Article pubs.acs.org/crystal
Cite This: Cryst. Growth Des. XXXX, XXX, XXX−XXX
Stability of Hydrogen Bonds in the Metal Guanidinium Formate Hybrid Perovskites: A Single-Crystal Neutron Diffraction Study M. Viswanathan* School of Physics and Astronomy, Queen Mary University of London, London E1 4NS, United Kingdom
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ABSTRACT: We report the direct probing of the N−H···O bonds in the metal guanidinium formate hybrid perovskites. N−H···O bonds are no strangers to promoting disorder, as found in supramolecules and in certain multiferroic compounds. While the demand for new multiferroic materials is continuing to increase, the predictions based on ab initio methods are not in a matured stage for multiferroic MOFs. This rare report for guanidinium-based materials, based on single-crystal neutron diffraction, establishes stable N−H···O bonds, providing no room for proton migration/disorder. Hence, facilitating potential theoretical/ experimental studies and by the simultaneous elimination of any disorder-induced physical phenomenon, this work increases the understanding of multiferroic attributions in a promised multiferroic metal−organic framework.
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INTRODUCTION Metal−organic frameworks (MOFs) are hybrid materials with most of them containing hydrogen. MOFs can be designed to have desired properties which rely on the nature of the linkers and the metal ions.1 Their structural diversity is regulated by the interactions between the linker and the metal ion, hence directly affecting the stability of the framework,2 as hydrogen bonding has an essential role in the structural stability.3 Formate (HCOO−) being the simplest carboxylic ligand with a short O−C−O bridge, can adopt different bridging modes resulting in the construction of different metal−organic structures. Few such examples are those with perovskite-type morphology: metal formates with dimethylammonium [(CH3)2NH2]+4 and ammonium [NH4]+5 as a guest exhibit order−disorder transition, attesting to the N−H···O bonds, and have sparked huge interest with their multiferroic characteristics. Guanidinium [C(NH2)3]+ is one of the simplest organic chemical units with a planar configuration and high symmetry D3h in a free state.6 With the discovery of ferroelectric properties in [C(NH2)3]Al(SO4)2·6H2O (guanidinium aluminum sulfate hexahydrate)7,8 as early as in the mid-1950s, guanidinium-based compounds have attracted interest by strengthening of their position in functional materials9 and with revived interest in ferroelectrics.10,11 A new series C(NH2)3 [MII(HCOO)3] (M = Mn, Fe, Co, Ni, Cu, and Zn) was reported with guanidinium positioning in the A-site. While all the members of the metal guanidinium formate (MeGFs) belong to the orthorhombic crystal system, it is only copper guanidinium formate (CuGF) that exhibits the polar space group Pna2 1 , while others belong to Pnna, a centrosymmetric class.12 © XXXX American Chemical Society
Ab initio studies on CuGF reported that the inversion symmetry is broken by cooperative interactions between the antiferrodistortions of Cu−O in the framework and the [C(NH2)3]+ cation via hydrogen bonding, which thus induces net dipole moment which is coupled to the weak ferromagnetism, resulting in a multiferroic MOF.13 Recent studies have provided deeper insights questioning such predictions.14 Although ab initio design and characterization of inorganic multiferroics is considered to be at a matured stage, it has been admitted that the situation is different for multiferroic MOFs.15 Yet, similar predictions based on ab initio studies have been extended not only to another family of perovskite-MOFs16 but also to a material that has remained nonexistent until recently: chromium guanidinium formate.17 In a continued spirit on the pursuit of finding new multiferroics, detailed experimental input is crucial. With the developing interests in multiferroic MOFs, there exist aspects that remain less explored, resulting in a void of many of the fundamental aspects, especially those that could be studied only with neutron scattering. While the ab initio studies reported that it is only the guanidinium atoms that firmly contribute to the polarization, it is noteworthy regarding the guest−host connectivity, i.e. the N−H···O bonds, a common route for order−disorder transitions accompanying multiferroic features in other families of metal-guest formates. By considering the following facts: (i) a similar class of MOFs with ABX3 morphology exhibit disorder via the guest-host connectivity: N−H···O; (ii) N−H··· O bonds are no strangers to providing room for disorder in Received: December 5, 2018 Revised: May 23, 2019
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DOI: 10.1021/acs.cgd.8b01809 Cryst. Growth Des. XXXX, XXX, XXX−XXX
Crystal Growth & Design
Article
While temperature-dependent migration is thought to be facilitated through a short hydrogen bond, representing a broad single-well potential, the phenomenon has so far remained little understood. This encourages an examination when single-crystal neutron diffraction data are available, especially when migration has never been reported in MOFs. Migration is a process with a simultaneous detachment of hydrogen from one end and attachment to another; H··· ( grows at the expense of + −H. The direct difference between H··· ( and + −H might reveal migration as expressed by Steiner et al.26 Yet, a clearer approach by expressing the ratio +−H Ḧ +··((%) = (+ − H) + (H ··· () × 100 is followed in this article.
supramolecules; (iii) single-crystal neutron diffraction studies on guanidinium-based materials are extremely rare,18,19 and to the best of our knowledge, there is no report based on singlecrystal neutron diffraction for any of the formate-based perovskite-type MOFs that provide direct information on the N−H···O bonds. In this article, we exclusively investigate the nature of N−H···O bonds and the plausibility of their attributions to a ferroic nature, if any, leading to being either an element of support or otherwise by providing valuable directions for future work, inclusive of theoretical predictions.
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EXPERIMENTAL SECTION
Fully deuterated CuGF single crystals and protonated MnGF single crystals were grown by solution method by slow evaporation based on the synthesis as described by Hu et al.12 However, for CuGF, guanidinium chloride along with potassium carbonate was used instead of guanidinium carbonate due to its unavailability in a deuterated form. Single-crystal data were collected in a heating run beginning at 30 K with the SXD diffractometer20 at the ISIS Pulsed Neutron and Muon Source, UK. Further experimental details are provided in ref 21. Structural refinements were performed using Jana200622 with no soft restraints implemented in the analysis. The corresponding crystallographic information was deposited with the CCDC.23 The difference Fourier maps (dFm) computed using Jana2006 were visualized using the VESTA program.24
The data from Steiner et al.26 are replotted and shown in Figure S1. The thermal response of dO···D/H and dN−D/H is due to thermal expansion with increase in temperature as no effect on the increase of one bond length at the expense of the other is observed (see Figure S2). Temperature dependence of Ḧ +··( ̈ MnGF as shown in Figure 2 reveals ΔḦ CuGF +··( < 0.4% and ΔH +··(
