Use of sterically hindered carboxylate ligands to model structural and functional features of dioxygen-activating centers in non-heme diiron enzymes

Abstract

Chapter I. Modeling Dioxygen-Activating Centers in Non-Heme Diiron Enzymes: Carboxylate Shifts in Diiron(II) Complexes Supported by Sterically Hindered Carboxylate Ligands General synthetic routes are described for a series of diiron(II) complexes supported by sterically demanding carboxylate ligands 2,6-di(p-tolyl)benzoate (ArTolCO2-) and 2,6-di(4-fluorophenyl)benzoate (Ar4-FPhCO2-). The interlocking nature of the m-terphenyl units in self-assembled [Fe2(p-O2CArTol)2(O2CArTol)2L2] (L = C5H5N (4); 1-MeIm (5)) promotes the formation of coordination geometries analogous to those of the non-heme diiron cores in the enzymes RNR-R2 and A9D. Magnetic susceptibility and MOssbauer studies of 4 and 5 revealed properties consistent with weak antiferromagnetic coupling between the high-spin iron(II) centers. Structural studies of several derivatives obtained by ligand substitution reactions demonstrated that the [Fe2([mu]O2CAr')2L2] (Ar'=ArTol; Ar 4-FPh) module is geometrically quite flexible. Details of the core rearrangement within the tetracarboxylate diiron framework, facilitated by carboxylate shifts, were probed by solution VT 19F-NMR spectroscopic studies of [Fe2(ap-O2CAr4-FPh)2(O2CAr4-FPh)2(THF)2] (8) and [Fe2(p-O2CAr4-FPh)4(4-tBuC5H4N)2] (12). The dynamic motion in the primary coordination sphere controls the positioning of open sites and regulates the access of exogenous ligands, processes that also occur at the catalytic sites of non-heme diiron enzymes. Chapter II. Structural Flexibility within a Sterically Hindered Ligand Platform: Mononuclear Iron(II) Carboxylate Complexes as Subsite Models for Diiron(II) Centers The synthesis and characterization of a series of mononuclear iron(II) carboxylate complexes are described.

(cont.) By using sterically hindered carboxylate ligands, 2,6-di(p-tolyl)benzoate (ArTolCO2-) and 2,6-di(4-tert-butylphenyl)benzoate (Ar4-tBPhCO2-), series of four-, five-, and six-coordinate iron(II) complexes were synthesized. The compounds are [Fe(O2CArTol)2(1-BnIm)2] (3), [Fe(O2CArTol)2(1-MeBzIm)2] (4), [Fe(02C-Ar4-tBuPh)2(2,2'-bipy)2] (5), [Fe(O2CArTol)2(TMEDA)] (6), and [Fe(O2CArTol)2(BPTA)] (7). Structural analyses of 3-7 revealed that the overall stereochemistry of the [Fe(O2CAr')2Ln] units is dictated by electronic and steric factors of the N-donor ligands (L), as well as by the flexible coordination of the carboxylate ligands. Distinctive MOss-Bauer parameters obtained for these and related compounds facilitated the spectral assignment of a diiron(II) complex having asymmetric metal sites, [Fe2(p-02CArTol)3(2CArTol)(2,6-lutidine)] (2). Well-defined mononuclear iron carboxylate complexes thus may serve as subsite models for higher nuclearity species in both synthetic and biological systems. Chapter III. Functional Mimic of Dioxygen-Activating Centers in Non-Heme Diiron Enzymes: Mechanistic Implications of Paramagnetic Intermediates in the Reactions between Diiron(II) Complexes and Dioxygen Tetracarboxylate diiron(II) complexes, [Fe2(-O02CArTOl)2(02CArToll)2(C5H5N)2] (la) and [Fe2(Pl-02CArTol)4(4-tBuC5H4N)2] (2a), where ArTloCO2- = 2,6-di(p-tolyl)benzoate, react with 02 in CH2C12 at -78 C to afford deep green intermediates ...