Synthesis and solid state structures of sterically crowded d0-metallocenes of magnesium, calcium, strontium, barium, samarium, and ytterbium

Article abstract of DOI:10.1021/om0108705

The base-free metallocenes [1,2,4-(Me₃C)₃C₅H₂]₂Ca, [1,2,4-(Me₃C)₃C₅H₂]₂Sr, and [1,2,4-(Me₃C)₃C₅H₂]

Full text of DOI:10.1021/om0108705

Organometallics 2002, 21, 3139-3146
3139
Syn th esis a n d Solid Sta te Str u ctu r es of Ster ica lly
Cr ow d ed d 0-Meta llocen es of Ma gn esiu m , Ca lciu m ,
Str on tiu m , Ba r iu m , Sa m a r iu m , a n d Ytter biu m
Frank Weber, Helmut Sitzmann,* Madeleine Schultz, Chadwick D. Sofield, and
Richard A. Andersen
FB Chemie der Universita¨t, Erwin-Schro¨dinger-Strasse, D-67663 Kaiserslautern, Germany,
and Chemistry Department and Chemical Sciences Division of Lawrence Berkeley National
Laboratory, University of California, Berkeley, California 94720
Received October 3, 2001
The base-free metallocenes [1,2,4-(Me3C)3C5H2]2Ca, [1,2,4-(Me3C)3C5H2]2Sr, and [1,2,4-
(Me3C)3C5H2]2Ba have been prepared by reaction of MI2 and the sodium or potassium salt
of the substituted cyclopentadiene anion in THF, while [1,2,4-(Me3C)3C5H2]2Mg was prepared
from the cyclopentadiene, 1,2,4-(Me3C)3C5H3, and dibutylmagnesium. The structures of the
magnesocene and calcocene as well as those of the previously prepared ytterbium and
samarium analgoues have been determined by X-ray crystallography. The metallocenes are
slightly bent in the solid state with centroid-metal-centroid angles ranging from 163° to
173°. Adducts of the strontocene and samarocene with THF and of the barocene, samarocene,
and ytterbocene with 2,6-Me2C6H3NC have also been prepared and characterized. The
massive bulk of the [1,2,4-(Me3C)3C5H2]- ligand leads to behavior quite different from that
previously observed for base-free metallocenes of the alkaline earths and divalent lan-
thanides.
In tr od u ction
a few adducts have been prepared. This contrasts with
the reactions of the sterically crowded (Me5C5)3Sm, in
which the steric repulsions at the metal do not inhibit
reactions with incoming ligands, but the relief of these
repulsions lowers the barriers to substitution reactions.7
We report here the synthesis and molecular struc-
The use of extremely bulky cyclopentadienide ligands
is an active area of metallocene research in which
previously unknown ligands have been prepared1 and
used to stabilize previously unknown metallocenes.2
Reports of new base-free metallocenes of the alkaline
earth metals3 and divalent lanthanides4-6 have recently
appeared, and the structures of these relatively simple
molecules continue to hold surprises. In this context and
for comparison with the corresponding divalent lan-
thanide metallocenes,6 we have prepared a series of
main group metallocenes bearing the bulky [1,2,4-
(Me3C)3C5H2]- (Cp‡) ligand. The coordination environ-
ment of this ligand leads to structures in which the
centroid-metal-centroid angle approaches 180°, but
the rings are not parallel in the solid state. The origin
of bending appears to be repulsive interactions between
the tert-butyl groups in the magnesium case. For
divalent metallocenes of calcium, strontium, barium,
samarium, or ytterbium, bending has previously been
described as the natural geometry.5 The steric bulk of
the Cp‡ ligand used in this study induces repulsion on
further bending of the metallocene sandwich, and only
tures of metallocenes of the form Cp‡ M as well as some
2
complexes with THF and xylyl isocyanide.
Resu lts a n d Discu ssion
Stirring a THF slurry of MI2 (M ) Ca, Sr, Ba) with
2 equiv of NaCp‡ or KCp‡ results in a rapid reaction
as two cyclopentadienide rings are transferred to the
divalent metal center (eq 1 using sodium salt). Either
the potassium or the sodium salt can be used in these
reactions with equally good yields of metallocenes. This
is in accord with the synthesis of many alkaline earth
and lanthanide metallocenes.5,6,8-12 However, it con-
trasts with the reactions of the related cyclopentadien-
ide salts, K[1,2,4-(Me2CH)3C5H2] and K[1,2,3,4-(Me2-
CH)4C5H], with CaI2 in THF, studied by Hanusa.13,14
Attempted synthesis of the corresponding calcocenes in
pure THF led to the mono-ring complexes, which was
attributed to kinetic stabilization due to the strong
(1) Quindt, V.; Saurenz, D.; Schmitt, O.; Scha¨r, M.; Dezember, T.;
Wolmersha¨user, G.; Sitzmann, H. J . Organomet. Chem. 1999, 579,
376-384.
(7) Evans, W. J .; Forrestal, K. J .; Ziller, J . W. J . Am. Chem. Soc.
1998, 120, 9273-9282.
(2) Sitzmann, H.; Dezember, T.; Ruck, M. Angew. Chem. 1998, 110,
3294-3296.
(8) Tilley, T. D.; Andersen, R. A.; Spencer, B.; Ruben, H.; Zalkin,
A.; Templeton, D. H. Inorg. Chem. 1980, 19, 2999-3003.
(9) Lappert, M. F.; Yarrow, P. I. W.; Atwood, J . L.; Shakir, R.;
Holton, J . J . Chem. Soc., Chem. Commun. 1980, 987-988.
(10) Burns, C. J .; Andersen, R. A. J . Organomet. Chem. 1987, 325,
31-37.
(3) Schumann, H.; Gottfriedsen, J .; Glanz, M.; Dechert, S.;
Demtschuk, J . J . Organomet. Chem. 2001, 617-618, 588-600.
(4) Visseaux, M.; Barbier-Baudry, D.; Blacque, O.; Hafid, A.;
Richard, P.; Weber, F. New J . Chem. 2000, 24, 939-942.
(5) Schultz, M.; Burns, C. J .; Schwartz, D. J .; Andersen, R. A.
Organometallics 2000, 19, 781-789.
(11) Tilley, T. D.; Boncella, J . M.; Berg, D. J .; Burns, C. J .; Andersen,
R. A. Inorg. Synth. 1990, 27, 146-149.
(12) Hitchcock, P. B.; Howard, J . A. K.; Lappert, M. F.; Prashar, S.
J . Organomet. Chem. 1992, 437, 177-189.
(6) Sitzmann, H.; Dezember, T.; Schmitt, O.; Weber, F.; Wolmer-
sha¨user, G.; Ruck, M. Z. Anorg. Allg. Chem. 2000, 626, 2241-2244.
10.1021/om0108705 CCC: $22.00 © 2002 American Chemical Society
Publication on Web 06/25/2002

3140 Organometallics, Vol. 21, No. 15, 2002
Weber et al.
Ta ble 1. P h ysica l P r op er ties of Ba se-F r ee
Meta llocen es [1,2,4-(Me3C)3C5H2]2M
phenomenon is most stark in the paramagnetic sama-
rocene case, in which the separation between the tert-
butyl groups of the base-free compound at room tem-
perature is 5.9 ppm. With 1 equiv of THF the separation
is 9 ppm, and the two resonances of the THF are
observed at 2.2 (ν1/2 ) 64 Hz) and -0.8 (ν1/2 ) 18 Hz)
ppm. The chemical shifts erroneously reported previ-
ously are for a solution containing 1 equiv of THF.6 The
shifted and broadened peaks in the proton NMR spec-
trum indicate clearly that the THF is bound to the
paramagnetic samarium center on average, although
chemical exchange is occurring in solution. Addition of
more THF results in further monotonic change in the
positions of the resonances of the tert-butyl protons of
the metallocene and of the THF, so that with 3.5 equiv
of THF, the spectrum consists of resonances at 16.4
M
color
melting point (°C)
reference
Mg
Ca
Sr
Ba
Sm
Eu
Yb
white
white
white
187-189
123-125
162
this work
this work
this work
white
115
this work
purple
red
brown-green
125
135
165
6
6
6
binding of THF to calcium, preventing substitution of
the second cyclopentadienide ring. It is somewhat
surprising, therefore, that the sterically even larger Cp‡
ligand leads directly to the calcocene in high yield, and
no mono-ring product is observed. One explanation for
this difference in reactivity is that the ligands used by
Hanusa are somewhat smaller than Cp‡, which means
that THF could bind more tightly to the mono-ring
intermediate. Also, the rate is expected to be slower for
the system studied by Hanusa as the isopropyl groups
are less nucleophilic than the tert-butyl groups.
After removal of the solvent under reduced pressure,
the base-free metallocenes are obtained and can be
recrystallized from pentane.
(18H, CMe3, ν1/2 ) 80 Hz), 5.5 (36H, (CMe3)2, ν1/2
)
21 Hz), 2.9 (14H, THF, ν1/2 ) 41 Hz), and 0.3 (14H,
THF, ν1/2 ) 16 Hz) ppm. With a large excess (20 equiv)
of THF, the tert-butyl resonances are observed at 17.6
(ν1/2 ) 54 Hz) and 5.6 (ν1/2 ) 21 Hz) ppm and do not
change further with the addition of more THF. At that
concentration the THF protons resonate close to the
frequencies for free THF at 3.4 (ν1/2 ) 27 Hz) and 1.3
(ν1/2 ) 34 Hz) ppm, but the resonances are still broader
than in a diamagnetic solution.
THF
2Na[1,2,4-(Me3C)3C5H2] + MI2 M ) Ca, Sr, Ba8
For the diamagnetic metallocenes of barium and
strontium, the changes in the 1H NMR spectra upon
addition of a donor ligand are less pronounced. None-
theless, electron donors have the effect of increasing
the separation between the two tert-butyl peaks (see
Experimental Section for details). A similar effect,
also caused by addition of THF, on the 1H NMR spectra
of alkaline earth metallocenes has been reported by
Hanusa for the isopropyl-substituted complexes, [1,2,4-
(Me2CH)3C5H2]2M (M ) Ca, Sr, Ba).13 This was not
observed for the calcium or ytterbium complexes in our
study, presumably because the metal center is too small
and shielded by the tert-butyl groups to allow the close
approach of a donor ligand.
[1,2,4-(Me3C)3C5H2]2M + 2NaI (1)
This contrasts with the synthesis of metallocenes bear-
ing less bulky cyclopentadienide ligands such as
[Me5C5]-, [1,3-(Me3Si)2C5H3]-, and [1,3-(Me3C)2C5H3]-.
For those metallocenes, THF is not the solvent of choice
for synthesis of base-free complexes because it inevitably
leads to a THF adduct from which the THF cannot
readily be removed.5,8 For some strontium and barium
examples, sublimation is effective to remove coordinated
solvent.10,15,16
The magnesium analogue is prepared by heating
dibutylmagnesium with the cyclopentadiene in heptane,
as previously reported for (Me5C5)2Mg17 and [1,3-
(Me3C)2C5H3]2Mg.18 The metallocenes reported here are
very soluble, crystalline solids. Table 1 summarizes
their physical properties and compares them to the
previously reported Sm, Eu, and Yb analogues. Note
that the NMR data for the paramagnetic samarium
analogue previously reported6 are incorrect as explained
in detail below; the correct chemical shifts are reported
in the Experimental Section.
The crystal structures of the base-free magnesium
and calcium complexes have been determined, as well
as those of the previously reported samarium and
ytterbium analogues. Unfortunately, the magnesocene
structure is the only one of high quality. This seems to
be a characteristic of base-free metallocenes of Cp‡; with
six tert-butyl groups altogether, the molecule is almost
spherical within the unit cell, and the packing is not
constrained to give a well-ordered structure, unless the
metal atom is very small, which reduces the degrees of
freedom of the molecule. In fact, all of the structures
reported here are isomorphous and have two very
similar independent molecules in the unit cell, as does
the previously reported europocene structure.6 Each
structure other than the magnesocene has two very
similar unit cell constants, which seems to be associated
with their poor ordering (Table 3). The crystals therefore
do not diffract well, and only low-angle data can be
obtained. In the presence of an additional ligand (THF
or xylyl isocyanide, see below) the ordering of the
compound within the crystal is far better and the
molecule packs better, giving good quality high-angle
data and well-resolved structures. Despite the disorder,
the overall geometry of the base-free molecules de-
Addition of a donor such as THF or DME to an NMR
tube containing a C6D6 solution of the strontium,
barium, or samarium complex results in a 1H NMR
spectrum in which the separation between the two
inequivalent tert-butyl resonances is larger than in the
spectra of the corresponding base-free molecules. This
(13) Burkey, D. J .; Williams, R. A.; Hanusa, T. P. Organometallics
1993, 12, 1331-1337.
(14) Burkey, D. J .; Alexander, E. K.; Hanusa, T. P. Organometallics
1994, 13, 2773-2786.
(15) Andersen, R. A.; Blom, R.; Burns, C. J .; Volden, H. V. J . Chem.
Soc., Chem. Commun. 1987, 10, 768-769.
(16) Engelhardt, L. M.; J unk, P. C.; Raston, C. L.; White, A. H. J .
Chem. Soc., Chem. Commun. 1988, 1500-1501.
(17) Andersen, R. A.; Blom, R.; Boncella, J . M.; Burns, C. J .; Volden,
H. V. Acta Chem. Scand. 1987, A41, 24-35.
(18) Sofield, C. D.; Andersen, R. A. J . Organomet. Chem. 1995, 501,
271-276.

Sterically Crowded d0-Metallocenes
Organometallics, Vol. 21, No. 15, 2002 3141
Ta ble 2. Selected Bon d Dista n ces (Å) a n d An gles (d eg) for Ba se-F r ee Meta llocen es [1,2,4-(Me3C)3C5H2]2M
M
Mg
Ca
Sm
Yb
Eua
M-C (ring) (Å, range)
M-C (ring) (Å, mean)
M-centroid (Å, mean)
centroid-M -centroid (deg)
twist angle (deg)
2.34-2.45
2.39
2.06
173
3
2.60-2.68
2.64
2.35
171
2
2.72-2.85
2.79
2.52
164
1
2.64-2.72
2.67
2.38
166
1
2.76-2.82
2.78
2.51
165
2
a
Reference 6.
Ta ble 3. Selected Cr ysta l Da ta a n d Da ta Collection P a r a m eter s for [1,2,4-(Me3C)3C5H2]2Mg,
[1,2,4-(Me3C)3C5H2]2Ca , [1,2,4-(Me3C)3C5H2]2Sm , a n d [1,2,4-(Me3C)3C5H2]2Yb
formula
fw
C34H58Mg
491.14
C34H58Ca
506.88
C34H58Sm
617.15
C34H58Yb
639.87
space group
a (Å)
b (Å)
P21/c (#14)
20.3699(5)
17.5802(3)
19.2997(4)
112.774(1)
6372.5(2)
8
P21/c (#14)
19.838(2)
18.220(1)
19.694(2)
110.41(1)
6671(1)
8
P21/c (#14)
19.5280(8)
18.3913(9)
19.8608(5)
109.895(2)
6707.2(5)
8
P21/c (#14)
19.482(1)
18.066(1)
19.526(1)
109.819(2)
6465.4(7)
8
c (Å)
â (deg)
V (Å3)
Z
dcalc (g/cm3)
µ(Mo KR)calc (cm-1
size (mm)
temperature (K)
diffractometera
scan type
2θ range (deg)
no. of reflns collected
no. of unique reflns
no. of obsd reflns
variables
1.024
0.7
1.01
2.06
1.222
17.69
1.31
29.11
)
0.32 × 0.28 × 0.14
0.21 × 0.19 × 0.12
0.35 × 0.25 × 0.20
0.19 × 0.14 × 0.08
149
293
293
136
SMART
ω
4-46
25 981
9451
5342 (I > 3σI)
631
Stoe IPDS
φ
4.3-52.1
23 297
7943
2954 (I > 2σI)
671
Enraf-Nonius
φ
3.1-55.1
22 324
13 936
7537 (I > 2σI)
667
SMART
ω
4-52
26 407
9617
2707 (I > 3σI)
291
abs corr
XPREP
0.978-0.947
0.0447
XPREP
0.964-0.854
0.095
none
SADABS
0.962-0.786
0.035
transmn range
b
R1
0.15
0.36
c
wR2
0.0511
0.24
0.033
wRall
GOF
0.0615
1.58
-0.19 and 0.25
0.20
0.94
-0.38 and 0.75
0.42
1.34
-2.02 and 4.04
0.057
0.88
-0.50 and 0.70
min. and max. RED (e-/Å3)
a
b
2
Radiation: graphite monochromated Mo KR (λ ) 0.71073 Å). R1 ) ∑||Fo| - |Fc||/∑|Fo|. c wR2 ) [∑w(Fo - Fc2)2/∑w(Fo2)2]1/2
.
scribed here, including the centroid-metal-centroid
angles and relative arrangement of the cyclopentadi-
enide rings, is clear. All of these metallocenes crystallize
as slightly bent sandwiches. Unsubstituted magneso-
cene is linear,19 as is decamethylmagnesocene,20 while
[(Me3Si)3C5H2]2Mg is also slightly bent.21 Thus, it ap-
pears that the steric bulk of the Cp‡ groups forces the
rings to bend back in the case of the small metal
magnesium. For Ca, Ba, Sm, Eu, and Yb, the cyclopenta-
dienide rings are closer to parallel in the structures
determined here than in those of their less-substituted
base-free metallocenes.5,12,22-24 Again, the steric bulk of
the Cp‡ groups seems to be responsible, in these cases
by not allowing the rings to bend back as much as their
natural geometry would require, due to an increase in
repulsion at the backside of the wedge. A similar effect
was noticed for the smaller ligand [(Me2CH)3C5H2]-.25,26
The arrangement of the cyclopentadienide rings in
the solid state is found to be eclipsed, so that, viewed
F igu r e 1. Schematic representation of eclipsed (A, D) and
staggered (B, C) conformations of metallocenes in which
the cyclopentadienide rings each bear three substituents.
U represents a substituent on the upper ring; L represents
a substituent on the lower ring.
(19) Bu¨nder, W.; Weiss, E. J . Organomet. Chem. 1975, 92, 1-6.
(20) Carmona, E. Structure of (Me5C5)2Mg, personal communication,
2001.
down the centroid-centroid axis, exactly one tert-butyl
group on one ring is directly over one on the other ring
(Figure 1A). Although the metallocenes are almost
linear (centroid-metal-centroid angles 163-173°), in
every case the two tert-butyl groups that are directly
over one another are located at the slightly open side
of the wedge (at the top of the diagram in the view
shown; Figure 1A). This geometry was also found for
(21) Morley, C. P.; J utzi, P.; Kru¨ger, C.; Wallis, J . M. Organo-
metallics 1987, 6, 1084-1090.
(22) Zerger, R.; Stucky, G. J . Organomet. Chem. 1974, 80, 7-17.
(23) Evans, W. J .; Hughes, L. A.; Hanusa, T. P. Organometallics
1986, 5, 1285-1291.
(24) Williams, R. A.; Hanusa, T. P.; Huffman, J . C. Organometallics
1990, 9, 1128-1134.
(25) Hanusa, T. P. Polyhedron 1990, 9, 1345-1362.
(26) Williams, R. A.; Tesh, K. F.; Hanusa, T. P. J . Am. Chem. Soc.
1991, 113, 4843-4851.

3142 Organometallics, Vol. 21, No. 15, 2002
Weber et al.
these metallocenes are not given and can be found in
the Supporting Information. Important data collection
and structure solution parameters are provided in Table
3, and full details of the crystallography are included
in the Supporting Information.
Despite the crowded environment, the mean metal-
ring carbon distances in the crystal structures of these
complexes are not elongated from the corresponding
values in the respective decamethylmetallocenes (2.64
for Ca,24 2.80 for Sm and Eu,23 and 2.67 for Yb5) (Table
2), other than for magnesium. The mean Mg-C distance
in both independent molecules of Cp‡ Mg, at 2.39(6) Å,
2
is slightly longer than in decamethylmagnesocene,
whose structure has been determined both in the solid
state (mean Mg-C 2.30 Å20) and in the gas phase (mean
Mg-C 2.34 Å17). Comparison with the reported struc-
ture of [1,2,4-(Me3Si)3C5H2]2Mg indicates that the steric
bulk of the tris(trimethylsilyl)cyclopentadienide ligand
is similar to that of Cp‡; as in the trimethylsilyl-
substituted case, the metal-ring carbon distances for
both molecules in the asymmetric unit lie between 2.32
and 2.40 Å (mean 2.37 Å).31 The tert-butyl groups are
expected to cause slightly more steric pressure due to
the shorter and less flexible C-C bond compared with
the C-Si bond. This is consistent with the bending in
these two magnesocenes being caused by steric pressure
as described above.
F igu r e 2. ORTEP diagram (50% probability ellipsoids) of
Cp‡ Mg. Only one of the two independent molecules in the
2
asymmetric unit is shown. Hydrogen atoms have been
omitted for clarity.
the corresponding europocene,6 as well as the bismo-
cenium and stibocenium cations of Cp‡.27,28 It has
also been observed for hexa(trimethylsilyl)metallocenes
of tin, germanium, magnesium, and iron and for the
corresponding ferrocenium cation.21,29-32 In this arrange-
ment, the other four tert-butyl (or trimethylsilyl) groups
are far apart from one another. This arrangement
(which can be achieved in two different ways) is appar-
ently energetically favored in the solid state over the
alternative, staggered arrangements, in which no group
lies directly over another, but each tert-butyl group has
a minimum of one near neighbor on the opposite ring
(Figure 1B,C). Two of the five possible staggered posi-
tions lead to three of these interactions (Figure 1B),
while three of these arrangements result in four near
neighbors on the opposite ring (Figure 1C). The latter
staggered geometry has been observed in structures in
which an additional ligand is present (see the THF
adducts below). The other three possible eclipsed ar-
rangements involve two or all three substituents being
located directly over each other and are presumably
Crystallization of the strontium, barium, and sa-
marium complexes from THF leads to the formation of
the corresponding THF adducts, but the THF is easily
lost upon exposure of the crystals to vacuum. This is in
stark contrast to several previously reported soluble
metallocenes, which must be subjected to heating under
vacuum in order to remove coordinated ethers.5,13 For
the calcium and ytterbium metallocenes, THF coordina-
tion has not been observed, even when the compound
was crystallized from neat THF. Clearly the metal
center is shielded from incoming ligands by the bulky
tert-butyl groups. However, unlike the heavily shielded
complexes [(Me2CH)4C5H]2Ca26 and [(Me2CH)5C5]2M
(M ) Ca, Sr, Ba),2 the base-free metallocenes reported
here are nonetheless extremely moisture sensitive.
much higher in energy. Nonetheless, the chloride Cp‡ -
2
BiCl adopts an arrangement with two sets of tert-butyl
groups directly over one another, Figure 1D.33 The
chloride ligand in that structure is located in the
position with no tert-butyl groups at the top, as shown
in Figure 1D. A similar eclipsed geometry was found
for isocyanide adducts in this work (see below).
The THF adducts of the strontocene and samarocene
were characterized by X-ray crystallography in order to
determine the structural changes upon adduct forma-
tion. The strontocene adduct contains two disordered
tert-butyl groups, which are the isolated tert-butyl
groups (in the 4-position) on each cyclopentadienide
ring. Figure 3 shows an ORTEP diagram of the stron-
An ORTEP diagram of Cp‡ Mg is shown in Figure 2,
2
while Table 2 gives selected bond distances and angles
of these structures (the mean of the two independent
molecules is given). The corresponding data for the
reported Eu structure are included for comparison.6 As
the calcium, samarium, and ytterbium compounds are
isostructural to the magnesocene, ORTEP diagrams of
tocene THF adduct Cp‡ Sr(THF), while important bond
2
distances and angles for both compounds are given
in Table 4. The complexes are isomorphous and iso-
structural, so no ORTEP diagram of the THF adduct of
the samarocene is given here; it can be found in the
Supporting Information. Table 5 contains relevant data
collection and structure solution parameters, and full
crystallographic details are given in the Supporting
Information.
(27) Sitzmann, H.; Wolmersha¨user, G. Z. Naturforsch. 1997, 52b,
398-400.
(28) Sitzmann, H.; Ehleiter, Y.; Wolmersha¨user, G.; Ecker, A.;
U¨ ffing, C.; Schno¨ckel, H. J . Organomet. Chem. 1997, 527, 209-213.
(29) Cowley, A. H.; J utzi, P.; Kohl, F. X.; Lasch, J . G.; Norman, N.
C.; Schlu¨ter, E. Angew. Chem. 1984, 96, 603-604.
(30) J utzi, P.; Schlu¨ter, E.; Hursthouse, M. B.; Arif, A. M.; Short,
R. L. J . Organomet. Chem. 1986, 299, 285-295.
(31) Okuda, J .; Herdtweck, E. Chem. Ber. 1988, 121, 1899-1905.
(32) Okuda, J .; Albach, R. W.; Herdtweck, E.; Wagner, F. E.
Polyhedron 1991, 10, 1741-1748.
It can be seen that the cyclopentadienide rings are
staggered; the twist angle is 26°.34 This is presumably
a consequence of the position in the middle of the open
wedge being occupied by the THF molecule, so it is no
longer energetically feasible for two tert-butyl groups
to lie above one another in that position. Instead the
(33) Sitzmann, H.; Wolmersha¨user, G. Chem. Ber. 1994, 127, 1335-
1342.

Sterically Crowded d0-Metallocenes
Organometallics, Vol. 21, No. 15, 2002 3143
Ta ble 4. Selected Bon d Dista n ces (Å) a n d An gles (d eg) of [1,2,4-(Me3C)3C5H2]2Sr (THF ),
[1,2,4-(Me3C)3C5H2]2Sm (THF ), [1,2,4-(Me3C)3C5H2]2Sm (2,6-Me2C6H3NC)‚Tolu en e,
[1,2,4-(Me3C)3C5H2]2Yb(2,6-Me2C6H3NC)‚Tolu en e, a n d [1,3-(Me3C)2C5H3]2Yb(2,6-Me2C6H3NC)2
Cp‡2Sm(2,6-
Cp‡2Yb(2,6-
[1,3-(Me3C)2C5H3]2Yb(2,6-
a
Cp‡2Sr(THF) Cp‡2Sm(THF) Me2C6H3NC)‚toluene Me2C6H3NC)‚toluene
Me2C6H3NC)2
M -C (ring) (Å, mean)
M-centroid (Å, mean)
centroid-M-centroid (deg)
twist angle (deg)
2.87
2.61
150
2.86
2.60
149
2.86
2.59
155
3
2.75
2.47
154
3
2.72
2.44
133
1
26
26
M-O or M-CCNR (Å)
2.587(3)
2.605(3)
2.749(4)
1.142(5)
1.399(5)
180
2.588(5)
1.150(6)
1.393(6)
180
2.61
1.15
1.40
172
CCNR-N (Å)
N-CR (Å)
M-CCNR-N (deg)
a
Reference 36.
2140 cm-1 (the barium analogue was characterized only
by NMR spectroscopy, which shows that the adducts
have identical stoichiometry). This value compares with
the single stretch at 2129 cm-1 observed for the 2:1
ytterbium analogue [1,3-(Me3C)2C5H3]2Yb(2,6-Me2C6H3-
NC)2, which has only two tert-butyl groups per cyclo-
pentadienide ligand and two isocyanide ligands. The
trimethylsilyl-substituted compound [1,3-(Me3Si)2C5H3]2-
Yb(2,6-Me2C6H3NC)2 has a single stretch at 2142 cm-1
,
so the electronic nature of the Cp‡ M (M ) Sm, Yb)
2
fragment bound to one xylylisocyanide ligand is similar
to that of [1,3-(Me3Si)2C5H3]2Yb bound to two such
ligands. These metallocenes do not engage in back-
donation of electron density in a manner similar to the
fragment [1,3-(Me3C)2C5H3]2Yb.36
The Sm and Yb isocyanide adducts have been char-
acterized by X-ray crystallography (with a toluene of
crystallization), which confirms that the single isocya-
nide ligand is bound in the middle of the open side of
the metallocene wedge. They are isostructural, so only
the ORTEP diagram for the samarium complex is shown
in Figure 4. Selected bond lengths and angles are
presented in Table 4 and compared with the corre-
sponding data for the reported structure of [1,3-
(Me3C)2C5H3]2Yb(2,6-Me2C6H3NC)2, an ORTEP diagram
of which is provided in Figure 5.36 Important data
collection and structure solution parameters are pro-
vided in Table 5, and full details of the crystallography
are included in the Supporting Information.
F igu r e 3. ORTEP diagram (50% probability ellipsoids) of
Cp‡ Sr(THF). Hydrogen atoms have been omitted for
2
clarity. Only one set of methyl atoms from each of the two
disordered tert-butyl groups (on C41 and C91) is shown.
molecule has adopted a staggered conformation similar
to that shown in Figure 1C, with somewhat elongated
metal-ring carbon distances due to the higher coordi-
nation number and therefore greater ionic radius.35 The
THF ligand is twisted so as to minimize steric interac-
tions between it and the neighboring tert-butyl groups.
The reaction of the ytterbium complex with carbon
monoxide was attempted, as carbonyl complexes of three
slightly less bulky ytterbocenes have recently been
reported.36 However, no change in the infrared spectrum
was observed in methylcyclohexane at room tempera-
ture. As mentioned above, this contrasts with the
reaction of the sterically encumbered complex (Me5C5)3-
Sm, which couples two CO groups to one of the cyclo-
pentadienide ligands.7
The metal atom in both the samarium and ytterbium
complexes lies on a crystallographic C2 axis, along with
the ipso-CNC part of the isocyanide ligand and its para
ring carbon atom, so the asymmetric unit consists of half
the molecule. Thus, the isocyanide ligand is crystallo-
graphically in the center of the wedge. The two cyclo-
pentadienide rings are eclipsed, and in two positions
tert-butyl groups are directly above one another, similar
To compare the new metallocenes with the well-
studied (Me5C5)2Yb, 2 equiv of the isocyanide 2,6-
Me2C6H3NC was added to toluene solutions of the base-
free metallocenes of Ba, Sm, and Yb. Instead of forming
the 2:1 compounds that have been reported for several
ytterbocenes with a series of isocyanides,36 1:1 products
to the geometry observed for Cp‡ BiCl and depicted in
2
Figure 1D.33 Note that the centroid-metal-centroid
angle of the bismuth complex is 145°, compared with
155° in these complexes, so the steric hindrance of the
eclipsed tert-butyl groups is less in that case than in
these complexes.
were formed. The infrared spectra of Cp‡ M(2,6-Me2C6H3-
2
NC) (M ) Sm, Yb) both contain a sharp CN stretch at
The most dramatic difference between Cp‡ Yb(2,6-
2
Me2C6H3NC) and [1,3-(Me3C)2C5H3]2Yb(2,6-Me2C6H3-
NC)2 is the number of isocyanide ligands that each
fragment binds. In the case of [1,3-(Me3C)2C5H3]2Yb, in
the presence of 1 or 2 equiv of isocyanide, only the bis-
isocyanide adduct can be isolated, with the remainder
of the ytterbocene remaining base-free if a deficiency
(34) The twist angle of a metallocene is measured by first creating
10 planes, each through the two calculated centroids and one of the
10 ring carbon atoms of the molecule. The smallest 5 angles of
intersection of pairs of planes, each pair having one plane with a ring
carbon on the top ring and one plane with a ring carbon on the bottom
ring, are summed, and the mean of these 5 angles is the twist angle.
(35) Shannon, R. D. Acta Crystallogr., Sect. A 1976, A32, 751-767.
(36) Schultz, M.; Burns, C. J .; Schwartz, D. J .; Andersen, R. A.
Organometallics 2001, 20, 5690-5699.
of isocyanide ligand is present.36 In stark contrast, Cp‡ -
2

3144 Organometallics, Vol. 21, No. 15, 2002
Weber et al.
Ta ble 5. Selected Cr ysta l Da ta a n d Da ta Collection P a r a m eter s for [1,2,4-(Me3C)3C5H2]2Sr (THF ),
[1,2,4-(Me3C)3C5H2]2Sm (THF ), [1,2,4-(Me3C)3C5H2]2Sm (2,6-Me2C6H3NC)‚Tolu en e, a n d
[1,2,4-(Me3C)3C5H2]2Yb(2,6-Me2C6H3NC)‚Tolu en e
Cp‡2Sm (2,6-
Me2C6H3NC)‚toluene
Cp‡2Yb (2,6-
Me2C6H3NC)‚toluene
Cp‡2Sr(THF)
38H66OSr
Cp‡2 Sm(THF)
C38H66OSm
formula
fw
C
C50H75NSm
840.46
C50H75NYb
863.15
626.53
689.26
color
space group
a (Å)
b (Å)
c (Å)
white
purple
dark green-blue
Pbcn (#60)
16.3115(6)
15.0697(6)
19.2592(6)
90
4734.1(3)
4
1.179
dark blue
Pbcn (#60)
16.2302(4)
14.8966(4)
19.2822(4)
90
4662.0(2)
4
1.230
P21/n (#14)
10.3389(6)
16.0479(8)
23.471(1)
94.595(7)
3881.8(4)
4
P21/n (#14)
10.3387(9)
16.0316(9)
23.435(2)
94.63(1)
3871.5(5)
4
â (deg)
V (Å3)
Z
d
calc (g/cm3)
1.072
1.413
1.183
1.541
µ (mm-1
)
1.271
2.037
size
0.60 × 0.45 × 0.42
0.42 × 0.31 × 0.18
0.43 × 0.40 × 0.33
0.40 × 0.30 × 0.20
temp (K)
293
293
200
200
diffractometera
scan type
Stoe IPDS
φ rotation
5.7-50.7
51 812
Stoe IPDS
φ rotation
5.7-51.4
46 948
SMART
ω
3.6-54.8
14 968
SMART
ω
3.7-54.9
15 124
2θ range (deg)
no. of reflns collected
unique
6937
7345
5279
5110
obsd (I > 2σI)
no. of variables
abs corr
5274
410
5612
379
3561
233
3104
233
numeric (XRED/STOE)
0.34-0.42
0.051
numeric (XRED/STOE)
0.54-0.79
0.042
SADABS43
0.67-0.73
0.027
SADABS43
0.63-0.72
0.027
transmn range
b
R1
c
wR2
0.105
0.097
0.063
0.066
wRall
0.119
0.101
0.074
0.077
GOF
1.13
1.076
-0.427 and 0.646
1.07
-0.51 and 0.51
1.04
-0.61 and 0.58
min. and max. RED (e-/Å3)
-0.350 and 0.458
Radiation: graphite-monochromated Mo KR (λ ) 0.71073 Å). R1 ) ∑||Fo| - |Fc||/∑|Fo|. c wR2 ) [∑w(Fo - Fc2)2/∑w(Fo2)2]1/2
.
a
b
2
F igu r e 5. ORTEP diagram of the published structure of
F igu r e 4. ORTEP diagram (50% probability ellipsoids) of
[1,3-(Me3C)2C5H3]2Yb(2,6-Me2C6H3NC)2.36
Cp‡ Sm(2,6-Me2C6H3NC)‚toluene. The whole molecule, con-
2
lowest energy geometry consistent with their ring
substitutions. The less-heavily substituted compound
can bend the metallocene wedge more easily, as no tert-
butyl groups are at the backside of the wedge. The
resulting smaller centroid-metal-centroid angle (133°
rather than 155°) allows the approach of two rather
than only one isocyanide ligand. The controlling factor
appears to be the reorganization energy on bending
the metallocene sandwich. The metal-ring carbon
bonds are the same length in all the ytterbocene
structures; variations in structure occur to maintain
these distances.37
sisting of two asymmetric units, is shown. Hydrogen atoms
and the toluene of crystallization have been omitted for
clarity.
Yb binds only one isocyanide ligand even when the
isocyanide ligand is present in excess. Figure 5 shows,
for comparison, an ORTEP diagram of the published
structure of [1,3-(Me3C)2C5H3]2Yb(2,6-Me2C6H3NC)2,
and it can be seen that the cyclopentadienide rings in
that molecule are also eclipsed, but no tert-butyl groups
are forced to lie above one another due to the lower
degree of substitution. The difference in structure
between these two compounds is presumably the result
of steric pressure; in each case the compounds adopt the
(37) Raymond, K. N.; Eigenbrot, C. W. Acc. Chem. Res. 1980, 13,
276-283.

Sterically Crowded d0-Metallocenes
Organometallics, Vol. 21, No. 15, 2002 3145
line salts Cp‡Na and Cp‡K were prepared from NaNH2 and
KH, in THF solvent, respectively. BaI2 was prepared in an
analogous manner to YbI2. CaI2 and SrI2 were purchased from
Strem and used without further purification. Bu2Mg was
purchased as a heptane solution from Aldrich, and its con-
centration was determined by titration. All NMR spectra were
recorded at ambient temperature in C6D6 solution.
It has been reported that alkyl isocyanides are reduc-
tively cleaved by decamethylsamarocene, with cyanide-
bridged trimers of Sm(III) being produced;38 the same
products were reportedly formed by reaction of the
isocyanides with (Me5C5)3Sm.7 In both of these reports,
the isocyanides were used without distillation. In our
experience, the purity of the isocyanide is crucial. For
[1,2,4-(Me3C)3C5H2]2Mg. Tri-tert-butylcyclopentadiene (1,2,4-
(Me3C)3C5H3) (12.8 g, 55 mmol) was added to dibutylmagne-
sium (31 mL, 0.87 M in heptane, 27 mmol), and the mixture
was heated to a vigorous reflux with stirring for 7 days. The
solution was cooled slowly to -20 °C, and colorless crystals
formed (2.0 g, 32% yield). The yield is low; additional crops of
crystals cannot be obtained from the oily mother liquor. Mp:
187-189 °C. The compound sublimes at 100-120 °C and 10-3
Torr. Anal. Calcd for C34H58Mg: C, 83.1; H, 11.9. Found: C,
82.9; H, 12.1. 1H NMR: δ 6.12 (4H), 1.51 (36H), 1.38 (18H).
[1,2,4-(Me3C)3C5H2]2Ca . A 1.08 g (4.21 mmol) sample of
[1,2,4-(Me3C)3C5H2]Na and 0.59 g (2.01 mmol) of CaI2 were
stirred in 20 mL of tetrahydrofuran for 2 days. After complete
evaporation of the solvent, the residue was extracted with
petroleum ether (3 × 10 mL) and the combined extracts were
filtered and evaporated to dryness to yield 0.99 g (1.95 mmol,
97%) of the white product. Mp: 123-125 °C. Anal. Calcd for
C34H58Ca: C, 80.6; H, 11.5. Found: C, 79.3; H, 11.4. 1H NMR:
δ 6.03 (4H), 1.48 (36H), 1.41 (18H). 13C NMR: δ 133.8 (m, 2C,
ring C(CMe3)), 131.1 (m, 4C, ring C(CMe3)2), 107.9 (dd, 4C,
t
example, addition of “old” BuNC to decamethylytter-
bocene in toluene led to the formation of a cyanide-
bridged dimer in which one of the Yb centers has been
oxidized to Yb(III).39 However, when freshly distilled
isocyanide was used, the simple adduct (Me5C5)2Yb-
(tBuNC)2 was isolated.36 Thus, it seems possible that
impurities, not the alkyl isocyanides themselves, may
be responsible for the formation of the products observed
by Evans. The isocyanide used in this work, 2,6-
Me2C6H3NC, is a solid that is readiliy purified by
sublimation. Using this pure isocyanide gave simple
adducts without the formation of the oxidized metal-
locene cyanide compounds.
Con clu sion s
Use of the bulky cyclopentadienide [1,2,4-(Me3C)2-
C5H2]- yields base-free metallocenes of s and f metals
upon removal of THF solvent under reduced pressure.
Base-free metallocenes bearing this ligand are crystal-
lographically problematic, presumably because the bulky
ligand leads to a molecule that is almost spherical, so
the packing is not well-ordered. The only exception is
the magnesocene, in which the extremely small metal
leads to a tight packing arrangement of the ligands. In
the solid state the base-free compounds are isostruc-
tural, with eclipsed cyclopentadienide rings in a slightly
bent metallocene; two tert-butyl groups lie above one
another at the open side of the metallocene wedge.
Enough space remains in the coordination environment
of Sm, Sr, Yb, and Ba to coordinate one more ligand,
but this is weakly bound compared with adducts of
the corresponding decamethylmetallocenes. With an
additional ligand, different ring conformations are
observed in the molecular structures; both staggered
and eclipsed, with two sets of eclipsed tert-butyl groups,
geometries have been found. The steric bulk of the
[1,2,4-(Me3C)2C5H2]- ligand is starkly illustrated by the
ytterbocene, which binds only a single isocyanide ligand,
in contrast to less-substituted ytterbocenes, which
always bind two.
1
3
ring CH, J C,H ) 158 Hz, J C,H ) 5 Hz ), 34.5 (q, 12C, CH3 of
1
(CMe3)2, J C,H ) 125 Hz), 33.8 (m, 4C, (CMe3)2), 33.8 (q, 6C,
1
CH3, J C,H ) 125 Hz), 32.2 (m, 2C, CMe3). EI-MS: m/z 506
(81, M+), 273 (100, M+ - C5H2(CMe3)3), 234 (48, C5H2(CMe3)3),
each with correct isotope pattern.
[1,2,4-(Me3C)3C5H2]2Sr . A 1.18 g (4.33 mmol) sample of
[1,2,4-(Me3C)3C5H2]K was stirred with 0.70 g (2.05 mmol) of
SrI2 in THF (25 mL) overnight at room temperature. Workup
similar to that for the calcium analogue led to the isolation of
0.84 g of the white product (1.52 mmol, 74%). Mp: 162 °C.
Anal. Calcd for C34H58Sr: C, 73.6; H, 10.5. Found: C, 72.2; H,
1
10.7. H NMR: δ 5.96 (4H), 1.43 (36H), 1.39 (18H). 13C NMR:
δ 133.0 (m, 2C, ring C(CMe3)), 129.9 (m, 4C, ring C(CMe3)),
1
3
107.2 (dd, 4C, ring CH, J C,H ) 155 Hz, J C,H ) 7 Hz), 34.5 (q,
1
12 C, CH3 of (CMe3)2, J C,H ) 125 Hz), 33.6 (m, 4C, (CMe3)2),
1
33.2 (q, 6C, CH3, J C,H ) 125 Hz), 32.1 (m, 2C, CMe3). EI-MS:
m/z 554 (17, M+), 321 (100, M+ - C5H2(CMe3)3), 234 (32, C5H2-
(CMe3)3)), each with correct isotope pattern.
[1,2,4-(Me3C)3C5H2]2Sr (THF ). Crystallization of [1,2,4-
(Me3C)3C5H2]2Sr from a minimum volume of THF leads to
isolation of the colorless mono-THF adduct. Upon heating in
a melting point capillary, THF is lost and the compound melts
at the same temperature as the base-free adduct (162 °C).
Anal. Calcd for C38H64SrO: C, 72.8; H, 10.6. Found: C, 70.7;
H, 10.7. 1H NMR: δ 6.01 (4H), 3.59 (m, 4H, O(CH2CH2)2), 1.56
(36H), 1.41 (m, 4H, O(CH2CH2)2), 1.34 (18H). 13C{1H} NMR:
δ 130.6 (2C, ring C(CMe3)), 130.0 (4C, ring C(CMe3)2), 106.5
(4C, ring CH), 68.3 (2C, O(CH2CH2)2), 35.1 (12 C, CH3 of
(CMe3)2), 33.8 (4C, (CMe3)2), 33.2 (6C, CH3), 32.2 (2C, CMe3),
25.6 (2C, O(CH2CH2)2).
Exp er im en ta l Section
All reactions and product manipulations were carried out
under dry nitrogen using standard Schlenk and drybox
techniques. Dry, oxygen-free solvents were employed through-
out. The elemental analyses and mass spectra were performed
at the analytical facility at the University of Kaiserslautern.
The following compounds were prepared as previously de-
scribed: NaNH2,40 SmI2,41 YbI2,11 1,2,4-Me3C5H3.42 The alka-
[1,2,4-(Me3C)3C5H2]2Ba . In a manner similar to the syn-
thesis of the calcium analogue, 0.52 g (2.03 mmol) of [1,2,4-
(Me3C)3C5H2]Na was stirred with 0.39 g (1.00 mmol) of BaI2
in THF overnight at room temperature. An analogous workup
led to 0.60 g (0.99 mmol, 99%) of the white product. Mp: 115
°C. Anal. Calcd for C34H58Ba: C, 67.6; H, 9.68. Found: C, 67.6;
H, 9.63. 1H NMR: δ 5.85 (4H), 1.42 (36H), 1.36 (18H). 13C
NMR: δ 132.9 (m, 2C, ring C(CMe3)), 130.4 (m, 4C, ring
(38) Evans, W. J .; Drummond, D. K. Organometallics 1988, 7, 797-
802.
1
3
C(CMe3)), 108.5 (dd, 4C, ring CH, J C,H ) 156 Hz, J C,H ) 8
(39) Schultz, M. Ph.D. Thesis, University of California, Berkeley,
1
2000.
Hz), 34.5 (q, 12 C, CH3 of (CMe3)2, J C,H ) 124 Hz), 33.5 (m,
(40) Greenlee, K. W.; Henne, A. L. Inorg. Synth. 1946, 2, 128-135.
(41) Namy, J . L.; Girard, P.; Kagan, H. B. Nouv. J . Chem. 1981, 5,
1
4C, (CMe3)2), 33.0 (q, 6C, CH3, J C,H ) 124 Hz), 32.0 (m, 2C,
479-484.
(42) Dehmlow, E. V.; Bollmann, C. Z. Naturforsch. 1993, 48b, 457-
(43) Sheldrick, G. M. SADABS; Siemens Industrial Automation,
Inc.: Madison, WI, 1996.
460.

3146 Organometallics, Vol. 21, No. 15, 2002
Weber et al.
CMe3). EI-MS: m/z 604 (4, M+), 371 (54, M+ - C5H2(CMe3)3),
234 (10, C5H2(CMe3)3), 57 (100, CMe3), each with correct
isotope pattern.
in THF. Workup analogous to the calcium analogue led to the
collection of 0.89 g (1.39 mmol, 76%) of the dark green-brown
product. Mp: 165 °C. Anal. Calcd for C34H58Yb: C, 63.8; H, 9.13.
Found: C, 63.0; H, 9.14. The NMR data were in agreement
with those reported previously.6
[1,2,4-(Me3C)3C5H2]2Ba (2,6-Me2C6H3NC). Two equivalents
of 2,6-Me2C6H3NC was stirred with the base-free compound
in the minimum volume of toluene for 2 h at room tempera-
ture. Colorless crystals were collected after reducing the
volume of the solution and cooling to -20 °C. Single crystals
suitable for X-ray diffraction were grown from pentane at
[1,2,4-(Me3C)3C5H2]2Yb(2,6-Me2C6H3NC). A 0.32 g (0.50
mmol) sample of the base-free [1,2,4-(Me3C)3C5H2]2Yb was
stirred in toluene with 0.13 g (0.99 mmol) of the isocyanide.
Cooling to -35 °C led to the formation of dark blue crystals,
yield 0.30 g (0.35 mmol, 70%). The compound did not melt,
but a colorless liquid appeared at 65 °C (presumably free
isocyanide) and the solid did not melt on further heating. The
1H NMR spectrum indicates that no toluene is present after
the product was dried under vacuum, although a toluene of
crystallization is observed in the crystal structure, the crystal
for which was mounted without exposure to vacuum. 1H
3
ambient temperature. 1H NMR: δ 6.74 (t, 1H, p-C6H3, J H,H
) 7.80 Hz), 6.57 (d, 2H, m-C6H3, 3J H,H ) 7.56 Hz), 6.10 (s, 4H,
ring H), 2.15 (s, 6H, (CH3)2C6H3NC), 1.59 (s, 36H, CH3 of
(CMe3)2), 1.41 (s, 18H, CH3 of (CMe3)).
[1,2,4-(Me3C)3C5H2]2Sm .6 Samarium iodide was generated
in situ by stirring 0.55 g (3.66 mmol) of Sm with 0.80 g (2.84
mmol) of ICH2CH2I in THF for 2 days. To the green slurry
was added 1.60 g (6.24 mmol) of Na(C5H2(CMe3)3), and the
mixture was stirred for a further day. A workup similar to
that of the calcium analogue led to the isolation of purple
crystals. Yield: 1.39 g (2.25 mmol, 79%), mp 125 °C. Anal.
Calcd for C34H58Sm: C 66.2; H 9.47. Found: C 66.2; H 9.81.
1H NMR: δ 10.59 (18H, ν1/2 ) 75 Hz), 10.15 (4H, ν1/2 ) 45
Hz), 4.72 (36H, ν1/2 ) 67 Hz). EI-MS: m/z 618 (75, M+), 385
(29, M+ - C5H2(CMe3)3), 234 (12, C5H2(CMe3)3), 57 (100, CMe3),
each with correct isotope pattern.
3
NMR: δ 6.77 (t, 1H, p-C6H3, J H,H ) 7.42 Hz), 6.60 (d, 2H,
3
m-C6H3, J H,H ) 7.45 Hz), 6.27 (s, 4H, ring H), 2.20 (s, 6H,
(CH3)2C6H3NC), 1.66 (36H), 1.41 (18H).
Cr ysta llogr a p h ic Stu d ies. Details of the crystallographic
studies are included in the Supporting Information. Selected
crystal data and data collection parameters are in Tables 3
and 5, while selected bond lengths and angles are in Tables 2
and 4.
[1,2,4-(Me3C)3C5H2]2Sm (THF ). Crystallization of the base-
free compound at -20 °C from a minimum of THF led to the
isolation of the THF adduct. 1H NMR: δ 14.16 (18H, ν1/2 ) 29
Ack n ow led gm en t. This work was partially sup-
ported by the Director, Office of Energy Research, Office
of Basic Energy Sciences, Chemical Sciences Division
of the U.S. Department of Energy under Contract
No. DE-AC03-76SF00098. We thank the Alexander
von Humboldt Foundation (M.S.) and the Deutsche
Forschungsgesellschaft (F.W.) for fellowships, and
Dr. Fred Hollander, Dr. Allen Oliver, Dr. Gotthelf
Wolmersha¨user, Mr. Seung Uk Son, and Prof. Young
Keun Chung for assistance with the crystallography.
Hz), 5.21 (36H, ν1/2 ) 15 Hz), 2.15 (s, 4H, O(CH2CH2)2, ν1/2
)
64 Hz), -0.79 (s, 4H, O(CH2CH2)2, ν1/2 ) 18 Hz). The cyclo-
pentadienide ring protons were not observed.
[1,2,4-(Me3C)3C5H2]2Sm (2,6-Me2C6H3NC). A 0.40 g (0.65
mmol) sample of the base-free (C5H2(CMe3)3)2Sm was stirred
with 0.17 g (1.30 mmol) of 2,6-Me2C6H3NC in toluene. Cooling
to -35 °C led to the isolation of dark green-blue crystals, 0.37
g (0.44 mmol, 68%) yield. The 1H NMR spectrum indicates that
no toluene is present after the product was dried under
vacuum, although a toluene of crystallization is observed in
the crystal structure, the crystal for which was mounted
Su p p or tin g In for m a tion Ava ila ble: Details of crystal-
lographic studies, atomic positions and anisotropic thermal
parameters, tables of bond lengths and angles, and an ORTEP
diagram showing the atom labeling scheme for each structure
are available. This material is available free of charge via the
Internet at http://pubs.acs.org. Structure factor tables are
available from the authors.
without exposure to vacuum. 1H NMR: δ 10.38 (18H, ν1/2
)
17 Hz), 7.48 (br, 2H, m-C6H3), 5.98 (br, 1H, p-C6H3), 5.80 (36H,
ν1/2 ) 19 Hz), 3.80 (s, 6H, (CH3)2C6H3NC, ν1/2 ) 14 Hz). The
cyclopentadienide ring protons were not observed.
[1,2,4-(Me3C)3C5H2]2Yb.6 A 0.95 g (3.71 mmol) sample of
Na(C5H2(CMe3)3) was stirred with 0.78 g (1.83 mmol) of YbI2
OM0108705