Spectroelectrochemical Investigation of the One-Electron Reduction of Nonplanar Nickel(II) Porphyrins

• Published in: Chemphyschem Vol. 17; no. 21; pp. 3480 - 3493
• Main Authors: Schindler, Julian, Kupfer, Stephan, Zedler, Linda, Wächtler, Maria, Gräfe, Stefanie, Ryan, Aoife A., Senge, Mathias O., Dietzek, Benjamin
• Format: Journal Article
• Language: English
• Published: Germany Blackwell Publishing Ltd 04-11-2016; Wiley Subscription Services, Inc
• Subjects: electrochemistry; porphyrinoids; quantum chemistry; reaction mechanisms; spectroscopic analysis; quantum chemistry; reaction mechanisms; porphyrinoids; spectroscopic analysis; electrochemistry
• ISSN: 1439-4235; 1439-7641
• DOI: 10.1002/cphc.201600698

The electrochemical reduction of a series of nickel porphyrins with an increasing number of substituents was investigated in acetonitrile. A one‐electron reduction of [5,15‐bis(1‐ethylpropyl)porphyrinato]nickel(II) leads to π‐anion radicals and to efficient formation of phlorin anions, presumably by disproportionation and subsequent protonation of the doubly reduced species. The phlorin anion was identified by using cyclic voltammetry and 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 intersystem crossing that populates the quartet state and thus lowers the energy barrier towards disproportionation channels. Structure–reactivity correlations are investigated by considering different substitution patterns of the investigated nickel(II) porphyrin cores, that is, 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 was observed under electrochemical conditions. This observation is correlated either to kinetic inhibition of the disproportionation reaction or to lower reactivity of the subsequently formed doubly reduced species towards protonation.
Porphyrin electrochemistry: The reactivity of singly reduced nickel porphyrins towards the formation of phlorin anions is investigated by using UV/Vis and resonance Raman spectroelectrochemistry and quantum‐chemical calculations. Structure–reactivity correlations are deduced by considering different substitution patterns of the investigated porphyrins.