TRICHLORO-2,6-PYRIDYLENE, A GOOD FERROMAGNETIC COUPLING UNIT BETWEEN 2 PERSISTENT CARBON RADICAL CENTERS

The synthesis, characterization, and physical properties of 2,6-bis(di phenylmethyl)pyridine-alpha,alpha'-ylene biradical (1) and alpha sachl oro-2,6-bis(diphenylmethyl)pyridine-alpha'-yl radical (2) which presen t exceptionally high stabilities are reported. The synthesis of 1 and 2 have been accomplished using different strategies in order to get th em separately, although 1 always contains 2 as an impurity. 1 is a mol ecule constituted by two trivalent carbon atoms linked by a trichloro- m-pyridylene spacer which has proved to be a good ferromagnetic coupli ng unit. Magnetic susceptibility measurements from 4.2 to 300 K show t hat the ground state of 1 is the triplet being 2J = 358 +/- 14 cm(-1) (1.03 kcal/mol), the triplet-singlet energy gap. At low temperatures, an antiferromagnetic interaction between intermolecular spines was obs erved (Curie temperature, theta(1) = -2.02 +/- 0.10 K). The EPR spectr um of 1 in frozen 2-methyltetrahydrofuran (133 K) in the Delta m(s) = +/-1 region affords three symmetric pairs of lines corresponding to a triplet species without axial symmetry with zfs parameters \D/hc\ = 0. 0081 cm(-1) and \E/hc\ = 0.00075 cm(-1). The Delta m(s) = +/-2 transit ion follows the Curie law over the range 10.0-62.0 K, supporting the h ighest multiplicity of the molecule in the ground state. The cyclic vo ltammogram of 1 exhibits a redox couple (E degrees = 0.18 V vs SCE) wh ich is attributed to the reduction of this biradical to the stable rad ical anion 12 (1(-)), and a second irreversible peak (E(p)(c) = -0.25 V vs SCE) which is ascribed to the formation of the unstable dianion 1 3(1(2-)). The electronic spectrum of 1 is presented and discussed.