Analysis of spin frustration in an FeIII7 cluster using a combination of computational, experimental, and magnetostructural correlation methods

Abstract

The synthesis, structure, and magnetic properties are reported for [Fe7O3(O2CtBu)9(mda)3(H2O)3] (1), where mdaH2 is N-methyldiethanolamine. 1 was prepared from the reaction of [Fe3O(O2CtBu)6(H2O)3](NO3) with mdaH2 in a 1:∼3 ratio in MeCN. The core of 1 consists of a central octahedral FeIII ion held within a non-planar Fe6 loop by three μ3-O2- and three μ2-RO- arms from the three mda2- chelates. Variable-temperature dc and ac magnetic susceptibility studies revealed dominant antiferromagnetic coupling, leading to a ground state spin of S = 5/2. The ground state was confirmed by a fit of magnetization data collected in the 0.1–7.0 T and 1.8–10.0 K ranges. The four Fe2 pairwise exchange parameters (J1-J4) were estimated by independent methods: theoretical calculations using either broken symmetry energy differences (−46.3, −16.2, −3.9, and − 28.1 cm−1, respectively) or Green’s function approximation methods (−41.4, −14.8, −13.2, and − 24.7 cm−1), and a magnetostructural correlation (MSC) previously developed for high nuclearity FeIII/O complexes (−39.5, −13.8, −6.7, and − 23.5 cm−1). Additionally, the J1-J4 obtained from the MSC and theoretical methods were used with the program PHI to both simulate χMT vs T as well as to serve as reasonable input values to fit the experimental data (−41.0, −11.4, −5.0, and − 27.3 cm−1). Analysis of the Jij led to identification of the spin frustration effects operative and the resultant spin vector alignments at each FeIII ion, thus allowing for the rationalization of the experimental ground state.