The Photochemistry of [Fe^IIIN₃(cyclam‐ac)]PF₆ at 266 nm

The photochemistry of iron azido complexes is quite challenging and poorly understood. For example, the photochemical decomposition of [Fe III N 3 (cyclam‐ac)]PF 6 ([ 1 ]PF 6 ), 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 [Fe V N(cyclam‐ac)]PF 6 ([ 3 ]PF 6 ) after cleavage of the azide N α N β bond, or to a photoreduced Fe II species after FeN azide 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N azide bond, as was suspected from earlier steady‐state irradiation studies. In argon‐purged solutions of [ 1 ]PF 6 , the solvent‐stabilized complex cation [Fe II (CH 3 CN)(cyclam‐ac)] + ( 2 red ) together with the azide radical (N 3 . ) 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 (N 3 − ) during the first 500 ns after photolysis, with a yield of 20 %. These azide ions can subsequently react thermally with 2 red to form [Fe II N 3 (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(O 2 )(cyclam‐ac)] + ( 6 ).