Hybrid organic–inorganic crystal structure of 4-(dimethylamino)pyridinium dimethylammonium tetrachloridolead(II)

The crystal structure of this hybrid organic–inorganic material exhibits chains of lead chloride capped by 4-(dimethylamino)pyridinium and dimethylammoium by hydrogen bonding. This creates a one-dimensional zipper-like structure down the a axis.


Chemical context
Hybrid organic-inorganic materials have been gaining attention due to their interesting optical properties and for their applications as semiconductors (Dobrzycki & Woźniak, 2008). Early materials such as lead halogen perovskites have been identified as having intrinsic white-light emission (Dohner et al., 2014) and as an inexpensive and high conversion material for solar cells (Baikie et al., 2013;Zhao & Zhu, 2014). By changing the size and structure of the organic portions of these materials, one can begin to assess the impact of these groups on the resulting crystal structures (Gillon et al., 2000). For organic cations containing groups capable of hydrogen bonding, these interactions may form the basis for deliberate crystal engineering of next generation materials. Herein we report the structure of a new hybrid organic-inorganic material that contains 1-D lead chloride chains capped by dimethylammonium (DMA) and 4-(dimethylamino)pyridinium (4DAP) groups through extensive hydrogenbonding interactions.

Structural commentary
This structure crystallizes in the centrosymmetric space group C2/c with half of a molecule of DMA, half of a [PbCl 4 ] 2À anion, ISSN 2056-9890 and half of a molecule of 4DAP in the asymmetric unit, shown in Fig. 1. The point groups of the two cations are C 2 . The lead metal center, with its six chloride ligands, exhibits a slightly distorted octahedral coordination geometry (formally C 2 symmetry). Of the two crystallographically unique Cl atoms, Cl1 and its symmetry equivalent produced through a C 2 rotation are the two terminal atoms [Pb-Cl = 2.8499 (4) Å ]. The other crystallographically unique Cl atom, Cl2, produces the four bridging atoms through a C 2 rotation and inversion operation that results in an additional two unique Pb-Cl bonds [2.9015 (5) and 2.9041 (5) Å ]. These bridging ligands form one-dimensional chains of [PbCl 4 ] 2À anions which extend approximately along the [001] direction. The net negative two charge of the lead chloride anion is balanced by the positive charges of the DMA and 4DAP cations.

Supramolecular features
The one-dimensional lead chloride chains are capped in the [010] direction by 4DAP and DMA molecules via hydrogen bonds (Table 1), which form between the NH groups on the 4DAP and DMA molecules and the terminal chloride ligands on each lead atom. The hydrogen-bond donor on the 4DAP forms a hydrogen bond with each of the two terminal chlorides on the nearest Pb atom, while each donor on the DMA forms a hydrogen bond with one terminal chloride on Pb atoms that are separated in the chain by one additional Pb. The hydrogen-bonding network of the title structure is shown in  Hydrogen-bonding network in a single PbCl 4 chain viewed down [100]. Displacement ellipsoids are drawn at the 50% probability level. Atom colors: carbon (gray), nitrogen (blue), hydrogen (white), lead (dark blue) and chlorine (green).

Database survey
The crystal structure of [PbCl 4 ] 2À and dimethylbenzene-1,4 diaminium was reported in 2008 (Dobrzycki & Woźniak, 2008). This compound crystallizes in the centrosymmetric space group P2 1 /n, and the structure contains two-dimensional lead chloride sheets that run parallel to [001]. Like in the title structure, each Pb center possesses approximately octahedral symmetry (formally C 1 ) and is coordinated to two terminal and four bridging chloride atoms. Two of the bridging atoms are crystallographically unique and each give rise to a symmetry-related atom to yield four bonds to the Pb center (Pb-Cl2 = 2.945 and 2.927 Å ; Pb-Cl3 = 3.095 and 2.765 Å ).
In this compound, hydrogen bonding occurs between the terminal chloride ligands and the protons on the diaminium groups of the organic cation, as shown in Fig. 4. Both structures are similarly charge-balanced in that the respective anionic PbCl 4 sheet or chain is balanced by the positive charge from organic cation molecules. The major difference between these two compounds is that the title structure contains onedimensional chains while this structure contains two-dimensional sheets.

Synthesis and crystallization
To a 20 mL scintillation vial was added 4DAP (1.008 g, 8.25 Â 10 À3 mol) and concentrated hydrochloric acid (2 ml) creating an acidic solution. To a 23 mL screw-top thick-walled vial was added PbCl 2 (0.5040 g, 1.81 Â 10 À3 mol), DMF (1 ml), and the 4DAP acid solution (1 ml). The thick-walled vial was placed in the oven for 11 days at 373 K. Clear, colorless crystals of the title compound that were suitable for single-crystal X-ray diffraction were obtained. The DMA is present in the lattice due to the in situ degradation of DMF, which can occur at the reaction temperature (Burrows et al., 2008).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The position of the ammonium hydrogen atom was determined from the difference-Fourier map, and all other hydrogen atoms were placed in idealized positions with bond lengths set to 0.98 Å for alkyl C-H protons, 0.95 Å for aliphatic C-H protons, and 0.88 Å for the pyridinium proton. These hydrogen atoms were refined using a riding model with U iso (H) = 1.2 U eq (N,C) for all N-H and aliphatic protons and 1.5 U eq (C) for methyl group protons. To appropriately model the ammonium hydrogen atom, which is complicated by the site of symmetry on which the nitrogen atom resides, the distance between the hydrogen atom and its symmetry-equivalent was restrained to 1.4 (2) Å . No other constraints were applied to the refinement model.

4-Dimethylaminopyridinium dimethylammonium tetrachloridolead(II)
Crystal data 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 )