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-chlor o-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 o f various triplet mononitrene, quintet dinitrene, and septet trinitrene spe cies. The quintet 2,4-dinitrenes derived from 1-2 and 3 have zero-field spl ittings (zfs) of \D/hc\ similar to 0.22-0.24 cm(-1), similar to values obse rved elsewhere for m-phenylene dinitrenes. 3,5-Dicyano-2,4,6-triazidopyridi ne (3) photolysis gives only a weak ESR signal corresponding to a mononitre ne with a zfs of \D/hc\ similar to 1.22 cm(-1). The spectrum from 1 also sh ows a 2,6-dinitrene with \D/hc\ = 0.283 cm(-1), \E/hc\ = 0.036 cm(-1), whic h illustrates the effects of heteroatom perturbation upon the zfs of a geom etrically rigid m-arylene dinitrene. Spectral peaks for unusual septet 2,4, 6-trinitrenopyridines derived from complete deazetation of 1-2 were also id entified by simulation of the septet spectra. The zfs parameters for the tr initrenes were estimated to be \D/hc\ similar to 0.1 cm(-1), with small E-v alues in both cases. Experimental and computational results support high sp in ground states for the dinitrene and trinitrene species.