Self-assembly of iodoacetylenyl-substituted nitronyl nitroxides via halogen bonding

Abstract

Novel nitronyl nitroxides, namely 2-(3-iodoethynylphenyl)- and 2-(4-iodoethynylphenyl)-4,4,5,5-tetramethyl-imidazoline-3-oxyl-1-oxides, were prepared by condensation of appropriate aldehydes with 2,3-bis(hydroxylamino)-2,3-dimethylbutane followed by oxidative treatment with NaIO₄. Crystal and molecular structures of the obtained paramagnets were studied by single-crystal X-ray diffraction. In the crystals, the radicals are assembled into zigzag chains in which the radical building blocks are linked by intermolecular I⋯N–O halogen bonding. Magnetic analyses revealed that in both nitronyl nitroxides, the radicals are weakly coupled. Nonetheless, spin–spin interactions in the 3-iodoethynyl isomer are antiferromagnetic, and the coupling is stronger than that in the 4-iodoethynyl derivative, in which the exchange interaction is ferromagnetic. DFT calculations in combination with MEP, NCIplot, and QTAIM analyses were used to evaluate and characterize the structure-directing halogen bonding interactions observed in the solid state of both compounds.