STABLE POLYRADICALS WITH HIGH-SPIN GROUND-STATES .1. SYNTHESIS, SEPARATION, AND MAGNETIC CHARACTERIZATION OF THE STEREOISOMERS OF 2,4,5,6-TETRACHLORO-ALPHA,ALPHA,ALPHA',ALPHA'-TETRAKIS(PENTACHLOROPHENYL)-META-XYLYLENE BIRADICAL

Biradical 3 and its corresponding monoradical 6 have been synthesized and isolated as stable solids at ambient conditions. Biradical 3 exists in three stereoisomeric forms due to restricted correlated rotation of the aryl groups. It has been obtained as a 60:40 equilibrated mixture of the meso form (C(s) symmetry) and the enantiomeric dl pair (C2 symmetry) which have been unambiguously identified by ESR spectroscopy. Recrystallization of the diastereoisomeric mixture afforded the pure meso form. Both diastereoisomers have been separated by HPLC chromatography and their interconversion rates have been measured at different temperatures. The diastereoisomerization barrier [DELTA-G-double-ended-dagger-298K (meso --> d(or l)) = 98 kJ mol-1] is comparable to that of related monoradicals with similar steric hindrances. Intense triplet ESR signals were obtained in rigid media, being (D/hc) = 0.0152 and (E/hc) = 0.0051 cm-1 for the meso and (D/hc) = 0.0085 and (E/hc) less-than-or-equal-to 0.003 cm-1 for the dl isomer. The observed zero-field splitting parameters for both stereoisomers are consistent with their symmetries. Curie plot studies on ESR signals are in agreement with triplet ground states for both diastereoisomers irrespective of their symmetries. Magnetic susceptibility and magnetization measurements of the diastereoisomers in solid state show quasi-ideal S = 1 paramagnetic behavior down to 4.2 K. For one representative case, the results presented here validate the current theories for the prediction of ground-state multiplicities in nondisjoint AH, regardless of the lack of planarity and changes in the symmetries.