Structural analysis of electrolyte solutions comprising magnesium-aluminate chloro-organic complexes by raman spectroscopy

Article abstract of DOI:10.1021/om061076s

We report herein on a rigorous analysis of unique electrolyte solutions for novel rechargeable magnesium batteries and nonaqueous magnesium electrochemistry, which contain organometallic complex electrolyte species, by Raman spectroscopy. These solutions comprise ethereal solvents and products of reactions between R₂Mg Lewis base species and AlCl₂R Lewis acid species that exist in solution in dynamic multiple equilibria. The reactions involve the exchange of ligands between the magnesium and the aluminum to form ions such as MgCl⁺, Mg₂Cl₃ ⁺, and AlCl_4-nR^-_n (n ≤ 4), stabilized by the ether molecules. The Raman peak assignments were based on a rigorous study of solutions containing reference compounds and some quantum-mechanical calculations. Raman spectroscopy enabled a quantitative analysis of the various species in solution.

Full text of DOI:10.1021/om061076s

3130
Organometallics 2007, 26, 3130-3137
Structural Analysis of Electrolyte Solutions Comprising
Magnesium-Aluminate Chloro-Organic Complexes by Raman
Spectroscopy
Yulia Vestfried, Orit Chusid, Yossi Goffer, Pinchas Aped, and Doron Aurbach*
Department of Chemistry, Bar-Ilan UniVersity, Ramat-Gan 52900, Israel
ReceiVed NoVember 27, 2006
We report herein on a rigorous analysis of unique electrolyte solutions for novel rechargeable magnesium
batteries and nonaqueous magnesium electrochemistry, which contain organometallic complex electrolyte
species, by Raman spectroscopy. These solutions comprise ethereal solvents and products of reactions
between R2Mg Lewis base species and AlCl2R Lewis acid species that exist in solution in dynamic
multiple equilibria. The reactions involve the exchange of ligands between the magnesium and the
+
aluminum to form ions such as MgCl+, Mg2Cl3 , and AlCl4-nR-n (n e 4), stabilized by the ether molecules.
The Raman peak assignments were based on a rigorous study of solutions containing reference compounds
and some quantum-mechanical calculations. Raman spectroscopy enabled a quantitative analysis of the
various species in solution.
understanding of these solution structures.3 Nevertheless, several
important questions were left unanswered, as the NMR analysis
could not shed light on the identity of the magnesium species
due to inherent resolution limitations.
Introduction
In 1999 a new class of nonaqueous electrolytic solutions was
developed, from which metallic magnesium could be reversibly
electrochemically deposited and magnesium ions could be
intercalated into appropriate compounds.1,2 These materials are
the reaction products of organomagnesium with organo-halo
aluminum compounds. In general, a whole range of such
compounds can be synthesized by the reaction of RxMgCl2-x
with R′yAlCl3-y (R and R′ are organic ligands, x ) 0-2, y )
0-3) at various proportions. The nature of R and R′, the number
of the organic and inorganic ligands, as expressed by x and y,
and the stoichiometric ratio of aluminum to magnesium are all
variables that determine the chemical and the electrochemical
properties of the system.
We report here on rigorous solution Raman spectroscopy
studies that enabled the identification of all the main species
formed in these systems. The peak assignment in the Raman
spectra of some of the relevant compounds was assisted by
quantum-mechanical calculations. The results prove that the
solutions contain various magnesium and aluminum species in
equilibrium.
Results and Discussion
Reference Spectra of Expected Important Compounds.
Among the wide spectra of different compounds that can be
synthesized from the reaction of RxMgCl2-x with R′yAlCl3-y
(R and R′ are organic ligands, x ) 0-2, y ) 0-3), one was
selected as the optimal composition from an electrochemical
point of view. The reaction product of 1 equiv of dibutylmag-
nesium (DBM) with 2 equiv of ethyl aluminum dichloride
possesses reasonable ionic conductivity, has a wide electro-
chemical window, and magnesium is deposited from its solution
with 100% Coulumbic reversibility.2 These solutions are denoted
as DCC (dichloro complex) solutions. In order to obtain the
best electrochemical results with these solutions in terms of low
overvoltage for Mg deposition and 100% reversibility of this
process, we had to add to them 2-10% v/v of 1 M DBM in
hexane. Thus, from a practical point of view, the constituents
of DCC/THF solutions are of special interest.
The equilibrium species in the solutions are the result of
transmetalation reactions in which the organic and inorganic
ligands are exchanged between the magnesium and the alumi-
num cores and reactions in which the aluminum species act as
Lewis acids and the magnesium species as bases.3 The solvent
also plays an important role in these reactions, both as an
efficient donor and as a polar medium that enables the
dissociation of the complex salts to yield solutions with high
ionic conductivity.
Despite the fact that the synthesis is simple, determination
of the species that exist in solution is not trivial. An attempt to
isolate species from the solutions for identification by single-
crystal XRD met with limited success, since it was revealed
that the isolated compounds in the solid state are not identical
with those in the solution phase.2 A thorough analysis of the
1
solutions using 25Mg, 27Al, 13C, and H NMR, in conjunction
In a previous study that combined the use of multinuclear
NMR and electrochemistry, it was revealed that for spectro-
scopic measurements a model complex salt system made up of
molecules with higher symmetry is desirable.3 We found that,
in terms of electrochemical properties, the use of diethylmag-
nesium as a reactant instead of DBM yields solutions with very
similar electrochemical properties, which can thus be used for
spectroscopic investigations. The reason that DBM has been
used so far in practical electrochemical systems is due to its
with electrochemical studies, provided a breakthrough in the
* To whom correspondence should be addressed. E-mail: aurbach@
mail.biu.ac.il.
(1) Aurbach, D.; Lu, Z.; Schechter, A.; Gofer, Y.; Gizbar, H.; Turgeman,
R.; Cohen, Y.; Moshkovich, M.; Levi, E. Nature 2000, 407, 724.
(2) Aurbach, D.; Gizbar, H.; Schechter, A.; Chusid, O.; Gottlieb, H. E.;
Gofer, Y.; Goldberg, I. J. Electrochem. Soc. 2002, 149, A115.
(3) Gizbar, H.; Vestfrid, Y.; Chusid, O.; Gofer, Y.; Gottlieb, H. E.; Marks,
V.; Aurbach, D. Organometallics 2004, 23, 3826.
10.1021/om061076s CCC: $37.00 © 2007 American Chemical Society
Publication on Web 05/15/2007

Magnesium-Aluminate Chloro-Organic Complexes
Organometallics, Vol. 26, No. 13, 2007 3131
Figure 1. Raman spectra of ethyl-butyl DCC and all-ethyl DCC solutions in THF.
Figure 2. Raman spectra of organo-halo aluminum compounds in THF.
availability and low cost. Figure 1 presents a comparison
between Raman spectra measured from DCC solutions based
on DBM and from all ethyl DCC solutions in which diethyl-
magnesium was used as a precursor.
of structures that we presume to be present in our solutions.
Moreover, the data are not consistent, and various authors report
on different spectral bands for the same molecules. In addition,
since the solvent, namely THF, has an important role in the
stabilization of the systems and influences the equilibria and
the molecular structures, it is very important to compare only
spectroscopic data that were acquired in THF solutions. Thus,
as a first step, Raman spectra for a collection of compounds to
be used as a database were systematically measured. For this
purpose we analyzed the spectra of solutions of pure, known
compounds, as well as a set of solutions of reaction products
whose structures had been previously identified by NMR. In
addition, we acquired spectra of THF solutions of reaction
products of simple model compounds, relevant to the studied
systems.
Figure 2 presents the Raman spectra of THF solutions of
Et3Al, Et2AlCl, and EtAlCl2, as well as that of pure THF for
comparison. The data are also given in Table 1 (together with
results from the quantum-mechanical calculations). Note that
these three compounds are all tetracoordinated in THF, with
one THF ligand attached (proven by NMR studies). The
spectrum of AlCl3 in THF was seriously disturbed by fluores-
cence and thus is not presented. In many respects, these spectra
The spectra in Figure 1 are indeed similar and allow a
conclusive study to be carried out with the all-ethyl DCC
solutions. (The peak assignments marked in the figure are based
on the spectral studies described later in this paper.) In our NMR
study we identified the aluminum complex structures to be
Et4Al-, Et3Al·THF, and Et4Al2Cl3- as the reaction products of
1 equiv of diethylmagnesium with 0.5, 1, and 2 equiv of
ethylaluminum dichloride, respectively.3 Interestingly, we no-
ticed that for 27Al NMR spectra, in general, the influence of
chloride ligands on the chemical shift was similar to that of
THF. This made discrimination between Et2AlCl2- and Et2AlCl·
THF (or their dimer), the hypothesized species in DCC solutions,
to be impossible. In that study, parallel 25Mg NMR studies were
not conclusive.
The scientific literature demonstrates some studies on the
vibrational spectroscopy of organo-halo aluminum compounds4
and fewer studies on organo and organo-halo magnesium
compounds.5 However, these data contain only a few spectra
(4) Crompton, T. R Chemical Analysis of Organometallic Compounds;
Academic Press: New York, 1977; Vol. V, p 115.
(5) Kress, J. J. Organomet. Chem. 1976, 111(1), 1.

3132 Organometallics, Vol. 26, No. 13, 2007
Vestfried et al.
Table 1. Summary of Peak Positions in Raman Spectra of a Variety of Organo-Halo Aluminum and Magnesium Speciesa
peak position, cm-1
M-R bond
(510 sym)
M-Cl bond
compd or species
Et3Al
490 s
553
Et2AlCl
(611 sym, 616 asym)
413
424
(408)
EtAlCl2
634
(642)
282
(292 sym)
333
(329 asym)
369
420 s
(420 sym)
Et4Al-
Et3AlCl-
460
490
539
611
(483 sym, 532 asym, w, 622 asym, w)
(500 sym, 580 asym, w)
(594 sym, 606 asym, w)
(621)
367 s
367 s
(342)
-
Et2AlCl2
(350 sym, 401 asym, w)
(179 bend)
-
EtAlCl3
181
346 w
367 s
(367 sym, 442 asym, w, 459 asym, w)
(344 sym, 483 asym, w)
-
AlCl4
346 s
a Calculated frequencies and assignments are given in parentheses. Legend: sym, symmetric stretching; asym, asymmetric stretching; w, low intensity.
-
Figure 3. Raman spectra of lithium organo-halo aluminum complexes with the general structure Li+[EtxAlCl4-x
]
in THF.
correlate with the published data on the same compounds in
diethyl ether.6
NMR study, it is clear that these three anionic complexes are
also tetracoordinated.3,7 The Raman spectra for these salt
solutions are presented in Figure 3, along with the data, which
are given in Table 1.
The strongest spectral bands in Figure 2 can be divided into
those between 250 and 430 cm-1, which relate mainly to
Al-Cl bonds, and those in the 450-650 cm-1 region, which
relate mainly to Al-R (well supported by the calculations).
Interestingly, the spectral bands of the Al-R bonds of this
homologous series show a trend that depends on the Cl/R ratio.
The higher this ratio, the higher the scattering wavenumber of
the Al-R bands. In a similar manner, the higher the R/Cl ratio,
the stronger the Al-R bands relative to the Al-Cl bands.
When the above compounds are reacted with an equimolar
The spectra in Figure 3 show several very interesting trends.
First, the principal peak that corresponds to an Al-Cl stretching
vibration appears in all cases at 367 cm-1. The positions of the
Al-R bands (stretching vibrations) show an almost linear
correlation to the Cl/R ratio, with the highest wavenumber for
-
the EtAlCl3 ion, at 611 cm-1. In this series, the Al-R vs
Al-Cl peak height ratios also correspond to the ratio between
the number of the chlorine ligands vs the number of organic
ligands. These results suggest that the strength of the Al-R
bonds increases with the Lewis acidity of the compound, which
in turn increases as the number of electron-withdrawing chlorine
ligands increases. This trend in bond strength gains further proof
from the electrochemical window measurements. In these
quantity of LiCl, an acid-base reaction takes place that results
-
in a complex salt formation. The generation of Li+[EtxAlCl4-x
]
is evident both from the dramatic increase in the ionic
conductivity of the solutions (LiCl alone is only slightly soluble
in THF, thus forming solutions with no practical ionic conduc-
tivity) and from 27Al NMR data for these materials.7 From the
(7) Gofer, Y.; Chusid, O.; Gizbar, H.; Viestfrid, Y.; Gottlieb, H. E.;
Marks, V.; Aurbach, D. Electrochem. Solid-State Lett. 2006, 9(5), A257.
(6) Tarao, R.; Takeda, S. Bull. Chem. Soc. Jpn. 1967, 40(3), 650.

Magnesium-Aluminate Chloro-Organic Complexes
Organometallics, Vol. 26, No. 13, 2007 3133
Figure 4. Raman spectra of organo-halo magnesium compounds in THF.
measurements, it was clearly observed that the higher the Cl/R
ligand ratio, the higher the electrochemical stability of the
solutions toward oxidation.
findings. Note that in Kress’ work the author stresses that the
Raman spectra of EtMgCl in diethyl ether reflect the existence
of dimers, while in a THF solution, as in our study, this material
is a single moiety.11 Salinger et al.12 report a single feature for
Et2Mg in THF at 512 cm-1 and do not report any peak around
210 cm-1 for MeMgCl in THF, which is expected for the
Mg-Cl band.
From the point of view of the objective of this study, although
the Mg-R bands we found lay in the same region as some of
the Al-Cl bands, this should not interfere with the analysis of
the solutions of interest (i.e., with the DCC solutions comprising
the Lewis acid-base reaction products), as we recognized from
the multinuclear NMR study that in these solutions no Mg-R
species exist.3
Identification of the Anion Structure in DCC-Type Solu-
tions. As mentioned in the Introduction, the main objective of
this study is the identification of the various species in the so-
called DCC solutions, which were developed as optimal
electrolyte solutions for use in rechargeable magnesium batteries
(reacting 2 equiv of the ethylaluminum dichloride with 1 equiv
of dibutylmagnesium). The basic understanding of the solution
structures in the family of DCC-like electrolytes (made by a
reaction in various proportions of the aluminum and magnesium
compounds) was based on a multinuclear NMR study, which
enabled the unambiguous identification of some of the aluminum-
based structures.3 Unfortunately, as was already mentioned, this
study could not give an unambiguous identification for the
aluminum species in the DCC solutions. However, on the basis
of Raman spectroscopy we can discriminate between the
different aluminum-based compounds present in these solu-
tions. Moreover, since the study necessitated the analysis of
whole spectra of DCC-based solutions, with various acid-base
proportions, as well as analyses of various reference solutions,
the present study achieved a thorough understanding of the
chemistry of these systems. The spectral identifications also
correlate well with the electrochemistry of these solutions:
namely, their appreciable ionic conductivity due to the existence
of dissociated aluminum-based anions and magnesium-based
cations.
For reference data on the possible magnesium compounds,
we acquired the Raman spectra of MgCl2, Bu2Mg, Et2Mg, and
EtMgCl, all as solutions in THF. Note that, from 25Mg NMR
analyses, it appears that in THF solutions all these magnesium
compounds are hexacoordinated.3,9 Nevertheless, we must stress
that the literature data on Grignard reagents are very inconsistent
and the actual species present in solution depend on the solvent,
concentration, purity, and temperature.8 Figure 4 presents the
Raman spectra of the above compounds. The data are also
compiled in Table 1.
As can be seen from these spectra in the 100-800 cm-1
region, the magnesium species show only a few features. MgCl2,
presumably coordinated by four THF molecules, shows only a
single sharp peak at 212 cm-1. Et2Mg, at a high concentration
in THF (2 M), shows one sharp and strong peak at 409 cm-1
and in two regions, 250-300 and 475-530 cm-1, a multitude
of small features can be discerned. The spectrum of Bu2Mg is
similar in this respect, although instead of a single strong peak
it possesses two large peaks at 370 and 394 cm-1, probably
due to the existence of the two isomers of butyl as the organic
ligands in the commercial product, namely n-butyl and 2-butyl.
It is evident here that the use of Et2Mg as the model system,
instead of Bu2Mg, is indeed highly advantageous for the
spectroscopic analysis. The spectrum of EtMgCl in THF, which
is expected to be a monomer in this solution, contains, as
reported by others,11 the features seen for both Et2Mg and
MgCl2. This is consistent with the Schlenk equilibrium (i.e.,
(2EtMgCl / Et2Mg + MgCl2). The spectra above are not
strictly consistent with some of the literature data on identical
or similar solutions (in other ethers). For instance, Kress et al.
reports on a multitude of features for Et2Mg in diethyl ether,
although one of the main peaks, that at 410 cm-1, and the
multitude of small peaks around 500 cm-1 are in line with our
(8) Kress, J.; Novak, A.; Hervieu, J. J. Organomet. Chem. 1976, 121(1),
7.
(9) Chabot, P. Infrared and Raman Spectroscopy [of Grignard Reagents];
Chemical Industries (Dekker); 1996.
(10) Kress, J. J. Organomet. Chem. 1975, 99, 23.
(11) Kress, J.; Novak, A. J. Organomet. Chem. 1975, 99(2), 199.
(12) Salinger, R. M.;. Mosher, H. S. J. Am. Chem. Soc. 1964, 86(9),
1782.

3134 Organometallics, Vol. 26, No. 13, 2007
Vestfried et al.
Figure 5. Raman spectrum of a 0.25 M all-ethyl DCC solution (top line) in comparison with appropriate reference spectra (from bottom
-
to top), all in THF: Et2AlCl, Et3Al, and Et2AlCl2
.
Figure 5 presents the spectra of the all-ethyl DCC species
(i.e., the reaction product of Et2Mg and AlCl2Et) in THF, as
well as the reference spectra of Et3Al, Et2AlCl, and LiEt2AlCl2
in THF.
Since our focus is on ionic conducting electrolyte solutions, the
exact equilibrium concentration in the hydrocarbon solutions
was not studied in detail. However, a knowledge of the actual
equilibrium concentration of the various species in the THF
solutions is of great importance, as it influences the electro-
chemical properties.
From these spectra, it is possible to identify in the DCC
solution the existence of the anion Et2AlCl2- (the large peak at
367 cm-1), the neutral molecule Et2AlCl-THF (the large
double peak at 413-424 cm-1), and, in a small concentration,
Et3Al-THF (a smaller feature at ca. 490 cm-1). It is also
discernible that the wide feature at around 540-550 cm-1 is a
superposition of the relevant peaks of Et2AlCl2- and Et2AlCl-
THF (the magnesium features, observed at ca. 212 and 239
cm-1, are not dealt with at this stage). This result supports the
NMR data which showed that dialkylmagnesium exchanges
ligands with the aluminum-based Lewis acid in such a way that
magnesium takes up the chlorides, giving up the alkyl groups
to which it is bound originally.3 Interestingly, from the Raman
spectra it is obvious that the DCC solution contains both the
postulated anion, Et2AlCl2-, as well as the neutral molecule,
Et2AlCl-THF. The Raman data show no indication of the
existence of a dimer of these two molecules, as was sug-
gested from the NMR data. Interestingly, it is also discernible
that in DCC solution there is also a small concentration of
Et3Al-THF.
Quantitative Analysis of the Aluminum Species in Equi-
librium. As described above, DCC-type complex salts are
prepared by reacting organo magnesium and organo-halo
aluminum precursors, both as solutions in hydrocarbons (e.g.,
hexane). These reactions are rapid and exothermic and are
accompanied by the immediate precipitation of solids. Elemental
analyses of the precipitate and the above hydrocarbon solutions
(the reaction medium) have shown that, in this preparative stage,
the precipitate includes mainly MgCl2 and the solution contains
mostly aluminum species, with both chloride and organic
ligands. Thus, it can be concluded that when the synthesis is
carried out in a nonpolar hydrocarbon medium, the first reaction
involves a rapid exchange of all the organic ligands of
magnesium by chlorides. Since DCC solution synthesis involves
the reaction of 2 equiv of ethyl aluminum chloride with 1 equiv
of dialkylmagnesium, it is obvious that, after the complete
exchange of organic for chlorine ligands on Mg, the aluminum
species will attain an equilibrium among Et2AlCl, EtAlCl2, and
Et3Al (AlCl3 was not detected by NMR and Raman analyses).
-
The main relevant aluminum species are the anion R2AlCl2
and the neutral molecule R2AlCl, as was concluded in the
previous section. Fortunately, the Raman spectra of these species
in THF have several unique peaks, adequate for calculating their
concentration ratio. For the quantitative analysis of these species,
we prepared solutions with a known ratio of R2AlCl to
R2AlCl2-. For this purpose, solutions of Li+R2AlCl2 were
-
prepared by reacting equimolar quantities of LiCl with R2AlCl
in THF. On the basis of the Raman spectrum of this solution,
it contains only the desired anionic species. Carefully weighed
amounts of R2AlCl were added at various stoichiometric
-
proportions to these Li+R2AlCl2 solutions (R ) Et). The
Raman spectra of these solutions are presented in Figure 6 and
the data summarized in Table 2.
From these spectra it can be judged that these solutions
contain only the desired anions and neutral aluminum spe-
cies. It can also be seen that there is a linear correlation be-
tween the species’ concentration ratio and the corresponding
peak height ratio (at 367 and 413 cm-1), as demonstrated in
Figure 7.
Identification of the Structures of the Cations in Solution.
Although the identities of the anions and the aluminum-based
neutral molecules in solution have an important influence on
their electrochemical properties, the magnesium species in the
solutions are of the greatest importance. In this respect, the
information that could be gathered from 25Mg NMR analysis
was poor, mainly due to the broad nature of the peaks, their
low intensities, and the very small chemical shifts that could
be observed in these spectra. Nevertheless, our previous studies
of DCC solutions by 25Mg NMR3 indicated that the magnesium
species in the various solutions tested are hexacoordinated.
Furthermore, from 13C and 1H NMR data, we could deduce that,
in solutions in which the reactant ratio is greater than 1:1
(Al/Mg), no organic ligands are bound to magnesium. Thus,
the remaining possibilities for the magnesium-based structures
are combinations of chloride and THF ligands, adding up to
six ligands per Mg ion.

Magnesium-Aluminate Chloro-Organic Complexes
Organometallics, Vol. 26, No. 13, 2007 3135
Figure 6. Raman spectra of THF solutions containing mixtures of Et2AlCl and Et2AlCl2- at different concentration ratios, as indicated in
the figure.
-
with varying ratios of MgCl2 to MgCl+ have shown the
Table 2. Correlation between Et2AlCl2 and Et2AlCl
anticipated trend in the peak height ratio.7
Concentration Ratio and the Corresponding Peak Height
Ratio (at 367 and 413 cm-1
)
Interestingly, the spectra of the DCC solutions invariably
organic ligand
ion-molecule ratio
peak height ratio
contain two peaks, at 212 and 239 cm-1. This provides evidence
+
e
thyl
1:1
2.06
1.80
1.76
1.47
1.25
1.13
1.03
for the existence of both MgCl2 and Mg2Cl3 moieties in
1:1.3
1:1.5
1:2
1:3
1:4
solution. The peak height ratio was found to be dependent both
on the exact reactant ratio (the exact Al to Mg ratio) and, to a
lesser degree, on the reaction conditions. Since MgCl+ does
not have a fingerprint in the Raman spectra that we could
measure, we deduce its existence on the basis of the reactant
ratio. For instance, in DCC solutions that were prepared with
an Al to Mg ratio of 1, only the peak at 212 cm-1 is observed,
testifying for the existence of only MgCl2 in the solution as a
single magnesium-based species. This is in line with the 27Al
NMR spectra, ionic conductivity data (minimum in conductiv-
ity), and the rest of the Raman spectra, which demonstrate the
existence of only the neutral aluminum species Et3Al.
1:5
methyl
1:1
2.00
1.35
1.31
1.21
1.14
1:1.3
1:1.5
1:2
1:3
Figure 8 presents the spectra of MgCl2, MgCl+AlEt4-, and
Mg2Cl3+AlEt4 in THF. MgCl+ was prepared by the reaction
-
To complete the picture, spectra were also acquired for
Et2Mg, Bu2Mg, BuMgCl, and EtMgCl solutions in THF. These
spectra are not presented, as they do not add to the identification
of the species in solution. Nevertheless, it is worth mentioning
that in THF solutions which contain species having organo-
magnesium bonds, significant peaks were seen around 350, 400,
and 490 cm-1, along with a peak at 212 cm-1 for the Grignard
solutions. These results are in agreement with literature data
and are in accordance with the expected Schlenk equilibrium
existing in all of these solutions, in which the presence of MgCl2
is postulated.9 The data related to these spectra are also given
in Table 1, along with a compilation of the data for other
solutions containing magnesium species.
of an equimolar quantity of EtMgCl with Et3Al, while
Mg2Cl3 was synthesized by reacting 2 equiv of EtMgCl with
1 equiv of Et2AlCl.
The expected reaction sequence is presented in eqs 1 and 2
(the THF presence was omitted for clarity).
+
EtMgCl + Et3Al / MgCl+ + Et4Al-
(1)
2EtMgCl + Et2AlCl / MgCl+ + MgCl2 +
Et4Al- T Mg2Cl3+ + Et4Al- (2)
An indication that reactions take place between the reactants
as described in eqs 1 and 2 was obtained from the dramatic
increase in the solution’s ionic conductivity compared to those
of the solutions of the reactants alone. The sharp peak at 460
cm-1 in the Raman spectra, measured from solutions in which
reactions (1) and (2) took place, testifies that from the viewpoint
of aluminum species only Et4Al- is present in the solution. No
other known spectral features were observed, which could
indicate the existence of any other aluminum species. From these
reference spectra, it is possible to learn that MgCl2 has a single
characteristic peak at 212 cm-1, that MgCl+ does not have
Conclusion
Raman spectroscopy was proven to be a powerful tool for
the identification of various organo-halo metallic species in
solutions. These spectroscopic measurements were used for the
identification of the various species in ethereal electrolyte
solutions for rechargeable magnesium batteries on the basis of
complexes obtained by reactions between RxMgCl2-x (Mg Lewis
base) and Al-based Lewis acidssR′yAlCl3-y (R and R′ are
organic ligands, x ) 0-2, y ) 0-3). From the Raman
spectroscopic studies presented herein, it was proven that
magnesium ion conducting ethereal solutions that comprise the
discernible features in the spectral window of our Raman setup,
and that MgCl+ and MgCl2 form a dimer, namely, Mg2Cl3
,
+
which shows a characteristic peak at 239 cm-1. Indeed, solutions

3136 Organometallics, Vol. 26, No. 13, 2007
Vestfried et al.
Figure 7. Correlation between various Et2AlCl2- and Et2AlCl concentration ratios and the corresponding ratio between the heights of their
main Raman peaks, at 367 and 413 cm-1, respectively, which belong to Et-Al stretching vibrations.
-
Figure 8. Raman spectra of MgCl2, MgCl+AlEt4-, and Mg2Cl3+AlEt4 solutions in THF.
(Aldrich, 97%) or to an appropriate amount of 2 M ethylmagnesium
chloride (EtMgCl) in THF (Aldrich, 99%). In these cases, a mildly
exothermic reaction took place, yielding immediately a powdery
white precipitate. The suspension thus formed was stirred at room
temperature for an additional 48 h, after which it was vacuum-
dried. Extra-dry tetrahydrofuran (THF, Merck, Selectipure) was then
added to the dry, white solid, usually to a desired concentration of
0.25 or 0.5 M. The solutions thus obtained were clear and colorless.
reaction products between R2Mg or RMgCl Lewis bases and
AlCl2R Lewis acids contain various species in multiple equi-
-
libria, including MgCl2, Mg2Cl3+, AlCl3-nRn, and AlCl4-nRn
(1 e n e 3). The exact composition of these solutions is
determined mostly by the Lewis base-Lewis acid reagent ratio.
It was demonstrated that the Raman spectroscopy of these
solutions enables their quantitative analysis. The exact species
and their relative concentration ratio, as determined by Raman
spectroscopy, come in line with the relatively high ionic
conductivity of the DCC solutions, and their appreciable anodic
stability. This Raman spectroscopy study enables accurate and
simple analysis for practical, DCC-like solutions, for battery
applications.
Another method of electrolyte synthesis consisted of the prepara-
tion of a 1 M ethylaluminum dichloride (EtAlCl2) solution in THF
by vacuum-drying hexane from an ethylaluminum dichloride
(EtAlCl2) solution in hexane (Aldrich, 97%) and then adding an
appropriate amount of THF (the same as above). After the obtained
solution was stirred fir 30 min, it was added dropwise to an
appropriate amount of 2 M ethylmagnesium chloride (EtMgCl) in
THF (Aldrich, 99%).
Experimental Section
Solutions of diethylmagnesium (Et2Mg) in hexane were prepared
by the addition of dry distilled 1,4-dioxane (Aldrich, 99.9%) to an
ethylmagnesium chloride (EtMgCl) solution in THF (Aldrich, 97%),
which caused the precipitation of MgCl2‚(dioxane), leaving out the
Et2Mg solution (which was obtained in a very pure form by filtration
through a fine glass frit). A MgCl2·4THF solution in THF was
prepared by reacting an excess of pure magnesium foil (Merck,
99.95%) with a dry solution of HgCl2 in THF.
All chemical preparations and electrochemical measurements
were carried out under a pure argon atmosphere in M. Braun, Inc.
gloveboxes (less than 1 ppm of water and oxygen).
Chemicals and Synthesis. One method of preparation of the
complex salt solution consisted of the dropwise addition of a chosen
amount of 1 M ethylaluminum dichloride (EtAlCl2) solution in
hexane (Aldrich, 97%) to a vigorously stirred, carefully measured
amount of filtered 1 M dibutylmagnesium (Bu2Mg) in heptane

Magnesium-Aluminate Chloro-Organic Complexes
Organometallics, Vol. 26, No. 13, 2007 3137
Triethylaluminum (Et3Al, Aldrich, 97%), diethylaluminum chlo-
ride (Et2AlCl, Aldrich, 97%), and ethylaluminum dichloride
(EtAlCl2, Aldrich, 97%) were all used as received after their former
solvent (hexane or heptane) was exchanged by THF, as described
for ethylaluminum dichloride. The solution of AlCl3 in THF was
obtained by dissolving solid AlCl3 (Aldrich, 99.99%) in this solvent.
Lithium chloride (LiCl, Aldrich) was dried under vacuum over P2O5
at 75 °C for 72 h.
Spectral Studies. Raman spectra were measured with a JY
Horiba spectrometer using an He-Ne excitation laser (632.817 nm)
in a standard screw-cap rectangular spectrozile quartz cuvette
(Aldrich). All of the experiments were performed at room temper-
ature (25 ( 2 °C).
The theoretical Raman spectra were calculated at the HF/6-31G*
level of theory, using the Gaussian 03 package.13 A solvent molecule
(THF) has been added to the neutral species as the fourth legand
on the aluminum atom. The solvent (THF, ꢀ ) 7.58) has been
accounted for via the IEFPCM14 model. The calculated vibrational
eigenvectors were used to create animation files, to assist in
assigning the vibrational modes.
This material is available free of charge via the Internet at
http://pubs.acs.org.
OM061076S
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N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.;
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Supporting Information Available: Tables giving QM cal-
culation data for the various compounds studied in this paper.