Decaaquadioxidobis[μ3-N-(phosphonatomethyl)iminodiacetato]dizinc(II)divanadium(IV) dihydrate

The title compound, [Zn2V2(C5H6NO7P)2O2(H2O)10]·2H2O, contains a [V2O2(pmida)2]4− dimeric anionic unit [where H4pmida is N-(phosphonomethyl)iminodiacetic acid] lying on a centre of symmetry which is exo-coordinated via the two deprotonated phosphonate groups to two Zn2+ cations, with the coordination environment of Zn completed by five water molecules. The crystal packing is mediated by an extensive network of strong and highly directional O—H⋯O hydrogen bonds involving the water molecules (coordinated and uncoordinated) and the functional groups of pmida4−, leading to a three-dimensional supramolecular network.


Comment
Research on highly crystalline organic-inorganic hybrids, in particular those belonging to the family of coordination polymers, has received a considerable attention over the last two decades. Such occurs as a direct consequence of the fascinating structural architectures achieved by assembling organic ligands and metal centres which, in many cases, can be allied with interesting potential applications (e.g. gas storage, separation, catalysis, guest exchange, magnetic or optical sensors).
The structure of (I) contains two crystallographically unique metal centres, Zn1 and V1, both exhibiting octahedral It is of considerable importance to emphasize that the geometrical aspects of this dimeric anionic unit are typical and in good agreement with those described in detail in our previous publications Shi, Almeida Paz, Trindade & Rocha, 2006;Paz, Rocha, Klinowski et al., 2005;Almeida Paz, Shi, Mafra et al., 2005;Shi et al., 2005;Paz et al., 2004). V1 is connected to one oxo group and to two pmida 4ligands, with the geometry of the first coordination sphere resembling a highly distorted octahedron, which is composed by one short V-O bond [1.6088 (16)  respectively. Noteworthy is the structural evidence of the notable trans effect of the oxo group over the long V-N distance (see Table of selected geometric parameters).
Individual [Zn 2 V 2 O 2 (pmida) 2 (H 2 O) 10 ] molecular units close pack with the water molecules of crystallization in a typical brick-wall-like fashion in the bc plane of the unit cell (Fig. 2), mediated by an extensive network of strong and highly directional O-H···O hydrogen bonding interactions (see Table summarizing the geometrical aspects of the hydrogen bonds).
A mixture containing 0.26 g of KVO 3 , 0.15 g of ZnO, 0.42 g of H 4 pmida, 0.13 g of imidazole and 0.27 g of adipic acid in ca 9 g of distilled water, was stirred thoroughly at ambient temperature for 30 minutes, yielding a suspension with a molar composition of ca 1:1:1:1:1:270, respectively, which was transferred to a PTFE-lined stainless steel reaction vessel (total volume ca 40 ml). The reaction vessel was placed inside a preheated oven at 473 K for one day, after which the temperature was decreased to 373 K allowing the reaction to proceed for another four days. After reacting, under autogeneous pressure and static conditions, the vessel was removed from the oven and left to cool to ambient temperature before opening. Small amounts of green and/or blue mixed powders of unknown phases were readily separated from the mother liquor by vacuum filtering. Large single crystals of the title compound were isolated by slow evaporation (in open air) of the mother liquor over the period of one week. It is of considerable importance to emphasize that similar reactions where imidazole and adipic acid were not included in the starting reactive mixture failed in the isolation of the title material.

Refinement
H atoms bound to carbon were placed at idealized positions and allowed to ride on their parent atoms with U iso fixed at 1.2×U eq (C). H atoms associated with the five coordinated water molecules were markedly visible in difference Fourier maps and were included in the structural model for subsequent least-squares refinement cycles with the O-H and H···H distances restrained to 0.90 (3) and 1.47 (3) Å, respectively, in order to ensure a chemically reasonable geometry for these chemical moieties. These H atoms were allowed to ride on their parent atoms with U iso fixed at 1.5×U eq (O).
The crystallographically unique O6W water molecule of crystallization was directly located from difference Fourier maps and refined assuming a full site occupancy and a thermal anisotropic displacement behaviour. The H atoms associated with this chemical moiety could not be unequivocally located from difference Fourier maps. Additionally, attempts to place the two H atoms in calculated positions did not produce a chemically reasonable structural model, in particular concerning the geometry of the resulting hydrogen bonding interactions. Therefore, these H atoms were omitted from the final structural model but were included in the empirical chemical formula. Fig. 1. Schematic representation of the tetranuclear centrosymmetric [Zn 2 V 2 O 2 (pmida) 2 (H 2 O) 10 ] molecular unit, showing the labelling scheme for all non-H atoms. Displacement ellipsoids are drawn at the 50% probability level and H atoms are represented as small spheres with arbitrary radii. The water molecule of crystallization O6W was omitted for clarity. Symmetry transformation used to generate equivalent atoms (in grey): 2x, 1 -y, 1 -z.

Special details
Experimental. See dedicated section in the main paper Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating Rfactors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.