Coupled Redox Reactions, Linkage Isomerization, Hydride Formation, and Acid−Base Relationships in the Decaphenylferrocene System

Abstract

Decaphenylferrocene (DPF) exists in two isomeric forms, blue Fe(η6-C6H5−C5Ph4)(η5-C5Ph5) (DPF1), which is known to readily protonate on the ligand giving orange [Fe(η6-C6H5−C5HPh4)(η5-C5Ph5)]+, [HDPF1]+, and insoluble pink Fe(η5-C5Ph5)2 (DPF2). The redox chemistry is unusually complex for a ferrocene system because acid−base chemistry, formation of the hydride [HFe(η5-C5Ph5)2]+, [HDPF2]+, together with isomerization and radical abstraction reactions are coupled with the electron-transfer processes. The use of a wide range of voltammetric techniques, time domains, and temperatures and the application of techniques which identify the species in bulk solution enable the reversible half-wave potentials of at least five-electron-transfer processes to be identified. These are [DPF2]+ + e- ⇌ DPF2 (Er1/2 = −40 mV); [HDPF2]+ + e- ⇌ HDPF2 (Er1/2 = −220 mV at −60 °C), [DPF1]+ + e- ⇌ DPF1 (Er1/2 = −170 mV), [HDPF1]2+ + e- ⇌ [HDPF1]+ (Er1/2 = 710 mV), [HDPF1]+ + e- ⇌ HDPF1 (Er1/2 = −1480 mV), and additionally, C5HPh5 is reversibly oxidized at 765 mV (all potentials vs Fc+/Fc (Fc = Fe(η5-C5H5)2) at 20 °C. Only the first two processes are metal based, while the other processes are believed to have considerable ligand character. The wide range of reversible potentials, in particular the similar potentials of the [HDPF2]+/0 and [DPF1]+/0 processes, enable an extensive series of cross-redox reactions to accompany the electron-transfer processes which occur either at the electrode surface or in the bulk solution. The reaction pathways identified include the following: in the presence of acid, DPF1 is protonated on the ligand to give [HDPF1]+, DPF2 forms a Fe(IV) hydride, [HDPF2]+, which rearranges to [HDPF1]+, thereby providing a route for isomerization. In very strong acid, DPF2 is oxidized to [DPF2]+, which in turn may be converted back to DPF2 by addition of base. Additionally, chemical oxidation or bulk electrolysis of DPF1 gives [HDPF1]+ via formation of [DPF1]+, which is followed by a hydrogen radical abstraction reaction. The electrochemical and chemical redox studies have been supported by NMR, ESR, and UV−vis spectroscopies and electrospray mass spectrometry.