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

The synthesis and magnetic properties of two new hexanuclear iron complexes [Fe6O2 (OH)(2) (O2CR)(10)L-2] (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 3.2CHCl(3).2H(2)O is presented. It consists of a planar array of six Fe3+ ions comprising two [Fe-3 (mu(3)-O)] subunits that are related by an inversion centre and linked at two of their apices, each linkage consisting of one mu-hydroxo and two mu-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 (greater than or equal to0.04 K), where no hysteresis characteristic of a single-molecule magnet was observed.