Synthesis, magnetic and vibrational properties of two novel mixed-valence iron(II)-iron(III) formate frameworks

Highlights

• Novel mixed-valence iron(II)-iron(III) MOF compounds have been synthesized.

• These compounds crystallize in niccolite-type architecture.

• IR and Raman spectra were measured and assignment of modes was proposed.

• Magnetization measurement revealed that EtMeAFeFe is N-type ferrimagnet.

Abstract

We report synthesis of two novel mixed-valence iron(II)-iron(III) formate frameworks templated by N-ethylmethylammonium (EtMeA⁺) and 2-Hydroxyethylammonium (HEA⁺) cations. X-ray diffraction shows that these compounds crystallize in the P $\overline{3}$1c niccolite-type structures. Optical studies confirm mixed-valence character of these compounds. Magnetic investigation reveals that both N-ethylmethylammonium iron formate (EtMeAFeFe) and 2-Hydroxyethylammonium iron formate (HEAFeFe) exhibit magnetic order at 36.5 and 37.0 K, respectively. EtMeAFeFe also exhibits large negative magnetization below 31.0 K indicating that this compound is an N-type ferrimagnet. We also present Raman and IR data for both compounds. In order to help assignment of modes corresponding to the EtMeA⁺ cation, we also performed quantum chemical calculations to derive the harmonic and anharmonic vibrational wavenumbers. The performed calculations revealed that protonation should affect most strongly modes related to vibrations of the N–H bonds as well as the stretching modes of the CH₂ and CH₃ groups attached to the N atom.

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][M^II(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][M^I_0.5M^III_0.5(HCOO)₃] (M^I=Na, K; M^III=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][Na_0.5M^III_0.5(HCOO)₃] (M^III=Fe, Cr, Al) [29], [30] whereas [CH₃CH₂NH₃][M^I_0.5Cr_0.5(HCOO)₃] (M^I=Na, K), [CH₃CH₂NH₃][Na_0.5Al_0.5(HCOO)₃]: Cr³⁺ and [(CH₃)₂NH₂][M^I_0.5Cr_0.5(HCOO)₃] ((M^I=Na, K) were shown to exhibit efficient chromium-based luminescence [28], [30], [32]. Second group constitute niccolite-type formates of general formula [cat][M^IIM^III(HCOO)₆] (M^II= Cd, Mg, Zn, Mn, Fe, Co, Ni, Cu; M^III=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][Fe^IIFe^III(HCOO)₆] (DMAFeFe), [(C₂H₅)₂NH₂][Fe^IIFe^III(HCOO)₆] (DEtFeFe) and [CH₃CH₂NH₃][Fe^IIFe^III(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 T_ord=39 K as well as negative magnetization below T_comp = 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][Fe^IIFe^III(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[M^II(HCOO)₄] instead of [NH₃C₂H₄OH][M^II(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.