Institut für Anorganische Chemie, Georg-August-Universität , Tammannstr. 4, 37077 Göttingen, Germany.
J Am Chem Soc. 2013 Sep 18;135(37):13892-9. doi: 10.1021/ja406176e. Epub 2013 Sep 10.
The Siamese-twin porphyrin (2H4) is a unique pyrazole-expanded porphyrin providing two adjacent cavities each offering an {N4} binding motif. It was previously found to form stable dicopper(II) or dinickel(II) complexes where both metal ions are nested in a porphyrin-like environment. This work addresses the rich redox chemistry of the dicopper complex 2Cu2 that originates from the redox synergy of two proximate metal ions in combination with the potentially non-innocent expanded porphyrin ligand. Complementing previous X-ray crystallographic and SQUID data for solid material, the electronic structure of parent 2Cu2 in solution was now investigated by MCD and EPR spectroscopy. This allowed the assignment of UV-vis absorptions and confirmed the drastic twist of the molecule with ferromagnetically coupled copper(II) ions. 2Cu2 was found to exhibit multiple redox events in the potential range from -2.4 to +1.7 V versus Fc/Fc(+), and singly oxidized 2Cu2 as well as doubly oxidized 2Cu2 were characterized in detail by various analytical and spectroscopic methods. 2Cu2 was found by EPR spectroscopy and DFT calculations to have an S = 1/2 ground state, while 2Cu2 is diamagnetic. Single crystal X-ray crystallography of 2Cu2(acetone)22 revealed that the 2Cu2 core is structurally invariant upon two-fold oxidation, while XAS measurements at the Cu K-edge for 2Cu2 and 2Cu2(acetone)22 showed that the copper ions remain in the +2 oxidation state throughout. The combined experimental and computational evidence identified the Siamese-twin porphyrin as a multi-electron redox-active ligand with hidden non-innocence. Each ligand subunit upon oxidation forms a ligand-centered radical, though the spin vanishes because of covalency and strong antiferromagnetic coupling between the ligand radical and the proximate metal ion. Complexes of the Siamese-twin porphyrin may thus serve as a valuable bioinspired platform that combines both metal-ligand and two-metal-ion cooperativities for use in multi-electron processes.
暹罗联体卟啉（2H4）是一种独特的吡唑啉扩展卟啉，提供两个相邻的腔，每个腔提供一个{N4}结合基序。先前发现它可以形成稳定的二铜（II）或二镍（II）配合物，其中两个金属离子都嵌套在卟啉样环境中。这项工作解决了二铜配合物 2Cu2 的丰富氧化还原化学，它源自两个相邻金属离子的氧化还原协同作用，以及潜在的非中性扩展卟啉配体。除了先前的固体材料 X 射线晶体学和 SQUID 数据外，现在还通过 MCD 和 EPR 光谱研究了母体 2Cu2 在溶液中的电子结构。这允许分配 UV-vis 吸收，并证实了分子的剧烈扭曲与铁磁耦合的铜（II）离子。在 -2.4 至 +1.7 V 相对于 Fc/Fc(+)的电势范围内，发现 2Cu2 表现出多个氧化还原事件，并且详细地通过各种分析和光谱方法表征了单氧化2Cu2和双氧化2Cu2。通过 EPR 光谱和 DFT 计算发现2Cu2具有 S = 1/2 的基态，而2Cu2是抗磁性的。2Cu2(丙酮)22 的单晶 X 射线晶体学表明，2Cu2 核在两倍氧化时结构不变，而对于 2Cu2 和2Cu2(丙酮)22 的 Cu K 边 XAS 测量表明，铜离子在整个过程中保持+2 氧化态。实验和计算相结合的证据表明，暹罗联体卟啉是一种具有隐藏非中性的多电子氧化还原活性配体。每个配体亚基在氧化时形成配体中心自由基，尽管由于共价键合和配体自由基与相邻金属离子之间的强反铁磁耦合，自旋消失。因此，暹罗联体卟啉的配合物可以作为一种有价值的仿生平台，结合金属-配体和双金属离子协同作用，用于多电子过程。
