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rz5032 scheme

Acta Cryst. (2013). E69, o56    [ doi:10.1107/S1600536812049872 ]

4-Methoxybenzamidinium bromide

S. Irrera and G. Portalone

Abstract top

The title salt, C8H11N2O+·Br-, was synthesized by the reaction between 4-methoxybenzamidine (4-amidinoanisole) and hydrobromic acid. In the cation, the amidinium group has two similar C-N bonds [1.304 (2) and 1.316 (2) Å], and its plane forms a dihedral angle of 31.08 (5)° with the benzene ring. The ions are associated in the crystal into a three-dimension hydrogen-bonded supramolecular network featuring N-H+...Br- interactions.

Comment top

As part of our ongoing interest in systematic structural analysis of proton-transfer adducts containing molecules of biological interest (Portalone, 2011; Portalone & Irrera, 2011) this study reports the single-crystal structure of the title molecular salt, 4-methoxybenzamidinium bromide, (I), which was obtained by a reaction between 4-methoxybenzamidine (4-amidinoanisole) and hydrobromic acid in water solution. Benzamidine derivatives, which have shown strong biological and pharmacological activity (Powers & Harper, 1999), are being used in our group as bricks for supramolecular construction (Portalone, 2010; Portalone, 2012). Indeed, these molecules are strong Lewis base and their cations can be easily anchored onto numerous inorganic and organic anions and polyanions, largely because of the presence of four potential donor sites for hydrogen-bonding.

The asymmetric unit of (I) comprises one non-planar 4-methoxybenzamidinium cation and one bromide anion (Fig. 1).

In the cation the amidinium group forms a dihedral angle of 31.08 (5)° with the benzene ring, which is close to the values observed in protonated benzamidinium ions (23.2-30.4°; Portalone, 2010; Portalone, 2012). The lack of planarity in all these systems is obviously caused by steric hindrances between the H atoms of the aromatic ring and the amidine moiety. This conformation is rather common in benzamidinium-containing small molecule crystal structures, with the only exception of benzamidinium diliturate, where the benzamidinium cation is planar (Portalone, 2010). The pattern of bond lengths and bond angles of the 4-methoxybenzamidinium cation agrees with that reported in previous structural investigations (Portalone, 2010; Portalone, 2012; Irrera et al., 2012; Irrera & Portalone, 2012a, 2012b, 2012c, 2012d, 2012e). In particular the amidinium group, true to one's expectations, features similar C—N bonds [1.304 (2) and 1.316 (2) Å], evidencing the delocalization of the π electrons and partial double-bond character.

Analysis of the crystal packing of (I), (Fig. 2), shows that each amidinium unit is bound to three bromide anions by four distinct weak N—H+···Br- hydrogen bonds (N+···Br- = 3.3163 (19)-3.4765 (17) Å; Table 1). The ion pairs of the asymmetric unit are joined by two N—H+···Br- hydrogen bonds in ionic dimers, where Br- anion acts as a bifurcated acceptor, thus generating an R12(6) motif (Bernstein et al., 1995). These subunits are then joined through the remaining N—H+···Br- hydrogen bonds to adjacent Br- anions leading to the formation of three-dimension hydrogen-bonded network.

Related literature top

For the biological and pharmacological relevance of benzamidine, see: Powers & Harper (1999). For structural analysis of proton-transfer adducts containing molecules of biological interest, see: Portalone (2011); Portalone & Irrera (2011). For the supramolecular association in proton-transfer adducts containing benzamidinium cations, see: Portalone (2010, 2012); Irrera et al. (2012); Irrera & Portalone (2012a,b,c,d,e). For hydrogen-bond motifs, see: Bernstein et al. (1995).

Experimental top

4-Methoxybenzamidine (1 mmol, Fluka at 96% purity) was dissolved without further purification in 6 ml of hot water and heated under reflux for 6 h. While stirring, HBr (2 mol L-1) was added dropwise until pH reached 2. After cooling the solution to an ambient temperature, colourless crystals suitable for single-crystal X-ray diffraction were grown by slow evaporation of the solvent after four weeks.

Refinement top

All H atoms were identified in a difference Fourier map, but for refinement all C-bound H atoms were placed in calculated positions, with C—H = 0.93 Å (phenyl) and 0.96 Å (methyl), and refined as riding on their carrier atoms. The Uiso values were kept equal to 1.2Ueq(C) or 1.5Ueq(C) for methyl H atoms. The hydrogen atoms of the methyl group were allowed to rotate with a fixed angle around the C–C bond to best fit the experimental electron density [HFIX 137 in the SHELX program suite]. Positional and isotropic thermal parameters of H atoms of the amidinium group were freely refined, giving N—H distances in the range 0.78 (2)-0.95 (3) Å.

Computing details top

Data collection: CrysAlis CCD (Oxford Diffraction, 2006); cell refinement: CrysAlis RED (Oxford Diffraction, 2006); data reduction: CrysAlis RED (Oxford Diffraction, 2006); program(s) used to solve structure: SIR97 (Altomare et al., 1999); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: WinGX (Farrugia, 2012); software used to prepare material for publication: WinGX (Farrugia, 2012).

Figures top
[Figure 1] Fig. 1. The asymmetric unit of the title compound, showing displacements ellipsoids drawn at the 50% probability level. H atoms are shown as small spheres of arbitrary radii and hydrogen bonds as dashed lines.
[Figure 2] Fig. 2. Crystal packing diagram of the title compound viewed approximately down c. All atoms are shown as small spheres of arbitrary radii. For clarity, H atoms not involved in hydrogen bonding (dashed lines) have been omitted.
4-Methoxybenzamidinium bromide top
Crystal data top
C8H11N2O+·BrF(000) = 464
Mr = 231.10Dx = 1.617 Mg m3
Orthorhombic, P212121Mo Kα radiation, λ = 0.71069 Å
Hall symbol: P 2ac 2abCell parameters from 10829 reflections
a = 7.5657 (6) Åθ = 3.2–32.5°
b = 10.8711 (7) ŵ = 4.29 mm1
c = 11.5419 (7) ÅT = 298 K
V = 949.29 (11) Å3Tablets, colourless
Z = 40.18 × 0.12 × 0.10 mm
Data collection top
Agilent Xcalibur Sapphire3
diffractometer
3278 independent reflections
Radiation source: Enhance (Mo) X-ray Source2903 reflections with I > 2σ(I)
Graphite monochromatorRint = 0.043
Detector resolution: 16.0696 pixels mm-1θmax = 32.0°, θmin = 3.2°
ω and φ scansh = 1111
Absorption correction: multi-scan
(CrysAlis PRO; Agilent, 2011)
k = 1616
Tmin = 0.513, Tmax = 0.674l = 1717
34724 measured reflections
Refinement top
Refinement on F2Secondary atom site location: difference Fourier map
Least-squares matrix: fullHydrogen site location: inferred from neighbouring sites
R[F2 > 2σ(F2)] = 0.027H atoms treated by a mixture of independent and constrained refinement
wR(F2) = 0.055 w = 1/[σ2(Fo2) + (0.0236P)2 + 0.1235P]
where P = (Fo2 + 2Fc2)/3
S = 1.09(Δ/σ)max < 0.001
3278 reflectionsΔρmax = 0.22 e Å3
126 parametersΔρmin = 0.31 e Å3
0 restraintsAbsolute structure: Flack (1983), 1387 Friedel pairs
Primary atom site location: structure-invariant direct methodsFlack parameter: 0.002 (9)
Crystal data top
C8H11N2O+·BrV = 949.29 (11) Å3
Mr = 231.10Z = 4
Orthorhombic, P212121Mo Kα radiation
a = 7.5657 (6) ŵ = 4.29 mm1
b = 10.8711 (7) ÅT = 298 K
c = 11.5419 (7) Å0.18 × 0.12 × 0.10 mm
Data collection top
Agilent Xcalibur Sapphire3
diffractometer
3278 independent reflections
Absorption correction: multi-scan
(CrysAlis PRO; Agilent, 2011)
2903 reflections with I > 2σ(I)
Tmin = 0.513, Tmax = 0.674Rint = 0.043
34724 measured reflectionsθmax = 32.0°
Refinement top
R[F2 > 2σ(F2)] = 0.027H atoms treated by a mixture of independent and constrained refinement
wR(F2) = 0.055Δρmax = 0.22 e Å3
S = 1.09Δρmin = 0.31 e Å3
3278 reflectionsAbsolute structure: Flack (1983), 1387 Friedel pairs
126 parametersFlack parameter: 0.002 (9)
0 restraints
Special details top

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

Refinement. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > 2σ(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/Ueq
Br10.35106 (3)0.611694 (17)0.195955 (16)0.04325 (6)
O10.3480 (2)0.16213 (12)0.17351 (11)0.0435 (3)
N10.4313 (3)0.31784 (17)0.13950 (15)0.0431 (4)
H1A0.431 (3)0.392 (3)0.169 (2)0.056 (7)*
H1B0.484 (3)0.261 (3)0.181 (2)0.062 (8)*
N20.3375 (3)0.40022 (16)0.02982 (15)0.0440 (4)
H2A0.329 (3)0.480 (2)0.003 (2)0.060 (7)*
H2B0.300 (3)0.394 (3)0.092 (2)0.054 (7)*
C10.3765 (2)0.18004 (16)0.01954 (14)0.0312 (3)
C20.3378 (3)0.07914 (17)0.04827 (15)0.0387 (4)
H20.31770.08980.12710.046*
C30.3284 (3)0.03770 (17)0.00107 (16)0.0407 (4)
H30.30340.10520.04770.049*
C40.3568 (3)0.05267 (15)0.11722 (15)0.0347 (3)
C50.3987 (2)0.04789 (17)0.18527 (16)0.0383 (4)
H50.42100.03700.26380.046*
C60.4076 (2)0.16333 (18)0.13810 (16)0.0364 (4)
H60.43440.23060.18480.044*
C70.3823 (2)0.30343 (17)0.03201 (15)0.0319 (4)
C80.3064 (3)0.26858 (18)0.1078 (2)0.0521 (6)
H8A0.19360.25780.07110.078*
H8B0.30210.33880.15810.078*
H8C0.39550.28110.04980.078*
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
Br10.05673 (11)0.03755 (9)0.03548 (8)0.00280 (10)0.00105 (9)0.01017 (8)
O10.0614 (8)0.0328 (6)0.0364 (7)0.0010 (7)0.0039 (7)0.0042 (5)
N10.0643 (11)0.0313 (8)0.0338 (8)0.0052 (8)0.0086 (8)0.0021 (7)
N20.0634 (10)0.0317 (8)0.0368 (8)0.0032 (10)0.0107 (8)0.0013 (7)
C10.0340 (9)0.0304 (8)0.0291 (7)0.0011 (7)0.0007 (7)0.0000 (6)
C20.0536 (11)0.0369 (9)0.0255 (7)0.0042 (9)0.0032 (8)0.0002 (6)
C30.0581 (13)0.0329 (8)0.0311 (8)0.0050 (9)0.0029 (9)0.0035 (7)
C40.0385 (8)0.0324 (8)0.0332 (8)0.0036 (9)0.0004 (9)0.0038 (6)
C50.0489 (10)0.0387 (9)0.0272 (8)0.0020 (7)0.0061 (8)0.0017 (7)
C60.0459 (10)0.0335 (9)0.0298 (8)0.0018 (8)0.0040 (7)0.0055 (7)
C70.0331 (9)0.0312 (8)0.0313 (8)0.0008 (7)0.0005 (7)0.0012 (6)
C80.0759 (17)0.0332 (10)0.0474 (11)0.0008 (10)0.0045 (11)0.0001 (8)
Geometric parameters (Å, º) top
O1—C41.357 (2)C2—C31.384 (3)
O1—C81.419 (2)C2—H20.9300
N1—C71.304 (2)C3—C41.392 (2)
N1—H1A0.87 (3)C3—H30.9300
N1—H1B0.88 (3)C4—C51.383 (3)
N2—C71.316 (2)C5—C61.370 (3)
N2—H2A0.95 (3)C5—H50.9300
N2—H2B0.78 (2)C6—H60.9300
C1—C21.379 (2)C8—H8A0.9600
C1—C61.400 (2)C8—H8B0.9600
C1—C71.468 (2)C8—H8C0.9600
C4—O1—C8118.05 (14)O1—C4—C3124.37 (16)
C7—N1—H1A118.6 (16)C5—C4—C3120.01 (16)
C7—N1—H1B124.6 (17)C6—C5—C4120.66 (16)
H1A—N1—H1B116 (2)C6—C5—H5119.7
C7—N2—H2A122.2 (16)C4—C5—H5119.7
C7—N2—H2B122 (2)C5—C6—C1119.93 (17)
H2A—N2—H2B115 (3)C5—C6—H6120.0
C2—C1—C6119.15 (17)C1—C6—H6120.0
C2—C1—C7120.22 (15)N1—C7—N2119.54 (18)
C6—C1—C7120.62 (16)N1—C7—C1120.25 (16)
C1—C2—C3121.18 (16)N2—C7—C1120.21 (16)
C1—C2—H2119.4O1—C8—H8A109.5
C3—C2—H2119.4O1—C8—H8B109.5
C2—C3—C4119.05 (16)H8A—C8—H8B109.5
C2—C3—H3120.5O1—C8—H8C109.5
C4—C3—H3120.5H8A—C8—H8C109.5
O1—C4—C5115.62 (15)H8B—C8—H8C109.5
C6—C1—C2—C30.4 (3)C3—C4—C5—C61.9 (3)
C7—C1—C2—C3178.8 (2)C4—C5—C6—C10.8 (3)
C1—C2—C3—C40.7 (3)C2—C1—C6—C50.3 (3)
C8—O1—C4—C5179.67 (18)C7—C1—C6—C5178.87 (17)
C8—O1—C4—C30.2 (3)C2—C1—C7—N131.3 (3)
C2—C3—C4—O1178.8 (2)C6—C1—C7—N1149.52 (19)
C2—C3—C4—C51.8 (3)C2—C1—C7—N2148.4 (2)
O1—C4—C5—C6178.63 (18)C6—C1—C7—N230.7 (3)
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
N1—H1A···Br10.87 (3)2.48 (3)3.3163 (19)159 (2)
N1—H1B···Br1i0.88 (3)2.49 (3)3.3676 (19)176 (2)
N2—H2A···Br10.95 (3)2.65 (3)3.4765 (17)145 (2)
N2—H2B···Br1ii0.78 (2)2.70 (3)3.4742 (17)175 (2)
Symmetry codes: (i) x+1, y1/2, z1/2; (ii) x+1/2, y+1, z+1/2.
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
N1—H1A···Br10.87 (3)2.48 (3)3.3163 (19)159 (2)
N1—H1B···Br1i0.88 (3)2.49 (3)3.3676 (19)176 (2)
N2—H2A···Br10.95 (3)2.65 (3)3.4765 (17)145 (2)
N2—H2B···Br1ii0.78 (2)2.70 (3)3.4742 (17)175 (2)
Symmetry codes: (i) x+1, y1/2, z1/2; (ii) x+1/2, y+1, z+1/2.
references
References top

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