Magnetism, Reactivity and Metal Ion Lability in Trigonal Iron Clusters

Important reactions are catalyzed by enzymes employing polynuclear cofactors, often characterized by weak-field ligands and transition metal ions within the sum of the van der Waals radii. While the overall stoichiometries and, in many cases, the structures, of the cofactors are known, the roles of the individual metal ions remain uncertain. Our approach is to investigate model clusters stabilized by a hexadentate, trinucleating ligand. The hexaamine ligand (MeC(CH2NHC6H4-o-NHPh)3) (PhLH6) allows facile synthesis of the clusters (PhL)Fe3(thf)3, (PhL)Fe3(py)3, and (PhL)Fe3(PMe2Ph)3 (thf = tetrahydrofuran, py = pyridine). The phenyl substituents on the ligand sterically prevent strong M-M bonding, but permit weaker M-M orbital interactions, with Fe-Fe distances near those found in Fe metal. The complex (PhL)Fe3(thf)3 exhibits a well-isolated S = 5 or S = 6 ground state over 5 - 300 K, as evidenced by magnetic susceptibility and reduced magnetization data. However, in the stronger-field pyridine and phosphine complexes, temperature dependent susceptibility is observed which is best modeled as a spin state transition from S = 2 to S = 4. Variable-temperature crystallography and Mössbauer spectroscopy reveal a whole-molecule, rather than site-isolated, spin transition.