Title

Pourbaix diagrams in weakly coupled systems: A case study involving acridinol and phenanthridinol pseudobases

Document Type

Article

Publication Date

4-1-2016

Publication Title

Journal of Physical Organic Chemistry

Volume

29

First Page

204

Last Page

208

Keywords

organic pseudobases, Pourbaix diagrams, proton-coupled electron transfer, water oxidation catalysis

Abstract

Copyright © 2015 John Wiley & Sons, Ltd. The thermodynamics of proton-coupled electron transfer (PCET) in weakly coupled organic pseudobases was investigated using 2,7-dimethyl-9-hydroxy-9-phenyl-10-tolyl-9,10-dihydroacridine (AcrOH) and 6-phenylphenanthridinol (PheOH) as model compounds. Pourbaix diagrams for two model compounds were constructed using the oxidation potentials and the pKa values obtained, respectively, from cyclic voltammetry and photometric titrations. Our comparative study reveals the importance of having the redox active -N center closer to -OH functionality on the thermodynamics of PCET process: PheOH exhibits a wider range of pH values (pH = 2.8 to 13.3) in which both the alcohol and the corresponding alkoxy radical are expected to coexist in solution. This result indicates that a concerted mechanism is more likely to be discovered in pseudobases analogous to PheOH. The thermochemical data also indicate that the concerted PCET mechanism cannot be achieved if water is used as the proton acceptor: assuming the pKa of hydronium ions as -1.7, the PCET involving PheOH or AcrOH as proton/electron donors and water as the proton acceptor is expected to follow the stepwise ET/PT mechanism.

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