Metallocene Complexes of Iron and Cobalt Derived from the 4,4‘-Bis(η⁵-cyclopentadienyl)octafluorobiphenyl Ligand¹

Abstract

The reaction of decafluorobiphenyl with 10 equiv of sodium cyclopentadienide (NaCp) in THF (65 °C, 2 h) afforded 4,4‘-bis(cyclopentadienyl)octafluorobiphenyl (1) as a mixture of double-bond isomers in 72% yield after workup and purification. Subsequent treatment with sodium hydride in THF (25 °C, 6 h) afforded disodium 4,4‘-bis(cyclopentadienyl)octafluorobiphenyl (2). Reaction of NaCp, ligand 2, and FeBr2 (21:1:10 mol ratio) in THF afforded mostly ferrocene as well as the diiron complex CpFe(η5-C5H4-4,4‘-C6F4C6F4-η5-C5H4)FeCp (3), the linear triiron complex Cp[Fe(η5-C5H4-4,4‘-C6F4C6F4-η5-C5H4)]2FeCp (4), and the linear tetrairon complex Cp[Fe(η5-C5H4-4,4‘-C6F4C6F4-η5-C5H4)]3FeCp (5), which were isolated by liquid chromatography. Solution voltammetric analysis of 3 showed a single, reversible two-electron oxidation at +200 mV, 4 showed two reversible oxidations (2:1 ratio) at +180 and +372 mV, respectively, and 5 showed two reversible oxidations (2:2 ratio) at +192 and +382 mV, respectively (all vs Cp2Fe|Cp2Fe+). A similar reaction of NaCp, ligand 2, and CoBr2 in THF was worked up with air and dilute aqueous HCl followed by aqueous KPF6 to afford a mixture of cobaltocenium hexafluorophosphates, from which [Cp2Co]+PF6- and the dicobalt complex [CpCo(η5-C5H4-4,4‘-C6F4C6F4-η5-C5H4)CoCp]2+[PF6-]2 (6) were isolated by liquid chromatography. CV analysis of 6 showed a single, reversible CoII|CoIII wave at +232 mV relative to Cp2Co|Cp2Co+. These voltammetric data suggest that no significant electronic communication exists between two metals bridged by this conjugated dibasic ligand (2). A crystal structure of diiron complex 3 demonstrated the conformational preferences of the CpC6F4C6F4Cp ligand in the solid state with respective torsional angles of 37.5°, 50.1°, and 32.1°.