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(μ-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 Δ9D. Magnetic susceptibility and Mössbauer 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(O2CAr‘)4L2] (Ar‘ = ArTol; Ar4-FPh) module is
geometrically flexible. Details of ligand migration within the tetracarboxylate diiron core, facilitated by carboxylate
shifts, were probed by solution variable-temperature 19F NMR spectroscopic studies of [Fe2(μ-O2CAr4-FPh)2-(O2CAr4-FPh)2(THF)2] (8) and [Fe2(μ-O2CAr4-FPh)4(4-tBuC5H4N)2] (12). Dynamic motion in the primary coordination
sphere controls the positioning of open sites and regulates the access of exogenous ligands, processes that also
occur in non-heme diiron enzymes during catalysis.

本文报道了一系列由位阻型羧酸配体2,6-二(对甲苯基)苯甲酸根(ArTolCO2⁻)和2,6-二(4-氟苯基)苯甲酸根(Ar4-FPhCO2⁻)支撑的二铁(II)配合物的通用合成路线。自组装得到的[Fe2(μ-O2CArTol)2(O2CArTol)2L2]（L = 吡啶(C5H5N, 4)；1-甲基咪唑(1-MeIm, 5)）中，间三联苯单元的互锁结构促进了配位几何构型的形成，该构型与酶RNR-R2和Δ9D中的非血红素二铁核心结构相似。对配合物4和5的磁化率与穆斯堡尔(Mössbauer)光谱研究表明，其性质与高自旋铁(II)中心之间存在弱反铁磁耦合的结论一致。通过配体取代反应获得的多种衍生物的结构研究表明，[Fe2(O2CAr')4L2]（Ar' = ArTol；Ar4-FPh）模块具有几何柔性。通过对配合物[Fe2(μ-O2CAr4-FPh)2(O2CAr4-FPh)2(四氢呋喃(THF))2] (8)和[Fe2(μ-O2CAr4-FPh)4(4-叔丁基吡啶(4-tBuC5H4N))2] (12)的溶液变温19F核磁共振(NMR)光谱研究，我们探究了由羧酸根位移驱动的四羧酸根二铁核心内配体迁移的详细过程。一级配位球内的动态运动控制着开放位点的排布，并调控外源性配体的进入过程，这一过程同样发生在催化过程中的非血红素二铁酶中。