57Fe nuclear resonance vibrational spectroscopic studies of tetranuclear iron clusters bearing terminal iron(iii)–oxido/hydroxido moieties

Abstract

⁵⁷Fe nuclear resonance vibrational spectroscopy (NRVS) has been applied to study a series of tetranuclear iron ([Fe₄]) clusters based on a multidentate ligand platform (L³⁻) anchored by a 1,3,5-triarylbenzene linker and pyrazolate or (tertbutylamino)pyrazolate ligand (PzNH^tBu⁻). These clusters bear a terminal Fe(III)–O/OH moiety at the apical position and three additional iron centers forming the basal positions. The three basal irons are connected with the apical iron center via a μ₄-oxido ligand. Detailed vibrational analysis via density functional theory calculations revealed that strong NRVS spectral features below 400 cm⁻¹ can be used as an oxidation state marker for the overall [Fe₄] cluster core. The terminal Fe(III)–O/OH stretching frequencies, which were observed in the range of 500–700 cm⁻¹, can be strongly modulated (energy shifts of 20–40 cm⁻¹ were observed) upon redox events at the three remote basal iron centers of the [Fe₄] cluster without the change of the terminal Fe(III) oxidation state and its coordination environment. Therefore, the current study provides a quantitative vibrational analysis of how the remote iron centers within the same iron cluster exert exquisite control of the chemical reactivities and thermodynamic properties of the specific iron site that is responsible for small molecule activation.