Photoinduced diradical formation and decay in uncomplexed and metal-bound benzotriazine systems: mechanistic implications to chemically and biologically relevant photochemistry

Abstract

The photochemical reactivities of 3-hydroxy-1,2,3-benzotriazine-4(3H)-one (1a) and tris[3-hydroxy-1,2,3-benzotriazine-4(3H)-one]iron(III) (1b) have been studied in solution and low temperature (5–77 K) glasses. Photoexcitation (λ345 nm) of 1a and 1b ultimately results in population of a ligand-centered excited state that releases N₂. Electron paramagnetic resonance reveals the presence of S  = 1 and S  = 0 diradical intermediates upon photolysis of 1a and 1b, respectively, at 4 K. These species convert to an S  = 1/2, nitrogen-centered monoradical species (a_N  = 23 G) upon warming to 77 K ia H-atom abstraction from the matrix. In solution, the first intermediates observed upon photolyses (λ  = 355 nm) of 1a and 1b are oxime ketenes (5: λ  = 385, 440 nm; 9: λ  = 390, 430, 740 nm) that are formed from collapse of the diradical to generate a 4-membered β-lactam ring. The decay kinetics for the oxime ketene 5 decay can be fitted to a biexponential expression representing a parallel reaction mechanism with an element of reversibility. Thus, the data proposes the existence of an equilibrium between the oxime ketene and the spectroscopically silent β-lactam intermediate, as well as a first or pseudo first-order solvent-dependent pathway for the oxime ketene. The kinetics for formation and decay of the ketene are strongly influenced by the presence of the Fe(III) center, which leads to an increase in the lifetime of the diradical in solution, and a retarded rate of formation for the oxime ketene 9. The solution lifetimes suggest that the diradical intermediates only persist sufficiently long to react with bound solution substrates, whereas the ketene intermediates have suitable kinetic viabilities to react bimolecularly in solution.