Two new hexanuclear iron(iii) complexes with S = 5 ground states

Abstract

The synthesis and magnetic properties of two new hexanuclear iron complexes [Fe₆O₂(OH)₂(O₂CR)₁₀L₂] (R = Bu^t (3), Me (4); LH = 2-(2-hydroxyethyl)pyridine (hepH) (3), 6-methyl-2-(hydroxymethyl)pyridine (Me-hmpH) (4)) are reported. Both compounds are prepared by treatment of [Fe₃O(O₂CR)₆(H₂O)₃]⁺ with three equivalents of LH in MeCN. The X-ray crystal structure of 3·2CHCl₃·2H₂O is presented. It consists of a planar array of six Fe³⁺ ions comprising two [Fe₃(μ₃-O)] subunits that are related by an inversion centre and linked at two of their apices, each linkage consisting of one μ-hydroxo and two μ-carboxylato groups. DC magnetic susceptibility measurements at 1.0 and 0.10 Tesla in the 2.0–300 K range show an increase in the effective magnetic moment with decreasing temperature, corresponding to a high spin (S) ground state. The spin of the ground state was established by magnetization measurements in the 1.0–7.0 T field range and 1.7–4.0 K temperature range. Fitting of the reduced magnetization data by full matrix diagonalization, incorporating both axial and rhombic anisotropy, gave S = 5, g = 1.96, D = 0.46 cm⁻¹ and |E| = 0.046 cm⁻¹ for 3, and S = 5, g = 2.07, D = 0.27 cm⁻¹ and |E| = 0 cm⁻¹ for 4. Alternative fits with a negative ZFS were rejected based on their relative fitting error as well as on measurements of the magnetization relaxation behaviour of the complexes at very low temperature (≥0.04 K), where no hysteresis characteristic of a single-molecule magnet was observed.