Pentabenzylcyclopentadienides of lithium

Article abstract of DOI:10.1021/om960375m

(Pentabenzylcyclopentadienyl)lithium (Li(η⁵-C₅Bz₅)) has been prepared by lithiation of pentabenzylcyclopentadiene with tert-butyllithium in toluene or benzene. This compound, which is the nonisolated starting material for most of the known pentabenzylcyclopentadienides, has been characterized by NMR and mass spectroscopy. The molecular structure of the benzene adduct of dimeric Li(η⁵-C₅Bz₅) has been determined by X-ray diffraction studies. A triple-decker-like structure, (η⁵-C₅Bz₅)Li(η⁵-C ₅Bz₅)Li(η²-C₆H₆), results. The reaction of Li(η⁵-C₅Bz₅) with Al(η⁵-C₅Me₅) leads to Al(η⁵-C₅Bz₅) and Li(η⁵-C₅Me₅), with [Li(η⁵-C₅Bz₅)₂][Al(η ⁵-C₅Me₅)₂] as a side product. The molecular structure of the last-named compound consists of the separated sandwich ions [Li(η⁵-C₅Bz₅]₂] and [Al(η⁵-C₅Me₅)₂]⁺.

Full text of DOI:10.1021/om960375m

4702
Organometallics 1996, 15, 4702-4706
P en ta ben zylcyclop en ta d ien id es of Lith iu m
Carsten Dohmeier, Elke Baum, Achim Ecker, Ralf Ko¨ppe, and
Hansgeorg Schno¨ckel*
Institute for Inorganic Chemistry, University of Karlsruhe, Box 6980,
D-76128 Karlsruhe, Germany
Received May 17, 1996X
(Pentabenzylcyclopentadienyl)lithium (Li(η5-C5Bz5)) has been prepared by lithiation of
pentabenzylcyclopentadiene with tert-butyllithium in toluene or benzene. This compound,
which is the nonisolated starting material for most of the known pentabenzylcyclopentadi-
enides, has been characterized by NMR and mass spectroscopy. The molecular structure of
the benzene adduct of dimeric Li(η5-C5Bz5) has been determined by X-ray diffraction studies.
A triple-decker-like structure, (η5-C5Bz5)Li(η5-C5Bz5)Li(η2-C6H6), results. The reaction of
Li(η5-C5Bz5) with Al(η5-C5Me5) leads to Al(η5-C5Bz5) and Li(η5-C5Me5), with [Li(η5-C5Bz5)2][Al(η5-
C5Me5)2] as a side product. The molecular structure of the last-named compound consists
of the separated sandwich ions [Li(η5-C5Bz5)2]- and [Al(η5-C5Me5)2]+.
In tr od u ction
The cyclopentadienide anion and its derivatives are
some of the most common ligands in main-group and
transition-metal chemistry. Replacement of the hydro-
gen atoms of the C5H5 ligand by bulky substituents
influences the physical and chemical properties of the
corresponding metal complexes.1 The better shielding
of the reactive metal atom by using alkyl, silyl, or aryl
groups allows the kinetic stabilization of thermody-
namically unstable compounds.1,2 Furthermore, the
increased electron donor ability of the substituted C5
ring alters the character of the metal-ligand bond
toward a higher degree of covalency.3 By modification
of the steric and electronic requirements the properties
of the complexes obtained can be tuned; e.g., a much
better solubility of bulky complexes in nonpolar aprotic
solvents is achieved. In addition, the variation of
intermolecular interactions caused by different substit-
uents often leads to new and unusual molecular struc-
tures.
These influences are reflected in the solid-state
structures of indium(I) and thallium(I) cyclopentadi-
enyls. While the C5H5 complexes form highly ionic
polymeric zigzag chains containing alternating rings
and metal atoms,4 the perbenzylated derivatives (M(η5-
C5Bz5)) are mainly covalent molecules arranged in a
dimeric fashion with weak metal‚‚‚metal interac-
tions.1,5,6 (Figure 1, left). A second modification of Tl(η5-
F igu r e 1. Schematic diagrams: (left) b ) In, Tl; (right)
b ) Tl.
C5Bz5), which is thermodynamically less stable, consists
of almost linear chains of monomeric Tl(η5-C5Bz5)
molecules7 (Figure 1, right).
The crystal structure of In(η5-C5Bz5) and those of both
modifications of Tl(η5-C5Bz5) seem to be mainly caused
by packing effects of the bulky ligands.5-7 Calculations
concerning the M(η5-C5Bz5) dimers show that weak
metal‚‚‚metal bonding interactions may be responsible
for an additional energy gain, but these interactions are
not necessarily structure-determining.6
It is of general interest to know what extent other
MI(η5-C5Bz5) structures are influenced by packing effects
of the C5Bz5 ligands. In this respect the examination
of compounds with and without M-M interactions is of
interest. Compounds without M-M interactions are
expected for alkali metals whereas M-M bonds are
predicted to be present in the complexes of BI, AlI, and
GaI in contrast to those shown by their heavier homo-
logues InI and TlI. For the alkali metals [K(η5-C5Bz5)-
(THF)3] is, as far as we know, the only example whose
molecular structure has been reported.8 However,
because of the coordinated THF molecules this complex
is not very helpful for a comparison in this field. So far
no structural data for C5Bz5 compounds are known for
BI, AlI and GaI.
X Abstract published in Advance ACS Abstracts, September 15, 1996.
(1) J aniak, C.; Schumann, H. Adv. Organomet. Chem. 1991, 33,
291-393 and references therein.
(2) For example, main-group-metal complexes containing the metal
in formally low oxidation states: (a) J utzi, P.; Kanne, D.; Kru¨ger, C.
Angew. Chem., Int. Ed. Engl. 1986, 25, 164-165. (b) Dohmeier, C.;
Robl, C.; Tacke, M.; Schno¨ckel, H. Angew. Chem., Int. Ed. Engl. 1991,
30, 564-565.
(3) See for example: J utzi, P.; Leffers, W. J . Chem. Soc., Chem.
Commun. 1985, 1735-1736.
(4) Frasson, E.; Menegus, F.; Panattoni, C. Nature 1963, 199, 1087-
1089.
(5) (a) Schumann, H.; J aniak, C.; Go¨rlitz, F.; Loebel, J .; Dietrich,
A. J . Organomet. Chem. 1989, 363, 243-251. (b) Schumann, H.;
J aniak, C.; Pickardt, J .; Bo¨rner, U. Angew. Chem., Int. Ed. Engl. 1987,
26, 789-790.
(6) J aniak, H.; Hoffmann, R. J . Am. Chem. Soc. 1990, 112, 5924-
5946.
In this paper we describe the properties and the
crystal structure of dimeric Li(η5-C5Bz5). In addition
the synthesis of Al(η5-C5Bz5) is reported. As a side
product, the double-sandwich compound [Li(η5-
C5Bz5)2][Al(η5-C5Me5)2] can be isolated.
(7) Schumann, H.; J aniak, C.; Khan, M. A.; Zuckermann, J . J . J .
Organomet. Chem. 1988, 354, 7-13.
(8) Lorberth, J .; Shin, S.-H.; Wocadlo, S.; Massa, W. Angew. Chem..,
Int. Ed. Engl. 1989, 28, 735-736.
S0276-7333(96)00375-5 CCC: $12.00 © 1996 American Chemical Society

Pentabenzylcyclopentadienides of Lithium
Organometallics, Vol. 15, No. 22, 1996 4703
F igu r e 2. Schematic representation of dimeric [Li2{C24H14
(SiMe3)2}]‚2THF.9
-
Resu lts a n d Discu ssion
[{Li(η5-C5Bz5)}2(C6D6)]. Li(η5-C5Bz5) is easily avail-
able by lithiation of pentabenzylcyclopentadiene with
tert-butyllithium in toluene. Single crystals were ob-
tained by recrystallization of the crude product from
benzene or, as presented here, from perdeuteriobenzene.
[{Li(η5-C5Bz5)}2(C6D6)](C6D6)2 crystallizes in the mono-
clinic space group P21/c. The X-ray structure determi-
nation demonstrates that the asymmetric unit contains
the benzene adduct of dimeric Li(η5-C5Bz5) and of two
additional benzene molecules, separated by normal van
der Waals distances.
F igu r e 3. Molecular structure of [{Li(η5-C5Bz5)}2(C6D6)]
(H and D atoms omitted for clarity). Li to ring centroid
distances and the closest Li-benzene distances are given
in the schematic representation. Li-C distances (Å): Li1
to ring A, 2.11(2)-2.19(2) (average 2.15); Li1 to ring B,
2.29(2)-2.42(2) (average 2.36); Li2 to ring B, 2.23(2)-
2.27(2) (average 2.25).
As expected for mainly ionic compounds, Li(η5-C5Bz5)
dimerizes in a head-to-tail manner, whereas in the
above-mentioned dimeric η5-C5Bz5 compounds of InI and
TlI the metal atoms point towards each other. Li(η5-
C5Bz5) exhibits two differently coordinated Li atoms,
very similar to the dimeric [Li2{C24H14(SiMe3)2}]‚2THF,
which contains two Li atoms sandwiched between the
Cp rings of two polycyclic dianions and two further THF-
solvated Li atoms situated on the outside of the sand-
wich units9 (Figure 2).
In [{Li(η5-C5Bz5)}2(C6D6)](C6D6)2 Li1 is sandwiched
between two η5-bonded C5Bz5 ligands (A and B) with
two significantly different Li1 to ring centroid distances
(1.78 and 2.03 Å; see Figure 3).10 The first one, which
is among the shortest known for cyclopentadienyl
complexes of lithium, is very close to the values observed
for the adducts of Li(C5H2(SiMe3)3) with monodentate
bases (e.g. 1.79 Å for Li(C5H2(SiMe3)3)-quinuclidine).11
The longer one is comparable to the corresponding
distances of [Li(C5R5)2]- anions (e.g. 2.01 Å for
[Li(C5H5)2]-).12a
The “external” Li2 atom is η5-bonded to ring B with
a Li2 to ring centroid distance of 1.90 Å. The coordina-
tion sphere of Li2 is completed by a benzene molecule,
which encloses an angle of 34.9° with the plane of ring
B.
The closest Li-C distances between Li2 and benzene,
2.50 and 2.63 Å, are in the typical range for weak Li-
arene interactions,13 while the closest Li-phenyl dis-
tances are much longer (3.32 Å). In the crystal the
[{Li(η5-C5Bz5)}2(C6D6)] units are arranged in columns.
Interestingly, very short C‚‚‚C distances between ben-
zene and phenyl C atoms of the two neighboring [{Li(η5-
C5Bz5)}2(C6D6)] units are observed (Figure 4). The
values, 3.48, 3.49, and 3.54 Å, are very close to the
shortest C‚‚‚C distances found in a high-pressure form
of solid benzene (C‚‚‚C ) 3.50 Å).14 Therefore, on the
one hand it might be possible that closer Li2-benzene
contacts are prevented by steric reasons. On the other
hand, nearly identical C‚‚‚C distances to both neighbor-
ing [{Li(η5-C5Bz5)}2] units suggest that mainly attrac-
tive van der Waals forces between benzene and phenyl
rings causes the column structure of [{Li(η5-C5Bz5)}2-
(C6D6)], with Li-benzene interactions being less impor-
tant.
It should be mentioned that solid [Fe(η5-C5Bz5)2](C6H5-
Me)2 forms the same column structure as [{Li(η5-
C5Bz5)}2(C6D6)](C6D6)2, with the exception that the
columns contain no arene molecules between the sand-
which molecules.15 This may support the significance
of Li-benzene interactions in [{Li(η5-C5Bz5)}2(C6D6)].
The observed dimeric structure of Li(η5-C5Bz5) may be
regarded as a special case of supramolecular chemistry
in which one molecule possesses host as well as guest
character.16
(13) Similar values have been reported previously for intramolecular
interactions of Li with ipso carbon atoms of mesityl or phenyl groups
in [LiC(SiMe2Ph)3(THF)] and [{Li(C6H3Mes2)}2]: Earborn, C.; Hitch-
cock, P. B.; Smith, J . D.; Sullivan, A. C. J . Chem. Soc., Chem. Commun.
1983, 1390-1391. Ruhlandt-Senge, K.; Ellison, J . J .; Wehmschulte,
R. J .; Pauer, F.; Power, P. P. J . Am. Chem. Soc. 1993, 115, 11353-
11357.
(14) Piermarini, G. J .; Mighell, A. D.; Weir, C. E.; Block, S. Science
1969, 165, 1250-1255.
(9) Malaba, D.; Chen, L.; Tessier, C. A.; Youngs, W. J . Organo-
metallics 1992, 11, 1007-1009.
(15) The packing of sandwich molecules and arene molecules
between the columns in solid [Fe(η5-C5Bz5)2](C6H5Me)2 is almost exactly
identical with that in [{Li(η5-C5Bz5)}2(C6D6)](C6D6)2, but the Fe(η5-
C5Bz5)2 molecules are stacked in columns without an arene between
them. On the basis of this fact and on shorter metal-C distances of
Fe(η5-C5Bz5)2 the lattice constant a of the Fe compound is smaller,
whereas the lattice constants b and c of the two compounds discussed
here are very similar. (Schumann, H.; J aniak, C.; Ko¨hn, R. D.; Loebel,
J .; Dietrich, A. J . Organomet. Chem. 1989, 365, 137-150).
(16) E.g.: (a) Cram, J . M. In Container Molecules and their Guests;
Stoddard, J . F., Ed.; The Royal Society of Chemistry: Cambridge, U.K.,
1994; Chapter 6, p 107. (b) Cram, D. J .; et al. J . Am. Chem. Soc. 1992,
114, 7748.
(10) For a comparison of distances of alkali-metal cyclopentadienyl
complexes, see for example: Stalke, D. Angew. Chem., Int. Ed. Engl.
1994, 33, 2168-2171.
(11) J utzi, P.; Schlu¨ter, E.; Pohl, S.; Saak, W. Chem. Ber. 1985, 118,
1959-1967. J utzi, P.; Leffers, W.; Pohl, S.; Saak, W. Chem. Ber. 1989,
122, 1449-1456.
(12) (a) Harder, S.; Prosenc, S. H. Angew. Chem., Int. Ed. Engl. 1994,
33, 1744-1745. (b) Wessel, J .; Lork, E.; Mews, R. Angew. Chem., Int.
Ed. Engl. 1995, 34, 2376-2377. (c) Zaegel, F.; Gallucci, J . C.; Meunier,
P.; Gautheron, B.; Sivik, M. R.; Paquette, L. A. J . Am. Chem. Soc. 1994,
116, 6466-6467.

4704 Organometallics, Vol. 15, No. 22, 1996
Dohmeier et al.
F igu r e 4. Left: unit cell of [{Li(η5-C5Bz5)}2(C6D6)]‚C6D6 viewed down the a axis. Right: side view of two columns of
[{Li(η5-C5Bz5)}2(C6D6)] molecules (a axis parallel to the plane of the paper); short benzene phenyl C‚‚‚C distances (see
text) are illustrated in the dashed area. Hydrogen atoms are omitted for clarity.
To gain a better insight into the bonding of dimeric
Li(η5-C5Bz5) and of the influences of benzene, we have
performed ab initio quantum-chemical calculations.17
The studies were restricted to unsubstituted Cp species,
since only trends need to be demonstrated. Dimeric
Li(η5-C5H5) with a C5v structure is formed from two
Li(η5-C5H5) molecules exothermically, with ∆ER ) -55
kJ mol-1 (eq 1).
(C6D6)], and the calculated formation energy of (η2-
C6H6)Li(η5-C5H5) depicts the significance of Li-arene
interactions in [{Li(η5-C5Bz5)}2(C6D6)]. However, re-
garding the dimerization as the first step in polymeri-
zation, it seems to be more favorable for Li(η5-C5H5) to
form polymeric chains than neutral oligomeric arene
adducts.
A structure consisting of (theoretically) infinite chains
of stacked lithium cyclopentadienyl dipoles is known for
[Li(η5-C5H4(SiMe3))]n. Each Li is located almost sym-
metrically between two cyclopentadienyl rings (average
Li to ring centroid distance 1.967 Å). Obviously an
To estimate the energetic situation for the addition
of a benzene molecule, we have calculated η6- and η2-
analogous polymeric structure of Li(η5-C5Bz5) with
similar Li to ring centroid distances is not possible for
steric reasons. A chainlike structure of M(η5-C5Bz5) as
observed for the second modification of Tl(η5-C5Bz5)
affords a much larger separation of the cyclopentadienyl
rings and is therefore unfavorable for ionic compounds
with attractive forces between the Mδ-(C5Bz5)δ- dipoles.
The structure of Li(η5-C5H5) is unknown so far, but
due to its low solubility it is very likely that it is
polymeric in the solid state and that dimeric or mono-
meric neutral arene adducts are unfavorable. In con-
trast, the formation of anionic arene complexes of
alkaline cyclopentadienides such as [Li(C5H5)2]- 12 and
[Cs2(C5H5)3]- 18 has been demonstrated experimentally.
This is in line with our ab initio calculations: the
formation of [Li(η5-C5H5)2]- from the free [C5H5]- anion
and Li(η5-C5H5) is highly exothermic (∆ER ) -172 kJ
mol-1; D5h symmetry). Our calculated Li to ring cen-
troid distance for [Li(η5-C5H5)2]- is 2.09 Å (d(Li-C) )
C6H6 adducts of Li(η5-C5H5) (eq 2).
The η6-C6H6 adduct of Li(η5-C5H5) is only 1.5 kJ mol-1
more stable than an η2-C6H6 adduct, with Li-C dis-
tances of 2.59 Å between Li and benzene.
The calculated Li-Cbenzene distances (η2-C6H6),19 which
are close to the experimental values of [{Li(η5-C5Bz5)}2-
(17) (a) Paquette, L. A.; Bauer, W.; Sivik, M. R.; Bu¨hl, M.; Feigel,
M.; Schleyer, P. v. R. J . Am. Chem. Soc. 1990, 112, 8776-8789. (b)
Bauer, W.; O’Doherty, G. A.; Schleyer, P. v. R.; Paquette, L. A. J . Am.
Chem. Soc. 1991, 113, 7093-7100. The semiempirical calculations
presented in (a) and (b) reveal the same trend as our ab initio
calculations.
(18) Harder, S.; Prosenc, M. H. Angew. Chem., Int. Ed. Engl. 1996,
35, 97-99.
(19) It should be mentioned that in general the calculated Li-C
distances are about 2-4% longer than those determined experimen-
tally. Furthermore, the experimentally observed Li-C distances are
shorter if substituted cyclopentadienyls are involved. This trend is in
accord with quantum-chemical calculations: [Li(η5-C5H5)2]-, d(Li-C)
) 2.406 Å; [Li(η5-C5Me5)2]-, d(Li-C) ) 2.374 Å.
(20) Evans, W. J .; Boyle, T. J .; Ziller, J . W. Organometallics 1992,
11, 3903-3907.

Pentabenzylcyclopentadienides of Lithium
Organometallics, Vol. 15, No. 22, 1996 4705
2.41 Å19). Especially if proper cations are present,
anionic arene complexes should be observed instead of
polymers. The formation of small neutral arene adducts
of alkaline cyclopentadienides such as [{Li(η5-C5Bz5)}2-
(C6D6)] seems to be restricted to special conditions in
which (i) the substituents of the ligand are bulky enough
to avoid the formation of polymers, (ii) no proper cation
can be generated (see below), and (iii) no strong donor
molecules are present, since they can stabilize mono-
meric species such as Li(C5H2(SiMe3)3)‚THF10,11
structural studies.22 [Li(η5-C5Bz5)2][Al(η5-C5Me5)2] is
insoluble in all common solvents.
2Li(C5Bz5) + 3Al(C5Me5) f Al0 + [Al(C5Me5)2]+ +
[Li(C5Bz5)2]- + Li(C5Me5) (4)
The crystallographic unit cell of [Li(η5-C5Bz5)2][Al(η5-
C5Me5)2] contains three almost identical entities of
[Al(η5-C5Me5)2]+ and [Li(η5-C5Bz5)2]- which are sepa-
rated by normal van der Waals distances. One of the
[Li(η5-C5Bz5)2]- species resides on a crystallographic
center of inversion.22 In addition, toluene molecules are
present. Since the experimental data are not sufficient
to solve the structure properly, we will not present
detailed structural data, besides the comments that the
structure of the cation [Al(η5-C5Me5)2]+ is comparable
to our former results23 and that the anion [Li(η5-
C5Bz5)2]- exhibits a slightly shorter Li to ring center
distance (1.90 Å) than was obtained for [Li(η5-C5H5)2]-
experimentally (2.01 Å).12 The distance between both
C5Bz5 rings is in the same range as in [{Li(η5-C5Bz5)}2-
(C6D6)] (3.80 Å; cf. Figure 3). In contrast to [{Li(η5-
C5Bz5)}2(C6D6)], the benzyl groups of [Li(η5-C5Bz5)2]- are
not oriented symmetrically.
In order to investigate the nature of Li(C5Bz5) in
solution, 7Li NMR spectra were recorded. When
Li(C5Bz5) is dissolved in benzene or toluene, only one
7Li NMR signal at -10.3 ppm is observed, indicating
either the presence of monomeric species or a fast
equilibrium between monomeric and dimeric species. In
view of the structure of solid Li(η5-C5Bz5) higher ag-
gregates seem to be unrealistic. Since the observed
shifts depend neither on the temperature (+60 to -70
°C for toluene) nor on the concentration (0.2 M solution,
melt) monomeric species are more likely.21 In OEt2 (0.2
7
M solution) again only one Li NMR signal is observed
(-7.6 ppm between +25 and -110 °C). The stronger
shielding of Li(η5-C5Bz5) in arene solutions might be a
result of the ring current of coordinated arene
molecules.17a
Exp er im en ta l Section
In THF below -10 °C a single resonance at -7.2 ppm
is accompanied by two new signals at -0.2 and -12.1
ppm. These results can be interpreted by the presence
of an equilibrium of THF-solvated, monomeric Li(η5-
C5Bz5) with Li+‚nTHF/[Li(η5-C5Bz5)2]-. Equilibrium
reactions of this type have been extensively studied for
Li(C5H5) and lithium isodicyclopentadienide.17 Further
cooling of the Li(η5-C5Bz5)/THF samples causes two
additional signals at -1.5 and -7.9 ppm, indicating a
more complex situation perhaps involving dimeric spe-
cies of Li(η5-C5Bz5).
Gen er a l Com m en ts. All operations were carried out
under dry nitrogen. Solvents were dried by conventional
procedures. NMR spectra were recorded by means of a Bruker
AC 250 (1H, 13C) or a Bruker AMX 300 spectrometer (7Li, 27Al).
Chemical shifts are reported in δ units (ppm) referenced to
C6D5H (7.15 ppm, 1H), C6D6 (128.0 ppm, 13C), external LiCl/
D2O (1 M, 0 ppm, 7Li) and external [Al(H2O)6]3+ (0.5 M, 0 ppm,
27Al). Mass spectra were obtained by electron impact. Melting
points are observed in sealed capillaries under vacuum and
are uncorrected. t-BuLi was obtained from Merck as a 1.6 M
hexane solution and used as purchased.
Syn th esis of C5Bz5H. The compound was synthesized by
a modification of the method by Hirsch and Bailey.24 The
solvents diisopropylbenzene and benzene were replaced by
isopropylbenzene and toluene, respectively. Originally the
final purification was carried out by distillation of the crude
product. Instead of this procedure, the resulting viscous
orange crude product was suspended with hot methanol and
treated with ultrasonic waves to dissolve impurities. After
cooling of the suspension to room temperature, the methanol
was decanted. This process was repeated several times until
removal of the last solvent by vacuum (10-3 mbar) left colorless
C5Bz5H (63% yield). Mp: 72-74 °C. MS (m/ e, ion, relative
intensity): 516, [M]+, 39%; 425, [M - Bz]+, 18%;, 91, [Bz]+,
100%.
Rea ction of Li(C5Bz5) w ith Al(C5Me5). Using the
lattice energy of Li(C5Me5), which is insoluble in aro-
matic hydrocarbons, as a driving force, the synthesis of
Al(C5Bz5) was attempted according to eq 3.
Li(C5Bz5) + Al(C5Me5) f Al(C5Bz5) + Li(C5Me5)V
(3)
The products of the reaction were separated by virtue
of their differing solubility in toluene, the soluble
compounds being Al(C5Bz5) and residual Li(C5Bz5).
Unfortunately, to date it has proved impossible to isolate
Al(C5Bz5) as a pure compound. However, NMR and
mass spectroscopic data give evidence that Al(C5Bz5) is
monomeric both in solution and in the gas phase.
Syn th esis of Li(C5Bz5). C5Bz5H was dissolved in toluene
to give a 0.2 M solution. An equimolar amount of a hexane
solution of t-BuLi was added (the reaction flask was connected
to a mercury overpressure valve). The slow metalation was
Li(η5-C5Me5) and the side products [Li(η5-C5Bz5)2][Al(η5-
C5Me5)2] and aluminum metal (maximum 3% of the
total Al content) are insoluble in toluene. [Li(η5-
C5Bz5)2][Al(η5-C5Me5)2], presumably formed as shown
in eq 4, is obtained as a colorless crystalline compound.
The identity of this product was confirmed by X-ray
(22) The experimental data obtained are not sufficient to solve the
structure properly. Data for the X-ray structural study of
[Li(η5-C5Bz5)2][Al(η5-C5Me5)2]: Mr ) 1335.7, triclinic, space group P1h,
a ) 13.712(10) Å, b ) 16.443(10) Å, c ) 29.86(3) Å, R ) 98.46(6)°, â )
99.63(7)°, γ ) 98.31(6)°, Z ) 3, V ) 6466(9) Å3, F(calcd) ) 1.100 g cm-3
,
F(000) ) 2298, µ ) 0.070 mm-1, θ range 1.53-22.50°, 15 857 unique
reflections, 6436 observed reflections, 1126 parameters, GOF ) 1.036,
R1 (I > 2σ(I)) ) 0.0951, wR2 ) 0.2881, w ) σ-2|F|, largest difference
peak 1.068 e Å-3, deepest difference peak -0.611 e Å-3
. While one
(21) The degree of aggregation (n) was also measured by cryoscopic
and osmometric molecular weight determinations in benzene (n ) 1.04
for a 0.013 M solution (cryoscopically), n ) 1.12 for a 0.036 M solution
(cryoscopically), and n ) 1.08 for a 0.04 M solution (osmometrically)).
These results confirm monomeric Li(η5-C5Bz5) to be the dominant
species over the measured concentration range.
[Al(η5-C5Me5)2]+ is normal, the second is split over two positions
(occupation number 0.25). Whether this is a disorder or an overstruc-
ture problem could not be decided because of the low crystal quality.
(23) Dohmeier, C.; Schno¨ckel, H.; Robl, C.; Schneider, U.; Ahlrichs,
R. Angew. Chem., Int. Ed. Engl. 1993, 32, 1655-1657.
(24) Hirsch, S. S.; Bailey, W. J . J . Org. Chem. 1978, 43, 4090-4094.

4706 Organometallics, Vol. 15, No. 22, 1996
Dohmeier et al.
completed after 7 days at 25 °C, as determined by 1H NMR
spectroscopy. During this period the originally colorless
mixture slowly turned red. The solution was concentrated to
ca. 30% of the original volume. After addition of a small
amount of benzene the product was obtained as a colorless
powder (75% yield). Single crystals were obtained by recrys-
tallization from benzene. The crystal used for the collection
of the X-ray diffraction data was obtained from a C6D6 solution.
Mp: 78-79 °C. MS (m/ e, ion, relative intensity; 80 °C): 84,
[C6D6]+, 100%. MS (m/ e, ion, relative intensity; 250 °C): 522,
[Li(C5Bz5)]+, 70%; 445, [Li(C5Bz5) - Ph]+, 22%; 91, [Bz]+, 100%;
84 [C6D6]+, 0%. 7Li NMR (0.2 M, C6D6): -10.3. 1H NMR (0.2
M, C6D6): 3.73 (s, 10H); 7.06 (m, 25H). 13C NMR (0.2 M,
C6D6): 32.4 (CH2); 113.9 (C5); 125.4 (Ph C4); 128.4, 128.8 (Ph
C2/3/5/6); 144.7 (Ph C1).
Syn th esis of Al(C5Bz5) a n d [Li(η5-C5Bz5)2][Al(η5-(C5-
Me5)2]. A 100 mg (0.62 mmol) amount of Al(η5-C5Me5)2b was
suspended in 20 mL of toluene. A solution containing 0.62
mmol of Li(η5-C5Bz5) in 8 mL of toluene was added, whereupon
slow precipitation of finely divided Li(η5-C5Me5) was observed.
Although the solution was heated to 50 °C, the metal exchange
is extremely slow (further heating leads to increased formation
of aluminum metal as a result of the disproportionation of AlI
species). After 12 days the toluene-insoluble products were
removed by filtration. The solid material consists of Li(η5-
C5Me5), trace amounts of Al0, and colorless single crystals of
[Li(η5-C5Bz5)2][Al(η5-C5Me5)2] (ca. 10 mg), which were used for
the X-ray diffraction studies.
Ta ble 1. Da ta for th e X-r a y Str u ctu r a l Stu d y of
[{Li(η5-C5Bz5)}2(C6D6)]‚2C6D6
formula
C98H70D18Li2
monoclinic
P21/c (No. 14)
1296.2
cryst syst
space group
fw
a, Å
b, Å
c, Å
14.018(9)
22.364(11)
24.087(14)
90
101.32(5)
90
R, deg
â, deg
γ, deg
Z
4
V, Å3
7404(7)
1.139
F(calcd), g cm-3
cryst dimens, mm
F(000)
0.5 × 0.3 × 0.3
2728
µ, mm-1
0.064
θ range, deg
scan width, deg
max scan time, s
no. of rflns collct
no. of unique rflns
no. of params
GOF
2.51-20.02
0.68, ω-scan
34 (per rfln)
6909
6909
901
0.963
wR2 (all data)
R1
0.1966
0.0606
+0.194
-0.181
largest diff peak, e Å-3
deepest diff peak, e Å-3
techniques. Hydrogen atoms were included in the refinement
at calculated positions. In [{Li(η5-C5Bz5)}2(C6D6)] all non-
hydrogen atoms were refined anisotropically; in [Li(η5-
C5Bz5)2][Al(η5-C5Me5)2] not all non-hydrogen atoms could be
refined anisotropically and the C5 and C6 rings were refined
as rigid groups.
Com p u ta tion a l Deta ils. Ab initio SCF calculations were
performed using basis sets of TZP-quality as implemented in
the TURBOMOLE26 program system. In order to save com-
putational time, the calculations were restricted to unsubsti-
tuted C5H5 systems.
The toluene-soluble products were isolated by removal of
the toluene under vacuum (10-3 mbar). Afterward the result-
ing light yellow oil was dissolved in pentane. Slow concentra-
tion of this solution afforded Al(η5-C5Bz5) as an off-white waxy
product. Spectroscopic studies of the product revealed small
amounts of Li(η5-C5Bz5) as an impurity. MS (m/ e, ion, relative
intensity; 160 °C): 542, [Al(C5Bz5)]+, 25%. 27Al NMR: -155
(w1/2 ) 1500 Hz). 1H NMR: 3.70 (s, 10H): 7.04 (m, 25H). 13C
NMR: 31.7 (CH2); 119.9 (C5); 126.2 (Ph C4); 128.6, 129.2 (Ph
C2/3/5/6); 141.2 (Ph C1).
X-r a y Da ta Collection a n d Solu tion a n d Refin em en t
of th e Str u ctu r e. The crystals were selected under nitrogen
in polyfluorinated polyethers. The oil-coated crystals were
manipulated onto a glass fiber and transferred to the cold
nitrogen stream of the Stoe Stadi IV diffractometer. The data
were collected at 210 K with Mo KR radiation (graphite
monochromated, λ ) 0.710 69 Å) under the conditions given
in Table 1. Calculations were carried out with the SHELX
program system. Atom form factors, including anomalous
scattering, were taken from ref 25. The structures were solved
by direct methods and refined by full-matrix least-squares
Ack n ow led gm en t. This work was supported by the
Deutsche Forschungsgemeinschaft and in part by the
Fonds der Chemischen Industrie.
Su p p or tin g In for m a tion Ava ila ble: Tables giving crys-
tal data and structure refinement details, positional and
thermal parameters, and bond distances and angles and an
additional ORTEP view of [{Li(η5-C5Bz5)}2(C6D6)]‚2C6D6 (13
pages). Ordering information is given on any current mast-
head page.
OM960375M
(25) International Tables for X-ray Crystallography; Kynoch Press:
Birmingham; U.K., 1974; Vol. IV.
(26) Ahlrichs, R.; Ba¨r, M.; Ha¨ser, M.; Horn, H.; Ko¨lmel, C. Chem.
Phys. Lett. 1989, 162, 165-169.