Syntheses and crystal structures of a new family of hybrid perovskites: C5H14N2·ABr3·0.5H2O (A = K, Rb, Cs)

The isostructural title compounds are a new family of hybrid perovskite hemihydrates.


Chemical context
Oxide perovskites of generic formula ABO 3 , where A and B are metal ions with a combined charge of +6, are probably the most-studied family of inorganic phases on account of their numerous physical properties and structural variety (Tilley, 2016). The aristotype (highest-possible symmetry) (Megaw, 1973) for this classic structure type is a three-dimensional network (space group Pm3m) of undistorted, vertex-sharing, BO 6 octahedra encapsulating the A cations in 12-coordinate dodecahedral cavities bounded by eight octahedra but lower symmetry ('hettotype') structures are very common, which can be systematically described in terms of tilting schemes of the octahedra (Woodward, 1997).
In this paper we describe the syntheses and structures of a new family of isostructural hybrid perovskite hemihydrates of formula C 5 H 14 N 2 ÁABr 3 Á0.5H 2 O (C 5 H 14 N 2 2+ = 1-methyl piperizinium cation) where A = K (I), Rb (II) and Cs (III).

Structural commentary
Compounds (I), (II) and (III) are isostructural as indicated by their orthorhombic unit cells, showing the expected trend of volume increase as a result of the increasing ionic radius (Shannon, 1976) of the alkali-metal cation on going from potassium (r = 1.52) to rubidium (r = 1.66) to caesium (r = 1.81 Å ). This description will focus on the structure of (I) and note significant differences for (II) and (III) where applicable.
The asymmetric unit of (I) (Fig. 1) contains two methylene groups (C1 and C2), an N1H 2 + grouping, an N2H + moiety and the carbon atom (C3) of a methyl group. N1, N2 and C3 lie on a (010) crystallographic mirror plane with y = 1 2 for the asymmetric atoms. The complete C 5 H 14 N 2 2+ cation is generated by the mirror to result in a typical (Dennington & Weller, 2018) chair conformation for this species with N1 and N2 deviating from the C1/C2/C1 i /C2 i [symmetry code: (i) x, 1 -y, z] plane by À0.623 (7) and 0.708 (6) Å , respectively. The pendant C3 methyl group adopts an equatorial orientation with respect to the ring. A water molecule with the O atom lying on the ( 1 2 , 1 2 , z) special position with mm2 symmetry (Wyckoff site 2a) is also present.

Supramolecular features
The linkage of the KBr 6 octahedra in (I) in the x, y and z directions through their bromide-ion vertices leads to an infinite network of corner-sharing KBr 6 octahedra akin to the network of BO 6 octahedra in the classical ABO 3 perovskite structure. Key features of the inorganic network are the K-Br-K bond angles (Table 1), with Br1 substantially bent from the nominal linear bond [K1-Br1-K1 ii = 157.98 (5) ; symmetry code (ii) x, 1 2 + y, z À 1 2 ], but Br2 and Br3 far less so. When the structure of (I) is viewed down [011], alternating (100) layers of K1-and K2-centred octahedra are apparent (Fig. 2). Within these (100) planes, the K1 atoms are linked by the Br1 ions and the K2 atoms are liked by the Br2 ions. Finally, Br3 provides the inter-layer linkages in the [100] direction.

Database survey
The title compounds and their significant analogue structures with their space groups and CCDC refcodes (Groom et al., 2016) are listed in Table 7. These compounds now represent a significant family of hybrid perovskites featuring several different cations -the protonated forms of piperazine, dabco,   physical properties such as ferroelectricity, which is of course a classic characteristic of oxide perovskites. An interesting structural comparison may be made between MEXMAG (an 'anhydrous' RAX 3 hybrid perovskite), (I) (an RAX 3 Á0.5H 2 O hybrid perovskite hemihydrate) and GUYMIX (an RAX 3 ÁH 2 O hybrid perovskite hydrate) (Fig. 3). It may be seen that the pendant methyl groups of the C 5 H 14 N 2 2+ cations in (I) both point towards an empty square site and their steric bulk presumably prevents water molecules from occupying that site. It is notable that the empty square site in (I) is associated with the reduced K1-Br1-K1 bond angles as noted above. Conversely, in MEXMAG, the iodide ions are perhaps too large to allow a water molecule to fit between them and the piperazinium cation is forced to form long N-HÁ Á ÁI hydrogen bonds (HÁ Á ÁI = 3.14 Å ) rather than N-HÁ Á ÁO w (w = water) links.

Synthesis and crystallization
To prepare (I), 0.3673 g (3.67 mmol) of 1-methyl piperazine and 0.4068 g (3.42 mmol) of KBr were added to 15.0 ml of 1.0 M aqueous HBr solution to result in a clear solution, which was left in a Petri dish to evaporate. After two or three days, colourless blocks of (I) were recovered, rinsed with acetone and dried in air. Compound (II) was prepared in the same way, with 0.4042 g (2.44 mmol) of RbBr replacing the KBr in (I) and (III) was prepared by using 0.4479 g (2.10 mmol) of CsBr in place of the KBr.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 8. For each structure, the N-and Cbond hydrogen atoms were located geometrically (C-H = 0.98-0.99, N-H = 0.91-1.00Å ) and refined as riding atoms. The water H atom was located in a difference map and refined as riding in its as-found relative position. The constraint U iso (H) = 1.2U eq (carrier) or 1.5U eq (methyl C) was applied in all cases.   used to refine structure: SHELXL2014/7 (Sheldrick, 2015); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012); software used to prepare material for publication: publCIF (Westrip, 2010).

Poly[1-methylpiperizine-1,4-diium [tri-µ-bromido-potassium] hemihydrate] (I)
Crystal data (C 5  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.