Interdependent Metal-Metal Bonding and Ligand Redox-Activity in a Series of Dinuclear Macrocyclic Complexes of Iron, Cobalt, and Nickel

Abstract

This report describes an isostructural series of dinuclear iron, cobalt, and nickel complexes bound by a redox-active macrocyclic ligand. The series spans five redox levels (34-38 e-/cluster core), allowing for a detailed investigation into both the degree of metal-metal interaction and the extent of ligand-based redox-activity. Magnetometry, electrochemistry, UV-vis-NIR absorption spectroscopy, and crystallography were used in conjunction with DFT computational analyses to extract the electronic structures of the six homodinuclear complexes. The isoelectronic, 34 e- species [(3PDI2)Fe2(PMe3)2(μ-Cl)](OTf) and [(3PDI2)Co2(PMe3)2(μ-Cl)](OTf)3 exhibit metal-metal single bonds, with varying amounts of electron density delocalization into the ligand as a function of the effective nuclear charge of the metal ions. One-and two-electron reductions of [(3PDI2)Co2(PMe3)2(μ-Cl)](OTf)3 lead to isolable products, which show successive increases in both the Co-Co distances and the extent of reduction of the ligand manifold. This trend results from reduction of a Co-Co σ∗ orbital, which was found to be heavily mixed with the redox-active manifold of the 3PDI2 ligand. A similar trend was observed in the 37 and 38 e- dinickel complexes [(3PDI2)Ni2(PMe3)2(μ-Cl)](OTf)2 and [(3PDI2)Ni2(PMe3)2(μ-Cl)](OTf); however, their higher electron counts lead to high-spin ground states that result from occupation of a high-lying δ/δ∗ manifold with significant Ni-NPDI σ∗ character. This change in ground state configuration reforms a M-M bonding interaction in the 37 e- complex, but formation of the 38 e- species again disrupts the M-M bond alongside the transfer of electron density to the ligand.