Synthesis and structure of poly[(μ₃-hydrogen phosphato)(pyridine)­zinc(II)]

The title compound, [Zn(HPO₄)(C₅H₅N)]_n or pyZn(HPO₄) (1), was prepared from the solvothermal reaction of [Mo₃O₂(O₂CCH₃)₆(H₂O)₃]ZnCl₄ and H₃PO₄ in a mixture of pyridine and water. It displays an infinite ladder structure built of alternately arranged ZnO₃N and PO₃(OH) tetra­hedra, linked O—H⋯O hydrogen bonds into supra­molecular sheets. C—H⋯O inter­actions between CH groups of the pyridine rings and phosphate groups connect the sheets into a three-dimensional framework structure.

1. Chemical context

Divalent metal phosphates such as hy­droxy­lapatite [Ca₅(PO₄)₃(OH)], the main component of human bones, play an essential role in body structure. Zinc phosphates are extensively involved in bone development, dental materials, environmentally friendly anti­corrosive and anti­rust pigments and industrial additives. They exhibit a vast structural diversity including cluster, chain, layer and open-framework structures (Mao et al., 2020; Amghouz et al., 2014; Chen et al., 2007; Lin et al., 2003b, 2007; Yang et al., 2009; Choudhury et al., 2000; Rayes et al., 2001). A number of LZn(H_xPO₄) where Zn²⁺ is datively coordinated to Lewis basic ligands [x = 0–2; L = Cl⁻ (Chen et al., 2007; Rayes et al., 2001); NH₃ (Amghouz et al., 2014); 5-(4-pyrid­yl)tetra­zolate) (Yang et al., 2009); 1,10-phenanthroline (Lin et al., 2003a); 4,4 -dimethyl-2,2-dipyridyl, 5,5-dimethyl-2,2-dipyridly (Lin et al., 2007) 1,2-di­methyl­imidazole (Mao et al., 2020); 4H-1,2,4-triazole-κN¹ (Aitenneite et al., 2012); CaZn₂Fe(PO₄)₃ (Khmiyas et al., 2016)] to form discrete or one-dimensional ladder structures. Herein, a new family member of zinc phosphates datively coordinated by an aromatic pyridine ligand, namely pyZn(HPO₄) (1), is reported including its synthesis, isolation and single-crystal structural characterization.

2. Structural commentary

As illustrated in Fig. 1, the asymmetric unit of the title compound, [pyZn(HPO₄)]_n, contains one Zn²⁺ cation, one (HPO₄)²⁻ anion and an pyridine ligand. The Zn²⁺ cation is coordinated by three O atoms from three phosphate ligands and the Lewis basic N atom from the pyridine ligand in a quite regular tetra­hedral geometry with bond angles in the range 110.2 (1)–116.6 (1)°. Each phosphate anion is connected to three Zn²⁺ cations with the strict alternation of ZnO₃N tetra­hedra and HPO₄ tetra­hedra giving rise to an extended ladder structure (Fig. 2) characteristic of Zn₂P₂O₄ eight-membered rings. The Zn—O bonds range from 1.911 (2) to 1.941 (3) Å, similar to the reported values, but at 2.043 (3) Å the Zn—N bond is markedly longer than those for example in Zn-mmim [1,2-di­methyl­imidazole, 1.988 (2) Å; Mao et al. 2020)], indicative of weaker Zn–py bonding. The P—O bond lengths fall in the range 1.505 (3)–1.579 (3) Å, similar to those [1.508 (2)–1.587 (2) Å] in mmimZnHPO₄ (Mao et al. 2020) with the longest P—O bond being assigned to the P—OH group.

Figure 1 The asymmetric unit of 1 with 50% probability displacement ellipsoids.

Figure 2 One-dimensional ladder structure of 1.

3. Supra­molecular features

Fig. 3 illustrates the hydrogen-bonded sheet of 1 (numerical details of the hydrogen bonds are given in Table 1). The short OH⋯O separation of 2.647 (3) Å and almost linear O—H⋯O angle [168 (4)°] indicate the significant hydrogen bonding inter­actions between the ladders. The hydrogen bonding network are characteristic of the P₂ZnO₅H₂ ten-membered ring as demonstrated in Fig. 3. The pyridine ligands are almost perpendicular to the hydrogen binding layers. There are significant inter­actions between the hydrogen-bonded sheets through C—H⋯O—P inter­actions (Table 1), which lead to the formation of three-dimensional supra­molecular framework as shown in Fig. 4.

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
O2—H⋯O3^i	0.94 (4)	1.73 (4)	2.647 (3)	168 (4)
C1—H1⋯O1^ii	0.98 (4)	2.58 (4)	3.452 (5)	149 (3)
C3—H3⋯O3^iii	0.96 (2)	2.61 (3)	3.465 (5)	148 (3)
C4—H4⋯O4^iv	0.95 (4)	2.49 (4)	3.354 (5)	152 (3)
Symmetry codes: (i) ; (ii) ; (iii) ; (iv) .

Figure 3 Two-dimensional hydrogen-bonded structure of 1.

Figure 4 Three-dimensional hydrogen-bonded framework of 1.

4. Database survey

A Cambridge Structural Database online search (July 17, 2025; Groom et al., 2016) for the [(μ₃-hydrogen­phosphato)(pyridine)­zinc] unit yielded no hits, indicating that no zinc phosphates coordinated by pyridine ligands have been reported. A search for zinc phosphates datively coordinated by N-donor ligands revealed several species containing NH₃ (Amghouz et al., 2014); 5-(4-pyrid­yl)tetra­zolate) (Yang et al., 2009); 1,10-phenanthroline (Lin et al., 2003a); 4,4-dimethyl-2,2 -dipyridyl, 5,5-dimethyl-2,2-dipyridyl (Lin et al., 2007) 1,2-di­methyl­imidazole (Mao et al., 2020); 4H-1,2,4-triazole-kN¹ (Aitenneite et al., 2012).

5. Synthesis and crystallization

[Mo₃O₂(O₂CCH₃)₆(H₂O)₃]ZnCl₄·8H₂O (0.1 g, 0.1 mmol) (Xu et al., 2018, 2025) was added to a mixture of H₃PO₄ (85%, 0.3 ml), pyridine (py, 6 ml) and water (4 ml). The resulting mixture was sealed in a 25 ml Teflon-lined steel autoclave and heated at 393 K for three days. The reactor was cooled to room temperature at a rate of 4 K h⁻¹ to produce colourless crystals of pyZn(HPO₄) (1), differing from the previous synthetic methodology of zinc phosphates wherein zinc sources were from zinc oxides or zinc salts such as Zn(O₂CCH₃)₂. The synthesis is shown in Fig. 5. Notably, the solvothermal reactions using zinc oxides or zinc salts instead of [Mo₃O₂(O₂CCH₃)₆(H₂O)₃]ZnCl₄ failed to produce pyZn(HPO₄) (1), indicative of some role of the dianionic group ZnCl₄²⁻ as zinc source.

Figure 5 Solvothermal synthesis of 1.

6. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 2. The crystal studied was refined as an inversion twin.

Table 2
Experimental details

Crystal data
Chemical formula	[Zn(HPO4)(C5H5N)]
Mr	240.45
Crystal system, space group	Monoclinic, P21
Temperature (K)	150
a, b, c (Å)	7.7394 (3), 5.3806 (2), 9.0929 (4)
β (°)	91.246 (2)
V (Å^3)	378.56 (3)
Z	2
Radiation type	Mo Kα
μ (mm^−1)	3.42
Crystal size (mm)	0.21 × 0.15 × 0.06
Data collection
Diffractometer	Bruker APEXII CCD
Absorption correction	Multi-scan (SADABS; Krause et al., 2015)
Tmin, Tmax	0.789, 1.000
No. of measured, independent and observed [I > 2σ(I)] reflections	2993, 1489, 1467
Rint	0.032
(sin θ/λ)max (Å^−1)	0.649
Refinement
R[F^2 > 2σ(F^2)], wR(F^2), S	0.023, 0.066, 0.87
No. of reflections	1489
No. of parameters	134
No. of restraints	7
H-atom treatment	All H-atom parameters refined
Δρmax, Δρmin (e Å^−3)	0.48, −0.42
Absolute structure	Refined as an inversion twin
Absolute structure parameter	0.051 (19)
Computer programs: APEX2 and SAINT (Bruker, 2014), SHELXT2014/5 (Sheldrick, 2015a), SHELXL2016/6 (Sheldrick, 2015b) and DIAMOND (Brandenburg, 2005).

Supporting information

Computing details

Poly[(µ₃-hydrogen phosphato)(pyridine)zinc(II)]

Crystal data

[Zn(HPO4)(C5H5N)]	F(000) = 240
Mr = 240.45	Dx = 2.109 Mg m^−3
Monoclinic, P21	Mo Kα radiation, λ = 0.71073 Å
a = 7.7394 (3) Å	Cell parameters from 2993 reflections
b = 5.3806 (2) Å	θ = 3.5–27.5°
c = 9.0929 (4) Å	µ = 3.42 mm^−1
β = 91.246 (2)°	T = 150 K
V = 378.56 (3) Å^3	Plate, white
Z = 2	0.21 × 0.15 × 0.06 mm

Data collection

Bruker APEXII CCD diffractometer	1467 reflections with I > 2σ(I)
φ and ω scans	Rint = 0.032
Absorption correction: multi-scan (SADABS; Krause et al., 2015)	θmax = 27.5°, θmin = 3.5°
Tmin = 0.789, Tmax = 1.000	h = −10→10
2993 measured reflections	k = −6→6
1489 independent reflections	l = −10→11

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)] = 0.023	w = 1/[σ^2(Fo^2) + (0.0516P)^2] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.066	(Δ/σ)max = 0.001
S = 0.87	Δρmax = 0.48 e Å^−3
1489 reflections	Δρmin = −0.42 e Å^−3
134 parameters	Absolute structure: Refined as an inversion twin
7 restraints	Absolute structure parameter: 0.051 (19)

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.

Refinement. Refined as an inversion twin.

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

	x	y	z	Uiso*/Ueq
Zn	0.84495 (4)	0.37175 (6)	0.86294 (3)	0.00820 (15)
P	0.75242 (9)	0.8661 (2)	1.02007 (8)	0.00782 (19)
O1	0.7596 (4)	0.7061 (6)	0.8821 (3)	0.0141 (5)
O2	0.5993 (3)	0.7752 (6)	1.1188 (3)	0.0133 (5)
H	0.496 (5)	0.724 (13)	1.073 (5)	0.041 (17)*
O3	0.7118 (3)	1.1366 (5)	0.9761 (3)	0.0105 (5)
O4	0.9091 (3)	0.8453 (7)	1.1202 (3)	0.0150 (6)
N	0.7918 (4)	0.3106 (6)	0.6448 (3)	0.0108 (7)
C1	0.6998 (5)	0.1149 (8)	0.5928 (4)	0.0134 (7)
H1	0.670 (5)	−0.016 (7)	0.662 (4)	0.010 (12)*
C2	0.6540 (5)	0.0929 (8)	0.4445 (4)	0.0178 (8)
H2	0.589 (7)	−0.050 (8)	0.416 (5)	0.035 (15)*
C3	0.7034 (6)	0.2769 (8)	0.3478 (4)	0.0176 (8)
H3	0.667 (5)	0.283 (9)	0.246 (3)	0.014 (12)*
C4	0.8002 (5)	0.4766 (8)	0.4008 (4)	0.0172 (8)
H4	0.831 (6)	0.623 (7)	0.349 (5)	0.021 (14)*
C5	0.8404 (5)	0.4859 (8)	0.5486 (4)	0.0150 (8)
H5	0.894 (6)	0.623 (7)	0.594 (5)	0.017 (12)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Zn	0.0081 (2)	0.0080 (2)	0.0085 (2)	0.00058 (18)	−0.00045 (13)	−0.00030 (19)
P	0.0070 (3)	0.0077 (4)	0.0087 (4)	0.0005 (5)	0.0001 (3)	0.0012 (5)
O1	0.0213 (13)	0.0094 (12)	0.0117 (13)	0.0050 (11)	0.0012 (10)	−0.0018 (11)
O2	0.0093 (12)	0.0185 (13)	0.0121 (12)	−0.0043 (11)	0.0003 (9)	0.0040 (11)
O3	0.0098 (11)	0.0081 (12)	0.0136 (12)	0.0004 (10)	0.0014 (9)	0.0000 (10)
O4	0.0091 (10)	0.0213 (16)	0.0144 (11)	−0.0005 (12)	−0.0015 (8)	0.0032 (13)
N	0.0100 (13)	0.0109 (17)	0.0116 (14)	0.0011 (10)	−0.0009 (10)	−0.0007 (11)
C1	0.0172 (18)	0.0097 (16)	0.0132 (17)	−0.0017 (15)	−0.0011 (14)	−0.0020 (14)
C2	0.023 (2)	0.016 (2)	0.0146 (18)	−0.0008 (15)	−0.0040 (16)	−0.0034 (16)
C3	0.022 (2)	0.0205 (18)	0.0107 (17)	0.0075 (16)	0.0001 (15)	−0.0025 (15)
C4	0.0170 (19)	0.020 (2)	0.0145 (17)	0.0021 (16)	0.0037 (15)	0.0030 (16)
C5	0.0152 (19)	0.015 (2)	0.0149 (18)	−0.0035 (15)	0.0010 (15)	−0.0021 (16)

Geometric parameters (Å, º)

Zn—O4^i	1.911 (2)	N—C1	1.351 (5)
Zn—O1	1.926 (3)	C1—C2	1.392 (5)
Zn—O3^ii	1.941 (3)	C1—H1	0.97 (2)
Zn—N	2.043 (3)	C2—C3	1.383 (6)
P—O4	1.505 (3)	C2—H2	0.95 (3)
P—O1	1.523 (3)	C3—C4	1.390 (6)
P—O3	1.540 (3)	C3—H3	0.96 (2)
P—O2	1.579 (3)	C4—C5	1.374 (6)
O2—H	0.93 (3)	C4—H4	0.95 (3)
N—C5	1.345 (5)	C5—H5	0.94 (2)
O4^i—Zn—O1	113.89 (14)	C5—N—Zn	117.7 (3)
O4^i—Zn—O3^ii	116.63 (13)	C1—N—Zn	124.0 (3)
O1—Zn—O3^ii	111.95 (11)	N—C1—C2	121.8 (4)
O4^i—Zn—N	104.18 (11)	N—C1—H1	118 (3)
O1—Zn—N	100.16 (12)	C2—C1—H1	120 (3)
O3^ii—Zn—N	108.16 (12)	C3—C2—C1	119.1 (4)
O4—P—O1	114.37 (17)	C3—C2—H2	124 (3)
O4—P—O3	112.61 (19)	C1—C2—H2	117 (3)
O1—P—O3	109.36 (16)	C2—C3—C4	119.2 (4)
O4—P—O2	103.77 (15)	C2—C3—H3	124 (3)
O1—P—O2	109.52 (18)	C4—C3—H3	117 (3)
O3—P—O2	106.80 (16)	C5—C4—C3	118.4 (4)
P—O1—Zn	128.43 (18)	C5—C4—H4	114 (3)
P—O2—H	119 (3)	C3—C4—H4	128 (3)
P—O3—Zn^iii	130.24 (16)	N—C5—C4	123.4 (4)
P—O4—Zn^iv	146.21 (16)	N—C5—H5	113 (3)
C5—N—C1	118.1 (3)	C4—C5—H5	123 (3)
O4—P—O1—Zn	40.3 (3)	C5—N—C1—C2	−0.5 (5)
O3—P—O1—Zn	167.62 (19)	Zn—N—C1—C2	174.2 (3)
O2—P—O1—Zn	−75.7 (2)	N—C1—C2—C3	−0.3 (6)
O4—P—O3—Zn^iii	64.0 (2)	C1—C2—C3—C4	1.2 (6)
O1—P—O3—Zn^iii	−64.4 (3)	C2—C3—C4—C5	−1.3 (6)
O2—P—O3—Zn^iii	177.22 (19)	C1—N—C5—C4	0.5 (6)
O1—P—O4—Zn^iv	55.2 (5)	Zn—N—C5—C4	−174.6 (3)
O3—P—O4—Zn^iv	−70.5 (4)	C3—C4—C5—N	0.4 (6)
O2—P—O4—Zn^iv	174.4 (4)

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

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O2—H···O3^v	0.94 (4)	1.73 (4)	2.647 (3)	168 (4)
C1—H1···O1^ii	0.98 (4)	2.58 (4)	3.452 (5)	149 (3)
C3—H3···O3^vi	0.96 (2)	2.61 (3)	3.465 (5)	148 (3)
C4—H4···O4^vii	0.95 (4)	2.49 (4)	3.354 (5)	152 (3)

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