Abstract
Bipyridinium salts are currently very popular due to their perspective applications in redox flow batteries. Hence, we designed and prepared a series of bipyridiniums based on 2,2′-, 3,3′-, and 4,4′-bipyridine and 2,2′-bipyrimidine. The straightforward synthesis utilizes commercially or readily available starting compounds and their direct N-alkylation, mostly using 1,3-propanesultone. All eleven target derivatives with systematically evolved structure were investigated by cyclic voltammetry, which allowed elucidating thorough structure-property relationships. The electrochemical behavior depends primarily on the parent scaffold, type of N-alkylation, number of quaternized nitrogen atoms, planarity, counter ion as well as the used media. Two derivatives featuring quasi-reversible redox processes were further tested on rotating disc electrode and in a flow battery half-cell. 4,4′-Bipyridinium derivative bearing two sultone residues showed better performance and stability in the flow half-cell with small capacity decays of 0.09/0.15% per reduction-oxidation cycle, based on the number of the utilized redox processes (one/two).
Highlights
Bipyridines represent simple yet very interesting and tempting heteroaromatic scaffold with tremendous use across various research fields
The synthetic strategy toward bipyridines and their salts is outlined in Scheme 1
Starting materials 5, 6, 8, 9, and 10 are commercially available, while precursor 7 was prepared by oxidation of 1,10phenantroline 6 using a mixture of sulfuric and nitric acids
Summary
Bipyridines represent simple yet very interesting and tempting heteroaromatic scaffold with tremendous use across various research fields. This mono-quaternized derivative showed one reduction with the potential lying between the two aforementioned redox processes seen for 1a. Oxalyl-bridged compound 1d showed two consecutive electrochemical processes in neutral media that belong to oneelectron reduction of the pyridinium and the carbonyl group.
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