Functional Elements from Redoxactive Coordination Network Compounds
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Functional Elements from Redoxactive Coordination Network Compounds
Coordination network compounds are inorganic polymers in which transition metal cations are linked together by bridging ligands. Cavities of a defined size form in the network. As this is of interest for a number of applications such as gas separation and storage, catalysis and energy conversion and storage, these compounds have attracted enormous interest in recent years. We are interested in depositing such compounds as films on electrodes so that their properties can be controlled by applying different potentials to the electrode.
Prussian Blue and its Analogs
One of the oldest compounds of this type is Prussian Blue, in which iron ions in the oxidation states 2+ and 3+ are linked in a cubic lattice by cyanide ligands (CN-). A large variety of metal hexacyanometallates (MHCMs) are derived from this compound by exchanging the transition metal cations. Figure 1 shows iron hexacyanoruthenate, also called ruthenium purple because of its intense color. In this case, Fe ions form a coordinative bond with the N atoms of the cyanide ion, while the C atom in cyanide is connected to the ruthenium ions. In this example, the charge neutrality of the compound is achieved by incorporating K+ ions into the cavities. The K+ ions can move in the channels. Other cations can also be absorbed. By applying different electrode potentials, the films can be converted into structurally related compounds in which the transition metal ions change their valence state (here between Fe2+ and Fe3+) and the charge is balanced by the uptake or release of the mobile K+ ions. The compounds therefore have a mixed conductivity, i.e. they conduct electrons and ions. They change in redox state is sometimes accompanied by significant color changes, changes in the electrocatalytic and magnetic properties and charge storage, which makes them interesting for various applications (absorption of radioactive Cs+ ions, electrochemical sensors, contact layers, electrochromic coatings, ...).
Characterization of Films from Coordination Network Compounds on Electrodes
Figure 1 also shows that the structures are much more complicated than the above description would suggest. This is due to the fact that defects such as those shown in Figure 1b are possible in the lattice. One Ru-center is missing with the six surrounding cyanaide ligands and water takes on the role of the ligand of the N-coordinated transition metal ion. The true structure of the material is a solid solution in which all the structures in Figure 1 may occur, which poses a challenge for structure determination. In many cases, different valence changes of the transition metals are also possible, which can be elucidated by the use of X-ray photoelectron spectroscopy. An additional complication is the presence of water (or another solvent) in the pores of the network compound, which is important for the function. The redox reactions can be observed by voltammetry. Spectroelectrochemistry is particularly informative, with simultaneous recording of reaction currents as evidence of chemical conversion and IR spectra to analyze structural changes (Figure 2).
Own contributions to the research area
Our group deals with the analysis of valence changes and the resulting switchable properties such as color, conductivity and catalytic activity. We have also developed new methods to locally deposit metal hexacyanometalates using the scanning electrochemical microscope and to prepare different layers of different metal hexacyanoferrates on top of each other. Recently, we discovered that the redox properties of films of iron hexacyanoruthenate in electrolyte solutions with heavy water (D2O) differ significantly from those in normal water (H2O). The reason for this surprising H/D isotope effect is probably the altered structure of the water in the pores and at the defect sites of the structure in Figure 1.