Structure-property relationships in trimethylenemethane-type biradicals. 2. Synthesis and EPR spectral characterization of dinitroxide biradicals

Several trimethylenemethane-type (TMM-type) dinitroxide biradicals have been prepared that differ in tert-butylaminoxylphenyl ring torsion angles by virtue of different steric demands of their 'spin-protecting groups'. In addition, the synthesis of a TMM-type dinitroxide having a planar π-system is reported. The synthetic route employed for the planar TMM-type biradical is general and should therefore expand the utility of TMM-type biradicals that are susceptible to bond torsions that attenuate exchange coupling. The compounds presented are designed to be effective to map out J- coupling/conformation space in TMM-type biradicals. EPR spectroscopic parameters (hfcc and D-values) were compared to determine if their values reflected differences in conformation in the series of biradicals. Interestingly, neither N- nor H-hfcc varied within our series of biradicals/monoradicals in a regular fashion, indicating that there is no apparent relationship between N- or H-hfcc and conformation (as judged by molecular mechanics calculations). However, D-values, estimated from relative intensities of Δm(s) = 1 and Δm(s) = 2 transitions, are consistent with a varied degree of delocalization in the dinitroxides: smaller D-values for biradicals having smaller aryl torsions (and greater delocalization), and larger D-values for dinitroxides having greater aryl torsions (and less delocalization). All the biradicals studied, except 5··, exhibited linear Curie plots. The linear Curie plots are consistent with both triplet ground- states and singlet-triplet degeneracies. Interestingly, dinitroxide 6·· exhibited a linear Curie plot, despite the lack of a π-coupling fragment. Biradical 5··, however, is a ground-state singlet species with the triplet lying about 140 cal/mol above the singlet.