No abstract

The generation of the 1,2,4-triazolyl cation (1) has been attempted by the thermolysis and photolysis of 1-(1,2,4-triazol-4-yl)-2,4,6-trimethylpyridinium tetrafluoroborate (2) and the thermolysis of 1- and 4-diazonium-1,2,4-triazoles, using mainly mesitylene as the trapping agent. Thermolysis of 2 gave mostly 1,2,4-triazole, together with 3-(1,2,4-triazol-4-yl)-2,4,6-trimethylpyridine, 4-(1,2,4-triazol-4-ylmethyl)-2,6-dimethylpyridine, and 4-(2,4,6-trimethylbenzyl)-2,6-dimethylpyridine. Thermolysis of each of the diazonium salts in the presence of mesitylene again gave mainly triazole together with very low yields of 1-(1,2,4-triazol-1-yl)-2,4,6-trimethylbenzene and the corresponding -4-yl isomer in about the same ratio. On the other hand, photolysis of 2 in mesitylene gave mainly 1-(1,2,4-triazol-1-yl)-2,4,6-trimethylbenzene. A photoinduced electron transfer from mesitylene to 2 has been observed and preliminary laser flash photolyses of 2 and the corresponding 2,4,6-triphenylpyridinium salt have been carried out. The observed transients are explained as arising from the first excited states of the pyridinium salts rather than from 1. Ab initio MO calculations are reported and indicate that the predicted electronic ground-state of the triazolyl cation is a triplet state of B1 symmetry with five pi electrons, which corresponds to a diradical cation (1c). Possible mechanisms for the formation of the various products are proposed.

Two series of isomeric, redox-responsive azacrown ethers based on ortho- and para-phenylenediamine (Wurster's crowns) have been synthesized and their properties explored through 13C NMR spectroscopy, electrospray ionization mass spectrometry, cyclic voltammetry, and X-ray crystallography. These crowns display strong affinity for alkali metal cations while maintaining comparable selectivity profiles to the parent crown ethers from which they are derived. Like Wurster's reagent (N,N,N',N'-tetramethyl-p-phenylenediamine or para-TMPD), the para-Wurster's crowns undergo two reversible one-electron oxidations. The integrity of the alkali metal ion complexes is maintained in the neutral and singly oxidized ligand states but not after removal of two electrons. In contrast, the oxidation of ortho-Wurster's crowns is scan rate dependent, occurring at potentials substantially higher than their para counterparts, with their complexes oxidizing irreversibly. X-ray crystal structures of representative complexes show, in all cases, participation of the redox-active phenylenediamine subunits in complex formation via direct bonding to the guest cation.