Stannocene as cyclopentadienyl transfer agent in transmetalation reactions with lanthanide metals for the synthesis of tris(cyclopentadienyl)lanthanides

Article abstract of DOI:10.1002/zaac.201000239

Transmetalation of the lanthanide metals samarium and ytterbium with bis(cyclopentadienyl)tin(II), stannocene, (η⁵-C₅H ₅)₂Sn as cyclopentadienyl transfer agent yields the tris(cyclopentadienyl)lanthanides (η⁵-C₅H ₅)₃Ln (in toluene), (η⁵-C₅H ₅)₃Ln·THF (Ln = Sm, Yb) and bis(cyclopentadienyl) ytterbium, (η⁵-C₅H₅)₂Yb· THF in a direct, facile, high-yield and halide-free route. In toluene the base-free forms (η⁵-C₅H₅)₃Ln are isolated directly. In THF the adducts (η⁵-C₅H ₅)₃Ln·THF are obtained in crystalline form. At a 1:1 molar ratio of Yb:Sn the ytterbium metal reduces the initially formed (η⁵-C₅H₅)₃Yb·THF to (η⁵-C₅H₅)₂Yb·THF.

Full text of DOI:10.1002/zaac.201000239

ARTICLE
DOI: 10.1002/zaac.201000239
Stannocene as Cyclopentadienyl Transfer Agent in Transmetalation
Reactions with Lanthanide Metals for the Synthesis of
Tris(cyclopentadienyl)lanthanides
Christoph Janiak*[a]
In Memory of Professor Herbert Schumann
Keywords: Stannocenes; Ligand transfer; Samarium; Ytterbium; Metallocenes; Cyclopentadienyl ligands
Abstract. Transmetalation of the lanthanide metals samarium and yt- high-yield and halide-free route. In toluene the base-free forms (η5-
terbium with bis(cyclopentadienyl)tin(II), stannocene, (η5-C5H5)2Sn as C5H5)3Ln are isolated directly. In THF the adducts (η5-C5H5)3Ln·THF
cyclopentadienyl transfer agent yields the tris(cyclopentadienyl)lantha- are obtained in crystalline form. At a 1:1 molar ratio of Yb:Sn the
nides (η5-C5H5)3Ln (in toluene), (η5-C5H5)3Ln·THF (Ln = Sm, Yb) and ytterbium metal reduces the initially formed (η5-C5H5)3Yb·THF to (η5-
bis(cyclopentadienyl)ytterbium, (η5-C5H5)2Yb·THF in a direct, facile, C5H5)2Yb·THF.
Introduction
The bent sandwich compound bis(cyclopentadienyl)tin(II)
[stannocene, (η5-C5H5)2Sn] [1] and the tris(cyclopentadi-
Unlike for cyclopentadienylthallium [19], we are not aware
enyl)lanthanides, (η5-C5H5)3Ln [2, 3] (also with one or two
of any use of stannocene as a cyclopentadienyl transfer agent.
donor [solvent] molecules) [4–6] were among the prototypical
There is one report on the reaction of the stannocene derivative
early cyclopentadienyl-metal compounds synthesized by
(η5-C5Me5)2Sn with YbI2 to give [(η5-C5Me5)2YbI(THF)2]
Fischer and Wilkinson in the 1950s following the discovery of
[20].
ferrocene [7].
Here we report the use of stannocene as a cyclopentadienyl
Complexes (η5-C5H5)3Ln are sublimable compounds with in-
transfer agent in transmetalation reactions with lanthanide met-
als for the synthesis of tris(cyclopentadienyl)lanthanides.
terest in their electronic structure [8], optical properties [9],
uses as synthetic starting materials [10] and dopants [11] for
semiconductive thin films [12], organic UV photocathodes
[13], or mesoporous activated carbon [14]. Compounds (η5-
C5H5)3Ln are typically synthesized from the metathesis of an-
hydrous lanthanide trichlorides with a large excess of sodium
cyclopentadienide in tetrahydrofuran (THF) [Equation (1)] [2–
5, 15].
Results and Discussion
Stannocene, (η5-C5H5)2Sn can be easily prepared in large
quantities and good yield from cyclopentadienylsodium and
tin(II) dichloride. It can be very well purified by sublimation,
is moderately air-sensitive and storable for prolonged times
under inert gas at room temperature.
Stannocene reacts with samarium and ytterbium metal pow-
der in toluene or tetrahydrofuran with formation of tris(cyclo-
pentadienyl)samarium and -ytterbium in high yield. In toluene
the base-free form (η5-C5H5)3Ln is isolated directly [Equation
(3)]. In THF, the adducts (η5-C5H5)3Ln·THF are obtained in
crystalline form [Equation (4)], from which THF-free com-
pounds would be available after vacuum sublimation above
200 °C [2, 3].
Also, molten (C5H5)2Be or (C5H5)2Mg were used as cyclo-
pentadienyl transfer reagents with LnX3 [16], and C5H5Tl [17]
or (C5H5)2Hg [18] in transmetalation reactions with rare earth
metal powders [Equation (2)] to yield (η5-C5H5)3Ln [4].
* Prof. Dr. C. Janiak
E-Mail: janiak@uni-freiburg.de
[a] Institut für Anorganische und Analytische Chemie
Universität Freiburg
Albertstr. 21
79104 Freiburg, Germany
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C. Janiak
ARTICLE
Experimental Section
Syntheses were routinely carried out under an inert gas of dried nitro-
gen or argon with Schlenk and vacuum techniques. The glass ware
was flame-dried prior to its usage. Solvents were dried with and dis-
tilled from (blue) sodium/benzophenone (ketyl) or liquid Na/K alloy
(25 wt.-% Na/75 wt.-% K) and stored under inert gas. Samarium and
ytterbium powders were obtained from Auer-Remy/Hamburg. NMR
spectra were obtained with samples in sealed NMR tubes with a Bruker
WP 80 SI and are referenced against TMS. The line width for the
paramagnetic samarium and ytterbium probes is given as the frequency
width at half-height maximum (FWHM). IR spectra were measured
with a Perkin–Elmer 580B infrared spectrometer in CsI (2–3 mg sam-
ple/300 mg CsI) with a CsI reference, Raman spectra with a Jobin–
Yvon spectrometer (Kr Laser, room temperature, 100–2000 cm–1). For
both the IR and Raman spectra only signals classified from weak (w),
medium (m) to strong (s) and shoulders (sh) but no very weak signals
are listed. C,H analyses were done with a Perkin–Elmer 240C CHN
elemental analyzer. Melting points were measured in capillaries, which
were sealed under vacuum. Cyclopentadienylsodium was prepared
from freshly cracked cyclopentadiene, C5H6 (42.0 mL, 0.51 mol, den-
sity ca. 0.8 g·mL–1), which was slowly added dropwise to a suspension
of finely cut sodium metal (14.0 g, 0.61 mol, excess) in tetrahydro-
furan (250 mL). After stirring for 18 h the slightly pink THF solution
was decanted from unreacted excess sodium and the solvent was re-
moved under vacuum to yield C5H5Na as a colorless powder (43.0 g,
96 %).
In THF and a 1:1 molar ratio of Yb:Sn the (excess) ytterbium
metal reduces the initially formed (η5-C5H5)3Yb·THF (verified
by its green color) to bis(cyclopentadienyl)ytterbium·which is
isolated as the yellow-orange mono-tetrahydrofuran complex,
(η5-C5H5)2Yb·THF complex [Equation (5)].
In toluene such a reduction with excess ytterbium metal is
not observed. An excess of samarium metal does not exhibit
such a reduction either, in line with the transmetalation with
C5H5Tl, which also gave solely (η5-C5H5)3Sm [21]. Thus, even
at a 1:1 molar ratio of samarium and (η5-C5H5)2Sn tris(cyclo-
pentadienyl)samarium is obtained in high yield. In analogy to
the transmetalation reactions of lanthanide metals with C5H5Tl
a tiny amount of mercury was added to the reactions of sama-
rium and ytterbium metal with stannocene [21]. The role of
mercury is to facilitate the start of the reaction by cleaning the
lanthanide metal surface through amalgamation. A summary of
the transmetalation reactions of samarium and ytterbium with
stannocene, (η5-C5H5)2Sn is provided in Table 1.
Bis(cyclopentadienyl)tin(II), Stannocene, (C5H5)2Sn: Solid C5H5Na
(43.0 g, 0.49 mol) and tin(II) dichloride (46.4 g, 0.245 mol) were
weighed into a 250-mL Schlenk flask. THF (150 mL) was added under
ice cooling to start an immediate exothermic reaction. Stirring was
continued for 1.5 h, afterwards the solvent was removed under vacuum
with gentle warming. The product was sublimed from the residue at
80 °C/10–1 Torr and obtained as colorless to slightly yellow crystals
(yield 33.4 g, 55 %).
Table 1. Transmetallation reactions of lanthanide metals with
stannocene.a)
Ln
Molar ratio
Solvent Temperature /°C; Product(yield /%)
time /d
Tris(cyclopentadienyl)samarium, (C5H5)3Sm: To a mixture of stan-
nocene (2.26 g, 9.08 mmol) and samarium metal powder (1.37 g,
9.08 mmol) was added a very small drop of mercury and toluene
(20 mL). The suspension was stirred at 60 °C for 4 d. Additional tolu-
ene (50 mL) was added to the off-white to yellow suspension and the
product was separated from the metal residue through a glass frit
equipped with a bypass to enable a continuous hot extraction. The
toluene solution was decanted from the precipitated product in the fil-
trate and discarded. The orange-yellow product powder was dried un-
der vacuum (yield 1.42 g, 68 %, based on stannocene). M. p. 346–
metal
Ln:Sn
Sm
Sm
Sm
Yb
Yb
Yb
1:1
2:3
1:1
1:1
2:3
1:1
toluene 60; 4
(C5H5)3Sm (68)b)
(C5H5)3Sm·THF (74)
(C5H5)3Sm·THF (87)b)
(C5H5)3Yb (77)b)
THF
THF
r.t.; 7
r.t.; 7
toluene 50; 7
THF
THF
r.t.; 2
r.t.; 3
(C5H5)3Yb·THF (76)
(C5H5)2Yb·THF (73)
a) R.t. = room temperature. b) Yield based on the minor stannocene component.
The color, melting points, IR-, Raman- and 1H NMR spectra
of the (η5-C5H5)3Ln, (η5-C5H5)3Ln·THF (Ln = Sm, Yb) and
(η5-C5H5)2Yb·THF products obtained here through transmeta-
lation agree with the literature values of the compounds [4].
348 °C (lit. 365 °C [3], ca. 330 °C) [24]. IR (CsI): ν = 3100 (m), 3085
˜
(m), 3070 (m), 1607 (m), 1442 (m), 1438 (m), 1363 (w), 1263 (w),
1238 (w), 1072 (w), 1008 (s), 914 (sh), 890 (m), 840 (m), 792 (sh),
770 (vs, br), 664 (m), 621 (w), 550 (w, br) cm–1 (lit. [24]). Raman:
1
1126 cm–1. H NMR (C6D6): δ = 12.8 (s, FWHM 22 Hz). C15H15Sm
(345.68): calcd. C 52.12, H 4.37; found C 51.37, H 4.36.
Conclusions
Tris(cyclopentadienyl)samarium·Tetrahydrofuran,
(C5H5)3Sm·THF: To a mixture of stannocene (1.49 g, 5.99 mmol) and
samarium metal powder (0.60 g, 3.99 mmol) was added a very small
drop of mercury and THF (20 mL). After 12 h a yellow, finely crystal-
line product residue has already been observed. The suspension was
stirred at room temperature for 7 d. Afterwards, additional THF
(30 mL) was added and the clear yellow solution was decanted from
the metal and product precipitate. Cooling (–20 °C) of the THF solu-
tion yielded yellow crystals whose yield was increased through re-
peated residue extraction and cooling of the bridge-connected residue
The transmetalation of the lanthanide metals samarium and
ytterbium is a facile, high-yield and halide-free route to the
tris(cyclopentadienyl)lanthanides
(η5-C5H5)3Ln,
(η5-
C5H5)3Ln·THF (Ln = Sm, Yb) and to bis(cyclopentadienyl)yt-
terbium, (η5-C5H5)2Yb·THF. The synthetic route should be ex-
tendable to (η5-C5H5–nRn)3Ln compounds with ring-substituted
cyclopentadienyl ligands [4, 22] for which a wide range of
stannocene derivatives is also available [23].
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Z. Anorg. Allg. Chem. 2010, 2387–2390

Stannocene as Cyclopentadienyl Transfer Agent in Transmetalation Reactions
and product flask. Finally the volume of the mother liquor was de- in vacuo and overlayering with hexane yielded yellow-orange crystals
creased in vacuo to further enhance the yield (1.24 g, 74 %). M.p. of the mono-THF complex and violet, feather-like crystals of the bis-
348–350 °C with irreversible darkening from 200 °C and softening at THF complex. The latter turned to a yellow powder upon drying in
335 °C.
vacuo by giving off one solvent molecule (yield 2.00 g, 73 %). M.p.
dec. at about 170 °C (no melting up to 350 °C). 1H NMR ([D8]THF):
δ = 5.67 (s, C5H5 no line broadening) (lit. in [D8]THF 5.64 ppm) [15b].
C14H18OYb (375.34): calcd. C 44.80, H 4.83; found C 44.96, H 5.03.
A 1:1 molar ratio of stannocene (1.85 g, 7.43 mmol) and samarium
metal powder (1.12 g, 7.43 mmol) yielded the same product (yield
1.81 g, 87 %, based on stannocene).
IR (CsI): ν = 3090 (w), 3075 (sh), 2980 (w), 2890 (w), 2875 (sh),
˜
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1
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Published Online: August 25, 2010
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