Benzdiamidine

The mol­ecule of benzene-1,4-dicarbox­amidine or benzdi­amidine, C₈H₁₀N₄, reveals C_i symmetry. Hydro­gen bonds utilize the amino groups as double donors, whereas the imino groups act as double acceptors. The network formed is similar to that observed in the crystal packing of terephthal­amide.

S1. Comment

In the successful design of molecular solids, it is of primary interest to identify molecular functionalities that will generate predictable intermolecular interactions (Nguyen et al., 1998). Many functional groups have been analysed with respect to their ability to form recognizable structural patterns (Desiraju, 1995). Among these, the amidine group appears to possess a very good functionality for hydrogen bonding, generating supramolecular aggregates. A high predominance of the protonated form has been detected in crystal structures. In the light of this, the molecular and crystal structures of benzdiamidine, (I), are presented here. \sch

The molecule of (I) has C_i symmetry with anti disposed amidine groups (Fig. 1). The dihedral angle between the benzene ring and the amidine group is 24.52 (9)°, close to the value observed in benzamidine, (II) (22.71°; Barker et al., 1996). In both molecules, deviations from planarity are a consequence of an overcrowding effect, i.e. steric hindrances between the H atoms of the aromatic ring and the amidine moieties.

In the crystal structure of (I), the molecules are connected into an infinite two-dimensional hydrogen-bonding network (Fig. 2). During formation of the hydrogen bonds, H atoms donated to the imino group are oriented toward its lone pairs (Ermer & Eling, 1994). Thus, the hybridization of the atomic orbitals of nitrogen N1 should also possess some sp³ character. As shown in Fig. 2, all bonds of the imino N atom, together with its hydrogen bonds, are oriented to the vertices of a distorted tetrahedron. The C1—N1—H1 angle of 111.0 (15)° is close to the sp³ hybridization value (Table 1). A similar situation was observed in the crystal structures of acetamidine (Norrestam et al., 1983) and (II).

The amino N atoms are double donors, while the imino N atoms are double acceptors and the H atoms of the imino groups are not involved in any hydrogen bonds. Generally, there are two types of hydrogen bonds in the three structures: N_amino-anti-H···N_imino and N_amino-syn-H···N_imino (Table 2). Motifs generated by the first type of hydrogen bond have different characters in these three structures. In the structure of acetamidine, the motif is a helical chain with a C(4) graph-set descriptor (Etter et al., 1990). In the structure of (II), the motif is a ring pattern, graph-set descriptor R₄⁴(16), and in (I) it is a centrosymmetrical ring, graph-set descriptor R₂²(8) (Fig. 2). Benzamidine, (II), does not form a centrosymmetrical R₂²(8) ring pattern; its imino H atom is in an anti position and it would be very close to the anti-amino H atom of a neigbouring molecule, so the formation of an R₂²(8) motif is not favoured.

The second type of hydrogen bond (with syn-amino H) for (I) and (II) is characterized by a one-dimensional C(4) chain. The combined first-level graph-set descriptor of the hydrogen-bonding network in (I) is C(4)R₂²(8), typical of primary amides (Etter et al., 1990). From this analogy, it can be expected that the hydrogen-bonding network in (I) is similar to that in the crystal structure of terephthalamide (Cobbledick & Small, 1972). In these two structures, the combined first-level graph-set descriptors and all higher level graph-set descriptors are identical. In spite of this, these two structures are not isostructural; terephthalamide crystallizes in space group P1 and (I) in P2₁/c. This example shows that identical graph-set descriptors for all levels of hydrogen-bonding patterns in two crystal structures do not necessarily imply isostructural crystals.

According to the concept of saturated hydrogen bonding (SHB) and complementarity in the number of donors and acceptors (Ermer & Eling, 1994; Loehlin et al., 1998), it is evident that, for molecules having amidine groups only, hydrogen-bond donors prevail. This implies that some donors (imino N) are not saturated, as observed in the crystal structures of acetamidine, (II) and (I). Only with additional acceptors in the molecule or in co-crystallized molecules is the imino H atom activated as a hydrogen-bond donor, as in the structure of the co-crystal of (II) with 2,6-diisopropyl-5,5-dimethyl-4-carboxymethoxide (Marsura et al., 1984).

Related literature

For related literature, see: Barker et al. (1996); Cobbledick & Small (1972); Desiraju (1995); Ermer & Eling (1994); Etter et al. (1990); Felix et al. (1997); Loehlin et al. (1998); Marsura et al. (1984); Nguyen et al. (1998); Norrestam et al. (1983).

S2. Experimental

Crystals of (I) suitable for the X-ray diffraction were grown from a solution of benzdiamidine hydrochloride (150 mg, 0.64 mmol) in water (5 ml) after addition of 1,8-diazabicyclo[5.4.0]undec-7-ene (0.2 ml). The benzdiamidine hydrochloride was prepared according to the procedure described by Felix et al. (1997).

S3. Refinement

All H atoms were refined freely.

Computing details

Data collection: CAD-4 EXPRESS (Enraf Nonius 1992); cell refinement: CAD-4 EXPRESS; data reduction: HELENA (Spek, 1997); program(s) used to solve structure: SIR97 (Altomare et al., 1999); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: PLATON (Spek, 1999); software used to prepare material for publication: PLATON.

Figures

	Fig. 1. The molecular structure of (I) with the atom-numbering scheme and 30% probability displacement ellipsoids. H atoms are shown as small spheres of arbitrary radii.
	Fig. 2. Part of the crystal structure of (I) showing the formation of the two-dimensional hydrogen-bonding network.

Benzene-1,4-dicarboxamidine

Crystal data

C8H10N4	F(000) = 172
Mr = 162.20	Dx = 1.400 Mg m^−3
Monoclinic, P21/c	Cu Kα radiation, λ = 1.54178 Å
Hall symbol: -P 2ybc	Cell parameters from 23 reflections
a = 5.0570 (4) Å	θ = 40.1–45.5°
b = 7.9646 (5) Å	µ = 0.74 mm^−1
c = 9.7226 (9) Å	T = 293 K
β = 100.702 (7)°	Prism, colourless
V = 384.79 (5) Å^3	0.20 × 0.15 × 0.10 mm
Z = 2

Data collection

Enraf-Nonius CAD-4 diffractometer	657 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.015
Graphite monochromator	θmax = 74.0°, θmin = 7.2°
ω/2θ scans	h = 0→6
Absorption correction: ψ-scan (PLATON; Spek, 1999)	k = 0→9
Tmin = 0.885, Tmax = 0.973	l = −12→11
870 measured reflections	3 standard reflections every 120 min
783 independent reflections	intensity decay: 2%

Refinement

Refinement on F^2	0 constraints
Least-squares matrix: full	All H-atom parameters refined
R[F^2 > 2σ(F^2)] = 0.040	w = 1/[σ^2(Fo^2) + (0.0747P)^2 + 0.0747P] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.118	(Δ/σ)max < 0.001
S = 1.03	Δρmax = 0.24 e Å^−3
783 reflections	Δρmin = −0.20 e Å^−3
76 parameters	Extinction correction: SHELXL97 (Sheldrick, 1997), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
0 restraints	Extinction coefficient: 0.011 (4)

Special details

Geometry. Bond distances, angles etc. have been calculated using the rounded fractional coordinates. All e.s.d.'s are estimated from the variances of the (full) variance-covariance matrix. The cell e.s.d.'s are taken into account in the estimation of distances, angles and torsion angles

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 > σ(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 (Ų)

	x	y	z	Uiso*/Ueq
N1	−0.0583 (3)	0.5380 (2)	0.18428 (10)	0.0439 (5)
N2	0.3073 (3)	0.4165 (2)	0.12014 (10)	0.0448 (5)
C1	0.1848 (3)	0.48545 (18)	0.21851 (10)	0.0318 (4)
C2	0.3474 (3)	0.49207 (16)	0.36385 (10)	0.0291 (4)
C3	0.2896 (3)	0.61100 (19)	0.45906 (10)	0.0340 (4)
C4	0.5593 (3)	0.38158 (18)	0.40677 (10)	0.0340 (4)
H1	−0.118 (5)	0.581 (3)	0.260 (2)	0.063 (6)*
H3	0.146 (4)	0.693 (3)	0.4318 (18)	0.044 (5)*
H4	0.602 (4)	0.296 (3)	0.3451 (19)	0.043 (5)*
H21	0.484 (5)	0.411 (3)	0.134 (2)	0.063 (6)*
H22	0.228 (4)	0.429 (3)	0.026 (2)	0.050 (5)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
N1	0.0284 (7)	0.0729 (10)	0.0287 (6)	0.0041 (6)	0.0006 (5)	−0.0033 (6)
N2	0.0364 (8)	0.0685 (10)	0.0273 (7)	0.0101 (7)	0.0000 (5)	−0.0066 (6)
C1	0.0286 (7)	0.0384 (8)	0.0275 (7)	−0.0036 (5)	0.0032 (5)	0.0014 (5)
C2	0.0260 (7)	0.0351 (7)	0.0259 (7)	−0.0030 (5)	0.0037 (5)	0.0020 (5)
C3	0.0315 (7)	0.0381 (8)	0.0314 (7)	0.0074 (6)	0.0030 (5)	0.0015 (6)
C4	0.0355 (8)	0.0363 (8)	0.0295 (7)	0.0051 (6)	0.0043 (5)	−0.0026 (5)

Geometric parameters (Å, º)

N1—C1	1.283 (2)	C2—C3	1.3929 (18)
N2—C1	1.3491 (18)	C2—C4	1.390 (2)
N1—H1	0.91 (2)	C3—C4^i	1.3852 (15)
N2—H22	0.934 (19)	C3—H3	0.98 (2)
N2—H21	0.88 (3)	C4—H4	0.96 (2)
C1—C2	1.4981 (15)
N1···N2^ii	3.298 (2)	H1···C3	2.56 (2)
N1···C4^ii	3.3948 (19)	H1···C4^ii	2.84 (2)
N1···N2^iii	3.0154 (15)	H1···H3	2.13 (3)
N2···C3^iv	3.354 (2)	H1···H21^ii	2.54 (3)
N2···N1^iii	3.0154 (15)	H3···N1	2.731 (19)
N2···N1^v	3.298 (2)	H3···H1	2.13 (3)
N1···H3	2.731 (19)	H3···N2^viii	2.87 (2)
N1···H21^ii	2.49 (3)	H3···C1^viii	3.07 (2)
N1···H22^iii	2.082 (19)	H4···N2	2.59 (2)
N2···H3^vi	2.87 (2)	H4···H21	2.23 (3)
N2···H4	2.59 (2)	H4···C1^iv	2.81 (2)
C3···N2^vii	3.354 (2)	H21···N1^v	2.49 (3)
C4···N1^v	3.3948 (19)	H21···C4	2.619 (19)
C1···H22^iii	2.94 (2)	H21···H1^v	2.54 (3)
C1···H3^vi	3.07 (2)	H21···H4	2.23 (3)
C1···H4^vii	2.81 (2)	H21···C3^iv	2.87 (2)
C3···H21^vii	2.87 (2)	H22···N1^iii	2.082 (19)
C3···H1	2.56 (2)	H22···C1^iii	2.94 (2)
C4···H1^v	2.84 (2)	H22···H22^iii	2.53 (3)
C4···H21	2.619 (19)
C1—N1—H1	111.0 (15)	C3—C2—C4	118.45 (10)
C1—N2—H22	118.8 (13)	C1—C2—C4	121.24 (11)
C1—N2—H21	119.9 (13)	C2—C3—C4^i	120.51 (13)
H21—N2—H22	113.2 (18)	C2—C4—C3^i	121.04 (12)
N1—C1—N2	119.54 (11)	C2—C3—H3	120.8 (11)
N1—C1—C2	124.40 (12)	C4^i—C3—H3	118.7 (11)
N2—C1—C2	116.05 (13)	C2—C4—H4	120.9 (12)
C1—C2—C3	120.31 (12)	C3^i—C4—H4	118.1 (12)
N1—C1—C2—C4	155.14 (16)	C3—C2—C4—C3^i	0.0 (2)
N1—C1—C2—C3	−25.3 (2)	C4—C2—C3—C4^i	0.0 (2)
N2—C1—C2—C3	155.72 (14)	C1—C2—C4—C3^i	179.51 (14)
N2—C1—C2—C4	−23.8 (2)	C2—C3—C4^i—C2^i	−0.1 (2)
C1—C2—C3—C4^i	−179.51 (13)

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

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N2—H21···N1^v	0.88 (3)	2.49 (3)	3.298 (2)	153 (2)
N2—H22···N1^iii	0.934 (19)	2.082 (19)	3.0154 (15)	178 (2)

Symmetry codes: (iii) −x, −y+1, −z; (v) x+1, y, z.