Quintet and Septet State Systems Based on Pyridylnitrenes:  Effects of Substitution on Open-Shell High-Spin States

Cryogenic matrix photolysis of 3,5-dichloro-2,4,6-triazidopyridine, 3-chloro-5-cyano-2,4,6-triazidopyridine, 3,5-dicyano-2,4,6-triazidopyridine, and 3,5-difluoro-2,4,6-triazidopyridine (1−4) gives rise to ESR spectral peaks of various triplet mononitrene, quintet dinitrene, and septet trinitrene species. The quintet 2,4-dinitrenes derived from 1−2 and 4 have zero-field splittings (zfs) of |D/hc| ∼ 0.22−0.24 cm-1, similar to values observed elsewhere for m-phenylene dinitrenes. 3,5-Dicyano-2,4,6-triazidopyridine (3) photolysis gives only a weak ESR signal corresponding to a mononitrene with a zfs of |D/hc| ∼ 1.22 cm-1. The spectrum from 1 also shows a 2,6-dinitrene with |D/hc| = 0.283 cm-1, |E/hc| = 0.036 cm-1, which illustrates the effects of heteroatom perturbation upon the zfs of a geometrically rigid m-arylene dinitrene. Spectral peaks for unusual septet 2,4,6-trinitrenopyridines derived from complete deazetation of 1−2 were also identified by simulation of the septet spectra. The zfs parameters for th...