Magnetic chiral ionic liquids derived from amino acidsw

Min Li,aSergio L. De Rooy,

aDavid K. Bwambok,

aBilal El-Zahab,

aJohn F. DiTusa

band

Isiah M. Warner*a

Received (in College Park, MD, USA) 27th August 2009, Accepted 8th October 2009

First published as an Advance Article on the web 26th October 2009

DOI: 10.1039/b917683g

Novel room temperature magnetic chiral ionic liquids derived

from amino acids were synthesized and their magnetic properties

as well as chiral discrimination abilities were investigated.

Ionic liquids (ILs) are low melting salts that consist solely of

ions. ILs are noted to have a number of advantages including

negligible vapor pressures, good thermal stabilities, wide

liquid temperature ranges, considerable electric conductivities,

wide electrochemical windows, and high solvation interactions

with both polar and nonpolar compounds.1 ILs may be

designed for specific applications through the judicious choice

of constituent ions. Recently the synthesis of a variety of chiral

ionic liquids (CILs), along with their applications have been

reported.2 CILs can be regarded as a subclass of ILs that have

been successfully used in asymmetric catalysis and synthesis3

as well as chiral discrimination and separation.4 Another

interesting and thriving area in IL research is the preparation

of transition metal containing ILs.5 These novel functional ILs

are regarded as promising new materials which can favorably

combine the advantages of ILs with catalytic,5c spectroscopic,5d

or magnetic5e properties that originate from the incorporated

metal ion. The discovery of magnetic ionic liquids (MILs)

was recently reported.6 These MILs are primarily based on

high-spin d5 iron(III) in the form of tetrachloro- or tetra-

bromoferrate(III) with various counter cations. Owing to the

high single-ion magnetic moments, these MILs exhibited a

strong response to magnetic fields. Other MILs containing

different transition metal ions such as gadolinium or dysprosium

have also been synthesized.7 So far most of the cations of the

MILs were limited to the traditional alkylimidazolium,

tetraalkylammonium, or tetraalkylphosphoniums.6,7 Gaertner

et al. reported a multiple-step synthesis of a chiral camphor-

derived imidazolium IL with an FeCl4� anion, however, its

magnetic properties were not investigated.3h Herein, we report

the synthesis, characterization, and property studies of amino

acid-derived magnetic chiral ionic liquids (MCILs) with

simultaneous chiral and magnetic properties. Both the

magneticity and chirality are investigated in this communication

with regard to employing these MCILs as new single-component

chiral and magnetic fluids.

MCILs were synthesized by reactions between various

L-amino acid methyl ester hydrochlorides and iron(III) chloride

hexahydrate in anhydrous methanol at room temperature

(Scheme 1). This synthesis is simple and straightforward

(ESIw). In addition, both of the MCIL enantiomers with

L- and D-configurations are readily available from the respective

enantiopure amino acid methyl ester hydrochlorides which are

abundant and relatively inexpensive. The presence of an amino

acid ester versus conventional imidazolium6a,b,7 as the cation

could also diminish the biological and environmental hazard

potential,8 while enhancing the biodegradability of the

MCILs. The synthesized MCILs include L-AlaOMeFeCl4(R=methyl), L-ValOMeFeCl4 (R= isopropyl), L-LeuOMeFeCl4(R = isobutyl), L-IleOMeFeCl4 (R = sec-butyl), and

L-PheOMeFeCl4 (R = benzyl) (Scheme 1). D-PheOMeFeCl4was also synthesized in order to investigate the enantiomeric

purity of the MCILs by circular dichroism (CD). The identities

and purities of the MCILs were characterized by the use of

UV-Vis, 1H and 13C NMR, FT-IR, and C,H,N elemental

analysis (ESIw). All products were light brown viscous liquids

(viscosity is 8.633 Pa s for L-PheOMeFeCl4) at room temperature

and were readily responsive to a conventional neodymium

magnet (1.3 T) (ESIw).Due to their polar functional groups, such as protonated

primary amines and esters, as well as the hydrophilic

tetrachloroferrate(III) anions, these MCILs are miscible with

water or polar solvents after vigorous shaking of the biphasic

mixture solution. In contrast, they are immiscible with

hydrophobic solvents such as hexane and carbon tetrachloride,

but partially miscible with chloroform.

The absorption spectra of the five MCILs were measured

using a UV-Vis-NIR spectrometer (Fig. 1). All MCILs had

similar absorption spectra with three major peaks at 532, 618,

and 685 nm. These peaks are well known to be characteristic

of tetrachloroferrate(III) ions.9 These very similar absorption

behaviors and peak wavelengths were also observed for

conventional MILs such as 1-butyl-3-methylimidazolium

tetrachloroferrate(III).6a,b

The thermal properties of these MCILs including thermal

stabilities and phase transition behaviors were investigated by

use of thermal gravimetric analysis (TGA) and differential

Scheme 1 Synthesis of magnetic chiral ionic liquids (MCILs).

aDepartment of Chemistry, Louisiana State University, Baton Rouge,LA 70803, USA. E-mail: iwarner@lsu.edu; Fax: +1 (225)5783971;Tel: +1 (225)5782829

bDepartment of Physics and Astronomy, Louisiana State University,Baton Rouge, LA 70803, USA

w Electronic supplementary information (ESI) available: Details of thesynthesis and characterization of MCILs by 1H, 13C NMR, FT-IR, andC,H,N elemental analysis, viscosity tests, magnetic property tests usingSQUID, pictures showing the response of MCILs to a magnet, TGA andDSC tests, steady-state fluorescence tests, chemicals, instrumentation andexperimental procedures. See DOI: 10.1039/b917683g

6922 | Chem. Commun., 2009, 6922–6924 This journal is �c The Royal Society of Chemistry 2009

COMMUNICATION www.rsc.org/chemcomm | ChemComm

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scanning calorimetry (DSC), respectively (Table 1). Observations

from the TGA curves (ESIw) showed that these MCILs are

thermally stable up to temperatures nearing 190 1C under a

nitrogen atmosphere. A two-stage decomposition process was

observed for all MCILs. The first stage was rapid and the loss

in mass was about 40–50% in nitrogen. This rapid degradation

suggests a dissociative nature of the decomposition without

participation of oxygen.10 In contrast, oxidation of the

residual carbon may be predominant in the second stage.

This two-step thermal decomposition behavior is in agreement

with previously observed degradation behavior of the MIL,

tetrabutylammonium tetrachloroferrate(III).10

Measurements using DSC showed low glass transition

temperatures between �30 and �50 1C for all MCILs (ESIw).However, no definitive melting points were observed. This is

clear evidence of the presence of a phase transition upon

decreasing the temperature, which takes the MCILs from the

readily stable liquid form to the glass form. It is worth noting

that normal molecular solvents generally do not show such

glass transition behavior. Therefore, the polymer-like behavior

and a high degree of order in the structures of MCILs may

lead to a significant transfer or induction of chirality from the

MCILs to reactants or products. As a general trend, the glass

transition temperature increased with an increase in molecular

weight of the MCILs. It was found that the glass transition

temperature steadily increased as the alkyl chain elongates from

butyl to octyl in the imidazolium cations in alkylimidazolium

tetrachloroferrate(III) MILs.5e

The enantiomeric purity of the MCILs was confirmed by the

use of circular dichroism (CD) measurements; (Fig. 2) shows

the CD spectra of L- and D-PheOMeFeCl4, measured in

methanol solution. L-PheOMeFeCl4 exhibits a positive

CD band at 218 nm, with a maximum of 24 mDeg. The fact

that D-PheOMeFeCl4 has a similar but negative CD band at

the same wavelength and similar magnitude further confirms

the optical purity of both L- and D-PheOMeFeCl4.4d,h

Steady state fluorescence spectroscopy was further used to

investigate the chiral recognition of fluorescent analytes in

ethanol solutions using L-AlaOMeFeCl4 as chiral selector

(Fig. 3). With (R)- and (S)-2,2,2-trifluoro-1-(9-anthryl)ethanol

(TFAE) as analytes, the emission intensity difference between

(R)- and (S)-TFAE confirmed the chiral discrimination ability

of L-AlaOMeFeCl4.4d,11 Similarly, with L-AlaOMeFeCl4 as

chiral selector, the chiral discrimination of 1,10-binaphthyl-

2,20-diamine (BNA) and 2-(6-methoxy-2-naphthyl)propionic

acid (naproxen) were also achieved (ESIw).All synthesized MCILs contained tetrachloroferrate(III)

anions with d5 configuration and exhibited paramagnetic

behavior at room temperature. The magnetic susceptibilities

of the MCILs were measured using a Quantum Design super-

conducting quantum interference device (SQUID). The

magnetization at 300 K was measured in the magnetic field

range of �50 000 to 50 000 Oe. The magnetization showed an

expected linear dependence on the applied magnetic field

(Fig. 4). From the slopes of the linear fits to the data,

the molar magnetic susceptibilities of L-AlaOMeFeCl4,

L-ValOMeFeCl4, L-IleOMeFeCl4, and L-PheOMeFeCl4 were

obtained as wM = 0.0134, 0.0129, 0.0127, and 0.0135 emu mol�1,

respectively (Table 2), which conformed well to the value

expected for Fe(III).5e The effective magnetic moments

(meff) of the four MCILs are determined as values between

5.52 and 5.66mB in accord with what is expected from the

Fig. 1 Visible absorption spectra of MCILs in acetonitrile.

Table 1 Thermal properties of MCILs determined by TGA and DSC

MCIL Tg/1Ca Tstart/1C

b Tonset/1Cc Tonset2/1C

d

L-AlaOMeFeCl4 �47.6 197.3 231.1 304.3L-ValOMeFeCl4 �44.8 215.7 235.5 325.5L-LeuOMeFeCl4 �39.2 196.1 220.4 311.7L-IleOMeFeCl4 �41.1 216.5 236.9 327.4L-PheOMeFeCl4 �30.5 195.3 217.5 296.5

a Glass transition temperature. b The beginning decomposition

temperature. c The temperature at which the sample loses weight at

the fastest speed. d The second beginning decomposition temperature.

Fig. 2 Circular dichroism spectra of L- and D-PheOMeFeCl4in methanol.

Fig. 3 Fluorescence emission spectral plots of (R)- and (S)-TFAE

enantiomers in the presence of L-AlaOMeFeCl4. The emission spectra

of TFAE were collected at an excitation wavelength of 365 nm at room

temperature.

This journal is �c The Royal Society of Chemistry 2009 Chem. Commun., 2009, 6922–6924 | 6923

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S = 5/2 high-spin electronic state of iron(III) (spin-only value

is 5.92mB). All MCILs show very weak antiferromagnetic

interactions as seen from the Curie–Weiss temperatures

(Table 2), confirming the lack of cluster formation in solution.7a

In conclusion, novel magnetic chiral ionic liquids (MCILs)

have been synthesized using a rapid and simple synthesis

procedure. These MCILs represent the first examples of

room temperature ionic liquids that contain both chiral

and magnetic properties. Their paramagnetic properties

were confirmed by use of SQUID measurements. In addition,

their chiral discrimination toward chiral analytes was

investigated by use of steady-state fluorescence spectroscopy.

These novel chiral magnetic fluid materials with dual

functionalities are expected to have great potential in a

number of applications such as asymmetric catalysis and

synthesis. Their strong response to a magnetic field, as well

as their extremely low vapor pressure could be particularly

beneficial for providing an easier approach to recovering

and recycling the MCILs used as asymmetric catalysts. In

addition, their magnetic properties could also provide a

method to manipulate the phase separation behavior of

two immiscible solvents, one of which is the MCIL. Thus, this

may find interesting applications in the field of chiral extraction

and enrichment of chiral compounds by the use of magnetic

fields.

We thank Drs Haiyun Liu and Qinglin Wu for assistance

with the TGA and DSC experiments. This work was supported

by the National Science Foundation (CHE-0911118).

Notes and references

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Fig. 4 Field dependence of the magnetization of MCILs at 300 K.

Table 2 Summary of magnetic properties of MCILs

MCIL Mwa

wMb/

emu mol�1wTc/emu K mol�1 meff

d/mB ye (T)

L-AlaOMeFeCl4 299.78 0.0134 3.90 5.585 �2.4L-ValOMeFeCl4 327.83 0.0129 3.86 5.556 �3.3L-IleOMeFeCl4 341.86 0.0127 3.81 5.520 �3.1L-PheOMeFeCl4 375.88 0.0135 4.01 5.663 �2.9a Molecular weight. b Molar magnetic susceptibility. c Measured at

300 K and H = 1000 G. d Effective magnetic moment. e Curie–Weiss

temperature.

6924 | Chem. Commun., 2009, 6922–6924 This journal is �c The Royal Society of Chemistry 2009

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