Chemical insight from crystallographic disorder: structural studies of a supramolecular β-cyclodextrin/coumarin photochemical system

Abstract

From a combination of X-ray crystallography, solid-state NMR, and theoretical calculations, a model of a β-cyclodextrin (β-CD)–coumarin inclusion complex has been developed which characterizes the complex as a 2∶3 β-CD–coumarin system with the coumarin molecules located in β-CD dimer ‘reaction nano-tubes’; the model explains the reported yield for the coumarin photodimerization reaction carried out in powdered solids.

References

1. For example see: V. Ramamurthy and K. Venkatesan, Chem. Rev., 1987, 87, 433 and references therein..
2. V. Ramamurthy, in Photoprocesses of Host–Guest Complexes in the Solid State, ed. V. Ramamurthy, VCH, New York, 1991, p. 303.
3. V. Ramamurthy, Tetrahedron, 1986, 42, 5753.
4. D. O. Cowan and R. L. Drisko, Elements of Organic Photochemistry, Plenum, New York, 1976.
5. K. Gnanaguru, N. Ramasubbu, K. Venkatesan and V. Ramamurthy, J. Org. Chem., 1985, 50, 2337.
6. J. N. Moorthy and K. Venkatesan, J. Org. Chem., 1991, 56, 6957.
7. J. N. Moorthy, K. Venkatesan and R. G. Weiss, J. Org. Chem., 1992, 57, 3292.
8. Crystal data for (C₄₂H₇₀O₃₅)·(C₉H₆O₂)_1.5·12H₂O: M_r= 1570.38, monoclinic, space group C2(no. 5), a= 19.322(2), b= 24.641(3), c= 16.050(2)Å, β= 108.759(8)°, Z= 4, D_c= 1.442 g cm^–3, crystal size 0.5 × 0.4 × 0.1 mm, λ= Mo-Kα, T= 293(2) K. Data were collected on an automated Siemens P4 diffractometer with a sealed tube source. 6778 unique reflections (R_int= 0.0403) were collected to 2θ_max= 50°. The structure was solved by molecular replacement of the β-CD coordinates from an isomorphous structure. Coumarin sites were located in difference electron density maps (F_o–F_c). Coumarin molecules in orientations determined from the lattice energy calculations were refined as rigid bodies. Least-squares refinement on F² of 835 parameters was carried out using SHELXL97 (G. M. Sheldrick, SHELXL97, Program for the Refinement of Crystal Structures. University of Göttingen, Germany, 1997) and converged to a final R₁= 0.0893, wR₂= 0.2037 and GOF = 1.239 for 3369 reflections with F_o > 4σ(F_o). All non-hydrogen atoms were treated anisotropically except those of the coumarin molecules and low occupancy waters. Hydrogens on carbon atoms were generated geometrically and were fixed in a riding model. A final difference electron density map showed no distinct features with ρ_max= 0.46 and ρ_min=–0.33 e Å^–3. CCDC 182/1260. See http://www.rsc.org/suppdata/cc/1999/1275/ for crystallographic data in .cif format..
9. ¹³C MAS spectra were obtained at 7.1 T with a standard Bloch decay pulse sequence on a sample of about 200 mg of hydrated crystalline complex. The dynamics of the guest made cross-polarization ineffective. 2048 transients were averaged, with an 80 s delay between acquisitions to allow spin relaxation. The spinning speed used was 4 kHz; at this speed, no rotational sidebands were observed. Assignments for coumarin resonances were deduced by comparison with a reference solution NMR spectrum (Aldrich). Assignments for β-CD resonances were deduced by comparison with previously published results (M. J. Gidley and S. M. Bociek, J. Chem. Soc., Chem. Commun., 1986, 1223). Overlapping peaks were deconvolved to obtain integrated spectral intensities. Spectral shifts were referenced to the high frequency resonance of adamantane, and converted to a TMS scale by adding 38.56 ppm. The shifts obtained (relative to TMS), along with intensities, are as follows: δ_C 160 (3C), 154 (3C), 143 (3C), 131 (3C), 128 (3C), 124 (3C), 119 (3C), 117 (3C), 116 (3C), 104 (14C), 81 (14C), 73 (42C), 60 (14C)..
10. Lattice energy calculations were performed using the CrystalPacker module in Cerius²(Molecular Simulations). Calculations performed were single point energy calculations with coumarin molecules located at positions from the crystallographic model. For the calculations, the orientations at site B and B′ were fixed as: an antiparallel pair, a parallel pair pointing in the +c direction, and a parallel pair pointing in the –c direction. With each of the fixed arrangements at site B, the orientation at site A was varied to each of the four orientations possible, and each time the energy was calculated. The low energy arrangements displayed an antiparallel pair at site B and B′ with site A aligned parallel to either of the molecules at site B or B′..
11. T. J. Brett, S. Liu, P. Coppens and J. J. Stezowski, Chem. Commun., 1999, 551.
12. D. Mentzafos, I. M. Mavridis, G. LeBas and G. Tsoucaris, Acta Crystallogr., 1991, B47, 746.
13. The syn-HH dimer was the reported product of the study in ref. 7. We verified formation of this product in our crystals as follows: the reaction product was extracted with CH₂Cl₂ and the crystal structure of the isolated photoproduct verified that it was the syn-HH dimer. The coordinates of this structure have also been deposited..