ANTIFERROMAGNETIC EXCHANGE INTERACTION AMONG THE 3 SPINS PLACED IN ANISOSCELES TRIANGULAR CONFIGURATION IN 2,4-DIMETHOXY-1,3,5-BENZENETRIYLTRIS(N-TERT-BUTYL NITROXIDE)

2,4-Dimethoxy-1,3,5-benzenetriyltris(N-tert-butyl nitroxide) (3) was p repared by lithiation of 2,4-dimethoxy-1,3,5-tribromobenzene followed by the reaction with 2-methyl-2-nitrosopropane and subsequent oxidatio n with Ag2O. The triradical crystallizes in the orthorhombic space gro up Pnma (no. 62) with four molecules in the unit cell of dimensions a = 18.200(3) Angstrom, b = 19.018(3) Angstrom, and c = 6.659(5) Angstro m. X-ray analysis for an orange plate single crystal of 3 revealed tha t the three nitroxide radicals in a molecule have large dihedral angle s (90 degrees and 79 degrees) between N-O bonds and the benzene ring p lane in the all-syn conformation and the distances between them are 5. 44 and 5.28 Angstrom. An EPR spectrum of triradical 3 in frozen toluen e solution showed quartet signals (g = 2.0062, \D/hc\ = 0.010, and \E/ hc\ = 0.0004 cm(-1)) in addition to the signals at g = 4.019 and 6.035 due to Delta m(s) = 2 and 3 transitions, respectively. Temperature de pendence of molar susceptibility for a microcrystalline sample obtaine d by SQUID measurements suggested that the antiferromagnetic interacti on produces a doublet ground state in triradical 3. Experimental data were fitted in terms of the equation derived from an isosceles triangu lar three spin system (H = -2J(SA1SB + SBSA2 + alpha SA1SA2) with nega tive J's to afford two sets of the best fitting parameters: J/k(B) = - 41.5 K, alpha = 0.31 and J/k(B) = -22.3 K, alpha = 2.29. The former J/ k(B) value is more consistent with J/k(B) = -39.7 K for the reference biradical, romo-2,4-dimethoxy-3,5-benzenediylbis(N-tert-butyl nitroxid e) (4). In triradical 3, therefore, the ground spin state would be a d oublet with the stronger antiferromagnetic interaction between the spi ns at 1 and 3 positions and 3 and 5 positions polarizing the ferromagn etic alignment of the 1 and 5 spins, providing the first demonstration of an organic triradical showing competing interactions.