Pyridyl-Substituted Thioaminyl Stable Free Radicals: Isolation, ESR Spectra, and Magnetic Characterization1

N -[(4-Nitrophenyl)thio]- ( 1a ) and N -[(2,4-dichlorophenyl)thio)]-2,6-diphenyl-4-(3-pyridyl)phenylaminyl ( 1b ), N -[(4-nitrophenyl)thio]- ( 2a ) and N -[(2,4-dichlorophenyl)thio]-4-phenyl-2,6-di(3-pyridyl)phenylaminyl ( 2b ), and N -[(2,4-dichlorophenyl)thio]-2,6-diphenyl(4-pyridyl)aminyl ( 3 ) were generated by PbO 2 oxidation of the corresponding precursors. Although 3 was not sufficiently stable to be isolated, both 1 and 2 persisted in solution without decomposition and could be isolated as radical crystals. X-ray crystallographic analysis of 1b revealed that the Ar−N−S−Ar‘ π-framework is approximately planar and the 2- and 6-phenyl groups are significantly twisted from this plane. The magnetic susceptibility measurements for the isolated radical crystals were carried out using a SQUID magnetometer in the temperature range 1.8−300 K. The susceptibilities of 1a and 2a were explained in terms of a one-dimensional (1D) antiferromagnetic (AFM) regular Heisenberg model with an exchange interaction of 2 J / k B = −63.4 and −17.8 K, respectively, and that of 1b was interpreted in terms of a 1D AFM alternating Heisenberg model with 2 J / k B = −12.8 K and α (alternation parameter) = 0.91. On the other hand, that of 2b was explained in terms of a 1D ferromagnetic regular Heisenberg model with 2 J / k B = 22.4 K.