Fe-N-O structure and bonding in six-coordinate {FeNO}6 porphyrinates containing imidazole: implications for reactivity of coordinated NO

We report density functional theory calculations on six-coordinate ferric-NO ({FeNO}6) porphyrinates that contain either imidazole or imidazolate as the trans axial ligand. Our results show that the sensitivities of the Fe-NO and N-O stretching frequencies to cis and trans influences are directly correlated. In other words, as one decreases so does the other for both the imidazole and the imidazolate complexes. This correlation is opposite that of the isoelectronic ferrous-CO systems, whose Fe-CO and C-O frequencies are well-known to be inversely correlated. Based on the results of our calculations, the molecular origin of the direct correlation in {FeNO}6 porphyrinates can be explained by trends in the electron density distributions within the HOMO or HOMO-1, which exhibits Fe-NO and N-O pi-antibonding character. Variability in the Fe-N-O pi-antibonding character of the HOMO or the HOMO-1 modulates the angleFeNO as well as the Fe-NO and N-O bond strengths in concert. Orbital interactions in the six-coordinate FeIIINO porphyrin complexes are compared and contrasted with those of the isoelectronic FeIICO analogues, and an overall view of {FeNO}6 bonding in these complexes is set forth.