The role of weak non-bonding interactions in the structure of 3-Methyl-5-(methylthio)-1,3,4-thiadiazole-2(3H)-thione

SEBASTIAN SUAREZ; SAROJ K. S. HAZARI; BIPLAB GANGULY; FABIO DOCTOROVICH; TAPASHI G. ROY; RICARDO BAGGIO

Congreso; American Crystallographic Association, Annual Meeting 2013; 2013

During a systematic trial intended to synthesize a molybdenum(VI) complex of our interest, excellent crystals were obtained, at the time thought to correspond to the expected product. A straightforward crystal structure determination disclosed that the compound was in fact the rather simple heterocyclic title compound, C4H6N2S3 (I), a readily available commercial product known to chemists since long [1-2], but which crystal structure had not been reported so far. In addition to the rather expectable molecular information the study revealed a surprising collection of varied non-bonding interactions which provide to the overall stability of the crystal structure, and to which analysis most of the following discussion will be devoted. The asymmetric unit of I includes two independent molecules (A and B) consisting of a 1,3,4-thiadiazole ring, with a methyl group attached at position 3 and a thiomethyl group at position 5. Both are extremely planar but not parallel (dihedral angle: 14.87(4)°). The two independent moieties present no significant differences between each other. Being the molecular details basically unexceptional, the most interesting aspect of the structure resides in its packing: in this respect this is a good example of very weak forces (London´s, dipole-induced dipole, etc.) expressed as a variety of usually borderline interactions of various types (S???S ; C-H???π ; π???π, C-S???π) which in the absence of stronger ones can become the basic synthons promoting molecular recognition and intermolecular interaction, and thus playing an essential constructive role in the crystal lattice. Both independent moieties form weakly linked planes parallel to (010) having only A or B molecules in them, or, in other words, A and B types are not mixed. A planes appear at y ~ 0.0, 0.50; B ones at y ~ 0.25, 0.75. Both substructures are interconnected (Fig 1) by a survey of S???S contacts of varied strengths. The final result is an extremely even spatial distribution of these cooperative interactions providing to the organization of a solid and stable 3D structure. The analysis of simple structures as the one herein reported might serve to have a better insight of the importance which these interactions have in crystal structure stability, and (eventually) induce the revision of perhaps too stringent, commonly accepted threshold values for the "acceptable ones". Trying to go beyond mere speculations, we are at present involved in some theoretical calculations regarding these type of weak S???S synthons.