Electronic Structure and Biologically Relevant Reactivity of Low-Spin {FeNO}8 Porphyrin Model Complexes: New Insight from a Bis-Picket Fence Porphyrin

Abstract

Because of HNO's emerging role as an important effector molecule in biology, there is great current interest in the coordination chemistry of HNO and its deprotonated form, the nitroxyl anion (NO(-)), with hemes. Here we report the preparation of four new ferrous heme-nitroxyl model complexes, {FeNO}(8) in the Enemark-Feltham notation, using three electron-poor porphyrin ligands and the bis-picket fence porphyrin H2[3,5-Me-BAFP] (3,5-Me-BAFP(2-) = 3,5-methyl-bis(aryloxy)-fence porphyrin dianion). Electrochemical reduction of [Fe(3,5-Me-BAFP)(NO)] (1-NO) induces a shift of ν(N-O) from 1684 to 1466 cm(-1), indicative of formation of [Fe(3,5-Me-BAFP)(NO)](-) (1-NO(-)), and similar results are obtained with the electron-poor hemes. These results provide the basis to analyze general trends in the properties of ferrous heme-nitroxyl complexes for the first time. In particular, we found a strong correlation between the electronic structures of analogous {FeNO}(7) and {FeNO}(8) complexes, which we analyzed using density functional theory (DFT) calculations. To further study their reactivity, we have developed a new method for the preparation of bulk material of pure heme {FeNO}(8) complexes via corresponding [Fe(porphyrin)](-) species. Reaction of [Fe(To-F2PP)(NO)](-) (To-F2PP(2-) = tetra(ortho-difluorophenyl)porphyrin dianion) prepared this way with acetic acid generates the corresponding {FeNO}(7) complex along with the release of H2. Importantly, this disproportionation can be suppressed when the bis-picket fence porphyrin complex [Fe(3,5-Me-BAFP)(NO)](-) is used, and excitingly, with this system we were able to generate the first ferrous heme-NHO model complex reported to date. The picket fence of the porphyrin renders this HNO complex very stable, with a half-life of ~5 h at room temperature in solution. Finally, with analogous {FeNO}(8) and {FeNHO}(8) complexes in hand, their biologically relevant reactivity toward NO was then explored.