Molecular spin frustration in mixed-chelate Fes5 and Fe6 oxo clusters with high ground state spin values

The synthesis, structures, and magnetic properties are reported of three new polynuclear Fe-III complexes containing the anions of picolinic acid (picH) and triethanolamine (teaH(3)) as chelates. The complexes [Fe6O2(OH)(2)(O2CR)(4)(pic)(4)(teaH)(2)] (R = Me (1), Ph (2)) and [FesO(2)(O2CBut)(4)(Pic)(3)(teaH)(2)] (3) were obtained from the reaction of [Fe3O(O2CR)(6)(H2O)(3)](NO3) (R= Me, Ph, Bu-t) with picH and teaH3 in a 1:2:1 ratio in MeCN. The core of 1 and 2 consists of an [Fe-4(mu(3)-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 [Fe-5(mu(3)-O)(2)](11+) unit comprising two near-perpendicular vertex-sharing [Fe-3(mu(3)-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. J(ij) exchange couplings were calculated by DFT and a magnetostructural correlation (MSC) for polynuclear Fe-III/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 chi T-M vs T data to obtain the J(ij) values. (C) 2019 Published by Elsevier Ltd.