INTERMOLECULAR PI-PI INTERACTIONS (PIMERIZATION) OF TERT-BUTYL-SUBSTITUTED PHENAZIN-5(10H)-YL RADICALS IN THE SOLID-STATE - SYNTHESES, CRYSTAL-STRUCTURES AND MAGNETIC-SUSCEPTIBILITY MEASUREMENTS

Introduction of tert-butyl groups into the phenazine framework was acc omplished by treatment of 5-acetyl-5,10-dihydrophenazine (2a) with ter t-butyl chloride in the presence of AlCl3. Starting from the 2,8- or 3 ,7-di-tert-butyl-substituted derivatives 2c and 2b, a series of phenaz in-5(10H)-yl radicals (1c-i) was synthesized and characterized by ESR and ENDOR spectroscopy. With the exception of 1c, all phenazin-5(10H)- yls were obtained in crystalline form, and for 1d-f the long-wavelengt h absorption band at lambda approximate to 870 nm indicates intermolec ular pi-pi interactions in the solid state, For 1d, 1e and 1h the crys tal structure could be determined. The unit cell of Id consists of eig ht phenazin-5(10H)-yls. Surprisingly, four of them are arranged in rad ical pairs, whereas the other four lie independently in the lattice. I n agreement with this structure, the magnetic susceptibility results c orrespond to a content of 50% monoradical and an almost complete spin- pairing in the radical pairs up to T = 220 K. In 1e, the four phenazin -5(10H)-yls in the unit cell are arranged in two independent radical p airs, A and B, which are characterized by close interplanar distances and short intermolecular contacts between atoms with significant spin populations. Accordingly, the susceptibility data indicate strong spin -pairing at low temperature. Due to extensive steric shielding of the phenazin-5(10H)-yl framework, the crystal structure of 1h gives no evi dence of any pi-pi interactions between adjacent radicals. As expected , the magnetic susceptibility of 1h corresponds to that of an ordinary monoradical.