Low-temperature UV-visible and NMR spectroscopic investigations of O2 binding to (6L)FeII, a ferrous heme bearing covalently tethered axial pyridine ligands

In this report, we describe the reversible dioxygen reactivity of (L-6)Fe-II (1) {L-6 = partially fluorinated tetraphenylporphyrin with covalently appended TMPA moiety; TMPA = tris(2-pyridylmethyl)amine} using a combination of low-temperature UV-vis and multinuclear (H-1 and H-2) NMR spectroscopies. Complex 1, or its pyrrole-deuterated analogue (L-6-d(8))Fe-II (1-d(8)), exhibits downfield shifted pyrrole resonances (delta 28-60 ppm) in all solvents utilized {CH2Cl2, (CH3)(2)C(O), CH3CN, THF}, indicative of a five-coordinate high-spin ferrous heme, even when there is no exogenous axial solvent ligand present (i.e., in methylene chloride). Furthermore, (L-6)Fe-II (1) exhibits non-pyrrolic upfield and downfield shifted peaks in CH2Cl2, (CH3)(2)C(O), and CH3CN solvents, which we ascribed to resonances arising from the intra- or intermolecular binding of a TMPA-pyridyl arm to the ferrous heme. Upon exposure to dioxygen at 193 K in methylene chloride, (L-6)Fe-II (1) {UV-vis: lambda(max) = 433 (Soret), 529 (sh), 559 nm} reversibly forms a dioxygen adduct {UV-vis: lambda(max) = 422 (Soret), 542 nm}, formulated as the six-coordinate low-spin {delta(pyrrole) 9.3 ppm, 193 K} heme-superoxo complex (L-6)Fe-III-(O-2(-)) (2). The coordination of the tethered pyridyl arm to the heme-superoxo complex as axial base ligand is suggested. In coordinating solvents such as THF, reversible oxygenation (193 K) of (L-6)Fe-II (1) {UV-vis: lambda(max) = 424 (Soret), 542 nm} also occurs to give a similar adduct (L-6)Fe-III-(O-2(-)) (2) {UV-vis: lambda(max) = 418 (Soret), 537 nm. H-2 NMR: delta(pyrrole) 8.9 ppm, 193 K}. Here, we are unable to distinguish between a bound solvent ligand or tethered pyridyl arm as axial base ligand. In all solvents, the dioxygen adducts decompose (thermally) to the ferric-hydroxy complex (L-6)Fe-III-OH (3) {UV-vis: lambda(max) = 412-414 (Soret), 566-575 nm; similar todelta(pyrrole) 120 ppm at 193 K}. This study on the O-2-binding chemistry of the heme-only homonuclear (L-6)Fe-II (1) system lays the foundation for a more complete understanding of the dioxygen reactivity of heterobinuclear heme-Cu complexes, such as [(L-6)(FeCuI)-Cu-II](+), which are models for cytochrome c oxidase.