New Insights on the Electronic and Molecular Structure of Cyanide-Ligated Iron(III) Porphyrinates

The preparation and characterization of several new cyano-ligated six-coordinate low-spin iron(III) porphyrinates
are reported. The synthesis and structure of the new bis(cyanide) derivative K(222)][Fe(TMP)(CN)2] (TMP =
tetramesitylporphyrinate) is described. Three mixed-ligand species of the general form [Fe(Porph)(CN)(L)], where
L = 1-methylimidazole or pyridine, have also been prepared and structurally characterized. All complexes have
been studied with EPR spectroscopy in frozen solution and in microcrystalline form. In some cases, especially
those of the bis(cyanide) derivative above and the previously reported [Fe(TPP)(CN)2]-, there are significant differences
in the EPR spectra as a result of the state change. These spectral differences can be correlated with changes in
the electron configuration that are the likely result of a differing environment of the coordinated cyanide ligands; the
core conformation and electronic structure of the porphyrin ligand are unlikely to play a role. All four new complexes
and [Fe(TPP)(CN)2]- have been studied by Mössbauer spectroscopy with variable-temperature and applied magnetic-field measurements. The sign of the quadrupole splitting value has been established as negative. These
measurements have allowed us to give estimates of the energy difference between the two close-lying dπ (dxz and
dyz) orbitals. These splitting values range from ∼267 cm-1 for [Fe(TPP)(CN)2]- to ∼614 cm-1 for [Fe(TPP)(CN)(Py)].