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Ghiladi RA, Karlin KD: Low-temperature UV-visible and NMR spectroscopic investigations of O (2) binding to ((6) L) Fe (II), a ferrous heme bearing covalently tethered axial pyridine ligands. Inorg Chem. 2002 May 6;41(9):2400-7.
In this report, we describe the reversible dioxygen reactivity of ((6) L) Fe (II) (1) [(6) L = partially fluorinated tetraphenylporphyrin with covalently appended TMPA moiety; TMPA = tris (2-pyridylmethyl) amine] using a combination of low-temperature UV-vis and multinuclear ((1) H and (2) H) NMR spectroscopies. Complex 1, or its pyrrole-deuterated analogue ((6) L-d (8)) Fe (II) (1-d (8)), exhibits downfield shifted pyrrole resonances (delta 28-60 ppm) in all solvents utilized [CH (2) Cl (2), (CH (3))(2) C (O), CH (3) CN, 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, ((6) L) Fe (II) (1) exhibits non-pyrrolic upfield and downfield shifted peaks in CH (2) Cl (2), (CH (3))(2) C (O), and CH (3) CN 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, ((6) L) 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 ((6) L) 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 ((6) L) Fe (II) (1) [UV-vis: lambda (max) = 424 (Soret), 542 nm] also occurs to give a similar adduct ((6) L) Fe (III)-(O (2)(-)) (2) [UV-vis: lambda (max) = 418 (Soret), 537 nm. (2) H 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 ((6) L) Fe (III)-OH (3) [UV-vis: lambda (max) = 412-414 (Soret), 566-575 nm; approximately delta (pyrrole) 120 ppm at 193 K]. This study on the O (2)-binding chemistry of the heme-only homonuclear ((6) L) Fe (II) (1) system lays the foundation for a more complete understanding of the dioxygen reactivity of heterobinuclear heme-Cu complexes, such as [((6) L) Fe (II) Cu (I)](+), which are models for cytochrome c oxidase. |
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