Transcript Electrochemistry

A.
Electrochemistry
Why are we doing this experiment?
1)
Transition metal ions are often stable in multiple oxidation states
2)
Ligand properties effect the stability of various oxidation states
a) Mn0 is stable in CO complexes (soft/soft)
b) Mn2+--Mn4+ is stable with Nitrogen Ligands (intermediate)
c) Mn7+ is stable with Oxide ligands (hard/hard)
3)
Bridged ligand complexes are very stable, a property useful for oxidation catalysts
4)
Catalytic complexes must be stable at multiple oxidation states
5)
The oxidation potential of a complex determines its ability to transfer electrons
Electrochemical Studies
I.
II.
Ligands stabilize metals in multiple
oxidation states
Cyclic Voltammetry of Me2B14N4 Complexes
CuII
Mn(Me2B14N4)Cl2 identified as
active catalyst
NiII
CoII
catalyst
H2O2
FeII
Patents: US 6,218,351
US 6,387,862
US 6,608,015
MnII
3
2
1
0
Potential (V) vs SHE
-1
-2
-3
B.
How are we doing this experiment?
1)
We are using a BAS Epsilon Electrochemical Analyzer
2)
It uses three electrodes to interact with the solution (TBAPF6 + complex)
a)
A button Platinum Working Electrode: this is where the redox process with
your complex actually takes place
b)
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
c)
A Silver wire Pseudo-Reference Electrode: the oxidation/reduction of this
electrode is what the voltage of the working electrode is measured against
d)
Supporting electrolyte carries the charge in the solution
TetraButylAmmonium Hexafluorophosphate = TBAPF6
1.
The BAS Epsilon sweeps through a voltage window at the working electrode
2.
If current flows to the complex in solution, a peak appears (TBAPF6 not redox active)
3.
Don’t stir, because we want oxidized complex to stay at electrode for further oxidation
and the return reduction
3.
An example for a modified Ferrocene Complex
a) 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.
b) 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.
c) 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.
d) 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.
e) Because this is a reversible system, the peak separation is 57mV.
57mV.
f) Animation of the data acquisition
4.
After we get a satisfactory CV of the complex, we add some Ferrocene
a) The Ferrocene acts as an internal reference
b) Its Oxidation Potential is know to be +0.400 V in acetonitrile (AcFc=+0.680)
c) It is highly reversible, giving us an estimate of our complex’s reversibility
C.
How do we work up the data?
1. You will save a txt file of your complex alone and with Ferrocene
2. You will import the files into Excel
D.
Workup of an Fe3+ complex data in Excel—Dr. Hubin
E.
How do we interpret the results?
1. Oxidation/Reduction potentials tell us how hard it is to add/remove an electron
a. Larger positive oxidation potentials mean it is difficult to remove the electron
b. Larger positive reduction potentials mean it is easy to add an electron
c. Smaller/negative oxidation potentials mean it is easy to remove the electron
d. Smaller/negative reduction potentials mean it is hard to add an electron
2. The characteristics of the ligands can help explain the ease of Oxidation/Reduction
a. Negatively charged and/or hard ligands favor higher oxidation states
b. Neutral and/or soft ligands favor lower oxidation states
3. The reversibility of the Ox/Red process tells us about processes after Ox/Red
a. Reversibility indicates that no ligands are lost or gained
b. Irreversibility indicates that ligands are lost or gained prior to the return wave
4. Reversibility tells us if multiple oxidation states can be reached with this complex
a. Multiple oxidation states are required for Redox Catalysts
b. Irreversibility indicates that the complex is not stable after redox
E.
Conclusions
1. The iron(III) complex is reduced to iron(II) at a mild potential of E½ = + 0.106 V
a. Addition and removal of an electron are fairly easy
b. Both the iron(II) and iron(III) complexes are stabilized by the ligand set
2.
The Bridged tetraazamacrocycle is a good electronic match for Fe2+ or Fe3+
a. Nitrogen donors are borderline hard/soft bases
b. Fe2+ is borderline and Fe3+ is hard
3.
The Reduction observed is Fairly Reversible
a. The ferrocene reversibility in this experiment was DE = 74 mV (57 mV)
b. The complex reversibility in this experiment was DE = 80-98 mV
4.
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)
5.
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)