Isostructural Bis-1,2,3-Thiaselenazolyl Dimers

Reduction of the N-methylated bis-1,2,3-thiaselenazolylium salts [2a,b,c][OTf] (with R1 = Me and R2 = H (2a), F (2b), Me (2c)) affords the corresponding bis-1,2,3-thiaselenazolyl radicals 2a,b,c. The radicals crystallize as centrosymmetric Se−Se σ-bonded dimers [2a,b,c]2, in which an intramolecular Se−S bond of the radical is cleaved and replaced by an intermolecular Se−Se bond. The crystal structures of the three dimers are isomorphous, all belonging to the monoclinic space group P21/c, and consist of interpenetrating, cross-braced, slipped π-stack arrays laced together by numerous short intermolecular Se---N′ and Se---S′ contacts. In the solid state the dimers are diamagnetic, and behave as small band gap semiconductors with a room temperature conductivity σRT near 10−6 S cm−1. Application of pressure leads to a dramatic increase in conductivity for all three compounds. At 5 GPa the value of σRT ranges from near 10−1 S cm−1 for [2c]2 to 1 S cm−1 for [2b]2 and 101 S cm−1 for [2a]2. Comparison of the three crystal structures suggests that the compressibility of [2a]2, and hence the response of its conductivity to pressure, is superior to that of [2b,c]2.