Phonon, optical and luminescent properties of novel heterometallic frameworks of [(NH4)(H2O)][CrIIIMII(HCOO)6] (MII=Mn, Zn, Co, Ni)
Graphical abstract
Introduction
Metal-organic framework (MOF) compounds received enormous interest in recent years because use of constituents with various sizes, geometries and physicochemical properties allows obtaining materials with various functionalities [1]. Metal formate frameworks constitute a subgroup of MOFs that attracted broad interest due to their magnetic, ferroelectric, multiferroic and luminescent properties. These compounds can be divided into three main groups. The first group of general formula [AmHnn+][MII(HCOO)3]n contains frameworks composed of MIIO6 octahedral units (MII = Mg, Cd, Zn, Mn, Fe, Co, Ni, Cu) linked by HCOO− ions. Protonated amines (AmHnn+) are located in the cavities or channels of the frameworks. The first members of this family were discovered in 2004 by Wang et al. [2] and later studies showed that many of these formates exhibit ferroelectric or even multiferroic properties that can be tuned by doping with alliovalent or organic cations [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]. The second group constitute heterometallic formates of general formula [AmHnn+]2[MIIIMI(HCOO)6]n, where MIII = Cr, Fe, Al and MI = Na, K. The first members of this family, [(CH3)2NH2]2[FeNa(HCOO)6] and [(CH3)2NH2]2[CrNa(HCOO)6], were discovered by us in 2014 and 2015, respectively [14], [15]. Later, ferroelectricity was discovered in [CH3CH2NH3]2[MIIINa(HCOO)6] (MIII = Fe, Cr, Al) [16], [17]. Recently, discovery of the first potassium analogues, [(CH3)2NH2]2[CrK(HCOO)6] and [CH3CH2NH3]2[CrK(HCOO)6], as well as a number of aluminium-sodium frameworks with mono, di, tri and tetravalent ammonium cations has been reported [18], [19]. It is worth adding that all compounds with monovalent cations crystallize in the perovskite-type structure, those with trivalent or tetravalent cations are chiral whereas divalent cations lead to either niccolite or layered structures. The third group of formates constitute heterometallic compounds of general formula [AmH][MIIIMII(HCOO)6], where MIII = Cr, Fe, Al and MII = Zn, Mg, Mn, Co, Fe, Ni, Cu. Synthesis of these compounds is more challenging and up to now analogues with only four AmH+ cations (methylammonium, dimethylammonium. ethylammonium and diethylammonium) are known [20], [21], [22], [23], [24], [25], [26], [27], [28]. All of these compounds crystallize in the niccolite type structures and they were shown to exhibit interesting magnetic, luminescent and dielectric properties as well as large magnetocaloric effects [20], [21], [22], [23], [24], [25], [26], [27], [28]. In contrast to the divalent and MIII/MI formate frameworks, phase transitions were found for very few [AmH][MIIIMII(HCOO)6] compounds, i.e., for iron(II)/iron(III) mixed valence formates [(CH3)2NH2][FeIIIFeII(HCOO)6] and [(C2H5)2NH2][FeIIIFeII(HCOO)6] [25], [26], [27], [28]. These formates are possible multiferroics and rare example of molecular-based magnets with large negative magnetization [25], [26], [27], [28].
The above examples show that structural and physicochemical properties of metal formate frameworks can be tuned by using different organic and metal cations. For instance, in case of [AmHnn+][MII(HCOO)3]n formates, employment of small NH4+ cation leads to formation of chiral structures for MII = Mn, Co, Zn, Mg, Fe, Ni and ferroelectric order at low temperatures [7], [8], whereas all analogues with large cations (ethylammonium, methylhydrazinium, dimethylammonium etc.) crystallize in perovskite-type structures [2], [3], [4], [5], [6], [10], [12], [13]. Depending on type of the organic cation, i.e., its size, shape and ability to form hydrogen bonds with the anionic framework, the perovskites exhibit switchable dielectric, ferroelectric or even multiferroic properties [3], [4], [5], [6], [11], [12]. Therefore, in order to obtain materials with various functional and tunable properties, it is important to synthesize novel formates with different chemical compositions. We decided, therefore, to attempt synthesis of novel heterometallic formates with the smallest AmH+ cation, NH4+, because by analogy with [AmHnn+][MII(HCOO)3]n formates, we expected that also in the family of [AmH][MIIIMII(HCOO)6] frameworks small size of NH4+ cation might significantly alter their crystal structure and properties. Interestingly, our results show that in spite of small size of NH4+ cation, the niccolite structure is preserved for NH4CrZn whereas NH4CrMn, NH4CrCo and NH4CrNi crystallize in chiral-type structures, not reported previously for any [AmH][MIIIMII(HCOO)6] formates. However, in contrast to these formates, all reported in the present paper ammonium compounds possess water molecules located in the cavities of the frameworks or channels.
Section snippets
Materials and instrumentation
All reagents (analytically grade) used for synthesis are commercially available and were used without further purification. Elemental analysis (C, H, N) was performed on an Elementar Vario EL CHNS analyzer. The content of metal elements was determined by an inductively coupled plasma (ICP) method, which was performed on an ARL 3410 ICP instrument. Metal ratios were also checked based on EDS spectra, which were acquired and analysed using an EDAX Pegasus XM4 spectrometer with SDD Apollo
Structural studies
NH4CrCo crystallizes in the hexagonal system with chiral space group of P6322, which is unchanged with the lowering of temperature from 295 (1) to 100 (1) K (Table S1). However, a small decrease of a parameter and increase of c parameter is observed. The asymmetric unit consists of one metal position located on 3.2 (D3) symmetry site that is statistically occupied by Cr3+ and Co2+ ions. The oxygen atoms from six formate ligands form coordination environment of the metal centre that can be
Conclusions
We have synthesized four novel heterometallic metal-organic frameworks. Three of them, NH4CrM (M = Co, Mn and Ni) crystallize in the in the hexagonal system with chiral space group of P6322. The same space group was reported for [NH4][M(HCOO)3] analogues but NH4CrM compounds are characterized by much smaller c/a ratios and presence of disordered water molecules in the hexagonal channels. It is worth adding that in contrast to [NH4][M(HCOO)3] compounds, NH4CrM formates do not exhibit any
Acknowledgements
This research was supported by the National Science Centre (NCN) in Poland under project No. DEC-2013/11/B/ST5/01058.
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