posted on 2008-03-12, 00:00authored byChuanjiang Hu, Bruce C. Noll, Paula M. B. Piccoli, Arthur J. Schultz, Charles E. Schulz, W. Robert Scheidt
The characterization of a new five-coordinate derivative of (2-methylimidazole)(tetraphenylporphinato)iron(II) provides new and unique information about the effects of forming a hydrogen bond
to the coordinated imidazole on the geometric and electronic structure of iron in these species. The complex
studied has two crystallographically distinct iron sites; one site has an axial imidazole ligand modified by
an external hydrogen bond, and the other site has an axial imidazole ligand with no external interactions.
The iron atoms at the two sites have distinct geometric features, as revealed in their molecular structures,
and distinct electronic structures, as shown by Mössbauer spectroscopy, although both are high spin (S =
2). The molecule with the external hydrogen bond has longer equatorial Fe−Np bonds, a larger displacement
of the iron atom out of the porphyrin plane, and a shorter axial bond compared to its counterpart with no
hydrogen bonding. The Mössbauer features are distinct for the two sites, with differing quadrupole splitting
and isomer shift values and probably differing signs for the quadrupole splitting as shown by variable-temperature measurements in applied magnetic field. These features are consistent with a significant change
in the nature of the doubly populated d orbital and are all in the direction of the dichotomy displayed by
related imidazole and imidazolate species where deprotonation leads to major differences. The results
points out the possible effects of strong hydrogen bonding in heme proteins.