Antiferromagnetic exchange interaction among the three spins placed in an isosceles 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 prepared by lithiation of 2,4-dimethoxy-1,3,5-tribromobenzene followed by the reaction with 2-methyl-2-nitrosopropane and subsequent oxidation with Ag2O. The triradical crystallizes in the orthorhombic space group 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) Angstrom. X-ray analysis for an orange plate single crystal of 3 revealed that the three nitroxide radicals in a molecule have large dihedral angles (90 degrees and 79 degrees) between N-O bonds and the benzene ring plane 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 toluene 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 dependence of molar susceptibility for a microcrystalline sample obtained by SQUID measurements suggested that the antiferromagnetic interaction produces a doublet ground state in triradical 3. Experimental data were fitted in terms of the equation derived from an isosceles triangular three spin system (H = -2J(SA1SB + SBSA2 + alpha SA1SA2) with negative 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, 1-bromo-2,4-dimethoxy-3,5-benzenediylbis(N-tert-butyl nitroxide) (4). In triradical 3, therefore, the ground spin state would be a doublet with the stronger antiferromagnetic interaction between the spins at 1 and 3 positions and 3 and 5 positions polarizing the ferromagnetic alignment of the 1 and 5 spins, providing the first demonstration of an organic triradical showing competing interactions.