Molecular and crystal structure, Hirshfeld analysis and DFT investigation of 5-(furan-2-ylmethylidene)thiazolo[3,4-a]benzimidazole-2-thione

The crystal structure of the title compound is stabilized by the presence of weak C—H⋯N hydrogen bonds and slipped π–π interactions. Hirshfeld surface analysis showed that H⋯H contacts are the dominant interactions.

The thiazolo[3,4-a]benzimidazole fused-ring system in the title compound, C 14 H 8 N 2 OS 2 , is nearly planar, the r.m.s. deviation being 0.0073 Å . The thiazolobenzimidazole-2-thione system is almost in the same plane as the furan-2-ylmethylene moiety, with a dihedral angle of 5.6 (2) between the two leastsquares planes. In the crystal, adjacent molecules are connected by weak intermolecular interactions (C-HÁ Á ÁN and slippedstacking) into a threedimensional network. The nature of the intermolecular interactions was also quantified by Hirshfeld surface analysis. DFT analysis indicates a good agreement of the experimentally determined and the theoretically calculated molecular structures.
We report in this communication the synthesis, molecular and crystal structures and Hirshfeld surface analysis of the title thiazolo derivative. In addition, the HOMO-LUMO ISSN 2056-9890 energies, molecular electrostatic potential and chemical reactivity descriptors are described on the basis of theoretical calculations.

Structural commentary
The molecular structure of the title compound is shown in Fig. 1. The tricyclic thiazolobenzimidazole group, consisting of a benzimidazole unit fused to a thiazole ring, is bonded to a furan-2-yl-methylene moiety at carbon atom C6. As expected, the thiazolo[3,4-a]benzimidazole group is planar with an r.m.s. deviation of 0.0073 Å for the thirteen (C6-C14/N1/N2/S1/S2) non H-atoms. The furan-2-yl-methylene moiety is also planar, with an r.m.s deviation of 0.0028 Å for the six (C1-C5/O1) non H-atoms. The two ring systems are almost in the same plane, their least-squares planes subtending a dihedral angle of 5.6 (2) . The molecule exists in a Z configuration with respect to the C5 C6 bond. The S1-C8 and S1-C6 distances, 1.739 (4) and 1.775 (3) Å , respectively, are in agreement with a C-S single bond of a thiazole ring . In comparison, the S2-C8 bond [1.612 (4) Å ] of the thione moiety is much shorter as a result of its double-bond character and the presence of a delocalized -electronic system throughout the entire thiazolobenzimidazole ring system (Liang et al., 2009). The bond lengths of the thiazolobenzimidazole and furan rings are similar than those in a series of thiazolo[3,2-a]benzimidazole and thiazolo[3,4-a]benzimidazole compounds (Bruno et al., 1996;Wang et al., 2011). The intramolecular C10-H10Á Á ÁS2 hydrogen-bonding interaction (Table 1) helps to stabilize the molecular conformation.

Supramolecular features and Hirshfeld surface analysis
In similar reported structures containing thiazole ring systems, the crystal packing is mainly based on short contacts and weak interactions . In the crystal packing of the title compound, weak C3-H3 aromatic Á Á ÁN2 i hydrogen bonds (Table 1) connect the molecules into dimers (Fig. 2).
Additionalstacking interactions between adjacent thiazolobenzimidazole ring systems link the dimers into a threedimensional network structure, with centroid-to-centroid distances of 3.6523 (18) Å (slippage 1.141 Å ) and 3.6515 (1) Å (slippage 1.137 Å ) between the thiazole ring and the benzene ring of one thiazolobenzimidazole ring system, and between the imidazole ring and the benzene ring of another thiazolobenzimidazole ring system, respectively.

Figure 2
Crystal packing diagram of the title compound with hydrogen bonds (dashed lines) viewed along the b axis.

Figure 1
The molecular structure of the title compound showing the atomnumbering scheme. Displacement ellipsoids are drawn at the 50% probability level.
CÁ Á ÁC contacts are shown in Fig. 4. HÁ Á ÁH contacts are the dominant interactions with a contribution of 29.8% to the overall HS. The SÁ Á ÁH/HÁ Á ÁS interactions appear as the next largest region of the FP plot, highly concentrated at the edges, characteristic of hydrogen-bond interactions with an overall HS contribution of 19.6%. The CÁ Á ÁH/HÁ Á ÁC interactions are illustrated by two symmetrical wings on the left and right sides (16.5% contribution). The CÁ Á ÁC contacts, which are the measure ofstacking interactions, occupy 9.1% of the HS and appear as a unique triangle. The NÁ Á ÁH/HÁ Á ÁN contacts are represented by a pair of sharp spikes and make a contribution of 6.6%. Other intermolecular contacts in the HS mapping contribution less than 5%.

Theoretical calculations
The hybrid functional B3LYP (Becke's three-parameter hybrid model using the Lee-Yang Parr correlation functional) with the 6-311G (d, p) basis set (Becke, 1993) were used in all calculations as implemented in Gaussian 09 (Frisch et al., 2009). Theoretical calculations were performed to obtain the optimized molecular structure of the title compound in the gas phase. The crystallographic information file was used as an input file in the GaussView 5 program (Frisch et al., 2000) to start structure optimization of the title compound. Comparison of the DFT-optimized molecular structure with the refined structure based on single crystal X-ray data revealed a good agreement (see supporting information for a detailed comparison of bond lengths and angles). Frontier molecular orbitals and the molecular electrostatic potential were calculated using the same level of theory.

Frontier molecular orbital and chemical reactivity
The frontier molecular orbitals, HOMO (highest occupied molecular orbital) and LUMO (lowest-unoccupied molecular orbital), are plotted to specify the distribution of electronic densities. The electron distribution of the HOMO-1, HOMO, LUMO and the LUMO+1 energy levels are shown in Fig. 5. As can be seen from the figure, the HOMO and LUMO are localized in the plane extending from the whole furan ring to the thiazolo-benzimidazole ring system. The frontier molecular orbital energies, EHOMO and ELUMO are À7.23 and À1.87 eV, respectively, and the HOMO-LUMO gap is 5.36 eV.
Since the gap energy is considered to be small, the molecule is defined as soft.    Global chemical reactivity descriptor (GCRD) parameters can be obtained as reported in the literature (Belkafouf et al., 2019). The calculated values of the GCRD parameters for the title molecule are summarized in Table 2. The chemical stability of the title molecule is explained by the chemical potential () value, which is À4.55 eV. On the other hand, the chemical hardness () value is 2.68 eV, indicating that the charge transfer occurs within the molecule. From Table 2, the electrophilic behaviour of the molecule is confirmed by the global electrophilicity (!), which has a value of 3.86 eV. The structure-property relationship can be also described by the hyper-hardness descriptor (À), which was introduced to investigate the reactivity or stability of molecules theoretically (Ghanavatkar et al., 2020). According to the results, the positive value of À (+4.30 eV) indicates stability of the molecule.

Molecular electrostatic potential analysis
To predict reactive sites for electrophilic and nucleophilic attack, molecular electrostatic potential (MEP) surfaces were computed at the B3LYP/6-311G (d,p) level with the optimized structure using GaussView (Frisch et al., 2000). The different values of the electrostatic potential at the MEP surface are represented by red, blue and green (Kourat et al., 2020). From Fig. 6, it is obvious that the negative potential regions (red) are associated with sulfur and nitrogen atoms whereas the positive potential regions (blue) are on the side of hydrogen atoms. It may also be seen in Fig. 6 that green areas cover parts of the molecule enveloping the system of the aromatic rings.

Synthesis and spectral characterization
The synthetic route preparation of the title compound is illustrated in Fig. 7. Initially, the tricyclic thiazolo(3,4a)benzimidazole (1) was obtained from amino phenylene dithiocarbamate and chloroacetic acid by the Hanztsch reaction. The title compound (3) was prepared by Knoevenagel condensation of furaldehyde 2 (2; 0.01 mol) and the tricyclic compound (1; 0.02 mol) in acetic acid (10 ml) buffered by sodium acetate (0.02 mol). The reaction was monitored by TLC (petroleum ether/ethyl acetate, 8/2). After 4 h of refluxing and stirring, the brown solid obtained was filtered off, dried and recrystallized from ethanol to give the title compound, m.p. 493 K, in a yield of 85%.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. H atoms were placed in calculated positions (C-H = 0.93 Å ) and allowed to ride on their parent atoms with U iso (H) = 1.2U eq (C).

Funding information
The authors gratefully acknowledge financial support via the PRFU project from the Algerian Ministry of Higher

Figure 6
Molecular electrostatic potential map of the title molecule.

5-(Furan-2-ylmethylidene)thiazolo[3,4-a]benzimidazole-2-thione
Crystal data where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.17 e Å −3 Δρ min = −0.17 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.