The Modular Nature of All-Ferrous Edge-Bridged Double Cubanes

Abstract

Two all-ferrous, edge-bridged 8Fe−8S clusters, one capped with carbenes (2) and the other with phosphenes (3), have been characterized by ⁵⁷Fe Mössbauer spectroscopy. The clusters have diamagnetic ground states that yield spectra consisting of one quadrupole doublet with a large splitting (25% of absorption) and one (3) or two (2) doublets with much smaller splittings (75% of absorption). These patterns closely resemble those observed for all-ferrous 4Fe−4S clusters. Structurally, the 4Fe−4S fragments of 2 and 3 are remarkably similar to all-ferrous 4Fe−4S clusters, sharing with them the characteristic 3:1 pattern of the iron sites, a differentiation that has been shown previously to reflect spontaneous distortions of the cluster core. These spectroscopic and geometric similarities suggest that the diamagnetic ground state of the 8Fe−8S cluster results from antiferromagnetic exchange coupling of two identical 4Fe−4S modules, each carrying spin S_4Fe = 4. The iron atoms with the largest quadrupole splittings are located at the opposite ends of the body diagonal containing the bridging sulfides.