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 i s a doubly negatively charged pentadentate ligand, providing five nitrogens for iron coordination) have been investigated by Mossbauer spectroscopy, m agnetic susceptibility and a theoretical treatment including the phenomena of double-exchange interaction. Two quadrupole doublets I and II with the r atio similar to 2:1 are observed at 4.2 K. By comparing the isomer shifts o f these doublets (delta(I) = 0.52 mm s (- 1) and delta(II) = 0.99 mm s (- 1 )) with those of other mixed metal complexes with comparable iron-ligand co ordination it is concluded in this study that [Fe3OL3] exhibits a partially delocalised ground state: Fe2.9 + -Fe2.9 + -Fe2.2 +. At elevated temperatu res the Mossbauer spectre of [Fe3OL3] exhibit two additional doublets III a nd IV (delta(III) = 0.60 mm s (- 1) and delta(IV) = 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 clust ers with different delocalised valence states, it is possible to consistent ly fit the Miissbauer 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 m odel with only two exchange parameters J = - 140 cm (- 1) and J(1) = - 49 c m (- 1), describing the Fe(III)-Fe(III) and Fe(III)-Fe(II) exchange interac tions and yielding a net antiferromagnetic coupling of the three paramagnet ic sites with a spin-triplet ground state. In order to explain the seemingl y contradictory situation that the Mossbauer spectra of [Fe3OL3] exhibit pa rtial valence delocalisation while it is possible to analyse the magnetic d ata with valence-trapped ground state and exited states a theoretical treat ment is presented, which includes antiferromagnetic exchange, double exchan ge and vibronic interactions. (C) 2000 Elsevier Science S.A. All rights res erved.