Department of Chemistry and Biochemistry, Rowan University, 201 Mullica Hill Rd, Glassboro, NJ 08028, USA.
Phys Chem Chem Phys. 2013 Feb 28;15(8):2669-83. doi: 10.1039/c2cp42876h.
The ferrocene/ferrocenium (Fc/Fc(+)) redox couple has been deemed one of the best candidates of the standard electrochemical reference for redox reactions in ionic liquids (ILs). To fully reveal the redox reaction mechanism in the IL condensed phase, solvation, as a prerequisite, apparently needs to be understood. As a preliminary attempt to study the solvation of Fc/Fc(+) in imidazolium-based ILs, using model complexes, systematic investigations of the intrinsic pair-wise interactions between the solute and solvent ions in the gas phase are performed with B97-D, a dispersion-corrected density functional method. B97-D is carefully benchmarked with available experimental values and high-level ab initio quantum mechanical results for Fc/Fc(+) alone and the solvated structures. Present calculations demonstrate dramatically different solvation features for Fc vs. Fc(+), e.g., Fc forms direct stable interactions with both solvent cations and anions with binding energies of -11 to -14 kcal mol(-1). As an interesting observation, no π-stacking interactions are captured between the imidazolium ring of the solvent cation and the cyclopentadienyl motif of the solute. However, due to the strong electrostatic interactions, Fc(+) can only be solvated by solvent anions in the first solvation shell with much stronger binding energies of -72 to -86 kcal mol(-1). Binding energies, structures, and electrostatic potentials are all characterized. The electronic structures are further analyzed with the Natural Bond Orbital method. The theoretical calculation results help explain several experimental observations obtained earlier or in the present work, including solubility, diffusion coefficient, and solution conductivity. Due to the involvement of the aromatic structures, the current study also sheds valuable light on π-interactions, in general.
二茂铁/二茂铁正离子（Fc/Fc(+)）氧化还原对被认为是离子液体（ILs）中氧化还原反应的最佳标准电化学参比之一。为了充分揭示 IL 凝聚相中氧化还原反应的机制，溶剂化显然需要被理解。作为在咪唑基 IL 中研究 Fc/Fc(+)溶剂化的初步尝试，使用模型配合物，在气相中系统地研究了溶质与溶剂离子之间的固有成对相互作用。采用具有色散校正的密度泛函方法 B97-D 对其进行了仔细的基准测试，可获得 Fc/Fc(+)单独和溶剂化结构的可用实验值和高级从头算量子力学结果。目前的计算表明 Fc 与 Fc(+)的溶剂化特征明显不同，例如，Fc 与溶剂阳离子和阴离子都形成直接稳定的相互作用，结合能为-11 至-14 kcal/mol。有趣的是，在溶剂阳离子的咪唑环和溶质的环戊二烯基结构之间没有捕捉到π堆积相互作用。然而，由于强静电相互作用，Fc(+)只能在第一溶剂化壳层中被溶剂阴离子溶剂化，其结合能为-72 至-86 kcal/mol。结合能、结构和静电势都进行了表征。进一步用自然键轨道方法分析了电子结构。理论计算结果有助于解释早期或当前工作中获得的几个实验观察结果，包括溶解度、扩散系数和溶液电导率。由于涉及芳香结构，当前的研究也普遍揭示了π相互作用。
