4,4 -Di- tert -butyl-2,2 -bipyridinium Triﬂuoromethanesulfonate

: 4,4 (cid:48) -di- tert -butyl-2,2 (cid:48) -bipyridinium triﬂuoromethanesulfonate was synthesized by stirring 4,4 (cid:48) -di- tert -butyl-2,2 (cid:48) -bipyridine with scandium(III) triﬂuoromethanesulfonate in acetonitrile, followed by precipitation with diethyl ether. The structure of the new compound was characterized by FT-IR, 1 H, 13 C{ 1 H} and 19 F{ 1 H} NMR spectroscopy and CHN elemental analysis. This is a safe and simple method to obtain mono-protonated bipyridinium triﬂuoromethanesulfonate without the direct use of triﬂuoromethanesulfonic acid.


Results and Discussion
Reaction of 1 with Sc(OTf) 3 in acetonitrile at ambient temperature afforded a pale pink solution. Subsequent addition of undried diethyl ether to this solution resulted in the formation of a white precipitate. Characterization of the compound revealed that the expected scandium bipyridine complex was not formed; instead, mono-protonated bipyridinium trifluoromethanesulfonate 2 was formed (Scheme 1).

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signals are shifted downfield with respect to those of 1 ( Figure 1). This suggests the existence of a protonated nitrogen atom in 2 [13] ( 1 H, 13 C{ 1 H} and 19 F{ 1 H} NMR spectra of 2 are included in the Supplementary Materials). We also obtained long plate-like crystals by slow diffusion of diethyl ether into the acetonitrile solution of 2. Although the obtained crystals were not of good quality, preliminary X-ray structure analysis of the crystals has confirmed the molecular structure of 2.  Although Sc(OTf)3 is often used as a stable Lewis acid even in water [22], this salt reversibly converts to TfOH through the hydrolysis of Sc 3+ [24,25]. In the present reaction, we speculate that Sc(OTf)3 reacts with the water in the reaction solvent to reversibly generate TfOH, followed by protonation of 1 to afford 2. Bipyridinium salt 2 has low solubility in an acetonitrile-diethyl ether mixture and readily precipitates in the mixed solvent, shifting the reaction equilibrium to the product 2 side. Although it has already been known that the protonation of a pyridine derivative occurs in the presence of Sc(OTf)3 and water in organic solvent [24], to our knowledge the preparation of protonated pyridinium trifluoromethanesulfonates based on this mechanism has not been explored. The protocol presented herein will provide safe and simple access not only to mono-protonated bipyridinium trifluoromethanesulfonates but also to a variety of protonated pyridinium trifluoromethanesulfonates without the direct use of TfOH.

General
All the reagents and solvents were purchased from chemical companies and used without further purification. 1 H NMR spectra were recorded on a JEOL JNM-ECS400 (400 MHz) FT NMR system or JEOL JMN-ECX400 (400 MHz) FT NMR system in CDCl3 with Scheme 1. Synthesis of bipyridinium salt 2.
The 1 H NMR spectrum of 2 in CDCl 3 shows three signals in the aromatic region at δ 8.88 (d, J = 5.6 Hz, 2H), 8.50 (d, J = 1.6 Hz, 2H), and 7.72 (dd, J = 5.6, 1.6 Hz, 2H), and these signals are shifted downfield with respect to those of 1 (Figure 1). This suggests the existence of a protonated nitrogen atom in 2 [13] ( 1 H, 13 C{ 1 H} and 19 F{ 1 H} NMR spectra of 2 are included in the Supplementary Materials). We also obtained long plate-like crystals by slow diffusion of diethyl ether into the acetonitrile solution of 2. Although the obtained crystals were not of good quality, preliminary X-ray structure analysis of the crystals has confirmed the molecular structure of 2.
Molbank 2021, 2021, x FOR PEER REVIEW 2 of 4 expected scandium bipyridine complex was not formed; instead, mono-protonated bipyridinium trifluoromethanesulfonate 2 was formed (Scheme 1). The 1 H NMR spectrum of 2 in CDCl3 shows three signals in the aromatic region at δ 8.88 (d, J = 5.6 Hz, 2H), 8.50 (d, J = 1.6 Hz, 2H), and 7.72 (dd, J = 5.6, 1.6 Hz, 2H), and these signals are shifted downfield with respect to those of 1 (Figure 1). This suggests the existence of a protonated nitrogen atom in 2 [13] ( 1 H, 13 C{ 1 H} and 19 F{ 1 H} NMR spectra of 2 are included in the Supplementary Materials). We also obtained long plate-like crystals by slow diffusion of diethyl ether into the acetonitrile solution of 2. Although the obtained crystals were not of good quality, preliminary X-ray structure analysis of the crystals has confirmed the molecular structure of 2.  Although Sc(OTf)3 is often used as a stable Lewis acid even in water [22], this salt reversibly converts to TfOH through the hydrolysis of Sc 3+ [24,25]. In the present reaction, we speculate that Sc(OTf)3 reacts with the water in the reaction solvent to reversibly generate TfOH, followed by protonation of 1 to afford 2. Bipyridinium salt 2 has low solubility in an acetonitrile-diethyl ether mixture and readily precipitates in the mixed solvent, shifting the reaction equilibrium to the product 2 side. Although it has already been known that the protonation of a pyridine derivative occurs in the presence of Sc(OTf)3 and water in organic solvent [24], to our knowledge the preparation of protonated pyridinium trifluoromethanesulfonates based on this mechanism has not been explored. The protocol presented herein will provide safe and simple access not only to mono-protonated bipyridinium trifluoromethanesulfonates but also to a variety of protonated pyridinium trifluoromethanesulfonates without the direct use of TfOH.

General
All the reagents and solvents were purchased from chemical companies and used without further purification. 1 H NMR spectra were recorded on a JEOL JNM-ECS400 (400 MHz) FT NMR system or JEOL JMN-ECX400 (400 MHz) FT NMR system in CDCl3 with Although Sc(OTf) 3 is often used as a stable Lewis acid even in water [22], this salt reversibly converts to TfOH through the hydrolysis of Sc 3+ [24,25]. In the present reaction, we speculate that Sc(OTf) 3 reacts with the water in the reaction solvent to reversibly generate TfOH, followed by protonation of 1 to afford 2. Bipyridinium salt 2 has low solubility in an acetonitrile-diethyl ether mixture and readily precipitates in the mixed solvent, shifting the reaction equilibrium to the product 2 side. Although it has already been known that the protonation of a pyridine derivative occurs in the presence of Sc(OTf) 3 and water in organic solvent [24], to our knowledge the preparation of protonated pyridinium trifluoromethanesulfonates based on this mechanism has not been explored. The protocol presented herein will provide safe and simple access not only to mono-protonated bipyridinium trifluoromethanesulfonates but also to a variety of protonated pyridinium trifluoromethanesulfonates without the direct use of TfOH.

General
All the reagents and solvents were purchased from chemical companies and used without further purification. 1 H NMR spectra were recorded on a JEOL JNM-ECS400 (400 MHz) FT NMR system or JEOL JMN-ECX400 (400 MHz) FT NMR system in CDCl 3 with Me 4 Si as an internal standard. 13 C{ 1 H} NMR spectrum was recorded on a JEOL JNM-ECS400 (100 MHz) FT NMR system in CDCl 3 . 19 F{ 1 H} NMR spectrum was recorded on a Bruker AVANCE NEO 400 spectrometer (376 MHz). The IR spectrum was recorded on a JASCO FT/IR-410 spectrometer.

Conclusions
The new mono-protonated bipyridinium trifluoromethanesulfonate 2 has been synthesized from 4,4 -t Bubpy using Sc(OTf) 3 without the direct use of TfOH. We consider that the present protocol will provide easy and safe access to a variety of protonated pyridinium trifluoromethanesulfonates.

Data Availability Statement:
The data presented in this study are available in the article and in its supplementary materials.