There have been many examples of the accelerating effects of acids in electron transfer (ET), oxygen atom transfer (OAT), and hydrogen atom transfer (HAT) reactions. Herein, we report a contrasting effect of acids in the ET, OAT, and HAT reactions of a nickel(III) complex, [NiIII(PaPy3*)]2+(1) in acetone/CH3CN (v/v 19:1). 1 was synthesized by reacting [NiII(PaPy3*)]+(2) with magic blue or iodosylbenzene in the absence or presence of triflic acid (HOTf), respectively. Sulfoxidation of thioanisole by 1 and H2O occurred in the presence of HOTf, and the reaction rate increased proportionally with increasing concentration of HOTf ([HOTf]). The rate of ET from diacetylferrocene to 1 also increased linearly with increasing [HOTf]. In contrast, HAT from 9,10-dihydroanthracene (DHA) to 1 slowed down with increasing [HOTf], exhibiting an inversely proportional relation to [HOTf]. The accelerating effect of HOTf in the ET and OAT reactions was ascribed to the binding of H+to the PaPy3∗ ligand of 2; the one-electron reduction potential (Ered) of 1 was positively shifted with increasing [HOTf]. Such a positive shift in the Eredvalue resulted in accelerating the ET and OAT reactions that proceeded via the rate-determining ET step. On the other hand, the decelerating effect of HOTf on HAT from DHA to 1 resulted from the inhibition of proton transfer from DHA+to 2 due to the binding of H+to the PaPy3ligand of 2. The ET reactions of 1 in the absence and presence of HOTf were well analyzed in light of the Marcus theory of ET in comparison with the HAT reactions.
Bibliographical noteFunding Information:
This work was supported by the NRF of Korea through CRI (NRF-2021R1A3B1076539 to W.N.) and the Basic Science Research Program (2020R1I1A1A01074630 to Y.-M.L. and 2019R1I1A1A01055822 to M.S.S.). This work was also supported by Grants-in-Aid (no. 16H02268 to S.F.) from MEXT, Japan.
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