Electron-Transfer Properties of Cp*FeP5: Evidence for Dimerization Reactions following both Oxidation and Reduction

Abstract

The redox reactions of Cp*Fe(η5-P5) (1; Cp* = η5-C5Me5) have been characterized in nonaqueous solvents by electrochemical methods. As anticipated by analogy with ferrocene, 1 may be both oxidized and reduced in one-electron processes. Both processes are irreversible by cyclic voltammetry but reversible by bulk electrolysis. In CH2Cl2 complex 1 oxidizes initially to 17-electron 1+ (Ep,a = 0.57 V vs Fc), which rapidly equilibrates to give the dimeric dication [12]2+. An ESR spectrum attributed to 1+ is consistent with a d5 iron sandwich complex. A dimerization rate constant for 1+ of kD(17) = 1.4 × 104 M-1 s-1 was determined from cyclic voltammetry (CV) data. The dimeric dication quantitatively re-forms neutral 1 upon rereduction. Complex 1 undergoes reduction (E1/2 = −2.00 V) to 19-electron 1-, which also appears to dimerize in THF; kD(19) = ca. 6 × 105 M-1 s-1. Reoxidation of the diamagnetic dimer [12]2- regenerates 1. The shifts in potential induced when replacing a cyclopentadienyl ring by a pentaphosphacyclopentadienyl ring, explicable in terms of the weaker electron-donating ability of the latter, are greater for the reductions than the oxidations, implying an increased P5 character to the LUMO of 1 compared to the HOMO. Possible structures of the dimeric ions are discussed in terms of known structural analogues and previously published molecular orbital descriptions.