Deuterated dimethyl sulfoxide as a good NMR solvent for the characterization of alkali metal cyclopentadienides, amides, alkoxides and phenoxides

Article abstract of DOI:10.1016/j.jorganchem.2011.07.038

A series of alkali metal cyclopentadienides, amides, alkoxides and phenoxides was characterized using NMR spectroscopy in deuterated dimethyl sulfoxide. Deuterated dimethyl sulfoxide showed very good stability and solubility for these compounds. Very nice

Full text of DOI:10.1016/j.jorganchem.2011.07.038

Journal of Organometallic Chemistry 696 (2011) 3431e3435
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Deuterated dimethyl sulfoxide as a good NMR solvent for the characterization
of alkali metal cyclopentadienides, amides, alkoxides and phenoxides
*
Boon-Ying Tay, Cun Wang , Ludger P. Stubbs, Chacko Jacob, Martin van Meurs
*
Institute of Chemical and Engineering Sciences, A STAR (Agency for Science, Technology and Research), 1 Pesek Road, Jurong Island, Singapore 627833, Singapore
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of alkali metal cyclopentadienides, amides, alkoxides and phenoxides was characterized using
NMR spectroscopy in deuterated dimethyl sulfoxide. Deuterated dimethyl sulfoxide showed very good
stability and solubility for these compounds. Very nice and well resolved spectra were obtained for most
compounds tested. The low cost of the solvent makes it possible to use it for the routine characterization
of these alkali salts as the ligands in organometallic synthesis.
Received 15 June 2011
Received in revised form
29 July 2011
Accepted 29 July 2011
Ó 2011 Elsevier B.V. All rights reserved.
Keywords:
Deuterated dimethyl sulfoxide
NMR solvent
NMR spectroscopy
Cyclopentadienides
Amides
Alkoxides
1. Introduction
crystallography, the most widely used characterization method is
NMR spectroscopy using deuterated tetrahydrofuran (THF-d8) as the
Organoalkali compounds play a prominent role as starting
materials for innumerable productsin synthetic chemistry, especially
in the formation of transition metal complexes which have a quite
broad range of applications in catalysis [1]. It is usually unfeasible to
develop the chemistry of organic functional groups on an assembled
transition metal complex due to the incompatibility of such systems
with the typical reaction conditions of organic chemistry. The
common route to these complexes includes ligand synthesis and the
transmetalation of the alkali metal ligands to transition metals, and
thus it is necessary to prove the success of ligand synthesis by various
characterization methods. Due to the prevailing electrostatic inter-
action between the alkali metal cation and the counteranion like
cyclopentadienide, amide, alkoxide, etc., there seems to be a high
tendency to form aggregates for such systems, which makes it diffi-
cult for the characterization of these ioniccompounds [2]. Addition of
donor ligand or solvent may stabilize smaller or even monomeric
structures, and a large number of organoalkali compounds have been
structurally characterized. Meanwhile, quite a lot of other such
compounds have not been studied, firstly because of the compounds’
insolubility and secondly because of the in situ preparation and the
use of these compounds directly in the reaction solvents. Besides
solvent, but in some cases, no satisfactory spectrum could be
obtained because THF was not strong enough to break the aggregates
into monomer units. In addition, the overlapping of THF and partially
deuterated THF peaks prevents the accurate calculation of the
number of THF molecules coordinated or co-crystallized. Another
drawback of THF-d8 as the solvent is its high cost.
DMSO is a polar aprotic solvent which is frequently used for
chemical reactions involving salts, most notably Finkelstein reac-
tions and other nucleophilic substitution reactions [3]. It is also
extensively used in biochemistry and cell biology because of its low
toxicity [4]. Because DMSO (pKa ¼ 35) is only weakly acidic, it
tolerates relatively strong bases and as such, it has been extensively
used in the study of carbanions. A set of non-aqueous pKa values
(CeH, OeH, SeH and NeH acidities) for thousands of organic
compounds have been determined in DMSO solution [5]. DMSO has
a dielectric constant of 47 which is much higher than that of THF
(7.6) [6]. That means DMSO is a better solvent for ionic organoalkali
compounds. Compared to the high price of deuterated THF, the cost
of deuterated DMSO is quite low [7].
We report here the use of deuterated dimethyl sulfoxide
(DMSO-d6) as the NMR solvent for the characterization of a series of
alkali metal of cyclopentadienides, amides, alkoxides and phen-
oxides. DMSO-d6 showed very good stability and solubility for
these compounds.
* Corresponding author. Tel.: þ65 6796 3838; fax: þ65 6316 6182.
E-mail address: wang_cun@ices.a-star.edu.sg (C. Wang).
0022-328X/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2011.07.038

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B.-Y. Tay et al. / Journal of Organometallic Chemistry 696 (2011) 3431e3435
Scheme 1. List of alkali metal of cyclopentadienides, amides, alkoxides and phenoxides.
2. Results and discussion
2.1. Characterization of alkali metal of cyclopentadienides in
DMSO-d6
The commercial deuterated dimethyl sulfoxide was first dried
with 4 Å molecular sieves and then distilled over calcium hydride
under argon to remove the water impurity according to the establish
method [8]. The samples to be tested were prepared in glovebox and
multi-nuclei NMR spectroscopy was recorded at room temperature
with a Bruker 400 MHz spectrometer. A series of alkali metal of
cyclopentadienides, amides, alkoxides and phenoxides shown in
Scheme 1 was obtained commercially, prepared according to liter-
atures or by ourselves. Other examples using DMSO-d6 as NMR
solvent can be found in the literatures [9].
A few selected cyclopentadienides were first tested in DMSO-
d6. Sodium cyclopentadienide (1a) was prepared by modifying the
literature method [10]. The final product was obtained in high
yield as colorless crystals with one third equivalent of THF by
heating the crude product in toluene with THF at 100 ꢀC for 2 h. In
DMSO-d6, compound 1a exhibited a very clean and finely resolved
NMR spectrum typical for a metallocene structure in which the
negative charge was equally shared by the five carbons coordi-
nated to the sodium cation symmetrically in a h5 mode (Fig. 1).
Fig. 1. 1H NMR spectrum of 1a.(THF)0.33 ((400 MHz, DMSO-d6, 295 K)).

B.-Y. Tay et al. / Journal of Organometallic Chemistry 696 (2011) 3431e3435
3433
Only one sharp singlet in each 1H and 13C NMR spectrum (
d
5.40,
substrates. To our surprise, even 2d (LDA, a widely used strong base)
doesn’t react with DMSO-d6 at room temperature. No high field shift
was found for the solvent peak since deprotonated by LDA should
lead to lower value for the carbon’s chemical shift. Oxides can also be
characterized in DMSO-d6. Clean and finely resolved spectra were
obtained for the 1H, 13C and 19F nuclei magnetic resonance in
compounds 3a and 3b.
103.0) was found for the C5H4 group despite the high viscosity of
the solvent. The coordinated THFs were replaced by the stronger
donor solvent and existed as free molecules as seen from their
chemical shifts (d d
1H 3.61, 1.77, 13C 67.1, 25.2) [11]. Unlike in THF-
d8, compound 1b [12], 1c [13] gave clean spectra in DMSO-d6
(supporting information). To the best of our knowledge, this is the
first report of the NMR studies of the two compounds which have
innumerable applications in organometallic chemistry. Limited
solubility and aggregate formation in THF-d8 may have prevented
their characterization. In our case, these two compounds were
very soluble in DMSO-d6, and they existed as uniform species.
Only at very high concentration for 1c (100 mg/mL), an additional
new species was formed (Fig. 2). The two species for 1c can’t be
sandwich lithonocenes since each lithonocene has two 7Li signals
of the same intensities [14]. Only two signals were found in the 7Li
NMR spectrum, and the ratio of the two integrations were corre-
sponding to that in the 1H NMR spectrum. The two species
exchanged with each other at high temperature as the two 7Li
signals coalescenced at 100 ꢀC. The compound 1d, normally used
without characterization or in situ preparation [15], also gave well
predicted 1H NMR spectrum in DMSO-d6 (supporting informa-
tion), but the low integrated value of the protons at the 1,3 posi-
2.3. Characterization of dilithium salts in DMSO-d6
Dilithium salts usually have even lower solubility in THF.
Compounds 4a [17], 4b [18] and 4c [19] were used as the ligands for
the formation of constrained geometry framework, but never char-
acterized. We find that they were quite soluble in DMSO-d6 and the
obtained NMR spectra match very well with their predicted struc-
tures. 4d [20] was used as the ligand to form an ansa-zirconocene
applicable in the stereoselective polymerization of propylene. It was
prepared by the treatment of the neutral precursor with two equiv-
alents of butyl lithium in ether and then used as pure compound free
of coordinated ether. We prepared this compound using the same
method, but we found that this dilithium salt could not be free of
diethyl ether even under high vacuum overnight. About 0.86 equiv-
alent of ether was detected in the compound by judging from its 1H
NMR spectrum in DMSO-d6.
tions and
a
complicated 13C NMR spectrum suggested the
existence of exchangeable isomeric structures due to the delo-
calization of the negative charge on the five-membered ring. The
negative charge is not shared equally by the five carbons, but is
much higher at the 1, 3 positions. 1e was isolated and character-
ized in THF-d8 [16], but in DMSO-d6 a very clean and finely
resolved 1H NMR spectrum was obtained, typical for a metallocene
structure in which even the coupling of phosphine with protons
from groups of CH and CH3 was easily found (supporting infor-
mation and experimental part).
3. Conclusion
In summary, DMSO-d6 is a good NMR solvent for the charac-
terization of alkali metal complexes of cyclopentadienides, amides,
alkoxides and phenoxides. It is a stronger donor solvent than
tetrahydrofuran and has much higher capability to solvate these
substrates, and usually finely resolved spectrum can be obtained
despite the high solvent viscosity. Coordinated solvents like THF or
ether in products can also be easily detected and quantified in
DMSO-d6. The deuterated dimethyl sulfoxide is also stable toward
these compounds. The low cost of the solvent makes it possible for
the routine characterization of these alkali salts as ligands in
organometallic chemistry. Although DMSO-d6 can be used for the
normal characterization in place of THF-d8, much stronger base like
n-BuLi, cannot be measured in DMSO-d6. n-BuLi can abstract
deuteron from DMSO-d6 and form an end-deuterated butane.
Another disadvantage of this solvent is its high melting point
(18 ꢀC). It needs to be used in combination with other solvents if
low temperature NMR is required.
2.2. Characterization of alkali metal of amides and oxides
in DMSO-d6
Four alkali amides were also dissolved in DMSO-d6 and their 1H
NMR and/or 13C NMR were recorded at room temperature. All of
them gave very clear spectra without evident peak broadening and
the couplings of nuclei were finely resolved. The coordinated ether
in compound 2c was replaced by DMSO-d6 and its amount was
easily determined by the calculation of related peak integrations.
Alkali amides are usually strong bases and react with a lot of
Fig. 2. (a) 1H NMR spectrum of 1c ((400 MHz, 100 mg/mL in DMSO-d6, 295 K)). (b) Dynamic 7Li NMR experiment of 1c (155.5 MHz, 100 mg/mL in DMSO-d6).

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B.-Y. Tay et al. / Journal of Organometallic Chemistry 696 (2011) 3431e3435
2
4. Experimental section
(d, 1JPC ¼ 19 Hz, PCH(CH3)2), 19.4(d, JPC ¼ 8.0 Hz, PCH(CH3)2). 31P
{1H}NMR:
d -3.68.
4.1. General considerations
4.2.6. Lithium tert-butylamide (2a)
All manipulations were carried out under a dry argon atmo-
sphere using standard Schlenk techniques or in a glovebox.
Solvents were dried and purified with MBRAUN solvent purifica-
tion system. DMSO-d6 was obtained from Cambridge Isotopes Ltd.
It was first dried with 4 Å molecular sieves and then distilled at low
pressure over calcium hydride under argon to remove the water
impurity [8]. 2b was purchased from SigmaeAldrich, and used as
received. 1a [10], 1b [12], 1c [13], 1d [15], 1e [16], 3a [21], 4a [17], 4b
[18], 4c [19] and 4d [20] were prepared according to the literature
procedures. NMR spectra were recorded in DMSO-d6 on a Bruker
400 MHz spectrometer (measurement frequencies: 1H: 400.2 MHz,
13C: 100.6 MHz, 19F: 376.4 MHz, 31P: 162.0 MHz, 7Li: 155.5 MHz) at
Freshly dried and distilled tert-butylamine (5.0 g, 68.4 mmol)
was dissolved in 10 mL of pentane and cooled with an ice-water
bath. n-BuLi (2.0 M in cyclohexane, 34 mL, 68 mmol) was added
dropwise under argon. All the volatiles were removed under
vacuum and the product was left as white powder. Yield: 5.23 g,
97%. 1H NMR:
was not found. 13C{1H}NMR:
d
1.02 (C(CH3)3). The signal of the proton on nitrogen
46.7 (C(CH3)3), 32.4 (C(CH3)3).
d
4.2.7. Potassium bis(trimethylsilyl)amide (2b)
1H NMR: -0.28 (Si(CH3)3). 13C{1H}NMR:
d 7.1(Si(CH3)3).
d
4.2.8. Lithium 4-vinylanilinide (2c)
295 K. Chemical shifts are reported in
d
, referenced to 1H, 13C
Similar to 2a, this compound was prepared as ether adduct (one
(DMSO-d6 as internal standard with residual protons at 2.50 ppm
and carbon at 39.5 ppm), 19F (CFCl3 as standard), 7Li (1.0 M of LiCl in
D2O as standard) and 31P (PPh3 as standard).
fourth equivalent) with a 97% yield from 4-vinylaniline (5.30 g,
44.5 mmol) and n-BuLi (44.0 mmol) in ether. 1H NMR:
d 6.63 (d,
3JHH ¼ 8.4 Hz, 2H, Ph), 6.25 (dd, 3JHH ¼ 10.6 Hz, 3JHH ¼ 17.6 Hz, 1H,
CH]CH2), 5.90 (d, 3JHH ¼ 8.4 Hz, 2H, Ph), 4.86 (dd, 3JHH ¼ 17.6 Hz,
2JHH ¼ 1.6 Hz, 1H, CH]CH2), 4.32 (dd, 3JHH ¼ 10.6 Hz, 2JHH ¼ 1.6 Hz,
1H, CH]CH2). 3.38 (q, OCH2CH3), 1.09 (t, OCH2CH3).
4.2. Synthesis and/or NMR spectroscopy of alkali metal
cyclopentadienides, amides, alkoxides and phenoxides
4.2.1. Sodium cyclopentadienide (1a)
4.2.9. Lithium diisopropylamide (2d)
Sodium cyclopentadienide was prepared by modifying the
literature method [10]. The trace of water and stabilizer in dicy-
clopentadiene were removed by passing 300 mL of dicyclopenta-
diene through a silica column. Sodium (7.0 g, 0.30 mmol) was cut in
small blocks and added to a Schlenk flask containing the dicyclo-
pentadiene. The reaction mixture was heated at 160 ꢀC under
stirring until all the metal disappeared. After cooling down the
reaction mixture, the crude product was isolated by filtration and
treated with 200 mL of toluene and 20 mL of THF at 100 ꢀC for 2 h.
Cooling of the mixture to room temperature led to the precipitation
of the product as colorless microcrystals with one third equivalent
of THF which was confirmed with the help of 1H NMR spectroscopy
Similar to 2a, LDA was prepared as white powder with a 98%
yield from diisoproplyamine (2.02 g, 20.0 mmol) and n-BuLi
3
(20.0 mmol). 1H NMR:
d
2.77 (sept, JHH ¼ 6.0 Hz, 2H, CH(CH3)2),
0.91 (d, 12H, CH(CH3)2). 13C{1H}NMR:
(CH(CH3)2).
d 44.3 (CH(CH3)2), 23.1
4.2.10. Lithium nanofluro-tert-butoxide (3a)
13C{1H}NMR:
19F{1H}NMR:
d
123.8 (q, 1JCF ¼ 298 Hz, C(CF3)3), 85.1 (m, C(CF3)3).
d
-75.3.
4.2.11. Lithium 2,6-Diformyl-bis(2,6-diisopropylanil)-4-tert-
butylphenoxide(3b)
in DMSO-d6. Yield: 28.2 g, 84%. 1H NMR:
1.3H, -THF), 1.78 (m, 1.3H,
-THF). 13C{1H}NMR:
-THF), 25.2 ( -THF).
d
5.40 (s, 5H, CpH), 3.61 (m,
103.0 (Cp), 67.1
The neutral ligand was prepared according to literature method
[22]. The lithium salt was obtained as yellow powder similar to 2c
by the treatment of the neutral ligand (1.05 g, 2.0 mmol) with n-
a
b
d
(a
b
BuLi (2.0 mmol) with a yield of 98%. 1H NMR:
d 8.38 (brs, 2H, HC]
3
4.2.2. Lithium tetramethylcyclopentadienide (1b)
1H NMR:
4.74 (s, 1H, CpH), 1.85, 1.79 (each s, each 6H, CpCH3).
13C{1H}NMR:
106.0, 105.5, 100.6 (Cp), 14.1, 11.7 (CpCH3).
O), 7.78 (brs, 2H, Ph), 7.08 (d, JHH ¼ 7.2 Hz, 4H, Ph), 7.00 (t,
d
3JHH ¼ 7.2 Hz, 2H, Ph), 2.98 (sept, 3JHH ¼ 6.8 Hz, 4H, CH(CH3)2), 1.27
d
(s, 9H, C(CH3)3), 1.10 (d, 24H, CH(CH3)2). 13C{1H}NMR:
d 150.9, 137.9,
124.5, 123.1, 122.5 (Ph), 33.2 (C(CH3)3), 31.6 (C(CH3)3), 27.1
(CH(CH3)2), 23.5 (CH(CH3)2). Four signals of the tertiary carbon of
phenyl rings were not detected.
4.2.3. Lithium pentamethylcyclopentadienide (1c)
When 1c was dissolved in DMSO-d6 at 10 mg/mL, only one
species was formed. 1H NMR:
d
1.83 (s, 15H, CpCH3). 13C{1H}NMR:
d
103.5 (Cp), 12.6 (CpCH3). When 1c was dissolved in DMSO-d6 at
4.2.12. Dilithium ((cyclopentadienyl)dimethylsilyl)(tert-butyl)
amide (4a)
100 mg/mL, an additional species was formed which had the
chemical shifts at 1H NMR:
d
1.75 (s, CpCH3). 13C{1H}NMR:
d
104.1
1H NMR:
d 5.62, 5.46 (each m, each 2H, CpH), 1.11 (s, 9H,
(Cp), 10.8 (CpCH3). 7Li{1H} NMR for the two species:
d
-1.26, -12.45
C(CH3)3), 0.08 (s, 6H, Si(CH3)2).
(each s).
4.2.13. Dilithium ((tetramethylcyclopentadienyl)dimethylsilyl)(tert-
4.2.4. Indenyl lithium (1d)
butyl)amide (4b)
1H NMR:
d
7.14 (m, 2H, 5-H/6-H), 6.42 (t, 3JHH ¼ 6.7 Hz, 1H, 2-H),
1H NMR:
d
2.00, 1.80 (each s, each 6H, CpCH3), 1.10 (s, 9H,
6.27 (m, 2H, 4-H/7-H), 5.77 (d, 3JHH ¼ 6.7 Hz, 2H,1-H/3-H). 13C{1H}
C(CH3)3), 0.13 (s, 6H, Si(CH3)2). 13C{1H}NMR:
d
114.8, 109.4, 100.8
NMR:
d
129.5, 129.4, 117.6, 117.4, 110.4, 92.8, 92.7.
(Cp), 48.4 (C(CH3)3), 33.8 (C(CH3)3), 15.3, 12.3 (CpCH3), 6.63
(Si(CH3)2).
4.2.5. (Diisopropylphosphino)cyclopentadienyl lithium (1e)
1H NMR:
d
5.60, 5.48 (each m, each 2H, CpH), 1.74 (d ꢁ sept,
4.2.14. Dilithium ((tetramethylcyclopentadienyl)
2JPH ¼ 2.0 Hz, 3JHH ¼ 6.8 Hz, 2H, CH(CH3)2), 0.89 (dd, 3JPH ¼ 12.0 Hz,
3JHH ¼ 6.8 Hz, 6H, CH(CH3)2), 0.87 (dd, 3JPH ¼ 8.4 Hz, 3JHH ¼ 6.8 Hz,
dimethylsilyl)(phenyl)amide (4c)
1H NMR:
d
6.91 (m, 2H, Ph), 6.65 (m, 2H, Ph), 6.38 (m, 1H, Ph),
2.02, 1.81 (each s, each 6H, CpCH3), 0.26 (s, 6H, Si(CH3)2). 13C{1H}
NMR: 128.3, 115.5, 114.3, 110.4, 15.0 (CpCH3), 12.2 (CpCH3), 3.27
6H, CH(CH3)2). 13C{1H}NMR:
d
112.0 (d, 1JPC ¼ 20 Hz, Cp), 105.5 (d,
2JPC ¼ 9.0 Hz, Cp), 100.5 (Cp), 23.5 (d, 2JPC ¼ 10 Hz, PCH(CH3)2), 20.7
d

B.-Y. Tay et al. / Journal of Organometallic Chemistry 696 (2011) 3431e3435
3435
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4.2.15. Dilithio-2-cyclopentadienyl-2-(10-indenyl)propane (4d)
1H NMR:
d
7.30 (d, 3JHH ¼ 8.0 Hz, 1H, Ph), 6.93 (d, 3JHH ¼ 7.6 Hz,
1H, Ph), 6.26 (s, 1H), 6.04, 5.95 (each m, each 1H, Ph), 5.40 (d,
3JHH ¼ 3.3 Hz, 1H), 5.30, 5.09 (each m, each 2H, CpH), 3.43 (q,
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a-ether), 1.65 (s, 6H, C(CH3)2), 1.30 (t,
3JHH ¼ 7.2 Hz, 5.1H,
b d 133.1, 130.0, 125.3,
-ether). 13C{1H}NMR:
120.9, 119.7, 117.0, 116.9, 116.7, 108.5, 107.5, 101.2, 99.7, 64.9 (ether),
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Acknowledgments
We thank the Institute of Chemical and Engineering Sciences,
A*STAR (Agency for Science, Technology and Research) Singapore
for its financial support.
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