B₃O₃Ph₃(7-azaindole):  Structure, Luminescence, and Fluxionality

Abstract

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 (λmax = 368 nm) and solid (λmax = 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 intermolecular 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 intermolecular 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.