catena-Poly[oxidanium [tris{μ-[amino(iminio)methyl]phosphonato}zincate(II)]]

The crystal structure of the anionic zinc–[amino(iminio)methyl]phosphonate one-dimensional coordination polymer, Zn-AIMP, is described.


Structure description
The chemistry of phosphonic acids was initiated by the need for hydrolysis-resistant replacements for polyphosphates. Synthetic access to a variety of phosphonic acid structures is possible through several well-established routes (Sevrain et al., 2017). To the inorganic chemist, phosphonic acids are a valuable synthetic tool as versatile ligands for generating a plethora of metal phosphonate compounds that present diverse structural architectures, from molecular complexes, to chains and layers, to framework structures (Clearfield & Demadis, 2012). Herein, we report a new Zn II phosphonate one-dimensional anionic coordination polymer that contains the ligand [amino(iminio)methyl]phosphonate ({[Zn(CH 4 N 2 PO 3 ) 3 ] À } n , Zn-AIMP) and an oxidanium (H 3 O + ) cation. The ligand AIMP was generated in situ during the synthesis by the decomposition of the hexaethyl 1,3,5-triazine-2,4,6-triyltris(phosphonate) ester upon dealkylation with trimethylbromosilane.
AIMP exists as a zwitterion in acidic solutions and it is neutral. However, at the pH of the reaction with Zn II , its second phosphonic acid group is deprotonated, thus generating the AIMP anion. The Zn:AIMP molar ratio in Zn-AIMP is 1:3. Upon careful examination, the +2 charge of Zn II is offset by three mono-anionic AIMP ligands, offering a total charge of À3. In the absence of any other cations in solution, the excess À1 charge per building unit is balanced by an oxidanium cation that is generated by protonation of water (from the solvent). Zn-AIMP is a one-dimensional coordination polymer, its chains extending parallel to the c axis. The Zn 2+ cation has a slightly distorted octahedral geometry, as illustrated in Fig. 1, coordinated exclusively by six phosphonate oxygen atoms from six different AIMP ligands. The Zn-O distance is 2.0927 (16) Å , which falls in the expected Zn-O(phosphonate) range (Colodrero et al., 2010). Each AIMP ligand bridges two neighbouring Zn 2+ cations, Fig. 1.
The phosphonate group in the AIMP ligand is fully deprotonated, while the N-C-N moiety is protonated, hence each N atom bears two H atoms. From symmetry, the C1-N1 bonds are equivalent, with the bond length at 1.310 (3) Å being intermediate between those of a single and a double bond. The C-N bond length is comparable to that found in 'free' AIMP [1.299 (5) Å and 1.314 (5) Å ; Yang et al., 2010].
The P-O bond lengths are 1.4957 (15) Å (coordinating) and 1.527 (2) Å (non-coordinating). It is reasonable to assume that the À2 charge on the phosphonate group is delocalized over all three O atoms. However, the P1-O2 bond (noncoordinating) is substantially longer than the P1-O1 bond (coordinating) and this can be rationalized by the formation of hydrogen bonds between O2 with two two N-H moieties and the oxidanium cation (see below). The packing of the chains in Zn-AIMP along the b-and c-axis directions is shown in Fig. 2 (left and middle). The linear chains (intra-chain Zn-Zn-Zn angle = 180 ) are packed parallel to the c axis. The oxidanium cation sits close to the non-coordinating P-O moiety of the chain and close to the N-C-N moiety of the neighbouring chain. The arrangement of the oxidanium cations (viewed down the c axis) is better described as staggered triangles that are $4.75 Å apart, see Fig    Packing of Zn-AIMP along the b axis (left) and along the c axis (middle). Arrangement of the H 3 O + staggered triangles (right). The disordered H 3 O + cations are shown as exaggerated green spheres. Colour coding is the same as in Fig. 1. Table 1 Hydrogen-bond geometry (Å , ). Symmetry codes: (i) Ày þ 1; x À y þ 1; z; (ii) y; Àx þ y; Àz À 1.
The presence of several hydrogen-bond donors and acceptors in the structure creates hydrogen-bonding schemes that deserve some discussion, see Fig. 3. First, the H 3 O + cation is located between the chains and utilizes all its H atoms to form three strong hydrogen bonds with three different non-coordinating phosphonate O atoms originating from three neighbouring chains [OÁ Á ÁO distance = 2.520 (3) Å , O3-H3Á Á ÁO2 angle = 155 , see Table 1 for symmetry codes]. Presumably, the H 3 O + cations fill the intra-chain void space and stabilize the packing of the one-dimensional chains. It is noted the oxidanium-O3 atom, which is statistically disordered (see Refinement), does not form a close interaction along the threefold axis it resides upon of less than 3.6 Å . In addition, the chains further interact via hydrogen bonds that include the cationic [H 2 N-C-NH 2 ] + moiety. Specifically, there are two intrachain hydrogen bonds with Zn-coordinating phosphonate O atoms [NÁ Á ÁO distance = 2.926 (3) Å , N1-H1B-O2 angle = 147 ] and two inter-chain hydrogen bonds with the noncoordinating phosphonate oxygen from a neighboring chain [NÁ Á ÁO distance = 2.925 (3) Å , N1-H1A-O1 angle = 143 ].

Synthesis and crystallization
Reagents and materials All starting materials were obtained from commercial sources and used without further purification. Ion-exchange column-deionized (DI) water was used for all syntheses. The starting reagents triethyl phosphite (98%), cyanuric chloride (98%) and zinc nitrate hexahydrate were from Alfa Aesar. The solvents petroleum ether, acetonitrile, methanol and nitric acid (70%) were from Scharlau. Trimethylbromosilane was from Flurochem.
Syntheses of [amino(iminio)methyl]phosphonate (AIMP). AIMP was synthesized from the dealkylation of the hexaethyl ester of 1,3,5-triazine-2,4,6-triyltris(phosphonate). The latter was synthesized based on the synthetic procedure reported in the literature (Morrison, 1957) with modifications (Maxim et al., 2010). Yield: 0.916 g, 92%. The 'as synthesized' solid ester (pure by NMR) was then dealkylated using trimethylbromosilane, as follows. In a dry vial the ester (0.490 g, 1.0 mmol) and trimethylbromosilane (1044 mL, 8.0 mmol) were dissolved in acetonitrile (10 ml). The solution was stirred for 24 h, and the colour changed from faint orange to dark orange. Then the homogenous orange solution was left to stand at ambient temperature to allow evaporation of the solvent, yielding an orange oil. Methanol (10 ml) was added to remove the trimethylsilyl group from the phosphonate moiety (as its methoxy ester), and the mixture was stirred for 1 h to allow precipitation of the desired AIMP product (Yield: 0.598 g, 60%). 13 C NMR (75.5 MHz, DMSO-d 6 ) 169.71 (d). 31 P NMR (121.5 MHz, DMSO-d 6 ) 2.85.

Synthesis of {(H 3 O)[Zn(CH 4 N 2 PO 3 ) 3 ]} n (Zn-AIMP).
The synthesis of Zn-AIMP was performed at ambient temperature. Specifically, AIMP (0.016 g, 0.071 mmol); an excess was used, as it was found to give a product with better crystallinity) was dissolved in DI water (7 ml), Zn(NO 3 ) 2 Á6H 2 O (0.005 g, 0.017 mmol, dissolved in 1 ml DI water) was added, and the pH was adjusted to $3.5 using nitric acid. After 30 days a crystalline precipitate appeared, which was isolated by filtration and rinsed with a small amount of water (Yield: 0.001 g, 13%). The crystal used for measurement was handled under inert conditions, being manipulated while immersed in a perfluoropolyether protecting oil, and was mounted on a MiTeGen Micromount 2 .

Refinement
Crystal data, data collection and structure refinement details are summarized in    data reports cation falls on a threefold axis and is disordered with respect to a mirror plane over two half-occupied O-atom positions. No further constraints were necessary to model the disorder. where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.28 e Å −3 Δρ min = −0.50 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. Refinement. All hydrogen atoms were located in difference Fourier maps and included as fixed contributions riding on attached atoms with isotropic thermal displacement parameters 1.2 or 1.5 times those of the respective carrier atom.