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

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 Mossbauer 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 (δ(I) = 0.52 mm s^-1 and δ(II) = 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: Fe^2.9+-Fe^2.9+-Fe^2.2+. At elevated temperatures the Mossbauer spectra of [Fe₃OL₃] exhibit two additional doublets III and IV (δ(III) = 0.60 mm s^-1 and δ(IV) = 0.80 mm s^-1 at 20 K), which are ascribed to an additional partially delocalised state, i.e. Fe^2.75+-Fe^2.75+-Fe^2.5+. By admitting the coexistence of two types of clusters with different delocalised valence states, it is possible to consistently fit the Mossbauer 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 Mossbauer 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. (C) 2000 Elsevier Science S.A.