Entropy-driven ligand exchange in a rotationally flexible dinuclear Fe(II)-Fe(II) complex

In metalloenzymes, precise control of metal–metal distance and coordination en- vironment enables challenging catalytic transformations. This often involves hydro- gen bonding in the second coordination sphere. Although many homogeneous sys- tems aim to mimic these features, the capture and characterization of transient co- ordination events remains a challenge. Exploring the first diiron complex of the rota- tionally flexible dinucleating ligand 1,1’,5,5’,6,6’-hexamethyl-4,4’-bis(picolinimino)-2,2’- bibenzimidazole, we uncover a counter-intuitive, entropy-driven ligand exchange equi- librium which we fully described by means of variable temperature studies in solution, key intermediate species trapped in crystallo, and a mechanistic study in silico. This comprehensive experimental and computational characterization of the ligand and com- plex revealed that the central C–C bond enables adaptation to different metal–metal distances. Cooling a methanol solution of the complex induces a color change from green to blue. Based on crystallographic and UV-vis studies, the color change is attributed to the reversible substitution of chlorido ligands by methanol. Density functional the- ory calculations suggest that this ligand exchange is driven by the change in entropy inherent to the reduction of temperature. Under controlled conditions, electrochemical analysis reveals two accessible redox events: a fully reversible redox process at -0.05 V vs. Fc/Fc+ and a series of irreversible reduction reactions at more negative potentials. The resemblance of these redox features with those in established iron catalysts high- lights the potential of this system to support a wide range of catalytic transformations.