COUPLED REDOX REACTIONS, LINKAGE ISOMERIZATION, HYDRIDE FORMATION, AND ACID-BASE RELATIONSHIPS IN THE DECAPHENYLFERROCENE SYSTEM

Decaphenylferrocene (DPF) exists in two isomeric forms, blue Fe(eta(6) -C6H5-C5Ph4)(eta(5)-C5Ph5) (DPF1), which is known to readily protonate on the ligand giving orange [Fe(eta(6)-C6H5-C5HPh4)(eta(5)-C5Ph5)](+) , [HDPF1](+), and insoluble pink Fe(eta(5)-C5Ph5)(2) (DPF2). The redox chemistry is unusually complex for a ferrocene system because acid-ba se chemistry, formation of the hydride [HFe(eta(5)-C5Ph5)(2)](+), [HDP F2](+), together with isomerization and radical abstraction reactions are coupled with the electron-transfer processes. The use of a wide ra nge 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(-) revers ible arrow DPFB (E-1/2(r) = -40 mV); [HDPF2](+) + e(-) reversible arro w HDPF2 (E-1/2(r) = -220 mV at -60 degrees C), [DPF1](+) + e(-) revers ible arrow DPF1 (E-1/2(r) = -170 mV), [HDPF1](2+) + e(-) reversible ar row [HDPF1](+) (E-1/2(r) = 710 mV), [HDPF1](+) + e(-) reversible arrow HDPF1 (E-1/2(r) = -1480 mV), and additionally, C5HPh5 is reversibly o xidized at 765 mV (all potentials vs Fc(+)/Fc (Fc = Fe(eta(5)-C5H5)(2) ) at 20 degrees C. Only the first two processes are metal based, while the other processes are believed to have considerable ligand characte r. The wide range of reversible potentials, in particular the similar potentials of the [HDPF2](+/0) and [DPF1](+/0) processes, enable an ex tensive series of cross-redox reactions to accompany the electron-tran sfer processes which occur either at the electrode surface or in the-b ulk solution. The reaction pathways identified include the following: in the presence of acid, DPF1 is protonated on the ligand to give [HDP F1](+), DPFB forms a Fe(IV) hydride, [HDPF2](+), which rearranges to [ HDPF1](+), thereby providing a route for isomerization. In very strong acid, DPFB is oxidized to [DPF2](+), which in turn may be converted b ack to DPFB by addition of base. Additionally, chemical oxidation or b ulk electrolysis of DPF1 gives [HDPF1](+) via formation of [DPF1](+), which is followed by a hydrogen radical abstraction reaction. The elec trochemical and chemical redox studies have been supported by NMR, ESR , and UV-vis spectroscopies and electrospray mass spectrometry.