Molecular spin frustration in mixed-chelate Fe5 and Fe6 oxo clusters with high ground state spin values

Abstract

The synthesis, structures, and magnetic properties are reported of three new polynuclear FeIII complexes containing the anions of picolinic acid (picH) and triethanolamine (teaH3) as chelates. The complexes [Fe6O2(OH)2(O2CR)4(pic)4(teaH)2] (R = Me (1), Ph (2)) and [Fe5O2(O2CBut)4(pic)3(teaH)2] (3) were obtained from the reaction of [Fe3O(O2CR)6(H2O)3](NO3) (R = Me, Ph, But) with picH and teaH3 in a 1:2:1 ratio in MeCN. The core of 1 and 2 consists of an [Fe4(µ3-O)2]8+ ‘planar-butterfly’ unit to which is attached an Fe atom on either side by bridging O atoms. The core of 3 consists of an [Fe5(µ3-O)2]11+ unit comprising two near-perpendicular vertex-sharing [Fe3(µ3-O)]7+ triangular units. Variable-temperature (T) and -field (H) solid-state dc and ac magnetization (M) studies in the 5.0–300 K temperature range revealed that 1 and 2 have an S = 5 ground state spin whereas 3 has an S = 5/2 ground state. Jij exchange couplings were calculated by DFT and a magnetostructural correlation (MSC) for polynuclear FeIII/O complexes. This allowed rationalization of the observed ground states from the analysis of the spin frustration effects operative, and provided good input values for fits of the experimental χMT vs T data to obtain the Jij values.