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Detailed Spectroscopic Studies on Metalloporphyrins
Vibrational and MCD spectroscopy have been proven to be key methods to gain insight into the geometric and
electronic structures of metalloporphyrins. Hence, these techniques have been widely applied to both proteins with heme
cofactors and corresponding model complexes [1]-[4]. However, the obtained data from these spectroscopic investigations are
not easy to analyze. Due to the size of the porphyrin ligand, a complete set of vibrational data including IR, non-resonance
and resonance Raman spectra for a simple tetraphenylporphyrin metal complex allow for the identification of more than 50 different
vibrations. Some of these modes are oxidation- and spin-state sensitive and hence, are also very useful for the investigation
of transition metal heme-NO complexes besides the vibrations of the M-NO subunit itself. However, in order to access the useful
information provided by the porphyrin core vibrations, detailed analyses of the vibrational spectra of corresponding
metalloporphyrins are necessary. Since
tetraphenylporphyrin (TPP)
is generally used in our investigations, we have performed detailed
vibrational and resonance Raman studies
on simple [MIII(TPP)Cl] (M = Fe, Mn, Co) complexes
to completely assign the normal modes of the [M(TPP)] core.
The optical spectra of metalloporphyrins are in general dominated by π to π*
transitions
of the porphyrin core, which give rise to the Soret, Q and Qv features in the absorption spectra [5]. Additional features are often
masked by these intense bands. This includes charge transfer (CT) transitions between the metal (M) and axially coordinated
ligands (L) or d-d transitions, which are diagnostic for the properties of the metal ion and the electronic structure of the
M-L bond. Hence, it is very important to identify these additional transitions, which, in the case of paramagnetic complexes,
is possible with the help of
magnetic circular dichroism (MCD) spectroscopy.
In order to investigate the MCD spectra of tetraphenylporphyrin (TPP) transition metal complexes and to explore the complexity
of the spectra as a function of the electron configuration of the metal, we have investigated the simple
[MIII(TPP)Cl] (M = Fe, Mn, Co) precursors and, additionally, [Co(TPP)], since [Co(TPP)Cl] is diamagnetic. These
studies are currently in progress.
The method calibration studies presented here are also very important for a different purpose. In many cases, DFT calculations
are used to assist in the assignments of vibrational or electronic spectra of transition metal complexes. However, it is not always
guaranteed that DFT actually works for the system studied. Hence, the detailed investigations of the vibrational and MCD spectroscopic
properties of simple metalloporphyrins performed here are also used to explore the reliability of DFT calculations to predict the
vibrational and electronic spectra of these compounds. From our experience, the vibrational properties of the metalloporphyrins of
type [MIII(TPP)Cl] (M = Mn, Fe, Co) are reproduced very well by the DFT calculations [6]. However, this does not seem to
hold true for the electronic spectra [7].
Literature:
[1] Spiro, T. G. in Iron porphyrins; Lever, A. B. P.; Gray, H. B., Eds.; Addison-Wesley: Massachusetts, 1983, part 2, pp 89-159.
[2] Spiro, T. G.; Li, X.-Y. in Resonance raman spectra of heme and metalloproteins; Spiro, T. G., Ed.; Wiley: New York, 1988, pp 1-37.
[3] Cheesman, M. R.; Greenwood, C.; Thomson, A. J. Adv. Inorg. Chem. 1991, 36, 201.
[4] Walker, F. A. Coord. Chem. Rev. 1999, 185-186, 471.
[5] Gouterman, M. in The Porphyrins; Dolphin, D., Ed.; Academic: New York, 1979, Vol. III, Part A, pp 1-156.
[6] Paulat, F.; Praneeth, V. K. K.; Näther, C.; Lehnert, N. Inorg. Chem. 2006, 45, 2835-2856
[7] Praneeth, V. K. K.; Näther, C.; Peters, G.; Lehnert, N. Inorg. Chem. 2006, 45, 2795-2811
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