N—H...π hydrogen bonding in 2-amino­fluorene

The amino group of the title compound, C₁₃H₁₁N, does not form N—H...N hydrogen bonds, but is engaged only in weaker N—H...Ph and C—H...N interactions. An aromatic ring acts as a double hydrogen-bond acceptor, leading to an infinite H—N—H...Ph...H—N—H...Ph array.

S1. Comment

The primary amino group (—NH₂) has a double-donor single-acceptor hydrogen-bond function. As a consequence, primary amines often have a surplus of strong hydrogen-bond donors that cannot be satisfied by conventional hydrogen bonding (N—H···X with X = O, N, S etc.). If π-bonded moieties are present, weaker N—H···π hydrogen bonds are often formed as a result (Hanton et al., 1992). Most important of all π acceptors is the phenyl group and today there is a large volume of experimental material on N—H···Ph hydrogen bonds (surveyed by Malone et al., 1997; Desiraju & Steiner, 1999). Even several neutron-diffraction studies have been performed (Allen et al., 1997; Steiner & Mason, 2000; also see Starikov & Steiner, 1998). Nevereless, the roles played by N—H···Ph interactions in hydrogen-bond networks are only poorly explored and deserve further investigation. In this context, 2-aminofluorene, (I), is an obviously interesting molecule. Therefore, its crystal structure was determined in order to characterize the hydrogen-bond pattern formed.

The molecular structure of (I) is shown in Fig. 1. The geometry of the fluorene moiety is unremarkable [for relevant references see, for example, the structure reports on fluorene by Gerkin et al. (1984), on 2-acetylaminofluorene by van Meersssche et al. (1980) and on 9-fluorenone by Luss & Smith (1972)]. The C—N bond length is 1.372 (4) Å. The amino group is significantly pyramidal, with the sum of angles at N equal to 347 (4)°; the H—N—H angle is 117 (4) °. Both N—H vectors are oriented out of the plane of the neighboring aromatic ring, with torsion angles C1—C2—N—H1N = 163 (3)° and C1—C2—N—H2N = 21 (3)°.

The crystal packing of (I) is depicted in Fig. 2. No N—H···N hydrogen bonds are formed, but both N—H vectors point at the faces of aromatic rings of neighboring molecules. The geometries of these contacts are typical for N—H···Ph hydrogen bonding [for N—H normalized to 1.01 Å (M = aromatic centre): N—H1···Ph(x − 1/2, 1/2 − y, z) with H···M = 2.55, N···M = 3.54 Å; N—H···M = 167°, H···C range = 2.60–3.17 Å; N—H2···Ph(1 − x, −y, z − 1/2) with H···M = 2.86, N···M = 3.82 Å, N—H···M = 160°, H···C range 2.53–3.71 Å]. Since the aromatic ring involved acts as a double acceptor, the resulting hydrogen-bond pattern is an infinite H—N—H···Ph···H—N—H···Ph chain. More commonly, Ph groups act only as single hydrogen-bond acceptors, X—H···Ph, and can then play only very simple roles in hydrogen-bond arrays (i.e. isolated bonds X—H···Ph, pairs Ph···H—X—H···Ph or chain terminators X—H···Y—H···Ph).

Due to its pyramidal geometry, the amino group of (I) can act also as a hydrogen-bond acceptor. Apart from the amino group, (I) contains only the weakly polar C—H groups as potential donors. The shortest intermolecular contact formed by the amino N atom is to C4—H(1 − x, 1 − y, 1/2 + z), with H···N = 2.65, C···N = 3.723 (6) Å, C—H···N = 170° (for C—H normalized to 1.08 Å; not shown in Fig. 2). This is a typical geometry for a weak C—H···N hydrogen bond (Steiner, 1998).

The crystal structure of 2-aminofluorene is an example of a structure containing no conventional hydrogen bonds, although they could be formed in principle. Instead, the amino group is involved only in weak hydrogen bonds of the types N—H···Ph and C—H···N, which have been generally neglected not so long ago.

S2. Experimental

2-Aminofluorene, (I), was obtained from Aldrich and was recrystallized from MeOH by slow evaporation of the solvent.

S3. Refinement

H atoms bonded to C atoms were treated with the default riding model, and the amino H atoms were located in difference Fourier calculations and refined isotropically. All H-atom displacement parameters refined to realistic values [H atoms bonded to C: 0.057–0.096 Ų; amino H atoms: 0.119 (16) and 0.100 (16) Ų for H1N and H2N, respectively]. At first glance, the U_eq values of the amino H atoms appear to be relatively high, but when compared with the U_eq of N [0.0836 (9) Ų] they can be considered as normal.

Computing details

Program(s) used to solve structure: SHELXS97 (Sheldrick, 1990); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: PLATON (Spek, 1999); software used to prepare material for publication: SHELXL97 (Sheldrick, 1997).

Figures

	Fig. 1. The molecular structure of (I). Displacement ellipsoids are drawn at the 30% probability level.
	Fig. 2. The crystal packing of (I) drawn in projection onto the xz plane. N—H···π hydrogen bonds are indicated by dashed lines.

2-Aminofluorene

Crystal data

C13H11N	Dx = 1.281 Mg m^−3
Mr = 181.23	Mo Kα radiation, λ = 0.71073 Å
Orthorhombic, Pna21	Cell parameters from 33 reflections
a = 14.790 (14) Å	θ = 8.1–17.3°
b = 5.681 (9) Å	µ = 0.08 mm^−1
c = 11.187 (11) Å	T = 293 K
V = 940 (2) Å^3	Rod, yellow
Z = 4	0.80 × 0.25 × 0.15 mm
F(000) = 384

Data collection

Stoe four-circle diffractometer	Rint = 0.033
Radiation source: fine-focus sealed tube	θmax = 27.5°, θmin = 2.8°
Graphite monochromator	h = −19→16
ω scans	k = −7→3
2226 measured reflections	l = −14→13
1358 independent reflections	3 standard reflections every 90 min
1037 reflections with I > 2σ(I)	intensity decay: none

Refinement

Refinement on F^2	Hydrogen site location: inferred from neighbouring sites
Least-squares matrix: full	H atoms treated by a mixture of independent and constrained refinement
R[F^2 > 2σ(F^2)] = 0.043	w = 1/[σ^2(Fo^2) + (0.0579P)^2 + 0.1454P] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.128	(Δ/σ)max < 0.001
S = 1.03	Δρmax = 0.13 e Å^−3
1358 reflections	Δρmin = −0.15 e Å^−3
145 parameters	Extinction correction: SHELXL97, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
1 restraint	Extinction coefficient: 0.019 (5)
Primary atom site location: structure-invariant direct methods	Absolute structure: Flack (1983)
Secondary atom site location: difference Fourier map	Absolute structure parameter: −1 (7)

Special details

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'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
N	0.3763 (2)	0.4268 (7)	0.0485 (3)	0.0836 (9)
H1N	0.341 (3)	0.529 (9)	0.101 (5)	0.119 (16)*
H2N	0.347 (3)	0.348 (8)	−0.005 (4)	0.100 (16)*
C1	0.48630 (18)	0.1156 (6)	0.0491 (3)	0.0636 (8)
H1	0.4643	0.0589	−0.0232	0.074 (10)*
C2	0.44602 (17)	0.3086 (6)	0.1026 (3)	0.0650 (8)
C3	0.4799 (2)	0.3882 (6)	0.2121 (3)	0.0687 (8)
H3	0.4525	0.5159	0.2494	0.076 (10)*
C4	0.55252 (19)	0.2823 (6)	0.2655 (3)	0.0653 (8)
H4	0.5745	0.3391	0.3379	0.070 (9)*
C5	0.72689 (19)	−0.0385 (7)	0.3438 (3)	0.0683 (8)
H5	0.7199	0.0808	0.4000	0.069 (10)*
C6	0.79407 (19)	−0.2008 (7)	0.3569 (3)	0.0725 (9)
H6	0.8333	−0.1905	0.4216	0.096 (13)*
C7	0.8042 (2)	−0.3789 (7)	0.2756 (3)	0.0750 (9)
H7	0.8493	−0.4911	0.2862	0.068 (9)*
C8	0.7471 (2)	−0.3920 (6)	0.1770 (3)	0.0710 (9)
H8	0.7544	−0.5117	0.1210	0.083 (10)*
C9	0.6116 (2)	−0.2014 (6)	0.0637 (2)	0.0638 (8)
H9A	0.6408	−0.1736	−0.0127	0.057 (8)*
H9B	0.5734	−0.3398	0.0574	0.087 (11)*
C10	0.55830 (16)	0.0084 (6)	0.1025 (3)	0.0578 (7)
C11	0.59276 (17)	0.0918 (5)	0.2120 (2)	0.0572 (7)
C12	0.66916 (17)	−0.0516 (6)	0.2468 (3)	0.0581 (7)
C13	0.68059 (18)	−0.2291 (6)	0.1627 (2)	0.0596 (7)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
N	0.0764 (17)	0.094 (2)	0.081 (2)	0.0064 (18)	−0.0077 (17)	0.000 (2)
C1	0.0626 (15)	0.077 (2)	0.0507 (16)	−0.0122 (15)	0.0037 (13)	−0.0035 (16)
C2	0.0570 (14)	0.078 (2)	0.0600 (17)	−0.0085 (15)	0.0058 (13)	0.0076 (18)
C3	0.0707 (17)	0.0733 (19)	0.0621 (18)	−0.0007 (16)	0.0121 (15)	−0.0057 (16)
C4	0.0687 (15)	0.076 (2)	0.0509 (15)	−0.0052 (16)	0.0079 (13)	−0.0111 (17)
C5	0.0687 (15)	0.084 (2)	0.0521 (16)	−0.0057 (17)	0.0026 (14)	−0.0090 (17)
C6	0.0672 (16)	0.095 (2)	0.0556 (17)	−0.0024 (18)	0.0013 (14)	0.001 (2)
C7	0.0764 (18)	0.088 (2)	0.061 (2)	0.0120 (19)	0.0095 (15)	0.0066 (19)
C8	0.0839 (18)	0.076 (2)	0.0528 (18)	0.0040 (19)	0.0139 (13)	−0.0015 (17)
C9	0.0773 (18)	0.0687 (19)	0.0453 (14)	−0.0061 (17)	0.0051 (12)	−0.0077 (15)
C10	0.0577 (12)	0.0696 (17)	0.0460 (14)	−0.0125 (14)	0.0102 (12)	−0.0004 (14)
C11	0.0589 (13)	0.0695 (19)	0.0430 (13)	−0.0113 (14)	0.0086 (11)	−0.0045 (14)
C12	0.0586 (13)	0.0715 (18)	0.0441 (14)	−0.0103 (14)	0.0098 (11)	−0.0029 (14)
C13	0.0648 (14)	0.0683 (18)	0.0456 (13)	−0.0118 (15)	0.0098 (11)	−0.0003 (15)

Geometric parameters (Å, º)

N—C2	1.372 (4)	C6—C7	1.368 (5)
N—H1N	0.97 (5)	C6—H6	0.9300
N—H2N	0.86 (4)	C7—C8	1.391 (5)
C1—C10	1.364 (4)	C7—H7	0.9300
C1—C2	1.384 (4)	C8—C13	1.360 (5)
C1—H1	0.9300	C8—H8	0.9300
C2—C3	1.398 (5)	C9—C10	1.494 (4)
C3—C4	1.369 (5)	C9—C13	1.514 (4)
C3—H3	0.9300	C9—H9A	0.9700
C4—C11	1.373 (4)	C9—H9B	0.9700
C4—H4	0.9300	C10—C11	1.409 (4)
C5—C6	1.364 (5)	C11—C12	1.446 (4)
C5—C12	1.383 (4)	C12—C13	1.390 (4)
C5—H5	0.9300
C2—N—H1N	115 (3)	C8—C7—H7	120.0
C2—N—H2N	115 (3)	C13—C8—C7	119.7 (3)
H1N—N—H2N	117 (4)	C13—C8—H8	120.1
C10—C1—C2	120.0 (3)	C7—C8—H8	120.1
C10—C1—H1	120.0	C10—C9—C13	103.1 (2)
C2—C1—H1	120.0	C10—C9—H9A	111.2
N—C2—C1	121.4 (4)	C13—C9—H9A	111.2
N—C2—C3	119.8 (4)	C10—C9—H9B	111.2
C1—C2—C3	118.8 (3)	C13—C9—H9B	111.2
C4—C3—C2	121.4 (3)	H9A—C9—H9B	109.1
C4—C3—H3	119.3	C1—C10—C11	120.9 (3)
C2—C3—H3	119.3	C1—C10—C9	129.9 (3)
C3—C4—C11	119.7 (3)	C11—C10—C9	109.3 (3)
C3—C4—H4	120.1	C4—C11—C10	119.2 (3)
C11—C4—H4	120.1	C4—C11—C12	131.7 (3)
C6—C5—C12	119.9 (3)	C10—C11—C12	109.1 (3)
C6—C5—H5	120.1	C5—C12—C13	119.7 (3)
C12—C5—H5	120.1	C5—C12—C11	131.6 (3)
C5—C6—C7	120.5 (3)	C13—C12—C11	108.7 (3)
C5—C6—H6	119.7	C8—C13—C12	120.1 (3)
C7—C6—H6	119.7	C8—C13—C9	130.1 (3)
C6—C7—C8	120.0 (3)	C12—C13—C9	109.7 (3)
C6—C7—H7	120.0