How to Design Fast Two-Electron Transfer Reagents: 37-Electron Mixed Valence FeIFeII-Bisandwiches as Key Intermediates

Abstract

The 38-electron complexes [(C5R5Fe)2(μ2,η12-biphenyl)] ++(PF6 -)2 (R = H, la; CH3, lb) are reduced by 2 electrons to the neutral bicyclohexadienylidene complexes [(C5R5Fe)2(η10-biphenyl) (R = H, 2a; CH3, 2b), structurally defined for 2b (double bond between the two phenyl rings: 1.37 A folding angle: 25°). The cyclic voltammetry shows a fast two-electron cathodic wave for la (Eo = - 1.12 V vs SCE, DMF, Pt) and two close one-electron waves for lb (EO: - 1.3 and ” 1.43 V vs SCE, DMF, Pt). The thermodynamically stable average valence 37-electron complex [(C5Me5-FeII)2(diphenyl)]+PF[6 - 3b can be isolated and its X-ray crystal structure shows that the diphenyl llgand is almost flat (folding angle: 5°) and coordinated in an hexahapto fashion to each ironas in the precursors 1. By comparison, the isomeric series [Fe2(fulvalene) (C6R6)2]++ (R = H, 4a; CH3, 4b) show fours well separated one-electron waves without structural rearrangement after at least two one-electron transfers separated by 0.5 V. Thus the gain of energy provided by the structural rearrangement FeIFeIdiphenyl → FeIIFeII bicyclohexadienylidene compensates the electrostatic factor responsible for the Eo value (0.5 V in the isomeric series 4) which brings the two-one electron waves into a single two-electron wave for 1a. The knowledge of the CV’s and of the exact structures of the three components 1, 2 and 3 together with the slight difference between the Cp and C5Me5 series allows for the first time to demonstrate how and why a fast-two electron transfer proceeds and thus how to design it rationally in any area of molecular chemistry.