Ligand dependence of binding to three-coordinate Fe(II) complexes

Area of Science:

• Inorganic Chemistry
• Organometallic Chemistry
• Biochemistry

Background:

• Iron complexes are crucial in biological systems, notably in enzymes like nitrogenase.
• Understanding the electronic and steric factors governing ligand binding in iron complexes is key to mimicking biological activity.
• Three-coordinate iron(II) intermediates are proposed in nitrogenase mechanisms, but their synthetic analogs are not fully understood.

Purpose of the Study:

• To synthesize and characterize three- and four-coordinate iron(II) complexes.
• To investigate the influence of neutral and anionic spectator ligands on the electronic properties and binding affinities of iron(II) complexes.
• To compare the binding trends of synthetic iron(II) complexes with proposed intermediates in the nitrogenase enzyme.

Main Methods:

• Synthesis of iron(II) complexes with varying nitrogen, chlorine, oxygen, and sulfur ligands.
• Measurement of association constants (K(eq)) for neutral ligand binding.
• Determination of reduction potentials for the iron(II) complexes.

Main Results:

• A significant variation in K(eq) was observed with different neutral ligands, following the order CN(t)Bu > pyridine > 2-picoline > DMF > MeCN > THF > PPh(3).
• Both steric and electronic effects (sigma-donation and pi-backbonding) contribute to the observed binding trends.
• Binding constants and reduction potentials showed minimal sensitivity to changes in anionic spectator ligands.

Conclusions:

• The electronic and steric properties of neutral ligands are primary drivers of binding affinity in these iron(II) complexes.
• The insensitivity to anionic spectator ligands suggests potential parallels between synthetic three-coordinate iron(II) complexes and proposed intermediates in the FeMoco of nitrogenase.
• These findings contribute to understanding iron coordination chemistry and its relevance to metalloenzyme function.