A metallo-supramolecular double-helix containing a major and a minor groove

Abstract

Control of the microarchitecture in a metallo-supramolecular double-helical array results from inter-strand edge–face π-stacking interactions which pull the ligand strands together thereby creating two distinct helical grooves (major and minor).

References

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2. M. J. Hannon, S. Bunce, A. J. Clarke and N. W. Alcock, Angew. Chem., Int. Ed., 1998, 38, 1277.
3. M. J. Hannon, C. L. Painting, A. Jackson, J. Hamblin and W. Errington, Chem. Commun., 1997, 1807.
4. For examples of such interactions, see E.-I. Kim, S. Paliwal and C. S. Wilcox, J. Am. Chem. Soc., 1998, 120, 11 192; Y. Umezawa, S. Tsuboyama, K. Honda, J. Uzawa and M. Nishio, Bull. Chem. Soc. Jpn., 1998, 71, 1207; M. Nishio, Y. Umezawa, M. Hirota and Y. Takeuchi, Tetrahedron, 1995, 51, 8665; C. A. Hunter, Chem. Soc. Rev., 1994, 23, 101 and references therein.
5. Selected data for [Ag₂(L²)₂][PF₆]₂: MS (FAB): m/z 585 {Ag(L²)}, 1061 {Ag(L²)₂}, 1168 {Ag₂(L²)₂}, 1313 {Ag₂(L²)₂(PF₆)}. MS (ESI): m/z 584 {Ag₂(L²)₂}²⁺, 1313 {Ag₂(L²)₂(PF₆)}⁺(Found: C, 52.7; H, 3.3; N, 7.5. Calc. for Ag₂C₆₆H₄₈N₈P₂F₁₂·2H₂O: C, 53.0; H, 3.5; N, 7.5%). ¹H NMR (CD₃CN, 250 MHz, 298 K): δ9.31 (2H, s, Hⁱ), 8.78 (2H, d, J 8.1 Hz, H^3/4), 8.18 (2H, d, J 8.1 Hz, H^3/4), 8.13 (2H, d, J 8.0 Hz, H^5/8), 8.00 (2H, d, J 8.1 Hz, H^5/8), 7.70 (4H, m, H^6,7), 7.51 (4H, d, J 8.4 Hz, Ph), 7.23 (4H, d, J 8.4 Hz, Ph), 3.89 (2H, s, CH₂).; ¹H NMR (CD₂Cl₂, 400 MHz, 183 K): δ 9.30 (3H, d, J 6.4 Hz, Hⁱrac), 9.24 (2H, d, J 6.9 Hz, Hⁱmeso), 8.72 (5H, m, H^3/4rac+meso), 8.14 (5H, m, H^3/4rac+meso), 8.06 (5H, m, H^5/8rac+meso), 7.94 (2H, d, J 8.0 Hz, H^5/8meso), 7.90 (3H, d, J 8.0 Hz, H^5/8rac), 7.65 (10H, m, H^6,7rac+meso), 7.55 (3H, d, J 8.4 Hz, Ph rac), 7.43 (2H, d, J 8.4 Hz, Ph meso), 7.29 (3H, d, J 8.4 Hz, Ph rac), 7.13 (2H, d, J 8.4 Hz, Ph meso), 3.80 (3H, s, CH₂rac), 3.86 (1H, d, J 9.8 Hz, CH₂meso), 3.75 (1H, d, J 9.8 Hz, CH₂meso).
6. Crystal data for C_45.5H_36.75AgF₆N_4.25P: M= 895.88, triclinic, space group P1̄, a= 13.351(3), b= 15.931(3), c= 20.478(3)Å, α= 86.741(5), β= 83.700(5), γ= 73.329(5)°, U= 4145.8(14)ų(by least squares refinement on 5622 reflection positions), Z= 4, µ(Mo-Kα)= 0.589 mm^–1, 16667 reflections measured on a Bruker AXS SMART system, 10674 unique (R_int= 0.0539). T= 180(2) K. Absorption correction by Φ-scans; minimum and maximum transmission factors: 0.73; 0.93. The lattice contains three fully occupied (as shown by test refinement) but highly mobile benzene molecules and one acetonitrile molecule (50% occupancy). Goodness-of-fit was 0.997, R1[for 5837 reflections with I> 2σ(I)]= 0.0744, wR2 = 0.2093. Refinement used SHELXTL (G. M. Sheldrick, 1997). CCDC 182/1403. See http: //www.rsc.org/suppdata/cc/1999/2023/ for crystallographic files in .cif format.
7. Such meso- and rac-systems have also been observed by other workers. See, for example: A. Bilyk, M. M. Harding, P. Turner and T. W. Hambley, J. Chem. Soc., Dalton Trans., 1994, 2783; C. O. Dietrich-Buchecker, J. F. Nierengarten, J. P. Sauvage, N. Armaroli, V. Balzani and L. DeCola, J. Am. Chem. Soc., 1993, 115, 11 237.
8. The temperature range over which the resonances of the two isomers are distinct is insufficient for accurate determination of thermodynamic parameters. Simple modelling reveals that the box conformation cannot accommodate inter-strand π-stacking interactions. If maintained in solution, the face–edge π-stacking interactions may contribute to the enthalpic preference for the helix and the concomitant restrictions on the free rotation of the phenyl rings in the helix might contribute to the entropic preference for the meso-isomer.