Electrosynthesis of Rh2(dpf)4(R) where dpf = N,N′-diphenylformamidinate anion and R = CH3, C2H5, C3H7, C4H9 or C5H11

Abstract

The electrosynthesis of Rh₂(dpf)₄(R) where dpf is the N,N′-diphenylformamidinate anion and R = CH₃, C₂H₅, C₃H₇, C₄H₉ or C₅H₁₁ was carried out in THF containing 0.2 M tetra-n-butylammonium perchlorate (TBAP) and one of several alkyl iodides represented as RI. The initial step in the reaction involved a one-electron reduction of the Rh₂⁴⁺ unit in Rh₂(dpf)₄ to its Rh₂³⁺ form followed by a homogeneous reaction involving electrogenerated [Rh₂(dpf)₄]⁻ and the alkyl iodide in solution to give Rh₂(dpf)₄(R). The homogeneously generated Rh₂⁵⁺ product was then immediately reduced by a second electron at the potential where [Rh₂(dpf)₄(R)]⁻ is generated, giving [Rh₂(dpf)₄(R)]⁻ which contains a Rh₂⁴⁺ center as a final product of an electrochemical ECE mechanism. The electrosynthesized [Rh₂(dpf)₄(CH₃)]⁻ derivative could be reoxidized to Rh₂(dpf)₄(CH₃) on the reverse potential sweep and both forms of the CH₃ bonded derivative were in situ characterized by cyclic voltammetry combined with UV-visible and/or ESR spectroscopy. The reversible Rh₂^4+/3+ process of Rh₂(dpf)₄ is located at E_1/2 = −1.11 V in THF, 0.2 M TBAP while the electrogenerated Rh₂(dpf)₄(R) products are substantially easier to reduce, with E_p values for the Rh₂^5+/4+ couples ranging from −0.50 to −0.54 V vs.SCE depending upon the specific R group.