Iron Complexes Capped with Dendrimer-Appended Triazacyclononanes as the Novel Spatially Encumbered Models of Non-Heme Iron Proteins

Poly(benzyl ether) dendrimers with a 1,4,7-triazacyclononane (TACN) focal core (Ln3TACN, 2a−4a) and nondendritic L13TACN (1a), upon reaction with FeCl2, followed by NaOAc and NH4PF6, afforded
mononuclear iron(II) complexes [FeII(η2-OAc)(Ln3TACN)]+ (1b−4b), which were oxidized under O2 to form
dinuclear (μ-O)(μ-OAc)2diiron(III) complexes (1c−4c) in 54−74% isolated yields. The formation of 1c−4c
obeyed second-order kinetics with respect to 1b−4b, respecitvely, where the observed rate constants (k2)
were clearly dependent on the generation number of the dendritic substituents. Photoirradiation of 1c−4c
in the presence of NaOAc gave diiron(II) complexes (1d−4d), which were reoxidized to 1c−4c by O2,
following first-order kinetics with respect to 1d−4d, respectively. The crystal structure of nondendritic 1c
showed that the diiron(III) center is surrounded by an aromatic wall of the six 3,5-dimethoxybenzyl
substituents, while spectroscopic profiles of dendritic 2c−4c suggested that the geometries of their diiron(III)
centers are little different from that of 1c. The diiron(III) center of the largest 4c was highly robust toward
alkaline hydrolysis and also insulated electrochemically.

以1,4,7-三氮杂环壬烷（1,4,7-triazacyclononane, TACN）为中心核的聚(苄醚)树状大分子（Ln3TACN，2a−4a）以及无树状结构的L13TACN（1a），先与氯化亚铁（FeCl₂）反应，再依次加入乙酸钠（NaOAc）与六氟磷酸铵（NH4PF6），可制得单核铁(II)配合物[FeII(η²-OAc)(Ln3TACN)]+（1b−4b）。上述配合物在氧气氛围下被氧化，以54%~74%的分离产率得到双核(μ-氧)(μ-乙酸根)₂-三(III)铁配合物（1c−4c）。1c−4c的生成对1b−4b分别遵循二级动力学规律，其表观速率常数(k₂)明显依赖于树状取代基的代数。对1c−4c在乙酸钠存在下进行光辐照，可得到双核铁(II)配合物（1d−4d）；该配合物可被氧气重新氧化为1c−4c，此过程对1d−4d分别遵循一级动力学规律。无树状结构的1c的晶体结构显示，其双核铁(III)中心被六个3,5-二甲氧基苄基取代基的芳香环壁所包裹；而树状大分子衍生物2c−4c的光谱特征表明，其双核铁(III)中心的几何构型与1c几乎无差异。尺寸最大的4c的双核铁(III)中心对碱性水解具有极高稳定性，同时具备电化学绝缘特性。