Synthesis and characterization of a highly reducing neutral extended viologen and the isostructural hydrocarbon 4,4′′′′-Di-n-octyl-p-quaterphenyl

The molecule 4,4-di-n-octyl-p-quaterphenyl was synthesized in one step by a nickel-catalyzed cross-coupling reaction. Powder X-ray diffraction shows that it crystallizes in a layered structure with the long axis of the molecule nearly perpendicular to the layer plane. Differential scanning calorimetry indicates a transition to a liquid-crystalline phase at 81 degrees C. Reaction of 4,4'-bis(4-pyridyl)biphenyl with 1-bromooctane yields the dication 2(2+)2Br(-), an extended viologen isostructural with 4,4-di-n-octyl-p-quaterphenyl. Reduction of 2(2+)2Br(-) with sodium amalgam in DMF yields 2, the first neutral extended viologen to be isolated. The molecule 2 is, to the best of our knowledge, the most reducing neutral organic molecule that has, been synthesized. Single-crystal X-ray diffraction shows that a diradical form, either singlet or triplet, makes an important contribution to the electronic structure of 2. The broadened H-1 NMR spectrum of 2 indicates the presence of a triplet, but it has not been possible to observe the triplet by ESR spectroscopy. The electronic structure of 2 appears to be closely related to that of a classic molecule, Chichibabin's hydrocarbon.