An ortho­rhom­bic polymorph of pyrazino­[2,3-f][1,10]phenanthroline-2,3-dicarbo­nitrile

The title compound, C₁₆H₆N₆, is a polymorph of the previously reported structure [Kozlov & Goldberg (2008). Acta Cryst. C64, o498–o501]. Unlike the previously reported monoclinic polymorph (space group P2₁/c, Z = 8), the title compound reveals ortho­rhom­bic symmetry (space group Pnma, Z = 4). The mol­ecule shows crystallographic mirror symmetry, while the previously reported structure exhibits two independent mol­ecules per asymmetric unit. In the title compound, adjacent mol­ecules are essentially parallel along the c axis and tend to be vertical along the b axis with dihedral angles of 72.02 (6)°. However, in the reported polymorph, the entire crystal structure shows an anti­parallel arrangement of adjacent columns related by inversion centers and the two independent mol­ecules are nearly parallel with a dihedral angle of 2.48 (6)°.

Related literature

For ligands based on 1,10-phenanthroline in coordination chemistry, see: Rabaca et al. (2008); Stephenson et al. (2008). For reports of the title compound in coordination chemistry, see: Kulkarni et al. (2004); Stephenson & Hardie (2006); Xiao et al. (2011); Xu et al. (2002). For examples of polymorphism, see: Demirtaş et al. (2011); Jiang et al. (2000); Okabe et al. (2001); Pan & Chen (2009); Ramos Silva et al. (2011); Thallapally et al. (2004). For the previously reported polymorph, see: Kozlov & Goldberg (2008). For related structures, see: Kozlov et al. (2008).

Experimental

Crystal data

• C₁₆H₆N₆

• M_r = 282.27

• Orthorhombic, P n m a

• a = 14.1055 (13) Å

• b = 16.3331 (14) Å

• c = 5.2694 (4) Å

• V = 1214.00 (18) ų

• Z = 4

• Mo Kα radiation

• μ = 0.10 mm⁻¹

• T = 293 K

• 0.45 × 0.30 × 0.26 mm

Data collection

• Bruker SMART CCD area-detector diffractometer

• Absorption correction: multi-scan (SADABS; Bruker, 2007) T_min = 0.956, T_max = 0.974

• 4543 measured reflections

• 1113 independent reflections

• 759 reflections with I > 2σ(I)

• R_int = 0.046

Refinement

• R[F² > 2σ(F²)] = 0.041

• wR(F²) = 0.119

• S = 1.06

• 1113 reflections

• 100 parameters

• H-atom parameters constrained

• Δρ_max = 0.20 e Å⁻³

• Δρ_min = −0.24 e Å⁻³

Data collection: SMART (Bruker, 2007); cell refinement: SAINT-Plus (Bruker, 2007); data reduction: SAINT-Plus; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008) and ORTEPIII (Burnett & Johnson, 1996); software used to prepare material for publication: SHELXTL.

Supporting information

S1. Comment

Ligands based on 1,10-phenanthroline are attractive building blocks for the formation of metal-organic complexes bearing diverse dimensionalities (Rabaca et al., 2008; Stephenson et al., 2008). Among 1,10-phenanthroline derivatives, the DICNQ (DICNQ = pyrazino[2,3-f][1,10]phenanthroline-2,3-dicarbonitrile) (Xu et al., 2002; Kulkarni et al., 2004) has a big conjugated system and multiple coordination including strong chelating phenanthroline ring, bridging pyrazine ring and cyanite groups, so it is good in synthesizing luminescent complexes containing multifunctional ligands. Up to date only a few complexes of DICNQ are reported, such as [Co(dicnq)₂Br₂] and [Cu(dicnq)₂Br₂][Cu(dicnq)₂Br]Br.(CH₃CN).2(H₂O) (Stephenson & Hardie, 2006). Very recently, we synthesized five Cu(I) complexes [Cu(PPh₃)(C₁₆H₆N₆)X] (C₁₆H₆N₆ = pyrazino[2,3-f][1,10]phenanthroline-2,3-dicarbonitrile; X = Cl, Br, I, CN, SCN) (Xiao et al., 2011). During our attempts to synthesize Cu(II) complexes of DICNQ and 4,4'-bipyridine, we obtained crystals of a new orthorhombic polymorph of the solvent-free DICNQ.

Polymorphism can potentially be found in any crystalline material including polymers, minerals, and metals, and is related to allotropy, which refers to elemental solids. When polymorphism exists as a result of difference in crystal packing, it is called packing polymorphism. The different crystal types probably are the result of hydration, solvation or the effect of synthesis methods. There are recently reported examples of organic polymorphs: 2-(pyrimidin-2-ylsulfanyl) acetic acid, which is able to form monoclinic and orthorhombic crystals (Ramos Silva et al., 2011; Pan & Chen, 2009). 3,4,5-Trihydroxybenzoic acid monohydrate is found to form three polymorphs (Demirtaş et al., 2011; Jiang et al., 2000; Okabe et al., 2001). We herein report a new orthorhombic polymorph of DICNQ (1).

ORTEPIII (Burnett & Johnson, 1996) representation of the title compound (1) is shown in Fig. 1. The title compound belongs to orthorhombic system, space group Pnma, consisting of one symmetric unit of single molecule, unlike the reported monoclinic polymorph (space group P2₁/c), (2), (Kozlov & Goldberg, 2008), which consists of two crystallographically independent molecules.

The phenanthroline fragment of both polymorphs is nearly planar, and a similar bending of the cyano groups from the plane of the phenanthroline residues can be observed. The deviating distance of the cyano groups from the plane in (1) (0.182 (7) Å) is obviously bigger than the corresponding deviating distances in (2) (0.0345 Å and 0.0374 Å).

The packing pattern differs from the reported polymorph. In the title compound, there are two packing arrangements (Fig. 2), adjacent molecules are essentially parallel along the c axis and tend to be vertical along the b axis with dihedral angles of 72.02 (6)°. Howerver, in the reported polymorph the entire crystal structure shows an antiparallel arrangement of the adjacent columns related by inversion centers, and the two independent molecules are nearly parallel with a dihedral angle of 2.48 (6)°.

In addition, the intermolecular assembly of (1) is dominated by π-π stacking of overlaps among phenanthroline fragments and π-π stacking between phenanthroline fragment and cyano group of adjacent molecules (Fig. 2), with an interplanar distance between the parallel phenanthroline rings in the two adjacent molecules of 3.030 (1) Å. In contrast, in (2) and the ethanol solvate of DICNQ (C₁₆H₆N₆).(C₂H₆O) (3) (Kozlov et al., 2008), adjacent molecules overlap only through their phenanthroline fragments with the shortest interplanar distance of 3.380 (3) Å for (2), 3.27 Å and 3.40 Å for (3).

Various methods and conditions of the crystallization process are the main reason responsible for the formation of different polymorphic forms (Thallapally et al., 2004). The two polymorphs (1) and (2) are prepared under different catalysts. (1) was prepared under the catalysis of CuCl₂.2H₂O and 4,4'-bipy, while (2) was prepared under the catalysis of [Pd(PPh₃)Cl₂].

Related literature

For ligands based on 1,10-phenanthroline in coordination chemistry, see: Rabaca et al. (2008); Stephenson et al. (2008). For reports of the title compound in coordination chemistry, see: Kulkarni et al. (2004); Stephenson & Hardie (2006); Xiao et al. (2011); Xu et al. (2002). For examples of polymorphism, see: Demirtaş et al. (2011); Jiang et al. (2000); Okabe et al. (2001); Pan & Chen (2009); Ramos Silva et al. (2011); Thallapally et al. (2004). For the previously reported polymorph, see: Kozlov & Goldberg (2008). For related structures, see: Kozlov et al. (2008).

S2. Experimental

A mixture of CuCl₂.2H₂O (0.0178 g, 0.1 mmol) in 5 ml CH₂Cl₂ and DICNQ (0.0290 g, 0.1 mmol) in 3 ml CH₃CN was stirred for 30 min, then a solution of 4,4'-bipyridine (0.0202 g, 0.1 mmol) in 2 ml CH₃CN was added under continuous stirring. The resulting solution was refluxed for 5 h and filtered. The filtrate was allowed to evaporate slowly at room temperature to yield yellow block crystals of the title complex. Crystals suitable for single-crystal X-ray diffraction were selected directly from the sample as prepared.

IR (KBr, disc: v/cm^-1): 3466.82 s, 1587.57m, 1571.17m, 1505.59m, 1459.84m, 1444.23m, 1384.77m, 1372.26m, 1331.97w, 1264.07w, 1219.29m, 1141.93m, 1123.84w, 1073.56w, 1028.64w, 975.09w, 817.56m, 743.23m, 688.54w, 618.37w, 527.98w, 441.00w, 420.02w, 409.42w.

S3. Refinement

H atoms were positioned geometrically and refined using a riding model with C—H = 0.93 Å and with U_iso(H) = 1.2U_eq(C).

Computing details

Data collection: SMART (Bruker, 2007); cell refinement: SAINT-Plus (Bruker, 2007); data reduction: SAINT-Plus (Bruker, 2007); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008) and ORTEPIII (Burnett & Johnson, 1996); software used to prepare material for publication: SHELXTL (Sheldrick, 2008).

Figures

	Fig. 1. Perspective view of a basic unit of the title complex. Atoms are displayed as elliposoids at the 50% probability level at ca 293 K.
	Fig. 2. The crystal packing of the title complex.

pyrazino[2,3-f][1,10]phenanthroline-2,3-dicarbonitrile

Crystal data

C16H6N6	F(000) = 576
Mr = 282.27	Dx = 1.544 Mg m^−3
Orthorhombic, Pnma	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ac 2n	Cell parameters from 1184 reflections
a = 14.1055 (13) Å	θ = 2.9–27.5°
b = 16.3331 (14) Å	µ = 0.10 mm^−1
c = 5.2694 (4) Å	T = 293 K
V = 1214.00 (18) Å^3	Block, yellow
Z = 4	0.45 × 0.30 × 0.26 mm

Data collection

Bruker SMART CCD area-detector diffractometer	1113 independent reflections
Radiation source: fine-focus sealed tube	759 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.046
phi and ω scans	θmax = 25.0°, θmin = 2.9°
Absorption correction: multi-scan (SADABS; Bruker, 2007)	h = −16→16
Tmin = 0.956, Tmax = 0.974	k = −19→7
4543 measured reflections	l = −6→6

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)] = 0.041	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.119	H-atom parameters constrained
S = 1.06	w = 1/[σ^2(Fo^2) + (0.0556P)^2 + 0.3018P] where P = (Fo^2 + 2Fc^2)/3
1113 reflections	(Δ/σ)max < 0.001
100 parameters	Δρmax = 0.20 e Å^−3
0 restraints	Δρmin = −0.24 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	0.56932 (11)	0.33317 (10)	−0.1078 (3)	0.0347 (5)
N2	0.77953 (11)	0.33585 (9)	0.6140 (3)	0.0322 (4)
N3	0.92249 (13)	0.37680 (13)	1.1146 (4)	0.0539 (6)
C1	0.61818 (12)	0.29508 (11)	0.0786 (3)	0.0279 (5)
C2	0.67134 (12)	0.33802 (11)	0.2603 (3)	0.0281 (5)
C3	0.67273 (13)	0.42373 (12)	0.2476 (4)	0.0349 (5)
H3	0.7060	0.4542	0.3673	0.042*
C4	0.62440 (14)	0.46176 (13)	0.0564 (4)	0.0381 (6)
H4	0.6249	0.5185	0.0422	0.046*
C5	0.57451 (13)	0.41428 (12)	−0.1161 (4)	0.0369 (5)
H5	0.5426	0.4410	−0.2463	0.044*
C6	0.72648 (12)	0.29328 (11)	0.4470 (3)	0.0285 (5)
C7	0.83106 (12)	0.29288 (12)	0.7773 (4)	0.0317 (5)
C8	0.88437 (14)	0.33918 (13)	0.9640 (4)	0.0372 (5)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
N1	0.0333 (9)	0.0368 (10)	0.0339 (10)	0.0021 (7)	−0.0025 (8)	0.0040 (8)
N2	0.0305 (9)	0.0357 (9)	0.0303 (9)	−0.0020 (7)	0.0014 (8)	−0.0007 (8)
N3	0.0476 (11)	0.0703 (14)	0.0437 (12)	−0.0084 (10)	−0.0041 (10)	−0.0142 (11)
C1	0.0249 (9)	0.0318 (10)	0.0271 (11)	0.0011 (8)	0.0041 (8)	0.0010 (9)
C2	0.0255 (9)	0.0291 (10)	0.0296 (11)	0.0002 (8)	0.0036 (8)	0.0018 (9)
C3	0.0335 (11)	0.0312 (10)	0.0401 (13)	−0.0031 (9)	−0.0007 (10)	−0.0031 (10)
C4	0.0353 (11)	0.0302 (10)	0.0488 (14)	0.0018 (9)	0.0026 (11)	0.0059 (10)
C5	0.0354 (11)	0.0370 (12)	0.0382 (12)	0.0052 (9)	−0.0005 (10)	0.0082 (10)
C6	0.0253 (9)	0.0319 (9)	0.0283 (10)	−0.0016 (8)	0.0028 (8)	−0.0029 (8)
C7	0.0264 (10)	0.0428 (11)	0.0261 (11)	−0.0027 (8)	0.0008 (9)	−0.0014 (9)
C8	0.0314 (11)	0.0480 (13)	0.0321 (12)	−0.0018 (9)	0.0013 (10)	−0.0013 (11)

Geometric parameters (Å, º)

N1—C5	1.327 (3)	C3—C4	1.366 (3)
N1—C1	1.352 (2)	C3—H3	0.9300
N2—C7	1.327 (2)	C4—C5	1.387 (3)
N2—C6	1.348 (2)	C4—H4	0.9300
N3—C8	1.138 (3)	C5—H5	0.9300
C1—C2	1.404 (2)	C6—C6^i	1.414 (4)
C1—C1^i	1.473 (4)	C7—C7^i	1.401 (4)
C2—C3	1.402 (3)	C7—C8	1.451 (3)
C2—C6	1.451 (2)
C5—N1—C1	117.07 (17)	C3—C4—H4	120.6
C7—N2—C6	117.02 (16)	C5—C4—H4	120.6
N1—C1—C2	122.55 (17)	N1—C5—C4	124.37 (19)
N1—C1—C1^i	117.41 (10)	N1—C5—H5	117.8
C2—C1—C1^i	119.97 (11)	C4—C5—H5	117.8
C3—C2—C1	118.29 (17)	N2—C6—C6^i	121.05 (10)
C3—C2—C6	121.85 (17)	N2—C6—C2	118.69 (16)
C1—C2—C6	119.80 (17)	C6^i—C6—C2	120.23 (10)
C4—C3—C2	118.85 (18)	N2—C7—C7^i	121.93 (10)
C4—C3—H3	120.6	N2—C7—C8	116.61 (17)
C2—C3—H3	120.6	C7^i—C7—C8	121.41 (11)
C3—C4—C5	118.85 (18)	N3—C8—C7	177.0 (2)
C5—N1—C1—C2	1.0 (3)	C3—C4—C5—N1	0.7 (3)
C5—N1—C1—C1^i	−175.87 (13)	C7—N2—C6—C6^i	−0.18 (19)
N1—C1—C2—C3	0.5 (3)	C7—N2—C6—C2	−178.16 (15)
C1^i—C1—C2—C3	177.37 (12)	C3—C2—C6—N2	0.7 (3)
N1—C1—C2—C6	−176.57 (15)	C1—C2—C6—N2	177.73 (15)
C1^i—C1—C2—C6	0.26 (19)	C3—C2—C6—C6^i	−177.26 (13)
C1—C2—C3—C4	−1.5 (3)	C1—C2—C6—C6^i	−0.26 (19)
C6—C2—C3—C4	175.52 (17)	C6—N2—C7—C7^i	0.18 (19)
C2—C3—C4—C5	1.0 (3)	C6—N2—C7—C8	−177.14 (15)
C1—N1—C5—C4	−1.7 (3)

Symmetry code: (i) x, −y+1/2, z.