A new tetranuclear iron complex with a [Fe4O6]6+ core: synthesis, structure, spectroscopic and magnetic properties

The reaction of 2,6-bis(hydroxymethyl)-4-tert-butylphenol (H3L) with [Fe(III)(acac)3] in acetonitrile in the presence of NEt3 yields [Fe(III)4(HL)6(acac)2](HNEt3)2·3CH3CN (1) or after subsequent addition of [NEt4]Cl [Fe(III)4(HL)6(acac)2](NEt4)2 (2) as reddish-brown crystalline solids. The molecular structure of 1 was established by single crystal X-ray diffraction. The dianion of complex 1 contains an [Fe4O6]6+ core with two μ3-bridging benzylalcoholato ligands and four μ2-bridging benzylalcoholato ligands. This is the first structurally characterized example for a benzylalcoholato ligand bound to an octahedrally coordinated iron ion. The core of 1 can be considered as two face-sharing cuboidal [Fe3O4]5+ units. Compound 2 has been examined by FTIR, UV–Vis–NIR-absorption, 57Fe Mössbauer, and magnetochemical measurements. Compound 2 has an St=0 ground state indicative of antiferromagnetic interactions between the high-spin Fe(III) metal ions. The spin topology resembles that of the known [Fe4O2]8+ core with a ‘butterfly’ disposition of the four iron ions. Fitting of the data to the appropriate spin-Hamiltonian yields exchange-coupling constants between the ‘inner’ (‘body’) iron ions of Jbb=−10.0±0.1 cm−1 and between the ‘inner’ iron ions and ‘outer’ (‘wing-tip’) iron ions of Jwb=−6.2±1.0 cm−1. The differences of the spectroscopic and magnetochemical properties between the [Fe4O6]6+ core and the [Fe4O2]8+ core with an inversion of the superexchange pathways are discussed.