A supramolecular polymer of nitroxide radicals via hydrogen bonding

2,2,6,6-Tetramethylpiperidinyl-N-oxy (TEMPO) is a robust nitroxide radical molecule under ambient conditions. We found that the TEMPO derivatives act as a proton acceptor to form an intermolecular hydrogen-bonding complex with many kinds of phenol or urea derivatives. ORTEP analysis of the crystals of TEMPO with the phenol derivatives indicated that hydrogen bonding could be formed between the oxygen of the nitroxide and the phenolic proton and the N-O bond of the hydrogen-bonded TEMPO was lengthened in comparison to that of the free N-O bond. The formation constant of the hydrogen-bonding complex of TEMPO with the phenol or urea derivatives in a chloroform solution was spectroscopically determined by IR to be 10-100 M^-1. Hydrogen bonding of the thelechelic bis-TEMPO derivatives with thelechelic bis-phenol or bis-urea derivatives provided a supramolecular structure. The estimated molecular weights of the supramolecules in the chloroform solution, based on DOSY-NMR spectroscopy, were 3000-4000. The potential of the nitroxide radical's supramolecule as a new functional material is also described.