Room temperature liquid salts of Cr and Mo as self-supported oxidants

Article abstract of DOI:10.1002/adsc.200404289

Room temperature liquid salts of Cr and Mo were synthesized and fully characterized including cyclic voltammetry of the neat Mo salt. These liquid salts were used as self-supported reagents for the oxidation of alcohols (under solvent-free and biphasic conditions) and their potential for biphasic self-supported catalytic applications was demonstrated.

Full text of DOI:10.1002/adsc.200404289

COMMUNICATIONS
Room Temperature Liquid Salts of Cr and Mo as Self-Supported
Oxidants
a
a
b
a,
Gladys Noguera, Jorge Mostany, Giuseppe Agrifoglio, Romano Dorta *
a
Universidad Sim o´ n Bolꢀvar, Departamento de Quꢀmica, Caracas 1080-A, Venezuela
Fax: (þ58)-212-906-3961, e-mail: rdorta@usb.ve
b
Instituto Venezolano de Investigaciones Cientꢀficas, Altos de Pipe, Edo. Miranda, Venezuela
Received: September 10, 2004; Accepted: December 6, 2004
Abstract: Room temperature liquid salts of Cr and
Mo were synthesized and fully characterized includ-
ing cyclic voltammetry of the neat Mo salt. These liq-
uid salts were used as self-supported reagents for the
oxidation of alcohols (under solvent-free and bipha-
sic conditions) and their potential for biphasic self-
supported catalytic applications was demonstrated.
cosity of h ¼0.1065 Pa·s (over a shear velocity range
0
ꢀ
1 [8]
between 0.5 and 300 s ) and a conductivity of 2.54
ꢀ
1
mS cm . The IR spectrum of neat 1 revealed two char-
ꢀ
1
ꢀ1
[9]
acteristic bands at 898 cm (s) and 945 cm (vs) and
1
the H NMR spectrum showed broadened signals of the
imidazolium cation.
Keywords: biphasic catalysis; chromium; cyclic vol-
tammetry; molybdenum; oxidation; room tempera-
ture ionic liquids
Room temperature ionic liquids (RTILs) are an attrac-
tive (often “green”) alternative to classic organic sol-
vents for stoichiometric and catalytic reactions as well
ð1Þ
[
1]
as separation processes. Furthermore, due to their ion-
ic nature, RTILs form ideal “immobilization” media for
polar and charged reagents, including catalysts, thus of-
fering the advantage of easy product separation. RTILs
are also emerging as competent solvents for oxidation
[2]
reactions. On the other hand, relatively few examples
of RTILs which integrate the function of solvent and re-
[
3]
agent have been published so far. Despite the fact that
transition metals exert crucial functions in synthesis and
catalysis, examples of transition metal salts that are liq-
[
4]
uid at room temperature are still rare. We present here
the synthesis and full characterization of Cr and Mo salts
ð2Þ
that are liquid at room temperature and their use as self- The molybdate [PPh ] [Mo(O) (NCS) ] was shown to
4
2
2
4
[
10]
supported molecular (catalytic) oxidants of alcohols and be a reversible oxo transfer agent. The corresponding
PPh3. room temperature liquid imidazolium molybdate 2 was
Pyridinium chlorochromate (Coreyꢁs reagent) is a synthesized by metathetical exchange of the cations in a
[
5]
[
6]
widely used oxidant which prompted us to synthesize biphasic H O/CH Cl system [Eq. (2)]. Compound 2 is a
2
2
2
[7]
an imidazolium analogue. Upon mixing solid CrO3 deep red viscous oil with a constant viscosity of h8–500
with solid 1-butyl-3-methylimidazolium chloride 0.865 Pa·s for a shear velocity ranging from 8 to
[BMI][Cl]) in a glove-box immediately a dark red 500 s . The considerably higher viscosity of 2 as com-
¼
ꢀ
1
(
brownish melt formed which was identified as 1 pared to 1 may explain the lower conductivity of pure
ꢀ
1
[
Eq. (1)]. Compound 1 turned out to be insoluble in al- 2 (0.42 mS cm ). Salt 2formed two phases with alkanes,
kanes, aromatics, ethers, and chloroform and, although aromatic solvents, ethers, chloroform, and water while it
it is slightly soluble, it forms two phases with water con- was soluble in CH Cl and DMSO. The NMR spectra
2
2
trasting the high water solubility of the starting materi- were as expected and the carbon atoms of the NCS li-
als. Liquid 1 displayed Newtonian behavior with a vis- gands resonated at 139.8 and 146.6 ppm. The IR spec-
Adv. Synth. Catal. 2005, 347, 231–234
DOI: 10.1002/adsc.200404289
ꢂ 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
231

COMMUNICATIONS
Gladys Noguera et al.
Table 1. Solvent-free oxidations of alcohols over 24 h with
equimolar amounts of 1 or 2.
Oxidant Substrate
Product
Yield
[
[
[
[
a]
a]
b]
b]
1
2
2
2
Benzyl alcohol (3) Benzaldehyde (4)
Benzyl alcohol (3) Benzaldehyde (4)
62%
40%
Cyclohexanol (5)
Cyclohexanone (6) 46%
n-Octanal (8)
[c]
n-Octanol (7)
30%
[a]
[b]
[c]
At room temperature.
At 708C in sealed vials.
6
6% as hemiacetal.
[a]
Table 2. Biphasic oxidations with equimolar amounts of 2.
[
b]
Entry Substrate Product
Partition [%]
Organic phase Ionic phase
7 (3), 10 (4) 50 (3), 33 (4)
[
12]
Figure 1. Cyclic voltammogram of neat 2 at a Pt electrode
ꢀ
1
[c]
and a scan rate n¼10 mV s . Dots: numerical simulation of
an ECE mechanism. Potentials are referred to a pseudo-ref-
erence Pt electrode.
1
2
3
4
3
5
7
4
6
8
[
[
[
c, d]
c, d]
f]
29 (5), 38 (6) <1 (5), 33 (6)
[e]
66 (7), 33 (8) <1 (7), <1 (8)
PPh (9) OPPh (10) 39 (9), 4 (10) <1 (9), 57 (10)
3
3
[
a]
trum confirmed the presence of the terminal cis-dioxo
At room temperature, 500 rpm.
Numbers are percent, bold numbers identify substrates
and products.
ꢀ
1
[b]
function exhibiting two absorption bands at 893 cm
ꢀ
1
(
s) and 928 cm (s).[10] Both salts showed shelf-lives
[
[
[
[
c]
d]
e]
f]
of at least several weeks at room temperature so far.
The steady state cyclic voltammogram of neat 2 is
shown in Figure 1, where two waves I and II are ob-
served. Negative going scans were initiated at the open
circuit potential (ca. ꢀ0.15 V). Peak IIa was not ob-
In toluene/alcohol¼3 (w/w), 24 h.
At 678C in sealed vials.
4
2
2% as hemiacetal.
5% solution of 9 in benzene/heptane¼2 (w/w), 10 h.
served in the first scan, because it was necessary to reach pletely insoluble. In order to demonstrate the feasibility
potentials below ꢀ1.25 V to observe the appearance of of a truly solvent-free oxidation, 1.5 g of benzyl alcohol
waves IIa and IIb. Process Ia is irreversible, and a net were oxidized using 1 as a solvent-reagent (36 h,
cathodic current is present in the reverse scan up to 400 rpm, room temperature, homogeneous solution)
ca. ꢀ0.75 V. This overall behavior could be explained followed by direct fractional high vacuum distillation af-
by an ECE mechanism in which the first reduction oc- fording 1.1 g of pure benzaldehyde.
curs at more negative potentials, followed by a fast irre-
RTIL 2 was then tested as a self-supported biphasic
versible chemical step that renders wave I practically ir- oxidant (Table 2). The initial biphasic system consisted
reversible. The product of the intermediate chemical of the heavier ionic phase of 2 and the lighter organic sol-
step undergoes further oxidation and reduction leading utions of the substrates. Table 2 presents the yields and
to waves IIa and IIb. Numerical simulation of the pro- partitions as percentage of substrate and product mole-
posed ECE mechanism was performed with the Digi- cules in the two phases at the end of the reactions. As in
ꢃ
sim 3.03 package, and a fairly consistent result was ob- Table 1, we note that 2 more readily oxidized cyclohex-
tained, as shown in Figure 1. The diffusion coefficient of anol (5) than benzyl alcohol (3) and that none of alcohol
2
, dissolved in 1-butyl-3-methylimidazolium hexafluor- 5 was detected in the ionic phase (entry 2). Likewise, lip-
1
ophosphate ([BMI][PF ]), was obtained from j vs. n ꢄ
2
ophilic octanol and octanal were not detected in the ion-
6
ꢀ
9
2
ꢀ1
analysis as D¼6.2·10 cm s . This value is consistent ic phase (entry 3). Oxygen atom transfer (OAT) from 2
[
11]
with previously reported data and reflects strong ionic to PPh (9, entry 4) was quite efficient and 94% of the
3
interactions which explain the low current values ob- product oxide 10 accumulated in the ionic phase, while
served in neat 2. To the best of our knowledge this is 99% of PPh partitioned into the organic phase. We
3
the first example of a well-behaved electroactive RTIL. wondered if reoxidation of the reduced RTIL (thought
IV
RTILs 1 and 2 were tested for the solvent-free oxida- to be [BMI] [Mo (O)(SCN) ]) with DMSO would re-
2
4
tion of alcohols to carbonyl compounds (Table 1). Dur- generate RTIL 2 and thus lead to the biphasic self-sup-
[
13]
ing the course of the reaction the homogeneous mix- ported catalytic system as outlined in Eq. (3). Indeed,
tures turned more viscous (mechanical stirring is advis- aPPh solution(25% w/w in benzene/heptane)was com-
3
able). At the end, the reaction mixtures were extracted pletely oxidized by 10 mol % 2 in the presence of 2
[14]
with diethyl ether in which the spent oxidants were com- equivalents of DMSO in 48 h at room temperature.
232
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Adv. Synth. Catal. 2005, 347, 231–234

Room Temperature Liquid Salts of Cr and Mo
COMMUNICATIONS
This catalytic OAT model reaction effectively and com- Filtration and evaporation of the volatiles under vacuum af-
forded a bright red oil; yield: 21.4 g (81%). Elemental analysis
pletely removed PPh from the organic phase and may
3
(
(
calculated for C H MoN O S ): C 38.20 (37.61), H 4.88
20 30 8 2 4
3
serve as a model reaction for the catalytic oxidative re-
moval of contaminants from organic phases (e.g., desul-
furization of fuels). Clearly, this model reaction suggests
that adequate design of the biphasic system allows one
to control the relative affinities of substrate and product
molecules for the ionic and organic phases. An RTIL
ꢀ
4.73), N 17.14 (17.54); d¼1.38 g·cm ; k¼0.42ꢁ0.01 mS·
ꢀ
1
1
cm ; h
(
(
¼0.865 Pa·s (all values at 258C); H NMR
8
–500
300.13 MHz, CD OD): d¼0.99 (t, J¼7.4 Hz, 3H), 1.30–1.45
3
m, 2H), 1.80–1.95 (m, 2H), 3.94 (s, 3H), 4.23 (t, J¼7.3 Hz,
3
H), 7.55–7.60 (m, 1H), 7.60–7.65 (m, 1H), 8.91 (s, br, 1H);
13
C NMR (75.47 MHz, CD Cl ): d¼14.99, 21.12, 33.61, 38.58,
2
2
catalyst such as salt 2 can conveniently be tailored 51.81, 124.28, 125.80, 137.50, 139.85, 146.57.
e.g., by modifying the cation) to suit specific applica-
tions.
(
Typical Biphasic Oxidation Procedure
To 2 (640 mg, 1.00 mmol) in a Schlenk tube were added 435 mg
(
1.01 mmol) of a solution of 3 in toluene (1:3 w/w). The bipha-
sic reaction mixture was stirred at 500 rpm for 24 h. The
H NMR spectrum of the organic phase showed the presence
of 10% aldehyde and 7% alcohol (internal toluene reference)
ð3Þ
1
In summary, we showed that chromate- and molybdate- while the spectrum of the diamagnetic ionic phase gave 33% al-
þ
based RTILs are accessible through Lewis acid-base and dehyde and 50% alcohol (internal reference: BMI ). No ben-
cation metathesis reactions, respectively. The neat RTIL zoic acid was detected.
2
was shown to be electrochemically well-behaved. Liq-
uids 1 and 2 were used as self-supported oxidants of al-
cohols. Finally, the RTIL molybdate 2 showed good po-
tential to act as a self-supported biphasic OAT catalyst.
We are currently investigating the electrochemical re-
Acknowledgements
We thank Prof. Henri Arzoumanian (University of Marseille)
oxidation of such liquid metallate phases and their use for carrying out the elemental analyses and Prof. Alejandro
as biphasic self-supported oxidation catalysts.
Mꢀller (USB) for measuring the viscosities. FONACIT (project
S1-2001000871) and NMR assistance by Ms Indira Vera (USB,
Project BID-FONACIT QF13) are acknowledged.
Experimental Section
References and Notes
[
1,3-Butylmethylimidazolium][CrClO ] (1)
3
[
1] a) Ionic Liquids in Synthesis, (Eds.: P. Wasserscheid, T.
Welton), Wiley-VCH, Weinheim, 2002; b) J. Dupont,
R. F. de Souza, P. A. Z. Suarez, Chem. Rev. 2002, 102,
Powdered [BMI][Cl] (2.97 g, 17.0 mmol) was added to pow-
dered CrO (1.70 g, 17.0 mmol) in a nitrogen-filled glove-
3
box. This resulted in the immediate formation of a brown oil
3
667; c) R. D. Rogers, K. R. Seddon, S. Volkov, (Eds.),
(
4.66 g, 99%) which was stirred for 2 h and then kept at
108C. Elemental analysis (calculated for C H ClCrN O ·
Green Industrial Applications of Ionic Liquids, Kluwer
Academic, Dodrecht, The Netherlands, 2003.
ꢀ
8
15
2
3
0
1
0
.6 H O): C 33.46 (33.66), H 5.74 (5.72), N 9.80 (9.81), Cl
2
ꢀ
3
ꢀ1
3.00 (12.42); d¼1.36 g·cm ; k¼2.54ꢁ0.07 mS cm ; h ¼
[2] a) C. E. Song, E. J. Roh, Chem. Commun. 2000, 837;
b) G. S. Owens, M. M. Abu-Omar, Chem. Comunn.
2000, 1165; c) V. Farmer, T. Welton, Green Chem. 2002,
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119; e) Q. Yao, Org. Lett. 2002, 4, 2197; f) R. Yanada,
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0
1
.1065 Pa·s (all values at 258C); H NMR (300.13 MHz,
DMSO-d , broadened signals): d¼0.90 (s, 3H), 1.25 (s, 2H),
6
1
1
1
.76 (m, 2H), 3.85 (s, 3H), 4.16 (s, 2H), 7.71 (s, 1H), 7.77 (s,
1
3
H), 9.16 (s, br, 1H); C NMR (75.47 MHz, CD Cl ): d¼
2
2
3.82, 19.34, 31.93, 36.34, 49.10, 122.83, 124.19, 137.13.
[
3] a) A. E. Visser, R. P. Swatloski, W. M. Reichert, R. May-
ton, S. Sheff, A. Wierzbicki, J. H. Davis, Jr., R. D. Rogers,
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ton, I. Ntai, J. H. Davies, Jr., J. Am. Chem. Soc. 2002,
[
1,3-Butylmethylimidazolium] [Mo(NCS) O ] (2)
2 4 2
Aqueous HCl (1.00 M, 166.1 mL) was added dropwise with a
burette over 15 min to a colorless solution of Na MoO ·
2
1
2
4
1
24, 926; c) O. Bartolini, M. Bottai, C. Chiappe, V. Conte,
H O (10.044 g, 41.518 mmol) and NaSCN (14.47 g,
2
D. Pieraccini, Green Chem. 2002, 4, 621; d) J. Salazar, R.
Dorta, Synlett 2004, 7, 1318.
78.5 mmol) in water (110 mL) affording a yellow solution. Af-
ter stirring for another 30 min, a solution of [BMI][Cl]
14.502 g, 83.025 mmol) in CH Cl (200 mL) was added drop-
[
4] a) S. A. Bolkan, J. T. Yoke, J. Chem. Eng. Data 1986, 31,
(
2
2
1
94; b) M. S. Sitze, E. R. Schreiter, E. V. Patterson, R. G.
wise under vigorous stirring giving a yellow emulsion which
separated into a bright orange organic phase and a yellowish
aqueous phase. The organic phase was separated, washed
with H O (150 mL, colorless washing), and dried over Na SO .
Freeman, Inorg. Chem. 2001, 40, 2298; c) R. J. C. Brown,
P. J. Dyson, D. J. Ellis, T. Welton, Chem. Commun. 2001,
1862; d) J. E. Ritchie, R. W. Murray, J. Am. Chem. Soc.
2
2
4
Adv. Synth. Catal. 2005, 347, 231–234
asc.wiley-vch.de
ꢂ 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
233

COMMUNICATIONS
Gladys Noguera et al.
2
000, 122, 2964; e) A. P. Abbott, G. Capper, D. L. Davies,
For the sake of comparison, the viscosity of olive oil is
0.1 Pa·s approximately.
[9] H. Stammreich, O. Sala, K. Kawai, Spectrochimica Acta
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[10] H. Arzoumanian, R. Lopez, G. Agrifoglio, Inorg. Chem.
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[
[
2
45.
[
7] 1,3-Dialkyl- and 1,2,3-trialkylimidazolium cations are
known for their “liquefying” (i. e., mp lowering) proper-
ties. For examples of solid transition metalate imidazoli-
um salts featuring X-ray structures, see: a) P. B. Hitch-
cock, K. R. Seddon, T. Welton, J. Chem. Soc. Dalton
Trans. 1993, 2641; b) J. E. L. Dullius, P. A. Z. Suarez, S.
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[12] CVs performed on an Au electrode gave the same result.
This precluded any electrode-specific surface processes.
3
1
[13] Experiments were conveniently followed by P NMR
spectroscopy. For the concept of self-supported mo-
lecular catalysts, see: a) C. Bianchini, E. Farnetti, M.
Graziani, F. Vizza J. Am. Chem. Soc. 1993, 115, 1753;
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[14] A control experiment where the RTIL [BMI]PF was
6
[8] H. A. Barnes, J. F. Hutton, K. Walters, An Introduction to
used instead of catalyst 2 did not produce any apprecia-
Rheology, Elsevier, Amsterdam, The Netherlands, 1989.
ble amount of phosphine oxide.
234
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asc.wiley-vch.de
Adv. Synth. Catal. 2005, 347, 231–234