Department of Chemistry, Simon Fraser University, 8888 University Drive, Burnaby, British Columbia, Canada.
Chemistry. 2013 Jul 15;19(29):9606-18. doi: 10.1002/chem.201300798. Epub 2013 Jun 7.
The geometric and electronic structure of an oxidized bimetallic Ni complex incorporating two redox-active Schiff-base ligands connected via a 1,2-phenylene linker has been investigated and compared to a monomeric analogue. Information from UV/Vis/NIR spectroscopy, electron paramagnetic resonance (EPR) spectroscopy, electrochemistry, and density functional theory (DFT) calculations provides important information on the locus of oxidation for the bimetallic complex. The neutral bimetallic complex is conformationally dynamic at room temperature, which complicates characterization of the oxidized forms. Comparison to an oxidized monomer analogue 1 provides critical insight into the electronic structure of the oxidized bimetallic complex 2. Oxidation of 1 provides 1˙, which is characterized as a fully delocalized ligand radical complex; the spectroscopic signature of this derivative includes an intense NIR band at 4500 cm(-1). Oxidation of 2 to the bis-oxidized form affords a bis-ligand radical species 2˙˙. Variable temperature EPR spectroscopy of 2˙˙ shows no evidence of coupling, and the triplet and broken symmetry solutions afforded by theoretical calculations are essentially isoenergetic. 2˙˙ is thus best described as incorporating two non-interacting ligand radicals. Interestingly, the intense NIR intervalence charge transfer band observed for the delocalized ligand-radical 1˙ exhibits exciton splitting in 2˙˙, due to coupling of the monomer transition dipoles in the enforced oblique dimer geometry. Evaluating the splitting of the intense intervalence charge transfer band can thus provide significant geometric and electronic information in less rigid bis-ligand radical systems. Addition of excess pyridine to 2˙˙ results in a shift in the oxidation locus from a bis-ligand radical species to the Ni(III) /Ni(III) derivative 2(py)4, demonstrating that the ligand system can incorporate significant bulk in the axial positions.
包含两个通过 1,2-亚苯基连接体连接的氧化还原活性希夫碱配体的氧化双金属 Ni 配合物的几何和电子结构已被研究，并与单体类似物进行了比较。来自紫外/可见/近红外光谱、电子顺磁共振（EPR）光谱、电化学和密度泛函理论（DFT）计算的信息提供了有关双金属配合物氧化位置的重要信息。中性双金属配合物在室温下构象动态，这使得氧化形式的特征复杂化。与氧化单体类似物 1 的比较为氧化双金属配合物 2 的电子结构提供了重要的见解。1 的氧化提供了1˙，其被表征为完全离域的配体自由基配合物；该衍生物的光谱特征包括在 4500 cm(-1)处的强近红外带。2 氧化为双氧化形式得到双配体自由基物种2˙˙。2˙˙的变温 EPR 光谱没有显示出偶合的证据，理论计算给出的三重态和破对称解基本上是等能量的。因此，2˙˙最好被描述为包含两个非相互作用的配体自由基。有趣的是，对于离域配体自由基1˙观察到的强近红外区间电荷转移带在强制斜二聚体几何形状下，由于单体跃迁偶极子的耦合，在2˙˙中表现出激子分裂。因此，评估强区间电荷转移带的分裂可以为不太刚性的双配体自由基体系提供重要的几何和电子信息。向2˙˙中添加过量吡啶会导致氧化位置从双配体自由基物种转移到 Ni(III)/Ni(III)衍生物2(py)4，表明配体系统可以在轴向位置容纳大量的物质。
