Nitronyl nitroxides and benzimidazole-3-oxide-1-oxyls as building blocks of organic magnetic materials
Abstract (Summary)We have synthesized several nitronyl nitroxides (NNs) to study their crystal packing and magnetic properties. These were characterized by ESR spectroscopy, and most by IR and mass spectrometry. Dominant magnetic interactions were studied by magnetic susceptibility studies performed on 2-(3,5-difluorophenyl)-NN and 2-[3,5-bis-(trifluoromethyl)-phenyl)-NN crystals. Structural information was used from X-ray studies to elucidate magnetic interactions present in the NN crystals. The synthesis of the precursors for nitronyl nitroxides was optimized in most of the cases by using triethylamine as the base during the condensation of aryl aldehydes with 2,3-dihydroxylamino-2,3-dimethylbutane (DHAB). 2-Aryl-substituted benzimidazole-3-oxide-1-oxyls (BNNs) were explored as possibly useful building blocks for organic magnetic materials. We analyzed spin delocalization in these systems by examining their ESR hyperfine coupling constants. These studies revealed substantial spin delocalization onto the benzenoid ring of the benzimidazole fragment, which may be responsible for their limited stability. B3LYP-6-31G* level density functional computations on 2-H-substituted BNN confirm that about 30% of spin is delocalized onto the benzenoid ring. The ESR studies also show that there is very little spin delocalization onto a phenyl ring attached at 2-position. Experimental hyperfine coupling constants are affected only by 2-4% with different substituents on the 2-phenyl ring nitrogen, further demonstrating the limited interaction between the 2-phenyl and the BNN unit. We synthesized new heterospin systems comprised of phenyl nitrene substituted NN or BNN molecules in order to investigate the effects of heteroatom substitution, connectivity and conformation. Low temperature ESR, UV-vis spectroscopic studies, and UB3LYP-6-31G* level computations were used to study 2-(4-nitrenophenyl)-NN, 2-(3-nitrenophenyl)-NN and 2-(4-nitrenophenyl)-BNN. The spectroscopic studies show the presence of quartet species resulted from arylnitrene-radical interaction. ESR zero-field-splittings reflected mostly the connectivity of the molecules, and varied only very little with the structure. The UV-vis studies supported the presence of some quinonoidal character in the para -linked nitreno-radicals. Computations show little difference in nitrene delocalization between the para - and meta -linked systems, but find a relatively higher stability of the high spin, planar structures over the low spin state in para-linked nitreno-radicals by comparison to meta -linked analogs.
School Location:USA - Massachusetts
Source Type:Master's Thesis
Date of Publication:01/01/2001