Crystal structure of non-centrosymmetric bis(4-methoxybenzylammonium) tetrachloridozincate

The crystal structure of the new non-centrosymmetric organic–inorganic hybrid salt (C8H12NO)2[ZnCl4], consists of 4-methoxybenzylammonium cations sandwiched between tetrachloridozincate anionic layers running parallel to the ac plane.


Chemical context
Non-linear optical (NLO) materials have received much attention in different research areas due to their potential applications in high-density optical data storage, electrooptical shutters, optical communication and signal processing (Maury & Le Bozec, 2005;Green et al., 2011;Evans & Lin, 2002). Mostly connected in the past to a few families of inorganic materials, the research was then extended to organic materials, generally salts of amino acids with organic acids, which are expected to have relatively strong NLO properties due to delocalized electrons at -* orbitals. More recently, organic-inorganic hybrid materials showing non-centrosymmetric structures started gaining attention in the field, since they are expected to offer enhanced properties, such as second harmonic generation efficiency, by combining the characteristic features of both organic and inorganic moieties. These materials are usually constituted by the crystal packing of inorganic anions (typically halogenidometalates) and organic ammonium cations ensured by hydrogen bonds and Coulombic interactions (Brammer et al., 2002). Herein we report the synthesis and crystal structure of a new organicinorganic hybrid compound, bis(4-methoxybenzylammonium) tetrachloridozincate. This salt crystallizes in a non-centrosymmetric space group and hence could be a potential candidate for second order non-linear optical properties.

Structural commentary
The asymmetric unit of the crystal structure consists of an isolated tetrachloridozincate anion, [ZnCl 4 ] 2À and two 4-methoxybenzylammonium cations, (C 8 H 12 NO) + , as shown in Fig. 1. One of the cations shows positional disorder of the methyleneammonium moiety. The lengths of the C-C, C-N and C-O bonds in the two independent 4-methoxybenzylammonium cations are in accordance with corresponding distances found in the literature (Groom et al., 2016). The Zn II atom is tetrahedrally coordinated by four chloride ligands with Zn-Cl bond lengths ranging from 2.249 (2) to 2.289 (2) Å and Cl-Zn-Cl bond angles varying between 107.25 (8) and 112.41 (10) .

Supramolecular features
The crystal structure consist of 4-methoxybenzylammonium cations sandwiched between tetrachloridozincate layers extending parallel to the ac plane, as shown in Fig. 2. The cationic units are linked into a two-dimensional network by weak C-HÁ Á Á interactions (Table 1). The crystal packing is assured by a complex hydrogen-bonding system, mostly involving the positively charged ammonium groups and the chloride ligands of the isolated tetrahedral [ZnCl 4 ] 2À units (Table 1), which reinforce the Coulombic interactions, as depicted in Fig. 3. Whereas the N2 atom is blocked by a very efficient hydrogen-bonding system involving five donorÁ Á Áacceptor distances ranging from 3.279 (8) to 3.452 (7) Å , the N1 ammonium group is disordered over two sets of sites as a consequence of a less efficient hydrogen bonding. The asymmetric unit of the title compound with displacement ellipsoids drawn at the 50% probability level. Only the major component of the disordered methyleneammonium group is shown for clarity.

Figure 2
Packing diagram of the title compound viewed along the b axis, showing the alternate stacking, along the c axis, of organic and inorganic layers.

Synthesis and crystallization
Single crystals of (C 8 H 12 NO) 2 [ZnCl 4 ] were synthesized starting from 4-methoxybenzylamine (Sigma-Aldrich, 98%), zinc chloride (Sigma-Aldrich, 98%) and HCl (37%), which were weighted in the stoichiometric proportion conforming to the equation reaction: 2 C 8 H 11 NO + 2 HCl + ZnCl 2 ! (C 8 H 12 NO) 2 [ZnCl 4 ] After mixing the reagents in 50 ml of water and stirring at room temperature for more 3 h, the resulting solution was placed in a Petri dish and allowed to evaporate slowly. Single crystals suitable for X-ray diffraction were obtained within a week (yield: 75%).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The crystals of bis(4-methoxybenzylammonium) tetrachloridozincate were systematically affected by non-merohedral polar twinning. The ratio of the twin components of the crystal selected for X-ray analysis was refined to 0.738 (2):0.262 (2). One methyleneammonium group was found to be disordered over two sets of sites with a refined occupancy ratio of 0.52 (2):0.48 (2). During the refinement of the disordered group, the C-C and C-N bond lengths were constrained to be 1.50 (2) and 1.47 (1) Å , respectively. EADP and ISOR restraints (Sheldrick, 2015b) were also applied. All H atoms were placed geometrically and refined using a riding-model approximation, with C-H = 0.93-0.97 Å , N-H = 0.89 Å , and with U iso (H) = 1.2U eq (C) or 1.5U eq (C, N) for methyl and ammonium H atoms, for which a rotating model was applied.    (Keller, 1999); software used to prepare material for publication: SHELXL2014 (Sheldrick, 2015b).

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. Refinement. Refined as a 2-component twin.