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

Abstract

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)](+) (2red) 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 2red to form [Fe(II)N(3)(cyclam-ac)] (1red) as a secondary product of the photochemical decomposition of 1. Molecular oxygen was further used to quench 1red and 2red to form what seems to be the elusive complex [Fe(O(2))(cyclam-ac)](+) (6).

The Photochemical Route to Octahedral Iron(V). Primary Processes and Quantum Yields from Ultrafast Mid-Infrared Spectroscopy

“…26 Thus, we are led to conclude further that upon absorption of a 450 nm photon, the axial azide ligand is dissociated from the metal center, and it is detached as an azide anion. The nascent N 3 − anion is initially formed in a vibrationally excited fashion.…”

“…The highly oxidized product was found to be formed with a relative quantum yield as high as 80%, to live for a few hundred microseconds only and to react with several substrates at nearly diffusion-limited rates. 26,27 did not provide any evidence so far for photooxidation and formation of the high-valent product [L 5 Fe…”

Observing the Formation and the Reactivity of an Octahedral Iron(V) Nitrido Complex in Real Time

“…As we have discussed in previous publications, [15] the mechanism of such a photoreaction may either be direct or it may involve a primary photo-reduction of iron(III) to iron(II) with formation of azide radicals followed by an electron transfer from Fe II to N 3 · with formation of 2 and N 3 À . As we have discussed in previous publications, [15] the mechanism of such a photoreaction may either be direct or it may involve a primary photo-reduction of iron(III) to iron(II) with formation of azide radicals followed by an electron transfer from Fe II to N 3 · with formation of 2 and N 3 À .…”

Photolysis of a High‐Spin Azidoiron(III) Complex Studied by Time‐Resolved Fourier‐Transform Infrared Spectroscopy

“…[4h, 6] According to Ref. [7] The electronic UV/Vis-absorption spectrumo f[ 1]PF 6 (Supporting Information) at room temperature in acetonitrile solution is showni nF igure 1a.I ti sc omposed of two very broad and structureless bands peaking at 443 and 247 nm. Such an electronic structure is in stark contrast to that of its non-methylatedd erivative, which adopts ad oublet ground state.…”