Electronic Configuration and Ligand Nature of Five-Coordinate Iron Porphyrin Carbene Complexes: An Experimental Study

The
five-coordinate iron porphyrin carbene complexes [Fe­(TPP) (CCl2)] (TPP = tetraphenylporphyrin), [Fe­(TTP) (CCl2)] (TTP = tetratolylporphyrin) and [Fe­(TFPP) (CPh2)] (TFPP
= tetra­(penta­fluoro­phenyl)­porphyrin), utilizing
two types of carbene ligands (CCl2 and CPh2),
have been investigated by single crystal X-ray, XANES (X-ray absorption
near edge spectroscopy), Mössbauer, NMR and UV–vis spectroscopies.
The XANES suggested the iron­(II) oxidation state of the complexes.
The multitemperature and high magnetic field Mössbauer experiments,
which show very large quadrupole splittings (QS, ΔEQ), determined the S = 0 electronic configuration.
More importantly, combined structural and Mössbauer studies,
especially the comparison with the low spin iron­(II) porphyrin complexes
with strong diatomic ligands (CS, CO and CN–) revealed
the covalent bond nature of the carbene ligands. A correlation between
the iron isomer shifts (IS, δ) and the axial bond distances
is established for the first time for these donor carbon ligands (:C–R).