Crystal structure, Hirshfeld surface analysis and spectroscopic characterization of the di-enol tautomeric form of the compound 3,3′-[(2-sulfanylidene-1,3-dithiole-4,5-diyl)bis(sulfanediyl)]bis(pentane-2,4-dione)

A new dithiolene derivative has been synthesized from [TBA]2[Zn(dmit)2] and 3-chloro-2,4-pentanedione. Crystals were obtained by slow evaporation of an acetonitrile solution of the title compound, which crystallizes in the triclinic space group P . The structure of the S—C heterocycle includes two pentadione moieties that are outside of the plane of the molecule. Intra- and intermolecular hydrogen bonds are observed, as well as C—H⋯S, S⋯S and O⋯H short contacts from other intermolecular interactions.

Herein, the reduction of the [Zn(dmit) 2 ] 2À dithiolene complex is utilized to aid the formation of a novel thiocarbonyl compound by its reaction with 3-chloro-2,4-penta- ISSN 2056-9890 nedione (Cl-acac) to yield the title compound (3E,3 0 E)-3,3 0 -[(2-sulfanylidene-1,3-dithiole-4,5-diyl)bis(sulfanediyl)]bis(4hydroxypent-3-en-2-one), the di-enol tautomer of 3,3 0 -[(2sulfanylidene-1,3-dithiole-4,5-diyl)bis(sulfanediyl)]bis (pentane-2,4-dione). The electrophilic nature of the acetylacetone (acac) motif and the high electron density on the sulfur atoms drive the nucleophilic substitution to completion. The title compound is a double -dicarbonyl compound that contains two acetylacetone moieties, which are found in their enolic form in the solid state. Concerning the reactivity of the title compound, it is able to undergo acid or base-catalyzedhydrogen substitution reactions, in which the rate-determining step is the formation of the enol or enolate anion (Shapet'ko et al., 1975). Compared to the acid-catalyzed process, the selfenolization of most ketones is negligible. The doubledicarbonyl compound described herein also undergoes tautomerization; however, in the solid phase, the enol tautomer predominates in this equilibrium as it is stabilized relative to the keto form via resonance through the conjugated -system and by intramolecular hydrogen bonding in the solid-state (Drexler et al. 1976;Seco et al. 1989). This aspect is confirmed by its FT-IR and NMR spectra.

Supramolecular features
The title compound exhibits numerous intermolecular interactions, namely four C-HÁ Á ÁO, three C-HÁ Á ÁS, three CÁ Á ÁO, one SÁ Á ÁC, and one SÁ Á ÁS interaction (Fig. 2, Tables 1 and 2). The five-membered thiocarbonyl-containing rings lie almost parallel to the c axis and extend in a sheet-like fashion, forming a network that propagates along the axis with all rings following the same orientation. The sheets are linked by outof-plane C13-H13BÁ Á ÁS1 short contacts, generating stacks along the a axis with SÁ Á ÁS short contacts between adjacent molecules [S5Á Á ÁS5 iv = 3.5688 (6) Å ]. In addition, the nucleophilic atom S3 is oriented towards the electrophilic C5, leading to an S3Á Á ÁC5 iii [3.471 (2) Å ] contact, further contributing to the extension of the network along the c-axis direction. Molecules of the title compound also associate with neighboring molecules above and below the thiocarbonyl ring planes through the acac backbone by C4-H4Á Á ÁS1 and C9-H9Á Á ÁS1 contacts. The acac backbone lies nearly perpendicular to the rings, and there are several key interactions between the 1428 Cordero Giménez et al.   The title compound with displacement ellipsoids drawn at 50% probability level and hydrogen bonds (O-HÁ Á ÁO) in the asymmetric unit indicated. carbonyl oxygen atoms (O1, O2, and O3) and neighboring methyl hydrogen atoms (H8A and H8C) with lengths in the range 2.56-2.66 Å . However, atom O4 is not involved in any interactions with hydrogen atoms, and instead makes short contacts with both C12 and C13.

Hirshfeld Surface Analysis
The Hirshfeld surface (Spackman & Jayatilaka, 2009) for the title compound mapped over d norm is shown in Fig. 3 while Fig. 4 shows the associated two-dimensional fingerprint plots (McKinnon et al., 2007), both generated with Crystal-Explorer17 (Turner et al., 2017). Red spots on the Hirshfeld surface mapped over d norm in the color range À0.0820 to 1.5568 arbitrary units confirm the above-mentioned primary intermolecular contacts. The fingerprint plots are given for all
stacking occurs between the benzene and thione rings) and YISBOR/YISBOR10 (where therestacking between the thione ring and one benzene ring).

Synthesis and crystallization
The synthesis of the title compound was carried out by refluxing 1 eq. of [TBA]

Spectroscopic Characterization
Without basic catalysis, the self-enolization of most ketones is negligible and the keto form is favored almost exclusively (Drexler et al., 1976). However, -dicarbonyl compounds, which can also undergo tautomerization, are stabilized in the enol tautomer via resonance of the conjugated -system and intramolecular hydrogen bonding. Furthermore, the enol is the less polar of the two tautomers because the intramolecular hydrogen bond reduces the dipole-dipole repulsion of the two carbonyls in the keto form. The equilibrium of -dicarbonyl compounds has been studied extensively and it has been shown that tautomeric interconversion between the diketo and enol forms is relatively slow and can be observed by NMR. Under normal conditions, the enolic form predominates in equilibrium (Egan et al., 1977). This effect was demonstrated to be solvent and concentration dependent. An NMR study of keto-enol tautomerism in -dicarbonyl compounds revealed that for the unsubstituted and symmetrical -dicarbonyl compound pentane-2,4-dione, the equilibrium constant at 310 K has a value of 2.95 with 93.3 enol % (acetone exists as 0.00025% enol) (Schubert, 1960). In addition, as these compounds are progressively diluted with nonpolar solvents, the enol content of the system increases. The progressive dilution with more polar solvents than the solute was observed to increase the stability of the keto form.
In the case of the 1 H NMR study of the title compound in deuterated chloroform at 298 K, the predominant form was observed to be the enol tautomer. NMR was used to confirm the underlying symmetry the title compound possesses in solution, in which the enol tautomer predominates, as can be observed in Fig. 5. The lowest frequency signal in the 1 H NMR spectrum integrates to twelve and corresponds to the methyl protons of the compound, indicating that the latter are chemically equivalent. Similarly, the enol form of the compound was observed crystallographically and in solution, exhibiting intramolecular hydrogen bonding and renders both methyl groups, as well as both carbonyls, chemically equivalent. When studying the proton spectrum, the conjugation in the six-membered pseudo-aromatic ring deshields the signal of the interchangeable proton, giving rise to a low field signal at 15.4 ppm that is lost in the baseline. Looking further into the baseline at higher fields, around 5.1 ppm, it reveals a wide signal that is almost lost in the noise and that can be assigned to the interchangeable proton in the keto tautomer (Fig. 6). The formation of this hydrogen-bridge bond is promoted by the planar structure of the enol-carbonyl resonance system because this leads to an ideal spatial orientation of the hydroxy group and carbonyl group in order to construct a strong hydrogen-bridge bond. Therefore, the monoenolic form of a -dicarbonyl compound has a planar, six-membered cyclic structure stabilized by resonance. Decreasing the concentration of the solute in non-polar solvents has been proven to increase the concentration of the enol tautomer. 13 C NMR spectrum displayed a single signal at 24.8 ppm for the methyl carbons, and a single signal at 197.7 ppm for the carbonyl carbons, supporting the statement that there is chemical equivalency between the methyl groups and, most importantly, between both carbonyl moieties. This effect has been previously demonstrated by comparing the 13 C NMR spectra of the enol forms of symmetrical and unsymmetrical derivatives of -diketones, where a different chemical shift was observed for the two carbonyls in the unsymmetrical case (Shapet'ko et al., 1975). It is possible to conclude that the three Tautomeric effect observed in the acetylacetonate portion of the title compound. Figure 6 signals of the 2,4-pentanedione portion of the title compound, as well as the chemical shifts observed, are indicative of a symmetrical system that results from intramolecular hydrogen-bonding in the enol tautomer.
IR peaks at 2,962 and 2,876 cm À1 are assigned to the C-H stretches (Fig. 7). The peaks between 1,575 and 1,402 cm À1 correspond to the C C bond in the enol form. Moreover, hidden under this peak there is also the C O stretch in the enol form, which is lowered by conjugation to the C C bond and the O atom of the -OH group, respectively. OH stretches for -diketones are tabulated from 3,200 to 2,400 cm À1 ; however, in the case of symmetric acac compounds where the enol form predominates and the interchangeable hydrogen is located between the two carbonyls, the dipole change associated to the symmetric OH stretch is null, and the signal is minimal to non-existent. Thus, evidence from NMR and IR spectroscopy indicates that the compound exists almost entirely in its enol form.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. H atoms were included in geometrically calculated positions for the alkyl groups while the hydrogen atoms from OH groups were located from the difference-Fourier map and refined as riding: O-H = 0.82 Å , C-H = 0.93-0.98 Å with U iso (H) =1.5U eq (O, C-methyl) and 1.2U eq (C) for other H atoms.

Figure 7
IR spectrum of the title compound.

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.