Consistent zincophosphite 4-ring ‘ladder’ chain structural motif with isomeric ligands

The same ‘ladder’ chain motif built up from ZnO3N and HPO3 units arises for zincophosphites templated by isomers of 2-amino-z-methylpyridine (z = 3, 4, 5).


Chemical context
Since the first report (Harrison et al., 2001) of zincophosphite (ZnPO) networks built up from vertex-sharing ZnO 4 or ZnO 3 N and HPO 3 building units templated or ligated by organic species, this family has grown to include well over 200 structures in the Cambridge Structural Database (CSD; Groom et al., 2016). Recent papers have described a ZnPO templated by a chiral amino acid, which displays non-linear optical behaviour (Mao et al., 2021) and a mixed-ligand ZnPO with promising gas sorption properties (Chen et al., 2022). As well as their potential applications, ZnPOs are of ongoing academic interest in terms of the challenge of designing rational and reproducible syntheses and the elucidation of the systematics of their crystal chemistry, for example, the effect of the Zn:P ratio, different polyhedral connectivities, hydrogen bonding and the 'dual role' (bonded ligand or protonated guest) of the organic template on the structure (Holmes et al., 2018).

Structural commentary
The asymmetric unit of (I) (Fig. 1), which crystallizes in the monoclinic space group C2/c, consists of a Zn 2+ ion, a [HPO 3 ] 2hydrogen phosphite anion, a C 6 H 7 N 2 2-amino-3methylpyridine molecule and a water molecule, the O atom of the last species lying on a crystallographic twofold axis. The zinc coordination polyhedron is a ZnO 3 N tetrahedron, i.e., the organic species is acting as a ligand bonding to the metal ion from its pyridine nitrogen atom and the Zn-O bonds (mean = 1.940 Å ) are notably shorter than the Zn1-N1 [2.0262 (14) Å ] link, as previously observed for related compounds (Holmes et al., 2018). The spread of bond angles about the metal ion [minimum = 104.23 (5) for O2-Zn1-N1, maximum = 113.89 (6) for O1-Zn1-N1] indicates a slight degree of distortion with 4 0 = 0.974 (Okuniewski et al., 2015). The [HPO 3 ] 2group adopts its usual tetrahedral (including the H atom) or pseudo-pyramidal (excluding H) geometry and the mean P-O separation is 1.522 Å with the O-P-O bond angles tightly clustered in the range 111.98 (7)-113.57 (7) ; the P atom is displaced by 0.4227 (8) Å from the plane of its attached O atoms. Each O atom is bonded to one Zn and one P atom [mean Zn-O-P = 130.2 ], thus there are no 'dangling' (Holmes et al., 2018) Zn-OH 2 , P O or P-OH bonds in this structure. The extended structure of (I) is discussed below.
Compound (V) is a simple molecular salt (Fig. 5), which crystallizes in the triclinic space group P1: its asymmetric unit consists of two 2-amino-3-methylpyridinium C 6 H 8 N 2 + cations protonated at their pyridine N atoms, a [ZnCl 4 ] 2anion and a water molecule of crystallisation. The tetrachlorozincate ion has a mean Zn-Cl separation of 2.2704 Å [range = 2.2536 (13)-2.2867 (13) Å ] and smallest and largest Cl-Zn-Cl bond angles of 104.48 (5) and 113.75 (5) , respectively. The synthetic intent here was to lower the pH with HCl and establish if a dihydrogen phosphite (H 2 PO 3 À ) anion containing a terminal P-OH moiety could be incorporated The asymmetric unit of (III) expanded to show the complete zinc-atom coordination sphere showing 50% displacement ellipsoids. Symmetry codes: (i) 1 2 + x, 3 2 À y, Àz; (ii) 1 + x, y, z.

Figure 5
The asymmetric unit of (V) showing 50% displacement ellipsoids. Hydrogen bonds are indicated by double-dashed lines.
into the structure (Lin et al., 2003) but the presence of excess chloride ions has led to a completely different and unwanted molecular salt containing the tetrachlorozincate complex ion, which has been reported many times before, with over 1000 matches in the CSD.

Supramolecular features
In the extended structure of (I), the constituent ZnO 3 N and HPO 3 polyhedra are linked by Zn-O-P bonds into [010] polyhedral 4-ring (two Zn and two P nodes) 'ladder' chains in which the zinc and phosphorus nodes strictly alternate (Fig. 6): the chains are built up by inversion symmetry at the centres of every 4-ring, as well as, of course, translation symmetry in the b-axis direction. Given that the Zn atom forms three bonds (via O atoms) to adjacent P atoms (and a fourth bond to the organic species) and that the P atom forms three links to zinc atoms (and a fourth P-H vertex), the 1:1 Zn:P stoichiometry is to be expected and hence no charge compensating, protonated template is needed. In (II), ladder chains similar to those seen in (I) arise in the extended structure ( Fig. 6) although they are more contorted: because Z 0 = 2, every other 4-ring is generated by inversion symmetry and translation in the [101] direction leads to the extended array. In (III), the 4ring ladder motif is again apparent (Fig. 6), although in this case, the combination of a 2 1 screw-axis parallel to the chain and a-translation symmetry generates the infinite [100] chains. In each structure, the organic molecules are pendant to the chains (Fig. 6). The extended structure of (IV) (Fig. 7) is quite different to those of (I)-(III) and features (010) sheets of ZnO 4 and HPO 3 polyhedra sharing corners. One way to visualize this rather complex arrangement (although this does not necessarily imply that the synthesis proceeds in such a step-by-step fashion) is in terms of contorted chains of 4-rings featuring atoms Zn1, Zn2, Zn3, P2, P3 and P4 as the nodes propagating in the [001] direction. One out of every three 4-rings in a chain is generated by inversion symmetry. These chains are crosslinked in the a-axis direction by the P1-centred hydrogen phosphite groups to form the (010) layers, which encapsulate 8-ring voids built up from four Zn and four P nodes although there is no suggestion of 'zeolitic' porosity. So far as stoichiometry is concerned, in this case the zinc nodes forming four bonds (via all their O atoms) to nearby phosphorus atoms and the P nodes forming three bonds to Zn atoms leads to the 3:4 ratio of zinc and phosphorus, which is the proportion most commonly seen in this family of phases (e.g., Phillips et al., 2002;Lin et al., 2009). In this case, the inorganic component bears a charge of À2 per [Zn 3 (HPO 3 ) 4 ] unit, hence the two protonated template molecules. The template cations and water molecules of crystallisation occupy the inter-layer regions.
Various classical (N-HÁ Á ÁO, N-HÁ Á ÁCl and O-HÁ Á ÁO) and non-classical (C-HÁ Á ÁO and C-HÁ Á ÁCl) hydrogen bonds occur in these structures. As is normal, the hydrogen phosphite P-H unit does not participate in hydrogen bonding (Katinaitė & Harrison, 2017). In (I), the water molecule of crystallization, which lies on a crystallographic twofold axis, appears to play an important role in consolidating the extended structure by accepting two N-HÁ Á ÁO hydrogen bonds (Table 1)    Comparison of the zincophosphite 4-ring ladder chains in the extended structures of (I) (left), (II) (centre) and (III) (right).  other hydrogen bond arising from the amine group is an intrachain N-HÁ Á ÁO link. There are no aromaticstacking interactions in (I), the shortest centroid-centroid separation being some 5.04 Å .

Database survey
A survey of the Cambridge Structural Database (Groom et al., 2016;updated to February 2023) revealed 213 crystal structures containing zinc cations and hydrogenphosphite anions (Zn-O-P-H fragment) of which 53 contain a ligated organic molecule (Zn-N bond). The only phase that bears a close chemical similarity to the structures described here is [C 5 (Liang et al., 2003), in which the ZnO 3 N and HPO 3 polyhedra assemble into (100) layers of 4-and 8-rings.
The fact that the N-bonded zinc ions and HPO 3 units in (I), (II) and (III) self assemble to form the same 4-ring ladder chain with different isomeric pyridine-based ligands suggests that it is a reasonably robust structural feature. However, it is not a particularly common motif in the wider ZnPO phase space: two other examples with very different ligating molecules to those in (I)-(III) are [C 4 H 8 N 2 O 3 ÁZn(HPO 3 )] n (C 4 H 8 N 2 O 3 = l-asparagine) (Gordon & Harrison, 2004) and [C 3 H 7 NO 2 ÁZn(HPO 3 )] n (C 3 H 7 NO 2 = racemic dl-alanine) (Mao et al., 2021); it is notable that these amino acids both bond to the zinc atom via one of their carboxylate O atoms rather than the pyridine N atoms in (I)-(III).
Compound (II), in which the C 6 H 8 N 2 organic molecule acts as a ligand (a Zn-N bond and a 1:1 Zn:P ratio) and (IV), in which the same organic species acts as a protonated C 6 H 9 N 2 + template (N-HÁ Á ÁO hydrogen bonds and a 3:4 Zn:P ratio) arose from similar syntheses, with the only difference being the source of zinc ions (zinc oxide and zinc acetate, respectively). Assuming that hydrothermal synthesis is not just an impenetrable 'black box' (Ursu et al., 2022), we may speculate that the acetate synthesis occurred at a lower pH, perhaps with some buffering action between acetic acid formed in situ and acetate ions, to allow for the protonation of the template.

Figure 11
The unit-cell packing in (IV) viewed down [100]. Hydrogen bonds are shown as dashed lines.

Figure 12
The unit-cell packing in (V) viewed approximately down [101]. Hydrogen bonds are shown as dashed lines.
week led to the same product, with a slight improvement in crystallinity, as indicated by sharper peaks in its IR spectrum and X-ray powder diffraction pattern.
Compound (II) was prepared from 0.75 g of ZnO, 0.81 g of H 3 PO 3 and 1.10 g of 2-amino-4-methylpyridine (Zn:P:template ratio ' 1:1:1); otherwise following the same procedure as for (I). A mass of blocky transparent crystals was recovered. IR: 2394, 2382 cm À1 (P-H stretch). Two peaks may arise because of the two different P-H groups in the asymmetric unit (Ma et al., 2007).
To prepare compound (III), 2.20 g of Zn(OAc) 2 , 0.86 g of H 3 PO 3 and 1.09 g of 2-amino-5-methylpyridine (Zn:P:template ratio ' 1:1:1) and 20 ml of water were placed in a 50 ml polypropylene bottle and heated to 353 K for three days. Upon cooling, the product consisted of a mass of colourless blocks. IR: 2406 cm À1 (P-H stretch).
Compound (IV) started from a mixture of 2.02 g Zn(OAc) 2 , 0.77 g of H 3 PO 3 and 1.03 g of 2-amino-4-methylpyridine (Zn:P:template ratio ' 1:1:1) and 20 ml of water. These components were placed in a 50-ml polypropylene bottle and heated to 353 K for 24 h. Upon cooling, the product consisted of a mass of colourless blocks. IR: 3000-3600 (broad) (O-H stretch), 2391, 2381 cm À1 (P-H stretch). The same product arises if the mixture is heated for one week.
Compound (V) was prepared from the same reagents as (I) and the same synthesis procedure but with the addition of 10 ml of 1 M HCl.
atoms were located in difference maps and their positions were freely refined with U iso (H) = 1.2U eq (N or O). The phosphite H atoms were geometrically placed (P-H = 1.32 Å ) and refined as riding atoms with U iso (H) = 1.2U eq (P). All the C-bound H atoms were located geometrically (C-H = 0.95-0.98 Å ) and refined as riding atoms with U iso (H) = 1.2U eq (C) or 1.5U eq (methyl C). The methyl groups were allowed to rotate, but not to tip, to best fit the electron density. Two peaks greater than 1 e Å À3 were found in the final difference map for (IV) in the vicinity of the C7 cation but they did not correspond to a plausible chemical feature. The data quality for (V) was notably poorer than for the other four crystals.

Poly[[(2-amino-3-methylpyridine)-µ 3 -phosphonato-zinc] hemihydrate] (I)
Crystal data 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.

Poly[(2-amino-4-methylpyridine)-µ 3 -phosphonato-zinc] (II)
Crystal data 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.

Crystal data
[Zn(HPO 3 )(C 6 H 8 N 2 )] M r = 253.49 Orthorhombic, P2 1 2 1 2 1 a = 5.1487 (9)  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.

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 (Å 2 )
x y z U iso */U eq

Refinement
Refinement on F 2 Least-squares matrix: full R[F 2 > 2σ(F 2 )] = 0.047 wR(F 2 ) = 0.111 S = 1.08 3383 reflections 203 parameters 0 restraints Primary atom site location: structure-invariant direct methods Hydrogen site location: mixed H atoms treated by a mixture of independent and constrained refinement w = 1/[σ 2 (F o 2 ) + (0.0281P) 2 + 3.4309P] where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 1.10 e Å −3 Δρ min = −1.01 e Å −3 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.