene
cation (1·CH3B(C6F5)−3 ) with adamantanone,
preparation of 2a
[(C4H6)TaCp2+][CH3B(C6F5)3−] (1, 300 mg, 340
Scheme 3.
mmol) was mixed with 55.6 mg (370 mmol) of adaman-

H.C. Strauch et al. / Journal of Organometallic Chemistry 584 (1999) 318–322
321
tanone. Bromobenzene (10 ml) was added and the
mixture stirred for 6 h at 60°C. After cooling to room
temperature 10 ml of pentane was added. The superna-
tant solvent was decanted from the resulting precipi-
and v scans, 20595 reflections collected (9h, 9k,
9l), [(sin q)/u]=0.72 A−1, 8881 independent and
,
8130 observed reflections [I]2|(I)], 551 refined
parameters, R=0.028, wR2=0.070, max. residual elec-
−3
,
tate, which was then dissolved in
5
ml of
tron density 1.91 (−1.29) e A
close to Ta, hydro-
dichloromethane. The product was again precipitated
by addition of 10 ml of pentane and collected by
filtration; yield of 2a 253 mg (72%), m.p. (dec.) 178°C.
1H-NMR (dichloromethane-d2, 599.8 MHz, 213 K):
l=6.40 (m, 1H, 4-H), 6.26, 6.20 (each s, 5H, Cp-H),
5.30 (m, 1H, 5-H), 2.78 (dd, 3JHH=7.9, 2JHH=12.9
Hz, 1H, 6-H%), 2.58 (dd, 3JHH=8.4, 2JHH=12.1 Hz,
gens calculated and refined as riding atoms.
3.2. Synthesis of 2,2-bis(p5-cyclopentadienyl)-3,6-
dihydro-7-(1-adamantyl)-2-tantala-2H-azepine
methyltris(pentafluorophenyl)borate (3a)
A solution containing 350 mg (390 mmol) of 1 and
70.0 mg (430 mmol) of 1-cyanoadamantane in bro-
mobenzene (10 ml) was kept at 60°C for 6 h. The
product was precipitated by adding pentane (10 ml) at
3
2
1H, 3-H%), 2.49 (dd, JHH=7.9, JHH=12.1 Hz, 1H,
3-H), 1.94–1.38 (m, 14H, adamantane-moiety), 0.91
(pt, JHH=12.9 Hz, 1H, 6-H), 0.40 (br s, 3H, Me–
1
B(C6F5)3) ppm; dynamic H-NMR spectra: Dw of the
ambient
temperature.
It
was
dissolved
in
Cp-resonances at 213 K: 35.3 Hz, coalescence tempera-
ture Tc=260 K, DG" (260 K)=12.590.3 kcal mol−1
with DG" =RTc ln((RTc21/2)/(2yNAhDw)). 1D-TOCSY-
NMR (dichloromethane-d2, 599.8 MHz, 213 K): irradi-
ation at l=5.25 (5-H) ppm: response at l=6.40 (4-H),
2.78 (6-H%), 2.58 (3-H%), 2.49 (3-H), 0.91 (6-H) ppm.
GCOSY-NMR (dichloromethane-d2, 599.8 MHz, 213
K): l=6.40/5.25, 2.58, 2.49 (4-H/5-H, 3-H%, 3-H), 5.25/
6.40, 2.78, 0.91 (5-H/4-H, 6-H%, 6-H), 2.78/5.25, 0.91
(6-H%/5-H, 6-H), 2.58/6.40, 2.49 (3-H%/4-H, 3-H), 2.49/
6.40, 2.58 (3-H/4-H, 3-H%), 0.91/5.25, 2.78 (6-H/5-H,
6-H%) ppm. 13C-NMR (dichloromethane-d2, 599.8 MHz,
dichloromethane (5 ml) and precipitated again with
pentane (10 ml) to give 303 mg (73%) of 3a, m.p. (dec.)
1
180°C. H-NMR (dichloromethane-d2, 599.8 MHz, 223
K): l=6.22 (m, 1H, 4-H), 5.96, 5.87 (each s, 5H,
3
2
Cp-H), 4.80 (m, 1H, 5-H), 3.90 (dd, JHH=8.5, JHH
=
3
2
13.1 Hz, 1H, 6-H%), 2.10 (dd, JHH=8.2, JHH=10.8
Hz, 1H, 3-H%), 1.72 (dd, 3JHH=6.9, 2JHH=13.1 Hz,
1H, 6-H), 2.04–1.95, 1.71–1.57 (m, 15H, adamantyl),
1.38 (pt, JHH=8.2 Hz, 1H, 3-H), 0.40 (br s, 3H,
1
Me–B(C6F5)3) ppm; dynamic H-NMR spectra: Dw of
the Cp-resonances at 223 K: 54.2 Hz, coalescence tem-
perature Tc=256 K, DG" (256 K)=12.190.3 kcal
mol−1. 1D-TOCSY-NMR (dichloromethane-d2, 599.8
MHz, 223 K): irradiation at l=4.80 (5-H) ppm: re-
sponse at l=6.22 (4-H), 3.90 (6-H%), 2.10 (3-H%), 1.72
(6-H), 1.38 (3-H) ppm. GCOSY-NMR (dichloro-
methane-d2, 599.8 MHz, 223 K): l=6.22/4.80, 2.10,
1.38 (4-H/5-H, 3-H%, 3-H), 4.80/6.22, 3.90, 1.72 (5-H/4-
H, 6-H%, 6-H), 3.90/4.80, 1.72 (6-H%/5-H, 6-H), 2.10/
6.22, 1.38 (3-H%/5-H, 3-H), 1.72/4.80, 3.90 (6-H/5-H,
6-H%), 1.38/6.22, 2.10 (3-H/5-H, 3-H%). 13C-NMR
(dichloromethane-d2, 150.8 MHz, 223 K): l=188.3
1
213 K): l=147.5 (d, JCF=235 Hz, o-B(C6F5)3), 136.9
(d, 1JCF=234 Hz, p-B(C6F5)3), 135.8 (d, 1JCF=234 Hz,
m-B(C6F5)3), 135.7 (C4), 127.8 (br m, ipso-B(C6F5)3),
124.1 (C5), 112.4, 109.9 (each C–Cp), 108.3 (C7), 39.3
(C3), 33.8 (C6), 37.5, 36.8, 34.2, 34.1, 34.0, 33.2, 32.3,
26.5, 26.3 (adamantane), 9.4 (br m, Me–B(C6F5)3)
ppm. GHSQC-NMR (dichloro-methane-d2, 150.8/599.8
MHz, 213 K): l=135.7/6.40 (C4/4-H), 124.1/5.25 (C5/
5-H), 112.4/6.20, 109.6/6.26 (C–Cp/Cp-H), 39.3/2.58
(C3/3-H%), 39.3/2.49 (C3/3-H), 33.8/2.78 (C6/6-H%),
33.8/0.91 (C6/6-H), 9.4/0.40 (Me–B(C6F5)3) ppm. ESI
MS (dichloromethane): cation: m/z=516 (100%), 462
(3%), 328 (95%). IR (KBr): w˜ =3123 (w), 2925 (m),
2904 (m), 2894 (s), 1641 (m), 1510 (vs), 1459 (vs), 1265
(s), 1101 (s), 1085 (vs), 993 (s), 963–937 (br m), 848
1
(C7), 147.6 (d, JCF=233 Hz, o-B(C6F5)3), 138.4 (C4),
1
1
137.3 (d, JCF=235 Hz, p-B(C6F5)3), 135.9 (d, JCF
236 Hz, m-B(C6F5)3), 127.9 (br m, ipso-B(C6F5)3), 113.3
(C5), 107.4, 105.0 (each C–Cp), 41.5 (Cquart
=
–
adamantyl), 37.6, 35.6, 27.3 (adamantyl), 29.5 (C3),
28.9 (C6), 9.4 (br m, Me–B(C6F5)3) ppm. GHSQC-
NMR (dichloromethane-d2, 150.8/599.8 MHz, 223 K):
l=138.4/6.22 (C4/4-H), 113.3/4.80 (C5/5-H), 107.4/
5.87, 105.0/5.96 (C–Cp/Cp-H), 29.5/2.10 (C3/3-H%),
29.5/1.38 (C3/3-H), 28.9/3.90 (C6/6-H%), 28.9/1.72 (C6/
6-H), 9.4/0.40 (Me–B(C6F5)3) ppm. ESI MS
(dichloromethane): cation: m/z=526 (100%), 472
(22%), 365 (10%), 311 (1%). IR (KBr): w=3127 (w),
3118 (w), 2914 (s), 2850 (m), 1677 (w), 1641 (m), 1604
(m), 1510 (vs), 1458 (vs), 1265 (m) 1086 (vs), 944–933
(br m), 844 (s), 802 (w) cm.−1 Anal. Calc. for
C44H34BNF15Ta (1053.5): C, 50.16; H, 3.25; N, 1.33.
Found: C, 49.17; H, 3.46; N, 1.41%.
(vs), 841 (vs), 802 (m) cm−1
. Anal. Calc. for
C43H33BOF15Ta (1042.4): C, 49.54; H, 3.19. Found: C,
48.52; H, 3.73%.
X-ray crystal structure analysis of 2a: diffusion of
pentane vapor into a dichloromethane solution (1.5 ml)
containing 30 mg of 2a gave single crystals suited for
X-ray diffraction. Formula C43H33BOF15Ta, Mr=
1042.45, yellow–orange crystal, 0.45×0.25×0.25 mm,
,
a=24.678(1), b=11.961(1), c=27.183(1) A, i=
3
110.03(1)°, V=7538.4(8) A , Dcalc=1.837 g cm−3
,
,
F(000)=4096 e, v=30.27 cm−1, absorption correction
via SORTAV (0.3435T50.518), Z=8, monoclinic,
,
space group C2/c (no. 15), u=0.71073 A, T=198 K, 8

322
H.C. Strauch et al. / Journal of Organometallic Chemistry 584 (1999) 318–322
X-ray crystal structure analysis of 3a: single crystals
senschaft, Forschung und Technologie (BMBF), and the
Deutsche Forschungsgemeinschaft is gratefully
acknowledged.
were obtained from dichloromethane:pentane by the
diffusion method. Formula C44H34BNF15Ta, Mr=
1053.48, red–orange crystal, 0.25×0.20×0.10 mm,
,
a=13.707(3), b=12.329(2), c=23.714(5) A, i=
3
103.30(2)°, V=3900.0(13) A , Dcalc=1.794 g cm−3
,
,
References
F(000)=2072 e, m=29.26 cm−1, absorption correction
via c scan data (0.5285T50.759), Z=4, monoclinic,
[1] H.C. Strauch, G. Erker, R. Fro¨hlich, Organometallics 17
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[2] (a) H. Yasuda, K. Tatsumi, T. Okamota, K. Mashima, K.
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Nakamura, J. Organomet. Chem. 502 (1995) 19.
,
space group P21/c (no. 14), u=0.71073 A, T=223 K,
v/2q scans, 15812 reflections collected (9h, −k, 9l),
[(sin q)/u]=0.62 A−1, 7913 independent and 4459 ob-
,
served reflections [I]2|(I)], 560 refined parameters,
R=0.031, wR2=0.058, max. residual electron density
[3] X. Yang, C.L. Stern, T. Marks, J. Am. Chem. Soc. 116 (1994)
10015.
−3
,
1.15 (−0.53) e A
close to Ta, hydrogens calculated
[4] (a) G. Erker, J. Wicher, K. Engel, C. Kru¨ger, Chem. Ber.
115 (1982) 3300. (b) G. Erker, K. Engel, C. Kru¨ger,
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and refined as riding atoms.
4. Supplementary material
Crystallographic data (excluding structural factors)
for the structures reported in this paper have been
deposited with the Cambridge Crystallographic Data
Centre as CCDC Nos. 114300 and 114301. Copies of the
data can be obtained free of charge on application to
The Director, CCDC, 12 Union Road, Cambridge, CB2
1EZ, UK (fax: int. code +44-1223-336-033, e-
mail: deposit@ccdc.cam.ac.uk or www:http://www.ccdc.
cam.ac.uk).
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Acknowledgements
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Financial support from the Fonds der Chemischen
Industrie, the Bundesministerium fu¨r Bildung, Wis-
.
.