Crystal structure, Hirshfeld surface analysis and corrosion inhibition study of 3,6-bis(pyridin-2-yl)-4-{[(3aS,5S,5aR,8aR,8bS)-2,2,7,7-tetramethyltetrahydro-5H-bis[1,3]dioxolo[4,5-b:4′,5′-d]pyran-5-yl)methoxy]methyl}pyridazine monohydrate

The title compound is built up by two dioxolo, two pyridine, one pyridazine and one pyran rings. The two dioxolo rings are in envelope conformations, while the pyran ring is in twisted-boat conformation. The pyradizine ring is oriented at dihedral angles of 9.23 (6) and 12.98 (9)° with respect to the pyridine rings, while the dihedral angle between the two pyridine rings is 13.45 (10)°. In the crystal, C—Hdioxolo⋯Odioxolo, O—Hwater⋯Opyran, O—Hwater⋯Omethoxymethyl and O—Hwater⋯Npyridazine hydrogen bonds link the molecules into a supramolecular structure. A weak C—Hmethoxymethyl⋯π interaction is also observed.


Chemical context
Given their importance in the pharmaceutical, chemical and industrial fields, the synthesis of 3,6-di(pyridin-2-yl)pyridazine and its derivatives has been a goal of chemists in recent years. 5-[3,6-Di(pyridin-2-yl)pyridazine-4-yl]-2 0 -deoxyuridine-5 0 -Otriphosphate can be used as a potential substrate for fluorescence detection and imaging of DNA (Kore et al., 2015). Systems containing this moiety have also shown remarkable corrosion inhibitory (Khadiri et al., 2016). Heterocyclic molecules such as 3,6-bis (2 0 -pyridyl)-1,2,4,5-tetrazine have been used in transition-metal chemistry (Kaim & Kohlmann, 1987). This bidentate chelate ligand is popular in coordination chemistry and complexes of a wide range of metals, including iridium and palladium (Tsukada et al., 2001). We report herein the synthesis and the molecular and crystal structures of the title compound, (I), along with the Hirshfeld surface analysis and its corrosion inhibition properties.

Structural commentary
The title molecule contains two dioxolo, two pyridine, one pyridazine and one pyran rings (Fig. 1). The pyridazine ring is linked to the pyran ring through the methoxymethyl moiety.

Hirshfeld surface analysis
In order to visualize the intermolecular interactions in the crystal of the title compound, a Hirshfeld surface (HS) analysis (Hirshfeld, 1977;Spackman & Jayatilaka, 2009) was carried out by using CrystalExplorer17.5 (Turner et al., 2017). In the HS plotted over d norm (Fig. 3), white indicates contacts with distances equal to the sum of van der Waals radii, while red and blue indicate distances shorter (in close contact) or longer (distinct contact) than the van der Waals radii, respectively (Venkatesan et al., 2016). The bright-red spots appearing near O1, O6, N2 and hydrogen atoms H2, H7A, H7B indicate their roles as the respective donors and/or acceptors. The shape-index of the HS is a tool to visualize the stacking by the presence of adjacent red and blue triangles; if these are absent, then there are nointeractions. Fig. 4 clearly suggest that there are nointeractions in (I). The overall two-dimensional fingerprint plot, Fig. 5a, and those delineated into HÁ Á ÁH, HÁ Á ÁC/CÁ Á ÁH, HÁ Á ÁO/OÁ Á ÁH, HÁ Á ÁN/N Á Á ÁH, CÁ Á ÁC and CÁ Á ÁN/NÁ Á ÁC contacts (McKinnon et al., 2007) are illustrated in Fig. 5b Table 1 Hydrogen-bond geometry (Å , ).

Figure 1
The molecular structure of the title compound with the atom-numbering scheme. Displacement ellipsoids are drawn at the 50% probability level. H atoms bonded to C atoms are not shown.

Figure 2
A The most important interaction is HÁ Á ÁH, contributing 55.7% to the overall crystal packing, which is reflected in Fig. 5b as widely scattered points of high density due to the large hydrogen content of the molecule with the tip at d e = d i $1.00 Å . In the presence of a weak C-HÁ Á Á interaction, the wings in the fingerprint plot delineated into HÁ Á ÁC/CÁ Á ÁH contacts (14.6% contribution to the HS) have a symmetrical distribution of points, Fig. 5c Hirshfeld surface of the title compound plotted over shape-index.

Figure 3
View of the three-dimensional Hirshfeld surface of the title compound plotted over d norm in the range À0.4555 to 1.4860 a.u. The Hirshfeld surface analysis confirms the importance of H-atom contacts in establishing the packing. The large number of HÁ Á ÁH, HÁ Á ÁC/CÁ Á ÁH, HÁ Á ÁO/OÁ Á ÁH and HÁ Á ÁN/NÁ Á ÁH interactions suggest that van der Waals interactions and hydrogen bonding play the major roles in the crystal packing (Hathwar et al., 2015).

Electrochemical measurements
The effect of the title compound as an inhibitor of the corrosion of mild steel (MS) were studied using electrochemical impedance spectroscopy in the concentration range of 10 À6 to 10 À3 M at 308 K. The electrochemical experiment consisted of a 3 electrode electrolytic cell consisting of platinum foil as counter-electrode, saturated calomel as reference electrode and MS as working electrode with an exposed area of 1 cm 2 . The MS specimen was immersed in a test solution for 0.5 h until a steady-state potential was achieved using a PGZ100 potentiostat (Bouayad et al., 2018). Electrochemical impedance spectroscopy (EIS) measurements were performed over a frequency range of 0.1 Â 10 À3 KHz to 10 mHz and an amplitude of 10 mV with 10 points per decade. The percentage inhibition efficiency is calculated from R t values as (Sikine et al., 2016) and R t(HCl) are the charge-transfer resistances for MS immersed in HCl, with the title compound and without inhibitor. Nyquist representations of mild steel in 1 M HCl in the absence and presence of the inhibitor system are shown in Fig. 7.
The impedance method provides information about the kinetics of the electrode processes and the surface properties of the investigated systems. The technique is based on the measurement of the impedance of the double layer at the MS/ solution interface, and represents the Nyquist plots of mild steel (MS) specimens in 1 M HCl without and with various concentrations of the inhibitor. The impedance diagrams obtained have an almost semicircular appearance. This indicates that the corrosion of mild steel in aqueous solution is mainly controlled by a charge-transfer process. The impedance parameters are given in   Nyquist plots of mild steel in 1M HCl in presence of different concentrations of 3,6-bis(pyridin-2-yl)-4-{[(3aS,5S,5aR,8aR,8bS)-2,2,7,7tetramethyltetrahydro-5H-bis[1,3]dioxolo[4,5-b:4 0 ,5 0 -d]pyran-5-yl)methoxy]methyl}pyridazine monohydrate.

Figure 8
EIS parameters for the corrosion of mild steel in 1M HCl with and without inhibitor 3,6-bis(pyridin-2-yl)-4-{[(3aS,5S,5aR,8aR,8bS)-2,2,7,7tetramethyltetrahydro-5H-bis[1,3]dioxolo[4,5-b:4 0 ,5 0 -d]pyran-5-yl)methoxy]methyl}pyridazine monohydrate at 308 K. plots that the impedance response of mild steel was significantly changed after addition of the inhibitor. R ct is increased to a maximum value of 185 cm 2 for the inhibitor, showing a maximum inhibition efficiency of 91% at 10 À3 M. The decrease in C dl from the HCl acid value of 200 mF cm À2 , may be due to the increase in the thickness of the electrical double layer or to a decrease in the local dielectric constant (Elmsellem et al., 2014). This is caused by the gradual displacement of water molecules by the adsorption of organic molecules on the mild steel surface (Hjouji et al., 2016). Apart from the experimental impedance (EIS) results, the following conclusion is drawn: the alternating impedance spectrum reveals that the double-layer capacitances decrease with respect to the blank solution when the title compound is added. This fact confirms the adsorption of inhibitor molecules on the surface of the MS.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. Water hydrogen atoms were located in a difference-Fourier map and refined with the distance constraint O-H = 0.80 (2) Å . Other H atoms were positioned geometrically with C-H = 0.93, 0.98, 0.97 and 0.96 Å , for aromatic, methine, methylene and methyl H atoms, respectively, and constrained to ride on their parent atoms, with U iso (H) = 1.5U eq (C-methyl) or 1.2U eq (C) for all other H atoms.

Funding information
TH is grateful to Hacettepe University Scientific Research Project Unit (grant No. 013 D04 602 004).

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.