Flexible coordination mode of a donor-functionalized terphenyl ligand in monomeric Cp-based lanthanocenes

Article abstract of DOI:10.1021/om900465c

Summary: The synthesis and structural characterization of monomeric Cp-based metallocene compounds of the rare-earth elements samarium, yttrium, and ytterbium of general composition Cp₂LnDanip (Danip = 2,6-di-o-anisyphenyl) is reported, featuring a donor-functionalized terphenyl moiety as a coligand. Depending on the size of the rare-earth element employed, different coordination modes of the terphenyl moiety are observed. In the case of the relatively large samarium cation both methoxy groups from the terphenyl ligand symmetrically coordinate to the metal cation. However, when smaller cations (yttrium and ytterbium) are employed, a flexible, asymmetric coordination mode of the terphenyl ligand is found.

Full text of DOI:10.1021/om900465c

Organometallics 2009, 28, 5277–5280 5277
DOI: 10.1021/om900465c
Flexible Coordination Mode of a Donor-Functionalized Terphenyl Ligand
in Monomeric Cp-Based Lanthanocenes
Gerd W. Rabe,*,† Florian A. Riederer,‡ Mei Zhang-Presse,‡ and Arnold L. Rheingold§
†Department of Chemistry, Texas A&M University, College Station, Texas 77842, ‡Department fu€r Chemie,
§
Technische Universitat Munchen, Lichtenbergstrasse 4, 85747 Garching, Germany, and Department of
€
Chemistry & Biochemistry, University of California, San Diego, La Jolla, California 92093
€
Received June 1, 2009
Summary: The synthesis and structural characterization of
monomeric Cp-based metallocene compounds of the rare-earth
elements samarium, yttrium, and ytterbium of general com-
position Cp2LnDanip (Danip=2,6-di-o-anisylphenyl) is re-
ported, featuring a donor-functionalized terphenyl moiety as
a coligand. Depending on the size of the rare-earth element
employed, different coordination modes of the terphenyl moi-
ety are observed. In the case of the relatively large samarium
cation both methoxy groups from the terphenyl ligand symme-
trically coordinate to the metal cation. However, when smaller
cations (yttrium and ytterbium) are employed, a flexible,
asymmetric coordination mode of the terphenyl ligand is found.
Danip (=2,6-di-o-anisylphenyl),10-12 which can easily be
prepared from rather inexpensive starting materials.13
We are now interested in probing the accessibility of unsolvated
monomeric metallocene compounds of the lanthanides based on
the unsubstituted Cp ligand on one hand, in combination with a
ꢀ
donor-functionalized terphenyl moiety vis-a-vis the metallocene
framework, thereby attempting to kinetically stabilize a mono-
meric metallocene unit by coordinative saturation through its
potentially reactive site. Previously Niemeyer et al. reported the
synthesis of nonfunctionalized terphenyl-based lanthanocene
compounds of the composition (CpMe)2LnDmp (Ln = Yb,
Er)14,15 and Cp2SmDmp14 (Dmp = 2,6-dimesitylphenyl).
These compounds were prepared from DmpLi and the
corresponding tris(cyclopentadienyl) precursor compounds.
Reaction of equimolar amounts of DanipLi with LnCl3
(Ln=Sm, Y, Yb) in THF at ambient temperature followed
by addition of 2 equiv of NaCp at -25 °C produces mono-
meric unsolvated metallocene compounds of the general
composition Cp2LnDanip (Ln =Sm (1), Y (2), Yb (3)) in
50-60% yield (Scheme 1). 1-3 are arene-soluble but inso-
luble in hexanes. We observed that the order of addition of
reagents (DanipLi/NaCp) cannot be reversed for the syn-
thesis of these particular metallocenes; otherwise, formation of
tris(cyclopentadienyl) compounds is observed instead. Single
crystals of 1 (yellow), 2 toluene (colorless), and 3 toluene
Metallocenes of the f elements are known to be excellent
catalysts for a variety of transformations, e.g., hydroamination,
hydrosilylation, and hydrogenation reactions.1 A very often
encountered problem in such compounds that are based on the
unsubstituted Cp ligand is dimerization of the obtained com-
plexes, which can be prevented by use of sterically more
demanding ligands such as the Cp* ligand and Cp ligands
bearing either tert-butyl or trimethylsilyl substituents or intro-
duction of donor groups to the Cp moiety.2,3 Naturally, use of
sterically more demanding or donor-functionalized Cp ligands
results in a lower catalytic activity of the resulting complexes.
Other concepts involve the use of ansa or constrained-geometry
type ligands in order to prevent dimerization while still retain-
ing desirable catalytic activity.4
3
3
(orange) were obtained by slow evaporation of toluene solu-
tions at ambient temperature inside a glovebox (Table 1). 1-3
show no visible signs of decomposition up to 200 °C.
Over the past two decades there has been much interest in
the chemistry of terphenyl compounds of both main-group
elements and transition metals.5-9 We previously reported a
number of stable terphenyl-based compounds of the f ele-
ments employing the donor-functionalized terphenyl ligand
We note that, as a minor side product, formation of a small
amount of DanipMe16 is observed, which was isolated from the
hexanes cut of the crude product of the reaction mixture and
identified by 1H NMR spectroscopy as well as GC/MS. For-
mation of DanipMe was seen earlier in the analogous reaction
using 1 or 2 equiv of KCp* instead of 2 equiv of NaCp.17
*To whom correspondence should be addressed. E-mail: rabe@
chem.tamu.edu.
(1) Molander, G. A.; Romero, J. A. C. Chem. Rev. 2002, 102, 2161.
(2) Hultzsch, K. C.; Spaniol, T. P.; Okuda, J. Organometallics 1997,
16, 4845.
Scheme 1. Formation of Danip-Based Monomeric
Lanthanocenes
(3) Schumann, H.; Rosenthal, E. C. E.; Demtschuk, J.; Molander, G.
A. Organometallics 1998, 17, 5324.
(4) Stern, D.; Sabat, M.; Marks, T. J. J. Am. Chem. Soc. 1990, 112, 9558.
(5) Wang, Y.; Robinson, G. H. Organometallics 2007, 26, 2.
(6) Nguyen, T.; Sutton, A. D.; Brynda, M.; Fettinger, J. C.; Long, G.
J.; Power, P. P. Science 2005, 310, 844.
(7) Twamley, B.; Haubrich, S. T.; Power, P. P. Adv. Organomet.
Chem. 1999, 44, 1.
(12) Rabe, G. W.; Zhang-Presse, M.; Riederer, F. A.; Golen, J. A.;
Incarvito, C. D.; Rheingold, A. L. Inorg. Chem. 2003, 42, 7587.
(13) Ionkin, A. S.; Marshall, W. J. Heteroat. Chem. 2003, 14, 360.
(14) Niemeyer, M.; Hauber, S.-O. Z. Anorg. Allg. Chem. 1999, 625, 137.
(15) Niemeyer, M. Acta Crystallogr. 2007, E63, m2188.
(16) Rabe, G. W.; Yap, G. P. A. Private communication, CCDC Dep.
No. 153825.
(8) Clyburne, J. A. C.; McMullen, N. Coord. Chem. Rev. 2000, 210, 73.
(9) Robinson, G. H. Acc. Chem. Res. 1999, 32, 773.
ꢁ
ꢁ
(10) Rabe, G. W.;Berube, C. D.;Yap, G. P.A. Inorg. Chem. 2001, 40, 4780.
(11) Rabe, G. W.; Zhang-Presse, M.; Riederer, F. A.; Yap, G. P. A.
Inorg. Chem. 2003, 42, 3527.
(17) Rabe, G. W.; Riederer, F. A.; Zhang-Presse, M.; Rheingold,
A. L. Organometallics 2007, 26, 5724.
r
2009 American Chemical Society
Published on Web 08/12/2009
pubs.acs.org/Organometallics

5278 Organometallics, Vol. 28, No. 17, 2009
Table 1. Crystallographic Data for Cp2LnDanip (1-3) and Danip[μ-Li(THF)]2I (4)a
Rabe et al.
param
1
2
3
4
formula
fw
space group
C20H20O2Sm
442.71
P41212
9.4197(3)
9.4197(3)
26.592(2)
C30H27O2Y
508.43
P21/n
11.5224(10)
13.4907(11)
15.4089(13)
93.8110(10)
2389.9(3)
4
1.413
100(2)
Mo KR (0.710 73)
24.65
1.036
C30H27O2Yb
592.56
P21/n
11.5714(8)
13.3246(9)
15.2776(10)
92.9300(10)
2352.5(3)
4
1.673
213(2)
Mo KR (0.710 73)
40.00
1.032
C28H33ILi2O4
574.32
P21/n
10.6788(13)
18.338(2)
14.1827(18)
100.334(2)
2732.3(6)
4
1.396
208(2)
Mo KR (0.710 73)
12.02
1.039
˚
a, A
˚
b, A
˚
c, A
β, deg
3
˚
V, A
2359.5(2)
Z
4
1.246
100(2)
D(calcd), g cm-3
temp, K
˚
radiation (λ, A)
Mo KR (0.710 73)
μ(Mo KR), cm-1
GOF
24.93
1.090
1.79
4.53
P
R1, % (obsd)
wR2, % (all data)
3.40
9.04
1.85
4.83
3.67
10.42
P
P
P
a The quantity minimized was wR2 = [w(F2o - Fc2)2]/ [(wFo2)2]1/2; R1 = Δ/ (Fo), Δ = |(Fo - Fc)|, w = 1/[σ2(Fo2) þ (aP)2 þ bP], P = [2F2c þ
Max(Fo, 0)]/3.
Figure 1. Molecular structure of 1.
Figure 2. Molecular structure of 3.
However, it should be pointed out that in the reactions reported
in this work formation of such ether-cleavage products clearly
is a side reaction only, with the main products being the Danip-
based metallocene compounds.
The molecular structure of 1 (Figure 1) is comprised of two
mode of the terphenyl moiety is seen, presumably due to
too much steric crowding around the somewhat smaller
yttrium and ytterbium cations. In both 2 and 3 only one of
the anisyl groups coordinates to the metal cation (M-O=
2.3899(16) A (2) and 2.3602(15) A (3)) with O-M-C(ipso)
angles of 77.25(6)° (2) and 78.89(6)° (3), respectively. In both
compounds this angle is noticeably larger than the corre-
sponding angle of 71.56(4)° in the symmetrically coordinated
samarium compound 1. Additionally, the Cnt-M-Cnt
angles of 128.6(5)° (2) and 128.8(5)° (3) are slightly larger
than the same angle in the samarium compound, as one
would expect as a consequence of the presence of only one
coordinating anisyl group. The average metal-ring carbon
distances of both Cp rings differ only little in both 2 (2.64
˚
Cp rings (Cnt-Sm-Cnt=124.4(5)°, Sm-Cnt=2.477(5) A,
˚
˚
˚
Sm-C(ring)=2.75 A (av)) and a terphenyl moiety coordi-
nating symmetrically in a d,l-type fashion to the metal
atom through both methoxy groups (Sm-C(ipso)-C=
122.75(14)°, C(ipso)-C(ortho)-C(ipso0) = 121.2(2)°). The
dihedral angle between the central phenyl ring and the rings
in the ortho positions is 43°. The Sm-C(ipso) distance of
˚
˚
2.520(3) A and the Sm-O distance of 2.5622(15) A are about
in the range of those in previously reported DanipSm-
˚
(THF)COT (Sm-C=2.543(3) A; Sm-O=2.5436(19) and
2.576(2) A) but are somewhat longer than those in five-
12
˚
˚
˚
˚
coordinate DanipSm[N(SiHMe2)2]2 (Sm-C = 2.484(4) A;
and 2.66 A) and 3 (2.61 and 2.62 A), with the Cnt-M dis-
tances being 2.365(5) and 2.379(5) A for 2 and 2.320(5) and
11
˚
˚
Sm-O=2.496(3) and 2.512(3) A) or in seven-coordinate
˚
˚
˚
[DanipSm(μ2-Cl)2(μ2-Cl)Li(THF)2]2 (Sm-C = 2.489(3) A;
2.330(5) A for 3. The M-C(ipso) distances (2, 2.4538(19) A; 3,
2.402(2) A) are shorter than the same distance in the samar-
Sm-O=2.503(2) and 2.520(2) A).10 The O-Sm-O angle of
˚
˚
143.12(7)° in 1 can be compared, e.g., with the corresponding
angles in the also d,l-type bonded DanipSm[N(SiHMe2)2]2
(144.51(9)°)11 and [DanipSm(μ2-Cl)2(μ2-Cl)Li(THF)2]2
(137.71(8)°),10 as well as in meso-type bonded DanipSm-
(THF)COT (103.56(6)°).12
ium compound. The relevant bond distances in the ytterbium
compound 3 can further be compared, for example, with those
˚
in DanipYb(Cp*)Cl (Yb-C=2.415(7) A; Yb-O=2.335(5)
and 2.340(6) A), featuring a meso-type bonded terphenyl
17
˚
moiety. As a consequence of the asymmetric bonding mode of
the terphenyl ligand, this moiety is slightly tilted in both 2 and 3
with M-C(ipso)-C angles of 119.44(14) and 124.71(14)° in 2
In the isomorphous molecular structures of compounds 2
and 3 (Figure 2) an asymmetric or flexible coordination

Note
Organometallics, Vol. 28, No. 17, 2009 5279
˚
Table 2. Comparison of Selected Interatomic Separations (A) and
Angles (deg) of 1-3
complex
1 (M = Sm) 2 (M = Y)
3 (M = Yb)
M-C(ipso)
M-O
C(ipso)-M-O 71.56(4)
M-Cipso-Cortho 122.75(14)
2.520(3)
2.5622(15)
2.4538(19)
2.3899(16)
77.25(6)
119.44(14) (C(21)) 118.91(14) (C(21))
124.71(14) (C(17)) 125.41(14) (C(17))
118.60(18) (C(17)) 119.60(18) (C(17))
121.60(18) (C(21)) 121.40(18) (C(21))
2.402(2)
2.3602(15)
78.89(6)
Cipso-Cortho
-
121.2(2)
0
Cipso
and 118.91(14) and 125.41(14)° in 3. Further bond distances
and angles can be derived from Table 2.
We note that both the proton and the carbon-13 NMR
spectra of diamagnetic 2 show only one signal for both
methoxy groups (a very broad signal in the carbon-13
NMR spectrum with ν1/2=ca. 800 Hz). Our efforts to resolve
the singlet in the proton NMR spectrum into two signals by
lowering the temperature, even to -80 °C in d8-toluene, did
not give the desired outcome, though.
For reasons of comparison with the Danip-based metal-
locenes we also wanted to investigate the accessibility and
stability of unsubstituted Cp-based metallocene compounds
bearing the nonfunctionalized terphenyl ligand Dpp (=2,6-
diphenylphenyl). However, in our hands such systems of
assumed composition Cp2Ln(THF)Dpp (Ln=Sm, Y, Yb),
obtained from the reaction of equimolar amounts of DppLi
and LnCl3 followed by addition of 2 equiv of NaCp at
-25 °C, are found to be of very limited stability only. Our
attempts to obtain single-crystal material yielded Cp3Ln-
(THF) instead, presumably as a result of a ligand redistribu-
tion reaction: i.e., ligand scrambling.
Figure 3. Molecular structure of 4.
overall d,l-type coordination of the ligand, with dihedral
angles between the central phenyl ring and the two rings in
the ortho positions of 50° (C2-C7 ring) and 56° (C14-C19
ring). The molecular structure of 4 can best be compared
with the solvent-modified dimeric double salt {Danip[μ-Li-
(THF)](μ-Li)I}2, featuring a four-step ladder-type structure
in the solid state.18 Both structures differ mainly in the
amount of THF per lithium atom: i.e., one (4) vs half a
molecule in {Danip[μ-Li(THF)](μ-Li)I}2, respectively, there-
by resulting in a monomeric vs a dimeric arrangement.
Further comparisons of compound 4 can be made with
another structurally characterized “halide rich” terphenyl
lithium compound, ArDbp2[Li(OEt2)]2I, in which the ipso
carbon atom of the central aryl ring is bound to two ether-
solvated lithium atoms which are also iodine-bridged.19
Finally, we were also interested in extending the idea of
ꢀ
employing a donor-functionalized terphenyl moiety vis-a-vis
a Cp ligand to divalent lanthanide chemistry. Our attempts
to obtain a divalent Danip-based terphenyl precursor com-
pound by reacting equimolar amounts of SmI2(THF)2 and
DanipLi in tetrahydrofuran yielded a brown powder. Our
repeated efforts to obtain single-crystal material of the
obtained crude product from toluene solutions reproducibly
gave colorless crystals (in yields of less than 10%) of Danip-
[μ-Li(THF)]2I (4), which can be described as a double salt or
as a tetrahydrofuran-solvated lithium iodide adduct of Da-
nipLi. We attempted a direct synthesis of compound 4 using
equimolar amounts of DanipLi and LiI in tetrahydrofuran
solution. However, in our hands the reaction did not yield
the expected product after evaporation of all volatiles,
followed by extraction with toluene, centrifugation, and slow
evaporation of the toluene solution at ambient temperature
inside a glovebox.
Experimental Section
The compounds described below were handled under nitro-
gen using Schlenk double-manifold, high-vacuum, and glove-
box (MBraun, Labmaster 130) techniques. Solvents were dried,
and physical measurements were obtained by following typical
laboratory procedures. NaCp was purchased from Aldrich as a
2.0 M solution in tetrahydrofuran. Prior to use it was crystal-
lized from the purple tetrahydrofuran solution (in a glovebox
refrigerator at -25 °C), filtered, washed with hexanes, and dried
under vacuum, yielding a colorless powder. DanipI13 was pre-
pared according to the literature. DanipLi was prepared as
reported earlier11,18 and was stored in a glovebox refrigerator.
SmCl3, YCl3, and YbCl3 were purchased from Aldrich
(packaged under argon in ampules) and were used as received.
SmI2(THF)2 was purchased from Aldrich as a 0.1 M solution in
tetrahydrofuran. Prior to use it was isolated by evaporation of
the solvent, washing with hexanes, and drying under vacuum
and was stored inside a glovebox refrigerator. NMR spectra
were recorded on a JMN-GX 400 instrument.
The molecular structure of 4 (Figure 3) features two four-
˚
coordinate lithium atoms (Li Li = 2.663(6) A) in dis-
torted-tetrahedral coordination environments, each of them
3 3 3
being complexed by one tetrahydrofuran molecule (Li-O=
1.930(4) and 1.901(5) A), and one methoxy group from the
terphenyl ligand (Li-O=1.960(4) and 1.988(5) A). The two
other positions of the coordination polyhedron are filled by
Cp2LnDanip (Ln=Sm, 1; Ln=Y, 2; Ln=Yb, 3). In a glovebox,
a freshly prepared solution of DanipLi (296 mg, 1.0 mmol)
dissolved in 5 mL of tetrahydrofuran was added to a suspension
of LnCl3 (1.0 mmol; 1, 257 mg; 2, 195 mg; 3, 279 mg) in 5 mL of
tetrahydrofuran at ambient temperature and stirred for 30 min,
which resulted in a yellow (Sm), colorless (Y), or purple (Yb)
˚
˚
the bridging ipso carbon atom from the central phenyl ring of
˚
the Danip ligand (Li-C=2.162(5) and 2.180(5) A; Li-C-Li
=75.65(17)°) and a bridiging iodine atom (Li-I=2.786(4)
(18) Rabe, G. W.; Zhang-Presse, M.; Yap, G. P. A. Acta Crystallogr.
2002, E58, m434.
(19) Hino, S.; Olmstead, M. M.; Fettinger, J. C.; Power, P. P. J.
˚
and 2.793(4) A; Li-I-Li=57.01(12)°). Each anisyl group of
the terphenyl ligand coordinates to a different lithium atom,
thereby forming a six-membered chelate ring, resulting in an
Organomet. Chem. 2005, 690, 1638.

5280 Organometallics, Vol. 28, No. 17, 2009
Rabe et al.
solution, respectively.10 The solution was cooled to -25 °C
(glovebox refrigerator). A freshly prepared solution of NaCp
(176 mg, 2.0 mmol) dissolved in 5 mL of tetrahydrofuran was
also cooled to -25 °C. Both solutions were then combined by
adding the NaCp solution dropwise to the other solution, and
the reaction mixture was then warmed to ambient temperature.
Centrifugation and removal of all volatiles followed by washing
of the residues several times with a few milliliters of hexanes
yielded a solid (1, yellow; 2, colorless; 3, orange), which was
extracted with toluene and then centrifuged. Storage of the
obtained solutions at -25 °C over several days produced
microcrystalline material of compounds 1-3 (1, 285 mg, 50%;
2, 280 mg, 55%; 3, 305 mg, 60%), after removal of the mother
liquor, washing of the residues with hexanes, and drying under
vacuum for several hours.
m/z(%) 304(100), 273(16), 258(7), 202(6), 152(4), 119(6), 101(6),
39 (2), 28 (6).
Danip[μ-Li(THF)]2I (4). In a glovebox, a freshly prepared
solution of DanipLi (296 mg, 1.0 mmol) dissolved in 5 mL of
tetrahydrofuran was cooled to -25 °C (glovebox refrigerator)
and then slowly added, upon stirring, to a blue solution of
SmI2(THF)2 (1.0 mmol, 548 mg) in 10 mL of tetrahydrofuran at
-25 °C. The reaction mixture was stirred for 30 min and warmed
to ambient temperature, resulting in a brown solution. Removal
of all volatiles, followed by washing of the residues several times
with a few milliliters of hexanes, yielded a brown solid, which
was extracted with toluene and then centrifuged. Slow evapori-
zation of the obtained solution inside a glovebox at ambient
temperature over a period of several days resulted in decoloriza-
tion of the solution and formation of colorless crystals of 4 in
yields of less than 10%.
1
Characterization Data. 1: H NMR (C6D6) δ 0.91 (br, 6H,
General Aspects of X-ray Data Collection, Structure Determi-
nation, and Refinement for Complexes 1-4. Crystal, data
collection, and refinement parameters are given in Table 1. Data
were collected on a Bruker Apex AXS CCD system. The
systematic absences in the diffraction data are uniquely consis-
tent for the reported space groups. All crystals were mounted on
glass fibers with Paratone-N oil and cooled to indicated tem-
peratures. The structures were solved using direct methods,
completed by subsequent difference Fourier syntheses, and
refined by full-matrix, least-squares procedures. SADABS
absorption corrections were applied to all data sets. All non-
hydrogen atoms, except those exhibiting disorder, were refined
with anisotropic displacement coefficients. All hydrogen atoms
were treated as idealized contributions.
OMe), 2.50 (br), 3.37 (v br), 5.59 (v br), 6.55 (v br), 8.20 (v br,
10H, Cp), 8.33 (d, aromatic H), 8.65 (t, aromatic H); we failed to
detect any signals in the carbon-13 NMR spectra in both
deuterated benzene and tetrahydrofuran, which is probably due
to both the paramagnetic nature of compound 1 and fluctiona-
lity of the molecule in the solution state; IR (Nujol, cm-1
)
ν 1461 m, 1387 s, 1340 w, 1242 w, 1023 m, 928 w, 860 m, 788
s, 753 s, 666 m, 452 vs, 412 vs. Anal. Calcd for C30H27O2Sm: C,
1
63.23; H, 4.78. Found: C, 63.43; H, 4.95. 2: H NMR (C6D6)
δ 3.29 (6H, s, OMe), 5.75 (10H, s, Cp), plus several multipletts in
the aromatic region ranging from 6.68 to 7.58 ppm; 13C NMR
(C6D6) δ 63.2 (v br, OMe, ν1/2=ca. 800 Hz), 110.7 (Cp), 124.7,
126.0, 126.7, 131.7, 139.8, 146.1, 156.6, 185.1 (d, JY-C=51 Hz,
ipso C); IR (Nujol, cm-1) ν 1291 s, 1232 s, 1175 m, 1158 m, 1107
m, 1046 m, 1008 s, 966 m, 796 m, 720 s, 415 vs. Anal. Calcd for
C30H27O2Y: C, 70.87; H, 5.35. Found: C, 71.02; H, 5.52. 3: IR
(Nujol, cm-1) ν 1479 m, 1431 m, 1370 s, 1294 m, 1178 m, 1161 s,
1119 m, 1107 s, 1048 m, 110 m, 970 s, 895 w, 849 m, 783 s, 728 s,
666 w, 630 w, 616 m, 584 s, 514 s, 460 s. Anal. Calcd for
C30H27O2Yb: C, 60.81; H, 4.59. Found: C, 61.01; H, 4.70.
Characterization Data for DanipMe. 1H NMR (C6D6): δ 2.19,
2.23 (s, 3H, Me, approximate ratio: 1:2), 3.20, 3.25 (s, 6H, OMe,
approximate ratio: 2:1), plus several multiplets in the aromatic
region ranging from 6.5 to 7.3 ppm (m, 11H). GC/MS (EI, 70 eV):
All software and sources of the scattering factors are con-
tained in the SHELXTL (5.10) program library (G. M. Sheldrick,
Siemens XRD, Madison, WI).
Acknowledgment. We thank the Deutsche Forschungs-
gemeinschaft for financial support.
Supporting Information Available: CIF files giving X-ray
crystallographic data for 1-4. This material is available free
of charge via the Internet at http://pubs.acs.org.