Crystal structure and Hirshfeld surface analysis of bis(6,7,8,9-tetrahydro-11H-pyrido[2,1-b]quinazolin-5-ium) tetrachloridozincate

N—H⋯Cl hydrogen bonds link two dihydroquinazolinium cations and a tetrachlorido zincate dianion into discrete aggregates. Neighbouring (C12H15N2)2[ZnCl4] units interact via non-classical C—H⋯π hydrogen bonds and π–π stacking.

The title compound, (C 12 H 15 N 2 ) 2 [ZnCl 4 ], is a salt with two symmetrically independent, essentially planar heterocyclic cations and a slightly distorted tetrahedral chlorozincate dianion. N-HÁ Á ÁCl hydrogen bonds link these ionic constituents into a discrete aggregate, which comprises one formula unit. The effect of hydrogen bonding is reflected in the minor distortions of the [ZnCl 4 ] 2À moiety: distances between the cation and chlorido ligands engaged in classical hydrogen bonds are significantly longer than the others. Secondary interactions comprise C-HÁ Á Á hydrogen bonding and weakstacking. A Hirshfeld surface analysis indicates that the most abundant contacts in packing stem from HÁ Á ÁH (47.8%) and ClÁ Á ÁH/HÁ Á ÁCl (29.3%) interactions.

Chemical context
Tricyclic quinazolines are counted among the most exciting quinazoline alkaloids. Specifically, the alkaloid mackinazoline was isolated from Mackinlaya sp. (Johns & Lamberton, 1965). Tricyclic quinazolines have several different reactive sites and can react with electrophilic and nucleophilic reagents to form various derivatives with potential biological activity (Michael, 2004). As quinazoline alkaloids are scarcely available from natural sources, multiple methods for their synthesis have been developed (Shakhidoyatov & Elmuradov, 2014). In the context of these synthetic efforts, reaction intermediates similar to the title compound have been studied (Sharma et al., 1993;Sargazakov et al., 1991;Tozhiboev et al., 2005). We investigated the crystal structure of bis (6,7,8,9-tetrahydro-11H-pyrido[2,1-b]quinazolin-5-ium) tetrachloridozincate, an intermediate in the synthesis of mackinazolinone, using highresolution diffraction data and Hirshfeld surface analysis. ISSN 2056-9890

Structural commentary
The title compound crystallizes in the P2 1 /n space group, with two [C 12 H 15 N 2 ] + cations and a [ZnCl 4 ] 2À counter-anion in the asymmetric unit (Fig. 1). The benzene and pyrimidine rings in either cation and the attached carbon atoms of the aliphatic ring (C9A and C12A for residue A and C9B and C12B for residue B) are essentially coplanar, with r.m.s. deviations of 0.0437 and 0.0168 Å for molecules A and B, respectively. The remaining atoms of the third ring are significantly displaced above the opposite faces of these planes with deviations of 0.3877 (12) Å for C10A and 0.3831 (11) Å for C11A in residue A and 0.4705 (11) Å for C10B and 0.2495 (11) Å for C11B in residue B. Fig. 2 shows that the independent cations are almost superimposable including the conformationally soft aliphatic ring.
The protonation of the ring occurs at the basic heteroatoms of the pyrimidine rings, N1A and N1B, respectively, and the acquired positive charge is delocalized within the -N-C-Nmoiety in the ring, where the N1A-C2A and N1B-C2B bonds are only slightly longer than C2A-N3A and C2B-N3B (Table 1). Similar differences were observed in related reported complexes (Sharma et al., 1993;Turgunov et al., 2003;Tozhiboev et al., 2005).
However, these C-N bond lengths are shorter than those in the related tricyclic protonated (PYQAZP: Reck et al., 1974) and non-protonated (GUCZUZ: Le Gall et al., 1999;LIZMOX: Zhang et al., 2008) quinazoline derivatives. In these three compounds, the sp 3 character of the carbon atom between the two nitrogen atoms and the lack of the C N double bond within the -N-C-N-moiety hampers the delocalization of the positive charge within this unit. It is Overlay (Spek, 2020) of the independent cations in the title compound in the least-squares (top) and most-squares plane (bottom); residue A is depicted in black, residue B in red.  Figure 1 Asymmetric unit of the title compound with the atom-numbering scheme (Spek, 2020). Displacement ellipsoids for non-hydrogen atoms are drawn at the 50% probability level.

Figure 3
Residual electron density in the planes through C2A, C4A and C8AA (top) and C2B, C4B and C4AB (bottom); contour lines are drawn at 0.2 e Å À3 . Covalent bonds in the heterocyclic cations clearly show up as local density maxima.
instead delocalized over the -N CH-C(phenylene) fragment (see Table S1 in the supporting information). Analysis of the residual electron density (Spek, 2020) reveals that the covalent bonds in the heterocyclic cations clearly show up as local density maxima (Fig. 3).
The Zn II centre in the dianion adopts a slightly distorted geometry, with 4 = 0.95 (Yang et al., 2007). The high resolution ( max = 109.6 , sin / = 1.150 Å À1 , d = 0.43 Å ) and the very favourable ratio between observations and variables (100:1) in our diffraction data result in small standard uncertainties for atomic coordinates and derived geometric parameters and allow to discuss more subtle details. The most acute angle of 103.33 (11) within the tetrachloridozincate dianion (Table 1) is subtended by Cl1 and Cl2. These atoms are associated with the longest Zn-Cl distances, which, in turn, are correlated with the most relevant intermolecular interactions in the structure: Cl1 is involved in the shortest and most linear N-HÁ Á ÁCl hydrogen bond (see Table 2) and represents the most distant ligand in the anion. Cl2 is signifi-cantly closer to Zn1 and is engaged in a longer and presumably weaker hydrogen bond. The remaining chlorido ligands are not associated with any classical short contacts. Similar features have been reported for structurally related compounds (Sharma et al., 1993;Sargazakov et al., 1991;Tozhiboev et al., 2005;Wang et al., 2017).

Supramolecular features
In the crystal structure, the protonated N1A and N1B nitrogen atoms in the cations interact with the chlorido ligands Cl2 and Cl1, respectively, via relatively short N-HÁ Á ÁCl bonds and generate a D 2 2 (5) graph-set motif (Bernstein et al., 1995) (Table 2 and Fig. 4).
The crystal packing is further stabilized by intermolecular C-HÁ Á Á interactions (Table 2) and additional short contacts between Cl3 and the N-C-N segment of the pyrimidine rings. The shortest contact distance occurs between Cl3 and C2B [3.5273 (9) Å ] and involves an interaction between the electron-rich equatorial region of the halogen atom and the ring atom attached to two N-atom neighbours, most probably the most electron-deficient atom in the heterocycle. These contacts link anions and cations into a three-dimensional network. Weakstacking interactions occur between pyrimidine (Cg1, Cg7) and benzene (Cg3, Cg9) rings of antiparallel pairs of cations and involve contact distances of Cg1Á Á ÁCg3 (Àx, Ày, Àz) = 3.6225 (5) Å (slippage 0.857 Å ) and of Cg7Á Á ÁCg9 (1 À x, Ày, 1 À z) = 3.6246 (7) Å (slippage 0.994 Å ).

Hirshfeld surface analysis
A Hirshfeld surface (HS) analysis (Spackman & Jayatilaka, 2009) was carried out using CrystalExplorer17.5 (Turner et al., 2017) to visualize interactions between the constituents of the title compound. The HS mapped with d norm is represented in Crystal packing and short contacts in the title compound. Colour code: N-HÁ Á ÁCl interactions light-blue dashed lines, intermolecular C-HÁ Á Á contacts red dashed lines, Zn-ClÁ Á Á contacts green dashed lines,stacking interactions dark-blue dashed lines. Centroid for the pyrimidine (Cg1, Cg7) and benzene rings (Cg3, Cg9) are shown as blue and red spheres, respectively.

Figure 5
Three-dimensional Hirshfeld surface of the title compound mapped with d norm . Table 2 Hydrogen-bond geometry (Å , ).

Database survey
A search in the Cambridge Structural Database (CSD, version 5.41, including the update of January 2020; Groom et al., 2016) confirmed that four related compounds had been structurally characterized in which similar cations interact with [ZnCl 4 ] 2À anions. They are associated with refcodes PODLUP (Sharma et al., 1993), PODLUP01 (Sargazakov et al., 1991) and SECFAI and SECFAI01 (Tozhiboev et al., 2005). An additional match for a similar cation interacting with a Cl À anion was identified: EYUHEL (Turgunov et al., 2003) and PYQAZP (Reck et al., 1974).

Synthesis and crystallization
3 g (0.015 mol) of 2,3-tetramethylenquinazoline-4-one (Fig. 7) were placed in a 300 mL flat-bottom flask equipped with a magnetic stirrer and a reflux condenser. 72 mL of hydrochloric acid (15%) were added under stirring. 12 g of Zn powder were added in small portions over a period of 1 h, and the mixture was heated in a water bath for 4 h. The hot reaction mixture was filtered and the filtrate was left to precipitate overnight. The precipitate corresponding to 2,3-tetramethylenquinazoline hydrochloride was removed by filtration (Fig. 7). Colourless single crystals of the title compound were obtained by slow evaporation of the resulting filtrate at room temperature.

Bis(6,7,8,9-tetrahydro-11H-pyrido[2,1-b]quinazolin-5-ium) tetrachloridozincate
Crystal data (C 12  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.