The heterogeneous thermal transformation of the yellow to the orange crystalline form of diethyl 2,5-diaminoterephthalate. X-Ray crystal structures of the two forms

Abstract

The structures of the yellow and orange forms of diethyl 2,5-diaminoterephthalate (1-Y) and (1-O) were determined by X-ray crystallography and the rearrangement of single crystals of (1-Y) to (1-O) studied. The structural results show that the reaction involves major changes in hydrogen bonding and also significant molecular reorientation. The reaction is shown by microscopic studies to be heterogeneous with nucleation at one or more sites followed by spread of a reaction front through the crystal. The nucleation process is difficult to influence. Yellow crystals of (1-Y) are monoclinic, a= 7.937(2), b= 12.721(4), c= 7.022(3)Å, β= 118.46(2)°; the space group is P2₁/c and the structure has been refined to R 0.043 on 989 non-zero reflections. There is two-dimensional hydrogen-bonded net. Orange crystals of (1-O) are trigonal, a=b= 21.859(8), c= 6.937(3)Å; the space group is R and the structure has been refined to R 0.077 on 3 063 non-zero reflections. In both forms, the molecules lie on crystallographic centres of symmetry. The three independently oriented molecules are nearly mutually orthogonal and there is a complex three-dimensional hydrogen bonding structure.