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Remarkable Sensitivity of the Electrochemical Reduction of Benzophenone to Proton Availability in Ionic Liquids
Author(s) -
Zhao ShuFeng,
Lu JiaXing,
Bond Alan M.,
Zhang Jie
Publication year - 2012
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201103365
Subject(s) - benzophenone , ionic liquid , chemistry , cyclic voltammetry , protonation , imide , ionic bonding , electrolyte , proton , electrochemistry , molecule , inorganic chemistry , photochemistry , ion , organic chemistry , electrode , catalysis , quantum mechanics , physics
The reduction of benzophenone was investigated in five different ionic liquids by using transient cyclic voltammetry, near steady‐state voltammetry, and numerical simulation. Two reversible, well‐resolved one‐electron‐reduction processes were observed in dry (≤20 ppm water, ca. 1 m M )) 1‐butyl‐1‐methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([Bmpyrd][NTf 2 ]) and 1‐butyl‐1‐methylpiperidinium bis(trifluoromethylsulfonyl)imide ([Bmpipd][NTf 2 ]), which did not contain any readily available proton source. Upon addition of water, the second process became chemically irreversible and shifted to a more positive potential by approximately 600 mV; moreover, the two reduction processes merged into a single two‐electron proton‐coupled process when about 0.6 M H 2 O was present. This large dependence of potential on water content, which was not observed in molecular solvents (electrolyte), was explained by a reaction mechanism that incorporated protonation and hydrogen‐bonding interactions of the benzophenone dianion with as many as seven water molecules. In the three imidazolium‐based ionic liquids used herein, the first benzophenone‐reduction process was again reversible, whilst the second reduction process became chemically irreversible owing to the availability of the C2‐H imidazolium protons in these ionic liquids. The reversible potentials for benzophenone reduction were remarkably independent of the identity of the ionic liquids, thereby implying either weak interactions with the ionic liquids or relatively insignificant differences in the levels of ion‐pairing. Thus, the magnitude of the separation of the potentials of the reversible first and irreversible second reduction processes mainly reflected the proton availability from either the ionic liquid itself or from adventitious water. Consequently, voltammetric reduction of benzophenone provides a sensitive tool for the determination of proton availability in ionic liquids.