Synthesis of phenols with potential biological activities and crystallography and magnetism of nitroxides
A series of 2,6-di-tert-butylphenols with para aza-heterocyclic rings and fluorinated was synthesized. These phenols were oxidized to the corresponding persistent phenoxyl radicals and their EPR spectra (spin density distribution) and kinetic decay rates were studied.
The varying electron spin delocalizations in the phenoxyl radicals were studied using EPR spectroscopy and UB3LYP/6-31G* spin density computations. Kinetic studies showed that the attachment of different heterocyclic rings results in varying degrees of the persistence of the 2,6-di-tert-butylphenoxyl radicals. The phenolic compounds themselves were tested for antioxidant activity, and were sent to the National Cancer Institute for in-vitro cell culture assays of their anticancer effect. The crystal structures and IR spectra of the phenols revealed phenolic OH---N hydrogen-bonding pattern despite the steric hindrance caused by tert-butyl groups.
Conjugated nitroxide radicals have been widely used for molecular magnetism. The delocalization of the spin density over the entire molecule makes a larger variety of magnetic interactions possible. The way the spins interact with each other can lead to paramagnetic, antiferromagnetic or ferromagnetic interactions. By examining the crystal structures of a number of nitroxides and nitronyl nitroxides bearing heteroatom substituents the relationship can be studied between close crystallographic contact spin sites and the resulting magnetic behavior is investigated. 2-(4,5,6,7-Tetrafluorobenzimidazol-2-yl)-4,4,5,5-tetramethyl-4,5,dihydro- 1H-imidazole-3-oxide-1-oxyl (F4BImNN) was synthesized and shown to be the first ferromagnetically coupled purely organic radical synthesized in the Lahti group. 1-(4-(N-[para-Methoxyphenyl]- N-hydroxylaminoxyl) phenyl)pyrrole (MNPP) was made and found to be a very rare example of a stable diarylnitroxide. It shows two-dimensional square planar antiferromagnetic exchange behavior, which is very unusual in purely organic electronic materials. 1-(4-[N-tert-Butylaminoxyl]-2,3,5,6-tetrafluorophenyl)pyrrole (BNPPF4) was made as the first fluorinated nitroxide subjected to magnetostructural analysis to our knowledge. Its crystal structure suggests possible one-dimensional antiferromagnetic interactions. However it interestingly shows magnetic behavior that is more consistent with a spin-pairing model.