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Institute of Chemistry, St. Petersburg State University, 199034 St. Petersburg, Russia
Author to whom correspondence should be addressed.
Academic Editor: Hidenori Tanaka
Molbank 2021, 2021(3), M1245;
Received: 8 June 2021 / Revised: 15 June 2021 / Accepted: 16 June 2021 / Published: 3 July 2021
(This article belongs to the Section Organic Synthesis)


Salen-type complexes with transition metals and corresponding polymers attract great scientific interest due to their high electrochemical properties and potential for use as part of next generation organic energy storage devices. Because of their good conductivity but relatively low capacity, energy-intensive additives such as quinones or TEMPO fragments can significantly enhance the capacitive characteristics of the electrode materials. Herein, we report a preparation of precursor for a modified Salen-type complex, the substituted 2,3-Dihydroxybenzaldehyde by butoxy linkers with TEMPO fragment using alkylation reaction. The resulting product was characterized by the 1H and 13C, COSY, HMBC, HSQC nuclear magnetic resonance (NMR), ESI–high resolution mass spectrometry (ESI–HRMS), and Fourier-transform infrared spectroscopy (FTIR). The reported approach opens the way for easy modification of Salen-type complexes in order to increase their specific characteristics.
Keywords: TEMPO; linkers; Salen precursor; alkylation TEMPO; linkers; Salen precursor; alkylation

1. Introduction

Polymeric metal complexes with Salen-type Schiff base ligands, poly[M(Schiff)] might be promising candidates for the creation of highly conducting polymer-based electrodes for energy storage devices [1,2,3,4,5,6]. The availability of modification for poly[M(Schiff)] precursors by changing the chemical structure of substitutes opens the path to the targeting adjustment of material performance. Usage of different substitutes significantly changes the properties of obtained polymers [7,8,9], even if the differences in the substituent structures are minimal [10]. Considering the relatively low capacity of poly[M(Schiff)] material, they can be combined with an energy bearing group such as TEMPO or quinone compound and used as a conductive polymer framework [11]. Evidence of the synergistic action of polymeric Ni-Salen with the nitroxyl polymer PTMA in a composite has been demonstrated [11]; however, such modification complicates the production material process due to the need for the exact ratio of the components. To avoid this, the direct modification of Salen complex precursors allow the monocomponent product to be obtained, which combines the advantages of several classes [12]. Usage of different linkers also allows the properties of the materials to be regulated [13,14].
Herein, we report the synthesis of a Ni-Salen precursor with butoxy TEMPO-containing fragment, namely 2-hydroxy-3-(4-oxy(2,2,6,6-tetramethylpiperidin-1-oxyl)butoxy)benzaldehyde, by the alkylation of 2,3-dihydroxybenzaldehyde with TEMPO-containing butyl bromide. The obtained product was characterized with nuclear magnetic resonance (NMR), high resolution mass spectrometry (HRMS) and Fourier-transform infrared spectroscopy (FTIR) spectra.

2. Results

The desired product was obtained by alkylation of 2,3-dihydroxybenzaldehyde with 4-(4-bromobutoxy)2,2,6,6-tetramethylpiperidine-1-oxyl (Scheme 1) using the typical alkylation conditions [15].
It is noteworthy that the product was purified by simple crystallization from a hexane–Et2O (3:1) mixture. Due to the paramagnetic nitroxyl fragment, NMR spectra could only be obtained after the reductive quenching of the radical center with ascorbic acid. The 1H-NMR spectrum of the product (Figure S1) shows a set of TEMPO-related signals: a multiplet around 3.53, two doublets at 1.84 and 1.25 and a pair of singlets at 1.06 and 1.04 ppm, along with the butoxy triplets at 4.04, 3.43, 1.77 and 1.63 ppm, aromatic signals at 7.25 and 6.9 ppm, accompanied by an aldehyde singlet at 10.24 ppm. The 13C-NMR spectrum (Figure S2) contains a complete set of signals attributed to the proposed structure of the product: linker signals at 25.6, 26.2, 66.9 and 68.6 ppm, TEMPOL peaks at 20.6, 32.3, 44.7, 58.0 and 68.5 ppm, aryl signals at 118.8, 119.3, 120.6, 122.4, 147.6, and 150.9 ppm, and a carbonyl carbon peak at 192.6 ppm. An unambiguous attribution of the signals was made on the basis of the COSY, HSQC and HMBC correlation spectra (Figure 1, Figures S3–S6).
The exact mass of [M+Na]+ ion, determined by ESI–HRMS (Figure S7), was found to be 387.2016 (387.2016 as calcd. for C20H30NO5Na+). The FTIR spectrum recorded in KBr (Figure S8) contains a strong peak at 1652 cm−1 (C=O). Vibration of the phenolic O-H, which typically occurs about 3400 cm−1, is shifted to ca. 3000 cm−1 due to the strong hydrogen bonding.

3. Materials and Methods

3.1. General Consideration

Reagents of “reagent grade” purity were purchased from Sigma–Aldrich (Darmstadt, Germany). 4-(4-bromobutoxy)-2,2,6,6-tetramethylpiperidine-1-oxyl was obtained using a known method [16] with minor modifications. The Fourier transform infrared spectra were recorded on a Shimadzu IRaffinity-1 FTIR spectrophotometer (Shimadzu Europa GmbH, Kyoto, Japan) in KBr pellets. 1H and 13C-NMR spectra were acquired on a Bruker Avance 400 spectrometer (Bruker Analytische Messtechnik GmbH, Rheinstetten, Germany) at 400 and 101 MHz, respectively, in DMSO-d6, as well as COSY, HMBC and HSQC 2D NMR spectra. Before NMR analysis, the paramagnetic center of nitroxyl radical residues was reduced in situ by ascorbic acid. The HRMS spectrum was recorded using electrospray ionization on a Bruker microTOF apparatus (Bruker Analytische Messtechnik GmbH, Rheinstetten, Germany) in positive mode.

3.2. Synthesis of 2-Hydroxy-3-(4-oxy(2,2,6,6-tetramethylpiperidin-1-oxyl)butoxy)benzaldehyde

To a stirred suspension of NaH (60% susp. in oil, 1 g, 25 mmol) in 25 mL of dry DMSO, 2,3-dihydroxybenzaldehyde (1.381 g, 10 mmol) was added (in one portion under Ar) dissolved in 5 mL of dry DMSO, and then, the mixture was stirred at RT for 1 h. At this point, 4-(4-bromobutoxy) 2,2,6,6-tetramethylpiperidine-1-oxyl (2.94 g, 9.5 mmol) dissolved in 2 mL dry DMSO was added in one portion and the mixture was stirred overnight at RT. Then, the mixture was poured into ice water (100 mL), acidified with 1M HCl solution (pH~3) and extracted with Et2O. The organic layer was washed with 1% NaOH, the aqueous layer was acidified with 1M HCl solution and again, extracted with DEE, dried over anhydrous Na2SO4 and evaporated by rotary evaporation in vacuum. Residue was purified by using crystallization from hexane-Et2O (3:1 v/v) solution in a fridge (+4 °C). The crystalline precipitate was separated by decantation and dried in a vacuum using an oil pump. The desired product is a pink-orange crystalline solid (0.8 g, 2.2 mol, 23%).
1H-NMR (400 MHz, DMSO-d6) δ, ppm: 10.24 (s, H, -CH=O), 7.22–7.25 (m, 2H, Ar), 6.87–6.91 (t, 1H, Ar), 4.04 (t, 2H, -CH2-), 3.5 (m, 1H, -CH-), 3.43 (t, 2H, -CH2-), 1.84 (d, 2H, -CH2-), 1.77 (t, 2H, -CH2-), 1.63 (t, 2H, -CH2-), 1.25 (d, 2H, -CH2-), 1.06 (s, 6H, -CH3), 1.04 (s, 6H, -CH3), 13C-NMR (101 MHz, DMSO-d6) δ, ppm: 192.6 (C=O), 150.9 (Ar), 147.6 (Ar), 122.4 (Ar), 120.6 (Ar), 119.3 (Ar), 118.8 (Ar), 68.6 (alkyl linker), 68.5 (alkyl TEMPO), 66.9 (alkyl linker), 58.0 (alkyl TEMPO), 44.7 (alkyl TEMPO), 32.3 (alkyl TEMPO), 26.2 (alkyl linker), 25.6 (alkyl linker), 20.6 (alkyl TEMPO). FTIR (KBr) ῦ, cm−1: 2850–3000 (O-H, C-H), 1652 (C=O). HRMS (ESI) m/z [M+Na]+ calcd for C20H30NO5Na+ 387.20, found 387.2016.

4. Conclusions

Substituted salicylaldehyde is a typical precursor to Salen-type materials, which is widely used in energy storage devices and electrocatalysis systems. We demonstrate the possibility for the direct alkylation of 2,3-dihydroxybenzaldehyde with 4-(4-bromobutoxy) 2,2,6,6-tetramethylpiperidine-1-oxyl, which provides the possibility to easily immobilize materials with TEMPO by using different linkers. Such approach allows significant change and improves the properties of materials, which is vital for the development of organic electrodes.

Supplementary Materials

The following are available online, 1H and 13C-NMR spectra, COSY, HMBC, HSQC, HRMS and FTIR data for 2-hydroxy-3-(4-oxy(2,2,6,6-tetramethylpiperidin-1-oxyl)butoxy)benzaldehyde.

Author Contributions

Conceptualization: D.A.L.; synthesis: A.A.V. and J.V.N.; writing—original draft preparation: A.A.V.; writing—review and editing: D.A.L.; Validation: A.Y.K.; visualization: D.A.L., A.A.V.; supervision: D.A.L.; funding acquisition: A.A.V. All authors have read and agreed to the published version of the manuscript.


This research was funded by Russian Foundation for Basic Research (RFBR) grant number 20-33-90122 and scholarship of the President of the Russian Federation No. SP-691.2021.1.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data available on request.


We thank the Research Center for Magnetic Resonance, the Center for Chemical Analysis and Materials Research of Saint Petersburg State University Research Park for the measurements provided.

Conflicts of Interest

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.


  1. Chen, C.; Zhu, Z.; Li, X.; Li, J. Electropolymerization and energy storage of poly[Ni(salphen)]/MWCNT composite materials for supercapacitors. J. Appl. Polym. Sci. 2017, 134. [Google Scholar] [CrossRef]
  2. Alekseeva, E.V.; Chepurnaya, I.A.; Malev, V.V.; Timonov, A.M.; Levin, O.V. Polymeric nickel complexes with salen-type ligands for modification of supercapacitor electrodes: Impedance studies of charge transfer and storage properties. Electrochim. Acta 2017, 225, 378–391. [Google Scholar] [CrossRef]
  3. Yan, G.; Li, J.; Zhang, Y.; Gao, F.; Kang, F. Electrochemical polymerization and energy storage for Poly[Ni(salen)] as supercapacitor electrode material. J. Phys. Chem. C 2014, 118, 9911–9917. [Google Scholar] [CrossRef]
  4. Gao, F.; Li, J.; Kang, F.; Zhang, Y.; Wang, X.; Ye, F.; Yang, J. Preparation and characterization of a Poly[Ni(salen)]/Multiwalled carbon nanotube composite by in situ electropolymerization as a capacitive material. J. Phys. Chem. C 2011, 115, 11822–11829. [Google Scholar] [CrossRef]
  5. Li, J.; Gao, F.; Zhang, Y.; Kang, F.; Wang, X.; Ye, F.; Yang, J. Electropolymerization of Ni(salen) on carbon nanotube carrier as a capacitive material by pulse potentiostatic method. Sci. China Chem. 2012, 55, 1338–1344. [Google Scholar] [CrossRef]
  6. Tedim, J.; Gonçalves, F.; Pereira, M.F.R.; Figueiredo, J.L.; Moura, C.; Freire, C.; Hillman, A.R. Preparation and characterization of Poly[Ni(salen)(crown receptor)]/multi-walled carbon nanotube composite films. Electrochim. Acta 2008, 53, 6722–6731. [Google Scholar] [CrossRef]
  7. Vereschagin, A.A.; Sizov, V.V.; Vlasov, P.S.; Alekseeva, E.V.; Konev, A.S.; Levin, O.V. Water-stable [Ni(salen)]-type electrode material based on phenylazosubstituted salicylic aldehyde imine ligand. New J. Chem. 2017, 41, 13918–13928. [Google Scholar] [CrossRef]
  8. Vilas-Boas, M.; Santos, I.C.; Henderson, M.J.; Freire, C.; Hillman, A.R.; Vieil, E. Electrochemical behavior of a new precursor for the design of Poly[Ni(salen)]-based modified electrodes. Langmuir 2003, 19, 7460–7468. [Google Scholar] [CrossRef]
  9. Tedim, J.; Carneiro, A.; Bessada, R.; Patrício, S.; Magalhães, A.L.; Freire, C.; Gurman, S.J.; Hillman, A.R. Correlating structure and ion recognition properties of [Ni(salen)]-based polymer films. J. Electroanal. Chem. 2007, 610, 46–56. [Google Scholar] [CrossRef]
  10. Eliseeva, S.N.; Alekseeva, E.V.; Vereshchagin, A.A.; Volkov, A.I.; Vlasov, P.S.; Konev, A.S.; Levin, O.V. Nickel-salen type polymers as cathode materials for rechargeable lithium batteries. Macromol. Chem. Phys. 2017, 218, 1700361. [Google Scholar] [CrossRef]
  11. Vereshchagin, A.A.; Vlasov, P.S.; Konev, A.S.; Yang, P.; Grechishnikova, G.A.; Levin, O.V. Novel highly conductive cathode material based on stable-radical organic framework and polymerized nickel complex for electrochemical energy storage devices. Electrochim. Acta 2019, 295, 1075–1084. [Google Scholar] [CrossRef]
  12. Vereshchagin, A.A.; Lukyanov, D.A.; Kulikov, I.R.; Panjwani, N.A.; Alekseeva, E.A.; Behrends, J.; Levin, O.V. The fast and the capacious: A [Ni(Salen)]-TEMPO redox-conducting polymer for organic batteries. Batter. Supercaps 2021, 4, 336–346. [Google Scholar] [CrossRef]
  13. Li, F.; Gore, D.N.; Wang, S.; Lutkenhaus, J.L. Unusual internal electron transfer in conjugated radical polymers. Angew. Chem. 2017, 129, 9988–9991. [Google Scholar] [CrossRef]
  14. Karlsson, C.; Huang, H.; Strømme, M.; Gogoll, A.; Sjödin, M. Impact of linker in polypyrrole/quinone conducting redox polymers. RSC Adv. 2015, 5, 11309–11316. [Google Scholar] [CrossRef]
  15. Han, S.; Zhang, F.-F.; Qian, H.-Y.; Chen, L.-L.; Pu, J.-B.; Xie, X.; Chen, J.-Z. Design, syntheses, structure–activity relationships and docking studies of coumarin derivatives as novel selective ligands for the CB2 receptor. Eur. J. Med. Chem. 2015, 93, 16–32. [Google Scholar] [CrossRef] [PubMed]
  16. Zheng, Z.; Wang, J.; Chen, H.; Feng, L.; Jing, R.; Lu, M.; Hu, B.; Ji, J. Magnetic superhydrophobic polymer nanosphere cage as a framework for miceller catalysis in biphasic media. ChemCatChem 2014, 6, 1626–1634. [Google Scholar] [CrossRef]
Scheme 1. Reaction conditions for the alkylation of 2,3-dihydroxybenzaldehyde with 4-(4-bromobutoxy) 2,2,6,6-tetramethylpiperidine-1-oxyl.
Scheme 1. Reaction conditions for the alkylation of 2,3-dihydroxybenzaldehyde with 4-(4-bromobutoxy) 2,2,6,6-tetramethylpiperidine-1-oxyl.
Molbank 2021 m1245 sch001
Figure 1. Attribution of the chemical shifts for 2-hydroxy-3-(4-oxy(2,2,6,6-tetramethylpiperidin-1-oxyl)butoxy)benzaldehyde.
Figure 1. Attribution of the chemical shifts for 2-hydroxy-3-(4-oxy(2,2,6,6-tetramethylpiperidin-1-oxyl)butoxy)benzaldehyde.
Molbank 2021 m1245 g001
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