4,6-DIMETHOXY-1,3-PHENYLENEBIS(N-TERT-BUTYL NITROXIDE) WITH A SINGLET GROUND-STATE - FORMAL VIOLATION OF A RULE THAT M-PHENYLENE SERVES AS A ROBUST FERROMAGNETIC COUPLING UNIT

4,6-Dimethoxy-1,3-phenylenebis(N-tert-butyl nitroxide) (1) was prepared by lithiation of the corresponding dibromo compound followed by reaction with 2-methyl-2-nitrosopropane and subsequent treatment with Ag2O. In a frozen toluene matrix, 1 showed EPR fine structures characteristic of triplet dinitroxides: g = 2.0068, D/hc = 0.0179 cm-1, and E/hc = 0.0008 cm-1. The variation in signal intensity vs reciprocal temperature deviated downward from a Curie relationship at temperatures lower than 45 K, suggesting a singlet ground state for 1. The effective magnetic moment obtained on a Faraday balance and a SQUID susceptometer showed a limiting value of 2.45 mu(B) at higher temperatures, a value consistent with degenerate singlet and triplet states. The temperature dependence for a sample of 1 in PVC film revealed a weak antiferromagnetic coupling, 2J/k(B) = -7.0 K, whereas the coupling was strongly antiferromagnetic for crystalline samples: 2J/k(B) = -73.8 K. An X-ray structure analysis of an orange-colored monoclinic single crystal of 1, P2(1), (No. 4), a = 9.8914 (8) angstrom, b = 7.904 (3) angstrom, c = 11.4547 (6) angstrom, beta = 106.258 (5)degrees, V = 859.8 (3) angstrom3, Z = 2, revealed the molecular structure 1b with two nitroxide groups out of the plane of the phenylene ring by 65.1-degrees and 75.3-degrees in a syn conformation. There is no interradical distance closer than 7.0 angstrom between neighboring molecules. Lower spin polarization on the m-phenylene ring and a possible antiferromagnetic through-space interaction may be responsible for the nonferromagnetic coupling in 1.