Characterization of Metalloporphines: Iron(II) Carbonyls and Environmental Effects on νCO

Research paper by Ming Li, Allen G. Oliver, W. Robert Scheidt

Indexed on: 27 Apr '18Published on: 26 Apr '18Published in: Inorganic Chemistry


The synthesis of two iron(II) porphine derivatives is reported. The new six-coordinate species have CO and 1-methylimidazole or 2-methylimidazole as the axial ligands. The two species have substantially different C−O stretching frequencies, with one found at the very low frequency of 1923 cm−1. Crystal structures reveal differing CO environments and differing Fe−C and C−O bond distances. These distances demonstrate the known π-backbonding relationships developed from vibrational spectroscopy but now revealed in the actual bond distances.The synthesis and characterization of two new iron(II) porphine complexes is described. Porphine, the simplest porphyrin derivative, has been studied less than other synthetic porphyrins owing to synthetic difficulties and solubility issues. The subjects of this study are two six-coordinate iron(II) species further coordinated by CO and an imidazole ligand (either 1-methylimidazole or 2-methylimidazole). The two species have very different CO stretching frequencies, with the 2-methylimidazole complex having a very low stretching frequency of 1923 cm–1 compared to the more usual 1957 cm–1 for the 1-methylimidazole derivative. The very low frequency is the result of environmental effects; the oxygen atom of the carbonyl forms a hydrogen bond with an adjacent coordinated imidazole with a hydrogen atom from the N–H group. The two species, with their differing C–O stretches, also display substantial differences in the values of the Fe–C and C–O bond distances, as determined by their X-ray structures. The two bond distances are strongly correlated (R = 0.98) in the direction expected for the classical π-backbonding model. The two bond distances are also strongly correlated with the C–O stretching frequencies. We can conclude that the Fe–C and C–O stretches are quite representative of the observed bond distances; their stretching frequencies are not affected by substantial mode mixing.

Figure 10.1021/acs.inorgchem.8b00599.1.jpg
Figure 10.1021/acs.inorgchem.8b00599.2.jpg
Figure 10.1021/acs.inorgchem.8b00599.3.jpg
Figure 10.1021/acs.inorgchem.8b00599.4.jpg
Figure 10.1021/acs.inorgchem.8b00599.5.jpg
Figure 10.1021/acs.inorgchem.8b00599.6.jpg
Figure 10.1021/acs.inorgchem.8b00599.7.jpg
Figure 10.1021/acs.inorgchem.8b00599.8.jpg
Figure 10.1021/acs.inorgchem.8b00599.9.jpg