supplementary materials


pv2618 scheme

Acta Cryst. (2013). E69, o368    [ doi:10.1107/S1600536813003504 ]

2-(4-Fluorophenyl)-1-(4-methoxyphenyl)-1H-phenanthro[9,10-d]imidazole

T. Mohandas, R. Sathishkumar, P. Sakthivel and J. Jayabharathi

Abstract top

In the title compound, C28H19FN2O, the phenanthrene fused with an imidazole ring, constituting an essentially planar tetracyclic system [maximum deviation = 0.032 (2) Å], makes dihedral angles of 60.83 (4) and 80.55 (4)° with the fluorobenzene and methoxybenzene rings, respectively. The dihedral angle between the the methoxybenzene and fluorobenzene rings is 69.45 (6)°. In the crystal, C-H...O hydrogen bonds connect the molecules into infinite strands along the b axis. The crystal structure is further consolidated by C-H...[pi] interactions.

Comment top

1H-phenanthro[9,10-d]imidazole can be used as a planar synthetic building block in supramolecular chemistry (Krebs & Spanggaard, 2002). Its derivatives are commonly used as ligands in metal complexes (Bian et al., 2002). Some phenanthrene derivatives have excellent coordinating abilities and have been used to build supramolecular architechture (Che et al., 2008; Stephenson & Hardie, 2006). We have synthesized the title compound in our laboratory and determined its crystal structure which is reported in this article.

The bond lengths and bond angles in the title compound (Fig. 1) are normal and agree very well with the corresponding bond lengths and bond angles reported for closely related compounds (Yuan et al., 2011; Krebs et al., 2001). The tetracyclic ring system is essentially planar (maximum deviation = 0.032 (2) Å). The dihedral angle between the fluorobenzene ring and the phenanthrene tetracyclic system is 60.83 (4)° and to that of the benzene ring of methoxybenzene is 80.55 (5)°. The dihedral angle between the methoxybenzene and the fluorobenzene rings is 69.45 (6)°. The maximum deviation of C8 atom from the mean plane of the methoxy benzene is -0.021° and that of C7 atom from that of fluorobenzene is 0.059 Å.

In the crystal packing, the molecules are linked by C—H···O intermolecular interactions into infinite chains running paralell to the b axis. The crystal structure is further stabilized by C—H···Cg interactions (Table 1), where Cg1 is the centre of gravity of N1/C7/N2/C15/C16, Cg2 is the center of gravity of C1—C6 and Cg3 is the center of gravity of C15/C16/C17/C22/C23/C28.

Related literature top

For background to the supramolecular architecture of phenanthrene derivatives, see: Krebs & Spanggaard (2002); Bian et al. (2002); Che et al. (2008); Stephenson & Hardie (2006). For the crystal structures of closely related compounds, see: Yuan et al. (2011); Krebs et al. (2001).

Experimental top

A mixture of phenanthrene-9,10-dione (1.0 g, 4.8 mmol), ammonium acetate (1.48 g, 19.2 mmol), 4-fluorobenzaldehyde (0.6 g, 4.3 mmol) and 4-methoxy aniline (2.95 g, 24 mmol) were refluxed in ethanol (20 ml) at 353 K. The reaction was monitored by TLC and purified by coloumn chromotography using petroleum ether:ethyl acetate (9:1) as the eluent. Yield: 0.84 g(50%). The compound was dissolved in dimethyl sulfoxide and allowed to evaporate slowly, until single crystals were grown.

Refinement top

The hydrogen atoms were placed in calculated positions with C–H = 0.93 Å to 0.96 Å, refined in the riding model with fixed isotropic displacement parameters: Uiso(H) = 1.5Ueq(C)for methyl group and Uiso(H) = 1.2Ueq(C)for other groups.

Computing details top

Data collection: APEX2 (Bruker, 2008); cell refinement: APEX2 and SAINT (Bruker, 2008); data reduction: SAINT and XPREP (Bruker, 2008); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: PLATON (Spek, 2009); software used to prepare material for publication: PLATON (Spek, 2009).

Figures top
[Figure 1] Fig. 1. The molecular structure of the title compound with displacement ellipsoids for non-H atoms drawn at the 30% probability level.
[Figure 2] Fig. 2. A unit cell packing diagram for the title compound. Intermolecular hydrogen bonds are shown as dashed lines.
2-(4-Fluorophenyl)-1-(4-methoxyphenyl)-1H-phenanthro[9,10-d]imidazole top
Crystal data top
C28H19FN2OZ = 2
Mr = 418.45F(000) = 436
Triclinic, P1Dx = 1.315 Mg m3
Hall symbol: -P 1Mo Kα radiation, λ = 0.71073 Å
a = 9.6430 (3) ÅCell parameters from 9741 reflections
b = 9.8980 (3) Åθ = 2.3–28.5°
c = 12.3070 (4) ŵ = 0.09 mm1
α = 77.432 (1)°T = 293 K
β = 71.621 (1)°Block, colourless
γ = 73.158 (1)°0.30 × 0.20 × 0.20 mm
V = 1056.57 (6) Å3
Data collection top
Bruker Kappa APEXII CCD
diffractometer
3723 independent reflections
Radiation source: fine-focus sealed tube3183 reflections with I > 2σ(I)
Graphite monochromatorRint = 0.026
ω and φ scansθmax = 25.0°, θmin = 2.2°
Absorption correction: multi-scan
(SADABS; Bruker, 2008)
h = 1111
Tmin = 0.975, Tmax = 0.983k = 1111
18612 measured reflectionsl = 1414
Refinement top
Refinement on F2Secondary atom site location: difference Fourier map
Least-squares matrix: fullHydrogen site location: inferred from neighbouring sites
R[F2 > 2σ(F2)] = 0.035H-atom parameters constrained
wR(F2) = 0.098 w = 1/[σ2(Fo2) + (0.0449P)2 + 0.2676P]
where P = (Fo2 + 2Fc2)/3
S = 1.03(Δ/σ)max < 0.001
3723 reflectionsΔρmax = 0.16 e Å3
290 parametersΔρmin = 0.22 e Å3
0 restraintsExtinction correction: SHELXL97(Sheldrick, 2008), Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4
Primary atom site location: structure-invariant direct methodsExtinction coefficient: 0.024 (2)
Crystal data top
C28H19FN2Oγ = 73.158 (1)°
Mr = 418.45V = 1056.57 (6) Å3
Triclinic, P1Z = 2
a = 9.6430 (3) ÅMo Kα radiation
b = 9.8980 (3) ŵ = 0.09 mm1
c = 12.3070 (4) ÅT = 293 K
α = 77.432 (1)°0.30 × 0.20 × 0.20 mm
β = 71.621 (1)°
Data collection top
Bruker Kappa APEXII CCD
diffractometer
3723 independent reflections
Absorption correction: multi-scan
(SADABS; Bruker, 2008)
3183 reflections with I > 2σ(I)
Tmin = 0.975, Tmax = 0.983Rint = 0.026
18612 measured reflectionsθmax = 25.0°
Refinement top
R[F2 > 2σ(F2)] = 0.035H-atom parameters constrained
wR(F2) = 0.098Δρmax = 0.16 e Å3
S = 1.03Δρmin = 0.22 e Å3
3723 reflectionsAbsolute structure: ?
290 parametersFlack parameter: ?
0 restraintsRogers parameter: ?
Special details top

Geometry. Bond distances, angles etc. have been calculated using the rounded fractional coordinates. All su'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 (Å2) top
xyzUiso*/Ueq
F10.87342 (15)0.53172 (11)0.19476 (9)0.0846 (5)
O10.75909 (13)0.28562 (13)0.05490 (9)0.0612 (4)
N10.77067 (12)0.21988 (11)0.62054 (9)0.0372 (3)
N20.75022 (11)0.02316 (11)0.49070 (9)0.0347 (3)
C10.8505 (2)0.44158 (16)0.27061 (12)0.0515 (5)
C20.95884 (18)0.45772 (16)0.32493 (13)0.0540 (5)
C30.93452 (16)0.36453 (15)0.40117 (12)0.0443 (4)
C40.80356 (14)0.25828 (13)0.42219 (10)0.0352 (4)
C50.69707 (16)0.24510 (15)0.36438 (11)0.0426 (4)
C60.71958 (18)0.33777 (16)0.28835 (12)0.0495 (5)
C70.77576 (14)0.16916 (13)0.51172 (11)0.0346 (4)
C80.75523 (14)0.05865 (13)0.37811 (10)0.0345 (4)
C90.62372 (15)0.12625 (15)0.34724 (12)0.0434 (4)
C100.62899 (16)0.20068 (15)0.23842 (12)0.0466 (5)
C110.76621 (16)0.20770 (15)0.15961 (11)0.0424 (4)
C120.89759 (16)0.13796 (17)0.18988 (12)0.0495 (5)
C130.89114 (15)0.06387 (15)0.29966 (12)0.0448 (5)
C140.8964 (3)0.2975 (3)0.02895 (15)0.0935 (9)
C150.72865 (13)0.02155 (13)0.59578 (11)0.0345 (4)
C160.74054 (14)0.10143 (13)0.67333 (11)0.0348 (4)
C170.72237 (14)0.10002 (14)0.79297 (11)0.0377 (4)
C180.73089 (16)0.22537 (16)0.87221 (12)0.0458 (4)
C190.71638 (18)0.22088 (18)0.98554 (12)0.0542 (5)
C200.69353 (18)0.09137 (19)1.02226 (13)0.0567 (5)
C210.68346 (17)0.03217 (18)0.94647 (12)0.0505 (5)
C220.69645 (14)0.03262 (15)0.82916 (11)0.0395 (4)
C230.68514 (14)0.16372 (14)0.74603 (12)0.0400 (4)
C240.65660 (17)0.29785 (16)0.78018 (14)0.0515 (5)
C250.64649 (19)0.42041 (17)0.70328 (15)0.0582 (6)
C260.66392 (18)0.41572 (16)0.58804 (15)0.0558 (6)
C270.69159 (16)0.28783 (15)0.55041 (13)0.0468 (5)
C280.70222 (14)0.15991 (14)0.62762 (11)0.0376 (4)
H21.046640.529600.310900.0648*
H31.007050.373210.438910.0532*
H50.609450.172850.377100.0511*
H60.647800.330010.250130.0594*
H90.531340.121460.400140.0520*
H100.540200.246350.217770.0559*
H120.990200.140750.136640.0594*
H130.979710.017260.320350.0538*
H14A0.875640.354330.098470.1400*
H14B0.950920.341950.000230.1400*
H14C0.955520.204280.045440.1400*
H180.746540.312280.847510.0550*
H190.721810.304391.037840.0651*
H200.684970.088551.099190.0680*
H210.667660.117970.972960.0606*
H240.644260.303390.857270.0618*
H250.627750.507590.728540.0698*
H260.656810.499570.536190.0670*
H270.703530.285370.472810.0562*
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
F10.1259 (10)0.0703 (7)0.0668 (7)0.0203 (7)0.0242 (6)0.0351 (5)
O10.0734 (8)0.0768 (8)0.0390 (6)0.0346 (6)0.0239 (5)0.0159 (5)
N10.0405 (6)0.0369 (6)0.0335 (6)0.0077 (5)0.0115 (5)0.0032 (4)
N20.0374 (6)0.0361 (6)0.0299 (5)0.0095 (4)0.0095 (4)0.0018 (4)
C10.0762 (11)0.0449 (8)0.0351 (7)0.0191 (8)0.0098 (7)0.0105 (6)
C20.0594 (9)0.0454 (8)0.0468 (8)0.0009 (7)0.0089 (7)0.0113 (7)
C30.0436 (8)0.0456 (8)0.0415 (7)0.0048 (6)0.0138 (6)0.0060 (6)
C40.0385 (7)0.0354 (7)0.0303 (6)0.0103 (5)0.0082 (5)0.0012 (5)
C50.0418 (7)0.0463 (8)0.0395 (7)0.0085 (6)0.0128 (6)0.0055 (6)
C60.0603 (9)0.0570 (9)0.0387 (7)0.0216 (8)0.0182 (7)0.0045 (7)
C70.0326 (6)0.0360 (7)0.0339 (7)0.0071 (5)0.0095 (5)0.0032 (5)
C80.0375 (7)0.0349 (7)0.0307 (6)0.0102 (5)0.0087 (5)0.0023 (5)
C90.0343 (7)0.0494 (8)0.0397 (7)0.0076 (6)0.0068 (6)0.0006 (6)
C100.0440 (8)0.0481 (8)0.0467 (8)0.0072 (6)0.0201 (6)0.0020 (6)
C110.0546 (8)0.0445 (8)0.0333 (7)0.0209 (6)0.0156 (6)0.0017 (6)
C120.0419 (8)0.0664 (10)0.0369 (7)0.0213 (7)0.0046 (6)0.0022 (7)
C130.0354 (7)0.0552 (9)0.0420 (8)0.0121 (6)0.0121 (6)0.0013 (6)
C140.0953 (15)0.150 (2)0.0433 (10)0.0706 (15)0.0220 (10)0.0309 (11)
C150.0314 (6)0.0392 (7)0.0331 (7)0.0088 (5)0.0088 (5)0.0047 (5)
C160.0326 (6)0.0390 (7)0.0330 (7)0.0086 (5)0.0095 (5)0.0044 (5)
C170.0331 (7)0.0463 (7)0.0333 (7)0.0092 (6)0.0094 (5)0.0045 (6)
C180.0495 (8)0.0493 (8)0.0370 (7)0.0104 (6)0.0136 (6)0.0018 (6)
C190.0598 (9)0.0637 (10)0.0354 (8)0.0143 (8)0.0147 (7)0.0031 (7)
C200.0605 (10)0.0765 (11)0.0333 (7)0.0145 (8)0.0139 (7)0.0086 (7)
C210.0499 (8)0.0625 (9)0.0419 (8)0.0117 (7)0.0119 (7)0.0162 (7)
C220.0324 (7)0.0495 (8)0.0376 (7)0.0090 (6)0.0095 (5)0.0092 (6)
C230.0324 (7)0.0450 (8)0.0452 (8)0.0089 (6)0.0112 (6)0.0112 (6)
C240.0526 (9)0.0509 (9)0.0567 (9)0.0097 (7)0.0176 (7)0.0184 (7)
C250.0623 (10)0.0434 (8)0.0745 (11)0.0118 (7)0.0198 (8)0.0190 (8)
C260.0601 (10)0.0392 (8)0.0688 (11)0.0164 (7)0.0181 (8)0.0017 (7)
C270.0513 (8)0.0422 (8)0.0478 (8)0.0154 (6)0.0133 (7)0.0023 (6)
C280.0325 (7)0.0395 (7)0.0418 (7)0.0100 (5)0.0101 (5)0.0059 (6)
Geometric parameters (Å, º) top
F1—C11.3544 (19)C20—C211.366 (2)
O1—C111.3612 (17)C21—C221.4084 (19)
O1—C141.417 (3)C22—C231.4673 (19)
N1—C71.3146 (17)C23—C241.405 (2)
N1—C161.3770 (16)C23—C281.4217 (19)
N2—C71.3737 (16)C24—C251.366 (2)
N2—C81.4388 (16)C25—C261.384 (2)
N2—C151.3920 (17)C26—C271.367 (2)
C1—C21.364 (3)C27—C281.407 (2)
C1—C61.367 (2)C2—H20.9300
C2—C31.378 (2)C3—H30.9300
C3—C41.382 (2)C5—H50.9300
C4—C51.386 (2)C6—H60.9300
C4—C71.4710 (18)C9—H90.9300
C5—C61.376 (2)C10—H100.9300
C8—C91.379 (2)C12—H120.9300
C8—C131.368 (2)C13—H130.9300
C9—C101.374 (2)C14—H14A0.9600
C10—C111.382 (2)C14—H14B0.9600
C11—C121.378 (2)C14—H14C0.9600
C12—C131.382 (2)C18—H180.9300
C15—C161.3742 (18)C19—H190.9300
C15—C281.4361 (18)C20—H200.9300
C16—C171.4296 (18)C21—H210.9300
C17—C181.401 (2)C24—H240.9300
C17—C221.407 (2)C25—H250.9300
C18—C191.367 (2)C26—H260.9300
C19—C201.386 (2)C27—H270.9300
C11—O1—C14117.86 (15)C24—C23—C28117.36 (13)
C7—N1—C16104.70 (10)C23—C24—C25121.83 (15)
C7—N2—C8123.87 (10)C24—C25—C26120.46 (15)
C7—N2—C15106.27 (10)C25—C26—C27120.02 (15)
C8—N2—C15129.77 (11)C26—C27—C28120.82 (14)
F1—C1—C2118.77 (15)C15—C28—C23116.05 (12)
F1—C1—C6118.08 (16)C15—C28—C27124.42 (12)
C2—C1—C6123.15 (15)C23—C28—C27119.53 (12)
C1—C2—C3118.13 (15)C1—C2—H2121.00
C2—C3—C4120.75 (15)C3—C2—H2121.00
C3—C4—C5119.16 (12)C2—C3—H3120.00
C3—C4—C7119.08 (13)C4—C3—H3120.00
C5—C4—C7121.60 (12)C4—C5—H5120.00
C4—C5—C6120.71 (14)C6—C5—H5120.00
C1—C6—C5118.09 (16)C1—C6—H6121.00
N1—C7—N2112.64 (11)C5—C6—H6121.00
N1—C7—C4123.91 (11)C8—C9—H9120.00
N2—C7—C4123.44 (11)C10—C9—H9120.00
N2—C8—C9120.21 (12)C9—C10—H10120.00
N2—C8—C13119.79 (12)C11—C10—H10120.00
C9—C8—C13119.93 (12)C11—C12—H12120.00
C8—C9—C10120.09 (14)C13—C12—H12120.00
C9—C10—C11120.04 (15)C8—C13—H13120.00
O1—C11—C10115.43 (14)C12—C13—H13120.00
O1—C11—C12124.75 (14)O1—C14—H14A109.00
C10—C11—C12119.82 (13)O1—C14—H14B109.00
C11—C12—C13119.72 (14)O1—C14—H14C109.00
C8—C13—C12120.40 (14)H14A—C14—H14B109.00
N2—C15—C16104.95 (11)H14A—C14—H14C109.00
N2—C15—C28132.25 (11)H14B—C14—H14C110.00
C16—C15—C28122.79 (12)C17—C18—H18120.00
N1—C16—C15111.44 (11)C19—C18—H18120.00
N1—C16—C17126.47 (11)C18—C19—H19120.00
C15—C16—C17122.10 (12)C20—C19—H19120.00
C16—C17—C18121.87 (12)C19—C20—H20120.00
C16—C17—C22117.60 (12)C21—C20—H20120.00
C18—C17—C22120.53 (12)C20—C21—H21119.00
C17—C18—C19120.48 (14)C22—C21—H21119.00
C18—C19—C20119.75 (15)C23—C24—H24119.00
C19—C20—C21120.61 (14)C25—C24—H24119.00
C20—C21—C22121.54 (15)C24—C25—H25120.00
C17—C22—C21117.08 (13)C26—C25—H25120.00
C17—C22—C23120.21 (12)C25—C26—H26120.00
C21—C22—C23122.71 (13)C27—C26—H26120.00
C22—C23—C24121.43 (13)C26—C27—H27120.00
C22—C23—C28121.21 (12)C28—C27—H27120.00
C14—O1—C11—C10179.46 (17)O1—C11—C12—C13178.88 (14)
C14—O1—C11—C120.7 (2)C11—C12—C13—C80.4 (2)
C7—N1—C16—C150.54 (16)N2—C15—C16—N10.91 (16)
C7—N1—C16—C17179.29 (14)C28—C15—C16—C172.1 (2)
C16—N1—C7—N20.08 (17)N2—C15—C28—C23179.92 (15)
C16—N1—C7—C4179.26 (14)N2—C15—C28—C270.4 (3)
C8—N2—C15—C16177.56 (13)C16—C15—C28—C231.5 (2)
C15—N2—C7—C4179.82 (13)C16—C15—C28—C27179.03 (15)
C7—N2—C15—C160.91 (15)N2—C15—C16—C17178.92 (13)
C8—N2—C7—N1177.54 (13)C28—C15—C16—N1178.04 (13)
C7—N2—C8—C9100.31 (16)N1—C16—C17—C181.4 (2)
C15—N2—C7—N10.64 (16)N1—C16—C17—C22178.10 (14)
C8—N2—C7—C43.3 (2)C15—C16—C17—C18178.38 (15)
C15—N2—C8—C1399.46 (17)C15—C16—C17—C222.1 (2)
C7—N2—C15—C28177.90 (15)C16—C17—C18—C19178.53 (15)
C8—N2—C15—C281.3 (2)C22—C17—C18—C191.0 (2)
C7—N2—C8—C1376.67 (18)C16—C17—C22—C21178.05 (14)
C15—N2—C8—C983.56 (18)C16—C17—C22—C231.5 (2)
F1—C1—C2—C3179.49 (14)C18—C17—C22—C211.5 (2)
F1—C1—C6—C5179.24 (13)C18—C17—C22—C23178.97 (14)
C6—C1—C2—C30.1 (2)C17—C18—C19—C200.2 (3)
C2—C1—C6—C50.3 (2)C18—C19—C20—C210.8 (3)
C1—C2—C3—C40.3 (2)C19—C20—C21—C220.3 (3)
C2—C3—C4—C7174.59 (13)C20—C21—C22—C170.9 (2)
C2—C3—C4—C50.8 (2)C20—C21—C22—C23179.60 (16)
C3—C4—C7—N2123.12 (15)C17—C22—C23—C24178.83 (15)
C5—C4—C7—N1117.51 (16)C17—C22—C23—C281.0 (2)
C3—C4—C7—N157.8 (2)C21—C22—C23—C241.7 (2)
C7—C4—C5—C6174.21 (13)C21—C22—C23—C28178.55 (15)
C5—C4—C7—N261.58 (19)C22—C23—C24—C25179.73 (16)
C3—C4—C5—C61.1 (2)C28—C23—C24—C250.5 (2)
C4—C5—C6—C10.8 (2)C22—C23—C28—C150.9 (2)
C9—C8—C13—C120.8 (2)C22—C23—C28—C27179.59 (14)
N2—C8—C13—C12177.75 (13)C24—C23—C28—C15178.94 (14)
C13—C8—C9—C101.0 (2)C24—C23—C28—C270.6 (2)
N2—C8—C9—C10178.00 (12)C23—C24—C25—C260.2 (3)
C8—C9—C10—C110.1 (2)C24—C25—C26—C270.1 (3)
C9—C10—C11—C121.1 (2)C25—C26—C27—C280.2 (3)
C9—C10—C11—O1179.12 (13)C26—C27—C28—C15179.01 (16)
C10—C11—C12—C131.3 (2)C26—C27—C28—C230.5 (2)
Hydrogen-bond geometry (Å, º) top
Cg1, Cg2 and Cg3 are the centroids of the N1/C7/N2/C15/C16, C1–C6 and C15–C17/C22/C23/C28 rings, respectively.
D—H···AD—HH···AD···AD—H···A
C21—H21···O1i0.932.593.426 (2)151
C9—H9···Cg1ii0.932.743.535143
C26—H26···Cg2iii0.932.813.590143
C13—H13···Cg3iv0.932.763.629152
Symmetry codes: (i) x, y, z+1; (ii) x+1, y, z+1; (iii) x, y+1, z; (iv) x+2, y, z+1.
Hydrogen-bond geometry (Å, º) top
Cg1, Cg2 and Cg3 are the centroids of the N1/C7/N2/C15/C16, C1–C6 and C15–C17/C22/C23/C28 rings, respectively.
D—H···AD—HH···AD···AD—H···A
C21—H21···O1i0.932.593.426 (2)151
C9—H9···Cg1ii0.932.743.535143
C26—H26···Cg2iii0.932.813.590143
C13—H13···Cg3iv0.932.763.629152
Symmetry codes: (i) x, y, z+1; (ii) x+1, y, z+1; (iii) x, y+1, z; (iv) x+2, y, z+1.
Acknowledgements top

The authors thank Dr Babu Varghese, SAIF, IIT-Madras, Chennai, India, for his help with the data collection.

references
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