Photogenerated Nitrenium Ions: A Search for Triplet-State Reactivity in the Chemistry of the Diphenylnitrenium Ion

Abstract

The N,N-diphenylnitrenium ion (Ph2N+) was generated from the photolysis of a stable precursor, the 1-(N,N-diphenylamino)-2,4,6-trimethylpyridinium ion. The behavior of this nitrenium ion was studied by laser flash photolysis (LFP) and the analysis of its stable products. In accord with an earlier communication, Ph2N+ is found to have absorption maxima at 425 and 660 nm, to live for ca. 1.5 μs in CH3CN, and to react with Cl- at or near the diffusion limit. However, no evidence for concurrent formation of the diphenylamine cation radical (Ph2NH+•) is found. Its formation in the earlier experiments is attributed to small amounts of accumulated photoproducts. The LFP experiments also show that Ph2N+ is quenched by various H atom donors, including 1,4-cyclohexadiene and tri(n-butyl)tin hydride. However, the radical cation that would be expected from a homolytic H atom transfer, Ph2NH+•, is not detected in these experiments. Moreover, the rate constants for trapping do not vary in the expected way with the bond dissociation energies of the traps. The rate constants do, however, show a reasonable correlation with the hydride affinities of the traps. It is argued that the quenching mechanism is a hydride transfer to the singlet state of Ph2N+ and that the triplet state of this nitrenium ion is not involved in these reactions.