B3O3Ph3(7-azaindole): Structure, luminescence, and fluxionality

Article abstract of DOI:10.1021/om990053t

A 7-azaindole adduct of boroxine, B₃O₃Ph₃-(7-azaindole), 1, was obtained from the reaction of PhB-(OH)₂ with 7-azaindole. The crystal structure of 1 shows that the 7-azaindole ligand is bonded to the boroxine molecule through a B-N bond and an H?O hydrogen bond. Compound 1 is fluorescent in solution and the solid state. There is, however, a dramatic difference in the emission maximum of the solution (λ_max = 368 nm) and solid (λ_max = 400 nm) spectra. The solution behavior of 1 was examined by ¹H NMR spectroscopic methods, which established that compound 1 is highly fluxional in solution, attributable to an intermodular 7-azaindole ligand dissociation/association process with an activation energy of 34 ± 1.5 kJ mol^-1. To determine the role of the hydrogen bond in the dynamic process of 1, the structure and solution behavior of B₃O₃Ph₃(Py), 2, was also examined. In solution compound 2 undergoes an intermodular exchange process similar to that of compound 1, with an activation energy of 39 ± 1.5 kJ mol^-1. The relatively small activation energy in 1 could be attributed to the hydrogen bond and the reduced base strength of 7-azaindole, relative to that of pyridine.