Phenazin‐5(10 H )‐yls, 5 . – Intermolecular π‐π Interactions (Pimerization) of tert ‐Butyl‐Substituted Phenazin‐5(10 H )‐yl Radicals in The Solid State: Syntheses, Crystal Structures and Magnetic Susceptibility Measurements

Abstract Introduction of tert ‐butyl groups into the phenazine frame‐work was accomplished by treatment of 5‐acetyl‐5,10‐dihydrophenazine ( 2a ) with tert ‐butyl chloride in the presence of AlCl 3 . Starting from the 2,8‐ or 3,7‐di‐ tert ‐butyl‐substituted derivatives 2c and 2b , a series of phenazin‐5(10 H )‐yl radicals ( 1c–i ) was synthesized and characterized by ESR and EN‐DOR spectroscopy. With the exception of 1c , all phenazin‐5(10 H )‐yls were obtained in crystalline form, and for 1d–f the long‐wavelength absorption band at λ ≈ 870 nm indicates intermolecular π‐π interactions in the solid state. For 1d , 1e and 1h the crystal structure could be determined. The unit cell of 1d consists of eight phenazin‐5(10 H )‐yls. Surprisingly, four of them are arranged in radical pairs, whereas the other four lie independently in the lattice. In agreement with this structure, the magnetic susceptibility results correspond to a content of 50% monoradical and an almost complete spinpairing in the radical pairs up to T = 220 K. In 1e , the four phenazin‐5(10 H )‐yls in the unit cell are arranged in two independent radical pairs, 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(10 H )‐yl framework, the crystal structure of 1h gives no evidence of any π‐π interactions between adjacent radicals. As expected, the magnetic susceptibility of 1h corresponds to that of an ordinary monoradical.