Synthesis, structure and solution dynamics of lithium salts of superbulky cyclopentadienyl ligands

Article abstract of DOI:10.1039/b302258g

The superbulky cyclopentadienes (CpAr₅)H (Ar^# = 3,5-Me₂C₆H₃ (1a); Ar* = 3,5-^tBu₂C₆H₃ (1b)) were prepared by the palladium-catalyzed reaction of metallocenes

Full text of DOI:10.1039/b302258g

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Synthesis, structure and solution dynamics of lithium salts of
superbulky cyclopentadienyl ligands
Garth R. Giesbrecht,*a John C. Gordon,b David L. Clarkc and Brian L. Scottb
a Nuclear Materials Technology Division, Los Alamos National Laboratory, Mail Stop J514,
Los Alamos, NM 87545. E-mail: garth@lanl.gov
b Chemistry Division, Los Alamos National Laboratory, Mail Stop J514, Los Alamos,
NM 87545
c Glenn T. Seaborg Institute for Transactinium Science, Los Alamos National Laboratory,
Mail Stop E500, Los Alamos, NM 87545
Received 26th February 2003, Accepted 6th May 2003
First published as an Advance Article on the web 27th May 2003
The superbulky cyclopentadienes (CpAr5)H (Ar# = 3,5-Me2C6H3 (1a); Ar* = 3,5-tBu2C6H3 (1b)) were prepared by the
palladium-catalyzed reaction of metallocenes with aryl bromides. NMR spectroscopy was consistent with Cs
symmetric solution structures with free rotation of the substituted aryl rings. The molecular structure of 1a is similar
to that previously determined for pentaphenylcyclopentadiene, with a planar Cp ring bisected by an approximate
mirror plane. Addition of BuLi to 1a,b yielded the lithium salts (CpAr#5)Li(THF)2 (2a) and (CpAr*5)Li (2b) in good
yield. 7Li NMR spectroscopy of 2a,b indicated more than one species in solution. Crystallization of 2a from toluene
revealed an infinite one-dimensional chain structure with alternating [(CpAr#5)2Li]Ϫ and [(THF)Li(CpAr#5)Li(CpAr# )-
5
Li(THF)]ϩ units; due to the sterics of the Cp rings, the lithium atom appears to be only loosely held in place with a
larger than usual Li–Cp distance. 7Li NMR spectroscopy suggested an equilibrium between the monomeric species
(CpAr# )Li(THF)x and the metallate [(CpAr#5)2Li][Li(THF)x] in C6D6 which was supported by variable temperature
5
and NOESY 7Li NMR experiments. Crystallization of 2a from THF/TMEDA yielded the solvent separated ion pair
[(CpAr# )2Li][Li(TMEDA)2] (3a). In this case, 7Li NMR spectroscopy pointed towards the presence of a single species
5
in solution.
members of this equilibrium by the addition of the appropriate
Introduction
Lewis base.11 In this manner, one can gain further insight into
The cyclopentadienide anion and its many variants are among
structural motifs postulated to occur in solution, but only
the most common ligands in organometallic chemistry.1
indirectly observed previously by methods such as NMR
Replacement of the hydrogen atoms of the C5H5 ligand with
spectroscopy. We present here the synthesis and characteriz-
alkyl, silyl or aryl groups provides the opportunity to system-
ation of lithium salts of two superbulky cyclopentadienyl
atically alter the electronic and steric properties of the ligand,
which directly influence the chemical properties of the resultant
metal complexes. A well known approach to the kinetic stabiliz-
ation of thermodynamically unstable compounds is the use of
ligands, the effect of the addition of different Lewis bases, and
the properties of these complexes in solution.
Results and discussion
large ancillary ligands. Unfortunately, the utilization of “super-
bulky” cyclopentadienyl derivatives has often been hampered
by arduous, multi-step synthetic procedures.2,3 Recent advances
in this area have made such target molecules more accessible4,5
and have already proven useful in the synthesis of new catalysts
for a number of enantioselective transformations.6
The structures of organolithium complexes in solution have
been the focus of much attention, due to the unique manner in
which the lithium center can interact with the organic moiety.
Extensive studies on enolates7 and lithium amide bases8,9 have
revealed a complex solution behavior, with numerous dynamic
processes and unusual solution structures being proposed. For
example, CpLi exists as a triple ion below Ϫ100 ЊC, with an
equilibrium between monomeric LiCp and a metallate-type
complex.10 The inherently fast rate of this exchange may be
slowed down by employing bulkier cyclopentadienyl groups.
Thus, with sterically encumbered ligands such as isodicyclo-
pentadienide,10 it is possible to structurally characterize both
Synthesis of (CpAr5)H (Ar# ؍ 3,5-Me2C6H3 (1a);
Ar* ؍ 3,5-tBu2C6H3 (1b))
The preparation of superbulky cyclopentadienyl derivatives
was accomplished according to the methodology developed by
Dyker and Muira via the multiple arylation of metallocenes
with aryl bromides in palladium-catalyzed reactions [eqn.
(1)].4,5 The dimethylphenyl derivative (CpAr# )H (Ar# = 3,5-
5
Me2C6H3) (1a) has been previously reported5 and has been
successfully utilized as a supporting ligand for a new class of
asymmetric catalysts.6 The bulkier di-tert-butylphenyl analogue
(CpAr*5)H (Ar* = 3,5-tBu2C6H3) (1b) was synthesized in a
similar fashion in good yield. The tert-butyl derivative is more
soluble in hydrocarbon solvents than the dimethyl version,
being soluble in solvents such as hexane and diethyl ether. Both
compounds may be isolated as pale yellow powders follow-
ing workup. The cyclopentadienes 1a,b are Cs symmetric as
(1)
D a l t o n T r a n s . , 2 0 0 3 , 2 6 5 8 – 2 6 6 5
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 3
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Table 1 Selected bond distances (Å) and angles (Њ) for (CpAr#5)H
(1a) (Ar# = 3,5-Me2C6H3)
C1–C2
C3–C4
C5–C1
C2–C14
C4–C30
1.504(4)
1.471(4)
1.508(4)
1.472(4)
1.485(4)
C2–C3
C4–C5
C1–C6
C3–C22
C5–C38
1.348(4)
1.337(4)
1.516(4)
1.481(4)
1.472(4)
C2–C1–C5
C5–C1–C6
C1–C2–C14
C2–C3–C4
C4–C3–C22
C3–C4–C30
C4–C5–C1
C1–C5–C38
103.2(2)
111.0(2)
122.6(3)
109.4(3)
123.1(3)
123.1(3)
109.3(3)
121.7(3)
C2–C1–C6
C1–C2–C3
C3–C2–C14
C2–C3–C22
C3–C4–C5
C5–C4–C30
C4–C5–C38
112.9(2)
108.7(3)
128.6(3)
127.3(3)
109.2(3)
127.7(3)
128.8(3)
1
Fig. 1 500 MHz H NMR spectrum of (CpAr*5)H (1b) (Ar* = 3,5-
tBu2C6H3) in C6D6. * denotes residual solvent.
indicated by their 1H NMR spectra; the 1H NMR spectrum of
1b is shown is Fig. 1. The tert-butyl resonances appear as sing-
lets at 1.28, 1.16 and 1.15 ppm in an 18 : 36 : 36 ratio, reflecting
the three distinct types of phenyl rings present in 1b. This inten-
sity ratio is mirrored in the aromatic region of the spectrum
where the para-protons of the phenyl rings appear as triplets in
Fig. 2 (a) Thermal ellipsoid view of (CpAr#5)H (1a) (Ar# = 3,5-
Me2C6H3) drawn with 30% probability ellipsoids, giving the atom
numbering scheme used in the tables. (b) Side view of (CpAr# )H (1a)
5
drawn with 30% probability ellipsoids.
while the corresponding angle at the sp3-hybridized carbon (C1)
is 103.2(2)Њ. The average of the C–C bond lengths in the Cp ring
is 1.434(4) Å, which is similar to that found in pentaphenyl-
cyclopentadiene (1.443 Å).12 The two bonds to the tetrahedral
carbon are longer than the other ring C–C bonds (1.504(4) and
1.508(4) Å), as is observed in pentaphenylcyclopentadiene12
and tetraphenylcyclopentadiene.13 The Cp ring is planar to
within 0.0125 Å, with the ipso-carbons of the alkenyl substi-
tuted phenyl rings located out of the Cp plane by 0.0206–0.0720
Å (Fig. 2b). The ipso-carbon of the remaining phenyl ring lies
1.255 Å out of the plane of the Cp ring. The phenyl rings are
canted at angles of 33.1, 90.9, 55.2 and 52.4Њ with respect to the
Cp ring (57.9Њ (ave)). These values compare with those pre-
viously observed for similar complexes (i.e. the four alkenyl
phenyl rings in pentaphenylcyclopentadiene are canted an
average of 49.7Њ).12 The slightly larger value for 1a is likely a
reflection of the larger steric bulk of the 3,5-dimethylphenyl
analogue.
4
a 1 : 2 : 2 ratio (7.29, 7.27, 7.23 ppm; JH–H = 1.5 Hz) and the
ortho-protons are evidenced by doublets of 2 : 4 : 4 intensity at
7.41, 7.18 and 7.13 ppm. The CpH proton is observed as a
singlet at 5.24 ppm. In spite of the fact that the molecule has
two distinct sides, due to the presence of the sp3 carbon of the
cyclopentadiene ring, only three sets of phenyl resonances are
observed, indicating that even with the extremely bulky di-tert-
butylphenyl group, free rotation is still possible. In contrast,
ortho-substituted derivatives such as penta(2,5-dimethylphenyl)-
cyclopentadiene consist of at least six rotamers in solution at
room temperature due to restricted movement of two adjacent
methyl groups which makes the rotation of the phenyl groups
difficult.5 It is difficult to remove residual DMF (DMF =
dimethylformamide) from compound 1b; even after prolonged
exposure to vacuum, the 1H NMR spectrum shows reson-
ances for DMF (see Fig. 1). Also, the elemental analysis of 1b
is consistent with the presence of one DMF molecule per
cyclopentadiene unit (see Experimental).
Molecular structure of (CpAr#5)H (1a) (Ar# ؍ 3,5-Me2C6H3)
Synthesis of lithium salts (CpAr5)Li (Ar# ؍ 3,5-Me2C6H3;
Ar* ؍ 3,5-tBu2C6H3)
Crystals of 1a that were suitable for an X-ray diffraction study
were grown by slow evaporation of a concentrated toluene solu-
tion. Selected bond lengths and angles are presented in Table 1.
A thermal ellipsoid plot giving the atom-numbering scheme
used in the tables is shown in Fig. 2a; complete details of the
structural analyses of compounds 1a, 2a and 3a are listed in
Table 4. The solid state structure consists of a planar cyclo-
pentadiene ring which is bisected by an approximate mirror
plane, containing the unique phenyl ring, the sp3 carbon atom
and the unique hydrogen atom. The structure is similar to that
previously determined for pentaphenylcyclopentadiene.12 The
C–C–C angles of the alkenyl carbons of the Cp ring are ∼109Њ,
The addition of BuLi to solutions of (CpAr#5)H (1a) or
(CpAr*5)H (1b) resulted in the formation of the lithium salts
(CpAr# )Li(THF)2 (2a) (THF = tetrahydrofuran) and (CpAr*5)-
5
Li (2b) in good yield [eqns. (2) and (3)].
(2)
(3)
D a l t o n T r a n s . , 2 0 0 3 , 2 6 5 8 – 2 6 6 5
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Table 2 Selected bond distances (Å) and angles (Њ) for {[(CpAr#5)2-
Li][Li(THF)](THF)}x (2a) (Ar# = 3,5-Me2C6H3)a
Deprotonation of the dimethylphenyl derivative 1a was per-
formed in THF (the starting cyclopentadiene has only limited
solubility in diethyl ether), yielding the lithium salt 2a as a yel-
low solid. Compound 2a is somewhat soluble in toluene and
THF and the 1H NMR spectrum (C6D6) indicates approx-
imately two coordinated molecules of THF per lithium center.
Exposure to vacuum results in removal of some of the co-
ordinated THF, which is further supported by the elemental
analysis of 2a, which corresponds to a formulation of 1.5 THF
molecules for every metal center. The 1H and 13C NMR spectra
of 2a were complicated by the presence of additional peaks due
to other lithium containing species; the nature of these species
was elucidated by 7Li NMR studies and will be discussed later.
However, NMR spectroscopy indicates that the major product
Li1–Cp1
Li1–C2
Li1–C4
Li2–Cp1
Li2–C2
Li2–C4
Li2–O1
Li2Ј–C94
Li1–Cp1–Li2
2.094
Li1–C1
Li1–C3
Li1–C5
Li2–C1
Li2–C3
Li2–C5
Li2Ј–C93
Cp1–Li1–Cp1*
Cp1–Li2–O1
2.444(5)
2.431(5)
2.397(5)
2.18(2)
2.28(2)
2.21(2)
1.99(2)
2.438(5)
2.403(5)
1.866
2.23(2)
2.27(2)
1.80(2)
2.23(2)
180.00
177.49
176.79
a Cp1 is defined as the centroid arising from atoms C1 – C5; Cp1* is
related to Cp1 by symmetry. The Ј designation refers to one of two
disordered sites for Li2.
upon workup is the monomeric species, (CpAr# )Li(THF)2. The
5
larger cyclopentadiene 1b reacts similarly with BuLi in diethyl
ether to yield the base-free complex (CpAr*5)Li as a colorless
solid; in this case there was no evidence for the presence of
coordinated Lewis base, likely as a result of the more crowded
coordination sphere around the lithium center in the di-tert-
butylphenyl derivative. Also, 1H and 13C NMR spectroscopy of
2b were consistent with a single product in solution. The aryl
region exhibited two peaks in a 2 : 1 ratio due to the ortho and
para-protons of the 3,5-tBu2C6H3 rings; these peaks appear as a
broad doublet and triplet, respectively (4JH–H = 1.5 Hz). As is
the case for the cyclopentadiene 1b, in spite of the asymmetry in
the molecule, a single doublet is observed for the ortho-protons,
indicating rapid rotation of the bulky di-t-butyl-phenyl rings.
Interestingly, the analogous resonances for the smaller dimethyl
analogue 2a are broad singlets, suggesting that the rate of
rotation of these substituted phenyl rings is slower, and perhaps
dependent on other steric factors, such as aggregation or
metallate formation in solution. Although the 1H and 13C NMR
spectra of 2b were consistent with a single species in solution,
7Li revealed the presence of more than one compound; this
feature of the solution behavior of 2b will be discussed later.
A persistent problem encountered when handling solutions
of the lithium salts was the gradual appearance of a dark blue
or violet contaminant which we attribute to the formation of
the penta-arylated cyclopentadienyl radicals, CpAr5 . We
found these radicals to rapidly react with THF to generate the
starting cyclopentadienes. Thus, it was important to handle the
lithium salts either in the absence of THF or for very short
periods when necessary. This tendency was more pronounced
with the di-tert-butylphenyl species 2b; for this reason, the
lithium salt was synthesized using diethyl ether as the reaction
solvent. However, the low solublity of the dimethylphenyl
analogue in diethyl ether necessitated the use of THF for the
lithiation reaction. We cannot rule out the possibility that the
ether adduct (CpAr*5)Li(ether)x is formed in solution also. We
were not able to obtain X-ray quality crystals of (CpAr*5)Li
or its ether adduct; thus we focused our attention on the less
soluble 3,5-dimethylphenyl derivative.
Fig. 3 Thermal ellipsoid view of the repeat unit of {[(CpAr#5)2Li]-
[Li(THF)](THF)}x (2a) (Ar# = 3,5-Me2C6H3) drawn with 30% prob-
ability ellipsoids, giving the atom numbering scheme used in the tables.
An interstitial molecule of THF is omitted for clarity. Cp1 is defined as
the centroid arising from atoms C1–C5; Cp1* is defined as the centroid
arising from atoms C1*–C5*. For the sake of clarity, only the centroid
designations are given. Only one of the two disordered Li2 sites is
shown.
14
ؒ
motif in alkali metal cyclopentadienides. However, solvated
17
alkali metal cyclopentadienides such as [(THF)K(µ-C5Me5)]x
18
or [(DME)Na(µ-C5H5)]x (DME = dimethoxyethane) exhibit
solid state structures in which the metal atom is sandwiched
between two Cp rings, with the alkali metal coordinated by the
Lewis base. In the case of 2a, the steric bulk of the 3,5-Me2-
C6H3 groups apparently prevents such an arrangement from
being obtained. As observed in Fig. 3, the (CpAr# )2Li2(THF)
5
unit contains two substituted Cp rings which are planar to each
other, in order to minimize steric repulsion. If Li1 were to
coordinate a molecule of THF, a bent structure would
likely result, as seen for [(THF)K(µ-C5Me5)]x or [(THF)2-
K(µ-C5Me5)]x. The large size of the penta-arylated cyclo-
pentadienyl rings makes this bent arrangement unfavorable.
Thus, the exogenous lithium atom (Li2), which is bound to a
single Cp ring, coordinates to the THF molecule. Li2 is dis-
ordered equally over two sites; for the sake of clarity only
one orientation is shown. In addition to the position shown in
Figs. 3 and 4, Li2 also occupies the vacancy between the THF
Molecular structures of {[(CpAr# )2Li][Li(THF)](THF)}x (2a)
5
and [(CpAr# )2Li][Li(TMEDA)2] (3a) (Ar# ؍ 3,5-Me2C6H3)
5
Colorless crystals of 2a that were amenable to X-ray diffraction
studies were grown by slow evaporation of a saturated toluene
solution. A thermal ellipsoid diagram of the repeat unit is
shown in Fig. 3; important bond lengths and angles are given in
Table 2. The molecule co-crystallizes with one molecule of
THF in the lattice. The two cyclopentadienyl rings within each
[(CpAr# )2Li]Ϫ unit are related by symmetry, and thus Table 2
5
contains only bond lengths between the lithium center and
one of the Cp groups. The solid state structure consists of an
infinite one-dimensional chain with alternating [(CpAr#5)2Li]Ϫ
and [(THF)Li(CpAr#5)Li(CpAr#5)Li(THF)]ϩ units (Fig. 4). This
extended chain structure is similar to that determined for CpM
(M = Li, Na, K)15 or (1,2,4-(SiMe3)3Cp)K16 and is a common
of the [(THF)Li(CpAr#5)Li(CpAr# )Li(THF)]ϩ units and the
5
Cp ring of the [(CpAr#5)2Li]Ϫ groups. In this case, the lithium
ion coordinates to the cyclopentadienyl ring and makes close
contact with two carbon atoms of the THF molecule (Li2Ј–C93
= 1.99(2) Å, Li2Ј–C94 = 2.23(2) Å). It is the combination of
these two disordered lithium sites that is responsible for the
D a l t o n T r a n s . , 2 0 0 3 , 2 6 5 8 – 2 6 6 5
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Fig. 4 Thermal ellipsoid view of the one-dimensional chain structure of {[(CpAr#5)2Li][Li(THF)](THF)}x (2a) (Ar# = 3,5-Me2C6H3) drawn with
30% probability ellipsoids. 3,5-dimethylphenyl rings omitted for clarity. Only one of the two disordered Li2 sites is shown.
Table 3 Selected bond distances (Å) and angles (Њ) for [(CpAr#5)2Li]-
[Li(TMEDA)2] (3a) (Ar# = 3,5-Me2C6H3)a
long range structure of 2a. The lithium atom that is sandwiched
between two Cp rings is only loosely held in place, as seen by the
large Li1–Cp1 distances of 2.094 Å. This is significantly longer
than the distance of 1.969 Å observed in polymeric LiCp,15 or
distances of 2.03 and 1.78 Å for [{Li(η5-C5Bz5)}2(C6D6)] (Bz =
benzyl).19 The Li1–C distances range from 2.397(5)–2.444(5) Å;
these larger than normal distances are the likely result of the
steric demands of the 3,5-Me2C6H3 rings. The substituted
phenyl rings are canted an average of 48.6Њ with respect to the
cyclopentadienyl groups for both rings, although in opposite
directions, resulting in a “gear-meshing” and a closer approach
of the two Cp rings. In contrast, the less crowded coordination
sphere around Li2 allows for a closer approach to the Cp ring
(Li2–Cp1 = 1.866 Å; Li2–C = 2.18(2)–2.28(2) Å). Within the
unit presented in Fig. 3, there is an almost linear O–Li–
Cpcentroid–Li–Cpcentroid linkage (O1–Li2–Cp1 = 177.49Њ; Li2–
Cp1–Li1 = 176.79Њ; Cp1–Li1–Cp1* = 180.00Њ), although the
disordered lithium results in an overall zig-zag pattern.
Colorless crystals of 3a were obtained by cooling a saturated
THF/TMEDA (TMEDA = N,N,NЈ,NЈ-tetramethylethylene-
diamine) solution of 2a to Ϫ30 ЊC. A thermal ellipsoid diagram
is shown in Fig. 5 with important bond lengths and angles
presented in Table 3. The solid state structure reveals a discrete
“triple ion” or metallate structure (see later), with a [(CpAr#5)2-
Li]Ϫ anion and a [Li(TMEDA)2]ϩ cation. Only one of two
independent molecules of each is shown; since the metric
parameters are similar in both, the bond angles and lengths of
only one will be discussed. Also, the two cyclopentadienyl rings
within each [(CpAr#5)2Li]Ϫ unit are related by symmetry, and
thus Table 3 contains only bond lengths between the lithium
center and one of the Cp groups. Although alkali metal cyclo-
Li1–Cp1
Li1–C2
Li1–C4
Li2–N1
Li2–N3
2.086
Li1–C1
Li1–C3
Li1–C5
Li2–N2
Li2–N4
2.445(4)
2.405(4)
2.400(4)
2.103(10)
2.112(10)
2.441(4)
2.377(4)
2.080(9)
2.169(11)
Cp1–Li1–Cp1*
N1–Li1–N3
N2–Li1–N3
N3–Li1–N4
180.00
N1–Li1–N2
N1–Li1–N4
N2–Li1–N4
88.2(4)
120.6(5)
122.6(5)
124.5(5)
118.5(4)
86.5(4)
a Cp1 is defined as the centroid arising from atoms C1 – C5; Cp1* is
related to Cp1 by symmetry.
pentadienide complexes commonly reveal ionic polymeric
chain structures, as observed for 2a, solvation of the metal
cation can break the chain into smaller units. The use of crown
ethers can result in cation–anion separation and the form-
ation of cyclopentadienide anions, as observed in [(1,2,4-
tris(trimethylsilyl)cyclopentadienide)][Li(12-crown-4)2] (A),20
or so-called “triple ions”, obtained by the specific solvation of
only half of the lithium cations. To date, the only cyclopenta-
dienyl derivative to exhibit this structural motif is the isodi-
cyclopentadienide species [(isodiCp)2Li][Li(12-crown-4)2] (B),11
although related porphyrin21 and carborane22 complexes have
been reported.
The formation of the triple ion 3a upon addition of TMEDA
to a THF solution of 2a differs from the pattern of reactivity
seen for lithium isodicyclopentadienide.11 In solution, the THF
solvated triple ion [(isodiCp)2Li][Li(THF)4] is postulated to
exist in equilibrium with the monomer, (isodiCp)Li(THF)x.
Addition of TMEDA to an ethereal solution of (isodiCp)Li
yields (isodiCp)Li(TMEDA). The triple ion B is only obtained
upon treating an equilibrium mixture of (isodiCp)Li(THF)x
and [(isodiCp)2Li][Li(THF)4] with an excess of 12-crown-4.
This difference in observed reactivity with Lewis bases is due to
the increased steric demands of the penta-arylated cyclo-
pentadienyl ligand.
As observed in 2a, the two Cp rings are planar (Cp1–Li1–
Cp1* = 180.00Њ) and the lithium ion is only loosely held between
the two. The Li1–centroid distance of 2.086 Å is essentially the
same as that determined for 2a (Li1–C = 2.377(4)–2.445(4) Å)
and longer than those in [(isodiCp)2Li][Li(12-crown-4)2]
(1.987 and 2.008 Å).11 Again, the Li–Cp distance is deter-
mined by the steric encumberence of the 3,5-Me2C6H3 rings,
which prevents the rings from any closer approach. The phenyl
rings are canted an average of 47.7Њ and have the same
opposing orientation that was seen in 2a. The bond lengths
and angles within the [Li(TMEDA)2]ϩ cation are unremark-
able and similar to those previously reported for related
compounds.11,22–24
Fig.
5
Thermal ellipsoid view of [(CpAr# )2Li] [Li(TMEDA)2]
5
(3a) (Ar# = 3,5-Me2C6H3) drawn with 30% probability ellipsoids,
giving the atom numbering scheme used in the tables. Only one of two
independent molecules is shown. Cp1 is defined as the centroid arising
from atoms C1–C5; Cp1* is defined as the centroid arising from atoms
C1*–C5*. For the sake of clarity, only the centroid designations are
given.
D a l t o n T r a n s . , 2 0 0 3 , 2 6 5 8 – 2 6 6 5
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7Li NMR spectroscopy and solution equilibria
The solid-state structures of 2a and 3a indicated that solvation
is important in these complexes, and generates different
structural types depending on the Lewis base employed. We
were interested in whether such structures were also present
in solution. To further elucidate the nature of “(CpAr#5)-
Li(solvent)x” in solution, we turned to 7Li NMR spectroscopy.
The room temperature 7Li NMR spectrum of {[(CpAr# )2-
5
Li][Li(THF)](THF)}x (2a) in C6D6 is shown in Fig. 6. The
spectrum consists of three distinct singlets at Ϫ0.21, Ϫ0.86 and
Ϫ1.96 ppm of intensity 4 : 1 : 1. These three peaks correspond
to two separate lithium-containing species: the monomer
(CpAr#5)Li(THF)x (Ϫ0.21 ppm) and the solvent-separated
ion pair, [(CpAr#5)2Li][Li(THF)x] (Ϫ0.86 ppm, THF-solvated
lithium; Ϫ1.96 ppm, “sandwiched” lithium), which are in equi-
librium at room temperature [eqn. (4)]. Such monomer–dimer
equilibria have been observed for CpLi and substituted cyclo-
pentadienyl lithium salts in d8-THF below Ϫ100 ЊC;10,25,26 the
dynamic exchange process is slowed considerably by the graft-
ing of five 3,5-Me2C6H3 moieties onto the Cp ring. There also
appears to be little effect of temperature (20 to Ϫ85 ЊC) on the
relative ratios of the two species in solution (2 monomer : 1
dimer). A similar example of a temperature independent equi-
librium was noted for CCpLi (CCp = camphorcyclopenta-
dienyl) in THF between Ϫ100 ЊC and room temperature.25 The
chemical shifts of the two Cp-containing compounds are in the
normal range for lithium chemical shifts (ca. ϩ2 to Ϫ2 ppm).
This contrasts with that reported for CpLi and other substi-
tuted lithium cyclopentadienides, for which resonances at
Ϫ7.64 (CpLi monomer) and Ϫ12.78 ppm ([(Cp)2Li]Ϫ sandwich)
were found. These anomalously large upfield shifts were
ascribed to magnetic anisotropy and ring current effects. In the
case of 2a, the lithium centers must reside in much more trad-
itionally shielded positions within the molecule.
Fig. 7 194 MHz 7Li NOESY spectrum of {[(CpAr#5)2Li][Li(THF)]-
(THF)}x (2a) (Ar# = 3,5-Me2C6H3) in C6D6 at room temperature.
[(CpAr# )2Li]Ϫ, it must also be exchanging with the THF sol-
5
vated cation. Changing the mixing time to 25 ms resulted in the
appearance of the resonance arising from [Li(THF)x]ϩ on the
diagonal, but still no cross-peaks were observed. This must be
due to an additional exchange process such as THF dissoci-
ation in [Li(THF)x]ϩ that relaxes the lithium atom to such an
extent that intermolecular exchange with this metal center is
not observed under these measurement conditions.
7
In d8-THF, the Li NMR spectrum exhibits two peaks of
equal intensity at 4.24 and Ϫ2.80 ppm at room temperature.
Heating the sample to 60 ЊC does not lead to any significant
change in the spectrum, suggesting that there is no exchange
between lithium sites. This was confirmed by 7Li NOESY
experiments, where no cross peaks were observed (tmix = 25 to
500 ms). These data are consistent with the solvent-separated
ion pair, [(CpAr#5)2Li][Li(THF)x] in THF solution. The lack of
lithium exchange in this species is supported by the absence
of cross peaks between the resonances due to [Li(THF)x]ϩ
(Ϫ0.86 ppm) and [(CpAr# )2Li]Ϫ (Ϫ1.96 ppm) when the
5
spectrum was taken in C6D6 (Fig. 6). Titration of a C6D6 solu-
tion of 2a with THF resulted in numerous additional signals
until the final spectrum observed in d8-THF was obtained. This
suggests that the situation is more complex in coordinating
solvents, possibly involving dimeric species, close-ion pairs, sol-
vent-separated ion pairs, super sandwiches or other more exotic
structures. Previous MNDO calculations on related systems
postulate that a super sandwich structure [(Cp)Li(Cp)Li(sol-
vent)x] (Fig. 3) is more stable than a solvent-separated ion pair
[(Cp)2Li][Li(TMEDA)2] (Fig. 5) and may play a large part in
describing the solution dynamics of such systems.26 Paquette
et al. have shown that many unusual structures may exist in
Fig. 6 194 MHz 7Li NMR spectrum of {[(CpAr# )2Li][Li(THF)]-
5
(THF)}x (2a) (Ar# = 3,5-Me2C6H3) in C6D6 at room temperature.
equilibrium in solution.10,25,26
7
Raising the temperature to 60 ЊC results in a single broad
resonance at Ϫ0.50 ppm, consistent with rapid lithium
The Li NMR spectrum of [(CpAr# )2Li][Li(TMEDA)2] (3a)
5
in C6D6 consists of a single sharp resonance at 6.80 ppm which
does not change upon cooling (C7D8). This suggests either (a)
fast exchange on the NMR timescale between a monomer such
7
exchange between compounds. This was confirmed by the Li
NOESY spectrum (Fig. 7), in which cross-peaks were found for
the resonances arising from the monomer (CpAr# )Li(THF)x
as (CpAr# )Li(TMEDA) and the solvent separated ion pair,
5
5
(Ϫ0.21 ppm) and the sandwiched lithium center in
[(CpAr#5)2Li][Li(TMEDA)2] or (b) the presence of a single
7
[(CpAr#5)2Li] (Ϫ1.96 ppm). At the mixing time employed (tmix
=
static structure in solution. The Li NMR chemical shift sup-
500 ms), no cross peak was present between the monomer
ports the latter, as the presence of only one CpAr#5 ring would
result in a downfield shift relative to the sandwich complex. It is
possible for a structure such as that observed for 3a to exist
(CpAr# )Li(THF)x and [Li(THF)x]ϩ (Ϫ0.86 ppm), although if
5
the lithium center in (CpAr#5)Li(THF)x is exchanging with
(4)
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2662

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without participating in an equilibrium with a solvated mono-
mer; the Li NMR spectrum of the lithiacarborane complex
techniques or under oxygen-free helium in a Vacuum Atmos-
7
pheres glovebox. Cp2ZrCl2 and dimethylformamide (<30 ppm
H2O) were purchased from Strem Chemical Co. and used as
received. 3,5-dimethyl-1-bromobenzene was obtained from
Lancaster and dried over molecular sieves prior to use. 3,5-di-
t-butyl-1-bromobenzene was obtained from Lancaster and
used as received. Caesium carbonate, tri-t-butyl-phosphine,
p-toluenesulfonic acid, sodium carbonate, sodium chloride,
magnesium sulfate, palladium acetate and BuLi were purchased
from Aldrich and used as received. Methylene chloride and
chloroform were purchased from Fisher and used as received.
N,N,NЈ,NЈ-tetramethylethylenediamine was purchased from
Aldrich and distilled over sodium prior to use. Hexanes, tolu-
ene, diethyl ether and tetrahydrofuran were de-oxygenated by
passage through a column of supported copper redox catalyst
(Cu-0226 S) and dried by passing through a second column of
activated alumina.27 C6D6 and d8-THF were dried over
Na–K alloy, trap-to-trap distilled and degassed before use. 1H,
[1,1Ј-Li{2,3-(SiMe3)2-2,3-C2B4H5}2][Li(TMEDA)2] exhibits two
distinct peaks, consistent with a single solution structure which
is the same as that determined in the solid state.22 We suggest
that in solution the formulation (CpAr# )Li(TMEDA) is
5
most likely, as found for (isodiCp)Li(TMEDA), although only
a small reorganization energy must be necessary to form the
triple ion observed in the solid state. This is supported by the
13C NMR spectrum of 3a, which is comprised of multiple
resonances, consistent with a more complex solution behavior.
7
The Li NMR spectrum of 2b consisted of two peaks of
roughly equal intensity at 1.18 and Ϫ4.43 ppm; the downfield
resonance was significantly broadened (∆1/2 = 120 Hz) as com-
pared to the upfield resonance (∆1/2 = 5 Hz). The presence of
two peaks points towards a monomer/dimer equilibrium in
solution. Recent studies on lithium pentabenzylcyclopenta-
dienides have revealed that bulky systems such as those
employed here often favor arene complexes over polymers
commonly observed with smaller cyclopentadienyl groups.
Schnöckel et al. have postulated that small neutral arene
adducts such as [{Li(η5-CpR5)}2(C6D6)] may be formed when
(1) R is bulky enough to prevent polymer formation, (2) no
proper cation can be generated and (3) no donor solvent is
present.19 The 7Li NMR spectrum of Li(η5-C5Bz5) (Bz =
CH2C6H5) in C6D6 revealed a single resonance, indicating either
the presence of a monomeric species, or a fast equilibrium
between monomeric and dimeric species. The presence of two
peaks in the 7Li NMR spectrum of 2b may be attributed to the
larger size of the di-tert-butylphenyl groups, which slows down
the exchange and allows for both species to be observed. The
broadened peak at 1.18 ppm is likely due to the more unsym-
metric environment about the lithium atom in the monomer
(CpAr*5)Li, while the sharper peak may be the result of a more
highly symmetric species such as a dimer [{(CpAr*5)Li}2-
(C6D6)] or higher oligomer {(CpAr*5)Li}x. We should also note
that the assignment of the upfield resonance to a dimeric
species is somewhat speculative since we were unable to obtain a
solid state structure for 2b i.e. a dimeric, trimeric or higher
order aggregate is possible. It should also be noted that
although two species were evident in the 7Li NMR spectrum of
7
13C{1H} and Li{1H} spectroscopy were performed at ambient
temperature on a Bruker DRX-500 spectrometer operating at
500.0, 125.1 and 194.4 MHz respectively. 1H chemical shifts are
given relative to residual C6D5H (δ = 7.15 ppm) or C4D7HO
(δ = 3.58 ppm). 13C chemical shifts are given relative to C6D6
(δ = 128.39 ppm). 7Li chemical shifts are given relative to
external LiCl in D2O (δ = 0.0 ppm). The 7Li NOESY spectrum
was obtained on a Bruker DRX-500 spectrometer operating at
194.4 MHz in C6D6 at 283 K; t1 was incremented in 512 steps,
and the data were zero-filled to 1024 words before Fourier
transformation. Sixteen scans were recorded for each increment
with tmix = 500 or 25 ms with a relaxation time of 800 ms.
Infrared spectra were recorded on a Nicolet Avatar 360 FT-IR
spectrometer; solid-state spectra were taken as Nujol mulls
between KBr plates. Elemental analyses were performed by the
Micro-Mass Facility at the University of California, Berkeley.
(CpAr5)H (Ar ؍ 3,5-Me2C6H3, 1a; 3,5-tBu2C6H3, 1b). This
compound was prepared by a modification of the original liter-
ature procedure5 as reported to us by Fu et al.28 For 1a: In the
glovebox, Cp2ZrCl2 (584 mg, 2.00 mmol), 3,5-dimethyl-1-
bromobenzene (4.44 g, 24.0 mmol), caesium carbonate (7.82 g,
24.0 mmol), palladium acetate (112 mg, 0.500 mmol), tri-t-
butylphosphine (404 mg, 2.00 mmol) and 50 mL DMF were
combined in a Schlenk tube. The contents were then heated to
130 ЊC in an oil bath for 24 h, forming a creamy brown slurry.
The Schlenk tube and its contents were then cooled to room
temperature and exposed to air. 150 mL CH2Cl2 was then
added, followed by p-toluenesulfonic acid (9.12 g, 48.0 mmol).
This was stirred at room temperature for 15 min, then passed
through a column of silica gel to yield a brown solution. The
column was additionally rinsed with small portions of CH2Cl2
and DMF until the washings were colorless (∼10 mL each). The
solvent was then removed under vacuum to give a brown tar.
The solid was dissolved in CHCl3 (150 mL) and extracted with
saturated aqueous NaHCO3 solution (3 × 150 mL) and satur-
ated aqueous NaCl solution (3 × 150 ml). The organic layer was
dried over MgSO4 and passed through a column of silica gel to
give a brown solution. The solvent was removed under vacuum
and the oily solid washed with hexanes until the washings
were colorless, yielding 1a as a yellow–orange solid (1.54 g, 66%
yield). The t-butyl derivative 1b was prepared in a similar
fashion from 3,5-di-t-butyl-1-bromobenzene (3.14 g, 78%
yield). For 1a: analytical data for this compound have been
1
2b, only a single set of resonances was observed in the H and
13C NMR spectra. This may be due to the different frequencies
of 7Li, 13C and 1H NMR spectroscopy, resulting in the observ-
ation of averaged signals when observing one nucleus but dis-
tinct species for another. Also, the protons observed in these
complexes are quite far removed from the metal center, and
thus the difference in chemical shift between monomeric and
oligomeric products may be negligible.
Conclusions
The superbulky cyclopentadienes, CpAr5H, are readily depro-
tonated by BuLi to give lithium salts of the general formula
(CpAr5)Li(solvent)x; the solution behavior observed is similar
to that reported for other bulky Cp derivatives such as lithium
isodicyclopentadienide. The large aryl groups in 2a slow down
the exchange between (CpAr#5)Li(THF)x and [(CpAr#5)2Li]-
[Li(THF)x] such that all three distinct lithium environments can
be observed under ambient conditions. The crystal structures
of 2a and 3a further lend credence to the proposed solution
structures although the presence of further more complicated
structures in solution cannot be discounted.
1
reported previously.5 For 1b: H NMR (C6D6): δ 7.41 (br d,
4JH–H = 1.5 Hz, 2H, o-H), 7.29 (br t, 4JH–H = 1.5 Hz, 1H, p-H), 7.27
and7.23(brt,4JH–H =1.5Hz,2H,p-H),7.18and7.13(brd,4JH–H
=
Experimental
1.5 Hz, 4H, o-H), 5.24 (s, 1H, CpH ), 1.28 (s, 18H, t-Bu–Ph) 1.16
and 1.15 (s, 36H, t-Bu–Ph). 13C{1H} NMR (C6D6): δ 151.23,
150.80, 150.10, 148.15, 146.06, 139.01, 137.42, 136.60, 125.27,
124.67, 123.76, 120.86, 120.32, 120.16, 63.81, 35.27, 35.07, 32.95,
32.38, 31.97, 31.54. IR (Nujol, cmϪ1): 1592 (s), 1362 (m), 1248
General considerations
All manipulations were carried out under an inert atmosphere
of oxygen-free UHP grade argon using standard Schlenk
D a l t o n T r a n s . , 2 0 0 3 , 2 6 5 8 – 2 6 6 5
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Table 4 Crystallographic dataa
Compound
1a
2a
3a
Formula
MW
Crystal system
Space group
C45H46
586.82
triclinic
C98H106Li2O2
1329.71
C110H138Li2N4O2
1562.18
triclinic
triclinic
¯
¯
¯
P1
P1
P1
Temp/K
a/Å
b/Å
c/Å
α/Њ
β/Њ
203
203
203
8.1689(19)
12.805(3)
17.290(4)
92.202(4)
93.823(5)
104.869(5)
1741.3(7)
2
13.997(4)
15.486(4)
20.198(5)
97.393(4)
102.993(5)
96.523(4)
4184.2(18)
2
14.216(4)
20.023(5)
22.771(6)
98.140(6)
106.914(4)
109.938(5)
5617(3)
2
γ/Њ
V/Å3
Z
µ/mmϪ1
0.063
0.060
0.056
Reflections
Independent reflections
8242
4694
20273
10298
28915
13878
Rint
R1
wR2
0.0266
0.0669
0.2181
0.0413
0.0933
0.2534
0.0454
0.0962
0.2744
2
2
a R1 =Σ||Fo|Ϫ|Fc||/Σ|Fo| and wR2 =[Σ[ω(Fo2 ϪFc2)2]/Σ[ω(Fo )2]]1/2; ω=1/[σ2(Fo )ϩ(aP)2], where a=0.1350, 0.1284 and 0.1776.
(m),1201(w),1018(w),910(w),872(w),711(m).Anal.Calcd.for
C78H113NO [1b(DMF)]: C, 86.68; H, 10.54%. Found: C, 86.52; H,
10.82%.
δ 6.80. IR (Nujol, cmϪ1): 1598 (s), 1351 (w), 1302 (w), 1168 (w),
1082 (w), 1033 (m), 909 (w), 897 (w), 850 (s), 841 (s), 775 (w),
754 (w), 725 (m), 704 (m), 692 (s). Anal. Calcd. for C90H90Li2
[3a Ϫ 2(TMEDA)]: C, 91.18; H, 7.65%. Found: C, 91.98; H,
8.13%.
{[(CpAr# )2Li][Li(THF)](THF)}x (2a) (Ar# ؍ 3,5-Me2C6H3).
5
nBuLi (0.39 mL of a 1.6 M hexane solution, 0.62 mmol) was
added dropwise to an orange solution of 1a (330 mg, 0.57
mmol) in 10 ml THF, causing a precipitate to form. This was
stirred for 15 min and the solvent was then removed under
vacuum. The yellow–orange solid was washed with toluene,
collected on a frit and dried under vacuum (290 mg, 68% yield).
Slow evaporation of a saturated toluene solution yielded
Crystallographic studies
The crystal structures of all compounds were determined as
follows, with exceptions noted in subsequent paragraphs: a
crystal was mounted onto a glass fiber using a spot of silicone
grease. Due to air sensitivity, the crystal was mounted from a
pool of mineral oil under argon gas flow. The crystal was placed
on a Bruker P4/CCD diffractometer, and cooled to 203 K
using a Bruker LT-2 temperature device. The instrument
was equipped with a sealed, graphite monochromatized MoKα
X-ray source (λ = 0.71073 Å). A hemisphere of data was col-
lected using φ scans, with 30 s frame exposures and 0.3Њ frame
widths. Data collection and initial indexing and cell refinement
were handled using SMART29 software. Frame integration,
including Lorentz-polarization corrections, and final cell par-
ameter calculations were carried out using SAINT30 software.
The data were corrected for absorption using the SADABS31
program. Decay of reflection intensity was monitored via
analysis of redundant frames. The structure was solved using
direct methods and difference Fourier techniques. All hydrogen
atom positions were idealized, and rode on the atom they were
attached to. The final refinement included anisotropic temper-
ature factors on all non-hydrogen atoms. Structure solution,
refinement, graphics, and creation of publication materials were
performed using SHELXTL NT.32 Additional details of data
collection and structure refinement are listed in Table 4.
1
large colorless crystals of 2a. H NMR (C6D6): δ 6.95 (br s,
10H, o-H), 6.69 (br s, 5H, p-H), 3.42 (m, 8H, THF), 2.06 (s,
30H, MePh), 1.28 (m, 8H, THF). 13C{1H} NMR (C6D6): the
spectrum of 2a was complicated by the presence of many
species in solution. 7Li{1H} NMR (C6D6, 20 ЊC): δ Ϫ0.21
((CpAr#5)Li(THF)x), Ϫ0.86 ([Li(THF)x]ϩ), Ϫ1.96 ([(CpAr#5)2-
Li]Ϫ); (C6D6, 60 ЊC): δ Ϫ0.50; (d8-THF, 20 ЊC): δ 4.24, Ϫ2.80. IR
(Nujol, cmϪ1): 1592 (s), 1300 (w), 1033 (s), 947 (w), 908 (m), 893
(m), 840 (s), 761 (m), 725 (m), 708 (m), 692 (m), 680 (m). Anal.
Calcd. for C102H114Li2O3: C, 87.39; H, 8.20%. Found: C, 87.20;
H, 8.30%.
n
(CpAr*5)Li (2b) (Ar* ؍ 3,5-tBu2C6H3). BuLi (0.13 mL of a
1.6 M hexane solution, 0.20 mmol) was added dropwise to an
orange solution of 1b (200 mg, 0.20 mmol) in 10 ml diethyl
ether, causing the solution to lighten slightly. This was stirred
for 3 h and the solution was then cooled to Ϫ30 ЊC, yielding 2b
as small colorless crystals (160 mg, 80% yield). 1H NMR
(C6D6): δ 7.19 (br t, 4JH–H = 1.5 Hz, 5H, p-H), 7.01 (br d, 4JH–H
=
1.5 Hz, 10H, o-H), 1.19 (s, 90H, t-Bu–Ph). 13C{1H} NMR
(C6D6): δ 149.82, 139.07, 126.37, 120.80, 117.65, 34.98, 32.05.
7Li{1H} NMR (C6D6): δ 1.18, Ϫ4.43. IR (Nujol, cmϪ1): 1591
(s), 1303 (m), 1248 (m), 1201 (w), 1168 (w), 1152 (w), 1058 (w),
1017 (m), 967 (w), 894 (m), 870 (m), 842 (w), 775 (w), 722 (s).
Anal. Calcd. for C75H105Li: C, 88.87; H, 10.44%. Found: C,
86.27; H, 11.05%.
(CpAr#5)H (1a). the unique hydrogen atom position was
located on the difference map, and refined with isotropic tem-
perature factor set to 0.08 Å2.
{[(CpAr# )2Li][Li(THF)](THF)}x (2a). the electron density
5
of a disordered THF molecule was refined with the aid of
PLATON/SQUEEZE33 (74 eϪ cellϪ1 and 838 Å3).
[(CpAr# )2Li][Li(TMEDA)2] (3a) (Ar# ؍ 3,5-Me2C6H3). 2a
5
(250 mg, 0.43 mmol) was dissolved in THF with heating and
TMEDA (0.5 mL) was added. The solution was cooled to Ϫ30
ЊC, resulting in large colorless crystals (310 mg, 90% yield). 1H
NMR (C6D6): δ 6.95 (br s, 20H, o-H), 6.68 (br s, 10H, p-H), 2.31
(s, 8H, TMEDA), 2.12 (s, 24H, TMEDA), 2.08 (s, 60H, MePh).
13C{1H} NMR (C6D6): the spectrum of 3a was complicated by
the presence of many species in solution. 7Li{1H} NMR (C6D6):
[(CpAr#5)2Li][Li(TMEDA)2] (3a). the electron density of
a disordered THF molecule was refined with the aid of
PLATON/SQUEEZE33 (209 eϪ cellϪ1 and 1728 Å3).
CCDC reference numbers 200149–200151.
See http://www.rsc.org/suppdata/dt/b3/b302258g/ for crystal-
lographic data in CIF or other electronic format.
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2664

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14 C. Janiak, R. Weimann and F. Gorlitz, Organometallics, 1997, 16,
Acknowledgements
4933.
15 R. E. Dinnebier, U. Behrens and F. Olbrich, Organometallics, 1997,
16, 3855.
16 M. J. Harvey, T. P. Hanusa and M. Pink, J. Chem. Soc.,
Dalton Trans., 2001, 1128.
17 W. J. Evans, J. C. Brady, C. H. Fujimoto, D. G. Giarikos and
J. W. Ziller, J. Organomet. Chem., 2002, 649, 252.
18 M. L. Cole, C. Jones and P. C. Junk, J. Chem. Soc., Dalton Trans.,
2002, 896.
19 C. Dohmeier, E. Baum, A. Ecker, R. Koppe and H. Schnockel,
Organometallics, 1996, 15, 4702.
We thank the research group of Dr. Greg Fu (MIT) for helpful
discussions regarding the isolation of compounds 1a and 1b,
and Dr. Kevin D. John (LANL) for acquiring the X-ray data for
compound 1a. This work was performed under the auspices of
the Laboratory Directed Research and Development Program.
Los Alamos National Laboratory is operated by the University
of California for the U.S. Department of Energy under
Contract W-7405-ENG-36.
20 H. Chen, P. Jutzi, W. Leffers, M. M. Olmstead and P. P. Power,
Organometallics, 1991, 10, 1282.
21 J. Arnold, J. Chem. Soc., Chem. Commun., 1990, 976.
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23 M. F. Lappert, A. Singh, L. M. Engelhardt and A. H. White,
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25 W. Bauer, G. OЈDoherty, P. von Rague Schleyer and L. A. Paquette,
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28 B. Tao and G. C. Fu, personal communication.
29 G. M. Sheldrick, SMART Version 4.210, Bruker Analytical X-ray
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D a l t o n T r a n s . , 2 0 0 3 , 2 6 5 8 – 2 6 6 5
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