2-Ethyl-4-methyl-1H-imidazol-3-ium bromide

Butcher, R.J.; Purdy, A.P.

doi:10.1107/S2414314622011725

organic compounds

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Volume 7| Part 12| December 2022| x221172

https://doi.org/10.1107/S2414314622011725

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2-Ethyl-4-methyl-1H-imidazol-3-ium bromide

Ray J. Butcher ^a ^* and Andrew P. Purdy ^b

^aDepartment of Chemistry, Howard University, 525 College Street NW, Washington DC 20059, USA, and ^bChemistry Division, Code 6123, Naval Research Laboratory, 4555 Overlook Av, SW, Washington DC 20375-5342, USA
^*Correspondence e-mail: rbutcher99@yahoo.com

Edited by W. T. A. Harrison, University of Aberdeen, United Kingdom (Received 27 October 2022; accepted 6 December 2022; online 15 December 2022)

In the title molecular salt, C₆H₁₁N₂⁺·Br⁻, the components are linked by N—H⋯Br⋯H—N hydrogen bonds into C(8)chains of alternating cations and anions propagating in the b-axis direction; these chains are cross-linked in the c-axis direction by weak C—H⋯Br hydrogen bonds.

Keywords: crystal structure; imidazolium cations; hydrogen bonding.

CCDC reference: 2224817

Structure description

The unique structure of imidazole, containing two N atoms in a five-membered ring, permits it to accept a proton on one of its N atoms to form a cation and simultaneously deliver another proton from the other N atom to a suitable acceptor. In fact, this sort of shuttling action has been proposed as part of the catalytic mechanism of a number of enzymes (Mikulski & Silverman, 2010 ), and is consistent with the proton-conductivity properties of imidazole in the solid state where long hydrogen-bonded chains are present (Kawada et al., 1970 ). These moieties and their derivatives have been implicated in proton-coupled electron-transfer processes (Huynh & Meyer, 2007 ; Onidas et al., 2010 ). Consequently, there have been many theoretical (Scheiner & Yi, 1996 ; Kumar & Venkatnathan, 2015 ) and structural studies (Purdy et al., 2007 ; Kim et al., 2016 ) investigating these species. In this paper, we report a crystal structure containing the 2-ethyl-4-methyl-1H-imidazol-3-ium (C₆H₁₁N₂⁺) cation. There have been four previous reports of structures containing this species (CSD refcode LEZSAL, Amanokura et al., 2007 ; POJFOL, Beckett et al., 2014 ; HOJJAT, Arici et al., 2014 ; UMALAX, Kazimierczuk et al., 2016 ).

The title salt, 1, crystallizes in the monoclinic space group P2₁/c with one ion pair in the asymmetric unit (Fig. 1) and consists of C₆H₁₁N₂⁺ cations and Br⁻ anions. The C8 methyl group is close to coplanar with the imidazole ring [N1—C2—C7—C8 = −8.03 (15)°]. Otherwise, the metrical parameters of the cation agree well with those observed in the other structures involving this species. In the extended structure, the component ions are linked by N—H⋯Br⋯H—N hydrogen bonds (Table 1) into C(8) (Etter et al., 1990 ) chains propagating in the b-axis direction. The chains are cross-linked in the c-axis direction by weak C—H⋯Br hydrogen bonds (Fig. 2).

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
N1—H1N⋯Brⁱ	0.829 (17)	2.446 (17)	3.2490 (9)	163.3 (16)
N3—H3N⋯Br	0.780 (16)	2.485 (16)	3.2642 (8)	176.6 (16)
C5—H5A⋯Brⁱⁱ	0.95	2.93	3.7842 (10)	151
C6—H6C⋯Brⁱⁱⁱ	0.98	3.08	3.8349 (11)	135
C7—H7B⋯Br^iv	0.99	2.93	3.8771 (11)	161
Symmetry codes: (i) $[-x+1, y+{\script{1\over 2}}, -z+{\script{1\over 2}}]$ ; (ii) $[x+1, -y+{\script{1\over 2}}, z+{\script{1\over 2}}]$ ; (iii) x+1, y, z; (iv) $[x, -y+{\script{1\over 2}}, z-{\script{1\over 2}}]$ .

Figure 1
The molecular structure of 1 showing 30% displacement ellipsoids. The hydrogen bond is shown with a dashed line.

Figure 2
Packing diagram of 1 viewed down [100] showing how the cations and anions are linked into C(8)chains propagating in the b-axis direction.

Synthesis and crystallization

The title compound resulted from an attempt to link two 2-ethyl-4-methylimidazole rings with a two-carbon chain by the reaction of 2-Et-4-Me-imidazole (6.20 g, 56.3 mmol) with BrCH₂CH₂Br (5.32 g, 28.3 mmol) in EtOH at 80°C overnight and several hours at 100°C. Ba(OH)₂·8H₂O (8.95 g, 28.3 mmol) was added with ethanol and water and heated to dissolve. On cooling, the mixture was rotovapped down and extracted between water and ether, and the ether layer was evaporated down to 3.1 g of an oil identified as primarily the starting imidazole by NMR. Recovery of about half of the starting imidazole must mean that the oligomer forms preferentially over the dimer. The barium ion was removed from the water layer by titration with H₂SO₄ followed by filtration. The solution was rotovapped down to an oil that precipitated a mass of salts on cooling. More crystals of 1 crystallized from the oil over time, and were washed with i-PrOH to remove the oil for NMR. NMR of 1 in D₂O, DSS ref: ¹H, 1.26 (t, 3H), 2.88 (q, 2H) (Et), 2.20 (s, 3H) (Me), 4.70 (s, 1H) (C—H), 6.95 (s, 2H) (N—H); ¹³C, 11.8 (Me), 13.3, 21.8 (Et), 117.1 (C—H), 131.4 (4-C), 150.7 (2-C).

Refinement

Crystal data, data collection and structure refinement details are summarized in Table 2.

Table 2
Experimental details

Crystal data
Chemical formula	C₆H₁₁N₂⁺·Br⁻
M_r	191.08
Crystal system, space group	Monoclinic, P2₁/c
Temperature (K)	100
a, b, c (Å)	6.8432 (6), 15.5962 (13), 7.5748 (7)
β (°)	94.360 (4)
V (Å³)	806.10 (12)
Z	4
Radiation type	Mo Kα
μ (mm⁻¹)	5.02
Crystal size (mm)	0.25 × 0.15 × 0.15

Data collection
Diffractometer	Bruker APEXII CCD
Absorption correction	Multi-scan (SADABS; Krause et al., 2015 )
T_min, T_max	0.571, 0.747
No. of measured, independent and observed [I > 2σ(I)] reflections	24466, 3936, 3324
R_int	0.027
(sin θ/λ)_max (Å⁻¹)	0.836

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.019, 0.042, 1.03
No. of reflections	3936
No. of parameters	92
H-atom treatment	H atoms treated by a mixture of independent and constrained refinement
Δρ_max, Δρ_min (e Å⁻³)	0.53, −0.36
Computer programs: APEX2 and SAINT (Bruker, 2014 ), SHELXT (Sheldrick 2015a ), SHELXL2018/3 (Sheldrick, 2015b ) and SHELXTL (Sheldrick 2008 ).

Structural data

CCDC reference: 2224817

Crystal structure: contains datablock I. DOI: https://doi.org/10.1107/S2414314622011725/hb4418sup1.cif

Structure factors: contains datablock I. DOI: https://doi.org/10.1107/S2414314622011725/hb4418Isup2.hkl

Supporting information file. DOI: https://doi.org/10.1107/S2414314622011725/hb4418Isup3.cml

checkCIF report

Computing details top

Data collection: APEX2 (Bruker, 2014); cell refinement: SAINT (Bruker, 2014); data reduction: SAINT (Bruker, 2014); program(s) used to solve structure: SHELXT (Sheldrick 2015a); program(s) used to refine structure: SHELXL2018/3 (Sheldrick, 2015b); molecular graphics: SHELXTL (Sheldrick 2008); software used to prepare material for publication: SHELXTL (Sheldrick 2008).

2-Ethyl-4-methyl-1H-imidazol-3-ium bromide top

Crystal data top

C₆H₁₁N₂⁺·Br⁻	F(000) = 384
M_r = 191.08	D_x = 1.574 Mg m⁻³
Monoclinic, P2₁/c	Mo Kα radiation, λ = 0.71073 Å
a = 6.8432 (6) Å	Cell parameters from 9911 reflections
b = 15.5962 (13) Å	θ = 3.0–36.4°
c = 7.5748 (7) Å	µ = 5.02 mm⁻¹
β = 94.360 (4)°	T = 100 K
V = 806.10 (12) Å³	Prism, colorless
Z = 4	0.25 × 0.15 × 0.15 mm

Data collection top

Bruker APEXII CCD diffractometer	3324 reflections with I > 2σ(I)
φ and ω scans	R_int = 0.027
Absorption correction: multi-scan (SADABS; Krause et al., 2015)	θ_max = 36.5°, θ_min = 2.6°
T_min = 0.571, T_max = 0.747	h = −11→11
24466 measured reflections	k = −26→26
3936 independent reflections	l = −12→12

Refinement top

Refinement on F²	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F² > 2σ(F²)] = 0.019	Hydrogen site location: mixed
wR(F²) = 0.042	H atoms treated by a mixture of independent and constrained refinement
S = 1.03	w = 1/[σ²(F_o²) + (0.0166P)² + 0.2838P] where P = (F_o² + 2F_c²)/3
3936 reflections	(Δ/σ)_max < 0.001
92 parameters	Δρ_max = 0.53 e Å⁻³
0 restraints	Δρ_min = −0.36 e Å⁻³

Special details top

Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Refinement. All hydrogen atoms were located in difference Fourier maps and those attached to N were refined isotropically. Those attached to carbon atoms were refined in idealized geometry using a riding model with with atomic displacement parameters of U_iso(H) = 1.2U_eq(C) [for CH₃, 1.5U_eq(C)] with C—H distances of 0.95 to 0.99 Å.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å²) top

	x	y	z	U_iso*/U_eq
Br	0.28697 (2)	0.12428 (2)	0.15497 (2)	0.01551 (3)
N1	0.63750 (12)	0.41897 (5)	0.31428 (11)	0.01531 (14)
H1N	0.631 (3)	0.4721 (11)	0.314 (2)	0.037 (5)*
C2	0.49850 (13)	0.36487 (6)	0.25237 (11)	0.01449 (16)
N3	0.56746 (12)	0.28564 (5)	0.28135 (11)	0.01568 (13)
H3N	0.503 (2)	0.2457 (9)	0.254 (2)	0.026 (4)*
C4	0.75365 (14)	0.28880 (6)	0.36671 (13)	0.01609 (15)
C5	0.79682 (13)	0.37312 (6)	0.38762 (13)	0.01694 (15)
H5A	0.914514	0.396452	0.442412	0.020*
C6	0.86990 (17)	0.21085 (7)	0.41511 (15)	0.02302 (19)
H6A	0.793013	0.172611	0.485620	0.035*
H6B	0.990828	0.227271	0.484469	0.035*
H6C	0.902634	0.181162	0.307207	0.035*
C7	0.30245 (14)	0.38674 (7)	0.16718 (13)	0.01933 (17)
H7A	0.201752	0.353879	0.225655	0.023*
H7B	0.296411	0.368801	0.041511	0.023*
C8	0.25416 (18)	0.48149 (8)	0.17584 (17)	0.0284 (2)
H8A	0.120458	0.491216	0.123853	0.043*
H8B	0.346648	0.514173	0.109621	0.043*
H8C	0.264154	0.500287	0.299648	0.043*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
Br	0.01508 (4)	0.01385 (4)	0.01734 (4)	0.00039 (3)	−0.00055 (3)	−0.00153 (3)
N1	0.0149 (3)	0.0141 (3)	0.0167 (3)	−0.0012 (3)	−0.0004 (3)	−0.0013 (3)
C2	0.0152 (4)	0.0147 (4)	0.0135 (3)	−0.0015 (3)	0.0006 (3)	−0.0010 (3)
N3	0.0159 (3)	0.0150 (3)	0.0162 (3)	−0.0026 (3)	0.0014 (3)	−0.0022 (3)
C4	0.0150 (4)	0.0173 (4)	0.0160 (4)	0.0012 (3)	0.0015 (3)	−0.0011 (3)
C5	0.0131 (3)	0.0187 (4)	0.0188 (4)	−0.0006 (3)	−0.0001 (3)	−0.0023 (3)
C6	0.0240 (5)	0.0210 (4)	0.0244 (5)	0.0076 (4)	0.0038 (4)	0.0006 (4)
C7	0.0166 (4)	0.0238 (4)	0.0168 (4)	−0.0007 (3)	−0.0036 (3)	−0.0005 (3)
C8	0.0238 (5)	0.0265 (5)	0.0331 (6)	0.0058 (4)	−0.0089 (4)	0.0014 (4)

Geometric parameters (Å, º) top

N1—C2	1.3299 (12)	C6—H6A	0.9800
N1—C5	1.3844 (13)	C6—H6B	0.9800
N1—H1N	0.829 (17)	C6—H6C	0.9800
C2—N3	1.3351 (12)	C7—C8	1.5167 (16)
C2—C7	1.4836 (13)	C7—H7A	0.9900
N3—C4	1.3851 (12)	C7—H7B	0.9900
N3—H3N	0.780 (16)	C8—H8A	0.9800
C4—C5	1.3545 (14)	C8—H8B	0.9800
C4—C6	1.4836 (14)	C8—H8C	0.9800
C5—H5A	0.9500

C2—N1—C5	109.52 (8)	H6A—C6—H6B	109.5
C2—N1—H1N	126.7 (12)	C4—C6—H6C	109.5
C5—N1—H1N	123.7 (12)	H6A—C6—H6C	109.5
N1—C2—N3	107.15 (8)	H6B—C6—H6C	109.5
N1—C2—C7	127.33 (8)	C2—C7—C8	113.45 (8)
N3—C2—C7	125.52 (8)	C2—C7—H7A	108.9
C2—N3—C4	110.17 (8)	C8—C7—H7A	108.9
C2—N3—H3N	120.7 (12)	C2—C7—H7B	108.9
C4—N3—H3N	129.1 (12)	C8—C7—H7B	108.9
C5—C4—N3	105.90 (8)	H7A—C7—H7B	107.7
C5—C4—C6	131.20 (10)	C7—C8—H8A	109.5
N3—C4—C6	122.89 (9)	C7—C8—H8B	109.5
C4—C5—N1	107.24 (8)	H8A—C8—H8B	109.5
C4—C5—H5A	126.4	C7—C8—H8C	109.5
N1—C5—H5A	126.4	H8A—C8—H8C	109.5
C4—C6—H6A	109.5	H8B—C8—H8C	109.5
C4—C6—H6B	109.5

C5—N1—C2—N3	−1.28 (10)	N3—C4—C5—N1	−0.29 (11)
C5—N1—C2—C7	178.55 (9)	C6—C4—C5—N1	178.35 (10)
N1—C2—N3—C4	1.11 (10)	C2—N1—C5—C4	0.98 (11)
C7—C2—N3—C4	−178.73 (9)	N1—C2—C7—C8	−8.03 (15)
C2—N3—C4—C5	−0.50 (11)	N3—C2—C7—C8	171.77 (10)
C2—N3—C4—C6	−179.27 (9)

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
N1—H1N···Brⁱ	0.829 (17)	2.446 (17)	3.2490 (9)	163.3 (16)
N3—H3N···Br	0.780 (16)	2.485 (16)	3.2642 (8)	176.6 (16)
C5—H5A···Brⁱⁱ	0.95	2.93	3.7842 (10)	151
C6—H6C···Brⁱⁱⁱ	0.98	3.08	3.8349 (11)	135
C7—H7B···Br^iv	0.99	2.93	3.8771 (11)	161

Symmetry codes: (i) −x+1, y+1/2, −z+1/2; (ii) x+1, −y+1/2, z+1/2; (iii) x+1, y, z; (iv) x, −y+1/2, z−1/2.

Funding information

RJB wishes to acknowledge the ONR Summer Faculty Research Program for funding in 2019 and 2020.

References

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https://doi.org/10.1107/S2414314622011725

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organic compounds\(\def\hfill{\hskip 5em}\def\hfil{\hskip 3em}\def\eqno#1{\hfil {#1}}\)

2-Ethyl-4-methyl-1H-imidazol-3-ium bromide

Structure description

Synthesis and crystallization

Refinement

Structural data

Funding information

References

organic compounds