Hydrogen-Bonding Effects in Five-Coordinate High-Spin Imidazole-Ligated Iron(II) Porphyrinates

The influence of hydrogen binding to the N-H group of coordinated imidazole in high-spin iron(II) porphyrinates has been studied. The preparation and characterization of new complexes based on [Fe(TPP)(2-MeHIm)] (TPP is the dianion of tetraphenylporphyrin) are reported. The hydrogen bond acceptors are ethanol, tetramethylene sulfoxide, and 2-methylimidazole. The last acceptor, 2-MeHIm, was found in a crystalline complex with two [Fe(TPP)(2-MeHIm)] sites, only one of which has the 2-methylimidazole hydrogen bond acceptor. This latter complex has been studied by temperature-dependent Mössbauer spectroscopy. All new complexes have also been characterized by X-ray structure determinations. The Fe-N_P and Fe-N_Im bond lengths, and displacement of the Fe atom out of the porphyrin plane are similar to, but marginally different than, those in imidazole-ligated species with no hydrogen bond. All the structural and Mössbauer properties suggest that these new hydrogen-bonded species have the same electronic configuration as imidazole-ligated species with no hydrogen bond. These new studies continue to show that the effects of hydrogen bonding in five-coordinate high-spin iron(II) systems are subtle and challenging to understand.