Synthesis and structure of poly[hydroxonium [μ₄-bromido-tetra-μ₃-bromido-diplumbate(II)]]

The title compound, {(H₃O)[Pb₂Br₅]}_n, crystallizes in the tetra­gonal space group I4/mcm. The asymmetric unit comprises one Pb, two Br and one O sites with site symmetry m.2m, 4/m.., ..m and 422, respectively. Bromide ions link the Pb^II atoms to form a two-periodic Pb₂Br₅⁻ anion extending parallel to the ab plane, whilst hydroxonium cations are embedded between neighbouring inorganic layers. Although the hydrogen positions of the hydroxonium cation could not be determined, close O⋯Br contacts suggest possible O—H⋯Br hydrogen-bonding inter­actions between cations and the anionic layers.

1. Chemical context

Lead(II) halide and halidoplumbate(II) compounds display various coordination environments and structural connectivities, both of which are influenced by the stereochemical activity of the Pb^II 6s² lone pair and by the terminal or bridging character of the halido ligands (Shimoni-Livny et al., 1998; Mitzi, 2001). Lead(II) halides are widely used as precursors for hybrid lead-halide perovskites. The latter have attracted considerable inter­est due to their semiconducting properties and potential applications in photovoltaics and optoelectronics (Yuan et al., 2025; Kucheriv et al., 2023, 2025). The chemistry of halidoplumbate species in solution is also central to the formation of lead-halide perovskites. Studies of lead-halide precursor solutions and solvent-dependent phase formation highlight the importance of halidoplumbate speciation and the coordination environment of Pb^II (Sharenko et al., 2017). Hydroxonium ions can exhibit different hydrogen-bonding arrangements in crystalline solids, with their local structure influenced by inter­actions with the surrounding atoms (Bernal, 2006).

In this paper, the crystal structure of the title compound, (H₃O)Pb₂Br₅, (I), containing a two-periodic Pb–Br network, is reported.

2. Structural commentary

The asymmetric unit of (I) comprises four sites, Pb1, Br1, Br2 and O1. They occupy the special Wyckoff positiosn 8 h, 4 c, 16 l and 4 a, with site symmetry m.2m, 4/m.., ..m and 422, respectively, in the tetra­gonal space group I4/mcm. The asymmetric unit, together with selected symmetry-generated bromide ions that complete the local environment of Pb1, is shown in Fig. 1. The Pb1 atom is eight-coordinated by bromide ions, with Pb1—Br2 and Pb1—Br1 distances of 2.8863 (10)/3.3470 (9) and 3.1287 (2) Å, respectively. The Br—Pb1—Br bond angles range from 66.989 (2)° for Br2( − x,  + y, 1 − z)—Pb1—Br2( + x,  − y, 1 − z) to 146.5601 (14)° for Br2—Pb1—Br2( + x,  − y, 1 − z). A continuous shape-measure analysis using SHAPE 2.1 (Llunell et al., 2013) identifies a distorted biaugmented trigonal prism (BTPR-8) as the closest ideal figure for the PbBr₈ coordination polyhedron (CShM = 1.278). Bromide anions link neighbouring Pb^II atoms into a two-periodic Pb₂Br₅⁻ anion extending parallel to the ab plane (Figs. 2, 3).

Figure 1 Fragment of the crystal structure showing the coordination environment of Pb1 in the title compound. Displacement ellipsoids are drawn at the 50% probability level. [Symmetry codes: (i)  + y, − + x, 1 − z; (ii) 1 + y, 1 − x, z; (iii) 1 − y, x, z; (iv)  − x,  + y, 1 − z; (v)  + x,  − y, 1 − z.]

Figure 2 Fragment of the two-periodic Pb–Br network extending parallel to the ab plane.

Figure 3 View of the crystal packing, showing the Pb2Br5^− anion layers and the O1-centred hydroxonium ion (small red spheres) between neighbouring layers.

3. Supra­molecular features

The O1-centred hydroxonium ion is embedded between two neighbouring Pb–Br layers and forms fourselected O1⋯Br contacts with bromide ions of symmetry-related Pb—Br units (Fig. 4). These contacts involve both the Br1 and Br2 sites. The observed O1⋯Br contacts (Table 1) suggest possible O—H⋯Br hydrogen-bonding inter­actions between the hydroxonium ion and the anionic layers. However, the hydrogen atom(s) could not be located from the available X-ray data and were therefore omitted from the refinement model. Consequently, the O—H bond orientations and further numerical details of the hydrogen-bonding scheme remain undetermined, and no specific hydrogen-bonding topology is assigned. A view of the overall crystal packing, including the two-periodic Pb–Br networks and the O1-centred hydroxonium ion, is shown in Fig. 3.

Table 1
Selected interatomic distances (Å)

O1⋯Br1^i	3.5848 (3)	O1⋯Br2^ii	3.5604 (1)
O1⋯Br2	3.5604 (1)	O1⋯Br2^iii	3.5604 (1)
Symmetry codes: (i) ; (ii) ; (iii) .

Figure 4 Local environment of the O1-centred hydroxonium ion, showing four selected O1⋯Br contacts with bromide ions of neighbouring Pb–Br layers. Dashed lines indicate O1⋯Br contacts associated with possible O—H⋯Br hydrogen-bonding; hydrogen atom positions remain undetermined. [Symmetry codes: (i) 1 − x, y,  − z; (ii) x − , y + , z - 1/2; (iii) x, 1 − y,  − z.]

4. Database survey

A search of the Crystallography Open Database (COD; Gražulis et al., 2012) identified related Pb^II bromide compounds, of which two were selected for structural comparison: the hydrated lead(II) bromide phase 3PbBr₂·2H₂O (Liu et al., 2018) and the oxyhy­droxy­bromide Pb₂OOHBr (Kunisawa et al., 2026). The former contains eight-coordinate Pb^II atoms with PbBr₈ or PbBr₇O coordination environments, whereas the latter contains PbBr₄O₄ coordination polyhedra within a Pb–O–Br framework.

5. Synthesis and crystallization

Lead(II) bromide (50 mg, 0.136 mmol) was dissolved in concentrated hydro­bromic acid (0.5 ml). After 2 days, light-yellow single crystals suitable for single-crystal X-ray diffraction were formed. They were stored in the mother liquor prior to measurement.

6. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 2. Due to the site symmetry (422) of the oxygen atom, the hydrogen-atom positions of the hydroxonium cation are orientationally disordered. This fact, combined with the high absorption coefficient of (I), has made it impossible to determine the relevant H positions from difference-Fourier syntheses. Therefore, they were not included in the final model; the contribution from the H atoms is accounted for in the molecular weight given in Table 2.

Table 2
Experimental details

Crystal data
Chemical formula	(H3O)[Pb2Br5]
Mr	832.95
Crystal system, space group	Tetragonal, I4/mcm
Temperature (K)	100
a, c (Å)	8.3948 (4), 14.3392 (10)
V (Å^3)	1010.52 (12)
Z	4
Radiation type	Mo Kα
μ (mm^−1)	52.98
Crystal size (mm)	0.20 × 0.17 × 0.06
Data collection
Diffractometer	XtaLAB Synergy, Dualflex, HyPix
Absorption correction	Analytical [CrysAlis PRO (Rigaku OD, 2024), based on expressions derived by Clark & Reid, 1995]
Tmin, Tmax	0.011, 0.094
No. of measured, independent and observed [I > 2σ(I)] reflections	1765, 368, 317
Rint	0.048
(sin θ/λ)max (Å^−1)	0.699
Refinement
R[F^2 > 2σ(F^2)], wR(F^2), S	0.028, 0.075, 1.09
No. of reflections	368
No. of parameters	16
H-atom treatment	H-atom parameters not defined
Δρmax, Δρmin (e Å^−3)	2.31, −1.59
Computer programs: CrysAlis PRO (Rigaku OD, 2024), SHELXT (Sheldrick, 2015a), SHELXL (Sheldrick, 2015b), OLEX2 (Dolomanov et al., 2009) and publCIF (Westrip, 2010).

Supporting information

Computing details

Poly[hydroxonium [µ₄-bromido-tetra-µ₃-bromido-diplumbate(II)]]

Crystal data

(H3O)[Pb2Br5]	Dx = 5.475 Mg m^−3
Mr = 832.95	Mo Kα radiation, λ = 0.71073 Å
Tetragonal, I4/mcm	Cell parameters from 1179 reflections
a = 8.3948 (4) Å	θ = 2.8–29.7°
c = 14.3392 (10) Å	µ = 52.98 mm^−1
V = 1010.52 (12) Å^3	T = 100 K
Z = 4	Plate, light yellow
F(000) = 1400	0.20 × 0.17 × 0.06 mm

Data collection

XtaLAB Synergy, Dualflex, HyPix diffractometer	368 independent reflections
Radiation source: micro-focus sealed X-ray tube, PhotonJet (Mo) X-ray Source	317 reflections with I > 2σ(I)
Mirror monochromator	Rint = 0.048
Detector resolution: 10.0000 pixels mm^-1	θmax = 29.8°, θmin = 2.8°
ω scans	h = −11→11
Absorption correction: analytical [CrysAlisPro (Rigaku OD, 2024), based on expressions derived by Clark & Reid, 1995]	k = −10→7
Tmin = 0.011, Tmax = 0.094	l = −18→13
1765 measured reflections

Refinement

Refinement on F^2	H-atom parameters not defined
Least-squares matrix: full	w = 1/[σ^2(Fo^2) + (0.037P)^2 + 12.0445P] where P = (Fo^2 + 2Fc^2)/3
R[F^2 > 2σ(F^2)] = 0.028	(Δ/σ)max = 0.001
wR(F^2) = 0.075	Δρmax = 2.31 e Å^−3
S = 1.09	Δρmin = −1.59 e Å^−3
368 reflections	Extinction correction: SHELXL2018/3 (Sheldrick 2015b), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
16 parameters	Extinction coefficient: 0.00046 (9)
0 restraints

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
Pb1	0.83338 (4)	0.33338 (4)	0.500000	0.0116 (2)
Br1	1.000000	0.000000	0.500000	0.0102 (4)
Br2	0.65560 (8)	0.15560 (8)	0.36269 (7)	0.0109 (3)
O1	0.500000	0.500000	0.250000	0.030 (4)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Pb1	0.0094 (3)	0.0094 (3)	0.0161 (4)	−0.0005 (2)	0.000	0.000
Br1	0.0078 (5)	0.0078 (5)	0.0150 (9)	0.000	0.000	0.000
Br2	0.0105 (4)	0.0105 (4)	0.0117 (5)	−0.0014 (4)	−0.0010 (3)	−0.0010 (3)
O1	0.034 (6)	0.034 (6)	0.023 (8)	0.000	0.000	0.000

Geometric parameters (Å, º)

Pb1—Br1	3.1287 (2)	Pb1—Br2^ii	2.8863 (10)
Pb1—Br1^i	3.1287 (2)	Pb1—Br2	2.8863 (10)
O1···Br1^iii	3.5848 (3)	O1···Br2^iv	3.5604 (1)
O1···Br2	3.5604 (1)	O1···Br2^v	3.5604 (1)
Br1—Pb1—Br1^i	143.113 (17)	Pb1—Br1—Pb1^vi	180.0
Br2^ii—Pb1—Br1^i	76.624 (9)	Pb1^vii—Br1—Pb1^viii	180.0
Br2^ii—Pb1—Br1	76.624 (9)	Pb1—Br1—Pb1^vii	90.0
Br2—Pb1—Br1	76.624 (9)	Pb1^vii—Br1—Pb1^vi	90.0
Br2—Pb1—Br1^i	76.624 (9)	Pb1^viii—Br1—Pb1^vi	90.0
Br2^ii—Pb1—Br2	86.02 (4)	Pb1—Br1—Pb1^viii	90.0

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