Magnetic monomers and polymers based on alkyl-imidazolium FeCl4: The effect of alkyl chain length
Graphical abstract
By introducing alkyl groups with different length into imidazolim FeCl4 based ionic liquid monomers and polymers, magnetic monomers and polymers were prepared. Magnetic studies showed that with increasing of the alkyl groups, the susceptibility increased and turned to ferromagnetism when increased to dodecyl and octaducyl groups. However, the polymers were paramagnetic, and showed lower susceptibility than the monomers.
Introduction
Magnetic ionic liquids (MILs) are unique room temperature ionic liquids (RTILs) containing transition metals [1,2], which not only possess excellent properties of RTILs [3], but also exhibit magneto-responsiveness in an external magnetic field. And the MILs showed great potential in applications in functional surfactant [1], catalyst [4], extracting agent [5], desulfurizer [6], CO2 [7] or protein separation [8], density measurement [9], and so on. Because of these superior properties and promising potential applications, MILs have attracted widespread interests in recent years.
The first MILs was reported by S. Hayashi and H. Hamaguchi [10]. They discovered a novel ionic liquid by mixing 1-butyl-3-methylimidazolium chloride ([bmim]Cl) and FeCl3, which showed paramagnetic and its magnetic susceptibility was 40 × 10−6 emu/g. Owing to cations and anions in the MILs being tunable and designable, numerous MILs were synthesized. For example, phosphonium-based [7], imidazolium-based [11], and ammonium-based [12] MILs were studied. Nevertheless, most MILs reported were paramagnetic at room temperature, a kind of magnetism weaker than superparamagnetism or ferromagnetism, which limits their potential applications. To broaden the field of applications, numerous efforts were made to improve the magnetic properties by designing cations or anions in the molecular structures. MILs containing lanthanide ions, such as Gd(III) [13], Ho(III) [14] and Dy(III) [15] were investigated in recent years. Compared with common MILs containing Fe(III), Co(II) and Mn(II) [1], their magnetic susceptibility values increased, which due to the higher spin orbital electrons of the lanthanide ions. On another hand, I. Pedro [16] et al. found that the paramagnetic 1-ethyl-3-methylimidazolium tetrachloroferrate transformed into antiferromagnetic at 3.8 K, which was due to long-range magnetic ordering at low temperature. Similarly, long-range antiferromagnetic ordering was also observed in 1-ethyl-3-methylimidazolium tetrachloroferrate at very low temperature [17].
At the same time, magnetic poly(ionic liquid)s (MPILs) were prepared. The MPILs showed similar paramagnetic property as the corresponding MILs [18,19]. Studies showed that with increasing of the magnetic tetrachloroferrate (FeCl4−) content in the polymer, the susceptibility of the MPILs increased [20]. Furthermore, block MPILs were prepared by post-modification. By means of bulk self-assembly, the magnetic FeCl4 anions was aligned orderly, and the magnetic susceptibility was increased [21]. Magnetic ordering was related to the magnetic property of the materials, which could be used in design of the magnetic materials, especially for improving the magnetic property.
As well known, with increase of the number of carbon atoms, the ordering of the alkyl groups was improved at room temperature. As the state of alkanes transformed from gas to liquid and further into solid with increasing of the carbon atom numbers. As reported, the imidazolium based ionic liquid crystals modified with longer alkyl groups with carbon atoms more than 12 showed ordered crystal phases at room temperature [22,23]. And, the melting temperature of the crystals increased with increasing length of the alkyl groups. Herein, alkyl groups with different length (n = 2, 4, 6, 12 and 18, n represents the number of the carbon atom of alkanes) were introduced in to the MILs modified with norbornenyl groups. The corresponding MPILs were prepared by ring-opening metathesis polymerization (ROMP) due to the high group tolerance of ROMP [24]. And, the effect of the alkyl length and molecules ordering on the magnetic property was studied to give clues on improving the magnetic property of MILs and MPILs, which is very important in design of new magnetic materials.
Section snippets
Materials
Cis-5-norbornene-exo-2,3-dicarboxylic anhydride (98%) and N-(3-aminopropyl)imidazole (98%) were purchased from Energy Chemical, Shanghai, China. 1-bromoethane (99%), 1-bromobutane (99%), 1-bromohexane (99%), 1-bromododecane (99%), and 1-bromooctadecane (99%) were purchased from Kemiou Chemical reagent, Tianjin, China. FeCl3·6H2O (99%) was purchased from Tianjin Shuangchuan Chemical Reagent Factory, Tianjin, China. Grubbs 2nd generation catalyst (G2, Sigma-Aldrich, USA) were used as received.
Methods
All structures of the compounds were verified by 1H NMR (400 MHz) and 13C NMR (101 MHz) on AvanceIII (Bruker) 400 MHz NMR spectrometer at room temperature, respectively. Deuterated chloroform (CDCl3) and dimethyl sulfoxide (DMSO‑d6) were used as solvent. The molecular weight of the MPILs was characterized by viscosity method. The intrinsic viscosity of Poly[nor-imid(Cl)-nC]s in methanol solution at 30 °C was measured by Ubbelohde capillary viscometer via extrapolation. The viscosity average
Synthesis and characterization of MILs and MPILs
The MILs and MPILs modified with different length of alkyl groups were prepared as shown in Scheme 1. Imidazole group was introduced in to the norbornene derivatives (Nor-imid). The structure of Nor-imid was confirmed by 1H NMR and 13 C NMR as shown in Fig. S1. Through quaternization of imidazole groups with different length of alkyl bromide, the Nor-imid(Cl)-nC were prepared. Fig. 1 showed the 1H NMR of Nor-imid(Cl)-nC. Compared with Nor-imid, the signals from imidazole (7.60 ppm, Fig. S1)
Conclusions
In summary, magnetic monomers and polymers based on imidazolium FeCl4 anions with different alkyl length (n = 2, 4, 6, 12 and 18) were prepared and characterized by NMR and Raman spectra. Magnetization measurements results showed that monomers with short alkyl chains (n = 2, 4 and 6) were paramagnetic, whereas monomers with long alkyl chains (n = 12 and 18) were ferromagnetic. The change of magnetism could be related to the length of alkyl. With increasing of the alkyl length the intermolecular
Acknowledgement
The project is supported by National Natural Science Foundation of China (No. 21304067).
References (31)
- et al.
Fe-containing magnetic ionic liquid as an effective catalyst for the glycolysis of poly(ethylene terephthalate)
Catal. Commun.
(2010) - et al.
Pyridinium-based temperature-responsive magnetic ionic liquid for oxidative desulfurization of fuels
Chem. Eng. J.
(2013) - et al.
Permeability modulation of supported magnetic ionic liquid membranes (SMILMs) by an external magnetic field
J. Membr. Sci.
(2013) - et al.
Tadpole-shaped magnetic block copolymer: self-assembly induced increase of magnetic susceptibility
Polymer
(2018) - et al.
Interactions of sodium polystyrene sulfonate with some imidazolium-based ionic liquids in aqueous solutions
J. Mol. Liq.
(2017) - et al.
Raman spectroscopy indications of the verwey transition in epitaxial Fe3O4(111) films on Pt(111) and Ru(0001)
Surf. Coating. Technol.
(2015) - et al.
Synthesis and characterization of magnetic ionic liquids (MILs) for CO2 separation
J. Chem. Technol. Biotechnol.
(2014) - et al.
Anionic rare-earth thiocyanate complexes as building blocks for low-melting metal-containing ionic liquids
J. Am. Chem. Soc.
(2006) - et al.
Magnetic ionic liquids: synthesis, properties and applications
RSC Adv.
(2014) - et al.
Application of iron-containing magnetic ionic liquids in extraction process of coal direct liquefaction residues
Ind. Eng. Chem. Res.
(2012)