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Frontiers of Chemical Science and Engineering

Front. Chem. Sci. Eng.    2019, Vol. 13 Issue (3) : 586-598
Deep eutectic ionic liquids based on DABCO-derived quaternary ammonium salts: A promising reaction medium in gaining access to terpyridines
Muhammad Faisal(), Azeem Haider, Quret ul Aein, Aamer Saeed(), Fayaz Ali Larik
Department of Chemistry, Quaid-I-Azam University, Islamabad, Pakistan
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Owing to the directional H-bonding, coordination and p-stacking abilities, terpyridines have been widely used as supramolecular tectons in molecular architectures, skeletons in molecular devices and metallopolymers, and are gaining importance in medicinal chemistry. In this paper, we have synthesized, characterized and applied deep eutectic ionic liquids (DEILs) based on 1,4-diazabicyclo[2.2.2]octane; triethylenediamine (DABCO)-derived quaternary ammonium salts for the preparation of terpyridines. These DEILs were synthesized through N-alkylation of DABCO with haloalkanes (1-bromopentane or 1-bromoheptane) followed by mixing and heating with methanol or polyethylene glycol as a hydrogen bond donor. The synthesized DEILs were structurally characterized by IR and NMR. The formation of deep eutectic solvent was confirmed by freezing point depression, it composition was investigated through phase diagram, and its thermal stability was determined through differential scanning calorimetry, derivative thermogravimetry and thermal gravimetric analysis studies. Further, these DEILs were investigated for their effectiveness towards synthesis of 2,2':6',2"-terpyridine, 3,2':6',3"-terpyridineand 4,2':6',4"-terpyridine derivatives through Kröhnke reaction. The results show that these three types of terpyridines can be obtained in reasonable yields (80%-97%) by the one-pot reaction of 2-, 3- or 4-acetylpyridine with a variety of aromatic aldehydes in the presence of DEIL as a reaction medium, sodium hydroxide as a base and ammonium acetate as a cyclizing agent. This methodology is highly efficient and cost-effective for synthesis of symmetrical as well as unsymmetrical terpyridines. Importantly, these DEILs can be reused several times without an obvious loss of activity and are non-toxic, low-volatile, biodegradable and highly thermally stable. Therefore, these DEILs as a non-conventional reaction medium for the synthesis of terpyridines provides appealing opportunities to be investigated in the domain of green synthesis.

Keywords terpyridine      deep eutectic solvent      ionic liquid      Kröhnke reaction      DABCO     
Corresponding Authors: Muhammad Faisal,Aamer Saeed   
Online First Date: 24 April 2019    Issue Date: 22 August 2019
 Cite this article:   
Muhammad Faisal,Azeem Haider,Quret ul Aein, et al. Deep eutectic ionic liquids based on DABCO-derived quaternary ammonium salts: A promising reaction medium in gaining access to terpyridines[J]. Front. Chem. Sci. Eng., 2019, 13(3): 586-598.
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Muhammad Faisal
Azeem Haider
Quret ul Aein
Aamer Saeed
Fayaz Ali Larik
Fig.1  Structure of terpyridine scaffolds
Fig.2  Scheme 1 Synthesis of deep eutectic ionic liquids
Fig.3  Scheme 2?Synthesis of 2,2':6',2"-terpyridines by using alkylated DABOC-PEG-300 based DEILs
Fig.4  List of synthesized 2,2′:6′,2″-terpyridines
Fig.5  List of synthesized 3,2':6',3''-terpyridines and 4,2':6',4''-terpyridines
Entry DABCO-ILs Refluxing time /h Yield of 9 /%
1 6a 3.5 86
2 6b 3.5 84
3 7a 3 87
4 7b 3 82
5 8a 3 96
6 8b 3 92
Tab.1  Yield of terpyridine 9 with various deep eutectic ionic liquids.
Molar ratio of PEG-300:5a Sample abbreviation Freezing point /°C
Freeze drying Rotary evaporation
1:1 DES1 ?61.1 ?61.4
1.5:1 DES2 ?65.2 ?65.3
2:1 DES3, 8a ?72.7 ?72.5
2.5:1 DES4 ?60.5 ?60.3
1:1.5 DES5 ?55.2 ?55.2
1:2 DES6 ?49.1 ?49.3
1:2.5 DES7 ?45.1 ?45.3
Tab.2  Freezing points of seven samples determined by rotary evaporation and freeze drying
Fig.6  Schematicphase diagram of5a in PEG-300.Tf: the freezing point; HBD:PEG-300; salt: 5a. The solid lines (red and blue) show the freezing point as a function of mixture composition, and the dashed black lines depictthe composition and temperature of the eutectic mixture
Fig.7  (a) Overlap of DTG (green) and TGA (red) graphs for DEIL8a; (b) Overlap of DSC (blue) and TGA (red) graphs for DEIL8a
Entry DEIL Product Yield/% of recovered DEIL
Cycle 1 Recycle 1 Recycle 2 Recycle 3 Recycle 4
1 8a 9 96 92 90 85 84
2 8b 9 92 86 84 83 80
3 8a 11 81 80 78 76 75
4 8b 11 78 72 72 71 70
Tab.3  Efficiency of recycled DEILs in synthesis of terpyridines
Entry Bases Solvent Reaction temp/°C Reaction time/h Yield/% Recyclability Ref.
1 KOH, NH4OAc MeOH r.t. 12 57 No [49]
2 KOH, aq. NH3 EtOH r.t. 2 53 No [50]
3 KOH, aq. NH3 EtOH r.t. 8 36 No [51]
4 KOH, NH4OH EtOH r.t. 4 62 No [52]
5 NaOH, aq. NH3 EtOH r.t. 12 69 No [53]
6 NaOH, AcOH, NH4OAc Solvent free conditions Reflux 2 89 No [54]
7 NH4OAc H2O 130 5 71 No [55]
8 NH4OAc H2O MWI 0.4 82 No [55]
9 NH4OAc Glycol MWI 0.33 80 No [22]
10 NaOH, NH4OAc Imidazolium-based IL 140 4 72 No [56]
11 NaOH, NH4OAc PEG-300 100 4 55 No [57]
12 NaOH, NH4OAc DEIL 8a 80 3 96 Yes This work
Tab.4  Synthesis of4'-phenyl-2,2':6',2''-terpyridine under various conditions
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