CRYSTAL-STRUCTURES AND MAGNETIC-PROPERTIES OF NITRONYL NITROXIDE AND IMINO NITROXIDE RADICALS ATTACHED TO THIENO[3,2-B]-THIOPHENE RINGS AND THIENO[2,3-B]THIOPHENE RINGS

Nitronyl nitroxide (ONC=NO) and imino nitroxide (NCNO) free radicals joined to isomeric thieno[3,a-b]thiophene and thieno[2,3-b]thiophene, respectively, were prepared and their crystal structures and magnetic properties were investigated. In all crystals of four free radical species, the ON-C-NO moieties are in the same plane as the thienothiophene ring. The coplanarity is associated with the extremely short intramolecular contact between the oxygen atom in the NO group and the sulfur atom in the thienothiophene ring. The free radicals attached to thieno[3,2-b]thiophene are arranged in a chain structure with short intermolecular NO-S contacts, while the free radicals linked to thieno[2,3-b]thiophenes make dimers with a face-to-face pairing by the use of the NO-S contacts. Among four radical species, thieno[3,2-b]thiophene-substituted nitronyl nitroxide exhibits a weak ferromagnetic interaction (J = +0.11 K/k(B), H = -2JS(i)S(j)) below 40 K as revealed by a SQUID susceptometer, while others are antiferromagnetic.