ZnPCl₇: a compositionally and structurally unprecedented metal–phospho­rus halide

The synthesis and crystal structure of an unprecedented compound, zinc phospho­rus hepta­chloride or catena-poly[phospho­rus tetra­chloride [[di­chlor­ido­zinc]-μ-chlorido]], ZnPCl₇ or {[PCl₄][ZnCl₃]}_n, are reported. The reaction of ZnCl₂ with PCl₅ in a 1:1 molar ratio at 623 K produced single crystals of ZnPCl₇, which crystallizes in the ortho­rhom­bic space group Ama2. The Zn and P atoms lie on crystallographic mirror planes, as do four of the Cl atoms. One Cl atom lies on a twofold axis and one occupies a general position. Its extended structure features isolated [PCl₄]⁺ tetra­hedra and one-dimensional chains of corner-sharing ZnCl₂Cl_2/2 tetra­hedra. Bond-valence sum calculations support the assignment of formal oxidation states. This discovery extends the compositional and structural landscape of mixed-metal halides.

1. Chemical context

Metal–phospho­rus compounds have garnered inter­est owing to their diverse structures and potential functional properties (Chen et al., 2023). In pursuit of discovering new compounds comprising a divalent metal and phospho­rus within a halide framework, we investigated the solid-state reaction between zinc chloride (ZnCl₂) and phospho­rus penta­chloride (PCl₅). Heating a 1:1 stoichiometric mixture yielded a previously unreported compound, ZnPCl₇ (I), confirmed by a new powder X-ray diffraction pattern. Single-crystal growth enabled full structure determination, revealing both a new stoichiometry and a previously unobserved structure in the Zn–P–Cl chemical system.

2. Structural commentary

Compound (I) crystallizes in the ortho­rhom­bic space group Ama2, with one Zn (site symmetry m), one P (site symmetry m) and six Cl atoms (four with site symmetry m, one with site symmetry 2 and one on a general position) in the asymmetric unit (Fig. 1). The structure can therefore be formulated as ZnPCl₇ or [ZnCl₃]⁻_n.n[PCl₄]⁺.

Figure 1 The crystal structure of (I) viewed approximately along the [100] direction. Displacement ellipsoids are drawn at the 50% probability level. Symmetry codes: (i) −x + , y, z; (ii) −x + , y, z.

The Zn²⁺ ion is tetra­hedrally coordinated by four chloride anions (Table 1), with an average Zn—Cl bond distance of 2.2900 Å and Cl—Zn—Cl bond angles in the range 98.49 (4) to 118.60 (6)°. The P⁵⁺ atom forms a discrete [PCl₄]⁺ tetra­hedron with a shorter average P—Cl bond distance of 1.9286 Å and Cl—P—Cl angles clustered between 109.51 (9) and 110.20 (6)°. These bond lengths are consistent with expectations based on the sums of ionic radii (Shannon, 1976).

Table 1
Selected geometric parameters (Å, °)

Zn1—Cl1	2.3689 (8)	P1—Cl4	1.937 (2)
Zn1—Cl2	2.2066 (14)	P1—Cl5	1.924 (2)
Zn1—Cl3	2.2223 (14)	P1—Cl6	1.9250 (13)
Cl1^i—Zn1—Cl1	98.49 (4)	Cl6^ii—P1—Cl6	107.69 (9)
Cl1—Zn1—Cl2	108.41 (3)	Cl4—P1—Cl6	110.20 (6)
Cl1—Zn1—Cl3	110.52 (3)	Cl5—P1—Cl6	109.61 (6)
Cl2—Zn1—Cl3	118.60 (6)	Zn1—Cl1—Zn1^iii	106.48 (5)
Cl4—P1—Cl5	109.51 (9)
Symmetry codes: (i) ; (ii) ; (iii) .

The extended structure of (I) consists of isolated PCl₄ units inter­spersed between infinite [100] chains of corner-sharing ZnCl₂Cl_2/2 tetra­hedra (Fig. 2), forming a structure that is, to our knowledge, unprecedented in metal–halide chemistry. Such a configuration – linking M²⁺ centered tetra­hedra into chains and combining them with isolated X⁵⁺-centered tetra­hedra – has not been previously reported. To further validate the structural model, bond-valence-sum (BVS) calculations were performed using the softBV program (Chen et al., 2019). The BVS values are in good agreement with the expected formal charges, further supporting the reliability of the refined structure: Zn +1.99, P +4.90, Cl1 −0.84, Cl2 −0.64, Cl3 −0.67, Cl4 −1.18, Cl5 −1.20 and Cl6 −1.17.

Figure 2 One-dimensional chains of corner-sharing ZnCl2Cl2/2 tetra­hedra (blue) extending along the a-axis direction in the crystal structure of (I). The isolated PCl4 tetra­hedra are depicted in yellow.

This type of one-dimensional chain of corner-sharing ZnCl₄ tetra­hedra found in (I) is rarely observed in the halide system. Most known zinc chlorides, including ZnCl₂, form extended networks or layered structures rather than chains (Winkler et al., 1959). Similar tetra­hedral chain motifs have been reported in some oxides such as Sr₂Fe₂O₅ (D'Hondt et al., 2008), but are extremely uncommon among halides.

3. Synthesis and crystallization

Anhydrous zinc chloride (Sigma-Aldrich, 98%) and phospho­rus(V) chloride (Sigma-Aldrich, 95%) were used as received. A 1:1 molar mixture of ZnCl₂ (0.3952 g) and PCl₅ (0.6039 g) was thoroughly ground in an agate mortar and pressed into a pellet. The pellet was placed in a dried fused-silica ampoule, sealed under vacuum (∼360 Pa), and heated from 303 K to 623 K at a rate of 5 K min⁻¹, and then slowly cooled to 373 K at a rate of 0.42 K min⁻¹, followed by natural cooling to room temperature.

Single crystals were isolated under an optical microscope in a dry room with a dew point of 223 K. ZnPCl₇ appears to be stable under dry-air conditions; therefore, all handling was carried out in a dry room. However, it is extremely sensitive to moisture and decomposed immediately upon exposure to humidity. A colourless crystal, approximately 0.1 mm in size, was placed into a 0.5 mm diameter glass capillary and sealed with capillary wax.

4. Refinement

Crystal data, data collection, and refinement parameters are summarized in Table 2.

Table 2
Experimental details

Crystal data
Chemical formula	[PCl4][ZnCl3]
Mr	344.51
Crystal system, space group	Orthorhombic, Ama2
Temperature (K)	293
a, b, c (Å)	7.1775 (5), 15.5212 (8), 9.0050 (4)
V (Å^3)	1003.19 (10)
Z	4
Radiation type	Mo Kα
μ (mm^−1)	4.39
Crystal size (mm)	0.10 × 0.10 × 0.10
Data collection
Diffractometer	Bruker D8 VENTURE
Absorption correction	Multi-scan (DENZO/SCALEPACK; Otwinowski & Minor, 1997)
Tmin, Tmax	0.64, 0.64
No. of measured, independent and observed [I > 2.0σ(I)] reflections	19802, 1627, 1467
Rint	0.050
(sin θ/λ)max (Å^−1)	0.712
Refinement
R[F^2 > 2σ(F^2)], wR(F^2), S	0.028, 0.060, 1.29
No. of reflections	1627
No. of parameters	52
No. of restraints	17
Δρmax, Δρmin (e Å^−3)	0.58, −0.55
Absolute structure	Parsons et al. (2013), 764 Friedel Pairs
Absolute structure parameter	0.016 (17)
Computer programs: APEX2 and SAINT (Bruker, 2006), SUPERFLIP (Palatinus & Chapuis, 2007), CRYSTALS (Betteridge et al., 2003) and VESTA (Momma & Izumi, 2011).

Supporting information

Computing details

(I)

Crystal data

[PCl4][ZnCl3]	F(000) = 656
Mr = 344.51	Dx = 2.281 Mg m^−3
Orthorhombic, Ama2	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: A 2 -2a	Cell parameters from 1767 reflections
a = 7.1775 (5) Å	θ = 3.9–31.3°
b = 15.5212 (8) Å	µ = 4.39 mm^−1
c = 9.0050 (4) Å	T = 293 K
V = 1003.19 (10) Å^3	Block, colourless
Z = 4	0.10 × 0.10 × 0.10 mm

Data collection

Bruker D8 VENTURE diffractometer	1467 reflections with I > 2.0σ(I)
Graphite monochromator	Rint = 0.050
ω/2θ scans	θmax = 30.4°, θmin = 3.9°
Absorption correction: multi-scan (DENZO/SCALEPACK; Otwinowski & Minor, 1997)	h = −10→10
Tmin = 0.64, Tmax = 0.64	k = −21→22
19802 measured reflections	l = −12→12
1627 independent reflections

Refinement

Refinement on F^2	Primary atom site location: other
Least-squares matrix: full	Method, part 1, Chebychev polynomial, (Watkin, 1994, Prince, 1982) [weight] = 1.0/[A0*T0(x) + A1*T1(x) ··· + An-1]*Tn-1(x)] where Ai are the Chebychev coefficients listed below and x = F /Fmax Method = Robust Weighting (Prince, 1982) W = [weight] * [1-(deltaF/6*sigmaF)^2]^2 Ai are: 20.3 28.2 14.3 4.23 0.510
R[F^2 > 2σ(F^2)] = 0.028	(Δ/σ)max = 0.0003
wR(F^2) = 0.060	Δρmax = 0.58 e Å^−3
S = 1.29	Δρmin = −0.55 e Å^−3
1627 reflections	Absolute structure: Parsons et al. (2013), 764 Friedel Pairs
52 parameters	Absolute structure parameter: 0.016 (17)
17 restraints

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

	x	y	z	Uiso*/Ueq
Zn1	0.2500	0.53981 (3)	0.06695 (9)	0.0283
P1	0.7500	0.80615 (8)	0.04103 (15)	0.0301
Cl1	0.5000	0.5000	−0.09048 (14)	0.0318
Cl2	0.2500	0.45321 (9)	0.26129 (16)	0.0370
Cl3	0.2500	0.68125 (8)	0.10522 (15)	0.0394
Cl4	0.7500	0.79444 (11)	0.25514 (17)	0.0538
Cl5	0.7500	0.69371 (10)	−0.04891 (18)	0.0503
Cl6	0.53345 (16)	0.86924 (7)	−0.02285 (15)	0.0535

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Zn1	0.0314 (3)	0.0250 (2)	0.0284 (3)	0.0000	0.0000	0.0023 (2)
P1	0.0314 (6)	0.0273 (6)	0.0316 (7)	0.0000	0.0000	0.0072 (5)
Cl1	0.0238 (5)	0.0414 (6)	0.0302 (5)	0.0025 (5)	0.0000	0.0000
Cl2	0.0380 (7)	0.0381 (7)	0.0351 (6)	0.0000	0.0000	0.0125 (6)
Cl3	0.0563 (9)	0.0257 (5)	0.0363 (7)	0.0000	0.0000	−0.0015 (5)
Cl4	0.0756 (12)	0.0546 (9)	0.0314 (7)	0.0000	0.0000	0.0058 (7)
Cl5	0.0740 (11)	0.0308 (7)	0.0461 (9)	0.0000	0.0000	0.0001 (6)
Cl6	0.0385 (5)	0.0465 (6)	0.0753 (8)	0.0087 (5)	−0.0050 (5)	0.0161 (5)

Geometric parameters (Å, º)

Zn1—Cl1^i	2.3689 (8)	P1—Cl6^ii	1.9250 (13)
Zn1—Cl1	2.3689 (8)	P1—Cl4	1.937 (2)
Zn1—Cl2	2.2066 (14)	P1—Cl5	1.924 (2)
Zn1—Cl3	2.2223 (14)	P1—Cl6	1.9250 (13)
Cl1^i—Zn1—Cl1	98.49 (4)	Cl6^ii—P1—Cl5	109.61 (6)
Cl1^i—Zn1—Cl2	108.41 (3)	Cl4—P1—Cl5	109.51 (9)
Cl1—Zn1—Cl2	108.41 (3)	Cl6^ii—P1—Cl6	107.69 (9)
Cl1^i—Zn1—Cl3	110.52 (3)	Cl4—P1—Cl6	110.20 (6)
Cl1—Zn1—Cl3	110.52 (3)	Cl5—P1—Cl6	109.61 (6)
Cl2—Zn1—Cl3	118.60 (6)	Zn1—Cl1—Zn1^iii	106.48 (5)
Cl6^ii—P1—Cl4	110.20 (7)

Symmetry codes: (i) −x+1/2, y, z; (ii) −x+3/2, y, z; (iii) x+1/2, −y+1, z.