The O–O bond length is often used as a structural indicator to determine the valence states of bound O₂ ligands in biological metal–dioxygen intermediates and related biomimetic complexes. Here, we report very distinct O–O bond lengths found for three crystallographic forms (1.229(4), 1.330(4), 1.387(2) Å at 100 K) of a side-on iron–dioxygen species. Despite their different O–O bond distances, all forms possess the same electronic structure of Fe(III)–O₂^•–, as evidenced by their indistinguishable spectroscopic features. Density functional theory and ab initio calculations, which successfully reproduce spectroscopic parameters, predict a flat potential energy surface of an η²-O₂ motif binding to the iron center regarding the O–O distance. Therefore, the discrete O–O bond lengths observed likely arise from differential intermolecular interactions in the second coordination sphere. The work suggests that the O–O distance is not a reliable benchmark to unequivocally identify the valence state of O₂ ligands for metal–dioxygen species in O₂-utilizing metalloproteins and synthetic complexes.

中文翻译：

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具有离散 O-O 键距离的侧面 Fe(III)-Superoxo 复合物的结构和光谱证据

O-O 键长通常用作结构指标来确定生物金属-双氧中间体和相关仿生复合物中结合的 O ₂配体的价态。在这里，我们报告了在侧面铁-双氧物种的三种结晶形式（1.229(4)、1.330(4)、1.387(2) Å，在 100 K）中发现的非常不同的 O-O 键长。尽管它们的 O-O 键距不同，但所有形式都具有相同的 Fe(III)-O ₂ ^•-电子结构，正如它们难以区分的光谱特征所证明的那样。密度泛函理论和 ab initio 计算成功地再现了光谱参数，预测了 η ² -O ₂的平坦势能面关于 O-O 距离的与铁中心结合的基序。因此，观察到的离散 O-O 键长可能来自第二配位球中的不同分子间相互作用。该工作表明，O-O 距离不是明确确定O ₂金属蛋白和合成复合物中金属-双氧物种的 O ₂配体价态的可靠基准。