Entropy-Driven Ligand Exchange in a Rotationally Flexible Dinuclear Fe(II)-Fe(II) Complex.

In metalloenzymes, precise control of metal-metal distance and coordination environment enables challenging catalytic transformations. This often involves hydrogen bonding in the second coordination sphere. Although many homogeneous systems aim to mimic these features, the capture and characterization of transient coordination events remains a challenge. Exploring the first di-iron complex of the rotationally flexible dinucleating ligand 1,1',5,5',6,6'-hexamethyl-4,4'-bis(picolinimino)-2,2'-bibenzimidazole, report an unexpected, entropy-driven ligand-exchange equilibrium which we describe by means of variable temperature studies in solution, key species fully structurally characterized in crystallo, and a mechanistic study in silico. This comprehensive experimental and computational characterization of the ligand and complex revealed how 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 which is attributed to the reversible substitution of chlorido ligands by methanol. Density functional theory 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 highlights the potential of this system to support catalytic transformations under certain conditions.