Comment

We report here the molecular structure and the supramolecular aggregation of (Z)-5-[4-(dimethylamino)benzylidene]-2-(piperidin-1-yl)-1,3-thiazolidin-4(5H)-one, (I) (Fig. 1), which we compare briefly with those of the analogues (II) (Low et al., 2007) and (III) (Insuasty et al., 2012) (see Scheme 1). Compound (I) was prepared by reaction of piperidine with the intermediate (Z)-5-[4-(dimethylamino)benzylidene]-1,3-thiazolidin-4-one (A) (see Scheme 2), which was itself prepared using a base-catalysed condensation reaction between rhodanine (2-sulfanylidene-1,3-thiazolidin-4-one) and 4-(dimethylamino)benzaldehyde. The structures of a number of substituted (Z)-benzylidene-2-sulfanylidene-1,3-thiazolidin-4-ones have been reported recently (Delgado et al., 2005, 2006); these compounds are of interest, both as potential intermediates for the synthesis of novel fused heterocyclic systems and as potential antifungal agents (Sortino et al., 2007).

As generally found for (Z)-5-benzylidene-1,3-thiazolidin-4-ones (Delgado et al., 2005, 2006; Low et al., 2007), the aryl and thiazolidine rings in compound (I) are only modestly displaced from coplanarity, as indicated by the relevant torsion angles (Table 1). The dihedral angle between these two ring planes is only 28.1 (2)° and the piperidine substituent adopts an almost perfect chair conformation with the N21-C2 bond occupying an equatorial site.

Perhaps the most striking feature of the molecular structure of (I) is the very wide C-C-C angle of 130.2 (2)° at the bridging C57 atom. Associated with this wide angle is a rather short intramolecular contact distance of 2.62 Å between atoms S1 and H56 (Fig. 1); this distance is significantly shorter than

the sum (2.89 Å) of the van der Waals radii for S and H (Bondi, 1964; Rowland & Taylor, 1996), and it may be compared with the corresponding value of 2.50 Å in compound (II) (Low et al., 2007) and 2.75 and 2.77 Å for the two independent molecules of compound (III) (Insuasty et al., 2012). In addition, the exocyclic angles S1-C5-C57 and C56-C51-C57 are both significantly larger than the corresponding angles C4-C5-C57 and C52-C51-C57, respectively (Table 1). These observations, taken together, indicate that the nonbonded intramolecular contact S1H56 is strongly repulsive. It appears that distortion of the bond angles at atoms C5, C51 and C57 is an energetically more favourable route for accommodating the repulsive SH contact than that provided by a significant rotation of the aryl ring around the C51-C57 bond. On the other hand, although the aryl-ring C52-C53 and C55-C56 bond lengths of 1.379 (4) and 1.389 (3) Å, respectively, are slightly shorter than the remaining C-C distances in this ring [range 1.397 (3)-1.412 (3) Å; mean 1.405 Å], none of the other bond lengths in (I) (Table 1) provides any evidence for the development of a polarized (zwitterionic) electronic structure of type (Ia) which might lead to restricted rotation about the C51-C57 bond.

The supramolecular aggregation in compound (I) is very simple as the crystal structure contains neither conventional hydrogen bonds nor aromatic - stacking interactions. Instead the molecules are linked in pairs by a single C-H(arene) hydrogen bond (Table 2) to form centrosymmetric dimers, with the reference dimer centred across (, , ) (Fig. 2). There are two such dimers per unit cell, but there are no direction-specific interactions between adjacent dimers. The only other potentially significant intermolecular contact is of the C-HS type between the molecules at (x, y, z) and (-x + 1, -y, -z + 1) (Table 2). However, not only is the HS distance greater than the sum (2.89 Å) of the van der Waals radii for H and S (Bondi, 1964; Rowland & Taylor, 1996), but two-connected sulfur has been shown (Allen et al., 1997) to be an extremely poor acceptor in hydrogen bonds, even from O-H and N-H as donors; accordingly, this contact cannot be regarded as structurally significant.

It is of interest to compare briefly the supramolecular aggregation reported here for compound (I) with that in compounds (II) and (III). In compound (II) (Low et al., 2007), symmetry-related pairs of molecules are again linked into cyclic dimers, but here the component molecules are related by a twofold rotation axis, rather than by inversion as in compound (I) and hydrogen bond concerned in (II) is of the C-HO type rather than of the C-H(arene) type found in (I), Compound (III) (Insuasty et al., 2012) crystallizes in the space group P with Z' = 2, and in one of the two independent molecules the chair-form morpholine ring is conformationally disordered over two sets of sites, with the two disorder forms exhibiting different orientations of the ring, while the other molecule is fully ordered. An extensive series of C-HO hydrogen bonds link the molecules of (III) into a complex two-dimensional structure; it is notable that, despite the molecular constitution, intermolecular hydrogen bonds of the C-HN type are absent from the structure of compound (III).

Experimental

To a mixture of 4-(dimethylamino)benzaldehyde (1.1 mmol) and rhodanine (1.0 mmol) in dry ethanol (10 ml) was added one drop of piperidine. This mixture was then heated under reflux for 6 h. After cooling the mixture to ambient temperature, the resulting solid precipitate was collected by filtration and washed with cold ethanol to provide intermediate (A). A mixture of (A) (1.0 mmol) and piperidine (2.0 mmol) in dry tetrahydrofuran (10 ml) was then heated under reflux for 20 h. After warming to ambient temperature, this mixture was poured onto an excess of crushed ice. The resulting solid product was collected by filtration and washed successively with water and hexane, and then crystallized from ethanol (yield 42%, m.p. 490-492 K). MS (EI, 70 eV) m/z (%): 315 (M⁺, 45), 179 (6), 178 (15), 177 (100), 176 (27), 162 (5), 161 (7).

All H atoms were located in difference maps and thereafter treated as riding atoms in geometrically idealized positions, with C-H = 0.95 (aromatic and methane), 0.98 (CH₃) or 0.99 Å (CH₂) and U_iso(H) = kU_eq(C), where k = 1.5 for the methyl groups, which were permitted to rotate but not to tilt, and k = 1.2 for all other H atoms.

Data collection: COLLECT (Hooft, 1999); cell refinement: DIRAX/LSQ (Duisenberg et al., 2000); data reduction: EVALCCD (Duisenberg et al., 2003); program(s) used to solve structure: SIR2004 (Burla et al., 2005); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: PLATON (Spek, 2009); software used to prepare material for publication: SHELXL97 and PLATON.

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Supplementary data for this paper are available from the IUCr electronic archives (Reference: YF3021 ). Services for accessing these data are described at the back of the journal.

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