Autocatalytic sonolysis of iron pentacarbonyl in room temperature ionic liquid [BuMeIm][Tf 2 N]

Abstract

The autocatalytic sonochemical reaction of Fe(CO)(5) decomposition in [BuMeIm][Tf(2)N] provides iron nanoparticles in higher yields than in tetralin. Such a difference is explained by the higher decomposition of the intermediate Fe(3)(CO)(12) according to the two-sites model of the sonochemical reactions and the specific properties of the ionic liquid.

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This journal is cthe Owner Societies 2011 Phys. Chem. Chem. Phys., 2011, 13, 2111–2113 2111
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Phys. Chem. Chem. Phys
., 2011, 13, 2111–2113
Autocatalytic sonolysis of iron pentacarbonyl in room temperature ionic
liquid [BuMeIm][Tf
2
N]w
Lenaı
¨c Lartigue,
a
Rachel Pflieger,
b
Sergey I. Nikitenko,*
b
Yannick Guari,*
a
Lorenzo Stievano,
c
Moulay T. Sougrati
c
and Joulia Larionova
a
Received 1st September 2010, Accepted 8th November 2010
DOI: 10.1039/c0cp01670e
The autocatalytic sonochemical reaction of Fe(CO)
5
decomposi-
tion in [BuMeIm][Tf
2
N] provides iron nanoparticles in higher
yields than in tetralin. Such a difference is explained by the
higher decomposition of the intermediate Fe
3
(CO)
12
according
to the two-sites model of the sonochemical reactions and the
specific properties of the ionic liquid.
Synthesis of nanoparticles (NPs) under the effect of power
ultrasound is a rapidly growing area of nanochemistry since
this approach offers a versatile synthetic tool for nanostructured
materials that are often unavailable by conventional methods.
1,2
Chemical activity of ultrasound originates from acoustic
cavitation: nucleation, growth, and implosion of microbubbles
in liquids subjected to the ultrasonic waves. Collapse of cavi-
tation bubbles under acoustic resonance creates extreme local
heating inside the bubble and a thin liquid shell surrounding
the bubble. The first example of the sonochemical synthesis of
NPs was a preparation of amorphous iron during sonolysis of
Fe(CO)
5
in alkanes.
3
In the absence of stabilizers, this reaction
yields agglomerated iron NPs while an addition of stabilizers
before sonolysis (e.g. oleic acid or polyvinyl-pyrrolidone)
causes formation of colloidal iron NPs with the average size
in the range of 3–8 nm. In general, size control of iron NPs is
considered to be relatively difficult.
4
Room-temperature ionic liquids (RTILs) are known as
alternative green solvents which present unique physico-
chemical properties such as high thermal stability, negligible
vapour pressure, good ionic conductivity, large electrochemical
window, and others.
5
For these reasons, ionic liquids are
actively being explored as an environmentally benign solvent
for organic chemical reactions,
6
separations,
7
electrochemical
applications,
8
biopolymers
9
and molecular self-assemblies.
10
In the recent few years, RTILs have also been discovered as an
excellent media in the formation and stabilization of inorganic
nanosized objects
11–13
but only few works have been devoted
to the synthesis of metallic nanoparticles.
14–18
According to
Derjaguin–Landau–Verwey–Overbeek theory,
15
ionic liquids
provide an electrostatic protection in the form of a protective
shell for nanoparticles and no additional ligands or stabilizing
agents are needed. Recently, various metallic NPs have been
prepared by thermal or photochemical decomposition of metal
carbonyls M
x
(CO)
y
in imidazolium RTILs.
16,17
However, this
method often provides NPs with broad size and shape distri-
bution that is particularly true for 3d metallic NPs. Among
several examples concerning the synthesis of inorganic nano-
particles in ionic liquids, only one is devoted to the synthesis of
iron nanoparticles performed by thermal flash decomposition
up to 250 1CofFe
2
(CO)
9
in 1-methyl-3-methyl imidazolium
tetrafluoroborate.
18
Highly aggregated NPs of ca. 5 nm were
obtained and no study of their magnetic properties have been
performed. To the best of our knowledge, iron NPs have never
been obtained in RTILs by using sonochemical synthesis.
This paper describes the autocatalytic sonochemical reaction
of Fe(CO)
5
decomposition in ionic liquid [BuMeIm][Tf
2
N]
(Scheme S1, ESIw), Tf
2
N

is bis(trifluorosulfonyl)imide,
providing iron NPs with narrow size distribution. The influence
of various parameters such as the ionic liquid counter ion
nature, the Fe(CO)
5
concentration and the reaction time was
considered. Several experiments were also performed using
tetralin as a conventional hydrocarbon solvent for comparison.
In a typical experiment 10 mL of 0.5 M Fe(CO)
5
solution in
anhydrous [BuMeIm][Tf
2
N] was placed in a round bottom
Schlenk tube (1 mm wall thickness) using an argon filled inert
glove-box. Then solution was sonicated with 20 kHz ultra-
sound at an absorbed specific acoustic power P
ac
equal to
0.53 W mL
1
with a cap-horn system (Fig. S1, ESIw) under
argon flow (100 mL min
1
). A steady-state temperature inside
the reaction vessel during sonolysis was maintained at 5 1Cby
circulation of the cooled silicon oil through a cap-horn cell.
The steady-state temperature is achieved after approximately
15 minutes of ultrasonic treatment. In a few minutes after
beginning of sonolysis the reaction mixture becomes black in
both studied solvents, RTILs and tetralin, indicating an
effective sonochemical reaction. However, integral curves of
a
Institut Charles Gerhardt Montpellier, UMR 5253,
Chimie Mole
´culaire et Organisation du Solide,
Universite
´Montpellier II, Place E. Bataillon, F-34095,
Montpellier Cx5, France. E-mail: yannick.guari@univ-montp2.fr;
Fax: +33 4 67 14 38 52
b
Institut de Chimie Se
´parative de Marcoule, UMR 5257,
Centre de Marcoule, BP 17171, F-30207 Bagnols sur Ce
`ze, France.
E-mail: serguei.nikitenko@cea.fr; Fax: +33 4 67 79 76 11
c
Institut Charles Gerhardt Montpellier, UMR 5253, Agre
´gats,
Interfaces et Mate
´riaux pour l’Energie, Universite
´Montpellier II,
Place E. Bataillon, F-34095, Montpellier Cx5, France
wElectronic supplementary information (ESI) available: Experimental
details, kinetic and magnetic curves. See DOI: 10.1039/c0cp01670e
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2112 Phys. Chem. Chem. Phys., 2011, 13, 2111–2113 This journal is cthe Owner Societies 2011
CO emission shown in Fig. 1 reveal the significant difference in
kinetics of CO release for these two kinds of solvent. For
tetralin solution concentration of CO in outlet gas increases
rapidly after beginning of sonolysis and reaches a steady-state
value in approximately 60 min of the process. After B100 min
of ultrasonic treatment the concentration of CO in outlet gas
decreases due to the Fe(CO)
5
consumption. By contrast, for
[BuMeIm][Tf
2
N] solution given as an example the kinetic
curve of CO emission exhibits a long induction period before
reaching a maximum after almost 4 hours of sonolysis. The
strong oscillations of CO emission during sonolysis of both
systems most probably are related to the temporal fluctuation
of bubbles coalescence which provides degassing of the liquids.
The total amount of CO released from [BuMeIm][Tf
2
N] is
almost twice (CO
RTIL
/CO
Tetralin
= 1.7) compared to that from
tetralin. Note here that the mass spectra of outlet gas also
indicate the presence of Fe and FeCO species originated
from Fe(CO)
5
evaporation. However, their concentration is
negligible compared to that of CO issued from Fe(CO)
5
sonolysis.
The sonicated solution of tetralin has a deep green colour
after the solids removal. The IR spectrum of this solution
shows the absorption bands at 2010–2050 and 1830–1870 cm
1
characteristic for C–O stretch vibrations of terminal and bridging
carbonyl groups, respectively, in a complex Fe
3
(CO)
12
.
19
This
observation is in line with the previously published data.
20,21
In
contrast to tetralin, [BuMeIm][Tf
2
N] solution presents a slight
yellow-green colour after sonolysis. Only a weak IR absorption
of Fe
3
(CO)
12
complex is observed in this case. Formation of
Fe
3
(CO)
12
complex during Fe(CO)
5
sonolysis in alkanes was
assigned to the process inside the cavitation bubbles:
3,20
4Fe(CO)
5
-))) -Fe + Fe
3
(CO)
12
+ 8CO (1)
where the symbol ‘‘-)))-’’ corresponds to the reactions under the
effect of acoustic cavitation. An increase in the bulk temperature
of the sonicated solution causes thermal decomposition of
Fe
3
(CO)
12
in solution even in the absence of ultrasound:
21
Fe
3
(CO)
12
-3Fe + 12CO (2)
Reaction (1) is known to be limited by the diffusion of
Fe(CO)
5
to the vicinity of the cavitation bubble.
21
Presumably
the diffusion coefficients of the species in [BuMeIm][Tf
2
N]
would be much lower than those in tetralin since the viscosity
of RTIL is almost 10 times higher than that of tetralin.
5
Moreover, we found that Fe
3
(CO)
12
has a low solubility in
[BuMeIm][Tf
2
N]. Therefore, the autocatalytic reaction in
RTIL can be explained by a slow reaction (1) at the first stage
of the process, accumulation of insoluble Fe
3
(CO)
12
inter-
mediate at the solution/bubble interface and its further
thermolysis in the liquid reaction zone of the cavitation
bubble according to the two-sites model of the sonochemical
reactions.
20
High solubility of Fe
3
(CO)
12
in tetralin and its low
volatility at 5 1C avoids the sonochemical decomposition of
this complex in tetralin.
About 120 (49%) and 72 mg (29%) of black magnetic solids
are removed after sonolysis of Fe(CO)
5
in [BuMeIm][Tf
2
N]
and tetralin respectively. The ratio in reaction yields of solid
products (1.7) fits very well with the ratio of released CO in
these systems. Note that decreasing the sonolysis time induces
a decrease of the nanoparticles yield (see ESIw). The proposed
reaction mechanism explains the higher reaction yield in
[BuMeIm][Tf
2
N] compared to ordinary alkanes by an addi-
tional sonochemical reaction with Fe
3
(CO)
12
.
The TEM images of the solids removed from RTIL reveal
the presence of non-aggregated uniform nanoparticles with a
mean size of 3.00(0.80) nm (Fig. 2a). Large agglomerates of
iron NPs usually observed in alkanes in the absence of
stabilizers are absent in the case of [BuMeIm][Tf
2
N]. The
X-ray diffraction pattern performed for these NPs clearly
shows the presence of bcc structure of iron (Fig. 2b).
22 57
Fe
Mo
¨ssbauer spectroscopy performed on this sample also
indicates the presence of iron nanoparticles of about 4 nm,
together with minor amounts of oxidised iron species which
might be present as oxidic iron species covering the surface of
the iron nanoparticles (Fig. S2 and Table S1, ESIw). The IR
spectra of the NPs obtained in RTIL and washed with
anhydrous THF clearly indicate the bands at 1572 cm
1
(nCQC), 1465 cm
1
(d
S
CH
3
), 1430 cm
1
(d
S
CH
3
(Me)),
1198 cm
1
(nN-Bu) and 1184 cm
1
(nN-Me), 1056 (nC–C) of
BuMeIm
+
cation from RTIL. This observation confirms that
RTILs are effective stabilizers for NPs. Moreover, these NPs
may be re-dispersed in RTILs (Fig. S3a, ESIw).
Using RTILs with [BF
4

]or[PF
6

] as counter anions instead
of [Tf
2
N

] also allows the synthesis of iron NPs with similar
sizes (Fig. S3b, ESIw) but with lower yields, which can be
explained by the lower solubility of Fe(CO)
5
in these RTILs.
Using higher Fe(CO)
5
concentration in [BuMeIm][Tf
2
N] leads
to the formation of iron cauliflowers (Fig. S3c, ESIw).
The magnetic properties of the NPs obtained in
[BuMeIm][Tf
2
N] studied by using static (dc) and dynamic
(ac) modes confirm the presence of iron NPs. The Zero
Fig. 1 (a) Integrated kinetic curves for CO emission during sonolysis
of 0.5 M Fe(CO)
5
in tetralin (grey) and [BuMeIm][Tf
2
N] (black)
(20 kHz, Pac = 0.53 W mL
1
,T=51C, Ar) and (b) the corres-
ponding raw data.
Fig. 2 (a) TEM image of iron NPs obtained in [BuMeIm][Tf
2
N]. The
TEM samples were prepared by suspending the NPs in ethanol and
deposition of a drop on a copper grid. (b) X-Ray diffraction of the NPs
obtained in [BuMeIm][Tf
2
N].
}
shows the lines originated from the
confined dome-like holder equipped with a stainless knife edge beam.
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Field-Cooled (ZFC) magnetization curve shows a peak at
T
max
= 184 K, which is typical for iron NPs of this size
(Fig. 3 and Fig. S4, ESIw).
23
The FC curve increases, reaches a
maximum value at 125 K then decreases suggesting the presence
of superspin-glass behaviour characteristic of interacting NPs.
24
The field dependence of the magnetization measured at 2.5 K
shows the presence of a hysteresis effect with a value of the
coercive field of 535 Oe and the value of the saturation magneti-
zation of 142 emu g
1
(inset of Fig. 3). As the temperature
increases, the coercive field decreases and at 300 K the hysteresis
loop is closed (Fig. S5, ESIw).
In order to determine the nature of the magnetic behaviour of
these nanoparticles, the dynamic ac magnetic measurements were
performed. The temperature dependence of the in-phase (w0)and
out-of-phase (w00) components of the ac susceptibility measured at
different frequencies shows a frequency dependent behaviour
(Fig. S6, ESIw). The thermal dependence of the relaxation time
can be described by a critical scaling law, t=t
0
[T
g
/(TT
g
)]
zv
(T
g
is the glass temperature, and zv is a critical exponent)
25
with
the satisfactory fit parameters: T
g
= 155 K, t
0
=1.2910
6
s
and zn= 11.5 (inset of Fig. S6, ESIw), indicating the presence of
superspin-glass behavior induced by strong magnetostatic inter-
actions between the nanoparticles.
26
The field dependence of
the spin-glass transition temperature, T
g
, taken from the field
dependence of the ac susceptibility (Fig. S7, ESIw)followsthede
Almeida–Thouless line,
27
HN[1 (T
max
/T
g
)]
3/2
(Fig. S8, ESIw)
confirming the existence of a spin-glass phase.
28
In summary, we describe the sonochemical autocatalytic
reaction of Fe(CO)
5
in various RTILs. This reaction produces
with a relatively high yield considering a sonolysis time of
5 h in [BuMeIm][Tf
2
N] non-agglomerated ultra small iron
nanoparticles of ca. 3.0 nm with a narrow size distribution.
The study of the magnetic properties of these nanoparticles by
using static and dynamic analyses reveals a superspin-glass like
behaviour of these iron nanoparticles induced by the presence
of interparticles interactions.
Notes and references
1 J. H. Bang and K. S. Suslick, Adv. Mater., 2010, 22, 1039.
2 A. Gedanken, Ultrason. Sonochem., 2004, 11, 47.
3 K. S. Suslick, M. Fang and T. Hyeon, J. Am. Chem. Soc., 1996,
118, 11960.
4 D. L. Huber, Small, 2005, 1, 482.
5(a)T.Welton,Chem. Rev., 1999, 99, 2071; (b) P. Wasserscheid and
W. Keim, Angew. Chem., Int. Ed., 2000, 39,3772;(c)L.A.Blanchard,
D. Hancu, E. J. Beckman and J. F. Brennecke, Nature, 1999, 399,28.
6(a) P. Wasserscheid and T. Welton, Ionic Liquids in Synthesis,
Wiley-VCH, Weinheim, 2003; (b) R. Scheldon, Chem. Commun.,
2001, 2399.
7 J. G. Huddleston, H. D. Willauer, R. P. Swatloski, A. E. Visser
and R. D. Rogers, Chem. Commun., 1998, 1765.
8(a) A. B. McEwen, S. F. McDevitt and V. R. Koch, J. Electrochem.
Soc., 1997, 144, L84; (b) E. V. Dickinson, M. E. Williams,
S. M. Hendrickson, H. Masui and R. W. Murray, J. Am. Chem.
Soc., 1999, 121, 613.
9(a) N. Kimizuka and T. Nakashima, Langmuir, 2001, 17, 6759;
(b) R. P. Swatloski, S. K. Spear, J. D. Holbrey and R. D. Rogers,
J. Am. Chem. Soc., 2002, 124, 4974.
10 T. Nakashima and N. Kimizuka, Chem. Lett., 2002, 1018.
11 G. Clavel, J. Larionova, Y. Guari and Ch. Gue
´rin, Chem.–Eur. J.,
2006, 12, 3798.
12 (a) C. W. Scheeren, G. Machado, J. Dupont, P. F. P. Fichtner and
S. R. Texeira, Inorg. Chem., 2003, 42, 4738; (b) J. Dupont and
J. D. Scholten, Chem. Soc. Rev., 2010, 39, 1780; (c) Y. Wang,
S. Maksimuk, R. Shen and H. Yang, Green Chem., 2007, 9, 1051.
13 (a) J. Huang, T. Jiang, B. Han, H. Gao, Y. Chang, G. Zhao and
W. Wu, Chem. Commun., 2003, 1654; (b) Y.-J. Zhu, W.-W. Wang,
R.-J. Qi and X.-L. Hu, Angew. Chem., Int. Ed., 2004, 43, 1410;
(c) T. Nakashima and N. Kimizuka, J. Am. Chem. Soc., 2003, 125,
6386.
14 M. Scariot, D. O. Silva, J. D. Scholten, G. Machado,
S. R. Teixeira, M. A. Novak, G. Ebeling and J. Dupont, Angew.
Chem., Int. Ed., 2008, 47, 9075.
15 E. J. W. Verwey and J. T. G. Overbeek, Theory of the Stability of
Lyophobic Colloids, Dover Publications, Mineola, NY, 2nd edn,
1999.
16 E. Redel, R. Thomann and C. Janiak, Chem. Commun., 2008,
1789.
17 D. O. Silva, J. D. Scholten, M. A. Gelesky, S. R. Teixeira, A. C. B.
Dos Santos, E. F. Souza-Aguiar and J. Dupont, ChemSusChem,
2008, 1, 291.
18 J. Kra
¨mer, E. Redel, R. Thomann and C. Janiak, Organometallics,
2008, 27, 1976.
19 K. Nakanishi and H. Solomon, Infrared Absorption Spectroscopy,
Holden-Day, Inc., San Francisco, 1982.
20 K. S. Suslick, S. B. Choe, A. A. Cichowlas and M. W. Grinstaff,
Nature, 1991, 353, 414.
21 S. I. Nikitenko, Yu. Koltypin, I. Felner, I. Yeshurun, A. I. Shames,
J. Z. Jiang, V. Markovich, G. Gorodetsky and A. Gedanken,
J. Phys. Chem. B, 2004, 108, 7620.
22 J. Carvella, E. Ayietaa, M. Johnsona and R. Cheng, Mater. Lett.,
2009, 63, 715.
23 (a) M. Yoona, Y. M. Kima, Y. Kima, V. Volkova, H. J. Songa,
Y. J. Park, S. L. Vasilyakc and I.-W. Park, J. Magn. Magn. Mater.,
2003, 265, 357; (b) G. Kataby, Y. Koltypin, A. Ulman, I. Felner
and A. Gedanken, Appl. Surf. Sci., 2002, 201, 191.
24 D. Parker, V. Dupuis, F. Ladieu, J.-P. Bouchaud, E. Dubois,
R. Perzynski and E. Vincent, Phys. Rev. B: Condens. Matter
Mater. Phys., 2008, 77, 104428.
25 J. A. Mydosh, Spin Glasses, Taylor and Francis, Washington, DC,
1993.
26 C. Djurberg, P. Svedlindh, P. Nordblad, M. F. Hansen, F. Bodker
and S. Morup, Phys. Rev. Lett., 1997, 79, 5154.
27 J. R. L. Almeida and D. J. Thouless, J. Phys. A: Math. Gen., 1978,
11, 983.
28 B. Martinez, X. Obradors, Ll. Balcells, A. Rouanet and C. Monty,
Phys. Rev. Lett., 1998, 80, 181.
Fig. 3 Field Cooled (FC) (grey)/Zero Field Cooled (ZFC) (black)
magnetization curves performed under an applied magnetic field of
1000 Oe. Inset: hysteresis loops of iron NPs performed at 2.5, 50, 150
and 300 K.
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