Magnetic Susceptibility Measurements
The number and nature of axial ligands are the primary determinants of spin state in
ferriporphyrins4. Thus, as the axial ligand field strength increases, the spin state adopted by an
Fe(III)(P), where P is any porphyrin, progresses from intermediate-spin (S = 3/2), to high-spin (S
= 5/2), and finally to low-spin (S = 1/2),. This is represented in Figure 1 with simple ligand-field
splitting diagrams. An Fe(III)(P) with very weak-field ligands invariably adopts a spin state that
is not pure S = 3/2 but rather quantum mechanically admixed S = 3/5, 5/2.
Figure 1. The variation of d-orbital energies with increasing axial field strength.
The effective magnetic moments of the iron porphyrins can be determined for the solid state
by the Faraday Balance method and in solution using Evans' method. For the solution
susceptibility (Evans’ Method), you will do :
1) one measurement of your group’s Fe(P)Cl in CHCl3 and
2) one measurement using in a solution of 0.1 M imidazole in CHCl3
The presence of the imidazole in (2) causes in situ formation of low-spin bis-imidazole
complexes, [Fe(P)(Im)2]Cl and provides another environment of the Fe atom found in biological
hemes.
(Note: in the case (2) the reference solution for the second solution must contain imidazole in
the same concentration). An Fe-porphyrin concentration of about 0.01 M works well for these
Evans’ experiments and you will need to calculate the appropriate amounts of Fe-P compound
needed for the experiments prior to coming to lab.
The results from each group will be shared with the rest of the class so that the magnetic
behavior of each iron-porphyrin in the series TPP, TTP and TClPP is examined and comparisons
of the magnetic state as a function of added imidazole and without imidazole can be made.
Magnetic moments for Fe(P)Cl by both the Faraday balance (solid state) and Evans (solution)
methods are typically in the range 5.3-6.0 µB, corresponding to a high-spin state (S = 5/2)
whereas the values obtained for [Fe(P)(Im)2]Cl are 2.4-2.7 µB corresponding to a low-spin state
(S = 1/2).
Samples will need to be corrected for diamagnetism using these values:
-700 x 10-6 cgs for the ligand TPP
-753 x 10-6 cgs for the ligand TTP
-760 x 10-6 cgs for the ligand TC1PP.
Electrochemical Characterization by Cyclic Voltammetry
The electrochemistry of iron porphyrins has been well characterized5, and the potentials for
various redox processes correlate with many aspects of iron-prophyrins: metal spin state,
coordination of axial ligands, the solvent system employed for the study, out-of-porphyrin-plane
displacement of the iron, counterion, and basicity of the porphyrin ring5c. In this lab,
experiments will focus only on the effect of porphyrin basicity and axial ligation on the Fe(III/II)
and Fe(II/I) redox potentials.
Cyclic voltammetry is used to obtain the half-wave reduction potentials for the Fe(III/II) and
Fe(II/I) couples in the absence and presence of imidazole. The results from each group will be
shared so that each porphyrin in the series is examined and comparisons of the E1/2’s can be
made. 0.1 M tetraethylammonium perchlorate, TEAP, in dimethylformamide, DMF, performs
well as a supporting electrolyte-solvent system combination. Complexities associated with the
displacement of anions from the coordination sphere are minimized in coordinating solvents such
as DMF.
Procedure
A three-electrode cell supplied by Bioanalytical Systems, Inc., and composed of a Pt disk
electrode, Pt wire auxiliary electrode and a Ag/AgCl reference electrode will be used.
A DMF solution 0.1 M in TEAP is prepared in the electrochemical cell and deaerated for 10
min with a stream of nitrogen. A voltammogram is obtained using a scan range of +0.3 to -1.4 V
vs. Ag/AgCl and a scan rate of 50 mV/s. A small amount of the Fe-prophyrin to be analyzed is
added to the electrolyte solution in the cell, deaerated for 5 minutes, then scanned over the same
potential range +0.3 to -1.4 V. Then 100 mg of imidazole is put into the DMF solution, which is
again deaerated. A second voltammogram is obtained. Finally, ferrocene is added to the
solution and the wave for the ferrocene-ferricinium couple is recorded for calibration of the cell.
Results
The data to be collected from each group is as follows:
1. E1/2 values for one of the three iron porphyrins , with and without imidazole.
2. The observed chemical shift and calculated magnetic susceptibility for one of the three
iron porphyrins, with and without imidazole.
Analysis of the half-wave reduction potentials should reveal a number of trends that you should
attempt to explain using concepts of electronic effects first introduced in organic chemistry
courses.
1. J. Chem. Ed. 1985, vol. 62, p 917.
2. the Bioinorganic chapter of any modern Inorganic text
3. J. Chem. Ed. 1989, vol. 66, p 854.
4. Scheidt and Reed Chem. Rev. 1981, vol. 81, p 543.
5. “Electrochemical and Spectrochemical Studies of Biological Redox Components”; currently
in my office.
6. Acc. Chem. Res. 1987, vol. 20, p 309.
7. Acc. Chem. Res. 1972, vol. 5, p 234.
8. Adler, J. Org. Chem. 1967 vol. 32, p 476.
9. Adler, J. Amer. Chem. Soc. 1975 vol. 97, p 5107.
10. Adler, J. Inorg. Nucl. Chem. 1970 vol. 32, p 2443.