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Free keywords:
kinetics; laser chemistry; photolysis; reaction mechanisms; time-resolved spectroscopy
Abstract:
The photochemistry of iron azido complexes is quite challenging and poorly understood. For example, the photochemical decomposition of [FeIIIN3(cyclam‐ac)]PF6 ([1]PF6), where cyclam‐ac represents the 1,4,8,11‐tetraazacyclotetradecane‐1‐acetate ligand, has been shown to be wavelength‐dependent, leading either to the rare high‐valent iron(V) nitrido complex [FeVN(cyclam‐ac)]PF6 ([3]PF6) after cleavage of the azide Nα-Nβ bond, or to a photoreduced FeII species after Fe-Nazide bond homolysis. The mechanistic details of this intriguing reactivity have never been studied in detail. Here, the photochemistry of 1 in acetonitrile solution at room temperature has been investigated using step‐scan and rapid‐scan time‐resolved Fourier transform infrared (FTIR) spectroscopy following a 266 nm, 10 ns pulsed laser excitation. Using carbon monoxide as a quencher for the primary iron‐containing photochemical product, it is shown that 266 nm excitation of 1 results exclusively in the cleavage of the Fe-Nazide bond, as was suspected from earlier steady‐state irradiation studies. In argon‐purged solutions of [1]PF6, the solvent‐stabilized complex cation [FeII(CH3CN)(cyclam‐ac)]+ (2 red) together with the azide radical (N3.) is formed with a relative yield of 80 %, as evidenced by the appearance of their characteristic vibrational resonances. Strikingly, step‐scan experiments with a higher time resolution reveal the formation of azide anions (N3−) during the first 500 ns after photolysis, with a yield of 20 %. These azide ions can subsequently react thermally with 2 red to form [FeIIN3(cyclam‐ac)] (1 red) as a secondary product of the photochemical decomposition of 1. Molecular oxygen was further used to quench 1 red and 2 red to form what seems to be the elusive complex [Fe(O2)(cyclam‐ac)]+ (6).
