Calcium octa­ammine dichloride

The redetermination of the crystal structure of calcium octa­ammine chloride, or octa­amminecalcium dichloride, [Ca(NH₃)₈]Cl₂, based on synchrotron X-ray diffraction powder data, revealed a more reasonable model in terms of N⋯N distances in comparison with the previous model [Westman et al. (1981). Acta Chem. Scand. Ser. A, 35, 467–472].

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Structure description

The reaction of CaCl₂ with NH₃ is promising for the energy efficiency improvement of automobiles and factories and is one form of thermal energy storage (TES) technology (Klerke et al., 2008). A detailed knowledge of the crystal structure of [Ca(NH₃)₈]Cl₂ is necessary for understanding the reaction mechanism associated with the uptake of ammonia from CaCl₂. In the current study, we developed in situ XRD equipment and redetermined the crystal structure of [Ca(NH₃)₈]Cl₂. The main difference from the structure model reported in the previous study (powder X-ray diffraction data; Westman et al., 1981) is the position of one N atom which had an unrealistically short N⋯N distances of 2.13 Å. Whereas this N atom was modelled in the previous study to be on a general position of space group Pnma (Wyckoff site 8d), it is now modelled to be split over two positions located on a mirror plane (Wyckoff site 4c), leaving to more reasonable N⋯N distances > 3.1 Å. The current structure model is supported by isotypism with [Sr(NH₃)₈]Cl₂ (Lysgaard et al., 2012), [Ca(NH₃)₈]Br₂ and [Ca(NH₃)₈]I₂ (Woidy et al., 2014). The coordination polyhedra around the alkaline earth ions are twofold-capped trigonal-prisms (Fig. 1; Table 1). Although no H-atom positions could be determined in the current synchrotron powder study, N⋯Cl contacts in the range 3.45–3.70 Å are evidence for hydrogen bonding between the complex cations and the chloride anions.

Table 1
Selected bond lengths (Å)

Ca1—N4	2.601 (4)	Ca1—N2	2.702 (7)
Ca1—N3	2.616 (4)	Ca1—N1	3.078 (7)
Ca1—N5	2.646 (4)

Figure 1 The crystal structure of [Ca(NH3)8]Cl2, viewed approximately along [010]. The blue and green ellipsoids represent Ca and Cl atoms, respectively, at the 50% probability level. Grey spheres indicate N atoms (arbitrary radius) of the NH3 mol­ecules. [Symmetry code (i) x, −y + , z.]

Synthesis and crystallization

A quartz glass capillary cell was developed for the in situ X-ray powder diffraction (XRD) under NH₃ gas pressure. The outside and inside diameters were 1.5875 mm (1/16 inch) and 1.0 mm, respectively. Carbon fiber was mixed with CaCl₂ powder to prevent breaking of the capillary by expansion of CaCl₂ powder during NH₃ adsorption. [Ca(NH₃)₈]Cl₂ was synthesized in situ in the capillary under 518 kPa of NH₃ gas pressure. The XRD experiments were performed at BL5S2 at Aichi Synchrotron Radiation Center in Aichi province, Japan.

Refinement

Crystal data, data collection and structure refinement details are summarized in Table 2. The crystal structure was modelled in the same space group (Pnma) as in the previous work by Westman et al. (1981). The coordinations of all atoms were estimated by application of direct methods for structure solution by using the EXPO2014 software (Altomare et al., 2013). Wyckoff positions of atoms Ca1, Cl1, Cl2 (on sites 4c with mirror symmetry), and N3, N4 and N5 (on general positions 8d) are the same as those reported in the previous study. In contrast to the previous model, sites N1 and N2 were modelled to be located on mirror planes, instead of as one atom on a general position. Mixing carbon fiber with CaCl₂ deteriorates the analytical accuracy by the overlap between diffraction peaks. Therefore, several parameters were constrained during the refinement as follows: (i) anisotropic displacement parameters of Cl2 were constrained to be the same as that of the Cl1 site; (ii) H atoms of the NH₃ mol­ecules were not positioned; (iii) isotropic displacement parameters were used for all N atoms. The Rietveld refinement (Fig. 2) was performed with the RIETAN-FP program (Izumi & Momma, 2007) using a split pseudo-Voigt profile function (Toraya, 1990).

Table 2
Experimental details

Crystal data
Chemical formula	[Ca(NH3)8]Cl2
Mr	247.23
Crystal system, space group	Orthorhombic, Pnma
Temperature (K)	301
a, b, c (Å)	12.0924 (2), 7.3293 (1), 15.1975 (2)
V (Å^3)	1346.94 (3)
Z	4
Radiation type	Synchrotron, λ = 0.9995754 Å
Specimen shape, size (mm)	Cylinder, 0.5 × 0.5
Data collection
Diffractometer	BL5S2 Debye-Scherrer Camera
Specimen mounting	Quartz capillary.
Data collection mode	Transmission
Scan method	Stationary detector
Refinement
R factors and goodness of fit	Rp = 0.021, Rwp = 0.028, Rexp = 0.024, RBragg = 0.039, R(F) = 0.030, χ^2 = 1.407
No. of parameters	43
H-atom treatment	H-atom parameters not refined
Computer programs: local data-collection software, EXPO2014 (Altomare et al., 2013), VESTA (Momma & Izumi, 2011), RIETAN-FP (Izumi & Momma, 2007) and publCIF (Westrip, 2010).

Figure 2 Rietveld refinement of [Ca(NH3)8]Cl2. 2θ ranges 16.50–17.80 and 47.48–48.18° are excluded because diffuse diffraction peaks of mixed carbon fiber appeared.

Structural data

S1. Experimental

A quartz glass capillary cell was developed for the in situ X-ray powder diffraction (XRD) under NH₃ gas pressure. The outside and inside diameters were 1.5875 mm (1/16 inch) and 1.0 mm, respectively. Carbon fiber was mixed with CaCl₂ powder to prevent breaking of the capillary by expansion of CaCl₂ powder during NH₃ adsorption. [Ca(NH₃)₈]Cl₂ was synthesized in situ in the capillary under 518 kPa of NH₃ gas pressure. The XRD experiment was performed at BL5S2 at Aichi Synchrotron Radiation Center in Aichi province, Japan.

S2. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 2. The crystal structure was modelled in the same space group (Pnma) as in the previous work by Westman et al. (1981). The coordinations of all atoms were estimated by application of direct methods for structure solution by using the EXPO2014 software (Altomare et al., 2013). Wyckoff positions of atoms Ca1, Cl1, Cl2 (on sites 4c with mirror symmetry), and N3, N4 and N5 (on general positions 8d) are the same as those reported in the previous study. In contrast to the previous model, sites N1 and N2 were modelled to be located on mirror planes, instead of as one atom on a general position. Mixing carbon fiber with CaCl₂ deteriorates the analytical accuracy by the overlap between diffraction peaks. Therefore, several parameters were constrained during the refinement as follows: (i) anisotropic displacement parameters of Cl2 were constrained to be the same as that of the Ca1 site; (ii) H atoms of the NH₃ molecules were not positioned; (iii) isotropic displacement parameters were used for all N atoms. The Rietveld refinement was performed with the RIETAN-FP program (Izumi & Momma, 2007) using a split pseudo-Voigt profile function (Toraya, 1990).

Structure description

The reaction of CaCl₂ with NH₃ is promising for the energy efficiency improvement of automobiles and factories and is one form of thermal energy storage (TES) technology (Klerke et al., 2008). A detailed knowledge of the crystal structure of [Ca(NH₃)₈]Cl₂ is necessary for understanding the reaction mechanism associated with the uptake of ammonia from CaCl₂. In the current study, we developed in situ XRD equipment and redetermined the crystal structure of [Ca(NH₃)₈]Cl₂. The main difference from the structure model reported in the previous study (powder X-ray diffraction data; Westman et al., 1981) is the position of one N atom which had an unrealistically short N···N distances of 2.13 Å. Whereas this N atom was modelled in the previous study to be on a general position of space group Pnma (Wyckoff site 8d), it is now modelled to be split over two positions located on a mirror plane (Wyckoff site 4c), leaving to more reasonable N···N distances > 3.1 Å. The current structure model is supported by isotypism with [Sr(NH₃)₈]Cl₂ (Lysgaard et al., 2012), [Ca(NH₃)₈]Br₂ and [Ca(NH₃)₈]I₂ (Woidy et al., 2014). The coordination polyhedra around the alkaline earth ions are twofold-capped trigonal-prisms (Fig. 1; Table 1). Although no H-atom positions could be determined in the current synchrotron powder study, N···Cl contacts in the range 3.45-3.70 Å are evidence for hydrogen bonding between the complex cations and the chloride anions.

Computing details

Data collection: BL5S2, Aichi Synchrotron Radiation Center (Aichi SR) local software; cell refinement: RIETAN-FP (Izumi & Momma, 2007); data reduction: RIETAN-FP (Izumi & Momma, 2007); program(s) used to solve structure: EXPO2014 (Altomare et al., 2013); program(s) used to refine structure: RIETAN-FP (Izumi & Momma, 2007); molecular graphics: VESTA (Momma & Izumi, 2011); software used to prepare material for publication: RIETAN-FP (Izumi & Momma, 2007) and publCIF (Westrip, 2010).

Figures

	Fig. 1. The crystal structure of [Ca(NH3)8]Cl2, viewed approximately along [010]. The blue and green ellipsoids represent Ca and Cl atoms, respectively, at the 50% probability level. Grey spheres indicate N atoms (arbitrary radius) of the NH3 molecules. [Symmetry code (i) x, -y + 1/2, z.]
	Fig. 2. Rietveld refinement of [Ca(NH3)8]Cl2. 2θ ranges 16.50–17.80 and 47.48–48.18° are excluded because diffuse diffraction peaks of mixed carbon fiber appeared.

Octaamminecalcium dichloride

Crystal data

[Ca(NH3)8]Cl2	F(000) = 440.00
Mr = 247.23	Dx = 1.100 Mg m^−3
Orthorhombic, Pnma	Synchrotron radiation, λ = 0.9995754 Å
Hall symbol: -P 2ac 2n	T = 301 K
a = 12.0924 (2) Å	Particle morphology: powder
b = 7.3293 (1) Å	white
c = 15.1975 (2) Å	cylinder, 0.5 × 0.5 mm
V = 1346.94 (3) Å^3	Specimen preparation: Prepared at 301 K and 518 kPa
Z = 4

Data collection

BL5S2 Debye-Scherrer Camera diffractometer	Data collection mode: transmission
Radiation source: synchrotron	Scan method: Stationary detector
Specimen mounting: Quartz capillary.

Refinement

Least-squares matrix: full	Profile function: split pseudo-Voigt function
Rp = 0.021	43 parameters
Rwp = 0.028	0 restraints
Rexp = 0.024	9 constraints
RBragg = 0.039	H-atom parameters not refined
R(F) = 0.030	Weighting scheme based on measured s.u.'s 1/yi
R(F^2) = 0.02947	(Δ/σ)max < 0.001
7419 data points	Background function: RIETAN-FP composite background function number 3.
Excluded region(s): 2θ ranges of 16.5 to 17.8 and 47.78 to 48.18 degrees were excluded because the diffraction of the carbon fiber appeared.

Special details

Experimental. The powder mounted in the quartz capillary that was filled by the NH₃ gas, 518 kPa (abs).

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Ca1	0.2599 (2)	0.25	0.3651 (2)	0.053 (2)
Cl1	0.1428 (3)	0.25	0.0556 (2)	0.067 (2)
Cl2	0.0589 (2)	0.25	0.6675 (2)	0.067 (2)
N1	0.3970 (5)	0.25	0.1944 (5)	0.029 (2)*
N2	0.3347 (6)	0.25	0.5326 (5)	0.029 (2)*
N3	0.1395 (3)	0.0237 (6)	0.4574 (3)	0.021 (1)*
N4	0.4130 (4)	0.0009 (6)	0.3684 (3)	0.021 (1)*
N5	0.1813 (3)	0.0106 (6)	0.2507 (3)	0.021 (1)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Ca1	0.068 (3)	0.050 (2)	0.042 (2)	0	−0.000 (2)	0
Cl1	0.079 (2)	0.086 (2)	0.036 (2)	0	0.009 (2)	0
Cl2	0.079 (2)	0.086 (2)	0.036 (2)	0	0.009 (2)	0

Geometric parameters (Å, º)

Ca1—N4	2.601 (4)	Ca1—N5	2.646 (4)
Ca1—N4^i	2.601 (4)	Ca1—N5^i	2.646 (4)
Ca1—N3	2.616 (4)	Ca1—N2	2.702 (7)
Ca1—N3^i	2.616 (4)	Ca1—N1	3.078 (7)
N4—Ca1—N4^i	89.2 (2)	N3—Ca1—N5	74.4 (1)
N4—Ca1—N3	86.6 (1)	N3—Ca1—N5^i	125.0 (2)
N4—Ca1—N3^i	146.2 (2)	N3—Ca1—N2	71.4 (2)
N4—Ca1—N5	78.6 (1)	N3—Ca1—N1	138.8 (1)
N4—Ca1—N5^i	137.2 (2)	N3^i—Ca1—N5	125.0 (2)
N4—Ca1—N2	75.1 (2)	N3^i—Ca1—N5^i	74.4 (1)
N4—Ca1—N1	68.5 (1)	N3^i—Ca1—N2	71.4 (2)
N4^i—Ca1—N3	146.2 (2)	N3^i—Ca1—N1	138.8 (1)
N4^i—Ca1—N3^i	86.6 (1)	N5—Ca1—N5^i	83.1 (2)
N4^i—Ca1—N5	137.2 (2)	N5—Ca1—N2	137.7 (1)
N4^i—Ca1—N5^i	78.6 (1)	N5—Ca1—N1	68.9 (1)
N4^i—Ca1—N2	75.1 (2)	N5^i—Ca1—N2	137.7 (1)
N4^i—Ca1—N1	68.5 (1)	N5^i—Ca1—N1	68.9 (1)
N3—Ca1—N3^i	78.7 (2)	N2—Ca1—N1	127.8 (2)

Symmetry code: (i) x, −y+1/2, z.