Gossypol clathrates: Structure and thermal behavior of gossypol solvates with two picoline isomers

Abstract

Gossypol forms stable solvates with 4- and 2-picolines at room temperature. The solvates are investigated by single crystal X-ray diffraction and thermal analysis. Solvate crystals of gossypol with 4-picoline (1) have the 1:3 composition (gossypol:4-picoline) and crystallize in the P21/c space group. This substance is isostructural to a trisolvate of gossypol with pyridine. Solvate crystals of gossypol with 2-picoline (2) have the 1:4 composition (gossypol:2-picoline) and crystallize in the P-1 space group. The unit cell parameters for the investigated structures are as follows: 1 monoclinic crystals, C30H30O8·3C6H7N, a = 10.7530(1) A, b = 20.7834(3) A, c = 19.1166(2) A, β = 95.537(1)°, V = 4252.32(9) A3, M = 797.92, Z = 4, dx = 1.246 g/cm3, and R = 0.0489 for 4102 reflections; 2 triclinic crystals, C30H30O8·4C6H7N, a = 11.467(1) A b = 14.962(2) A, c = 15.570(3) A, α = 75.62(1)°, β = 69.83(1)°, γ = 79.58(1)°, V = 2414.6(7) A3, M = 891.04, Z = 2, dx = 1.226 g/cm3, and R = 0.0528 for 3779 reflections. The results of the single crystal XRD and thermal analysis confirm that gossypol with 4-picoline forms a trisolvat, and a tetrasolvate with 2-picoline. The transition from 4-picoline to 2-picoline proves to change the type of the host-guest association from one-dimensional to zero-dimensional, i.e., to lead to a new crystal structure. Desolvation of compound 2 begins at a lower temperature than that for compound 1, which is explained by their different crystal structures. Keywords: gossypol, 4-picoline, 2-picoline, clathrate formation, crystal structure.