Ligand Field Tuning of Photoreactivity: Contrasting Low-spin and High-spin Fe(III)-Azido Complexes

We systematically investigate the electronic structure factors underpinning the distinct photochemical behaviors in a series of structurally related Fe(III)–azido complexes [Fe(III)(cy)(N3 )2]+ (1), [Fe(III)(cy-ac)(N3 )]+ (2), and [Fe(III) (Me3cy-ac)(N3)]+ (3), using density functional theory (DFT), time-dependent DFT (TDDFT), and ab initio ligand field theory together with the angular overlap model. It had been shown in experimental studies that the ground spin state of these complexes influences the photochemical reaction pathways. Low-spin complexes 1 and 2 exhibit wavelength-dependent competition between photoreductive and redox-neutral ligand dissociation, and photooxidation to yield high-valent nitrenoid species. More specifically, excitation of 1 and 2 with visible light promotes photoreductive and redox-neutral ligand dissociation pathways, whereas excitation with UV light allows accessing the photooxidation channel via excited states that effectively funnel energy into azide-centered low-frequency vibrational modes. In contrast, the high-spin ground state of 3 with a significantly altered excited state manifold precludes photooxidation, instead favoring photoreduction and a unique acetato-ligand decarboxylation channel not observed in the low-spin analogs. In this first comparative in silico study of the three complexes, we show how the steric demands of the methylated ligand in 3 lead to a weakened equatorial ligand field resulting in a high spin ground state which in turn influences the excited state manifold. With a simple approach wherein we explore the evolution of the electronically excited states along specific vibrational modes, we find the onset of dissociative photooxidation paths in 1 and 2 which are absent in the high-spin species 3. Similarly, we can rationalise why photoinduced decarboxylation is only observed in 3.