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(pentafluoro-phenyl)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), Mossbauer, NMR and UV-vis spectroscopies. The XANES suggested the iron(II) oxidation state of the complexes. The multitemperature and high magnetic field Mossbauer experiments, which show very large quadrupole splittings (QS, Delta E-Q), determined the S = 0 electronic configuration. More importantly, combined structural and Mossbauer 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, delta) and the axial bond distances is established for the first time for these donor carbon ligands (:C-R).