Bipyridinium-bis(carboxylate) Radical Based Materials: X-ray, EPR and Paramagnetic Solid-State NMR Investigations

作者：Toma Oksana; Leroux Maxime; Mercier Nicolas; Allain Magali; Kassiba Abdel Hadi; Swamy Shashi Kumar Kumara; Dittmer Jens

来源：European Journal of Inorganic Chemistry, 2016, 2016(7): 1036-1043.

DOI：10.1002/ejic.201501206

• Solid-state structures
• Thermochromism
• Viologen
• Radicals
• NMR spectroscopy

The zwitterionic 1,1'-bis(4-carboxyphenyl)-4,4'-bipyridinium (bp4pc) has been synthesized and crystals of its hydrated form bp4pc center dot 2H(2)O and of its protonated reduced form H-bp4pc have been obtained. Upon heating, bp4pc center dot 2H(2)O undergoes partial dehydration, leading to bp4pc center dot H2O at 160 degrees C, together with a color change from yellow (room temperature) to green (140 degrees C) and finally to brown (160-180 degrees C). Analysis of bond lengths in the solid state reveals the expected short (d = 1.425 angstrom) and long (d = 1.485 angstrom) C-C central bond lengths in the all-radical salt H-bp4pc and bp4pc center dot 2H(2)O, respectively, whereas the distance of 1.475 angstrom in bp4pc center dot H2O does not allow a conclusion to be drawn regarding the presence of radicals in this compound. EPR and solid-state paramagnetic NMR experiments of H-bp4pc and the hydrated zwitterion bp4pc center dot 2H(2)O at different temperatures, however, show that the color change of the latter upon heating is due to the presence of bipyridinium radicals, the concentration of which, although low, increases with increasing temperature. The nature of the electron donor involved in this thermal-induced electron transfer is not fully understood. Most plausible is the possibility that it is the carboxylate group with an intramolecular electron-transfer process; on the other hand it, cannot be excluded that the electron stems from the water molecule, which decomposes into O-2, H+, and e(-) giving H-bp4pc entities.

• 出版日期2016-3