Vanadium oxide complexes in room-temperature chloroaluminate molten salts

Article abstract of DOI:10.1021/ic990693o

The dissolution of vanadium(V) oxide (V₂O₅) in various ionic liquids has been studied to determine the complexes formed with respect to melt composition and V₂O₅ concentration. Vanadium oxide did not dissolve in either 1-n-butyl-3-methylimidazolium tetrafluoroborate or 1-n-butyl-3-methylimidazolium trifluoromethanesulfonate ionic liquids. V₂O₅ was found to dissolve at temperatures greater than 70 °C in 1-ethyl- and 1-n-butyl-3-methylimidazolium tetrachloroaluminate ionic liquids. Analyses of vanadium-containing melts by ⁵¹V, ¹H, and ¹³C NMR and infrared spectroscopy indicate the emergence of different species as a function of melt acidity. In basic and neutral melts, VO₂Cl₂^- and a metavanadate species of the form [(VO₃)_n]^n- are observed. The species VO₂Cl₂^- is the prominent product in basic melts, but as the melt becomes neutral or as the concentration of V₂O₅ is increased, the concentration of the metavanadate species is found to increase. However, V₂O₅ has been found to react in acidic melts to form volatile VOCl₃.

Full text of DOI:10.1021/ic990693o

Inorg. Chem. 1999, 38, 5709-5715
5709
Vanadium Oxide Complexes in Room-Temperature Chloroaluminate Molten Salts
R. C. Bell and A. W. Castleman, Jr.*
Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802
D. L. Thorn
Central Research and Development Department, Central Science and Engineering Laboratory,
E. I. DuPont de Nemours and Company, Inc., Experimental Station, P.O. Box 80328,
Wilmington, Delaware 19880
ReceiVed June 15, 1999
The dissolution of vanadium(V) oxide (V2O5) in various ionic liquids has been studied to determine the complexes
formed with respect to melt composition and V2O5 concentration. Vanadium oxide did not dissolve in either
1-n-butyl-3-methylimidazolium tetrafluoroborate or 1-n-butyl-3-methylimidazolium trifluoromethanesulfonate ionic
liquids. V2O5 was found to dissolve at temperatures greater than 70 °C in 1-ethyl- and 1-n-butyl-3-
1
methylimidazolium tetrachloroaluminate ionic liquids. Analyses of vanadium-containing melts by 51V, H, and
13C NMR and infrared spectroscopy indicate the emergence of different species as a function of melt acidity. In
-
basic and neutral melts, VO2Cl2 and a metavanadate species of the form [(VO3)n]n- are observed. The species
VO2Cl2- is the prominent product in basic melts, but as the melt becomes neutral or as the concentration of V2O5
is increased, the concentration of the metavanadate species is found to increase. However, V2O5 has been found
to react in acidic melts to form volatile VOCl3.
Introduction
insight into the mechanisms of these reactions.12,13 Room-
temperature chloroaluminate molten salts provide an ideal
There has been a growing interest in the study of ambient-
temperature ionic liquids as an amiable solvent for the study of
transition metal chloro complexes. Ionic liquids represent an
ideal nonaqueous environment for studying the reactions of these
transition metal complexes free from the effects of solvation
and solvolysis phenomena.1 Many transition metal chloro
complexes1-4 and their reactivities toward various hydrocar-
bons3,5 have been investigated in this unique media. In addition
to metal chlorides, stable oxochloro complexes have been
observed in these liquids.6-9 Oxygen-containing transition metal
compounds have been used extensively in industry as catalysts
or as supports for other catalytic materials. For example,
vanadium oxides and other related vanadium-containing com-
pounds are widely used as catalysts,10 especially for oxygen-
transfer reactions.11 Vanadium compounds have been studied
extensively to determine their catalytic properties and to gain
aprotic environment in which vanadium compounds may be used
and studied as catalysts or as battery cathodes. Thus, it is
necessary that the stability of V2O5 or that of the reaction
products in these melts be understood. The objective of the
present study is to determine the reactions of V2O5 in Lewis
basic, neutral, and acidic chloroaluminate ionic liquids of
varying vanadium(V) oxide content.
Several studies have shown a variety of vanadium species to
be stable in Lewis basic 1-ethyl-3-methylimidazolium tetra-
chloroaluminate ionic liquids, EMIC/AlCl3.14-16 Hanz and
Riechel have shown that chloride can be added to VCl3 to form
3-
higher order chloride complexes up to VCl6 and that each
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P. B.; Mohammed, T. J.; Seddon, K. R.; Zora, J. A.; Hussey, C. L.;
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Chem. Soc., Chem. Commun. 1999, 25. (b) Crofts, D.; Dyson, P. J.;
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Suarez, P. A. Z.; De Cian, A. Organometallics 1998, 17, 815.
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103, 3754.
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Catalysis; VCH: New York, 1994; pp 251-253. (b) Satterfield, C.
N. Heterogeneous Catalysis in Industrial Practice, 2nd ed.; McGraw-
Hill: New York, 1991; pp 267-334.
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Somorjai, G. A. J. Phys. Chem. 1990, 94, 5022. (d) Zhang, Y.; Holm,
R. H. Inorg. Chem. 1990, 29, 911.
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A 1999, 103, 2992. (b) Bell, R. C.; Zemski, K. A.; Castleman, A. W.,
Jr. J. Phys. Chem. A 1999, 103, 1585. (c) Bell, R. C.; Zemski, K. A.;
Castleman, A. W., Jr. J. Phys. Chem. A 1998, 102, 8293. (d) Bell, R.
C.; Zemski, K. A.; Kerns, K. P.; Deng, H. T.; Castleman, A. W., Jr.
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Seddon, K. R.; Welton, T. J. Chem. Soc., Chem. Commun. 1987, 1643.
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10.1021/ic990693o CCC: $18.00 © 1999 American Chemical Society
Published on Web 11/12/1999

5710 Inorganic Chemistry, Vol. 38, No. 25, 1999
Bell et al.
evaporated to yield the crude products. All experiments were performed
in inert-atmosphere gloveboxes under dry nitrogen or argon. Glassware
was oven dried under reduced pressure prior to use.
vanadium(III) complex can be reversibly oxidized to a corre-
sponding vanadium(IV) complex.14 However, not all chloride-
containing vanadium compounds are stable in ionic liquids, as
many react with the medium to form new species. For example,
VOCl3 and VCl62- react in basic chloroaluminate melts to form
Results and Discussion
VOCl4 and VCl63-, respectively.15 Welton and co-workers
2-
The dissolution of vanadium oxide occurred in acidic, neutral,
and basic EMIC/AlCl3 and BMIC/AlCl3 melts at temperatures
above 70 °C. At no time did V2O5 or the products formed during
the dissolution process precipitate out of solution after the ionic
liquid reached room temperature. V2O5 was found to be rather
soluble in the chloroaluminate ionic liquids. For example, 0.15
g of V2O5 was found to dissolve in 1.0 g of basic EMIC/AlCl3
at a mole ratio of 1.0:0.8 (EMIC:AlCl3). No formal solubility
constants were determined due to the reaction of the vanadium
oxide with the solvent. The Lewis acidity of the melts may be
expressed in terms of the chloride concentration,21 which
depends on the molar ratio of the constituent 1-alkyl-3-
methylimidazolium chloride salt and AlCl3. Neutral melts
contain an equal molar ratio of AlCl3 and EMIC (or BMIC)
with AlCl4- as the predominant anion, although trace amounts
formed the salt 1-ethyl-3-methylimidazolium tetrachlorooxo-
vanadate(IV) to further explore the chemistry of oxygen-transfer
reactions of vanadium compounds by studying both the X-ray
crystal structure of the salt and conducting a spectroscopic
investigation of its structure in a basic EMIC/AlCl3 melt.16 To
further understand the nature by which oxychloro vanadium
complexes are formed, the dissolution of V2O5 in EMIC/AlCl3
and 1-n-butyl-3-methylimidazolium tetrachloroaluminate, BMIC/
AlCl3, ionic liquids was examined in the present study through
a spectroscopic investigation. To gain more insight into these
processes, an investigation of the effects of melt acidity and
vanadium concentration on the types and stability of the products
formed was undertaken.
Experimental Section
of Cl- and Al2Cl7 are present.1b,22 Acidic melts contain an
-
Instrumentation. 51V NMR spectra were recorded at 105.2 MHz
and 25 ( 1 °C, using a Varian/NOVA 400 MHz spectrometer equipped
with a 10 mm broad band probe. Typically, spectral widths of 150
kHz, pulse widths of 8 µs (90° pulse angle), and line broadening of 50
Hz were used. All 51V NMR spectra were collected using neat samples
in 5 mm sample tubes which were placed inside 10 mm tubes containing
CO(CD3)2 for deuterium locking. All 51V NMR chemical shift literature
values and those for the present experiments are reported relative to
neat VOCl3 (0 ppm).
1H and 13C NMR spectra were recorded using a Bruker 500
spectrometer. These spectra were collected neat or in CD3CN solvent
with chemical shifts measured in ppm relative to the external reference
tetramethylsilane (TMS). Infrared spectra were collected on a Perkin-
Elmer 1600 spectrometer using both NaCl salt plates and polypropylene
film. The mass spectra were collected using a PerSeptive Biosystems
delayed extraction MALDI time-of-flight mass spectrometer. An N2
laser was used to thermally excite the ionic liquid to expel the ions
into the time-of-flight region of the spectrometer for mass analysis.
excess of AlCl3, thus increasing the concentration of the acidic
species (Al2Cl7-) in the melt, while basic ionic liquids have
higher concentrations of Cl- due to excess EMIC in the melt.
Although the dissolution of vanadium oxide occurred in
chloroaluminate melts, it did not appear to dissolve in the
inherently neutral ionic liquids 1-n-butyl-3-methylimidazolium
tetrafluoroborate, [BMI][BF4], or 1-n-butyl-3-methylimidazo-
lium trifluoromethanesulfonate, [BMI][CF3SO3], at temperatures
up to 100 °C with constant stirring for 24 h.
The 1-alkyl-3-methylimidazolium cation was not affected by
the dissolution process nor did it appear to react with the
vanadium compounds formed over the range of compositions
1
examined, as indicated by H and 13C NMR spectra. The 51V
NMR results displayed distinctly different products for the
dissolution of vanadium oxides in acidic melts as compared to
those formed in basic and neutral melts. The dissolution of
vanadium oxide was first attempted in open vials within an inert
atmosphere drybox. However, the samples prepared in this
manner displayed no 51V NMR signals in acidic melts.
Therefore, all subsequent samples were prepared in closed
containers and transferred to NMR tubes prior to obtaining the
spectra. Samples of V2O5 dissolved in acidic melts in this
manner displayed a single peak at -1 ppm, indicating that V2O5
reacts with the chloroaluminate species to form volatile VOCl3
in acidic melts.
Reagents and Synthesis. All compounds were synthesized and
purified using published methods. The 1-ethyl-3-methyl-1H-imidazo-
lium chloride (Aldrich) was dissolved in a minimal amount of
acetonitrile and recrystallized from toluene.17 Residual solvent was
driven off by heating the powder to 90 °C under reduced pressure.
Aluminum(III) chloride (Aldrich) was purified by vacuum sublimation.18
The reagents 1-chlorobutane, 1-methylimidazole, 99.99% anhydrous
vanadium oxide, sodium metavanadate, sodium tetrafluoroborate, and
sodium trifluoromethanesulfonate were used as supplied by the Aldrich
Chemical Co. VOCl3 (Aldrich) was distilled in vacuo prior to use.
Acetonitrile and toluene (Aldrich) were dried over activated molecular
sieves in a nitrogen atmosphere. All other solvents were distilled from
appropriate drying agents under nitrogen.
The dissolution of V2O5 in acidic melts was further examined
in a modified bulb-to-bulb vacuum transfer apparatus, as shown
in Figure 1. In the drybox, the vanadium oxide and chloroalu-
minate melts were placed inside the larger vessel. The apparatus
was sealed via a threaded stopcock, removed from the drybox,
and evacuated. It was then closed to form a self-contained
system. The large vessel was used to heat and stir the V2O5-
containing acidic melt while open to the small vessel, which
was immersed in liquid N2 to collect volatile products formed
during the dissolution process. The dark red liquid was then
The 1-n-butyl-3-methylimidazolium chloride (BMIC) was prepared
by refluxing 1-methylimidazole in an excess of 1-chlorobutane for 24
h.17 The excess chlorobutane was removed by evaporation and the crude
product recrystallized from acetonitrile. The 1-n-butyl-3-methylimida-
zolium tetrafluoroborate was prepared by adding equal molar amounts
of sodium tetrafluoroborate to a solution of BMIC in acetone at room
temperature.19 The 1-n-butyl-3-methylimidazolium trifluoromethane-
sulfonate20 was prepared in the same manner using sodium trifluo-
romethanesulfonate. Both mixtures were stirred for 24 h, filtered, and
(20) (a) Bonhoˆte, P.; Dias, A.; Papageorgiou, N.; Kalyanasundaram, K.;
Gra¨tzel, M. Inorg. Chem. 1996, 35, 1168. (b) Cooper, E. I.; Sullivan,
E. J. M. Proceedings of the 8th International Symposium on Moten
Salts; The Electrochemical Society: Pennington, NJ, 1992; Proceeding
Vol. 92-16, pp 386-396.
(21) Gale, R. J.; Osteryoung, R. A. Inorg. Chem. 1979, 18, 1603.
(22) Hussey, C. L.; Scheffler, T. B.; Wilkes, J. S.; Fannin, A. A. J.
Electrochem. Soc. 1986, 133, 1389.
(17) Wilkes, J. S.; Levisky, J. A.; Wilson, R. A.; Hussey, C. L. Inorg.
Chem. 1982, 21, 1263.
(18) Sun, I.-W.; Ward, E. H.; Hussey, C. L.; Seddon, K. R.; Turp, J. E.
Inorg. Chem. 1987, 26, 2140.
(19) Suarez, P. A.; Dullius, J. E. L.; Einloft, S.; De Souza, R. F.; Dupont,
J. Polyhedron 1996, 15, 1217.

V2O5 Complexes in Chloroaluminate Molten Salts
Inorganic Chemistry, Vol. 38, No. 25, 1999 5711
Figure 1. Diagram of the adapted bulb-to-bulb vacuum transfer
apparatus. The large vessel is used to heat and stir the sample. The
smaller vessel serves as the collection reservoir and is cooled in liquid
nitrogen.
transferred to a NMR tube for further analysis. The volatile
liquid thus collected displayed a single 51V NMR peak at -3
ppm. It is believed that small amounts of moisture in the system
caused partial hydrolysis of VOCl3, resulting in the dark red
appearance of the liquid.23 To confirm this, pure VOCl3 was
subjected to a small amount of water vapor and quickly turned
dark red in color. The 51V NMR of this sample also displayed
a peak at -3 ppm, while pure VOCl3 in the acidic melts
produced a chemical shift at -1 ppm. Infrared studies of the
vanadium-containing acidic melts indicate that no other products
are formed during the dissolution process.
Figure 2. 51V NMR spectrum of EMIC/AlCl3 ionic liquid containing
V2O5 with a mole ratio of 1:0.80:0.190 (EMIC:AlCl3:V2O5).
Neutral and basic chloroaluminate melts form similar species
but in varying amounts. The dissolution of vanadium oxide in
both neutral and basic melts produces two major 51V NMR
signals at -367 and -574 ppm. In addition, a minor peak at
-721 ppm is observed in the basic melts, while neutral melts
contain minor products indicated by peaks at +45 and -398
ppm. The exact chemical shifts for these peaks depend on both
the acidity of the melt and on V2O5 concentration. For example,
chemical shifts from -367 to -365 ppm and from -574 to
-570 ppm are observed as the vanadium concentration in a
basic melt (BMIC:AlCl3 ) 1:0.60) is changed from a V2O5:
BMIC ratio of 0.010 to 0.362. A similar ∆δ from -367 to -365
is observed for a change in melt composition of BMIC:AlCl3
) 1:0.38 to 1:1.00 with a constant V2O5:BMI ratio of 0.05.
However, a greater ∆δ is observed for the signal at ca. -574
ppm. This peak shifts from -577 ppm (BMIC:AlCl3 ) 1:0.38)
to -572 (1:0.81) and then back up to -574 in neutral melt
(1:1.00). The 51V NMR spectra for basic (BMIC:AlCl3:V2O5
) 1:0.80:0.190) and neutral (1:1.00:0.050) vanadium oxide-
containing melts are shown in Figures 2 and 3, respectively. In
addition, the dissolution of sodium metavanadate (NaVO3) in
basic chloroaluminate melts was found to form the same 51V
NMR signals as those of V2O5, but was not pursued in any
further detail.
Figure 3. 51V NMR spectrum of BMIC/AlCl3 ionic liquid containing
V2O5 with a mole ratio of 1:1.00:0.050 (BMIC:AlCl3:V2O5).
acetonitrile/water26 displayed chemical shifts at -364 and -365
ppm, respectively. Attempts to make [Et4N][VO2Cl2] by meth-
ods suggested in the literature15 were unsuccessful, and we did
not directly compare 51V NMR spectra of independently
-
synthesized VO2Cl2 salts with spectra of V2O5-containing
chloroaluminate melts. However, extended X-ray absorption fine
structure (EXAFS) of the solid salt [NEt4][VO2Cl2] and a
solution formed when this salt is dissolved in a basic EMIC/
AlCl3 melt are reported in good agreement with one another,
indicating that the structure is unchanged upon dissolution in
the chloroaluminate ionic liquid.15 These results provide no
evidence for the coordination of the chloroaluminate species at
the oxygen atoms nor for any increase in the number of chloride
ions in the coordination sphere, which would result in a
The 51V NMR signal at -367 ppm indicates the presence of
VO2Cl2-. Similar shifts have been observed for M[VO2Cl2] (M
) AsPh4+ in CH2Cl2/THF, PPh4+ in CH2Cl2, and NEt4+ in THF
(THF ) tetrahydrofuran)), all of which displayed signals at
-359 ppm relative to VOCl3.24 VO2Cl2 in acetonitrile25 and
-
(23) Mellor, J. W. A ComprehensiVe Treatise on Inorganic and Theoretical
Chemistry; Longmans, Green and Co.: New York, 1929; Vol. IX, p
806.
(24) Hanich, J.; Krestel, M.; Mu¨ller, U.; Dehnicke, K.; Rehder, D. Z.
Naturforsch 1984, 39B, 1686.
(25) Hibbert, R. C. J. Chem. Soc., Dalton Trans. 1986, 751.
(26) Hibbert, R. C.; Logan, N.; Howarth, O. W. J. Chem. Soc., Dalton
Trans. 1986, 369.

5712 Inorganic Chemistry, Vol. 38, No. 25, 1999
Bell et al.
lengthening of all the V-Cl bonds and particularly of the Vd
O bond. Lengthening of the VdO bond would be expected to
appear in the infrared spectra as a decrease in the VdO
2-
stretching frequency, but is not seen for VOCl4 in the basic
melt.16 Therefore, it is expected that the 51V NMR chemical
-
shift for VO2Cl2 in chloroaluminate melts should not be
substantially different from those found in other solvents.
The peak at -574 ppm falls within the regime of iso- and
heteropolyvanadates and of VO(OR)3 species.27,28 Slightly basic
aqueous solutions of dissolved vanadium oxide produce 51V
NMR chemical shifts in the area of -571 to -577 ppm, which
have been assigned as the metavanadate species [(VO3)n]n- with
n equal to 3 or 4.29-32 More recently, the signals at -571 and
3-
4-
-574 ppm have been assigned to the V3O9 and V4O12
3-
species, respectively.33 It is generally accepted that V3O9 is
the prominent metavanadate ion in dilute solutions,34 and
4-
at higher vanadium concentrations the V4O12
species
dominates.35-37 The interconversion of these species could be
fast and could result in the presence of a single averaged peak,
but no chemical shift was observed as the VO3- concentration
changed from 0.002 to 0.249 M in a slightly basic aqueous
solution.38 As stated previously, a shift in chemical shift from
-574 to -570 ppm was observed in the chloroaluminate melts
as the mole ratio of V2O5 increased from 0.01 to 0.36 in a basic
melt (1:0.60). A similar shift in chemical shift from -577 to
-572 ppm was observed for melts containing a constant mole
ratio of V2O5 (0.05) as the melt became less basic (1:0.38 to
1:0.81). Both situations should result in a lower concentration
of Cl-; that is, as the concentration of V2O5 is increased, more
VO2Cl2 would be formed lowering the Cl- concentration.
-
Therefore, the shift of the -574 ppm peak is probably due to
subtle changes in the local environment at vanadium with
changes in chloride concentration rather than a change between
vanadate trimer and tetramer.
The neutral melts also display minor peaks at +46 and -398
ppm. The peak at +46 is most likely due to the presence of a
small amount of VOCl4-. The 51V NMR chemical shift for
Figure 4. IR spectra (335-2000 cm-1) of (a) basic EMIC/AlCl3
with a mole ratio of 1:0.80 and (b) basic EMIC/AlCl3 containing V2O5
with a mole ratio of 1:0.80:0.190 (EMIC:AlCl3:V2O5).
-
VOCl4 has been reported as +50 in a mixture of dichlo-
romethane and tetrahydrofuran,24 as +46 in tetrahydrofuran,24,39
-
and as +43 in acetonitrile.25 However, in basic melts, VOCl4
display 51V NMR signals within the δ-range of -409 to -580
ppm.27 The minor signal at -398 ppm in neutral melts and
possibly the signal at -721 ppm in basic melts could be due to
aluminum heteropolyvanadate species or a chlorine-containing
isopolyvanadate.
reacts with excess Cl- in the melt to form the reduced species
VOCl4 .
2- 15 In neutral melts, without excess Cl-, the persistence
-
of VOCl4 in the melt is not surprising. It is possible that
VOCl42- exists in equilibrium with VOCl4- in the neutral melt,
but due to its being V4+, it is not detected by 51V NMR. An
important characteristic of vanadium is its ability to form
polymeric oxoanions.40,41 Most iso- and heteropolyvanadates
The infrared spectra of the ionic liquids were recorded neat
using both polypropylene film and NaCl salt plates. The IR
spectra for neutral and basic vanadium-containing melts display
strong absorbances at 997 and 432 cm-1. A small shoulder
indicates the presence of a peak in the area of 953 cm-1 but is
obscured by the N-H and C-H in-plane bending band of the
(27) Kidd, R. G.; Goodfellow, R. J. In NMR and the Periodic Table; Harris,
R. K., Mann, B. E., Eds.; Academic Press: New York, 1978; p 206.
(28) Howarth, O. W. Prog. Nucl. Magn. Reson. Spectrosc. 1990, 22, 453.
(29) Heath, E.; Howarth, O. W. J. Chem. Soc., Dalton Trans. 1981, 1105.
(30) Howarth, O. W.; Richards, R. E. J. Chem. Soc. 1965, 864.
(31) Rehder, D. Z. Naturforsch 1977, 32B, 771.
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22, 254.
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J. Mol. Catal. 1991, 69, 61.
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Robinson, R. A.; Sinclair, D. A. J. Chem. Soc. 1934, 642.
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1-ethyl-3-methylimidazolium cation (EMI+) at 959 cm-1 42
. In
addition, a strong absorbance occurs in the area of 900 cm-1
but is broad and overlaps many of the weaker signals of the
imidazole cation. The IR spectra for a basic EMIC/AlCl3 melt
and one containing V2O5 are displayed in Figure 4.
The ν(VdO) bond stretch is found in the range of 935-
1035 cm-1 as reported for a large set of oxovanadium
complexes.43,44 The infrared VdO and VsCl stretching fre-
(40) Douglas, B.; McDaniel, D.; Alexander, J. Concepts and Models of
Inorganic Chemistry, 3rd ed.; Wiley: New York, 1994; pp 744-745.
(41) Pope, M. T.; Dale, B. W. Q. ReV. Chem. Soc. 1968, 22, 527.
(42) Tait, S.; Osteryoung, R. A. Inorg. Chem. 1984, 23, 4352.
(43) Selbin, J. Coord. Chem. ReV. 1966, 1, 293.
(44) Selbin. J. Chem. ReV. 1965, 65, 153.

V2O5 Complexes in Chloroaluminate Molten Salts
Inorganic Chemistry, Vol. 38, No. 25, 1999 5713
Table 1. Infrared VdO and VsCl Stretching Frequencies for Various Vanadium Compounds
salt
ν(VdO)/cm-1
ν(VsCl)/cm-1
ref
[Et4N][VO2Cl2]
[Ph3PMe][VO2Cl2]
[AsPh4][VO2Cl2]
995 (sym), 946 (asym)
970 (sym), 959 (asym)
971 (sym), 960 (asym)
1021
1023
993
1001
1000
1042
437
15
45
46
47
48
16
16
49
50
438 (sym), 431 (asym)
435
422 (sym), 366 (asym)
420, 403, 363
obscured by melt
361 (asym), 340 (sym)
350
[VCl2(15-crown-5)] [VOCl4]a
[N(PCl3)2][VOCl4]
[EMI]2[VOCl4] in basic EMIC/AlCl3
[EMI]2[VOCl4]
[Et4N]2[VOCl4]
VOCl3
509
a 15-crown-5 ) 1,4,7,10,13-pentaoxacyclopentadecane.
Table 2. Species Observed for Vanadium Oxide Containing Neutral
quencies for various oxochloro vanadium compounds are shown
in Table 1. Infrared spectra are sensitive to both the oxidation
state of the vanadium and to the coordination geometry of the
EMIC/AlCl3 Ionic Liquidsa
possible
species
rel
possible
species
rel
complex. The ion VO2Cl2 has C2V symmetry45,46 and would
-
m/z
abundance m/z
abundance
-
35 Cl-
3
11
9
41
100
27
19
12
14
236 V2O4Cl2
11
7
17
21
10
21
7
be expected to show two VdO stretches (A1 and B1) and two
VsCl stretches (A1 and B2).45 The absorbances at 997 and the
shoulder in the area of 953 cm-1 are characteristic of the VdO
stretching frequencies, and absorbance at 432 cm-1 for the V-Cl
99 [(VO3)n]n-
118 V2O4Cl2-
245 Al2OCl5
-
-
265 V3O7
-
-
153 VO2Cl2
167 AlCl4
172 VOCl3
271 V2O4Cl3
-
-
281 V3O8
-
stretch of VO2Cl2 supports the 51V NMR chemical shift
-
-
285 AlCl5VO2
-
-
assignment for this species.
182 V2O5
335 V3O7Cl2
217 V2O5Cl-
364 V4O10
9
-
The two symmetry-allowed V-Cl stretching modes for
-
VOCl42- are expected at 361 and 340 cm-1 48,51
The absorbance
.
231 AlCl3VO3
at 340 cm-1 is below the range available to us. The peak at
361 cm-1 is obscured by the melt itself, but the IR spectra
display a slightly greater absorbance in this area for both neutral
and basic vanadium-containing melts, which might indicate the
presence of a small amount of VOCl42-. The VdO stretch of
a The m/z ratios are for those species containing 35Cl isotopes
exclusively. The relative abundance for each species is given with
-
respect to AlCl4 and represents all isotopic species. Several of these
species may arise from the laser desorption process and may not be
contained in the ionic liquids as such.
-
VOCl4 is expected to occur at a higher wavenumber (∼1022
In an attempt to provide further insight into the products
formed during the dissolution of vanadium oxide in the ionic
liquids, laser desorption (LD) mass spectra of the melts were
obtained. The samples were prepared and placed on a gold
substrate, which was then transferred from the drybox to the
mass spectrometer in a dry argon atmosphere. A large plastic
tent was constructed around the sample port, and dry nitrogen
was allowed to flow into the tent for approximately 1 h.
Although this procedure greatly reduced the contamination of
the samples, small amounts of moisture were inevitably present.
The positive-ion laser desorption time-of-flight (LD-TOF) mass
spectra of the ionic liquids obtained in these studies are in good
agreement with those collected by fast atom bombardment
(FAB) mass spectrometry.56 The LD-TOF mass spectra for both
basic and neutral EMIC/AlCl3 melts display peaks at 83 and
111 m/z. These represent the 1-methylimidazole cation and
EMI+, respectively. The positive ion mass spectra for all melts
of varying acidity and vanadium content display similar mass
spectra.
Negative-ion LD-TOF mass spectra were more difficult to
acquire with the laser power increased to obtain these spectra.
However, by increasing the intensity of the laser, desorption of
the ionic liquid and vanadium-containing species resulted in
fragmentation and recombination reactions in the plume,
generating many species not believed to be contained in the
melt itself. Therefore, the results presented should be taken at
face value due to the pitfalls associated with this technique.
Both the neutral and basic vanadium-containing melts display
many anionic species. The prominent peaks that appear in the
majority of the spectra for vanadium-containing melts are shown
in Tables 2 and 3 for the neutral and basic melts, respectively.
The peaks at 35, 167, and 245 m/z represent the mass-to-charge
cm-1 47,48
and is not seen in either basic or neutral vanadium-
)
containing melts.
The strong absorbance that occurs in the area of 900 cm-1 is
very broad, overlapping many of the weaker signals of the
imidazole cation. This cannot be explained by a change in melt
acidity and therefore could be due to the presence of another
vanadium oxide species. The expected frequency range for
M-O-M bonds is in the range of 650-920 cm-1 43,52
Therefore,
.
the broad band at 900 cm-1 could be an indication of the
presence of a metavanadate species.53 The major species found
in slightly basic aqueous solutions of V2O5 are [(VO3)n]n- ions,
4-
where n is taken as 3 or 4.54,55 The infrared spectrum of V4O12
in an aqueous solution displays a strong band at 905 and another
less intense band at 952 cm-1 55
This corresponds well with
.
the broad band at 900 cm-1 and the shoulder at 953 cm-1
.
However, it is not certain if the band at 953 cm-1 is due to this
-
species or to VO2Cl2 or a combination of both.
(45) Fenske, V. D.; Shihada, A.-F.; Schwab, H.; Dehnicke, K. Z. Anorg.
Allg. Chem. 1980, 471, 140.
(46) Ahlborn, E.; Diemann, E.; Mu¨ller, A. J. Chem. Soc., Chem. Commun.
1972, 378.
(47) Frenzen, G.; Massa, W.; Ernst, T.; Dehnicke, K. Z. Naturforsch 1990,
45B, 1393.
(48) Zinn, A.; Patt-Siebel, U.; Muller, U.; Dehnicke, K. Z. Anorg. Allg.
Chem. 1990, 591, 137.
(49) Collison, D.; Gahan, B.; Garner, C. D.; Mabbs, F. E. J. Chem. Soc.,
Dalton Trans. 1980, 667.
(50) Filgueira, R. R.; Fournier, L. L.; Varetti, E. L. Spectrochim. Acta,
Part A 1982, 38A, 965.
(51) Haase, W.; Hoppe, H. Acta Crystallogr. 1968, B24, 282.
(52) Mathew, M.; Carty, A. J.; Palenik, G. J. J. Am. Chem. Soc. 1970, 92,
3197.
(53) Frederickson, L. D., Jr.; Hausen, D. M. Anal. Chem. 1963, 35, 818.
(54) (a) Jahr, K. F.; Fuchs, J. Z. Naturforsch. 1959, 14B, 468. (b)
Schwarzenbach, G.; Parissakis, G. HelV. Chim. Acta 1958, 41, 2425.
(c) Souchay, P. Bull. Soc. Chim. Fr. 1947, 14, 914. (d) Jander, G.;
Aden, T. Z. Phys. Chem. 1933, 144, 197.
(56) Abdul-Sada, A. K.; Greenway, A. M.; Seddon, K. R.; Welton, T. Org.
Mass Spectrom. 1993, 28, 759.
(55) Griffith, W. P. J. Chem. Soc., A 1967, 905.

5714 Inorganic Chemistry, Vol. 38, No. 25, 1999
Bell et al.
Table 3. Species Observed in Basic EMIC/AlCl3 Ionic Liquids
Containing Vanadium Oxidea
possible
species
rel
possible
species
rel
m/z
abundance m/z
abundance
35 Cl-
5
3
31
100
45
231 AlCl3VO3
245 Al2OCl5
14
21
18
17
-
99 [(VO3)n]n-
-
-
153 VO2Cl2
167 AlCl4
172 VOCl3
267 AlCl4VO3H-
-
-
285 AlCl5VO2
-
a The m/z ratios are for those species containing 35Cl isotopes
exclusively. The relative abundance for each species is given with
-
respect to AlCl4 and represents all isotopic species. Several of these
species may arise from the laser desorption process and may not be
contained in the ionic liquids as such.
ratio for compounds containing the 35Cl isotope, which cor-
respond to Cl-, AlCl4-, and Al2OCl5-, respectively. The peaks
at 99 and 153 m/z represent [(VO3)n]n- and VO2Cl2-, respec-
tively.
Figure 5. Plot displaying the ratio of the areas under the 51V NMR
signals at -367 ppm divided by that of -574 as the mole ratio of
V2O5 increases (BMIC:AlCl3:V2O5).
There are two prominent peaks at 231 and 285 m/z in these
-
spectra, which represent AlCl3VO3 and AlCl5VO2-, respec-
tively. These species may be better represented as adducts of
aluminum chloride and the more prevalent anionic species in
the melt to form AlCl3‚VO3- and AlCl3‚VO2Cl2-. We speculate
that these products are anomalies of the laser desorption
process.57,58 That is, the pulsed laser creates a dense plume of
neutral and ionic species with the neutral aluminum chloride
generated by the dissociation of the aluminum tetrachloride
anion to form AlCl3 and Cl- in the plume. As this plume
-
expands, recombination of neutral AlCl3 with the species VO3
-
and VO2Cl2 may occur to create these adduct products. The
large number of species observed in the mass spectra are due
to such processes as fragmentation and recombination reactions,
in addition to chloride- and oxygen-transfer reactions that are
presumed to have occurred in the plume.58,59 This technique
has proven to be less reliable for the exact determination of the
anionic species in the melt as the desorption laser has obviously
created many gas-phase species that are not presumed to be in
the melt as indicated by the IR and 51V NMR studies.
Had these species been present in the ionic liquids they would
be expected to appear in the 51V NMR spectra at these
concentrations, but only two major 51V NMR signals are
observed in the neutral and basic melts which have been
assigned to [(VO3)n]n- and VO2Cl2-. Therefore, it is apparent
that the laser desorption process has caused many reactions,
creating species that are not present in the ionic liquids as such.
However, there is a minor 51V NMR signal at -721 ppm in
basic melts and one at -398 ppm in the neutral melts that have
not been assigned and could possibly represent one of the species
found in these spectra.
Figure 6. Plot of the ratio of 51V NMR peaks for the area under -367
divided by that of -574 ppm as a function of melt acidity (BMIC:
AlCl3:V2O5).
so does the relative amount of metavanadate species. This can
be seen in the plot in Figure 5 that shows the ratio for the 51
V
NMR peak at -367 (VO2Cl2-) with respect to that at -574
ppm (metavanadate) in a basic melt (1:0.60) as the mole ratio
of V2O5 is increased. A similar trend occurs when the mole
ratio of V2O5 remains constant and the melt composition is
changed from basic to acidic (Figure 6). Both of these results
are consistent with an increase in the metavanadate species as
a decrease in the Cl- concentration in the melt occurs.
There are several possible reaction mechanisms that may
account for the formation of the observed species for the
dissolution of vanadium oxide in the chloroaluminate melts. At
low concentrations of V2O5 in a basic melt, there exists an
-
Gas-phase reactions of VO3 and HCl have been examined
in a flow tube apparatus.60 The reaction between the metavana-
-
date anion and HCl produced VO2Cl2 and water as follows:
-
excess of free Cl- and the primary products are VO2Cl2 and
metavanadates. However, as the concentration of V2O5 increases,
VO3- + HCl f VO2(OH)Cl-
(1)
(2)
VO2(OH)Cl- + HCl f VO2Cl2- + H2O
(57) Beavis, R. C.; Chait, B. T. Rapid Commun. Mass Spectrom. 1989, 3,
432.
(58) Dopke, N. C.; Treichel, P. M.; Vestling, M. M. Inorg. Chem. 1998,
37, 1272.
Upon formation of VO2Cl2-, no further reaction is observed
up to an increased concentration of 50.9 cm3 (STP) min-1 HCl
(59) (a) Dale, M. J.; Dyson, P. J.; Suman, P.; Zenobi, R. Organometallics
1997, 16, 197. (b) Dale, M. J.; Dyson, P. J.; Johnson, B. F. G.;
Langridge-Smith, P. R. R.; Yates, N. T. J. Chem. Soc., Dalton Trans.
1996, 771. (c) Bjarnason, A.; DesEnfants, R. E., II.; Barr, M. E.; Dahl,
L. F. Organometallics 1990, 9, 657. (d) Bjarnason, A. Rapid Commun.
Mass Spectrom. 1989, 3, 373.
in 0.3 Torr of helium. In contrast, NbO3 (at 15.1 cm3 (STP)
-
(60) Sigsworth, S. W.; Castleman, A. W., Jr. J. Am. Chem. Soc. 1992, 114,
10471.

V2O5 Complexes in Chloroaluminate Molten Salts
Inorganic Chemistry, Vol. 38, No. 25, 1999 5715
min-1 of HCl) and TaO3- (at 6.1 cm3 (STP) min-1 of HCl) do
-
VO2Cl2- + Al2Cl7- / VOCl3 + AlOCl2- + AlCl4
(6)
-
not truncate at MO2Cl2 but continue to react with HCl in the
-
-
Again, there is no direct experimental evidence that AlOCl2
is the actual aluminum species generated in reaction 5 or 6 and
is considered simply for convenience in balancing the reaction.
The VOCl3 species appears to be stable in acidic melts, for there
is no evidence from the IR and NMR studies that would indicate
the presence of other species in the ionic liquid.
same manner as shown in reactions 1 and 2 to form NbOCl4
and TaOCl4-, respectively.60 However, in highly acidic environ-
ments, the reversible reaction of V2O5 with HCl to form vanadyl
chloride and water is observed.61
V2O5 + 6HCl / 2VOCl3 + 3H2O
(3)
A possible mechanism for the dissolution of V2O5 may
Conclusion
involve the reaction with Cl- (as Cl-, AlCl4-, or Al2Cl7
,
-
It has been found that V2O5 does not dissolve in the
chloroaluminate ionic liquids EMIC/AlCl3 and BMIC/AlCl3 to
form a stable species. Instead, the dissolution of V2O5 at
temperatures greater than ∼70 °C results in the formation of
depending on the melt acidity) to form the major species in
these melts.
V2O5 + 2Cl- f VO2Cl2- + VO3
(4)
-
-
VO2Cl2 and [(VO3)n]n- in basic and neutral melts. The ratio
of the species formed depends on both melt acidity and V2O5
concentration with VO2Cl2- more prominent in basic melts with
low concentration of V2O5 and [(VO3)n]n- more prevalent in
neutral melts or with high concentrations of V2O5. Both species
appear to be stable in the ionic liquid and do not precipitate
out upon cooling to room temperature. It is believed that these
At low concentrations of vanadium, a single oxygen is lost from
V2O5 or NaVO3 to form VO2Cl2-. This is possibly an oxygen-
-
-
transfer reaction between VO3 and AlCl4 (or Al2Cl7-):
-
VO3- + AlCl4- / VO2Cl2- + AlOCl2
(5)
-
species further react with the acidic species Al2Cl7 to form
The oxochloroaluminate species AlOCl2- has been shown here
for convenience as it is still unknown what oxochloroaluminate
volatile VOCl3 in acidic melts. However, the existence of other
low-concentration species with V4+ such as VOCl42- is quite
possible. The dissolution of V2O5 did not appear to occur in
the inherently neutral ionic liquids [BMI][CF3SO3] or [BMI]-
[BF4].
-
species are present in these melts.8 The VO2Cl2 species has
been found to be stable in basic chloroaluminate melts15 and is
unlikely to react with a chloroaluminate species to remove any
additional oxygen atoms at these melt compositions. In acidic
melts, a possible mechanism for the formation of VOCl3 is the
initial dissolution of V2O5 in the melt to form VO2Cl2- as seen
for the reactions of V2O5 in basic and neutral melts. However,
Acknowledgment. Financial support from the DuPont Co.
and a Goali grant from the National Science Foundation, Grant
No. CHE-9632771, is greatly appreciated. We wish to thank
Fred Davidson of the DuPont Co. for helpful discussions during
the course of this work. We also wish to thank Dan Jones of
Penn State for the use of mass spectrometry facilities.
-
VO2Cl2 may react further with the acidic chloroaluminate
-
species Al2Cl7 as follows:
(61) Malygin, A. A.; Dergachev, V. F. Russ. J. Appl. Chem. 1988, 61,
1128.
IC990693O