Crystal structure of a new hybrid compound based on an iodidoplumbate(II) anionic motif

The inorganic part of the crystal structure of the 1-D hybrid compound (C4N2H12)2[PbI5]I·H2O contains corner-sharing [PbI6]4− octahedra running as zigzag chains along the a axis. The organic (piprazineH2)2+ cations are lodged around the anionic framework. Water molecules and isolated iodine ions play an important role in the structure connectivity.


Chemical context
Organic-inorganic hybrid materials offer the opportunity to combine the desirable properties of the organic moiety such as processability, toughness and impact strength with the typical properties of the inorganic part such as high temperature stability and durability. The opto-electronic characteristics of hybrid materials are closely related to the metal cluster size. In recent years, a significant number of organic-inorganic hybrid materials based on lead halide units have been prepared and studied Rayner & Billing, 2010), in particular with self-organized low-dimensional families of lead iodide-based crystals where the [PbI 6 ] octahedra form one-, two-or three-dimensional networks (Elleuch et al., 2007;Trigui et al., 2011). In one-dimensional lead halide hybrid compounds, the inorganic chains may be formed by one, two or three bridging halides, referred to as corner-, edge-and face-sharing polyhedra, respectively. Thanks to their anticipated electroluminescence, photoluminescence and nonlinear optical properties, these compounds are the most desired ones . Lead iodide-based hybrid materials are studied extensively for their excitonic and magneto-optical properties. In this work we report the synthesis and crystal structure determination of a new lead iodide hybrid, (C 4 N 2 H 12 ) 2 [PbI 5 ]ÁIÁH 2 O, (I).

Structural commentary
The structural units of (I) consist of one piperazine molecule, one water molecule, one isolated iodine and one [PbI 6 ] unit ISSN 2056-9890 ( Fig. 1). The electrical neutrality is ensured by two organic molecules of doubly protonated piperazine.
The main part of the inorganic moiety is composed by the lead Pb 2+ cation which adopts a distorted octahedral coordination. The angles between cis-related I À ions range from 85.022 (12) to 96.89 (3) at most, whereas the trans angles deviate from 180 by 12.95 (3) ( Table 1). Two adjacent corners connect the [PbI 6 ] octahedron to its neighbours, leading to zigzag chains running parallel to the a axis (Fig. 2). This one-dimensional anionic network leaves empty spaces in which the organic cations are located. The [PbI 6 ] octahedra establish two strong hydrogen bonds (Table 2), N2-H4NÁ Á ÁI3 and N2 i -H4N i Á Á ÁI3, via the I3 corners [symmetry code: (i) x, 1 2 À y, z] as illustrated in Fig. 3.

Figure 3
Linkage around one [PbI 6 ] octahedron formed by two similar octahedra and two protonated piperazine cations. Hydrogen bonds are drawn as dashed green lines. [Symmetry codes: The six-membered piperazinium cation ring adopts a chair conformation. It interacts with the inorganic chain via strong N2-H4NÁ Á ÁI3 hydrogen bonds with a 2.85 Å bond length (Table 2 and Fig. 6). In the crystal structure, the piperazinium cations are also linked to the water molecule by an N1-H1NÁ Á ÁOW iii hydrogen bond and to the iodine anion by N1-H2NÁ Á ÁI5 iii hydrogen bonds.
Compared to its homologous hybrids, the structure of the title compound exhibits an original arrangement of the inorganic layers. It is composed by two parts: the first are the [PbI 6 ] octahedra sharing adjacent corners and so assembling into chains running along the [100] direction. The second original feature is the structural cohesion by water molecules and isolated iodide anions. This structural arrangement will probably have an impact on the dielectric behavior of the material. Luminescence and UV-visible spectroscopy measurements of this compound, coupled to theoritical calculation of the Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO) electronic transitions are in progress.
As shown in Fig. 7, the structure of (I) is self-assembled into alternating organic and inorganic layers parallel to the ac plane. The organic part is made up of (C 4 H 12 N 2 ) 2+ cations located in the voids around the corner-sharing [PbI 6 ] 4À octahedra. The iodine anions and the water molecules connect the organic and inorganic sheets by strong hydrogen-bond interactions. The hydrogen bonding environment of the cation of the title compound.

Figure 7
A packing diagram of (I), viewed along the a axis showing the alternating organic and inorganic layers. Hydrogen bonds are omitted for clarity.

Figure 5
Water molecule hydrogen bonding interactions in (I). [Symmetry codes:

Database survey
Using the piperazine-1,4-diium cation scheme in the similarity option of the WEBCSD interface (Groom & Allen, 2014), more than 90 records are found in the CCDC database. Only 24 are inorganic-organic hybrid compounds with several metals Cu, Zn, Co, Bi, Cd, Sb, Au etc. The closest chemical composition found is a bismuth-based compound (II): (C 4 N 2 H 12 ) 2 [BiCl 6 ]ÁClÁH 2 O (Gao et al., 2011). In spite of the chemical formula similarity, it seems that the orthorhombic (Pnma) title structure is much more regular than the monoclinic (P2 1 /c) compound (II) with approximately the same cell volume, where the small difference is probably due to the chlorine/iodine substitution. In contrast to the structure of (I), the anionic network in the structure of (II) is 0-D, built up by isolated [BiCl 6 ] octahedra. The water molecule and the isolated halogen play, in both cases, the same crucial role in the structural cohesion, linking the anionic part to the organic moieties.

Synthesis and crystallization
Crystals of the title compound were prepared by slow evaporation at room temperature by mixing 1,4-diazacyclohexane (C 4 H 10 N 2 ) (2 mol) with a solution of lead iodide PbI 2 (1 mol) in an equimolar mixture of ethanol and DMF. After several weeks, the obtained crystals were isolated and dried.