Crystal structure and Hirshfeld surface analysis of poly[[bis[μ4-N,N′-(1,3,5-oxadiazinane-3,5-diyl)bis(carbamoylmethanoato)]nickel(II)tetrapotassium] 4.8-hydrate]

The complex nickel(II) anion comprises a pseudomacrocyclic hydrazide-based ligand with an L shape. In the crystal, such anions are connected with the potassium cations and the water solvent molecules, forming a three-dimensional polymeric framework, which is stabilized by an extensive system of hydrogen bonds.

The title compound, { [K 4 Ni 2 (C 7 H 6 N 4 O 7 ) 2 ]Á4.8H 2 O} n , was obtained as a result of a template reaction between oxalohydrazidehydroxamic acid, formaldehyde and nickel(II) nitrate followed by partial hydrolysis of the formed intermediate. The two independent [Ni(C 7 H 6 N 4 O 7 )] 2complex anions exhibit pseudo-C S symmetry and consist of an almost planar metal-containing fragment and a 1,3,5-oxadiazinane ring with a chair conformation disposed nearly perpendicularly with respect to the former. The central Ni II atom has a square-planar N 2 O 2 coordination arrangement formed by two amide N and two carboxylate O atoms. In the crystal, the nickel(II) complex anions form layers parallel to the ab plane. Neighboring complex anion layers are connected by layers of potassium cations for which two of the four independent cations are disordered over two sites [ratios of 0.54 (3):0.46 (3) and 0.9643 (15):0.0357 (15)]. The framework is stabilized by an extensive system of hydrogen bonds where the water molecules act as donors and the carboxylic O atoms, the amide O atoms and the oxadiazinane N atoms act as acceptors.

Chemical context
Coordination compounds of paramagnetic metal ions based on polydentate ligands comprising amide, hydrazide and hydroxamate functional groups are of great interest as they often form novel oligonuclear structures with interesting supramolecular features (Mezei et al., 2007;Strotmeyer et al., 2003). Frequently, these compounds exhibit unusual magnetic properties (Pavlishchuk et al., 2010;Gumienna-Kontecka et al., 2007;Pavlishchuk et al., 2011;Huang et al., 2014) and have potential biological activity (Raja et al., 2012). The use of hydrazide metal complexes as synthons for template reactions has allowed coordination compounds with more complicated, sometimes unpredictable molecular structures to be obtained (Clark et al., 1976). In particular, for ring-closure reactions, aldehydes (especially formaldehyde) can be used successfully as capping reagents for template condensation, as has been shown in several studies (Fritsky et al., 1998(Fritsky et al., , 2006Tomyn et al., 2017). Importantly, depending on the nature and coordination preference of the metal ion, the products of the ring-closure reactions can be both macrocyclic or pseudomacrocyclic (Ni 2+ , Cu 2+ ; Fritsky et al., 1998Fritsky et al., , 2006  The asymmetric unit of (1) with displacement ellipsoids shown at the 50% probability level. The potassium cations K3 and K4 and the solvate water molecule O4W are disordered over two positions, namely K3A and K3B, K4A and K4B, O4WA and O4WB, respectively.

Supramolecular features
In the crystal, the nickel(II) complex anions [Ni(L-2H)] 2À form layers parallel to ab plane (Fig. 3). Neighboring complex anion layers are sandwiched by layers of potassium countercations (Fig. 4). Thus, complex anion layers and potassium layers are stacked along the c-axis direction (Fig. 5).
The potassium cations are bound to the nickel(II) complex anions through the amide and the carboxylic O atoms (K1, K4A) or through the amide O and the oxadiazinane N atoms (K2, K3B). In addition, the potassium cations have contacts with the O atoms of the water molecules, with the amide and the carboxylic O atoms, and with the oxadiazinane N atoms of neighboring complex anions. For definition of the coordination spheres around the cations, K-O and K-N contacts that do not exceed the sum of the ionic radii by more than 0.2 Å were defined as bonding contacts [the values of the ionic radii were taken from Shannon (1976)  Layers formed by the anionic nickel(II) complexes.

Figure 5
Crystal packing of the title compound in a stick model, showing the coordination polyhedra of the potassium cations in lilac. H atoms of the C-H groups and minor disordered components (K3A and K4B, O4WB water molecule) are omitted for clarity.
The polymeric framework is stabilized by an extensive system of hydrogen-bonding interactions where the water molecules act as donors and the carboxylic O atoms, the amide O atoms and the oxadizdinane N atoms act as acceptors (Table 3, Fig. 7).
sum of the van der Waals radii) through white to blue (distances longer than the sum of the van der Waals radii). The Hirshfeld surface mapped over d norm , in the colour range À0.6411 to 0.9651 a.u. for the anion centred by Ni1 (A) and À0,6382 to 0.9607 a.u. for the anion centred by Ni1B (B) is shown in Fig. 8. Both complex anions are connected to the other moieties of the crystal structure mainly through the amide and the carboxylic O atoms. A two-dimensional fingerprint plot contains information related to specific intermolecular interactions. The blue colour refers to the frequency of occurrence of the (d i , d e ) pair with the full fingerprint plot outlined in gray. Figs. 9a and 10a show the two-dimensional fingerprint plots for the anion centred by Ni1 (A) and by Ni1B (B), represented by the sum of the contacts contributing to the Hirshfeld surface in normal mode. The most significant contribution to the Hirshfeld surface is from OÁ Á ÁH/HÁ Á ÁO contacts (41.3% for complex A and 41.0% for complex B, respectively; Fig. 9b and 10b). In addition, OÁ Á ÁK/KÁ Á ÁO (15.8% for complex anions A and B; Fig. 9c and 10c) and HÁ Á ÁH (13.7% for complex anion A and 15.1% for complex anion B; Fig. 9d and 10d) are other significant contributions to the total Hirshfeld surface.

Database survey
A search of the Cambridge Structural Database (CSD version 5.41, update of November 2019; Groom et al., 2016) for complexes obtained by hydrazide, aldehyde and 3d-metal salt interactions gave eleven hits for structures with full atomic coordinates. All these compounds include macrocyclic or pseudo-macrocyclic ligands formed by template binding of several hydrazide groups by aldehyde molecules. The 3d-metal ions of these complexes are often in high oxidation states: Cu III (Oliver et al., 1982;Fritsky et al., 1998Fritsky et al., , 2006 and Fe IV (Tomyn et al., 2017) complexes have been described.     resulting light-green mixture was stirred with heating (320-330 K) for 20 min, and then 1 ml of a 4M KOH solution was added. As a result, the color of the solution changed to pink. After 5 min of stirring, 0.03 g of paraformaldehyde (1 mmol) were added, followed by stirring with heating (320-330 K) for 30 min. The resulting orange solution was left for crystallization by slow diffusion of methanol vapor. After two months, orange crystals suitable for X-ray diffraction studies were obtained. The crystals were filtered off, washed with diethyl ether and dried in air.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4. The potassium cations K3 and K4 were found to be disordered over two positions with occupancy factors for the major disorder component of 0.54 (3) (K3B) and 0.9643 (15) (K4A). The solvate water molecule O4W appeared to be disordered over two positions with relative occupancies of 0.805 (4) (O4WA) and 0.195 (4) (O4WB). The solvate water molecule O5W was found to be incompatible with the second positions of the water molecule O4W and thus was refined with the same occupancy factor as the major fraction of O4W as they are linked by a hydrogen bond. The O-H hydrogen atoms were located from a difference-Fourier map and constrained to ride on their parent atoms with U iso (H) = 1.5U eq (O). The methylene C-H hydrogen atoms were positioned geometrically and were constrained to ride on their parent atoms, with C-H = 0.99 Å , and U iso (H) = 1.2U eq (C).

Funding information
This project has received funding from the European Union's Horizon 2020 Research and Innovation Programme under the Marie Skłodowska-Curie grant agreement No. 778245. MOP, AOH and TSI acknowledge funding received from the Ministry of Education and Science of Ukraine (grant No. 19BF037-04).  program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2018/1 (Sheldrick, 2015); molecular graphics: DIAMOND (Brandenburg, 2009); software used to prepare material for publication: publCIF (Westrip, 2010).

Poly[[bis[µ 4 -N,N′-(1,3,5-oxadiazinane-3,5-diyl)bis(carbamoylmethanoato)]nickel(II)tetrapotassium] 4.8-hydrate]
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.