Microwave-assisted generation of lanthanide(II) halides in THF and simple quantitative determination

Article abstract of DOI:10.1002/ejic.200400309

Lanthanide(II) reagents (SmI₂, SmBr₂, YbI ₂, EuI₂) have been prepared rapidly in high yields using microwave-assisted heating. Samarium diiodide is obtained as a saturated solution in THF (0.13 M) within 5 min and ytterbium diiodide (0.065 M) after 45 min. A simple method for quantitative determination of LnX₂ in THF is also described by utilizing the instantaneous reaction with ketones in the presence of an amine and water. This method establishes the concentration of the active single-electron-donor species, i.e. Ln²⁺. Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004.

Full text of DOI:10.1002/ejic.200400309

SHORT COMMUNICATION
Microwave-Assisted Generation of Lanthanide(II) Halides in THF and Simple
Quantitative Determination
[a]
[a]
´
Anders Dahlen and Göran Hilmersson*
Keywords: Microwave-assisted reactions / Lanthanides / Halides / Titrations / Quantitative determinations
Lanthanide(II) reagents (SmI2, SmBr2, YbI2, EuI2) have been
prepared rapidly in high yields using microwave-assisted
heating. Samarium diiodide is obtained as a saturated solu-
instantaneous reaction with ketones in the presence of an
amine and water. This method establishes the concentration
of the active single-electron-donor species, i.e. Ln2+
.
tion in THF (0.13
M) within 5 min and ytterbium diiodide
(0.065 ) after 45 min. A simple method for quantitative de- ( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
M
termination of LnX2 in THF is also described by utilizing the
Germany, 2004)
Introduction
from several different batches from commercial sources
showed that the concentration of SmI2 varied dramatically.)
Today there are only a few volumetric titration methods
known for e.g. SmI2; however, these methods only establish
the total concentration of IϪ or Sm2ϩ and Sm3ϩ (i.e. they
also measure inactive reagents such as SmI3).[16Ϫ18] There
is, to the best of our knowledge, no method known for a
direct and quantitative method for the titration of the sin-
gle-electron-transfer reagent Ln2ϩ itself. Recently, we re-
ported on the SmI2/amine/water instantaneous reduction of
ketones.[19] This reaction constitutes a key for quantitative
determination of the concentration of various LnX2 solu-
tions, since the reduction of simple ketones is very clean
and proceeds extremely fast. Furthermore, the progress of
the reaction may be monitored by the disappearance of the
characteristic dark blue colour of SmI2 in THF (Scheme 1).
The introduction of the versatile and highly selective sin-
gle-electron-transfer reagent SmI2 has had a great impact
on the development of organic chemistry during the last
10Ϫ15 years.[1Ϫ4] The reduction potential of SmI2 is in the
range of Ϫ1.3 to Ϫ2.2 V,[5] depending on solvents and li-
gands, which makes SmI2 particularly useful for selective
reduction and various coupling reactions.[6,7] Since SmI2
has become commercially available, the development of
SmI2-mediated reactions has rapidly entered different areas
of organic synthesis. Recently, there have also been reports
on the use of other anions (BrϪ and amides)[8Ϫ10] and lan-
thanides (Yb, Nd and Dy)[11Ϫ13] for various selective re-
duction reactions. However, reagents such as SmBr2, YbI2,
EuI2 and ThI2 are still far from being widely used, much
due to the lack of simple methods and long reaction times
needed for their preparation. Recent progress in microwave-
assisted synthesis is particularly attractive due to dramati-
cally shorter reaction times and higher yields relative to
conventional reaction conditions.[14,15]
Scheme 1. The colour changes from dark blue to white during the
reduction of ketones
We have realised that research groups that currently ex-
plore the chemistry mediated by e.g. SmI2 are lacking a re-
liable method for a simple, rapid and quantitative determi-
nation of the concentration of the reducing reagent itself. It
is important that the concentration of the reducing reagent
is known before the success of a new SmI2-mediated reac-
tion can be fully evaluated. The concentration of the com-
mercially available SmI2 reagent (0.1 ) has been found to
vary from 0.05 to 0.10 . (Evaluating the concentrations
Results and Discussion
Generation of LnX2
Microwave-assisted heating of THF mixtures of sam-
arium metal and iodine, iodoform or diiodoethane for 5
min (at 180 °C) results in rapid formation of dark blue solu-
tions of SmI2. The resulting SmI2 solutions are of similar
quality whether they are made from iodine, iodoform or
diiodoethane. However, the use of iodine is preferred since
this source of IϪ does not involve the generation of gaseous
[a]
Organic Chemistry, Department of Chemistry, Göteborg
University,
41296 Göteborg, Sweden
Fax: (internat.) ϩ 46-31-772-3840
E-mail: hilmers@chem.gu.se
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
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DOI: 10.1002/ejic.200400309
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Microwave-Assisted Generation of Lanthanide(II) Halides in THF
SHORT COMMUNICATION
by-products (i.e. acetylene or ethene)[20,21] that may cause can be stored for extended periods of time inside the glove
the maximum pressure allowed in the microwave reaction box without any observable change in colour, concentration
vessel to be exceeded during heating. Crystals of iodine are or reactivity, as long as there is a slight excess of ytterbium
also more easily handled inside the glove box. There is a metal present. The UV/Vis spectrum of the microwave-gen-
significant difference in the rate of SmI2 formation from erated YbI2 is identical to that previously reported (Sup-
samarium metal if it is used as chunks or as a powder. The porting Information).[22]
formation of SmI2 using powder of mesh Ϫ40 is rapid; the
We were also interested in the preparation of SmBr2 since
reaction is complete within 3Ϫ5 min, while chunks require it is known to have a considerably larger oxidation potential
about 20Ϫ30 min. When SmI2 is generated from more than (Ϫ2.07 V) than SmI2 in THF.[23] Recently, Namy and co-
the necessary amount of I2 and Sm, the excess SmI2 precipi- workers reported on the preparation of SmBr2 from tetra-
tates leaving a saturated THF solution. After centrifugation bromoethane.[10] We found that the predicted concentration
of excess undissolved SmI2, the maximum concentration of of SmBr2 has to be significantly higher than that of SmI2,
SmI2 in THF is 0.13 . The UV/Vis spectrum of the micro- otherwise the preparation failed; the use of approximately
wave-generated SmI2 was identical to that previously re- 0.45 mmol of SmBr2 per mL of THF gives reproducible re-
ported (Supporting Information; for Supporting Infor- sults. The samarium bromide could be prepared in a micro-
mation see also the footnote on the first page of this arti- wave oven from samarium powder and either bromine or
cle).[22]
tetrabromoethane. However, we recommend preparation of
Ytterbium diiodide (YbI2) has not been used much in SmBr2 from tetrabromoethane since it gives somewhat cle-
organic synthesis since the first report by Kagan.[21] This is aner reactions relative to SmBr2 prepared from bromine.
in part due to its reported low solubility in THF (0.040 ) (The GC chromatograms of reduced 2-heptanone with
and lower reactivity than that of SmI2. In contrast to the SmBr2/triethylamine/water shows no impurities for tetra-
large number of reports on SmI2-mediated reactions, the bromoethane-generated SmBr2, while SmBr2 generated
development of YbI2 as a single-electron-transfer reagent is from bromine gives several smaller, unidentified peaks.) In
still in its infancy. It was, therefore, with great interest that agreement with previous studies we conclude that the solu-
we prepared YbI2 from ytterbium metal and iodine crystals bility of SmBr2 is very low in THF, and it exists predomi-
using microwave-assisted heating. The saturated clear yel- nantly as a suspension. The UV/Vis spectrum agrees with
low THF solution of YbI2 (maximum solubility 0.065  in the spectrum for SmBr2 prepared from SmI2 and LiBr pre-
THF) is formed in less than 45 min under microwave heat- viously reported by Flowers and co-workers (Supporting
ing at 180 °C, which results in a pressure of 13 bar. YbI2 Information).[23]
Table 1. Microwave-assisted generation of LnX2 and the resulting concentration in THF[a]
Entry
Ln (mmol)
X2 (mmol)
V [mL]
[LnX2]theor. []
[LnX2]titrated []
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
Sm (1.3)[b]
Sm (1.0)
Sm (1.0)
Sm (1.0)
Sm (1.1)
Sm (13)[b]
Sm (5.0)
Sm (5.0)
Sm (5.0)
Sm (5.0)
Sm (5.0)
Sm (5.0)
Sm (5.0)[b]
Sm (5.0)[b]
Sm (3.0)
Sm (3.0)[b]
Sm (2.3)[d]
Sm (2.3)[d]
Yb (2.9)
Yb (2.9)
Yb (2.9)
Yb (2.9)
Yb (2.9)
Yb (2.9)
I2 (0.10)
I2 (0.32)
I2 (0.39)
I2 (0.47)
I2 (0.50)
I2 (11.0)
I2 (0.25)
I2 (0.50)
I2 (0.75)
I2 (1.00)
I2 (1.50)
I2 (2.00)
I2 (1.50)
I2 (2.00)
C2H2Br4 (0.75)
C2H2Br4 (0.75)
C2H2Br4 (0.75)
Br2 (1.50)
I2 (0.16)
I2 (0.276)
I2 (0.33)
I2 (0.355)
I2 (0.48)
I2 (0.59)
10.0
5.0
5.0
5.0
5.0
100.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
3.5
3.5
3.5
3.5
5.0
5.0
5.0
5.0
5.0
5.0
0.010
0.063
0.078
0.094
0.100
0.110
0.050
0.100
0.15
0.20
0.30
0.40
0.30
0.010
0.063
0.076
0.091
0.099
0.108
0.048
0.102
0.15[c]
0.21[c]
0.31[c]
0.36[c]
0.29[c]
0.35[c]
0.42[c]
0.43[c]
0.44[c]
0.40[c]
0.031
0.057
0.064
0.069[c]
0.108[c]
0.119[c]
0.40
0.43
0.43
0.43
0.43
0.032
0.055
0.063
0.071
0.100
0.118
[a]
The starting materials were mixed inside a glove box. YbI2 was microwave-irradiated for 45 min, and SmI2 and SmBr2 for 5 min.
[LnX2] was titrated according to the method described below to yield the concentration with an error of approximately Ϯ2%. [b] Prepared
in ultrasonic bath for 2Ϫ4 h. Suspension, maximum solubility is exceeded, and the given value is an expected concentration provided
that all reagents would dissolve. Prepared inside a glove box with magnetic stirring for at least 15 h.
[c]
[d]
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´
A. Dahlen, G. Hilmersson
SHORT COMMUNICATION
The use of microwave heating was also explored in the
It may be argued that SmI2 decomposes rapidly in the
preparation of EuI2. We found that EuI2 is prepared within presence of excess water and amine. The stability of the re-
45 min (this reaction time was not optimised) to yield a agent mixture of SmI2 with excess water and amine in THF
clear, light yellow solution. The UV/Vis spectrum is ident- was therefore also studied (Figure 1). There is an observed
ical to that previously reported (Supporting Infor- decomposition of a 0.075  solution of SmI2 in THF in the
mation).[22]
presence of 4 equiv. of water and 3 equiv. of amine. How-
ever, the decomposition of SmI2 after 1 h is only about 5%.
We therefore conclude that the decomposition of SmI2 is
only of marginal consequence for the analysis. The volu-
metric addition of the ketone to the THF solution should
only require approximately 1Ϫ2 min, which corresponds to
a maximum error of about 1% in the analysis.
Volumetric Titration of SmI2 Solutions
The reduction of 2-heptanone has been studied using
mixtures of SmI2, amine and water. There is no side-reac-
tion, thus two molecules of SmI2 are consumed for every
reduced molecule of ketone. An excess of IϪ, I2, Sm0 or
Sm3ϩ is not capable of reducing even trace amounts of ke-
tone, hence the reduction of the ketone proceeds only by
reaction with the single-electron-transfer reagent SmI2, and
the amount of reduced ketone is directly proportional to
the molar concentration of SmI2 in THF according to
Equation (1).
(1)
Triethylamine (approximately 3 equiv.) and water (ap-
proximately 4 equiv.) were added to a solution of unknown
concentration of SmI2 in THF (10Ϫ130 m). The resulting
mixture of unknown concentration of the single-electron-
transfer reagent SmI2 was volumetrically titrated with a ke-
tone (2-heptanone) dissolved in THF until the end point,
i.e. when the characteristic dark blue colour of SmI2 had
disappeared and a white precipitate and colourless liquid
remained. The reduction is instantaneous, even when there
is excess precipitated SmI2 present, which therefore allows
rapid titration. The titrant reacts immediately, and there is
no delay time during the titration. The added titrant volume
was read when the colour changed from light green to
white (Table 2).
Figure 1. Decomposition of SmI2 in the presence of triethylamine
(3 equiv.) and water (4 equiv.) over an extended period of time
(30 h)
GC-Based Titration of SmI2 and YbI2
Since the other lanthanide() halides do not have such a
characteristic colour as SmI2, another method for the deter-
mination of their concentrations was developed. Triethyl-
amine (3 equiv.), water (4 equiv.) and 2-heptanone (1 equiv.)
were added to a 5 mL solution or suspension of SmI2 or
YbI2 in THF (10Ϫ400 m). Provided that the amount of
ketone is more than half of the actual amount of LnX2,
the instantaneous reaction of the excess ketone with SmI2/
amine/water results in a mixture of 2-heptanol and 2-hep-
tanone (Scheme 2). Analysis of the amounts of 2-heptanol
and 2-heptanone by GC chromatography (verified by an
internal standard) yields the initial concentration of LnX2
(Tables 1 and 2).
Table 2. Comparison of volumetric titration with GC-based deter-
minations of [SmI2][a]
[SmI2]Volumetric titration
[SmI2]Conversion
[SmI2]Internal standard
[][b]
[][c]
[][d]
0.077
0.077
0.074
0.074
0.075
0.076
0.074
0.075
0.074
0.075
0.075
0.074
Scheme 4. Unbalanced formula for the LnX2/amine/H2O re-
ductions
[a]
SmI2 (5 mL) was added to a dry Schlenk tube, then Et3N (3
[b]
equiv.) and water (4 equiv.) were added by syringe. 2-heptanone
was dissolved in dry THF (0.070 ) and added dropwise to the GC-Based Titration of Suspensions of SmBr2, SmI2 and
SmI2/H2O/amine mixture. The colour of the solution changed from
dark blue to green and finally white. The added titrant volume was
YbI2
Initially, we did not expect to be able to determine the
amount of precipitated SmI2 or SmBr2 utilizing the re-
duction of 2-heptanone in the presence of amine and water.
On the contrary, the reduction of the ketone is extremely
fast (instantaneous), even with suspensions of YbI2, SmI2
and SmBr2; this allows the quantitative titration of suspen-
noted when the colour changed from light green to white. [SmI2]
was calculated according to the balanced Equation (1). The de-
termined [SmI2] calculated from the conversion of 2-heptanone to
2-heptanol after integration of the separate peaks of the GC chro-
matogram. [d] The determined [SmI2] based on the concentration of
unreduced 2-heptanone relative to an internal standard (1-hexanol)
after integration of the separate peaks of the GC chromatogram.
[c]
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Eur. J. Inorg. Chem. 2004, 3020Ϫ3024

Microwave-Assisted Generation of Lanthanide(II) Halides in THF
SHORT COMMUNICATION
bar, located inside a glove box and fitted with a septum. Triethyl-
amine (approximately 3 equiv.) and water (approximately 4 equiv.)
were then added to the SmI2 solution by syringe. While vigorously
stirring, 2-heptanone diluted in THF (0.07 ) was added dropwise
until the dark blue solution turned colourless with the formation
of a white suspension, and the volume of added 2-heptanone was
read. The final calculation of [SmI2] could then be carried out using
the balanced formula [Equation (1)].
sions of up to at least 0.4  (Table 1). These results also
indicate that suspensions of either YbI2, SmI2 or SmBr2 can
be used as single-electron-transfer reagents in the instan-
taneous reduction of, for example, ketones by the amine/
water method.
Titration of EuI2
Titrations of EuI2 in THF were unsuccessful. Apparently,
the oxidation potential of Eu2ϩ is too low to mediate the
reduction of, for example, 2-heptanone in the presence of
triethylamine and water.
General GC-Based Determination of the Active Amount of SmBr2,
SmI2 and YbI2: A solution or suspension of SmI2 (5 mL) was added
to a test tube containing a magnetic stirrer bar, located inside a
glove box and fitted with a septum. Triethylamine (approximately
3 equiv.) and water (approximately 4 equiv.) were added into the
SmI2 solution by syringe. While vigorously stirring, excess 2-hep-
tanone diluted in THF (0.070 ) (i.e. more ketone than would be
possible to reduce with the estimated amount of the prepared lan-
thanide reagent) was added in one portion, which caused the solu-
tion to decolourise. With the use of gas chromatography, the
amount of consumed 2-heptanone was determined with an internal
standard added, e.g. 1-hexanol. The final calculation of [SmI2]
could then be performed according to Equation (1).
Conclusion
There should be numerous reduction reactions for which
the oxidation potential of SmI2 is nonideal, and in which
other lanthanide halides and also samarium with different
counter anions may play a crucial role. We believe that the
exploration of the chemistry mediated by these reagents is
dependent on simple methods for their preparation. Due to
the very low oxidation potential of EuI2 its synthetic utility
as a single-electron-reagent is somewhat limited, neverthe-
less its use may increase once this rapid and reliable method
of preparation is known. Microwave-assisted generation of
LnX2 from metal (Ln ϭ Sm, Yb and Eu) and halides (I2
and Br2) appear to be a viable alternative to the recently
reported fast sonication technique.[20] Preliminary results
indicate that LnX2 reagents can be prepared in situ in the
presence of reducible substrates. In an ideal situation that
would allow the use of less solvent as the actual concen-
tration of LnX2 could be kept low during the reaction.
The direct method of determining the concentration of
LnX2 solutions by utilizing the instantaneous reaction be-
tween, for example, SmI2 and ketones in the presence of
excess amine and water has proven to be a very powerful
and reliable method. The major advantage with this method
is that the determination is made only on the active single-
electron-donor species, i.e. LnX2. Previously reported meth-
ods also include inactive species such as LnX3 and therefore
tend to overestimate the concentration of the reagent. This
simple method of determining accurate concentrations of
LnX2 will therefore become useful among those working
with single-electron-transfer reagents.
Supporting Information Available (see also the footnote on the first
page of this article): UV/Vis spectra of THF solutions of SmI2,
YbI2, SmBr2 and EuI2. Further experimental details are also in-
cluded.
Acknowledgments
˚
We thank the Swedish Research Council (Vetenskapsradet) for fin-
ancial support, and Knut and Alice Wallenberg for funding of the
glove box. We also thank Personal Chemistry AB and Astra-
Zeneca R&D in Mölndal for the microwave equipment.
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