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Acta Cryst. (2012). E68, o3277    [ doi:10.1107/S1600536812044911 ]

4-Methoxybenzamidinium acetate

S. Irrera and G. Portalone

Abstract top

The title compound, C8H11N2O+·CH3CO2-, was synthesized by a reaction between 4-methoxybenzamidine (4-amidinoanisole) and acetic acid. In the cation, the amidinium group forms a dihedral angle of 11.65 (17)° with the mean plane of the benzene ring. The ionic components are associated in the crystal via N-H+...O- hydrogen bonds, resulting in a one-dimensional structure consisting of dimers and catemers and orientated approximately along the c axis.

Comment top

The present work is part of a general structural analysis of proton-transfer adducts containing molecules of biological interest (Portalone, 2011a; Portalone & Irrera, 2011). In this context, 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, 2011b, 2012). Indeed, the bidentate hydrogen-bonding interaction between the amidinium and the carboxylate functional groups can be a powerful organizing force in solution and in the solid-state.

We report here the single-crystal structure of the title molecular salt, 4-methoxybenzamidinium acetate, (I), which was obtained by a reaction between 4-methoxybenzamidine (4-amidinoanisole) and acetic acid.

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

In the cation the amidinium group forms dihedral angle of 11.65 (17)° with the mean plane of the phenyl ring, which agrees with the values observed in protonated benzamidinium ions (14.4 (1) - 32.7 (1)°, Portalone, 2010, 2012; Irrera et al., 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 (Irrera et al., 2012; Portalone, 2010, 2012; Irrera & Portalone, 2012a, 2012b). In particular the amidinium group, true to one's expectations, features C—N bonds within experimental error [1.312 (2) and 1.306 (2) Å], evidencing the delocalization of the π electrons and double-bond character.

In the acetate moiety the C—O bond lengths indicate delocalization of the negative charge on both O atoms, since the C—O bond lengths [1.250 (2) and 1.249 (2) Å] are intermediate between single Csp2—O and double Csp2O, and correlate well with values for carboxylate anions [1.247 - 1.262 Å, Allen et al., 1987].

Analysis of the crystal packing of (I), (Fig. 2), shows that each amidinium unit is bound to three acetate anions by four distinct N—H+···O- strong intermolecular hydrogen bonds (N+···O- = 2.776 (2) - 2.847 (2) Å, Table 1) into a one-dimensional structure. The ion pairs of the asymmetric unit are joined by two N+—H···O- (±) hydrogen bonds to form ionic dimers with graph-set motif R22(8) (Bernstein et al., 1995). These subunits are then joined as catemers into linear chains approximately along the crystallographic c axis through the remaining N+—H···O- hydrogen bonds to adjacent anti-parallel dimers.

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, (2011a); Portalone & Irrera (2011). For the supramolecular association in proton-transfer adducts containing benzamidinium cations, see; Portalone (2010, 2011b, 2012); Irrera & Portalone (2012a,b); Irrera et al. (2012). For hydrogen-bond motifs, see Bernstein et al. (1995). For standard bond lengths, see: Allen et al. (1987).

Experimental top

4-Methoxybenzamidine (0.1 mmol, Fluka at 96% purity) was dissolved without further purification in 8 ml of a 20% solution of acetic acid and heated under reflux for 3 h. 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 one week.

Refinement top

All H atoms were identified in difference Fourier maps, but for refinement all C-bound H atoms were placed in calculated positions, with C—H = 0.93 Å (phenyl) and 0.89 - 1.01 Å (methyl), and refined as riding on their carrier atoms. The Uiso values were kept equal to 1.2Ueq(C, phenyl). and to 1.5Ueq(C, methyl). Positional and displacement parameters of H atoms of the amidinium group were refined, giving N—H distances in the range 0.88 (2) - 0.94 (3) Å.

Computing details top

Data collection: CrysAlis PRO (Agilent, 2011); cell refinement: CrysAlis PRO (Agilent, 2011); data reduction: CrysAlis PRO (Agilent, 2011); program(s) used to solve structure: SIR97 (Altomare et al., 1999); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 (Farrugia, 1997); software used to prepare material for publication: WinGX (Farrugia, 1999).

Figures top
[Figure 1] Fig. 1. The asymmetric unit of (I), showing the atom-labelling scheme. Displacements ellipsoids are at the 50% probability level. The asymmetric unit was selected so that the two ions are linked by N+.—H···O- hydrogen bonds. H atoms are shown as small spheres of arbitrary radii. Hydrogen bonding is indicated by dashed lines.
[Figure 2] Fig. 2. Crystal packing diagram for (I), viewed approximately down b. Displacements ellipsoids are at the 50% probability level. H atoms are shown as small spheres of arbitrary radii. For the sake of clarity, H atoms not involved in hydrogen bonding have been omitted. Hydrogen bonding is indicated by dashed lines.
4-Methoxybenzamidinium acetate top
Crystal data top
C8H11N2O+·C2H3O2F(000) = 448
Mr = 210.23Dx = 1.282 Mg m3
Monoclinic, P21/nMo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ynCell parameters from 3220 reflections
a = 8.7591 (14) Åθ = 2.8–28.9°
b = 6.5478 (8) ŵ = 0.10 mm1
c = 19.456 (3) ÅT = 298 K
β = 102.580 (14)°Tablets, colourless
V = 1089.0 (3) Å30.21 × 0.18 × 0.15 mm
Z = 4
Data collection top
Oxford Diffraction Xcalibur S CCD
diffractometer
2365 independent reflections
Radiation source: Enhance (Mo) X-ray Source1834 reflections with I > 2σ(I)
Graphite monochromatorRint = 0.040
Detector resolution: 16.0696 pixels mm-1θmax = 27.0°, θmin = 3.3°
ω and φ scansh = 1111
Absorption correction: multi-scan
(CrysAlis PRO; Agilent, 2011)
k = 88
Tmin = 0.980, Tmax = 0.986l = 2424
14433 measured reflections
Refinement top
Refinement on F2Primary atom site location: structure-invariant direct methods
Least-squares matrix: fullSecondary atom site location: difference Fourier map
R[F2 > 2σ(F2)] = 0.056Hydrogen site location: inferred from neighbouring sites
wR(F2) = 0.141H atoms treated by a mixture of independent and constrained refinement
S = 1.08 w = 1/[σ2(Fo2) + (0.0561P)2 + 0.3698P]
where P = (Fo2 + 2Fc2)/3
2365 reflections(Δ/σ)max < 0.001
156 parametersΔρmax = 0.21 e Å3
0 restraintsΔρmin = 0.15 e Å3
Crystal data top
C8H11N2O+·C2H3O2V = 1089.0 (3) Å3
Mr = 210.23Z = 4
Monoclinic, P21/nMo Kα radiation
a = 8.7591 (14) ŵ = 0.10 mm1
b = 6.5478 (8) ÅT = 298 K
c = 19.456 (3) Å0.21 × 0.18 × 0.15 mm
β = 102.580 (14)°
Data collection top
Oxford Diffraction Xcalibur S CCD
diffractometer
2365 independent reflections
Absorption correction: multi-scan
(CrysAlis PRO; Agilent, 2011)
1834 reflections with I > 2σ(I)
Tmin = 0.980, Tmax = 0.986Rint = 0.040
14433 measured reflectionsθmax = 27.0°
Refinement top
R[F2 > 2σ(F2)] = 0.056H atoms treated by a mixture of independent and constrained refinement
wR(F2) = 0.141Δρmax = 0.21 e Å3
S = 1.08Δρmin = 0.15 e Å3
2365 reflectionsAbsolute structure: ?
156 parametersFlack parameter: ?
0 restraintsRogers parameter: ?
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
O30.2977 (2)1.0070 (2)0.35136 (8)0.0624 (5)
N10.0153 (2)0.1780 (3)0.42774 (8)0.0453 (4)
H1A0.074 (3)0.064 (4)0.4296 (12)0.063 (7)*
H1B0.054 (3)0.217 (4)0.4677 (13)0.063 (7)*
N20.1170 (2)0.2005 (3)0.31128 (9)0.0468 (4)
H2A0.175 (3)0.081 (4)0.3149 (12)0.070 (7)*
H2B0.133 (3)0.261 (3)0.2696 (13)0.055 (6)*
C10.0643 (2)0.4636 (3)0.36478 (9)0.0343 (4)
C20.1370 (2)0.5636 (3)0.42564 (10)0.0449 (5)
H20.13180.50770.46900.054*
C30.2173 (2)0.7446 (3)0.42381 (10)0.0472 (5)
H30.26590.80880.46550.057*
C40.2248 (2)0.8294 (3)0.35969 (10)0.0429 (5)
C50.1539 (3)0.7298 (4)0.29855 (10)0.0584 (6)
H50.15940.78560.25520.070*
C60.0756 (3)0.5502 (3)0.30090 (10)0.0510 (5)
H60.02930.48490.25910.061*
C70.0247 (2)0.2737 (3)0.36775 (9)0.0356 (4)
C80.3954 (3)1.0997 (4)0.41088 (13)0.0624 (6)
H8A0.3323 (10)1.130 (2)0.4474 (7)0.094*
H8B0.4395 (17)1.231 (2)0.3962 (3)0.094*
H8C0.4838 (17)1.0040 (17)0.4315 (6)0.094*
O10.20216 (18)0.1693 (2)0.44118 (7)0.0548 (4)
O20.29092 (18)0.1444 (2)0.32662 (7)0.0550 (4)
C90.2818 (2)0.2339 (3)0.38404 (9)0.0390 (4)
C100.3743 (3)0.4258 (3)0.38417 (12)0.0606 (6)
H10A0.3120 (10)0.5337 (17)0.3841 (9)0.091*
H10B0.4519 (17)0.4292 (13)0.3458 (7)0.091*
H10C0.4143 (17)0.4291 (12)0.4226 (7)0.091*
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
O30.0827 (11)0.0586 (9)0.0449 (9)0.0302 (8)0.0113 (8)0.0040 (7)
N10.0564 (11)0.0443 (10)0.0298 (8)0.0120 (8)0.0020 (7)0.0052 (7)
N20.0613 (11)0.0439 (10)0.0293 (9)0.0095 (8)0.0034 (8)0.0040 (7)
C10.0389 (10)0.0354 (9)0.0271 (9)0.0018 (7)0.0039 (7)0.0009 (7)
C20.0616 (13)0.0460 (11)0.0263 (9)0.0063 (9)0.0076 (8)0.0021 (8)
C30.0616 (13)0.0474 (11)0.0302 (10)0.0119 (9)0.0049 (9)0.0056 (8)
C40.0485 (11)0.0423 (10)0.0379 (10)0.0043 (8)0.0092 (8)0.0012 (8)
C50.0798 (16)0.0647 (14)0.0292 (10)0.0221 (12)0.0083 (10)0.0075 (9)
C60.0686 (14)0.0545 (12)0.0270 (10)0.0158 (10)0.0037 (9)0.0019 (8)
C70.0411 (10)0.0358 (9)0.0279 (9)0.0021 (7)0.0030 (7)0.0007 (7)
C80.0673 (15)0.0587 (14)0.0587 (14)0.0234 (11)0.0086 (12)0.0064 (11)
O10.0657 (10)0.0627 (9)0.0293 (7)0.0192 (7)0.0045 (6)0.0027 (6)
O20.0742 (10)0.0551 (9)0.0281 (7)0.0154 (7)0.0053 (7)0.0019 (6)
C90.0376 (10)0.0436 (10)0.0330 (10)0.0009 (8)0.0014 (8)0.0003 (8)
C100.0608 (14)0.0558 (13)0.0603 (15)0.0123 (11)0.0023 (12)0.0026 (11)
Geometric parameters (Å, º) top
O3—C41.353 (2)C3—H30.9300
O3—C81.418 (3)C4—C51.380 (3)
N1—C71.312 (2)C5—C61.367 (3)
N1—H1A0.91 (3)C5—H50.9300
N1—H1B0.91 (2)C6—H60.9300
N2—C71.306 (2)C8—H8A1.0093
N2—H2A0.94 (3)C8—H8B1.0093
N2—H2B0.88 (2)C8—H8C1.0093
C1—C21.381 (2)O1—C91.250 (2)
C1—C61.389 (3)O2—C91.249 (2)
C1—C71.475 (2)C9—C101.495 (3)
C2—C31.383 (3)C10—H10A0.8930
C2—H20.9300C10—H10B0.8930
C3—C41.380 (3)C10—H10C0.8930
C4—O3—C8119.11 (16)C5—C6—C1121.01 (18)
C7—N1—H1A120.0 (14)C5—C6—H6119.5
C7—N1—H1B121.9 (15)C1—C6—H6119.5
H1A—N1—H1B118 (2)N2—C7—N1118.69 (18)
C7—N2—H2A119.1 (14)N2—C7—C1120.84 (16)
C7—N2—H2B123.6 (14)N1—C7—C1120.45 (16)
H2A—N2—H2B117 (2)O3—C8—H8A109.5
C2—C1—C6117.64 (17)O3—C8—H8B109.5
C2—C1—C7120.99 (16)H8A—C8—H8B109.5
C6—C1—C7121.36 (16)O3—C8—H8C109.5
C1—C2—C3121.77 (17)H8A—C8—H8C109.5
C1—C2—H2119.1H8B—C8—H8C109.5
C3—C2—H2119.1O2—C9—O1123.45 (18)
C4—C3—C2119.52 (17)O2—C9—C10117.82 (16)
C4—C3—H3120.2O1—C9—C10118.71 (17)
C2—C3—H3120.2C9—C10—H10A109.5
O3—C4—C5116.00 (17)C9—C10—H10B109.5
O3—C4—C3124.78 (17)H10A—C10—H10B109.5
C5—C4—C3119.22 (18)C9—C10—H10C109.5
C6—C5—C4120.83 (18)H10A—C10—H10C109.5
C6—C5—H5119.6H10B—C10—H10C109.5
C4—C5—H5119.6
C6—C1—C2—C30.6 (3)C3—C4—C5—C60.6 (4)
C7—C1—C2—C3178.24 (18)C4—C5—C6—C10.5 (4)
C1—C2—C3—C40.4 (3)C2—C1—C6—C51.1 (3)
C8—O3—C4—C5169.3 (2)C7—C1—C6—C5177.8 (2)
C8—O3—C4—C310.9 (3)C2—C1—C7—N2167.00 (19)
C2—C3—C4—O3178.89 (19)C6—C1—C7—N211.9 (3)
C2—C3—C4—C51.0 (3)C2—C1—C7—N111.3 (3)
O3—C4—C5—C6179.3 (2)C6—C1—C7—N1169.9 (2)
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
N1—H1A···O10.91 (3)1.94 (3)2.847 (2)175 (2)
N1—H1B···O1i0.91 (2)1.98 (2)2.832 (2)155 (2)
N2—H2A···O20.94 (3)1.83 (3)2.776 (2)176 (2)
N2—H2B···O2ii0.88 (2)1.95 (2)2.817 (2)168 (2)
Symmetry codes: (i) x, y, z+1; (ii) x1/2, y+1/2, z+1/2.
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
N1—H1A···O10.91 (3)1.94 (3)2.847 (2)175 (2)
N1—H1B···O1i0.91 (2)1.98 (2)2.832 (2)155 (2)
N2—H2A···O20.94 (3)1.83 (3)2.776 (2)176 (2)
N2—H2B···O2ii0.88 (2)1.95 (2)2.817 (2)168 (2)
Symmetry codes: (i) x, y, z+1; (ii) x1/2, y+1/2, z+1/2.
references
References top

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