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

The crystal structure of [ZnCl₂(NH₃)₂], diamminedi­chlorido­zinc, was re-investigated at low temperature, revealing the positions of the hydrogen atoms and thus a deeper insight into the hydrogen-bonding scheme in the crystal packing. In comparison with previous crystal structure determinations [MacGillavry & Bijvoet (1936). Z. Kristallogr. 94, 249–255; Yamaguchi & Lindqvist (1981). Acta Chem. Scand. 35, 727–728], an improved precision of the structural parameters was achieved. In the crystal, tetra­hedral [Zn(NH₃)₂Cl₂] units (point-group symmetry mm2) are linked through N—H⋯Cl hydrogen bonds into a three-dimensional network.

1. Chemical context

Zn(NH₃)₂Cl₂ is found in discharged zinc–air batteries. It is formed by dissolution of the zinc electrode in an ZnCl₂–NH₄Cl electrolyte. At high Zn²⁺ concentrations, the cations subsequently undergo complex formation with NH₃ and Cl⁻ (Clark et al., 2017). Moreover, in the last century, Zn(NH₃)_nCl₂ 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(NH₃)₂Cl₂ was to this day incomplete, lacking the positions of hydrogen atoms. The crystal structure of Zn(NH₃)₂Cl₂ 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°/2θ, which is unsuitable for Rietveld refinement.

Large single crystals of Zn(NH₃)₂Cl₂ 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.

2. Structural commentary

As shown in Fig. 1, the new structure refinement of Zn(NH₃)₂Cl₂ (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(NH₃)₂Cl₂ 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 NH₃ group and the Cl ligands of adjacent tetra­hedra lead to formation of a three-dimensional network, as shown in Fig. 3.

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
N1—H1A⋯Cl1^i	0.825 (17)	2.806 (18)	3.4552 (9)	137.0 (17)
N1—H1A⋯Cl1^ii	0.825 (17)	2.897 (18)	3.5505 (9)	137.6 (16)
N1—H1B⋯Cl1^iii	0.74 (4)	2.95 (3)	3.5556 (10)	141 (1)
N1—H1B⋯Cl1^iv	0.74 (4)	2.95 (3)	3.5556 (10)	141 (1)
Symmetry codes: (i) -x+1, -y+1, -z+1; (ii) ; (iii) ; (iv) .

Figure 1 The tetra­hedral [Zn(NH3)2Cl2] group. H atoms (cyan circles) are plotted with arbitrary size. [Symmetry code: (A) 1 − x,  − y, z.].

Figure 2 Comparison of the [ZnN2Cl2] tetra­hedra from the room temperature data (left; Yamaguchi & Lindqvist, 1981) and the present 100 K data (right). Bond lengths for the room-temperature data were calculated with DIAMOND (Brandenburg, 2006).

Figure 3 Packing view along the b axis (left) and the c axis (right), showing N—H⋯Cl inter­actions between adjacent [Zn(NH3)2Cl2] tetra­hedra.

3. Synthesis and crystallization

Zn(NH₃)₂Cl₂ was obtained during the course of a chemical vapour transport experiment. Prior to use, a silica tube sealed at one end (Ø = 15 mm, wall thickness = 2 mm) was heated under vacuum to 1073 K overnight. Zinc oxide (317 mg) and previously dried ammonium chloride (190 mg) were pressed into pellets and put into the pretreated silica tube. After evacuation to 10⁻⁶ mbar, the tube was sealed and placed into a horizontal tube furnace. The temperature at the source was slowly heated to 1173 K and maintained at this value for the whole duration of growth. At the low-temperature zone (873 K), a colourless powder mixed with high-quality single crystals of Zn(NH₃)₂Cl₂ with an irregular morphology was collected after complete cooling to room temperature. Since the thermal stability of Zn(NH₃)₂Cl₂ is limited as it already decomposes at 463 K (Gardner et al., 1989), it is very likely that Zn(NH₃)₂Cl₂ crystallized at the very end of the cooling process.

4. 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.

Table 2
Experimental details

Crystal data
Chemical formula	[ZnCl2(NH3)2]
Mr	170.34
Crystal system, space group	Orthorhombic, Imma
Temperature (K)	100
a, b, c (Å)	7.7077 (2), 8.0226 (2), 8.4526 (3)
V (Å^3)	522.67 (3)
Z	4
Radiation type	Mo Kα
μ (mm^−1)	5.56
Crystal size (mm)	0.26 × 0.18 × 0.16
Data collection
Diffractometer	Rigaku SuperNOVA
Absorption correction	Analytical (CrysAlis PRO; Rigaku OD, 2019)
Tmin, Tmax	0.277, 0.454
No. of measured, independent and observed [I > 2σ(I)] reflections	2303, 708, 678
Rint	0.017
(sin θ/λ)max (Å^−1)	0.833
Refinement
R[F^2 > 2σ(F^2)], wR(F^2), S	0.015, 0.036, 1.04
No. of reflections	708
No. of parameters	25
H-atom treatment	All H-atom parameters refined
Δρmax, Δρmin (e Å^−3)	0.50, −0.55
Computer programs: CrysAlis PRO (Rigaku OD, 2019), SHELXT (Sheldrick, 2015a), SHELXL2018 (Sheldrick, 2015b), DIAMOND (Brandenburg, 2006) and publCIF (Westrip, 2010).

Supporting information

Computing details

Data collection: CrysAlis PRO (Rigaku OD, 2019); cell refinement: CrysAlis PRO (Rigaku OD, 2019); data reduction: 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

[ZnCl2(NH3)2]	Dx = 2.165 Mg m^−3
Mr = 170.34	Mo Kα radiation, λ = 0.71073 Å
Orthorhombic, Imma	Cell parameters from 1597 reflections
a = 7.7077 (2) Å	θ = 3.6–37.4°
b = 8.0226 (2) Å	µ = 5.56 mm^−1
c = 8.4526 (3) Å	T = 100 K
V = 522.67 (3) Å^3	Irregular, colourless
Z = 4	0.26 × 0.18 × 0.16 mm
F(000) = 336

Data collection

Rigaku SuperNOVA diffractometer	678 reflections with I > 2σ(I)
Radiation source: micro-focus sealed X-ray tube	Rint = 0.017
ω scans	θmax = 36.3°, θmin = 3.5°
Absorption correction: analytical (CrysAlis PRO; Rigaku OD, 2019)	h = −12→8
Tmin = 0.277, Tmax = 0.454	k = −13→12
2303 measured reflections	l = −12→14
708 independent reflections

Refinement

Refinement on F^2	Hydrogen site location: difference Fourier map
Least-squares matrix: full	All H-atom parameters refined
R[F^2 > 2σ(F^2)] = 0.015	w = 1/[σ^2(Fo^2) + (0.0188P)^2] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.036	(Δ/σ)max = 0.001
S = 1.04	Δρmax = 0.50 e Å^−3
708 reflections	Δρmin = −0.55 e Å^−3
25 parameters	Extinction correction: SHELXL2018 (Sheldrick, 2015b), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
0 restraints	Extinction coefficient: 0.0043 (6)
Primary atom site location: structure-invariant direct methods

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 (Ų)

	x	y	z	Uiso*/Ueq
Zn1	0.500000	0.250000	0.61168 (2)	0.01163 (6)
Cl1	0.500000	0.47954 (3)	0.76915 (3)	0.01375 (6)
N1	0.28237 (15)	0.250000	0.47949 (13)	0.01603 (17)
H1A	0.274 (3)	0.3281 (19)	0.416 (2)	0.045 (4)*
H1B	0.206 (5)	0.250000	0.533 (4)	0.090 (11)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Zn1	0.01002 (9)	0.01352 (9)	0.01134 (9)	0.000	0.000	0.000
Cl1	0.01288 (11)	0.01346 (11)	0.01490 (11)	0.000	0.000	−0.00253 (7)
N1	0.0136 (4)	0.0211 (4)	0.0134 (4)	0.000	−0.0016 (3)	0.000

Geometric parameters (Å, º)

Zn1—Cl1^i	2.2722 (3)	N1—H1A^ii	0.825 (17)
Zn1—Cl1	2.2722 (3)	N1—H1A	0.825 (17)
Zn1—N1^i	2.0155 (11)	N1—H1B	0.74 (4)
Zn1—N1	2.0155 (11)
Cl1^i—Zn1—Cl1	108.282 (14)	Zn1—N1—H1A^ii	115.0 (13)
N1—Zn1—Cl1	108.951 (17)	Zn1—N1—H1A	115.0 (13)
N1^i—Zn1—Cl1^i	108.951 (17)	Zn1—N1—H1B	109 (3)
N1—Zn1—Cl1^i	108.951 (17)	H1A—N1—H1A^ii	99 (2)
N1^i—Zn1—Cl1	108.950 (17)	H1A—N1—H1B	109 (2)
N1^i—Zn1—N1	112.66 (7)	H1B—N1—H1A^ii	109 (2)

Symmetry codes: (i) −x+1, −y+1/2, z; (ii) x, −y+1/2, z.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N1—H1A···Cl1^iii	0.825 (17)	2.806 (18)	3.4552 (9)	137.0 (17)
N1—H1A···Cl1^iv	0.825 (17)	2.897 (18)	3.5505 (9)	137.6 (16)
N1—H1B···Cl1^v	0.74 (4)	2.95 (3)	3.5556 (10)	141 (1)
N1—H1B···Cl1^vi	0.74 (4)	2.95 (3)	3.5556 (10)	141 (1)

Symmetry codes: (iii) −x+1, −y+1, −z+1; (iv) −x+1/2, −y+1, z−1/2; (v) −x+1/2, −y+1/2, −z+3/2; (vi) x−1/2, y, −z+3/2.