Crystal structures of three zinc(II) halide coordination complexes with quinoline N-oxide

The structures of the three related compounds dichloridobis(quinoline N-oxide-κO)zinc(II); dibromidobis(quinoline N-oxide-κO)zinc(II) and diiodidobis(quinoline N-oxide-κO)zinc(II) are presented.

Herein we report the crystal structures of three complexes of quinoline N-oxide (QNO) with zinc(II) chloride, bromide and iodide. All three were obtained by 1:2 stoichiometric reaction of the zinc(II) halide with QNO in methanol and found to be mononuclear ZnX 2 (QNO) 2 complexes with a distorted tetrahedral environment around the zinc ion.

Structural commentary
Compound (I) crystallizes in the monoclinic space group P2 1 (Fig. 1), whereas compounds (II) (Fig. 2) and (III) (Fig. 3) both crystallize in the monoclinic space group P2 1 /c. Each structure contains one symmetrically independent molecule, the coordination sphere around each Zn atom being a distorted tetrahedron. Selected bond lengths and angles in these complexes are shown in Table 1. Compounds (II) and (III) are isostructural in both the molecular conformation and crystal packing, while (I) differs in both aspects, as illustrated by an overlay of molecules (I) and (II) (Fig. 4a) on one hand, and molecules (II) and (III) on the other (Fig. 4b). Most notably, (I) differs in the orientation of the QNO rings relative to each other, the C2-N1-N2-C11 torsion angles being À16.9 (5) in (I) versus À113.9 (3) in (II) and À111.6 (3) in (III).

Hirshfeld surface analysis
The intermolecular interactions were further investigated by quantitative analysis of the Hirshfeld surface, and visualized with Crystal Explorer 21 (Spackman et al., 2021)  atoms, normalized by the van der Waals (vdW) radii of the corresponding atoms (r vdW ). Contacts shorter than the sums of vdW radii are shown in red, those longer in blue, and those approximately equal to vdW as white spots. For (I), the most intense red spots correspond to the intermolecular contacts O1Á Á ÁC9(1 À x, y À 1 2 , 1 À z) [3.048 (9) Å ] and the hydrogen bond C18-H18Á Á ÁCl2(x, y + 1, z). The latter has the distances HÁ Á ÁCl = 2.53 Å (for the C-H distance normalized to 1.083 Å ) and CÁ Á ÁCl = 3.416 (9) Å within the previously observed range but shorter than the average values of 2.64 and 3.66 Å , respectively (Steiner, 1998). The other chloride ligand, Cl2, forms four HÁ Á ÁCl contacts of 2.83-2.98 Å , more typical for van der Waals interactions (Rowland & Taylor, 1996). For (II) and (III), the red spots correspond to C-HÁ Á ÁX interactions, viz. C18-H18Á Á ÁX1, C5-H5Á Á ÁX1, C16-H16Á Á ÁX2, and C9-H9Á Á ÁX2, which can be also regarded as weak hydrogen bonds (Steiner, 1998       Analysis of the two-dimensional fingerprint plots (Table 2) indicates that HÁ Á ÁH contacts are the most common in all three structures. XÁ Á ÁH contacts make the second highest contribution, which increases in the succession (I) < (II) < (III), together with the size of the halogen atoms and hence their share of the molecular surface (16.9, 18.5 and 20.6%, respectively). Interestingly, -stacking in the structures of (II) and (III) gives only a modest increase of CÁ Á ÁC contacts compared to (I), probably because it is counterbalanced by an overall decrease of carbon atoms' share of the surface (21.4 > 19.5 > 18.3%). No halogenÁ Á Áhalogen contacts are observed in any of the three structures.

Synthesis and crystallization
The water content of QNO and ZnBr 2 have been determined by Thermal Gravimetric Analysis. The formulation for each was found to be QNOÁ0.28H 2 O (M W = 150.21 g mol À1 ) and ZnBr 2 Á0.86H 2 O (F W = 240.69 g mol À1 ).
The title compounds were all synthesized in a similar manner. Compound (I) was synthesized by dissolving 0.0986 g of QNOÁ0.28H 2 O (0.656 mmol, purchased from Aldrich) in 33 mL of methanol to which 0.0440 g of ZnCl 2 (0.176 mmol, purchased from Strem Chemicals) were added at 295 K. The solution was covered with parafilm then allowed to sit; X-ray quality crystals were grown by slow evaporation at 295 K. Yield, 0.0822 g (60.2%). Selected IR bands (ATR-IR, cm   Hirshfeld surface for (I) mapped over d norm .

Figure 9
Hirshfeld surface for (II) mapped over d norm .

Figure 10
Hirshfeld surface for (III) mapped over d norm . Compound (II) was synthesized by dissolving 0.0983 g of QNOÁ0.28H 2 O (0.654 mmol), in 40 mL of methanol to which 0.0778 g of ZnBr 2 Á0.86H 2 O (0.323 mmol, purchased from Alfa Aesar) were added at 295 K. The solution was covered with parafilm then allowed to sit; X-ray quality crystals were grown by slow evaporation at 295 K. Yield, 0.0866 g (46.7%). Compound (III) was synthesized by dissolving 0.0517 g of QNOÁ0.28H 2 O (0.352 mmol) in approximately 36 mL of methanol to which 0.0524 g of ZnI 2 (0.164 mmol, purchased from Aldrich) were added at 295 K. The solution was covered with parafilm then allowed to sit; X-ray quality crystals were grown by slow evaporation at 295 K. Yield, 0.0910 g (52.3%). Infrared spectroscopy confirms the presence of the QNO ligand in all three complexes. Characteristic IR bands include weak C-H aromatic stretches observed from 3020-3107 cm À1 and N-O stretches of the bound N-oxide in the range 1350-1150 cm À1 ; notably, a medium band observed in the ligand at 1311 cm À1 , appears at between 1225-1227 cm À1 in the three metal complexes. Finally, a broad absorbance in the free ligand from 3100-3500 cm À1 (assigned to the water O-H stretch) is absent in all of the metal complexes (Mautner et al., 2016).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. All carbon-bound H atoms were positioned geometrically and refined as riding: C-H = 0.95-0.98 Å with U iso (H) = 1.2U eq (C).   (Dolomanov et al., 2009); software used to prepare material for publication: OLEX2 (Dolomanov et al., 2009).

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.

Dibromidobis(quinoline N-oxide-κO)zinc(II) (II)
Crystal data where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.55 e Å −3 Δρ min = −0.35 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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Zn1 0.25508 (4) 0.26213 (9) 0.37264 (4) 0.0514 (2) (11) 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.