Synthesis, magnetic and vibrational properties of two novel mixed-valence iron(II)-iron(III) formate frameworks
Graphical abstract
Disordered EtMeA+ (a) and HEA+ (b) templates in the niccolite cavity
Introduction
Dense metal-formate frameworks are compounds composed of metal cations linked by HCOO- ligands to form extended three-dimensional structures with cavities occupied by ammonium cations or protonated amines. These compounds received great interest in recent years because combination of different metal centers and cations located at cavities of the framework allows obtaining huge number of compounds exhibiting various functionalities. Most extensively studied compounds of general formula [cat][MII(HCOO)3] (M=Cd, Mg, Zn, Mn, Fe, Co, Ni, Cu) crystallize in perovskite- or chiral-type structures and they have attracted attention mainly due to their magnetic [1], [2], [3], [4], [5], [6], [7], ferroelectric [8], [9], [10], multiferroic [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], negative thermal expansion [25] and negative linear compressibility [26] properties.
Heterometallic and mixed-valence formate frameworks are much less common. One group constitute compounds of general formula [cat][MI0.5MIII0.5(HCOO)3] (MI=Na, K; MIII=Cr, Fe, Al.) that also crystallize in perovskite-type structures [27], [28], [29], [30], [31], [32]. Among these compounds, ferroelectric properties were discovered for [CH3CH2NH3][Na0.5MIII0.5(HCOO)3] (MIII=Fe, Cr, Al) [29], [30] whereas [CH3CH2NH3][MI0.5Cr0.5(HCOO)3] (MI=Na, K), [CH3CH2NH3][Na0.5Al0.5(HCOO)3]: Cr3+ and [(CH3)2NH2][MI0.5Cr0.5(HCOO)3] ((MI=Na, K) were shown to exhibit efficient chromium-based luminescence [28], [30], [32]. Second group constitute niccolite-type formates of general formula [cat][MIIMIII(HCOO)6] (MII= Cd, Mg, Zn, Mn, Fe, Co, Ni, Cu; MIII=Fe, Cr, Al) [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44]. These compounds exhibit interesting magnetic and luminescent properties [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44]. Especially interesting properties were, however, observed for mixed-valence iron(II)-iron(III) frameworks. Up to now only three such compounds are known, i.e., [(CH3)2NH2][FeIIFeIII(HCOO)6] (DMAFeFe), [(C2H5)2NH2][FeIIFeIII(HCOO)6] (DEtFeFe) and [CH3CH2NH3][FeIIFeIII(HCOO)6] (EtFeFe) [33], [35], [36], [38], [41], [43], [44]. All of them are extensively colored (dark blue or even black) and they show magnetic order near Tord=39 K as well as negative magnetization below Tcomp = 22–29 K [34], [38]. DMAFeFe and DEtFeFe undergo order-disorder phase transitions at 152–155 and 240 K, respectively, and are regarded as mixed-valence multiferroic materials [35], [38], [41], [43], [44]. Very recent studies of DEtFeFe showed that this compound has four different switchable physical properties, i.e., in addition to the known previously reversible phase transition and switchable dielectric constant, it also exhibits magnetic poles reversal and tunable positive and negative exchange bias fields at low temperatures [44]. It is worth adding here that interesting magnetic properties have also been reported for iron and iron oxide nanoparticles, including mixed-valence Fe3O4 [45], [46], [47], [48].
Motivated by recent discovery of interesting properties of mixed-valence formate frameworks, we have attempted to synthesize novel mixed-valence compounds of general formula [cat][FeIIFeIII(HCOO)6] by using not yet employed protonated amines. In family of metal formate frameworks, shape and size of the protonated amine play a crucial role in stability of the structure, electric and even magnetic properties [1], [35], [38]. We have expected, therefore, that employment of new protonated amines will lead to discovery of new compounds exhibiting interesting physicochemical properties. An important limitation in the synthesis of metal formate frameworks is size of the protonated amine since it cannot be larger than size of the available cavities of the metal formate frameworks. In case of niccolite-type structures, characteristic for majority of heterometallic and mixed-valence formates, the crystal cavities form channels expanding along the c axis that may accommodate even large cations such as (C2H5)2NH2+ [38]. In this article, we will show that successful synthesis of two novel mixed-valence formate frameworks could be performed using N-ethylmethylamine, which was not yet employed in synthesis of any formate frameworks, and 2-aminoethanol (ethanolamine, 2-hydroxyethylamine), which was previously employed in synthesis of divalent metal formate frameworks but with different chemical composition ([NH3C2H4OH]2[MII(HCOO)4] instead of [NH3C2H4OH][MII(HCOO)3]) and structure (layered, not perovskite) [49]. The obtained compounds were characterized for structural, optical and magnetic properties by x-ray diffraction, diffuse reflectance and SQUID, respectively. Our aim was also to understand phonon properties of the studied compounds by Raman and IR spectroscopy as well as DFT calculations.
Section snippets
Materials and instrumentation
All reagents (analytically grade) used for synthesis are commercially available and used without further purification. Elemental analysis (C, H, N) was performed on a Elementar Vario EL CHNS analyzer. Magnetization of a large number of freely oriented single crystals of HEAFeFe and EtMeAFeFe (about 30 and 20 mg in total) were measured using a commercial Quantum Design SQUID (superconducting quantum interference device) magnetometer at temperatures ranging from room temperature down to 2 K and in
Formation of EtMeAFeFe and HEAFeFe
Formation of the studied compounds involves a few steps. In the first step, HCOO- anions react with FeII and FeIII cations to form extended framework composed of the iron ions bridged by formate linkers. Such anionic framework contains cavities that are templated in the second step by protonated amines to maintain electrical neutrality. It is important to use a non-coordinating solvent to avoid its incorporation into the structure. It is also important to use protonated amines with the
Conclusions
We have synthesized two novel mixed-valence iron(II)-iron(III) formate frameworks employing EtMeA+ and HEA+ as templating cations. These MOFs crystallize in the niccolite-type structure (space group P1c), in which metal centers are octahedrally coordinated by formate oxygen atoms and connected by formate linkers in the anti-anti configuration mode whereas organic cations are disordered. Raman and IR data confirm that these compounds crystallize in the niccolite-type structure. They also show
Acknowledgements
This research was supported by the National Science Center (Narodowe Centrum Nauki) in Poland under project No. DEC-2013/11/B/ST5/01058.
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