organic compounds\(\def\hfill{\hskip 5em}\def\hfil{\hskip 3em}\def\eqno#1{\hfil {#1}}\)

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ISSN: 2056-9890

3,3′-Di­ethyl-1,1′-[anthracene-9,10-diylbis(oxyethyl­ene)]diimidazolium diiodide

aSchool of Materials Science and Engineering, Tianjin Polytechnic University, No. 63 Chenglin Road, Tianjin 300160, People's Republic of China
*Correspondence e-mail: houyh1977@163.com

(Received 11 September 2009; accepted 22 September 2009; online 30 September 2009)

In the title centrosymmetric compound, C28H32N4O22+ 2I, the two midazole rings are approximately perpendicular to the central anthracene ring system [dihedral angle = 86.6 (2)°]. The ionic units are linked into a two-dimensional network parallel to ([\overline{1}]01) by C—H⋯I hydrogen bonds and ππ inter­actions involving the anthracene ring system and imidazole rings [centroid–centroid distance = 3.717 (3) Å].

Related literature

For general background to N-heterocyclic carbenes and their transition metal complexes, see: Bourissou et al. (2000[Bourissou, D., Guerret, O., Gabbai, F. P. & Bertrand, G. (2000). Chem. Rev. 100, 39-91.]); Herrmann & Kocher (1997[Herrmann, W. A. & Kocher, C. (1997). Angew. Chem. Int. Ed. Engl. 36, 2162-2187.]); Cavell & McGuinness (2004[Cavell, K. J. & McGuinness, D. S. (2004). Coord. Chem. Rev. pp. 248-671.]); Baker et al. (2004[Baker, M. V., Brown, D. H., Haque, R. A., Skelton, B. W. & White, A. H. (2004). J. Chem. Soc. Dalton Trans. pp. 3756-3764.]); Melaiye et al. (2004[Melaiye, A., Simons, R. S., Milsted, A., Pingitore, F., Wesdemiotis, C., Tessier, C. A. & Youngs, W. J. (2004). J. Med. Chem. 47, 973-977.]).

[Scheme 1]

Experimental

Crystal data
  • C28H32N4O22+·2I

  • Mr = 710.38

  • Monoclinic, P 21 /n

  • a = 11.4733 (13) Å

  • b = 10.6692 (12) Å

  • c = 13.1553 (15) Å

  • β = 112.725 (2)°

  • V = 1485.3 (3) Å3

  • Z = 2

  • Mo Kα radiation

  • μ = 2.15 mm−1

  • T = 298 K

  • 0.24 × 0.22 × 0.22 mm

Data collection
  • Bruker SMART CCD area-detector diffractometer

  • Absorption correction: multi-scan (SADABS; Sheldrick, 1996[Sheldrick, G. M. (1996). SADABS. University of Göttingen, Germany.]) Tmin = 0.627, Tmax = 0.650

  • 8914 measured reflections

  • 2603 independent reflections

  • 1989 reflections with I > 2σ(I)

  • Rint = 0.035

Refinement
  • R[F2 > 2σ(F2)] = 0.044

  • wR(F2) = 0.102

  • S = 1.07

  • 2603 reflections

  • 163 parameters

  • H-atom parameters constrained

  • Δρmax = 0.81 e Å−3

  • Δρmin = −0.27 e Å−3

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A D—H H⋯A DA D—H⋯A
C3—H3A⋯I1i 0.93 2.89 3.771 (5) 159
C4—H4A⋯I1ii 0.93 2.97 3.893 (5) 172
C5—H5A⋯I1iii 0.93 3.05 3.957 (6) 167
Symmetry codes: (i) x, y+1, z; (ii) [x+{\script{1\over 2}}, -y+{\script{1\over 2}}, z+{\script{1\over 2}}]; (iii) [-x+{\script{3\over 2}}, y+{\script{1\over 2}}, -z+{\script{1\over 2}}].

Data collection: SMART (Bruker, 1998[Bruker (1998). SMART. Bruker AXS Inc., Madison, Wisconsin, USA.]); cell refinement: SAINT (Bruker, 1999[Bruker (1999). SAINT. Bruker AXS Inc., Madison, Wisconsin, USA.]); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008[Sheldrick, G. M. (2008). Acta Cryst. A64, 112-122.]); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008[Sheldrick, G. M. (2008). Acta Cryst. A64, 112-122.]); molecular graphics: SHELXTL (Sheldrick, 2008[Sheldrick, G. M. (2008). Acta Cryst. A64, 112-122.]); software used to prepare material for publication: SHELXTL.

Supporting information


Comment top

As ancillary ligands, N-heterocyclic carbenes (NHCs) have received considerable attention due to their strong σ-donor ability, and are attractive alternatives to the widely utilized phosphine ligands in metal coordination chemistry (Bourissou et al., 2000; Herrmann & Kocher, 1997). A number of N-heterocyclic carbene transition metal complexes have been synthesized and isolated, and some of them have been successfully applied as homogeneous catalysts (Cavell & McGuinness, 2004; Baker et al., 2004; Melaiye et al., 2004). Herein, the synthesis and crystal structure of a new biscarbene analogue, (I), is reported.

The asymmetric unit contains one-half of the cation and a iodide anion. The cation of (I) lies across a crystallographic inversion centre (Fig. 1). The two imidazole ring planes are approximately perpendicular to the central anthracene ring system, the dihedral angle between them being 86.6 (2)°.

In the crystal, the ionic units are linked via C—H···I interactions. In addition, the anthracene ring system and imidazole rings of two adjacent molecules are stacked, with a centroid-to-centroid separation of 3.717 (3) Å indicating weak π-π interactions. The C—H···I and π-π interactions link ionic units into a two-dimensional network parallel to the (101) [Fig.2].

Related literature top

For general background to N-heterocyclic carbenes and their transition metal complexes, see: Bourissou et al. (2000); Herrmann & Kocher (1997); Cavell & McGuinness (2004); Baker et al. (2004); Melaiye et al. (2004).

Experimental top

A mixture of 1,2-bis(2-chloroethoxy)anthracene (6.7 g, 20 mmol) and 1-ethylimidazole (4.22 g, 44 mmol) was refluxed in THF (100 ml) for 24 h, giving a pale yellow precipitate, which was filtered and washed with THF and recrystallized from methanol and ethyl ether. The obtained solid was dissolved in methanol (200 ml) and an aqueous solution of NH4I (4.64 g, 32 mmol) was added to the solution. The precipitate formed was collected by filtration and recrystallized from CH3CN and diethyl ether (1:6 v/v) to give the title compound (yield 95%). Analysis found: C 32.49, H 3.22, N 5.36%; calculated for C28H32N4O2I2: C 32.64, H 3.13, N, 5.44%. 1H NMR (300 M, d6-DMSO): δ 9.51 (s, 2 H), 8.03 (s, 2 H), 7.97 (s, 2 H), 7.89–7.86 (m, 4 H), 7.55–7.51 (m, 4 H), 4.85 (t, J = 4.5 Hz, 4 H), 4.49 (t, J = 4.3 Hz, 4 H), 4.35 (q, J = 7.4 Hz, 4 H), 1.51 (t, J = 7.3 Hz, 6 H) p.p.m..

Refinement top

H atoms were placed in calculated positions [C-H = 0.93 (aromatic) or 0.97 Å (methylene)] and included in the final cycles of refinement using a riding-model approximation, with Uiso(H) = 1.2Ueq(carrier atom).

Computing details top

Data collection: SMART (Bruker, 1998); cell refinement: SAINT (Bruker, 1999); data reduction: SAINT (Bruker, 1999); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXTL (Sheldrick, 2008).

Figures top
[Figure 1] Fig. 1. The cation of (I), showing the atom-numbering scheme. Displacement ellipsoids are drawn at the 30% probability level. Atoms labeled with the suffix A are generated by the symmetry operation (1-x, 1-y, -z).
[Figure 2] Fig. 2. The packing diagram of (I). Dashed lines indicate C—H···I interactions.
3,3'-Diethyl-1,1'-[anthracene-9,10-diylbis(oxyethylene)]diimidazolium diiodide top
Crystal data top
C28H32N4O22+·2IF(000) = 700
Mr = 710.38Dx = 1.588 Mg m3
Monoclinic, P21/nMo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ynCell parameters from 5650 reflections
a = 11.4733 (13) Åθ = 0.9–28.4°
b = 10.6692 (12) ŵ = 2.15 mm1
c = 13.1553 (15) ÅT = 298 K
β = 112.725 (2)°Block, colourless
V = 1485.3 (3) Å30.24 × 0.22 × 0.22 mm
Z = 2
Data collection top
Bruker SMART CCD area-detector
diffractometer
2603 independent reflections
Radiation source: fine-focus sealed tube1989 reflections with I > 2σ(I)
Graphite monochromatorRint = 0.035
ϕ and ω scansθmax = 25.0°, θmin = 2.0°
Absorption correction: multi-scan
(SADABS; Sheldrick, 1996)
h = 713
Tmin = 0.627, Tmax = 0.650k = 1211
8914 measured reflectionsl = 1515
Refinement top
Refinement on F2Primary atom site location: structure-invariant direct methods
Least-squares matrix: fullSecondary atom site location: difference Fourier map
R[F2 > 2σ(F2)] = 0.044Hydrogen site location: inferred from neighbouring sites
wR(F2) = 0.102H-atom parameters constrained
S = 1.07 w = 1/[σ2(Fo2) + (0.0505P)2]
where P = (Fo2 + 2Fc2)/3
2603 reflections(Δ/σ)max = 0.001
163 parametersΔρmax = 0.81 e Å3
0 restraintsΔρmin = 0.27 e Å3
Crystal data top
C28H32N4O22+·2IV = 1485.3 (3) Å3
Mr = 710.38Z = 2
Monoclinic, P21/nMo Kα radiation
a = 11.4733 (13) ŵ = 2.15 mm1
b = 10.6692 (12) ÅT = 298 K
c = 13.1553 (15) Å0.24 × 0.22 × 0.22 mm
β = 112.725 (2)°
Data collection top
Bruker SMART CCD area-detector
diffractometer
2603 independent reflections
Absorption correction: multi-scan
(SADABS; Sheldrick, 1996)
1989 reflections with I > 2σ(I)
Tmin = 0.627, Tmax = 0.650Rint = 0.035
8914 measured reflections
Refinement top
R[F2 > 2σ(F2)] = 0.0440 restraints
wR(F2) = 0.102H-atom parameters constrained
S = 1.07Δρmax = 0.81 e Å3
2603 reflectionsΔρmin = 0.27 e Å3
163 parameters
Special details top

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 F^2^ against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F^2^, conventional R-factors R are based on F, with F set to zero for negative F^2^. The threshold expression of F^2^ > σ(F^2^) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F^2^ 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
I10.42215 (3)0.07079 (3)0.19395 (3)0.06624 (18)
O10.6771 (3)0.6579 (3)0.1484 (2)0.0520 (8)
N10.6819 (4)0.7779 (4)0.4677 (3)0.0628 (11)
N20.7451 (3)0.8096 (3)0.3355 (3)0.0503 (9)
C10.5148 (7)0.6942 (7)0.5210 (6)0.115 (2)
H1A0.47190.70750.56990.172*
H1B0.55440.61320.53490.172*
H1C0.45500.69810.44610.172*
C20.6093 (6)0.7894 (7)0.5390 (5)0.100 (2)
H2A0.66770.78600.61550.120*
H2B0.56840.87080.52660.120*
C30.6648 (5)0.8459 (5)0.3797 (4)0.0603 (13)
H3A0.60550.90960.35270.072*
C40.7780 (5)0.6961 (5)0.4802 (4)0.0723 (16)
H4A0.81070.63750.53650.087*
C50.8164 (5)0.7143 (5)0.3988 (4)0.0651 (14)
H5A0.88000.67050.38680.078*
C60.7488 (5)0.8552 (5)0.2318 (4)0.0637 (13)
H6A0.83150.83920.23070.076*
H6B0.73420.94490.22580.076*
C70.6479 (4)0.7891 (4)0.1346 (3)0.0577 (12)
H7A0.56460.80480.13450.069*
H7B0.64970.81890.06560.069*
C80.5860 (4)0.5813 (4)0.0729 (3)0.0443 (10)
C90.4887 (4)0.5346 (4)0.0997 (3)0.0432 (10)
C100.4733 (5)0.5667 (4)0.1992 (4)0.0534 (12)
H10A0.53100.62040.24950.064*
C110.3758 (5)0.5200 (5)0.2213 (4)0.0642 (13)
H11A0.36760.54170.28670.077*
C120.2873 (5)0.4392 (5)0.1468 (4)0.0655 (14)
H12A0.22050.40830.16280.079*
C130.2987 (5)0.4058 (4)0.0511 (4)0.0557 (12)
H13A0.23960.35170.00270.067*
C140.6018 (4)0.5485 (4)0.0243 (3)0.0426 (10)
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
I10.0748 (3)0.0667 (3)0.0669 (2)0.00388 (17)0.0379 (2)0.00174 (16)
O10.0521 (18)0.0522 (19)0.0433 (16)0.0023 (14)0.0090 (14)0.0128 (14)
N10.073 (3)0.064 (3)0.048 (2)0.011 (2)0.020 (2)0.013 (2)
N20.051 (2)0.052 (2)0.0430 (19)0.0073 (18)0.0127 (18)0.0170 (17)
C10.124 (6)0.129 (7)0.115 (5)0.023 (5)0.072 (5)0.023 (5)
C20.128 (6)0.118 (5)0.076 (4)0.036 (4)0.064 (4)0.031 (4)
C30.066 (3)0.056 (3)0.056 (3)0.003 (2)0.020 (3)0.012 (2)
C40.081 (4)0.067 (4)0.047 (3)0.002 (3)0.001 (3)0.005 (3)
C50.059 (3)0.067 (3)0.055 (3)0.009 (3)0.008 (3)0.013 (3)
C60.081 (4)0.060 (3)0.050 (3)0.019 (3)0.025 (2)0.011 (2)
C70.072 (3)0.050 (3)0.045 (2)0.005 (2)0.016 (2)0.007 (2)
C80.044 (3)0.045 (2)0.039 (2)0.005 (2)0.0102 (19)0.0063 (19)
C90.049 (3)0.043 (2)0.034 (2)0.012 (2)0.0113 (19)0.0012 (17)
C100.061 (3)0.055 (3)0.043 (2)0.002 (2)0.018 (2)0.007 (2)
C110.085 (4)0.069 (3)0.050 (3)0.004 (3)0.039 (3)0.007 (2)
C120.067 (3)0.071 (4)0.069 (3)0.003 (3)0.037 (3)0.005 (3)
C130.057 (3)0.052 (3)0.057 (3)0.001 (2)0.021 (2)0.007 (2)
C140.043 (2)0.040 (2)0.041 (2)0.0060 (19)0.0121 (19)0.0009 (18)
Geometric parameters (Å, º) top
O1—C81.395 (5)C6—H6A0.97
O1—C71.434 (5)C6—H6B0.97
N1—C31.315 (6)C7—H7A0.97
N1—C41.365 (6)C7—H7B0.97
N1—C21.482 (7)C8—C91.387 (6)
N2—C31.323 (6)C8—C141.402 (6)
N2—C51.368 (6)C9—C101.428 (6)
N2—C61.464 (6)C9—C14i1.432 (5)
C1—C21.437 (8)C10—C111.355 (7)
C1—H1A0.96C10—H10A0.93
C1—H1B0.96C11—C121.402 (7)
C1—H1C0.96C11—H11A0.93
C2—H2A0.97C12—C131.363 (7)
C2—H2B0.97C12—H12A0.93
C3—H3A0.93C13—C14i1.407 (7)
C4—C51.321 (7)C13—H13A0.93
C4—H4A0.93C14—C13i1.407 (7)
C5—H5A0.93C14—C9i1.432 (5)
C6—C71.525 (6)
C8—O1—C7114.0 (3)N2—C6—H6B109.7
C3—N1—C4107.4 (5)C7—C6—H6B109.7
C3—N1—C2125.7 (5)H6A—C6—H6B108.2
C4—N1—C2127.0 (5)O1—C7—C6106.3 (4)
C3—N2—C5107.7 (4)O1—C7—H7A110.5
C3—N2—C6126.0 (4)C6—C7—H7A110.5
C5—N2—C6126.0 (4)O1—C7—H7B110.5
C2—C1—H1A109.5C6—C7—H7B110.5
C2—C1—H1B109.5H7A—C7—H7B108.7
H1A—C1—H1B109.5C9—C8—O1119.0 (4)
C2—C1—H1C109.5C9—C8—C14122.8 (4)
H1A—C1—H1C109.5O1—C8—C14118.0 (4)
H1B—C1—H1C109.5C8—C9—C10122.9 (4)
C1—C2—N1114.2 (5)C8—C9—C14i119.0 (4)
C1—C2—H2A108.7C10—C9—C14i118.1 (4)
N1—C2—H2A108.7C11—C10—C9120.8 (4)
C1—C2—H2B108.7C11—C10—H10A119.6
N1—C2—H2B108.7C9—C10—H10A119.6
H2A—C2—H2B107.6C10—C11—C12120.8 (5)
N1—C3—N2109.5 (4)C10—C11—H11A119.6
N1—C3—H3A125.3C12—C11—H11A119.6
N2—C3—H3A125.3C13—C12—C11120.3 (5)
C5—C4—N1108.3 (5)C13—C12—H12A119.9
C5—C4—H4A125.8C11—C12—H12A119.9
N1—C4—H4A125.8C12—C13—C14i121.2 (4)
C4—C5—N2107.1 (5)C12—C13—H13A119.4
C4—C5—H5A126.4C14i—C13—H13A119.4
N2—C5—H5A126.4C8—C14—C13i123.0 (4)
N2—C6—C7110.0 (4)C8—C14—C9i118.2 (4)
N2—C6—H6A109.7C13i—C14—C9i118.8 (4)
C7—C6—H6A109.7
C3—N1—C2—C1102.4 (7)C7—O1—C8—C991.0 (5)
C4—N1—C2—C177.6 (7)C7—O1—C8—C1493.1 (4)
C4—N1—C3—N20.6 (5)O1—C8—C9—C103.5 (6)
C2—N1—C3—N2179.3 (4)C14—C8—C9—C10179.2 (4)
C5—N2—C3—N10.1 (5)O1—C8—C9—C14i177.3 (3)
C6—N2—C3—N1174.7 (4)C14—C8—C9—C14i1.6 (7)
C3—N1—C4—C50.9 (6)C8—C9—C10—C11179.1 (4)
C2—N1—C4—C5179.0 (5)C14i—C9—C10—C110.1 (6)
N1—C4—C5—N20.8 (6)C9—C10—C11—C120.2 (8)
C3—N2—C5—C40.5 (5)C10—C11—C12—C130.5 (8)
C6—N2—C5—C4175.2 (4)C11—C12—C13—C14i0.4 (7)
C3—N2—C6—C780.2 (6)C9—C8—C14—C13i179.1 (4)
C5—N2—C6—C793.6 (5)O1—C8—C14—C13i3.3 (6)
C8—O1—C7—C6173.3 (4)C9—C8—C14—C9i1.6 (6)
N2—C6—C7—O160.2 (5)O1—C8—C14—C9i177.3 (3)
Symmetry code: (i) x+1, y+1, z.
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
C3—H3A···I1ii0.932.893.771 (5)159
C4—H4A···I1iii0.932.973.893 (5)172
C5—H5A···I1iv0.933.053.957 (6)167
Symmetry codes: (ii) x, y+1, z; (iii) x+1/2, y+1/2, z+1/2; (iv) x+3/2, y+1/2, z+1/2.

Experimental details

Crystal data
Chemical formulaC28H32N4O22+·2I
Mr710.38
Crystal system, space groupMonoclinic, P21/n
Temperature (K)298
a, b, c (Å)11.4733 (13), 10.6692 (12), 13.1553 (15)
β (°) 112.725 (2)
V3)1485.3 (3)
Z2
Radiation typeMo Kα
µ (mm1)2.15
Crystal size (mm)0.24 × 0.22 × 0.22
Data collection
DiffractometerBruker SMART CCD area-detector
diffractometer
Absorption correctionMulti-scan
(SADABS; Sheldrick, 1996)
Tmin, Tmax0.627, 0.650
No. of measured, independent and
observed [I > 2σ(I)] reflections
8914, 2603, 1989
Rint0.035
(sin θ/λ)max1)0.595
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.044, 0.102, 1.07
No. of reflections2603
No. of parameters163
H-atom treatmentH-atom parameters constrained
Δρmax, Δρmin (e Å3)0.81, 0.27

Computer programs: SMART (Bruker, 1998), SAINT (Bruker, 1999), SHELXS97 (Sheldrick, 2008), SHELXL97 (Sheldrick, 2008), SHELXTL (Sheldrick, 2008).

Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
C3—H3A···I1i0.932.893.771 (5)159
C4—H4A···I1ii0.932.973.893 (5)172
C5—H5A···I1iii0.933.053.957 (6)167
Symmetry codes: (i) x, y+1, z; (ii) x+1/2, y+1/2, z+1/2; (iii) x+3/2, y+1/2, z+1/2.
 

Acknowledgements

This work was supported by the Startup Fund for PhDs of Natural Scientific Research of Tianjin Polytechnic University (grant No. 029312).

References

First citationBaker, M. V., Brown, D. H., Haque, R. A., Skelton, B. W. & White, A. H. (2004). J. Chem. Soc. Dalton Trans. pp. 3756–3764.  CrossRef Google Scholar
First citationBourissou, D., Guerret, O., Gabbai, F. P. & Bertrand, G. (2000). Chem. Rev. 100, 39–91.  Web of Science CrossRef PubMed CAS Google Scholar
First citationBruker (1998). SMART. Bruker AXS Inc., Madison, Wisconsin, USA.  Google Scholar
First citationBruker (1999). SAINT. Bruker AXS Inc., Madison, Wisconsin, USA.  Google Scholar
First citationCavell, K. J. & McGuinness, D. S. (2004). Coord. Chem. Rev. pp. 248–671.  Google Scholar
First citationHerrmann, W. A. & Kocher, C. (1997). Angew. Chem. Int. Ed. Engl. 36, 2162–2187.  CrossRef CAS Web of Science Google Scholar
First citationMelaiye, A., Simons, R. S., Milsted, A., Pingitore, F., Wesdemiotis, C., Tessier, C. A. & Youngs, W. J. (2004). J. Med. Chem. 47, 973–977.  Web of Science CSD CrossRef PubMed CAS Google Scholar
First citationSheldrick, G. M. (1996). SADABS. University of Göttingen, Germany.  Google Scholar
First citationSheldrick, G. M. (2008). Acta Cryst. A64, 112–122.  Web of Science CrossRef CAS IUCr Journals Google Scholar

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