New refinement of the crystal structure of Zn(NH3)2Cl2 at 100 K

The crystal structure of Zn(NH3)2Cl2 was redetermined at low temperature, revealing the positions of the hydrogen atoms.


Chemical context
Zn(NH 3 ) 2 Cl 2 is found in discharged zinc-air batteries. It is formed by dissolution of the zinc electrode in an ZnCl 2 -NH 4 Cl electrolyte. At high Zn 2+ concentrations, the cations subsequently undergo complex formation with NH 3 and Cl À (Clark et al., 2017). Moreover, in the last century, Zn(NH 3 ) n Cl 2 phases with n = 0.167-6 have been intensively investigated since these materials appear to be by-products of hydrocracking of heavy-oil fractions. The monoammine salt with n = 1 and the diammine salt with n = 2 are the most studied members in this family. Thermal stability, thermomechanical and thermogravimetric behaviour are well documented for these systems (Gardner et al., 1989).
The crystal-structure model of Zn(NH 3 ) 2 Cl 2 was to this day incomplete, lacking the positions of hydrogen atoms. The crystal structure of Zn(NH 3 ) 2 Cl 2 was originally determined by MacGillavry & Bijvoet (1936) without refinement of atomic positions [COD number ID 1010197; ICSD number 26136; PDF4 number 01-074-0506]. This incompleteness was rather normal considering the X-ray techniques and measurement conditions at that time. An improved structure model was obtained decades later from a room-temperature data set collected on a four-circle diffractometer by Yamaguchi & Lindqvist (1981), however without hydrogen-atom positions [PDF4 number 04-015-4717]. One more record of this phase in PDF4 [number 00-024-1435] can be found, however with a very limited X-ray powder diffraction pattern range from 5-63 /2, which is unsuitable for Rietveld refinement.
Large single crystals of Zn(NH 3 ) 2 Cl 2 were obtained as a side product of a chemical vapour transport (CVT) reaction, which allowed the reinvestigation of the crystal structure at low temperature with a particular focus on determination of the H-atom positions.

Structural commentary
As shown in Fig. 1, the new structure refinement of Zn(NH 3 ) 2 Cl 2 (100 K data) confirms the former model by Yamaguchi & Lindqvist (1981). The small shrinkage of the unit cell at low temperature is mainly due to lattice dynamics. However, the bond lengths within the Zn(NH 3 ) 2 Cl 2 tetrahedron refined from 100 K data are similar to the ones obtained from room-temperature data, but with higher precision. This confirms that the tetrahedron is a very rigid building block of the structure (Fig. 2). Two independent H atoms, one in a general position (H1A) and one on a mirror plane (H1B), bond to the nitrogen atom with refined bond lengths of N1-H1A = 0.825 (17) Å and N1-H1B = 0.74 (4) Å , respectively. Hydrogen-bonding interactions of weak-to-medium strengths (Table 1) between the NH 3 group and the Cl ligands of adjacent tetrahedra lead to formation of a three-dimensional network, as shown in Fig. 3.

Figure 1
The tetrahedral [Zn(NH 3 ) 2 Cl 2 ] group. H atoms (cyan circles) are plotted with arbitrary size. [Symmetry code: (A) 1 À x, 1 2 À y, z.]. the thermal stability of Zn(NH 3 ) 2 Cl 2 is limited as it already decomposes at 463 K (Gardner et al., 1989), it is very likely that Zn(NH 3 ) 2 Cl 2 crystallized at the very end of the cooling process.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. Unlike the two previous refinements in space group Imam, the unit cell was chosen to be in the standard setting (Imma) for space group No. 74. Three reflections (020, 002, 004) affected by the incident beam-stop were omitted. The two H atoms were refined freely. CrysAlis PRO (Rigaku OD, 2019); program(s) used to solve structure: SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2018 (Sheldrick, 2015b); molecular graphics: DIAMOND (Brandenburg, 2006); software used to prepare material for publication: publCIF (Westrip, 2010).

Diamminedichloridozinc
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.