Catalytic "triangles": Binding of iron in task-specific ionic liquids

Article abstract of DOI:10.1039/c2cc36741f

A new class of task-specific ionic liquids (ILs) which contain septuply positively charged {Fe₃^IIIO(RCOO)₆L ₃}⁷⁺ triangles has been synthesized and structurally characterized. Such metal-containing IL

Full text of DOI:10.1039/c2cc36741f

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Catalytic “Triangles”: Binding of Iron in Task-Specific Ionic Liquids†
Denis Prodius,*a,b Fliur Macaev,b Eugenia Stingaci,b Vsevolod Pogrebnoi,b Valeriu Mereacre,a Ghenadie
Novitchi,b,d George E. Kostakis,c Christopher E. Anson,a and Annie K. Powell*a,c
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
A new class of task-specific ionic liquids (ILs) which contain
septuply positively charged {FeIII3O(RCOO)6L3}7+ triangles
has been synthesized and structurally characterized. Such
metal-containing ILs can be repeatedly used as alternative
10 catalysts in the synthesis of 2-pyrrolo-3’-yloxindole or the
condensation of indoles with various aldehydes.
motif in ILs is unprecedented. The new compounds were
50 characterized by single-crystal X-ray diffraction, elemental
analyses, IR, temperature-dependent thermal analyses
(differential
scanning
calorimetry),
susceptibility
measurements and temperature-dependent Mössbauer spectra
for compound 3
55
Compounds 1–6 can be obtained in moderate yields via
optimized synthetic procedures (Scheme S1, molecular
structure of 1 in Fig. S1). The important solid-glass/liquid
transition temperatures of compounds 1–6 (DSC) are
presented in Table 1 (more detailed data in ESI†, Fig. S2).
Salts consisting of ions and with melting points below 100˚C
are called “Ionic Liquids” (ILs). They have receive attention
as potential green solvents because they are non-flammable,
15 non-volatile, possess good electrolytic properties with a large
electrochemical window, have tunable polarity and are easy to
recycle.1 Also these salts may be designed for specific
applications by incorporation of functionalities (task-specific
ionic liquids)2 in one or both ions as well as by the choice of
20 cation–anion combination. Metal-ion-containing ILs represent
a promising subclass of ILs since they may have interesting
magnetic, luminescence or catalytic properties.3 Apart from a
few examples of ILs based on rare earth metal ions,4 most
investigations on metal-based ILs have been limited to
60 Quaternized salts 2-6 have ionic structures composed of
[FeIII3O(RCOO)6(L)3]7+ complex cations and a combination of
FeCl4 , Cl− and Tf2N− anions with water or methanol
−
molecules as terminal ligands plus lattice water molecules. An
example of the complex cation (for compound 2) and seven
65 FeCl4− counter-anions is shown in Fig. 1.
Table 1. Melting Points/or Glass Transition Points (onset temperature,
˚C) and catalyst activity parameters of investigated compounds (formulas
presented without solvent water molecules).
25 transition metals (MZ+
)
or Al3+ in [MXn](n-z)− anions
containing compounds (where X = halide−, SCN−, Tf2N−,
(OC(CF3)3)−, (F6-acac)−)3d, 5 or as inorganic ionic liquids with
Yield/time
(%/hours)
T1)
(˚C)
Compound
CS12)
CS23)
82/5
92/5
-
polyoxometalate anions [MTiW11O39]5-/6- 6
A drawback of
.
42b
92a
-
86/2
94/2
-
[mcmmim]FeCl4 (1)
these compounds is their undesired properties such as poor
30 solubility in water (or hydrolytic instability) and high
viscosity. The examples of ionic liquids containing transition
[Fe3O(cmmim)6(H2O)3](FeCl4)7 (2)
[Fe3O(cmmim)6(MeOH)3](FeCl4)7 (3)
[Fe3O(cmmim)6(H2O)3](FeCl4)3Cl4 (4)
[Fe3O(cmmim)6(H2O)3](Tf2N)7 (5)
[Fe3O(cepy)6(H2O)3](FeCl4)6Cl (6)
103a
92b
94a
98/2
98/2
96/2
85/5
70/5
89/5
metal-based cations7 are limited to derivatives of N-
7c
alkylimidazoles,7b,
R1-NH-R2 or metallocenium-based
ligands, where R1 or R2 can be H, alkyl or –CH2-CH2-OH
35 groups.8 It has been widely demonstrated that the physical and
chemical properties of ILs and their catalytic activity can be
significantly influenced by the presence of small amounts of
impurities. Therefore the challenge is to develop synthetic
protocols which give high-purity ILs with the goal of utilizing
40 the diversity of possible cation-anion combinations available.
Herein we present the synthesis, structural characterization
and catalytic properties of a new class of ionic liquids which
contain septuply positively charged antiferromagnetically
70 1) Tg (glass transition point)a/Tm.p. (melting point)b at heating (˚C);
2) synthesis of ethyl 2-methyl-4-(2-oxo-2,3-dihydro-1H-3-indolyl)-5-
phenyl-1H-3-pyrrolecarboxylate 7a using ~0.65 mol% of co-catalyst at
RT; 3) synthesis of 3,3'-((4-methoxyphenyl)methylene)bis(1H-indole) 8a;
mcmmim (C1ImC1CO2CH3)
75 yl)acetate; cmmim (C1ImC1CO2)
methylimidazolium; Tf2N−
bis(trifluromethane)sulfonamide; cepy
≡
methyl 2-(3-methyl-1H-imidazol-1-
≡
1-carboxymethyl-3-
≡
(PyC2CO2) ≡ N-(2-carboxyethyl)pyridinium.
At the centre of each triangle there is a ꢀ3-oxo oxygen atom
80 connecting the three iron atoms (see ESI†, Table S1, Figures S3-
S7). Each iron(III) has an octahedral O6 coordination
environment formed by the ꢀ3-oxo atom, four oxygens from the
carboxylate ligands and the oxygen atom from the water ligand.
Pairs of irons are further linked by double bidentate-bridging
85 carboxylate groups with Fe-O distances between 1.99-2.06 Å.
coupled {FeIII3O(RCOO)6L3}7+ cationic triangles 2-6.
A
45 survey of the Cambridge Crystallographic Database (CSD) up
to Nov. 2011 reveals that more than 1100 crystal structures of
metal carboxylates containing ꢀ3-oxo bridged metal triangle
motifs have been deposited. However, so far the use of this
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Mössbauer spectra of polycrystalline samples of 3 at T = 50
are typical for paramagnetic behavior with an
asymmetric quadrupole doublet which can be fitted using two
doublets with isomer shifts ( ) 0.43 (0.43) and 0.32 (0.33) mm/s
Thus the structures are very similar to those of the “classic”
40
trinuclear transition metal(III) carboxylate triangles.9
and 3 K
δ
and quadrupole splittings (ꢁEQ) 0.60 (0.62) and 0.17 (0.19)
45 mm/s, respectively for {Fe3O}7+ and 7[FeCl4]-, typical of high
spin S=5/2, FeIII ions (details in Table S2, Figure S9).
Pyrroles and oxindoles are important heterocyclic scaffolds
of medicinal and therapeutic relevance.14 Devising reactions to
achieve one-step (concerted) multi-bond formation is a challenge
50 in heterocycle and natural product synthesis. Recently, one-pot
procedures have been developed for InCl3 (20 mol%) catalyzing
the synthesis of 2-methyl-4-(2-oxo-2,3-dihydro-1H-3-indolyl)-5-
phenyl-1H-3-pyrrolecarboxylates.15 This procedure does not
require the isolation of the initial Michael addition product prior
55 to the Paal-Knorr condensation. However, InCl3 is harmful,
corrosive and, moreover, expensive. Thus the development of
eco-friendly alternatives, such as task-specific ionic liquids,
would extend the scope of this approach. The new compounds 1,
2 and 4-6 can be used as promoters for two useful organic
60 reactions. Compound 3 was excluded from this study since its
thermal instability and technologically inefficient, time-
consuming synthesis (crystallization takes about 2-3 month at
0°C) preclude useful applications. At the outset of the study, the
synthesized compounds 1, 2 and 4-6 were tested (the results are
65 summarized in Table S3) in the selected reaction of ammonium
acetate, ethyl acetoacetate with 3-phenacylideneoxindole in
EtOAc at 77°C for the synthesis of ethyl 2-methyl-4-(2-oxo-2,3-
dihydro-1H-3-indolyl)-5-phenyl-1H-3-pyrrolecarboxylate (7a).
Figure 1. Molecular fragment for decanuclear "double” (cation-anion)
iron based [Fe3O(C1ImC1CO2)6(H2O)3](FeCl4)7 cluster (2). Hydrogen
atoms and solvent water molecules were omitted for clarity. Polyhedra
correspond to octahedral FeO6 (brown) and tetrahedral FeCl4 (green)
environments.
5
10
The results of magnetic susceptibility measurements on the
polycrystalline compound
3 are shown in Figure 2. For
compound
3 the room temperature χT product is 34.75
cm3·K·mol-1, lower than the expected value (43.75 cm3·K·mol-1)
for the presence of ten FeIII ions (S = 5/2, g = 2). On decreasing
15 the temperature, the χT product at 0.1T decreases smoothly until
30 K (30.43 cm3·K·mol-1) and then undergoes a rapid drop down
to 14.04 cm3·K·mol-1 at 1.8 K indicating antiferromagnetic
interactions in 3. Based on the X-ray structure, the magnetic
susceptibility data can be modeled as the sum of the susceptibility
70
Scheme 1. Synthesis of 2-pyrrolo-3'-yloxindole (7a).
Compounds 4 and 5 gave the best results for these
catalysts with product yields of 98 % (Table 1). By optimizing
the reaction conditions, it was found that ~0.313 mol% of 1 was
20 of the trinuclear clusters {
seven isolated FeIII ions with SFe = 5/2):
} with contributions from the
χFe3O
( )
T
75 sufficient for the completion of the reaction with good yield.
Another aim of our study was to investigate the recycling of
catalysts. It is worth noting that the catalysts can be reused at
least ten times without significant loss of activity for 4-6
(89÷92%) and a maximum of five times for 1 and 2 (65÷69%).
Ng2β 2
3kT




χ0
(T
)
=
χFe3O
(
T
)
+ 7
SFe (SFe +1)


The following definition of the susceptibility, which accounts for
contributions from the intermolecular interactions, has been used
80
Among various indole derivatives, bis(indolyl)methanes are
found in cruciferous plants and known to promote beneficial
oestrogen metabolism and induce apoptosis in human cancer
cells.16 In order to study the scope of the properties of the
compounds, these were also investigated as promoters for the
χo
[1− (2zJ ' χo
( )
T
25
χ
(
T =
)
(
T
Ng 2 β 2 )]
)
where χo is the susceptibility of the noninteracting paramagnetic
centers in 3 (Fe3O and 7 FeIII), z is the number of nearest
neighbors, and J’ is the magnetic interactions between units.10
The magnetic susceptibilities {χFe3O(T)} of the spin frustrated
85 condensation of indole with 4-methoxybenzaldehyde.17 Refluxing
of indole with 4-methoxybenzaldehyde in the presence of the
synthesized ionic liquids as catalyst resulted in the selective
formation of substance 8a (Table S4). In all cases, the product
was isolated by decantation from the catalyst followed by silica
90 gel chromatography. Optimum results were obtained using
~0.315 mol % of 2 (Table S4). After a few cycles it was found
necessary to increase the reaction time in order to achieve this
optimum yield. The best overall yield (92%) was obtained using
2 in benzene (Table 1). In particular, all the catalysts apart from 5
95 are more effective than the known FeCl3/[omim]PF6 system
which has an optimum yield of 78 %.17
30 {FeIII3O} cluster were calculated using the Hamiltonian:
H=-2Ja(S1S2+S2S3)-2JbS1S3 where Ja and Jb are exchange
parameters and S1 = S2 = S3 = 5/2 the spin on the individual FeIII
ions. The eigenvalue of the spin-Hamiltonian was determined by
using the Kambe vector coupling method.11 Fits of the
35 experimental data to the final expression of the susceptibility
yielded one satisfactory set of parameters with Ja = -22.8(6) cm-1,
Jb = -41.9(9) cm-1, g = 2.02(1) and zJ’ = -0.27(2) (Fig. S8). These
values are in good agreement with other reported {FeIII3O}
cases.9, 12, 13
2
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1H-indole (8a).
5
In summary, a series of ionic liquids containing
antiferromagnetically coupled {FeIII3O(RCOO)6L3}7+ cationic
triangles has been synthesized and characterized by spectroscopic
methods, magnetism and X-ray crystallography. It was
demonstrated that this combination of metal ions (iron) with task-
6
7
10 specific ionic liquid ligands can be used as new efficient catalysts
for the successful high-yield synthesis of heterocyclic compounds
within a green chemical protocol.
Acknowledgements
15 We thank the DFG Center for Functional Nanostructures
(F.M.) and Alexander von Humboldt Foundation (D.P) for
financial support.
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20 Engesserstrasse 15, 76128 Karlsruhe, Germany. Fax: +49 721-608-
48142; Tel: +49 721-608-42135; E-mail: annie.powell@kit.edu
bInstitute of Chemistry of the Academy of Sciences of Moldova, Academiei
str. 3, MD-2028 Chisinau, Moldova. Fax: +373 22 739722; Tel: +373 22
739954; E-mail: denis.prodius@kit.edu
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Hermann-von-Helmholtz Platz 1, 76344 Eggenstein-Leopoldshafen,
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[
(
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− Jb
[S13
(
S13 +1
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Table of Contents (TOC)
Catalytic “Triangles”: Binding of Iron in Task-Specific Ionic Liquids
Authors: Denis Prodius, Fliur Macaev, Eugenia Stingaci, Vsevolod Pogrebnoi, Valeriu
Mereacre, Ghenadie Novitchi, George E. Kostakis, Christopher E. Anson, and Annie K. Powell
A new class of catalytically active task-specific ionic liquids with antiferromagnetically
coupled {FeIII3O(RCOO)6L3}7+ cations has been synthesized and structurally characterized.