Magnetostructural Correlation for High-Nuclearity Iron(III)/Oxo Complexes and Application to Fe5, Fe6, and Fe8 Clusters.

Abstract

The synthesis and characterization are reported of two new polynuclear Fe(III) complexes containing the anion of 8-hydroxyquinoline (hqnH), an N,O-chelating ligand. The complexes are [Fe8O4(O2CPh)10(hqn)4(OMe)2] (1) and [Fe6O2(OH)2(O2CPh)10(hqn)2] (2) and were obtained from reactions in MeOH (1) or H2O (2) using either low-nuclearity preformed clusters or simple metal salts as starting materials. Variable-temperature, solid-state dc and ac magnetic susceptibility studies were carried out and indicate S = 0 and S = 5 ground states for 1 and 2, respectively. In order to rationalize the ground states of these and other higher-nuclearity Fe(III)/O clusters, a magnetostructural correlation (MSC) has been developed specifically for polynuclear Fe(III)/O systems that predicts the exchange interaction constant (Jij) between two Fe(III) atoms based on the Fe-O distances and Fe-O-Fe angles at monoatomically bridging ligands. This correlation was refined using selected tri- and tetranuclear complexes in the literature for which both crystal structures and reliable experimentally determined Jij values were available. The predictive capability of the MSC was evaluated by rationalizing the ground-state spins of 1, 2, and other Fe5-Fe8 clusters, simulating the dc magnetic susceptibility data of polynuclear Fe(III) complexes, and fitting experimental dc magnetic susceptibility vs T data. The latter fits were evaluated to identify and eliminate systematic errors, and this allowed a protocol to be developed for application of this MSC to other polynuclear Fe(III)/oxo clusters.