1,3-Bis(4-bromo­phen­yl)-1H-imidazol-3-ium tetra­fluoro­borate

Ibrahim, H.; Zamisa, S.J.; Bala, M.D.; Ntola, P.

doi:10.1107/S2414314626002531

organic compounds

IUCrDATA

ISSN: 2414-3146

Volume 11| Part 3| March 2026| x260253

https://doi.org/10.1107/S2414314626002531

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1,3-Bis(4-bromophenyl)-1H-imidazol-3-ium tetrafluoroborate

Halliru Ibrahim,^a ^* Sizwe J. Zamisa,^b Muhammad D. Bala ^b and Pinkie Ntola ^a

^aDepartment of Chemistry, Durban University of Technology, PO Box 1334, Durban, 4000, South Africa, and ^bSchool of Agriculture and Science, Discipline of Chemistry, University of KwaZulu-Natal, Private Bag X54001, Westville, Durban, 4000, South Africa
^*Correspondence e-mail: [email protected]

(Received 5 March 2026; accepted 10 March 2026; online 17 March 2026)

The crystal of the title salt, C₁₅H₁₁Br₂N₂⁺·BF₄⁻, contains one half of a 1,3-bis(4-bromophenyl)imidazolium cation and one half of a tetrafluoroborate anion in the asymmetric unit; the complete ions are generated by crystallographic twofold symmetry. The imidazolium cation adopts a syn-periplanar conformation, with the 4-bromophenyl rings inclined to the central imidazolium ring by 36.04 (4)°. In the crystal, the tetrafluoroborate anion participates in structure-directing halogen-bonding contacts through F⋯Br and F⋯π(phenyl) interactions. These are complemented by intermolecular π–π and Br⋯π interactions, which assemble the ions into two-dimensional supramolecular sheets lying parallel to the ac plane. The sheets are further linked by C—H⋯F hydrogen bonds, forming ring motifs of graph sets R²₂(7) and R²₁(4) within a three-dimensional supramolecular network.

Keywords: crystal structure; halogen bonds; imidazolium; tetrafluoroborate salt.

CCDC reference: 2536403

Structure description

The title compound is a 1,3-diarylsubstituted imidazolium salt, which was originally synthesized via a method involving mechano-grinding and acidification (Ikhile et al., 2011 ). The synthetic method employed herein is a `green' modification of the initial method (Arduengo et al., 1992 ). While the synthesis of the title compound has been reported (Ikhile et al., 2011), crystallographic details and those of analogous 1,3-bis(4-halophenyl)imidazolium tetrafluoroborates are not available. The title compound and its related analogues bearing ferrocenyl moieties have found application as green catalysts for the transfer hydrogenation of ketones (Ikhile et al., 2012 , 2013 ). Recent work on similar 1,3-diarylimidazoliumm salts (Ndlovu et al., 2017 ) and reviews on the biological activity of non-heteroatom functionalized azolium salts (Patil et al., 2020 ; Fletcher et al., 2018 ; Mercs & Albrecht, 2010 ) have also provided evidence on the structure/activity trends in their well established potential as anti-fungal, anti-bacterial and anti-proliferative agents. As part of our work in developing new imidazolium derivatives with impressive anti-microbial activities (Kadafour et al., 2022 ; Ndlovu et al., 2017), we synthesized the title compound and determined its crystal structure.

The asymmetric unit of the title compound comprises half a cationic 1,3-bis(4-bromophenyl)imidazolium species and half a tetrafluoroborate counter-ion, with the complete ions being generated by a C₂ rotation axis that runs parallel to the C5—H5 bond (Fig. 1). The cationic species adopts a syn-periplanar conformation with the dihedral angle between the mean planes of the central imidazolium ring and the 4-bromophenyl wingtip being 36.04 (4)°, which is wider than that observed in the hydrated chloride analogue of the title compound, i.e. 2.9 (1)° (Garden et al., 2010 ).

Figure 1
Molecular structure of the title compound showing the atom-numbering scheme and displacement ellipsoids drawn at the 50% probability level. Symmetry code: (i) −x + 1, y, −z + $[{3\over 2}]$ .

The extended structure features halogen bonding that is driven by the tetrafluoroborate moiety via F1⋯π(phenyl) [F1⋯Cg(phenyl) = 3.5669 (12) Å, symmetry operation: $[{3\over 2}]$ − x, $[{1\over 2}]$ + y, $[{3\over 2}]$ − z and F2⋯Br1 (F2⋯Br1 = 2.8890 (12) Å, symmetry code: $[{1\over 2}]$ + x, $[{3\over 2}]$ − y, − $[{1\over 2}]$ + z] interactions. In conjunction with the halogen-bonding patterns involving F atoms, intermolecular π(phenyl)–π(phenyl) [Cg⋯Cg = 3.6720 (9) Å, symmetry operation: 1 − x, 1 − y, 1 − z] and π(phenyl)⋯Br1 [Br⋯π = 3.9061 (6) Å, symmetry operation: 1 − x, 1 − y, 1-z; x, 1 − y, − $[{1\over 2}]$ − z) interactions occur within two-dimensional supramolecular networks parallel to the ac plane (Fig. 2). Finally, intermolecular C—H⋯F hydrogen bonds with graph-set descriptors R₂²(7) and R₁²(4), link the two-dimensional supramolecular networks along the b axis within a three-dimensional framework (Table 1, Fig. 3).

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
C5—H5⋯F1ⁱ	0.95	2.29	3.156 (2)	151
C6—H6⋯F2ⁱⁱ	0.95	2.39	3.1854 (19)	140
Symmetry codes: (i) ; (ii) $[-x+1, y, -z+{\script{3\over 2}}]$ .

Figure 2
Representation of π(phenyl)–π(phenyl), π(phenyl)⋯Br and F⋯Br interactions in the crystal of the title compound.

Figure 3
Representation of C—H⋯F hydrogen bonds in the crystal of the title compound.

Synthesis and crystallization

The details on the synthesis of the title compound have been reported (Ikhile et al., 2011). The starting materials were 1,3-bis(4-bromophenyl)ethylenediimine (1.5 g; 4.10 mmol), paraformaldehyde (0.123 g: 4.10 mmol) and HBF₄ (0.54 ml; 0.36 g; 4.10 mmol). Brown precipitate: 0.9603 g (yield: 62%); m.p. 181 °C; IR (ATR cm⁻¹): 3126, 1590, 1491, 1401, 1295, 1122, 1059, 1015, 988, 960, 827, 617, 592, 523. ¹H NMR (400 MHz, DMSO-d₆): δ 7.88 (4H, d, J = 8.9, Ar—H), 7.97 (4H, d, J = 8.9, Ar—H), 8.57 (2H, s, Imid-CH=CH) and 10.37 p.p.m. (1H, s, NCHN). All other spectroscopic data matched those previously reported. Suitable crystals for X-ray diffraction analysis were grown by layering the DMSO solution with diethyl ether.

Refinement

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

Table 2
Experimental details

Crystal data
Chemical formula	C₁₅H₁₁Br₂N₂⁺·BF₄⁻
M_r	465.89
Crystal system, space group	Monoclinic, C2/c
Temperature (K)	100
a, b, c (Å)	9.5407 (2), 9.8272 (2), 17.0451 (3)
β (°)	102.878 (1)
V (Å³)	1557.92 (5)
Z	4
Radiation type	Mo Kα
μ (mm⁻¹)	5.25
Crystal size (mm)	0.37 × 0.28 × 0.21

Data collection
Diffractometer	Bruker SMART APEXII area detector
Absorption correction	Multi-scan (SADABS; Krause et al., 2015 )
T_min, T_max	0.618, 0.746
No. of measured, independent and observed [I > 2σ(I)] reflections	9293, 1922, 1748
R_int	0.023
(sin θ/λ)_max (Å⁻¹)	0.667

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.017, 0.038, 1.06
No. of reflections	1922
No. of parameters	110
H-atom treatment	H-atom parameters constrained
Δρ_max, Δρ_min (e Å⁻³)	0.37, −0.29
Computer programs: APEX2, COSMO and SAINT (Bruker, 2009 ), SHELXS2013 (Sheldrick, 2008 ), SHELXL2018/3 (Sheldrick, 2015 ) and OLEX2 (Dolomanov et al., 2009 ).

Structural data

CCDC reference: 2536403

Crystal structure: contains datablock I. DOI: https://doi.org/10.1107/S2414314626002531/tk4122sup1.cif

Structure factors: contains datablock I. DOI: https://doi.org/10.1107/S2414314626002531/tk4122Isup2.hkl

Supporting information file. DOI: https://doi.org/10.1107/S2414314626002531/tk4122Isup3.cml

checkCIF report

Computing details top

1,3-Bis(4-bromophenyl)-1H-imidazol-3-ium tetrafluoroborate top

Crystal data top

C₁₅H₁₁Br₂N₂⁺·BF₄⁻	F(000) = 904
M_r = 465.89	D_x = 1.986 Mg m⁻³
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71073 Å
a = 9.5407 (2) Å	Cell parameters from 5032 reflections
b = 9.8272 (2) Å	θ = 2.5–28.2°
c = 17.0451 (3) Å	µ = 5.25 mm⁻¹
β = 102.878 (1)°	T = 100 K
V = 1557.92 (5) Å³	Block, colourless
Z = 4	0.37 × 0.28 × 0.21 mm

Data collection top

Bruker SMART APEXII area detector diffractometer	1922 independent reflections
Radiation source: microfocus sealed X-ray tube, Incoatec Iµs	1748 reflections with I > 2σ(I)
Mirror optics monochromator	R_int = 0.023
Detector resolution: 7.9 pixels mm^-1	θ_max = 28.3°, θ_min = 2.5°
ω and φ scans	h = −12→12
Absorption correction: multi-scan (SADABS; Krause et al., 2015)	k = −12→13
T_min = 0.618, T_max = 0.746	l = −22→22
9293 measured reflections

Refinement top

Refinement on F²	0 restraints
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F² > 2σ(F²)] = 0.017	H-atom parameters constrained
wR(F²) = 0.038	w = 1/[σ²(F_o²) + (0.0133P)² + 2.086P] where P = (F_o² + 2F_c²)/3
S = 1.06	(Δ/σ)_max = 0.002
1922 reflections	Δρ_max = 0.37 e Å⁻³
110 parameters	Δρ_min = −0.29 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.

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

	x	y	z	U_iso*/U_eq
Br1	0.79285 (2)	0.44748 (2)	0.42461 (2)	0.01694 (5)
F1	0.60495 (11)	1.28959 (10)	0.79615 (6)	0.0298 (2)
F2	0.43907 (13)	1.12447 (11)	0.79918 (7)	0.0379 (3)
N1	0.54845 (13)	0.66867 (12)	0.69843 (7)	0.0130 (2)
C1	0.71272 (15)	0.51725 (16)	0.50861 (9)	0.0153 (3)
C2	0.63143 (15)	0.63529 (15)	0.49678 (9)	0.0150 (3)
H2	0.613229	0.680767	0.446339	0.018*
C3	0.57701 (15)	0.68594 (15)	0.56002 (9)	0.0145 (3)
H3	0.520537	0.766483	0.553292	0.017*
C4	0.60593 (15)	0.61777 (15)	0.63310 (9)	0.0139 (3)
C5	0.500000	0.5896 (2)	0.750000	0.0140 (4)
H5	0.500002	0.492896	0.750001	0.017*
C6	0.53051 (16)	0.80363 (15)	0.71783 (9)	0.0161 (3)
H6	0.556228	0.881331	0.690995	0.019*
C7	0.73926 (16)	0.44820 (16)	0.58138 (9)	0.0175 (3)
H7	0.793608	0.366355	0.587808	0.021*
C8	0.68604 (16)	0.49934 (16)	0.64455 (9)	0.0165 (3)
H8	0.704252	0.453812	0.694975	0.020*
B1	0.500000	1.2063 (3)	0.750000	0.0185 (5)

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
Br1	0.01930 (8)	0.01754 (8)	0.01556 (8)	−0.00009 (6)	0.00730 (5)	−0.00169 (6)
F1	0.0342 (6)	0.0190 (5)	0.0305 (5)	−0.0001 (4)	−0.0052 (5)	−0.0008 (4)
F2	0.0569 (7)	0.0275 (6)	0.0381 (6)	−0.0040 (5)	0.0292 (6)	0.0094 (5)
N1	0.0148 (6)	0.0119 (6)	0.0123 (6)	−0.0011 (5)	0.0028 (5)	−0.0002 (5)
C1	0.0145 (7)	0.0179 (7)	0.0140 (7)	−0.0039 (6)	0.0045 (5)	−0.0027 (6)
C2	0.0148 (7)	0.0161 (7)	0.0138 (7)	−0.0035 (6)	0.0027 (5)	0.0014 (6)
C3	0.0142 (7)	0.0127 (7)	0.0162 (7)	−0.0015 (5)	0.0024 (6)	0.0008 (5)
C4	0.0137 (6)	0.0157 (7)	0.0129 (6)	−0.0027 (6)	0.0039 (5)	−0.0024 (5)
C5	0.0167 (10)	0.0115 (9)	0.0138 (9)	0.000	0.0031 (8)	0.000
C6	0.0202 (7)	0.0110 (7)	0.0163 (7)	−0.0005 (6)	0.0025 (6)	−0.0002 (5)
C7	0.0174 (7)	0.0166 (7)	0.0184 (7)	0.0024 (6)	0.0038 (6)	0.0002 (6)
C8	0.0185 (7)	0.0167 (7)	0.0138 (7)	0.0006 (6)	0.0030 (6)	0.0023 (6)
B1	0.0280 (13)	0.0122 (11)	0.0173 (11)	0.000	0.0096 (10)	0.000

Geometric parameters (Å, º) top

Br1—C1	1.8953 (14)	C3—H3	0.9500
F1—B1	1.3930 (18)	C3—C4	1.387 (2)
F2—B1	1.3799 (18)	C4—C8	1.382 (2)
N1—C4	1.4364 (18)	C5—H5	0.9500
N1—C5	1.3311 (17)	C6—C6ⁱ	1.351 (3)
N1—C6	1.3867 (19)	C6—H6	0.9500
C1—C2	1.385 (2)	C7—H7	0.9500
C1—C7	1.387 (2)	C7—C8	1.383 (2)
C2—H2	0.9500	C8—H8	0.9500
C2—C3	1.389 (2)

C5—N1—C4	123.87 (13)	N1ⁱ—C5—H5	125.7
C5—N1—C6	108.76 (13)	N1—C6—H6	126.5
C6—N1—C4	127.35 (12)	C6ⁱ—C6—N1	106.97 (8)
C2—C1—Br1	120.01 (11)	C6ⁱ—C6—H6	126.5
C2—C1—C7	121.65 (14)	C1—C7—H7	120.2
C7—C1—Br1	118.34 (12)	C8—C7—C1	119.54 (14)
C1—C2—H2	120.6	C8—C7—H7	120.2
C1—C2—C3	118.77 (13)	C4—C8—C7	118.90 (14)
C3—C2—H2	120.6	C4—C8—H8	120.5
C2—C3—H3	120.3	C7—C8—H8	120.5
C4—C3—C2	119.35 (14)	F1ⁱ—B1—F1	108.01 (18)
C4—C3—H3	120.3	F2—B1—F1	110.28 (7)
C3—C4—N1	119.62 (13)	F2ⁱ—B1—F1ⁱ	110.28 (7)
C8—C4—N1	118.58 (13)	F2ⁱ—B1—F1	109.77 (7)
C8—C4—C3	121.78 (14)	F2—B1—F1ⁱ	109.77 (7)
N1ⁱ—C5—N1	108.53 (18)	F2ⁱ—B1—F2	108.73 (19)
N1—C5—H5	125.7

Br1—C1—C2—C3	−178.42 (11)	C4—N1—C5—N1ⁱ	−178.58 (15)
Br1—C1—C7—C8	177.89 (12)	C4—N1—C6—C6ⁱ	178.37 (15)
N1—C4—C8—C7	178.69 (13)	C5—N1—C4—C3	142.28 (12)
C1—C2—C3—C4	0.3 (2)	C5—N1—C4—C8	−36.09 (19)
C1—C7—C8—C4	0.7 (2)	C5—N1—C6—C6ⁱ	−0.25 (19)
C2—C1—C7—C8	−1.3 (2)	C6—N1—C4—C3	−36.1 (2)
C2—C3—C4—N1	−179.22 (13)	C6—N1—C4—C8	145.49 (15)
C2—C3—C4—C8	−0.9 (2)	C6—N1—C5—N1ⁱ	0.10 (7)
C3—C4—C8—C7	0.4 (2)	C7—C1—C2—C3	0.8 (2)

Symmetry code: (i) −x+1, y, −z+3/2.

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
C5—H5···F1ⁱⁱ	0.95	2.29	3.156 (2)	151
C6—H6···F2ⁱ	0.95	2.39	3.1854 (19)	140

Symmetry codes: (i) −x+1, y, −z+3/2; (ii) x, y−1, z.

Acknowledgements

The authors thank the University of KwaZulu-Natal for the research facilities. DUT/HANT is acknowledged for funding the postdoctoral fellowship of HI.

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