Electrochemistry and Infrared Spectroelectrochemistry of [(η5-C5R5)Fe(CO)2]2 (R = H, Me): Generation and Characterization of [(η5-C5R5)Fe(CO)2]2(PF6) Complexes

John P. Bullock, Kent R. Mann, Michael C. Palazotto

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Abstract

The electrochemistry of [CpFe(CO)2]2 (Cp = η5-cyclopentadienyl) was reexamined under a variety of conditions. This compound, and the related species [Cp*Fe(CO)2]2 (Cp* = η5-pentamethylcyclopentadienyl), exhibit irreverisble net two-electron oxidations in CH3CN/TBAH (Ep,a = 0.68 and 0.33 V vs AgCl/Ag, respectively) but undergo two, one-electron oxidations in CH2Cl2/TBAH. For [CpFe(CO)2]2, these occur at 0.68 and 1.28 V, while in the [Cp*Fe(CO)2]2 complex they occur at 0.34 and 1.16 V. The first process is quasi-reversible for each compound and generates the corresponding binuclear radical cations. The binuclear radical cations were generated and characterized via UV-vis and infrared spectroelectrochemistry. [CpFe(CO)2]2+ exists as a mixture of the cis and trans carbonyl-bridged isomers in solution with v(CO) at 2023, 2055, and 1934 cm−1. [Cp*Fe(CO)2]2+ exists only as the trans isomer in solution with λmax = 518 nm and v(CO) at 1987 and 1884 cm−1. The binuclear radical-cation species [CpFe(CO)2]2+ has long been invoked as an intermediate in the oxidation chemistry of [CpFe(CO)2]2 but has never been cleanly generated or characterized prior to this work. EPR spectra were observed for both radical cations. [CpFe(CO)2]2+ has g = 2.004 and g= 2.084, while [Cp*Fe(CO)2]2+ has g= 1.999 and g = 2.088. These cations are susceptible to rapid, ligand-induced disproportionation reactions. When the ligand is acetonitrile, the proposed mechanisms of these reactions involve the generation and subsequent decomposition of acetonitrile adducts of the radical cation species, e.g., {[CpFe(CO)2]2(NCCH3))+. For the Cp* compound, the intermediate is short-lived. Stopped-flow measurements of the reaction of [Cp*Fe(CO)2]2+ with CH3CN showed that the reaction is first order in [[Cp*Fe(CO)2]2+] and [CH3CN] with k=118 (±2) M−1 s−1. Cyclic voltammetry studies detected the Cp-containing intermediate [CpFe(CO)2]2(NCCH3)}+. The initially formed decomposition products, in the case of [CpFe- (CO)2]2+, are the 18-electron cationic species [CpFe(CO)2(NCCH3)]+ and the 17-electron radical CpFe(CO)2. In subsequent reactions, the latter species generates [CpFe(CO)2]2 and, to a lesser extent, CpFe(CO)2Cl.

Original languageEnglish (US)
Pages (from-to)1284-1293
Number of pages10
JournalInorganic chemistry
Volume30
Issue number6
DOIs
StatePublished - Mar 1 1991

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