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, Δ<i>E</i>_Q), determined the <i>S</i> = 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)