Crystal structure of poly[tetrakis(4-methylanilin-ium) [octa-l -chlorido-dichloridotricadmium(II)]]: a two-dimensional organic–inorganic hybrid perovskite

The title polymeric compound, (C7H10N)4[Cd3Cl10], involves a centrosymmetric [Cd3Cl10]4− tetra-anion, which is made up of three face-sharing CdCl6 octahedra, linked by four corner Cl atoms, forming layers propagating in the ab plane. The p-methylanilinium cations, situated between the layers, form N—H...Cl hydrogen bonds to the layers, which stack up the c-axis direction. There are no π–π or C—H...π interactions involving the aromatic rings, which are inclined to each other by 42.3 (1) ° in the asymmetric unit.


Chemical context
There are numerous reports of the structures of polymeric structures involving transition metal halide networks with organic cations to provide charge compensation [Cambridge Structural Database (CSD), Version 5.43, last update September 2022; Groom et al., 2016]. They include a number of layer-like structures that have been described as organicinorganic two-dimensional hybrid perovskites. The structure, properties and applications, especially optoelectronic applications, of such compounds have been reviewed recently by Zhu and collaborators (Zhang et al., 2020).
Beatty and collaborators (Costin-Hogan et al., 2008) reported on a number of complexes formed by the reaction of ortho-substituted phenylamines with cadmium halide salts. They showed, for example, that the reaction of an acidified solution in methanol of CdCl 2 with aniline led to the formation of the [Cd 3 Cl 10 ] 4linear tetra-anion in the compound poly-[tetrakis (anilinium) [decachlorotricadmium(II)]] (CSD refcode EGUFUI). In the present work an analogous reaction has been studied using a para-substituted derivative of aniline, 4-methylaniline. The resulting structure of the title compound, (I), is isostructural with that of EGUFUI.
A search of the CSD for polymeric compounds involving the title cation, 4-methylanilinium, gave only four hits. One in particular is of interest, namely bis(4-methylanilinium) pentamolybdate (YIKLIP; Oszajca et al., 2013), whose structure was determined by powder X-ray diffraction analysis. It is composed of layers of inorganic {[Mo 5 O 16 ] 2-} n polyanions alternating with layers of 4-methylanilinium cations. The latter are linked to the inorganic polyanions by N-HÁ Á ÁO hydrogen bonds, involving both terminal and shared O atoms.

Structural commentary
The asymmetric unit of the title compound, {[Cd 3 Cl 10 ] 4À Á4[(C 7 H 10 N) + ]} n (I), is composed of half of a centrosymmetric [Cd 3 Cl 10 ] 4tetra-anion, with the central Cd2 atom being situated on a crystallographic inversion centre, and two 4-methylanilinium cations (Fig. 1). The complete [Cd 3 Cl 10 ] 4unit is made up of three face sharing CdCl 6 octahedra. They are linked by four corner Cl À ions (Cl2, Cl2 i , Cl2 iv and Cl2 v ; Fig. 1) to form a layer-like structure lying parallel to the ab plane ( Figs. 2 and 3). The octahedral environment of atom Cd1 is slightly distorted with one short contact to a terminal Cl atom (Cl1) of 2.5051 (5) Å , while the other five Cd-Cl bond lengths vary from 2.6329 (5) to 2.7220 (5) Å . The Cd2-Cl bond lengths vary from 2.5764 (5) to 2.6750 (4) Å ( Table 1). The cadmium atoms are separated by 3.4082 (2) Å . These bond lengths and the metalÁ Á Ámetal distance are very similar to those observed for the three compounds involving cadmium chloride mentioned below in x 6. Database survey.    Symmetry code: (i) Àx þ 3 2 ; y À 1 2 ; z.

Figure 1
A view of the structure of the polymeric unit of compound I. The displacement ellipsoids are drawn at the 50% probability level.

Figure 3
A view along the b-axis of the crystal packing of compound I. The N-HÁ Á ÁCl hydrogen bonds (see Table 2) are shown as dashed lines. For clarity, the C-bound H atoms have been omitted. Colour code as in Fig. 2.

Supramolecular features
In the crystal of I, the p-methylanilinium cations that are situated between the layers are N-HÁ Á ÁCl hydrogen bonded to the front and back of the layers that stack up the c-axis (Fig. 3). All six ammonium H atoms are involved in hydrogen bonding with all five chloride ions (Table 2). However, there are no identifiedor C-HÁ Á Á interactions involving the aromatic rings (C1-C6 and C8-C13) of the p-methylanilinium cations. The rings are inclined to each other by 42.3 (1) in the asymmetric unit and by ca 71.8 and 73.8 , respectively, to the ab plane in which lies the anionic {[Cd 3 Cl 10 ] 4À } n layer-like structure.

Thermal analyses
Differential thermal analysis (DTA) and thermogravimetric analysis (TGA) were recorded in the temperature range 25-650 C, at a heating rate of 10 C min À1 under a nitrogen atmosphere, using an SDT Q600 simultaneous thermo analytical system. The alumina crucible was loaded with 6.191 mg of compound I. It can be seen in the TGA and DTA curves for I (Fig. 4), that the sample begins to decompose before reaching the melting point. In the TGA curve, the first weight loss (198-216 C) is due to the loss of two methylanilinium cations and two chloride anions: calculated 25.5%, observed 24.8%. The second weight loss (216-250 C) is due to the loss of the two remaining methylanilinium cations: calculated 19.2%, observed 18.2%. The third weight loss (553-559 C) involves the loss of two equivalents of HCl: calculated 6.3%, experimental 6.3%. The residual cadmium chloride (CdCl 2 ) begins to evaporate at 559 C as observed from the DTA curve ( Fig. 4), and it continues slowly up to 650 C. There is a small residue (0.83%) remaining at 650 C.

FT-IR and FT-Raman spectroscopy
A Perkin Elmer-paragon-500 Fourier transform infrared (FT-IR) was used to record the FT-IR spectrum (KBr pellet) in the wavelength range of 450-4000 cm À1 . A Varian FT-Raman spectrometer was used to record the FT-Raman spectrum in the wavelength range 400-4000 cm À1 .
The FT-IR and FT-Raman spectra of I are illustrated in Fig. 5, and the assignment of the vibrational frequencies are presented in Table 3. The intermolecular N-HÁ Á ÁCl stretching vibration is observed at 3129 cm À1 (FT-IR) and 3126 cm À1 (FT-Raman) (Haigh et al., 1967). The band at 637 cm À1 in the FT-IR and 638 cm À1 in the FT-Raman corresponds to the NH 2 twisting frequency. The asymmetric NH stretching frequency is observed at 3510 cm À1 in the FT-IR spectrum. The peak at 1243 cm À1 in the FT-IR spectrum is due to the C-N vibration. The frequencies of the FT-IR spectrum agree well with the corresponding values of the FT-Raman spectrum and also when compared with those of pmethylaniline (Altun et al., 2003).  The TGA (blue) and DTA (black) curves for I.

Database survey
All four compounds crystallize at room temperature in the orthorhombic space group Pbca, as does the title compound (I). Hence, all five compounds are isostructural. As noted by Gagor et al. (2011) and Liao et al. (2014), some layered organic-inorganic hybrids have been shown to show reversible structural phase transitions because cooling and heating can induce reorientation of the organic cations and deformation of the anionic framework. Such changes were observed for compounds IPEMASS01 and QOHGUR, which undergo two phase transitions. At low temperature they transform into the non-centrosymmetric orthorhombic space group P2 1 2 1 2 1 [structures IPEMAS02 (275 K) and QOHGUR01 (93 K)], while at high temperature they transform to the centrosymmetric orthorhombic space group Cmca [structures IPEMAS (320 K) and QOHGUR02 (343 K)]. As reported by Gagor et al. (2011), the transition from Pbca to P2 1 2 1 2 1 is type I: translationengleiche; the crystal class changes from mmm to 222. The change from Cmca to Pbca is type IIA: klassengleiche; the crystal class does not change (mmm to mmm). For further details concerning subgroups and supergroups of space groups, see Mü ller (2013).

Synthesis and crystallization
Concentrated HCl (1 ml) was added dropwise to a mixture of cadmium chloride dihydrate (1 g, 0.009 mol) and p-methylaniline (1.71 g, 0.009 mol) in methanol (30 ml) until the solution was clear. The solution was then stirred and heated under reflux at 353 K for 6 h and filtered. The solution was allowed to evaporate slowly at room temperature, yielding small, orange block-like crystals of I after ca 21 days.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 5. The ammonium H atoms were located in difference-Fourier maps and freely refined. The Cbound H atoms were included in calculated positions (C-H = 0.95 Å ) and treated as riding atoms with U iso (H) = 1.2U eq (C). The average hkl measurement multiplicity was low, hence an empirical absorption correction was applied.  Table 3 Assignment of FT-IR and FT-Raman vibrational frequencies (cm À1 ) for I and p-methylaniline.

Poly[tetrakis(4-methylanilinium) [octa-µ-chlorido-dichloridotricadmium(II)]]
Crystal data (C 7  Hydrogen site location: mixed H atoms treated by a mixture of independent and constrained refinement w = 1/[σ 2 (F o 2 ) + (0.019P) 2 ] where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 0.22 e Å −3 Δρ min = −0.34 e Å −3 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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq