A neutral triple-helical trinuclear oxo-centered mixed-valent iron complex: Mössbauer spectroscopic and magnetic susceptibility studies

Abstract

The electronic properties of the trimeric oxo-centred [Fe3OL3] complex (L is a doubly negatively charged pentadentate ligand, providing five nitrogens for iron coordination) have been investigated by Mössbauer spectroscopy, magnetic susceptibility and a theoretical treatment including the phenomena of double-exchange interaction. Two quadrupole doublets I and II with the ratio ∼2:1 are observed at 4.2 K. By comparing the isomer shifts of these doublets (δI=0.52 mm s−1 and δII=0.99 mm s−1) with those of other mixed metal complexes with comparable iron–ligand coordination it is concluded in this study that [Fe3OL3] exhibits a partially delocalised ground state: Fe2.9+-Fe2.9+-Fe2.2+. At elevated temperatures the Mössbauer spectra of [Fe3OL3] exhibit two additional doublets III and IV (δIII=0.60 mm s−1 and δIV=0.80 mm s−1 at 20 K), which are ascribed to an additional partially delocalised state, i.e. Fe2.75+-Fe2.75+-Fe2.5+. By admitting the coexistence of two types of clusters with different delocalised valence states, it is possible to consistently fit the Mössbauer spectra over the whole temperature range (4.2–332 K) being investigated. Temperature-dependent magnetic susceptibility data of [Fe3OL3] from 2 to 295 K can be analysed by the Heisenberg–Dirac–Van Vleck model with only two exchange parameters J=−140 cm−1 and J1=−49 cm−1, describing the Fe(III)-Fe(III) and Fe(III)-Fe(II) exchange interactions and yielding a net antiferromagnetic coupling of the three paramagnetic sites with a spin-triplet ground state. In order to explain the seemingly contradictory situation that the Mössbauer spectra of [Fe3OL3] exhibit partial valence delocalisation while it is possible to analyse the magnetic data with valence-trapped ground state and exited states a theoretical treatment is presented, which includes antiferromagnetic exchange, double exchange and vibronic interactions.