1,1 ',3,3 '-tetramethyl-2,2 '-diimidazolium dication: Its bromide salt andthe tetraphenylborates of its ketone adducts

The crystal structures of several 1,1',3,3'-tetramethyl-2,2'-diimidazolium dication salts were determined at low temperatures. The dihedral angles bet ween the planes of the two five-membered rings vary between 64 degrees in t he bromide salt and up to 80 degrees in the ketone adduct tetraphenylborate s, indicating some conformational flexibility around the central C-C bond o f the 1.1',3,3'-tetramethyl-2,2'-diimidazolium dication. Semiempirical calc ulations predict the conformation with a dihedral angle of 90 degrees to be most favourable and the conformations within the crystal structures to be results of packing effects. In tetraphenylborate salts of ketone and diketo ne adducts, the oxygen centers coordinate to the dication from above as wel l as below the defined molecular least squares plane and form chains of alt ernating dications and ketones. Since this packing motif is found in tetrap henylborate salts with different ketones, it might prove to be a robust sup ramolecular synthone in crystal engineering.