Electron-transfer properties of Cp*FeP5: Evidence for dimerization reactions following both oxidation and reduction

The redox reactions of Cp*Fe(eta(5)-P-5) (1; Cp* = eta(5)-C5Me5) have been characterized in nonaqueous solvents by electrochemical methods. As anticip ated by analogy with ferrocene, 1 may be both oxidized and reduced in one-e lectron processes. Both processes are irreversible by cyclic voltammetry bu t reversible by bulk electrolysis. In CH2Cl2 complex 1 oxidizes initially t o 17-electron 1(+) (E-p,E-a = 0.57 V vs Fc), which rapidly equilibrates to give the dimeric dication [1(2)](2+). An ESR spectrum attributed to 1(+) is consistent with a d(5) iron sandwich complex. A dimerization rate constant for 1(+) of k(D(17)) = 1.4 X 10(4) M-1 s(-1) was determined from cyclic vo ltammetry (CV) data. The dimeric dication quantitatively re-forms neutral 1 upon rereduction. Complex 1 undergoes reduction (E-1/2 = -2.00 V) to 19-el ectron 1-, which also appears to dimerize in THF; k(D(19)) = ca. 6 X 10(5) M-1 s(-1).(.) Reoxidation of the diamagnetic dimer [1(2)](2-) regenerates 1 . The shifts in potential induced when replacing a cyclopentadienyl ring by a pentaphosphacyclopentadienyl ring, explicable in terms of the weaker ele ctron-donating ability of the latter, are greater for the reductions than t he oxidations, implying an increased P-5 character to the LUMO of 1 compare d to the HOMO. Possible structures of the dimeric ions are discussed in ter ms of known structural analogues and previously published molecular orbital descriptions.