Electrochemistry Why are we doing this experiment? Transition metal ions are often stable in multiple oxidation states Ligand properties effect the stability of various oxidation states Mn0 is stable in CO complexes (soft/soft) Mn2+--Mn4+ is stable with Nitrogen Ligands (intermediate) Mn7+ is stable with Oxide ligands (hard/hard) Bridged ligand complexes are very stable, a property useful for oxidation catalysts Catalytic complexes must be stable at multiple oxidation states The oxidation potential of a complex determines its ability to transfer electrons

Electrochemical Studies Ligands stabilize metals in multiple oxidation states Mn(Me2B14N4)Cl2 identified as active catalyst Cyclic Voltammetry of Me2B14N4 Complexes CuII NiII CoII FeII Patents: US 6,218,351 US 6,387,862 US 6,608,015 MnII Potential (V) vs SHE

B. How are we doing this experiment? We are using a BAS Epsilon Electrochemical Analyzer It uses three electrodes to interact with the solution (TBAPF6 + complex) A button Platinum Working Electrode: this is where the redox process with your complex actually takes place A Platinum wire Auxiliary Electrode (or counter electrode): which serves as a source or sink for electrons so that current can be passed from the external circuit through the cell A Silver wire Pseudo-Reference Electrode: the oxidation/reduction of this electrode is what the voltage of the working electrode is measured against Supporting electrolyte carries the charge in the solution TetraButylAmmonium Hexafluorophosphate = TBAPF6

The BAS Epsilon sweeps through a voltage window at the working electrode If current flows to the complex in solution, a peak appears (TBAPF6 not redox active) Don’t stir, because we want oxidized complex to stay at electrode for further oxidation and the return reduction

An example for a modified Ferrocene Complex The ferrocene compound is initially present in the reduced state.  The voltage scan starts at a potential negative of the Eo value for this couple and hence, there is no flow of current. As the voltage approaches the Eo value (320mV) a positive current begins to flow indicating that the ferrocene molecule is being oxidized.  The current continues to rise (exponentially).  This is known as the kinetic region of the voltammogram. As the voltage increases, the rate of reaction also increases until a point is reached when the process becomes limited by the mass transfer of ferrocene from the bulk to the electrode surface.  The current then begins to fall and a peak is produced. As the voltage sweep is reversed (when the switching potential is reached), the oxidized material that is in the vicinity of the electrode is reduced resulting in a reduction peak of similar magnitude. Because this is a reversible system, the peak separation is 57mV. Animation of the data acquisition After we get a satisfactory CV of the complex, we add some Ferrocene The Ferrocene acts as an internal reference Its Oxidation Potential is know to be +0.400 V in acetonitrile It is highly reversible, giving us an estimate of our complex’s reversibility An example for a modified Ferrocene Complex The ferrocene compound is initially present in the reduced state.  The voltage scan starts at a potential negative of the Eo value for this couple and hence, there is no flow of current. As the voltage approaches the Eo value (320mV) a positive current begins to flow indicating that the ferrocene molecule is being oxidized.  The current continues to rise (exponentially).  This is known as the kinetic region of the voltammogram. As the voltage increases, the rate of reaction also increases until a point is reached when the process becomes limited by the mass transfer of ferrocene from the bulk to the electrode surface.  The current then begins to fall and a peak is produced. As the voltage sweep is reversed (when the switching potential is reached), the oxidized material that is in the vicinity of the electrode is reduced resulting in a reduction peak of similar magnitude. Because this is a reversible system, the peak separation is 57mV. Animation of the data acquisition After we get a satisfactory CV of the complex, we add some Ferrocene The Ferrocene acts as an internal reference Its Oxidation Potential is know to be +0.400 V in acetonitrile (AcFc=+0.680) It is highly reversible, giving us an estimate of our complex’s reversibility

How do we work up the data? You will save a txt file of your complex alone and with Ferrocene You will import the files into Excel Workup of an Fe3+ complex data in Excel—Dr. Hubin How do we interpret the results? Oxidation/Reduction potentials tell us how hard it is to add/remove an electron Larger positive oxidation potentials mean it is difficult to remove the electron Larger positive reduction potentials mean it is easy to add an electron Smaller/negative oxidation potentials mean it is easy to remove the electron Smaller/negative reduction potentials mean it is hard to add an electron The characteristics of the ligands can help explain the ease of Oxidation/Reduction Negatively charged and/or hard ligands favor higher oxidation states Neutral and/or soft ligands favor lower oxidation states The reversibility of the Ox/Red process tells us about processes after Ox/Red Reversibility indicates that no ligands are lost or gained Irreversibility indicates that ligands are lost or gained prior to the return wave Reversibility tells us if multiple oxidation states can be reached with this complex Multiple oxidation states are required for Redox Catalysts Irreversibility indicates that the complex is not stable after redox

Conclusions The iron(III) complex is reduced to iron(II) at a mild potential of E½ = + 0.106 V Addition and removal of an electron are fairly easy Both the iron(II) and iron(III) complexes are stabilized by the ligand set The Bridged tetraazamacrocycle is a good electronic match for Fe2+ or Fe3+ Nitrogen donors are borderline hard/soft bases Fe2+ is borderline and Fe3+ is hard The Reduction observed is Fairly Reversible The ferrocene reversibility in this experiment was DE = 74 mV (57 mV) The complex reversibility in this experiment was DE = 80-98 mV The reversibility indicates that no ligands are gained or lost under the time window of the CV experiment, even after reduction (Cl- might want to leave) Both Fe2+ and Fe3+ are accessible to this complex. Redox catalysts would likely need a wider range of oxidation states to function well. (And in fact, this complex turned out not to be an effective catalyst)