Covalent immobilization, high-potential stability, and synthetic manipulation of catalytically relevant iron complexes at oxide electrodes [electronic resource]
Keywords: Chemistry

Immobilizing transition metal complexes to electrode surfaces offers a variety of advantages for developing and characterizing new molecular architectures. The copper-catalyzed alkyne-azide cycloaddition (CuAAC) is a useful method for such covalent immobilization. This work presents a new technique for applying the CuAAC reaction to covalent surface immobilization. In this technique, a drop of an alkyne solution is enclosed between an azide-terminated electrode surface and a freshly etched copper plate, which provides the copper(I) catalyst for the cycloaddition reaction. Using this convenient benchtop procedure, a full monolayer of alkyne is covalently immobilized to an azide-terminated electrode surface in 10 seconds. Cyclic voltammetry and X-ray photoelectron spectroscopy are used to characterize both conducting and non-conducting surfaces modified in this way. This method is effective in aerobic conditions using either water or aprotic organic solvents. The copper plate and the alkyne are the only reagents required to rapidly immobilize dense coverages of alkyne-terminated molecules using the rapid, additive-free, CuAAC surface immobilization method. Electrocatalysis is an important application of immobilized metal complexes. Discrete iron complexes supported by pyridine-based ligands are promising candidates for surface-immobilized electrocatalysts. This study characterizes the stability of catalytically relevant iron complexes covalently immobilized to oxide electrodes when the eletrodes are held at high potential. Several homoleptic polypyridyl iron complexes are covalently immobilized onto electrode surfaces using the CuAAC reaction and characterized electrochemically. The decay rate of each complex is quantified using its iv faradaic wave as the electrode is posed at +1.5 V vs. NHE, mimicking the high-potential conditions relevant for substrate oxidation. Several strategies to enhance the highpotential stability of surface-bonded complexes are explored. These include higher-denticity ligands, lower electrolyte pH, and buffering the electrolyte with iron. In their oxidized (ferric) forms, the surface-bound complexes are unstable to common electrochemical buffers. Conditions that enhance stability at high potentials include acidic conditions, iron-containing electrolyte, and longer alkyl linkers between the surface and the immobilized complex. These immobilized complexes are also used to explore the synthetic opportunities of immobilization. Voltammetry of modified electrodes indicates that when a surface-immobilized Fe(terpyridine)2 complex is exposed to a solution of Fe(bipyridine)3, a new complex is generated on the electrode surface. This new species is identified by its unique reduction potential. The incorporation of a bipyridine ligand is confirmed by using a series of bipyridine derivatives and monitoring the effect on the potential of the surface complex. The new species is assigned as a heteroleptic terpyridine-bipyridine complex immobilized to the surface through the terpyridine ligand. The ability to generate a heteroleptic polypyridyl Fe complex contrasts with solution conditions, in which thermodynamic equilibria disfavor the formation of such a species. Immobilization of the complex to the electrode surface therefore facilitates the ligand exchange. The reduction potential of the new complex is altered by the choice of substituent on the bipyridine ligand introduced onto the metal center, demonstrating the ability to modulate the redox potential of an immobilized complex through convenient solution processing.