Synthesis, hydrogen bond interactions and crystal structure elucidation of some stable 2H-imidazolium salts

1 Introduction

Hydrogen bonds are important in chemistry and biochemistry. It is fundamental to the double helix arrangement of the DNA molecule [1], the α-helix in protein [2] and β-pleated sheet formation [3] in a polypeptide chain. Also many amino acids contain alcohol or thiol groups that additionally may form hydrogen bonds and stabilize a particular protein conformation.

In 2H-imidazolium salts which are considered as ionic liquid compounds, the imidazolium ion is commonly linked to the counter anion by hydrogen bonds.

In continuing our investigation of the hydrogen bonds formed by the interaction between 2H-imidazolium ion with sulfur, oxygen and nitrogen atoms [4], [5], [6], [7], [8], [9], [10], [11], [12], we report the synthesis and crystal structure elucidation of some stable 2H-imidazolium salts.

2 Results and discussion

The imidazolium phthalimide (2) and imidazolium quinazolinedione (3) are formed readily by the reaction of the carbene 1 with phthalimide and quinazolinedione in THF (Scheme 1). Single crystals of compounds 2 and 3 were obtained by recrystallization from Acetone-Et₂O by vapor diffusion. Salt 2 crystallizes in the monoclinic space group P2₁/c with Z = 4, while salt 3 crystallizes in the triclinic space group P 1 ‾ with Z = 2 (Tables 1 –3, Figures 1 and 2).

Scheme 1:

Synthesis of 2H-imidazolium salts 2–5. Reagents and conditions: i. phthalimide, THF, −50 °C, overnight; ii. quinazolinedione, THF, −50 °C, overnight; iii. thiophenole, Et₂O, −50 °C, overnight; iv. pyridinethiole, Et₂O, −50 °C, overnight.

Figure 1:

Molecular structure of the ion pair of imidazolium phthalimide (2) in the crystal.

Figure 2:

Molecular structure of the ion pair of imidazolium quinazolinedione (3) in the crystal.

The crystal structure analyses of 2 and 3 reveal that the heterocyclic anions and imidazolium cations are linked by hydrogen bonds. A summary of important hydrogen bond interactions is given in Table 6 below. In crystals of 2, the imidazolium ion and phthalimide anion are linked by C2–H2···O30 = 2.216(3) Å, while in crystals of 3, the imidazolium cation and quinazolinedione anion are linked by C24–H24···N10 = 2.247(2) Å. In both compounds 2 and 3, the interionic hydrogen bonds are close to linearity (2: C2–H2···O30 = 177.8(9)° and 3: C24–H24···N10 = 173.6(9)°).

The bond lengths of phthalimide anion in 2 [C(28)–O(30) = 1.24(2) Å, C(26)–O(29) = 1.229(2) Å, C(28)–N(27) = 1.36(2) Å and C(26)–N(27) = 1.377(2) Å] indicate that there is a symmetrical distribution of negative charge around N(27). The bonds lengths of quinazolinedione anion in 3 [C(9)–O(12) = 1.248(2) Å, C(7)–O(11) = 1.23(2) Å, C(9)–N(10) = 1.339(2) Å, C(9)–N(8) = 1.40(2) Å and C(7)–N(8) = 1.36(2) Å] indicate that the negative charge is mainly delocalized over O(12)–C(9)–N(10).

The packing of 2 in the crystal shows no interaction between the carbonyl oxygen of the phthalimide anion and methyl protons of a neighboring imidazolium cation. While in 3, the molecule packing gives evidence to very weak interactions between the carbonyl oxygen atom of the quinazolinedione anion and methyl protons of a neighboring imidazolium cation [O(11)···H(28c) = 2.520(2) Å and O(11)···H(29b) = 2.550(2) Å].

The imidazolium thiophenolate 4 and imidazolium 4-pyridinethiolate 5 are obtained from the reaction of carbene 1 with thiophenol and 4-pyridinethiol, respectively, in diethyl ether (Scheme 1). Both salts 4 and 5 crystallize in the monoclinic space group P2₁/c with Z = 4 (Tables 1, 4 –6, Figures 3 and 4).

Figure 3:

Molecular structure of the ion pair of imidazolium thiophenolate (4) in the crystal.

Figure 4:

Molecular structure of the ion pair of imidazolium 4-pyridinethiolate (5) in the crystal.

The crystal structure analyses of 4 and 5 reveal that the heterocyclic anions and imidazolium cations are linked by hydrogen bonds. In crystals of 4, the imidazolium cation and thiophenolate anion are linked by C10–H10···S1 = 2.492(10) Å, while in crystals of 5, the imidazolium cation and 4-pyridinethiolate anion are linked by C1–H1···N18 = 2.283(2) Å. In compound 4 the interionic hydrogen bond angle is C10–H10···S1 = 148.5(9)°, while in 5 the interionic hydrogen bond is close to linearity [C1–H1···N18 = 171.7(10)°]. The C–S bond length in the 4-pyridinethiolate ion [C(15)–S(14) = 1.723(2) Å] is shorter than the C–S bond length in thiophenolate ion [C(1)–S(1) = 1.749(3) Å].

A correlation between the H···X distance (X: O, N or S) and the electronegativity of X was observed. It has been found that H···X distance decreases with the increase in electronegativity of X. Also, there is a correlation between the H···X distance and the C–H···X angle. As H···X distance decreases, the C–H···X angle becomes closer to linear (Table 6).

3 Conclusions

Stable 2H-imidazolium salts (2, 3, 4 and 5) have been prepared and characterized through NMR spectroscopy, elemental analyses and single-crystal X-ray diffraction. The structure analyses of the prepared salts revealed that anions and cations are linked by strong hydrogen bonds. Correlation between the H···X distance and the electronegativity of X atoms has been observed. The H···O distance in 2 was shorter than H···N distance in 3 and 5 and the H···S distance in 4.

4 Experimental section

All reactions were performed in purified solvents under argon. 1,3-Diisopropyl-4,5-dimethyl-4,5-dimethylimidazol-2-ylidene (1) was obtained according to a published procedure [13].