Electrooxidation of [(η5-C5H5)Fe(CO)2]2 As a Probe of the Nucleophilic Properties of Ionic Liquid Anions

详细信息    查看全文

• 作者：Angel A. J. Torriero ; Muhammad J. A. Shiddiky ; John P. Bullock ; John F. Boas ; Douglas R. MacFarlane ; Alan M. Bond
• 刊名：Inorganic Chemistry
• 出版年：2010
• 出版时间：March 1, 2010
• 年：2010
• 卷：49
• 期：5
• 页码：2502-2511
• 全文大小：1034K
• 年卷期：v.49,no.5(March 1, 2010)
• ISSN：1520-510X

文摘

The oxidative electrochemistry of [CpFe(CO)₂]₂, 1 (Cp = [ηp>5p>-C₅H₅]p>–p>), 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 1p>+p> followed by attack from the IL anion. In [NTf₂]p>–p>-based ILs ([NTf₂]p>–p> = bis(trifluoromethylsulfonyl)amide), the process shows nearly ideal chemical reversibility as the reaction between 1p>+p> and [NTf₂]p>–p> is very slow. This is highly significant, as 1p>+p> 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][PF₆], the oxidation of 1 is still largely reversible, but there is more pronounced evidence of [PF₆]p>–p> 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 [NTf₂]p>–p>-based ILs as a supporting electrolyte in CH₂Cl₂ 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]₂ (Cp* = [ηp>5p>-C₅(CH₃)₅]p>–p>). These cleanly yielded the corresponding binuclear radical-cation species, 1p>+p> and 1*p>+p>, 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.