Magnetostructural Correlation for High-Nuclearity Iron(III)/Oxo Complexes and Application to Fe5, Fe6, and Fe8Clusters

The synthesis and characterization are reported of two new polynuclear FeIIIcomplexes 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 1and 2, respectively. In order to rationalize the ground states of these and other higher-nuclearity FeIII/O clusters, a magnetostructural correlation (MSC) has been developed specifically for polynuclear FeIII/O systems that predicts the exchange interaction constant (Jij) between two FeIIIatoms 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 Jijvalues were available. The predictive capability of the MSC was evaluated by rationalizing the ground-state spins of 1, 2, and other Fe5–Fe8clusters, simulating the dc magnetic susceptibility data of polynuclear FeIIIcomplexes, and fitting experimental dc magnetic susceptibility vs Tdata. 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 FeIII/oxo clusters.