The asymmetric unit of compound 1 contains four Zn atoms, two P atoms, 11 O atoms (three of which belong to water molecules) and two C₅H₁₄N₂ (dap) molecules (Figure 1).

The zinc ions in 1 adopt two coordination modes: Zn1 and Zn2 (which both lie on crystallographic twofold axes) are coordinated by two O and two N atoms and Zn3 and Zn4 are bonded to three O and one N atom in tetrahedral geometries (Table 1). The global mean Zn-O and Zn-N separations are 1.937 Å and 2.038 Å, respectively, which are very similar to the equivalent geometrical data for related compounds [11]. The XO₄ tetrahedral angular variances, defined as where ϑ is the O-X-O bond angle (°) [12], indicate that all the Zn-centred tetrahedra in 1 are significantly distorted: values of 98.4°², 79.8°², 81.2°² and 85.9°² arise for Zn1, Zn2, Zn3 and Zn4, respectively. All the O atoms also link to an adjacent P atom (mean Zn-O-P = 124.5°) and the N atoms are all parts of neutral dap molecules. Both PO₄ groups in 1 are close to regular tetrahedra (mean P1-O = 1.538 Å, angular variance = 2.7°²; mean P2-O = 1.541 Å, angular variance = 1.4°²) and all the O atoms link to an adjacent Zn atom, thus there are no terminal or “dangling” P=O or P-OH bonds [13] in 1. The geometrical parameters for the two unique dap molecules are unexceptional [14], and both molecules are in essentially extended conformations (mean absolute values of the N-C-C-C and C-C-C-C torsion angles = 177.6° and 174.7°, respectively).

The polyhedral connectivity of the Zn- and P-centred moieties leads to (100) sheets of vertex-sharing tetrahedra (Figure 2), in which there is prefect alternation of the Zn and P nodes (i.e., no Zn-O-Zn or P-O-P links). These sheets can be decomposed into four-ring ladders [15] propagating in [010] built up from the Zn3- and Zn4-centred species and the phosphate groups, with Zn1 and Zn2 (i.e., the ZnN₂O₂ species) providing inter-chain links, as parts of polyhedral 8-rings. For any [010] stack of 8-rings, all the Zn1 (or Zn2) tetrahedra point in the same direction.

The dap molecules form bridges (via both their N atoms) to the adjacent (100) polyhedral sheets to generate a distinctive porous network (Figure 3), in which elongated [010] channels are bounded by the methylene groups of the dap molecules on their “long” sides and by the ZnPO framework on their “short” sides. Measured atom-to-atom, their dimensions are about 4.5 Å × 11.3 Å. It remarkable that there are three distinct types of channel in this structure: one contains four water molecules per b unit-cell repeat distance (type-A), one contains two water molecules (type-B) and the third (type-C) is empty. It is apparent from Figure 3 that the type-A channels “bulge out” to accommodate the water molecules and the type-C channels are consequently compressed inwards and are empty.

Within the type-A channel, the water molecules interact via O-H∙∙∙O hydrogen bonds (Table 2), such that a C(2) chain of O12-H6W∙∙∙O12 bonds propagates up the central region of the channel. The other H atom attached to O12 forms a link to O11, which in turn forms two hydrogen bonds to framework O atoms. The -NH₂ groups of the dap molecules form N-H∙∙∙O hydrogen bonds to framework O atoms as well as the water molecules to reinforce this rather intricate network, which occurs in a largely hydrophobic environment.

The asymmetric unit of 2 contains three Zn atoms, two PO₄ groups, one HPO₄ group, a water molecule and a doubly-protonated H₂dap dication (Figure 4).

The three Zn atoms form the centers of ZnO₄ tetrahedra (mean Zn1-O =1.943 Å, mean Zn2-O = 1.939 Å, mean Zn3-O = 1.939 Å). These are somewhat less distorted than the zinc polyhedra in 1 as indicated by the angular variances of 32.2°², 37.4°², and 47.2°², for Zn1, Zn2 and Zn3, respectively. Atoms P1 and P2 (mean P-O = 1.534 Å and 1.536 Å, respectively; angular variances = 2.9°² and 2.4°², respectively) are the central atoms of essentially regular phosphate groups and all their O atoms link to nearby zinc atoms. Atom P3 (mean P-O = 1.534 Å, angular variance = 14.9°²) forms three links to Zn but also possesses a terminal bond to O12. Its length of 1.587 (4) Å indicates that it must by protonated (i.e., a P-OH species) and the corresponding H atom could indeed be located in a difference map. Geometrical data for 2 are summarized in Table 3.

In the extended structure of 2, atom O4 plays an important role in the linking of the tetrahedra, as it forms bonds to Zn1, Zn2 and P1 (bond-angle sum = 357.6°). This results in “dimers” of the zinc species (Figure 4), but the Zn-O-Zn connectivity does not extend any further than this [16]. The mean value of the Zn-O-P bond angle for the other O atoms is 133.7°, almost 10 degrees larger than the corresponding value in 1.

In the extended structure of 2, the inorganic layers form infinite (100) sheets containing 4- and 10-rings (Figure 5). The sheets sandwich the H₂dap cations and the extra-layer water molecule (O13) (Figure 6). The water molecule accepts an O-H∙∙∙O hydrogen bond from the hydrogen phosphate group and also probably forms O-H∙∙∙O links to framework oxygen atoms. The protonated -NH₃⁺ groups of the H₂dap cation form three N-H∙∙∙O hydrogen bonds each (Table 4), which is a thoroughly typical bonding mode for a protonated amine in a ZnPO [5]. The conformation of the carbon chain of the H₂dap species is somewhat uncertain, but its contorted geometry is clearly different to the extended conformation found for the bridging dap molecules in 1.

Compound 1 was prepared from0.798 g ZnO, 4.9 mL 85% H₃PO₄, 1.00 g dap and 10 mL water (Zn:PO₄:dap ratio = 2:1:2). The components were placed in a 60-mL HDPE bottle and sealed (starting pH ≈ 6.0). The bottle was shaken for five minutes and placed in an 80 °C oven for four days. The bottle was removed from the oven and cooled to room temperature over 30 minutes and the solid product, consisting of colorless plates of 1 accompanied by white and brownish powders was recovered by vacuum filtration and rinsing with water and acetone. Compound 2 was prepared from 0.794 g ZnO, 9.8 mL 85% H₃PO₄, 0.51 g dap and 10 mL water (Zn:PO₄:dap ratio = 2:2:1; starting pH ≈ 2.0) and subjected to the same heating and product recovery protocol. Pale brown blocks of 2 were recovered, accompanied by some white powder.

The single-crystal data for 1 (colorless slab, 0.10 × 0.10 × 0.02 mm) were collected using a Rigaku Saturn CCD diffractometer at 93 K (Mo Kα radiation, λ = 0.71073 Å); the data for 2 (pale brown block, 0.40 × 0.30 × 0.24 mm) were collected using a Bruker Kappa APEX II CCD diffractometer (Mo Kα radiation, λ = 0.71073 Å) at room temperature. After data reduction, the structures were solved by direct methods with SHELXS and the resulting atomic models were developed and refined against |F|² with SHELXL [17]. The “observed data” threshold for calculating the R(F) residuals was set as I > 2σ(I).

For 1, the C- and N-bound bound H atoms were placed in idealised locations (C-H = 0.99 Å, N-H = 0.92 Å) and refined as riding atoms. The O-bound (water) H atoms were located in difference maps and refined as riding atoms in their as-found relative locations. The constraint U_iso(H) = 1.2U_eq(carrier) was applied in all cases. The structural model was analysed and validated with PLATON [18] and full refinement details are given in the deposited cif. PLATON indicated considerable pseudo-symmetry corresponding to space group C2/c. Trial refinements in this space group, which revealed a number of systematic absence violations, led to essentially the same structure for the framework (albeit with dubious anisotropic displacement parameters) but un-resolvable disorder of the water molecules in the channels. Thus it appears that the symmetry lowering is due to ordering of the water molecules in the channels.

The methylene chain in the H₂dap species in 2 was found to be severely disordered although the terminal -NH₃⁺ groupings were well defined. The application of C-C bond distance restraints led to a geometrically plausible conformation but this cannot be regarded as certain. The H atoms of the water molecule could not be located from difference maps and may be disordered. The C- and N-bound bound H atoms in 2 were placed in idealised locations (C-H = 0.97 Å, N-H = 0.89 Å) and refined as riding atoms. The P-OH H atom was located in a difference map and refined as riding in its as-found relative location. The constraint U_iso(H) = 1.2U_eq(carrier) was applied in all cases. Further refinement details are given in the deposited cif.

Crystal data for 1: C₁₀H₃₄N₄O₁₁P₂Zn₃, M_r = 644.46, monoclinic, C2 (No. 4), Z = 4, a = 25.302 (7) Å, b = 4.9327 (13) Å, c = 19.808 (6) Å, β = 107.377 (8)°, V = 2359.4 (12) ų, F(000) = 1320, T = 93 (2) K, ρ_calc = 1.814 g cm⁻³, µ = 3.216 mm⁻¹, 7612 reflections recorded (3.4° ≤ 2θ ≤ 50.7°; −30 ≤ h ≤ 30, −5 ≤ k ≤ 3, −23 ≤ l ≤ 18), R_Int = 0.047, 3310 merged reflections, 2728 with I > 2σ(I), 273 variable parameters, Flack absolute structure parameter = 0.46 (5), R(F) = 0.054, wR(F²) = 0.139, min./max. Δρ = –1.23, +1.01 e Å⁻³. Cambridge Structural Database deposition number: CCDC-883458.

Crystal data for 2: C₅H₁₉N₂O₁₃P₃Zn₃, M_r = 604.24, monoclinic, P2₁/c (No. 14), Z = 4, a = 11.3275 (15) Å, b = 8.3235 (11) Å, c = 18.588 (2) Å, β = 96.979 (3)°, V = 1739.6 (4) ų, F(000) = 1208, T = 293 (2) K, ρ_calc = 2.307 g cm⁻³, μ = 4.447 mm⁻¹, 10734 reflections recorded (8.7° ≤ 2θ ≤ 60.0°; −14 ≤ h ≤ 15, −11 ≤ k ≤ 11, −20 ≤ l ≤ 26), R_Int = 0.076, 4954 merged reflections, 3105 with I > 2σ(I), 237 variable parameters, R(F) = 0.056, wR(F²) = 0.119, min./max. Δρ = −0.85, +1.97 eÅ^–3. Cambridge Structural Database deposition number: CCDC-883459.