The asymmetric unit of 1contains one 2-amino-1-phenyleneammonium cation, two Zn atoms (one of which lies on a crystallographic twofold axis), two (HPO₄)²⁻ hydrogen phosphate groups and one water molecule of crystallisation (Figure 1). This leads to the 3:4 ratio of Zn:P seen in this compound.

Both the zinc atoms in 1 adopt tetrahedral ZnO₄ geometries: mean Zn–O separations of 1.949 Å and 1.947 Å occur for Zn1 and Zn2, respectively. The bond angles around Zn2 (site symmetry 2) are noticeably more distorted from ideal tetrahedral values than those for Zn1: angular variances as defined by Robinson et al. [5] of 5.25°² (Zn1) and 37.42°² (Zn2) arise. The Brown–Altermatt [6] bond valence sum (BVS) values for Zn1 and Zn2 are 2.07 and 2.08, respectively (expected value = 2.00). All the Zn–O vertices form a bi-coordinate bridge to an adjacent P atom (mean Zn–O–P = 127.3°), thus there are no Zn–O–Zn links in 1.

The hydrogen phosphate anions possess typical geometrical parameters [7]: mean P1−O = 1.539 Å; O–P1–O angular variance = 4.14°², mean P2–O = 1.536 Å; O–P1–O angular variance = 9.03°². The long P1–O4 and P2–O8 bonds (Table 1) indicate that they are protonated [8], and the H atoms were visible in difference maps. These terminal bonds do not link to zinc atoms. The geometrical parameters for the organic cation in 1 are typical [9].

The connectivity of the Zn- and P-centred building units in 1 leads to infinite sheets of tetrahedra, which propagate in the (100) plane, with the organic cations and water molecules occupying the inter-layer regions (Figure 2). Each Zn node bonds to four different P atoms and each P atom bonds to three different Zn atoms (as well as its terminal P–OH vertex), thus a topological (3,4) net [10] is formed. The environments of the phosphorus atoms are quite different: pairs of HP1O₄ groups are connected by three ZnO₄ tetrahedra to form distinctive “lantern” motifs, whereas the HP2O₄ groups are linked into pairs bridged by Zn1-centred groups as well as a link to a Zn2 species.

In terms of polyhedral circuits [11], this connectivity leads to 4-, 6- and 8-ring loops (see Figure 3 below) in the sheets. The 4-ring is a simple grouping of two Zn1- and two P2-centred polyhedra and is generated by inversion symmetry. The 6-ring (generated by twofold rotation symmetry) is more complex and could be described as “bifurcated” [12] due to the presence of a lantern motif in the ring. Finally, the inversion-generated 8-ring contains two lantern motifs as part of its alternating array of Zn- and P-centred polyhedra. As discussed below, these different rings seem to strongly correlate with the hydrogen bonds present in this structure.

Hydrogen bonds are an important feature of most organically-templated networks [13] and the various interactions in 1 are summarised in Table 2. Atom N1 of the APA cation is protonated (as an NH₃⁺ group) and forms two N–H...O links to bridging O atoms and one link to a terminal P–OH group. All these O atoms occur within one particular (100) sheet, i.e., there are no inter-sheet N–H...O links. Based on the short (<2.0 Å) H...O separations and large N–H...O angles (>150°), we may assume that these hydrogen bonds are relatively strong.

The situation for the un-protonated N2 group is quite different. This forms one weak (based on its much longer H...O separation of 2.30 Å) N–H...O link to a bridging O atom in the same sheet as the acceptor O atoms for the N1 bonds (Figure 1). Its other H atom does not form a hydrogen bond and points into the inter-layer region. An unusual feature of 1 is the presence of a framework-to-template O–H...N hydrogen bond arising from the P1–O4H group, which points towards the region of the presumed lone-pair of electrons of atom N2 (Figure 1).

The P2–O8H group forms an O–H...O hydrogen bond to the water molecule and the water molecule in turn forms two O–H...O hydrogen bonds to framework O atom. Again, these framework atoms are in the same (100) sheet. In terms of the graph-set notation of Bernstein et al. [14], the smallest circuits involving the N1H₃⁺ and N2H₂ moieties are R²₂(10) and R²₂(8) loops, respectively, whereas the water molecule is involved in an R²₂(8) loop.

These hydrogen bonds appear to exert a significant template effect, as shown in Figure 3. The polyhedral 8-ring is templated by no fewer than eight N–H...O links (four each from the two symmetry-related cations lying to either side of the inorganic sheet) as well as an O–H...N “back bond” from a framework P–OH grouping to each of the template N-atoms. Each O atom in the ring accepts one hydrogen bond. The 6-ring is templated by a pair of water molecules (one above the plane of the framework and one below) but again P–OH...O framework-to-water hydrogen bonds consolidate this effect.

It is notable that there are no classical hydrogen bonds that provide inter-layer connectivity in this phase, i.e., as a topological L...T...L linkage [15], which is commonly seen in layered ZnPOs templated by linear-chain, protonated diamines [16]. However, two C–H...O bonds (Table 2, Figure 4) may provide this cohesion in 1: there is no ambiguity in the location of the aromatic C–H atoms in question and the H...O separations (around 2.50 Å) and C–H...O angles (>150°) fall well within the accepted criteria for such “non-classical” bonds [17]. Conversely, there are no aromatic π–π stacking interactions in 1, as the shortest centroid–centroid separation is over 4.24 Å.