A second solvatomorph of poly[[μ4-N,N′-(1,3,5-oxadiazinane-3,5-diyl)bis(carbamoylmethanoato)]nickel(II)dipotassium]: crystal structure, Hirshfeld surface analysis and semi-empirical geometry optimization

The title compound is a second solvatomorph of the earlier reported compound. The complex nickel(II) anions exhibit an L-shaped geometry. The central Ni atom is in a square-planar N2O2 coordination arrangement. In crystal, the complex nickel(II) anions and the potassium cations form layers stacked along the a-axis direction.


Chemical context
In 1976, the products of the metal-templated reaction of hydrazide and aldehyde were separated and structurally described (Clark et al., 1976). It was further shown that such a synthetic strategy makes it possible to obtain complexes with 3d metals in high oxidation states. In particular, there are several works devoted to copper(III) complexes obtained by this method (Oliver & Waters, 1982;Fritsky et al., 1998Fritsky et al., , 2006. Moreover, the preparation of an unprecedentedly stable iron(IV) clathrochelate complex was reported (Tomyn et al., 2017). Some such compounds are promising redox catalysts, as has been shown by Pap et al. (2011) and Shylin et al. (2019). Thus, the study of the conditions and peculiarities of hydrazide-aldehyde template interactions, as well as the isolation and characterization of their products, is an important task in modern coordination chemistry.
Similarly to 1, the complex anion [Ni(L-2H)] 2À has an Lshaped geometry and consists of two almost flat fragments perpendicular to one another: the 1,3,5-oxadiazinane fragment and the fragment including other atoms of the anion. The dihedral angles between the mean planes formed by the non-hydrogen atoms of these fragments are 95.06 (8) and 94.06 (8) for Ni1 and Ni1B, respectively. The ligand molecule is coordinated in a tetradentate {O carboxyl ,N amide ,N amide , O carboxyl }-mode. The central atom of the complex anion exhibits a square-planar coordination arrangement with the N 2 O 2 chromophore. The deviation of the Ni II atom from the mean plane defined by the donor atoms is 0.0073 (13) and 0.0330 (12) Å for Ni1 and Ni1B, respectively.

Supramolecular features
In the crystal, the nickel(II) complex anions [Ni(L-2H)] 2À form layers parallel to the bc plane (Fig. 3a). Neighbouring complex anion layers are sandwiched by layers of potassium counter-cations (Fig. 4). Thus, negatively charged complex anion layers and positively charged potassium cationic layers are stacked along the a-axis direction. It is useful to note that a similar layered structure motif was observed in the crystal of the previously published compound 1. However, in the crystal of 1 the NiN 2 O 2 plane is almost perpendicular to the complex anion layer plane (Fig. 3b)  The asymmetric unit of 2 with displacement ellipsoids shown at the 50% probability level.

Figure 1
A plausible mechanism for the formation of the [Ni(L-2H)] 2complex anion.
respectively. In contrast, in the crystal of 2 the angle between NiN 2 O 2 and the bc plane is 78.30 (8) and 86.29 (7) for Ni1 and Ni1B, respectively.
The demarcation of bonded and non-bonded K-X interactions (X = N or O) is still an unclear and debatable problem (Alvarez, 2013). Therefore, the criteria of such demarcation used in this paper need to be detailed. Based on the aforementioned publication (Alvarez, 2013), we propose 3.7 Å as the maximal distance for K-N bonds. Recently, it was shown (Gagné & Hawthorne, 2016) that K-O main and maximal bond distances depend on the coordination number of K. The results of this work permits 3.4, 3.5 and 3.6 Å to be proposed as the maximal distances for K-O bonds in the case of potassium coordination numbers 7, 8 and 9, respectively. In addition, KÁ Á ÁN amide interactions were determined as nonbonding because the existence of such bonds would lead to the presence of unstable three-membered KN amide N oxadiazinane rings with extremely small N-K-N 0 angles. For an evaluation of the coordination geometry of each potassium cation, SHAPE 2.1 software (Llunell et al., 2013) was used. A SHAPE analysis of the potassium coordination sphere (      The polymeric framework of 2 is stabilized by an extensive system of hydrogen-bonding interactions in which the water molecules act as donors and the carboxylic, the amide and the water O atoms act as acceptors (Table 2). Similarly to 1, the hydrogen bonds are localized mainly at the potassium cation layers (Fig. 6). Moreover, in comparison to 1, the unit cell of 2 contains a smaller number of water molecules, which causes a smaller number of hydrogen-bond interactions in the crystal structure.

Hirshfeld analysis
The Hirshfeld surface analysis (Spackman & Jayatilaka, 2009) and the associated two-dimensional fingerprint plots (McKinnon et al., 2007) were performed with Crystal-Explorer17 (Turner et al., 2017). The Hirshfeld surfaces of the complex anions are colour-mapped with the normalized contact distance (d norm ) from red (distances shorter than the 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 of the title compound is mapped over d norm , in the colour ranges À0.6388 to 0.9164 a.u. and À0.6768 to 0.7286 a.u. for Ni1 and Ni1B complex anions, respectively ( Fig. 7). Similarly to 1, the complex anions of 2 are connected to the other elements of the crystal packing mainly via the amide and the carboxylic O atoms. However, in contrast to 1, one of the oxadiazinane O atoms of 2 is also involved in intermolecular bond formation.
A fingerprint plot delineated into specific interatomic contacts 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. Fig. 8a and 9a show the two-dimensional fingerprint plots of the sum of the contacts contributing to the Hirshfeld surface represented in normal mode for the Ni1 and Ni1B complex anions, respectively.

Figure 6
Crystal packing of the title compound. C-H hydrogen atoms are omitted for clarity. Hydrogen bonds are indicated by dashed lines.

Figure 7
The Hirshfeld surfaces of the Ni1 (A) and Ni1B (B) complex anions mapped over d norm . Fig. 8d and 9d) make very significant contributions to the total Hirshfeld surface. This indicates that there are more KÁ Á ÁO contacts and fewer OÁ Á ÁH contacts compared to the crystal of 1.

Geometry optimization
The searching of computationally 'cheap' but still sufficiently accurate methods of transition-metal complex geometry optimization is an important task of modern computational chemistry. The geometry optimization calculations were carried out with three semi-empirical methods: PM7, DFTB and GFN2-xTB. The PM7 (Stewart, 2013) calculations were performed with MOPAC2016 software (Stewart, 2016). The DFTB calculations were carried out with the DFTB+ software package (Hourahine et al., 2020) using the 'mio-1-1' (Elstner et al., 1998) and the 'trans3d-0-1' (Zheng et al., 2007) Slater-Koster parameterization sets. The GFN2-xTB (Bannwarth et al., 2019) calculations were applied with xtb 6.4 package (Grimme, 2019). The geometry of the Ni1 complex anion obtained from the crystal structure was used as the starting geometry for the calculations.
In general, for all described semi-empirical methods, the calculated geometric parameters of the oxadiazinane ring are in reasonable agreement with experimental values (see Table 3). On the other hand, the accuracy of the nonoxadiazinane fragment geometry prediction varies greatly depending on the method. The worst agreement with experiments is from the PM7 method, mainly because of the pyramidalization of the amide nitrogen atom (Table 3). Such nonplanarity of the amide fragment is a well-known problem of the PMx methods (Feigel & Strassner, 1993). In contrast, the DFTB method predicts the amide geometric parameters with high accuracy but demonstrates longer than experimental carboxylate C-O bonds and a slight tetragonal distortion of the nickel(II) coordination polyhedra (Table 3). The best results were obtained with the GFN2-xTB method for which the calculated geometric parameters correlate nicely with experimental values (Table 3). The maximal difference between the calculated and the experimental bond lengths concerns the C-O lengths (shorter than the experimental values within 0.024-0.033 Å ). A superimposed analysis of the Ni1 complex anion with its optimized structure gives an research communications     RMSD of 0.131 Å (Fig. 10). Thus, the GFN2-xTB method is a promising geometry prediction method for transition-metal complexes based on hydrazide and carboxylate ligands.

Database survey
A search in the Cambridge Structural Database (CSD version 5.39, update of May 2018; Groom et al., 2016) resulted in 11 hits dealing with 3d-metal complexes with macrocyclic or pseudo-macrocyclic ligands formed by template binding of several hydrazide groups by formaldehyde molecules. These complexes contain the following 3d metals: Ni II (Fritsky et al., 1998), Cu II (Clark et al., 1976;Fritsky et al., 2006), Cu III (Oliver & Waters, 1982;Fritsky et al., 1998, Fritsky et al., 2006 and Fe IV (Tomyn et al., 2017). Thus, such macrocyclic and pseudomacrocyclic ligand systems exhibit a tendency to stabilize the high oxidation states of 3d metals.

Synthesis and crystallization
A solution of Ni(ClO 4 ) 2 Á6H 2 O (0.091 g, 0.25 mmol) in 5 ml of water was added to a warm solution of oxalohydrazidehydroxamic acid (0.06 g, 0.5 mmol) in 5 ml of water. The resulting light-green mixture was stirred with heating (320-330 K) for 20 min and then 1 ml of 4M KOH solution was added. As a result, the colour of the solution changed to pink.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4  Structural overlay between the experimental (blue) and optimized (orange) structures.

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.