Stabilization of 3:1 Site-Differentiated Cubane-Type Clusters in the [Fe4S4]1+ Core Oxidation State by Tertiary Phosphine Ligation: Synthesis, Core Structural Diversity, and S = ½ Ground States

An extensive series of 3:1 site-differentiated cubane-type clusters [Fe 4 S 4 (PPr i 3 ) 3 L] (L = Cl − , Br − , I − , RO − , RS − , RSe − ) has been prepared in 40–80% yield by two methods: ligand substitution of [Fe 4 S 4 (PPr i 3 ) 4 ] 1+ in THF/acetonitrile by reaction with monoanions, and reductive cleavage of ligand substrates (RSSR, RSeSeR, I 2 ) by the all-ferrous clusters [Fe 8 S 8 (PPr i 3 ) 6 ]/ [Fe 16 S 16 (PPr i 3 ) 8 ] in THF. These neutral clusters are stable and do not undergo ligand redistribution reactions involving charged species in benzene and THF solutions. X-ray structural studies confirm the cubane stereochemistry but with substantial and variable distortions of the [Fe 4 S 4 ] 1+ core from idealized cubic core geometry. Based on Fe-S bond lengths, seven clusters were found to have compressed tetragonal distortions (4 short and 8 long bonds) and the remaining seven display other types of distortions with different combinations of long, short and intermediate bond lengths. These results further emphasize the facile deformabililty of this core oxidation state previously observed in [Fe 4 S 4 (SR) 4 ] 3− clusters. The Fe 2.25+ mean oxidation state was demonstrated from 57 Fe isomer shifts and the appearance of two quadrupole doublets arises from the spin-coupled | 9/2,4,1/2〉 state. The S = 1/2 ground state was further supported by EPR spectra and magnetic susceptibility data.