Ammonium bis(salicylaldehyde thiosemicarbazonato)ferrate(III), a supramolecular material containing low-spin FeIII

The FeIIIS2N2O2 chromophore of ammonium bis[salicylaldehyde thiosemicarbazonato(2−)]iron(III) contains two O,N,S-donating salicylaldehyde thiosemicarbazonate(2−) ligands in perpendicular planes, with the O and S atoms in cis and the N atoms in trans positions. The FeIII ion is in the low-spin state at 100 K. Systematic twinning by metric pseudomerohedry is explained by application of the order–disorder (OD) theory.


Introduction
The study of the coordination chemistry of thiosemicarbazones is an attractive research area. Thiosemicarbazones display a wide range of pharmacological uses based, for example, on their antineoplastic, antibacterial, antiviral and antifungal activities (Beraldo & Gambino, 2004;. This pharmacological action is often related to coordination of the thiosemicarbazone to metal ions (Farrell, 2002). On the other hand, the magnetic properties of iron(III) compounds of thiocarbazone derivatives have attracted attention, particularly as switching behaviour was displayed for iron(III) bound to particular salicylaldehyde thiosemicarbazone derivatives (van Koningsbruggen et al., 2004;Phonsri et al., 2017;Powell et al., 2014Powell et al., , 2015Zelentsov et al., 1973, Ryabova et al., 1978, 1981a,b, 1982Floquet et al., 2003Floquet et al., , 2006Floquet et al., , 2009Li et al., 2013).
This type of magnetic interconversion between the low-spin (S = 1 2 ) and high-spin (S = 5 2 ) state in Fe III (3d 5 ) systems has now been found to be triggered by a change in temperature or pressure, or by light irradiation (Hayami et al., 2000(Hayami et al., , 2009 and may be used in a functional way in research and technology (Lé tard et al., 2004;Gü tlich & Goodwin 2004;Nihei et al., 2007;Halcrow, 2013;Harding et al., 2016).
In recent years, particular interest has focused on Fe III complexes of substituted derivatives of R-salicylaldehyde 4R 0thiosemicarbazone (Powell et al., 2014(Powell et al., , 2015Floquet et al., 2003Floquet et al., , 2006Floquet et al., , 2009Li et al., 2013) for generating Fe III spin crossover. In solution, free R-salicyl-aldehyde 4R 0 -thiosemicarbazone (H 2 L) exists in two tautomeric forms, i.e. the thione and thiol forms, as illustrated in Scheme 1. The chemistry of the Fe III compounds is rather complicated as it is possible for the tridentate R-salicylaldehyde 4R 0 -thiosemicarbazone ligand (H 2 L) to exist in tautomeric forms; moreover, the ligand may also be present in its neutral, anionic or dianionic form. However, the formation of a particular type of Fe III complex unit, i.e. neutral, monocationic or monoanionic, can be achieved by tuning the degree of deprotonation of the ligand through pH variation of the reaction solution during the synthesis (Powell et al., 2014(Powell et al., , 2015Powell, 2016;Floquet et al., 2009). We have been particularly skilled in preparing anionic Fe III complexes of the general formula (cation + )[Fe(L 2À ) 2 ]Áx(solvent), for which it became evident that the electronic state of the Fe III ion is dependent on the nature of the counter-ion, the nature and degree of solvation and the nature of the R,R 0substituted ligands (Powell et al., 2014(Powell et al., , 2015Powell, 2016;. We report here a novel Fe III compound, ammonium bis-[salicylaldehyde thiosemicarbazonato(2À)-3 O,N 1 ,S]iron(III), (I) (see Scheme 2), containing two dianionic tridentate ligands, i.e. salicylaldehyde thiosemicarbazonate(2À), abbreviated as thsa 2À , whose structure was determined at 100 K. Ryabova et al. (1981a) reported the crystallographic data of the related compound Cs[Fe(thsa) 2 ] at 103 and 298 K, which contains Fe III in the high-spin electronic state (S = 5 2 ). The main difference between NH 4 [Fe(thsa) 2 ] and Cs[Fe(thsa) 2 ] is the associated outer-sphere cation. This article describes that the variation in the cation leads to a modification of the structure of the Fe III compound, also changing the crystal packing, and being responsible for the Fe III in the present NH 4 [Fe(thsa) 2 ] compound exhibiting the low-spin electronic state (S = 1 2 ). Compound (I) systematically crystallizes as twins. The twinning will be interpreted in the light of order-disorder (OD) theory (Dornberger-Schiff & Grell-Niemann, 1961). The OD theory was created in the 1950s to explain the common occurrence of polytypism and stacking faults. It has since been developed into a comprehensive theory of local/ partial symmetry. According to OD theory, if a structure is composed of layers and the layers are related by partial symmetry that is not valid for the whole structure, then the stacking becomes ambiguous. This means that the layers can be arranged in different ways, which are nevertheless all locally equivalent. Owing to the short range of interatomic interactions, these different stacking arrangements are also energetically equivalent. Consequently, OD structures often feature stacking faults. In OD twins, such as the title compound, the stacking faults lead to domains with different spacial orientations and are sporadic, i.e. the resulting domains are macroscopic and do not diffract coherently.

Spectroscopic measurements
Room-temperature IR spectra within the range 4000-400 cm À1 were recorded on a PerkinElmer FT-IR spectrometer Spectrum RXI using KBr pellets. Variable-temperature FT-IR spectra were measured with the attenuated total reflectance (ATR) technique using a PerkinElmer spectrum 400 with a Harrick diamond ATR equipped with a thermostatable temperature-control device. 1 H and 13 C{ 1 H} NMR spectra were recorded on a Bruker 200 spectrometer with a broadband probe head. All NMR chemical shifts are reported in ppm; 1 H and 13 C shifts are established on the basis of the residual solvent resonance.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1. The crystal was modelled as twinned by reflection at (100). The positions of the H atoms on the amine N atoms were located in difference Fourier maps and were refined with restrained N-H distances of 0.87 (2) Å . The NH 4 + cation is disordered around a pseudo-twofold axis. The ammonium N atom was refined as disordered about two positions (N7 and N7 0 ). The sum of the occupancy parameters was constrained to 1. The atomic displacement parameters (ADPs) were constrained to the same value, resulting in a significant decrease of the estimated standard uncertainty on the occupancy parameters. Even though residual electron density in the difference Fourier maps could be attributed to the H atoms of the disordered ammonium positions, a reliable refinement was not possible. The ammonium H atoms were therefore ultimately omitted from the refinement. Other H atoms were included in the refinement in calculated positions and allowed to ride on their parent atoms.

Results and discussion
3.1. Crystal structure The structure of NH 4 [Fe(thsa) 2 ], (I) (Fig. 1), was determined at 100 K and was found to crystallize in the monoclinic space group P2 1 /n. The asymmetric unit consists of one formula unit, i.e. NH 4 [Fe(thsa) 2 ], with no atom on a special position. The Fe III cation is coordinated by two dianionic O,N,S-tridentate chelating thsa 2À ligands, displaying a distorted octahedral Fe III O 2 N 2 S 2 geometry. Selected geometric parameters are listed in Table 2. The twofold deprotonated ligands are coordinated to the Fe III atom via the phenolate O, thiolate S and imine N atoms. These donor-Fe III bonds are located in two perpendicular planes, with the O and S atoms in cis positions, whereby the S1-Fe-S2 and O1-Fe-O2 angles are 92.07 (3) and 88.53 (12) , respectively. In addition, the N atoms are situated in trans positions, which is evidenced by the N1-Fe-N4 bond angle of 176.71 (11) . The FeO 2 N 2 S 2 coordination core is distorted; the Fe-donor atom distances fall within the range expected for Fe III in the low-spin state (van Koningsbruggen et al., 2004).
The incorporation of a monovalent NH 4 + cation could be corroborated by variable-temperature FT-IR spectroscopy, which revealed the sharpening of the N-H stretching vibrational mode of the NH 4 + cation at 3240 cm À1 upon cooling from ambient temperature to 173 K, which is in line with the freezing of the rotation of the NH 4 + cation in the cavity. The presence of the trivalent iron cation is supported by the coordination of two doubly deprotonated ligands to the Fe III  Computer programs: APEX2 (Bruker, 2012), SAINT-Plus (Bruker, 2012), SUPERFLIP (Palatinus & Chapuis, 2007), SHELXL2018 (Sheldrick, 2015) and ORTEP-3 (Farrugia, 2012).

Figure 1
The molecular structure and atom-numbering scheme for NH 4 [Fe(thsa) 2 ], (I). The N atom of the NH 4 + cation has been omitted for clarity. Displacement ellipsoids are drawn at the 50% probability level.
ion. In addition, the presence of both dianionic ligands is confirmed by the C-S, C-N and N-N bond lengths (Table 2) obtained for NH 4 [Fe(thsa) 2 ], which show characteristics of a bond order between single and double bonds. Ryabova et al. (1981a) reported the structure of the related high-spin compound Cs[Fe(thsa) 2 ] at 103 and 298 K, which crystallizes in the space group Pna2 1 with an asymmetric unit consisting of a Cs + cation and an [Fe(thsa) 2 ] À anionic unit. The C-S, C-N and N-N bond lengths [at 103 K: C-S = 1.749 (9) and 1.761 (9) Å ; C-N = 1.314 (10) and 1.303 (11) Å ; N-N = 1.371 (11) and 1.380 (11) Å ; at 298 K: C-S = 1.743 (14)  The embedding of the NH 4 + cation is, therefore, essentially different from that of the Cs + cation in Cs[Fe(thsa) 2 ] at 103 and 298 K, where the nearest-neighbour coordination sphere of the Cs + cation is constituted by O, N and C atoms, which form a seven-pointed polyhedron with Cs-(ligand donor atom) separations between 3.06 and 3.82 Å (Ryabova et al., 1981a). This feature shows some similarity with the Cs + cation in Cs[Fe(5-Br-thsa) 2 ] (Powell et al., 2015) that is at the centre of an irregular seven-donor-atom polyhedron, the donor atoms of which originate from symmetry-related equivalents of both symmetry-independent 5-bromosalicylaldehyde thiosemicarbazonate(2À) (5-Br-thsa) ligands. Several donor atoms coordinated to the Fe III atom of Cs[Fe(5-Br-thsa) 2 ] form interactions with the Cs + cation in the second coordination sphere; this is likely to modulate the electron density of the Fe-(donor atom) bonds and hence influence the electronic state of the Fe III cation. The latter is also prone to be affected by the assembly of Fe III units in the unit cell. The presence of the Br substituent on the salicylaldehyde group of the ligand in Cs[Fe(5-Br-thsa) 2 ] is a factor in determining the crystal packing, as the Br substituent of one Cs[Fe(5-Br-thsa) 2 ] unit provides a hydrogen-bonding interaction with an amino group of a neighbouring Fe III unit, creating ring systems.
A higher point symmetry of the lattice compared to the point symmetry of the crystal is often associated with twinning. However, it is not a sufficient precondition for its existence. In fact, many polar structures do not form twins by inversion despite inversion being an intrinsic symmetry of any lattice. One common and often overlooked cause of twinning is partial symmetry, which may lead to a twin interface that is locally equivalent to the twin individuals. The order-disorder (OD) theory (Dornberger-Schiff & Grell-Niemann, 1961) was introduced in the 1950s to deal precisely with these kinds of structures.
In the light of OD theory, the crystal structure of NH 4 [Fe(thsa) 2 ] can be decomposed into OD layers A n (n being a sequential number) parallel to (010), which, in this case, also correspond to layers in the crystallochemical sense (Fig. 3). The crucial point of an OD structure is that partial symmetry operations relate individual layers, yet need not be valid for the whole structure. In the case of NH 4 [Fe(thsa) 2 ], the A n layers possess (idealized) P2(n)a symmetry (Fig. 4). In this layer-group notation, which is commonly used in the OD literature, the parentheses indicate the direction missing translational symmetry. The [Fe(thsa) 2 ] À ions are located on the twofold axes of the A n layers, whereas the ammonium ions are disordered about these axes.
The set of partial symmetry operations of any possible stacking of NH 4 [Fe(thsa) 2 ] is described by the OD groupoid family symbol (Dornberger-Schiff & Grell-Niemann, 1961). P 2 (n) a n 2,r 2 2 2 r-1 OD groupoid families are the analogue of space group types in classical crystallography. They abstract from metric parameters and additionally of the particular stacking. The first line of the symbol indicates the layer symmetry, the second line one set of operations relating adjacent layers. Since the intrinsic translations are not limited to those found in space groups, a generalization of the Hermann-Mauguin notation is used. For example, n 2,r represents a glide reflection with the intrinsic translation b/2 + rc/2, whereby r is one of the metric parameters the OD groupoid family abstracts from. The crystal structure of NH 4 [Fe(thsa) 2 ], viewed down [100]. The names of the OD layers are indicated to the right and a dotted line indicates the interface between the OD layers, which in this case is not planar. Owing to the partial symmetry, layers can be arranged in different ways while keeping pairs of adjacent layers geometrically equivalent. These stacking possibilities can be enumerated using the NFZ relationship (Ď urovič, 1997), which reads as Z = N/F = [G n :G n \G n+1 ]. G n = P1(1)a is the group of operations of the A n layer that do not invert A n with respect to the stacking direction. Since the a-glide planes of adjacent layers do not overlap, G n \G n+1 = P1(1)1. The possible layer arrangements are determined by coset decomposition of the latter in the former. In other words, given the A n layer, the adjacent A n+1 layer can be placed in Z = [P1(1)a:P1(1)1] = 2 ways, which are related by the a-glide reflection of the A n layer.
Of the infinity of the thus obtained locally equivalent polytypes, a finite number is especially simple in the sense that they cannot be decomposed into fragments of even simpler polytypes. In these polytypes, which are said to be of a maximum degree of order (MDO), not only pairs but also triples, quadruples and generally n-tuples of adjacent layers are equivalent (for a more rigorous definition, see Dornberger-Schiff, 1982). Polytypes of the MDO type play a special role in OD theory, because all other polytypes can be decomposed into fragments of MDO polytypes. Moreover, experience shows that ordered bulk polytypes are in most cases of the MDO type. The OD family of NH 4 [Fe(thsa) 2 ] contains two MDO polytypes: where b 0 is the vector perpendicular to the layer lattices with the length of one layer width. Both MDO polytypes are shown schematically in Fig. 5. The twin individuals of NH 4 [Fe(thsa) 2 ] correspond to the MDO 2 polytype. A fragment of the MDO 1 polytype is located at the twin interface.
Thus, the OD theory plausibly explains the formation of the observed twins, as the twin interface is geometrically and, if interactions over one layer width are ignored, also energetically equivalent to the twin individuals. Moreover, it explains the pseudo-oP metrics of the lattice. Such a metric pseudosymmetry has often been considered as 'accidental'. However, here it is clearly very much intrinsic to the structure family.
Finally, it should be noted that an OD description is usually based on a certain degree of idealization. Ordered polytypes are desymmetrized with respect to the ideal description (Ď urovič, 1979). Indeed, in the actual MDO 2 polytypes of NH 4 [Fe(thsa) 2 ], the symmetry of the A n layers is reduced by an index of 2 from P2(n)a to P1(n)1. Accordingly, the site symmetry of the [Fe(thsa) 2 ] À ion is reduced from 2 to 1. Moreover, the unit-cell parameters deviate slightly from orthorhombic metrics [ = 90.052 (4) , according to singlecrystal diffraction]. Finally, the desymmetrization is also observed by a splitting of the single disordered ammonium position into two independent positions, which are now not forcibly disordered in a 1:1 manner. Indeed, the ratio of the occupancies of both positions refines to 52.7 (9):47.3 (9). However, collectively the deviations from the idealized partial symmetry are minute and the OD description can be considered as correct.

Computing details
Data collection: APEX2 (Bruker, 2012); cell refinement: APEX2 (Bruker, 2012); data reduction: SAINT-Plus (Bruker, 2012); program(s) used to solve structure: SUPERFLIP (Palatinus & Chapuis, 2007); program(s) used to refine structure: SHELXL2018 (Sheldrick, 2015); molecular graphics: ORTEP-3 (Farrugia, 2010); software used to prepare material for publication: SHELXL2018 (Sheldrick, 2015). Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. Refinement. Refined as a 2-component twin. Crystal data collection for NH 4 [Fe(thsa) 2 ] was carried out after attaching a single crystal to a Kapton micro mount by using perfluorinated oil and mounted on a Bruker KAPPA APEX II diffractometer equipped with a CCD detector. Data were collected at 100 K in a dry stream of nitrogen with MoKα radiation (λ = 0.71073 Å). Data were reduced to intensity values using SAINT-Plus (Bruker, 2012) and absorption correction was applied using the multi-scan method implemented by SADABS (Bruker, 2012). The structure was solved using charge-flipping implemented by SUPERFLIP (Palatinus & Chapuis, 2007) and refined against F 2 with SHELX (Sheldrick, 2014