Proton-coupled reduction of an iron nitrosyl porphyrin in the protic ionic liquid nanodomain

Abstract

The one-electron reduction of many molecules becomes much more favorable if combined with proton transfers or strong hydrogen bonding. Protic room temperature ionic liquids (RTILs), which can form nanodomains in solutions with molecular solvents (MS), can provide an efficient avenue for this process. In this work, we report on the voltammetry, UV/visible and resonance Raman spectroelectrochemistry of Fe(TPP)(NO) in the presence of aprotic/protic ammonium-based ionic liquids. While aprotic RTILs did shift the reduction to more positive potentials, similar shifts could be observed at much lower concentrations of diethylmethylammonium triflate (HAmOTf, a protic ionic liquid). Deconvolution of the rotating ring-disk electrode (RRDE) voltammetry revealed the partitioning of the reduced species into the ionic liquid nanodomains at low concentrations. The potential shift was substantially in excess of the value expected based on the pKa of the weak acid. Upon the addition of small amounts of the protic RTIL, the electrochemically or chemically generated anion, Fe(TPP)(NO)-, reacted rapidly with the HAm+ acid, forming a Fe(TPP)(HNO) complex. Further reduction to Fe(TPP)(NH2OH) could be observed on the spectroelectrochemical time scale. The outcome of this work revealed the advantageous role of protic RTIL nanodomains in accelerating the proton-coupled reductions to form more energetically favorable product.