Gautam Ritika, Astashkin Andrei V, Chang Tsuhen M, Shearer Jason, Tomat Elisa
Department of Chemistry and Biochemistry, The University of Arizona , Tucson, Arizona 85721, United States.
Department of Chemistry, University of Nevada , Reno, Nevada 89577, United States.
Inorg Chem. 2017 Jun 5;56(11):6755-6762. doi: 10.1021/acs.inorgchem.7b01030. Epub 2017 May 12.
The ability of tetrapyrrolic macrocycles to stabilize unpaired electrons and engage in π-π interactions is essential for many electron-transfer processes in biology and materials engineering. Herein, we demonstrate that the formation of π dimers is recapitulated in complexes of a linear tripyrrolic analogue of naturally occurring pigments derived from heme decomposition. Hexaethyltripyrrindione (HTD1) coordinates divalent transition metals (i.e., Pd, Cu, Ni) as a stable dianionic radical and was recently described as a robust redox-active ligand. The resulting planar complexes, which feature a delocalized ligand-based electronic spin, are stable at room temperature in air and support ligand-based one-electron processes. We detail the dimerization of neutral tripyrrindione complexes in solution through electron paramagnetic resonance (EPR) and visible absorption spectroscopic methods. Variable-temperature measurements using both EPR and absorption techniques allowed determination of the thermodynamic parameters of π dimerization, which resemble those previously reported for porphyrin radical cations. The inferred electronic structure, featuring coupling of ligand-based electronic spins in the π dimers, is supported by density functional theory (DFT) calculations.
四吡咯大环稳定未成对电子并参与π-π相互作用的能力对于生物学和材料工程中的许多电子转移过程至关重要。在此，我们证明了在源自血红素分解的天然色素的线性三吡咯类似物的配合物中重现了π二聚体的形成。六乙基三吡咯二酮（HTD1）作为稳定的二价阴离子自由基配位二价过渡金属（即Pd、Cu、Ni）并且最近被描述为一种强大的氧化还原活性配体。所得平面配合物具有基于配体离域的电子自旋，在室温下空气中稳定并支持基于配体的单电子过程。我们通过电子顺磁共振（EPR）和可见吸收光谱方法详细研究了溶液中中性三吡咯二酮配合物的二聚化。使用EPR和吸收技术进行的变温测量允许确定π二聚化的热力学参数，其类似于先前报道的卟啉自由基阳离子的数据。密度泛函理论（DFT）计算支持了所推断的电子结构，其特征是π二聚体中基于配体的电子自旋耦合。
