Synthetic aspects of (C5H4SiMe3) 3Ln rare-earth chemistry: Formation of (C5H 4SiMe3)3Lu via [(C5H 4SiMe3)2Ln]+ m

Article abstract of DOI:10.1021/om400116d

New analogues of Cp′₃Ln (Cp′ = C₅H ₄SiMe₃, Ln = rare earth) complexes have been synthesized with the largest and smallest lanthanides, La and Lu. Due to the smaller size of Lu, the reaction of LuCl₃ w

Full text of DOI:10.1021/om400116d

Article
pubs.acs.org/Organometallics
Synthetic Aspects of (C5H4SiMe3)3Ln Rare-Earth Chemistry: Formation
of (C5H4SiMe3)3Lu via [(C5H4SiMe3)2Ln]+ Metallocene Precursors
Jeffrey K. Peterson, Matthew R. MacDonald, Joseph W. Ziller, and William J. Evans*
Department of Chemistry, University of California, Irvine, California 92697-2025, United States
S
* Supporting Information
ABSTRACT: New analogues of Cp′3Ln (Cp′ = C5H4SiMe3,
Ln = rare earth) complexes have been synthesized with the
largest and smallest lanthanides, La and Lu. Due to the smaller
size of Lu, the reaction of LuCl3 with 3 equiv of KCp′
produced only traces of Cp′3Lu, yielding [Cp′2Lu(μ-Cl)]2 as
the major product. An alternative route was developed in
which [Cp′2Lu(μ-Cl)]2 reacts with (C3H5)MgCl to form the tetrameric allyl complex [Cp′2Lu(μ-η1:η1-C3H5)]4. The allyl
complex reacts with [HNEt3][BPh4] to generate [Cp′2Lu(THF)2][BPh4], which reacts with KCp′ to produce Cp′3Lu in good
yield. The reaction of LaCl3 with 3 equiv of KCp′ in Et2O gave the unsolvated Cp′3La, while the same reaction in rigorously dry
THF gave solvated Cp′3La(THF). The reaction of LaCl3 and KCp′ in improperly dried THF gave the mixed-ligand complex
Cp′2CpLa(THF) (Cp = C5H5), rather than the expected hydrolysis product, [Cp′2La(μ-OH)]2.
unknown whether these complexes could be obtained with all
the rare earth metals, as the steric requirements for thermal
stability can often differ across this series. For example, due to
steric unsaturation, the unsolvated [(C5Me5)2LnMe]n com-
plexes of the larger rare-earth metals are less stable than those
of the smaller metals.7 Conversely, due to steric crowding,
(C5Me5)3Ln and (C5Me4H)3Ln complexes of the smaller rare
earths are much more reactive and difficult to access than those
of the larger metals.8−11
We report here the synthesis of the Cp′3Ln complexes of the
largest and smallest lanthanides, La and Lu. The synthesis of
the lanthanum complex from LaCl3 and 3 equiv of KCp′ is
straightforward, but an alternative synthesis was developed for
lutetium, since the reaction between LuCl3 and 3 equiv of KCp′
gives primarily [Cp′2Lu(μ-Cl)]2.
INTRODUCTION
■
Recent studies of the reductive chemistry of the rare-earth
metals have shown that trimethylsilylcyclopentadienyl com-
plexes are excellent starting materials for the synthesis of
complexes of metals in novel low oxidation states.1,2 Lappert
and co-workers showed that the bis(trimethylsilyl)-
cyclopentadienyl complexes Cp″3Ln (Cp″ = 1,3-(Me3Si)2C5H3;
Ln = La, Ce) can be reduced to form crystalline La2+ and Ce2+
molecular complexes, as shown in eq 1. More recently, the
EXPERIMENTAL SECTION
■
monosubstituted trimethylsilylcyclopentadienyl ligand
C5H4SiMe3 (Cp′) was useful with the smaller rare earths in
providing the first crystallographic data on molecular complexes
of Y2+, Ho2+, and Er2+, [(18-crown-6)K][Cp′3Ln], using
Cp′3Ln as precursors (eq 2).3,4
All syntheses and manipulations described below were conducted
under argon or nitrogen with rigorous exclusion of air and water using
glovebox, Schlenk, and vacuum line techniques, except where noted.
Solvents were sparged with UHP grade argon (Airgas) and passed
through columns containing Q-5 and molecular sieves before use.
NMR solvents (Cambridge Isotope Laboratories) were dried over
sodium−potassium alloy, degassed by three freeze−pump−thaw
cycles, and vacuum-transferred prior to use. Trimethylsilyl chloride
(Alfa Aesar) was dried over CaH2 and vacuum-transferred before use.
Potassium bis(trimethylsilyl)amide (Sigma-Aldrich) was purified by
dissolving in minimal toluene, centrifuging to remove insoluble
material, and removing solvent from the supernatant. Allylmagnesium
chloride (2.0 M solution in THF, Sigma-Aldrich) was used as received.
KCp (Cp = C5H5) was made from potassium bis(trimethylsilyl)amide
and freshly cracked C5H6. [HNEt3][BPh4] was synthesized by
treatment of NEt3·HCl with NaBPh4 in water, followed by filtration
In order to attempt the synthesis of other Ln2+ ions along
similar lines, the synthesis of previously unknown tris-
(trimethylsilylcyclopentadienyl) complexes was necessary.
Although Cp′3Ln (Ln = Ce,5 Nd,6 Y,4 Ho,3 Er3) complexes
can be readily prepared from LnCl3 and 3 equiv of KCp′, it was
Received: February 8, 2013
Published: April 22, 2013
© 2013 American Chemical Society
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and drying under high vacuum (10−5 Torr) for 48 h. Anhydrous LaCl3
stopcock and heated to reflux for 2 h. The solvent was removed under
vacuum, and the resulting solids were stirred in hexane (40 mL) for 1
h and then heated to reflux in the sealed flask for an additional 2 h.
The solvent was removed under vacuum, and the resulting solids were
transferred to a glovebox free of coordinating solvents. The solids were
extracted with hexane (40 mL) and the extracts filtered to remove
white insoluble material, presumably KBPh4 and excess KCp′. The
solvent was removed from the yellow filtrate under vacuum, and the
resulting solids were extracted with pentane. Removal of pentane
under vacuum yielded 3 as a yellow microcrystalline solid (0.950 g,
86%). Pale yellow crystals of 3 suitable for X-ray diffraction were
12
13
and LuCl31 and [Cp′2Lu(μ-Cl)]2 were prepared as previously
described. H NMR (500 MHz) and 13C NMR (125 MHz) spectra
were recorded at 25 °C with a Bruker GN500 or CRYO500
spectrometer. Infrared spectra were recorded as KBr pellets on a
Varian 1000 FT-IR spectrometer. Elemental analyses were performed
on a Perkin-Elmer 2400 Series II CHNS analyzer.
KCp′. Following an adaptation of the procedure previously
reported,14 in an argon-filled glovebox, trimethylsilyl chloride (5.87
g, 54.0 mmol) was added dropwise to a stirred solution of KCp (5.63
g, 54.0 mmol) in Et2O (300 mL) in a 500 mL round-bottom flask. The
resulting mixture was stirred for 4 h. Hexane (200 mL) was added, the
mixture was filtered with a medium frit to remove KCl, and the filtrate
was transferred to a new 1 L round-bottom flask. Potassium
bis(trimethylsilyl)amide (10.8 g, 54.1 mmol) was dissolved in Et2O
(50 mL), the solution was added dropwise to the stirred filtrate, and
the mixture was stirred for 4 h. Solvent was removed from the
colorless solution under vacuum, and the resulting white solids were
stirred in hexane (100 mL) overnight. The white insoluble material
was collected on a medium frit, washed with hexane, and dried under
1
grown from a concentrated pentane solution at −35 °C. H NMR
(C6D6): δ 6.60 (m, C5H4SiMe3, 6H), 6.18 (m, C5H4SiMe3, 6H), 0.21
(s, C5H4SiMe3, 27H). 13C NMR (C6D6): δ 125.5 (C5H4SiMe3), 119.2
(C5H4SiMe3), 114.4 (C5H4SiMe3), 0.45 (C5H4SiMe3). IR: 3069 w,
2953 m, 2895 m, 2081 w, 1932 w, 1763 w, 1662 w, 1559 w, 1443 m,
1415 m, 1367 m, 1314 m, 1243 s, 1199 s, 1177 s, 1063 m, 1042 s, 906
s, 836 s, 791 s, 779 s, 752 m, 686 m, 632 m cm−1. Anal. Calcd for
C24H39Si3Lu: C, 49.12; H, 6.70. Found: C, 48.73; H, 6.59.
Cp′3La (4). In a nitrogen-filled glovebox, a solution of KCp′ (970
mg, 5.50 mmol) in Et2O (20 mL) was added to a stirred slurry of
LaCl3 (440 mg, 1.79 mmol) in Et2O (20 mL). After it was stirred
overnight, the colorless mixture was sealed in a 100 mL side arm
Schlenk flask fitted with a greaseless stopcock and heated to reflux for
3 h. The solvent was removed from the mixture under vacuum, and
the resulting solids were stirred in hexane (30 mL) while heating to
reflux for 3 h, after which the mixture was stirred at room temperature
overnight. The solvent was removed under vacuum, and the resulting
solids were transferred to a glovebox free of coordinating solvents. The
solids were extracted with hexane (30 mL) and filtered to remove
white insoluble material, presumably KCl and excess KCp′. The
solvent was removed from the colorless filtrate to give 4 as a white
powder (530 mg, 54%). Colorless crystals of 4 suitable for X-ray
diffraction were grown from a concentrated pentane solution at −35
°C. 1H NMR (C6D6): δ 6.48 (m, C5H4SiMe3, 6H), 6.29 (m,
C5H4SiMe3, 6H), 0.21 (s, C5H4SiMe3, 27H). 13C NMR (C6D6): δ
124.8 (C5H4SiMe3), 123.5 (C5H4SiMe3), 118.1 (C5H4SiMe3), 0.00
(C5H4SiMe3). IR: 3061 w, 2954 m, 2894 m, 2077 w, 1931 w, 1735 w,
1640 w, 1544 w, 1441 m, 1412 m, 1362 m, 1311 m, 1243 s, 1193 m,
1177 s, 1058 m, 1040 s, 902 s, 833 s, 769 s, 684 m, 629 m, 530 w cm−1.
Anal. Calcd for C24H39Si3La: C, 52.34; H, 7.14. Found: C, 51.98; H,
7.15.
1
vacuum to yield a white solid determined to be KCp′ by H NMR
spectroscopy15 (7.02 g, 74%). 1H NMR (THF-d8): δ 5.78 (m,
C5H4SiMe3, 2H), 5.67 (m, C5H4SiMe3, 2H), 0.06 (s, C5H4SiMe3, 9H).
[Cp′2Lu(μ-η1:η1-C3H5)]4 (1). In a nitrogen-filled glovebox, a 100
mL round-bottom flask was charged with a solution of [Cp′2Lu(μ-
Cl)]2 (1.62 g, 1.67 mmol) in toluene (50 mL). Allylmagnesium
chloride (2.0 M solution in THF, 1.80 mL, 3.60 mmol) was added
dropwise via syringe to the stirred solution. The reaction mixture was
stirred for 2 h, during which time the colorless mixture became yellow.
The solvent was removed under vacuum, hexane (40 mL) and 1,4-
dioxane (1 mL) were added, and the reaction mixture was stirred
overnight. The yellow mixture was centrifuged to remove white solids,
and the supernatant was filtered and solvent removed under vacuum to
give 1 as a light yellow powder (1.40 g, 85%). Yellow crystals of 1
suitable for X-ray diffraction were grown via slow evaporation from
1
C6D6 at room temperature over several weeks. H NMR (C6D6): δ
3
7.52 (quint, JHH = 10.0 Hz, C3H5, 1H), 6.68 (m, C5H4SiMe3, 4H),
6.15 (m, C5H4SiMe3, 4H), 3.06 (d, 3JHH = 10.0 Hz, C3H5, 4H), 0.05 (s,
C5H4SiMe3, 18H). 13C NMR (C6D6): δ 159.1 (C3H5), 120.8
(C5H4SiMe3), 119.1 (C5H4SiMe3), 113.2 (C5H4SiMe3), 63.0 (C3H5),
−0.43 (C5H4SiMe3). IR: 3068 w, 2952 m, 2894 m, 1543 m, 1485 m,
1443 m, 1403 m, 1364 m, 1310 w, 1247 s, 1180 s, 1042 s, 909 m, 834
m, 785 m, 752 m, 687 m, 645 m, 632 m cm−1. Anal. Calcd for
C19H31Si2Lu: C, 46.52; H, 6.37. Found: C, 46.78; H, 6.31.
Cp′3La(THF) (5). Complex 5 was synthesized as described for 4,
with the exception that THF was used instead of Et2O; KCp′ (1.78 g,
10.1 mmol) and LaCl3 (0.798 g, 3.25 mmol) were combined to
produce 5 as a white powder (1.70 g, 84%). Colorless crystals of 5
suitable for X-ray diffraction were grown from a concentrated pentane
[Cp′2Lu(THF)2][BPh4] (2). In a nitrogen-filled glovebox, a
suspension of [HNEt3][BPh4] (860 mg, 2.04 mmol) in toluene (10
mL) was added to a stirred solution of 1 (1.10 g, 2.24 mmol) in
toluene (40 mL). THF (2 mL) was added, and the resulting pale
yellow mixture was stirred overnight, during which time the color
changed to white. Hexane (20 mL) was added, and the resulting slurry
was centrifuged to collect the insoluble material. The solids were
washed with hexane and dried under vacuum to give 2 as a white
powder (1.85 g, 99%). Colorless crystals of 2 suitable for X-ray
diffraction were obtained by gas-phase diffusion of hexane into a
saturated solution of 2 in THF at room temperature. 1H NMR (THF-
1
solution at −35 °C. H NMR (C6D6): δ 6.50 (m, C5H4SiMe3, 6H),
6.23 (m, C5H4SiMe3, 6H), 3.48 (m, THF, 4H), 1.31 (m, THF, 4H),
0.33 (s, C5H4SiMe3, 27H). 13C NMR (C6D6): δ 123.3 (C5H4SiMe3),
120.6 (C5H4SiMe3), 116.1 (C5H4SiMe3), 71.5 (THF), 26.0 (THF), 1.4
(C5H4SiMe3). IR: 3076 w, 2952 m, 2894 m, 2712 w, 2360 w, 2084 w,
1931 w, 1872 w, 1710 w, 1601 w, 1528 w, 1443 m, 1403 m, 1363 m,
1340 m, 1309 m, 1295 w, 1247 s, 1178 s, 1139 w, 1065 m, 1039 s,
1018 s, 924 m, 904 s, 841 s, 765 m, 687 m, 628 m, 526 w cm−1. Anal.
Calcd for C28H47OSi3La: C, 54.00; H, 7.61. Found: C, 53.78; H, 7.40.
Cp′2CpLa(THF) (6). An initial attempt to synthesize 4 from LaCl3
and 3 equiv of KCp′ using THF containing up to 50 ppm of water led
to the isolation of colorless X-ray-quality single crystals of 6 from
3
d8): δ 7.30 (m, BPh4, 8H), 6.88 (t, JHH = 7.5 Hz, BPh4, 8H), 6.74 (t,
3JHH = 7.0 Hz, BPh4, 4H), 6.72 (m, C5H4SiMe3, 4H), 6.26 (m,
C5H4SiMe3, 4H), 0.31 (s, C5H4SiMe3, 18H). 13C NMR (THF-d8): δ
165.2 (BPh4), 137.1 (BPh4), 125.8 (BPh4), 123.1 (C5H4SiMe3), 122.2
(C5H4SiMe3), 121.9 (BPh4), 116.0 (C5H4SiMe3), 0.3 (C5H4SiMe3).
1
pentane at −35 °C. H NMR (C6D6): δ 6.49 (m, C5H4SiMe3, 4H),
1
The H and 13C resonances of the THF ligands were not observed
6.14 (s, C5H5, 5H), 6.13 (m, C5H4SiMe3, 4H), 3.42 (m, THF, 4H),
1.26 (m, THF, 4H), 0.39 (s, C5H4SiMe3, 18H).
because of exchange with THF-d8. IR: 3053 m, 2999 m, 2952 m, 2902
m, 1943 w, 1880 w, 1817 w, 1581 m, 1496 w, 1481 m, 1444 m, 1427
m, 1365 m, 1345 w, 1312 w, 1250 s, 1176 s, 1067 w, 1043 s, 992 m,
907 m, 836 s, 804 m, 732 m, 707 m, 631 w, 605 w cm−1. Anal. Calcd
for C48H62BO2Si2Lu: C, 63.15; H, 6.85. Found: C, 63.31; H, 6.49.
Cp′3Lu (3). In a nitrogen-filled glovebox, a solution of KCp′ (335
mg, 1.90 mmol) in Et2O (20 mL) was added to a stirred slurry of 2
(1.72 g, 1.88 mmol) in Et2O (40 mL). The pale yellow mixture was
sealed in a 100 mL side arm Schlenk flask fitted with a greaseless
A more direct approach for the synthesis of 6 was subsequently
attempted by reacting KCp′ (400 mg, 2.27 mmol) and KCp (117 mg,
1.12 mmol) with LaCl3 (278 mg, 1.13 mmol) in dry THF (20 mL).
After 12 h, the solvent was removed from the white mixture under
vacuum and the resulting solids were stirred in hexane (10 mL) for 1
h. The mixture was filtered and the colorless filtrate stored at −35 °C
overnight. This produced colorless crystals (68 mg) that were found to
contain both 6 and Cp′3La(THF) (5) in a 14:1 ratio as well as other
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1
(C5Me4H)3Lu9 in good yield. A key step in Scheme 1 is the
use of the tetraphenylborate complex [(C5Me4H)2Ln][(μ-
Ph)2BPh2]. This is a better precursor than the chloride
analogue since the (BPh4)− ion is more easily substituted
than chloride from the metallocene cation.
Although the synthetic approach in Scheme 1 is well
established for pentamethylcyclopentadienyl and tetramethyl-
cyclopentadienyl complexes,8−10,19−23 it has not been exten-
sively used for other types of cyclopentadienyl ligands until
now. We describe the use of this approach to Cp′3Lu and the
characterization of the first examples of Cp′ analogues of the
allyl and tetraphenylborate complexes in Scheme 1, including
the structural characterization of an unusual tetrametallic allyl
complex containing μ-η1:η1-allyl bridges.24−31
unidentified products by H NMR spectroscopy. Unfortunately, an
analytically pure sample of 6 could not be obtained due to similar
solubilities of the products.
Hydrolysis Studies. An NMR tube capped with a rubber septum
was charged with a solution of 5 (40 mg, 0.064 mmol) in THF-d8. A
solution of deionized water in THF-d8 (1.3 M) was sparged with argon
for 5 min. An aliquot of this solution (50 μL, 0.065 mmol) was added
via syringe to the NMR tube, which was immediately shaken. Cp′H
(6.51, 6.40, 2.99, −0.03 ppm) and “[Cp′2La(μ-OH)]n” (6.29, 6.09,
5.30, 0.26 ppm) were the only species observed in the 1H NMR
spectrum.
When the same procedure was followed to add 1 equiv of water (3.5
μL, 0.20 mmol) to a solution of KCp′ (35 mg, 0.20 mmol) in THF-d8,
resonances for Cp′H, KOH (4.75 ppm), KCp (5.67 ppm), and
1
Me3SiOSiMe3 (0.07 ppm) were observed in the H NMR spectrum.
The 1H NMR resonances of these products were assigned according to
the literature14,16,17 and verified by collecting their spectra
independently in THF-d8.
[Cp′2Lu(μ-Cl)]2 reacts with the allyl Grignard reagent
(C3H5)MgCl in toluene to form a pale yellow solid that was
identified by X-ray diffraction as the tetrameric allyl complex
[Cp′2Lu(μ-η1:η1-C3H5)]4 (1) (Scheme 2). Complex 1 was
RESULTS AND DISCUSSION
■
Synthesis of Cp′3Lu. In the case of the smallest lanthanide,
lutetium, the reaction of LuCl3 with 3 equiv of KCp′ produces
primarily the bis(trimethylsilylcyclopentadienyl) chloride dimer
[Cp′2Lu(μ-Cl)]2 (eq 3).
Scheme 2. Synthetic Route to Analytically Pure Cp′3Lu (3)
Trace amounts of Cp′3Lu are formed in this reaction, but
they are not easily separated from [Cp′2Lu(μ-Cl)]2, and
addition of excess KCp′ to the chloride complex has not proven
to be an effective route to Cp′3Lu. A similar situation was
observed with the (C5Me4H)3Ln series: the reactions of ScCl3
and LuCl3 with 3 equiv of KC5Me4H form primarily
(C5Me4H)2ScCl(THF)18 and (C5Me4H)2LuCl2K(THF)2,9 re-
spectively, and the (C5Me4H)3Ln complexes are not generated
in significant yields.
One successful strategy for forming tris(cyclopentadienyl)
complexes of the small rare earths is to use the synthetic route
shown in Scheme 1, involving allyl and tetraphenylborate
intermediates. This provided (C5Me4H)3Sc18 and
isolated in 85% yield and has a solid-state structure (Figure 1)
that differs significantly from the typical monomeric rare-earth
allyl metallocene complexes in which the allyl ligands bind in a
η3 fashion to a single metal.32 In 1, the allyl ligands bridge two
Lu centers in a trans bis-η1 mode to form a 16-membered ring.
Scheme 1. Multistep Synthesis of Sterically Crowded (C5Me4R)3Ln Complexes (R = Me, H)
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but not equivalent at each metal center (Lu1−C17 = 2.508(3)
Å and Lu1−C20 = 2.482(3) Å; Lu2−C19 = 2.485(3) Å and
Lu2−C22 = 2.509(3) Å). The allylic carbons that have more
alkene character (C17 and C22) have the numerically longer
distances to Lu than the allylic carbons that have more alkyl
character (C20 and C19). The 1H NMR spectrum of 1 shows a
single set of (Cp′)− and (C3H5)− resonances, with a quintet
observed at 7.52 ppm for the methine CH proton due to
equivalent coupling to the four allylic protons. Hence, the allyl
ligands are equivalent by NMR in solution. Variable-temper-
ature 1H NMR studies showed only one species over a
temperature range of −75 to +100 °C (spectra are given in the
Supporting Information).
The allyl complex 1 reacts with [HNEt3][BPh4] in toluene/
THF to produce the tetraphenylborate salt [Cp′2Lu(THF)2]-
[BPh4] (2) in nearly quantitative yield (Scheme 2). The
structure of 2 (Figure 2a) was determined by single-crystal X-
ray diffraction to confirm the presence of two THF ligands
coordinated to lutetium. The low quality of the crystal data,
however, does not allow a detailed analysis of metrical
parameters.
The tetraphenylborate complex 2 proved to be a much better
precursor than the chloride complex [Cp′2Lu(μ-Cl)]2 for a salt
metathesis reaction with KCp′. Complex 2 reacts with 1 equiv
of KCp′ to produce analytically pure Cp′3Lu (3) as a
microcrystalline yellow solid in 86% yield, (Scheme 2). It
should be noted that, in contrast to the more sterically crowded
(C5Me5)3Ln complexes which can ring-open THF,8,38−40
rigorous exclusion of THF is not necessary for isolation of
Cp′3Lu. Hence, there is no need to ensure full desolvation of 2
in order to obtain the THF-free analogue of 2 prior to
synthesizing 3.
Figure 1. Molecular structure of [Cp′2Lu(μ-η1:η1-C3H5)]4 (1) with
thermal ellipsoids drawn at the 50% probability level and hydrogen
atoms omitted for clarity. Only one component of a disordered
trimethylsilyl group is shown.
The μ-η1:η1-allyl ligand has been observed previously in a
handful of transition-metal,24−28,33 alkali/alkaline-earth
metal,29−31 and main-group34,35 complexes, but to our
knowledge, the closest example to the bridging allyl structure
in 1 in a rare-earth complex is the μ-η1:η1-cyclohexadienyl
ligand in the polyyttrium compound (C5Me4SiMe3)4-
Y4H7(C6H9).36
The molecular structure of 3, crystallized at −35 °C from
pentane (Figure 2b), was determined to be isomorphous with
the La analogue 4 (see below), as well as with the Ce, Nd, Y,
Ho, and Er analogues previously reported.3,4,6,41 The average
metal−(Cp′ centroid) distance of 2.361 Å in 3 is similar to that
in 4 when the difference in ionic radii of nine-coordinate Lu3+
and La3+ are considered. The fact that 3 contains the smallest
lanthanide, Lu, suggests that the rest of the lanthanide
analogues should be accessible, whether by simple reaction of
LnCl3 with 3 equiv of KCp′ or via the allyl and
tetraphenylborate intermediates, as shown in Scheme 2.
The tetrameric 1 sits about an inversion center such that
there are two crystallographically unique Cp′2Lu(μ-η1:η1-C3H5)
units in the structure. In each allyl ligand, there is a small
difference in carbon−carbon bond lengths (C17−C18 =
1.355(4) Å and C18−C19 = 1.406(4) Å; C20−C21 =
1.400(4) Å and C21−C22 = 1.386(4) Å). This suggests that
there is only a slight localization of the allyl π electrons in the
solid state, as these bond distances are still intermediate
between the typical values for C(sp3)−C(sp2) single bonds and
C(sp2)C(sp2) double bonds: 1.51 and 1.32 Å, respectively.37
Consistent with this, the Lu−C(allyl) bond lengths are similar
Figure 2. Molecular structures of (a) [Cp′2Lu(THF)2][BPh4] (2) and (b) Cp′3Lu (3), with thermal ellipsoids drawn at the 50% probability level
and hydrogen atoms omitted for clarity. For 2, only one of the two independent molecules in the asymmetric unit is shown.
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Synthesis of Cp′3La. The unsolvated complex Cp′3La (4)
can be isolated from LaCl3 and 3 equiv of KCp′ in 54% yield
when Et2O is used as a solvent. The solvated complex
Cp′3La(THF) (5) can be obtained similarly but in THF in 84%
yield. Complex 4 converts quantitatively to 5 with the addition
of THF (Scheme 3). Complexes 4 and 5 were both
characterized by X-ray crystallography (Figure 3), but disorder
in 5 prevented any detailed structural discussion.
Initial attempts to synthesize Cp′3La (4) from LaCl3 and 3
equiv of KCp′ in THF obtained from a drying column that was
no longer active in removing water led to the isolation of
Cp′2CpLa(THF) (6; Cp = C5H5) which was identified by X-
ray crystallography. The structure of 6 is very similar to that of
5 (Figure 3). Complex 6 has an average La−(Cp′ centroid)
distance of 2.592 Å that is 0.033 Å longer than that in 4, as
might be expected for 10-coordinate vs 9-coordinate La3+.42 An
additional difference is that the La metal center in 6 is displaced
0.415 Å out of the plane defined by the three ring centroids,
while the La metal center in 4 lies within 0.127 Å of this plane.
This is consistent with the addition of THF that moves the
geometry from trigonal in 4 toward tetrahedral in 6. Addition
of 2 equiv of KCp′ and 1 equiv of KCp to a mixture of LaCl3 in
anhydrous THF gave NMR data consistent with the formation
of 6, but it was formed as a mixture with other products,
including 5.
Scheme 3. Synthetic Routes to Complexes 4−6
Hydrolysis. The serendipitous formation of a Cp− ligand in
the synthesis of 6 in inadequately dried THF was surprising,
because hydrolysis of rare-earth metallocenes more commonly
cleaves a cyclopentadienyl ligand rather than a SiMe3 group.
Formation of the [Cp′2La(μ-OH)]n lanthanum analogue of the
13
known [Cp′2Lu(μ-OH)]2 would have been expected.
Although organometallic rare-earth complexes typically react
with water to give protonated ligands and metal oxides or
hydroxides, there have been exceptions where cyclopentadienyl
ligands remain intact. For example, the reactions of Cp3Ln (Ln
= Y, Ho) and (C5H4Me)3Ho with water produce the aquo
complexes Cp3Ln(H2O) and (C5H4Me)3Ho(H2O), respec-
tively.43 The mixed-ligand complex Y(OAr)2(bpzcp) (OAr =
OC6H3Me2-2,6; bpzcp = 2,2-bis(3,5-dimethylpyrazol-1-yl)-1,1-
Figure 3. Molecular structures of (a) Cp′3La (4), (b) Cp′3La(THF) (5), and (c) Cp′2CpLa(THF) (6), with thermal ellipsoids drawn at the 50%
probability level and hydrogen atoms omitted for clarity. For 5 and 6, only one of the two independent molecules in the asymmetric unit is shown.
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Organometallics
Article
diphenylethylcyclopentadienyl) forms [{Y(bpzcp)}(μ-
OH)2(μ3-OH){Y(OAr)2}]2 in the presence of 1.5 equiv of
water.44
[Cp′2Lu(THF)2][BPh4] intermediates. The latter synthesis
demonstrates that this chloride/allyl/tetraphenylborate route
that has been so successful with the larger cyclopentadienyl
ligands, (C5Me5)− and (C5Me4H)−, can also be used with
monosubstituted (C5H4SiMe3)−. The unusual bis-η1 allyl
bridging mode found in the [Cp′2Lu(μ-η1:η1-C3H5)]4 tetramer
demonstrates how the metallocenes of the (C5H4SiMe3)−
ligand can differ from those of the more substituted analogues.
It was of interest to determine if Cp′3La (4) was initially
formed and underwent hydrolysis at the C−SiMe3 bond in one
of the Cp′ rings to form 6. However, when a solution of
Cp′3La(THF) (5) in THF-d8 was treated with 1 equiv of water,
Cp′H16 was observed by H NMR spectroscopy as well as
1
resonances consistent with a [Cp′2La(μ-OH)]n complex.13 In
contrast, when a solution of KCp′ in THF-d8 was treated with 1
equiv of water, a colorless crystalline precipitate formed and the
1H NMR spectrum of the remaining solution contained
ASSOCIATED CONTENT
* Supporting Information
■
S
Figures, tables, and CIF files giving NMR spectra, crystallo-
graphic data collection, structure solution, and refinement
details, and X-ray diffraction details of compounds 1−6
(CCDC Nos. 923589−923594). This material is available
free of charge via the Internet at http://pubs.acs.org.
17
resonances for Cp′H,16 KCp,45 and Me3SiOSiMe3 as well
as a broad resonance at 4.75 ppm that matched that observed
for KOH in wet THF-d8. When the isolated precipitate was
1
dissolved in additional THF-d8, it was found to be KCp by H
NMR spectroscopy.45
AUTHOR INFORMATION
Corresponding Author
*E-mail for W.J.E.: wevans@uci.edu.
These products suggest that KCp′ reacts with water in two
ways: (1) protonation of the ligand to form Cp′H and metal
hydroxide, as seen with Cp′3La(THF) (5) above, and (2) C−
SiMe3 cleavage to form KCp and Me3SiOH, which most likely
undergoes a condensation reaction46 with another 1 equiv of
Me3SiOH to produce Me3SiOSiMe3 and H2O (Scheme 4).
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Scheme 4. Possible Formation of Cp′2CpLa(THF) (6) via
We thank the U.S. National Science Foundation (Grant No.
CHE-1010002) for support of this research and Jordan F.
Corbey for assistance with X-ray crystallography.
Hydrolysis
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