Utilization of hydrophobic ligands for water-insoluble Fe(II) water oxidation catalysts – Immobilization and characterization

Herein, we compare the electrochemical and electrocatalytic properties of two selected, water-insoluble Fe(II) coordination complexes made with the non-symmetric, bidentate ligands, 2-(2′-pyridyl)benzimidazole (PBI) in [Fe(PBI)₃](OTf)₂ (1, OTf⁻ = trifluoromethyl sulfonate anion) and 2-(2′-pyridyl)benzoxazole (PBO) in [Fe(PBO)₂(OTf)₂] (2) . Cyclic voltammetry in water/acetonitrile mixture indicates considerable activity for both compounds. However, only 1 acts as homogeneous catalyst. The complexes have been successfully immobilized on indium-tin-oxide (ITO) electrode surface. The hydrophobic ligands allowed for a simple dip-coating and drop-casting of 1 and 2 onto ITO. Both 1/ITO and 2/ITO showed increased activity in electrocatalytic O₂ evolution in borate buffer at pH 8.3. According to scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), moreover, re-dissolution tests, the Fe remains in complex with PBI during electrolysis in the drop-casted, nano-porous films of 1/ITO. In contrast, the PBO complex in 2/ITO undergoes a rapid in situ decomposition yielding a mineralized form that is responsible for catalysis.