School of Chemistry, Monash University, Clayton, Victoria 3800, Australia.
Inorg Chem. 2010 Mar 1;49(5):2502-11. doi: 10.1021/ic9025238.
The oxidative electrochemistry of CpFe(CO)(2), 1 (Cp = eta(5)-C(5)H(5)), was examined in detail in ionic liquids (ILs) composed of ions of widely varying Lewis acid-base properties. Cyclic voltammetric responses were strongly dependent on the nucleophilic properties of the IL anion, but all observations are consistent with the initial formation of 1(+) followed by attack from the IL anion. In NTf(2)-based ILs (NTf(2) = bis(trifluoromethylsulfonyl)amide), the process shows nearly ideal chemical reversibility as the reaction between 1(+) and NTf(2) is very slow. This is highly significant, as 1(+) is known to be highly susceptible to nucleophilic attack and its stability indicates a remarkable lack of coordinating ability of these ILs. In 1-methyl-3-butylimidazolium hexafluorophosphate, [bmim][PF(6)], the oxidation of 1 is still largely reversible, but there is more pronounced evidence of PF(6) coordination. In contrast, 1 exhibits an irreversible two-electron oxidation process in a dicyanamide-based IL. This overall oxidation process is thought to proceed via an ECE mechanism, details of which are presented. Rate constants were estimated by fitting the experimental data to digital simulations of the proposed mechanism. The use of NTf(2)-based ILs as a supporting electrolyte in CH(2)Cl(2) was examined by using this solvent/electrolyte as a medium in which to perform bulk electrolyses of 1 and 1*, the permethylated analogue Cp*Fe(CO)(2) (Cp* = eta(5)-C(5)(CH(3))(5)). These cleanly yielded the corresponding binuclear radical-cation species, 1(+) and 1*(+), which were subsequently characterized by electron paramagnetic resonance (EPR) spectroscopy. In addition to the above oxidation studies, the reduction of 1 was studied in each of the ILs; differences in cathodic peak potentials are attributed, in part, to ion-pairing effects. This study illustrates the wide range of electrochemical environments available with ILs and demonstrates their utility for the investigation of the redox properties of metal carbonyls and other organometallic compounds.
CpFe(CO)(2)，1（Cp = eta(5)-C(5)H(5)) 的氧化电化学在由路易斯酸碱性质差异很大的离子组成的离子液体（ILs）中被详细研究。循环伏安响应强烈依赖于 IL 阴离子的亲核性质，但所有观察结果都与 1(+)的初始形成一致，然后是 IL 阴离子的攻击。在基于 NTf(2)的 ILs(NTf(2) = 双(三氟甲烷磺酰)酰胺)中，由于 1(+)与 NTf(2)之间的反应非常缓慢，因此该过程表现出几乎理想的化学可逆性。这是非常重要的，因为 1(+)已知容易受到亲核攻击，其稳定性表明这些 ILs缺乏配位能力。在 1-甲基-3-丁基咪唑六氟磷酸盐，[bmim][PF(6)]中，1 的氧化仍然在很大程度上是可逆的，但有更明显的证据表明 PF(6)的配位。相比之下，1 在基于二氰胺的 IL 中表现出不可逆的二电子氧化过程。该总氧化过程被认为是通过 ECE 机制进行的，详细信息已提出。通过将实验数据拟合到所提出机制的数字模拟，估计了速率常数。通过使用该溶剂/电解质作为 1 和 1*（CpFe(CO)(2)（Cp = [eta(5)-C(5)(CH(3))(5)](-）的均相电解的介质，检查了基于 NTf(2)的 IL 在 CH(2)Cl(2)中的作为支持电解质的用途。这两种方法都得到了相应的双核自由基阳离子物种 1(+)和 1*(+)，它们随后通过电子顺磁共振（EPR）光谱进行了表征。除了上述氧化研究外，还在每种 IL 中研究了 1 的还原；阴极峰电位的差异部分归因于离子对效应。该研究说明了 IL 提供的广泛电化学环境，并展示了它们在研究金属羰基和其他有机金属化合物的氧化还原性质方面的实用性。
