Halogen Bonding in Diaza-Triisopropylsilyl-Tetracene Crystals?

Four
halogenated triisopropylsilylethynyl (TIPS)-tetracene derivatives
were synthesized, their single crystal structures and optical and
electronic properties were examined. Both, diiodo-TIPS-diazatetracene
and dibromo-TIPS-diazatetracene showed herringbone arrangements, in
which the halogen atoms are directed toward the nitrogen atoms of
a neighboring molecule with slightly lower distances than the van
der Waals radii between halogen and nitrogen atoms. Computations of
the electrostatic potentials showed the availability and the strength
of σ holes, so that we assume a weak halogen–nitrogen
bonding. Tribromo-TIPS-diazatetracene delivered a 2D-brick wall structure
with no sign of a halogen bonding. Dibromo-TIPS-tetracene crystallized
neither in a herringbone nor in a brick wall motif. These results
show that for halogenated azaacenes at least weak halogen–nitrogen
interactions exist.