Spectroelectrochemical Investigation of the One-Electron Reduction of Nonplanar Nickel(II) Porphyrins
in: ChemPhysChem (2016)
The electrochemical reduction of a series of nickel porphyrins with increasing number of substituents is investigated in acetonitrile. One-electron reduction of [5,15-bis(1-ethylpropyl)porphyrinato]nickel(II) leads to π-anion radicals and to an efficient formation of phlorin anions presumably via disproportionation and subsequent protonation of the doubly-reduced species. The phlorin anion is identified by cyclic voltammetry, UV/Vis- and resonance Raman spectroelectrochemistry, complemented by quantum chemical calculations to assign the spectral signatures. The theoretical analysis of the potential energy landscape of the singly-reduced species suggests a thermally activated inter-system crossing which populates the quartet state and thus, lowers the energy barrier towards disproportionation channels. Structure-reactivity correlations are investigated by considering different substitution pattern of the investigated nickel(II) porphyrin cores, i.e. for the porphyrin with additional β-aryl ([5,15-bis(1-ethylpropyl)-2,8,12,18-tetra(p-tolyl)-porphyrinato]nickel(II)) and meso-alkyl substitution ([5,10,15,20-tetrakis(1-ethylpropyl)porphyrinato]nickel(II)) no phlorin anion formation is observed under electrochemical conditions. This observation is correlated either to kinetic inhibition of the disproportionation reaction or to a lower reactivity of the subsequently formed doubly-reduced species towards protonation.