Remarkable π–π stacking of dipyrido[f,h]quinoxaline-6,7-dicarbonitrile in its ethanol solvate

The title compound, C₁₆H₆N₆·C₂H₆O, is an ethanol solvate of an aromatic phenanthroline-based flat ligand. The latter exhibits a remarkable π–π stacking in the crystal structure, with inter­planar distances of 3.27 and 3.40 Å, which directs the columnar organization of the ligands. The ethanol solvent mol­ecule is located in channels between these columns, being hydrogen bonded to one of the N-atom sites of the phenanthroline fragment.

S1. Comment

6,7-Dicyanodipyridoquinoxaline (DICNQ) has been widely used as a coordination multidentate ligand in the synthesis of various transition metal complexes (Stephensen & Hardie, 2006; Xu et al., 2002; Liu et al., 2001). It has also been employed as an efficient antenna chromophore in the design of photonic and biochemical sensors (Arounaguiri & Maiya, 1999; Ambroize & Maiya, 2000; van der Tol et al., 1998). Redox chemistry of Ru^I complexes of DICNQ has also been investigated (Kulkarni et al., 2004). During our attempts to synthesize new metal-organic frameworks based on 1,10-phenanthroline and its derivatives with transition metal ions we synthesized DICNQ by a literature procedure (Arounaguiri & Maiya, 1999; van der Tol et al., 1998). Surprisingly, the structure of this important ligand has not been characterized before in its uncomplexed form. Correspondingly, we report here the crystal structure of DICNQ at ca 110 K, which crystallized as an ethanol solvate, (I), with an emphasis on its supramolecular self-organization. The latter is a measure of optimal ligand–ligand interactions in the absence of foreign metal ions, the coordination preference of which dominates the topology of the metal complexes of DICNQ in the previously published structures.

An ORTEPIII (Burnett & Johnson, 1996) representation of (I) is shown in Fig. 1. The molecular framework consists of four fused six-membered rings and is aromatic. This 18-membered delocalized system (excluding the two –CN substituents) is essentially planar, the deviations of the individual atoms from its mean plane not exceeding ±0.07 Å (with an r.m.s. deviation of the fitted atoms of 0.040 Å). The cyano groups are bent to a minor extent with respect to this plane. As commonly observed in crystals of large aromatic molecules, the intermolecular assembly is dominated by π–π stacking of overlapping flat molecular entities. Thus, the crystal structure of (I) can be best described as composed of columns of tightly stacked DICNQ ligands. The stacking direction is along the a axis, though the molecular units are slightly inclined with respect to a (the angle between the normal to the molecular plane and a is about 15°). Along the stacks, the individual species are oriented in alternating directions; the –CN dipoles of adjacent overlapping units related by inversion are aligned in an antiparallel manner. Fig. 2 illustrates the two modes of intermolecular overlap along the stacks. Molecules paired around the inversion center at x = 0, y = 0, z = 1.0 at an interplanar distance of 3.269 (3) Å exhibit a more extensive overlap. Those paired around the x = 1/2, y = 0, z = 1.0 inversion with an interplanar distance of 3.397 (3) Å overlap only through their phenathroline fragments. The almost equidistant intermolecular separation of up to 3.4 Å along these supramolecular arrays indicate that strong π–π stacking interactions assisted by the antiparallel arrangements of the polar species hold together the columnar structure (Fig. 3a).

The packing of the oval stacks in the b and c directions is stabilized mostly by dispersion, including long-range electrostatic (dipolar) interactions between the laterally oriented cyano substituents and van der Waals C—H···NC contacts. The packing leaves channel voids centered at (x, 1/2, 1/2). These channels contain the ethanol solvent molecules, which hydrogen bond to one of the N-atom sites of the phenanthroline fragments (Table 1 and Fig. 3b). The tight packing of DICNQ along one direction and the loose packing in another, associated with the incorporation of the solvent into the crystal structure, illustrates the significance of π–π stacking as a structure directing interaction. Similar stacking patterns have been observed in a large number of crystal structures that contain similar extended aromatic fragments (e.g. Gupta et al., 2004; Gut et al., 2002; Bergman et al., 2002).

Related literature

For related literature, see: Arounaguiri & Maiya (1999); Bergman et al. (2002); Gupta et al. (2004); Gut et al. (2002); Kulkarni et al. (2004); Liu et al. (2001); Stephensen & Hardie (2006); van der Tol et al. (1998); Xu et al. (2002).

S2. Experimental

DICNQ was synthesized by previously reported procedures (Arounaguiri & Maiya, 1999; van der Tol et al., 1998) and crystallized from ethanol by slow evaporation.

S3. Refinement

H atoms bound to C atoms were located in calculated positions and were constrained to ride on their parent atoms, with C—H distances of 0.95, 0.98 and 0.99 Å and with U_iso(H) values of 1.2 and 1.5 times U_eq(C). That bound to the O atom was located in a difference Fourier map; its atomic coordinates were refined freely with a U_iso(H) value of 1.5 U_eq(O).

Computing details

Data collection: Collect (Nonius, 1999); cell refinement: DENZO (Otwinowski & Minor, 1997); data reduction: DENZO (Otwinowski & Minor, 1997); program(s) used to solve structure: SIR97 (Altomare et al., 1999); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ORTEPIII (Burnett & Johnson, 1996) and Mercury (Macrae et al., 2006); software used to prepare material for publication: SHELXL97 (Sheldrick, 1997).

Figures

	Fig. 1. The molecular structure of the title compound, showing the atom labeling scheme. The atomic ellipsoids represent displacement parameters at the 50% probability level at ca 110 K.
	Fig. 2. Modes of overlap between neighboring DICNQ molecules within the stacked arrays: (a) molecules related by inversion at (0, 0, 1), spaced at 3.269 (3) Å; (b) molecules related by inversion at (1/2, 0, 1), spaced at 3.397 (3) Å.
	Fig. 3. The crystal packing of (I). (a) An illustration of two adjacent stacks of the DICNQ species (wireframe). The alternating interplanar distances along the stacks are indicated, and the ethanol solvent molecule had been omitted. (b) The intermolecular organization projected down the a axis, showing molecules of the ethanol solvent located in the channels between the stacks (DICNQ is given in wireframe and ethanol in ball-and-stick form). Hydrogen bonds are denoted by dashed lines.

dipyrido[f,h]quinoxaline-6,7-dicarbonitrile ethanol solvate

Crystal data

C16H6N6·C2H6O	Z = 2
Mr = 328.34	F(000) = 340
Triclinic, P1	Dx = 1.381 Mg m^−3
a = 7.1090 (4) Å	Mo Kα radiation, λ = 0.71073 Å
b = 10.2326 (5) Å	Cell parameters from 2590 reflections
c = 11.1591 (7) Å	θ = 2.0–25.7°
α = 93.2852 (19)°	µ = 0.09 mm^−1
β = 102.9380 (18)°	T = 110 K
γ = 90.9640 (18)°	Plate, brown
V = 789.48 (8) Å^3	0.25 × 0.20 × 0.10 mm

Data collection

Nonius KappaCCD diffractometer	1842 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.049
Graphite monochromator	θmax = 25.7°, θmin = 2.0°
Detector resolution: 12.8 pixels mm^-1	h = −8→8
1 deg. ϕ & ω scans	k = −12→12
6564 measured reflections	l = −13→13
2958 independent reflections

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)] = 0.058	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.155	H atoms treated by a mixture of independent and constrained refinement
S = 1.00	w = 1/[σ^2(Fo^2) + (0.0815P)^2] where P = (Fo^2 + 2Fc^2)/3
2958 reflections	(Δ/σ)max = 0.001
230 parameters	Δρmax = 0.32 e Å^−3
0 restraints	Δρmin = −0.25 e Å^−3

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
N1	0.3721 (3)	−0.25824 (17)	1.03785 (19)	0.0316 (5)
C2	0.3360 (4)	−0.3510 (2)	0.9460 (3)	0.0359 (6)
H2	0.3750	−0.4372	0.9646	0.043*
C3	0.2449 (4)	−0.3301 (2)	0.8247 (2)	0.0344 (6)
H3	0.2250	−0.3999	0.7628	0.041*
C4	0.1850 (3)	−0.2072 (2)	0.7967 (2)	0.0298 (6)
H4	0.1232	−0.1900	0.7147	0.036*
C5	0.2158 (3)	−0.1067 (2)	0.8907 (2)	0.0254 (5)
C6	0.1481 (3)	0.02402 (19)	0.8684 (2)	0.0234 (5)
C7	0.1738 (3)	0.12118 (19)	0.9664 (2)	0.0232 (5)
C8	0.2740 (3)	0.0922 (2)	1.0894 (2)	0.0243 (5)
C9	0.3035 (3)	0.1862 (2)	1.1888 (2)	0.0282 (6)
H9	0.2571	0.2722	1.1777	0.034*
C10	0.4009 (4)	0.1512 (2)	1.3026 (2)	0.0339 (6)
H10	0.4232	0.2127	1.3716	0.041*
C11	0.4666 (4)	0.0239 (2)	1.3152 (2)	0.0340 (6)
H11	0.5359	0.0020	1.3942	0.041*
N12	0.4385 (3)	−0.06891 (17)	1.22374 (18)	0.0301 (5)
C13	0.3107 (3)	−0.1369 (2)	1.0102 (2)	0.0252 (5)
C14	0.3431 (3)	−0.0348 (2)	1.1112 (2)	0.0251 (5)
N15	0.0568 (3)	0.04955 (17)	0.75295 (17)	0.0266 (5)
N16	0.1019 (3)	0.24223 (16)	0.94864 (18)	0.0262 (5)
C17	−0.0108 (3)	0.1685 (2)	0.7370 (2)	0.0261 (5)
C18	0.0100 (3)	0.2647 (2)	0.8349 (2)	0.0253 (5)
C19	−0.1092 (4)	0.1961 (2)	0.6136 (2)	0.0315 (6)
C20	−0.0752 (4)	0.3912 (2)	0.8144 (2)	0.0320 (6)
N21	−0.1868 (3)	0.2187 (2)	0.5159 (2)	0.0436 (6)
N22	−0.1468 (3)	0.48930 (19)	0.7943 (2)	0.0414 (6)
C23	0.7334 (5)	−0.4303 (3)	1.4540 (3)	0.0500 (8)
H23A	0.8690	−0.4280	1.4484	0.075*
H23B	0.6764	−0.5178	1.4265	0.075*
H23C	0.7254	−0.4100	1.5396	0.075*
C24	0.6255 (4)	−0.3314 (3)	1.3739 (3)	0.0461 (7)
H24A	0.4870	−0.3370	1.3756	0.055*
H24B	0.6759	−0.2423	1.4055	0.055*
O25	0.6470 (3)	−0.35486 (16)	1.25190 (17)	0.0373 (5)
H25	0.562 (5)	−0.304 (3)	1.203 (3)	0.056*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
N1	0.0297 (12)	0.0272 (10)	0.0386 (13)	0.0065 (8)	0.0074 (10)	0.0079 (9)
C2	0.0356 (15)	0.0254 (12)	0.0484 (18)	0.0070 (10)	0.0115 (13)	0.0072 (11)
C3	0.0340 (15)	0.0319 (13)	0.0386 (16)	0.0040 (10)	0.0117 (12)	−0.0017 (11)
C4	0.0281 (14)	0.0306 (12)	0.0321 (14)	0.0044 (9)	0.0086 (11)	0.0043 (10)
C5	0.0206 (12)	0.0264 (12)	0.0313 (14)	0.0028 (9)	0.0099 (10)	0.0028 (10)
C6	0.0198 (12)	0.0268 (12)	0.0243 (13)	−0.0005 (9)	0.0053 (10)	0.0060 (9)
C7	0.0195 (12)	0.0262 (11)	0.0255 (13)	0.0011 (8)	0.0083 (10)	0.0035 (9)
C8	0.0211 (13)	0.0277 (11)	0.0255 (13)	0.0032 (9)	0.0073 (10)	0.0052 (9)
C9	0.0255 (13)	0.0285 (12)	0.0310 (14)	0.0026 (9)	0.0060 (11)	0.0051 (10)
C10	0.0316 (14)	0.0414 (14)	0.0275 (14)	0.0017 (10)	0.0042 (11)	0.0016 (11)
C11	0.0327 (15)	0.0440 (14)	0.0240 (14)	0.0032 (11)	0.0016 (11)	0.0088 (11)
N12	0.0303 (12)	0.0329 (10)	0.0279 (12)	0.0040 (8)	0.0063 (9)	0.0099 (9)
C13	0.0203 (13)	0.0277 (11)	0.0286 (14)	0.0029 (9)	0.0060 (10)	0.0084 (10)
C14	0.0189 (12)	0.0327 (12)	0.0253 (14)	0.0003 (9)	0.0069 (10)	0.0084 (10)
N15	0.0235 (11)	0.0315 (10)	0.0250 (11)	0.0006 (8)	0.0054 (9)	0.0047 (8)
N16	0.0256 (11)	0.0255 (10)	0.0279 (12)	0.0030 (8)	0.0060 (9)	0.0068 (8)
C17	0.0232 (13)	0.0280 (12)	0.0275 (14)	0.0032 (9)	0.0045 (11)	0.0089 (10)
C18	0.0225 (13)	0.0259 (11)	0.0279 (14)	0.0012 (9)	0.0049 (11)	0.0075 (10)
C19	0.0310 (14)	0.0313 (13)	0.0309 (15)	0.0033 (10)	0.0027 (12)	0.0071 (10)
C20	0.0326 (14)	0.0310 (13)	0.0302 (14)	0.0006 (10)	0.0015 (11)	0.0052 (10)
N21	0.0497 (15)	0.0469 (13)	0.0321 (14)	0.0082 (10)	0.0029 (12)	0.0084 (10)
N22	0.0466 (15)	0.0334 (12)	0.0425 (14)	0.0085 (10)	0.0042 (11)	0.0105 (9)
C23	0.063 (2)	0.0504 (16)	0.0401 (18)	0.0223 (14)	0.0141 (15)	0.0143 (13)
C24	0.0472 (18)	0.0557 (16)	0.0369 (17)	0.0188 (13)	0.0105 (14)	0.0082 (13)
O25	0.0406 (11)	0.0395 (10)	0.0344 (11)	0.0142 (8)	0.0109 (9)	0.0105 (7)

Geometric parameters (Å, º)

N1—C2	1.334 (3)	C11—N12	1.332 (3)
N1—C13	1.352 (3)	C11—H11	0.9500
C2—C3	1.394 (4)	N12—C14	1.355 (3)
C2—H2	0.9500	C13—C14	1.469 (3)
C3—C4	1.365 (3)	N15—C17	1.323 (3)
C3—H3	0.9500	N16—C18	1.326 (3)
C4—C5	1.404 (3)	C17—C18	1.409 (3)
C4—H4	0.9500	C17—C19	1.444 (3)
C5—C13	1.407 (3)	C18—C20	1.447 (3)
C5—C6	1.445 (3)	C19—N21	1.145 (3)
C6—N15	1.348 (3)	C20—N22	1.144 (3)
C6—C7	1.413 (3)	C23—C24	1.494 (3)
C7—N16	1.356 (3)	C23—H23A	0.9800
C7—C8	1.448 (3)	C23—H23B	0.9800
C8—C9	1.402 (3)	C23—H23C	0.9800
C8—C14	1.412 (3)	C24—O25	1.411 (3)
C9—C10	1.373 (3)	C24—H24A	0.9900
C9—H9	0.9500	C24—H24B	0.9900
C10—C11	1.395 (3)	O25—H25	0.91 (3)
C10—H10	0.9500
C2—N1—C13	116.8 (2)	C11—N12—C14	116.88 (19)
N1—C2—C3	124.5 (2)	N1—C13—C5	122.8 (2)
N1—C2—H2	117.7	N1—C13—C14	117.5 (2)
C3—C2—H2	117.7	C5—C13—C14	119.77 (19)
C4—C3—C2	118.5 (2)	N12—C14—C8	122.7 (2)
C4—C3—H3	120.7	N12—C14—C13	117.15 (19)
C2—C3—H3	120.7	C8—C14—C13	120.1 (2)
C3—C4—C5	119.2 (2)	C17—N15—C6	116.76 (19)
C3—C4—H4	120.4	C18—N16—C7	116.57 (19)
C5—C4—H4	120.4	N15—C17—C18	122.2 (2)
C4—C5—C13	118.2 (2)	N15—C17—C19	116.9 (2)
C4—C5—C6	121.9 (2)	C18—C17—C19	120.84 (19)
C13—C5—C6	119.9 (2)	N16—C18—C17	121.93 (19)
N15—C6—C7	121.25 (19)	N16—C18—C20	117.7 (2)
N15—C6—C5	118.4 (2)	C17—C18—C20	120.3 (2)
C7—C6—C5	120.3 (2)	N21—C19—C17	179.6 (3)
N16—C7—C6	121.2 (2)	N22—C20—C18	177.5 (3)
N16—C7—C8	118.3 (2)	C24—C23—H23A	109.5
C6—C7—C8	120.40 (19)	C24—C23—H23B	109.5
C9—C8—C14	118.5 (2)	H23A—C23—H23B	109.5
C9—C8—C7	122.10 (19)	C24—C23—H23C	109.5
C14—C8—C7	119.4 (2)	H23A—C23—H23C	109.5
C10—C9—C8	118.6 (2)	H23B—C23—H23C	109.5
C10—C9—H9	120.7	O25—C24—C23	109.8 (2)
C8—C9—H9	120.7	O25—C24—H24A	109.7
C9—C10—C11	119.0 (2)	C23—C24—H24A	109.7
C9—C10—H10	120.5	O25—C24—H24B	109.7
C11—C10—H10	120.5	C23—C24—H24B	109.7
N12—C11—C10	124.3 (2)	H24A—C24—H24B	108.2
N12—C11—H11	117.8	C24—O25—H25	107.9 (19)
C10—C11—H11	117.8

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O25—H25···N1	0.91 (3)	2.11 (3)	2.958 (3)	155 (3)