Tuning Metal-Metal Interactions through Reversible Ligand Folding in a Series of Dinuclear Iron Complexes

A dinucleating macrocyclic ligand with two redox-active, pyridyldiimine components was shown to undergo reversible ligand folding to accommodate various substitution patterns, metal ion spin states, and degrees of Fe-Fe bonding within the cluster. An unfolded-ligand geometry with a rectangular Fe₂(μ-Cl)₂ core and an Fe-Fe distance of 3.3262(5) Å served as a direct precursor to two different folded-ligand complexes. Chemical reduction in the presence of PPh₃ resulted in a diamagnetic, folded ligand complex with an Fe-Fe bonding interaction (d_Fe-Fe = 2.7096(17) Å) between two intermediate spin (S_Fe = 1) Fe(II) centers. Ligand folding was also induced through anion exchange on the unfolded-ligand species, producing a complex with three PhS^- ligands and a temperature-dependent Fe-Fe distance. In this latter example, the weak ligand field of the thiolate ligands led to a product with weakly coupled, high-spin Fe(II) ions (S_Fe = 2; J = -50.1 cm^-1) that form a bonding interaction in the ground state and a nonbonding interaction in the excited state(s), as determined by SQUID magnetometry and variable temperature crystallography. Finally, both folded-ligand complexes were shown to reform an unfolded-ligand geometry through convergent syntheses of a complex with an Fe-Fe bonded Fe₂(μ-SPh)₂ core (d_Fe-Fe = 2.7320(11) Å). Experimentally validated DFT calculations were used to investigate the electronic structures of all species as a way to understand the origin of Fe-Fe bonding interactions, the extent of ligand reduction, and the nature of the spin systems that result from multiple, weakly interacting spin centers.