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 the trinitrenes were estimated to be | D / hc | ∼ 0.1 cm - 1, with small E -values in both cases. Experimental and computational results support high spin ground states for the dinitrene and trinitrene species.