Syntheses and structures of piperazin-1-ium ABr2 (A = Cs or Rb): hybrid solids containing ‘curtain wall’ layers of face- and edge-sharing ABr6 trigonal prisms

The isostructural title compounds feature striking layers of ABr6 (A = Cs, Rb) trigonal prisms sharing faces and edges.


Chemical context
Oxide perovskites of generic formula ABO 3 , where A and B are metal ions, have been studied for decades because of their physical properties and structural variety (Tilley, 2016). The aristotype (highest-possible symmetry) for this familiar structure type is a cubic network (space group Pm3m) of vertex-sharing, regular, BO 6 octahedra encapsulating the A cations in 12-coordinate cavities bounded by eight octahedra, but lower symmetry structures are very common (Woodward, 1997). More recently, 'hybrid' RMX 3 perovskites containing organic cations and MX 3 (M = Pb, Sn . . . ; X = halide ion) octahedral networks have attracted intense interest because of their remarkable photophysical properties (Xu et al., 2019;Stylianakis et al., 2019;Zuo et al., 2019). A number of different organic cations occur in these hybrid structures, one of which is the doubly protonated C 4 H 12 N 2 2+ piperizinium (or piperazin-1,4-diium) ion as found in the C 4 H 12 N 2 ÁACl 3 ÁH 2 O (A = K, Rb, Cs) family (Paton & Harrison, 2010) and C 4 H 12 N 2 ÁNaI 3 (Chen et al., 2018).
As an extension of these studies, we now describe the title hybrid compounds, containing the singly protonated C 4 H 11 N 2 + piperazin-1-ium cation, which have a generic formula of RMX 2 and totally different crystal structures to RMX 3 hybrid perovskites. ISSN 2056-9890 2. Structural commentary Compounds (I) and (II) are isostructural and crystallize in the orthorhombic space group Pbcm. The smaller unit-cell volume (by 5.3%) of (II) presumably reflects the smaller ionic radius (Shannon, 1976) of the Rb + cation (r = 1.66 Å ) compared to Cs + (r = 1.81 Å ). This structure description will focus on (I) and note significant differences for (II) where applicable.
The asymmetric unit of (I) consists of two methylene groups, an NH group and an NH 2 + group; both nitrogen atoms and their attached H atoms lie on a (001) crystallographic mirror plan (at z = 1/4 for the asymmetric atoms). The structure is completed by a caesium atom [site symmetry m(001), Wyckoff site 4d] and two bromine atoms: Br1 [m(001); 4d] and Br2 (2[100]; 4c). The structure of (I) is shown in (Fig. 1).
The complete C 4 H 11 N 2 + cation is generated by reflection to result in a typical (Brü ning et al., 2009) chair conformation for the ring: N1 and N2 deviate from the mean plane of C1/C2/ C1 i /C2 i [symmetry code: (i) x, y, 1 2 À z] by 0.656 (5) and À0.682 (4) Å , respectively. The H atom of the neutral N2-H3N group has an equatorial orientation with respect to the ring.
The caesium coordination polyhedron in (I) is completed by crystal symmetry, resulting in a distinctive CsBr 6 trigonal prism ( Fig. 1): the prism has longitudinal (001) mirror symmetry, with the Br1 atoms and the metal atom lying on the mirror. The mean Cs-Br bond length based on four distinct Cs-Br bonds (Table 1) is 3.573 Å [mean Rb-Br bond length for (II) = 3.461 Å ; Table 2]. These data may be compared with the shortest Cs-Br separation of 3.716 Å in CsBr (8-coordinate caesium chloride structure) and the shortest Rb-Br separation of 3.427 Å in RbBr (6-coordinate rocksalt structure).
In (I), the prism ends (Br1/Br2/Br2 i and Br1 iii /Br2 ii /Br2 iii ; see Fig. 1 for symmetry codes) are parallel by symmetry and separated by 4.5787 (8) Å , i.e., the a unit-cell parameter, hence there is no twisting of the end faces and the BrÁ Á ÁBrÁ Á ÁBr angles vary from 56.65 (1)-61.68 (1) [the equivalent prismend separation for (II) is 4.4675 (13) Å ]. The caesium cation in (I) is not quite equidistant from the prism-ends mentioned in the previous sentence, being displaced from them by 2.3177 (6) and 2.2605 (5) Å , respectively. The equivalent data for the Rb atom in (II) are 2.2581 (9) and 2.2091 (9) Å , respectively. The bond-valence sum (BVS) for Cs1 (in valence units) using the formalism of Brese & O'Keeffe (1991) in (I) is 1.12 and the equivalent value for Rb1 in (II) is 0.95 (expected value in both cases = 1.00). This indicates that the bond valences of these cations are satisfied without notable underbonding or overbonding in these unusual coordination environments.

Supramolecular features
The extended structure of (I) is consolidated by hydrogen bonds (Fig. 2 Table 1 Selected bond lengths (Å ) for (I).

Figure 1
The asymmetric unit of (I) showing 50% displacement ellipsoids expanded to show the complete organic cation and the caesium coordination polyhedron. The N-HÁ Á ÁBr hydrogen bond is shown as a double-dashed line. The purple lines linking the bromine atoms emphasize the trigonal-prismatic shape of the CsBr 6 polyhedron. Symmetry codes: (i) x, y, 1 adjacent molecule links the organic cations into [100] chains with adjacent cations related by translation symmetry and the N1-H2NÁ Á ÁBr1 bond connects the organic cation to the inorganic network. The neutral N2-H3N moiety forms a bifurcated N-HÁ Á Á(Br2,Br2) hydrogen bond; the HÁ Á ÁBr contacts are long at 3.07 (3) Å but given their apparent role in bridging the (010) CsBr 2 layers we judge them to be structurally significant. The hydrogen-bonding scheme for (II) ( Table 4) is almost identical to that in (I).

Database survey
So far as we are aware, the RABr 2 topology of the title compounds is a novel one. A search of the Cambridge Structural Database (CSD, version 5.40, last update 19 May 2019; Groom et al., 2016) for the mono-protonated C 4 H 11 N 2 + cation returned 55 crystal structures but none of them bear a close resemblance to the title compound. As noted in the chemical context section, the doubly protonated C 4 H 12 N 2 2+ species occurs in several hybrid RMX 3 perovskites including C 4 H 12 N 2 ÁACl 3 ÁH 2 O with A = K (CSD refcode GUYMIX), Rb (GUYMOD) and Cs (GUYMUJ) (Paton & Harrison, 2010) and C 4 H 12 N 2 ÁNaI 3 (MEXMAG; Chen et al., 2018).

Synthesis and crystallization
Compound (I) was prepared by adding 0.213 g (1.0 mmol) of CsBr and 0.086 g (1.0 mmol) of piperazine to 11.0 ml (1.1 mmol) of a 0.1 M HBr solution in a Petri dish to result in a clear solution. Colourless rods of (I) formed after a few days as the water evaporated. Colourless rods of (II) were prepared in the same way, with 0.165 g (1.0 mmol) of RbBr replacing the CsBr. The quantity of acid appears to be critical to the syntheses of (I) and (II): smaller amounts lead to recrystallized CsBr and RbBr and larger amounts lead to different structures containing doubly protonated C 4 H 12 N 2 2+ cations.

Figure 3
Polyhedral view of part of an (010) layer of CsBr 6 trigonal prisms in (I).
were freely refined, for (II) they were refined as riding atoms in their as-found relative positions. The C-bound H atoms were placed geometrically (C-H = 0.99 Å ) and refined as riding atoms for both structures. The constraint U iso (H) = 1.2U eq (carrier) was applied in all cases. The displacement ellipsoids for the C and N atoms in (II) refined to somewhat elongated shapes suggestive of positional disorder of the C 4 H 11 N 2 + cations but attempts to model this did not lead to a significant improvement in fit.   used to refine structure: SHELXL2018 (Sheldrick, 2015); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012) and ATOMS (Shape Software, 2005); software used to prepare material for publication: publCIF (Westrip, 2010).

Poly[piperazin-1-ium [di-µ-bromido-rubidium]] (II)
Crystal data (C 4  where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.74 e Å −3 Δρ min = −0.47 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.