Electronically-coupled redox centers in trimetallic cobalt complexes
Electronically-coupled redox centers in trimetallic cobalt complexes
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DOI：
10.1039/d1dt03404a
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发表时间：
2022-03-11
影响因子：
4
通讯作者：
Marinescu, Smaranda C.
中科院分区：
文献类型：
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作者：
Intrator, Jeremy A.;Orchanian, Nicholas M.;Marinescu, Smaranda C.
金属 - 有机框架（MOFs）分子构建单元的合成与分离可为表征提供独特的机会，否则由于MOFs的异质性这些机会是无法获得的。在此，我们报道了一系列含二硫烯配体、三亚苯基 - 2,3,6,7,10,11 - 六硫醇盐（THT）（1(3+)）和苯六硫醇盐（BHT）（2(3+)）的三核钴配合物，其中1,1,1 - 三（二苯基膦甲基）乙烷（三膦）用作封端配体。1(3+)和2(3+)的单晶X射线分析显示三个五配位钴中心以扭曲的四方锥几何构型与三膦和二硫烯配体键合。1(3+)和2(3+)的循环伏安研究揭示了由于氧化还原活性金属中心（Co - III/II）的顺序还原而形成混合价态相关的三个氧化还原特征。利用这些电化学数据，确定了1和2的共比例值（1的log K - c = 1.4和1.5，2的log K - c = 4.7和5.8），表明金属中心有强的共振稳定耦合，且与配合物1相比，配合物2观察到更强的电子耦合。还在不同极性的溶剂中进行了循环伏安研究，结果发现1和2的标准电位差（ΔE - 1/2）随溶剂极性而变化，表明电化学介质的介电常数与混合价态物质的稳定性之间呈负相关。对配合物1和2原位生成的多价（MV）态的光谱电化学研究显示出特征性的近红外区间电荷转移（IVCT）带，对配合物2的IVCT跃迁分析表明是一种弱耦合的II类多价物质以及相对较大的电子耦合因子（2(2+)的第一个多价态为1700 cm(-1)，2(+)的第二个多价态为1400和4000 cm(-1)）。密度泛函理论（DFT）计算表明配合物1(3+)、2(3+)和3(+)前沿轨道的相对能量有显著偏差，这与使用五甲基环戊二烯基（Cp*）作为封端配体的类似三核钴二硫烯配合物（分别为Co3Cp*3THT和Co3Cp*3BHT）计算结果形成对比，并且可能是配合物1(3+)、2(3+)和3(+)阳离子性质的结果。
Synthesis and isolation of molecular building blocks of metal-organic frameworks (MOFs) can provide unique opportunities for characterization that would otherwise be inaccessible due to the heterogeneous nature of MOFs. Herein, we report a series of trinuclear cobalt complexes incorporating dithiolene ligands, triphenylene-2,3,6,7,10,11-hexathiolate (THT) (1(3+)), and benzene hexathiolate (BHT) (2(3+)), with 1,1,1,-tris(diphenylphosphinomethyl)ethane (triphos) employed as the capping ligand. Single crystal X-ray analyses of 1(3+) and 2(3+) display three five-coordinate cobalt centers bound to the triphos and dithiolene ligands in a distorted square pyramidal geometry. Cyclic voltammetry studies of 1(3+) and 2(3+) reveal three redox features associated with the formation of mixed valence states due to the sequential reduction of the redox-active metal centers (Co-III/II). Using this electrochemical data, the comproportionality values were determined for 1 and 2 (log K-c = 1.4 and 1.5 for 1, and 4.7 and 5.8 for 2), suggesting strong resonance-stabilized coupling of the metal centers, with stronger electronic coupling observed for complex 2 compared to that for complex 1. Cyclic voltammetry studies were also performed in solvents of varying polarity, whereupon the difference in the standard potentials (Delta E-1/2) for 1 and 2 was found to shift as a function of the polarity of the solvent, indicating a negative correlation between the dielectric constant of the electrochemical medium and the stability of the mixed valence species. Spectroelectrochemical studies of in situ generated multi-valent (MV) states of complexes 1 and 2 display characteristic NIR intervalence charge transfer (IVCT) bands, and analysis of the IVCT transitions for complex 2 suggests a weakly coupled class II multi-valent species and relatively large electronic coupling factors (1700 cm(-1) for the first multi-valent state of 2(2+), and 1400 and 4000 cm(-1) for the second multi-valent state of 2(+)). Density functional theory (DFT) calculations indicate a significant deviation in relative energies of the frontier orbitals of complexes 1(3+), 2(3+), and 3(+) that contrasts those calculated for the analogous trinuclear cobalt dithiolene complexes employing pentamethylcyclopentadienyl (Cp*) as the capping ligand (Co3Cp*3THT and Co3Cp*3BHT, respectively), and may be a result of the cationic nature of complexes 1(3+), 2(3+), and 3(+).