Heme Carbonyls:  Environmental Effects on ν_C-O and Fe−C/C−O Bond Length Correlations

The synthesis and characterization of four low-spin (carbonyl)iron(II) tetraphenylporphyrinates, [Fe(TPP)(CO)(L)], where L = 1-methylimidazole, 2-methylimidazole, 1,2-dimethylimidazole (unsolvated), and 1,2-dimethylimidazole (toluene solvate) are reported. The complexes show nearly the same value of ν_C-O in toluene solution (1969−72 cm^-1) but a large range of CO stretching frequencies in the solid-state (1926−1968 cm^-1). The large solid-state variation results from CO interactions in the solid state, as shown by an examination of the crystal structures of the four complexes. The high precision of the four structures obtained allows us to make a number of structural and spectroscopic correlations that describe the Fe−C−O and N_Im-Fe−CO units. The values of ν_C-O and the Fe−C and C−O bond distances are strongly correlated and provide a structural, as well as a spectroscopic, correlation of the π back-bonding model. The interactions of CO described are closely related to the large range of CO stretching frequencies observed in heme proteins and specific interactions observed in carbonylmyoglobin (MbCO).