The Synthesis of Gemini Ionic Salt in the Form of a (Tetra)Fluoborate of 1,4-Bis- ((1-Decyloimidazol-3-Yl)Methyl)Benzene


 Ionic liquids as anticorrosion agents have a wide spectrum of unique properties, which make them less harmful than the majority of typical organic corrosion inhibitors in antifreeze. The synthesis of ionic liquids enables them to be provided with the desired physicochemical and thermodynamical properties by the appropriate selection of cations, anions, and alkyl substituents on the cations. The aim of this paper is the synthesis of the gemini ionic salt (ionic liquid) made up of two imidazole rings with a linear alkyl substituent with 10 carbon atoms, bonded by a 1,4-dimethylphenyl moiety. Such compounds are studied in view of steel protection against oxidizing and corrosive effects. The compound was obtained in a satisfactory yield.


Introduction and aim of the article
In HVAC systems (heating, ventilation, and cooling), glycol-based antifreeze is a typical thermodynamic factor that mediates heat exchange. Antifreeze dedicated to HVAC systems is usually aqueous glycol solution with a concentration slightly higher than 33% (m/m). Under natural conditions, most structural metals will corrode strongly under the impact of the aggressive action of the mixture of water and glycol. For this reason, the fluid basic mixture for the production of the fluid is supplemented with a package of improvers, mainly corrosion and oxidation inhibitors, but also buffering substances and antifoam agents [10].
Glycol-based antifreeze is classified on the basis of its production technology; more precisely, it is classified on the basis of the character of chemical compounds used in the production process as improvers, especially as corrosion inhibitors. Packages of anticorrosion additives for glycol-based antifreeze can be composed of inorganic or organic compounds or may contain the so-called mixed package. Today, HVAC is developing rapidly and along with it, the popularity of antifreeze-containing organic compounds is also on the increase.
In liquids for HVAC systems, organic compounds that contain, e.g., nitrogen, sulfur, or phosphorus atoms are used as corrosion inhibitors. Typical organic compounds that inhibit corrosion processes are heterocyclic compounds, e.g., triazoles that, in aqueous solutions, show anticorrosive activity against a wide group of metals. Nowadays, there is a tendency to reduce the usage of this type of substances as corrosion inhibitors in liquids dedicated for HVAC systems due to numerous complaints concerning its harmful effect on health and the environment. Substances that are in line with the assumptions of 'green chemistry' and can be designed to achieve the desired anticorrosion properties are ionic liquids [11].
The physicochemical and thermodynamical properties of ionic liquids can be modified in a controlled manner, e.g., by changing the length of the substituent or the introduction of functional groups. In this way, it is possible to synthesize ionic liquids in view of particular applications.
In the last decade, the design of ionic liquids with different performance properties has been the subject of numerous scientific papers [1,3,4,5,7,12,13]. Gemini ionic liquids contain two cation groups bonded by rigid or flexible bonds. They are characterized by high thermal stability and high surface activity compared to traditional monocation salts. It is claimed that derivatives of imidazole have anticorrosion properties on steel. It is frequently attributed to the properties of chemical structures, e.g., electrophilic (hydrophilic) and hydrophobic groups. An imidazole ring is a hydrophilic element, and an alkyl group is treated as a hydrophobic group. Generally, it is adopted that a free electron pair on the nitrogen atom creates strong bonds with the free orbital of the iron atom [8,9], while the alkyl group is directed outwards. In this way, a protective layer is formed that prevents the surface from contact with the aggressive environment. The influence of the structure of rings as a place of direct adsorption on steel and the interaction between steel and inhibitor molecules may affect the efficiency of corrosion inhibition. Ionic liquids based on imidazole cations containing steric hindrance in the form of phenol and benzotriazale groups were investigated, i.e., by M. Cai. et al. [2].
The aim of this paper is the synthesis of the gemini ionic salt (ionic liquid) made up of two imidazole rings with a linear alkyl substituent on 10 carbon atoms, linked by a 1,4-dimethylphenyl moiety. Compounds with such complex molecule structures aren't commonly available on the market. Compounds with similar structure were examined in view of steel protection against oxidizing and corrosive effects [2,3]. The compound obtained will be applied in further research on the corrosive effect of antifreeze on elements of HVAC systems in industry and construction.

Methodology
The pathway for the synthesis of ionic salts was developed in two main steps on the basis of the literature that describes the synthesis of compounds with similar structure. In the first stage, 1-decyloimidazole (Lewis base) was obtained from imidazole and 1-bromodecane. The obtained compound was isolated and purified with the column chromatography method to use it as a starting material in the next step of the process. The isolation and purification method of the obtained substance was developed by the authors of this paper. Then, quaternization of nitrogen atoms using 1,4-(dibromomethyle)benzene and ion exchange reaction using sodium tetrafluoroborate were carried out. The expected final product was 1,4-bis -((1-decylimidazol-3-yl) methyl) benzene tetrafluoroborate.
In practice, the synthesis process was conducted in one step according to the following scheme:
The reaction progress was controlled using thin-layer chromatography (TLC). The precipitate of potassium bromide (KBr) formed in the reaction flask was filtered, and then the solvent was evaporated.
The obtained product was purified using a chromatography column. Silica gel (Sigma Aldrich, high-purity, 60 Å) was used as a stationary phase. Chloroform (CHEMPUR, cz.d.the.) was used as a mobile phase.
Then 0.005 M of 1,4-dibromoparaxylene (Sigma Aldrich, 98.0%) and 0.01 M of sodium tetrafluoroborate (Alfa Aesar, 97%) were placed in a three-neck flask. Reaction was carried out without solvent in an inert gas shield. 0,01 M of 1-decyloimidazole obtained in the first step of synthesis was added and heated for 6 hours. The NaBr precipitate formed during the reaction was filtered.

Identification of the structure
The infrared spectrum was recorded on the Thermo Scientific NICOLET iS10 FT-IR spectrometer allowing analysis in the range of 4000 -400 cm -1 .
NMR spectra were obtained on a Bruker AVANCE III HD 500 MHz spectrometer. The samples were dissolved in deuterated chloroform (CDCl3). Measurements were made at 25°C.
Measurements with high-resolution mass spectrometry (HRMS) were conducted on a Synapt G2-Si (Waters) mass spectrometer fitted with an ESI source and a quadrupole time of flight (TOF) analyzer. Mass spectrometer operated in positive ion mode.

Results
The structure of 1-decyloimidazole obtained in the first step of the synthesis was confirmed by instrumental methods: FT-IR, MS, and NMR. The IR spectrum obtained for 1-decyloimidazole was compared with the reference spectrum of 1-decyloimidazole available also in Wiley's Spectral Libraries [6]. Functional groups, which characterize chemical compounds, absorb infrared radiation within a certain range, and it occurs very rarely that two substances have identical spectra over the entire range. As a result, in practice, unambiguous identification is possible.   The yield of the process was calculated from the following formula:    The peaks of m/z 509 observed in the spectra correspond to the proton structure of 1,4bis-((1-decyl-imidazol-3-yl)methyl)benzene. The mass calculated for such an ion is 519.4427, but the experimental value is 519.4410, which indicates an ion with the composition of C34H55N4. The next peak, observed in the positive ion mode, is a peak corresponding to the ion of 1,4-bis-((1-decyl-imidazol-3-yl)methyl)benzene with one counterion BF4 -. The measured mass of this ion is 607.4537, and the calculated mass is 607.4537.
NMR analyzes confirm the expected compound structure C34H56N4(BF4)2. The size of the integrals obtained indicates a symmetrical compound structure with one benzene ring substituted in the para position -by the CH2 group with two decyloimidazole groups.

Conclusions
The aim of this paper was to conduct a multiple-phase synthesis of a substance with the potential anti-corrosive effect. This resulted in obtaining the expected product in satisfactory yield.
In the first reaction, 1-decyloimidazole (Lewis base) was obtained from imidazole and 1-bromodecane in a yield of 91,2%. The obtained structure of 1-decylimidazole was confirmed by IR method by comparison with a reference spectrum, by the MS technique (mass spectrometry), and by 1H and 13C NMR techniques.
In the second process of quaternizing nitrogen atoms with 1,4-(dibromomethyl) benzene and simultaneous ion exchange reaction with sodium tetrafluoroborate, the compound C34H56N4 (BF4) 2 was obtained in a yield of 80.2%. The structure of the salt C34H56N4 (BF4) 2 was confirmed by instrumental techniques: MS, 1H, 13C NMR. The compound was obtained in satisfactory yield.
Ionic liquids are a valuable research material. Because of the possibility of designing their structures towards unique properties, they can be used in many industries.