Curtis Clayton J, Astashkin Andrei V, Conradie Jeanet, Ghosh Abhik, Tomat Elisa
Department of Chemistry and Biochemistry, The University of Arizona, 1306 East University Blvd., Tucson, Arizona 85721, United States.
Department of Chemistry, University of the Free State, P.O. Box 339, Bloemfontein 9300, Republic of South Africa.
Inorg Chem. 2021 Aug 16;60(16):12457-12466. doi: 10.1021/acs.inorgchem.1c01691. Epub 2021 Aug 4.
Oligopyrroles form a versatile class of redox-active ligands and electron reservoirs. Although the stabilization of radicals within oligopyrrolic π systems is more common for macrocyclic ligands, bidentate dipyrrindiones are emerging as compact platforms for one-electron redox chemistry in transition-metal complexes. We report the synthesis of a bis(aqua) palladium(II) dipyrrindione complex and its deprotonation-driven dimerization to form a hydroxo-bridged binuclear complex in the presence of water or triethylamine. Electrochemical, spectroelectrochemical, and computational analyses of the binuclear complex indicate the accessibility of two quasi-reversible ligand-centered reduction processes. The product of a two-electron chemical reduction by cobaltocene was isolated and characterized. In the solid state, this cobaltocenium salt features a folded dianionic complex that maintains the hydroxo bridges between the divalent palladium centers. X-band and Q-band EPR spectroscopic experiments and DFT computational analysis allow assignment of the dianionic species as a diradical with spin density almost entirely located on the two dipyrrindione ligands. As established from the EPR temperature dependence, the associated exchange coupling is weak and antiferromagnetic ( ≈ -2.5 K), which results in a predominantly triplet state at the temperatures at which the measurements have been performed.
寡聚吡咯形成了一类多功能的氧化还原活性配体和电子储存库。尽管自由基在寡聚吡咯π体系中的稳定化在大环配体中更为常见，但双齿二吡咯二酮正成为过渡金属配合物中单电子氧化还原化学的紧凑平台。我们报道了一种双（水合）钯（II）二吡咯二酮配合物的合成及其在水或三乙胺存在下通过去质子化驱动的二聚反应，形成一种羟基桥连的双核配合物。对该双核配合物的电化学、光谱电化学和计算分析表明，存在两个准可逆的以配体为中心的还原过程。分离并表征了由二茂钴进行双电子化学还原的产物。在固态中，这种二茂钴盐具有一种折叠的双阴离子配合物，该配合物在二价钯中心之间保持着羟基桥。X波段和Q波段电子顺磁共振光谱实验以及密度泛函理论计算分析表明，该双阴离子物种为双自由基，自旋密度几乎完全位于两个二吡咯二酮配体上。根据电子顺磁共振对温度的依赖性确定，相关的交换耦合较弱且为反铁磁性（≈ -2.5 K），这导致在进行测量的温度下主要为三重态。
