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Volume 67 
Part 8 
Pages o288-o293  
August 2011  

Received 28 April 2011
Accepted 20 June 2011
Online 5 July 2011

Extensive hydrogen and halogen bonding, and absence of intramolecular hydrogen bonding between alcohol and nitro groups in a series of endo-nitronorbornanol compounds

aMolecular Sciences Institute, School of Chemistry, University of the Witwatersrand, Johannesburg, PO Wits 2050, South Africa
Correspondence e-mail: andreas.lemmerer@wits.ac.za

The influence of the substituent at the C2 position on the hydrogen-bonding patterns is compared for a series of five related compounds, namely (±)-3-exo,6-exo-dibromo-5-endo-hydroxy-3-endo-nitrobicyclo[2.2.1]heptane-2-exo-carbonitrile, C8H8Br2N2O3, (II), (±)-3-exo,6-exo-dibromo-6-endo-nitro-5-exo-phenylbicyclo[2.2.1]heptan-2-endo-ol, C13H13Br2NO3, (III), (±)-methyl 3-exo,6-exo-dibromo-5-endo-hydroxy-3-endo-nitrobicyclo[2.2.1]heptane-2-exo-carboxylate, C9H11Br2NO5, (IV), (±)-methyl 3-exo,6-exo-dibromo-7-diphenylmethylidene-5-endo-hydroxy-3-endo-nitrobicyclo[2.2.1]heptane-2-exo-carboxylate, C22H19Br2NO5, (V), and (±)-methyl 3-exo,6-exo-dibromo-5-endo-hydroxy-3-endo-nitro-7-oxabicyclo[2.2.1]heptane-2-exo-carboxylate, C8H9Br2NO6, (VI). The hydrogen-bonding motif in all five compounds is a chain, formed by O-H...O hydrogen bonds in (III), (IV), (V) and (VI), and by O-H...N hydrogen bonds in (II). All compounds except (III) contain a number of Br...Br and Br...O halogen bonds that connect the chains to each other to form two-dimensional sheets or three-dimensional networks. None of the compounds features intramolecular hydrogen bonding between the alcohol and nitro functional groups, as was found in the related compound (±)-methyl 3-exo,6-exo-dichloro-5-endo-hydroxy-3-endo-nitrobicyclo[2.2.1]heptane-2-exo-carboxylate, (I) [Boeyens, Denner & Michael (1984b). J. Chem. Soc. Perkin Trans. 2, pp. 767-770]. The crystal structure of (V) exhibits whole-molecule disorder.

Comment

Although hydrogen bonding between hydroxy and nitro groups is not uncommon (Desiraju, 2002[Desiraju, G. R. (2002). Acc. Chem. Res. 35, 565-573.]), intramolecular hydrogen bonding between these groups is largely confined to systems in which they find themselves in enforced proximity, as in 2-nitrophenols (Baitinger et al., 1964[Baitinger, W. F., Schleyer, P. R., Murty, T. S. S. R. & Robinson, L. (1964). Tetrahedron, 20, 1635-1647.]; Heintz et al., 2007[Heintz, A., Kapteina, S. & Verevkin, S. P. (2007). J. Phys. Chem. A, 111, 6552-6562.]; Litwinienko et al., 2009[Litwinienko, G., DiLabio, G. A., Mulder, P., Korth, H.-G. & Ingold, K. U. (2009). J. Phys. Chem. A, 113, 6275-6288.]). We have been interested in hydrogen bonding in nitronorbornanol systems for several years (Boeyens et al., 1984a[Boeyens, J. C. A., Denner, L. & Michael, J. P. (1984a). J. Chem. Soc. Perkin Trans. 2, pp. 1569-1573.]; Michael et al., 1994[Michael, J. P., Billing, D. G. & Maqutu, T. L. (1994). J. Chem. Crystallogr. 24, 311-314.]). In particular, when both groups are constrained to occupy the endo cavity of the norbornane skeleton, the likelihood of intramolecular hydrogen bonding is high, as we have found, for example, in 3-exo,6-exo-dichloro-5-endo-hydroxy-3-endo-nitrobicyclo[2.2.1]heptane-2-exo-carbonitrile, (I) (Boeyens et al., 1984b[Boeyens, J. C. A., Denner, L. & Michael, J. P. (1984b). J. Chem. Soc. Perkin Trans. 2, pp. 767-770.]). We previously determined the room-temperature crystal structure of the corresponding dibromo compound 3-exo,6-exo-dibromo-5-endo-hydroxy-3-endo-nitrobicyclo[2.2.1]heptane-2-exo-carbonitrile, (II)[link] (Blom et al., 1980[Blom, N. F., Edwards, D. M. F., Field, J. S. & Michael, J. P. (1980). J. Chem. Soc. Chem. Commun. pp. 1240-1241.]), but owing to the limitations of the techniques available at the time, we were unable to locate H atoms and to establish unambiguously whether or not the hydrogen bonding was intramolecular. We report here a redetermination of the crystal structure of compound (II)[link] at low temperature, as well as the structures of three analogous dibrominated endo-nitronorbornanols, (III)-(V), and the related 7-oxanorbornanol, (VI)[link], in order to elucidate their hydrogen-bonding patterns and to establish whether there is any intra- or intermolecular hydrogen bonding between the alcohol and nitro functionalities.

[Scheme 1]

The distances and angles within the five compounds reported (Fig. 1[link]) are generally as expected (Allen et al., 1987[Allen, F. H., Kennard, O., Watson, D. G., Brammer, L., Orpen, A. G. & Taylor, R. (1987). J. Chem. Soc. Perkin Trans. 2, pp. S1-19.]). In all five structures, hydrogen bonds play a part in controlling the supramolecular assembly of the molecules (Desiraju, [Desiraju, G. R. (1996). Acc. Chem. Res. 29, 441-449.], 2002[Desiraju, G. R. (2002). Acc. Chem. Res. 35, 565-573.]). All five compounds contain an alcohol group and a number of good hydrogen-bonding acceptor functional groups including nitro, ester and ether units as well as Br atoms. Furthermore, a number of halogen-type C-Br...A (A = Br or O; Metrangelo et al., 2005[Metrangelo, P., Neukirch, H., Pilati, T. & Resnati, G. (2005). Acc. Chem. Res. 38, 386-395.]) interactions are also present (Fig. 2[link]).

Compound (II)[link] crystallizes in the polar space group Cc. The O1-H1...N2 hydrogen bond forms a C(8) (Bernstein et al., 1995[Bernstein, J., Davis, R. E., Shimoni, L. & Chang, N. L. (1995). Angew. Chem. Int. Ed. Engl. 34, 1555-1573.]) chain along the [010] direction. Adjacent chains of this type are connected by a Br2...O2 halogen interaction along the [001] direction (Fig. 3[link]) and by a Br1...Br2 halogen interaction (Table 6[link]) along the [100] direction to form a three-dimensional network.

In compound (III)[link], the O1-H1...O2 hydrogen bond forms C(7) chains along the [010] direction, containing molecules related by the twofold screw axis along (0.5, y, 0.75) (Fig. 4[link]). Compound (III)[link] has no short Br...Br contacts and does not form a higher-dimensional network.

In the crystal structure of compound (IV)[link], C(8) chains are formed along the [1[\overline{1}]0] direction, utilizing the O1-H1...O4 hydrogen bond (Fig. 5[link]a). Adjacent chains of this type are connected to form a three-dimensional network by Br2...Br2 interactions along the [001] direction and by Br1...O1 interactions along the [010] direction (Table 6[link], and Figs. 5[link]a and 5b).

The entire molecule of compound (V)[link] is disordered over two sets of atomic positions and the two parts, labelled A and B (Fig. 1[link]), have equal site-occupancy factors. The only substantial conformational difference between the two disorder components is the orientations of the aromatic rings relative to the nitronorbornanol unit. Molecule A has torsion angles of -46.9 (15)° (C7A-C10A-C17A-C22A) and 120.9 (12)° (C7A-C10A-C11A-C16A), as compared to angles of -61.3 (14)° (C7B-C10B-C17B-C22B) and 139.4 (12)° (C7B-C10B-C11B-C16B) in molecule B. Nonetheless, the intermolecular hydrogen and halogen bonding is similar between the two molecules (Tables 4[link] and 6[link]). The O1A-H1A...O3A hydrogen bond in molecule A forms C(7) chains from alcohol atom O1A to nitro atom O3A. The chains run along the [010] direction (Fig. 6[link]), generated by the twofold screw axis in the space group P21/c. The molecules within the chains are further connected by Br2A...O3A halogen bonds (Table 6[link] and Fig. 6[link]). (V)[link] contains no Br...Br halogen bonds. The hydrogen bonding of molecule B is not shown in Fig. 6[link].

In the crystal structure of compound (VI)[link], the O1-H1...O4 hydrogen bond forms C(8) chains along the [[\overline 1]01] direction (Fig. 7[link]a). Adjacent hydrogen-bonded chains are connected by Br1...O1 interactions along the [100] direction to form sheets (Fig. 7[link]a). Two adjacent sheets are then connected by Br2...Br2 halogen bonds along [010] (Table 6[link]) to form bilayers of sheets (Fig. 7[link]b).

Compound (II)[link], which is the dibromo analogue of (I), does not contain an intramolecular O-H...O(nitro) hydrogen bond as observed in (I). Instead, it forms a C(8) hydrogen-bonded chain with the nitrile N atom as acceptor on a neighbouring molecule. Nonetheless, the O atoms of the nitro group are utilized in intermolecular interactions, in this case halogen bonding with the Br atoms to form two-dimensional sheets which are further linked into a three-dimensional network via Br...Br interactions. Compound (III)[link] has the nitrile group replaced by a phenyl group, and this seems to have an influence on the lack of any halogen bonding observed in (III)[link] because of the steric increase of the phenyl group next to one of the Br atoms. The absence of any good hydrogen-bonding acceptor at the 2-position leaves only the nitro group or the alcohol O atom as candidates and, indeed, in (III)[link], there is an intermolecular O-H...O(nitro) hydrogen bond forming C(7) chains. Similar chains are formed by (V)[link], which at the same time uses the second O atom of the nitro group in halogen bonding to strengthen the chain motif. Compounds (IV)[link] and (VI)[link] have the same intermolecular hydrogen bonding from the alcohol to the ester carbonyl group, and similar packing of the chains into larger architectures. (IV)[link] has chains connected in three dimensions by the halogen-bond interactions, whereas (VI)[link] has bilayers of hydrogen-bonded sheets using similar Br...Br and Br...O interactions. The halogen bonds observed in these compounds all have X...A distances less than the van der Waals radii sum (3.70 Å for Br...Br contacts and 3.37 Å for Br...O contacts).

[Figure 1]
Figure 1
The molecular structures and atom-labelling schemes for (a) (II)[link], (b) (III)[link], (c) (IV)[link], (d) molecule A of (V)[link], (e) molecule B of (V)[link] and (f) (VI)[link]. Displacement ellipsoids are drawn at the 50% probability level and H atoms are drawn as small spheres of arbitrary radii.
[Figure 2]
Figure 2
The three types of halogen bonding observed in this study of nitronorbornanols.
[Figure 3]
Figure 3
The C(8) hydrogen-bonded chain of (II)[link], showing the Br...O halogen-bonded interactions along the [001] direction. The Br...Br halogen bonds along the [100] direction are not shown. Atoms marked with the superscripts `i' and `ii' are at the symmetry positions (x - [{1\over 2}], y + [{1\over 2}], z) and (x + 1, -y + 1, z + [{1\over 2}]), respectively. H atoms not involved in hydrogen-bonding interactions have been omitted for clarity.
[Figure 4]
Figure 4
The C(7) hydrogen-bonded chain of (III)[link]. Atoms marked with the superscript `i' are at the symmetry position (-x + 1, y + [{1\over 2}], -z + [{3\over 2}]). H atoms not involved in hydrogen-bonding interactions have been omitted for clarity.
[Figure 5]
Figure 5
(a) The C(8) hydrogen-bonded chain of (IV)[link], as well as the Br...O halogen bonds forming a two-dimensional sheet. (b) The sheets are then connected into a three-dimensional network by Br...Br halogen bonds. Atoms marked with the superscripts `i', `ii' and `iii' are at the symmetry positions (x - 1, y + 1, z), (x, y - 1, z) and (-x + 1, -y + 2, -z + 1), respectively. H atoms not involved in hydrogen-bonding interactions have been omitted for clarity.
[Figure 6]
Figure 6
The C(7) hydrogen-bonded chain of (V)[link]. Note how the halogen bonding connects every second molecule involved in hydrogen-bonded interactions within the chain (by translation only). Only the hydrogen bonding of molecule A is shown. Molecule B has similar interactions but is not shown in the figure. Atoms marked with the superscripts `i' and `ii' are at the symmetry positions (-x + 1, y + [{1\over 2}], -z + [{1\over 2}]) and (x, y + 1, z), respectively. H atoms not involved in hydrogen-bonding interactions have been omitted for clarity.
[Figure 7]
Figure 7
(a) The C(8) hydrogen-bonded chains of (VI)[link] connected by Br...O halogen bonds to form two-dimensional sheets. (b) The sheets form bilayers through further Br...Br halogen bonding. Atoms marked with the superscripts `i', `ii' and `iii' are at the symmetry positions (x - 1, y, z - 1), (x + 1, y, z) and (-x + 2, -y, -z + 2), respectively. H atoms not involved in hydrogen-bonding interactions have been omitted for clarity.

Experimental

The syntheses and spectroscopic characterization of the five compounds (II)-(VI) by bromination of the corresponding endo-nitronorbonenes have been reported previously (Michael et al., 1991[Michael, J. P., Blom, N. F. & Glintenkamp, L. A. (1991). J. Chem. Soc. Perkin Trans. 1, pp. 1855-1862.]). In these syntheses, transannular neighbouring group participation by the nitro group during bromination of the alkene bond is responsible for the introduction of the endo-hydroxy group in a regiospecific and totally stereoselective manner. Crystals of (II)[link] were grown from methanol, (III)[link] from benzene, (IV)[link] from benzene, (V)[link] from ethyl acetate/hexane (1:1 v/v) and (VI)[link] from acetone, all by slow evaporation.

Compound (II)[link]

Crystal data
  • C8H8Br2N2O3

  • Mr = 339.98

  • Monoclinic, C c

  • a = 6.6517 (8) Å

  • b = 16.084 (2) Å

  • c = 9.8254 (14) Å

  • [beta] = 91.825 (6)°

  • V = 1050.6 (2) Å3

  • Z = 4

  • Mo K[alpha] radiation

  • [mu] = 7.70 mm-1

  • T = 173 K

  • 0.6 × 0.2 × 0.2 mm

Data collection
  • Bruker SMART 1K CCD area-detector diffractometer

  • Absorption correction: integration (XPREP; Bruker, 1999[Bruker (1999). SAINT-Plus (including XPREP). Version 6.02. Bruker AXS Inc., Madison, Wisconsin, USA.]) Tmin = 0.075, Tmax = 0.301

  • 3346 measured reflections

  • 2038 independent reflections

  • 1945 reflections with I > 2[sigma](I)

  • Rint = 0.067

Refinement
  • R[F2 > 2[sigma](F2)] = 0.045

  • wR(F2) = 0.116

  • S = 1.04

  • 2038 reflections

  • 139 parameters

  • 2 restraints

  • H atoms treated by a mixture of independent and constrained refinement

  • [Delta][rho]max = 1.20 e Å-3

  • [Delta][rho]min = -0.94 e Å-3

  • Absolute structure: Flack (1983[Flack, H. D. (1983). Acta Cryst. A39, 876-881.]), 765 Friedel pairs

  • Flack parameter: 0.003 (19)

Table 1
Hydrogen-bond geometry (Å, °) for (II)[link]

D-H...A D-H H...A D...A D-H...A
O1-H1...N2i 0.86 (12) 2.01 (12) 2.858 (8) 169 (10)
Symmetry code: (i) [x-{\script{1\over 2}}, y+{\script{1\over 2}}, z].

Compound (III)[link]

Crystal data
  • C13H13Br2NO3

  • Mr = 391.06

  • Monoclinic, P 21 /c

  • a = 15.945 (2) Å

  • b = 6.7578 (10) Å

  • c = 13.194 (2) Å

  • [beta] = 107.655 (9)°

  • V = 1354.7 (4) Å3

  • Z = 4

  • Mo K[alpha] radiation

  • [mu] = 5.99 mm-1

  • T = 173 K

  • 0.4 × 0.3 × 0.14 mm

Data collection
  • Bruker SMART 1K CCD area-detector diffractometer

  • Absorption correction: integration (XPREP; Bruker, 1999[Bruker (1999). SAINT-Plus (including XPREP). Version 6.02. Bruker AXS Inc., Madison, Wisconsin, USA.]) Tmin = 0.110, Tmax = 0.467

  • 18725 measured reflections

  • 3256 independent reflections

  • 2786 reflections with I > 2[sigma](I)

  • Rint = 0.042

Refinement
  • R[F2 > 2[sigma](F2)] = 0.022

  • wR(F2) = 0.052

  • S = 1.04

  • 3256 reflections

  • 175 parameters

  • H atoms treated by a mixture of independent and constrained refinement

  • [Delta][rho]max = 0.34 e Å-3

  • [Delta][rho]min = -0.64 e Å-3

Table 2
Hydrogen-bond geometry (Å, °) for (III)[link]

D-H...A D-H H...A D...A D-H...A
O1-H1...O2i 0.76 (3) 2.17 (3) 2.927 (2) 171 (3)
Symmetry code: (i) [-x+1, y+{\script{1\over 2}}, -z+{\script{3\over 2}}].

Compound (IV)[link]

Crystal data
  • C9H11Br2NO5

  • Mr = 373.01

  • Triclinic, [P \overline 1]

  • a = 6.7221 (2) Å

  • b = 7.7353 (3) Å

  • c = 12.1546 (5) Å

  • [alpha] = 88.296 (3)°

  • [beta] = 80.595 (3)°

  • [gamma] = 69.323 (3)°

  • V = 583.08 (4) Å3

  • Z = 2

  • Mo K[alpha] radiation

  • [mu] = 6.96 mm-1

  • T = 173 K

  • 0.28 × 0.12 × 0.04 mm

Data collection
  • Bruker SMART 1K CCD area-detector diffractometer

  • Absorption correction: integration (XPREP; Bruker, 1999[Bruker (1999). SAINT-Plus (including XPREP). Version 6.02. Bruker AXS Inc., Madison, Wisconsin, USA.]) Tmin = 0.318, Tmax = 0.770

  • 7104 measured reflections

  • 2805 independent reflections

  • 2244 reflections with I > 2[sigma](I)

  • Rint = 0.083

Refinement
  • R[F2 > 2[sigma](F2)] = 0.039

  • wR(F2) = 0.07

  • S = 0.96

  • 2805 reflections

  • 155 parameters

  • H-atom parameters constrained

  • [Delta][rho]max = 1.44 e Å-3

  • [Delta][rho]min = -1.41 e Å-3

Table 3
Hydrogen-bond geometry (Å, °) for (IV)[link]

D-H...A D-H H...A D...A D-H...A
O1-H1...O4i 0.84 1.96 2.752 (3) 157
Symmetry code: (i) x-1, y+1, z.

Compound (V)[link]

Crystal data
  • C22H19Br2NO5

  • Mr = 537.2

  • Monoclinic, P 21 /c

  • a = 15.8724 (8) Å

  • b = 9.0341 (4) Å

  • c = 15.0064 (7) Å

  • [beta] = 98.219 (2)°

  • V = 2129.71 (17) Å3

  • Z = 4

  • Mo K[alpha] radiation

  • [mu] = 3.84 mm-1

  • T = 173 K

  • 0.3 × 0.1 × 0.06 mm

Data collection
  • Bruker SMART 1K CCD area-detector diffractometer

  • Absorption correction: integration (XPREP; Bruker, 1999[Bruker (1999). SAINT-Plus (including XPREP). Version 6.02. Bruker AXS Inc., Madison, Wisconsin, USA.]) Tmin = 0.509, Tmax = 0.817

  • 17671 measured reflections

  • 5119 independent reflections

  • 3059 reflections with I > 2[sigma](I)

  • Rint = 0.078

Refinement
  • R[F2 > 2[sigma](F2)] = 0.033

  • wR(F2) = 0.057

  • S = 0.82

  • 5119 reflections

  • 467 parameters

  • 83 restraints

  • H-atom parameters constrained

  • [Delta][rho]max = 0.45 e Å-3

  • [Delta][rho]min = -0.43 e Å-3

Table 4
Hydrogen-bond geometry (Å, °) for (V)[link]

D-H...A D-H H...A D...A D-H...A
O1A-H1A...O3Ai 0.84 2.24 3.03 (2) 157
O1B-H1B...O3Bi 0.84 2.14 2.90 (3) 151
Symmetry code: (i) [-x+1, y+{\script{1\over 2}}, -z+{\script{1\over 2}}].

Compound (VI)[link]

Crystal data
  • C8H9Br2NO6

  • Mr = 374.98

  • Monoclinic, P 21 /c

  • a = 7.8071 (13) Å

  • b = 22.760 (4) Å

  • c = 6.7673 (10) Å

  • [beta] = 110.32 (1)°

  • V = 1127.7 (3) Å3

  • Z = 4

  • Mo K[alpha] radiation

  • [mu] = 7.21 mm-1

  • T = 173 K

  • 0.42 × 0.4 × 0.2 mm

Data collection
  • Bruker SMART 1K CCD area-detector diffractometer

  • Absorption correction: integration (XPREP; Bruker, 1999[Bruker (1999). SAINT-Plus (including XPREP). Version 6.02. Bruker AXS Inc., Madison, Wisconsin, USA.]) Tmin = 0.092, Tmax = 0.290

  • 12168 measured reflections

  • 2715 independent reflections

  • 2453 reflections with I > 2[sigma](I)

  • Rint = 0.085

Refinement
  • R[F2 > 2[sigma](F2)] = 0.032

  • wR(F2) = 0.076

  • S = 1.22

  • 2715 reflections

  • 158 parameters

  • H atoms treated by a mixture of independent and constrained refinement

  • [Delta][rho]max = 0.47 e Å-3

  • [Delta][rho]min = -0.92 e Å-3

Table 5
Hydrogen-bond geometry (Å, °) for (VI)[link]

D-H...A D-H H...A D...A D-H...A
O1-H1...O4i 0.89 (4) 1.94 (4) 2.774 (3) 156 (4)
Symmetry code: (i) x-1, y, z-1.

Table 6
Br...Br and Br...O geometries in four of the five title compounds (Å, °)

Compound Interaction X...X [theta]1 [theta]2 Type
(II) C3-Br1...Br2i 3.663 (2) 138 99 II
  C6-Br2...O2ii 3.205 (2) 155   III
(IV) C6-Br2...Br2iii 3.453 (4) 155 155 I
  C3-Br1...O1iv 3.017 (3) 176   III
(V) C6A-Br2A...O3Av 3.159 (2) 158   III
  C6B-Br2B...O3Bv 3.258 (2) 152   III
(VI) C6-Br2...Br2vi 3.608 (10) 149 149 I
  C3-Br1...O1vii 3.097 (5) 168   III
Symmetry codes: (i) x - [{1\over 2}], y - [{1\over 2}], z; (ii) x + 1, -y + 1, z + [{1\over 2}]; (iii) -x + 1, -y + 2, -z + 1; (iv) x, y - 1, z; (v) x, y + 1, z; (vi) -x + 2, -y, -z + 2; (vii) x + 1, y, z.

The whole-molecule disorder of (V)[link] was modelled by finding alternative positions for all the atoms in the molecule. The corresponding bonded distance and the one-angle nonbonded distances in the two disorder components were restrained to have the same values, subject to s.u. values of 0.005 and 0.01 Å, respectively. The atomic displacement parameters were restrained to be equal for each of the atom pairs C1A/C1B, C2A/C2B, C3A/C3B, C5A/C5B, O1A/O1B, C6A/C6B, C7A/C7B, C8A/C8B, C9A/C9B, O2A/O2B, O3A/O3B, C10A/C10B and C11A/C11B. Refinement of the site occupancies gave values of 0.501 (8) and 0.499 (8): the occupancies were thereafter both fixed at 0.50. For all compounds, all C-H atoms were refined using a riding model, with distances of 0.95 (aromatic), 1.00 (aliphatic CH), 0.99 (CH2) and 0.98 Å (CH3), and with Uiso(H) = 1.2Ueq(C) or 1.5Ueq(C). H atoms on O atoms which are involved in hydrogen-bonding interactions were located in difference maps for all compounds except (IV)[link] and (V)[link] (which were refined using a riding model) and their positions allowed to refine freely, with Uiso(H) = 1.5Ueq(O) for all compounds. The value of the Flack x parameter (Flack, 1983[Flack, H. D. (1983). Acta Cryst. A39, 876-881.]) for (II)[link], viz. 0.003 (19), confirms the correct orientation of the structure with respect to the two polar-axis directions in the space group Cc.

For all compounds, data collection: SMART-NT (Bruker, 1998[Bruker (1998). SMART-NT. Bruker AXS Inc., Madison, Wisconsin, USA.]); cell refinement: SAINT-Plus (Bruker, 1999[Bruker (1999). SAINT-Plus (including XPREP). Version 6.02. Bruker AXS Inc., Madison, Wisconsin, USA.]); data reduction: SAINT-Plus; 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: ORTEP-3 for Windows (Farrugia, 1997[Farrugia, L. J. (1997). J. Appl. Cryst. 30, 565.]) and DIAMOND (Brandenburg, 1999[Brandenburg, K. (1999). DIAMOND. Crystal Impact GbR, Bonn, Germany.]); software used to prepare material for publication: WinGX (Farrugia, 1999[Farrugia, L. J. (1999). J. Appl. Cryst. 32, 837-838.]) and PLATON (Spek, 2009[Spek, A. L. (2009). Acta Cryst. D65, 148-155.]).


Supplementary data for this paper are available from the IUCr electronic archives (Reference: GD3390 ). Services for accessing these data are described at the back of the journal.


Acknowledgements

This material is based upon work supported financially by the National Research Foundation, Pretoria (GUN 65559). Any opinions, findings and conclusions or recommendations expressed in this material are those of the authors and therefore the NRF does not accept any liability in regard thereto. This work was also supported by the University of the Witwatersrand, which is thanked for providing the infrastructure required to do this work.

References

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Acta Cryst (2011). C67, o288-o293   [ doi:10.1107/S0108270111024140 ]