A new polymorph of benzene-1,2-diamine: isomorphism with 2-amino­phenol and two-dimensional iso­structurality of polymorphs

A new crystalline form of benzene-1,2-diamine, C₆H₈N₂, crystallizing in the space group Pbca, has been identified during screening for cocrystals. The crystals are constructed from mol­ecular bilayers parallel to (001) that have the polar amino groups directed to the inside and the aromatic groups, showing a herringbone arrangement, directed to the outside. The known monoclinic form and the new ortho­rhom­bic polymorph exhibit two-dimensional isostructurality as the crystals consist of nearly identical bilayers. In the monoclinic form, neighbouring bilayers are generated by a unit translation along the a axis, whereas in the ortho­rhom­bic form they are generated by a c-glide. Moreover, the new form of benzene-1,2-diamine is essentially isomorphous with the only known form of 2-amino­phenol.

S1. Comment

Increased interest in co-crystals has resulted in several recent reports of the serendipitous formation of a new polymorph by one of the substrates during attempted co-crystallization experiments (Callear & Hursthouse, 2009; Dey et al., 2006; Rafilovich & Bernstein, 2006; Wenger & Bernstein, 2007), with the most famous case represented by a new polymorph of aspirin (Vishweshwar et al., 2005; Bond et al., 2007). Our attempts to prepare new co-crystals have already resulted in obtaining the elusive β form of phenazine (Herbstein & Schmidt, 1955; Jankowski & Gdaniec, 2002) and the hydrated form of 1,4-benzenediamine (Czapik et al., 2010). Crystal structures of all three benzenediamine isomers are known (Betz et al., 2008; Poveteva et al., 1975; Stalhandske, 1981) and polymorphism has not been observed for any of these compounds so far. Recently, we have shown that unstable 1,4-benzamine dihydrate converts on dehydration to its known anhydrous form (Czapik et al., 2010). Polymorphism and hydrate formation are rather exceptional among aromatic amines, as shown by our survey of the Cambridge Structural Database (CSD, Version 5.31; Allen, 2002), which gave 48 structures of aromatic amines of the general formula [C_nH_m(NH₂)_x] with at least one primary amino group attached to the aromatic ring and with only C and H atoms in the remaining part of the molecule. Among these amines only benzidine has been shown to exhibit polymorphism, with four polymorphic forms characterized so far (Rafilovich & Bernstein, 2006). Another aspect that emerged from this CSD search was the unusually high percentage of structures with Z' ≠ 1. Out of the 48 structures analysed, 15 had Z' > 1, 21 had Z' = 1 and 12 had Z' < 1.

In this paper, we report the crystal structure of a new polymorph, (II), of benzene-1,2-diamine with Z' = 1, produced unexpectedly during an attempt to synthesize co-crystals of phenazine with benzene-1,2-diamine. This new form was obtained from a 1:1 mixture of the above two components dissolved in methanol–diethyl ether (1:1 v/v). Slow evaporation of the solution resulted in precipitation of two types of crystals, yellow needles (subsequently identified as the 2:1 phenazine–benzene-1,2-diamine complex) and a pale-pink tabloid crystal which has been shown to be a new orthorhombic form, (II), of benzene-1,2-diamine. The known form, (I), of benzene-1,2-diamine is monoclinic in space group P2₁/c and crystallizes with one molecule in the asymmetric unit (Stalhandske, 1981; CSD Refcode BAGFIY).

The molecular structure of (II), with the atom-numbering scheme, is shown in Fig. 1. In the two polymorphs the molecules have very similar geometries and exhibit non-crystallographic C₂ symmetry, with both amino N atoms sp³ hybridized and one of the N—H bonds of each NH₂ group virtually in the plane of the benzene ring, resulting in two short intramolecular H···N contacts of 2.508 (14) and 2.538 (14) Å between the ortho-situated amino substituents. The C—N bond lengths of 1.4004 (16) and 1.4047 (15) Å are consistent with the N hybridization state, and the endocyclic bond angles at C1 and C2, both smaller than 120°, are consistent with the electron-donating character of the amine groups (Domenicano et al., 1975).

A projection of the crystal structure of (II) along the a axis is shown in Fig. 2(a). This crystal structure can be seen as constructed from bilayers (centred at z = 0, 1/2, 1.0 etc.) that are parallel to (001) and have the polar amino groups directed to the inside and the aromatic groups, which have a herringbone arrangement, directed to the outside. The amino groups, each acting as a single donor and a single acceptor, are involved in two weak intermolecular N—H···N hydrogen bonds (Table 1, Fig. 3) operating between monolayers. Additionally, one of the amino groups forms an N2—H22···π interaction [H22···Cgⁱⁱⁱ = 2.59 Å and N2—H22···Cgⁱⁱⁱ = 141°, where Cg is the centroid of the aromatic ring; symmetry code: (iii) 1/2 - x, 1/2 + y, z] with the aromatic ring of the b-glide-related molecule within the same monolayer (Fig. 3). A closer look at the structure of form (I) (Stalhandske, 1981; Fig. 2b) reveals that the two polymorphs of benzene-1,2-diamine exhibit two-dimensional isostructurality (Fábián & Kálmán, 2004) because they are constructed from virtually identical bilayers with similar metric parameters [b = 7.544(s.u.?) Å, c = 7.716(s.u.?) Å and α = 90° in (I), and a = 7.533 (2) Å, b = 7.835 (2) Å and γ = 90° in (II)] and the same layer group symmetry [p12₁/c1 in (I) and p2₁/b11 in (II)]. The two polymorphs differ in the stacking arrangement of the bilayers: in form (I), neighbouring bilayers are generated by a unit translation along the a axis, whereas in form (II) they are generated by a c-glide. In both crystalline forms the interactions between the bilayer surface atoms are only of the van der Waals type, with all interlayer contacts being longer than the sum of the van der Waals radii of the appropriate atoms.

The melting temperatures measured for both polymorphs using a Boetius apparatus were 368–370 K for (I) (reference value 375.5 K; CRC Handbook of Chemistry and Physics, 1998) and 366–368 K for (II), and no phase changes were observed under a polarizing microscope when the crystals were heated from room temperature to the m.p. Interestingly, the two forms also have very similar volumes for a single diamine molecule [147.9 and 148.3 ų for (I) and (II), respectively] and thus also similar crystal densities, 1.214 and 1.211 Mg m^-3 for forms (I) and (II), respectively. However, simulated powder diffractograms show that the two forms can be easily discerned from their diffraction patterns (Fig. 4)

Since one (N1—H12) of the N—H groups in (II) is not involved in any specific intermolecular interactions, we decided to check whether the monohydroxy congener of benzene-1,2-diamine, 1,2-aminophenol, formed a similar structure. Aminophenols have been extensively studied (Allen et al., 1997; Dey et al., 2005, 2006) in the context of the saturated hydrogen-bond principle introduced by Ermer & Eling (1994) and in relation to crystal structure prediction using the supramolecular synthon approach. Indeed, the structure for form (II) reported here for benzene-1,2-diamine is essentially isomorphous with the only known orthorhombic form of 1,2-aminophenol (Ashfaquzzaman & Pant, 1979, CSD Refcode AMPHOM10; Korp et al., 1981, CSD Refcode AMPHOM02; Allen et al., 1997, CSD Refcode AMPHOM03) and very similar in structure to 2-amino-4-methylphenol (Kashino et al., 1988; Dey et al., 2005), which has the unit-cell parameter c elongated by ca 2.8 Å compared with (II) due to the presence of the methyl substituents on the bilayer surface. Moreover, the monoclinic polymorph, (I), closely resembles, in a similar manner, the structure of the monoclinic compounds 2-amino-4-chlorophenol (Ashfaquzzaman & Pant, 1979) and 2-amino-4-ethylphenol (Dey et al., 2006). These examples show that ortho-aminophenols and aromatic ortho-diamines show extensive similarities in their crystal packing. However, these similarities do not extend to the meta and para isomers of these compounds

S2. Experimental

Polymorph (II) of benzene-1,2-diamine was formed during co-crystallization in a 1:1 molar ratio of benzene-1,2-diamine (6 mg) with phenazine (10 mg). The two components were dissolved in a methanol–diethyl ether (6 ml, 1:1 v/v) solvent mixture and the solution was slowly evaporated. As only one relatively large crystal of form (II) (2 × 1 × 0.5 mm) was found in the crystallization vial, it was cut into small pieces, one of which was used for structure determination. Unfortunately, our attempts to produce more crystals of form (II) by repeating the crystallization conditions were unsuccessful. Crystallization of benzene-1,2-diamine carried out under a wide range of different sets of conditions (different solvents, different co-solute molecules) led systematically to polymorph (I).

S3. Refinement

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, and refined as riding on their carrier atoms, with U_iso(H) = 1.2U_eq(C). All N-bound H atoms were freely refined (coordinates and isotropic displacement parameters). [Range of N—H distances?]

Computing details

Data collection: CrysAlis PRO (Oxford Diffraction, 2009); cell refinement: CrysAlis PRO (Oxford Diffraction, 2009); data reduction: CrysAlis PRO (Oxford Diffraction, 2009); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 for Windows (Farrugia, 1997) and Mercury (Macrae et al., 2006); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008).

Figures

	Fig. 1. The molecular structure of (II), with the atom-numbering scheme. Displacement ellipsoids are drawn at the 50% probability level and H atoms are shown as small spheres of arbitrary radii.
	Fig. 2. Two-dimensional isostructurality among polymorphs of benzene-1,2-diamine. (a) The crystal packing in polymorph (II), viewed down the a axis. (b) The crystal packing in polymorph (I), viewed down the b axis. Hydrogen bonds are shown as dotted lines.
	Fig. 3. The structure of the (001) layer in (II), centred around z = 1/2. Hydrogen bonds and N—H···π interactions are shown as dotted lines. For symmetry codes, see Table 1 and the Comment.
	Fig. 4. Simulated diffraction patterns for (a) (II), (b) aminophenol and (c) (I).

benzene-1,2-diamine

Crystal data

C6H8N2	F(000) = 464
Mr = 108.14	Dx = 1.211 Mg m^−3
Orthorhombic, Pbca	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ac 2ab	Cell parameters from 1955 reflections
a = 7.5330 (17) Å	θ = 2.9–28.8°
b = 7.8350 (15) Å	µ = 0.08 mm^−1
c = 20.098 (4) Å	T = 293 K
V = 1186.2 (4) Å^3	Block, pale-pink
Z = 8	0.5 × 0.25 × 0.1 mm

Data collection

Oxford XcaliburE CCD area-detector diffractometer	1204 independent reflections
Radiation source: fine-focus sealed tube	817 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.029
ω scans	θmax = 26.4°, θmin = 4.3°
Absorption correction: multi-scan (CrysAlis PRO; Oxford Diffraction, 2009)	h = −9→8
Tmin = 0.776, Tmax = 1.000	k = −9→9
6421 measured reflections	l = −25→25

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.036	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.088	H atoms treated by a mixture of independent and constrained refinement
S = 0.94	w = 1/[σ^2(Fo^2) + (0.054P)^2] where P = (Fo^2 + 2Fc^2)/3
1204 reflections	(Δ/σ)max < 0.001
89 parameters	Δρmax = 0.11 e Å^−3
0 restraints	Δρmin = −0.20 e Å^−3

Special details

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 > 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	0.54191 (17)	0.26495 (14)	0.49593 (5)	0.0443 (3)
H11	0.6074 (18)	0.1881 (17)	0.5225 (6)	0.053 (4)*
H12	0.440 (2)	0.2877 (18)	0.5127 (7)	0.067 (5)*
N2	0.30911 (16)	0.42760 (13)	0.41182 (6)	0.0463 (3)
H21	0.361 (2)	0.4984 (18)	0.4408 (7)	0.064 (4)*
H22	0.253 (2)	0.4824 (18)	0.3806 (7)	0.067 (4)*
C1	0.53540 (15)	0.21377 (13)	0.42921 (6)	0.0346 (3)
C2	0.42111 (15)	0.30052 (13)	0.38596 (5)	0.0352 (3)
C3	0.41203 (18)	0.24953 (15)	0.32035 (6)	0.0457 (4)
H3	0.3335	0.3038	0.2916	0.055*
C4	0.5182 (2)	0.11875 (18)	0.29683 (6)	0.0566 (4)
H4	0.5110	0.0861	0.2524	0.068*
C5	0.6342 (2)	0.03709 (17)	0.33868 (6)	0.0556 (4)
H5	0.7070	−0.0495	0.3227	0.067*
C6	0.64204 (16)	0.08434 (14)	0.40470 (6)	0.0435 (3)
H6	0.7201	0.0285	0.4332	0.052*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
N1	0.0459 (8)	0.0443 (6)	0.0427 (6)	0.0065 (6)	−0.0047 (6)	−0.0011 (5)
N2	0.0476 (8)	0.0395 (6)	0.0518 (7)	0.0092 (5)	−0.0031 (5)	0.0042 (5)
C1	0.0335 (7)	0.0292 (6)	0.0410 (6)	−0.0051 (5)	0.0004 (5)	0.0001 (4)
C2	0.0336 (7)	0.0298 (6)	0.0423 (6)	−0.0051 (5)	0.0028 (5)	0.0050 (5)
C3	0.0507 (9)	0.0452 (7)	0.0412 (7)	−0.0053 (6)	−0.0032 (6)	0.0079 (6)
C4	0.0690 (10)	0.0596 (8)	0.0411 (7)	−0.0037 (8)	0.0053 (7)	−0.0081 (6)
C5	0.0538 (10)	0.0545 (8)	0.0584 (8)	0.0062 (7)	0.0064 (7)	−0.0150 (6)
C6	0.0377 (7)	0.0388 (7)	0.0541 (7)	0.0028 (6)	−0.0037 (6)	−0.0017 (5)

Geometric parameters (Å, º)

N1—C1	1.4004 (16)	C2—C3	1.3795 (16)
N1—H11	0.944 (14)	C3—C4	1.3833 (18)
N1—H12	0.855 (15)	C3—H3	0.9300
N2—C2	1.4047 (15)	C4—C5	1.3711 (18)
N2—H21	0.893 (15)	C4—H4	0.9300
N2—H22	0.868 (15)	C5—C6	1.3788 (17)
C1—C6	1.3843 (15)	C5—H5	0.9300
C1—C2	1.3996 (16)	C6—H6	0.9300
C1—N1—H11	112.2 (8)	C2—C3—C4	120.83 (12)
C1—N1—H12	113.9 (10)	C2—C3—H3	119.6
H11—N1—H12	112.2 (13)	C4—C3—H3	119.6
C2—N2—H21	114.8 (10)	C5—C4—C3	120.29 (11)
C2—N2—H22	112.0 (10)	C5—C4—H4	119.9
H21—N2—H22	111.9 (14)	C3—C4—H4	119.9
C6—C1—C2	119.46 (11)	C4—C5—C6	119.50 (12)
C6—C1—N1	122.01 (11)	C4—C5—H5	120.3
C2—C1—N1	118.50 (11)	C6—C5—H5	120.3
C3—C2—C1	118.92 (11)	C5—C6—C1	120.95 (12)
C3—C2—N2	121.95 (11)	C5—C6—H6	119.5
C1—C2—N2	118.94 (11)	C1—C6—H6	119.5
C6—C1—C2—C3	2.82 (16)	C2—C3—C4—C5	0.2 (2)
N1—C1—C2—C3	−179.13 (11)	C3—C4—C5—C6	1.1 (2)
C6—C1—C2—N2	177.82 (9)	C4—C5—C6—C1	−0.4 (2)
N1—C1—C2—N2	−4.13 (16)	C2—C1—C6—C5	−1.53 (17)
C1—C2—C3—C4	−2.20 (18)	N1—C1—C6—C5	−179.50 (12)
N2—C2—C3—C4	−177.05 (11)

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N1—H11···N2^i	0.944 (14)	2.207 (14)	3.1250 (16)	163.8 (11)
N2—H21···N1^ii	0.893 (15)	2.365 (15)	3.2404 (16)	166.6 (13)

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