Synthesis and Crystal Structure of 1-Methyl-3-( 2-pyridyl ) imidazolium Hexafluorophosphate

The chemistry ofN-heterocyclic carbene (NHC) has become emerged field after the crystal structure of free 1,3-diadamantyl imidazol-2-ylidene was first reported in 1991 by Arduengo III et al. [1–3]. The chemical durability of NHC encouraged the chemist to study their reactivity with inorganic and organic compounds. The synthesis of NHC normally had been done by removal of C-2 proton of imidazolium salt with a suitable base. Many examples of imidazolium salts were synthesized and their corresponding NHC and NHC-metal compounds were studied [4, 5]. The interesting criteria of the imidazole are that, by tuning the substitution on nitrogen center at the imidazole ring, different types of functionalized imidazolium salts have been synthesized [6–9]. Herein, the modified synthesis of 1-Methyl-3-(2-pyridyl)imidazolium hexafluorophosphate (1) and its crystal structure have been discussed.


Introduction
The chemistry of N-heterocyclic carbene (NHC) has become emerged field after the crystal structure of free 1,3-diadamantyl imidazol-2-ylidene was first reported in 1991 by Arduengo III et al. [1][2][3].The chemical durability of NHC encouraged the chemist to study their reactivity with inorganic and organic compounds.The synthesis of NHC normally had been done by removal of C-2 proton of imidazolium salt with a suitable base.Many examples of imidazolium salts were synthesized and their corresponding NHC and NHC-metal compounds were studied [4,5].The interesting criteria of the imidazole are that, by tuning the substitution on nitrogen center at the imidazole ring, different types of functionalized imidazolium salts have been synthesized [6][7][8][9].Herein, the modified synthesis of 1-Methyl-3-(2-pyridyl)imidazolium hexafluorophosphate (1) and its crystal structure have been discussed.

Materials and Methods
A mixture 2-chloropyridine (1.35 g, 10 mmole) and 1-methylimidazole (0.821 g, 10 mmole) were heated at 150 ∘ C for 20 hours.To this mixture saturated aqueous solution of KPF 6 was added, while the mixture was hot and it results in the rapid precipitation of white color substance.The obtained white color precipitate was filtrated off and dried in vacuum.The colorless block type crystals of (MeIMPy)(PF 6 ) were obtained after one week from methanolic solution.Yield (based on 1-methylimidazole): 2-Chloropyridine (99.0%), 1-methylimidazole (99.0%), and KPF 6 (98.0%) were purchased from Sigma-Aldrich and used as received. 1H NMR spectra were obtained on a JEOL-JNM LAMBDA 400 model spectrometer operating at 400 MHz.The spectra were recorded in CD 3 OD solution, and the chemical shifts were referenced with respect to TMS.
Crystal was coated with light hydrocarbon oil and mounted in the 100 K dinitrogen stream of Bruker SMART APEX CCD diffractometer equipped with CRYO Industries low-temperature apparatus and intensity data were collected using graphite monochromated Mo K radiation ( = 0.71073 Å).The data integration and reduction were processed with SAINT software [10].An absorption correction was applied [11].Structure was solved by the direct method using SHELXS-97 and was refined on F 2 by full-matrix least-squares technique using the SHELXL-97 software package [12].Nonhydrogen atoms were refined anisotropically.In the refinement, hydrogen atoms were

Results and Discussion
1-Methyl-3-(2-pyridyl)imidazolium hexafluorophosphate (MeIMPy)(PF 6 ) has been synthesized by slight modification from reported procedure [13].The 2-chloropyridine and 1-methylimidazole were heated for 20 hours with neat condition.During the course of reaction, the reaction mixture slowly turned into brown viscous liquid.When the reaction mixture was in hot condition, saturated solution of KPF 6 was added.Immediately, white color precipitate appeared.The precipitates were filtered and dried in high vacuum.
The precipitates were filtered and dried in high vacuum (Scheme 1).
Single crystals of 1 were grown in methanolic solution at room temperature and a suitable crystal was subjected to the X-ray structural analysis.The crystal data reveals that the compound 1 crystallizes in the monoclinic system and centrosymmetric space group P2(1)/c.The solid state structure of 1 consists of one cation and one anion (Figure 1).The unit cell contains four of each cation and anion.The important bond lengths and bond angles are shown in Tables 2 and 3.
In the anion unit, the bond distances between phosphorous and fluorine atom are shown in the range of 1.590(19) Å to 1.608(17) Å and the bond angles suggest that the anion unit is almost perfect octahedron (Table 2).From the cation unit, the bond angle between (N1-C1-N2 108.3(2) ∘ ) is similar to a typical bond angle of imidazolium unit and it is in plane.
Table 2: Bond distances and angles of hexafluorophosphate unit.
Bond distances ( Å) P(1)-F(1) 1.608( 2 The pyridyl ring has slightly twisted from plane of imidazolium moiety. The solid state structure of 1 shows that the cations and anions are connected through intermolecular CH⋅ ⋅ ⋅ F type hydrogen bond interaction (Table 4).Each cation unit is hydrogen bonded to five separate anion units (Figure 2).CH⋅ ⋅ ⋅ F interaction between C2 position of five membered imidazolium rings and F4, C8 position of pyridyl moiety and F1 results in the 1D supramolecular polymer chains (Figures 3 and 4).

Summary
The solid state structure of 1-Methyl-3-(2-Pyridyl)imidazolium hexafluorophosphate has been discussed by using single crystal X-ray diffraction study.The different types of CH⋅ ⋅ ⋅ F supramolecular interaction and the formation of 1D supramolecular polymer chain between cation and anion unit by CH⋅ ⋅ ⋅ F weak interaction of titled compound have been investigated.

Table 1 :
Crystal data and data collection parameters for the compound (MeIMPy)(PF 6 ).
treated as riding atoms using SHELXL default parameters.The crystallographic figures, except Figure1, were generated using Diamond 3.1f software.Crystal data and data collection parameters for the compound 1 are given in Table1.CCDC 851202 contains the supplementary crystallographic data for 1.These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via http:// www.ccdc.cam.ac.uk/Community/Requestastructure/Pages/DataRequest.aspx.

Table 4 :
Details of inter-and intramolecular hydrogen bond.