Isomeric and Isostructural Oligothienylsilanes–Structurally Similar, Physicochemically Different: The Effect of Interplay between C–H···C(π), S···C(π), and Chalcogen S···S Interactions

The solid state and solution properties of tris­(2-thienyl)­methylsilane, <b>I</b>, tetrakis­(2-thienyl)­silane, <b>III</b>, and their positional isomers bearing 3-thienyl groups (<b>II</b> and <b>IV</b>) were investigated and compared. The tris­(thienyl)­silanes (<b>I</b>, <b>II</b>) crystallize in different space groups, but their respective structural motifs are very comparable. In turn, the tetrathienyl isomers are isostructural. Furthermore, in all studied systems the same set of C–H···C­(π), S···C­(π), S···S, C–H···S interactions are engaged in supramolecular structure formation. These interactions are interchangeable as thienyl rings (excluding structure <b>II</b>) are affected by 2-fold positional disorder. Despite the high level of structural similarity, the studied thienylsilanes show very different physicochemical behavior: (1) much higher melting points and larger enthalpies of fusion for <b>II</b> (mp = 71.3 °C, Δ<i>H</i> = 20.9 kJ mol<sup>–1</sup>) and <b>IV</b> (mp = 221.2 °C, Δ<i>H</i> = 29.1 kJ mol<sup>–1</sup>) with respect to their isomeric counterparts <b>I</b> (mp = 28.6 °C, Δ<i>H</i> = 16.0 kJ mol<sup>–1</sup>) and <b>III</b> (mp = 131.5 °C, Δ<i>H</i> = 27.0 kJ mol<sup>–1</sup>), (2) different temperature-dependence unit-cell evolution, and (3) much lower solubility of <b>IV</b> compared to <b>III</b>. The computations show that the strength of interactions decreases in the series C­(α)–H···C­(π) > C­(β)–H···C­(π) > S···C­(π) ≫ S···S. In a combination with crystal symmetry, this leads to a different distribution of energy within the corresponding crystal structures, and as a consequence, results in their different macroscopic behaviors. In addition, the solid–liquid equilibrium studies suggest that the specific S···S chalcogen bonding between molecules of <b>IV</b> is responsible for decreased solubilities of this compound. To characterize the specific interactions involving sulfur atoms (S···S and S···C­(π)), the quantum theory of atoms in molecules has been successfully applied.